TWI903037B - Surface Protective Sheets and Treatment Methods - Google Patents

Abstract

This invention provides a surface protective sheet that maintains the required adhesion even when the object to be protected is treated in a liquid while being adhered to it, and can be peeled off without damaging or deforming the adhered object. The surface protective sheet provided by this invention has an adhesion force F1 of 0.5 N/20 mm or more after immersion in warm water for 30 minutes, and the water peel force FW1 [N/20 mm] after immersion in warm water for 30 minutes is less than 50% of the adhesion force F1 [N/20 mm] after immersion in warm water for 30 minutes.

Description

Surface Protective Sheets and Treatment Methods

This invention relates to a surface protection sheet and a treatment method. This application claims priority based on Japanese Patent Application No. 2021-52064, filed March 25, 2021, and Japanese Patent Application No. 2021-151207, filed September 16, 2021, the entire contents of which are incorporated herein by reference.

It is known that techniques exist for protecting the surfaces of various articles during processing or transportation by attaching protective sheets (adhesive sheets) to them to prevent surface damage (scratches, contamination, corrosion, etc.). For example, in various processes such as chemical treatment of glass, semiconductor wafers, metal plates, etc., using etching solutions, or physical treatments such as cutting or grinding, surface protection sheets are attached to the untreated surfaces of the object to protect those surfaces. Patent 1 is an example of prior art related to protective sheets for chemical treatment. Furthermore, Patent 2 relates to water-peelable adhesive sheets. [Previous Art Documents] [Patent Documents]

Patent Document 1: Japanese Patent Application Publication No. 2015-193688 Patent Document 2: Japanese Patent Application Publication No. 2020-23656

[The problem that the invention aims to solve]

After achieving the protective purpose, the surface protective sheet is removed from the adherend (the protected object) at an appropriate time. Therefore, the surface protective sheet is required to have the adhesion necessary for protecting the protected object during the protection period, such as during chemical treatment, and to be easy to peel off from the protected object. If the peeling force on the protected object is large, for example, in the case of a thin protected object, there is a risk that the protected object may be damaged or deformed when the surface protective sheet is peeled off from the protected object due to the peeling force.

In recent years, the miniaturization and thinning of electronic devices such as smartphones, tablets, and various wearable devices (e.g., portable electronic devices) have been continuously advancing. Consequently, the semiconductor components or optical components such as glass used in these electronic devices also tend to be thinner. Therefore, the surface protective sheet used to protect the above-mentioned components must also have the property of being easy to peel off without damaging or deforming the protected object when the surface protective sheet is peeled off from the thinner protected object.

For example, the glass panel used in the aforementioned optical components can be thinned by glass slimming treatment using solutions such as hydrofluoric acid. In this glass slimming process, a surface protective sheet can be used to protect the untreated surfaces of the glass. However, due to increased peeling force or peeling patterns during the treatment, the thinned glass may crack when peeled off from the glass panel after treatment, resulting in reduced yield. In particular, the viewing glass or cover glass used in foldable or rollable displays is thinned to approximately 100 μm or less to impart flexibility. Therefore, the risk of breakage during the peeling of the surface protective sheet is even greater. Setting the peel strength of the surface protective sheet to a lower value can reduce the load applied to the adherend during peeling, thus minimizing the risk of breakage or deformation. However, this approach carries the following risks: reduced adhesion to the protected object, penetration of the protective solution into the protected area, or, in severe cases, bulging or peeling from the adherend during the protection period, failing to achieve the intended protective purpose. For thin, brittle materials such as thin glass, it is even more difficult to balance the required adhesion for protection with the ease of peeling without damaging the adherend.

This invention was created in view of the above-mentioned circumstances, and its purpose is to provide a surface protective sheet that maintains the necessary adhesion even when used in a manner where the object to be protected is treated in a liquid, and that can be peeled off without causing damage or deformation to the adhered object. Another related objective is to provide a processing method using the aforementioned surface protective sheet. [Technical Means for Solving the Problem]

According to this specification, a surface protective sheet having an adhesive surface is provided. The adhesive force F1 of the surface protective sheet after immersion in warm water for 30 minutes is 0.5 N/20 mm or more, and the water peeling force FW1 [N/20 mm] after immersion in warm water for 30 minutes is less than 50% of the adhesive force F1 [N/20 mm] after immersion in warm water for 30 minutes. Here, the adhesion force F1 after 30 minutes of warm water immersion is the peel strength F1 [N/20 mm] measured under the conditions of temperature 23°C, peel angle 180°C and speed 300 mm/min, after the surface of the alkaline glass with a water contact angle of less than 20 degrees is attached to the aforementioned bonding surface and immersed in warm water at 60°C±2°C for 30 minutes, then lifted out of the warm water and wiped off the water. Furthermore, the water peeling force FW1 after 30 minutes of warm water immersion is determined by immersing the surface of the alkaline glass with a water contact angle of less than 20 degrees in warm water at 60°C±2°C for 30 minutes, then removing it from the warm water and wiping off the water, and supplying 20 μL of distilled water between the alkaline glass and the interface so that the distilled water enters one end of the interface between the alkaline glass and the interface, under the conditions of a temperature of 23°C, a peeling angle of 180 degrees and a speed of 300 mm/min [N/20 mm].

The inventors are currently developing an adhesive sheet (water-peelable adhesive sheet) that can be easily peeled off using water or other aqueous liquids, and whose water resistance reliability during bonding is improved. According to this water-peelable adhesive sheet, by using water or other aqueous liquids for peeling, the adhesive sheet can be removed from the adhered object with minimal damage or physical load. The technology disclosed herein utilizes the aforementioned water peeling method. Specifically, the aforementioned surface protective sheet exhibits a water peeling force FW1 that decreases to less than 50% of the adhesion force F1 after 30 minutes of warm water immersion. Therefore, it can achieve peeling without damaging or deforming the adhered object (water peeling). Furthermore, surface protective sheets with an adhesion force F1 of 0.5 N/20 mm or higher after 30 minutes of warm water immersion can maintain the required adhesion even when the object being protected is treated in a liquid while being adhered to it. The surface protective sheet with an adhesion strength F1 after 30 minutes of warm water immersion, as described above, exhibits water-peelability. Furthermore, even when used in liquids (typically aqueous solutions) or warm water, it does not show a decrease in adhesion strength based on water peelability, or the decrease in adhesion strength is suppressed, thereby maintaining a close bond to the adherend. This type of surface protective sheet offers excellent protection, for example, by preventing peeling from the ends during the aforementioned liquid treatment. According to this surface protective sheet, the required adhesion can be achieved, and even when the protected object is a thin, brittle material such as thin glass, peeling can be performed without damaging the adherend.

In several preferred embodiments, the water peeling force FW1 of the surface protective sheet after 30 minutes of warm water immersion is the same as or less than the normal water peeling force FW0 [N/20 mm]. Here, the normal water peeling force FW0 is measured by bonding the surface of an alkaline glass with a water contact angle of less than 20 degrees to the bonding surface, maintaining it in an environment of 23°C and 50%RH for 1 hour, and then supplying 20 μL of distilled water between the alkaline glass and the bonding surface, allowing the distilled water to enter one end of the interface between the alkaline glass and the bonding surface. The water peeling force FW0 [N/20 mm] is measured under the conditions of temperature 23°C, peeling angle 180 degrees, and speed 300 mm/min. The surface protective sheet constructed in this way will not show an increase in water peeling force due to aging even after 30 minutes of immersion in warm water. Therefore, during the protection period, such as in liquid treatments like chemical treatments, even if exposed to temperatures higher than normal (e.g., above 40°C), the adhesion of the surface protective sheet to the adherend does not increase, or the increase in adhesion is suppressed. When peeling off the surface protective sheet, based on the required water peelability, it is easy to achieve peeling without damaging the adherend.

In several preferred embodiments, the surface protective sheet exhibits a moisture permeability of less than 24 g/( ^m² ·day) as measured by the cupping method. By designing a structure with such limited moisture permeability, even when the surface protective sheet is immersed in a liquid or comes into contact with an aqueous liquid while still attached to the substrate, the aqueous liquid does not easily penetrate to the interface between the surface protective sheet and the substrate. This prevents a decrease in adhesion based on water peelability, or suppresses the decrease in adhesion, thereby maintaining a close bond with the substrate.

Several types of surface protective sheets include an adhesive layer constituting the aforementioned adhesive surface and a substrate layer supporting the adhesive layer. With this configuration, the surface protective sheet can possess the protective function provided by the substrate layer located on the back side and the adhesion to the protected object provided by the adhesive layer. Furthermore, by selecting an appropriate substrate layer material, the characteristics of the adhesive layer (adhesion and peelability) can be stably and effectively represented.

In several preferred embodiments, the adhesive layer comprises a hydrophilic agent. Based on the adhesive layer comprising a hydrophilic agent, it is easy to obtain an adhesive that better balances adhesion and water peelability under normal conditions (normal state), and it is easy to obtain an adhesive in which the water peelability FW1 after immersion in warm water for 30 minutes is less than 50% of the adhesion F1 after immersion in warm water for 30 minutes.

In several preferred embodiments, the adhesive layer includes an adhesive preform. By including an adhesive preform in the adhesive layer, removability from the adhered object using water peeling is maintained, and the adhesion F1 after 30 minutes of warm water immersion is improved. Therefore, even when used in a state where the protected object is treated in a liquid while being adhered to it, the surface protective sheet can maintain a better adhesion to the protected object, and can become less prone to end peeling during or after the aforementioned treatment. Typically, surface protection sheets are designed to limit adhesion because they are designed to allow for the removal of the adhered material. However, according to the technology disclosed in this paper, by using water peeling technology, unlike previous surface protection sheets, an adhesive premix as an adhesion enhancer can be added, which can achieve a high level of balance between adhesion to the protected object and peelability.

In several preferred embodiments, the aforementioned adhesive layer is formed of a light-curing or solvent-based adhesive compound. In configurations having an adhesive layer formed of a light-curing or solvent-based adhesive compound, the effects of the techniques disclosed herein can be better achieved.

In several preferred embodiments, the aforementioned substrate layer comprises a resin film. By appropriately selecting a material comprising a resin film for use in the substrate layer, the properties of the adhesive layer (adhesion and water peeling force) can be stably and well represented, and the effects brought about by the technology disclosed herein can be better realized.

In several embodiments, the aforementioned substrate layer includes a layer containing inorganic materials. By employing a substrate layer containing a layer containing inorganic materials, the effects brought about by the technology disclosed herein can also be achieved.

In several preferred embodiments, the thickness (total thickness) of the surface protective sheet is 20–100 μm. Surface protective sheets with the above-mentioned total thickness tend to provide good protective functions. For example, they tend to easily provide protection against the penetration of pharmaceutical solutions.

The surface protective sheet disclosed herein is suitable, for example, for use in the process of chemically and/or physically treating glass or semiconductor wafers in a liquid. In the aforementioned applications, the surface protective sheet disclosed herein possesses the adhesion required to protect the object being protected during the aforementioned treatment, and upon peeling after treatment, it allows for smooth peeling of the glass or semiconductor wafer (the object being protected, the adhered material) using water. Even when the object being protected is a thin, brittle material such as thin glass, the surface protective sheet with the aforementioned water-peeling power after immersion in warm water can achieve peeling without damaging the object being protected, based on its water-peeling properties. For example, in the process of thinning glass or semiconductor wafers as described above, the protective object during peeling is thinner than during attachment, increasing the risk of damage. By utilizing the surface protective sheet disclosed herein for such applications, both excellent adhesion-based protection and excellent peelability (water-peelability) that prevents damage to the protected object can be achieved.

Furthermore, according to this specification, a treatment method is provided. This treatment method includes: a step of attaching a surface protective sheet to the surface of a treatment object having a water contact angle of 20 degrees or less; a step of treating the treatment object to which the surface protective sheet is attached, wherein the treatment object comes into contact with a liquid during the treatment; and a step of removing the surface protective sheet from the treatment object (treated object) by water peeling. Moreover, the water peeling force FW1 [N/20 mm] of the surface protective sheet after 30 minutes of warm water immersion is less than 50% of the adhesion force F1 [N/20 mm] after 30 minutes of warm water immersion. [Adhesion Strength F1 after 30-minute Warm Water Immersion] The aforementioned adhesion strength F1 after 30-minute warm water immersion is the peel strength [N/20 mm] measured under the following conditions: the adhesive surface of an alkaline glass surface with a water contact angle of less than 20 degrees is immersed in warm water at 60℃±2℃ for 30 minutes, then removed from the warm water and wiped dry; the surface is then peeled at a temperature of 23℃, a peel angle of 180 degrees, and a peel speed of 300 mm/min. [Water Peeling Force FW1 After 30-Minute Warm Water Immersion] The aforementioned water peeling force FW1 after 30-minute warm water immersion is determined by immersing the surface of an alkaline glass substrate with a surface having a water contact angle of less than 20 degrees, onto the bonding surface of the substrate, in warm water at 60℃±2℃ for 30 minutes. After being removed from the warm water and the water was wiped off, 20 μL of distilled water was supplied between the alkaline glass and the bonding surface, allowing the distilled water to penetrate to one end of the interface between the alkaline glass and the bonding surface. The water peeling force [N/20 mm] was measured under the conditions of a temperature of 23℃, a peeling angle of 180 degrees, and a peeling speed of 300 mm/min. According to the above method, by using a surface protective sheet that meets the aforementioned characteristics (FW1≦F1×0.5), when the object to be treated with the surface protective sheet attached is placed in a liquid sample for treatment, the portion with the surface protective sheet attached can be protected while the target treatment is performed. Furthermore, the surface protective sheet is easily removed from the treated object using water peeling after treatment. Therefore, even when the adhered object is a thin, brittle material such as thin glass, removal can be achieved without damaging the adhered object during peeling.

In several preferred embodiments, the liquid is an aqueous solution. The techniques disclosed herein are well utilized for the treatment of aqueous solutions.

In several preferred embodiments, the surface protective sheet has an adhesive layer comprising an adhesive preform. By including an adhesive preform in the adhesive layer, removability from the adhered object using water peeling is maintained, and the adhesion F1 after 30 minutes of warm water immersion is improved. Therefore, in embodiments where the object is treated in a liquid while being adhered to, the surface protective sheet adheres sufficiently to the protective surface of the object, firmly protecting the protective surface.

The preferred embodiment of this invention is described below. Furthermore, for matters necessary for implementing this invention other than those specifically mentioned in this specification, practitioners can understand them based on the instructions for implementation of the invention recorded in this specification and the technical common sense used when filing an application. This invention can be implemented based on the content disclosed in this specification and the technical common sense in the field. Also, in the following figures, components and parts that perform the same function are sometimes labeled with the same symbols for explanation, and sometimes repeated explanations are omitted or simplified. Furthermore, the embodiments described in the figures are standardized for the purpose of clearly illustrating this invention and may not accurately represent the dimensions or scale of the actual product provided.

<Example of Surface Protective Sheet Composition> Figure 1 shows a cross-sectional structure of a surface protective sheet of one form. As shown in Figure 1, the surface protective sheet 1 is a single-sided adhesive sheet having an adhesive surface 1A, and an adhesive layer 20 is provided on one side 10A of a sheet-like substrate layer (support substrate) 10. The surface protective sheet 1 is used to attach the adhesive layer 20A, which serves as its adhesive surface 1A, to the adhered object (the object to be protected). The back side 10B of the substrate layer 10 (the side opposite to one side 10A) is also the back side 1B of the surface protective sheet 1, forming the outer surface of the surface protective sheet 1. Before use (i.e., before being attached to the substrate), the surface protection sheet 1 can be in the form of a surface protection sheet 50 with a peelable liner, where the bonding surface 1A is protected by a peelable liner 30 that forms a peelable surface from at least the adhesive layer 20 side. Alternatively, it can be a surface protection sheet in which the other side (back side) 10B of the substrate layer 10 becomes the peelable surface, and the adhesive layer 20 abuts against the back side because the surface protection sheet 1 is rolled into a roll, thereby protecting its surface (bonding surface 1A).

Furthermore, as shown in Figure 2, the substrate layer 10 in the surface protection sheet 2 can also have a multi-layer structure. In this embodiment, the surface protection sheet 2 has an adhesive layer 20 disposed on one side 10A of the sheet-like substrate layer (supporting substrate) 10, and the substrate layer 10 has a laminated structure of a first layer 11 and a second layer 12. Specifically, the substrate layer 10 has a first layer 11 as the main layer of the substrate layer 10, and a second layer 12 constituting one side surface (back surface) 10B of the substrate layer 10. In this embodiment, the second layer 12 is a layer containing inorganic materials. The adhesive layer 20 is closely attached to the side surface 10A of the first layer 11 of the substrate layer 10. Before use (i.e., before being attached to the substrate), the surface protection sheet 2 can be in the form of a surface protection sheet 50 with a peelable liner, wherein the contact surface 2A is protected by a peelable liner 30 that forms a peelable surface from at least the adhesive layer 20 side. Alternatively, it can be a surface protection sheet in the form where the other side (back side) 10B of the substrate layer 10 becomes the peelable surface, and the adhesive layer 20 abuts against the back side because the surface protection sheet 2 is rolled into a roll, thereby protecting its surface.

<Characteristics of Surface Protective Sheets> (Adhesion F1 after 30-minute warm water immersion) In several samples, the adhesion F1 of the surface protective sheet after 30-minute warm water immersion is 0.5 N/20 mm or higher. Surface protective sheets that meet the above characteristics maintain the required adhesion even when used in samples where the object to be protected is treated in a liquid while being adhered to it. For example, even when used in a pharmaceutical solution (typically an aqueous solution) or warm water, there is no reduction in adhesion based on water peelability, or the reduction in adhesion is suppressed, thereby maintaining a close bond to the adhered object. Such surface protective sheets can be considered superior in terms of protection, for example, by not peeling off from the ends during the aforementioned liquid treatment. In several preferred embodiments, the adhesion force F1 after immersion in warm water for 30 minutes is preferably 1.0 N/20 mm or more, more preferably 1.5 N/20 mm or more, and even more preferably 2.0 N/20 mm or more, or 2.5 N/20 mm or more, or 3.0 N/20 mm or more (e.g., 3.5 N/20 mm or more). A higher adhesion force F1 after immersion in warm water for 30 minutes tends to maintain higher adhesion reliability, even when used in embodiments treated with chemicals or warm water. In several preferred embodiments, the adhesion force F1 after immersion in warm water for 30 minutes is 4.0 N/20 mm or more, or 5.0 N/20 mm or more, or 6.0 N/20 mm or more (e.g., 7.5 N/20 mm or more). The upper limit of the adhesion force F1 after 30 minutes of warm water immersion can be appropriately set according to the required adhesion, and is therefore not limited to a specific range. For example, it can be approximately 15 N/20 mm or less, or approximately 10 N/20 mm or less, or approximately 5 N/20 mm or less.

The adhesion force F1 after 30 minutes of warm water immersion is the peel strength [N/20 mm] measured at a temperature of 23°C, a peel angle of 180°C, and a speed of 300 mm/min after the surface of an alkaline glass with a water contact angle of less than 20 degrees is bonded to the surface protective sheet. The surface is then immersed in warm water at 60°C ± 2°C for 30 minutes, removed from the warm water, and the water is wiped off. More specifically, the adhesion force F1 after 30 minutes of warm water immersion can be measured using the method described in the embodiments below.

(Reduction rate of water peeling force FW1/F1 after 30-minute warm water immersion) One of the characteristics of the surface protection sheet disclosed in this article is that the water peeling force FW1 [N/20 mm] after 30-minute warm water immersion is less than 50% of the adhesion force F1 [N/20 mm] after 30-minute warm water immersion. In other words, the characteristic of the surface protection sheet is that the reduction rate of water peeling force [%] after 30-minute warm water immersion, expressed by formula: FW1/F1×100, is less than 50%. When a surface protective sheet, after being soaked in warm water for 30 minutes, has a water peeling force FW1 reduced to less than 50% of the adhesion force F1 after 30 minutes of warm water soaking, is attached to the object being protected and, as described above, has an adhesion force above a certain value after 30 minutes of warm water soaking, and can be peeled off without damaging or deforming the object being protected (water peeling). More specifically, by using water-based liquids such as water, the surface protective sheet can be easily peeled off from the object being protected. For example, by supplying a small amount of aqueous liquid between the protected object and the adhesive layer, the aqueous liquid penetrates to the interface between the protected object and the adhesive layer, forming the starting point for peeling. This significantly reduces the peel strength of the adhesive layer from the protected object. Utilizing this property, the surface protective sheet can be peeled off without damaging or deforming the protected object by using water-based peeling with an aqueous liquid such as water. Such a surface protective sheet can provide the necessary adhesion for protection, and even when the protected object is a thin, brittle material such as thin glass, peeling can be achieved without damaging the adhered object. In several embodiments, the reduction rate of water peeling force after immersion in warm water for 30 minutes is preferably 30% or less, more preferably 20% or less, further preferably 10% or less, and even more preferably 5% or less (e.g., 3% or less). Surface protective sheets exhibiting this reduction rate of water peeling force after immersion in warm water for 30 minutes can achieve a better balance between adhesion reliability during protection and ease of peeling. In several preferred embodiments, the reduction rate of water peeling force after immersion in warm water for 30 minutes is 2% or less, or 1.5% or less, or 1.0% or less. Theoretically, the lower limit of the reduction in water peeling force after 30 minutes of warm water soaking is 0%, but in practice it can be more than 1% (e.g., more than 2%).

(Water peeling force FW1 after 30-minute warm water immersion) The water peeling force FW1 after 30-minute warm water immersion is not particularly limited to the range showing the rate of reduction in water peeling force after 30-minute warm water immersion. For example, the water peeling force FW1 after 30-minute warm water immersion may be less than 0.5 N/20 mm, preferably less than 0.4 N/20 mm, and most preferably less than about 0.3 N/20 mm. In several preferred embodiments, the water peeling force FW1 after 30-minute warm water immersion may be less than 0.2 N/20 mm, less than 0.15 N/20 mm, or less than 0.10 N/20 mm. The surface protective sheet exhibiting the above-mentioned water peeling force FW1 after 30 minutes of warm water immersion demonstrates good water peelability even after treatment with liquids such as pharmaceuticals or warm water. The lower limit of the water peeling force FW1 after 30 minutes of warm water immersion can be appropriately set to achieve the desired water peelability and is not limited to a specific range. The lower limit of the water peeling force FW1 after 30 minutes of warm water immersion can be 0.0 N/20 mm, or it can be 0.01 N/20 mm or higher (e.g., 0.05 N/20 mm or higher).

The water peeling force FW1 after 30 minutes of warm water immersion is determined by immersing the surface of an alkaline glass with a surface having a water contact angle of less than 20 degrees, on the interface between the alkaline glass and the surface protective sheet, in warm water at 60°C ± 2°C for 30 minutes. After being removed from the warm water and the water adhering thereto, 20 μL of distilled water is supplied between the alkaline glass and the interface, allowing the distilled water to enter one end of the interface between the alkaline glass and the interface. The water peeling force [N/20 mm] is measured under the conditions of a temperature of 23°C, a peeling angle of 180 degrees, and a peeling speed of 300 mm/min. More specifically, the water peeling force FW1 after 30 minutes of warm water immersion can be measured using the method described in the embodiments below.

Furthermore, among several samples, the surface protective sheet preferably exhibits a water peeling force FW1 after immersion in warm water for 30 minutes that is the same as or less than the normal water peeling force FW0 described later. The surface protective sheet thus constructed will not experience an increase in water peeling force due to aging even after immersion in warm water for 30 minutes. Therefore, during the protection period, such as in liquid treatments like chemical treatments, even if exposed to temperatures higher than normal (e.g., above approximately 40°C), the adhesion of the surface protective sheet to the adherend will not increase, or the increase in adhesion will be suppressed. When peeling the surface protective sheet, based on the required water peelability, peeling without damaging the adherend can be easily achieved. After immersion in warm water for 30 minutes, the water peeling force FW1 can be less than 70%, less than 50%, less than 30%, or less than 10% of the normal water peeling force FW0. There is no particular limitation on the water peeling force FW1 after immersion in warm water for 30 minutes; it can be more than 0% of the normal water peeling force FW0, or more than 1% (e.g., more than 3%).

(Adhesion F2 after 1 hour of warm water immersion) Among several samples, the surface protective sheet preferably has an adhesion F2 of 0.5 N/20 mm or more after 1 hour of warm water immersion. Surface protective sheets that meet the above characteristics can better maintain the required adhesion even when used in samples where the object to be protected is treated in a liquid while being adhered to it. For example, even when used in pharmaceutical solutions (typically in the form of aqueous solutions) or warm water, it is less likely to experience a decrease in adhesion due to water peeling, and peeling from the ends is less likely to occur during the aforementioned liquid treatment. In several preferred embodiments, the adhesion force F2 after 1 hour of warm water immersion is 1.0 N/20 mm or more, or 1.5 N/20 mm or more, or 2.0 N/20 mm or more, or 2.5 N/20 mm or more (e.g., 3.0 N/20 mm or more). There is a tendency that the greater the adhesion force F2 after 1 hour of warm water immersion, the easier it is to maintain excellent adhesion reliability, even when used in embodiments treated with chemicals or warm water. The upper limit of the adhesion force F2 after 1 hour of warm water immersion can be appropriately set according to the required adhesion, and is therefore not limited to a specific range; for example, it can be about 15 N/20 mm or less, or about 10 N/20 mm or less, or about 5 N/20 mm or less.

The adhesion force F2 after 1 hour of warm water immersion is the peel strength [N/20 mm] measured at a temperature of 23°C, a peel angle of 180°C, and a speed of 300 mm/min after the surface of an alkaline glass with a water contact angle of less than 20 degrees is bonded to the surface of a protective sheet. The surface is immersed in warm water at 60°C ± 2°C for 1 hour, then removed from the warm water and wiped dry. More specifically, the adhesion force F2 after 1 hour of warm water immersion can be measured using the method described in the embodiments below.

(Reduction rate of water peeling force after 1 hour of immersion in warm water FW2/F2) Furthermore, in several samples, the surface protective sheet preferably has a water peeling force FW2 [N/20 mm] after 1 hour of immersion in warm water that is less than 50% of the adhesion force F2 [N/20 mm] after 1 hour of immersion in warm water. In other words, the surface protective sheet preferably has a water peeling force reduction rate [%] of less than 50% as expressed by the formula: FW2/F2×100 after 1 hour of immersion in warm water. When a surface protective sheet, after being soaked in warm water for one hour, has a water peeling force FW2 reduced to less than 50% of the adhesion force F2 after one hour of soaking in warm water, is attached to the object being protected, it exhibits an adhesion force above a certain value after one hour of soaking in warm water, as described above, and can achieve peeling without damaging or deforming the object being protected (water peeling). More specifically, during the above-mentioned peeling, by using water-based liquids such as water for water peeling, the surface protective sheet can be peeled off without damaging or deforming the object being protected. In several preferred embodiments, the reduction rate of water peeling force after 1 hour of immersion in warm water is preferably 30% or less, more preferably 20% or less, and even more preferably 10% or less (e.g., 1% or less). Surface protective sheets exhibiting this reduction rate of water peeling force after 1 hour of immersion in warm water can maintain both adhesion reliability during protection and ease of peeling even under harsher conditions such as prolonged immersion in liquid. Theoretically, the lower limit of the reduction rate of water peeling force after 1 hour of immersion in warm water is 0%, but practically it can be approximately 1% or more (e.g., 2% or more).

(Water peeling force FW2 after 1 hour of warm water immersion) The water peeling force FW2 after 1 hour of warm water immersion is not particularly limited. For example, it can be less than 0.5 N/20 mm, appropriately less than 0.4 N/20 mm, and preferably less than about 0.3 N/20 mm. In several preferred embodiments, the water peeling force FW2 after 1 hour of warm water immersion can be less than 0.2 N/20 mm, less than 0.15 N/20 mm, or less than 0.10 N/20 mm. This demonstrates that the surface protective sheet with the above-mentioned water peeling force FW2 after 1 hour of warm water immersion exhibits good water peeling properties even after treatment with liquids such as pharmaceuticals or warm water. The lower limit of the water peeling force FW2 after 1 hour of warm water immersion can be appropriately set to achieve the desired water peeling performance, and is not limited to a specific range. The lower limit of the water peeling force FW2 after 30 minutes of warm water immersion can be 0.0 N/20 mm, or it can be 0.01 N/20 mm or higher (e.g., 0.03 N/20 mm or higher).

The water peeling force FW2 after 1 hour of warm water immersion is determined by immersing the surface of an alkaline glass with a surface having a water contact angle of less than 20 degrees, on the interface between the alkaline glass and the surface protective sheet, in warm water at 60°C ± 2°C for 1 hour. After being removed from the warm water and the water adhering thereto, 20 μL of distilled water is supplied between the alkaline glass and the interface, allowing the distilled water to enter one end of the interface between the alkaline glass and the interface. The water peeling force [N/20 mm] is measured under the conditions of a temperature of 23°C, a peeling angle of 180 degrees, and a peeling speed of 300 mm/min. More specifically, the water peeling force FW2 after 1 hour of warm water immersion can be measured using the method described in the embodiments below.

Furthermore, among the various samples, the surface protective sheet preferably exhibits a water peeling force FW2 after immersion in warm water for 1 hour that is the same as or less than the normal water peeling force FW0 described later. The surface protective sheet thus constructed will not experience an increase in water peeling force due to aging even after immersion in warm water for 1 hour. Therefore, during the protection period, such as in liquid treatments like chemical treatments, even with prolonged exposure to temperatures above room temperature (e.g., above approximately 40°C), the adhesion of the surface protective sheet to the adherend does not increase, or the increase in adhesion is suppressed. When peeling off the surface protective sheet, based on the required water peelability, peeling can be easily achieved without damaging the adherend. After 1 hour of warm water soaking, the water peeling force FW2 can be less than 70%, less than 50%, less than 30%, or less than 10% of the normal water peeling force FW0. There is no particular limitation on the water peeling force FW2 after soaking in warm water for 1 hour. It can be more than 0% of the normal water peeling force FW0, or more than 1%.

(Normal Adhesion Force F0) In several embodiments, the surface protective sheet preferably has a normal adhesion force F0 of 0.5 N/20 mm or higher. Surface protective sheets with a normal adhesion force F0 of a certain value or higher tend to exhibit good adhesion to the protected object. In several preferred embodiments, the normal adhesion force F0 is 1.0 N/20 mm or higher, more preferably 3.0 N/20 mm or higher (e.g., exceeding 3.0 N/20 mm), further preferably 5.0 N/20 mm or higher (e.g., exceeding 5.0 N/20 mm), and may also be 7.0 N/20 mm or higher, or may be 10 N/20 mm or higher. The higher the normal adhesion force F0, the easier it is to obtain higher adhesion reliability. According to the technology disclosed herein, even when a surface protective sheet is attached to the object being protected with high adhesion, it can be peeled off smoothly using water without damaging or deforming the object. Therefore, compared to previous surface protective sheets that achieved peelability by limiting adhesion, the adhesion force (normal adhesion force F0) can be set higher. This means that even in harsher environments, sufficient protection can be ensured due to the higher adhesion reliability, which is beneficial in practical applications. The upper limit of the normal bonding force F0 can be appropriately set according to the required bonding strength, and is therefore not limited to a specific range. For example, it can be less than approximately 20 N/20 mm, less than approximately 15 N/20 mm, less than approximately 10 N/20 mm, or less than approximately 6 N/20 mm.

The normal adhesion force F0 is the peel strength [N/20 mm] measured at 23°C, 50%RH, on the bonding surface of an alkaline glass having a water contact angle of less than 20 degrees with a surface protective sheet, after being kept in an environment of 23°C and 50%RH for 1 hour, at a peeling angle of 180 degrees and a peeling speed of 300 mm/min. More specifically, the normal adhesion force F0 can be measured using the method described in the embodiments below.

(Reduction rate of normal water peeling force FW0/F0) Furthermore, in several samples, the surface protective sheet preferably has a normal water peeling force FW0 [N/20 mm] of 50% or less of the aforementioned normal adhesion force F0 [N/20 mm]. In other words, the surface protective sheet preferably has a normal water peeling force reduction rate [%] of 50% or less, expressed by the formula: FW0/F0×100. A surface protective sheet satisfying this characteristic can adhere well to the substrate, and during peeling, it can be easily peeled off from the protected object by using water-based liquids such as water. According to this type of surface protective sheet, the required adhesion can be achieved, and peeling can be performed without damaging the adhered substrate. In several preferred embodiments, the reduction rate of the above-mentioned normal water peel force is 30% or less, more preferably 20% or less, even more preferably 10% or less, and may also be 5% or less (e.g., 3% or less). Surface protective sheets exhibiting this reduction rate of normal water peel force provide a better balance between adhesion reliability during protection and ease of peeling. The lower limit of the above-mentioned reduction rate of normal water peel force is theoretically 0%, but in practice it can also be about 1% or more (e.g., 2% or more).

(Normal Water Peeling Force FW0) The above-mentioned normal water peeling force FW0 is not particularly limited. For example, it may be less than 3 N/20 mm, less than 1.5 N/20 mm, or about 1.0 N/20 mm or less. Surface protective sheets that meet the above-mentioned water peeling characteristics can easily peel off the adhesive layer by applying water or other aqueous liquids to the interface between the surface protective sheet and the protected object. In several preferred embodiments, the normal water peeling force FW0 may be less than 0.7 N/20 mm, less than 0.5 N/20 mm, or less than 0.3 N/20 mm (e.g., less than 0.1 N/20 mm). The aforementioned lower limit of the normal water stripping force FW0 can be appropriately set to achieve the desired water stripping performance and is not limited to a specific range. The lower limit of the normal water stripping force FW0 can be 0.0 N/20 mm, or it can be 0.01 N/20 mm or higher (e.g., 0.1 N/20 mm or higher).

The normal water peel force FW0 is measured at 23°C and 50%RH on the interface between the alkaline glass and the surface protective sheet, after being kept at 23°C and 50%RH for 1 hour. Then, 20 μL of distilled water is supplied between the alkaline glass and the interface, allowing the distilled water to enter one end of the interface. The water peel force [N/20 mm] is measured at 23°C, a peel angle of 180°, and a peeling speed of 300 mm/min. More specifically, the normal water peel force FW0 can be measured using the method described in the embodiments below.

(Underwater Peel-Off Force) While not specifically limited, in several samples, the underwater peel-off force of the surface protection sheet is preferably 0.2 N/10 mm or higher. This underwater peel-off force was measured in water at room temperature (23–25°C) under conditions of a peel angle of 20 degrees and a tensile speed of 1000 mm/min. Surface protection sheets meeting this characteristic tend to exhibit excellent resistance to end peeling. External forces such as vibrations that may cause end peeling of the surface protection sheet during handling and other manufacturing processes can be considered as high-speed peeling loads applied to the protected object at relatively small angles. The surface protective sheet exhibits a peeling force of 0.2 N/10 mm or more at a peeling angle of 20 degrees and a peeling speed of 1000 mm/min. Therefore, even when the object being protected is subjected to external forces such as vibration during the process of treating the object in a liquid such as medicine or water while the surface protective sheet is attached to it, it can still exhibit excellent anti-end peeling properties against such external forces. From the perspective of improving end-peel resistance, the aforementioned underwater initial peel force is preferably 0.3 N/10 mm or more, more preferably 0.5 N/10 mm or more, and even more preferably 0.6 N/10 mm or more (e.g., 0.7 N/10 mm or more). There is no particular limitation on the upper limit of the aforementioned underwater initial peel force; for example, it can be 3 N/10 mm or less, or 2 N/10 mm or less (e.g., 1 N/10 mm or less). The aforementioned underwater initial peel force can be preferably achieved primarily by setting the 25° bending stiffness value of the surface protective sheet to a specific range. Furthermore, the aforementioned underwater initial peel force can also be improved by the composition of the adhesive (e.g., the use, amount, and type of adhesive additive). More specifically, the aforementioned in-water stripping force can be measured using the method described in the embodiments below.

(Humidity) Among several samples, the surface protective sheet preferably has a humidity permeability of 24 g/( ^m² ·day) or less, as measured by the cupping method. By configuring it with such a limited humidity permeability, even when the surface protective sheet is immersed in a liquid or comes into contact with an aqueous liquid while still attached to the substrate, the aqueous liquid does not easily penetrate to the interface with the substrate, and there is no reduction in adhesion based on water peelability, or the reduction in adhesion is suppressed. As a result, the adhesion to the substrate is maintained, and the surface protective sheet can maintain a close bond with the substrate. Such a surface protective sheet can be used in liquid treatments, such as pharmaceutical processing, without peeling from its ends. In several preferred embodiments, the moisture permeability of the surface protective sheet is approximately 20 g/( ^m² ·day) or less, more preferably approximately 16 g/( ^m² ·day) or less, even more preferably approximately 12 g/( ^m² ·day) or less, particularly preferably approximately 8 g/( ^m² ·day) or less, and may also be approximately 5 g/( ^m² ·day) or less, for example, approximately 3 g/( ^m² ·day) or less. Furthermore, if the moisture permeability is excessively low when the surface protective sheet is exposed to heat such as warm water, there is a risk that the aging caused by the heating may prevent it from effectively exhibiting water peelability. From this perspective, in several samples, the moisture permeability of the surface protective sheet is preferably 1 g/(m ²・day) or more, more preferably about 3 g/(m ²・day) or more, even more preferably more than 5 g/(m ²・day), and for example, more than 6 g/(m ²・day).

More specifically, the moisture permeability of the surface protective sheet can be, for example, 23 g/( ^m² ·day) or less, 22 g/( ^m² ·day) or less, 21 g/( ^m² ·day) or less, 20 g/( ^m² ·day) or less, 19 g/( ^m² ·day) or less, 18 g/( ^m² ·day) or less, 17 g/( ^m² ·day) or less, 16 g/( ^m² ·day) or less, 15 g/( ^m² ·day) or less, 14 g/( ^m² ·day) or less, 13 g/( ^m² ·day) or less, 12 g/( ^m² ·day) or less, 11 g/( ^m² ·day) or less, 10 g/( ^m² ·day) or less, 9 g/( ^m² ·day) or less, 8 ... g/(m ²・day) or more or less, 7 g/(m ²・day) or more or less, 6 g/(m ²・day) or more or less, 5 g/(m ²・day) or more or less, 4 g/(m ²・day) or more or less, 3 g/(m ²・day) or more or less, 2 g/(m ²・day) or more or less, or 1 g/(m ²・day) or more or less.

The aforementioned moisture permeability of the surface protective sheet can be obtained by selecting an appropriate material with opacity or low moisture permeability (typically, a substrate). More specifically, the moisture permeability of the surface protective sheet can be measured using the methods described in the embodiments described later.

(25°C Bending Stiffness Value) In several samples, the surface protective sheet preferably has a bending stiffness value (25°C bending stiffness value) within the range of 1.0× ^10⁻⁶ to 1.0× ^10⁻² Pa・^m³ . A surface protective sheet that meets this characteristic will not easily peel off from its ends even when subjected to external forces such as vibration during a process in which the object being protected is treated in a liquid such as a pharmaceutical solution or water while the surface protective sheet is attached to it. Specifically, by giving the surface protection sheet a specific range of stiffness (25°C bending stiffness), the stress (peeling stress) against external forces such as vibration that could cause end peeling of the surface protection sheet during the aforementioned manufacturing process is increased. Even when external forces such as vibration are applied during the aforementioned process, end peeling can be prevented or the risk of end peeling can be reduced. Furthermore, by setting the aforementioned 25°C bending stiffness value to a specific value (e.g., ^10⁻² Pa· ^m³ or less), the surface protection sheet possesses stiffness suitable for surface protection applications, easily achieving good peeling operability and handling. There is also a tendency to improve the surface tracking of the protected object.

From the perspective of preventing end-stripping, the aforementioned 25°C bending stiffness value D can be 5.0 × ^10⁻⁶ Pa· ^m³ or more, preferably 1.0 × ^10⁻⁵ Pa· ^m³ or more, even more preferably 5.0 × ^10⁻⁵ Pa· ^{m³ or} more, and further preferably 1.0 × ^10⁻⁴ Pa· ^m³ or more, or it can also be 3.0 × ^10⁻⁴ Pa· ^m³ or more. From the perspective of preventing end-stripping, peeling workability, and operability, the aforementioned 25°C bending stiffness value D is preferably 5.0 × ^10⁻³ Pa· ^m³ or less, more preferably 1.0 × ^10⁻³ Pa· ^m³ or less, even more preferably 5.0 × ^10⁻⁴ Pa· ^m³ or less, or it can also be 1.0 × ^10⁻⁵ Pa· ^m³ or less. The aforementioned 25°C bending stiffness value D is relatively low within a certain range, which is advantageous in terms of improving the surface tracking of the protected object.

The aforementioned 25°C bending stiffness value D [Pa・^m³ ] is calculated using the formula: D＝Eh³/12(1－ ^ν² ), where the thickness of the substrate layer is h [m], the Poisson's ratio of the substrate is ν, and the tensile modulus of elasticity of the surface protection sheet at 25°C (25°C tensile modulus of elasticity) is E [Pa ^] . Furthermore, the bending stiffness value of the adhesive layer is very small compared to that of the substrate layer; therefore, the bending stiffness of the surface protection sheet can depend on the bending stiffness of the substrate layer. Therefore, in this specification, the bending stiffness value D of the surface protection sheet refers to the value converted into the cross-sectional area per unit of the substrate layer constituting the surface protection sheet. The cross-sectional area of the substrate layer is calculated based on the thickness of the substrate layer. The thickness h of the substrate layer is assumed to be the value obtained by subtracting the thickness of the adhesive layer from the measured thickness of the surface protection sheet. Poisson's ratio ν is a value determined by the material of the substrate layer (dimensionless), and when the material is resin, 0.35 is typically used as the value of ν. The aforementioned 25°C bending stiffness value D [Pa・m ³ ] can be obtained by substituting the 25°C tensile modulus of elasticity E [Pa] obtained from the tensile test described in the following embodiments and the substrate thickness h [m] into the above formula. The aforementioned 25°C bending stiffness value can be the 25°C bending stiffness value in the length direction (MD: Machine Direction), or it can be the 25°C bending stiffness value in the width direction (TD: Transverse Direction, the direction orthogonal to MD). Therefore, it can be the 25°C bending stiffness value of at least one of the 25°C bending stiffness values of MD and TD, or it can be the 25°C bending stiffness value in any direction of MD or TD. The 25°C bending stiffness value of the surface protection sheet can be obtained mainly by selecting the material and setting the thickness of the substrate layer that makes up the surface protection sheet.

(Tensive Modulus at 25°C) While not particularly limited, in several embodiments, the tensile modulus at 25°C of the surface protective sheet may be 100 MPa or higher, or 500 MPa or higher. In several preferred embodiments, the aforementioned tensile modulus at 25°C is 1000 MPa or higher, more preferably 3000 MPa or higher, even more preferably 5000 MPa or higher, and may also be 6000 MPa or higher. A higher tensile modulus at 25°C results in a higher 25°C bending stiffness value. There is no particular upper limit to the aforementioned tensile modulus at 25°C; for example, it may be below 30 GPa, below 15 GPa, below 10 GPa, below 8000 MPa, below 6000 MPa, or below 4500 MPa. A lower tensile modulus of elasticity at 25°C results in a lower 25°C flexural stiffness value. Furthermore, surface protective sheets with a tensile modulus of elasticity at 25°C within the aforementioned range tend to exhibit good peelability, operability, and surface tracking properties.

(Stress at 100% elongation at 25°C) While not particularly limited, in several samples, the stress at 100% elongation of the surface protective sheet at 25°C may be 10 N/ ^mm² or higher, preferably 30 N/ ^mm² or higher, more preferably 50 N/ ^mm² or higher, even more preferably 80 N/ ^mm² or higher, and may also be 120 N/ ^mm² or higher. There is a tendency that the higher the stress at 100% elongation, the easier it is for the surface protective sheet to possess a certain level of stiffness and to achieve end-peel resistance. The upper limit of the stress at 100% elongation may be, for example, 300 N/ ^mm² or lower, or may be 200 N/ ^{mm² or} lower, or may be 100 N/ ^mm² or lower. Surface protective sheets with 100% elongation stress within the above range tend to exhibit good peelability, operability, and surface traceability.

(Tear stress at 25°C) While not particularly limited, in several samples, the tear stress of the surface protective sheet at 25°C may be 10 N/ ^mm² or higher, preferably 30 N/ ^mm² or higher (e.g., 50 N/ ^mm² or higher), more preferably 100 N/ ^mm² or higher, even more preferably 120 N/ ^mm² or higher, and may also be 150 N/ ^mm² or higher. There is a tendency that the higher the tear stress, the easier it is for the surface protective sheet to possess a certain or higher stiffness, and the easier it is to obtain end-peel resistance. The upper limit of the tear stress may be, for example, 500 N/ ^mm² or lower, or may be 300 N/ ^mm² or lower, or may be 200 N/ ^mm² or lower, or may be 150 N/ ^mm² or lower. Surface protective sheets with fracture stress within the above range tend to exhibit good peelability, operability, and surface tracking.

(Tear Strain at 25°C) While not specifically limited, in several samples, the tear strain at 25°C for the surface protective sheet can be less than 500%, preferably less than 300%, more preferably less than 250%, and may also be less than 200%. There is a tendency that the smaller the tear strain, the easier it is for the surface protective sheet to possess a certain level of stiffness and to achieve resistance to end peeling. The lower limit of the tear strain is, for example, 120% or more, or more than 150%, or more than 200%. Surface protective sheets with tear strain within the above range tend to exhibit good peelability, operability, and surface followability.

The aforementioned tensile modulus of elasticity at 25°C is derived by linear regression of the stress-strain curve obtained from the tensile test described in the embodiments described later. Furthermore, the stress at 100% elongation [N/ ^mm² ], the stress at break [N/ ^mm² ], and the strain at break [%] can also be determined by the tensile test described in the embodiments described later. Moreover, the mechanical property values (tensile modulus of elasticity, stress at 100% elongation, stress at break, and strain at break) of the adhesive layer are very small compared to those of the substrate layer, and the mechanical properties of the surface protective sheet can depend on the mechanical properties of the substrate layer. Therefore, in this specification, the tensile modulus of elasticity, stress at 100% elongation, and tensile stress of the surface protection sheet refer to values converted into per unit cross-sectional area of the substrate layer constituting the surface protection sheet. The cross-sectional area of the substrate layer can be calculated based on the thickness of the substrate layer. The thickness of the substrate layer is assumed to be the value obtained by subtracting the thickness of the adhesive layer from the measured thickness of the surface protection sheet. The aforementioned tensile modulus of elasticity at 25°C can be the tensile modulus of elasticity at 25°C of MD, or it can be the tensile modulus of elasticity at 25°C of TD. Therefore, it can be the tensile modulus of elasticity at 25°C of at least one of the tensile modulus of elasticity at 25°C of MD and the tensile modulus of elasticity at 25°C of TD, or it can be the tensile modulus of elasticity at 25°C in any direction of MD or TD. Similarly, the stress at 100% elongation, the fracture stress, and the fracture strain mentioned above can be the measured values of MD (stress at 100% elongation, fracture stress, or fracture strain), or the measured values of TD. Therefore, it can be the measured value of at least one of the measured values of MD and TD, or it can be the measured value of MD or TD in any direction.

The aforementioned mechanical properties of the surface protective sheet (tensile modulus of elasticity at 25°C, stress at 100% elongation at 25°C, breaking stress at 25°C, and breaking strain at 25°C) can be set and adjusted primarily by selecting the material of the substrate layer that constitutes the surface protective sheet.

<Adhesive Layer> The surface protection sheets disclosed herein typically possess an adhesive layer. This adhesive layer may, for example, be composed of one or more adhesives selected from acrylic adhesives, rubber adhesives (natural rubber, synthetic rubber, mixtures thereof), polysiloxane adhesives, polyester adhesives, urethane adhesives, polyether adhesives, polyamide adhesives, fluorinated adhesives, etc. Here, acrylic adhesives refer to adhesives whose main component is an acrylic polymer. Rubber adhesives and other adhesives have the same meaning.

Furthermore, in this specification, the term "acrylic polymer" refers to a polymer derived from a monomeric component containing more than 50% by weight of an acrylic monomer. The aforementioned acrylic monomer refers to a monomer having at least one (meth)acrylic group in one molecule. Also, in this specification, the term "(meth)acrylic" means including both acrylonitrile and methacrylic. Similarly, the term "(meth)acrylate" means including both acrylate and methacrylate, and the term "(meth)acrylic acid" means including both acrylic acid and methacrylic acid. The aforementioned acrylic polymer can be an acrylic polymer. For example, the aforementioned acrylic polymer can be an acrylic polymer contained in a water-dispersible or solvent-based adhesive as a base polymer (main constituent polymer). In this case, "monomer components constituting acrylic polymers" in this specification may be changed to "monomer components constituting acrylic polymers". Furthermore, the content of added components expressed in this specification as a relative amount to "monomer components constituting polymers" or "monomer components constituting acrylic polymers" may be changed to a relative amount to "acrylic polymers".

(Acrylic Adhesives) From the perspective of weather resistance and other factors, acrylic adhesives are preferred as the constituent material of the adhesive layer in several applications.

As an acrylic adhesive, it is preferably an acrylic polymer comprising a monomeric component consisting of more than 35% by weight of an alkyl (meth)acrylate containing a linear or branched alkyl group having 1 or more carbon atoms at the ester terminus. Hereinafter, an alkyl (meth)acrylate containing an alkyl group having X or more carbon atoms at the ester terminus is sometimes referred to as "(meth)acrylate C _XY alkyl ester". The alkyl (meth)acrylate containing the above-mentioned linear (including linear and branched) alkyl groups can be used alone or in combination of two or more.

In several states, to facilitate a balance of properties, the proportion of (meth)acrylic acid _C1-20 alkyl esters in the total monomer component may be, for example, 40% by weight or more, 45% by weight or more, or 50% by weight or more (e.g., 55% by weight or more). For the same reason, the proportion of (meth)acrylic acid _C1-20 alkyl esters in the total monomer component may be, for example, 90% by weight or less, 70% by weight or less, or 65% by weight or less (e.g., 55% by weight or less). In several other states, to facilitate a balance of properties, the proportion of (meth)acrylic acid _C1-20 alkyl esters in the total monomer component may be, for example, 70% by weight or more, 80% by weight or more, or 90% by weight or more. For the same reason, the percentage of (meth)acrylic acid _C1-20 alkyl ester in the monomer component may be, for example, 99.9% by weight or less, 99.5% by weight or less, or 99% by weight or less.

As non-limiting specific examples of _C1-20 alkyl esters of (meth)acrylate, the following can be cited: methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, dibutyl methacrylate, terbutyl methacrylate, pentyl methacrylate, isoamyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, and isooctyl methacrylate. Nonyl methacrylate, isononyl methacrylate, decyl methacrylate, isodecanyl methacrylate, undecyl methacrylate, dodecyl methacrylate, tridecyl methacrylate, tetradecyl methacrylate, pentadecyl methacrylate, hexadecyl methacrylate, heptadecanyl methacrylate, stearyl methacrylate, isostearyl methacrylate, nonadecanyl methacrylate, eicosyl methacrylate, etc.

Among these, it is preferable to use at least a _C4-20 alkyl ester of (meth)acrylate, and more preferably at least a _C4-18 alkyl ester of (meth)acrylate. For example, it is preferable to use an acrylic adhesive containing one or both of n-butyl acrylate (BA) and 2-ethylhexyl acrylate (2EHA) as the above-mentioned monomer components, and even more preferably an acrylic adhesive containing at least BA. Other examples of _C4-20 alkyl esters of (meth)acrylate that can be preferably used include: isononyl acrylate, n-butyl methacrylate (BMA), 2-ethylhexyl methacrylate (2EHMA), isostearyl acrylate (iSTA), etc.

In several embodiments, the monomer component constituting the acrylic polymer may contain (meth)acrylate _C4-18 alkyl esters at a ratio of 40% by weight or more. Based on such a relatively large amount of (meth)acrylate alkyl esters with alkyl groups having 4 or more carbon atoms at the ester terminus, there is a tendency to form acrylic polymers with higher oleophilicity. Acrylic polymers with higher oleophilicity tend to form adhesive layers whose adhesion is not easily reduced even when immersed in warm water or other water. The percentage of (meth)acrylate _C4-18 alkyl esters in the monomer component may, for example, be 60% by weight or more, 70% by weight or more, 75% by weight or more, or 80% by weight or more. It is also possible to have a monomer component containing (meth)acrylate _C6-18 alkyl esters at a ratio of any of the above lower limits. Furthermore, from the viewpoint of improving the cohesiveness of the adhesive layer and preventing aggregation failure, the proportion of (meth)acrylate _C4-18 alkyl ester in the monomer component is appropriately set to 99.5% by weight or less, or 99% by weight or less, or 98% by weight or less, or 97% by weight or less. From the viewpoint of improving the cohesiveness of the adhesive layer, in some samples, the proportion of (meth)acrylate _C4-18 alkyl ester in the above-mentioned monomer component is 95% by weight or less, for example, appropriately 90% by weight or less. In other samples, the proportion of (meth)acrylate _C4-18 alkyl ester in the monomer component may be 85% by weight or less, or 75% by weight or less. It is also possible to have a monomer component containing (meth)acrylate _C6-18 alkyl ester at a proportion below any of the above upper limits.

Among several embodiments, an acrylic polymer formed by a monomer component comprising more than 50% by weight of a _C1-4 alkyl methacrylate (preferably BA) in a (meth)acrylate containing the aforementioned chain alkyl group is preferably used. Based on this acrylic polymer, an adhesive with adhesion and cohesive strength suitable for surface protection applications can be readily obtained. One _C1-4 alkyl methacrylate may be used alone or in combination of two or more. The proportion of _C1-4 alkyl methacrylate in the (meth)acrylate containing the aforementioned chain alkyl group is preferably 70% by weight or more, more preferably 85% by weight or more, and for example, 90% by weight or more. The maximum percentage of (meth)acrylate _C1-4 alkyl esters in (meth)acrylate alkyl esters having the above-mentioned chain alkyl group is 100% by weight, or it can be 99% by weight or less, for example, less than 97% by weight.

In several preferred embodiments, the proportion of (meth)acrylate _C2-4 alkyl ester in the (meth)acrylate alkyl ester having the aforementioned chain alkyl group is greater than 50% by weight (e.g., 70% by weight or more, or 85% by weight or more, or 90% by weight or more). Specific examples of (meth)acrylate _C2-4 alkyl esters include: ethyl acrylate, propyl acrylate, isopropyl acrylate, BA, isobutyl acrylate, dibutyl acrylate, and tributyl acrylate. One (meth)acrylate _C2-4 alkyl ester can be used alone or in combination of two or more. Using acrylic polymers composed of such monomers, it is easy to achieve surface protective sheets with good adhesion to the adherend. Among them, a preferred state is an example in which the proportion of BA in the alkyl (meth)acrylate containing the above-mentioned chain alkyl group is more than 50% by weight (e.g., more than 70% by weight, or more than 85% by weight, or more than 90% by weight). The proportion of (meth)acrylate _C2-4 alkyl ester in the alkyl (meth)acrylate containing the above-mentioned chain alkyl group is 100% by weight, or it can be 99% by weight or less, for example, it can be less than 97% by weight.

Among several preferred embodiments, an acrylic polymer formed from a monomer component comprising more than 30% by weight of a (meth)acrylate _C7-12 alkyl ester in a (meth)acrylate having the aforementioned chain alkyl group can be preferably used. Based on this acrylic polymer, a surface protective sheet with good adhesion to the adherend can be readily achieved. As for the aforementioned (meth)acrylate _C7-12 alkyl ester, _C8-9 alkyl ester is preferred, more preferably _C8-9 alkyl acrylate, and especially preferably 2EHA. One type of (meth)acrylate _C7-12 alkyl ester can be used alone or in combination of two or more types. The proportion of (meth)acrylate _C7-12 alkyl ester (preferably 2EHA) in the (meth)acrylate containing the above-mentioned chain alkyl group can be 40% by weight or more, appropriately 50% by weight or more, preferably 70% by weight or more, and can also be 85% by weight or more, for example, 90% by weight or more, or 95% by weight or more. The upper limit of the proportion of (meth)acrylate _C7-12 alkyl ester in the (meth)acrylate containing the above-mentioned chain alkyl group is 100% by weight, and can also be 99% by weight or less, for example, less than 97% by weight.

In several preferred embodiments, the monomer component comprises one or more alkyl methacrylates as (meth)acrylates. By using alkyl methacrylates, acrylic polymers suitable for surface protection applications can be better designed. Preferably, the alkyl methacrylate is a _C1-10 alkyl methacrylate, more preferably a _C1-4 (and even more preferably _C2-4 ) alkyl methacrylate. The alkyl methacrylate can preferably be used in combination with alkyl acrylates. When alkyl methacrylates and alkyl acrylates are used together, the weight ratio ( _CAM ) of one or more alkyl methacrylates (e.g., _C2-4 alkyl methacrylates) to the weight ( _CAA) of one or more alkyl acrylates ( _CAM : _CAA ) is not particularly limited. In some samples, it is usually about 1:9 to 9:1, appropriately about 2:8 to 8:2, preferably about 3:7 to 7:3, and even more preferably about 4:6 to 6:4. In other samples, the weight CAM of alkyl methacrylates (e.g., _C1 alkyl methacrylates, i.e., methyl methacrylate (MMA)) in the total amount of (meth) _acrylates ( _CAM + _CAA ) is usually about 30% by weight or less, appropriately about 10% by weight or less, and possibly about 5% by weight or less, and even more preferably about 3% by weight or less. On the other hand, its lower limit is usually above about 0.1% by weight or above about 0.5% by weight.

The monomer components constituting acrylic polymers may also include alkyl (meth)acrylates, and other monomers (copolymeric monomers) that can copolymerize with alkyl (meth)acrylates as needed. As copolymeric monomers, monomers with polar groups (e.g., carboxyl, hydroxyl, nitrogen-containing rings, etc.) are preferred. Monomers with polar groups can help introduce crosslinking sites into acrylic polymers or improve the cohesiveness of adhesives. One copolymeric monomer may be used alone or in combination of two or more.

As non-limiting examples of copolymerizable monomers, the following can be cited: Monomers containing carboxyl groups: such as acrylic acid, methacrylic acid, carboxyethyl acrylate, carboxypentyl acrylate, itconic acid, maleic acid, fumaric acid, butenoic acid, isobutenoic acid, etc. Monomers containing anhydride groups: such as maleic anhydride, itconic anhydride. Hydroxyl-containing monomers: such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, methyl (4-hydroxymethylcyclohexyl)acrylate, and other hydroxyalkyl (meth)acrylates. Monomers containing sulfonic acid or phosphoric acid groups: Examples include styrene sulfonic acid, allyl sulfonic acid, sodium vinyl sulfonate, 2-(meth)acrylamide-2-methylpropanesulfonic acid, (meth)acrylamide propanesulfonic acid, (meth)acrylate sulfonylpropyl ester, (meth)acryloxynaphthalene sulfonic acid, and 2-hydroxyethyl acrylphosphate. Monomers containing epoxy groups: Examples include (meth)acrylate glycidyl ester or (meth)acrylate-2-ethyl glycidyl ether, as well as epoxy-containing acrylates, allyl glycidyl ether, and (meth)acrylate glycidyl ether. Monomers containing cyano groups: Examples include acrylonitrile and methacrylonitrile. Monomers containing isocyanate groups: Examples include (meth)acrylate 2-isocyanate ethyl ester. Monomers containing amide groups: Examples include (meth)acrylamide; N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N,N-dipropyl(meth)acrylamide, N,N-diisopropyl(meth)acrylamide, N,N-di(n-butyl)(meth)acrylamide, N,N-di(tert-butyl)(meth)acrylamide, etc. N-Dialkyl(meth)acrylamide; N-ethyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N-butyl(meth)acrylamide, N-n-butyl(meth)acrylamide, and other N-alkyl(meth)acrylamides; N-vinylacetamide and other N-vinylcarboxylic acid amides; monomers having hydroxyl and amide groups, such as N-(2-hydroxyethyl)( N-(2-Hydropropyl)(meth)acrylamide, N-(1-Hydropropyl)(meth)acrylamide, N-(3-Hydropropyl)(meth)acrylamide, N-(2-Hydrobutyl)(meth)acrylamide, N-(3-Hydrobutyl)(meth)acrylamide, N-(4-Hydrobutyl)(meth)acrylamide, etc., are N-hydroxyl groups. Alkyl (meth)acrylamide; monomers having alkoxy and amide groups, such as N-methoxymethyl (meth)acrylamide, N-methoxyethyl (meth)acrylamide, N-butoxymethyl (meth)acrylamide, and other N-alkoxyalkyl (meth)acrylamides; as well as N,N-dimethylaminopropyl (meth)acrylamide, N-(meth)acrylyl α-line, etc. Amino-containing monomers: such as aminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, and tributylaminoethyl (meth)acrylate. Epoxy-containing monomers: such as glycidyl (meth)acrylate, methyl glycidyl (meth)acrylate, and allyl glycidyl ether. Monomers having a ring containing a nitrogen atom: for example, N-vinyl-2-pyrrolidone, N-methylvinylpyrrolidone, N-vinylpyridine, N-vinylpiperidone, N-vinylpyrimidine, N-vinylpiperidine, N-vinylpyrrole, N-vinylimidazol, N-vinylpyrazole, N-(meth)propenyl-2-pyrrolidone, N-(meth)propenylpiperidine, N-(meth)propenylpiperidine, etc. (N-)Acryloylpyrrolidone, N-vinylpyrrolidone, N-vinyl-3-vinylpyrrolidone, N-vinyl-2-caprolactone, N-vinyl-1,3-vinyl-2-one, N-vinyl-3,5-vinylpyrrolidone, N-vinylpyrazole, N-vinylisothiazole, N-vinylisothiazole, N-vinylpyrrolidone, etc. (e.g., lactamines such as N-vinyl-2-caprolactone). Monomers having a succinimide skeleton: e.g., N-(meth)acryloyloxymethylenesuccinimide, N-(meth)acryloyl-6-oxyhexamethylenesuccinimide, N-(meth)acryloyl-8-oxyhexamethylenesuccinimide, etc. Cisyldiamides: such as N-cyclohexylcisyldiamide, N-isopropylcisyldiamide, N-laurylcisyldiamide, N-phenylcisyldiamide, etc. Iconamides: such as N-methyliconamide, N-ethyliconamide, N-butyliconamide, N-octyliconamide, N-2-ethylhexyliconamide, N-cyclohexyliconamide, N-lauryliconamide, etc. (Meth)acrylate aminoalkyl esters: Examples include aminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, and tributylaminoethyl (meth)acrylate. Alkoxy-containing monomers: Examples include 2-methoxyethyl (meth)acrylate, 3-methoxypropyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, propoxyethyl (meth)acrylate, butoxyethyl (meth)acrylate, ethoxypropyl (meth)acrylate, and other (meth)acrylate alkoxyalkyl esters; methoxyethylene glycol (meth)acrylate, methoxypolypropylene glycol (meth)acrylate, and other alkoxyalkyl glycol (meth)acrylates. Alkoxysilyl monomers: Examples include 3-(meth)propenyloxypropyltrimethoxysilane, 3-(meth)propenyloxypropyltriethoxysilane, 3-(meth)propenyloxypropylmethyldimethoxysilane, and 3-(meth)propenyloxypropylmethyldiethoxysilane. Ethylene esters: Examples include vinyl acetate and vinyl propionate. Ethylene ethers: Examples include vinylalkyl ethers such as methyl vinyl ether or ethyl vinyl ether. Aromatic vinyl compounds: Examples include styrene, α-methylstyrene, and vinyltoluene. Alkenes: Examples include ethylene, butadiene, isoprene, and isobutene. (Meth)acrylates containing alicyclic hydrocarbon groups: Examples include cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, isocyanate (meth)acrylate, dicyclopentyl (meth)acrylate, and tetraalkyl (meth)acrylate. (Meth)acrylates containing aromatic hydrocarbon groups: Examples include phenyl (meth)acrylate, phenoxyethyl (meth)acrylate, and benzyl (meth)acrylate. Furthermore, (meth)acrylates containing heterocyclic groups such as tetrahydrofurfuryl (meth)acrylate, (meth)acrylates containing halogen atoms such as vinyl chloride or fluorine-containing (meth)acrylates, (meth)acrylates containing silicon atoms such as polysiloxane (meth)acrylates, and (meth)acrylates obtained from terpene compound derivative alcohols, etc.

When using this type of copolymer, there is no particular limitation on its amount, which is appropriately set to 0.01% by weight or more of the total monomer component. From the viewpoint of better utilizing the copolymer, the amount of copolymer can also be set to 0.1% by weight or more of the total monomer component, or even 0.5% by weight or more. Furthermore, from the viewpoint of easily obtaining a balance of adhesive properties, the amount of copolymer is appropriately set to 50% by weight or less of the total monomer component, preferably 40% by weight or less.

In several embodiments, the monomer components constituting acrylic polymers may include monomers having nitrogen atoms. By using monomers having nitrogen atoms, the cohesive force of the adhesive can be improved, and the adhesion force can be better enhanced. Monomers having nitrogen atoms may be used alone or in combination of two or more. As a preferred example of a monomer having nitrogen atoms, a monomer having a ring containing nitrogen atoms can be cited. As a monomer having a ring containing nitrogen atoms, those exemplified above can be used, for example, the general formula (1) can be used: [Chemical 1] The N-vinylcyclic acetylamine is represented here. In general formula (1), ^R1 is a divalent organic group, specifically -( _CH2 ) _n- . n is an integer from 2 to 7 (preferably 2, 3 or 4). N-vinyl-2-pyrrolidone is preferably used. As another preferred example of a monomer having a nitrogen atom, (meth)acrylamide is exemplified.

The amount of nitrogen-containing monomers (preferably monomers with nitrogen-containing rings) used is not particularly limited; for example, it can be 1% or more, 3% or more, 5% or more, or 7% or more of the total monomer component. In some samples, from the viewpoint of enhancing adhesion, the amount of nitrogen-containing monomers used can be 10% or more, 12% or more, 15% or more, or 20% or more of the total monomer component. Furthermore, the amount of nitrogen-containing monomers used is appropriately set to, for example, 40% or less, 35% or less, 30% or less, or 25% or less of the total monomer component. In other samples, the amount of nitrogen-containing monomers used can be set to, for example, 20% or less, or 15% or less of the total monomer component. In several other states, the amount of monomers containing nitrogen atoms used may be set to, for example, 12% by weight or less, 8% by weight or less, or 4% by weight or less of the total monomer component.

In several embodiments, the monomer component includes a carboxyl-containing monomer. Preferred examples of carboxyl-containing monomers include acrylic acid (AA) and methacrylic acid (MAA). AA and MAA may also be used together. When AA and MAA are used together, the weight ratio (AA/MAA) is not particularly limited, and may be set in the range of about 0.1 to 10. In several embodiments, the aforementioned weight ratio (AA/MAA) may be, for example, about 0.3 or more, or about 0.5 or more. Furthermore, the aforementioned weight ratio (AA/MAA) may be, for example, about 4 or less, or about 3 or less.

By using carboxyl-containing monomers, aqueous liquids such as water can quickly adhere to the surface of the adhesive layer. This helps reduce water peeling forces. The amount of carboxyl-containing monomers used can be, for example, 0.05% by weight or more, 0.1% by weight or more, 0.3% by weight or more, 0.5% by weight or more, 0.8% by weight or more, 1.2% by weight or more, or 1.5% by weight or more of the total monomer component. By using a certain amount or more of carboxyl-containing monomers, the cohesiveness or crosslinking density of the adhesive layer can be improved. The proportion of the aforementioned carboxyl-containing monomers can be, for example, 15% by weight or less, 10% by weight or less, 5% by weight or less, 4.5% by weight or less, 3.5% by weight or less, 3.0% by weight or less, or 2.5% by weight or less. It is preferable to use a moderate amount of carboxyl-containing monomers to suppress the diffusion of water into the adhesive layer as a whole, and to suppress the reduction of adhesion when in contact with aqueous liquids, such as during warm water immersion. Furthermore, using a moderate amount of carboxyl-containing monomers is also advantageous from the viewpoint of preventing the water used to measure water peeling force from being absorbed by the adhesive layer, thus avoiding insufficient water during peeling. Furthermore, the techniques disclosed herein can also be preferably implemented in monomeric compositions that substantially do not contain carboxyl-containing monomers. In this view, the proportion of carboxyl-containing monomers in the monomeric composition may, for example, be less than 1% by weight, less than 0.3% by weight, or less than 0.1% by weight.

In several formulations, the monomer component may include hydroxyl-containing monomers. By using hydroxyl-containing monomers, the cohesive force or crosslinking density of the adhesive can be adjusted, thereby improving adhesion. Hydroxyl-containing monomers can be those exemplified above, such as preferably 2-hydroxyethyl acrylate (HEA) or 4-hydroxybutyl acrylate (4HBA). One hydroxyl-containing monomer may be used alone or in combination of two or more.

There are no particular limitations on the amount of hydroxyl-containing monomers used, for example, it can be 0.01% by weight or more, 0.1% by weight or more, or 0.5% by weight or more of the total monomer component. In several preferred embodiments, the amount of hydroxyl-containing monomers used is 1% by weight or more of the total monomer component, more preferably 5% by weight or more, and even more preferably 10% by weight or more, for example, 12% by weight or more. In several other embodiments, the amount of hydroxyl-containing monomers used can be 15% by weight or more, 20% by weight or more, or 25% by weight or more of the total monomer component. Furthermore, from the viewpoint of suppressing the water absorption of the adhesive layer, in several samples, the amount of hydroxyl-containing monomers used is appropriately set to, for example, 40% by weight or less of the total monomer component, or 30% by weight or less, or 20% by weight or less, or 15% by weight or less, 10% by weight or less, 5% by weight or less, or 3% by weight or less. The technology disclosed herein can also be implemented in samples in which hydroxyl-containing monomers are not substantially used as monomer components of the adhesive layer.

In several preferred embodiments, the monomer component of the acrylic polymer combines a nitrogen-containing monomer (e.g., monomers containing amide groups such as (meth)acrylamide, monomers containing nitrogen-containing rings such as NVP) and a hydroxyl-containing monomer (e.g., HEA, 4HBA) as a polar monomer. This effectively enhances adhesion. In embodiments combining nitrogen-containing and hydroxyl-containing monomers, the weight ratio ( _AN /AOH ₎ of the amount of nitrogen-containing monomer ( _AN) to the amount of hydroxyl-containing _monomer (AOH) is not particularly limited, and can be, for example, 0.1 or more, 0.5 or more, 0.8 or more, 1.0 or more, or 1.2 or more. Furthermore, the aforementioned weight ratio (A _N / A _OH ) may be 10 or less, or 5 or less, or 3 or less, or 1.5 or less.

In several embodiments, the monomer component may include alkoxysilyl-containing monomers. Typically, an alkoxysilyl-containing monomer is an ethylene-unsaturated monomer having at least one (preferably two or more, e.g., two or three) alkoxysilyl groups within a molecule, examples of which are described above. The aforementioned alkoxysilyl-containing monomers may be used alone or in combination of two or more. By using alkoxysilyl-containing monomers, cross-linking structures formed by the condensation reaction of silanol groups (silanol condensation) can be introduced into the adhesive layer. Furthermore, alkoxysilyl-containing monomers can also be understood as silane coupling agents as described below.

In monomeric components comprising alkoxysilyl-containing monomers, the proportion of alkoxysilyl-containing monomers in the total monomeric component may be set to 0.005% by weight or more, and appropriately to 0.01% by weight or more. Furthermore, from the viewpoint of improving the adhesion of the adherend, the proportion of the aforementioned alkoxysilyl-containing monomers may be 0.5% by weight or less, or 0.1% by weight or less, or 0.05% by weight or less.

Furthermore, from the perspective of inhibiting gelation, in several preferred embodiments of the acrylic polymer, the monomer composition limits the total ratio of (meth)acrylate alkoxyalkyl esters and alkoxy polyalkylene glycol (meth)acrylate to less than 20% by weight. More preferably, the total ratio of (meth)acrylate alkoxyalkyl esters and alkoxy polyalkylene glycol (meth)acrylate is less than 10% by weight, further preferably less than 3% by weight, and even more preferably less than 1% by weight. In several embodiments, the monomer composition substantially does not contain (meth)acrylate alkoxyalkyl esters and alkoxy polyalkylene glycol (meth)acrylate (content 0-0.3% by weight). Similarly, the monomer composition of the acrylic polymers disclosed herein may include or exclude alkoxy-containing monomers at a ratio of less than 20% by weight. The alkoxy-containing monomer constitutes less than 10% by weight, more preferably less than 3% by weight, and even more preferably less than 1% by weight in the above-mentioned monomer components. In a particularly preferred embodiment, the above-mentioned monomer components substantially do not contain alkoxy-containing monomers (content 0-0.3% by weight).

Furthermore, in several preferred embodiments, the monomer composition of the acrylic polymer has the hydrophilic monomer ratio set within an appropriate range. This allows for better water exfoliation. Here, "hydrophilic monomer" in this specification refers to monomers containing carboxyl groups, monomers containing anhydride groups, monomers containing hydroxyl groups, monomers with nitrogen atoms (typically, monomers containing amide groups such as (meth)acrylamide, and monomers with rings containing nitrogen atoms such as N-vinyl-2-pyrrolidone), and monomers containing alkoxy groups (typically, alkoxyalkyl esters of (meth)acrylate and alkoxy polyalkylene glycol (meth)acrylate). In this sample, the proportion of the hydrophilic monomer in the monomer component of the acrylic polymer is preferably 40% by weight or less (e.g., 35% by weight or less), more preferably 32% by weight or less, for example, 30% by weight or less, or 28% by weight or less. There is no particular limitation; the proportion of the hydrophilic monomer in the monomer component of the acrylic polymer can be 1% by weight or more, 10% by weight or more, or 20% by weight or more.

In several embodiments, the monomer components constituting the acrylic polymer may include (meth)acrylates containing alicyclic hydrocarbons. This improves the cohesiveness of the adhesive and enhances adhesion. One type of (meth)acrylate containing alicyclic hydrocarbons may be used alone or in combination of two or more. As alicyclic hydrocarbon (meth)acrylate, those exemplified above can be used, such as preferably cyclohexyl acrylate or isopropyl acrylate. There are no particular limitations on the amount of (meth)acrylate containing alicyclic hydrocarbons used; for example, it may be set to 1% or more, 3% or more, or 5% or more of the total monomer component. In several formulations, the amount of (meth)acrylate containing alicyclic hydrocarbons may be 10% by weight or more, or 15% by weight or more, of the total monomer component. The upper limit of the amount of (meth)acrylate containing alicyclic hydrocarbons is appropriately set at about 40% by weight or less, for example, 30% by weight or less, or 25% by weight or less (e.g., 15% by weight or less, and further 10% by weight or less).

In several preferred embodiments, the acrylic polymer comprises 0.05 mol to 0.45 mol of a polar-group-containing monomer per 100 g of the acrylic polymer as a monomer component. This improves the adhesion to polar substrates, for example, maintaining higher adhesion after immersion in warm water. It can be considered that by introducing the aforementioned polar groups into the acrylic polymer, for example, based on hydrogen bonds targeting polar substrates such as glass, interfacial adhesion is improved. As a monomer containing a polar group, one or more of the above-mentioned monomers containing carboxyl groups (typically AA, MAA, etc.), monomers containing hydroxyl groups (typically HEA, 4HBA, etc.), and monomers having nitrogen atoms (typically (meth)acrylamide and other monomers containing amide groups, NVP and other monomers having nitrogen-containing rings) can be used. Regarding the ratio of monomers containing polar groups in the monomer component of the acrylic polymer, from the viewpoint of effectively exerting the role of monomers containing polar groups, it is appropriate to set the ratio to be 0.10 mol or more per 100 g of acrylic polymer, preferably 0.15 mol or more, more preferably 0.20 mol or more, and for example, 0.24 mol or more. Furthermore, regarding the upper limit of the proportion of polar monomers in the monomer composition of acrylic polymers, it is appropriate to set it to less than 0.40 mol per 100 g of acrylic polymer, preferably less than 0.35 mol, and for example, less than 0.30 mol.

The composition of the monomer components can be set such that the glass transition temperature (hereinafter, also referred to as the "glass transition temperature of the polymer") calculated by the Fox formula based on the composition of the monomer components is above -75°C and below -10°C. In several embodiments, the glass transition temperature (Tg) of the aforementioned polymer (e.g., acrylic polymers, typically acrylic polymers) is preferably below -15°C, more preferably below -20°C, even more preferably below -25°C, and further preferably below -30°C, and may also be below -40°C (e.g., below -55°C). If the Tg of the aforementioned polymer is lower, there is a tendency for the adhesion of the adhesive layer to the substrate layer or the overall adhesion of the adherend to the substrate to be improved. According to this adhesive layer, water penetration to the interface between the adherend and the adhesive layer is easily suppressed even when the adhesive layer is not intended to be peeled off. This is advantageous from the viewpoint of suppressing the decrease in adhesion when in contact with aqueous liquids, such as when immersed in warm water. Furthermore, from the viewpoint of easily improving adhesion, the Tg of the polymer can be, for example, above -70°C or above -65°C. In several other embodiments, the aforementioned Tg can be, for example, above -60°C, above -50°C, above -45°C, or above -40°C.

Here, as shown below, the Fox formula above relates the glass transition temperature (Tg) of the copolymer to the glass transition temperature (Tgi) of the homopolymer formed by the individual polymerization of the monomers constituting the copolymer. 1/Tg＝Σ(Wi/Tgi) Furthermore, in the Fox formula above, Tg represents the glass transition temperature of the copolymer (unit: K), Wi represents the weight fraction of monomer i in the copolymer (weight-based copolymerization ratio), and Tgi represents the glass transition temperature (unit: K) of the homopolymer of monomer i.

The glass transition temperature (Tg) of the homopolymer used to calculate the glass transition temperature is the value recorded in known sources. For example, for the monomers listed below, the following values are used for the glass transition temperature of the homopolymer of those monomers. 2-Ethylhexyl acrylate: -70℃ n-Butyl acrylate: -55℃ n-Butyl methacrylate: 20℃ Isostearic acid acrylate: -18℃ Methyl methacrylate: 105℃ Methyl acrylate: 8℃ Cyclohexyl acrylate: 15℃ N-Vinyl-2-pyrrolidone: 54℃ 2-Hydroethyl acrylate: -15℃ 4-Hydrobutyl acrylate: -40℃ Dicyclopentyl methacrylate: 175℃ Isopropyl acrylate: 94℃ Acrylic acid: 106℃ Methacrylamide: 228℃

For the glass transition temperatures of homopolymers of monomers other than those illustrated above, the values recorded in the "Polymer Handbook" (3rd edition, John Wiley & Sons, Inc., 1989) shall be used. In cases where multiple values are recorded in this literature, the highest value shall be used.

Regarding the monomer whose glass transition temperature of the homopolymer is not recorded in the aforementioned Polymer Handbook, the value obtained by the following determination method was used (see Japanese Patent Application Publication No. 2007-51271). Specifically, in a reactor equipped with a thermometer, a stirrer, a nitrogen inlet pipe, and a reflux cooling pipe, 100 parts by weight of the monomer, 0.2 parts by weight of azobisisobutyronitrile, and 200 parts by weight of ethyl acetate as a polymerization solvent were added, and the mixture was stirred for 1 hour while nitrogen was flowing through it. After removing oxygen from the polymerization system, the temperature was raised to 63°C and the reaction was carried out for 10 hours. Then, the mixture was cooled to room temperature to obtain a homopolymer solution with a solid content of 33% by weight. Next, the homopolymer solution was cast onto a release liner and dried to produce a test sample (sheet-shaped homopolymer) with a thickness of approximately 2 mm. The test sample was punched into a disc shape with a diameter of 7.9 mm, clamped between parallel plates, and the viscoelasticity was measured in shear mode using a viscoelasticity testing machine (ARES, manufactured by Rheometrics) while applying shear strain at a frequency of 1 Hz in the temperature range of -70 to 150 °C at a heating rate of 5 °C/min. The peak temperature of tanδ was set as the Tg of the homopolymer.

The polymers (e.g., acrylic polymers, typically acrylic polymers) contained in the adhesive layer disclosed herein are not particularly limited, but preferably have an SP value of 23.0 (MJ/ ^m³ ) ^1/2 or less. Adhesives containing polymers with such SP values, for example by containing the water affinity agent described later, can preferably achieve adhesives with sufficient adhesive strength and excellent water peelability. The aforementioned SP value is more preferably 21.0 (MJ/ ^m³ ) ^1/2 or less (e.g., 20.0 (MJ/ ^m³ ) ^1/2 or less). There is no particular limitation on the lower limit of the above SP value, for example, it is about 10.0 (MJ/m ³ ) ^1/2 or more, and appropriately it is about 15.0 (MJ/m ³ ) ^1/2 or more, preferably it is 18.0 (MJ/m ³ ) ^1/2 or more.

Furthermore, the SP value of the aforementioned polymer can be calculated using Fedors' method [see "Polymer Engineering and Science", Vol. 14, No. 2 (1974), pp. 148-154], i.e., the formula: SP value δ = (ΣΔe/ΣΔv) ^{^(1/2} ) (where Δe is the evaporation energy of each atom or group at 25°C, and Δv is the mole volume of each atom or group at the same temperature). Polymers with the aforementioned SP value can be obtained by appropriately determining the monomer composition based on the industry's technical expertise.

The adhesive layer can be formed using an adhesive composition comprising a monomeric component as described above, in the form of a polymer, a non-polymer (i.e., a polymerizable functional group in an unreacted form), or a mixture thereof. The adhesive composition can take various forms, including: a water-dispersible adhesive composition in which the adhesive (adhesive component) is dispersed in water; a solvent-based adhesive composition in which the adhesive is contained in an organic solvent; an active energy line-curing adhesive composition (e.g., a light-curing adhesive composition) prepared by curing with active energy lines such as ultraviolet light or radiation to form an adhesive; and a hot-melt adhesive composition that forms an adhesive when applied in a heated molten state and cooled to near room temperature.

During polymerization, depending on the polymerization method or polymerization state, known or commonly used thermal polymerization initiators or photopolymerization initiators may be used. Such polymerization initiators may be used alone or in combination of two or more as appropriate.

There are no particular limitations on its use as a thermal polymerization initiator. For example, azo-based polymerization initiators, peroxide-based initiators, redox initiators obtained by combining peroxides and reducing agents, and substituted ethane-based initiators can be used. More specifically, examples include: 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylpropane)disulfate, 2,2'-azobis(2-amylpropane)dihydrochloride, 2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis(N,N'-dimethyleneisobutylamidine), 2,2'-azobis[N-(2-... Azo-based initiators such as [-carboxyethyl]-2-methylpropanedione hydrate; persulfates such as potassium persulfate and ammonium persulfate; peroxide-based initiators such as benzoyl peroxide, tributyl hydroperoxide, and hydrogen peroxide; substituted ethane-based initiators such as phenyl-substituted ethane; redox initiators such as combinations of persulfates and sodium bisulfite, and combinations of peroxides and sodium ascorbate, etc., but not limited to these. Furthermore, thermal polymerization is preferably carried out at temperatures of, for example, around 20–100°C (typically 40–80°C).

There are no particular limitations on the type of photopolymerization initiator used. For example, ketone-based, acetophenone-based, benzoin ether-based, phosphine oxide-based, α-ketool-based, aromatic sulfonyl chloride-based, photoactive oxime-based, benzoin-based, benzoin-based, benzoin-based, benzoyl acetone-based, and 9-oxosulfuron-methyl-2-ethylhexylene ... Photopolymerization initiators, etc.

The amount of such thermal polymerization initiator or photopolymerization initiator used can be set to the usual amount corresponding to the polymerization method or polymerization state, and there is no particular limitation. For example, about 0.001 to 5 parts by weight of polymerization initiator can be used relative to 100 parts by weight of monomer of the polymerization target (typically about 0.01 to 2 parts by weight, for example about 0.01 to 1 part by weight).

In the above polymerization, various previously known chain transfer agents (which can also be understood as molecular weight regulators or degree of polymerization regulators) may be used as needed. Thiols such as n-dodecyl mercaptan, ter-dodecyl mercaptan, and thioglycolic acid can be used as chain transfer agents. Alternatively, chain transfer agents that do not contain sulfur atoms (non-sulfur chain transfer agents) may also be used. Specific examples of non-sulfur chain transfer agents include: anilines such as N,N-dimethylaniline and N,N-diethylaniline; terpenes such as α-pinene and isoprene; styrene compounds such as α-methylstyrene and α-methylstyrene dimer; compounds containing benzylidene groups such as dibenzylacetone, cinnamyl alcohol, and cinnamaldehyde; hydroquinones such as hydroquinone and dihydroxynaphthalene; quinones such as benzoquinone and naphthoquinone; alkenes such as 2,3-dimethyl-2-butene and 1,5-cyclooctadiene; alcohols such as phenol, benzyl alcohol, and allyl alcohol; and benzylhydrogen compounds such as diphenylbenzene and triphenylbenzene. Chain transfer agents can be used alone or in combination of two or more. When using a chain transfer agent, the amount used, relative to 100 parts by weight of the monomer component, can be set to, for example, about 0.01 to 1 part by weight. The technique disclosed herein can also be preferably implemented in samples without the use of a chain transfer agent.

There are no particular limitations on the molecular weight of polymers (e.g., acrylic polymers, typically acrylic polymers) suitable for the various polymerization methods described above, and they can be set to an appropriate range according to the required performance. The weight average molecular weight (Mw) of the above polymers is preferably about 10 × ^10⁴ or higher, for example, about 15 × ^10⁴ or higher. By using polymers with a specific or higher Mw value (e.g., acrylic polymers), a good balance between cohesiveness and adhesion can be achieved. In some cases, from the viewpoint of obtaining good adhesion reliability, the above Mw can be 20 × ^10⁴ or higher, or 30 × ^10⁴ or higher (e.g., exceeding 30 × ^10⁴ ), or about 40 × ^10⁴ or higher, or about 50 × ^10⁴ or higher, for example, about 55 × ^10⁴ or higher. There is no particular upper limit to the Mw of the aforementioned polymers; for example, it can be less than approximately 500 × ^10⁴ (e.g., less than approximately 150 × ^10⁴ ) or less than approximately 75 × ^10⁴ . In some preferred embodiments, the aforementioned Mw can be less than 50 × ^10⁴ , less than 40 × ^10⁴ , or less than 35 × ^10⁴ (e.g., less than 30 × ^10⁴ ). With polymers having such Mw, there is a tendency to easily adjust the elastic modulus of the adhesive at 60°C to a specific range. Here, Mw refers to the value converted from standard polystyrene obtained by gel osmosis chromatography (GPC). As a GPC device, for example, the model name "HLC-8320GPC" (pipe column: TSKgelGMH-H(S), manufactured by Tosoh Corporation) can be used. The same applies to the embodiments described later.

Several types of surface protective sheets have an adhesive layer formed by a water-dispersible adhesive composition. As a representative example of a water-dispersible adhesive composition, an emulsion adhesive composition can be cited. Emulsion adhesive compositions typically contain a polymer of monomeric components and additives used as needed.

Emulsion polymerization of monomer components is usually carried out in the presence of an emulsifier. There are no particular restrictions on the emulsifier used in emulsion polymerization; known anionic emulsifiers, nonionic emulsifiers, etc., can be used. One emulsifier can be used alone or in combination of two or more.

Examples of non-ionic emulsifiers include: sodium lauryl sulfate, ammonium lauryl sulfate, sodium dodecylbenzene sulfonate, polyoxyethylene sodium lauryl sulfate, polyoxyethylene alkyl ether sodium sulfate, polyoxyethylene alkylphenyl ether ammonium sulfate, polyoxyethylene alkylphenyl ether sodium sulfate, and polyoxyethylene alkyl sulfosuccinate. Examples of non-ionic emulsifiers include: polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene fatty acid esters, and polyoxyethylene polyoxypropylene block polymers. Emulsifiers with reactive functional groups (reactive emulsifiers) may also be used. Examples of reactive emulsifiers include free radical polymerizable emulsifiers, which incorporate free radical polymerizable functional groups such as propylene or allyl ether groups into the structure of the aforementioned anionic or nonionic emulsifiers.

The amount of emulsifier used in emulsion polymerization relative to 100 parts by weight of the monomer component can be, for example, 0.2 parts by weight or more, 0.5 parts by weight or more, 1.0 parts by weight or more, or 1.5 parts by weight or more. Furthermore, from the viewpoint of suppressing the decrease in adhesion after soaking in warm water, in several samples, the amount of emulsifier used relative to 100 parts by weight of the monomer component is appropriately set to 10 parts by weight or less, preferably 5 parts by weight or less, and can also be set to 3 parts by weight or less. Moreover, the emulsifier used in emulsion polymerization here can also function as a hydrophilic agent in the adhesive layer.

According to emulsion polymerization, a polymerization reaction solution in which the polymer of the monomer components is dispersed in water in the form of an emulsion can be obtained. Water-dispersible adhesive compositions used to form adhesive layers can preferably be manufactured using the above-mentioned polymerization reaction solution.

In several preferred embodiments, the surface protective sheet has an adhesive layer formed by a solvent-based adhesive composition. The solvent-based adhesive composition typically contains a solution polymer of monomeric components and, as needed, additives. The effects of the techniques disclosed herein can be effectively achieved in the form of having a solvent-based adhesive layer. The solvent used for solution polymerization (polymerization solvent) can be appropriately selected from previously known organic solvents. For example, solvents selected from aromatic compounds such as toluene (typically aromatic hydrocarbons); esters such as ethyl acetate or butyl acetate; aliphatic or cyclohexane such as hexane or cyclohexane; halogenated alkane such as 1,2-dichloroethane; lower alcohols such as isopropanol (e.g., monohydric alcohols with 1 to 4 carbon atoms); ethers such as tributyl methyl ether; ketones such as methyl ethyl ketone, or any one or a mixture of two or more solvents can be used. According to solution polymerization, a polymerization reaction solution in which the polymer of the monomer components is dissolved in the polymerization solvent can be obtained. Solvent-type adhesive compositions used to form adhesive layers can preferably be manufactured using the above-mentioned polymerization reaction solution.

In other preferred embodiments, the surface protective sheet has an adhesive layer formed by an active energy line curing adhesive composition. Here, in this specification, "active energy line" refers to an energy line with the energy to initiate chemical reactions such as polymerization, crosslinking, and initiator decomposition. Examples of active energy lines include light such as ultraviolet, visible, and infrared rays, or radiation such as alpha, beta, and gamma rays, electron beams, neutron beams, and X-rays. A preferred example of an active energy line curing adhesive composition is a photocurable adhesive composition. Photocurable adhesive compositions have the advantage of being easily formed even into thicker adhesive layers. Among these, ultraviolet-curing adhesive compositions are preferred. Furthermore, the effects of the technology disclosed herein can be effectively achieved in morphologies with light-curing adhesive layers.

Photocurable adhesive compositions typically contain at least a portion (which may be a portion of the type or quantity of monomers) of the composition in polymer form. The polymerization method used to form the polymer is not particularly limited, and various previously known polymerization methods may be suitable. For example, thermal polymerization such as solution polymerization, emulsion polymerization, and block polymerization (typically carried out in the presence of a thermal polymerization initiator) may be suitable; photopolymerization carried out by irradiation with ultraviolet light (typically carried out in the presence of a photopolymerization initiator); and radiation polymerization carried out by irradiation with beta rays, gamma rays, etc. Photopolymerization is preferred.

Several preferred forms of photocurable adhesive compositions comprise a partial polymer of the monomer component. This partial polymer is typically a mixture derived from the polymer of the monomer component and unreacted monomers, preferably in a slurry (a viscous liquid). Hereinafter, the partial polymer with the above properties is sometimes referred to as a "monomer slurry" or simply "slurry." There are no particular limitations on the polymerization method used to partially polymerize the monomer component; various polymerization methods as described above can be appropriately selected. From the viewpoint of efficiency or simplicity, photopolymerization is preferable. According to photopolymerization, the polymerization conversion rate (monomer conversion rate) of the monomer component can be easily controlled by polymerization conditions such as the amount of light irradiation (light intensity).

The polymerization conversion rate of the monomer mixture in the aforementioned polymer is not particularly limited. The polymerization conversion rate can be set, for example, to about 70% by weight or less, preferably to about 60% by weight or less. From the viewpoint of ease of preparation or coatability of the adhesive composition containing the aforementioned polymer, the polymerization conversion rate is appropriately to be about 50% by weight or less, preferably about 40% by weight or less (e.g., about 35% by weight or less). The lower limit of the polymerization conversion rate is not particularly limited, typically to be about 1% by weight or more, appropriately set to about 5% by weight or more.

Adhesive compositions containing a partial polymer of monomer components can be obtained, for example, by partially polymerizing a mixture of monomers containing the entire amount of the monomer components used to prepare the adhesive composition using a suitable polymerization method (e.g., photopolymerization). Furthermore, adhesive compositions containing a partial polymer of monomer components can also be a mixture of a partial or complete polymer of a monomer mixture containing a portion of the monomer components used to prepare the adhesive composition, and the remaining monomer components or their partial polymers. Moreover, in this specification, "complete polymer" refers to a polymerization conversion rate exceeding 95% by weight.

Other components (such as photopolymerization initiators, multifunctional monomers, crosslinking agents, hydrophilic agents, etc.) may be incorporated into the adhesive composition containing the aforementioned polymers as needed. There are no particular limitations on the method of incorporating such other components; for example, they may be pre-contained in the aforementioned monomer mixture or added to the aforementioned polymers.

(Hydrophilic Agents) Hydrophilic agents may be included in the adhesive layer as needed. By including hydrophilic agents in the adhesive layer, peeling forces can be effectively reduced using water or other aqueous liquids. The reason for this is not particularly definitive, but it is generally believed that hydrophilic agents, by possessing hydrophilic regions, tend to aggregate on the surface of the adhesive layer, thereby efficiently increasing the water affinity of the adhesive layer surface. This effectively reduces peeling forces when the adhesive layer comes into contact with water. Hydrophilic agents can be used alone or in combination of two or more. One type of hydrophilic agent can be used alone or in combination.

In several samples, at least one compound A selected from surfactants and compounds having a polyoxyalkylene backbone may be used as a water affinity agent. One or more known surfactants and compounds having a polyoxyalkylene backbone may be used without particular restriction as surfactants and compounds having a polyoxyalkylene backbone. Furthermore, among the aforementioned surfactants, there may be compounds having a polyoxyalkylene backbone, and vice versa.

As a surfactant that can be used as compound A, known nonionic surfactants, anionic surfactants, and cationic surfactants can be used. Among these, nonionic surfactants are preferred. One surfactant can be used alone or in combination of two or more.

Examples of nonionic surfactants include: polyoxyethylene alkyl ethers such as polyoxyethylene lauryl ether, polyoxyethylene cetyl ether, polyoxyethylene stearyl ether, and polyoxyethylene oleyl ether; polyoxyethylene alkylphenyl ethers such as polyoxyethylene octylphenyl ether and polyoxyethylene nonylphenyl ether; sorbitan fatty acid esters such as polyoxyethylene sorbitan monocinnamate, polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan tristearate, polyoxyethylene sorbitan triisostearate, polyoxyethylene sorbitan monooleate, and polyoxyethylene sorbitan trioleate; polyoxyethylene glycerol ether fatty acid esters; and polyoxyethylene-polyoxypropylene block copolymers. These nonionic surfactants can be used alone or in combination of two or more.

Examples of anionic surfactants include: alkylbenzene sulfonates such as nonylbenzene sulfonate and dodecylbenzene sulfonate (e.g., sodium dodecylbenzene sulfonate); alkyl sulfates such as lauryl sulfate (e.g., sodium lauryl sulfate, ammonium lauryl sulfate) and octadecyl sulfate; fatty acid salts; polyoxyethylene alkyl ether sulfates such as polyoxyethylene octadecyl ether sulfate and polyoxyethylene lauryl ether sulfate (e.g., sodium polyoxyethylene alkyl ether sulfate), and polyoxyethylene alkylphenyl ether sulfates such as polyoxyethylene lauryl phenyl ether sulfate (e.g., sodium polyoxyethylene alkyl ether sulfate). Examples of polyether sulfates include: polyoxyethylene alkylphenyl ether ammonium sulfate, polyoxyethylene alkylphenyl ether sodium sulfate, etc.; polyoxyethylene styrene phenyl ether sulfate, etc.; polyoxyethylene stearyl ether phosphate, polyoxyethylene lauryl ether phosphate, etc.; sodium salts, potassium salts, etc., of the above-mentioned polyoxyethylene alkyl ether phosphates; lauryl sulfosuccinate, polyoxyethylene lauryl sulfosuccinate (e.g., polyoxyethylene alkyl sulfosuccinate sodium), etc.; polyoxyethylene alkyl ether acetate, etc. When anionic surfactants form salts, these salts can be, for example, metallic salts such as sodium salts, potassium salts, calcium salts, magnesium salts (preferably monovalent metal salts), ammonium salts, amine salts, etc. Anionic surfactants can be used alone or in combination of two or more.

Among several types, anionic surfactants having at least one of the following groups (-POH, -COH, and -SOH) are preferably used. Among these, surfactants having the -POH group are preferred. Such surfactants typically comprise a phosphate ester structure, such as a monoester of phosphate (ROP(=O)(OH) _₂ ; where R is a monovalent organic group), a diester ((RO) _₂P (=O)OH; where R is the same or different monovalent organic groups), or a mixture comprising both monoesters and diesters. A preferred example of a surfactant having the -POH group is a polyoxyethylene alkyl ether phosphate. The number of carbon atoms in the alkyl group of polyoxyethylene alkyl ether phosphate can be, for example, 6 to 20, 8 to 20, 10 to 20, 12 to 20, or 14 to 20.

Examples of cationic surfactants include polyoxyethylene laurylamine and polyoxyethylene stearylamine. Cationic surfactants can be used alone or in combination of two or more.

As compounds with a polyoxyalkylene backbone that can be used as compound A, examples include polyalkylene glycols such as polyethylene glycol (PEG) and polypropylene glycol (PPG); polyethers containing polyoxyethyl units, polyethers containing polyoxypropyl units, compounds containing both oxyethyl and oxypropyl units (these units can be arranged randomly or in a block configuration); and derivatives thereof. Furthermore, compounds with a polyoxyalkylene backbone among the above-mentioned surfactants can also be used. One of these can be used alone or in combination of two or more. Preferably, a compound containing a polyoxyethylene backbone (also known as a polyoxyethylene segment) is used, and more preferably, PEG.

The molecular weight (chemical formula weight) of compounds having a polyoxyalkylene backbone (e.g., polyethylene glycol) is not particularly limited, but is preferably less than 1000, and preferably about 600 or less (e.g., 500 or less) for the purposes of adhesive composition preparation. The lower limit of the molecular weight of compounds having a polyoxyalkylene backbone (e.g., polyethylene glycol) is not particularly limited, but those with a molecular weight of about 100 or more (e.g., about 200 or more, and further about 300 or more) are preferably used.

Other examples of water-soluble polymers include polyvinyl alcohol, polyvinylpyrrolidone, and polyacrylic acid. A single water-soluble polymer can be used alone, or in combination of two or more. In the art disclosed herein, one or more compounds A, one or more water-soluble polymers, or combinations thereof can be used as water-soluble polymers.

The HLB value of the hydrophilic agent is not particularly limited, but can be, for example, 3 or more, preferably about 6 or more, or 8 or more (e.g., 9 or more). In several preferred embodiments, the HLB value of the hydrophilic agent is 10 or more. This results in a better tendency to exhibit water peelability. More preferably, the HLB value is 11 or more, even more preferably 12 or more, and particularly preferably 13 or more (e.g., 14 or more). By incorporating a hydrophilic agent (typically a surfactant) with an HLB value within the above range into the adhesive layer, water peelability can be exhibited more effectively. The upper limit of the above HLB value is 20 or less, but can also be 18 or less, 16 or less, or 15 or less.

Furthermore, the HLB value in this manual is based on Griffin's Hydrophilic-Lipophile Balance, which represents the degree of affinity of a surfactant for water or oil. It is expressed as a value between 0 and 20, indicating the ratio of hydrophilicity to lipophilicity. The definition of HLB is as described in W. C. Griffin: J. Soc. Cosmetic Chemists, 1,311 (1949), or in "Surfactant Handbook" co-authored by Takahashi Koshimitsu, Namba Yoshiro, Koike Motoo, and Kobayashi Masao, 3rd edition, Kogaku Shusho Publishing, November 25, 1972, pp. 179-182. Water-loving agents with the aforementioned HLB value can be selected based on the industry's technical knowledge, referring to the above references as needed.

This hydrophilic agent is preferably contained in the adhesive layer in a free form. As a hydrophilic agent, it is preferable to use one that is liquid at room temperature (about 25°C) in terms of the ease of preparation of the adhesive composition.

The adhesive layer containing a hydrophilic agent is typically formed from an adhesive composition containing a hydrophilic agent. This adhesive composition can be any of the water-dispersible adhesive composition, solvent-based adhesive composition, active energy line curing adhesive composition, or hot-melt adhesive composition mentioned above. In several preferred embodiments, the adhesive layer containing the hydrophilic agent can be an adhesive layer formed from a photocurable or solvent-based adhesive composition. In such an adhesive layer, the added effect of the hydrophilic agent can be better utilized. The adhesive layer may also have photocurability.

The content of the hydrophilic agent in the adhesive layer is not particularly limited and can be set in a way that appropriately utilizes the effect of the hydrophilic agent. In several samples, the content of the hydrophilic agent relative to the monomer component of the polymer (e.g., acrylic polymer) contained in the adhesive layer per 100 parts by weight can be, for example, 0.001 parts by weight or more, appropriately 0.01 parts by weight or more, 0.03 parts by weight or more, 0.07 parts by weight or more, or 0.1 parts by weight or more. In several preferred embodiments, the content of the hydrophilic agent relative to 100 parts by weight of the monomer component may be, for example, 0.2 parts by weight or more, and from the viewpoint of obtaining a higher effect, it may be 0.3 parts by weight or more, or 0.4 parts by weight or more, or 0.5 parts by weight or more, or 1.0 parts by weight or more, or 1.5 parts by weight or more. Furthermore, from the viewpoint of inhibiting excessive diffusion of water into the overall adhesive layer, in several embodiments, the amount of hydrophilic agent used relative to 100 parts by weight of the monomer component may be, for example, 20 parts by weight or less, appropriately set to 10 parts by weight or less, preferably set to 5 parts by weight or less, or may be set to 3 parts by weight or less. It is preferable to use a low concentration of hydrophilic agent to minimize the decrease in adhesion when in contact with aqueous liquids, such as during warm water immersion. For example, in several samples, the concentration of hydrophilic agent relative to 100 parts by weight of the monomer component may be less than 2 parts by weight, less than 1 part by weight, less than 0.7 parts by weight, less than 0.3 parts by weight, or less than 0.2 parts by weight. Hydrophilic agents with an HLB of 10 or higher tend to exhibit good water-peeling properties even with small amounts.

(Cross-linking agents) The adhesive compositions disclosed herein primarily aim at cross-linking within the adhesive layer or between the adhesive layer and its adjacent surfaces, and may contain cross-linking agents as needed. Typically, the cross-linking agent is incorporated into the adhesive layer in its post-cross-linking form. By using cross-linking agents, the cohesive strength of the adhesive layer can be appropriately adjusted.

There are no particular limitations on the type of crosslinking agent. It can be selected from previously known crosslinking agents, for example, based on the composition of the adhesive compound, in a way that allows the crosslinking agent to perform its appropriate crosslinking function within the adhesive layer. Examples of usable crosslinking agents include: isocyanate-based crosslinking agents, epoxy-based crosslinking agents, aziridine-based crosslinking agents, carbodiimide-based crosslinking agents, melamine-based crosslinking agents, urea-based crosslinking agents, metal alkoxide-based crosslinking agents, metal chelate-based crosslinking agents, metal salt-based crosslinking agents, hydrazine-based crosslinking agents, and amine-based crosslinking agents. One of these can be used alone, or two or more can be used in combination. In water-dispersible adhesive compositions, it is preferable to use crosslinking agents that are soluble or dispersible in water.

As an isocyanate crosslinking agent, multifunctional isocyanate compounds with two or more functions can be used. Examples include aromatic isocyanates such as toluene diisocyanate, xylene diisocyanate, polymethylene polyphenyl diisocyanate, tris(p-isocyanophenyl) thiophosphate, and diphenylmethane diisocyanate; alicyclic isocyanates such as isophorone diisocyanate; and aliphatic isocyanates such as hexamethylene diisocyanate. Examples of commercially available products include: trihydroxymethylpropane/toluene diisocyanate trimer adduct (manufactured by Tosoh Corporation, trade name "Coronate L"), trihydroxymethylpropane/hexamethylene diisocyanate trimer adduct (manufactured by Tosoh Corporation, trade name "Coronate HL"), hexamethylene diisocyanate isocyanurate (manufactured by Tosoh Corporation, trade name "Coronate HX"), and trihydroxymethylpropane/phenylenedimethyl diisocyanate adduct (manufactured by Mitsui Chemicals, trade name "Takenate D-110 N"), etc. In water-dispersible adhesive compositions, isocyanate-based crosslinkers that are soluble or dispersible in water are preferred. For example, water-soluble, water-dispersible, or self-emulsifying isocyanate crosslinkers are preferred. Isocyanate-type isocyanate crosslinkers with isocyanate groups capped are preferred.

As an epoxy crosslinker, it can be used without particular restriction on those having two or more epoxy groups per molecule. Preferably, it is an epoxy crosslinker having three to five epoxy groups per molecule. Specific examples of epoxy crosslinkers include: N,N,N',N'-tetraglycidyl m-phenylenediamine, 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane, 1,6-hexanediol diglycidyl ether, polyethylene glycol diglycidyl ether, polyglycerol polyglycidyl ether, etc. Commercially available epoxy crosslinking agents include: Mitsubishi Gas Chemical Co., Ltd.'s "TETRAD-X" and "TETRAD-C", DIC Co., Ltd.'s "EPICLON CR-5L", Nagase ChemteX Co., Ltd.'s "DENACOL EX-512", and Nissan Chemical Industries Co., Ltd.'s "TEPIC-G".

As an acezoline crosslinker, it can be used without particular restriction on those having one or more acezoline groups per molecule. In water-dispersible adhesive compositions, acezoline crosslinkers that are soluble or dispersible in water are preferred. The acezoline group can be any of 2-acezoline, 3-acezoline, or 4-acezoline. Generally, acezoline crosslinkers having a 2-acezoline group are preferred. For example, water-soluble copolymers or water-dispersible copolymers obtained by copolymerizing addition polymerizable acezoline species such as 2-vinyl-2-acezoline, 2-vinyl-4-methyl-2-acezoline, 2-vinyl-5-methyl-2-acezoline, 2-isopropenyl-2-acezoline, 2-isopropenyl-4-methyl-2-acezoline, and 2-isopropenyl-5-ethyl-2-acezoline with other monomers can be used as acezoline crosslinking agents. Commercially available acezoline crosslinking agents include, for example, the "Epocros WS" series and "Epocros K" series manufactured by Nippon Shokubai Co., Ltd.

Examples of aziridine crosslinkers include trihydroxymethylpropanetrios[3-(1-aziridine)propionate] and trihydroxymethylpropanetrios[3-(1-(2-methyl)aziridine propionate)]. As carbodiimide crosslinkers, low-molecular-weight or high-molecular-weight compounds having two or more carbodiimide groups can be used.

In several formulations, peroxides can also be used as crosslinking agents. Examples of peroxides include: di(2-ethylhexyl) percarbonate, di(4-tert-butylcyclohexyl) percarbonate, dibutyl percarbonate, tert-butyl neodecanoate, tert-hexyl peroxypentanoate, tert-butyl peroxypentanoate, dilauryl peroxide, dioctyl peroxide, 1,1,3,3-tetramethylbutyl peroxide, and dibenzoxyl peroxide. Among these, peroxides with particularly high crosslinking efficiency include: di(4-tert-butylcyclohexyl) percarbonate, dilauryl peroxide, and dibenzoxyl peroxide. Furthermore, when using peroxide as the aforementioned polymerization initiator, the residual peroxide that was not used in the polymerization reaction can also be used in the crosslinking reaction. In this case, the residual amount of peroxide is measured, and if the peroxide ratio does not reach a specific amount, peroxide can be added as needed to achieve a specific amount. The measurement of the peroxide can be performed by the method described in Japanese Patent No. 4971517.

There is no particular limitation on the amount of crosslinking agent used (when using two or more crosslinking agents, the total amount of each is considered). From the perspective of adhesives that achieve a good balance of adhesive properties such as bonding force or cohesion, the amount of crosslinking agent used relative to 100 parts by weight of the monomer component (e.g., the monomer component of acrylic polymer) contained in the adhesive composition is, for example, about 10 parts by weight or less, appropriately set to about 5 parts by weight or less, or 3 parts by weight or less, or 2 parts by weight or less, or 1 part by weight or less, or even less than 1 part by weight. In several samples, the amount of crosslinking agent (e.g., an isocyanate-based crosslinking agent) used relative to 100 parts by weight of the aforementioned monomer component may be, for example, 0.50 parts by weight or less, 0.40 parts by weight or less, 0.30 parts by weight or less, or 0.20 parts by weight or less. There is no particular limitation on the lower limit of the amount of crosslinking agent used, as long as it is an amount greater than 0 parts by weight relative to 100 parts by weight of the aforementioned monomer component. In several samples, the amount of crosslinking agent used relative to 100 parts by weight of the aforementioned monomer component may, for example, be 0.001 parts by weight or more, 0.01 parts by weight or more, 0.05 parts by weight or more, or 0.10 parts by weight or more. In several other samples, the amount of crosslinking agent used relative to 100 parts by weight of the above-mentioned monomer component may be, for example, 0.5 parts by weight or more, 1 part by weight or more, or 1.5 parts by weight or more.

Alternatively, it may be an adhesive composition that does not contain the crosslinking agent described above. When a light-curing adhesive composition is used as the adhesive composition disclosed herein, the adhesive composition may be substantially free of crosslinking agents such as isocyanate-based crosslinking agents. Here, "substantially free of crosslinking agents (typically isocyanate-based crosslinking agents)" means that the amount of crosslinking agent relative to 100 parts by weight of the aforementioned monomer component is less than 0.05 parts by weight (e.g., less than 0.01 parts by weight).

To make the crosslinking reaction more effective, a crosslinking catalyst may also be used. Examples of crosslinking catalysts include tetrabutyl titanium oxide, tetraisopropyl titanium oxide, ferric acetone, butyltin oxide, and dioctyltin dilaurate, among other metal-based crosslinking catalysts. Tin-based crosslinking catalysts, such as dioctyltin dilaurate, are preferred. There is no particular limitation on the amount of crosslinking catalyst used. The amount of crosslinking catalyst used relative to 100 parts by weight of the monomer component (e.g., the monomer component of an acrylic polymer) contained in the adhesive composition can be, for example, about 0.0001 parts by weight or more, about 0.001 parts by weight or more, about 0.005 parts by weight or more, or about 1 part by weight or less, about 0.1 parts by weight or less, or about 0.05 parts by weight or less.

In adhesive compositions used to form adhesive layers, compounds that generate keto-enol tautomers may be included as crosslinking retarders as needed. For example, compounds that generate keto-enol tautomers are preferably used in adhesive compositions containing isocyanate-based crosslinkers or adhesive compositions incorporating isocyanate-based crosslinkers. This extends the shelf life of the adhesive composition. Various β-dicarbonyl compounds can be used as compounds that generate keto-enol tautomers. Specific examples include: β-diketones such as acetoacetone and 2,4-hexanedione; acetyl acetates such as methyl acetate and ethyl acetate; acetyl acetates such as ethyl propionate; isobutyl acetates such as ethyl isobutylate; and malonates such as methyl malonate and ethyl malonate. Among these, acetoacetone and acetyl acetates are considered preferred compounds. Compounds that produce keto-enol tautomerism can be used alone or in combination of two or more. The amount of the compound that produces the ketone-enol tautomer isomer is approximately 100 parts by weight relative to the monomer component (e.g., the monomer component of an acrylic polymer) contained in the adhesive composition. For example, it can be 0.1 parts by weight to 20 parts by weight, appropriately 0.5 parts by weight to 15 parts by weight, for example, 1 part by weight to 10 parts by weight, or 1 part by weight to 5 parts by weight.

(Multifunctional Monomers) Multifunctional monomers may be used as needed in adhesive compositions (and consequently adhesive layers). Multifunctional monomers can aid in purposes such as adjusting cohesive strength. During the formation of the adhesive layer or after attachment to the substrate, the vinyl unsaturated groups of the multifunctional monomer react under light (e.g., ultraviolet light) to form a crosslinked structure with suitable flexibility. Therefore, in this specification, "multifunctional monomer" may be referred to as a crosslinking agent. For example, multifunctional monomers are preferably used in adhesive layers formed from light-curing adhesive compositions. Compounds having two or more vinyl unsaturated groups can be used as multifunctional monomers. One multifunctional monomer may be used alone or in combination of two or more.

Examples of vinyl unsaturated groups in multifunctional monomers include acrylonitrile, methacrylonitrile, vinyl, and allyl, but are not limited to these. From the viewpoint of photoreactivity, acrylonitrile and methacrylonitrile are preferred vinyl unsaturated groups. Among them, acrylonitrile is preferred.

As a multifunctional monomer, it is preferably a compound having 2 to 10 ethylene unsaturated groups within the molecule, more preferably a compound having 2 to 8 ethylene unsaturated groups within the molecule, and even more preferably a compound having 2 to 6 ethylene unsaturated groups within the molecule. In several embodiments, as a multifunctional monomer, it can be used in compounds having 4 or fewer ethylene unsaturated groups within the molecule (specifically 2 to 4, for example 2 or 3, preferably 2). By using such a multifunctional monomer with a limited number of ethylene unsaturated groups, it is easy to obtain an adhesive layer that balances elongation and strength.

As multifunctional monomers, examples include: ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, ethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,12-dodecanediol di(meth)acrylate, trihydroxymethylpropane tri(meth)acrylate, tetrahydroxymethylmethane tri(meth)acrylate, allyl methacrylate, ethylene methacrylate, divinylbenzene, epoxy acrylate, polyester acrylate, urethane acrylate, butanediol (meth)acrylate, hexanediol di(meth)acrylate, etc. Among them, the preferred materials are trihydroxymethylpropane tri(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and dipentaerythritol hexa(meth)acrylate, and the most preferred material is 1,6-hexanediol diacrylate.

The amount of multifunctional monomers used varies depending on their molecular weight or functional groups. For example, relative to 100 parts by weight of the monomeric component of the polymer contained in the adhesive layer (typically, an acrylic polymer or a monomeric component of such polymer), it is appropriately set to a range of about 0.01 parts by weight to 3.0 parts by weight. In some samples, the amount of multifunctional monomers used relative to 100 parts by weight of the aforementioned monomeric component may be, for example, 0.02 parts by weight or more, or 0.1 parts by weight or more, or 0.5 parts by weight or more, 1.0 parts by weight or more, or 2.0 parts by weight or more. By increasing the amount of multifunctional monomers used, there is a tendency to obtain higher cohesive strength. On the other hand, from the viewpoint of avoiding a decrease in adhesion between the layers adjacent to the adhesive layer due to excessive enhancement of cohesion, the amount of the multifunctional monomer used relative to 100 parts by weight of the aforementioned monomer component can be, for example, 10 parts by weight or less, or 5.0 parts by weight or less, or 3.0 parts by weight or less. In several embodiments, the amount of the multifunctional monomer used relative to 100 parts by weight of the aforementioned monomer component is appropriately set to 1.0 parts by weight or less, preferably 0.5 parts by weight or less, more preferably 0.3 parts by weight or less, or possibly 0.2 parts by weight or less.

(Adhesive Assignment) In several preferred embodiments, the adhesive layer includes an adhesive assignment. By including an adhesive assignment in the adhesive layer, removability from the adhered object using water peeling is maintained, and the adhesion F1 after 30 minutes of warm water immersion is improved. Therefore, even when used in a state where the protected object is treated in a liquid while being adhered to it, the surface protective sheet can maintain a better adhesion to the protected object, and can become less prone to end peeling during or after the aforementioned treatment. By utilizing the water peeling technology disclosed herein and adding an adhesive preformer as an adhesion enhancer, a high level of balance can be achieved between adhesion to the protected substrate and peelability. Various adhesion enhancers can be used without particular restriction. Preferred examples of adhesive preformers include adhesive-enhancing resins or acrylic oligomers. One adhesive preformer can be used alone or in combination of two or more.

While there are no particular limitations, it is preferable to use adhesives with acid values as adhesive enhancers. By using adhesive enhancers with acid values above a certain level, for example, the adhesion of polar substrates can be improved, and the adhesion after soaking in warm water can be maintained at a higher level. The acid value of the adhesive enhancer is, for example, greater than 10 mgKOH/g, preferably greater than 15 mgKOH/g, more preferably greater than 20 mgKOH/g, and even more preferably greater than 23 mgKOH/g. The upper limit of the above acid value is usually, for example, below 200 mgKOH/g, and from the viewpoint of water stripping, it can also be below 100 mgKOH/g, below 50 mgKOH/g, or below 40 mgKOH/g. The acid value of the adhesive assignor can be determined by potentiometric titration as specified in JIS K 0070:1992.

In the case of using adhesive preformers, there is no particular limitation on the amount of adhesive preformers used. From the viewpoint of improving the adhesion force F1 after 30 minutes of warm water immersion, the amount of adhesive preformers used relative to 100 parts by weight of the monomer component of the polymer contained in the adhesive layer can be, for example, 0.1 parts by weight or more, 0.3 parts by weight or more, appropriately 1 part by weight or more, 3 parts by weight or more, 5 parts by weight or more, or 10 parts by weight or more. In several preferred embodiments, the amount of adhesive agent used relative to 100 parts by weight of the aforementioned monomer component may exceed 10 parts by weight, or may be about 11 parts by weight or more, or about 12 parts by weight, more preferably 15 parts by weight or more, further preferably 18 parts by weight or more, especially 20 parts by weight or more (e.g., 22 parts by weight or more), or 25 parts by weight or more, or 28 parts by weight or more, or 32 parts by weight or more, or 35 parts by weight or more. Alternatively, the amount of adhesive agent used relative to 100 parts by weight of the aforementioned monomer component may be appropriately set to less than 100 parts by weight, or may be about 80 parts by weight or less, or 70 parts by weight or less, or 50 parts by weight or less. By limiting the amount of adhesive preformer used to an appropriate range, the adhesive preformer and the adhesive are well miscible, and the additive effect of the adhesive preformer (adhesive properties such as adhesion) can be easily and effectively exerted. In several preferred embodiments, the amount of adhesive preformer used relative to 100 parts by weight of the above-mentioned monomer component is less than 50 parts by weight, more preferably less than 40 parts by weight, further preferably less than 35 parts by weight, especially less than 32 parts by weight, and may also be less than 30 parts by weight or less, or may also be less than 25 parts by weight. In several other embodiments, the amount of adhesive preformer used relative to 100 parts by weight of the above-mentioned monomer component may be less than 20 parts by weight, or may be less than 10 parts by weight or less than 5 parts by weight.

In the samples using the aforementioned adhesive enhancer, the adhesion after immersion in warm water can be improved by using the adhesive enhancer. Therefore, even without limiting the amount of hydrophilic agent used, which may cause a decrease in adhesion in situations involving contact with aqueous liquids such as warm water immersion, the target adhesion after immersion in warm water can still be obtained even by increasing the amount of hydrophilic agent. Therefore, by using a specific amount or more of hydrophilic agent for the purpose of better water peelability and more reliable water peel removal, and by using the adhesive enhancer, the adhesion after immersion in warm water can be improved. In summary, by using adhesive assigners and hydrophilic agents, a high level of adhesion and peelability to the protected object can be achieved. In the embodiment using adhesive assigners and hydrophilic agents, the ratio ( _CA / _CB ) of the amount of hydrophilic agent ( _CA ) in the adhesive layer to the amount of adhesive assigner ( _CB ) is not particularly limited. For example, it can be 0.0001 or more, preferably 0.001 or more, more preferably 0.01 or more, even more preferably 0.02 or more, and even more preferably 0.03 or more, or 0.05 or more, or 0.1 or more. By increasing the amount of hydrophilic agent relative to the amount of adhesive additive, adhesion based on the use of adhesive additive (typically, adhesion after warm water immersion) can be maintained within a specific range, and water peeling force can be suppressed to a lower level, maintaining or improving water peeling removal properties. There is no particular upper limit to the above ratio ( _CA / _CB ), for example, it can be below 10, preferably below 1, more preferably below 0.5, even more preferably below 0.3, and may be less than 0.15 or less than 0.1. By limiting the amount of hydrophilic agent used within a specific range relative to the amount of adhesive additive, water peeling properties can be maintained, and adhesion after warm water immersion can be maintained or improved.

(Acrylic Oligomers) The adhesive layer disclosed herein may contain acrylic oligomers from the viewpoints of improving cohesiveness, adhesion to the substrate layer, or adhesion to the adherend. According to the technology disclosed herein, even when a surface protection sheet is adhered to the protected object with high adhesion, it can be peeled off using water without damaging or deforming the protected object. Therefore, adhesives containing adhesion-enhancing components such as acrylic oligomers can improve adhesion reliability, thereby enhancing protective functionality. The adhesive layer containing acrylic oligomers can be formed using an adhesive composition containing such acrylic oligomers. As an acrylic oligomer, it is preferable to use one with a higher Tg than the aforementioned acrylic polymers (e.g., acrylic polymers).

The Tg of the aforementioned acrylic oligomers is not particularly limited, and can be, for example, above about 20°C and below 300°C. The aforementioned Tg can be, for example, above about 30°C, above about 40°C, above about 60°C, above about 80°C, or above about 100°C. If the Tg of the acrylic oligomer increases, the overall effect of improving cohesiveness tends to increase. Furthermore, from the viewpoint of adhesion to the substrate layer or impact absorption, the Tg of the acrylic oligomers can be, for example, below about 250°C, below about 200°C, below about 180°C, or below about 150°C. Moreover, the Tg of the acrylic oligomers, like the Tg of the aforementioned acrylic polymers, is a value calculated based on the Fox formula.

There is no particular limitation on the Mw of acrylic oligomers. For example, it can be above approximately 1000, appropriately above approximately 1500, or above approximately 2000, or above approximately 3000. Furthermore, the Mw of acrylic oligomers can be less than approximately 30000, appropriately less than approximately 10000, less than approximately 7000, or less than approximately 5000. If the Mw is within the above range, the cohesiveness or adhesion enhancement effect of the adhesive layer can be better utilized. The Mw of acrylic oligomers can be determined by GPC and converted to a value equivalent to standard polystyrene. Specifically, for example, the HPLC8020 manufactured by Tosoh Corporation uses two TSKgel GMH-H (20) tubes as columns and performs the determination using tetrahydrofuran solvent at a flow rate of about 0.5 mL/min.

Examples of monomeric components constituting acrylic oligomers include: various _C1-20 alkyl esters of (meth)acrylates as described above; various (meth)acrylates containing alicyclic hydrocarbons as described above; various (meth)acrylates containing aromatic hydrocarbons as described above; and (meth)acrylate monomers obtained from terpene compound derivative alcohols. One of these may be used alone or in combination of two or more.

From the perspective of improving continuity, acrylic oligomers are preferably composed of acrylic monomers with relatively large volumes, such as alkyl methacrylates with branched alkyl groups (e.g., isobutyl methacrylate or tributyl methacrylate), methacrylates containing alicyclic hydrocarbons, or methacrylates containing aromatic hydrocarbons. Furthermore, when using ultraviolet light during the synthesis of acrylic oligomers or the preparation of adhesive layers, monomers with saturated hydrocarbon groups at the ester terminus are preferred to minimize polymerization inhibition. For example, alkyl methacrylates with branched alkyl groups or methacrylates containing saturated alicyclic hydrocarbons are preferred.

The proportion of (meth)acrylate monomers in the total monomer composition of the acrylic oligomer is typically more than 50% by weight, preferably more than 60% by weight, and more preferably more than 70% by weight (e.g., more than 80% by weight, and further more than 90% by weight). In several preferred embodiments, the acrylic oligomer has a monomer composition that substantially contains only one or more (meth)acrylate monomers. When the monomer composition includes (meth)acrylates containing alicyclic hydrocarbons and (meth)acrylate _C1-20 alkyl esters, their weight ratio is not particularly limited. In several samples, the weight ratio of (meth)acrylate containing alicyclic hydrocarbons to (meth)acrylate _C1-20 alkyl esters can be set to, for example, 10/90 or more, 20/80 or more, or 30/70 or more, or 90/10 or less, 80/20 or less, or 70/30 or less.

In addition to the (meth)acrylate monomers mentioned above, functionalized monomers may also be used as constituent monomers in acrylic oligomers, depending on the requirements. Examples of functionalized monomers include: N-vinyl-2-pyrrolidone, N-acryloyl terephthaloline, and other monomers with nitrogen-containing heterocyclic rings; N,N-dimethylaminoethyl (meth)acrylate and other monomers containing amino groups; N,N-diethyl(meth)acrylamide and other monomers containing amide groups; AA, MAA, and other monomers containing carboxyl groups; and 2-hydroxyethyl (meth)acrylate and other monomers containing hydroxyl groups. One of these functionalized monomers may be used alone or in combination of two or more. When using functionalized monomers, the percentage of functionalized monomers in the total monomer composition of the acrylic oligomer can be, for example, 1% or more by weight, 2% or more by weight, or 3% or more by weight, or, for example, 15% or less by weight, 10% or less by weight, or 7% or less by weight. Acrylic oligomers may also be those that do not use functionalized monomers.

Examples of preferred acrylic oligomers include: dicyclopentyl methacrylate (DCPMA), cyclohexyl methacrylate (CHMA), isopropyl methacrylate (IBXMA), isopropyl acrylate (IBXA), dicyclopentyl methacrylate (DCPA), 1-dalcanyl methacrylate (ADMA), 1-dalcanyl acrylate (ADA), and copolymers of DCPMA and MMA, DCPMA and IBXMA, ADA and methyl methacrylate (MMA), CHMA and isobutyl methacrylate (IBMA), CHMA and IBXMA, CHMA and acrylonitrile methacrylate (ACMO), CHMA and diethylacrylamide (DEAA), and CHMA and AA.

Acrylic oligomers can be formed by polymerizing their constituent monomer components. There are no particular limitations on the polymerization method or polymerization state; various previously known polymerization methods (e.g., solution polymerization, emulsion polymerization, bulk polymerization, photopolymerization, radiation polymerization, etc.) can be used as appropriate. The types of polymerization initiators (e.g., azo polymerization initiators) that may be used as needed are generally as illustrated in the synthesis of acrylic polymers. The amount of polymerization initiator or the amount of chain transfer agent (e.g., thiols) used arbitrarily to achieve the desired molecular weight is appropriately determined based on common sense, and therefore detailed descriptions are omitted.

When the adhesive layer or adhesive composition contains acrylic oligomers, their content relative to 100 parts by weight of the monomer component of the polymer (typically an acrylic polymer) contained in the adhesive layer can be, for example, 0.01 parts by weight or more, and from the viewpoint of obtaining a higher effect, can be 0.05 parts by weight or more, or 0.1 parts by weight or more, or 0.2 parts by weight or more. In several embodiments, the content of acrylic oligomers relative to 100 parts by weight of the aforementioned monomer component is, for example, appropriately 0.5 parts by weight or more, preferably 1 part by weight or more, or can be 2 parts by weight or more. Furthermore, from the viewpoint of compatibility with the aforementioned polymers (typically acrylic polymers), the content of acrylic oligomers relative to 100 parts by weight of the aforementioned monomer components is appropriately set to less than 50 parts by weight, preferably less than 30 parts by weight, more preferably less than 25 parts by weight, for example, less than 10 parts by weight, or less than 5 parts by weight or less than 1 part by weight.

(Adhesive-enhancing resins) Adhesive layers can also contain adhesive-enhancing resins. According to the technology disclosed herein, even when a surface protection sheet is adhered to the object being protected with high adhesion, it can be peeled off using water without damaging or deforming the object. Therefore, adhesives containing adhesive-enhancing components such as adhesive-enhancing resins can improve adhesion reliability, thereby enhancing protective functionality. Examples of adhesives that impart adhesion to resins include: rosin-based adhesives, rosin derivative adhesives, petroleum-based adhesives, terpene-based adhesives, phenolic adhesives, and ketone-based adhesives. These can be used alone or in combination of two or more.

As adhesives to resins, the aforementioned rosin-based adhesives include, for example, rosin glue, wood rosin, tall oil rosin, and other rosins, as well as stabilized rosin (e.g., stabilized rosin obtained by disproportionation or hydrogenation of the aforementioned rosin), polymerized rosin (e.g., polymers of the aforementioned rosin, typically dimers), and modified rosin (e.g., unsaturated acid-modified rosin obtained by modification with unsaturated acids such as maleic acid, fumaric acid, and (meth)acrylic acid). Examples of rosin derivatives that act as adhesives to resins include: esters of the aforementioned rosin-based adhesives (e.g., rosin esters such as stabilized rosin esters or polymerized rosin esters), phenol-modified rosin resins (phenol-modified rosin), and their esters (phenol-modified rosin esters). Examples of petroleum-based adhesives that act as adhesives to resins include: aliphatic petroleum resins, aromatic petroleum resins, copolymer petroleum resins, alicyclic petroleum resins, and their hydrogenates. Examples of terpene-based adhesive resins include α-pinene resins, β-pinene resins, aromatic modified terpene resins, and hydrogenated terpene resins. The aforementioned terpene phenol resins refer to polymers containing terpene and phenolic residues. This concept includes both copolymers of terpenes and phenolic compounds (terpene-phenol copolymer resins) and those obtained by phenolic modification of terpene homopolymers or copolymers (phenol-modified terpene resins). Terpene phenol resins include hydrogenated terpene phenol resins. Examples of phenolic adhesive resins include alkylphenol resins obtained from alkylphenols and formaldehyde. Examples of alkylphenol resins include phenolic varnish-type and soluble phenolic resins. Examples of ketone-based adhesive resins include ketone resins obtained by the condensation of ketones (e.g., aliphatic ketones such as methyl ethyl ketone, methyl isobutyl ketone, and acetophenone; alicyclic ketones such as cyclohexanone and methyl cyclohexanone) with formaldehyde.

Among several types of resins, one or more selected from rosin-based adhesive resins, rosin derivative adhesive resins, and terpene phenol resins may preferably be used as adhesive resins. Among these, rosin derivative adhesive resins are preferred, and rosin esters such as stabilized rosin esters and polymerized rosin esters may be cited as examples.

In the aqueous adhesive composition, it is preferable to use an aqueous adhesive resin in the form of an adhesive resin dispersed in an aqueous solvent as described above. For example, by mixing an aqueous dispersion of an acrylic polymer with an aqueous adhesive resin, an adhesive composition containing these components in the desired ratio can be easily prepared. Among several embodiments, as an aqueous adhesive resin, from the viewpoint of environmental hygiene considerations, it is preferable to use one that is at least substantially free of aromatic hydrocarbon solvents. More preferably, an aqueous adhesive resin that is substantially free of aromatic hydrocarbon solvents and other organic solvents is used.

Commercially available products that contain water-dispersible adhesive resins containing rosin esters include, for example, products manufactured by Arakawa Chemical Industry Co., Ltd. under the trade names "SUPER ESTER E-720", "SUPER ESTER E-730-55", "SUPER ESTER E-865NT", and "SUPER ESTER NS", or products manufactured by Halima Chemical Co., Ltd. under the trade names "HARIESTER SK-90D", "HARIESTER SK-70D", "HARIESTER SK-70E", and "NEOTOL 115E". Furthermore, commercially available products that are terpene phenol resins (which may be in the form of water-dispersible terpene phenol resins) include, for example, the products manufactured by Arakawa Chemical Industry Co., Ltd. under the trade names "TAMANOL E-100", "TAMANOL E-200", and "TAMANOL E-200NT".

There is no particular limitation on the softening point of the adhesive resin. From the viewpoint of suppressing the reduction of the cohesiveness of the adhesive layer, an adhesive resin with a softening point of 80°C or higher is preferable. The softening point of the adhesive resin may also be 90°C or higher, 100°C or higher, 110°C or higher, or 120°C or higher. Adhesive resins with a softening point of 130°C or higher or 140°C or higher may also be used. Furthermore, from the viewpoint of adhesion to the substrate layer and adhesion to the adhered object, adhesive resins with a softening point of 200°C or lower or 180°C or lower are preferable. Furthermore, the softening point of the resin, referred to here as the adhesive softening point, can be the nominal value recorded in literature or catalogues. In the absence of a nominal value, the softening point of the resin can be determined based on the softening point test method (circular method) specified in JIS K5902 or JIS K2207.

From the viewpoint of better exerting its effect, the amount of adhesive resin used is appropriately set to 1 part by weight or more, or 5 parts by weight or more, or 10 parts by weight or more, relative to 100 parts by weight of the monomer component of the polymer contained in the adhesive layer. In several preferred embodiments, the amount of adhesive resin used relative to 100 parts by weight of the aforementioned monomer component exceeds 10 parts by weight, more preferably 15 parts by weight or more, further preferably 18 parts by weight or more, especially 20 parts by weight or more (e.g., 22 parts by weight or more), or 25 parts by weight or more, or 28 parts by weight or more, or 32 parts by weight or more, or 35 parts by weight or more. Furthermore, from the perspective of achieving a good balance between adhesion and cohesion to the substrate layer or the adherend, the amount of adhesive resin used relative to 100 parts by weight of the monomer component can be appropriately set to less than 100 parts by weight, for example, it can be 70 parts by weight or less, or 50 parts by weight or less, or 40 parts by weight or less, or 30 parts by weight or less, or 20 parts by weight or less. By limiting the amount of adhesive resin used to an appropriate range, the adhesive resin and the adhesive can be well miscible, and the additive effect of the adhesive resin (adhesive properties such as adhesion) can be effectively exerted. Alternatively, an adhesive layer that does not actually contain adhesive resin can also be used.

(Silane Coupling Agent) In several formulations, the adhesive layer may contain a silane coupling agent. Depending on the adhesive layer containing the silane coupling agent, a surface protection sheet with higher adhesion can be achieved. One silane coupling agent may be used alone or in combination of two or more.

Examples of silane coupling agents include: 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, which are silicon compounds with an epoxy structure; 3-aminopropyltrimethoxysilane, N-(2-aminoethyl)3-aminopropyltrimethoxysilane, and N-(2-aminoethyl) Silica compounds containing amino groups, such as 3-aminopropylmethyldimethoxysilane; silane coupling agents containing (meth)acrylyl groups, such as acetyltrimethoxysilane, 3-propenyloxypropyltrimethoxysilane, and 3-methpropenyloxypropyltriethoxysilane; and silane coupling agents containing isocyanate groups, such as 3-isocyanopropyltriethoxysilane. Among these, 3-glycidyloxypropyltrimethoxysilane and acetyltrimethoxysilane are preferred examples.

The amount of silane coupling agent used can be set in a way that achieves the desired effect and is not particularly limited. In several samples, the amount of silane coupling agent used relative to 100 parts by weight of the monomeric component of the polymer contained in the adhesive layer can be, for example, 0.001 parts by weight or more, and from the viewpoint of obtaining a higher effect, it can be 0.005 parts by weight or more, or 0.01 parts by weight or more, or 0.015 parts by weight or more. Furthermore, from the viewpoint of improving adhesion, in several samples, the amount of silane coupling agent used relative to 100 parts by weight of the monomeric component constituting the adhesive layer can be, for example, 3 parts by weight or less, or 1 part by weight or less, or 0.5 parts by weight or less. Furthermore, the techniques disclosed herein can be implemented in adhesive compositions that substantially do not contain silane coupling agents. By limiting or eliminating the use of silane coupling agents, the increase in adhesive strength over time can be suppressed, and good water peelability can be easily obtained.

Furthermore, in the case where the monomer component includes a monomer containing alkoxysilyl groups, the aforementioned monomer containing alkoxysilyl groups can also be used as part or all of the silane coupling agent contained in the adhesive layer.

(Photopolymerization Initiator) The adhesive compositions and photocurable adhesive layers disclosed herein may contain a photopolymerization initiator (also known as a photocatalyst) as needed for the purpose of imparting photocurability. As photopolymerization initiators, similar to those exemplified for use in the synthesis of acrylic polymers, ketone-based, acetophenone-based, benzoin ether-based, phosphine oxide-based, α-ketool-based, aromatic sulfonylurea-chloro-based, photoactive oxime-based, benzoin-based, benzoin-based, benzoin-based, benzoin-based, benzoyl ketone-based, benzophenone-based, and 9-oxosulfuron-methyl-2-ethylhexylene ... Photopolymerization initiators, etc. A single photopolymerization initiator can be used alone, or two or more can be used in combination as appropriate.

The content of photopolymerization initiator in the adhesive layer is not particularly limited and can be set in a way that appropriately achieves the desired effect. In several samples, the content of photopolymerization initiator relative to 100 parts by weight of the polymer monomer component (typically an acrylic polymer) contained in the adhesive layer can be, for example, set to approximately 0.005 parts by weight or more, appropriately set to 0.01 parts by weight or more, preferably set to 0.05 parts by weight or more, or also set to 0.10 parts by weight or more, or 0.15 parts by weight or more, or even 0.20 parts by weight or more. By increasing the content of photopolymerization initiator, the photocurability of the adhesive layer is improved. Furthermore, the content of the photopolymerization initiator at 100 parts by weight of the aforementioned monomer component is appropriately set to 5 parts by weight or less, preferably 2 parts by weight or less, or 1 part by weight or less, or 0.7 parts by weight or less, or 0.5 parts by weight or less. Maintaining a low content of the photopolymerization initiator is advantageous from the perspective of improving the stability of the surface protection sheet (e.g., stability against photodegradation).

An adhesive layer containing a photopolymerization initiator is typically formed using an adhesive composition (e.g., a solvent-based adhesive composition) containing the photopolymerization initiator. The adhesive composition containing the photopolymerization initiator can be prepared, for example, by mixing other components used in the composition with the photopolymerization initiator. Furthermore, when preparing an adhesive composition using a polymer synthesized (photopolymerized) in the presence of a photopolymerization initiator (typically an acrylic polymer), the residue (unreacted product) of the photopolymerization initiator used in the synthesis of the aforementioned polymer can also be used as part or all of the photopolymerization initiator contained in the adhesive layer. The same applies when using an acrylic oligomer synthesized in the presence of a photopolymerization initiator as needed. From the perspective of ease of manufacturing management, the adhesive layer disclosed herein can be preferably formed in adhesive compositions prepared by adding the aforementioned amount of photopolymerization initiator to other components.

(Other Components) The adhesive composition used to form the adhesive layer may contain, as needed, acids or alkalis (such as ammonia) used for purposes such as pH adjustment. Examples of other arbitrary components that may be included in this composition include: viscosity modifiers (e.g., thickeners), leveling agents, plasticizers, fillers, colorants such as pigments or dyes, stabilizers, preservatives, anti-aging agents, and various additives commonly found in the field of adhesive compositions. Such additives can be used by conventional methods and are not specifically characterized by this invention; therefore, detailed descriptions are omitted. Furthermore, although not particularly limited, the technology disclosed herein is preferably implemented in samples having an adhesive layer with the aforementioned polymer (e.g., an acrylic polymer) as the main component. In several samples, the proportion of the aforementioned polymer (e.g., an acrylic polymer) in the adhesive layer is approximately 85% by weight or more (e.g., 85-100% by weight), or more than 90% by weight, or more than 95% by weight.

(Formation of the Adhesive Layer) The adhesive layer can be a hardened layer of the adhesive composition. That is, the adhesive layer can be formed by applying (e.g., coating) the adhesive composition to a suitable surface and then performing a curing treatment. When two or more curing treatments (drying, crosslinking, polymerization, etc.) are performed, these can be carried out simultaneously or in multiple stages. In adhesive compositions using a monomeric polymer (acrylic polymer slurry) as a partial polymer, typically, as a curing treatment, a final copolymerization reaction is performed. That is, a portion of the polymer is fed to a further copolymerization reaction to form a complete polymer. For example, if it is a photocurable adhesive composition, light irradiation is performed. Crosslinking, drying, and other curing treatments can also be performed as needed. For example, when drying is necessary with a photocurable adhesive composition, photocuring can be performed after drying. In adhesive compositions using a complete polymer, typically, as a curing treatment, drying (heat drying), crosslinking, or other processes are performed as needed. Multilayer adhesive layers with two or more layers can be fabricated by bonding pre-formed adhesive layers. Alternatively, an adhesive composition can be applied to a pre-formed first adhesive layer, and the adhesive composition can be cured to form a second adhesive layer.

The application of the adhesive composition can be carried out using conventional coating machines such as gravure roller coating machines, reverse roller coating machines, contact roller coating machines, dip roller coating machines, rod coating machines, doctor blade coating machines, and spray coating machines. For example, as a method for setting an adhesive layer on a substrate layer, a direct method can be used to directly apply the adhesive composition to the substrate layer to form the adhesive layer, or a transfer method can be used to transfer the adhesive layer formed on the peel surface to the substrate layer.

The thickness of the adhesive layer is not particularly limited, and can range from approximately 3 μm to 1000 μm. From the perspective of improving water resistance by ensuring the adhesive layer adheres tightly to the substrate or the adherend, in several embodiments, the thickness of the adhesive layer is preferably 5 μm or more (e.g., exceeding 5 μm), more preferably 10 μm or more (e.g., exceeding 10 μm), further preferably 15 μm or more, and especially preferably 20 μm or more. The surface protection sheet disclosed herein has water-peelability, allowing it to be smoothly peeled off from the adherend. Therefore, the thickness of the adhesive layer can be increased, improving adhesion (which can be expressed as normal adhesion or adhesion after warm water immersion), maintaining or enhancing protective properties. Furthermore, from the viewpoint of preventing paste residue due to the aggregation and destruction of the adhesive layer, in several embodiments, the thickness of the adhesive layer may be, for example, 500 μm or less, 300 μm or less, 200 μm or less, or 150 μm or less. In several preferred embodiments, the thickness of the adhesive layer is 100 μm or less, more preferably 60 μm or less, and even more preferably 50 μm or less, for example, 40 μm or less, or 30 μm or less. By limiting the thickness of the adhesive layer, water penetration from the ends of the adhesive layer can be limited, and the decrease in adhesion when immersed in aqueous liquids or warm water can be suppressed. Furthermore, the adhesive layer can be a single-layer structure or a multi-layer structure with two or more layers.

In several samples, the loss modulus of elasticity G" (loss modulus of elasticity G" at 60°C) of the adhesive layer is preferably in the range of 10 kPa or more and 50 kPa or less. Surface protective sheets with an adhesive layer having a loss modulus of elasticity G" at 60°C within the aforementioned range exhibit improved resistance to hot water due to the adhesive properties (loss modulus of elasticity G" at 60°C). For example, even when used in liquids (typically aqueous solutions) or warm water, it easily maintains a close bond to the adherend, does not exhibit a decrease in adhesion based on water peelability, or easily suppresses a decrease in adhesion. Therefore, even when the object to be protected is treated in a liquid while the aforementioned surface protective sheet is attached to it, the required adhesion can be maintained better. Furthermore, it can be considered that when a peeling load is applied to the ends of the surface protective sheet in warm water due to the expansion and contraction of the substrate layer, the peeling load is converted into heat energy and reduced in adhesives with a specific viscosity at 60°C, thus easily maintaining a stable adhesion. This type of surface protective sheet can be considered superior in terms of protection, for example, by not peeling from the ends during the aforementioned liquid treatment.

In several preferred embodiments, from the viewpoint of adhesion after immersion in the adhesive solution or warm water, the elastic modulus G" of the adhesive layer at 60°C is 12 kPa or higher, more preferably 15 kPa or higher, and can also be 18 kPa or higher, 22 kPa or higher, 25 kPa or higher, 28 kPa or higher, 30 kPa or higher, or 32 kPa or higher. By setting a higher elastic modulus G" at 60°C, the adhesion force F1 after immersion in warm water for 30 minutes can be maintained at a higher level. In several embodiments, the upper limit of the above-mentioned elastic modulus loss at 60°C, G", may be 45 kPa or less, or 40 kPa or less, or 35 kPa or less. By limiting the elastic modulus loss at 60°C, G", of the adhesive layer to a specific value or less, it is easy to obtain an adhesive with good adhesive properties suitable for surface protection applications. In other embodiments, the elastic modulus loss at 60°C, G", of the above-mentioned adhesive layer may be 30 kPa or less, or 25 kPa or less, or 20 kPa or less.

More specifically, the elastic modulus G" of the adhesive layer at 60°C can be 11 kPa or less, 12 kPa or less, 13 kPa or less, 14 kPa or less, 15 kPa or less, 16 kPa or less, 17 kPa or less, 18 kPa or less, 19 kPa or less, 20 kPa or less, 21 kPa or less, 22 kPa or less, 23 kPa or less, 24 kPa or less, 25 kPa or less, 26 kPa or less, 27 kPa or less, 28 kPa or less, 29 kPa or less, 30 kPa or less, 31 kPa or less, 32 kPa or less, 33 kPa or less, 34 kPa or less, 35 kPa or less, 36 kPa or less, 37 kPa or less. 38 kPa or above, 39 kPa or above, 40 kPa or below, 41 kPa or above, 42 kPa or below, 43 kPa or above, 44 kPa or above, 45 kPa or above, 46 kPa or below, 47 kPa or above, 48 kPa or above, and 49 kPa or above.

The aforementioned loss modulus of elasticity G at 60°C can be obtained primarily by adjusting the molecular weight or molecular weight distribution of the polymer contained in the adhesive. In addition, it can also be adjusted by the crosslinking density in the adhesive. The loss modulus of elasticity G at 60°C of the adhesive layer can be determined using the method described in the embodiments below.

<Substrate Layer> The surface protection sheet disclosed herein may include a substrate layer. Non-limiting examples of materials for the substrate layer include: various resin films such as polyolefin films, polyester films, and polyvinyl chloride films; foamed sheets containing foams such as polyurethane foam, polyethylene foam, and polychloroprene foam; woven and non-woven fabrics obtained by spinning or blending various fibrous materials (such as natural fibers such as hemp and cotton, synthetic fibers such as polyester and vinylon, and semi-synthetic fibers such as acetate); paper such as Japanese paper, woodfree paper, kraft paper, and wrinkled paper; and metal foils such as aluminum foil, copper foil, and stainless steel (SUS). A suitable material can be selected from one or more of these composites to form a layer and used as the substrate layer material. Examples of substrate layers in the aforementioned composite structures include: laminated substrates (multilayer substrates) consisting of metal foil and the aforementioned resin film, and resin sheets reinforced with inorganic fibers such as glass cloth.

As the material for the substrate layer, various membranes (hereinafter also referred to as substrate membranes) can be preferably used. The aforementioned substrate membrane can be a porous membrane such as a foamed membrane or a non-woven sheet, or a membrane with a structure of porous layers and non-porous layers. Among several embodiments, as the aforementioned substrate membrane, a resin membrane comprising a shape that can independently maintain its shape (self-supporting or independent) can preferably be used as the base membrane. Here, "resin membrane" refers to a non-porous structure, typically a resin membrane that is substantially free of bubbles (pore-free). Therefore, the aforementioned resin membrane is a concept distinct from foamed membranes or non-woven sheets. The resin film described above can be a single-layer structure or a multi-layer structure with two or more layers (e.g., a three-layer structure).

Resin materials used to form resin films include, for example, polyesters such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN); polyolefins such as polyethylene (PE) or polypropylene (PP), and ethylene-propylene copolymer; polycyclic cyclic olefins derived from monomers with aliphatic ring structures such as norethene; polyamides (PA) such as nylon 6, nylon 66, and some aromatic polyamides; polyimides (PI) such as transparent polyimide (CPI), and polyamide-imide (PAI); and polyetheretherketone (PEE). K); polyether sulfide (PES); polyphenylene sulfide (PPS); polycarbonate (PC); polyurethane (PU); ethylene-vinyl acetate copolymer (EVA); polyvinyl alcohol (PVA); polystyrene; ABS resin; polyvinyl chloride; polyvinylidene chloride; polytetrafluoroethylene (PTFE) and other fluoropolymers; polymethyl methacrylate and other acrylic resins; diacetyl cellulose or triacetyl cellulose (TAC) and other cellulose polymers; vinyl butyral polymers; aromatic ester polymers; polyoxymethylene polymers; epoxy polymers and other resins. The substrate layer disclosed herein may have its surface composed of the above-mentioned resin materials. The resin film that can be used as a substrate layer can be selected from those formed using a resin material that contains only one of the aforementioned resins, or those formed using a mixture of two or more resin materials. The resin film can also be a composite resin film formed by laminating a resin layer containing one or more resin materials, or a resin layer containing one or more resin materials of the same or different type as the resin layer. The resin film can be unstretched or stretched (e.g., uniaxially or biaxially stretched).

In several preferred embodiments, a polyolefin resin film is used as the substrate layer. By using a polyolefin resin film, a surface protective sheet can be obtained with appropriate thickness to achieve better properties (e.g., adhesion F1 after 30 minutes of warm water immersion and reduction rate of water peeling force after 30 minutes of warm water immersion). Here, polyolefin resin refers to a resin containing polyolefins at a ratio of more than 50% by weight. As a polyolefin resin, one type of polyolefin can be used alone or in combination with two or more types of polyolefins. The polyolefin can be, for example, a homopolymer of α-olefin, a copolymer of two or more α-olefins, or a copolymer of one or more α-olefins with other vinyl monomers. Specific examples include: ethylene-propylene copolymers such as PE, PP, poly-1-butene, poly-4-methyl-1-pentene, and ethylene-propylene rubber (EPR), ethylene-propylene-butene copolymers, ethylene-butene copolymers, ethylene-vinyl alcohol copolymers, and ethylene-ethyl acrylate copolymers. Both low-density (LD) and high-density (HD) polyolefins can be used. Examples of polyolefin resin films include: unstretched polypropylene (CPP) films, biaxially oriented polypropylene (OPP) films, low-density polyethylene (LDPE) films, linear low-density polyethylene (LLDPE) films, medium-density polyethylene (MDPE) films, high-density polyethylene (HDPE) films, polyethylene (PE) films blended with two or more types of polyethylene (PE), and PP/PE blended films blended with polypropylene (PP) and polyethylene (PE). Among these, OPP films are preferred from the perspective of moisture permeability.

Other preferred examples of resin materials constituting resin films include: polyvinylidene chloride resin, PPS resin, polyurethane resin, EVA resin, and fluoropolymers such as PTFE. Here, polyvinylidene chloride resin refers to a resin containing more than 50% by weight of polyvinylidene chloride. Similarly, PPS resin refers to a resin containing more than 50% by weight of PPS. The same applies to polyurethane resin, EVA resin, and fluoropolymers. The polyolefin resins (PE, PP), or polyvinylidene chloride resin, PPS resin, polyurethane resin, EVA resin, and fluororesin exemplified above can be used in combination with other materials. However, by using each of them alone as a substrate layer, a surface protective sheet with a specific value of adhesion F1 after 30 minutes of warm water immersion and a specific reduction rate of water peeling force after 30 minutes of warm water immersion can also be formed.

In resin films, known additives such as light stabilizers, antioxidants, antistatic agents, colorants (dyes, pigments, etc.), fillers, lubricants, and antiblocking agents can be added as needed. There are no particular limitations on the amount of additives added, which can be appropriately set according to the application.

There are no particular limitations on the manufacturing method of resin films. For example, commonly known resin film forming methods such as extrusion molding, blow molding, T-die casting, and burnishing roll forming can be used.

The aforementioned substrate layer may be substantially composed of such a resin film. Alternatively, the aforementioned substrate layer may also include auxiliary layers in addition to the aforementioned resin film. Examples of such auxiliary layers include: optical property adjustment layers (e.g., coloring layers, anti-reflective layers), printed layers or laminates for imparting the desired appearance, antistatic layers, primer layers, release layers, and other surface treatment layers.

In several other embodiments, the substrate layer has a layer containing inorganic materials (a layer containing inorganic materials). By using a substrate layer containing an inorganic material layer, the effects brought about by the technology disclosed herein can also be achieved. By configuring a layer containing inorganic materials, there is a tendency to improve barrier properties (moisture-proof properties). In several embodiments, a substrate layer having a layer containing inorganic materials can be exemplified as having a main substrate layer including the aforementioned resin film, and having a layer containing inorganic materials disposed on at least one surface of the main substrate layer. In several other embodiments, the substrate layer may also substantially contain a layer containing inorganic materials.

The inorganic material used in the aforementioned inorganic material-containing layer can be various metallic materials containing transition metal elements or half-metal elements, alloys, or inorganic compounds such as inorganic oxides that can form hydrophilic surfaces. One type of inorganic material can be used alone, or two or more can be used in combination. Preferred examples of inorganic materials include oxides (inorganic oxides, typically metal oxides) such as titanium oxide, zinc oxide, magnesium oxide, aluminum oxide, silicon oxide, cerium oxide, chromium oxide, zirconium oxide, manganese oxide, zinc oxide, iron oxide, tin oxide, and niobium oxide. Among these, inorganic oxides such as silicon oxide are preferred. Other preferred examples of inorganic materials include aluminum foil, copper foil, and stainless steel (SUS) foil (metallic materials). In addition to the aforementioned inorganic materials, the layer containing inorganic materials may or may not contain various organic materials containing organic polymer compounds that can be used as coatings or adhesives.

The amount of inorganic material (e.g., inorganic oxides such as silicon oxide) in the layer containing inorganic material can be set to an appropriate amount to obtain the target hydrophilic surface, and is not limited to a specific range. For example, the content ratio of inorganic material in the layer containing inorganic material can be set to about 30% by weight or more, appropriately about 50% by weight or more (e.g., more than 50% by weight), or about 70% by weight or more. In several preferred embodiments, the content ratio of inorganic material in the layer containing inorganic material is about 90 to 100% by weight (e.g., about 95% by weight or more).

The method for forming the aforementioned inorganic material layer is not particularly limited, and can be formed using appropriate methods depending on the target thickness. For example, layered inorganic materials can be formed using known film-forming methods such as vacuum evaporation, sputtering, or coating. When using inorganic compounds as the inorganic material, various evaporation methods can be used, such as physical evaporation (PVD) methods such as vacuum evaporation, sputtering, and ionic coating, or chemical evaporation (CVD) methods such as atomic layer deposition. The formation of coatings containing inorganic polymers such as polysiloxanes can be carried out by selecting a suitable coating agent from known options that can produce a surface with the desired water contact angle, using conventional methods.

The thickness of the inorganic material layer is not particularly limited. In embodiments where the substrate layer has a main substrate layer and an inorganic material layer, from the viewpoint of not impairing the function of the substrate layer itself (the main substrate layer), the thickness of the inorganic material layer is specifically preferably about 5 μm or less (e.g., less than 5000 nm), or about 2 μm or less (e.g., less than 2000 nm). In some embodiments, the thickness of the inorganic material layer is less than 1000 nm, more preferably less than 500 nm, even more preferably less than 100 nm, and most preferably less than 50 nm, or about 30 nm or less, or about 20 nm or less, or about 15 nm or less (e.g., less than 10 nm). By designing this thin inorganic material layer, the required properties such as barrier properties (moisture-proofing) can be imparted to the substrate layer (the main layer of the substrate layer) without compromising its function. A thin inorganic material layer is also advantageous from the perspectives of lightweight and optical properties. Furthermore, the thickness of the inorganic material layer is appropriately 1 nm or more (e.g., 3 nm or more), and for example, from the perspective of reducing moisture permeability, it can be about 5 nm or more, or about 10 nm or more (e.g., 15 nm or more).

In the aforementioned substrate layer having a main substrate layer and a layer containing inorganic materials, the thickness of the main substrate layer (in the case of having multiple layers other than the layer containing inorganic materials, the total thickness of the layers other than the layer containing inorganic materials) is appropriately set to be 50% or more of the total thickness of the substrate layer, preferably 70% or more, more preferably 90% or more, or 97% or more (e.g., 99% or more).

The substrate layer can be a single layer or a multi-layer structure with two or more layers. Examples of a single-layer substrate layer include a substrate layer comprising a resin film. Substrate layers composed of resin films are suitable for surface protection sheets used in applications involving etching solutions and other chemical treatments. They also tend to have excellent flexibility and bendability. Examples of multi-layer substrate layers include those comprising a multi-layer resin film and those comprising a main substrate layer and layers containing inorganic materials.

In several samples, the substrate layer (the substrate film used as the substrate layer) preferably has a moisture permeability of 24 g/( ^m² ·day) or less, as measured by the cupping method. By designing a structure with such limited moisture permeability, even when used in samples that come into contact with aqueous liquids, such as those treated with chemicals or immersed in warm water, the presence of a low-permeability substrate layer can adequately prevent the penetration of aqueous liquids into the adhesive layer, thus preventing a decrease in adhesion based on water peelability, or suppressing the decrease in adhesion. As a result, adhesion to the substrate is maintained, and the surface protective sheet can maintain a close bond with the substrate. In several preferred embodiments, the aforementioned moisture permeability of the substrate layer is approximately 18 g/( ^m² ·day) or less, more preferably approximately 14 g/( ^m² ·day) or less, even more preferably approximately 10 g/( ^m² ·day) or less, particularly preferably approximately 8 g/( ^m² ·day) or less, and may also be approximately 5 g/( ^m² ·day) or less (e.g., approximately 3 g/( ^m² ·day) or less). Furthermore, when the surface protective sheet is exposed to heat such as warm water, if the aforementioned moisture permeability is excessively low, there is a risk that heat-induced aging may prevent effective water peeling. From this perspective, in several samples, the moisture permeability of the substrate layer is preferably 1 g/(m ²・day) or more, preferably about 3 g/(m ²・day) or more, and for example, it can also be more than 5 g/(m ²・day).

More specifically, the aforementioned moisture permeability of the substrate layer may be, for example, 23 g/( ^m² ·day) or less, 22 g/( ^m² ·day) or less, 21 g/( ^m² ·day) or less, 20 g/( ^m² ·day) or less, 19 g/( ^m² ·day) or less, 18 g/( ^m² ·day) or less, 17 g/( ^m² ·day) or less, 16 g/( ^m² ·day) or less, 15 g/( ^m² ·day) or less, 14 g/( ^m² ·day) or less, 13 g/( ^m² ·day) or less, 12 g/( ^m² ·day) or less, 11 g/( ^m² ·day) or less, 10 g/( ^m² ·day) or less, 9 g/( ^m² ·day) or less, 8 ... g/(m ²・day) or more or less, 7 g/(m ²・day) or more or less, 6 g/(m ²・day) or more or less, 5 g/(m ²・day) or more or less, 4 g/(m ²・day) or more or less, 3 g/(m ²・day) or more or less, 2 g/(m ²・day) or more or less, or 1 g/(m ²・day) or more or less.

The aforementioned moisture permeability of the substrate layer can be obtained by selecting an appropriate impermeable or low-permeability substrate material. More specifically, the moisture permeability of the substrate layer can be measured using the methods described in the embodiments below.

The 25°C flexural stiffness value of the substrate layer (the substrate film used as the substrate layer) can be set to the same range as the 25°C flexural stiffness value obtainable by the surface protection sheet described above, therefore, repeated explanations are omitted. Similarly, the ranges of the 25°C tensile modulus of elasticity, stress at 100% elongation at 25°C, fracture stress at 25°C, and fracture strain at 25°C obtainable by the substrate layer can also be set to the same ranges as the ranges of the 25°C tensile modulus of elasticity, stress at 100% elongation at 25°C, fracture stress at 25°C, and fracture strain at 25°C for the surface protection sheet, therefore, repeated explanations are omitted. Regarding the 25°C flexural stiffness, 25°C tensile modulus of elasticity, 25°C stress at 100% elongation, 25°C breaking stress, and 25°C breaking strain of the substrate layer, in addition to using the substrate layer (the substrate film used as the substrate layer) as a test piece, the same methods as those used for the surface protection sheet can be employed to determine these parameters. The thickness and cross-sectional area of the test piece used to calculate the 25°C flexural stiffness, 25°C tensile modulus of elasticity, 25°C stress at 100% elongation, and 25°C breaking stress can be obtained using the same materials as the substrate layer. Furthermore, the 25°C bending stiffness value of the substrate layer and the 25°C bending stiffness value of the surface protective sheet can both be the 25°C bending stiffness value of the MD or the 25°C bending stiffness value of the TD. Therefore, it can be the 25°C bending stiffness value of at least one of the 25°C bending stiffness values of the MD and the 25°C bending stiffness values of the TD, or it can be the 25°C bending stiffness value in any direction of the MD or TD. Similarly, the 25°C tensile modulus of elasticity of the substrate layer can be either the 25°C tensile modulus of elasticity of the substrate (MD) or the 25°C tensile modulus of elasticity of the substrate (TD). Therefore, it can be the 25°C tensile modulus of elasticity of at least one of the 25°C tensile modulus of elasticity of the substrate (MD) and the 25°C tensile modulus of elasticity of the substrate (TD), or it can be the 25°C tensile modulus of elasticity of the substrate (MD). Likewise, the stress at 100% elongation, the breaking stress, and the breaking strain of the substrate layer can be the measured values of the substrate (MD) (stress at 100% elongation, breaking stress, or breaking strain), or they can be the measured values of the substrate (TD). Therefore, they can be the measured values of the substrate (MD) and the TD, or they can be the measured values of the substrate (MD) and the substrate (TD).

The thickness of the substrate layer is not particularly limited and can be selected according to the purpose of protection or usage. The thickness of the substrate layer can be, for example, less than approximately 1000 μm, or less than approximately 300 μm. From the perspective of lightweighting or thinning, it is suitable to be less than approximately 100 μm, preferably less than approximately 75 μm (typically less than 75 μm), even more preferably less than approximately 50 μm, and can also be less than 40 μm or less, or less than 30 μm. If the thickness of the substrate layer is reduced, there is a tendency to improve the flexibility of the surface protection sheet or the ability to follow the surface shape of the adherend. Furthermore, by limiting the thickness of the substrate layer, deformation (expansion and contraction) of the substrate layer caused by heating is suppressed. Therefore, even when used in heated conditions such as immersion in warm water, there is a tendency to easily maintain the adhesion to the substrate. Also, from the viewpoint of operability or processability, the thickness of the substrate layer can be, for example, 2 μm or more, or even more than 5 μm. In several cases, the thickness of the substrate layer is preferably about 10 μm or more, more preferably about 15 μm or more, even more preferably about 20 μm or more, and can also be about 30 μm or more, or 40 μm or more, or even 50 μm or more. There is a tendency that the greater the thickness of the substrate layer, the better the protection against the penetration of pharmaceutical solutions into the substrate.

For the adhesive layer side surface of the substrate layer, previously known surface treatments such as corona treatment, plasma treatment, ultraviolet irradiation treatment, acid treatment, alkali treatment, and primer application can be applied as needed. Such surface treatments can be used to improve the adhesion between the substrate layer and the adhesive layer, in other words, the adhesion of the adhesive layer to the substrate layer. The composition of the primer is not particularly limited and can be appropriately selected from known sources. The thickness of the primer layer is not particularly limited; for example, it is suitable to be around 0.01 μm to 1 μm, and preferably around 0.1 μm to 1 μm. Furthermore, the surface of the substrate main layer (typically, the side surface containing inorganic materials) can also be treated with the above-mentioned surface treatments, antistatic treatments, and other surface treatments.

For the side of the substrate layer opposite to the adhesive layer (hereinafter also referred to as the back side), previously known surface treatments such as peeling and antistatic treatments can be applied as needed. For example, by surface treating the back side of the substrate layer with a peeling agent, the unwinding force of surface protective sheets wound into rolls can be reduced. As peeling agents, polysiloxane-based peeling agents, long-chain alkyl-based peeling agents, olefin-based peeling agents, fluorinated peeling agents, fatty acid amide-based peeling agents, molybdenum sulfide, silica powder, etc., can be used.

<Total Thickness> The thickness of the surface protective sheet (including the adhesive layer and substrate layer, but excluding the peel-off pad) disclosed herein is not particularly limited. It can be 3 μm or more, 5 μm or more, appropriately 10 μm or more, and preferably 20 μm or more, more preferably 30 μm or more, and even more preferably 40 μm or more, and can also be 45 μm or more, from the viewpoint of step followability and adhesion to the adherend. There is a tendency that the greater the thickness of the surface protective sheet, the better the protection against drug penetration and other adverse reactions against the adherend. In several embodiments, the thickness of the surface protective sheet is greater than 50 μm, and can be 60 μm or more, 70 μm or more, or 80 μm or more. The maximum thickness of the surface protective sheet is, for example, 5 mm or less, but can be 3 mm or less, or even 1 mm or less. In several embodiments, the thickness of the surface protective sheet is preferably 300 μm or less (e.g., 200 μm or less), more preferably 100 μm or less, even more preferably 75 μm or less, and further preferably 65 μm or less, for example, 55 μm or less. By limiting the thickness of the surface protective sheet to a specific value, the surface protective sheet tends to: suppress deformation (expansion and contraction) due to heating, and easily maintain the adhesive state to the substrate. Thinning the adhesive sheet is also advantageous in terms of thinning, miniaturization, weight reduction, and resource conservation.

<Peel-Off Liner> The peel-off liner used in the surface protection sheet disclosed herein is not particularly limited. For example, a peel-off liner with a surface treated by peeling the liner substrate such as a resin film or paper, or a peel-off liner containing a low-adhesion material such as a fluorinated polymer (polytetrafluoroethylene, etc.) or a polyolefin resin (polyethylene, polypropylene, etc.) may be used. In the above-mentioned peeling treatment, for example, polysiloxane-based or long-chain alkyl-based peeling agents may be used. In several embodiments, a peel-treated resin film may preferably be used as the peel-off liner.

<Peeling Method> According to this specification, a method for peeling a surface protective sheet adhered to an adhesive (the object being protected) is provided. The peeling method includes a water-peeling step, in which, while the surface protective sheet is at the interface between the adhesive and the surface protective sheet at a peeling line from the adhesive, an aqueous liquid is present. The aqueous liquid moves along the peeling line and enters the interface, thereby peeling the surface protective sheet from the adhesive. According to the water-peeling step, the aqueous liquid can be effectively used to peel the surface protective sheet from the adhesive.

As an aqueous liquid, it can be used in water or a mixture of solvents with water as the main component, containing a small amount of additives as needed. As a solvent other than water constituting the aforementioned mixture, lower alcohols (e.g., ethanol) or lower ketones (e.g., acetone) that are homogeneous with water can be used. As the aforementioned additives, known surfactants can be used. From the viewpoint of avoiding contamination of adherents, in several aspects, an aqueous liquid that is substantially free of additives is preferable. From an environmental hygiene perspective, water is particularly preferred as the aqueous liquid. There are no particular limitations on the type of water; considering the purity required for the intended use or ease of acquisition, distilled water, ionized water, tap water, etc., can be used, for example.

In several cases, the above-described peeling method can preferably be performed, for example, in the following case: similar to the determination of normal water peeling force FW0, an aqueous liquid is supplied to the adherend near the outer edge of the surface protective sheet, so that the aqueous liquid enters from the outer edge of the surface protective sheet to the interface between the surface protective sheet and the adherend, and no new water is supplied (i.e., only the aqueous liquid supplied to the adherend before peeling begins) to peel the surface protective sheet. Furthermore, if the water that follows the movement of the peeling line to the interface between the surface protective sheet and the adherend dries up midway through the water peeling process, water can be supplied intermittently or continuously after the water peeling process begins. For example, this approach of supplying water after the water peeling process is preferable when the adherend is absorbent or when aqueous liquids are easily retained on the surface or interface of the adherend after peeling.

The amount of aqueous liquid supplied before peeling begins is only sufficient to introduce the aqueous liquid from outside the adhesion area of the surface protective sheet to the interface between the surface protective sheet and the adhered object; there is no particular limitation. The amount of the aqueous liquid may be, for example, 5 μL or more, typically suitable as 10 μL or more, or even 20 μL or more. Furthermore, there is no particular upper limit on the amount of the aqueous liquid. In some cases, from the viewpoint of improving workability, the amount of the aqueous liquid may be, for example, 10 mL or less, 5 mL or less, 1 mL or less, 0.5 mL or less, 0.1 mL or less, or 0.05 mL or less. By reducing the amount of the aforementioned aqueous liquid, the process of removing the aqueous liquid by drying or wiping after peeling off the surface protective sheet can be omitted or simplified.

The operation of allowing the aqueous liquid to enter the interface between the surface protective sheet and the substrate at the start of peeling can be performed as follows: inserting the tip of a cutting tool or needle into the interface at the outer edge of the surface protective sheet; scraping and lifting the outer edge of the surface protective sheet using a hook or fingernail; or attaching a strong adhesive tape or suction cup to the back side near the outer edge of the surface protective sheet and lifting the end of the surface protective sheet. By forcibly allowing the aqueous liquid to enter the interface from the outer edge of the surface protective sheet in this way, a state in which the aqueous liquid exists at the interface between the substrate and the surface protective sheet can be formed efficiently. Furthermore, it can better balance the good water peelability after the operation of forcibly introducing an aqueous liquid into the interface to form the starting point of peeling, and the high water resistance reliability when the operation is not carried out.

<Applications> The surface protection sheet disclosed herein can be used for various applications. For example, in various processes such as chemical treatment of glass or semiconductor wafers, metal plates, etc., or physical treatments such as cutting or grinding, the surface protection sheet disclosed herein can be applied to the untreated surface of the object being protected.

There is no particular limitation on the type of object to be protected. The surface protective sheet disclosed herein can be used to protect various components or materials. The surface protective sheet disclosed herein achieves peeling without damaging or deforming the adhered material during peeling by utilizing water peeling, thus making it suitable for protecting glass materials such as alkaline glass or semiconductor wafers. These materials are typically brittle materials (also known as hard and brittle materials) with limited thickness, easily prone to cracking, breakage, or fissures due to external forces during handling or peeling. By applying water peeling to such adhered materials, damage to the adhered material during peeling can be better prevented. The glass material used to protect the aforementioned object can be, for example, a glass plate with a surface partially covered with a transparent conductive film (e.g., ITO (indium tin oxide) film) or FPC (flexible printed circuit) used in tablet computers, mobile phones, or organic LEDs (light-emitting diodes). Furthermore, as a preferred example of the object to be protected, glass plates used for window glass or cover glass in foldable or rollable displays can be cited. These glass plates are thinner (e.g., less than 100 μm thick), and therefore have a greater risk of breakage. However, according to the technology disclosed herein, even when the object to be protected is a thin, brittle material as described above, damage to the object during peeling can be prevented.

There is no particular limitation on the water contact angle of the surface of the object to which the surface protection sheet is attached. In some embodiments, the surface of the object to be protected may be a hydrophilic surface exhibiting a water contact angle of, for example, 60 degrees or less, preferably 50 degrees or less. In some preferred embodiments, the water contact angle of the aforementioned surface may be, for example, 45 degrees or less, 40 degrees or less, 35 degrees or less, or 30 degrees or less. If the aforementioned water contact angle is smaller, water can easily wet and diffuse along the surface of the adherend, and the desired water peelability can be easily obtained. The surface protection sheet disclosed herein is preferably used to protect components having a surface with a water contact angle of approximately 20 degrees or less (e.g., less than 15 degrees, and further less than 10 degrees) (e.g., alkaline glass or alkali-free glass). The lower limit of the water contact angle of the surface to be protected is, in principle, 0 degrees. In some embodiments, the water contact angle of the surface to be protected may exceed 0 degrees, or may exceed 1 degree, or may exceed 3 degrees, or may exceed 5 degrees. In other embodiments, the water contact angle of the surface to be protected may exceed 30 degrees, or may exceed 50 degrees, or may exceed 60 degrees (e.g., more than 70 degrees). The surface protection sheet disclosed herein can be used for various materials with different water contact angles. The water contact angle of the surface of the object to be protected can be measured by the same method as the contact angle measurement method described in the embodiments described later.

The thickness of the protected object (e.g., a glass plate or semiconductor wafer) is not particularly limited; for example, it can be about 1 mm or less, or about 500 μm or less, or about 300 μm or less. For the effect of the technology disclosed herein (preventing damage during peeling) to be more effective for thinner protected objects, the aforementioned thickness can be, for example, about 150 μm or less, or about 100 μm or less. The lower limit of the aforementioned thickness is, for example, about 10 μm or more (e.g., 30 μm or more).

In several preferred embodiments, the surface protection sheet is suitable for use in steps involving the chemical and/or physical treatment of objects such as glass or semiconductor wafers in a liquid. In the aforementioned applications, the surface protection sheet disclosed herein possesses the adhesion required to protect the object during the aforementioned treatment, and upon peeling after treatment, allows for smooth peeling of the object (adhesive) using water. The aforementioned chemical treatment includes treatment using etching solutions containing acids or alkalis, such as aqueous solutions of hydrofluoric acid. For example, the surface protection sheet disclosed herein is preferably used in etching processes that dissolve glass with a chemical solution (etching solution) to adjust the thickness of glass or remove burrs or microcracks formed on the cut ends of glass; anti-glare processing; etching processes that locally corrode the surface of metals with a chemical solution (etching solution); and coating processes that locally coat the connection terminals of circuit boards (printed circuit boards, flexible printed circuit boards (FPCs), etc.) with a chemical solution (coating solution). It is particularly suitable for applications where etching is performed using acidic solutions such as hydrofluoric acid. Furthermore, physical treatments include grinding or cutting to protect the surface of the object.

The surface protection sheet disclosed herein is preferably used for glass thinning processes. For example, glass plates used as optical components can be thinned by glass thinning processes using solutions such as hydrofluoric acid aqueous solutions. In this glass thinning process, the surface protection sheet can be used to protect the untreated surfaces of the glass. Although not particularly limited, in the glass thinning process, the glass plate is thinned to, for example, about 150 μm or less (for example, about 100 μm or less). Furthermore, the thickness of the glass plate before the glass thinning process can be, for example, about 0.15 mm to 5 mm, or about 300 μm or more (for example, about 500 μm to 1000 μm). Thinned glass is prone to breakage due to external forces during peeling. However, by using the surface protection sheet disclosed in this article, the problem of glass breakage during the peeling of the surface protection sheet can be eliminated, or the risk can be significantly reduced.

Furthermore, surface protection sheets can also be preferably used in the manufacture of semiconductors. The aforementioned semiconductor wafers can be, for example, silicon wafers, silicon carbide (SiC) wafers, nitride semiconductor wafers (silicon nitride (SiN), gallium nitride (GaN), etc.), gallium arsenide wafers, and other compound semiconductor wafers. In the manufacture of the aforementioned semiconductors, the surface protection sheets disclosed herein can be preferably used in semiconductor wafer processing steps (typically, silicon wafer processing), such as steps to thin the semiconductor wafer (more specifically, back-side grinding steps to grind the back side of the semiconductor wafer) or steps to dicing the semiconductor wafer (e.g., dicing). In the back-side grinding step, the semiconductor wafer is thinned to, for example, less than 150 μm (e.g., less than 100 μm), but there is no particular limitation. Furthermore, the thickness of the semiconductor wafer before back-side grinding can be more than 300 μm (e.g., around 500 μm to 1000 μm). The aforementioned semiconductor manufacturing steps may expose the wafer to temperatures above room temperature (e.g., 40°C to 90°C, preferably 40°C to 60°C), therefore, the use of a surface protection sheet that maintains good properties (adhesion and water peelability) under heat is particularly meaningful. Furthermore, the surface protection sheet used in this application is sometimes simply referred to as a back-side grinding sheet or a dicing sheet.

In several embodiments, the surface protection sheets used in the various surface protection applications described above can be used in the following manner: with one or more objects to be treated having a surface protection sheet adhered to one side, the objects are continuously or individually transported into a chemical tank or washing tank, or other water, using a conveying mechanism such as rollers, for targeted treatment. During or after the aforementioned transport process (including placement or installation on a device), the objects to be protected may sometimes be subjected to unavoidable or unintentional external forces such as impact, vibration, or deformation. For example, as a conveying mechanism used in liquid treatment or cleaning, multiple rollers configured with specific intervals can be used. However, when conveying using these rollers, a peeling load with a small peeling angle can be continuously applied due to the height difference between the rollers, vibration, etc. The surface protective sheet disclosed herein has the following advantages: excellent end-peeling prevention against external forces such as vibration. Therefore, even when used in a process including a step of treating the object to be protected in a liquid while it is attached to the object, it is not easy to peel off from the ends due to external forces such as vibration during the process.

Furthermore, the objects of protection mentioned above may be components constituting machines such as liquid crystal display devices, organic EL (electroluminescent) display devices, PDP (plasma display panels), electronic paper and other display devices (image display devices), or input devices such as touch panels (optical machines), especially portable electronic machines such as foldable or rollable displays.

Examples of portable electronic devices mentioned above include: mobile phones, smartphones, tablets, laptops, various wearable devices (e.g., wrist-worn types like watches, modular types worn on the body via clips or straps), and eyewear (including monocular or binocular types, and headband types). (Wear) type, such as clothing-type accessories attached to shirts, socks, hats, etc., and ear-worn types like headphones, etc.), digital cameras, digital video cameras, audio equipment (portable music players, IC recorders, etc.), calculators (electronic desktop calculators, etc.), portable game consoles, electronic dictionaries, electronic notebooks, e-books, in-vehicle information equipment, portable radios, portable televisions, portable printers, portable scanners, portable modems, etc. Furthermore, in this manual, the term "portable" does not simply mean "able to carry," but rather refers to a level of portability that is relatively easy for a standard adult to move.

<Protection Method> As described above, according to this specification, a surface protection method using the surface protection sheet disclosed herein is provided. The surface protection method includes: a step of attaching the surface protection sheet to at least a portion of the object to be protected (the surface to be protected or the portion to be protected); a step of treating the object to which the surface protection sheet is attached (e.g., chemical treatment, warm water immersion treatment, water immersion treatment, physical treatment such as cutting or grinding); and a step of peeling the surface protection sheet from the object to be protected after the above treatment. Preferably, the step of peeling the surface protection sheet from the object to be protected includes the water peeling step described above. The above-described protection method, because it includes the treatment of the object to be protected (the object to be treated), is also referred to as a treatment method. Details regarding the surface protection sheet, the water peeling process, the object to be protected, the treatment (glass thinning, semiconductor wafer thinning, etc.), and other matters (applications, etc.) are as described above, and therefore, repeated explanations are omitted. Typical examples of objects to be protected include glass plates and semiconductor wafers to which glass thinning and other treatments are to be performed. Therefore, according to this specification, a glass thinning method and a semiconductor manufacturing method including the above-described steps can be provided.

<Processing Method> Furthermore, as described above, according to this specification, a processing method using the surface protection sheet disclosed herein is provided. This processing method includes: a step of attaching the surface protection sheet to the surface of the object to be processed; a step of processing the object to which the surface protection sheet is attached; and a step of removing the surface protection sheet from the object to be processed (the processed object) after the above processing. Typically, the surface of the object to which the surface protection sheet is attached is a surface with a water contact angle of 20 degrees or less. Also, in the above processing step, the object to be processed is typically in contact with a liquid (e.g., an aqueous solution). Furthermore, the step of removing the surface protective sheet from the treated object (treated object) is preferably a step of removing the surface protective sheet from the treated object (treated object) by water peeling (peeling in the presence of water). The treatment method disclosed herein may include at least one step of the above-mentioned protection method. As the surface protective sheet, the surface protective sheet disclosed herein may be used. Specifically, a surface protective sheet that satisfies the requirement that the water peeling force FW1 [N/20 mm] after immersion in warm water for 30 minutes is less than 50% of the adhesion force F1 [N/20 mm] after immersion in warm water for 30 minutes. As an example of processing, glass thinning or semiconductor processing can be cited. Typical examples of the objects to be processed include glass plates and semiconductor wafers to be subjected to glass thinning or similar processes. Therefore, according to this specification, a glass thinning method and a semiconductor manufacturing method including the above-described steps can be provided. Details regarding the surface protection sheet, water peeling, removal steps, objects to be processed, processing (glass thinning, semiconductor wafer thinning, etc.), and other matters (applications, etc.) are as described above, and therefore, repeated descriptions are omitted. [Example]

The following describes several embodiments related to the present invention, but it is not intended to limit the present invention to those embodiments. Furthermore, in the following description, unless otherwise stated, "parts" and "%" are based on weight.

<Evaluation Method> [Normal Adhesion F0] The surface protection sheet of the test object was cut into specimens with a width of 20 mm and a length of 100 mm to prepare test pieces. At 23°C and 50%RH, the peeling pad covering the adhesive surface (adhesive layer surface) was peeled off from the test piece. A 2 kg rubber roller was used to press the exposed adhesive surface against an alkaline glass plate (a surface of alkaline glass having a water contact angle of less than 20 degrees) serving as the bonded object. The obtained evaluation sample was then subjected to autoclave treatment (50°C, 0.5 MPa, 15 minutes). The evaluation sample taken from the autoclave was kept at 23°C and 50%RH for 1 hour. Then, under the same conditions, according to JIS Z0237:2009, 10.4.1, Method 1: For the 180° peel adhesion of the test plate, the peel strength of the specimen from the adherend was measured using a tensile testing machine at a tensile speed of 300 mm/min and a peel angle of 180 degrees (wherein, the peel strength refers to the period before transitioning to the water peel strength measurement below, i.e., before distilled water is supplied to the peel interface). The measurement was performed 3 times, and the average value of the equal values was set as the normal adhesion force F0 [N/20 mm]. The above-mentioned normal adhesion force F0 is measured by peeling the specimen attached to the substrate from bottom to top. The substrate can be an alkaline glass plate (product name "Micro Slide Glass" S200423, manufactured by Matsunami Glass Co., Ltd.). The tensile testing machine can be a universal tensile-compression testing machine (device name "Tensile-Compression Testing Machine, TCM-1kNB", manufactured by Minebea Co., Ltd.) or an equivalent. During the test, a suitable backing material can also be attached to the opposite side of the surface protective sheet (the surface opposite to the bonding surface) to reinforce the specimen, if necessary. For example, a polyethylene terephthalate (PET) film with a thickness of approximately 25 μm can be used as the backing material.

[Normal Water Peeling Force FW0] In the determination of the normal adhesion force F0 described above, during the determination of the peel strength of the specimen from the adhered body, 20 μL of distilled water is supplied to the part of the specimen from the adhered body (the peel line), and the peel strength after the supply of distilled water is measured. The measurement is performed three times for each measurement of the normal adhesion force F0, and the average value is set as the normal water peeling force FW0 [N/20 mm]. The measurement conditions for the peel strength after the supply of distilled water are based on JIS Z0237:2009, 10.4.1, Method 1: For the 180° peel adhesion of the test plate. Specifically, the tensile testing machine is set to a tensile speed of 300 mm/min and a peel angle of 180 degrees at a test temperature of 23°C. Furthermore, the determination of the normal water peel force FW0 can be performed continuously on each specimen, measuring both the normal adhesion force FW0 and the normal water peel force FW0. Alternatively, different specimens can be used for these measurements. For example, in cases where it is difficult to prepare specimens of sufficient length for continuous testing, different specimens can be used. The same applies to the adhesive, tensile testing machine, and other matters as for the determination of the normal adhesion force FW0.

[Adhesion Strength F1 After 30-Minute Warm Water Immersion] The adhesion strength F1 after 30-minute warm water immersion is determined by immersing the evaluation sample (an alkaline glass plate with a test piece (surface protective sheet) attached) in warm water at 60℃±2℃ for 30 minutes, then removing it from the warm water and wiping off the water, and measuring the peel strength. Otherwise, it is measured using the same method as the normal adhesion strength F0. Specifically, similar to the determination of the normal adhesion strength F0, the surface protective sheet of the test object is cut into test pieces with a width of 20 mm and a length of 100 mm to prepare the test pieces. At 23°C and 50%RH, the peeling pad covering the adhesive layer was peeled off from the test piece. A 2 kg rubber roller was then used to press the exposed adhesive layer onto an alkaline glass plate serving as the bonded object. The resulting evaluation sample was then subjected to autoclaving (50°C, 0.5 MPa, 15 minutes). The evaluation sample removed from the autoclave was then immersed in a water bath containing warm water at a set temperature of 60°C ± 2°C for 30 minutes. Ionized water or distilled water was used as the warm water. In the warm water, the evaluation sample was kept horizontal with the adhesive layer side facing upwards. The distance from the upper surface of the evaluation sample to the water surface (immersion depth) is set to 10 mm or more (e.g., approximately 10 mm to 100 mm). Then, the evaluation sample is lifted from the warm water, and the water adhering to the sample is gently wiped away. Under conditions of 23°C and 50%RH, the peel strength of the test piece from the adhered body is measured using a tensile testing machine at a tensile speed of 300 mm/min and a peel angle of 180 degrees (the peel strength refers to the period before the water peel strength measurement described below, i.e., before distilled water is supplied to the peel interface). The test was performed three times, and the average value was set as the adhesion force F1 [N/20 mm] after immersion in warm water for 30 minutes. The time from when the evaluation sample was removed from the warm water to when the peel strength was measured was set to be within 10 minutes. Regarding the adherend, tensile testing machine, and other matters, the same procedures applied as for the determination of the normal adhesion force F0.

[Water Peeling Force After 30-Minute Warm Water Immersion (FW1)] The water peeling force after 30-minute warm water immersion (FW1) is determined by immersing the evaluation sample (an alkaline glass plate with a test piece (surface protective sheet) attached) in warm water at 60℃±2℃ for 30 minutes, then removing it from the warm water and wiping off the water. The water peeling force is then measured. Otherwise, it is measured using the same method as the normal water peeling force (FW0). Specifically, in the determination of adhesion force F1 after 30 minutes of warm water immersion, after the 30-minute warm water immersion, in the determination of the peel strength of the specimen from the adhered body, 20 μL of distilled water was supplied to the part of the specimen from the adhered body (the peel line) and the peel strength after the supply of distilled water was measured. The measurement was performed three times for each determination of adhesion force F1 after 30 minutes of warm water immersion, and the average value of these measurements was set as the water peel force FW1 [N/20 mm] after 30 minutes of warm water immersion. The peel strength after distillation was determined according to JIS Z0237:2009, 10.4.1, Method 1: 180° peel adhesion of the test plate. Specifically, a tensile testing machine was used at a test temperature of 23°C, a tensile speed of 300 mm/min, and a peel angle of 180 degrees. Furthermore, the determination of the water peeling force FW1 after 30 minutes of warm water immersion can be performed continuously on each specimen, including both the adhesion force F1 and the water peeling force FW1 after 30 minutes of warm water immersion. Alternatively, different specimens can be used to perform the determination of both the adhesion force F1 and the water peeling force FW1 after 30 minutes of warm water immersion. Regarding the substrate, tensile testing machine, and other matters, the same applies as for the determination of the normal adhesion force F0.

[Adhesion Force F2 after 1-Hour Warm Water Immersion] The adhesion force F2 after 1-hour warm water immersion is determined by setting the warm water immersion time to 1 hour. Otherwise, it is measured using the same method as the adhesion force F1 after 30-minute warm water immersion. Specifically, similar to the determination of the normal adhesion force F0, the surface protective sheet of the test object is cut into test pieces with a width of 20 mm and a length of 100 mm. At 23°C and 50%RH, the peeling pad covering the bonding surface (adhesive layer surface) is peeled off from the test piece. A 2 kg rubber roller is then used to press the exposed bonding surface against an alkaline glass plate serving as the bonded object. The obtained evaluation samples were subjected to autoclave treatment (50°C, 0.5 MPa, 15 minutes). The evaluation samples removed from the autoclave were then immersed in a water bath containing warm water at a set temperature of 60°C ± 2°C for 1 hour. The warm water immersion conditions were the same as those for the adhesion force F2 after 30 minutes of warm water immersion. Next, the evaluation sample was removed from the warm water, and the water adhering to the sample was gently wiped off. The sample was then tested at 23°C and 50%RH according to JIS Z0237:2009, 10.4.1, Method 1: 180° peel strength of the test plate. The peel strength of the specimen from the adherend was measured using a tensile testing machine at a tensile speed of 300 mm/min and a peel angle of 180 degrees (the peel strength refers to the period before transitioning to the water peel strength measurement described below, i.e., before distilled water is supplied to the peel interface). The measurement was performed three times, and the average value was set as the adhesion force F2 [N/20 mm] after 1 hour of warm water immersion. Regarding the adherend, tensile testing machine, and other matters, the same procedures apply as for determining the adhesion force F2 after 30 minutes of warm water immersion.

[Water Peeling Force FW2 After 1-Hour Warm Water Immersion] The water peeling force FW2 after 1-hour warm water immersion is determined by setting the warm water immersion time to 1 hour, and otherwise using the same method as the water peeling force FW1 after 30-minute warm water immersion. Specifically, in the above-mentioned determination of adhesion force F2 after 1-hour warm water immersion, after 1 hour of warm water immersion, in the determination of the peeling strength of the specimen from the adhered body, 20 μL of distilled water is supplied to the part of the specimen from the adhered body that begins to separate (the peeling line), and the peeling strength after the supply of distilled water is measured. The measurement was performed three times (i.e., after each 1-hour warm water immersion) for the adhesion force F2, and the average value of these measurements was set as the water peeling force FW2 [N/20 mm] after 1-hour warm water immersion. Regarding the adherend, tensile testing machine, and other matters, the same procedures apply as for the measurement of the water peeling force FW1 after 30-minute warm water immersion.

In the above measurements (F0, FW0, F1, FW1, F2, and FW2), the substrate used is one in which the contact angle between the surface of the test piece and the distilled water is 20 degrees or less (e.g., 5 to 10 degrees). Specifically, an alkaline glass plate can be used as the substrate, which is manufactured using the float glass method, and the contact angle between the surface of the test piece and the distilled water is 20 degrees or less (e.g., 5 to 10 degrees). While the alkaline glass plate manufactured by Matsunami Glass Industry Co., Ltd. can be used as such a substrate, it is not limited to this; equivalents of the alkaline glass plate manufactured by Matsunami Glass Industry Co., Ltd., or other alkaline glass plates, can also be used.

Furthermore, the contact angle of the aforementioned alkaline glass plate was measured using the following method: Under a measuring atmosphere of 23°C and 50%RH, a contact angle meter (manufactured by Kyowa Interface Science Co., Ltd., trade name "DMo-501", control box "DMC-2", control/analysis software "FAMAS (version 5.0.30)") was used to measure the angle using the droplet method. The amount of distilled water added was set to 2 μL, and the contact angle was calculated using the Θ/2 method based on the image obtained 5 seconds after addition (implemented at N5).

Furthermore, the inventors have confirmed that the adhesion and water peeling forces after immersion in warm water exhibit a relatively high correlation with those after immersion in hydrofluoric acid aqueous solution. Based on this observation, the adhesion and water peeling forces after immersion in warm water are set as evaluation indicators for the applicability of surface protective sheets in liquid treatment applications, including chemical immersion.

[Moisture Permeability] The moisture permeability of the substrate (layer) and surface protective sheet was determined according to the moisture permeability test (cup method) of JIS Z0208. The method for determining the moisture permeability of the substrate is as follows: The substrate of each example was cut into a circle with a diameter of 7 cm and used as an evaluation sample. Then, a specific amount of calcium chloride was added to the inside of the test cup (made of aluminum, a moisture permeability cup as specified in JIS Z0208), and the cup was sealed with the above-mentioned evaluation sample. Specifically, the evaluation sample is placed on the test cup to cover its opening. A ring-shaped gasket and cap with the same shape as the edge of the test cup's opening (6 cm inner diameter, 9 cm outer diameter, and 1.5 cm edge width) are then placed on top and secured with a special screw plug to seal the interior of the test cup. The cup covered by the evaluation sample is then stored at 40°C and 92%RH for 24 hours, and the change in total weight before and after storage is measured (specifically, the weight change based on the water absorption of calcium chloride). The humidity permeability [g/( ^m² ·day)] is then determined. The moisture permeability of the surface protective sheet is measured by placing the surface protective sheet with the cup side as the contact surface instead of the substrate and sealing the cup opening. Otherwise, it is measured using the same method as the moisture permeability measurement method for the substrate.

[60°C Loss of Elastic Modulus G"] The 60°C loss of elastic modulus G" [Pa] of the adhesive layer is determined by dynamic viscoelasticity measurement. Specifically, several layers of the adhesive layer to be measured are overlapped to create an adhesive layer with a thickness of approximately 2 mm. The adhesive layer was punched into a disc shape with a diameter of 7.9 mm to obtain a sample. The sample was fixed using parallel plates and its dynamic viscoelasticity was measured using a viscoelastic testing machine (e.g., TA Instruments, ARES, or equivalent) under the following conditions to determine the loss modulus of elasticity G" [Pa] at 60°C. ・Measurement mode: Shear mode ・Temperature range: -70°C to 150°C ・Heating rate: 5°C/min ・Measurement frequency: 1 Hz Furthermore, the adhesive layer used for testing can be formed by applying the corresponding adhesive composition in layers and then drying or hardening it.

[Tensile Test] Surface protective sheet was cut into short strips with a width of 10 mm to prepare specimens. The specimens were stretched under the following conditions according to JIS K 7161 to obtain the stress-strain curve. (Stretching Conditions) Measurement Temperature: 25°C; Tensile Speed: 300 mm/min; Grip Distance: 50 mm. A universal tensile-compression testing machine (device name "Tensile-Compression Testing Machine, TCM-1kNB", manufactured by Minebea Corporation) or an equivalent can be used as the tensile testing machine. The tensile modulus of elasticity [Pa] at 25°C was determined by linear regression of the stress-strain curve described above. Furthermore, the tensile modulus of elasticity at 25°C is calculated by subtracting the thickness of the adhesive layer from the measured thickness of the surface protective sheet, or by measuring the thickness of the substrate layer itself, and then converting it into the value per unit cross-sectional area of the substrate layer. Also, based on the above tensile test, the stress [N/ ^mm² ] at 100% elongation, the breaking stress [N/ ^mm² ], and the breaking strain [%] at 25°C were measured. The stress at 100% elongation was obtained by dividing the load [N] measured at 100% elongation of the specimen in the above tensile test by the cross-sectional area [ ^mm² ] of the substrate layer of the specimen. The aforementioned fracture stress is obtained by dividing the load [N] at which the specimen breaks in the tensile test by the cross-sectional area [ ^mm² ] of the substrate layer of the specimen, and the aforementioned fracture strain [%] is the elongation [%] of the specimen at fracture. Furthermore, the measured values (tensile modulus of elasticity, stress at 100% elongation, fracture stress, and fracture strain) in this embodiment are the measured values of MD obtained by aligning the MD of the surface protective sheet (more specifically, the substrate layer) with the tensile direction of the aforementioned tensile test. However, in addition to performing the tensile test on the MD of the surface protective sheet, the measured value of TD can also be obtained by performing the aforementioned tensile test on the TD of the surface protective sheet by changing the cutting method of the specimen. Alternatively, the above tensile test can be performed in either direction of MD or TD to obtain the measured value.

[25℃ Bending Stiffness Value] The 25℃ bending stiffness value D [Pa・m ³ ] of the surface protective sheet is calculated according to the formula: D＝Eh ³ /12(1－ν ² ); In the above formula, E is the tensile modulus of elasticity of the surface protective sheet at 25℃ [Pa], h is the thickness of the substrate layer [m]. ν is Poisson's ratio, which is set to ν＝0.35 in the above formula. Furthermore, the 25°C bending stiffness value in this embodiment is the 25°C bending stiffness value of MD. However, as mentioned above, by changing the cutting method of the test piece, not only the 25°C bending stiffness value of MD can be obtained, but also the 25°C bending stiffness value of TD can be obtained, or the 25°C bending stiffness value of MD or TD in any direction can be obtained.

[Waterborne Peeling Force] A test piece was prepared by cutting the surface protective sheet of the test object into pieces 10 mm wide and 100 mm long. At 23°C and 50%RH, a peeling pad covering the adhesive layer was peeled off the test piece. A 2 kg rubber roller was used to press the exposed adhesive surface against an alkaline glass plate (a surface of alkaline glass having a water contact angle of less than 20 degrees) serving as the substrate. The test piece was then fitted so that one end of its length extended from the substrate. The resulting evaluation sample was then subjected to autoclaving (50°C, 0.5 MPa, 15 minutes). After the evaluation sample is removed from the autoclave and kept in an environment of 23°C and 50%RH for 1 hour, it is immersed in water at room temperature (23°C to 25°C). Ionized water or distilled water is used as the water. In the water, the evaluation sample is kept horizontal with the side with the test piece attached facing upwards. The distance from the upper surface of the evaluation sample to the water surface (immersion depth) is set to be more than 10 mm (e.g., about 10 mm to 100 mm). Thus, with the evaluation sample placed in water, a peeling test was conducted using a tensile testing machine at 23°C and 50% RH within one minute of the start of immersion, starting from one end of the specimen along its length (the end extending from the adherend). The test was performed at a tensile speed of 1000 mm/min and a peeling angle of 20 degrees. The maximum stress applied at the initial stage of peeling was recorded. The test was performed three times, and the average of the maximum stress was set as the initial peeling force in water [N/10 mm]. If the initial peeling force in water is 0.2 N/10 mm or higher, it is considered to have excellent end-peeling performance against external forces such as vibration during the conveying process. External forces such as vibration, which may cause end peeling of surface protective sheets during handling and other manufacturing processes, can be considered as high-speed peeling loads applied to the protected object at relatively small angles. It can be determined that surface protective sheets exhibiting an underwater initial peeling force of 0.2 N/10 mm or more under the aforementioned conditions of a peeling angle of 20 degrees and a peeling speed of 1000 mm/min have greater stress before peeling under the aforementioned peeling load, demonstrating excellent end peeling resistance against external forces such as vibration. In the above tests, an alkaline glass plate (product name "Micro Slide Glass" S200423, manufactured by Matsunami Glass Co., Ltd.) can be used as the substrate. A universal tensile-compression testing machine (device name "Tensile-Compression Testing Machine, TCM-1kNB", manufactured by Minebea Co., Ltd.) or its equivalent can be used as the tensile testing machine. During the measurement, a suitable substrate can also be attached to the opposite side of the surface protective sheet (the surface opposite to the bonding surface) to reinforce the specimen, as needed. For example, a PET film with a thickness of approximately 25 μm can be used as the substrate.

≪Experiment 1≫ ＜Preparation of Adhesive＞ (Adhesive S1) In a reaction vessel equipped with a cooler, nitrogen inlet, thermometer, and stirring device, add 60 parts of n-butyl acrylate (BA), 6 parts of cyclohexyl acrylate (CHA), 18 parts of N-vinyl-2-pyrrolidone (NVP), 15 parts of 4-hydroxybutyl acrylate (4HBA), and 1 part of isostearate acrylate (iSTA) as monomer components; 0.125 parts of α-thioglycerol as a chain transfer agent; 122 parts of ethyl acetate as a polymerization solvent; and 0.2 parts of 2,2'-azobisisobutyronitrile (AIBN) as a thermal polymerization initiator. Solution polymerization is carried out under a nitrogen atmosphere to obtain a solution containing acrylic polymers. The proportion of polar monomers in 100 g of this acrylic polymer is 0.27 mol.

In the solution obtained above, for every 100 parts of the monomeric component used to prepare the solution, the following are added: an isocyanate-based crosslinking agent (trihydroxymethylpropane/phenylenedimethyl diisocyanate adduct, manufactured by Mitsui Chemicals, trade name: Takenate D-110N, solids concentration 75%) at a solids concentration of 0.05 parts; dioctyltin dilaurate (manufactured by Tokyo Fine Chemicals, trade name: EMBILIZER OL-1) as a crosslinking promoter; acetoacetone as a crosslinking retarder; and a nonionic surfactant (polyoxyethylene sorbitan monolaurate, HLB16.7, trade name: RHEODOL) as a hydrophilic agent. 0.1 parts of TW-L120 (manufactured by Kao Corporation) were mixed evenly to prepare solvent-based adhesive composition S1.

A 38 μm thick release liner (manufactured by Mitsubishi Resin Corporation, MRF#38) with one side of a polyester film serving as the release surface was prepared, as well as another 38 μm thick release liner (manufactured by Mitsubishi Resin Corporation, MRE#38) with one side of a polyester film serving as the release surface. The solvent-based adhesive composition S1 prepared above was applied to the release surface of one release liner (MRF#38), dried at 60°C for 3 minutes, and then dried at 120°C for 3 minutes to form an adhesive layer with a thickness of 25 μm. The release surface of the other release liner (MRE#38) was then bonded to this adhesive layer for protection. Thus, an adhesive layer S1 with a surface protected by two peel-off films is obtained. The elastic modulus G" of the adhesive layer S1 at 60°C is 32.5 kPa.

(Adhesive S2) Without using a water-affinity agent, an adhesive layer S2 with a thickness of 25 μm is obtained in the same manner as the adhesive layer S1 described above.

(Adhesive U1) 100 parts of a monomer mixture containing BA, CHA, and 4HBA in a weight ratio of 67/14/19, along with 0.09 parts of Irgacure 651 (manufactured by Ciba Specialty Chemicals) and Irgacure 184 (manufactured by Ciba Specialty Chemicals) as photopolymerization initiators, were added to a four-necked flask and photopolymerized under a nitrogen atmosphere by irradiation with ultraviolet light until the viscosity (BH viscometer, No. 5 rotor, 10 rpm, measurement temperature 30°C) reached approximately 15 Pa·s, thereby preparing a monomer syrup containing a portion of the polymer of the above monomer mixture. To 100 parts of the monomer syrup, 5 parts of NVP, 9 parts of 2-hydroxyethyl acrylate (HEA), 8 parts of 4HBA, 0.15 parts of dipentaerythritol hexaacrylate (DPHA) as a multifunctional monomer, and 0.3 parts of a nonionic surfactant (polyoxyethylene sorbitan monolaurate, HLB16.7, trade name: RHEODOL TW-L120, manufactured by Kao Corporation) as a hydrophilic agent were added and uniformly mixed to prepare UV-curable adhesive composition U1. The ratio of polar-group-containing monomers (the monomer components that form acrylic polymers) in 100 g of this adhesive composition U1 is 0.25 mol.

The adhesive composition U1 obtained above was coated onto a 38 μm thick release film R1 (manufactured by Mitsubishi Materials Corporation, MRF#38) with one side of the polyester film as the release surface. Another 38 μm thick release film R2 (manufactured by Mitsubishi Materials Corporation, MRE#38) was then applied to the polyester film to block air exposure and irradiated with ultraviolet light to harden it, thereby obtaining an adhesive layer U1 with a thickness of 25 μm. The elastic modulus G" of the adhesive layer U1 at 60°C was 19.7 kPa.

(Adhesive U2) Without using a water-affinity agent, an adhesive layer U2 with a thickness of 25 μm is obtained in the same manner as the adhesive layer U1 described above.

(Adhesive E1) 85 parts of 2-ethylhexyl acrylate (2EHA), 13 parts of methyl acrylate (MA), 1.2 parts of acrylic acid (AA), 0.75 parts of methacrylic acid (MAA), 0.01 parts of 3-glycidyloxypropyltrimethoxysilane (manufactured by Shin-Etsu Chemical Industry Co., Ltd., KBM-403), 0.05 parts of tri-dodecyl mercaptan (as a chain transfer agent), and 1.9 parts of emulsifier (manufactured by Kao Corporation, Latemul E-118B) were mixed with 100 parts of ion-exchanged water and emulsified to prepare an aqueous emulsion of the monomer mixture (monomer emulsion). The monomer emulsion was added to a reaction vessel equipped with a coolant, a nitrogen inlet, a thermometer, and a stirring device. Nitrogen gas was introduced while stirring at room temperature for at least 1 hour. Next, the system was heated to 60°C, and 0.1 parts of 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropanediamine] hydrate (manufactured by Wako Junya Pharmaceutical Co., Ltd., VA-057) as a polymerization initiator were added. The reaction was carried out at 60°C for 6 hours to obtain an aqueous dispersion of acrylic polymer e1. After the system was cooled to room temperature, 10 parts of an adhesive-improving resin emulsion (manufactured by Arakawa Chemical Co., Ltd., SUPER ESTER E-865 NT, aqueous dispersion of polymeric rosin ester with a softening point of 160°C) was added per 100 parts of the solids content of the above aqueous dispersion of acrylic polymer e1. Furthermore, by using 10% ammonia as a pH adjuster and polyacrylic acid (an aqueous solution with 36% nonvolatile components) as a tackifier, the pH was adjusted to approximately 7.5 and the viscosity to approximately 9 Pa·s, thereby preparing an emulsion-type adhesive compound E1.

A 38 μm thick release liner (Mitsubishi Resin Co., Ltd., MRF#38) with one side of a polyester film as the release surface was prepared, and another 38 μm thick release liner (Mitsubishi Resin Co., Ltd., MRE#38) with one side of a polyester film as the release surface was prepared. An adhesive composition E1 was applied to the release surface of one release liner (MRF#38), and dried at 120°C for 3 minutes to form an adhesive layer E1 with a thickness of 25 μm. The release surface of the other release liner (MRE#38) was then bonded to this adhesive layer for protection. This resulted in an adhesive layer E1 with its surface protected by two release liner sheets. The elastic modulus G" of the adhesive layer E1 at 60°C is 12.3 kPa.

<Example 1> The substrate layer material is a 25 μm thick extended polypropylene (OPP) film (product name "Torayfan #25A-KW37", manufactured by Toray Industries, Inc., biaxially extended PP film). The peeling pad covering one surface of the adhesive layer S1 with the peeling pad obtained above is peeled off, and a 2 kg rubber roller is used to press the exposed surface (adhesive surface) onto the surface of the OPP film by moving it back and forth twice. This results in a surface protection sheet (a single-sided adhesive sheet with a substrate layer) whose adhesive surface is protected by the peeling pad. The surface protective sheet in this example has a flexural stiffness of 9.3 × ^10⁻⁶ Pa· ^m³ at 25°C, a tensile modulus of elasticity of 6.3 × ^10⁹ Pa at 25°C, a stress of 85 N/ ^mm² at 100% elongation at 25°C, a breaking stress of 146 N/ ^mm² at 25°C, and a breaking strain of 239% at 25°C.

<Example 2> The surface protective sheet of this example is obtained in the same manner as in Example 1, except that adhesive layer U1 is used instead of adhesive layer S1.

<Example 3> The substrate layer material is a 25 μm thick polyphenylene sulfide (PPS) film (product name "TORELINA #25-3030", manufactured by Toray Industries, Inc., biaxially stretched PPS film) instead of an OPP film. Furthermore, the surface protective sheet of this example is obtained in the same manner as in Example 2.

<Example 4> The substrate layer material is a 12 μm thick OPP film (product name "Torayfan #12D-KW37", manufactured by Toray Industries, Inc., biaxially stretched PP film). Furthermore, the surface protective sheet of this example is obtained in the same manner as in Example 2.

<Example 5> The substrate layer material is a 60 μm thick OPP film (product name "Torayfan #60-2500", manufactured by Toray Industries, Inc., biaxially stretched PP film). Furthermore, the surface protective sheet of this example is obtained in the same manner as in Example 2.

<Example 6> Except for using adhesive layer E1 instead of adhesive layer U1, the surface protective sheet of this example is obtained in the same manner as in Example 2.

<Comparative Example 1> The surface protective sheet of this example is obtained in the same manner as in Embodiment 1, except that adhesive layer S2 is used instead of adhesive layer S1.

<Comparative Example 2> The surface protective sheet of this example is obtained in the same manner as in Embodiment 2, except that adhesive layer U2 is used instead of adhesive layer U1.

<Comparative Examples 3-5> A 25 μm thick PET film (product name "Lumirror S10", manufactured by Toray Industries, Inc.) was used as the substrate layer material. Furthermore, surface protective sheets for each example were obtained in the same manner as in Examples 1-3.

<Example 7> The substrate layer material is a 40 μm thick PP film (product name "Torayfan NO 3701J", manufactured by Toray Industries, Inc., unstretched polypropylene (CPP) film) instead of an OPP film. Furthermore, the surface protective sheet of this example is obtained in the same manner as in Example 1.

<Example 8> The substrate layer material is a 55 μm thick polyethylene (PE) film (product name "Polytop NSM-M", manufactured by Omori Chemical Co., Ltd.) instead of an OPP film. Furthermore, the surface protective sheet of this example is obtained in the same manner as in Example 1.

<Example 9> The substrate layer material is a PET film coated with silicon dioxide, used instead of an OPP film. Furthermore, the surface protection sheet of this example is obtained in the same manner as in Example 1. The PET film coated with silicon dioxide is manufactured using the following method: A silicon dioxide film is deposited on a PET film (manufactured by Mitsubishi Chemical Corporation, trade name "LC-N50JBN", thickness 50 μm) using RF (Radio-Frequency) magnetron sputtering with a Si metal target manufactured by Mitsui Metals Corporation. Argon and oxygen are used as the sputtering gases, with an oxygen/argon ratio of 5.3 vol%. Under the above conditions, the film formation time is adjusted to form a silicon oxide film ( _SiO2 film) with a thickness of 2nm on the above PET film (main layer), thereby obtaining a PET film coated with silicon dioxide.

<Example 10> The substrate layer material is a 25 μm thick vapor-deposited aluminum-coated PET film (product name "METALMY 25S", manufactured by Toray Industries, Inc.) instead of an OPP film. Furthermore, the surface protective sheet of this example is obtained in the same manner as in Example 1.

<Example 11> The substrate layer material is a 12 μm thick aluminum foil (manufactured by UACJ Corporation) instead of an OPP film. Furthermore, the surface protection sheet of this example is obtained in the same manner as in Example 1.

<Performance Evaluation> For each example of surface protective sheet, the following parameters were measured: normal adhesion force F0 [N/20 mm], normal water peel force FW0 [N/20 mm], adhesion force F1 [N/20 mm] after 30 minutes of warm water immersion, water peel force FW1 [N/20 mm] after 30 minutes of warm water immersion, adhesion force F2 [N/20 mm] after 1 hour of warm water immersion, and water peel force FW2 [N/20 mm] after 1 hour of warm water immersion. Furthermore, for each stage (normal state, after 30 minutes of warm water immersion, and after 1 hour of warm water immersion), the water peel force reduction rate [%] was calculated using the formula: FW/F×100. In the above formula, F represents the adhesion force [N/20 mm], which is any one of F0, F1, and F2. FW represents the water peeling force [N/20 mm], which is any one of FW0, FW1, and FW2. If the adhesion force F1 is 0.5 N/20 mm or more after immersion in warm water for 30 minutes, it is considered to have sufficient adhesion force for surface protection. Furthermore, if the water peeling force reduction rate (FW1/F1) after immersion in warm water for 30 minutes is less than 50%, it is considered to have sufficient easy peelability (easy water peelability). The results are shown in Tables 1 and 2. In addition, the moisture permeability [g/( ^m2・day)] of the substrate layer material and the surface protection sheet in each example was measured. The moisture permeability of the substrate [g/( ^m² ·day)] is shown in Tables 1 and 2. Each table also includes a summary of the examples.

[Table 1] Table 1 Implementation Example 1 Implementation Example 2 Implementation Example 3 Implementation Example 4 Implementation Example 5 Implementation Example 6 Comparative example 1 Comparative example 2 Comparative example 3 Comparative example 4 Comparative example 5 Adhesive layer Kind S1 U1 U1 U1 U1 E1 S2 U2 S1 U1 E1 Aggregation methods solvent UV UV UV UV lotion solvent UV solvent UV lotion Water affinity agent contain contain contain contain contain contain Does not contain Does not contain contain contain contain Thickness [μm] 25 25 25 25 25 25 25 25 25 25 25 Substrate layer Kind OPP OPP PPS OPP OPP OPP OPP OPP PET PET PET Moisture permeability [g/( ^m² ·day)] 6.4 6.4 12.7 10.3 2.1 6.4 6.4 6.4 29.0 29.0 29.0 Thickness [μm] 25 25 25 12 60 25 25 25 25 25 25 normal Then apply force F0 [N/20 mm] 8.0 4.9 6.5 3.8 5.6 5.8 8.8 5.0 10.8 7.6 8.6 Water peeling force FW0 [N/20 mm] 0.5 0.5 0.3 0.9 0.2 0.1 8.8 5.0 0.2 0.3 0.6 FW0/F0[%] 6.3 10.2 4.6 23.7 3.6 1.7 100 100 1.9 3.9 7.0 Soak in warm water for 30 minutes Then apply force F1 [N/20 mm] 3.3 1.7 1.6 0.5 4.2 0.5 8.9 5.1 0.07 0.1 0.2 Water peeling force FW1 [N/20 mm] 0.1 0.3 0.3 0.07 0.3 0.07 8.9 5.1 0 0.04 0.05 FW1/F1[%] 3.0 17.6 18.8 15.6 7.1 14.0 100 100 0.0 40.0 25.0 Soak in warm water for 1 hour Then apply force F2 [N/20 mm] 0.7 1.3 0.1 0.3 2.9 0.2 9.0 5.2 0 0.01 0.03 Water peeling force FW2 [N/20 mm] 0.05 0.3 0.09 0.07 0.7 0.03 9.0 5.2 0 0 0.02 FW2/F2[%] 7.1 23.1 90.0 23.3 24.1 20.0 100 100 - 0.0 66.7

[Table 2] Table 2 Implementation Example 7 Implementation Example 8 Implementation Example 9 Implementation Example 10 Implementation Example 11 Adhesive layer Kind S1 S1 S1 S1 S1 Aggregation methods solvent solvent solvent solvent solvent Water affinity agent contain contain contain contain contain Thickness [μm] 25 25 25 25 25 Substrate layer Kind PP PE Evaporation of PET with silicon dioxide Steam-coated PET with aluminum Aluminum foil Moisture permeability [g/( ^m² ·day)] 10.8 13.7 12.8 2.4 1.2 Thickness [μm] 40 55 50 25 12 normal Then apply force F0 [N/20 mm] 11.1 9.0 8.8 8.0 6.9 Water peeling force FW0 [N/20 mm] 1.4 1.1 0.2 0.2 0.5 FW0/F0[%] 12.6 12.2 2.3 2.5 7.2 Soak in warm water for 30 minutes Then apply force F1 [N/20 mm] 1.4 1.3 1.0 7.8 6.8 Water peeling force FW1 [N/20 mm] 0.02 0.04 0.04 0.1 1.1 FW1/F1[%] 1.4 3.1 4.0 1.3 16.2 Soak in warm water for 1 hour Then apply force F2 [N/20 mm] 0.08 0.08 0.1 2.2 6.4 Water peeling force FW2 [N/20 mm] 0 0 0 0.06 1.2 FW2/F2[%] 0 0 0 2.7 18.8

As shown in Tables 1-2, the surface protective sheets of Examples 1-11 exhibit an adhesion strength F1 of 0.5 N/20 mm or higher after 30 minutes of warm water immersion, and a water peeling force reduction rate (FW1/F1) of less than 50% after 30 minutes of warm water immersion. They demonstrate acceptable levels of adhesion and water peelability after this treatment. Among these, the surface protective sheets of Examples 1-2, 5, and 10-11 also exhibit an adhesion strength F2 of 0.5 N/20 mm or higher after 1 hour of warm water treatment, and a water peeling force reduction rate (FW2/F2) of less than 50%, demonstrating even superior performance. Furthermore, it can be argued that the reason why the water peeling force FW2 after 1 hour of warm water immersion in Examples 5 and 11 is higher than the normal or 30-minute warm water immersion values FW0 and FW1 is that the surface protective sheet has lower moisture permeability. Therefore, compared to water peelability, the increase in peeling force due to aging during warm water immersion is dominant. On the other hand, in Comparative Examples 1 and 2, no water peelability was observed, and the water peeling force did not decrease relative to adhesion. Also, in Comparative Examples 3 to 5, the adhesion decreased due to warm water immersion, and the adhesion force F1 after 30 minutes of warm water immersion did not reach 0.5 N/20 mm. Furthermore, the moisture permeability of the surface protective sheet in each example is within ±1.5 g/(m ²・day) of the moisture permeability of the substrate (layer) used in each example.

<Experiment 2> <Preparation of Adhesive> (Adhesive E2) The amount of adhesive-imparting resin emulsion (manufactured by Arakawa Chemical Industry Co., Ltd., SUPER ESTER E-865NT, an aqueous dispersion of polymeric rosin ester with a softening point of 160°C, sometimes referred to as "Adhesive-imparting Resin A") added was changed from 100 parts (solids content) to 20 parts. Furthermore, an adhesive layer E2 with a thickness of 25 μm was obtained using the same method as for adhesive layer E1.

(Adhesive E3) The amount of adhesive-enriched resin A added to the aqueous dispersion of acrylic polymer e1 was changed to 30 parts per 100 parts (solids). Otherwise, an adhesive layer E3 with a thickness of 25 μm was obtained using the same method as for adhesive layer E2.

(Adhesive E4) The monomer composition of the acrylic polymer was changed to 49 parts of 2EHA, 49 parts of n-butyl methacrylate (BMA), and 2 parts of AA. An anionic reactive emulsifier (manufactured by Daiichi Kogyo Pharmaceutical Co., Ltd., AQUALON BC2020) was used as an emulsifier for 100 parts of the aforementioned monomer components. Furthermore, an aqueous emulsion (monomer emulsion) of the monomer mixture was prepared using the same method as that used for preparing acrylic polymer e1 in adhesive E1, and a polymerization reaction was carried out to obtain an aqueous dispersion of acrylic polymer e2. After cooling the system to room temperature, for every 100 parts of the solids content of the aqueous dispersion of the aforementioned acrylic polymer e2, 20 parts of adhesive resin A (based on solids content) and 2 parts of acezoline crosslinking agent (Epocros WS-500, manufactured by Nippon Shokubai Co., Ltd.) were mixed. Then, using 10% ammonia as a pH adjuster and polyacrylic acid (36% non-volatile aqueous solution) as a tackifier, the pH was adjusted to approximately 7.5 and the viscosity to approximately 9 Pa·s, thereby preparing an emulsion-type adhesive compound E4. Using the above emulsion-type adhesive compound E4, except for the preparation of adhesive layer E2, an adhesive layer E4 with a thickness of 25 μm was obtained.

(Adhesive E5) An aqueous dispersion of adhesive-imparting resin B (manufactured by Arakawa Chemical Industry Co., Ltd., SUPER ESTER NS-121, a rosin-based resin with a softening point of 120°C (acid value imparted)) was used, at 20 parts by solids, instead of 20 parts by adhesive-imparting resin A. Furthermore, an adhesive layer E5 with a thickness of 25 μm was obtained using the same method as that used to prepare adhesive layer E4.

(Adhesive E6) Without using adhesive to coat the resin, an adhesive layer E6 with a thickness of 25 μm is obtained using the same method as adhesive layer E2.

(Adhesive S3) In a reaction vessel equipped with a coolant, nitrogen inlet, thermometer, and stirring device, 72 parts of 2EHA, 14 parts of NVP, 13 parts of HEA, and 1 part of methyl methacrylate (MMA) as monomer components, 0.12 parts of α-thioglycerol as a chain transfer agent, and ethyl acetate as a polymerization solvent were added. 0.2 parts of AIBN as a thermal polymerization initiator were also added, and solution polymerization was carried out under a nitrogen atmosphere to obtain a solution containing an acrylic polymer s1 with a Mw of 300,000. In the solution obtained above, for every 100 parts of the monomeric component used to prepare the solution, 20 parts of rosin ester dibutylide (manufactured by Halima Chemical Co., Ltd., HARITACK 4740, softening point 115-125°C, sometimes referred to as "adhesion-enhancing resin C") as an adhesive binder are added on a solids basis; 0.75 parts of isocyanate crosslinking agent (trimethylolpropane/phenylenedimethyl diisocyanate adduct, manufactured by Mitsui Chemicals Co., Ltd., trade name: Takenate D-110N, solids concentration 75%) as a solids basis are added on a solids basis; and dioctyltin dilaurate (manufactured by Tokyo Fine Chemical Co., Ltd., trade name: EMBILIZER) as a crosslinking accelerator is added. Solvent-based adhesive compound S3 was prepared by uniformly mixing 0.01 parts of OL-1, 3 parts of acetoacetone as a crosslinking delay agent, and 0.5 parts of a nonionic surfactant (polyoxyethylene sorbitan monolaurate, HLB16.7, trade name: RHEODOL TW-L120, manufactured by Kao Corporation) as a hydrophilic agent. Using the obtained solvent-based adhesive compound S3, an adhesive layer S3 with a thickness of 25 μm was obtained in the same manner as the adhesive layer S1.

(Adhesive S4) Three parts of an acrylic oligomer were used to replace 20 parts of the adhesive-improving resin C. Furthermore, adhesive layer S4 was obtained in the same manner as adhesive layer S3. The acrylic oligomer was prepared using the following method: Specifically, in a reaction vessel equipped with a mixer, thermometer, nitrogen inlet, reflux cooler, and dropping funnel, 95 parts of cyclohexyl methacrylate (CHMA), 5 parts of AA, 10 parts of AIBN as a polymerization initiator, and toluene as a polymerization solvent were added. The mixture was stirred in a nitrogen stream for 1 hour to remove oxygen from the polymerization system, then heated to 85°C and reacted for 5 hours to obtain an acrylic oligomer with a solid content of 50%. The obtained acrylic oligomer has a molecular weight (Mw) of 3600.

<Examples 12-18> As the substrate layer material, a 60 μm thick OPP film (product name "Torayfan #60-2500", manufactured by Toray Industries, Inc., biaxially stretched PP film) is prepared. The peeling pad covering one of the surfaces of the adhesive layers E2-E6 and S3-4 with peeling pads obtained above is peeled off, and a 2 kg rubber roller is used to press the exposed surface (adhesive surface) onto the surface of the OPP film by moving it back and forth twice. In this way, surface protection sheets (single-sided adhesive sheets with substrate layers) are obtained in various examples where the adhesive surface is protected by the peeling pad. The surface protective sheet of each example has a flexural stiffness of 1.2× ^10⁻⁴ Pa・^m³ , a stress of 83 N/ ^mm² at 100% elongation at 25°C, a fracture stress of 131 N/ ^mm² at 25°C, and a fracture strain of 232% at 25°C.

<Performance Evaluation> For each example of surface protective sheet, the following parameters were measured: normal adhesion force F0 [N/20 mm], normal water peel force FW0 [N/20 mm], adhesion force F1 [N/20 mm] after 30 minutes of warm water immersion, and water peel force FW1 [N/20 mm] after 30 minutes of warm water immersion. Furthermore, for each stage (normal state, after 30 minutes of warm water immersion), the water peel force reduction rate [%] was calculated using the formula: FW/F×100. In the above formula, F represents the adhesion force [N/20 mm], which is either F0 or F1. FW represents the water peel force [N/20 mm], which is either FW0 or FW1. The results, along with summaries of each example, are presented in Table 3.

[Table 3] Table 3 Implementation Example 12 Implementation Example 13 Implementation Example 14 Implementation Example 15 Implementation Example 16 Implementation Example 17 Implementation Example 18 Adhesive layer Kind E2 E3 E4 E5 E6 S3 S4 polymer e1 e1 e2 e2 e1 s1 s1 Aggregation methods lotion lotion lotion lotion lotion solvent solvent Water affinity agent contain contain contain contain contain contain contain Adhesion enhancer [parts]* Adhesion to resin A 20 30 20 Adhesion is given to resin B 20 Adhesion is given to resin C 20 acrylic oligomers 3 Thickness [μm] 25 25 25 25 25 25 25 Substrate layer Kind OPP OPP OPP OPP OPP OPP OPP Thickness [μm] 60 60 60 60 60 60 60 normal Then apply force F0 [N/20 mm] 8.1 8.2 5.5 10.4 5.9 8.8 4.1 Water peeling force FW0 [N/20 mm] 0.1 0.1 0.1 0.4 0.3 0.1 0.7 FW0/F0[%] 1.2 1.2 1.8 3.8 0.1 0.9 17.1 Soak in warm water for 30 minutes Then apply force F1 [N/20 mm] 2.6 5.4 4.7 8.5 1.5 2.2 1.5 Water peeling force FW1 [N/20 mm] 0.1 0.1 0.1 0.1 0.0 0.1 0.1 FW1/F1[%] 2.7 0.9 1.3 1.2 0.0 2.3 6.7 *Parts relative to 100 parts of acrylic polymer

As shown in Table 3, the surface protective sheets of Examples 12-18 exhibit an adhesion strength F1 of 0.5 N/20 mm or higher after 30 minutes of warm water immersion, and a water peel strength reduction rate (FW1/F1) of less than 50% after 30 minutes of warm water immersion. They demonstrate acceptable levels of adhesion and water peelability after this treatment. In Examples 12-15, which incorporate adhesive resin in a water-dispersible adhesive, the water peel strength FW1 after 30 minutes of warm water immersion is maintained at a lower level, and the adhesion strength F1 after 30 minutes of warm water immersion is higher than that in Example 16, which does not use adhesive resin. Although no comparative results are shown, in Examples 17-18, which use solvent-based adhesives, adhesive preambles were also added. It was observed that the more preambles were used, the stronger the adhesion F1 tended to be after 30 minutes of warm water soaking.

The above details specific examples of the present invention, but these are merely illustrative and do not limit the scope of the patent application. The technology described in the patent application includes various variations and modifications of the above-illustrative examples.

1: Surface Protective Sheet 1A: Adhesive Side 1B: Back Side 2: Surface Protective Sheet 2A: Adhesive Side 2B: Back Side 10: Substrate Layer 10A: One Side 10B: The Other Side 11: First Layer 12: Second Layer (including inorganic material layer) 20: Adhesive Layer 20A: Adhesive Side 30: Peel-off Liner 50: Surface Protective Sheet with Peel-off Liner

Figure 1 is a cross-sectional view schematically illustrating one form of surface protection sheet. Figure 2 is a cross-sectional view schematically illustrating another form of surface protection sheet.

1: Surface Protective Sheet

1A: Next step

1B: Back side

10: Substrate Layer

10A: One side

10B: The Other Side

20: Adhesive layer

20A: Next step

30: Peeling off the pad

50: Surface protective sheet with peel-off backing

Claims (17)

A surface protective sheet having an adhesive surface, wherein the peel strength (adhesion force after 30 minutes of warm water immersion) F1 [N/20 mm] of the surface protective sheet is 0.5 N/20 mm or higher. This peel strength (adhesion force after 30 minutes of warm water immersion) F1 [N/20 mm] is measured by attaching the adhesive surface to the surface of an alkaline glass having a water contact angle of less than 20 degrees, immersing it in warm water at 60°C ± 2°C for 30 minutes, then removing it from the warm water and wiping off the water, under conditions of a temperature of 23°C, a peel angle of 180 degrees, and a peel speed of 300 mm/min. The water peeling force (water peeling force after 30 minutes of warm water immersion) FW1 [N/20 mm] of the aforementioned surface protective sheet is less than 50% of the aforementioned peel strength F1. This water peeling force (water peeling force after 30 minutes of warm water immersion) FW1 [N/20 mm] is obtained by adhering the surface of an alkaline glass having a water contact angle of less than 20 degrees to the bonding surface, immersing it in warm water at 60℃±2℃ for 30 minutes, then removing it from the warm water and wiping off the water, and then supplying 20... The distilled water was introduced into one end of the interface between the alkaline glass and the bonding surface, and the result was measured at a temperature of 23°C, a peeling angle of 180 degrees, and a speed of 300 mm/min. For the surface protective sheet of claim 1, wherein the water peeling force FW1 is the same as or less than the water peeling force (normal water peeling force) FW0 [N/20 mm] measured by the following method, wherein the normal water peeling force FW0 is obtained by bonding the surface of an alkaline glass having a water contact angle of less than 20 degrees to the bonding surface, maintaining it in an environment of 23°C and 50%RH for 1 hour, supplying 20 μL of distilled water between the alkaline glass and the bonding surface, so that the distilled water enters one end of the interface between the alkaline glass and the bonding surface, and then at a temperature of 23°C, a peeling angle of 180 degrees and a speed of 300 The water stripping force FW0 [N/20 mm] measured under the condition of mm/min. For example, the surface protective sheet of Request 1 or 2 shall have a moisture permeability of less than 24 g/(m ²・day) as measured by the cup method. The surface protection sheet of claim 1 or 2 includes an adhesive layer constituting the aforementioned bonding surface and a substrate layer supporting the adhesive layer. For example, the surface protective sheet of claim 4, wherein the adhesive layer contains a hydrophilic agent. As in claim 4, the surface protective sheet, wherein the adhesive layer comprises an adhesive preamble. For example, the surface protective sheet of claim 4, wherein the adhesive layer is formed of a light-curing or solvent-based adhesive composition. As in claim 4, the surface protective sheet, wherein the substrate layer comprises a resin film. As in claim 4, the surface protective sheet, wherein the substrate layer includes a layer containing inorganic materials. The surface protective sheet for request item 1 or 2 has a thickness of 20 to 100 μm. The surface protective sheet, as requested in item 1 or 2, is used for the process of thinning glass or semiconductor wafers in a liquid by chemical and/or physical means. For example, the surface protective sheet of claim 4, wherein the adhesive layer is an acrylic adhesive layer. As in claim 5, the surface protective sheet wherein the adhesive layer contains a hydrophilic agent selected from at least one of surfactants and compounds having a polyoxyalkylene backbone. For example, the surface protective sheet of claim 4, wherein the moisture permeability of the substrate layer, as measured by the cupping method, is less than 24 g/(m ²・day). A method for treating an object includes: a step of attaching a surface protective sheet to the surface of the object having a water contact angle of 20 degrees or less; a step of treating the object to which the surface protective sheet is attached, wherein the object comes into contact with a liquid during the treatment; and a step of removing the surface protective sheet from the treated object by water peeling; wherein the water peeling force FW1 [N/20 mm] of the surface protective sheet after 30 minutes of warm water immersion is less than 50% of the adhesion force F1 [N/20 mm] after 30 minutes of warm water immersion, [adhesion force F1 after 30 minutes of warm water immersion]. The adhesion force F1 after 30 minutes of warm water immersion is the peel strength [N/20 mm] measured at a temperature of 23°C, a peel angle of 180°C, and a speed of 300 mm/min after the surface of the alkaline glass bonded to the protective sheet having a water contact angle of less than 20 degrees was immersed in warm water at 60°C ± 2°C for 30 minutes, then removed from the warm water and wiped dry; [Water peel force FW1 after 30 minutes of warm water immersion] The water peeling force FW1 after 30 minutes of warm water immersion is the water peeling force [N/20 mm] measured under the following conditions: the surface of the alkaline glass with a water contact angle of less than 20 degrees is immersed in warm water at 60℃±2℃ for 30 minutes, then lifted out of the warm water and wiped off the water, and 20 μL of distilled water is supplied between the alkaline glass and the surface of the adhesive, so that the distilled water enters one end of the interface between the alkaline glass and the surface of the adhesive. The treatment method described in claim 15, wherein the liquid is an aqueous solution. The processing method of claim 15 or 16, wherein the surface protective sheet has an adhesive layer, the adhesive layer comprising an adhesive preamble.