Sealing agent for optical semiconductor devices, and optical semiconductor device

An encapsulant for optical semiconductor devices, which comprises:

• a first organopolysiloxane having an alkenyl group and a methyl group bonded to a silicon atom;
• a second organopolysiloxane having a hydrogen atom bonded to a silicon atom and a methyl group bonded to a silicon atom;
• and
• a catalyst for hydrosilylation reaction, wherein
• the first organopolysiloxane is a first organopolysiloxane represented by the following formula (1A) and the second organopolysiloxane is a second organopolysiloxane represented by the following formula (51A), or the first organopolysiloxane is a first organopolysiloxane represented by the following formula (1B) and the second organopolysiloxane is a second organopolysiloxane represented by the following formula (51B), and
• the content ratios of methyl groups bonded to silicon atoms in the first organopolysiloxane and the second organopolysiloxane each are 80 mol % or more.
• [Formula 1]
• (R1R2R3SiO1/2)a(R4R5SiO2/2)b(R6SiO3/2)c Formula (1A)
• In the formula (1A), a, b and c satisfy a/(a+b+c)=0 to 0.30, b/(a+b+c)=0.70 to 1.0 and c/(a+b+c)=0 to 0.10, at least one of R1 to R6 represents an alkenyl group, at least one of R1 to R6 represents a methyl group, and R1 to R6 other than an alkenyl group and a methyl group represent a hydrocarbon group having 2 to 8 carbon atoms.
• [Formula 2]
• (R51R52R53SiO1/2)p(R54R55SiO2/2)q(R56SiO3/2)r Formula (51A)
• In the formula (51A), p, q and r satisfy p/(p+q+r)=0.10 to 0.50, q/(p+q+r)=0 to 0.40 and r/(p+q+r)=0.40 to 0.90, at least one of R51 to R56 represents a hydrogen atom, at least one of R51 to R56 represents a methyl group, and R51 to R56 other than a hydrogen atom and a methyl group represent a hydrocarbon group having 2 to 8 carbon atoms.
• [Formula 3]
• (R1R2R3SiO1/2)a(R4R5SiO2/2)b(R6SiO3/2)c Formula (1B)
• In the formula (1B), a, b and c satisfy a/(a+b+c)=0.10 to 0.50, b/(a+b+c)=0 to 0.40 and c/(a+b+c)=0.40 to 0.90, at least one of R1 to R6 represents an alkenyl group, at least one of R1 to R6 represents a methyl group, and R1 to R6 other than an alkenyl group and a methyl group represent a hydrocarbon group having 2 to 8 carbon atoms.
• [Formula 4]
• (R51R52R53SiO1/2)p(R54R55SiO2/2)q(R56SiO3/2)r Formula (51B)
• In the formula (51B), p, q and r satisfy p/(p+q+r)=0 to 0.30, q/(p+q+r)=0.70 to 1.0 and r/(p+q+r)=0 to 0.10, at least one of R51 to R56 represents a hydrogen atom, at least one of R51 to R56 represents a methyl group, and R51 to R56 other than a hydrogen atom and a methyl group represent a hydrocarbon group having 2 to 8 carbon atoms.