WO2026004869A1 - Olefin-based heat-shrinkable multilayer film - Google Patents

Abstract

Provided is an olefin-based heat-shrinkable multilayer film that is substantially free of a cyclic olefin-based resin, is easy to use for solvent bonding and gravity separation, and has excellent heat shrinkability and transparency.　The olefin-based heat-shrinkable multilayer film comprises a first surface layer and a second surface layer, which are derived from a polystyrene-based resin, and an intermediate layer derived from a polyolefin-based resin, wherein (a) the polystyrene-based resin contains at least 50 wt% of a styrene-butadiene copolymer, (b) the crystal melting enthalpy of the intermediate layer is set to a value of 135 mJ/mg or less, (c) the thermal shrinkage rate in the TD direction is 50-80% under the shrinkage conditions of 100°C and 10 seconds, (d) the haze value is 10% or less, and (e) the specific gravity is 0.95 or less.

Description

Olefin-based heat-shrinkable multilayer film

The present invention relates to an olefin-based heat-shrinkable multilayer film (hereinafter, sometimes simply referred to as a heat-shrinkable film).
In particular, the present invention relates to an olefin-based heat-shrinkable multilayer film that is substantially free from cyclic olefin-based resins, and that is easy to solvent bond and gravity separate, and that is excellent in heat shrinkability, transparency, etc.

BACKGROUND ART In recent years, a wide variety of heat-shrinkable films have been widely used for the purpose of packaging glass bottles or plastic bottles with heat-shrinkable films that both protect the bottles and display the product in order to improve the appearance of the packaged products.
Known base resins for such heat-shrinkable films include polyvinyl chloride resins, polystyrene resins, polyester resins, polyolefin resins, and the like.

However, although heat-shrinkable films made of polyvinyl chloride resins have excellent heat shrinkability, they have environmental problems such as being prone to generating chlorine gas when burned.
On the other hand, heat-shrinkable films made from polystyrene resins or polyester resins as the main raw materials have relatively good heat shrinkability, but because the difference in specific gravity between them and the PET resin that makes up PET bottles is small, gravity separation using water or the like is difficult, making them poorly recyclable.
In contrast, heat-shrinkable films made of polyolefin resins have a large difference in specific gravity from PET bottles, and have the advantage that gravity separation using water or the like is relatively easy. However, since the film itself is difficult to solvent bond, there is a problem that conventional cylindrical heat-shrinkable film manufacturing equipment cannot be used as is.

Therefore, it has been proposed to incorporate, for example, a cyclic olefin resin as one of the constituent components of a polyolefin resin (see Patent Document 1).
More specifically, the heat-shrinkable laminated film is a laminate formed by providing an intermediate layer made of an olefin resin and an adhesive resin, the intermediate layer being a layer having a density of less than 0.94 g/ ^cm3 , a surface layer and a back layer made of a styrene resin on the front and back surfaces of the intermediate layer, respectively, and the thicknesses of the layers of the laminate satisfy the relationship (surface layer + back layer)/intermediate layer = 1/1 to 1/6, and the laminate is stretched at least uniaxially by 2 to 6 times.

Also proposed is a heat-shrinkable laminated porous film having a porous polyolefin resin layer (see Patent Document 2).
More specifically, the heat-shrinkable laminated porous film comprises a pair of front and back layers whose main component is at least one selected from the group consisting of copolymer resins (A) of styrene-based hydrocarbons and conjugated diene-based hydrocarbons, and a porous layer disposed between the pair of front and back layers and whose main component is a resin composition containing a polyolefin-based resin (B) and a filler (C), and is stretched in at least one direction and has a porosity within a predetermined range.

JP 2000-309071 A (Claims, etc.) JP 2018-153984 A (Claims, etc.)

However, in the case of the heat-shrinkable laminate film of Patent Document 1 and the like, when a cyclic olefin resin is blended, the resulting film becomes hard and brittle, and there are problems in that the film is prone to tearing and interlayer delamination during secondary processing such as printing.
Furthermore, when a cyclic olefin resin is blended into the base resin of a heat-shrinkable film, the compatibility with other resins decreases, and the transparency of the film tends to decrease.
Furthermore, since cyclic olefin resins are relatively expensive, the use of such resins as constituent materials increases the production costs of heat-shrinkable films, resulting in an economical disadvantage.

Furthermore, in the case of the heat-shrinkable laminated porous film of Patent Document 2 and the like, there is a problem in that it is difficult to control the porosity value and hence the specific gravity during production.
This results in variations in specific gravity, making it difficult to separate the films by specific gravity during recycling, and further causes problems such as a decrease in the transparency, heat shrinkability, and mechanical properties of the film.

The present invention was made in response to the above-mentioned problems, and its purpose is to provide a heat-shrinkable multilayer film that is easy to solvent bond and gravity separate, and has excellent heat shrinkability and transparency, without substantially incorporating a cyclic olefin resin.

According to the present invention, there is provided an olefin-based heat-shrinkable multilayer film comprising an intermediate layer derived from a polyolefin-based resin, a first surface layer (usually the top surface side) derived from at least a polystyrene-based resin on one surface side of the intermediate layer, and a second surface layer (usually the back surface side) derived from at least a polystyrene-based resin on the other surface side of the intermediate layer, and which is characterized by satisfying the following configurations (a) to (e), thereby solving the above-mentioned problems.
(a) The polystyrene resin contains 50% by weight or more of a styrene-butadiene copolymer based on the total weight.
(b) The intermediate layer has a crystalline melting enthalpy (ΔH) of 135 mJ/mg or less as measured by DSC in accordance with JIS K 7121:2012 (corresponding to ISO 11357-2:2013).
(c) When the film is shrunk in boiling water at 100°C for 10 seconds, the thermal shrinkage rate in the main shrinkage direction is defined as A1, and A1 is set to a value within the range of 50 to 80%.
(d) The haze value measured in accordance with JIS K 7136:2000 (equivalent to ISO 14782:1999) is 10% or less.
(e) The specific gravity measured in accordance with JIS K 7112-1:2023 (corresponding to ISO 1183-1:2023) is 0.95 or less.
In this way, by satisfying at least the components (a) to (e), solvent adhesion and gravity separation are easy, and excellent heat shrinkability, transparency, etc. can be obtained without substantially blending a cyclic olefin resin.

In constructing the olefin-based heat-shrinkable multilayer film of the present invention, it is preferable that the molecular weight distribution (Mw/Mn) of the polyolefin-based resin constituting the intermediate layer as the structure (f) is 4.0 or less.
By limiting the molecular weight distribution (Mw/Mn) of the polyolefin resin constituting the intermediate layer to a predetermined range in this manner, it is possible to obtain excellent heat shrinkability, lightweight properties, transparency (low haze), and the like.

In constructing the olefin-based heat-shrinkable multilayer film of the present invention, it is preferable that the intermediate layer contains linear low-density polyethylene as the polyolefin-based resin.
By including linear low-density polyethylene as the polyolefin resin constituting the intermediate layer, it is possible to suppress variations in the specific gravity of the film and to achieve further weight reduction.

In constructing the olefin-based heat-shrinkable multi-layer film of the present invention, it is preferable that the weight average molecular weight of the polyolefin-based resin constituting the intermediate layer is set to a value within the range of 150,000 to 250,000.
By limiting the weight average molecular weight (Mw) of the polyolefin resin constituting the intermediate layer to a predetermined range in this way, it is possible to obtain excellent heat shrinkability, light weight, transparency, and the like.

In constructing the olefin-based heat-shrinkable multilayer film of the present invention, the polystyrene-based resin constituting the first surface layer and/or the second surface layer preferably contains a styrene-based elastomer.
In this way, by containing a styrene-based resin, which constitutes the first surface layer and the second surface layer, a styrene-based elastomer, in particular a hydrogenated styrene-based elastomer, the transparency is further improved and interlayer delamination can be effectively suppressed.

In constructing the olefin-based heat-shrinkable multilayer film of the present invention, the polystyrene-based resin constituting the first surface layer and/or the second surface layer preferably contains a compatibilizer.
By containing a compatibilizer in the styrene-based resin constituting the first surface layer and the second surface layer, the transparency is excellent and delamination can be effectively suppressed.

In constructing the olefin-based heat-shrinkable multi-layer film of the present invention, it is preferable that the thickness of the film after stretching treatment is set to a value within the range of 10 to 60 μm.
By setting the thickness of the heat-shrinkable film to a value within the specified range in this way, not only is the film easy to use, but the film also has better transparency, and excellent heat shrinkability and mechanical properties.

In constructing the olefin-based heat-shrinkable multilayer film of the present invention, when the thickness of the intermediate layer before stretching is t1, the thickness of the first surface layer before stretching is t2, and the thickness of the second surface layer before stretching is t3, it is preferable that the thickness ratio (t2+t3)/(t1) is a value within the range of 0.1 to 0.5.
By setting the ratio of the thickness of the intermediate layer (t1) to the sum of the thicknesses of the first surface layer and the second surface layer before the stretching treatment (t2 + t3) within a predetermined range in this way, not only does the heat-shrinkable film become easier to use, but it also has excellent heat-shrinkability and mechanical properties.

1(a) to 1(c) are diagrams provided to explain the form and method of use of the heat-shrinkable film, respectively. FIG. 2 is a chart showing the molecular weight distribution of the resin constituting the intermediate layer in the heat-shrinkable film. FIG. 3 is a DSC chart of the resin constituting the intermediate layer in the heat-shrinkable film. FIG. 4 is a diagram provided for explaining the relationship between the crystalline melting enthalpy of the intermediate layer in the heat-shrinkable film and the heat shrinkage rate in the TD direction. FIG. 5 is a diagram provided for explaining the relationship between the crystalline melting enthalpy of the intermediate layer in the heat-shrinkable film and the haze. FIG. 6 is a diagram provided for explaining the relationship between the crystalline melting enthalpy and the specific gravity of the intermediate layer in the heat-shrinkable film. FIG. 7 is a diagram provided for explaining the relationship between the SBC content in the surface layer of a heat-shrinkable film and the haze. FIG. 8 is a diagram provided for explaining the relationship between the ratio of the total thickness of the surface layers (t2+t3) to the thickness of the intermediate layer (t1) in a heat-shrinkable film and the haze.

[First embodiment]
As illustrated in Figures 1(a) to 1(c), the first embodiment is an olefin-based heat-shrinkable multilayer film 10 including an intermediate layer 10a derived from a polyolefin-based resin, a first surface layer 10b derived from at least a polystyrene-based resin on one surface side of the intermediate layer 10a, and a second surface layer 10c derived from at least a polystyrene-based resin on the other surface side of the intermediate layer 10a, and is characterized in that the olefin-based heat-shrinkable multilayer film 10 satisfies the following configurations (a) to (e):
(a) The polystyrene resin contains 50% by weight or more of styrene-butadiene copolymer based on the total weight.
(b) The intermediate layer has a crystalline melting enthalpy (ΔH) measured by DSC in accordance with JIS K 7121:2012 (corresponding to ISO 11357-2:2013) of 135 mJ/mg or less.
(c) When the film is shrunk in boiling water at 100°C for 10 seconds, the thermal shrinkage rate in the main shrinkage direction is defined as A1, and A1 is set to a value within the range of 50 to 80%.
(d) The haze value measured in accordance with JIS K 7136:2000 (equivalent to ISO 14782:1999) is 10% or less.
(e) The specific gravity measured in accordance with JIS K 7112-1:2023 (corresponding to ISO 1183-1:2023) is 0.95 or less.
Fig. 1(a) shows the basic three-layer structure of the olefin-based heat-shrinkable multilayer film 10. Fig. 1(b) shows a structure in which a decorative layer 10d is provided on the outer side (the bottom layer in the drawing) of the second surface layer 10c of the olefin-based heat-shrinkable multilayer film 10. Fig. 1(c) shows the olefin-based heat-shrinkable multilayer film 10 provided with the decorative layer 10d of Fig. 1(b) applied to a PET bottle.

1. Basic Structure (1) Intermediate Layer As illustrated in FIGS. 1(a) to 1(c), the olefin-based heat-shrinkable multilayer film 10 is characterized by having an intermediate layer 10a derived from a polyolefin-based resin between a first surface layer 10b and a second surface layer 10c.
The reason for this is that polyolefin resins can exhibit the properties required for a heat-shrinkable film by utilizing crystallization due to stretching or the like.
Furthermore, polyolefin resins have excellent transparency (haze) and light weight (specific gravity), and therefore recycling can be carried out accurately and quickly using a specified cyclone device or the like to separate them by specific gravity.
Therefore, among polyolefin resins, polyethylene resins in particular are superior in transparency, crystallinity (thermal shrinkage rate), light weight (specific gravity), and the like compared to polypropylene and the like.

(2) First Surface Layer/Second Surface Layer As illustrated in Figures 1(a) to (c), the olefin-based heat-shrinkable multilayer film 10 is characterized by having a first surface layer 10b derived from at least a polystyrene-based resin on one surface side of the intermediate layer 10a, and a second surface layer 10c derived from at least a polystyrene-based resin on the other surface side of the intermediate layer 10a.
The reason for this is that an intermediate layer made of a polyolefin resin has low surface energy, and as it is, it is difficult to achieve solvent adhesion or paintability, and may be difficult to use.
In other words, by forming a first surface layer and a second surface layer made of a polystyrene-based resin, which has excellent solvent adhesion and paintability, on both sides of an intermediate layer made of a polyolefin-based resin, the solvent adhesion and paintability of the intermediate layer made of a polyolefin-based resin can be easily improved.

(3) Multilayer Structure As illustrated in FIGS. 1(a) to 1(c), it is advantageous to use an olefin-based heat-shrinkable multilayer film 10 having a multilayer structure that basically includes an intermediate layer 10a derived from a polyolefin-based resin, a first surface layer 10b derived from a polystyrene-based resin, and a second surface layer 10c also derived from a polystyrene-based resin.
The reason for this is that although a two-layer structure may be acceptable in practice for an olefin-based heat-shrinkable multilayer film, a three-layer structure is generally easier to manufacture and, furthermore, makes it easier to obtain uniform heat shrinkability, etc., regardless of whether it is on the front or back.

In constructing the olefin-based heat-shrinkable multilayer film of the present invention, it is preferable that no adhesive layer (including a primer layer) is included between the intermediate layer and the first and second surface layers.
The reason for this is that the olefin-based heat-shrinkable multilayer film must be able to achieve the desired specific gravity, thickness, and heat shrinkage rate.
Conversely, if an adhesive layer (including a primer layer) is included between the intermediate layer and the first and second surface layers, it may become difficult to control the specific gravity, resulting in a decrease in lightness, or the thickness may become excessively large, making it difficult to manage and control the thermal shrinkage rate.
Moreover, the number of manufacturing steps may increase, making it difficult to provide a stable and economical olefin-based heat-shrinkable multi-layer film.

2. Specific Configuration (1) Components of Intermediate Layer The type of polyolefin resin constituting the intermediate layer is preferably determined taking into consideration the transparency, heat shrinkage rate, molecular weight distribution, etc. of the resulting film, but basically, a polymer containing an olefin hydrocarbon such as ethylene or propylene as a monomer component can be used.
The polyolefin resin may be a homopolymer or a copolymer. In the case of a copolymer, the copolymerization ratio of olefin hydrocarbons such as ethylene, butene, hexene, etc. is preferably 50% by weight or more (same as mass %, and the same applies hereinafter), and may be 70% by weight or more, or 90% by weight or more.
Therefore, examples of such polyolefin resins include polyethylene resin, 1-hexene copolymer, ethylene-propylene copolymer, etc., and linear low density polyethylene (LLDPE) is particularly preferred.

There are also no particular limitations on the catalyst used in polymerizing the polyolefin resin, and Ziegler-Natta catalysts, metallocene catalysts, etc. can be used.
However, it is also preferable to use polyolefin resins (isotactic, syndiotactic, etc.) produced using metallocene catalysts, since this makes it easier to obtain polyolefin resins that are excellent in stereoregularity, mechanical strength, etc.
Furthermore, the polyolefin resin used may have any crystallinity or melting point, and a polyolefin resin composition containing two polyolefin resins with different properties blended in specific ranges may be used depending on the physical properties and application of the resulting film.

(2) Molecular Weight Distribution of Polyolefin Resin Constituting the Intermediate Layer It is preferable that the weight average molecular weight (Mw) of such an olefin resin is set to a value within the range of 150,000 to 250,000.
The reason for this is that by having such a weight average molecular weight (Mw), it is easy to obtain an olefin-based heat-shrinkable multilayer film with low haze and a controlled heat shrinkage rate.
Furthermore, it is preferable that the number average molecular weight (Mn) of such an olefin-based resin is set to a value within the range of 50,000 to 100,000.
The reason for this is that when the olefin-based resin has such a number average molecular weight (Mn), it is easy to obtain an olefin-based heat-shrinkable multilayer film that similarly has low haze and a controlled heat shrinkage rate.
FIG. 2 shows a chart (Types A to E) showing the molecular weight distribution of the resin constituting the intermediate layer.

Therefore, since the haze, heat shrinkage rate, and the like can be easily controlled by the olefin-based resin having such a weight-average molecular weight (Mw) and number-average molecular weight (Mn), it is preferable that the molecular weight distribution (Mw/Mn) also be a value of 4.0 or less.
The reason for this is that when the molecular weight distribution (Mw/Mn) of the polyolefin resin constituting the intermediate layer is controlled, the transparency, crystallinity (thermal shrinkage rate), etc. are further improved.

As shown in FIG. 3, the weight average molecular weight (Mw) and number average molecular weight (Mn) of the olefin resin, as well as the molecular weight distribution (Mw/Mn) based on these, can be measured using high-temperature GPC.
More specifically, for example, the weight average molecular weight (Mw) of the olefin resin can be calculated using HLC-8321GPC/HT (manufactured by Tosoh Corporation) at a column temperature of 145°C using dichlorobenzene as a solvent, in comparison with a calibration curve using standard styrene particles.

(3) Crystal fusion enthalpy (ΔH) of the intermediate layer measured by DSC
The intermediate layer is characterized by having a crystalline fusion enthalpy (ΔH) of 135 mJ/mg or less as measured by DSC.
The reason for this is that by controlling the crystalline fusion enthalpy, which is usually called the heat of fusion, it becomes easier to obtain a preferable heat shrinkage rate and also good transparency.
More specifically, if the crystalline melting enthalpy exceeds 135 mJ/mg, the thermal shrinkage rate under shrinkage conditions of, for example, 100° C. for 10 seconds will be significantly reduced.
Moreover, if the crystalline melting enthalpy exceeds 135 mJ/mg, the transparency also tends to decrease.
However, if the crystalline melting enthalpy is too small, the crystallinity will be low, which may make it difficult to control the thermal shrinkage rate.
Therefore, the crystalline melting enthalpy of the intermediate layer measured by DSC is preferably set to a value within the range of 100 to 130 mJ/mg, and more preferably within the range of 110 to 125 mJ/mg.
The crystalline melting enthalpy of the intermediate layer can be measured using a DSC (differential scanning calorimeter) in accordance with JIS K 7121:2012 (corresponding to ISO 11357-2:2013).

Here, with reference to FIG. 4 , based on the data of Example 1 and the like, the relationship between the crystalline melting enthalpy (ΔH) of the polyolefin resin constituting the intermediate layer and the thermal shrinkage rate in the TD direction (100°C, 10 seconds) will be described.
That is, the horizontal axis indicates the crystalline melting enthalpy of the polyolefin resin, and the vertical axis indicates the thermal shrinkage rate in the TD direction (%, 100° C., 10 seconds).
From the characteristic curve in FIG. 4, it can be said that there is a negative linear correlation between the crystalline melting enthalpy and the thermal shrinkage rate (%) in the TD direction, and that by controlling the crystalline melting enthalpy to 135° C. or less, the thermal shrinkage rate in the TD direction can be controlled within a desired range.

Also, with reference to FIG. 5, the relationship between the crystalline fusion enthalpy (ΔH) of the polyolefin resin constituting the intermediate layer and the haze (%) will be described based on the data of Example 1 and the like.
That is, the horizontal axis indicates the crystalline fusion enthalpy of the polyolefin resin, and the vertical axis indicates the haze (%).
From the characteristic curve in FIG. 5, it can be seen that there is no significant correlation between the crystalline melting enthalpy and the haze (%), and that if the crystalline melting enthalpy is in the range of 110 to 140° C., approximately the same haze value can be obtained.

Furthermore, with reference to FIG. 6, the relationship between the crystalline fusion enthalpy (ΔH) and the specific gravity of the polyolefin resin constituting the intermediate layer will be described based on the data of Example 1 and the like.
That is, the horizontal axis indicates the crystalline fusion enthalpy of the polyolefin resin, and the vertical axis indicates the specific gravity.
From the characteristic curve in FIG. 6, it can be seen that there is a positive linear correlation between the crystalline melting enthalpy and the specific gravity, and that by controlling the crystalline melting enthalpy to 135° C. or less, the specific gravity can be controlled to a low value of 0.95 or less.
Characteristic curves A and B were obtained based on the data of Example 1, etc., and it is presumed that characteristic curve A is due to the fact that the total thickness (t2 + t3) of the surface layer made of polystyrene-based resin is relatively thick, and the relative amount of polystyrene-based resin, which has a relatively high specific gravity, is greater than the relative amount of polyolefin-based resin, which has a relatively low specific gravity, in the entire film.
Conversely, it is presumed that characteristic curve B is due to the fact that the total thickness (t2 + t3) of the polystyrene-based resin is relatively thin, and the relative amount of polystyrene-based resin, which has a relatively high specific gravity, is smaller than the relative amount of polyolefin-based resin, which has a relatively low specific gravity, in the entire film.

(4) Thickness of Intermediate Layer When the thickness of the intermediate layer before stretching is (t1), it is usually preferable that the thickness (t1) is set to a value within the range of 50 to 200 μm.
The reason for this is that by controlling the thickness of such an intermediate layer to a value within a specified range, not only is it easy to use, but the specific gravity is controlled to a value below the desired value, and excellent heat shrinkability, transparency, etc. are obtained.
More specifically, if the thickness of the intermediate layer is less than 50 μm, it may become difficult to adjust the specific gravity, and handling may be significantly impaired.
On the other hand, if the thickness of such an intermediate layer exceeds 200 μm, the transparency may decrease, it may become difficult to achieve a uniform thickness, and delamination may become more likely.
Therefore, it is more preferable that the thickness of the intermediate layer is set to a value within the range of 80 to 150 μm, and even more preferable that it is set to a value within the range of 100 to 130 μm.

(5) Compounding component 1 of the first surface layer and the second surface layer
The polystyrene resin constituting the first surface layer and/or the second surface layer preferably contains 50% by weight or more of styrene-butadiene copolymer (SBC) relative to the total amount (100% by weight).
The reason for this is that if the SBC content is less than 50% by weight, the transparency of the resulting film may be significantly reduced.
Therefore, it is more preferable to set the SBC content to 60% by weight or more, and even more preferable to set it to 70% by weight or more, relative to the total amount (100% by weight) of at least one of the first surface layer and the second surface layer.
The types of residual components other than SBC in the first surface layer and the second surface layer are not particularly limited, but typically include polymers other than styrene-butadiene copolymer (SBC) and compatibilizers described below.

It is also preferable that the polystyrene resin constituting the first surface layer and/or the second surface layer contains a styrene elastomer.
The reason for this is that the inclusion of a styrene-based elastomer as a type of styrene-butadiene copolymer (SBC) improves adhesion with the intermediate layer, effectively suppresses interlayer delamination, and furthermore, makes it easier to obtain a good thermal shrinkage rate.
Therefore, when a styrene-based elastomer is contained, the content of the styrene-based elastomer is preferably set to a value within the range of 1 to 30% by weight, more preferably a value within the range of 2 to 20% by weight, and even more preferably a value within the range of 3 to 10% by weight, relative to the total amount (100% by weight) of at least one of the first surface layer and the second surface layer.
As the styrene-based elastomer, for example, a styrene-butadiene block copolymer, a styrene-butadiene/butylene-styrene triblock copolymer, a styrene-ethylene-butylene-styrene block copolymer, a styrene-ethylene/butylene-styrene triblock copolymer (each of the styrene-based copolymers includes a thermoplastic elastomer), etc., are preferred, either alone or in combination of two or more.

(6) Compounding component 2 of the first surface layer and the second surface layer
It is also preferable that the polystyrene resin constituting the first surface layer and/or the second surface layer is a polymer other than the above-mentioned styrene elastomer, and further contains a compatibilizer.
The reason for this is that the inclusion of a compatibilizer improves the adhesion with the intermediate layer, effectively suppresses delamination between layers, and furthermore, makes it easier to obtain a good heat shrinkage rate.
Therefore, when a compatibilizer is contained, the content of the compatibilizer is preferably set to a value within the range of 1 to 30 wt %, more preferably a value within the range of 2 to 20 wt %, and even more preferably a value within the range of 3 to 10 wt %, relative to the total amount (100 wt %) of at least one of the first surface layer and the second surface layer.

As the compatibilizer, various compounds and various polymers can be used as long as they exhibit the desired compatibility. For example, a polymer other than the above-mentioned styrene-based elastomer is preferred because it is compatible with the styrene-based elastomer and exhibits good compatibility, and is therefore a styrene-based compatibilizer (such as a hydrogenated elastomer resin).
That is, as the styrene-based compatibilizer (hydrogenated elastomer resin), hydrogenated elastomer of styrene-butadiene block copolymer, hydrogenated elastomer of styrene-butadiene/butylene-styrene triblock copolymer, hydrogenated elastomer of styrene-ethylene-butylene-styrene block copolymer, hydrogenated elastomer of styrene-butadiene block copolymer having a functional group (carboxyl group, hydroxyl group, etc., hereinafter the same), hydrogenated elastomer of styrene-butadiene/butylene-styrene triblock copolymer having a functional group, hydrogenated elastomer of styrene-ethylene-butylene-styrene block copolymer having a functional group, Hydrogenated elastomers of polymers, hydrogenated elastomers of styrene-ethylene/butylene-styrene triblock copolymers having functional groups, hydrogenated elastomers of styrene-ethylene/butylene-styrene triblock copolymers, oligomers of styrene-butadiene block copolymers, oligomers of styrene-butadiene/butylene-styrene triblock copolymers, oligomers of styrene-ethylene-butylene-styrene block copolymers, oligomers of styrene-ethylene/butylene-styrene triblock copolymers, and further, these various tackifiers, etc., are preferably used singly or in combination of two or more kinds.
Therefore, when these styrene-based compatibilizers contain a butadiene moiety in the molecule, it is more preferable that the butadiene moiety is hydrogenated, and further, it is preferable that the styrene-based compatibilizer has a functional group (a carboxyl group, a hydroxyl group, etc., the same applies hereinafter), because this makes it possible to more effectively adjust compatibility, heat resistance, transparency, etc.
Furthermore, when these styrene-based compatibilizers are used, the compatibility, heat resistance, flowability, and the like can be more effectively adjusted, and therefore the styrene content contained therein is preferably set to a value within a range of 10 to 80% by weight, more preferably a value within a range of 30 to 75% by weight, and even more preferably a value within a range of 50 to 70% by weight, based on the total weight.
In order to facilitate plasticization and improve compatibility in the styrene-based compatibilizer, it is preferable that a tackifier is blended in an amount of usually 10 to 100 parts by weight per 100 parts by weight of the main component of the compatibilizer.

(7) Thickness of the First Surface Layer and the Second Surface Layer When the thickness of the first surface layer before the stretching treatment is t2 and the thickness of the second surface layer before the stretching treatment is t3, it is usually preferable that the total thickness (t2 + t3) of the first surface layer and the second surface layer be a value within the range of 10 to 80 μm.
The reason for this is that by controlling the total thickness to a value within a predetermined range, not only is the usability improved, but the specific gravity is controlled to a value below a desired value, and excellent heat shrinkability, transparency, etc. are obtained.
More specifically, if the total thickness is less than 10 μm, it may become difficult to adjust the specific gravity, or the handling may be significantly impaired.
On the other hand, if the total thickness exceeds 80 μm, the transparency may decrease, it may become difficult to achieve a uniform thickness, and it may become difficult to adjust the specific gravity.
Therefore, it is more preferable that the total thickness of the first surface layer before the stretching treatment and the second surface layer before the stretching treatment is a value within the range of 20 to 60 μm, and even more preferable that it is a value within the range of 30 to 50 μm.

The thickness (t2) of the first surface layer and the thickness (t3) of the second surface layer before the stretching treatment may be different from each other, taking into consideration the application and ease of use of the film.
Therefore, when the thickness (t3) of the second surface layer is made thicker than the thickness (t2) of the first surface layer in the ratio (thickness (t2) of the first surface layer) to the thickness (t3) of the second surface layer, the ratio (t2/t3) is preferably set to a value within the range of 0.1 to 0.9, more preferably to a value within the range of 0.2 to 0.8, and even more preferably to a value within the range of 0.3 to 0.7.
Of course, when the thickness of the second surface layer (t3) is thinner than the thickness of the first surface layer (t2) in the ratio of the thickness of the first surface layer (t2) to the thickness of the second surface layer (t3), it is preferable that the ratio exceeds 1.1, more preferably a value within the range of 1.2 to 5, and even more preferably a value within the range of 1.5 to 4.

However, taking into consideration the application and ease of use of the film, as well as the uniformity of the thermal shrinkage rate and ease of production, it is also preferable that the thickness of the first surface layer (t2)/the thickness of the second surface layer (t3) before the stretching treatment is substantially equal.
That is, it is preferable to set the ratio (t2/t3) to a value in the range of more than 0.9 to 1.1, more preferably to a value in the range of 0.95 to 1.05, and even more preferably to a value in the range of 0.99 to 1.01.

(8) Additives The olefin-based heat-shrinkable multilayer film has the above-described structure. However, if necessary, known resins other than those described above, antioxidants, heat stabilizers, antistatic agents, antiblocking agents, slip agents, nucleating agents, ultraviolet absorbers, colorants, and the like may be appropriately added within the scope of the above-described object of the present invention.
Among these, the addition of antistatic agents, antiblocking agents, slip agents, etc. is particularly effective in improving blocking resistance, but excessive addition impairs solvent adhesion, so careful consideration must be given to the amount added.

3. Specific Configuration (1) Heat Shrinkage Ratio When shrunk in boiling water at 100°C for 10 seconds, the heat shrinkage ratio in the main shrinkage direction (TD direction) is defined as A1, and the A1 is a value within the range of 50 to 80%.
The reason for this is that if the heat shrinkage rate A1 is outside this range, the types and average molecular weights of usable polyolefin-based resins and the types and average molecular weights of usable polystyrene-based resins may be excessively limited.
Therefore, when shrunk in boiling water at 100°C for 10 seconds, the heat shrinkage rate A1 in the main shrinkage direction (TD) is more preferably set to a value within the range of 55 to 75%, and even more preferably set to a value within the range of 58 to 65%.

(2) Haze Value The haze value of the olefin-based heat-shrinkable multilayer film before heat shrinking, measured in accordance with JIS K 7136:2000 (equivalent to ISO 14782:1999), is characterized by being 10% or less.
By specifically limiting the haze value to a value within a predetermined range in this manner, the transparency of the shrink film can be easily controlled quantitatively, and since the transparency is good, the versatility of the film can be further enhanced.
More specifically, if the haze value of the film before heat shrinkage exceeds 10%, the transparency decreases, which may make it difficult to apply the film to decorative purposes and the like.
On the other hand, if the haze value of the film before heat shrinkage becomes too small, it becomes difficult to control it stably, and the production yield may decrease significantly.
Therefore, in the configuration (m), the haze value of the film before heat shrinkage is more preferably set to a value within the range of 1 to 9%, and even more preferably set to a value within the range of 2 to 8%.

Here, the relationship between the content (wt %) of SBC blended in the polystyrene resin of the surface layer and the haze value (%) will be described with reference to FIG.
That is, based on the data of Example 1 and the like, the relationship between the SBC content (wt %) and the haze value (%) will be explained.
That is, the horizontal axis indicates the SBC content, and the vertical axis indicates the haze.
From the characteristic curve in FIG. 7 , it can be seen that there is a curvilinear correlation between the amount of SBC (wt %, the remainder being general-purpose polystyrene) blended into the total amount of polystyrene resin (100 wt %) and the haze value, and that by controlling the SBC content to 50 wt % or more, more preferably 60 wt % or more, it is possible to control the haze to a low value of 10% or less.

(3) Specific Gravity The specific gravity of the olefin-based heat-shrinkable multilayer film after stretching is measured in accordance with JIS K 7112-1:2023 (corresponding to ISO 1183-1:2023), and is characterized in that the value is 0.95 or less.
The reason for this is that if the specific gravity exceeds 0.95, it may take excessive time and effort to separate the material by specific gravity.
However, if the specific gravity is too small, the types and average molecular weights of usable polyolefin resins and polystyrene resins may be excessively limited.
Therefore, it is more preferable that the specific gravity is set to a value within the range of 0.91 to 0.945, and even more preferable that the specific gravity is set to a value within the range of 0.92 to 0.94.

(4) Thickness It is generally preferable that the thickness of the olefin-based heat-shrinkable multi-layer film after stretching is set to a value within the range of 10 to 60 μm.
The reason for this is that by setting the thickness of such a heat-shrinkable multilayer film to a value within a specified range, not only is it easy to use, but it also has even better transparency and excellent heat shrinkability and mechanical properties.
Therefore, it is more preferable that the thickness of such a heat-shrinkable multilayer film is set to a value within the range of 20 to 55 μm, and even more preferably to a value within the range of 30 to 50 μm.

(5) Relationship 1 between the total thickness of the first surface layer and the second surface layer and the thickness ratio of the intermediate layer
When the thickness of the intermediate layer before stretching is t1, the thickness of the first surface layer before stretching is t2, and the thickness of the second surface layer before stretching is t3, it is preferable that the thickness ratio (t2+t3)/(t1) is a value within the range of 0.1 to 0.5.
The reason for this is that, although it depends on the type of raw material, by setting the ratio of the thickness of the intermediate layer (t1) to the total thickness of the first surface layer and the second surface layer (t2 + t3) within a predetermined range, the haze value and specific gravity can be controlled to be small, and not only can ease of use be improved, but excellent heat shrinkability can also be obtained.
Therefore, it is more preferable that the thickness ratio (t2+t3)/(t1) is set to a value within the range of 0.2 to 0.48, and even more preferable that it is set to a value within the range of 0.25 to 0.45.

(6) Relationship 2 between the total thickness of the first surface layer and the second surface layer and the thickness ratio of the intermediate layer
Here, with reference to FIG. 8, the relationship between the ratio of the thickness (t2+t3)/(t1) of each layer before stretching treatment and the haze (%) of the heat-shrinkable multilayer film will be described.
That is, the horizontal axis shows the values obtained by changing the ratio of thickness (t2+t3)/(t1) in the heat-shrinkable multilayer film (corresponding to Example 18) before stretching treatment, and the vertical axis shows the haze value of the heat-shrinkable multilayer film before stretching treatment.
Judging from the characteristic curve in FIG. 8, it can be seen that when the ratio of thickness (t2+t3)/(t1) is increased, the obtained haze value tends to increase significantly.
Therefore, in order to make the haze value of such a heat-shrinkable multilayer film 10% or less, it is preferable to make the ratio of thickness (t2+t3)/(t1) about 0.5 or less.
Although not shown, it has been found that the specific gravity of the heat-shrinkable multilayer film can be easily controlled to a value of 0.95 or less by setting the ratio of thickness (t2+t3)/(t1) to a value within the range of 0.1 to 0.5.

Second Embodiment
The second embodiment is a method for producing an olefin-based heat-shrinkable multilayer film according to the first embodiment, characterized in that it comprises at least first to third steps.

1. Preparation of Raw Materials and Melting Step First, as raw materials, an olefin-based resin is prepared for the intermediate layer, and a polystyrene-based resin is prepared for the first surface layer.
Therefore, it is preferable to prepare, as such raw materials, linear low-density polyethylene (LLDPE), which is suitable as an olefin-based resin, or polystyrene-butadiene copolymer (containing a predetermined amount of SBC, etc.), which is suitable as a polystyrene-based resin.
Next, it is preferable that the raw materials are weighed and charged into the two stirring vessels, and are heated in each stirring vessel until they become homogeneous and in a molten state.

2. Process for Producing Raw Sheets Next, the uniformly mixed raw materials are each dried to an absolute dry state, and then typically extrusion-molded to produce raw sheets of a predetermined thickness.
More specifically, for example, extrusion molding is performed using an extruder (manufactured by Tanabe Plastic Machinery Co., Ltd.) with an L/D of 24 and an extrusion screw diameter of 50 mm under conditions of an extrusion temperature of 220°C, and a raw sheet of a predetermined thickness (usually 100 to 250 μm) can be obtained.
That is, molten LLDPE or the like was placed in an extruder for forming the intermediate layer and melt-kneaded at 180 to 220°C.
On the other hand, a polystyrene-butadiene copolymer containing a predetermined amount of SBC and the like was placed in an extruder for forming the first surface layer on the front side and an extruder for forming the second surface layer on the back side, and melt-kneaded at 180 to 220°C.

Next, the extrusion rates of the extruders are set so that the thickness ratio of each layer, (t2/t3)/(t1), is usually 0.1 to 0.5, and the mixture is co-extruded downward through a three-layer die maintained at 210°C to preferably form a raw sheet consisting of the desired laminate.
It is also preferable that the thickness of the first surface layer and the second surface layer are substantially equal to each other, but it is also preferable that they be different in consideration of the intended use and the like.

3. Process for Producing Olefin-Based Heat-Shrinkable Multilayer Film Next, the obtained raw sheet (multilayer laminate) was stretched, for example, in a shrink film production device by moving it over and between rolls while being heated and pressed, so that the stretch ratio in the TD direction was 200 to 800% and the stretch ratio in the MD direction was 90 to 120%, to produce an olefin-based heat-shrinkable multilayer film.
More specifically, the standard stretching ratios are usually set to about 500% in the TD direction and about 100% in the MD direction.

4. Inspection process for olefin-based heat-shrinkable multilayer film (optional process)
It is preferable to provide a predetermined inspection step (optional step) in which the following properties are measured continuously or intermittently for the produced olefin-based heat-shrinkable multilayer film.
That is, by measuring the following properties through a predetermined inspection process and confirming that the values fall within the predetermined ranges, an olefin-based heat-shrinkable multilayer film having more uniform specific gravity separation properties, heat shrinkability, etc. can be obtained.
1) Visual inspection of the appearance of olefin-based heat-shrinkable multilayer film 2) Measurement of thickness variation 3) Measurement of tensile modulus 4) Measurement of tear strength 5) Measurement of viscoelastic properties using SS curve

[Third embodiment]
The third embodiment relates to a method for using an olefin-based heat-shrinkable multilayer film.
Therefore, any known method for using shrink films can be suitably applied.
For example, when using the olefin-based heat-shrinkable multilayer film, first, the olefin-based heat-shrinkable multilayer film is cut to an appropriate length and width and formed into a long cylindrical object.
The long cylindrical object is then fed to an automatic label attachment device (shrink labeler) and further cut to the required length.
Next, the container is fitted onto a PET bottle or the like filled with the contents.

Next, the olefin-based heat-shrinkable multi-layer film fitted onto the outside of a PET bottle or the like is subjected to a heat treatment by passing through a hot air tunnel or steam tunnel at a predetermined temperature.
These tunnels provide radiant heat such as infrared rays or heated steam at about 90°C to 100°C is blown onto the olefin-based heat-shrinkable multi-layer film from the surrounding area, thereby uniformly heating the film and causing it to shrink.
Therefore, as shown in FIG. 1(c), the heat-shrinkable olefin-based multilayer film 10 after heat shrinkage can be adhered to the outer surface of a PET bottle 20, thereby quickly obtaining a labeled container.

That is, according to the olefin-based heat-shrinkable multilayer film of the present invention, as described in detail in the first embodiment, it is an olefin-based heat-shrinkable multilayer film including a surface layer derived from a polystyrene-based resin, and is characterized by satisfying at least the configurations (a) and (b).
This improves the handling properties of the heat-shrinkable film, and for example, improves the resistance of the label to breakage during transportation and storage after it has been attached to a PET bottle.
Furthermore, the olefin-based heat-shrinkable multilayer film of the present invention is characterized by a low haze value and high transparency.
Therefore, as shown in Figure 1(c), even if a predetermined decorative layer is provided directly or indirectly on the second surface layer of an olefin-based heat-shrinkable multilayer film, which is the surface facing a PET bottle or the like, it is possible to achieve the effect of making the letters, symbols, figures, patterns, etc. on such decorative layer fully recognizable.

The olefin-based heat-shrinkable multilayer film of the present invention will be described in detail below based on examples. However, the scope of the present invention is not limited by the description of Example 1 or the like without any particular reason.
The polyolefin resins, polystyrene resins, etc. used in Example 1 etc. are as follows.

(1) Olefin-based resin 1) Type A
Ethylene-1-hexene copolymer produced using a metallocene catalyst Density: 0.92 g/cm ³ , MFR: 2.0 g/10 min Melting enthalpy (ΔH): 126 mJ/mg
Mn: 6.6×10 ⁴ , Mw: 20.2×10 ⁴ , Mw/Mn: 3.1

2) Type B
Ethylene-1-hexene copolymer produced using a metallocene catalyst. Density: 0.91 g/cm ³ , MFR: 1.8 g/10 min, melt tension: 104 mN
Melting enthalpy (ΔH): 115 mJ/mg
Mn: 8.2×10 ⁴ , Mw: 21.6×10 ⁴ , Mw/Mn: 2.6

3) Type C
Ethylene-1-hexene copolymer produced using a metallocene catalyst. Density: 0.92 g/cm ³ , MFR: 2.0 g/10 min, melt tension: 200 mN
Melting enthalpy (ΔH): 110 mJ/mg
Mn: 7.0×10 ⁴ , Mw: 23.8×10 ⁴ , Mw/Mn: 3.4

4) Type D
Ethylene-1-butene copolymer produced with a Ziegler-Natta catalyst. Density: 0.92 g/cm ³ , MFR: 0.2 g/10 min, molecular weight distribution: 19.5
Melting enthalpy (ΔH): 140 mJ/mg
Mn: 2.0×10 ⁴ , Mw: 44.2×10 ⁴ , Mw/Mn: 22.1

5) Type E
Ethylene-1-butene copolymer produced using a Ziegler-Natta catalyst Density: 0.92 g/cm ³ , MFR: 1 g/10 min Melting enthalpy (ΔH): 138 mJ/mg
Mn: 6.4×10 ⁴ , Mw: 26.2×10 ⁴ , Mw/Mn: 4.1

(2) Polystyrene resin and other compounding components 1) Type F
A styrene-butadiene block copolymer with a weight ratio of styrene/butadiene of 85/15 and molecular weights of the styrene block portions of 24,000 and 125,000.

2) Type K
A mixture of styrene-butadiene block copolymer and polystyrene resin (weight ratio: 50:50)

3) Type AB
High impact polystyrene resin (HIPS)

4) Type L
Compatibilizer (SEBS1): styrene-butadiene block copolymer (hydrogenated elastomer resin), styrene content 53% by weight,

5) Type M
Compatibilizer (SEBS2): Styrene-butadiene/butylene-styrene triblock copolymer (hydrogenated elastomer resin), styrene content 68% by weight

6) Type N
Compatibilizer (SEBS3): styrene-ethylene-butylene-styrene block copolymer (hydrogenated elastomer resin), styrene content 43% by weight

7) Type P
Compatibilizer (SEBS4): styrene-ethylene/butylene-styrene triblock copolymer (hydrogenated elastomer resin), styrene content 68% by weight

[Example 1]
1. Preparation of an olefin-based heat-shrinkable multilayer film In a first stirring vessel, 100 parts by weight (same meaning as parts by mass, and the same applies hereinafter) of an ethylene-1-hexene copolymer produced using a Type A metallocene catalyst was placed as an olefin-based resin for an intermediate layer, and the mixture was stirred while being heated at 220°C to form a uniform solution.
Meanwhile, 87.7 parts by weight of Type F as a polystyrene-based resin for the surface layer, 10 parts by weight of Type L as a compatibilizer, and 2.3 parts by weight of AB as an antiblocking agent were placed in a second stirring vessel, and the mixture was stirred while being heated at 220°C to form a uniform solution.

Next, each was fed into an extruder (manufactured by Tanabe Plastic Machinery Co., Ltd.) with an L/D of 24 and an extrusion screw diameter of 50 mm at an extrusion temperature of 220°C, and extrusion molding was carried out to obtain raw sheets of the three structures each having a thickness of 160 μm.
That is, the thickness of each of the obtained raw sheets before the stretching treatment was 16 μm for the surface layers and 128 μm for the intermediate layer, for a total thickness of 160 μm.

Then, using a shrink film manufacturing device, the obtained raw sheet (multilayer laminate) was moved over and between rolls while being heated and pressed, and stretched to a stretch ratio of 500% in the TD direction and 100% in the MD direction, producing a 50 μm thick olefin-based heat-shrinkable multilayer film.

2. Evaluation of Olefin-Based Heat-Shrinkable Multilayer Films (1) Evaluation 1: Amount of Styrene-Butadiene Copolymer The amount of styrene-butadiene copolymer (SBC) blended into the olefin-based resin, which was the main component used in the obtained heat-shrinkable film, was evaluated according to the following criteria.
⊚: The amount of SBC blended is 60% by weight or more.
Good: The amount of SBC blended is 50% by weight or more.
Δ: The amount of SBC blended is 40% by weight or more.
×: The amount of SBC blended is less than 40% by weight.

(2) Evaluation 2: Crystal Melting Enthalpy (ΔH)
In accordance with JIS K 7121:2012 (corresponding to ISO 11357-2:2013), the crystalline melting enthalpy of the intermediate layer constituting the olefin-based heat-shrinkable multilayer film was measured using a DSC (DSC7000X, manufactured by Hitachi High-Tech Corporation) during the second heating process at a heating rate of 10°C/min.

(3) Evaluation 3: Specific Gravity The specific gravity of the olefin-based heat-shrinkable multilayer film was determined from the ratio of the density measured by a density gradient tube method in accordance with JIS K 7112-1999 to the density of water at a temperature of 23°C.

(4) Evaluation 4: Haze Value The haze value of the obtained heat-shrinkable film was measured in accordance with JIS K 7105.

(5) Evaluation 5: Heat shrinkage rate (100°C, 10 seconds)
The film is cut into a 100 mm x 100 mm square so that one side is parallel to the flow direction of the film, and this is immersed in a hot water bath maintained at 100°C for 10 seconds.
Then, immediately after 10 seconds had elapsed, the sample was immersed in a separately prepared water bath at 25° C. for 10 seconds.
Next, the film was quickly removed from the hot water bath, and the length of the film in the main shrinkage direction was measured to determine the thermal shrinkage rate.

(6) Rating 6: Mw/Mn
The weight average molecular weight (Mw) and number average molecular weight (Mn) of the olefin resin, which was the main component used in the obtained heat-shrinkable film, were measured, and the molecular weight distribution Mw/Mn was calculated.

(7) Evaluation 7: Peelability Using the same resin as that used to prepare the above-mentioned olefin-based heat-shrinkable multilayer film, a single-layer surface layer sheet having a thickness of 60 μm and a single-layer intermediate layer sheet having a thickness of 160 μm were prepared using an extruder with an L/D of 24 and an extrusion screw diameter of 50 mm at an extrusion temperature of 220° C.
Next, the obtained surface layer sheet and intermediate surface layer sheet were cut into two kinds of long samples each having a length of 100 mm in the MD direction and a length of 15 mm in the TD direction.
Next, the two types of long samples were heat sealed using a heat sealer under conditions of 200°C, 0.2 MPa, and 2 seconds, and then left at room temperature for 24 hours to obtain test samples.
The obtained test samples were subjected to a T-test peel strength measurement at a speed of 200 mm/min using a precision universal testing machine (Shimadzu Corporation: Autograph AGX-10kNV2D), and the releasability was evaluated according to the following criteria. The obtained results are shown in Table 4.
○: Peel strength is 3.0 N/mm ² or more.
Δ: Peel strength is less than 3.0 N/mm ² .
×: The peel strength is less than 0.3 N/mm ² .

[Example 2]
In Example 2, 100 parts by weight of Type A, an ethylene-1-hexene copolymer produced using a metallocene catalyst, was used as the olefin resin for the intermediate layer, 87.7 parts by weight of Type F as the polystyrene resin for the surface layer, 10 parts by weight of Type L as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used.
Then, for each thickness of the obtained raw sheet, the surface layer was 24 μm and the middle layer was 112 μm, for a total thickness of 160 μm, and a heat-shrinkable film was prepared in the same manner as in Example 1, except that it was subjected to a stretching treatment, and evaluated in the same manner as in Example 1.

[Example 3]
In Example 3, 100 parts by weight of Type A, an ethylene-1-hexene copolymer produced using a metallocene catalyst, was used as the olefin resin for the intermediate layer, 87.7 parts by weight of Type F as the polystyrene resin for the surface layer, 10 parts by weight of Type N as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. In the same manner as in Example 1, except for this, a raw sheet having a three-layer structure with a thickness of 16/128/16 μm was prepared, and this was stretched to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.

[Example 4]
In Example 4, 100 parts by weight of Type A, an ethylene-1-hexene copolymer produced using a metallocene catalyst, was used as the olefin resin for the intermediate layer, 87.7 parts by weight of Type F as the polystyrene resin for the surface layer, 10 parts by weight of Type N as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. In the same manner as in Example 2, except for this, a three-layer raw sheet having a thickness of 24/112/24 μm was prepared, and this was stretched to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.

[Example 5]
In Example 5, 100 parts by weight of Type A, an ethylene-1-hexene copolymer produced using a metallocene catalyst, was used as the olefin resin for the intermediate layer, 87.7 parts by weight of Type F as the polystyrene resin for the surface layer, 10 parts by weight of Type P as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. In the same manner as in Example 1, except for this, a raw sheet having a three-layer structure with a thickness of 16/128/16 μm was prepared, and this was stretched to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.

[Example 6]
In Example 6, 100 parts by weight of Type A, an ethylene-1-hexene copolymer produced using a metallocene catalyst, was used as the olefin resin for the intermediate layer, 87.7 parts by weight of Type F as the polystyrene resin for the surface layer, 10 parts by weight of Type P as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. In the same manner as in Example 2, except for this, a three-layer raw sheet having a thickness of 24/112/24 μm was prepared, and this was stretched to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.

[Example 7]
In Example 7, 100 parts by weight of Type A, an ethylene-1-hexene copolymer produced using a metallocene catalyst, was used as the olefin resin for the intermediate layer, 87.7 parts by weight of Type F as the polystyrene resin for the surface layer, 10 parts by weight of Type M as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. In the same manner as in Example 1, except for this, a three-layer raw sheet having a thickness of 16/128/16 μm was prepared, and this was stretched to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.

[Example 8]
In Example 8, 100 parts by weight of Type A, an ethylene-1-hexene copolymer produced using a metallocene catalyst, was used as the olefin resin for the intermediate layer, 87.7 parts by weight of Type F as the polystyrene resin for the surface layer, 10 parts by weight of Type M as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. In the same manner as in Example 2, except for this, a three-layer raw sheet having a thickness of 24/112/24 μm was prepared, and this was stretched to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.

[Example 9]
In Example 9, 100 parts by weight of Type A, an ethylene-1-hexene copolymer produced using a metallocene catalyst, was used as the olefin resin for the intermediate layer, 92.7 parts by weight of Type F as the polystyrene resin for the surface layer, 5 parts by weight of Type M as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. In the same manner as in Example 1, except for this, a three-layer raw sheet having a thickness of 16/128/16 μm was prepared, and this was stretched to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.

[Example 10]
In Example 10, 100 parts by weight of Type A, an ethylene-1-hexene copolymer produced using a metallocene catalyst, was used as the olefin resin for the intermediate layer, 92.7 parts by weight of Type F as the polystyrene resin for the surface layer, 5 parts by weight of Type M as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. Except for this, a three-layer raw sheet having a thickness of 24/112/24 μm was prepared in the same manner as in Example 2, and this was then stretched to prepare a heat-shrinkable film, which was then evaluated in the same manner as in Example 1.

[Example 11]
In Example 11, 100 parts by weight of Type A, an ethylene-1-hexene copolymer produced using a metallocene catalyst, was used as the olefin resin for the intermediate layer, 92.7 parts by weight of Type F as the polystyrene resin for the surface layer, 5 parts by weight of Type P as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. Except for this, a three-layer raw sheet having a thickness of 16/128/16 μm was prepared in the same manner as in Example 1, and this was stretched to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.

[Example 12]
In Example 12, 100 parts by weight of Type A, an ethylene-1-hexene copolymer produced using a metallocene catalyst, was used as the olefin resin for the intermediate layer, 92.7 parts by weight of Type F as the polystyrene resin for the surface layer, 5 parts by weight of Type P as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. Except for this, a three-layer raw sheet having a thickness of 24/112/24 μm was prepared in the same manner as in Example 2, and this was subjected to a stretching treatment to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.

[Example 13]
In Example 13, 100 parts by weight of Type B produced using a metallocene catalyst was used as the olefin resin for the intermediate layer, 92.7 parts by weight of Type F as the polystyrene resin for the surface layer, 5 parts by weight of Type P as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. Except for this, a three-layer raw sheet having a thickness of 16/128/16 μm was prepared in the same manner as in Example 1, and this was stretched to prepare a heat-shrinkable film, which was then evaluated in the same manner as in Example 1.

[Example 14]
In Example 14, 100 parts by weight of Type B produced using a metallocene catalyst was used as the olefin resin for the intermediate layer, 92.7 parts by weight of Type F as the polystyrene resin for the surface layer, 5 parts by weight of Type P as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. Except for this, a three-layer raw sheet having a thickness of 24/112/24 μm was prepared in the same manner as in Example 2, and this was then subjected to a stretching treatment to prepare a heat-shrinkable film, which was then evaluated in the same manner as in Example 1.

[Example 15]
In Example 15, 100 parts by weight of Type C produced using a metallocene catalyst was used as the olefin resin for the intermediate layer, 92.7 parts by weight of Type F as the polystyrene resin for the surface layer, 5 parts by weight of Type P as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. Except for this, a three-layer raw sheet having a thickness of 16/128/16 μm was prepared in the same manner as in Example 1, and this was then subjected to a stretching treatment to prepare a heat-shrinkable film, which was then evaluated in the same manner as in Example 1.

[Example 16]
In Example 16, 100 parts by weight of Type C produced using a metallocene catalyst was used as the olefin resin for the intermediate layer, 92.7 parts by weight of Type F as the polystyrene resin for the surface layer, 5 parts by weight of Type P as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. Except for this, a three-layer raw sheet having a thickness of 24/112/24 μm was prepared in the same manner as in Example 2, and this was then subjected to a stretching treatment to prepare a heat-shrinkable film, which was then evaluated in the same manner as in Example 1.

[Example 17]
In Example 17, 100 parts by weight of Type A produced using a metallocene catalyst was used as the olefin resin for the intermediate layer, 46.35 parts by weight of Type F and 46.35 parts by weight of Type K were used as polystyrene resins for the surface layer, 5 parts by weight of Type P was used as the compatibilizer, and 2.3 parts by weight of Type AB was used as the antiblocking agent. Except for this, a three-layer raw sheet having a thickness of 16/128/16 μm was prepared in the same manner as in Example 1, and this was used for stretching treatment to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.

[Example 18]
In Example 18, 100 parts by weight of Type A produced using a metallocene catalyst was used as the olefin resin for the intermediate layer, 46.35 parts by weight of Type F and 46.35 parts by weight of Type K were used as polystyrene resins for the surface layer, 5 parts by weight of Type P was used as the compatibilizer, and 2.3 parts by weight of Type AB was used as the antiblocking agent. Except for this, a three-layer raw sheet having a thickness of 24/112/24 μm was prepared in the same manner as in Example 2, and this was then stretched to prepare a heat-shrinkable film, which was then evaluated in the same manner as in Example 1.

[Example 19]
In Example 19, 100 parts by weight of Type A produced using a metallocene catalyst was used as the olefin resin for the intermediate layer, 97.2 parts by weight of Type F was used as the polystyrene resin for the surface layer, and 2.3 parts by weight of Type AB was used as the antiblocking agent. A three-layer raw sheet having a thickness of 24/112/24 μm was prepared, and this was stretched to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.

[Example 20]
In Example 20, 100 parts by weight of Type A produced using a metallocene catalyst was used as the olefin resin for the intermediate layer, 23.18 parts by weight of Type F and 69.53 parts by weight of Type K were used as polystyrene resins for the surface layer, 5 parts by weight of Type P was used as the compatibilizer, and 2.3 parts by weight of Type AB was used as the antiblocking agent. A three-layer raw sheet having a thickness of 24/112/24 μm was prepared, and this was then stretched to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.

[Example 21]
In Example 21, 100 parts by weight of Type A produced using a metallocene catalyst was used as the olefin resin for the intermediate layer, 95.2 parts by weight of Type F as the polystyrene resin for the surface layer, 2.5 parts by weight of Type P as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. A three-layer raw sheet having a thickness of 24/112/25 μm was prepared, and this was stretched to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.

[Comparative Example 1]
In Comparative Example 1, 100 parts by weight of Type D produced using a Ziegler-Natta catalyst was used as the olefin resin for the intermediate layer, 92.7 parts by weight of Type F as the polystyrene resin for the surface layer, 5 parts by weight of Type P as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. A three-layer raw sheet having a thickness of 24/112/24 μm was prepared, and this was subjected to a stretching treatment to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.
As a result, in Comparative Example 1, unevenness occurred due to the stretching process and the specific gravity exceeded 0.95, which is presumably because the intermediate layer used Type D, an olefin-based resin with a crystalline melting enthalpy (ΔH) exceeding 135 mJ/mg and a wide molecular weight distribution.

[Comparative Example 2]
In Comparative Example 2, 100 parts by weight of Type E produced using a Ziegler-Natta catalyst was used as the olefin resin for the intermediate layer, 92.7 parts by weight of Type F as the polystyrene resin for the surface layer, 5 parts by weight of Type P as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. A three-layer raw sheet having a thickness of 16/128/16 μm was prepared, and this was subjected to a stretching treatment to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.
As a result, in Comparative Example 2, the thermal shrinkage rate in the TD direction tended to be significantly low at 45% (less than 50%), which is presumably due to the use of Type E, an olefin-based resin with a crystalline melting enthalpy (ΔH) exceeding 135 mJ/mg and a wide molecular weight distribution, in the intermediate layer.

[Comparative Example 3]
In Comparative Example 3, 100 parts by weight of Type E produced using a Ziegler-Natta catalyst was used as the olefin resin for the intermediate layer, 92.7 parts by weight of Type F as the polystyrene resin for the surface layer, 5 parts by weight of Type P as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent were used. A three-layer raw sheet having a thickness of 24/112/24 μm was prepared, and this was subjected to a stretching treatment to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.
As a result, in Comparative Example 2, the thermal shrinkage rate in the TD direction tended to be significantly low at 46% (less than 50%), which is presumably due to the use of Type E, an olefin-based resin with a crystalline melting enthalpy (ΔH) exceeding 135 mJ/mg and a wide molecular weight distribution, in the intermediate layer.

[Comparative Example 4]
In Comparative Example 4, 100 parts by weight of Type A was used as the olefin resin for the intermediate layer, 92.7 parts by weight of Type K as the polystyrene resin for the surface layer, 5 parts by weight of Type P as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent. A three-layer raw sheet having a thickness of 16/128/16 μm was prepared, and this was subjected to a stretching treatment to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.
As a result, it is presumed that Comparative Example 4 uses Type K, which contains a small amount of SBC as the main component of the polystyrene resin, but the stretching treatment causes whitening, resulting in a considerably high haze value of 12.1%.

[Comparative Example 5]
In Comparative Example 5, 100 parts by weight of Type A was used as the olefin resin for the intermediate layer, 92.7 parts by weight of Type K as the polystyrene resin for the surface layer, 5 parts by weight of Type P as the compatibilizer, and 2.3 parts by weight of Type AB as the antiblocking agent. A three-layer raw sheet having a thickness of 24/112/24 μm was prepared, and this was subjected to a stretching treatment to prepare a heat-shrinkable film, which was evaluated in the same manner as in Example 1.
As a result, it is presumed that Comparative Example 5 uses Type K, which contains a small amount of SBC as the main component of the polystyrene resin, but the stretching treatment causes whitening, resulting in a considerably high haze value of 19.1%.

      

The olefin-based heat-shrinkable multilayer film of the present invention is substantially free of cyclic olefin resins, and by controlling the resin composition of the three layers that make up the olefin-based heat-shrinkable multilayer film to fall within specific ranges, it has excellent heat shrinkability, transparency, and interlayer peelability, and when used in PET bottles, it shrinks uniformly and exhibits excellent decorative properties, while also preventing breakage during printing or center sealing.

Furthermore, because its specific gravity is low at 0.95 or less, even if it is still used as heat-shrinkable film for PET bottles, it can be easily and quickly recovered and recycled by separating it by specific gravity using a specified cyclone device or the like to separate only the olefin-based heat-shrinkable multilayer film from the PET bottle.

Furthermore, when solvents such as cyclohexane and tetrahydrofuran are used as adhesive solvents, the solvent adhesion is good, and center sealing can be easily performed using standard solvent adhesion methods, eliminating the need to install additional secondary processing equipment.
Furthermore, the polystyrene resin constituting the first surface layer and/or the second surface layer contains a predetermined amount of styrene elastomer, thereby making it possible to effectively increase the peel strength (T-peel strength).

As explained above, the olefin-based heat-shrinkable multilayer film of the present invention can be suitably applied to various PET bottles, outer wrapping materials for boxed lunches, and the like, significantly expanding its versatility and making it highly applicable in industry.

10, 10', 10": Olefin-based heat-shrinkable multilayer film 10a: First surface layer 10b: Intermediate layer 10c: Second surface layer 10d: Decorative layer 20: PET bottle

Claims (8)

An olefin-based heat-shrinkable multilayer film comprising an intermediate layer derived from a polyolefin-based resin, a first surface layer derived from at least a polystyrene-based resin on one surface side of the intermediate layer, and a second surface layer derived from at least a polystyrene-based resin on the other surface side of the intermediate layer, and characterized in that the film satisfies the following configurations (a) to (e):
(a) The polystyrene resin contains 50% by weight or more of a styrene-butadiene copolymer based on the total weight.
(b) The intermediate layer has a crystalline melting enthalpy (ΔH) measured by DSC in accordance with JIS K 7121:2012 (corresponding to ISO 11357-2:2013) of 135 mJ/mg or less.
(c) When the film is shrunk in boiling water at 100°C for 10 seconds, the thermal shrinkage rate in the main shrinkage direction is defined as A1, and A1 is set to a value within the range of 50 to 80%.
(d) The haze value measured in accordance with JIS K 7136:2000 (equivalent to ISO 14782:1999) is 10% or less.
(e) The specific gravity measured in accordance with JIS K 7112-1:2023 (corresponding to ISO 1183-1:2023) is 0.95 or less. The olefin-based heat-shrinkable multilayer film of claim 1, characterized in that, as component (f), the molecular weight distribution (Mw/Mn) of the polyolefin-based resin constituting the intermediate layer is 4.0 or less. The olefin-based heat-shrinkable multilayer film according to claim 1 or 2, characterized in that the intermediate layer contains linear low-density polyethylene as the polyolefin-based resin. The olefin-based heat-shrinkable multilayer film according to claim 1 or 2, characterized in that the weight-average molecular weight of the polyolefin-based resin constituting the intermediate layer is within the range of 150,000 to 250,000. The olefin-based heat-shrinkable multilayer film according to claim 1 or 2, characterized in that the number-average molecular weight of the polyolefin-based resin constituting the intermediate layer is within the range of 50,000 to 100,000. The olefin-based heat-shrinkable multilayer film according to claim 1 or 2, characterized in that the polystyrene-based resin constituting the first surface layer and/or the second surface layer contains a styrene-based elastomer. The olefin-based heat-shrinkable multilayer film according to claim 1 or 2, characterized in that the thickness after stretching is within the range of 10 to 60 μm. The olefin-based heat-shrinkable multilayer film according to claim 1 or 2, characterized in that the thickness ratio (t2+t3)/(t1) is set to a value within a range of 0.1 to 0.5, where t1 is the thickness of the intermediate layer before stretching treatment, t2 is the thickness of the first surface layer before stretching treatment, and t3 is the thickness of the second surface layer before stretching treatment.