Process for making silicone rubber base compositions

A continuous method for the preparation of a silicone rubber base composition, the composition comprising:

• (A) one or more polyorganosiloxanes containing at least two unsaturated groups per molecule selected from alkenyl groups and alkynyl groups, and
• (B′) a hydrophobically treated reinforcing silica filler;
• said method comprising the steps of:
• (a) introducing (C) at least one hydrophobing treating agent, and component (A), and optionally (D) water, into a first static mixer (25) to form a step (a) mixture and then introducing the step (a) mixture on to a first twin-screw extruder (4);
• (b) introducing (B) reinforcing silica filler into the step (a) mixture via a reinforcing silica filler (B) entry port (27) in the first twin-screw extruder (4) while maintaining the temperature in a range of between 20 to 80° C. to form a viscous paste and providing at least one vent (5, 8) to the atmosphere up and/or downstream of the reinforcing silica filler (B) entry port (27) to allow gases present to escape;
• (c) mixing the viscous paste resulting from step (b) in a dispersive mixing and kneading zone in the first twin-screw extruder (4) to form a silica dispersion;
• (d) further mixing the silica dispersion produced in step (c) in a residence zone (17) downstream of the first twin-screw extruder (4) to provide an unstripped silicone rubber base composition;
• (e) stripping the unstripped silicone rubber base composition with vacuum stripping to provide a silicone rubber base composition at a temperature of at least 100° C.;
• and
• (f) introducing component (C) and optionally one or both of components (A) and (D) either into the first twin-screw extruder (4) between steps (c) and (d) and/or during step (d) to dilute and further hydrophobically treat silica from the silica dispersion of step (c) and subsequently form a diluted silica dispersion.