1. Compositions which are heat-curable to give elastomers with improved resistance to hydrolytic degradation and are based on organopolysiloxanes, characterised in that they contain the following constituents a) an organopolysiloxane polymer having a viscosity of 1 000 000 to 200 000 000 mPas at 25 degrees C and comprising the structural unit (R)a SiO4-a /2 wherein R represents a monovalent hydrocarbon radical which can optionally be cyano- or halogen-substituted, and a is between 1.95 and 2.01, b) polycarbodiimide-polysiloxane copolymers or polycarbodiimidepolysiloxane mixtures, it being possible for the polycarbodiimide content t to be from 0.1 to 70% by weight - relative to total polymer, c) curing catalysts and d) fillers.
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