Curable resin composition

1. A curable resin composition comprising:

(A) a straight-chain organopolysiloxane having at least two silicon-bonded hydrogen atoms and at least one aryl group in one molecule and having an average degree of polymerization of greater than 10;

(B) a branched-chain organopolysiloxane having at least three alkenyl groups and at least one aryl group in one molecule; and

(C) a hydrosilylation reaction catalyst;

wherein a proportion of diphenylsiloxane units relative to an amount of all siloxane units is 10 mol % or greater, and the all siloxane units containing siloxane units derived from the straight-chain organopolysiloxane (A) and the branched-chain organopolysiloxane (B);

the curable resin composition further comprising a straight-chain organopolysiloxane (G) represented by formula (6) below:

HR 2 2 SiO(R 2 2 SiO) k SiR 2 2 H  (6)

wherein, each R 2 moiety is independently an alkyl group, aryl group, hydroxy group, or alkoxy group, and at least one of the R 2 moieties is an aryl group; and

k is a positive number of 1 to 5.

2. The curable resin composition according to claim 1 , wherein a content of a cyclic organosiloxane having a molecular weight of 150 to 1,200 is 7.5 mass % or less.

3. The curable resin composition according to claim 1 , wherein the average degree of polymerization of the straight-chain organopolysiloxane (A) is greater than 30.

4. The curable resin composition according to claim 1 , wherein the branched-chain organopolysiloxane (B) is an organopolysiloxane represented by average unit formula (4) below:

(R 3 SiO 3/2 ) a (R 3 2 SiO 2/2 ) b (R 3 3 SiO 1/2 ) c (SiO 4/2 ) d (X 1 O 1/2 ) e   (4)

wherein, each R 3 moiety is independently a substituted or unsubstituted monovalent hydrocarbon group; however, in one molecule, at least three of the R 3 moieties are alkenyl groups, and at least one R 3 moiety is an aryl group;

X 1 is a hydrogen atom or alkyl group;

a is a positive number; b is 0 or a positive number; c is 0 or a positive number; d is 0 or a positive number; e is 0 or a positive number; b/a is a number in a range of 0 to 10; c/a is a number in a range of 0 to 0.50; d/(a+b+c+d) is a number in a range of 0 to 0.3; and e/(a+b+c+d) is a number in a range of 0 to 0.4.

5. The curable resin composition according to claim 1 , wherein the composition is a composition for sealing an optical semiconductor element.

6. The curable resin composition according to claim 2 , wherein the average degree of polymerization of the straight-chain organopolysiloxane (A) is greater than 30.

7. The curable resin composition according to claim 2 , wherein the branched-chain organopolysiloxane (B) is an organopolysiloxane represented by average unit formula (4) below:

(R 3 SiO 3/2 ) a (R 3 2 SiO 2/2 ) b (R 3 3 SiO 1/2 ) c (SiO 4/2 ) d (X 1 O 1/2 ) e   (4)

wherein, each R 3 moiety is independently a substituted or unsubstituted monovalent hydrocarbon group; however, in one molecule, at least three of the R 3 moieties are alkenyl groups, and at least one R 3 moiety is an aryl group;

X 1 is a hydrogen atom or alkyl group;

a is a positive number; b is 0 or a positive number; c is 0 or a positive number; d is 0 or a positive number; e is 0 or a positive number; b/a is a number in a range of 0 to 10; c/a is a number in a range of 0 to 0.50; d/(a+b+c+d) is a number in a range of 0 to 0.3; and e/(a+b +c+d) is a number in a range of 0 to 0.4.

8. The curable resin composition according to claim 3 , wherein the branched-chain organopolysiloxane (B) is an organopolysiloxane represented by average unit formula (4) below:

(R 3 SiO 3/2 ) a (R 3 2 SiO 2/2 ) b (R 3 3 SiO 1/2 ) c (SiO 4/2 ) d (X 1 O 1/2 ) e   (4)

wherein, each R 3 moiety is independently a substituted or unsubstituted monovalent hydrocarbon group; however, in one molecule, at least three of the R 3 moieties are alkenyl groups, and at least one R 3 moiety is an aryl group;

X 1 is a hydrogen atom or alkyl group;

a is a positive number; b is 0 or a positive number; c is 0 or a positive number; d is 0 or a positive number; e is 0 or a positive number; b/a is a number in a range of 0 to 10; c/a is a number in a range of 0 to 0.50; d/(a+b+c+d) is a number in a range of 0 to 0.3; and e/(a+b+c+d) is a number in a range of 0 to 0.4.