JPH10338763A - Polyester film containing microfine voids - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the subject microfine voids-containing polyester film that has both light weight and flexibility and shows excellent heat resistance by admixing at least three-kinds of thermoplastic resins incompatible to polyester to the polyester film in a specific proportion. SOLUTION: The objective film is obtained by admixing at least three kinds of thermoplastic resins incompatible to polyester [for example, poly(ethylene terephthalate) or poly(butylene terephthalate)], namely (a) a polystyrene resin (for example, a homopolymer as atactic polystyrene), (b) a polymethylpene resin (for example, a polymer having >=90 mol.% of the monomer units derived from 4-methyl-pentene-1) and (c) a polypropylene resin (for example, a homopolymer as isotactic homopolymer). The proportion of these thermoplastic resins is 0.01<=a/(b+c)<=1, c/b<=1 (preferably 0.1<=c/b<=0.5), 5<=a+b+c<=30.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a uniform fine void-containing polyester film containing a large number of fine voids.

[0002]

2. Description of the Related Art Synthetic paper, which is a paper substitute made of synthetic resin as a main raw material, has higher water resistance, moisture absorption, dimensional stability, surface gloss, glossiness of printed matter, and the like than paper made of natural pulp as a main raw material. Excellent in sharpness, mechanical strength, etc. For this reason, in recent years, various applications have been developed utilizing the characteristics.
The main raw materials for synthetic paper include polyolefin resins and polyester resins. Among these, polyester resins represented by polyethylene terephthalate are important requirements for synthetic paper, such as high heat resistance and strong stiffness. Because of its characteristics, it is still widely used at present.

The most effective method for obtaining a film having a function similar to that of natural paper using a polyester-based resin as a main raw material at present is to form an infinite number of minute cavities inside the polyester-based resin film. In this method, the film itself can be given an appropriate degree of flexibility, can be reduced in weight, and can have excellent writing properties and clear printing / transfer properties.
As a means for forming a cavity inside the film, a resin incompatible with the polyester resin is mixed as a cavity-forming agent in the raw material resin, formed on the film, and then stretched to form a cavity at the resin interface. Is a method for generating

Examples of the cavity forming agent used for forming the cavity include polyolefin resins represented by polypropylene resin and polymethylpentene resin (JP-A-49-34755), and polystyrene resins (for example, 49
-2016, Japanese Patent Publication No. 54-29550, etc.).

[0005] Among them, polyolefin resins, especially polymethylpentene, have excellent cavity-forming ability.
It is very good at reducing the weight of the film.
On the other hand, due to poor compatibility with polyester,
The cavity forming agent is coarsely dispersed in the polyester, and the size of the cavity becomes very large. Then, there is a problem that the flexibility of the film is significantly reduced due to the combination of the large cavity and the strength of the polyester itself. This problem becomes evident during the manufacturing and processing of the film, or during the handling of the final product such as printed matter, and the film is bent only by slightly bending the film, or the surface of the film is easily wrinkled. Cause problems. Flexibility is
This is one of the extremely excellent properties of a synthetic resin film, and losing this property is a fatal defect.

[0006] On the other hand, the polystyrene-based resin has a low degree of incompatibility with the polyester-based resin as compared with the polyolefin-based resin, and the fine dispersion of the cavity-forming agent is possible, so that many fine cavities can be formed. It becomes possible. Therefore, it is possible to minimize a decrease in the flexibility of the film, which is a fatal defect when using a polyolefin-based resin. However, when the film is stretched, the cavity-forming agent tends to be crushed in the film thickness direction, and the growth of the cavity is inhibited. As a result, the cavity-forming ability is inferior, and the film is insufficient in weight reduction. Was.

On the other hand, there have been proposed several methods for maintaining the excellent void-forming ability of a polyolefin resin and improving the dispersibility in polyester. For example, a method in which a surfactant (Japanese Patent Publication No. 7-17779), polyethylene glycol (JP-A-2-235942), or a polyetherester copolymer (JP-A-4-264141) is added to the polyester is used.

However, there is a limit in finely dispersing a polyolefin resin by these methods, and it is difficult to obtain fine dispersibility equivalent to that of a polystyrene resin. Further, it has been extremely difficult to obtain a stable fine dispersion effect without impairing other properties (eg, film strength and whiteness).

That is, in the method of adding a surfactant, the surfactant is deteriorated in the melt line due to insufficient heat resistance, and a stable fine dispersion effect cannot be obtained.

In addition, in the method of adding a polyether-based resin, the polyether is extremely susceptible to thermal degradation. Therefore, in the step of drying the raw material or the step of collecting and recycling the resin melt line or film waste, the polyether is used. Causes decomposition. Decomposition of polyether not only causes variation in the effect of finely dispersing the polyolefin resin,
The molecular weight of the polyester may be reduced to cause a significant decrease in film strength, a problem that the film is colored yellow, and a problem such as a pungent odor due to the generation of aldehyde.

[0011] As described above, in the prior art, the fine cavities having both the excellent cavity-forming ability of polyolefin-based resins, especially polymethylpentene, and the excellent flexibility of polystyrene-based resins, and having excellent heat resistance. No polyester-containing film was obtained.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a microcavity containing both lightness and flexibility and having excellent heat resistance. An object of the present invention is to provide a polyester film.

[0013]

The polyester film containing fine voids according to the present invention, which can solve the above problems, comprises a biaxial polymer mixture obtained by mixing a polyester with a thermoplastic resin incompatible with the polyester. A polyester film containing fine voids obtained by stretching and heat-treating, comprising at least a polystyrene resin, a polymethylpentene resin and a polypropylene resin as a thermoplastic resin incompatible with polyester, and a polystyrene resin. (A wt%), the content of the polymethylpentene resin (b wt%), and the content of the polypropylene resin (c wt%) are in the following relationship: 0.01
≦ a / (b + c) ≦ 1, c / b ≦ 1, 5 ≦ a + b + c ≦
The point is that the point that satisfies 30 is satisfied.

Hereinafter, the constituent components of the present invention will be sequentially described. First, the polyester used in the present invention is terephthalic acid, isophthalic acid, an aromatic dicarboxylic acid such as naphthalenedicarboxylic acid or an ester thereof and ethylene glycol, diethylene glycol, 1,4-butanediol, and a glycol such as neopentyl glycol. Polyester produced by polycondensation.
These polyesters can be prepared by directly reacting an aromatic dicarboxylic acid and a glycol, or by subjecting the alkyl ester of an aromatic dicarboxylic acid to a transesterification reaction with a glycol followed by polycondensation, or a diglycol ester of an aromatic dicarboxylic acid. Can be produced by a method such as polycondensation. Representative examples of such polyesters include polyethylene terephthalate, polyethylene butylene terephthalate and polyethylene-2,6.
-Naphthalate and the like. This polyester may be a homopolymer or a copolymer of the third component. In any case, in the present invention,
7 units of ethylene terephthalate unit, butylene terephthalate unit or ethylene-2,6-naphthalate unit
A polyester having 0 mol% or more, preferably 80 mol% or more, more preferably 90 mol% or more is preferable.

Next, the thermoplastic resin incompatible with the polyester, which characterizes the present invention (hereinafter sometimes abbreviated as a cavity-forming agent), includes polystyrene-based resin, polymethylpentene-based resin and polypropylene. A system resin is an essential component. For the first time, a polystyrene-based resin and a specific polyolefin-based resin are mixed and used at a specific ratio, and only for the first time, is obtained a polyester film containing fine cavities having both lightness and flexibility, and having excellent heat resistance. Can do things.

On the other hand, when polystyrene is used alone as a cavity-forming agent, an excellent film is obtained in terms of film flexibility as described above. However, when polyolefin resin is used as a cavity-forming agent, Compared with, there is a limit in terms of lightness.

In the present invention, the content (a wt%) of the polystyrene resin is defined as the content (b wt%) of the polymethylpentene resin and the content (c) of the polypropylene resin.
Weight%), it is necessary to satisfy the relationship of 0.01 ≦ a / (b + c) ≦ 1. By setting the content of the polystyrene-based resin in the above range, the cavity-forming agent can be finely dispersed in the film, and the weight of the film can be sufficiently reduced while securing the flexibility of the film. When the content of the polystyrene-based resin is smaller than the above range, the dispersion effect of the polystyrene-based resin on the polyolefin-based resin (polypropylene-based resin and polymethylpentene-based resin) becomes unstable, resulting in unevenness and flexibility of the film. Becomes defective. Conversely, if the content of the polystyrene resin is larger than the above range, a sufficient weight reduction effect cannot be obtained.

The content of the polystyrene resin can be arbitrarily adjusted within the above range.
The range of /(b+c)≦0.5 is more preferable. this is,
By setting the content of the polystyrene resin within this range, the weight of the film can be reduced more easily.

In the polyester film containing fine voids of the present invention, in addition to polystyrene resin, polymethylpentene resin and polypropylene resin are essential components as polyolefin resin. When a polypropylene resin is used alone as the polyolefin resin, the effect of reducing the weight of the film is hardly exhibited.

On the other hand, when the polymethylpentene resin is used alone as the polyolefin resin, it is relatively easy to reduce the weight of the film. However, dispersion unevenness of the polymethylpentene resin in the film is exhibited, and the film surface exhibits canvas-like unevenness. However, if the content of the polystyrene-based resin is larger than the content of the polymethylpentene-based resin, the unevenness of the canvas ground shape is greatly reduced to fall within the practical range, but in this case, the weight reduction effect of the present invention cannot be obtained. .

In the polyester film containing fine voids of the present invention, the polymethylpentene resin and the polypropylene resin are used in combination as the polyolefin resin, so that the polymethylpentene resin is used alone as the polyolefin resin. It succeeded in obtaining excellent uniformity while obtaining a weight reduction effect comparable to the case.

The mixing ratio of the polymethylpentene-based resin and the polypropylene-based resin to obtain the above-mentioned effects is in the range of c / b ≦ 1 when the content of each resin is b weight% and c weight%. Must be present, preferably 0.01 ≦ c / b ≦ 0.5, particularly preferably 0.1 ≦ c
/B≦0.5. The desired purpose can be achieved by mixing a small amount of the polypropylene resin. Conversely, if the mixing ratio exceeds 1, the effect of reducing the weight of the polymethylpentene resin will be greatly impaired.

The polystyrene-based resin of the present invention refers to a thermoplastic resin having a polystyrene structure as a basic component, and includes other components in addition to homopolymers such as atactic polystyrene, syndiotactic polystyrene, and isotactic polystyrene. Modified resins graft- or block-copolymerized, such as impact-resistant polystyrene resins and modified polyphenylene ether resins, and mixtures of these polystyrene-based resins with thermoplastic resins, such as polyphenylene ether, are also included.

The polymethylpentene resin in the present invention is a polymer having at least 80 mol%, preferably at least 90 mol%, of units derived from 4-methylpentene-1. Ethylene unit, propylene unit, butene-1 unit, 3-methylbutene-1
And the like.

The melt flow rate of the polymethylpentene is preferably 200 g / 10 minutes or less,
More preferably 30 g / 10 min or less, particularly preferably 1 g / min.
0 g / 10 minutes or less. This is because if the melt flow rate exceeds 200 g / 10 minutes, it is difficult to obtain the effect of reducing the weight of the film.

The polypropylene resin in the present invention also includes a modified resin obtained by grafting or block copolymerizing other components in addition to homopolymers such as isotactic polypropylene and syndiotactic polypropylene.

Although the present invention comprises the above components as essential elements, other resins can be used in combination as the cavity-forming agent. In this case, as a resin that can be added, for example, polyphenylene ether-based resin, polyolefin-based resin, polyacryl-based resin, polycarbonate resin, polysulfone-based resin, cellulose-based resin, polyphenylene sulfide-based resin, cyclic olefin-based polymer or ring-opening polymer thereof And the like, but are not limited to these.

The mixing amount of these void-forming agents, that is, the thermoplastic resin incompatible with the polyester, to the polyester varies depending on the amount of the target voids, but is in the range of 5 to 20% by weight based on the whole film. And a more preferable range is 8 to 16% by weight.
If it is less than 5% by weight, there is a limit to increasing the amount of cavities formed. 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.

In addition, inorganic or organic particles may be added to the film, if necessary, in the polyester or the cavity-forming agent in order to improve the concealing property and the like. Examples of the particles that can be added include silica, kaolinite, talc, calcium carbonate, zeolite, alumina, barium sulfate, carbon black, zinc oxide, titanium oxide, zinc sulfide, and organic white pigments, but are particularly limited. is not.

The polyester film containing fine cavities of the present invention comprises the above-mentioned constituent components, and may have a single-layer structure or a multilayer structure of two or more layers. It is preferable that at least one surface of the film has a skin layer substantially free of a polyolefin-based resin, and it is particularly preferable that the film has the skin layer on both surfaces. This is because bleeding of the polyolefin resin to the surface of the film, which causes process contamination and contamination, can be effectively suppressed. Further, it is preferable to provide a skin layer to form a multilayer structure of the film from the viewpoint of imparting a characteristic function to the film surface.

As a method for forming the skin layer, a co-extrusion method is preferably used. This is because there is an effect of effectively suppressing bleeding of the polyolefin-based resin that occurs during cooling and solidification immediately after the melt extrusion of the resin.

From the viewpoint of suppressing bleeding of the polyolefin resin, the thickness of each skin layer is preferably 0.5% or more of the total thickness of the film. On the other hand, the upper limit of the thickness of the skin layer is set at 30 times the total thickness of the film in order to secure the lightness of the entire film.
% Is preferable.

Although the lightweight property of the polyester film containing fine cavities of the present invention is arbitrary, the apparent specific gravity is preferably in the range of 0.6 to 1.2, particularly 0.7 The range of -1.1 is more preferable. If the apparent specific gravity is less than 0.6, the void content is too large, and the film tends to be insufficient in strength or insufficient in flexibility. Conversely, if the specific gravity is higher than 1.2, the void content is insufficient, and the properties provided by the void formation, that is, the weight reduction, such as cushioning and flexibility, cannot be effectively exhibited.

The color tone of the polyester film containing fine voids of the present invention is optional, but the b value is preferably less than 3. If the b value exceeds 3, the film will be yellowish and its quality will be greatly impaired.

The method for producing the polyester film containing fine cavities of the present invention is arbitrary and not particularly limited, but the mixture having the above composition is formed into a film to obtain an unstretched film. A general method of stretching a stretched film can be used.

The conditions for stretching and orienting the unstretched sheet are closely related to the formation of cavities. In the following, the sequential biaxial stretching method most preferably used, in particular, a method of stretching an unstretched sheet in the longitudinal direction and then in the width direction is taken as an example.
The alignment conditions will be described. 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 transition temperature Tg + 50 ° C. or lower.
Next, the uniaxially stretched film is introduced into a tenter, and is heated in the width direction at a temperature not higher than the melting point Tm-10 ° C. of the polyester at 2.5 to 5 °
Stretch twice.

The biaxially stretched film thus obtained is subjected to a heat treatment if necessary. The heat treatment is preferably performed in a tenter, and the melting point of the polyester is Tm-50 ° C.
It is preferably performed in the range of -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 polyester resin is preferable, and in addition, a compound disclosed as a means for improving the adhesiveness of a normal polyester film such as a polyurethane resin, a polyester urethane resin, and an acrylic resin. Etc. are applicable.

As a method for 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, a reverse roll coating method, etc. Applicable. As the 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 surface of the film after the orientation treatment is possible.

The polyester film containing fine cavities obtained in this way achieves both high flexibility and light weight of the film. Furthermore, it does not require a surfactant or polyether-based resin as a dispersant for polyolefin-based resins, so it has excellent heat resistance and has a small change in color tone even when self-recovered raw materials are reused, stabilizing film production. Also excellent in nature. When the self-recovered raw material is reused, a preferable usage ratio is 5 to 50% by weight.

Next, examples of the present invention and comparative examples will be described. The measurement / evaluation method used in the present invention will be described below.

1) Apparent Specific Gravity A film was accurately cut into a square of 10 cm × 10 cm, and its thickness was measured at 50 points to determine the average thickness t (unit: μm).
Ask for. Next, the weight of the sample is measured up to 0.1 mg and is referred to as w (unit: g). Then, the apparent specific gravity was calculated by the following equation. (Equation 1) Apparent specific gravity (−) = (w / t) × 100

2) Flexibility of the film The film was cut into a strip having a length of 5 cm and a width of 1 cm.
Wrap it around a stainless steel rod with a diameter of 2 mm and squeeze it. Thereafter, the sample was stretched again, and the state of wrinkles generated on the surface was observed using a stereoscopic microscope.

3) Color tone of film The b value was evaluated according to JIS Z8729-1994.

Example 1 As a raw material, a polystyrene resin having a melt flow index of 2.0 was added to 70% by weight of a polyethylene terephthalate resin having an intrinsic viscosity of 0.64 (Toporex 57 manufactured by Mitsui Toatsu Co., Ltd.).
0-57U) 3% by weight, melt flow index 1.7 polypropylene resin (Noblen FO-5 manufactured by Mitsui Toatsu Co., Ltd.)
0F) Polymethylpentene resin having a melt flow index of 6% by weight (TPX, DX manufactured by Mitsui Petrochemical Co., Ltd.)
-845) 21% by weight of pellets are mixed, supplied to a twin-screw extruder and sufficiently kneaded, and the strand is cast in water, cooled, and then cut with a strand cutter to obtain a master pellet containing a cavity-forming agent (-845). A) was prepared.

The obtained master pellet was dried with hot air (1
After 70 ° C. × 3 hours), 43% by weight of master pellets, 55% by weight of polyethylene terephthalate resin having an intrinsic viscosity of 0.62 similarly dried with hot air, and anatase type titanium dioxide having an average particle diameter of 0.3 μm (electron microscopic method) 2% by weight of TA-300) was used as a raw material for the film.

These raw materials were supplied to a twin-screw extruder, kneaded, and extruded using a T-die onto a cooling drum adjusted to 30 ° C. to produce an unstretched sheet having a thickness of about 650 μm. Subsequently, the obtained unstretched sheet was heated to 86 ° C. using a heating roll, and longitudinally stretched 3.4 times between the rolls. Subsequently, it is heated to 120 ° C with a tenter, stretched laterally 3.7 times, fixed in width, subjected to a heat treatment at 230 ° C for 5 seconds, and further relaxed by 4% in the width direction at 220 ° C to obtain a thickness of 75%.
μm microvoided polyester film (Example
1) got.

Example 2 and Comparative Examples 1 to 3 The composition of the master pellet and the composition of the film were changed as shown in Tables 1 and 2, respectively, so that the film thickness after biaxial stretching was 75 μm. A film was produced in the same manner as in Example 1 except that the thickness of the stretched sheet was adjusted.

COMPARATIVE EXAMPLE 4 15% by weight of polyethylene glycol having a molecular weight of 8,000 was added at a stage during the production of polyester (after transesterification reaction and before polycondensation) to give a polyethylene terephthalate-polyether copolymer having an intrinsic viscosity of 0.83. Obtained. After drying the obtained chips with hot air (170 ° C. × 3 hours),
Polymethylpentene (TPX, RT-18, manufactured by Mitsui Petrochemical Co., Ltd.) chips having a melt flow rate of 26 were blended by 10% by weight as chips to obtain film raw materials.

Using this raw material, a film was produced in the same manner as in Example 1 except that the thickness of the unstretched sheet was adjusted so that the film thickness after biaxial stretching was 75 μm.

Example 3, Comparative Example 5 A part of the film obtained in Example 1 and Comparative Example 4 was cut into fluff, and then supplied to a vented twin-screw extruder to produce recovered pellets. The obtained recovered pellets were used in an amount of 30% by weight, and the remaining 70% by weight was mixed with the raw materials at the same composition ratio as in Example 1 and Comparative Example 4. Example 1 was compared with Example 1 except that a raw material containing 30% by weight of the self-recovered raw material thus obtained was used and the thickness of the unstretched sheet was adjusted so that the film thickness after biaxial stretching was 75 μm. In the same manner as in Example 4, films of Example 3 and Comparative Example 5 were produced. The film of Comparative Example 5 was produced by reducing the transverse stretching ratio to 2.5 times because the film was frequently broken during the transverse stretching under the same stretching conditions as in Comparative Example 1.

Example 4 The raw material used in Example 1 was used as a raw material for the film core layer. Separately, 65% by weight of polyethylene terephthalate resin having an intrinsic viscosity of 0.64 and 35% by weight of anatase type titanium dioxide (TA-300 manufactured by Fuji Titanium) Was used as a skin layer raw material. The raw material for the core layer was supplied to a twin-screw extruder, and the raw material for the mixed skin layer was supplied to a vented twin-screw extruder, and supplied to a feed block to bond the skin layers to both surfaces of the core layer. At this time, the discharge amount was controlled using a gear pump so that the volume ratio of the skin layer material and the core layer material was 1: 9. Then use a T-die for 30
Extruded on a cooling drum adjusted to a temperature of
A μm unstretched sheet was prepared. The subsequent steps were performed in exactly the same manner as in Example 1 to obtain a 75 μm-thick fine void-containing polyester film having a skin layer (Example 4).

Table 3 shows the properties of the film thus obtained. In addition, since the stretching ratio of the film of Comparative Example 5 was significantly reduced, the thickness unevenness of the film was remarkable, and reliable evaluation of specific gravity and flexibility could not be performed. Therefore, Table 3 does not describe the evaluation results.

From Table 3, the following can be considered. First, in Examples 1 and 2 which satisfy the constitutional requirements of the present invention, sufficient flexibility is obtained despite low specific gravity, and excellent color tone and uniform surface state are stably obtained. be able to. Also, it can be seen that in Example 3 in which a large amount of the self-recovered raw material is contained, excellent quality can be obtained without impairing the color tone and film forming stability.

On the other hand, when the polypropylene-based resin which is an essential component of the present invention is not contained at all, the effect of lowering the specific gravity of the film becomes insufficient (Comparative Example 1) or the uniformity of the film surface or It can be seen that the flexibility is insufficient (Comparative Example 2).

In Comparative Example 3 where the mixing ratio of the polypropylene resin is larger than the range regulated by the present invention, the effect of reducing the specific gravity is insufficient.

In the conventional method in which a polymethylpentene resin and a polyether are added in combination, flexibility and surface uniformity become insufficient (Comparative Example 4). (Comparative Example 5) shows that not only the film-forming properties of the film become extremely poor, but also that various qualities are significantly reduced.

Furthermore, in Example 4, which has a skin layer and satisfies the constitutional requirements of the present invention, not only the expected effect can be obtained, but also the contamination in the film production process is very excellent. .

[0059]

The polyester film containing fine voids of the present invention not only has good lightness, flexibility and color tone, but also
It can be seen that the surface condition is uniform and the productivity is excellent.

[0060]

[Table 1]

[0061]

[Table 2]

[0062]

[Table 3]

Continued on the front page (51) Int.Cl. ⁶ Identification code FI // B29K 23:00 25:00 67:00 105: 04 B29L 7:00

Claims (6)

[Claims] 1. A microvoided polyester film obtained by biaxially stretching and heat-treating a polymer mixture obtained by mixing a polyester with a thermoplastic resin incompatible with the polyester, wherein the polyester film is incompatible with the polyester. The thermoplastic resin contains at least a polystyrene-based resin, a polymethylpentene-based resin, and a polypropylene-based resin. The content of the polystyrene-based resin (a wt%), the content of the polymethylpentene-based resin (b wt%), and the polypropylene A polyester film containing fine cavities, wherein the content (c% by weight) of the system resin satisfies the following relationship. 0.01 ≦ a / (b + c) ≦ 1 c / b ≦ 15 5 ≦ a + b + c ≦ 30 2. The film according to claim 1, further comprising a skin layer substantially free of a polyolefin-based resin on at least one side of the film, wherein the thickness of each skin layer is 0.5 to 30% of the total thickness of the film. A polyester film containing fine cavities, characterized in that: 3. A polyester film containing fine cavities, wherein the apparent specific gravity of the entire film according to claim 1 is 0.6 to 1.2. 4. A polyester film containing fine cavities, wherein the content of the self-collecting raw material in the film raw material according to any one of claims 1 to 3 is 5% by weight or more. 5. A fine void-containing polyester film obtained by mixing a polyester and an incompatible thermoplastic resin with the polyester, the flexibility of the film being free from wrinkles in a test specified in [0043] in the text. And a film containing fine cavities, wherein the apparent specific gravity of the whole film is 0.6 to 1.2. 6. A polyester film containing fine cavities, wherein the color tone of the film according to claim 5 is less than 3 in b value.