TRANSVERSE ORIENTATION DIES FOR SOLID-STATE EXTRUSION OF POLYMERS .2. ORIENTATION AND PROPERTIES OF EXTRUDATES

Biaxial orientation in polymer products is normally brought about by the application of planar tensile stresses to sheets or films in either the "plastic" or "rubbery" state. More recently, other techniques have been explored as, for instance, the superimposition of rotational components in planes perpendicular to compressive forces. The present work is concerned with the solid-state extrusion of very high molecular mass crystalline polymers, such as poly(tetra)fluoroethylene (PTFE) and ultrahigh-molecular-weight polyethylene (UHMWPE), using dies featuring converging and diverging walls perpendicular to each other and with a cross-section area at the entry being the same as at the exit. Measurements of birefringence and tensile strength on solid section extrudates have shown that dies with small converging angles, known as fish-tail dies, produce a uniaxial type of orientation along the transverse direction, while dies with large converging angles at the entry region give rise to an unbalanced biaxial orientation.