Abstract: Adding silica particles having specified collective characteristics to a fluororubber formulation enhances direct adhesion between the fluororubber formulation and a copolymer upon curing. Specifically, as measured before being mixed into the fluororubber formulation, the silica particles have an average value of the product “(particle size)×(circularity)” that is 17.5 nm or higher and 500 ?m or lower. The silica particles may also have an average particle size of between 25.0 nm inclusive and 500 ?m inclusive, and an average circularity of 0.80 or higher. Alternatively, as measured after being incorporated into the fluororubber formulation and then cured, the silica particles have an apparent average value of the product “(particle size)×(circularity)” that is 32.0 nm or higher and 500 ?m or lower. The silica particles may also have an apparent average particle size of between 40.0 nm inclusive and 500 ?m inclusive as measured after being incorporated into the fluororubber formulation and then cured.

Abstract: Adding silica particles having specified collective characteristics to a fluororubber formulation enhances direct adhesion between the fluororubber formulation and a copolymer upon curing. Specifically, as measured before being mixed into the fluororubber formulation, the silica particles have an average value of the product “(particle size)×(circularity)” that is 17.5 nm or higher and 500 ?m or lower. The silica particles may also have an average particle size of between 25.0 nm inclusive and 500 ?m inclusive, and an average circularity of 0.80 or higher. Alternatively, as measured after being incorporated into the fluororubber formulation and then cured, the silica particles have an apparent average value of the product “(particle size)×(circularity)” that is 32.0 nm or higher and 500 ?m or lower. The silica particles may also have an apparent average particle size of between 40.0 nm inclusive and 500 ?m inclusive as measured after being incorporated into the fluororubber formulation and then cured.