Preparation of compounded rubber composition from bubble-containing ethylene/.alpha.-olefin rubber

A process for preparing a compounded rubber composition, which comprises (I) preparing a non-vulcanized bubble-containing ethylene/.alpha.-olefin copolymer rubber by the steps of

• (A) heating an ethylene/.alpha.-olefin copolymer rubber obtained by polymerizing:
• (1) 50-90 mol % of ethylene,
• (2) 5-50 mol % of .alpha.-olefin having at least 3 carbon atoms;
• and
• (3) 0-10 mol % of non-conjugated polyene, to at least the softening temperature of the rubber,
• (B) kneading the softened rubber while blowing a gaseous substance thereinto;
• and
• (C) extruding the gaseous substance-containing rubber, followed by cooling;
• wherein said rubber contains from 5 to 1000 parts by volume, based on 100 parts by volume of said ethylene/.alpha.olefin copolymer rubber of step (A), of bubbles and providing a water absorption volume as measured in accordance with ASTM D 1506 of less than 20% by volume of the total volume of bubbles, and (II) kneading the resulting ethylene/.alpha.-olefin copolymer rubber with a compounding agent selected from reinforcing materials, fillers, softeners, vulcanizing accelerators and vulcanizers, said bubbles being dissipated in said compound rubber.