Silicone rubber compositions and cured products

Abstract: A RTV silicone rubber composition comprising (A) an organopolysiloxane composed predominantly of a silalkylenesiloxane unit and containing an aliphatic unsaturated group in a molecule, (B) an organohydrogenpolysiloxane having at least two SiH groups in a molecule, and (C) a platinum group compound cures into a transparent elastomer which is increased in rubbery strength without adding a filler.

Description: This invention relates to silicone rubber compositions and cured products thereof. In particular aspects, it relates to room temperature vulcanisable (RTV) silicone rubber compositions which readily cure to rubbery products having good tensile strength and good transparency, by standing at room temperature or by moderate heating. Prior Art Nowadays silicone rubber compositions are widely used in a variety of fields because their cured products have good properties including heat resistance, low-temperature resistance, mold release and weathering resistance. However RTV silicone rubber compositions of the type in which linear organopolysiloxane polymers as a base component cure by peroxide, condensation or addition reaction suffer from the problem that cured products have low tensile strength, of the order of several kg/cm 2 . To overcome this problem, the silicone rubber compositions are generally filled with silica fine powder for reinforcement whereby the tensile strength of cured products is increased to from 10 to 140 kg/cm 2 . The filler loading in turn brings some inconvenient problems in that the loaded compositions are increased in viscosity and thus less good in flow; furthermore the cured products lose transparency. In the prior art, EP-A-154 898 describes addition-curable silicone rubber compositions in which, with the aim of enhancing tensile strength without the use of filler, the alkenyl-bearing organopolysiloxane has end groups of the formulae -Z-SiR 1 2 -O-Si(R 1 3-a ) (OSiRR 1 2 ) a where R is alkenyl, R 1 is any monovalent organic group, a is 2 or 3 and Z is alkylene, particularly ethylene or propylene. The general problem addressed herein is to provide novel vulcanisable silicone rubber compositions (as well as their use and preparation) which can give high tensile strength and cured products e.g. without a filler being required, and which can also give good transparency. According to the present invention, there is provided a silicone rubber composition, preferably room temperature vulcanisable, comprising (A) organopolysiloxane containing at least 50 mol% of silalkylenesiloxane unit of the general formula: wherein R 1 , R 2 , R 3 and R 4 are independently selected from substituted or unsubstituted monovalent hydrocarbon groups and n is an integer of 4 to 16, said organopolysiloxane containing at least one aliphatic unsaturated group per molecule, (B) organohydrogenpolysiloxane having at least two SiH groups in a molecule, and (C) platinum group catalyst. The inventors found that by using an organopolysiloxane composed predominantly of a silalkylenesiloxane unit as defined above as a silicone rubber component, they could obtain RTV silicone rubber compositions which cured to elastomers which were increased in tensile strength, without the need to add fillers such as silica powder to the composition. The cured elastomers remain transparent. Another aspect is the blending of components (A), (B) and (C) to make a silicone rubber composition. DETAILED DESCRIPTION The first essential component of the silicone rubber composition according to the present invention is (A) an organopolysiloxane comprising silalkylenesiloxane units of formula (1) and containing an aliphatic unsaturated group in a molecule. In formula (1), n is an integer of 4 to 16 and R 1 , R 2 , R 3 and R 4 are independently selected from substituted or unsubstituted monovalent hydrocarbon groups, preferably monovalent hydrocarbon groups having 1 to 8 carbon atoms, for example, alkyl groups such as methyl, ethyl and propyl groups, cycloalkyl groups such as a cyclohexyl group, alkenyl groups such as vinyl, allyl and isopropenoxy groups, aryl groups such as phenyl and tolyl groups, and substituted ones of these groups in which some hydrogen atoms are replaced by halogen atoms or the like, such as 3,3,3,-trifluoropropyl, 6,6,6,5,5,4,4,3,3-nonafluorohexyl, chloromethyl and 3-chloropropyl groups. Groups R 1 to R 4 may be identical or different. In addition to the silalkylenesiloxane unit of formula (1), the organopolysiloxane may contain another unit or units such as R 5 R 6 SiO and R 7 R 8 R 9 SiO 1/2 units wherein R 5 to R 9 are as defined for R 1 to R 4 and may be identical or different. The organopolysiloxane contains at least 50 mol%, preferably at least 70 mol% of the silalkylenesiloxane unit of formula (1) based on the entire siloxane units. With a silalkylenesiloxane content of less than 50 mol%, cured products tend to have lower strength. More specifically, the organopolysiloxane may comprise 50 to 100 mol%, preferably 70 to 100 mol% of the silalkylenesiloxane unit of formula (1), 0 to 50 mol%, preferably 0 to 30 mol% of R 5 R 6 SiO unit and 0.01 to 2 mol%, preferably 0.01 to 1 mol% of R 7 R 8 R 9 SiO 1/2 unit. The organopolysiloxane contains at least one aliphatic unsaturated group per molecule, either intermediate or terminal on its molecular chain. The content of aliphatic unsaturated group is preferably not more than 0.5 mol, more preferably up to 0.1 mol per 100 grams of the organopolysiloxane. An aliphatic unsaturated group content of more than 0.5 mol/100 grams tends to give an elastomer which is increased in crosslinking density and thus reduced in elongation. Preferably, the organopolysiloxane has a viscosity of 2.5 x 10 -5 to 5 x 10 -1 m 2 /s (25 to 500,000 centistokes (cs)) at 25°C, more preferably 10 -3 to 10 -1 m 2 /s (1,000 to 100,000 cs) at 25°C. Some illustrative, non-limiting, examples of organopolysiloxane (A) are given below. In the following formulae, Me is a methyl group and Ph is a phenyl group and this abbreviation is used throughout the specification. The organopolysiloxane may be synthesized, for example, by the following reaction scheme. The starting reactant, that is, compound of formula (2), may be selected in accordance with the silalkylenesiloxane unit of the organopolysiloxane. The compound of formula (2) may be hydrolyzed in a conventional procedure. The hydrolyzed compound is then polymerized in a conventional procedure to form a compound of formula (3) as an intermediate. The compound of formula (3) is further copolymerized with disilanols or their analogues to synthesize silalkylenesiloxane polymers. Disilanols and analogues usable in this silylation are shown below. wherein ℓ is 2 to 100. Preferably, the silylation reaction may be promoted by using a silylating agent. Exemplary silylating agents are of the following formulae. R 10 3 SiX X is a halogen, -NCO, etc. (R 10 3 Si ) 2 NH R 10 to R 12 are as defined for R 1 to R 4 . Where the silylation reaction takes place in the presence of such a silylating agent, any of the following groups can be introduced at the terminal of the resulting polymer. It will be understood that the silylation reaction can be performed under conventional conditions. The second essential component is (B) an organohydrogenpolysiloxane having at least two SiH groups in a molecule. It functions as a crosslinking agent. The organohydrogenpolysiloxane is not particularly limited in form, molecular weight or the like and may be linear or cyclic and have a low or high molecular weight. Often those having a relatively low molecular weight of lower than 300,000 are preferred because of ease of manufacture. The conventional organohydrogenpolysiloxanes can be used as component (B). The organohydrogenpolysiloxane as a crosslinking agent should preferably be compatible with component (A) in order to provide a uniformly cured product. Examples of the organohydrogenpolysiloxane are compounds of the following structures. The organohydrogenpolysiloxane may be synthesized by any conventional procedure. For example, it is synthesized (1) by equilibration among the components: wherein e is a number of 3 to 8 in the presence of an acid catalyst such as sulfuric acid, (2) by co-hydrolysis reaction of corresponding silanes or siloxanes, or (3) by partial addition reaction where a special functional group is present in accordance with the teaching of Japanese Patent Publication (JP-B) No. 33540-1976. Preferably, component (B) or hydrogenpolysiloxane is blended in an amount of 0.1 to 50 parts, more preferably 1 to 10 parts by weight per 100 parts by weight of component (A). In particular, the organohydrogenpolysiloxane is preferably blended in such amounts that 0.5 to 5 mol, especially 1.2 to 3 mol of SiH group is present per mol of the aliphatic unsaturated group in the entire composition. An organohydrogenpolysiloxane content below the above-defined range tends to lead to a lower degree of crosslinking whereas above the range, foaming is liable to occur and some properties like heat resistance and compression set may be adversely affected. As the third essential component, the composition contains (C) a catalyst, desirably a platinum group catalyst which serves for promoting addition reaction or hydrosilylation between components (A) and (B). Any of platinum group catalysts may be used although they are generally expensive because they are noble metal catalysts. Among others, platinum compounds are preferred because of ease of availability. Examples of platinum compounds include chloroplatinic acid, complexes of chloroplatinic acid with olefins (e.g., ethylene), alcohols, and vinylsiloxanes, Examples of solid catalysts are platinum on silica, alumina and carbon supports. To obtain more uniformly cured products, chloroplatinic acid or a complex thereof is desirably dissolved in a suitable solvent so that it may be miscible with component (A). Besides the platinum compounds, compounds of rhodium, ruthenium, iridium and palladium may also be used as component (C). Examples are RhCl(C 2 H 4 ) 2 , Ru 3 (CO) 12 , IrCl(CO)(PPh 3 ) 2 , Pd(PPh 3 ) 4 , RhCl(PPh 3 ) 3 , and RhCl(CO)(PPh 3 ) 2 . Component (C) is used in a catalytic amount, although the amount of component (C) blended is preferably about 1 to 1,000 ppm, more preferably about 10 to 500 ppm especially when the cost is taken into account. In addition to the above-mentioned essential components (A) to (C), the composition may contain further components consistent with useful properties. Exemplary of such additives are organopolysiloxanes of the resin structure comprising SiO 2 , CH 2 =CH(R') 2 SiO 1/2 and R' 3 SiO 1/2 units wherein R' is a monovalent hydrocarbon group free of an aliphatic unsaturated double bond which are added to enhance the strength of the resulting cured elastomers (see JP-B 26771/1963 and 9476/1970); polysiloxanes comprising a CH 2 =CH(R)SiO unit wherein R is a substituted or unsubstituted monovalent hydrocarbon group which are added to control the curing rate of the compositions (see JP-B 10947/1973); acetylene compounds (see USP 3,445,420 and JP-B 3774/1979); and ionic heavy metal compounds (see USP 3,532,649). It is also acceptable to add non-functional organopolysiloxanes for the purposes of improving thermal shock resistance and flexibility. Where these additives are blended, their amount is preferably adjusted such that 0.5 to 5 hydrogen atoms directly attached to silicon atoms are present per unsaturated bond given as the total of alkenyl and ethynyl groups directly attached to silicon atoms in the entire composition. Insofar as the desirable properties of the composition are still achieved, a filler may be included e.g. for the purposes of reducing thermal shrinkage upon curing, reducing the thermal expansion, improving the thermal stability, weather resistance, chemical resistance, flame retardancy, and mechanical strength, and reducing the gas permeability of cured products or elastomers. Examples of the filler include fumed silica, quartz powder, glass fibers, carbon, metal oxides such as iron oxide, titanium oxide, and cerium oxide, and metal carbonates such as calcium carbonate and magnesium carbonate. Fumed silica is preferred since it can maintain cured products transparent. If neccessary, any suitable one of pigments, dyes and anti-oxidants may be added. Depending on the particular application and purpose, the silicone rubber composition may be dissolved in a suitable organic solvent such as toluene and xylene to a desired concentration before use. The silicone rubber composition may cure at room temperature depending on the type of functional group on component (A) and the type of catalyst (C). Alternatively, the composition is cured by moderate heating, for example, heating at 100 to 150°C for a relatively short time of several minutes to several hours. There has been described a RTV silicone rubber composition which can be embodied as a cured transparent elastomer having high rubbery strength. EXAMPLE Examples of the present invention are given below by way of illustration and not by way of limitation. All parts are by weight. Example 1 and Comparative Example 1 Silicone rubber composition A was prepared by blending the following components: 100 parts of an organopolysiloxane containing a silalkylenesiloxane unit (viscosity 1.23 x 10 -2 m 2 /s (12,300 cs), vinyl group content 0.008 mol/100 g) of formula (4): 2.5 parts of methylhydrogenpolysiloxane (viscosity 1.1 x 10 -5 m 2 /s (11 cs)) of formula (5): 0.1 parts of a toluene solution of a chloroplatinic acid modified with catalyst (platinum concentration 1.0% by weight), and 0.1 parts of cyclotetrasiloxane of formula (6): Composition A was deaerated in vacuum, cast into a mold cavity dimensioned 13 × 17 × 0.20 cm, and cured at 100°C for 60 minutes, obtaining a transparent elastomer (Example 1). Silicone rubber composition B was prepared by blending the same components as composition A except that the compound of formula (4) was replaced by a dimethylpolysiloxane blocked with a vinyl group at each end (viscosity 5 x 10 -3 m 2 /s (5,000 cs)). It was similarly cured, obtaining a transparent elastomer (Comparative Example 1). Example 2 Silicone rubber composition C was prepared by blending the following components: 100 parts of an organopolysiloxane containing a silalkylenesiloxane unit (viscosity 2.23 x 10 -2 m 2 /s (22,300 cs), vinyl group content 0.007 mol/100 g) of formula (7): 2.2 parts of methylhydrogenpolysiloxane of formula (5) used in Example 1, 0.05 parts of a 2-ethylhexanol-modified chloroplatinic acid catalyst (platinum concentration 2.0% by weight), and 0.05 parts of cyclotetrasiloxane of formula (6) used in Example 1. Composition C was cured as in Example 1, obtaining a transparent elastomer. The elastomers of Examples 1 and 2 and Comparative Example 1 were measured for physical properties in accordance with JIS K-6301. The results are shown in Table 1. Example 1 Comparative Example 1 Example 2 Silicone rubber composition A B C Specific gravity 0.89 0.98 0.88 Hardness, JIS A scale 18 20 23 Elongation, % 260 180 280 Tensile strength, kg/cm 2 23 3 16 As is evident from Table 1, those silicone rubber compositions containing an organopolysiloxane having a silalkylenesiloxane unit and an aliphatic unsaturated group (Examples 1 and 2) cure into elastomers having higher strength than the silicone rubber composition containing dimethylpolysiloxane (Comparative Example 1). Example 3 and Comparative Example 2 Silicone rubber composition D was prepared by blending the following components: 100 parts of an organopolysiloxane containing a silalkylenesiloxane unit (viscosity 1.23 x 10 -2 m 2 /s (12,300 cs), vinyl group content 0.008 mol/100 g) of formula (8): 2.5 parts of methylhydrogenpolysiloxane of formula (5) used in Example 1, 0.05 parts of a 2-ethylhexanol-modified chloroplatinic acid catalyst (platinum concentration 2.0% by weight), 0.05 parts of cyclotetrasiloxane of formula (6) used in Example 1, and 15 parts of fumed silica treated with a trimethylsiloxy group having a specific surface area of 200 m 2 /g. Composition D was deaerated in vacuum, cast into a mold cavity dimensioned 13 × 17 × 0.20 cm, and cured at 120°C for 20 minutes, obtaining a transparent elastomer (Example 3). Silicone rubber composition E was prepared by blending the same components as composition D except that the compound of formula (8) was replaced by a dimethylpolysiloxane blocked with a vinyl group at each end (viscosity 5 x 10 -3 m 2 /s (5,000cs)). It was similarly cured, obtaining a milky white elastomer (Comparative Example 2). These elastomers were measured for physical properties in accordance with JIS K-6301. The results are shown in Table 2. Example 3 Comparative Example 2 RTV Silicone rubber composition D E Specific gravity 0.96 1.06 Hardness, JIS A scale 45 37 Elongation, % 360 320 Tensile strength, kg/cm 2 165 22 As is evident from Table 2, the silicone rubber composition containing an organopolysiloxane having a silalkylenesiloxane unit and an aliphatic unsaturated group (Example 3) cures into an elastomer which remains transparent, despite the blending of fumed silica, because of the proximity of the respective refractive indexes. The reinforcement by fumed silica is more significant than with the silicone rubber composition containing dimethylpolysiloxane (Comparative Example 2). While the invention has been described with reference to what are presently considered to be preferred embodiments, other variations and modifications will become apparent to those skilled in the art. It is intended, therefore, that the invention not be limited to the illustrative embodiments.

Claims: A vulcanisable silicone rubber composition comprising (A) organopolysiloxane comprising at least 50 mol% of silalkylenesiloxane units of the general formula: wherein R 1 , R 2 , R 3 and R 4 are independently selected from substituted or unsubstituted monovalent hydrocarbon groups and n is an integer of 4 to 16, said organopolysiloxane containing at least one aliphatic unsaturated group per molecule, (B) organohydrogenpolysiloxane having at least two SiH groups in a molecule, and (C) platinum group catalyst. A composition of claim 1 wherein organopolysiloxane (A) contains at least 70 mol% of silalkylenesiloxane units of formula (1). A composition of claim 1 or 2 wherein organopolysiloxane (A) contains not more than 0.5 mol of aliphatic unsaturated group per 100 grams of the organopolysiloxane. A composition of any preceding claim wherein organohydrogenpolysiloxane (B) has a molecular weight of up to 300,000. A composition of any preceding claim which contains 100 parts by weight of organopolysiloxane (A), 0.1 to 50 parts by weight of organohydrogenpolysiloxane (B), and a catalytic amount of platinum group catalyst (C). A cured product of the composition of any one of claims 1 to 5. A method of making a vulcanisable silicone rubber composition, comprising blending (A) organopolysiloxane comprising at least 50 mol% of silalkylenesiloxane units of the general formula: wherein R 1 , R 2 , R 3 and R 4 are independently selected from substituted or unsubstituted monovalent hydrocarbon groups and n is an integer of 4 to 16, said organopolysiloxane containing at least one aliphatic unsaturated group per molecule, (B) organohydrogenpolysiloxane having at least two SiH groups in a molecule, and (C) platinum group catalyst.

Inventors: KOIKE NORIYUKI, MATSUDA TAKASHI, SATO SHINICHI, TAKAGO TOSHIO, KOIKE, NORIYUKI, MATSUDA, TAKASHI, SATO, SHINICHI, TAKAGO, TOSHIO
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1992
Main classification: C08L83
IPC codes: C08L83