Heat-shrinkable polyester film
Patent Information
- Application Number
- JP2025531515
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Priority Date
- 2023-07-06
- Filing Date
- 2024-06-27
- Publication Date
- 2025-01-09
Abstract
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 as labels for beverage bottles and for banding containers such as lunch boxes by shrinking in the longitudinal direction, and is particularly excellent in productivity and shrink finish.
[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] Heat-shrinkable films that shrink significantly in the width direction are widely used. These heat-shrinkable films are often stretched in the width direction. Labels for beverage bottles, for example, are typically made by gluing the widthwise edges of the film into a ring (sleeve-like shape), then applying the film to the object to be labeled and then heat-shrinking it in the circumferential direction. However, when using labels made of heat-shrinkable films that heat-shrink in the width direction on bottles and lunch boxes, speeding up the label application has been a challenge. In recent years, due to the aging population and the increase in dual-income households, sales of boxed lunches and noodles at convenience stores and other locations have increased. Therefore, a film that heat-shrinks in the lengthwise direction can be unwound from a roll and directly wrapped around bottles, lunch boxes, and other containers and heat-sealed, thereby speeding up label application and automating the process of applying ring-shaped labels, which was previously performed manually.
[0004] For example, a technology for producing a polyester film that heat-shrinks in the machine direction is disclosed in Patent Document 1. However, Patent Document 1 discloses a film that is stretched only in the machine direction, and the stretching ratio described in the examples is at most 5 times, leaving room for improvement in productivity. Furthermore, such heat-shrinkable films that are uniaxially stretched only in the machine direction tend to shrink at a high rate, leaving room for improvement in shrink finish.
[0005] Patent Document 2 discloses a polyester film that is stretched approximately four times in the transverse direction, then stretched approximately two times in the longitudinal direction, and then heat-shrunk in the longitudinal direction. Patent Document 2 achieves an area ratio (the product of transverse and longitudinal stretching) of approximately 12 times, improving productivity. However, the equipment required for the transverse-longitudinal sequential biaxial stretching method disclosed in Patent Document 2 differs from the longitudinal-transverse sequential biaxial stretching machine typically used to produce films. After transverse-longitudinal stretching, the film must be passed through a heater such as a tenter or oven for heat treatment, which tends to result in a large-scale facility. Patent Document 3 discloses a polyester film that shrinks in the longitudinal direction by longitudinal-longitudinal sequential biaxial stretching. However, in the examples of Patent Document 3, the area ratio of stretching is approximately 4.5, resulting in inferior productivity compared to Patent Document 2, which stretches only in the longitudinal direction. Furthermore, there is still room for improvement in shrinkage finish due to the high shrinkage rate.
[0006] Japanese Patent Publication No. 2021-127377 Japanese Patent Publication No. 4411556 Japanese Patent Publication No. 7101542
[0007] In view of the above problems, an object of the present invention is to provide a heat-shrinkable polyester film which is excellent in productivity and shrink finish and shrinks in the longitudinal direction.
[0008] The present invention comprises the following configurations: 1. A heat-shrinkable polyester film containing ethylene terephthalate as a main constituent component, the longitudinal direction of the film being the main shrinkage direction, and satisfying the following (1) to (5): (1) The shrinkage rate in the longitudinal direction of the film when immersed in 80°C hot water for 10 seconds is 20 to 70% (2) The difference between the shrinkage rate in the longitudinal direction of the film when immersed in 80°C hot water for 10 seconds and the shrinkage rate when immersed in 80°C hot water for 3 seconds is 5 to 15% (3) The seal strength when the film is heat-sealed to itself at 130°C is 3 N / 15 mm to 20 N / 15 mm (4) The degree of planar orientation of the film is 0.03 to 0.08 (5) The difference in refractive index between the longitudinal direction and the width direction of the film is 0.01 to less than 0.04 2. A heat-shrinkable polyester film characterized in that the shrinkage rate in the width direction of the film when immersed in 80°C hot water for 10 seconds is -2 to 18%. 3. The heat-shrinkable polyester film according to 1. or 2., characterized in that the shrinkage rate in the longitudinal direction when the film is immersed in 90°C hot water for 10 seconds is 40% or more and 75% or less. 4. The heat-shrinkable polyester film according to any one of 1. to 3., characterized in that the reversible heat capacity difference ΔCp of the film is 0.15 J / g·K or more and 0.28 J / g·K or less. 5. The heat-shrinkable polyester film according to any one of 1. to 4., 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. 6. The heat-shrinkable polyester film according to any one of 1. to 5., characterized in that the raw materials constituting the film are mechanically recycled and / or chemically recycled polyester raw materials. 7. The heat-shrinkable polyester film according to 1. to 6. 7. A heat-shrinkable label, at least in part of which is made of the heat-shrinkable polyester film described in any one of items 1 to 6. 8. A package, characterized by using the heat-shrinkable polyester film described in any one of items 1 to 6. 9. A package, characterized by using the label described in item 7.
[0009] According to the present invention, a heat-shrinkable polyester film that shrinks in the longitudinal direction and has excellent productivity and shrink finish can be provided.
[0010] Overview of the container and label used to evaluate shrinkage finish in the examples
[0011] 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.
[0012] [1-1] 80°C 10-Second Heat Shrinkage in the Longitudinal Direction When the heat-shrinkable polyester film of the present invention is 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, the heat shrinkage in the longitudinal direction of the film (hereinafter sometimes simply referred to as 80°C 10-second heat shrinkage in the longitudinal direction) calculated from the lengths before and after shrinkage using the following formula 1 must be 20% to 70%: Heat shrinkage = {(length before shrinkage - length after shrinkage) / length before shrinkage} × 100 (%) Formula 1 If the 80°C 10-second heat shrinkage in the longitudinal direction is less than 20%, the shrinkage for the target object will be insufficient, which is undesirable from the viewpoint of finish quality. If the 80°C 10-second heat shrinkage rate in the longitudinal direction exceeds 70%, the value described below in [1-3] (difference between the 80°C 10-second heat shrinkage rate in the longitudinal direction and the 80°C 3-second heat shrinkage rate) is likely to fall below 5% (the shrinkage rate becomes faster), which is undesirable. The 80°C 10-second heat shrinkage rate in the longitudinal direction is preferably 30% or more and 65% or less, and more preferably 35% or more and 60% or less. That is, the 80°C 10-second heat shrinkage rate in the longitudinal direction is 20 to 70%, preferably 30 to 65%, and more preferably 35 to 60%.
[0013] [1-2] Width Direction 80°C 10-Second Heat Shrinkage 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 width direction heat shrinkage (hereinafter sometimes simply referred to as width direction 80°C 10-second heat shrinkage) calculated from the length before and after shrinkage using the above formula 1 is -2% to 18%. A width direction 80°C 10-second heat shrinkage of less than -2% is undesirable because it reduces the ability to remove small wrinkles (grain wrinkles) that occur during shrinkage finishing, thereby deteriorating the finish. On the other hand, a width direction 80°C 10-second heat shrinkage of more than 18% is undesirable because it increases jumping and necking during shrinkage finishing. The width direction 80°C 10-second heat shrinkage of more than -1% to 17% is more preferable, and 0% to 16% is even more preferable. That is, the heat shrinkage rate at 80° C. for 10 seconds in the width direction is preferably −2 to 18%, more preferably −1 to 17%, and even more preferably 0 to 16%.
[0014] [1-3] Difference between the longitudinal heat shrinkage rate at 80°C for 10 seconds and the longitudinal heat shrinkage rate at 80°C for 3 seconds The heat-shrinkable polyester film of the present invention is measured for the longitudinal heat shrinkage rate at 80°C for 10 seconds and the longitudinal heat shrinkage rate at 80°C for 3 seconds, and the difference calculated using the following formula 2 (hereinafter, sometimes simply referred to as the heat shrinkage rate difference) must be 5% to 15%. Heat shrinkage rate difference = 80°C for 10 seconds heat shrinkage rate - 80°C for 3 seconds heat shrinkage rate (%) Formula 2 A heat shrinkage rate difference of less than 5% means that the heat shrinkage rate at 3 seconds of immersion is close to the heat shrinkage rate at 10 seconds of immersion, and therefore the shrinkage rate during shrink finishing is fast. The inventors have found that the difference in heat shrinkage rate has a significant impact on the appearance of the finished shrinkage. A fast shrinkage rate is undesirable because it is prone to problems such as distortion, wrinkles, and folds during shrink finishing. On the other hand, if the difference in heat shrinkage rate exceeds 15%, the shrinkage rate becomes slower, which tends to result in a better finish, but it becomes difficult to achieve a necessary and sufficient shrinkage rate in a short period of time, making it difficult to achieve a shrink finish at high speed. The difference in heat shrinkage rate is preferably 5.5% or more and 14.5% or less, and more preferably 6% or more and 14% or less. That is, the difference in heat shrinkage rate is 5 to 15%, preferably 5.5 to 14.5%, and more preferably 6 to 14%.
[0015] [1-4] Longitudinal 90°C 10-Second Heat Shrinkage Rate The heat-shrinkable polyester film of the present invention is preferably immersed in 90°C warm water for 10 seconds under no load, followed immediately by immersion in 25°C ± 0.5°C water for 10 seconds, and then has a longitudinal heat shrinkage rate (hereinafter sometimes simply referred to as longitudinal 90°C 10-second heat shrinkage rate) calculated from the lengths before and after shrinkage using the above formula 1 of 40% to 75%. If the longitudinal 90°C 10-second heat shrinkage rate is less than 40%, the shrinkage rate for the target object will be insufficient, which is undesirable from the viewpoint of finish quality. If the longitudinal 90°C 10-second heat shrinkage rate exceeds 75%, the value described above in [1-3] (the difference between the longitudinal 80°C 10-second heat shrinkage rate and the 80°C 3-second heat shrinkage rate) is likely to fall below 5% (the shrinkage rate will increase), which is undesirable. The heat shrinkage rate at 90°C for 10 seconds in the longitudinal direction is preferably 45% to 70%, and more preferably 50% to 65%. That is, the heat shrinkage rate at 90°C for 10 seconds in the longitudinal direction is preferably 40 to 75%, more preferably 45 to 70%, and even more preferably 50 to 65%.
[0016] [1-5] Width Direction 90°C 10-Second Heat Shrinkage The heat-shrinkable polyester film of the present invention is preferably immersed in 90°C warm water for 10 seconds under no load, followed immediately by immersion in 25°C ± 0.5°C water for 10 seconds. The width direction heat shrinkage (hereinafter sometimes simply referred to as the width direction 90°C 10-second heat shrinkage) calculated from the length before and after shrinkage using the above formula 1 is -4% to 20%. A width direction 90°C 10-second heat shrinkage of less than -4% is undesirable because it reduces the ability to remove small wrinkles (grain wrinkles) that occur during shrinkage finishing, thereby deteriorating the finish. On the other hand, a width direction 90°C 10-second heat shrinkage of more than 20% is undesirable because it increases jumping and neck-in during shrinkage finishing. The width direction 90°C 10-second heat shrinkage of more than -2% to 19% is more preferable, and -1% to 18% is even more preferable. That is, the heat shrinkage rate at 90° C. for 10 seconds in the width direction is preferably −4 to 20%, more preferably −2 to 19%, and even more preferably −1 to 18%.
[0017] [1-6] 130°C Seal Strength The heat-shrinkable polyester film of the present invention must have a seal strength (hereinafter sometimes simply referred to as 130°C seal strength) of 3 N / 15 mm or more and 20 N / 15 mm or less, as determined by heat-sealing the films together at 130°C, 0.2 MPa, and 2 seconds using a test sealer, followed by peeling using a tensile tester. A 130°C seal strength of less than 3 N / 15 mm is undesirable because the sealed portion after labeling tends to peel easily. On the other hand, a 130°C seal strength of more than 20 N / 15 mm is undesirable because, for example, when used as a strip label for a lunch box container, the peel force required becomes too great, making it impossible to open the container. The 130°C seal strength is preferably 4 N / 15 mm or more and 19 N / 15 mm or less, and more preferably 5 N / 15 mm or more and 18 N / 15 mm or less. That is, the 130° C. seal strength is 3 to 20 N / 15 mm, preferably 4 to 19 N / 15 mm, and more preferably 5 to 18 N / 15 mm.
[0018] [1-7] Refractive Index Difference Between Longitudinal and Width Directions The heat-shrinkable polyester film of the present invention must have a refractive index difference between the longitudinal and width directions (hereinafter sometimes simply referred to as the refractive index difference), calculated using the following formula 3, of 0.01 or more and less than 0.04. Refractive Index Difference Between Longitudinal and Width Directions = (Refractive Index in Longitudinal Direction) - (Refractive Index in Width Direction) Formula 3. By setting the refractive index difference to 0.01 or more and less than 0.04, the difference between the 80°C 10-second heat shrinkage rate and the 80°C 3-second heat shrinkage rate (shrinkage rate) described in [1-3] above is likely to be 5% or more and 15% or less. This is thought to be because, when shrinkage occurs in the longitudinal direction (main shrinkage direction), alignment of molecules in the width direction (orthogonal to the longitudinal direction) temporarily inhibits shrinkage in the longitudinal direction. If the refractive index difference exceeds 0.04, the degree of molecular orientation in the width direction is low, and the difference in the heat shrinkage rate (shrinkage rate) is likely to be less than 5%. Furthermore, a refractive index difference of more than 0.04 means that the stretch ratio in the width direction is low, resulting in extremely poor productivity. On the other hand, a refractive index difference of less than 0.01 is undesirable because the degree of molecular orientation in the width direction is high, making the heat shrinkage in the width direction at 80°C or 90°C more likely to exceed a predetermined value. The refractive index difference is preferably 0.011 or more and 0.039 or less, and more preferably 0.012 or more and 0.038 or less. That is, the refractive index difference is 0.01 to less than 0.04, preferably 0.011 to 0.039, and more preferably 0.012 to 0.038.
[0019] [1-8] Planar Orientation Degree The heat-shrinkable polyester film of the present invention must have a planar orientation degree of 0.03 or more and 0.08 or less, as calculated by the following formula 4: Planar orientation degree = {(longitudinal refractive index) + (widthwise refractive index)} / 2} - (thicknesswise refractive index) Formula 4 A planar orientation degree of less than 0.03 means that the sum of the stretching ratios in the longitudinal and widthwise directions (area ratio) is less than 5.0, resulting in poor productivity. On the other hand, a planar orientation degree of more than 0.08 improves productivity, but the reversible heat capacity difference (described below) tends to fall below 0.15 g / J·K, resulting in a decrease in 130°C seal strength, which is undesirable. The planar orientation degree is preferably 0.032 or more and 0.068 or less, and more preferably 0.034 or more and 0.066 or less. That is, the degree of planar orientation is 0.03 to 0.08, preferably 0.032 to 0.068, and more preferably 0.034 to 0.066.
[0020] [1-9] 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 0.15 J / g·K or more and 0.28 J / g·K or less. If the reversible heat capacity difference exceeds 0.28 J / g·K, this means that the molecules constituting the finished film (after stretching) are not oriented, which is undesirable because the planar orientation degree tends to fall below 0.03 and productivity deteriorates. On the other hand, if the reversible heat capacity difference is less than 0.15 J / g·K, the 130°C seal strength tends to fall below 3 N / 15 mm, which is undesirable. The reversible heat capacity difference is more preferably 0.17 J / g·K or more and 0.26 J / g·K or less, and even more preferably 0.18 J / g·K or more and 0.25 J / g·K or less. That is, the reversible heat capacity difference is preferably 0.15 to 0.28 J / g·K, more preferably 0.17 to 0.26 J / g·K, and even more preferably 0.18 to 0.25 J / g·K.
[0021] [1-10] 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. If the glass transition temperature is lower than 50°C, the heat resistance required for a heat-shrinkable label is poor, and the film may not be able to maintain its shape during finishing shrinkage. On the other hand, if the glass transition temperature exceeds 100°C, the longitudinal 80°C 10-second heat shrinkage rate tends to fall below 25%, which is undesirable. 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.
[0022] [1-11] 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 thickness of less than 4 μm is undesirable because the 130°C seal strength is likely to fall below 3 N / 15 mm. On the other hand, a thickness of more than 100 μm is undesirable because it unnecessarily increases chemical costs. The thickness is more preferably 6 μm or more and 98 μm or less, and even more preferably 8 μm or more and 96 μm or less. That is, the thickness is preferably 4 to 100 μm, more preferably 6 to 98 μm, and even more preferably 8 to 96 μm.
[0023] [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 whose main constituent is ethylene terephthalate units. "Mainly" means 50 mol% or more, and 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. 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 contained in the polyester, the content is preferably less than 3 mol% (based on 100 mol% of the dicarboxylic acid component). Furthermore, it is preferable that the polyester does not contain a polycarboxylic acid having a valence of 3 or more (e.g., trimellitic acid, pyromellitic acid, and anhydrides thereof). Heat-shrinkable polyester films obtained using polyesters containing these polycarboxylic acids have difficulty achieving the required shrinkage ratio.
[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 the above, neopentyl glycol, 1,4-cyclohexanedimethanol, 1,4-butanediol, and diethylene glycol are preferred. It is preferable to contain from 1 mol % to 30 mol % of the above dicarboxylic acid component other than terephthalic acid and / or diol component other than ethylene glycol. Of the above constituent components, it is more preferable to contain at least one of isophthalic acid, neopentyl glycol, 1,4-cyclohexanedimethanol, 1,4-butanediol, and diethylene glycol.
[0025] In addition to the polyester raw materials having 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. Furthermore, in addition to the above-mentioned 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 environmentally friendly perspective of the present invention. For example, 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). The 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 preferred. They may be appropriately oriented, and colorless containers are preferred, but they may contain a small amount of coloring components. The content of the recycled raw materials is preferably 5% by weight or more and 100% by weight or less, within the range of 1 mol% to 30 mol% of the dicarboxylic acid components other than terephthalic acid and / or diol components other than ethylene glycol. If the recycled raw materials are less than 5% by weight, the proportion of recycled raw materials contained in the raw materials used for film formation will be 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 recycled raw materials used for film formation is 100% by weight.The higher the content of recycled materials, the greater the effect on reducing the environmental load, which is preferable. However, it is also preferable to use a lubricant as a masterbatch to impart slipperiness to the film, or to use materials other than recycled materials to adjust the shrinkage rate, so even if the upper limit of recycled materials is 95% by weight, it is sufficient for practical purposes.
[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.
[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, there is no limitation on the stretching method as long as the properties of the heat-shrinkable polyester film [1] described above are satisfied, but the following describes an example of sequential biaxial stretching in which longitudinal stretching is followed by transverse stretching. The heat-shrinkable polyester film of the present invention may have a single-layer structure or a multi-layer structure of two or more layers. When a multi-layer structure is used, a structure of five or less 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. If the raw material is not dried in advance, moisture can be removed by vacuuming the extruder via a vent. The melting temperature when extruding the polyester raw material is preferably 200°C or higher and 300°C or lower. A temperature below 200°C is undesirable because it significantly increases the extrusion pressure, causing problems such as filter deformation. On the other hand, a melting temperature above 300°C promotes thermal decomposition of the raw material, potentially leading to problems such as an increase in foreign matter in the film. The melting 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 melting 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 Film After melt-extrusion of the raw material resin, an unstretched film can be obtained by quenching the sheet-like molten resin. As a method for quenching the molten resin, a method can be suitably adopted in which the molten resin is cast from a die onto a chill roll and rapidly cooled and solidified to obtain a substantially unoriented resin sheet (hereinafter, sometimes referred to as an unstretched film).
[0032] [2-2-3] Control of Stretching Conditions in the Longitudinal (Machine) Direction In the production of the heat-shrinkable polyester film of the present invention, the stretching ratio in the machine direction (hereinafter sometimes referred to as machine stretching) is preferably 3 to 6 times. If the machine stretching ratio is less than 3 times, it becomes difficult to achieve a machine direction 80°C heat shrinkage rate of 25% or more. On the other hand, if the machine stretching ratio exceeds 6 times, the machine direction 80°C heat shrinkage rate difference (shrinkage rate) tends to fall below 5%, which is undesirable. The machine stretching ratio is more preferably 3.2 to 5.8 times, and even more preferably 3.4 to 5.6 times. That is, the machine stretching ratio is preferably 3 to 6 times, more preferably 3.2 to 5.8 times, and even more preferably 3.4 to 5.6 times. When machine stretching, it is preferable to preheat a substantially unoriented film to a temperature above Tg and below Tg + 60°C. If the temperature is lower than Tg, the risk of breakage during machine stretching increases, which is undesirable. On the other hand, if the temperature during longitudinal stretching exceeds Tg + 60°C, problems such as the film sticking to the rolls are likely to occur. The longitudinal stretching temperature is more preferably Tg + 5°C or higher and Tg + 55°C or lower, and even more preferably Tg + 10°C or higher and Tg + 50°C or lower. That is, the stretching temperature is preferably Tg to Tg + 60°C, more preferably Tg + 5°C to Tg + 55°C, and even more preferably Tg + 10°C to Tg + 50°C. Furthermore, either one-stage stretching or two or more stage multi-stage stretching can be used for longitudinal stretching.
[0033] [2-2-4] Control of Stretching Conditions in the Width (Transverse) Direction In film production, increasing the stretching ratio in the width direction (hereinafter sometimes referred to as transverse stretching) can improve productivity, but an increase in the thermal shrinkage in the width direction is a concern. The inventors have discovered that by dividing the transverse stretching zone into two parts, a first half and a second half, and setting the stretching temperature higher in the second half than in the first half, it is possible to increase the transverse stretching ratio and improve productivity while reducing the thermal shrinkage in the width direction and keeping it within a predetermined range. Specifically, the first half stretching zone is preferably Tg+5°C or higher and Tg+25°C or lower, and the second half stretching zone is preferably above Tg+25°C and Tg+50°C or lower. When a film is transversely stretched, the stretching stress tends to be higher in the second half of stretching than in the first half. It is believed that increasing the temperature in the second half of transverse stretching zone as described above can suppress this increase in stretching stress, thereby suppressing the degree of molecular orientation in the width direction. If the temperature in the transverse stretching first half zone is below Tg + 5°C or if the temperature in the transverse stretching second half zone is below Tg + 25°C, the degree of orientation in the width direction will be high, and the difference in refractive index between the longitudinal and width directions will tend to fall below 0.01, which is undesirable. On the other hand, if the temperature in the transverse stretching first half zone is above Tg + 25°C or if the temperature in the transverse stretching second half zone is above Tg + 50°C, the film will be excessively heated, and the longitudinal shrinkage at 80°C or 90°C will tend to fall below the specified range, which is undesirable. The stretching first half zone is more preferably Tg + 7°C or higher and Tg + 23°C or lower, and even more preferably Tg + 9°C or higher and Tg + 21°C or lower. That is, the stretching first half zone is preferably Tg + 5°C to Tg + 25°C, more preferably Tg + 7°C to Tg + 23°C, and even more preferably Tg + 9°C to Tg + 21°C. The temperature in the latter stretching zone is more preferably Tg+27° C. or higher and Tg+48° C. or lower, and even more preferably Tg+29° C. or higher and Tg+46° C. or lower. That is, the temperature in the latter stretching zone is preferably from Tg+25° C. to Tg+50° C., more preferably from Tg+27° C. to Tg+48° C., and even more preferably from Tg+29° C. to Tg+46° C. Before entering transverse stretching, it is preferable to preheat the film to a temperature of Tg° C. or higher and Tg+40° C. or lower.
[0034] The lengths of both the first and second half transverse stretching zones are preferably 30% to 70% of the total transverse stretching zone. If the length of the first or second half zone is less than 30% of the total transverse stretching zone, it becomes difficult to raise the actual film temperature in that zone to the set temperature, which is undesirable. Note that a zone length of either the first or second half being less than 30% is equivalent to a zone length of the other exceeding 70%. The length of the first or second half zone is more preferably 35% to 65% of the total transverse stretching zone, even more preferably 40% to 60%, and particularly preferably 50% (the lengths of the first and second half are the same). That is, the length of the first or second half zone is preferably 30% to 70%, more preferably 35% to 65%, and even more preferably 40% to 60% of the total transverse stretching zone.
[0035] The product of the transverse stretching ratios in the first half and the second half is preferably 1.2 times or more and 2.4 times or less. A product of the transverse stretching ratios less than 1.2 times is undesirable because it drastically reduces the productivity of the film. On the other hand, a product of the transverse stretching ratios exceeding 2.4 times is undesirable because it tends to increase the 80°C heat shrinkage in the width direction by more than 18%. The stretching ratio in the first half of transverse stretching is preferably 1.1 times or more and 1.8 times or less. If the stretching ratio in the first half of transverse stretching is less than 1.1 times, the film will be substantially barely stretched, making it meaningless to separate the stretching zones. On the other hand, if the stretching ratio in the first half of transverse stretching exceeds 1.8 times, the stretching stress in the subsequent second half zone tends to increase, and the refractive index difference between the longitudinal direction and the width direction is likely to fall below 0.01, which is undesirable. The stretching ratio in the first half of transverse stretching is more preferably 1.11 times or more and 1.79 times or less, and even more preferably 1.12 times or more and 1.78 times or less. That is, the stretch ratio in the first half of transverse stretching is preferably 1.1 to 1.8 times, more preferably 1.11 to 1.79 times, and even more preferably 1.12 to 1.78 times. The stretch ratio in the second half of transverse stretching zone is preferably 1.1 times or more and 2.1 times or less. If the stretch ratio in the second half of transverse stretching is less than 1.1 times, as described in the explanation of the first half of stretching zone, there is no point in separating the stretching zones. If the stretch ratio in the second half of transverse stretching exceeds 2.1 times, even if the temperature of this zone is set to above Tg+25°C and not more than Tg+50°C, the refractive index difference between the longitudinal direction and the width direction is likely to fall below 0.01, which is undesirable. The stretch ratio in the second half of transverse stretching is more preferably 1.11 times or more and 2.09 times or less, and even more preferably 1.12 times or more and 2.08 times or less. That is, the stretching ratio in the latter half of the transverse stretching is preferably 1.1 to 2.1 times, more preferably 1.11 to 2.09 times, and even more preferably 1.12 to 2.08 times.
[0036] The area ratio, obtained by multiplying the ratios after sequential stretching in the longitudinal and width directions as described above, is preferably 5.5 times or more and 14 times or less. By setting the area ratio to 5.5 times or more, film productivity can be improved. On the other hand, attempting to set the area ratio to more than 14 times is undesirable because it tends to cause the stretching ratios in the longitudinal and width directions to exceed the specified range, which is likely to adversely affect various physical properties such as shrinkage. The area ratio is more preferably 5.6 times or more and 13.9 times or less, and even more preferably 5.7 times or more and 13.8 times or less. That is, the area ratio is preferably 5.5 to 14 times, more preferably 5.6 to 13.9 times, and even more preferably 5.7 to 13.8 times.
[0037] [2-2-5] Heat Treatment The film sequentially biaxially stretched as described above may be heat-treated in a tenter while both edges in the width direction are held with clips, if necessary. Heat treatment here refers to heat treatment at a temperature of 60°C to 120°C for a time of 1 to 9 seconds. This type of heat treatment is preferred because it allows for adjustment of the thermal shrinkage rate. 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 thermal shrinkage rate in the longitudinal direction is likely to fall below the specified range, 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 will become large, so it is preferable to control it to 1 to 10 seconds. It is more preferable to set it to 2 to 9 seconds, and even more preferably 3 to 8 seconds. 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 in order to adjust the thermal shrinkage rate in the width direction. However, since the improvement of productivity is an important point in the present invention, the relaxation rate is preferably 0% or more and 10% or less. A relaxation rate of 0% means no relaxation.
[0038] [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.
[0039] [3] Labels The labels of the present invention comprise at least a portion of the heat-shrinkable polyester film of the present invention. The heat-shrinkable polyester film of the present invention may be bonded to itself or to other films. The length (circumferential length) of the label and the number of bonded portions may be arbitrary. Any conventionally known film may be used in any length as the other film constituting the label of the present invention. The film material may also be arbitrary, including polyesters, polyolefins, polyamides, etc. Adhesion methods required to form the label of the present invention include solvent-based center sealing, heat sealing, ultrasonic sealing, hot glue (hot melt), and cold glue. Among these, solvent-based center sealing, heat sealing, and ultrasonic sealing are preferred. For 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 sealing temperature is increased, the actual temperature applied to the film will be lower if the sealing time is shortened. For example, the heat sealing temperature is preferably 100° C. to 180° C., more preferably 110° C. to 170° C., and even more preferably 120° C. to 160° C. The heat sealing pressure is preferably 0.05 MPa to 0.6 MPa, more preferably 0.1 MPa to 0.55 MPa, and even more preferably 0.15 MPa to 0.5 MPa. If the pressure is below 0.05 MPa, the seal bar will have difficulty transferring sufficient heat to the film.
[0040] The heat sealing time is preferably from 0.1 to 3 seconds, more preferably from 0.2 to 2.9 seconds, and even more preferably from 0.3 to 2.8 seconds. There is no limitation 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.
[0041] This application claims priority based on Japanese Patent Application No. 2023-111489, filed on July 6, 2023. The entire contents of the specification of Japanese Patent Application No. 2023-111489, filed on July 6, 2023, are incorporated herein by reference.
[0042] 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.
[0043] <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.
[0044] [80°C 3-Second Heat Shrinkage Percentage] A heat-shrinkable film was cut into a 10 cm x 10 cm square, and the film was immersed in warm water at 80±0.5°C for 3 seconds without load to cause heat shrinkage, and then immersed in water at 25°C±0.5°C for 10 seconds. The film was then pulled out of the water, and the longitudinal and transverse dimensions of the film were measured, and the heat shrinkage percentages were calculated according to the following formula 1. Heat shrinkage percentage = {(length before shrinkage - length after shrinkage) / length before shrinkage} × 100 (%) Formula 1
[0045] [Heat shrinkage rate at 80°C for 10 seconds] A heat-shrinkable film was cut into a 10 cm x 10 cm square, and the cut piece was immersed in warm water at 80±0.5°C for 10 seconds under no load to cause heat shrinkage. The cut piece was then immersed in water at 25°C±0.5°C for 10 seconds, and the film was then pulled out of the water. The dimensions of the film in the longitudinal and transverse directions were measured, and the heat shrinkage rate was calculated according to the above formula 1.
[0046] [Difference in heat shrinkage between 10 seconds and 3 seconds at 80°C] Using the longitudinal shrinkage rates determined from the above [heat shrinkage rate at 80°C for 3 seconds] and [heat shrinkage rate at 80°C for 10 seconds], the difference in heat shrinkage between 10 seconds and 3 seconds was calculated according to the following formula 2. Difference in heat shrinkage rate = heat shrinkage rate at 80°C for 10 seconds - heat shrinkage rate at 80°C for 3 seconds (%) Formula 2
[0047] [Heat shrinkage rate at 90°C for 10 seconds] A heat-shrinkable film was cut into a 10 cm x 10 cm square, and the cut piece was immersed in warm water at 90±0.5°C for 10 seconds under no load to cause heat shrinkage. The cut piece was then immersed in water at 25°C±0.5°C for 10 seconds, and the film was then pulled out of the water. The dimensions of the film in the longitudinal and transverse directions were measured, and the heat shrinkage rate was calculated according to the above formula 1.
[0048] [130°C Seal Strength] In accordance with JIS Z1707, the seal strength was measured when polyester films were sealed together at 130°C for 2 seconds. Specifically, a sample was cut out of the film to a length of 100 mm in the longitudinal direction and a length of 15 mm in the width direction, and the film was bonded with a heat sealer so that the seal line was parallel to the width direction. The heat seal strength was measured using a universal testing machine 5965 (manufactured by Instron) at a chuck distance of 50 mm and a pulling speed of 200 mm / min, and the seal strength was calculated as the strength per 15 mm (N / 15 mm). Note that if the sample cannot be cut out to the above size due to film size limitations, it may be cut to a length shorter than that (e.g., 20 mm in the longitudinal direction and 10 mm in the width direction). If the longitudinal size is insufficient, the distance between the chucks may be 50 mm or less as long as one chuck is provided with a length of at least 5 mm to hold the sample (for example, if the sample length is 20 mm, the distance between the chucks may be 10 mm). If the width is less than 15 mm, a 5 mm section may be cut out in the width direction, for example, to measure the seal strength, and convert it to the strength per 15 mm width (multiply by 3 for a 5 mm width). If the longitudinal and width directions of the sample are unknown, the direction in which the seal strength is measured may be any.
[0049] [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 of this sample was measured in the longitudinal direction, width direction, and thickness direction of the film 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.
[0050] [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.
[0051] 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.
[0052] [Finished Shrinkage Properties] Two sheets of film, each measuring 240 mm in length and 50 mm in width, were prepared and marked with 10 mm x 10 mm grid lines. As shown in Figure 1, a bowl-shaped noodle container with a lid (bottom diameter 110 mm, upper lid diameter 180 mm, height from bottom to lid 80 mm) was placed on the top and bottom of the container so that the two films held the body and lid of the container together. The film edges (two locations) were heat-sealed at 130°C / 0.2 MPa / 2 seconds. This resulted in a ring-shaped film covering the container. The film's shrinkage direction (longitudinal direction) was aligned with the circumferential direction of the ring-shaped film, and the ring-shaped film was positioned so that the long sides of the noodle container were held together by the ring-shaped film, with the widthwise midpoint of the ring-shaped film coinciding with the midpoint of the long sides of the container. The ring-shaped film sagged 25% relative to the container. The noodle container and annular film were shrunk by heating them for 10 seconds with hot air at a set temperature of 150°C using a hot air shrink tunnel (TORNAD 2500 manufactured by Japan Technology Solutions), and the shrinkage finish was evaluated.
[0053] <Wrinkles> The number of wrinkles in the heat-shrunk film was visually counted and rated according to the following criteria: ○: 5 or fewer wrinkles ×: 6 or more wrinkles
[0054] <Insufficient shrinkage> The label attached to the container was touched with a hand, and the state of adhesion between the container 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)
[0055] <Distortion> The degree to which the grid lines in the film before heat shrinking were distorted after shrinkage was measured. Specifically, the length by which the grid lines deviated from a straight line was measured with a ruler and judged according to the following criteria: ○: Distortion of 2 mm or less ×: Distortion of more than 2 mm
[0056] <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 alkaline 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, filtered out using a filter with the smallest mesh size (50 μm), to obtain polyester raw material A with an intrinsic viscosity of 0.75 dl / g and an isophthalic acid content of 2 mol%.
[0057] [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 produced methanol. Subsequently, a polycondensation reaction was carried out at 280°C under reduced pressure of 26.7 Pa to obtain a polyester raw material B with an intrinsic viscosity of 0.72 dl / g. The composition is shown in Table 1.
[0058] [Synthesis of Polyester Raw Materials C, D, and G to I] Polyester raw materials C, D, and G to I having the compositions shown in Table 1 were obtained in the same manner as in polyester raw material B. Note that silica (Sylysia 266 manufactured by Fuji Silysia Corporation; average particle size 1.5 μm) was added as a lubricant to polyester raw material C in an amount of 10,000 ppm.
[0059] [Polyester Raw Material E] As polyester raw material E, 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 in a reducing agent aqueous solution (100°C) containing 1.0 wt% hydrosulfite and 0.3 wt% thiourea dioxide as reducing agents for 60 minutes, thereby carrying out a deinking treatment 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 passed through a water chute and pelletized using 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 E. The composition is shown in Table 1.
[0060] [Polyester Raw Material F] Chemically recycled polyester raw material F having the composition shown in Table 1 was obtained in the same manner as polyester raw material E. 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: 0.72 dl / g, D: 0.75 dl / g, E: 0.75 dl / g, F: 0.75 dl / g, G: 0.75 dl / g, H: 0.78 dl / g, and I: 1.20 dl / g, respectively. Each polyester was appropriately cut into chips.
[0061]
[0062] Example 1 Polyester C and polyester F were mixed in a mass ratio of 5:95 and heated to 270°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 25°C to obtain an unstretched film. The obtained unstretched film was introduced into a longitudinal stretching machine having a plurality of roll groups arranged in series, preheated with a preheating roll until the film temperature reached 105°C, and then stretched 4.8 times in the longitudinal direction by utilizing the difference in rotation speed between a low-speed rotating roll set at a surface temperature of 105°C and a high-speed rotating roll set at a surface temperature of 30°C. The film after longitudinal stretching was introduced into a transverse stretching machine (tenter) and, with both ends of the film held by clips, preheated until the film temperature reached 95°C. Thereafter, the film was stretched in the width direction at 80°C in the first half stretching zone and at 105°C in the second half stretching zone to 1.4 times, and transversely stretched to a total of 1.8 times in the first and second half stretching zones. The length of each stretching zone was set to 50% of the total length of the zone. After stretching, the film was heat-treated at 95°C for 5 seconds with the clip spacing kept constant. The biaxially stretched film was then wound into a roll with the main shrinkage direction of approximately 30 μm as the longitudinal direction, while both edges of the film were cut and removed. The properties of the obtained film were evaluated using the methods described above. The film-forming conditions and evaluation results are shown in Table 2.
[0063] (Examples 2 to 7) Films were produced in the same manner as in Example 1, except that various conditions were changed, and biaxially stretched films with thicknesses of 13 μm or 30 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.
[0064] Comparative Example 1: Polyester B, Polyester C, Polyester D, and Polyester I were mixed in a mass ratio of 10:5:60:25 and heated to 275°C in an extruder to melt. The molten resin was extruded through a T-die and taken up while cooled on a chill roll set at a surface temperature of 25°C to obtain an unstretched film. The obtained unstretched film was introduced into a longitudinal stretching machine having a series of rolls arranged in a row. It was preheated with a preheating roll until the film temperature reached 95°C, and then stretched twice 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. Next, both edges of the film were trimmed and removed, and the uniaxially stretched film with a main shrinkage direction of approximately 12 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.
[0065] Comparative Examples 2 to 4 A uniaxially stretched film roll was obtained in the same manner as in Comparative Example 1, except that the longitudinal stretching temperature and longitudinal stretching ratio were changed. Table 2 shows the production conditions and evaluation results.
[0066] (Comparative Examples 5 to 7) Films were produced in the same manner as in Example 1, except that various conditions were changed, and biaxially stretched films with a thickness of 30 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.
[0067]
[0068]
[0069]
[0070]
[0071] All of the polyester heat-shrinkable films of the examples of the present invention satisfied the required properties and were excellent in productivity and shrink finish. On the other hand, Comparative Example 1 was not only poor in productivity but also lacked the necessary heat shrinkability, making it unsuitable as a heat-shrinkable film. Comparative Examples 2 to 4 were uniaxially stretched in the machine direction, so not only were they poor in productivity, but the difference in heat shrinkage rate in the machine direction at 80°C for 10 seconds and 3 seconds was low, resulting in films with poor shrink finish.
[0072] In Comparative Example 5, although the film was longitudinally and transversely sequentially biaxially stretched, the difference in refractive index between the longitudinal direction and the width direction exceeded the specified range, resulting in a film with poor shrink finish, similar to Comparative Examples 2 to 4. In Comparative Example 6, the 130°C seal strength was low, so it was not possible to produce a circular label for evaluating shrink finish. In Comparative Example 7, the required heat shrinkability was insufficient, making it unsuitable as a heat shrinkable film.
[0073] According to the present invention, it is possible to provide a heat-shrinkable polyester film that has high productivity and excellent shrink finish.
Claims
1. A heat-shrinkable polyester film whose main component is ethylene terephthalate, whose longitudinal direction is the main shrinkage direction, and which satisfies the following (1) to (5): (1) The shrinkage rate in the longitudinal direction of the film when immersed in 80°C hot water for 10 seconds is 20 to 70% (2) The difference between the shrinkage rate in the longitudinal direction of the film when immersed in 80°C hot water for 10 seconds and the shrinkage rate when immersed for 3 seconds is 5 to 15% (3) The seal strength when the film is heat-sealed to itself at 130°C is 3 N / 15 mm to 20 N / 15 mm (4) The degree of planar orientation of the film is 0.03 to 0.08 (5) The difference in refractive index between the longitudinal direction and the transverse direction of the film is 0.01 to less than 0.04 2. The heat-shrinkable polyester heat-shrinkable film according to claim 1, characterized in that the shrinkage rate in the width direction when the film is immersed in 80° C. hot water for 10 seconds is -2% or more and 18% or less.
3. A heat-shrinkable polyester film according to claim 1, characterized in that the shrinkage rate in the longitudinal direction when the film is immersed in 90°C hot water for 10 seconds is 40% or more and 75% or less.
4. The heat-shrinkable polyester film according to claim 1, characterized in that the reversible heat capacity difference ΔCp of the film is 0.15 J / g·K or more and 0.28 J / g·K or less.
5. 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.
6. 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.
7. The heat-shrinkable polyester film according to claim 1, characterized in that the shrinkage rate in the width direction when the film is immersed in 90° C. hot water for 10 seconds is from -4% to 20%.
8. The heat-shrinkable polyester film according to claim 1, which has a glass transition temperature of 50 to 100°C.
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.