The influence of a new processing additive (fine particles of boron nitride
) on the processability of polyolefins and fluoropolymers in extrusion is s
tudied. The equipment used includes an Instron capillary rheometer with two
types of dies, namely capillary dies and special annular dies (Nokia Maill
efer wire coating crosshead) attached to the rheometer, and an extruder. Tw
o metallocene polyethylenes and several Teflon(R) fluoropolymers were teste
d using these two pieces of equipment. The additive had a significant effec
t on the extrudate appearance of polyethylene and fluoropolymer particularl
y in the crosshead dies. It was found to eliminate surface melt fracture an
d to postpone the critical shear rate for the onset of gross melt fracture
to significantly higher values depending on resin type, temperature, and ad
ditive concentration (typically 0.005% to 0.5%). To explain the possible me
chanism for the effect of the additive on the processability of the resins,
rheological measurements using both parallel-plate and sliding-plate rheom
eters were carried out. The rheology of the resins did not seem to change s
ignificantly with the addition of boron nitride except for the low-shear-ra
te (low-frequency) range, where the behavior of the filled resin was found
to be similar to that of a crosslinked polymer or a phase-separated entangl
ed blend. Practical wire coating and tubing extrusion studies for these res
ins were also carried out.