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The effect of boron nitride on the rheology and processability of molten polymers Yip, Franky Kam Yin


Experiments were carried out using capillary, sliding plate, and parallel plate rheometer and a blow moulding machine in order to assess the effect of boron nitride (BN) powders on the rheology and processability of a metallocene polyethylene, a Teflon FEP polymer and a polypropylene. In particular, to assess the processability of these resins in the presence of BN or not, the critical conditions for the onset of melt fracture is determined. The influence of the average size of boron nitride particles, its agglomeration if present, its concentration in the resins and the quality of the attained dispersion on the melt fracture of the metallocene polyethylene was particularly investigated. For this a number of boron nitride powders were examined having various characteristics, i.e. agglomeration, different particle size etc. It was found that for BN to be efficient in eliminating surface and gross melt fracture, the particle size should be about 10-20 microns in size, and free of agglomeration. Concentration was also found to be a critical factor, with the optimum value to fall in the range between 200 and lOOOppm. The mechanism by which BN acts to eliminate melt fracture was also investigated. Six different capillary dies with various entrance angles were used to study the entrance effect. A new apparatus was also designed during the course of this study in order to visualize the flow patterns developed at the entrance of the capillary. It was found that BN changes the entrance flow pattern of a PP by eliminating the onset of unstable flow. This is done by providing the condition for a smoother transfer of momentum from a polymer layer to the next. The effect of BN type on two blow molding grade high-density polyethylene was also studied by using a blow moulding machine. It was found that BN can eliminate the surface melt fracture of blow moulding grade HDPE depending on the properties of resins.

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