Heat-shrinkable polyester-based film for laser printing

Abstract

To provide a heat-shrinkable polyester-based film for laser printing that causes no unevenness, wrinkle, or deformation with respect to laser printing on a shrunken product.SOLUTION: A heat-shrinkable polyester-based film for laser printing includes at least one laser printing layer that undergoes a color change upon irradiation with laser, and satisfies the following conditions (1) to (3): (1) a shrinkage percentage in a main shrinkage direction at 80°C is 30% or more and 80% or less; (2) a decline rate in the shrinkage percentage in the main shrinkage direction at 80°C, after having undergone a 10% shrinkage in the main shrinkage direction in hot air at 100°C, is 11% or more and 30% or less; and (3) a transmittance rate at 355 nm in the ultraviolet-visible spectrum is 65% or more and 90% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a heat-shrinkable polyester film suitable for laser marking and suitable for label applications. [Background technology]

[0002] In recent years, heat-shrinkable films have come to be widely used for label packaging, cap seals, and integrated packaging, which combine the protection of glass bottles or plastic bottles 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 generation of dioxins. Polystyrene films, on the other hand, have poor solvent resistance, require the use of inks with special compositions when printing, and require incineration at high temperatures, resulting in the generation of large amounts of black smoke accompanied by an unpleasant odor when incinerated. For these reasons, polyester heat-shrinkable films, which have high heat resistance, are easy to incinerate, and have excellent solvent resistance, tend to be widely used as shrink labels.

[0003] Laser-markable heat-shrinkable polyester films made of plastic are widely used in distribution goods such as food, pharmaceuticals, and industrial products. Laser-markable heat-shrinkable polyester films not only protect the contents but also display information such as the product name, manufacturing date, and ingredients.

[0004] In recent years, heat-shrinkable polyester films for laser marking have been disclosed that can display information by laser marking. Laser marking allows for efficient fine marking. It has been disclosed that laser-markable films can be obtained by incorporating a laser-sensitive color former into the substrate or by applying a coating layer containing a laser-sensitive color former to the substrate film (Patent Documents 1, 2, and 3).

[0005] However, when attempting to apply laser printing to shrinkable films, there is a problem in that unevenness, wrinkles, distortion, etc. are likely to occur due to shrinkage, making the laser-printed content difficult to read. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6268873 [Patent Document 2] Japanese Patent Publication No. 2021-148978 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-66722 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 having a laser-printable layer that is free from unevenness, wrinkles, and distortion in laser printing on the product after shrinkage. [Means for solving the problem]

[0008] The present invention comprises the following configurations. [1] A heat-shrinkable polyester film for laser marking, which contains at least one laser-markable layer that changes color upon laser irradiation and satisfies the following (1) to (3): (1) The 80°C shrinkage rate in the main shrinkage direction is 30% or more and 80% or less. (2) After shrinking 10% in the main shrinkage direction with 100°C hot air, the reduction in shrinkage rate in the main shrinkage direction at 80°C is 11% or more and 30% or less. (3) The transmittance at 355 nm in the ultraviolet-visible spectrum is 65% or more and 90% or less. [2] The laser-markable heat-shrinkable polyester film according to item 1, characterized in that the maximum shrinkage stress measured in 90°C hot air in the main shrinkage direction is 6 MPa or more and 20 MPa or less. [3] The laser-markable heat-shrinkable polyester film according to 1 or 2, characterized in that it has a thickness of 20 μm or more and 80 μm or less. [4] The laser-markable heat-shrinkable polyester film according to any one of 1 to 3, wherein the laser-markable layer, which changes color upon laser irradiation, contains one or more metals or metal compounds that can be printed upon laser irradiation, selected from the group consisting of bismuth, gadolinium, neodymium, titanium, antimony, tin, aluminum, calcium, and barium. [5] The laser-markable heat-shrinkable polyester film according to any one of 1 to 4, wherein the laser-markable layer that changes color upon laser irradiation contains a polyester containing 50 mol % or more of ethylene terephthalate units. [6] The laser-markable heat-shrinkable polyester film according to any one of 1 to 5, characterized in that the laser-markable heat-shrinkable polyester film contains, as a polyester-constituting component, a polyester containing 5 mol % to 30 mol % of at least one of butanediol, diethylene glycol, tetramethylene glycol, and ε-caprolactone. [7] A display comprising the laser-markable heat-shrinkable polyester film according to any one of 1 to 6, wherein the difference in color L* value between the laser-marked portion and the non-laser-marked portion is 1.0 or more and 10 or less. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a heat-shrinkable polyester film for laser marking that is free from unevenness, wrinkles, and distortion in laser marking of the product after shrinkage. [Brief explanation of the drawings]

[0010] [Figure 1] Schematic diagram of the metal frame used when shrinking film by 10% [Figure 2] A schematic diagram of the film with gauge lines prepared before the film is shrunk by 10% DETAILED DESCRIPTION OF THE INVENTION

[0011] The laser-markable heat-shrinkable polyester film of the present invention will now be described. [1] Composition of heat-shrinkable polyester film for laser marking [1-1] Layer structure and thickness The laser-markable heat-shrinkable polyester film of the present invention must have at least one film layer that can be printed by laser irradiation (hereinafter, sometimes referred to as a laser-printable layer that changes color when irradiated with a laser, or simply as a laser-printable layer). In the present invention, a preferred embodiment is one in which the laser-markable heat-shrinkable polyester film has a laser-printable layer over the entire area in the planar direction. Furthermore, in order to improve design, the laser-markable heat-shrinkable polyester film of the present invention may be provided with a printed layer bearing characters or designs other than the laser-printed characters.

[0012] The thickness of the laser-markable heat-shrinkable polyester film of the present invention is not particularly limited, but is preferably 5 μm or more and 100 μm or less. A laser-markable heat-shrinkable polyester film thinner than 5 μm is undesirable because not only does it reduce the visibility of the laser marking, but it may also reduce the mechanical strength and welding seal strength. On the other hand, a laser-markable heat-shrinkable polyester film with a thickness exceeding 100 μm is economically wasteful for display purposes. The thickness of the laser-markable heat-shrinkable polyester film is more preferably 10 μm or more and 90 μm or less, and even more preferably 20 μm or more and 80 μm or less.

[0013] The thickness of the laser-printable layer constituting the laser-markable heat-shrinkable polyester film of the present invention is preferably 5 μm or more and 100 μm or less. If this thickness is less than 5 μm, the visibility of the laser marking may be reduced even if the concentration of the laser-markable pigment described below is increased. On the other hand, if the thickness of the printable layer exceeds 100 μm, it is economically wasteful for display purposes. The thickness of the laser-printable layer is more preferably 10 μm or more and 90 μm or less, and even more preferably 20 μm or more and 80 μm or less.

[0014] Furthermore, all layers constituting the laser-markable heat-shrinkable polyester film of the present invention may be provided with a layer that has been subjected to corona treatment, coating treatment, flame treatment, or the like in order to improve the printability of the surface.

[0015] [1-2] Laser-printed layer that changes color when exposed to laser light To make the printable layer of the present invention laser-printable, it is necessary to add a laser-printable pigment that changes color upon laser irradiation. The plastics that make up the laser-printable heat-shrinkable polyester film typically have almost no reaction to laser light, making it impossible to print on them by laser irradiation. The laser-printable pigment is excited by the energy of the laser light, carbonizing the surrounding plastic, thereby making printing possible. In addition to the carbonization of the plastic, some types of laser-printable pigments themselves turn black. The carbonization and the color-changing effect of the laser-printable pigment, either alone or in combination, enable printing on the printable layer. From the standpoint of print density, it is preferable to select a laser-printable pigment that both carbonizes the plastic and changes color itself.

[0016] Specific types of laser-markable pigments include any of the elements or oxides of bismuth, gadolinium, neodymium, titanium, antimony, tin, aluminum, calcium, and barium. Among these, titanium oxide, calcium carbonate, bismuth trioxide, antimony trioxide, and barium sulfate are preferred, with titanium oxide, calcium carbonate, and bismuth trioxide being more preferred. Furthermore, the particle size of the laser-markable pigment is preferably 0.1 μm or more and 10 μm or less. If the particle size of the laser-markable pigment is less than 0.1 μm, there is a risk that the color change upon laser irradiation will be insufficient. On the other hand, if the particle size of the laser-markable pigment exceeds 10 μm, there is a risk of accelerated clogging of the filter during the extrusion process when producing a film. The particle size of the laser-markable pigment is more preferably 1 μm or more and 9 μm or less, and even more preferably 2 μm or more and 8 μm or less.

[0017] The amount of laser-marking pigment added to the laser-printable layer is preferably 0.05% by mass or more and 50% by mass or less. An amount of pigment added of less than 0.05% by mass is undesirable because the laser print density will be insufficient. On the other hand, an amount of pigment added of more than 50% by mass results in a relative decrease in the amount (volume) of carbonized plastic, which may also result in insufficient print density. The amount of laser-marking pigment added is more preferably 0.1% by mass or more and 49% by mass or less, even more preferably 0.15% by mass or more and 48% by mass or less, and particularly preferably 0.2% by mass or more and 47% by mass or less. When the laser-markable layer is made up of multiple layers, the amount of laser-marking pigment added to the entire laser-printable layer can be determined by proportionally dividing the thickness ratio of each layer and the amount of laser-marking pigment added.

[0018] The laser-markable pigment can be added at any stage in the production of the resin that will serve as the raw material for the laser-markable layer or the film that will serve as the laser-markable layer. For example, in the resin production stage, a vented kneading extruder can be used to blend a slurry of particles dispersed in a solvent with a plastic raw material, or a kneading extruder can be used to blend dried particles with a plastic resin (masterbatch formation). Among these, the method of using a masterbatch containing the laser-markable pigment as the raw material for the film is preferred.

[0019] The laser-printable heat-shrinkable polyester film of the present invention may contain various additives other than the laser-printable pigment, such as waxes, antioxidants, antistatic agents, nucleating agents, viscosity reducers, heat stabilizers, coloring pigments, color inhibitors, and UV absorbers, if necessary. Furthermore, when the laser-printable layer is the outermost layer, it is preferable to add fine particles as a lubricant to improve slipperiness. Any fine particles can be selected. For example, inorganic fine particles include silica, alumina, kaolin, white lead, titanium white, zeolite, zinc oxide, and lithopone. Organic fine particles include acrylic particles, melamine particles, silicone particles, crosslinked polystyrene particles, carbon black, and iron oxide. 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. The lower limit of the fine particle content is preferably 0.01% by mass, more preferably 0.015% by mass, and even more preferably 0.02% by mass. If the content is less than 0.01% by mass, the slipperiness may decrease. The upper limit is preferably 1% by mass, more preferably 0.2% by mass, and even more preferably 0.1% by mass. If the content exceeds 1% by mass, the surface smoothness may decrease, causing problems such as blurred printability, which is not preferable. The particles can be incorporated into the laser-printable layer at any stage in the production of the plastic raw material, and the same methods as those described above in "1.2.1. Types, amounts, and methods of addition of laser-printable pigments" can be used.

[0020] [1-3] Other layers The laser-markable heat-shrinkable polyester film of the present invention may have a printing layer, a lubricating layer, a surface protective layer, an antistatic layer, etc., in addition to the laser-markable layer described above.

[0021] [1-3-1] Printing layer In addition to the printing by laser irradiation, the laser-markable heat-shrinkable polyester film of the present invention may be provided with letters or patterns to improve the design. Known materials for forming these letters and patterns, such as inks for gravure printing and flexographic printing, can be used. The number of printed layers may be one or more. To improve the design by printing in multiple colors, it is preferable to have a printed layer consisting of multiple layers. The printed layer may be located either as the outermost layer or as an intermediate layer.

[0022] [1-3-2] Easy slip layer The laser-markable heat-shrinkable polyester film of the present invention is intended for purposes other than laser marking, such as optimizing the film's slipperiness. The slippery layer is a surface layer, and may be formed on one or both sides. It is preferable to add fine particles as a lubricant to the slippery layer. Any fine particles can be selected. For example, inorganic fine particles include silica, alumina, kaolin, white lead, titanium white, zeolite, zinc oxide, and lithopone. Organic fine particles include acrylic particles, melamine particles, silicone particles, cross-linked polystyrene particles, carbon black, and iron oxide. 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. The lower limit of the fine particle content is preferably 0.01% by mass, more preferably 0.015% by mass, and even more preferably 0.02% by mass. A content of less than 0.01% by mass may result in reduced slipperiness. The upper limit is preferably 1% by mass, more preferably 0.2% by mass, and even more preferably 0.1% by mass. If the amount exceeds 1% by mass, the surface smoothness may decrease, causing problems such as blurred printing, which is not preferable.

[0023] [2] Characteristics of heat-shrinkable polyester film for laser marking Next, the properties of the laser-markable heat-shrinkable polyester film of the present invention will be described. [2-1] Shrinkage rate in the main shrinkage direction at 80°C and 90°C The laser-markable heat-shrinkable polyester film of the present invention is heat-treated in 80°C warm water for 10 seconds under no load, and then immediately immersed in water at 25°C±0.5°C for 10 seconds. The heat shrinkage rate in the width direction (main shrinkage direction) of the film (i.e., the 80°C heat shrinkage rate in the main shrinkage direction) calculated from the lengths before and after shrinkage using the following formula 1 is 30% or more and 80% or less. Heat shrinkage rate = {(length before shrinkage - length after shrinkage) / length before shrinkage} x 100 (%) Formula 1

[0024] If the 80°C heat shrinkage rate in the main shrinkage direction is less than 30%, when the film is used for beverage labels or as a film for lunch box packaging, the shrinkage amount is so small that the label will wrinkle or sag after heat shrinkage, which is undesirable. The 80°C shrinkage rate is more preferably 35% or more, particularly preferably 40% or more, and most preferably 45% or more. There is no problem if the 80°C heat shrinkage rate in the main shrinkage direction is higher than 80%, but if the 80°C shrinkage rate is 80% or less, the film can be produced with good productivity.

[0025] The 90°C heat shrinkage in the main shrinkage direction, measured in the same manner as the 80°C heat shrinkage, is preferably 35% to 80%. If the 90°C heat shrinkage in the main shrinkage direction is less than 35%, the shrinkage amount is small and, when used as a film for beverage labels or lunch box packaging, wrinkles and sagging of the label after heat shrinkage are generated, which is undesirable. The 90°C heat shrinkage in the main shrinkage direction is more preferably 40% or more, and particularly preferably 45% or more. There is no problem if the 90°C heat shrinkage rate in the main shrinkage direction is higher than 80%, but if the 90°C heat shrinkage rate in the main shrinkage direction is 80% or less, the film can be produced efficiently.

[0026] [2-2] Decrease in shrinkage rate at 80℃ in the main shrinkage direction after shrinking 10% in the main shrinkage direction with 100℃ hot air The laser-markable heat-shrinkable polyester film of the present invention exhibits a reduction in shrinkage at 80°C in the main shrinkage direction of 11% to 30% after shrinking 10% in the main shrinkage direction with 100°C hot air. Typically, when a heat-shrinkable film is shrunk 10%, a reduction in the heat shrinkage is observed by the amount of shrinkage, i.e., 10%. In this case, the heat shrinkage (heat shrinkability) of the label remains even after heat shrinking onto the object to be covered. That is, the heat generated when the label is irradiated with a laser increases the heat shrinkage of the corresponding area, making it more susceptible to deformation and perforation. Conventionally, to suppress the shrinkage after 10% shrinkage, it was necessary to reduce the shrinkage before shrinkage. This made the heat-shrinkable film difficult to use for applications requiring a high shrinkage, such as irregular-shaped bottles, and potentially resulted in a heat-shrinkable film that was not versatile. Therefore, the inventors have found that in order to achieve both a high shrinkage rate before shrinkage and a low shrinkage rate after shrinkage (high heat resistance), it is preferable to set the reduction rate of the 80°C heat shrinkage rate in the main shrinkage direction after 10% shrinkage to 11% or more. The conditions necessary to achieve this property will be described later. If the reduction in the 80°C heat shrinkage rate in the main shrinkage direction after 10% shrinkage is less than 11%, this is not preferred because the heat resistance after shrinkage may be insufficient. On the other hand, the higher the reduction in this shrinkage rate, the better, but if it is 30% or less, the film can be produced with good production efficiency. The reduction in the 80°C heat shrinkage rate in the main shrinkage direction after 10% shrinkage is preferably 12% or more and 29% or less, and more preferably 13% or more and 28% or less.

[0027] [2-3] 80℃ heat shrinkage rate in the main shrinkage direction after shrinking 10% in the main shrinkage direction with 100℃ hot air The laser-markable heat-shrinkable polyester film of the present invention preferably has an 80°C heat shrinkage rate in the main shrinkage direction of 20% to 60% after shrinking 10% in the main shrinkage direction with 100°C hot air. This characteristic corresponds to the heat resistance described above in terms of the decrease in 80°C heat shrinkage rate in the main shrinkage direction after 10% shrinkage. If the 80°C heat shrinkage rate in the main shrinkage direction after 10% shrinkage is less than 20%, the 80°C heat shrinkage rate in the main shrinkage direction tends to be less than 30%, which is undesirable. On the other hand, if the 80°C heat shrinkage rate in the main shrinkage direction after 10% shrinkage is more than 60%, the heat resistance after shrinkage tends to be insufficient, which is undesirable. This shrinkage rate is more preferably 21% to 59%, and even more preferably 22% to 58%.

[0028] [2-4] Maximum shrinkage stress measured in 90°C hot air in the main shrinkage direction The laser-markable heat-shrinkable polyester film of the present invention preferably has a maximum shrinkage stress of 6 MPa to 20 MPa as measured in 90°C hot air in the main shrinkage direction. A maximum shrinkage stress of less than 6 MPa is undesirable because it is likely that the decrease in 80°C shrinkage in the main shrinkage direction after 10% shrinkage in 100°C hot air in the main shrinkage direction will be less than 10%. On the other hand, a maximum shrinkage stress of more than 20 MPa may result in excessive force being applied to the object to be covered during shrinkage of the film, potentially causing deformation of the object, or may result in poor finish (so-called sink marks) when the film is shrunk into a label. The maximum shrinkage stress is more preferably 6.5 MPa to 19.5 MPa, and even more preferably 7 MPa to 19 MPa.

[0029] [2-5] Transmittance of 355 nm in the UV-visible spectrum The laser-markable heat-shrinkable polyester film of the present invention has a transmittance of 65% to 90% at 355 nm in the ultraviolet-visible spectrum. The transmittance at 355 nm in the ultraviolet-visible spectrum indicates the ability to absorb ultraviolet (UV) lasers. The lower this transmittance, the more easily the film absorbs laser energy and discolors, and the higher the laser print density tends to be. If the transmittance at 355 nm in the ultraviolet-visible spectrum exceeds 90%, it becomes difficult to achieve a color L* value difference of 1.0 or more in the laser-printed portion. On the other hand, if this transmittance is 65% or less, the haze tends to exceed 40%, which is undesirable. The transmittance at 355 nm in the ultraviolet-visible spectrum is preferably 66% to 89% and more preferably 67% to 88%.

[0030] [2-6] Hayes The haze of the film was measured in accordance with JIS K7136 using a haze meter "500A" (manufactured by Nippon Denshoku Industries Co., Ltd.) The measurement was carried out five times, and the average value was taken as the haze.

[0031] [2-6] Absolute difference in color L* value between the printed and non-printed areas after laser marking The laser-markable heat-shrinkable polyester film of the present invention preferably has an absolute difference in color L* value between the printed and non-printed areas after laser printing (hereinafter, sometimes simply referred to as the "L* value difference") of 1.0 to 10.0. If this difference is less than 1.0, the color tones of the printed and non-printed areas will be similar, making the print difficult to see. On the other hand, if the L* value difference exceeds 10.0, the print will be more visible, but the laser irradiation power will need to be increased accordingly, which is undesirable because it will cause greater damage to the laser-markable heat-shrinkable polyester film and make it more susceptible to problems such as perforation and deformation. The L* value difference is more preferably 1.5 to 9.5, and even more preferably 2.0 to 9.0.

[0032] [3] Manufacturing method for heat-shrinkable polyester film for laser marking [3-1] Polyester raw material for heat-shrinkable polyester film for laser printing The polyester used in the laser-markable heat-shrinkable polyester film of the present invention has ethylene terephthalate units, which preferably account for 40 mol % or more, more preferably 50 mol % or more, and even more preferably 55 mol % or more of 100 mol % of the constituent units of the polyester. The constituent units refer to monomer units of polyhydric alcohol and polycarboxylic acid that constitute the copolymer.

[0033] Furthermore, the laser-markable heat-shrinkable polyester film of the present invention preferably contains at least one of butanediol, diethylene glycol, and ε-caprolactone in an amount of 5 mol% to 30 mol% of the total polyester resin components (100 mol%). The inventors have discovered that by including 5 mol% or more of these components and performing the width direction relaxation described below, the reduction in shrinkage at 80°C in the main shrinkage direction after shrinking 10% in the main shrinkage direction with 100°C hot air is likely to be 10% or more. Although the detailed mechanism is unclear, it is thought that this is because butanediol, diethylene glycol, and ε-caprolactone have longer molecular chains than ethylene glycol and ester units composed of monomers that can become amorphous components described below. In other words, when relaxing in the width direction (the main shrinkage direction) during the film formation process, the presence of these long-chain components can presumably cause components with relatively short molecular chain lengths, such as ethylene glycol, to relax preferentially. As a result, it is thought that while the heat shrinkage rate of the finished film (before shrinkage) in the main shrinkage direction at 80°C is maintained high (30% or more), it will be easier to make the reduction in the 80°C shrinkage rate in the main shrinkage direction after shrinking it by 10% in the main shrinkage direction with 100°C hot air 100°C easier. If the content of at least one component selected from butanediol, diethylene glycol, and ε-caprolactone is less than 5 mol%, the relaxation of short-chain components such as ethylene glycol will be relatively large, and the 80°C heat shrinkage in the main shrinkage direction (before 10% shrinkage in the main shrinkage direction with 100°C hot air) may be less than 30%. If the content of the long-chain components exceeds 30 mol%, the amount of ethylene glycol and other short-chain components that can become amorphous components will be relatively small, and it may be difficult to achieve a 10% or greater reduction in the 80°C shrinkage in the main shrinkage direction after 10% shrinkage in the main shrinkage direction with 100°C hot air. The content of these long-chain components is more preferably 6 mol% to 29 mol%, and even more preferably 7 mol% to 28 mol%.

[0034] The polyester raw material used in the laser-markable heat-shrinkable polyester film of the present invention preferably contains 18 mol% or more of units (total amount) derived from one or more monomers that can become amorphous components other than the above-mentioned units derived from butanediol, diethylene glycol, and ε-caprolactone, based on 100 mol% of the total polyester resin components. If the amorphous component content is less than 18 mol%, the heat shrinkage properties will be poor. The monomers that can become amorphous components are preferably 20 mol% or more and 25 mol% or less, based on 100 mol% of the polyhydric alcohol components or 100 mol% of the polycarboxylic acid components in the total polyester resin.

[0035] Specific examples of monomers that can become amorphous components include neopentyl glycol, 1,4-cyclohexanedimethanol, isophthalic acid, 1,4-cyclohexanedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,3-propanediol, 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, and isophthalic acid are preferred.

[0036] When the monomer that can become the amorphous component is isophthalic acid, and terephthalic acid and isophthalic acid are used in combination as the dicarboxylic acid component, and ethylene glycol, butanediol, and ε-caprolactone are used in combination as the diol component, the polyester resin that constitutes the film will contain a mixture of constituent units consisting of terephthalic acid and butanediol, constituent units consisting of isophthalic acid and butanediol, constituent units consisting of isophthalic acid and ethylene glycol, etc.

[0037] Here, the constituent unit consisting of isophthalic acid and butanediol is a constituent unit derived from butanediol and also a constituent unit derived from one or more monomers that can become an amorphous component. Therefore, in the present invention, the content of the constituent unit consisting of isophthalic acid and butanediol is counted as both a constituent unit derived from butanediol and a constituent unit derived from one or more monomers that can become an amorphous component. Therefore, the content of the constituent unit derived from butanediol refers to the total content of the constituent unit consisting of isophthalic acid and butanediol and the content of the constituent unit consisting of terephthalic acid and butanediol. The content of the constituent unit derived from one or more monomers that can become an amorphous component refers to the total content of all constituent units derived from one or more monomers that can become an amorphous component, including the content of the constituent unit consisting of isophthalic acid and butanediol and the content of the constituent unit consisting of isophthalic acid and ethylene glycol. The same applies to the relationship between the content of constituent units derived from ε-caprolactone and the content of constituent units derived from one or more monomers that can become amorphous components.

[0038] Other dicarboxylic acid components constituting the polyester used in the present invention include aromatic dicarboxylic acids such as orthophthalic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid, and decanedicarboxylic acid; and alicyclic dicarboxylic acids.

[0039] When an aliphatic dicarboxylic acid (e.g., adipic acid, sebacic acid, decanedicarboxylic acid, etc.) is contained in the polyester, the content is preferably less than 3 mol % (based on 100 mol % of the dicarboxylic acid component). Laser-markable 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.

[0040] It is also preferable that the polyester does not contain trivalent or higher polycarboxylic acids (e.g., trimellitic acid, pyromellitic acid, and anhydrides thereof), as laser-markable heat-shrinkable polyester films obtained using polyesters containing these polycarboxylic acids will have difficulty achieving the required high shrinkage percentage.

[0041] Other polyhydric alcohol components constituting the polyester used in the present invention include aromatic diols such as bisphenol A.

[0042] The polyester used in the present invention is preferably a polyester whose glass transition temperature (Tg) is adjusted to 50 to 80° C. by appropriately selecting the amounts of butanediol, diethylene glycol, and ε-caprolactone, and the amount of monomers that can become amorphous components. Tg is more preferably 52° C. or higher and 78° C. or lower.

[0043] The polyester used in the present invention preferably does not contain diols having 8 or more carbon atoms (e.g., octanediol, etc.) or trihydric or higher polyhydric alcohols (e.g., trimethylolpropane, trimethylolethane, glycerin, diglycerin, etc.). Laser-markable heat-shrinkable polyester films obtained using polyesters containing these diols or polyhydric alcohols are unlikely to achieve the required high shrinkage rate. It is also preferable that the polyester not contain triethylene glycol or polyethylene glycol as much as possible.

[0044] The most preferred polyester is a polyester in which, out of 100 mol% of all polyester constituent units, it contains 1 to 25 mol% butylene terephthalate units, 1 to 25 mol% units composed of ε-caprolactone and terephthalic acid, a total of 2 to 50 mol%, 18 to 25 mol% units composed of terephthalic acid and a monomer that can become an amorphous component, and the remainder is ethylene terephthalate units. Note that amorphous units in which part of the terephthalic acid is replaced with isophthalic acid may also be included.

[0045] 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 laser-markable heat-shrinkable polyester film of the present invention, as needed.

[0046] It is preferable to add fine particles as a lubricant to the resin forming the laser-markable heat-shrinkable polyester film of the present invention, which improves 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 is preferably in the range of 0.05 to 3.0 μm. The average particle size of the fine particles is measured using a Coulter counter.

[0047] The particles can be incorporated into the resin that forms the laser-markable heat-shrinkable polyester film of the present invention, for example, by adding them at any stage in the production of the polyester resin, but it is preferred to add them as a slurry dispersed in ethylene glycol or the like at the esterification stage or after the completion of the transesterification reaction and before the start of the polycondensation reaction, and then proceed with the polycondensation reaction. Alternatively, it is also preferred to blend a slurry of the particles dispersed in ethylene glycol or water or the like with the polyester resin raw material using a vented kneading extruder, or to blend dried particles with the polyester resin raw material using a kneading extruder.

[0048] [3-2] Method for producing heat-shrinkable polyester film for laser marking The laser-markable 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 by the following predetermined method. In the present invention, the general shrink film manufacturing method, transverse uniaxial stretching or sequential biaxial stretching in which longitudinal stretching is performed after transverse stretching, will be described as an example, but these may also be longitudinal uniaxial stretching or sequential biaxial stretching in which longitudinal stretching is performed after transverse stretching, or simultaneous biaxial stretching in which stretching is performed simultaneously in the longitudinal and transverse directions.

[0049] [3-2-1] Extrusion conditions, unstretched film production conditions 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 extrusion, any existing method such as a T-die method or a tubular method can be used.

[0050] 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.

[0051] [3-2-2] Control of longitudinal stretching conditions In the production of the laser-markable heat-shrinkable polyester film of the present invention, if the longitudinal direction is not the main shrinkage direction, the stretching ratio in the longitudinal direction is preferably 2 times or less so as not to inhibit the relaxation of short-chain components as described above. The lower limit of the longitudinal stretching ratio is 1 time (unstretched). A longitudinal stretching ratio of more than 2 times is undesirable because it is thought that molecular chains will be oriented not only in the transverse direction, which is the main shrinkage direction, but also in the longitudinal direction, which inhibits the movement of molecular chains that attempt to relax in the transverse direction. The upper limit of the longitudinal stretching ratio is more preferably 1.8 times or less, and even more preferably 1.6 times or less. The lower limit of 1 time is particularly preferable. When longitudinal stretching is the main shrinkage direction, the stretching ratio in the longitudinal direction is preferably 2 to 5 times. If the stretching ratio in the longitudinal direction is less than 2 times, it becomes difficult to achieve an 80°C heat shrinkage rate of 30% or more and a 90°C heat shrinkage rate of 35% or more in the main shrinkage direction. On the other hand, if the stretching ratio exceeds 5 times, the stretching stress becomes too high, making the film prone to breakage, which is not preferable. The stretching ratio in the longitudinal direction is more preferably 2.5 to 4.5 times, and even more preferably 3 to 4 times. When stretching in the machine direction, it is preferable to preheat a substantially unoriented film to a temperature of Tg or higher and Tg+30° C. In addition, the machine direction stretching may be either a one-stage stretching or a multi-stage stretching method (two or more stages). When longitudinal stretching is the main shrinkage direction, relaxation in the longitudinal direction is required after stretching in the longitudinal direction. Detailed conditions are explained in [3-2-4] Heat treatment and relaxation conditions after stretching in the shrinkage direction below.

[0052] [3-2-3] Control of transverse stretching conditions In the present invention, transverse stretching is performed in a tenter with both ends in the width direction held by clips at a temperature of Tg+10°C or higher and Tg+30°C or lower, at a stretching ratio of 3 to 6 times. By performing transverse stretching under such predetermined conditions, a good film can be obtained. The transverse stretching temperature is more preferably Tg+12°C or higher, even more preferably Tg+14°C or higher, more preferably Tg+28°C or lower, and even more preferably Tg+26°C or lower. Meanwhile, the transverse stretching ratio is more preferably 3.5 times or higher, even more preferably 3.7 times or higher, more preferably 5.5 times or lower, and even more preferably 5 times or lower.

[0053] When stretching in the transverse direction, if the stretching temperature exceeds Tg+30°C, the thickness precision of the film tends to be low. However, by controlling the stretching temperature to Tg+30°C or less, the thickness precision of the film can be increased, which is preferable.

[0054] On the other hand, if the stretching temperature is lower than Tg+10° C., the orientation in the width direction becomes too large, and the film is likely to break during transverse stretching, which is undesirable.

[0055] [3-2-4] Heat treatment and relaxation conditions after stretching in the shrinkage direction After being stretched in the main shrinkage direction, the film must be relaxed in the corresponding direction. In the following description, the transverse direction is assumed to be the main shrinkage direction. After transverse stretching, the film must be relaxed in the transverse direction, which is an operation of narrowing the widthwise gap between clips while holding both widthwise edges with clips in a tenter, at a temperature equal to or lower than the transverse stretching temperature. As described above in the explanation of polyester raw materials, by containing 5 mol% or more of long-chain components such as butanediol, diethylene glycol, and ε-caprolactone and relaxing in the transverse direction, it becomes easier to achieve a reduction in the 80°C shrinkage rate in the main shrinkage direction of 10% or more after shrinking 10% in the main shrinkage direction with 100°C hot air. It has been conventionally believed that heat treatment is meaningless when the temperature at which the film is relaxed in the width direction is lower than the transverse stretching temperature. However, the present invention has discovered a novel finding that, when a molecular structure is formed in which the short-chain components are preferentially relaxed by mixing long-chain and short-chain components, the molecular chain relaxation effect can be achieved even if the relaxation temperature is lower than the transverse stretching temperature. On the other hand, when the relaxation temperature is higher than the transverse stretching temperature, it becomes difficult to achieve a heat shrinkage rate (before shrinkage) in the main shrinkage direction of 30% or more at 80°C. The relaxation temperature is preferably −10°C or higher and 0°C or lower, more preferably −9°C or higher and −1°C or lower, relative to the transverse stretching temperature. The relaxation rate in the width direction is preferably 3% to 20%. If the relaxation rate is less than 3%, the short-chain components will not be sufficiently relaxed, and it may be difficult to achieve a reduction in the 80°C shrinkage rate in the main shrinkage direction of 10% or more after shrinking 10% in the main shrinkage direction with 100°C hot air. On the other hand, if the relaxation rate exceeds 20%, it will be difficult to achieve a heat shrinkage rate in the main shrinkage direction at 80°C (before shrinkage) of 30% or more. The relaxation rate in the width direction is preferably 4% to 19%, and more preferably 5% to 18%. The residence time in the relaxation step is preferably 1 second or more and 9 seconds or less. If the relaxation time is less than 1 second, the short-chain components will not be sufficiently relaxed, making it difficult to achieve a 10% reduction in the 80°C shrinkage rate in the main shrinkage direction after shrinking 10% in the main shrinkage direction with 100°C hot air. On the other hand, if the relaxation time exceeds 9 seconds, the equipment will become large, which is not preferable. The relaxation time is more preferably 1.5 seconds or more and 8.5 seconds or less, and even more preferably 2 seconds or more and 8 seconds or less.

[0056] [4] Laser printing conditions Examples of laser types (wavelengths) that can be used for laser printing on the laser-markable heat-shrinkable polyester film of the present invention include CO2 lasers (10,600 nm), YAG lasers (1064 nm), YVO4 lasers (1064 nm), fiber lasers (1064 and 1090 nm), green lasers (532 nm), and UV lasers (355 nm). While there are no particular limitations on the type of laser used for laser printing in the present invention, CO2 lasers are often used to burn through plastics and are therefore not preferred as laser sources because they are often used for purposes other than the printing intended by the present invention. YAG lasers, YVO4 lasers, fiber lasers, green lasers, and UV lasers are preferred as laser sources, with YAG lasers, fiber lasers, and UV lasers being more preferred. UV lasers are particularly preferred because they cause less thermal damage. Commercially available equipment can be used for laser marking, with representative examples including the Brother Industrial Printing LM-2550 (YAG laser), Omron MX-Z2000H-V1 (fiber laser), Trotec 8028 Trotec Speedy 100 flexx (fiber laser), Keyence MD-X1000 (YVO4 laser), and MD-U1000C (UV laser). Laser marking conditions vary depending on the equipment manufacturer and model, with specifications and configurable conditions differing depending on the film being printed, so it's difficult to generalize, but the Keyence MD-U1000C (UV laser, wavelength 355 nm) is an example, and is as follows:

[0057] The laser power is preferably 20% to 80% of the maximum 13W specified by the device specifications. An output of less than 20% is undesirable because it reduces print density and reduces visibility. An output of 80% or more is undesirable because it can cause holes in the display. An output of 25% to 75% is more preferable, and 30% to 70% is even more preferable. The pulse frequency is preferably 10 kHz to 100 kHz. A frequency below 10 kHz is undesirable because the laser energy per irradiation is high and the thickness reduction rate of the printed area tends to exceed 80 vol%. Conversely, a frequency above 100 kHz makes it easier to achieve a thickness reduction rate of 80 vol% or less in the printed area, but it may be difficult to achieve a difference in color L* value of 1 or more in the printed area. A frequency of 15 kHz to 95 kHz is more preferable, and a frequency of 20 kHz to 90 kHz is even more preferable. The scan speed is preferably 10 mm / s to 3000 mm / s. A scan speed below 10 mm / sec is undesirable because the printing speed drops dramatically, slowing down the production speed of the display. On the other hand, a scan speed above 3000 mm / sec is undesirable because it reduces the print density and makes it difficult to achieve a color L* value difference of 1 or more. A scan speed of 100 mm / sec or more and 2900 mm / sec or less is more preferable, and a scan speed of 200 mm / sec or more and 2800 mm / sec or less is even more preferable.

[0058] [5] Display body The display of the present invention is formed by covering at least a portion of the outer periphery of an object to be packaged with the laser-markable heat-shrinkable polyester film of the present invention and then heat-shrinking it. Examples of objects to be packaged include plastic bottles for food storage, polyethylene containers used for shampoos and conditioners, various bottles, cans, plastic containers for confectionery and lunch boxes, and paper boxes. Furthermore, the label obtained from the heat-shrinkable polyester film need not cover the entire container; it may cover only a portion, such as a bottle cap (cap seal), and may be a preform before being completely attached to the container. Typically, when a label obtained from the heat-shrinkable polyester film is heat-shrunk to cover such an object to be packaged, the label is heat-shrunk by approximately 2 to 15% before being attached to the object to be packaged. It is preferable that the difference in color L* value between the laser-printed and non-printed portions after laser printing is 1.0 or more and 10 or less.

[0059] 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]

[0060] Next, the present invention will be specifically explained using examples and comparative examples, but the present invention is not limited to the embodiments of these examples and can be appropriately modified within the scope of the present invention. The film evaluation method and polyester synthesis method are described below.

[0061] <Evaluation method for heat-shrinkable polyester film for laser marking> The evaluation method for laser-markable heat-shrinkable polyester film is as follows: A sample of the non-printed area was cut out at least 1 mm away from the printed area or the heat-sealed area and used as the sample.

[0062] Thickness Using a micrometer (Militron 1254D manufactured by Feinprüf), measurements were taken at five points, and the average value was calculated.

[0063] [Heat shrinkage rate at 80℃ and 90℃] The laser-markable heat-shrinkable 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 heat shrink, then immersed in water at 25°C ±0.5°C for 10 seconds and pulled out of the water, the dimensions of the film in the longitudinal and transverse directions were measured, and the heat shrinkage rate was calculated according to the following formula 1. The direction with the largest heat shrinkage rate was defined as the main shrinkage direction (width direction). Heat shrinkage rate = {(length before shrinkage - length after shrinkage) / length before shrinkage} x 100 (%) Formula 1

[0064] [80℃ shrinkage rate and shrinkage rate reduction rate after 10% shrinkage at 100℃] A heat-shrinkable film for laser printing was attached to a metal frame (Fig. 1) with inner dimensions of 160 mm square and a frame thickness of 20 mm, and then subjected to a 10% shrinkage treatment. Specifically, after cutting the film to 150 mm lengthwise and 200 mm widthwise, marks were made so that the spacing in the widthwise direction was 178 mm (Figure 2), and the film was attached so that the marks were aligned with the inner dimensions of the metal frame (the spacing between the marks on the film was 178 mm, while the spacing between the metal frame was 160 mm, and the relaxation rate was 10%). The metal frame with the film attached was inserted through a small window measuring 25 mm high and 200 mm wide into a hot air oven (Espec STPH-102) with a door that had been opened, causing the film to shrink by 10% in the width direction. A 10 cm x 10 cm square sample was cut from the center of the above-mentioned 10% shrunk film, and immersed in warm water at 80°C ± 0.5°C for 10 seconds without load to allow heat shrinkage, then immersed in water at 25°C ± 0.5°C for 10 seconds, and pulled out of the water, and the dimensions in the vertical and horizontal directions were measured, and the heat shrinkage rate for each was calculated according to the above-mentioned formula 1. The direction with the larger heat shrinkage rate was considered to be the main shrinkage direction.

[0065] [Glass transition temperature (Tg)] The sample film was weighed at 5.0 ± 0.2 mg and placed in a TA Instruments T-zero pan or aluminum pan (flat plate-shaped). The sample was then melted by heating on a hot plate heated to 300 °C for 30 seconds. The sample was then removed from the hot plate with tweezers and immediately immersed in liquid nitrogen for 1 minute to prepare a melt-quenched sample. To ensure good adhesion to the pan bottom, the film was punched into a circular shape (4.5 mm diameter) using a punch. "Good adhesion to the pan bottom" refers to the film sample being placed in the pan without any creases, and the film was firmly pressed down with the lid, leaving no gaps between the overlapping films. While wrinkles are acceptable before punching, it is preferable to smooth out any wrinkles before punching. The shape and size of the punched sample (punch) are not limited to the above; it is sufficient for the sample to fit neatly into the pan bottom. The melt-quenched samples were measured using a temperature-modulated differential scanning calorimeter (DSC) "DSC250" (manufactured by TA Instruments) in MDSC (registered trademark) heat-only mode at an average heating rate of 2°C / min and a modulation period of 40 seconds to obtain reverse heat flow. In the reverse heat flow obtained by the measurement, Tg was determined as a signal that appears as a step change from the baseline. Specifically, extensions of the baselines of each heat flow were drawn on both the lower and higher sides of Tg, and the intersections with the tangents at the inflection point (Tg) were determined. The value on the horizontal axis at this intersection was read, and the average of the lower and higher sides was taken as Tg.

[0066] [Hayes] The haze of the film was measured in accordance with JIS K7136 using a haze meter "500A" (manufactured by Nippon Denshoku Industries Co., Ltd.) The measurement was carried out twice, and the average value was taken as the haze.

[0067] [Shrinkage finish] The edges of the laser-markable heat-shrinkable film were welded with dioxolane to obtain a cylindrical label (a label with the main shrinkage direction of the heat-shrinkable film as the circumferential direction). This label was placed over a commercially available PET bottle (containing the contents; Ito En Co., Ltd.'s "Oi Ocha") and heat-shrunk by passing it through steam adjusted to 85°C (tunnel passage time: 30 seconds). The shrinkage finish of the label was visually evaluated on a 5-point scale according to the following criteria. The defects described below refer to lifting, wrinkles, insufficient shrinkage, folding of the label edge, whitening from shrinkage, etc. 5: Best finish (no defects) 4: Good finish (one defect) 3: There are two flaws 2: 3 to 5 defects 1: Many defects (6 or more)

[0068] [Laser marking evaluation: Film deformation (holes, distortion)] A 1 cm × 1 cm grid was printed on a laser-printable heat-shrinkable film, and the edges were welded with dioxolane to obtain a cylindrical label (a label with the main shrinkage direction of the heat-shrinkable film aligned circumferentially). This label was placed on a commercially available PET bottle (containing the contents; Ito En Co., Ltd.'s "Oi Ocha") and passed through a steam tunnel adjusted to 85°C to cause heat shrinkage (tunnel passage time: 30 seconds). A laser was used to print the letters "ABC123" on a label heat-shrunk onto a bottle, and the label deformation was evaluated visually. A 355 nm ultraviolet (UV) laser marker (MD-U1000, Keyence Corporation) was used as the printer, and the laser was irradiated under the following conditions: laser power 40%, scan speed 1000 mm / sec, pulse frequency 40 kHz, and spot variable -20. Label deformation was evaluated according to the following criteria. Judgment: No holes or distortions in the laser irradiated area Evaluation: Poor: There is a hole or distortion in the laser irradiation area.

[0069] [Laser printing evaluation: Color L* value (printed area, non-printed area)] The edges of a laser-printable heat-shrinkable film (plain) were welded with dioxolane to obtain a cylindrical label (a label with the main shrinkage direction of the heat-shrinkable film aligned circumferentially). This label was placed over a commercially available PET bottle (containing the contents; Ito En Co., Ltd.'s "Oi Ocha") and passed through a steam tunnel adjusted to 85°C to cause thermal shrinkage (passing through the tunnel for 30 seconds). A laser was used to print the letters "ABC123" on a label heat-shrunk onto a bottle, and the print density was evaluated based on color L*. An ultraviolet (UV) laser marker (MD-U1000, Keyence Corporation) with a wavelength of 355 nm was used as the printer, and the laser was irradiated under the following conditions: laser power 40%, scan speed 1000 mm / sec, pulse frequency 40 kHz, and spot variable -20.

[0070] To evaluate the color L* value, a spectrophotometer (ZE-6000, manufactured by Nippon Denshoku Co., Ltd.) was used to measure the L* value of each printed and unprinted portion of a single laser-printed label using the reflection method. The flat portion of the label was used as the sample for measurement. The measurement method for the printed portion was as follows: A 3cm square sample was cut out so that the entire "B" of the characters "12345ABCDE" was included (characters other than "B" may also be included). The colorimeter's measurement light source was a 6mm diameter sample stage (the opening where the measurement light hits is approximately 1cm in diameter) and a 6mm diameter sight glass, so that the character "B" was included in the opening of the sample stage. If the print does not fit completely within the opening of the sample stage (it extends beyond the opening), the sample stage can be changed as needed (e.g., 10mm diameter, 30mm diameter, etc.). Even if the print extends beyond the opening, it is sufficient that part of the print is within the opening of the sample stage and is exposed to the measurement light. For the non-printed area, a 3cm square sample was cut out from the unprinted area and the color L* value was measured using a 6φ color difference meter sight glass and sample stage. Note that the color difference meter sight glass and sample stage may be changed to 10φ, 30φ, etc. as needed, and in that case the sample size may be any size as long as it covers the opening of the sample stage (so that measurement light does not leak).

[0071] [Laser printing evaluation: Print size] The height and width of the "345ABC" part of the printed text "12345ABCDE" was measured visually in 0.5mm increments using a stainless steel straightedge (Kokuyo Co., Ltd. TZ-RS15), and the average value was taken as the print size. If the print size was less than 0.5mm, the print size was measured separately using a HIROX RH-2000 digital microscope. The software included with the HIROX RH-2000 digital microscope was used to measure the print size.

[0072] [Laser marking evaluation: visibility of characters] The visibility of the characters "12345ABCDE" printed on the laminate was judged according to the following criteria. Judgment: The characters can be visually recognized. Evaluation: Fair: Characters can be visually recognized but are unclear. Evaluation: Unable to visually recognize the characters.

[0073] <Preparation of polyester raw materials> [Synthesis of polyester raw material A] 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 A with an intrinsic viscosity of 0.75 dL / g. The composition is shown in Table 1.

[0074] [Synthesis of polyester raw materials B to H and J] Polyesters B to J having the compositions shown in Table 1 were obtained in the same manner as in polyester raw material A. When producing Polyester D, a composite oxide pigment of bismuth and neodymium (TOMATEC COLOR 42-920A manufactured by TOMATEC Corporation; average particle size 1 μm) was added as a laser printing pigment at a ratio of 5% by mass to the polyester. When producing polyester E, a mica pigment coated with antimony-doped tin oxide (Iriotec® 8825 manufactured by MERCK; average particle size less than 15 μm) was added as a laser printing pigment at a ratio of 5 mass % to the polyester. When producing polyester F, SiO2 (Silysia 266 manufactured by Fuji Silysia Ltd.; average particle size 1.5 μm) was added as a lubricant at a ratio of 7,000 ppm to the polyester.

[0075] In the table, TPA is terephthalic acid, IPA is isophthalic acid, EG is ethylene glycol, BD is 1,4-butanediol, NPG is neopentyl glycol, CHDM is 1,4-cyclohexanedimethanol, TMG is polytetramethylene glycol, CL is ε-caprolactone, and DEG is diethylene glycol (which may contain by-products from the polymerization). Chemically recycled terephthalic acid was used for the polymerization of polyester raw material J. The intrinsic viscosities of the polyesters were B: 0.72 dL / g, C: 0.80 dL / g, D: 1.20 dL / g, E: 0.77 dL / g, F: 0.75 dL / g, G: 0.78 dL / g, H: 0.78 dL / g, and J: 0.68 dL / g. Each polyester was cut into chips.

[0076] [Polyester raw material I] Polyester raw material I was a PET copolymerized with 10 mol % isophthalic acid obtained by mechanically recycling PET bottles, and had an intrinsic viscosity of 0.75 dl / g.

[0077] [Table 1]

[0078] Example 1 The raw materials for the printing layer (Layer A) were a mixture of Polyester A, Polyester D, and Polyester I in a mass ratio of 85:3:12, while the raw materials for Layer B were a mixture of Polyester A, Polyester F, and Polyester I in a mass ratio of 830:7:10. The mixed raw materials for Layers A and B were melted in separate extruders. The molten resins were joined by a feed block midway through the flow path and extruded through a T-die. The unstretched laminated film was then drawn off while cooled on a chill roll set at 30°C. The laminated film was configured so that the center layer was Layer A and both outer layers were Layer B (a three-layer structure consisting of two types of layers: B / A / B). The extrusion rate was adjusted so that the thickness ratio of Layer A to Layer B was 90 / 10 (B / A / B = 5 / 90 / 5). The Tg of the unstretched film was 65°C.

[0079] The resulting unstretched film was introduced into a transverse stretching machine (tenter), and while both ends of the film were held with clips, the film was preheated to a temperature of 90°C. It was then stretched 4.5 times in the width direction at a temperature of 85°C. It was then heat-treated at 80°C and relaxed 10% in the width direction. Both edges of the film were then trimmed and removed, and a uniaxially stretched film of approximately 40 μm was wound into a roll. The properties of the resulting film were evaluated using the methods described above. The film-forming conditions and evaluation results are shown in Table 2. The film presented no practical problems in terms of shrink finish and laser printing.

[0080] Example 2 In the same manner as in Example 1, an unstretched film having a thickness ratio of layer A to layer B of 90 / 10 (B / A / B=5 / 90 / 5) was obtained. The obtained unstretched film was introduced into a longitudinal stretching machine having multiple roll groups arranged in series, and preheated using a preheating roll until the film temperature reached 80°C.Then, using the difference in rotation speed between a low-speed rotating roll set to a surface temperature of 80°C and a high-speed rotating roll set to a surface temperature of 80, the film was stretched 1.3 times in the longitudinal direction.

[0081] After longitudinal stretching, the film was introduced into a transverse stretching machine (tenter), and while both ends of the film were held with clips, the film was preheated to a temperature of 105°C. It was then stretched 4.5 times in the width direction at a temperature of 88°C. It was then heat-treated at 82°C and relaxed 15% in the width direction. Both edges of the film were then trimmed and removed, and a biaxially stretched film of approximately 20 μm was wound into a roll. The properties of the resulting film were evaluated using the methods described above. The film-forming conditions and evaluation results are shown in Table 2. The film presented no practical problems in terms of shrink finish and laser printing evaluation.

[0082] (Examples 3, 4, and 5) In the same manner as in Example 1, an unstretched film having a thickness ratio of 90 / 10 (B / A / B=5 / 90 / 5) for layers A and B was obtained using the mixed polyester raw materials for layers A and B shown in Table 2. A uniaxially stretched film roll was obtained under the transverse stretching conditions shown in Table 2 in the same manner as in Example 2. The properties of the obtained film were evaluated by the methods described above. The film formation conditions and evaluation results are shown in Table 2. The film presented no practical problems in terms of both shrink finish and laser printability.

[0083] Example 6 In the same manner as in Example 2, an unstretched film having a thickness ratio of 90 / 10 (B / A / B=5 / 90 / 5) for layers A and B was obtained using the mixed polyester raw materials for layers A and B shown in Table 2. As in Example 1, a biaxially stretched film roll was obtained under the longitudinal stretching conditions and transverse stretching conditions shown in Table 2. The properties of the obtained film were evaluated by the methods described above. The film formation conditions and evaluation results are shown in Table 2. The film presented no practical problems in terms of both shrink finish and laser printing evaluation. Example 7 In the same manner as in Example 1, an unstretched film having a thickness ratio of layer A to layer B of 90 / 10 (B / A / B=5 / 90 / 5) was obtained. The resulting unstretched film was introduced into a longitudinal stretching machine having a plurality of roll groups arranged in series, and after preheating with a preheating roll until the film temperature reached 82°C, it was stretched 3.6 times in the longitudinal direction by utilizing the difference in rotation speed between a low-speed rotating roll set to a surface temperature of 82°C and a high-speed rotating roll set to a surface temperature of 78°C. Subsequently, a 10% relaxation treatment in the longitudinal direction was performed at 78°C by utilizing the difference in speed between the high-speed rotating roll and another low-speed rotating roll. Next, both edges of the film were cut and removed, and the resulting longitudinally uniaxially stretched film of approximately 40 μm was wound into a roll. The properties of the resulting film were evaluated using the methods described above. The results of the film formation condition evaluation are shown in Table 2. The film presented no practical problems in terms of shrinkage finish and laser printability.

[0084] (Comparative Example 1) A uniaxially stretched film roll was obtained in the same manner as in Example 1, except that the relaxation treatment in the width direction after transverse stretching in the transverse stretching device was not performed. The production conditions and evaluation results are shown in Table 2.

[0085] (Comparative Example 2) A uniaxially stretched film roll was obtained in the same manner as in Example 1, except that after transverse stretching in the transverse stretching device, heat treatment at 110°C and 10% relaxation treatment in the width direction were performed. The production conditions are shown in Table 2, and the evaluation results are shown in Table 2.

[0086] (Comparative Example 3) The polyester raw materials shown in Table 2 were melt-extruded in the same manner as in Example 1 to obtain an unstretched film. Under the conditions shown in Table 3, the unstretched film was preheated to 95°C in a tenter without being longitudinally stretched, then stretched 4.5 times in the width direction at 85°C, followed by heat treatment at 83°C and a 10% relaxation treatment in the width direction. Next, both edges of the film were trimmed and removed, and a uniaxially stretched film of approximately 40 μm was wound into a roll. The properties of the obtained film were evaluated using the methods described above. The evaluation results are shown in Table 2.

[0087] Comparative Example 4 The polyester raw materials shown in Table 2 were melt-extruded in the same manner as in Example 1 to obtain unstretched films. Under the conditions shown in Table 3, the unstretched films were longitudinally stretched 3.6 times at 85°C, preheated to 140°C in a tenter, and then stretched 5.3 times in the width direction at 110°C. Subsequently, they were heat-treated at 95°C and relaxed 10% in the width direction. Both edges of the film were then trimmed and removed, and a uniaxially stretched film of approximately 40 μm was wound into a roll. The properties of the obtained films were evaluated using the methods described above. The evaluation results are shown in Table 2.

[0088] [Table 2A]

[0089] [Table 2B]

[0090] The laser-markable heat-shrinkable film of the present invention was a stretched film that used a predetermined amount of at least one of butanediol, diethylene glycol, and ε-caprolactone and underwent a predetermined relaxation process, and the laser marking on the product after shrinkage was free of unevenness, wrinkles, and distortion.

[0091] In Comparative Example 1, the relaxation in the transverse direction was 0% (no relaxation), and in Comparative Example 3, the total amount of the long-chain molecules butanediol, ε-caprolactone, and diethylene glycol was 1.8 mol%, and in Comparative Example 4, the stretching ratio in the longitudinal direction, which is not the main shrinkage direction, was 3.6. As a result, the reduction in the shrinkage rate at 80°C after 10% shrinkage was lower than 11% in both cases, resulting in inferior laser printing quality. In Comparative Example 2, the shrinkage rate in the main shrinkage direction before shrinkage did not satisfy the predetermined range, and the film had poor shrink finish. [Industrial Applicability]

[0092] The laser-printed heat-shrinkable polyester film of the present invention has no unevenness, wrinkles, or distortion in the laser printing on the product after shrinkage, and can therefore be widely used in applications such as label packaging, cap seals, and integrated packaging that combine the protection of glass bottles or plastic bottles and the like with product labeling.

Claims

1. A heat-shrinkable polyester film for laser marking, which contains at least one laser-markable layer that changes color upon laser irradiation and satisfies the following (1) to (3): (1) The shrinkage rate at 80°C in the main shrinkage direction is 30% or more and 80% or less. (2) After shrinking the film by 10% in the main shrinkage direction with hot air at 100°C, the reduction in shrinkage rate at 80°C in the main shrinkage direction is 11% or more and 30% or less. (3) The transmittance at 355 nm in the ultraviolet-visible spectrum is 65% or more and 90% or less.

2. 2. The heat-shrinkable polyester film for laser marking according to claim 1, wherein the maximum shrinkage stress measured in a hot air atmosphere at 90° C. in the main shrinkage direction is 6 MPa or more and 20 MPa or less.

3. 2. The laser-markable heat-shrinkable polyester film according to claim 1, wherein the thickness is 20 μm or more and 80 μm or less.

4. 2. The laser-markable heat-shrinkable polyester film according to claim 1, wherein the laser-markable layer, which changes color upon laser irradiation, contains one or more metals or metal compounds that can be printed upon laser irradiation, selected from the group consisting of bismuth, gadolinium, neodymium, titanium, antimony, tin, aluminum, calcium, and barium.

5. 2. The laser-markable heat-shrinkable polyester film according to claim 1, wherein the laser-markable layer, which changes color upon laser irradiation, contains a polyester containing 50 mol % or more of ethylene terephthalate units.

6. 2. The laser-markable heat-shrinkable polyester film according to claim 1, wherein the laser-markable heat-shrinkable polyester film contains, as a polyester-constituting component, a polyester containing 5 mol % to 30 mol % of at least one of butanediol, diethylene glycol, tetramethylene glycol, and ε-caprolactone.

7. A display comprising the laser-printable heat-shrinkable polyester film according to any one of claims 1 to 6, wherein the difference in color L* value between the laser-printed portion and the non-laser-printed portion is 1.0 or more and 10 or less.

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