Room temperature curable organopolysiloxane composition and gas separation membrane

1. A room temperature curable organopolysiloxane composition comprising

(I) 100 parts by weight of an organopolysiloxane which is a condensation product of 80 to 20 parts by weight of the following component (A) and 20 to 80 parts by weight of the following component (B) (with the proviso that total of the components (A) and (B) are 100 parts by weight):

(A) an organopolysiloxane comprising R 3 SiO 1/2 unit (wherein R independently represents a substituted or unsubstituted monovalent hydrocarbon group containing 1 to 6 carbon atoms) and SiO 4/2 unit, with the R 3 SiO 1/2 unit being present at 0.6 to 1.2 mol per mole of the SiO 4/2 unit; optionally further comprising R 2 SiO 2/2 unit and RSiO 3/2 unit (wherein R is as defined above) respectively at 0 to 1.0 mol per mole of the SiO 4/2 unit, and containing 0.02 to 0.12 mol/100 g of hydroxy group bonded to the silicon atom, and

(B) a diorganopolysiloxane raw rubber having hydroxy group on opposite ends thereof and having a degree of polymerization of at least 5,000,

(II) 10 to 50 parts by weight of an organosilane compound having at least 2 hydrolyzable groups bonded to the silicon atom on average per molecule, and/or its partial hydrolytic condensate,

(III) 500 to 1,500 parts by weight of a solvent, and

(IV) 10 to 150 parts by weight of silica nano particles having their surface modified by a branch structure.

2. A composition according to claim 1 wherein the component (B) is a straight chain diorganopolysiloxane represented by the following formula (1):

HO—(R 1 2 SiO) n —H  (1)

wherein R 1 is independently a substituted or unsubstituted monovalent hydrocarbon group containing 1 to 10 carbon atoms and n is an integer of at least 5,000.

3. A composition according to claim 1 wherein the component (II) is an organosilane compound having 2 hydrolyzable groups bonded to the silicon atom on average per molecule represented by the following formula (2):

R 2 a SiX 4-a   (2)

wherein R 2 is a substituted or unsubstituted monovalent hydrocarbon group, x is a hydrolyzable group, and a is 0, 1, or 2, and/or its partial hydrolytic condensate.

4. A composition according to claim 3 wherein R 2 in the component (II) is vinyl group.

5. A composition according to claim 1 wherein the component (IV) is the one produced by treating surface of a silica nano particle with a compound having amino group at its end and represented by the following formula (3):

(wherein R 5 represents methyl group or ethyl group, R 6 represents an alkylene group containing 1 to 5 carbon atoms and optionally having an intervening —NH— group) to modify the silica nano particle surface with the amino group, and then reacting a hyperbranched monomer represented by the following formula (4):

(wherein R 7 represents a trivalent saturated aliphatic group or an aromatic group) with the amino group-modified silica nano particles.

6. A composition according to claim 1 further comprising

(V) 0.1 to 5 parts by weight of a condensation catalyst.

7. A composition according to claim 6 wherein the condensation catalyst (V) is an aminosilane coupling agent represented by the following formula (5):

H 2 N—(CH 2 ) b —SiR 8 c (OR 9 ) 3-c   (5)

wherein R 8 is a monovalent hydrocarbon group containing 1 to 6 carbon atoms, R 9 is methyl group or ethyl group, b is an integer of 1 to 3, and c is 0 or 1.

8. A gas separation membrane produced by curing the composition of claim 1 .