(706c) 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.