Patent ID: 6875534

Claim:
A seal member for sealing a periphery of at least one side of a polymer electrolyte fuel-cell separator, comprising a rubber composition in a cured state, said rubber composition comprising: (A) a polyisobutylene polymer having an allyl radical on at least one molecular chain-end, (B) an organopolysiloxane of the average compositional formula (1): R 1 n SiO (4-n)/2 (1) wherein R 1 is a substituted or unsubstituted monovalent hydrocarbon radical and may be the same or different and n is a positive number of 1.9 to 2.1, a weight ratio of components (A) to (B) being between 100:0 and 5:95, (C) an organohydrogenpolysiloxane having at least two hydrogen atoms each attached to a silicon atom per molecule, in such an amount as to give 0.2 to 10 silicon-attached hydrogen atoms per aliphatic unsaturated bond in components (A) and (B), said organohydrogenpolysiloxane having an average compositional formula (4) R 4 a H b SiO (4-a-b)/2 (4) wherein R 4 is a substituted or unsubstituted monovalent hydrocarbon atom, a is a positive number of 0.7 to 2.2, b is a positive number of 0.5 to 1, and a+b is from 1.2 to 3, and (D) a catalytic amount of a platinum group metal catalyst, wherein the rubber composition in a cured state has a weight change of up to 3.4% when immersing it in a 10% aqueous sulfuric acid solution at 100° C. for 500 hours.