JPH059319A - Polyester film reduced in weight and its production - Google Patents

Abstract

PURPOSE:To obtain a polyester film excellent in lightweight and masking properties and useful as labels, etc., and a method for producing the aforementioned film. CONSTITUTION:The subject polyester film reduced in weight is a polyester film composed of a composition composed of (A) a crystalline polyester resin and (B) a copolymer composed of at least one cyclic olefin selected from the group consisting of ethylene/bicycloalkenes and tricycloalkenes. The aforementioned component (B) is dispersed in the form of particles. Fine voids are present and the weight is reduced. The aforementioned polyester film is obtained by orienting an unoriented film prepared from the composition of both at a temperature within the range of the glass transition temperature or above and the crystallization temperature or below of the above-mentioned component (A) at >=1.5 times in at least one axial direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyester film which contains fine voids and has a low density (small specific gravity) and is excellent in strength, heat resistance, hiding power and writability, and a method for producing the same. Due to its excellent properties, the lightweight polyester film of the present invention is used for labels, information paper, drawing paper, printing paper and the like.

[0002]

2. Description of the Related Art As an attempt to reduce the weight of a polyester film, (a) a method of adding a gas or a vaporizable substance (for example, JP-A-50-38765, JP-B-57-46456, JP-A-57-46456). 57-34931), (b) a method of adding a substance that generates a gas by a chemical reaction (for example, JP-A-52-43871, JP-B-58-50625), (C) Soluble in a solvent. Method of adding various substances and then extracting this with a solvent (for example, JP-A-51-34963 and JP-B-52-27666).
No. publication) is known. However, none of these methods has been put into practical use. The reason is that it is possible to generate voids in the polyester film by these methods, but it is very difficult to control the size, and not only coarse voids are likely to occur, but also the spatial distribution thereof. It is also mentioned that the polyester film is apt to become non-uniform, and accordingly, a portion having extremely low strength is generated in the polyester film and the film is apt to be cut during stretching. Further, the film obtained by such a method is not satisfactory in its lightness, strength, hiding power, writability and uniformity of performance.

There is also a method of adding inorganic particles (for example, Japanese Patent Laid-Open No. 63-137927), but it is difficult to disperse the fine particles because the inorganic particles are likely to secondary aggregate.
Although a method of adding inorganic particles at the time of polymerization is taken (for example, JP-A-63-66222), when a large amount of inorganic particles are added, secondary aggregation similarly occurs or at least the degree of polymerization of the thermoplastic polyester resin is increased. May be affected. Further, even when trying to obtain a film by melt-extruding the inorganic particle-mixed thermoplastic polyester resin, since the inorganic particles are clogged in the screen, a coarse mesh screen is used, and large dust may be mixed in the film. The film may be cut during stretching. Further, in the case of inorganic particles, there is a drawback that the lip of the molding die is worn. In addition, the presence of inorganic particles in the film significantly shortens the life of the knife in the cutting process such as slitting the film, resulting in reduced productivity and in handling such as scratches at the edges of the film. There was a problem. Further, when it is used for a magnetic tape, the head of the recorder is worn, which is a problem.

In contrast to these methods, a method has been proposed in which a composition obtained by blending a void-forming polymer with a polyester resin is melt-extruded to form an unstretched film, and the void is generated by stretching the unstretched film (for example, a special method). Kaisho 6
3-168441, JP-A-2-235942). Specific examples of such a void-forming polymer include polypropylene, polystyrene, polymethylmethacrylate, polymethylpentene, polyphenylene sulfide, polyphenylene oxide, and liquid crystalline polyester. In this method, the void-forming polymer is finely dispersed in the polyester film, and the dispersed particle size depends on the affinity between the void-forming polymer and the polyester and the difference in melt viscosity between them. By stretching the unstretched film composed of this composition at least uniaxially, peeling occurs at the interface between the void-forming polymer dispersed in the form of fine particles and the polyester, and a void is formed. The degree of void formation depends on the glass transition temperature of the void-forming polymer and the affinity between the void-forming polymer and the polyester, in addition to the stretching conditions.
The method for obtaining a lightweight polyester film by blending this void-forming polymer is as described in (a) above.
It is an excellent method without the drawbacks mentioned in the methods (b) and (c). However, even in the case of a lightweight polyester film using the above specifically proposed polymer as a void-forming polymer, its performance is not always satisfactory.

When a polymer having a relatively low glass transition temperature such as polypropylene, polystyrene, polymethylmethacrylate and polymethylpentene is used as the void-forming polymer, for example, the melt viscosity of these void-forming polymers at the melt extrusion temperature of the polyester resin. Therefore, the void-forming polymer in the unstretched film obtained by melt-extruding the composition tends to become rod-shaped particles oriented in the flow direction of the polyester due to shear stress during melt-extrusion, and is difficult to disperse in spherical particles. Therefore, there has been a problem that the unstretched film and the stretched film made of such a composition have large anisotropy. Further, when an unstretched film composed of such a composition is stretched at a temperature not lower than the glass transition temperature of the polyester resin, the void-forming polymer has a low glass transition temperature, and thus the dispersed void-forming polymer is easily plastically deformed, resulting in void formation. Occurrence is inhibited. In addition, the degree of fine irregularities on the surface of the stretched film becomes insufficient. Therefore, a film made of such a composition is not satisfactory in terms of weight saving, hiding power and writability. Further, since the glass transition temperature of the void-forming polymer is low, it has been pointed out that a film made of such a composition has a large change in performance at a temperature higher than that. On the other hand, when polyphenylene sulfide and polyphenylene oxide are used as the void-forming polymer, these polymers have a relatively high glass transition temperature, so that the above problems do not occur. However, these polymers are colored in themselves, and the film formed from such a composition has a disadvantage of poor whiteness.

The method of using a liquid crystalline polyester as the void-forming polymer has problems associated with that it tends to be rod-shaped particles oriented in the flow direction in an unstretched film, and problems caused by its extremely low affinity with polyester. Every point occurs. It goes without saying that the whiteness is inferior.

As described above, since a suitable void-forming polymer has not been found even in the method of blending the void-forming polymer with the polyester resin, a lightweight polyester film having satisfactory properties has not been obtained. Met.

[0008]

Under such circumstances, the object of the present invention is to find a void-forming polymer having an appropriate affinity with polyester and a difference in melt viscosity, and using this, it is possible to obtain a low density, strength and heat resistance. To provide a polyester film having excellent properties, hiding power and writability, and a method for producing the same.

[0009]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that the composition comprises a crystalline polyester resin (A) and a cyclic olefin copolymer (B). Finds that a polyester film having a specific structure satisfies all the problems of the present invention,
The present invention has been reached.

That is, the first gist of the present invention is that the crystalline polyester resin (A) is 50% by weight or more and less than 95% by weight, and the cyclic olefin copolymer (B) is 5% by weight or more 50% by weight.
A polyester film having a composition of less than 10% by weight, wherein the cyclic olefin copolymer (B) is dispersed in the polyester film in the form of particles having a diameter of 0.1 to 20 μm, and fine voids are formed. It is a lightweight polyester film that is present and has an apparent specific gravity of 0.3 to 1.1. However, the cyclic olefin copolymer (B) is a copolymer of ethylene and at least one cyclic olefin selected from the group consisting of bicycloalkenes and tricycloalkenes, and the glass of the crystalline polyester resin. It has a glass transition temperature at least 10 ° C. above the transition temperature.

A second aspect of the present invention is that the crystalline polyester resin (A) is 50% by weight or more and less than 95% by weight,
An unstretched polyester film obtained by melt-extruding a composition comprising a cyclic olefin copolymer (B) in an amount of 5% by weight or more and less than 50% by weight, having a glass transition temperature of the crystalline polyester resin (A) or more, The method for producing a lightweight polyester film is characterized in that it is stretched at least 1.5 times in a uniaxial direction in a temperature range not higher than the crystallization temperature.

In the present invention, the crystallinity means that the heat of fusion of the crystal measured by a differential thermal analyzer at a temperature rising rate of 20 ° C./min shows a value of 1 cal / g or more. The crystallization temperature is defined as the crystallization temperature peak when measured with a differential thermal analyzer at a temperature rising rate of 20 ° C./min. The glass transition temperature means the freezing onset temperature of thermal motion of a polymer main chain as generally defined. This glass transition temperature can also be measured using a differential thermal analyzer.

The crystalline polyester resin (A) used in the present invention is a polyester mainly containing an aromatic dicarboxylic acid residue and an aliphatic diol residue and / or an alicyclic diol residue. Representative examples of such aromatic dicarboxylic acid include terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid and the like. The aromatic ring of these aromatic dicarboxylic acids may be substituted with halogen, an alkyl group or other substituents. Typical examples of the aliphatic diol or alicyclic diol are ethylene glycol, diethylene glycol, propylene glycol, butylene glycol, neopentyl glycol,
There is cyclohexylene dimethanol. Two or more kinds of these aromatic dicarboxylic acids, aliphatic diols and / or alicyclic diols can be mixed and used.

In the present invention, a particularly preferable crystalline polyester resin is polyethylene terephthalate (PE
T), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), and polycyclohexylene dimethylene terephthalate (PCT). The glass transition temperature (Tg), crystallization temperature (Tc), and crystal melting temperature (Tm) of these crystalline polyester resins measured with a differential thermal analyzer are as follows. PET (Tg = 70 ° C., Tc = 160 ° C., Tm = 263 ° C.) PBT (Tg = 45 ° C., Tc = 130 ° C., Tm = 220 ° C.) PEN (Tg = 115 ° C., Tc = 220 ° C., Tm = 272 ° C. ) PCT (Tg = 90 ° C, Tc = 180 ° C, Tm = 295 ° C)

In the present invention, two or more kinds of such crystalline polyester resins may be mixed and used, or a crystalline polyester resin obtained by copolymerizing other components may be used.

These crystalline polyester resins are produced by directly reacting an aromatic dicarboxylic acid with an aliphatic diol and / or an alicyclic diol, as well as an alkyl ester of an aromatic dicarboxylic acid and an aliphatic diol and / or Alternatively, it is produced by a method such as transesterification with an alicyclic diol followed by polycondensation, or polycondensation of a diglycol ester of an aromatic dicarboxylic acid.

There is no particular limitation on the molecular weight of the crystalline polyester resin used in the present invention, and it is sufficient if the crystalline polyester resin has a film forming ability, but at 25 ° C. in a mixed solvent of phenol / tetrachloroethane = 1/1 (weight ratio). The measured intrinsic viscosity is preferably 0.4 (dl / g) or more.

The cyclic olefin copolymer (B) used in the present invention is ethylene and at least one selected from the group consisting of bicycloalkenes and tricycloalkenes.
It is a copolymer composed of one kind of cyclic olefin and has a glass transition temperature which is at least 10 ° C. higher than the glass transition temperature of the crystalline polyester constituting the composition.

Typical examples of such cyclic olefins include:
Bicyclo [2,2,1] hept-2-ene, 6-methylbicyclo [2,2,1] hept-2-ene, 5,6-dimethylbicyclo [2,2,1] hept-2-ene, 1-
Methylbicyclo [2,2,1] hept-2-ene, 6-
Ethylbicyclo [2,2,1] hept-2-ene, 6-
n-butylbicyclo [2,2,1] hept-2-ene,
6-i-butylbicyclo [2,2,1] hept-2-ene, 7-methylbicyclo [2,2,1] hept-2-ene, tricyclo [4,3,0,1 ^2.5] -3 Decene,
2-methyl - tricyclo [4,3,0,1 ^2.5] -3
Decene, 5-methyl-tricyclo [4,3,0,
1 ^2.5 ] -3-decene, tricyclo [4,4,0,
1 ^2.5] -3-decene, 10-methyl - there is tricyclo [4,4,0,1 ^2.5] -3-decene.

The cyclic olefin copolymer used in the present invention is a copolymer of ethylene and the cyclic olefins exemplified above. The content of the cyclic olefin component in the copolymer is usually 10 mol%. The content of the ethylene component is in the range of 40 mol% or more and less than 60 mol% and the content of the ethylene component is usually 40 mol% or more and less than 90 mol%. In addition to the essential two components, another copolymerizable unsaturated monomer component may be copolymerized, if necessary, within a range not impairing the object of the present invention. As the copolymerizable unsaturated monomer, propylene, 1-butene, 4-methyl-1-pentene, 1-
Hexene, 1-octene, 1-decene, 1-dodecene,
Α having 3 to 20 carbon atoms such as 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene
-Olefin, cyclopentene, cyclohexene, 3-
Methylcyclohexene, cyclooctene, 1,4-hexadiene, 4-methyl-1,4-hexadiene, 5-methyl-1,4-hexadiene, 1,7-octadiene,
Examples thereof include dicyclopentadiene, 5-ethylidene-2-norbornene, 5-vinyl-2-norbornene, tetracyclododecene, 2-methyltetracyclododecene, and 2-ethyltetracyclododecene. The cyclic olefin copolymer used in the present invention can be produced by a known liquid phase polymerization method. For example, JP-A-6
The cyclic olefin copolymer can be produced according to the method exemplified in JP-A 1-271308.

The cyclic olefin copolymer used in the present invention has a glass transition temperature which is at least 10 ° C. higher than the glass transition temperature of the crystalline polyester resin, and more preferably at least 20 ° C. higher. When the glass transition temperature of the cyclic olefin copolymer is equal to or lower than that of the crystalline polyester resin, stretching the film made of the composition causes the cyclic olefin copolymer to plastically deform and inhibit the formation of voids. It is not preferable. In addition, the degree of fine irregularities on the surface of the stretched film becomes insufficient. When a cyclic olefin copolymer having such a low glass transition temperature is used, the resulting stretched polyester film is not satisfactory in lightness, hiding power and writability.

The molecular weight of the cyclic olefin copolymer used in the present invention is not particularly limited, and the molecular weight giving a melt viscosity of at least 1/10 or more of the melt viscosity of the crystalline polyester resin at the temperature of melt extrusion of the composition. That's enough.

The amount of the crystalline polyester resin as the component (A) of the composition constituting the lightweight polyester of the present invention is 50% by weight or more and less than 95% by weight, and the cyclic olefin copolymer as the component (B). It is 5% by weight or more and less than 50% by weight. When the content of the cyclic olefin copolymer as the component (B) is less than 5% by weight, the amount of fine voids generated in the final stretched polyester film obtained is small, resulting in insufficient lightness, hiding power and writability. Becomes On the contrary, if the content is 50% by weight or more, not only the strength of the film is lowered and breakage during stretching is likely to occur, but also the thermal stability during melt extrusion of the composition is lowered and thermal decomposition or the like occurs, which is not preferable. .

In the lightweight polyester film of the present invention, the cyclic olefin copolymer as the component (B) is dispersed in the form of particles having a diameter of 0.1 to 20 μm in a matrix composed of the crystalline polyester resin as the component (A). It is necessary that the thickness be in the range of 1 to 15 μm.
In this case, the fine particles of the cyclic olefin copolymer are preferably spherical as much as possible. The spherical shape is preferred because the anisotropic shape of the film is unlikely to occur, uniform voids are formed by stretching, and a film having excellent strength can be obtained. When the fine particle diameter of the cyclic olefin copolymer is less than 0.1 μm, the amount of voids formed in the stretched polyester film is small, and the lightness, hiding power and writability are insufficient. On the other hand, if it exceeds 20 μm, the amount of voids generated is large, and although the lightness and the concealing property are excellent, the strength of the film is lowered and the breakage during stretching tends to occur, which is not preferable.

In the present invention, the unstretched film is a crystalline polyester resin (A) and a cyclic olefin copolymer (B).
It is obtained by melt-extruding a composition comprising In this case, the crystalline polyester resin (A) and the cyclic olefin copolymer (B) are melt-kneaded using an extruder or the like to prepare pellets of the composition in advance, and this is melt-extruded to form an unstretched film. You may get it. Alternatively, the crystalline polyester resin (A) and the cyclic olefin copolymer (B) may be blended and directly melt-extruded to obtain an unstretched film. The melt extrusion temperature is usually a crystal melting temperature of the crystalline polyester resin or 50 from the temperature.
Selected in the range of ℃ higher temperature. The melted composition is extruded from, for example, a T die or a circular die and cooled to a temperature not higher than the glass transition temperature of the crystalline polyester resin to obtain a substantially amorphous film. The unstretched film is virtually free of voids and is often transparent or translucent.

Then, the unstretched film is stretched in at least a uniaxial direction. The stretching is performed at a temperature in the range of the glass transition temperature of the crystalline polyester resin or higher and the crystallization temperature or lower. By stretching in this temperature range, the crystalline polyester forming the matrix is oriented and crystallized, and at the same time voids are effectively generated at the interface with the cyclic olefin copolymer forming the fine particles. The draw ratio has a great influence on the properties of the obtained film. The higher the draw ratio, the more lightweight, hiding and strength the resulting film will be. Although it is possible to select the draw ratio according to the purpose, it is necessary to draw at least uniaxially at least 1.5 times in order to achieve the purpose of the present invention. Furthermore, it is also possible to stretch in the longitudinal and transverse biaxial directions. In this case, the characteristics can be further improved. Also in this case, it is preferable to stretch by 1.5 times or more in each direction.

The stretched polyester film is heat-fixed if necessary. This treatment further increases the dimensional stability of the stretched polyester film. Usually, the temperature for heat setting is about 10 ° C higher than the crystal melting temperature of the crystalline polyester resin.
To 30 ° C lower.

When the lightweight polyester film of the present invention is stretched, its apparent specific gravity becomes 1.1 or less. The apparent specific gravity can be made 0.3 or less by further increasing the draw ratio. However, the productivity and strength of the stretched film decrease, so that the apparent specific gravity of the lightweight polyester film is preferably in the range of 0.3 to 1.1. From the viewpoint of productivity and performance, the most preferable range of specific gravity is 0.5 to 1.0.

If desired, other polymers may be added to the composition of the present invention as long as the characteristics are not significantly impaired. Examples of such a polymer include polyethylene,
Polypropylene, polystyrene, polymethylpentene,
Polymethylmethacrylate, ABS resin, polycarbonate, polyamide, polyphenylene oxide, polyarylate, polyethylene glycol, polysulfone,
There are polyether sulfone, polyether ether ketone, polyphenylene sulfide, phenoxy resin, polyvinyl chloride, polyvinylidene chloride, liquid crystal polyester, fluororesin, phenol resin, melamine resin, urea resin, epoxy resin and the like.

If necessary, the composition of the present invention may contain inorganic fillers and pigments such as silica, titanium oxide, calcium carbonate, alumina, kaolin, mica and talc, as long as the characteristics are not significantly impaired. It is possible.

It is also possible to add other additives. Such examples include antioxidants, heat-resistant agents, antistatic agents, ultraviolet absorbers, lubricants, colorants and the like.

The lightweight polyester film of the present invention is used alone or after being subjected to treatments such as painting, printing and metallizing, as packaging materials, labels, information papers, drawing papers, printing papers and the like. Further, the lightweight polyester film of the present invention can be used as one of the constituent components of the multilayer structure. Examples of such multi-layer structures include laminate films, composite sheets, composites and the like.

[0033]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto.

Reference Example 1 6-Ethylbicyclo [2.
2.1] VO (OC ₂ H ₅ ) C as a vanadium compound in the ratio of 60 g of hept-2-ene and 1 L of toluene.
l ₂ was added to 1 mmol / L and ethylaluminum sesquichloride was added to 4 mmol / L, and ethylene was added to 40
L / hour, nitrogen was supplied at 80 L / hour, and the copolymerization reaction was continuously carried out at 10 ° C. Then, the polymerization was stopped with a small amount of methanol, and the mixture was precipitated with a large amount of acetone / methanol to obtain a cyclic olefin copolymer (A-1). Other cyclic olefin copolymers were similarly obtained. These are shown in Table 1. The glass transition temperature is shown in Table 3.

[0035]

[Table 1]

(Examples 1 to 7) Using a direct-heating vacuum dryer manufactured by Tabai Co., Ltd., vacuum drying was carried out at 120 ° C. for 48 hours, and cyclic olefin copolymer pellets having a water content of 0.001% and ultimate Polyethylene terephthalate pellets (a-1 in Table 2 below) having a viscosity of 0.78 were mixed by a Henschel mixer in a mixing ratio shown in Table 3, and were mixed at 280 ° C. using a 50 mmφ extruder manufactured by Nippon Steel Co., Ltd. It was melt extruded by a T-die to obtain a sheet having a thickness of 500 μm. Next, at 90 ℃, MD (vertical direction) 3 times, TD (horizontal direction) 3 times
The film was double-stretched to obtain a film having a thickness of 55 μm. Dispersion particle size and stretchability of cyclic olefin copolymer in unstretched film, apparent specific gravity of stretched film, three-dimensional surface roughness, haze (H
Table 4 shows z), parallel light transmittance (Tp), glossiness (G), whiteness, tensile strength, and writability. In addition, Example 7 using a polybutylene terephthalate resin (A-2 in Table 2) as the crystalline polyester resin was carried out at a film forming temperature of 260 ° C and a stretching temperature condition of 60 ° C. Similarly, the performance of the film is shown in Table 4.

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

[Table 4]

(Comparative Examples 1 to 7) In the same manner as in Examples,
The pellets were dried and melt extruded to obtain a film having a thickness of 500 μm. Stretching temperatures are 90 ° C, 60 ° C, and 20
It was carried out under the condition of 0 ° C. The stretching ratio in Comparative Example 6 was 1.
It was carried out at 3 × 3 times, except for 2 × 1.0. In Comparative Example 7, a polyphenylene oxide resin (PPO-534, Nagase & Co., Ltd.) was used as the additive polymer for comparison with the cyclic olefin copolymer. Experimental conditions are shown in Table 5,
The performance of the film is shown in Table 6.

[0041]

[Table 5]

[0042]

[Table 6]

The following can be seen from Examples 1 to 7 and Comparative Examples 1 to 7. It can be seen that when the ethylene component of the cyclic olefin copolymer constituent is 84 mol%, the crystallized polyester resin and the cyclic olefin copolymer composition film lack in lightness, matteness and whiteness. It can be seen that the crystalline polyester resin containing 3% by weight of the cyclic olefin copolymer and the cyclic olefin copolymer composition film are similarly lacking in lightness, matteness and whiteness. Further, the crystalline polyester resin containing 55% by weight of the cyclic olefin copolymer and the cyclic olefin copolymer composition film have no confirmed dispersed particle size, and it is clear that they lack in lightness, matteness and whiteness.

Regarding the stretching temperature, 90 ° C. is preferable.
At 60 ° C, the film breaks and the stretchability is poor. 200 ° C
It can be seen that when the film is stretched with, the film thickness is largely uneven and the film quality is significantly deteriorated, and the film lacks in lightness, matteness and whiteness. 1.2 × for the draw ratio
It can be seen that at 1.0 times, lightness, mattness and whiteness are lacking. Further, in Comparative Example 7, it can be seen that the strength of the obtained film remarkably decreases when the dispersed particles exceed 20 μm.

(Measurement Method) The measurement method used in the examples is as follows. 1. Particle size A JSM-15 scanning electron microscope manufactured by JEOL Ltd. is used to read the dimensions from the electron microscopic photograph of the fracture surface of the sample. 2. Apparent specific gravity It was measured by the water displacement method according to JIS K6758. 3. Surface roughness The surface roughness was measured using a surface roughness measuring instrument SE-3AK type manufactured by Kosaka Laboratory Ltd. according to the method of JIS B-0601-1976. Stylus diameter 2 μm, Stylus pressure 10 mg. 4. Haze (Hz), parallel light transmittance (Tp) Using a haze meter manufactured by Tokyo Denshoku Co., Ltd., ASTM D10
It measured according to 03-61. 5. G% Murakami Color Research Laboratory Gloss Meter G
Using a M-30 type, the glossiness at an incident angle of 20 ° was measured. 6. The whiteness was measured using a Suga Tester SM-4 type color difference meter. 7. Strength (tensile strength) Using Shimadzu DSS-500 type autograph,
It was measured according to STMD882 and displayed as the average value in the vertical and horizontal directions. 8. Writable pencil Scratch value test pencil UNI manufactured by Mitsubishi Corp. was used to write with a pressure of 500 g, and when the character was written darkly, it was marked with O, when it was written very darkly, and when it was thin, it was marked with X.

[0046]

As described in detail in the examples, the polyester film provided by the present invention, by using the specific void-forming polymer, has excellent lightness, strength, heat resistance, and hiding power. Writability and productivity are imparted, and in these respects, the performance of conventional lightweight polyesters is greatly exceeded.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical display location B29K 67:00 105: 04 B29L 7:00 4F C08L 67:02 (72) Inventor Minoru Kishida Kyoto Prefecture 23, Uji Kozakura, Uji-shi, Central Research Institute of Unitika Co., Ltd. (72) Inventor Yoshiaki Kozuka 23, Uji-Kozakura, Uji-shi, Kyoto Pref.

Claims (2)

[Claims] 1. A polyester film comprising a composition comprising a crystalline polyester resin (A) of 50% by weight or more and less than 95% by weight and a cyclic olefin copolymer (B) of 5% by weight or more and less than 50% by weight. And the cyclic olefin copolymer (B) has a diameter of 0.1 in the polyester film.
To 20 μm in the form of particles, fine voids are present, and the apparent specific gravity is 0.3 to 1.1. A lightweight polyester film. However, the cyclic olefin copolymer (B) is a copolymer of ethylene and at least one cyclic olefin selected from the group consisting of bicycloalkenes and tricycloalkenes, and the glass of the crystalline polyester resin. It has a glass transition temperature at least 10 ° C. above the transition temperature. 2. A composition comprising a crystalline polyester resin (A) of 50% by weight or more and less than 95% by weight and a cyclic olefin copolymer (B) of 5% by weight or more and less than 50% by weight is melt extruded. The unstretched polyester film obtained is stretched at least uniaxially by 1.5 times or more in a temperature range from the glass transition temperature of the crystalline polyester resin (A) to the crystallization temperature thereof. A method for producing a lightweight polyester film according to claim 1.