The mechanism of orientation of poly(ethylene terephthalate) film in two stages

The mechanism of uniaxial orientation of PETP film in two stages is discussed. It is shown that drawing in two stages enables stress relaxation to occur during the intermediate, “rest” period. The stress on overloaded chains is thus reduced and the nature of the dependence of stress on draw ratio is altered in comparison with single stage orientation. In multi-stage orientation the mechanical field is more evenly distributed over the segments of the macromolecules and it thus becomes possible to increase the draw ratio. Stress relaxation occurs most extensively in the region of low. draw ratios, less than 2·5 which is the “pre-orientation period”.