Boron nitride powders: New processing aids for molten polymers

The effect of a new processing additive (boron nitride powder) on the rheology and processability of molten polymers was studied. The equipment used included both an Instron capillary rheometer equipped with a special annular die (Nokia Maillefer wire coating cross-head), and a parallel-plate rheometer. Metallocene polyethylenes with several types of boron nitrides (BN) varying in particle size and morphology were tested at various concentration levels. The BN additive with the finest particle size, and thus best dispersed finely and uniformly into the polymer matrix, provided the greatest improvement in the melt fracture performance of polymers. This BN powder eliminates surface melt fracture and postpones the critical shear rate for the onset of gross melt fracture to significantly higher values with an optimum additive concentration. It was also demonstrated that boron nitride is a superior processing aid compared to conventional fluoropolymer ones. Finally, it was found that a combination of a BN powder with a small amount of a fluoropolymer improves even fur ther the processability of molten polymers (melt fracture performance).