Water Absorption of Elastomers

Abstract

Abstract This study considers high temperature (160° C) water absorption in the absence of oxygen. Examination of the freezing behavior of water-swollen butyl gum vulcanizates shows the water to be disposed as discrete droplets of the order of 3µ diameter, corresponding in this case with soluble residues based on zinc. Data are shown for peroxide gum vulcanizates of natural rubber, SBR, NBR, ethylene-propylene rubber, and cis-1,4 poly butadiene. Equilibrium absorption occurs, the degree being dependent on polymer impurities and state of cure. Model systems based on cis-polybutadiene show the effects of typical emulsion polymerization residues and the nature of the equilibrium balance between osmotic pressure and the retractive pressure exerted by the rubber. Classes of absorption behavior, whether low equilibrium, moderate to high equilibrium, or nonequilibrium, are discussed in terms of osmotic and rubber pressures. The temperature dependence of absorption rate is shown for the various polymers for the range 25° to 92° C. Five decades are required to accomodate the data, ranging from low rates for butyl rubber to high rates for NBR and natural rubber. Curing systems are compared, with cis-polybutadiene and butyl rubber as base polymers. Common classes of fillers and reinforcing agents are compared, particular consideration being given to the hydrophylic calcium silicate and silica fillers, which appear to facilitate leaching of electrolytes when present in sufficient loading. From the combined experience, illustrative water-resistant formulations are derived.