JP2007276338A - Multi-layered heat-shrinkable film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multi-layered heat-shrinkable film excellent in low temperature processability and solvent resistance in printing with an oily ink, low in degradation of properties after contact with a solvent and excellent in mechanical strength and practical shrinkability. <P>SOLUTION: A multi-layered heat-shrinkable film comprises an intermediate resin composition made of a mixture of (a) a copolymer resin of a vinyl aromatic hydrocarbon and a C12-18 aliphatic unsaturated carboxylic acid alkyl ester, (b) a polymer block resin containing a polymer block mainly composed of a vinyl aromatic hydrocarbon, and a polymer block mainly composed of a conjugated diene derivative and a polystyrene resin (c), and both outer layers are made of a mixture of (b) and (a) and/or (c). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a copolymer of a vinyl aromatic hydrocarbon and a high molecular weight aliphatic unsaturated carboxylic acid acrylic ester having a specific alkyl group carbon number, or a specific amount of liquid paraffin added to the copolymer. Low-temperature processability (film stretchability, shrinkability), oiliness, using a mixture of resin, vinyl aromatic hydrocarbon and conjugated diene block copolymer, or a mixture of polystyrene resin in the intermediate layer or both outer layers The present invention relates to a multilayer heat shrinkable film excellent in solvent resistance at the time of printing with an ink, and further having little deterioration in physical properties after contact with a solvent and excellent in mechanical strength and practical shrinkage.

  As a heat shrink film for shrink labels such as PET bottles, a copolymer of a vinyl aromatic hydrocarbon and an aliphatic unsaturated carboxylic acid alkyl ester, a vinyl aromatic hydrocarbon having a relatively high vinyl aromatic hydrocarbon content, A multilayer film using a mixture of a conjugated diene block copolymer as an intermediate layer and a resin mainly composed of a vinyl aromatic hydrocarbon having a relatively high vinyl aromatic hydrocarbon content and a conjugated diene block copolymer as both outer layers, Several proposals have been made for its excellent properties such as transparency, impact resistance, and low-temperature shrinkage.

  For example, in Patent Document 1 below, in order to obtain a multilayer polystyrene heat-shrinkable film having reduced natural shrinkage and improved strength, surface characteristics, waist strength, low-temperature shrinkage, etc., both outer layers are made of styrene. -Both outer layers using a mixture of butadiene-styrene block copolymer and styrene-butyl acrylate copolymer, and using a mixture of styrene-butadiene-styrene block copolymer and styrene-butyl acrylate copolymer for the intermediate layer A multilayer polystyrene heat-shrinkable film is disclosed in which the ratio of the mixture and the butadiene content are defined for each of the intermediate layers and the butadiene content of the entire resin.

  In Patent Document 2 below, both outer layers are used in order to obtain a multilayer polystyrene-based heat-shrinkable film that reduces the natural shrinkage ratio and improves the strength, surface characteristics, and waist strength, and particularly improves the low-temperature shrinkability. A styrene-butadiene-styrene block copolymer and a styrene-butyl acrylate copolymer, or a mixture of polystyrene, and a styrene-butadiene-styrene block copolymer and a styrene-butyl acrylate copolymer in the intermediate layer, and Disclosed is a three-layer polystyrene heat-shrinkable film using a styrene-butadiene-styrene type block copolymer hydrogenated mixture and defining the ratio of the mixture and the butadiene content in each of the outer layer and the intermediate layer. Yes.

  In Patent Document 3 below, in order to obtain a multilayer polystyrene-based heat-shrinkable film having reduced natural shrinkage and improved strength, surface characteristics, waist strength, low-temperature shrinkage, etc., both outer layers have SBS blocks. A mixture of polymer and polystyrene was used, a mixture of SBS block copolymer and styrene-butyl acrylate copolymer was used for the intermediate layer, and the ratio of the mixture and the butadiene content were defined for each of the outer layer and the intermediate layer. Furthermore, a multilayer polystyrene-based heat shrink film is disclosed in which the butadiene content of the entire resin is defined.

Patent Document 4 listed below discloses a polystyrene-based heat-shrinkable film that has good shrinkage finish even in a field that requires heat shrinkage specification at a low temperature, has a low natural shrinkage, and does not block between films when heated. In order to obtain a styrene-acrylic acid ester copolymer having a Vicat softening point defined in the intermediate layer or a styrene-acrylic acid ester copolymer as a main component, a styrene having a Vicat softening point defined in both outer layers. -A multilayer polystyrene-based heat-shrinkable film is disclosed which is a multi-layer film mainly composed of a conjugated butadiene block copolymer and which defines a heat shrinkage rate in the main stretching direction. In addition, the styrene-acrylic acid ester copolymer used in Examples and the like is styrene-n-butyl acrylate (carbon number C _{4 of} alkyl group).

However, the acrylic acid ester of the styrene-acrylic acid ester copolymer used in Patent Documents 1 to 4 is a butyl acrylate (n-butyl acrylate) having a relatively small number of carbon atoms in the alkyl group and a C ₄ carbon number. It is. In general, a multilayer polystyrene heat-shrink film used for containers and packaging is subjected to gravure printing from the viewpoint of design, content display, advertisement, and the like. When the styrene-butyl acrylate copolymer is printed with an oil-based printing ink, the styrene-butyl acrylate copolymer is relatively free from solvents used in the printing ink (generally esters such as ethyl acetate and alcohols such as isopropyl alcohol). Films that are easily eroded and use styrene-butyl acrylate copolymer for both outer layers (or inner and outer layers) and intermediate layers may cause breakage when winding the film after printing, or poor appearance due to rough surface of the printed surface. In some cases, improvements are desired.
JP 2000-185373 A JP 2000-238192 A JP 2000-94598 A JP 2002-46231 A

  The present invention relates to a copolymer of a vinyl aromatic hydrocarbon and a high molecular weight aliphatic unsaturated carboxylic acid acrylic ester having a specific alkyl group carbon number, or a specific amount of liquid paraffin added to the copolymer. Low-temperature processability (film stretchability, shrinkability), oiliness, using a mixture of resin, vinyl aromatic hydrocarbon and conjugated diene block copolymer, or a mixture of polystyrene resin in the intermediate layer or both outer layers An object of the present invention is to provide a multilayer heat-shrinkable film which has excellent solvent resistance during printing with ink, and has little deterioration in physical properties after contact with a solvent, and has excellent mechanical strength and practical shrinkage.

As a result of diligent research in view of the above problems, the present inventor has obtained a copolymer of a vinyl aromatic hydrocarbon, an alkyl group having a carbon number C _{12 to} C ₁₈ and an aliphatic unsaturated carboxylic acid alkyl ester having a large molecular weight. Or a specific amount of liquid paraffin added to the copolymer, a resin having an aliphatic unsaturated carboxylic acid alkyl ester content and a Vicat softening temperature in a specific range, and a vinyl aromatic carbonization having a Vicat softening temperature in a specific range. A mixture of a block resin composed of hydrogen and a conjugated diene, or a polystyrene resin, and further a solvent swelling ratio in a 25 ° C. solvent (ethyl acetate 40 wt% and isopropyl alcohol 60 wt%) immersion (10 minutes). It is found that a multilayer heat-shrinkable film having excellent properties that could not be expected until now can be obtained by using Which resulted in the completion of the invention.

That is, the present invention has the following configuration.
(1) The resin of the intermediate layer is a resin (a) comprising 10 to 95% by weight of a copolymer of a vinyl aromatic hydrocarbon and an alkyl unsaturated aliphatic carboxylic acid alkyl ester having C _{12 to} C ₁₈ carbon atoms. , A mixture of 90 to 5% by weight of a resin (b) consisting of a polymer block mainly composed of vinyl aromatic hydrocarbon and a polymer block mainly composed of a conjugated diene derivative, and 0 to 5% by weight of polystyrene resin (c) And (b) 90 to 100% by weight of a resin (b) consisting of a polymer block mainly composed of vinyl aromatic hydrocarbon and a polymer block mainly composed of a conjugated diene derivative; And a resin (a) and / or a polystyrene resin (c) 10 comprising an aliphatic unsaturated carboxylic acid alkyl ester having an alkyl group of C _{12 to} C ₁₈ carbon atoms.
A multilayer heat-shrinkable film consisting of a mixture with 0% by weight.
(2) The vinyl aromatic hydrocarbon forming the resin (a) is made of styrene, and the alkyl group having an alkyl group having C _{12 to} C ₁₈ aliphatic unsaturated carboxylic acid alkyl ester is lauryl (meth) acrylate, The multilayer heat-shrinkable film according to (1), comprising at least one compound selected from tridecyl acrylate).
(3) The vinyl aromatic hydrocarbon forming the resin (b) is made of styrene, and the conjugated diene derivative is at least one selected from 1,3-butadiene and 2-methyl-1,3-butadiene (isoprene). The multilayer heat-shrinkable film according to (1) or (2), comprising a compound.
(4) The resin of the intermediate layer is immersed in a solvent (ethyl acetate 40% by weight and isopropyl alcohol 60% by weight) at 25 ° C. (10 minutes), and the solvent swelling ratio is less than 6% by weight (1 The multilayer heat-shrinkable film according to any one of (3) to (3).
(5) A copolymer in which the resin (a) has a vinyl aromatic hydrocarbon content of 78 to 95% by weight and an alkyl unsaturated carbonic acid alkyl ester having a carbon number of C _{12 to} C ₁₈ of 22 to 5% by weight. The multilayer heat shrinkage according to any one of (1) to (4), wherein 0 to 5 parts by weight of liquid paraffin is added to 100 parts by weight, and the Vicat softening temperature is 60 to 85 ° C. Sex film.
(6) The resin (b) has a polymer block mainly composed of at least one vinyl aromatic hydrocarbon and a polymer block polymerized mainly based on at least one conjugated diene derivative, and is a vinyl aromatic hydrocarbon. The multilayer according to any one of (1) to (5), wherein the content is 60 to 95% by weight, the conjugated diene derivative content is 40 to 5% by weight, and the Vicat softening temperature is 65 to 93 ° C. Heat shrinkable film.
(7) The multilayer heat shrinkage according to any one of (1) to (6), wherein an absolute value of a difference in refractive index between the resin (a) and the resin (b) is 0.01 or less. Sex film.
(8) Stretching in the main stretching direction is 3 to 8 times, stretching in the orthogonal direction to the main stretching direction is 1 to 1.5 times, and shrinkage rate in 10 seconds of 75 ° C. hot water in the main stretching direction is 15 to 80%. Any one of (1) to (7), wherein the ratio of the shrinkage in 10 seconds of 90 ° C. hot water / shrinkage in 10 seconds of 75 ° C. hot water is 1.5 to 4.0. The multilayer heat-shrinkable film described.
(9) A container in which the multilayer heat-shrinkable film according to any one of (1) to (8) is heat-shrink mounted.

  The multilayer heat-shrinkable film of the present invention is excellent in low-temperature processability and solvent resistance during printing with oil-based ink, and further has little deterioration in physical properties after contact with a solvent, and is excellent in mechanical strength and practical shrinkability. The multilayer heat-shrinkable film of the present invention is particularly excellent in low-temperature shrinkability, so that it can be mounted on PET bottles (such as for aseptics) and polyethylene bottles that are inferior in heat resistance without deforming the bottles. is there.

  Hereinafter, the present invention will be described in detail.

In the present invention, the resin (a) mainly used for the intermediate layer includes styrene, α-methyl styrene, o-methyl styrene, m-methyl styrene, p-methyl styrene and the like as the vinyl aromatic hydrocarbon. Particularly common is styrene. These may be used alone or in combination of two or more. Examples of the aliphatic unsaturated carboxylic acid alkyl ester having C _{12 to} C ₁₈ carbon atoms of the alkyl group include lauryl acrylate, tridecyl acrylate, tetradecyl acrylate, pentadecyl acrylate, hexadecyl acrylate, palmitic acid acrylate, stearyl acrylate, Examples include lauryl methacrylate, tridecyl methacrylate, tetradecyl methacrylate, pentadecyl methacrylate, hexadecyl methacrylate, palmitic acid methacrylate, and stearyl methacrylate. Of these, particularly preferred are lauryl (meth) acrylate and tridecyl (meth) acrylate. These alkyl (meth) acrylates may be used alone or in combination of two or more. Further, if the Vicat softening temperature is within a desired range and the solvent swelling rate is less than 6% by weight (10 minutes) after immersion in a solvent (ethyl acetate 40% by weight and isopropyl alcohol 60% by weight) at 25 ° C., the vinyl aroma The (meth) acrylic acid alkyl ester having an alkyl group with a carbon number of C _{1 to} C ₁₁ of less than 1% by weight based on the total amount of the group hydrocarbon and the (meth) acrylic acid alkyl ester can be added. Furthermore, in addition to vinyl aromatic hydrocarbons and (meth) acrylic acid alkyl esters, other vinyl monomers copolymerizable therewith, such as acrylonitrile and methacrylic acid, can be added as appropriate.

  The Vicat softening temperature of the resin (a) is preferably 60 to 85 ° C, more preferably 65 to 83 ° C, and still more preferably 70 to 80 ° C. If it is less than 60 ° C., the stretched film obtained from the mixture of the resin (a) and the resin (b) may not be used because the stretch film has a large natural shrinkage rate at a relatively high temperature. Furthermore, it becomes difficult to cool the resin during the production of the resin (a), and the productivity may be lowered. On the other hand, when the temperature exceeds 85 ° C., the amount of the aliphatic unsaturated carboxylic acid alkyl ester introduced is relatively small, and the processability at low temperature is lowered and the low temperature shrinkability is remarkably lowered. The addition of liquid paraffin can reduce the amount of aliphatic unsaturated carboxylic acid alkyl ester introduced when the Vicat softening temperature is kept constant, and has the effect of improving solvent resistance. However, if it is added too much, the plasticizer may be deposited at the outlet of the die of the extruder at the time of extrusion and molding for a long period of time, which is not preferable. The amount of liquid paraffin added is preferably 0 to 5 parts by weight, more preferably 1 to 4 parts by weight with respect to 100 parts by weight of the resin (a). The composition of the resin (a) for obtaining the above-mentioned Vicat softening temperature by combining the liquid paraffin addition amount and the aliphatic unsaturated carboxylic acid alkyl ester content preferably has an aliphatic unsaturated carboxylic acid alkyl ester content of 22-5. The vinyl aromatic hydrocarbon content is 78 to 95% by weight, more preferably 80 to 94% by weight, more preferably 20 to 6% by weight.

  The liquid paraffin used in the present invention is a known liquid paraffin (also referred to as white oil) which is defined as a mixture of liquid saturated hydrocarbons of extremely high purity belonging to the lubricating oil fraction in terms of boiling point. There is no problem. For example, trade names White Rex 307, 309, 326, 335 made by Mobil Oil, Prime All N382 made by Exxon Mobil, trade names Smoyl Moresco Biores P-260, P-300, P-made by MORESCO As an example, 350, trade names of Plastic Oil 250, 350, 360, 380, and 550 manufactured by Chemtura can be given. In use, the initial distillation temperature under a reduced pressure of 1.3 kPa is preferably 220 ° C. or higher, more preferably 230 ° C. or higher from the viewpoint of contamination at the die outlet.

In the resin (a), the weight average molecular weight excluding a molecular weight of 1000 or less is preferably from 200 to 470,000, more preferably from 2 to 450,000, still more preferably from 25 to 420,000. The ratio of weight average molecular weight / number average molecular weight is preferably 1.5 to 3.8, more preferably 1.7 to 3.5, and still more preferably 1.8 to 3.3. The ratio of Z average molecular weight / weight average molecular weight is preferably 1.4 to 3.6, more preferably 1.5 to 3.2, and still more preferably 1.8 to 2.8. The melt flow rate is preferably 0.5 to 50 g / 10 minutes, more preferably 2 to 40 g / 10 minutes, and further preferably 3 to 30 g / 10 minutes. By setting the molecular weight, each molecular weight ratio, and the melt flow rate within these ranges, a high-strength stretched film with less thickness unevenness can be obtained by low-temperature processing from the film of the present invention. Further, even in the heat shrinkage, the film has a uniform shrinkage property, and the obtained film has an excellent mechanical strength, which is preferable. The film of the present invention exhibits excellent paintability (printability) and solvent resistance with almost no change in surface appearance even when coated with an oil-based printing ink or immersed in an ink solvent. In addition, when a weight average molecular weight exceeds 470,000, the dispersibility of resin (b) tends to deteriorate with a mixture with resin (b), and it may be difficult to obtain a film with excellent surface appearance. Furthermore, the difference in heat shrinkage between the low temperature and the high temperature becomes large, and when a film is attached to a container such as a PET bottle and heat shrinks, wrinkles are likely to occur, which is not preferable. On the other hand, if it is less than 200,000, the decrease in mechanical strength is unfavorable.

  The method for producing the vinyl aromatic hydrocarbon and the aliphatic unsaturated carboxylic acid alkyl ester resin of the present invention is a known method for producing a styrenic resin, such as bulk polymerization, solution polymerization, suspension polymerization, emulsion weight Legal or the like can be used.

  The resin (b) used for the intermediate layer and both outer layers is obtained by polymerizing a vinyl aromatic hydrocarbon and a conjugated diene in an organic solvent using an organolithium compound as an initiator. Examples of the vinyl aromatic hydrocarbon used in the resin (b) include styrene, α-methyl styrene, o-methyl styrene, m-methyl styrene, and p-methyl styrene. Particularly common is styrene. Can be mentioned. These may be used alone or in combination of two or more.

  The conjugated diene is a diolefin having a pair of conjugated double bonds, such as 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene), 2,3-dimethyl-1,3-butadiene. 1,3-pentadiene, 1,3-hexadiene, etc. These may be used alone or in combination of two or more. Particularly preferred are 1,3-butadiene and isoprene, and the weight ratio of 1,3-butadiene to isoprene in the block copolymer is 97/3 to 20/80, preferably 90/10 to 25/75, Preferably 85/15 to 30/70. By setting the amount within this range, a copolymer with little fish eye can be obtained.

With respect to the resin (b), a block bonding method represented by the following formula is exemplified. Assuming that a block segment mainly composed of vinyl aromatic hydrocarbons is S and a block segment mainly composed of conjugated dienes is D, one group thereof is:
(DS) _{n + 1} , (DS) _n- D, S- (DS) _n (where n = 1 to 10)
A linear block copolymer having a basic structure represented by formula (1), and its production method is based on a means for block copolymerization in a hydrocarbon solvent using an organic lithium-based polymerization initiator.

The next group includes [(DS) _n ] _{m + 2-} X, [(SD) _n ] _{m + 2-} X,
[(SD) _n- S] _{m + 2-} X, [(DS) _n- D] _{m + 2-} X
Non-linear block copolymers having a branched basic structure such as (Where n = 1 to 10, m = 1 to 10), (X represents a residue of a polyfunctional initiator, for example, the initiator is SiCl ₄ , SnCl ₄ polyfunctional organolithium compound, polyepoxide, polyisocyanate. , Polyaldehyde, polyketone, tetraallyl Sn, etc.).

  A preferable aspect among the above is n = 1 to 5, preferably n = 1 to 3, and more preferably n = 1 to 2 in the linear block copolymer. In the case of a non-linear block copolymer, m = 1 to 5 and n = 1 to 5, preferably m = 1 to 3, n = 1 to 3, more preferably m = 1 to 2 and n = 1 to 2. 2. The resin (b) may be an arbitrary mixture of block copolymers having a structure represented by the above general formula.

  These production methods are disclosed in, for example, Japanese Patent Publication Nos. 36-19086, 43-14799, 48-2423, 48-4106, 49-36957, and 51-27701. Although known, in the present application, the above-mentioned specific range is used. In addition, at least a part of the polymer molecular structure of each group described above has a random structure or a tapered random structure or block structure in which a component composed of both monomers gradually decreases or increases the ratio of both. It also includes a copolymer having a copolymer, a copolymer containing another copolymerizable monomer, or a modified polymer.

  In the resin (b), the number average molecular weight of the block mainly composed of vinyl aromatic hydrocarbon is preferably 5000 or more, more preferably 7000 to 100,000, and still more preferably about 10,000 to 80,000. The number average molecular weight of the block mainly composed of conjugated diene is preferably about 5,000 to 200,000, more preferably about 7,000 to 100,000, and still more preferably about 10,000 to 100,000. Furthermore, the number average molecular weight of the whole block copolymer is preferably 20,000 to 500,000, more preferably 20,000 to 400,000, and still more preferably 30,000 to 300,000. By using a block copolymer having a molecular weight in such a range, a sheet or film excellent in mechanical strength, workability, and low-temperature shrinkage can be obtained. A particularly preferred block copolymer has a melt flow rate (200 ° C., load 49 N) of 0.1 to 50 g / 10 minutes, preferably 1 to 20 g / 10 minutes from the viewpoint of molding.

  The molecular weight was measured by gel permeation chromatography (hereinafter referred to as GPC) in terms of polystyrene. The molecular weight of the resin (b) block mainly composed of vinyl aromatic hydrocarbon is obtained by GPC from the vinyl aromatic hydrocarbon polymer block component obtained by the oxidative decomposition method using osmium tetroxide and peroxide described later. It was.

  The composition of the resin (b) has a conjugated diene content of 40 to 5% by weight, preferably 37 to 10% by weight, more preferably 32 to 15% by weight, and a vinyl aromatic hydrocarbon content of 60 to 95% by weight, preferably Is 63 to 90% by weight, more preferably 68 to 85% by weight. When the conjugated diene content exceeds 40% and the vinyl aromatic hydrocarbon content is less than 60% by weight, the mechanical strength is excellent, but the elastic modulus is low, and gel formation in an extruder or the like increases, The rough surface of the film is noticeable and undesirable. Furthermore, the compatibility with the resin (a) is poor, and the transparency tends to decrease. On the other hand, when the conjugated diene content is less than 5% by weight and the vinyl aromatic hydrocarbon content exceeds 95% by weight, the rubber component is decreased, and the mechanical strength such as film strength and waist strength is unfavorable.

  The Vicat softening temperature of the resin (b) is preferably 65 to 93 ° C, more preferably 70 to 90 ° C, and even more preferably 75 to 85 ° C. If it is less than 65 degreeC, a tensile elasticity modulus will become low too much with the film which consists of resin (a). On the other hand, when it exceeds 93 ° C., the low-temperature shrinkage is inferior, which is not preferable. In particular, the outer layer is preferably 75 ° C. or higher from the viewpoint of blocking resistance.

  The residual amount of the vinyl aromatic hydrocarbon monomer in the multilayer film of the present invention is preferably 0.1% by weight or less, more preferably 0.05% by weight or less, and further preferably 0.02% by weight or less. The total remaining amount of the aliphatic unsaturated carboxylic acid alkyl ester monomer is preferably 1% by weight or less, more preferably 0.55% by weight or less, and still more preferably 0.2% by weight or less. When the residual amount of vinyl aromatic hydrocarbon monomer exceeds 0.1% by weight, or when the residual amount of aliphatic unsaturated carboxylic acid alkyl ester monomer exceeds 1% by weight, film extrusion Odor may be felt at the time of injection or injection molding. Note that the difference in concentration between the vinyl aromatic hydrocarbon monomer and the aliphatic unsaturated carboxylic acid alkyl ester monomer with respect to odor is largely due to the difference in vapor pressure.

The resin of the intermediate layer mainly composed of the resin (a) and the resin (b) is immersed in a solvent (40% by weight of ethyl acetate and 60% by weight of isopropyl alcohol) at 25 ° C. (10 minutes), and preferably has a solvent swelling ratio of 6 Less than 1% by weight. More preferably, it is 5.5 weight% or less, More preferably, it is 5.0 weight% or less. When the solvent swell ratio is 6% by weight or more, when printing or coating on a film with inks using these solvents, mechanical defects occur due to appearance defects due to rough skin on the printed or painted surface and fine cracks due to solvent penetration. It may cause a decrease in strength, which is not preferable. For example, when winding a thin film while printing, there is a case where it breaks, which is not preferable. Even when the outer layer is coated with a resin (b) that is relatively strong against these solvents, the physical properties are deteriorated due to the penetration of the solvent, and the more the both outer layers are thinner, the more remarkable. Moreover, when adding resin (a) to both outer layers, the range of the above-mentioned solvent swelling rate is preferable. In general, an ester solvent or an alcohol solvent is used as an ink solvent. In particular, ethyl acetate is frequently used in the ester system, and isopropyl alcohol is frequently used in the alcohol system.

  The resin composition of the intermediate layer is preferably resin (a) 10 to 95% by weight, resin (b) 90 to 5% by weight, polystyrene resin (c) 0 to 5% by weight, more preferably resin (a) 35 to 90%. % By weight, resin (b) 65 to 10% by weight, polystyrene resin (c) 0 to 3% by weight. When the resin (a) exceeds 95% by weight, the rigidity is excellent, but the mechanical strength of the film is lowered, which is not preferable. On the other hand, if the resin (a) is less than 10% by weight, the elastic modulus becomes too low, which may be a problem in automatic packaging of shrink labels. Polystyrene resin (c) is effective in adjusting the stiffness and shrinkage of the film, but the refraction of resin (a) and resin (b) is greatly different, and the transparency of the film exceeds 5% by weight. Tends to decrease.

  The absolute value of the difference in refractive index between the resin (a) and the resin (b) that constitutes the intermediate layer is preferably 0.01 or less and 0.00 or more, more preferably 0.008 or less, and still more preferably 0.006. It is as follows. When the absolute value of the difference in refractive index exceeds 0.01, the transparency of mixing tends to be remarkably lowered, which is not preferable. In many cases, a shrink film such as a PET bottle and a label are printed and used, but this printing is generally performed on the back surface (the surface of the PET bottle main body) so that the printed display does not disappear. For this reason, transparency is an important factor for availability.

  Various polymers and additives can be added to the resin of the intermediate layer or both outer layers depending on the purpose. Suitable polymers include block copolymer elastomers of vinyl aromatic hydrocarbons and conjugated dienes or hydrogenated products thereof, rubber-modified styrene polymers, non-rubber-modified styrene polymers, rubber-modified styrene- (meth) acrylic. Acid ester polymers and the like.

  The resin composition of both outer layers is resin (b) 90-100% by weight, resin (a) and / or polystyrene resin (c) 10-0% by weight, preferably resin (b) 93-100% by weight, resin (a ) And / or polystyrene resin (c) 7 to 0% by weight, more preferably resin (b) 95 to 100% by weight, and resin (a) and / or polystyrene resin (c) 5 to 0% by weight. Polystyrene resin (c) is effective in adjusting the waist strength of the film, preventing blocking, etc., but is significantly different from resin (b) in refraction, and if it exceeds 10% by weight, the transparency of the film is lowered, which is not preferable. . The polystyrene resin (c) has a weight average molecular weight of 200,000 to 450,000, more preferably 230,000 to 400,000, and still more preferably 250,000 to 350,000. The addition amount of the resin (a) was such that the resin in both outer layers was immersed in a solvent (ethyl acetate 40 wt% and isopropyl alcohol 60 wt%) at 25 ° C. (10 minutes), and the solvent swelling rate was less than 6 wt% 1 wt. % Can be suitably added within the range of at least%.

  In addition, when using resin (a) for an intermediate | middle layer and both outer layers at the time of production of a multilayer film, the thing of the same composition or a different composition can be used in an intermediate | middle layer and both outer layers. Moreover, the same layer and different compositions can be mixed and used. The same applies when the resin (b) is used for the intermediate layer and both outer layers.

  In the heat-shrinkable film of the present invention, the ratio of the thickness of the intermediate layer to the whole film is preferably 50 to 95%, more preferably 55 to 90%, and still more preferably 55 to 85%. If the ratio of the intermediate layer is less than 50%, the natural shrinkage rate is undesirably increased. On the other hand, if it exceeds 95%, the elongation at low temperature is undesirably lowered. Further, the thickness of the resin layer (b) as a main component of both outer layers is reduced, which is not preferable because it is easily affected by the ink solvent. The total thickness of the film is selected in the range of 20 to 70 μm, preferably 30 to 60 μm.

The stretching ratio of the film is preferably 3 to 8 times the stretching in the main stretching direction (referred to as a direction orthogonal to the film extrusion direction) and the stretching in the orthogonal direction (referred to as the film extrusion direction) in the main stretching direction. It is 1 to 1.5 times, more preferably, the stretching in the main stretching direction is 4 to 6 times, and the stretching in the orthogonal direction to the main stretching direction is 1.05 to 1.2 times. When the draw ratio in the main drawing direction exceeds 8 times, the thickness unevenness of the intermediate layer and both outer layers tends to increase, which is not preferable. On the other hand, if it is less than 3 times, the contracted film does not sufficiently adhere to the container and tends to lack adhesion. Further, when the stretching in the direction perpendicular to the main stretching direction exceeds 1.5 times, the film shrinkage rate in the height direction of the container becomes large after being attached to a container such as a PET bottle and thermally shrinking, which is not preferable.

  The shrinkage rate of the film is preferably 15% to 80% in the main stretching direction for 10 seconds in 75 ° C. hot water, 10 seconds in 90 ° hot water in the main stretching direction / 10 seconds in 75 ° C. hot water. The ratio of shrinkage ratio is 1.5 to 4.0, more preferably, the shrinkage ratio of 10 seconds in 75 ° C. hot water is 25 to 75% in the main stretching direction, and the shrinkage ratio in 10 seconds of 90 ° C. hot water in the main stretching direction / The ratio of shrinkage in 10 seconds of 75 ° C. hot water is 1.5 to 3.0, and even more preferably, the shrinkage of 10 seconds in 75 ° C. hot water is 30 to 75% in the main stretching direction and 90 ° C. in the main stretching direction. The ratio of shrinkage in hot water 10 seconds / shrinkage in 70 ° C. hot water 10 seconds is 1.5 to 2.4. When the heat shrinkage rate in 10 seconds of 75 ° C. hot water is less than 15%, the shrunken film does not sufficiently adhere to the container and tends to lack adhesion. On the other hand, when it exceeds 80%, it shrinks too much and the container may be deformed, which is not preferable. In addition, when the ratio of the heat shrinkage ratio of 10 seconds of 90 ° C. hot water / heat shrinkage ratio of 10 seconds of 75 ° C. hot water exceeds 4, wrinkles occur at the constricted portions where the outer diameters of the shrunken film containers are different. It is easy and not preferable.

  The method for producing the multilayer heat-shrinkable film of the present invention is not particularly limited, and after producing a multilayer film by a method conventionally used in the production of a multilayer polystyrene-based heat-shrinkable film, for example, a coextrusion method, uniaxial stretching It is possible to use a stretching method by a method, a sequential biaxial stretching method, a simultaneous biaxial stretching method, a tube stretching method, or the like. In order to produce a multilayer film by the coextrusion method, first, a styrene resin composition for forming both outer layers and a styrene resin composition for forming an intermediate layer are prepared.

  The resin of the intermediate layer is a blender such as a ribbon blender or a Henschel mixer, in which a resin (a), a resin (b) and / or polystyrene (c) and various additive components used as required are blended in a predetermined ratio. By mixing using, or after mixing, by melt-kneading using a single screw extruder or a multi-screw extruder, or by kneading using a kneader such as a kneader, Banbury mixer, roll, Can be prepared. Further, as the resin of both outer layers, resin (b) and optionally used resin (a) and / or polystyrene resin (c) are blended at a predetermined ratio, and mixed or melt-kneaded in the same manner as described above. Can be prepared.

  The film of the present invention can be produced by a known method such as a flat method or a tubular method. For example, in the case of the flat method, the resin is melted using a plurality of extruders, co-extruded from a T die, taken up by a take-up roll, stretched in the vertical direction, stretched in the tenter direction in the horizontal direction, annealed, and cooled. And the method of obtaining a film by winding up with a winder can be illustrated. There is no restriction | limiting in particular as extending | stretching temperature, Generally it is the range of 70-110 degreeC.

Resins used for the intermediate layer and both outer layers include 2- [1- (2-hydroxy-3,5-di-t-pentylphenyl) ethyl] -4,6-di-t-pentylphenyl acrylate, 2-t -Butyl-6- (3-tert-butyl-2-hydroxy-5-methylbenzyl) -4-methylphenyl acrylate, 2,4-bis [(octylthio) methyl] -o-cresol, 5,7-di- tert-Butyl-3- (3,4-dimethylphenyl) -3H-benzofuran-2-one, 3,4-dihydro-2,5,7,8-tetramethyl-2- (4,8,12-trimethyl) (Tridecyl) -2H-benzopyran-6-ol, at least one stabilizer selected from 0.02 to 3 parts by weight, more preferably 100 parts by weight of the resin composition of each of the intermediate layer and the outer layer. Is 0 By adding 05 to 2 parts by weight, it is possible to suppress fisheyes that occurs when creating a sheet, a film extruder.

  Further, n-octadecyl 3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate, 2,4-bis [(octylthio) methyl] -o-cresol, tetrakis [methylene-3- (3, 5-di-t-butyl-4-hydroxyphenyl) propionate] methane, 1,3,5-trimethyl-2,4,6-tris (3,5-di-t-butyl-4-hydroxybenzyl) benzene, etc. At least one phenol-based stabilizer of 0.02 to 3 parts by weight, tris- (nonylphenyl) phosphite, 2,2-methylenebis (4,6-di-t-) per 100 parts by weight of the resin At least one organic phosphate stabilizer such as butylphenyl) octyl phosphite, tris (2,4-di-t-butylphenyl) phosphite, Relative to 100 parts by weight each of the resin composition layer may be added 0.02 to 3 parts by weight.

  Examples of suitable additives for the resin used for the intermediate layer and both outer layers include coumarone-indene resins, terpene resins, softeners such as oil, and plasticizers. Various stabilizers, pigments, antiblocking agents, antistatic agents, lubricants and the like can also be added. Examples of the antiblocking agent, antistatic agent, and lubricant include fatty acid amide, ethylene bisstearamide, sorbitan monostearate, saturated fatty acid ester of fatty acid alcohol, pentaerythritol fatty acid ester, etc. -T-butylphenyl salicylate, 2- (2'-hydroxy-5'-methylphenyl) benzotriazole, 2- (2'-hydroxy-3'-t-butyl-5'-methylphenyl) -5-chlorobenzo Triazole, 2,5-bis- [5′-t-butylbenzoxazolyl- (2)] thiophene, and the like, compounds described in “Practical Handbook for Additives for Plastics and Rubber” (Chemical Industry Co., Ltd.) can be used. . These are generally used in the range of 0.01 to 5% by weight, preferably 0.05 to 3% by weight.

  The container referred to in the present invention refers to various plastic bottles and glass bottles as well as various molded containers and other containers to which the heat-shrinkable film of the present invention can be applied. However, application to a container having poor heat resistance or a container filled with a medium product for which high temperatures are to be avoided is preferable from the viewpoint of best utilizing the characteristics of the film of the present invention.

  EXAMPLES Next, although an Example and a comparative example demonstrate this invention in detail, this invention is not limited to these Examples.

In the present invention, the following measurement method and evaluation method were used. (1) Measurement of Vicat softening temperature Measurement was performed according to ASTM D-1525. The load was 9.8 N, and the heating rate was 2 ° C./min.
(2) Measurement of melt flow rate It was measured according to ISO 1133 (200 ° C., load 49 N).
(3) Measurement of refractive index The refractive index was measured at 25 ° C. according to JIS K7015 using an appe refractometer.
(4) Preparation of liquid paraffin in resin (a) Sample preparation: 2 g of copolymer is dissolved in 15 ml of methyl ethyl ketone, and then 15 ml of ethanol is added and further dissolved. 10 ml of the supernatant was collected, 10 ml of tetrahedrofuran (THF) was added, stirred and measured.
Instrument: High-performance liquid chromatograph HLC-802A, Column: JASCO Finepak
Two GEL 101, solvent: tetrahydrofuran (THF), temperature: column thermostat 38 ° C., RI detector 38 ° C., devolatilization tank 45 ° C., flow rate: 0.6 ml / min, injection amount: 500 μl.
(5) Measurement of molecular weight and molecular weight distribution The molecular weight of the resin (a) and the resin (b) was measured by dissolving the resin in a tetrahydrofuran solvent and using gel permeation chromatography (GPC). The weight average molecular weight, number average molecular weight, and MZ molecular weight were determined in terms of polystyrene based on a calibration curve composed of monodispersed polystyrene. The molecular weight of the styrene block in the resin (b) is determined by a method in which the block copolymer is oxidatively decomposed with osmium tetroxide and tertiary butyl hydroperoxide (IM KOLTHOFF, et al., J Polym.Sci.1, 429 (1946)) was obtained by measuring the vinyl aromatic hydrocarbon polymer block component obtained by GPC.
Measurement conditions Sample preparation: About 1000 ppm of the copolymer was dissolved in tetrahydrofuran, and the injection amount was 100 μl.
Equipment: Shodex System 21 manufactured by Showa Denko Co., Ltd. Column: Sample: KF-806L 2 pieces, Reference: KF-800RL 2 pieces, Temperature: 40 ° C., Carrier: THF 1 ml / min (6) Solvent swelling rate measurement Used for intermediate layer A disk-shaped test piece having a diameter of 25 mmφ and a thickness of 0.8 mm was prepared from the resin composition with a compression molding machine, and immersed in a solvent at 25 ° C. (ratio of ethyl acetate and isopropyl alcohol 40/60) for 10 minutes. The solvent swelling ratio was measured by the following formula.

Solvent swelling rate (%) = (W2 / W1-1) × 100
W1: Specimen weight before immersion, W2: Specimen weight after immersion (7) Printed film making method for measuring tensile modulus and tensile fracture nominal strain Table 3-4 as resins for the intermediate layer and both outer layers Were blended at a composition blending ratio (resin (a), resin (b), other resin) and extruded with a twin screw extruder to produce pellets. Using this pellet, a three-layer sheet having a thickness of about 300 μm was prepared by coextrusion so that the ratio of the thickness of the outer layer / intermediate layer / outer layer was 10/80/10. Subsequently, this sheet was heated with a batch type tenter (EX6-S1 manufactured by Toyo Seiki Co., Ltd.) at a temperature of Vicat softening temperature of resin (a) + 15 ° C., 5 times in the direction perpendicular to the sheet extrusion direction, and 1. in the sheet extrusion direction. A film having a thickness of about 50 μm stretched twice was obtained. In addition, when Vicat softening temperature +15 degreeC exceeded 110 degreeC, 110 degreeC was made into the upper limit of heating temperature. The coating was applied to one side of this film once using a bar coater (RDS # 5: leveling to 10 μm when wet), dried at 35 ° C. for 3 hours under reduced pressure of 1.3 kPa (10 mmHg), and then the desired size. A test piece for evaluation was cut out (film after printing). Similarly, a test piece was cut out from an untreated film before printing (film before printing).

The coating ink used was a mixture of 72 parts by weight of paint SY390 manufactured by Toyo Ink Co., Ltd. and 28 parts by weight of ethyl acetate / isopropyl alcohol. The ratio of 28 parts by weight of ethyl acetate / isopropyl alcohol was determined by mixing the acetic acid after mixing. The weight ratio of ethyl / isopropyl alcohol was adjusted to 40/60.
(8) Measurement of tensile elastic modulus and tensile fracture nominal strain Using the test specimen for evaluation obtained in (7), measurement was performed at 25 ° C. in accordance with JIS K7127. The tensile speed was 5 mm / min, the specimen shape was type 5, and a tensile test was performed in the direction perpendicular to the main stretching. The retention rate of tensile fracture nominal strain after printing was calculated from the following equation.

Tensile fracture nominal strain retention (%) = E2 / E1 × 100
E1: Tensile fracture nominal strain before printing, E2: Tensile fracture nominal strain after printing (9) Judgment of change in appearance after printing Judgment of visual change in appearance after printing and film before printing obtained in (7) did. Compared with the film before printing, ◎: No change is observed, ○: A slight change is observed, ×: A change is observed (10) A method for preparing a solvent-coated film for measuring haze and measurement of haze The coated film was applied once to the stretched film obtained in (7) (untreated) with a brush at 25 ° C. having a ratio of ethyl acetate and isopropyl alcohol of 40/60, and thoroughly soaked at 35 ° C. It was obtained by drying under reduced pressure of 1.3 kPa for 3 hours (film after solvent application). In addition, the untreated film before solvent application is called a film before solvent application. The haze was measured according to JIS K7015.
(11) Measurement of natural shrinkage rate The untreated film obtained in (7) was allowed to stand at 37 ° C for 7 days, and the shrinkage rate in the main stretching direction was calculated by the following formula.

Natural shrinkage (%) = (1−L2 / L1) × 100
L1: Length before being left, L2: Length after being left.
(12) Measurement of Shrinkage The untreated film obtained in (7) was immersed in hot water at 75 ° C. and 90 ° C. for 10 seconds, and the shrinkage in the main stretching direction was calculated by the following formula.

Shrinkage rate (%) = (1−L2 / L1) × 100
L1: Length before immersion, L2: Length after immersion.
(13) Dirt Exit Method for Extruding Dice at the Time of Extruding Resin The oil adhesion (dirt) at the die (T die) resin exit was visually determined after extrusion for 4 hours continuously with a 30 mmφ short shaft sheet extruder. A: Almost no adhesion, O: Slight adhesion, X: Severe adhesion.
(14) Evaluation of stretchability when a 300 μm thick sheet obtained by the low temperature workability determination method (7) is stretched biaxially with a batch type tenter, and the Vicat softening temperature of the resin (a) Five films heated at + 15 ° C. and stretched 5 times in the direction perpendicular to the sheet extrusion direction and 1.5 times in the sheet extrusion direction were measured, and the low temperature workability was determined by the following method. In addition, about the thing whose Vicat softening temperature +15 degreeC exceeds 110 degreeC, it extended | stretched by making 110 degreeC into the upper limit of heating temperature.
◎: When there is no tear, and the thickness unevenness with respect to the average thickness of the effective portion (excluding the edge portion) of the stretched film can be stretched to an absolute value of less than 10%. ○: There is no tear and is slightly uneven and average. When the thickness unevenness with respect to the thickness is 10-30% in absolute value x: When there is a tear, or when the thickness unevenness with respect to the average thickness is more than 30% in absolute value: x (15) Method for determining practical shrinkage After putting water into an aseptic PET bottle and capping it, a film having a length of 1.06 times the outer periphery of the PET bottle and a margin of 5 mm was cut out from the untreated film obtained in the film (7), and this was enveloped Then, the bottle was put on a bottle, passed through a steam tunnel at 80 ° C. for 10 seconds, and judged by the following method.
◎: Very good finished appearance ○: Good finished appearance ×: Insufficient shrinkage (Poor finished appearance)
[Production example]
Styrenic resins used in Examples and Comparative Examples were produced by the following method.

First, a resin (a) comprising a copolymer of styrene and an alkyl unsaturated aliphatic carboxylic acid alkyl ester having ₁₂ to ₁₈ carbon atoms and a vinyl aromatic hydrocarbon block used in Examples and Comparative Examples; A method for producing a resin (b) comprising a conjugated diene block will be described.
◎ Resin (a)
Table 1 shows the composition of styrene and the aliphatic unsaturated carboxylic acid ester copolymer and the amount of liquid paraffin added.

As for the (meth) acrylic acid alkyl ester monomer used in the present invention, lauryl methacrylate alone, a mixture of lauryl methacrylate and tridecyl methacrylate and stearyl methacrylate are LMA (GE manufactured by Mitsubishi Gas Chemical Company, Inc.). -410), SLMA (GE-420), SMA, and other monomers used commercially available reagents.
The resin A2 was produced as follows.

  A polymerization apparatus in which two fully mixed reactors (capacity: 4 liters) equipped with a stirrer and two laminar flow reactors (capacity: 2 liters) are connected in series is connected to 87.7 parts by weight of styrene and 4.3 weights of lauryl methacrylate. Parts, ethylbenzene 8 parts by weight, organic peroxide is 1,25 bis (t-butyl baroxy) cyclohexane 0.025 parts by weight, liquid paraffin (Exxon Mobil, trade name Primeall N382) 3.0 parts by weight The raw material solution was fed at a rate of 1 liter / hr, and polymerization was carried out at a reaction temperature of 110 ° C. in a complete mixing reactor and a reaction temperature of 120 ° C. to 140 ° C. in a laminar flow reactor. The obtained polymerization solution was continuously supplied to a devolatilizing extruder with a two-stage vent, and the temperature of the extruder was 200 to 240 ° C., and the single-stage vent and the two-stage vent were reduced to 2.0 kPa under an unreacted single amount. The body and solvent were recovered to obtain a resin. The polymerization rate of the monomer was 75%, and the loss rate during devolatilization of liquid paraffin was 25%. The composition of the copolymer was measured by NMR.

  Resins A1, A3 to A8 and B1 to B3 were carried out in the same manner as Resin A2, but in order to obtain the resin composition, molecular weight and molecular weight distribution shown in Table 1, the type of monomer, the ratio of monomers, the ethylbenzene The amount, polymerization temperature and the like were adjusted as appropriate. The liquid paraffin-containing resin other than the resin A2 was kneaded with a 20 mmφ twin screw extruder. The number of parts of liquid paraffin added in Table 1 is a ratio of the amount added relative to 100 parts by weight of the copolymer in the resin (a), and was obtained from analysis of the resin.

The obtained resins A1 to A8 had a melt flow rate within a range of 3 to 40 g / 10 min, and a volatile content calculated from heat loss of hot air gear oven 180 ° C. for 1 hour was 0.1% by weight or less. . Table 1 shows the compositions of the resins A1 to A8 and B1 to B3.
◎ Resin (b)
Table 2 shows a block copolymer having at least one polymer block mainly composed of vinyl aromatic hydrocarbon and at least one polymer block mainly composed of conjugated diene. The block copolymer was obtained by polymerizing normal butyl lithium in cyclohexane using a polymerization initiator. The non-linear block copolymer used silicon tetrachloride as a coupling agent.

  Resin SB1 was produced as follows.

  Using an autoclave equipped with a stirrer, 0.07 part by weight of n-butyllithium and 1/10 mole of tetramethylethylenediamine of n-butyllithium were added to a cyclohexane solution containing 42.5 parts by weight of styrene under a nitrogen gas atmosphere. After polymerization at 76 ° C. for 30 minutes, a cyclohexane solution containing 15 parts by weight of 1,3-butadiene was continuously added, and polymerization was performed at 75 ° C. for 40 minutes. Next, a cyclohexane solution containing 42.5 parts by weight of styrene was continuously added and polymerized at 75 ° C. for 30 minutes. Thereafter, methanol was added to the polymerization vessel at a 0.9-fold molar ratio with respect to n-butyllithium to stop the polymerization, and the solvent was removed to obtain a block copolymer.

  Resins SB2 to SB6 were carried out in the same manner as resin SB1, but in order to obtain the copolymer composition, molecular weight and molecular weight distribution of Table 2, the types of monomers, the ratio of monomers, the amount of polymerization initiator, etc. Adjusted accordingly.

Table 2 shows the resin compositions of the resins SB1 to SB6.
[Examples 1-14] and [Comparative Examples 1-8]
Tables 3 to 4 show the types of resins used in the respective mixtures composed of the resin (a), the resin (b), and other resins, the mixing ratio, and the measurement results of the physical properties. It should be noted that 2- [1- (2-hydroxy-3,5-di-t-pentylphenyl) ethyl] -4,6-di-t-pentylphenyl acrylate as a stabilizer is added in an amount of 0. 0 to 100 parts by weight of the mixture. 3 parts by weight were added.
(Examples 1-14)
From Tables 3 to 4, the physical properties after printing (tensile modulus, tensile fracture nominal strain, tensile fracture nominal strain retention, appearance change), solvent resistance (printability) such as transparency after solvent application, and low temperature Excellent workability, practical shrinkage, and natural shrinkage.
(Comparative Example 1)
Compared with Example 4, the Vicat softening temperature of the resin (a) in the intermediate layer is greatly different. In contrast to the Vicat softening temperature of 79 ° C. of the resin (a) of Example 4, this comparative example is as high as 100 ° C. (the amount of the (meth) acrylic acid alkyl ester component is small), and is particularly inferior in low-temperature workability and practical shrinkability.
(Comparative Example 2)
Compared to Example 2, the content of liquid paraffin in the resin (a) of the intermediate layer is greatly different. Compared to 3.5 parts by weight of the liquid paraffin of the resin (a) in Example 2, this comparative example is as much as 6.5 parts by weight, which is not preferable because the liquid paraffin is deposited at the time of resin extrusion and adheres to the die outlet part. .
(Comparative Example 3)
Compared with Example 4, the Vicat softening temperature of the resin (a) in the intermediate layer is greatly different. In contrast to the Vicat softening temperature of 79 ° C. of the resin (a) in the intermediate layer of Example 4, this comparative example is very low at 51 ° C. (the amount of the (meth) acrylic acid alkyl ester component is large). Distortion (retention rate), appearance after printing, transparency after solvent application, natural shrinkage, low temperature workability, and practical shrinkage are inferior.
(Comparative Example 4)
Compared with Example 4, the Vicat softening temperature of the resin (b) in the intermediate layer is greatly different. In contrast to the Vicat softening temperature of 82 ° C. of the resin (b) of the intermediate layer of Example 4, this comparative example is as low as 50 ° C. (the amount of conjugated diene component is large), and in particular, the natural shrinkage rate and the transparency before solvent application are poor. Further, the difference in refractive index between the resin (a) in the intermediate layer and the resin (b) in the intermediate layer is as large as 0.023 in this comparative example compared to 0.005 in Example 4, and the transparency is poor. In the case of back printing, if the transparency is inferior, it is not preferable because printed pictograms and the like are difficult to see.
(Comparative Examples 5 and 6)
Compared with Example 4, the kind of (meth) acrylic acid alkyl ester in the resin (a) of the intermediate layer is different. In contrast to lauryl methacrylate (alkyl group C ₁₂ ) and tridecyl methacrylate (alkyl group C ₁₃ ) of Example 4, Comparative Example 5 was 2-ethyl-hexyl acrylate (alkyl group C ₈ ), and Comparative Example 6 was acrylic. using acid n- butyl (alkyl _{C 4).} In contrast to Example 4, Comparative Examples 5 and 6 are particularly inferior in solvent swelling ratio, nominal tensile strain after printing, appearance after printing, and transparency after solvent application.
(Comparative Example 7)
Compared to the examples, the Vicat softening temperature of the resin (a) of the intermediate layer is as high as 92 ° C., which is inferior in low-temperature workability and practical shrinkage compared to the examples.
(Comparative Example 8)
Compared with the comparative example 6, it is the same except that the mixing ratio of the resin (a) and the resin (b) in the intermediate layer is changed from 75/25 to 50/50. Compared to the Examples, this Comparative Example is inferior in tensile fracture nominal strain after printing, appearance after printing, and transparency after solvent application, as in Comparative Examples 5 and 6.

The multilayer heat-shrinkable film of the present invention is excellent in low-temperature processability, excellent solvent resistance during printing with oil-based ink, and has little deterioration in physical properties after contact with solvent, and excellent in mechanical strength and practical shrinkage. Taking advantage of this, it can be suitably used for heat-shrinkable films for packaging materials such as food containers and food and beverage containers, or for laminated film applications such as wrapping films and foam containers.

Claims (9)

The resin of the intermediate layer is a resin (a) made of a copolymer of a vinyl aromatic hydrocarbon and an alkyl unsaturated aliphatic carboxylic acid alkyl ester having ₁₂ to ₁₈ carbon atoms, a vinyl aromatic A resin block (b) comprising 90 to 5% by weight of a polymer block mainly comprising a group hydrocarbon and a polymer block mainly comprising a conjugated diene derivative, and a mixture of polystyrene resin (c) 0 to 5% by weight, The resin of both outer layers is a resin block (b) consisting of a polymer block mainly composed of vinyl aromatic hydrocarbons and a polymer block mainly composed of conjugated diene derivatives, vinyl aromatic hydrocarbons and alkyl groups. multi of a mixture of the resin (a) and / or a polystyrene resin (c) 10.about.0 wt% consisting of aliphatic unsaturated carboxylic acid alkyl ester of carbon number _C 12 _{-C 18} Heat-shrinkable film. The vinyl aromatic hydrocarbon forming the resin (a) is made of styrene, and the alkyl group having C _{12 to} C ₁₈ aliphatic unsaturated carboxylic acid alkyl ester is lauryl (meth) acrylate or (meth) acrylic acid. The multilayer heat-shrinkable film according to claim 1, comprising at least one compound selected from tridecyl.   The vinyl aromatic hydrocarbon forming the resin (b) is made of styrene, and the conjugated diene derivative is made of at least one compound selected from 1,3-butadiene and 2-methyl-1,3-butadiene (isoprene). The multilayer heat-shrinkable film according to claim 1 or 2.   The resin of the intermediate layer is immersed in a solvent (ethyl acetate 40% by weight and isopropyl alcohol 60% by weight) at 25 ° C. (10 minutes), and the solvent swelling ratio is less than 6% by weight. The multilayer heat-shrinkable film according to any one of the above. 100 parts by weight of a copolymer in which the resin (a) has a vinyl aromatic hydrocarbon content of 78 to 95% by weight and an aliphatic unsaturated alkyl carboxylic acid alkyl ester having an alkyl group of C _{12 to} C ₁₈ carbon atoms of 22 to 5% by weight The multilayer heat-shrinkable film according to any one of claims 1 to 4, wherein 0 to 5 parts by weight of liquid paraffin is added and the Vicat softening temperature is 60 to 85 ° C.   The resin (b) has a polymer block mainly composed of at least one vinyl aromatic hydrocarbon and a polymer block polymerized mainly based on at least one conjugated diene derivative, and has a vinyl aromatic hydrocarbon content of 60 The multilayer heat-shrinkable film according to any one of claims 1 to 5, wherein the multilayer heat-shrinkable film has a conjugated diene derivative content of 40 to 5% by weight and a Vicat softening temperature of 65 to 93 ° C.   The multilayer heat-shrinkable film according to any one of claims 1 to 6, wherein an absolute value of a difference in refractive index between the resin (a) and the resin (b) is 0.01 or less.   Stretching in the main stretching direction is 3 to 8 times, stretching in the direction perpendicular to the main stretching direction is 1 to 1.5 times, and the shrinkage ratio in 10 seconds of 75 ° C. hot water in the main stretching direction is 15 to 80%, 90 ° C. The ratio of shrinkage ratio of 10 seconds in hot water / shrinkage ratio of 10 seconds in 75 ° C hot water is 1.5 to 4.0, and the multilayer heat shrinkage according to any one of claims 1 to 7 Sex film. A container in which the multilayer heat-shrinkable film according to any one of claims 1 to 8 is mounted by heat shrinkage.