STABILIZED SILICA GEL AND PROCESS FOR MAKING SAME

Abstract: Process for producing a silica gel having improved pore volume relative to conventional silica gels which comprises the steps of gelling an aqueous alkali metal silicate with an aqueous acid, washing the resulting gel to remove salt or salts and acid, and soaking the gel in an aqueous solution of a water soluble aluminum salt to impart from 0.1% to .7% by weight aluminum on the dry basis to said gel, drying and reducing the particle size of the dried gel to from 2.0 to 15 microns, said particles having a pore volume in the range of 1.25 to 2.25 cm<3>/gram. and a product produced by said process.

Description: -1 - STABILIZED SILICA GEL AND PROCESS FOR MAKING SAME This invention relates to an improved silica gel, and more particularly an improved silica gel which i characterized by an enhanced ability to retain its por volume during processing to a better degree tha conventionally treated silica gels. The process i characterized by treatment of the silica gel afte neutralization and chemical treatment ("aging") with a aqueous solution of an aluminum salt, e.g., aluminum nitrat or aluminum sulfate. BACKGROUND OP THE INVH-JTION AND PRIOR ART Silica gels, whether hydrogels or aerogels, ar well known. (See Her, "The Chemistry of Silica", Joh Wiley & Sons 1979, ISBN 0-471-02404-X) . Reference may als be had to U. S. Patent 2,856,268 to L. 0. Young and date 14 October 1958 for preparation of hydrogels suitable fo the treatment described herein. Aerogels, or xerogels a they may be called, are dried hydrogels. They are als amorphous. Reference may also be had to the article b Mitchell "The Surface Properties of Amorphous Silicas printed in Chemistry and Industry, June 4, 1966 at page 924-932. These references are incorporated herein b reference thereto. Experience has shown that almost any physical treatmen performed on a silica gel after its formation results in ■ 2- decrease in pore volume (PV) if we consider pore volume to be the difference between the volume of the gel and the volume of Siθ2 in this gel. Silica gels as initialy formed may have a wide range of pore volumes from about 2 to about 50 cm*Vgm. depending on many factors as shown in the art. Those of particular commercial interest have pore volumes less than 10 cm-Vgram, e.g., about 4.5 cm-Vgm. Thus, washing to remove acid results in a decrease in pore volume. Aging results in a still further reduction in pore volume. By the time a silica gel with an initial pore volume of, e.g., 4.5 cm 3 /g. is dried and milled to its final particle size in the range of 2 to 15 microns, its pore volume has decreased to from 20% to 39% of its original pore volume. According to the present invention, we are able to stabilize the pore volume so that it decreases substantially less during physical treatment than that experienced with similar gels not treated in accordance with this invention. This is commercially significant in at least one area of use of the gels hereof; for example, the flatting of coating compositions, e.g., paints, varnishes, lacquers, enamels, and the like. The higher the pore volume, the lower the density of the gel particles. Hence, a smaller weight of silica particles of a predetermined size can be used effectively to produce a desired degree of flatting. BRIEF STATEMENT OF THE INVENTION Briefly stated, the present invention is in a process for producing a silica gel having improved pore volume relative to conventional silica gels which comprises the steps of gelling an aqueous alkali metal silicate with an -3- aqueous acid, washing the resulting gel to remove salt or salts and acid, and soaking the gel in an aqueous solution of a water soluble aluminum salt to impart from 0.1% to 7% by weight aluminum on the dry basis to said gel, drying and reducing the particle size of the dried gel to from 2.0 to 15 microns, said particles having a pore volume in the range of 1.25 to 2.25cπ.*Vgram. Tne invention also contemplates a product produced in accordance with the foregoing process. DETAILED DESCRIPTION OF THE INV--NTI0N AND SPECIFIC EXAMPLES The formation of fine particle silica gels, of either the- hydrogel or aerogel type is ' well known and involves acidifying an aqueous solution of an alkali meta silicate with a mineral acid, e.g., sulphuric acid, breakin up the gel, washing and aging, followed by drying an milling, or simultaneously drying and milling as in a flui energy mill. The present invention inserts an additiona step after the aging step, i.e., soaking the gel for period of 1/2 to 8 hours in an aqueous solution of a water soluble aluminum salt, e.g., aluminum nitrate, aluminu sulfate, aluminum acetate, aluminum formate, aluminu oxalate, aluminum chloride. Salts containing anioni aluminum, such as, sodium aluminate do not work. The fina products contain a small amount of aluminum, calculated a the metal and in the range of 0.1 to 0.7% aluminum. Thi small amount of aluminum in a form as yet unknown to us, appears to strengthen the micropores in the gel so as t enable it better to withstand physical treatments such a -4- drying and size reduction without as great a shrinkage in pore volume. This we term "stabilization". The following discussion will have reference to aluminum 'stabilized hydrogels (ASH) and aluminum stabilized aerogels (ASA) . It was found that the ASH gels provided somewhat higher pore volumes than ASA gels and were also free of overgrind sensitivity previously observed in ASA gels. It was also found to be desirable to wash the aged gel to remove residual ammonia to prevent reaction with the aluminum salt to form ammonium salts, and to eliminate variations in pore volume of a given gel in subsequent treatment. After treatment of the gel with the aqueous aluminum salt solution, another wash was found to be desirable to remove and ' excess aluminum salt anion for enviromental reasons. Very rapid or flash drying was found to favor retention of porosity. Loss of pore volume during milling was largely eliminated by using a more lightly aged gel, e.g., 9 hours vs. 18 hours or more. The pore volume of the ASH products which result from the improved process was about 2.0 cc/gm. (cubic centimeters per gram) compared to pore volumes of 1.1 to 1.7 for standard prior hydrogels and aerogels, respectively. The pore volumes obtained by rapid drying were found to be independent of the aluminum salt soak solution pH between 1.0 and 4.5 making unnecessary close control of pH. Two drying methods were found to give consistently higher pore volumes: 1) heating a thin layer of the gel in a preheated dish in a muffle furnace at 500° to 600°C. and 2) direct exposure of the gel to burner flames as the particles pass along an iron channel or flight as in a direct fired rotary kiln. Hot fluid energy milling of ASH * to reduce the particle size was found usually to cause smaller losses i -5- PV compared to normal operation of the mill which showed a larger loss of PV. Shorter aging times, i.e., down to about 3 hours, can be achieved depending upon the temperature and pH of the aging solution. This ASH product was the best ASH for flatting a polyester coil coating. (See Example I below) . The following Table I shows the properties of examples of ASA gels. All examples were soaked in aluminum nitrate solutions containing 5.1 grams/liter aluminum nitrate nonahydrate. 1000 grams of wet gel/10 liters of aluminum nitrate solution were used. TABLE I AGING MZR INJ/ POKE TIME SIZE RENG FEED VOL. MPLE (EERS) (in.) RATE EMARKS 9 4-1/2 100/80 43 1.82 Oil Absorption = 260 9 4-1/2 100/80 41 1.77 Specific Resistance = 170 9 4-1/2 100/80 40 1.86 Washed sample with water 9 4-1/2 100/80 41 1.81 Adjusted pH of aluminum n solution to pH = 3.9. 9 4-1/2 100/80 40 1.79 Washed with water. 18 4-1/2 100/80 37 1.56 9 4-1/2 100/80 38 . 1.75 Same as 4, except washed NH4OH solution (3X) (pH ~ 9. 9 4-1/2 100/80 38 1.80 Same as 4, except washed water (3X) . 9 4-1/2 100/90 46 , 1.54 Washed with water (3X) luid energy mill ring size. TABLE I (Continued) S0RF7__E (,-ΞNTRIFUGE MEDIAN AREA EBUSITY PARTICLE NO3 PLE i__^___L K_-Hf_ιH*wKt~_-_-~ (q/∞? SIZE, U % AL 289 0.17 3.25 0.20 1800 Oil Absorption = 260. 286 0.17 2.90 0.20 3500 Specific Resistance = 1700 292 0.18 3.50 0.17 1000 Washed sample with water. 290 0.18 3.25 0.29 2000 Adjusted pH of aluminum " ni solution to pH = 3.9. 277 0.17 3.35 0.21 700 Washed with water 220 0.18 2.80 0.26 11800 273 0.18 3.45 0.21 539 Same as 4, washed with NH4OH solution (3X) ( ~ fl=9. 268 0.18 3.55 0.19 740 Same as 4, except washed w water (3X) . 280 0.17 2.45 0.15 430 Washed with water (3X) . -8- The following Table II shows the properties of AS and ASA gels treated in accordance herewith compared to commercially available gel in a polyester coil coating. TABLE II PLANT REN Cc-flparision of --"proved ASH with improved ASA and a Standard Flatting Silica Oil Pore Surface Median Flatted Polyester Ooil Coating Abe. Vol. Area Particle Gloss Gloss ica Samples %A1 g/lOOg. cc/g. π_Vg. Size, u Hegman 60° 85° mercial A (STD) 4-3/4 STD STD roved (ASH) 0.18 258 1.74 289 4.05 5-V4 -12 -35 γ > roved (ASA) 0.13 248 1.71 296 3.65 5-3/4 -10 -27 0.18 274 1.65 274 4.30 5-1/2 -15 -38 0.17 258 1.76 301 5.00 5-1/4 -15 -38 0.17 271 1.71 300 4.55 5-1/2 -12 -30 0.14 281 1.68 281 3.90 5-1/2 -14 -30 -10- Preferred ASA materials had the following physical properties: SAMPLE 1 SAMPLE 2 Pore Volume 1.8 cm * Vg. 1.85 cm 3 /g. Surface Area 283 m 2 /gm. 252 m 2 /gm. Centrifuge Density 0.17 g/cc. 0.18 g/cc. Aluminum 0.20 % 0.40% Particle Size (Elzone) 3.25 microns 9.1 mircons In general, the products of this invention have the following physical characteristics: The pore .volume is high compared to standard commercially available gels, (for example "Silcrons" products of SCM Corp. - Pigments which are, i.e., broadly 1.2 to 1.6 cc/gm.) and is most usually 1.54 to 1.86 cc/gm. The surface area is broadly in the range of 150 to 400 square meters/gram and desirably 220 to 292 m 2 /gm. The acidity of the products is broadly in the range of 3.0 to 4.5 pH in a 5% aqueous slurry, and preferably 3.0 to 3.6 pH. The median particle size is 2.0 to 15 microns and preferably 5 to 8 microns. The density (centrifuge) is .14 to .25 grams /cc. The oil absorption is broadly 175 to 350 grams linseed oil/lOO grams of silica (or pounds of linseed oil/lOO pounds of silica product) , and desirably 225 to 300. Chemically, these products are a -11- least 99% silica on dry basis (dried at 200°C. under vacuum) They contain 0.28 to 0.8% aluminum in unwashed samples an from .10 to 0.30% in washed samples. The amount of aluminu in the best samples is 0.17 to 0.19%. The present method for making aerogels require the reaction of a 35° or 36° Baume solution of sulfuric aci with a 32° Baume solution of sodium silicate (3.22/1 Rati of Siθ2/Na2θ) to form a mixture that polymerizes on standin to form a gel. The gel is allowed to set for approximatel one hour, then sized and washed with water to remove th excess acid and the sodium sulfate. The water is replace with an ammonia solution and the gel is aged for from 3 t 36, e.g., 9 or 18 hours. The solution is than drained an the wet gel is fluid energy milled with hot air (or steam) a a relatively high temperature above 500°C to rapidly remov the water and replace it with air without collapsing th gel. This results in a pore volume of 1.5 to 1.6 cc/g Soaking the sample with aluminum nitrate solution prior t fluid energy milling does not increase the pore volume However, reducing the ammonia aging time of the gel increase the pore volume to 1.74 cc/g, and treating the sample wit aluminum nitrate solution increases the pore volume to 1.8 cc/g. , or higher. EXAMPLE I 3420 cc of 35° Baume Sulfuric acid and 8000 cc o 32° Baume Sodium Silicate were reacted at room temperatur over a period of 19 minutes, the temperature reaching 43°C The acid and silicate solutions were blended by pumpin through an aspirator type plastic tube, the acid bein -12- flowed at a rate of 297 cc/minute and the silicate at th rate of 814 cc/minute. The running time of the acid wa 11.5 minutes, and that of the silicate was 9 minutes 5 seconds. The gel was allowed to set, cut and as i conventional it was broken up in a chopper to a manageabl size for washing. The resulting hydrosol was clear. It was washe with water at 120°F. to a pH of 6.5. The gel was washe twice with aqueous • ammonia solution at 182°F. and 9.75 p and 180°F. and 9.6 pH, respectively. The pH of the was solution after two washes was 9.05 at 45°C. The gel wa then overflow washed with 150°F city water for 15 minute after which the pH taken from the top was 8.3 but afte returning through the gel, a 9.3 pH was found. The gel wa then aged. After aging for 18 hours, the wet gel was soake for 4 hours in an aqueous solution of. aluminum nitra nonahydrate in city water. The following Table A shows th various samples concentrations of Al2(N03)3 • 9H20 temperature of soak and blot pore volume. In determinin blot pore volume a weighed sample of wet gel (deionize water) is placed on a filter paper surface and moved aroun to dry areas on the paper, until the gel is dry. The sampl is again weighed and the water weight loss determined. Thi gives a measure of the pore volume as the water loss/gram o sample and converts readily to cc/gm. which is pore volum as determined by the blot method. TABLE A Sample Al(HO 3 ) 3 Wash Temperature of Blot P.V. Number grams/liter Procedure Alflθ3) 3 Sol. oc/gram 1-1 5.1 None 23°C. 1.52 1-2 8.6 None 23°C. 1.56 1-3 5.1 9x 23°C. 2.34 1-4 8.6 9x 23°C. 2.38 1-5 5.1 None 66°C. 1.59 1-6 8.6 None 66°C. 1.61 1-7 5.1 9x 66°C. 2.38 I 1-8 8.6 9x 66°C. 2.40 1-9 5.1 15 min. Ove flow 23°C. 2.27 1-10 8.6 15 min. Overflow 23°C. 2.41, 2.3 Soaked large sample (6400 g) wet gel using indicated concentration of A1(N03)3 • 9H-0 in city water for 4 hours. The soaking was done using a pump for a flow thru system. After 4 hrs. the pump is shut off and the Al0-103)3 solution drained back into the holding tank. The sample sat for 14 hours after the soaking before being dried. -14- The soaking in aluminum nitrate solution was done using pump. After 4 hours, the pump is shut off and the aluminu nitrate solution drains back into a holding tank. The ge sample remained untouched for 14 hours after the soakin before being dried. The material taken from the soaking tank (as is) had a blot PV of 2.30. After 2 water washes the blot PV wa 2.31. The fluid energy mill feed sample had on aluminu content of 0.30%, a PV of 2.03 and a surface area of 23 m /gm. A specific sample of fluid energy mill feed stoc was an 18 hour aged gel, washed for 15 minutes after agin with 66°C. city water and then soaked in Al( θ3)3 • 9H2O, at 8.6 gms./liter, by.. flowing through the gel at 1 gal pe minute for 2 hours. Small, thin batches were flash dried a 600°C. The average blot PV at this point was 2.20 cc/gm. The specific sample immediately above-describe was then milled in a Sturtevant "Micronizer" mill usin steam at 600°C. for a period of 1 minute 10 seconds in first sample and 1 minute 8 seconds for a second sample The mill temperature was 530°C. The flow rate was 5.2 and 5.4 cu. ft./sec. at a inlet pressure of 80 psi and a pressure drop of 10 psi. Th feed rate of gel was 111 g/15 seconds. The ring diamete was 4.5" and thimble 1/4". The by-pass was closed. Th finished product had a median particle size of 5.2 microns a Digisorb PV of 1.97, a specific area of 237 m 2 /gm. an analyzed 0.30% Al. The centrifuge density was 0.1 grams/cubic centimeter. (Digisorb is a commerciall available meter for measuring pore volume and surface area Digisorb 2500, from Micromeritics Corp.). -15- In a replicative sample, the steam temperature wa 600°C. and the fluid energy mill temperature was 510°C. Th flow rate was 4.4 cubic ft./sec. and the inlet/outle pressures were 60/50. The gel feed rate was 13.5 g./15 sec. Otherwise the conditions were the same. The resultin product had a median particle size of 6.1 microns, Digisorb PV of 2.03 cc/gm. a specific area of 238 m /gm.. an a centrifuge density of 0.18 gm/cc. The Al content wa 0.30% Al. This represents the best mode now known to us fo carrying out our invention. EXAMPLE II A silica hydrosol was prepared by combining a 35 Baume sulfuric acid solution with a 32° Baume sodium silicat solution (Na2θ:Siθ2 mole ratio 1:3.22). The hydrosol wa allowed to " set to a hydrogel, sized, and washed with ho water then aged with a hot ammonia solution for 18 hours The solution was drained and the silicate was converted t an aerogel in a heated fluid energy mill at 500°C. plus The pore volume of this commercial aerogel, which is used a SCM's Silcron G-100, (standard) is 1.37 cc/g, and has a Hegman in our Nitrocellulose Lacquer formulation. EXAMPLE III A hydrogel prepared in a manner similar to Exampl I was washed, sized and then aged in a hot ammonia solutio for 9 hours, drained and milled in a heated fluid energ mill at 530°C. to yield a silica aerogel with a pore volum of 1.74 cc/g, and was a better flatting agent i nitrocellose -16- lacquer than the commerical standard shown in Example II. This product has a 5-1/4 Hegman and has gloss readings which are 12 points flatter at 60°, and 30 points flatter at 85° than the standard (Example II) . EXAMPLE IV A silica hydrosol was prepared by combining 36° Baume sulfuric acid solution with a 32° Baume soldium silicate solution (Na2θ:Siθ2 mole ratio 1:3.22). The hydrosol was allowed to set to a hydrogel, sized, and washed with hot water then aged with hot ammonia solution for 18 hours. The hydrogel was drained. To 2000 grams of the drained hydrogel were added 20 liters of aluminum nitrate nonahydrate solution (5.1 g/1) . This mixture was • soaked for 2 hours with occasional stirring. The aluminum treated hydrogel was drained, and the sample was milled in a heated fluid energy mill at approximately 510°C. The aluminum stabilized aerogel has a pore volume of 1.56 cc/g. In nitrocellulose lacquer formulation the aerogel had a 3 Hegman and was glossy compared to the standard in Example II, namely +3 at 60°, and +8 at 85°. EXAMPLE V The hydrogel was prepared as shown in Example III with an ammonia age of 9 hours. To 2000 grams of the drained hydrogel was added 20 liters of aluminum nitrate nonahydrate solution (5.1 g/1) and soaked with occasional stirring for 2 hours. The aluminum stabilized hydrogel was -17- drained and micronized in a heated fluid energy mill. Th pore volume was 1.86 cc/g, and in a nitrocellulose lacquer formula had a 5-1/4 Hegman with 4 points flatter at 60°, and equal to standard at 85° using a high shear system (Cowles Dissolver) . However, high pore volumes should give th silica gels better stir-in qualities at low shear. This was proven when stir-in qualities were measured for 15 minutes with high shear vs. low shear stirrings. The standar (Example II) yielded a 3-1/2 Hegman while this product had 5 Hegman, with gloss readings 11 points flatter at 60° and points flatter at 85°. EXAMPLE VI A standard Silcron type material was made i accordance with Ex'ample II above. Prior to drying an milling, 4000 gms. of silica gel was tumbled in a roller mil for 2 hours with 20 liters of Aluminum Sulphate wate solution containing 2.7 grams per liter of the sulfate salt. The product was separated from the solution and air drie and showed a loss on drying of 70.0%. The feed stock had Pore Volume of 2.2. After milling at a temperature of fro 310°C. to 240°C. for 6 minutes at air inlet pressure of 6 psi and exit pressure 50 psi, by-pass open 5.0. The fee rate was 48-50 grams/minute. The ring size was 4.5", +0.2 timble position and +1/4 injector position. The product ha a pore volume of 1.74 cm-Vgm. and a surface area of 246 gms linseed oil/lOO gms. of aerogel. The Hegman grind was 4.5D. Eleven grams wer blended into 140 gms. nitrocellulose lacquer. The texture o the film was equal to standard (untreated Silcron G-100) Gloss at 60° was 3 units better than standard and at 85° wa -18- also 3 units better (flatter) than standard. In using the high pore volume silicas of the present invention the techniques of addition of silica flatting agents to various paints, varnishes, lacquers, enamels, etc., which are currently in use and are well known may be used. These silicas stir into the coating composition readily. Reference may be had to the patent to Cohen et al 4,097,302 for details of adding silica flatting agent to a coating composition.

Claims: -19- WHAT IS CLAIMED IS: 1. A process for producing a silica gel having improved pore volume relative to conventional silica gels which process comprises the steps of gelling an aqueous alkali metal silicate with an aqueous acid, washing the resulting gel to remove salt or salts and acid, aging said gel from 3 to 36 hours with aqueous ammonia solution and soaking the gel in an aqueous solution of a water soluble aluminum salt to impart from 0.1% to .7% by weight aluminum on the dry basis to said gel, drying and reducing the particle size of the dried gel to from 2.0 to 15 microns, said particles having a pore volume in the range of 1.25 to 2.25 cm 3 /gram 2. A product produced by the process of claim 1. 3. A process as defined in Claim 1 wherein the alkali metal silicate is sodium silicate. 4. A process as defined in Claim 1 wherein the aqueous acid is a water solution of a mineral acid. 5. A process as defined in Claim 4 wherein the mineral acid is hydrochloric acid. 6. A process as defined in Claim 4 wherein the mineral acid is sulphuric acid. 7. A process as defined in Claim 6 wherein the acid is 35° Baume. -20- 8. A process as defined in Claim 1 wherein the aluminum salt is aluminum nitrate. 9. A process as defined in Claim 1 wherein the gel is soaked in the aqueous solution of aluminum salt for a period of at least about 2 hours. 10. A process as defined in Claim 9 wherein the aluminum salt is aluminum nitrate nonahydrate. 11. A process as defined in Claim 10 wherein the aluminum salt is aluminum sulfate dodecahydrate. 12. A process as defined in Claim 1 wherein the silica gel is a silica hydrogel. 13. A process as defined in Claim 1 wherein the silica gel is a silica aerogel. 14. A process as defined in Claim 1 wherein the temperature of drying the aluminum salt treated gel is from 200-600°C. 15. A process as defined in Claim 14 wherein the temperature of drying the aluminum salt treated gel is from 500-600°C. 16. A process as defined in Claim 1 wherein drying and milling occur simultaneously in a fluid energy mill.

Inventors: COHEN HOWARD J, FITCH STEVEN J, VANDER HEEM PETER, COHEN, HOWARD, J., FITCH, STEVEN, J., VANDER HEEM, PETER
Applicants: SCM CORP, SCM CORPORATION
Application year: 1986
Main classification: C01B33
IPC codes: C01B33, C08K7, C09D7