Title of Invention

"AN IMPROVED PROCESS FOR PREPARATION OF SILICA SOL WITH SILICA PARTICLES OF SUBMICRON SIZE"

Abstract

This invention relates to an improved process for the preparation of silica sol with silica particles of submicron size. According to the process a solution of ethyl silicate is homogenised by stirring at the speed varying from 70 to 200 rpm. A catalyst for example ammonia solution is added in a dropwise manner to said homogenized solution at a temperature of 25 to 100°C. The reaction mixture is kept for period of 30 to 126 hours so as to complete the reaction, if reaction temperature is in the range of 25 to 100°C and for 40 to 50 hours if reaction temperature is in the range of 50 to 100°C.

Full Text

This invention relates to an improved process for preparation of silica sol with silica particles of submicron size. Silica sol with silica particles of submicron size has gained importance as low-cost damage resisistant anti-reflection coating material. It improves the efficiency of optical components of laser systems. Silica sol is known to be prepared by base catalysed hydrolysis cum polycondensation of Tetraethoxysilane (TEOS) using dry ammonia gas as catalyst and ethanol as solvent. The reaction proceeds via homogeneous nucleation and growth process in which silica particles are initially formed. When saturation point is reached, silica sol with silica particles of submicron size are formed. The disadvantage of the processes known in the art is that Tetraethoxysilane (TEOS) used in these processes is a costly chemical which is not produced/available in some countries like India due to nonavailability of absolute alcohol which is required for its preparation. Another disadvantages of the processes known in the art is that dry ammonia gas used in these processes, as catalyst poses storage problems as it requires storage under pressure in cylinders. Another problem of the known processes is that the dry ammonia gas used as catalyst is not eco-friendly and is health-hazardous as its leakage can cause breathing problems to the workers who may inhale this gas. Further limitation of the known processes is that these processes used ethanol as solvent. The use of this solvent by itself cannot yield large silica particle size of 100-250nm dia. Another major drawback of the processes known in the art is that in the solution of TEOS in Ethanol, water is added drop wise which is followed by bubbling of dry ammonia gas through the solution. These processes, therefore,, require careful monitoring at two stages,, Firstly,, during addition water,, is to be added drop wise,, in a controlled manner and has to be carefully monitored,, If water content, is less,, gelling takes place and silica particles are not formed,, Even if water content is higher,, phase separation takes place and silica particles are not formed. Similary addition off ammonia in the next step has to be monitored carefully. If ammonia is in lesser proportion and if pH of the solution falls below the value of 9, then also gelling takes place and silica particles are not formed,. The primary object of the present invention is to propose an improved process for preparation of silica sol with silica particles of sutamicron size. Another object of the present invention is to propose an improved process for preparation of silica sol with silica particles of submicron size using relatively easily available reagent. ethyl silicate 40 (ES 40) „ A further object of the present invention is to propose an improved process for preparation of silica sol with silica particles of submicron size wherein using an ammonia solution as catalyst instead of dry ammonia gas thereby eleminating the problems associated with storage/leakage of ammonia gas. Yet a further object of the present invention is to propose a simpler and shorter process for preparation of silica sol with silica particles of submicron size involving mainly two steps as against known process which are essentially three-steps processes. Still further object of the present invention is to propose a process for preparation of silica sol with silica particles size from small particles of size 50-100nm diameter to the large particles of size 200-250nm diameter depending upon the requirement. According to this invention there is provided an improved process for preparation of silica sol with silica particles of submicron size of 50-250 nm which comprises in the step of:- a) dissolving ethyl-silicate in solvent selected from ethanol, 2- propanol or an equimolar mixture of methanol and n-butanol and homogenizing by stirring at speed varying from 70 to 200 rpm, b) adding 25% aqueous ammonia solution, as a catalyst in a drop wise manner upto 2 to 10 ml to said homogenized solution at temperature of 25 to 100°C, c) allowing the reaction to complete by allowing the reaction mixture to stand for period of time of 30 to 126 hours, when the reaction temperature is 25 to 100°C preferably 40 to 50 hours at reaction temperature is 50 to 100°C, to obtain a turbid suspension of silica solution of sub micron size. The present invention propose a process for the preparation of silica sol by base catalysed-hydrolsis-cum polycondensation of ethyl silicate-40 (ES-40). Depending upon the requirement of particle size, ES-40, is dissolved in ethanol or in a mixture of methanol arid or n-butanol. For obtaining silica particles of large size of 100-250nm dia, ES-40 is dissolved in 2-propanol. For particle size of 50-100nm, either ethanol or an equimolar mixture of methanol and n-butanol is used. This is followed by addition of 25% ammonia solution. A turbid suspension of silica sol is obtained sonsisting of monodispersed nearsperical silica particles of diameter between 50 to lOOnm or between 100 to 250nm depending upon the solvent closen. The silica content of silica sol varies from 1.5 to 4% by weight. The refractive index of silica sol varies from 1.20 to 1.25. Hiqh water and low base (Ammonia solution) catalyst concentration also favour large particles size a nd vice v ers a„ The solution of ethyl acetate-40 is stirred at speeds varying from 7O to 2OO rpm to make it homogeneous solution. 25%, ammonia solution is added drop wise ensuring that each drop get dissolved before the next drop i. s poured into the solution. The reaction temperature can be within the temperature range of 25 to 100 C. The reaction is allowed to complete by keeping it undisturbed for a period varying from 30 to 126 hours,, After an initial hydrolysis resulting in the formation of silicic acid,, the condensation of supersaturated silicic acid occurs and is indicated by an increasing turbicity of the reaction mixture. Finally, a 'turbid suspension of silica sol is obtained,, The invention will now be illustrated with the following example which is intended to be a typical example without limiting in any way on the scope of the present invention. EXAMPLES Example-l: A reaction vessel fitted with thermometer, dropping funnel and water condenser is charged with 4 to lOml of Ethylsilicate-40 (ES-40) in 100ml of Ethanol . Thou 7. to 10ml Ammonia solution 25% A.R. is added slowly and pH of the solution is controlled so that it does not fall bo low the value 9. Ammonia solution functions as a homogenous base catalyst. The reaction temperature is varied from 25 to 100 degree celcius preferably within 50 to 100 degree celcius. The stirring speed is varied from 70 to 200 r.p.m. The reaction period is varied from 30 to 126 hours depending upon the processing temperature. If the reaction is carried out at the temperature from 50 to 100 degree celcius, the reaction period is from 40 to 50 hours. Example-2 The reaction vessel is charged with 4 to 10 ml Ethylsilicate-40 (ES-40) in 100ml of 2 - Propano.l. . The reaction is carried out as illustrated in Example-l. Example -3 The reaction vessel is charged with 4 to 10ml of: Ethyl silicate-40 (ES-40) in 100ml of equ-imolar ratio of Methanol and l-Butanol. The reaction is then carried out as illustrated in example 1. It is to be understood that the above description of the invention is susceptible to considerable change and adaptations by those skilled in the art such modifications are intended to be within the ncope ^ of he present invention, which is set forth by the following claims: - I CLAIM; 1. An improved process for preparation of silica sol with silica particles of submicron size of 50-250 nm which comprises in the step of:- a) dissolving ethyl-silicate in solvent selected from ethanol, 2- propanol or an equimolar mixture of methanol and n-butanol and homogenizing by stirring at speed varying from 70 to 200 rpm, b) adding 25% aqueous ammonia solution, as a catalyst in a drop wise manner upto 2 to 10 ml to said homogenized solution at temperature of 25 to 100°C, c) allowing the reaction to complete by allowing the reaction mixture to stand for period of time of 30 to 126 hours, when the reaction temperature is 25 to 100°C preferably 40 to 50 hours at reaction temperature is 50 to 100°C, to obtain a turbid suspension of silica solution of sub micron size. 2. A process as claimed in claim 1 wherein said ammonia solution is added drop-wise in a manner such that each drop gets dissolved before the next drop is added to the solution. 3. An improved process for preparation of silica sol with silica particles of submicron size substantially as herein described and illustrated.

Documents:

55-del-1997-abstract.pdf

55-del-1997-claims.pdf

55-DEL-1997-Correspondence-Others-(16-12-2009).pdf

55-del-1997-correspondence-others.pdf

55-del-1997-correspondence-po.pdf

55-del-1997-description (complete).pdf

55-del-1997-form-1.pdf

55-del-1997-form-13.pdf

55-DEL-1997-Form-15-(16-12-2009).pdf

55-del-1997-form-19.pdf

55-del-1997-form-2.pdf

55-del-1997-form-3.pdf

55-del-1997-gpa.pdf