|Title of Invention||
A PROCESS FOR PRODUCING POLYCARBOSILANE, A CERAMIC PRECURSOR FROM POLYSILAHYDROCARBON
|Abstract||This invention relates to a process for producing polycarbosilanes which are ceramic precursors. Polysilahydrocarbon is prepared by polywlerising styrene with dialkyl dichlorosilarte and alkyl aryl dichlorosilane. This is theft heat treated in an inert atwlosphere to obtain the precursor.|
|Full Text||This invention relates to a process for producing polycarbosilane, a ceramic precursor from polysilahydrocarbon. The ceramic precursor produced by this process exhibits better process ability and yields ceramics with high ceramic residue.
In recent years, considerable interest has been shown on the synthesis of poiycarbosilanes and their evaluation as precursors for SiC ceramics. Polycarbosilane is synthesized by a two step process, the first step involving the synthesis of polydimethylsilane through dechlorination of dimethyldichlorosilane and the second step involving the heat treatment of the polydimethylsilane obtained there from under pressure in an autoclave. Polycarbosilane may also be prepared by the heat treatment of polydimethylsilane in the presence of a catalyst.
Yet another method of preparing polycarbosilane is by dechlorination of a mixture of vinyl group containing dichlorosilanes with other chlorosilane monomers in the presence of an alkali metal. The silyl radical or anion formed under dechlorination conditions initiates the vinyl polymerization resulting in the formation of a polycarbosilane in which silicon atoms are separated by hydrocarbon units containing more than one
carbon unit. These polymers are referred to as polysilahydrocarbons. Polysilahydrocarbons may also be synthesized by dechlorination of diorganodichlorosilane in the presence of diene or vinylic monomers- which do not contain silicon. An advantage of this method is that the monomers are relatively cheaper and the yield is relatively high. However, polysilahydrocarbons synthesized from vinylic or diene monomer is that the ceramics produced there from are of poor quality. The reason for the poor ceramic quality is attributed to the presence of thermally less stable disilyl or polyvinyl linkage. One way of improving the thermal stability and ceramic residue from polysilahydrocarbons is by converting the disilyl linkage to carbosilane linkage and to introduce branching in the polymer. However, process ability and high yielding ceramics could not be achieved together from these samples.
The object of this invention is to develop a ceramic precursor from polysilahydrocarbons which are process able and has high ceramic contents. Reducing the evolved volatiles minimizes the residual porosity of the final product which in turn improves the mechanical property of the ceramics derived from these polymers.
Incorporation of alkyl arylsilylene units in the polysyllabic hydrocarbon provides the opportunity for both alKyiene and
arsine insertion between the silicon atoms in the polymer backbone. This improves the process ability and ceramic yield of the polycarbosilane prepared there from. Polysilahydrocarbons obtained by such incorporation are poly(dialkylsilylene-Co-alkylarylsilylene-co-styrene) and can be prepared by known methods of polymerizing the monomers. The polymerization is preferably carried out in a hydrocarbon solvent selected from benzene, toluene, xylene or mixture thereof. The preferred solvent is however toluene. The monomers are dialkyldichlorosilane, alkylaryldichlorosilanes and styrene. The preferred monomers are dimethyl dichlorosilane, methyl phenyldichlorosilane,, ethyl phenyldichlorosilane and p-toiyl methyl dichlorosilane. Two or more of these monomers may be taken in any ratio preferred. The polymerizations is preferably carried out at a temperature between 100-135 C and for a period of 5 to 10 hours.
Polysilahydrocarbons obtained by this method is subjected to heat treatment in a vacuum furnace in an inert atmosphere at a temperature between 380-400 0. Initial pressure of the reaction chamber is maintained at 8-12 psi at room temperature and the final pressure on completion of treatment is between 40 to 50 psi depending on the heat treatment temperature. Polycarbosilanes obtained by such heat treatment are excellent ceramic precursors having good process ability and high ceramic yield.
This invention relates to a process for producing polycarbosilane, a ceramic precursor from polysilahydrocarbon comprising the steps of polymerizing dialkyl dichlorosilane, alkyl aryl dichlorosilane and styrene to produce poly(dialkylsilylene-co-alkyl arylsilylene-co-styrene), recovering said polymer in a known manner and subsequently heat treating the same in an inert atmosphere to obtain polycarbosilane.
Polycarbosilane produced by this process find excellent use as a binder for ceramic compounds. The polymer may be dissolved in any solvent, and the solution coated on any desired article before subjecting it for further heat treatment to obtain a ceramic coating thereon.
The following example illustrates the invention without limiting its scope:
Synthesis of Poly(dimethylsilvlene-co-methvl phenyl silvlene-co-styrene
A three necked flask fitted with a stirrer mantle, condenser inlet and outlet for inert gas and a calcium chloride guard tube is changed with 211.4 gm of freshly cut sodium in 3.32 litre toluene. Sodium is melted under reflux conditions with stirring. A monomer mixture consisting of 361.36 of dimethyl
dichlorosilane, 229.34 gm of methyl phenyl dichlorosilane and 49.92 gm of styrene is added to the sodium content in the flask. This addition is carried out with the help of a pressure equilising funnel over a period of 1 hour. The reaction mixture is refluxed for a period of 10 hours. Unreacted sodium is then neutralised by the addition of ethanol/toluene mixture. The reaction mixture is then poured into 10 litres of alcohol and the precipitate obtained is collected and dried in vacuum at 60*^0 for 3 hours. The product washed with ethanol/water mixture to free it from contaminants. The polymer is finally dried in vacuum at lOO^C for 10 hours.
10 gm of poly(dimethyl silylene-co-methy1 phenylsilylene-co-styrene) prepared by the above process is taken in a china dish and placed in a vacuum furnace. Air is completely removed by flushing with argon and the chamber is filled with argon at 8 to 12 psi. The chamber is then heated to 420°C at a heating ratio l-4"^C,/min and then maintained at this temperature for a periodi. The pressure is then increased to 40-50"""psi whereafter the chamber is cooled to room temperature at a cooling ratio of 2-4°C/min. Sample is taken out, weighed and analysed.
Conversion of the polysilahydrocarbon to polycarbosilane is shown here in after
Scheme 1 Conversion of polysilahydrocarbon to polycarbosilane (PCS-1) described In example 1.
The above example is only illustrative. Obvious modifications and alterations known to persons skilled in the art are within the scope of the appended claims.
1. A process for producing polycarbosilane, a ceramic precursor from polysilahydrocarbon comprising the steps of polymerising dialkyl dichlorosilane, alkylaryl dichlorosilane and styrene to produce poly(dialkylsilylene-co-alkyl arylsilylene-co-styrene), recovering said polymer in a known manner and subsequently heat treating the same in an inert atmosphere to obtain polycarbosilane.
2. The process as claimed in claim 1, wherein the polymerisation is carried out in a hydrocarbon solvent medium.
3. The process as claimed in claim 2, wherein the solvent medium is selected from benzene, toluene, xylene or mixtures thereof
4. The process as claimed in claims 1 to 3, wherein the polymerisation
reaction is carried out at 100-135°C for a period of 5 to 10 hours.
5. The process as claimed in claims 1 to 4, wherein the monomers are
dimethyl dichlorosilane, methyl phenyl dichlorosilane and styrene.
6. The process as claimed in claims 1 to 4, wherein said polymer
is washed and dried prior to the step of heat treatment.
7. The process as claimed in claims 1 to 5, wherein said polymer
iS heat treated at a temperature ranging from 380"C to 460"C.
8. The process as claimed in claim 7, wherein the heat treatment
is carried out in an inert atmosphere.
9. The process as claimed in claim 7, wherein the heat treatment
is carried out at an initial pressure of 8-12 psi and at a final
pressure of 40 to 50 psi.
10. A process for producing polycarbosilane,, a ceramic precursor
substantially as described hereinabove.
|Indian Patent Application Number||741/MAS/1999|
|PG Journal Number||13/2008|
|Date of Filing||14-Jul-1999|
|Name of Patentee||INDIAN SPACE RESEARCH ORGANISATION|
|Applicant Address||ANTARIKSH BHAVAN, NEW BEL ROAD, BANGALORE - 560 094,|
|PCT International Classification Number||C08 G 77/60|
|PCT International Application Number||N/A|
|PCT International Filing date|