THERMALLY STABLE TRANSPARENT SILICONE RESIN COMPOSITIONS AND METHODS FOR THEIR PREPARATION AND USE

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of U.S. Provisional Application Serial No.

60/759,501 filed on 17 January 2006. U.S. Provisional Application Serial No. 60/759,501 is hereby incorporated by reference.

BACKGROUND OF THE INVENTION [0002] Optically transparent silicone resin compositions having refractive index (RI) greater than 1.40 are useful for packaging light emitting diode (LED) devices. LEDs generally operate at 400 to 700 nm. In this application, an important characteristic of silicone materials is their transparent nature in the near visible region of 400 to 800 nanometers (nm). Silicone encapsulants generally do not adversely affect the emission spectrum of light from the LED and do not attenuate light output. [0003] The RI of silicones increases as aromatic group content increases. Increasing aromatic group content is one approach to obtaining curable silicone compositions with RI > 1.40 for LED packaging applications. However, known silicone resins having refractive index of greater than 1.40 can compromise the LED performance because optical transmission of these silicone resins degrades over time. It is undesirable for light in the wavelength range from ultra-violet (UV) to yellow to be absorbed over time. This phenomenon is called yellowing because the silicone resin appears to have transformed from transparent to a yellow color after thermal aging.

Problem to be Solved [0004] There is a need for curable silicone resin compositions that form cured silicone resins having RI > 1.40 and minimal yellowing after thermal aging.

BRIEF SUMMARY OF THE INVENTION

[0005] This invention relates to a curable silicone resin composition that upon cure forms a cured silicone resin having a refractive index greater than 1.40. The composition forms a cured silicone resin with an optical transparency > 95% at a thickness of 2.0 millimeters (mm) or less at 400 nm to 700 nm wavelength after heating at 200 ⁰ C for 14 days.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 shows a cross section of an LED device with a cured silicone resin prepared from the composition of this invention.

Reference Numerals 101 dome

102 wire

103 soft silicone

104 LED chip

105 lead frame DETAILED DESCRIPTION OF THE INVENTION

Definitions and Usage of Terms

[0007] All amounts, ratios, and percentages are by weight unless otherwise indicated. The articles "a", "an", and "the" each refer to one or more. "Combination" means two or more items put together by any method. A "silylated acetylenic inhibitor" means any reaction product of an acetylenic alcohol inhibitor and a silane.

[0008] This invention relates to a curable silicone resin composition. The composition comprises:

(A) a polydiorganosiloxane having an average, per molecule, of at least two aliphatically unsaturated organic groups and at least one aromatic group; (B) a branched polyorganosiloxane having an average, per molecule, of at least one aliphatically unsaturated organic group and at least one aromatic group;

(C) a polyorganohydrogensiloxane having an average per molecule of at least two silicon-bonded hydrogen atoms and at least one aromatic group,

(D) a hydrosilylation catalyst, and (E) a silylated acetylenic inhibitor.

The curable silicone resin composition cures to form a cured silicone resin having a refractive index greater than 1.40. The composition cures to form a cured silicone resin with an optical transparency greater than 95% at a thickness of 2.0 mm or less at 400 nm wavelength after heating at 200 ⁰ C for 14 days, alternatively optical transparency greater than 95% at a thickness of 1.8 mm at 400 nm wavelength after heating at 200 °C for 14 days.

Ingredient (A) Polydiorganosiloxane

[0009] Ingredient (A) is a polydiorganosiloxane having an average per molecule of at least two aliphatically unsaturated organic groups and at least one aromatic group. Ingredient (A) can be a single polydiorganosiloxane or a combination comprising two or more polydiorganosiloxanes that differ in at least one of the following properties: structure, viscosity, average molecular weight, siloxane units, and sequence. The viscosity of ingredient (A) is not critical, however, viscosity may range from 10 to 1,000,000 mPa-s at 25 ⁰ C, alternatively 100 to 50,000 mPa-s, to improve handling properties of the cured silicone resin prepared from the composition of this invention. The amount of ingredient (A) in the composition may range from 10 to 40, alternatively 15 to 30, parts by weight based on the total weight of the composition.

[0010] The unsaturated organic groups in ingredient (A) may be alkenyl exemplified by, but not limited to, vinyl, allyl, butenyl, pentenyl, and hexenyl, alternatively vinyl. The unsaturated organic groups may be alkynyl groups exemplified by, but not limited to, ethynyl, propynyl, and butynyl. The unsaturated organic groups in ingredient (A) may be located at terminal, pendant, or both terminal and pendant positions. The aromatic group or groups in ingredient (A) may be located at terminal, pendant, or both terminal and pendant positions. The aromatic group is exemplified by, but not limited to, phenyl, tolyl, xylyl, benzyl, styryl, and 2-phenylethyl, alternatively phenyl. Ingredient (A) contains an average of at least one aromatic group per molecule. However, ingredient (A) may contain more than 40 mol%, alternatively more than 45 mol% aromatic groups.

[0011] The remaining silicon-bonded organic groups in ingredient (A), if any, may be monovalent substituted and unsubstituted hydrocarbon groups free of aromatics and free aliphatic unsaturation. Monovalent unsubstituted hydrocarbon groups are exemplified by, but not limited to alkyl groups such as methyl, ethyl, propyl, pentyl, octyl, undecyl, and octadecyl and cycloalkyl groups such as cyclohexyl. Monovalent substituted hydrocarbon groups are exemplified by, but not limited to halogenated alkyl groups such as chloromethyl, 3- chloropropyl, and 3,3,3-trifluoropropyl, fiuoromethyl, 2-fluoropropyl, 3,3,3-trifluoropropyl, 4,4,4-trifluorobutyl, 4,4,4,3,3 -pentafluorobutyl, 5,5,5,4,4,3,3-heptafluoropentyl, 6,6,6,5,5,4,4,3,3-nonafluorohexyl, and 8,8,8,7,7-pentafluorooctyl.

[0012] Ingredient (A) may have general formula (I): R ¹ ₃ SiO-(R ² ₂ SiO) _a -SiR ¹ ₃ , where each R ¹ and each R ² are independently selected from the group consisting of aliphatically unsaturated organic groups, aromatic groups, and monovalent substituted and unsubstituted hydrocarbon groups described above, and subscript a is an integer having a value sufficient to provide ingredient (A) with a viscosity ranging from 10 to 1,000,000 mPa-s at 25 ⁰ C, with the proviso that on average at least two of R ¹ and/or R ² are unsaturated organic groups and at least one of R ¹ and/or R ² is an aromatic group. Alternatively, at least two of R ¹ are unsaturated organic groups, at least one of R ² is an aromatic group, and subscript a has a value ranging from 5 to 1,000. Alternatively, formula (I) is an a, ω-dialkenyl-functional polydiorganosiloxane.

Ingredient (B) Branched Polyorganosiloxane

[0013] Ingredient (B) is a branched polyorganosiloxane having an average, per molecule, of at least one unsaturated organic group and at least one aromatic group. Ingredient (B) can be a single polyorganosiloxane or a combination comprising two or more polydiorganosiloxanes that differ in at least one of the following properties: structure, viscosity, average molecular weight, siloxane units, and sequence. The molecular weight of ingredient (B) is not critical, however, weight average molecular weight (Mw) may range from 500 to 10,000, alternatively 700 to 3,000. Ingredient (B) may be added to the composition in an amount of 35 to 75 parts by weight based on the total weight of the composition. [0014] Ingredient (B) comprises units of formula R ³ Siθ _3/2 , where each R ³ is independently selected from the group consisting of aliphatically unsaturated organic groups, aromatic groups, and monovalent substituted and unsubstituted hydrocarbon groups described above, with the proviso that on average, per molecule, at least one of R ³ is an aliphatically unsaturated organic group and at least one R ³ is an aromatic group. [0015] Ingredient (B) may have unit formula (II):

(R ³ SiO _3/2 ) _b (R ³ ₂ SiO _2/2 ) _c (R ³ ₃ SiO _1/2 ) _d (SiO _4/2 ) _e (XO _1/2 ) _f , where R ³ is as described above, X is a hydrogen atom or a monovalent hydrocarbon group such as an alkyl group, b is a positive number, c is 0 or a positive number, d is 0 or a positive number, e is 0 or a positive number, f is 0 or a positive number, c/b is a number ranging from 0 to 10, άlb is a number ranging from 0 to 0.5, e/(b+c+d+e) is a number ranging from 0 to 0.3, and f/(b+c+d+e) is a number ranging from 0 to 0.4. In formula (II), the polyorganosiloxane contains an average of at least one

unsaturated organic group per molecule, however, 0.1 mol% to 40 mol% of R may be unsaturated organic groups. In formula (II), the polyorganosiloxane contains an average of at least one aromatic group per molecule, however, at least 10 mol% of R ³ may be aromatic groups. Furthermore, in the D unit of formula R ₂ SiO ₂ ^, at least 30 mol% of R may be aromatic groups. The composition may contain 0 to 17%, alternatively 2 to 17% of ingredient (B) where f is a positive number.

Ingredient (C) Polyorganohydrogensiloxane

[0016] Ingredient (C) is a polyorganohydrogensiloxane having an average, per molecule, of at least two silicon bonded hydrogen atoms and at least one aromatic group. Ingredient (C) can be a single polyorganohydrogensiloxane or a combination comprising two or more polyorganohydrogensiloxanes that differ in at least one of the following properties: structure, viscosity, average molecular weight, siloxane units, and sequence. The viscosity of ingredient (C) is not critical and may range from 1 to 1,000 mPa-s at 25 ⁰ C, alternatively 2 to 500 mPa-s. The aromatic groups are as exemplified above. Ingredient (C) may contain at least 15 mol%, alternatively at least 30 mol% aromatic groups.

[0017] Ingredient (C) may comprise a linear polyorganohydrogensiloxane of general formula (III): HR ⁴ ₂ SiO-(R ⁴ ₂ SiO) _g -SiR ⁴ ₂ H, where each R ⁴ is independently a hydrogen atom, an aromatic group as exemplified above, or a monovalent substituted or unsubstituted hydrocarbon group free of aromatics and free aliphatic unsaturation as exemplified above, with the proviso that on average at least one R ⁴ per molecule is an aromatic group, and g is an integer with a value of 1 or more. Alternatively, at least one R ⁴ per molecule is phenyl and g may range from 1 to 20, alternatively 1 to 10.

[0018] Alternatively, ingredient (C) may comprise a branched polyorganohydrogensiloxane of unit formula (IV): (R ⁵ Siθ _3/2 ) _h (R ⁵ ₂ Siθ _2/2 )i(R ⁵ ₃ SiO _1/2 ) _j (SiO _4/2 ) _k (XO) _m where X is as described above. Each R ⁵ is independently a hydrogen atom, an aromatic group as exemplified above, or a monovalent substituted or unsubstituted hydrocarbon group free of aromatics and free aliphatic unsaturation as exemplified above, with the proviso that an average of at least two per molecule of R ⁵ are hydrogen atoms. In formula (IV), the polyorganohydrogensiloxane contains an average of at least two silicon bonded hydrogen atoms per molecule, however, 0.1 mol% to 40 mol% of R ⁵ may be hydrogen atoms. In formula (IV), the polyorganosiloxane contains an average of at least one aromatic group per

molecule, however, at least 10 mol% of R ⁵ may be aromatic groups. Furthermore, in the D unit of formula R ⁵ ₂ Si0 _2/2 , at least 30 mol% of R ⁵ may be aromatic groups. In formula (IV), h is a positive number, i is 0 or a positive number, j is 0 or a positive number, k is 0 or a positive number, m is 0 or a positive number, i/h has a value ranging from 0 to 10, j/h has a value ranging from 0 to 5, k/(h+i+j+k) has a value ranging from 0 to 0.3, and m/(h+i+j+k) has a value ranging from 0 to 0.4.

[0019] The amount of ingredient (C) added to the composition may range from 10 to 50 parts by weight based on the total weight of the composition. The amount of ingredient (C) may be selected such that the amount of silicon bonded hydrogen atoms in the composition ranges from 0.1 mol to 10 mol, alternatively 0.1 to 5 mol, and alternatively 0.5 to 2 mol, per 1 mol of unsaturated organic groups in the composition.

Ingredient (D) Hydrosilylation Catalyst

[0020] Ingredient (D) is a hydrosilylation catalyst. Ingredient (D) is added in an amount sufficient to promote curing of the composition of this invention. However, the amount of ingredient (D) may range from 0.01 to 1,000 ppm, alternatively 0.01 to 100 ppm, and alternatively 0.01 to 50 ppm of platinum group metal based on the weight of the composition. [0021] Suitable hydrosilylation catalysts are known in the art and commercially available. Ingredient (D) may comprise a platinum group metal selected from the group consisting of platinum, rhodium, ruthenium, palladium, osmium or iridium metal or organometallic compound thereof, and a combination thereof. Ingredient (D) is exemplified by compounds such as chloroplatinic acid, chloroplatinic acid hexahydrate, platinum dichloride, and complexes of said compounds with low molecular weight organopolysiloxanes or platinum compounds microencapsulated in a matrix or coreshell type structure. Complexes of platinum with low molecular weight organopolysiloxanes include 1,3-diethenyl-l, 1,3,3- tetramethyldisiloxane complexes with platinum. These complexes may be microencapsulated in a resin matrix. Alternatively, the catalyst may comprise 1 ,3 -diethenyl- 1,1,3,3- tetramethyldisiloxane complex with platinum. When the catalyst is a platinum complex with a low molecular weight organopolysiloxane, the amount of catalyst may range from 0.02 to 0.2 parts by weight based on the total weight of the composition. [0022] Suitable hydrosilylation catalysts for ingredient (D) are described in, for example, U.S. Patents 3,159,601; 3,220,972; 3,296,291; 3,419,593; 3,516,946; 3,814,730; 3,989,668;

4,784,879; 5,036,117; and 5,175,325 and EP 0 347 895 B. Microencapsulated hydrosilylation catalysts and methods of preparing them are also known in the art, as exemplified in U.S. Patent No. 4,766,176; and U.S. Patent No. 5,017,654.

Ingredient (E) Silylated Acetylenic Inhibitor [0023] Ingredient (E) in the composition of this invention is a silylated acetylenic inhibitor. Without wishing to be bound by theory, it is thought that adding a silylated acetylenic inhibitor reduces yellowing of the cured silicone resin prepared from the composition of this invention as compared to a cured silicone resin prepared from a hydrosilylation curable composition that does not contain an inhibitor or that contains a conventional organic acetylenic alcohol inhibitor. Examples of conventional organic acetylenic alcohol inhibitors are disclosed, for example, in EP 0 764 703 A2 and U.S. Patent 5,449,802 and include 1- butyn-3-ol, l-propyn-3-ol, 2-methyl-3-butyn-2-ol, 3 -methyl- l-butyn-3-ol, 3 -methyl- 1- pentyn-3-ol, 3 -phenyl- l-butyn-3-ol, 4-ethyl-l-octyn-3-ol, 3,5-diemthyl-l-hexyn-3-ol, and 1- ethynyl-1-cyclohexanol. The composition of this invention may be free of conventional organic acetylenic alcohol inhibitors. "Free of conventional organic acetylenic alcohol inhibitors" means that if any organic acetylenic alcohol is present in the composition, the amount present is insufficient to reduce optical transparency of the cured silicone resin to < 95% at a thickness of 2.0 mm or less at 400 nm wavelength after heating at 200 ⁰ C for 14 days. [0024] Ingredient (E) may be added in an amount ranging from 0.001 to 1 parts by weight based on the total weight of the composition, alternatively 0.01 to 0.5 parts by weight. Suitable silylated acetylenic inhibitors for ingredient (E) may have general formula (V):

general formula (VI):

or a combination thereof;

where each R ⁶ is independently a hydrogen atom or a monovalent organic group, and n is 0, 1, 2, or 3, q is 0 to 10, and r is 4 to 12. Alternatively n is 1 or 3. Alternatively, in general formula (V), n is 3. Alternatively, in general formula (VI), n is 1. Alternatively q is 0.

Alternatively r is 5, 6, or 7, and alternatively r is 6. Examples of monovalent organic groups for R ⁶ include an aliphatically unsaturated organic group, an aromatic group, or a monovalent substituted or unsubstituted hydrocarbon group free of aromatics and free aliphatic unsaturation, as described above. [0025] Ingredient (E) is exemplified by (3 -methyl- l-butyn-3-oxy)trimethylsilane, ((1,1- dimethyl-2-propynyl)oxy)trimethylsilane, bis(3 -methyl- 1 -butyn-3 -oxy)dimethylsilane, bis(3 - methyl- 1 -butyn-3 -oxy)silanemethylvinylsilane, bis(( 1 , 1 -dimethyl-2- propynyl)oxy)dimethylsilane, methyl(tris(l,l-dimethyl-2-propynyloxy))silane, methyl(tris(3- methyl- 1 -butyn-3 -oxy))silane, (3 -methyl- 1 -butyn-3 -oxy)dimethylphenylsilane, (3 -methyl- 1 - butyn-3 -oxy)dimethylhexenylsilane, (3 -methyl- 1 -butyn-3 -oxy)triethylsilane, bis(3 -methyl- 1 - butyn-3 -oxy)methyltrifluoropropylsilane, (3 , 5-dimethyl- 1 -hexyn-3 -oxy)trimethylsilane, (3 - phenyl- 1 -butyn-3 -oxy)diphenylmethylsilane, (3 -phenyl- 1 -butyn-3-oxy)dimethylphenylsilane, (3 -phenyl- 1 -butyn-3 -oxy)dimethylvinylsilane, (3 -phenyl- 1 -butyn-3 - oxy)dimethylhexenylsilane, (cyclohexyl- 1 -ethyn- 1 -oxy)dimethylhexenylsilane, (cyclohexyl- 1 -ethyn- 1 -oxy)dimethylvinylsilane, (cyclohexyl- 1 -ethyn- 1 -oxy)diphenylmethylsilane, (cyclohexyl- 1 -ethyn- 1 -oxy)trimethylsilane, and combinations thereof. Alternatively, ingredient (E) is exemplified by methyl(tris(l,l-dimethyl-2-propynyloxy))silane, ((1,1- dimethyl-2-propynyl)oxy)trimethylsilane, or a combination thereof. [0026] Ingredient (E) may be prepared by methods known in the art for silylating an alcohol such as reacting a chlorosilane of formula R ⁶ _n SiCl _4-n with an acetylenic alcohol of

formula or

in the presence of an acid receptor. In these formulae, n, q, r, and R ⁶ are as described above and R ⁷ is a bond or a divalent hydrocarbon group. Examples of silylated acetylenic inhibitors and methods for their preparation are disclosed, for example, in EP 0 764 703 A2 and U.S. Patent 5,449,802.

Optional Ingredients

[0027] The composition of this invention may further comprise one or more additional ingredients selected from the group consisting of (F) a mold release agent, (G) an optically active agent, (H) a filler, (I) an adhesion promoter, (J) a heat stabilizer, (K) a flame retardant, (L) a reactive diluent, and a combination thereof, provided however that the additional ingredients and amounts added do not render the curable silicone composition incapable of curing to form a cured silicone resin with an optical transparency > 95% at a thickness of 2.0 mm or less at 400 nm wavelength after heating at 200 °C for 14 days.

Ingredient (F) Mold Release Agent [0028] Ingredient (F) is an optional mold release agent. Ingredient (F) may comprise an α,£ϋ-dihydroxy-functional polydiorganosiloxane that may be added to the composition in an amount ranging from 0% to 5%, alternatively 0.25% to 2% based on the weight of the composition. Ingredient (F) can be a single α,ω-dihydroxy-functional polydiorganosiloxane or a combination comprising two or more α,co-dihydroxy-functional polydiorganosiloxanes that differ in at least one of the following properties: structure, viscosity, average molecular weight, siloxane units, and sequence. The viscosity of ingredient (F) is not critical and may range from 50 to 1,000 mPa-s at 25 ⁰ C. Ingredient (F) may contain at least one aromatic group per molecule, and the aromatic groups are as exemplified above. Ingredient (F) may contain at least 15 mol%, alternatively at least 30 mol% aromatic groups.

[0029] Ingredient (F) may comprise an α,ω-dihydroxy-functional polydiorganosiloxane of general formula (V): HOR ⁸ ₂ SiO-(R ⁸ ₂ SiO) ₀ -SiR ⁸ ₂ OH, where each R ⁸ is independently an aromatic group as exemplified above, or a monovalent substituted or unsubstituted hydrocarbon group free of aromatics and free aliphatic unsaturation as exemplified above, with the proviso that on average at least one R ⁸ per molecule is an aromatic group, and o is an integer with a value of 1 or more. Alternatively, at least one R ⁸ per molecule is phenyl and o may range from 2 to 8.

Ingredient (G) Optically Active Agent [0030] Optional ingredient (G) is an optically active agent. Examples of ingredient (G) include optical diffusants, phosphor powders, photonic crystals, quantum dots, carbon nanotubes, dyes such as fluorescent dyes or absorbing dyes, and combinations thereof. The exact amount of ingredient (G) depends on the specific optically active agent selected, however, ingredient (G) may be added in an amount ranging from 0% to 20%, alternatively 1% to 10% based on the weight of the composition. Ingredient (H) Filler

[0031] Optional ingredient (H) is a filler. Suitable fillers are known in the art and are commercially available. For example, ingredient (H) may comprise an inorganic filler such as silica, glass, alumina, zinc oxide, or a combination thereof. The filler may have an average particle diameter of 50 nanometers or less and does not lower the percent transmittance by scattering or absorption. Alternatively, ingredient (H) may comprise an organic filler such as poly(meth)acrylate resin particles. Ingredient (H) may be added in an amount ranging from 0% to 50%, alternatively 1% to 5% based on the weight of the composition.

Methods of Making the Composition [0032] The curable silicone composition described above may be prepared by any convenient means, such as mixing all ingredients at ambient or elevated temperature. The compositions may be prepared as one-part compositions or multiple part compositions. In a multiple part composition, such as a two part composition, ingredients (C) and (D) are stored in separate parts. For example, a base part may be prepared by mixing ingredients comprising: 30 to 60 parts ingredient (A), 30 to 65 parts ingredient (B), and 0.0005 to 0.005 parts ingredient (D). The base part may optionally further comprise 0.2 to 5 parts ingredient (F). A curing agent part may be prepared by mixing ingredients comprising: 0 to 10 parts

ingredient (A), 42 to 67 parts by weight ingredient (B), 20 to 50 parts by weight ingredient (C), and 0.001 to 1 part by weight ingredient (E). The base part and the curing agent part may be stored in separate containers until just prior to use, when the two parts are mixed together in a ratio of 1 to 10 parts base per 1 part curing agent. Methods of Using the Composition

[0033] The composition of this invention may be used to form a cured silicone resin. The composition may be cured at room temperature or with heating, however, heating the composition may accelerate curing. The composition may be heated at a temperature ranging from 50 to 200 ⁰ C for several minutes to several hours. The cured product obtained is a cured silicone resin.

[0034] The composition of this invention may be used for packaging optoelectronic devices, such as LED devices. For example, curable silicone composition may be molded and cured such that the cured silicone resin is formed as a hard lens. The lens may be tack free and resistant to dirt pick up. The composition of this invention may be used to form lenses in, for example, an injection molding process such as that disclosed in WO 2005/017995.

[0035] Alternatively, the composition of this invention may be used to encapsulate an LED device, such as that disclosed in U.S. Patent 6,204,523 or WO 2005/033207. Figure 1 shows a cross section of an LED device with a cured silicone resin prepared from the composition of this invention. The LED device includes an LED chip 104 encapsulated in a soft silicone 103, such as a rubber or gel. The LED chip 104 is bonded to lead frame 105 by wire 102. The LED chip 104, wire 102, and soft silicone 103 are surrounded by dome 101, which is made of a cured silicone resin of this invention.

EXAMPLES [0036] The following examples are included to demonstrate the invention to one of ordinary skill in the art. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention set forth in the claims. [0037] In these examples, M ^Vl ₂ D ^Ph ' ^Me _x is dimethyl, vinyl siloxy-terminated methyl, phenyl siloxane, with a viscosity ranging from 500 to 50,000 cSt and x ranging from 50 to 500.

Alternatively, viscosity may range from 2,000 to 25,000 cSt, and x may range from 100 to 250. M ^Vl o.2 ₅ T ^Ph o. ₇ 5 is a solution of 45 parts toluene and 55 parts vinyl terminated phenylsilsesquioxane resin. The resin is a solid. M ^H o. ₆ T ^pll _o . ₄ is a mixture of 98 parts hydrogen-terminated phenylsilsesquioxane and 2 parts phenyltris(dimethylhydrogensiloxy)silane with a viscosity ranging from 1 to 100 mPa-s. Mold Release Agent is hydroxy-terminated methyl, phenyl polysiloxane of formula:

, where p is an integer ranging from 3 to 10. Pt is a mixture of 38 parts tetramethyldivinyldisiloxane and 62 parts 1,3-diethenyl-l, 1,3,3 -tetramethyldisiloxane complexes with platinum. Etch is 1-ethynyl-l-cyclohexanol. Inhibitor is methyl(tris(l,l - dimethyl-2-propynyloxy))silane. In the formulae above, Me represents a methyl group, Ph represents a phenyl group, Vi represents a vinyl group, M represents a monofunctional siloxane unit, D represents a difunctional siloxane unit, and T represents a trifunctional siloxane unit.

Comparative Example I — Effect of Inhibitor [0038] A sample was prepared by mixing the following ingredients: 54 parts M ^Vl ₀ . ₂₅ T ^ph ₀ . ₇₅ , 24 parts M ^Vi ₂ D ^Ph ' ^Me _x , 21 parts M ^H ₀ . ₆ T ^Ph ₀ . ₄ , 1 part Mold Release Agent, 2 parts Pt, and 750 parts per million Etch. The sample was cured by heating at 200 ⁰ C to form 1.8 mm thick specimens. The specimens were aged at 200 ⁰ C for up to 14 days, and per cent transmission at 400 nm was measured at various times. The results are in Table 1. Example 1 - Effect of Inhibitor

[0039] A sample was prepared by mixing the following ingredients: 54 parts M ^Vl _o . ₂₅ T ^Ph o. ₇₅ , 24 parts M ^vi ₂ D ^Ph ' ^Me _x , 21 parts M ^H ₀ . ₆ T ^ph _α4 , 1 part Mold Release Agent, 2 parts Pt, and 250 parts per million Inhibitor. The sample was cured by heating at 200 ⁰ C to form 1.8 mm thick specimens. The specimens were aged at 200 °C for up to 14 days, and per cent transmission at 400 nm was measured at various times. The results are in Table 1.

Table 1

[0040] Example 1 and comparative example 1 show that this invention may be used to prepare a cured silicone resin with an optical transparency > 95% at a thickness of 1.8 mm at 400 nm wavelength after heating at 200 °C for 14 days. Comparative example 1 shows that conventional organic acetylenic alcohol inhibitors for hydrosilylation curable silicone compositions, such as 1-ethynyl-l-cyclohexanol, may be unsuitable for preparing cured silicone resins with an optical transparency > 95% at a thickness of 1.8 mm at 400 nm wavelength after heating at 200 °C for 14 days.