Foam Extrusion of Elastomers using Water as Physical Blowing Agent

Abstract

An alternative to chemical blowing agents is the physical blowing agent water which can be adsorbed by fillers and thus easily incorporated into rubber compounds. The phase transition of the water during heating of the rubber in the vulcanisation unit is used to initiate the blowing reaction. In order to achieve the required quality parameters (e.g. mechanics and cell structure), the cross-linking and blowing reaction have to be balanced. This can be achieved by optimising the compound recipe and the processing. This paper examines the influence of the extrusion and vulcanisation process on the foam structure and the mechanical properties of water-foamed rubber compounds based on ethylene propylene diene terpolymer rubber (EPDM) and nitrile butadiene rubber (NBR). In the extrusion process, the barrel temperature and screw speed are systematically varied and the foamed profiles are evaluated with regard to surface, foam structure, tension set and an optimal process point was identified. For the subsequent vulcanisation process, the effect of the hot air temperature on the tension set, the spring constant and characteristic values of the cell structure are analysed.