DE2163963C3 - Process for the production of a biaxially oriented and post-heat treated, electrically insulating film made of polyethylene-2,6-naphthalate - Google Patents

Description

The invention relates to a method for producing a biaxially oriented and post-heat treated, electrically insulating film made of polyethylene-2,6-naphthalate.

It is known that polyethylene-2,6-naphthalate is an orientable, crystalline polymer which is suitable for shaping into threads or film and that its second transition point is about UO "C, ie about 50 ^J C higher than that The second transition point of polyethylene terephthalate is that its melting point is about 270 ³ C, ie about 10 ⁰ C higher than the melting point of polyethylene terephthalate (cf. British patent specification 604 073).

Although this polymer has been known for more than 20 years, hardly any proposals have been made for made the production of oriented films from this polymer.

As far as is known, only the production of a film from polyethylene terephthalate, the 2,6-naphthalenecarboxylate with a proportion of less than 25% of the repeating structural units of the polymer in U.S. Patent Specification 3,161,710. However, it can no insulating film can be made from such a copolyester, which at the same time has excellent electrical properties Has insulating properties and sufficient physical properties to withstand continued use to withstand high temperatures.

US Pat. No. 3,501,344 describes a magnetic recording tape which is used for polyethylene-2,6-naphthalate is used as a carrier material. Here, one by biaxial stretching becomes biaxial oriented film made of polyethylene-2,6-naphthaIate, a thermosetting or crystallization process with protection against shrinkage at a temperature of usually more than 120 up to about 250.degree or in some foils also above, preferably at 200 to 240 ° C. The heat-hardened Film can be thermally relaxed within the same temperature range without any anti-shrinkage measures will. It is suggested that said biaxially stretching the unstretched film by stretching the film at 105 to 150 C (preferably 115 to 140 C) in the longitudinal direction from about 3.5 to about fi.Ofuehen (preferably from 4.0 to 5.5 times) of its original length and then stretching the film (from one part to the next at 105 to 150 C) in the latitude of about 0.9 to about 1.4 times (preferably about 1.0 to about 1.3 times) its original width, i.e. H. from about 0 ", ', transverse extension to about 40", transverse extension, is performed. The film that is biaxially oriented and post-heat treated in this way is then with a non-magnetic hinder, m the magnetizable particle of Fe /) -, or the like magnetically sensitive material are dispersed, coated.

Investigations were carried out with regard to an insulating mass made of polyethylene-2. (I-naphihalai. Das not only electrical insulating properties but also satisfactory physical properties with continuous long-term use under high temperatures has carried out, and it has been found that in the case of polyethylene-2, fi-naphthalate the stretching mode during the biaxial stretching. the method of heating during post-heat treatment and the post heat treatment conditions the physical properties of the obtained biaxially oriented and post-heat treated film, especially those desired from an insulating film Properties, very strongly affect and that known process for the production of biaxially stretched Films that were used for other polymers, such as polyethylene terephthalate, do not can be applied directly to polyethylene-2,6-naphthalate.

It was z. B. found that the refractive index (nz) in the thickness direction of the biaxially stretched and post-heat treated film by the heating method, the stretching method and the post-heat treatment conditions are greatly changed, with the result that the elongation at break of the film to tear, if the same is another vV'ärmenachbehandlung at 200 ⁰ C, for a period of 200 hours, is subjected to conditions that permit the free shrinkage of the film is greatly affected.

The object of the invention is to create a method for the production of a biaxially oriented and post-heat treated electrically insulating film Made of polyethylene-2,6-naphthalate with excellent electrical insulating properties and satisfactory physical properties of good suitability for long-term continuous Has been used under high temperatures.

According to the invention, a method for producing a biaxially oriented and post-heat treated, electrically insulating film made of polyethylene-2,6-naphthalate, which is characterized is that an unstretched Polyäthykn-2,6-naphthalatnlm, which consists essentially of polyethylene-2,6-naphthalate, in which at least

of the structural units ethylene-2,6-naphthylate units are. Accordingly, under the expression "polyethylene-2,6-naphthalate" used here, not just one Homopolymer of ethylene 2,6-naphthalate but rather

also a modified polyethylene-2,6-naphlhalat in a proportion of less than 10 mole percent, preferably understood less than 5 mole percent, of comonomers or modifiers.

Production of polyethylene - ?. 6-naphthalate

ίο is generally known to the technology. If a comonomer or a modifier in an amount less than 10 mole percent prior to termination of the Reaction to form polyethylene-2,6-naphthalate is used, one or more suitable

t) QMul Percentage of Structural UnitsAilVjlen-2, f) -miphlhulateinheeilen are, with a. Stretching ratio either in the width direction or in the longitudinal direction, ung in the range from 3 to about 4 biaxially stretched, where the stretch ratio in the width direction is at least two times the stretching ratio in the longitudinal direction, the stretching in the width direction at 115 to 150 C and stretching in the longitudinal direction is carried out at 120 to 160 C, and then the hi ixul stretched film one Subsequent heat treatment at 170 to 240 C under the same conditions at which the f-ilni length kept constant and no shrinkage above 10 "; is permitted.

The stretching ratio in the 15 comonomers or modifiers is preferably 1 to 1.2 times and in particular the merization system added to the poly-width system, and then 1 to 1 times the stretching ratio is added in the longitudinal reaction through the formation of a copolyester or direction. When said biaxially oriented and a blended polyester finishes,
post-heat treated film of a further heat- as such comonomer or modifier-

Luckily at 200 C for a period of nn medium, compounds that have a zvjiweitige

ester-forming functional group are used will. 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-hydroxyäiho \ yhcnzoäure and lower alkyl esters of these hydroxycarboxylic acids and

589 ηιμ, is within the range of 30 iveiwertigc alcohols such as propylene glycol and Tii formula below melhylene glycol. Polyethylene ^ .6-naphthalate or be

modified polymer can be used as terminal hydroxyl and or carboxyl groups those through

1,499 S (nz) g 1,5j0 a monofunctional compound like benzoic acid,

Benzoylbenzoic acid, benzyloxybenzoic acid and methoxypolyalkylene glycol are capped. It is also possible with a very small amount of a polyfunctional compound, such as Glycerin and pentaerythritol to such an extent

1,500 S (nz) < 1,515 40 modified polyethylene-2.6-naphth.alate that the

Linearity of the polymer is not significantly impaired to use.

It is possible to use such starting polyethylene

defined range, and the basic molar viscose 2,6-naphthalate various additives used in the film or film number U]) of the said film, measured during manufacture. At -35 ° C in a mixed solvent of phenol games for such additives are matting agents,

such as titanium dioxide, stabilizers such as phosphoric acid, phosphorous acid and esters of these acids, as well Lubricants such as finely divided silica (silicon dioxide) and finely divided China clay.

It is desirable that the polyethylene-2,6-naphthalate used in the present invention be one intrinsic molar viscosity (»/) in the range from 0.50 to 0.80. since polymers with such a basic SS molar viscosity number (»,) can be used in foils leave.

200 hours, under conditions that there- free shrinkage of the! .! never allow, is subjected, the elongation at break (elongation at break j of the film obtained should not be less than 10 ", preferably not less than 15 ° "and expediently 20" ". The Thickness direction refractive index (Hr) of the aforesaid biaxially oriented and post-heat treated film, measured at 20 C with an Ab, -> e refractometer using light of the wavelength of

preferably

and 2,4,6-trichlorophenol in a weight ratio of 6: 4, is within the range given by the following formula

0.48 ύ Ui) ύ 0.70

is defined, with the proviso that the values of (n :) and (η) satisfy the following condition:

0.3-Oj) + 1.361.

The polyethylene-2,6-naphthalate used as the base polymer according to the invention can be any Be polymer in which at least 90 mol percent The intrinsic molar viscosity mentioned in the invention (> /) is determined by measurements with respect to the polymer in a mixed solvent,

that from phenol and 2,4,6-trichlorophenol (phenol to

2,4,6-trichlorophenol weight ratio = 6: 4)

35 ° C is carried out, determined, the unit being 100 cmVg.

The refractive index Uu mentioned in the invention

in the direction of the thickness is the value that is determined by the ^{measurement made at 20 r} C with an Abbe refractometer using a beam of wavelength 589 ΐημ (center of the D line).

An unstretched film that is used to make a biaxially stretched and post-heat treated film can be used with the help of the Technique for film production usual methods are produced. Usually the starting material is polyethylene-2,6-naphthalene dried in order to remove the water contained therein, is then through a Extruder or the like, melted, extruded, from waste or freed dirt and then shaped into a film.

cited Amordernisse on the intrinsic molar viscosity in :) and (i /) in addition, the stretching ratio should also meet certain requirements so that the film obtained has sufficient physical properties to be suitable for continuous long-term use at high temperatures. More precisely, it has been found that in the biaxially stretched film according to the invention, the stretching ratio in transverse or longitudinal

During this deformation process io direction decreased within a range of about., - b, s the degree of polymerization of the polymer should be 4 times and the stretching ratio in the transverse direction at least 1 times, preferably 1 to 1.2 times, preferably 1 to 1, l times that

Stretch ratio in the longitudinal direction should be.

the intrinsic viscosity (η) of the unstretched
Film fluctuates greatly depending on the drying and extrusion conditions of the starting polymer and other factors. Furthermore, if the stretch ratio in the transverse direction changes less, the refractive index («ζ) in the thickness direction is 1 times the stretch ratio in the longitudinal direction, there is when the obtained film lasts for a long period of time under high temperatures

is used, the tendency that the elongation at break

The preferred range listed above is when z. B. the stretch ratio in the transverse direction more than 1.3 times the stretching ratio in the longitudinal direction

purpose-oriented biaxially oriented and post-heat treated film made of polyethylene-2,6-naphthalate inside one by the following formula

of the biaxially oriented and post-heat treated film depending on the change in the intrinsic molar viscosity (η) mentioned above

of the unstretched starting film and depending on ao in the transverse direction is very greatly reduced. If the stretching and post-heat treatment ibedin stretching ratio in the transverse direction is higher than the above. Studies of the relationship between changes in the basal molar
Viscosity number (//) and the refractive index (wr) in
Thickness direction have shown that to; · Achievement is 25, so the elongation at break often decreases very sharply in the longitudinal direction of its biaxially stretched and heat-treated direction. In order for the obtained film to have both polyethylene-2,6-naphthalate film having sufficient physical properties to withstand physical properties to withstand long-term continuous use under high temperature conditions, and excellent electro -isolate- 30 excellent electro-insulating properties, holds a specific, required, it is important that the stretching ratio in longitudinal relation between the intrinsic viscosity or transverse direction within the above (η) and the refractive index {nz ) in the direction of the thickness and that the resulting film must be set up between the stretchers. Proportions in the longitudinal and transverse directions

More specifically, it was found that the above-mentioned relationship exists.

Refractive index (nz) in the direction of thickness in the biaxially stretched and post-heat treated poly-

Ethylene-2,6-naphthalate insulating films which meet requirements (i) and (iii) above generally have an elongation at break of not less than 10 " _o after they have undergone a further heat treatment at 2 (X) C for a period of time have been of 200 hours, under conditions which permit shrinking subjected. In order to achieve the purposes of the invention are biaxially stretched and wärmenachbehandelte films having an elongation at break of not less than 10 "O, preferably not less than 15 ',' _o, especially suitable of not less than 20 "" after the further heat treatment mentioned above.

To achieve the biaxially oriented and post-heat treated polyethylene-2,6-naphthalate insulating film composition of the invention, conditions, such as the intrinsic viscosity (η) of the unstretched film, the production conditions of the unstretched film, the stretching conditions during biaxial stretching, the temperature conditions during biaxial stretching, the temperature conditions during biaxial stretching, can be used , the temperature conditions for post-heat treatment and the degree of restriction of free shrinkage during post-heat treatment

should be freely selected and experimentally using the above (nz) and (tj) meet the following requirements of the above conditions (i), (ii) and (iii): be combined as criteria , although this Be

conditions cannot be determined independently in a monistic manner. For a better visual

(nz) S 0.3 - (η) 4- 1.361. Examples of implementation of the invention are given below

given for embodiments.

Schnitzel of polyethylene-2,6-naphthalate with a It has also been shown that over the above intrinsic viscosity (η) of z. B. 0.50 to

preferably

1.499 ^ (nz) g 1.520

1,500 ^ (nz) <, 1,515),

expressed range and intrinsic viscosity (η) within one by the following formula

0.48 ^ Ui) S 0.70

0.80 are heated with hot air for several hours is heated to 170 to 200 C, dried, melted and held at 290 to 300 C by a Extruder measured and extruded. The obtained exudate is then made with the help of usual Film forming devices formed into an unstretched film. The unstretched film is stretched lengthways to about 3 to about 4 times the original length at about 120 to about 160 C, and in Transverse direction (width direction) to about 3 to about 4 times the original width at about 115 to about 150 ° C, preferably at about 135 to about 150 ° C, stretched such that the stretch ratio in the transverse direction is at least 1 times the stretch ratio is in the longitudinal direction. Then the biaxially stretched film undergoes the heat treatment at about 170 to about 240 C under conditions that do not allow free shrinkage of the film. Generally will this heat treatment is carried out under such shrinkage-limiting conditions that the film remains constant in the longitudinal direction or the shrinkage does not exceed 10 ".

The biaxially oriented and post-heat treated polyethylene-2, (i-naphthalate insulating film material according to the invention has excellent electrical properties such as high dielectric strength, high surface resistivity and high volume resistivity under temperatures as high as 155 ° C or more and almost constant dielectric properties at temperatures in the range from 60 ° C. to 180 ° C. In addition, the films according to the invention have excellent mechanical properties such as tensile strength, Young's modulus, ultimate breaking strength, tear and impact strength, low thermal shrinkage without any Precautions against shrinkage are taken, and excellent heat resistance sufficient for continuous use at 155 C. (Grade F heat resistance for insulating materials according to the test proposed by the International Organization for Standardization - ISO.) In addition, they are sufficiently resistant to various insulating oils, freon. Oils used in refrigeration machines, organic solvents and plasticizers and, compared to polyethylene terephthalate films. they are not easily hydrolyzed. Accordingly, biaxially oriented and post-heat treated films according to the invention are excellent insulating materials.

The films according to the invention can be used directly as insulating materials for various electrical and electronic machines can be used, e.g. B. tape material for electrical and electronic machines, as materials for slot insulation, for step-phase insulation and as construction material for capacitors. You can also use other insulating materials such as cloth, paper, glass and other inorganic substances are laminated and used in the form of layered products will.

The following examples and comparative examples are intended to illustrate the invention.

The tensile mechanical properties reported in the examples were measured at 23 "C in a 65 "relative humidity environment using an Instron tensile tester determined under the following conditions:

Shape of the sample. Stripes (15
Span: IC cm,
Pulling extent. 10 cm / min.

1 cm),

Between the intrinsic molar viscosity (j /) mentioned in the invention, namely the intrinsic viscosity (»/), - /, measured at 35 ^J C in a mixed solvent of phenol and 2,4, ίο 6-trichlorophenol in the weight ratio of 6: 4, and the intrinsic viscosity ( »/) ο <Λ measured at ³⁵ C in o-chlorophenol, following relationship is established:

¹ S U ₁ ) 1.008 · (»/) '>(·'': 0.0128.

B e i s ρ i e 1 e 1 to 9

and Comparative Examples 1 and 2

Polyethylene-2,6-naphthalate chips with an intrinsic viscosity (i /) of 0.64 were dried at 180 ° C. for a sufficiently long time to achieve the water content given in Table 1 and melted at 290 to 300 ° C. The melt was extruded with the dwell set as shown in Table 1, and the extrudal was rapidly cooled and solidified on a casting drum at 50 ° C., and an unstretched film having a thickness of about 600 μm was obtained. The unstretched film was subjected to the stepwise biaxial stretching and heat treatment under the conditions shown in Table 1. The biaxially oriented film was wärmenachbehandelte and bcabsichliglcrwcise deteriorated state of relaxation at 200 ¹ C in air, and the change in mechanical properties in the longitudinal direction was examined. The results are shown in Table III.

Table I.

50 example Deformation conditions for Enamel Temporary unstretched films temperature time Water content of (C) (Minutes; Exit Schnilzci 1 (Weight percent) 290 15th 55 2 293 10 3 0.001 290 10 4th 0.001 295 15th 5 0.001 300 10 6th 0.002 300 20th 60 7th 0.003 300 15th 8th 0.002 290 10 9 0.004 295 15th Comparison 0.03 example 1 0.012 295 10 «5 Comparative example 2 0.012 300 15th 0.025

409 638/315

Table I (continued)

10

Stretching conditions (condition i) t (L) *) Direction (B) *) Stretch
ratio Stretch Heat treatment conditions time
(Seconds) Conditions, Richtuni Stretch
ratio Tempe
rature
Γ C) 3.7 ratio (B) / 10 the no free
Shrink
allow example Tempe
rature
("C) 3.4 133 Stretch
ratio (L)
* »*) temperature
( ⁰ C) 3% in crush 1 133 4.0 1.1 235 15th direction 3.7 138 3% in crush 2 135 3.8 1.07 240 5 direction 3.5 135 3.6 10 undervoltage 3 140 3.3 130 3.7 1.08 238 18th undervoltage 4th 135 3.5 130 3.8 1.1 230 10 undervoltage 5 135 3.6 135 3.5 1.06 227 15th undervoltage 6th 130 3.5 125 3.6 1.06 220 15th undervoltage 7th 140 2.5 135 3.6 1.00 225 15th undervoltage 8th 140 3.5 135 1.03 240 undervoltage 9 140 3.5 1.03 240 10 Comparison 3.6 120 undervoltage example 1 135 3.5 0.97 250 15th Comparison 3.5 125 undervoltage example 2 140 1.00 240

Remarks:

·) Direction (B) denotes the width direction, which also applies to the following tables. * ♦) Direction (L) denotes the longitudinal direction, which also applies to the following tables.

* · ♦) All values were determined by rounding up fractions from 0.005 and above to 0.01 and neglecting the rest would.

Table Il

example Request
(nz) ■ nisse an dit
Ci) ; obtained films
0.3 * (>;) + 1.361 Requirements (ii)
to the films obtained:
Elongation at break (Vo)
after another
Heat treatment at 200 ° C
200 hours at fieier
shrinkage Breaking strength
(kg / cm *) of the obtained
Slides after another
Heat treatment
200 ⁰ C, 200 hours
with free shrinkage warmth
persistence
test·) 1 1.507 0.56 1.529 38 1540 passed 2 1.515 0.58 1.535 40 1620 passed 3 1.519 0.60 1.541 51 1590 passed 4th 1.504 0.55 1.526 37 1510 passed 5 1.502 0.54 1.523 40 1580 passed 6th 1,499 0.53 1.520 43 1590 passed 7th 1.505 0.52 1.517 27 1480 passed 8th 1.509 0.50 1.511 30th 1550 passed 9 1.507 0.49 1.508 22nd 1480 passed Comparison example 1 1.514 0.50 1.511 3 1180 failed Comparison example 2 1.506 0.47 1.502 4th 1050 failed

·) According to the attempt proposed by the International Organization for Standardization

Table IH

example Dir
Tempe
rature
CC) Stretching bed
ng (L)
Stretch
ver
ratio ingings
Dir
Tempe
rature
CQ ng (B)
Stretch
ver
ratio W.
Tempe
rature
("C) ärmebehan
time
(Se
customers) Terms of use
Conditions,
the free
Allow shrinking Raise!
Fo
(η) iltene
lien
(nz) 10
11th
Comparison
example 3
12th
Comparison
example 4
Comparison
example 5 140
130 3.5
3.6 135
145 3.5
3.8 225
225 15th
10 undervoltage
4% in the direction of break 0.60
0.53 1.514
1.520 140
130 3.6
3.5 145
135 3.5
3.5 240
225 15th
10 undervoltage
undervoltage 0.53
0.70 1.522
1.520 140 3.6 145 3.5 230 8th undervoltage 0.70 1.522 135 3.6 120 3.5 245 15th undervoltage 0.50 1.520

Examples 10 to 12 and Comparative Examples 3 to 5

Chips of polyethylene-2,6-naphthalate having an intrinsic viscosity (η) as shown in Table III were melted and extruded in the same manner as in Example 1, and an unstretched film having a thickness of about 6 (X ) μ obtained. The unstretched film became biaxial

stretched and heat-treated under the conditions shown in Table III, and there was a biaxially stretched and post-heat treated film having an intrinsic viscosity (η) and a Refractive index (nz) in the thickness direction, both of which are given in Table III.

The physical properties of each film before and after further heat treatment at 200 ° C for 200 hours without taking precautions against the

ίο Shrinkages are shown in Table 4.

Table IV

Before further heat treatment Elongation at break After further heat treatment Elongation at break Heat resistance test *) example Breaking strength («/.) Breaking strength C /.) (kg / cm «) 70 (kß / cm «) 50 passed 10 2300 55 1680 35 passed 11th 2150 1400 Comparison 50 6th failed example 3 2150 73 1200 38 passed 12th 2350 1750 Comparison 70 7th failed example 4 2250 1250 Comparison 51 5 failed example 5 2100 UOO

*) According to the test proposed by the ISO.

Examples 13-15 and Comparative Examples 6 and 7

An unstretched film made of an ethylene-2,6-naphthalate copolymer with an intrinsic viscosity {η) of 0.60, which is obtained by copolymerization using 97 mol percent naphthalene-2,6-dicarboxylic acid as the dicarboxylic acid component and 3 mol percent terephthalic acid was prepared was stretched in the longitudinal and the transverse direction (width direction) simultaneously at a stretch ratio of 3.5 in each direction at 130 ⁰ C, and the stretched film was carried out at 23O ⁰ C under tension wärmenachbehandelt (example 13).

An unstretched film made of an ethylene-2,6-naphthalate copolymer with an intrinsic viscosity (jj) of 0.56, which is produced by copolymerization using 96.5 mole percent naphthalene-2,6-dicarboxylic acid as the dicarboxylic acid component and from 3.5 mole percent terephthalic acid. The unstretched film was made in the longitudinal and transverse directions with a stretch ratio of 3.6 in each Direction stretched at 135 ° C and under at 235 ° C Tension post-heat treated for 10 seconds (Example 14).

A comparative product was prepared in the same manner as in Example 13, except that the starting polymer was an ethylene-2,6-naphthalate copolymer prepared by copolymerization using 85 mole percent naphthalene-2,6-dicarboxylic acid as the dicarboxylic acid component and 15 mole percent terephthalic acid was used (Comparative Example 6).

A commercially available biaxially stretched and post-heat treated polyethylene terephthalate film (comparative example 7) was used as a further comparative product.

These films were subjected to the further heat treatment at 200 ° C. for 200 or 400 hours under conditions which allowed free shrinkage. After this further heat treatment, the breaking strength, the breaking elongation and the dielectric breakdown resistance of the foils is determined. The results are shown in Table V, where values are also produced for the film from Example IS, which was obtained from the polymer which did not contain any terephthalate acid comonomers would.

Table V (A)

(»)) (nz) Tested foils before further heat treatment Stretching ratio CL) (B) OW (L) Fracture-
festijkert fracture
strain example 0.3- (7Y) 3.5 3.6 1.03 (kg / cm ») (V.) 0.68 1.497 ~ t ~ A »JD1 3.5 3.5 1.00 2300 76 15th 0.68 1,500 1.541 3.6 3.6 1.00 2300 75 13th 0.56 1.510 1.541 2210 73 14th 1.529 3.6 3.6 1.00 Comparative 0.56 1.506 1700 80 example 1.529 - - Comparison - - 2210 126 example 7 -

13th

200 C, 200 hours Table V (B) Heat treatment Elongation at break Break strength wedge After another ("/") example (kg / cm-) with free shrinkage 21 1680 29 *) 1840 *) 23 *) 15th 1660 *) 4th 13th 810 2 14th 790 Comparative example 6 Comparative example 7 Elongation at break (": ,,) 200 C. 400 hours of free shrinkage 53 Breaking strength 56 *) (kg 'cm' I 53 *) 1 590 7th «90 *) 5 770 *) 620 5K0

*) The treatment process "ar 220 ° C

Examples 16-18
and comparative examples 8 to 10

Unstretched Holte from Polyäthylcn-2.6-naphthalat with an intrinsic molar viscosity (//) of 0.55 in the longitudinal and transverse directions under the conditions listed in Table VI using the method of biaxially stretched and postheated under tension at 225 ° C. for 10 seconds. I :? wutdc obtained a biaxially oriented film. The he The film was subjected to the thermal aging treatment in the relaxed state in air subjected at 200 C. The mechanical tensile strength of the film longitudinally and transversely direction were determined before and after the deterioration * treatment. The results are in Table V. shown.

Table Vl

Stretch conditions Stretch B direction Stretch L-direction! ver Tem ver example Tem ratio pera ratio pera 3.5 door 3.8 door 3.2 <C) 3.5 (C) 3.7 135 3.9 16 130 3.5 130 1.4 17th 130 138 18th 135 3.8 125 3.6 Comparative 140 example 8 2.8 135 3.6 Comparison 135 example 9 135 Comparison 130 example 10

B-Slreck ratio l.-Slreck ratio.

1.08

1.09

1.05

0.4

0.95

1.28

to you) Rich tion 1.504 L. B. 1,508 L. B. 1,502 L. B. 1,510 L. B. 1,504 L. B. 1,507 L. B.

Mechanical tensile strength fasteners

Before thermal sealing
Break strength ^{Iru: h} -I ₄ C I d hn mg

(kg / cm ² ). ("'")

2310
2400
2280
2330
2390
2410
2320
1980
2420
2380
2250
2420

75
70
83
75
74
73
74
115
71
80
95
72

After the thermal deterioration

Elongation at break-break

strength

(kg / cm ² )

1460 1500 1480 1510 1520 1550 1470 940 1480 1100 1050 1530

45 48 31 30 42 45 41 5

Claims (2)

Sponsorship claims: 1. Process for the production of a biaxially oriented and heat-treated, electrically insulating film made of polyethylene-2,6-naphthalate, characterized in that an unstretched polyethylene ^ o-naphthalate film, consisting essentially of polyethylene-2,6-naphthalate in which at least 90 mole percent of the structural units are ethylene-2,6-napthalate units, at a stretching ratio either in the width direction or in the length direction stretched in the range from about 3 to about 4 biaxially, the stretching ratio in the Width direction at least 1 times the stretching ratio in the lengthwise direction, the stretching in the widthwise direction is 115 to 150C and the stretching in the longitudinal direction is carried out at 120 to 160 ° C, and then the biaxially stretched film a post heat treatment at 170 to 240 C under such conditions subject, at which the film length is kept constant and no shrinkage above 10 ", ', is allowed. 2. The method according to claim 1, characterized in that the draw ratio in the width direction 1 to 1.2 times the stretching ratio in the longitudinal direction.