Heat-setting process for polyester film

Claim 1

In a process for production of an asymmetrically oriented, heat-set polyethylene terephthalale film, oriented more in the machine direction than in the transverse direction, the film having a machine direction tensile strength at 5% elongation (F5) of at least 20,000 psi (14.1 kg/mm 2 ) and a machine direction shrinkage of not more than 2% at 105° C., by stretching the film in the transverse direction and in the machine direction, heat setting the biaxially oriented film in successive stages by contacting it in a first stage with a heat set roll at 150° to 210° C. and in a second stage with a roll at least 5° C. below the temperature of the first stage heat setting roll, but above the second order transition temperature, and thereafter contacting the film with a roll to quench it to below the second order transition temperature, the improvement comprising: relaxing the film in the machine direction up to 2% between the first and second stage heat setting; heat setting the film in a third stage following quenching by contacting it with a heat setting roll at the same temperature to 5° C. below the temperature of the second stage heat setting roll; then quenching the film to less than 40° C. after the third stage heat setting by contacting with a second quench roll; and relaxing the film in the machine direction from 0.5 to 6.5% between the third heat set roll and the second quench roll; whereby a film having less than 0.7% shrinkage at 105° C. in the transverse direction and a reduced tear tendency is produced without significant loss of machine direction tensile strength at 5% elongation (F5) or increase in machine direction shrinkage.