Heat-shrinkable polyester film with the main shrinkage direction being the machine (longitudinal) direction

Abstract

[Problem] To provide a heat-shrinkable polyester film that has excellent shrink finishing properties by having sufficient heat shrinkability in the lengthwise direction and low shrinkage stress in the lengthwise direction, and undergoing little change (so-called neck in) in the film non-shrinkage direction. [Solution] Disclosed is a polyester film that is heat-shrinkable in the longitudinal (lengthwise) direction, the heat-shrinkable polyester film being characterized by satisfying the following requirements (1) to (6). (1) The shrinkage rate in the lengthwise direction after immersion for 10 seconds in hot water of 80°C is from 45% to 70%; (2) the shrinkage rate in a direction (width direction) orthogonal to the lengthwise direction after immersion for 10 seconds in hot water of 80°C is from -8% to 7%; (3) the rate of change in the film width direction as found by fixing the film only in the lengthwise direction in a constant-length state in the lengthwise direction and gripping the film for 10 seconds under hot air of 90°C is from 5% to 22%; (4) the rate of change in the film width direction as found by fixing the film only in the lengthwise direction in a state with 10% slack in the lengthwise direction and gripping the film for 10 seconds under hot air of 90°C is from 5% to 20%; (5) the maximum heat shrinkage stress measured in the lengthwise direction under hot air of 90°C is from 5 MPa to 15 MPa; and (6) the stress at 10% elongation (so-called F10) measured under hot air of 90°C is from 1 MPa to 5 MPa in the lengthwise direction and from 0.5 MPa to 3 MPa in the width direction.

Description

[Technical Field]

[0001] The present invention relates to a polyester film that heat-shrinks in the longitudinal (longitudinal) direction and has an excellent appearance after shrinkage, and to a package. More specifically, the present invention relates to a heat-shrinkable polyester film that is suitable for use in packaging strip labels for lunch boxes, noodle containers, and the like in supermarkets and convenience stores, and that shrinks in the longitudinal direction of the film roll, has a rate of change in the width direction after shrinking in the longitudinal direction within a certain range, and has a shrinkage stress within a certain range when heated, allowing for a good shrink finish. [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 that both protects glass bottles and PET bottles and displays product labels, as well as for banding and ribbon label packaging used to fasten the containers and lids of convenience store lunch boxes. Among these heat-shrinkable films, polyvinyl chloride films have problems such as low heat resistance, generating hydrogen chloride gas when incinerated, and being a source of dioxins. Polystyrene films, on the other hand, have poor solvent resistance, require the use of inks with special compositions for printing, and require incineration at high temperatures, resulting in the generation of large amounts of black smoke accompanied by an unpleasant odor during 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.

[0003] Furthermore, typical heat-shrinkable polyester films widely used are those that shrink significantly in the width direction. When used as a label film for bottles or a strip label for lunch boxes, the film must be formed into a ring and attached to the bottle or lunch box, and then heat-shrunk in the circumferential direction. Therefore, when attaching a heat-shrinkable film that heat-shrinks in the width direction, a ring must be formed so that the width direction of the film is the circumferential direction, and the ring must then be cut to a predetermined length and attached to the bottle or lunch box by hand-sealing or other methods. Therefore, it is difficult to attach a label film or strip label made of a heat-shrinkable film that heat-shrinks in the width direction to a bottle or lunch box at high speed. Therefore, there has been a recent demand for a film that heat-shrinks in the longitudinal direction, which can be directly wound around a bottle or lunch box after being unwound from a film roll. This eliminates the need for a center-sealing process for forming and sealing a film ring, as well as cutting, hand-sealing, and other processes, and allows for high-speed attachment.

[0004] In recent years, convenience stores and other establishments have seen an increase in the number of prepared bento lunches and noodle products such as udon and ramen. To improve the appearance of bento lunches, packaging that completely covers the entire lunch box has been replaced with a side shrink label that shrinks and covers only the sides of the lunch box and container, allowing the contents to be clearly seen from the top of the lunch box. Another type of packaging is known as strip label packaging, in which cooked noodles are placed in a deep plastic container, a lid is placed on top, and a ring of shrink film secures the container and lid from above and below (so-called strip label packaging). Strip label packaging also requires automated packaging, in which shrink film is unwound from two film rolls and heat-sealed widthwise. The packaged noodle container is inserted between the two films, and the film opposite the heat-sealed portion is heat-sealed to form a ring of film. The ring of film is then heated with hot air or other means to shrink, adhering to the packaged item and forming a strip label that secures the container and lid. The performance of shrink film used in side shrink labels and strip labels not only secures the lid to the container, but also enhances product value by minimizing distortion in the non-shrink direction during shrinkage, resulting in a clean appearance. It is also important that the container does not deform due to shrinkage stress. Container distortion not only negatively impacts the product's appearance, but also poses problems such as spillage of the contents or the inclusion of foreign matter. Furthermore, if the shrinkage stress is high, a force perpendicular to the shrinkage direction (the so-called neck-in force) acts after the film shrinks, resulting in significant distortion in the width direction and an undesirable appearance. Furthermore, if the shrinkage stress is low, the lid and container are not securely fastened, and vibrations during transportation can create gaps between the lid and container, potentially allowing foreign matter to enter.

[0005] For example, Patent Document 1 describes that heat-shrinkable films used for battery cell coatings, etc., achieve favorable results in heat cycle tests by suppressing deformation (neck-in) after shrinkage in the non-shrinkage direction. However, the heat-shrinkable film described in Patent Document 1 has a widthwise primary shrinkage direction, and does not describe heat-shrinkable films with a longitudinal primary shrinkage direction. Furthermore, because the heat shrinkage rate in the primary shrinkage direction at 80°C is low, a high shrinkage temperature is required when shrinking the film as a label on a lunch box or delicatessen container. High shrinkage temperatures are undesirable because they affect the contents and the container. Furthermore, because the shrinkage stress is low, when used as a side shrink label or a strip label, the fastening of the lid and the container becomes loose, which can lead to gaps between the lid and the container due to vibration during transportation, potentially allowing foreign objects to enter. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. WO2018 / 003994 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a heat-shrinkable polyester film that is suitable for side shrink labels and strip labels, has sufficient heat shrinkability in the longitudinal direction of the film, and has a widthwise deformation ratio and a longitudinal shrinkage stress that fall within predetermined ranges, resulting in a label with excellent appearance after shrinkage. [Means for solving the problem]

[0008] The present invention, which has solved the above problems, comprises the following components. 1. A heat-shrinkable polyester film whose main shrinkage direction is in the longitudinal (longitudinal) direction, characterized in that it satisfies the following requirements (1) to (6): (1) The shrinkage rate in the longitudinal direction after immersion in 80°C hot water for 10 seconds is 35% to 70%. (2) After immersion in 80°C hot water for 10 seconds, the shrinkage rate in the direction perpendicular to the longitudinal direction (width direction) is between -8% and 7%. (3) The film is held at a constant length in the longitudinal direction, fixed only in the longitudinal direction, and held in a 90°C hot air atmosphere for 10 seconds. The change in the film width direction is 5% or more and 22% or less. (4) The film is fixed only in the longitudinal direction with 10% slack in the longitudinal direction, and the change in the film width direction measured by holding it in hot air at 90°C for 10 seconds is 5% or more and 20% or less. (5) The maximum thermal shrinkage stress in the longitudinal direction measured under hot air at 90°C is 2 MPa or more and 10 MPa or less. (6) The stress at 10% elongation (so-called F10) measured with hot air at a temperature of 90°C is 1 MPa or more and 5 MPa or less in the longitudinal direction, and 0.5 MPa or more and 3 MPa or less in the transverse direction. 2. The heat-shrinkable polyester film according to 1, having a film thickness of 6 μm or more and 30 μm or less. 3. A heat-shrinkable polyester film according to any one of 1. to 2., characterized in that the butanediol component is contained in an amount of 8 mol % to 40 mol % of the total glycol components of the polyester constituting the film. 4. A heat-shrinkable polyester film according to any one of 1. to 3., having a maximum heat shrinkage stress of 1 MPa or more and 6 MPa or less, measured in a state of 10% slack in the longitudinal direction in hot air at 90°C. 5. The heat-shrinkable polyester film according to any one of 1. to 4., which is used for packaging labels for plastic containers. 6. A package comprising a strip label covered with the heat-shrinkable polyester film according to any one of 1. to 5. above, attached in a ring shape by heat sealing. [Effects of the Invention]

[0009] As a result of extensive research, the inventors have discovered that by uniaxially stretching an unstretched sheet having a specific polyester composition in the longitudinal direction and then performing a relaxation treatment in the longitudinal direction, not only is a sufficient shrinkage rate achieved in the longitudinal direction, but the rate of change in the width direction and the shrinkage stress in the longitudinal direction can be kept within a certain range, thereby preventing deformation of the packaged item during shrinkage and resulting in an excellent appearance of the label after shrinkage, and have completed the present invention.

[0010] In other words, the heat-shrinkable polyester film of the present invention has sufficient heat shrinkage properties in the longitudinal direction of the film and can be suitably used for side shrink labels or band labels for containers such as lunch boxes. Since the main shrinkage direction is the longitudinal direction, it can be attached very efficiently in a short time. In addition, when heat-shrunk after attachment, there is no insufficient shrinkage, and since the shrinkage stress is within a predetermined range, there is very little deformation of the container or slack in the label after attachment to the container. Furthermore, since the deformation rate in the width direction is low, it is possible to obtain a good finished appearance. DETAILED DESCRIPTION OF THE INVENTION

[0011] The configuration of the heat-shrinkable polyester film according to the present invention will be described in detail below.

[0012] [Characteristics of the Heat-Shrinkable Polyester Film of the Present Invention, the Main Shrinkage Direction of which is the Machine (Longitudinal) Direction] When the heat-shrinkable polyester film of the present invention is treated in 80°C hot water for 10 seconds under no load, the heat shrinkage rate in the longitudinal direction of the film (i.e., the hot water heat shrinkage rate at 80°C) calculated from the lengths before and after shrinkage using the following formula 1 is 35% or more and 70% or less. Heat shrinkage rate = {(length before shrinkage - length after shrinkage) / length before shrinkage} x 100 (%) Formula 1

[0013] If the longitudinal hot water shrinkage rate at 80°C is less than 35%, when used as a side shrink label or band label, the small shrinkage amount will result in wrinkles and sagging in the label after heat shrinkage, which is undesirable. Furthermore, shrinking at a high temperature to increase the shrinkage rate is undesirable because it can deform the container and heat the contents, making them more susceptible to damage, so a high longitudinal hot water shrinkage rate at 80°C is preferable. A longitudinal hot water shrinkage rate of 38% or more is more preferable, and a rate of 41% or more is even more preferable. On the other hand, if the hot water shrinkage rate in the longitudinal direction at 80°C is greater than 70%, the shrinkage stress also increases, causing a problem of deformation of the packaged container. The hot water shrinkage rate in the longitudinal direction is more preferably 67% or less, and even more preferably 64% or less.

[0014] When the heat-shrinkable polyester film of the present invention is treated in 80°C hot water for 10 seconds under no load, the heat shrinkage rate in the width direction of the film (i.e., the hot water heat shrinkage rate at 80°C) calculated from the lengths before and after shrinkage using the above formula 1 is -8% or more and 7% or less. A hot water heat shrinkage rate of less than -8% in the width direction at 80°C is acceptable, but -8% was set as the lower limit in the present invention. Furthermore, if it is higher than 7%, when used as a side shrink label or band label, the rate of change in the non-shrink direction after shrinkage becomes large, which impairs the appearance of the label after shrinkage, and is therefore undesirable. The hot water heat shrinkage rate in the width direction at 70°C is more preferably 6% or less, and even more preferably 4% or less.

[0015] When the heat-shrinkable polyester film of the present invention is fixed only in the longitudinal direction in hot air at 90°C and treated for 10 seconds, the change rate in the width direction of the film calculated by the following formula 2 is 5% or more and 22% or less. Change rate in the width direction of the film = {(length before heat treatment - length after heat treatment) / length before heat treatment} × 100 (%) Formula 2 If the deformation rate calculated by the above formula 2 is higher than 22%, when used as a side shrink label or a band label, the deformation rate in the non-shrink direction after shrinkage becomes large, which is undesirable because it impairs the appearance of the label after shrinkage. The deformation rate in the width direction is more preferably 20% or less, and even more preferably 18% or less. While a lower limit of 0% is preferable, this was not possible in the present invention, so the lower limit was set to 5%.

[0016] When the heat-shrinkable polyester film of the present invention is fixed only in the longitudinal direction with 10% slack in the longitudinal direction and treated in hot air at 90°C for 10 seconds (i.e., when shrunk by 10% in the longitudinal direction), the change rate in the width direction of the film calculated by the above formula 2 is 5% or more and 20% or less. If the deformation rate calculated by the above formula 2 is higher than 20%, when used as a side shrink label or a band label, the deformation rate in the non-shrink direction after shrinkage becomes large, which is undesirable because it impairs the appearance of the label after shrinkage. The deformation rate in the width direction is more preferably 20% or less, and even more preferably 18% or less. While a lower limit of 0% is preferable, this was not possible in the present invention, so the lower limit was set to 5%.

[0017] The heat-shrinkable polyester film of the present invention preferably has a stress at 10% elongation (so-called F10) measured with hot air at a temperature of 90°C of 1 MPa or more and 5 MPa or less in the longitudinal direction. If the F10 in the longitudinal direction is higher than 5 MPa, the elastic modulus in the longitudinal direction is high during shrinkage, causing a phenomenon in which the film is pulled in the width direction due to necking, resulting in large width-direction deformation during shrinkage, which is undesirable. The F10 in the longitudinal direction is more preferably 4.5 MPa or less, and even more preferably 4 MPa or less. The smaller the F10 in the longitudinal direction at a temperature of 90°C, the better, but if it is less than 1 MPa, the heat shrinkage rate will be insufficient and the film will be unsuitable as a heat-shrinkable film, so the lower limit in the present invention is 1 MPa. The stress at 10% elongation (so-called F10) measured with hot air at a temperature of 90°C is preferably 0.5 MPa or more and 3 MPa or less in the width direction. If the F10 in the width direction is lower than 0.5 MPa, the elastic modulus during shrinkage is low, causing a phenomenon in which the film is pulled in the longitudinal direction, resulting in large deformation in the width direction during shrinkage, which is undesirable. The F10 in the width direction is more preferably 1 MPa or more, and even more preferably 1.5 MPa or more. It is preferable if the F10 in the width direction at a temperature of 90°C is equal to the F10 in the longitudinal direction, but in the present invention, it could not be made higher than 3 MPa, so 3 MPa was set as the upper limit.

[0018] The heat-shrinkable polyester film of the present invention preferably has a maximum shrinkage stress in the film's longitudinal direction measured under hot air at 90°C of 2 MPa or more and 10 MPa or less. If the shrinkage stress under hot air at 90°C is less than 2 MPa, the film will not be tightly fitted as a strip label after shrinkage, and its original purpose of fastening a container and a lid will not be achieved, which is problematic. The shrinkage stress in the film's longitudinal direction measured under hot air at 90°C is more preferably 2.2 MPa or more, and even more preferably 2.4 MPa or more. If the shrinkage stress exceeds 10 MPa, the force will cause deformation of the container, which will not only be undesirable in appearance but will also cause problems such as spillage of the contents or contamination with foreign matter. The shrinkage stress in the film's longitudinal direction measured under hot air at 90°C is more preferably 9.8 MPa or less, and even more preferably 9.6 MPa or less.

[0019] The heat-shrinkable polyester film of the present invention preferably has a maximum shrinkage stress in the longitudinal direction of the film measured in a state in which the film is slackened by 10% in the longitudinal direction under hot air at 90°C (i.e., measured after shrinking by 10% in the longitudinal direction) of 1 MPa or more and 6 MPa or less. If the shrinkage stress in a 10% slack state under hot air at 90°C is less than 1 MPa, the film will not be tightly finished as a strip label after shrink finishing, and will not be able to fulfill its original purpose of fastening a container and a lid, which is a problem. Furthermore, products containing cooked noodles may be reheated together with the label strip in a microwave oven, etc., and the reheated label strip will shrink again, generating shrinkage stress. The shrinkage stress in the film's longitudinal direction, measured under 10% slack in the longitudinal direction under 90°C hot air, is preferably 5.8 MPa or less, and even more preferably 5.6 MPa or less.

[0020] The thickness of the heat-shrinkable polyester film of the present invention is preferably 6 to 30 μm for use as a heat-shrinkable film for labels and banding applications including strip labels. It is more preferably 8 to 28 μm, and particularly preferably 10 to 26 μm. If the thickness is too large, the absolute value of the shrinkage stress increases, which may cause deformation of the container during banding applications.

[0021] [Polyesters used in the heat-shrinkable polyester film of the present invention] The polyester used in the heat-shrinkable polyester film of the present invention preferably contains terephthalic acid as the main dicarboxylic acid component. "Containing terephthalic acid as the main dicarboxylic acid component" means that terephthalic acid accounts for 50 mol % or more of 100 mol % of the dicarboxylic acid components constituting the polyester. The terephthalic acid content is more preferably 60 mol % or more, and even more preferably 70 mol % or more.

[0022] Examples of dicarboxylic acid components other than terephthalic acid contained in the polyester constituting the heat-shrinkable polyester 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.

[0023] When an aliphatic dicarboxylic acid (for example, 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).

[0024] It is also preferable that the polyester does not contain trivalent or higher polycarboxylic acids (e.g., trimellitic acid, pyromellitic acid, and anhydrides thereof), since heat-shrinkable polyester films obtained using polyesters containing these polycarboxylic acids have difficulty achieving the required high shrinkage ratio.

[0025] Examples of polyhydric alcohol components constituting the polyester used in the heat-shrinkable polyester film of the present invention include aliphatic diols such as ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol, and hexanediol, alicyclic diols such as 1,4-cyclohexanedimethanol, and aromatic diols such as bisphenol A. Of the above, it is preferred that ethylene glycol be contained in the largest amount as the polyhydric alcohol component. The content of ethylene glycol is preferably 40 mol% or more, more preferably 45 mol% or more, based on 100 mol% of the polyhydric alcohol component.

[0026] The polyester used in the heat-shrinkable polyester film of the present invention preferably contains 1,4-butanediol. The inclusion of 1,4-butanediol can lower the glass transition temperature (Tg) of the film, which in turn reduces the stretching stress during stretching, resulting in a lower shrinkage stress of the film. The lower glass transition temperature also increases the peel strength after heat sealing. This is thought to be due to the increased mobility of molecular chains due to the lower glass transition temperature, which enhances the entanglement of molecular chains under the heat and pressure during heat sealing. The content of the 1,4-butanediol component is preferably 8 mol% to 40 mol% of the polyhydric alcohol component (100 mol%). A 1,4-butanediol component content of less than 8 mol% is undesirable because low shrinkage stress and high heat sealability cannot be achieved. A content of more than 40 mol% is undesirable because the stretching stress is too low, resulting in poor thickness accuracy of the stretched film. The content of 1,4-butanediol is preferably 10 mol % or more and 38 mol % or less, more preferably 12 mol % or more and 36 mol % or less, and particularly preferably 14 mol % or more and 34 mol % or less. As described above, by containing 8 mol % or more and 40 mol % or less of 1,4-butanediol, the heat seal strength is increased and the shrinkage stress is reduced, which is preferable because tearing of the heat seal portion due to shrinkage when the film is actually made into a label and shrunk is less likely to occur.

[0027] Furthermore, in order to provide high shrinkage properties, it is preferable that the polyester contains at least 10 mol% of one or more monomer components capable of forming amorphous components out of 100 mol% of the polyhydric alcohol components or 100 mol% of the polycarboxylic acid components in the entire polyester resin. If the content is less than 10 mol%, the required shrinkage rate cannot be obtained, resulting in insufficient shrinkage during finishing. The content of the monomer components capable of forming amorphous components is at least 10 mol%, preferably at least 11 mol%, more preferably at least 12 mol%, and particularly preferably at least 13 mol%. The upper limit of the total amount of monomer components capable of forming amorphous components is not particularly limited, but a preferred upper limit is 40 mol%. Here, the interpretation of the above-mentioned term "can be an amorphous component" will be explained in detail.

[0028] In the present invention, the term "amorphous polymer" specifically refers to a polymer that does not exhibit an endothermic peak due to melting when measured using a DSC differential scanning calorimeter. Amorphous polymers are polymers that have not substantially progressed in crystallization and are unable to assume a crystalline state, or even if they do crystallize, the degree of crystallization is extremely low.

[0029] Generally, a polymer in which many monomer units are bonded will be an amorphous polymer if it meets various conditions, such as low stereoregularity, poor polymer symmetry, large polymer side chains, extensive branching, and weak intermolecular cohesive forces between polymers. However, depending on the state of existence, crystallization may progress sufficiently to form a crystalline polymer. For example, even if a polymer has a large side chain, if the polymer is composed of a single monomer unit, crystallization may progress sufficiently to become crystalline. Therefore, even if the same monomer unit is used, the polymer may be crystalline or amorphous. Therefore, in the present invention, the expression "monomer-derived unit that can become an amorphous component" is used.

[0030] In the present invention, the term "monomer unit" refers to a repeating unit constituting a polymer derived from one polyhydric alcohol molecule and one polycarboxylic acid molecule, and in the case of ε-caprolactone, refers to a constituent unit obtained by ring-opening of the lactone ring.

[0031] When a monomer unit consisting of terephthalic acid and ethylene glycol is the main monomer unit constituting the polymer, examples of the monomer-derived unit that can become the amorphous component include a monomer unit consisting of isophthalic acid and ethylene glycol, a monomer unit consisting of terephthalic acid and neopentyl glycol, a monomer unit consisting of terephthalic acid and 1,4-cyclohexanedimethanol, and a monomer unit consisting of isophthalic acid and butanediol.

[0032] Examples of monomers that can become amorphous components include neopentyl glycol, 1,4-cyclohexanedimethanol, isophthalic acid, 1,4-cyclohexanedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,2-diethyl-1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2-isopropyl-1,3-propanediol, 2,2-di-n-butyl-1,3-propanediol, and hexanediol. Among these, neopentyl glycol, 1,4-cyclohexanedimethanol, or isophthalic acid are preferred. ε-caprolactone is also preferred. Neopentyl glycol or 1,4-cyclohexanedimethanol are more preferred, and neopentyl glycol is even more preferred.

[0033] 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.

[0034] 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, but examples of inorganic fine particles include silica, alumina, titanium dioxide, calcium carbonate, kaolin, and barium sulfate, and 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 can be appropriately selected as needed within the range of 0.05 to 3.0 μm (as measured with a Coulter counter).

[0035] 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.

[0036] [Method of producing a polyester film that heat-shrinks in the machine direction (longitudinal direction) and has excellent appearance after shrinkage] The heat-shrinkable polyester film of the present invention is not limited in any way by its production method, but can be obtained, for example, by melt-extruding the above-mentioned polyester raw material using an extruder to form an unstretched film, and then producing the unstretched film by the method described below.

[0037] 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 200 to 300°C using an extruder and extruded into a film. For such extrusion, any existing method such as a T-die method or a tubular method can be used.

[0038] 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.

[0039] Furthermore, the obtained unstretched film can be stretched in the longitudinal direction under predetermined conditions as described below, thereby obtaining the heat-shrinkable polyester film of the present invention.

[0040] Ordinary heat-shrinkable polyester films are produced by stretching an unstretched film in the direction of desired shrinkage. In the present invention, uniaxial stretching is preferably performed in the longitudinal direction, which is the main shrinkage direction. When uniaxially stretching in the longitudinal direction, the unstretched film is introduced into a longitudinal stretching machine having a plurality of rolls arranged in series, and the film is heated to a predetermined temperature on a preheating roll (a low-speed roll). After that, a roll (a high-speed roll) having a speed faster than the preheating roll is provided downstream of the preheating roll, and the film is stretched in the longitudinal direction by the speed difference between the low-speed roll and the high-speed roll. Note that the production method using uniaxial stretching in the longitudinal direction is preferred because it has the advantage of being able to be produced with simple equipment since it does not require transverse stretching equipment.

[0041] The method of longitudinal stretching is not particularly limited, but multi-stage stretching such as two-stage stretching is preferred, as multi-stage stretching disperses stress during stretching and reduces residual stress in the film.

[0042] The stretching ratio at this time is not particularly limited, but is preferably 2 to 6 times. If the stretching ratio is less than 2 times, it is difficult to obtain a high shrinkage rate in terms of material balance, and thickness accuracy deteriorates. Furthermore, if the stretching ratio exceeds 6 times, planar orientation is promoted, not only reducing the heat seal peel strength but also increasing the shrinkage stress, which is undesirable. Furthermore, biaxial stretching is not preferred for the film of the present invention. For example, a method can be considered in which an unstretched film is stretched in the width direction using a tenter and then stretched in the longitudinal direction, which is the main shrinkage direction. However, this method increases the stretching stress during longitudinal stretching, resulting in increased shrinkage stress.

[0043] The method for heating the film before and during stretching is not particularly limited, but in addition to the above-mentioned method of heating on the rolls, the film may also be heated between a low-speed roll and a high-speed roll using an infrared heater or a focused infrared heater.

[0044] Furthermore, if the longitudinal stretching temperature is less than Tg+5°C, breakage is likely to occur during stretching, which is undesirable. Furthermore, if the temperature is higher than Tg+40°C, thermal crystallization of the film progresses, which is undesirable, resulting in a decrease in the shrinkage rate. The temperature is more preferably Tg+8°C or higher and Tg+37°C or lower, and even more preferably Tg+11°C or higher and Tg+34°C or lower.

[0045] In addition, in the longitudinal direction or after the longitudinal stretching, it is preferable to perform longitudinal relaxation and heat treatment in order to adjust the longitudinal shrinkage rate so that it does not become too high and to reduce the shrinkage stress. The method of longitudinal relaxation is not particularly limited, but an example is a method in which the roll after the longitudinal stretching step is heated to heat the film. Another example is to introduce the longitudinally stretched film into a tenter device that can heat the film by holding both ends of the film with clips, and then perform a heat treatment. In the present invention, it is preferable to heat-treat the film after longitudinal stretching with the rolls after stretching at a temperature higher than that of the preheating rolls, and to relax the film in the longitudinal direction using the speed difference between the rolls. The rolls used for relaxation are preferably used at a location where the distance between the rolls is increased from 500 mm to 1500 mm. Furthermore, during relaxation, it is preferable to heat the film by applying hot air heated to 90 to 120°C. The relaxed film is cooled with a cooling roll having a surface temperature of 30°C. When relaxing, if the distance between the rolls is less than 500 mm, the deformation speed during relaxation becomes too fast, which is undesirable as wrinkles and the like occur due to insufficient relaxation. It is preferably 550 mm or more, and more preferably 600 mm or more. Furthermore, if the distance between the rolls is longer than 1500 mm, shrinkage of the film in the width direction (so-called neck-in) becomes large, which is undesirable as it deteriorates the difference in film properties and thickness accuracy in the width direction. It is preferably 1450 mm or less, and more preferably 1400 mm or less. Although not particularly limited, the relaxation rate in the longitudinal direction is preferably 2% or more and 20% or less. If the relaxation rate is less than 2%, the relaxation in the longitudinal direction is insufficient, resulting in high shrinkage stress, which is undesirable. It is preferably 4% or more, and more preferably 6% or more. If the relaxation rate is higher than 20%, the relaxation in the longitudinal direction is too great, resulting in insufficient shrinkage rate in the longitudinal direction, which is undesirable. It is preferably 18% or less, and more preferably 16% or less. [Example]

[0046] The present invention will be described in more detail below using examples, but the present invention is not limited to the aspects of these examples and can be modified as appropriate within the scope of the invention.

[0047] The film was evaluated as follows. [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.

[0048] [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.

[0049] [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 thermally. It was then immersed in water at 25°C ±0.5°C for 10 seconds and removed from the water, and the dimensions of the film in the longitudinal and transverse directions were measured, and the thermal shrinkage rate was calculated 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

[0050] [Stress at 10% elongation (F10)] The furnace of a Shimadzu Thermo Static Chamber (model TCH-220) was heated to 90°C. After the temperature stabilized, a sample measuring 200 mm in length in the film measurement direction and 20 mm in width was cut out and attached with a chuck distance of 100 mm to prevent the film from slackening. After heating the film for 20 seconds, it was stretched in the Thermo Static Chamber using an Autograph (model AG-I) in accordance with JIS K7127. The stress at 10% elongation at this time was taken as the stress at 10% elongation at 90°C (F10).

[0051] [Shrinkage stress] A sample measuring 200 mm in length in the main shrinkage direction and 20 mm in width was cut out from the heat-shrinkable film and measured using a heating furnace-equipped strength and elongation tester (Tesilon (registered trademark of Orientec)) manufactured by Toyo Baldwin Co., Ltd. (now Orientec). The heating furnace was preheated to 90°C, and the distance between chucks was set to 100 mm. The air flow to the heating furnace was stopped temporarily, the door of the heating furnace was opened, and the sample was attached to the chuck. The door of the heating furnace was then quickly closed and the air flow was resumed. The contractile stress was measured for 30 seconds or more, and the maximum value during the measurement was taken as the maximum contractile stress (MPa).

[0052] [Contraction stress (10% sag)] A sample measuring 200 mm in length in the main shrinkage direction and 20 mm in width was cut from the heat-shrinkable film and measured using a Toyo Baldwin Co., Ltd. (now Orientec) heating furnace-equipped strength and elongation tester (Tesilon (registered trademark of Orientec)). The heating furnace was preheated to 90°C, and the distance between the chucks was 100 mm. The film length between the chucks was 111.1 mm, which was 11.1 mm longer than the distance between the chucks, making it possible to measure the shrinkage stress at a 10% slack state. The airflow through the heating furnace was stopped temporarily, the furnace door was opened, and the sample was attached to the chuck. The door was then quickly closed and the airflow was resumed. The contractile stress was measured for 30 seconds or more, and the maximum value during the measurement was taken as the maximum contractile stress (MPa).

[0053] [Rate of change in film width direction] A sample measuring 260 mm in the main shrinkage (longitudinal) direction and 150 mm in the direction perpendicular to the main shrinkage direction (width direction) was cut out from the heat-shrinkable film and attached to a frame measuring 200 mm in length and width so that the film was in a tensioned state (no slack) in the longitudinal direction. The non-shrinkable direction was attached to the center of the frame so that there was a gap of 25 mm on both sides of the frame. The frame with the attached film was placed through a small window into a hot air oven (ESPEC, Model PHH-102) heated to 90°C, heated for 10 seconds, and then removed. The length of the narrowest point in the width direction of the film attached to the frame was measured and calculated according to Equation 2. Change rate in film width direction = {(length before heat treatment - length after heat treatment) / length before heat treatment} × 100 (%) Formula 2

[0054] [Change rate in film width direction (10% slack)] A sample measuring 290 mm in the main shrinkage (longitudinal) direction and 150 mm in the direction perpendicular to the main shrinkage (width direction) was cut from the heat-shrinkable film and attached to a 200 mm frame so that the film length within the frame in the longitudinal direction was 222 mm (with a slack of 22 mm). The non-shrinkage direction was attached to the center of the frame, leaving a 25 mm gap on each side of the frame. The attached frame was placed through a small window in a hot air oven (ESPEC Corporation, Model PHH-102) heated to 90°C, heated for 10 seconds, and then removed. At this time, the film attached to the frame had shrunk longitudinally and was in a state of no slack. The length of the film attached to the frame at its narrowest point in the width direction was measured and calculated according to Equation 2.

[0055] [Shrinkage finish] Two 100mm-wide films were placed on the top and bottom of a plastic noodle container (220mm long x 150mm short x 50mm high) available at convenience stores to secure the body and lid of the container. The film edges were then heat-sealed at 130°C. This resulted in a circular film that was then placed over the noodle container. The film's shrinkage direction (longitudinal direction) was aligned with the circumferential direction of the circular film, and the circular film was positioned to secure the long sides of the noodle container together, with the midpoint of the circular film's width aligned with the midpoint of the long sides of the container. The circular film was allowed to sag 10% relative to the container. The noodle container and circular film were then heated and shrunk using hot air at 100°C in a hot-air shrink tunnel (TORNAD 2500, manufactured by Japan Technology Solutions) and evaluated for shrinkage quality. The residence time in the oven was 8 seconds. The shrinkage finish in the oven was evaluated based on three points: the deformation rate of the label width, the deformation of the container, and the breakage of the heat-sealed portion.

[0056] (Label width variation) The rate of change in label width was calculated using the above formula 2. The results were judged according to the following criteria. ○: 0% ≦ rate of change ≦ 20% △: 20% < change rate ≦ 25% ×: 25% < change rate

[0057] (Container deformation) The deformation of the container was determined as the change in the distance A from the midpoint of one long side of the container to the midpoint of the other long side before and after contraction, which was defined as the deformation amount R (Equation 3 below). Deformation R = A (before contraction) - A' (after contraction) Equation 3 The larger the amount of change, the greater the container deformation was judged to be, and the criteria were as follows: ○: 0mm≦ R < 3mm △: 3mm≦ R < 5mm : 5mm≦ R

[0058] (Heat seal tear) The labels after heat shrinkage were visually inspected to check for tears in the heat-sealed areas. The criteria were as follows: 〇: No tears ×: There is some tearing

[0059] <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 0.69 dL / g. The composition is shown in Table 1. [Synthesis Examples 2 to 4] Polyesters 2 to 5 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 10,000 ppm relative to the polyester. In the table, TPA stands for terephthalic acid, EG stands for ethylene glycol, NPG stands for neopentyl glycol, and BD stands for 1,4-butanediol. The intrinsic viscosity of each polyester was 0.69 dL / g. Each polyester was appropriately cut into chips. The composition of each polyester is shown in Table 1. [Synthesis Example 5] Polyester 5 shown in Table 1 was obtained. During the production of Polyester 5, it contained 500 ppm of uniform, fine precipitated particles containing calcium, lithium, and phosphorus elements with a particle size of approximately 0.5 to 1 μm, had an intrinsic viscosity of 0.67 dl / g, and a glass transition temperature of 63°C. The polyester was suitably cut into chips. The composition of each polyester is shown in Table 1.

[0060] [Table 1]

[0061] [Example 1] The above-mentioned polyester 1, polyester 2, polyester 3, and polyester 4 were mixed in a mass ratio of 20:5:57:18 and charged into an extruder. The mixed resin was then melted at 270°C, extruded through a T-die, and quenched by being wound around a rotating metal roll cooled to a surface temperature of 25°C at a speed of 20 m / min, to obtain an unstretched film with a thickness of 48 μm. The Tg of the unstretched film was 69°C. The unstretched film was introduced into a longitudinal stretching machine with multiple roll groups arranged in series, heated in a preheated roll form to a film temperature of 85°C (Tg + 16°C), and then longitudinally stretched by roll stretching to a stretch ratio of 4.4 times in the longitudinal direction so that the stretched film had a thickness of 11 μm. After longitudinal stretching, the film was heated with a heat treatment roll set to a surface temperature of 90°C (Tg + 21°C), and then relaxed by 10% by varying the speed between the rolls while applying 110°C hot air at a gap of 1000 mm between the rolls. Both edges of the film were then cut and removed, and the film was wound into a roll with a thickness of 12 μm and a width of 900 mm. 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. Labels made from the resulting film had excellent shrink finish.

[0062] [Example 2] The same procedures as in Example 1 were carried out except that the stretching ratio in the longitudinal direction was set to 3.3 times and the production conditions were adjusted so that the thickness of the obtained film would be 16 μm. The production conditions are shown in Table 2 and the evaluation results in Table 3. The labels made from the obtained film had excellent shrink finish.

[0063] [Example 3] The mass ratio of Polyester 1, Polyester 2, Polyester 3, and Polyester 4 was changed from Example 2 to 5:5:65:25. The Tg of the resulting unstretched film was 65°C. The same procedure as in Example 2 was repeated, except that the preheating roll temperature was changed from 85°C to 81°C (Tg + 16°C) and the heat treatment roll temperature was changed from 90°C to 86°C (Tg + 21°C), to obtain a film with a thickness of 16 μm. 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. Labels made from the resulting film had excellent shrink finish.

[0064] [Example 4] The mass ratio of Polyester 1, Polyester 2, Polyester 3, and Polyester 4 was changed from Example 1 to 28:5:57:10. The Tg of the resulting unstretched film was 71°C. The same procedure as in Example 1 was repeated, except that the preheating roll temperature was changed from 85°C to 87°C (Tg + 16°C), the heat treatment roll temperature was changed from 90°C to 92°C (Tg + 21°C), the longitudinal stretching ratio was changed from 4.4 times to 4.2 times, and the longitudinal relaxation rate was changed from 10% to 5%, to obtain a 12 μm-thick 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. Labels made from the resulting film had excellent shrink finish.

[0065] [Example 5] The conditions for longitudinal stretching were changed to two-stage stretching from Example 1. In the longitudinal stretching process, in the first stage, stretching was performed at 1.5 times with a preheat roll temperature of 85°C, and in the second stage, stretching was performed at 2.2 times with a preheat roll temperature of 95°C (Tg + 24°C). The total stretching ratio in the longitudinal direction was 3.3 times. The film stretched 3.3 times in the longitudinal direction was heated with heat treatment rolls set to a surface temperature of 90°C (Tg + 21°C), and then hot air at 90°C was applied at a location with a 1000mm gap between the rolls to obtain a film with a thickness of 30µm. 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. Labels made from the resulting film had excellent shrink finish.

[0066] [Comparative Example 1] The above-mentioned polyester 1, polyester 2, polyester 3, and polyester 4 were mixed in a mass ratio of 40:5:45:10 and charged into an extruder. The mixed resin was then melted at 270°C, extruded through a T-die, and quenched by being wound around a rotating metal roll cooled to a surface temperature of 25°C at a speed of 20 m / min, to obtain an unstretched film with a thickness of 50 μm. The Tg of the unstretched film was 71°C. The unstretched film was introduced into a longitudinal stretching machine equipped with a series of rolls, heated in a preheated roll state to a film temperature of 87°C (Tg + 16°C), and then longitudinally stretched by roll stretching to a stretch ratio of 4.4 times in the longitudinal direction to a thickness of 11 μm after stretching. After longitudinal stretching, the film was heated with heat treatment rolls set to a surface temperature of 92°C (Tg + 21°C), and then relaxed by 5% by varying the speed between the rolls while applying hot air at 100°C at a gap of 1000 mm between the rolls. Both edges of the film were then cut and removed, and the film was wound into a roll with a thickness of 12 μm and a width of 900 mm. 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 in Table 3. Labels made from the resulting film had poor shrink finish.

[0067] Comparative Example 2 The same procedure as in Comparative Example 1 was repeated, except that the thickness of the unstretched film was changed from 50 μm to 55 μm and the longitudinal stretching ratio was changed from 4.4 times to 4.8 times. 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. The labels made from the resulting film had poor shrink finish.

[0068] Comparative Example 3 The same method as in Example 4 was used, except that the thickness of the unstretched film was changed to 58 μm, the longitudinal stretching ratio was changed to 4.8 times, and no heat treatment or relaxation in the longitudinal direction was performed. 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. The labels made from the resulting film had poor shrink finish.

[0069] Comparative Example 4 Polyester raw material 5 was stretched 3.5 times in the longitudinal direction and then heat-treated by contacting it with a heated roll at 80°C for 0.5 seconds to obtain a film with a thickness of 60 μm. The production conditions are shown in Table 2, and the evaluation results are shown in Table 3. The labels made from the obtained film had poor shrinkage finish. In addition, because the film was thick, the shrinkage force increased, causing container deformation.

[0070] Comparative Example 5 Polyester 1, polyester 2, polyester 3, and polyester 4 were mixed in a mass ratio of 25:5:60:10 to prepare a resin mixture for the skin layer. The above-mentioned polyester 1, polyester 2, polyester 3, and polyester 4 were mixed in a mass ratio of 25:5:30:40 to prepare a resin mixture for the core layer. The above resin mixtures for the skin and core layers were co-extruded at 280°C using two twin-screw extruders with a T-die mold equipped with a two-layer multi-manifold and rapidly cooled on a cooling roll to obtain an unstretched film with a skin / core layer thickness of 48 μm. The Tg of the unstretched film was 62°C. The thickness ratio of the skin layer to the core layer was skin layer:core layer = 1:4. The unstretched film was then introduced into a longitudinal stretching machine equipped with multiple roll groups arranged in series. It was heated in a preheated roll state until the film temperature reached 95°C, after which it was longitudinally stretched by a stretching ratio of 4.0 times using a roll stretching method. After longitudinal stretching, it was cooled on a cooling roll set at a surface temperature of 25°C and wound into a roll. It was then co-extruded at a temperature of 25°C and rapidly cooled on a cooling roll to obtain an unstretched film with a skin layer / core layer thickness of 48 μm. The Tg of the unstretched film was 62°C. The co-extrusion was performed so that the thickness ratio of the skin layer to the core layer was skin layer:core layer = 1:4. The unstretched film was then introduced into a longitudinal stretching machine equipped with multiple roll groups arranged in series. It was heated in a preheated roll state until the film temperature reached 95°C, after which it was longitudinally stretched by a stretching ratio of 4.0 times using a roll stretching method to obtain a film thickness of 12 μm. After longitudinal stretching, the film was cooled on a cooling roll set to a surface temperature of 25°C and then wound into a roll. A two-type, two-layer film with a thickness of 12 μm was obtained. The evaluation results are shown in Table 3. Labels made from the resulting film had poor shrink finish.

[0071] Comparative Example 6 Polyester 1, polyester 2, polyester 3, and polyester 4 were mixed in a mass ratio of 25:5:60:10 and charged into an extruder. The mixed resin was then melted at 280°C, extruded through a T-die, and quenched by being wound around a rotating metal roll cooled to a surface temperature of 30°C, yielding an unstretched film with a thickness of 42 μm. The Tg of the unstretched film was 75°C. The unstretched film was introduced into a longitudinal stretching machine with multiple roll groups arranged in series, heated in a preheated roll state until the film temperature reached 80°C, and then stretched in two stages by roll stretching, with the first stage stretching at 1.5x and the second stage stretching at 3.0x, with the first stage stretching distance being 160mm and the second stage stretching distance being 31mm. After longitudinal stretching, the film was cooled on a cooling roll set at a surface temperature of 25°C, then wound up into a roll, melted, extruded through a T-die, and quenched by winding around a rotating metal roll cooled to a surface temperature of 30°C, yielding an unstretched film with a thickness of 42 μm. The Tg of the unstretched film was 75°C. The unstretched film was introduced into a longitudinal stretching machine with multiple roll groups arranged in series, heated in a preheated roll state until the film temperature reached 80°C, and then longitudinally stretched by one stage by roll stretching, with a longitudinal stretch ratio of 3.5x and a thickness of 12 μm after stretching. The stretching distance was 31 mm. After longitudinal stretching, the film was cooled on a cooling roll set to a surface temperature of 25°C and then wound into a roll. A film with a thickness of 12 μm was obtained. The evaluation results are shown in Table 3. Labels made from the obtained film had poor shrink finish.

[0072] [Table 2]

[0073] [Table 3]

[0074] As a result of the evaluation, the films of Examples 1 to 5 had sufficient shrinkage finish, and did not cause label deformation, container deformation, or heat seal peeling during shrinkage finish, making them high-quality and practically excellent films for use as ribbon labels.

[0075] On the other hand, the films of Comparative Examples 1, 2, and 6 had high shrinkage stress and a high deformation rate in the width direction, resulting in poor results in label width deformation and container deformation.Furthermore, the films of Comparative Examples 3, 4, and 5 had low F10 at 90°C in the film width direction, and a large rate of change in the width direction after 10% shrinkage, resulting in poor results in label width deformation. [Industrial Applicability]

[0076] The heat-shrinkable polyester film having excellent heat-sealing properties of the present invention has the above-mentioned excellent properties and can therefore be suitably used for band labels of lunch boxes, noodle containers, etc.

Claims

1. A uniaxially stretched heat-shrinkable polyester film having a longitudinal (longitudinal) direction as the main shrinkage direction, The polyester constituting the heat-shrinkable polyester film contains 50 mol % or more of terephthalic acid based on 100 mol % of a dicarboxylic acid component, 40 mol % or more of ethylene glycol, and 8 mol % to 40 mol % of 1,4-butanediol based on 100 mol % of a polyhydric alcohol component, and 10 mol % or more of one or more monomer components selected from the group consisting of neopentyl glycol, 1,4-cyclohexanedimethanol, isophthalic acid, 1,4-cyclohexanedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,2-diethyl-1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2-isopropyl-1,3-propanediol, 2,2-di-n-butyl-1,3-propanediol, and hexanediol, A heat-shrinkable polyester film characterized by satisfying the following requirements (1) to (6): (1) The shrinkage rate in the longitudinal direction after immersion in 80°C hot water for 10 seconds is 35% or more and 70% or less. (2) The shrinkage rate in the direction perpendicular to the longitudinal direction (width direction) after immersion in 80°C hot water for 10 seconds is between -8% and 7%. (3) The film is held in a constant length in the longitudinal direction, fixed only in the longitudinal direction, and held in a 90°C hot air atmosphere for 10 seconds. The change in the film width direction is 5% or more and 22% or less. (4) The film is fixed only in the longitudinal direction with a 10% slack in the longitudinal direction, and is held in a 90°C hot air atmosphere for 10 seconds. The change in the film width direction is 5% or more and 20% or less. (5) The maximum heat shrinkage stress in the longitudinal direction measured under hot air at 90°C is 2 MPa or more and 10 MPa or less. (6) The stress at 10% elongation (so-called F10) measured with hot air at a temperature of 90°C is 1 MPa or more and 5 MPa or less in the longitudinal direction, and 0.5 MPa or more and 3 MPa or less in the transverse direction.

2. 2. The heat-shrinkable polyester film according to claim 1, wherein the film thickness is from 6 μm to 30 μm.

3. 3. The heat-shrinkable polyester film according to claim 1, wherein the maximum heat shrinkage stress measured in a state of 10% slack in the longitudinal direction by hot air at 90°C is 1 MPa or more and 6 MPa or less.

4. 4. The heat-shrinkable polyester film according to claim 1, which is used for packaging labels for plastic containers.

5. 5. A package comprising a strip label covered with the heat-shrinkable polyester film according to claim 1, which is attached in a ring shape by heat sealing.

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