Solid-state forming of polypropylene

Solid-state forming processes can induce a permanent deformation of the internal structure, namely, the conversion of an initially isotropic and spherulitic structure to a fibrillar structure. The fibrils are made of oriented and extended molecular chains which ensure mechanical connection between crystals and load transfer. It can be realized that, for maximum mechanical performance, all polymeric chains should be 

Mechanical calculations for the extrusion process showed that PP can prematurely leave contact with the die wall, particularly in decreasing strain rate dies at high drawing speeds. Increased friction of the die wall and increased drawing speed cause this loss of contact point to move 

The response of spherulites to a deformation appeared to result from a number of mechanisms including slip, tilt (rotation in the plane of orientation) and twinning of the crystalline lamellae. Besides this complexity, which is inherent in all spherulitic polymers, the plastic deformation of PP is affected by the early occurrence of a necking process. Ultrahigh molecular weight PP has also been studied and its colddrawing orientational behavior was observed to be very different from that of conventional PP. 

The drawing behavior of several syndiotactic polypropylenes (sPP) was also examined as a function of molecular weight and initial morphology. It was found that these materials can be drawn to a much lesser extent than comparable isotactic pol5q3ropylene. This limited drawability was attributed to the absence of an effective crystalline c-shp process. 

Oriented PP multidirectional (OPP) laminates (with (0 2, (0/90)s and (0/ 45)3 lay-ups) with improved properties in more than one direction in the plane obtained using a hot-plate welding technique were also 

---