Millable silicone rubber composition, millable silicone rubber sponge, and method for producing said sponge

A millable silicone rubber sponge with an open cell ratio of not lower than 20%, a cell aspect ratio of 1.0 to 1.3 and an average cell diameter of not larger than 250 μm, the millable silicone rubber sponge being a heat-treated product of a silicone rubber composition containing:

• (A) 100 parts by mass of an organopolysiloxane having at least two alkenyl groups in one molecule and a polymerization degree of not lower than 3,000, the organopolysiloxane being represented by the following average composition formula (I)
• R1aSiO4-a/2 (I)
• wherein R represents an identical or different monovalent hydrocarbon group, and a represents a positive number of 1.95 to 2.04;
• (B) a reinforcing silica in an amount of 10 to 100 parts by mass per 100 parts by mass of the component (A);
• (C) thermally expandable microcapsules exhibiting an expansion starting temperature of 90 to 150° C., and contracting from a maximum expansion volume by 20% or more when heated at 200° C. for 5 min, the thermally expandable microcapsules being in an amount of 0.1 to 20 parts by mass per 100 parts by mass of the component (A);
• (D) a curing agent which is an organic peroxide in an amount of 0.01 to 20 parts by mass per 100 parts by mass of the component (A);
• and
• (E) an open cell-forming agent which is a solid high temperature decomposition-type organic foaming agent having a decomposition starting temperature of not lower than 180° C., and starting to decompose after the component (A) is cured, the open cell-forming agent being in an amount of 0.1 to 30 parts by mass per 100 parts by mass of the component (A).