Curable resin composition

1. A curable resin composition containing:

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

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

a hydrosilylation reaction catalyst (C); and

a straight-chain organopolysiloxane (G) represented by the following formula (16)

HR 2 2 SiO(R 2 2 SiO) k SiR 2 2 H  (16)

In formula (16), each R 2 moiety is independently an alkyl group, an aryl group, a hydroxy group (—OH), or an alkoxy group, and at least one of the R 2 moieties is an aryl group;

in formula (16), k is a positive number from 1 to 5.

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

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

(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)

in formula (4), each R 3 moiety is independently a substituted or unsubstituted monovalent hydrocarbon group, however in each molecule, at least three of the R 3 moieties are alkenyl groups, and at least one R 3 is an aryl group;

in formula (4), X 1 is a hydrogen atom or an alkyl group;

in formula (4), 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 within a range of from 0 to 10; c/a is a number within a range of from 0 to 5; d/(a+b+c+d) is a number within a range of from 0 to 0.3; and e/(a+b+c+d) is a number within a range of from 0 to 0.4.

4. The curable resin composition according to claim 1 , further containing a low-viscosity organopolysiloxane (D) having a viscosity of at most 50,000 mPa·s at 25° C.

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 branched-chain organopolysiloxane (B) is an organopolysiloxane represented by the following average unit formula (4)

(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)

in formula (4), each R 3 moiety is independently a substituted or unsubstituted monovalent hydrocarbon group, however in each molecule, at least three of the R 3 moieties are alkenyl groups, and at least one R 3 is an aryl group;

in formula (4), X 1 is a hydrogen atom or an alkyl group;

in formula (4), 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 within a range of from 0 to 10; c/a is a number within a range of from 0 to 5; d/(a+b+c+d) is a number within a range of from 0 to 0.3; and e/(a+b+c+d) is a number within a range of from 0 to 0.4.

7. The curable resin composition according to claim 2 , further containing a low-viscosity organopolysiloxane (D) having a viscosity of at most 50,000 mPa·s at 25° C.

8. The curable resin composition according to claim 3 , further containing a low-viscosity organopolysiloxane (D) having a viscosity of at most 50,000 mPa·s at 25° C.

9. The curable resin composition according to claim 6 , further containing a low-viscosity organopolysiloxane (D) having a viscosity of at most 50,000 mPa·s at 25° C.