JP2009214471A - Polyester film for thermal transfer ink ribbon - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polyester film for a thermal transfer ink ribbon which exhibits an extremely small shrinkage of the ribbon and is suitable for preventing wrinkles from being generated in the time of thermal transfer even in the thermal transfer printer which has a weak back tension on the forwarding side of the thermal transfer ink ribbon and tends to store heat. <P>SOLUTION: The polyester film for the thermal transfer ink ribbon is characterized in that its pore volume is 0.50-1.50 ml/g, a porous silica particle with a mean particle diameter of 2.6-4.5 μm with 0.04-0.15 wt.% is incorporated therein, the maximum shrinking stress in the film longitudinal direction at 23-230°C is 10-24 g/cm, and the heat shrinkage factor of the film in the width direction at 200°C is -2.0 to 2.5%. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a polyester film for a thermal transfer ink ribbon in a thermal transfer recording system.

  The thermal transfer recording method is excellent in cost performance and maintenance, has a high printing speed and is easy to operate, and is used in the fields of FAX, barcode printing, and digital photo printing using the thermal transfer recording method. In particular, since digitalization of images such as photographs has progressed, the development for digital image printing and the like is remarkable, and the market is rapidly expanding.

  In the thermal transfer recording method, the thermal transfer ink ribbon and the thermal transfer ink surface of the thermal transfer ink ribbon are overlapped with the recording body, the thermal transfer ink ribbon is heated by pressing the thermal head from the opposite side of the thermal transfer ink surface, and the thermal transfer ink is melted or sublimated on the recording medium. In this method, an image or the like is formed on a recording medium by transferring.

  Some conventional polyester films for thermal transfer ink ribbons have improved workability and printability by defining the thermal shrinkage rate and protrusions on the film surface (see, for example, Patent Document 1). In addition, there is one that prevents printing wrinkles by defining a temperature dimensional change rate in the width direction (see Patent Document 2).

However, in the conventional polyester film for thermal transfer ink ribbon, the back tension on the thermal transfer ink ribbon feeding side is weak, and it is difficult to suppress wrinkles to a high degree in an inexpensive thermal transfer printer that easily stores heat.
JP 2002-36736 A JP-A-10-264337

  The present invention has been made in view of the above circumstances, and the problem to be solved is that, even in a thermal transfer printer where the back tension on the thermal transfer ink ribbon feeding side is weak and heat storage is easy, the ribbon shrinkage is as small as possible, and the thermal transfer is An object is to provide a polyester film for a thermal transfer ink ribbon suitable for preventing the occurrence at a low cost.

  As a result of intensive studies in view of the above circumstances, the present inventor has found that the above-mentioned problems can be easily solved by using a polyester film having a specific configuration, and has completed the present invention.

  That is, the gist of the present invention is that 0.04 to 0.15% by weight of porous silica particles having a pore volume of 0.50 to 1.50 ml / g and an average particle diameter of 2.6 to 4.5 μm. And the maximum shrinkage stress in the film longitudinal direction at 23 to 230 ° C. is 10 to 24 g / cm, and the film heat shrinkage in the width direction at 200 ° C. is −2.0 to 2.5%. It exists in the polyester film for thermal transfer ink ribbons.

Hereinafter, the present invention will be described in more detail.
In the present invention, the polyester film constituting the polyester film for the thermal transfer ink ribbon may be a single layer structure or a laminated structure. For example, as long as it does not exceed the gist of the present invention other than a two-layer or three-layer structure. There may be four or more layers, and there is no particular limitation.

  In the present invention, the polyester used for the biaxially oriented polyester film may be a homopolyester or a copolyester. In the case of a homopolyester, those obtained by polycondensation of an aromatic dicarboxylic acid and an aliphatic glycol are preferred. Examples of the aromatic dicarboxylic acid include terephthalic acid and 2,6-naphthalenedicarboxylic acid, and examples of the aliphatic glycol include ethylene glycol, diethylene glycol, and 1,4-cyclohexanedimethanol. Representative polyester includes polyethylene terephthalate (PET) and the like.

  On the other hand, the dicarboxylic acid component of the copolyester includes isophthalic acid, phthalic acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid, oxycarboxylic acid (for example, P-oxybenzoic acid, etc.), etc. 1 type or 2 types or more are mentioned, As a glycol component, 1 type or 2 types or more, such as ethylene glycol, diethylene glycol, propylene glycol, butanediol, 1, 4- cyclohexane dimethanol, neopentyl glycol, is mentioned.

  In any case, the polyester referred to in the present invention refers to a polyester which is polyethylene terephthalate or the like in which 60 mol% or more, preferably 80 mol% or more is an ethylene terephthalate unit.

  The intrinsic viscosity of such polyester is usually 0.50 or more, preferably 0.55 or more, and more preferably 0.58 or more with respect to the lower limit. When the intrinsic viscosity is less than 0.50, there may be a problem that productivity at the time of film production is lowered or the mechanical strength of the film is lowered. On the other hand, when the intrinsic viscosity exceeds 1.0, melt extrusion of the polymer becomes unstable, and the thickness unevenness of the film tends to deteriorate. The intrinsic viscosity is preferably 1.0 or less, more preferably 0.80 or less.

  In the biaxially oriented polyester film of the present invention, the layer that forms at least the outermost layer on the thermal head side contains particles, but is not necessarily limited to a single layer, and may be a multilayer. For example, the outermost layer is the A layer, and the A layer is a layer other than the single layer, and the B layer having a small content even if it contains substantially no particles in order to increase the film strength (A / B). Moreover, a particle | grain can also be made to contain in C layer as a structure of (A / B / C).

  The film thickness of the present invention is generally 12 μm or less, preferably 10 μm or less, more preferably 8 μm, in order to efficiently transfer the heat of the thermal head to the thermal transfer ink and to make the thermal transfer ink ribbon cassette compact. It is as follows. On the other hand, if it is less than 1 μm, the thermal transfer ink ribbon tends to be easily broken. The minimum of the preferable film thickness is 1 micrometer or more, More preferably, it is 2 micrometers or more.

  The polyester layer forming the A layer needs to contain porous silica particles mainly for the purpose of dispersing the slide and running tension of the thermal head during thermal transfer. As long as it does not exceed the gist of the present invention, it may contain a plurality of particles and is not particularly limited. There is no particular limitation as long as the particle diameter is smaller than that of porous silica particles and the generation of voids is small, and specific examples include particles of silica, calcium carbonate, aluminum oxide, titanium oxide, and the like. . Further, the heat-resistant organic particles described in JP-B-59-5216, JP-A-59-217755 and the like may be used. Examples of other heat-resistant organic particles include thermosetting urea resins, thermosetting phenol resins, thermosetting epoxy resins, benzoguanamine resins, and the like. Furthermore, precipitated particles obtained by precipitating and finely dispersing a part of a metal compound such as a catalyst during the polyester production process can also be used.

  In the present invention, the smaller the pore volume of the porous silica particles, the harder the particles, and the lower limit of the pore volume is 0.50 ml / g or more, preferably 0.80 ml / g or more, more preferably Is 1.00 ml / g or more. When the pore volume is less than 0.50 ml / g, voids generated during film formation become larger, and particles fall off and adhere to the thermal head during thermal transfer. On the other hand, if the pore volume exceeds 1.50 ml / g, the particles become flat at the time of film formation, making winding difficult, so the upper limit of the pore volume is 1.50 ml / g.

  The lower limit of the average particle size of the porous silica particles is 2.6 μm or more, preferably 2.8 μm or more, and more preferably 3.0 μm or more. When the average particle size is less than 2.6 μm, it is difficult to wind the film. On the other hand, when the average particle size exceeds 4.5 μm, voids generated during film formation become larger, and particles drop off during thermal transfer and adhere to the thermal head, so the upper limit of the average particle size is 4.5 μm. More preferably, it is 4.0 μm or less.

  In the present invention, the content of the porous silica particles is 0.04% by weight or more, preferably 0.05% by weight or more, and more preferably 0.06% by weight or more. When the content is less than 0.04% by weight, it is difficult to wind the film. On the other hand, if the content exceeds 0.15% by weight, the gloss of the surface of an image formed by thermal transfer is deteriorated, so the upper limit of the content is 0.15% by weight, preferably 0.13% by weight.

  In the production of the polyester containing the additive particles, the particles may be added during the polyester synthesis reaction or directly to the polyester. When added during the synthesis reaction, a method in which the particles are dispersed in ethylene glycol or the like as a slurry and added at any stage of the polyester synthesis is preferable. On the other hand, when added directly to the polyester, a method of adding and mixing to the polyester using a twin-screw kneading extruder as dry particles or as a slurry dispersed in water or an organic solvent having a boiling point of 200 ° C. or less is preferable. . In addition, the particles to be added may be subjected to a treatment such as crushing, dispersion, classification, and filtration before being necessary.

  As a method for adjusting the content of particles, a master raw material containing particles at a high concentration is prepared by the above-described method, and at the time of film formation, it is diluted with a raw material that does not substantially contain particles, and the particle content is reduced. It is effective to adjust this.

  Further, as additives other than the above-described protrusion forming agent, an antistatic agent, a stabilizer, a lubricant, a crosslinking agent, an antiblocking agent, an antioxidant, a colorant, a light blocking agent, an ultraviolet absorber, etc. May be contained within a range not impairing the effects of the present invention.

  In the film of the present invention, the maximum shrinkage stress in the film longitudinal direction at 23 to 230 ° C. is 10 g / cm or more, preferably 15 g / cm or more, and more preferably 17 g / cm or more. When the maximum shrinkage stress is less than 10 g / cm, the thermal transfer ink ribbon is stretched in the longitudinal direction due to the winding tension of the thermal transfer ink ribbon, easily contracts in the width direction, and wrinkles are likely to occur. On the other hand, if the maximum shrinkage stress exceeds 24 g / cm, the thermal transfer ink ribbon tends to shrink on the thermal transfer ink ribbon feed side, and wrinkles are likely to occur. The upper limit of the maximum shrinkage stress is preferably 22 g / cm, more preferably 20 g / cm.

  The heat shrinkage rate in the film width direction after treating the film of the present invention at 200 ° C. for 15 minutes is −2.0% or more, preferably −1.0% or more, more preferably −0.5% or more. When the thermal shrinkage rate is less than −2.0, the thermal transfer ink ribbon is easily damaged in the width direction on the printing paper, and wrinkles are easily generated. On the other hand, if the thermal shrinkage rate exceeds 2.5%, the thermal transfer ink ribbon is reduced in width on the thermal transfer ink ribbon feed side, and wrinkles are likely to occur. The upper limit of the heat shrinkage is preferably 2.0%, more preferably 1.0%.

Next, the manufacturing method of the film of this invention is demonstrated concretely.
First, a polyester raw material is supplied to an extrusion apparatus. That is, it extrudes as a molten sheet from a slit-shaped die. Next, the molten sheet is rapidly cooled and solidified on the rotary cooling drum so as to have a temperature equal to or lower than the glass transition temperature to obtain a substantially amorphous non-oriented sheet. In this case, in order to improve the flatness of the sheet, it is preferable to improve the adhesion between the sheet and the rotary cooling drum. In the present invention, the electrostatic application adhesion method and / or the liquid application adhesion method are preferably employed.

  In the present invention, the sheet thus obtained is stretched biaxially to form a film. Specifically describing the biaxial stretching conditions, the unstretched sheet is stretched in the longitudinal direction (MD direction) at a stretching temperature of 80 to 150 ° C. and a stretching ratio of 2.5 to 4.5 times. Stretching can be performed in one step or in two or more steps. Next, the uniaxially oriented film is once cooled below the glass transition point in the width direction, that is, in the direction orthogonal to the first axis direction, or preheated to a temperature range of, for example, 80 to 150 ° C. The film is stretched 3.5 to 5 times, preferably 4.0 to 4.7 times under the same temperature, and stretched within 30%, limited shrinkage, or constant length, 200 to 250 ° C., 0.3 seconds. The maximum shrinkage stress of the biaxially oriented film can be kept within the above range by performing the heat treatment for ˜5 minutes.

  In addition, it is preferable to perform the stretching in the first axial direction in two or more stages because good thickness uniformity can be achieved. In addition, a method of re-stretching in the longitudinal direction after transverse stretching is also possible. Also, so-called simultaneous biaxial stretching, in which stretching in the first axial direction and the direction perpendicular thereto, is simultaneously possible.

  Moreover, after heat processing, it is set as the range of 1-10% relaxation in the range of 190-170 degreeC, and relaxation can be performed in one step or two steps or more. Then, the heat shrinkage rate of the film can be brought close to zero by holding and cooling the film in the width direction until the film temperature becomes 100 ° C. or lower.

  A film satisfying the requirements of the present invention can be obtained by appropriately selecting the stretching conditions, heat treatment conditions, and relaxation conditions described above.

  In addition, in the range which does not inhibit the effect of this invention, according to various end uses, in order to provide adhesiveness with the layer which contacts, a coating layer can be provided in the film surface. In order to impart this adhesion, a polymer (binder polymer) comprising at least one selected from the group consisting of an aqueous polyester polymer and an aqueous acrylic polymer is particularly useful.

  Also, urea, melamine, guanamine, and acrylamide that are methylolated or alkylolated as a crosslinkable compound to improve adhesion resistance (blocking resistance), water resistance, solvent resistance, and mechanical strength of the coating layer Compounds, polyamide compounds, epoxy compounds, aziridine compounds, block polyisocyanates, silane coupling agents, titanium coupling agents, zircoaluminate coupling agents, peroxides, thermal and photoreactive vinyl compounds Or a photosensitive resin. Further, if necessary, it may contain an antifoaming agent, a coating property improver, a thickener, an antistatic agent, an organic lubricant, an antioxidant, an ultraviolet absorber, a foaming agent, a dye, a pigment, and the like. .

  As a method of applying the above-mentioned coating solution to a polyester film, Yuji Harasaki, Tsuji Shoten, published in 1979, reverse roll coater, gravure coater, rod coater, air doctor coater or other coating apparatus shown in "Coating system" Can be used.

  In the present invention, from the viewpoint of cost, in order to provide a coating layer, a so-called in-line coating method in which coating is performed in the film manufacturing process is recommended.

  As a method of coating within the film production process, a coating solution is applied to an unstretched polyester film, and a method of biaxially stretching sequentially or simultaneously, coating to a uniaxially stretched polyester film, and further perpendicular to the previous uniaxially stretched direction. There is a method of stretching in the direction of the above, or a method of applying to a biaxially stretched polyester film and then stretching in the transverse and / or longitudinal direction.

  According to the film of the present invention, a polyester film for a thermal transfer ink ribbon that has high resistance to environment and has few defects in printed matter such as photographs due to wrinkles at high speed even in thermal transfer at any environmental temperature that has been desired in recent years. And its industrial value is high.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a following example, unless the summary is exceeded. The measuring method used in the present invention is as follows.

(1) Measurement of intrinsic viscosity of polyester 1 g of polyester from which other polymer components and pigments incompatible with polyester have been removed are precisely weighed, and 100 ml of a mixed solvent of phenol / tetrachloroethane = 50/50 (weight ratio) is added. It was dissolved and measured at 30 ° C.

(2) Average particle diameter of porous silica particles (d50) (μm)
It was measured with a Shimadzu laser diffraction particle size distribution analyzer (SALD-2000), and the median diameter (μm) was defined as the average particle diameter (d50) of the silica particles.

(3) Pore volume of porous silica particles (ml / g)
Samples 0.10 to 0.15 g were weighed and measured with a SA-1100 type rapid surface area measuring device manufactured by Shibata Kagaku. Measurement method = BET single point method

(4) Oil quantity of porous silica particles (ml / 100g)
It measured by JIS-K5101.

(5) Maximum shrinkage stress in the film flow direction (g / cm)
The film sample is sampled so as to have a size of 300 mm in the film longitudinal direction (MD direction) and 10 mm in the width direction. A sample was set on an INTESCO micro constant load device so that the distance between chucks was 200 mm, the load was set so that the initial load was 5 g / 10 mm width, and the temperature was increased to 230 ° C. at a temperature increase rate of 4 ° C./min. The stress change at the time was measured, and the maximum shrinkage stress in the range of 23 to 230 ° C. was read.

(6) Film heat shrinkage in the width direction at 200 ° C. (%)
A strip-shaped test piece is cut out in the width direction (TD direction) of the sample film, placed in a hot-air circulating thermostat, heat-treated at 200 ° C. for 15 minutes, taken out, air-cooled, and the total length is measured with a stainless steel scale. Then, the heat shrinkage in the width direction was determined.
Heat shrinkage rate (%) = | [(original length−length after heating) / (original length)] | × 100

(7) Preparation of thermal transfer ink ribbon A non-defective roll prepared as a polyester film for thermal transfer ink ribbon was unwound, and the back coat layer coating was applied to the entire surface by the gravure coating method and then placed in an oven at 55 ° C. For 5 days and cured to form a backcoat layer. The coating material for the backcoat layer is 5.0 parts by weight of Denkabutyral # 3000K (manufactured by Denki Kagaku Kogyo Co., Ltd.) as the polyvinyl acetal resin and 0.5 of Coronate L (manufactured by Nippon Polyurethane Industry Co., Ltd.) as the isocyanate. Parts by weight, 20 parts by weight of phosphanol GB520 (manufactured by Toho Chemical Co., Ltd.) as the phosphate ester, 0.5 parts by weight of Nipsil E-200A (manufactured by Nippon Silica Industry Co., Ltd.) as the silica, and methyl ethyl ketone It was prepared by mixing 37 parts by weight and 37 parts by weight of toluene. Subsequently, the roll-shaped film after the backcoat layer is cured is rewound again, and yellow, yellow, magenta (red), cyan (blue) dyes and images are sequentially formed on the opposite surface of the backcoat layer. The protective layer (overcoat) coating was repeated, applied by gravure coating, and dried. About yellow (yellow) dye, it apply | coated so that it may become final thickness of 1.0 micrometer, and the coating material is 2.0 parts of macrolex yellow 6G (made by Bayer), and polyvinyl acetoacetal KS-5D (made by Sekisui Chemical Co., Ltd.). Was prepared by mixing 47.5 parts of methyl ethyl ketone and 47.5 parts of toluene. About magenta (red) dye, it apply | coats so that it may become final thickness of 1.0 micrometer, and the coating material is 3.0 parts of bimicron VPSN2670 (made by Bayer) and polyvinyl acetoacetal KS-5D (made by Sekisui Chemical Co., Ltd.). It was prepared by mixing 4.0 parts, 46.5 parts of methyl ethyl ketone and 46.5 parts of toluene. The cyan (blue) dye was applied to a final thickness of 1.0 μm, and the paint was Kayaset Blue 714 (Nippon Kayaku Co., Ltd.) and polyvinyl acetoacetal KS-5D (Sekisui Chemical Co., Ltd.) Prepared by mixing 4.0 parts of methyl ethyl ketone, 46.0 parts of methyl ethyl ketone, and 46.0 parts of toluene. About the coating material for screen protective layers, it apply | coated so that it might become final thickness 2.0micrometer, and a coating material is 20.0 weight part of CAB500-0.5 (made by Eastman Chemical Co.) as cellulose acetate butyrate resin, It was prepared by mixing 40.0 parts by weight of methyl ethyl ketone and 40.0 parts by weight of toluene.

(8) Image production (transfer of each color and transfer of protective layer)
For printing on the dye receiving layer of photographic paper and thermal transfer, a part of the ink ribbon of sublimation type CONPACT PHOTO PRINTER SELPHY CP750 series genuine media manufactured by Canon Inc. was cut and produced by the method described in (7) above. Used by joining ribbons together. Sublimation type CONPACT PHOTO PRINTER SELPHY CP750 manufactured by Canon Inc.

(8-1) Preparation of wrinkle determination image (black solid image) at the time of thermal transfer Specifically, first, a black solid image is created using Adobe Photo Shop made by Adobe, and the data is obtained. Transfer to a printer CP750 installed in a room controlled at 0 ° C. ± 1 ° C., and use the ink ribbon in which the thermal transfer film of the present invention is replaced as a part of the ink ribbon to print a solid black image on the photographic paper Then, the protective layer was solid-transferred onto the image, and the final black image was repeated 300 times.

(8-2) Preparation of Glossiness Determination Image (White Solid Image) After a white solid image is transferred and printed on photographic paper by the same method as in (8-1) above, a protective layer is formed on the image. The solid image was transferred to obtain a final image of white solid.

(9) Wrinkles (defects) during thermal transfer
Image defects on the surface of the final image produced by the method described in (8-1) above were visually observed. In other words, when the ribbon is wrinkled during the transfer of each color, the color of the wrinkle portion is not transferred, and the final color is not black (= combination of yellow + red + blue), but becomes another color. Can be judged. Image defects were determined according to the following criteria.
◎: Image defects due to wrinkles are not recognized at all, and excellent ○: Slight wrinkles are observed on the thermal transfer ink ribbon after thermal transfer, but no image defects due to wrinkles are observed, and good Δ: Image defects due to wrinkles are slight Although it is recognized, it is a level that is not a problem for practical use.

(10) Gloss of Thermal Transfer Image The glossiness of the surface of the final image (white solid) produced by the method described in (8-2) above was visually determined based on the following criteria.
Image gloss;
◎: Excellent gloss, excellent ○: Glossy, good △: Slightly lower gloss, but practically no problem level ×: Insufficient gloss, unusable, unsatisfactory

(11) Head Waste After the transfer of the final image in (8-1) above, the head residue was determined based on the following criteria by observing the deposit on the thermal head with a microscope.
◎: Excellent with no deposit on the thermal head, excellent ○: No deposit due to polyester film on the thermal head, good △: Some deposit on the thermal head is observed, but there is no practical problem ×: There is a deposit on the thermal head, it cannot be used practically, and is defective

(Polyester raw material a)
86 parts of terephthalic acid and 70 parts of ethylene glycol were placed in a reactor, and an esterification reaction was carried out at about 250 ° C. under a pressure of 0.5 kg / mm ² for 4 hours. Next, 0.015 part of antimony trioxide, an average particle diameter of 3.2 μm, porous silica particles having a pore volume of 1.25 ml / g and 0.01 part of phosphoric acid were added. While gradually raising the temperature from 250 ° C. to 285 ° C., the pressure was gradually reduced from normal pressure to 0.5 mmHg. After 4 hours, the polycondensation reaction was stopped to obtain a polyester (a) having an intrinsic viscosity of 0.66. This polyester (a) contained 0.50% by weight of porous silica particles.

(Polyester raw material b)
In the polyester raw material a, in place of the porous silica particles, porous silica particles having an average particle size of 4.0 μm and a pore volume of 1.25 ml / g are added, and the intrinsic viscosity is 0.66 in the same manner as for the polyester a. Polyester (b) was obtained. This polyester (b) contained 0.50% by weight of porous silica particles.

(Polyester raw material c)
The intrinsic viscosity 0.66 is the same as polyester a except that porous silica particles having an average particle size of 2.7 μm and a pore volume of 0.80 ml / g are added in place of the porous silica particles in the polyester raw material a. Polyester (c) was obtained. This polyester (c) contained 0.50% by weight of porous silica particles.

(Polyester raw material d)
In the polyester raw material a, in place of the porous silica particles, porous silica particles having an average particle diameter of 2.5 μm and a pore volume of 1.25 ml / g are added, and the intrinsic viscosity is 0.66 in the same manner as for the polyester a. Polyester (d) was obtained. This polyester (d) contained 0.50% by weight of porous silica particles.

(Polyester raw material e)
In the polyester raw material a, in place of the porous silica particles, porous silica particles having an average particle diameter of 5.0 μm and a pore volume of 0.44 ml / g are added, and the intrinsic viscosity is 0.66 in the same manner as for the polyester a. Polyester (e) was obtained. This polyester (e) contained 0.50% by weight of porous silica particles.

(Polyester raw material f)
In the polyester raw material a, in place of the porous silica particles, porous silica particles having an average particle diameter of 3.2 μm and a pore volume of 1.60 ml / g are added, and the intrinsic viscosity is 0.66 in the same manner as for the polyester a. Polyester (f) was obtained. This polyester (f) contained 0.50% by weight of porous silica particles.

(Polyester raw material g)
A polyester for dilution (g) having an intrinsic viscosity of 0.66 was obtained in exactly the same manner as for polyester a except that no porous silica particles were added to polyester raw material a.

Example 1:
Polyester (a) and polyester (g) were mixed to obtain a raw material containing 0.096% by weight of porous silica particles having an average particle diameter of 3.2 μm and a pore volume of 1.25 ml / g. The mixed raw material was dried by a conventional method, supplied to an extruder, melted at 290 ° C., extruded into a sheet, and rapidly cooled on a cooling roll using an electrostatic application adhesion method to obtain an amorphous sheet. The obtained sheet was stretched 2.70 times at 102 ° C. in the machine direction (MD direction) using a roll stretching method, and further stretched 1.56 times at 80 ° C. Then, it was led to a tenter, stretched 4.40 times in the transverse direction at 130 ° C, heat treated at 235 ° C, relaxed 10% at 195 ° C, and biaxially stretched polyester film (master roll) with a thickness of 4.5 µm. ) While being trimmed to a width of 500 mm from this master roll, it was wound into a roll shape to obtain a roll film.

Example 2:
Except for changing the content of the porous silica particles to 0.060% by weight, the stretching ratio in the second stage in the machine direction (MD direction) to 1.50 times, the heat treatment to 234 ° C., and the relaxation rate to 3%. In the same manner as in Example 1, a roll-shaped biaxially stretched polyester film having a thickness of 4.5 μm was obtained.

Example 3:
The average particle diameter of the porous silica particles is 4.0 μm, the content is 0.040% by weight, the draw ratio in the second stage in the machine direction (MD direction) is 1.60 times, the heat treatment is 234 ° C., and the relaxation rate is 13%. A roll-shaped biaxially stretched polyester film having a thickness of 6 μm was obtained in the same manner as in Example 1 except that it was changed to.

Example 4:
Except that the average particle size of the porous silica particles was changed to 2.7 μm, the pore volume was 0.80 ml / g, the content was 0.098 wt%, the heat treatment temperature was 232 ° C., and the relaxation rate was changed to 7%. In the same manner as in Example 1, a roll-shaped biaxially stretched polyester film having a thickness of 4.5 μm was obtained.

Comparative Example 1:
The thickness (4.5 μm) is the same as in Example 1, except that the stretching temperature in the second stage in the machine direction (MD direction) is 1.67 times at 88 ° C., the heat treatment temperature is 230 ° C., and the relaxation rate is 5%. A roll-shaped biaxially stretched polyester film was obtained.

Comparative Example 2:
The thickness of the porous silica particles was 2.5 μm, the content was 0.12 wt%, the heat treatment temperature was 238 ° C., and the relaxation rate was changed to 5% in the same manner as in Example 3, except that the thickness was 4. A 5-μm roll-shaped biaxially stretched polyester film was obtained.

Comparative Example 3:
Except for changing the average particle size of the porous silica particles to 5.0 μm, the pore volume to 0.44 ml / g, and the content to 0.03% by weight, the thickness is 4.5 μm. A roll-shaped biaxially stretched polyester film was obtained.

Comparative Example 4:
A roll-shaped biaxially stretched polyester film having a thickness of 4.5 μm was obtained in the same manner as in Example 1 except that the pore volume of the porous silica particles was changed to 1.60 ml / g.

  The results obtained for Examples 1-4 and Comparative Examples 1-4 are summarized in Table 1 below.

  The film of the present invention can be suitably used, for example, in applications such as sublimation type thermal transfer system and melt type thermal transfer system, especially when printing is repeated with an inexpensive thermal transfer printer for general household use.

Claims (1)

Containing 0.04 to 0.15 wt% of porous silica particles having a pore volume of 0.50 to 1.50 ml / g and an average particle size of 2.6 to 4.5 μm at 23 to 230 ° C. A polyester film for a thermal transfer ink ribbon, wherein the maximum shrinkage stress in the longitudinal direction of the film is 10 to 24 g / cm, and the film heat shrinkage in the width direction at 200 ° C. is −2.0 to 2.5%. .