Functional fluid systems containing alkoxysilane cluster compounds

#The present invention is directed to functional fluid systems containing specified fluids. More particularly, the present invention is directed to hydraulic and heat transfer fluid systems containing an effective amount of an alkoxysilane cluster compound, the compound being one having the general formula: ##STR1## wherein R is hydrogen, an alkyl, aryl or aralkyl and each R' is independently selected from the same group as R with the proviso that at least a majority of R' radicals are sterically hindered alkyl groups having at least 3 carbon atoms. This general Formula (I) may also be written in an abbreviated form as RSi[OSi(OR').sub.3 ].sub.3 wherein R and R' are as defined.

#Silicate esters, silanes and oxysilanes are well known for their utility as functional fluids and many of these compounds have been proposed for use as heat transfer fluids, hydraulic fluids, brake fluids, transmission fluids and the like. Novel alkoxysilane compounds with desirable functional fluid properties, which have heretofore not been described in the literature, are described in the above-identified parent application. The present invention is directed to the use of these compounds in functional fluid systems as more fully set forth below.

#As mentioned, the compounds used in the functional fluid systems of the present invention are those represented by the Formula (I) above wherein R is hydrogen, an alkyl, alkenyl, aryl or aralkyl. Desirably, R is hydrogen, an alkyl or alkenyl having about 1 to about 18 carbon atoms or an aryl or aralkyl having about 6 to about 24 carbon atoms. Preferably, R is hydrogen, an alkyl having about 1 to about 8 carbon atoms or an aryl or aralkyl having about 6 to about 14 carbon atoms. In Formula (I), each R' is independently selected from the same group as R, with the proviso that at least a majority of the R' radicals are sterically hindered alkyl groups having at least 3 carbon atoms. The desired and preferred groups for R' are the same as for R subject to the preceeding proviso. Desirably, at least a majority of the R' radicals are sterically hindered alkyl groups having about 3 to about 24 carbon atoms and preferably are sterically hindered alkyl groups having about 4 to about 12 carbon atoms. By sterically hindered alkyl groups is meant alkyl radicals which contribute to the hydrolytic stability of the molecule, i.e. which inhibit the reaction of water with the silicon-oxygen or the carbon-oxygen bonds in the molecule. Exemplary of sterically hindered alkyl radicals are non-linear primary alkyl radicals having a beta position side chain of at least 2 carbon atoms, secondary alkyl radicals and tertiary alkyl radicals. Particularly useful sterically hindered alkyl groups include sec. butyl, isobutyl, 2-ethyl butyl, 2-ethyl pentyl, 3-ethyl pentyl, 2-ethyl hexyl, 3-ethyl hexyl, and 2,4-dimethyl-3-pentyl, etc.

#A functional fluid was prepared comprising a compound having the formula:

#CH.sub.3 Si[OSi(sec. C.sub.4 H.sub.9 O).sub.3 ].sub.3

#the compound fluid was tested for various functional fluid properties. To examine an important aspect of its heat transfer properties, the fluid was tested to determine its ASTM slope (ASTM test no. D 341--43). The ASTM slope is based on viscosity measurements at 100.degree. F. and 210.degree. F. and is used as an indication of change in viscosity in response to temperature changes. This fluid exhibited an ASTM slope of 0.45, one which is very good for functional fluids and particularly for one containing no viscosity controlling additives. Also, the lubricity properties of the fluid were tested by subjecting the fluid to a wear scar test in which a four ball 40 kg load apparatus is used at 1800 rpm and 168.degree. F. for 1 hour. This fluid yielded a wear scar test result of 0.73 mm total scar, illustrating very good lubricity for a fluid containing no lubricity improver. One significant test for a hydraulic fluid is its stability in the presence of water. This fluid was subjected to a hydrolysis solids test which was carried out at 210.degree. F. in the presence of 33% by weight water and copper metal catalyst for 100 hours. Only 0.002% solids was found to be present at the end of the test. In addition to the above desirable characteristics, this fluid exhibited a flash point of 390.degree. F. suggesting its use as a hydraulic fluid under severe high thermal conditions. Low temperature functional fluid utility was also suggested by the fact that the fluid was still liquid at temperatures lower than -40.degree. F. (Viscosity measurements: 10 centistokes at 210.degree. F.; 37.2 cs at 100.degree. F.; 1050 cs at -40.degree. F.) The ASTM extended viscosity index (ASTM test no. D 22 70) was found to be 310. Weight loss in air was found to be only 13.96% at 1 atmosphere, 400.degree. F. for 1 hour.