Filler Reinforcement of Liquid Elastomers

To realize the potential processing benefits of liquid rubbers, methods must be developed for producing acceptable reinforced compounds which retain the inherent rheological advantages of the uncured polymer. The most promising commercially available liquid rubbers are functionally terminated low molecular weight analogs of common general-purpose elastomers such as poly-(butadiene) or butadiene copolymers. In principle, these low molecular weight polymers can be vulcanized by a combination of chain extension and cross-linking to yield vulcanizates comparable to those from solid elastomers. Both cure reactions can be carried out simultaneously with a chemically specific polyfunctional coupling agent, or alternatively, chain extension with a difunctional reactant can be followed by use of accelerated sulfur or equivalent crosslinking agents. In contrast to certain polar polymers, liquid hydrocarbon rubbers yield relatively weak vulcanizates which are unsuitable for mechanical and tire applications. Physical properties are greatly improved by addition of up to 60 phr of reinforcing fillers such as carbon black or precipitated silica, but at the expense of flow behavior. Important filler-dependent properties of representative liquid rubbers will be described, with emphasis on the rheology of reinforced compounds.