Title of Invention	A PROCESS FOR SOLVENTLESS SYNTHESIS OF RESINOUS BOROSILOXANE OLIGOMER PRECURSORS FOR CERAMICS
Abstract	This invention relates to a process for the synthesis of borosiloxane oligomers in the resinous form which serve as precursors for boron containing siliconoxycarbide (SiOBC), silicon carbide (SiC) or silicon carbide-boron carbide (SiC-B4C) mixed non-oxide ceramics or a mixture of all the three ceramic systems. In the disclosed process, Boric acid and organotriailkoxysilanes are reacted in different monomer feed ratios at temperatures ranging from 50°C to 100°C for 1 to 10 hrs, without using any solvent or catalyst. The invention also includes a process for controlling the viscosity of the resin from 2 cps to 1000 cps either by changing the organotrialkoxysilane and/or monomer feed ratio or by distilling out by-product alkylalchol. The invention further describes the processes for the conversion of borosiloxane oligomers to ceramics and their end-use as matrix resins and infiltrating resins for ceramic matrix composites.

Title of Invention

A PROCESS FOR SOLVENTLESS SYNTHESIS OF RESINOUS BOROSILOXANE OLIGOMER PRECURSORS FOR CERAMICS

Abstract

This invention relates to a process for the synthesis of borosiloxane oligomers in the resinous form which serve as precursors for boron containing siliconoxycarbide (SiOBC), silicon carbide (SiC) or silicon carbide-boron carbide (SiC-B4C) mixed non-oxide ceramics or a mixture of all the three ceramic systems. In the disclosed process, Boric acid and organotriailkoxysilanes are reacted in different monomer feed ratios at temperatures ranging from 50°C to 100°C for 1 to 10 hrs, without using any solvent or catalyst. The invention also includes a process for controlling the viscosity of the resin from 2 cps to 1000 cps either by changing the organotrialkoxysilane and/or monomer feed ratio or by distilling out by-product alkylalchol. The invention further describes the processes for the conversion of borosiloxane oligomers to ceramics and their end-use as matrix resins and infiltrating resins for ceramic matrix composites.

Full Text	The subject matter of the present invention relates to synthesis of resinous borosiloxane oligomer precursors for ceramics from boric acid and organotrialicoxysilanes by a solventless process without using any catalyst and their end-use as matrix resin and infiltrating resin for ceramic matrix composites. These oligomers give ceramic residue of 50 to 90% when subjected to pyrolysis at 900°C in inert atmosphere. On heat treatment in inert atmosphere at 1200-2000°C, the oligomers give boron containing siliconoxycarbide (SiOBC), silicon carbide (SiC) or SiC-B4C mixed non-oxide ceramics. Background and prior art: Borosiloxane oligomers and polymers refer to compounds in which the repeating units contain B-O-Si linkages. They find use as high temperature resistant materials, insulating materials and precursors for borosilicate glass and ceramics. The general procedure adopted for the synthesis of borosiloxane oligomers or polymers are the following: i) through the reaction of organoalkoxysilane with boric acid, ii) by reacting organochlorosilane with boric acid or alkoxyboranes and iii) by reacting silanols with boric acid. US Patent No. 2434953 (1948) describes the method of preparing a water repellant material, viz., lower alkylpolysiloxane borates by reacting boric acid and a lower alkyl halogenopolysiloxane in glacial acetic acid. US Patent No. 2644805 (1953) deals with the preparation of boron containing polysiloxane compositions by reacting boric acid and tetramethyldisiloxane-l,3-diol, which at normal temperatures (25-35°C) resembles a thin paste and at 50-55°C gets converted into a putty like composition. US Patent No.3519670 (1970) relates to the preparation of liquid borosiloxanes through the reaction of alkoxyfunctional organosilicone compounds with boric acid or its derivatives followed by distillation of aikyi alcohol at 110-193°C and the end-use of such liquid boroslloxane as curatives for epoxy resins to produce materials having low dissipation factor and good corona resistance. The use of boroslloxane as precursor for ceramics was first reported by Yajima et al. [US Patent No. 4152609 (1979)], where they described the synthesis of polyborosiloxanes through the polycondensation of boric acid with diphenyidichlorosllane In n-butyiether. The product obtained was further heat treated at SOOC under vacuum to get polyboroslloxane. This polymer on pyrolysis under nitrogen at 1000°C gave an amorphous ceramic in 46% yield. This upon further heating to 17000 in argon yields p-SIC, glassy carbon and BC. The ease of handling of these polyborodlphenyisiioxanes, maice them useful as binders for SIC and SljN powders and as adhesives for joining SIC bodies [Nature 266 (1977) p522]. Polyboroslloxane prepared from boric acid and diphenyldlchiorosiiane also serves as a catalyst for the conversion of poiydimethylsiiane to polycarbosllane at atmospheric pressure [Nature 273 (1978) p5251. European Patent No. 0437934 (1991) describes the process of obtaining high-density silicon carbide sintered bodies using polyboroslloxane as binder. Polyboroslloxane used as a binder was prepared by refluxing a mixture of methyl borate, phenyltriethoxysllane and tetramethyldivinyi-disiloxane in presence of water and trifluoroacetic add. US Patent No. 4228270 (1980) deals with the preparation of borodiphenylsiloxanol by the condensation reaction of boric add and diphenylsilane did and then polycondensing the borodiphenylsiloxanol in the absence of solvent at a temperature from 140 to about 400'C to obtain borosiloxanes with a weight average molecular weight of 800-5000. Indian Patent No. 208583 (2007) discloses the synthesis of boroslloxane oligomers through the reaction of boric add with phenyltrialkoxysilane or vlnyltrialkoxysilane at a relatively low temperature (ISO-ieO'C) using diglyme as the reaction medium. These oligomers on pyroiysis at 900'C give ceramic residue of 75-85% which is very much higher than that reported by Yajima et ai. [US Patent No. 4152509(1979)]. Babonneau et al. p. Sol-Gel Scl. Tech., 18 (2000) p11 and Chem. Mater. 11 (1999) p910] in their study reported the synthesis of borosiioxane sol-gels by reacting boric acid and arylallcoxysilane at eO'C in the absence of solvent and leaving the sol in air for more than ten days to gel. The hybrid borosilicate gel was prepared by adding required amount of boric acid to the liquid silicon alkoxide, and by stining until its complete dissolution. The dissolution time varies from 1 to 10 days depending upon the type of the arylaikoxysilane used. In this approach, there exists the possibility of moisture to react with aikoxysiiane resulting in the formation of SI-OH groups which would react with B-OH or self condense resulting in the fomiation of B-O-Si and Si-O-Si linkages respectively, i-iere it is found that the boron atoms are present in the silica matrix as a separate B2O3 and/or B(0I)3 phase. Thus, by following such a method for the synthesis of borosiloxanes there is a possibility of the fomnation of Si02 rich or B2O3 rich phase In the final ceramic. The processes described in the prior art suffer from one or more of the following siiortcomings: 1) the borosiloxane oligomers are not suitable as precursors for ceramics as they give poor ceramic residue or they have not been designed for this specific end-use, ii) high boiling solvent and catalyst are used for the synthesis and reaction is carried out at high temperatures (>150°C) and attempts to remove high boiling solvent render the oligomers insoluble. Hi) the monomers are reacted and left in air for days to promote gelation through reaction with moisture and this procedure results in the formation of SIO2 and BaOa ricli phases in the final ceramic obtained from the borosiloxane oligomer and iv) water is used for promoting get formation and this procedure invariably promotes the formation of Si02 and B2O3 rich phases. Objects of the invention: The primary object of the invention is to synthesize high ceramic residue (50 to 90 wt%) yielding borosiloxane preceramic oligomers in the resinous fonri of desired viscosity by directly reacting boric acid with aikoxysllanes at 50-100'C for 1 to 10 hrs without using any solvent and catalyst for end-use as matrix resin and Infiltrating resin for ceramic matrix composite. The borosiloxane oligomers synthesized by the procedure disclosed in the present invention overcome the limitations listed above for the borosiloxanes described in the prior art. The borosiloxane oligomers disclosed in the present Invention l}ave the following advantages: i) As the reactants, viz., boric acid and organotrlalkoxysiianes are directly reacted in the absence of any catalyst or water, the fomiatlon of B-OSi linkage is ensured. The presence of B-O-Si linkages In the oligomers ensures uniform composition of ceramic when the oligomers are subjected to pyrolysis and sintering, thus eliminating the formation of SiOa or BsOa rich phases. As no catalyst is used for the synthesis the oligomer is free from impurities when compared to a similar system where residual quantity of catalyst used remains in the system as an impurity. ■ ji) The process is simple as the monomers are reacted directly without using any solvent and catalyst at a relatively lower temperature (50-1OCC). This procedure eliminates the need to remove high boiling solvents used for the synthesis to obtain polyborositoxane. Hence, this process can be scaled-up easily. Boroslloxane oligomers produced in this process remain as low viscous resin in alkyl alcohol which is produced as a by-product. ill) The viscosity of boroslloxane oligomers can be varied from low viscous to high viscous resin by regulating the amount of alkyl alcohol (by-product) distilled out. Boroslloxane oligomers can also be obtained as a solid by complete removal of alcohol. The viscosity of boroslloxane oligomers can also be tailored by proper choice of organotrialkoxysilane and the mole ratio of boric acid to organotrialkoxysilane. Iv) As the boroslloxane oligomers are obtained In the resinous form of desired viscosity it can be used as matrix resin for preparing ceramic matrix composites. v) When polyboroeiioxanes synthesized in solvent medium (usually iilgh boiling) are used for Infiltration of porous ceramic matrix composite, solvent removal becomes difficult. This problem can be overcome by using boroslloxane oligomers synthesized by soiventless process disclosed in this patent application. As the viscosity of boroslloxane oligomer is low, the infiltration of ceramic matrix composite can be canied out without application of external pressure. After infiltration, by slowly heating the composite at relatively low temperatures (50-80C) alkyl alcohol by-product can be removed. By applying temperature, further molecular weight build-up of the boroslloxane resin takes place inside the pores of the composite. vi) The borosiloxane oligomers disclosed in the present invention are obtained In 1 to 10 hrs. as against 1 to 10 days when boric acid and aikoxysiianes are reacted and left in air, as reported in the literature. Summary of the invention: The invention provides processabie borosiloxane oligomers, containing structural units represented by general stmcture given below, whose viscosity can be varied from low viscous (~2 cps) to high viscous resin (-1000 cps) depending on the requirement. The invention provides borosiloxane oligomers capable of giving ceramic residue In the range of 50 to 90% at 900"C in inert atmosphere. The resinous borosiloxane oligomers can be used as matrix resin for ceramic matrix composites. The low viscous borosiloxane oligomers can be effectively used for the infiitration/densification of ceramic matrix composites at nonnal pressure. It also provides the scope for converting the liquid borosiloxane ojigomer to solid material, which can be converted to boron containing siiiconoxycarbide (SIOBC), silicon carbide (SiC) or SiC-B4C mixed non-oxide ceramics by heat treatment In Inert atmosphere in the temperature range 12000 to aooO'C. Description of the invention: Boric acid is condensed with organotrialkoxysilane to produce borosiloxane oligomer in the absence of a solvent and catalyst. The molar ratio of boric acid to alkoxysilane monomer may vary from 3:1 to 1:3. The organic group attached to silicon may be alkyl, alkenyl, alkynyl, aryl, styrylalkyl, cyclopentyl, cyclohexyl, cyclopentadienyl, acryloxyalkyl or methacryloxyalkyl. Aryl groups may be phenyl or tolyl. Alkenyl group may be vinyl, allyl and butenyl group and alkynyl group may be acetyleinic or propargyl. Alkoxy groups may be methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy or t-butoxy. Condensation takes place in the temperature range of 50 to 100°C. The reaction is carried out for a period of 1-10 hrs. Alkyl alcohol produced as a by-product during the reaction serves as a solvent medium for the borosiloxane oligomer and the borosiloxane oligomer in alkyl alcohol can be directly used or alternately the alkyl alcohol can be partly or completely removed. Depending on the quantity of the removal of alkyl alcohol by-product the viscosity and the nature of the product can be altered. The viscosity of borosiloxane oligomers also depends on the nature of alkoxysilanes used for the synthesis. Thus, the viscosity of resinous borosiloxane can be tailored by the proper choice of organoalkoXysilane and by controlling the amount of alkyl alcohol by-product removed from the system. The borosiloxane oligomers obtained can be stored in airtight containers and used directly as resinous material. The present invention is illustrated with reference to the accompanying drawings:- Fig. 1 shows 'Si-NMR of vinylborosiloxane (boric acid: vinyltriethoxysilane ratio-1:2). Fig. 2 shows the XRD pattern of vinylborosiloxane oligomer (boric acid: vinyltriethoxysilane ratio-1:2) heat treated at three different temperatures namely, 900, 1500 and 1650°C. The borosiloxane oligomers described in the present invention were characterized by solid content, elemental composition, viscosity, number average molecular weight, weight average molecular weight and ceramic residue at 900°C in inert atmosphere. They were also characterized by infrared spectra, H-, 'C- and SiNMR spectra and thermogravimetric analysis. The above studies confirm the formation of borosiloxanes having Si-O-B linkages. The peak at -68 ppm corresponds to RSi(0R')2(0B) [Ti] at -73 and -74 ppm corresponds to RSi(0R')(0B)2 [T2] and at -78 and -81 ppm corresponds to RSi(0B)3 [T3] structures with Si-O-B linkages. The formation of different types of structures depends upon the number of alkoxygroups replaced during the reaction. The borosiloxane oligomers can be subjected to heat treatment in inert atmosphere at 1200-2000°C to obtain boron containing siliconoxycarbide, silicon carbide or SiC-B4C mixed non-oxide ceramics. The ceramics obtained were characterized by IR spectra, XRD and elemental analysis. These studies suggest that the ceramic yield and nature of ceramic formed depend on the molar ratio of boric acid to organotrialkoxysilane used for the synthesis of the oligomers and heat treatment temperature. The ceramics obtained after the ceramic conversion of the vinylborosiloxane at 1650°C is purely crystalline and nanosized and no distinct phases of SiOa or B2O3 are seen in the XRD pattern. Borosiloxane oligomers in the resinous form can be used as matrix resin for the preparation of ceramic matrix composites using carbon fiber/fabric, silicon carbide fiber/fabric or silica fiber/fabric as reinforcement. The borosiloxane oligomers in the resinous form can also be used for infiltration of ceramic matrix composites to reduce the porosity or in other words to irpprove the density. No external pressure is required for infiltration as the viscosity of the resin Is very low (as low as 2 ops). The following non-limitfve examples illustrate the process for synthesizing the preceramic borosiloxane oligomers as precursor for ceramics which can be used for the preparation and densiflcation of ceramic matrix composites. Example 1: A mixture containing 6.18 g (0.10 mol) of boric acid and 48.08 g (0.20 mol) of phenyltriethoxysitane was taken in a three-necked round bottom flask equipped with a mechanical stirrer and an inlet and an outlet for nitrogen. The contents of the flask were heated at 50-100°C under stining for a period of 5 hrs. The resinous product obtained has a viscosity of 19 cps at 25'C. The resin has JA„ and (by GPC) of 1300 and 1000 respectively. 10 g of the resin was taken in a petridish and dried at ITSC for 2 hrs in an air oven to determine the solid content. The solid content was found to be 58%. A ceramic residue of 69% (at 900°C) was obtained when this solid phenylborosiloxane oligomer was subjected to themfiogravlmetric analysis (TGA) up The synthesis as described above was repeated following the same experimental procedure with molar ratio of boric acid: phenyitriethoxysilane 1:1.5 to get borosiloxane oligomer having viscosity of 22 cps at 25'C, M of 1270 and J of 98Q. The solid content of the resin Is 62% and the ceramic residue of the solid borosiloxane at QOCC (by TGA) Is 71% in inert atmosphere. The synthesis was also repeated with boric acid: phenyKrlethoxysilane molar ratio of 1:1 to get borosiloxane oligomer having viscosity of 34 cps at 25°C, % of 1150 and H, of 910. The solid content of the resin is 65% and the ceramic residue of the solid borosiloxane at QOO'C (by TGA) is 68% in inert atmosphere. Example 2: Following the experimental procedure given in Example 1, 9.27 g (0.15 mol) of boric acid and 59.50 g (0.30 mol) of phenyltrlmethoxysilane was reacted for 3 hrs to produce a borosiloxane resin having a viscosity of 120 cps at 25°C. The following scheme represents the reaction: GPC analysis shows that the oligomer has M„of 2300 and M„of 2100. The resin has a solid content of 78%. The solid borosiloxane oligomer gave a ceramic residue of 68% at 900"C in inert atmosphere. Example 3: A mixture of 9.27g (0.15 mol) of boric add and 57.10 g (0.30 mol) of vinyltriethoxysilane was reacted following the procedure given In Example 1 for 3 hrs. The product was obtained as a resinous material having a viscosity of 4.3 cps at 25'C, Mw of 1300 and Mn of 980. The following scheme represents the reaction: The resin has a solid content of 49%. it was flasii evaporated to partly remove the by-product ethanoi to obtain a resin having viscosity of 260 cps and solid content of 83 %. The solid borosiloxane oligomer gave a ceramic residue of 86% (by TGA) at SOO'C In inert atmosphere. Example 4: Following the experimental procedure given in Example 1, 6.18 g (0,10 mol) of boric acid and 35.70 g (0.20 mol) of methyltrlettioxysllane were reacted for 3 hrs to produce a resinous borosiloxane having a viscosity of 23 cps at 25"C, M of 2160 and ,of 1460 respectively. The solid borosiloxane oligomer obtained from this resin gave a ceramic residue of 84% (by TGA) at eoO'C in Inert atmosphere. Example 6: The synthesis of phenylborositoxane oligomer as described in Example 1 was scaled-up to get 12.8 kg of resin. A mixture containing 1462.97 g (23.66 mol) of boric add and 11371.94 g (47.32 mol) of phenyltrlethoxysllane was reacted to obtain a low viscous resin. The resin has a viscosity of 9.2 cps at 25"C and a solid content of 60%. The resin was flash evaporated to partly remove the by-product ethanoi to obtain a resin having viscosity of 120 cps at 25"C and a solid content of 80%. The solid borosiloxane oligomer gave a ceramic residue of 63 % at QOCC in inert atmosphere. Example 6: Tlie synthesis of vinylborosiloxane oligomer as described in Example 3 was scaled-up to get ~5 kg of resin. A mixture containing 698.7 g (11.30 moi) of boric acid and 4301.23 g (22.60 mol) of vinyitiiethoxysilane was reacted to obtain a low viscous resin. The resin has a viscosity of 3.7 cps at 25'C, M of 4200 and J!H„o1 1430. The resin has a solid content of 46%. The resin was flash evaporated to partly remove the by-product ethanol to obtain a resin having viscosity of 670 cps at 25'C, M«, of 9870 and 1, of 2060 and solid content of 85%. The solid borosiloxane oligomer gave a ceramic residue of 86 % (by TGA) at OOO'C in Inert atmosphere. Example 7: Ceramic conversion of phenylborosiloxane oligomer, synthesized as per the procedure given in Example 1, was carried out at 900, 1500 and 16500 under argon atmosphere. The resinous product obtained after synthesis was dried at 175'C for 2 hrs and the solid material obtained as flakes obtained was powdered and used for ceramic conversion. Pyrolysis of the solid borosiloxane oligomer was performed in an inconel furnace at 900'C under the flow of argon. In a typical experiment, 5 g of the oligomer was taien in a graphite crucible and heated at a rate of 3"C/min up to 900C and maintained at this temperature for 3 hrs. The furnace was then cooled to room temperature at a rate of 3(3/min. Ceramic conversion studies at 1500'C were canied out In ah alumina tubular furnace. The 900'C pyrolyzed sample was heated to ISOO'C at a heating rate of 3'C/min and maintained at 1500'C for 3 hrs. The furnace was cooled to room temperature at a rate of S'C/min. Ceramic conversion at 1650'C was also canied out in an alumina furnace. The 9O0'C pyrolyzed sample was heat treated at lescc for 3 hrs at a heating/cooling rate of S'C/min. The furnace was cooled to room temperature at a rate of S'C/mln. Ail the oligomers gave amorphous SiOBC ceramic powdar at GOCC. XRD analysis showed that crystallization of p-SIC starts at ISOO'C and sample heat treated at lesO'C shows crystalline p-SiC peaks. Examples: Ceramic conversion of the phenyiborosiloxane oligomer from Example 2 was canled out at 900, 1500 and lesO'C under argon atmosphere following the procedure given in Example 9. The furnace was cooled to room temperature at a rate of S'C/min. All the oligomers gave amorphous SiOBC ceramic powder at 900'C. XRD analysis showed that crystallization of p-SiC starts at 1S00C and sample heat treated at 1650C shows crystalline p-SiC peal Example 9: Phenyiborosiloxane synthesized as per Example 2 was used as the matrix resin and infiltrating resin for the preparation of C/SiC ceramic matrix composite. Carbon T-300 fabric was cut into pieces of 150x150 mm size. The carbon fabric piece was coated on both sides with phenyiborosiloxane resin. The coated febric piece was left in air overnight for drying. The resin up-take for each ply was about 12 g for 8 g of fabric. 6 Nos. of phenyiborosiloxane coated C-fabric pieces were stacked in O'-gO" orientation inside a mould with top and bottom plates with Teflon releasing sheets in between. The whole assembly was kept inside a hydraulic press and was hot pressed as per the following schedule: Heating up to 150*0 from room temperature in steps of 25'C at rc/min, holding at each step for 60 min, under a negHgible holding pressure. Then a pressure of 750 psi was applied on the sample and subsequently the temperature was raised to ITS'C. A dwell time of 120 min. was given at 175'C after which the heating is turned off. The sample was unloaded from the press after cooling to room temperature. The precursor composite has thicltness of 3 mm. About 26 % weight loss was observed due to oozing out of resin and curing of borosiloxane. The 150x150x3mm composites obtained were pyrolyzed at GOO'C under argon flow at a slow heating/cooling rate of 3'C/mln with a holding time of 2 hrs. The pyrolyzed composite showed a large number of voids formed due to the thermal degradation of phenylborosiloxane. A weight loss of 26% was observed for the composite (Density = 1.15g/cc). The composite was then Infiltrated with phenylborosiloxane resin, cured at ITS'C in air oven and coupons for flexural strength evaluation were machined out from it using diamond cutter. The test coupons were further heat treated at ISOCC for 2 hrs under argon flow at a heating/cooling rate of 3°C/min to complete the ceramic conversion. Further densification of the porous composite v\»s carried out by infiltration with phenyl borosiloxane followed by pyrolysis at 900'C. Finally, the CMC samples were heat treated at ISOO'C to get specimens of density 1.78 g/cc. A ftexural strength of ~147 MPa (average) was obtalr\ed for the specimens. Infiltration of the composite with phenylborosiloxane has resulted in 55% increase in the density of the composite. We claim: 1. A process for solventless synthesis of resinous borosiioxane oligomer precursors for ceramics having a general structure of wherein R is alkyl, aryl, alkenyl, alkynyl, styrylalkyl, cyclopentyl, cyclohexyl, cyclopentadienyl, acryloxyalkyl or methacryloxyalkyl comprising reacting boric acid and organotrialkoxysilane at temperature ranging from 50 to 100°C in an inert atmosphere in the absence of any solvent or catalyst and distilling the alkyl alcohol by product from the reaction mixture. 2. The process as claimed in claim 1, wherein the molar ratio of boric acid to organotrialkoxysilane is from 3:1 to 1:3 and the reaction is carried out for 1 to 10 hours prior to distillation of alkyl alcohol. 3. The process as claimed in claim 1, wherein the viscosity of the borosiioxane resin produced is from 2 cps to 200 cps (at 25°C) based on the organotrialkoxysilane, monomer feed ratio, reaction temperature and duration. 4. The process as claimed in claim 1, wherein the viscosity of the borosiioxane resin is from 2 cps to 1000 cps (at 25°C) and is achieved by regulating the quantam of distillation of alkyl alcohol by product from the reaction mixture. 5. The process as claimed in claims 1 to 4, wherein the monomer feed ratio, the reaction temperature and duration are regulated to produce resinous borosiioxane oligomer which is dried at a temperature of about n5°C to obtain solid borosiloxane oligomer having a ceramic residue of 50% to 90% when heated in an inert atmosphere. 6. The process as claimed in claim 1, wherein the borosiloxane oligomer is heat treated in an inert atmosphere at a temperature ranging from 1200 to 2000°C to produce siliconoxycarbide (SiOBC), siliconcarbide (SiC), silicon carbide-boron carbide, (SiC-B4C) or a mixture thereof having a particle size of 5 to 90 nm. 7. The process as claimed in claim 1, wherein said resinous borosiloxane oligomer is infiltrated into ceramic matrix composites reinforced with carbon-fiber/fabric, silicon carbide fiber/fabric or silica-fiber/fabric. 8. The process as claimed in claim 1, wherein the alkoxy group of the trialkoxysilane is methoxy, ethoxy, 2-ethoxymethoxy, propoxy or butoxy.

Full Text

The subject matter of the present invention relates to synthesis of resinous borosiloxane oligomer precursors for ceramics from boric acid and organotrialicoxysilanes by a solventless process without using any catalyst and their end-use as matrix resin and infiltrating resin for ceramic matrix composites. These oligomers give ceramic residue of 50 to 90% when subjected to pyrolysis at 900°C in inert atmosphere. On heat treatment in inert atmosphere at 1200-2000°C, the oligomers give boron containing siliconoxycarbide (SiOBC), silicon carbide (SiC) or SiC-B4C mixed non-oxide ceramics.
Background and prior art:
Borosiloxane oligomers and polymers refer to compounds in which the repeating units contain B-O-Si linkages. They find use as high temperature resistant materials, insulating materials and precursors for borosilicate glass and ceramics.
The general procedure adopted for the synthesis of borosiloxane oligomers or polymers are the following: i) through the reaction of organoalkoxysilane with boric acid, ii) by reacting organochlorosilane with boric acid or alkoxyboranes and iii) by reacting silanols with boric acid. US Patent No. 2434953 (1948) describes the method of preparing a water repellant material, viz., lower alkylpolysiloxane borates by reacting boric acid and a lower alkyl halogenopolysiloxane in glacial acetic acid. US Patent No. 2644805 (1953) deals with the preparation of boron containing polysiloxane compositions by reacting boric acid and tetramethyldisiloxane-l,3-diol, which at normal temperatures (25-35°C) resembles a thin paste and at 50-55°C gets converted into a putty like composition. US Patent No.3519670 (1970) relates to the preparation of liquid borosiloxanes through the reaction of alkoxyfunctional organosilicone compounds with boric acid or its derivatives followed by distillation of

aikyi alcohol at 110-193°C and the end-use of such liquid boroslloxane as curatives for epoxy resins to produce materials having low dissipation factor and good corona resistance.
The use of boroslloxane as precursor for ceramics was first reported by Yajima et al. [US Patent No. 4152609 (1979)], where they described the synthesis of polyborosiloxanes through the polycondensation of boric acid with diphenyidichlorosllane In n-butyiether. The product obtained was further heat treated at SOOC under vacuum to get polyboroslloxane. This polymer on pyrolysis under nitrogen at 1000°C gave an amorphous ceramic in 46% yield. This upon further heating to 1700*0 in argon yields p-SIC, glassy carbon and BC. The ease of
handling of these polyborodlphenyisiioxanes, maice them useful as binders for SIC and SljN powders and as adhesives for joining SIC bodies [Nature 266 (1977)
p522]. Polyboroslloxane prepared from boric acid and diphenyldlchiorosiiane also serves as a catalyst for the conversion of poiydimethylsiiane to polycarbosllane at atmospheric pressure [Nature 273 (1978) p5251. European Patent No. 0437934 (1991) describes the process of obtaining high-density silicon carbide sintered bodies using polyboroslloxane as binder. Polyboroslloxane used as a binder was prepared by refluxing a mixture of methyl borate, phenyltriethoxysllane and tetramethyldivinyi-disiloxane in presence of water and trifluoroacetic add. US Patent No. 4228270 (1980) deals with the preparation of borodiphenylsiloxanol by the condensation reaction of boric add and diphenylsilane did and then polycondensing the borodiphenylsiloxanol in the absence of solvent at a temperature from 140 to about 400*'C to obtain borosiloxanes with a weight average molecular weight of 800-5000. Indian Patent No. 208583 (2007) discloses the synthesis of boroslloxane oligomers through the reaction of boric add with phenyltrialkoxysilane or

vlnyltrialkoxysilane at a relatively low temperature (ISO-ieO'C) using diglyme as the reaction medium. These oligomers on pyroiysis at 900'C give ceramic residue of 75-85% which is very much higher than that reported by Yajima et ai. [US Patent No. 4152509(1979)].
Babonneau et al. p. Sol-Gel Scl. Tech., 18 (2000) p11 and Chem. Mater. 11 (1999) p910] in their study reported the synthesis of borosiioxane sol-gels by reacting boric acid and arylallcoxysilane at eO'C in the absence of solvent and leaving the sol in air for more than ten days to gel. The hybrid borosilicate gel was prepared by adding required amount of boric acid to the liquid silicon alkoxide, and by stining until its complete dissolution. The dissolution time varies from 1 to 10 days depending upon the type of the arylaikoxysilane used. In this approach, there exists the possibility of moisture to react with aikoxysiiane resulting in the formation of SI-OH groups which would react with B-OH or self condense resulting in the fomiation of B-O-Si and Si-O-Si linkages respectively, i-iere it is found that the boron atoms are present in the silica matrix as a separate B2O3 and/or B(0I)3 phase. Thus, by following such a method for the synthesis of borosiloxanes there is a possibility of the fomnation of Si02 rich or B2O3 rich phase In the final ceramic.
The processes described in the prior art suffer from one or more of the following siiortcomings: 1) the borosiloxane oligomers are not suitable as precursors for ceramics as they give poor ceramic residue or they have not been designed for this specific end-use, ii) high boiling solvent and catalyst are used for the synthesis and reaction is carried out at high temperatures (>150°C) and attempts to remove high boiling solvent render the oligomers insoluble. Hi) the monomers are reacted and left in air for days to promote gelation through reaction with moisture and this

procedure results in the formation of SIO2 and BaOa ricli phases in the final ceramic obtained from the borosiloxane oligomer and iv) water is used for promoting get formation and this procedure invariably promotes the formation of Si02 and B2O3 rich phases.
Objects of the invention:
The primary object of the invention is to synthesize high ceramic residue (50 to 90 wt%) yielding borosiloxane preceramic oligomers in the resinous fonri of desired viscosity by directly reacting boric acid with aikoxysllanes at 50-100'*C for 1 to 10 hrs without using any solvent and catalyst for end-use as matrix resin and Infiltrating resin for ceramic matrix composite.
The borosiloxane oligomers synthesized by the procedure disclosed in the present invention overcome the limitations listed above for the borosiloxanes described in the prior art.
The borosiloxane oligomers disclosed in the present Invention l}ave the following advantages:
i) As the reactants, viz., boric acid and organotrlalkoxysiianes are directly reacted in the absence of any catalyst or water, the fomiatlon of B-OSi linkage is ensured.
The presence of B-O-Si linkages In the oligomers ensures uniform composition of ceramic when the oligomers are subjected to pyrolysis and sintering, thus eliminating the formation of SiOa or BsOa rich phases. As no catalyst is used for the synthesis the oligomer is free from impurities when compared to a similar system where residual quantity of catalyst used remains in the system as an impurity.

■
ji) The process is simple as the monomers are reacted directly without using any solvent and catalyst at a relatively lower temperature (50-1OCC). This procedure eliminates the need to remove high boiling solvents used for the synthesis to obtain polyborositoxane. Hence, this process can be scaled-up easily. Boroslloxane oligomers produced in this process remain as low viscous resin in alkyl alcohol which is produced as a by-product.
ill) The viscosity of boroslloxane oligomers can be varied from low viscous to high viscous resin by regulating the amount of alkyl alcohol (by-product) distilled out. Boroslloxane oligomers can also be obtained as a solid by complete removal of alcohol. The viscosity of boroslloxane oligomers can also be tailored by proper choice of organotrialkoxysilane and the mole ratio of boric acid to organotrialkoxysilane.
Iv) As the boroslloxane oligomers are obtained In the resinous form of desired viscosity it can be used as matrix resin for preparing ceramic matrix composites.
v) When polyboroeiioxanes synthesized in solvent medium (usually iilgh boiling) are used for Infiltration of porous ceramic matrix composite, solvent removal becomes difficult. This problem can be overcome by using boroslloxane oligomers synthesized by soiventless process disclosed in this patent application. As the viscosity of boroslloxane oligomer is low, the infiltration of ceramic matrix composite can be canied out without application of external pressure. After infiltration, by slowly heating the composite at relatively low temperatures (50-80*C) alkyl alcohol by-product can be removed. By applying temperature, further molecular weight build-up of the boroslloxane resin takes place inside the

pores of the composite.
vi) The borosiloxane oligomers disclosed in the present invention are obtained In 1 to 10 hrs. as against 1 to 10 days when boric acid and aikoxysiianes are reacted and left in air, as reported in the literature.
Summary of the invention:
The invention provides processabie borosiloxane oligomers, containing structural units represented by general stmcture given below, whose viscosity can be varied from low viscous (~2 cps) to high viscous resin (-1000 cps) depending on the requirement.

The invention provides borosiloxane oligomers capable of giving ceramic residue In the range of 50 to 90% at 900"*C in inert atmosphere. The resinous borosiloxane oligomers can be used as matrix resin for ceramic matrix composites. The low viscous borosiloxane oligomers can be effectively used for the infiitration/densification of ceramic matrix composites at nonnal pressure. It also provides the scope for converting the liquid borosiloxane ojigomer to solid material, which can be converted to boron containing siiiconoxycarbide (SIOBC), silicon carbide (SiC) or SiC-B4C mixed non-oxide ceramics by heat treatment In Inert atmosphere in the temperature range 12000 to aooO'C.

Description of the invention:
Boric acid is condensed with organotrialkoxysilane to produce borosiloxane oligomer in the absence of a solvent and catalyst. The molar ratio of boric acid to alkoxysilane monomer may vary from 3:1 to 1:3. The organic group attached to silicon may be alkyl, alkenyl, alkynyl, aryl, styrylalkyl, cyclopentyl, cyclohexyl, cyclopentadienyl, acryloxyalkyl or methacryloxyalkyl. Aryl groups may be phenyl or tolyl. Alkenyl group may be vinyl, allyl and butenyl group and alkynyl group may be acetyleinic or propargyl. Alkoxy groups may be methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy or t-butoxy. Condensation takes place in the temperature range of 50 to 100°C. The reaction is carried out for a period of 1-10 hrs. Alkyl alcohol produced as a by-product during the reaction serves as a solvent medium for the borosiloxane oligomer and the borosiloxane oligomer in alkyl alcohol can be directly used or alternately the alkyl alcohol can be partly or completely removed. Depending on the quantity of the removal of alkyl alcohol by-product the viscosity and the nature of the product can be altered. The viscosity of borosiloxane oligomers also depends on the nature of alkoxysilanes used for the synthesis. Thus, the viscosity of resinous borosiloxane can be tailored by the proper choice of organoalkoXysilane and by controlling the amount of alkyl alcohol by-product removed from the system. The borosiloxane oligomers obtained can be stored in airtight containers and used directly as resinous material.
The present invention is illustrated with reference to the accompanying drawings:-
Fig. 1 shows 'Si-NMR of vinylborosiloxane (boric acid: vinyltriethoxysilane
ratio-1:2).
Fig. 2 shows the XRD pattern of vinylborosiloxane oligomer (boric acid:
vinyltriethoxysilane ratio-1:2) heat treated at three different temperatures namely, 900,
1500 and 1650°C.

The borosiloxane oligomers described in the present invention were characterized by solid content, elemental composition, viscosity, number average molecular weight, weight average molecular weight and ceramic residue at 900°C in inert atmosphere. They were also characterized by infrared spectra, *H-, 'C- and SiNMR spectra and thermogravimetric analysis. The above studies confirm the formation of borosiloxanes having Si-O-B linkages. The peak at -68 ppm corresponds to RSi(0R')2(0B) [Ti] at -73 and -74 ppm corresponds to RSi(0R')(0B)2 [T2] and at -78 and -81 ppm corresponds to RSi(0B)3 [T3] structures with Si-O-B linkages. The formation of different types of structures depends upon the number of alkoxygroups replaced during the reaction.
The borosiloxane oligomers can be subjected to heat treatment in inert atmosphere at 1200-2000°C to obtain boron containing siliconoxycarbide, silicon carbide or SiC-B4C mixed non-oxide ceramics. The ceramics obtained were characterized by IR spectra, XRD and elemental analysis. These studies suggest that the ceramic yield and nature of ceramic formed depend on the molar ratio of boric acid to organotrialkoxysilane used for the synthesis of the oligomers and heat treatment temperature.
The ceramics obtained after the ceramic conversion of the vinylborosiloxane at 1650°C is purely crystalline and nanosized and no distinct phases of SiOa or B2O3 are seen in the XRD pattern.
Borosiloxane oligomers in the resinous form can be used as matrix resin for the preparation of ceramic matrix composites using carbon fiber/fabric, silicon carbide fiber/fabric or silica fiber/fabric as reinforcement. The borosiloxane oligomers in the resinous form can also be used for infiltration of ceramic matrix composites to

reduce the porosity or in other words to irpprove the density. No external pressure is required for infiltration as the viscosity of the resin Is very low (as low as 2 ops).
The following non-limitfve examples illustrate the process for synthesizing the preceramic borosiloxane oligomers as precursor for ceramics which can be used for the preparation and densiflcation of ceramic matrix composites.
Example 1:
A mixture containing 6.18 g (0.10 mol) of boric acid and 48.08 g (0.20 mol) of phenyltriethoxysitane was taken in a three-necked round bottom flask equipped with a mechanical stirrer and an inlet and an outlet for nitrogen. The contents of the flask were heated at 50-100°C under stining for a period of 5 hrs. The resinous product obtained has a viscosity of 19 cps at 25'C. The resin has JA„ and (by GPC) of 1300 and 1000 respectively. 10 g of the resin was taken in a petridish and dried at ITSC for 2 hrs in an air oven to determine the solid content. The solid content was found to be 58%. A ceramic residue of 69% (at 900°C) was obtained when this solid phenylborosiloxane oligomer was subjected to themfiogravlmetric analysis (TGA) up

The synthesis as described above was repeated following the same experimental procedure with molar ratio of boric acid: phenyitriethoxysilane 1:1.5 to get borosiloxane oligomer having viscosity of 22 cps at 25'C, M of 1270 and J of

98Q. The solid content of the resin Is 62% and the ceramic residue of the solid borosiloxane at QOCC (by TGA) Is 71% in inert atmosphere.
The synthesis was also repeated with boric acid: phenyKrlethoxysilane molar ratio of 1:1 to get borosiloxane oligomer having viscosity of 34 cps at 25°C, % of 1150 and H, of 910. The solid content of the resin is 65% and the ceramic residue of the solid borosiloxane at QOO'C (by TGA) is 68% in inert atmosphere.
Example 2:
Following the experimental procedure given in Example 1, 9.27 g (0.15 mol) of boric acid and 59.50 g (0.30 mol) of phenyltrlmethoxysilane was reacted for 3 hrs to produce a borosiloxane resin having a viscosity of 120 cps at 25°C. The following scheme represents the reaction:

GPC analysis shows that the oligomer has M„of 2300 and M„of 2100. The resin has a solid content of 78%. The solid borosiloxane oligomer gave a ceramic residue of 68% at 900"*C in inert atmosphere.
Example 3:
A mixture of 9.27g (0.15 mol) of boric add and 57.10 g (0.30 mol) of vinyltriethoxysilane was reacted following the procedure given In Example 1 for 3 hrs. The product was obtained as a resinous material having a viscosity of 4.3 cps at 25'C, Mw of 1300 and Mn of 980. The following scheme represents the reaction:

The resin has a solid content of 49%. it was flasii evaporated to partly remove the by-product ethanoi to obtain a resin having viscosity of 260 cps and solid content of 83 %. The solid borosiloxane oligomer gave a ceramic residue of 86% (by TGA) at SOO'C In inert atmosphere.
Example 4:
Following the experimental procedure given in Example 1, 6.18 g (0,10 mol) of boric acid and 35.70 g (0.20 mol) of methyltrlettioxysllane were reacted for 3 hrs to produce a resinous borosiloxane having a viscosity of 23 cps at 25"C, M of 2160 and ,of 1460 respectively. The solid borosiloxane oligomer obtained from this resin gave a ceramic residue of 84% (by TGA) at eoO'C in Inert atmosphere.

Example 6:
The synthesis of phenylborositoxane oligomer as described in Example 1 was scaled-up to get 12.8 kg of resin. A mixture containing 1462.97 g (23.66 mol) of boric add and 11371.94 g (47.32 mol) of phenyltrlethoxysllane was reacted to obtain a low viscous resin. The resin has a viscosity of 9.2 cps at 25"C and a solid content of 60%. The resin was flash evaporated to partly remove the by-product ethanoi to obtain a resin having viscosity of 120 cps at 25"C and a solid content of 80%. The

solid borosiloxane oligomer gave a ceramic residue of 63 % at QOCC in inert atmosphere.
Example 6:
Tlie synthesis of vinylborosiloxane oligomer as described in Example 3 was scaled-up to get ~5 kg of resin. A mixture containing 698.7 g (11.30 moi) of boric acid and 4301.23 g (22.60 mol) of vinyitiiethoxysilane was reacted to obtain a low viscous resin. The resin has a viscosity of 3.7 cps at 25'C, M of 4200 and J!H„o1 1430. The resin has a solid content of 46%. The resin was flash evaporated to partly remove the by-product ethanol to obtain a resin having viscosity of 670 cps at 25'C, M«, of 9870 and 1, of 2060 and solid content of 85%. The solid borosiloxane oligomer gave a ceramic residue of 86 % (by TGA) at OOO'C in Inert atmosphere.
Example 7:
Ceramic conversion of phenylborosiloxane oligomer, synthesized as per the procedure given in Example 1, was carried out at 900, 1500 and 1650*0 under argon atmosphere. The resinous product obtained after synthesis was dried at 175'C for 2 hrs and the solid material obtained as flakes obtained was powdered and used for ceramic conversion. Pyrolysis of the solid borosiloxane oligomer was performed in an inconel furnace at 900'C under the flow of argon. In a typical experiment, 5 g of the oligomer was taien in a graphite crucible and heated at a rate of 3"C/min up to 900*C and maintained at this temperature for 3 hrs. The furnace was then cooled to room temperature at a rate of 3*(3/min. Ceramic conversion studies at 1500'C were canied out In ah alumina tubular furnace. The 900'C pyrolyzed sample was heated to ISOO'C at a heating rate of 3'C/min and maintained at 1500'C for 3 hrs. The furnace was cooled to room temperature at a rate of S'C/min. Ceramic conversion at 1650'C was also canied out in an alumina furnace.

The 9O0'C pyrolyzed sample was heat treated at lescc for 3 hrs at a heating/cooling rate of S'C/min. The furnace was cooled to room temperature at a rate of S'C/mln. Ail the oligomers gave amorphous SiOBC ceramic powdar at GOCC. XRD analysis showed that crystallization of p-SIC starts at ISOO'C and sample heat treated at lesO'C shows crystalline p-SiC peaks.
Examples:
Ceramic conversion of the phenyiborosiloxane oligomer from Example 2 was canled out at 900, 1500 and lesO'C under argon atmosphere following the procedure given in Example 9. The furnace was cooled to room temperature at a rate of S'C/min. All the oligomers gave amorphous SiOBC ceramic powder at 900'C. XRD analysis showed that crystallization of p-SiC starts at 1S00*C and sample heat treated at 1650*C shows crystalline p-SiC peal Example 9:
Phenyiborosiloxane synthesized as per Example 2 was used as the matrix resin and infiltrating resin for the preparation of C/SiC ceramic matrix composite. Carbon T-300 fabric was cut into pieces of 150x150 mm size. The carbon fabric piece was coated on both sides with phenyiborosiloxane resin. The coated febric piece was left in air overnight for drying. The resin up-take for each ply was about 12 g for 8 g of fabric. 6 Nos. of phenyiborosiloxane coated C-fabric pieces were stacked in O'-gO" orientation inside a mould with top and bottom plates with Teflon releasing sheets in between. The whole assembly was kept inside a hydraulic press and was hot pressed as per the following schedule: Heating up to 150*0 from room temperature in steps of 25'C at rc/min, holding at each step for 60 min, under a negHgible holding pressure. Then a pressure of 750 psi was applied on the sample

and subsequently the temperature was raised to ITS'C. A dwell time of 120 min. was given at 175'C after which the heating is turned off. The sample was unloaded from the press after cooling to room temperature. The precursor composite has thicltness of 3 mm. About 26 % weight loss was observed due to oozing out of resin and curing of borosiloxane.
The 150x150x3mm composites obtained were pyrolyzed at GOO'C under argon flow at a slow heating/cooling rate of 3'C/mln with a holding time of 2 hrs. The pyrolyzed composite showed a large number of voids formed due to the thermal degradation of phenylborosiloxane. A weight loss of 26% was observed for the composite (Density = 1.15g/cc). The composite was then Infiltrated with phenylborosiloxane resin, cured at ITS'C in air oven and coupons for flexural strength evaluation were machined out from it using diamond cutter. The test coupons were further heat treated at ISOCC for 2 hrs under argon flow at a heating/cooling rate of 3°C/min to complete the ceramic conversion.
Further densification of the porous composite v\»s carried out by infiltration with phenyl borosiloxane followed by pyrolysis at 900'C. Finally, the CMC samples were heat treated at ISOO'C to get specimens of density 1.78 g/cc. A ftexural strength of ~147 MPa (average) was obtalr\ed for the specimens. Infiltration of the composite with phenylborosiloxane has resulted in 55% increase in the density of the composite.

We claim:
1. A process for solventless synthesis of resinous borosiioxane oligomer
precursors for ceramics having a general structure of

wherein R is alkyl, aryl, alkenyl, alkynyl, styrylalkyl, cyclopentyl, cyclohexyl, cyclopentadienyl, acryloxyalkyl or methacryloxyalkyl comprising reacting boric acid and organotrialkoxysilane at temperature ranging from 50 to 100°C in an inert atmosphere in the absence of any solvent or catalyst and distilling the alkyl alcohol by product from the reaction mixture.
2. The process as claimed in claim 1, wherein the molar ratio of boric acid to organotrialkoxysilane is from 3:1 to 1:3 and the reaction is carried out for 1 to 10 hours prior to distillation of alkyl alcohol.
3. The process as claimed in claim 1, wherein the viscosity of the borosiioxane resin produced is from 2 cps to 200 cps (at 25°C) based on the organotrialkoxysilane, monomer feed ratio, reaction temperature and duration.
4. The process as claimed in claim 1, wherein the viscosity of the borosiioxane resin is from 2 cps to 1000 cps (at 25°C) and is achieved by regulating the quantam of distillation of alkyl alcohol by product from the reaction mixture.
5. The process as claimed in claims 1 to 4, wherein the monomer feed ratio, the reaction temperature and duration are regulated to produce resinous borosiioxane

oligomer which is dried at a temperature of about n5°C to obtain solid borosiloxane oligomer having a ceramic residue of 50% to 90% when heated in an inert atmosphere.
6. The process as claimed in claim 1, wherein the borosiloxane oligomer is heat treated in an inert atmosphere at a temperature ranging from 1200 to 2000°C to produce siliconoxycarbide (SiOBC), siliconcarbide (SiC), silicon carbide-boron carbide, (SiC-B4C) or a mixture thereof having a particle size of 5 to 90 nm.
7. The process as claimed in claim 1, wherein said resinous borosiloxane oligomer is infiltrated into ceramic matrix composites reinforced with carbon-fiber/fabric, silicon carbide fiber/fabric or silica-fiber/fabric.
8. The process as claimed in claim 1, wherein the alkoxy group of the
trialkoxysilane is methoxy, ethoxy, 2-ethoxymethoxy, propoxy or butoxy.

Documents:

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Patent Number

277874

Indian Patent Application Number

113/CHE/2010

PG Journal Number

51/2016

Publication Date

09-Dec-2016

Grant Date

02-Dec-2016

Date of Filing

18-Jan-2010

Name of Patentee

INDIAN SPACE RESEARCH ORGANISATION

Applicant Address

ISRO HEADQUARTERS, DEPARTMENT OF SPACE, ANTARIKSH BHAVAN, NEW BEL ROAD, BANGALORE-560 094

Inventors:

#	Inventor's Name	Inventor's Address
1	DEEPA DEVAPAL	C/O VIKRAM SARABHAI SPACE CENTRE, THIRUVANANTHAPURAM-695 022
2	SHANMUGAM PACKIRISAMY	C/O VIKRAM SARABHAI SPACE CENTRE, THIRUVANANTHAPURAM-695 022
3	PAYYADAKKAM VEEDU PRABHAKARAN	C/O VIKRAM SARABHAI SPACE CENTRE, TRIVANDRUM-695 022
4	KRISHNAN NAIR JAYAKUMARI SREEJITH	C/O VIKRAM SARABHAI SPACE CENTRE, THIRUVANANTHAPURAM-695 022
5	ANISH PAUL	C/O VIKRAM SARABHAI SPACE CENTRE, THIRUVANANTHAPURAM-695 022
6	ANIL PAINULY	C/O VIKRAM SARABHAI SPACE CENTRE, THIRUVANANTHAPURAM-695 022

PCT International Classification Number

C04B 35/58 ; C04B 35/571

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA