|Title of Invention||
A PROCESS FOR THE SYNTHESIS OF BORON AND SILICON CONTAINING PRECERAMIC OLIGOMERS
|Abstract||This invention relates to a novel process for synthesising boron and silicon containing oligomers which are ceramic precursors. Boric acid and substituted alkoxysilanes are condensed in the presence of an inorganic acid catalyst at a temperature ranging from 100 and above. The oligomer is separated from the reaction mixture by knowrJ means and ~Iay be stored in an inert atmosphere. These oligomers yield a high percentage of ceramic residue on pyrolysis and ~Iay be used for preparation of ceramic bodies and ceramic coatings.|
This invention relates to a process for the synthesis of boron and silicon containing preceramic oligomers.
Ceramic materials are widely used in aerospace environments particularly in fabricating heat engines, high speed cutting tools and electronic sensors operable under extreme environmental conditions. Advanced ceramic materials which provide oxidation resistant coatings are preferred in aerospace industry. However, inherent brittleness and difficult processing conditions of many ceramics limit the scope of their application in this field. Organometallic polymers that yield ceramics on pyrolysis are now considered as a viable alternative to conventional ceramics. These polymers may be drawn into fibres, moulded to form any desired shape, dissolved in solvents and applied as coatings on substrates of choice and may then be subjected to pyrolysis to obtain corresponding ceramic end products. These polymers find use as binders to conventional ceramic powders which are difficult to sinter and as matrix resins for making ceramic matrix composites and for obtaining ultrafine ceramic powders. Idealy, the polymer chosen as a ceramic precursor must be meltable or soluble in organic solvents and must yield a high ceramic residue on pyrolysis.
Over the last two decades, many preceramic polymers have been developed from SiC, B4C Si3N4, Si-C-N, and BN. Ceramic fibres, coatings, components and matrix composites are made using these polymers. Considerable interest is now shown in
muIticomponent ceramics as they possess improved thermomechanical properties when compared to single component ceramic systems. However, multicomponent ceramic materials are difficult to synthesise by the conventional high temperature synthesis such as solid state reaction due to the thermodynamic instability of their solid solutions. On the other hand, multicomponent ceramics can be synthesised in the form of matastable single phase solid solutions through the polymer to ceramic transformation route. Boron and silicon containing multicomponent ceramics are reported to have high thermal shock resistance, oxidation resistance and high crystallization temperature. Synthesis of preceramic polymers containing boron and silicon are known in the art. Major impediments involved in the existing synthetic processes of preceramic oligomers containing boron and silicon are the high cost of components and low ceramic residue. Preceramic polymers yielding high ceramic residues are preferred as the volatiles evolved during pyrolysis are lower when compared to polymers yielding low ceramic residues. Reduction in the yield of volatiles during pyrolysis reduces the porosity of the final ceramic product. Considering this* it is clear that a high ceramic yielding polymer having low volatile content is preferred over a low ceramic yielding prepolymer. Conventional processes for synthesising silicon and boron preceramic polymers involve high reaction temperatures.
The objective of this invention is to develop a process for synthesising boron and silicon preceramic polymers
avoiding the above discussed drawbacks. Boric acid which is a cheap source of boron when compared with other sources boron is used as one of the starting materials. It is surprisingly found that use of boric acid as a starting material has definite advantages over other aspects also. Preceramic oligomers preparedd from boric acid yields very high ceramic residues. Reaction temperature is also considerably lower when compared with the other known synthetic processes where a temperature of 1200° or more is often required.
Boric acid is polycondensed with substituted alkoxysilanes in a high boiling organic solvents to produce preceramic oligomer. The molar ratio of boric acid to the alkoxysilane monomer may vary from 3:1 to 1:3. Solvents used may be selected from diglyme, dioxane, n-butyl ether or a mixture of two or more of these solvents. The proportion of the solvents in the mixture is not particularly relevant. The preferred solvent is diglyme. The solvent to monomer ratio may vary from 0.5 to 2.0 by weight. Alkoxysilane may be substituted mono, di or tri substituted alkyl, alkenyl, aryl, styrylalkyl, cyclopentyl, cyclohexyl> cyclopentadienyl, acryloxyalkyl or methacryloxyalkyl. Alkoxysilane may have more than one of the above substituents. Alkoxy groups may be methoxy, ethoxy, propoxy or butoxy. Alkyl groups may be methyl, ethyl, propyl, or isopropyl. Aryl groups may be phenyl or tolyl group and the alkenyl groups may be vinyl, allyl and butenyl groups. Alkoxysilanes may be substituted with
cyclopenty1, cyclohexyl, cyclohexenyl, cyclohenyl ethyl, cyolopentadienyl propyl trialkoxysilnes. Some examples of substituted alkoxysilanes are acryloxyalkyl, methacryloxyalkyl trialkoxysilane, di(acryloxyalkyl) or di (methacryloxy alkyl) dialkoxysilane, di(methacryloxylalkyl) dialkoxysilane, di(methacryloxyl alkyl) dialkoxysilane, tri(acryloxyalkyl) or tri(methacryloxyalkyl) alkoxysilane. The styrylalkyl substituted alkoxysilanes are styrylethyltriaIkyIsilane, di(styrylethyl) dialkoxysilane or tri (styrylethylalkoxysilane. A mixture of two or more of these substituted monomers may be used in any desired porportion. Polycondensation takes place at a temperature range of 100 to 160°C, An inorganic acid selected from hydrochloricacid, sulphuric acid, nitric acid or phosphoric acid is used as a catalyst. The preferred catalyst may be added in the range of 0.1 to 2% by wt of the monomers. The reaction is carried out for a period of 3-5 hours. Alkyl alcohol produced as a by-product during the polycondensation reaction is distilled off during the reaction. The oligomer produced may be separated from the organic solvent medium by distillation of the solvent and may be stored in an inert atmosphere.
The process for the synthesis of boron and silicon containing preceramic oligomers according to this invention comprises copolycondensation of boric acid with at least one substituted alkoxysilane monomer in the presence of an inorganic acid catalyst under known polymerisation conditions and isolating the resultant ologomer from the reaction mixture by known means.
The oligomer may be converted into ceramics by heating in an inert atmosphere to a temperature of 900° or above. Ceramic components may be prepared from these oligomers by dissolving in a suitable organic solvents and admixing with a nonoxide ceramic powder. Nonoxide ceramic powder may be selected from SiC, Si3N4 , B4C TiC or TiN or mixtures thereof. On removal of the solvent by drying under vacuum at 80 - 100°C for about 30 hours, on oligomers-non-xide-ceramic blend is obtainedd which may be cast into a green body of any desired shape. This green body may be converted into a ceramic components by heating in a tubular furnace at a heating rate of 2.5%°C/min to 1400°C.
The following non-limitive examples illustrates illustrates the process of synthesising the preceramic oligomers.
A mixture of 57.14g (o.30 mol) of vinyltriethoxysilane, 9.27g (0.15mol) boric acid, 50 ml of diglyme and 1 ml of concentrated hydrochloric acid is taken in a three necked round bottom flask equipped with mechanical stirrer and an inlet and an outlet for nitrogen. The contents of the flask are heated under a flow of nitrogen in an oil bath at a temperature of about 150°C under stirring for a period of three hours. The reaction mixture is then allowed to cool and ethanol formed as a by-product is distilled off solvent digylm is distilled thereafter under reduced pressure to obtain a colourless residue which is dried
under vaccum at 60°C for about 10 hours. The oligomer is obtained in 75 to 85% of the theoretical yield and is stored in a dessioator in nitrogen atmosphere.
A mixture containing 57.7g of phenyltrimethoxysilane, 9.27g of boric acid, 50 ml of diglyme and 1 ml of concentrated hydrochloric acidd is heated and the steps detailed in Example I is carried out. The resultant polymer is found to be stable upto 400°C in an atmosphere of nitrogen. This preceramic polymer yields about 67 to 70% of ceramic residue when heated to 900°C or above.
The boron and silicon preceramic oligomers synthesised by the above described process have the following advantages:
1. They are easily synthesised,
2. They are prepared at a relatively low temperatures,
3. Less volatiles are given out on pyrolysis,
4. High ceramic residue is obtained on pyrolysis,
5. Ceramic powders, components and coatings may be
prepared from these oligomers.
1. A process for the synthesis of boron and silicon containing preceramic oligomers comprising copolycondensation of boric acid with atleast one substituted alkoxysilane monomer in the presence an inorganic acid catalyst under known polymerisation condition and isolating the resultant oligomer from the reaction mixture by known means.
2. The process as claiamed in claim 1, wherein the substituted alkoxysilanes are mono, di or trisubstituted alkyl, alkenyl, aryl, styrylalkyl, oyclopentyl, oyclohexyl, cyclopentadienyl, aeryloxyalkyl, or methacryloxyalkyl alkoxysilano.
3. The process as claimed in claims 1 and 2 wherein the alkoxy groups of the alkoxysilane are methoxy, ethoxy, propoxy and butoxy groups.
4. The process as claimed in claim 1, wherein the alkyl substituted alkoxysilanes are alkyltrialkoxysilane, dialkyldialkyloxysilane or trialkulalkoxysilane and the alkyl groups are selected from methylethyl, ptopyl, idoptopyl, butyl, or isobutyl groups.
5. The process as claimed in claim 1, wherein the styrylalkyl substituted alkoxysilanes are styrylethyltrialkoxysilane, di(styrylethyl) dialkoxysilane, or tri(styrylethylalkoxysilane.
6. The process as claimed in claim 1, wherein the acryloxy substituted alkoxysilanes are acryloxy alkyl, methacryloxyalkyltrialkyloxysilane, di(acryloxyalkyl> or di(roethacryloxyalkyl) dialkoxysilane, tri(acryloxyalkyl) or tri(methacryloxyalkyl) alkoxysilane.
7. The process as claimed in claims 1-6, wherein the ratio of boric acid to alkoxysilane monomer is in the range of 3:1 to 1:3.
8 The process as claimed in any of the preceding claims
wherein the inorganic acid catalyst is selected from hydrochloric acid, sulphuric acid, nitric acid or phosphoric acid.
9. The process as claimed in claim 8, wherein 0.1 to 2% by
wt of said catalyst per the weight of the monomers is added to
the reaction mixture.
10. The process as claimed in any one of the preceding
claims wherein the polycondensation is carried out at a
temperature ranging from 10 to 160°C for 3 to 5 hours.
1.1. The process as claimed in any of the preceding claims wherein the polycondensor is carried out in an inert atmosphere.
12. The process as claimed in claim 11, wherein alcohol generated during polycondensation is distilled off from the reaction mixture.
13. The process as claimed in claim 1, wherein said
polycondensation solvent is carried out in a solvent medium such
as an ether.
14. The process as claimed in claim 13, wherein the ether
is selected from diglyme, dioxane, n-butyl ether or mixtures
15. The process for the synthesis of boron and silicon
containing preceramic oligomers substantially as herein
described and exemplified.
|Indian Patent Application Number||798/MAS/1999|
|PG Journal Number||35/2007|
|Date of Filing||06-Aug-1999|
|Name of Patentee||INDIAN SPACE RESEARCH ORGANISATION|
|Applicant Address||DEPARTMENT OF SPACE ANTARIKSH BHAVAN, NEW BEL ROAD, BANGALORE|
|PCT International Classification Number||C04B35/589|
|PCT International Application Number||N/A|
|PCT International Filing date|