Dispersive Mixing in Rubber and Plastics

Abstract

Abstract Most useful plastics and rubber products are mixtures of pure polymers or copolymers with a variety of additives such as pigments, stabilizers, antioxidants, flame retardants, crosslinking agents, fillers, reinforcing agents, plasticizers, foaming agents, etc. Moreover, much attention is being placed recently on the preparation of blends of polymers, leading to products of unique properties. The distribution of additives into the polymeric matrix and the detailed topological structure of the blends depend on the nature of the mixing mechanisms, hence they are intimately related to the mixing equipment configuration and mixing operating conditions. It is convenient to distinguish between two mixing mechanisms extensive and intensive or dispersive. The former, in the very viscous polymeric systems, is achieved by and large by convection. It may either be distributive or laminar. Distributive mixing may involve an ordered or random rearrangement process. Laminar mixing is achieved by imposing on the material permanent deformation in various laminar flow patterns (e.g., shearing, squeezing, or elongational flows). The objective of extensive mixing is to bring about composition uniformity throughout the mixture. For extensive mixing, large strains have to be imposed upon the liquid. This requirement, however, must be complemented by the equally important requirement of distribution of interfacial elements (or solid additives) throughout the system. Initial orientation and spatial location may be critical in laminar mixing. Dispersive mixing generally involves rupture of agglomerates formed by a solid phase and separation of closely packed particles after rupture. This is followed by the distribution of the separated particles throughout the polymeric matrix, which of course, is an extensive mixing step. The most investigated dispersive mixing operation is that of carbon black into rubber. The mechanical properties of the rubber are directly affected by carbon black dispersion. In this review, dispersive mixing mechanism and equipment are reviewed. Although the emphasis is on rubber-carbon black systems, the discussion is relevant to dispersive mixing operations of other solid fillers in rubber and plastics as well as to dispersion of one liquid polymeric phase in another (homogenization) as in blending of polymers. First, mixing equipment and its hydrodynamic analysis are reviewed. Next, the chemical and physical characteristics of some of the solid additives, specifically carbon black, are discussed. The dispersion of the solid particles takes place in various deformation fields. Hence, the behavior of single particles in flow field, of doublets and swarms of particles is relevant, as well as the interaction between particle and matrix. Finally, various postulated dispersive mixing mechanisms, consequent scaleup techniques, and the quantitative characterization of the mixture are discussed in some detail.