Modulus Enhancement and Orientation In Die-Drawn PP-Talc Composites

Solid-phase drawing of heated polymer composites through a heated converging die leads to debonding of filler particles and produces expanded or voided composites with a high level of molecular orientation in the matrix polymer.  The expanded and oriented composite has potential applications in the construction industry. It is well known that die-drawing of unfilled semi-crystalline isotactic polypropylene at elevated temperatures can produce 10-20 fold increase in the modulus along with a very high degree of crystalline c-axis orientation along the machine direction. The objective of this work was to study the extent of molecular orientation and modulus enhancement achieved at various draw rates in die-drawing of polypropylene (PP)-talc composite.

Billets were extruded from PP-talc composite containing 20 wt% (7 vol %) talc in a 2.4 MFR isotactic PP. These billets were drawn at 145⁰C through a heated converging die with an inlet to outlet area ratio of 2 at draw rates ranging from 1m/min to 6m/min. Extruded billets of the corresponding neat PP matrix were also drawn for comparison. The actual or final draw ratio or the ratio of final velocity to inlet velocity is typically much greater than this area ratio and reached values of 8 to 9 in our drawing experiments. The orientation distribution of crystal plane normals (040) and (110) in the drawn specimens was characterized by pole figures from X-Ray diffractometry with WIMV corrections and inverse pole figures were obtained for orientation of the equivalent orthogonal crystal axes a*, b (the long dimension) and c, the chain axis. The b-axis or the long crystal dimension appears to be much more strongly oriented along the normal direction in the case of the drawn composite than in the case of neat PP where the b-axis was more distributed in the plane of the transverse and normal directions, normal to the machine direction.   A Herman orientation parameter f_cM for the c-axis with respect to the machine direction was evaluated from this data; this parameter leveled off at an intermediate draw ratio along with the void fraction or density of the expanded composite. The tensile modulus of the drawn composite was increasing over the entire range of draw rates and draw ratios to over three times the modulus of the undrawn matrix. These trends point to other microstructural features that continue to evolve with increasing draw rate.