JP7232009B2 - heat shrinkable multilayer film - Google Patents

Description

TECHNICAL FIELD The present invention relates to a heat-shrinkable multilayer film in which defects such as scratches and foreign matter present on the film surface can be easily detected, and defects such as ink loss after printing can be prevented.

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

In a polystyrene heat-shrinkable film whose main material is styrene-butadiene block copolymer (SBS), butadiene in the butadiene block copolymer develops thermal cross-linking during heat-melt processing, generating cross-linked gels (fish eyes). When the film is printed, the ink in that portion is removed due to unevenness caused by migration to the surface of the crosslinked gel, especially during stretching.
On the other hand, Patent Document 1 discloses a heat-shrinkable film that can be used for applications such as shrink packaging, shrink binding packaging, and shrink labels, and that has little crosslinked gel (fish eyes).

In addition, heat-shrinkable films are expected to have a better surface profile in order to accommodate increasingly diverse print designs. It is monitored using a detector or the like, and the resulting gel or the like is removed before shipment.

The means for inspecting defects in the heat-shrinkable film is to use a white fluorescent lamp as a light source, illuminate the heat-shrinkable film from behind through a polarizing filter, and image it with a CCD camera with a polarizing filter placed on the opposite side. A method of judging quality by detecting leakage (transmission) of light from a defect through processing, or using infrared light as a light source and using a CCD camera with a visible light cut filter to similarly image and process the image to detect defects. method is used.

However, conventional heat-shrinkable films have the problem that interference fringes appear when photographed by a CCD camera with a polarizing filter, and defects can be overlooked, resulting in ink loss during printing. rice field.

JP 2005-28736 A

In view of the above-mentioned current situation, the present invention provides a heat-shrinkable multilayer film in which defects such as scratches and foreign matter existing on the film surface can be easily detected, and defects such as ink loss after printing can be prevented. for the purpose.

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

In the heat-shrinkable multilayer film, the present inventors use a polyester resin as the resin constituting the front and back layers, use a polystyrene resin as the resin constituting the intermediate layer, and set the retardation to a predetermined range. Thus, the inventors have found that the accuracy of defect detection can be improved, and have completed the present invention.

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

(front and back layers)
The front and back layers contain a polyester-based resin.
The polyester-based resin constituting the front and back layers can be obtained by condensation polymerization of a dicarboxylic acid and a diol.
The dicarboxylic acid is not particularly limited, and examples thereof include 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, their anhydrides and lower alkyl esters.
The diol is not particularly limited, and examples 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 diols, 2,5-hexanediol, 2-methyl-2,4-pentanediol, 3-methyl-1,3-pentanediol and 2-ethyl-1,3-hexanediol;2, Alicyclic diols such as 2-bis(4-hydroxycyclohexyl)propane, alkylene oxide adducts of 2,2-bis(4-hydroxycyclohexyl)propane, 1,4-cyclohexanediol, and 1,4-cyclohexanedimethanol etc.

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

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

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

As the polyester-based resin, it is preferable to use a resin having a relatively low crystalline melting temperature or no crystalline melting temperature. In the production of a heat-shrinkable multilayer film, trimming pieces and recycled films from stretching may be reused as return materials. Normally, such a return material is mixed with polystyrene resin as a raw material for the intermediate layer. When film molding was performed, the polyester resin was sometimes extruded in an unmelted state. However, by using a polyester resin with a relatively low crystalline melting temperature or no crystalline melting temperature, it is possible to prevent unmelted polyester resin from becoming foreign matter in the film after molding.

Antioxidants, heat stabilizers, ultraviolet absorbers, light stabilizers, lubricants, antistatic agents, antiblocking agents, flame retardants, antibacterial agents, fluorescent brighteners, colorants, etc. may contain additives.

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

As used herein, 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 examples thereof include 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, and examples include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, and 1,3-hexadiene. etc. These may be used alone or in combination of two or more.

The above aromatic vinyl hydrocarbon-conjugated diene copolymer preferably contains a styrene-butadiene copolymer (SBS resin) because it is particularly excellent in heat shrinkability. In order to produce a heat-shrinkable multi-layer film with less fish eyes, the aromatic vinyl hydrocarbon-conjugated diene copolymer contains 2-methyl-1,3-butadiene (isoprene) as the conjugated diene. It preferably contains the styrene-isoprene copolymer (SIS resin), styrene-isoprene-butadiene copolymer (SIBS), etc. used.
The above aromatic vinyl hydrocarbon-conjugated diene copolymer may contain any one of SBS resin, SIS resin and SIBS resin alone, or may contain a plurality of them in combination. In addition, when using a plurality of SBS resins, SIS resins and SIBS resins, each resin may be dry blended, and a compound resin obtained by kneading each resin with a specific composition using an extruder and pelletizing is used. may

When the aromatic vinyl hydrocarbon-conjugated diene copolymer contains the SBS resin, the SIS resin and the SIBS resin singly or in combination, a heat-shrinkable multilayer film having particularly excellent heat-shrinkability can be obtained. It is preferable that the aromatic vinyl hydrocarbon-conjugated diene copolymer has a styrene content of 65 to 90% by weight and a conjugated diene content of 10 to 35% by weight based on 100% by weight. If 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 may be easily cut when tension is applied, or may be difficult to handle during processing such as printing. It may break without twisting. If the styrene content is less than 65% by weight or the conjugated diene content is more than 35% by weight, foreign substances such as gels are likely to occur during molding, and the stiffness of the heat-shrinkable multilayer film becomes weak. and the handleability may deteriorate.

The preferred lower limit of the Vicat softening temperature of the polystyrene resin is 60°C, and the preferred upper limit is 85°C. When the Vicat softening temperature is 60° C. or higher, the low-temperature shrinkability of the heat-shrinkable multilayer film can be improved, and wrinkles can be prevented when the film is attached to 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 enhanced, and the generation of unshrunk portions can be prevented when the film is mounted on a container. A more preferable lower limit of the Vicat softening temperature is 65°C, and a more preferable upper limit is 80°C.
The Vicat softening temperature can be measured by a method conforming to ISO 306.

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

Examples of commercially available polystyrene resins constituting the intermediate layer include "Clearen" (manufactured by Denki Kagaku Kogyo), "Asaflex" (manufactured by Asahi Kasei Chemicals), "Styrolux" (manufactured by BASF), and "PSJ." -Polystyrene” (manufactured by PS Japan Co., Ltd.).

The preferred lower limit of the styrene component content in the polystyrene resin constituting the intermediate layer is 60% by weight, the more preferred lower limit is 70% by weight, the preferred upper limit is 90% by weight, and the more preferred upper limit is 85% by weight.
When the polystyrene resin constituting the intermediate layer is a mixed resin in which a plurality of polystyrene resins are combined, the styrene component content in the polystyrene resin constituting the intermediate layer is It is calculated by dividing the sum of the resin content multiplied by the styrene component content of each polystyrene resin by 100.

The preferable lower limit of the content of the conjugated diene component in the polystyrene resin constituting the intermediate layer is 10% by weight, the more preferable lower limit is 15% by weight, the preferable upper limit is 40% by weight, and the more preferable upper limit is 30% by weight.
In addition, when the polystyrene-based resin constituting the intermediate layer is a mixed resin in which a plurality of polystyrene-based resins are combined, the content of the conjugated diene component in the polystyrene-based resin constituting the intermediate layer is It is calculated by dividing by 100 the total obtained by multiplying the content of the polystyrene resin by the content of the conjugated diene component in each polystyrene resin.

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

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

When two or more of the above polystyrene resins are used to form a mixed resin, the difference in Vicat softening temperature between the 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 less.

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 one as that constituting the front and back layers may be used.

The content of the polyester resin in the intermediate layer preferably has a lower limit of 1% by weight, a more preferred lower limit of 5% by weight, a preferred upper limit of 30% by weight, and a more preferred upper limit of 20% by weight.
When the content of the polyester-based resin is within this range, the retardation value can be lowered, which is preferable.

Additives such as antioxidants, heat stabilizers, ultraviolet absorbers, light stabilizers, lubricants, antistatic agents, flame retardants, antibacterial agents, fluorescent whitening agents, and colorants may be added to the intermediate layer as necessary. may contain.

(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 polyester-based elastomers, modified products thereof, mixed resins of polyester-based resins and polystyrene-based resins (herein also referred to as mixed resin (a)), and polystyrene. A mixed resin of a base resin and a polyester-based elastomer (also referred to as a mixed resin (b) in this specification) is preferred. Such an adhesive layer has a high affinity with both the polyester-based resin forming the front and back layers and the polystyrene-based resin forming the intermediate layer, and can bond the two with high strength. In addition, since it dissolves or swells in a solvent that dissolves the polyester resin constituting the front and back layers, the solvent can penetrate into the inside of the heat-shrinkable multilayer film at the time of center sealing, and during the subsequent heat shrinkage, the interlayer It is possible to prevent peeling from occurring. Furthermore, since the intermediate layer and the front and back layers can be molded by a coextrusion method, the productivity is also 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 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. Furthermore, polyester polyether block copolymers having polyalkylene ether glycol as soft segments are more preferred.

The polyester polyether block copolymer includes (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 raw materials, and an oligomer obtained by an esterification reaction or a transesterification reaction is preferably polycondensed.

The aliphatic and/or alicyclic diol having 2 to 12 carbon atoms, and the aromatic dicarboxylic acid and/or aliphatic dicarboxylic acid or alkyl ester thereof are used as raw materials for polyester, particularly as raw materials for polyester-based elastomers. Any commonly used one 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 preferable lower limit of the number average molecular weight of the polyalkylene ether glycol is 400, and a preferable upper limit thereof is 6,000. By setting the number average molecular weight to 400 or more, the blockiness of the copolymer is enhanced. By setting the number average molecular weight to 6000 or less, phase separation in the system is less likely to occur, and polymer physical properties are likely to be exhibited. A more preferable lower limit of the number average molecular weight is 500, a more preferable upper limit is 3000, and a further preferable lower limit is 600.
In addition, the said number average molecular weight means the thing measured by the gel permeation chromatography (GPC). Further, the GPC calibration can be performed by using, for example, a POLYTETRAHYDROFURAN calibration kit (manufactured by POLYMER LABORATORIES, UK).

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

The preferable lower limit of the durometer hardness of the polyester-based elastomer is 10, and the preferable upper limit thereof is 80. By setting the durometer hardness to 10 or more, the mechanical strength of the adhesive layer is improved. A durometer hardness of 80 or less improves the flexibility and impact resistance of the adhesive layer. The lower limit of durometer hardness is more preferably 15, the upper limit is 70, the lower limit is 20, and the upper limit is 60.
The durometer hardness can be measured by using a durometer type D in accordance with ISO18517.

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

The modified polyester-based elastomer (hereinafter also referred to as modified polyester-based elastomer) is obtained by modifying the above-described polyester-based elastomer using a modifier.
The modification reaction for obtaining the modified polyester elastomer is carried out, for example, by reacting the polyester elastomer with α,β-ethylenically unsaturated carboxylic acid as a modifier. It is preferable to use a radical generator in the modification reaction.

In the modification reaction, a graft reaction in which the α,β-ethylenically unsaturated carboxylic acid or derivative thereof is added to the polyester elastomer mainly occurs, but a decomposition reaction also occurs. As a result, the modified polyester elastomer has a lower molecular weight and a lower melt viscosity.
In addition, in the above-mentioned modification reaction, other reactions such as transesterification are usually considered to occur, and the reaction product obtained is generally a composition containing unreacted raw materials. In this case, the preferred lower limit for the content of the modified polyester-based elastomer in the resulting reaction product is 10% by weight, and the more preferred lower limit is 30% by weight. Also, the content of the modified polyester-based restomer is preferably closer to 100% by weight.

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

The modification rate (graft amount) of the modified polyester-based elastomer can be determined according to the following formula (2) from the spectrum obtained by H1-NMR measurement. As a device used for the H1-NMR measurement, for example, "GSX-400" (manufactured by JEOL Ltd.) or the like can be used.
Grafting amount (% by weight) = 100 x [(C/3 x 98)/{(A x 148/4) + (B x 72/4) + (C/3 x 98)}] (2)
In formula (2), A represents the integrated value at 7.8 to 8.4 ppm, B represents the integrated value at 1.2 to 2.2 ppm, and C represents the 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 the mixed resin (a) in this specification), the same polyester-based resin as the polyester-based resin constituting the front and back layers described above can be used. You can use one or another. In particular, it is preferable to contain 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. Such a polyester resin may further contain 0 to 30 mol %, preferably 1 to 25 mol %, more preferably 2 to 20 mol % of a component derived from diethylene glycol.

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

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

As the polystyrene-based resin constituting the mixed resin (a), the same polystyrene-based resin as that constituting the intermediate layer described above may be used, or a different one 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 is less than 50% by weight. preferable. If the conjugated diene content 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), the preferred lower limit of the content of the polystyrene resin is 10% by weight, and the preferred upper limit is 95% by weight. If 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 overlapped portion is scratched after mounting the container, or the perforation Delamination of the heat-shrinkable multilayer film can occur when torn. 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 mounting the container. A more preferable lower limit of the polystyrene resin content is 20% by weight, a more preferable lower limit is 25% by weight, a more preferable upper limit is 90% by weight, and a further preferable 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 mixed resin (b) in this specification), the same polystyrene-based resin as that constituting the above-described intermediate layer can be used. It may be used, or another one may be used, but one that is softer than the polystyrene-based resin constituting the intermediate layer is preferable.

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

In the mixed resin (b), the preferable lower limit of the content of the polystyrene resin is 10% by weight, and the preferable upper limit is 95% by weight. If the content of the polystyrene resin is less than 10% by weight, when the heat-shrinkable multilayer film is folded strongly, whitening (whitening phenomenon) occurs at the creases, which may impair the appearance. If the content of the polystyrene resin exceeds 95% by weight, the interlaminar strength of the heat-shrinkable multilayer film may decrease, and when the overlapped portion is scratched after mounting the container, or the perforation is torn. Delamination of the heat-shrinkable multi-layer film may occur when the heat-shrinkable multilayer film is used. A more preferable lower limit of the polystyrene resin content is 20% by weight, a more preferable lower limit is 25% by weight, and a more preferable upper limit is 90% by weight, and a further preferable upper limit is 85% by weight.

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

The melting point of the polyester elastomer constituting the mixed resin (b) is preferably 120 to 200°C. If the melting point is less than 120°C, the heat resistance of the heat-shrinkable multilayer film is lowered, and peeling may occur 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 preferable lower limit of the melting point is 130°C, and a more preferable upper limit is 190°C.
The melting point of the polyester-based elastomer that constitutes the mixed resin (b) is, for example, due to the copolymerization ratio of the aromatic polyester that is the hard segment and the polyalkylene ether glycol that is the soft segment, the structure, and the like. Among others, the melting point of the polyester-based elastomer tends to depend on the amount of polyalkylene ether glycol copolymerized.

In the mixed resin (b), the preferred lower limit of the content of the polyester elastomer is 5% by weight, and the preferred upper limit is 80% by weight. If 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 mounting the container, or the perforation is torn. Delamination of the heat-shrinkable multi-layer film may occur when the heat-shrinkable multilayer film is used. If the content of the polyester-based elastomer exceeds 80% by weight, when the heat-shrinkable multilayer film is folded strongly, whitening (whitening phenomenon) occurs at the creases, which may impair the appearance. A more preferable lower limit of the polyester-based elastomer content is 10% by weight, a more preferable lower limit is 15% by weight, and a more preferable upper limit is 75% by weight, and a further preferable upper limit is 70% by weight.

Antioxidants, heat stabilizers, lubricants, antistatic agents, and the like may be added to the adhesive layer as necessary.

(Characteristics of heat-shrinkable multilayer film)
The heat-shrinkable multilayer film of the present invention has a retardation lower limit of 0.1 nm and an upper limit of 2900 nm.
When the retardation is within the above range, it is possible to improve the accuracy of film defect detection and prevent defects from being overlooked.
The preferable lower limit of the retardation is 90 nm, the more preferable lower limit is 100 nm, the preferable upper limit is 2800 nm, and the more preferable upper limit is 2700 nm.
The retardation is calculated from the product of the film thickness and the difference in refractive index between the main shrinkage direction and the direction perpendicular thereto in the plane of the film. Specifically, the retardation is measured by an optical method. It is a value measured by a phase difference measuring device (for example, KOBRA series manufactured by Oji Scientific Instruments Co., Ltd.).

The heat-shrinkable multilayer film of the present invention has a heat shrinkage rate in the main shrinkage direction (TD direction) when immersed in hot water at 70°C for 10 seconds (hereinafter referred to as "TD heat shrinkage rate (70°C x 10 seconds)" Also referred to as), the preferred lower limit is 15%, and the preferred upper limit is 50%.
When the TD heat shrinkage rate (70° C.×10 seconds) is 15% or more, the shrink finish can be improved. When the TD heat shrinkage rate (70° C.×10 seconds) is 50% or less, spontaneous shrinkage can be suppressed.
The TD heat shrinkage rate (70° C.×10 seconds) has a more preferable lower limit of 20%, a more preferable lower limit of 25%, a particularly preferable lower limit of 30%, a more preferable upper limit of 47%, and a further preferable upper limit of 45%. .
The heat shrinkage ratio can be calculated by measuring the length after heat shrinking at a predetermined temperature and time and calculating the ratio of the length before heat shrinking.

The heat shrinkable multilayer film of the present invention has a heat shrinkage rate in the main shrinkage direction (TD direction) when immersed in hot water at 100 ° C. for 10 seconds (hereinafter referred to as "TD heat shrinkage rate (100 ° C. x 10 seconds)" Also referred to as), the preferred lower limit is 60%, and the preferred upper limit is 85%.
When the TD heat shrinkage rate (100° C.×10 seconds) is 60% or more, the occurrence of wrinkles or the like during shrinkage can be prevented. When the TD heat shrinkage rate (100° C.×10 seconds) is 85% or less, it is possible to prevent appearance defects such as shifting of the label during shrinkage.
The TD heat shrinkage rate (100° C.×10 seconds) has a more preferable lower limit of 65%, a more preferable lower limit of 70%, a more preferable upper limit of 83%, and a further preferable upper limit of 82%.

The heat-shrinkable multilayer film of the present invention preferably has a maximum shrinkage stress of 2.0 MPa as a lower limit and 11 MPa as an upper limit when immersed in warm water of 80° C. for 30 seconds. When the maximum shrinkage stress is 2.0 MPa or more, it is possible to sufficiently improve the finish of shrinkage when attached to a container. When the maximum shrinkage stress is 11 MPa or less, it is possible to suppress interlayer displacement at the center seal portion of the label. Moreover, deformation of the container when it is attached to the container can be suppressed. A more preferable lower limit of the maximum shrinkage stress is 2.5 MPa, and a more preferable upper limit is 10 MPa.
The maximum shrinkage stress is the maximum value of 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 adjusting the stretching conditions (stretching ratio, stretching temperature, etc.), the Vicat softening temperature of the polystyrene resin used in the intermediate layer, and the like.
A higher stretching temperature tends to lower the shrinkage stress, and a lower stretching temperature tends to increase the shrinkage stress. The stretching temperature must be adjusted according to the Vicat softening temperature of the polystyrene resin used in the intermediate layer.
Also, shrinkage stress decreases when the draw ratio in the transverse (TD) direction is low, and shrinkage stress increases when the draw ratio is high.

A preferable lower limit of the thickness of the entire heat-shrinkable multilayer film of the present invention is 20 µm, and a preferable upper limit thereof is 80 µm. By setting the thickness of the entire heat-shrinkable multilayer film within the above range, it is economical and easy to handle.
In the heat-shrinkable multilayer film of the present invention, the thickness ratio between the front and back layers and the intermediate layer (thickness of front and back layers/thickness of intermediate layer) has a preferred lower limit of 1/12, and a more preferred lower limit of 1. /10, the preferred upper limit is 1/3, and the more preferred upper limit is 1/4.
In addition, the thickness of the said front and back layer means each thickness of a surface layer and a back layer.

The ratio of the thickness of the front and back layers to the thickness of the entire heat-shrinkable multilayer film of the present invention has a preferred lower limit of 7%, a more preferred lower limit of 8%, a preferred upper limit of 18%, and a more preferred upper limit of 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 can be easily peeled off from the container.
For example, when the total thickness of the heat-shrinkable multilayer film of the present invention is 40 μm, the lower limit of the thickness of the front and back layers is preferably 2.8 μm, more preferably 3.2 μm, and preferably 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 has a preferred lower limit of 60%, a more preferred lower limit of 65%, a preferred upper limit of 84%, and a more preferred upper limit of 82%. . When the ratio is 60% or more, the label can be easily peeled off 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, the preferred lower limit of the thickness of the intermediate layer is 24 µm, the preferred lower limit is 26 µm, the preferred upper limit is 33.6 µm, and the more preferred upper limit is 32 µm. 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 has a preferred lower limit of 0.5%, a more preferred lower limit of 1%, a preferred upper limit of 5%, and a more preferred upper limit of 4%. is.
For example, when the total thickness of the heat-shrinkable multilayer film of the present invention is 40 μm, the lower limit of the thickness of the adhesive layer is preferably 0.2 μm, more preferably 0.4 μm, more preferably 2 μm, more preferably 2 μm. is 1.6 μm.

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

The application 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 high interlaminar strength, and is resistant to scratching of the overlapped portion and tearing of perforations after mounting the container. Since it suppresses delamination and has excellent transparency, it is suitably used as a base film for heat-shrinkable labels attached to containers such as PET bottles and metal cans. A heat-shrinkable label using the heat-shrinkable multilayer film of the present invention is also one aspect of the present invention.

According to the present invention, it is possible to provide a heat-shrinkable multilayer film in which defects such as scratches and foreign matter present on the film surface can be easily detected, and defects such as ink loss after printing can be prevented. .

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

The following raw materials were used in Examples , Reference 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- Component derived from cyclohexanedimethanol: 15 mol%) polyester resin (glass transition temperature: 69 ° C.)
- Polyester resin B: polyester resin (melting point: 223 ° C.) consisting of a dicarboxylic acid component (component derived from terephthalic acid: 100 mol%) and a diol component (component derived from 1,4-butanediol: 100 mol%) )
· 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% ) made of polyester resin (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- Component derived from cyclohexanedimethanol: 23 mol%) polyester resin (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 (ethylene glycol Derived component: 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 relative to oil in ethylene glycol: 68 mol%, component derived from 1,4-cyclohexanedimethanol: 32 mol %) polyester resin (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-based elastomer A: 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: Modified polyester/polyether block copolymer composed of polyester as a hard segment and polyether as a soft segment, graft-modified with α,β-ethylenically unsaturated carboxylic acid (Mitsubishi Kagaku Co., Ltd., Primalloy AP, durometer hardness: 40)

( Reference example 1)
A mixed resin containing 85% by weight of polyester-based resin A and 15% by weight of polyester-based resin C was used as the resin constituting the front and back layers.
As the resin constituting the intermediate layer, a mixed resin containing 50% by weight of polystyrene resin A and 50% by weight of polystyrene resin B was used.
A mixed resin containing 59% by weight of polyester-based resin A, 10% by weight of polyester-based resin C, and 31% by weight of polystyrene-based resin D was used as the resin constituting the adhesive layer.
These resins were put into an extruder with a barrel temperature of 140 to 230°C, extruded into a five-layer sheet through a multi-layer die at 225 to 230°C, and cooled and solidified by a take-up roll at 21°C. Next, after stretching in a tenter stretching machine with a preheating zone of 100° C., a stretching zone of 84 to 89° C., and a heat setting zone of 84° C., at a stretching ratio of 6.3, the film is wound up with a winding machine, thereby perpendicularly to the main shrinking direction. A heat-shrinkable multilayer film was obtained in which the shrinking direction was MD and the main shrinking direction was TD. The resulting heat-shrinkable multilayer film had a total thickness of 50 µm and had a five-layer structure of surface layer (7 µm)/adhesive layer (1 µm)/intermediate layer (34 µm)/adhesive layer (1 µm)/back layer (7 µm). Met.

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

( Reference example 3)
A mixed resin containing 80% by weight of polyester-based resin A and 20% by weight of polyester-based resin B was used as the resin constituting the front and back layers.
As the 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.
100% by weight of polyester-based elastomer B was used as the resin constituting the adhesive layer.
These resins were put into an extruder with a barrel temperature of 190 to 255°C, extruded into a five-layer sheet through a multi-layer die at 240°C, and cooled and solidified by a take-up roll at 30°C. Next, after stretching at a stretching ratio of 5.9 times in a tenter stretching machine with 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 with a winding machine, thereby perpendicularly to the main shrinking direction. A heat-shrinkable multilayer film was obtained in which the shrinking direction was MD and the main shrinking direction was TD. The resulting heat-shrinkable multilayer film had a total thickness of 40 μm, and consisted of 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 5-layer structure of layers (5.7 μm).

( Reference example 4)
As the resin constituting the front and back layers, 100% by weight of 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.
A mixed resin containing 30% by weight of polyester-based elastomer A and 70% by weight of polystyrene-based resin D was used as the resin constituting the adhesive layer.
These resins were put into an extruder with a barrel temperature of 140 to 225°C, extruded into a five-layer sheet through a multi-layer die at 240°C, and cooled and solidified by a take-up roll at 25°C. Next, after stretching at a stretching ratio of 6.2 times in a tenter stretching machine with a preheating zone of 106° C., a stretching zone of 87 to 90° C., and a heat setting zone of 83° C., the film is taken up by a take-up machine so that the film is perpendicular to the main shrinkage direction. A heat-shrinkable multilayer film was obtained in which the shrinking direction was MD and the main shrinking direction was TD. The resulting heat-shrinkable multilayer film had a total thickness of 40 μm, and consisted of 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 5-layer structure of layers (5.7 μm).

(Example 5)
As the resin constituting the front and back layers, 100% by weight of polyester resin A was used.
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 intermediate layer.
A mixed resin containing 30% by weight of polyester-based elastomer A and 70% by weight of polystyrene-based resin D was used as the resin constituting the adhesive layer.
These resins were put into an extruder with a barrel temperature of 140 to 225°C, extruded into a five-layer sheet through a multi-layer die at 240°C, and cooled and solidified by a take-up roll at 25°C. Next, after stretching at a stretching ratio of 6.2 times in a tenter stretching machine with a preheating zone of 106° C., a stretching zone of 87 to 90° C., and a heat setting zone of 83° C., the film is taken up by a take-up machine so that the film is perpendicular to the main shrinkage direction. A heat-shrinkable multilayer film was obtained in which the shrinking direction was MD and the main shrinking direction was TD. The resulting heat-shrinkable multilayer film had a total thickness of 40 μm, and consisted of 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 5-layer structure of layers (5.7 μm).

( Reference example 6)
As the resin constituting the front and back layers, 100% by weight of 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 F was used.
A mixed resin containing 60% by weight of polyester-based elastomer B and 40% by weight of polystyrene-based resin D was used as the resin constituting the adhesive layer.
These resins were put into an extruder with a barrel temperature of 170 to 230°C, extruded into a five-layer sheet through a multi-layer die at 230°C, and cooled and solidified by a take-up roll at 28°C. Next, after stretching at a stretch ratio of 5.8 times in a tenter stretching machine with a preheating zone of 109° C., a stretching zone of 81 to 100.5° C., and a heat setting zone of 74° C., the film is wound with a winder to obtain the main shrinkage direction. A heat-shrinkable multi-layer film was obtained in which the direction perpendicular to the direction was MD and the main shrinkage direction was TD. The resulting heat-shrinkable multilayer film had a total thickness of 40 μm, and consisted of 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 5-layer structure of layers (5.7 μm).

( Reference example 7)
100% by weight of polyester-based resin D was used as the resin constituting the front and back layers.
As the resin constituting the intermediate layer, a mixed resin containing 80% by weight of polystyrene resin G and 20% by weight of polyester resin D was used.
These resins were put into an extruder with a barrel temperature of 140 to 225°C, extruded into a five-layer sheet through a multi-layer die at 240°C, and cooled and solidified by a take-up roll at 25°C. Next, after stretching in a tenter stretching machine with 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 taken up by a winder to make the film perpendicular to the main shrinkage direction. A heat-shrinkable multilayer film was obtained in which the shrinking direction was MD and the main shrinking direction was TD. The resulting heat-shrinkable multilayer film had a total thickness of 40 μm and had a three-layer structure of surface layer (4.4 μm)/intermediate layer (31.2 μm)/back layer (4.4 μm).

(Comparative example 1)
A mixed resin containing 50% by weight of polyester-based resin E and 50% by weight of polyester-based resin F was used as the resin constituting the front and back layers.
A mixed resin containing 50% by weight of polyester resin E and 50% by weight of polyester resin F was used as the resin constituting the intermediate layer.
These resins were put into an extruder with a barrel temperature of 200 to 210°C, extruded into a five-layer sheet through a multi-layer die at 240°C, and cooled and solidified by a take-up roll at 35°C. Next, after stretching at a stretching ratio of 6.5 times in a tenter stretching machine with a preheating zone of 88° C., a stretching zone of 85 to 86° C., and a heat setting zone of 86° C., the film is taken up by a winder, so that the film is perpendicular to the main shrinkage direction. A heat-shrinkable multilayer film was obtained in which the shrinking direction was MD and the main shrinking direction was TD. The resulting heat-shrinkable multilayer film had a total thickness of 40 μm and had a three-layer structure of surface layer (6.6 μm)/intermediate layer (26.8 μm)/back layer (6.6 μm).

(evaluation)
The heat-shrinkable films obtained in Examples , Reference Examples and Comparative Examples were evaluated by the following methods.
Table 1 shows the results.

(1) Phase difference (retardation)
The heat-shrinkable films obtained in Examples , Reference Examples , and Comparative Examples were cut into a size of 35 mm in the main shrinkage direction (TD direction) × 35 mm in the direction perpendicular to the main shrinkage direction (MD direction), and used as a measurement sample. .
The sample for measurement is set in the phase difference measuring device (KOBRA-WR manufactured by Oji Scientific Instruments Co., Ltd.) so that the angle defined in this measuring device is 0 °, and the incident angle is 0 ° (film surface ) was measured at a wavelength of 590 nm.

(2) Heat shrinkage A heat-shrinkable multilayer film was cut into a size of 100 mm in the main shrinkage direction (TD) x 100 mm in the direction perpendicular to the main shrinkage direction (MD), and immersed in hot water at 70°C 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 in TD of this heat-shrinkable multilayer film was measured, and the heat shrinkage rate in the TD direction was obtained according to the following formula (1). In addition, the heat shrinkage rate after immersion in warm water of 80° C. for 10 seconds was similarly measured.
Thermal shrinkage rate (%) = {(100-L) / 100} x 100 (1)

For the shrinkage rate, the average value was used with the number of samples (n)=3. The heat shrinkage rate was similarly measured for hot water at 100°C.

(3) Defect Confirmation A heat-shrinkable multilayer film of 20 cm width×100 m length was sandwiched between two polarizing films of 20 cm×20 cm, placed on a white LED light source, and illuminated from below.
While rotating the polarizing film on the upper side of the heat shrinkable film so that the absorption axes of the two polarizing films are orthogonal (cross Nicol condition), confirm whether the defect is visible for 100 m of the heat shrinkable film, Evaluation was made according to the following criteria.
◯: Defects were visible.
x: Interference fringes (rainbow color) were strong, and defects could not be visually observed even when the polarization direction was changed.

According to the present invention, it is possible to provide a heat-shrinkable multilayer film in which defects such as scratches and foreign matter present on the film surface can be easily detected, and defects such as ink loss after printing can be prevented. .

Claims (4)

A heat-shrinkable multilayer film having front and back layers containing a polyester resin and an intermediate layer containing a polystyrene resin,
The heat shrinkable multilayer film has a retardation (Re) of 0.1 to 2900 nm,
The intermediate layer further contains a polyester resin,
The content of the polyester resin in the intermediate layer is 15% by weight or more and 30% by weight or less,
A heat- shrinkable multilayer film, wherein the polystyrene-based resins constituting the intermediate layer contain at least two types of polystyrene-based resins having a Vicat softening temperature difference of 2° C or more and 20°C or less. 2. The heat-shrinkable multilayer film according to claim 1, wherein the Vicat softening temperature of the polystyrene resin constituting the intermediate layer is 60[deg.] C. or more and 85[deg.] C. or less. The front and back layers and the intermediate layer are laminated via an adhesive layer,
3. The heat-shrinkable multilayer film according to claim 1, wherein the adhesive layer contains a polyester elastomer. 3. The heat-shrinkable multilayer film according to claim 1, wherein the heat-shrinkable multilayer film has a heat shrinkage rate of 37% or more and 50% or less in the main shrinkage direction when immersed in warm water of 70[deg.] C. for 10 seconds.