JP4890958B2 - Biaxially oriented polyester film - Google Patents

Description

  The present invention relates to a biaxially oriented polyester film excellent in heat discoloration and slipperiness. More specifically, the present invention relates to a biaxially oriented polyester film containing calcium carbonate particles that suppresses deterioration in color even when subjected to heat history over a plurality of times or has a long heat history time and also has slipperiness.

  Polyester films, especially biaxially oriented films made of polyethylene terephthalate and polyethylene naphthalene dicarboxylate, have excellent mechanical properties, heat resistance, chemical resistance, etc., so magnetic tape, ferromagnetic thin film tape, photographic film, packaging It is widely used as a material for films, films for electronic parts, electrical insulating films, metal laminating films, protective films and the like. As an electrical insulating material, a polyethylene naphthalene dicarboxylate film having high heat resistance (Patent Document 1) has come to be used due to an increase in calorific value of electrical equipment accompanying performance improvement, including a polyethylene terephthalate film.

  As a method for inserting an electric insulating film into a wedge or slot of a motor, an automatic insertion method using a machine is adopted, and high slipperiness is required for the film. Therefore, particles having a relatively large particle size are used as a lubricant that imparts high slipperiness, and calcium carbonate particles are used as an example.

  Calcium carbonate particles have been conventionally used as a lubricant, and a usage form such as a large amount in a polyester film is known for the purpose of enhancing whiteness. However, simply adding a large amount of calcium carbonate causes the interaction between the particles and the polyester due to the surface activity of the particles, resulting in a decrease in heat resistance and foreign matters and foaming, and the resulting film has insufficient whiteness Therefore, it has been proposed to use a phosphorus compound such as trimethyl phosphate together (Patent Document 2, Patent Document 3, and Patent Document 4). Furthermore, using a specific phosphonate compound for calcium carbonate has been proposed as a method for further improving heat resistance stability, discoloration, foaming, and the like (Patent Document 5).

  On the other hand, when adding a relatively small amount of calcium carbonate as a lubricant, Patent Document 6 discloses that a phosphorus compound such as trimethyl phosphate is used in combination mainly for the purpose of improving the dispersibility of calcium carbonate. In addition, in order to suppress the aggregation of inert particles such as calcium carbonate, and to increase the thermal stability of polyester, a titanium compound is used as a catalyst, and a specific phosphonate compound is used in a range where the difference from the titanium catalyst amount is small. Is disclosed in Patent Document 7.

  As described above, when the calcium carbonate particles are contained in a large amount, there has been a problem with the color tone. However, the color tone is not regarded as a problem until only about several weight% is contained for the purpose of the lubricant. However, as plastic products are collected and reused in recent years, the problem that coloring occurs when a film containing a small amount of calcium carbonate is subjected to multiple heat histories or when the heat history time is long becomes apparent. It has become like this. Therefore, the present situation is that a biaxially oriented polyester film that suppresses the deterioration of color tone and has slipperiness even when subjected to such a thermal history is desired.

Japanese Patent Laid-Open No. 01-23220 JP 07-331038 A Japanese Patent Laid-Open No. 08-143756 JP 09-272793 A JP 10-237275 A Japanese Patent Laid-Open No. 06-157877 JP 2004-043539 A

  The object of the present invention is to solve such problems of the prior art, even when a polyester film containing calcium carbonate particles is subjected to heat history over multiple times or when the heat history time is long, color deterioration is suppressed, and The object is to provide a biaxially oriented polyester film that also has slipperiness.

  As a result of intensive studies to solve the above problems, the present inventors have found that a specific phosphonate is used in order to suppress color deterioration even when the total time of heat history in the polyester film containing calcium carbonate particles as a lubricant is long. It has been found that sufficient heat discoloration can be obtained for the first time by using a phosphonate compound in a high ratio range that has not been conventionally used for calcium carbonate particles, and has completed the present invention.

（上記式（Ｉ）中、Ｒ_１およびＲ_２は、それぞれ互いに独立に１〜４個の炭素原子を有するアルキル基を表し、Ｘは、−ＣＨ_２−または−ＣＨ（Ｙ）−を表す（Ｙはフェニル基を示す））
かつ炭酸カルシウム粒子とホスホネート化合物との含有比およびホスホネート化合物の含有量がそれぞれ下記式（１）、下記式（２）
１≦Ｃａ／Ｐ≦３０ ・・・・・（１）
（上式（１）中、Ｃａはフィルム中の炭酸カルシウム粒子のカルシウム元素としての濃度（単位：ｐｐｍ）、Ｐはフィルム中のホスホネート化合物のリン元素としての濃度（単位：ｐｐｍ）をそれぞれ示す）
８０≦Ｐ≦１５０ ・・・・・（２）
（上式（２）中、Ｐはフィルム中のホスホネート化合物のリン元素としての濃度（単位：ｐｐｍ）を示す）
を満たしており、下記式（３）で表わされる３００℃、３０分加熱処理後のフィルムの変色指数
変色指数(％)＝(300℃,30分熱処理後のフィルムのYID)/(300℃,1分熱処理後のフィルムのYID)×100 ・・・（３）
が１５０以下である二軸配向ポリエステルフィルムによって達成される。 That is, according to the present invention, an object of the present invention is to provide a biaxially oriented polyester film containing calcium carbonate particles and a phosphonate compound, wherein the polyester is polyethylene naphthalene dicarboxylate and the content of calcium carbonate is based on the weight of the polyester film. 0.02 to 1.5% by weight, and the phosphonate compound is a phosphonate compound represented by the following general formula (I):

(In the above formula (I), R ₁ and R ₂ each independently represent an alkyl group having 1 to 4 carbon atoms, and X represents —CH ₂ — or —CH (Y) — ( Y represents a phenyl group))
And the content ratio of the calcium carbonate particles and the phosphonate compound and the content of the phosphonate compound are the following formula (1) and the following formula (2), respectively.
1 ≦ Ca / P ≦ 30 (1)
(In the above formula (1), Ca represents the concentration of calcium carbonate particles in the film as calcium element (unit: ppm), and P represents the concentration of phosphonate compound in the film as phosphorus element (unit: ppm))
80 ≦ P ≦ 150 (2)
(In the above formula (2), P represents the concentration (unit: ppm) of the phosphonate compound in the film as phosphorus element)
The have been met, discoloration index of the film 300 ° C., after 30 minutes heat treatment represented by the following formula (3)
Discoloration index (%) = (YID of film after heat treatment at 300 ° C. for 30 minutes) / (YID of film after heat treatment at 300 ° C. for 1 minute) × 100 (3)
Is achieved by a biaxially oriented polyester film having a 150 or less .

In addition, the biaxially oriented polyester film of the present invention has, as a preferred embodiment thereof, that the polyester is polyethylene naphthalene dicarboxylate, contains an antimony compound as a main catalyst in the amount of 60 to 300 ppm as an antimony element in the film, The content is 0.02 to 1.5% by weight based on the weight of the polyester film, the color change index of the film after heat treatment at 300 ° C. for 30 minutes represented by the following formula (2), the color change index (%) = (YID of film after heat treatment at 300 ° C. for 30 minutes) / (YID of film after heat treatment at 300 ° C. for 1 minute) × 100 (2)
Including at least one of the following is included: the film surface is less than 180, and the ten-point average roughness Rz of the film surface is 120 to 200 nm.
Moreover, the biaxially oriented polyester film of the present invention is suitably used as an electrical insulating film.

  According to the present invention, it is possible to provide a calcium carbonate particle-containing biaxially oriented polyester film that is prevented from deterioration in color when subjected to heat history over a plurality of times or when the heat history time is long, and also has slipperiness. And its industrial value is extremely high.

The present invention will be described in detail below.
<Polyester>
The polyester constituting the polyester film of 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, naphthalenedicarboxylic acid, 4,4′-diphenyldicarboxylic acid, adipic acid and sebacic acid, and diols include, for example, ethylene glycol, 1,4-butanediol, 1 , 4-cyclohexanedimethanol and 1,6-hexanediol. Among these polyesters, specifically, polyethylene terephthalate, polyethylene isophthalate, polytetramethylene terephthalate, poly (1,4-cyclohexanedimethylene terephthalate), polyethylene naphthalene dicarboxylate are exemplified, and particularly from the viewpoint of heat resistance, Polyethylene terephthalate and polyethylene naphthalene dicarboxylate are preferred, and polyethylene naphthalene dicarboxylate is particularly preferred when higher heat resistance is required. In addition, since the problem of color deterioration due to thermal history of the present invention is particularly remarkable in polyethylene naphthalene dicarboxylate having a high processing temperature, the improvement effect is high when polyethylene naphthalene dicarboxylate is used.

  The polyester resin of the present invention may be used alone or as a copolymer with other polyesters and as a mixture with two or more kinds of polyesters. The other component in the copolymer or mixture is preferably 20 mol% or less, more preferably 10 mol% or less, based on the number of moles of the repeating structural unit.

  Polyethylene naphthalene dicarboxylate (hereinafter sometimes abbreviated as PEN) uses naphthalene dicarboxylic acid as the dicarboxylic acid component and ethylene glycol as the diol component. Examples of naphthalenedicarboxylic acid include 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, and 1,5-naphthalenedicarboxylic acid. Among these, 2,6-naphthalenedicarboxylic acid is preferable. When the polyethylene naphthalene dicarboxylate is a copolymer, a compound having two ester-forming functional groups in the molecule can be used as the copolymer component constituting the copolymer. Examples of such compounds include oxalic acid and adipic acid. Phthalic acid, sebacic acid, dodecanedicarboxylic acid, isophthalic acid, terephthalic acid, 1,4-cyclohexanedicarboxylic acid, 4,4′-diphenyldicarboxylic acid, phenylindanedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, tetralindicarboxylic acid Dicarboxylic acids such as decalin dicarboxylic acid and diphenyl ether dicarboxylic acid; oxycarboxylic acids such as p-oxybenzoic acid and p-oxyethoxybenzoic acid; or trimethylene glycol, tetramethylene glycol, hexamethyleneglycol Le, cyclohexane methylene glycol, neopentyl glycol, ethylene oxide adduct of bisphenol sulfone, ethylene oxide adduct of bisphenol A, diethylene glycol, can be preferably used such diol polyethylene oxide glycol. These copolymer components may be used alone or in combination of two or more. Among these copolymer components, preferred acid components are isophthalic acid, terephthalic acid, 4,4′-diphenyldicarboxylic acid, 2,7-naphthalenedicarboxylic acid or p-oxybenzoic acid, and preferred diol components include An ethylene oxide adduct of trimethylene glycol, hexamethylene glycol, neopentyl glycol or bisphenol sulfone.

  When polyethylene naphthalene dicarboxylate is a polymer blend, the blend components include polyethylene terephthalate, polyethylene isophthalate, polytrimethylene terephthalate, polyethylene 4,4'-tetramethylene diphenyl dicarboxylate, polyethylene-2,7-naphthalene. Carboxylate, polytrimethylene-2,6-naphthalene dicarboxylate, polyneopentylene-2,6-naphthalene dicarboxylate, poly (bis (4-ethyleneoxyphenyl) sulfone) -2,6-naphthalene dicarboxylate Mention may be made of polyesters such as rate. These blend components may be used alone or in combination of two or more.

  The polyester of the present invention may be one in which a part or all of the terminal hydroxyl group and / or carboxyl group is blocked with a monofunctional compound such as benzoic acid or methoxypolyalkylene glycol. It may be copolymerized with a trifunctional or higher functional ester-forming compound such as glycerin and pentaerythritol within a range where a substantially linear polymer is obtained.

  The intrinsic viscosity of the polyester of the present invention is preferably 0.40 dl / g or more at 35 ° C. in o-chlorophenol, and more preferably 0.40 to 0.85 dl / g. When the intrinsic viscosity is less than the lower limit, cutting often occurs during film formation, or the strength of the product after forming may be insufficient. On the other hand, when the intrinsic viscosity exceeds the upper limit, productivity during polymerization may be lowered. Moreover, it is preferable that the intrinsic viscosity of polyester after forming into a biaxially oriented film is 0.38 dl / g or more and 0.83 dl / g or less.

  The polyethylene naphthalene dicarboxylate of the present invention is a conventionally known method, for example, a method of directly obtaining a low-polymerization degree polyester by reaction of dicarboxylic acid and glycol, or a transesterification catalyst of a lower alkyl ester of dicarboxylic acid and glycol. After the reaction, it can be obtained by a method of conducting a polymerization reaction in the presence of a polymerization catalyst. This transesterification catalyst can use a well-known thing. Moreover, a well-known thing can also be used for a polymerization catalyst, and an antimony compound is illustrated as a main catalyst. The antimony compound is preferably used in the range of 60 to 300 ppm as an antimony element in the film. When the amount of the polymerization catalyst is less than the lower limit, the productivity during the production of the polyester is delayed, while when the amount of the polymerization catalyst exceeds the upper limit, the thermal deterioration at the time of melting is severe, and the color deterioration due to the polymerization catalyst may occur. . Specific examples of the antimony compound include antimony trioxide and antimony pentoxide. In addition, a titanium compound may be used in a very small range, and examples include tetraethoxy titanium, tetrabutoxy titanium, trimellitic acid titanium, titanium sulfate, and titanium chloride. In the present invention, an antimony compound is preferably used as the polymerization catalyst from the viewpoint of preventing deterioration in color derived from the polymerization catalyst. The antimony compound is more preferably used in the range of 70 to 250 ppm.

<Calcium carbonate particles>
The polyester film of the present invention contains calcium carbonate particles as a main lubricant. When using calcium carbonate particles as a lubricant, since it is possible to impart high slip properties among commonly used lubricants, for example, when a biaxially oriented polyester film containing calcium carbonate particles is used as an electrical insulating film, Film can be automatically inserted into the motor wedge or slot.

  The content of the calcium carbonate particles is preferably 0.02 to 1.5% by weight based on the weight of the polyester film. When the content of calcium carbonate particles is less than the lower limit, high slipperiness cannot be obtained. For example, when used as an electrical insulating film, it is clogged when the film is automatically inserted into the motor wedge or slot by a machine. The defect rate may be high. On the other hand, even if calcium carbonate particles are included in a range exceeding the upper limit, not only the slipperiness can be obtained, but the color in remelting tends to deteriorate.

  The average particle size of the calcium carbonate particles is preferably 0.01 to 10 μm, more preferably 0.1 to 5.0 μm, and particularly preferably 0.2 to 2.0 μm. Here, the average particle diameter refers to an “equivalent sphere diameter” corresponding to 50% by volume from an integrated curve of calcium carbonate particles of each particle diameter and their abundance. If the average particle size is less than the lower limit, high slipperiness cannot be obtained.For example, when used as an electrical insulating film, the defective rate is reduced due to clogging when the film is automatically inserted into the wedge or slot of the motor. May be high. On the other hand, when the average particle size exceeds the upper limit, the film forming property may be poor during film formation.

（上記式（Ｉ）中、Ｒ_１およびＲ_２は、それぞれ互いに独立に１〜４個の炭素原子を有するアルキル基を表し、Ｘは、−ＣＨ_２−または−ＣＨ（Ｙ）−を表す（Ｙはフェニル基を示す））
で表されるホスホネート化合物を用い、かつフィルム中のホスホネート化合物のリン元素としての濃度が８０〜１５０ｐｐｍであることが必要である。 <Phosphonate compound>
The polyester film of the present invention has the following general formula (I) as a phosphonate compound.

(In the above formula (I), R ₁ and R ₂ each independently represent an alkyl group having 1 to 4 carbon atoms, and X represents —CH ₂ — or —CH (Y) — ( Y represents a phenyl group))
And the concentration of the phosphonate compound in the film as a phosphorus element is 80 to 150 ppm.

  By using such a phosphonate compound, it can be contained in the film in a high concentration range of 80 to 150 ppm. When conventional phosphorus compounds such as phosphoric acid, phosphonic acid and their alkyl ester compounds are used in such a high concentration range, the polymerization reaction rate is delayed and the heat resistance stability of the phosphorus compound itself is not sufficient. Discoloration derived from phosphorus compounds may occur or the thermal stability of the polyester may be reduced. When the phosphonate compound of the present invention is used, if the concentration of the phosphonate compound as a phosphorus element is less than the lower limit, it is not possible to suppress color deterioration due to thermal history. On the other hand, when the concentration of the phosphonate compound as a phosphorus element exceeds the upper limit, the polymerization reaction rate is delayed in the polyester production process.

  Such a phosphonate compound is not particularly limited as long as it is a compound represented by the formula (I). For example, methyl diethylphosphonoacetate, ethyldimethylphosphonoacetate, triethylphosphonoacetate, ethyldiisopropylphosphonoacetate, isopropyldiethyl Examples thereof include phosphonoacetate, tert-butyldiethylphosphonoacetate, and dimethyl (phenylmethoxycarbonylmethyl) phosphonate. Of these, triethylphosphonoacetate is particularly preferable. The content of the phosphonate compound is more preferably 90 to 140 ppm.

  The addition time of the phosphonate compound is not particularly limited as long as it is any stage from the end of the transesterification reaction or esterification reaction to the stretch film-forming process. The addition of the phosphonate compound may be performed at once, or may be performed in two or more divided portions. Examples of the method of adding the phosphonate compound include a method of directly adding a predetermined amount of the phosphonate compound, or a method of adding a so-called master polymer (master batch) containing a high concentration of the phosphonate compound.

  As a method of directly adding a predetermined amount of phosphonate compound, i) a method of directly adding a predetermined amount of phosphonate compound to the molten polyester before the polymerization step or film formation, ii) medium with the molten polyester before the polymerization step or film formation Examples thereof include a method of adding a predetermined amount of diluted phosphonate compound, or iii) a method of adding a predetermined amount of phosphonate compound to solid polyester, mixing them, and melt-kneading.

As a method of adding a master polymer (master batch), i) a method of adding a master polymer containing a predetermined amount of a phosphonate compound to a molten polyester before the polymerization step or film formation, ii) a predetermined amount of solid polyester Examples thereof include a method of adding a master polymer containing a phosphonate compound, mixing them, and then melt-kneading them.
Among these, the preferred addition time of the phosphonate compound includes the end of the transesterification reaction.

<Ca / P ratio>
One of the characteristics of the present invention is that when the content ratio of the calcium carbonate particles and the phosphonate compound in the biaxially oriented polyester film is in a specific range, deterioration of color due to thermal history is suppressed, The content ratio with the phosphonate compound needs to satisfy the following formula (1).
1 ≦ Ca / P ≦ 40 (1)
(In the above formula (1), Ca represents the concentration of calcium carbonate particles in the film as calcium element (unit: ppm), and P represents the concentration of phosphonate compound in the film as phosphorus element (unit: ppm))
As represented by the above formula (1), when a phosphonate compound is contained in a very high ratio with respect to the calcium carbonate particles, the polyester film is subjected to a heat history over a plurality of times or a heat history over a long time for the first time. It is possible to suppress the deterioration of color tone.

The lower limit of Ca / P is preferably 7 or more, more preferably 10 or more, particularly preferably 20 or more, while the upper limit of Ca / P is preferably 35 or less, more preferably 33 or less, particularly preferably 30 or less. . When Ca / P exceeds the upper limit, the activity of the calcium carbonate particles cannot be sufficiently suppressed because the blending ratio of the phosphonate compound with respect to the calcium carbonate particles is small, and the degree of film discoloration due to heat history increases. On the other hand, when Ca / P is less than the lower limit, not only the calcium carbonate activity suppression effect by the phosphonate compound cannot be expected any more, but also the polymerization reaction rate may be delayed by the excess phosphonate compound.
When a calcium compound is used as a transesterification catalyst in addition to the calcium carbonate particles, the calcium element as the catalyst is not included in the calcium concentration of the above formula (1).

<Other additives>
In the biaxially oriented polyester film of the present invention, particles having a small surface activity and good heat stability may be used in combination as a lubricant other than calcium carbonate particles. Here, the particles having good heat stability indicate inert particles, and examples include silica particles and particles made of a polymer having high heat resistance such as crosslinked silicone resin, crosslinked polystyrene, crosslinked acrylic resin particles, and the like. Spherical silica particles are preferred. The average particle diameter of the particles other than the calcium carbonate particles is preferably 0.1 μm or more smaller than the average particle diameter of the calcium carbonate particles, and is preferably contained in the range of 0.5% by weight or less based on the weight of the polyester film. .
Moreover, the biaxially oriented polyester film of the present invention may contain a small amount of an antistatic agent, a light-shielding agent, a stabilizer and the like as required.

<Discoloration index>
The biaxially oriented polyester film of the present invention is characterized by small discoloration during remelting. Specifically, the discoloration index of the film after heat treatment at 300 ° C. for 30 minutes represented by the following formula (3) is 180. It is preferable that it is less than.
Color change index (%) = (YID of film after heat treatment at 300 ° C. for 30 minutes) / (YID of film after heat treatment at 300 ° C. for 1 minute) × 100 (3)
The discoloration index represented by the above formula (3) is measured by the following procedure. A 300 μm thick spacer having a space of 80 mm × 80 mm square is placed on the heating plate of a heating press (model IMC-16CE, manufactured by Imoto Seisakusho), and the heating plate is set to 300 ° C. A biaxially oriented film finely cut into flakes is placed in the space of the spacer, pressurized at a pressure of 0.5 MPa for 2 minutes, then increased to 2.0 MPa and pressurized for 1 minute, and then the sample is taken out. Quench in water to obtain a 300 μm thick sheet. A yellow index (YID) is obtained by the following formula (II) using a color difference meter (manufactured by Nippon Denshoku Industries Co., Ltd., Σ-90), and the obtained sheet is defined as YID of the film after heat treatment at 300 ° C. for 1 minute. . Here, X, Y, and Z are tristimulus values of light defined in the international CIE standard.
YID = 100 / Y * (1.28 * X−1.06 * Z) (II)

Similarly, the yellow index (YID) of the sheet obtained by repeating the same operation except that the pressing time at 2.0 MPa is changed from 1 minute to 30 minutes is obtained by the above formula (II), and is 300 ° C., 30 The YID of the film after the fractional heat treatment is used.
The discoloration index (%) is obtained by the above equation (3) using YID at a pressure time of 30 minutes at 2.0 MPa and YID at a pressure time of 1 minute at 2.0 MPa.
The discoloration index represents the degree of discoloration when the melt processing time at a high temperature is long, and the smaller the discoloration index, the smaller the discoloration of the film after the melt processing. The discoloration index is more preferably 150 or less. When the discoloration index exceeds the upper limit, the film becomes strongly yellowish and gives a recognition of a hue different from the hue inherent in the polyester film.

  The reason for the discoloration is that the heat resistance of the polyester decreases due to the surface activity of calcium carbonate, so it is likely that the polyester will be easily colored due to thermal history, especially when polyethylene naphthalene dicarboxylate is used as the polyester, because the processing temperature is high, usually When the phosphorous compound is added or when the amount of the phosphonate compound of the present invention is small relative to the calcium carbonate particles, the degree of discoloration tends to increase. In order to make the discoloration index into the above-mentioned range, it is achieved by blending the specific phosphonate compound of the present invention at a high blending ratio of 1 ≦ Ca / P ≦ 40 with respect to the calcium carbonate particles.

<10-point average roughness Rz of film surface>
The biaxially oriented polyester film of the present invention preferably has a ten-point average roughness Rz of 100 to 200 nm on the film surface. The lower limit of the ten-point average roughness Rz on the film surface is more preferably 120 nm or more. Here, the ten-point average roughness Rz of the film surface is a value determined according to JIS standard B0601. Such film surface roughness is achieved by using calcium carbonate particles as the main lubricant and using a specific phosphonium compound in a specific ratio, and when used as an electrical insulating film, the film is automatically machined in the motor wedge or slot. Can be inserted. When the ten-point average roughness Rz of the film surface is less than the lower limit, high slipperiness may not be obtained. For example, when used as an electrical insulating film, the film is automatically inserted into the wedge or slot of the motor by a machine. The defect rate may increase due to clogging. On the other hand, when the ten-point average roughness Rz exceeds the upper limit, not only the slipperiness beyond that can be obtained, but the amount of calcium carbonate particles is too much to deteriorate the color, or the excessive phosphonate compound causes the polymerization reaction rate to be reduced. There may be a delay.

<Film forming method>
The biaxially oriented polyester film of the present invention can be produced using a known film forming method as long as it is biaxially stretched. For example, a sufficiently dried polyester is melted at a temperature of melting point to (melting point + 70) ° C. It can be produced by melt extrusion, quenching on a castin drum to form an unstretched film, and then subjecting the unstretched film to sequential or simultaneous biaxial stretching and heat setting. When forming a film by sequential biaxial stretching, the unstretched film is stretched 2.5 to 5.0 times in the longitudinal direction, and then stretched 2.5 to 5.5 times in the transverse direction by a stenter. The heat setting is preferably carried out under tension or limited shrinkage at a temperature below the melting point. In the case of simultaneous biaxial stretching, the above stretching temperature, stretching ratio, heat setting temperature and the like can be applied.

The biaxially oriented polyester film thus obtained preferably has a film thickness in the range of 0.5 to 400 μm, more preferably 25 to 380 μm, and particularly preferably 150 to 350 μm.
The biaxially oriented polyester film of the present invention is usually used as a single layer, but may be a laminate in which a coating layer or the like is laminated as long as the effects of the invention are not impaired.

  According to the present invention, the biaxially oriented polyester film of the present invention is suitably used for an electrical insulation film and the like, and when inserting the electrical insulation film into a wedge or a slot of a motor, an automatic insertion method by a machine is used. At the same time, when a reused film is included, when the number of melting heat histories is large and when the heat history time is long, coloring and discoloration are small, and deterioration of the color of the film is suppressed.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited only to these Examples. Each characteristic value was measured by the following method. Moreover, unless otherwise indicated, the part and% in an Example mean a weight part and weight%, respectively.

(1) Amount of P and Ca in a polyester film 5 g of a film is heated and melted to 300 ° C. to make a circular disk with a diameter of 40 mm and a thickness of 5 mm, using a fluorescent X-ray apparatus 3270 type manufactured by Rigaku Electric Industry Co., The amounts of P and Ca in the film were determined, respectively, and the ratio (Ca / P) ratio between calcium atoms (Ca) and phosphorus atoms (P) was determined. In addition, Ca shows the density | concentration (unit: ppm) as a calcium element of the calcium carbonate particle in a film, P shows the density | concentration (unit: ppm) as a phosphorus element of the phosphorus compound in a film, respectively.

(2) Average Particle Size of Lubricant The average particle size of the lubricant was measured using a trade name “SACP-4L Centrifuggle Particle Size Analyzer” manufactured by Shimadzu Corporation. A particle diameter “equivalent sphere diameter” corresponding to 50 mass percent is read from an integrated curve of particles of each particle diameter calculated based on the obtained centrifugal sedimentation curve and the abundance thereof, and this value is taken as the above average particle diameter. (See the book “Particle Size Measurement Technology” published by Nikkan Kogyo Shimbun, 1975, pages 242-247).

(3) Heat-resistant discoloration A 300 μm thick spacer having a space of 80 mm × 80 mm square was placed on the heating plate of a heating press (model IMC-16CE, manufactured by Imoto Seisakusho), and the heating plate was set at 300 ° C. The biaxially oriented film finely cut into flakes was put into the space of the spacer, pressurized at a pressure of 0.5 MPa for 2 minutes, and then increased to 2.0 MPa and pressurized for 1 minute. The sample was taken out and quenched in water to obtain a 300 μm thick sheet. Similarly, the same operation was performed except that a biaxially oriented film finely cut into flakes was used and the pressurization time at 2.0 MPa was changed to 30 minutes. Using a color difference meter (Nippon Denshoku Industries Co., Ltd., Σ-90), the yellow index (YID) of the two sheets was determined by the following formula (II). Here, X, Y, and Z are tristimulus values of light defined in the international CIE standard.
YID = 100 / Y * (1.28 * X−1.06 * Z) (II)
Using the YID value at a pressure time of 30 MPa at 2.0 MPa and the YID value at a pressure time of 1 minute at 2.0 MPa, the discoloration index (%) is obtained by the following formula (3), Sex was evaluated. A lower color change index is less likely to change color and is judged to be good.
Discoloration index (%) = (YID of film after heat treatment at 300 ° C. for 30 minutes) / (YID of film after heat treatment at 300 ° C. for 1 minute) × 100 (3)
○: Discoloration index 150 or less Δ: Discoloration index greater than 150 and less than 180 ×: Discoloration index 180 or more (discolored to brown)

(4) Ten-point average roughness Rz of the film surface
Using a surface roughness measuring device SE-3FAT manufactured by Kosaka Laboratory Ltd., the ten-point average roughness Rz of the film surface was determined by the measuring method of JIS B0601.

[Example 1]
100 parts of dimethyl 2,6-naphthalenedicarboxylate and 60 parts of ethylene glycol were used as a transesterification catalyst and 0.03 part of manganese acetate tetrahydrate was used. As a lubricant, calcium carbonate particles having an average particle diameter of 0.5 μm were added to polyethylene-2, 6-Naphthalene dicarboxylate resin composition based on the weight of 0.8% by weight, 0.06% by weight of spherical silica particles having an average particle size of 0.2 μm, and 0% of spherical silica particles having an average particle size of 0.1 μm The mixture was added so as to contain 1% by weight and subjected to a transesterification according to a conventional method, and then 0.109 part of triethylphosphonoacetate was added to substantially complete the transesterification.
Next, 0.024 part of antimony trioxide was added, and then a polymerization reaction was carried out in a conventional manner at a high temperature and high vacuum to obtain polyethylene-2,6-naphthalenedicarboxylate (PEN) having an intrinsic viscosity of 0.60 dl / g. Got. The PEN polymer was dried at 175 ° C. for 5 hours, then supplied to an extruder, melted at a melting temperature of 300 ° C., extruded through a die slit, and then cooled and solidified on a casting drum set at a surface temperature of 60 ° C. An unstretched film was created.

This unstretched film is stretched 3.3 times in the continuous film-forming direction at 140 ° C. Thereafter, the film was biaxially stretched 3.7 times sequentially at 145 ° C. in the width direction, and further heat-set at 235 ° C. for 10 seconds to obtain a biaxially oriented film having a thickness of 100 μm and wound on a roll.
The evaluation results of the obtained biaxially oriented film are shown in Table 1. This film had a small discoloration due to heat history and was excellent in heat discoloration.

[Examples 2 to 11 and Comparative Examples 1 to 7 ]
A biaxially oriented film was obtained in the same manner as in Example 1 except that the concentration of calcium carbonate particles as calcium element and the concentration of triethylphosphonoacetate as phosphorus element were changed as shown in Table 1. The evaluation results of the obtained biaxially oriented film are shown in Table 1. In Comparative Example 2, the polymerization rate was delayed, and the intrinsic viscosity of the polymer was less than 0.40 dl / g.

  The biaxially oriented polyester film of the present invention is excellent in slipperiness, and even if it contains calcium carbonate particles, deterioration in color due to thermal history is suppressed, and it is excellent in heat discoloration. For example, it is suitably used as a film for electrical insulation. It is done.

Claims (4)

In the biaxially oriented polyester film containing calcium carbonate particles and a phosphonate compound, the polyester is polyethylene naphthalene dicarboxylate , and the content of calcium carbonate is 0.02 to 1.5% by weight based on the weight of the polyester film, The phosphonate compound is a phosphonate compound represented by the following general formula (I):

(In the above formula (I), R ₁ and R ₂ each independently represent an alkyl group having 1 to 4 carbon atoms, and X represents —CH ₂ — or —CH (Y) — ( Y represents a phenyl group))
And the content ratio of the calcium carbonate particles and the phosphonate compound and the content of the phosphonate compound are the following formula (1) and the following formula (2), respectively.
1 ≦ Ca / P ≦ 30 (1)
(In the above formula (1), Ca represents the concentration of calcium carbonate particles in the film as calcium element (unit: ppm), and P represents the concentration of phosphonate compound in the film as phosphorus element (unit: ppm))
80 ≦ P ≦ 150 (2)
(In the above formula (2), P represents the concentration (unit: ppm) of the phosphonate compound in the film as phosphorus element)
The have been met, discoloration index of the film 300 ° C., after 30 minutes heat treatment represented by the following formula (3)
Discoloration index (%) = (YID of film after heat treatment at 300 ° C. for 30 minutes) / (YID of film after heat treatment at 300 ° C. for 1 minute) × 100 (3)
Is a biaxially oriented polyester film, characterized in that it is 150 or less . The biaxially oriented polyester film according to claim 1, comprising an antimony compound as a main catalyst in an amount of 60 to 300 ppm as an antimony element in the film. The biaxially oriented polyester film according to claim 1 or 2 , wherein the film surface has a ten-point average roughness Rz of 100 to 200 nm. The film for electrical insulation which consists of a biaxially-oriented polyester film in any one of Claims 1-3 .