Heat-shrinkable polyester film

Abstract

To provide a heat-shrinkable polyester-based film which has a high heat shrinkage in the main shrinkage direction, has a low shrinkage stress, and hardly causes a decrease in tensile elongation at break with time.SOLUTION: A heat-shrinkable polyester film satisfying the following requirements (1) to (6) and having a main shrinkage direction in a width direction: (1) The shrinkage factor (hot-water shrinkage factor) when the film is immersed in hot water at 98 °C for 10 sec is ≥ 40% in the main shrinkage direction of the film. (2) The plane orientation coefficient of the film is 0.035 to 0.070. The amount of diethylene glycol (DEG) is 100mol% or more and 6mol% or less of 25mol% of polyhydric alcohol components in all polyester resins constituting the film, and the content of polyhydric alcohol components other than ethylene glycol and diethylene glycol in 100 mol% of the polyhydric alcohol components is 0mol% or more and 15mol% or less (4) the maximum shrinkage stress in the main shrinkage direction of the film measured in hot air at 90 °C is 2MPa or more and 17MPa or less (5) after the film is left for 28 days under an atmosphere of an atmosphere temperature of 40 °C and a humidity of 85%. (6) The shrinkage (hot-water shrinkage) when the film is immersed in hot water at 70 °C for 10 seconds is 10% or more in the main shrinkage direction of the film SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a heat-shrinkable polyester film, and more particularly to a polyester heat-shrinkable film that is less likely to break during processing steps such as printing and bag making because it does not lose its tensile elongation even after long-term storage after film formation, and does not lose its shrinkage rate over time, making it suitable for use as labels for beverage bottles and outer packaging for convenience store lunches. [Background technology]

[0002] In recent years, stretched films (so-called heat-shrinkable films) made of polyvinyl chloride resins, polystyrene resins, polyester resins, etc. have come into widespread use for label packaging, cap seals, and stacked packaging, which combine the protection of glass bottles, PET bottles, etc. with product labeling. 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 production of dioxins. Polystyrene films, on the other hand, 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 upon incineration. Therefore, polyester-based heat-shrinkable films, which have high heat resistance, are easy to incinerate, and have excellent solvent resistance, have come to be widely used as shrink labels, and their usage is on the rise as the distribution volume of PET containers increases.

[0003] Conventional heat-shrinkable polyester films that shrink significantly in the width direction are widely used. This film is stretched using a tenter stretching method or similar to produce a wide master roll. The master roll is then slit to the desired width and wound into a roll of the desired length to produce a film roll product. This film is then subjected to a printing process in roll form to impart design features or to display products. After printing, the film is re-slit to the required width and wound into a roll. It then undergoes a center-sealing process using solvent bonding, is made into a tubular bag, and is wound into a roll (to produce a label roll).

[0004] The labels are rolled up into a tube, cut to the required length as they are unwound from the roll, and then attached to the packaged item by hand or other methods, and passed through a steam tunnel or hot air tunnel to shrink and become a label.

[0005] Heat-shrinkable polyester films are required to have further improved shrinkage properties. If the shrinkage stress during shrinkage is too high, the adhesive bond of the tube may peel off, resulting in a significant deterioration in appearance and even a loss of the protective function of the packaged items. Furthermore, in recent years, in order to reduce waste, thin containers have been used for lunch boxes and prepared foods sold at convenience stores and supermarkets. However, when shrink labels are used on thin containers, problems such as deformation of the container can occur if the shrinkage stress is high. It is important that the shrinkage stress is not too high, nor too low, in order to tightly protect the packaged items.

[0006] Several methods for improving the shrinkage stress of labels have been reported in the past. Patent Document 1 describes a method for improving the shrinkage stress of labels containing a diethylene glycol-derived constituent unit as a polyhydric alcohol component. , and shrinkage stress is reduced. However, the film described in Patent Document 1, which is uniaxially stretched only in the width direction, has a problem in that the tensile elongation in the longitudinal direction of the film significantly decreases when stored for a long period after film formation. In addition, it is known that the inclusion of a structural unit derived from diethylene glycol makes the hydrolysis of polyester more likely to proceed, and the decrease in molecular weight due to hydrolysis also causes a decrease in tensile elongation.

[0007] After being produced and wound into a roll, the above-mentioned films are not immediately subjected to post-processing such as printing, but are usually stored and transported at room temperature, although in some cases they may be subjected to the printing process after a long period of time, such as six months or more. If the tensile elongation of the shrink film decreases during this long storage period, it will be prone to breakage when tension is applied in the longitudinal direction in the post-processing, causing process troubles, and if the elongation decreases significantly, processing will become impossible, which is a problem.

[0008] To improve this decrease in tensile elongation due to long-term storage (over time), there is a method of biaxial stretching in which the film is stretched not only in the width direction but also in the longitudinal direction, but this method is not preferred because it necessarily requires long and large equipment. Also, it is possible to maintain elongation by increasing the intrinsic viscosity of the polyester that constitutes the film, but the range that can be improved by the intrinsic viscosity is small, and in order to increase the intrinsic viscosity, the polymerization time must be extended to increase the degree of polymerization of the polyester, which is costly, and in addition, the filtration pressure increases greatly during melt extrusion, making high-precision filtration difficult, which is not preferred. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. WO2018 / 147249 Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to provide a heat-shrinkable polyester film that has a high heat shrinkage rate in the main shrinkage direction, a low shrinkage stress, and is less likely to experience a decrease in tensile elongation at break over time. [Means for solving the problem]

[0011] The present inventors have conducted extensive research to solve the above problems and have completed the present invention, which comprises the following features.

[0012] 1. A heat-shrinkable polyester film that satisfies the following requirements (1) to (5): (1) When the film is immersed in hot water at 98°C for 10 seconds, the shrinkage rate (hot water shrinkage rate) is 40% or more in the main shrinkage direction of the film. (2) The film plane orientation coefficient is 0.035 or more and 0.070 or less. (3) The main component is ethylene terephthalate, and the amount of diethylene glycol (DEG) component is 100 mol% of the polyhydric alcohol component in the total polyester resin that constitutes the film. Of which, 6 mol% or more and 25 mol% or less (4) The maximum shrinkage stress in the main shrinkage direction of the film measured in 90°C hot air is 2 MPa or more and 17 MPa or less. (5) After the film is left in an atmosphere of 40°C and 85% relative humidity for 28 days, the tensile elongation at break in the direction perpendicular to the main shrinkage direction is 20% or more. 2. A heat-shrinkable polyester film according to 1, characterized in that the tensile elongation at break in the direction perpendicular to the main shrinkage direction is 100% or more after the film is aged for 28 days in an atmosphere at an ambient temperature of 40°C and a relative humidity of 85%. 3. A heat-shrinkable polyester film according to 1 or 2, characterized in that the difference between the shrinkage rate in the main shrinkage direction when the film is immersed in 70°C hot water for 10 seconds after aging for 28 days in an atmosphere at an ambient temperature of 30°C and a relative humidity of 85% and the shrinkage rate before aging is less than 5%. 4. The heat-shrinkable polyester film according to any one of 1. to 3., characterized in that the solvent adhesive strength is 4 N / 15 mm width or more and 15 N / 15 mm width or less. 5. A heat-shrinkable polyester film according to any one of 1. to 4., characterized in that the natural shrinkage rate in the main shrinkage direction after the film is aged for 28 days in an atmosphere at an atmospheric temperature of 40°C and a relative humidity of 85% is less than 1.0%. 6. The heat-shrinkable polyester film according to any one of 1. to 5., wherein the intrinsic viscosity of the film is 0.60 dl / g or more and 0.75 dl / g or less. 7. The heat-shrinkable polyester film according to any one of 1. to 6., wherein the main shrinkage direction is the transverse direction. 8. The heat-shrinkable polyester film according to any one of 1. to 7., which is a uniaxially stretched film. 9. A label obtained from the heat-shrinkable polyester film described in any one of 1. to 8. 10. A package formed by covering at least a portion of the outer periphery of an object to be packaged with the label described in 9 above and then heat-shrinking it. [Effects of the Invention]

[0013] The heat-shrinkable polyester film of the present invention not only has a high heat shrinkage rate, but also has a low shrinkage stress, so that when it shrinks, the adhesive portion of the label does not peel off, and even if a thin container is used for the packaged item, the container does not deform.In addition, since the tensile elongation of the film does not decrease even after aging, it is possible to reduce problems in post-processing after film formation. DETAILED DESCRIPTION OF THE INVENTION

[0014] The heat-shrinkable polyester film of the present invention will be described in detail below. The method for producing the heat-shrinkable polyester film will be described in detail later, but the heat-shrinkable film is usually obtained by transporting and stretching the film using rolls or the like. Here, the transport direction of the film is referred to as the longitudinal direction (or machine direction), and the direction perpendicular to the longitudinal direction is referred to as the width direction (or transverse direction) of the film. Therefore, the width direction of the heat-shrinkable polyester film described below refers to the direction perpendicular to the unwinding direction of the roll, and the longitudinal direction of the film refers to the direction parallel to the unwinding direction of the roll. In the present invention, it is preferable that the main shrinkage direction of the film is the width direction.

[0015] To achieve high heat shrinkability in heat-shrinkable polyester films, it is common to copolymerize other polycarboxylic acid components or other polyhydric alcohol components with a homopolymer of ethylene terephthalate (PET). While neopentyl glycol and 1,4-cyclohexanediethanol are widely considered and widely used polyhydric alcohol components for use as copolymerization components, films copolymerized with these components have been found to exhibit a significant decrease in heat shrinkability at low temperatures below 70°C over time in ambient temperatures ranging from room temperature to approximately 40°C. However, the present inventors have discovered that films copolymerized with PET and diethylene glycol suppress this decrease in heat shrinkage over time, exhibit high heat shrinkability, and exhibit excellent solvent adhesion. Furthermore, when producing raw resins copolymerized with diethylene glycol, diethylene glycol is a liquid at room temperature, eliminating the need for a melting step, which is essential for powder raw materials such as neopentyl glycol. Furthermore, compared to neopentyl glycol, diethylene glycol offers the advantages of higher polymerization activity and less foaming during polymerization, which can lead to reduced productivity.

[0016] The heat-shrinkable polyester film of the present invention is one that contains ethylene terephthalate as a main component. Here, "main component" means that ethylene terephthalate accounts for 50 mol% or more of all polymer components that make up the film. It is more preferable that the film contains 70 mol% or more of ethylene terephthalate. By using ethylene terephthalate as a main component, it is possible to achieve excellent mechanical strength and transparency.

[0017] Polyethylene terephthalate (hereinafter sometimes simply referred to as PET) can be produced by any of a variety of polymerization methods, including a direct polymerization method in which terephthalic acid is directly reacted with ethylene glycol, and, if necessary, other dicarboxylic acid components and diol components, and an ester exchange method in which a dimethyl ester of terephthalic acid (containing, if necessary, a methyl ester of another dicarboxylic acid) is subjected to an ester exchange reaction with ethylene glycol (containing, if necessary, another diol component).

[0018] Examples of dicarboxylic acid components other than terephthalic acid that constitute the polyester used in the film of the present invention include aromatic dicarboxylic acids such as isophthalic acid, naphthalenedicarboxylic acid, and orthophthalic acid, aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid, and decanedicarboxylic acid, and alicyclic dicarboxylic acids. The content of these dicarboxylic acid components other than terephthalic acid is preferably 0 mol % to 15 mol %, more preferably 0 mol % to 10 mol %, and particularly preferably 0 mol % to 4 mol %, based on 100 mol % of the polycarboxylic acid component.

[0019] When an aliphatic dicarboxylic acid (such as adipic acid, sebacic acid, or decadicarboxylic acid) is contained, the content is preferably 0 mol % or more and less than 3 mol %. Heat-shrinkable polyester films obtained using polyesters containing 3 mol % or more of these aliphatic dicarboxylic acids have insufficient film stiffness when applied at high speed.

[0020] It is also preferable not to include trivalent or higher polycarboxylic acids (e.g., trimellitic acid, pyromellitic acid, and anhydrides thereof), as heat-shrinkable polyester films obtained using polyesters containing these polycarboxylic acids will have difficulty achieving the required shrinkability.

[0021] It is necessary that diethylene glycol accounts for 6 mol % or more and 25 mol % or less of 100 mol % of the polyhydric alcohol component constituting the polyester used in the film of the present invention.

[0022] Examples of polyhydric alcohol components other than ethylene glycol and diethylene glycol that constitute the polyester used in the present invention include aliphatic diols such as 1-3 propanediol, 1-4 butanediol, neopentyl glycol, and hexanediol, alicyclic diols such as 1,4-cyclohexanedimethanol, and aromatic diols such as bisphenol A. The content of these polyhydric alcohol components other than ethylene glycol and diethylene glycol is preferably 0 mol % or more and 15 mol % or less, more preferably 0 mol % or more and 10 mol % or less, and particularly preferably 0 mol % or more and 4 mol % or less, based on 100 mol % of the polyhydric alcohol components.

[0023] It is preferable not to include diols having 8 or more carbon atoms (e.g., octanediol, etc.) or trihydric or higher polyhydric alcohols (e.g., trimethylolpropane, trimellitolethane, glycerin, diglycerin, etc.). Heat-shrinkable polyester films obtained using polyesters containing these diols or polyhydric alcohols are unlikely to achieve the required high shrinkage.

[0024] Various additives, such as waxes, antioxidants, antistatic agents, crystal nucleating agents, viscosity reducers, heat stabilizers, coloring pigments, color inhibitors, and ultraviolet absorbers, may be added to the resin forming the heat-shrinkable polyester film of the present invention, as needed.

[0025] 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, including inorganic fine particles such as silica, alumina, titanium dioxide, calcium carbonate, kaolin, and barium sulfate, and organic fine particles such as acrylic resin particles, melamine resin particles, silicone resin particles, and cross-linked polystyrene particles. The average particle size of the fine particles can be selected appropriately within the range of 0.05 to 3.0 μm (as measured with a Coulter counter) as needed. The lower limit of the fine particle content in the film is preferably 0.01 wt %, more preferably 0.015 wt %, and even more preferably 0.02 wt %. If the content is less than 0.01 wt %, the slipperiness may decrease. The upper limit is preferably 1 wt %, more preferably 0.2 wt %, and even more preferably 0.1 wt %. If the content exceeds 1 wt %, the transparency may decrease, which is undesirable.

[0026] The method of incorporating the particles into the resin that forms the heat-shrinkable polyester film can be, for example, adding them at any stage in the production of the polyester resin, but it is preferable to add them as a slurry dispersed in ethylene glycol or the like at the stage of esterification or after the completion of the transesterification reaction and before the start of the polycondensation reaction, and then proceed with the polycondensation reaction. Also preferred is a method of blending a slurry of particles dispersed in ethylene glycol or water or the like with a polyester resin raw material using a vented kneading extruder, or a method of blending dried particles with a polyester resin raw material using a kneading extruder.

[0027] To prevent processing problems even after long-term storage, it is preferable for heat-shrinkable polyester films to exhibit minimal deterioration in tensile elongation over time. Generally, molecular orientation plays a major role in the tensile elongation of a film. When polymers are stretched, their molecular chains are stretched (molecular orientation), increasing their rigidity. Furthermore, the entanglement of the molecules forms a strong network, increasing their strength against tension and improving their elongation. However, it is known that over a long period of time after film formation (aging), molecular orientation relaxes, resulting in a decrease in tensile elongation. It is also known that hydrolysis of polyester over time reduces the molecular weight and decreases tensile elongation. We have found that by sufficiently increasing the molecular orientation in the plane direction of the film after film formation (planar orientation), it is possible to prevent this deterioration over time and prevent the decrease in elongation over time. High planar orientation is believed to result in strong rigidity and entanglement of the molecular chains, making them less likely to relax over time and reducing elongation even when hydrolysis occurs. A method for evaluating the decrease in elongation over time will be described later. Methods for increasing planar orientation will also be described later.

[0028] Furthermore, it is preferable that the heat-shrinkable polyester film of the present invention has a heat shrinkage rate in the main shrinkage direction of the film (i.e., hot water heat shrinkage rate at 98°C) calculated from the lengths before and after shrinkage using the following formula 1 when treated in 98°C hot water for 10 seconds under no load, of 40% or more. Heat shrinkage rate = {(length before shrinkage - length after shrinkage) / length before shrinkage} x 100 (%) Formula 1

[0029] If the hot water shrinkage rate in the main shrinkage direction at 98°C is less than 40%, when the film is used for beverage labels or lunch box packaging, the shrinkage amount is so small that the label will wrinkle or sag after heat shrinkage, which is undesirable. A hot water shrinkage rate at 98°C of 42% or more is more preferable, and a rate of 45% or more is particularly preferable.

[0030] The hot water shrinkage in the main shrinkage direction at 70°C is not particularly limited, but is preferably 10% to 40%. If it is less than 10%, when used as a beverage label or lunch box packaging film, the shrinkage amount is small, which is undesirable as it can cause wrinkles and sagging in the label after heat shrinkage. If it is 40% or more, it is undesirable as it can cause rapid shrinkage inside a shrink tunnel using steam, resulting in wrinkles and distortion. More preferably, it is 15% to 35%, and particularly preferably 20% to 30%.

[0031] The plane orientation coefficient of the film is preferably 0.035 or more and 0.070 or less. The plane orientation coefficient can be calculated by the following formula 2. Plane orientation coefficient ΔP=(Nx+Ny) / 2-Nz Equation 2 Nx: Refractive index in the longitudinal direction of the film Ny: Refractive index in the film width direction Nz: Refractive index in the film thickness direction

[0032] If the planar orientation coefficient is less than 0.035, the initial breakage over time (described later) deteriorates, and the tensile elongation decreases when the film is stored for a long period after production, which is undesirable. If the planar orientation coefficient is more than 0.070, the crystallinity becomes too high, which is undesirable because the solvent adhesive strength decreases when the film is made into a tubular bag for a beverage label. The planar orientation coefficient is more preferably 0.040 or more and 0.065 or less, and even more preferably 0.045 or more and 0.060 or less.

[0033] Of the 100 mol% of polyhydric alcohol components constituting the polyester used in the film of the present invention, diethylene glycol is preferably 6 mol% to 25 mol%. If the diethylene glycol content is less than 6 mol%, the shrinkage rate at 70°C for 10 seconds over time, as described below, will decrease significantly, which is undesirable. If the diethylene glycol content exceeds 25 mol%, the planar orientation coefficient will decrease, which is undesirable as it will tend to cause a decrease in elongation over time. The diethylene glycol content is more preferably 10 mol% to 23 mol%, and even more preferably 12 mol% to 21 mol%.

[0034] The heat-shrinkable polyester film of the present invention preferably has a maximum shrinkage stress in the main shrinkage direction measured under hot air at 90° C. of 2 MPa to 17 MPa. The shrinkage stress is measured by the method described in the examples. If the maximum shrinkage stress at 90°C in the main shrinkage direction exceeds 17 MPa, the label may lift or peel off at the label attachment point, or the shrinkage stress may cause the label to collapse during shrinkage in thin-walled containers, which is undesirable. The maximum shrinkage stress at 90°C is more preferably 15 MPa or less, and even more preferably 13 MPa or less. Furthermore, if the maximum shrinkage stress at 90°C is less than 2 MPa, the label may slacken and not adhere tightly to the container when used as a label for the container, which is undesirable. The maximum shrinkage stress at 90°C is more preferably 3 MPa or more, and even more preferably 4 MPa or more.

[0035] The polyester used in the film of the present invention preferably has a tensile breaking elongation in the longitudinal direction of the film of 20% or more after aging the film in an atmosphere of 40°C and 85% relative humidity for 28 days. If the breaking elongation after aging is below 20%, problems such as breaking due to tension in the process when the film is aged after long-term storage are likely to occur, which is undesirable. The breaking elongation before and after aging is more preferably 100% or more, even more preferably 200% or more, and particularly preferably 300% or more. The higher the breaking elongation, the better, but in the case of the product of the present invention, the upper limit is 700%.

[0036] The heat-shrinkable polyester film of the present invention preferably has a difference of 0% to 5% in the shrinkage rate in the main shrinkage direction when the film is immersed in 70°C warm water for 10 seconds after aging for 28 days in an atmosphere at a temperature of 40°C and a relative humidity of 85%. If the shrinkage rate difference exceeds 5%, the shrinkage rate at low temperatures decreases when the film is stored for a long period of time, which is undesirable because wrinkles and shrinkage spots are likely to occur when the film is shrunk as a label, etc. More preferably, the shrinkage rate difference is 4% or less, and particularly preferably 3% or less. The lower limit of the shrinkage rate difference is preferably as low as possible, but since it is unlikely that the shrinkage rate will increase over time, 0% is considered to be the lower limit.

[0037] The heat-shrinkable polyester film of the present invention preferably has a solvent adhesive strength of 4 (N / 15 mm) or more. If the solvent adhesive strength is less than 4 (N / 15 mm), the solvent-bonded portion is likely to peel off due to the shrinkage force when the label is heat-shrunk, which is undesirable. The solvent adhesive strength is more preferably 5 (N / 15 mm) or more, and particularly preferably 7 (N / 15 mm) or more. The higher the solvent adhesive strength, the better, but the upper limit of the solvent adhesive strength is thought to be about 15 (N / 15 mm). If the solvent adhesive strength is too high, when two films are solvent-bonded to form a label, unnecessary film adhesion is likely to occur, which may reduce label productivity. Therefore, a solvent adhesive strength of 10 (N / 15 mm) or less is not a problem in practice.

[0038] The heat-shrinkable polyester film of the present invention preferably has a natural shrinkage rate of 1.0% or less in the main shrinkage direction after aging for 28 days in an atmosphere at a temperature of 40°C and a humidity of 85% RH. The method for evaluating the natural shrinkage rate is shown in the Examples. If the natural shrinkage rate exceeds 1.0%, the film roll is likely to wrinkle when the rolled product is stored, which is undesirable. The smaller the natural shrinkage rate, the better. The natural shrinkage rate is preferably 0.9% or less, and 0.8% or less. More preferable.

[0039] The heat-shrinkable polyester film of the present invention preferably has an intrinsic viscosity (IV) of 0.60 dL / g or more and 0.75 dL / g or less. When the film has an intrinsic viscosity (IV) of 0.60 dL / g or more, the tensile elongation does not decrease even when the film is stored for a long period of time, and the occurrence of problems and defects such as breakage during processing can be reduced. Furthermore, increasing the intrinsic viscosity increases the degree of polymerization of the polyester, which increases the polymerization time, which is costly, and also increases the filtration pressure during melt extrusion, making high-precision filtration difficult. Therefore, the upper limit is preferably 0.75 dL / g.

[0040] The thickness of the heat-shrinkable polyester film of the present invention is not particularly limited, but is preferably 8 to 100 μm, more preferably 10 to 60 μm, for use as a heat-shrinkable film for labels or lunch box packaging. A film thickness of less than 8 μm is undesirable because the film's stiffness is significantly reduced, making the roll prone to wrinkling. On the other hand, although a thicker film does not pose a problem for a film roll, a thinner film is preferable from a cost perspective. The film thickness is more preferably 10 to 58 μm, and particularly preferably 12 to 56 μm.

[0041] The heat-shrinkable polyester film of the present invention can be obtained by melt-extruding the above-mentioned polyester raw material using an extruder to form an unstretched film, and then stretching the unstretched film in the width direction. The polyester can be obtained by polycondensing the above-mentioned suitable dicarboxylic acid component and diol component using a known method. In addition, chip-like polyester is usually used as the raw material for the film.

[0042] When melt-extruding the raw material resin, it is preferable to dry the polyester raw material using a dryer such as a hopper dryer or a paddle dryer, or a vacuum dryer. After drying the polyester raw material in this way, it is melted at a temperature of 230 to 270°C using an extruder and extruded into a film. For extrusion, any existing method such as a T-die method or a tubular method can be used.

[0043] The extruded sheet-like molten resin can then be rapidly cooled to obtain an unstretched film. A suitable method for rapidly cooling the molten resin is to cast the molten resin from a die onto a rotating drum and rapidly cool and solidify it to obtain a substantially unoriented resin sheet.

[0044] The unstretched film thus obtained can be stretched in the width direction under predetermined conditions to obtain the heat-shrinkable polyester film of the present invention. Preferred stretching methods for obtaining the heat-shrinkable polyester film of the present invention are described below.

[0045] Ordinary heat-shrinkable polyester films are produced by stretching an unstretched film in the direction of desired shrinkage. In the present invention, the film is uniaxially stretched in the width direction, which is the main shrinkage direction. The production method using uniaxial stretching in the width direction has the advantage that it can be produced with simple equipment because it does not require equipment for stretching in the longitudinal direction.

[0046] For widthwise stretching, the unstretched film is introduced into a tenter device that can heat the film by holding both ends with clips, and the film is preheated to a temperature of Tg+10°C or higher and 25°C or lower with hot air. The film is then stretched by increasing the distance between the clips while being transported in the longitudinal direction. The stretching ratio is preferably 4.5 times or more and 6 times or less. A stretching ratio of less than 4.5 is not preferred because it becomes difficult to obtain the required planar orientation coefficient. A stretching ratio of more than 6 times is not preferred because it increases the risk of breakage during film production and requires long and large equipment. A more preferred ratio is 4.7 times or more and 5.8 times or less.

[0047] The stretching strain rate is preferably different between the first and second halves of stretching, and the ratio of the stretching strain rates in the first and second halves of stretching (stretching strain rate ratio) is expressed by the following formula, and the stretching strain rate ratio is preferably 2.5 to 5.0. The stretching strain rate is expressed as nominal strain per unit time (second) (% / s). Stretching strain rate ratio = (Stretching strain rate in the second half of stretching) ÷ (Stretching strain rate in the first half of stretching) Equation 3

[0048] Here, the first half of stretching refers to stretching up to a ratio equal to the square root of the final stretch ratio, and the second half of stretching refers to stretching thereafter. For example, if the final stretch ratio is 5.0, the first half of stretching refers to stretching up to a ratio of 2.2, and the remaining stretching of 2.3 is the second half of stretching. A stretching strain rate ratio of less than 2.5 is undesirable because the required planar orientation coefficient cannot be obtained. A stretching strain rate ratio of more than 5.0 is undesirable because breakage during stretching is more likely to occur and the shrinkage stress of the film increases. A more preferable range is 2.7 to 4.8, and even more preferably 2.9 to 4.6.

[0049] The total stretching strain rate from the start to the end of stretching is preferably 10% / sec or more and 30% / sec or less. A rate less than 10% / sec is undesirable because it reduces molecular orientation and increases the likelihood of initial breakage over time. A rate greater than 30% / sec is undesirable because it increases the shrinkage stress of the film too much. A rate of 13% or more and 27% or less is more preferable, and a rate of 16% or more and 24% or less is even more preferable.

[0050] As described above, a stretching strain rate ratio of 2.5 to 5.0 means that the stretching rate is faster in the latter half of stretching than in the first half, and the strain rate is accelerated. The inventors' research has revealed that increasing the strain rate in the latter half of stretching enhances the molecular orientation of the film, increasing the planar orientation coefficient, and reducing the likelihood of a decrease in elongation over time. It is known that the stress value in the stress-strain curve of an unstretched film in a hot tensile test increases significantly in the latter half of stretching, suggesting that molecular orientation advances significantly in the latter half of stretching. Increasing the stretching rate in the latter half of stretching is thought to increase the stretching stress and further enhance molecular orientation. Conventionally, tenter stretching is performed at a nearly constant speed from the start to the end of stretching (i.e., the stretching strain rate ratio is approximately 1). While increasing the stretching strain rate is effective in enhancing molecular orientation, it undesirably increases the energy imparted to the film by stretching, resulting in excessively high shrinkage stress in the stretched film. In the present invention, it is important to reduce the stretching strain rate in the first half of stretching and to increase it in the second half in order to reduce the shrinkage stress in the main shrinkage direction of the film while suppressing a decrease in elongation in the direction perpendicular to the main shrinkage direction of the film over time.

[0051] Specifically, the stretching strain rate is adjusted by adjusting the film transport speed in the tenter and the tenter pattern, and the stretching strain rate ratio is adjusted by adjusting the tenter pattern. It is preferable not to stretch in the longitudinal direction, as this requires large-scale equipment. Furthermore, although not particularly limited, heat treatment may be performed after stretching in the width direction to adjust the shrinkage rate. The heat treatment temperature is preferably 70°C or higher and 110°C or lower. Heat treatment at less than 70°C is not preferred because the shrinkage rate in the width direction at 70°C exceeds 40%. Furthermore, heat treatment temperatures higher than 110°C are not preferred because the shrinkage rate in the width direction at 98°C falls below 40%. A more preferred range is 75°C or higher and 105°C or lower, and even more preferably 80°C or higher and 100°C or lower.

[0052] The film temperature during width direction stretching is preferably at least Tg+5°C and not more than Tg+40°C. If the film temperature is less than Tg+5°C, the stretching force becomes too high, which is undesirable as it increases the shrinkage stress of the film. If the film temperature exceeds Tg+40°C, the stretching force is too low, which results in molecular orientation not being imparted, reducing planar orientation and making it more likely that the elongation will decrease over time, which is undesirable. Although not particularly limited, the film temperature may be the same in the first and second half of stretching, but it is preferable that the temperature in the second half of stretching is lower than that in the first half, as this increases planar orientation.

[0053] The packaging of the present invention is formed by covering at least a portion of the outer periphery of an object to be packaged with a label obtained from the heat-shrinkable polyester film of the present invention and then heat-shrinking the label. Examples of the object to be packaged include PET beverage bottles, various bottles, cans, plastic containers for confectionery and lunch boxes, and paper boxes. Typically, when covering such an object to be packaged with a label obtained from the heat-shrinkable polyester film by heat-shrinking, the label is heat-shrunk by about 5 to 70% to be tightly attached to the packaging. The label to be covered on the object to be packaged may or may not be printed.

[0054] Labels can be produced by coating one side of a rectangular film with an organic solvent slightly inward from the edge, then immediately rolling the film and overlapping and gluing the edges together to form a label; alternatively, by coating one side of a rolled film with an organic solvent slightly inward from the edge, then immediately rolling the film and overlapping and gluing the edges together to form a tubular body, which can then be cut into labels. Preferred organic solvents for adhesive use are cyclic ethers such as 1,3-dioxolane or tetrahydrofuran. Other suitable organic solvents include aromatic hydrocarbons such as benzene, toluene, xylene, and trimethylbenzene; halogenated hydrocarbons such as methylene chloride and chloroform; and phenols such as phenol; or mixtures of these. [Example]

[0055] The present invention will be described in more detail below using examples, but the present invention is not limited to the embodiments of these examples and can be modified as appropriate within the scope of the invention.

[0056] The film was evaluated as follows.

[0057] [Tg (glass transition temperature)] Using a differential scanning calorimeter (DSC220, manufactured by Seiko Instruments Inc.), 5 mg of unstretched film was placed in a sample pan, the pan was covered, and the temperature was increased from -40°C to 120°C at a rate of 10°C / min under a nitrogen gas atmosphere. Tg (°C) was determined in accordance with JIS-K7121-1987.

[0058] [Intrinsic viscosity (IV)] 0.2 g of polyester was dissolved in 50 ml of a mixed solvent of phenol / 1,1,2,2-tetrachloroethane (60 / 40 (weight ratio)), and the viscosity was measured using an Ostwald viscometer at 30° C. The unit is dl / g.

[0059] [Heat shrinkage rate (hot water heat shrinkage rate)] The film was cut into a 10 cm x 10 cm square and immersed in warm water of a specified temperature ±0.5°C for 10 seconds without load to allow it to shrink, then immersed in water at 25°C ±0.5°C for 10 seconds and removed from the water, measuring the longitudinal and transverse dimensions of the film, and calculating the thermal shrinkage rate according to the following formula (1). The direction with the largest thermal shrinkage rate was defined as the main shrinkage direction. Heat shrinkage rate = {(length before shrinkage - length after shrinkage) / length before shrinkage} x 100 (%) Formula 1

[0060] Maximum shrinkage stress A strip-shaped film sample measuring 150 mm in length in the main shrinkage direction and 20 mm in width was cut from the heat-shrinkable film, and the shrinkage stress was measured using a Tesilon universal testing machine PTM-250 (a registered trademark of Orientec) with a heating furnace and strength / strain tester manufactured by Toyo Baldwin Co., Ltd. (now Orientec). The heating furnace of the strength / strain tester was preheated to 90°C, and the distance between the chucks for holding the film sample was 100 mm. To attach the sample to the chucks of the strength / strain tester, the air flow to the heating furnace was stopped, the door of the heating furnace was opened, and 25 mm of each end of the 150 mm long sample was clamped between the chucks. The distance between the chucks was 100 mm, and the sample was secured tightly so that the distance between the chucks was aligned with the length of the sample and the sample was horizontal. After attaching the sample to the chucks, the door of the heating furnace 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 shrinkage stress measured from the starting point for measuring the shrinkage stress until 30 seconds after the start of the measurement was taken as the maximum value of the shrinkage stress (maximum shrinkage stress (MPa)).

[0061] [Change in shrinkage rate over time] After leaving the film in an atmosphere of 40°C x 85% RH for 28 days (672 hours) (aging), the hot water shrinkage was measured in 70°C hot water to determine the longitudinal 70°C hot water shrinkage, and the difference from the longitudinal 70°C hot water shrinkage before aging was determined and recorded as the "difference in shrinkage before and after aging."

[0062] [Solvent adhesive strength] 1,3-Dioxolane was applied to two heat-shrinkable films at a coating weight of 5±0.3 g / m2 and a coating width of 5±1 mm, and two sheets were laminated together to form a seal. Then, a 15 mm wide strip was cut perpendicular to the sealing direction, and the strip was placed in a Baldwin STM-50 universal tensile tester with a chuck distance of 20 mm. The strip was then subjected to a tensile peel test at a tensile speed of 200 mm / min to measure the peel resistance. The strength measured at this point was taken as the solvent adhesive strength.

[0063] [Natural shrinkage rate] The film was cut into a size of 200 mm x 30 mm (main shrinkage direction x orthogonal direction), and a 150 mm long benchmark was drawn in the main shrinkage direction. After leaving the film for 28 days (aging) in an atmosphere of 40°C and 85% RH, the length of the benchmark was measured and the natural shrinkage was calculated using the following formula. Natural shrinkage rate = (length of gauge line before aging (150 mm) - length of gauge line after aging) ÷ (length of gauge line before aging (150 mm) × 100 (%) Formula 4

[0064] [Face orientation coefficient] The refractive indexes in the longitudinal, transverse, and thickness directions of the film were measured using an Abbe Refractometer Model 4T manufactured by Atago Co., Ltd., after leaving each sample film in an atmosphere of 23°C and 65% RH for at least 2 hours. The plane orientation coefficient was calculated from the measurement results using the following equation 2. Plane orientation coefficient ΔP=(Nx+Ny) / 2-Nz Equation 2 Nx: Refractive index in the longitudinal direction of the film Ny: Refractive index in the film width direction Nz: Refractive index in the film thickness direction

[0065] [Tensile elongation at break] In accordance with JIS-K-7127, rectangular specimens measuring 50 mm in the direction perpendicular to the main shrinkage direction (film longitudinal direction) and 20 mm in the main shrinkage direction (film width direction) were prepared. Tensile tests were performed using a universal tensile tester (Shimadzu Corporation, Autograph®) at a tension speed of 200 mm / min, with the elongation at break being taken as the breaking elongation. Tensile tests were performed immediately after film formation and after leaving the specimens for 28 days in an atmosphere at 40°C and 85% RH.

[0066] [Shrinkage finish (before and after aging)] A three-color print was applied to the heat-shrinkable film using Toyo Ink Mfg. Co., Ltd.'s grass, gold, and white inks. Both ends of the printed film were then glued together with dioxolane to create cylindrical labels (labels with the main shrinkage direction of the heat-shrinkable film aligned circumferentially). These were then cut. The diameter of the label in the shrinking direction was 70 mm. The labels were then attached to 500 ml PET bottles (body diameter 62 mm, minimum neck diameter 25 mm) by heat shrinking them in a Fuji Astec Inc. steam tunnel (model: SH-1500-L) at a zone temperature of 90°C for 4 seconds. The neck was adjusted so that the 30 mm diameter was at one end of the label. The finished appearance after shrinkage was evaluated visually, using the following criteria:

[0067] Label shrinkage distortion To evaluate the finish after shrinkage, the distortion of the top of the attached label in the 360-degree direction was measured using a gauge to determine the maximum distortion. Evaluation was made according to the following criteria. ○: Maximum distortion less than 2.0 mm ×: Maximum distortion 2.0mm or more

[0068] [Label shrinkage is insufficient] The above-mentioned shrinkage state of the label was evaluated according to the following criteria. ○: There is no slack between the attached label and the container and it has shrunk. ×: There is slack between the label and the container due to insufficient shrinkage.

[0069] [Wrinkles on label] Under the same conditions as those for the shrinkage distortion of the label described above, the occurrence of wrinkles was evaluated according to the following criteria. ○: The number of wrinkles 2 mm or larger is 2 or less. ×: Three or more wrinkles of 2 mm or larger.

[0070] [Peeling of adhesive joints] The above-mentioned shrinkage state of the label was evaluated according to the following criteria. ○: No peeling at the adhesive joints between labels. ×: Peeling occurred at the adhesive portion between labels.

[0071] <Preparation of polyester raw materials> [Synthesis Example 1] A stainless steel autoclave equipped with a stirrer, thermometer, and partial reflux condenser was charged with 100 mol% dimethyl terephthalate (DMT) as the dicarboxylic acid component and 100 mol% ethylene glycol (EG) as the 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) zinc acetate was added as a transesterification catalyst, and 0.225 mol% (relative to the acid component) antimony trioxide was added as a polycondensation catalyst. The resulting methanol was distilled off while the transesterification reaction was carried out. Polycondensation was then carried out at 280°C under reduced pressure of 26.7 Pa, yielding Polyester 1 with an intrinsic viscosity of 0.75 dL / g. The composition is shown in Table 1.

[0072] [Synthesis Examples 2 to 5] Polyesters 2 to 4 shown in Table 1 were obtained in the same manner as in Synthesis Example 1. When producing polyester 2, SiO2 (Sylysia 266 manufactured by Fuji Silysia Corporation; average particle size 1.5 μm) was added as a lubricant at a ratio of 7200 ppm to the polyester. In the table, NPG stands for neopentyl glycol, and CHDM stands for 1,4-cyclohexanedimethanol. The intrinsic viscosities of the polyesters were 2:0.75 dl / g, 3:0.75 dl / g, 4:0.75 dl / g, and 5:0.75 dl / g, respectively. Each polyester was appropriately cut into chips. The composition of each polyester is shown in Table 1.

[0073] [Table 1]

[0074] [Example 1] The above-mentioned polyester 1, polyester 2, and polyester 3 were mixed in a mass ratio of 80:5:15 and charged into an extruder. The mixed resin was then melted at 280°C, extruded through a T-die, and rapidly cooled by being wound around a rotating metal roll cooled to a surface temperature of 30°C, yielding an unstretched film with a thickness of 144 μm. The Tg of the unstretched film was 72°C. The unstretched film was introduced into a tenter, and while both ends of the film were held with clips, it was preheated to a film temperature of 82°C (Tg + 10°C). It was then stretched 4.8 times in the transverse direction at a film temperature of 82°C (Tg + 10°C). The stretching strain rate in the first half of the stretching (up to a stretch ratio of 2.2 times) was 12.5% / s, and the stretching strain rate in the second half of the stretching was 37.3% / s. The overall stretching strain rate was 23% / sec, and the stretching speed ratio was 3.0. After stretching, the film was heated at 85°C inside the tenter while maintaining the film width at a fixed length. Both edges of the stretched film were cut and removed, resulting in a continuous uniaxially stretched film of approximately 30 μm over the specified length, yielding a film roll made of heat-shrinkable polyester film. The properties of the resulting film were evaluated using the methods described above. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0075] [Example 2] The conditions of Example 1 were changed such that the stretching ratio in the transverse direction was 5.3 times, the film temperature during preheating and stretching was 83°C, and the resin extrusion amount was adjusted so that the film thickness after stretching would be approximately 30 μm. The stretching strain rate in the first half of stretching (up to a stretch ratio of 2.3) was 14.3% / sec, and the stretching strain rate in the second half of stretching was 40.4% / sec. The overall stretching strain rate was 26% / sec, and the stretching speed ratio was 2.8. Other than the above, the same conditions were used as in Example 1. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0076] [Example 3] The conditions of Example 1 were changed such that the stretching ratio in the transverse direction was 5.8 times, the film temperature during preheating and stretching was 87°C, and the resin extrusion amount was adjusted so that the film thickness after stretching would be approximately 30 μm. The stretching strain rate in the first half of stretching (up to a stretch ratio of 2.4) was 16.0% / sec, and the stretching strain rate in the second half of stretching was 43.9% / sec. The overall stretching strain rate was 29% / sec, and the stretching speed ratio was 2.7. Other than the above, the same conditions were used as in Example 1. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0077] [Example 4] The same procedures as in Example 1 were carried out except that the above polyester 1, polyester 2, and polyester 3 were mixed in a mass ratio of 70:5:25 and charged into an extruder, and the film temperature during preheating and stretching was set to 76°C. At this time, the Tg of the unstretched film was 68°C. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0078] [Example 5] The conditions of Example 4 were changed such that the stretching ratio in the transverse direction was 5.3 times, the film temperature during preheating and stretching was 77°C, and the resin extrusion amount was adjusted so that the film thickness after stretching would be approximately 30 μm. The stretching strain rate in the first half of stretching (up to a stretch ratio of 2.3) was 14.3% / sec, and the stretching strain rate in the second half of stretching was 40.4% / sec. The overall stretching strain rate was 26% / sec, and the stretching speed ratio was 2.8. Other than the above, the same conditions were used as in Example 4. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0079] [Example 6] The conditions of Example 4 were changed such that the stretching ratio in the transverse direction was 5.8 times, the film temperature during preheating and stretching was 81°C, and the resin extrusion amount was adjusted so that the film thickness after stretching would be approximately 30 μm. The stretching strain rate in the first half of stretching (up to a stretch ratio of 2.4) was 16.0% / sec, and the stretching strain rate in the second half of stretching was 43.9% / sec. The overall stretching strain rate was 29% / sec, and the stretching speed ratio was 2.7. Other than the above, the same conditions were used as in Example 4. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0080] [Example 7] The same procedures as in Example 2 were carried out except that the above polyester 1, polyester 2, and polyester 3 were mixed in a mass ratio of 58:5:37 and charged into an extruder, and the film temperature during preheating and stretching was 75°C. At this time, the Tg of the unstretched film was 62°C. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0081] [Example 8] The conditions of Example 7 were changed such that the stretching ratio in the transverse direction was 5.8 times, the film temperature during preheating and stretching was 77°C, and the resin extrusion amount was adjusted so that the film thickness after stretching would be approximately 30 μm. The stretching strain rate in the first half of stretching (up to a stretch ratio of 2.4) was 16.0% / sec, and the stretching strain rate in the second half of stretching was 43.9% / sec. The overall stretching strain rate was 29% / sec, and the stretching speed ratio was 2.7. Other than the above, the same conditions were used as in Example 7. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0082] [Example 9] The conditions of Example 7 were changed such that the stretching ratio in the transverse direction was 4.8 times, the film temperature during preheating and stretching was 77°C, and the resin extrusion amount was adjusted so that the film thickness after stretching would be approximately 30 μm. The stretching strain rate in the first half of stretching (up to a stretch ratio of 2.2) was 12.0% / sec, and the stretching strain rate in the second half of stretching was 39.5% / sec. The overall stretching strain rate was 23% / sec, and the stretching speed ratio was 3.3. Other than the above, the same conditions were used as in Example 7. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0083] [Example 10] The stretching strain rate in the first half of stretching (up to a stretch ratio of 2.2) was 11.5% / sec, and the stretching strain rate in the second half of stretching was 42.3% / sec. The overall stretching strain rate was 23% / sec, and the stretching speed ratio was 3.7, but the same procedures as in Example 9 were followed. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0084] [Comparative Example 1] The same procedures as in Example 1 were carried out except that the above polyester 1, polyester 2, and polyester 3 were mixed in a mass ratio of 87:5:8 and charged into an extruder, and the film temperature during preheating and stretching was 84°C. At this time, the Tg of the unstretched film was 73°C. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0085] Comparative Example 2 The same procedures as in Example 3 were carried out except that the above polyester 1, polyester 2, and polyester 3 were mixed in a mass ratio of 87:5:8 and charged into an extruder, and the film temperature during preheating and stretching was 84°C. At this time, the Tg of the unstretched film was 73°C. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0086] Comparative Example 3 The same procedures as in Example 3 were carried out except that the above polyester 1, polyester 2, and polyester 3 were mixed in a mass ratio of 45:5:50 and charged into an extruder, and the film temperature during preheating and stretching was set to 72°C. At this time, the Tg of the unstretched film was 60°C. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0087] Comparative Example 4 The stretching strain rate in the first half of stretching (up to a stretch ratio of 2.3) was 30.0% / sec, and the stretching strain rate in the second half of stretching was 24.6% / sec, resulting in an overall stretching strain rate of 29% / sec and a stretching rate ratio of 0.8, all of which were the same as in Example 7. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0088] Comparative Example 5 The same procedures as in Example 1 were carried out except that the above polyester 1, polyester 2, and polyester 4 were mixed in a mass ratio of 45:5:50 and charged into an extruder, and the film temperature during preheating and stretching was set to 76°C. At this time, the Tg of the unstretched film was 75°C. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0089] Comparative Example 6 The same procedures as in Example 1 were carried out except that the above polyester 1, polyester 2, and polyester 5 were mixed in a mass ratio of 45:5:60 and charged into an extruder, and the film temperature during preheating and stretching was set to 76°C. At this time, the Tg of the unstretched film was 73°C. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0090] Comparative Example 7 The above polyester 1, polyester 2, polyester 3, and polyester 4 were mixed in a weight ratio of 4:5:25:66 and fed into an extruder. An unstretched sheet having a thickness of 153 μm was obtained in the same manner as in Example 1. The Tg of the unstretched film was 68°C. The unstretched film was introduced into a tenter, and while both ends of the film were held with clips, the film was preheated to a temperature of 73°C (Tg + 5°C). It was then stretched 5.1 times in the transverse direction at a film temperature of 73°C (Tg + 5°C). The stretching strain rate in the first half of the stretching (up to a stretch ratio of 2.2 times) was 23.0% / sec, and the stretching strain rate in the second half of the stretching was 23.0% / sec. The overall stretching strain rate was 23% / sec, and the stretching speed ratio was 1.0. After stretching, the film was heat-treated at a film temperature of 74°C while being relaxed by 5% in the transverse direction inside the tenter. After the tenter process, both edges of the film were cut and removed to continuously produce a uniaxially stretched film of approximately 30 μm over a predetermined length, thereby obtaining a film roll made of heat-shrinkable polyester film. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3.

[0091] [Table 2]

[0092] [Table 3]

[0093] As a result of the evaluation, the films of Examples 1 to 10 had sufficient shrinkability and low shrinkage stress, resulting in good shrinkage finish. In addition, because the film plane orientation was high, the initial breakage rate after aging was good at 0%, and because the 70°C shrinkage rate remained almost unchanged even after aging, the shrinkage finish after aging was also good.

[0094] The film of Comparative Example 1 had a low diethylene glycol ratio, so it had low amorphousness, could not obtain a sufficient shrinkage rate, and showed a large change in shrinkage rate at 70°C over time. The shrinkage finish was poor, with shrinkage distortion and insufficient shrinkage both before and after aging.

[0095] The film of Comparative Example 2 had a low diethylene glycol ratio, which resulted in low amorphousness, insufficient shrinkage, high shrinkage stress, and large changes in shrinkage over time. The high planar orientation coefficient resulted in low solvent adhesive strength, and the shrinkage finish was poor, with shrinkage distortion and insufficient shrinkage both before and after aging, and peeling of the sealed area also occurred.

[0096] The films of Comparative Examples 3 and 4 had sufficient shrinkage and low shrinkage stress, but because the degree of planar orientation was low, the longitudinal elongation after aging was low and the films were prone to problems during post-processing.

[0097] The film of Comparative Example 5 was highly amorphous due to the neopentyl glycol component, had sufficient shrinkage, a high degree of planar orientation, and low shrinkage stress, but due to the low diethylene glycol ratio, the 70°C shrinkage rate changed significantly over time and the natural shrinkage rate was high. The shrinkage finish was good before aging, but after aging, the decrease in 70°C shrinkage rate caused shrinkage distortion and wrinkles, resulting in poor results.

[0098] The film of Comparative Example 6 was highly amorphous due to the cyclohexanedimethanol component, had sufficient shrinkage, high planar orientation, and low shrinkage stress, but due to the low diethylene glycol ratio, the 70°C shrinkage rate changed significantly over time and the natural shrinkage rate was high. The shrinkage finish was good before aging, but after aging, the decrease in 70°C shrinkage rate caused shrinkage distortion and wrinkles, resulting in poor results.

[0099] The film of Comparative Example 7 had sufficient shrinkage and low shrinkage stress, but because the degree of planar orientation was small, the elongation in the longitudinal direction after aging was low and the film was prone to problems during post-processing. [Industrial Applicability]

[0100] The heat-shrinkable polyester film of the present invention has sufficient shrinkability and low shrinkage stress as described above, resulting in good shrinkage finish. Furthermore, because the shrinkage rate changes little over time, the shrinkage finish after aging is also good. Furthermore, because the degree of planar orientation is high, the initial break rate over time is low, and even after long-term storage, the film is less susceptible to breakage problems during post-processing, such as printing. As described above, the film has excellent shrinkage finish and processability both immediately after production and after long-term storage, making it suitable for use in applications such as beverage bottle labels and bento box banding packaging.

Claims

1. A heat-shrinkable polyester film that satisfies the following requirements (1) to (6) and whose main shrinkage direction is the width direction: (1) The shrinkage rate (hot water shrinkage rate) when the film is immersed in hot water at 98°C for 10 seconds is 40% or more in the main shrinkage direction of the film. (2) The plane orientation coefficient of the film is 0.035 or more and 0.070 or less. (3) The main constituent is ethylene terephthalate, and the amount of diethylene glycol (DEG) component is 6 mol% to 25 mol% of 100 mol% of the polyhydric alcohol components in the total polyester resin constituting the film, and the content of polyhydric alcohol components other than ethylene glycol and diethylene glycol is 0 mol% to 15 mol% of 100 mol% of the polyhydric alcohol components. (4) The maximum shrinkage stress in the main shrinkage direction of the film measured in 90°C hot air is 2 MPa or more and 17 MPa or less. (5) After the film is left in an atmosphere at an atmospheric temperature of 40°C and a relative humidity of 85% for 28 days, the tensile elongation at break in the direction perpendicular to the main shrinkage direction is 20% or more. (6) After the film is left in an atmosphere of 40°C and 85% relative humidity for 28 days, the film is immersed in 70°C hot water for 10 seconds, and the shrinkage rate in the main shrinkage direction of the film (hot water shrinkage rate) is 14.2% or more and 27.8% or less.

2. 2. The heat-shrinkable polyester film according to claim 1, wherein the tensile elongation at break in a direction perpendicular to the main shrinkage direction is 100% or more after the film is aged for 28 days in an atmosphere at an atmospheric temperature of 40°C and a relative humidity of 85%.

3. 3. The heat-shrinkable polyester film according to claim 1, wherein the solvent adhesive strength is 4 N / 15 mm width or more and 15 N / 15 mm width or less.

4. 4. The heat-shrinkable polyester film according to claim 1, wherein the natural shrinkage rate in the main shrinkage direction after aging the film for 28 days in an atmosphere at an atmospheric temperature of 40°C and a relative humidity of 85% is less than 1.0%.

5. 5. The heat-shrinkable polyester film according to claim 1, wherein the intrinsic viscosity of the film is 0.60 dl / g or more and 0.75 dl / g or less.

6. The heat-shrinkable polyester film according to any one of claims 1 to 5, which is a uniaxially stretched film.

7. A label obtained from the heat-shrinkable polyester film according to any one of claims 1 to 6.

8. A package formed by covering at least a part of the outer periphery of an object to be packaged with the label according to claim 7 and then heat-shrinking the label.

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