Optically clear silicone compositions curable to elastomers

A method for producing an optically clear composition curable to an elastomer which comprises the steps of

• (I) mixing
• (A) 100 parts by weight of at least one polydiorganosiloxane consisting essentially of siloxane units selected from the group consisting of siloxane units of the unit formula ##STR16## wherein each R radical is selected from the group consisting of alkyl radicals of from 1 to 6 inclusive carbon atoms, cyclohexyl radicals, phenyl radicals, halogenated alkyl radicals of from 1 to 10 inclusive carbon atoms and alkenyl radicals of from 2 to 6 inclusive carbon atoms, each X being selected from the group consisting of hydroxyl radicals, hydrogen radicals and alkoxy radicals of from 1 to 6 inclusive carbon atoms, at least 50 percent of the total amount of R radicals and ##STR17## radicals present in said polydiorganosiloxane being methyl radicals, a having a value of from 0 to 1 inclusive, b having a value of from 1 to 3 inclusive and c having a value of from 0 to 1 inclusive, the sum of a+b having a value of from 1 to 3 inclusive, the sum of a+c having a value of from 0 to 1 inclusive, the values of a, b and c being such that the ratio of total R radicals and ##STR18## radicals to total silicon atoms present in said polydiorganosiloxane is in the range of from 1.98/1 to 2.02/1 inclusive, said polydiorganosiloxane having a viscosity of at least 0.1 pascal.seconds at 25.degree. C., and
• (B) a sufficient amount of a wet, gelled filler composition to provide from 15 to 120 parts by weight per 100 parts by weight of (A) of a hydrophobic reinforcing silica filler consisting essentially of surface-treated silica particles wherein said particles consist essentially of SiO.sub.4/2 units containing a sufficient amount of organosiloxy units chemically bonded to the surface of said particles to render the silica filler hydrophobic, said organosiloxy units being selected from the group consisting of R.sub.3 SiO.sub.1/2 units, R.sub.2 SiO units, O.sub.1/2 R.sub.2 SiO(R.sub.2 SiO).sub.d SiR.sub.2 O.sub.1/2 units, XR.sub.2 SiO(R.sub.2 SiO).sub.d SiR.sub.2 O.sub.1/2 units and mixtures thereof where each R and each X are as above defined and d has an average value of from 1 to 12 inclusive, wherein substantially all of said particles have an aggregate particle size of no greater than 4,000 Angstroms in their largest dimension and are of an overall particle size distribution which is sufficiently small such that when a sufficient amount of said wet, gelled filler composition to provide 60 parts by weight of said filler is homogeneously mixed with 100 cubic centimeters at 23.degree..+-.2.degree. C. of a polydiorganosiloxane of the type described in (A) to form a test blend, wherein (1) the refractive index (at 25.degree. C., sodium D line) of the polydiorganosiloxane chosen for use in said blend differs from the refractive index (at 25.degree. C., sodium D line) of the filler by at least 0.025 units and (2) the organosiloxy units employed to render said filler hydrophobic are primarily the same as the R radicals and the ##STR19## radicals present in the polydiorganosiloxane chosen for use in said blend, then the test blend possesses a haze value of less than 4% per 2.54 millimeter thickness of said blend at 23.degree..+-.2.degree. C. according to the procedure set out in ASTM D1003-61, to form a mixture, and
• (II) removing the volatile portion of said mixture whereby an optically clear composition curable to an elastomer is obtained, and
• wherein, when the volatile portion has been removed, the mixture of (A) and (B) is an optically clear composition possessing a luminous transmittance value of at least 85% and a haze value of no greater than 4% per 2.54 millimeter thickness of said composition at 23.degree..+-.2.degree. C. according to the procedure set out in ASTM D1003-61.

A method for producing an optically clear composition curable to an elastomer which comprises the steps of

• (I) mixing
• (A) 100 parts by weight of at least one polydiorganosiloxane consisting essentially of siloxane units selected from the group consisting of siloxane units of the unit formula ##STR20## wherein each R radical is selected from the group consisting of alkyl radicals of from 1 to 6 inclusive carbon atoms, cyclohexyl radicals, phenyl radicals, halogenated alkyl radicals of from 1 to 10 inclusive carbon atoms and alkenyl radicals of from 2 to 6 inclusive carbon atoms, each X being selected from the group consisting of hydroxyl radicals, hydrogen radicals and alkoxy radicals of from 1 to 6 inclusive carbon atoms, at least 50 percent of the total amount of R radicals and ##STR21## radicals present in said polydiorganosiloxane being methyl radicals, a having a value of from 0 to 1 inclusive, b having a value of from 1 to 3 inclusive and c having a value of from 0 to 1 inclusive, the sum of a+b having a value of from 1 to 3 inclusive, the sum of a+c having a value of from 0 to 1 inclusive, the values of a, b and c being such that the ratio of total R radicals and ##STR22## radicals to total silicon atoms present in said polydiorganosiloxane is in the range of from 1.98/1 to 2.02/1 inclusive, said polydiorganosiloxane having a viscosity of at least 0.1 pascal.seconds at 25.degree. C., and
• (B) a sufficient amount of a wet, fluid filler composition to provide from 15 to 120 parts by weight per 100 parts by weight of (A) of a hydrophobic reinforcing silica filler consisting essentially of surface-treated silica particles wherein said particles consist essentially of SiO.sub.4/2 units containing a sufficient amount of organosiloxy units chemically bonded to the surface of said particles to render the silica filler hydrophobic, said organosiloxy units being selected from the group consisting of R.sub.3 SiO.sub.1/2 units, R.sub.2 SiO units, O.sub.1/2 R.sub.2 SiO(R.sub.2 SiO).sub.d SiR.sub.2 O.sub.1/2 units, XR.sub.2 SiO(R.sub.2 SiO).sub.d SiR.sub.2 O.sub.1/2 units and mixtures thereof where each R and each X are as above and d has an average value of from 1 to 12 inclusive, wherein substantially all of said particles have an aggregate particle size of no greater than 4,000 Angstroms in their largest dimension and are of an overall particle size distribution which is sufficiently small such that when a sufficient amount of said wet, fluid filler composition to provide 60 parts by weight of said filler is homogeneously mixed with 100 cubic centimeters at 23.degree..+-.2.degree. C. of a polydiorganosiloxane of the type described in (A) to form a test blend, wherein (1) the refractive index (at 25.degree. C., sodium D line) of the polydiorganosiloxane chosen differs from the refractive index (at 25.degree. C., sodium D line) of the filler by at least 0.025 units and (2) the organosiloxy units employed to render said filler hydrophobic are primarily the same as the R radicals and the ##STR23## radicals present in the polydiorganosiloxane chosen for use in said test blend, then the test blend possesses a haze value of less than 4% per 2.54 millimeter thickness of said blend at 23.degree..+-.2.degree. C. according to the procedure set out in ASTM D1003-61, to form a mixture, and
• (II) removing the volatile portion of said mixture whereby an optically clear composition curable to an elastomer is obtained, and
• wherein, when the volatile portion has been removed, the mixture of (A) and (B) is an optically clear composition possessing a luminous transmittance value of at least 85% and a haze value of no greater than 4% per 2.54 millimeter thickness of said composition at 23.degree..+-.2.degree. C. according to the procedure set out in ASTM D1003-61.