JP4778683B2 - Biaxially stretched polyester film and film capacitor comprising the same - Google Patents

Description

  The present invention relates to a biaxially stretched polyester film suitable for a capacitor and a film capacitor comprising the same, and more specifically, for a capacitor having excellent withstand voltage characteristics and heat resistance, and excellent handling properties such as film forming properties. The present invention relates to a suitable biaxially stretched polyester film and a film capacitor comprising the same.

  Conventionally, a film capacitor is manufactured by a method in which a film such as a biaxially oriented polyethylene terephthalate film or a biaxially oriented polypropylene film and a metal thin film such as an aluminum foil are overlapped and wound or laminated. In recent years, with the demand for miniaturization of electric or electronic circuits, film capacitors have also been miniaturized and mounted, and further heat resistance has been demanded in addition to electrical characteristics. In automotive applications, the range of use extends not only in the cab, but also in the engine room. In addition to electrical characteristics, film capacitors that are suitable for environments with higher temperatures and higher humidity are required. Yes.

  For the purpose of solving the heat resistance, a method using a polyethylene-2,6-naphthalate film is disclosed in Japanese Patent Application Laid-Open No. 2000-173855, and the crystalline state, intrinsic viscosity, etc. are improved for the purpose of improving the electrical characteristics. A method of controlling has been proposed. In addition, as thermoplastic resins having excellent electrical characteristics, JP-A-2-143951, JP-A-3-124750, and JP-A-5-008588 disclose a resin composition mainly composed of a syndiotactic polystyrene polymer. A film excellent in heat resistance and electrical properties obtained by biaxially stretching a product is disclosed. However, in the former method, since it is a polar polymer, there is a limit to further improvement in electrical characteristics. On the other hand, in the latter method, it is difficult to form a film as compared with a conventionally used polyester film. Since the obtained film is also easy to tear, an improvement in handling property at the time of manufacturing the capacitor is demanded.

  Moreover, as a film in which syndiotactic polystyrene and a polyester film are laminated in two layers, Japanese Patent Application Laid-Open No. 8-39974 discloses that moisture resistance and self-healing properties are improved. Japanese Patent No. 195327 discloses that the temperature range having excellent heat resistance and electrical characteristics is made wider. However, these two-layered film structures are excellent in both heat resistance and electric characteristics because the film tends to curl, and when the thickness ratio design is inappropriate, variation in electric characteristics is large. The present condition is that the polyester-type film excellent in film forming property and suitable for a film capacitor use is not yet provided.

JP 2000-173855 A Japanese Patent Laid-Open No. 2-143851 Japanese Patent Laid-Open No. 3-124750 Japanese Patent Laid-Open No. 5-200858 Japanese Patent Laid-Open No. 8-39741 JP-A-8-195327

  An object of the present invention is to provide a biaxially stretched polyester film excellent in heat resistance and withstand voltage characteristics and excellent in film formability and a film capacitor comprising the same.

As a result of intensive studies in view of the prior art, the present inventors have found that polyethylene-2,6-naphthalenedicarboxylate (a) is 50 to 95% by weight and syndiotactic polystyrene resin (b) is 5 to 50% by weight. the biaxially oriented polyester film comprising a mixture of, even with a small amount of the syndiotactic polystyrene resin having the syndiotactic polystyrene resin equivalent to the withstand voltage characteristics Surprisingly, at the same time the heat resistance and The present inventors have found that the film-forming property is excellent and have completed the present invention.

Thus, according to the present invention, the object of the present invention is to provide a mixture of polyethylene-2,6-naphthalenedicarboxylate resin (a) 50 to 95% by weight and syndiotactic polystyrene resin (b) 5 to 50% by weight. This is achieved by a biaxially stretched polyester film consisting of

Further, biaxially oriented polyester film of the present invention includes, as its preferred embodiments, while others breakdown voltage exceeds 400V / [mu] m, and comprises a this heat-resistant temperature is 110 ° C. or higher.

  Further, according to the present invention, the biaxially stretched polyester film of the present invention is used for a film capacitor, a polyester film composite in which a metal layer is formed on at least one surface of the polyester film, and further a polyester film composite. What comprises at least one of the film capacitors to be used is also included as a preferred embodiment.

According to the present invention, by forming a biaxially stretched polyester film in which a polyethylene-2,6-naphthalene dicarboxylate resin and a syndiotactic polystyrene resin are mixed at a specific blending ratio, a conventional polyester film, syndiotactic Compared to tic polystyrene film and film made of polyester and syndiotactic polystyrene, it has extremely high heat resistance and withstand voltage characteristics and has excellent film-forming properties, so it is suitable for biaxial stretching for film capacitors A polyester film and a film capacitor comprising the same can be provided, and its industrial value is extremely high.

The present invention will be described in detail below.
<Polyester resin>
The polyester resin (a) in the present invention is a polymer obtained by polycondensation of a diol and a dicarboxylic acid. Examples of such dicarboxylic acids include terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 4,4′-diphenyldicarboxylic acid, adipic acid and sebacic acid. Examples of diols include ethylene glycol, 1,4- Examples include butanediol, 1,4-cyclohexanedimethanol, and 1,6-hexanediol. In particular, polyethylene-2,6-naphthalenedicarboxylate is used from the viewpoint of heat resistance.

  The polyester resin in the present invention may be a single resin, a copolymer with other polyesters, or a mixture with two or more kinds of polyesters. The other component in the copolymer or mixture is preferably 10 mol% or less, more preferably 5 mol% or less, based on the number of moles of the repeating structural unit. Examples of the copolymer component include diol components such as diethylene glycol, neopentyl glycol, and polyalkylene glycol, and dicarboxylic acid components such as adipic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, and 5-sodium sulfoisophthalic acid.

  The intrinsic viscosity of the polyester resin in the present invention is preferably 0.40 or more at 35 ° C. in o-chlorophenol, and more preferably 0.40 to 0.80. If the intrinsic viscosity is less than 0.4, cutting may occur frequently during film formation, or the strength of the product after forming may be insufficient. On the other hand, when the intrinsic viscosity exceeds 0.8, the productivity during polymerization decreases.

  The melting point of the polyester resin in the present invention is preferably 200 to 300 ° C, and more preferably 260 to 290 ° C. If the melting point is less than the lower limit, the heat resistance of the polyester film may be insufficient. Moreover, when melting | fusing point exceeds an upper limit, mixing with a thermoplastic resin (b) may become difficult.

  The dielectric constant of the polyester resin in the present invention is preferably 2.7 to 3.4 under the conditions of 23 ° C. and 1 MHz. Such a dielectric constant is a characteristic characteristic of a polyester resin.

  The content of the polyester resin used in the biaxially stretched polyester film of the present invention is 50 to 95% by weight, preferably 55 to 94% by weight, based on the weight of the film. When the content of the polyester resin is less than the lower limit, the stretched film formation may not be sufficiently improved. Moreover, when the content of the polyester resin exceeds the upper limit, the withstand voltage characteristics may be difficult.

<Thermoplastic resin>
As the thermoplastic resin (b) in the present invention, poly-3-methylbutene-1, poly-4-methylpentene-1, polyvinyl t-butane, 1,4-trans-poly-2,3-dimethylbutadiene, Polyhalogenated styrene such as polyvinylcyclohexane, polystyrene, polymethylstyrene, polydimethylstyrene, polybutylstyrene, polyalkylstyrene, polyfluoroethylene, polychloroethylene, polybromostyrene, poly-2-methyl-4-fluorostyrene, Examples include polyvinyl-t-butyl ether, cellulose triacetate, cellulose tripropionate, polyvinyl fluoride, and polychlorotrifluoroethylene. In the present invention, among the thermoplastic resins, polyolefin resins are preferable, and syndiotactic polystyrene is particularly preferable from the viewpoint of heat resistance.

The syndiotactic polystyrene in the present invention is a polystyrene whose stereochemical structure has a syndiotactic structure, and the tacticity measured by a nuclear magnetic resonance method ( ¹³ C-NMR method) is dyad (two constituent units). 75% or more, preferably 85% or more, and pentad (5 structural units) is 30% or more, preferably 50% or more.

  Examples of such syndiotactic polystyrene include polystyrene, poly (alkylstyrene), poly (methylstyrene), poly (ethylstyrene), poly (propylstyrene), poly (butylstyrene), poly (phenylstyrene), poly (vinyl). Naphthalene), poly (vinylstyrene) and poly (acenaphthylene), poly (chlorostyrene) as poly (halogenated styrene), poly (bromostyrene) and poly (fluorostyrene), poly (methoxystyrene) as poly (alkoxystyrene) And poly (ethoxystyrene). Among these, polystyrene, poly (p-methylstyrene), poly (m-methylstyrene), and poly (p-tertiarybutylstyrene) are preferably exemplified.

  The syndiotactic polystyrene in the present invention may be a single substance or a copolymer with another polystyrene and a mixture with two or more kinds of polystyrene as long as the syndiotacticity is within the above range.

  The syndiotactic polystyrene in the present invention preferably has a polymerization average molecular weight of 10,000 or more, more preferably 50,000 or more. When the polymerization average molecular weight is less than the lower limit, heat resistance and mechanical properties are insufficient. On the other hand, the upper limit of the polymerization average molecular weight is preferably 500,000 or less. When this upper limit is exceeded, film-forming property may become poor.

  The thermoplastic resin in the present invention is not necessarily a single compound, and may be a mixture of two or more thermoplastic resins. The other components in the mixture are preferably 10% by weight or less, more preferably 5% by weight or less, based on the weight of the thermoplastic resin.

Melting point of the thermoplastic resin in the present invention, Ri 230 ° C. to 280 ° C. der, more preferably a two hundred and forty to two hundred and seventy-five ° C.. If the melting point is less than the lower limit, the heat resistance of the polyester film may be insufficient. When the melting point exceeds the upper limit, mixing with the polyester resin may be difficult.

  The dielectric constant of the thermoplastic resin in the present invention is less than 3.0 under the conditions of 23 ° C. and 1 MHz. The dielectric constant of the thermoplastic resin in the present invention is preferably 2.2 to 2.9. When the dielectric constant exceeds the upper limit, the withstand voltage characteristic of the polyester film is not sufficiently improved. In addition, a resin whose dielectric constant is less than the lower limit may have poor processability.

  The dielectric loss of the thermoplastic resin in the present invention is represented by a dielectric loss tangent (tan δ) at 23 ° C. and 1 MHz, and is less than 0.001. In the case where the dielectric loss is 0.001 or more, the insulation properties are lowered, and the withstand voltage characteristics of the resulting polyester film are not sufficiently improved.

  Content of the thermoplastic resin used for the biaxially stretched polyester film of this invention is 5 to 50 weight% on the basis of the weight of a film, Preferably it is 6 to 45 weight%. When the content of the thermoplastic resin is less than the lower limit, the withstand voltage characteristics may not be sufficiently improved. Moreover, when content of a thermoplastic resin exceeds an upper limit, extending | stretching film formation may become difficult.

<Inert particles>
The biaxially stretched polyester film of the present invention may be added with inert particles in order to impart winding properties during film formation. Such inert particles are used within a range not impairing the effects of the invention, but the inert particles having an average particle diameter of 0.001 to 5 μm are contained in the finally obtained polyester film in an amount of 0.01 to 10% by weight. It is preferable to add so as to. As the inert particles to be added, for example, inorganic particles such as calcium carbonate, silica, talc and clay, organic particles composed of any of silicone, thermoplastic resin and thermosetting resin, pigments such as barium sulfate and titanium oxide are at least included. One type can be used.

  Before adding these inert particles to the polyester resin, it is preferable to adjust the particle size and remove coarse particles using a purification process. Industrial means of the purification process include, for example, a jet mill and a ball mill as pulverizing means, and a dry or wet centrifuge as a classification means. In addition, you may refine | purify these means combining 2 or more types in steps.

Various methods can be used as a method for incorporating the inert particles into the polyester resin, and examples thereof include the following methods.
(A) A method of adding inactive particles before the end of the transesterification or esterification reaction or before starting the polycondensation reaction in the polyester synthesis process.
(A) A method in which inert particles are added to a polyester resin obtained by a polycondensation reaction and melt-kneaded.
(C) A method of producing a master pellet to which a large amount of inert particles is added in the methods (a) and (b) above, kneading with a polyester resin not containing inert particles, and containing inert particles at a predetermined concentration. .

  In addition, when using the method of said (a), it is preferable to add to a reaction system as a slurry which disperse | distributed the inert particle in glycol.

<Additives>
The biaxially stretched polyester film of the present invention may contain a small amount of additives such as ultraviolet absorbers, antioxidants, antistatic agents, light stabilizers, and heat stabilizers as necessary.

  Moreover, you may add a compatibilizing agent for the purpose of making a polyester resin and a thermoplastic resin compatible and improving a stretch characteristic. Examples of such a compatibilizing agent include oxazoline group-containing reactive polymers.

  Furthermore, the biaxially stretched polyester film of the present invention may contain a phosphorus compound. The phosphorus compound is not particularly limited as long as it is a phosphorus compound that acts as a heat stabilizer, and examples thereof include phosphoric acid, phosphoric acid ester such as methyl phosphate and ethyl phosphate, phosphorous acid, and phosphorous acid ester. Of these phosphorus compounds, triethylphosphonoacetate is particularly preferred.

  A preferable content of the phosphorus compound is 20 to 300 ppm, further 30 to 250 ppm, particularly 50 to 200 ppm as a molar concentration of the phosphorus element in the phosphorus compound with respect to the total polyester dicarboxylic acid component. When the content of the phosphorus compound is less than 20 ppm, the transesterification reaction catalyst is not completely deactivated, the thermal stability is poor, and the mechanical strength may be lowered. On the other hand, if the content of the phosphorus compound exceeds 300 ppm, the thermal stability may be poor and the mechanical strength may decrease.

<Coating layer>
The biaxially stretched polyester film of the present invention may have a coating layer on at least one surface of the outermost layer. Such a coating layer is obtained by applying a coating agent comprising a binder resin and a solvent to a biaxially stretched polyester film. As the binder resin, various resins such as a thermoplastic resin or a thermosetting resin can be used. For example, polyester, polyimide, polyamide, polyesteramide, polyolefin, polyvinyl chloride, poly (meth) acrylic acid ester, polyurethane and polystyrene, And copolymers and mixtures thereof. Among these binder resins, polyester copolymers are particularly preferable. Examples of the solvent include organic solvents and mixtures such as toluene, ethyl acetate, and methyl ethyl ketone, and may be water.

  The coating film layer of the present invention may further contain a crosslinking agent, a surfactant and inert particles as components for forming the coating film. An example of such a surfactant is polyalkylene oxide.

  In addition to the above components, the coating film layer of the present invention includes other resins such as melamine resin, soft polymer, filler, heat stabilizer, weathering stabilizer, anti-aging agent, labeling agent, antistatic agent, slip agent, and anti-blocking agent. Further, an antifogging agent, a dye, a pigment, a natural oil, a synthetic oil, a wax, an emulsifier, a curing agent, a flame retardant, and the like may be further included, and the blending ratio thereof is appropriately selected within a range not impairing the object of the present invention.

  The coating layer of the present invention is a method of laminating on a polyester film, a method of applying a coating on at least one side of a biaxially stretched polyester film and drying, a method of applying a coating on a stretchable polyester film, and then drying Any method of stretching and heat-treating as necessary may be used. Here, the stretchable polyester film is an unstretched polyester film, a uniaxially stretched polyester film, or a biaxially stretched polyester film, and among these, a longitudinally stretched polyester film that is uniaxially stretched in the film extrusion direction (longitudinal direction) is particularly preferable. Preferably exemplified.

  Moreover, when apply | coating a coating agent to a polyester film, application | coating in a clean atmosphere, ie, the application | coating in a film forming process, is preferable, and the adhesiveness to the polyester film of a coating film improves. When a normal coating process, that is, biaxial stretching, is performed in a heat-set polyester film in a process separated from the film manufacturing process, dust, dust, and the like are likely to be caught.

  As a method for applying the coating agent to the polyester film, any known coating method can be used, for example, roll coating method, gravure coating method, roll brush method, spray coating method, air knife coating method, impregnation method and curtain. The coating methods can be used alone or in combination.

<Film formation>
The biaxially stretched polyester film of the present invention is provided with 50 to 95% by weight of a polyester resin (a) and 5 to 50% by weight of a thermoplastic resin (b) for the purpose of imparting excellent film forming properties together with heat resistance and voltage resistance characteristics. % Biaxially stretched polyester film consisting of a mixture of Due to the mixture, the properties of the polyester resin with relatively low withstand voltage characteristics are complemented by the thermoplastic resin with good withstand voltage characteristics despite the low blending amount of the thermoplastic resin, Also has membrane properties.

  In addition, the biaxially stretched polyester film of the present invention may be a laminate having at least one layer composed of the above-mentioned mixture as long as the effects of the present invention are not impaired.

  The biaxially stretched polyester film of the present invention is made from the above-described polyester resin (a) and thermoplastic resin (b) as raw materials, extruded into a sheet in a molten state, and then a known product such as a tenter method or an inflation method. For example, a predetermined amount of polyester resin (a) and thermoplastic resin (b) are mixed, dried, and supplied to an extruder heated to 300 ° C. The method of shape | molding in a sheet form is mentioned.

  The sheet-like molded product extruded from the die is cooled and solidified with a cooling drum having a surface temperature of 10 to 60 ° C., and this unstretched film is heated by, for example, roll heating or infrared heating, and then stretched in the longitudinal direction to form a longitudinally stretched film. obtain. Such longitudinal stretching is preferably performed by utilizing the difference in peripheral speed between two or more rolls. The longitudinal stretching temperature is preferably higher than the glass transition point (Tg) of the polyester resin, more preferably 20 to 40 ° C. higher than Tg. The longitudinal draw ratio may be adjusted as appropriate according to the requirements of the application to be used, but is preferably 2.5 times or more and 5.0 times or less, more preferably 2.8 times or more and 3.9 times or less. When the longitudinal draw ratio is 2.5 or less, the thickness unevenness of the film is deteriorated and a good film may not be obtained. Further, when the longitudinal draw ratio is 5.0 times or more, breakage tends to occur during film formation.

  The obtained longitudinally stretched film is successively subjected to transverse stretching, heat setting, and thermal relaxation to form a biaxially oriented film, which is performed while the film is running. The transverse stretching treatment is performed while starting from a temperature 20 ° C. higher than the glass transition point (Tg) of the polyester resin and increasing the temperature to a temperature lower by 120 to 30 ° C. than the melting point (Tma) of the polyester resin. The transverse stretching start temperature is preferably (Tg + 40) ° C. or lower. The maximum transverse stretching temperature is preferably (100 to 40) ° C. lower than Tma. When the transverse stretching start temperature is too low, the film is easily broken. If the maximum transverse stretching temperature is lower than (Tma-120) ° C., the thermal shrinkage rate of the obtained film increases, and the uniformity of physical properties in the width direction tends to decrease. On the other hand, when the maximum transverse stretching temperature is higher than (Tma-30) ° C., the film becomes too soft and the film is easily broken during film formation.

  Although the temperature increase in the transverse stretching process may be continuous or stepwise (sequential), the temperature is usually increased stepwise. For example, the transverse stretch zone of the stenter is divided into a plurality along the film running direction, and the temperature is raised by flowing a heating medium having a predetermined temperature for each zone.

  The transverse draw ratio may be adjusted as appropriate according to the requirements of the application to be used, but is preferably 2.5 times or more and 5.0 times or less, more preferably 2.8 times or more and 4.0 times or less. When the longitudinal draw ratio is 2.5 or less, the thickness unevenness of the film is deteriorated and a good film may not be obtained. Further, when the longitudinal draw ratio is 5.0 times or more, breakage tends to occur during film formation.

  The biaxially stretched polyester film is then heat set. By performing heat setting, the dimensional stability of the obtained film under high temperature conditions is improved.

  The preferable range of the heat shrinkage rate at 200 ° C. of the biaxially stretched polyester film of the present invention is −3 to 3%, further −2 to 2%, and particularly −1 to 1%. When the thermal shrinkage at 200 ° C. does not satisfy the above range, the film may be wrinkled when a metal film is deposited on the film to form a polyester composite and a capacitor. In order to make the heat shrinkage rate at 200 ° C. within the above-mentioned range, the heat setting treatment is performed at (Tma-100 ° C.) or more, and further in the range of (Tma-50) ° C. to (Tma-20) ° C. preferable.

  In addition, the biaxially stretched polyester film of the present invention is further annealed in an offline process at 150 to 220 ° C. for 1 to 60 seconds and then gradually cooled in a temperature atmosphere of 50 to 80 ° C. in order to suppress thermal shrinkage. May be applied.

  The total thickness of the biaxially stretched polyester film of the present invention thus obtained is preferably 0.1 to 20 μm, more preferably 0.5 to 15 μm, and particularly preferably 1.0 to 10 μm. When the film thickness is less than 0.1 μm, it is difficult to form a film, and the withstand voltage characteristic may be deteriorated. On the other hand, if the film thickness exceeds 20 μm, it may be difficult to reduce the size of the capacitor.

  The dielectric breakdown voltage of the biaxially stretched polyester film of the present invention exceeds 400 V / μm, preferably 410 V / μm or more, more preferably 460 V / μm or more, and particularly preferably 470 V / μm or more. If the dielectric breakdown voltage is 400 V / μm or less, the electrical characteristics when used in a capacitor are not sufficient.

  The heat resistance of the biaxially stretched polyester film of the present invention is 110 ° C. or higher, preferably 115 ° C. or higher, more preferably 120 ° C. or higher. If the heat resistance is less than 110 ° C., the heat resistance when used in a capacitor is not sufficient. Here, the heat resistance conforms to the temperature index of IEC60216, and is defined as a temperature that can withstand 20000 hours by Arrhenius plotting the relationship between the half-life time of the dielectric breakdown voltage and the temperature.

The biaxially stretched polyester film of the present invention may be a laminate in which another layer is further laminated on at least one surface for the purpose of imparting another function. For example, for the purpose of further improving the self-healing property, the polyester film may further have a layer containing an oxygen atom-containing compound on at least one side, and the oxygen atoms relative to the carbon atoms on the surface measured by X-ray photoelectron spectroscopy. The ratio is preferably 10% or more, more preferably 15% or more. Examples of the oxygen atom-containing compound include cellulose and SiO ₂ . In the case of SiO ₂ , lamination may be performed by any of vacuum deposition, ion plating, or sputtering.

  The polyester film composite of the present invention is obtained by forming a metal layer on at least one side of the above-described biaxially stretched polyester film. The material of the metal layer is not particularly limited, and examples thereof include aluminum, zinc, nickel, chromium, tin, copper, and alloys thereof.

  Such a polyester film composite can be suitably used for a film capacitor.

  EXAMPLES The present invention will be described in detail below using examples, but the scope of the present invention is not limited by these examples. The melting point, heat resistance, dielectric constant, and dielectric breakdown voltage in the examples were measured and evaluated by the following methods. Moreover, unless otherwise indicated, the part and% in an Example mean a weight part and weight%, respectively.

(1) Melting point 10 mg each of the polyester resin (a) and the thermoplastic resin (b) are enclosed in an aluminum pan for measurement, and 20 ° C./min from 25 ° C. to 300 ° C. using a differential calorimeter DSC2920 manufactured by TAinstruments. The respective melting points (Tma, Tmb) were determined by measuring the temperature rise rate.
(2) Heat resistance Using the obtained biaxially stretched polyester film, according to the temperature index of IEC60216, Arrhenius plots the relationship between the half-life time of the dielectric breakdown voltage and the temperature, and obtains a temperature that can withstand 20000 hours. It was.
(3) Dielectric constant Using a thermoplastic resin, the dielectric constant at 23 ° C. and 1 MHz was measured in accordance with JIS C2151.
(4) Dielectric loss Using a thermoplastic resin, the dielectric loss at 23 ° C. and 1 MHz was measured in accordance with JIS C2151.
(5) Dielectric breakdown voltage Using the obtained biaxially stretched polyester film, in accordance with the plate electrode method described in JIS C2151, using ITS-6003 made by Tokyo Seiden Co., Ltd., the dielectric breakdown was caused by a direct current of 160 V / s. The voltage was measured.

[Example 1]
After transesterification of naphthalene-2,6-dicarboxylate dimethyl and ethylene glycol in the presence of manganese acetate by a conventional method, triethylphosphonoacetate was added. Subsequently, antimony trioxide was added and polycondensed by a conventional method to obtain a polyethylene-2,6-naphthalenedicarboxylate resin. As a result of measuring the concentration of each element in the resin by the atomic absorption method, Mn = 50 ppm, Sb = 300 ppm, and P = 50 ppm.

  90% by weight of the obtained polyethylene-2,6-naphthalenedicarboxylate resin (inherent viscosity 0.62) and syndiotactic polystyrene (manufactured by Idemitsu Petrochemical Co., Ltd., grade: 130ZC) were uniformly blended, After drying at 180 ° C. for 6 hours, the mixture was supplied to an extruder heated to 300 ° C. and formed into a sheet from a 290 ° C. die. Further, the unstretched film obtained by cooling and solidifying the sheet with a cooling drum having a surface temperature of 60 ° C. was led to a roll group heated to 140 ° C., and stretched 3.6 times in the longitudinal direction (longitudinal direction), and then the roll group at 60 ° C. It was cooled with.

  Next, while holding both ends of the longitudinally stretched film with clips, it is guided to a tenter and stretched by 4.0 times in the direction perpendicular to the longitudinal direction (lateral direction) in an atmosphere heated to a maximum transverse stretching temperature of 150 ° C. did. Thereafter, heat setting was performed at 220 ° C. for 5 seconds in the tenter, and further heat relaxation was performed at 200 ° C. for 1%, followed by uniform cooling and cooling to room temperature to obtain a biaxially stretched polyester film having a thickness of 5 μm. .

  Table 1 shows the characteristics of the polyester resin (a) and the thermoplastic resin (b) used and the characteristics of the obtained biaxially stretched polyester film. The polyester film of this example was excellent in heat resistance and dielectric breakdown voltage, and at the same time, could be formed under constant conditions for 48 hours, and had excellent film forming properties.

[Example 2]
70% by weight of polyethylene-2,6-naphthalenedicarboxylate resin prepared and prepared in the same manner as in Example 1 and 30% by weight of syndiotactic polystyrene (made by Idemitsu Petrochemical Co., Ltd., grade: 130ZC) were used. Except for the above, the same operation as in Example 1 was repeated to obtain a biaxially stretched polyester film having a thickness of 5 μm.

  Table 1 shows the characteristics of the polyester resin (a) and the thermoplastic resin (b) used and the characteristics of the obtained biaxially stretched polyester film. The polyester film of this example was excellent in heat resistance and dielectric breakdown voltage, and at the same time, could be formed under constant conditions for 48 hours, and had excellent film forming properties.

[Comparative Example 1]
The same operation as in Example 1 was repeated except that 100% by weight of polyethylene-2,6-naphthalenedicarboxylate resin obtained by the same manner as in Example 1 was used and syndiotactic polystyrene was not used. A biaxially stretched polyester film having a thickness of 5 μm was obtained.

  Table 1 shows the characteristics of the polyester resin (a) used and the characteristics of the obtained biaxially stretched polyester film. Although the polyester film of this comparative example was excellent in heat resistance, the dielectric breakdown voltage was not satisfactory.

[Comparative Example 2]
Instead of the polyethylene-2,6-naphthalenedicarboxylate resin of Comparative Example 1, a polyethylene terephthalate resin was used, dried at 170 ° C. for 3 hours, and then supplied to an extruder heated to 280 ° C. From a die at 290 ° C. Molded into a sheet. Further, the unstretched film obtained by cooling and solidifying the sheet with a cooling drum having a surface temperature of 20 ° C. was led to a roll group heated to 90 ° C., and stretched 3.6 times in the longitudinal direction (longitudinal direction), and then the 20 ° C. roll group. It was cooled with.

  Next, while holding both ends of the longitudinally stretched film with clips, it is guided to a tenter, and stretched by 4.0 times in the direction perpendicular to the longitudinal direction (lateral direction) in an atmosphere heated to a maximum transverse stretching temperature of 120 ° C. did. Thereafter, heat setting was performed at 220 ° C. for 5 seconds in the tenter, and further heat relaxation was performed at 200 ° C. for 1%, followed by uniform cooling and cooling to room temperature to obtain a biaxially stretched polyester film having a thickness of 5 μm. .

  Table 1 shows the characteristics of the polyester resin (a) used and the characteristics of the obtained biaxially stretched polyester film. Although the polyester film of this example satisfied the dielectric breakdown voltage, the heat resistance was not satisfactory.

[Comparative Example 3]
10% by weight of polyethylene-2,6-naphthalenedicarboxylate resin obtained by the same manner as in Example 1 was used, and 90% by weight of syndiotactic polystyrene (made by Idemitsu Petrochemical Co., Ltd., grade: 130ZC) was used. Except for the above, the same operation as in Example 1 was repeated to obtain a biaxially stretched polyester film having a thickness of 5 μm.

  Table 1 shows the characteristics of the polyester resin (a) used and the characteristics of the obtained biaxially stretched polyester film. Although the polyester film of this comparative example was excellent in heat resistance and dielectric breakdown voltage, breakage occurred several times during film formation for 48 hours, and the film forming property was not satisfactory.

  The biaxially stretched polyester film obtained by the present invention is excellent in heat resistance and withstand voltage characteristics expressed by dielectric breakdown voltage, and at the same time, is excellent in film forming properties. It is suitably used for. A polyester film composite in which a metal layer is formed on at least one side of the biaxially stretched polyester film of the present invention is also suitably used as a film capacitor.

Claims (6)

A biaxially stretched polyester film comprising a mixture of polyethylene-2,6-naphthalenedicarboxylate (a) 50 to 95% by weight and syndiotactic polystyrene resin (b) 5 to 50% by weight.   The biaxially stretched polyester film according to claim 1, wherein the dielectric breakdown voltage exceeds 400 V / µm and the heat resistant temperature is 110 ° C or higher. The biaxially stretched polyester film according to claim 1 or 2 , wherein the film thickness is 0.1 to 20 µm. Polyester film biaxially oriented polyester film for film capacitors, characterized in that used in the film capacitor according to any one of claims 1-3. The polyester film composite by which a metal layer is formed in the at least single side | surface of the polyester film in any one of Claims 1-4 . A film capacitor comprising the polyester film composite according to claim 5 .