Fundamentals of Filler Mixing in Rubber

Fundamental understanding of the chemical and physical factors that govern filler dispersion represents a crucial step for process optimization and control of final material properties. Dispersion occurs when the cohesive forces that hold the filler particles together are overcome by the hydrodynamic and drag forces that tend to pull particles apart. The hydrodynamic dispersion of fine-particle agglomerates into smaller fragments is affected by both the physical and chemical nature of the particle-fluid system as well as processing conditions.  Particle characteristics, agglomerate packing structure, and the presence of interstitial liquids primarily affect the cohesive strength of the agglomerate, whereas the flow geometry and intensity determine the hydrodynamic forces that act to disrupt the agglomerate.  Fluid infiltration into agglomerates affects both agglomerate cohesivity and the distribution of hydrodynamic stress acting on and within the agglomerate.  Traditionally, dispersion has been controlled by optimizing the factors that directly affect the counterbalance of cohesive and hydrodynamic forces. Filler surface treatment has been used either to augment or reduce the cohesivity of particle clusters. A new approach to control fine particle dispersion is through variables which are not directly related to the solid or dispersing fluid properties. This is achieved through the use of chemical systems that respond to external stimuli. By adding a responsive material to the surface of a particle or within the void space between particles, it is possible to adjust the degree of interparticle interaction by changing the variable to which the additive is responsive.