The cold drawing of high polymers

Abstract

Like many other polymers, amorphous polyethyleneterephthalate cold draws in the shoulders of a neck when stretched at room temperature. The draw ratio across the shoulders is not a constant for the polymer, but is found to depend on the temperature and speed of stretching, and on the initial specimen cross-section and birefringence. A local temperature rise is also observed in the shoulders. Isothermal load-extension curves have therefore been measured from 20 to 140°C, and from them an adiabatic load-extension curve has been calculated. This has a negative slope range which appears to be unstable. An alternative process of extension at constant tension in a shoulder, coupled with exchanges of heat along the specimen by conduction, appears to be possible at a lower tension than that needed for pure adiabatic extension. For this second process the length of the shoulder, the tension at which it operates, and the draw ratio across it, can be calculated from the simple adiabatic curve and are found to agree with experimental data within a factor of two. This theory is then used to discuss cold drawing in some other polymers.