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Principles of Mixing with Filled Polymers
The underlying science behind polymer compounding operations depends not only on the chemical and physical nature of the materials requiring combination but also on the application of specially designed process technology to effect the optimum composition and microstructure required. In the case of introducing fillers into thermoplastics, as mentioned earlier, there are many different factors and situations which can arise, each posing their own challenges and often requiring tailored solutions.
Of central importance is the need to disperse and distribute the additive without thermal or mechanical damage to either the filler or polymer. The energy required to achieve this depends on the extent of particle agglomeration; the interparticle strength of crystalline aggregates, which generally form the substructure of agglomerates; and the way in which these aggregates combine together to produce the agglomerate. The magnitude of agglomeration forces between powders results from surface interactions and may involve electrostatic, van der Waals, or liquid-bridge forces, of which the latter can be up to four times larger than electrostatic forces due to the presence of moisture. In practice with many mineral fillers, this reinforces the need to ensure that powders are fully dry before compounding. Adhesion forces between particles may also depend on their contact geometry and surface roughness and, in some cases, their ability to undergo plastic deformation. When subjected to powder compaction, rearrangement of the particles may occur, increasing their packing efficiency and hence interparticle adhesion forces. Such a situation can occur on the rotor blades of a high-speed mixer used to pre-blend powders of filler with polymer before melt compounding and during the early stages of extrusion compounding, where powdered filler is compacted prior to, or during, polymer melting.
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