FR2675809A1 - ABLATION COATING COMPOSITION AND SUBSTRATE CARRYING THE CURED COMPOSITION - Google Patents

Abstract

Disclosed is a composition which exhibits, in the cured state, ablation properties and which comprises (A) a curable polydiorganosiloxane, (B) a curing agent therefor, (C) silicon carbide and ( E) a load. The silicon carbide is in the form of fibers of which at least 70% by weight have a length between 0.5 and 9 mm and the composition also contains (D) carbon fibers. Application to the protection of aerospace vehicles and rocket engines.

Description

The present invention relates to ablation coating compositions for

  siloxane base and the use of these compositions to protect surfaces against

  effects of exposure to hot gases and the flame.

  Ablation coatings are known and used to protect the surfaces of metals and other substrates against erosion resulting from exposure to very high temperatures, particularly those occurring in ignited fuels and hot gases. particular application of such coatings is the protection of the internal and external surfaces of aerospace and propellant-powered lower-altitude missiles. During flight, surfaces may be subjected to temperatures exceeding 2000 OC which are generated, for example, by high speed entry of the vehicle into the atmosphere and / or exposure to propulsion engine exhaust gases A type of ablation coating composition has been proposed in GB-A-1 255 132 which relates to a method for protecting an inner or outer surface of an aerospace machine against the effect of gas at temperatures above 16500 C, said method comprising coating said surface with a composition of (A) a silicone elastomer, (B) 1.0 to 100 parts by weight of silicon carbide, (C) 1.0 to 250 parts by weight of silica and (D) 0.1 to 15 parts by weight of a high decomposition temperature fiber that melts at a temperature greater than 1650 ° C but is not a silica fiber Examples of high temperature decomposition fibers comprise carbon fibers, graphite, nitrides, borides, oxides and silicates, and the length of these fibers is specified in the range of 30 μm to 2.5 cm. The silicon carbide component (B) of the described composition is mentioned as being preferably a powdered silicon carbide having

a high degree of purity.

  Although the compositions of the type specifically described in GB-A-1,255,132 are generally satisfactory in their ablation behavior, they have shown a tendency to crack in the event of exposure to extreme temperatures, e.g. temperatures on the interior surfaces of the rocket engine tubes. Compositions which exhibit reduced cracking at

  Extremely high temperatures have therefore been requested.

  According to the present invention, there is provided a composition having hardened ablative properties, which comprises, by weight, (A) 100 parts of a curable polydiorganosiloxane whose organic groups are selected from alkyl groups having 1 to 10 carbon atoms, alkenyl groups having 2 to 8 carbon atoms and aryl, aralkyl and alkaryl groups having 6 to 8 carbon atoms, at least 30 percent of the total substituent groups being methyl groups, (B) a curing agent for the polydiorganosiloxane (A), (C) 1.0 to 50 parts of silicon carbide in the form of fibers, o at least 70 percent by weight of the fibers are 0.5 to 9 mm in length, ( D) 1.0 to 40 parts carbon fiber and (E) 5 to 150

  parts by weight of a charge different from (C) and (D).

  The polydiorganosiloxanes (A) used in the compositions of the present invention are those which can be cured in the presence of a suitable curing agent to form an organosiloxane elastomer. The organic substituents attached to the silicon atoms are selected from alkyl groups having 1 to to 10 carbon atoms, for example methyl, propyl, hexyl and decyl, alkenyl groups having 2 to 8 carbon atoms, for example vinyl, allyl and hexenyl, and aryl, alkaryl and aralkyl groups having 6 to 8 carbon atoms; for example phenyl, tolyl and phenethyl At least 30 percent of the total substituents should be methyl groups Preferred polydiorganosiloxanes are those in which the silicon-bonded substituents consist of methyl and phenyl groups with, optionally, a small proportion (<4% ) vinyl groups, the presence of phenyl groups contributing to the high temperature resistance of the pol Among the preferred polydiorganosiloxanes, there may be used, 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. after, the polydiorganosiloxanes (A) contain groups by means of which the required curing may be effected. These groups include, for example, hydroxyl, alkoxy or acyloxy groups which are normally attached to the terminal silicon atoms of the polydiorganosiloxane, and alkenyl groups which may be present at terminal and / or non-terminal positions in the chain of

polydiorganosiloxane.

  Component (B) of the compositions of the present invention is a curing agent, i.e. an agent for converting the polydiorganosiloxane (A) to a solid elastic state at or near normal room temperature, or at elevated temperatures, for example at a temperature of about 15 to 150 ° C or higher. The polydiorganosiloxane (A) and the curing agent (B) thus constitute a curable elastomer-forming system. Various compositions based on such systems are The examples of these compositions are: (i) compositions based on a polyorganosiloxane the molecule of which contains oxime radicals bonded to silicon and / or a mixture of a polyorganosiloxane comprising silanol groups with a silane having at least 3 silicon-bonded oxime groups Compositions of this type eg are described in United Kingdom Patents N 975 603 and

990,107;

  (ii) compositions based on a polyorganosiloxane comprising silicon-bonded end-acyloxy groups and / or a mixture of a silanol-terminated polyorganosiloxane with a silane having at least 3 silicon-bonded acyloxy groups per molecule Such compositions are described, for example, in United Kingdom Patent Nos. 862,576, 894,758 and 920,036; (iii) compositions based on a polyorganosiloxane having silicon-bonded amido or amino end groups and / or a mixture of a silanol-terminated polyorganosiloxane with silylamine or silylamide. Vulcanizable compositions of this type are described, for example, in United Kingdom Patent Nos. 1,078,214 and 1,175,794; (iv) compositions based on a polyorganosiloxane whose molecule contains silicon-bonded alkoxy groups and / or a mixture of a polyorganosiloxane having silanol groups with a silane having alkoxy groups or a partial hydrolysis product said silane, for example an ethyl polysilicate. Such compositions are described in United Kingdom Patents Nos. 957,255, 962,061 and 841,825; (v) compositions comprising a polyorganosiloxane having silicon-bonded alkenyl groups, for example vinyl, a polyorganohydrogensiloxane and a catalyst for the addition of SiH to unsaturated groups, for example compounds and complexes of metals of the platinum group. Such as platinum and rhodium Siloxane elastomers based on such a system are well known and widely described in the prior art; see, for example, Noll's "Chem and Tech of Silicone", page 398 (published in 1968 by Academic Press); and (vi) heat-vulcanizable compositions which comprise a polyorganosiloxane and an organic peroxide or perester, for example

  dicumyl or p-dichlorobenzoyl peroxide.

  Those skilled in the art will therefore understand that the curing agent (B) may be a peroxide or perester, a silane or siloxane crosslinking agent, a catalyst such as an organic metal compound, for example octoate stanneux, dibutyltin dilaurate or a titanium chelate, or that the curing agent may comprise both a crosslinking agent and a catalyst The proportion of the curing agent (B) used in the compositions depends on the type of the For example, when the curing agent is a catalyst such as a metal compound, it is generally used in catalytic amounts, i.e., about 0.05 to 5 parts by weight. Per 100 parts of (A) When a silane or siloxane is used as a crosslinking agent, it is normally incorporated in the composition in an amount of from about 0.2 to about 20 parts per 100 parts of (A). Although the compositions of the present invention may involve any curing reaction, the preferred compositions are those of the type described in (v) above and comprise a mixture of a polydiorganosiloxane having alkenyl groups, usually vinyl of an organosiloxane having silicon-bonded hydrogen atoms, for example a polymethylhydrogenosiloxane or a copolymer of dimethylsiloxane and methylhydrogensiloxane units, and a catalyst which is a compound or complex of a platinum-metal metal. as is well known, these compositions are normally prepared and stored as two components, the components being mixed at the time of use. The curing of the compositions can take place at normal ambient temperatures of about 20 to 250 C, but it can be accelerated by exposure to higher temperatures. Component (C) of the compositions of this

  The invention is silicon carbide in the form of fibers.

  At least 70 percent by weight of the fibers should have a length of between 0.5 mm and 9 mm, preferably between 1.0 and 2.5 mm. The silicon carbide fibers can be used in an amount of 1.0. to 50 parts by weight, preferably 2 to 15 parts by weight, per 100 parts by weight of the polydiorganosiloxane (A) The fibers may optionally have undergone a treatment of

area.

  Component (D) of the compositions of the invention consists of carbon fibers which are used in an amount of 1.0 to 40 parts by weight, preferably 2 to parts by weight, per 100 parts by weight of the polydiorganosiloxane (A) It is preferred that at least 80 percent by weight of the carbon fibers be less than 1.0 mm in length and at least 50 percent by weight of the fibers be less than 0.6 mm in length. Desired length characteristics can be obtained by grinding spun carbon fibers to reduce their size to the desired degree. In addition to the additives (C) and (D), the compositions of the present invention comprise a filler (E). The filler can consist of one or more of the diluent or reinforcing fillers that can be used to modify the physical aspect or cost of the final elastomer Examples of these fillers are fumed silica, silica aerogels, clays, crushed quartz, calcium carbonate, titanium dioxide, magnesium silicate, silicate The charge constituting Component (E) is used in an amount of from 5 to 150 parts by weight, preferably 10 to 100 parts by weight, of zirconium and ferric oxide.

  per 100 parts by weight of the polydiorganosiloxane (A).

  In addition to the essential components (A) to (E), the compositions may contain other ingredients that are useful or customary in the preparation of silicone rubbers, for example, siloxanes and silazanes as charge-treating agents, caustic agents and adhesiveness to improve the adhesion of the cured elastomer to the substrate, pigments and inhibitors for

  delay the curing reaction.

  The compositions of the present invention can be prepared using standard preparation techniques for elastomer-forming compositions

  silicone, for example mixing or grinding.

  For their use, the compositions may be preformed and cured, preferably under pressure, prior to application to the substrate. When large areas are to be covered, it is preferable to apply to the surface a composition which cures at low temperature or room temperature and then allow the

composition "in situ".

  The compositions of the invention find use in any application requiring the protection of a substrate surface against the effects of the impact of hot gases They are particularly useful for the protection of surfaces of rocket engines and their surroundings , and external surfaces of aerospace machines. The following non-limiting example illustrates the invention

  and all parts are expressed by weight.

Example

  Using a conventional rubber mixer, 100 parts of a vinyl-terminated copolymer composed of 70 mole percent of dimethylsiloxane units and 30 mole percent of methylphenylsiloxane units, 100 parts of silica, 10 parts of carbide fibers are mixed. of silicon (Nikalon) of which at least 70 percent by weight is between 0.5 and 3.0 mm in length, 8 parts of crushed carbon fiber (GM 300) having a length distribution of up to about 1 0 mm and at least 50 percent by weight have a length between 0.015 and 0.6 mm, and platinum-siloxane complex (as catalyst). The resulting composition is called Composition A. A second composition is prepared (Composition B) in a similar manner by mixing together 74 parts of a mixture of vinyl-containing polydimethylsiloxane and a resinous copolymer of methylvinylsiloxane, parts of a methylhydroxy-methylhydroxylsiloxane copolymer rofosiloxane, dimethylsiloxane units and trimethylsiloxane units, and 1.0 part of a

  methylvinylsiloxane of low molecular weight.

  Composition A (10 parts) and Composition B (1.0 part) were then mixed together and the resulting composition was cured in a press at 1500 C for 15 minutes to form a sheet having a thickness of 2.0. The sheet is then post-cured at 700 ° C. for 4 hours. Pieces of the cured sheet are tested by heating in a conical calorimeter under a heat flow of 0.1 MW / m 2 for 10 minutes. , the pieces being subjected three times to the heating conditions It is found that the heated pieces are

substantially free of cracks.

  By way of comparison, the test on a mixture of A and B is repeated as described above, but the silicon carbide fibers have been replaced by the same amount of silicon carbide powder and the carbon fibers by the same amount of carbon fibers having a wide distribution of lengths up to 6.3 mm After heating, it is found that the comparison composition exhibits significant cracking.

Claims (4)

  A composition comprising, by weight, (A) 100 parts of a curable polydiorganosiloxane whose organic groups are selected from alkyl groups having 1 to 10 carbon atoms, alkenyl groups having 2 to 8 carbon atoms and aryl groups, aralkyls and alkaryls having 6 to 8 carbon atoms, at least 30 percent of the total substituent groups being methyl groups, (B) a curing agent for the polydiorganosiloxane (A), (C) 1.0 to 50 parts of carbide of silicon and (E) 5 to 150 parts of a filler, characterized in that the silicon carbide is present in the form of fibers of which at least 70 percent by weight have a length of between 0.5 and 9 mm, and what the composition also contains (D) 1 to 40 parts of carbon fibers.   Composition according to Claim 1, characterized in that the silicon-bonded substituents in the polydiorganosiloxane (A) comprise groups methyl and phenyl.   Composition according to Claim 1 or 2, characterized in that the polydiorganosiloxane (A) has silicon-bonded alkenyl groups and the hardening agent (B) comprises an organosiloxane having silicon-bonded hydrogen atoms and a compound or complex of a metal from the platinum mine.   4 Composition according to any one of   preceding claims, characterized in that at least   percent by weight of silicon carbide fibers   have a length of between 1.0 and 2.5 mm.   Substrate carrying an ablation surface, characterized in that said ablation surface is the   hardening product of a composition according to any one of the preceding claims.