Title of Invention

HIGH PURITY POROUS SILICA FIBER - SILICA MATRIX COMPOSITE AND A METHOD OF MANUFACTURING THEREOF

Abstract

This invention describes a novel process for the preparation of high purity porous silica-silica composite for applications as a thermal protection material in the temperature range 1000 degree celcius to 1400 degree celcius.Silica/silica composite are prepared by leaching glass cloth with hydrochloric acid,rinsing to remove acid therefrom,vaccum drying,moulding and treating with ethyl silicate and cationic starch binder further drying and sintering.

Full Text

This invention relates to a process for producing high purity porous silica, silica composites. The composite material obtained by this process is particularly useful in making high temperature insulating materials having high thermal shock resistance that can withstand a temperature of 1400°C Background and prior art: Good thermal insulators with minimum density are required in various applications like insulation for high temperature furnaces etc. It is also one of the prime subsystem requirements for reusable space vehicle. During the ascent and descent phases of a re-entry vehicle, it is subjected to very high temperatures. This can be as high as 1800°C on its nose region and leading edges and relatively less on the rest of its surface. In order to protect the structure from such a high temperature, it is necessary to provide suitable thermal insulation system for the re-entry vehicle so that the structure does not encounter a temperature greater than 150°C As no single material can fulfil the requirements of the entire thermal protection system for a given vehicle, the need for identifying various materials having desirable properties for use at different locations is called for. Silica tiles with a reradiating coating have been employed for the temperature range between 1000 and 1400°C. The main characteristics of such a material include ability to withstand high temperature gradients, resist devitrification, maintain structural integrity, possess low density (0.2 gm/cc to OAgm/cc) and can be used for repeated missions without degradation in any of its thermo-mechanical properties. The development of reusable space vehicle has created need for high temperature lightweight insulating material. The important criterion of such material is that it should be reusable without deterioration in any of its thermo-mechanical properties. The present invention addresses this need and satisfies this reusability requirement. US patent 3,952,083 on Silica Re-usable Surface Insulation describes the development of insulating tile based on high purity 95 to 98% by weight of silica fibres and the remaining colloidal silica binder. Besides reagent grade starch is used as a temporary binder. The fibre along with the binder and starch is mixed in a V blender. The resulting slurry is poured into a mould. Suitable pressure is applied to remove the excess solution and also control the density of the finished product. The insulating tile is then subjected to careful drying and firing conditions. The tile experiences a large shrinkage of the order of 25-45% by volume, US patent 4,148,962 on Fibrous Refractory Composite Insulation describes the development of a refractory composite based on silica fibres and aluminosilicate fibres. Boron oxide in the range 0.5% to 30 % by weight of the total fibre weight constitutes the binder. The optical property of the composite insulation tiles is modified by adding up to 3 % opacifiers like chromium oxide, cobalt oxide and silicon carbide etc. The procedure involved in processing the tiles includes cleaning of the silica fibres and aluminosilicate fibres by hydrochloric acid/deionised water. The cleaned fibres are then dispersed in a V blender with the required amount of water and binder. The resulting slurry is poured into a mould and pressed into the desired shape by applying pressure. After moulding the tile is subjected to controlled drying and sintering conditions. US patent 5,569,423 describes a one-component silica fibre based heat insulating material. The silica fibre is mixed with an organosiloxane binder namely polyethoxysiloxane in the range 5- 25 wt % in relation to the weight of the quartz fibre. Boron (0.5 to 3 wt %) is added as a sintering aid. A non-ionic surface-active agent is also added. The fibre is dispersed in water using a blender equipped with a propeller mixer. A fibrous block is moulded in a mould. An emulsion is formed by mixing an alcoholic solution of polyethoxysiloxane, a non-ionic surface-active agent and amorphous boron powder. The fibrous block is again dispersed with water containing the emulsion in a blender. A green moulded billet is formed from this mixture. Firing is then carried out. The density that can be obtained by this process is 0.1 to 0,4 gm/cc. All the above processes incorporate drawn silica fibres. Obtaining high purity fibres is highly essential as the presence of even trace amounts of alkali content can cause devitrification. The use of binder matrix like colloidal silica limits the retention of amorphous phase of silica tile during processing and application. This is because colloidal silica incorporates sodium ions for its stabilisation. Beyond a certain limit ( say 50 ppm ) sodium ions can play a vital role in conversion of amorphous silica to alpha cristobalite which is a crystalline form of silica. This has a coefficient of thermal expansion, which is an order of magnitude greater than the amorphous phase. The formation of a crystalline phase can render the insulation unfit for use, This invention uses E-glass cloth, which is leached with hot hydrochloric acid to remove all traces of alkali metal containments therefrom to provide silica cloth of about 99.6% purity. The present invention is unique in that it dispenses with the colloidal silica binder and instead uses a silica matrix formed as a result of hydrolysis of ethyl silicate by a sol-gel process. This process ensures high purity and fine particles of silica, which is used as a matrix in the present invention. By employing a matrix and reinforcement made of the same material namely silica, the coefficient of thermal expansion mismatch, which is a problem when a two-component system is used, is totally avoided. A two-component system will also increase the overall thermal conductivity of the system. An object of the invention is to produce and use silica fibre of high purity that is devoid of any alkali metal content in the silica matrix. This process prevents conversion of amorphous silica to crystalline state and provides high temperature resistant light weight reusable insulation material. High purity silica-silica porous composite tiles have been processed using high purity silica fibre obtained by leaching E glass cloth in hot hydrochloric acid as reinforcement and high purity and fine silica is produced in-situ by the hydrolysis of tetra-ethyl-ortho silicate. The density of the final porous silica-silica composite can be varied by varying the parameters like fibre volume, matrix volume, load applied for compaction and the time-temperature profile during the sintering process. The following description relates to the preparation of high purity silica felt: i) Preparation of High purity (>99 %) Silica felt: High Silica cloth (Si02 content: 99% plus) is produced by hot hydrochloric acid leaching of E-glass cloth and washing the leached cloth by DM water. The silica fibres of length 10-15 mm are cut from high silica cloth. The fibres are then placed in a tank and compressed air is admitted into the tank to split the fibres to form a felt. ii) Making of green Preform: Green preform is made out of silica felt by vacuum moulding technique. In this process a water tank is filled with approximately 150 cc.per gm of felt of DM water. Two stirrers placed sufficiently far from each other are switched on. The compressed air is let in. The silica felt is dispersed in water. The stirrers along with compressed air provide sufficient agitation to keep the fibres uniformly suspended in the water. To improve the dispersion further, cationic starch (5-10% wt of the felt) is added to the water-silica felt mixture. The agitation is continued for 20-30 minutes. After ensuring uniform dispersion, compressed air agitation is stopped and the stirrers are put off The mould box is then immediately lowered into the water tank. The vacuum is applied till all the fibres are captured in the mould box. The valve is closed and additional water about 50 cc/gm of felt is added to the tank. 0.1 to 0.2 cc/gm of felt of hydrochloric acid (LR grade) is added. Now the vacuum valve is opened partially so that the acidified water passes through the silica felt in the mould. This washes the felt completely. Again water is added to the tank and the felt is washed thoroughly until it becomes free of acid. The mould box with fibres is then raised and vacuum is applied after placing a MS plate with rubber sheet under the mould box in order to achieve required compaction of the preform and removal of excess water from it. The preform is ejected out from the mould box on to a tray by admitting the compressed air into the mould. The preform is then compressed to the required thickness by placing it in between two plates and using thickness spacer. iii) Drying and Conditioning: The green preform is dried at 250-300°C for 24 hours in clamped condition in an oven to remove all the water and volatile matters. Density of the dry preform is measured and is maintained at 0.15 ± 0.03 g/cc. iv) Impregnation of dried preform: The dried preform is impregnated with binder solution based on alkyl silicate (C=l to 5 where C represents Carbon). The details of raw materials required for this processing step, equipments and the processing parameters are given below: Cold water from a water cooler (below 20°C) is circulated through a stainless steel jacketed vessel and the vessel is allowed to cool. 45-50% (vol) of ethyl silicate - 40 is added into this jacketed vessel. 45-50% (vol) of ethyl alcohol is added to ethyl silicate and the solution is stirred well until it reaches about 20°C. Concentrated hydrochloric acid (LR) 0.1-0.2% (vol) is slowly added to 4.8-4.9% (vol) of water with stirring in a separate vessel. While continuously stirring, hydrochloric acid/water solution and boron oxide (2-3 gms/lit of binder solution) are then slowly added into the vessel. Stirring is continued till the solution attains the required pH value of 2 to 3. The resulting solution is the required binder solution for impregnation of the dried preform. The composition of the slurry is so prepared so that 6-8cc of slurry is used to impregnate 1 gm of felt. The dried preform is impregnated with binder solution so prepared and using equipment/tools namely vacuum bagging unit which consists of a reservoir for holding the binder solution, perforated top plate for distribution of binder, bottom plate and isolation valves. The bypass drain line is connected to a cylindrical tank (with liquid level indicator) for collecting the excess binder. The tank is connected to a mechanical vacuum pump and fitted with a vacuum dial gauge and drain valve. The impregnation unit is housed in an air-conditioned room. (Temperature 20 ± 2 ° C and relative humidity 80 = 5%). The dried and weighed preform is placed in the vacuum bag and the bag is sealed. The required quantity of binder solution is poured in the reservoir. Then the valve of the reservoir is opened and the binder solution flows through the preform uniformly. The preform is kept in fully soaked condition for 5 minutes. The excess binder solution is removed by applying vacuum 10 to 20 mm of Hg for 15 to 20 minutes. The final product is maintained as 60-70% silica felt and 40-30 % matrix by weight. v) Curing of impregnated preform: The impregnated preform is allowed to cure (by hydrolysis and condensation process) under controlled temperature (20 ± 2°C) and humidity (80 ± 5 %) in an AC room for 10-12 days. vi) Drying and sintering of silica tile billet: After curing, the billets are dried at 250°-300°C for 24 hours to expel all the volatiles. The dried billet is then placed in a furnace for sintering. The sintering cycle was at 800°c in 5 hours, dwell for 0.5 hours, 1275°C in 5 hours and a dwell for 1 hour. After the sintering cycle was completed, the silica tile was allowed to cool in the furnace under normal furnace cooling. The invention is described in greater detail with the help of the following example. i) Preparation of High purity (>99 %) Silica felt: High Silica cloth (Si02 content: 99% plus) is produced by hot hydrochloric acid leaching of E-glass cloth and washing the leached cloth by DM water. The silica fibres of length 10-15 mm are cut from high silica cloth. The fibres are then placed in a tank and compressed air is admitted into the tank to split the fibres to form a felt. ii) Making of green Preform: Green preform is made out of silica felt by vacuum moulding technique. The details of material specifications are as follows: 200 to 250 litres/tile 1000 to 1250 g/tile 100 to 125 g/tile Material: i) De-mineralised water : ii) High silica felt : iii) Dispersing agent : Vacuum Moulding process: The water tank is filled with approximately 140 liters of DM water. Both the stirrers are switched on. The compressed air is let in. The silica felt is dispersed in water. The stirrers along with compressed air provide sufficient agitation to keep the fibre uniformly suspended in the water. To improve the dispersion further, 100 to 125 g of cationic starch is added to the water -silica felt mixture. The agitation is continued for 30 -60 minutes. After ensuring uniform dispersion, compressed air agitation is stopped and the stirrers are switched off The mould box is then immediately lowered into the water tank. The vacuum is applied till all the fibres are captured in the mould box. The valve is closed and additional water about 50-75 litres are added to the tank. 100 to 125 cc of concentrated hydrochloric acid (LR grade) is added. Now the vacuum valve is opened partially so that the acidified water passes through the silica felt in the mould. This washes the felt completely. Again water is added to the tank and the felt is washed thoroughly until it becomes free of acid. The mould box with fibres is then raised and vacuum is applied after placing a MS plate with rubber sheet under the mould box in order to achieve required compaction of the preform and removal of excess water from it. The preform is ejected from the box mould onto a tray by admitting compressed air into the mould. The preform is then compressed to the required thickness, 60-75 mm, by placing it in between two plates and using thickness spacer. The preform is finally clamped at the required thickness level. The dimensions of the green preform (under clamped condition) is maintained as: 300 ± 25/0 x 300 ± 25/0 x 70 ± 15 mm. This green preform is then taken for drying. iii) Drying and Conditioning: The green preform is dried at 300°C for 24 hours in clamped condition in an oven to remove all the water and volatile matters. iv) Impregnation of dried preform: The dried preform is impregnated with alkyl silicate based binder solution. The details of raw materials required for this process step and the process details are given below: The binder solution is prepared as per the composition and process given below: Ethyl silicate 40 Ethyl alcohol Water Hydrochloric acid Boron oxide 4000 to 5000 cc 3500 to 4000 cc 30 to 50 cc 10 to 20 cc 2,5 to 5 gms/1 of binder solution Note : Ethyl Silicate may be substituted by other alkyl silicates and Ethyl Alcohol by other alkyl alcohols (Ol to 5 where C represents Carbon). The cold water from the water cooler (below 20°C) is circulated through the jacketed vessel and the vessel is allowed to cool. The sequential process steps for preparation of binder solution are as given below: a) Required quantity of alkyl silicate is added into the jacketed vessel which is kept cool by circulating cold water, b) Required amount of alkyl alcohol is added to alkyl silicate and the solution stirred well until it reaches about 20 ° C. c) Concentrated hydrochloric acid (LR) is slowly added to the required amount of water with stirring in a separate vessel. d) While continuously stirring, hydrochloric acid/water solution and boron oxide are then slowly added into the vessel e) Stirring is continued till the solution attains the required pH value of 2 to 3. The resulting binder solution is needed for impregnation of the dried preform. Impregnation of preform: The dried preform is impregnated with binder solution so prepared as stated above and using equipment/tools namely vacuum bagging unit, which consists of a reservoir for holding the binder solution, perforated top plate for distribution of binder, bottom plate and isolation valves. The bypass drain line is connected to a cylindrical tank (with liquid level indicator) for collecting the excess binder. The tank is connected to a mechanical vacuum pump and fitted with a dial type vacuum gauge and drain valve. The impregnation unit is housed in an air conditioned room. (Temperature 20 ± 2° C and relative humidity 80 ± 5%). The dried and weighed preform is placed in the vacuum bag and the bag is sealed. The required quantity of binder solution ( 8 to 10 litres) is poured in the reservoir. Then the valve of the reservoir is opened and the binder solution flows through the preform uniformly. The preform is kept in fully soaked condition for 5 minutes. The excess binder solution is removed by applying vacuum 10 to 20 mm of Hg for 15 to 20 minutes. The final product is maintained as 60-70% silica felt and 40-30 % matrix by weight. v) Curing of impregnated preform: The impregnated preform is allowed to cure (by hydrolysis and condensation process) under controlled temperature (20 ± 2°C) and humidity (80 ± 5%) in an AC room for 10-12 days. vi) Drying and sintering of silica tile billet: After curing, the billets are dried at 300°C for 24 hours to expel all the volatiles. The dried billet is then placed in a furnace for sintering at 1275°C for 6 hours. After the sintering cycle was completed, the billets are allowed to cool in the furnace itself This invention relates to a process for producing high purity > 99 % silica and porous silica composite tiles therefrom, comprising the steps of leaching glass cloth with hydrochloric acid, subsequent washing with demineralised water, treating silica fibres obtained thereby with compressed air to split the fiber into felt, producing performs therefrom by vacuum moulding, drying the same and impregnating thereafter with an alkyl silicate binder solution containing boron oxide, curing the thus impregnated perform, drying and sintering the billets thus obtained. The composite material prepared by this method is used in re-entry space vehicles as thermal protection material. It also has various other applications for insulation in high temperature furnaces and heating systems. We claim: 1. A process for producing high purity > 99 % silica and porous silica composite tiles therefrom, comprising the steps of leaching glass cloth with hydrochloric acid, subsequent washing with demineralised water, treating silica fibres obtained thereby with compressed air to split the fiber into felt, producing performs therefrom by vacuum moulding, drying the same and impregnating thereafter with an alkyl silicate binder solution containing boron oxide, curing thus impregnated perform, drying and sintering the billets thus obtained. 2. The process as claimed in claim 1, wherein in E-glass cloth cloth is leached with hot hydrochloric acid, silica fibres obtained thereby are cut and then treated with compressed air to obtain silica felt. 3. The process as claimed in claims 1 and 2, wherein said perform is obtained by vaccum moulding in a tank having demineralized water provided with atleast two spaced apart stirrers supplying compressed air under agitation to uniformly suspend said felt in water, adding a solution of cationic starch thereto, introducing a mould box into said tank, applying vacuum till the dispersed fibres are captured in said mould box, thereafter introducing and passing acidified water through the silica encapsulated in the mould, washing the same till free of acid, removing the mould from the tank, compacting the same under vacuum, ejecting the perform from the mould and further compressing the same, if desired, before drying and impregnating with binder solution. 4. The process as claimed in claims 1 to 3, wherein said dried perform is impregnated with a binder solution based on ethyl silicate having 1-15 carbon content obtained by treating 50% by volume of ethyl silicate with 40% by (vol) of ethyl alcohol and thereafter with 0.15 % (vol) of HC1 contained in 4.85% (vol) water under stirring along with about 2.5 gm/1 of binder solution of boron oxide till a pH value of 2-3 is obtained, said binder solution being used for impregnating said perform with cationic starch solution. 5. The process as claimed in claims 1 to 4, wherein the thus impregnated perform is cured under controlled temperature of 20 ± 2 and humidity of 80 ± 5 under air conditioning. 6. The process as claimed in claims 1 to 5, wherein said cured perform is dried at 300° C till all volatiles are expelled and then sintered at 800°C for 5 hours, 1275°C for 4.7 hours followed by a dwell time of 1 hour and then cooled.

Documents:

047-che-2005-abstract.pdf

047-che-2005-claims.pdf

047-che-2005-correspondnece-others.pdf

047-che-2005-description(complete).pdf

047-che-2005-form 1.pdf

047-che-2005-form 3.pdf

047-che-2005-form 8.pdf

47-CHE-2005 CORRESPONDENCE OTHERS 11-07-2012.pdf

47-CHE-2005 AMENDED CLAIMS 12-07-2012.pdf

47-CHE-2005 AMENDED CLAIMS 29-03-2012.pdf

47-CHE-2005 AMENDED PAGES OF SPECIFICATION 29-03-2012.pdf

47-CHE-2005 CORRESPONDENCE OTHRES 12-07-2012.pdf

47-CHE-2005 EXAMINATION REPORT REPLY RECEIVED 29-03-2012.pdf

47-CHE-2005 FORM-1 29-03-2012.pdf

47-CHE-2005 POWER OF ATTORNEY 29-03-2012.pdf