Process for the preparation of a polyimide film

Abstract: A polyimide film essentially consisting of polyimide having recurring units of the formula ( I ): which has a density of 1.335 to 1.390 g/cm³ at 23 °C and/or a refractive index of 1.605 to 1.680 at 23 °C in the direction of thickness and is transparent; and a preparation process of the polyimide film by extruding the polyimide having recurring units of the above formula ( I ) throught a common melt-extrusion process, casting an a chill-roller to obtain an unstratched film, uniaxially or biaxially stretching the unstretched film to cause molecular orientation, and successively setting the stretched film through heat-­ treatment; is disclosed.

Description: The present invention relates to a process for the preparation of polyimide film having stability in mechanical properties and dimensions over a broad range of temperatures. Industrial materials for electric and electronic industries, automotive industry and atomic industry have recently been intended to improve reliability over a long period, miniaturize the products and reduce the weight of the products. Consequently, plastic films for use in these products have been required to be excellent in heat-resistance, mechanical properties and dimensional stability. Thermoplastic polymers having heat resistance, for example, polyphenylene sulfide, polyaryl ketone, polyaryl sulfone, polyallylate, polyester and polycarbonate have conventionally been known and investigated to prepare plastic films in order to meet with the above requirement for heat resistance. However, these films have insufficient heat-resistance and their properties are also unsatisfactory for industrial materials. Hence, the field of application for these films has been restricted. At present, film materials having the highest heat-resistance designed to meet the above requirement are various polyimide films such as Upilex™ (a product of Ube Industries Ltd.) and Kapton ™ (a product of E.I. Du Pont De Nemours & Co.). Since hot molding of these polyimides is difficult, films are prepared by a so-called solution casting method where a polyamic acid precursor solution is cast, solvent is removed, and the residual film is heat-treated. The method is, however, poor in productivity and high in cost, see e.g. JP-A-62214927 and EP-A-235 294. Polyimide having recurring structural units represented by the formula (I): has been disclosed as a hot-moldable polyimide in, for example, Japanese Patent Laid-Open Publication SHO 62-205124(1987). However, the film obtained by the disclosed process has low mechanical strength and becomes soft above its glass transition temperature of about 240 °C, and additionally changes from transparent to opaque and brittle at temperatures exceeding 280 °C. Thus heat-resistance of the film is unsatisfactory. EP-A-235 294 also discloses polyimide film as adhesive layer. The present invention aims to provide a polyimide film which is prepared from polyimide raw material having recurring structural units represented by the formula (I) above but which is excellent in heat resistance, mechanical properties and dimensional stability. Thus, the present invention provides a process for the preparation of a stretched polyimide film from an unstretched polyimide film comprising polyimide having recurring structural units of the formula I: which process comprises the steps of heat-stretching the unstretched film and successively heat-setting the stretched film under tension or shrinkage restriction so as to obtain a stretched polyimide film having a density of from 1.335 to 1.390 g/cm³ at 23°C and a refractive index of from 1.605 to 1.680 at 23°C in the direction of thickness. Preferably, an unstretched film of the polyimide, prepared by a conventional melt-extrusion process from polyimide having the recurring units of the formula (I), is stretched from 1.5 to 10 times in the temperature range of from 230° to 320°C either uniaxially or biaxially in directions which are at right angles to each other, and is successively heat-set under tension in the temperature range of from 250°C to less than the melting point of the film. Specific embodiments of the process for preparing the polyimide film are as follows: (1) The unstretched film is stretched from 1.5 to 5.0 times in one direction in the temperature range of from 230° to 300°C, and is successively heat-set under tension in the temperature range of from 250°C to less than the melting point of the film. (2) The unstretched film is stretched from 1.5 to 3.0 times in one direction in the temperature range of from 240° to 300°C and from 1.5 to 3.0 times in the direction which is at right angles to said first stretching direction in the temperature range of from 250° to 320°C, and is successively heat-set under tension in the temperature range of from 250°C to less than the melting point of the film. (3) The unstretched film is stretched so that its area is magnified from 2 to 10 times by stretching simultaneously in two directions which are at right angles to each other in the temperature range of 250° to 300°C, and is successively heat-set under tension in the temperature range of 250°C to less than the melting point of the film. (4) The film can also be stretched in the temperature range of from 150°C to less than the glass transition temperature while maintaining the moisture content of the film from 0.1 to less than 3% during the stretching step. By so adjusting the moisture content of the film during stretching, the film stretching can be conducted at a temperature lower than the glass transition temperature, which method has conventionally been considered very difficult. The preparation process in accordance with preferred embodiments of the present invention is very effective for the production of practical films having heat-resistance and mechanical properties superior to those of conventional polyimide film. The polyimide film made by the process of the present invention is excellent in heat and chemical resistance and additionally in mechanical strength and dimensional stability. Hence the polyimide film is suitable for use in fields such as wire covering, liner insulation material for motors and transformers, flexible substrates for printed circuits, capacitors and other electric and electronic parts, and precision parts such as base films for recording medium. As previously mentioned, the polyimide made by the process of the present invention has recurring structural units represented by the above formula (I). The polyimide can be prepared by reacting pyromellitic dianhydride (hereinafter referred to as acid anhydride) with 4,4'-bis(3-aminophenoxy)biphenyl(hereinafter referred to as diamine) and imidizing the resultant polyamic acid represented by the formula (II): The polyimide of the formula (I) is preferably comprised of 95 % by mole or more of the acid anhydride and the diamine, respectively. A polyimide containing more than 5% by mole of other acid dianhydrides or diamines decreases crystallinity and increases the amorphous portion. Consequently, even after molecular orientation by stretching, heat-treatment gives insufficient set and the dimensional stability of the film lowers unfavorably. The polyimide film made by the process of the present invention can be prepared by using the above polyimide as the raw material, preparing the amorphous film generally by a melt-extrusion process and casting on a chill-roller to give an unstretched film. The unstretch film is then stretched uniaxially or biaxially and heat-treated to crystallize the film, as described above. The polyimide film made by the process of the present invention is obtained by conducting molecular orientation through stretching in the interior of the film. The degree of molecular orientation can be judged by each refractive index n x , n y and n z in the two rectangular directions (x and y directions) and in the thickness direction (z direction), respectively, and particularly by the refractive index in the thickness direction of the film interior. For example, the orientation to x direction in the film interior increases n x and decreases n y , n z , and then increases the birefringence(n x - n y ) in the film interior. On the other hand, the orientation to both x and y directions increases both n x and n y , and n z alone. Thus, the refractive index in the thickness direction indicates the degree of molecular orientation in the film interior. In the present invention, the refractive index n z in the thickness direction is from 1.605 to 1.680 at 23°C. Typically, n z is 1.690 in the non-oriented state, that is, unstretched film. A value of n z exceeding 1.680 leads to unfavorable problems such as loss of transparency and tendency of embrittlement at high temperatures. On the other hand, when n z is less than 1.605, the film causes remarkable fibrillation and becomes unpractical because cracks and tears are liable to occur. The preferred refractive index in the thickness direction is in the range of from 1.615 to 1.675. The polyimide film made by the process of the present invention has a density in the range of from 1.335 to 1.390 g/cm³ at 23°C. The density is measured by a density gradient tube method using a mixed organic solvent system such as carbon tetrachloride-toluene or an aqueous inorganic salt solution such as an aqueous sodium sulfate solution. A density less than 1.335 g/cm³ leads to unfavorably large dimensional changes at high temperatures, above 250 °C in particular. On the other hand, a density of 1.390 g/cm³ or more causes remarkable crystallization and unfavorably results in cracks, tears and inferior mechanical properties. The preferred range of the density is from 1.340 to 1.370 g/cm³. The process of the invention will hereinafter be described in more detail. The polyimide used for the raw material of the invention can be obtained by conducting a polymerization reaction of the above acid anhydride with diamine and then imidizing the resulting intermediate. The polyimide particularly preferred for use has a melt viscosity in the range of usually from 10-60,000 Pa.s (100 to 600,000 poise) at 400°C under a shear rate of 200 sec⁻¹. A melt viscosity less than 10 Pa.s (100 poise) is liable to cause gelation in the melt-extrusion step, give adverse effects on the surface appearance of the film and also lower the mechanical strength of the film. On the other hand, a melt viscosity exceeding 60,000 Pa.s (600,000 poise) makes melt-extrusion difficult and causes problems such as embrittlement and difficult stretching of the film. Consequently, a polyimide having the melt viscosity of the above range is frequently used. By using the polyimide mentioned above, an unstretched film is prepared by melt-extrusion, calendering and other processes. The preparation process is most preferably carried out by melting the polyimide in an extruder at a temperature of 300° to 450 °C, delivering through a slit nozzle, and cooling on a casting roller having a surface temperature of 100° to 250 °C to obtain an unstretched film (amorphous film). The characteristic feature of the process of the present invention is that the above obtained unstretched film is stretched and heat-set. The unstretched film used for the stretching and heat setting desirably has a density of 1.350 g/cm³ or less. A density exceeding 1.350 g/cm³ causes embrittlement of the film and film breakage unfavorably occurs in the successive step for stretching. The stretching and heat setting of the unstretched film is preferably conducted by stretching the film 1.5 to 10 times in one direction or two directions which are at right angles to each other in the temperature range of 230° to 320 °C, and successively by heat setting the film under tension in the temperature range of from 250 °C to less than the melting point of the film. As previously mentioned, the stretching and heat setting process can be carried out in the following embodiments according to the specific conditions of stretching treatment: (1) A method for stretching the film 1.5 to 5.0 times in one direction in the temperature range of 230° to 300 °C and successively conducting heat setting under tension in the temperature range of from 250°C to less than the melting point of the film (hereinafter referred to as uniaxial stretching method). (2) A method for stretching the film 1.5 to 3.0 times in one direction in the temperature range of 240° to 300 °C, successively stretching 1.5 to 3.0 times in the direction which is at right angles to said first stretching direction in the temperature range of 250° to 320 °C, and then conducting heat setting under tension in the temperature range of from 250 °C to less than the melting point of the film (hereinafter referred to as successive-biaxial stretching method). (3) A method for stretching the film to obtain an area magnification of from 2 to 10 times by stretching simultaneously in two directions which are at right angles to each other in the temperature range of 250° to 300 °C, and successively carrying out heat setting under tension in the temperature range of from 250 °C to less than the melting point of the film (hereinafter referred to as simultaneous-biaxial stretching method). (4) A method for stretching the film in the temperature range of 150 °C to less than the glass transition temperature of the film by maintaining the moisture content of the film from 0.1 to less than 3 % during the stretching step. Each embodiment will hereinafter be further described in detail. (1) Uniaxial stretching method: This embodiment is a method for preparing a uniaxially stretched film by stretching the above unstretched film in one direction in a specific temperature range, and successively heat setting the stretched film. Practical conditions of the uniaxial stretching method are stretching of 1.5 to 5.0 times in one direction in the temperature range of 230° to 300 °C preferably 240° to 290 °C. The unstretched film is uniaxially oriented under these conditions. A stretching temperature lower than 230 °C leads to difficulty in stretching. On the other hand, a temperature exceeding 300 °C is also unfavorable because the film loses transparency, embrittles and cannot be stretched. A stretching magnification less than 1.5 unfavorably leads to development of creases in the successive heat-treatment step or to embrittlement of the film. On the other hand, a stretching magnification exceeding 5.0 results in high orientation in the course of the stretching step and causes unfovarable problems such as film breakage. The film can be stretched using conventional techniques, eg using a pair or more of rollers or a tenter, or by rolling out. The rate of stretching is preferably in the range of 1 to 100,000 %/min. Preheating of the film before stretching to prevent crystallization is preferable for carrying out smooth stretching. The uniaxially stretched film thus obtained is successively heat-set for 1 to 5,000 seconds under tension or shrinkage restriction in the temperature range of from 250 °C to less than the melting point of the film, preferably from 270° to 370 °C. (2) Successive-biaxial stretching method: This embodiment is a method for preparing a biaxially stretched film by stretching the unstretched polyimide film in one direction in a specific temperature range, successively stretching the resultant film in the direction which is at right angles to said direction in a specific temperature range, and then heat-setting the stretched film. Practical conditions of the successive-biaxial stretching method are the first step stretching of 1.5 to 3.0 times in one direction in the temperature range of 240° to 300 °C, preferably 250° to 290 °C, and the second step stretching of 1.5 to 3.0 times in the direction which is at right angles to the direction of the first step stretching in the temperature range of 250° to 320 °C, preferably 260° to 310 °C. The unstretched film is biaxially oriented under these conditions. As for the uniaxial stretching method, the film can be stretched by using conventional techniques, eg using a pair or more of rollers or a tenter, or by rolling out. A first step stretching temperature lower than 240 °C leads to difficulty in stretching. On the other hand, a temperature exceeding 300 °C is also unfavorable because the film loses transparency, embrittles and cannot be stretched. A stretching magnification less than 1.5 unfavorably leads to development of creases in the successive heat-treatment step or to embrittlement of the film. On the other hand, a stretching magnification exceeding 3.0 results in high orientation in the course of the stretching step and causes unfavorable problems such as film breakage in the second stretching step. A second step stretching temperature lower than 250 °C leads to difficulty in stretching and frequent occurrence of film breakage. On the other hand, a temperature exceeding 320 °C results in troubles such as embrittlement and breakage of the film. The preferred stretching magnification is in the range of 1.5 to 3.0 in view of orientation effect and film breakage. The rate of stretching is preferably in the range of 1 to 100,000 %/min. Preheating of the film before stretching to prevent crystallization is preferable for carrying out smooth stretching. The biaxially stretched film thus obtained is successively heat-set for 1 to 5,000 seconds under tension or shrinkage restriction in the temperature range of from 250 °C to less than melting point of the film, preferably from 270 to 370 °C. (3) Simultaneous-biaxial stretching method: This embodiment is a method for preparing a biaxially stretched polyimide film by stretching the above unstretched film to advise an area magnification of from 2 to 10 by simultaneously stretching in two directions which are at right angles to each other in a specific temperature range, and successively heat-setting the stretched film in the temperature range of 250 °C to less than the melting point of the film. Practical conditions of the simultaneous-biaxial stretching are to stretch the unstretched film in the area magnification range of 2 to 10 and in the temperature range of 250° to 300 °C, preferably 260 to 290 °C. The unstretched film is biaxially oriented under these conditions. A stretching temperature lower than 250 °C causes high stretching stress and makes stretching impossible. On the other hand, a stretching temperature exceeding 300 °C is unfavorable for the same reasons as explained above in connection with the first step of the successive-biaxial stretching method. A stretching magnification less than 2 by area ratio is unfavorable because creases or embrittlement of the film develop in the successive heat-treatment step. Stretching cannot be carried out to give a stretching magnification exceeding 10. The rate of stretching is preferably in the range of 1 to 100,000 %/min. Preheating of the film before stretching to prevent crystallization is preferred for carrying out stretching smoothly. The biaxially stretched film thus obtained must be successively heat-set for 1 to 5,000 seconds under tension or shrinkage restriction in the temperature range of 250 °C to less than the melting point of the film, preferably from 270° to 370 °C. (4) Streching method under specific moisture content of the unstretched film: This embodiment is a method for preparing a polyimide film having excellent properties, in the process of forming, stretching and heat-setting the polyimide film, by adjusting the moisture content of the film in a specific range and stretching the film at a temperature lower than the glass transition temperature, which method has conventionally been considered very difficult. That is, an unstretched polyimide film having a moisture content of at least 0.1 % is stretched in the temperature range of 150 °C to less than glass transition temperature of the film. The polyimide film for use in the process of the present invention has a moisture content of 0.1 % or more. A moisture content less than 0.1 % increases stiffness of the film and makes stretching operation difficult due to increase in tension. Other unfavorable problems also occur, for example, loss of clarity and remarkable decrease in mechanical strengths due to development of micro voids. No particular limitation is imposed upon the maximum moisture content. The maximum moisture content is usually less than 3 %, preferably from 0.2 to 2 % in particular. Polyimide for use in the present invention absorbs 0.5 % or more of moisture in the usual atmosphere and can absorb about 3 % of water when immersed in warm water. In the process of the present invention, the stretching temperature is preferably in the range of from 150 °C to the glass transition temperature and the stretching magnification is preferably from 1.2 to 2.5. Stretching at a temperature exceeding the glass transition temperature leads to reduction of moisture content and development of extreme orientation in the stretching step, whereby rendering the stretching operation difficult. In the stretching process of the present invention, the above film can be uniaxially or biaxially stretched with a roller stretching system using at least a pair of rollers, or with a tenter system. The rate of stretching is preferably in the range of 1 to 100,000 %/min. In order to increase density and to improve dimensional change characteristic and mechanical properties, the stretched film thus obtained is heat-set for 1 to 5,000 seconds under tension or shrinkage restriction in the temperature range of from 250 °C to less than the melting point of the film, preferably from 270 to 370 °C. The heat treatment enhances dimensional stability and other properties of the film. The film is substantially transparent unless processed into rough surface in particular. The polyimide film thus obtained is useful for electric insulation films for high temperature, substrates for recording medium and dielectric films. The thickness of the film of the invention varies depending upon application. No particular restriction is put upon the thickness. The film thickness is usually from 0.1 µm to less than 5 mm. The present invention is illustrated by the Examples which follow. The properties of the polyimides in the Examples were measured by the following methods. (1) Refractive index (thickness direction) Abbe's refractometer 4-T™ (a product of ATAGO Co., Ltd.) was used. Measurement was conducted by deflection method at 23 °C by using sulfur/methylene iodide intermediate liquid. (2) Density Density gradient tube was used. Measurement was conducted at 23 °C in toluene-carbon tetrachloride. (3) Glass transition temperature Du Pont 1090™ thermal analyser (a product of E.I. Du Pont De Nemours & Co.) was used. Measurement was conducted according to DSC method at a temperature rise rate of 4 °C/min. (4) Tensile test Tensile test was carried out respectively in the x and y directions of the sample in accordance with ASTM D-882. Tensile strength at break and initial elastic modulus were measured. (5) Heat shrinkage Shrinkage after heating at 280 °C for 2 hours was measured in accordance with JIS C-2318. (6) Solder heat resistance Appearance of the specimen was observed after floating on a solder bath at 260 °C for 30 seconds. ○ ... ... ... No deformation was found. X ... ... ... Deformation occurred. Preparation of an unstretched film, comparative film To a reaction vessel equipped with a stirrer, reflux condenser, and a nitrogen inlet tube, 368.4 g (1 mole) of 4,4'-bis(3-aminophenoxy)biphenyl and 2,500 g of N,N-dimethylacetamide were charged. In the nitrogen atmosphere, 213.7 g(0.98 mole) of pyromellitic dianhydride was added by portions with caution to prevent temperature rise of the solution and 5.92 g (0.04 mole) of phthalic anhydride was further added. The mixture obtained was stirred for about 20 hours at room temperature and successively 30.3 g (0.3 mole) of triethylamine and 30.6 g (0.3 mole) of acetic anhydride were added over about 30 minutes. After stirring the reaction mixture for 30 minutes, 2,500 g of methanol was added and precipitated polyimide was filtered at 30 °C. Filtered polyimide was washed with methanol and acetone, and dried at 300 °C for 8 hours in the nitrogen atmosphere. Polyimide powder thus obtained was 528 g. Yield was 96 %. Melt viscosity of the polyimide powder was measured with a KOKA model flow tester CFT-500™(a product of SHIMADZU Seisakusho Co. Ltd.). The apparent melt viscosity was 640 Pa.s (6,400 poise) at 400 °C under apparent shear rate of 200/seconds. Polyimide powder thus obtained was dried at 180 °C for 24 hours, charged in a vented extruder of 25 mm in diameter, melted at 410 °C, extruded through a nozzle of 2 mm in diameter, and allowed to cool to obtain a strand of about 1.8 mm in diameter. The strand was cut into a length of about 3 mm to obtain pellets. The pellets had a melt viscosity of 650 Pa.s (6,500 poise). The pallets were dried at 180 °C for 24 hours, fed to a 25 mm extruder, heat-melted at 410 °C, delivered from a slit die having a width of 150 mm and an opening of 1.0 mm and taken up on a roller at 220 °C to obtain a film of about 0.5 mm in thickness. The film thus obtained had a density of 1.327 g/cm³. The X-ray diffraction diagram of the film had no peak assigned to crystal and indicated that the film was amorphous. Examples 1 to 4 An unstretched film of 10 cm square in size was fitted on a stretcher X4HDHT™ (a product of TOYO Seiki Manufacturing Co., Ltd.). The film was stretched, under the conditions summarized in Table 1, only in the X direction which was defined as the film flow direction in the melt-extrusion process. The stretched film was held as intact, heated to 300 °C and heat-treated for 30 minutes. In all examples, the stretching and heat-treatment steps were carried out in good conditions and breakage or non-uniform stretching was not observed. The properties of the film obtained were excellent in heat resistance, dimensional stability and mechanical strengths, as shown in Table 1. Comparative Example 1 The unstretched film had a density of 1.327 g/cm³, and a refractive index of 1.690 in the thickness direction. Mechanical properties of the unstretched film was unsatisfactory. Dimensional stability and solder heat resistance were also poor. Results are given in Table 1. Comparative Examples 2 to 4 The unstretched film was stretched under conditions illustrated in Table 1. When stretching was carried out at 220 °C, the stretching magnification became non-uniform, whitening occurred in part, and mechanical strengths were poor. When stretching was conducted at 310 °C as in Comparative Example 3, crystallization occurred in the preheating step and caused breakage in the stretching step. When stretching magnification was 5.5 as in Comparative Example 4, the physical properties of the stretched film were good. The film, however, had extremely high tearability and was liable to cause breakage in the stretching step. Results are illustrated in Table 1. Examples 5 to 12 and Comparative Examples 5 to 12 The unstretched film was biaxially stretched and heat-treated under the conditions given in Table 1. Under the conditions of these examples, all films could be uniformly stretched without breakage, and all stretched films obtained were good in heat resistance, mechanical properties and dimensional stability. However, under the conditions of the comparative examples, the stretching state was irregular or the dimensional stability and physical properties of the stretched film were poor. Results are given in Table 1. Examples 13 to 17 and Comparative Examples 13 to 15 The unstretched film was humidified under the conditions shown in Table 2. The moisture content of the humidified film was measured with a HIRANUMA-Model Moisture Tester. The humidified film was previously subjected to the preheating conditions given in Table 2, thereafter removed quickly and its moisture content immediately measured. The value obtained was considered as the moisture content during stretching step. The humidified film was stretched under the preheating and stretching conditions illustrated in Table 2. The stretched sample was held as such and heated to 300 °C, and heat treatment was carried out at 300 °C for 30 minutes. Under the conditions of the examples, all films could be stretched uniformly and all stretched films obtained were satisfactory in physical properties. However, under the moisture content or the stretching conditions of the comparative examples, the stretched films exhibited loss of transparency, breakage and insufficient mechanical strengths. Results are given in Table 2. Example or Comparative Example Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 17 Compa. Ex. 13 Compa. Ex. 14 Compa. Ex. 15 Humidificaton Condition not humidify in water 40°C×24 hrs. not humidify Moisture Content (%) 0.72 1.8 0.72 Preheating Temperature (%) 150 240 200 200 240 240 140 260 Time (sec) 300 30 120 60 30 120 300 20 Moisture content (%) 0.31 0.20 0.25 0.65 0.43 0.08 0.32 0.12 Stretching uniaxial biaxial uniaxial Temperature (°C) 150 240 200 200 240 200 140 260 Rate (%/min) 100 200 100 200 200 100 100 200 Magnification 1.5 2.0 2.0 2.5 2.0 x2.0 1.5 1.5 1.5 Film appearance after stretching somewhat ununiform good good good good loss of clarify, ununiform breakage loss of clarity, ununiform Tensile strength (kg/mm²) 18.5 24.8 25.5 30.5 24.1 2.1 - 1.1 Refractive index (thickness direction) 1.635 1.630 1.628 1.621 1.618 - - - Density (g/cm³) 1.352 1.361 1.359 1.370 1.369 - - -

Claims: A process for the preparation of a stretched polyimide film from an unstretched polyimide film comprising polyimide having recurring structural units of the formula I: which process comprises the steps of heat-stretching the unstretched film and successively heat-setting the stretched film under tension or shrinkage restriction so as to obtain a stretched polyimide film having a density of from 1.335 to 1.390 g/cm³ at 23°C and a refractive index of from 1.605 to 1.680 at 23°C in the direction of thickness. A process as claimed in Claim 1 in which the unstretched film is stretched from 1.5 to 10 times in one direction or in two directions which are at right angles to each other, and the stretched film is successively heat-set under tension at a temperature of from 250°C to less than the melting point of said film. A process as claimed in Claim 2 in which the unstretched film is stretched from 1.5 to 5.0 times in one direction in the temperature range of from 230 to 300°C. A process as claimed in Claim 2 in which the unstretched film is stretched from 1.5 to 3.0 times in one direction in the temperature range of from 240 to 300°C and successively stretched from 1.5 to 3.0 times in the direction which is at right angles to said first stretching direction in the temperature range of from 250 to 320°C. A process as claimed in Claim 2 in which the unstretched film is stretched so that its area is magnified from 2 to 10 times by stretching simultaneously in two directions which are at right angles to each other at a temperature of from 250 to 300°C. A process as claimed in Claim 1 in which the unstretched film is stretched at a temperature of from 150°C to less than the glass transition temperature of the film while maintaining the moisture content of the film in the range of from 0.1 to 3% during the stretching step.

Inventors: FUJII YASUHIRO MITSUI TOATSU T, HONJI YASUKO HIGASHI DAIDO, MORIYA SHINOBU, OHTA YASUHIKO MITSUI TOATSU TA, SARUWATARI MASUMI MITSUI APART, TSUJI SHOICHI, FUJII, YASUHIRO MITSUI TOATSU TAKIHARU-RYO, HONJI, YASUKO, HIGASHI DAIDO 84, MORIYA, SHINOBU, OHTA, YASUHIKO MITSUI TOATSU TAKIHARU-RYO, SARUWATARI, MASUMI MITSUI APARTMENT J-508, TSUJI, SHOICHI
Applicants: MITSUI TOATSU CHEMICALS, MITSUI TOATSU CHEMICALS, INC.
Application year: 1990
Main classification: C08J5
IPC codes: H05K1, H05K3, B29K79, C08J5, C08G73, B29C55, B29C67, B29L7