JPH09278916A - White polyester film, its production and receiving sheet for heat-sensitive recording - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a white polyester film, comprising a polyester and a polymer incompatible wit the polyester, having a high void ratio by adding a small amount of the incompatible polymer thereto, good in film-forming operating efficiency and useful as a receiving paper for heat-sensitive recording. SOLUTION: This white polyester film comprises (A) a polyester and (B) a polymer incompatible with the component A and has a relationship between the cross-sectional area fraction (Vb ) and the void cross-sectional area fraction (Vv ) satisfying 20>=Vv /Vb >=2 and the heat shrinkage factor at 100 deg.C regulated to <=0.5%. The polyester film has >=55% whiteness degree, 1-1.6 optical density based on 150μm thickness and the component B is preferably a polymer having >=200 deg.C melting point such as polymethylpentene. The polyester film is preferably prepared by longitudinally stretching an unstretched film, previously heat- treating the stretched film at a higher temperature than the longitudinal stretching temperature and then transversely stretching the heat-treated film at a lower temperature than the previous heat-treating temperature by 6-50 deg.C when stretching the film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a white polyester film having a low specific gravity and a high hiding property, a method for producing the same, and a heat-sensitive recording receiving sheet. Further, a coating layer may be provided on the surface layer of the white polyester film, or the white polyester film and the paper may be attached and used. The present invention also relates to a white polyester film which is used as a base material such as a heat-sensitive recording receiving sheet, printing base material, packaging, card, label, receiving paper for video printer, photographic paper, and display board. .

[0002]

2. Description of the Related Art It is well known that a large amount of titanium dioxide, calcium carbonate or the like is added to polyester and stretched to obtain a white polyester (for example, Japanese Patent Publication No. 6-86).
No. 537). It is also well known to add a polyolefin to polyester to obtain a white polyester film (for example, Japanese Patent Publication No. 64-2141).

[0003]

However, in a method in which a large amount of an inorganic substance such as titanium dioxide or calcium carbonate or a thermoplastic polymer such as polypropylene is added to polyester and simply biaxially stretched, the porosity is increased and the high hiding property is improved. It was necessary to add a large amount of an inorganic substance or a thermoplastic polymer in order to give it. Generally, when a large amount of incompatible resin or inorganic particles is added to polyester, the phenomenon of die swell is likely to occur, which causes deterioration of ejection stability during extrusion, and thus film thickness unevenness and breakage during film formation, resulting in extremely film formation. The workability was poor. Further, if the addition amount is reduced, it becomes difficult to increase the porosity.

Since the voids are formed around the incompatible polymer by stretching, the size of the void is determined by the addition amount of the incompatible polymer and the stretching conditions. Generally, stretching at a high temperature can increase the draw ratio, but it is difficult to form voids. Conversely, stretching at a low temperature tends to create voids, but the draw ratio cannot be increased. Therefore, a high porosity can be achieved by adding a small amount of incompatible polymer. It was difficult. Further, when stretched at a high ratio, the heat shrinkage rate at 100 ° C. becomes too large, which may cause problems such as curling when used in a card and misalignment and curling when used in a heat-sensitive transfer receiving sheet.

That is, there was no white polyester film having a high porosity with a small amount of incompatible polymer, a good workability for film formation, a high hiding property and a good dimensional stability. Further, there was no method for producing the same and a heat-sensitive recording receiving sheet using the same.

An object of the present invention is to add a small amount of an incompatible polymer, a white polyester film having a high porosity and good workability for film formation, a high hiding property and a good dimensional stability, and a method for producing the same. Another object of the present invention is to provide a heat-sensitive recording receiving sheet based on the white polyester film.

[0007]

In order to obtain a white polyester film having a high porosity, good workability for film formation, and good dimensional stability, the polyester (a) is used in the present invention. And a polymer (b) that is incompatible with the polyester, and the relationship between the cross-section integration rate (Vb) and the void cross-section integration rate (Vv) of the incompatible polymer satisfies the expression (1) and heat shrinkage at 100 ° C. It is a white polyester film characterized by having a rate of 0.5% or less. 20 ≧ Vv / Vb ≧ 2 (1)

Further, in the method for producing a white polyester film as described above according to the present invention, after longitudinal stretching, preliminary heat treatment is performed at a temperature higher than the longitudinal stretching temperature, and then transversely at a temperature 6 to 50 ° C. lower than the preliminary heat treatment temperature. The method comprises stretching.

Further, the heat-sensitive recording receiving sheet according to the present invention is based on the white polyester film as described above.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The polyester referred to in the present invention means
A polymer obtained by condensation polymerization from a diol and a dicarboxylic acid, and as the dicarboxylic acid, terephthalic acid, isophthalic acid, phthalic acid naphthalene dicarboxylic acid,
Typical examples are adipic acid and sebacic acid.
The diol is represented by ethylene glycol, trimethylene glycol, tetramethylene glycol, cyclohexanedimethanol and the like. Specifically, for example, polyethylene terephthalate (PET), polytetramethylene terephthalate (PBT), polyethylene-p-oxybenzoate, poly-1,4-cyclohexylene dimethylene terephthalate (PCHDM).
T), polyethylene-2,6-naphthalene dicarboxylate (PEN) and the like. In the case of the present invention, especially PE
T and PEN are preferred. Various known additives such as antioxidants and antistatic agents may be added to the polyester.

The incompatible polymer referred to in the present invention is dispersed in the polyester by melt extrusion molding, and in the polyester having the effect of causing interfacial peeling at the interface of the incompatible polymer dispersion in the subsequent stretching step to form voids. Although it may be a thermoplastic resin dispersed in, preferably poly-3-methylbutene-1, poly-4-methylpentene-1, polyvinyl-t-butane, 1,4-trans-poly-2,3-dimethyl. Butadiene, polyvinyl cyclohexane, polystyrene, polymethylstyrene, polyfluorostyrene,
A polymer having a melting point of 200 ° C. or higher selected from poly-2-methyl-4-fluorostyrene, polyvinyl-t-butyl ether, cellulose triacetate, cellulose tripropionate, polyvinyl fluoride, polychlorotrifluoroethylene and the like can be used. . In the case of the present invention, poly-4-methylpentene-1, cellulose triacetate and modified products thereof are particularly preferable in terms of price, heat stability, dispersibility with polyester and the like.

The melting point of the incompatible polymer is preferably 20.
The temperature is 0 ° C or higher, more preferably 210 ° C or higher, and most preferably 220 ° C or higher. This is because when the melting point is less than 200 ° C., the dispersed shape of the incompatible polymer in the polyester film does not take a spherical shape but often takes a layered or flat shape, and voids are not effectively generated. Further, the melting point of the incompatible polymer is 300 ° C. or lower, preferably 280 ° C.
Hereafter, it is preferably 260 ° C. or lower. This is because the incompatible polymer does not melt unless it is lower than the melt extrusion temperature of the polyester.

The amount of the incompatible polymer used is 1 to 21% by weight, preferably 2 to 11% by weight. More preferably, it is 2 to 6 parts by weight. Addition amount is 1%
When it is less than 1, it is difficult to obtain a white polyester having a high hiding property. On the other hand, when the amount of the incompatible polymer added exceeds 21% by weight, not only the mechanical properties of the polyester film of the present invention are inferior, but also the thermal dimensional stability is inferior and the film is broken during film formation. Problems such as frequent occurrence occur.

The present invention comprises a composition mixture comprising a polyester (a) and a polymer (b) which is incompatible with the polyester, but a compatibilizing agent may be added to make the voids finer. Polyester polyether copolymer (c) is preferable as the compatibilizer. For example, the polyester used as the polyester polyether copolymer is polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene-p-oxybenzoate, poly-1,4-cyclohexylene dimethylene terephthalate (PCHDMT), polyethylene- 2,6
-Naphthalene dicarboxylate (PEN) or the like can be used. The polyether used as the copolymer preferably has a molecular weight of 300 to 20,000, and examples thereof include polyethylene glycol (PEG), methoxy polyethylene glycol, polytetramethylene glycol (PTM).
G), polypropylene glycol (PPG) and the like.
PE as the preferred polyester polyether copolymer
There are T-PEG, PBT-PTMG, and PCT-PPG. The copolymerization ratio of the polyester and the polyether is preferably 1: 9 to 9: 1 by weight, more preferably 4: 6 to 9: 1, further preferably 4: 6 to 6: 4.
Is good. As a production method, a polyester-polyether copolymer can be produced by adding a polyether at an arbitrary stage when producing a polyester composed of an acid component and a glycol component. Further, the molecular weight of the polyether used is preferably 400 to 20,000. It is preferably 1,000 to 10,000.

The amount of the polyester polyether copolymer used is 0.5 to 20% by weight, preferably 1 to 5% by weight. If the addition amount is less than 0.5% by weight, the incompatible polymer is not finely dispersed, the generated voids are large, and wrinkles are likely to occur in the film. On the other hand, when it exceeds 20% by weight, voids are less likely to be formed and the cushioning property becomes insufficient.

The polyester as the main component is preferably 75% by weight or more. If it is less than 75% by weight, the mechanical strength and heat resistance, which are the advantages of the polyester film, are difficult to be exhibited.

Further, it is desirable that the amount of the incompatible polymer is larger than that of the polyester polyether copolymer. The addition amount is preferably twice or more. When the amount of the incompatible polymer is smaller than the amount of the polyester polyether copolymer added, voids are less likely to occur at the interface of the incompatible polymer particles, and it becomes difficult to obtain a white polyester film having low specific gravity and high optical density.

It is composed of polyester (a) and a polymer (b) which is incompatible with polyester, and the relationship between the cross-section integration rate (Vb) and the void cross-section integration rate (Vv) of the incompatible polymer is expressed by the formula (1) Need to be satisfied. 20 ≧ Vv / Vb ≧ 2 (1)

Although stretching causes peeling from the interface of the incompatible polymer to generate voids, in the present invention, as a method for achieving a high porosity by adding a small amount of incompatible polymer, for example, preliminary heat treatment after longitudinal stretching is performed. And then transverse stretching at a temperature lower by 6 ° C. or more than the preheat treatment temperature. Further, longitudinal stretching, preliminary heat treatment, and then longitudinal stretching again are also preferably used.

The method for measuring the cross-section integration rate (Vb) and void cross-section integration rate (Vv) of the incompatible polymer in the film will be explained in the section of the evaluation method.
The relationship between b) and the porosity (Vv) is preferably 15 ≧
Vv / Vb ≧ 2, more preferably 10 ≧ Vv / V
b ≧ 3, and most preferably 8 ≧ Vv / Vb ≧ 4.

If this value is less than 2, it is necessary to add a large amount of the incompatible polymer in order to achieve a high porosity, and therefore a phenomenon of die swell is likely to occur during extrusion, and therefore extrusion is performed. In this case, ejection stability is deteriorated, and unevenness of film thickness and breakage during film formation occur, resulting in extremely poor film forming workability. On the other hand, when it exceeds 20, not only the heat shrinkage at 100 ° C. increases but also the film breaks frequently.

The heat shrinkage at 100 ° C. is 0.5% or less. It is preferably 0.3% or less. More preferably, it is 0.1% or less. If it is more than 0.5, problems such as curling when used for a card and misalignment and curling when used for a heat-sensitive transfer receiving sheet occur.

The preferred specific gravity is 0.5 or more and 1.3 or less. If it is less than 0.5, the film tends to have wrinkles.
When it exceeds 3, the cushioning property and flexibility of the heat-sensitive recording receiving sheet are insufficient.

Next, in order to make the present invention more preferable, it is desirable that the whiteness of the film of the present invention is 55% or more, preferably 80% or more, more preferably 90% or more. When the whiteness is less than 55%, it is difficult to take advantage of the special features of the white film.

Although the whiteness is changed by the addition of the above-mentioned incompatible polymer or polyester polyether copolymer,
If necessary, a fluorescent whitening agent containing an inorganic fluorescent agent may be added. As a fluorescent whitening agent, the trade name is "Ubitech" O
B, MD (made by Ciba-Geigy), "OB-1" (made by Eastman), etc. are mentioned.

In order to make the present invention more preferable, the cushion ratio of the film of the present invention is 10% or more, preferably 1%.
It is desired to be 5% or more, and more preferably 20% or more. When the cushion ratio is less than 10%, when used as a receiving paper for a video printer, for example, when the pressure of the thermal recording head is lowered, the head hits hard and missing dots occur and a clear image can be transferred. And the feeling when touched by the hand becomes worse.

In order to make the present invention more preferable, the thickness of the white polyester film of the present invention is 150 μm.
The optical density in terms of m is preferably 1 or more and 1.6 or less. When the optical density is less than 1, the hiding property of the film is insufficient and the back side is transparent, which is not preferable.

The white polyester film of the present invention can be used by laminating resin, paper, non-woven fabric, cloth and the like.

Next, although the present invention is not particularly limited, in the present invention, the incompatible polymer has a shape close to a sphere in the white polyester, that is, the shape factor is within the range of 1 to 20. Is desirable. That is,
When the shape of the immiscible polymer is close to a sphere, not only the specific gravity can be reduced as compared with the case where the polymer is dispersed in a layered or flat shape, but also a film having a high cushion rate and good thermal dimension can be obtained. The shape close to a sphere means that the shape factor of the incompatible polymer dispersed in the film, that is, the ratio of the major axis to the minor axis is 1 to 20, preferably 1 to 5.

Next, a preferred method for producing the white polyester film of the present invention will be described. Polymethylpentene as an incompatible polymer was mixed with polyethylene terephthalate (PET), thoroughly mixed and dried.
Feed to an extruder heated to a temperature of 0-300 ° C. Here, the polymer mixture can be extruded with a vented twin-screw extruder to remove water during extrusion.

The melted sheet is electrostatically adhered to the drum cooled at a drum surface temperature of 10 to 60 ° C. to be solidified by cooling, and the unstretched film is guided to a roll group heated to 75 to 120 ° C. 2 to 5 times in the machine direction, 2
Cool with rolls at 0-50 ° C.

In order to obtain a target white polyester film having a relationship between the cross-sectional integration rate (Vb) and the porosity (Vv) of the incompatible polymer, the heat shrinkage rate at 100 ° C. is 0.5% or less. Next, pre-heat treatment (100 ℃ ~ 1
50 ° C.) is preferable. Further, the uniaxially stretched film obtained at this time may be subjected to corona discharge treatment for coating. Next, while holding both ends of the uniaxially stretched film with clips, the uniaxially stretched film is led to a tenter and 90 to 144
The film is transversely stretched in a direction perpendicular to the machine direction in an atmosphere heated to ° C. The stretching ratio is 2 to 5 times in each of the length and width, and the area ratio is preferably 6 to 15 times.
If the area magnification is less than 6 times, the whiteness is insufficient and conversely 15
If it exceeds 2 times, tearing tends to occur during stretching, and the film-forming paper tends to be defective. Further, after the preliminary heat treatment, the longitudinal stretching and the lateral stretching may be performed again. The preheat treatment temperature is preferably higher than either longitudinal stretching temperature. Further, the tenter temperature is preferably 6 ° C to 50 ° C lower than the preliminary heat treatment temperature.
When the preheat treatment temperature is lower than the tenter temperature by less than 6 ° C., voids may not be effectively generated, and it may be difficult to obtain the target white polyester. When the preheat treatment temperature is higher than the tenter temperature by 50 ° C. or more, void formation tends to be nonuniform.

In order to impart the flatness and dimensional stability of the biaxially stretched film in this manner, 150 in a tenter.
After heat fixing at ˜230 ° C., the material is gradually cooled to a uniform temperature, then cooled to room temperature and wound up. During this heat setting, 150 ℃ to 190 ℃
Relaxation of 10% or less can be added to reduce the heat shrinkage in the temperature range of ° C. Thus, the white polyester film of the present invention is obtained.

If necessary, adhesiveness, antistatic property,
During the production of the film for the purpose of imparting releasability and the like,
For example, a function can be imparted to the surface by using a so-called in-line coating method of coating on the film surface before transverse stretching and then drying and then transversely stretching, or by coating on the film after biaxial stretching. Examples of the material to be applied include polyester, acrylic, polyurethane, acrylic graft polyester, ion conductive substances such as anions and cations, and silicone compositions. Mixtures of two or more of these and / or crosslinks with melamine, epoxy, isocyanate, etc. can be used.

The white polyester film of the present invention comprises:
Fine particles such as calcium carbonate, amorphous zeolite particles, anatase type titanium dioxide, calcium phosphate, silica, kaolin, talc and clay may be used in combination. The addition amount of these is preferably 0.005 to 1 part by weight with respect to 100 parts by weight of the polyester composition. In addition to the fine particles, fine particles precipitated by the reaction between the catalyst residue and the phosphorus compound in the polyester polycondensation reaction system can be used in combination. As the deposited particles, for example, those composed of calcium, lithium and phosphorus compounds, or those composed of calcium, magnesium and phosphorus compounds can be used, and the content of these particles in the polyester is 100 parts by weight of the polyester. On the other hand, it is preferably 0.05 to 1 part by weight.

The heat-sensitive receiving sheet used in the present invention can be obtained by providing the white polyester film obtained above with a heat-sensitive receiving layer by a known method.

[Measurement of Physical Properties and Evaluation Method of Effect] (1) Specific Gravity The film was cut into 100 × 100 mm squares, and a dial gauge (No. 2109-10, manufactured by Mitoyo Seisakusho Co., Ltd.) was used to measure a 10 mm diameter probe (No. 7002) is attached to measure the thickness at a minimum of 10 points, and the average thickness d (μ
m) is calculated. Further, this film is weighed with a direct balance and the weight w (g) is read up to the unit of 10 ⁻⁴ g. At this time, specific gravity = w / d × 100.

(2) Average Diameter and Shape Coefficient of Dispersion A cross section of the white film cut in the machine direction (MD) or in the direction perpendicular to it (TD) was measured with a scanning electron microscope to obtain 1000
A photograph magnified 5000 times is taken, and at least 100 or more incompatible polymer dispersions having a specified thickness range are subjected to an image analyzer to obtain a distribution of diameters of circles corresponding to the areas of the dispersions. The area average diameter of this distribution is defined as the average diameter (dispersion diameter) of the dispersion. The ratio of the major axis / minor axis of 100 pieces is defined as the shape factor of the dispersion.

(3) Cross-section integration ratio (V
b) and void cross-section integration ratio (Vv) and its ratio A cross-section of the film cut in the longitudinal stretching direction (MD) is magnified 1000 to 5000 times with a scanning electron microscope, and the cross-sectional area (Sdm) of the film is taken. Ask. Further, all the incompatible polymer dispersions in the film cross section of the photograph were subjected to an image analyzer, and the areas corresponding to the incompatible polymer dispersion portions were totaled (Sbm). Further, all the void portions in the film cross section of the same photograph were applied to an image analyzer, and the areas corresponding to the voids were totaled (Svm). The cross-section integration rate (Vbm) and void cross-section integration rate (Vvm) of the incompatible polymer and their ratios are as follows. Vbm = Sbm / Sdm Vvm = Svm / Sdm Vvm / Vbm = Svm / Sbm Similarly, the cross-section integration ratio (Vbt) and void cross-section integration ratio (Vbt) of the incompatible polymer are also obtained for the cross section of the film cut in the transverse stretching direction (TD). Vvt) is calculated. Vbt = Sbt / Sdt Vvt = Svt / Sdt Vvt / Vbt = Svt / Sbt The larger value of Vvm / Vbm and Vvt / Vbt is Vv / Vb.

(4) Optical density An optical densitometer (TR9
27, manufactured by Macbeth Co.) is used to measure the transmission density. The thickness of the film and the optical density are plotted, and the optical density corresponding to the thickness of 150 μm is determined by the interpolation method or the extrapolation method.

(5) Cushion rate (%) Dial gauge No. manufactured by Mitoyo Manufacturing Co., Ltd. 2109-1
The standard gauge head 900030 is used for the dial gauge stand No. 0. It is determined by the following formula from the thicknesses d50 and d500 of the respective films when the dial gauge pressing partial loads of 50 g and 500 g are applied using 7001DGS-M. Cushion rate = 100 × (d50−d500) / d50

(6) Flexibility About 1 parts of acrylic acid ester type adhesives KP-1405 and CK-102 (crosslinking agent) manufactured by Nippon Carbide Industry Co., Ltd.
00: 1 to mix, and one side of the film is coated with this adhesive mixture solution in a dry state to a thickness of 25 μm,
It was left to cure for 1 week. The adhesive strength is adjusted by increasing the amount of the curing agent when the adhesive strength is strong and decreasing the amount of the curing agent when the adhesive strength is weak. As described above, the film is adhered to stainless steel (SUS304, mirror surface) and bent to 180 °. At this time, the angle at which wrinkles start to develop is measured.
The flexibility was judged as follows. ○ and ◎ were passed. Folding wrinkles were found within the range of 0 ° to less than 60 °: × Folding wrinkles were found within the range of 60 ° to less than 120 °: ○ Folding wrinkles were found within the range of 120 ° to less than 180 ° Given: ◎

(7) Void Length after Stretching (x) The cross section of the stretched film is observed by SEM to read the length of the void. At this time, the cross section cut in the MD direction was taken as the MD cross section, and the cross section cut in the TD direction was taken as the TD cross section. The void length read from each cross-section observation photograph was defined as x (μm) and y (μm).

(8) Whiteness U manufactured by Shimadzu Corporation according to JIS-L-1015
When V-260 is used and the reflectances at wavelengths of 450 nm and 550 nm are B% and G%, respectively, whiteness (%) = 4B + 3G. The thickness of the film and the whiteness are plotted, and the whiteness corresponding to the thickness of 150 μm is determined by the interpolation method or the extrapolation method.

(9) Printing quality The film was cut into A4 size sheets and CX manufactured by Sharp Corp.
Thermal transfer recording was performed using a -5000 color printer, and the obtained hard copy was visually evaluated for print density, print unevenness, and degree of contrast in the following five grades.

(10) Heat shrinkage rate at 100 ° C. The film was cut in the MD or TD direction to have a width of 10 mm and a length of 300 mm, marked at intervals of 200 mm, and fixed on a support plate under a constant tension (5 g). The original length a (mm) is measured. Next, a load is applied using a clip of 3 g and the treatment is performed for 30 minutes while rotating in a hot air oven at 100 ° C., and the marking interval b (mm) is measured in the same manner as the measurement of the original length. The heat shrinkage is determined by the following equation, and the average value of five pieces is used. Heat shrinkage rate (%) = (ab) / a

[0047]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described based on embodiments. Example 1 Polybutylene terephthalate (PBT) and polytetramethylene glycol (PTMG, molecular weight 400 were previously prepared.
0) PBT-PTMG copolymer 1 produced by adding PTMG during the polymerization of PBT so that the weight ratio thereof is 1: 1.
% By weight, 94% by weight of polyethylene terephthalate (PET) and polymethylpentene (Mitsui Petrochemical Co., Ltd.)
Manufactured by TPX, DX820) 5% by weight is mixed, and
A sheet was formed from a T-die extruded while being degassed and dried with a vacuum pump so that the vent pressure was 10 (mmHg) from a twin-screw vented extruder heated to 300 ° C. Further, this sheet-shaped formed product is cooled and solidified by a cooling drum having a surface temperature of 25 ° C., guided to a roll group heated to 100 ° C., longitudinally stretched twice in a machine axis direction, and then the longitudinally stretched film is heated to 130 ° C. The sample was introduced into a tunnel oven at the ambient temperature of and preheated. Subsequently, this preheat-treated film was introduced into a roll group heated to 120 ° C., re-longitudinal stretched twice in the machine axis direction, and cooled with a roll group at 25 ° C. Subsequently, both ends of this film were guided to a tenter while gripping both ends with clips, and transversely stretched in a direction perpendicular to the machine axis direction in an atmosphere heated to 100 ° C. The ratio between the entrance width of the tenter and the maximum width (mechanical magnification of the tenter) was 3.5 times. After that, heat setting was carried out at 230 ° C. in a tenter, and the film was gradually cooled to room temperature, then cooled to room temperature and wound up to obtain a thickness of 10
A 0 μm film was obtained. The physical properties of the obtained film are as shown in Table 1. When the white film was used as an image-receiving sheet and the printing quality was evaluated, good images with a print density of 3, a print unevenness of 3 and a contrast of 3 were obtained, and it showed good practical suitability without paper jams and running defects during the image receiving operation. .

Example 2 90% by weight of polyethylene terephthalate (PET) and PP (Mitsui Toatsu Chemical Co., Ltd., Noblen, J3H)
Vent pressure is 10 (mmHg) from an extruder with a twin-screw vent that is mixed with 10 wt% and heated to 270 to 300 ° C.
While being degassed and dried by a vacuum pump, a sheet was formed from an extruded T-die, and a film was obtained in the same manner as in Example 1. The thickness was 50 μm. The physical properties of the obtained film are as shown in Table 1. When a heat-sensitive transfer receiving layer was applied to the obtained film and the print quality was evaluated as an image receiving sheet, good images were obtained with a print density of 4, print unevenness of 5, and contrast of 5, and paper jams and running defects during image receiving operation were obtained. Soon it showed good practical suitability.

Comparative Example 1 The sheet extruded in Example 1 had a surface temperature of 25 ° C.
It is cooled and solidified by the cooling drum of, and guided to a group of rolls heated to 100 ° C., longitudinally stretched twice in the machine axis direction, and subsequently, this film is guided to a group of rolls heated to 120 ° C. without preliminary heat treatment, It was re-longitudinal stretched twice in the machine axis direction and cooled by a roll group at 25 ° C. Subsequently, both ends of this film were guided to a tenter while gripping both ends with clips, and transversely stretched in a direction perpendicular to the machine axis direction in an atmosphere heated to 100 ° C. The tenter mechanical magnification was 3.5. Then, heat setting was carried out at 230 ° C. in a tenter, and the film was slowly annealed and then cooled to room temperature to obtain a film having a thickness of 100 μm.

The physical properties of the obtained film are shown in Table 1. Vv / Vb is 2 without preliminary heat treatment after longitudinal stretching
It did not grow larger and void formation did not occur effectively.

Comparative Example 2 The sheet extruded in Example 1 had a surface temperature of 25 ° C.
It was cooled and solidified by the cooling drum of No. 2, introduced into a roll group heated at 80 ° C., longitudinally stretched 4 times in the machine axis direction, and cooled by a roll group at 25 ° C. Subsequently, both ends of this film were guided to a tenter while gripping both ends with clips, and transversely stretched in a direction perpendicular to the machine axis direction in an atmosphere heated to 100 ° C. The tenter mechanical magnification was 3.5. Then in the tenter 2
The film was heat-set at 30 ° C., uniformly cooled and then cooled to room temperature to obtain a film having a winding thickness of 100 μm.

The physical properties of the obtained film are as shown in Table 1. Even if Vv / Vb was increased to more than 2 by increasing the draw ratio without pre-heat treatment after the longitudinal drawing, the heat shrinkage ratio at 100 ° C. was not 0.5 or less.

Comparative Example 3 1% by weight of the PBT-PTMG copolymer used in Example 1,
A sheet extruded in the same manner as in Example 1 except that a mixture of 97% by weight of PET and 2% by weight of polymethylpentene was used, was cooled and solidified by a cooling drum having a surface temperature of 25 ° C., and heated to 100 ° C. It was led to a roll group and longitudinally stretched twice in the machine axis direction. Subsequently, this longitudinally stretched film was introduced into a tunnel oven having an ambient temperature of 160 ° C. and preheated. Then, this film is 120 ℃
The film was guided to a roll group heated to 1, re-stretched twice in the machine axis direction, and cooled by a roll group at 25 ° C. Then, while holding both ends of this film with clips, guide the film to a tenter.
The film was transversely stretched in a direction perpendicular to the machine axis direction in an atmosphere heated to 0 ° C. The film was easily broken, and the workability of forming the film was extremely poor. The tenter mechanical magnification was 3.6. When the film was observed, the thickness was uneven. The physical properties of the obtained film are as shown in Table 1.

[0054]

[Table 1]

[0055]

EFFECTS OF THE INVENTION According to the white polyester film of the present invention, a white polyester film having a high porosity and good workability for film formation can be obtained by adding a small amount of an incompatible polymer, and it has high concealing property and uneven thickness. It is possible to provide an inexpensive film, which has less film forming workability. Further, when used as a heat-sensitive transfer receiving paper, the heat dimensional property is good and the flexibility is excellent, so that the heat-sensitive transfer receiving paper can be easily handled in the film processing step and has excellent image quality.

Continuation of the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical display location B29L 7:00 C08L 67:02

Claims (7)

[Claims] 1. A relationship between the cross-section integration rate (Vb) and the void cross-section integration rate (Vv) of polyester and a polymer incompatible with the polyester, which satisfies the expression (1), 0.5% heat shrinkage at ℃
A white polyester film characterized in that: 20 ≧ Vv / Vb ≧ 2 (1) 2. The white polyester film according to claim 1, which has a whiteness of 55% or more. 3. A white polyester film having a thickness of 150.
The white polyester film according to claim 1, having an optical density of 1 or more and 1.6 or less per μm. 4. The polymer incompatible with the polyester is a polymer having a melting point of 200 ° C. or higher selected from methylpentene polymer, methylbutene polymer, styrene-based polymer, fluorine-based polymer, cellulose acetate, and cellulose propionate polymer. Characterized by that
The white polyester film according to any one of claims 1 to 3. 5. The white polyester film according to claim 4, wherein the polyester-incompatible polymer is polymethylpentene. 6. After the longitudinal stretching, a preliminary heat treatment is performed at a temperature higher than the longitudinal stretching temperature, and then 6 ° C. to 50 ° C. from the preliminary heat treatment temperature.
The method for producing a white polyester film according to any one of claims 1 to 5, wherein the white polyester film is transversely stretched at a low temperature. 7. A receiving sheet for heat-sensitive recording, which comprises the white polyester film according to claim 1 as a base material.