THERMALLY STABLE SILICONE POTTING COMPOSITIONS

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention pertains to improved silicone gel compositions which are suitable, e.g. for the potting of electronic components and components in the automotive and aerospace fields, and as thermally stable adhesives.

2. Description of the Related Art

[0002] Electronic components such as power transistors are frequently potted. The potting serves to geometrically position the electronic component as well as to isolate it and its electrical comments from the environment. During operation, the electronic devices emit a considerable amount of heat. Even though often mounted on cooling devices such as finned aluminum heat sinks, the temperatures achieved during operation can easily exceed 200°C, and often reach higher temperatures. As the use of high power electronic components has increased, so also has the thermal stress imposed upon the polymer potting compositions, as well as adhesives used in such applications.

[0003] A variety of potting compositions and adhesives are known in the art, including epoxy-based compositions and silicone compositions, among many others which are less often used. Regardless of what type of polymer is used, the polymer must meet very stringent performance requirements.

[0004] The visual observable effects of thermal stress on electronic potting compositions is first, a tendency toward discoloration; second, measurable shrinkage; and third, a tendency towards a decrease in elastomericity, e.g. embrittlement. Discoloration is most commonly evidenced by a yellow or brown discoloration. Development of discoloration is perceived as evidencing a defective encapsulant, and may decrease the amount of light emitted by optoelectrical components containing light sources such as LEDs. Due to absorbtion of a portion of the light emitted by the optoelectronic source, the developed discoloration generates yet more heat, resulting in further thermal stress.

[0005] Thermal stress generally results in increased embrittlement. Embrittlement increases the modulus of the potting composition and also decreases tensile strength. As a result, upon thermal cycling, such as turning a power device on and off, the potting composition may not be able to accommodate the accompanying thermal expansion and contraction, resulting in failure, not only of the potting composition itself, but also the electrical leads to the power device which the potting composition is designed to protect.

[0006] Silicones are also useful in other applications where parts are to be exposed to high temperatures for extended periods. Thus, electrical connectors, sensors, relays, etc., are frequently encapsulated for uses within vehicular engine compartments, and in the aerospace field. These applications require retention of properties after long term exposure to high temperatures, often also with exposure to functional fluids such as oils, greases, water, fuel, alcohols (which may be present in fuels, for example), antifreeze, and the like. Such silicones are also frequently useful as adhesives, both with respect to electronic components as well as others. Very useful applications of silicones include the encapsulation of integrated circuits, and potting of photovoltaic junction boxes in solar cell modules and "farms."

[0007] Silicone potting compositions have been widely used, and have the advantages of chemical inertness, relatively high hydrophobicity, thermal stability, and high dielectric breakdown strength, which distinguishes them from epoxy-functional potting agents. However, the thermal stability of potting compositions is still in need of improvement, and the art has long sought methods of improving these characteristics, not only in silicone potting compositions, but in epoxy- functional and other potting compositions as well. For example, silicone potting compositions may exhibit excellent performance at use temperatures in the range of 180°C or lower, but may rapidly degrade at higher use temperatures over extended periods of time. [0008] Silicone gels are to be distinguished from silicone elastomers generally, and from other silicone polymers, by their very considerable softness and penetrability. These characteristics are sometimes necessary to accommodate the high thermal expansion experienced during use. If, for example, the modulus ("stiffness") is too high, the tensile stress associated with thermal expansion may cause adhesive or cohesive parting of the silicone gel, or may induce such stresses in the associated components, that these components or their connections may fail. The very low modulus of silicone gels allows for thermal expansion and contraction without exerting large stress to other components.

[0009] Such very soft, low modulus silicone gels are produced by limiting the degree of crosslinking in the gel polymer. Silicone gels have a very low crosslink density. However, this same low crosslink density limits the thermal stability of silicone gels relative to other, harder, silicone elastomers. Thermal and/or chemical destruction of some of the already low number of crosslinking sites will severely lower physical properties such as tensile strength. On the other hand, thermally induced further crosslinking will increase hardness and modulus much more rapidly than in elastomers which are already more extensively crosslinked. The result is that thermal ageing frequently results in hard, brittle polymers with low tensile strength and very low elongation.

[0010] In conventional silicone elastomers, very fine fillers, with average particle sizes less than about 1 μιη, and BET surface areas generally higher than 50 m /g are often added to increase physical properties. In contrast to larger particle size fillers and pigments, such "reinforcing fillers" actually significantly increase mechanical properties. Some such reinforcing fillers, for example fumed silica, finely divided iron oxides, and zinc oxide, have been shown to also increase thermal stability when used in relatively large amounts.

[0011] Unfortunately, the addition of the large amounts of the reinforcing fillers customary in conventional silicone elastomers to silicone gels, is not possible, for several reasons. First, because of their high surface area, such fillers have a pronounced effect on viscosity of uncured gel systems. Some compositions may even become thixotropic, which is often not desired. A curable gel system of high viscosity will not flow well, and encapsulation of components such as electronic devices may be incomplete, or there may be voids or bubbles in the cured structure.

[0012] Second, some reinforcing fillers, such as carbon blacks for example, endow the silicone gel with conductive properties, which is undesirable. Even essentially non-conductive fillers such as pyrogenic silica, depending upon its means of production, may contain appreciable ionic species such as chloride ions, which lower the resistivity of the gel.

[0013] Third, the presence of any substantial amount of reinforcing filler will dramatically increase the modulus and hardness of the gel, properties which are ordinarily undesirable. Large quantities of fillers may increase light scattering to such a degree so as to prohibit their use in compositions for potting optoelectronic devices. Moreover, high levels of fillers raise abrasion issues with respect to wire bonds of electronic components.

[0014] Thus, it would be desirable to achieve an increase in the thermal stability of soft silicone gels and also silicone adhesives, without significantly increasing the uncured viscosity of the silicone gel or adhesive formulation, nor the modulus and/or hardness of the cured silicone gel or adhesive. It would be further desirable to be able to maintain substantial transparency for use with optoelectronic devices, when such transparency is desired.

[0015] The physicochemical properties of silicone gels and adhesives may be assessed by measuring physical properties such as hardness, tensile strength, elongation at break, tear strength, lap shear strength, color (discoloration), shrinkage, adhesive failure mode (adhesive or cohesive), weight loss, observance of visible cracks, etc. Some of these physicochemical properties are more important than others in certain applications, and all can be measured either qualitatively or quantitatively, by well-established test methods, or in some cases, by visual or tactile observation.

[0016] It would be desirable to extend the temperature range in which silicone gels are useful. It would be further desirable to offer such improvements without sacrificing other admirable qualities of silicones. SUMMARY OF THE INVENTION

[0017] It has now been surprisingly and unexpectedly discovered that the addition of minor quantities of finely divided titanium dioxide can dramatically improve the thermal stability of addition-curable, soft silicone gels and adhesives. The stabilized silicone gels and adhesives are suitable for the potting and/or encapsulation of components which operate at high temperatures, particularly electronic components, and due to the small amounts of filler, can also be formulated for use in optoelectronic devices.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIGURE 1 illustrates the improvement in weight loss achieved by the present invention.

[0019] FIGURE 2 illustrates the reduction in embrittlement achieved by the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The soft silicone gels of the invention are addition-curable organopolysiloxane compositions containing at least one organopolysiloxane bearing aliphatically unsaturated groups, preferably ethylenically unsaturated groups, and at least one organopolysiloxane bearing silicon- bonded hydrogen ("Si-H") groups, and at least one catalyst which promotes the addition of Si-H groups onto aliphatically unsaturated groups. The silicone gels are thus cured by hydro silylation.

[0021] The organopolysiloxane(s) bearing aliphatically unsaturated functional groups may be linear, branched, and/or cyclic, and contain, on average, at least 1.5 and preferably at least 2 aliphatically unsaturated groups per molecule. Mixtures of such organopolysiloxanes may be used. These include the use of two or more "unsaturated" organopolysiloxanes with the same functionality (by "functionality" here is meant the number of hydrosilylatable groups) with different molecular weights, and the use of unsaturated organopolysiloxanes with different functionalities and either the same or different molecular weights. For example, monofunctional unsaturated organopolysiloxanes can be included to lower the modulus and/or hardness of the resulting silicone gels obtained after curing.

[0022] Organopolysiloxanes bearing Si-H functionality similarly have an average functionality (number of Si-H groups per molecule) of at least 1.5, preferably at least 2, and more preferably 3 or more. As with the unsaturated organopolysiloxanes, the Si-H functional organopolysiloxanes may be supplied as a mixture of two or more Si-H functional organopolysiloxanes with the same or different functionalities, and/or the same or different molecular weights.

[0023] The "hardness" of a silicone gel is related to the degree of crosslinking, which in turn is related to the average functionalities of the unsaturated and Si-H functional organopolysiloxanes. In general, the higher the combined functionality, the greater the crosslink density and the higher the hardness, at a given molecular weight (chain length) of the respective organopolysiloxanes. The crosslink density is also influenced by the molecular weight (chain length) of the respective organopolysiloxanes. At a given functionality of one or both of the unsaturated organopolysiloxane(s) and the Si-H functional organopolysiloxanes(s), the crosslink density will decrease with increasing molecular weight.

[0024] The hardness of the inventive soft silicone gels is Shore A 15 or lower, preferably

Shore A 10 or lower. Most preferably the hardness, measured on the Shore 00 scale, is less than 90, preferably 5 to 80, and more preferably 10-80. Since the silicone gels of the invention are quite soft, another appropriate measurement is the penetration depth. The gels preferably have a penetration depth of 10-100 mm/10 as measured with a texture analyzer with 12.7 mm spherical probe, more preferably 20-80 mm mm/10, and most preferably 30-75 mm/10.

[0025] Preparation of such soft gels is known, and one skilled in the art can select a combination of unsaturated and Si-H functional organopolysiloxanes to alter the hardness appropriately. In addition, reactive or non-reactive plasticizers, to be later described, can also be added to lower gel hardness. [0026] In addition to potting and encapsulation, the inventive silicone compositions can also be used as thermally stable adhesives. For this use, the silicones generally are elastomeric, with their hardness measured on the Shore A scale. The soft gels generally have a modulus less than 0.1 MPa, and potting elastomers, a modulus of up to 0.5 MPa or thereabouts, while adhesives have a modulus > 1 MPa. While the gel and soft elastomer encapsulants and potting compounds are generally supplied as two-component systems, adhesives are usually 1 part systems, and for this purpose, contain a significant amount of hydrosilylation inhibitor, or are UV-cured systems (for applications where UV cure is feasible).

[0027] The preferred organopolysiloxanes and will now be described in greater detail.

[0028] The unsaturated organopolysiloxanes are preferably organopolysiloxane(s) (A) which contain aliphatically unsaturated groups, preferably alkenyl groups, and preferably correspond to the average formula (1)

R ¹ _x R ² ySiO(4_ _x _y)/2 (1), where

R ¹ is a monovalent, unsubstituted or halogen- or cyano-substituted d-Cio-hydrocarbon radical which contains an aliphatic carbon-carbon multiple bond and is bound directly or via an organic divalent group to the silicon atom,

2

R is a monovalent, unsubstituted or halogen- or cyano-substituted Ci-Cio-hydrocarbon radical free of aliphatic carbon-carbon multiple bonds,

x is an integer such that on average, 1.5 or more radicals R ¹ are present per molecule and y is an integer such that (x+y) is in the range from 1.8 to 2.5.

[0029] The aliphatically unsaturated groups R ¹ are preferably alkenyl groups containing from 2 to 6 carbon atoms, but may also, less preferably, be alkynyl groups. Examples include vinyl, allyl, methallyl, 1-propenyl, 5-hexenyl, ethynyl, butadienyl, hexadienyl, cyclopentenyl, cyclopentadienyl, cyclohexenyl, and propargyl, preferably vinyl and allyl. It is possible also for the unsaturated groups to be present in an aliphatically unsaturated heteroatom-containing group, preferably bonded through a carbon atom or an oxygen atom to a silicon atom. Examples include (meth)acryloyl groups.

[0030] Organic divalent groups via which the alkenyl groups R ¹ can be bound to silicon of the polymer chain may consist, for example, of oxyalkylene units such as those of the formula (2) where

m is 0 or 1, preferably 0,

n is from 1 to 4, preferably 1 or 2, and

o is from 1 to 20, preferably from 1 to 5.

The oxyalkylene units of the formula (2) are bound at the left-hand end to a silicon atom. Preferably, no oxyalkylene units are present. Further examples include divalent aliphatic groups such as C MO alkylene groups, urethane groups, urea groups, and the like.

[0031] The radicals R ¹ can be located on any position on the polymer chain, for example along the chain (pendent), or preferably on terminal silicon atom(s).

[0032] Examples of radicals R include C _1-2 o hydrocarbon radicals, more preferably C _1-6 hydrocarbon radicals, which may optionally be substituted. Examples of unsubstituted radicals R are alkyl radicals such as the methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n- pentyl, isopentyl, neopentyl, and tert-pentyl radicals, hexyl radicals such as the n-hexyl radical, heptyl radicals such as the n-heptyl radical, octyl radicals such as the n-octyl radical and isooctyl radicals such as the 2,2,4-trimethylpentyl radical, nonyl radicals such as the n-nonyl radical, decyl radicals such as the n-decyl radical; alkenyl radicals such as the vinyl, allyl, n-5-hexenyl, 4- vinylcyclohexyl and the 3-norbornenyl radical; cycloalkyl radicals such as cyclopentyl, cyclohexyl, 4-ethylcyclohexyl, cycloheptyl radicals, norbornyl, and methylcyclohexyl radicals; aryl radicals such as the phenyl, biphenylyl, and naphthyl radicals; alkaryl radicals such as the o-, m-, p-tolyl radicals and ethylphenyl radicals; and aralkyl radicals such as the benzyl radical, and the alpha and the β-phenylethyl radicals. For reasons of economy, availability of starting materials, and thermal stability, methyl and/or phenyl R groups are preferred.

[0033] Examples of substituted hydrocarbon radicals R are halogenated hydrocarbons such as the chloromethyl, 3-chloropropyl, 3-bromopropyl, 3,3,3-trifluoropropyl and 5,5,5,4,4,3,3- hexafluoropentyl radicals and also the chlorophenyl, dichlorophenyl and trifluorotolyl radicals. Fluorine-substituted hydrocarbon radicals are useful with respect to thermal stability when substituted hydrocarbon radicals R are present.

[0034] Organopolysiloxane (A) can also be a mixture of various organopolysiloxanes which contain alkenyl groups and differ, for example, in the alkenyl group content, the type of alkenyl group, or structurally. For example, the structure of the organopolysiloxane(s) (A) can be linear, cyclic or branched. The content of trifunctional and/or tetrafunctional units leading to branched organopolysiloxanes is typically very low, preferably not more than 5 mol%, more preferably not more than 1 mol%, and most preferably not more than 0.1 mol%, all based on total moles of siloxy units. Branching tends to increase the uncured viscosity of the inventive compositions.

[0035] Particular preference is given to using polydimethylsiloxanes which contain vinyl groups as at least one organopolysiloxane (A), which correspond to the formula (3)

(ViMe ₂ SiOi/2)2(ViMeSiO)p(Me ₂ SiO) _q (3), where

p is from 0 to 100, preferably 0 to 10, and more preferably 0-5, and q is from 1 to 20,000, preferably 10-1000, and "Vi" represents a vinyl group.

[0036] The viscosity of the organopolysiloxane (A) is preferably from 10 to 500,000 mPa-s, preferably 100-100,000 mPa-s, and more preferably 100-20,000 mPa-s, all measured at 25°C. [0037] The content of organopolysiloxane(s) (A) in the curable mixture is preferably selected so that the curable mixture has a content of compound (A) of 30-95% by weight, preferably 50-90% by weight, and most preferably 60-90% by weight, relative to the total weight of the unsaturated organopolysiloxane and Si-H functional organopolysiloxane.

[0038] The organopolysiloxane(s) containing SiH functions are preferably organopolysiloxanes (B) having a composition of the average formula (4)

H _a R ³ _b SiO(4-a-b)/2 (4), where

R is a monovalent, unsubstituted or halogen- or cyano-substituted Ci-Cig-hydrocarbon radical which is free of aliphatic carbon-carbon multiple bonds, and

a and b are integers,

with the proviso that 0.5<(a+b)<3.0 and 0<a<2 and that at least 1.5 silicon-bonded hydrogen atoms are present per molecule, preferably at least two silicon-bonded hydrogen atoms are present per molecule.

[0039] Examples of R 3 are the same radicals previously disclosed for R 2. R 3 preferably has from 1 to 6 carbon atoms. R are preferably methyl or phenyl radicals.

[0040] Organopolysiloxane(s) (B) which contain on average three or more SiH groups per molecule are preferred. When using an organosilicon compound (B) averaging only two or fewer SiH bonds per molecule, it is advisable to use a organopolysiloxane (A) which has at least three aliphatically unsaturated groups per molecule. The average numbers of aliphatically unsaturated groups in organopolysiloxanes (A) and Si-H groups in organopolysiloxane(s) (B) should be such that upon curing, a soft, dimensionally stable silicone gel meeting the softness requirements previously described, can be obtained. By "dimensionally stable" is meant that the gel, while it may distort or sag, will not flow as a liquid on its own. [0041] The hydrogen content of the organopolysiloxane(s) (B) based exclusively on the hydrogen atoms bound directly to silicon atoms is preferably in the range from 0.002 to 1.7% by weight, preferably from 0.1 to 1.7% by weight.

[0042] The organopolysiloxane(s) (B) preferably contain at least three and preferably not more than 600 silicon atoms per molecule. Preferred organosilicon compounds (B) contain from 4 to 200 silicon atoms per molecule, and have a structure which can be linear, branched, cyclic or network-like.

[0043] Particularly preferred organopolysiloxane(s) (B) are linear organopolysiloxanes of the formula (5)

(HR ⁴ ₂ Si0 _1/2 ) _c (R ⁴ ₃ Si0 _1/2 ) _d (HR ⁴ Si0 _2/2 ) _e (R ⁴ ₂ Si0 _2/2 ) _f (5), where

R ⁴ is as previously defined for R ³ , and

the nonnegative integers c, d, e and are such that: c and d are 0, 1, or 2, and total 2, the sum of c and e is greater than 1.5, preferably 2 or more, and more preferably 3 or more, and the sum of "D" units denoted by subscripts e and f is between 5 and 200.

[0044] The SiH-functional organopolysiloxane(s) (B) are preferably present in the crosslinkable silicone gel composition in such an amount that the molar ratio of SiH groups to alkenyl groups is from 0.1 to 3, more preferably from 0.2 to 1.5.

[0045] A hydrosilylation catalyst (C) is required for curing the inventive soft silicone gels.

As catalyst(s) (C), it is possible to use all catalysts which catalyze the hydrosilylation reactions occurring in the crosslinking of addition-crosslinking silicone compositions, preferably a catalyst containing a platinum group metal. Such catalyst(s) (C) contain at least one metal or compound or complex thereof, of platinum, rhodium, palladium, ruthenium and iridium, preferably platinum. [0046] Examples of catalysts (C) include metallic and finely divided platinum which can be present on supports such as silicon dioxide, aluminum oxide or activated carbon, compounds or complexes of platinum, for example platinum halides, e.g. PtC , H ₂ PtCl ₆ -6H ₂ 0, Na2PtC14-4H20, platinum-olefin complexes, platinum- alcohol complexes, platinum-alkoxide complexes, platinum- ether complexes, platinum- aldehyde complexes, platinum-ketone complexes, including reaction products of H ₂ PtCl ₆ -6H ₂ 0 and cyclohexanone, platinum-vinylsiloxane complexes, in particular platinum-divinyltetramethyldisiloxane complexes with or without detectable inorganically bound halogen, bis(gamma-picoline)platinum dichloride, trimethylenedipyridineplatinum dichloride, dicyclopentadieneplatinum dichloride, (dimethyl sulfoxide)ethyleneplatinum(II) dichloride and also reaction products of platinum tetrachloride with olefin(s) and primary amine or secondary amine, or primary and secondary amine, for example the reaction product of platinum tetrachloride dissolved in 1-octene with sec-butylamine, or ammonium-platinum complexes.

[0047] A particularly preferred catalyst (C) is the so-called Karstedt catalyst, i.e. a Pt(0) complex, in particular the platinum(0)-l,3-divinyl-l,l,3,3-tetramethyldisiloxane complex of the formula Pt ₂ [[(CH ₂ =CH)(CH ₃ ) ₂ Si] ₂ 0] ₃ .

[0048] The soft silicone gels of the present invention are preferably thermally cured, through use of one of the foregoing hydrosilylation catalysts. However, in another embodiment, the catalysts (C) can be activated by means of light, preferably light having a wavelength or wavelengths in the range of from 200 to 500 nm. Particularly suitable light-activatable catalysts (C) are cyclopentadienyl complexes of platinum, preferably of the formula (6)

(6)

where g = 1 to 8, h = 0 to 2, i = 1 to 3,

[0049] the radicals R are, independently of one another, identical or different and are each a monovalent, unsubstituted or substituted, linear, cyclic or branched hydrocarbon radical which contains aliphatically saturated or unsaturated or aromatically unsaturated radicals and has from 1 to 30 carbon atoms and in which individual carbon atoms can be replaced by O, N, S or P atoms, the radicals R are each, independently, hydrolyzable functional groups, preferably selected from among carboxy -0-C(0)R ¹⁰ , oxime -0-N=CFr ^u ₂ , alkoxy -OR ¹⁰ , alkenyloxy -O-R ¹² , amide -NR ¹⁰ -C(O)R ⁿ , amine -NR ¹⁰ R ⁿ , and aminoxy groups, where

[0050] the radicals R ¹⁰ are, independently of one another, H, alkyl, aryl, arylalkyl, or alkylaryl,

[0051] the radicals R ¹¹ are, independently of one another, alkyl, aryl, arylalkyl, or alkylaryl,

[0052] the radicals R 12 are each a linear or branched, aliphatically unsaturated organic radical,

[0053] the radicals R ^9a are, independently of one another, alkyl, aryl, arylalkyl, or alkylaryl having from 1 to 30 carbon atoms, and at least 6 carbon atoms when aryl groups are present, where the hydrogens may be replaced by -Hal or -SiR ₃ ⁹ , where

[0054] the radicals R ⁹ are identical or different and are each a monovalent, unsubstituted or substituted, linear, cyclic or branched hydrocarbon radical,

[0055] the radicals R ^9b are identical or different and are hydrogen or a monovalent, unsubstituted or substituted, linear or branched hydrocarbon radical which contains aliphatically saturated or unsaturated or aromatically unsaturated radicals and has from 1 to 30 carbon atoms and in which individual carbon atoms can be replaced by O, N, S or P atoms and which can form rings fused to the cyclopentadienyl radical. Other photo-activated hydrosilylation catalysts may also be used. [0056] A titanium dioxide filler (D) is a necessary ingredient, preferably pyrogenic titanium dioxide. Pyrogenic titanium dioxide is commercially available in a wide range of particle sizes and BET surface areas. In the inventive compositions, the titanium dioxide, preferably pyrogenic titanium dioxide, preferably has a mean, weight average particle size of between 10 nm and 2 μιη, more preferably between 20 nm and 1.5 μιη, and yet more preferably between 50 nm and 1 μιη. Alternatively, the pyrogenic titanium dioxide may have a BET surface area of from 30-400 m7g,

2 2

preferably from 40-200 m /g, and more preferably from 40-100 m /g. The particle size and/or BET surface area may be selected such that the filled compositions, in the absence of any additional filler, to the eye, are visually transparent or translucent, rather than opaque in the amounts used, in particular with amounts ranging up to 4-5 weight percent. However, opaque compositions are also contemplated and preferred.

[0057] The titanium dioxide filler may be substantially of rutile crystal orientation, anastase crystal orientation, or any mixture of these. Fillers which are of substantially of the anastase orientation are preferred. The titanium dioxide pigments may be substantially pure titanium dioxide, for example with purities, relative to Ti0 ₂ , of > 95%, > 98%, > 99%, and/or greater than 99.5% (all percents by weight), or may contain unintentional or intentional "impurities" such as additional metal oxides.

[0058] Non-limiting examples of other metal oxides which may be present in the titanium dioxide particles include those of the alkaline earth metals, aluminum, tin, antimony, bismuth, and the transition metals. Less preferred, with the exception of cerium, are the lanthanide and actinide metals. However, lanthanides in particular may be desired as "phosphors" in optoelectronic devices such as LEDs to alter the wavelength distribution of light emitted by the encapsulated device. When metals other than titanium are present, preferred metals include magnesium, aluminum, cerium, zirconium, zinc, and iron. Additionally, oxides of silicon may be present. These additional ingredients may be present in very small amounts, for example 0.01 to 1.0 weight percent based on the total weight of the particles, and may then be alternatively described as "dopants," or may be present in much larger amounts, e.g. up to 10 weight percent, or up to 15, 20, 30, 40 wt. %, but less than 50 wt. %. The additional metal oxides just described are physiochemically bound, and thus cannot be separated by purely mechanical means, as opposed to mere physical mixture of pyrogenic titantium dioxide and other finely divided pigments, for example the so-called "transparent iron oxides" and the like. Such fillers may be obtained, for example, by flame hydrolyzing a titanium precursor such as titanium tetrachloride, together with a precursor of the other metal, for example anhydrous ferric or ferrous chloride.

[0059] The preferred pyrogenic titanium dioxide is contained in an amount of less than 15% by weight relative to the total weight of the curable soft silicone gel, preferably less than 10% by weight, more preferably less than 5% by weight, and preferably in the range of 0.1 to 4 weight percent, more preferably 0.2 to 4 weight percent, yet more preferably 0.3 to 2 weight percent, and most preferably 0.5 to 2 weight percent. Amounts of 0.75, 1.5, and 3.0 weight percents have all been found to be satisfactory. Combinations of an upper limit of any of these ranges with a lower limit of any of these ranges are also suitable. It is preferable not to substantially exceed 3 wt. %, as beyond this value, no significant increase in thermal stability is obtained, and the thermal stability may actually decrease. Moreover, the viscosity, and hence flowability of the curable composition will decrease as the particulate titanium content increased, and the modulus of the cured system will also frequently increase. An increased modulus is often undesirable, as it can promote adhesive failure (as opposed to cohesive failure).

[0060] In addition to the titanium dioxide fillers, similar amounts of other metal oxides, whether pyrogenic or precipitated, may be used. The preferred particle sizes and BET surface areas are the same as previously described for the pyrogenic titanium dioxide fillers. However, care must be taken to ensure that the viscosity of the uncured composition does not unduly increase, and that the modulus and hardness of the cured gel is still within the targeted range. Thus, it is preferred that the total amount of fine particle size fillers, including the particulate titanium dioxide, does not exceed 15-20% by weight, and preferably does not exceed 10%, and more preferably does not exceed 5% by weight. If an opaque composition can be tolerated, the same or larger weight percentages of coarse fillers, for example those having BET surface areas appreciably below 30 m /g, and or particle sizes > 2 μιη, preferably > 5 μιη, can be added, since particles with a very low surface area have minimal effect on viscosity. [0061] Examples of other small size particle fillers (E) include the previously mentioned transparent iron oxides, cerium oxide, zirconium oxide, zinc oxide, aluminum oxide, silicon dioxide, and other metal oxides. These metal oxides should be non-catalytic with regard to accelerating thermal instability of the gel, which can be easily assessed by simple thermal aging tests, and should, at least for most purposes, be dielectric in nature. Conductive particles such as those of carbon black, and semiconductive particles such as indium tin oxide, etc., should usually, and preferably, be absent. Thermally conductive particles such as A1BN, BN, and SiC may also be present. When other oxides, nitrides or other inorganic substances which are not physiochemically bonded to the titanium oxide are present, these may be supplied in an admixture with the titanium dioxide fillers.

[0062] The silicone gel compositions of the invention, in addition to the silicone gel- forming components, catalyst, and titanium dioxide filler, may contain other ingredients customary in silicone gel and also in silicone elastomer adhesive compositions. Such ingredients include light stabilizers; further thermal stabilizers ("secondary thermal stabilizers"), both inorganic or organic in nature, such as antioxidants; silicone resins; adhesion promoters; pigments; dyes; plasticizers; rheology control agents; additives which establish pot life or curing temperature; hollow glass, ceramic, or polymer beads (microballoons); inorganic and polymer fibers; biocides; and/or solvents This list should not be viewed as limiting in any way.

[0063] Light stabilizers (F) are well known to one skilled in the art, and are particularly useful when an optoelectronic device such as an LED is to be encapsulated, or where an encapsulated ("potted") component or electrical connection is to be exposed to light, particularly blue or violet light, or ultraviolet light. Preferred light stabilizers include hindered amine light stabilizers, or "HALS."

[0064] The light stabilizers (F), when present, are generally present in amounts of 0.01 to 5 weight percent, more preferably 0.05 to 4 weight percent, yet more preferably 0.1 to 3 weight percent, and most preferably 0.2 to 2 weight percent. [0065] Secondary thermal stabilizers (G) are compounds or compositions which provide an improvement in tensile strength, elongation, hardness, tear strength, lap shear strength, reduced discoloration, reduced cracking, reduced shrinkage or change in other physical or chemical properties as compared to an otherwise identical composition not containing the secondary thermal stabilizer after exposure to temperatures > 200°C for an extended period, for example but not by limitation, for 500, 1000, or 2000 hours. The secondary thermal stabilizers may be tested for effectiveness, for example, by the methods used in the examples, but other test methods are likewise suitable.

[0066] One class of secondary thermal stabilizers (G') includes ferrocene and ferrocene derivatives. Preferred ferrocene compounds are ferrocene ((di(cyclopentadienyl)iron), acetylferrocene, vinylferrocene, ethynyl ferrocene, ferrocenylmethanol, tetrachloroferrate(III), bis(.eta.-cyclopentadienyl)iron(III), tetracarbonylbis(.eta.-cyclo-pentadienyl)diiron(I), 1,1'- bis(trimethylsilyl)ferrocene, l,l'-(dimethylphenoxysilyl)ferrocene and l,l'-bis(dimethylethoxy- silyl)ferrocene. Particular preference is given to ferrocene and acetylferrocene. The ferrocene compound can also be a mixture of various ferrocene compounds.

[0067] The content of ferrocene compound is preferably selected so that the silicone mixture has a content of ferrocene compound of 0.001 to 5 percent by weight, preferably 0.01 to 4 percent by weight, based on the total weight of the uncured silicone gel composition.

[0068] A second class of secondary thermal stabilizers (G") includes, preferably, organic antioxidants known to those skilled in the art, preferably hindered amine-type and hindered phenol- type antioxidants. Examples include hindered phenols (such as BHT), organophosphites, secondary aromatic amines, hindered amines such as HALS and HAS (hindered amine stabilizers) amines, and many others. More than one stabilizer may be used, of the same or different type.

[0069] When a second class G" secondary thermal stabilizer G is present, the amounts, in wt. %, range from 0.01 to 4, preferably 0.05 - 3, more preferably 0.1 to 2 weight percent. Higher and lower amounts may also be used. The weight percent is based on the total weight of the uncured silicone gel composition. [0070] The optional silicone resins (H) are well known, and comprise highly crosslinked organopolysiloxanes containing M, D, T, and Q units. Examples include MT, MQ, MDT, MDQ, MDTQ, DT, DQ, and T resins. The terminology defining M, D, T, and Q units is well known, and may be described in a non-limiting fashion as follows:

[0071] M: R ₃ Si0 _1/2

[0072] D: R ^a ₂ Si0 _2/2 ,

[0073] T: R ^a Si0 _3/2 , and

[0074] Q: Si0 _4/2 ,

[0075] where R ^a is H, OH, R ^b O, or R, where R ^b is generally lower alkyl or alkenyl, such as

Ci_4 alkyl or vinyl, and R has the definitions previously given for R ² . Preferably, R ^a is R, most preferably methyl or phenyl or mixtures thereof.

[0076] In silicone resins, M, T, and Q units generally predominate, such that the respective compounds are highly crosslinked, or network like. M groups serve as end groups. D units are generally present in quantities of less than 20 mol percent based on the sum of M, D, T, and Q units, more preferably less than 15 mol percent, yet more preferably less than 10 mol percent, still more preferably less than 5 mol percent, and, most preferably, D units are not present in the silicone resin, except for their presence as unavoidable impurities.

[0077] If the silicone resins contain silicon-bonded hydrogen, it is preferred that no silicon- bonded OH groups are present. The silicone resins may contain both silicon-bonded hydrogen and silicone-bonded alkenyl groups such as vinyl, allyl, 2-propenyl, and higher ω-alkenyl groups such as 5-hexenyl, 7-octenyl, and the like, but this is not preferred. Alkoxy groups R ^b O are preferably present in only small amounts, for example 2 weight percent or less, preferably 1 weight percent or less, based on the total weight of the silicone resin, or may be essentially absent, only unavoidable amounts being present due to the method of preparation. While not illustrated in the foregoing formulae, the silicone resins may also contain very small quantities of silicon-bonded chlorine, also as a result of their method of preparation, but this is not preferred. Silicon-bonded chlorine is preferably absent.

[0078] Preferred silicone resins include MT, MQ, and T resins, preferably containing methyl and/or phenyl groups as R ^a . Where water ingress might be a problem, silicone resins containing long chain alkyl groups, preferably Cg-Cig alkyl groups such as n-octyl, 2-ethylhexyl, dodecyl, octadecyl, and the like, may be present.

[0079] When silicone resins are present, they are preferably present in amounts of from 0.1

- 20 weight percent, more preferably 0.5 - 10 weight percent, and most preferably 0.5 - 5 weight percent, based on the total weight of the uncured silicone gel composition. Silicone resins are preferably absent from the silicone gel composition.

[0080] Adhesion promoters (I) enhance the adhesion of the silicone gel composition to substrates such as, but not limited to metals, glass, sapphire, plastics. Suitable adhesion promoters (I) include functional silanes and partial hydrolysates thereof, epoxy resins, amine-functional polymers, and the like. The adhesion promoters generally contain as "functional" groups, polar groups, reactive groups, or mixtures thereof. For polymers, such polar and/or reactive groups include well known chemical groups such as primary, secondary, and tertiary amine groups, halo, hydroxyl, sulfhydryl, carboxyl, carboxylate, (meth) acrylic, (meth)acrylate, and epoxy groups. In the case of silane adhesion promoters, preferred functional groups include Si-bonded hydroxyl, alkoxy, alkenyl, (meth)acryl and meth(acryloyl), aminoalkyl, and epoxy groups. Examples of suitable silane adhesion promoters, which are generally monosilanes, are vinyltrialkoxysilanes such as vinyltrimethoxy silane; organoalkoxysilanes such as alkylalkoxysilanes and arylalkoxysilanes, examples being methyltrimethoxysilane and phenyltrimethoxysilane; epoxy-functional silanes such as glycidoxypropyltrimethoxysilane, and the like. Partial hydrolysates of such silanes are also preferred. Such partial hydrolysates are prepared by hydrolyzing the alkoxysilane with a limited amount of water such that hydrolysis is incomplete, and generally low molecular weight oligomers still having alkoxy functionality are obtained. [0081] The amount of adhesion promoter (I), when used, is preferably from 0.1 to 5 wt. %, more preferably 0.2 to 5 wt. %, and most preferably 0.2 to 2 weight percent, based on the total weight of the uncured silicone gel composition.

[0082] Pigments (J) are solid particles which increase opacity or alter color. "Pigments" do not include thermal stabilizing Ti0 ₂ fillers as defined herein. Pigments may be "transparent" pigments of very small particle size, or opacity-increasing pigments of larger particle size. The ability of pigments to scatter visible light is well known, as are the particle sizes of pigments which facilitate scattering, especially in the visible range, e.g. 400 - 700 nm. Examples of pigments include those of any color, including white pigments and black pigments. Examples include, but are not limited to, oxides, carbonates, phosphates, sulfides, silicates, molybdates, tungstates, etc., of numerous metals, examples including iron oxides, ground quartz, ground limestone, ground dolomite, precipitated inorganic pigments, and carbon black, among others. Organic pigments, which are similar to organic dyestuffs but are substantially insoluble in the silicone gel composition, may also be used. The amount of pigment (J) is not necessarily limited, and depends upon the desired degree of opacity and/or color and/or color saturation. In general, the amount of pigments is less than 20 wt. %, preferably less than 10 wt. %, and more preferably less than 5 wt. % based on the total weight of the uncured silicone gel composition. Pigments are preferably absent.

[0083] Dyes (K) are colored synthetic organic compounds or colored natural products which are soluble in the silicone gel compositions. Dyes do not affect the opacity of the composition, but decrease its light transmission at various portions of the visible UV, or IR ranges. The amounts of dyes which are useful spans a wide range depending upon the degree of absorbtion and/or color and/or color saturation desired, and is preferably less than 2 weight percent, based on the total weight of the uncured silicone gel composition. Dyes are preferably absent.

[0084] Plasticizers (L) are organic substances which generally decrease the viscosity of the uncured composition, and increase the softness and flexibility of the cured silicone gel or adhesive compositions. Such plasticizers are well known, and preferably have boiling points under standard pressure (1 bar) of 250°C or greater. Examples include paraffinic oils such as mineral oil and petrolatum, organophosphate esters such as tricresylphosphate, and esters based on alcohols and di- or tricarboxylic acids, or based on carboxylic acids and di- or triols. Esters of polyoxyalkylene polyethers may also be useful. Examples include diethylhexylphthalate, bis(2-ethylhexyl)adipate, bis(n-octyl)sebacate, ethylene glycol bis(octoate), triethyleneglycol bis(2-ethylhexanoate), and the like. Plasticizers also include non-functional organopolysiloxanes such as trimethoxysilyl- terminated polydimethylsiloxanes.

[0085] Reactive plasticizers (L') can also be used. Reactive plasticizers (L') are preferably hydrocarbons, esters, acrylates, or oligomeric or polymeric compounds containing an ethylenically or ethylynically unsaturated group such as a vinyl, allyl, or propargyl group which can participate in a hydro silylation reaction. Organopolysiloxanes bearing a single aliphatically unsubstituted group are also suitable, but are not included herein in the list of reactive plasticizers, as these have been previously described as a possible alkenyl-functional organopolysiloxane (A). An advantage of reactive plasticizers over conventional plasticizers is that the reactive plasticizers become chemically bound, and cannot exude, vaporize, or be extracted by organic solvents.

[0086] When either type of plasticizer (L or L') is used, the amounts are preferably less than

20 wt. % based on the total weight of the uncured silicone gel composition, and in order of increasing preference, less than 15 wt. %, less than 10 wt. %, and less than 5 wt. %. Mixtures of reactive and conventional plasticizers may be used. Plasticizers are preferably absent.

[0087] Rheology control agents (M) are compositions, other than low molecular weight organic solvents, which are added to increase or decrease viscosity, or to alter the thixotropy of the uncured composition. Preferred rheology control agents are soluble organic polymers which have a viscosity-altering effect, or very fine particle size fillers, commonly known as "reinforcing fillers."

Such fillers have a high BET surface area, 50 m 2 /g, to about 400 m 2 /g or more, preferably > 100 m 2 /g and more preferably > 200 m 2 /g. Preferred fillers of this type include fumed alumina, fumed silica, and fumed zirconia. In general, the higher the BET surface area and the smaller the particle size, the greater the thickening effect. Such rheology control additives are particularly useful in uncured compositions where low viscosity gel forming compounds are used, and where the particular application requires a higher viscosity or even thixotropy, for example when applied to level surfaces without boundaries, or to vertical or angled surfaces, where otherwise the composition could run or sag. It should be remembered that the silicone gel compositions are generally, but not always, cured at elevated temperatures. At such temperatures, viscosity is further reduced until such a time that crosslinking causes a rise in viscosity or gelling.

[0088] Both hydrophilic fillers or hydrophobic filers, particularly hydrophobic fillers, may be used. Examples of hydrophobic fillers are HDK® H20 and HDK® H30 hydrophobic fumed

2 2 silicas from Wacker Chemie, with nominal BET surface areas of 200 m7g and 300 m7g, respectively.

[0089] When rheology control agents (M) are used, they are used in amounts necessary to cause the desired effect, preferably in amounts of less than 20 weight percent, based on the total weight of the uncured silicone gel composition, more preferably less than 15 wt. %, yet more preferably less than 10 wt. %, and still more preferably less than 5 wt. %. Most preferably, no rheology control agents are used. It is noted that the rheology control agents may impart a slight increase in physical property retention which is desirable. Rheology control additives are preferably absent.

[0090] Additives (N) which regulate the pot life or "latency" of the silicone gel compositions, or alter the cure temperature or time, are frequently termed "inhibitors." These function to lower the activity of the hydro silylation catalyst at lower temperatures, e.g. room temperature, but are not effective at doing so at elevated temperatures. Inhibitors (N) are well known, and include a variety of organic sulfides, hydrosulfides, phosphines and phosphites such as substituted aryl triarylphosphines and phosphites, and especially acetylenic alcohols. One particularly common inhibitor is dehydrolinalool, 3-hydroxy-3,7-dimethylocta-6-ene- l-yne.

[0091] Inhibitors are optional, and can be used in the curable silicone gel compositions when long pot life and prevention of premature curing is desired, or when it is desired to provide the silicone gel composition as a storage stable, one-component composition. One-component compositions are sometimes preferable, as no metering or mixing is required. When inhibitors are used, they are used in customary amounts which provide the desired inhibition, keeping the projected cure temperature in mind, and also whether a one-component or two- or multi-component curable composition is desired. Amounts of from about 0.1 wt. % to 2 wt. % are typical, preferably 0.5 wt. % to 1.5 wt. %, based on the total weight of the uncured silicone gel composition.

[0092] Hollow beads (O) of polymer, ceramic, or glass may be added. These function primarily as a composition extender, reducing the expense of the formulation. Such "microballoons" are well known and are commercially available. The amounts are preferably less than 40 volume percent, more preferably, in order of increasing preference, less than 30 vol. %, less than 20 vol. %, and less than 10 vol. %. Preferably, no hollow beads are used. When used, they are often used for encapsulating components of relatively high volume, such as pumps and relays.

[0093] The silicone gel compositions may contain short fibers (P) such as but not limited to glass fibers, carbon fibers, basaltic fibers, or polymer fibers. Fibers may significantly affect flowability, and their use is not preferred. When used, fiber lengths are generally between 0.1 mm and 2 mm, more preferably from 0.1 mm to 1 mm. The amounts used, when fibers are present, is preferably below 5 wt. %, more preferably below 2 wt. %, based on the weight of the silicone gel composition. When fibers are used, they are preferably used for potting or encapsulating larger components. Short fibers are preferably absent.

[0094] Solvents (Q) are desirably absent. However, solvents (Q) may be added to lower the viscosity of the composition when constituents of high viscosity are used, or when larger amounts of particulate inorganic fillers are present. When solvents are used, it is preferable to cure the curable silicone gel compositions under conditions which allow the escape of the solvent without the formation of bubbles in the cured product. Preferred solvents are low molecular weight, essentially monomeric compounds, which include alkyl esters such as methyl acetate and ethylacetate; aliphatic and aromatic hydrocarbon solvents such as ligroin, petroleum ether, naphtha, benzene, toluene and xylenes; ketones such as methylethylketone; ethers such as diethylether, glyme, diglyme, and tetrahydrofuran; sulfolane; dimethlformamide; dimethylsulfoxide; and the like. Preferably, no solvents are used. When solvents are present, they are preferably used in amounts of, in order of increasing preference, less than 20 wt. %, less than 10 wt. %, less than 5 wt. %, and less than 2 wt. %, all based on the total weight of the curable silicone gel. [0095] Other ingredients such as biocides ® may also be present, but these are generally not necessary. Examples of suitable biocides (R) are the parabens, thiaoxazolidinones, and the like. Additional biocides are well known and commercially available. When biocides are present, the amount, in order of increasing preference, is < 5 wt. %, < 2 wt. %, and less than 1 wt. %, all based on the weight of the silicone gel composition. Biocides are preferably absent.

[0096] The invention also pertains to a process of preparing the curable silicone gel composition, wherein the curable gel components, Ti0 ₂ , and hydrosilylation catalyst, optionally also including any or all of the previously mentioned optional ingredients, are blended together in any order, to form a homogenous mixture. Preferably, the catalyst is blended in following the blending of the other components.

[0097] Preferred silicone gel and elastomer formulations complying with the prior description of the respective components of SEMICOSIL® 915 HT, SEMICOSIL® 920 LT, SEMICOSIL® 988/1K, and ELASTOSIL® RT-745S, all products of Wacker Chemie AG, Munich, Germany.

[0098] SEMICOSIL® 920 LT is a pourable two-component addition curing silicone which cures to a silicone gel upon addition of a Pt catalyst and curing at room temperature or at an elevated temperature (depending upon the catalyst chosen). Preferred catalysts are catalyst PT, catalyst PT-F, and catalyst UV. Catalysts PT and PT-F exhibit 2 mm curing times of 30 minutes and 10 minutes, respectively. Catalyst UV is a UV curing catalyst. When cured (10: 1 mixing ratio of the two components), SEMICOSIL® 915 HT exhibits a Shore 00 hardness (ASTM D2240) of 10, a penetration (9.38 g hollow cone; ISO 2137) of 20 mm/10, a volume resistivity (IEC 93) of 10 ¹⁵ Ω cm, a dielectric strength (IEC 243) of > 30 KV/mm, and a modulus of 0.1 MPa.

[0099] SEMICOSIL® 920 LT is a two-component, addition curing composition designed for a 1: 1 mix ratio. The silicone component(s) bear phenyl groups in part. The penetration hardness when cured is 70 mm/10. Curing time is adjustable by choice and concentration of catalyst and inhibitor. [0100] SEMICOSIL® 988/1K is a one-component composition with a curing time of 10 min (10 mm thickness) at 150°C. Upon curing, the elastomer has a Shore A hardness (ISO 868) of 35, a modulus of 0.5 MPa, a dielectric strength of 23 kV/mm, and a volume resistivity of 10 ¹⁵ Ω cm.

[0101] ELASTOSIL® RT-745 S is a two -component, pourable primerless silicone encapsulant designed for a 1: 1 mix ratio. The cured Shore 00 hardness is 37, the dielectric strength

23 kV/mm, volume resistivity 10 ¹⁵ Ω cm, and dielectric constant (VDE 0303 T4/50 Hz) is 2.9

[0102] The invention further pertains to a method for encapsulating electronic and electrical components, including optoelectronic components, comprising applying the inventive curable silicone gel composition to the respective component or components and curing to form a silicone gel.

[0103] A preferred composition contains:

a) an organopolysiloxane bearing aliphatically unsaturated groups, b) an organopolysiloxane bearing Si-H functionality

c) a hydrosilylation catalyst,

d) fine particle size titanium dioxide e) optionally,

dioxide;

f) optionally,

g) optionally,

h) optionally,

i) optionally,

j) optionally,

k) optionally,

1) optionally,

m) optionally,

n) optionally,

o) optionally,

P) optionally, q) optionally, one or more solvents (Q), and

r) optionally, one or more biocides (R).

[0104] Further preferred compositions contain,

a) at least one organopolysiloxane (A),

b) at least one organopolysiloxane (B),

c) a hydrosilylation catalyst,

d) fine particle size titanium dioxide

e) optionally, one or more inorganic fillers (E) other than finely divided titanium dioxide;

f) optionally, one or more light stabilizers (F);

g) optionally, one or more secondary thermal stabilizers (G);

h) optionally, one or more silicone resins (H);

i) optionally, one or more adhesion promoters (I);

j) optionally, one or more pigments (J);

k) optionally, one or more dyes (K);

1) optionally, one or more plasticizers (L);

m) optionally, one or more rheology control additives (M);

n) optionally, one or more inhibitors (N);

o) optionally, hollow glass, ceramic, or polymer beads (O);

p) optionally, short inorganic or organic fibers (P);

q) optionally, one or more solvents (Q), and

r) optionally, one or more biocides (R).

[0105] Yet further preferred compositions contain:

a) at least one organopolysiloxane of the formula (3)

b) at least one organopolysiloxane of the formula (4)

c) a hydrosilylation catalyst

d) fine particle size pyrogenic titanium dioxide e) optionally, one or more inorganic fillers (E) other than finely divided titanium dioxide;

f) optionally, one or more light stabilizers (F);

g) optionally, one or more secondary thermal stabilizers (G);

h) optionally, one or more silicone resins (H);

i) optionally, one or more adhesion promoters (I);

j) optionally, one or more pigments (J);

k) optionally, one or more dyes (K);

1) optionally, one or more plasticizers (L);

m) optionally, one or more rheology control additives (M);

n) optionally, one or more inhibitors (N);

o) optionally, hollow glass, ceramic, or polymer beads (O);

p) optionally, short inorganic or organic fibers (P);

q) optionally, one or more solvents (Q), and

r) optionally, one or more biocides (R).

[0106] Most preferred compositions contain:

a) an organopolysiloxane bearing aliphatically unsaturated groups, b) an organopolysiloxane bearing Si-H functionality

c) a hydrosilylation catalyst,

d) fine particle size titanium dioxide

g) optionally, one or more secondary thermal stabilizers, preferably ferrocene or a substituted ferrocene.

Examples

[0107] The examples below were, unless otherwise specified, produced by homogeneously blending the polymer components and the filler on a DISPERMAT® dissolver available from VMA Getzmann GmbH Verfahrenstechnik, Relchshof, Germany, at 2000 min ^"1 for 20 minutes. Neither the type of mixer nor mixing time are critical, so long as a homogenous mixture is obtained. Mixing takes place at room temperature, or about 20-25° C or at the temperature established during mixing, preferably without additional cooling or heating. The polymer components can be supplied as a premixed composition. The platinum catalyst is then added, preferably as a solution of platinum catalyst in component A, in a suitable ratio. In the examples, the catalyst is blended with the blend of A, B, and filler in the recommended weight ratio for the particular silicone. Blending also took place on the DISPERMAT® dissolver at 2000 min ^"1 .

[0108] The catalyzed mixture is then degassed under vacuum, and added to a glass jar with a tare weight of about 116 g capable of holding about 45 g of the silicone mixture, and the weight of silicone mixture determined. Curing is effected at 150°C for 12 hours, followed by cooling to room temperature and maintained for 16 hours. For mechanical property testing, the mixtures are cured at 150°C for 10 minutes or at 150°C for 30 minutes to assess lap shear strength. For testing tensile strength and elongation, suitable test plaques were prepared by casting a sheet of appropriate thickness.

[0109] Test Methods

[0110] Durometer Hardness

Durometer hardness is measure in accordance with ASTM D 2240-05. For specimens having a Shore A hardness greater than 10, a Type A durometer is used. For Shore A hardnesses of 10 or less, a Type 00 durometer is used. If multiple readings are made, the median is reported.

[0111] Tensile Strength and Elongation

Tensile strength and elongation at break are measured in accordance with ATM D 412-06a on standard dumbbell (Die C) specimens. A United E-VI instrument was used for the tests.

[0112] Tear Strength

Tear strength is measured according to ASTM D 412-06a on a United E-VI instrument, using a right angled specimen (tear, die B) where more than one measurement is performed, median values are reported. [0113] Lap Shear

Lap shear is measured using overlapping 25.4 mm wide aluminum substrates. The formulation is applied between the substrates with an overlap of 12.7 mm, at a preload rate of 1.27 mm/min (0.05 in/min) and testing rate of 1.27 mm/min, in accordance with ASTM D 1002-10, using a United Smart Load Frame.

[0114] Penetration Test

Gel hardness by the penetration test was measured by a Brookfield CT3 texture analyzer using a 35 mm deep probe with a spherical end of 12.7 mm diameter (TA 23 probe) unless otherwise specified. The force in grams necessary to achieve a penetration of 10 mm, or 5 mm where necessary, is measured.

Dielectric strength is measured using an AC Dielectric Test St, with a Phoenix Model 6CC50 - 2/D149 as per ASTM D 149-09.

[0115] Comparative Examples C1-C3

Potting compositions were prepared by blending 500 g SEMICOSIL® 915 HT (W acker Chemie) with 11 g of filler (see Table 1). The blend, in a 10: 1 weight ratio was then catalyzed with CAT PT (W acker Chemie) and deaired. Representative samples were exposed to various temperatures above 200°C for 1000 hours, and weight loss, discoloration, and other physical properties observed. The results are presented in Table 1.

[0116] TABLE 1

Ex. Filler Temp. Time % % Yellowing? Cracking? Feel Notes

(°C) (Hr) wt. Shrinkage

Loss

C2 ZnO ² 232 1000 4.1 2.2 yes yes rubbery pulls away

C3 ZnO ³ 232 500 6.4 n.m. yes yes hard pulls away fragments observed

CI ZnO ¹ 260 1000 21.7 33.3 yes yes brittle complete failure

C2 ZnO ² 260 1000 32.0 14.6 yes yes brittle crystallized zinc oxide 35 from AkroChem, 2 wt. % zinc oxide Zn 2605 from Nanophase, 2 wt. % zinc oxide, Elementis, 1 wt. %

[0117] Comparative Examples C1-C3 show that various commercially available zinc oxides fail to provide acceptable thermal stability in soft silicone gels.

[0118] Comparative Examples C4-C6 and Examples 7 and 8

[0119] Comparative Example C4

A soft silicone gel was prepared as in the previous examples, but using SEMICOSIL® 988 (Wacker Chemie) as the curable organopolysiloxane, blended with platinum catalyst as described previously. No filler was added. One set of test plaques were prepared from the composition and cured at 150°C for 10 minutes for testing tensile strength, elongation at break, and tear strength, as previously described. The composition was also applied to aluminum substrates, cured at 150°C for 30 minutes, and tested for lap shear strength, also as previously described. The testing followed storage at elevated temperatures over various periods of time. The results are presented below in Table 2.

[0120] Comparative Example C5

The procedure of Comparative Example C4 was followed, but 3 weight percent of Elementis Nanox 200 zinc oxide pigment was blended prior to blending in the catalyst. Cured specimens were tested in the same manner as for Comparative Example C4. Results are presented in Table 2.

[0121] Comparative Example C6

Comparative Example C5 was repeated, except that the Elementis Nanox® 200 zinc oxide was replaced by AkroChem Zinc Oxide 35. Test results may be found in Table 2.

[0122] Example 7

Comparative Example C5 was repeated, but the Elementis Nanox® 200 zinc oxide was replaced by titanium dioxide Aeroxide P25 (Evonik Industries). Test results may be found in Table 2.

[0123] Example 8

Example 7 was repeated, but the Aeroxide P25 was replaced by titanium dioxide Aeroxide PF2 (Evonik Industries). Test results may be found in Table 2.

[0124] In Table 2 below, test results from the foregoing Examples and Comparative

Examples are tabulated. The composition of Comparative Example C4 was clear (transparent). The compositions containing zinc oxide were white, while the compositions containing titanium dioxide were cream colored. There was no change of color in these examples after heat treatment. All physical properties reported are an average from three samples, rounded to one decimal place. [0125] TABLE 2

Example C4 C5 C6 7 8

204°C, 1000 H 4.6 6.0 6.4 10.7 7.5

232°C, 500 H 2.3 3.2 3.2 4.6 5.8

232°C, 1000 H 2.6 3.8 3.7 4.9 4.6

Lap Shear, MPa

Initial 1.98 2.3 1.8 1.7 2.6

204°C, 1000 H 3.5 3.0 3.3 2.4 3.3

232°C, 500 H 1.6 2.0 1.4 2.3 2.8

232°C, 1000 H 1.3 2.0 1.8 2.8 2.7

[0126] Table 2 shows that the inventive compositions 7 and 8, containing Ti0 ₂ filler, exhibited far better physical properties after thermal ageing at 204°C and 232°C than either the unfilled silicone gel C4 or the silicone gels containing zinc oxide fillers (C5, C6). While all physical properties declined after thermal ageing, the inventive examples retained considerably more of each physical property, sometimes from 3 to 8 times that of the Comparative Examples. It should be noted that the physical properties retained are more important than the decrease in physical properties as compared to the initial physical properties, as failure will be dictated by retained physical properties. Retention of significant elongation is particularly surprising and unexpected.

[0127] Comparative Examples C7 and Examples 9 and 10

These examples were prepared as for Comparative Example C4 and Example 8. In Comparative Example C7, ferrocene at 3% by weight is added as a stabilizer, while in Examples 9 and 10, ferrocene at 3 wt. % and 3 wt. % and 1.5 wt. %, respectively, of titanium dioxide was used as the stabilizer. The results of testing after thermal ageing are presented in Table 3 below.

TABLE 3

Example C4 C7 9 10

Filler Ferrocene 3% Aeroxide PF2 Aeroxide PF2

Ti0 ₂ 3%, and Ti0 ₂ 1.5%, and Ferrocene 3% Ferrocene 3%

Tensile Strength,

N/mm ²

Initial 5.8 4.6 5.5 5.3

204°C, 1000 H 1.3 1.0 1.7 1.5

204°C, 2000 H 0.81 0.8 1.7 1.5

232°C, 500 H 0.88 0.7 2.8 3.1

232°C, 1000 H 0.92 0.9 2.0 1.7

Elongation, %

Initial 323.7 269.7 286.0 280.0

204°C, 1000 H 48.6 50.3 88.0 84.0

232°C, 500 H 15.3 12.7 119.0 118.0

232°C, 1000 H 6.3 8.3 85.0 66.0

Durometer Tear

N/mm ² Example C4 C7 9 10

Initial 14.2 10.0 9.3 10.1

204°C, 1000 H 4.6 4.2 9.1 7.9

232°C, 500 H 2.3 2.2 6.4 5.5

232°C, 1000 H 2.6 2.3 5.6 5.8

Lap Shear, MPa

Initial 1.98 6.3 5.7 6.8

204°C, 1000 H 3.5 8.9 9.0 10.1

232°C, 500 H 1.6 3.7 6.2 8.1

232°C, 1000 H 1.3 4.0 5.1 6.1

[0128] Table 3 shows that addition of ferrocene, indicated by U.S. Patent 9,150,726 to improve thermal stability, offers some improvement in thermal stability when used in amounts of 3 weight percent, as compared to the same composition without any thermal stabilizer. However, greater thermal stability has been a long sought need. The failure mode in lap shear strength of the unstabilized composition, Comparative Example C4, was cohesive, except after thermal ageing at 232°C for 1000 H, where the failure mode was adhesive. With ferrocene alone, Comparative Example C7, the initial failure mode was cohesive, but all 204°C and 232°C failure modes were adhesive. In contrast cohesive failure was experienced in Examples 9 and 10 which employed ferrocene plus an inventive Ti0 ₂ stabilizer.

[0129] In addition, average tensile strength after 232°C ageing for 1000 H was 200% greater in the inventive examples (9, 10) as compared to ferrocene alone (C7), durometer tear strength was improved by considerably more than 200%, and elongation was improved, on average, by over 1700%! Lap shear was also improved, by about 50%. These results are surprising and unexpected. Even more surprising is that Example 10, with only 1.5 wt. % of Ti0 ₂ stabilizer, exhibited comparable properties when compared to Example 9, using twice as much Ti0 ₂ stabilizer. Tensile strength and Elongation, after ageing at 232°C for 1000 H, were slightly lower, but both durometer tear strength and lap shear strength were higher. These examples illustrate that a considerable improvement in thermal stability is achievable even with relatively low amounts of Ti0 ₂ stabilizer.

[0130] Example 11

A master batch was prepared by mixing 800 g of Semicosil® 915 HT silicone with 13.2 g Aeroxide PF 25 (1.5 wt. %) in a dispermat mixer at 2000 min ^"1 for 20 minutes, followed by de-airing under vacuum. The catalyst, CAT PT was added and mixed in at a ratio of 1 part catalyst to 10 parts silicone. The mixture was added to a 116 g tare weight glass jar and cured overnight at 150°C. Each jar contained approximately 45 g of silicone. The jars were stored at 232°C for various periods of time, and the percent weight loss and penetration hardness (5 mm) were measured. The results are presented in Tables 4 and 5.

[0131] Example 12

The procedure of Example 11 was followed, but Aeroxide PF2 was substituted for Aeroxide PF 25 (1.5%). The results may be found in Tables 4 and 5, and Figures 1 and 2.

[0132] Example 13

The procedure of Example 12 was repeated, but with Aeroxide PF2 and a loading of 0.75 wt. %. Results may be found in Tables 4 and 5 and in Figures 1 and 2.

[0133] Comparative Example 14

The procedure of Example 11 was repeated, but no titanium dioxide was added. TABLE 4

Weight Loss After Thermal Storage at Elevated Temperature

TABLE 5

Indentation Hardness After Thermal Storage at Elevated Temperature

Composition has a hard crust.

At 1000 hours, the composition has become fully crystallized. [0134] Tables 4 and 5 document the surprising and unexpected increase in property retention by the compositions according to the invention. After 3000 hours at 232°C, titanium dioxide-filled soft gels exhibited weight losses of 6.9 and 7.8 %, the latter with only 0.75 wt. % filler, as compared to a 20% weight loss of the unfilled composition. At 1.5 wt. % filler, a composition according to the invention exhibited a weight loss of only 7.8% after 1000 hours at 250°C. These examples contained no other thermal stabilizers, the addition of which would be expected to further increase property retention. Ferrocenes are one type of such additional thermal stabilizer.

[0135] While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.