Addition curing silicone rubber composition and its cured product

1. An addition curing silicone rubber composition comprising:

(A) 100 parts by weight of a polyorganosiloxane containing, on average, at least two alkenyl groups which are bonded to silicon atom, which is represented by the following average compositional formula (1):

R 1 a SiO (4-a)/2   (1)

wherein R 1 is independently a monovalent moiety selected from the group consisting of alkyl, aryl, aralkyl, alkenyl and trifluoropropyl, and a is a positive number of 1.5 to 2.8;

(B) 0.3 to 20 parts by weight of an organohydrogenpolysiloxane having 11 to 300 silicon atoms per molecule and containing at least two hydrogen atoms which are bonded to silicon atom (SiH functional groups) per molecule, which is represented by the following average compositional formula (3):

R 2 b H c SiO (4-b-c)/2   (3)

wherein R 2 is a monovalent moiety selected from the group consisting of alkyl, aryl, aralkyl and trifluoropropyl, b is a positive number of 0.7 to 2.1, and c is a positive number of 0.001 to 1.0, with the proviso that b+c is 0.8 to 3.0; and

(C) a catalytic amount of an addition reaction catalyst;

wherein ratio of the SiH functional group in the component (B) to the alkenyl group bonded to the silicon atom in the component (A) (SiH/Alkenyl group) is in the range of 0.6 to 3.0 and

ratio of the total number of hydrogen atoms which are bonded to the silicon atom to the total number of carbon atoms in the component (B) ([hydrogen atom]/[carbon atom]) is less than 0.6, and content of the polyorganosiloxane having a degree of polymerization of up to 10 and containing at least one SiH functional group per molecule is up to 0.11% by weight in relation to the entire composition,

wherein the polyorganosiloxane having a degree of polymerization of up to 10 and containing at least one SiH functional group is a cyclic polyorganosiloxane derived from the organohydrogenpolysiloxane (B).

2. The addition curing silicone rubber composition according to claim 1 wherein the polyorganosiloxane having a degree of polymerization of up to 10 and containing at least one SiH functional group per molecule is an organohydrogen siloxane-diorganosiloxane cyclic copolymer.

3. The addition curing silicone rubber composition according to claim 2 wherein the polyorganosiloxane having a degree of polymerization of up to 10 and containing at least one SiH functional group per molecule is a cyclic organohydrogenpolysiloxane represented by the following formula (5):

wherein m is an integer of 1 to 9 and n is an integer of 1 to 9 with the proviso that n+m is an integer of 3 to 10; R is independently an optionally substituted monovalent hydrocarbon group containing 1 to 10 carbon atoms.

4. The addition curing silicone rubber composition according to claim 3 wherein, in the formula (5), m is an integer of 1 to 3 and n is an integer of 1 to 9 with the proviso that n+m is an integer of 3 to 10.

5. The addition curing silicone rubber composition according to claim 1 wherein component (A) comprises up to 0.5% by weight of a polyorganosiloxane having a degree of polymerization of up to 10 and containing no SiH functional group.

6. The addition curing silicone rubber composition according to claim 5 wherein the polyorganosiloxane having a degree of polymerization of up to 10 and containing no SiH functional group is a cyclic polyorganosiloxane represented by the following formula (2):

wherein x is an integer of 3 to 10, and R is independently an optionally substituted monovalent hydrocarbon group containing 1 to 10 carbon atoms.

7. The addition curing silicone rubber composition according to claim 1 wherein the component (B) is an organohydrogenpolysiloxane having a straight chain, cyclic, branched, or three dimensional network structure with a degree of polymerization of at least 11.

8. The addition curing silicone rubber composition according to claim 7 wherein the component (B) is an organohydrogenpolysiloxane represented by the following formula (4):

wherein y is an integer of 1 to 98 and z is an integer of 2 to 50 with the proviso y+z is 9 to 100, R 2 is independently an optionally substituted monovalent hydrocarbon group containing 1 to 10 carbon atoms, and R 3 is R 2 or hydrogen atom.

9. The addition curing silicone rubber composition according to claim 1 wherein component (B) comprises up to 10.0% by weight of the polyorganosiloxane having a degree of polymerization of up to 10 and containing at least one SiH functional group per molecule.

10. A cured silicone rubber produced by a method comprising curing an addition curing silicone rubber composition comprising:

(A) 100 parts by weight of a polyorganosiloxane containing, on average, at least two alkenyl groups which are bonded to silicon atom, which is represented by the following average compositional formula (1):

R 1 a SiO (4-a)/2   (1)

wherein R 1 is independently a monovalent moiety selected from the group consisting of alkyl, aryl, aralkyl, alkenyl and trifluoropropyl, and a is a positive number of 1.5 to 2.8;

(B) 0.3 to 20 parts by weight of an organohydrogenpolysiloxane having 11 to 300 silicon atoms per molecule and containing at least two hydrogen atoms which are bonded to silicon atom (SiH functional groups) per molecule, which is represented by the following average compositional formula (3):

R 2 b H c SiO (4-b-c)/2   (3)

wherein R 2 is a monovalent moiety selected from the group consisting of alkyl, aryl, aralkyl and trifluoropropyl, b is a positive number of 0.7 to 2.1, and c is a positive number of 0.001 to 1.0, with the proviso that b+c is 0.8 to 3.0; and

(C) a catalytic amount of an addition reaction catalyst;

wherein ratio of the SiH functional group in the component (B) to the alkenyl group bonded to the silicon atom in the component (A) (SiH/Alkenyl group) is in the range of 0.6 to 3.0 and

ratio of the total number of hydrogen atoms which are bonded to the silicon atom to the total number of carbon atoms in the component (B) ([hydrogen atom]/[carbon atom]) is less than 0.6, and content of the polyorganosiloxane having a degree of polymerization of up to 10 and containing at least one SiH functional group per molecule is up to 0.11% by weight in relation to the entire composition,

wherein the polyorganosiloxane having a degree of polymerization of up to 10 and containing at least one SiH functional group is a cyclic polyorganosiloxane derived from the organohydrogenpolysiloxane (B).

11. The cured silicone rubber according to claim 10 , wherein the component (B) comprises up to 10.0% by weight of the polyorganosiloxane having a degree of polymerization of up to 10 and containing at least one SiH functional group per molecule.

12. The cured silicone rubber according to claim 10 , wherein the method includes, prior to the curing, the steps of reducing a low-molecular weight siloxane of the component (B).

13. The cured silicone rubber according to claim 12 , wherein the steps of reducing the low molecular weight siloxane of the component (B) includes:

heating the component (B) at 180° C. for 6 hours at a reduced pressure of about 100 Pa with stirring to remove the low-molecular weight siloxane component;

bubbling nitrogen gas through the component (B) for 4 hours to remove the low-molecular weight siloxane component; and

allowing the component (B) to flow over a hot plate heated to a temperature of 200° C. at a reduced pressure of up to 20 Pa in the form of a thin film having at a thickness of up to 100 μm.