Repeated Cold Drawing and Annealing of Polypropylene

Abstract

Samples of polypropylene were cold drawn to a maximum extension of five times their original length and then annealed, under no load, at 160°C. Eighty percent of the extension was recovered on annealing. Samples were then redrawn and annealed again repeating the procedure through five drawing cycles. While the first drawing proceeded by neck formation and propagation, subsequent drawings were homogeneous. One hundred percent of the extension in the second and subsequent cold drawings was recovered on annealing. In general the polypropylene became more rubbery with each subsequent cold drawing in the sense that less of the drawing extension was retained after removing the drawing load and the modulus of drawn material decreased while its elastic extensibility increased. Micrographs of sections cut from the polypropylene showed a spherulite structure which was retained throughout the drawing and annealing. After several cycles a distinct Maltese-cross pattern was seen when viewing the sections between crossed polars. Such a pattern is not generally seen with polypropylene spherulites.