JP2001334570A - Pore-containing polyester film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a film having both excellent characteristics (light weight, cushionability and the like) originated from a pore-containing structure and good handleability (conveyability). SOLUTION: In a pore-containing polyester film comprising many pores originated from a thermoplastic resin non-compatible with a polyester resin, a curl value after heat treating at a (Tg+30) deg.C for 30 min in a no load state is 1.0 mm or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a void-containing polyester film having a low curl, and more particularly to a void-containing polyester film which is excellent in handling properties in post-processing and paper feeding / discharging properties and is suitable as a film for various printing and cards. It relates to a polyester film.

[0002]

2. Description of the Related Art Synthetic papers made of synthetic resins as main raw materials have been developed for various applications because of their excellent water resistance, surface gloss, and suitability for printing on smooth surfaces. In particular, polyester resin represented by polyethylene terephthalate,
Among synthetic paper raw materials, they are characterized by high heat resistance and high rigidity, and their use range is expanding.

[0003] Further, due to the cushioning property exhibited by the void-containing structure, it is widely used as various printing films, including thermal transfer printing disclosed in, for example, JP-A-63-280687.

[0004] As the void-containing polyester film used for such an application, a film in which voids are formed around the particles by mixing and stretching inorganic fine particles in polyester, or a thermoplastic resin incompatible with the polyester resin is used. There is known a method in which a resin or the like is mixed and dispersed in polyester and used as a core for forming a cavity. Especially the latter,
Widely used because it can reduce the weight of the film.

[0005] Examples of the cavity-forming agent used for forming the cavity include polyolefin resins represented by polypropylene resin and polymethylpentene resin (for example, JP-A-49-34755), and polystyrene resins (for example, JP-B-49-2016, JP-B-54-2016
No. 2955) has been proposed.

[0006] In particular, the use of base films for IC cards and various cards requires rigidity and high concealment, so that relatively thick films of 50 µm or more, preferably 100 µm or more are suitably used. . It is desired that films used for such applications have excellent flatness and little curl.

This is because, when a curled film is used at the time of high-speed handling in various printing methods and the like, problems such as poor printing due to poor feeding properties and poor transportability such as paper jam occur.

[0008] Usually, the film is stored in a roll form and stored. However, if the film is stored for a long period of time, the curl gradually becomes tight inside the roll, that is, a so-called curl curl is formed. Although this curl curl can be removed by heat treatment while keeping it flat, it is desirable to use it without heat treatment in terms of cost.

[0009] A white polyester film containing a white pigment such as titanium oxide or calcium carbonate and containing no void caused by a thermoplastic resin incompatible with the polyester resin, or an ordinary transparent polyester film has a curl curl. Occurrence is very modest and is of little concern.

On the other hand, in the case of a void-containing polyester film containing a large number of voids derived from a thermoplastic resin that is incompatible with the polyester resin inside the film, it is very difficult to obtain a film with less curl by the conventional technology. Have difficulty.

In the production of a thick biaxially stretched film, first, the unstretched film becomes very thick, so that the cooling by the cooling roll is clearly different between the cooling surface and the opposite side. And other structures will be different on the front and back of the film.

Further, since the film is a void-containing polyester film containing fine voids therein, the size, shape, and volume fraction of the voids easily change in the thickness direction of the film. It is extremely difficult to produce a film that makes the same.

As described above, the difference in structure easily occurs between the front and back of the film, and the structure containing voids therein easily causes curl due to the slight difference in structure between the front and back of the film. Since it increases at a high speed, the passability in the printing process is significantly deteriorated. Such a thick void-containing polyester film is liable to curl, and it has been very difficult to stably produce the film at a low curl value.

[0014]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to obtain excellent characteristics (light weight, cushioning properties, etc.) and good handling properties (transportability) derived from the structure containing cavities. An object of the present invention is to provide a film having the above characteristics.

[0015]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and a void-containing polyester film capable of solving the above-mentioned problems comprises:
It is as follows.

The first invention of the present invention is a cavity-containing polyester film containing a large number of cavities derived from a thermoplastic resin incompatible with a polyester resin inside the film, wherein (Tg + 30) ) A void-containing polyester film having a curl value of 1.0 mm or less after heat treatment at 30 ° C for 30 minutes.

A second invention is the void-containing polyester film according to the first invention, wherein the thickness of the void-containing polyester film is 50 to 500 μm.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The polyester in the present invention includes aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid and naphthalenedicarboxylic acid or esters thereof and ethylene glycol, diethylene glycol, 1,4-butanediol and neopentyl glycol. It is a polyester produced by subjecting a glycol to an esterification or transesterification reaction followed by polycondensation. These polyesters may be subjected to a direct esterification reaction of an aromatic dicarboxylic acid and a glycol followed by polycondensation, or may be subjected to a transesterification reaction between an alkyl ester of an aromatic dicarboxylic acid and a glycol followed by polycondensation, or It can be produced by a method such as polycondensation of a diglycol ester of an aromatic dicarboxylic acid.

Representative examples of the polyester include polyethylene terephthalate, polyethylene butylene terephthalate and polyethylene 2,6-naphthalate. This polyester may be a homopolymer or a copolymer of the third component. In any case, in the present invention, a polyester containing ethylene terephthalate units, butylene terephthalate units or ethylene-2,6-naphthalate units in an amount of 70 mol% or more, preferably 80 mol% or more, more preferably 90 mol% or more. preferable.

The voids in the void-containing polyester film of the present invention are obtained by mixing and dispersing a polyester resin and an incompatible thermoplastic resin or the like in polyester, extruding the unstretched film into a sheet shape, at least in a uniaxial direction. It can be formed by stretching.

When a thermoplastic resin incompatible with the polyester resin is used as a core for forming the cavity, the thermoplastic resin incompatible with the polyester resin may be a polystyrene resin, a polyphenylene ether resin, a polyolefin resin, or the like. Although illustrated, it is not limited to these.

The term "polystyrene resin" refers to a thermoplastic resin having a polystyrene structure as a basic structural unit. In addition to homopolymers such as atactic polystyrene, syndiotactic polystyrene and isotactic polystyrene, other components are grafted or blocked. It includes copolymerized modified resins such as impact-resistant polystyrene resins and modified polyphenylene ether resins, as well as mixtures of these polystyrene resins with thermoplastic resins such as polyphenylene ether.

The polymethylpentene resin is a polymer having at least 80 mol%, preferably at least 90 mol%, of units derived from 4-methylpentene-1. Other components include ethylene units and propylene units. , Butene-1 unit, a derivative unit derived from 3-methylbutene-1 and the like. The melt flow rate of such polymethylpentene is preferably 200 g / 10 min or less, particularly preferably 30 g / 10 min or less. This means that if the melt flow rate exceeds 200 g / 10 minutes,
This is because it is difficult to obtain the effect of reducing the weight of the film.

The polypropylene-based resin includes not only homopolymers such as isotactic polypropylene and syndiotactic polypropylene but also modified resins obtained by grafting or block copolymerizing other components.

As for the presence state of the polypropylene resin, the above-mentioned polypropylene resin may be used separately from the above-mentioned polymethylpentene, or propylene units may be introduced into the polymethylpentene resin as a copolymer component. A thing may be used.

The mixing amount of these cavity-forming agents, that is, the thermoplastic resin incompatible with the polyester resin, to the polyester varies depending on the amount of the target cavity, but is 3 to 20% by weight based on the whole film. The content is preferably in the range, and more preferably 5 to 18% by weight. If the amount is less than 3% by weight, there is a limit in increasing the amount of cavities generated. Conversely, if the content is 20% by weight or more, the stretchability of the film is significantly impaired, and the heat resistance, strength, and stiffness are impaired.

The film may contain inorganic or organic particles as necessary in the polyester resin or the immiscible resin in order to improve the concealing property and the like. Examples of the particles include silica, kaolinite, talc, calcium carbonate, zeolite, alumina, barium sulfate, carbon black, zinc oxide, titanium oxide, zinc sulfide, and organic white pigments, but are not particularly limited. Absent.

The void-containing polyester film of the present invention has an apparent specific gravity of 1.30 or less, more preferably 1.2.
0 or less, and most preferably 1.15 or less. When the apparent specific gravity exceeds 1.30, the amount of voids existing in the film is too small, and the excellent properties (for example, light weight, cushioning properties, etc.) obtained by the void-containing structure become insufficient.

On the other hand, the lower limit of the apparent specific gravity is not particularly limited. However, if the apparent specific gravity is less than 0.80, the handling property of the film is remarkably reduced, which is not preferable.

As a more preferable film raw material for controlling the apparent specific gravity of the film to 1.30 or less, for example, polyethylene terephthalate resin is 80.0 to 98.0.
% By weight, polystyrene resin is 0.5 to 10.0% by weight,
Examples of the composition include 0.5 to 10.0% by weight of a polypropylene resin, 0.5 to 10.0% by weight of a polymethylpentene resin, and 0.5 to 20.0% by weight of titanium oxide particles.

The void-containing polyester film of the present invention may have a single-layer structure or a multilayer structure in which the same or different synthetic resin film layers are combined. The synthetic resin film layer used for such a composite is obtained, for example, by a co-extrusion method, but can also be formed by a coating method, a lamination method via an adhesive layer or the like.

As the synthetic resin film, for example, a film mainly containing one or more of polyethylene terephthalate, polyethylene naphthalate, polyphenylene sulfide, polyarylate and polyimide can be used, but is not limited thereto. Not something.

Further, the synthetic resin film layer may contain a coloring agent, a light-fast agent, a fluorescent agent, an antistatic agent, and the like, if necessary.

The method for producing the void-containing polyester film of the present invention is arbitrary, and is not particularly limited. For example, a mixture having the above-described composition is formed into a film to obtain an unstretched film. A general method of stretching a stretched film at least in a uniaxial direction can be used.

The conditions for stretching and orienting the unstretched sheet are closely related to the formation of cavities. Hereinafter, the stretching and orientation conditions will be described by taking the most suitable sequential biaxial stretching method, particularly a method of stretching an unstretched sheet in the longitudinal direction and then in the width direction.

First, in the first-stage longitudinal stretching step, stretching is performed between two or many rolls having different peripheral speeds. As a heating means at this time, a method using a heating roll, a method using a non-contact heating method may be used, or both may be used. However, in order to develop a large number of cavities at the interface of the incompatible resin, the film is stretched 3 to 5 times at a polyester secondary transition temperature Tg to (Tg + 50 ° C.). Next, the uniaxially stretched film is introduced into a tenter, and stretched 2.5 to 5 times in the width direction at a temperature of the polyester second transition temperature Tg to (melting point Tm−10 ° C. or less).

The biaxially stretched film thus obtained is optionally subjected to a heat treatment. The heat treatment is preferably performed in a tenter, and the polyester (melting point Tm-5)
(0 ° C.) to Tm.

The polyester film containing fine voids of the present invention may have a coating layer on at least one of the surfaces. By providing a coating layer, it is possible to improve the wettability and adhesion of ink, a coating agent, and the like.

As the compound constituting the coating layer, a copolymerized polyester resin is preferable. In addition, as a compound for improving the adhesiveness of a general polyester film, such as a polyurethane resin, a polyester urethane resin, and an acrylic resin, may be used. The disclosed compounds and the like are applicable.

As a method of providing a coating layer, a commonly used method such as a gravure coating method, a kiss coating method, a dip method, a spray coating method, a curtain coating method, an air knife coating method, a blade coating method, and a reverse roll coating method is applied. it can.

As a step of applying, any method such as a method of applying before stretching the film, a method of applying after longitudinal stretching, and a method of applying to the film surface after the orientation treatment is possible.

The polyester film containing fine voids thus obtained has excellent properties (light weight, cushioning properties, etc.) derived from the void containing structure and good handling properties (transportability).

Further, since a surfactant or a polyether-based resin is not required as a dispersant for the polyolefin-based resin, it has excellent heat resistance, and its color change is small even when self-recovered raw materials are reused. Also has excellent manufacturing stability. When the self-recovered raw material is reused, a preferable usage ratio is 5 to 50% by weight of the whole film.

In a void-containing polyester film containing a large number of voids derived from a thermoplastic resin incompatible with the polyester resin inside the film, the void-containing structure inside the film easily causes a slight difference in structure between the front and back sides. Since the curl appears and the curl increases at a high speed, the passability in the printing step, which is the object of the present invention, is significantly deteriorated.

However, since the film of the present invention has a curl value of 1 mm or less after being heat-treated at (Tg + 30) ° C. for 30 minutes under no load, curl before storage naturally suppresses curl curl due to roll storage. can do. In other words, by increasing the production of the void-containing polyester film in a strictly controlled state of essentially zero curl, the subsequent increase in curl can be suppressed.

Even if the curl immediately after stretching is large, it can be rolled and stored in the opposite direction, and the curl can be made in the reverse direction to make it seemingly zero curl. However,
Even in the case of apparently zero curl by such a method, the curl component disappears due to the heat of the printing process, and a large initial curl may result in poor printing. From this point, the initial curl value is strictly controlled. It is important to keep.

In order to produce the film of the present invention, a special production method is required in addition to the production method described above.

As a method of suppressing the curl immediately after the film production, (1) the volume fraction of the cavities is made small and the size of each cavity is made small so as to withstand the internal strain and suppress the occurrence of the curl. (2) a method of providing a distribution of cavities in the film thickness direction, (3) a difference in crystallinity in the film thickness direction due to a difference in crystallinity in the film thickness direction due to a cooling difference during extrusion. In order to control the resulting curl, a method of positively generating a structural difference between the front and back surfaces of the film, compensating for the necessary structural difference, and bringing the curl value close to zero is preferable.

Specifically, in the stretching step such as longitudinal stretching or transverse stretching and the heat setting step, the temperature or the amount of heat on the front and back of the film is made different to independently control the degree of orientation on the front and back of the film. By adopting conditions that balance the structure and physical properties of the front and back, zero-curl film formation is realized.

Further, as a basic requirement for stably producing curl in a low state over the entire width, it is also important to form a cavity with little change in the film thickness direction by a stretching recipe with little unevenness in thickness.

More specifically, for the curl in the vertical direction immediately after film formation, the structural difference between the front and the back of the film during longitudinal stretching is controlled, and for the curl in the horizontal direction, the structural difference between the front and the back of the film is controlled during horizontal stretching and heat setting. By doing so, an internal distortion in the opposite direction is created, and the internal distortion due to the inevitable structural difference between the front and back of the film is balanced to suppress curling.

In the case of the film of the present invention, even if the film is stored in a roll shape, the progress of curl curl can be suppressed, so that excellent properties derived from the void-containing structure and good handling properties can be obtained. .

[0053]

Next, the method for producing a film of the present invention will be described in detail with reference to examples and comparative examples. The method for evaluating the characteristics used in the present invention is described below.

(1) Glass transition temperature (Tg) A sample (10 mg) was heated at a rate of 10 ° C./min using a DSC (V4.OB2000, manufactured by DuPont), and the glass transition temperature was measured. I do.

(2) Curl value The film was cut into a length of 100 mm and a width of 50 mm, and conditioned for 1 day at a temperature of 23 ° C. and a humidity of 55 RH%. After standing on a glass plate, the vertical distance between the glass plate and the lower end of the four corners of the raised film was measured using a ruler with a minimum scale of 0.5 mm, using a ruler. did. Three samples were prepared and repeatedly measured, and the average value was defined as a curl value.

(3) Film Handling (Sheet Transfer Properties) A mixed solution of water and isopropyl alcohol (70/30; weight ratio) was prepared, and (A) a copolymerized polyester resin (manufactured by Toyobo Co., Ltd. (Bironal MD1200) 92 parts by weight (B) 2 parts by weight of an antistatic agent (Matsumoto Yushi Co., Ltd., Fcol 214) (C) 5 parts by weight of organic particles (Nippon Shokubai Co., Ltd., Eposter S6) in a mixture using a homogenizer Well dispersed,
A coating liquid was prepared so that the total solid concentration consisting of (A), (B), and (C) was 3% by weight with respect to the total amount of the coating liquid.

This coating solution was applied to the film surface by a bar coating method so as to have a coating weight of 8 g / m ² , then dried at 160 ° C. for 2 minutes, and 100 sheets of A4 size were prepared. This sample was prepared using a laser beam printer (Canon
LBP430) and conducted a transport test. The handling property was evaluated based on the following three criteria based on the frequency of occurrence of transport failures such as folding, wrinkles, and paper jams caused by this test.

：: No conveyance failure occurred in 100 sheets. Δ: One or more conveyance failures occurred in 100 sheets. ×: Five or more conveyance failures occurred in 100 sheets.

(4) Apparent specific gravity According to JIS K-7112 floating / sinking method.

(5) Intrinsic viscosity of polyester Polyester is dissolved in a mixed solvent of phenol 60% by weight and 1,1,2,2-tetrachloroethane 40% by weight.
After the solid content was substantially filtered, it was measured at 30 ° C.

Example 1 (Manufacture of master pellet)
A polystyrene resin having a melt flow rate of 2.0 g / 10 min (manufactured by Mitsui Toatsu Co., Ltd., Toporex 570) is added to a 62 dl / g polyethylene terephthalate resin (70 wt%).
-57U) 6% by weight, melt flow rate 1.7 g / 1
6% by weight of 0 minute polypropylene resin (Noblen FO-50F, manufactured by Mitsui Toatsu Co., Ltd.) and 18% by weight of polymethylpentene resin (TPX, DX-845, manufactured by Mitsui Petrochemical Co., Ltd.) having a melt flow rate of 8 g / 10 minutes. %, And the mixture was supplied to a twin-screw extruder and kneaded sufficiently. The strand was cooled and cut to produce a master pellet (A) containing a cavity-forming agent.

In addition, an intrinsic viscosity of 0.1 produced by a known method.
A mixture of 50 wt% of 62 dl / g polyethylene terephthalate resin and 50 wt% of anatase type titanium dioxide particles (TA-300, manufactured by Fuji Titanium Co., Ltd.) having an average particle diameter of 0.3 μm is supplied to a vented twin-screw extruder. And pre-mixed. The molten resin was continuously supplied to a vent-type single-screw kneader, kneaded and extruded. The obtained strand was cooled and cut to produce a titanium dioxide-containing master pellet (B).

Then, 63% by weight of a polyethylene terephthalate resin having an intrinsic viscosity of 0.62 dl / g, 25% by weight of the above-mentioned cavity-forming agent-containing master pellet (A), and 12% by weight of the master pellet (B) were mixed by pellet mixing. And vacuum dried to obtain a film material.

(Production of Unstretched Film) Next, the raw material of the above-mentioned film is supplied to an extruder, and cooled to 30 ° C. by using a T-die using an extrusion line provided with a static mixer immediately before the T-die. It was extruded on a drum to produce an unstretched film having a thickness of about 1900 μm.
The residence time after kneading was 5.3 minutes, and the opposite surface of the cooling drum was cooled by blowing cold air adjusted to 20 ° C.

(Production of Biaxially Stretched Film) The obtained unstretched film is uniformly heated to 65 ° C. by using a heating roll, and two pairs of nip rolls having different peripheral speeds (low-speed roll: 1).
(m / min, high-speed roll: 3.4 m / min). At this time, as an auxiliary heating device for the film, an infrared heater (rated: 40 W / cm) having a gold reflection film in the middle of the nip roll was installed opposite to both surfaces of the film (a distance of 1 cm from the film surface). 18W /
cm and the opposite side was heated at 12 W / cm. After longitudinal stretching,
The film was quenched evenly in a water bath controlled at 15 ° C. The uniaxially stretched film thus obtained is guided to a tenter, heated to 140 ° C., stretched 3.6 times in the transverse direction, fixed in width, subjected to a heat treatment at 230 ° C. for 5 seconds, and further heated at 210 ° C. in the width direction. To obtain a void-containing polyester film having a thickness of 188 μm. The temperature difference between the front and back of the film in the stretching section was controlled to 10 ° C., and the temperature difference between the front and back of the film in the heat fixing section was controlled to within 3 ° C. The film has a Tg of 78 ° C
Met. (Storage) The film was stored immediately after film formation, cut into a square of 500 mm, and stored at room temperature for 6 months. The curl value and the handling property after the heat treatment were examined for those that had undergone the above storage.

Comparative Example 1 At the time of preparing an unstretched film, the film surface on the opposite side of the cooling drum was not air-cooled. During longitudinal stretching, the roll preheating temperature was set to 60 ° C., and infrared heating was performed at 21 W / cm on one side and 21 W / cm on the opposite side. The film after longitudinal stretching is
The film front and back were alternately cooled by a cooling roll having a roll surface temperature of 45 ° C. The transverse stretching temperature was 130 ° C., and the heat treatment temperature after fixing the width was 210 ° C. The maximum temperature difference between the front and back of the film in the tenter was 10 ° C. Otherwise, the same procedure as in Example 1 was carried out to obtain a void-containing polyester film. The Tg of the film was 78 ° C. Immediately after film formation, it was cut into a square of 500 mm and stored at room temperature for 6 months. The curl value and the handleability of the film after the heat treatment were examined for those having undergone the above storage.

Example 2 The preheating temperature of the roll in longitudinal stretching was set to 70 ° C., and the film was immediately stretched 3.4 times. The infrared heating was performed at 15 W / cm on one side and 10 W / cm on the opposite side. After longitudinal stretching, the film
The mixture was rapidly cooled evenly in a water bath adjusted to 5 ° C. Except for this, a void-containing polyester film was obtained in the same manner as in Example 1. The Tg of the film was 78 ° C. Immediately after the film formation, 1000 rolls are formed into 6 inch diameter tubes.
The m rolls were stored at room temperature for 3 months. The curl value and the handling property after the heat treatment were examined for those that had undergone the above storage.

Comparative Example 2 At the time of producing an unstretched film, no air cooling was performed on the film surface opposite to the cooling drum. During longitudinal stretching, the roll preheating temperature was 62 ° C., and infrared heating was performed at 19 W / cm on both sides. The film after longitudinal stretching was alternately cooled on both sides of the film by a cooling roll having a roll surface temperature of 48 ° C. The transverse stretching temperature is 125 ° C, and the heat treatment temperature after fixing the width is 215 ° C.
And The temperature difference between the front and back of the film in the tenter is up to 10
° C. Otherwise, the same procedure as in Example 1 was carried out to obtain a void-containing polyester film. The Tg of the film was 78 ° C. Immediately after completion of the film formation, a tube having a diameter of 6 inches and wound in a roll of 1000 m was stored at room temperature for 6 months. The curl value and the handling property after the heat treatment were examined for those that had undergone the above storage.

Comparative Example 3 At the time of preparing an unstretched film, no air cooling was performed on the film surface opposite to the cooling drum. During longitudinal stretching, the roll preheating temperature was set to 75 ° C., and infrared heating was performed at 17 W / cm on both sides. The film after longitudinal stretching was alternately cooled on both sides of the film by a cooling roll having a roll surface temperature of 48 ° C. The transverse stretching temperature is 150 ° C, and the heat treatment temperature after fixing the width is 230 ° C.
And The temperature difference between the front and back of the film in the tenter is up to 15
° C. Otherwise, the same procedure as in Example 1 was carried out to obtain a void-containing polyester film. The Tg of the film was 78 ° C. Immediately after the completion of the film formation, a tube having a diameter of 6 inches wound in a roll of 1000 m was stored at room temperature for 3 months. The curl value and the handling property after the heat treatment were examined for those that had undergone the above storage.

Table 1 shows the measurement results of the void-containing polyester film obtained by the above method.

[0071]

[Table 1]

From the measurement results in Table 1, the following can be considered. It can be seen that the films of Examples 1 and 2 satisfying the requirements defined in the present invention can obtain good handling properties while maintaining excellent properties (light weight: apparent specific gravity) due to the void-containing structure. On the other hand, in the films of Comparative Examples 1, 2, and 3 having a curl value of 1 mm or more after heat treatment at (Tg + 30) ° C. for 30 minutes under no load, transport failures occurred frequently in the transportability test by the printing press. .

[0073]

As described above, the void-containing polyester film of the present invention reduces the difference in physical properties due to the difference in structure between the front and back of the film, and (Tg + 30) with no load.
C) after heating at 5 ° C for 5 minutes is 1 mm or less, so that the curling curl is small even if the film is stored in a roll form for a long period before storing the film. Therefore, it is possible to obtain good handling properties (transportability) while maintaining excellent characteristics (light weight, cushioning property, printability, etc.) derived from the cavity-containing structure, and particularly, to include a cavity having a thickness of 50 to 500 μm. It is suitable as an application using a polyester film (base film for various printing, card, etc.).

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C08L 67:00 C08L 67:00 F term (Reference) 4F074 AA24 AA26 AA32 AA65 AA66 AA97 AB05 AC17 AE05 CA02 CA03 CA04 CA05 CA06 CA07 CC02Y CC02Z CC04Y CC04Z CC05Z CC22X CC32Y CC32Z CC49 DA02 DA23 DA24 DA52 DA54 DA59 4F210 AA24 AG01 AG20 QC01 QC05 QC06 QG01 QG18 QW12

Claims (2)

[Claims] A cavity-containing polyester film containing a large number of cavities derived from a thermoplastic resin which is incompatible with a polyester resin, inside the film, and in a state of no load,
A void-containing polyester film having a curl value of 1.0 mm or less after heat treatment at (Tg + 30) ° C. for 30 minutes. 2. The void-containing polyester film according to claim 1, wherein the thickness of the void-containing polyester film is 50 to 500 μm.