ORGANIC SI·LICON COMPOUNDS Presented before the Division of Industrial and Engineering Chemistry at the 111th Meeting of the American Chemical Socjety, Atlantic City, N. J.
Ethyl Silicates H. D. Cogan and C. A. Setterstrolll
. . . . . . . . . .
lerl-Alkoxyaminosilane~
C. S. l\Iiner, Jr., L. A. Bryan, 1{.P. Ilolssz, J f., and C;. \V. Pedlow, Jr.
. .....
l368
.
1372
Polysiloxane Elastomers C. IVI.Doede and Ahmed Panagrossi . . . . . . Silicone Resins.
Use in Protective and Decorative Finishes
. . . . . . . . . . . . . . . . . . . .
l376
Polysilicic Acid Esters. Preparatioll from Sodiulll Silicate R. K. Iler and P. S. l)inkney .
1379
James R. Patterson
Butoxychlorosilanes.
Hydrolysis and Condensation
R. K. Iler
1384
.
Properties of Polyor~anosiloxane Surfaces on Glass ~t1. J. Hunter, 1\1. S. Gordon, A. J. Barry, J. l~". llyde, and 11. D. I-feidenreich Polvmethvlsiloxanes.
Thermal an.d ()xidation Stabilities
·D. C. Atkins, C. 1\'1. 1\'1urpl~y, and C. E. Saunders
1395
Polyorganosiloxanes. Surface Active Properties H. "T. Fox, Paula "T. Taylor, and \\T. A. Zisrnan
T
1389
1401
interest \vhere high purity of the deposited silica is ilnportant. Condensed ethyl silicate and A. SETTERSTROMI ethyl silicate 40, alPITTSBURGH. PA. though undistilled, are of greater industrial importance because of lower initial cost and higher available silica content. Ethyl silicate 40 is the most economical; condensed ethyl silicate is of special interest as an investment binder for prerisioll casting.
ETHYL SILICATES
HE esters of silicic acid are old-tirners chen1ically ('I') and Inay lack the glan10r of the newer H. D. COGAN AND C. organic silicon comMELLON INSTITUTE, pounds. Kevertheless, they are among the most important commercially, and serve many useful purposes. Son1c of the esters are of interest as heat transfer liquids (16) or as chelnical intermediates (l~, 27), but in general the organic silicates are of industrial importance because they are a convenient source of silica. 1Iethyl silicate would be the logical ester for such applications because it contains the highest percentage of silicon, but exposure to the vapors .under certain conditions causes a necrosis of the corneal cells \vhi('h may lead even to blindness. The ethyl silicates, on the other hand, SUfffr frolll no serious toxicological handicaps (39, 4-0) and have becoll1e the esters of major industrial ill1portance. PHYSICAL PROPERTIES
Three types of ethyl silicate are available cOIllIllere.iully. Tetraethyl orthosilicate, (C 2 H fJ O )48i, is a colorless liquid with a Inild esterlike odor. Condensed ethyl silicate is a light bnnvIl liquid consisting predominantly of tetraethyl orthosilicate together with some polysilicates. Ethyl silicate 40 is a nlixture of ethyl polysilicates. Its name comes from the fact that its available siJica content is approximately 40S:~. Table I lists SODle of the physical properties of the tetraethylorthosilicate. Table II gives specifications of the three types. Tetraethyl orthosilicate is a distilled product of particular
METHODS OJl' MANUFACTURE
In the usual batch process the esters are made by charging silicon tetrachloride to a glass-lined jacketed reactor and adding ethanol at some predetermined rate. The initial addition of ethanol is accornpanied by vigorous evolution of hydrogen chloride and a dropping of the reactor temperature. \Vhen the theoretical alnount of ethanol has been charged, the heat of reaction begins to outweigh tpe heat of vaporization of the hydrogen chloride, and the reactor telnperature rises. After the alcohol has been added, the n1ixture Inust be stripped promptly to avoid formation of undesirable high boiling polYll1ers. Variations in the stripping technique and in the subsequent hydrolysis, to obtain the pari ially condensed esters, affect the structure and size of the resultant polynlers, with resultant variations in performance (82,85).
In the continuous process carefully controlled feeding rates, together with integrated continuous stripping, hydrolysis, and distillation procedures, assure'more consistent and uniform products than are possible by the batch process. These products, in turn, make possible the use of standardized techniques for deposition of silica.
1 Present address, Carbide & Carbon Chemicals Corporation, 30 East 42nd Street, New York, N. Y. The work reported here was carried out at l\lellon Institute under a Multiple Industrial Fellowship of Carbide & Carbon Chemicals Corporation.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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l\IETHODS OF HYDROLYSIS
TIlE esters of silicic acid were the first organic silicon
All of the ethyl silicates hydrolyze slowly in the presence of. \\Fatel' to yield silicic acids. These acids then undergo a sinlultaneous dehydration and polymerization \vhich eventually yield an amorphous silica. The hydrolysis of a typical Jnolecul{~ of ethyl silicate 40 can be pictured as fo11o\\'s:
cOlllpounds to achieve commercial importance. ]\/Ionomeric ethyl silicate and its polymers are large-tonnage industrial chenlicals lllanufactured frolll silicon tetrachloride and ethanol by a con tinuous process which elilllinates batch-to-batch variation. 'The illlportant COlllmercial applications hinge upon the ability of ethyl silicate to deposit silica from solution, so the techniques of hydrolysis and polylnerization are illlportant. Il.ecent ad'"ances include the developnlent of useful aqueous systellls without lllutual solvents, and the large-tonnage availability of a stable liquid polymer with an equivalent silica content of 40%. The ethyl silicates are used as adhesives for investlllents in preci!i'lipn casting, binders for ceralllics, gelling agents for alcohol fuels, sources of finely divided amorphous silica, building stone illlpregnants for weatherproofing, and in the preparation of glass adherent lacquers.
susceptible to catalysis and to gelling than are t he ethanol solutions descrihed in the follo\ving paragraphs. ALCOHOL SOLUTIONS
The complex silicic acid, in turn, reacts with other silllilar molecules or ester ll10lecules, splits out water or alcohol, and polylllerizes finall:v to yi~ld an adhesive forrn of silica, (Si0 2 ),£. Xone of the ethyl silicates is Iniscible ,,'ith water, but hydrolysi~ ran be made to proceed fairly rapidl~T in an aqueous mediulll I).\" usillg vigorous agitation or an f'lnulsifying agent, and large anlounts of a catalyst. For exalnple, if 20 volu1l1es of ethyl silicate 40 are vigorously lllixed with 4 voluilles of 5% hydrochloric acid, the lllixture which is at first nonholnogeneous beCOllles clear in 5 to 10 lllinutes. After about an hour this silicate solution can be diluted \,'ith the desired aillount of \vater. 'The addition of 76 volullles of water gives an opalescent solution, containing about 8Sj silica, which n\nlains fluid for several da~Ts. Such solutions deposit adhesive silica, but are sOlllewhat nl0l'("
For lnany industrial uses a lnutual solvent sueh as ethanol or isopropanol is employed. Figure 1 shows the solubility eharaeteristics of the three-conlponent systenl: ethyl silicate 40, water, and Synasol solvent. (Synasol consists of proprietary solvent based on Specially I)enatured Alcohol Formula l\ o. 1.) Similar diagrams for tetraethyl orthosilicate and "eondelu;;ed" ethyl silicate have already been described in detail (4). Line B~," represents conlpositions containing the correct theoretical ratio of \yater to ethyl silicate 40 needed to give complete hydrolysis of the ester to SiC}~. Line .:LY represents useful conlpositions containing less than the theoretical water. In such systerns a portion of the ester is converted only to a condensed resinous rnaterial which tends to irnprove the adhesive nature and to preserve seHne temporary flexibility of the silica filn1 as the
Production of .\rolds for the Precision Casting of Stainless Steel Exhaust Couplings At left the wax patterns are being dipped into a suspension of fine silica in a dilute lllediulll containing ethyl silicate. The cOlllpleted lllolds, shown below, with wax patterns set in the ethyl silicate investlllent, are loaded upside down in a continuous furnace to llleit out the wax and further dry and set the inve~tlllent.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 39, No. 11
vestment binder in the so-called lost wax process (6, 8, 10, 25, 28, 29). Prior to the \yar tho products of this process \verc chiefl; dentures and metal appliances for bone surgery. During the \val' huge quantities of buckets or blades for airplane turbosuperchargers were rnanufactured by this technique. Since the war this industry has successfully cast nlany diverse products, particularly from I-Iayne~ Rtellite cobalt-chrornium-tungsten alloys, thf' IIastelloy nickel-base alloys, lVlonel Inetal alloys, and other high melting n1aterials. Somf' of t he advantages claimed (15) over older lnf>t hods of casting are reduction in direct lahor charges, reduction in capital costs ,vitb 10'70 (·limination of tooling up, and greater versatility. The investment Inixture for the lost wa\ r90 \ ~o process usually consists of a thick slurry of -+-----¥----4. j silica, nlagnesia, and brick dust in a water x-- --:~~-"'-~~'--~~·_=_=_---"-~_"___::~~~-----Y...--\L.~-.:L-......:...IO~~OIOO WA TE R solution containing ethyl silicate, ethanol, and SYNASOL SOLVENT hydrochloric acid. Such an investInent ha:several advantages~for example, controlled Figure 1. ~oluhility Charaeteristies of Three-ConlponenL S)stf"ll1 ('old setting, dimensional accuracy, toleranet:' Ethyl Silicate 'iO-190-Proof Ethanol-Water of trmperatures up to 1600° C., adequate and Si02 is given as perc.entage by weight; the other relations are by volullle. casH:\' varied mechanical stTength, and read~ parting from the cast object. The ethyl silicates have also been suggested solvent evaporates. The relatively srnall aInount of water reas binde'I's for Ow "semidry piece mold" method (33) where tht' quired to complete the hydrolysis is subsequently absorbed fron1 wax nH)(h-l is eliIninated and the n10ld is Inade directl.y· from a the surroundings. InaRh~r pattern. In investment casting the Iuold must be readil.\ The maximum possible silica content obtainable by a one-step brokpll away, but in piece molding a stronger and harder mold hydrolysis is about 240/0 (indicated on chart at point 1). By is required. It is believed that these properties can be ohfirst preparing a partially hydrolyzed solution fronl the ternary tained by using higher percentages of one of the ethyl silicates. described by point 2, and subsequently adding enough water so REFRACTORY BINDERS. Cerallliclike articles may be producee i the total constituents before reaction are described by point by a simple rl10lding and drying process. Sillimanite, an alun11·· 2H, a stable solution containing 32% Si0 2 is obtained. l\Iisnunl silicate, for exaruple, may be shaped in this \vay by using cibility is achieved by allowing the components of the first step enough h:vdrolyzed ethyl silicate to yield ,5% silica after baking to react, fornl ethanol, and, in effect, shift the Iniseihility line (36). Ot 11('1' rnaterials which can be bonded \vith ethyl sili(~ate to the right. are: refractory crucibles for high nlcHing alloys; refractory and acid-resistant brick and mortar; cold-set electrical insulator~. RATES OF REACTION porous silica articles; ferrochrome; activated carbon; sawdust. In a neutral solution the hydrolysis of ethyl silicate is very \vood flour, bagasse, etc., to make insulating material; and slow. In the presence of ammonia the reaction proceeds rapidly asbestos electrical insulation. Some \vork has been done t{1 to form gels or gelatinous precipitates. Hydrolysis is more determine the suitability of ethyl silicate for bonding refractor., easily controlled \vhen hydrogen chloride is employed, and it is lllaterials to produce refractory and acid-resistant brick and common practice to use a concentration of 0.3 to 5.00/0 hydrogen mortars. If pulverized silica is mixed with the hydrolyzed ethyl chloride in the ;vater for hydrolysis. Even with the .5% hysilicate and then allowed to air-dry, a hard, strong, slight!.' drogen chloride, however, it is necessary to allow the solutions porous refractory and acid-resisting material is formed. Airto stand 8-12 hours, not only between steps in the t\vo-step dried blocks produced by this method withstood a crushing load process, but also between the final mixing and use in both the of over 2000 pounds per square inch and a temperature of 1500 one- and two-step methods. C. On heating and subsequent cooling, the strength of tllt~ The one-step solution can be stored only for several days before blocks \vas reduced somewhat, but no spalling or serious deterigelling. In the two-step procedure the partially hydrolyzed oration developed. solution can be stored for an indefinite period, and the completely From the standpoint of price it is impractical to use ethyl sili-· hydrolyzed solution ca~ be stored as long as a week \vithout cate for the manufacture of ordinary refractory brick. I t ma~' marked change in viscosity. Deposition of silica occurs upon be practical, however, to use the solution for making special reevaporation of the water and ethanol. fractory objects and mortar which must be resistant to high temperature acid, and might therefore command the higher price. USES IMPREGNATING POROUS l\IATERIALS. Many porous material... can be impregnated with the ethyl silicate solutions _and thus bt, One of the most important applicatiolls has been as a bonding strengthened, hardened, or stiffened by the deposited silica. The· agent for comminuted materials (29). The sticky, colloidal porous body nlay be impregnated cOlnpletely or only superficially silicate dries to a hard, vitreouslike material which \viII \vithstand If a smooth surface is desired, SOllle filler may be incorporated iIH high temperatures, is insoluble in water, and has no chemical the ethyl silicate solution. Some of the applications in this field action upon the surrounding material. Ethyl silicate is of parare: hardening of foundry molds; surface hardening of porowticular value as a mold binder in casting metals, alloys, or other silica brick and of graphite molds used for making special meta] materials that cannot readily be machined to form objects with exact dimensions. casting~; stiffening of asbestos lllaterials, paper, straw, leather, The ethyl sili~atp~ havp heen used for 111any years as the incork, and t(-'xtile products; produC'tion of a polished watPfL
~~. ~_---+--1:-_~-~~-:'-~-:;-~~~~-=~~~~~;--*-_--ii--~1,----_8-l.0--~_-:~~
' __
~:"(_--'\----fr-----------J\;;--""---;Il(--k---1l;--*---\-----*---:'r---
~
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INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1947
resistant surface coating for porous articles such as stone, tilp. and plaster (37'). "TEATHERPROOFI~G. The oldest application of ethyl silicato has been for the preservation of artistic and architectural stone"rork (1 '1', 20). Properly prepared solutions penetrate the pores of stone, briek, or concrete surfaces. Upon evaporation of the solvent, they deposit an adhesive silica ,,"hich sets and a(·ts as a hardening and bonding agent" and thereby increases t hp n'''''istance of structu!al stone to spalling and crunlbling caused by exposure to the elenlents. The ethyl silicate first penetrat('~. the' surface only slightly; then, as it hydrolyzes and dries, it fonTl~ a gelatinous silicate which dries further with shrinkage to give the· ultilnate tenacious silica bond. Because of the shrinkage, eOll}plete ,vaterproofing is not obtained, but this tn'airnent dccn'asc~ porosity, retards the penetration of 1l1oist un'~ and definitely increas('s hardness. \Veatherproofing \yith ethyl sili('atc had it:-start in England, but experinlental studies are now undcnva~ in this country ,vith such buildings and lllonunlent s as t lw chapel at \~aIIey Forge, Pa., the Eternal Light Peace l\Ionlllnent at Get tysburg, Pa., adobe huts in the \Yyonling X ational Parks, and the I\:enncth Taylor Galleries at ~antucket, i\Iass. In England during the 'war larg(~· quantities of the ethyl silicates were used to danlpproof brick clnergency constructions (85). GELS. Stable gels of liquids such as ethanol, isopropanol, and acetone can be prepared by the addition of ethyl silirate, water, and catalyst such as caustic soda (80). The ethyl silicate h:vdrolyzes to produce, and distribute uniforrnly throughout the liquid, a relatively stiff franle\vork or body of silicic acid and silica. The resultant gels are tough, elastic, and highly resistant to deterioration by aging, and tenaciously retain the liquid \vithin the gel bod.y. Because these gels burn unifonnly at an ignited surface ,vithout observable liquefaction, they are of particular value a:s ~olid fuels. During the ,val' carload quantities of elhyl silicate were used to make isopropanol and ethanol solid fuels for the l\Iarine Corps and the Office of the Quartennaster General. Because of 1nore unifornl burning, these fuels produced H10rf' usable heat than equivalent nitrocellulose gelled alcohols. In preliminary studies ethyl silicate gels of dilute organic acid~ have shown sonle promise for the debridenlpnt of burns. FILMS. So far it has not been possible to prepare filrns of ethyl silicate alone ,,'hich do not suffer fronl the disadvantages of brittleness and lack of adhesion. Pigmented ethyl silicate solution~ deposit filIns of greater adhesion. In order to obtain Inaxirnunl adhesion, it is necessary to usc lanlinar fillers such as finely divided Inica. A. typical forn1ula is 10 parts of conlpletely hydrolyzed ethyl silicate, 3 parts of finely divided rniea, and 3 parts of pigments by weight. Pigments nlost suitable for use \\'ith pthyl
TABLE 1. PROPERTIES OF TETRAETHYL ORTHOSILICATE 1Iolecular weight Specific gravity at 20/20° C. Boiling point at 760 mm. Hg, 0 C. Vapor pressure at 20° C., mm. Hg Freezing point, 0 C. Viscosity at 20° C., centipoises Refractive index, n ~o Flash point (open cup), 0 F.
TABLE II. SPECIFICATIONS Tetraethyl Orthosilicate Sp. gr. at 20/20° C. Boiling range at 760 mm. Hg, 0 C.
A vailable silica as Si0 2 • % Max. acidity as Hel,
% Av. wt./gal. at 20° C., lb.
OF
208.30 0.9356 168.1 1.8 -77 0.60 1,3832 125
COl\IMERCIAL l\fATERIAI--J Condensed Ethyl Silicate
Ethyl Silicate 40
0.933 to 0.938 O. 9~0 to 0.950 Below 160, not Below 90, not more than 5%; more than 5%; above 170, above 210, not more than not more than 27.9% 15% N at les~ than X ot less than 30 27.9 0.05 0.20
0.10
7.78
8.82
7.78
1. 050 to 1. 070 Below 80, none; below 110, not
more than 5% 38-42
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silicate are generally those that are cheluieally inert, such a~ oehrt', sienna, or chromium oxide (18, 19). 'Titaniuln oxide is a good white, and carbon black can be used as a black piglnent (2). Such paints formulated "rith ethyl silicate are extremely resistant to heat and chemical fumes, do not darken on aging, and are fire retardant. They may prove useful as decorative and protectiv(~ coatings for furnace· castings, infrared lamps, asbestos blocks, ('onrretc, and other special surfaces ,\'here inflexibility and porosity are not deterrents. In England during the ,var large tonnagps of the ethyl silicates ,yere used to camouflage concrete surfac('s, particularly when subject to abrasion (85). Rha,,' recently reported from England that cellulose l,thpr and (',-;tp)' ('oating compositions can be made fire resistant by the' U:--t· of (,thyl silicate-ammonium phosphate solutions (,S.n. Ethyt silicate· alone, however, is not particularly effective a~ a flrc /'j'tardaut for flammable materials b>ecause the deposited silica i~ infusible and nonvolatile. IXCREASINr; .ADHESION TO GLASS. In 1936 Van IIul'ckerol It CP) noted the usefulness of ethyl silicate in incTeasing the adhesion of cellulose nitrate lacquers to glass. Since that tinw ct byl silicate has also been employed to increase the adhesion (It' vinyl acetate (22), urea-formaldehyde (88), vinyl acetal, and villYI. hut yral (6) resins. Sufficient ethyl silicate is used to supply :3() to 50 parts of silica per 100 parts of resin in a Inixture, and i.. . applied as an undercoat. The completely hydrolyzed solut illllS of an~r of the ethyl silicates may be used for this application. Sizable quantities of ethyl silicate have been used as bindt'r~ for, or components of, the' fluorescent po~yders or phospho!'.,,; ('oared on the inside of fluorescent tubes (9, 11, 24). In England t honnomctcr gradations arc filled 'with ethyl silicate bound pi~ t11ents (35). So FReE OF PURE SILICA. Ilecently there has b(\(,ll eonsidt'rahle interest in ethyl silicate as a source of "whHe carbon black" for the rubber industry. One company (28) announced a Illcth()( I of burning ethyl silicate and collecting the resultant silica which is a white, partly translucent po\vder nleeting the specific rt·~ quil'cnlents of particle shape and degree of division necessary for this application. The cost is such, however, that no large tonnag(' use is anticipated. Nevertheless, ethyl silicate ,viII probably continue to be used as a reference standard for the product ion of ,vhite carbon black from more eeonoluical sources sueh as silicon tetrachloride (21), and as a source of specially purifieJ nlaterial \vhere high electrical resistance or some other propcrt:\ \\'ould justify a premium. Ethyl silicate and other volatile silicon eonlpounds, such a . . trichlorosilane, have also been found useful for the production of transparent and heat-resistant articles of silica (12, 13). Tht' silica deposited on burning is subsequently vitrified by heatill~ at 1
