Heat-shrinkable film

Abstract

The heat-shrinkable film has a layer containing a polystyrene resin, and when the thermal shrinkage rates in a direction orthogonal to the main shrinkage direction at 70°C, 80°C, 90°C, and 98°C are respectively represented by R70, R80, R90, and R98, R70≤R80≤R90≤R98, R70<R98, R70≥0%, and R98≤25%.

Description

heat shrinkable film

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

[0002] Japanese Patent No. 2972571 (Patent Document 1) discloses a heat-shrinkable film having a layer of high-impact polystyrene (HIPS), also known as high impact polystyrene, and used as a label for covering a container.

[0003] Patent No. 2972571

[0004] Consider a case where a heat-shrinkable film is used, for example, as a label to be attached to a container. In this case, if the container to which the label is attached is dropped, there is a risk that the label will break in a direction (generally the vertical direction of the container) perpendicular to the main shrinkage direction of the heat-shrinkable film (generally the horizontal direction of the container). In particular, in recent years, labels have become thinner from the standpoint of economic efficiency and environmental friendliness, and therefore, it is becoming increasingly important to impart breakage resistance to heat-shrinkable films in consideration of various situations, such as the above-mentioned dropping of a container.

[0005] The present inventors have considered increasing the stretch ratio in the direction perpendicular to the main shrinkage direction of the heat-shrinkable film as a method for preventing the breakage. However, this generally increases the heat shrinkage rate of the heat-shrinkable film in the same direction. This can lead to distortion or slippage of the label when it is attached to a container by thermally shrinking the heat-shrinkable film, resulting in poor finish of the label.

[0006] An object of the present invention is to provide a heat-shrinkable film that is excellent in finish and break resistance when used as a label.

[0007] Item 1. A film having a layer containing a polystyrene resin, wherein the thermal shrinkage rates in a direction perpendicular to the main shrinkage direction at 70°C, 80°C, 90°C, and 98°C are respectively R ７０ , R ８０ , R ９０ , R ９８ When this is done, R ７０ ≦R ８０ ≦R ９０ ≦R９８ , R ７０ <R ９８ , R ７０ ≧0%, and R ９８ 25%≦Heat-shrinkable film.

[0008] Item 2. R ７０ Item 2. The heat-shrinkable film according to item 1, wherein the elongation is ≦7%.

[0009] Item 3. R ８０ Item 3. The heat-shrinkable film according to item 1 or 2, wherein the elongation coefficient is ≦13%.

[0010] Item 4. R ９０ Item 4. The heat-shrinkable film according to any one of Items 1 to 3, wherein the heat-shrinkable film has a modulus of elasticity of 17% or less.

[0011] Item 5. The heat-shrinkable film according to any one of Items 1 to 4, wherein the stretching ratio in the direction perpendicular to the main shrinkage direction is 120% or more.

[0012] Item 6. The heat-shrinkable film according to any one of Items 1 to 5, further comprising a layer containing a polyester-based resin on at least one side of the layer containing a polystyrene-based resin.

[0013] Item 7. The heat-shrinkable film according to any one of Items 1 to 6, wherein the layer containing a polystyrene-based resin contains at least one of general-purpose polystyrene (GPPS) and hyperbranched polystyrene.

[0014] Item 8. The heat-shrinkable film according to any one of Items 1 to 7, having a film thickness of 35 μm or less.

[0015] According to the present invention, it is possible to provide a heat-shrinkable film that is excellent in finish and break resistance when used as a label.

[0016] 1A and 1B are diagrams illustrating a cross section of a heat-shrinkable film according to an embodiment of the present invention, and a flow chart illustrating a method for manufacturing a heat-shrinkable film and a packaging container according to an embodiment of the present invention.

[0017] Hereinafter, a heat-shrinkable film according to one embodiment of the present invention will be described with reference to the drawings. 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 in a schematic manner with objects appropriately omitted or exaggerated.

[0018] [1. Configuration of Heat-Shrinkable Film] The heat-shrinkable film 1 according to this embodiment is used as a substrate for labels used in various fields such as beverages, toiletries, foods, pharmaceuticals, medical products, chemicals, cosmetics, industrial products, etc. For ease of understanding, the following description may take as an example a case where the heat-shrinkable film 1 is used as a substrate for labels attached to containers.

[0019] 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 60 μm or less, more preferably 50 μm or less, even more preferably 40 μm or less, even more preferably 35 μm or less, even more preferably 30 μm or less, and even more preferably 25 μm or less.

[0020] FIG. 1 is a schematic diagram showing a cross section of a heat-shrinkable film 1 according to the present embodiment. In this example, the heat-shrinkable film 1 has a three-layer structure, including 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 (the first surface layer 10, the second surface layer 20, the intermediate layer 30, and the adhesive layer, if any) into an extruder and co-extruding them.

[0021] 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 amount of resin contained 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. 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.

[0022] 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 of the material. The amount of the primary resin contained 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 contained 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.

[0023] The compositions of the first surface layer 10 and the second surface layer 20 may be the same or different. For example, the first surface layer 10 and the second surface layer 20 may contain different amounts of the same type of resin or different types of resin. Also, for example, the first surface layer 10 and the second surface layer 20 may contain different amounts of the same type of additive or different types of additive.

[0024] 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 15 μm or less, more preferably 10 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.

[0025] The intermediate layer 30 contains a polystyrene-based resin, preferably containing a polystyrene-based resin as a main component. The polystyrene-based resin contained in the intermediate layer 30 may be one type or multiple types. The amount of polystyrene-based resin contained in the intermediate layer 30 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. The intermediate layer 30 may further contain additives. Examples of additives include antiblocking agents, heat stabilizers, antioxidants, UV absorbers, light stabilizers, lubricants, antistatic agents, flame retardants, antibacterial agents, and fluorescent brighteners.

[0026] 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 40 μm or less, more preferably 30 μm or less, and even more preferably 25 μm or less.

[0027] For example, if a container to which a heat-shrinkable film is attached as a label is dropped, the label is more likely to break. Therefore, the heat-shrinkable film 1 is designed to prevent the label from breaking even in situations where label breakage is relatively likely, such as when a container is dropped. The heat-shrinkable film 1 according to this embodiment is a uniaxially stretched film. Generally, uniaxially stretched heat-shrinkable films have a relatively high stretch ratio in the main shrinkage direction and a relatively high strength in the main shrinkage direction, and therefore tend to break more easily in a direction perpendicular to the main shrinkage direction than in the main shrinkage direction. Therefore, the heat-shrinkable film 1 according to this embodiment is designed to improve the break resistance in the direction perpendicular to the main shrinkage direction.

[0028] Although not limited thereto, in this embodiment, the main shrinkage direction of the heat-shrinkable film 1 is the TD (Transverse Direction) of the heat-shrinkable film 1, and the direction perpendicular to the main shrinkage direction is the MD (Machine Direction) of the heat-shrinkable film 1. Furthermore, although not limited thereto, in this embodiment, when the heat-shrinkable film 1 is attached to a container as a label, the TD corresponds to the horizontal direction of the container, and the MD corresponds to the vertical direction of the container. Therefore, the heat-shrinkable film 1 according to this embodiment is devised to improve its MD break resistance.

[0029] Increasing the MD stretch ratio of the heat-shrinkable film is considered as a way to improve the MD break resistance. However, this generally increases the MD heat shrinkage rate of the heat-shrinkable film. This can lead to distortion or slippage during label attachment, such as when the heat-shrinkable film is thermally shrunk to fit the label to a container, resulting in poor label finish. In other words, simply adjusting the stretch ratio can result in a trade-off between the break characteristics of the heat-shrinkable film and the finish quality of the label when used as a label. However, through extensive research, including experiments described below as examples, the present inventors have conceived a heat-shrinkable film that is excellent in both finish quality and break resistance.

[0030] The heat-shrinkable film 1 satisfies the following conditions C1 to C4 for improving finishability and break resistance. ７０ , R ８０ , R ９０ , R ９８ are the heat shrinkage rates of the heat-shrinkable film 1 in a direction perpendicular to the main shrinkage direction (MD in this embodiment) at 70°C, 80°C, 90°C, and 98°C, respectively. The heat shrinkage rate is defined by a measurement method included in the explanation of the examples described later. Condition C1: R ７０ ≦R ８０ ≦R ９０ ≦R ９８ Condition C2: R ７０ <R ９８ Condition C3: R ７０ ≧0% Condition C4: R ９８ ≦25%

[0031] Conditions C1 and C2 may indicate that the heat shrinkage ratio of the heat-shrinkable film 1 increases monotonically with increasing heat shrinkage temperature in the range of 70°C to 98°C. Generally, the heat shrinkage ratio of a uniaxially stretched film in a direction perpendicular to the main shrinkage direction does not increase monotonically with increasing heat shrinkage temperature, but tends to decrease along the way. This is because heat shrinkage begins earlier in the main shrinkage direction than in the direction perpendicular to the main shrinkage direction, and elongation occurs in the direction perpendicular to the main shrinkage direction in response to heat shrinkage in the main shrinkage direction. However, as the stretch ratio in the direction perpendicular to the main shrinkage direction increases (approaching biaxial orientation), the heat shrinkage ratio in the direction perpendicular to the main shrinkage direction increases to a certain extent, the influence of heat shrinkage in the main shrinkage direction decreases, and the heat shrinkage ratio of the heat-shrinkable film increases monotonically. Therefore, conditions C1 and C2 mean that the stretch ratio in the direction perpendicular to the main shrinkage direction is relatively high and that strength in the same direction is maintained to a certain extent. Therefore, conditions C1 and C2 ensure the break resistance in the direction perpendicular to the main shrinkage direction.

[0032] On the other hand, if the heat shrinkage rate increases excessively due to an increase in the stretch ratio, the finish quality may be degraded. In this regard, condition C4 ensures that the stretch ratio in the direction perpendicular to the main shrinkage direction is relatively high while the heat shrinkage rate in the direction perpendicular to the main shrinkage direction is suppressed, ensuring the finish quality of the label when attached to a container. Furthermore, if the heat shrinkage rate becomes negative (expands), problems such as poor break resistance may occur. However, condition C3 prevents the heat shrinkage rate from becoming negative, making problems less likely to occur.

[0033] More preferably, the heat-shrinkable film 1 satisfies at least one of the following conditions C5 to C7. Even more preferably, the heat-shrinkable film 1 satisfies all of the conditions C5 to C7. Condition C5: R ７０ ≦7% Condition C6: R ８０ ≦13% Condition C7: R ９０ ≦17%

[0034] Regarding condition C5, R ７０ More preferably, R ７０ It is more preferable that R ≦3%. ８０ More preferably, R ８０ It is more preferable that R≦7%. ９０ More preferably, R ９０ It is more preferred that it is ≦11%.

[0035] The heat shrinkage rate of the heat-shrinkable film 1 in the main shrinkage direction (TD in this embodiment) at 98°C is U ９８ In this case, U ９８ ≧60%, and U ９８ More preferably, U is ≧65%. ９８ More preferably, it is ≧70%.

[0036] The stretching ratio in the direction perpendicular to the main shrinkage direction of the heat-shrinkable film 1 (MD in this embodiment) 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 main shrinkage direction (TD in this embodiment) 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 same direction is preferably 700% or less, more preferably 650% or less, and even more preferably 600% or less.

[0038] Examples of polystyrene resins contained in the mid layer 30 include styrene-butadiene block copolymer (SBC), general-purpose polystyrene (GPPS), and hyperbranched polystyrene. There are no particular limitations on the method for synthesizing hyperbranched polystyrene, but hyperbranched polystyrene generally refers to polystyrene having two or more branches in the polymer chain. An example of hyperbranched polystyrene is one obtained by polymerizing a styrene monomer and a hyperbranched macromonomer, and it may also be one copolymerized with other monomers such as an acrylic monomer.

[0039] The intermediate layer 30 preferably contains at least one of general-purpose polystyrene and hyperbranched polystyrene. General-purpose polystyrene and hyperbranched polystyrene have relatively high glass transition temperatures and relatively hard structures. Therefore, by including at least one of general-purpose polystyrene and hyperbranched polystyrene in the intermediate layer 30, the MD heat shrinkage rate of the heat-shrinkable film 1 can be suppressed. Furthermore, by including at least one of general-purpose polystyrene and hyperbranched polystyrene in the intermediate layer 30, the rigidity and stiffness of the heat-shrinkable film 1 can be maintained, thereby maintaining suitability for application machines (machines that apply the heat-shrinkable film to containers) and improving productivity. From this perspective, the amount of general-purpose polystyrene, the amount of hyperbranched polystyrene, or the total amount of general-purpose polystyrene and hyperbranched polystyrene contained in the intermediate layer 30 is preferably 10 wt % or more, more preferably 15 wt % or more, even more preferably 20 wt % or more, even more preferably 25 wt % or more, and even more preferably 30 wt % or more.

[0040] On the other hand, the amount of general-purpose polystyrene, the amount of hyperbranched polystyrene, or the total amount of general-purpose polystyrene and hyperbranched polystyrene contained in the intermediate layer 30 is preferably 80 wt % or less, more preferably 70 wt % or less, even more preferably 60 wt % or less, even more preferably 50 wt % or less, and even more preferably 40 wt % or less. If the amount of general-purpose polystyrene and / or hyperbranched polystyrene is too large, problems may arise, such as a decrease in the break resistance of the heat-shrinkable film 1, an excessively low heat shrinkage percentage in the main shrinkage direction, difficulty in stretching in the main shrinkage direction during production (e.g., tearing), and the like.

[0041] The intermediate layer 30 preferably contains SBC in addition to at least one of general-purpose polystyrene and hyperbranched polystyrene. The amount of SBC relative to the total amount of general-purpose polystyrene and / or hyperbranched polystyrene and SBC is preferably 20 wt% or more, more preferably 30 wt% or more, even more preferably 40 wt% or more, even more preferably 50 wt% or more, and even more preferably 60 wt% or more. Furthermore, the amount of SBC relative to the total amount of general-purpose polystyrene and / or hyperbranched polystyrene and SBC is preferably 90 wt% or less, more preferably 80 wt% or less, and even more preferably 70 wt% or less.

[0042] One method for improving the break resistance is to use a softer resin. However, in this case, the rigidity of the heat-shrinkable film decreases, causing it to lose its stiffness, which in turn may worsen its suitability for the mounting machine and reduce productivity. From the viewpoint of preventing this, the Young's modulus of the heat-shrinkable film 1 in the direction perpendicular to the main shrinkage direction (MD in this embodiment) is preferably 1.1 GPa or more, more preferably 1.3 GPa or more, and even more preferably 1.5 GPa or more. The Young's modulus here is defined by the measurement method included in the explanation of the examples described below.

[0043] 2. Method for Manufacturing Heat-Shrinkable Film and Packaging Container An example of a method for manufacturing a heat-shrinkable film 1 and a packaging container 2 will be described with reference to FIG. 2. The packaging container 2 is a container packaged with the heat-shrinkable film 1.

[0044] First, heat-shrinkable film 1 is produced by film production apparatus 50. As shown in FIG. 2 , 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. T-die 300 includes a T-die main body 301 and raw material input sections 330, 331, and 332. Raw material for first surface layer 10 is input into raw material input section 330, raw material for second surface layer 20 is input into raw material input section 332, and raw material for intermediate layer 30 is input into raw material input section 331. If an adhesive layer is inserted between first surface layer 10 and intermediate layer 30 and / or between second surface layer 20 and 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 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.

[0045] The longitudinal stretching machine 51 stretches the heat-shrinkable film 1 (molten material) cooled by the cast rolls 310, 320 in the MD at the stretching ratio described above. The transverse stretching machine 52 stretches the heat-shrinkable film 1 stretched in the MD at the stretching ratio described above in the TD. The heat-shrinkable film 1 stretched in various directions is taken up as a film roll F1.

[0046] 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.

[0047] Next, the film roll F2 is set in the cutter 70. In the cutter 70, the heat-shrinkable film 1 unwound from the film roll F2 is cut along the MD (longitudinal direction) by a cutter, thereby dividing the film into TD (transverse direction) pieces. The width of the division of the heat-shrinkable film 1 in the transverse direction corresponds to the 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 transverse direction are wound up as separate film rolls F3.

[0048] 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 (longitudinal direction) and formed into a cylindrical shape. Specifically, both ends of the heat-shrinkable film 1 in the TD (transverse 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 wound up as a film roll F4. The sealing method is not particularly limited, and possible methods include solvent sealing, heat sealing, and ultrasonic sealing.

[0049] 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 each container. The tubular unit labels are then placed on the containers so as to cover them from the outside, and the containers are inserted inside the tubular unit labels. The containers 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 closely to the outer surfaces of the containers, conforming to the outer shapes of the containers.

[0050] For example, a container covered with a tubular unit label is passed through a tunnel, which serves as a heating space. During this time, the tubular unit label is heated in various ways within the heating space, such as by spraying steam or hot air (hereinafter referred to as "steam, etc."). It is preferable that the portions of the tubular unit label covering the container with smaller cross-sectional diameters (such as the mouth and shoulders) are heated to higher temperatures, and the portions covering the container with larger cross-sectional diameters (such as the body) are heated to lower temperatures. Therefore, the temperature distribution within the heating space is not uniform, and different temperatures can be set depending on the zone. Although not limited thereto, it is generally preferable that the steam, etc., is approximately 100°C, and the container covered with the tubular unit label is heated to various temperatures between approximately 70°C and approximately 98°C depending on the zone. Adjusting the temperature distribution within the heating space in this manner makes it possible to reduce the occurrence of wrinkles and misalignment in the heat-shrinkable film 1 attached to the container. As a result, the heat-shrinkable film 1 is beautifully attached along the outer shape of the container. Through the above heating process, a packaging container 2 is produced.

[0051] In addition, the center sealing machine 80 may appropriately form perforations in the unit labels or cylindrical unit labels. The perforations are formed to make it easier to peel off the labels attached to the containers. Although not limited to this, for example, the center sealing machine 80 forms perforations extending in the MD.

[0052] [3. Features] The heat-shrinkable film 1 according to this embodiment has the above-described improvements made to the MD heat shrinkage rate, and therefore has excellent finish and break resistance when used as a label.

[0053] [4. Other Embodiments] Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment and various modifications are possible without departing from the spirit of the present invention. For example, the following modifications are possible.

[0054] 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, the heat-shrinkable film 1 may have a four-layer or more structure in which one or more additional layers are formed on the outer side of one or both of the first surface layer 10 and the second surface layer 20. 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.

[0055] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0056] [1. Examples and Comparative Examples] Heat-shrinkable films of Examples 1 to 5 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 primarily composed of polyester-based 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. These heat-shrinkable films were produced by co-extruding the raw materials for each layer shown in Tables 1 and 2 into an extruder in the weight ratios shown in the tables (the numbers listed next to the raw material names), and then stretching them in the longitudinal and transverse directions. The longitudinal stretch ratio was set as shown in Tables 1 and 2, and the transverse stretch ratio was 500%.

[0057]

[0058]

[0059] Details of the raw materials shown in Tables 1 and 2 are as follows: Polyester resin: manufactured by Eastman Chemical Company, amorphous polyethylene terephthalate resin, glass transition temperature 82°C Polystyrene resin A: manufactured by Denka Company, SBC, butadiene content 25%, Vicat softening point 84°C Polystyrene resin B: manufactured by Denka Company, SBC, butadiene content 22%, Vicat softening point 72°C Polystyrene resin C: manufactured by DIC Corporation, styrene homopolymer, general-purpose polystyrene, Vicat softening point 95°C Polystyrene resin D: manufactured by DIC Corporation, hyperbranched polystyrene, Vicat softening point 96°C

[0060] [2. Various Measurement and Test Methods] <2-1. Heat Shrinkage Rate> Each heat-shrinkable film of Examples 1 to 5 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 warm water at 70°C, 80°C, or 90°C or 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 rate was calculated according to the following formula (1), and the TD heat shrinkage rate was calculated according to the following formula (2). In formula (1), LMD is the MD length (mm) of the test piece after heat shrinkage, and LTD is the TD length (mm) of the test piece after heat shrinkage. The heat shrinkage rate 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} x 100 (1) Heat shrinkage rate (%) = {(100 - LTD) / 100} x 100 (2)

[0061] <2-2. Young's Modulus> Each of the heat-shrinkable films of Examples 1 to 5 and Comparative Examples 1 to 4 was cut into a sample measuring 250 mm lengthwise (measurement direction) × 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.

[0062] <2-3. Break Resistance> Each heat-shrinkable film of Examples 1 to 5 and Comparative Examples 1 to 4 was molded into a cylindrical shape using a center-sealing machine. Continuous perforations were formed in the MD on both sides of the sealed portion of each cylindrically molded heat-shrinkable film. The perforations were 0.5 mm in size and 4.0 mm in pitch. Each cylindrically molded heat-shrinkable film was then manually cut into cylindrical unit labels, which were then manually placed around beverage containers (PET bottles) and passed through a heating space to produce packaging containers. A Fuji Astec steam tunnel (SH-5000) was used as the heating space, with the first zone temperature set to 75°C, the second zone temperature set to 85°C, and the third zone temperature set to 95°C. The manufactured packaging containers were then air-dried, and then placed horizontally and allowed to freely fall horizontally onto a drop surface. The bottom of the packaging container before the drop was positioned 120 cm above the drop surface. The presence or absence of fracture defects such as breaks in the perforations or cracks was then judged. This test was carried out on five packaging containers, and they were classified into three levels: good (◯) if there were no defects, fair (△) if there were 1 to 3 defects, and unacceptable (×) if there were 4 to 5 defects.

[0063] <2-4. Finish> Labels attached to packaging containers manufactured as described in the previous section (breakage resistance) were judged for the presence or absence of attachment defects such as the lower part slipping up or the upper part slipping down. This test was carried out on five packaging containers, and the results were classified into three levels: good (◯) if there were no defects, fair (△) if there were 1 to 3 defects, and poor (×) if there were 4 to 5 defects.

[0064] [3. Discussion] Tables 1 and 2 show that excellent label finish and break resistance can be obtained by satisfying conditions C1 to C4. Note that in Example 3, the finish was slightly inferior compared to Examples 1, 2, 4, and 5, but was still within an acceptable range. It is believed that Examples 1, 2, 4, and 5 achieved particularly excellent finish because the MD heat shrinkage at 98°C was suppressed.

[0065] Comparative Example 1 is R ７０is negative, condition C3 is not satisfied, and break resistance is impaired. In Comparative Examples 2 and 3, condition C1 is not satisfied, and the heat shrinkage does not increase monotonically in the range of 70°C to 98°C, so break resistance is significantly impaired. On the other hand, in Comparative Example 4, although the heat shrinkage increases monotonically in the range of 70°C to 98°C, the MD heat shrinkage at 98°C is high, so condition C4 is not satisfied, and finishability is significantly impaired.

[0066] DESCRIPTION OF SYMBOLS 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 to F4 Film roll

Claims

1. A film having a layer containing a polystyrene resin, the thermal shrinkage rate in the direction perpendicular to the main shrinkage direction at 70°C, 80°C, 90°C, and 98°C is R ７０ , R ８０ , R ９０ , R ９８ When this is done, R ７０ ≦R ８０ ≦R ９０ ≦R ９８ , R ７０ <R ９８ , R ７０ ≧0%, and R ９８ 25%≦A.

2. R ７０ 2. The heat-shrinkable film of claim 1, wherein the elongation is ≦7%.

3. R ８０ 3. The heat-shrinkable film according to claim 1, wherein the elongation coefficient is ≦13%.

4. R ９０ 3. The heat-shrinkable film of claim 1, wherein the elongation is ≦17%.

5. A heat-shrinkable film according to claim 1 or 2, wherein the stretching ratio in the direction perpendicular to the main shrinkage direction is 120% or more.

6. The heat-shrinkable film according to claim 1 or 2, further comprising a layer containing a polyester-based resin on at least one side of the layer containing a polystyrene-based resin.

7. The heat-shrinkable film according to claim 1 or 2, wherein the layer containing a polystyrene-based resin contains at least one of general-purpose polystyrene (GPPS) and hyperbranched polystyrene.

8. The heat-shrinkable film according to claim 1 or 2, having a film thickness of 35 μm or less.

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