Product and process of producing biaxially oriented insulating film of polyethylene-2,6-naphthalate

Claim 2

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 215*C. to 240*C. 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 direction is within a range of from about 3 to about 4 times based on the original length of the unstretched film, with the proviso that the stretch ratio in the widthwise direction to the stretch ratio in the lengthwise direction is from 1 to 4/3; (ii) the elongation at breakage of the resulting film is not less than 15 % when said biaxially oriented and post-heat-treated film is subjected to further heat treatment at 200*C. for 200 hours under such conditions as will allow free shrinkage of the film; and (iii) the thickness-direction refractive index (nz) of said biaxially oriented and post-heat-treated film, measured at 20*C. by Abbe''s refractometer with use of a ray of a wavelength of 589 m Mu , is within a range expressed by the following formula: 1.499 < or = (nz) < or = 1.520 and the intrinsic viscosity ( eta ) of said film, measured at 35*C. in a mixed solvent of phenol: 2,4,6-trichlorophenol at a weight ratio of 6:4, is within a range expressed by the following formula: 0.48 < or = ( eta ) < or = 0.70 with a proviso that above (nz) and ( eta ) satisfy the following condition: (nz) 0.3 X ( eta ) + 1.361

Claim 3

A biaxially oriented and post-heat-treated 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 %, and said thickness-direction refractive index (nz) is within a range expressed by the following formula: 1.500 < or = (nz) < or = 1.515

Claim 4

A PROCESS FOR THE PREPARATION OF A BIAXIALLY ORIENTED AND POST-HEAT-TREATED POLYETHYLENE-2,6-NAPHTHALATE INSULATING FILM COMPOSITION, WHICH COMPRISES 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-2,6NAPHTHALATE UNITS, AND AT MOST 10 MOL PERCENT OF THE STRUCTURAL UNITS ARE UNITS DERIVED FROM A COMONOMER OR COMONOMERS HAVING A DIVALENT, ESTER-FORMING 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 IN THE LENGTHWISE DIRECTION IS WITHIN A RANGE OF FROM ABOUT 3 TO 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 115* - 150*C. AND THE STRETCHING IN THE LENGTHWISE DIRECTION BEING CONDUCTED AT 120* - 160*C., AND THEN SUBJECTING THE BIAXIALLY STRETCHED FILM TO THE POST-HEAT-TREATMENT AS 215* - 240*C 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 200*C. FOR 200 HOURS UNDER SUCH CONDITIONS AS WILL ALLOW FREE SHRINKAGE OF THE FILM; AND THE THICKNESS-DIRECTION REFRACTIVE INDEX (NZ) OF SAID BIAXIALLY ORIENTED AND POST-HEATED-TREATED FILM MEASURED AT 20*C. BY ABBE''S REFRACTOMETER, WITH USE OF A RAY OF A WAVELENGTH OF 589 MU, IS WITHIN A RANGE EXPRESSED BY THE FOLLOWING FORMULA:

Claim 5

The process of claim 3 wherein the stretch ratio in the widthwise direction is from 1 to 1.2 times the stretch ratio in the lengthwise direction.

Claim 6

The process of claim 3 wherein said post-heat-treatment is conducted under such conditions which keep the film length constant.