Heat-shrinkable polyester film

Abstract

The present invention addresses the problem of providing a heat-shrinkable polyester film having improved shrinkage finish properties without limiting the raw material composition. The present invention is a heat-shrinkable polyester film characterized by containing ethylene terephthalate as a main constituent component and satisfying items (1)-(4) below. (1) A skirt ratio of 1-8% when the film is measured after being fixed in a state of being shrunk by 10% in a main shrinkage direction in 90°C hot air and heat-treated for one minute (2) A maximum heat shrinkage stress in the main shrinkage direction of 1-10 MPa when the film is fixed in a state of being shrunk by 10% in the main shrinkage direction in 90°C hot air and is measured while being heat-treated for one minute (3) A shrinkage rate of 20-72% in the main shrinkage direction when the film is immersed in 80°C hot water for 10 seconds (4) The average refractive index of the film is 1.57-1.585.

Description

Heat-shrinkable polyester film

[0001] The present invention relates to a heat-shrinkable polyester film suitable for label applications and container packaging. Specifically, the film is suitable for use in labels for beverage bottles and banding of containers such as lunch boxes, and is particularly environmentally friendly and has excellent finish properties.

[0002] In recent years, heat-shrinkable films have come into widespread use for applications such as label packaging that combines the protection of glass or plastic bottles and product labeling, cap seals, stacked packaging, and banding applications for bundling lunch boxes and the like. Among these heat-shrinkable films, polyvinyl chloride films have problems such as low heat resistance, the generation of hydrogen chloride gas when incinerated, and the generation of dioxins. Polystyrene films have poor solvent resistance, require the use of inks with special compositions when printed, and require incineration at high temperatures, resulting in the generation of large amounts of black smoke accompanied by an unpleasant odor when incinerated. For these reasons, polyester-based heat-shrinkable films, which have high heat resistance, are easy to incinerate, and have excellent solvent resistance, tend to be widely used as shrink labels.

[0003] Among these, polyester-based heat-shrinkable films have room for improvement in finish compared to other materials, and the present inventors, as manufacturers of polyester-based heat-shrinkable films, have conducted extensive research (see, for example, Patent Document 1). Patent Document 2 also discloses a technology that can improve shrinkage finish when used as a label, particularly when shrinking onto a container with a fixed point, such as a square container. Patent Document 2 specifies a diethylene glycol content of 1 to 20 mol%, and states that a diethylene glycol content of 3 mol% or more improves the skirt ratio suppression effect. Patent Document 2 also states that the skirt ratio can be suppressed if the maximum stress generated by immersion in 90°C hot water for 10 seconds is 6N or less. Meanwhile, in recent environmental efforts, there is a method of using recycled materials from PET bottles in heat-shrinkable polyester films to reduce environmental impact (see, for example, Patent Document 3). Patent Document 2 requires a limited raw material composition to suppress the skirt ratio, making it difficult to increase the recycled material content. Furthermore, the skirt ratio of the film described in the examples of Patent Document 2 was in the range of 10 to 13%, and the appearance after shrinkage was unsatisfactory.

[0004] JP 2011-94159 A Japanese Patent No. 5971897 A Japanese Patent No. 6802535 A

[0005] FJ Balta Calleja; MC Garcia Gutierrez; DR Rueda; S. Piccarolo. Structure development in poly (ethyleneterephthalate) quenched from the melt at high cooling rates: X-ray scattering and microhardness study. Polymer. 2000, vol. 41, p. 4143-4148.Atsushi Taniguchi; Miko Cakmak. The suppression of strain induced crystallization in PET through sub-micron TiO2 particle incorporation. Polymer. 2004, vol. 45, p. 6647-6654.

[0006] In view of the above problems, an object of the present invention is to provide a heat-shrinkable polyester film which has improved shrink finish without limiting the raw material composition.

[0007] The present invention comprises the following configurations: 1. A heat-shrinkable polyester film containing ethylene terephthalate as a main constituent component, and characterized by satisfying the following (1) to (4): (1) a skirt ratio of 1% to 8% measured after the film is shrunk by 10% in the main shrinkage direction in 90°C hot air, fixed in that state, and heat-treated for 1 minute; (2) a maximum heat shrinkage stress of 1 MPa to 10 MPa in the main shrinkage direction measured after the film is shrunk by 10% in the main shrinkage direction in 90°C hot air, fixed in that state, and heat-treated for 1 minute; (3) a shrinkage rate of 20% to 72% in the main shrinkage direction when the film is immersed in 80°C hot water for 10 seconds; and (4) an average refractive index of the film of 1.57 to 1.585. 2. The heat-shrinkable polyester film according to 1., characterized in that the storage modulus at 90°C in the direction perpendicular to the main shrinkage direction of the film is 8 MPa to 100 MPa. 3. The heat-shrinkable polyester film according to 1. or 2., characterized in that the shrinkage rate in a direction perpendicular to the main shrinkage direction when the film is immersed in 80°C hot water for 10 seconds is between -10% and 10%. 4. The heat-shrinkable polyester film according to any one of 1. to 3., characterized in that the polyester components constituting the film contain between 1 mol% and 30 mol% of one or more monomer components selected from the group consisting of isophthalic acid, neopentyl glycol, 1,4-cyclohexanedimethanol, 1,4-butanediol, and diethylene glycol. 5. The heat-shrinkable polyester film according to any one of 1. to 4., characterized in that the raw materials constituting the film contain mechanically recycled and / or chemically recycled polyester raw materials. 6. A heat-shrinkable label, at least a part of which is made of the heat-shrinkable polyester film according to 1. to 5. 7. A package, characterized in that the heat-shrinkable polyester film according to 1. to 5. is used. 8. A package, characterized in that the label according to 6. is used.

[0008] According to the present invention, it is possible to provide a heat-shrinkable polyester film having improved shrink finish without limiting the raw material composition.

[0009] Schematic diagram of a sample when measuring shrinkage stress after 10% shrinkage and skirt ratio after 10% shrinkage

[0010] The heat-shrinkable polyester film of the present invention will be described below. [1] Properties of the heat-shrinkable polyester film Next, the properties of the heat-shrinkable polyester film of the present invention will be described.

[0011] [1-1] Skirt Ratio After 10% Shrinkage The heat-shrinkable polyester film of the present invention must have a skirt ratio (hereinafter sometimes simply referred to as skirt ratio) of 1% to 8% measured after shrinking for 1 minute in 90°C hot air with a 10% sag in the main shrinkage direction. A skirt ratio exceeding 8% is undesirable because it increases the amount of sink marks after the heat-shrinkable film is wrapped around an object and shrunk. While a lower skirt ratio is preferable in light of the gist of the present invention, a skirt ratio below 1% is undesirable because the 80°C shrinkage rate in the main shrinkage direction is likely to fall below 20%. The skirt ratio is more preferably 1.5% to 7.5%, and even more preferably 2% to 7%. That is, the skirt ratio is 1 to 8%, more preferably 1.5 to 7.5%, and even more preferably 2 to 7%.

[0012] [1-2] Maximum 90°C Heat Shrinkage Stress After 10% Shrinkage The heat-shrinkable polyester film of the present invention must have a maximum heat shrinkage stress in the main shrinkage direction (hereinafter sometimes simply referred to as heat shrinkage stress) of 1 MPa to 10 MPa when shrunk 10% in the main shrinkage direction in 90°C hot air, and then shrunk at a fixed length in the shrinkage direction in 90°C hot air. A heat shrinkage stress exceeding 10 MPa is undesirable because it tends to result in a skirt ratio exceeding 8%. From the perspective of the present invention, a lower heat shrinkage stress is preferable because it suppresses the skirt ratio, but a heat shrinkage stress below 1 MPa is undesirable because it tends to result in the 80°C hot water shrinkage rate in the main shrinkage direction (described below) falling below 20%. The relationship between heat shrinkage stress and skirt ratio will be explained assuming that the heat-shrinkable film is used as a bottle label.

[0013] Bottle labels are made from a ring-shaped heat-shrinkable film, and their circumference is typically about 10% larger than the bottle diameter, assuming they will be placed over the bottle. That is, when the label shrinks, it first shrinks to eliminate the space between the bottle and the label (the 10% excess circumference), and then continues to shrink while adhering to the bottle until heating is complete. Shrinking the film while adhering to the bottle means that both ends of the film in the main shrinkage direction are fixed and shrink, which is the same as the application of a tensile force in the main shrinkage direction. Generally, when a tensile force is applied to a film, stress and strain are generated in a direction perpendicular to the tensile force due to the Poisson effect. When a label shrinks, this tensile force corresponds to the thermal shrinkage stress, and the greater the thermal shrinkage stress, the greater the strain in the perpendicular direction, i.e., the skirt ratio. The thermal shrinkage stress is preferably 1.5 MPa or more and 9.5 MPa or less, and even more preferably 2 MPa or more and 9 MPa or less. That is, the heat shrinkage stress is 1 to 10 MPa, more preferably 1.5 to 9.5 MPa, and even more preferably 2 to 9 MPa.

[0014] [1-3] 80°C Heat Shrinkage in the Main Shrinkage Direction The heat-shrinkable polyester film of the present invention is required to have a heat shrinkage in the main shrinkage direction of 20% or more and 72% or less when immersed in 80°C warm water for 10 seconds under no load and then immediately immersed in water at 25°C ± 0.5°C for 10 seconds, as calculated from the lengths before and after shrinkage using the following formula 1: Heat shrinkage = {(length before shrinkage - length after shrinkage) / length before shrinkage} × 100 (%) Formula 1

[0015] If the 80°C heat shrinkage rate in the main shrinkage direction exceeds 72%, the skirt ratio is likely to exceed 8%, which is undesirable. This is for the same reason as the Poisson effect explained in [1-2] Maximum 90°C heat shrinkage stress after 10% shrinkage. On the other hand, if the 80°C heat shrinkage rate in the main shrinkage direction is less than 20%, it is undesirable when used for bottle labels or as a film for lunch box packaging, as wrinkles may occur after heat shrinkage. The 80°C heat shrinkage rate in the main shrinkage direction is more preferably 20% to 70%, and even more preferably 20% to 68%. That is, the 80°C heat shrinkage rate in the main shrinkage direction is 20 to 72%, more preferably 20 to 70%, and even more preferably 20 to 68%.

[0016] [1-4] Average Refractive Index The heat-shrinkable polyester film of the present invention must have an average refractive index of 1.57 or more and 1.585 or less. The inventors have discovered that the average refractive index represents the rigidity of the film and acts as an antagonistic force to the Poisson effect that occurs when a film to be used as a label is heated and shrunk, as described in [1-2] Maximum 90°C Heat Shrinkage Stress After 10% Shrinkage. Therefore, if the average refractive index is below 1.57, it is believed that the film will be overwhelmed by strain stress in the direction perpendicular to the main shrinkage direction, resulting in a skirt ratio exceeding 8%. On the other hand, if the average refractive index exceeds 1.585, there is a risk that the 80°C heat shrinkage rate in the main shrinkage direction will fall below 20% and the heat shrinkage stress will likely exceed 10 MPa, which is undesirable. The average refractive index is more preferably 1.5705 or more and 1.5845 or less, and even more preferably 1.571 or more and 1.584 or less. That is, the average refractive index is 1.57 to 1.585, more preferably 1.5705 to 1.5845, and even more preferably 1.571 to 1.584.

[0017] [1-5] Storage Modulus at 90°C in a Direction Perpendicular to the Main Shrinkage Direction The heat-shrinkable polyester film of the present invention preferably has a storage modulus at 90°C (hereinafter sometimes simply referred to as storage modulus) of 8 MPa or more and 100 MPa or less. A storage modulus below 8 MPa is undesirable because the skirt ratio is likely to exceed 8%. The storage modulus is thought to act as an antagonistic force to the Poisson effect that occurs when a film to be used as a label is heated and shrunk, as explained in [1-2] Maximum 90°C Heat Shrinkage Stress After 10% Shrinkage. Therefore, if the storage modulus is below 8 MPa, it is thought that the film will be overwhelmed by the strain stress in the direction perpendicular to the main shrinkage direction, resulting in a skirt ratio exceeding 8%. A higher storage modulus is preferable because skirt formation can be suppressed, but a storage modulus of 100 MPa or more is undesirable because the 80°C heat shrinkage rate in the main shrinkage direction is likely to fall below 20%. A storage modulus of 9 MPa or more and 90 MPa or less is more preferable, and a storage modulus of 10 MPa or more and 80 MPa or less is even more preferable. That is, the storage modulus is 8 to 100 MPa, more preferably 9 to 90 MPa, and even more preferably 10 to 80 MPa.

[0018] [1-6] 80°C Shrinkage in the Direction Perpendicular to the Main Shrinkage Direction The heat-shrinkable polyester film of the present invention is preferably immersed in 80°C warm water for 10 seconds under no load, followed immediately by immersion in 25°C±0.5°C water for 10 seconds. The heat shrinkage in the direction perpendicular to the main shrinkage direction of the film, calculated using the above formula 1 from the lengths before and after shrinkage, is preferably -10% to 10%. An 80°C heat shrinkage in the orthogonal direction exceeding 10% is undesirable because the skirt ratio is likely to exceed 8%. On the other hand, an 80°C heat shrinkage in the orthogonal direction of less than -10% is undesirable because it means that the film stretches in the orthogonal direction during heat shrinkage, making it prone to wrinkles during shrinkage finishing. The 80°C heat shrinkage in the orthogonal direction is more preferably -10% to 10%, and even more preferably -9% to 9%. That is, the 80°C heat shrinkage in the orthogonal direction is -10 to 10%, more preferably -9 to 9%.

[0019] [1-7] Reversible Heat Capacity Difference The heat-shrinkable polyester film of the present invention preferably has a reversible heat capacity difference before and after the glass transition temperature of 0.2 J / g·K or more and 0.3 J / g·K or less. A reversible heat capacity difference of 0.3 J / g·K or more means that the mobile amorphous content of the finished film (after stretching) is high, which is undesirable because the average refractive index is likely to fall below 1.57 and the skirt ratio is likely to exceed 8%. On the other hand, a reversible heat capacity difference of 0.2 J / g·K or less is undesirable because the 80°C heat shrinkage rate in the main shrinkage direction is likely to fall below 20%. The reversible heat capacity difference is more preferably 0.21 J / g·K or more and 0.3 J / g·K or less, and even more preferably 0.22 J / g·K or more and 0.29 J / g·K or less. That is, the reversible heat capacity difference is preferably 0.2 to 0.3 J / g·K, more preferably 0.21 to 0.3 J / g·K, and even more preferably 0.22 to 0.29 J / g·K.

[0020] [1-8] Glass Transition Temperature The heat-shrinkable polyester film of the present invention preferably has a glass transition temperature of 50°C or higher and 100°C or lower. A glass transition temperature lower than 50°C is undesirable because not only is the heat resistance of the heat-shrinkable label poor, but the storage modulus at 90°C is likely to fall below 8 MPa and the skirt ratio is likely to exceed 8%, which is not desirable. On the other hand, a glass transition temperature higher than 100°C is undesirable because the 80°C heat shrinkage rate in the main shrinkage direction is likely to fall below 20%. The glass transition temperature is more preferably 55°C or higher and 95°C or lower, and even more preferably 60°C or higher and 90°C or lower. That is, the glass transition temperature is preferably 50 to 100°C, more preferably 55 to 95°C, and even more preferably 60 to 90°C.

[0021] [1-9] Thickness The heat-shrinkable polyester film of the present invention preferably has a thickness of 4 μm or more and 100 μm or less. A thinner thickness is preferable because it increases the cooling efficiency of the molten resin during the casting process of film production (described below), thereby increasing the storage modulus and making it easier to suppress the skirt ratio. However, a thickness below 4 μm is undesirable because the film becomes extremely thin, making it difficult to handle (poor handling) in processes such as label production. On the other hand, a thickness above 100 μm is undesirable because the cooling efficiency of the molten resin during the casting process of film production is insufficient, making the storage modulus at 90°C less than 8 MPa and the skirt ratio more likely to exceed 8%. The thickness is more preferably 8 μm or more and 96 μm or less, and even more preferably 12 μm or more and 92 μm or less. That is, the thickness is preferably 4 to 100 μm, more preferably 8 to 96 μm, and even more preferably 12 to 92 μm.

[0022] [2] Method for producing heat-shrinkable polyester film [2-1] Polyester raw material The polyester raw material used in the heat-shrinkable polyester film of the present invention is one having ethylene terephthalate units as the main constituent component. "Mainly" means 50 mol% or more, and the ethylene terephthalate units preferably account for 50 mol% or more, more preferably 55 mol% or more, and even more preferably 60 mol% or more of 100 mol% of the constituent units of the polyester.

[0023] Examples of dicarboxylic acid components other than terephthalic acid that constitute the polyester of the present invention include aromatic dicarboxylic acids such as isophthalic acid, orthophthalic acid, and 2,6-naphthalenedicarboxylic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid, and decanedicarboxylic acid; and alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid. Among these, isophthalic acid and orthophthalic acid are preferred. When an aliphatic dicarboxylic acid (e.g., adipic acid, sebacic acid, decanedicarboxylic acid, etc.) is incorporated into the polyester, the content is preferably less than 3 mol% (based on 100 mol% of the dicarboxylic acid component). Furthermore, it is preferable not to incorporate trivalent or higher polycarboxylic acids (e.g., trimellitic acid, pyromellitic acid, and anhydrides thereof) into the polyester. Heat-shrinkable polyester films obtained using polyesters containing these polycarboxylic acids are less likely to achieve the required shrinkage percentage.

[0024] Examples of diol components other than ethylene glycol that constitute the polyester of the present invention include alicyclic diols such as neopentyl glycol and 1,4-cyclohexanedimethanol, aliphatic diols such as 1,4-butanediol, diethylene glycol, 1,3-propanediol, and hexanediol, and aromatic diols such as bisphenol A. Of these, neopentyl glycol, 1,4-cyclohexanedimethanol, 1,4-butanediol, and diethylene glycol are preferred. It is preferable to contain 1 mol % to 30 mol % of the above-mentioned dicarboxylic acid component other than terephthalic acid and / or diol component other than ethylene glycol. It is more preferable to contain at least one of the above-mentioned components: isophthalic acid, neopentyl glycol, 1,4-cyclohexanedimethanol, 1,4-butanediol, and diethylene glycol. In addition to polyester raw materials containing the above-mentioned components, polyester elastomers may also be used in the film of the present invention. Examples of polyester elastomers include copolymers of ε-caprolactone and 1,4-butanediol and polytetramethylene glycol.

[0025] In addition to the above polyester raw materials, the use of recycled polyester raw materials from collected used waste such as PET bottles is also a preferred embodiment from the environmental perspective of the present invention. Examples of polyester resins recycled from PET bottles include polyester resins obtained by a physical regeneration method in which used PET bottles collected from the market or society are sorted, crushed, and washed to thoroughly remove surface dirt and foreign matter, then exposed to high temperatures to thoroughly remove contaminants remaining inside the resin, and then re-pelletized (hereinafter sometimes referred to as "mechanically recycled polyester resin"); and polyester resins obtained by decomposing polyester resins contained in used packaging containers to the monomer level, removing contaminants, and then polymerizing them again (hereinafter sometimes referred to as "chemically recycled polyester resin"). Recycled raw materials that can be preferably used in the present invention are primarily recycled containers made primarily of polyethylene terephthalate. For example, recycled containers for beverages such as tea drinks and soft drinks are preferably used. These may be appropriately oriented, and colorless bottles are preferred, although they may contain a small amount of coloring components. The content of recycled raw materials is preferably 5% by weight or more and 100% by weight or less, within the range of 1 mol% or more and 30 mol% or less of the dicarboxylic acid component other than terephthalic acid and / or diol component other than ethylene glycol. If the content of recycled raw materials is less than 5% by weight, the proportion of recycled raw materials contained in the raw materials used for film production is very small, resulting in little effect on reducing environmental impact. The content of recycled raw materials is more preferably 6% by weight or more, and even more preferably 7% by weight or more. On the other hand, the upper limit of the raw materials used for film production is 100% by weight. A higher content of recycled raw materials is preferable because it is more effective in reducing environmental impact. However, since it is also preferable to use a lubricant as a masterbatch to impart slip properties to the film or to use raw materials other than recycled raw materials to adjust the shrinkage rate, an upper limit of 95% by weight of recycled raw materials is sufficient for practical use.

[0026] Recycled raw materials from the market or society, including PET bottles, are preferably polyesters produced and molded by conventional polymerization and solid-state polymerization, preferably composed primarily of polyethylene terephthalate, and may contain other polyester components or copolymer components. They may contain metal compounds such as antimony, germanium, and titanium as catalysts, and phosphorus compounds as stabilizers. Polyesters for PET bottles typically contain germanium as a catalyst, and when recycled PET bottle raw materials are used to produce a film, the germanium content in the film will be 1 ppm or more. However, since this refers solely to the catalyst content, it is usually no more than 100 ppm, and usually no more than 50 ppm. Various additives, such as waxes, antioxidants, antistatic agents, nucleating agents, viscosity reducers, heat stabilizers, coloring pigments, color inhibitors, and UV absorbers, may be added to the resin forming the heat-shrinkable polyester film of the present invention as needed.

[0027] It is preferable to add fine particles as a lubricant to the resin forming the heat-shrinkable polyester film of the present invention to improve the workability (slipperiness) of the film. Any fine particles can be selected as the fine particles. Examples of inorganic fine particles include silica, alumina, titanium dioxide, calcium carbonate, kaolin, and barium sulfate. Examples of organic fine particles include acrylic resin particles, melamine resin particles, silicone resin particles, and cross-linked polystyrene particles. The average particle size of the fine particles is preferably in the range of 0.05 to 3.0 μm. The average particle size of the fine particles is measured using a Coulter counter. If the particle size is less than 0.05 μm, the smoothness required for handling may not be achieved. On the other hand, if the particle size exceeds 3.0 μm, the particles may aggregate and become too coarse, resulting in foreign matter and significantly degrading the appearance quality. The particle size is more preferably 0.06 μm to 2.99 μm, and even more preferably 0.07 μm to 2.98 μm. That is, the average particle size is preferably 0.05 to 3.0 μm, more preferably 0.06 to 2.99 μm, and even more preferably 0.07 to 2.98 μm.

[0028] The method for incorporating the particles into the resin that forms the heat-shrinkable polyester film of the present invention can be, for example, addition at any stage in the production of the polyester resin. Preferably, the particles are added as a slurry dispersed in ethylene glycol or the like at the esterification stage, or after the completion of the transesterification reaction and before the start of the polycondensation reaction, and the polycondensation reaction is then carried out. Alternatively, a method of blending a slurry of particles dispersed in ethylene glycol or water with the polyester resin raw material using a vented kneading extruder, or a method of blending dried particles with the polyester resin raw material using a kneading extruder, is also preferred. The method for incorporating the fine particles into the polyester raw material is not particularly limited. For example, the particles can be added at any stage in the production of the polyester resin. Preferably, the particles are added as a slurry dispersed in ethylene glycol or the like at the esterification stage, or after the completion of the transesterification reaction and before the start of the polycondensation reaction, and the polycondensation reaction is then carried out. Alternatively, the method may be such that a slurry of fine particles dispersed in ethylene glycol or water is blended with a polyester-based resin raw material using a vented kneading extruder; or a method in which dried fine particles are blended with a polyester-based resin raw material using a kneading extruder.

[0029] [2-2] Method for Producing Heat-Shrinkable Polyester Film The heat-shrinkable polyester film of the present invention can be obtained by melt-extruding the above-described polyester raw material using an extruder to form an unstretched film, and then stretching the unstretched film by the following predetermined method. In the present invention, the typical shrink film manufacturing method, transverse uniaxial stretching or sequential biaxial stretching in which longitudinal stretching is performed after transverse stretching, will be described as an example. However, these may be longitudinal uniaxial stretching or sequential biaxial stretching in which longitudinal stretching is performed after transverse stretching, or simultaneous biaxial stretching in which longitudinal and transverse stretching are performed simultaneously. The heat-shrinkable polyester film of the present invention may have a single-layer structure or a multilayer structure of two or more layers. When a multilayer structure is used, a structure of five or fewer layers is preferred.

[0030] [2-2-1] Extrusion Conditions When melt-extruding the raw resin, it is preferable to dry the polyester raw material using a dryer such as a hopper dryer or paddle dryer, or a vacuum dryer. After drying the polyester raw material in this manner, the polyester raw material is melted and extruded into a film using an extruder. The melt temperature when extruding the polyester raw material is preferably 200°C or higher and 300°C or lower. As described below, the key to the present invention is to increase the cooling efficiency of the molten resin in the cooling process after the extrusion process. Therefore, if the extrusion temperature exceeds 300°C, subsequent cooling may be insufficient, and the skirt ratio may easily exceed 8%. Furthermore, if the extrusion temperature exceeds 300°C, thermal decomposition of the resin may be excessively accelerated, which may lead to adhesion of foreign matter to the film or breakage during the film-forming process. On the other hand, from the perspective of the present invention, a lower extrusion temperature is preferable because it can suppress the skirt ratio, but an extrusion temperature below 200°C is undesirable because the extrusion pressure of the molten resin becomes too high, which may cause deformation of the filter in the melt line. The extrusion temperature is more preferably 205° C. or higher and 295° C. or lower, and even more preferably 210° C. or higher and 290° C. or lower. That is, the extrusion temperature is preferably 200 to 300° C., more preferably 205 to 295° C., and even more preferably 210 to 290° C. For extrusion, any existing method such as a T-die method or a tubular method can be used.

[0031] [2-2-2] Conditions for Producing Unstretched Films (Quenching of Molten Resin) After melt-extrusion of the raw resin, an unstretched film can be obtained by quenching the sheet-like molten resin. A suitable method for quenching the molten resin is to cast the molten resin from a die onto a chill roll and rapidly solidify it to obtain a substantially unoriented resin sheet (hereinafter, sometimes referred to as an unstretched film). The inventors have discovered that these quenching conditions for the molten resin improve the finished quality of the heat-shrinkable polyester film after the stretching process described below. Specifically, they found that the quenching conditions improve the average refractive index of the film and the storage modulus, thereby maintaining a low skirt ratio. The detailed mechanism is described below.

[0032] It is known that when polyester is cooled after melting, its molecular structure changes depending on the cooling rate. For example, Figure 4 of Non-Patent Document 1 shows the relationship between crystallinity and Vickers hardness when the cooling rate is changed. It can be seen that when the cooling rate is increased to a certain value (approximately 5%) of crystallinity, the crystallinity does not fall below that value, while the Vickers hardness further decreases. According to the authors of Non-Patent Document 1, samples with high Vickers hardness in this range exhibit the local presence of a relatively ordered structure (but not reflected in the crystallinity) that could serve as a precursor to crystals. Furthermore, apart from the description in Non-Patent Document 1, the molecular structure of polyester includes rigid amorphous and mobile amorphous structures in addition to crystals, and the former is considered to exist as an amorphous structure existing between crystals or as a precursor to crystals. Taking this into consideration, the present inventors speculated that when the cooling rate from the molten state is increased, a large amount of mobile amorphous structures will be produced in the unstretched film.

[0033] Next, we will explain the molecular structure changes that occur during the stretching process of the unstretched film produced in this manner. Non-Patent Document 2 examines the molecular structure of a polyester film containing titanium oxide particles when stretched. Figure 9 of this document shows the relationship between stretching ratio and crystallinity for each titanium oxide content, indicating that the greater the titanium oxide content, the more suppressed crystallization is with increasing stretching ratio. The authors explain this as follows: In plain films (no titanium oxide added), stretching promotes molecular chain orientation, while in the presence of fine particles such as titanium oxide, the presence of these particles inhibits orientation, resulting in a lower crystallinity. In other words, it is suggested that any substance, not just fine particles, that inhibits orientation during stretching will reduce the crystallinity and rigid amorphous content of the finished film after stretching. Based on the above mechanism, the present inventors conducted a detailed study to determine a molecular structure that can resist the Poisson effect when shrinking in the main shrinkage direction, i.e., improve the storage modulus in the direction perpendicular to the main shrinkage direction, while maintaining the shrinkage required for heat-shrinkable polyester films. The inventors then realized that the presence of crystals and rigid amorphous phases formed during the cooling process in the production of unstretched films inhibits the formation of orientation during stretching. Furthermore, they believed that if the formation of crystals and rigid amorphous phases could be suppressed by increasing the cooling rate, the formation of orientation could be promoted after stretching, resulting in the formation of more crystals and rigid amorphous phases, and the necessary properties could be obtained without limiting the raw material type. As a result, the average refractive index of the film could be increased, increasing the storage modulus, and ultimately succeeding in reducing the skirt ratio, leading to the realization of the present invention.

[0034] The cooling rate for cooling the molten resin is preferably 40°C / s or more and 300°C / s or less, as the average from immediately after the molten resin is extruded (extruded resin temperature) until it leaves the chill roll (hereinafter sometimes simply referred to as the average cooling rate). An average cooling rate of less than 40°C / s is undesirable because it results in little mobile amorphous formation in the unstretched film, and poor crystalline and rigid amorphous formation after stretching, making the skirt ratio more likely to exceed 8%. On the other hand, from the perspective of the present invention, a higher cooling rate is preferable because it can reduce the skirt ratio. However, increasing the cooling rate requires lowering the chill roll temperature or increasing the air blowing speed, which can lead to adverse effects such as condensation on the chill roll and disruption of the catenary of the molten resin (details will be described later). An upper limit of the average cooling rate of 200°C / s is sufficient for practical purposes. In order to increase the average cooling rate to 40°C / sec or more, it is effective to reduce the distance between the molten resin outlet and the chill roll, lower the chill roll temperature, blow air onto the molten resin, or immerse the chill roll in a water tank. These measures are explained in detail below.

[0035] [2-2-2-1] Distance between the Molten Resin Discharge Port and the Chill Roll Increasing the distance between the molten resin discharge port and the chill roll is effective for increasing the cooling efficiency of the molten resin, and this distance is preferably 10 mm or more and 150 mm or less. If this distance exceeds 150 mm, the cooling efficiency of the molten resin may be insufficient, and desired film properties such as skirt ratio may not be achieved. On the other hand, the closer the distance between the molten resin discharge port and the chill roll, the higher the cooling efficiency, which is preferable, but this is undesirable because there is a risk of equipment damage due to contact between the discharge port and the chill roll. The distance between the molten resin discharge port and the chill roll is more preferably 12 mm or more and 148 mm or less, and even more preferably 14 mm or more and 146 mm or less. That is, the distance between the molten resin discharge port and the chill roll is preferably 10 to 150 mm, more preferably 12 to 148 mm, and even more preferably 14 to 146 mm.

[0036] [2-2-2-2] Chill Roll Temperature Lower chill roll temperatures are preferable because they increase the cooling efficiency of the molten resin, making it easier to achieve a skirt ratio of 8% or less. The cooling efficiency of the molten resin is determined by balancing other measures (such as the distance between the molten resin outlet and the chill roll in [2-2-2-1] above and the air blowing onto the molten resin in [2-2-2-3] below), making it difficult to determine an upper limit for the chill roll temperature. However, a preferred example is 30°C or less. Meanwhile, the lower limit of the chill roll temperature is preferably at least 1°C above the dew point in the unstretched film production chamber. Lower chill roll temperatures are preferable because they increase the cooling efficiency of the molten resin. However, if the temperature falls below 1°C above the dew point, condensation may form on the chill roll, causing the unstretched film to wrap around the chill roll. Reducing the indoor dew point is effective by reducing the temperature and humidity. For example, a preferred method is to control the dew point using an air-conditioning dehumidifier. A dew point of 5°C or higher and 25°C or lower is preferred. A dew point exceeding 25°C is undesirable because condensation easily occurs on the chill roll. A dew point below 5°C is preferable from the viewpoint of lowering the chill roll temperature, but is undesirable because it results in the continued use of a large amount of power when using an air-conditioning dehumidifier, as described above. Because the indoor dew point is determined by the balance between temperature and humidity, it is not possible to determine a general range for the temperature and humidity. However, as an example, the indoor temperature is preferably 5°C or higher and 25°C or lower, and the humidity is preferably 25% or higher and 75% or lower. Temperatures above 25°C or humidity above 75% are undesirable because the dew point is likely to exceed 25°C. Temperatures below 5°C or humidity below 25% are undesirable because they result in the continued use of a large amount of power by the air-conditioning dehumidifier.

[0037] [2-2-2-3] Blowing Air to the Molten Resin In the present invention, blowing air to the molten resin is effective for increasing cooling efficiency. Blowing air to the surface opposite to the surface in contact with the chill roll is particularly preferable because it reduces the temperature difference between the front and back of the molten resin (unstretched film) and further increases cooling efficiency. A multi-duct or other blower can be used as the blowing method. The blowing location is preferably behind the point where the molten resin lands on the chill roll. Blowing air to the molten resin before it lands on the chill roll is undesirable because the wind disrupts the catenary line of the molten resin before it reaches the chill roll, which can change the width of the unstretched resin film or the adhesion state between the molten resin and the chill roll. The blowing speed is preferably 2 m / s or more and 20 m / s or less. If the blowing speed is less than 2 m / s, the cooling efficiency of the molten resin will be insufficient. On the other hand, if the air blowing speed exceeds 20 m / sec, even if the air blowing location is appropriate, the catenary of the molten resin is likely to be disturbed, which is undesirable.

[0038] [2-2-3] Control of longitudinal stretching conditions In the production of the heat-shrinkable polyester film of the present invention, when the longitudinal direction is not the main shrinkage direction, the stretching ratio in the longitudinal direction is preferably 2 times or less. The lower limit of the longitudinal stretching ratio is 1 times (unstretched). A longitudinal stretching ratio greater than 2 times is undesirable because the 80°C shrinkage in the main shrinkage direction is likely to exceed 10%. The upper limit of the longitudinal stretching ratio is more preferably 1.8 times or less, and even more preferably 1.6 times or less. A particularly preferable longitudinal stretching ratio is the lower limit of 1 times. When longitudinal stretching is the main shrinkage direction, the stretching ratio in the longitudinal direction is preferably 3 times or more and 5.8 times or less. If the longitudinal stretching ratio is less than 3 times, it becomes difficult to achieve an 80°C heat shrinkage in the main shrinkage direction of 20% or more. On the other hand, if the stretching ratio exceeds 5.8 times, the 80°C heat shrinkage in the main shrinkage direction and the heat shrinkage stress become higher than the specified range, and the skirt ratio is likely to exceed 8%, which is undesirable. When the longitudinal direction is the main shrinkage direction, the stretching ratio is more preferably 3.2 to 5.6 times, and even more preferably 3.4 to 5.4 times. That is, when the longitudinal direction is the main shrinkage direction, the stretching ratio is preferably 3 to 5.8 times, more preferably 3.2 to 5.6 times, and even more preferably 3.4 to 5.4 times. When stretching in the longitudinal direction, it is preferable to preheat a substantially unoriented film to a temperature of Tg or higher and Tg + 60°C or lower. Furthermore, longitudinal stretching can be either one-stage stretching or two or more multi-stage stretching.

[0039] [2-2-4] Control of Transverse Stretching Conditions In the present invention, when the main shrinkage direction is the transverse direction, transverse stretching is preferably performed in a tenter with both ends in the width direction held by clips at a temperature of Tg or higher and Tg + 50°C or lower, at a stretching ratio of 3.5 to 6 times. A transverse stretching temperature of Tg or lower is undesirable because the heat shrinkage stress tends to exceed 10 MPa. On the other hand, a transverse stretching temperature higher than Tg + 50°C is undesirable because the 80°C shrinkage rate in the main shrinkage direction tends to fall below 20%. The transverse stretching temperature is more preferably Tg + 2°C or higher and Tg + 48°C or lower, and even more preferably Tg + 4°C or higher and Tg + 46°C or lower. That is, the transverse stretching temperature is preferably Tg to Tg + 50°C, more preferably Tg + 2°C to Tg + 48°C, and even more preferably Tg + 4°C to Tg + 46°C. Furthermore, a stretching ratio of less than 3.5 times is undesirable because the 80°C shrinkage rate in the main shrinkage direction is likely to fall below 20%, which is not desirable. On the other hand, a stretching ratio of more than 6 times is undesirable because the heat shrinkage stress is likely to exceed 10 MPa. The stretching ratio in the transverse direction is more preferably 3.6 times or more and 6.1 times or less, and even more preferably 3.7 times or more and 6.2 times or less. That is, the stretching ratio in the transverse direction is preferably 3.5 to 6 times, more preferably 3.6 to 6.1 times, and even more preferably 3.7 to 6.2 times. When the longitudinal direction is the main shrinkage direction, the stretching ratio in the transverse direction is preferably 1 time or more and 2.4 times or less. A stretching ratio in the transverse direction of more than 2.4 times is undesirable because the 80°C heat shrinkage rate in the direction perpendicular to the main shrinkage direction is likely to exceed 10%, which is not desirable. The lower limit of the transverse stretching ratio is 1 time (unstretched). The transverse stretching temperature is preferably Tg or more and Tg + 80°C or less. If the transverse stretching temperature is lower than Tg, not only will the 80°C heat shrinkage rate in the direction perpendicular to the main shrinkage direction easily exceed 10%, but there is also the possibility of problems with film formation stability, such as breakage. On the other hand, if the transverse stretching temperature exceeds Tg + 80°C, the 80°C shrinkage rate in the main shrinkage direction (in this case, the longitudinal direction) will easily fall below 20%, which is undesirable. The transverse stretching temperature is more preferably Tg + 5°C or higher and Tg + 75°C or lower, and even more preferably Tg + 10°C or higher and Tg + 70°C or lower.That is, the transverse stretching temperature is preferably Tg to Tg + 80° C., more preferably Tg + 5° C. to Tg + 75° C., and even more preferably Tg + 10° C. to Tg + 70° C. When the longitudinal direction is the main shrinkage direction, the timing of transverse stretching may be either before or after the longitudinal stretching.

[0040] [2-2-5] Heat Treatment and Relaxation Conditions After Stretching in the Shrinkage Direction If necessary, the film stretched as described above may be heat-treated in a tenter while both edges in the width direction are held with clips. Here, "heat treatment" refers to heat treatment at a temperature of 60°C or higher and 120°C or lower for 1 to 10 seconds. This type of heat treatment is preferred because it allows for adjustment of the 80°C shrinkage rate and heat shrinkage stress. If the heat treatment temperature is lower than 60°C, the above-mentioned effects of the heat treatment are not effectively exhibited. On the other hand, if the heat treatment temperature is higher than 120°C, the 80°C heat shrinkage rate in the main shrinkage direction is likely to fall below 20%, which is undesirable. The longer the heat treatment time, the more likely the effect is to be exhibited, but if it is too long, the equipment becomes large. Therefore, it is preferable to control the heat treatment time to 1 to 10 seconds. More preferably, it is 2 to 9 seconds, and even more preferably, it is 3 to 8 seconds. That is, the heat treatment time is preferably 1 to 10 seconds, more preferably 2 to 9 seconds, and even more preferably 3 to 8 seconds.

[0041] Furthermore, in the heat treatment step, relaxation in the width direction can be performed by shortening the distance between the holding clips in the tenter for the purpose of adjusting the 80°C shrinkage rate and the heat shrinkage stress. However, since relaxation can decrease the average refractive index and storage modulus, which are the technical points of the present invention, and there is a concern that the skirt ratio may exceed 8%, it is preferable that the relaxation rate is 5% or less.

[0042] [2-2-6] Surface Treatment One or both surface layers of the heat-shrinkable polyester film of the present invention may be subjected to a surface treatment. Examples of surface treatments include corona treatment, plasma treatment, flame treatment, sandblasting, and coating. These surface treatments are intended to improve adhesion, electrostatic charge characteristics, and the like, and can be applied within the scope of the present invention. The timing of the surface treatment can be selected as desired, either during or after film formation.

[0043] [3] Label The label of the present invention uses at least a portion of the heat-shrinkable polyester film of the present invention, and the heat-shrinkable polyester film of the present invention may be made of itself or may be bonded to another film. The length (circumferential length) of the label and the number of bonded portions may be arbitrary. As the other film constituting the label of the present invention, a conventionally known film may be used in any length. The material of the film may also be arbitrary, and examples thereof include polyester, polyolefin, and polyamide.

[0044] Adhesion methods required to produce the label of the present invention include solvent-based center sealing, heat sealing, ultrasonic sealing, and methods using hot glue (hot melt) or cold glue. Of these, solvent-based center sealing, heat sealing, or ultrasonic sealing is preferred. When using solvent-based center sealing, solvents such as 1,3-dioxolane and tetrahydrofuran can be used, but are not limited to these. Heat seal strength varies depending on the temperature, pressure, and sealing time of the heat seal bar (the surface that contacts the film), and is determined by the combination of these conditions. For example, even if the heat seal temperature is increased, if the sealing time is shortened, the actual temperature applied to the film will be lower. As an example, the heat seal temperature is preferably 100°C or higher but 180°C or lower, more preferably 110°C or higher but 170°C or lower, and even more preferably 120°C or higher but 160°C or higher. That is, the heat seal temperature is preferably 100 to 180°C, more preferably 110 to 170°C, and even more preferably 120 to 160°C. If the heat sealing temperature is below 100°C, the heat seal strength is likely to fall below 20 N / 15 mm. If the heat sealing temperature is above 180°C, the heat seal strength is improved and is preferable, but the film is more likely to heat shrink, which may cause changes in the label dimensions at the heat-sealed portion and deteriorate the appearance. The heat sealing pressure is preferably 0.05 MPa or more and 0.6 MPa or less, more preferably 0.1 MPa or more and 0.55 MPa or less, and even more preferably 0.15 MPa or more and 0.5 MPa or less. That is, the heat sealing pressure is preferably 0.05 to 0.6 MPa, more preferably 0.1 to 0.55 MPa, and even more preferably 0.15 to 0.5 MPa. If the pressure is below 0.05 MPa, the seal bar will not transfer sufficient heat to the film, and the heat seal strength will likely fall below 20 N / 15 mm. As far as the inventors have investigated, there is no significant change in the heat seal strength if the pressure is higher than 0.05 MPa, but if the pressure is higher than 0.6 MPa, there is a concern that the film may break.

[0045] The heat sealing time is preferably 0.1 seconds or more and 3 seconds or less, more preferably 0.2 seconds or more and 2.9 seconds or less, and even more preferably 0.3 seconds or more and 2.8 seconds or less. That is, the heat sealing time is preferably 0.1 to 3 seconds, more preferably 0.2 to 2.9 seconds, and even more preferably 0.3 to 2.8 seconds. If the sealing time is less than 0.1 seconds, the heat seal strength is likely to fall below 20 N / 15 mm. If the sealing time exceeds 3 seconds, the heat seal strength is improved, which is preferable, but this is not preferable because it goes against the purpose of the present invention of improving productivity. There are no limitations on the objects to be packaged with the label of the present invention, and examples include plastic containers and PET bottles with square or round shapes.

[0046] This application claims priority based on Japanese Patent Application No. 2023-110168, filed on July 4, 2023. The entire contents of the specification of Japanese Patent Application No. 2023-110168, filed on July 4, 2023, are incorporated herein by reference.

[0047] Next, the present invention will be specifically explained using examples and comparative examples, but the present invention is not limited to the aspects of these examples and can be appropriately modified within the scope of the present invention.

[0048] <Method for Evaluating Heat-Shrinkable Polyester Films> [Thickness] Using a micrometer (Militron 1254D manufactured by Finepfluh), measurements were taken at five points, and the average value was calculated.

[0049] [90°C Shrinkage Stress After 10% Shrinkage] A strip-shaped film sample measuring 150 mm in the main shrinkage direction (sample length) and 20 mm in the orthogonal direction (sample width) was cut from the heat-shrinkable film, and a benchmark was drawn so that the length in the main shrinkage direction was 100 mm. The skirt ratio was measured using a Tensilon universal testing machine RTC-1210A (manufactured by Orientec Co., Ltd.) equipped with a heating furnace. The furnace interior was preheated to 90°C, and the distance between the chucks for holding the film sample was 90 mm. When attaching the sample to the chuck, the air flow to the heating furnace was temporarily stopped, the furnace door was opened, and 25 mm of each end of the 150 mm long sample was clamped between the chucks to secure it. After attaching the sample to the chuck, the furnace door was quickly closed and the air flow was resumed. The point at which the door of the heating furnace was closed and air flow was resumed was taken as the starting point for measuring the shrinkage stress, and the maximum value of the measured shrinkage stress from the start of the measurement until 60 seconds after the start of the measurement was taken as the maximum value of the shrinkage stress (maximum shrinkage stress (MPa)). Figure 1 shows a schematic diagram of the sample before shrinkage.

[0050] [Skirt ratio after 10% shrinkage] In the above-mentioned [90°C shrinkage stress after 10% shrinkage], the sample was quickly removed from the heating furnace after the measurement of the shrinkage stress was completed. The width of the narrowest part of the removed sample was measured, and the skirt ratio was calculated according to the following formula 2. Figure 1 shows a schematic diagram of the sample before and after shrinkage. Skirt ratio = (sample width before shrinkage - sample width after shrinkage) / chuck distance x 100 (%) = (20 - sample width after shrinkage) / 90 x 100 (%) Formula 2

[0051] [Heat Shrinkage at 80°C] A heat-shrinkable film was cut into a 10 cm x 10 cm square and immersed in warm water at a predetermined temperature ±0.5°C for 10 seconds without load to allow it to heat shrink. The film was then immersed in water at 25°C ±0.5°C for 10 seconds, and after being pulled out of the water, the dimensions of the film in the longitudinal and transverse directions were measured, and the heat shrinkage ratio was calculated according to the following formula 1. The direction with the larger heat shrinkage ratio was defined as the main shrinkage direction. Heat shrinkage ratio = {(length before shrinkage - length after shrinkage) / length before shrinkage} × 100 (%) Formula 1

[0052] [Average Refractive Index] The refractive index was measured in accordance with JIS K7105. The film was cut into a width of 2 cm and a length of 3 cm to prepare a sample. The sample was prepared by cutting the longitudinal direction of the sample parallel to the longitudinal direction of the film. The refractive index (Ny) of this sample in the film width direction was measured using an Abbe refractometer 4T manufactured by Atago Optical Co., Ltd. The solvent used for the measurement was diiodomethane, and the measurement conditions were 23°C and 60% RH. The number of measurements was n=3.

[0053] [Storage Modulus] Samples measuring 15 mm in the main shrinkage direction and 4 mm in the direction perpendicular to the main shrinkage direction were cut out and measured using an RSA-G2 (manufactured by TA Instruments Japan). Storage modulus (E') was measured by raising the temperature from 20°C to 160°C at a rate of 10°C / min under a nitrogen atmosphere with a chuck distance of 10 mm, a load of 10 g, and a frequency of 10 Hz. From the obtained E' profile, the value at 90°C was adopted as the storage modulus. Note that if there is no E' measurement data at 90°C due to sampling interval issues (for example, if E' is obtained at 89.5°C and then at 91°C), the slope and intercept can be calculated from the data between two points before and after the 90°C boundary to determine E' corresponding to a temperature of 90°C.

[0054] [Reversible Heat Capacity (ΔCp)] The sample film was weighed at 5.0±0.2 mg and placed in a TA Instruments T-zero pan or an aluminum pan (flat dish shape). When the sample was placed in the pan, the film was stacked and punched into a circle (4.5 mm diameter) to ensure good adhesion to the pan bottom. "Good adhesion to the pan bottom" means that the film sample is not bent inside the pan when the sample is placed inside the pan, and there is no space between the stacked films when firmly pressed down with the lid. While wrinkles may be present in the film before punching, it is preferable to smooth out the wrinkles before punching the sample. The shape and size of the punched sample (punch) are not limited to the above, as long as the sample fits into the pan bottom without bending. The prepared samples were measured using a temperature-modulated differential scanning calorimeter (DSC) "DSC250" (manufactured by TA Instruments) in MDSC (registered trademark) heat-only mode at an average heating rate of 2°C / min and a modulation period of 40 seconds to obtain reverse heat flow. In the reverse heat flow obtained by the measurement, ΔCp was calculated from the signal that changed stepwise from the baseline. Specifically, at two inflection points that changed stepwise from the baseline, an extension line of each heat flow baseline was drawn, and the intersection with the tangent at each inflection point (Tg) was determined. The difference in the horizontal axis value between these intersections was defined as ΔCp.

[0055] Glass Transition Temperature (Tg) A sample film was weighed at 5.0±0.2 mg and placed in a TA Instruments T-zero pan or aluminum pan (flat dish shape). The sample was then melted by heating for 30 seconds on a hot plate heated to 300°C. The sample was then removed from the hot plate with tweezers and immediately immersed in liquid nitrogen for 1 minute to prepare a melt-quenched sample. When sealing the sample in the pan, the film was punched into a circular shape (4.5 mm in diameter) using a punch to ensure good adhesion to the pan bottom. "Good adhesion to the pan bottom" means that the film sample is not bent when sealed in the pan, and there is no space between the overlapping films when firmly pressed down with the lid. While wrinkles may be present in the film before punching, it is preferable to smooth out the wrinkles before punching the sample. The shape and size of the punched sample (punch) are not limited to the above, as long as the sample fits into the pan bottom without bending. The prepared melt-quenched samples were measured using a temperature-modulated differential scanning calorimeter (DSC) "DSC250" (manufactured by TA Instruments) in MDSC (registered trademark) heat-only mode at an average heating rate of 2°C / min and a modulation period of 40 seconds to obtain reverse heat flow. In the reverse heat flow obtained by the measurement, Tg, which appears as a signal that changes stepwise from the baseline, was determined. Specifically, extensions of the baseline of each heat flow were drawn on the lower and higher sides of Tg, and the intersections with the tangents at the inflection point (Tg) were determined. The value on the horizontal axis at this intersection was read, and the average value of the lower and higher sides was taken as Tg.

[0056] [Shrinkage Finish] The edges of the heat-shrinkable film were welded with dioxolane to obtain a cylindrical label with a folded diameter of 115 mm and a height of 157 mm (the label's main shrinkage direction was the circumferential direction of the heat-shrinkable film). This label was placed over a commercially available rectangular PET bottle (containing contents; "iMUSE" manufactured by Kirin Beverage Co., Ltd.) and heat-shrunk by passing it through steam adjusted to 85°C (tunnel passage time: 30 seconds). The shrinkage finish of the label was evaluated according to the following criteria.

[0057] <Sink (Skirt)> The difference between the bottom of the label and the part with the largest sink (skirt) was measured as the skirt amount, and was evaluated according to the following criteria: ○: Skirt amount 2 mm or less ×: Skirt amount more than 2 mm

[0058] <Jump-up> The distance between the bottom of the bottle (the part where the bottle comes into contact with the floor when the bottle is placed on a horizontal floor) and the bottom of the label was measured as the jump-up amount, and was evaluated according to the following criteria: ○: Jump-up amount 3 mm or less ×: Jump-up amount more than 3 mm

[0059] <Insufficient shrinkage> The label attached to the bottle was touched with a hand and the state of adhesion between the bottle and the label was judged according to the following criteria: ○: No label lift (adheres to the bottle) ×: Label lifts (does not adhere to the bottle)

[0060] <Preparation of Polyester Raw Material> [Polyester Raw Material A] A mechanically recycled polyester raw material was prepared as polyester raw material A by the following procedure. After washing out residual beverages and other foreign matter from a PET beverage bottle, the bottle was crushed to obtain flakes. The resulting flakes were washed by stirring in the presence of a 3.5 wt. % sodium hydroxide solution at a flake concentration of 10 wt. % at 85°C for 30 minutes. After the alkali washing, the flakes were removed and washed with distilled water under stirring at a flake concentration of 10 wt. % at 25°C for 20 minutes. This water washing was repeated two more times, with the distilled water replaced. After washing, the flakes were dried and melted in an extruder. Then, finer foreign matter was further filtered out twice using filters with successively smaller mesh sizes, and finally, a third time, using a filter with the smallest mesh size of 50 μm, to obtain polyester raw material A with an intrinsic viscosity of 0.75 dl / g and an isophthalic acid content of 2.0 mol%.

[0061] [Polyester Raw Material B] A stainless steel autoclave equipped with a stirrer, thermometer, and partial reflux condenser was charged with 100 mol% dimethyl terephthalate (DMT) as a dicarboxylic acid component and 100 mol% ethylene glycol (EG) as a polyhydric alcohol component, with the ethylene glycol being 2.2 times the molar ratio of dimethyl terephthalate. 0.05 mol% (relative to the acid component) of zinc acetate as a transesterification catalyst and 0.225 mol% (relative to the acid component) of antimony trioxide as a polycondensation catalyst were added, and a transesterification reaction was carried out while distilling off the resulting methanol. Subsequently, a polycondensation reaction was carried out at 280°C under reduced pressure of 26.7 Pa. During this process, silica (Sylysia 266 manufactured by Fuji Silysia Corporation; average particle size 1.5 μm) was added as a lubricant to a concentration of 10,000 ppm, yielding a polyester raw material B with an intrinsic viscosity of 0.72 dL / g. The composition is shown in Table 1.

[0062] [Synthesis of Polyester Raw Materials C, and FH] Polyester raw materials C and FH having the compositions shown in Table 1 were obtained in the same manner as in the synthesis of polyester raw material B.

[0063] [Polyester Raw Material D] As polyester raw material D, a chemically recycled polyester raw material was prepared by the following procedure. Recovered bales were obtained by removing accessories made of materials other than polyester from recovered polyester products. These recovered bales were placed in a wet grinder and immersed for 60 minutes in an aqueous reducing agent solution (100°C) containing 1.0 wt% hydrosulfite and 0.3 wt% thiourea dioxide as reducing agents, for deinking by reduction washing. The bales were then finely shredded and fed into a melt extruder, where they were melted at 260-270°C. Unnecessary residues were removed through a screen changer, and the polymer was discharged from the extruder nozzle. The strand-like polymer was then passed through a water chute and pelletized in a pelletizer. The first batch reactor was charged with 13 wt% of the recovered pellets, 61 wt% of polyester resin, 25.93 wt% of ethylene glycol (EG), and 0.07 wt% of tetraethylammonium hydroxide (EAH). The temperature was increased simultaneously with the charging of the raw materials, and a depolymerization reaction was carried out under normal pressure. The polyester low polymer produced by this depolymerization reaction was transferred to a second batch reactor, where a polycondensation reaction was carried out at 290°C under reduced pressure (1 mmHg or less), yielding chemically recycled polyester raw material D. The composition is shown in Table 1.

[0064] [Polyester Raw Material E] Polyester raw material E having the composition shown in Table 1 was obtained in the same manner as polyester raw material D. In Table 1, TPA represents terephthalic acid, IPA represents isophthalic acid, EG represents ethylene glycol, NPG represents neopentyl glycol, CHDM represents 1,4-cyclohexanedimethanol, DEG represents diethylene glycol (which may contain by-products from polymerization), and BD represents 1,4-butanediol. The intrinsic viscosities of each polyester were A: 0.75 dl / g, B: 0.72 dl / g, C: 0.75 dl / g, D: 0.75 dl / g, E: 0.75 dl / g, F: 0.75 dl / g, G: 0.78 dl / g, and H: 1.20 dl / g, respectively. Each polyester was appropriately formed into chips.

[0065]

[0066] Example 1 Polyester A, polyester B, polyester C, and polyester H were mixed in a mass ratio of 12:5:72:11 and heated to 275°C in an extruder to melt. The molten resin was extruded from a T-die and taken up while cooled on a chill roll set at a surface temperature of 15°C to obtain an unstretched laminated film. The temperature and humidity in the unstretched film production chamber were kept constant using a dehumidifying air conditioner, with the temperature and humidity being 20°C / 50% RH and the dew point being 9°C. The distance between the molten resin outlet of the die and the chill roll was 20 mm, and the average cooling rate at this time was 86°C / sec. The obtained unstretched film was introduced into a transverse stretching machine (tenter), and with both ends of the film held with clips, the film was preheated to a temperature of 90°C. It was then stretched 4.2 times in the width direction at a film temperature of 88°C. Next, it was heat-treated at 92°C for 5 seconds. The edges of the film were cut and removed, and the uniaxially stretched film having a thickness of about 30 μm was wound into a roll. The properties of the obtained film were evaluated by the above-mentioned methods. The film-forming conditions and evaluation results are shown in Table 2.

[0067] (Examples 2 and 3) A film was produced in the same manner as in Example 1, and a uniaxially stretched film with a thickness of approximately 40 μm was wound into a roll. The properties of the obtained film were evaluated by the methods described above. The film production conditions and evaluation results are shown in Table 2.

[0068] Example 4: Polyester A, polyester B, and polyester E were mixed in a mass ratio of 25:5:70 and heated to 275°C in an extruder to melt. The molten resin was extruded from a T-die and taken up while cooled on a chill roll set at a surface temperature of 15°C to obtain an unstretched laminated film. The temperature and humidity in the unstretched film production chamber were maintained constant using a dehumidifying air conditioner, with the temperature and humidity being 20°C / 50% RH and the dew point being 9°C. The distance between the molten resin outlet of the die and the chill roll was 30 mm. Immediately after the molten resin landed on the chill roll, air was blown from a multi-duct at a speed of 10 m / s to actively cool the molten resin on the side opposite the chill roll contact surface. The average cooling rate was 105°C / s. The resulting unstretched film was introduced into a longitudinal stretching machine with multiple roll groups arranged in series, preheated using a preheating roll until the film temperature reached 80°C, and then stretched 1.2 times in the longitudinal direction by utilizing the difference in rotation speed between a low-speed rotating roll set at a surface temperature of 80°C and a high-speed rotating roll set at a surface temperature of 30°C. The longitudinally stretched film was introduced into a transverse stretching machine (tenter), preheated to a film temperature of 90°C while both ends of the film were held with clips, and then stretched 4.2 times in the width direction at a film temperature of 85°C. Next, a heat treatment was performed at 86°C for 5 seconds. Both edges of the film were then trimmed and removed, and the biaxially stretched film of approximately 40 μm was wound into a roll. The properties of the resulting film were evaluated using the methods described above. The film formation conditions and evaluation results are shown in Table 2.

[0069] (Example 5) A film was produced in the same manner as in Example 4, and a uniaxially stretched film with a thickness of approximately 30 μm was wound into a roll. The properties of the obtained film were evaluated by the methods described above. The film production conditions and evaluation results are shown in Table 2.

[0070] (Examples 6 to 8) Films were produced in the same manner as in Example 1 or 4, and uniaxially stretched films with a thickness of approximately 40 μm were wound into rolls. The properties of the obtained films were evaluated by the methods described above. The film production conditions and evaluation results are shown in Table 2.

[0071] Example 9 An unstretched film was prepared in the same manner as in Example 3, then introduced into a longitudinal stretching machine with multiple rolls arranged in series. The film was preheated using a preheating roll until the film temperature reached 100°C, and then stretched 5.2 times in the longitudinal direction by utilizing the difference in rotation speed between a low-speed rotating roll set at a surface temperature of 95°C and a high-speed rotating roll set at a surface temperature of 30°C. The longitudinally stretched film was introduced into a transverse stretching machine (tenter) and, with both ends of the film held by clips, preheated until the film temperature reached 110°C. It was then stretched 1.8 times in the width direction at a film temperature of 85°C and heat-treated at 92°C for 5 seconds. Next, both edges of the film were trimmed and removed, and the biaxially stretched film with a main shrinkage direction of approximately 30 μm in the longitudinal direction was wound into a roll. The properties of the obtained film were evaluated using the methods described above. The film formation conditions and evaluation results are shown in Table 2.

[0072] (Comparative Example 1) A uniaxially stretched film roll was obtained in the same manner as in Example 1, except that the indoor temperature and humidity and the distance between the die and the chill roll were changed in the casting process. The average cooling rate in the casting process of Comparative Example 1 was 31°C / sec. The production conditions and evaluation results are shown in Table 2.

[0073] Comparative Example 2 A uniaxially stretched film roll was obtained in the same manner as in Example 1, except that the transverse stretching ratio was set to 5.7 times in the transverse stretching step.

[0074] Comparative Example 3 A uniaxially stretched film roll was obtained in the same manner as in Example 1, except that the temperature in the heat treatment step was 125° C. The production conditions and evaluation results are shown in Table 2.

[0075] Comparative Example 4 A biaxially stretched film roll was obtained in the same manner as in Example 4, except that the longitudinal stretching ratio was set to 3.5 times.

[0076] Comparative Example 5: The polyester raw material was melt-extruded and introduced onto a chill roll in the same manner as in Example 1. The temperature and humidity in the unstretched film production chamber were 30°C / 80% RH, the dew point was 26°C, and the chill roll temperature was 15°C. Condensation was observed on the chill roll, and the molten resin wrapped around the chill roll during film formation, so it was not possible to evaluate each parameter. The production conditions and evaluation results are shown in Table 2.

[0077] (Comparative Example 6) A uniaxially stretched film roll was obtained in the same manner as in Example 3, except that the indoor temperature and humidity, the distance between the die and the chill roll, etc. in the casting process were changed, and the transverse stretching ratio in the transverse stretching process and the temperature in the heat treatment process were changed. The production conditions and evaluation results are shown in Table 2.

[0078]

[0079]

[0080] All of the polyester heat-shrinkable films of the examples of the present invention satisfied the required characteristics and achieved excellent finishes. On the other hand, in Comparative Example 1, the average cooling rate of the molten resin was slow at 31°C / sec, resulting in the average refractive index and storage modulus falling below their respective required ranges, resulting in a skirt ratio exceeding 8%. As a result, the shrinkage finish was evaluated as having significant sink marks, making it unsuitable for use as a label. In Comparative Example 2, the 80°C shrinkage rate and heat shrinkage stress in the main shrinkage direction exceeded the required ranges, resulting in a skirt ratio exceeding 8%, and similarly to Comparative Example 1, the film produced had poor label finish. In Comparative Example 3, the 80°C shrinkage rate in the main shrinkage direction was below 20%, resulting in an insufficient shrinkage rating in the shrinkage finish evaluation. In Comparative Example 4, not only did the heat shrinkage stress exceed 10 MPa, but the 80°C shrinkage rate in the direction perpendicular to the main shrinkage direction also exceeded 10%, resulting in a skirt ratio exceeding 8%. As a result, Comparative Example 4 also produced a film with poor finish, similar to Comparative Examples 1 and 2.

[0081] According to the present invention, it is possible to provide a heat-shrinkable polyester film that has excellent shrink finish even when using recycled raw materials without limiting the raw material composition.

Claims

1. A heat-shrinkable polyester film whose main component is ethylene terephthalate and which satisfies the following (1) to (4): (1) The skirt ratio measured after the film is shrunk by 10% in the main shrinkage direction in 90°C hot air, fixed in that state, and heat-treated for 1 minute is 1% to 8% (2) The maximum heat shrinkage stress in the main shrinkage direction measured while the film is shrunk by 10% in the main shrinkage direction in 90°C hot air, fixed in that state, and heat-treated for 1 minute is 1 MPa to 10 MPa (3) The shrinkage rate in the main shrinkage direction when the film is immersed in 80°C hot water for 10 seconds is 20% to 72% (4) The average refractive index of the film is 1.57 to 1.585 2. The heat-shrinkable polyester film according to claim 1, characterized in that the storage modulus at 90° C. in the direction perpendicular to the main shrinkage direction of the film is 8 MPa or more and 100 MPa or less.

3. A heat-shrinkable polyester film according to claim 1, characterized in that the shrinkage rate of the film when immersed in 80°C hot water for 10 seconds is -10% or more and 10% or less in the direction perpendicular to the main shrinkage direction.

4. The heat-shrinkable polyester film according to claim 1, characterized in that the polyester components constituting the film contain 1 mol % or more and 30 mol % or less of one or more monomer components selected from the group consisting of isophthalic acid, neopentyl glycol, 1,4-cyclohexanedimethanol, 1,4-butanediol, and diethylene glycol.

5. The heat-shrinkable polyester film according to claim 1, characterized in that the raw material constituting the film contains mechanically recycled and / or chemically recycled polyester raw materials.

6. The heat-shrinkable polyester film according to claim 1, wherein the difference in reversible heat capacity before and after the glass transition temperature is 0.2 J / g·K or more and 0.3 J / g·K or less.

7. The heat-shrinkable polyester film according to claim 1, having a glass transition temperature of 50°C or higher and 100°C or lower.

8. The heat-shrinkable polyester film according to claim 1, wherein the ethylene terephthalate accounts for 60 mol % or more of 100 mol % of the polyester constituent units.

9. A heat-shrinkable label, comprising at least a portion thereof made of the heat-shrinkable polyester film according to any one of claims 1 to 8.

10. A packaging material comprising the heat-shrinkable polyester film according to any one of claims 1 to 8.

11. A package comprising the label according to claim 9.

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