Method of Foaming a Siloxane Composition Using Microwave Energy

1. Field of the Invention

This invention relates to a method of foaming and curing silicone gum stocks by using microwave.

2. Description of the Prior Art

It is known that silicone rubber can be foamed or sponged by using various blowing agents which release nitrogen or carbon dioxide gas. Berridge in U.S. Patent No. 2,857,343 describes using blowing agents such as dinitrosopentamethylenetetramine and N,N' -dimethyl-N,N'-dinitrosoterephthalamide with a combination of two peroxide catalysts, tertiary butyl perbenzoate and either benzoyl peroxide or bis-(2,4-dichlorobenzoyl)peroxide to make silicone rubber sponge. Berridge in U.S Pat. No. 2,875,163 describes using a heating process before adding the catalysts and blowing agent to improve the final product. Wade and Blanchard in U.S. Pat. No. 2,951,819 describe making cellular rubber-like articles using stearic acid and alkyl metal borohydrides as the blowing agent. Wade et al. also describe using sodium carbonate and p,p-oxybis(benzene sulfonyl hydrazide) as blowing agents for natural and synthetic rubber-like materials. Smith in U.S. Pat. No. 3,238,157 describe making cellular silicone rubber by mixing silicone gums with particulate, solid, void-producing matter, by curing the mixture to a silicone elastomer and thereafter leaching out the particulate matter thereby forming the cellular silicone rubber.

Bond and Tomita in U.S. Pat. No. 3,271,332 describe making a room temperature vulcanizable siloxane foam by adding a basic vulcanization catalyst, such as dibutyltindilaurate to a mixture of 60 to 80 parts by weight of a diorganopolysiloxane fluid gum free of hydroxyl termination having a viscosity of 1,000 to 50,000 cps. at 25.degree. C., 40 to 20 parts by weight of a benzenesoluble silicone resin composed of R.sub.3 SiO.sub.0.5 and SiO.sub.2 units, sufficient methyl hydrogen polysiloxane to provide from 0.017 to 0.17 part by weight silanic hydrogen and between 0.1 to 2 parts by weight of a basic vulcanization catalyst active to split off silanic hydrogen.

Modic in U.S. Pat. No. 3,425,967 describes a foamable mixture comprising 100 parts of a vinyl chain-stopped polysiloxane, 10 to 100 parts organopolysiloxane copolymer of R.sub.3 SiO.sub.0.5 and SiO.sub.2 units having 2.5 to 10 mole percent silicon-bonded vinyl groups, 10 to 100 parts asbestos or fibrous potassium titanate, 0 to 50 parts of a finely divided inorganic filler, a platinum catalyst, an amount of liquid organohydrogenpolysiloxane to provide from 0.5 to 1.0 silicon-bonded hydrogen atoms per silicon-bonded vinyl group and 1 to 5 parts of a blowing agent, such as azo-isobutyronitrile, dinitrosopentamethylenetetramine, benzenesulfonhydrazide, N,N'-dinitroso-N,N'-dimethylterephthalamide, p,p'-oxy-bis-(benzenesulfonhydrazide), terephthalazide and azodicarbonamide.

Wada, Itoh and Kuga in U.S. Pat. No. 3,677,981 describe foamable silicone elastomeric compositions comprising 80 to 99 parts diorganopolysiloxane gum having a viscosity of at least 1,000,000 cs. at 25.degree. C. and 0.025 to 0.25 moles of vinyl group, 1 to 20 parts of diorganopolysiloxane having a viscosity of at least 10 cs. at 25.degree. C. and at least ten times as many moles of vinyl group as the gum, an amount of organohydrogenpolysiloxane such that the Si-H linkages are from 50 to 200 mole percent of the total vinyl groups and there are at least three Si-H per molecule, 20 to 100 parts of silica, a catalytic amount of a platinum catalyst and 1 to 10 parts blowing agent such as azobisisobutyronitrile, dinitrosopentamethylenetetramine, N,N' -dimethyldinitrosoterephthalamide and diaminobenzene.

Other than Bond et al. cited above, these references describe heat activated and curing foams. Siloxane foams have also been prepared at room temperature by combining hydroxylated organopolysiloxanes, silicon-bonded hydrogen and tin salts of carboxylic acid such as described by Bruner in U.S. Pat. No. 3,070,555, by Nitzsche et al. in U.S. Pat. No. 3,338,847, Hersh in U.S. Pat. No. 3,429,838, Joyce in U.S. Pat. No. 2,956,032 and Nitzsche et al. in U.S. Pat. No. 3,428,580. More recently, Schuyler B. Smith in an application Ser. No. 519,380, filed Oct. 30, 1974, now U.S. Pat. No. 3,923,705 and assigned to the same assignee as this application described a method of making organosiloxane foams by mixing an organohydrogensiloxane having at least three silicon-bonded hydrogen atoms per molecule, a hydroxylated organosiloxane having from greater than 1.0 to 2.5 silicon-bonded hydroxyl radicals per molecule and a platinum catalyst wherein the molar ratio of silicon-bonded hydrogen atoms to silicon-bonded hydroxyl radicals is from 2.5 to 40 and the viscosity of the mixture is less than 100,000 cps. at 25.degree. C. Smith also describes a method wherein there is also present a benzene-soluble vinyl containing triorganosiloxy endblocked polydiorganosiloxane.

Foams or sponges described above use external blowing agents or use tin salts with silicon-bonded hydrogen to make the foam. The external blowing agents are useful but many are dangerous to handle or there is difficulty in regulating the gas formation from the blowing agent and the curing at the same time. The prior art patents have therefore made various contributions to overcome this problem. Additionally the resulting foam properties required improvement and composition variations were also described by these references to improve the foam properties. Foams could be prepared using tin salts but these foams were not as useful as hoped for, because of properties and stability over broad temperature ranges. Smith in Ser. No. 519,380 describes a more suitable foam which could be used in areas where fire retardancy was required, however, Smith's method required use within a short period of time, once the ingredients were all mixed. It was therefore unexpected that a platinum catalyst could be used to make gum based stocks foam because they require substantial mixing, such as on mills, to prepare the compositions and premature foaming and curing would certainly not be desirable, especially if it occured on the mill.

Where these spongeable compositions are described in an application Ser. No. 595,918, filed on even date herewith July 14, 1975, by CHi-Long Lee and Gary M. Ronk entitled "Spongeable Silicone Gum Stocks" which is assigned to the same party as the present application. This application discloses compositions comprising 100 parts by weight of a polydiorganosiloxane gum base stock, 15 to 60 parts by weight of an organosiloxane having silicon-bonded hydroxyl radicals, 3 to 25 parts by weight of an organohydrogensiloxane, 0.05 to 0.5 parts by weight acetylenic alcohol inhibitor and 1 to 200 p.p.m platinum as a platinum catalyst. Foamable gum base stocks are also described in an application Ser. No. 595,920, filed July 14, 1975 by Ronald E. Kittle entitled "Foamable Silicone Gum Stocks Having Less Skin Thickness" assigned to the same party as the present application. The compositions described by Kittle are similar to those described by Lee and Ronk except the 15 to 60 parts by weight of organosiloxane having silicon-bonded hydroxyl radical is replaced by 5 to 30 parts by weight of an organic alcohol having one C--OH per molecule.

It is an object of this invention to provide a method of foaming a silicone gum based composition using microwave.

This invention relates to a method of exposing to a microwave source to foam and cure a composition comprising a polydiorganosiloxane gum base stock, an organohydrogensiloxane, a platinum catalyst, an acetylenic alcohol inhibitor and a microwave sensible material.

This invention relates to a method of foaming a silicone composition comprising mixing (A) 100 parts by weight of a polydiorganosiloxane gum base stock, (B) from 0 to 60 parts by weight of an organosiloxane having an average of at least 2 silicon-bonded hydroxyl radicals per molecule, where the organosiloxane (B) has from 0.5 to 3 parts by weight silicon-bonded hydroxyl radicals in the amount of organosiloxane (B) present, only when (F) is an organic alcohol can (B) be zero, (C) from 3 to 25 parts by weight of an organohydrogensiloxane having an average of at least 2.5 silicon-bonded hydrogen atoms per molecule, where the organohydrogensiloxane has from 0.01 to 0.5 parts by weight silicon-bonded hydrogen atoms in the amount of organohydrogensiloxane (C) present, (D) from 0.05 to 0.5 parts by weight of an acetylenic alcohol inhibitor, (E) from 1 to 200 parts by weight platinum per one million parts by weight of composition wherein the platinum is in the form of a platinum catalyst, and (F) from 0.25 to 30 parts by weight of a microwave sensible material and thereafter exposing the mixture to a microwave source to obtain a silicone foam.

The polydiorganosiloxane gum base stock can be any of the gum stocks available commercially and well known in the art. These gum stocks basically comprise a polydiorganosiloxane gum and a reinforcing filler, such as fume silica, and often contain other additives such as plasticizers, heat stability additives, pigments and colorants, extending fillers, flame retardant additives, compression set additives, anti-bloom additives, handling additives and the like. The polydiorganosiloxane gums can generally be described as benzene soluble and having a Williams plasticity of greater than 0.030 inch. The organic groups on the polydiorganosiloxane gums can be any of those found in the commercial gum stocks including methyl, phenyl, vinyl, 3,3,3-trifluoropropyl, among others. These gums are most commonly endblocked with triorganosiloxy units or hydroxyl radicals. The polydiorganosiloxane gum base stocks can be either uncatalyzed or catalyzed with a vulcanizing agent such as an organic peroxide.

The organosiloxane, (B), can be any organosiloxane or mixture of organosiloxanes which have an average of at least two silicon-bonded hydroxyl radicals per molecule. These organosiloxanes can be any of the monovalent organic groups which do not interfere with the curing or foaming action of the composition. Preferably, the organic groups are alkyl radicals having from 1 to 6 carbon atoms per radical, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, isopropyl, tertiary butyl, and cyclohexyl, phenyl radicals or 3,3,3-trifluoropropyl radicals. In the amount of the organosiloxanes of (B) present in the composition, there is from 0.5 to 3 parts by weight of silicon-bonded hydroxyl radical, preferably from 0.5 to 2.5 parts by weight. These organosiloxanes can be homopolymers, copolymers or mixtures thereof. They can be mixtures of different molecular weight species and varying hydroxyl contents as long as the average falls within the defined range. These organosiloxanes preferably contain an average of at least one organic radical per silicon atom. Examples of the organosiloxane, (B), include hydroxyl endblocked polydimethylsiloxane, hydroxyl endblocked polydiorganosiloxanes having siloxane units of dimethylsiloxane and phenylmethylsiloxane, hydroxyl endblocked polymethyl-3,3,3-trifluoropropylsiloxane and hydroxyl endblocked polyorganosiloxane having siloxane units of monomethylsiloxane, dimethylsiloxane, monophenylsiloxane and diphenylsiloxane. The organosiloxanes can also include mixtures of hydroxylated organosiloxane polymers and hydroxylated organosilanes, such as a mixture of hydroxyl endblocked polydimethylsiloxane and diphenylmethylsilanol. The molecular weight of these organosiloxanes will be limited by the amount of silicon-bonded hydroxyl radical as defined by the limits recited above. The organosiloxane is preferably a hydroxyl endblocked polydimethylsiloxane having a viscosity of from 20 to 200 cps. at 25.degree. C.

The organohydrogensiloxane, (C), can be any siloxane having an average of at least 2.5 silicon-bonded hydrogen atoms per molecule and an average of no more than one silicon-bonded hydrogen atom per silicon atom. The remaining valences of the silicon atoms are satisfied by divalent oxygen atoms or by monovalent radicals of alkyl having one to six carbon atoms per radical, such as methyl, ethyl, propyl, isopropyl, butyl, tertiary butyl, pentyl, hexyl, cyclohexyl, or phenyl or 3,3,3-trifluoropropyl. The organohydrogensiloxanes can be homopolymers, copolymers and mixtures thereof which contain siloxane units of the following types:

RsiO.sub.1.5, R.sub.2 SiO, R.sub.3 SiO.sub.0.5,

RhsiO, HSiO.sub.1.5, R.sub.2 HSiO.sub.0.5,

H.sub.2 siO, RH.sub.2 SiO.sub.0.5 and SiO.sub.2

where R is the monovalent radical defined above. Some specific examples include polymethylhydrogensiloxane cyclics, copolymers of trimethylsiloxy and methylhydrogensiloxane, copolymers of dimethylhydrogensiloxy and methylhydrogensiloxane, copolymers of trimethylsiloxy, dimethylsiloxane and methylhydrogensiloxane and copolymers of dimethylhydrogensiloxane, dimethylsiloxane and methylhydrogensiloxane. Preferably, the organohydrogensiloxanes have an average of at least five silicon-bonded hydrogen atoms per molecule. In the amount of the organohydrogensiloxanes of (C) present in the composition, there is from 0.01 to 0.5 parts by weight silicon-bonded hydrogen atoms. The organohydrogensiloxane is preferably a trimethylsiloxy endblocked polymethylhydrogensiloxane having a viscosity of from 10 to 200 cps. at 25.degree. C.

The acetylenic alcohol inhibitors, (D), are those acetylenic alcohols which are inhibitors for platinum catalyzed compositions at room temperature as described in U.S. Pat. No. 3,445,420 which is hereby incorporated by reference to show this class of inhibitors. Examples of these inhibitors include 3-methyl-1-pentyn-3-ol, 1-ethynylcyclo-hexan-1-ol, 3,5-dimethyl-1-hexyn-3-ol, 3-phenyl-1-butyn-3-ol and 3-methyl-1-butyn-3-ol.

Platinum catalyst, (E), can be any of the known forms, ranging from platinum as deposited on carriers such as silica gel or powdered charcoal, to platinic chloride, salts of platinum and chloroplatinic acid. Any of these forms will function in the instant curing system. A preferred form of platinum is the chloroplatinic acid either as the commonly obtainable hexahydrate or the anhydrous form, on account of its easy dispersability in organosilicon systems and its non-effect on color of the mixture. Another preferred platinum catalyst is a chloroplatinic acid catalyst complex as prepared by the method described in U.S. Pat. No. 3,419,593 which is hereby incorporated by reference, where chloroplatinic acid hexahydrate is mixed with symetrical divinyltetramethyldisiloxane to provide the complex. Another similar complex is one prepared from the chloroplatinic acid hexahydrate, symetrical divinyltetramethyldisiloxane, symetrical tetramethyldisiloxane and alcohol solvent. Additional platinum compounds which include,

platinum bromides, a complex of platinous halide and an olefin such as ethylene, propylene, butylene, cyclohexene and styrene,

Pt(CH.sub.3 CN).sub.2 Cl.sub.2, {Pt(CH.sub.2 CN).sub.2 (CH.sub.3).sub.4 }Cl.sub.2,

Pt(NH.sub.3).sub.2 Cl.sub.2, K{PtCl.sub.3 CH.sub.2 CH.sub.2 CH.sub.2 OH} ,

PtBr.sub.2 (C.sub.2 H.sub.4).sub.2, K{PtBr.sub.3 (C.sub.2 H.sub.4)} ,

PtCl.sub.2 (C.sub.2 H.sub.4), (CH.sub.3).sub.2 C=CH.sub.2.sup.. PtCl.sub.2,

H.sub.2 pt(CN).sub.4.sup.. 5H.sub.2 O, H{PtCl.sub.3 (CH.sub.2 CN)} ,

Pt(NH.sub.3).sub.2 (CNS).sub.2, PtCl.sub.2.sup.. PCl.sub.3, {Pt(NH.sub.3).sub.4 }.sup.. {PtCl.sub.4 } ,

PtCl.sub.2 {P(CH.sub.2 CH.sub.3).sub.3 }.sub.2, PtCl.sub.2.sup.. P(OH).sub.3,

PtCl.sub.2.sup.. P(OCH.sub.2 CH.sub.3).sub.3, PtCl.sub.2.sup.. { P(OCH.sub.2 CH.sub.3).sub.3 }.sub.2 ,

Pt(OOCH.sub.2 SCH.sub.2 CH.sub.3).sub.2, Pt(CN).sub.3, (CH.sub.3).sub.4 Pt,

(CH.sub.3).sub.3 Pt-Pt(CH.sub.3).sub.3, ##STR1## PtCl.sub.2 CO and PtBr.sub.2 CO.

The microwave sensible material can be any material which is activated by microwaves. These materials are known in the art and as long as they are stable in the silicone composition so as not to cause premature reaction or to inhibit the foam formation, these materials will be suitable for this invention. Some of these microwave sensible materials are organic compounds such as the organic alcohols, including methanol, ethanol, isopropanol, propanol, butanol, cyclohexanol, hexanol, and the like, carbon black and metal oxides such as iron oxide.

The amounts of ingredients are based on 100 parts by weight of the polydiorganosiloxane gum base stock, (A). The organosiloxane, (B), can be present in amounts from 15 to 60 parts by weight, preferably from 20 to 50 parts by weight. The organohydrogensiloxane, (C), can be present in amounts from 3 to 25 parts by weight, preferably from 5 to 15 parts by weight. The acetylenic alcohol inhibitor, (D), can be present in amounts from 0.05 to 0.5 parts by weight, the platinum catalyst (E) can be present in amounts of from 1 to 200 parts by weight platinum per one million parts by weight silicone composition and the microwave sensible material, (F), can be present in amounts of from 0.25 to 30 parts by weight. When an organic alcohol is used as the microwave sensible material, it is not necessary to use any organosiloxane (B), if one desires (B) can be used when an organic alcohol is used as the microwave sensible material. Where the microwave sensible material is not a source for hydrogen gas formation, such as where metal oxides are used as the microwave sensible material, the organosiloxanes (B) must be used.

The silicone compositions of this invention are prepared by thoroughly mixing a polydiorganosiloxane gum base stock with a mixture of (B), (C), (D), (E) and (F). The mixing can be done by conventional mixers such as dough type mixers or on a rubber mill. The only precaution should be that the mixture should not see a temperature above the boiling point of any ingredient or above the activation temperature which is about 125.degree. C., preferably the temperature is maintained below 100.degree. C. After the composition is prepared, it can be exposed to a microwave source to obtain a foamed and cured product. The duration of the exposure will depend upon the particular microwave sensible material and the intensity of the microwave dosage. Conventional sources for microwave preparation can be used. There are many such instruments available commercially.

The method of this invention allows one to foam silicone gum stocks which are generally uneffected by microwave. This is an advantage because there are many places where heating is not practical or desired. Other advantages are that the composition is uniformly and instantaneously heated throughout which is desired in foaming applications. The heat can be turned on and off instantly and results in an energy savings. The foam products are useful as pads, cushions, pillows, insulation and the like and find particular use where fire retardant properties are useful. The fire retardant properties can be improved by adding small amounts of carbon black such as less than 2 weight percent. The foams or sponges are useful for use in buses, trains and airplanes.

The following example is presented for illustrative purposes only and should not be construed as limiting the present invention which is properly delineated in the claims.

A commercially available gum base stock comprising a reinforcing silica filled polydimethylsiloxane gum was mixed on a cooled rubber mill which reached a temperature between 50.degree. and 60.degree. C. with mixtures as defined in the Table. The amounts of ingredients shown in the Table were parts by weight and were added as a mixture to 100 parts of the gum base stock.

Ingredient A was a hydroxyl endblocked polydimethylsiloxane having about 4 weight percent silicon-bonded hydroxyl radical.

Ingredient B was a trimethylsiloxy endblocked polymethylhydrogensiloxane having about 1.6 weight percent silicon-bonded hydrogen atom.

Ingredient C was 1-ethynylcyclohexan-1-ol.

Ingredient D was a mixture of 6 parts by weight chloroplatinic acid hexahydrate in 94 parts by weight isopropanol.

Ingredient E was a reinforcing fume silica.

Ingredient F was methanol.

Ingredient G was an equal weight mixture of iron oxide in polydimethylsiloxane gum.

All compositions defined by the Table were placed in a microwave oven and exposed to the microwave source. Compositions 1 and 2 formed uniform foams in 10 minutes whereas Composition 3 did not foam or cure.