Product and process of producing biaxially oriented insulating film of polyethylene-2,6-naphthalate
United States Patent [191 Aoki et al.  
 PRODUCT AND PROCESS OF PRODUCING BIAXIALLY ORIENTED INSULATING FILM OF POLYETIIYLENE-2,6-NAPHTI&#39;IALATE Inventors: Hiroshi Aoki, 2-20, Nakamachi,  
 Koganei-shi, Tokyo; Sakae Shimotsuma, 23-12, 6-chome, Higashi-rinkan, Sagamihara-shi, Kanagawa-ken; Masanori Masuda, 1200-6, Kamitsuruma, Sagamihara-shi, Kanagawa-ken; Takeo Asai, 16-25, 6-chome. Seishin, Sagamihara-shi, Kanagawa-ken; Hujio Matsiimoto, 7- l2, S-chome, Seishin, Sagamihara-shi, Kanagawa-ken, all of Japan Filed: Feb. 15, 1974 Appl. No.: 443,033  
  Related U.S. Application Data &#39;Continuation-in-part of Scr. No. 2l0,54l, Dec. 21,  
 197 l abandoned.  
 Foreign Application Priority Data Field of Search 264/289, 29l, 210 R, 235, 264/346, 342 RE; 260/75 T, 47 C [56] References Cited UNITED STATES PATENTS 2,968,065 l/l96l Gronholz 264/289 2,975,484 3/196] Amborski..... 260/75 T 3,l6l.7ll 12/1964 Tassler 264/289 3,50l,344 3/1970 Watson et al. ll7/l38.8 F 3,683,060 8/1972 Tanabe et al. 264/289 Primary Examiner-Donald J. Arnold Assistant ExaminerJames B. Lowe Attorney, Agent, or FirmSherman &amp; Shalloway 5 7 ABSTRACT A biaxially oriented insulating film of polyethylene- 2,6-naphthalate containing up to 10 mol percent of a comonomer which can withstand continuous use at high temperatures and has excellent insulating properties and other physical properties. The film has an.  
 elongation at breakage of not less than 10 after heating at 200C. for 200 hours, a thickness-direction refractive index [11,] of the film within a range of 1.499 [n 1.520 and an intrinsic viscosity [1 of 0.48 [1;] 0.70, with the proviso that [n,] and [n] satisfy the condition that [12,] 0.3 X [171+ l.36l. The film is produced by biaxially orienting the film at stretch ratios of 3-4 in each direction, wherein the stretch ratio in the transverse direction is at least equal to that in the lengthwise direction. The film is then post-heat-treated at 2l5240C under such conditions as will not allow a shrinkage greater than 10 percent.  
 5 Claims, No Drawings PRODUCT AND PROCESS OF PRODUCING BIAXIALLY ORIENTED INSULATING FILM OF POLYETHYLENE-2,6-NAPHTHALATE This application is a continuation-in-part application of HIROSHI AOKI, et al., Ser. No. 210,541, filed Dec. 21, 1971, now abandoned.  
 BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION This invention relates to an insulating film composition which can withstand continuous use under high temperature conditions and has excellent electrical insulating characteristics in combination with satisfactory physical properties.  
 2. DISCUSSION OF THE PRIOR ART It is known that polyethylene-2,6-naphthalate is an orientable, crystalline polymer capable of being shaped into filament or film and that its second transition point is about 1 C. which is higher by about 50C. than the second transition point of polyethylene terephthalate and its melting point is about 270C. which is higher by about 10C. than the melting point of polyethylene terephthalate.  
  More than years have passed since this polymer was first discovered (see the specification of British Pat. No. 604,073), but hardly any proposals have been made as to the preparation of oriented films from this polymer. As far as applicants are aware, until very recently the prior art only taught the preparation of film from a polyethylene terephthalate containing 2,6- naphthalene dicarboxylate in an amount of less than of the recurring structural units of the polymer (see the specification of US. Pat No. 3,161,710). However,  
 v film prepared from such copolyester fails to provide an insulating film possessing in combination excellent electrical-insulating characteristics and sufficient physical properties to stand continuous use under high temperature conditions, such as intended in this invention.  
  More recently, however, US. Pat. No. 3,683,060 dis closed a method for producing biaxially drawn polyethylene-2,6-naphthalate films having superior physical properties in the machine direction such as tensile strength and Youngs modulus, particularly useful as a base of magnetic recording tape, prepared by stretching an undrawn film of polyethylene 2,6- naphthalate, consisting essentially of ethylene 2,6- naphthalate units (which may be modified by up to 10 mol of a third component or third components) and having an intrinsic viscosity of at least 0.35, first in the machine direction to 3.5-5 times the original length and then in the transverse direction at a draw ratio 50-90 of that of the machine direction stretching. However, this reference only discloses heat treatment subsequent to the biaxial drawing at a temperature of 210C. or less, with the lengthwise draw ratio and widthwise draw ratio less than 4 times their original respective lengths. Moreover, this reference suggests that contrary to the discovery made by the present inventors, unsatisfactory results are obtained when the respective lengthwise draw ratio and widthwise draw ratio are less than 4 times their original lengths. As will be seen below in connection with the examples and comparative examples of the present specification, when the post-heat-treatment temperature falls below 215C. or above 240C. or when the lengthwise draw ratio and/or widthwise draw ratio are above about 4,  
 polyethylene-2,6-naphthalate films having the desired physical properties for the purposes of the present invention cannot be obtained. In particular, it will be seen that such films have a lower elongation at breakage after strenuous heat treatment and do not pass the heat resistance test as set forth below.  
  The specification of US. Pat. No. 3,501,344 proposes a magnetic recording tape supported on polyethylene-2,6-naphthalate. According to this proposal, a film of polyethylene-2,6-naphthalate biaxially oriented by the biaxial stretching is subjected to a heatsetting or crystallizing treatment under restraint against shrinkage at a temperature usually above 120C. and up to about 250C. or more in some cases, preferably 200240C. The heat-set film can be heat-relaxed within the same temperature range without restraint against shrinkage. It is proposed that the biaxial stretching of unstretched film is performed by stretching the film at 150C. (preferably C.) in the lengthwise direction from about 3.5 to about 6.0 (preferably from 4.0 to 5.5) times its original length and then stretching the film advantageously at 105 C.) in the transverse or widthwise direction anywhere from about 0.9 to about 1.4 (preferably about 1.0 to about 1.3) times its original width, i.e., from about zero stretch up to about a 40% widthwise stretch. This biaxially oriented and post-heat-treated film is then coated with a nonmagnetic binder in which are dispersed magnetizable particles of Fe O or similar magnetically susceptible material.  
 SUMMARY OF THE INVENTION We have conducted research with a view to providing an insulating film composition formed from polyethylene-2,6-naphthalate which exhibits not only electrical -insulating characteristics but also satisfactory physical properties when used continuously for a long time under high temperature conditions, and we have now found that in the case of polyethylene-2,6- naphthalate, the stretch history during the biaxial stretching, the heat history during the post heat treatment and the post heat treatment conditions very greatly influence the physical properties of the resulting biaxially oriented and post-heat-treated film, especially properties desired in insulating films, and that known techniques of preparing biaxially oriented films which have been applied to other polymers such as polyethylene terephthalate cannot be directly applied to polyethylene-2,6-naphthalate.  
  For instance, it has now been found that the thick ness-direction refractive index [n,] of the biaxially oriented and post-heat-treated film is considerably changed by the heat history, the stretch history and the post heat treatment conditions, with the result that the elongation at breakage of the film measured when it is subjected to further heat treatment at 200C. for 200 hours under such conditions as will allow free shrinkage of the film, is greatly influenced. Thus, we have now found that only when the above-mentioned factors satisfy certain requirements, can there be obtained a biaxially oriented and post-heat-treated polyethylene- 2,6-naphthalate insulating film which is very useful because of a combination of excellent electricalinsulating characteristics and physical properties sufficient to withstand continuous use for a long time under high temperature conditions.  
  Accordingly, a primary object of this invention is to provide a biaxially oriented and post-heat-treated polyethylene-2,6-naphthalate film which possesses in combination physical properties sufficient to withstand continuous use under high temperature conditions and excellent electrical-insulating characteristics.  
  An additional primary object of this invention is to provide a process for preparing such a film.  
  Other objects and advantages of this invention will be apparent from the description given hereinbelow.  
 DETAILED DESCRIPTION OF THE INVENTION The film possessing the desired properties is obtained according to the present invention with a biaxially oriented and post-heat-treated polyethylene-2,6- naphthalate insulating film composition consisting essentially of polyethylene-2,6-naphthalate in which at least 90 mol percent of the structural units are ethylene-2,6-naphthalate units, and at most mol per cent of the structural units are derived from a comonomer having a divalent ester-forming functional group, the film being produced by biaxially stretching an unstretched film under heating conditions and post-heattreating the film to a temperature of from 215C. to 240C. under such conditions as will not allow free shrinkage of the film; characterized in that (i) the stretch ratio in the widthwise direction and in the lengthwise direction are both within a range of from about 3 to about 4 times based on the original width or length respectively, of the unstretched film, with the proviso that the stretch ratio in the widthwise direction is at least 1 time, preferably 1 4/3 times, more preferably l 1.2 times, and especially preferably 1 1.1 times, the stretch ratio in the lengthwise direction; (ii) when the biaxially-oriented and postheat-treated film is subjected to further heat treatment at 200C. for 200 hours under such conditions as will allow free shrinkage of the film, the elongation at breakage of the resulting film is not less than especially preferably not less than and (iii) the thickness-direction refractive index [n,] of the biaxially-oriented and post-heattreated film, measured at 20C. by Abbe s refractometer with use of a ray of a wavelength of 589 mu, is within a range expressed by the following formula:  
 preferably and the intrinsic viscosity [1 of the film, measured at 35C. in a mixed solvent of phenol:2,4,6- trichlorophenol of a weight ratio of 6:4, is within a range expressed by the following formula:  
 with a proviso that above [n and [n] satisfy the following condition:  
  According to the process of the present invention biaxially oriented and post-heat-treated polyethylene- 2,6-naphthalate insulating film composition is prepared by biaxially stretching an unstretched polyethylene- 2,6-naphthalate film composition consisting essentially of polyethylene-2,6-naphthalate in which at least 90 mol percent of the structural units are ethylene-2,6- naphthalate units, and at most 10 mol percent of the structural units are units derived from a comonomer or comonomers having a divalent, esterforming functional group or groups, at a stretch ratio in the widthwise direction within a range of from about 3 to about 4 times based on the original width of the unstretched film, and the stretch ratio&#39;in the lengthwise direction is within a range of from about 3 to about 4 times based on the original length of the unstretched film, with a proviso that the stretch ratio in the widthwise direction is l 4/3 times the stretch ratio in the lengthwise direction, the stretching in the widthwise direction being conducted at C. and the stretching in the lengthwise direction being conducted at 120 C., and then subjecting the biaxially stretched film to the post-heat-treatment at 215 240C. under such conditions as will not allow a shrinkage greater than 10 and preferably 0 i.e. constant film length, to thereby obtain a biaxially oriented and post-heat-treated film wherein the elongation at breakage of the resulting film is not less than 15 when the film is subjected to further heat treatment at 200C. for 200 hours under such conditions as will allow free shrinkage of the film; and the thickness-direction refractive index [n of the biaxially oriented and post-heat-treated film, measured at 20C. by Abbes refractometer, with use of a ray of a wavelength of 589 mm, is within a range expressed by the following formula:  
 with a proviso that about [n and [17] satisfy the following condition:  
  As the polyethylene-2,-naphthalate to be used as the base polymer in this invention, there may be employed any polymer in which at least 90 mol percent of the structural units are ethylene-2,6-naphthalate units. Accordingly, by the term polyethylene-2,6- naphthalate used herein is meant not only a homopolymer of ethylene-2,6-naphthalate but also a polyethylene-2,6-naphthalate modified with no more than 10 mol percent, preferably less than 5 mol percent, of a comonomer or comonomers having a divalent, esterforming functional group or groups or a modifier. Since a film of polyethylene-2,-naphthalate modified with more than 10 mol percent of such comonomer or comonomers or modifier is likely to be degraded during the heat treatment, such modified polymer cannot be used in this invention.  
  In general, polyethylene-2,6-naphthalate is prepared by reacting naphthalene-2,6-dicarboxylic acid or its functional derivative with ethylene glycol or its functional derivative in the presence of a catalyst or catalysts. Reaction conditions and operation procedures are well known in the art and are disclosed in, for instance, the above-mentioned specifications. In case a comonomer or comonomers or modifier is used in an amount of less than 10 mol percent, prior to completion of the reaction of forming polyethylene-2,6- naphthalate, one or more suitable comonomers or modifiers are added to the polymerization system and then the reaction is completed to form a copolyester or mixed polyester.  
  As such comonomer or modifier, there may be employed compounds having a divalent, ester-forming, functional group. Examples of such compounds are dicarboxylic acids such as oxalic acid, adipic acid, phthalic acid, isophthalic acid, terephthalic acid, naphthalene-2,7-dicarboxylic acid, succinic acid, diphenyl ether dicarboxylic acid and lower alkyl esters of these dicarboxylic acids; hydroxycarboxylic acids such as p-hydroxybenzoic acid and p-hydroxyethoxybenzoic acid, and lower alkyl esters of these hydroxycarboxylic acids; and dihydric alcohols such as neopentyl glycol diethyleneglycol and trimethylene glycol. Polyethylene-2,6-naphthalate or its modified polymer may have the terminal hydroxyl and/or carboxyl groups capped with a monofunctional compound such as benzoic acid, benzoyl benzoic acid, benzyloxybenzoic acid and methoxypolyalkylene glycol. It is also possible to use polyethylene-2,6-naphthalate modified with a very small amount of a polyfunctional compound such as glycerin and pentaerythritol to such an extent that the linearity of the polymer is not substantially lost.  
  It is possible to incorporate in such starting polyethylene-2,6-naphthalate various additives known in the film manufacture field. Examples of such additives are delustering agents such as titanium dioxide, stabilizers such as phosphoric acid, phosphorous acid and esters of these acids, and slip agents such as finely divided silica and china clay.  
  It is preferred that the polyethylene-2,6-naphthalate to be used in this invention has an intrinsic viscosity [&#39;17] in the range of from 0.50 to 0.80, because polymers having such intrinsic viscosity [1 exhibit good processability when shaped into films.  
  The intrinsic viscosity [1;] referred to in this invention is one determined by the measurement conducted with respect to the polymer in a mixed solvent consisting of phenol and 2,4,6-trichlorophenol (phenol: 2,4,6- trichlorophenol weight ratio 6:4) at 35C., the unit being 100 cc/g.  
  The thickness-direction refractive index [n referred to in this invention is the value determined by the measurement conducted at 20C. by means of Abbes refractometer with use of a ray having a wavelength of 589 mp.(the center of D-line).  
  An unstretched film suitable for use in the formation of the biaxially oriented and post-heat-treated film may be prepared according to customary methods adopted for film formation in the art. Usually, the starting polyethylene-2,6-naphthalate is dried to remove water therefrom, melted by an extruder or the like, extruded, ridded of trash or dirt, and then shaped into a film. During such shaping process the degree of polymerization of the polymer is reduced, and the intrinsic viscosity [1 of the unstretched film greatly varies depending on conditions of drying and extruding the starting polymer and other factors. Still further, the thickness-direction refractive index [11,] of the biaxially oriented and postheat-treated film varies depending on the abovementioned change in the intrinsic viscosity [1 of the starting unstretched film and on the stretching and post-heat-treatment conditions. As a result of our researches on the relation between the changes of the intrinsic viscosity [n] and the thickness-direction refractive index [n,], it has been found that in order to provide the resulting biaxially oriented and post-heat treated polyethylene-2,6-naphthalate film with a combination of physical properties sufficient to withstand continuous use under high temperature conditions and excellent electrical-insulating characteristics, it is necessary that a specific relationship be established between the intrinsic viscosity [n] and thicknessdirection refractive index [n;] in the resulting film.  
  More specifically, we have now found that in the intended biaxially oriented and post-heat treated insulating film derived from polyethylene-2,6-naphthalate, the thickness-direction refractive index [n,] should be within a range expressed by the following formula:  
 1.499 [n 1.520 (preferably 1.500 [n 5 1.515), the intrinsic viscosity [1 should be within a range expressed by the following formula:  
 and the above [n,,] and [1;] should satisfy the following condition:  
  It has also been found that in addition to the above requirementsfor the intrinsic viscosity [n,] and [1 an additional requirement must be satisfied with respect to the stretch ratio in order for the resulting film to have suitable physical properties sufficient to withstand continuous use under high temperature conditions. More specifically, it has now been found that in the biaxially stretched film of the present invention, it is essential that the stretch ratio in either the widthwise direction or lengthwise direction should be within a range of from about 3 to about 4 and the stretch ratio in the widthwise direction should be at least 1 time (i.e. equal), preferable l 4/3 times, more preferably 1 1.2 times, especially preferably 1 1.1 times the stretch ratio in the lengthwise direction. In case the stretch ratio in the widthwise direction is less than 1 time the stretch ratio in the lengthwise direction, when the resulting film is subjected to high temperature conditions for a long period of time, there is a tendency that the elongation at breakage in the widthwise direction is disadvantageously lowered. In case the stretch ratio in the widthwise direction is above the previous range, for instance, the stretch ratio in the widthwise direction is more than 4/3 times the stretch ratio in the lengthwise direction, the elongation at breakage in the lengthwise is likewise disadvantageously reduced. In order to provide the resulting film with a combination of physical properties sufficient to withstand continuous use under high temperature conditions and excellent electricalinsulating characteristics, it is important that the stretch ratio in either the lengthwise or the widthwise direction should be within the above range and the above proportional relation should be established between the stretch ratios in both the lengthwise and widthwise directions.  
  Biaxially stretched and post-heat-treated polyethylene-2,6-naphthalate insulating films meeting the above requirements (i) and (iii) usually have an elongation at breakage of not less than 10 after they have been subjected to further heat treatment at 200C. for 200 hours under such conditions as will allow free shrinkage. In order to obtain the desired properties according to the present invention, biaxially stretched and post-heat-treated films having an elongation at breakage of not less than 15 preferably not less than 20 after the above-mentioned additional heat treatment are suitable.  
  In the present invention the elongation at breakage of the resulting post-heat-treated film is much more balanced both in the lengthwise direction and the widthwise direction.  
  In order to obtain the biaxially oriented and postheat-treated polyethylene-2,6-naphthalate insulating film composition of this invention, such conditions as the intrinsic viscosity [n] of the unstretched film, the unstretched film-preparing conditions, the stretch ratios in the biaxial stretching, the temperature conditions in the biaxial stretching, the post-heat treatment temperature conditions and the degree of restraint of free shrinkage in the post-heat treatment may be readily chosen and combined experimentally with above requirements (i), (ii) and (iii) as criteria to be satisfied although these conditions cannot be determined independently in an absolute manner. For better illustration, selection of such conditions and preparation of the film composition of this invention will now by described with reference to the following detailed description.  
  Chips of polyethylene-2,6-naphthalate having an intrinsic viscosity [1;] of, for example, 0.50 0.80 are dried for several hours by hot air maintained at 170 200C., and melted, metered and extruded by an extruder maintained at 285 310C. The resulting extrudate is then shaped into an unstretched film by a customary film-shaper. The unstretched film is stretched in the lengthwise direction about 3 to about 4 times the original length at about 120 to about 160C. and stretched in the widthwise direction from about 3 to about 4 times the original width at about 115 to about 150C., preferably about 125 to about 150C. in such a manner that the stretch ratio in the widthwise direction will be at least equal to the stretch ratio in the lengthwise direction. Then, the biaxially stretched film is subjected to heat treatment at about 215 to about 240C. under such conditions as will not allow free shrinkage of the film. In general, this heat treatment is conducted under such restraint conditions as will keep the film length constant or as will not allow a shrinkage greater than The period of time for the above post-heat-treatment is usually less than about 10 minutes, and generally can be selected within the range of from about 3 200 seconds. In the process of the present invention, the postheat-treatment residence time is not critical, and may vary over a wide range depending on the draw ratio and temperature of heat treatment. The optimum residence time, depending on the above parameters, can be easily determined by a person of ordinaryskill in the art; and, in general, when the post-heat-treatment temperature is within the range of 215 240C., residence time of less than about 10 minutes will produce satisfactory results.  
  The biaxially oriented and post-heated-treated polyethylene-2,6-naphthalate insulating film composition of the present invention exhibits excellent electrical characteristics, for instance, high dielectric breakdown strength, high surface resistivity and high volume resistivity at such high temperatures as 155C. or higher, and almost constant dielectric characteristics at temperatures ranging from 60C. to +1 80C. Further, the films of this invention are excellent in mechanical properties such as tensile strength, Youngs modulus,  
 end-breaking resistance, tear resistance and impact strength, and have a low thermal shrinkage without any restraint against shrinkage and an excellent heat resistance sufficient to withstand continuous use at C. (heat resistance grade F for insulating materials according to the test recommended by the International Organization for Standardization ISO). Still further, they are sufficiently resistant against various insulating oils, Freon, refrigerator oils, organic solvents and plasticizers, and as compared with polyethylene terephthalate films, the films of this invention are not readily hydrolyzed. Accordingly, biaxially oriented and postheat-treated film compositions of this invention are very excellent as insulating materials.  
  The films of this invention may be directly used as insulating materials for various electric and electronic machines and Instruments, i.e. for layer/barrier insulation and phase insulation for instance, as tapings for electric and electronic machines and Instruments, materials for slot liner, coil separator, and as dielectric materials for condensers. Further, they may be laminated with other insulating materials such as cloth, paper, glass and other inorganic substances and they may be utilized in the form of composite products.  
 This invention will now be illustrated by the following non-limiting examples and comparative examples.  
  Tensile mechanical properties given in the examples were determined at 23C. in an atmosphere of 65 relative humidity by employing an Instron tensile tester under the following conditions:  
 Sample form: strip (15 cm X 1 cm) Chuck distance: 10 cm Pulling rate: 10 cm/min According to the present invention, there is measured each in the lengthwise direction and the widthwise direction the elongation at breakage of the resulting film when the film is subjected to further heat treatment at 200C. for 200 hours under such conditions as will allow free shrinkage of the film. The elongation at breakage is expressed as the smaller of the two values measured, i.e. the worse result.  
  The intrinsic viscosity [17] referred to above, namely the intrinsic viscosity measured at 35C. in a mixed solvent of phenol:2,4,6-trichlorophenol of 6:4 weight ratio and the intrinsic viscosity [11], measured at 35C. in o-chlorophenol, satisfies the following relation:  
 EXAMPLES 1 TO 9 AND COMPARATIVE EXAMPLES 1 TO 2 Chips of polyethylene-2,6-naphthalate having an intrinsic viscosity [1 of 0.64 was dried at 180C. for a period sufficient to reach the water content as indicated in Table l, and were melted at 290 300C. The melt was extruded while adjusting the residence time as indicated in Table l, and the extrudate was quenched and solidified on a casting drum maintained at 50C. to obtain an unstretched film having a thickness of about 600 w. The unstretched film was subjected to step-wise &#39;biaxial stretching and heat treatment under conditions indicated in Table 1. The resulting biaxially oriented and post-heat-treated film was positively deteriorated under the relax state at 200C. in air, and the change in the mechanical properties in the lengthwise direction was examined. Results are shown in Table 2.  
 Table 1 Conditions for Forming Stretching Conditions Post Heat Treatment Unstretched Films Requirement i Conditions Water Melting Resi- Direction (L)** Direction (W)* Stretch Tempe- Time Conditions content 01 temperadence Tcmpe- Stretch Tcmpe- Stretch ratio rature allowing starting ture time rature ratio rature ratio (W no free chips (71 stretch shrinkage by weight) (C.) (min) (C.) (C.) ratio 1C.) (sec) Ex. 1 0.001 290 133 3.4 133 3.7 1 l 235 10 3% in W Ex. 2 0.001 293 10 135 3.7 138 4.0 1.07 240 15 3% in W Ex. 1 0001 290 10 140 3.5 135 3.8 1.08 238 5 under tension Ex. 4 0.002 295 15 135 3.3 130 3.6 1.1 230 10 do. Ex. 5 0.003 300 10 135 3.5 130 3.7 1.06 227 18 do Ex. 6 0.002 300 130 3.6 135 3.8 1.06 220 10 do Ex. 7 0.004 300 15 140 3.5 125 3.5 1.00 225 15 do Ex. 8 0.03 290 10 140 3.5 135 3.6 1.03 240 15 do Ex. 9 (1.012 295 15 140 3.5 135 3.6 1.03 240 15 do. gomp. 0.012 295 10 135 3.6 3.5 0.97 250 10 do.  
  x.l Comp. 0.025 300 15 140 3 5 3. 1 ()0 240 15 do. Ex. 2  
 Notes:  
  Direction (&#39;1 designates the widthwisc direction. which will apply to &#39;l&#39;ahles given below. Direction (1.) designates the lengthwise direction. which will apply to Tables given below. Each value was determined by counting fractions of 0.005 and over as 0.01 and disregarding the rest.  
 Table 2 Requirement (iii) of Requirement (ii) of Breaking Strength Resulting Films: Resulting Films: (kglcm of Elongation at Resulting Films Breakage (7r) after after further further heat treatheat treatment merit 200C., 200 200C., 200 hrs. hrs. under free under free shrin- [nz] [i 0.3 [-r;]+1.361 shrinkage kage Heat Resistance Test* Ex. 1 1.507 0.56 1.529 38 1540 passed Ex. 2 1.515 0.58 1.535 40 1620 d Ex. 3 1.519 0.60 1.541 51 1590 do. Ex. 4 1.504 0.55 1.526 37 1510 do. Ex. 5 1.502 0.54 1.523 40 1580 do. Ex. 6 1.499 0.53 1.520 43 1590 do. Ex. 7 1.505 0.52 1.517 27 1480 do. Ex. 8 1.509 0.50 1.511 30 1550 do. Ex. 9 1.507 0.49 1.508 22 1480 do. gomp. 1.514 0.50 1.51 1 3 1180 not passed x. 1 Comp. 1.506 0.47 1.502 4 1050 do. Ex. 2  
  According to the test recommended by the lnternational Organization for Standardization.  
 EXAMPLES 10 TO 12 AND COMPARATIVE 5 3 to obtain a biaxially stretched and post-heat-treated EXAMPLES 3 TO 5 film having an; intrinsic viscosity [1 and a thickness- Chips of polyethylene-2,o-naphthalate having an indirection refractive index [n each of which is inditrinsic viscosity [1 indicated in Table 3 were melted cated in Table 3. and extruded in the same manner as in Example 1 to obtain an unstretched film having a thickness of about 50 v Physical properties of each film before and after the 600 a. The unstretched film was biaxially stretched and further heat treatment at 200C. for 200 hours without post-heat-treated under conditions indicated in Table ny r r in gainst shrinkage are shown in Table 4.  
 Table 3 Post Heat Resulting Stretching Conditions Treatment Conditions Films Direction (L) Direction (W) Tempera- Time Conditions Tcmpera- Stretch Tempcra- Stretch ture allowing ture ratio ture ratio free shrinkage 1C.) (C.) (C.) (see) [1;] [n2] Example 10 140 3.5 135 3.5 225 15 under 0.60 1.514 Example 1 l 3.6 145 3.8 225 10 47: in W 0.53 1.520 Comparative 140 3.6 145 3.5 240 15 under 0.53 1.522 Example 3 tension Example 12 130 3.5 3.5 225 10 do. 0.70 1.520 Comparative 3.6 3 5 230 8 do. 0.70 1.522 Example 4 Comparative 135 3.6 120 3.5 245 15 do. 0.50 1.520  
 Example 5 Table 4 Before Further After Further Heat Treatment Heat Treatment Breaking Elongation Breaking Elongation Strength at Breakage Strength at Breakage (kg/cm l (71) (kg/cm (71) Heat Resistance Test* Example 2300 70 1680 passed Example 1 l 2150 1400 35 passed Comparative l50 50 1200 6 not passed Example 3 Example I2 2350 73 1750 38 passed Comparative 250 1250 7 not passed Example 4 Comparative 2100 51 l 100 5 not passed Example 5 According to the test recommended by ISO.  
 EXAMPLES 13 TO 15 AND COMPARATIVE EXAMPLES 6 TO 7 ment, the breaking strength, the elongation at breakage and the dielectric breakdown strength of the films were An unstretched film of an ethylene-2,6-naphthalate determined. Results are shown in Table 5, where data copolymer of an intrinsic viscosity [1 of 0.60 formed by copolymerization employing as the dicarboxylic acid component 97 mol percent of naphthalene-2,6-  
 of the film of Example 15 derived from the polymer free of any terephthalic acid comonomer are also shown.  
 Table 5 (A) Tested Films Before Further Heat Treatment Stretch Ratio Breaking Elongation Strength at Breakage [nz] 0.3 [-q]+l.36l (L) (W) (\\&#39;l/(LI (kg/6m (7) Example 15 0.68 1.497 1.54l 3.5 3.6 1.03 2300 &#39;6 Example 13 0.0.x l..&#39; )0 1.541 3.5 3.5 l.00 2300 Example 14 0.56. l.5l0 1. 29 3.6 3.6 l.00 22I0 T3 Comparative 0.56 1. 06 1.529 3.6 3.6 I .00 l&#39;.00 Example 6 a 1 Comparative 2210 I26 Example 7 dicarboxylic acid and 3 mol percent of terephthalic Table 5 (B) acid was stretched in both the lengthwise and widthwise directions coincidentally at a stretch ratio of 3.5 40 0 After Further Heat Treatment 1 a e in each direction at C., and the stretched film was 200 i 400 i 0 under free shrinkage under free shrinkage post-heat-treated at 230 C. under tenslon (Example Breaking Elongation Breaking Elongation 13 Strength at Br akage Strength at Breakage 2 J C An unstretched film of an ethylene-2.6-naphthalate g (kg/cm (kg/cm l copolymer of an intrinsic viscosity [1 of 0.56 formed Example 15 1680 53 1590 21 by copolymerization employing as the dicarboxylic ggg: $8: acid component 96.5 mol percent of naphthalene-L6- fl 810 630 &#34;.1 dlcarboxyllc acid and 3.5 mol percent of terephthalic E p 790 0 a ompara we 5 I. acld. The unstretched film was stretched in both the 50 Example 7 lengthwise and widthwise directions at a stretch ratio of 3.6 in each direction at C. and post-heat-treated at 235C. under tension for 10 seconds (Example 14).  
  A comparison product was prepared in the same manner as in Example 13 except that an ethylene-2.6- naphthalate copolymer obtained by the copolymerization using as the dicarboxylic acid component 85 mol percent of nathphalene-2.6-dicarboxylic acid and 15 mol percent of terephthalic acid was employed as the starting polymer (Comparative Example 6).  
 ment at 200C. for 200 or 400 hours under conditions allowing free shrinkage. After such further heat treat- Treatment temperature was 220C.  
 EXAMPLES 16 TO 18 AND COMPARATIVE EXAMPLES 8 TO I l the lengthwise and widthwise directions under conditions indicated in Table 6 by the subsequent biaxial stretching method and post-heat-treated at 225C. for 10 seconds under tension to obtain a biaxially oriented film. The resulting film was subjected to the thermal deterioration treatment under the relaxed state in the air at 200C. Tensile mechanical properties in the lengthwise and widthwise directions of the film were determined before and after the above deterioration treatment. Results are shown in Table 6.  
 Table 6 Stretching Conditions Tensile Mechanical Properties W L direction W direction stretch Before thermal After thermal ratio/L deterioration deterioration Tcmpcra- Stretch Tcmpcra- Stretch stretch Breaking Elonga- Breaking Elongaturc ratio turc ratio ratio Direcstrength tion at strength tion at (C.) (C.) [n l tion breakage breakage (kg/m (kg/cm (7) Ex. 16 130 3.5 135 3.8 1.08 1.504 L 2310 75 1460 45 W 2400 70 1500 48 Ex. 17 130 3.2 130 3.5 1.09 1.508 L 2280 83 1480 31 W 2330 75 1510 30 Ex. 18 135 3.7 138 3.9 1.05 1.502 L 2390 74 1520 42 W 2410 73 1550 45 Comp. 140 3.5 125 1.4 0.4 1.510 L 2320 74 1470 41 Ex. 8 W 1980 115 940 Comp. 135 3.8 135 3.6 0.95 1.504 L 2420 71 1480 38 Ex. 9 W 2380 80 l 100 8 Comp. 130 2.8 135 3.6 1.28 1.507 L 2250 95 1050 9 Ex. W 2420 72 1530 33 Comp. 140 3.8 130 3.4 0.84 1.503 L 2400 70 1500 41 Expll W 2310 87 1020 7 EXAMPLE 19 the stepwise biaxial stretching and heat treatment were EXPERIMENT I changed as shown in Table 7.  
  Run 2&#39; Run 1(a) Run 2 was repeated except that the heat-treatment Molten polythylene-2,-naphthalate was extruded and the extrudate was quenched and solidified on a casting drum maintained at 60C. to obtain an unstretched film having a thickness of about 330 microns.  
  The unstretched film was subjected to the stepwise biaxial stretching and heat treatment under the conditions indicated in Table 7. The resulting biaxially oriented and post-heat-treated film was positively deteriorated in the relaxed state at 200C. in air, and changes in mechanical properties in the lengthwise direction were examined.  
 Run 1(b) Run 1(a) was repeated except that the heat treatment of the unstretched film was carried out at the temperature indicated in Table 7.  
 Run 1&#39; Run 1(a) was repeated except that the heat treatment of the unstretched film was carried out at the temperature indicated in Table 7 to form a film as a comparison sample.  
 Run 2 Run 1( a) was repeated except that the conditions for temperature was changed as shown in Table 7 to form a film and as a comparison sample.  
 RESULTS OF EXPERIMENT I The results obtained in Run Nos. 1(a), 1(b), 1&#39;, 2 and 2&#39; are shown in Table 7 below.  
 EXPERIMENT 11 RESULTS OF EXPERIMENT II The results obtained in Run Nos. 3 and 3&#39; are shown in Table 8.  
  17 EXPERIMENT 111 Runs 4-14 Run 2 was repeated except that polyethylene-2,6- naphthalate containing units derived from thirdcomponents in the amount indicated in Table 9 were used.  
 Runs 4&#39;-7&#39; (Comparisons) Runs 4-14 were repeated except that polyethylene- 2,6-naphtha1ates containing units derived from third components in the amounts indicated in Table 9 were used.  
 RESULTS OF EXPERIMENT 111 The results obtained in Runs 4-14 and Runs 4&#39;-7 are shown in Table 9.  
  Molten polyethylene-2,6-naphthalate was extruded and the extrudate was quenched and solidified on a casting drum maintained at 60C. to obtain an unstretched film having a thickness of about 330 microns.  
  The unstretched film was subjected to the stepwise biaxial stretching and heat treatment under the following conditions, LD drawing temperature 135C., LD draw ratio =.-3.5 times, WD drawing temperature 130C., WD draw ratio 3.8 times, and the post-heattreatment temperature was 227C. and its time was as shown in Table 10. The resulting biaxially oriented and post-heat treated film was positively deteriorated in the relaxed state at 200C. in air, and changes in mechanical properties in the lengthwise direction were examined.  
 Table 9 (Part 1) Stretching Conditions (Requirement i) Stretch 3rd Components Direction (L) Direction (W) we,  
 . Amount Tempe- Stretch Tempe Stretch stretch of units rature ratio ture ratio ratio Runs Names (mo17z) (C.) (C.) (L) 4 Dimcthyl adipate 3.5 135 3.5 130 3.8 1.08 5 lsophthalic acid 3.5 do. do. do. do. do. 6 Naphthalcnc-2.7-dicarboxylic acid 3.5 do.. do. do. do. do. 7 Naphthalene-2,7-dicarboxy1ic acid 7.0 do. do. do. do. do. 4&#39; NaphthaIenc-ZJ-dicarboxylic acid 15.0 do. do. do. do. do. 8 Terephthalic acid 3.5 do. do. do. do. do. 9 Terephthalic acid 7.0 do. do. do. do. do. 5&#39; Terephthalic acid 15.0 do. do. do. do. do. 10 Methyl p-hydroxybenzoatc 3.5 do. do. do. do. do. 1 1 Mcthyl-p-hydroxyethoxybenzoate 3.5 do. do. do. do. do. 12 Propylene glycol 3.5 do. do. do. do. do. 6&#39; Propylene glycol 15.0 do. do. do. do. do. 13 Trimethylene glycol 3.5 do. do. do. do. do. 14 Trimethylenc glycol 7.0 do. do. do. do. do. 7&#39; Trimethylene glycol 15.0 do. do. do. do. do. 15 Napththalene-2.7-dicarboxylic acid 1.5 do. do. do. do. do. Diethyleneglycol 2.2 do. do. do. do. do.  
 Table 9 (Part 2) Requirement (iii) of Resulting Films: Post Heat Treatment Requirement (iii) of Elon ation at Conditions Resulting Films Brea age (70) after furhter heat treat- Tempement 200C., 200 Heat rature Time Conditions allowing I hrs. under free Resistance Runs (C.) (see) no free shrinkage [n,] [1;] 0.3X[1;]+1.361 shrinkage Test 4 225 10 Maintaining a constant 1.503 0.55 1.526 passed length both in the longitudinal and widthwise directions 5 do. do. do. 1.506 0.56 1.529 36 do. 6 do. do. do. 1.505 0.58 1.535 53 do. 7 do. do. do. 1.515 0.59 1.538 35 do. 4 do. do. do. 1.525 0.54 1.523 7 not passed 8 do. do. do. 1.509 0.60 1.541 54 passed 9 do. do. do. 1.514 0.55 1.526 33 do.  
  5&#39; do. do. do. 1.533 0.56 1.529 8 not passed 10 do. do. do. 1.51 1 0.56 1.529 32 passed 1 1 do. do. do. 1.509 0.55 1.526 33 do. 12 do. do. do. 1.508 0.56 1.529 29 do.  
  6&#39; do. do. do. 1.530 0.51 1.514 5 not passed 13 do. do. do. 1.505 0.59 1.538 48 passed 14 do. do. do. 1.515 0.54 1.523 23 do.  
  7&#39; do. do. do. 1.528 0.54 1.523 8 not passed 15 do. do. do. 1.508 0.58 1.535 30 passed EXAMPLE 20 RESULTS OF EXPERIMENT 1 EXPERIMENT I The results obtain in Runs 1-5 are shown In Table 10 Runs 1-5 below.  
 Table Runs Post heat n; [1 0.3X[-q]+ Elongational Heat treatment 1.361 breakage (71) Resistance timc(scc) after further test heat treat- 7 merit 200C., under free shrinkage l- 5 1.501 0.53 1.520 47 passed .2. l5 1.500 0.53 1.520 48 do. j  
 3 50 1.501 0.53 1.520 49 do. 4 300 1.502 0.53 1.520 46 do. 5 500 1.501 0.53 1.520 48 do.  
 EXPERIMENT H trichlorophenol at a weight ratio of 6:4,.1s within a RESULTS OF EXPERIMENT 11 The results obtained in Run 2 are shown in Table l l.  
 range expressed by the following formula:  
 with a proviso that above [n,,] and [1;] satisfy the following condition:  
 Table l 1 Run n, [1;] 0.3 [1i]+1.361 Elongation at Heat breakage (71) after Resistance fruther heat treatment 200C., 200 hrs. under free shrinkage 2&#39; 1.515 0.53 1.520 3 not passed While the present invention has been described pri- 2. A biaxially oriented and post-heat-treated marily in connection with the foregoing exemplification, it should be understood that the present invention is to be in no way deemed as limited thereto, but, rather should be construed as broadly as any or all equivalence thereto.  
 What we claim is:  
  l. A biaxially oriented and post-heat-treated polyethylene-2,6-naphthalate insulating film composition consisting essentially of polyethylene-2,6- naphthalate in which at least 90 mol percent of the structural units are ethylene-2,6-naphthalate units and at most 10 mol percent of the structural units derived from a comonomer or comonomers having a divalent, ester-forming functional group or groups, said film being produced by biaxially stretching an unstretched film under heating conditions and post-heat-treating said film to a temperature of from 215C. to 240C. under such conditions as will not allow free shrinkage of the film; characterized in that (i) the stretch ratio in the widthwise direction is within a range of from about 3 to about 4 times based on the original width of the unstretched film, and the stretch ratio in the lengthwise I and (iii) the thickness-direction refractive index [n,,] ofsaid biaxially oriented and post-heat-treated film, measured at 20C. by Abbes refractometer with use of a ray of a wavelength of 589 mg, is within a range expressed by the following formula;  
 &#39; and the intrinsic viscosity [1 of said film, measured at 35C. in a mixed solvent of phenol: 2,4,6-  
 polyethylene-2,6-naphthalate insulating film composition as set forth in claim 1, wherein-the stretch ratio in the widthwise direction is 1 1.2 times the stretch ratio in the lengthwise direction, the elongation at breakage after said further heat treatment is not less than 20 %,l and said thickness-direction refractive index [n,] is} within a range expressed by the following formula:  
 structural units are units derived from a comonomer or comonomers having a divalent, ester-forming functional group or groups, ata stretch ratio in the width-- wise direction within a range of from about 3 to about 4 times based on the original width of the unstretched film, and the stretch ratio in the lengthwise direction is within a range of from about 3 to about 4 times based! on the original length of the unstretched film, with a proviso that the stretch ratio in the widthwise direction is 1 4/3 times to the stretch ratio in the lengthwise direction, the stretching in the widthwise direction being conducted at C. and the stretching in the lengthwise direction being conducted at 120 C., and then subjecting the biaxially stretched film to the&#39; post-heat-treatment at 215 240C. under such conditions as will not allow a shrinkage greater than 10 to thereby obtain a biaxially oriented and post-heattreated film wherein the elongation at breakage of the resulting film is not less than 15 when said film is subjected to further heat treatment at 200C. for 200 hours under such conditions as will allow free shrinkage of the film; and the thickness-direction refractive 21 22 index [n,] of said biaxially oriented and post-heatwith a proviso that above [n and [n] satisfy the foltreated film, measured at 20C. by Abbes refractomelowing condition: ter, with use of a ray of a wavelength of 589 mu, is within a range expressed by the following formula: [&#34;z] X [&#39;0] 1361 1.499 [n 1.520 5 4. The process of claim 3 wherein the stretch ratio in the widthwise direction is from 1 to 1.2 times the and the intrinsic viscosity [1;] of said film, measured at Stretch ratio in the lengthwise direction 35C. in a mixed solvent of phenol: 2,4,6- trichlorophenol at a weight ratio of 6:4, is within a The Process of clalm 3 Wherem f i range expressed by the foll wi g for la; 10 treatment is conducted under such condrtlons WhlCh keep the film length constant. 0.485 [1;] 0.70 =1:  
 UNITED STATES PATENT OFFICE CERTIFICATE OF CORECTICN PATENT NO. 3,875,119 DATED April 1, 1975 INVENTOR(S) HIROSHI AOKI, ET AL.  
  It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 In the Heading, insert the following:  
 -- Assignors to&#39;Teij in Limited of Osaka, Japan Signed and sealed this 1st day of July 1&#34;?75.  
 (SEAL) Attest:  
  C. E&#39;iskRSEZALL DANE RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks