Film having fine voids and manufacture thereof

Abstract

A polyester film having fine voids, which comprises a polymer mixture comprising a polyester resin and a thermoplastic resin (B) incompatible with said polyester resin, and which has an apparent specific gravity of 0.6-1.3 and an inplane birefringence as determined from the equation: inplane birefringence=refractive index along lengthwise principal axis-refractive index along crosswise principal axis, of from -0.02 to +0.04, and manufacture thereof. A thermoplastic resin film having fine voids, which comprises a polymer mixture comprising a thermoplastic resin (A) and a thermoplastic resin (B) incompatible with said thermoplastic resin (A), and which has a ratio (L1/L2) of an average length (L1) of the voids at a cross section in the lengthwise direction of the film, to an average length (L2) of the voids at a cross section in the direction forming a right angle with said lengthwise direction, i.e., crosswise direction of the film, of 2/3-3/2, a thickness variation of the film of not more than 5%, and a void content of 5-50% by volume, and manufacture thereof. The polyester film having fine voids of the present invention has an extremely superior isotropy, excellent tear resistance, extremely superior water resistance, moisture absorption, dimensional stability, surface stability, gloss and vividness of printed matters, and mechanical strength, which have been particularly achieved by making apparent specific gravity and inplane birefringence fall within a specific range mentioned above. The thermoplastic resin film having fine voids of the present invention suffers less from curling by heat, which has been achieved by making (L1/L2), thickness variation and void content fall within a specific range mentioned above.

Dependencies are inferred from the claim text.