JP2020049895A - Heat-shrinkable multilayer film - Google Patents

Abstract

Provided is a heat-shrinkable multilayer film which can easily detect defects such as scratches and foreign substances present on a film surface and can prevent defects such as ink loss after printing.
A heat-shrinkable multilayer film having a front and back layer containing a polyester-based resin and an intermediate layer containing a polystyrene-based resin, wherein the heat-shrinkable multilayer has a retardation (Re) of 0.1 to 2900 nm. the film.
[Selection diagram] None

Description

The present invention relates to a heat-shrinkable multilayer film that can easily detect defects such as scratches and foreign matter present on the film surface and can prevent defects such as ink loss after printing.

Many containers such as plastic bottles and metal cans are provided with a heat-shrinkable label based on a heat-shrinkable resin film.
The heat-shrinkable label is provided with a printed layer for applying characters and patterns for distinguishing from products of other companies such as a product name, a logo mark and a manufacturer name. Detailed designs are increasing to differentiate from other companies and appeal to consumers. Therefore, the heat-shrinkable resin film is required to have an appropriate surface shape.

In a heat-shrinkable polystyrene-based film mainly composed of styrene-butadiene block copolymer (SBS), butadiene in the butadiene block copolymer develops thermal crosslinking during heat-melt processing to generate a crosslinked gel (fish eye). When printing on a film, there has been such a problem that the ink in that portion comes off due to the unevenness of the crosslinked gel, particularly due to migration to the surface during stretching.
On the other hand, Patent Literature 1 discloses a heat-shrinkable film that can be used for applications such as shrink wrapping, shrink bundling wrapping, and shrink label, and that has little crosslinked gel (fish eye).

Further, in order to respond to increasingly diversified printing designs, further improvement of the surface shape of the heat-shrinkable film is desired, and the heat-shrinkable film is disadvantageous in producing defects (foreign matter such as fish eyes) during production. It is monitored using a detector and the like, and the generated gel and the like are removed before shipping.

Inspection means for defects in the heat-shrinkable film used a white fluorescent lamp as a light source, illuminated it from behind the heat-shrinkable film through a polarizing filter, and captured an image with a CCD camera with a polarizing filter placed on the opposite side. A method of detecting the leakage (transmission) of light from defects by processing and judging pass / fail, or using infrared light as a light source, similarly imaging with a CCD camera equipped with a visible light cut filter, image processing, and detecting defects. Is used.

However, the conventional heat-shrinkable film has a problem in that, when an image is taken by a CCD camera equipped with a polarizing filter, interference fringes appear, and defects are overlooked, and as a result, ink loss occurs during printing. Was.

JP 2005-28736 A

The present invention provides a heat-shrinkable multilayer film that can easily detect defects such as scratches and foreign substances present on the film surface and can prevent defects such as ink loss after printing in view of the above-mentioned current situation. The purpose is to:

The present invention relates to a heat-shrinkable multilayer film having a front and back layer containing a polyester resin and an intermediate layer containing a polystyrene resin, wherein the heat-shrinkable multilayer has a retardation (Re) of 0.1 to 2900 nm. Film.
Hereinafter, the present invention will be described in detail.

The present inventors, in the heat-shrinkable multilayer film, using a polyester-based resin as a resin constituting the front and back layers, using a polystyrene-based resin as a resin constituting the intermediate layer, further, the retardation is a predetermined range As a result, it has been found that the accuracy of defect detection can be improved, and the present invention has been completed.

The heat shrinkable multilayer film of the present invention has a structure in which the front and back layers and the intermediate layer are laminated.
In addition, in this specification, a front and back layer means both a surface layer and a back layer. Therefore, the heat-shrinkable multilayer film of the present invention has a structure in which the intermediate layer is sandwiched between the front surface layer and the back surface layer.

(Front and back layers)
The front and back layers contain a polyester resin.
The polyester resin constituting the front and back layers can be obtained by polycondensing a dicarboxylic acid and a diol.
The dicarboxylic acid is not particularly limited, for example, o-phthalic acid, terephthalic acid, isophthalic acid, succinic acid, adipic acid, sebacic acid, azelaic acid, octylsuccinic acid, cyclohexanedicarboxylic acid, naphthalenedicarboxylic acid, fumaric acid, Maleic acid, itaconic acid, decamethylenecarboxylic acid, anhydrides and lower alkyl esters thereof, and the like.
The diol is not particularly limited, and examples thereof include ethylene glycol, 1,3-propanediol, 1,4-butanediol, diethylene glycol, 1,5-pentanediol, 1,6-hexanediol, dipropylene glycol, and triethylene. Glycol, tetraethylene glycol, 1,2-propanediol, 1,3-butanediol, 2,3-butanediol, neopentyl glycol (2,2-dimethylpropane-1,3-diol), 1,2-hexane Aliphatic diols such as diol, 2,5-hexanediol, 2-methyl-2,4-pentanediol, 3-methyl-1,3-pentanediol, 2-ethyl-1,3-hexanediol; 2-bis (4-hydroxycyclohexyl) propane, 2,2-bis (4-h B carboxymethyl cyclohexyl) alkylene oxide adducts of propane, 1,4-cyclohexane diol, alicyclic diols such as 1,4-cyclohexanedimethanol.

Among the polyester-based resins, among others, those containing a component derived from terephthalic acid as a dicarboxylic acid component, and those containing components derived from ethylene glycol and 1,4-cyclohexanedimethanol as a diol component are preferable. is there. By using such a polyester resin, particularly high heat resistance and solvent resistance can be imparted to the obtained heat-shrinkable multilayer film of the present invention.

In the polyester resin, a preferable lower limit of the content of the component derived from terephthalic acid is 60 mol%, a more preferable lower limit is 65 mol%, a preferable upper limit is 100 mol%, and a more preferable upper limit is 100 mol% of the dicarboxylic acid component. 95 mol%.
The polyester resin has a preferable lower limit of the content of the component derived from isophthalic acid of 0 mol%, a more preferable lower limit is 5 mol%, a preferable upper limit is 45 mol%, and a more preferable upper limit is 100 mol% of the dicarboxylic acid component. 40 mol%.

In the polyester-based resin, a preferable lower limit of the content of the component derived from ethylene glycol is 100% by mole, a more preferable lower limit is 60% by mole, a preferable upper limit is 100% by mole, and a more preferable upper limit is 95% in 100% by mole of the diol component. Mol%.
In the polyester resin, a preferable lower limit of the content of the component derived from 1,4-cyclohexanedimethanol is 0 mol%, a more preferable lower limit is 10 mol%, and a preferable upper limit is 40 mol%, based on 100 mol% of the diol component. A more preferred upper limit is 35 mol%.
In the polyester-based resin, a preferable lower limit of the content of the component derived from diethylene glycol is 0 mol%, a more preferable lower limit is 2 mol%, a preferable upper limit is 30 mol%, and a more preferable upper limit is 25 mol in 100 mol% of the diol component. %.
In the polyester-based resin, the preferable lower limit of the content of the component derived from butanediol is 0 mol%, the more preferable lower limit is 5 mol%, the preferable upper limit is 100 mol%, and the more preferable upper limit is 95 mol% in 100 mol% of the diol component. Mol%.
As the polyester resin constituting the front and back layers, a polyester resin having the above composition may be used alone, or two or more polyester resins having the above composition may be used in combination.

As the polyester resin, it is preferable to use a resin having a relatively low crystal melting temperature or having no crystal melting temperature. In the production of a heat-shrinkable multilayer film, a trimmed piece at the time of stretching or a recycled film may be used again as a return material. Usually, such a return material is mixed with a polystyrene resin as a raw material of the intermediate layer, but since the polystyrene resin and the polyester resin have different properties such as melting points, the return material is at a temperature suitable for molding the polystyrene resin. When a film was formed, the polyester resin was sometimes extruded in an unmelted state. However, by using a polyester resin having a relatively low crystal melting temperature or having no crystal melting temperature, it is possible to prevent an unmelted polyester resin from being formed as a foreign substance in a formed film.

The front and back layers are, if necessary, an antioxidant, a heat stabilizer, an ultraviolet absorber, a light stabilizer, a lubricant, an antistatic agent, an antiblocking agent, a flame retardant, an antibacterial agent, a fluorescent brightener, a coloring agent, and the like. May be contained.

(Intermediate layer)
The heat-shrinkable multilayer film of the present invention contains the above-mentioned intermediate layer.
The intermediate layer contains a polystyrene resin.
Examples of the polystyrene resin include, for example, an aromatic vinyl hydrocarbon-conjugated diene copolymer, an aromatic vinyl hydrocarbon-conjugated diene copolymer and an aromatic vinyl hydrocarbon-aliphatic unsaturated carboxylic acid ester copolymer. And a rubber-modified impact-resistant polystyrene. By using the polystyrene resin, the heat shrinkable multilayer film of the present invention can start shrinking at a low temperature and has high shrinkage.

In the present specification, the aromatic vinyl hydrocarbon-conjugated diene copolymer refers to a copolymer containing a component derived from an aromatic vinyl hydrocarbon and a component derived from a conjugated diene.
The aromatic vinyl hydrocarbon is not particularly limited, and includes, for example, styrene, o-methylstyrene, p-methylstyrene and the like. These may be used alone or in combination of two or more. The conjugated diene is not particularly limited. For example, 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene And the like. These may be used alone or in combination of two or more.

The aromatic vinyl hydrocarbon-conjugated diene copolymer preferably contains a styrene-butadiene copolymer (SBS resin) because it is particularly excellent in heat shrinkability. Further, in order to produce a heat-shrinkable multilayer film having less fish eyes, the aromatic vinyl hydrocarbon-conjugated diene copolymer may be 2-methyl-1,3-butadiene (isoprene) as the conjugated diene. It is preferable to use the used styrene-isoprene copolymer (SIS resin), styrene-isoprene-butadiene copolymer (SIBS), and the like.
The aromatic vinyl hydrocarbon-conjugated diene copolymer may contain any one of SBS resin, SIS resin and SIBS resin alone, or may contain a combination of two or more. When a plurality of SBS resins, SIS resins and SIBS resins are used, each resin may be dry-blended, and each resin is kneaded using an extruder with a specific composition and a compound resin pelletized is used. You may.

When the aromatic vinyl hydrocarbon-conjugated diene copolymer contains one or more SBS resin, SIS resin and SIBS resin, a heat-shrinkable multilayer film having particularly excellent heat-shrinkability is obtained. Preferably, the styrene content and the conjugated diene content in the aromatic vinyl hydrocarbon-conjugated diene copolymer are 65 to 90% by weight and 10 to 35% by weight, respectively, based on 100% by weight. When the styrene content is more than 90% by weight or the conjugated diene content is less than 10% by weight, the heat-shrinkable multilayer film is easily cut when tension is applied, or may be unexpectedly generated during processing such as printing. It may be broken without depending on it. When the styrene content is less than 65% by weight or the conjugated diene content exceeds 35% by weight, foreign substances such as gels are easily generated during molding, and the heat-shrinkable multilayer film becomes weak. In some cases, the handleability may deteriorate.

A preferred lower limit of the Vicat softening temperature of the polystyrene resin is 60 ° C, and a preferred upper limit is 85 ° C. When the Vicat softening temperature is 60 ° C. or higher, the heat-shrinkable multilayer film can have good low-temperature shrinkability and can prevent wrinkles from being generated when the film is mounted on a container. When the Vicat softening temperature is 85 ° C. or lower, the low-temperature shrinkability of the heat-shrinkable multilayer film can be sufficiently increased, and the generation of an unshrinkable portion when mounted on a container can be prevented. A more preferred lower limit of the Vicat softening temperature is 65 ° C, and a more preferred upper limit is 80 ° C.
The Vicat softening temperature can be measured by a method based on ISO 306.

A preferable lower limit of the MFR (melt flow rate) at 200 ° C. of the polystyrene resin is 2 g / 10 minutes, and a preferable upper limit is 15 g / 10 minutes. If the MFR at 200 ° C. is less than 2 g / 10 minutes, it becomes difficult to form a film. If the MFR at 200 ° C. exceeds 15 g / 10 minutes, the mechanical strength of the film becomes low, and the film cannot be put to practical use. A more preferred lower limit of the MFR at 200 ° C. is 4 g / 10 minutes, and a more preferred upper limit is 12 g / 10 minutes. Note that the MFR can be measured by a method based on ISO1133.

Commercial products of the polystyrene resin constituting the intermediate layer include, for example, “Clearylene” (manufactured by Denki Kagaku Kogyo), “Asaflex” (manufactured by Asahi Kasei Chemicals), “Styrolux” (manufactured by BASF), and “PSJ”. -Polystyrene "(manufactured by PS Japan) and the like.

A preferable lower limit of the styrene content in the polystyrene resin constituting the intermediate layer is 60% by weight, a more preferable lower limit is 70% by weight, a preferable upper limit is 90% by weight, and a more preferable upper limit is 85% by weight.
When the polystyrene-based resin constituting the intermediate layer is a mixed resin obtained by combining a plurality of polystyrene-based resins, the styrene component content in the polystyrene-based resin constituting the intermediate layer is determined by the content of each polystyrene-based resin in the mixed resin. It is calculated by dividing the sum of the product of the resin content by the content of the styrene component of each polystyrene resin by 100.

A preferred lower limit of the content of the conjugated diene component in the polystyrene resin constituting the intermediate layer is 10% by weight, a more preferred lower limit is 15% by weight, a preferred upper limit is 40% by weight, and a more preferred upper limit is 30% by weight.
When the polystyrene-based resin constituting the intermediate layer is a mixed resin obtained by combining a plurality of polystyrene-based resins, the content of the conjugated diene component in the polystyrene-based resin constituting the intermediate layer is determined in accordance with each of the contents of the mixed resin. It is calculated by dividing the sum of the content of the polystyrene resin by the content of the conjugated diene component in each polystyrene resin by 100.

The lower limit of the content of the polystyrene resin in the intermediate layer is preferably 70% by weight and the upper limit is preferably 100% by weight.
When the content of the polystyrene resin is within this range, it is possible to confirm a defect using a polarizing filter.
A more preferred lower limit of the content of the polystyrene resin in the intermediate layer is 80% by weight, and a more preferred upper limit is 95% by weight.

As the polystyrene-based resin constituting the intermediate layer, the polystyrene-based resin may be used alone, or may be a mixed resin of two or more kinds.

When a mixed resin is used by using two or more of the above polystyrene resins, the difference between the Vicat softening temperatures of the respective polystyrene resins is preferably 2 ° C or more, more preferably 5 ° C or more, and 30 ° C or less. And more preferably 20 ° C. or lower.

The intermediate layer may further contain a polyester resin.
Examples of the polyester-based resin include those similar to those constituting the front and back layers.
As the polyester-based resin, the same resin as that constituting the front and back layers may be used.

The lower limit of the content of the polyester resin in the intermediate layer is preferably 1% by weight, more preferably 5% by weight, more preferably 30% by weight, and more preferably 20% by weight.
When the content of the polyester resin is in this range, the retardation value can be reduced, which is preferable.

The intermediate layer may contain additives such as an antioxidant, a heat stabilizer, an ultraviolet absorber, a light stabilizer, a lubricant, an antistatic agent, a flame retardant, an antibacterial agent, an optical brightener, and a colorant, if necessary. May be contained.

(Adhesive layer)
The heat-shrinkable multilayer film of the present invention may have a structure in which the front and back layers and the intermediate layer are laminated via an adhesive layer.

Examples of the resin constituting the adhesive layer include a polyester-based elastomer, a modified product thereof, a mixed resin of a polyester-based resin and a polystyrene-based resin (also referred to as a mixed resin (a) in the present specification), and polystyrene. A mixed resin of a base resin and a polyester-based elastomer (also referred to herein as a mixed resin (b)) is preferable. Such an adhesive layer has a high affinity for both the polyester-based resin constituting the front and back layers and the polystyrene-based resin constituting the intermediate layer, and can bond both with high strength. In addition, since the polyester resin constituting the front and back layers dissolves or swells in a solvent that dissolves the polyester resin, the solvent can penetrate into the heat-shrinkable multilayer film at the time of center sealing, and when the heat shrinks thereafter, the interlayer can be formed. Peeling can be prevented. Furthermore, since the intermediate layer and the front and back layers can be molded by a co-extrusion method, productivity is excellent.

The polyester-based elastomer is preferably a saturated polyester-based elastomer, and particularly preferably a saturated polyester-based elastomer containing a polyalkylene ether glycol segment.
As the saturated polyester-based elastomer containing the polyalkylene ether glycol segment, for example, a block copolymer composed of an aromatic polyester as a hard segment and a polyalkylene ether glycol or an aliphatic polyester as a soft segment is preferable. Further, a polyester polyether block copolymer having a polyalkylene ether glycol as a soft segment is more preferred.

Examples of the polyester polyether block copolymer include (i) an aliphatic and / or alicyclic diol having 2 to 12 carbon atoms, and (ii) an aromatic dicarboxylic acid and / or an aliphatic dicarboxylic acid or an alkyl ester thereof. And (iii) a polyalkylene ether glycol as a raw material, and an oligomer obtained by an esterification reaction or a transesterification reaction is preferably polycondensed.

As the aliphatic and / or alicyclic diol having 2 to 12 carbon atoms, and the aromatic dicarboxylic acid and / or aliphatic dicarboxylic acid or its alkyl ester, as a raw material of polyester, particularly, a raw material of polyester elastomer Generally used ones can be used.
Examples of the polyalkylene ether glycol include polyethylene glycol, poly (1,2- and / or 1,3-propylene ether) glycol, poly (tetramethylene ether) glycol, poly (hexamethylene ether) glycol, and the like. .

A preferred lower limit of the number average molecular weight of the polyalkylene ether glycol is 400, and a preferred upper limit is 6,000. By setting the number average molecular weight to 400 or more, the blockability of the copolymer increases. When the number average molecular weight is 6000 or less, phase separation in the system hardly occurs, and the polymer properties are easily developed. A more preferred lower limit of the number average molecular weight is 500, a more preferred upper limit is 3,000, and a still more preferred lower limit is 600.
In addition, the said number average molecular weight means what was measured by gel permeation chromatography (GPC). The calibration of the GPC can be performed, for example, by using a POLYTETRAHYDROFURAN calibration kit (manufactured by POLYMER LABORATORIES, UK).

The preferable lower limit of the content of the polyalkylene ether glycol component in the polyester polyether block copolymer is 5% by weight, and the preferable upper limit is 90% by weight. When the content of the polyalkylene ether glycol component is 5% by weight or more, the block copolymer becomes excellent in flexibility and impact resistance. When the content of the polyalkylene ether glycol component is 90% by weight or less, the block copolymer becomes excellent in hardness and mechanical strength. A more preferred lower limit of the content of the polyalkylene ether glycol component is 30% by weight, a more preferred upper limit is 80% by weight, and a still more preferred lower limit is 55% by weight.
The content of the polyalkylene ether glycol component can be calculated by using nuclear magnetic resonance spectroscopy (NMR) based on the chemical shift of hydrogen atoms and the content thereof.

A preferred lower limit of the durometer hardness of the polyester-based elastomer is 10, and a preferred upper limit is 80. By setting the durometer hardness to 10 or more, the mechanical strength of the adhesive layer is improved. By setting the durometer hardness to 80 or less, the flexibility and impact resistance of the adhesive layer are improved. A more preferred lower limit of the durometer hardness is 15, a more preferred upper limit is 70, a still more preferred lower limit is 20, and a still more preferred upper limit is 60.
The durometer hardness can be measured by using a durometer type D according to a method based on ISO18517.

Commercially available polyester elastomers include "Primalloy" (manufactured by Mitsubishi Chemical Corporation), "Pelprene" (manufactured by Toyobo Co., Ltd.), "Hytrel" (manufactured by Toray DuPont) and the like.

The modified product of the polyester-based elastomer (hereinafter also referred to as a modified polyester-based elastomer) is obtained by modifying the polyester-based elastomer with a modifying agent.
The modification reaction for obtaining the modified polyester elastomer is performed, for example, by reacting the polyester elastomer with an α, β-ethylenically unsaturated carboxylic acid as a modifier. In the modification reaction, it is preferable to use a radical generator.

In the above modification reaction, a graft reaction in which the α, β-ethylenically unsaturated carboxylic acid or a derivative thereof is added to the polyester elastomer mainly occurs, but a decomposition reaction also occurs. As a result, the modified polyester-based elastomer has a lower molecular weight and a lower melt viscosity.
In addition, in the above-mentioned denaturation reaction, it is generally considered that a transesterification reaction or the like occurs as another reaction, and the obtained reactant is generally a composition containing unreacted raw materials and the like. In this case, a preferable lower limit of the content of the modified polyester elastomer in the obtained reaction product is 10% by weight, and a more preferable lower limit is 30% by weight. The content of the modified polyester-based relastomer is preferably as close to 100% by weight as possible.

A preferred lower limit of the modification rate (graft amount) of the modified polyester elastomer is 0.01% by weight, and a preferred upper limit is 10.0% by weight. When the modification ratio is 0.01% by weight or more, the affinity between the modified polyester-based elastomer and the polyester is increased. When the modification ratio is 10.0% by weight or less, a decrease in strength due to molecular deterioration during modification can be reduced. A more preferred lower limit of the modification rate is 0.03% by weight, a more preferred upper limit is 7.0% by weight, a still more preferred lower limit is 0.05% by weight, and a still more preferred upper limit is 5.0% by weight.

The modification ratio (graft amount) of the modified polyester elastomer can be determined from the spectrum obtained by the H1-NMR measurement according to the following formula (2). In addition, as an apparatus used for the H1-NMR measurement, for example, “GSX-400” (manufactured by JEOL Ltd.) or the like can be used.
Graft amount (% by weight) = 100 × [(C ÷ 3 × 98) / {(A × 148 ÷ 4) + (B × 72 ÷ 4) + (C ÷ 3 × 98)}] (2)
In the formula (2), A represents an integrated value at 7.8 to 8.4 ppm, B represents an integrated value at 1.2 to 2.2 ppm, and C represents an integrated value at 2.4 to 2.9 ppm.

As the polyester-based resin constituting the mixed resin of the polyester-based resin and the polystyrene-based resin (also referred to as a mixed resin (a) in the present specification), the same resin as the polyester-based resin constituting the front and back layers described above It may be used or another one may be used. Particularly, those containing a component derived from terephthalic acid as a dicarboxylic acid component and a component derived from ethylene glycol and 1,4-cyclohexanedimethanol as a diol component are preferred. Such a polyester resin may further contain a component derived from diethylene glycol in an amount of 0 to 30 mol%, preferably 1 to 25 mol%, more preferably 2 to 20 mol%.

When using a polybutylene terephthalate-based resin as the polyester-based resin, in addition to a polybutylene terephthalate-based resin consisting of only a component derived from terephthalic acid and a component derived from 1,4-butanediol, a component derived from terephthalic acid A polybutylene terephthalate resin containing a dicarboxylic acid component other than the above and / or a diol component other than the component derived from 1,4-butanediol may be used.
The content of the dicarboxylic acid component other than the component derived from terephthalic acid is preferably 50 mol% or less based on 100 mol% of the dicarboxylic acid component. When the content is 50 mol% or less, the heat resistance of the polybutylene terephthalate-based resin can be maintained. Further, the content of the diol component other than the component derived from 1,4-butanediol is preferably 50 mol% or less based on 100 mol% of the diol component.

In the mixed resin (a), a preferable lower limit of the content of the polyester resin is 5% by weight, and a preferable upper limit is 90% by weight. When the content of the polyester resin is less than 5% by weight, when the overlapped portion is scratched or the perforation is torn after the container is mounted, delamination of the heat-shrinkable multilayer film may occur. . If the content of the polyester-based resin exceeds 90% by weight, the interlayer strength of the heat-shrinkable multilayer film may decrease, and when the overlapping portion is scratched after the container is attached, or the perforation is broken. In some cases, delamination of the heat-shrinkable multilayer film may occur. A more preferred lower limit of the content of the polyester resin is 10% by weight, a still more preferred lower limit is 25% by weight, a more preferred upper limit is 80% by weight, and a still more preferred upper limit is 75% by weight.

As the polystyrene resin constituting the mixed resin (a), the same one as the above-mentioned polystyrene resin constituting the intermediate layer may be used, or another resin may be used.
When the polystyrene resin constituting the mixed resin (a) is a styrene-conjugated diene copolymer, the content of the conjugated diene in 100% by weight of the styrene-conjugated diene copolymer may be less than 50% by weight. preferable. When the content of the conjugated diene is 50% by weight or more, delamination of the heat-shrinkable multilayer film may occur when the overlapped portion is scratched or the perforation is torn after the container is mounted.

In the mixed resin (a), a preferable lower limit of the content of the polystyrene resin is 10% by weight, and a preferable upper limit is 95% by weight. When the content of the polystyrene resin is less than 10% by weight, the interlayer strength of the heat-shrinkable multilayer film may decrease, and when the overlapping portion is scratched after the container is attached, or When torn, delamination of the heat-shrinkable multilayer film may occur. If the content of the polystyrene-based resin exceeds 95% by weight, delamination of the heat-shrinkable multilayer film may occur when the overlapped portion is scratched or the perforation is torn after the container is mounted. A more preferred lower limit of the content of the polystyrene resin is 20% by weight, a still more preferred lower limit is 25% by weight, a more preferred upper limit is 90% by weight, and a still more preferred upper limit is 80% by weight.

As the polystyrene-based resin constituting the mixed resin of the polystyrene-based resin and the polyester-based elastomer (also referred to as a mixed resin (b) in the present specification), the same polystyrene-based resin constituting the above-mentioned intermediate layer can be used. It may be used or another one may be used, but a softer material than the polystyrene resin constituting the intermediate layer is preferable.

When the polystyrene resin constituting the mixed resin (b) is a styrene-conjugated diene copolymer, the conjugated diene content in the styrene-conjugated diene copolymer may be less than 50% by weight based on 100% by weight. preferable. When the content of the conjugated diene is 50% by weight or more, peeling may occur from the solvent sealing portion at the time of mounting the container, and when the overlapping portion is scratched after mounting the container, or the perforation is torn. Occasionally, delamination of the heat-shrinkable multilayer film may occur.

In the mixed resin (b), a preferable lower limit of the content of the polystyrene resin is 10% by weight, and a preferable upper limit is 95% by weight. When the content of the polystyrene resin is less than 10% by weight, when the heat-shrinkable multilayer film is strongly folded, whiteness (whitening phenomenon) occurs at the fold portion, and the appearance may be impaired. If the content of the polystyrene-based resin exceeds 95% by weight, the interlayer strength of the heat-shrinkable multilayer film may decrease, and when the overlapping portion is scratched after the container is mounted, or when the perforation is broken. In some cases, delamination of the heat-shrinkable multilayer film may occur. A more preferred lower limit of the content of the polystyrene resin is 20% by weight, a still more preferred lower limit is 25% by weight, a more preferred upper limit is 90% by weight, and a still more preferred upper limit is 85% by weight.

As the polyester-based elastomer constituting the mixed resin (b), it is preferable to use the same one as the polyester-based elastomer constituting the above-mentioned adhesive layer.

The melting point of the polyester elastomer constituting the mixed resin (b) is preferably from 120 to 200C. When the melting point is lower than 120 ° C., the heat resistance of the heat-shrinkable multilayer film is reduced, and the heat-shrinkable multilayer film may be peeled off from the solvent-sealed portion when the container is mounted. If the melting point exceeds 200 ° C., sufficient adhesive strength may not be obtained. A more preferred lower limit of the melting point is 130 ° C, and a more preferred upper limit is 190 ° C.
The melting point of the polyester-based elastomer constituting the mixed resin (b) results from, for example, the copolymerization ratio, structure, and the like of the aromatic polyester as the hard segment and the polyalkylene ether glycol as the soft segment. Above all, the melting point of the polyester-based elastomer tends to depend on the copolymerization amount of the polyalkylene ether glycol. The higher the copolymerization amount of the polyalkylene ether glycol, the lower the melting point.

In the mixed resin (b), a preferable lower limit of the content of the polyester elastomer is 5% by weight, and a preferable upper limit is 80% by weight. When the content of the polyester-based elastomer is less than 5% by weight, the interlayer strength of the heat-shrinkable multilayer film may decrease, and when the overlapped portion is scratched after the container is attached, or the perforation is broken. In some cases, delamination of the heat-shrinkable multilayer film may occur. If the content of the polyester-based elastomer is more than 80% by weight, when the heat-shrinkable multilayer film is strongly folded, whiteness (whitening phenomenon) occurs at the fold portion, and the appearance may be impaired. A more preferred lower limit of the content of the polyester elastomer is 10% by weight, a still more preferred lower limit is 15% by weight, a more preferred upper limit is 75% by weight, and a still more preferred upper limit is 70% by weight.

The adhesive layer may optionally contain an antioxidant, a heat stabilizer, a lubricant, an antistatic agent, and the like.

(Characteristics of heat shrinkable multilayer film)
In the heat-shrinkable multilayer film of the present invention, the lower limit of the retardation is 0.1 nm and the upper limit is 2900 nm.
When the retardation is within the above range, the accuracy of detecting a defect of the film can be improved, and the defect can be prevented from being overlooked.
In the retardation, a preferable lower limit is 90 nm, a more preferable lower limit is 100 nm, a preferable upper limit is 2800 nm, and a more preferable upper limit is 2700 nm.
The retardation is calculated from the product of the film thickness and the difference between the refractive index in the main shrinkage direction and the direction orthogonal to the main shrinkage direction in the plane of the film. Specifically, the retardation is measured by an optical method. This is a value measured by a phase difference measuring device (for example, KOBRA series manufactured by Oji Scientific Instruments).

The heat shrinkable multilayer film of the present invention has a heat shrinkage in the main shrinkage direction (TD direction) when immersed in warm water at 70 ° C. for 10 seconds (hereinafter, “TD heat shrinkage (70 ° C. × 10 seconds)”). Is also 15%, and the preferable upper limit is 50%.
When the TD heat shrinkage (70 ° C. × 10 seconds) is 15% or more, the shrink finish can be improved. When the TD heat shrinkage (70 ° C. × 10 seconds) is 50% or less, natural shrinkage can be suppressed.
The TD heat shrinkage (70 ° C. × 10 seconds) has a more preferred lower limit of 20%, a still more preferred lower limit of 25%, a particularly preferred lower limit of 30%, a more preferred upper limit of 47%, and a still more preferred upper limit of 45%. .
In addition, the said heat shrinkage rate can measure the length after heat-shrinking at predetermined temperature and time, and can calculate it from the ratio with the length before heat shrinkage.

The heat shrinkable multilayer film according to the present invention has a heat shrinkage in the main shrinkage direction (TD direction) when immersed in hot water of 100 ° C. for 10 seconds (hereinafter, “TD heat shrinkage (100 ° C. × 10 seconds)”). Is also 60%, and the preferable upper limit is 85%.
When the TD heat shrinkage (100 ° C. × 10 seconds) is 60% or more, it is possible to prevent the occurrence of wrinkles and the like when shrunk. When the TD heat shrinkage (100 ° C. × 10 seconds) is 85% or less, it is possible to prevent poor appearance such as an increase in the displacement of the label when shrunk.
The lower limit of the TD heat shrinkage (100 ° C. × 10 seconds) is more preferably 65%, further preferably 70%, more preferably 83%, and still more preferably 82%.

In the heat-shrinkable multilayer film of the present invention, the preferred lower limit of the maximum shrinkage stress when immersed in warm water at 80 ° C. for 30 seconds is 2.0 MPa, and the preferred upper limit is 11 MPa. When the maximum shrinkage stress is 2.0 MPa or more, the finish of shrinkage when attached to a container can be sufficiently improved. When the maximum shrinkage stress is 11 MPa or less, it is possible to suppress the displacement between layers at the center seal portion of the label. In addition, deformation of the container when mounted on the container can be suppressed. A more preferred lower limit of the maximum shrinkage stress is 2.5 MPa, and a more preferred upper limit is 10 MPa.
The maximum shrinkage stress refers to the maximum value of the shrinkage stress when the heat shrinkable multilayer film is immersed in hot water at 80 ° C. for 30 seconds and the shrinkage stress is measured.

The maximum shrinkage stress can be adjusted, for example, by the stretching conditions (stretching ratio, stretching temperature, etc.), the Vicat softening temperature of the polystyrene resin used for the intermediate layer, and the like.
When the stretching temperature is increased, the shrinkage stress tends to decrease, and when the stretching temperature is decreased, the shrinkage stress tends to increase. The stretching temperature needs to be adjusted according to the Vicat softening temperature of the polystyrene resin used in the intermediate layer.
When the stretching ratio in the transverse (TD) direction is reduced, the shrinkage stress decreases, and when the stretching ratio is increased, the shrinkage stress increases.

The preferable lower limit of the thickness of the entire heat-shrinkable multilayer film of the present invention is 20 μm, and the preferable upper limit is 80 μm. By setting the total thickness of the heat-shrinkable multilayer film within the above range, it is excellent in economy and easy to handle.
In the heat-shrinkable multilayer film of the present invention, the ratio of the thickness of the front and back layers to the thickness of the intermediate layer (the thickness of the front and back layers / the thickness of the intermediate layer) is preferably 1/12, more preferably 1 / 10, a preferred upper limit is 1/3, and a more preferred upper limit is 1/4.
The thickness of the front and back layers means the thickness of each of the front and back layers.

As for the ratio of the thickness of the front and back layers to the total thickness of the heat-shrinkable multilayer film of the present invention, a preferable lower limit is 7%, a more preferable lower limit is 8%, a preferable upper limit is 18%, and a more preferable upper limit is 16%. . When the above ratio is 7% or more, the solvent resistance and heat resistance of the heat-shrinkable multilayer film can be sufficiently improved. When the ratio is 18% or less, the label is easily peeled from the container.
For example, when the total thickness of the heat-shrinkable multilayer film of the present invention is 40 μm, the preferable lower limit of the thickness of the front and back layers is 2.8 μm, the more preferable lower limit is 3.2 μm, and the preferable upper limit is 7.2 μm. A preferred upper limit is 6.4 μm.

The ratio of the thickness of the intermediate layer to the total thickness of the heat-shrinkable multilayer film of the present invention is preferably 60% at a lower limit, 65% at a more preferable lower limit, 84% at a preferable upper limit, and 82% at a more preferable upper limit. . When the ratio is 60% or more, the label is easily peeled from the container. When the above ratio is 84% or less, the heat resistance of the heat-shrinkable multilayer film can be sufficiently improved.
For example, when the total thickness of the heat-shrinkable multilayer film of the present invention is 40 μm, a preferable lower limit of the thickness of the intermediate layer is 24 μm, a preferable lower limit is 26 μm, a preferable upper limit is 33.6 μm, and a more preferable upper limit is 32. 8 μm.

The ratio of the thickness of the adhesive layer to the total thickness of the heat-shrinkable multilayer film of the present invention is preferably 0.5% for a lower limit, 1% for a more preferable lower limit, 5% for a preferable upper limit, and 4% for a more preferable upper limit. It is.
For example, when the total thickness of the heat-shrinkable multilayer film of the present invention is 40 μm, a preferable lower limit of the thickness of the adhesive layer is 0.2 μm, a more preferable lower limit is 0.4 μm, a preferable upper limit is 2 μm, and a more preferable upper limit. Is 1.6 μm.

The said heat-shrinkable multilayer resin film can be especially manufactured by a co-extrusion-stretching method.
In the co-extrusion-stretching method, by changing the extrusion conditions such as the resin temperature and the cooling temperature and the stretching conditions such as the stretching temperature and the stretching ratio (particularly by adjusting the stretching ratio and the total thickness), Since the degree of orientation between the resin constituting the front and back layers and the resin constituting the intermediate layer can be adjusted, the retardation value can be adjusted.

The use of the heat-shrinkable multilayer film of the present invention is not particularly limited, but the heat-shrinkable multilayer film of the present invention has a high interlayer strength, when the overlapped portion is scratched and the perforation is torn after the container is mounted. Since it suppresses delamination and is also excellent in transparency, it is suitably used, for example, as a base film of a heat-shrinkable label attached to a container such as a PET bottle or a metal can. A heat-shrinkable label using the heat-shrinkable multilayer film of the present invention is also one of the present invention.

According to the present invention, it is possible to provide a heat-shrinkable multilayer film that can easily detect defects such as scratches and foreign substances present on the film surface and can prevent defects such as ink loss after printing. .

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to only these Examples.

The following raw materials were used in Examples and Comparative Examples.
(Polyester resin)
Polyester resin A: dicarboxylic acid component (component derived from terephthalic acid: 100 mol%) and diol component (component derived from ethylene glycol: 65 mol%, component derived from diethylene glycol: 20 mol%, 1,4- Polyester resin composed of a component derived from cyclohexanedimethanol: 15 mol% (glass transition temperature: 69 ° C)
Polyester resin B: polyester resin composed of a dicarboxylic acid component (a component derived from terephthalic acid: 100 mol%) and a diol component (a component derived from 1,4-butanediol: 100 mol%) (melting point: 223 ° C.) )
Polyester resin C: dicarboxylic acid component (component derived from terephthalic acid: 70 mol%, component derived from isophthalic acid: 30 mol%) and diol component (component derived from 1,4-butanediol: 100 mol%) ) (Melting point: 170 ° C)
Polyester resin D: dicarboxylic acid component (component derived from terephthalic acid: 100 mol%) and diol component (component derived from ethylene glycol: 64 mol%, component derived from diethylene glycol: 13 mol%, 1,4- Polyester resin composed of a component derived from cyclohexanedimethanol: 23 mol% (glass transition temperature: 83 ° C.)
Polyester resin E: dicarboxylic acid component (component derived from terephthalic acid: 83 mol%, component derived from isophthalic acid: 11 mol%, component derived from adipic acid: 6 mol%) and diol component (to ethylene glycol) (Derived from: 100 mol%) polyester resin (melting point: 205 ° C.)
Polyester resin F: dicarboxylic acid component (component derived from terephthalic acid: 100 mol%) and diol component (component based on ethylene glycol: 68 mol%, component derived from 1,4-cyclohexanedimethanol: 32 mol) %) (Glass transition temperature: 82 ° C.)

(Polystyrene resin)
-Polystyrene resin A: styrene-butadiene copolymer (styrene content: 81.3% by weight, butadiene content: 18.7% by weight, Vicat softening temperature: 81 ° C)
Polystyrene resin B: styrene-butadiene copolymer (styrene content: 79% by weight, butadiene content: 21% by weight, Vicat softening temperature: 59 ° C.)
Polystyrene resin C: styrene-butadiene copolymer (styrene content: 77.7% by weight, butadiene content: 22.3% by weight, Vicat softening temperature: 71 ° C.)
Polystyrene resin D: styrene-butadiene copolymer (styrene content: 72% by weight, butadiene content: 28% by weight, Vicat softening temperature: 76 ° C.)
Polystyrene resin E: styrene-butadiene copolymer (styrene content: 80% by weight, butadiene content: 20% by weight, Vicat softening temperature: 74 ° C.)
・ Polystyrene resin F: styrene-butadiene copolymer (styrene content: 82% by weight, butadiene content: 18% by weight, Vicat softening temperature: 75 ° C.)
-Polystyrene resin G: styrene-butadiene copolymer (styrene content: 75.6% by weight, butadiene content: 24.4% by weight, Vicat softening temperature: 83 ° C)

(Polyester elastomer)
Polyester elastomer A: an unmodified polyester / polyether block copolymer composed of polyester as a hard segment and polyether as a soft segment (manufactured by Toray Dupont, Hytrel 2521, durometer hardness: 55)
Polyester elastomer B: a modified polyester / polyether block copolymer (Mitsubishi) composed of a polyester as a hard segment and a polyether as a soft segment and graft-modified with an α, β-ethylenically unsaturated carboxylic acid Chemical Co., Primalloy AP, Durometer hardness: 40)

(Example 1)
As a resin constituting the front and back layers, a mixed resin containing 85% by weight of a polyester-based resin A and 15% by weight of a polyester-based resin C was used.
As a resin constituting the intermediate layer, a mixed resin containing 50% by weight of a polystyrene-based resin A and 50% by weight of a polystyrene-based resin B was used.
As a resin constituting the adhesive layer, a mixed resin containing 59% by weight of the polyester resin A, 10% by weight of the polyester resin C, and 31% by weight of the polystyrene resin D was used.
These resins were introduced into an extruder having a barrel temperature of 140 to 230 ° C., extruded from a multilayer die of 225 to 230 ° C. into a five-layered sheet, and cooled and solidified by a take-off roll at 21 ° C. Next, the film is stretched at a draw ratio of 6.3 times in a tenter stretching machine having a preheating zone of 100 ° C., a stretching zone of 84 to 89 ° C., and a heat fixing zone of 84 ° C., and then wound up by a winder, so as to be orthogonal to the main shrinkage direction. Thus, a heat-shrinkable multilayer film having a direction of MD and a direction of main shrinkage of TD was obtained. The obtained heat-shrinkable multilayer film has a total thickness of 50 μm and has a five-layer structure of a surface layer (7 μm) / adhesive layer (1 μm) / intermediate layer (34 μm) / adhesive layer (1 μm) / backside layer (7 μm). Met.

(Example 2)
As the resin constituting the front and back layers, 100% by weight of a polyester resin A was used.
As the resin constituting the intermediate layer, a mixed resin containing 30% by weight of polystyrene resin A and 70% by weight of polystyrene resin C was used.
As the resin constituting the adhesive layer, a mixed resin containing 65% by weight of a polyester-based elastomer A and 35% by weight of a polystyrene-based resin D was used.
These resins were introduced into an extruder having a barrel temperature of 140 to 225 ° C, extruded from a multilayer die at 240 ° C into a sheet having a five-layer structure, and cooled and solidified by a take-up roll at 25 ° C. Then, the film is stretched in a tenter stretching machine having a preheating zone of 108 ° C., a stretching zone of 88.5 to 90.5 ° C., and a heat setting zone of 82 ° C. at a stretching ratio of 6.4 times. A heat-shrinkable multilayer film was obtained in which the direction perpendicular to the shrinkage direction was MD and the main shrinkage direction was TD. The obtained heat-shrinkable multilayer film has a total thickness of 50 μm and has a five-layer structure of a surface layer (7 μm) / adhesive layer (1 μm) / intermediate layer (34 μm) / adhesive layer (1 μm) / backside layer (7 μm). Met.

(Example 3)
As the resin constituting the front and back layers, a mixed resin containing 80% by weight of the polyester resin A and 20% by weight of the polyester resin B was used.
As a resin constituting the intermediate layer, a mixed resin containing 32% by weight of polystyrene resin D and 68% by weight of polystyrene resin E was used.
As the resin constituting the adhesive layer, 100% by weight of a polyester elastomer B was used.
These resins were introduced into an extruder having a barrel temperature of 190 to 255 ° C., extruded from a multilayer die at 240 ° C. into a sheet having a five-layer structure, and cooled and solidified with a take-off roll at 30 ° C. Next, after stretching at a stretching ratio of 5.9 in a tenter stretching machine having a preheating zone of 99 ° C, a stretching zone of 80 to 85 ° C, and a heat setting zone of 80 ° C, the film is wound up by a winder to be orthogonal to the main shrinkage direction. Thus, a heat-shrinkable multilayer film having a direction of MD and a direction of main shrinkage of TD was obtained. The obtained heat-shrinkable multilayer film has a total thickness of 40 μm, and has a surface layer (5.7 μm) / adhesive layer (0.7 μm) / intermediate layer (27.2 μm) / adhesive layer (0.7 μm) / back surface. It had a five-layer structure of layers (5.7 μm).

(Example 4)
As the resin constituting the front and back layers, 100% by weight of a polyester resin A was used.
As the resin constituting the intermediate layer, a mixed resin containing 30% by weight of polystyrene resin A and 70% by weight of polystyrene resin C was used.
As the resin constituting the adhesive layer, a mixed resin containing 30% by weight of the polyester-based elastomer A and 70% by weight of the polystyrene-based resin D was used.
These resins were introduced into an extruder having a barrel temperature of 140 to 225 ° C, extruded from a multilayer die at 240 ° C into a sheet having a five-layer structure, and cooled and solidified by a take-up roll at 25 ° C. Next, the film is stretched in a tenter stretching machine having a preheating zone of 106 ° C., a stretching zone of 87 to 90 ° C., and a heat fixing zone of 83 ° C. at a stretching ratio of 6.2, and then wound up by a winder so as to be orthogonal to the main shrinkage direction. Thus, a heat-shrinkable multilayer film having a direction of MD and a direction of main shrinkage of TD was obtained. The obtained heat-shrinkable multilayer film has a total thickness of 40 μm, and has a surface layer (5.7 μm) / adhesive layer (0.7 μm) / intermediate layer (27.2 μm) / adhesive layer (0.7 μm) / back surface. It had a five-layer structure of layers (5.7 μm).

(Example 5)
As the resin constituting the front and back layers, 100% by weight of a polyester resin A was used.
As the resin constituting the intermediate layer, a mixed resin containing 25.5% by weight of polystyrene resin A, 59.5% by weight of polystyrene resin C, and 15% by weight of polyester resin A was used.
As the resin constituting the adhesive layer, a mixed resin containing 30% by weight of the polyester-based elastomer A and 70% by weight of the polystyrene-based resin D was used.
These resins were introduced into an extruder having a barrel temperature of 140 to 225 ° C, extruded from a multilayer die at 240 ° C into a sheet having a five-layer structure, and cooled and solidified by a take-up roll at 25 ° C. Next, the film is stretched in a tenter stretching machine having a preheating zone of 106 ° C., a stretching zone of 87 to 90 ° C., and a heat fixing zone of 83 ° C. at a stretching ratio of 6.2, and then wound up by a winder so as to be orthogonal to the main shrinkage direction. Thus, a heat-shrinkable multilayer film having a direction of MD and a direction of main shrinkage of TD was obtained. The obtained heat-shrinkable multilayer film has a total thickness of 40 μm, and has a surface layer (5.7 μm) / adhesive layer (0.7 μm) / intermediate layer (27.2 μm) / adhesive layer (0.7 μm) / back surface. It had a five-layer structure of layers (5.7 μm).

(Example 6)
As the resin constituting the front and back layers, 100% by weight of a polyester resin A was used.
As a resin constituting the intermediate layer, a mixed resin containing 30% by weight of a polystyrene-based resin A and 70% by weight of a polystyrene-based resin F was used.
As the resin constituting the adhesive layer, a mixed resin containing 60% by weight of the polyester-based elastomer B and 40% by weight of the polystyrene-based resin D was used.
These resins were introduced into an extruder having a barrel temperature of 170 to 230 ° C., extruded from a multilayer die at 230 ° C. into a five-layered sheet, and cooled and solidified by a take-up roll at 28 ° C. Next, the film is stretched at a stretching ratio of 5.8 in a tenter stretching machine having a preheating zone of 109 ° C., a stretching zone of 81 to 100.5 ° C., and a heat setting zone of 74 ° C., and then wound up by a winder to obtain a main shrinkage direction. A heat-shrinkable multilayer film was obtained in which the direction perpendicular to the direction was MD and the main shrinkage direction was TD. The obtained heat-shrinkable multilayer film has a total thickness of 40 μm, and has a surface layer (5.7 μm) / adhesive layer (0.7 μm) / intermediate layer (27.2 μm) / adhesive layer (0.7 μm) / back surface. It had a five-layer structure of layers (5.7 μm).

(Example 7)
As the resin constituting the front and back layers, 100% by weight of a polyester resin D was used.
As a resin constituting the intermediate layer, a mixed resin containing 80% by weight of a polystyrene-based resin G and 20% by weight of a polyester-based resin D was used.
These resins were introduced into an extruder having a barrel temperature of 140 to 225 ° C, extruded from a multilayer die at 240 ° C into a sheet having a five-layer structure, and cooled and solidified by a take-up roll at 25 ° C. Next, after stretching in a tenter stretching machine having a preheating zone of 106 ° C., a stretching zone of 87 to 90 ° C., and a heat setting zone of 83 ° C. at a stretching ratio of 6.1 times, the film is wound up by a winder to be orthogonal to the main shrinkage direction. Thus, a heat-shrinkable multilayer film having a direction of MD and a direction of main shrinkage of TD was obtained. The obtained heat-shrinkable multilayer film had a total thickness of 40 μm, and had a three-layer structure of a surface layer (4.4 μm) / intermediate layer (31.2 μm) / backside layer (4.4 μm).

(Comparative Example 1)
As the resin constituting the front and back layers, a mixed resin containing 50% by weight of a polyester resin E and 50% by weight of a polyester resin F was used.
As the resin constituting the intermediate layer, a mixed resin containing 50% by weight of the polyester resin E and 50% by weight of the polyester resin F was used.
These resins were introduced into an extruder having a barrel temperature of 200 to 210 ° C., extruded from a multilayer die at 240 ° C. into a five-layered sheet, and cooled and solidified by a take-off roll at 35 ° C. Next, the film is stretched at a stretching ratio of 6.5 in a tenter stretching machine having a preheating zone of 88 ° C, a stretching zone of 85 to 86 ° C, and a heat fixing zone of 86 ° C. Thus, a heat-shrinkable multilayer film having a direction of MD and a direction of main shrinkage of TD was obtained. The obtained heat-shrinkable multilayer film had a total thickness of 40 μm, and had a three-layer structure of a surface layer (6.6 μm) / an intermediate layer (26.8 μm) / a back layer (6.6 μm).

(Evaluation)
The heat shrinkable films obtained in Examples and Comparative Examples were evaluated by the following methods.
The results are shown in Table 1.

(1) Phase difference (retardation)
The heat-shrinkable films obtained in the examples and comparative examples were cut out into a size of 35 mm in the main shrinkage direction (TD direction) × 35 mm in the direction (MD direction) orthogonal to the main shrinkage direction to obtain a measurement sample.
The measurement sample is set on a phase difference measuring device (KOBRA-WR, manufactured by Oji Scientific Instruments) so that the angle defined by the measuring device is 0 °, and the incident angle is 0 ° (film surface). (In the direction perpendicular to the vertical direction) at a wavelength of 590 nm.

(2) Heat Shrinkage The heat shrinkable multilayer film is cut into a size of 100 mm in the main shrinkage direction (TD) × 100 mm in a direction perpendicular to the main shrinkage direction (MD), and immersed in 70 ° C. warm water for 10 seconds. The heat-shrinkable multilayer film was taken out and immediately immersed in tap water for 10 seconds. The length (L) of one side of the TD of the heat shrinkable multilayer film was measured, and the heat shrinkage in the TD direction was obtained according to the following equation (1). The heat shrinkage after immersion in hot water of 80 ° C. for 10 seconds was measured in the same manner.
Heat shrinkage (%) = {(100−L) / 100} × 100 (1)

The average value was used as the shrinkage rate assuming that the number of samples (n) was 3. The heat shrinkage was similarly measured for hot water at 100 ° C.

(3) Defect Confirmation A heat-shrinkable multilayer film having a width of 20 cm and a length of 100 m was sandwiched between two polarizing films of 20 cm x 20 cm, placed on a white LED light source, and illuminated from below.
It was confirmed for 100 m of the heat-shrinkable film whether the defect was visible while rotating the polarizing film on the upper side of the heat-shrinkable film so that the absorption axes of the two polarizing films were orthogonal (cross Nicol condition), Evaluation was made according to the following criteria.
：: The defect was visible.
X: The interference fringe (rainbow color) was strong, and the defect was not visible even when the polarization direction was changed.

According to the present invention, it is possible to provide a heat-shrinkable multilayer film that can easily detect defects such as scratches and foreign substances present on the film surface and can prevent defects such as ink loss after printing. .

Claims (1)

A heat-shrinkable multilayer film having a front and back layer containing a polyester resin and an intermediate layer containing a polystyrene resin,
A heat-shrinkable multilayer film having a retardation (Re) of 0.1 to 2900 nm.