Thixotropic foamable organosiloxane compositions

Abstract: The presence in foamable, curable organosiloxane compositions of a hydrogenated glyceride of an ethylenically unsaturated carboxylic acid imparts sufficient thixotropy to the uncured composition to inhibit flow without adversely affecting desirable properties of the cured foam such as density and texture.

Description: This invention relates to foamable organosiloxane compositions containing an additive that imparts thixotropy without adversely affecting other properties or affecting cured foams prepared from these compositions. The additive is particularly useful for preparing low density foams of small cross-sectional area. Foamable compositions, comprising a curable polyorganosiloxane, an organohydrogensiloxane, a hydroxyl-containing compound and a platinum group metal or a compound thereof as a foaming and curing catalyst, have been described in the patent and journal literature. Relevant patents include US Patents 3,923,705; 4,026,843; 4,189,545, 4,590,222; 4,599,367 and 5,252,627. The curable polyorganosiloxane used to prepare the foam contains at least two alkenyl or silanol groups per molecule. If the polyorganosiloxane does not contain silanol groups a low molecular weight hydroxyl compound such as an alcohol, silanol or siloxanol is included in the reaction mixture to generate the hydrogen required for foaming of the composition. For certain end use applications such as gaskets, it is desirable that the cured foam have a density less than 20 lb/ft 3 (0.32 g/cm 3 ). The viscosity of the curable organosiloxane compositions used to prepare these foams is typically from 20 to 70 Pa·s. These compositions flow to the extent that it is difficult to prepare three dimensional objects in the absence of dams or other barriers to prevent flowing of the composition prior to curing. A variety of additives have been used to impart thixotropic character to such compositions, thereby reducing the tendency of these compositions to flow in the absence of shearing forces. These additives include finely divided forms of untreated silica, silica that has been treated with silanes or other hydroxyl-containing organosilicon compounds, asbestos fibers, carbon black and other forms of finely divided carbon, clays that have been treated with quaternary ammonium compounds, phthalocyanine and other organic pigments, finely divided metal carbonates, oxides, hydroxides and silicates, lithium hydroxystearate, soaps of aluminum and other metals, cellulose esters, colloidal forms of alkali or alkaline earth metal aluminum silicates and hydrogenated glycerides of unsaturated fatty acids, such as those present in castor oil. These thixotropic agents are disclosed in one or more of the following references: US Patent 4,497,919; JP-A 2-038309; and 3rd edition Kirk Othmer Encyclopedia of Chemical Technology, volume 19, page 530 or volume 20, page 262. We have found that some of the known thixotropic agents, particularly treated and untreated forms of finely divided silicas, adversely affect the processability of foamable organosiloxane compositions. They also alter the texture, density and other physical properties of the cured foam prepared from these compositions such that the foam is no longer suitable for its intended use. An objective of this invention is to provide foamable compositions that can be formed into three dimensional low density cured articles by casting or extrusion on to unobstructed surfaces. The objectives of the present invention are achieved by the presence in our foamable organosiloxane compositions of a hydrogenated castor oil or other hydrogenated glyceride of an ethylenically unsaturated fatty acid. This invention provides a foamable organosiloxane composition comprising: A. a polyorganosiloxane containing at least two reactive groups per molecule selected from silanol groups or alkenyl radicals; B. an amount sufficient to foam and cure said composition of an organohydrogensiloxane containing at least three silicon-bonded hydrogen atoms per molecule wherein there is no more than one of said hydrogen atoms per silicon atom; C. a compound containing at least one hydroxyl or silanol group per molecule; and D. as the catalyst, an amount of a metal from the platinum group of the periodic table or a compound of said metal sufficient to promote foaming and curing of said composition, characterized by the presence in said composition of a thixotropic additive in an amount sufficient to prevent substantial flowing of said composition prior to curing, said additive comprising the hydrogenated glyceride of at least one ethylenically unsaturated carboxylic acid containing from 12 to 20 carbon atoms, wherein the composition further comprises a resinous organosiloxane copolymer containing triorganosiloxy and SiO 4/2 units, provided that at least a portion of the silicon atoms in this copolymer contains ethylenically unsaturated hydrocarbon radicals. An important feature of our foamable compositions is the presence as a thixotropic agent of a hydrogenated glyceride derived from at least one ethylenically unsaturated carboxylic acid containing from 12 to 20 carbon atoms and at least one hydroxyl group as an optional substituent. These acids are also referred to as unsaturated fatty acids and include dodecanoic, oleic, linoleic and ricinoleic (12-hydroxy-9-octadecenoic) acids. Hydrogenated castor oil, a mixture of the glycerides of ricinoleic, oleic and linoleic acids, is a preferred thixotropic agent based on cost and availability. Hydrogenated fatty acid glycerides are considered unique among the known thixotropic agents for use in our compositions based on their ability to prevent substantial flow of an otherwise flowable composition before cure when the composition is applied to an inclined or vertical surface. Unlike other thixotropic additives for these compositions, such as finely divided forms of silica, our thixotropic agents do not adversely affect the processability of the composition or the desired properties of the cured foam and most especially its texture and density. The thixotropic agent constitutes from 0.5 to 5 weight percent, preferably from 1 to 4 weight percent, of the present compositions. Concentrations greater than 5 weight percent make the compositions difficult to process in conventional mixing equipment. Two types of curable organosiloxane compositions are suitable for use with our thixotropic additives. In both types, the hydrogen required to form the cellular structure of the foam is generated by the reaction of silanol or hydroxyl groups with silicon-bonded hydrogen atoms in the presence of a suitable catalyst, typically a metal from the platinum group of the periodic table or a compound of one of these metals. The reaction occurs concurrently with curing of the organosiloxane composition. One type of curing reaction involves silicon-bonded hydrogen atoms present on a curing agent and silanol groups present on the curable polyorganosiloxane. The hydrogen required as a blowing agent is generated as a by-product of this reaction. The second type of curing reaction is a hydrosilation reaction between alkenyl radicals present on the curable polyorganosiloxane and silicon-bonded hydrogen atoms present on the curing agent. Depending upon the desired cured foam properties, our curable composition contains at least one silanol-functional polyorganosiloxane, alkenyl-functional polyorganosiloxane or a mixture of both types of polyorganosiloxanes. If this polyorganosiloxane does not contain a sufficient concentration of silanol groups to generate the hydrogen required for a satisfactory foam, additional compounds, containing hydroxyl or silanol groups (ingredient C), must be present in the curable composition. The silanol groups can be present on silanes or low molecular weight organosiloxanes and the hydroxyl groups are typically present as water or as the reactive groups of a mono- or polyhydric alcohol. Suitable alcohols contain from 1 to 12 carbon atoms and are methanol, ethanol, propanol, benzyl alcohol, lauryl alcohol, ethylene glycol, propylene glycol and 1,4-butanediol. The use of alcohols to modify the physical properties of foams prepared using curable organosiloxane compositions is more completely described in US Patent 4,590,222. Suitable curable polyorganosiloxanes contain at least two alkenyl radicals or silanol groups per molecule and exhibit a viscosity of from 0.1 to 100 Pa·s at 25°C. At least 90 percent of the repeating units in this polyorganosiloxane are diorganosiloxane units represented by the general formula R 1 R 2 SiO where R 1 and R 2 are individually selected from monovalent unsubstituted and substituted hydrocarbon radicals that contain from 1 to 10 carbon atoms. The alkenyl radicals and silanol groups that characterize the curable polyorganosiloxanes are preferably located at the terminal positions of the molecule; however, one or more can be bonded to non-terminal siloxane units as well. In preferred curable polyorganosiloxanes, at least one of the hydrocarbon radicals bonded to each silicon atom is preferably methyl and any remaining ones are phenyl, 3,3,3-trifluoropropyl or alkenyl. The present compositions contain at least one organohydrogensiloxane that supplies the silicon-bonded hydrogen atoms required to cure the composition and generate the hydrogen gas that creates the cellular structure of the final foam. The organohydrogensiloxane can have a linear or branched structure and can be homopolymers, copolymers or mixtures of these types of polymers. A preferred linear type of organohydrogensiloxane is a linear organosiloxane copolymer exhibiting a viscosity of from 0.01 to 10 Pa·s at 25°C. and comprising dialkylsiloxane and alkylhydrogensiloxane units with trialkylsiloxy terminal units. The alkyl radicals contain from 1 to 4 carbon atoms and are most preferably methyl. Preferred compositions contain two types of organohydrogensiloxanes, one of which is a polymethylhydrogensiloxane and the second which is a copolymer containing methylhydrogensiloxane and dimethylsiloxane units. According to a preferred embodiment said linear organosiloxane copolymer constitutes at least half of the total weight of organohydrogensiloxane present in said foamable organosiloxane composition according to the present invention. The relative concentration of curable polyorganosiloxane (ingredient A) and organohydrogensiloxane (Ingredient B) in the present compositions is equivalent to a molar ratio of silicon-bonded hydrogen atoms to the combination of alkenyl radicals, silanol groups and hydroxyl groups that is typically from 3.5:1 to 5.5:1, and preferably 5:1. The reactions used to foam and cure the present compositions are catalyzed by metals from the platinum group of the periodic table and compounds of these metals. Preferred catalysts include platinic chloride, chloroplatinic acid and coordination complexes of these compounds with ethylenically unsaturated organosiloxanes such as divinyltetramethyldisiloxane. These coordination complexes can be prepared as described in US Patent 3,419,593. The concentration of catalyst is typically equivalent to from 3200 to 6000 parts of platinum group metal per one million parts by weight of curable composition. In addition to ingredients A, B, C, D and E, our compositions can also contain various ingredients to modify the properties of either the curable composition or the cured foam. These optional ingredients include finely divided reinforcing and non-reinforcing mineral fillers such as quartz and calcium carbonate; metal oxides such as alumina, hydrated alumina, ferric oxide and titanium dioxide; pigments such as carbon black and zinc oxide; organic pigments and dyes; anti-oxidants; heat stabilizers; ultraviolet stabilizers; flame retarding agents and catalyst inhibitors, such as cyclic methylvinylsiloxanes, to increase the working time of the curable composition. A further ingredient of these compositions is a resinous organosiloxane copolymer containing triorganosiloxy and SiO 4/2 units. The triorganosiloxy units in this type of copolymer are represented by the formula R 3 3 SiO 1/2 , where R 3 represents a monovalent unsubstituted or substituted hydrocarbon radical. In preferred copolymers, the hydrocarbon radicals represented by R 3 are a combination of C 1 - C 5 alkyl groups, most preferably methyl. The molar ratio of triorganosiloxy units to SiO 4/2 units in the copolymer is typically from 0.7 to 1.2, inclusive. The copolymer constitutes up to 30 percent of the combined weight of the curable alkenyl-substituted polyorganosiloxane and the copolymer. At least a portion of the silicon atoms in this copolymer contains ethylenically unsaturated hydrocarbon radicals such as vinyl. Our curable compositions are prepared by blending the ingredients A-E to homogeneity. To maximize the effect of the thixotropic additive, these compositions are heated to a temperature of at least 65°C., preferably 70°C. and mixed using a high rate of shear, which activates the thixotropic additive. The desired shear rate can be achieved by using a stirring blade rotating at a speed of 200 revolutions per minute. This shear rate is maintained while the composition is allowed to cool to a temperature of 40°C. The present compositions begin to cure and foam when ingredients A, B, C and D are combined. If it is desired to store the compositions prior to curing, this can be achieved by packaging the ingredients in two or more containers with the organohydrogensiloxane and curing catalyst in separate containers. Both the curing and foaming reactions are accelerated by heating the curable composition. Reaction temperatures from 30 to 65°C. are preferred. The time required for foaming and curing of these compositions is typically from 3 to 30 minutes, based on the reactivity of the ingredients and the temperature. The present compositions are particularly useful for preparing low density foams exhibiting a soft, resilient texture. Preferred foams have densities less than 20 pounds per cubic inch (320 kg/m 3 ) and can be compressed to 50 percent of their height under a loading of 4 psi (27.6 KPa). The following examples describe preferred embodiments of the present compositions and should not be interpreted as limiting the scope of the invention defined in the accompanying claims. All parts and percentages in the examples are by weight and viscosities were measured at 25°C. Example 1 This example demonstrates that while both hydrogenated castor oil and fumed silica will impart thixotropic flow properties to foamable compositions, both the untreated and treated forms of silica adversely affect the desired cell structure and texture for the cured foam. Two curable organosiloxane compositions of this invention (compositions 1 and 2) were prepared as two-part compositions consisting of a base and a curing agent. The base and curing agent portions of each composition were prepared by blending the ingredients to homogeneity, heating the resultant mixture to 70°C. and then cooling to ambient temperature using a circulating water bath. The base portion (Part I) contained the following ingredients: 60 parts of a liquid dimethylvinylsiloxy terminated polydimethylsiloxane exhibiting a viscosity of 55 Pa·s at 25°C.; 24 parts of a benzene-soluble resinous copolymer containing triorganosiloxy units and SiO 2 units in the molar ratio of 0.7 moles of triorganosiloxy units per mole of SiO 2 units, where the triorganosiloxy units were trimethylsiloxy and dimethylvinylsiloxy and the copolymer contained 1.8 weight percent of vinyl radicals; 15 parts of alumina trihydrate; 7 parts of benzyl alcohol; 1 part of a reaction product of hexachloroplatinic acid and sym-tetramethyldivinyldisiloxane that had been diluted with a liquid dimethylvinylsiloxy-terminated polydimethylsiloxane in an amount sufficient to achieve a platinum content of 0.7 weight percent; 0.6 part of carbon black, and a hydrogenated castor oil available as Thixin R(R) from Rheox, Inc. The curing agent portion (Part II) of each composition contained: 45 parts of the liquid polydimethylsiloxane present in the base portion; 10 parts of the resinous copolymer present in the base portion; 25 parts of quartz with an average particle size of 5 micrometers; 12 parts of a mixture containing equal parts by weight of (1) a trimethylsiloxy-terminated organohydrogensiloxane containing 1.6 percent silicon-bonded hydrogen and (2) an organosiloxane copolymer comprised of (CH 3 ) 3 SiO 1/2 and SiO 2 units with a molar ratio of (CH 3 ) 3 SiO 1/2 to SiO 2 units of 0.75:1. The resin can be prepared as described in US Patent 2,676,182 which contains about 1.7 weight percent of hydroxyl groups. 5 parts of a trimethylsiloxy-terminated polydiorganosiloxane containing an average of five methylhydrogensiloxane units and three dimethylsiloxane units per molecule with a silicon-bonded hydrogen atom content of 0.8 weight percent; 0.4 part of cyclic methylvinylsiloxanes; and the hydrogenated castor oil present in the base portion. The base and curing agent portions each contained 1 part (composition 1) or 3 parts (composition 2) of the hydrogenated castor oil. Foamable compositions were prepared by blending equal weights of the base and curing agent portions of each composition in a cup for 1 minute using a spatula. The resultant foam was allowed to rise under ambient conditions for at least one hour, at which time the density of the foam was determined. The flow characteristics of the foamable compositions were determined using a test device described in U.S. Military Specification (Mil Spec) 46146. The device was a block containing a circular cavity in one surface. The cavity measured 1.5 inches (3.8 cm.) in diameter and 0.375 inch (0.95 cm) in depth. A ram having substantially the same diameter as the cavity and a thickness of 1/4 inch (0.64 cm) was located at the bottom of the cavity. With the surface containing the cavity in a horizontal position, the cavity was filled with the curable composition to be evaluated. Any material extending above the cavity was removed and the surface containing the cavity was placed in a vertical position. The material in the cavity was then ejected using the ram and allowed to flow down the vertical surface containing the cavity until the foam cured, which usually required from 0.5 to 1 hour. The maximum distance over which the composition flowed during this period was measured and recorded in Table 1. Samples for determining compression deflection, a measure of compressibility and compression set were prepared by dispensing a foamable composition between two sheets of polyester film separated by 0.062 inch (0.16 cm) - thick shims. The composition was then allowed to foam and cure, at which time the polyester sheets were removed. Compression set was determined by compressing a cured foam sample of known thickness to 50% of its initial height using the procedure and formula described in American Society for Testing and Materials (ASTM) procedure D1056, a temperature of 80°C. for 5 days and a recovery time of 30 minutes. Compression deflection was determined by measuring the force required to compress the foam to 50 percent of its initial height. Comparative compositions were prepared and evaluated as foams using the same types and amounts of ingredients as compositions 1 and 2 of the present invention, but omitting the thixotropic additive and the carbon black (composition 3). Three additional comparative compositions (4, 5 and 6) were prepared by adding 3, 5 and 7 weight percent, respectively, of an untreated fumed type silica having a surface area of 90 m 2 /g to composition 3. Comparative compositions 7 and 8 contained 3 and 5 weight percent, respectively, of a silica with a surface area of 255 m 2 /g in place of the silica of compositions 4 and 5. Comparative compositions 9, 10 and 11 were prepared by adding to composition 3 concentrations of 3, 4 and 5 weight percent, respectively, of a fumed type silica that had been treated with a hydroxyl-containing polydimethylsiloxane. The density, flow, compression deflection and compression set exhibited by these compositions are summarized in Table 1. Compositions 3-11 were prepared for comparative purposes and are not within the scope of the present invention. Density Flow Compression/Deflection Compress. Set (%) Comp. lbs/ft 3 //Kg/m 3 in / cm lb/in 2 //KPa 1 16.9/270.4 1.95/4.95 19.90/137.3 8.19 2 14.5/232 2.00/5.1 10.25/70.7 6.51 3 11.7/187.2 >4/>9 2.10/14.5 5.35 4 20.2/323.2 3.60/9.1 5.00/34.5 11.54 5 22.6/361.6 3.20/8.1 Not Determined 6 >30.6/>490 3.20/8.1 Not Determined 7 20.7/331.2 >4/>9 6.50/44.9 5.64 8 >30.6/>490 >4/>9 Not Determined 9 22.5/360 Not Determined Not Determined 10 19.7/315.2 0.50/1.3 9.75/67.3 13.02 Example 2 This example demonstrates the utility of our compositions for preparing foamed-in-place gaskets. The ingredients of composition 2 described in Example 1 were blended by passing them through a static mixer that was 8.75 inches (22.2 cm.) long, 0.25 inch (0.64 cm) in internal diameter and contained 32 Kenics™ type elements. The composition was extruded from the static mixer through a nozzle having a diameter of 0.240 in (6.1 mm) and a thickness of 0.12 in (3.0 mm). The pressure required to transport the composition through the mixer and nozzle was generated using compressed air. The equipment used to extrude the composition is described in the examples of US Patent 5,222,627. The composition was extruded onto a horizontal surface as a continuous bead. The height of the bead was 8 mm and the width was 12.5 mm. The composition cured in 2 minutes when heated at a temperature of 50°C. to a bead exhibiting a width of 12.57 mm and a height of 9.51 mm. For purposes of comparison a bead was extruded and cured using composition 3 of Example 1, which did not contain the thixotropic additive and carbon black present in composition 1. The cured bead was 12.68 mm. wide and 4.69 mm. high, indicative of substantial flow prior to curing.

Claims: A foamable organosiloxane composition comprising A. a liquid polyorganosiloxane containing at least two reactive groups per molecule selected from silanol groups or alkenyl radicals; B. an amount sufficient to foam and cure said composition of an organohydrogensiloxane containing at least three silicon-bonded hydrogen atoms per molecule wherein there is no more than one of said hydrogen atoms per silicon atom; C. a compound containing at least one hydroxyl or silanol group per molecule, and D. as the catalyst, an amount of a metal from the platinum group of the periodic table or a compound of said metal sufficient to promote foaming and curing of said composition, characterized by the presence in said composition of a thixotropic additive in an amount ranging from 0.5 to 5 weight percent of said compositions to prevent substantial flowing of said composition prior to curing, said additive comprising the hydrogenated glyceride of at least one ethylenically unsaturated hydroxycarboxylic acid containing from 12 to 20 carbon atoms, wherein the composition further comprises a resinous organosiloxane copolymer containing triorganosiloxy and SiO 4/2 units, provided that at least a portion of the silicon atoms in this copolymer contains ethylenically unsaturated hydrocarbon radicals. A composition according to claim 1 wherein said polyorganosiloxane is a polydiorganosiloxane wherein at least 90 percent of the repeating units are diorganosiloxane units represented by the general formula R 1 R 2 SiO, where R 1 and R 2 are individually selected from monovalent unsubstituted or substituted hydrocarbon radicals that typically contain from 1 to 10 carbon atoms, said alkenyl radicals are vinyl radicals, any silanol-containing polyorganosiloxane constitutes at least a portion of ingredient A; said compound containing at least one hydroxyl group per molecule as an alcohol and said thixotropic additive is a hydrogenated castor oil. A composition according to claim 2 wherein said monovalent hydrocarbon radicals present in said polyorganosiloxane or said organohydrogensiloxane are selected from alkyl, phenyl and 3,3,3-trifluoropropyl; said composition cures by a hydrosilation reaction; said organohydrogensiloxane comprises 1) a linear organosiloxane copolymer exhibiting a viscosity of from 0.01 to 10 Pa·s at 25°C. and comprised of dialkylsiloxane or alkylhydrogensiloxane units with trialkylsiloxy terminal units, and 2) a linear polyorganohydrogensiloxane; the molar ratio of silicon-bonded hydrogen atoms to the combination of alkenyl radicals, silanol groups and hydroxyl groups is from 3.5:1 to 5.5:1; and said composition further comprises at least one non-reinforcing filler. A composition according to claim 3 where at least one of the hydrocarbon radicals bonded to each silicon atom of said polyorganosiloxane or said organohydrogensiloxane is a methyl radical and said linear organosiloxane copolymer constitutes at least half of the total weight of organohydrogensiloxane present in said composition. A composition according to claim 1 wherein said composition is packaged in at least two containers, said organohydrogensiloxane and said catalyst being located in different containers.

Inventors: JACOBS THOMAS GEORGE JR, LOISELLE BRIAN PAUL, JACOBS, THOMAS GEORGE, JR., LOISELLE, BRIAN PAUL
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1995
Main classification: C08L83
IPC codes: C08K5, C08J9, C08L83