Boron nitride as a processing aid for the extrusion of polyolefins and fluoropolymers

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.