Separator and separator seal for polymer electrolyte fuel cells

A separator seal providing electric insulation of a volume resistivity of at least 1.0 TΩ·m for use in polymer electrolyte fuel cells which is formed of a sealing composition in the cured state, said sealing composition comprising:

• (A) 100 parts by weight of an organopolysiloxane containing at least two alkenyl groups each attached to a silicon atom in a molecule, said organopolysiloxane having the following average compositional formula (I):
• R1aSiO(4-a)/2 (I)
• wherein R1 which may be the same or different is a monovalent hydrocarbon group selected from the groups consisting of alkyl groups, aryl groups, aralkyl groups, alkenyl groups, and substituted forms of the foregoing groups in which some or all hydrogen atoms are replaced by halogen atoms or cyano groups, and “a” is a positive number in the range 1.5 to 2.8,
• (B) 5 to 50 parts by weight of a resinous copolymer composed mainly of R3SiO1/2 units and SiO2 units in a molar ratio between 0.5:1 and 1.5:1, wherein R is a substituted or unsubstituted monovalent hydrocarbon group, said resinous copolymer containing 5×10−3 to 1×10−4 mol/g of vinyl groups and having a weight average molecular weight of 1000 to 10,000,
• (C) 0.5 to 20 parts by weight of an organohydrogenpolysiloxane containing at least three hydrogen atoms each attached to a silicon atom in a molecule,
• (D) 10 to 30 parts by weight of fumed silica having a BET specific surface area of 50 to 400 m2/g,
• (E) 0.8 to 1.9 parts by weight of carbon powder comprising carbon black having a BET specific surface area of 39 to 65 m2/g, an iodine adsorption value of 52 to 88 mg/g, and a DBP absorption value of 100 to 200 ml/100 g, and
• (F) 0.5 to 1,000 ppm of an addition reaction catalyst as platinum group metal based on the weight of component (A).