Creep behavior of biaxial cold‐rolled polypropylene

Polypropylene was biaxially rolled up to 60% at ambient temperature, and the tensile creep behavior over the temperature range 27 to 60°C was investigated using a dead‐load apparatus. The degrees of crystallinity of the as‐molded and rolled PP were determined using a differential scanning calorimeter (DSC) and density bottle. The DSC showed a slight change in the crystallinity during the early stage of the rolling process, while the density bottle indicated a continuous drop of the density with increasing rolling reduction. The elongation due to rolling was found almost fully recoverable when the samples were thrown into hot silicon oil at 180°C. The effects of cold rolling on creep strain, secondary creep strain rate, and creep activation energy were investigated. Cold rolling led to an increase in the creep strain and secondary creep strain rate. The creep activation energy was found to increase with increasing rolling reduction. Within the secondary creep stage, the creep process in polypropylene is mainly due to the α‐relaxation process and most of the creep strain was recoverable. © 1995 John Wiley & Sons, Inc.