JP7501207B2 - Polyester film for thermal stencil printing base paper - Google Patents

Description

The present invention relates to a polyester film for thermal stencil printing base paper.

Conventionally, heat-sensitive stencil printing base paper has been known to have a structure in which a porous support made of natural fibers, chemical fibers, synthetic fibers, or mixtures of these, such as tissue paper, nonwoven fabric, or gauze, is bonded with an adhesive to a thermoplastic resin film such as vinylidene chloride film, polyester, or polypropylene film. These heat-sensitive stencil printing base paper films are perforated by flash irradiation from a halogen lamp, xenon lamp, flash lamp, etc., infrared irradiation, pulsed irradiation from a laser beam, or a thermal head, etc., to become a printing plate through which ink passes through the porous support.

In addition, in printing methods using thermal heads, etc., attempts have been made to make each head smaller and increase the number of heads per unit area in order to obtain high resolution. However, even if the energy supplied to each head is reduced by making the heads smaller and the energy supplied to the head per unit area is the same as that of conventional heads, the life of each head is reduced due to the density of the heads. In order to maintain the head life at the same level as conventional heads, it is necessary to further reduce the energy supplied to each head. There is a demand for a film for thermal stencil printing base paper that can be perforated sensitively with low energy and that allows the ink held in the porous tissue paper to pass reliably through the perforated holes, thereby providing good resolution, print quality, and density during printing.

In order to improve this sensitivity, methods have been disclosed in which the melting point of the film is controlled to be lowered (Patent Documents 1 and 2), and the thermal shrinkage rate is controlled to make it easier for holes to expand when punched (Patent Document 3).

WO 00/020490 JP 2005-349586 A JP 2015-208944 A

However, the above-mentioned conventional techniques have the drawback that the perforation sensitivity is insufficient in the low energy region, resulting in a decrease in print clarity of character printing and solid printing. That is, although perforation is required to be sensitive to temperature differences, conventional films have a problem of slow response to temperature.
Therefore, an object of the present invention is to solve such problems and to provide a polyester film for thermal stencil printing base paper which, when used as a film for thermal stencil printing base paper, has good perforation properties even with low energy and excellent curl resistance.

In order to solve the above problems, the present invention has the following configuration: A polyester film for thermal stencil paper, the melting point of which is 180°C or more and 220°C or less, and the molecular weight distribution (weight average molecular weight/number average molecular weight (Mw/Mn)) is 2.0 or more and 3.5 or less.

When used as a polyester film for thermal stencil printing base paper, the present invention can provide a film for thermal stencil printing base paper that has good perforation properties even with low energy.

The polyester used in the polyester film of the present invention is a polyester whose main components are an aromatic dicarboxylic acid or an aliphatic dicarboxylic acid and a diol. Examples of aromatic dicarboxylic acids 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'-diphenylsulfonedicarboxylic acid. Examples of aliphatic dicarboxylic acids that can be used include adipic acid, suberic acid, sebacic acid, and dodecanedioic acid. Among these, terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, and adipic acid are preferably used. Examples of the diol component that can be used include 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. Among these, ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, and 1,4-cyclohexanedimethanol are preferably used.

The polyester used in the present invention may be copolymerized with other components than the dicarboxylic acid component and diol component constituting the present invention, as long as the effect of the present invention is not impaired. Examples of such components that can be used include polyfunctional compounds such as trimellitic acid, trimesic acid, pyromellitic acid, tricarballylic acid, trimethylolpropane, glycerin, and pentaerythritol, and oxycarboxylic acids such as p-oxybenzoic acid, lactic acid, and 3-hydroxybutanoic acid.

As the main polyester resin constituting the polyester film of the present invention, polyethylene terephthalate is preferred. The polyethylene terephthalate may be homopolyethylene terephthalate or copolymerized polyethylene terephthalate. In the present invention, copolymerized polyethylene terephthalate refers to a polyester in which the ratio of ethylene glycol to the total diol components is 50 mol% or more, and the ratio of terephthalic acid to the total dicarboxylic acid components is 50 mol% or more. Examples of copolymerized polyethylene terephthalate include polyethylene terephthalate/isophthalate copolymers containing isophthalic acid as an acid component, polyethylene terephthalate/butylene isophthalate copolymers containing butylene diol as a diol component, polyethylene terephthalate/butylene-2,6-naphthalate copolymers containing 2,6-naphthalene acid as an acid component and butylene diol as a diol component, and polyethylene terephthalate/cyclohexanedimethylene terephthalate copolymers containing cyclohexanediol as a diol component. The main polyester resin constituting the polyester film is particularly preferably a polyethylene terephthalate/isophthalate copolymer, and the copolymerization amount is preferably a molar ratio of terephthalic acid component/isophthalic acid component of 99/1 to 60/40, and more preferably 98/2 to 70/30. The main polyester resin constituting the polyester film refers to a polyester resin that accounts for 50% by mass or more of the polyester resins constituting the film.

The polyester used in the present invention can be produced, for example, by the following methods. That is, a method in which a dicarboxylic acid component is directly esterified with a diol component, and then the product of this reaction is heated under reduced pressure to remove excess diol component while polycondensing, or a method in which a dialkyl ester is used as the dicarboxylic acid component, and this is subjected to an ester exchange reaction with a diol component, and then polycondensed in the same manner as above. In this case, a reaction catalyst can be appropriately used as necessary.

The polyester film of the present invention preferably has a melting point of 180°C or more and 220°C or less. If the melting point is higher than 220°C, a large amount of energy is required to obtain a perforation of the desired size, so that the perforation ability at low energy is reduced and the high perforation sensitivity aimed at by the present invention cannot be obtained. If the melting point is lower than 180°C, the heat resistance dimensional stability of the film is deteriorated, or adjacent holes are connected to each other during perforation, and the desired perforation cannot be obtained, resulting in a decrease in resolution and print quality. More preferably, the melting point is 183°C or more and 217°C or less, and even more preferably, 185°C or more and 215°C or less. The melting point in the present invention represents the peak top temperature of the crystal melting peak in the differential scanning calorimetry chart measured using a differential scanning calorimetry device in the measurement method described below. When there are multiple crystal melting peaks, the peak top temperature of the crystal melting peak with the highest temperature is taken as the melting point. Methods for making the melting point of the polyester film fall within the above range include using a polyester having a melting point within the above range as the film raw material, as well as using two or more polyesters having different melting points as raw materials and melt-mixing the two or more polyesters to obtain a polyester film. For example, if a polyester film is obtained after thoroughly melt-mixing two polyester raw materials having different melting points, the melting point of the polyester film will be between the two melting points of the polyester raw materials or lower than either of the melting points of the polyester raw materials.

The polyester film of the present invention preferably contains a polyethylene terephthalate/isophthalate copolymer and polybutylene terephthalate and/or polytrimethylene terephthalate in the polyester resin constituting the polyester film. By adopting such a configuration, it is possible to particularly improve the perforation property, resolution, and printing quality. The polybutylene terephthalate referred to here may be homopolybutylene terephthalate or copolymerized polybutylene terephthalate. It is more preferable that the polybutylene terephthalate is a polybutylene terephthalate having 10 mol% or less of diol components other than butylene diol and 10 mol% or less of dicarboxylic acid components other than terephthalic acid, and it is even more preferable that the polybutylene terephthalate is homopolybutylene terephthalate. In addition, the polytrimethylene terephthalate may be homopolytrimethylene terephthalate or copolymerized polytrimethylene terephthalate. Polytrimethylene terephthalate having 10 mol% or less of diol components other than 1,3-propanediol and 10 mol% or less of dicarboxylic acid components other than terephthalic acid is more preferable, and homopolytrimethylene terephthalate is even more preferable.

In addition, when the mass of the entire resin in the polyester resin composition constituting the polyester film is taken as 100 mass%, it is preferable that the polyethylene terephthalate/isophthalate copolymer is 40 mass% or more and 90 mass% or less, and the polybutylene terephthalate and/or polytrimethylene terephthalate is 10 mass% or more and 60 mass% or less. When the content of the polyethylene terephthalate/isophthalate copolymer is 40 mass% or more, perforations having a certain size and a low energy can be easily obtained, and when it is 90 mass% or less, the heat resistance dimensional stability of the film is good, and curling during the process of manufacturing the base paper or during storage of the base paper, and deterioration of the resolution and printing quality can be suppressed. In addition, when the content of the polybutylene terephthalate and/or polytrimethylene terephthalate is 10 mass% or more, perforations having a certain size and a low energy can be easily obtained, and when it is 60 mass% or less, deterioration of the stretchability during film manufacturing and breakage during production can be suppressed.

The polyester film of the present invention preferably has a molecular weight distribution (weight average molecular weight/number average molecular weight (Mw/Mn)) of 2.0 or more and 3.5 or less. The smaller the Mw/Mn, the narrower the molecular weight distribution. More preferably, it is 2.0 or more and 3.2 or less, and even more preferably, it is 2.0 or more and 3.0 or less. As a result of intensive research by the present inventors, it was found that when a polyester film is used as a polyester film for a thermal stencil base paper, Mw/Mn has a large effect on perforation properties. By making Mw/Mn 2.0 or more, that is, by having a certain distribution of the molecular weight of the polyester constituting the polyester film (having a certain amount of low molecular weight polyester and high molecular weight polyester, respectively), the high molecular weight polyester contained in the film suppresses curling that occurs during the process of manufacturing the base paper and during storage of the base paper, while the low molecular weight polyester creates a trigger for perforation, making it easier to obtain perforations that are reliable and of a moderate size with low energy. On the other hand, by setting the Mw/Mn to 3.5 or less, i.e., by keeping the molecular weight distribution of the polyester that makes up the polyester film below a certain level, the amount of low molecular weight polyester components does not become too high, and the variation in perforation can be suppressed.

In addition, the "Mw" and "Mn" values in the present invention can be measured as polymethyl methacrylate equivalent values using GPC (gel permeation chromatography) in the measurement method described below.

The polyester film of the present invention preferably has a weight average molecular weight (Mw) of 12,000 or more and 25,000 or less. When Mw is 12,000 or more, the heat resistance dimensional stability of the film is improved, and it is possible to suppress a decrease in resolution and print quality due to adjacent holes being connected to each other during perforation, which prevents the desired perforation from being obtained. In addition, by setting Mw to 25,000 or less, it is possible to suppress a decrease in perforation ability at low energy. More preferably, it is 12,500 or more and 22,400 or less, and even more preferably, it is 13,000 or more and 23,000 or less.

The polyester film of the present invention preferably has a number average molecular weight (Mn) of 4,000 or more and 8,000 or less. When Mn is 4,000 or more, the heat resistance dimensional stability of the film is improved, and it is possible to suppress a decrease in resolution and print quality due to adjacent holes being connected to each other during perforation and not being able to obtain the desired perforation. Furthermore, by setting Mn to 8,000 or less, it is possible to suppress a decrease in perforation ability at low energy. It is more preferably 4,100 or more and 7,500 or less, and even more preferably 4,300 or more and 7,000 or less.

There are no particular limitations on the method for setting the "Mw/Mn", "Mw", and "Mn" of the polyester film within the above ranges, but examples include a method of using a polyester with adjusted "Mw" and "Mn" as the film raw material, as well as a method of using two or more polyesters with adjusted "Mw" and "Mn" as raw materials, melt-mixing the two or more polyesters to obtain a polyester film.

The method for obtaining the polyester resin composition used in the polyester film of the present invention is not particularly limited, and can be obtained by a conventionally known polymerization method. When using multiple polyester resins, a method of mixing other polyester resins in a polymerization reaction vessel containing the main polyester resin and then continuing polymerization, or a method of dry blending multiple polyester resins and melt extruding them, etc. can be used.

The polyester resin composition used in the polyester film of the present invention may contain, as necessary, flame retardants, heat stabilizers, plasticizers, antioxidants, ultraviolet absorbers, antistatic agents, pigments, organic lubricants such as fatty acid esters and waxes, or defoamers such as polysiloxanes, or two or more of these may be used in combination.

Furthermore, slipperiness can be imparted to the polyester resin composition used in the polyester film of the present invention as necessary. There are no particular limitations on the method for imparting slipperiness, but examples include a method of blending inorganic particles such as clay, mica, titanium oxide, calcium carbonate, kaolin, talc, wet or dry silica, organic particles containing acrylic acid polymers, polystyrene, etc. as components, a method using so-called internal particles formed by the inactivation of a catalyst added during the polyester polymerization reaction, and a method of applying a surfactant.

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 thickness of the film 1.0 μm or more, it is possible to maintain printing durability while maintaining good perforation sensitivity in the low energy region, thereby suppressing damage to the film that becomes the plate when printing a large number of copies, improving film formation stability and winding properties in the production of the film, and also suppressing deterioration of yield in the lamination process of the obtained film and a porous support such as tissue paper. On the other hand, by making the thickness 4.0 μm or less, it is possible to improve perforation properties in the low energy region. More preferably, it is 1.0 μm or more and 3.0 μm or less, and even more preferably, it is 1.2 μm or more and 2.5 μm or less.

The polyester film of the present invention is preferably a biaxially oriented film obtained by biaxially stretching the above-mentioned polymer. Unstretched films tend to have poor perforation characteristics because they melt when perforated but do not form holes, and have poor printing durability due to their low film strength. As for the stretching method, the film is biaxially stretched by any of the following methods: inflation simultaneous biaxial stretching method, stenter simultaneous biaxial stretching method, and stenter sequential biaxial stretching method. Among these, films formed by the stenter sequential biaxial stretching method are preferably used in terms of film formation stability and thickness uniformity.

The polyester film of the present invention can be produced by the following method using the above polyester resin composition. That is, an unstretched film can be produced by extruding the resin composition onto a casting drum by a T-die extrusion method. Any of the electrostatic application method, the adhesion method using the surface tension of water, the air knife method, the press roll method, the underwater casting method, etc. may be used as a method for adhering to the casting drum, but the adhesion casting method using the surface tension of water or the electrostatic application method is particularly effective as a method for 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 combining the adhesion casting method using the surface tension of water and the electrostatic application method in order to suppress crystallization at the cast film stage and not to reduce the 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 casting drum.

There are no particular limitations on the stretching method, but in the case of the stenter sequential biaxial stretching method, the longitudinal stretching is performed at a temperature equal to or higher than the glass transition point of the polyester resin composition that constitutes the film, and the stretching ratio is appropriately determined depending on the type of resin composition used, but is preferably about 2 to 5 times.

The stretching ratio and stretching temperature in the width direction are not particularly limited and are appropriately determined depending on the type of resin composition used, but a stretching ratio of about 2 to 5 times is preferable, and the stretching temperature is preferably equal to or higher than the longitudinal stretching temperature, which results in good stretchability.

After biaxial stretching, the film may be stretched again in the longitudinal direction, the transverse direction, or both.

Furthermore, it is preferable that the polyester film of the present invention, after biaxial stretching, is heat-treated at a constant length and/or while being slightly stretched in the width direction by 10% or less of the total width of the film, from the viewpoint of the 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 within the range of the following formula (1), since it satisfies both the dimensional stability in the low temperature range and the flatness of the film.

Ttd≦Ths≦Ttd+30 (1)
(where Ttd is the stretching temperature in the width direction (°C), and Ths is the heat treatment temperature (°C))
The heat treatment temperature is more preferably Ttd≦Ths≦Ttd + 20. The heat treatment time is preferably 0.5 to 60 seconds.
[Methods for evaluating physical properties and effects]
The characteristics used in the present invention were measured and evaluated by the following methods.

(1) Melting Point According to JIS K7121 (1999), the measurement is carried out using a differential scanning calorimeter "ROBOT DSC-RDC220" manufactured by Seiko Instruments Inc. and a disk session "SSC/5200" for data analysis, as follows.

5 mg of sample is weighed into a sample pan, the sample is heated from 25°C to 300°C at a heating rate of 10°C/min, held in that state for 5 minutes, and then cooled to 25°C at a rate of 10°C/min to obtain differential scanning calorimetry charts (with thermal energy on the vertical axis and temperature on the horizontal axis) for the heating process from 25°C to 300°C (1st RUN) and the cooling process from 300°C to 25°C (2nd RUN). In the differential scanning calorimetry chart for the 1st RUN, the peak top temperature of the crystal melting peak is determined and this is taken as the melting point (Tm) (°C). Note that when there are multiple crystal melting peaks, the peak top temperature of the highest crystal melting peak is taken as the melting point.

(2) Film thickness (μm)
The film thickness was measured using a dial gauge at any five points on ten stacked films in accordance with JIS K7130 (1992) A-2 method, and the average value was divided by 10 to obtain the film thickness.

(3) Mw (weight average molecular weight) / Mn (number average molecular weight)
The weight average molecular weight (Mw) and number average molecular weight (Mn) were measured using GPC under the following conditions: A differential refractive index detector was used and was calibrated with a monodisperse polymethyl methacrylate standard sample.
Equipment: Gel permeation chromatography (GPC)
Detector: Differential refractive index detector RI (Waters RI-2410, sensitivity 256)
Column: Shodex HFIP-LG (1), HFIP-806M (2)
(φ8.0mm x 30cm, made by Showa Denko)
Solvent: hexafluoroisopropanol containing 5 mM sodium trifluoroacetate Flow rate: 0.5 mL/min
Column temperature: 40°C
Sample preparation: 5 mL of solvent was added to 3 mg of sample and stirred at room temperature.
Then, filtration was carried out using a 0.20 μm filter.
Injection volume: 0.2 mL
Standard sample: Monodisperse polymethyl methacrylate (PMMA) manufactured by Showa Denko
(4) Perforation characteristics A thermal stencil printing base paper was prepared by bonding a Japanese paper with 100% natural fibers made from Manila hemp and a fiber weight of 10 g/ ^m2 to the obtained film using vinyl acetate as an adhesive. The thermal stencil printing base paper was fed to RISOGRAPH "GR377" manufactured by Riso Kagaku Corporation, and a lattice-shaped 5 mm square black solid was made by a thermal head type plate making method (600 dpi). At this time, the energy input to the thermal head was 12 μJ and 8 μJ per dot. In this state, perforations were made, and 100 perforated parts of the film were observed at a magnification of 200 times with a scanning microscope, and the area of the perforated parts of the film was measured. The average value and standard deviation of the perforation area per dot were obtained, and the perforation characteristics were evaluated in the following items. Sensitivity and variation are both suitable for practical use with ◎, ○, and △.
A. Perforation sensitivity : Average perforation area is ⁴⁵⁰ μm2 or more.
◯: Average perforation area is 300 μm ² or more and less than 450 μm ² .
Δ: The average perforation area is 150 μm ² or more and less than 300 μm ² .
×: Average perforation area is less than 150 μm ² .
B. Perforation Variability Perforation Variability = 10 x log (average perforation area ² / standard deviation of perforation area ² )
⊚: The degree of variation is 15 or more. ◯: The degree of variation is 10 or more and less than 15.
Δ: The degree of variation is 5 or more and less than 10.
x: The degree of variation is less than 5.

(5) Evaluation of Transportability (Curl) The prepared stencil sheets were treated in a thermo-hygrostat at 50° C. and 90% RH for one week, and then a transport test of the stencil sheets was carried out using a printing machine and evaluated according to the following criteria.
◯: Almost no curl, or some curl but still able to be conveyed well. Δ: Some curl but still able to be conveyed without practical problems. ×: Curl is significant and conveying problems occur frequently.

(6) Film-forming ability The film-forming ability of the film was evaluated according to the following criteria.
◯: No film breakage occurred for 48 hours or more, and film formation was stable. △: Film breakage occurred 1 to 3 times within 48 hours, and film formability was slightly poor. ×: Film breakage occurred 4 or more times within 48 hours, and film formability was poor.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited thereto.
(material)
・PET-I (polyethylene terephthalate/isophthalate copolymer)
A polyester having a dicarboxylic acid component of dimethyl terephthalate/dimethyl isophthalate (molar ratio: 75/25) and a glycol component of ethylene glycol was used as the main component, and polymerized by a conventional method to obtain a polyester resin (PET-I) having Mw: 14700, Mn: 5000 and containing 0.5 mass% of particles having an average particle size of 1.5 μm as contained silica particles.

- PET/CHDC (polyethylene terephthalate/cyclohexane dimethylene terephthalate copolymer)
A polyester having a dicarboxylic acid component of dimethyl terephthalate/dimethyl 1,4-cyclohexanedicarboxylate (molar ratio: 75/25) and a glycol component of ethylene glycol was used as the main component, and polymerized by a conventional method to obtain a polyester resin (PET/CHDC) having Mw: 20,000, Mn: 6,500 and containing 0.5 mass% of particles having an average particle size of 1.5 μm as contained silica particles.

・PBT (Polybutylene terephthalate)
The polyester, which is the main component, was polymerized in a conventional manner using a dicarboxylic acid component of dimethyl terephthalate and a glycol component of 1,4-butanediol to obtain a polyester resin (PBT) having Mw: 15600, Mn: 5200 and a melting point of 220°C.

・PTT (Polytrimethylene terephthalate)
The polyester, which is the main component, was polymerized in a conventional manner using a dicarboxylic acid component of dimethyl terephthalate and a glycol component of 1,3-propanediol to obtain a polyester resin (PTT) having Mw: 14,800, Mn: 5,800 and a melting point of 225°C.

[Example 1]
50% by mass of PET-I and 50% by mass of PBT were blended to obtain the types and amounts of the components shown in Table 1, and treated in boiling water for 3 hours to crystallize the surface, and then vacuum dried at 125°C for 24 hours. Thereafter, the mixture was fed to an extruder and melted at 240°C, and was then cooled and solidified in a sheet-like manner on a casting drum at 25°C by electrostatic application from a T-shaped die to obtain an unstretched film. The unstretched film was then heated to 85°C and stretched 3.5 times in the longitudinal direction. It was then heated to 90°C and stretched 3.7 times in the transverse direction, and then heat-treated at 100°C for 5 seconds, and cooled to obtain a biaxially stretched film of 1.7 μm.

The obtained film was laminated with Japanese paper, made of 100% natural fibers made from Manila hemp and having a fiber weight of 10 g/ ^m2 , using vinyl acetate as an adhesive to prepare a thermal stencil printing base paper. The amount of adhesive applied was 1 g/ ^m2 . Perforation tests and curl evaluations were carried out according to the above-mentioned methods, and the results are shown in Table 1 together with the properties of the obtained film.
[Examples 2 to 11, Comparative Examples 1 to 4]
Biaxially oriented polyester films were obtained in the same manner as in Example 1, except that the raw material types and mixing ratios in Example 1 were changed to have Mw and Mn, and the final thickness of the polyester film was changed as shown in Table 1.
[Summary of evaluation results]
The films of Examples 1 to 11 had Mw/Mn in a suitable range and were excellent in perforation sensitivity, perforation variation and curl resistance.

In Comparative Examples 1 to 4, the mixing ratio of Example 1 was changed, and as a result, the perforation sensitivity, perforation variation, and curl were insufficient.

The polyester film of the present invention can be used for heat-sensitive stencil sheets, but the range of applications is not limited thereto.

Claims (6)

A polyester film for thermal stencil paper having a melting point of 180°C or higher and 220°C or lower, and a molecular weight distribution (weight average molecular weight/number average molecular weight (Mw/Mn)) of 2.0 or higher and 3.5 or lower. The polyester film for thermal stencil paper according to claim 1, wherein the weight average molecular weight (Mw) of the polyester film is 12,000 or more and 25,000 or less. The polyester film for thermal stencil paper according to claim 1 or 2, wherein the number average molecular weight (Mn) of the polyester film is 4,000 or more and 8,000 or less. The polyester film for thermal stencil paper according to any one of claims 1 to 3, wherein the polyester resin composition constituting the polyester film contains a polyethylene terephthalate/isophthalate copolymer, polybutylene terephthalate and/or polytrimethylene terephthalate. A polyester film for thermal stencil paper according to claim 4, in which, when the total mass of the resin in the polyester resin composition constituting the polyester film is taken as 100 mass%, the polyethylene terephthalate/isophthalate copolymer is 40 mass% or more and 90 mass% or less, and the polybutylene terephthalate and/or polytrimethylene terephthalate is 10 mass% or more and 60 mass% or less. 6. The polyester film for heat-sensitive stencil paper according to claim 1, wherein the total thickness of said polyester film is from 1.0 μm to 4.0 μm.