Optically clear silicone compositions curable to elastomers

• US 4,418,165 A
• Filed: 10/07/1981
• Issued: 11/29/1983
• Est. Priority Date: 06/03/1980
• Status: Expired due to Term

• Current Assignee: Dow Corning Corporation (Dow, Inc.)
• Original Assignee: Dow Corning Corporation (Dow, Inc.)
• Inventors: Polmanteer, Keith E., Chapman, Harry L.
• Primary Examiner(s): BRIGGS SR, WILBERT
• Application Number:
  US06/309,302
• Time in Patent Office: 783 Days
• Field of Search: 523/107, 523/209, 523/212, 523/210
• US Class Current: 523/210
• CPC Class Codes: C01P 2004/03 obtained by SEM
• CPC Class Codes: C01P 2004/32 Spheres
• CPC Class Codes: C01P 2004/50 Agglomerated particles
• CPC Class Codes: C01P 2004/51 Particles with a specific particle size distribution
• CPC Class Codes: C01P 2004/60 Particles characterised by their size
• CPC Class Codes: C01P 2004/62 Submicrometer sized, i.e. from 0.1-1 micrometer
• CPC Class Codes: C01P 2006/10 Solid density
• CPC Class Codes: C01P 2006/12 Surface area
• CPC Class Codes: C01P 2006/14 Pore volume
• CPC Class Codes: C01P 2006/60 Optical properties, e.g. expressed in CIELAB-values
• CPC Class Codes: C01P 2006/90 Other properties not specified above
• CPC Class Codes: C08G 77/02 Polysilicates
• CPC Class Codes: C08G 77/12 containing silicon bound to hydrogen C08G77/045 takes precedence
• CPC Class Codes: C08G 77/16 to hydroxyl groups
• CPC Class Codes: C08G 77/18 to alkoxy or aryloxy groups
• CPC Class Codes: C08G 77/20 containing silicon bound to unsaturated aliphatic groups C08G77/045 takes precedence
• CPC Class Codes: C08G 77/24 halogen-containing groups
• CPC Class Codes: C08G 77/70 Siloxanes defined by use of the MDTQ nomenclature
• CPC Class Codes: C08L 2666/54 Inorganic substances
• CPC Class Codes: C08L 83/04 Polysiloxanes
• CPC Class Codes: C09C 1/3081 : Treatment with organo-silicon compounds
• CPC Class Codes: G02B 1/04 : made of organic materials, e.g. plastics G02B1/08 takes precedence

Abstract

Optically clear silicone compositions comprising 100 parts by weight of at least one polydiorganosiloxane and 15 to 120 parts by weight of a hydrophobic reinforcing silica filler wherein substantially all of the surface-treated silica filler particles are sufficiently small in their largest dimensions such that a uniform 2.54 mm thickness of the composition possesses a haze value of less than 4% and a luminous transmittance value of at least 85% according to ASTM D1003-61. The refractive index of the polydiorganosiloxane does not have to be matched with that of the silica filler to obtain optically clear compositions. The haze value of certain of such compositions does not increase by more than 1% per 2.54 mm thickness when the composition is heated from 23°±2° C. to 100°±5° C. Optionally, curing agents and other additives can be present in the compositions. Optically clear cured elastomers useful as lenses or tubing can be prepared from such elastomers. Methods for the preparation of such compositions are also claimed.

Claims

1. An optically clear composition curable to an elastomer which comprises:

(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 ##STR12## 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 ##STR13## 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 ##STR14## 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°

C., and

(B) 15 to 120 parts by weight of a hydrophobic reinforcing silica filler consisting essentially of surface-treated silica particles wherein said particles consist essentially of SiO_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 R₃ SiO_1/2 units, R₂ SiO units, O_1/2 R₂ SiO(R₂ SiO)_d SiR₂ O_1/2 units, XR₂ SiO(R₂ SiO)_d SiR₂ O_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 60 parts by weight of said filler is homogeneously mixed with 100 cubic centimeters at 23°

±

2°

C. of a polydiorganosiloxane of the type described in (A) to form a test blend, wherein (1) the refractive index (at 25°

C., sodium D line) of the polydiorganosiloxane chosen for use in said blend differs from the refractive index (at 25°

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 ##STR15## 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°

±

2°

C. according to the procedure set out in ASTM D1003-61;

wherein 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°

±

2°

C. according to the procedure set out in ASTM D1003-61.

2. The composition as claimed in claim 1 wherein the haze value of a composition consisting of a mixture of 100 cubic centimeters of (A) at 23°

±

2°

C. and 60 grams of (B) does not increase by more than 1% per 2.54 millimeter thickness of said mixture upon heating the mixture from 23°

±

2°

C. to 100°

±

5°

C.

3. The composition as claimed in claims 1 or 2 wherein each R radical is selected from the group consisting of methyl radicals, vinyl radicals and 3,3,3-trifluoropropyl radicals and wherein no more than 5 mole percent of the siloxane units contain vinyl radicals, there being no more than one vinyl radical per siloxane unit, the mole percent of said siloxane units being based upon the total moles of siloxane units present in said polydiorganosiloxane.

4. The composition as claimed in claims 1 or 2 wherein the refractive index of (A) at 25°

C., Sodium D wavelength, differs from the refractive index of (B) by at least 0.025 units.

5. The composition as claimed in claims 1 or 2 wherein the amount of (B) present is in a range of from 40 to 90 parts by weight per 100 parts by weight of (A) and said organosiloxy units of (B) are derived from organosilicon hydrophobing compounds selected from the group consisting of R_n SiY_4-n, (R₃ Si)₂ NH, (R₃ Si)₂ O, (R₂ SiO)_e, (R₂ SiNH)_e, R'"'"'O(R₂ SiO)_d R'"'"', (R₃ Si)₂ NR", (R₂ SiNR")_e, and mixtures of the same where each R'"'"' is hydrogen or R", each R" is an alkyl radical of from 1 to 4 inclusive carbon atoms, each Y is --OH, OR'"'"', --NHR" or --NR"₂, e has an average value of from 3 to 6 inclusive, and n has an average value of from 2 to 3 inclusive.

6. The composition as claimed in claim 1 wherein said organosiloxy units of (B) are derived from organosilicon hydrophobing compounds selected from the group consisting of R_n SiY_4-n, (R₃ Si)₂ NH, (R₃ Si)₂ O, (R₂ SiO)_e, (R₂ SiNH)_e, R'"'"'O(R₂ SiO)_d R'"'"', (R₃ Si)₂ NR", (R₂ SiNR")_e, and mixtures of the same where each R'"'"' is hydrogen or R", each R" is an alkyl radical of from 1 to 4 inclusive carbon atoms, each Y is --OH, --OR'"'"', --NHR" or --NR"₂, e has an average value of from 3 to 6 inclusive, and n has an average value of from 2 to 3 inclusive and wherein (B) is a hydrophobic reinforcing silica filler obtained from the alkaline hydrolysis of Si(OCH₃)₄ in the presence of at least 70% of one-half of the stoichiometric amount of water required to completely hydrolyze the theoretical amount of -- OCH₃ radicals present in the Si(OCH₃)₄, at least one alcohol selected from the group consisting of methanol, ethanol, n-propanol and isopropanol, at least one organosilicon hydrophobing compound, and at least one basic catalyst.

7. The composition as claimed in claim 6 wherein the refractive index of (A) at 25°

C., sodium D wavelength, differs from the refractive index of (B) by at least 0.025 units.

8. The composition as claimed in claim 6 wherein each R radical is selected from the group consisting of methyl radicals, vinyl radicals and 3,3,3-trifluoropropyl radicals and wherein no more than 5 mole percent of the siloxane units contain vinyl radicals, there being no more than one vinyl radical per siloxane unit, the mole percent of said siloxane units being based upon the total moles of siloxane units present in said polydiorganosiloxane.

9. The composition as claimed in claim 5 wherein (B) is a hydrophobic reinforcing silica filler obtained from the alkaline hydrolysis of Si(OCH₃)₄ in the presence of at least 70% of one-half of the stoichiometric amount of water required to completely hydrolyze the theoretical amount of --OCH₃ radicals present in the Si(OCH₃)₄, at least one alcohol selected from the group consisting of methanol, ethanol, n-propanol and isopropanol, at least one basic catalyst, and at least one of said organosilicon hydrophobing compounds.

10. The composition as claimed in claim 9 wherein said organosilicon hydrophobing compound is at least one compound selected from the group consisting of {(CH₃)₃ Si}₂ NH, {(CH₃)₂ (CF₃ CH₂ CH₂)Si}₂ NH, {(CH₃)R"'"'"'SiO}_e, CH₃ O{(CH₃)R"'"'"'SiO}_d R"" and (CH₃)R"'"'"'Si(OR")₂ where R"'"'"' is a methyl radical, a 3,3,3-trifluoropropyl radical or a vinyl radical, each R"" is hydrogen or a methyl radical and the average value of e is 3.

11. The composition as claimed in claim 10 wherein the refractive index of (A) at 25°

C., sodium D wavelength, differs from the refractive index of (B) by at least 0.025 units.

12. The composition as claimed in claim 10 wherein each R radical is selected from the group consisting of methyl radicals, vinyl radicals and 3,3,3-trifluoropropyl radicals and wherein no more than 5 mole percent of the siloxane units contain vinyl radicals, there being no more than one vinyl radical per siloxane unit, the mole percent of said siloxane units being based upon the total moles of siloxane units present in said polydiorganosiloxane.

13. The composition as claimed in claim 9 wherein each R radical present in said polydiorganosiloxane is selected from the group consisting of methyl radicals and vinyl radicals, wherein no more than 5 mole percent of the siloxane units contain vinyl radicals, there being no more than one vinyl radical per siloxane unit, the mole percent of said siloxane units being based upon the total moles of siloxane units present in said polydiorganosiloxane and wherein said organosiloxy units of (B) consist primarily of organosiloxy units wherein each R radical is selected from the group consisting of methyl radicals and vinyl radicals.

14. The composition as claimed in claim 13 wherein said (A) is a polydiorganosiloxane gum having a viscosity of at least 10 kilopascal.seconds at 25°

C. and wherein at least two siloxane units present in said (A) contain vinyl radicals, any X radicals present in said (A) being hydroxyl radicals.

15. The composition as claimed in claim 9 wherein each R radical present in said polydiorganosiloxane is selected from the group consisting of methyl radicals, vinyl radicals and 3,3,3-trifluoropropyl radicals, wherein at least 7 mole percent of the siloxane units present in said polydiorganosiloxane contain 3,3,3-trifluoropropyl radicals and no more than 5 mole percent of the siloxane units present in said polydiorganosiloxane contain vinyl radicals, there being no more than one vinyl radical per siloxane unit and no more than one 3,3,3-trifluoropropyl radical per siloxane unit, the mole percent of said siloxane units being passed upon the total moles of siloxane units present in said polydiorganosiloxane and wherein said organosiloxy units of (B) consist primarily of organosiloxy units containing one 3,3,3-trifluoropropyl radical per silicon atom, each of the remaining R radicals being selected from the group consisting of methyl radicals and vinyl radicals.

16. The composition as claimed in claim 15 wherein said (A) is a polydiorganosiloxane gum having a viscosity of at least 10 kilopascal.seconds at 25°

C., wherein at least 95 mole pecent of the siloxane units present in said gum contain 3,3,3-trifluoropropyl radicals and at least two siloxane units present in said (A) contain vinyl radicals, any X radicals present in said (A) being hydroxyl radicals.

17. The composition as claimed in claim 8 wherein the method by which the filler is obtained comprises the steps of

(I) mixing said water, alcohol and any basic catalyst not provided by the organosilicon hydrophobing compounds together to form a solution,

(II) mixing the total amount of said organosilicon hydrophobing compounds into the solution to form a mixture,

(III) mixing the Si(OCH₃)₄ into the mixture to form a composition containing a silica filler, and

(IV) aging the composition for a sufficient amount of time to render the silica filler present in said composition hydrophobic, thereby obtaining a composition containing a hydrophobic reinforcing silica filler.

18. The composition as claimed in claim 8 wherein the method by which the filler is obtained comprises the steps of

(I) mixing said water, alcohol and any basic catalyst not provided by the organosilicon hydrophobing compounds together to form a solution,

(II) adding a solution composed of the Si(OCH₃)₄ and the total amount of said organosilicon hydrophobing compounds to the solution of step (I) to form a composition containing a silica filler, and

(III) aging the composition for a sufficient amount of time to render the silica filler present in said composition hydrophobic, thereby obtaining a composition containing a hydrophobic reinforcing silica filler.

19. The composition as claimed in claim 12 wherein the method by which the filler is obtained comprises the steps of

(I) mixing said water, alcohol and any basic catalyst not provided by the organosilicon hydrophobing compounds together to form a solution,

(II) mixing the total amount of said organosilicon hydrophobing compounds into the solution to form a mixture,

(III) mixing the Si(OCH₃)₄ into the mixture to form a composition containing a silica filler, and

(IV) aging the composition for a sufficient amount of time to render the silica filler present in said composition hydrophobic, thereby obtaining a composition containing a hydrophobic reinforcing silica filler.

20. The composition as claimed in claim 12 wherein the method by which the filler is obtained comprises the steps of

(I) mixing said water, alcohol and any basic catalyst not provided by the organosilicon hydrophobing compounds together to form a solution,

(II) adding a solution composed of the Si(OCH₃)₄ and the total amount of said organosilicon hydrophobing compounds to the solution of step (I) to form a composition containing a silica filler, and

(III) aging the composition for a sufficient amount of time to render the silica filler present in said composition hydrophobic, thereby obtaining a composition containing a hydrophobic reinforcing silica filler.

21. The composition as claimed in claims 1 or 2 which additionally contains at least one curing agent.

22. The composition as claimed in claim 3 which additionally contains at least one curing agent.

23. The composition as claimed in claim 4 which additionally contains at least one curing agent.

24. The composition as claimed in claim 7 which additionally contains at least one curing agent.

25. The composition as claimed in claim 8 which additionally contains at least one curing agent.

26. The composition as claimed in claim 9 which additionally contains at least one curing agent.

27. The composition as claimed in claim 11 which additionally contains at least one curing agent.

28. The composition as claimed in claim 12 which additionally contains at least one curing agent.

29. The composition as claimed in claim 14 which additionally contains at least one curing agent.

30. The composition as claimed in claim 16 which additionally contains at least one curing agent.

31. The composition as claimed in claim 17 which additionally contains at least one curing agent.

32. The composition as claimed in claim 18 which additionally contains at least one curing agent.

33. The composition as claimed in claim 19 which additionally contains at least one curing agent.

34. The composition as claimed in claim 20 which additionally contains at least one curing agent.

35. An optically clear elastomer which comprises the product formed upon curing the composition of claims 1 or 2.

36. An optically clear elastomer which comprises the product formed upon curing the composition of claim 3.

37. An optically clear elastomer which comprises the product formed upon curing the composition of claim 4.

38. An optically clear elastomer which comprises the product formed upon curing the composition of claim 7.

39. An optically clear elastomer which comprises the product formed upon curing the composition of claim 8.

40. An optically clear elastomer which comprises the product formed upon curing the composition of claim 9.

41. An optically clear elastomer which comprises the product formed upon curing the composition of claim 11.

42. An optically clear elastomer which comprises the product formed upon curing the composition of claim 12.

43. An optically clear elastomer which comprises the product formed upon curing the composition of claim 14.

44. An optically clear elastomer which comprises the product formed upon curing the composition of claim 16.

45. An optically clear elastomer which comprises the product formed upon curing the composition of claim 17.

46. An optically clear elastomer which comprises the product formed upon curing the composition of claim 18.

47. An optically clear elastomer which comprises the product formed upon curing the composition of claim 19.

48. An optically clear elastomer which comprises the product formed upon curing the composition of claim 20.

49. An optically clear elastomer which comprises the product formed upon curing the composition of claim 21.

50. An optically clear elastomer which comprises the product formed upon curing the composition of claim 22.

51. An optically clear elastomer which comprises the product formed upon curing the composition of claim 23.

52. An optically clear elastomer which comprises the product formed upon curing the composition of claim 24.

53. An optically clear elastomer which comprises the product formed upon curing the composition of claim 25.

54. An optically clear elastomer which comprises the product formed upon curing the composition of claim 26.

55. An optically clear elastomer which comprises the product formed upon curing the composition of claim 27.

56. An optically clear elastomer which comprises the product formed upon curing the composition of claim 28.

57. An optically clear elastomer which comprises the product formed upon curing the composition of claim 29.

58. An optically clear elastomer which comprises the product formed upon curing the composition of claim 30.

59. An optically clear elastomer which comprises the product formed upon curing the composition of claim 31.

60. An optically clear elastomer which comprises the product formed upon curing the composition of claim 32.

61. An optically clear elastomer which comprises the product formed upon curing the composition of claim 33.

62. An optically clear elastomer which comprises the product formed upon curing the composition of claim 34.

69. An optically clear elastomer which comprises the product formed upon curing the composition of claim 1 wherein said product is in the form of a biocompatible prosthetic device.

70. An optically clear elastomer which comprises the product formed upon curing the composition of claim 2 wherein said product is in the form of a biocompatible prosthetic device.

71. An optically clear elastomer which comprises the product formed upon curing the composition of claim 6 wherein said product is in the form of a biocompatible prosthetic device.

72. An optically clear elastomer which comprises the product formed upon curing the composition of claim 8 wherein said product is in the form of a biocompatible prosthetic device, each R radical present in said polydiorganosiloxane being selected from the group consisting of methyl radicals and vinyl radicals, wherein no more than 5 mole percent of the siloxane units contain vinyl radicals, there being no more than one vinyl radical per siloxane unit, the mole percent of said siloxane units being based upon the total moles of siloxane units present in said polydiorganosiloxane and wherein said organosiloxy units of (B) consist primarily of organosiloxy units wherein each R radical is selected from the group consisting of methyl radicals and vinyl radicals.

63. 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°

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_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 R₃ SiO_1/2 units, R₂ SiO units, O_1/2 R₂ SiO(R₂ SiO)_d SiR₂ O_1/2 units, XR₂ SiO(R₂ SiO)_d SiR₂ O_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°

±

2°

C. of a polydiorganosiloxane of the type described in (A) to form a test blend, wherein (1) the refractive index (at 25°

C., sodium D line) of the polydiorganosiloxane chosen for use in said blend differs from the refractive index (at 25°

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°

±

2°

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°

±

2°

C. according to the procedure set out in ASTM D1003-61.

64. The method as claimed in claim 63 wherein the haze value of a composition consisting of a mixture of 100 cubic centimeters of (A) at 23°

±

2°

C. and 60 grams of (B) does not increase by more than 1% per 2.54 millimeter thickness of said mixture upon heating the mixture from 23°

±

2°

C. to 100°

±

5°

C.

65. The method as claimed in claims 63 or 64 which additionally includes the step (III) of mixing at least one curing agent into the composition.

73. The method as claimed in claim 63 wherein said organosiloxy units of said filler are derived from organosilicon hydrophobing compounds selected from the group consisting of R_n SiY_4-n, (R₃ Si)₂ NH, (R₃ Si)₂ O, (R₂ SiO)_e, (R₂ SiNH)_e, R'"'"'O(R₂ SiO)_d R'"'"', (R₃ Si)₂ NR", (R₂ SiNR")_e, and mixtures of the same where each R'"'"' is hydrogen or R", each R" is an alkyl radical of from 1 to 4 inclusive carbon atoms, each Y is --OH, --OR'"'"', --NHR" or --NR"₂, e has an average value of from 3 to 6 inclusive, and n has an average of from 2 to 3 inclusive and wherein the wet, gelled filler composition is obtained from the alkaline hydrolysis of Si(OCH₃)₄ in the presence of at least 70% of one-half of the stoichiometric amount of water required to completely hydrolyze the theoretical amount of --OCH₃ radicals present in the Si(OCH₃)₄, at least one alcohol selected from the group consisting of methanol, ethanol, n-propanol and isopropanol, at least one of said organosilicon hydrophobing compounds, and at least one basic catalyst.

74. The method as claimed in claim 64 wherein said organosiloxy units of said filler are derived from organosilicon hydrophobing compounds selected from the group consisting of R_n SiY_4-n, (R₃ Si)₂ NH, (R₃ Si)₂ O, (R₂ SiO)_e, (R₂ SiNH)_e, R'"'"'O(R₂ SiO)_d R'"'"', (R₃ Si)₂ NR", (R₂ SiNR")_e, and mixtures of the same where each R'"'"' is hydrogen or R", each R" is an alkyl radical of from 1 to 4 inclusive carbon atoms, each Y is --OH, --OR'"'"', --NHR" or --NR"₂, e has an average value of from 3 to 6 inclusive, and n has an average value of from 2 to 3 inclusive and wherein the wet, gelled filler composition is obtained from the alkaline hydrolysis of Si(OCH₃)₄ in the presence of at least 70% of one-half of the stoichiometric amount of water required to completely hydrolyze the theoretical amount of --OCH₃ radicals present in the Si(OCH₃)₄, at least one alcohol selected from the group consisting of methanol, ethanol, n-propanol and isopropanol, at least one of said organosilicon hydrophobing compounds, and at least one basic catalyst.

66. 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°

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_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 R₃ SiO_1/2 units, R₂ SiO units, O_1/2 R₂ SiO(R₂ SiO)_d SiR₂ O_1/2 units, XR₂ SiO(R₂ SiO)_d SiR₂ O_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°

±

2°

C. of a polydiorganosiloxane of the type described in (A) to form a test blend, wherein (1) the refractive index (at 25°

C., sodium D line) of the polydiorganosiloxane chosen differs from the refractive index (at 25°

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°

±

2°

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°

±

2°

C. according to the procedure set out in ASTM D1003-61.

67. The method as claimed in claim 66 wherein the haze value of a composition consisting of a mixture of 100 cubic centimeters of (A) at 23°

±

2°

C. and 60 grams of (B) does not increase by more than 1% per 2.54 millimeter thickness of said mixture upon heating the mixture from 23°

±

2°

C. to 100°

±

5°

C.

68. The method as claimed in claims 66 or 67 which additionally includes the step (III) of mixing at least one curing agent into the composition.

75. The composition as claimed in claim 66 wherein said organosiloxy units of said filler are derived from organosilicon hydrophobing compounds selected from the group consisting of R_n SiY_4-n, (R₃ Si)₂ NH, (R₃ Si)₂ O, (R₂ SiO)_e, (R₂ SiNH)_e, R'"'"'O(R₂ SiO)_d R'"'"', (R₃ Si)₂ NR", (R₂ SiNR")_e, and mixtures of the same where each R'"'"' is hydrogen or R", each R" is an alkyl radical of from 1 to 4 inclusive carbon atoms, each Y is --OH, --OR'"'"', --NHR" or --NR"₂, e has an average value of from 3 to 6 inclusive, and n has an average value of from 2 to 3 inclusive and wherein the wet, fluid filler composition in obtained from the alkaline hydrolysis of Si(OCH₃)₄ in the presence of at least 70% of one-half of the stoichiometric amount of water required to completely hydrolyze the theoretical amount of --OCH₃ radicals present in the Si(OCH₃)₄, at least one alcohol selected from the group consisting of methanol, ethanol, n-propanol and isopropanol, at least one of said organosilicon hydrophobing compounds, and at least one basic catalyst.

76. The composition as claimed in claim 67 wherein said organosiloxy units of said filler are derived from organosilicon hydrophobing compounds selected from the group consisting of R_n SiY_4-n, (R₃ Si)₂ NH, (R₃ Si)₂ O, (R₂ SiO)_e, (R₂ SiNH)_e, R'"'"'O(R₂ SiO)_d R'"'"', (R₃ Si)₂ NR", (R₂ SiNR")_e, and mixtures of the same where each R'"'"' is hydrogen or R", each R" is an alkyl radical of from 1 to 4 inclusive carbon atoms, each Y is --OH, --OR'"'"', --NHR" or --NR"₂, e has an average value of from 3 to 6 inclusive, and n has an average value of from 2 to 3 inclusive and wherein the wet, fluid filler composition is obtained from the alkaline hydrolysis of Si(OCH₃)₄ in the presence of at least 70% of one-half of the stoichiometric amount of water required to completely hydrolyze the theoretical amount of --OCH.sub. 3 radicals present in the Si(OCH₃)₄, at least one alcohol selected from the group consisting of methanol, ethanol, n-propanol and isopropanol, at least one of said organosilicon hydrophobing compounds, and at least one basic catalyst.