Development of dispersion in the mixing of calcium carbonate into polymer blends in an internal mixer

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Abstract
An experimental study of agglomeration tendency and the compounding of calcium carbonates of varying particle size into polypropylene and polystyrene melts in an internal mixer and in a two roll mill is described. The tendency of particles to agglomerate was studied using the sedimentation volume method. Agglomeration increased with decreasing particle size. It was sharply reduced by coating with stearic acid. Agglomerate size distributions in thermoplastic matrices were measured using optical microscopy. The greatest problems in dispersing fillers were with the smallest particles. One unexpected result with the polystyrene compounds was that the number of large agglomerates of the smallest particles actually increased with time. This was associated with uncoated small CaCO3 particles clinging to the chamber walls of the internal mixer and being only gradually incorporated. The phenomenon was less pronounced in the polypropylene compounds. Generally large agglomerates were broken down with increased mixing. Stearic acid coating reduced the number of large agglomerates.