Silicate Esters and Related Compounds Physical Properties of Certain Tetraalkoxysilanes, Polyalkoxypolysiloxanes,Bis( trialkoxysilyl) ethanes, Bis( trialkoxysiloxy)alkanes, and Compounds Containing Nitrogen, Phosphorus, and Halogens
A. DOYLE ABBOTT, JAMES R. WRIGHT’, ALFRED GOLDSCHMIDT, WILLIAM T. STEWART, and ROBERT 0. BOLT, California Research Corp., Richmond, Calif.
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E R T A I N SILICATE ESTERS exhibit good viscositytemperature characteristics and hydrolytic stability. As a result, these materials are finding increasing use as high temperature hydraulic fluids and lubricants (3). Such silicates were discovered by extensive work on the synthesis of new compounds and the study of their properties. The first article of this series (14) describes the methods used t o synthesize the silcate esters. T h e second (12) presents a convenient empirical method for the calculation of normal boiling points of certain of these materials. This third article gives data of physical and chemical properties o f t h e old as well as new compounds. SYNTHESIS
The compounds in Table I were synthesized, as noted in the last column, by the methods previously described (14). These procedures, as applied to the silicates in Table I, are typified by the following equations: Method IA (symmetrical tetraalkoxysilanes) 4 ROH + SiCL4+ (RO)8i
+ 4 HCl T
(1)
where R = sec-alkyl; also applicable to ROH + (CH2SiC13)2. Method IB (unsymmetrical tetraalkoxysilanes, disiloxanes, and trisiloxanes) (RO)3SiC1+ R’OH + CbHsN + (RO)3R’OSi+ CsH5N .HCl 1 (2) R = see- or tert-alkyl and R’ = p - or sec-alkyl; or R =p- or sec-alkyl and R’ = tert-alkyl; also applicable to (R0)2SiC12 + 2 R’OH. Or 2(R0)3SiCl+2 CsHsN + [(R0)3Si]20 + 2 C5H5N.HC1 R = p - or sec-alkyl (13). Or
1
(3)
(RO)ZSiC12+ (R’0)3SiOH+ 2csHsN -. [(R’O)SS~O]ZS~(OR)Z + 2CsH5N.HC11
(4)
R and R’ = p - or see-alkyl ( 1 5 ) ;also applicable to R2SiC12+ (R’0)3SiOH where R = ethyl or phenyl, Method IIA (tetraalkoxysilanes with tert-alkyl group)
NaOMe
(RO)$iOH + R’NHz (RO)aR’OSi+ NH3 T R = p - or sec-alkyl and R’ = tert-alkyl. Method llB (tetrabenzyloxysilane) (EtO)aSi+ ROH
Na
(R0)4Si+ 4EtOH T
(5)
(6)
R = PhCH2. The silicate derivatives of aliphatic and aromatic diols in Table I1 [bis(trialkoxysiloxy)alkanes or benzenes], were synthesized by Method I B according to Equation 7 : 2(RO)1SiCl+ R’(OH)z+ 2CbHsN+
R’[OSi(OR)3]2 + 2C5HbN.HCl 1 (7) R = sec- or tert-alkyl and R’ = alkyl or aryl diradical ( 1 ) . The miscellaneous silicate derivatives in Table I11 were Present address, National Bureau of Standards, Washington, D. C.
VOL. 6 , NO. 3,JULY 1961
prepared by a somewhat more complicated synthesis than the compounds in Tables I and 11. T h e materials containing phosphorus (compounds 1, 2, and 3) involved the following reactions! 2(RO)zP(0)H+ 2 Na-+2(R0)zP(0)Na+ HzT (RO)ZP(O)Na+ R’ (OH) (Cl)+ (RO)zP(O)R’OH+NaCl T
(8)
(9)
(RO)zP(O)R’OH+ (Rf’0)3SiC1--1 (RO)zP(O)R’OSi(OR”)3+ HC1-1
(10)
R = Bu, R‘ = alkyl diradical, R ” = see- or tert-alkyl. The materials containing nitrogen in Table 111 (compounds 4 and 5) were prepared by alcoholysis of tetraethoxysilane with a dialkylamino alcohol (Method IIA). The aromatic silicate derivatives (compounds 6, 7, and 8) were made by reacting the trialkoxychlorosilane with the appropriate halogen-containing aromatic diol (Method IB) . The last compound in Table 111, tetra( 1-carbethoxyethoxy) silane, was prepared by reacting silicon tetrachloride with ethyl lactate (Method IA) . IDENTIFICATION OF COMPOUNDS
Relatively large laboratory preparations were made of all the compounds studied, so that ample quantities of “plateau” distillation fractions were available for analysis. Identities of most of the compounds in Tables I, 11, and 111 were established by elemental analyses. Molar refractions were also used. Theoretical values were calculated from the bond refraction values of Denbigh ( 4 ) and Warrick (11). With few exceptions, agreement was excellent between the experimental and calculated molar refractions for all classes of compound? PHYSICAL PROPERTIES Boiling Points. A convenient method for determining the normal boiling points of silicate esters using the Kinney equation (6) was given previously (12). T h e same technique is applicable to many of the compounds shown in Tables I, 11, and 111. Viscosity- I ernperature Characteristics. These relationships with the silicate derivatives are affected by the structure of the alkyl group in a manner similar to that observed with hydrocarbons (10). I n tetraalkoxysilanes, the dependence of viscosity on temperature increases markedly with increased degree of branching:
Compound
tetra(1-butoxy)silane (8)
tetra(2-butoxy)silane 2-Butoxytri(tert-butoxy)silane
ASTM Slope (100-210” F.)
0.74 0.87 0.92
The transition from silanes to disiloxanes and trisiloxanes is marked by a significant decrease in the viscositytemperature slope: (Continued on page 442)
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Compound Tetra(2-butoxy)silane Hexa(2-butoxy)disiloxane
Octa (2-butoxy)trisiloxane Dimethyl silicone (20-cs. Dow Corning-200)
ASTM Slope (100-210” F.) 0.87 0.64 0.53 0.38
This effect of the growing -Si-0-Si “backbone” is further illustrated by the outstanding viscosity-temperature behavior of the silicone polymers. As the backbone becomes larger, its influence on the slope overshadows the effect of the alkyl groups, as can be seen in Table IV. These results are in agreement with, and extend, the findings of Morgan and Olds (8). HYDROLYTIC STABILITY Test Method. The hydrolytic stability test was conducted in the presence of air as follows: A borosilicate glass vial of 10-ml. capacity, fitted to a 4-mm. diameter glass tube condenser by a standard-taper joint, was charged with 2 ml. of silicate ester and 2 ml. of distilled water or acid solution. The vial was then placed on a steam plate operating a t 110” C. The briskly boiling water bubbled through the supernatant layer of silicate, thus establishing good contact between the two phases. The beginning of the hydrolysis was marked by a drop in viscosity and, simultaneously, by haze formation. This was ordinarily followed, after a short time, by voluminous precipitation of gelatinous silica, a sign of massive decomposition. The time (in hours) required to produce the latter was the criterion of relative hydrolytic stability. T h e reproducibility of the test was i10%. Results. Tetraalkoxysilanes containing more than one tert-alkoxy group are remarkably stable (7, 9). This stabilizing effect is not limited to tert-alkoxy groups. As shown in Table V, all the alkyl groups which impart some degree of steric hindrance give a corresponding degree of resistance to hydrolysis. Thus, the structural features which make certain silicate esters more difficult to synthesize also promote their hydrolytic stability. This effect is attributed to shielding of the Si-0 bond by nearby alkyl groups. Straight-chain p-alkyl silicates were easily hydrolyzed. Primary alkyl groups with branching on the second carbon atom from the Si-0 bond impart a significantly higher stability. Secondary alkyl silicates and compounds containing two or three tert-alkyl groups can withstand prolonged contact with boiling water. These considerations apply equally well to the alkoxysilanes, disiloxanes, and trisiloxanes. The increase in the number of S i - 0 units has little or no effect on hydrolytic stability; the important factor is the nature of the alkyl group. The gelation of silicates in contact with water is accelerated by acid and atmospheric oxygen. Times to gelation were longer when air was excluded from the system, and aromatic amine oxidation inhibitors did not delay gelation under this condition ( 5 ) . When air was present, these oxidation inhibitors retarded gelation, as shown in Table VI for phenyl-1-naphthylamine. A similar effect was observed with phenothiazine. T h e accelerated rate of decomposition in the presence of acid is illustrated by Table VII.
ACKNOWLEDGMENT
The authors express gratitude t o P. Noble, who synthesized some of the compounds included in this paper. They are also indebted to the U. S. Air Force, Wright Air Development Division, for financial assistance under Contract AF 33(038)-9831with part of the work. LITERATURE CITED (1) Abbott, A.D., Bolt, R.O., U. S. Patent 2,776,307 (Jan. 1,
1957).
442
Table V. Stability of Silicate Esters in Boiling Water in Presence of Air
Hours to Gelation
Compound Silanes Tetra(ethoxy) Tetra(1-butoxy) (8) Tetra(1-hexoxy) (8) Tetra(1-octoxy) (8) Tetra(2-methyl-1-butoxy) (9) Tetra(2-ethyl-1-butoxy) (9) Tetra(2-ethyl-1-hexoxy) Di (2-butoxy)di(2-ethyl-l-butoxy) Tetra(2-butoxy) Tetra (2-octoxy)
500 700 1150 > 236 > 236
Tetra(4-methyl-2-pentoxy)
1-Butoxytri(tert-butoxy) 2-Butoxytri(tert-butoxy)
tert-Butoxytri (2-butoxy) Di (tert-butoxy)di(2-ethyl-l-hexoxy)
> 600
1850 700 > 750
tert-Butoxytri(2-ethyl-1-hexoxy)
tert-Butoxytri (2-octoxy) Disiloxanes Hexa(2-butoxy) Hexa(2-ethyl-1-hutoxy)(9) Hexa(2-ethyl-1-hexoxy)
> 300 120 95
Others Octa(2-butoxy)trisiloxane Octa(2-ethyl-1-butoxy)trisiloxane 1,2-Bis[tri(2-ethyl-l-hutoxy) silyllethane Tri(tert-butoxy)silanol (14)
1600 126 140 > 300
Table VI. Effect of 1 Yo Phenyl-1-naphthylamine on Stability of Silicate Esters in Boiling Water in Presence of Air
Hours t o Gelation No With Compounds additive additive Di(2-hutoxy)di(2-ethyl-l-butoxy)silane 150 280 Tetra(2-ethyl-1-butoxy)silane(9) 75 240 Tetra(2-ethyl-1-hexoxy)silane 72 180 Table VII. Stability of Silicate Esters in Presence of Air in Boiling 0.5N Hydrochloric Acid and in Water
Compounds Tetra(2-butoxy)silane 2-Butoxytri (tert-butoxy)silane tert-Butoxytri(2-butoxy)silane
Hours to Gelation Water Acid > 500 21 > 236 45
> 600
45
Am. SOC.Testing Materials., Philadelphia, Pa., “1958 Book of ASTM Standards,” p. 483,1958. Chem. Erg. News 30,4517 (1952). 36,936 (1940). Denbigh, K.G., Trans. Faraday SOC. Harle, O.L., California Research Corp., Richmond, Calif. unpublished work, 1951. Kinney, C.R., J . A m . Chem. SOC. 60,3032 (1938). Minor, C.S., Byran, L.A., Holysz, R.P., Pedlow, G.W., Ind. Erg. Chem. 39,1368 (1947). Morgan, C.R., Olds, W.F., Ibid.,45,2593 (1953). Peeler, R.L., Kovacich, S.A., Ibid., 51, 749 (1959). Schiessler, R.W., Sutherland, H., Proc. A m . Petrol. Inst. 32 I, 74 (1952). Warrick, E.L., J . A m . Chem. Soc 68, 2455 (1946). Wright, J.R., J . CHEM.ENG.DATA5 , 206 (1960). Wright, J.R., U. S. Patent 2,870,184, (Jan. 20, 1959). Wright, J.R., Bolt, R.O., Goldschmidt, A., Abbott, A.D., J . A m . Chem. Soc. 80, 1733 (1958). Wright, J.R., Goldschmidt, A,, U. S. Patent 2,780,636, (Feb. 5, 1957). RECEIVED for review June 10, 1960. Accepted August 16, 1960. JOURNAL OF CHEMICAL AND ENGINEERING DATA
