Heat-shrinkable film

Abstract

To provide a relatively thin heat-shrinkable film having an impact resistant property and stiffness more than a predetermined level.SOLUTION: There is provided a heat-shrinkable film that is a heat-shrinkable film having a thickness of 40 μm or less. The heat-shrinkable film has a first layer and a second layer. The first layer is composed of a first resin composition containing a polystyrene-based resin having a hyperbranched structure. The second layer is laminated on at least one surface of the first layer and is composed of a second resin composition different from the first resin composition. In the first layer, a content of the polystyrene-based resin having a hyperbranched structure is 10 wt.% or more and 65 wt.% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a heat-shrinkable film. [Background technology]

[0002] Japanese Patent Laid-Open Publication No. 2007-291264 (Patent Document 1) discloses a heat-shrinkable film. The resin composition used to produce this heat-shrinkable film contains a styrene-(meth)acrylic copolymer (A) and a block copolymer (B) having a styrene-based polymer block (b1) and a conjugated diene polymer block (b2). The styrene-(meth)acrylic copolymer (A) contains a hyperbranched styrene-(meth)acrylic copolymer (a1) (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-291264 Summary of the Invention [Problem to be solved by the invention]

[0004] From the viewpoint of reducing environmental impact, there is a demand for thinner heat-shrinkable films. Simply reducing the thickness of a heat-shrinkable film reduces the impact resistance and stiffness of the heat-shrinkable film. Heat-shrinkable films are used, for example, for packaging labels on containers such as PET beverage bottles. The packaging labels are attached to containers such as PET beverage bottles, for example, by a label attachment device (labeler). During the process of attaching the packaging labels to the containers, for example, the packaging labels are processed into a tubular shape and then stood upright. When the impact resistance of the heat-shrinkable film decreases, for example, the packaging labels are more likely to break when the product is dropped. Furthermore, when the stiffness of the heat-shrinkable film decreases, for example, the tubular packaging labels do not stand upright during the process of attaching the packaging labels to the containers, resulting in reduced productivity. Patent Document 1 does not disclose a solution to these problems.

[0005] The present invention has been made to solve these problems, and its object is to provide a relatively thin heat-shrinkable film that has impact resistance and stiffness at or above a predetermined level. [Means for solving the problem]

[0006] The heat-shrinkable film according to the present invention has a thickness of 40 μm or less. This heat-shrinkable film comprises a first layer and a second layer. The first layer is made of a first resin composition containing a polystyrene-based resin having a hyperbranched structure. The second layer is laminated on at least one surface of the first layer and is made of a second resin composition different from the first resin composition. In the first layer, the content of the polystyrene-based resin having a hyperbranched structure is 10% by weight or more and 65% by weight or less.

[0007] The present inventors have found that when the content of the polystyrene-based resin having a hyperbranched structure in the first layer is 10% by weight or more and 65% by weight or less, the impact resistance and stiffness of the heat-shrinkable film are both at or above predetermined levels. With this heat-shrinkable film, because the content of the polystyrene-based resin having a hyperbranched structure in the first layer is 10% by weight or more and 65% by weight or less, it is possible to achieve performance at or above predetermined levels in terms of both impact resistance and stiffness even if the thickness is 40 μm or less.

[0008] In the heat-shrinkable film, the content of the polystyrene-based resin having a hyperbranched structure in the first layer may be 15% by weight or more and 50% by weight or less.

[0009] In the heat-shrinkable film, the first resin composition may further contain a styrene-butadiene block copolymer.

[0010] In the heat-shrinkable film, the first resin composition may further contain a polyester-based resin.

[0011] The present inventors have found that the stiffness of the heat-shrinkable film is further improved when the first resin composition further contains a polyester-based resin. According to this heat-shrinkable film, the stiffness of the heat-shrinkable film can be further improved because the first resin composition further contains a polyester-based resin.

[0012] In the heat-shrinkable film, the second resin composition may contain a polyester-based resin.

[0013] In the heat-shrinkable film, the second layer may be laminated on both sides of the first layer. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a relatively thin heat-shrinkable film that has impact resistance and stiffness at or above a predetermined level. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 2 is a diagram schematically illustrating a cross section of a heat-shrinkable film. [Figure 2] 1A to 1C are diagrams illustrating a manufacturing procedure for a heat-shrinkable film and a packaging container. DETAILED DESCRIPTION OF THE INVENTION

[0016] An embodiment according to one aspect of the present invention (hereinafter also referred to as "the present embodiment") will be described in detail below with reference to the drawings. Note that the same or corresponding parts in the drawings are designated by the same reference numerals, and their description will not be repeated. Furthermore, for ease of understanding, each drawing is drawn schematically with objects appropriately omitted or exaggerated.

[0017] [1. Composition of heat-shrinkable film] 1 is a diagram schematically illustrating a cross section of a heat-shrinkable film 1 according to the present embodiment. The heat-shrinkable film 1 is used as a base material for labels used in various fields such as beverages, toiletries, foods, pharmaceuticals, medical products, chemicals, cosmetics, and industrial products. For ease of understanding, the following description may mainly focus on an example in which the heat-shrinkable film 1 is used as a base material for labels attached to containers.

[0018] When the above-mentioned uses are assumed, from the viewpoint of strength, the thickness of the heat-shrinkable film 1 is preferably 10 μm or more, more preferably 15 μm or more, and even more preferably 20 μm or more. From the viewpoints of economy and environment, the thickness of the heat-shrinkable film 1 is preferably 40 μm or less, more preferably 35 μm or less, even more preferably 30 μm or less, even more preferably 27 μm or less, and even more preferably 25 μm or less.

[0019] Referring to FIG. 1 , the heat-shrinkable film 1 in this example has a three-layer structure and includes a first surface 10, a second surface 20, and an intermediate layer 30. One of the first surface 10 and the second surface 20 forms one surface (outermost surface) of the heat-shrinkable film 1, and the other forms the other surface (outermost surface) of the heat-shrinkable film 1. The intermediate layer 30 is formed between the first surface 10 and the second surface 20 in the thickness direction of the heat-shrinkable film 1. In other words, the first surface 10 is laminated on one surface of the intermediate layer 30, and the second surface 20 is laminated on the other surface of the intermediate layer 30. The first surface 10 and the intermediate layer 30 may be bonded via an adhesive layer. The second surface 20 and the intermediate layer 30 may also be bonded via an adhesive layer. The heat-shrinkable film 1 can be produced, for example, by feeding the raw materials of each layer contained in the heat-shrinkable film 1 (first surface layer 10, second surface layer 20, intermediate layer 30, and adhesive layer, if any) into an extruder and co-extruding them.

[0020] Each of the first surface layer 10 and the second surface layer 20 contains a resin. The resin contained in each of the first surface layer 10 and the second surface layer 20 may be one type or multiple types. The resin content of each of the first surface layer 10 and the second surface layer 20 is preferably 50 wt% (weight %) or more, more preferably 60 wt% or more, even more preferably 70 wt% or more, even more preferably 80 wt% or more, even more preferably 90 wt% or more, and even more preferably 95 wt% or more. Each of the first surface layer 10 and the second surface layer 20 may further contain an additive. Examples of additives include antiblocking agents, heat stabilizers, antioxidants, UV absorbers, light stabilizers, lubricants, antistatic agents, flame retardants, antibacterial agents, and fluorescent brighteners.

[0021] Each of the first surface layer 10 and the second surface layer 20 may contain a specific type of resin as a primary component. In this specification, the term "primary component" refers to the component that accounts for the largest proportion of the total weight. The content of the primary resin in each of the first surface layer 10 and the second surface layer 20 is preferably 50 wt% or more, more preferably 60 wt% or more, even more preferably 70 wt% or more, even more preferably 80 wt% or more, even more preferably 90 wt% or more, and even more preferably 95 wt% or more. Examples of the primary resin in each of the first surface layer 10 and the second surface layer 20 include polyester-based resins such as PET (Poly-Ethylene-Terephthalate) and polystyrene-based resins.

[0022] The compositions of first surface layer 10 and second surface layer 20 may be the same or different. For example, first surface layer 10 and second surface layer 20 may contain different amounts of the same type of resin or different types of resin. Furthermore, for example, first surface layer 10 and second surface layer 20 may contain different amounts of the same type of additive or different types of additive.

[0023] The thickness of each of the first surface layer 10 and the second surface layer 20 is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 3 μm or more. The thickness of each of the first surface layer 10 and the second surface layer 20 is preferably 10 μm or less, more preferably 9 μm or less, and even more preferably 8 μm or less. The thicknesses of the first surface layer 10 and the second surface layer 20 do not need to be the same, but are preferably the same.

[0024] The intermediate layer 30 contains a polystyrene-based resin having a hyperbranched structure (hereinafter also referred to as "hyperbranched polystyrene"). While the synthesis method of hyperbranched polystyrene is not particularly limited, hyperbranched polystyrene generally refers to a polystyrene-based resin having two or more branches in the polymer chain. Hyperbranched polystyrene is obtained, for example, by polymerizing a styrene-based monomer and a hyperbranched macromonomer, and may also be copolymerized with other monomers such as acrylic monomers. The content of hyperbranched polystyrene in the intermediate layer 30 is 10 wt% or more and 65 wt% or less, preferably 15 wt% or more and 50 wt% or less, and more preferably 20 wt% or more and 40 wt% or less.

[0025] The mid layer 30 may contain one type of resin or multiple types. In addition to hyperbranched polystyrene, the mid layer 30 may contain at least one of a styrene-butadiene block copolymer (SBC) and a polyester resin such as PET. When the mid layer 30 contains SBC, the SBC content in the mid layer 30 is 35 wt% or more and 90 wt% or less, preferably 50 wt% or more and 85 wt% or less, and more preferably 60 wt% or more and 80 wt% or less. When the mid layer 30 contains a polyester resin, the polyester resin content in the mid layer 30 is 20 wt% or less, preferably 17 wt% or less, and more preferably 15 wt% or less.

[0026] The intermediate layer 30 may further contain additives, such as antiblocking agents, heat stabilizers, antioxidants, ultraviolet absorbers, light stabilizers, lubricants, antistatic agents, flame retardants, antibacterial agents, and fluorescent brighteners.

[0027] The thickness of the intermediate layer 30 is preferably 10 μm or more, more preferably 12 μm or more, and even more preferably 15 μm or more. The thickness of the intermediate layer 30 is preferably 30 μm or less, more preferably 28 μm or less, and even more preferably 25 μm or less.

[0028] In general, from the viewpoint of reducing environmental impact, there is a demand for thinner heat-shrinkable films. Simply reducing the thickness of a heat-shrinkable film reduces the impact resistance and stiffness of the heat-shrinkable film. Heat-shrinkable films are used, for example, for packaging labels on containers such as PET beverage bottles. The packaging labels are attached to containers such as PET beverage bottles, for example, by a label attachment device (a labeler 90 (FIG. 2) described below). During the process of attaching the packaging labels to the containers, for example, the packaging labels are processed into a cylindrical shape and then stand upright. If the impact resistance of the heat-shrinkable film decreases, for example, the packaging labels are more likely to break when the product is dropped. Furthermore, if the stiffness of the heat-shrinkable film decreases, for example, the cylindrical packaging labels do not stand upright during the process of attaching the packaging labels to the containers, resulting in reduced productivity.

[0029] The present inventor(s) have found that when the content of hyperbranched polystyrene in the intermediate layer 30 is 10% by weight or more and 65% by weight or less, both the impact resistance and stiffness of the heat-shrinkable film 1 are at or above predetermined levels. According to the heat-shrinkable film 1 of the present embodiment, the content of hyperbranched polystyrene in the intermediate layer 30 is 10% by weight or more and 65% by weight or less, so that even if the thickness of the heat-shrinkable film 1 is 40 μm or less, it is possible to achieve performance at or above predetermined levels in terms of both impact resistance and stiffness.

[0030] If the heat-shrinkable film 1 does not break during a drop test (a test in which a product equipped with the heat-shrinkable film 1 is dropped from a height of 120 cm), the impact resistance of the heat-shrinkable film 1 is determined to be at or above a predetermined level. If the compressive strength of the heat-shrinkable film 1 is 1.2 N or more and the Young's modulus of the heat-shrinkable film 1 is 1.2 GPa or more, the stiffness of the heat-shrinkable film 1 is determined to be at or above a predetermined level.

[0031] The impact puncture strength of the heat-shrinkable film 1 is preferably 0.45 J or more, more preferably 0.55 J or more, even more preferably 0.60 J or more, even more preferably 0.65 J or more, and even more preferably 0.70 J or more. The compression strength of the heat-shrinkable film 1 is preferably 1.25 N or more, more preferably 1.3 N or more, even more preferably 1.35 N or more, even more preferably 1.4 N or more, and even more preferably 1.5 N or more. The Young's modulus of the heat-shrinkable film 1 is preferably 1.3 GPa or more, more preferably 1.5 GPa or more, even more preferably 1.6 GPa or more, and even more preferably 1.7 GPa or more.

[0032] The present inventors have also found that when the intermediate layer 30 further contains a polyester-based resin, the stiffness of the heat-shrinkable film 1 is further improved. According to the heat-shrinkable film 1 according to the present embodiment, when the intermediate layer 30 further contains a polyester-based resin, the stiffness of the heat-shrinkable film 1 can be further improved.

[0033] [2. Method for manufacturing heat-shrinkable film and packaging container] 2 is a diagram illustrating a manufacturing procedure for the heat-shrinkable film 1 and the packaging container 2. The packaging container 2 is a container packaged with the heat-shrinkable film 1.

[0034] Referring to FIG. 2, first, a heat-shrinkable film 1 is produced by a film production apparatus 50. As shown in FIG. 2, the film production apparatus 50 includes a T-die 300, cast rolls 310 and 320, a longitudinal stretching machine 51, and a transverse stretching machine 52. The T-die 300 includes a T-die main body 301 and raw material input sections 330, 331, and 332. The raw material for the first surface layer 10 is input into the raw material input section 330, the raw material for the second surface layer 20 is input into the raw material input section 332, and the raw material for the intermediate layer 30 is input into the raw material input section 331. If an adhesive layer is inserted between the first surface layer 10 and the intermediate layer 30 and / or between the second surface layer 20 and the intermediate layer 30, a separate raw material input section for the adhesive layer is provided, and the raw material for the adhesive layer is input thereto. The T-die body 301 co-extrudes the raw materials fed through the raw material feed sections 330, 331, and 332 (and the raw material feed section for the adhesive layer, if any) to fuse the molten raw materials fed into each raw material feed section together to form a single integrated heat-shrinkable film 1 (molten material). The casting rolls 310 and 320 cool the extruded molten material and send it downstream.

[0035] The longitudinal stretching machine 51 stretches in the MD (Machine Direction) the heat-shrinkable film 1 (molten material) cooled by the cast rolls 310 and 320. The transverse stretching machine 52 stretches in the TD (Transverse Direction) the heat-shrinkable film 1 that has been stretched in the MD. The heat-shrinkable film 1 that has been stretched in various ways is taken up as a film roll F1.

[0036] The stretching ratio in MD of the heat-shrinkable film 1 is preferably 120% or more, more preferably 125% or more, and even more preferably 130% or more. The stretching ratio in the same direction is preferably 180% or less, more preferably 170% or less, even more preferably 160% or less, even more preferably 150% or less, and even more preferably 140% or less.

[0037] The stretching ratio in the TD of the heat-shrinkable film 1 is preferably 300% or more, more preferably 400% or more, and even more preferably 500% or more. The stretching ratio in the TD is preferably 800% or less, more preferably 700% or less, and even more preferably 600% or less.

[0038] Next, the film roll F1 is set in the printing machine 60. In the printing machine 60, the heat-shrinkable film 1 is unwound from the film roll F1 and passes sequentially through one or more impression cylinders. At this time, ink is transferred from the impression cylinder to one side of the heat-shrinkable film 1 (for example, the outer surface of the first surface 10), and a printed layer is laminated. Note that the printing method is not limited to this. Thereafter, the heat-shrinkable film 1 with the printed layer laminated thereon (hereinafter, the heat-shrinkable film 1 including the printed layer) is taken up as a film roll F2.

[0039] Next, the film roll F2 is set in the cutting machine 70. In the cutting machine 70, the heat-shrinkable film 1 unwound from the film roll F2 is cut by a cutter along the MD (machine direction) and divided into pieces in the TD (transverse direction). The width of the division of the heat-shrinkable film 1 in the lateral direction corresponds to the lateral width of the label (hereinafter referred to as the unit label) to be attached to one container in its unfolded state. However, the cutting method is not limited to this. Thereafter, the multiple rows of heat-shrinkable film 1 divided in the lateral direction are wound up as separate film rolls F3.

[0040] Next, the film roll F3 is set in a center sealing machine 80. In the center sealing machine 80, the heat-shrinkable film 1 unwound from the film roll F3 is sealed in the MD (machine direction) and formed into a cylindrical shape. Specifically, both ends of the heat-shrinkable film 1 in the TD (machine direction) are overlapped, and the overlapped portion is continuously sealed vertically. In this way, a cylindrical heat-shrinkable film 1 is produced, which is then taken up as a film roll F4. The sealing method is not particularly limited, and possible methods include solvent sealing, heat sealing, and ultrasonic sealing.

[0041] Next, the film roll F4 is set in a labeler (applying machine) 90. In the labeler 90, the tubular heat-shrinkable film 1 unwound from the film roll F4 is applied to individual containers. Specifically, the tubular heat-shrinkable film 1 unwound from the film roll F4 is cut along the TD (transverse direction) at predetermined intervals in the MD (longitudinal direction) to separate into tubular labels (hereinafter referred to as tubular unit labels) to be applied to individual containers. The tubular unit labels are then erected, and the container is inserted inside the erected tubular unit labels. Because the heat-shrinkable film 1 has sufficient stiffness, the tubular unit labels stand firmly. The containers covered with the tubular unit labels are sent to a heating space. In the heating space, the tubular unit labels are heat-treated. As a result, the tubular unit labels are thermally shrunk and adhere to the outer surface of the container, conforming to the outer shape of the container. This completes the packaging container 2.

[0042] [3. Features] As described above, the heat-shrinkable film 1 according to the present embodiment is a heat-shrinkable film having a thickness of 40 μm or less. The heat-shrinkable film 1 includes a first surface layer 10, a second surface layer 20, and an intermediate layer 30. The intermediate layer 30 is made of a first resin composition containing a polystyrene-based resin having a hyperbranched structure. Each of the first surface layer 10 and the second surface layer 20 is laminated to the intermediate layer 30 and is made of a second resin composition different from the first resin composition. The content of the polystyrene-based resin having a hyperbranched structure in the intermediate layer 30 is 10% by weight or more and 65% by weight or less. With the heat-shrinkable film 1, since the content of the polystyrene-based resin having a hyperbranched structure in the intermediate layer 30 is 10% by weight or more and 65% by weight or less, it is possible to achieve performance above predetermined levels in terms of both impact resistance and stiffness even when the thickness is 30 μm or less.

[0043] 4. Other Embodiments The concept of the above embodiment is not limited to the embodiment described above. Hereinafter, examples of other embodiments to which the concept of the above embodiment can be applied will be described.

[0044] In the above embodiment, the heat-shrinkable film 1 has a three-layer structure. However, the heat-shrinkable film 1 may have a two-layer structure in which one of the first surface layer 10 and the second surface layer 20 is omitted. Conversely, one or more additional layers may be formed on the outer side of one or both of the first surface layer 10 and the second surface layer 20, making the heat-shrinkable film 1 have a four or more layer structure. The one or more additional layers may each have the same structure as the first surface layer 10, the second surface layer 20, or the intermediate layer 30 described above, or may have a different structure.

[0045] The above describes exemplary embodiments of the present invention. That is, the detailed description and the accompanying drawings are disclosed for the purpose of illustrative explanation. Therefore, some of the components described in the detailed description and the accompanying drawings may be non-essential components for solving the problems. Therefore, just because these non-essential components are described in the detailed description and the accompanying drawings, it should not be immediately recognized that these non-essential components are essential.

[0046] Furthermore, the above-described embodiments are merely illustrative of the present invention in all respects. Various improvements and modifications to the above-described embodiments are possible within the scope of the present invention. For example, at least a portion of the configuration of any of the embodiments may be combined with at least a portion of the configuration of any of the other embodiments. In other words, when implementing the present invention, specific configurations can be appropriately adopted depending on the embodiment. [Example]

[0047] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0048] [1. Examples and Comparative Examples] Heat-shrinkable films of Examples 1 to 7 and Comparative Examples 1 to 4 shown in Tables 1 and 2 were produced. All of these heat-shrinkable films had a three-layer structure, with a first surface layer and a second surface layer formed on both sides of an intermediate layer. The intermediate layer was bonded to the first and second surface layers with adhesive layers made of polyester elastomer. All of these heat-shrinkable films had a thickness of 23 μm. The thickness of the first surface layer was 3.3 μm, the thickness of the second surface layer was 3.3 μm, and the thickness of the intermediate layer was 16.4 μm. Each of these heat-shrinkable films was produced by feeding the raw materials for each layer shown in Tables 1 and 2 into an extruder in the weight ratios shown in these tables, co-extruding, and stretching in the transverse and longitudinal directions.

[0049] [Table 1] [Table 2]

[0050] Of the raw materials shown in Tables 1 and 2, "polyester resin" is amorphous polyethylene terephthalate resin (PETG) (glass transition temperature: 79°C) manufactured by SK Chemicals, "polystyrene resin A" is SBC (Vicat softening temperature: 46°C, butadiene content: 25% by weight) manufactured by INEOS, and "polystyrene resin B" is SBC (Vicat softening temperature: 84°C, butadiene content: 25% by weight) manufactured by Denka. "polystyrene resin C" is hyperbranched polystyrene (Vicat softening temperature: 96°C) manufactured by DIC, and "polystyrene resin D" is styrene homopolymer (Vicat softening temperature: 95°C) manufactured by Denka.

[0051] [2.Various measurement methods] <2-1. Bag broken by falling> The heat-shrinkable films of Examples 1 to 7 and Comparative Examples 1 to 4 were attached as labels to beverage containers (PET bottles) using a center sealing machine and a labeler in the same manner as described in the above embodiment to produce packaging containers. Drop tests were then conducted on the produced packaging containers. The drop tests were conducted according to the following procedure.

[0052] (1) Allow the label to dry naturally while still attached to the plastic bottle. (2) Place the plastic bottle on its side (horizontally) and allow it to fall freely from a height of 120 cm. (3) Check for defects such as cracks.

[0053] The above test was carried out once, and if the film broke, it was marked as "fail" (×), and if it did not break, it was marked as "pass" (◯).

[0054] <2-2. Impact Drilling Strength> Each heat-shrinkable film of Examples 1 to 7 and Comparative Examples 1 to 4 was cut into a sample measuring 100 mm long x 100 mm wide to obtain a test piece. The impact puncture strength of the obtained test piece was measured using a film impact tester manufactured by Toyo Seiki Seisakusho Co., Ltd., in accordance with JIS P8134. Specifically, the test piece was fixed on a table. Next, the hook fixing the arm of the film impact tester was removed, causing the fan-shaped arm to rotate around its axis, and the impact head attached to the tip of the arm broke through the test piece. The energy required for the impact head to break through the test piece was quantified as impact strength. Impact strength was measured five times, and the average value was calculated.

[0055] <2-3. Compressive strength> The compressive strength of each of the heat-shrinkable films of Examples 1 to 7 and Comparative Examples 1 to 4 was measured according to a method in accordance with JIS P8126. Specifically, the following method was used. Each of the heat-shrinkable films of Examples 1 to 7 and Comparative Examples 1 to 4 was cut into a strip measuring 152.4 mm in length and 12.7 mm in width, and the strip was set in a cylindrical shape on a previously prepared support. The support was then placed on the base of a ring crush tester (manufactured by Toyo Seiki Seisaku-sho, Model D) and the compressive strength was measured. Measurement was performed only on the compressive strength in the machine direction (direction of film flow), n=8, and the average value was calculated.

[0056] <2-4.Young's Modulus> Each heat-shrinkable film of Examples 1 to 7 and Comparative Examples 1 to 4 was cut into a sample measuring 250 mm length x 25 mm width to obtain a test piece. The Young's modulus of the obtained test piece was measured using a Strograph VE-1D manufactured by Toyo Seiki Seisaku-sho, Ltd., in accordance with a method in accordance with ASTM D882. The Young's modulus was measured using four test pieces for each Example and Comparative Example, and the average value was calculated.

[0057] <2-5. Heat shrinkage rate> Each heat-shrinkable film of Examples 1 to 7 and Comparative Examples 1 to 4 was cut into a sample measuring 100 mm in length (MD) × 100 mm in width (TD) to obtain a test piece. The obtained test piece was immersed in boiling water (98°C) for 10 seconds, then removed and immediately immersed in tap water at 15°C for 10 seconds. The MD heat shrinkage was calculated according to the following formula (1), and the TD heat shrinkage was calculated according to the following formula (2). In formula (1), LMD is the MD length of the test piece after heat shrinkage, and LTD in formula (2) is the TD length of the test piece after heat shrinkage. The heat shrinkage was measured using three test pieces for each heat-shrinkable film of each Example and Comparative Example, and the average value was used. Heat shrinkage rate (%) = {(100 - LMD) / 100} × 100 (1) Heat shrinkage rate (%) = {(100 - LTD) / 100} × 100 (2)

[0058] [3. Measurement results] The results of each measurement are shown in Tables 3 and 4.

[0059] [Table 3] [Table 4]

[0060] Each of the heat-shrinkable films of Examples 1 to 7 did not break in the drop test (evaluated as "good" (◯)). On the other hand, each of the heat-shrinkable films of Comparative Examples 1 to 4 was evaluated as "poor" (×) in the drop test. Each of the heat-shrinkable films of Examples 1 to 7 also had a compressive strength of 1.2 N or more and a Young's modulus of 1.2 GPa or more. It was confirmed that each of the heat-shrinkable films of Examples 1 to 7 had impact resistance and stiffness at or above the predetermined levels. [Explanation of symbols]

[0061] 1 heat shrinkable film, 2 packaging container, 10 first surface layer, 20 second surface layer, 30 intermediate layer, 50 film manufacturing apparatus, 51 longitudinal stretching machine, 52 transverse stretching machine, 60 printing machine, 70 cutting machine, 80 center sealing machine, 90 labeler, 300 T-die, 301 T-die body, 310, 320 cast roll, 330, 331, 332 raw material input section, F1, F2, F3, F4 film roll.

Claims

1. A heat-shrinkable film having a thickness of 40 μm or less, a first layer formed of a first resin composition containing a polystyrene-based resin having a hyperbranched structure; a second layer laminated on at least one surface of the first layer and made of a second resin composition different from the first resin composition; A heat-shrinkable film, wherein the content of the polystyrene-based resin having a hyperbranched structure in the first layer is 10% by weight or more and 65% by weight or less.

2. 2. The heat-shrinkable film according to claim 1, wherein the content of the polystyrene-based resin having a hyperbranched structure in the first layer is 15% by weight or more and 50% by weight or less.

3. The heat-shrinkable film according to claim 1 or claim 2, wherein the first resin composition further contains a styrene-butadiene block copolymer.

4. The heat-shrinkable film according to claim 3 , wherein the first resin composition further comprises a polyester-based resin.

5. The heat-shrinkable film according to claim 3 , wherein the second resin composition comprises a polyester-based resin.

6. The heat-shrinkable film of claim 5 , wherein the second layer is laminated to both sides of the first layer.

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