Polyester film for thermal stencil paper

Abstract

To provide a film for thermal stencil printing that exhibits good dimensional stability and good perforation even at low energy levels. [Solution] A polyester film for thermal stencil paper in which the movable amorphous content of the polyester film is 40% or more and 85% or less.

Description

[Technical Field]

[0001] This invention relates to a polyester film for heat-sensitive stencil paper used in digital plate making, which is perforated by a thermal head or laser irradiation, or by flash irradiation using a halogen lamp, xenon lamp, flash lamp, etc. [Background technology]

[0002] Conventionally, thermal stencil printing bases have been known to have a structure in which a porous support made of natural fibers, chemical fibers, or synthetic fibers, or a mixture thereof, is bonded with an adhesive to a thermoplastic resin film such as polyvinylidene chloride film, polyester, or polypropylene film. These thermal stencil printing base films are perforated by flashing light irradiation from halogen lamps, xenon lamps, flash lamps, infrared irradiation, pulsed irradiation from laser beams, or by thermal heads, and become printing plates through which ink passes through the porous support.

[0003] Furthermore, in printing methods using thermal heads, attempts have been made to increase the number of heads per unit area by making individual heads smaller in order to obtain high resolution. However, by making the heads smaller, the energy supplied to each head is reduced, and even if the energy supplied to the heads per unit area is kept the same as conventional heads, the lifespan of individual heads decreases due to the increased density of the heads. In order to maintain the same lifespan as conventional heads, it is necessary to further reduce the energy supplied to each head, and there is a need for a thermal perforation printing base film that can be perforated sensitively with low energy, and in which ink held in a porous support can be reliably passed through the perforated holes, resulting in good resolution, print quality, and density during printing.

[0004] To improve sensitivity in this way, methods have been disclosed for controlling the melting point of the film to lower it (see Patent Documents 1 and 2), and for controlling the thermal shrinkage rate to make it easier to widen holes during perforation (see Patent Document 3). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 00 / 020490 [Patent Document 2] Japanese Patent Publication No. 2005-349586 [Patent Document 3] Japanese Patent Publication No. 2015-208944 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, the conventional technologies described above have drawbacks, such as insufficient perforation sensitivity in the low-energy range, resulting in reduced print clarity for both text and solid color printing. In other words, despite the requirement for perforation to be sensitive to temperature differences, conventional films have the problem of being slow to respond to temperature. In addition, those with a high thermal shrinkage rate have low dimensional stability, leading to problems with transportability and flatness when used as base paper for thermal stencil printing.

[0007] Therefore, the present invention aims to solve these problems and provide a film for thermal stencil printing that has good dimensional stability and good perforation even at low energy when used as a thermal stencil printing base paper. [Means for solving the problem]

[0008] To solve the above problems, a preferred embodiment of the present invention has the following configuration. (1) A polyester film for thermal stencil paper in which the movable amorphous content of the polyester film is 40% or more and 85% or less. (2) The polyester film for thermal stencil paper according to (1), wherein the intrinsic viscosity (IV) of the polyester film is 0.50 dL / g or more and 0.75 dL / g or less. (3) The polyester film for thermal stencil paper according to (1) or (2), wherein the resin in the polyester resin composition constituting the polyester film contains recovered raw materials and / or recycled raw materials. (4) The polyester film for thermal stencil paper according to (3), wherein the total mass of the resin in the polyester resin composition constituting the polyester film is 100% by mass, and the recovered raw material and / or recycled raw material having an intrinsic viscosity (IV) of 0.50 dL / g or more and 0.75 dL / g or less comprises 20% by mass or more and 50% by mass or less. (5) A polyester film for thermal stencil paper according to any one of (1) to (4), comprising a polyethylene terephthalate / isophthalate copolymer in the polyester resin composition constituting the polyester film. (6) A polyester film for thermal stencil paper according to any one of (1) to (5), wherein the total mass of the resin in the polyester resin composition constituting the polyester film is 100% by mass, and the content of polyethylene terephthalate / isophthalate copolymer is 40% by mass or more and 90% by mass or less. (7) A polyester film for heat-sensitive stencil paper according to any one of (1) to (6), wherein the heat shrinkage rate of the polyester film at 65°C for 60 minutes is 0.5% or more and 5.0% or less, and the heat shrinkage rate at 50°C for 24 hours is 1.0% or less. (8) A polyester film for thermal stencil paper according to any one of (1) to (7), wherein the total thickness of the polyester film is 1.0 μm or more and 4.0 μm or less. [Effects of the Invention]

[0009] The present invention provides a film for thermal stencil printing that exhibits good dimensional stability and good perforation even at low energy levels. [Modes for carrying out the invention]

[0010] A preferred embodiment of the present invention is a polyester film for a thermal stencil master paper, wherein the amount of mobile amorphous content of the polyester film is 40% or more and 85% or less. By adopting this embodiment, a film for a thermal stencil printing master paper with good perforability even at low energy can be obtained.

[0011] In view of the above problems, the inventors of the present invention conducted intensive studies and as a result, newly discovered the perforation mechanism.

[0012] Conventionally, in the polyester film for thermal stencil master paper, the perforation mechanism has been considered as follows as described in WO 00 / 20490: 1. The heating element touches the film. 2. The crystals of the film soften and melt. 3. Perforation starts and the perforation expands due to the thermal contraction of the film. 4. The film cools and solidifies, and the perforation ends. This has been the conventional understanding.

[0013] However, when observing the area where the film is locally heated using a micro heat source of the actual size used, it was newly discovered that the perforation proceeds by the following mechanism. Actually, 1. The heating element touches the film. 2. Multiple local strains occur due to the softening of the film. 3. Cracks occur due to the strain of the film and perforation starts. 4. The cracks of the film connect and the perforation expands due to the thermal contraction of the film. 5. The perforation ends due to the cooling and solidification of the film. It was found that the process proceeds through the above steps.

[0014] These cracks and perforations occurred when the film temperature was above the glass transition temperature (Tg) but below the melting point. Therefore, it was found that this differs from the previously thought scenario of perforation occurring after the film softened and then reached its melting point. Based on this, it was found that creating strain is extremely important for the initiation of perforation, and that film softening is a contributing factor. Of the crystalline and amorphous phases that make up the film, the amorphous phase plays a particularly important role in film softening, and it is thought that how the molecular motion of the amorphous phase occurs at temperatures above Tg but below the melting point is crucial. It is known that there are rigid amorphous and mobile amorphous phases (Wunderlich B, Thermal Analysis of Polymeric Materials, Springer, Berlin (2005)), but it is thought that the amount of mobile amorphous, where molecular motion becomes active at temperatures above Tg, holds the key to softening.

[0015] Polyester films, when subjected to biaxial stretching or heat treatment (thermal fixation), develop microcrystals and form nodal points, thereby constraining the molecular chains. Movable amorphous regions are the parts of the amorphous region where polymer chains are not constrained at nodal points and can move freely. When the film is heated, the amorphous chains begin to move, and because the movable amorphous regions have a higher degree of freedom of movement compared to amorphous chains constrained at nodal points, a larger amount of movable amorphous regions leads to faster softening, resulting in a film for heat-sensitive stencil printing with good perforation properties even at low energy.

[0016] The polyester film for thermal stencil paper of the present invention preferably has a movable amorphous content of 40% or more and 85% or less. It is believed that having a movable amorphous content within this range promotes the softening of the film.

[0017] The method for measuring the movable amorphous content shall be determined using temperature-modulated differential scanning calorimetry (mDSC) as described below.

[0018] From the viewpoint of achieving good perforation even at low energy, it is preferable that the amount of movable amorphous material be between 40% and 85%, and more preferably between 55% and 85%. If the amount of movable amorphous material is less than 40%, the amount of movable amorphous material in the film is insufficient, making it difficult to soften, and sufficient perforation may not be obtained at low energy. If the amount of movable amorphous material is greater than 85%, the film softens too much when heated, resulting in a deterioration of the heat-resistant dimensional properties of the film after heating. Furthermore, the flatness also deteriorates.

[0019] There are no particular limitations on how to achieve a movable amorphous content of 40% or more and 85% or less in a polyester film, but the following methods can be used. For example, this could involve performing biaxial stretching during film formation, lowering the heat treatment temperature and shortening the heat treatment time, setting the intrinsic viscosity of the polyester film to 0.50 dL / g or more and 0.75 dL / g or less, using a highly amorphous polyester resin, particularly a polyethylene terephthalate / isophthalate copolymer described later with a high copolymer concentration of isophthalic acid, in an amount of 40% to 90% by mass, and further using recovered raw materials with low molecular weight and a relatively narrow molecular weight distribution.

[0020] The polyester used in the polyester film of the present invention is a polyester whose main components are an aromatic dicarboxylic acid component or an aliphatic dicarboxylic acid component and a diol component. Examples of aromatic dicarboxylic acid components include terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, and 4,4'-diphenylsulfondicarboxylic acid. Examples of aliphatic dicarboxylic acid components include adipic acid, suberic acid, sebacic acid, and dodecanedionic acid. Terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, and adipic acid are particularly preferred. Furthermore, as diol components, for example, ethylene glycol, 1,2-propanediol, 1,3-propanediol, neopentyl glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, diethylene glycol, triethylene glycol, polyalkylene glycol, 2,2′bis(4′-β-hydroxyethoxyphenyl)propane, etc. can be used. Among these, ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, and 1,4-cyclohexanedimethanol are preferred.

[0021] The polyester used in the present invention may be copolymerized with other components besides the dicarboxylic acid and diol components that constitute the present invention, to the extent that they do not impede the effects of the present invention. Examples of such components include polyfunctional compounds such as trimellitic acid, trimesic acid, pyromellitic acid, tricarbaryl acid, trimethylolpropane, glycerin, and pentaerythritol, as well as oxycarboxylic acids such as p-oxybenzoic acid, lactic acid, and 3-hydroxybutanoic acid.

[0022] The main polyester resin constituting the polyester film of the present invention is preferably polyethylene terephthalate (PET). Polyethylene terephthalate may be homopolyethylene terephthalate or copolymerized polyethylene terephthalate. Examples of copolymerized polyethylene terephthalate include a copolymerized polyester containing isophthalic acid and terephthalic acid as dicarboxylic acid components and ethylene glycol as a diol component; a copolymerized polyester containing isophthalic acid as a dicarboxylic acid component and butylenediol as a diol component; a copolymerized polyester containing 2,6-naphthalene acid as a dicarboxylic acid component and butylenediol as a diol component; and a copolymerized polyester containing terephthalic acid as a dicarboxylic acid component and cyclohexanediol as a diol component. Furthermore, the main polyester resin constituting the polyester film refers to the polyester resin that accounts for 50% or more by mass of the polyester resin constituting the film.

[0023] In particular, the polyester film of the present invention preferably contains polyethylene terephthalate / isophthalate copolymer (PET-I). PET-I has the property of being less prone to crystallization and having a large amount of amorphous components. Therefore, even when subjected to biaxial stretching or heat treatment, microcrystals are less likely to form, the amorphous components can be secured, and molecular mobility can be maintained.

[0024] Furthermore, the copolymerization amount of PET-I is preferably such that the molar ratio of terephthalic acid component to isophthalic acid component is 90 / 10 to 60 / 40, and more preferably 85 / 15 to 70 / 35. This embodiment ensures the molecular mobility that is key to the movable amorphous mass.

[0025] Furthermore, when the total mass of the resin in the polyester resin composition constituting the polyester film is taken as 100% by mass, it is preferable that the content of PET-I copolymer is between 40% by mass and 90% by mass. A PET-I copolymer content of 40% by mass or more makes it easier to obtain perforations of appropriate size reliably with low energy, while a content of 90% by mass or less improves the heat-resistant dimensional stability of the film, thereby suppressing curling, resolution, and a decrease in print quality during the paper manufacturing process and storage of the paper.

[0026] In other words, in the polyester film for heat-sensitive stencil paper of the present invention, it is preferable that the isophthalic acid component is contained in an amount of 10 mol% to 40 mol% of the total dicarboxylic acid component.

[0027] The polyester used in the present invention can be produced, for example, by the following methods. Specifically, it can be produced by directly esterifying a dicarboxylic acid component with a diol component, then heating the product under reduced pressure to remove excess diol component while polycondensing, or by using a dialkyl ester as the dicarboxylic acid component, transesterifying it with the diol component, and then polycondensing it in the same manner as above. In this case, a reaction catalyst can be used as appropriate if necessary.

[0028] Furthermore, the polyester film of the present invention may optionally contain flame retardants, heat stabilizers, plasticizers, antioxidants, ultraviolet absorbers, antistatic agents, pigments, fatty acid esters, organic lubricants such as waxes, or defoaming agents such as polysiloxanes, and two or more of these may be used in combination.

[0029] Furthermore, the polyester film of the present invention may be given slipperiness as needed. There are no particular limitations on the method of imparting slipperiness, but examples include a method of blending inorganic particles such as clay, mica, titanium dioxide, calcium carbonate, kaolin, talc, wet or dry silica, acrylic acid polymers, organic particles composed of polystyrene, etc., a method using so-called internal particles formed by the deactivation of catalysts added during the polyester polymerization reaction, and a method of coating with a surfactant.

[0030] The polyester film of the present invention may be blended with polybutylene terephthalate (PBT) to form a polymer alloy. When PBT is included, it is preferable to include it in an amount of 40% by mass or less. When PBT is included, it acts like a plasticizer in PET-I, improving molecular mobility and increasing the amount of movable amorphous material. If the amount of PBT exceeds 40% by mass, the polyester film may experience increased shrinkage and decreased flatness, and furthermore, the crystallinity of the film increases due to the crystallinity of PBT, leading to a decrease in the amount of movable amorphous material.

[0031] The method of incorporating PBT into the polyester film of the present invention is not particularly limited, and for example, a method of blending PET and PBT can be used. Blending as used in the present invention means polymerizing the resins of the composition constituting the film and mixing the polymerized resins by melt kneading. It is also possible to include PBT-derived components in the polyester film by adding and copolymerizing components that make up PBT during the polymerization of PET, which is used as the main component of the polyester film, but this makes it difficult to express the resin properties such as crystallinity and flexibility that are characteristic of PBT. Therefore, by mixing by blending rather than copolymerization, the different properties of each resin can be expressed even when it is made into a film.

[0032] The polyester film of the present invention preferably has an intrinsic viscosity IV of the polyester resin constituting the film of 0.50 dL / g or more and 0.75 dL / g or less. Setting IV to 0.50 dL / g or more improves the heat-resistant dimensional stability of the film, suppresses the reduction in productivity due to film tearing and the adhesion of molten resin composition residue to thermal heads, etc., thereby suppressing the occurrence of perforation irregularities, and also suppresses the occurrence of curling during the paper manufacturing process and paper storage. Furthermore, setting IV to 0.75 dL / g or less increases the amount of movable amorphous material, which improves perforation performance at low energy. This is thought to be because the low molecular weight of the polymer chains reduces entanglement of the polymer chains, thereby improving the mobility of the amorphous chains.

[0033] From the above viewpoint, the intrinsic viscosity IV of the polyester resin constituting the film is more preferably 0.57 dL / g or more and 0.63 dL / g or less.

[0034] The polyester film of the present invention preferably contains 20% to 50% by mass of recovered and / or recycled raw materials, when the total mass of the resin in the polyester resin composition constituting the polyester film is taken as 100% by mass, and the intrinsic viscosity IV is 0.50 dL / g or more and 0.75 dL / g or less. More preferably, it is 20% to 40% by mass. By keeping the recovered and / or recycled raw materials within the above range, polymers with low molecular weight and a relatively narrow molecular weight distribution can be introduced into the polyester film, improving porosity at low energy.

[0035] If the recovered and / or recycled material accounts for more than 50% by mass, it may be undesirable because it not only reduces thermal shrinkage between 100°C and 130°C but also lowers productivity. Recovered material refers to waste film recovered and recycled within the company's own process (in-house recycled material), while recycled material refers to recycled products processed outside the company from waste film (pre-consumer recycled material), recycled products from used film by users, and recycled products from PET beverage bottles that have been distributed to the market (post-consumer recycled material). When using post-consumer recycled material, recycled material obtained through mechanical recycling is preferable from a cost perspective.

[0036] The polyester film of the present invention preferably has a heat shrinkage rate of 0.5% to 5.0% in both the MD direction (longitudinal direction of the film) and the TD direction (width direction of the film) at 65°C for 60 minutes. The heat shrinkage rate when heated to 65°C is related to the film's softening onset temperature and crystallinity. By setting the film's heat shrinkage rate within the above range, the processability of the film can be improved. If the heat shrinkage rate when heated to 65°C exceeds 5.0%, deformation at low temperatures becomes more likely, which may reduce the film's flatness and durability in practical use. In particular, flatness tends to deteriorate, and the film is more prone to sagging.

[0037] Furthermore, the thermal shrinkage rate at 65°C for 60 minutes is related to the curling of the printing master. In particular, when a porous support such as tissue paper is laminated onto the polyester film of the present invention to create a two-layer printing master (master), if the dimensions of the film change, the printing master will easily curl. If the curl becomes large, the handling of the printing master deteriorates, the transportability of the master in the stencil printing machine becomes poor, and it can cause problems such as master jams. Curling can occur due to distortion when laminating the film and the porous support, or during storage at room temperature or during transport. Also, because the master is not uniformly set on the printing cylinder, uneven printing is likely to occur. To improve the transportability of the master, it is better to reduce the curl in the MD direction, so it is preferable that the thermal shrinkage rate at 65°C for 60 minutes is particularly small in the MD direction. Hereinafter, the thermal shrinkage rate at 65°C for 60 minutes may be abbreviated as the 65°C thermal shrinkage rate.

[0038] Preferably, the polyester film of the present invention has a heat shrinkage rate of 1.0% or less at 50°C for 24 hours. The heat shrinkage rate at 50°C for 24 hours is assumed to be the heat shrinkage that occurs during aging treatment. When creating the master, lamination is performed by applying heat of about 40°C to 60°C. In addition, curling may occur due to shrinkage of the film due to thermal history such as long-term storage including the hot and humid summer period, or during transportation. To suppress heat shrinkage due to these factors, aging treatment may be performed. If the heat shrinkage rate at 50°C for 24 hours is 1.0% or less, a master that is less prone to curling can be obtained even after aging treatment. Hereinafter, the heat shrinkage rate at 50°C for 24 hours may be abbreviated as the 50°C heat shrinkage rate.

[0039] There are no particular limitations to the methods for keeping the thermal shrinkage rate within the above range, but some examples include not setting the longitudinal stretching temperature too low (not creating too much longitudinal orientation), balancing the longitudinal stretching ratio with the transverse stretching ratio, increasing the heat treatment temperature, and using the aforementioned resins.

[0040] The polyester film of the present invention preferably has a total thickness of 1.0 μm or more and 4.0 μm or less. By making the total film thickness 1.0 μm or more, print durability can be maintained while maintaining good perforation sensitivity in the low-energy region, thereby suppressing damage to the film used as a printing plate when printing a large number of copies, resulting in good film formation stability and winding properties in the film manufacturing process, and also suppressing deterioration of yield in the lamination process between the obtained film and a porous support such as tissue paper. On the other hand, by making it 4.0 μm or less, it is possible to improve perforation at low energy. More preferably, it is 1.0 μm or more and 3.0 μm or less, and even more preferably 1.2 μm or more and 2.5 μm or less.

[0041] The polyester film of the present invention is preferably a biaxially oriented film that has been biaxially stretched using the polymer described above. Unstretched films tend to have poor perforation characteristics and low film strength, resulting in poor print resistance. The stretching method is to obtain a biaxially stretched film using one of the following formulations: simultaneous inflation biaxial stretching, simultaneous stenter biaxial stretching, or sequential stenter biaxial stretching. Among these, films produced by sequential stenter biaxial stretching are preferred in terms of film formation stability and thickness uniformity.

[0042] The polyester film of the present invention can be manufactured using the above-mentioned polyester resin composition by the following method. Specifically, an unstretched film can be manufactured by extruding the resin composition onto a cast drum using a T-die extrusion method. Any of the following methods can be used for adhesion to the cast drum: electrostatic application method, adhesion method utilizing the surface tension of water, air knife method, press roll method, underwater casting method, etc. However, the adhesion casting method utilizing the surface tension of water or the electrostatic application method is particularly effective in obtaining a film with good flatness and few surface defects. When the polyester resin constituting the film contains a resin composition represented by polybutylene terephthalate, it is preferable to rapidly cool the molten resin composition by using a combination of the adhesion casting method utilizing the surface tension of water and the electrostatic application method to suppress crystallization at the cast film stage and not reduce subsequent stretchability. An unstretched film of the desired thickness can be produced by adjusting the slit width of the die, the discharge amount of the resin composition, and the rotation speed of the cast drum.

[0043] The stretching method is not particularly limited, but in the case of the Stentor sequential biaxial stretching method, the longitudinal stretching is performed at a temperature above the glass transition point of the polyester resin composition constituting the film, and the stretching ratio is appropriately determined depending on the type of resin composition used. Preferably, the unstretched film is preheated to a temperature of 40°C to 110°C, and then stretched in one or more stages to a length of 2.0 to 6.0 times, more preferably 3.5 to 5.0 times, in the longitudinal direction (MD direction) of the film while heating with a heating roll at a temperature of 90°C to 110°C, more preferably 95°C to 110°C. After that, it is cooled with a group of cooling rolls at 20 to 50°C.

[0044] Furthermore, the stretching ratio in the width direction and the stretching temperature are not particularly limited and are appropriately determined depending on the type of resin composition used. From the viewpoint of ensuring sufficient orientation during stretching and improving flatness, the stretching temperature is preferably 80°C to 130°C, more preferably 95°C to 130°C, and stretched to a ratio of 3.0 to 6.0 times, and more preferably 3.5 to 5.0 times.

[0045] Furthermore, after biaxial stretching, the film may be stretched again in the longitudinal direction, the width direction, or both.

[0046] Furthermore, after biaxial stretching, the polyester film of the present invention is preferably subjected to heat treatment while undergoing micro-stretching of 10% or less of the total width of the film in the constant length and / or width direction, from the viewpoint of dimensional stability of the film in the low-temperature range near room temperature and the flatness of the film. The heat treatment temperature is preferably in the range of 90°C to 140°C because it simultaneously satisfies dimensional stability in the low-temperature range, the flatness of the film, and the amount of movable amorphous material. A more preferable heat treatment temperature is 100°C to 135°C. The heat treatment time is preferably short, between 0.5 and 20 seconds. More preferably, it is between 0.5 and 10 seconds. If the heat treatment is performed at a high temperature for a long time, crystal formation is promoted and the amorphous component decreases. Furthermore, the movable amorphous component changes to a rigid amorphous component, which can reduce molecular mobility and prevent sufficient perforation performance from being obtained. Based on the above, it is preferable that the minute endothermic peak temperature (Tmeta) of the polyester film for heat-sensitive perforated paper of the present invention, as determined by differential scanning calorimetry (DSC), is 140°C or lower. The method for measuring Tmeta is as follows: Measurement is performed using a differential scanning calorimeter (DA Instruments DSC Q100) in the range of 30°C to 300°C at a heating rate of 20°C / min. The minute endothermic peak temperature before the PET crystal melting peak in the differential scanning calorimetry chart obtained by this measurement is defined as Tmeta (°C). Note that Tmeta appears as the history of the heat treatment temperature for the polyester film.

[0047] The polyester film of the present invention has good thermal perforation properties and can therefore be suitably used for heat-sensitive perforation paper. In particular, because it has good perforation properties even at low energy, when the energy supplied to the thermal head is 12 μJ per dot, the average perforation area per dot is 300 μm². 2 The above-mentioned thermal perforation paper can be used more suitably. In addition, when the energy supplied to the thermal head is 8 μJ per dot, the average perforation area per dot is 150 μm².2 The above-mentioned thermal stencil paper can be used more suitably.

[0048] [Methods for evaluating physical properties and effects] Each characteristic used in this invention is measured and evaluated by the following method.

[0049] (1) Movable amorphous amount Using a temperature-modulated DSC (TA Instrument Q1000), the temperature is increased on average by repeatedly heating and cooling at a constant period and amplitude under the following conditions. The overall DSC signal (total heat flow) is separated into reversible components such as glass transition and irreversible components such as enthalpy relaxation and desolvation. The glass transition temperature (Tg) and the specific heat difference around Tg (ΔCp) are read from the heating process of the resulting curve, and the movable amorphous mass (Xa) is determined by substituting these values ​​into the following equation. Xa(%) = ΔCp / ΔCp0 × 100 ΔCp0: Difference in specific heat around Tg of a perfectly amorphous material (theoretical value distributed according to composition ratio) Theoretical value: PET 0.4052 J / g / ℃ PBT 0.3497 J / g / ℃ Atmosphere: Nitrogen (50 mL / min) Temperature and heat calibration: High-purity indium (Tm 156.61℃, ΔHm = 28.71 J / kg) Temperature range: -10℃ to 280℃ Heating rate: 2°C / min Cycle: 60s Temperature amplitude: ±1℃ Sample quantity: approximately 5 mg Sample container: Standard aluminum container

[0050] The theoretical value of ΔCp0 was calculated by distributing the literature values ​​according to their composition ratios, and PET-I was also treated as PET in the calculation. The following literature was consulted. Reference: Wunderlich B, “Thermal Analysis of Polymeric Materials”, Springer, Berlin (2005)

[0051] For film sampling, if using a film roll, samples are taken from three locations (center and both ends) at 5m intervals, and the movable amorphous amount (Xa) is calculated for each sample using the method described above. The arithmetic mean of N=15 is then taken to obtain the movable amorphous amount. If using a cut sample, five samples are taken from one cut sample, from three locations (center and both ends), using different films each time. The movable amorphous amount (Xa) is calculated for each sample using the method described above, and the arithmetic mean of N=15 is then taken to obtain the movable amorphous amount.

[0052] (2) Intrinsic viscosity (IV) The film of the present invention is dissolved in 100 mL of orthochlorophenol (solution concentration C = 1.2 g / dL), and the viscosity of the solution at 25°C is measured using an Ostwald viscometer. The viscosity of the solvent is also measured in the same manner. Using the obtained solution viscosity and solvent viscosity, [η] (dL / g) is calculated using the following formula (a), and the obtained value is taken as the intrinsic viscosity (IV). ηsp / C = [η] + K[η] 2 ·C ···(a) (Here, ηsp = (solution viscosity (dl / g) / solvent viscosity (dl / g)) - 1, and K is the Huggins constant (assumed to be 0.343).)

[0053] (3) Film thickness (μm) The film thickness is determined by using a dial gauge and measuring the thickness at five arbitrary points on a stack of 10 films, in accordance with JIS K7130 (1992) A-2 method. The average value is then divided by 10 to obtain the film thickness.

[0054] (4) Heat shrinkage rate at 50°C Cut the film into strips 10 mm wide and 100 mm long in both an arbitrary direction and a direction perpendicular to it, and measure the length of each test piece. Heat-treat the test pieces by placing them in a hot air oven set to 50°C for 24 hours in a tension-free state, and measure the length of the test pieces after heating. A load of 3 g is applied only when measuring the length of the test pieces. Calculate the 50°C heat shrinkage rate from the average value of the dimensional change of the test pieces. 50°C heat shrinkage rate [%] = 100 × (AB) / A A: Dimensions of the sample piece before heat treatment B: Dimensions of the sample piece after heat treatment

[0055] (5) Heat shrinkage rate at 65°C Cut the film into pieces with a width of 10 mm and a length of 100 mm in each of an arbitrary direction and the direction perpendicular to it, and measure the length of the test piece. Place it in a hot air oven set at a temperature of 65°C for 1 hour in a tension-free state for heat treatment, and measure the length of the test piece after heating. Apply a load of 3 g only when measuring the length of the test piece. Calculate the heat shrinkage rate at 65°C from the average value of the dimensional changes of the test piece. Heat shrinkage rate at 65°C [%] = 100×(A - B) / A A: Dimensions of the sample piece before heat treatment B: Dimensions of the sample piece after heat treatment

[0056] (6) Perforation characteristics Attach Japanese paper with a basis weight of 10 g / m² made of 100% natural fibers with Manila hemp as the raw material to the obtained film using vinyl acetate as an adhesive to produce a base paper for thermal stencil printing. Supply such a base paper for thermal stencil printing to "RISOGRAPH" (registered trademark) "GR377" manufactured by Ideal Scientific Industry Co., Ltd., and make a 5 mm square solid black plate in a grid pattern by a thermal head plate-making method (600 dpi). At this time, set the energy input to the thermal head to 12 μJ and 8 μJ per dot. Perforate under each condition, observe 100 perforated parts of the film at a magnification of 200 times with a scanning microscope, and measure the area of the perforated parts of the film. Obtain the average value and standard deviation of the perforated area per dot, and evaluate the perforation characteristics (also referred to as perforability) using the following items. Both sensitivity and variation are ◎, ○, and △ are suitable for practical use. A. Perforation sensitivity ◎: Average perforated area is 450 μm 2 or more ○: Average perforated area is 300 μm 2 or more and less than 450 μm 2 △: Average perforated area is 150 μm 2 or more and less than 300 μm 2 ​​​​ ×: Average perforation area is 150 μm 2 Less than B. Variation in perforation Perforation variability = 10 × log(average perforation area) 2 Standard deviation of perforation area 2 ) ◎: Variation score of 15 or higher ○: Variability of 10 or more but less than 15 △: Variability of 5 or more but less than 10 ×: Variability is less than 5

[0057] (7) Flatness The prepared stencil paper is cut to 300mm vertically and 300mm horizontally, and the lifting of the four corners is evaluated in the following three stages when it is placed on a horizontal surface at room temperature with the film side facing up. ○: The lift at all four corners is less than 5mm. △: The corners are lifted by 5mm or more but less than 10mm. ×: The corners are lifted by more than 10mm.

[0058] (8) Transportability (resistance to curling) After treating the prepared stencil paper in a constant temperature and humidity chamber at 50°C and 90% RH for one week, a paper transport test is performed using a printing press, and the results are evaluated according to the following criteria. ○: Almost no curl, or slight curl, but can be transported well. △: Although there is some curling, it can be transported without any practical problems. ×: The curl is large, causing frequent transport problems.

[0059] (9) Film forming properties The film-forming properties of the film will be evaluated according to the following criteria. ○: No film tears occurred for more than 48 hours, indicating stable film formation. △: Film tearing occurred 1-2 times in 48 hours, indicating slightly poor film formation. ×: More than 3 film tears occurred in 48 hours, indicating poor film formation. [Examples]

[0060] The present invention will be described in more detail below with reference to examples, etc., but the present invention is not limited thereto.

[0061] (raw materials) • Polyester resin A A polyester resin (PET-I) with an intrinsic viscosity of 0.65 was obtained by polymerization using dimethyl terephthalate / dimethyl isophthalate (molar ratio: 75 / 25) as the dicarboxylic acid raw material and ethylene glycol as the glycol, by conventional methods.

[0062] • Polyester resin A' A polyester resin (PET-I) with an intrinsic viscosity of 0.75 was obtained by polymerizing dimethyl terephthalate / dimethyl isophthalate (molar ratio: 75 / 25) as the dicarboxylic acid raw material and ethylene glycol as the glycol using a conventional method, resulting in a product containing 0.5% by mass of silica particles with an average particle size of 1.5 μm.

[0063] • Polyester resin B A polyester resin (PET-I) with an intrinsic viscosity of 0.70 was obtained by polymerization using dimethyl terephthalate / dimethyl isophthalate (molar ratio: 83 / 17) as the dicarboxylic acid raw material and ethylene glycol as the glycol, by conventional methods.

[0064] • Polyester resin B' A polyester resin (PET-I) with an intrinsic viscosity of 0.85 was obtained by polymerizing dimethyl terephthalate / dimethyl isophthalate (molar ratio: 83 / 17) as the dicarboxylic acid raw material and ethylene glycol as the glycol using a conventional method, resulting in a product containing 0.5% by mass of silica particles with an average particle size of 1.5 μm.

[0065] • Polyester resin C A polyester resin (PET-I) with an intrinsic viscosity of 0.55 and terminal carboxyl groups of 35 equivalents / ton was polymerized using dimethyl terephthalate / dimethyl isophthalate (molar ratio: 65 / 35) as the dicarboxylic acid raw material and ethylene glycol as the glycol by a conventional method.

[0066] • Polyester resin D Dimethyl terephthalate was used as the dicarboxylic acid raw material, and ethylene glycol was used as the glycol. Polymerization was carried out by a conventional method to obtain a polyester resin (PET) containing 0.5% by mass of silica particles with an average particle size of 1.5 μm, an intrinsic viscosity of 0.65, and a melting point of 270°C.

[0067] • Polyester resin E A polyester resin (PBT) with an intrinsic viscosity of 0.60 and a melting point of 220°C was obtained by polymerization using dimethyl terephthalate as the dicarboxylic acid raw material and 1,4-butanediol as the glycol by a conventional method.

[0068] • Recovered raw material F Film edges and unusable films (which did not become products for any reason) generated during the production of a film consisting of 90% by mass of polyester resin A and 10% by mass of polyester resin D were collected, finely shredded, melted, and granulated in a pelletizer to obtain a recovered raw material F with an intrinsic viscosity of 0.58.

[0069] ·Recovered raw materials G Film edges and unusable films (which did not become products for any reason) generated during the production of a film consisting of 90% by mass of polyester resin B and 10% by mass of polyester resin D were recovered, finely shredded, melted, and granulated in a pelletizer to obtain recovered raw material G with an intrinsic viscosity of 0.65.

[0070] [Example 1] Polyester resin A (hereinafter simply referred to as A; the same applies to others) was blended at 70% by mass, D at 10% by mass, and recovered raw material F (hereinafter simply referred to as F; the same applies to others) at 20% by mass, and dried under reduced pressure at 125°C for 24 hours, according to the types and amounts of components listed in Table 1. The mixture was then fed into an extruder and melted at 250°C. It was then cooled and solidified in a sheet form using electrostatic application on a casting drum at 25°C using a T-type die, obtaining an unstretched film. Next, the unstretched film was heated to 95°C and stretched 4.6 times in the longitudinal direction. Then, it was heated to 95°C and stretched 4.5 times in the width direction, followed by heat treatment at 100°C for 4 seconds, and cooled to obtain a 1.5 μm biaxially oriented film.

[0071] The resulting film is bonded to vinyl acetate as an adhesive, using 100% natural fibers made from Manila hemp, with a fiber basis weight of 10g / m². 2 A master sheet for thermal stencil printing was prepared by laminating it with Japanese paper. The amount of adhesive applied was 1 g / m². 2 Based on the above method, a perforation test and curl evaluation were performed, and the results, along with the characteristics of the obtained film, are shown in Table 1.

[0072] [Example 2] The materials were blended at 58% by mass of B, 10% by mass of D, and 32% by mass of G, according to the types and amounts of components listed in Table 1, and dried under reduced pressure at 125°C for 24 hours. The mixture was then fed into an extruder and melted at 260°C. The mixture was then cooled and solidified in a sheet form using electrostatic application on a casting drum at 25°C via a T-type die, obtaining an unstretched film. Next, the unstretched film was heated to 105°C and stretched 4.8 times in the longitudinal direction. Then, it was heated to 100°C and stretched 4.0 times in the width direction, followed by heat treatment at 110°C for 3 seconds and cooling to obtain a 1.7 μm biaxially oriented film. A master paper for thermal stencil printing was prepared from the obtained film in the same manner as in Example 1. A perforation test and curl evaluation were performed based on the method described above, and the results, along with the characteristics of the obtained film, are shown in Table 1.

[0073] [Example 3] The materials were blended at 125°C under reduced pressure for 24 hours, with B at 42% by mass, D at 8% by mass, and G at 50% by mass, according to the types and amounts of components listed in Table 1. The mixture was then fed into an extruder and melted at 260°C. The mixture was then cooled and solidified in a sheet form using electrostatic application on a casting drum at 25°C using a T-type die, obtaining an unstretched film. Next, the unstretched film was heated to 110°C and stretched 4.8 times in the longitudinal direction. Then, it was heated to 100°C and stretched 4.0 times in the width direction. After heat treatment at 110°C for 3 seconds and cooling, a 1.7 μm biaxially oriented film was obtained. A master paper for thermal stencil printing was prepared from the obtained film in the same manner as in Example 1. A perforation test and curl evaluation were performed based on the method described above, and the results, along with the characteristics of the obtained film, are shown in Table 1.

[0074] [Example 4] The materials were blended with 50% by mass of C, 30% by mass of E, and 20% by mass of F, according to the types and amounts of components listed in Table 1, and dried under reduced pressure at 125°C for 24 hours. The mixture was then fed into an extruder and melted at 250°C. The mixture was then cooled and solidified in a sheet form using electrostatic application on a casting drum at 25°C via a T-type die, obtaining an unstretched film. Next, the unstretched film was heated to 90°C and stretched 4.0 times in the longitudinal direction. Then, it was heated to 85°C and stretched 4.1 times in the width direction, followed by heat treatment at 100°C for 4 seconds and cooling to obtain a 1.5 μm biaxially oriented film. A master paper for thermal stencil printing was prepared from the obtained film in the same manner as in Example 1. A perforation test and curl evaluation were performed based on the method described above, and the results, along with the characteristics of the obtained film, are shown in Table 1.

[0075] [Example 5] The materials were blended at 30% by mass of A', 50% by mass of E, and 20% by mass of F, according to the types and amounts of components listed in Table 1, and dried under reduced pressure at 125°C for 24 hours. The mixture was then fed into an extruder and melted at 250°C. The mixture was then cooled and solidified in a sheet form using electrostatic application on a casting drum at 25°C via a T-type die, obtaining an unstretched film. Next, the unstretched film was heated to 95°C and stretched 4.0 times in the longitudinal direction. Then, it was heated to 85°C and stretched 4.2 times in the width direction, followed by heat treatment at 95°C for 4 seconds and cooling to obtain a 1.5 μm biaxially oriented film. A master paper for thermal stencil printing was prepared from the obtained film in the same manner as in Example 1. A perforation test and curl evaluation were performed based on the method described above, and the results, along with the characteristics of the obtained film, are shown in Table 1.

[0076] [Example 6] The materials were blended at 60% by mass of A', 20% by mass of E, and 20% by mass of F, according to the types and amounts of components listed in Table 1, and dried under reduced pressure at 125°C for 24 hours. The mixture was then fed into an extruder and melted at 250°C. The mixture was then cooled and solidified in a sheet form using electrostatic application on a casting drum at 25°C via a T-type die, obtaining an unstretched film. Next, the unstretched film was heated to 100°C and stretched 4.0 times in the longitudinal direction. Then, it was heated to 95°C and stretched 4.0 times in the width direction, followed by heat treatment at 100°C for 3 seconds and cooling to obtain a 1.5 μm biaxially oriented film. A master paper for thermal stencil printing was prepared from the obtained film in the same manner as in Example 1. A perforation test and curl evaluation were performed based on the method described above, and the results, along with the characteristics of the obtained film, are shown in Table 1.

[0077] [Comparative Example 1] The materials were blended at 125°C for 24 hours under reduced pressure, using the types and amounts of components listed in Table 1, with 10% by mass of A', 65% by mass of E, and 25% by mass of F. The mixture was then fed into an extruder and melted at 250°C. The mixture was then cooled and solidified in a sheet form using electrostatic application on a casting drum at 25°C via a T-type die, obtaining an unstretched film. Next, the unstretched film was heated to 80°C and stretched 3.5 times in the longitudinal direction. Then, it was heated to 90°C and stretched 4.5 times in the width direction. After heat treatment at 100°C for 4 seconds, it was cooled to obtain a 1.5 μm biaxially oriented film. A master paper for thermal stencil printing was prepared from the obtained film in the same manner as in Example 1. A perforation test and curl evaluation were performed based on the method described above, and the results, along with the characteristics of the obtained film, are shown in Table 1.

[0078] [Comparative Example 2] The materials were blended at 125°C under reduced pressure for 24 hours, using the types and amounts of components listed in Table 1, with 80% by mass of B' and 20% by mass of E. The mixture was then fed into an extruder and melted at 250°C. The mixture was then cooled and solidified in a sheet form using electrostatic application on a casting drum at 25°C via a T-type die, obtaining an unstretched film. Next, the unstretched film was heated to 73°C and stretched 4.0 times in the longitudinal direction. Then, it was heated to 80°C and stretched 3.7 times in the width direction. After heat treatment at 100°C for 5 seconds, it was cooled to obtain a 1.5 μm biaxially oriented film. A master paper for thermal stencil printing was prepared from the obtained film in the same manner as in Example 1. A perforation test and curl evaluation were performed based on the method described above, and the results, along with the characteristics of the obtained film, are shown in Table 1.

[0079] [Comparative Example 3] The materials were blended at 22% by mass of B, 3% by mass of D, and 75% by mass of G, according to the types and amounts of components listed in Table 1, and dried under reduced pressure at 125°C for 24 hours. The mixture was then fed into an extruder and melted at 260°C. The mixture was then cooled and solidified in a sheet form using electrostatic application on a casting drum at 25°C via a T-type die, obtaining an unstretched film. Next, the unstretched film was heated to 90°C and stretched 4.8 times in the longitudinal direction. Then, it was heated to 100°C and stretched 4.0 times in the width direction. After heat treatment at 145°C for 5 seconds and cooling, a 4.5 μm biaxially oriented film was obtained. A master paper for thermal stencil printing was prepared from the obtained film in the same manner as in Example 1. A perforation test and curl evaluation were performed based on the method described above, and the results, along with the characteristics of the obtained film, are shown in Table 1.

[0080] [Comparative Example 4] A' was dried under reduced pressure at 125°C for 24 hours at a concentration of 100% by mass to achieve the types and amounts of components listed in Table 1. It was then fed into an extruder and melted at 260°C. The film was then cooled and solidified in a sheet form using electrostatic application on a casting drum at 25°C via a T-type die to obtain an unstretched film. Next, the unstretched film was heated to 105°C and stretched 4.8 times in the longitudinal direction. Then, it was heated to 82°C and stretched 4.0 times in the width direction, after which it was cooled without heat treatment to obtain a 1.7 μm biaxially oriented film. A master paper for thermal stencil printing was prepared from the obtained film in the same manner as in Example 1. A perforation test and curl evaluation were performed based on the method described above, and the results, along with the characteristics of the obtained film, are shown in Table 1.

[0081] [Summary of evaluation results] Examples 1 to 6 were films with excellent perforation sensitivity, perforation variability, transportability (curl resistance), and film formation properties (productivity) because their movable amorphous mass, heat shrinkage rate at 50°C for 24 hours, and heat shrinkage rate at 65°C for 60 minutes were within a suitable range.

[0082] Comparative Examples 1-3 were films that exhibited poor puncture properties at low energy, were prone to curling due to high thermal shrinkage, and had poor film-forming properties. Comparative Example 4 was a film with poor transportability and flatness due to an excessively high amount of movable amorphous material.

[0083] Table 1

Claims

1. A polyester film for thermal stencil paper, wherein the movable amorphous content of the polyester film is 40% or more and 85% or less.

2. The polyester film for thermal stencil paper according to claim 1, wherein the intrinsic viscosity (IV) of the polyester film is 0.50 dL / g or more and 0.75 dL / g or less.

3. The polyester film for thermal stencil paper according to claim 1 or 2, wherein the resin in the polyester resin composition constituting the polyester film comprises recovered raw materials and / or recycled raw materials.

4. The polyester film for thermal stencil paper according to claim 3, wherein, when the total mass of the resin in the polyester resin composition constituting the polyester film is taken as 100% by mass, the recovered raw material and / or recycled raw material having an intrinsic viscosity (IV) of 0.50 dL / g or more and 0.75 dL / g or less comprises 20% by mass or more and 50% by mass or less.

5. The polyester film for heat-sensitive perforated paper according to claim 1 or 2, wherein the polyester resin composition constituting the polyester film comprises a polyethylene terephthalate / isophthalate copolymer.

6. The polyester film for heat-sensitive stencil paper according to claim 5, wherein, when the total mass of the resin in the polyester resin composition constituting the polyester film is taken as 100% by mass, the content of the polyethylene terephthalate / isophthalate copolymer is 40% by mass or more and 90% by mass or less.

7. The polyester film for heat-sensitive stencil paper according to claim 1 or 2, wherein the heat shrinkage rate of the polyester film at 65°C for 60 minutes is 0.5% or more and 5.0% or less, and the heat shrinkage rate at 50°C for 24 hours is 1.0% or less.

8. The polyester film for thermal stencil paper according to claim 1 or 2, wherein the total thickness of the polyester film is 1.0 μm or more and 4.0 μm or less.

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