Abstract A biaxially oriented polyethylene 2,6-naphthalate film having superior physical properties in the machine direction such as tensile strength and Young''s modulus, particularly useful as a base of magnetic recording tape, is prepared by stretching an undrawn film of polyethylene 2,6-naphthalate consisting essentially of ethylene 2,6-naphthalate units and having an intrinsic viscosity of at least 0.35, first in the machine direction to 3.5-5 times the original length at a temperature ranging from 10*C. above the second order transition temperature of said polyethylene 2,6-naphthalate to 170*C., and then in the transverse direction at a draw ratio 50-90 percent of that of the machine direction stretching at a temperature ranging from 3*C. above said second order transition temperature to 160*C. and satisfying the following relationship
in the transverse direction at a draw ratio 50-90 percent of that at which the film was stretched in the machine direction, at a temperature not higher than the temperature of stretching in the machine direction ranging between 30*C. above said second order transition temperature and 160*C., said temperature further satisfying the following relationship 25X-770n+1200 < or = T < or = 22X-400n+700 wherein T is the drawing temperature (* C.) when stretching the film in the transverse direction, X is the draw ratio when stretching the film in the transverse direction, and n is the refractive index in the thickness direction of the film after stretching in the machine direction.
Claim 2
Dependent Claim
46 words
The method of claim 1 wherein said undrawn film is stretched at a draw ratio of 4-5 to its original length in the machine direction and thereafter stretched at a draw ratio of 60-70 percent of that at which it was stretched in the machine direction.
Claim 3
Dependent Claim
32 words
stretched in the transverse direction at a temperature in the range of 120*-140*C. at a draw ratio of 60-70 percent of that at which the film was stretched in the machine direction.
Claim 4
Dependent Claim
10 words
The method of claim 1 wherein said undrawn film is
Claim 5
Dependent Claim
34 words
The method of claim 1 wherein the film is heat treated at a temperature in the range of 170*-250*C. for a period of 2 seconds to 5 minutes after stretching in the transverse direction.