Heat-shrinkable foam film and labels

Abstract

Provided are a heat-shrinkable foamed film having improved surface smoothness and a label obtained using the heat-shrinkable foamed film. The heat-shrinkable foamed film includes a foam layer formed from a resin composition including a block copolymer comprising units of a vinylaromatic hydrocarbon monomer and units of a conjugated diene monomer, and is a film stretched in the MD and / or TD of the heat-shrinkable foamed film. When the resin composition is examined for uniaxial elongation viscosity ƞ at 110°C and an elongation rate of 0.1 / sec and when the elongation viscosity at Hencky strain 1 and the elongation viscosity at Hencky strain 2 are expressed by ƞ1 and ƞ2, respectively, then the value of ƞ2-ƞ1 is 10 MPa·s or greater and the ƞ2 is 50 MPa·s or greater.

Description

Technical Field

[0001] The present invention relates to a heat-shrinkable foamed film and a label obtained using the heat-shrinkable foamed film.

Background Art

[0002] Labels are used for PET bottle beverages and the like, and there is a demand to improve the recyclability of PET bottles by enhancing the separability between the bottle and the label. Patent Document 1 discloses the use of a foamed film as a label separable in water.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When using a foamed film as a label for PET bottle beverages and the like, since printing is applied to the surface of the foamed film, improvement in surface smoothness is desired.

[0005] An object of the present invention is to provide a heat-shrinkable foamed film with improved surface smoothness and a label obtained using the heat-shrinkable foamed film.

Means for Solving the Problems

[0006] Through diligent research, the inventors discovered that, when measuring the uniaxial extensional viscosity η of the resin composition constituting the heat-shrinkable foam film at 110°C and an extension rate of 0.1 / sec, and setting the value of η2-η1 to 10 MPa·s or more, and setting η2 to 50 MPa·s or more, a heat-shrinkable foam film with improved surface smoothness can be obtained, as η1 is the extensional viscosity at Hencky strain 1 and η2 is the extensional viscosity at Hencky strain 2. This led to the completion of the present invention.

[0007] In other words, the present invention is as follows: (1) A heat-shrinkable foamed film comprising a foamed layer composed of a resin composition containing a block copolymer having vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units, The heat-shrinkable foamed film is a stretched film that is stretched in at least one direction of the MD direction and the TD direction of the heat-shrinkable foamed film. The uniaxial extension viscosity η of the resin composition, measured at 110°C and an extension rate of 0.1 / sec, is given by η1 as the extension viscosity at Hencky strain 1 and η2 as the extension viscosity at Hencky strain 2. The value of η2-η1 is 10 MPa·s or more. The above η2 is 50 MPa·s or more. Heat-shrinkable foam film. (2) The heat-shrinkable foamed film described in (1), having a specific gravity of 0.7 or more and less than 1.0. (3) A heat-shrinkable foamed film according to (1) or (2), wherein the heat shrinkage rate at least in one direction at 100°C for 10 seconds is 50 to 90%. (4) A single-layer heat-shrinkable foamed film consisting only of the foamed layer, as described in any one of (1) to (3). (5) The surface roughness Ra of the heat-shrinkable foam film, measured in a direction perpendicular to the direction of maximum shrinkage of the heat-shrinkable foam film, is 0.05 to 0.70 μm. A heat-shrinkable foamed film as described in any one of (1) to (4). (6) A label printed on the surface of a heat-shrinkable foam film as described in any one of (1) to (5). (7) A label comprising a laminate layer with printing applied to the surface, on a heat-shrinkable foam film as described in any one of (1) to (5). (8) The label described in (6) has a specific gravity of 0.7 or more and less than 1.0. (9) The label described in (7) has a specific gravity of 0.7 or more and less than 1.0. [Effects of the Invention]

[0008] The heat-shrinkable foamed film of the present invention exhibits excellent surface smoothness. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic diagram illustrating an example of a laminated structure of a heat-shrinkable foam film. [Figure 2] This is a schematic diagram illustrating an example of a single-layer structure of a heat-shrinkable foam film. [Figure 3] This is a log-log graph showing the resin compositions obtained in the examples and comparative examples, with Hencky strain plotted on the horizontal axis and extensional viscosity on the vertical axis. [Modes for carrying out the invention]

[0010] <Explanation of Terms> In this specification, for example, the description "A to B" means that it is greater than or equal to A and less than or equal to B.

[0011] The embodiments of the present invention will be described in detail below. The present invention is not limited thereto, and various modifications are possible without departing from its essence. The various features shown in the embodiments below can be combined with each other. Furthermore, each feature constitutes an invention independently.

[0012] <Heat-shrinkable foam film> The heat-shrinkable foamed film according to an embodiment of the present invention is composed of a resin composition containing a block copolymer having vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units, and includes a foamed layer stretched in at least one of the MD direction and the TD direction of the heat-shrinkable foamed film. In the present invention, the MD direction of the heat-shrinkable foamed film means the film feeding direction (Machine Direction) in the line for producing the heat-shrinkable foamed film, and the TD direction of the heat-shrinkable foamed film means the direction (Transverse Direction) orthogonal to the MD direction. Also, among the MD direction and the TD direction in which the heat-shrinkable foamed film is stretched, the direction in which it is stretched more is sometimes referred to as the main stretching direction.

[0013] The heat-shrinkable foamed film may be a heat-shrinkable foamed film having a multilayer structure formed by "coextrusion" in which a plurality of resins are simultaneously extruded using, for example, a multilayer T-die. In the case of a heat-shrinkable foamed film having a multilayer structure, as shown in FIG. 1, for example, a structure in which the above foamed layer is used as the first layer (2) and a second layer (1) is provided as the surface layer on the foamed layer can be cited. A three-layer structure in which the second layer (1) is provided on both sides of the first layer (2) may also be used. Preferably, from the viewpoint of simplifying the manufacturing equipment and manufacturing process, the heat-shrinkable foamed film is a single-layer heat-shrinkable foamed film composed only of the foamed layer (2), as shown in FIG. 2.

[0014] <Foamed layer> The foamed layer according to an embodiment of the present invention is formed from a resin composition containing a block copolymer having vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units. As will be described later, the resin composition according to an embodiment of the present invention may contain a component other than the block copolymer having vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units, which has an effect of adjusting the elongational viscosity (hereinafter referred to as an elongational viscosity adjusting component) for the purpose of adjusting the elongational viscosity of the resin composition within a desired range.

[0015] The elongation viscosity adjusting component is not particularly limited as long as it can adjust the elongation viscosity of the resin composition to a desired range. Examples thereof include polystyrene, styrene-butadiene block copolymer, polypropylene, polyethylene, and the like. From the viewpoints of transparency and film-forming property when blended with a copolymer of vinyl aromatic hydrocarbon and conjugated diene, polystyrene and styrene-butadiene block copolymer are preferred. These elongation viscosity adjusting components may be used alone or in combination of two or more. The content of the elongation viscosity adjusting component in 100% by mass of the resin composition is preferably 0 to 40% by mass, more preferably 1 to 40% by mass, and still more preferably 10 to 20% by mass. Specific examples of the preferred content of the elongation viscosity adjusting component in 100% by mass of the resin composition are 0, 1, 3, 5, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, 40% by mass, and it may be within the range between any two of the values exemplified herein. In one embodiment, the resin composition may substantially contain no elongation viscosity adjusting component. When the elongation viscosity adjusting components are used in combination, the content of the elongation viscosity adjusting component means the total content of the elongation viscosity adjusting components used in combination.

[0016] <Block copolymer having vinyl aromatic hydrocarbon monomer unit and conjugated diene monomer unit> The block copolymer having vinyl aromatic hydrocarbon monomer unit and conjugated diene monomer unit contained in the resin composition according to one embodiment of the present invention is a copolymer synthesized by block copolymerizing a vinyl aromatic hydrocarbon monomer and a conjugated diene monomer. The block copolymer is a copolymer having one or more block chains composed of vinyl aromatic hydrocarbon monomer units and / or conjugated diene monomer units. The block copolymer may be used alone or in combination of two or more.

[0017] The content of block copolymer in 100% by mass of the resin composition is preferably 60 to 100% by mass, and more preferably 80 to 100% by mass. The preferred content of block copolymer in 100% by mass of the resin composition is specifically 60, 65, 70, 75, 80, 85, 90, 95, 99, and 100% by mass, and may be within the range of any two of the values ​​exemplified herein. Furthermore, in one embodiment, the resin composition may contain substantially only block copolymer. When block copolymers are used in combination, the block copolymer content refers to the total content of all block copolymers used.

[0018] <Vinyl aromatic hydrocarbon monomer units> Vinyl aromatic hydrocarbon monomer units are constituent units of block copolymers derived from vinyl aromatic hydrocarbon monomers used in the copolymerization of block copolymers. Examples of vinyl aromatic hydrocarbon monomers include styrene, o-methylstyrene, p-methylstyrene, p-tert-butylstyrene, 2,4-dimethylstyrene, 2,5-dimethylstyrene, α-methylstyrene, vinylnaphthalene, and vinylanthracene. In one embodiment, the vinyl aromatic hydrocarbon monomer is preferably styrene. These monomers may be used individually or in combination of two or more.

[0019] <Conjugated diene monomer units> Conjugated diene monomer units are constituent units of block copolymers derived from conjugated diene monomers used in the copolymerization of block copolymers. Examples of conjugated diene monomers include 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene), 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, and 1,3-hexadiene. In one embodiment, the conjugated diene monomer is preferably 1,3-butadiene or isoprene. These monomers may be used individually or in combination of two or more.

[0020] The content of vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units in the block copolymer contained in the resin composition according to one embodiment of the present invention is not particularly limited, but when the block copolymer is considered to be 100% by mass, it is preferable that the content of vinyl aromatic hydrocarbon monomer units is 70 to 90% by mass and the content of conjugated diene monomer units is 10 to 30% by mass, and it is more preferable that the content of vinyl aromatic hydrocarbon monomer units is 75 to 85% by mass and the content of conjugated diene monomer units is 15 to 25% by mass. The preferred content of vinyl aromatic hydrocarbon monomer units in the block copolymer is, for example, 70, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, and 90% by mass, when the block copolymer is considered to be 100% by mass, and may be within the range of any two of the values ​​exemplified here. The preferred content of conjugated diene monomer units in the block copolymer is, for example, 10, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, and 30% by mass, when the block copolymer is considered to be 100% by mass, and may be within the range of any two of the values ​​exemplified here. When vinyl aromatic hydrocarbon monomer units are used in combination, the content of vinyl aromatic hydrocarbon monomer units refers to the total content of all vinyl aromatic hydrocarbon monomer units used in combination. When conjugated diene monomer units are used in combination, the content of conjugated diene monomer units refers to the total content of all conjugated diene monomer units used in combination.

[0021] <Structure of polymer block> The overall structure of the block copolymer and the structure of the polymer blocks constituting the block copolymer are not particularly limited. The structure of the polymer blocks may be a polymer block mainly composed of vinyl aromatic hydrocarbon monomer units (e.g., styrene units), a polymer block mainly composed of conjugated diene monomer units (e.g., 1,3-butadiene units), a polymer block with a random structure in which vinyl aromatic hydrocarbon monomer units (e.g., styrene units) and conjugated diene monomer units (e.g., 1,3-butadiene units) are arranged randomly, or a polymer block with a tapered structure in which vinyl aromatic hydrocarbon monomer units (e.g., styrene units) and conjugated diene monomer units (e.g., 1,3-butadiene units) are arranged in a tapered shape with a gradient in the distribution density. The overall structure of the block copolymer has a structure having at least two or more types of polymer blocks. There are no particular limitations on the types of polymer blocks or the order in which the polymer blocks are linked. Furthermore, the way in which the polymer blocks are linked may be linear, branched, or star-shaped. For example, a polymer block with a random structure containing styrene units and 1,3-butadiene units can be obtained by polymerizing styrene and 1,3-butadiene by simultaneously adding small amounts of each to the polymerization active end. Similarly, a polymer block with a tapered structure containing styrene units and 1,3-butadiene units can be obtained by polymerizing styrene and 1,3-butadiene by simultaneously adding excess amounts of each to the polymerization active end. Furthermore, by using polyfunctional initiators and coupling agents, the overall structure of the block copolymer can be made branched or star-shaped.

[0022] <Structure of block copolymer> The structure of the block copolymer according to one embodiment of the present invention is not particularly limited.

[0023] The number-average molecular weight of the block copolymer is preferably 40,000 to 500,000, and more preferably 80,000 to 300,000. A number-average molecular weight of 40,000 or more provides sufficient rigidity and impact resistance to the block copolymer composition, while a number-average molecular weight of 500,000 or less is preferable because it results in a block copolymer composition with good processability. The number-average molecular weight of the block copolymer can be measured using gel permeation chromatography (hereinafter abbreviated as GPC).

[0024] <Method for manufacturing block copolymers> A method for producing a block copolymer according to one embodiment of the present invention is not particularly limited, but for example, it is a method of polymerizing the vinyl aromatic hydrocarbon monomer and the conjugated diene monomer in an organic solvent using an organolithium compound as an initiator.

[0025] Examples of organic solvents include aliphatic hydrocarbons such as butane, pentane, hexane, isopentane, heptane, octane, and isooctane; alicyclic hydrocarbons such as cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane, and ethylcyclohexane; and aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and xylene.

[0026] Organolithium compounds are compounds in which one or more lithium atoms are bonded to the molecule. Examples of organolithium compounds include monofunctional organolithium compounds such as ethyllithium, n-propyllithium, isopropyllithium, n-butyllithium, sec-butyllithium, and tert-butyllithium, and polyfunctional organolithium compounds such as hexamethylenedisitium, butadienyldilithium, and isoprenyldilithium.

[0027] In so-called living anionic polymerization using organolithium compounds as initiators, almost the entire amount of vinyl aromatic hydrocarbons and conjugated dienes subjected to the polymerization reaction can be converted into polymers.

[0028] Furthermore, a randomizing agent may be added to control the polymerization state. Tetrahydrofuran (THF) is mainly used as a randomizing agent, but other ethers, amines, thioethers, phosphoramides, alkylbenzene sulfonates, potassium or sodium alkoxides, etc., can also be used. Suitable ethers other than THF include dimethyl ether, diethyl ether, diphenyl ether, diethylene glycol dimethyl ether, and diethylene glycol dibutyl ether. As for amines, tertiary amines such as trimethylamine, triethylamine, and tetramethylethylenediamine, as well as cyclic amines, can also be used. Other randomizing agents that can be used include triphenylphosphine, hexamethylphosphoramide, potassium or sodium alkylbenzene sulfonate, potassium or sodium butoxide, etc.

[0029] The amount of randomizing agent to add can be, for example, 0.001 to 10 parts by mass per 100 parts by mass of total monomer. The timing of addition may be before the start of the polymerization reaction or before the polymerization of the copolymer chain. Additional additions may also be made as needed.

[0030] The block copolymer obtained in this way is deactivated by adding a polymerization inhibitor such as water, alcohol, or carbon dioxide in an amount sufficient to deactivate the active ends. Any method can be used to recover the copolymer from the obtained block copolymer solution, such as (A) precipitation using a poor solvent such as methanol, (B) precipitation by evaporating the solvent using a heated roll or the like (drum dryer method), (C) a method of concentrating the solution with a concentrator and then removing the solvent with a vented extruder, or (D) a method of dispersing the solution in water and recovering the copolymer by heating and removing the solvent by blowing in steam (steam stripping method).

[0031] <Resin composition constituting the foamed layer> The resin composition constituting the foamed layer according to one embodiment of the present invention is obtained by mixing at least one block copolymer obtained by the above-mentioned manufacturing method, etc., with extension viscosity adjusting components and other additives as needed.

[0032] Other additives include, for example, various stabilizers, lubricants, processing aids, anti-blocking agents, antistatic agents, anti-fogging agents, lightfastness enhancers, softeners, plasticizers, and pigments. Each additive may be added to the block copolymer solution, or it may be blended with the recovered copolymer and melt-mixed.

[0033] Examples of stabilizers include 2-tert-butyl-6-(3-tert-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenyl acrylate, 2-[1-(2-hydroxy-3,5-di-tert-pentylphenyl)ethyl]-4,6-di-tert-pentylphenyl acrylate, phenolic antioxidants such as octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate and 2,6-di-tert-butyl-4-methylphenol, and phosphorus-based antioxidants such as trisnonylphenyl phosphite. Examples of antiblocking agents include organic fillers such as high-impact polystyrene and cross-linked vinyl aromatic hydrocarbon copolymer beads, silica beads, and quartz beads. Examples of other additives include fatty acid amides, ethylene bisstearoamide, sorbitan monostearate, saturated fatty acid esters of aliphatic alcohols, and pentaerythritol fatty acid esters. These additives are preferably used in an amount of 5% by mass or less relative to the block copolymer.

[0034] A resin composition according to one embodiment of the present invention may contain two or more block copolymers, and known methods can be used to mix these block copolymers. For example, they may be dry blended using a Henschel mixer, ribbon blender, super mixer, V blender, etc., or they may be further melted and pelletized in an extruder. In one embodiment, melt mixing is preferred. Alternatively, a method can be used in which the polymer solutions are mixed and then the solvent is removed.

[0035] <Extension viscosity of the resin composition constituting the foam layer> The extensional viscosity of the resin composition constituting the foamed layer according to one embodiment of the present invention, measured at 110°C and an extension rate of 0.1 / sec, satisfies the following requirements. That is, in the uniaxial extensional viscosity η measured at 110°C and an extension rate of 0.1 / sec, when η1 is the extensional viscosity at Hencky strain 1 and η2 is the extensional viscosity at Hencky strain 2, the value of η2-η1 is 10 MPa·s or more, preferably 20 Pa·s or more, and more preferably 30 Pa·s or more. By setting the value of η2-η1 to 10 MPa·s or more, a heat-shrinkable foamed film with improved surface smoothness can be obtained. The upper limit of the value of η2-η1 is not particularly limited, but from the viewpoint of film-forming properties, it is preferably 100 Pa·s or less. The values ​​of η2-η1 are, specifically, for example, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, and 100 Pa·s, and may also be within the range of any two of the values ​​exemplified here.

[0036] In the uniaxial extension viscosity η of the resin composition constituting the foamed layer according to one embodiment of the present invention, measured at 110°C and an extension rate of 0.1 / sec, the extension viscosity η2 at Hencky strain 2 is 50 MPa·s or more, preferably 60 Pa·s or more, and more preferably 70 Pa·s or more. By setting η2 to 50 MPa·s or more, a heat-shrinkable foamed film with improved surface smoothness can be obtained. The upper limit of η2 is not particularly limited, but from the viewpoint of film-forming properties, it is preferably 150 Pa·s or less. Specifically, η2 may be, for example, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150 Pa·s, and may be within the range of any two of the values ​​exemplified here.

[0037] Extensional viscosity η is a value obtained by the following measurement method: using the DHR-2 instrument (manufactured by TA Instruments) with the extensional viscosity jig SER3, at 110°C and a strain rate of 0.1 / sec.

[0038] Using the extensional viscosity η measured by the method described above, a log-log graph is created with Hencky strain on the x-axis and extensional viscosity η on the y-axis. The value of η2-η1 is calculated, where η1 is the extensional viscosity at Hencky strain 1 and η2 is the extensional viscosity at Hencky strain 2.

[0039] Extensional viscosity η can be increased, for example, by introducing branched structures into the polymer chains of a block copolymer, or by introducing structures that facilitate entanglement between polymer chains to form pseudo-branched structures. It can also be increased by increasing the weight-average molecular weight of the block copolymer or by mixing it with other resins that have high extensional viscosity. In addition, as described above, the extensional viscosity of the resin composition can also be adjusted by including an extensional viscosity adjusting component in the resin composition. The extensional viscosity adjusting component is not particularly limited as long as it is a component that can adjust the extensional viscosity of the resin composition to a desired range, but examples include polystyrene, styrene-butadiene copolymer, polypropylene, polyethylene, etc. From the viewpoint of transparency and film-forming properties when blended with a copolymer of vinyl aromatic hydrocarbon and conjugated diene, polystyrene and styrene-butadiene copolymer are preferred. These extensional viscosity adjusting components may be used individually or in combination of two or more. The method for increasing the extensional viscosity of the resin composition may be one of the above-mentioned methods alone, or two or more methods may be used in combination.

[0040] <Resin composition constituting the second layer> The resin composition constituting the second layer according to one embodiment of the invention is not particularly limited, as long as it can be formed into a sheet or film by stretching. Examples include resin compositions containing block copolymers used in the foamed layer described above, and polyester resins such as polystyrene, polyethylene, polypropylene, and glycol-modified polyethylene terephthalate. From the viewpoint of film-forming properties, shrinkage finish, and specific gravity, resin compositions containing block copolymers of vinyl aromatic hydrocarbons and conjugated dienes are preferred. The resin composition constituting the second layer may contain various additives, similar to the resin composition constituting the foam layer.

[0041] <Method of manufacturing foamed sheets> The method for manufacturing a foamed sheet is not particularly limited, but for example, a method can be used in which a foamed layer is formed by extruding a foamed layer using a resin composition. Alternatively, the foamed sheet may be made by extruding the resin composition that constitutes the layers other than the foamed layer together with the foamed layer during the extrusion process, thereby producing a foamed sheet composed of multiple layers, including the layers other than the foamed layer. In this specification, the terms "sheet" and "film" are not distinguished based on differences in thickness; however, if the thickness changes (becomes thinner) due to operations such as stretching, the material before thinning may be referred to as a "sheet."

[0042] The method for forming the foamed layer is not limited, and commonly used methods can be used, such as the chemical foaming method, in which the resin is foamed with gas generated by the thermal decomposition of the chemical foaming agent when the resin and chemical foaming agent are melt-kneaded together, and the physical foaming method, in which gas is injected into the molten resin in an extruder to cause foaming.

[0043] Specific examples of chemical blowing agents used in the chemical blowing method include sodium bicarbonate, organic acids such as citric acid, azodicarbonamide, azobisisobutyronitrile, diazoaminobenzene, N,N'-dinitrosopentamethylenetetraamine, N,N'-dimethyl-N,N'-dinitroterephthalamide, benzenesulfonyl hydrazide, p,p'-oxybisbenzenesulfonyl hydrazide, carbonates, etc. Two or more of these may be used in mixture. Sodium bicarbonate or a mixture of sodium bicarbonate and sodium citrate is preferably used, and the resulting foaming gas is carbon dioxide. There are no particular limitations on the method of adding the chemical blowing agent, but it may be by dry blending it into the resin pellets, by adding it using a quantitative feeder in the hopper of the extruder, or by creating a masterbatch (blowing agent masterbatch) based on the same resin as the main raw material and adding it. The amount of chemical blowing agent added is adjusted as appropriate depending on the desired foaming ratio and the amount of foaming gas generated by the blowing agent.

[0044] Specific examples of physicoblasting agents used in the physicoblasting method include carbon dioxide, propane, butane, n-pentane, dichlorodifluoromethane, dichloromonofluoromethane, trichloromonofluoromethane, methanol, ethanol, and water, but carbon dioxide is preferred for safety reasons. Methods for adding the physicoblasting agent include supplying it to the central zone of the extruder or, in the case of a tandem extruder, to the central zone of the first stage extruder. Alternatively, resin pellets impregnated with foaming gas may be fed into the extruder to obtain a foamed film. The amount of physicoblasting agent added is adjusted as appropriate according to the desired foaming ratio.

[0045] <Heat-shrinkable foam film> The heat-shrinkable foamed film according to one embodiment of the present invention is a film using the foamed sheet described above. The heat-shrinkable foamed film is the foamed sheet that has been stretched. Generally, due to the presence of bubbles in the foamed layer, the stress distribution during stretching is not necessarily uniform, and the smoothness of the original foamed film tends to decrease. In contrast, the heat-shrinkable foamed film according to one embodiment of the present invention is a heat-shrinkable foamed film that maintains surface smoothness suitable for printing even after stretching.

[0046] Stretching may be uniaxial, biaxial, or multiaxial. Examples of uniaxial stretching include stretching an extruded foam sheet in a direction perpendicular to the extrusion direction (TD direction) using a tenter, stretching an extruded tubular foam film in the circumferential direction (TD direction), and stretching an extruded foam sheet in the extrusion direction (MD direction) using a roll. Examples of biaxial stretching include stretching an extruded foam sheet in the extrusion direction (MD direction) using a roll, and then stretching it in a direction perpendicular to the extrusion direction (TD direction) using a tenter or the like, and stretching an extruded tubular foam film simultaneously or separately in the extrusion direction (MD direction) and the circumferential direction (TD direction).

[0047] The stretching temperature is preferably, for example, 60 to 120°C. A temperature of 60°C or higher makes the film less likely to break during stretching, while a temperature of 120°C or lower is preferable because it yields a film with good shrinkage characteristics. Particularly preferable is a range of Tg+5°C to Tg+20°C relative to the glass transition temperature (Tg) of the composition constituting the film. In the case of a multilayer film, a range of Tg+5°C to Tg+20°C relative to the Tg of the polymer composition of the layer with the lowest Tg is particularly preferable. The glass transition temperature (Tg) can be determined, for example, from the temperature of the peak of the loss modulus.

[0048] While there are no particular restrictions on the stretching ratio in the main stretching direction, where the stretching is greater, a ratio of 1.5 to 8.0 is preferred. A stretching ratio of 1.5 or more in the main stretching direction is preferred because it allows for the production of a film with good shrinkage characteristics, while a ratio of 8.0 or less allows for the easy production of a stretched film.

[0049] <Surface roughness Ra> In the present invention, the surface roughness Ra of the heat-shrinkable foam film is preferably 0.05 to 0.70 μm. Here, surface roughness Ra refers to the surface roughness Ra measured horizontally to the surface of the heat-shrinkable foam film and perpendicular to the direction of maximum shrinkage of the heat-shrinkable foam film. The following description will focus on, but is not limited to, the case in which maximum shrinkage of the heat-shrinkable foam film occurs in the main stretching direction (the direction of greater stretching) of the heat-shrinkable foam film. In the present invention, the surface roughness Ra of the heat-shrinkable foam film refers to the value measured in the MD direction on the surface of the heat-shrinkable foam film when the main stretching direction of the heat-shrinkable foam film (the direction in which it is stretched more) is the TD direction (see Figures 1 and 2). On the other hand, when the main stretching direction of the heat-shrinkable foam film is the MD direction, it refers to the value measured in the TD direction on the surface of the heat-shrinkable foam film.

[0050] When the main stretching direction of the heat-shrinkable foam film according to one embodiment of the present invention is the TD direction (see Figures 1 and 2), the surface roughness Ra measured in the MD direction of the surface of the heat-shrinkable foam film is 0.05 to 0.70 μm, and more preferably 0.05 to 0.5 μm. The preferred surface roughness Ra measured in the MD direction of the surface of the heat-shrinkable foam film is specifically, for example, 0.05, 0.10, 0.15, 0.20, 0.30, 0.40, 0.50, 0.60, and 0.70 μm, and may be within the range of any two of the values ​​exemplified here. If the surface roughness Ra measured in the MD direction is within this range, it becomes possible to suitably print on the surface of the heat-shrinkable foam film.

[0051] In the case where the main stretching direction of the heat-shrinkable foam film according to one embodiment of the present invention is the MD direction, the surface roughness Ra measured in the TD direction of the surface of the heat-shrinkable foam film is 0.05 to 0.70 μm, and more preferably 0.05 to 0.5 μm. The preferred surface roughness Ra measured in the TD direction of the surface of the heat-shrinkable foam film is specifically, for example, 0.05, 0.10, 0.15, 0.20, 0.30, 0.40, 0.50, 0.60, and 0.70 μm, and may be within the range of any two of the values ​​exemplified here. If the surface roughness Ra measured in the TD direction is within this range, it becomes possible to suitably print on the surface of the heat-shrinkable foam film. Surface roughness Ra is measured, for example, using a device called "SurfCorder ET4000" (manufactured by Kosaka Laboratory). Surface roughness Ra can be adjusted, for example, by lowering the foaming ratio of the foamed layer, introducing a branched structure to the polymer chains of the block copolymer, or introducing a structure that makes the polymer chains more likely to intertwine. It can also be adjusted by lowering the stretching ratio.

[0052] <Specific gravity of heat-shrinkable foamed film> The specific gravity of the heat-shrinkable foam film according to one embodiment of the present invention is preferably 0.70 or more and less than 1.00, and more preferably 0.70 to 0.95. The preferred specific gravities of the heat-shrinkable foam film are, specifically, for example, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, and 0.99, and may be within the range of any two of the values ​​exemplified herein. A specific gravity of less than 1.0 is preferable because it floats on water, allowing for specific gravity separation by water from films with a specific gravity of 1.0 or higher. Specific gravity is measured, for example, according to JIS Z8807:2012. The specific gravity can be adjusted, for example, by adjusting the degree of foaming of the foamed layer, adjusting the composition of the resin composition, or designing the block copolymer.

[0053] <Thermal shrinkage rate of heat-shrinkable foamed film> In the present invention, the heat shrinkage rate of the heat-shrinkable foam film is preferably 50 to 90% in at least one direction. The following describes, but is not limited to, cases in which the specified heat shrinkage rate is satisfied in the main stretching direction (the direction in which the film is stretched more) of the heat-shrinkable foam film. In the present invention, the thermal shrinkage rate of the heat-shrinkable foam film refers to the value measured in the TD direction of the heat-shrinkable foam film when the main stretching direction of the heat-shrinkable foam film (the direction in which it is stretched more) is the TD direction (see Figures 1 and 2). On the other hand, when the main stretching direction of the heat-shrinkable foam film is the MD direction, it refers to the value measured in the MD direction of the heat-shrinkable foam film. In one embodiment, when the main stretching direction (the direction in which the film is stretched more) of the heat-shrinkable foam film according to one embodiment is the TD direction, the heat shrinkage rate of the heat-shrinkable foam film in the TD direction is preferably 50-90% at 100°C for 10 seconds, and preferably 60-80% at 100°C for 10 seconds. When the heat shrinkage rate is 50% or more, high temperatures are not required during shrinkage, thus minimizing the impact on the covered article. In one embodiment, it is preferable that the shrinkage rate is 65-80% at 100°C for 10 seconds. In another embodiment, it is preferable that the natural shrinkage rate is 3% or less at 40°C for 7 days. In one embodiment, when the main stretching direction of the heat-shrinkable foam film is the MD direction, the heat shrinkage rate of the heat-shrinkable foam film in the MD direction is preferably 50-90% at 100°C for 10 seconds, and preferably 55-80% at 100°C for 10 seconds. When the heat shrinkage rate is 50% or more, high temperatures are not required during shrinkage, thus minimizing the impact on the covered article. In one embodiment, it is preferable that the shrinkage rate is 65-80% at 100°C for 10 seconds. In another embodiment, it is preferable that the natural shrinkage rate is 3% or less at 40°C for 7 days. The thermal shrinkage rate is calculated, for example, by immersing a stretched film in hot water for a certain period of time and measuring the difference in length before and after shrinkage. The thermal shrinkage rate can be adjusted, for example, by adjusting the degree of stretching or the stretching temperature.

[0054] The thickness of the heat-shrinkable foam film according to one embodiment of the present invention is preferably 20 to 100 μm, and more preferably 50 to 95 μm.

[0055] Heat-shrinkable foamed film can be used as labels, cap seals, and other packaging materials.

[0056] A label using a heat-shrinkable foam film according to one embodiment of the present invention can be manufactured by known methods. For example, it can be manufactured by printing on a heat-shrinkable foam film and then solvent sealing it with the direction of greatest shrinkage (the TD direction if the film is greatly stretched in the TD direction) as the circumferential direction. Alternatively, it can be manufactured by laminating a printed laminate layer onto the surface of a heat-shrinkable foam film and then laminating it with the direction of greatest shrinkage (the TD direction if the film is greatly stretched in the TD direction) as the circumferential direction. There are no particular restrictions on the lamination temperature, but a preferred temperature range is less than 70°C. The resin constituting the printed label to be laminated onto the surface of the heat-shrinkable foam film is not particularly limited, but from the viewpoint of low specific gravity, ensuring rigidity, and providing smoothness, it is preferable to use one or more resins selected from polyethylene, polypropylene, polyethylene terephthalate, styrene-butadiene block copolymer, polystyrene, and high-impact polystyrene, with polystyrene and styrene-butadiene block copolymer being more preferable.

[0057] <Specific gravity of the label> The specific gravity of the label according to one embodiment of the present invention is preferably 0.7 or more and less than 1.0, and more preferably 0.7 to 0.95. A specific gravity of less than 1.0 is preferable because it allows for separation by water from labels with a specific gravity of 1.0 or more, as the label will float on water. The specific gravity of the label can be controlled by adjusting the specific gravity of the heat-shrinkable foam film or the specific gravity of the printed label. Specific gravity is measured, for example, according to JIS Z8807:2012.

[0058] The labels are not particularly limited, but can be used as labels for metal can containers such as tin, TFS (Tin Free Steel), and aluminum (3-piece and 2-piece cans, or bottle cans with lids, etc.), glass containers, or resin containers such as polyethylene terephthalate (abbreviated as PET) and polyethylene.

[0059] Furthermore, since heat-shrinkable foam film and labels made using it have a specific gravity of less than 1, when used as labels for PET bottles, they can be separated from the container in water, offering the advantage of excellent recyclability. [Examples]

[0060] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited thereto.

[0061] The materials used in the examples and comparative examples are as follows:

[0062] <Preparation of block copolymer: (P-1)> (1) 234.0 kg of cyclohexane and 37.0 g of tetrahydrofuran (THF) were placed in the reaction vessel. (2) 654 mL of a 10% by mass cyclohexane solution of n-butyllithium was added to this as a polymerization initiator solution, and the temperature was maintained at 30°C. (3) 16 kg of styrene was added, and the styrene was subjected to anionic polymerization. The internal temperature rose to 60°C. (4) After the styrene was completely consumed, the internal temperature of the reaction system was lowered to 30°C, and 33.4 kg of styrene and 8.6 kg of 1,3-butadiene were added simultaneously. The internal temperature was raised to 70°C. (5) After the styrene was completely consumed, the internal temperature of the reaction system was lowered to 50°C, and 33.4 kg of styrene and 8.6 kg of 1,3-butadiene were added simultaneously in one go. The internal temperature was raised to 84°C. (6) After the styrene and 1,3-butadiene have been completely consumed, all polymerization active ends are finally deactivated with water to obtain a polymerization solution containing a block copolymer having polystyrene blocks and tapered blocks of styrene and butadiene. (7) This polymerization solution was pre-concentrated and then devolatilized and extruded using a twin-screw extruder with a vacuum vent to obtain the desired pelletized block copolymer (P-1).

[0063] <Preparation of block copolymer: (P-2)> (1) 234.0 kg of cyclohexane and 37.0 g of tetrahydrofuran (THF) were placed in the reaction vessel. (2) 787 mL of a 10% by mass cyclohexane solution of n-butyllithium was added to this as a polymerization initiator solution, and the mixture was kept at 30°C. (3) 16 kg of styrene was added, and the styrene was subjected to anionic polymerization. The internal temperature rose to 60°C. (4) After the styrene was completely consumed, the internal temperature of the reaction system was lowered to 30°C, and 33.4 kg of styrene and 8.6 kg of 1,3-butadiene were added simultaneously in one go. The internal temperature was raised to 73°C. (5) After the styrene was completely consumed, the internal temperature of the reaction system was lowered to 50°C, and 33.4 kg of styrene and 8.6 kg of 1,3-butadiene were added simultaneously in one go. The internal temperature was raised to 85°C. (6) After the styrene and 1,3-butadiene have been completely consumed, all polymerization active ends are finally deactivated with water to obtain a polymerization solution containing a block copolymer having polystyrene blocks and tapered blocks of styrene and butadiene. (7) This polymerization solution was pre-concentrated and then devolved using a twin-screw extruder with a vacuum vent to obtain the desired pelletized block copolymer (P-2).

[0064] <Number average molecular weight> Furthermore, the number-average molecular weight was measured using the GPC method under the following conditions. Device name: HLC-8220GPC (manufactured by Tosoh Corporation) Columns: Four ShodexGPCKF-404 (manufactured by Showa Denko) were connected in series. Temperature: 40℃ Detection: Ultraviolet-Vis spectroscopy (254 nm) Solvent: tetrahydrofuran Concentration: 2% by mass Calibration curve: Created using standard polystyrene (VARIAN).

[0065] Table 1 shows the conditions under which block copolymers P-1 and P-2 were prepared.

[0066] [Table 1]

[0067] <Method for manufacturing heat-shrinkable foamed film> [Raw materials used] • Block copolymer: P-1, P-2, 631M (manufactured by Denka Co., Ltd.) obtained by the polymerization described above. • Extension viscosity modifier: HMT-PS (manufactured by Toyo Styrene Co., Ltd.) • Antiblocking agent: E640N (manufactured by Toyo Styrene Co., Ltd.) • Chemical blowing agent: Polystyrene ES405 (manufactured by Eiwa Chemical Industries Co., Ltd.)

[0068] <Example 1> The method for producing the heat-shrinkable foamed film of Example 1 is described below.

[0069] The heat-shrinkable foamed film of Example 1 was prepared by the method described below. For the sake of explanation, the process will be described in two parts: (1) extrusion of the foamed sheet and (2) stretching of the sheet (film).

[0070] (1) Extrusion of foamed sheet An extruder equipped with a T-die with a lip width of 300 μm, capable of extruding sheets, was used. A block copolymer of SBC-A, 1.3 parts by mass of "E640N" (manufactured by Toyo Styrene Co., Ltd.) as an antiblocking agent, and 0.5 parts by mass of "Polyslene ES405" (manufactured by Eiwa Chemical Industries, Ltd.) as a foaming agent were melt-mixed and extruded into a sheet according to the mass ratios shown in the table. The extruder used to melt the resin and supply it to the T-die was a 65 mmφ short-shaft extruder, with a set temperature of 200°C. The T-die temperature was set to 180°C. The thickness of the resulting sheet was 0.30 mm.

[0071] (2) Sheet (film) stretching The obtained foamed sheet was subjected to a longitudinal stretcher having two rolls with different rotation speeds at 80°C without stretching in the MD direction (i.e., a stretching ratio of 1.0x), and then subjected to a tenter-type transverse stretcher at 90°C and stretched 4.5x in the TD direction to finally obtain the heat-shrinkable foamed film of Example 1 with a thickness of 70 μm.

[0072] Examples 1-5 and Comparative Example 1 were prepared in the same manner as in Example 1, under the conditions shown in Table 2. In Example 5, the raw materials were melt-mixed, and the film was prepared in the same manner as in Example 1.

[0073] In addition, the multilayer heat-shrinkable foamed film of Example 2 was obtained by using a multilayer T-die in the (1) foamed sheet extrusion process described above to obtain a multilayer sheet, and then following the same process as the (2) sheet (film) stretching process described above.

[0074] <Elongational viscosity η> Measurements were performed using the "DHR-2" device (manufactured by TA Instruments) and the "SER3" extensional viscosity measurement fixture, at a measurement temperature of 110°C and a strain rate of 0.1 / sec. A 0.3 mm thick resin test piece was created using a press molding machine (manufactured by Shoji Co., Ltd.), cut to 20 mm x 3 mm, and used for measurement. The number of measurements was set to 1 (n=1), and the representative value of extensional viscosity was calculated as follows. η2: Extensional viscosity at Hencky strain 1 η1: Extensional viscosity at Hencky strain of 0.1

[0075] <Thermal shrinkage rate> The thermal shrinkage rate at 100°C was calculated by immersing the stretched film in warm water adjusted to 100°C for 10 seconds and using the following formula. Thermal shrinkage rate (%) = (L1 - L2) / L1 × 100 L1: Length before shrinkage (in the main stretching (TD) direction) L2: Length after contraction (in the main stretching (TD) direction)

[0076] <Specific gravity> The specific gravity of the heat-shrinkable foam film was measured by cutting a test piece from the foam film and using the "liquid weighing method" of JIS Z8807:2012 with the apparatus shown below. Device name: MD-200S (manufactured by Alpha Mirage Corporation)

[0077] <Surface smoothness> Using the "SurfCorder ET4000" device (manufactured by Kosaka Laboratory), three-dimensional surface roughness measurement (i.e., measurement of surface smoothness) was performed in a 1mm x 1mm area. Since the main stretching direction in the example was the TD direction, the surface roughness measurement was performed in the MD direction of the film. For evaluation, a reference length (L) was extracted from the roughness curve obtained by scanning the film in the MD direction. The x-axis was taken in the direction of the mean line for this extracted portion, and the y-axis was taken in the direction of the vertical magnification. The roughness curve was then represented as y=f(x), and the arithmetic mean surface roughness (Ra) was obtained by integrating the absolute value of f(x) from 0 to L and dividing by L. [Measurement conditions] Measurement speed: 0.2 mm / s MD direction pitch: 0.5 μm TD direction pitch: 10 μm × 10¹ lines Measurement procedure: The needle was moved in the MD direction at a speed of 0.2 mm / s while in contact with the film, and the roughness was measured at 0.5 μm intervals. Then, the needle was removed from the film, and the needle was moved 10 μm in the TD direction, and the roughness was obtained in the MD direction as described above.

[0078] <Examples 1-6 and Comparative Example 1> Similar to Example 1, heat-shrinkable foamed films for Examples 1-6 and Comparative Example 1 were prepared according to Table 2. The physical properties of the heat-shrinkable foamed films are shown in Table 2.

[0079] <Rating> The performance of the heat-shrinkable foam films of Examples 1-6 and Comparative Example 1 was evaluated as follows.

[0080] <Surface smoothness> For the heat-shrinkable foam films of Examples 1-6 and Comparative Example 1, the arithmetic mean surface roughness (Ra) was determined using the procedure described above, and the surface smoothness was evaluated. The following criteria were used for evaluation. Excellent: Ra is less than 0.30 μm Good: Ra is between 0.30 μm and less than 0.50 μm Acceptable: Ra is between 0.50 μm and less than 0.70 μm Unacceptable: Ra is 0.70 μm or higher

[0081] <Expansion suppression> For the heat-shrinkable foam films of Examples 1-6 and Comparative Example 1, the average film thickness of the sheets and films was measured using a micrometer. The expansion rate of the film thickness when a film was made from a sheet was calculated using the following formula, and the expansion suppression effect was evaluated. The following criteria were used for evaluation. Film thickness expansion coefficient = (Average sheet thickness × Stretching ratio in the main stretching direction / Average film thickness) × 100 (%) Excellent: Less than 120% Good: 120% or more, less than 135% Acceptable: 135% or more but less than 150% Impossible: 150% or more

[0082] <Separability> The heat-shrinkable foam films and PET bottles from Examples 1-6 and Comparative Example 1 were each cut into approximately 1 cm squares. Both were placed in water and stirred, and after standing for 1 minute, the separation state (i.e., whether the heat-shrinkable foam film pieces floated in the water and separated from the PET pieces) was evaluated. The following criteria were used for evaluation. OK: All heat-shrinkable foam film pieces float in water and are completely separate from the PET pieces. Unacceptable: Contains sunken heat-shrinkable foam film fragments.

[0083] <Contractile> For the heat-shrinkable foam films of Examples 1-6 and Comparative Example 1, the heat shrinkage rate at 100°C was calculated by immersing the stretched film of the test specimen in warm water adjusted to 100°C for 10 seconds and using the following formula. The stretched film of the test specimen used was a 10cm x 10cm heat-shrinkable foam film test specimen with two reference points on its surface spaced at L1 (10cm) apart. Thermal shrinkage rate (%) = (L1 - L2) / L1 × 100 L1: Length before shrinkage (in the main stretching (TD) direction) L2: Length after contraction (in the main stretching (TD) direction) The following criteria were used for evaluation. Excellent: 75% or higher Good: 65% or more, less than 75% Acceptable: 55% or more but less than 65% Not possible: Less than 55%

[0084] <Film forming properties> The film-forming properties of the heat-shrinkable foamed films of Examples 1-6 and Comparative Example 1 were evaluated according to the following criteria. Excellent: Good film formation. Possible: Film formation is possible, but the following defects ((1) to (3)) may occur in terms of film formation. (1) Foaming is not stable (2) There is a tendency for FE (foreign matter due to poor mixing) to increase. (3) It may break. Not possible: Film cannot be formed.

[0085] [Table 2]

[0086] <Consideration> The heat-shrinkable foam film according to the embodiment of the present invention has excellent surface smoothness, and therefore, good printing can be expected when printing is applied to its surface. On the other hand, heat-shrinkable foam films outside the scope of the present invention have poor surface smoothness. Therefore, printing defects may occur when printing is applied to their surface. Although some aspects of the mechanism by which the heat-shrinkable foamed film according to the embodiment of the present invention exhibits excellent surface smoothness are not yet clear, it is believed that by controlling the value of η2-η1 to 10 MPa·s or more, when the uniaxial extension viscosity η of the resin composition constituting the heat-shrinkable foamed film is measured at 110°C and an extension rate of 0.1 / sec, with η1 being the extension viscosity at Hencky strain 1 and η2 being the extension viscosity at Hencky strain 2, the expansion of the heat-shrinkable foamed film during main stretching is suppressed, and thus the surface smoothness of the heat-shrinkable foamed film is improved. This is further supported by the fact that, when comparing the specific gravity of the sheet (before main stretching) and the specific gravity of the film (after main stretching), in the comparative example the specific gravity of the film is lower, causing the heat-shrinkable foamed film to expand during the main stretching process, whereas in the example the difference between the two is reduced, and the expansion of the heat-shrinkable foamed film during the main stretching process is suppressed.

[0087] Furthermore, in the uniaxial extension viscosity η of the resin composition constituting the heat-shrinkable foam film, measured at 110°C and an extension rate of 0.1 / sec, the extension viscosity η2 at Hencky strain 2 was controlled to 50 MPa·s or higher. This suppressed the expansion of the heat-shrinkable foam film during stretching, and thus improved the surface smoothness of the heat-shrinkable foam film. [Industrial applicability]

[0088] The heat-shrinkable foam film according to the present invention, and the labels obtained using said heat-shrinkable foam film, have improved surface smoothness. The heat-shrinkable foam film according to the present invention, and the labels obtained using said heat-shrinkable foam film, have excellent printability and industrial applicability. [Explanation of Symbols]

[0089] 1. Second layer 2. First layer (foamed layer)

Claims

1. A heat-shrinkable foamed film comprising a foamed layer and a non-foamed layer, comprising a resin composition containing a block copolymer having vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units, The heat-shrinkable foamed film is a stretched film that is stretched in at least one direction of the MD direction and the TD direction of the heat-shrinkable foamed film. The uniaxial extension viscosity η of the resin composition, measured at 110°C and an extension rate of 0.1 / sec, is given by η1 as the extension viscosity at Hencky strain 1 and η2 as the extension viscosity at Hencky strain 2. The value of η2 - η1 is 10 MPa·s or more. The above η2 is 50 MPa·s or more, The non-foamed layer is composed of the resin composition, polystyrene, polyethylene, or polypropylene. Heat-shrinkable foam film.

2. The heat-shrinkable foamed film according to claim 1, wherein the specific gravity is 0.7 or more and less than 1.

0.

3. The heat-shrinkable foamed film according to claim 1, wherein the heat shrinkage rate at least in one direction at 100°C for 10 seconds is 50 to 90%.

4. The surface roughness Ra of the heat-shrinkable foam film, measured in a direction perpendicular to the direction of maximum shrinkage of the heat-shrinkable foam film, is 0.05 to 0.70 μm. The heat-shrinkable foamed film according to claim 1.

5. A label having been printed on the surface of the heat-shrinkable foam film according to claim 1.

6. A label comprising a laminate layer on which printing is applied to the surface, on a heat-shrinkable foam film according to claim 1.

7. The label according to claim 5 or claim 6, wherein the specific gravity is 0.7 or more and less than 1.0.

Images