WO2019159381A1 - Pressure-sensitive adhesive sheet, surface protective sheet, pressure-sensitive adhesive sheet roll and method for producing pressure-sensitive adhesive sheet - Google Patents

Abstract

[Problem] Provided is a PSA sheet that gives excellent tightness of adhesion to adherend. [Solving Means] A PSA sheet is provided. The PSA sheet comprises a substrate layer and a PSA layer provided to at least one face of the substrate layer. The PSA sheet has a width of about 1 m or greater. The PSA sheet shows at least a 4 % 60 °C thermal contraction in its width direction.

Description

PRESSURE-SENSITIVE ADHESIVE SHEET, SURFACE PROTECTIVE SHEET, PRESSURE-SENSITIVE ADHESIVE SHEET ROLL AND METHOD FOR PRODUCING PRESSURE-SENSITIVE ADHESIVE SHEET

The present invention relates to a pressure-sensitive adhesive sheet, a surface protective sheet, pressure-sensitive adhesive sheet roll and a method for producing a pressure-sensitive adhesive sheet.
The present application claims priority to U. S. Provisional Patent Application No. 62/629,808 filed on February 13, 2018, the entire contents of which are incorporated herein by reference.

In general, pressure-sensitive adhesive (or PSA; the same applies hereinafter) has characteristics of being in a soft solid (viscoelastic) state in a room temperature range and easily adhering to adherend under some pressure. Because of these characteristics, for instance, as a PSA sheet having a substrate, PSA is widely used for purposes such as bonding, fixing and protecting various parts. For instance, a protective sheet using PSA typically has a PSA layer on one face of a substrate sheet formed of a material such as resin and is constituted so as to achieve a protection purpose when applied via the PSA layer to an adherend (an object to be protected). Conventional art documents disclosing PSA sheets usable as surface protective sheets include JP2017-186517A (Patent Document 1), JP5719194B2 (Patent Document 2), JP2012-131976A (Patent Document 3) and JP3571460B2 (Patent Document 4).

In typical, a PSA sheet used as a protective sheet is applied to an adhered for a temporal period including time during which the adherend needs to be protected (e.g. while being transported, processed, etc.). After serving the protection purpose, it is removed from the adherend. A PSA sheet used in such an embodiment requires properties to adhere well and conforms to the adherend to protect the adherend surface while being used. When a gap is present between the adherend and the protective sheet, due to lifting, air bubbles, wrinkling, etc., tearing of the PSA sheet may take place from the gap, leading to possible damage to the adherend. Also, undesired matters such as water may enter the open space, causing the adherend surface to undergo deterioration and degradation of appearance.

This invention has been made in view of these circumstances with an objective to provide a PSA sheet, a surface protective sheet and a PSA sheet roll that provide excellent tightness of adhesion to adherend, by means other than selection of the PSA composition and the substrate material (by means of thermal contraction of the PSA sheet as a whole). Another objective of this invention is to provide a method for producing the PSA sheet.

The present application provides a PSA sheet comprising a substrate layer and a PSA layer provided to at least one face of the substrate layer. The PSA sheet may typically have a long length. The PSA sheet has a width of about 1 m or greater. The PSA sheet shows at least a 4 % 60 °C thermal contraction in the width direction of the PSA sheet. As used herein, the percent 60 °C thermal contraction refers to the thermal contraction ratio determined under the condition of 60 °C for 30 minutes. Details are provided later.

Unlike conventional PSA sheets, the PSA sheet having such constitution undergoes thermal contraction in the width direction; and therefore, by applying a piece having at least a certain width to an adherend and heating it or by applying it to a heated adherend, it contracts in the width direction, enabling its tight adhesion to the adherend. In the length direction, for instance, by stretching and applying the PSA sheet through its stretching properties, contraction upon release of the stretching tension allows tight adhesion of the PSA sheet. In summary, the aforementioned constitution allows the entire PSA sheet to tightly adhere to adherend through thermal contraction in the width direction.

The PSA sheet disclosed herein and the initial PSA sheet described later are typically identified by having a long side and a short side with respect to its plane (sheet face). By definition, the long side is longer than the short side and the short side is shorter than the long side. For instance, the short side may be approximately perpendicular to the long side. The length direction of the PSA sheet is in the direction along the long side and the width direction is in the direction perpendicular to the length direction. Thus, as used herein, the “width” is defined as the length in the direction perpendicular to the length direction. Typical examples of the PSA sheet disclosed herein include a PSA sheet that is described as having a long length, a band shape, and a rectangular shape. The long side is a line segment that runs almost linearly. The short side is not limited to a straight line and can be a curve, zig-zag, etc. The same applies to the initial PSA sheet described later.

In a preferable embodiment of the art disclosed herein (including the PSA sheet, surface protective sheet, PSA sheet roll and PSA sheet production method; the same applies hereinafter) satisfies at least one of the following: the PSA sheet shows a 60°CHS_LD % 60 °C thermal contraction in the length direction and a 60°CHS_WD % 60 °C thermal contraction in the width direction with a difference 60°CHS_WD - 60°CHS_LD of 3 or greater; and the PSA sheet shows a less than 2 % 60 °C thermal contraction in the length direction. According to an embodiment that satisfies one or each of these conditions, the effects of the art disclosed herein are preferably obtained.

In a preferable embodiment of the art disclosed herein, the PSA sheet satisfies at least one of the following: having a width of about 2.6 m or greater; and showing an initial peel strength of 0.01 N/20mm to 5 N/20mm to a glass plate. The art disclosed herein is preferably applied in an embodiment that satisfies one or each of these features.

In a preferable embodiment of the art disclosed herein, the substrate layer is formed from resin film, a rubber sheet or a foam sheet. The substrate layer is more preferably formed from at least one species of resin selected from the group consisting of polyolefinic resin, polyvinyl chloride-based resin and polyurethane-based resin.

In a preferable embodiment of the art disclosed herein, at least one of the following is satisfied: the substrate layer has a thickness of 25 μm to 150 μm; and the PSA layer has a thickness of 1 μm to 20 μm. According to an embodiment that satisfies one or each of the above, a PSA sheet showing a prescribed thermal contraction can be preferably obtained.

In a preferable embodiment of the art disclosed herein, the length direction of the PSA sheet corresponds to the MD (machine direction) of the substrate layer and the width direction of the PSA sheet corresponds to the TD (transverse direction) of the substrate layer. In this embodiment, a PSA sheet having at least a certain percent 60 °C thermal contraction in the width direction can be preferably obtained. In a more preferable embodiment, the length direction of the PSA sheet is in the substrate layer’s MD and the width direction of the PSA sheet is in the substrate layer’s TD. In the present description, that one direction (e.g. the length direction of the PSA sheet) corresponds to another direction (e.g. MD of the substrate layer) means that when the first direction is the baseline (0°), the second direction is in a range between -30° and +30° from the baseline.

In a preferable embodiment of the art disclosed herein, the PSA layer comprises at least one species selected from the group consisting of acrylic polymers, urethane-based polymers, polyester-based polymers, polyether-based polymers, rubber-based polymers, silicone-based polymers, polyamide-based polymers and fluoropolymers, with the at least one species accounting for 50 % by weight or greater. Among them, the PSA layer is more preferably an acrylic PSA layer comprising an acrylic polymer as the base polymer or a rubber-based PSA layer comprising a rubber-based polymer as the base polymer.

The present description provides a PSA sheet roll. In the PSA sheet roll, a PSA sheet disclosed herein is wound in the length direction (the length of the PSA sheet is wound). From the PSA sheet roll in this embodiment, the PSA sheet is unwound (withdrawn) and used, enabling tight adhesion to adherend through its thermal contraction in the width direction. The PSA sheet withdrawn from the roll tends to thermally shrink in the length direction upon release from the tension, whereas it generally tends to thermally expand in the width direction. According to the art disclosed herein, because the PSA sheet shows at least a 4 % 60 °C thermal contraction in its width direction, issues related to thermal expansion of conventional PSA sheet rolls can be solved.

The present description provides a surface protective sheet formed from a PSA sheet disclosed herein. In the surface protective sheet, the PSA sheet is an adhesively single-faced PSA sheet wherein the PSA layer is provided solely to one face of the substrate layer. The PSA sheet disclosed herein is preferably used as a surface protective sheet.

This description provides a method for producing the PSA sheet according to an embodiment. The PSA sheet obtained by the production method is a PSA sheet disclosed herein. The production method comprises a step of preparing (obtaining) an initial PSA sheet (or PSA sheet-workpiece, or workpiece-PSA sheet) having a substrate layer and a PSA layer provided to at least one face of the substrate layer (preparation step); and a step of stretching the initial PSA sheet in its width direction while heating the same (stretching step). The initial PSA sheet may typically have a long length. According to this method, a PSA sheet that has a width of about 1 m or greater and shows at least a 4 % 60 °C thermal contraction in the width direction can be produced.

The present description provides a PSA sheet production method according to another embodiment. This method comprises a step of preparing (obtaining) an initial PSA sheet having a substrate layer and a PSA layer provided to at least one face of the substrate layer (preparation step); and a step of stretching the initial PSA sheet in its width direction while heating the same (stretching step). The initial PSA sheet may typically have a long length. According to this method, a PSA sheet having a greater width than that of the initial PSA sheet can be produced. This method allows effective use of a PSA sheet that has been prepared (obtained) in advance with a smaller width than required. Even when a large-scale system is not available for manufacturing a wide PSA sheet, after a PSA sheet is fabricated with an available system, a desirably-wide PSA sheet can be produced by the aforementioned method.

In a preferable embodiment of the PSA sheet production method disclosed herein, in the stretching step, the initial PSA sheet is stretched by a factor of 1.1 or greater and less than 1.45 in its width direction. According to this method, a PSA sheet that shows at least the prescribed percent widthwise 60 °C thermal contraction can be preferably produced. When the stretch factor is 1.1 or greater, at least a 4 % widthwise 60 °C thermal contraction can be preferably obtained. The PSA sheet can be efficiently made wider. When the stretch factor is less than 1.45, it is possible to produce a PSA sheet that is unsusceptible to quality degradation and is readily available for practical use.

In a preferable embodiment of the PSA sheet production method disclosed herein, after the stretching step, the method further comprises a step of maintaining the width of the initial PSA sheet 0 % to 10 % shorter than the stretched width (width maintenance step). This step can reduce uneven contraction and inhibit undesirable contraction arising from the thermal history from its production to its use.

In a preferable embodiment of the PSA sheet production method disclosed herein, the width maintenance step is carried out at a temperature ±10 °C from the temperature at which the stretching step is carried out or at a temperature that is more than 10 °C higher than the temperature in the stretching step. This can reduce uneven contraction and also unintentional contraction from its production up to its use.

In a preferable embodiment of the PSA sheet production method disclosed herein, the method further comprises a step of cooling down the stretched initial PSA sheet. Natural or forced cooling carried out after thermal stretching or thermal width maintenance can preferably bring about a desirable PSA sheet.

Fig. 1 shows a cross-sectional diagram schematically illustrating embodiments of the PSA sheet and PSA sheet roll.

Preferred embodiments of the present invention are described below. Matters necessary to practice this invention other than those specifically referred to in this description can be understood by a person skilled in the art based on the disclosure about implementing the invention in this description and common technical knowledge at the time of application. The present invention can be practiced based on the contents disclosed in this description and common technical knowledge in the subject field.

<Constitution of PSA sheet>
As used herein, the term "PSA" refers to, as described earlier, a material that exists as a soft solid (a viscoelastic material) in a room temperature range and has a property to adhere easily to an adherend with some pressure applied. As defined in C. A. Dahlquist, "Adhesion : Fundamental and Practice" (McLaren & Sons (1966), P. 143), the PSA referred to herein is a material that has a property satisfying complex tensile modulus E* (1Hz) < 10⁷ dyne/cm² (typically, a material that exhibits the described characteristics at 25 °C). The concept of PSA sheet herein may encompass so-called PSA tape, PSA labels, PSA film, etc. The PSA sheet disclosed herein can be in a roll or in a flat sheet. Alternatively, the PSA sheet may be processed into various shapes.

The PSA sheet disclosed herein has a PSA layer on a substrate layer (support substrate). Fig. 1 shows a cross-sectional structure of the PSA sheet according to an embodiment. PSA sheet 10 has a long shape (a band shape) and is in an embodiment where a PSA layer 2 is provided on one face (first face) 1A of a substrate-layer sheet 1; for use, the surface (adhesive face) 2A of PSA layer 2 is applied to an adherend. When PSA sheet 10 is used as a surface protective sheet, the surface 2A of PSA layer 2 is applied to an object to be protected. The back face (second face) 1B (on the reverse side of the first face 1A) of substrate layer 1 is also the back face of PSA sheet 10, forming the outer surface of PSA sheet 10. PSA layer 2 is placed to entirely cover the surface of the first face 1A of substrate layer 1. The PSA sheet can be a substrate-bearing double-faced PSA sheet having a PSA layer on each face of the substrate layer. In this case, each face of the substrate layer is entirely covered with a PSA layer.

PSA sheet 10 may be embodied as a PSA sheet roll 100 as shown in Fig. 1. PSA sheet roll 100 is in a form that PSA sheet 10 is wound in the length direction, the PSA sheet 10 having a substrate layer 1 that has the first and second faces 1A and 1B and a PSA layer 2 placed on the first face 1A. In PSA sheet roll 100, with respect to PSA sheet 10, the second face 1B of substrate layer 1 is in contact with the surface (adhesive face) 2A of PSA layer 2 to protect the surface 2A. In this embodiment, the second face (back face) 1B of substrate layer 1 is a release face. Alternatively, prior to use (i.e. before applied to the adherend), PSA sheet 10 can be in a form where the face 2A (adhesive face, i.e. the bonding surface to the adherend) of PSA layer 2 is protected with a release liner (not shown in the drawing) having a release face at least on the PSA layer side.

As the release liner, commonly-used release paper and the like can be used without particular limitations. For instance, a release liner having a release layer on a surface of a liner substrate such as plastic film and paper, a release liner formed from a low-adhesive material such as fluorinated polymer (polytetrafluoroethylene, etc.) and polyolefinic resin, and the like can be used. The release layer can be formed by subjecting the liner substrate to surface treatment with various release agents including silicone-based, long-chain alkyl-based, and fluorinated kinds as well as molybdenum sulfide.

The PSA sheet disclosed herein has a width of about 1 m or greater. In an application using such a wide PSA sheet, the adhesion-tightening effect of the art disclosed herein can be preferably obtained. In such an application, it may be preferable to use a PSA sheet having a width of, for instance, about 2 m or greater, or even about 2.6 m or greater. The width of the PSA sheet may be greater than 2.6 m (e.g. 3 m or greater). The maximum width of the PSA sheet is not particularly limited. From the standpoint of the productivity, handling properties, etc., it is usually suitably about 5 m or less, for instance, about 4 m or less. The long PSA sheet has a length (the distance in the length direction) equal to or greater than the width.

The thickness of the PSA sheet disclosed herein is not particularly limited. From the standpoint of the handling properties, the lightness of weight, etc., it is usually suitably about 1000 μm or less (typically about 300 μm or less, e.g. about 150 μm or less). In an embodiment, the thickness of the PSA sheet is preferably about 120 μm or less, more preferably about 100 μm or less, yet more preferably about 75 μm or less, or possibly, for instance, less than 60 μm. The thickness of the PSA sheet can be typically greater than 20 μm, preferably greater than 30 μm, or more preferably greater than 40 μm, for instance, greater than 45 μm.
As used herein, the thickness of the PSA sheet includes the thicknesses of the PSA layer and the substrate layer, but excludes the thickness of the release liner.

The thickness of the substrate layer constituting the PSA sheet disclosed herein is not particularly limited. The thickness of the substrate layer can be, for instance, about 800 μm or less (typically about 250 μm or less). In an embodiment, the thickness of the substrate layer (typically, non-foamed resin film) is preferably about 150 μm or less, more preferably about 100 μm or less, or yet more preferably less than 65 μm, for instance, less than 55 μm. With decreasing thickness of the substrate layer, the PSA sheet tends to exhibit greater conformability to the adherend shape while its lifting and peeling tend to be inhibited. From the standpoint of adherend protection and handling properties, etc., the thickness of the substrate layer can be typically about 10 μm or greater, preferably about 25 μm or greater, more preferably greater than about 30 μm or greater, or yet more preferably greater than 40 μm, or possibly, for instance, greater than 45 μm.

No particular limitations are imposed on the thickness of the PSA layer constituting the PSA sheet disclosed herein. From the standpoint of preventing adhesive transfer to the adherend, the thickness of the PSA layer is usually about 50 μm or less, suitably about 30 μm or less, preferably about 15 μm or less, or more preferably about 8 μm or less (e.g. less than 6 μm). In another embodiment, from the standpoint of the ease of removal, etc., the thickness of the PSA layer is suitably about 5 μm or less, about 4 μm or less, or possibly, for instance, 3 μm or less. From the standpoint of the adhesion, the thickness of the PSA layer is usually suitably about 0.5 μm or greater, preferably about 1 μm or greater, or more preferably greater than 2 μm. The thickness of the PSA layer is greater than 3 μm, for instance, greater than 4 μm.

With respect to a PSA sheet roll in which the PSA sheet disclosed herein is wound in the length direction, the diameter is not particularly limited. From the standpoint of the ease of winding, it is advantageous that the diameter of the PSA sheet roll is not excessively large. From such a standpoint, the diameter of the PSA sheet roll is usually suitably about 1 m or smaller, or preferably about 50 cm or smaller. In view of the use, storage, efficient transportation, etc., the art disclosed herein can be favorably implemented in an embodiment of the PSA sheet roll having a diameter of about 5 cm or larger (e.g. about 15 cm or larger).

<Properties of PSA sheet>
(60 °C thermal contraction)
The PSA sheet disclosed herein shows at least a 4 % 60 °C thermal contraction in the width direction. With this, heating the PSA sheet after applied to an adherend or applying the PSA sheet to a heated adherend allows it to contract in the width direction and tightly adhere to the adherend. The widthwise 60 °C thermal contraction can be 4.5 % or greater, 5 % or greater, or even 5.5 % or greater. The maximum widthwise 60 °C thermal contraction is not particularly limited. It can be about 10 % or less (e.g. 8 % or less).

The PSA sheet disclosed herein is not particularly limited in 60 °C thermal contraction in the length direction. It is usually suitably about 10 % or less, for instance, possibly about 6 % or less, or about 4 % or less. In a preferable embodiment, the lengthwise 60 °C thermal contraction is less than 2 %, more preferably less than 1 %, yet more preferably less than 0.5 %, or particularly preferably 0 % or less. In such an embodiment, the PSA sheet is likely to be obtained with desirable widthwise 60 °C thermal contraction and the adhesion-tightening effect of the art disclosed herein is likely to be obtained. The lengthwise 60 °C thermal contraction can be below 0 % (e.g. -0.2 %), indicating that it is thermally expandable.

In a preferable embodiment, the PSA sheet disclosed herein shows a 60°CHS_WD % 60 °C thermal contraction in the width direction and a 60°CHS_LD % 60 °C thermal contraction in the length direction, and the difference of 60°CHS_WD from 60°CHS_LD (i.e. 60°CHS_WD - 60°CHS_LD) is 3 or greater. In such an embodiment, the adhesion-tightening effect of the art disclosed herein is likely to be obtained. The 60°CHS_WD - 60°CHS_LD value is more preferably 4 or greater, for instance, possibly 5 or greater, or even 6 or greater. The maximum 60°CHS_WD - 60°CHS_LD value can be about 10 or less (e.g. about 8 or less).

The widthwise and lengthwise 60 °C thermal contractions of the PSA sheet are determined from changes in width and length of the PSA sheet before and after heating at 60 °C for 30 minutes. In particular, a PSA sheet (typically a long PSA sheet) for the measurement is cut to have a 100 mm width and 100 mm length, each corresponding to the width and length of the PSA sheet. The PSA sheet’s widthwise 60 °C thermal contraction (60°CHS_WD %) is determined with respect to the 100 mm by 100 mm piece of the PSA sheet by substituting the initial width W0 (i.e. 100 mm) of the piece measured at room temperature (25 °C) before heating and the final width W1 after heating into the equation:
60°CHS_WD % = (W0 - W1)/W0 × 100
The PSA sheet’s lengthwise 60 °C thermal contraction (60°CHS_LD %) is determined with respect to the piece of the PSA sheet by substituting the initial length L0 (i.e. 100 mm) of the piece measured at room temperature (25 °C) before heating and the final length L1 after heating into the equation:
60°CHS_LD % = (L0 - L1)/L0 × 100
The same method is used in the working examples described later.

(80 °C thermal contraction properties)
In a preferable embodiment, the PSA sheet shows at least about a 7 % 80 °C thermal contraction in the width direction. With this, heating the PSA sheet at 80 °C after applied to an adherend or applying the PSA sheet to an adherend heated at a similar temperature allows it to contract in the width direction and tightly adhere to the adherend. The widthwise 80 °C thermal contraction is more preferably about 10 % or greater, or can be for instance, about 11 % or greater, possibly about 12 % or greater, or even about 13 % or greater. The maximum widthwise 80 °C thermal contraction is not particularly limited. It can be about 20 % or less (e.g. about 15 % or less).

The PSA sheet disclosed herein is not particularly limited in 80 °C thermal contraction in the length direction. It is usually suitably about 10 % or less, for instance, about 6 % or less, or even about 4 % or less. In a preferable embodiment, the lengthwise 80 °C thermal contraction is less than 2 %, for instance, possibly less than 1 %, or even less than 0.5 %. In such an embodiment, the PSA sheet is likely to be obtained with desirable widthwise 80 °C thermal contraction and the adhesion-tightening effect of the art disclosed herein is likely to be obtained. The lengthwise 80 °C thermal contraction is suitably about -1 % or greater, possibly about 0 % or greater, about 0.5 % or greater, or even about 1 % or greater.

In a preferable embodiment, the PSA sheet disclosed herein shows a 80°CHS_WD % 80 °C thermal contraction in the width direction and a 80°CHS_LD % 80 °C thermal contraction in the length direction, and the difference of 80°CHS_WD from 80°CHS_LD (i.e. 80°CHS_WD - 80°CHS_LD) is about 6 or greater. In such an embodiment, the adhesion-tightening effect of the art disclosed herein is likely to be obtained. The 80°CHS_WD - 80°CHS_LD value is more preferably about 8 or greater, yet more preferably about 9 or greater, for instance, possibly 10 or greater, or even 12 or greater. The maximum 80°CHS_WD - 80°CHS_LD value can be about 20 or less (e.g. about 15 or less).

The widthwise and lengthwise 80 °C thermal contractions of the PSA sheet are determined from changes in width and length of the PSA sheet before and after heating at 80 °C for 30 minutes. In particular, a PSA sheet (typically a long PSA sheet) for the measurement is cut to have a 100 mm width and 100 mm length, each corresponding to the width and length of the PSA sheet. The PSA sheet’s widthwise 80 °C thermal contraction (80°CHS_WD %) is determined with respect to the 100 mm by 100 mm piece of the PSA sheet by substituting the initial width W0 (i.e. 100 mm) of the piece measured at room temperature (25 °C) before heating and the final width W2 after heating into the equation:
80°CHS_WD % = (W0 - W2)/W0 × 100
The PSA sheet’s lengthwise 80 °C thermal contraction (80°CHS_LD %) is determined with respect to the piece of the PSA sheet by substituting the initial length L0 of the piece measured at room temperature (25 °C) before heating and the final length L2 after heating into the equation:
80°CHS_LD % = (L0 - L2)/L0 × 100
The same method is used in the working examples described later.

The PSA sheet having such 60 °C thermal contraction properties and 80 °C thermal contraction properties can be preferably fabricated by a stretching process (typically a thermal stretching process) carried out in the width direction as described later. Accordingly, the PSA sheet disclosed herein can be a PSA sheet stretched in the width direction (a widthwise-stretched PSA sheet).

(Adhesive properties)
The PSA sheet disclosed herein suitably exhibits an initial peel strength of about 0.01 N/20mm or greater to a glass plate, determined at a tensile speed of 0.3 m/min at 180° peel angle. The PSA sheet showing such initial peel strength adheres well to an adherend in relatively short time and is less likely to lift off the adherend. When the PSA sheet disclosed herein is used as a surface protective sheet, it may provide good protection. In an embodiment, the initial peel strength can be about 0.05 N/20mm or greater (e.g. about 0.1 N/20mm or greater). In another embodiment, the initial peel strength can be about 0.5 N/20mm or greater (e.g. about 1 N/20mm or greater). The maximum initial peel strength is not particularly limited. From the standpoint of light peel, it is usually suitably about 5 N/20mm or less, or preferably about 2.5 N/20mm or less (e.g. about 2 N/20mm or less). In an embodiment, the initial peel strength can be about 1 N/20mm or less (e.g. 0.4 N/20mm or less). The to-glass-plate initial peel strength is determined by the method described below. The same method is used in the working examples described later.

〔Initial peel strength to glass plate〕
The PSA sheet to be measured is cut to a 20 mm wide by 100 mm long strip to prepare a test piece. In a standard environment at 23 °C, 50 % RH, with a 2 kg rubber roller moved back and forth twice, the test piece is press-bonded to a glass plate as the adherend. The sample is stored in the standard environment for 30 minutes. In the same standard environment, using a universal tensile tester, the initial peel strength (N/20mm) is determined at a tensile speed of 0.3 m/min, at 180° peel angle. As the glass plate, a cut blue plate available from Matsunami Glass Ind. (1.35 mm thick, 100 mm by 100 mm) can be used. The initial peel strength can also be determined, using a similar product or other commercial glass plate as the adherend.

After applied to a glass plate and stored at 50 °C for seven days, the PSA sheet disclosed herein preferably exhibits an aged peel strength less than about 11 N/20mm, determined at a tensile speed of 0.3 m/min at 180° peel angle. With the PSA sheet satisfying this property, even when it is applied to the adherend for a relatively long time, the aged adhesive strength is sufficiently suppressed and its light peel from adherend can be maintained. Thus, it shows excellent efficiency of removal from adherends. This is particularly meaningful when the PSA sheet disclosed herein is used as a surface protective sheet. According to the PSA sheet showing an aged peel strength of about 5 N/20mm or less (more preferably about 2 N/20mm or less), greater efficiency of removal can be achieved. From the standpoint of inhibiting lifting and peeling while the adherend is protected (e.g. during processing of the adherend with the PSA sheet applied thereon), the aged peel strength is usually suitably about 0.05 N/20mm or greater, preferably about 0.1 N/20mm or greater, or more preferably about 0.3 N/20mm or greater. The aged peel strength is determined by the method described below. The same method is used in the working examples described later.

〔Aged peel strength to glass plate〕
The PSA sheet to be measured is cut to a 20 mm wide by 100 mm long strip to prepare a test piece. In a standard environment at 23 °C, 50 % RH, with a 2 kg rubber roller moved back and forth twice, the test piece is press-bonded to a glass plate as the adherend. The sample is stored in an environment at 50 °C for seven days and then in a standard environment at 23 °C, 50 % RH for one hour. Subsequently, in the same standard environment, using a universal tensile tester, the aged peel strength (N/20mm) is determined at a tensile speed of 0.3 m/min, at 180° peel angle. The glass plate used as the adherend is the same as the one used in the initial peel strength measurement.

The PSA sheet disclosed herein has an initial peel strength P₁ (N/20mm) and an aged peel strength P₂ (N/20mm), preferably showing an increase in aged adhesive strength (P₂ - P₁, the difference between the aged peel strength P₂ and the initial peel strength P₁) of 8.5 N/20mm or less. A limited increase in aged adhesive strength may suggest, in addition to suppression of the increase in aged adhesive strength, the absolute value of the aged adhesive strength is limited to a level that does not compromise the efficiency of removal when the initial adhesive strength is limited. The PSA sheet satisfying this property is likely to provide excellent efficiency of removal. The increase (P₂ - P₁) in aged adhesive strength is more preferably 5 N/20mm or less, yet more preferably 3.5 N/20mm or less, or particularly preferably 1 N/20mm or less (typically 0.5 N/20mm or less). P₂ - P₁ (the increase in aged adhesive strength) is usually 0 N/20mm or greater, but the PSA sheet disclosed herein is not limited to an embodiment that shows an increase in aged adhesive strength.

While no particular limitations are imposed, the PSA sheet disclosed herein may have a ratio of aged peel strength P₂ (N/20mm) to initial peel strength P₁ (N/20mm) (i.e. a P₂/P₁ ratio value) of 5 or lower. A small P₂/P₁ ratio value indicates a small increase in peel strength with aging. By this, initial adhesion and light peel during removal are favorably combined. From such a standpoint, the P₂/P₁ ratio is preferably 4 or lower, more preferably 3 or lower, or yet more preferably 2 or lower, for instance, 1.8 or lower, 1.5 or lower, or even 1.3 or lower. The P₂/P₁ ratio is typically 0.8 or higher; it can be, for instance, 1 or higher.

The PSA sheet disclosed herein preferably exhibits an aged peel strength in humidity (aged in-humidity peel strength) less than about 11 N/20mm, determined at a tensile speed of 0.3 m/min at 180° peel angle, after applied to a glass plate and stored at 40 °C and 92 % RH for seven days. With the PSA sheet satisfying this property, even when it is used in an embodiment where it is exposed to a highly-humid environment, the aged adhesive strength is sufficiently suppressed and its light peel from adherend can be maintained. With the PSA sheet having an aged in-humidity peel strength of about 5 N/20mm or less (more preferably about 2 N/20mm or less, e.g. about 1.5 N/20mm or less), greater efficiency of removal can be achieved. From the standpoint of inhibiting lifting and peeling to prevent permeation of water such as moisture while the adherend is protected (e.g. during processing of the adherend with the PSA sheet applied thereon) with a chance of exposure to a highly-humid environment, the aged in-humidity peel strength is usually suitably about 0.05 N/20mm or greater, preferably about 0.1 N/20mm or greater, or more preferably about 0.3 N/20mm or greater. The aged peel strength is determined by the method described below. The same method is used in the working examples described later.

〔Aged in-humidity peel strength to glass plate〕
The PSA sheet to be measured is cut to a 20 mm wide by 100 mm long strip to prepare a test piece. In a standard environment at 23 °C and 50 % RH, the test piece is press-bonded to a glass plate as the adherend, with a 2 kg rubber roller moved back and forth twice. The sample is stored in an environment at 40 °C and 92 % RH for seven days and then in a standard environment at 23 °C and 50 % RH for one hour. Subsequently, in the same standard environment, using a universal tensile tester, the aged in-humidity peel strength (N/20mm) is determined at a tensile speed of 0.3 m/min, at 180° peel angle. The glass plate used as the adherend is the same as the one used in the initial peel strength measurement.

<PSA layer>
The type of PSA forming the PSA layer disclosed herein is not particularly limited. The PSA layer may be formed from a PSA composition including, as the base polymer (the primary component among the polymers, i.e. a component accounting for 50 % by weight or more), one, two or more species selected among various polymers (adhesive polymers), such as acrylic, polyester-based, urethane-based, polyether-based, rubber-based, silicone-based, polyamide-based, and fluorinated polymers. The art disclosed herein can be preferably made, for instance, as a PSA sheet having an acrylic PSA layer or a rubber-based PSA layer.

The “acrylic PSA layer” here refers to a PSA layer including an acrylic polymer as the base polymer. Similarly, the “rubber-based PSA layer” refers to a PSA layer including a rubber-based polymer as the base polymer. The “acrylic polymer” refers to a polymer whose primary monomer (the primary component among the monomers, i.e. a component that accounts for 50 % by weight or more of the total amount of the monomers forming the acrylic polymer) is a monomer having at least one (meth)acryloyl group per molecule. Such a monomer may be referred to as an “acrylic monomer” hereinafter. As used herein, the “(meth)acryloyl group” comprehensively refers to acryloyl group and methacryloyl group. Similarly, the “(meth)acrylate” comprehensively refers to acrylate and methacrylate. Acrylic and rubber-based PSA layers are described below as favorable examples, but the PSA layer used in the art disclosed herein is not limited to these.

(Acrylic polymer)
A preferable example of the acrylic polymer is a polymer of a starting monomer mixture that includes an alkyl (meth)acrylate (or a monomer A hereinafter) and may further include another monomer (or a monomer B hereinafter) that is copolymerizable with the alkyl (meth)acrylate. The acrylic polymer typically has a monomer unit composition corresponding to the monomer composition of the starting monomer mixture.

A preferable monomer A is an alkyl (meth)acrylate represented by the next general formula (1):
CH₂=C(R¹)COOR² (1)
Here, R¹ in the formula (1) is a hydrogen atom or a methyl group. R² is an alkyl group having 1 to 20 carbon atoms. Hereinafter, such a range of the number of carbon atoms may be indicated as “C_1-20.” From the standpoint of the polymerization reactivity, polymerization stability, etc., an alkyl (meth)acrylate wherein R² is a C_1-16 alkyl group is preferable, and an alkyl (meth)acrylate wherein R² is a C_1-12 (typically C_1-10, e.g. C_1-8) alkyl group is more preferable.

Examples of an alkyl (meth)acrylate with R² being a C_1-20 alkyl group include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, n-nonyl (meth)acrylate, isononyl (meth)acrylate, n-decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate, eicosyl (meth)acrylate, etc. These alkyl (meth)acrylates can be used solely as one species or in a combination of two or more species.

Examples of compounds that can be used as the monomer B may include functional group-containing monomers such as carboxy group-containing monomers (e.g. acrylic acid (AA)), acid anhydride group-containing monomers, hydroxy group-containing monomers (e.g. 2-hydroxyethyl (meth)acrylate), amide group-containing monomers, imide group-containing monomers, amino group-containing monomers, epoxy group-containing monomers, cyano group-containing monomers, keto group-containing monomers, monomers having nitrogen-containing rings, and alkoxysilyl group-containing monomers. These functional group-containing monomers may be useful for introducing crosslinking points into the acrylic polymer or for increasing the cohesiveness of the acrylic polymer. Functional group-containing monomers can be used solely as one species or in a combination of two or more species.

Other examples of compounds that can be used as the monomer B include vinyl ester-based monomers such as vinyl acetate; aromatic vinyl compounds; non-aromatic ring-containing (meth)acrylates; aromatic ring-containing (meth)acrylates; olefinic monomers; chlorine-containing monomers; isocyanate group-containing monomers; alkoxy group-containing monomers; and vinyl ether-based monomers. These can be used singly as one species or in a combination of two or more species. As the monomer B, one, two or more species can be used among polyfunctional monomers such as 1,6-hexanediol di(meth)acrylate. When using such a polyfunctional monomer, its amount used is not particularly limited. It is usually suitably about 2 % by weight or less (more preferably about 1 % by weight or less) of the total monomer content.

The monomer A content in the total monomer content can be, but is not particularly limited to, for instance, about 50 % by weight or greater; it is suitably about 60 % by weight or greater, preferably about 70 % by weight or greater, more preferably about 80 % by weight or greater, or yet more preferably about 85 % by weight or greater. With the inclusion of the monomer A in a prescribed amount, a PSA sheet having good adhesiveness can be favorably obtained. The art disclosed herein can be preferably implemented, for instance, in an embodiment where the monomer A content in the total monomer content is about 90 % by weight or greater. In an embodiment, the monomer A content can be about 95 % by weight or greater, or even about 97 % by weight or greater. In an embodiment using a monomer A and a monomer B together, from the standpoint of suitably obtaining the effects of the monomer B, the monomer A content in the total monomer content can be, for instance, 99.9 % by weight or less; it is usually preferably 99.5 % by weight or less, more preferably 99 % by weight or less, or about 97 % by weight or less (e.g. 95 % by weight or less).

When an aforementioned functional group-containing monomer is copolymerized in the acrylic polymer, the ratio of the functional group-containing monomer to all the monomers forming the acrylic polymer is usually preferably about 0.1 % by weight or higher (typically about 0.5 % by weight or higher, e.g. about 1 % by weight or higher), and preferably about 40 % by weight or lower (typically about 30 % by weight or lower, e.g. about 20 % by weight or lower).

(Rubber-based polymer)
In another preferable embodiment, the PSA layer can be a rubber-based PSA layer. Examples of the base polymer include natural rubber; styrene-butadiene rubber (SBR); polyisoprene; butene-based polymer synthesized with a butene (1-butene or cis- or trans-2-butene) and/or 2-methylpropene (isobutylene) as the primary monomer(s); A-B-A block copolymer rubber and a hydrogenation product thereof, e.g. styrene-butadiene-styrene block copolymer rubber (SBS), styrene-isoprene-styrene block copolymer (SIS), styrene-isobutylene-styrene block copolymer rubber (SIBS), styrene-vinyl-isoprene-styrene block copolymers (SVIS), hydrogenated SBS (styrene-ethylene/butylene-styrene block copolymer (SEBS)), and hydrogenated SIS ( styrene-ethylene-propylene-styrene block copolymers (SEPS)). These rubber-based polymers can be used singly as one species or in a combination of two or more species.

(Tg of base polymer)
The Tg value of the PSA layer’s base polymer (an acrylic polymer in an acrylic PSA layer) is not particularly limited. The Tg of the base polymer can be, for instance, about -70 °C or higher. In the PSA sheet according to a preferable embodiment, the base polymer of the PSA layer has a Tg of about -65 °C or higher. According to a base polymer having such a Tg, a PSA layer having good adhesive properties can be favorably formed. In an embodiment where the base polymer has a Tg of about -50 °C or higher (more preferably about -35 °C or higher), greater effects can be obtained. The Tg of the base polymer is usually suitably 0 °C or lower, preferably about -5 °C or lower, more preferably about -15 °C or lower, or yet more preferably about -20 °C or lower (e.g. about -25 °C or lower). In the PSA sheet according to another preferable embodiment, from the standpoint of the adhesion, ease of elongation, etc., the base polymer of the PSA layer has a Tg of about -35 °C or lower, more preferably about -40 °C or lower, or yet more preferably about -45 °C or lower (e.g. about -55 °C or lower). The base polymer’s Tg can be adjusted by suitably changing the monomer composition (i.e. the monomer species used in the synthesis of the polymer and their ratio).

In the present description, the Tg of a polymer refers to the value determined by the Fox equation based on the Tg values of homopolymers of the respective monomers forming the polymer and the weight fractions (copolymerization ratio by weight) of the monomers. As shown below, the Fox equation is a relational expression between the Tg of a copolymer and glass transition temperatures Tgi of homopolymers of the respective monomers constituting the copolymer.
1/Tg = Σ(Wi/Tgi)
In the Fox equation, Tg represents the glass transition temperature (unit: K) of the copolymer, Wi the weight fraction (copolymerization ratio by weight) of a monomer i in the copolymer, and Tgi the glass transition temperature (unit: K) of homopolymer of the monomer i.

For the glass transition temperatures of homopolymers used for determining the Tg, the values found in known documents are used. For instance, with respect to the monomers listed below, for the glass transition temperatures of their homopolymers, the following values are used:
2-ethylhexyl acrylate: -70 °C
n-butyl acrylate: -55 °C
ethyl acrylate: -20 °C
methyl acrylate: 8 °C
n-butyl methacrylate: 20 °C
methyl methacrylate: 105 °C
2-hydroxyethyl acrylate: -15 °C
4-hydroxybutyl acrylate: -40 °C
vinyl acetate: 32 °C
styrene: 100 °C
acrylic acid: 106 °C
methacrylic acid: 228 °C
acrylonitrile: 104 °C

With respect to the Tg values of homopolymers other than the examples listed above, the values given in Polymer Handbook (3rd edition, John Wiley & Sons, Inc., Year 1989) are used. With respect to a monomer for which two or more values are listed in the Polymer Handbook, the highest value is used. When no values are given in the Polymer Handbook, values obtained by the measurement method described in Japanese Patent Application Publication No. 2007-51271 are used.

(Synthesis of base polymer)
The method for obtaining the base polymer (e.g. an acrylic polymer) is not particularly limited. Known polymerization methods can be suitably employed, such as solution polymerization, emulsion polymerization, bulk polymerization, and suspension polymerization. Alternatively, it is also possible to employ photopolymerization involving irradiation of light such as UV (typically carried out in the presence of a photopolymerization initiator) and active energy ray irradiation polymerization such as radiation polymerization involving irradiation of radioactive rays such as β rays and γ rays. As the monomer supply method in solution polymerization and emulsion polymerization, a suitable method can be employed among the all-at-once method where all the starting monomer mixture is supplied in one portion, gradual supply method, portion-wise supply method, etc. The polymerization temperature can be suitably selected in accordance with the monomer species, the solvent species, and the polymerization initiator species used, etc. The polymerization temperature is usually suitably about 20 °C or higher, preferably about 40 °C or higher, more preferably about 50 °C or higher; it can also be about 60 °C or higher, about 65 °C or higher, or even about 70 °C or higher. The polymerization temperature is usually suitably about 170 °C or lower (typically about 140 °C or lower), or preferably about 95 °C or lower (e.g. about 85 °C or lower).

The solvent (polymerization solvent) used in solution polymerization can be suitably selected among heretofore known organic solvents. For instance, it is preferable to use aromatic compounds (typically aromatic hydrocarbons) such as toluene, acetic acid esters such as ethyl acetate, aliphatic or alicyclic hydrocarbons such as hexane and cyclohexane, and the like.

The initiator used in the polymerization can be suitably selected among known or commonly-used polymerization initiators in accordance with the monomer species and the type of polymerization method. For instance, azo-based polymerization initiators such as 2,2’-azobisisobutyronitrile can be preferably used. Other examples of the polymerization initiator include persulfates such as potassium persulfate; peroxide-based initiators such as benzoyl peroxide; substituted ethane-based initiators; and aromatic carbonyl compounds. Yet other examples of the polymerization initiator include redox initiators by the combination of a peroxide and a reducing agent. These polymerization initiators can be used singly as one species or in a combination of two or more species. The polymerization initiator can be used in a usual amount. For instance, it can be selected from a range of about 0.005 part to 1 part by weight (typically about 0.01 part to 1 part by weight) to 100 parts by weight of the total monomer content.

The surfactant (emulsifier) used in emulsion polymerization is not particularly limited. Commonly-known anionic surfactants, nonionic surfactants and the like can be used. A surfactant having a radically polymerizable functional group can also be used. For the surfactant, solely one species or a combination of two or more species can be used. The amount of surfactant is usually preferably about 0.1 part by weight or greater (e.g. about 0.5 part by weight or greater) to 100 parts by weight of the total monomer content; and it is preferably about 10 parts by weight or less (e.g. about 5 parts by weight or less) to 100 parts by weight of the total monomer content.

In the emulsion polymerization, as necessary, various heretofore known chain transfer agents (which can be considered also as a molecular weight-adjusting agent or polymerization degree-adjusting agent) can be used. For the chain transfer agent, solely one species or a combination of two or more species can be used. As the chain transfer agent, mercaptans can be preferably used, such as n-dodecyl mercaptan, t-dodecyl mercaptan, and thioglycolic acid. When using a chain transfer agent, its amount can be, for instance, about 0.01 part to 1 part by weight to 100 parts by weight of the total monomer content. The art disclosed herein can also be preferably practiced in an embodiment that uses no chain transfer agent.

<PSA composition>
The PSA layer of the PSA sheet disclosed herein can be formed from various forms of PSA compositions. Examples of the forms of PSA compositions include a solvent-based PSA composition containing the PSA (adhesive component(s)) in an organic solvent, a water-dispersed PSA composition containing at least part of the PSA dispersed in an aqueous solvent, an active energy ray-curable PSA composition formulated so as to cure with active energy rays such as UV rays and radioactive rays to form PSA, and a hot-melt PSA composition which is applied in the molten state by heating and forms PSA when it cools to near room temperature.

(Crosslinking agent)
In the art disclosed herein, the PSA composition used to form the PSA layer preferably includes a crosslinking agent. With the use of crosslinking agent, the cohesive strength can be suitably adjusted. The type of crosslinking agent used is not particularly limited. Examples include oxazoline-based crosslinking agents, aziridine-based crosslinking agents, isocyanate-based crosslinking agents, epoxy-based crosslinking agents, melamine-based crosslinking agents, peroxide-based crosslinking agents, urea-based crosslinking agents, metal alkoxide-based crosslinking agents, metal chelate-based crosslinking agents, metal salt-based crosslinking agents, carbodiimide-based crosslinking agents, hydrazine-based crosslinking agents, amine-based crosslinking agents, and silane coupling agents. These can be used solely as one species or in a combination of two or more species. For instance, it is preferable to use one, two or more species selected from a group consisting of oxazoline-based crosslinking agents, aziridine-based crosslinking agents, isocyanate-based crosslinking agents and epoxy-based crosslinking agents.

The crosslinking agent content (the total amount of crosslinking agent) in the PSA composition disclosed herein is not particularly limited and can be suitably selected in view of the composition and the molecular weight of the base polymer so as to obtain favorable properties after crosslinked. While no particular limitations are imposed, the amount of the crosslinking agent used to 100 parts by weight of the base polymer (typically an acrylic polymer) is usually about 0.01 part by weight or greater, suitably about 0.1 part by weight or greater, or preferably about 1 part by weight or greater (e.g. about 2 parts by weight or greater). From the standpoint of the adhesion, etc., the amount of the crosslinking agent is usually suitably about 15 parts by weight or less (preferably about 10 parts by weight or less, e.g. about 5 parts by weight or less) to 100 parts by weight of the base polymer.

The PSA composition may include, as necessary, various optional additives generally known in the field of PSA compositions, such as tackifier such as rosin-based tackifier, peel-adjusting agent such as a phosphate, viscosity-adjusting agent (viscosifier, etc.), crosslinking accelerator, plasticizer, softener, filler, anti-static agent, anti-aging agent, UV-absorber, antioxidant and photo-stabilizing agent. With respect to these various optional additives, heretofore known species can be used by typical methods. Because these additives do not characterize the present invention in particular, details are omitted.

(Formation of PSA layer)
As for the method for providing the PSA layer to a support substrate which forms the substrate layer, it is possible to employ a direct method where the PSA composition as described above is directly provided (typically applied) to the support substrate and subjected to a curing treatment; a transfer method where the PSA composition is applied to a suitable release face (e.g. a releasable surface of a transfer sheet) and subjected to a curing treatment to form a PSA layer on the surface followed by applying and transferring the PSA layer to the support substrate; and so on. The curing treatment may include one, two or more processes selected among drying (heating), cooling, crosslinking, supplemental copolymerization reaction, aging, etc. The curing treatment referred to herein also encompasses, for instance, a process (heating process, etc.) simply to allow a PSA composition containing a solvent to dry, a process simply to cool down (solidify) a heat-melted PSA composition. When the curing treatment includes two or more processes (e.g. drying and crosslinking), these processes may be performed at once or stepwise.

The PSA composition can be applied, for instance, using a commonly used coater such as a gravure roll coater, reverse roll coater, kiss roll coater, dip roll coater, bar coater, knife coater and spray coater. From the standpoint of accelerating the crosslinking reaction, increasing the productivity, etc., the PSA composition is preferably dried with heat. The drying temperature may vary depending on the object (a support substrate, etc.) to which the PSA composition is applied, but it can be, for instance, about 40 °C to 150 °C.

(Gel fraction)
The weight fraction (gel fraction) of the ethyl acetate-insoluble portion of the PSA layer disclosed herein is not particularly limited. It can be, for instance, about 40 % or higher (typically about 50 % or higher). In an embodiment, from the standpoint of obtaining at least certain cohesive strength, the gel fraction of the PSA layer is suitably about 60 % or higher, preferably about 80 % or higher, or more preferably about 90 % or higher. The gel fraction of the PSA layer can be, for instance, about 95 % or higher (e.g. about 98 % or higher). With increasing gel fraction, the cohesion of the PSA tends to increase while the aged adhesive strength tends to be suppressed. The maximum gel fraction is theoretically 100 %. In some embodiments, the gel fraction can be, for instance, about 98 % or lower, or even about 95 % or lower (e.g. about 90 % or lower). The gel fraction can be adjusted by the selection of, for instance, the base polymer composition, the polymerization method and conditions for the base polymer, the molecular weight of the base polymer, the presence of a crosslinking agent as well as its type and amount used if any, and so on. The gel fraction is determined by the method described below.

(Degree of swelling)
The degree of swelling of the PSA layer disclosed herein is not particularly limited and can be usually about 30-fold or less. From the standpoint of obtaining at least certain cohesive strength, the degree of swelling is suitably about 20-fold or less, preferably about 15-fold or less, or more preferably about 12-fold or less, for instance, about 10-fold or less, or even about 8-fold or less. The minimum degree of swelling is theoretically 1-fold; it can be usually about 3-fold or greater, for instance, about 5-fold or greater. The degree of swelling can be adjusted, for instance, through the molecular weight of the base polymer, the type pf crosslinking agent (distances among functional groups) and its amount used, etc. The degree of swelling is determined by the method described below.

〔Determination of gel fraction and degree of swelling〕
A PSA layer sample (weight: W₁) weighing approximately 0.1 g is wrapped into a pouch with a porous polytetrafluoroethylene membrane (weight: W₂) having an average pore diameter of 0.2 μm, and the opening is tied with twine (weight: W₃). As the porous polytetrafluoroethylene membrane, trade name NITOFLON (registered trademark) NTF 1122 (product of Nitto Denko Corp.; 0.2 μm average pore diameter, 75 % porosity, 85 μm thickness) or an equivalent product can be used. The resulting package is immersed in 50 mL of ethyl acetate and stored at room temperature (typically 23 °C) for 7 days. Subsequently, the package is taken out, and any residual ethyl acetate is wiped off the outer surface. The package weight (W₄) is measured. The package is then dried at 130 °C for 2 hours and the package weight (W₅) is measured. The gel fraction and the degree of swelling of the PSA layer can be determined by substituting the respective values into the following equation:
Gel fraction (%) = [(W₅-W₂-W₃)/W₁] × 100
Degree of swelling (fold) = (W₄-W₂-W₃)/(W₅-W₂-W₃)

<Substrate layer>
As the substrate layer of the PSA sheet disclosed herein, resin film, a rubber sheet, a foam sheet, a composite of these, etc., can be used. Examples of the rubber sheet include natural rubber sheets and butyl rubber sheets. Examples of the foam sheet include polyurethane foam sheets, and polychloroprene rubber foam sheets.

The art disclosed herein can be preferably applied to a PSA sheet wherein the substrate layer is resin film. The concept of “resin film” here refers to film typically obtained by molding a thin layer from a resin composition primarily including resin components as described below; it should be distinguished from so-called non-woven and woven fabrics. In other words, the concept of resin film excludes non-woven and woven fabrics. A resin film (non-foamed resin film) which is essentially not foamed can be preferably used. Here, the non-foamed resin film refers to resin film that has not been deliberately subjected to a foaming process. In particular, the resin film may have an expansion ratio lower than about 1.1 (e.g. lower than 1.05, typically lower than 1.01).

Examples of the resin components forming the resin film include polyolefinic resins (polyethylene, polypropylene, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, etc.), poly(vinyl chloride)-based resins (typically soft poly(vinyl chloride)-based resin); poly(vinyl acetate)-based resin, polyurethane-based resins (ether-based polyurethane, ester-based polyurethane, carbonate-based polyurethane, etc.), urethane (meth)acrylate-based resin, thermoplastic elastomers (olefinic elastomer, styrene-based elastomer, acrylic elastomer, etc.), polyester-based resins (polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, etc.), polycarbonate-based resin, polyamide-based resin, and polyimide-based resin. Among these resins, solely one species or a combination of two or more species can be used.

While no particular limitations are imposed, in the PSA sheet according to an embodiment, it is preferable to use a substrate layer that includes, as its primary component(s), one, two or more species of resin selected from the group consisting of polyolefinic resin, poly(vinyl chloride)-based resin, polyurethane-based resin, thermoplastic elastomer and polyester-based resin (typically a substrate layer including such resin in an amount exceeding 50 % by weight). In another embodiment, in view of the performance, ease of handling, costs, etc., a substrate layer including a polyolefinic resin layer, polyester-based resin layer or polyvinyl chloride-based resin layer can be preferably used. Among the resin materials, in view of the heat stability, the lightness of weight, etc., polyolefinic resins, polyurethane-based resins and olefinic elastomers are preferable; in view of the handling properties, etc., polyolefinic resins and olefinic elastomers are particularly preferable.

The PSA sheet disclosed herein can be preferably made in an embodiment having a substrate layer that includes a polyolefinic resin as the primary component, that is, an embodiment wherein the substrate layer is polyolefinic resin film. For instance, it is preferable to use polyolefinic resin film in which 50 % by weight or more of the entire substrate layer is polyethylene (PE) resin or polypropylene (PP) resin. In other words, in the polyolefinic resin film, the combined amount of PE resin and PP resin may account for 50 % by weight or more of the entire substrate layer.

The PP resin may include, as the primary component, various polymer species (propylene-based polymers) that include propylene as a monomer unit. The PP resin may be formed essentially of one, two or more species of propylene-based polymer. The concept of propylene-based polymer here includes homopolypropylene as well as a random copolymer of propylene and other monomer(s) (random polypropylene) and a block copolymer (block polypropylene). The concept of propylene-based polymer here includes, for instance, the following species:
Propylene homopolymer (homopolypropylene), for instance, isotactic polypropylene;
Random copolymer (random polypropylene) of propylene and other α-olefin(s) (typically, one, two or more species selected from ethylene and α-olefins having 4 to 10 carbon atoms), preferably random polypropylene synthesized with propylene as the primary monomer (i.e. the monomer accounting for 50 % by weight or more of the total monomer content);
Block copolymer (block polypropylene) of propylene and other α-olefin(s) (typically, one, two or more species selected from ethylene and α-olefins having 4 to 10 carbon atoms), preferably block polypropylene synthesized with propylene as the primary monomer (i.e. the monomer accounting for 50 % by weight or more of the total monomer content).

The PE resin can be various types of polymer (ethylene-based polymer) synthesized with ethylene as a monomer. The PE resin may be essentially formed of one, two or more species of ethylene-based polymer. The ethylene-based polymer can be an ethylene homopolymer or a copolymer (random copolymer, block copolymer, etc.) of ethylene as the primary monomer and other α-olefin(s) as secondary monomer(s). Favorable examples of the α-olefins include α-olefins having 3 to 10 carbon atoms such as propylene, 1-butene (which can be a branched 1-butene), 1-hexene, 4-methyl-1-pentene and 1-octene. For instance, it is preferable to use PE resin that includes, as the primary component, an ethylene-based polymer in which an α-olefin as the secondary monomer is copolymerized up to about 10 % by weight (typically up to about 5 % by weight).

The PE resin may include a copolymer of ethylene and a monomer (functional monomer) containing other functional group(s) in addition to a polymerizable functional group, copolymer of an ethylene-based polymer copolymerized with such a functional monomer, or the like. Examples of a copolymer of ethylene and a functional monomer include ethylene-vinyl acetate copolymers (EVA), ethylene-acrylic acid copolymers (EAA), ethylene-methacrylic acid copolymers (EMAA), ethylene-methyl acrylate copolymers (EMA), ethylene-ethyl acrylate copolymers (EEA), ethylene-methyl methacrylate copolymers (EMMA), and copolymers of ethylene and (meth)acrylic acid (i.e. acrylic acid and/or methacrylic acid) crosslinked by metal ions.

The PE resin is not particularly limited in density. The concept of PE resin here includes all of the following: high density polyethylene (HDPE), medium density polyethylene (MDPE), low density polyethylene (LDPE) and linear low density polyethylene (LLPDE). In an embodiment, the density of the PE resin can be, for instance, about 0.90 g/cm³ to 0.94 g/cm³. Preferable PE resins include LDPE and LLDPE. The PE resin may include one, two or more species of LDPE and one, two or more species of LLDPE. There are no particular limitations to the respective blend ratios of LDPE and LLDPE, or to the LDPE to LLDPE blend ratio. They can be suitably selected to form a PE resin having desirable properties. As the substrate layer of the PSA sheet disclosed herein, it is preferable to use polyethylenic resin film such as LLDPE film whose LLDPE content is higher than 50 % by weight (preferably about 75 % by weight or higher, e.g. about 90 % by weight or higher) and LDPE film whose LDPE content is higher than 50 % by weight (preferably about 75 % by weight or higher, e.g. about 90 % by weight or higher). Laminate resin film including such polyethylenic resin film as a component can be used as well.

The resin film (e.g. polyolefinic resin film) used as the substrate layer of the PSA sheet disclosed herein may include, as necessary, suitable components allowable in the substrate layer. Examples of additives that can be suitably added include filler, colorant (pigment such as inorganic pigment, dye), antioxidant, photostabilizer (including radical scavenger and UV absorber), antistatic agent, plasticizer, slip agent, and anti-blocking agent. Each additive can be added, for instance, in an amount similar to a typical amount in the field of resin film used as substrate layers and the like of PSA sheets.

The substrate layer may have a mono-layer structure or a multi-layer structure formed of two, three or more layers. In a multi-layer structure, it is preferable that at least one layer (preferably each layer) is formed of aforementioned resin film. For instance, in a preferable substrate layer, 75 % or more (more preferably 90 % or more) of the thickness is attributed to mono-layer or multi-layer (typically mono-layer) polyolefinic resin film. The substrate layer may be entirely formed of mono-layer or multi-layer polyolefinic resin film. From the standpoint of the cost-effectiveness, it is preferable to use a substrate layer formed of mono-layer resin film (e.g. LLDPE film, LDPE film, etc.).

The method for producing the substrate layer can be suitably selected among heretofore known methods and is not particularly limited. For instance, when resin film is used as the substrate layer, it is possible to use resin film fabricated by suitably employing a heretofore known general film-forming method such as inflation molding, extrusion, T-die cast molding, and calendar roll molding.

When the substrate layer is formed by an aforementioned method, the substrate layer usually has a MD and a TD perpendicular to the MD. In such an embodiment, the PSA sheet is preferably fabricated so that the substrate layer’s MD corresponds to the length direction of the PSA sheet and the substrate layer’s TD corresponds to the width direction of the PSA sheet. In other words, the substrate layer’s MD and the length direction of the PSA sheet are within a range between -30° and +30° (preferably between -15° and +15°, more preferably between -10° and +10°, or yet more preferably between -5° and +5°) from each other. The same is true with the substrate layer’s TD and the PSA sheet’s width direction. In a typical embodiment, the PSA sheet’s length direction is in the substrate layer’s MD and the PSA sheet’s width direction is in the substrate layer’s TD. In such an embodiment, a PSA sheet showing at least the prescribed widthwise 60 °C thermal contraction can be preferably obtained.

In an embodiment where at least one face (the PSA layer-side face) of the substrate layer is a resin film surface, the resin film surface can be subjected to a heretofore known surface treatment such as corona discharge treatment, plasma treatment, ozone exposure, flame exposure, UV irradiation, acid treatment, alkali treatment, and primer coating. These surface treatments may enhance the tightness of adhesion between the substrate layer and the PSA layer, or the anchoring of the PSA layer onto the substrate layer. In an embodiment using polyolefinic resin film as the substrate layer, it is particularly meaningful to provide these surface treatments.

<Method for producing PSA sheet>
Described next is a method for producing the PSA sheet disclosed herein. The production method includes a step of preparing (obtaining) an initial PSA sheet having a substrate layer and a PSA layer provided to at least one face of the substrate layer (preparation step); and a step of stretching the initial PSA sheet in its width direction while heating the same (stretching step). According to this method, a PSA sheet that has a width of about 1 m or greater and shows at least a 4 % 60 °C thermal contraction in the width direction can be produced. The method preferably further includes, after the stretching step, a step of maintaining the width of the initial PSA sheet 0 % to 10 % shorter than the stretched width (width maintenance step); it preferably further includes a step of cooling down the stretched initial PSA sheet (cool-down step). The production method disclosed herein is described in detail below.

First, an initial PSA sheet is obtained, the initial PSA sheet having a substrate layer and a PSA layer provided to at least one face of the substrate layer. The initial PSA sheet can be typically long. In typical, the initial PSA sheet may thermally contract in the length direction and thermally expand in the width direction. Thus, the initial PSA sheet’s 60 °C thermal contraction can be above 0 % (e.g. about 1 % or greater) in the length direction and below 0 % (e.g. about -0.5 % or less) in the width direction. The 60 °C thermal contraction is determined by the method described earlier. The thermal contraction properties (typically at 60 °C and 80 °C) of the initial PSA sheet are thus different from the PSA sheet disclosed herein. The initial PSA sheet usually has a greater overall thickness than the final PSA sheet produced. With respect to other features, the initial PSA sheet is basically the same as the final PSA sheet and the same applies to the substrate layer and the PSA layer. Thus, details are omitted here.

Subsequently, in the stretching step, the initial PSA sheet is stretched in the width direction while it is heated. In the stretching step, the initial PSA sheet is preferably stretched by a factor of 1.1 or greater and less than 1.45 in its width direction. The stretch factor can be, for instance, 1.2 or greater, or even 1.3 or greater. When the stretch factor is at least the prescribed value, at least a 4 % widthwise 60 °C thermal contraction is preferably achieved. The PSA sheet can be efficiently made wider. The stretch factor is preferably 1.4 or less (e.g. 1.35 or less). When the stretch factor is limited to or below the prescribed value, a PSA sheet that is unsusceptible to degradation of quality and is practical for use can be preferably produced.

The means of stretching is not particularly limited. For instance, with the two edges of the width direction of the long initial PSA sheet fastened with a fixture (or chucks, clips), the initial PSA sheet is stretched by pulling it at a constant rate in the width direction. The stretching speed (the tensile speed in the width direction of the initial PSA sheet) can be suitably set in accordance with the material and thickness of the PSA sheet, etc. For instance, from the standpoint of the productivity, it is suitably about 1 mm/s or higher (e.g. about 3 mm/s or higher). From the standpoint of inhibiting degradation of quality, it is suitably about 40 mm/s or lower (e.g. about 30 mm/s or lower, preferably 15 mm/s or lower, more preferably 10 mm/s or lower).

A preferable fixture has several chucks placed equidistantly in the length direction of the initial PSA sheet. As for the stretching, the fixture may be movable (able to be pulled) in the width direction only by its parts (e.g. chucks) placed on one side of the width direction of the initial PSA sheet or by its parts (e.g. chucks) placed on each side of the width direction of the initial PSA sheet. To deal with thermal expansion in the length direction of the initial sheet, such several chucks may be placed on rails so that they can freely move in the length direction.

The stretching step is carried out in a heated state. Thus, this step is also called the thermally-stretching step. In particular, the initial PSA sheet at room temperature (e.g. 20 °C, 50 % RH) is placed in an oven heated to a certain temperature and then stretched. The heating temperature (typically the temperature inside the oven) can be suitably selected in accordance with the resin species forming the initial PSA sheet, etc. For instance, it can be in a range of about 60 °C to 120 °C (preferably about 70 °C to 110 °C, typically about 80 °C to 100 °C).

The time for carrying out the stretching step in a heated state (the thermally stretching time) is suitably about 10 seconds or more, preferably about 15 seconds or more (e.g. about 20 seconds or more), or possibly about 30 seconds or more. The thermally stretching time is usually suitably about 3 minutes or less, for instance, about 1 minute or less. From the standpoint of smooth stretching, it is preferable to subject the initial PSA sheet, before the stretching step, to a pre-heating step at a temperature similar (about ±10 °C) to the heating temperature in the stretching step. The time for carrying out the pre-heating step is suitably about 10 seconds or more, preferably about 15 seconds or more, and suitably about 1 minute or less, for instance, preferably about 30 seconds or less.

After the stretching step, it is preferable to maintain the width of the initial PSA sheet 0 % to 10 % shorter than the stretched width. In a preferable embodiment, the fixture placed across the initial PSA sheet is set and maintained to have a width more than 0 % up to 10 % narrower than the stretched width to release the tension (relaxing width maintenance). This can reduce uneven contraction of the initial PSA sheet and unintentional contraction by the thermal history from its production to its use. In another embodiment, the fixture is placed to maintain the width of the initial PSA sheet after stretched for a prescribed amount of time after completion of the stretching process (non-relaxing width maintenance).

The maintenance step can typically be carried out at an elevated temperature (while heating the initial PSA sheet). Accordingly, the width maintenance step can be a thermal width maintenance step. The temperature can be about the same as that in the stretching step (within a range of ±10 °C from the temperature in the stretching step), or can be higher by more than 10 °C (e.g. by about 20 °C up to 40 °C) than the temperature in the stretching step. Such a heating condition can be applied to produce a higher-quality PSA sheet that shows a prescribed widthwise 60 °C thermal contraction. The time for carrying out the thermal width maintenance step is suitably about 10 seconds or more, preferably about 15 seconds or more (e.g. about 20 seconds or more), or even about 30 seconds or more. The time for the thermal width maintenance is usually suitably about 3 minutes or less, for instance, about 1 minute or less.

The initial PSA sheet stretched as described above is allowed to cool down by natural cooling at room temperature (e.g. 20 °C) or forced cooling using a cooling device (cool-down step). When natural cooling is employed, a PSA sheet of higher quality can be obtained. When forced cooling is employed, a PSA sheet can be produced efficiently. At an appropriate point of time before, during or after the cool-down step, from the stretched PSA sheet, one edge (typically each edge) of the width direction may be or may not be cut off for width adjustment, etc.

A PSA sheet showing at least a 4 % widthwise 60 °C thermal contraction can be preferably produced. The 60 °C thermal contraction and the 80 °C thermal contraction can be adjusted by suitably selecting the stretch factor, stretching conditions, width maintenance conditions, etc. According to this method, a PSA sheet having a greater width than that of the initial PSA sheet can be produced. This method allows effective use of a PSA sheet that has been prepared (obtained) in advance with a smaller width than required. Even when a large-scale system is not available for manufacturing a wide PSA sheet, after a PSA sheet is fabricated with an available system, a desirably-wide PSA sheet can be produced by the aforementioned method.

The PSA sheet produced as described above can be wound in the length direction in an easy-to-handle PSA sheet roll and the roll is stored, transported and so on until its use.

<Applications>
The PSA sheet disclosed herein provides excellent tightness of adhesion to adherend; and therefore, it is used as a surface protective sheet that is to be applied to surfaces of a metal plate, a coated steel plate, a synthetic resin plate, a glass plate and the like so as to prevent damage (scratches, contamination, etc.) to these surfaces while they are being processed or transported and to be eventually removed from the adherend at the end of the protection period. The surface protective sheet encompasses shrink film for product wrapping. The surface protective sheet is typically formed of an adhesively single-faced PSA sheet having a PSA layer provided to one face of a substrate. On an uneven adherend surface, the PSA sheet disclosed herein can conform well to the unevenness with application of heat to provide good tightness of adhesion. The PSA sheet disclosed herein may have been stretched in the width direction; and therefore, it is preferably used in an embodiment where it is applied to an adherend having a relatively large surface area. For instance, the PSA sheet disclosed herein is preferably used in an embodiment where it covers the entire adherend surface having a width of about 1 m or greater, for instance, about 2.6 m or greater.

Matters disclosed by this description include the following:
(1) A PSA sheet (typically a long PSA sheet) comprising a substrate layer and a PSA layer provided to at least one face of the substrate layer, with the PSA sheet
having a width of about 1 m or greater, and
showing at least a 4 % 60 °C thermal contraction in its width direction.
(2) The PSA sheet according to (1) above, having 60°CHS_LD % 60 °C thermal contraction in its length direction and 60°CHS_WD % 60 °C thermal contraction in its width direction, with 60°CHS_WD - 60°CHS_LD of 3 or greater.
(3) The PSA sheet according to (1) or (2) above, showing a less than 2 % 60 °C thermal contraction in its length direction.
(4) The PSA sheet according to any of (1) to (3) above, having a width of about 2.6 m or greater.
(5) The PSA sheet according to any of (1) to (4) above, wherein the substrate layer is formed from resin film, a rubber sheet or a foam sheet.
(6) The PSA sheet according to (5) above, wherein the substrate layer is formed from at least one species of resin selected from the group consisting of polyolefinic resin, polyvinyl chloride-based resin and polyurethane-based resin.
(7) The PSA sheet according to (5) or (6) above, wherein its length direction corresponds to the substrate layer’s MD and its width direction corresponds to the substrate layer’s TD.
(8) The PSA sheet according to (7) above, wherein its length direction is in the substrate layer’s MD and its width direction is in the substrate layer’s TD.
(9) The PSA sheet according to any of (1) to (8) above, wherein the substrate layer has a thickness of 25 μm to 150 μm.
(10) The PSA sheet according to any of (1) to (9) above, wherein the PSA layer comprises at least one species selected from the group consisting of acrylic polymers, urethane-based polymers, polyester-based polymers, polyether-based polymers, rubber-based polymers, silicone-based polymers, polyamide-based polymers and fluoropolymers, with the at least one species accounting for 50 % by weight or more.
(11) The PSA sheet according to (10) above, wherein the PSA layer is an acrylic PSA layer comprising an acrylic polymer as its base polymer or a rubber-based PSA layer comprising a rubber-based polymer as its base polymer.
(12) The PSA sheet according to any of (1) to (11) above, wherein the PSA layer has a thickness of 1 μm to 20 μm.
(13) The PSA sheet according to any of (1) to (12) above, showing an initial peel strength of 0.01 N/20mm to 5 N/20mm to a glass plate.
(14) A PSA sheet roll comprising the pressure-sensitive adhesive sheet according to any of (1) to (13) above, wherein the pressure-sensitive adhesive sheet is wound in its length direction.
(15) A surface protective sheet comprising the PSA sheet according to any of (1) to (13) above, wherein the PSA sheet is an adhesively single-faced PSA sheet having the PSA layer provided only to one face of the substrate layer.

(16) A method for producing the PSA sheet according to any of (1) to (13) above, the method comprising
a step of preparing (obtaining) an initial PSA sheet (typically a long initial PSA sheet) that comprises a substrate layer and a PSA layer provided to at least one face of the substrate layer, and
a step of stretching the initial PSA sheet in its width direction while heating the same.
(17) A method for producing a PSA sheet, the method comprising
a step of preparing (obtaining) an initial PSA sheet (typically a long initial PSA sheet) that comprises a substrate layer and a PSA layer provided to at least one face of the substrate layer, and
a step of stretching the initial PSA sheet in its width direction while heating the same.
(18) The production method according to (16) or (17) above, wherein in the stretching step, the initial PSA sheet is stretched in its width direction by a factor of 1.1 or greater and less than 1.45.
(19) The production method according to any of (16) to (18) above, further comprising, after the stretching step, a step of maintaining the initial PSA sheet at a width 0 % to 10 % shorter than its width after stretched.
(20) The production method according to any of (16) to (19) above, wherein the width maintenance step is carried out at a temperature in a range of ±10 °C from or higher by more than 10 °C than the temperature at which the stretching step is carried out.
(21) The production method according to any of (16) to (20) above, further comprising a step of cooling down the stretched initial PSA sheet.

Several tested Examples related to the present invention are described below, but the present invention should not be limited to these tested Examples. In the description below, "part(s)" and "%" are by weight unless otherwise specified.

<Example 1>
〔Fabrication of initial PSA sheet〕
Into a reaction vessel equipped with a stirrer, a thermometer, a nitrogen inlet, a condenser and an addition funnel, was placed a monomer mixture formed of 45 parts of 2-ethylhexyl acrylate, 51 parts of n-butyl methacrylate and 4 parts of 2-hydroxyethyl acrylate as monomers; along with ethyl acetate as the polymerization solvent. The resulting mixture was allowed to stir under a nitrogen flow for two hours. Oxygen was thus eliminated from the polymerization system. Subsequently, to 100 parts of the monomer mixture, was added 2,2'-azobisisobutyronitrile at a ratio of 0.2 part and solution polymerization was carried out at 60 °C for 6 hours to obtain an acrylic polymer solution according to this Example.
To the acrylic polymer solution, for 100 parts of the acrylic polymer in the solution, was added 3 parts of isocyanate-based crosslinking agent (product of Covestro, trade name DESMODUR RFE, tris(p-isocyanatophenyl)thiophosphate, 27 % ethyl acetate solution) and mixed with stirring to prepare a PSA composition according to this Example.
Two sheets of 50 μm thick resin film formed of LDPE having one corona-treated face were obtained. The acrylic PSA composition was applied to the corona-treated face of the first sheet of resin film and allowed to dry at 90 °C for 1 minute to form a 5 μm thick PSA layer and obtain a 500 mm wide initial PSA sheet having an overall thickness of 55 μm. The resin film was oriented so that its TD is in the width direction of the PSA sheet. The non-corona-treated face (second face) of the second sheet of resin film was applied to the PSA layer and used as release liner.

〔Production of PSA sheet〕
The two edges of the width direction of the resulting initial PSA sheet were fastened with several chucks of a tensile tester. The several chucks are placed equidistantly in the length direction at each end of the width direction of the initial PSA sheet. This was pre-heated in an oven at 80 °C for 18 seconds and then stretched in the width direction at a speed of 4.9 mm/s at the same temperature. After stretched, at the same temperature, the tension in the width direction was released by a length equivalent to 7.5 % of the stretched width and the resulting state was maintained for 36 seconds (relaxing width maintenance). Subsequently, the stretched initial PSA sheet was removed from the oven and cooled down to room temperature (20 °C, 50 % RH) using a cooling nozzle. A PSA sheet according to this Example was thus obtained, stretched by a factor of 1.12 in the width direction. The chucks fastening the two edges of the width direction of the initial PSA sheet were configured so that they freely move on rails placed in the length direction of the initial PSA sheet; the inter-chuck distance increased 1.25 times from the initial value to the final value before and after the stretching. Certain portions were cut off at the two edges of the width direction of the PSA sheet fastened with the chucks. The resulting PSA sheet was wound to obtain a PSA sheet roll.

<Examples 2 to 5>
The production conditions for PSA sheets were modified as shown in Table 1. Otherwise in the same manner as Example 1, PSA sheets were produced and PSA sheet rolls were obtained according to the respective Examples. In Example 5, the relaxing width maintenance (releasing the tension in the width direction by a length equivalent to 7.5 % of the stretched width) was not carried out; instead, the PSA sheet was held at the stretched width for 36 seconds (non-relaxing width maintenance).

〔Appearance after stretched〕
The stretched PSA sheet according to each Example was visually inspected. In particular, the non-stretched initial PSA sheet was stamped entirely on the back with a grid pattern; after stretched, it was inspected for the presence of deformation in the grid pattern as an indicator of uneven stretching. One with no uneven stretching was graded “E” (excellent); one with some uneven stretching, but to an extent not hindering practical use, “G” (good); one with uneven stretching similar to cracking or with cracking, “P” (poor). The results are shown in Table 1.

〔Adhesion tightness test〕
With respect to the PSA sheet according to each Example and the initial PSA sheet (Reference Example) obtained in Example 1, the conformability to an uneven surface was tested. In particular, the PSA sheet was applied to a whole surface frost glass slide (100 mm by 100 mm, available from Matsunami Glass Ind., Ltd.) as the adherend with unevenness of 3.0 μm in arithmetic surface roughness Ra, with a 2 kg roller moved back and forth once. The resultant was left standing in an environment at 60 °C for 30 minutes. With respect to the heated PSA sheet, the state of tightness of adhesion to the adherend was inspected. One with no air bubble or lifting of at least 2 mm in diameter or width was graded “G” (good); one with such air bubble or lifting, “P” (poor). The results are shown in Table 1.

With respect to the PSA sheet according to each Example and the initial PSA sheet (Reference Example) obtained in Example 1, Table 1 shows the lengthwise and widthwise 60 °C thermal contractions (%), 80 °C thermal contractions (%), 60°CHS_WD - 60°CHS_LD, 80°CHS_WD - 80°CHS_LD, initial peel strength (N/20mm), aged peel strength (N/20mm), and aged in-humidity peel strength (N/20mm) along with the production conditions, appearance after stretched, and adhesion tightness test result.

As shown in Table 1, with respect to the PSA sheets according to Examples 1 to 4 showing at least a 4 % widthwise 60 °C thermal contraction, they all appeared acceptable for practical use with sufficient levels of tightness of adhesion to the adherend. On the other hand, with respect to Example 5 and Reference Example with less than 4 % 60 °C thermal contractions showed lower levels of adhesion tightness as compared to Examples 1 to 4. These results indicate that according to a PSA sheet that has a width of about 1 m or greater and shows at least a 4 % widthwise 60 °C thermal contraction, excellent tightness of adhesion to adherend can be achieved.

Although specific embodiments of the present invention have been described in detail above, these are merely for illustrations and do not limit the scope of the claims. The art according to the claims includes various modifications and changes made to the specific embodiments illustrated above.

1: substrate layer
1A: first face
1B: second face
2: PSA layer
2A: adhesive face
10: PSA sheet
100: PSA sheet roll

Claims (20)

1. A pressure-sensitive adhesive sheet comprising a substrate layer and a pressure-sensitive adhesive layer provided to at least one face of the substrate layer, the pressure-sensitive adhesive sheet,
   wherein the pressure-sensitive adhesive sheet has a width of about 1 m or greater, and
   shows a 60 °C thermal contraction of 4 % or more in its width direction.
2. The pressure-sensitive adhesive sheet according to Claim 1, having a 60°CHS_LD % 60 °C thermal contraction in its length direction and a 60°CHS_WD % 60 °C thermal contraction in its width direction, with a difference 60°CHS_WD - 60°CHS_LD of 3 or greater.
3. The pressure-sensitive adhesive sheet according to Claim 1 or 2, wherein the 60 °C thermal contraction is less than 2 % in the length direction of the pressure-sensitive adhesive sheet.
4. The pressure-sensitive adhesive sheet according to any one of Claims 1 to 3, having a width of about 2.6 m or greater.
5. The pressure-sensitive adhesive sheet according to any one of Claims 1 to 4, wherein the substrate layer is formed from resin film, a rubber sheet or a foam sheet.
6. The pressure-sensitive adhesive sheet according to Claim 5, wherein the substrate layer is formed from at least one species of resin selected from the group consisting of polyolefinic resin, polyvinyl chloride-based resin and polyurethane-based resin.
7. The pressure-sensitive adhesive sheet according to Claim 5 or 6, wherein the length direction of the pressure-sensitive adhesive sheet corresponds to the machine direction of the substrate layer and the width direction of the pressure-sensitive adhesive sheet corresponds to the transverse direction.
8. The pressure-sensitive adhesive sheet according to Claim 7, wherein the length direction of the pressure-sensitive adhesive sheet is in the machine direction of the substrate layer and the width direction of the pressure-sensitive adhesive sheet is in the transverse direction of the substrate layer.
9. The pressure-sensitive adhesive sheet according to any one of Claims 1 to 8, wherein the substrate layer has a thickness of 25 μm to 150 μm.
10. The pressure-sensitive adhesive sheet according to any one of Claims 1 to 9, wherein the pressure-sensitive adhesive layer is an acrylic pressure-sensitive adhesive layer comprising an acrylic polymer as its base polymer or a rubber-based pressure-sensitive adhesive layer comprising a rubber-based polymer as its base polymer.
11. The pressure-sensitive adhesive sheet according to any one of Claims 1 to 10, wherein the pressure-sensitive adhesive layer has a thickness of 1 μm to 20 μm.
    
12. The pressure-sensitive adhesive sheet according to any one of Claims 1 to 11, exhibiting an initial peel strength of 0.01 N/20mm to 5 N/20mm to a glass plate.
13. A pressure-sensitive adhesive sheet roll comprising the pressure-sensitive adhesive sheet according to any one of Claims 1 to 12, wherein the pressure-sensitive adhesive sheet is wound in its length direction.
14. A surface protective sheet comprising the pressure-sensitive adhesive sheet according to any one of Claims 1 to 12, wherein the pressure-sensitive adhesive sheet is an adhesively single-faced pressure-sensitive adhesive sheet having the pressure-sensitive adhesive layer provided solely to one face of the substrate layer.
15. A method for producing the pressure-sensitive adhesive sheet according to any one of Claims 1 to 12, the method comprising:
    a step of preparing an initial pressure-sensitive adhesive sheet that comprises a substrate layer and a pressure-sensitive adhesive layer provided to at least one face of the substrate layer, and
    a step of stretching the initial pressure-sensitive adhesive sheet in its width direction while heating the same.
16. A method for producing a pressure-sensitive adhesive sheet, the method comprising:
    a step of preparing an initial pressure-sensitive adhesive sheet that comprises a substrate layer and a pressure-sensitive adhesive layer provided to at least one face of the substrate layer, and
    a step of stretching the initial pressure-sensitive adhesive sheet in its width direction while heating the same.
17. The method according to Claim 15 or 16, wherein in the stretching step, the initial pressure-sensitive adhesive sheet is stretched in its width direction by a factor of 1.1 or greater and less than 1.45.
18. The method according to any one of Claims 15 to 17, the method further comprising, after the stretching step, a step of maintaining the initial pressure-sensitive adhesive sheet at a width 0 % to 10 % shorter than its width after stretched.
19. The method according to any one of Claims 15 to 18, wherein the width maintenance step is carried out at a temperature in a range of ±10 °C from or higher by more than 10 °C than the temperature at which the stretching step is carried out.
20. The method according to any one of Claims 15 to 19, the method further comprising a step of cooling down the stretched initial pressure-sensitive adhesive sheet.

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