Heat-shrinkable polyester 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 for laser printing suitable for label applications and the like.

Background Art

[0002] In recent years, heat-shrinkable films have been widely used for applications such as label packaging, cap sealing, and integrated packaging that combine the protection of glass bottles or plastic bottles and the display of products. Among such heat-shrinkable films, polyvinyl chloride-based films have problems such as low heat resistance, generation of hydrogen chloride gas during incineration, and causing dioxins. In addition, polystyrene-based films are inferior in solvent resistance, require the use of inks with special compositions during printing, and need to be incinerated at high temperatures, resulting in problems such as the generation of a large amount of black smoke with a strange odor during incineration. For this reason, polyester-based heat-shrinkable films with high heat resistance, easy incineration, and excellent solvent resistance tend to be widely used as shrink labels.

[0003] On the other hand, heat-shrinkable polyester films for laser printing made of plastic films are widely used for distribution items represented by foods, pharmaceuticals, and industrial products. The heat-shrinkable polyester film for laser printing not only protects the contents but also plays a role in displaying information such as product names, manufacturing dates, raw materials, and the like.

[0004] In recent years, heat-shrinkable polyester films for laser printing that can display information by laser printing have been disclosed. Laser printing can efficiently perform fine printing. It has been disclosed that laser-printable films can be obtained by incorporating a coloring agent that reacts to a laser into the base material or by applying a coating layer containing a coloring agent that reacts to a laser to the base film (Patent Documents 1, 2, and 3).

[0005] However, when attempting to apply laser printing to shrinkable film, there was a problem in that unevenness, wrinkles, and distortions due to shrinkage were likely to occur, 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 Publication No. 2015-66722 [Overview of the project] [Problems that the invention aims to solve]

[0007] The object of the present invention is to provide a heat-shrinkable polyester film having a laser-printed layer that is free from unevenness, wrinkles, and distortion in the laser printing of the product after shrinkage. [Means for solving the problem]

[0008] The present invention consists of the following configuration. [1] A heat-shrinkable polyester film for laser printing that includes at least one laser-printable layer that changes color upon laser irradiation and satisfies the following conditions (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 by 10% in the main shrinkage direction with 100°C hot air, the decrease in the 80°C shrinkage rate in the main shrinkage direction is 11% or more and 30% or less. (3) The transmittance at 355 nm in the ultraviolet-visible spectral spectrum is 65% or more and 90% or less. [2] The heat-shrinkable polyester film for laser printing according to claim 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] A heat-shrinkable polyester film for laser printing according to claim 1 or 2, characterized in that it has a thickness of 20 μm or more and 80 μm or less. [4] A laser-printable heat-shrinkable polyester film according to any one of claims 1 to 3, characterized in that the laser-printable layer that changes color upon laser irradiation contains one or more elements or compounds selected from the group consisting of bismuth, gadolinium, neodymium, titanium, antimony, tin, aluminum, calcium, and barium as a metal or metal compound that can be printed upon laser irradiation. [5] A heat-shrinkable polyester film for laser printing according to any one of 1 to 4, characterized in that the laser-printed layer that changes color upon laser irradiation contains polyester containing 50 mol% or more of ethylene terephthalate units. [6] The heat-shrinkable polyester film for laser printing according to any one of claims 1 to 5, characterized in that the heat-shrinkable polyester film for laser printing contains a polyester in which at least one of butanediol, diethylene glycol, tetramethylene glycol, and ε-caprolactone is present in an amount of 5 mol% to 30 mol% as a component of the polyester. [7] A display body made of a laser-printed heat-shrinkable polyester film as described in any of 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. [Effects of the Invention]

[0009] The present invention provides a heat-shrinkable polyester film for laser printing that is free from unevenness, wrinkles, and distortion in laser printing on the product after shrinkage. [Brief explanation of the drawing]

[0010] [Figure 1] Schematic diagram of a metal frame used when shrinking film by 10% [Figure 2] Schematic diagram of a marked film prepared before shrinking it by 10%. [Modes for carrying out the invention]

[0011] The following describes the heat-shrinkable polyester film for laser printing according to the present invention. [1] Composition of heat-shrinkable polyester film for laser printing [1-1] Layer structure, thickness The laser-printable heat-shrinkable polyester film of the present invention must have at least one film layer that can be printed by laser irradiation (hereinafter, this may be abbreviated as a laser-printable layer that changes color by laser irradiation, or simply a laser-printable layer). In the present invention, it is preferable that the laser-printable layer is present over the entire area in the planar direction of the laser-printable heat-shrinkable polyester film. Furthermore, in order to improve the design, the laser-printable heat-shrinkable polyester film of the present invention may be provided with a printed layer containing characters or designs other than those formed by the laser.

[0012] The thickness of the heat-shrinkable polyester film for laser printing of the present invention is not particularly limited, but is preferably 5 μm or more and 100 μm or less. If the thickness of the heat-shrinkable polyester film for laser printing is less than 5 μm, not only will the visibility of the laser printing decrease, but the mechanical strength and heat-seal strength may also decrease, which is undesirable. On the other hand, if the thickness of the heat-shrinkable polyester film for laser printing exceeds 100 μm, it is economically wasteful for the purpose of display. The thickness of the heat-shrinkable polyester film for laser printing 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 printing layer that constitutes the heat-shrinkable polyester film for laser printing of the present invention is preferably 5 μm or more and 100 μm or less. If this thickness is less than 5 μm, there is a risk that the visibility of laser printing will decrease even if the concentration of the laser printing pigment described later is increased. On the other hand, if the thickness of the printing layer exceeds 100 μm, it is economically wasteful for display purposes. The thickness of the laser printing 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] In addition, for all the layers that constitute the heat-shrinkable polyester film for laser printing of the present invention, it is also possible to provide a layer that has been subjected to corona treatment, coating treatment, flame treatment, etc. in order to improve the printability of the surface and the like.

[0015] [1-2] Laser printing layer that changes color by laser irradiation In order to make the printing layer that constitutes the present invention capable of laser printing, it is necessary to add a laser printing pigment having a discoloration function by laser irradiation. The plastic that constitutes the heat-shrinkable polyester film for laser printing usually hardly reacts to laser light, so it cannot be printed by laser irradiation. The laser printing pigment is excited by the energy of the laser light, and printing becomes possible by carbonizing the surrounding plastic. In addition to the carbonization action of the plastic, depending on the type of laser printing pigment, it itself may change to black. Due to the single or combined effects of this carbonization action and the discoloration action of the laser printing pigment, printing on the printing layer becomes possible. From the perspective of printing density, it is preferable to select a laser printing pigment that has both the carbonization action of the plastic and its own discoloration action.

[0016] Specific types of laser printing pigments include any single element or oxide of bismuth, gadolinium, neodymium, titanium, antimony, tin, aluminum, calcium, or barium. Among these, titanium oxide, calcium carbonate, bismuth trioxide, antimony trioxide, and barium sulfate are preferred, and titanium oxide, calcium carbonate, and bismuth trioxide are more preferred. Also, the particle size of the laser printing pigment is preferably 0.1 μm or more and 10 μm or less. If the particle size of the laser printing pigment is less than 0.1 μm, there is a risk that the color change during laser irradiation will not be sufficient. On the other hand, if the particle size of the laser printing pigment exceeds 10 μm, there is a concern that it will accelerate the clogging of the filter in the extrusion process when forming the film. The particle size of the laser printing 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 addition amount of the laser printing pigment in the laser printing layer is preferably 0.05% by mass or more and 50% by mass or less. If the addition amount of the pigment is less than 0.05% by mass, the printing density by laser will not be sufficient, which is not preferable. On the other hand, if the addition amount of the pigment exceeds 50% by mass, the amount (volume) of the plastic to be carbonized will relatively decrease, so there is also a risk that the printing density will not be sufficient. The addition amount of the laser printing pigment 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 printing layer has multiple layers, the addition amount of the laser printing pigment for the entire laser printing layer can be obtained by proportionally dividing the thickness ratio of each layer and the addition amount of the laser printing pigment.

[0018] Laser printing pigments can be added at any stage in the manufacturing process of the resin that will serve as the raw material for the laser printing layer, or the film that will serve as the laser printing layer. For example, in the resin manufacturing stage, methods include blending a slurry of particles dispersed in a solvent with a plastic raw material using a vented kneading extruder, or blending dried particles with plastic resin using a kneading extruder (masterbatch formation). Among these methods, the method of using a masterbatch containing laser printing pigment as the raw material for the film is preferred.

[0019] The laser printing layer constituting the heat-shrinkable polyester film for laser printing of the present invention may contain various additives other than laser printing pigments, such as waxes, antioxidants, antistatic agents, nucleating agents, viscosity reducers, heat stabilizers, coloring pigments, color inhibitors, and ultraviolet absorbers, as needed. Furthermore, if the laser printing 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 can be silica, alumina, kaolin, lead white, titanium white, zeolite, zinc oxide, lithopone, etc., while organic fine particles can be acrylic particles, melamine particles, silicone particles, cross-linked polystyrene particles, carbon black, iron oxide, etc. The average particle size of the fine particles can be appropriately selected as needed within the range of 0.05 to 3.0 μm when 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 amount 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 it exceeds 1% by mass, the surface smoothness may decrease, which may cause problems such as blurred printability and is therefore undesirable. As for the method of incorporating particles into the laser printing layer, they can be added at any stage in the manufacturing of the plastic raw material, and the same method as described in "1.2.1. Type, amount, and method of addition of laser printing pigment" above can be used.

[0020] [1-3] Other layers The heat-shrinkable polyester film for laser printing of the present invention may have layers other than the laser printing layer described above, such as a printing layer, a smoothing layer, a surface protection layer, an antistatic layer, etc.

[0021] [1-3-1] Printing layer In addition to laser printing, the heat-shrinkable polyester film for laser printing of the present invention may be provided with characters or designs to improve its aesthetic appeal. Known materials such as gravure printing inks or flexographic printing inks can be used to constitute these characters and designs. The number of printing layers may be one or multiple. To improve aesthetic appeal by using multiple colors, it is preferable to have multiple printing layers. The printing layers may be located at the outermost layer or in the middle layers.

[0022] [1-3-2] Easy slip layer In addition to laser printing, the heat-shrinkable polyester film for laser printing of the present invention may have a slippery layer on the surface, which may be on one side or both sides, for the purpose of optimizing the film's slipperiness. 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, lead white, titanium white, zeolite, zinc oxide, lithopone, etc., and organic fine particles include acrylic particles, melamine particles, silicone particles, cross-linked polystyrene particles, carbon black, iron oxide, etc. The average particle size of the fine particles can be appropriately selected as needed within the range of 0.05 to 3.0 μm when 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 it 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 amount exceeds 1% by mass, it may lead to problems such as reduced surface smoothness and blurred printability, which is undesirable.

[0023] [2] Characteristics of heat-shrinkable polyester film for laser printing Next, the properties of the heat-shrinkable polyester film for laser printing of the present invention will be described. [2-1] Shrinkage rates at 80°C and 90°C in the principal shrinkage direction The heat-shrinkable polyester film for laser printing of the present invention is subjected to a heat treatment in 80°C hot water for 10 seconds under no load, followed immediately by immersion of the film in 25°C ± 0.5°C water 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 length before and after shrinkage using the following formula 1, is 30% or more and 80% or less. Thermal shrinkage rate = {(Length before shrinkage - Length after shrinkage) / Length before shrinkage} × 100 (%) Equation 1

[0024] If the heat shrinkage rate at 80°C in the main shrinkage direction is less than 30%, it is undesirable when used as a film for beverage labels or bento box packaging because the amount of shrinkage is too small, resulting in wrinkles and sagging of the labels after heat shrinkage. A heat shrinkage rate of 35% or more at 80°C is more preferable, 40% or more is particularly preferable, and 45% or more is most preferable. While it is not a problem if the 80°C heat shrinkage rate in the main shrinkage direction is higher than 80%, if the 80°C shrinkage rate is 80% or less, the film can be manufactured efficiently.

[0025] The 90°C heat shrinkage rate in the main shrinkage direction, measured similarly to the 80°C heat shrinkage rate, is preferably 35% to 80%. If the 90°C heat shrinkage rate in the main shrinkage direction is less than 35%, it is undesirable when used as a film for beverage labels or bento box packaging because the amount of shrinkage is too small, resulting in wrinkles and sagging of the label after heat shrinkage. A 90°C heat shrinkage rate of 40% or more in the main shrinkage direction is more preferable, and 45% or more is particularly preferable. While a 90°C heat shrinkage rate in the main shrinkage direction that is higher than 80% is not a problem, if the 90°C heat shrinkage rate in the main shrinkage direction is 80% or less, the film can be manufactured with good production efficiency.

[0026] [2-2] Percentage decrease in the 80°C shrinkage rate in the main shrinkage direction after shrinking by 10% in the main shrinkage direction with 100°C hot air. The heat-shrinkable polyester film for laser printing of the present invention exhibits a reduction in the 80°C shrinkage rate in the main shrinkage direction of the present invention of 11% to 30% after shrinking by 10% in the main shrinkage direction with 100°C hot air. Normally, when a heat-shrinkable film is shrunk by 10%, a reduction in the heat shrinkage rate is observed by the amount of shrinkage, i.e., 10%. In this case, even after heat shrinking the object to be coated, the label retains some heat shrinkage rate (heat shrinkage performance). That is, the heat generated when the label is irradiated with a laser causes significant heat shrinkage in the relevant area, making deformation and perforation more likely. Conventionally, in order to suppress the shrinkage rate after 10% shrinkage, it was necessary to reduce the shrinkage rate before shrinkage, which made it difficult to use in applications requiring high shrinkage rates, such as irregularly shaped bottles, and there was a risk that the heat-shrinkable film would not be 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 reduce the 80°C heat shrinkage rate in the main shrinkage direction by 11% or more after 10% shrinkage. The conditions necessary to achieve this characteristic will be described later. If the decrease in the 80°C heat shrinkage rate in the main shrinkage direction after 10% shrinkage is less than 11%, it is undesirable because it may result in insufficient heat resistance after shrinkage. On the other hand, a higher decrease in this shrinkage rate is preferable, but if it is 30% or less, the film can be manufactured efficiently. The decrease in the 80°C heat shrinkage rate in the main shrinkage direction after 10% shrinkage is preferably between 12% and 29%, and more preferably between 13% and 28%.

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

[0028] [2-4] Maximum shrinkage stress measured in 90°C hot air in the principal shrinkage direction The heat-shrinkable polyester film for laser printing of the present invention preferably has a maximum shrinkage stress of 6 MPa or more and 20 MPa or less, measured in 90°C hot air in the main shrinkage direction. If this maximum shrinkage stress is less than 6 MPa, it is undesirable because the decrease in the 80°C shrinkage rate in the main shrinkage direction after shrinking by 10% in the main shrinkage direction with 100°C hot air tends to be less than 10%. On the other hand, if the maximum shrinkage stress exceeds 20 MPa, there is a risk that excessive force will be applied to the object to be covered during the shrinkage of the film, causing deformation of the object, or that the finish defects (so-called sink marks) when shrunk as a label will increase. The maximum shrinkage stress is more preferably 6.5 MPa or more and 19.5 MPa or less, and even more preferably 7 MPa or more and 19 MPa or less.

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

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

[0031] [2-6] The absolute difference in color L* values ​​between the printed and unprinted areas after laser printing. The heat-shrinkable polyester film for laser printing of the present invention preferably has an absolute difference in the color L* value between the printed and unprinted areas after laser printing (hereinafter sometimes simply referred to as "difference in L* value") of 1.0 or more and 10.0 or less. If this difference is less than 1.0, the color tones of the printed and unprinted areas become similar, making it difficult to see the print. On the other hand, if the difference in L* value exceeds 10.0, the print becomes easier to see, but it is necessary to increase the laser irradiation power accordingly, which increases the damage to the heat-shrinkable polyester film for laser printing and makes it more prone to problems such as holes and deformation, so this is undesirable. The difference in L* value is more preferably 1.5 or more and 9.5 or less, and even more preferably 2.0 or more and 9.0 or less.

[0032] [3] Method for manufacturing heat-shrinkable polyester film for laser printing [3-1] Polyester raw material for heat-shrinkable polyester film for laser printing The polyester used in the heat-shrinkable polyester film for laser printing of the present invention has ethylene terephthalate units. The ethylene terephthalate units are preferably present in an amount of 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 aforementioned constituent units refer to monomeric units of polyhydric alcohols and polyhydric carboxylic acids that constitute the copolymer.

[0033] Furthermore, the heat-shrinkable polyester film for laser printing 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 component (100 mol%). The inventors have found that by including 5 mol% or more of these components and performing relaxation in the width direction as described later, it becomes easier to achieve a reduction of 10% or more in the 80°C shrinkage rate in the main shrinkage direction after shrinking by 10% in the main shrinkage direction with 100°C hot air. Although the detailed mechanism is not clear, 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 as described later. In other words, when relaxing in the width direction, which is the main shrinkage direction, during the film formation process, it can be estimated that the presence of the above-mentioned long-chain components causes components with relatively short molecular chain lengths, such as ethylene glycol, to be preferentially relaxed. As a result, it is thought that the heat shrinkage rate in the main shrinkage direction at 80°C of the finished film (before shrinkage) will be kept high (30% or more), while the decrease in the 80°C shrinkage rate in the main shrinkage direction after shrinking by 10% in the main shrinkage direction with 100°C hot air will be 10% or more. If at least one component selected from butanediol, diethylene glycol, and ε-caprolactone is present in less than 5 mol%, the relaxation of short-chain components such as ethylene glycol will be relatively large, and the 80°C heat shrinkage rate 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 above long-chain components exceed 30 mol%, the amount of ethylene glycol and short-chain components that can become amorphous components will be relatively small, and it may be difficult to reduce the reduction rate of the 80°C shrinkage rate in the main shrinkage direction by 10% or more after 10% shrinkage in the main shrinkage direction with 100°C hot air. It is more preferable that these long-chain components be between 6 mol% and 29 mol%, and even more preferable that they be between 7 mol% and 28 mol%.

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

[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 isophthalic acid is a monomer that can be an amorphous component, 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 constituting 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, and so on.

[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 amorphous components. Therefore, in this 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 amorphous components. Accordingly, the content of the constituent unit derived from butanediol is the sum of the content of the constituent unit consisting of isophthalic acid and butanediol and the content of the constituent unit consisting of terephthalic acid and butanediol. Furthermore, the content of the constituent unit derived from one or more monomers that can become amorphous components is the sum of the content of all constituent units derived from one or more monomers that can become amorphous components, 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 that make up 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 aliphatic dicarboxylic acids (e.g., adipic acid, sebacic acid, decanedicarboxylic acid, etc.) are incorporated into polyester, the content is preferably less than 3 mol% (out of 100 mol% of the dicarboxylic acid component). Heat-shrinkable polyester films for laser printing obtained using polyester containing 3 mol% or more of these aliphatic dicarboxylic acids have insufficient stiffness when mounted at high speed.

[0040] Furthermore, it is preferable not to include polycarboxylic acids with a valency of 3 or higher (for example, trimellitic acid, pyromellitic acid, and their anhydrides) in the polyester. Laser-printable heat-shrinkable polyester films obtained using polyesters containing these polycarboxylic acids tend to have difficulty achieving the required high shrinkage rate.

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

[0042] The polyester used in this invention is preferably one in which the amount of butanediol, diethylene glycol, and ε-caprolactone, as well as the amount of monomers that can become amorphous components, are appropriately selected to adjust the glass transition temperature (Tg) to 50-80°C. A Tg of 52°C to 78°C is more preferable.

[0043] In the present invention, it is preferable that the polyester used does not contain diols with 8 or more carbon atoms (e.g., octanediol) or polyhydric alcohols with a valency of 3 or higher (e.g., trimethylolpropane, trimethylolethane, glycerin, diglycerin). Laser printing heat-shrinkable polyester films obtained using polyesters containing these diols or polyhydric alcohols are less likely to achieve the required high shrinkage rate. It is also preferable that the polyester contains as little triethylene glycol and polyethylene glycol as possible.

[0044] The most preferred polyester is one in which, out of 100 mol% of the total polyester constituent units, 1 to 25 mol% are butylene terephthalate units, 1 to 25 mol% are units consisting of ε-caprolactone and terephthalic acid, totaling 2 to 50 mol%, 18 to 25 mol% are units consisting of monomers that can become amorphous components and terephthalic acid, and the remainder are ethylene terephthalate units. Note that amorphous units in which some of the terephthalic acid is replaced with isophthalic acid may also be included.

[0045] The resin forming the heat-shrinkable polyester film for laser printing of the present invention may contain various additives as needed, such as waxes, antioxidants, antistatic agents, nucleating agents, viscosity reducers, heat stabilizers, coloring pigments, color inhibitors, and ultraviolet absorbers.

[0046] In the resin forming the heat-shrinkable polyester film for laser printing of the present invention, it is preferable to add fine particles as a lubricant 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, while 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 a value measured with a Coulter counter.

[0047] As for how to incorporate the above particles into the resin forming the heat-shrinkable polyester film for laser printing of the present invention, for example, they can be added 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 esterification stage, or after the completion of the transesterification reaction but before the start of the polycondensation reaction, in order to proceed with the polycondensation reaction. It is also preferable to blend the slurry of particles dispersed in ethylene glycol or water with the polyester resin raw material using a kneading extruder with a vent, or to blend the dried particles with the polyester resin raw material using a kneading extruder.

[0048] [3-2] Method for manufacturing heat-shrinkable polyester film for laser printing The heat-shrinkable polyester film for laser printing 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 predetermined method shown below. In this invention, the method of sequential biaxial stretching, in which transverse uniaxial stretching or longitudinal stretching is performed followed by transverse stretching, which is a common method of manufacturing shrinkable films, is described as an example. However, these may also be sequential biaxial stretching in which longitudinal uniaxial stretching or transverse stretching is performed followed by longitudinal stretching, or simultaneous biaxial stretching in which the longitudinal and transverse directions are stretched simultaneously.

[0049] [3-2-1] Extrusion conditions, unstretched film formation 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 paddle dryer, or a vacuum dryer. After drying the polyester raw material in this way, it is melted at a temperature of 200-300°C using an extruder and extruded into a film. Any existing method such as the T-die method or the tubular method can be used for extrusion.

[0050] Then, an unstretched film can be obtained by rapidly cooling the sheet-like molten resin after extrusion. As a method for rapidly cooling the molten resin, a method of casting the molten resin from a die onto a rotating drum and rapidly cooling and solidifying it to obtain a substantially unoriented resin sheet can be suitably employed.

[0051] [3-2-2] Control of longitudinal extension conditions In the production of the heat-shrinkable polyester film for laser printing according to the present invention, if the longitudinal direction is not the primary shrinkage direction, it is preferable to set the longitudinal stretching ratio to 2 times or less in order to avoid inhibiting the relaxation of the short-chain components as described above. The lower limit of the longitudinal stretching ratio is 1 time (unstretched). It is undesirable to set the longitudinal stretching ratio to more than 2 times because it is thought that the molecular chains will be oriented not only in the transverse direction, which is the primary shrinkage direction, but also in the longitudinal direction, which will inhibit the movement of the molecular chains that try to relax in the transverse direction. It is more preferable that the upper limit of the longitudinal stretching ratio be 1.8 times or less, and even more preferable that be 1.6 times or less. The most preferred longitudinal stretching ratio is the lower limit of 1 time. When longitudinal stretching is the primary shrinkage direction, it is preferable that the stretching ratio in the longitudinal direction be between 2 and 5 times. If the stretching ratio in the longitudinal direction is less than 2 times, it becomes difficult to achieve a thermal shrinkage rate of 30% or more at 80°C and 35% or more at 90°C in the primary shrinkage direction. On the other hand, if the stretching ratio exceeds 5 times, the tensile stress becomes too high, making it prone to fracture, which is undesirable. It is more preferable that the stretching ratio in the longitudinal direction be between 2.5 and 4.5 times, and even more preferable that it be between 3 and 4 times. When stretching in the longitudinal direction, it is preferable to preheat a substantially unoriented film to a temperature of Tg or above Tg+30°C. Furthermore, both single-stage and multi-stage stretching (two or more stages) can be used for longitudinal stretching. When longitudinal stretching is the primary direction of shrinkage, longitudinal relaxation is required after longitudinal stretching. Detailed conditions are explained below in [3-2-4] Heat treatment and relaxation conditions after stretching in the shrinkage direction.

[0052] [3-2-3] Control of lateral extension 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 to Tg+30°C, to achieve a magnification of 3 to 6 times. By performing transverse stretching under these predetermined conditions, it is possible to obtain a good film. The temperature for transverse stretching 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. On the other hand, the magnification of transverse stretching 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 accuracy of the film tends to decrease. However, by controlling the stretching temperature to Tg+30°C or lower, it is possible to improve the thickness accuracy of the film, which is preferable.

[0054] On the other hand, if the stretching temperature falls below Tg+10℃, the orientation in the width direction becomes too large, making it prone to fracture during transverse stretching, which is undesirable.

[0055] [3-2-4] Heat treatment and relaxation conditions after stretching in the shrinkage direction After stretching in the direction of primary shrinkage, the film needs to relax in that direction. In the following explanation, the transverse direction will be considered the primary shrinkage direction. After transverse stretching, the film needs to be relaxed in the width direction at a temperature equal to or lower than the transverse stretching temperature, while both ends in the width direction are held with clips inside the tenter, thereby reducing the distance between the clips in the width direction. As described in the explanation of the polyester raw materials above, by including 5 mol% or more of long-chain components such as butanediol, diethylene glycol, and ε-caprolactone, and by relaxing in the width direction, it becomes easier to achieve a reduction of 10% or more in the 80°C shrinkage rate in the main shrinkage direction after shrinking by 10% in the main shrinkage direction with 100°C hot air. Conventionally, it was thought that heat treatment was meaningless if the temperature at which the material relaxes in the width direction was lower than the transverse stretching temperature. However, in this invention, we have found a novel finding that when a molecular structure is adopted 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 temperature at which the material relaxes is lower than the transverse stretching temperature. On the other hand, if the relaxation temperature is higher than the transverse stretching temperature, it becomes difficult to achieve a thermal shrinkage rate (before shrinkage) of 30% or more in the main shrinkage direction at 80°C. The relaxation temperature is preferably -10°C or higher and 0°C or lower relative to the transverse stretching temperature, and more preferably -9°C or higher and -1°C or lower. Furthermore, a relaxation rate in the width direction of 3% to 20% is preferable. If the relaxation rate is less than 3%, the relaxation of the short-chain components will not be sufficient, and it may become difficult to achieve a reduction of 10% or more in the 80°C shrinkage rate in the main shrinkage direction after shrinking by 10% in the main shrinkage direction with 100°C hot air. On the other hand, if the relaxation rate exceeds 20%, it becomes difficult to achieve a thermal shrinkage rate (before shrinkage) of 30% or more in the main shrinkage direction at 80°C. A relaxation rate in the width direction of 4% to 19% is preferable, and a relaxation rate of 5% to 18% is more preferable. The residence time in the relaxation process is preferably between 1 second and 9 seconds. If the relaxation time is less than 1 second, the relaxation of the short-chain components will not be sufficient, making it difficult to achieve a 10% reduction in the 80°C shrinkage rate in the main shrinkage direction after 10% shrinkage in the main shrinkage direction with 100°C hot air. On the other hand, if the relaxation time exceeds 9 seconds, the equipment becomes too large, which is undesirable. The relaxation time is more preferably between 1.5 seconds and 8.5 seconds, and even more preferably between 2 seconds and 8 seconds.

[0056] [4] Laser printing conditions Examples of laser types (wavelengths) that can be used for laser printing on the heat-shrinkable polyester film for laser printing of the present invention include CO2 lasers (10600 nm), YAG lasers (1064 nm), YVO4 lasers (1064 nm), fiber lasers (1064, 1090 nm), green lasers (532 nm), and UV lasers (355 nm). Among these, the type of laser used for laser printing in the present invention is not particularly limited, but CO2 lasers are often used to burn through plastics and are often used for purposes other than printing, which is the essence of the present invention, so they are not preferred as a laser source. YAG lasers, YVO4 lasers, fiber lasers, green lasers, and UV lasers are preferred as laser sources, YAG lasers, fiber lasers, and UV lasers are more preferred, and UV lasers are particularly preferred because they cause less thermal damage. Commercially available laser printing equipment can be used, 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 printing conditions vary depending on the equipment manufacturer and model, as well as the type of film being printed on, so it is difficult to generalize. However, using the Keyence MD-U1000C (UV laser, wavelength 355nm) as an example, the conditions are as follows.

[0057] The laser power is preferably 20% to 80% of the maximum device specification of 13W. An output of less than 20% is undesirable because it reduces print density and visibility. An output of more than 80% is undesirable because it can cause holes in the display surface. An output of 25% to 75% is more preferable, and 30% to 70% is even preferable. The pulse frequency is preferably 10kHz to 100kHz. A frequency below 10kHz is undesirable because the laser energy per pulse becomes high, making it easy for the thickness reduction rate of the printed area to exceed 80 vol%. Conversely, if the frequency exceeds 100kHz, it is easier to keep the thickness reduction rate of the printed area below 80 vol%, but it may be difficult to keep the difference in color L* values ​​of the printed area at 1 or more. A frequency of 15kHz to 95kHz is more preferable, and 20kHz to 90kHz is even preferable. The scan speed is preferably 10mm / sec to 3000mm / sec. If the scan speed falls below 10 mm / second, the printing speed decreases drastically, which is undesirable as it slows down the production speed of the display units. On the other hand, if the scan speed exceeds 3000 mm / second, the print density decreases, making it difficult to maintain a color L* value difference of 1 or more, which is also undesirable. A scan speed of 100 mm / second to 2900 mm / second is more preferable, and 200 mm / second to 2800 mm / second is even more preferable.

[0058] [5] Display body The label of the present invention is formed by covering at least a portion of the outer circumference of the object to be packaged with the laser-printable 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 shampoo and conditioner, various bottles and cans, plastic containers for confectionery and bento boxes, and paper boxes. Furthermore, the label obtained from the heat-shrinkable polyester film does not need to cover the entire container; it may cover only a portion, such as a cap seal covering the bottle lid, and may even be a preform before being completely adhered to the container. When heat-shrinking a label obtained from a heat-shrinkable polyester film to cover such objects to be packaged, the label is usually heat-shrinked by about 2 to 15% to ensure close adhesion to the object to be packaged. Preferably, the difference in color L* value between the laser-printed portion and the unprinted portion of the label of the present invention is 1.0 or more and 10 or less.

[0059] The method for producing the labels involves applying an organic solvent slightly inward from one edge of a rectangular film, immediately rolling the film and overlapping and bonding the edges to form a label, or applying an organic solvent slightly inward from one edge of a roll of film, immediately rolling the film and overlapping and bonding the edges to form a tube, which is then cut to form a label. Preferred organic solvents for bonding are cyclic ethers such as 1,3-dioxolane or tetrahydrofuran. Other usable solvents include aromatic hydrocarbons such as benzene, toluene, xylene, and trimethylbenzene, halogenated hydrocarbons such as methylene chloride and chloroform, phenols, or mixtures thereof. [Examples]

[0060] Next, the present invention will be specifically described using examples and comparative examples. However, the present invention is not limited in any way to the embodiments of these examples, and can be modified as appropriate without departing from the spirit of the invention. The film evaluation method and polyester synthesis method are shown below.

[0061] <Evaluation method for heat-shrinkable polyester film for laser printing> The evaluation method for heat-shrinkable polyester films for laser printing is as follows: For non-printed samples, portions were cut out at least 1 mm away from the printed and heat-sealed areas and used as samples.

[0062] [thickness] Five points were measured using a micrometer (Millitron 1254D, manufactured by Feinpluf), and the average value was calculated.

[0063] [Heat shrinkage rate at 80°C and 90°C] A heat-shrinkable film for laser printing was cut into 10cm x 10cm squares, immersed in warm water at a specified temperature ±0.5°C for 10 seconds without load to induce heat shrinkage, then immersed in water at 25°C ±0.5°C for 10 seconds. After removing the film from the water, its lengthwise and widthwise dimensions were measured, and the heat shrinkage rate was calculated according to Equation 1 below. The direction with the greater heat shrinkage rate was defined as the primary shrinkage direction (width direction). Thermal shrinkage rate = {(Length before shrinkage - Length after shrinkage) / Length before shrinkage} × 100 (%) Equation 1

[0064] [Shrinkage rate and reduction in shrinkage rate at 80°C after 10% shrinkage at 100°C] A heat-shrinkable film for laser printing was applied to a metal frame (Figure 1) with an inner dimension of 160 mm square and a frame thickness of 20 mm, and then subjected to a 10% shrinkage treatment. Specifically, the film was cut to 150mm in the longitudinal direction and 200mm in the width direction, and then markings were made so that the spacing in the width direction was 178mm (Figure 2). The film was then attached so that the markings aligned with the inner dimensions of the metal frame (the film markings were spaced 178mm apart, the metal frame spacing was 160mm, and the relaxation rate was 10%). The metal frame with the film attached was inserted through a small window (ESPEC STPH-102) in the door of a hot air oven (ESPEC STPH-102) with a 25mm high x 200mm wide opening, causing the film to shrink by 10% in the width direction. A 10cm x 10cm square sample was cut from the center of the film that had undergone the 10% shrinkage treatment described above. This sample was then immersed in 80°C ± 0.5°C hot water for 10 seconds under no load to induce heat shrinkage, followed by immersion in 25°C ± 0.5°C water for 10 seconds. After being removed from the water, the lengthwise and widthwise dimensions were measured, and the respective heat shrinkage rates were determined according to Equation 1 described above. The direction with the largest heat shrinkage rate was designated as the primary shrinkage direction.

[0065] [Glass transition temperature (Tg)] A sample film was weighed to 5.0 ± 0.2 mg and placed in a TA Instruments T-zero pan or aluminum pan (flat dish shape). The sample was then heated on a hot plate at 300°C for 30 seconds to melt it. After that, the sample was removed from the hot plate with tweezers and immediately immersed in liquid nitrogen for 1 minute to prepare a rapidly thawed sample. When sealing the sample in the pan, a circular punch (4.5 mm in diameter) made by overlapping films was used to ensure good adhesion to the bottom of the pan. "Good adhesion to the bottom of the pan" means that when the sample is sealed in the pan, there is no bending of the film sample inside the pan, and there are no gaps between the overlapping films when the lid is firmly pressed down. It is acceptable for the film to have wrinkles before punching, but it is preferable to smooth out the wrinkles when punching the sample. The shape and size of the punched sample (punch) are not limited to the above, as long as the sample does not bend and fits on the bottom of the pan. The prepared melt-quenched samples were measured using a temperature-modulated differential scanning calorimeter (DSC) "DSC250" (TA Instruments) in MDSC® heat-only mode with an average heating rate of 2°C / min and a modulation period of 40 seconds to obtain the reverse heat flow. In the reverse heat flow obtained from the measurement, the Tg (temperature change) was determined, which appears as a stepwise change in the signal from the baseline. Specifically, extensions of the baseline of each heat flow were drawn on the lower and higher temperature sides of Tg, the intersection point with the tangent line at the inflection point (Tg) was found, the value on the horizontal axis at this intersection point was read, and the average value of the lower and higher temperature sides was taken as Tg.

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

[0067] [Shrinkage properties] The edges of a heat-shrinkable film for laser printing were welded with dioxolane to obtain a cylindrical label (a label with the main shrinkage direction of the heat-shrinkable film being circumferential). This label was placed over a commercially available PET bottle (filled with contents; "Oi Ocha" manufactured by Ito En) and heat-shrinked 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 listed below include jumping up, wrinkling, insufficient shrinkage, folding of the label edges, shrinkage whitening, etc. 5: Excellent finish (no flaws) 4: Good finish (one flaw) 3: There are two drawbacks. 2: There are 3 to 5 flaws. 1: Has numerous flaws (more than 6)

[0068] [Laser printing evaluation: Film deformation (perforations, distortion)] A 1cm x 1cm grid was drawn onto a heat-shrinkable film for laser printing, and then the edges were welded with dioxolane to obtain a cylindrical label (a label with the main shrinkage direction of the heat-shrinkable film being circumferential). This label was placed over a commercially available PET bottle (filled with contents; "Oi Ocha" manufactured by Ito En) and heat-shrinked by passing it through steam adjusted to 85°C (tunnel passage time: 30 seconds). A heat-shrinkable label attached to a bottle was irradiated with a laser to print the letters "ABC123," and the deformation of the label was visually evaluated. A 355nm ultraviolet (UV) laser marker (MD-U1000, manufactured by Keyence Corporation) was used as the printing machine, 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 judged according to the following criteria. Result: No holes or distortions in the laser irradiation area. Judgment ×: There are holes or distortions 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 being circumferential). This label was placed over a commercially available PET bottle (filled with contents; "Oi Ocha" manufactured by Ito En) and heat-shrinked by passing it through steam adjusted to 85°C (tunnel passage time: 30 seconds). A heat-shrinkable label attached to a bottle was irradiated with a laser to print the letters "ABC123," and the print density was evaluated using color L*. A 355nm ultraviolet (UV) laser marker (MD-U1000, manufactured by Keyence Corporation) was used as the printing machine, 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, and the L* values ​​of both the printed and unprinted areas of a single laser-printed label were measured using the reflection method. The sample used for measurement was the flat portion of the label. The measurement method for the printed area was as follows: A 3cm square sample was cut out so that all the "B"s in the printed letters "12345ABCDE" were included in the measurement (other letters may be included). A 6φ sample stage (with an opening approximately 1cm in diameter where the measurement light hits) and a 6φ eyepiece were used as the measurement light source for the colorimeter, and the letter "B" was placed within the opening of the sample stage. If the printing does not fit within the opening of the sample stage (overflows), the sample stage may be changed as needed (e.g., 10φ, 30φ, etc.). Even if the printing overflows, it is sufficient if a part of the printing enters the opening of the sample stage and is illuminated by the measurement light. Furthermore, for the non-printed areas, a 3cm square sample was cut from the unprinted portion, and the color L* value was measured using a 6φ eyepiece and sample stage of the colorimeter. Note that the eyepiece and sample stage of the colorimeter may be changed to 10φ, 30φ, etc., as needed, and in that case, the sample size can be any size as long as it covers the opening of the sample stage (preventing leakage of measurement light).

[0071] [Laser printing evaluation: Print size] Of the characters printed as "12345ABCDE", the height and width of "345ABC" were measured visually in 0.5mm increments using a stainless steel straight ruler (KOKUYO TZ-RS15), and the average value was used 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 printing evaluation: Character legibility] The legibility of the letters "12345ABCDE" printed on the laminate was judged according to the following criteria. Judgment: ○ Characters can be recognized visually. Judgment: △ The characters can be recognized visually, but they are unclear. Judgment: Unable to recognize characters by visual inspection.

[0073] <Preparation of polyester raw materials> [Synthesis of polyester raw material A] In a stainless steel autoclave equipped with a stirrer, thermometer, and partial reflux condenser, 100 mol% dimethyl terephthalate (DMT) as the dicarboxylic acid component and 100 mol% ethylene glycol (EG) as the polyhydric alcohol component were charged so that the molar ratio of ethylene glycol was 2.2 times that of dimethyl terephthalate. 0.05 mol% zinc acetate (relative to the acid component) was added as a transesterification catalyst, and 0.225 mol% antimony trioxide (relative to the acid component) was added as a polycondensation catalyst. The transesterification reaction was carried out while distilling off the resulting methanol. Subsequently, a polycondensation reaction was carried out at 280°C under reduced pressure of 26.7 Pa to obtain 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-H and J] Polyesters B to J with the compositions shown in Table 1 were obtained using the same method as for polyester raw material A. During the production of polyester D, a bismuth-neodymium composite oxide pigment (TOMATEC COLOR 42-920A, manufactured by TOMATEC Corporation; average particle size 1 μm) was added to the polyester at a ratio of 5% by mass as a laser printing pigment. During the production of polyester E, a mica pigment coated with antimony-doped tin oxide (Iriotec(R) 8825, manufactured by MERCK; average particle size less than 15 μm) was added to the polyester at a ratio of 5% by mass as a laser printing pigment. During the production of polyester F, SiO2 (Silysia 266, manufactured by Fuji Silysia Co., Ltd.; average particle size 1.5 μm) was added as a lubricant at a ratio of 7,000 ppm relative 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 polymerization). Chemically recycled terephthalic acid was used for the polymerization of polyester raw material J. The intrinsic viscosities of each polyester were as follows: 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 processed into chips as appropriate.

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

[0077] [Table 1]

[0078] (Example 1) For the printing layer (layer A), polyester A, polyester D, and polyester I were mixed in a mass ratio of 85:3:12. For the layer B, polyester A, polyester F, and polyester I were mixed 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 from a T-die. The extruded resins were then cooled on a chill roll set to a surface temperature of 30°C to obtain an unstretched laminated film. The laminated film was configured so that the central layer was layer A and both surface layers were layer B (a 2-type, 3-layer structure of B / A / B), and 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 obtained unstretched film was guided to a transverse stretcher (tenter), and with both ends of the film held with clips, it was preheated until the film temperature reached 90°C. Then, at a film temperature of 85°C, it was stretched 4.5 times in the width direction. Next, it underwent heat treatment at 80°C and a 10% relaxation treatment in the width direction. Then, while cutting off both edges of the film, a uniaxially oriented film of approximately 40 μm was wound into a roll. The properties of the obtained film were evaluated using the method described above. The film formation conditions and evaluation results are shown in Table 2. The film was practically acceptable in terms of both shrinkage finish and laser printability evaluation.

[0080] (Example 2) An unstretched film with a thickness ratio of 90 / 10 (B / A / B = 5 / 90 / 5) between layers A and B was obtained in the same manner as in Example 1. The obtained unstretched film was guided to a longitudinal stretcher with multiple rolls arranged in a series. After preheating with a preheating roll until the film temperature reached 80°C, it was stretched 1.3 times in the longitudinal direction by utilizing the difference in rotational 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°C.

[0081] The longitudinally stretched film was guided to a transverse stretcher (tenter), and with both ends of the film held by clips, it was preheated until the film temperature reached 105°C. Then, at a film temperature of 88°C, it was stretched 4.5 times in the width direction. Next, heat treatment and a 15% relaxation treatment in the width direction were performed at 82°C. Then, while cutting off both edges of the film, a biaxially oriented film of approximately 20 μm was wound into a roll. The properties of the obtained film were evaluated using the method described above. The film formation conditions and evaluation results are shown in Table 2. The film showed no practical problems in terms of shrinkage finish and laser printability evaluation.

[0082] (Examples 3, 4, and 5) In the same manner as in Example 1, an unstretched film with a thickness ratio of 90 / 10 (B / A / B = 5 / 90 / 5) between layers A and B was obtained using the mixed polyester raw materials of layers A and B shown in Table 2. In the same manner as in Example 2, a uniaxially stretched film roll was obtained under the transverse stretching conditions shown in Table 2. The properties of the obtained film were evaluated using the method described above. The film formation conditions and evaluation results are shown in Table 2. The film was practically satisfactory in terms of both shrinkage finish and laser printability.

[0083] (Example 6) In the same manner as in Example 2, an unstretched film with a thickness ratio of 90 / 10 (B / A / B = 5 / 90 / 5) between layers A and B was obtained using the mixed polyester raw materials of layers A and B shown in Table 2. In the same manner as in Example 1, biaxially oriented film rolls were obtained under the longitudinal and transverse stretching conditions shown in Table 2. The properties of the obtained films were evaluated using the method described above. The film formation conditions and evaluation results are shown in Table 2. The films were practically acceptable in terms of both shrinkage finish and laser printability. (Example 7) An unstretched film with a thickness ratio of 90 / 10 (B / A / B = 5 / 90 / 5) between layers A and B was obtained in the same manner as in Example 1. The obtained unstretched film was guided to a longitudinal stretcher with multiple rolls arranged in a series. After preheating with a preheating roll until the film temperature reached 82°C, it was stretched 3.6 times in the longitudinal direction using the difference in rotational 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 was performed in the longitudinal direction at 78°C using the speed difference between the high-speed rotating roll and another low-speed rotating roll. Next, the 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 method described above. The results of the film formation condition evaluation are shown in Table 2. The film was deemed to have 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 a widthwise relaxation process was not performed after transverse stretching in the transverse stretching apparatus. The manufacturing 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 a transverse stretching apparatus, heat treatment was performed at 110°C and a 10% relaxation treatment was performed in the width direction. The manufacturing conditions and 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 longitudinal stretching, then stretched 4.5 times in the width direction at 85°C, followed by heat treatment and a 10% relaxation treatment in the width direction at 83°C. Next, the uniaxially stretched film of approximately 40 μm was wound into a roll while cutting off both edges of the film. The properties of the obtained film were evaluated using the method 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 an unstretched film. Under the conditions shown in Table 3, the unstretched film was longitudinally stretched 3.6 times at 85°C, then preheated to 140°C in a tenter, stretched 5.3 times in the width direction at 110°C, and then heat-treated at 95°C and relaxed by 10% in the width direction. Next, the uniaxially oriented film of approximately 40 μm was wound into a roll while cutting off both edges of the film. The properties of the obtained film were evaluated using the method described above. The evaluation results are shown in Table 2.

[0088] [Table 2A]

[0089] [Table 2B]

[0090] The heat-shrinkable film for laser printing in the embodiment of the present invention is a stretched film obtained by using a predetermined amount of at least one of butanediol, diethylene glycol, and ε-caprolactone and undergoing a predetermined relaxation process, and the laser printing on the product after shrinkage was free from unevenness, wrinkles, and distortion.

[0091] Comparative Example 1 had 0% lateral relaxation (no relaxation), Comparative Example 3 had a total amount of long-chain molecules butanediol, ε-caprolactone, and diethylene glycol of 1.8 mol%, and Comparative Example 4 had a stretching ratio of 3.6 in the longitudinal direction, which is not the main shrinkage direction. As a result, the reduction in shrinkage rate at 80°C after 10% shrinkage was lower than 11% in all 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, resulting in a film with inferior shrinkage finish. [Industrial applicability]

[0092] The laser-printed, heat-shrinkable polyester film of the present invention exhibits no unevenness, wrinkles, or distortion in the laser printing on the product after shrinkage, making it widely usable for applications such as label packaging, cap sealing, and bulk packaging that serve to protect glass bottles or plastic bottles while also displaying product information.

Claims

1. A heat-shrinkable polyester film for laser printing, comprising at least one laser-printable layer that changes color upon laser irradiation, wherein the laser-printable layer contains polyester with 50 mol% or more of ethylene terephthalate units, and satisfying the following conditions (1) to (4). (1) The shrinkage rate at 80°C in the main shrinkage direction is 30% or more and 80% or less. (2) After shrinking by 10% in the main shrinkage direction with 100°C hot air, the decrease in the 80°C shrinkage rate in the main shrinkage direction is 11% or more and 30% or less. (3) The transmittance at 355 nm in the ultraviolet-visible spectral spectrum is 65% or more and 90% or less. (4) A polyester comprising at least one of butanediol, diethylene glycol, tetramethylene glycol, and ε-caprolactone in an amount of 5 mol% to 30 mol% as a component of the polyester.

2. The heat-shrinkable polyester film for laser printing according to claim 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 heat-shrinkable polyester film for laser printing according to claim 1, characterized in that its thickness is 20 μm or more and 80 μm or less.

4. The laser-printable heat-shrinkable polyester film for laser printing according to claim 1, characterized in that the laser-printable layer that changes color upon laser irradiation contains one or more elements or compounds selected from the group consisting of bismuth, gadolinium, neodymium, titanium, antimony, tin, aluminum, calcium, and barium as a metal or metal compound that can be printed upon laser irradiation.

5. A display body made of a laser-printed heat-shrinkable polyester film according to any one of claims 1 to 4, wherein the difference in color L* values ​​between the laser-printed portion and the non-laser-printed portion is 1.0 or more and 10 or less.

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