JP2000230065A - Void-containing polyester film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide films having excellent properties (e.g. cushioning properties, low dielectric properties, and lightweight properties) derived from a void- containing structure and good handling properties as well. SOLUTION: In the void-containing polyester films composed of a composition comprising a polyester resin and a thermoplastic resin incompatible with the polyester resin and having a number of voids derived from the incompatible thermoplastic resin which has been dispersed in the polyester resin in the form of particles within the films, the films contain a polyolefin resin as the incompatible thermoplastic resin and meet all three requisites of (1) an apparent specific gravity of not greater than 1.3; (2) an average diameter d of the dispersed particles of the polyolefin resin of not greater than 15 μm; and (3) a ring crush strength G (unit: kg/mm) and a film thickness t (unit: mm) which meet the following formula: G>15×t3.

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 both properties derived from a void-containing structure (for example, cushioning properties, low dielectric properties, light weight) and handling properties in post-processing. More specifically, a void-containing polyester film that is excellent in low dielectric properties and has good handling properties (such as mounting properties) and is suitable as an electrical insulating material, and has excellent cushioning properties and good handling properties (such as transportability). The present invention relates to a void-containing polyester film suitable as various printing films.

[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 have the characteristics of high heat resistance and high rigidity, and their use range is expanding. For example, utilization as an insulating material for an electric motor by utilizing the low dielectric property derived from the cavity-containing structure (JP-A-9-149576, etc.) is being studied. Further, due to the cushioning property derived from the cavity-containing structure, it can be used for thermal transfer printing (Japanese Patent Application Laid-Open No. 63-2806).
No. 87 gazette) and various printing films.

[0003] As the void-containing polyester film used for such an application, an inorganic fine particle is mixed with a polyester resin and stretched to form a void around the particle or an incompatible polyester film. It is known that a thermoplastic resin or the like is mixed in a polyester resin, dispersed into particles, and stretched to form cavities around the particles. In particular, the latter is widely used because it can reduce the weight of the film.

[0004] Examples of the thermoplastic resin incompatible with the polyester resin (hereinafter also referred to as a cavity-forming agent) used for forming the cavity include polypropylene and polymethylpentene (JP-A-49-34755). Representative polyolefin resins and polystyrene resins (for example, JP-B-49-2016, JP-B-54-295)
No. 5) has been proposed.

[0005] Among them, polyolefin resins, especially polymethylpentene, have excellent cavity-forming ability.
It is very excellent in that cavities can be efficiently formed inside the film. On the other hand, it is difficult to finely disperse the polyester resin in the polyester resin, and the cavities tend to be coarse and uneven, and the flexibility of the film (the ability of the film to follow the deformation) is reduced. Greatly decreases. Specifically, problems such as poor punchability, poor transportability, and poor mechanical insertion suitability (mountability) when used as an insulating material for electric motors occur. In order to solve these problems, it is necessary to limit the void content in the film, and characteristics derived from the void-containing structure (eg, cushioning, low dielectric properties, light weight)
I have to sacrifice.

[0006] Such a decrease in handleability is caused not only by the use as the above-mentioned electric insulating material but also by the transportability (poor sheet feeding, paper jam, etc.) and the wrinkle position even at the time of high-speed handling in various printing methods. This causes problems such as poor printing. In order to solve these problems, the void content must also be limited, and the properties (for example, cushioning properties, low dielectric properties, and light weight) derived from the void containing structure must be sacrificed.

On the other hand, when a polystyrene-based resin is used as a cavity-forming agent, it can be finely dispersed in a polyester resin more easily than a polyolefin-based resin, and a film excellent in handleability can be obtained. Can be However, the polystyrene-based resin has a lower ability to form cavities than the polyolefin-based resin, and cannot sufficiently exhibit characteristics (for example, cushioning properties, low dielectric properties, and light weight) derived from the cavity-containing structure.

Some attempts have been made to finely disperse a polyolefin resin (for example, polymethylpentene) in a polyester resin. For example, use of a surfactant (JP-B-7-17779), use of polyethylene glycol (JP-A-2-235942), or use of a polyetherester copolymer (JP-A-4-26)
No. 4141) has been proposed.

Certainly, these methods reduce the coarsening and heterogeneity of the cavities, improve the flexibility of the film to some extent, and thus improve the handling of the film to some extent. However, in the method using a surfactant,
The adhesion between the polyester resin and the polyolefin-based resin (for example, polymethylpentene) is increased, and the formation of voids when stretched is hindered. Also, in the method using polyethylene glycol or polyetherester copolymer,
Since these components have a property of acting as a plasticizer with respect to the polyester resin, the rigidity (toughness) inherent in the polyester film is significantly reduced. Then, the reduction in rigidity causes a problem that the handleability of the film is reduced. Also,
These ether components are easily oxidatively degraded, and have a problem that they cause coloring of the film by themselves and, in some cases, accelerate deterioration of the polyester resin.

[0010] As described above, in the above prior art,
A film having both excellent properties (for example, cushioning properties, low dielectric properties, and light weight) derived from the void-containing structure and good handling properties has not been obtained.

[0011]

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

[0012]

SUMMARY OF THE INVENTION The present invention is as follows. (1) A film comprising a composition containing a polyester resin and a thermoplastic resin incompatible with the polyester resin, and forming a film formed by the incompatible thermoplastic resin dispersed in the polyester resin into particles. A void-containing polyester film containing a large number of voids therein, comprising a polyolefin resin as the incompatible thermoplastic resin, and satisfying all of the following requirements: The apparent specific gravity is 1.3 or less; the average dispersed particle diameter d of the polyolefin resin is 15 μm
The ring crush strength G (unit: kg / mm) and the film thickness t (unit: mm) satisfy the following expression. G> 15 × t ³ (2) Aspect (1), wherein the aspect ratio of the dispersed particles of the thermoplastic resin incompatible with the polyester resin in the cut surface parallel to the longitudinal direction of the film is 1 to 10. A void-containing polyester film. (3) Buckling limit radius r (unit: mm) specified in the text
And the film thickness t (unit: mm) satisfies the following expression: the void-containing polyester film according to the above (1). r <25 × t (4) The void-containing polyester film according to the above (1), wherein the dielectric constant is less than 2.9. (5) The void-containing polyester film according to the above (1), wherein the polyolefin resin is polymethylpentene and / or polypropylene. (6) The void-containing polyester film according to (5), further comprising a polystyrene resin as a thermoplastic resin incompatible with the polyester resin. (7) Content a of polystyrene resin in film
(6) The cavity-containing polyester film according to (6), wherein the content (% by weight), the content b (% by weight) of polymethylpentene, and the content c (% by weight) of polypropylene satisfy the following formulas: . 0.01 ≦ a / (b + c) ≦ 1 c / b ≦ 13 3 ≦ a + b + c ≦ 20

(8) The void-containing polyester film according to the above (1), wherein the content of the cyclic trimer in the polyester resin is 0.5% by weight or less based on the total weight of the film. (9) The void-containing polyester film according to (8), which is an insulating material for an electric motor. (10) The limiting viscosity of the polyester resin is 0.68 to 1.
The void-containing polyester film according to the above (8), which is 0 dl / g. (11) The cavity according to (8) above, wherein the polyester resin is a polyester resin in which the amount of increase of the cyclic trimer when melted at a temperature of 290 ° C. for 60 minutes is 0.5% by weight or less. Containing polyester film. (12) The polyester resin is a polyester resin which has been subjected to water treatment in a chip form after polycondensation, and the polyester resin is mainly composed of an aromatic dicarboxylic acid or an ester-forming derivative thereof and ethylene glycol as raw materials for a Ge compound and / or a Ti compound. The void-containing polyester film according to the above (8), which is obtained by using as a catalyst.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The polyester resin in the present invention is preferably a crystalline polyester resin mainly obtained from an aromatic dicarboxylic acid component and a glycol component. It is a polyester resin containing at least 85 mol% of the components, particularly preferably a polyester resin containing an aromatic dicarboxylic acid component at 90 mol% or more of the total acid component.

The aromatic dicarboxylic acid component constituting the polyester resin includes aromatic dicarboxylic acids such as terephthalic acid, 2,6-naphthalenedicarboxylic acid, diphenyl-4,4'-dicarboxylic acid, diphenoxyethanedicarboxylic acid and the like. Ester-forming derivatives thereof and the like.

Examples of the glycol component constituting the polyester resin include aliphatic glycols such as ethylene glycol, trimethylene glycol and tetramethylene glycol; and alicyclic glycols such as cyclohexanedimethanol.

The acid components other than those described above which are used by copolymerization in the polyester resin include aromatic dicarboxylic acids such as isophthalic acid and ester-forming derivatives thereof;
Oxyacids such as oxybenzoic acid and oxycaproic acid and ester-forming derivatives thereof; aliphatic dicarboxylic acids such as adipic acid, sebacic acid, succinic acid, glutaric acid and dimer acid and ester-forming derivatives thereof; Alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid and 1,3-cyclohexanedicarboxylic acid, and ester-forming derivatives thereof;

Other glycol components used by copolymerization in the polyester resin include aliphatic glycols such as diethylene glycol and neopentyl glycol; aromatic glycols such as bisphenol A and alkylene oxide adducts of bisphenol A; And polyalkylene glycols such as polyethylene glycol and polybutylene glycol.

Further, as long as the polyester resin is substantially linear, a polyfunctional compound such as trimellitic acid, trimesic acid, pyromellitic acid, tricarballylic acid, glycerin, pentaerythritol, trimethylolpropane or the like is copolymerized. Or a monofunctional compound such as benzoic acid or naphthoic acid.

A preferred example of the polyester resin is
A polyester resin whose main repeating unit is composed of ethylene terephthalate, more preferably a linear polyester resin containing 85 mol% or more of ethylene terephthalate units, and particularly preferably a linear polyester resin containing 90 mol% or more of ethylene terephthalate units. It is.

Another preferred example of the polyester resin is a polyester resin whose main repeating unit is composed of ethylene-2,6-naphthalate.
A linear polyester resin containing at least 5 mol%, particularly preferably a linear polyester resin containing at least 90 mol% of ethylene-2,6-naphthalate units.

The limiting viscosity of the polyester resin, particularly the polyester resin whose main repeating unit is composed of ethylene terephthalate, is preferably 0.50 to 1.3.
0 dl / g, more preferably 0.55 to 1.20 dl /
g, more preferably in the range of 0.60 to 0.90 dl / g. When the intrinsic viscosity is less than 0.50 dl / g, the mechanical properties of the obtained film may be deteriorated. On the other hand, when the intrinsic viscosity exceeds 1.30 dl / g, the resin temperature becomes high during melt extrusion and the thermal decomposition is severe. This may cause problems such as an increase in low molecular weight compounds such as cyclic trimers and the like, and the film may be colored yellow, which is not preferable.

The intrinsic viscosity of the polyester resin, particularly the polyester resin whose main repeating unit is composed of ethylene-2,6-naphthalate, is preferably 0.1.
40 to 1.00 dl / g, more preferably 0.42 to dl / g
0.95 dl / g, more preferably 0.45 to 0.9
The range is 0 dl / g. Intrinsic viscosity 0.40dl / g
If it is less than 1, the mechanical properties of the obtained film may be deteriorated. Conversely, if it exceeds 1.00 dl / g, the resin temperature becomes high during melt extrusion and the thermal decomposition becomes severe, so that the cyclic trimer or the like may be formed. Problems such as an increase in the number of low molecular weight compounds and coloring of the film in yellow may occur, which is not preferable.

Further, when the void-containing polyester film of the present invention is used as an insulating material for an electric motor, the polyester resin is mainly composed of an aromatic dicarboxylic acid or an ester-forming derivative thereof and ethylene glycol as raw materials for a Ge compound and / or It is preferable to use a polyester resin obtained by using a Ti compound as a catalyst. The limiting viscosity in this case is 0.68 dl / g or more.
In the range of 1.0 dl / g, especially 0.68 dl / g to 0.9
A range of 0 dl / g is preferred. The limiting viscosity is 0.68dl
If it is less than / g, it is not preferable because the strength of the insulating material for an electric motor may be significantly reduced during a long-term use at a high temperature. If the intrinsic viscosity exceeds 1.0 dl / g, the load on the extruder and the pressure loss of the filter in the polymer melt extrusion molding step become remarkable, and it becomes necessary to greatly restrict the polymer discharge amount. As a result, the residence time in the melt line is prolonged, and a large amount of the cyclic trimer precipitates, which may cause a reduction in the reliability of the motor due to the release of the cyclic trimer into the electric motor system. Not preferred.

Further, when the void-containing polyester film of the present invention is used as an insulating material for an electric motor, the content of the cyclic trimer of the polyester resin may be 0.5% by weight or less based on the total weight of the film. Preferably, 0.4
It is more preferable that the content be not more than weight%. Here, when the content of the cyclic trimer of the polyester resin exceeds 0.5% by weight with respect to the total weight of the film, the release of the cyclic trimer to the electric motor system sharply increases, and Is not preferred because of the possibility that the reliability may decrease.

When the void-containing polyester film of the present invention is used as an insulating material for an electric motor, the amount of the polyester resin increases by 0.5% by weight when melted at a temperature of 290 ° C. for 60 minutes. % Or less, and more preferably 0.2% by weight or less. Here, when the increase of the cyclic trimer exceeds 0.5% by weight, the amount of the cyclic trimer precipitated in the melt extrusion molding step of the polymer increases, and the content of the cyclic trimer of the polyester resin is increased. To 0.5% by weight or less with respect to the total weight of the film. The amount of increase of the cyclic trimer is measured by the method described later.

The polyester resin having the above properties can be produced, for example, by subjecting a polyester chip to contact treatment with treated water in a treatment tank. As a method of the contact treatment, a method of immersing in water may be mentioned. The time for performing the contact treatment with water is 5 minutes to 2 days, preferably 10 minutes to 1 day, more preferably 30 minutes to 10 hours, and the water temperature is 20 to 180 ° C, preferably 40 to 150 ° C. ° C, more preferably 50 to 120 ° C.

The voids in the void-containing polyester film of the present invention are obtained by mixing a thermoplastic resin incompatible with the polyester resin into the polyester resin and dispersing the thermoplastic resin into particles.
By stretching, it is formed at the interface between the dispersed particles and the polyester resin.

As the thermoplastic resin incompatible with the polyester resin used in the present invention, it is indispensable to include a polyolefin resin, but as other components, a polystyrene resin, a polyacrylic resin, and a polycarbonate resin are used. It may contain a resin, a polysulfone resin, a cellulose resin, a polyphenylene ether resin, or the like. Examples of the polyolefin-based resin include, but are not limited to, polyethylene, polypropylene, polymethylpentene, ring-opened polymers of cyclic olefins, copolymers of cyclic olefins and other olefins, and the like.

The content of the thermoplastic resin incompatible with the polyester resin varies depending on the amount of the target cavity, but is preferably 3 to 20% by weight, more preferably 5 to 18% by weight in the film. is there. If this content is less than 3% by weight, there is a limit to increasing the amount of cavities formed, and properties derived from the cavities-containing structure (for example, low dielectric properties,
Cushioning, lightweight) may not be able to be exhibited.
Conversely, when the content exceeds 20% by weight, the properties inherent in the polyester resin, that is, the stretchability, heat resistance, strength,
Rigidity may be impaired, which is not preferable.

In a particularly preferred embodiment, the polystyrene-based resin and the polyolefin-based resin have a specific ratio, that is, the ratio of polystyrene-based resin / polyolefin-based resin is 0.1%.
Examples of using a mixture of 01 to 1 are exemplified. Further, it is preferable to use polymethylpentene and polypropylene as the polyolefin resin, that is, to use a mixture of polystyrene resin, polymethylpentene and polypropylene.

In the present invention, the term “polystyrene resin” 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. (Eg, impact-resistant polystyrene, graft copolymers of polystyrene and polyphenylene ether, etc.), and thermoplastic resins compatible with these polystyrene resins, for example, polyphenylene ether And mixtures thereof.

In the present invention, polymethylpentene is used in an amount of 80 mol% or more, preferably 90 mol% or more of 4-methylpentene.
It is a polymer containing units derived from methylpentene-1, and examples of other units include units derived from ethylene, propylene, butene-1, 3-methylbutene-1, and the like.

In the present invention, the polypropylene includes not only homopolymers such as isotactic polypropylene and syndiotactic polypropylene, but also modified resins obtained by grafting or block copolymerizing other components. The polypropylene may be used in a state where it is copolymerized in the polymethylpentene, that is, in a state where propylene units are introduced as copolymerized units.

These resins are preferably used in a mixture at a specific ratio. The content a (% by weight) of the polystyrene resin in the film and the content b of the polymethylpentene
(% By weight) and polypropylene content c (% by weight)
Particularly preferably satisfies the following expression. 0.01 ≦ a / (b + c) ≦ 1 c / b ≦ 13 3 ≦ a + b + c ≦ 20

If a / (b + c) is less than 0.01, the dispersing effect of the polystyrene resin on the polyolefin resin (polymethylpentene and / or polypropylene) becomes unstable, resulting in unevenness and flexibility of the film. There is a possibility that the handling property is deteriorated and the handling property is deteriorated, which is not preferable. Conversely, if a / (b + c) exceeds 1, there is a limit to increasing the amount of cavities generated, and characteristics (eg, low dielectric properties, cushioning properties, and light weight) derived from the cavity-containing structure cannot be exhibited. There is a possibility that it is not preferable. The upper limit of the value of a / (b + c) is 0.5, and the lower limit is 0.
1 is particularly preferred.

Similarly, when c / b exceeds 1, it is not preferable because there is a limit in increasing the amount of cavities generated. Although the lower limit of c / b is not particularly limited, if it is less than 0.01, there is a possibility that unevenness and flexibility of the film may be poor and handling properties may be inferior, which is not preferable. c /
The upper limit of the value of b is particularly preferably 0.5, and the lower limit thereof is particularly preferably 0.1.

When a + b + c is less than 3% by weight, there is a limit in increasing the amount of cavities formed, and characteristics derived from the structure containing cavities (eg, low dielectric properties, cushioning properties, light weight) are exhibited. It may not be possible, and conversely, a +
If b + c exceeds 20% by weight, the inherent properties of the polyester resin, that is, the stretchability, heat resistance, strength, rigidity, etc. of the film may be impaired, resulting in poor handling properties, which is not preferable. The upper limit of a + b + c is particularly preferably 18% by weight, and the lower limit is particularly preferably 5% by weight.

Further, in order to improve the concealing property and the like, inorganic or organic particles may be contained in the void-containing polyester film of the present invention as required. Examples of usable particles include, but are not limited to, silica, kaolinite, talc, calcium carbonate, zeolite, alumina, barium sulfate, carbon black, zinc oxide, titanium oxide, zinc sulfide, and organic white pigments. is not. These particles are previously contained in a polyester resin and / or
Alternatively, it can be contained in a film by being added to a thermoplastic resin incompatible with a polyester resin.

The method for producing the void-containing polyester film of the present invention is arbitrary and not particularly limited. For example, after forming a mixture having the above-mentioned composition into a film to obtain an unstretched film, A general method of stretching the unstretched film can be used.

The conditions for stretching and orienting the unstretched film are closely related to the formation of cavities. In the following, the stretching and orientation conditions will be described by taking as an example a sequential biaxial stretching method most preferably used, particularly a method of stretching an unstretched film in a longitudinal direction (longitudinal direction) and then in a transverse direction (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 between the polyester resin and the thermoplastic resin incompatible with the polyester resin, the film is stretched 3 to 5 times at a second transition temperature Tg + 50 ° C. or lower of the polyester resin. Next, the uniaxially stretched film was introduced into a tenter, and the melting point Tm
The film is stretched 2.5 to 5 times at a temperature of −10 ° C. or less.

The biaxially stretched film thus obtained is subjected to a heat treatment as required. The heat treatment is preferably performed in a tenter, and the melting point Tm-
It is preferable to carry out in the range of 50 ° C. to Tm.

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 can be formed not only by a co-extrusion method but also by a coating method, a laminating method via an adhesive layer or the like.

Further, as such a synthetic resin film,
For example, a film containing one or more of polyethylene terephthalate, polyethylene naphthalate, polyphenylene sulfide, polyarylate and polyimide as a main component can be used, but is not limited thereto.

Further, a coloring agent, a light-proofing agent, a fluorescent agent, an antistatic agent, and the like can be added to the synthetic resin film layer as needed. Further, the above-mentioned inorganic particles and organic particles may be added.

The void-containing polyester film 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 coating method, a spray coating method, a curtain coating method, an air knife coating method, a blade coating method, a reverse roll coating method, etc. Can be applied. 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 surface of the film after the stretching treatment is possible.

The thus obtained void-containing polyester film has excellent properties derived from the void-containing structure (for example, cushioning, low dielectric properties, light weight) and good handling.

The void-containing polyester film of the present invention has an apparent specific gravity of 1.3 or less, preferably 1.2 or less.
It is more preferably 1.15 or less. The apparent specific gravity is 1.
If it is larger than 3, the amount of cavities existing in the film is too small, and the properties (for example, low dielectric property, cushioning property, light weight) derived from the structure containing cavities become insufficient. Although the lower limit of the apparent specific gravity is not particularly limited, if the apparent specific gravity is less than 0.8, the buckling limit radius r described later cannot be set to a suitable range, and good handling properties cannot be obtained. There is a possibility that it is not preferable. In the present invention, in order to reduce the apparent specific gravity to 1.3 or less, a method of appropriately selecting the mixing amount of the thermoplastic resin incompatible with the polyester resin, the method including the polyolefin-based resin, A method of appropriately selecting a combination of thermoplastic resins, a method of appropriately selecting film production conditions (stretch ratio, stretching temperature, heat treatment temperature, and the like), and the like can be adopted.

The void-containing polyester film of the present invention has an average dispersed particle diameter d of a polyolefin resin.
(Unit: μm) is 15 μm or less, preferably 10 μm or less. In a film in which the average dispersed particle diameter d of the polyolefin-based resin exceeds 15 μm, the dispersed particles are coarse, the flexibility of the film is poor, and the handling properties are poor. In the present invention, the average dispersed particle diameter d of the polyolefin resin refers to an average value of the maximum diameter of the dispersed particles. In the present invention, in order to reduce the average dispersed particle diameter d of the polyolefin resin to 15 μm or less, a method of including a polystyrene resin together with the polyolefin resin, a method of appropriately selecting a mixing ratio of the polyolefin resin and the polystyrene resin. (The d can be reduced by increasing the proportion of the polystyrene resin), and controlling the shear history applied to the polymer in the melt line at the film production conditions (by applying a larger shear for a longer time, Can be reduced).

Furthermore, the void-containing polyester film of the present invention has a ring crush strength G (unit: kg / m
m) and the film thickness t (unit: mm) satisfy the following expression. G> 15 × t ^{3 In the} present invention, the ring crush strength G is defined by the method described in Examples described later. G is 15x
For t ³ below, rigidity is lost with polyester film originally, improvement of mounting property by a machine, becoming resistant like for Re seat refraction under load is poor, good handling property can not be obtained. In the present invention, G is preferably 20 × t ³ or more. In the present invention, in order for the ring crush strength G and the film thickness t to satisfy the above formula, the polystyrene-based resin and the polyolefin-based resin are added at a specific ratio, that is, the polystyrene-based resin / polyolefin-based resin is 0.01%. It can be achieved by mixing and using a ratio of 1 to 1. Specifically, polymethylpentene and polypropylene are used as the polyolefin resin, and as described above, the content a (% by weight) of the polystyrene resin in the film, the content b (% by weight) of the polymethylpentene and It is particularly preferable that the content c (% by weight) of the polypropylene satisfies the following expression. 0.01 ≦ a / (b + c) ≦ 1 c / b ≦ 13 3 ≦ a + b + c ≦ 20

The void-containing polyester film of the present invention has the above requirements, an apparent specific gravity of 1.3 or less, and an average dispersed particle diameter d of the polyolefin resin of 15 or less.
It is not more than μm, and only when the ring crush strength G (unit: kg / mm) and the film thickness t (unit: mm) satisfy the above formula, characteristics derived from the cavity-containing structure (for example, , Cushioning, low dielectric,
(Light weight) and a void-containing polyester film having good handling properties can be obtained.
If any one of these requirements is not satisfied, the cavity-containing polyester system having good handling properties while ensuring the properties derived from the cavity-containing structure (eg, cushioning property, low dielectric property, light weight). No film is obtained.

In addition, the void-containing polyester film of the present invention has a cross section parallel to the longitudinal direction of the film.
The dispersed particles of the thermoplastic resin incompatible with the polyester resin preferably have an aspect ratio of 1 to 10, and more preferably 2 to 8. If the aspect ratio exceeds 10, the amount of cavities generated is small, and it may be difficult to keep the apparent specific gravity within the above range. In the present invention, the aspect ratio means an average value of (the major axis / the minor axis of the dispersed particles of the thermoplastic resin incompatible with the polyester resin). In the present invention, the aspect ratio is 1 to 10
In order to use, a method of appropriately combining a plurality of polyolefin resins having different melting points or glass transition points,
A method in which the mixing ratio of the polyolefin-based resin and the polystyrene-based resin is appropriately selected, a method in which the heat treatment temperature is appropriately selected in the film production conditions (the higher the heat treatment at a higher temperature, the larger the aspect ratio) can be adopted.

Further, in the void-containing polyester film of the present invention, the buckling limit radius r (unit: mm) and the film thickness t (unit: mm) preferably satisfy the following expression: r <25 × t. In the present invention, the buckling limit radius r is defined by a method described in Examples described later. When the buckling limit radius r is 25 × t or more, the rigidity inherent in the polyester film is impaired, the mountability by a machine is improved, and the resistance to buckling under load is deteriorated. There is a possibility that handling properties cannot be obtained. In the present invention, the buckling limit radius r is more preferably less than 20 × t. In the present invention,
In order for the buckling limit radius r and the film thickness t to satisfy the above formula, the same method as the method for setting the average dispersed particle diameter d of the polyolefin-based resin to 15 μm or less, that is, polystyrene together with the polyolefin-based resin is used. A method of including a polyresin, a method of appropriately selecting a mixing ratio of a polyolefin-based resin and a polystyrene-based resin (r can be reduced by increasing the ratio of the polystyrene-based resin), and a method of producing a resin in a melt line under film production conditions. A method of controlling the shear history applied to the polymer in the above (where r can be reduced by applying larger shear for a long time) or the like can be adopted.

Further, the void-containing polyester film of the present invention has a dielectric constant of preferably less than 2.9, more preferably 2.7 or less, and most preferably 2.6 or less. If the dielectric constant is 2.9 or more, when used as an electrical insulating material, the effect of reducing the leakage current (the reduction rate with respect to the leakage current of the standard PET film) may be insufficient, which is not preferable. In the present invention, the dielectric constant is set to 2.9.
In order to make it less than 1, the apparent specific gravity of the film needs to be 1.3 or less.

The polyester film containing voids according to the present invention is particularly useful as an electrical insulating material (for example, an insulating material for an electric motor, particularly an insulating material for a Hermetec motor) and a printing film (for example, for thermal transfer). Become.

The void-containing polyester film of the present invention does not require a surfactant, a polyalkylene glycol, a polyether resin or the like as a dispersant for the polyolefin resin. Even when the raw materials are reused, the change in color tone is small and the stability during film production is excellent. When the self-recovered raw material is reused, a preferable usage ratio is 5 to 50% by weight.

In the void-containing polyester film of the present invention, a polymer recovered from a PET bottle or a polymer recovered from a magnetic card, an IC card or the like may be reused.

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

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

2) An average dispersed particle diameter d film of the polyolefin resin was cut into 1.0 g pieces, placed in a 50 ml volumetric flask, and then hexafluoroisopropanol heated to 40 ° C. was added. The flask is kept for 2 hours 4
Heating to 0 ° C. dissolved the film. 0.45μ
Filtration was performed using a membrane filter of m, and undissolved substances (polyolefin-based resin particles) were taken out. After this was dried, 20 points were randomly selected, magnified 1500 times using a scanning electron microscope (SEM), the maximum diameter was measured as the diameter, and the average value was d.

3) Aspect Ratio of Dispersed Particles of Thermoplastic Resin Incompatible with Polyester Resin The film is cut in parallel in the longitudinal direction, and the cross section is exposed with a microtome. Resin dispersed particles are randomly selected at 50 points, arbitrarily magnified 300 to 1000 times using a scanning electron microscope (SEM), and the major axis / minor axis of each dispersed particle is measured. The aspect ratio was used.

4) Critical buckling radius r (unit: mm) Two films each of which is 10 m in the longitudinal direction / width direction.
It was cut into mx 100 mm. Using a caliper having a beak length of 50 mm or more, the film was slightly curved and inserted into a U-shape between the beaks of the caliper, and the size was reduced in the direction of pinching and crushing the film. The bending width at the time when the film did not withstand the bending and buckled with the occurrence of corners was read on a caliper scale, and half the value was defined as the buckling limit radius r (unit: mm) of the film. Values were averaged over four points.

5) Ring crush strength G The method for testing the compressive strength of paperboard (ring crush method) shown in JIS P-8126 was used. Use tester: The stress value was read using a load cell at 100 kg full scale using a universal compression / tensile tester manufactured by Toyo Baldwin. Test speed 13mm / mi
compressed by n. Specimen support: 50m in diameter shown in JIS P-8126
A stainless jig having a groove of m and a depth of 6.35 mm was used. Test piece: The film was cut out into two pieces each in the longitudinal direction / width direction to a length of 152.4 mm and a width of 12.07 mm. Measuring method: The test piece was inserted into a test piece support, this was set on a tester, and the stress was read while compressing at a test speed of 13 mm / min. The value obtained by dividing the stress at the time when the buckling occurred in the test piece by the test piece length of 152.4 mm was defined as the ring crush strength G (unit: kg / mm). Values were averaged over four points.

6) Film thickness t Sony Precision Technology
Inc. Digital Micrometer
Using M-30, the thickness of the 20-point film was randomly measured, and the average value was taken as the film thickness t (mm).

7) Handling properties of film-1 (easiness of mounting on slot insulated portion of low-voltage induction motor) 100 samples of each sample were cut out to the size shown in FIG. 1 and each was bent into the shape shown in FIG. 3 was inserted into the film insertion part 4 of the motor slot model 3 shown in FIG. The criteria are as follows. ：: No insertion failure due to crushing or bending in 100 sheets △: Less than 5 insertion failures due to crushing or bending in 100 sheets ×: 5 or more insertion failures due to crushing or bending in 100 sheets

8) Handling property of film-2 (transportability of sheet film) After applying antistatic and antiblocking treatment to the film surface, 100 samples of A6 postcard size were prepared. This sample was supplied to an offset printing machine (AR-010, manufactured by Texel) and a transport test was performed. Handling performance was evaluated based on the frequency of occurrence of transport failures due to folding, wrinkles, paper jams, and the like caused by this test. ：: No transport failure occurred in 100 sheets. Δ: Less than 5 transport failures occurred in 100 sheets. ×: 5 or more transport failures occurred in 100 sheets.

9) Dielectric constant According to JIS C 2151-1990 “Testing method for plastic film for electric”.

10) Thermal transfer sensitivity characteristics Coating liquid having the following composition Water-dispersible copolymerized polyester resin: 2 parts by weight Water-dispersible acrylic / styrene copolymer resin: 5 parts by weight Water-dispersible isocyanate-based crosslinking agent: 0.5 part by weight Part Water: 67.4 parts by weight Isopropyl alcohol: 25 parts by weight Surfactant: 0.1 parts by weight After drying, apply to the film surface to a weight of 4 g / m ² , fix the dimensions, and at 160 ° C. Heat treatment was performed for 30 seconds to form a recording layer, and a thermal transfer image-receiving sheet was prepared.

The thermal transfer image-receiving sheet thus obtained was
The commercially available ink ribbon (print set P-PS100 for sublimation transfer printer manufactured by Caravelle Data System Co., Ltd.) and a commercially available thermal transfer printer (thermal transfer type label printer BLP-323 manufactured by Bonn Electric Co., Ltd.) were used for the samples cut to 6 sizes. And printing speed 100
mm / sec and printing was performed at a head voltage of 18V. Print patterns include C (cyan), M (magenta), Y (yellow), and K (black) obtained by superimposing them.
Regarding the colors, a pattern was used in which 7 (9 in total) square solid characters each having a size of 9 mm × 9 mm were arranged in an A6 sheet.

After printing, Macbeth densitometer (TR-927)
Was used to measure the reflection density of each color of CMYK, and the average reflection density of four colors (total of 28 places) was determined. Similarly, a commercially available image receiving paper (print set P-PS100 for a sublimation transfer printer manufactured by Caravelle Data System Co., Ltd .: a sheet obtained by laminating a foamed polypropylene film on both sides of natural paper and forming a recording layer) in the same manner. The average reflection density was determined, and the thermal transfer sensitivity characteristics were evaluated by the ratio (%) of the average reflection density of the sample to the average reflection density of the commercially available image-receiving paper.

11) Intrinsic viscosity of polyester resin (I
V) It was determined from the solution viscosity at 30 ° C. in a 1,1,2,2-tetrachloroethane / phenol (2: 3 weight ratio) mixed solvent.

12) Content of cyclic trimer of polyester resin (hereinafter referred to as "CT content") A sample (300 mg) was dissolved in 3 ml of a hexafluoroisopropanol / chloroform mixed solution (volume ratio = 2/3),
Further, 30 ml of chloroform was added for dilution. After adding 15 ml of methanol to precipitate the polymer,
Filtered. The filtrate was evaporated to dryness and dimethylformamide 1
The volume was adjusted to 0 ml and the cyclic trimer was quantified by high performance liquid chromatography.

13) Ring 3 when polyester resin is melted
Amount of increase in monomer (ΔCT) 3 g of the dried polyester chip was put into a glass test tube, and immersed in a 290 ° C. oil bath for 60 minutes in a nitrogen atmosphere to be melted. The increased amount of the cyclic trimer at the time of melting was determined by the following equation. Cyclic trimer increase during melting (% by weight) = Cyclic 3 after melting
Monomer content (% by weight) -cyclic trimer content before melting (% by weight)

14) Retention rate of elongation HFC-134a (CH ₂ F—CF ₃ ) 20 as a refrigerant
g, 50 g of POE (polyol ester) synthetic oil was used as a lubricant, and a measurement sample was put in a 120 cc autoclave at 140 ° C. and 30 atm for 2000 hours.
Prior to the introduction of the refrigerant / oil, the measurement sample was subjected to a dehydration treatment at 140 ° C. and a vacuum of 26 Pa (0.2 Torr) for 3 hours in an autoclave. The oil used was dried and conditioned in advance to control the water content to 300 ppm. The elongation at break before and after this test was measured, and the ratio of the elongation at break of the sample after the test to the elongation at break of the sample before the test was evaluated as elongation retention. In addition, the measurement of the breaking elongation followed the method prescribed | regulated to JIS-C2318.

15) Reduction rate of leakage current HFC-134a as a refrigerant and POE oil as a lubricant were used in combination. An initial value (X ₁ ) of leakage current was measured by inserting a standard electric insulating PET film into the motor of the hermetic compressor for refrigerator. Then
The sample was replaced with a standard PET film for electrical insulation and inserted into an electric motor, and the initial value (X ₂ ) of the leakage current of the sample was measured. The reduction rate of the leakage current was calculated from the following equation. Leakage current reduction rate (%) = [1− (X ₂ / X ₁ )] × 1
00

Example 1 (Preparation of Cavity-forming Agent)
1 / g polyethylene terephthalate 70% by weight, melt flow rate 2.0 polystyrene (Mitsui Toatsu Co., Ltd. Toporex 570-57U) 6% by weight, melt flow rate 1.7 polypropylene (Mitsui Toatsu Co., Ltd.) 6% by weight of Noblen F0-50F) and 18% by weight of polymethylpentene having a melt flow rate of 8 (TPX, DX-845, manufactured by Mitsui Petrochemical Co., Ltd.), and the mixture was fed to a twin-screw extruder. Knead,
The strand was cooled and cut to prepare a master pellet (Pa) containing a cavity-forming agent. Subsequently, the limiting viscosity is 0.
75% by weight of 62 dl / g polyethylene terephthalate
And the above-mentioned cavity-forming agent-containing master pellet (Pa) 2
5% by weight and a pellet were mixed and vacuum dried to obtain a film raw material.

(Preparation of Unstretched Film) Then, the above-mentioned raw materials for the film were supplied to an extruder, and the extruder was used for 30 minutes using a T-die.
Extruded onto a cooling drum adjusted to
A 0 μm unstretched film was made.

(Preparation of Biaxially Stretched Film) The obtained unstretched film was uniformly heated to 65 ° C. using a heating roll, and two pairs of nip rolls having different peripheral speeds (low speed roll = 1 m / min, high speed roll) (Speed = 3.4m / min)
Stretched 3.4 times. At this time, as an auxiliary heating device for the film, an infrared heater (rated 20 W / cm) having a gold reflection film in the middle of the nip roll is installed facing both surfaces of the film (a distance of 1 cm from the film surface),
Heated. The uniaxially stretched film thus obtained was guided to a tenter, heated to 150 ° C., stretched transversely 3.7 times, fixed in width, subjected to a heat treatment at 220 ° C. for 5 seconds, and further heated at 210 ° C. By relaxing by 4% in the width direction, a 188 μm-thick void-containing polyester film (Example 1) was obtained.

Comparative Example 1 As a raw material, the limiting viscosity obtained by vacuum drying was 0.62 dl /
g of 90% by weight of polyethylene terephthalate and 10% by weight of polystyrene having a melt flow rate of 2.0 (Toporex 570-57U manufactured by Mitsui Toatsu Co., Ltd.) were used. Otherwise by the same method as in Example 1, a void-containing polyester film (Comparative Example 1) was obtained.

Comparative Example 2 The raw material was vacuum dried and had an intrinsic viscosity of 0.62 dl /
g of 90% by weight of polyethylene terephthalate and 10% by weight of polypropylene (Noblen F0-50F manufactured by Mitsui Toatsu Co., Ltd.) having a melt flow rate of 1.7 were used. Otherwise by the same method as in Example 1, a void-containing polyester film (Comparative Example 2) was obtained.

Comparative Example 3 The raw material was vacuum dried and had an intrinsic viscosity of 0.62 dl /
g of polyethylene terephthalate (83% by weight), melt flow rate 180 polymethylpentene (TPX, DX-820, manufactured by Mitsui Petrochemical Co., Ltd.) 7% by weight chips and polyethylene glycol flake 10% by weight of 4000 are mixed. Was. Otherwise by the same method as in Example 1, a void-containing polyester film (Comparative Example 3) was obtained.

Example 2 As raw materials, 49.5% by weight of polyethylene terephthalate having an intrinsic viscosity of 0.62 dl / g and an average particle diameter of 0.3 μm
A mixture obtained by mixing 50% by weight of anatase type titanium dioxide (TA-300 manufactured by Fuji Titanium Co., Ltd.) and 0.5% by weight of a fluorescent whitening agent (OB-1 manufactured by Eastman Chemical Co., Ltd.) (venting method 2) After supplying to a shaft extruder and preliminarily kneading, the molten polymer was continuously supplied to a vent type single screw kneader and kneaded to prepare a master pellet (Pb) containing fine particles (titanium oxide). Next, the cavity-forming agent-containing master pellet (Pa) prepared in Example 1
30% by weight, the fine-particle-containing master pellet (Pb)
5% by weight and 65% by weight of polyethylene terephthalate having an intrinsic viscosity of 0.64 dl / g were mixed in a pellet and dried under vacuum to obtain a film raw material (A). On the other hand, 50% by weight of the fine particle-containing master pellet (Pb) and polyethylene terephthalate 50 having an intrinsic viscosity of 0.64 dl / g were used.
% By weight and dried under vacuum to obtain a film material (B)
And These film materials (A) and (B) are supplied to separate extruders, and the film material (B) is evenly laminated on both sides of the film material (A) using a feed block (B / A / B = 5). / 90/5) and a thickness of about 700 μm
Was produced. Then, a thickness of 100 μm was obtained by the same method as in Example 1 (however, low-speed roll speed = 2 m / min, high-speed roll speed = 6.8 m / min).
Thus, a void-containing polyester film (Example 2) was obtained.

Comparative Example 4 10% by weight of the fine-particle-containing master pellet (Pb) prepared in Example 2 was mixed with 80% by weight of polyethylene terephthalate having an intrinsic viscosity of 0.62 dl / g, followed by vacuum drying. Polystyrene with a rate of 2.0 (Toporex 570-57U manufactured by Mitsui Toatsu Co., Ltd.)
10 wt% was added to obtain a film raw material. Then, the same method as in Example 1 (however, low-speed roll speed = 2 m /
min, high-speed roll speed = 6.8 m / min)
A void-containing polyester film having a thickness of 100 μm (Comparative Example 4) was obtained.

Comparative Example 5 10% by weight of the fine-particle-containing master pellet (Pb) prepared in Example 2 was mixed with 78% by weight of polyethylene terephthalate having an intrinsic viscosity of 0.62 dl / g, and vacuum-dried. 12% by weight of a polypropylene having a rate of 1.7 (Noblen F0-50F manufactured by Mitsui Toatsu Co., Ltd.) was added to obtain a film raw material. Then, Example 1
(However, low-speed roll speed = 2 m / mi
n, high-speed roll speed = 6.8 m / min) to obtain a 100 µm-thick void-containing polyester film (Comparative Example 5).

Comparative Example 6 The raw material was vacuum dried and had an intrinsic viscosity of 0.62 dl /
g of polyethylene terephthalate and 80 wt% of polyethylene terephthalate, and 10 wt% of polymethylpentene chips (TPX, DX-820 manufactured by Mitsui Petrochemical Co., Ltd.) having a melt flow rate of 180.
And a mixture of 10% by weight of polyethylene glycol flake having a molecular weight of 4000 was used. Next, the same method as in Example 1 (however, low-speed roll speed = 2 m / mi)
n, high-speed roll speed = 6.8 m / min) to obtain a 100 µm-thick void-containing polyester film (Comparative Example 6).

Example 3 (Preparation of Cavity Forming Agent) As raw materials, 20% by weight of polystyrene having a melt flow rate of 2.0 (Toporex 570-57U manufactured by Mitsui Toatsu Co., Ltd.) and polypropylene having a melt flow rate of 1.7 were used. 20% by weight of Noblen F0-50F manufactured by Mitsui Toatsu Co., Ltd. and 60% by weight of polymethylpentene having a melt flow rate of 180 (TPX, DX-820 manufactured by Mitsui Petrochemical Co., Ltd.) are pellet-mixed, and the mixture is extruded into a twin screw extruder. The mixture was supplied and kneaded sufficiently, and the strand was cooled and cut to prepare a master pellet (Pc) containing a cavity-forming agent.

(Production of Film) Next, 10% by weight of a master pellet (Pc) containing the cavity-forming agent was mixed with 90% by weight of polyethylene terephthalate having an intrinsic viscosity of 0.62 dl / g manufactured using an Sb catalyst. And vacuum dried to obtain a film raw material (C). On the other hand, the intrinsic viscosity of 0.62 dl /
g of polyethylene terephthalate alone was used as film raw material (D). These film materials (C) and (D) are supplied to separate extruders, melted at 290 ° C., and the film material (D) is evenly laminated on both sides of the film material (C) using a feed block (D). / C / D =
7.5 / 85 / 7.5) to produce an unstretched film having a thickness of about 2600 μm. At this time, the residence time of the molten resin until the raw material was put into the extruder and the unstretched film was formed was about 10 minutes. Next, the unstretched film was stretched and heat-treated in the same manner as in Example 1 to obtain a void-containing polyester film having a thickness of 250 μm and an apparent specific gravity of 1.05 (Example 3).

Example 4 An intrinsic viscosity of 0.75 dl was produced using a Ge catalyst in place of the polyethylene terephthalate used in Example 3.
/ G, a void-containing polyester film (Example 4) in the same manner as in Example 3 except that a polyethylene terephthalate chip having a cyclic trimer amount of 0.30% by weight and ΔCT = 0.6% by weight was used. I got

Example 5 Instead of the polyethylene terephthalate used in Example 3, the polyethylene terephthalate chip used in Example 4 was treated with water (95 ° C., 5 hours).
75 dl / g, cyclic trimer amount 0.30% by weight, ΔCT =
A void-containing polyester film (Example 5) was obtained in the same manner as in Example 3 except that a 0.07% by weight polyethylene terephthalate chip was used.

[0091]

[Table 1]

[0092]

[Table 2]

[0093]

[Table 3]

Tables 1 to 3 show a list of the measurement results of the void-containing polyester film obtained by the above method.
From the measurement results of Tables 1 to 3, it can be considered as follows. Since the films of Examples 1 to 5 satisfy the requirements defined in the present invention, it is understood that excellent characteristics and good handling properties can be obtained due to the void-containing structure.

On the other hand, in Comparative Example 1 where the apparent specific gravity exceeds the requirement specified in the present invention, the dielectric constant is high, and similarly, in Comparative Example 4 where the apparent specific gravity exceeds the requirement specified in the present invention, the thermal transfer sensitivity characteristics are increased. It can be seen that the excellent properties derived from the cavity-containing structure were impaired in each case.

In Comparative Example 2 in which the average dispersed particle size of the polyolefin resin exceeds the requirement specified in the present invention, and in Comparative Example 3 in which the ring crush strength does not satisfy the requirement specified in the present invention, both motors were used. Insufficient insertion due to crushing or bending during the mountability test on the slot insulating part occurred frequently.

Similarly, in Comparative Example 5 in which the average dispersed particle diameter of the polyolefin resin exceeds the requirement specified in the present invention, and in Comparative Example 6 in which the ring crush strength deviates from the requirement specified in the present invention, all of the single wafers were used. Many transport failures occurred during the transport test of the film.

[0098]

According to the above description, the void-containing polyester film of the present invention has both properties (for example, cushioning property, low dielectric property, light weight) derived from the void-containing structure and handling properties in post-processing. Since it has both properties, it can be used as an electrical insulating material (for example, an insulating material for electric motors) that requires low dielectric properties and handling properties (such as mounting properties), and for various types of printing that require cushioning properties and handling properties (such as transportability). It is extremely useful as a film.

[Brief description of the drawings]

FIG. 1 is a cut film, and a broken line indicates a folding position. The unit of the length in the drawing is mm.

FIG. 2 shows a state in which the film of FIG. 1 is folded with a broken line and bent in a u-shape.

FIG. 3 is a motor slot model for inserting the film of FIG. 2; The unit of the length in the drawing is mm.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Film 2 Film bent 3 Motor slot model 4 Film insertion part

Claims (12)

[Claims] 1. A cavity formed of a polyester resin and a composition containing a thermoplastic resin that is incompatible with the polyester resin, the cavity being caused by the incompatible thermoplastic resin dispersed in particles in the polyester resin. A void-containing polyester film containing a large number of inside the film,
A void-containing polyester film containing a polyolefin resin as the incompatible thermoplastic resin and satisfying all of the following requirements: an apparent specific gravity of 1.3 or less; Average dispersed particle diameter d is 15 μm
The ring crush strength G (unit: kg / mm) and the film thickness t (unit: mm) satisfy the following expression. G> 15 × t ³ 2. The void-containing polyester system according to claim 1, wherein an aspect ratio of a dispersed particle of a thermoplastic resin incompatible with the polyester resin in a cut surface parallel to a longitudinal direction of the film is 1 to 10. the film. 3. The void-containing polyester according to claim 1, wherein the buckling limit radius r (unit: mm) and the film thickness t (unit: mm) defined in the text satisfy the following expressions. System film. r <25 × t 4. The void-containing polyester film according to claim 1, wherein the dielectric constant is less than 2.9. 5. The void-containing polyester film according to claim 1, wherein the polyolefin resin is polymethylpentene and / or polypropylene. 6. The void-containing polyester film according to claim 5, further comprising a polystyrene resin as a thermoplastic resin incompatible with the polyester resin. 7. The polystyrene resin content a (% by weight), polymethylpentene content b (% by weight), and polypropylene content c (% by weight) in the film satisfy the following formula: The void-containing polyester film according to claim 6, wherein: 0.01 ≦ a / (b + c) ≦ 1 c / b ≦ 13 3 ≦ a + b + c ≦ 20 8. The void-containing polyester film according to claim 1, wherein the content of the cyclic trimer in the polyester resin is 0.5% by weight or less based on the total weight of the film. 9. The void-containing polyester film according to claim 8, which is an insulating material for an electric motor. 10. The polyester resin has an intrinsic viscosity of 0.6.
The void-containing polyester film according to claim 8, wherein the content is 8 to 1.0 dl / g. 11. The polyester resin according to claim 8, wherein the amount of increase of the cyclic trimer when melted at a temperature of 290 ° C. for 60 minutes is 0.5% by weight or less. Void-containing polyester film. 12. The polyester resin is a polyester resin which has been subjected to water treatment in a chip form after polycondensation, and the polyester resin is mainly composed of an aromatic dicarboxylic acid or an ester-forming derivative thereof and ethylene glycol as raw materials for a Ge compound and / or The void-containing polyester film according to claim 8, which is obtained by using a Ti compound as a catalyst.