Patent ID: 8373196

Claim:
A composition for encapsulating an optical semiconductor, comprising: (A) 100 parts by mass of an organopolysiloxane component composed of a component (A1) and a component (A2) described below, wherein a mass ratio of component (A1)/component (A2) is within a range from 50/50 to 95/5, the component (A) contains 0.005 to 0.05 mols of alkenyl groups per 100 g, and the component (A) contains not more than 0.01 mols of silanol groups per 100 g, (A1) an alkenyl group-containing linear organopolysiloxane represented by a formula (1) shown below and having a viscosity at 25° C. of 10 to 1,000,000 mPa·s: wherein R 1 represents an alkenyl group, each R 2 independently represents an unsubstituted or substituted monovalent hydrocarbon group containing no aliphatic unsaturated bonds, x represents an integer of 0 or greater that yields a viscosity at 25° C. of 10 to 1,000,000 mPa·s, and each y independently represents an integer of 1 to 3, (A2) an organopolysiloxane resin comprising at least one SiO 2 unit and an alkenyl group within each molecule, (B) an organohydrogenpolysiloxane component composed of a component (B1) and a component (B2) described below, wherein a molar ratio of [SiH groups within component (B1)/SiH groups within component (B2)] is within a range from 50/50 to 90/10, and an amount of the organohydrogenpolysiloxane component yields a molar ratio of [SiH groups within component (B)/alkenyl groups within component (A)] within a range from 0.9 to 4, (B1) a linear organohydrogenpolysiloxane represented by a formula (2) shown below and having two or more SiH groups per molecule: wherein R 4 , R 5 and R 6 each independently represents a monovalent hydrocarbon group of 1 to 10 carbon atoms, p and q each represents an integer of 0 to 100, r and s each independently represent an integer of 0 to 3, provided that p+q is an integer of 3 or greater, and p+r+s is an integer of 2 or greater, (B2) a branched organohydrogenpolysiloxane that is liquid at 25° C., (C) a platinum group metal catalyst in an effective amount, and (D) a linear or cyclic organopolysiloxane of 4 to 50 silicon atoms, having at least two functional groups selected from the group consisting of silicon atom-bonded alkenyl groups, alkoxysilyl groups and epoxy groups: in an amount within a range from 0.01 to 10 parts by mass per 100 parts by mass of the component (A).