New boron nitride processing aids for the extrusion of molten polymer

It is already known that the use of Boron Nitride (BN) in the extrusion of molten polymers may eliminate surface melt fracture and postpone the critical shear rate for the onset of gross melt fracture to significantly higher values depending on resin type and additive content. In this work several new Boron Nitride powders (Carborandum Co.) are tested that exhibit superior behavior from the previous studied ones. Critical parameters for Us unique behavior (elimination of gross melt fracture) are good dispersion, small average particle size and free of agglomeration. Ile equipment used for the testing of the new powders include both an Instron capillary rheometer with special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer, and a parallel-plate rheometer. A metallocene polyethylene with all types of boron nitride is tested at various additive concentrations. The additive having the finest particle size and that is free of agglomeration was found to have the greatest influence on the gross melt fracture performance of the polymer tested in crosshead dies and tips. Moreover, one of the additives was found to enhance melt slippage and as a result relieved internal shear stresses. This action is believed to eliminate surface melt fracture and postpone the critical shear rate for the onset of gross melt fracture to significantly higher values depending on the additive concentration. The influence of the boron nitride type and its concentration on the polymer rheology is also discussed.