A study of the long-term crystallization process in polypropylene and acetal

Over the years there has been a great deal of discussion regarding the length of time required for certain semicrystalline materials to achieve full potential crystallinity. Many suppliers of flexible chain crystalline materials have studied the phenomenon of post mold shrinkage. In addition, many suppliers of these materials recommend an extended conditioning period for molded test specimens before conducting physical property tests. It is supposed that it can take two to three weeks for properties to fully develop. Yet even within supplier organizations there seems to be no consensus on the necessity for this practice and little documentation exists to support either a long-term or a short-term approach to sample conditioning. This study uses tensile stress-strain properties and dynamic mechanical analysis to track the property development of test specimens molded in polypropylene and acetal. Unfilled homopolymers and copolymers were used in both material families.