Alkyl silicate
Aviation Hydraulic Fluids with excellent hydrolytic stability R. L. PEELER and S. A. KOVACICH California Research Corp., Richmond, Calif.
silicate derivatives, notably tetraalkoxysilanes and hexaalkoxydisiloxanes, are being used as hydraulic fluids in several supersonic aircraft within the temperature range of -65' to 400' F. For this application the alkyl silicates combine excellent viscositytemperature properties and good oxidation and thermal stability with low volatility and adequate lubricating qualities. Currently available silicates hydrolyze on storage with water by a mechanism which has been postulated (7) as follows:
A L K Y L
/
SiOR
+ H a 0 4 ROH + LI SiOH+
--.I + '/z HzO
'/~-SioSi~ /
As a result of this reaction, the corresponding alcohol and polymeric siloxanes are produced, changing the properties of the fluid and leading to precipitation of insoluble polymers or, ultimately, of silica. Current silicate-base hydraulic fluids are handled with considerable care to avoid contamination with water. This article covers the results of work aimed at increasing the hydrolytic stability of alkyl silicate derivatives while retaining their other desirable properties. Earlier work in this laboratory (6) on the effect of the alkoxy group structure on hydrolytic stability of alkyl silicate derivatives at 200' F. showed that order of stability was tertiary > secondary > 2-alkyl primary > normal primary. Miner, Bryan, Holysz, and Pedlow (8) have shown the remarkable stability of tetraalkoxysilanes containing up to three tert-alkoxy groups. Ryskiewicz (9) and Aelion, Loebel, and Eirich ( 2 ) also have demonstrated the beneficial effect of increasing steric hindrance on hydrolytic stability. Increasing the length of the alkoxy groups in a homologous series (2, 7) increases stability. Likewise, in alkyltriethoxysilanes ( 3 ) increasing the chain length of the alkyl group increases the resistance of the compound to hy-
This article covers methods of producing silicate derivatives that have 400" F. hydrolytic stability far superior to current available esters drolysis. The proposed program, therefore, consisted of synthesis of silanes and disiloxanes containing sterically hindered alkoxy groups and alkyl groups directly attached to silicon. As most of these compounds had not been previously prepared, appropriate synthetic methods were devised, a 400' F. hydrolytic stability test method was developed, and tests were run to evaluate hydrolytic stability. Synthesis
Methods of preparing silicate derivatives used earlier in this laboratory have recently been summarized ( 7 I). Starting materials used in the present work were the appropriate alcohols and silicon tetrachloride or commercially available alkylchlorosilanes. Products were rectified at reduced pressure in a spinning band column; the purest fractions available were used for evaluation. The
compounds synthesized and their physical properties are listed in the table.
400' F. Hydrolytic Stability Test Equipment
The performance of 400' F. hydrolytic stability tests in the presence of liquid water requires the use of pressure equipment. T o allow screening of a large number of compounds, small sample size and ease of operation are essential. The 22-ml. Parr flame ignition peroxide bomb was chosen as the most practical available pressure vessel. Procedure. Ten milliliters of fluid, along with a weighed copper strip (0.25 X 0.50 X 0.012 inch) and water (usually 0.60 ml.), are placed in the bomb, which is then sealed by using a Teflon gasket. The bomb assembly is weighed before and after the run to determine whether any leakage has occurred. The bomb is then placed in a rack and
Hydrolytic stability tests were run in Parr flame ignition peroxide bombs rotated in an oven VOL. 51, NO. 6
JUNE 1959
749
TYPE OF COMPOUND
NO. COMP'S.
V 1SCOSIT Y ,G HAN G E AT 21 F. %
INSOLUBLES. % 0
I
I
I
2
3-
I
4
5
0
+?O
-
l b
3 I I 2 2 3 I I I 2
t
I I
+
2 I 3 I I I
I
I
2
I 2 I 2 I
I 6
2 3 I 2
I
tl
0
D
I
2
I
I
3
4 I
5
+20
D 0
-20
I
-40
-qO
I
I
I
-
I
I I
a 3 I LDESIRED MAXIMUM
Figure 1.
750
LDESIRED~ MAXIMUM
Hydrolytic stability was dependent on the number and type of alkyl and alkoxy groups 20-hour tests with 6% water at 400' F.
INDUSTRIAL AND ENGINEERING CHEMISTRY
A V I A T I O N H Y D R A U L I C FLUIDS Propeirties of Cornpounds Synthesized Boiling Refractive Density, Index, Point at 1 Mm., a C. 20/4 nY
Compound Silanes Tetra(2-ethylbutoxy) Tetra(2-pentoxy) tert-Butoxytri(2-ethylhexoxy) tert-Butoxytri(2-octoxy) tert-Butoxytri(5-ethyl-2-nonoxy) Di(tert-butoxy)di(2-ethylhexoxy) Di(tert-pentoxy)di(2-ethylhexoxy) Di(tert-butoxy)di(2-pentoxy) Di(tert-butoxy)di(2-heptoxy) Di(lert-butoxy)di(2-octoxy) Di(tert-butoxy)di(5-ethyl-2-nonoxy) Tri(tert-butoxy) Methyltri(2,Z-d Methyltri(2-oct Methyl-tert-butoxydi(2-ethylhexoxy)
153-5 105-6 170-3 177-80 1 98-202a 139-43
1.4305 1.4168 1.4320 1.4286 1.4408 1.4238 1.4311 1.4101 1.4182 1.4220 1.4325 1.4134. 1.4280 1.4313 I . 4272 1.4267 1.4350 1.4004 1.4302 1.4372 1.4342 1.4300 1.4278 1.4330 1.4347 1.4573 1.4384
194.1 121.1 1,176 1,644 48,645 1,228 1,204 353.7 1,006 1,580 25,213 1,981 1,470 707 41 1
211-12 263-9 179-83 204-13= 196-207b 147-50 158-62 1 76-9OC 167-71 175-86 179-81 185-90 211-14 190-3 213 222-7 158-67 172-4 161-6 156-60 186-90 214-28
0.9217 0.8933 0.9197 0.9031 0.9272 0.9168
1.4329 1.4369 1.4261 1.4327 1.4308 1.4179
0.9074 0.8926 0.9083 0.8916 0.8954 0.9035 0.8856 0.8898 0.9970 0.8991 0.8963 0.8967 0.8961 0.8918
150-62 169-76
0.8996 0.9012 0.8909 0.8916 0.8971 0.9100 0.9023 0.9951
73-4 126-37 144-8 181-2 102-3 122-7 167-78 121-8 152-5 160-8 39-41 134-7 162-9 178-80 139-71 112-17 159-60 148-52 108-14 161-3
Methyl-tert-butoxydi(1-octoxy)
Pentyl-tert-butoxydi(2-ethylhexox y)
Phenyl-terl-butoxydi(2-butoxy) Dimethyldi(5-ethyl-2-nonoxy) Disiloxanes Hexa1.2-ethylbutoxy) Hexa(2-octoxy) 1,3-Di(tert-butoxy)-I, 1,3,3-tetra(2-ethylbutoxy) 1,3-Di(krt-butoxy) -1,1,3,3-tetra(2-ethylhexoxy) 1,3-Di(tert-pentoxy) -1,1,3,3-tetra(2-ethylbutoxy) 1 ,3-Di(tert-butoxy)- II1,3,3-tetra(2-pentoxy) 1,1,3,3-Tetra(tert-butoxy) -1,3-di(2-ethylbutoxy) 1,1,3,3-Tetra(twt-pentoxy)-1,3-di(2-ethylbutoxy) 1,3-Dimethyl-1,1,3,3-tetra(2-ethylbutoxy) 1,3-Dimethyl-l, 1,3,3-tetra(2,2-dimethylpentoxy) 1,3-Diethyl-l, 1,3,3-tetra(2-ethylbutoxy) 1,3-Diethyl-1,1,3,3-tetra(2,2-dimethylpentoxy) 1,3-Diethyl-l,1,3,3-tetra(2-ethylhexoxy) 1,3-Dipentyl-l, 1,3,3-tetra(2-ethylbutoxy) 1,3-Dimethyl-l,1,3,3-tetra(2-octoxy) 1,3-Diethyl-1,1,3,3-tetra(2-octoxy) 1,3-Diphenyl-l, 1,3,3-tetra(Z-butoxy) I,3-Dimethyl-l,3-di(tert-butoxy)-l,3-di( I-octoxy) 1,3-Dimethyl-1,3-di(tert-buloxy)-1,3-di(iso-octoxy) 1 ,3-Dimethyl-l,3-di(tert-butoxy) -1 ,3-di(2-ethylhexoxy) I ,3-Dimethyl-l,3-di(tert-butoxy) -l,3-di(isodecoxy) 1,3-Dimethyl-l,d-di(tert-butoxy) -1,3-di(isotridecoxy) 1,3-Dimethyl-l,3-di(tert-pentoxy)-1,3 pentoxy) 1,3-Dimethyl-l,3-di(tert-pentoxy) -1,3 1,3-Dimethyl-l,3-di(terl-butoxy) -1,31,3-Dimethyl-1,3-di(tert-butoxy)-1,3-di(5-ethyl-2-nonoxy) 1,3-Diethyl-1,3-di(tert-butoxy)-1,3-di(2-ethylhexoxy) 1 ,3-Diethyl-1,3-di(tert-pentoxy)-1,3-di(2-ethylbutoxy) l,3-Diethyl-1,3-di(terf-pentoxy)-1,3-di(l-octoxy) 1,3-Diphenyl-1,3-di(tert-butoxy)-1,3-di(2-butoxy) a At 0.1 mm. At 0 3 mm. At 0.2 mm.
rotated at 5 r.p.m. in an oven operated at 400' F. The rack is also used to hold beverage bottles during 200" F. hydrolytic stability tests ( 5 ) . At the completion of the test, the hydrolysis mixture is centrifuged. Any insolubles are washed with chloroform, dried, and weighed. The viscosity change of the fluid at 100' and 210' F. is measured. The change in weight of the copper strip is also determined.
0.8920 0.8800 0.8799 0.8689 0.8735 0.8777 0.8850 0.8815 0.8711 0.8712 0.8740 0.8705 0.8627 0.8633 0.8665 0.8688 0.8661 0.8662 0.8792 0.8721 0.8639 0.8735 0.8785 0.8680 0.8668 0.9379 0.8512
160-4
Methyl-tert-butoxydi(5-ethyl-2-nonoxy) Methyltri(tert-butoxy) Ethyltri(2-ethylbutoxy) Ethyltri(2-ethylhexoxy) Ethyltri(2-octoxy) Ethyl-tert-butoxydi(2-ethylhexoxy) Ethyl-tert-pentoxydi(2-ethylbutoxy'i Ethyl-tert-pentoxydi( I-octoxy)
Viscosity, Centistokes at F. - 65 100 210
160-61
194-9 171-8 150-62 187-8 175-85
... ...
Solid 1,177 804 3,216
3.897 2.592 6.200 6.135 15.29 5.563 6.862 3.36 4.694 5.722 11.44 5.456 4.887 4.757 3.77 4.041 7.774 2.460 2.927 4.940 5.150 4.338 3.111 4.343 5.641 4.161 5.623
1.542 1.102 2.027 1.921 3.450 1.836 2.176 1.201 1.568 1.789 2.794 1.705 1.684 1.654 1.41 1.523 2.186 0.9217 1.246 1.772 1.737 1.521 1.280 1.591 1.849 1.445 1.823
1.4289 1.4330 1.4327 1 * 4393 1.4402 1.4329 1.4375 1.4787 1.4244 1.4249 1.4247 1.4314 1.4378
560.3 3,521 1,225 3,348 1,458 898.2 Solid Solid 219.5 4,530 251.8 2,899 1,053 1,194 1,477 1,537 10,402 420 734.6 880 3,576 29,822
9.273 12.31 11.43 12.35 12.74 9.444 solid Solid 4.478 8.292 5.852 11.07 8.397 11.37 7.100 8.67 15.52 5.47 5 * 893 5.785 9.126 16.17
3.365 3.491 3.715 3.553 3.687 3.123 4.879 Solid 1.787 2.582 2.285 3.384 2.801 3.652 2.300 2.71 4.160 1.94 2.038 2.013 2.745 3.913
1.4257 1.4301 1.4229 1.4322 1.4301 1.4304 1.4343 1.4781
1,406 820.3 870.5 12,767 1,177 596 607 Solid
7.706 6.58 5.61 10.45 7.234 6.46 7.589
2.499 2.27 1.93 2.887 2.201 1.96 2.527 5.533
... ... 1.4288
Hydrolytic Stability Test Results The hydrolytic stability of the silicates synthesized was determined by the method discussed. The degree of hydrolysis was determined by the viscosity change and the quantity of insoluble material formed. The goal of this work was the synthesis of a compound which could withstand hydrolysis as long as possible at 400' F. with insolubles for-
...
9,972 Solid
... ... ... ... 117
25.61
Pour Point, ' F.
