GB2255098A - Siloxane ablative compositions - Google Patents

Description

1 SILOXANE ABLATIVE COMPOSITIONS This invention relates to siloxane

ablative compo- sitions and to the use of such compositions for protecting surfaces from the effects of exposure to hot gases and flame.

Ablative coatings are known and are employed to protect the surfaces of metals and other substrates from erosion resulting from exposure to very high temperatures, in particular those arising in burning fuels and hot gases.

such coatings is the protection A particular application of.

of the internal and external surfaces of aerospace vehicles and lower altitude rocket fuel-powered missiles. During flight the surfaces can be exposed to temperatures in excess of 2,OOOOC resulting from, for example, the high speed re-entry of the vehicle into the atmosphere and/or exposure to the exhaust gases of the propulsion motor. one type of ablative coating composition has been disclosed in G.B.-A-1 255 132 which is concerned with a method for protecting an internal or external surface of an aerospace vehicle from the effect of gases at temperatures above 16500C which comprises coating said surface with a compo sition consisting of (A) a silicone elastomer, (B) from 1.0 to 100 parts by weight of silicon carbide, (C) from 1.0 to 250 parts by weight of silica and (D) from 0.1 to 15 parts by weight of a high temperature decomposing fibre which melts at a temperature above 16500C, other than fibrous silica. Examples of high temperature decomposing fibres include carbon, graphite, nitrides, borides, oxides and silicates, and the length of such fibres is indicated to range from 30 microns (gm) to 2.5cm. The silicon carbide component (B) of the disclosed composition is stated to be preferably powdered silicon carbide having a high degree of purity.

- 2 Although compositions of the kind specifically disclosed in G.B.-A-1 255 132 are generally satisfactory in their ablative performance it has been found that they have a tendency to crack when exposed to extreme temperatures, for example those existing at the interior surfaces of the tubes of rocket propulsion engines. There has thus existed a need for ablative coating compositions which exhibit reduced cracking under extremely high temperatures.

According to the present invention there is provided a composition which when cured exhibits ablative properties which comprises by weight (A) 100 parts of a curable polydiorganosiloxane wherein the organic groups are selected from alkyl groups having from 1 to 10 carbon atoms, alkenyl groups having from 2 to 8 carbon atoms and aryl, ara.1kyl and alkaryl groups having from 6 to 8 carbon atoms at least percent of total substituent groups being methyl, (B) a curing agent for polydiorganosiloxane (A), (C) from 1.0 to parts of silicon carbide in the form of fibres wherein at least 70 percent by weight of the fibres have a length in the range from 0.5 to 9mm, (D) from 1.0 to 40 parts of carbon fibres and (E) from 5 to 150 parts by weight of a filler other than (C) and (D).

The polydiorganosiloxanes (A) employed in the compo sitions of this invention are those which are curable in the presence of a suitable curing agent to an organo siloxane elastomer. The organic substituents attached to the silicon atoms are selected from alkyl groups having from 1 to 10 carbon atoms, for example methyl, propyl, hexyl and decyl, alkenyl groups having from 2 to 8 carbon atoms, for example vinyl, allyl and hexenyl, and aryl, alkaryl and aralkyl groups having from 6 to 8 carbon atoms, for example phenyl, tolyl and phenylethyl. At least 30 percent of the total substituents should be methyl.

Preferred are polydiorganosiloxanes in which the siliconbonded substituents comprise methyl and phenyl groups with optionally a small proportion (<4%) of vinyl groups, the presence of phenyl groups contributing to the high tempe- rature resistance of the polydiorganosiloxane. As the preferred polydiorganosiloxanes there may be employed for example copolymers of dimethylsiloxane and methylphenylsiloxane units, copolymers of dimethylsiloxane and diphenylsiloxane units, copolymers of dimethylsiloxane, methylphenylsiloxane and dimethylvinylsiloxane units and copolymers of dimethylsiloxane, methylphenylsiloxane and methylvinylsiloxane units. As hereinafter described polydiorganosiloxanes (A) will contain groups by means of which the desired curing can be affected. Such groups include for example hydroxyl, alkoxy or acyloxy which are normally attached to the terminal silicon atoms of the polydiorganosiloxane, and alkenyl which may be present in terminal or non-terminal positions in the polydiorganosiloxane chain, or in both.

Component (B) of the compositions of this invention is a curing agent, that is means for converting polydiorganosiloxane (A) to the solid elastic state either at or about normal ambient or at elevated temperatures, for example from about 15 to 1500C or higher. Polydiorgano siloxane (A) and curing agent (B) thus comprise a curable elastomer- forming system. A variety of compositions based on such systems are well- known in the art and any of these can be employed as the basis of the compositions of the present invention. Examples of such compositions are:

(i) compositions based on an organosiloxane polymer having in the molecule silicon-bonded oxime radicals, and/or a mixture of an organosiloxane polymer having silanol groups (iv) (v) and a silane having at least 3 silicon-bonded oxime groups. Such compositions are described for example in U.K. Patents 975 603 and 990 107; (ii) compositions based on an organosiloxane polymer having terminal silicon-bonded acyloxy groups, and/or a mixture of a silanolterminated organosiloxane polymer and a silane having at least 3 silicon-bonded acyloxy groups per molecule. Such compositions are described for example in U.K. Patents 862 576, 894 758 and 920 036; (iii) compositions based on an organosiloxane polymer having terminal silicon-bonded.an'Lde or amino groups, and/or a mixture of silanol- terminated organosiloxane polymer and a silylamine or silylamide. Such vulcanisable compositions are described for example in U.K. Patents 1 078 214 and 1 175 794; compositions based on an organosiloxane polymer having in the molecule silicon-bonded alkoxy groups, and/or a mixture of an organosiloxane polymer having silanol groups with a silane having alkoxy groups or a partial hydrolysis product of said silane, for example ethyl polysilicate. Compositions of this type are described in U.K. Patents 957 255, 962 061 and 841 825; compositions comprising an organosiloxane polymer having silicon-bonded alkenyl e.g. vinyl groups, an organohydrogen siloxane polymer and a catalyst for the addition of SiH to unsaturated groups, such as compounds and is complexes of metals of the platinum group, for example platinum and rhodium. Siloxane elastomers employing such a system are well-known and documented in the art, see for example Noll, Chem. and Tech. of the Silicones, page 398 (published 1968 by Academic Press), and (vi) compositions vulcanisable by heat and comprising an organosiloxane polymer and an organic peroxide or per-ester, for example dicumyl peroxide or p-dichlorobenzoyl peroxide.

It will thus be understood by those skilled in the art that the curing agent (B) may be a peroxide or perester, a silane or siloxane crosslinking agent, a catalyst such as an organo metal compound, for example stannous octoate, dibutyltin dilaurate or a titanium chelate, or the curing agent may comprise both a crosslinking agent and a catalyst. The proportion of curing agent (B) employed in the compositions will depend on the type of curing reaction desired. For example when the curing agent is a metal compound catalyst it will generally be employed in catalytic quantities, that is from about 0.05 to 5 parts by weight based on 100 parts of (A). When a silane or siloxane crosslinking agent is employed it is normally incorporated into the composition in an amount of from about 0.2 to about 20 parts per 100 parts of (A).

Although the compositions of this invention may utilise any curing reaction the preferred compositions are those of the type described under (v) above and comprise a mixture of a polydiorganosiloxane having alkenyl, usually vinyl, groups, an organosiloxane having silicon-bonded hydrogen atoms, for example a polymethylhydrogensiloxane or a copolymer of dimethyl-siloxane and methylhydrogensiloxane units, and a catalyst which is a compound or complex of a platinum group metal. As is well known such compositions are normally prepared and stored as two packages, the packages being mixed the the point of use. Cure of the compositions can occur at normal ambient temperatures of about 20 to 250C but can be accelerated by exposure to higher temperatures.

Component (C) of the compositions of this invention is silicon carbide in the form of fibres. At least 70 percent by weight of the fibres should have a length in the range-from 0.5mm to 9mm, preferably from 1.0 to 2. 5mm. The silicon carbide fibres may be employed in an amount of from 1.0 to 50 parts by weight, preferably 2 to 15 parts, per 100 parts by weight of polydiorganosiloxane (A). If desired the fibres may be surface-treated.

Component (D) of the compositions of the invention is carbon fibre which is employed in an amount of from 1.0 to 40 parts, preferably 2 to 10 parts, by weight per 100 parts by weight of polydiorganosiloxane (A). It is preferred that at least 80 percent by weight of the carbon fibres have a length of less than 1.Omm and that at least 50 percent by weight of the fibres have a length of less than 0.6mm. Fibres of the desired length characteristics can be obtained by milling spun carbon fibres to reduce their dimensions to the desired level.

In addition to additives (C) and (D) the compositions of this invention comprise a filler (E). The filler component may be one or more of the reinforcing or extending fillers which can be employed for modifying the physical properties, appearance or cost of the finished elastomer. Examples of such fillers are fume silicas, silica aerogels, clays, crushed quartz, calcium carbonate, titanium dioxide, magnesium silicate, zirconium silicate and ferric oxide. The filler component (E) is employed in an amount of from 5 to 150 parts, preferably 10 to 100 parts, by weight per 100 parts by weight of polydiorganosiloxane (A).

In addition to the essential components (A) to (E) the compositions may contain other ingredients, useful or conventional in the compounding of silicone rubbers, for example siloxane and silazane treating agents for fillers, adhesion promoting agents for improving the adhesion of the cured elastomer to the substrate, pigments and inhibitors for delaying the curing reaction.

The compositions of the invention may be prepared employing conventional compounding technique for silicone elastomer-forming compositions, for example by mixing or milling.

In use the compositions may be preformed and cured, preferably under pressure, prior to application to the substrate. Where large surface areas are to be coated it is preferable to apply to the surface a composition which cures at low or ambient temperatures and thereafter allow the composition to cure in situ.

The compositions of the invention find use in any application requiring the protection of a substrate surface from the effects of impinging hot gases. They are particularly useful for the protection of surfaces of the rocket engine and its environs and the external surfaces of aerospace vehicles.

The following Example in which the parts are expressed by weight illustrates the invention.

ExamDle parts of a vinyl end-stopped copolymer of 70 mole percent dimethylsiloxane units and 30 mole percent of is 8 - methylphenylsiloxane units, 100 parts of silica, 10 parts of silicon carbide (Nikalon) fibres in which at least 70 percent by weight have a length in the range from 0.5 to 3.Omm, 8 parts of milled carbon fibres (GM 300) having a length distribution up to about 1.Omm with at least 50 percent by weight having a length within the range from 0.015 to 0.6mm and a platinum siloxane complex (as catalyst) were mixed using a conventional rubber mixer. The resulting composition was designated Composition A.

A second composition (Composition B) was prepared in similar manner by mixing together 74 parts of a blend of vinyl-containing polydimethylsiloxane, and a methylvinylsiloxane resin copolymer, 25 parts of a copolymer of methylhydrogensiloxane units, dimethylsiloxane units and trimethylsiloxane units and 1.0 part of a low molecular weight methylvinylsiloxane.

Composition A (10 parts) and Composition B (1.0 part) were then mixed together and the resulting composition cured in a press at a temperature of 1500C for 15 minutes to form a sheet having a thickness of 2.Omm. The sheet was then given an aftercure at 700C for 4 hours. Pieces of the cured sheet were tested by firing in a cone calorimeter under a heat flux of 0.1 MW/M2 for 10 minutes, the pieces being exposed to the firing conditions three times. The fired pieces were found to be substantially free of cracks.

For comparison the test was repeated on a mixture of A and B as described above except that the silicon carbide fibres were replaced with the same quantity of silicon carbide powder and the carbon fibres with the same quantity of carbon fibres having a broad distribution of length extending up to 6. 3mm. After firing the comparison composition was found to exhibit significant cracking.

1 j11,!:'

Claims (6)

1. A composition which comprises by weight (A) 100 parts of curable polydiorganosiloxane wherein the organic groups are selected from alkyl groups having from 1 to 10 carbon atoms, alkenyl groups having from 2 to 8 carbon atoms and aryl, aralkyl and alkaryl groups having from 6 to 8 carbon atoms at least 30 percent of total substituent groups being methyl, (B) a curing agent for polydiorganosiloxane (A), (C) from 1.0 to 50 parts of silicon carbide in the form of fibres wherein at least 70 percent by weight of the fibres have a length in the range from 0.5 to 9mm, (D) from 1.0 to 40 parts of carbon fibres and (E) from 5 to 150 parts by weight of a filler other than (C) and (D).
2. 2. A composition as claimed in Claim 1 in which the siliconbonded substituents in polydiorganosiloxane (A) comprise methyl and phenyl groups.
3. 3. A composition as claimed in Claim 1 or Claim 2 wherein polydiorganosiloxane (A) has silicon-bonded alkenyl groups and curing agent (B) comprises an organosiloxane having siliconbonded hydrogen atoms and a compound or complex of a platinum group metal.
4. 4. A composition as claimed in any one of the preceding claims wherein the silicon carbide fibres (C) are present in an amount of from 2 to 15 parts by weight per 100 parts by weight of polydiorganosiloxane (A).
5. 5. A composition as claimed in any one of the preceding claims wherein the carbon fibre (D) is present in an amount of from 2 to 10 parts by weight per 100 parts by weight of polydiorganosiloxane (A).
6. 6. A substrate having thereon the product of curing a conpo- sition according to any one of the preceding claims.