CA1083593A

CA1083593A - Silane ester ether derivatives for hydraulic fluid

Info

Publication number: CA1083593A
Application number: CA265,706A
Authority: CA
Inventors: Herbert F. Askew; Colin J. Harrington; Gerald J. J. Jayne
Original assignee: Castrol Ltd
Current assignee: Castrol Ltd
Priority date: 1975-11-21
Filing date: 1976-11-15
Publication date: 1980-08-12
Also published as: IT1121743B; AU1982376A; NL7612915A; CH629250A5; DE2652719A1; IE44233B1; JPS6046155B2; ATA862776A; NL174467C; NZ182641A; JPS5265227A; GB1577715A; NO763957L; IE44233L; DK154024B; NO155623B; AU506008B2; DE2652719C2; FR2332281A1; AT358701B

Abstract

ABSTRACT OF THE DISCLOSURE

Novel silane derivatives having the general formula:- I

wherein:
(a) R is a group of the formula R4-(OR5)m-OR6-;
(b) each of R1 and R2 lo independently alkyl, alkenyl, aryl, alkaryl or aralkyl, a group of the formula -OR3 or a group of the formula R4-(OR5)m-OR6-:
(c) R3 is a group of the formula R4-(OR5)m- or a group of the formula:- (d) R4 is alkyl, alkenyl, aryl, alkaryl or aralkyl;
(e) R5 is alkylene;
(f) R6 is alkylene;
(g) m is zero or an integer;
(h) each of R7 and R8 is independently alkyl, alkenyl, aryl, alkaryl or aralkyl, a group of the formula -OR9 or a group of the formula R4-(OR5)m-OR6-; and (i) R is a group of the formula R4-(OR5)m-.
Also disclosed are hydraulic fluids containing the novel compounds.

Description

Thls lnvention relates to certaln novel silane derivatlves which are useful as components of hydraulic flulds and to hydraulic fluids containing such compounds.
Hydraulic 1uids based on glycol ethers have been used in, for example, vehicle brake and clutch systems for many years and still xemain the mo~t commonly used type of fluid. However, specifications of required quality standards laid down by hyaraulic systems manufacturers and non-commercial organisations such as the Society o~ Automotive Engineers and the U S Department of Transportation have become progressively more severe. In particular, a need has arisen ~or fluias having highe~ boiling points and, more importantly, higher vapour lock temperatures both ~or the fluid as formulated by the manufacturers and also for the fluid in the presence o~
water. Glycol ether based fluids are known to be deficient in this respect due to the hygroscopicity of the fluid which results in the absorption of water from the atmosphere.
This in turn reduces the boiling point and vapour lock temperature of the fluid and with extended use the water content of the fluid can build up to a level at which the boiling point and vapour lock temperatures are reduced to a dangerou~ extent. When subjected to heat, e.g. generated by heavy braking, the fluid may boil or vaporise to a su~ficient extent to cause a serious brake malfunction.
Hydraulic fluids having low hygroscopicity have been developed, based on glycol esters, to deal with this problem.
Such fluids are relatively insensitve to the effect of atmospheric moisture, but are more expensive than ~lycol ether based fluids and have certain technical disadvantages, e.g. their viscosity properties are inferior to those of glycol ether based 1uids. Consequently, use of these low hygroscopicity fluids has been mainly limlted to where the

2.

:
desirable properties such as high boiling point and vapour lock temperatures are deemed to outweigh their disadvantages.
Other types of water insensitive fluids have also been developed. Nevertheless, manufacturers are still seeking new fluids which combine as many as possible o~ the desirable properties of both glycol ether based and low hygroscopicity fluids and, desirably, have even higher boiling points and/or vapour lock temperatures than the low hygroscopicity fluids.
Recently, there has emerged a growing tendency in vehicle design to use a single hydraulic system to operate equipment, such as power-steering, shock absorbers and brakes, which hitherto were provided with separate hydraulic systems.
This has created serious problems in the formulation of suitable fluids. The mineral oil based fluids hitherto used in power-steering systems and shock absorbers are satisfactory with respect to the nitrile and chloroprene rubber used for the seals and gaskets in such systems but are highly detrimental to the natural and styrene/butadiene rubbers used in the constructian of hydraulic brake and clutch systems. This ~; 20 results in excessive swelling of the latter seals which can lead to a serious malfunction of the brake or clutch system.
Conver~ely, the fluids hitherto used in brake and clutch systems, which are normally based on glycols, glycol ethers and/or glycol ether esters, and which have operated satisfactorily in such systems, have a detrimenkal effect on the nitrile and chloroprene rubber gaskets used in power-steering systems and shock absorbers which can also lead to malfunctioning.
In the case of vehicle operation the characteristic of reliability in operation, which i~ generally de~irable in all mechanical devices, is increased in importance to an absolutely essential re~uirement by virtue of safety considerations.
The need has therefore arisen for a fluid which can be used .. .,. . , ~ : . . . .

.. . . . ..

-' ~Lo~3593 satisfactorily in a central system controlling the operation of a number of different items of equipment.
We have now found certain novel silicon compounds which are useful -as components of hydraulic fluids, for hydraulic brake and clutch systems and also for central hydraulic systems. These compounds exhibit improved rubber swell properties with respect to a variety of natural and synthetic rubbers used in the construction of hydraulic systems and they are also relatively water insensitive.
Accordingly, the present invention provides as novel ccmpounds, ; 10 silane derivatives having the general formula:
:. Rl ::
R4 - (oR5) - oR6 _ Si - R
OR
wherein:
(a) each Rl and R2 is independently aLkyl, alkenyl, aryl, alk æyl or ` aralkyl, a group of the formula -OR or a group of t~e formLla R -(OR )m~OR ~;
(b) R is a group of the form~la R ~(OR5)m - or a group of the formula:
. R

I g OR
and each R3 may be the same as or different from any other group R3;
(c) R4 is aLkyl, alkenyl, æyl, aLkaryl or æalkyl and each R4 may be the same as or different from any other group R4;
(d) R5 is an alkylene group~and each R5 may be the same as or different from any other group R ;
(e) R6 is an alkylene group and each R6 may be the same as or dlEferent from any other group R ;
(f) m is zero or an integer and each m may be the same as or different from any other m;
(g) each of R7 and R is independently alkyl, alkenyl, aryl, alk æyl or aralkyl L~ , ,, 5g;~

a group of -the fonmula R4-(oR5) _oR6 or a group of the formwla -OR9 pro~ided that only one of R7 and R8 may be a group of the formula -OR9, and (h) R is a group of the formula R4-(oR5) - and each R9 may be the same as or different from any other group R9;
provided that when Rl and R2 are ethoxy or methyl, and R3 is ethyl; R4-(oR5)m-OR - is not a group of the formula C4HgO-C2H4-OC3~I6~ and when R and R are ethoxy and R3 is ethyl, R4-toR5) -oR6- is not a group of the formula In another aspect, of the present invention there is provided a hydraulic fluid comprising from 0.5 to 99~ of at least one compound having the general formNla:

Rl R4-(oR5) - oR6 _ Si - R2 Ia OR
, wherein:
ta) each of Rl and R2 is independently alkyl, aIkenyl, aryl, alkaryl, or araIkyl, a group of the formula -oR3 or a group of the formula R - toR5)m - oR6 -;
(b) R is a group of the formula R - (OR )m ~ or a group of the : formNla:

R
R8 _ Si - tOR )m~

OR
20 and each R3 may be the same as or different from any other group R3;
(c) R4 is aIkyl, aIkenyl, aryl, alkaryl or araIkyl and each R4 may be -.; .
the same as or different from any other group R4;
(d) R5 is an alkylene growp and each R5 may be the same as or different from any other ~oup R ;
- (e) R6 is an aIkylene group and each R6 may be the same as or different from any other group R6;
(f) m is zero or an integer and each m may be the same as or different - 5 - :
~3 L(18~S~;~

from any other m;
(g) each of R7 and R8 is independently alkyl, alkenyl, aryl, aIkaryl or araLkyl, a group of the formula R4 - (oR5) - OR - or a group of the formula -OR9 pro~ided that only one of R7 and R8 may be a group of the ; formula -OR ; and (h) R9 is a group of the formula R - (oR5) - and each R9 m~y be the same as or different from any other group R9.
Preferably, Rl and R2 are independently aIkyl containing from 1 to 18 carbon atoms (especially methyl) alkenyl containing from 1 to 18 ~-carbon atcms, phenyl, alkyl substituted phenyl in which ~he alkyl substituent contains from 1 to 12 carbon atoms, benzyl, a group of the formula -oR3, or a group of the formula R4 - (OR5)m - oR6.
Preferably R4 iS aLkyl containing from 1 to 18 carbon atoms, aLkenyl containing from 1 to 18 carbon atoms, phenyl, alkyl substit~d phenyl in which the aIkyl substituent contains from 1 to 12 carbon atoms or benzyl.
Preferably R iS an alkylene group containing from 1 to 15 ` OE bon atoms, more preferably from 1 to 4 carbon atoms, and especially ethylene or propylene.
Preferably R6 is an alkylene group containing from 1 to 15 carbon atcms, and more preferably from 1 to 6 carbon atoms.
Preferably m is zero or an integer of from 1 to 4.
Preferably R7 and R8 areindependently aIkyl containing from 1 to 18 carbon atoms (especially methyl), alkenyl containing from 1 to 18 carbon atoms, phenyl, alkyl substituted phenyl in which the alkyl substituent con-tains from 1 to 12 carbon atoms, benzyl, a group of the formula R4 - (OR5)m -oR6-, or a group of the formula -OR9 provided that only one of R7 and R8 may be a group of the formwla -OR9.
It is preferred that the silane derivatives of the invention con-tain no more than 2 silicon atoms.
' ~:

- 5a -~`''~
.
: . .
.

513~;~3 In the case of silane derivatives for use ln hydraulic brake and clutch systems it is preferable for any terminal alkyl groups present to be relatively short chain alkyl groups, e.g. containlng from 1 to ~, more pre~erably 1 or 2, carbon atoms, in order to minimise the rubber swelling effect on the seals and gaskets used in such systems. However when used in a central system it may be more desirable to effect a compromise between the requirements, often confllcting, for each of the various seal and gasket materials. In this case some, ox all, of the terminal alkyl groups may be longer chain alkyl groups, e.g. up to 6, or even 8, carbon atoms.
Furthexmore, in the case of fluids based on mineral oil, even longer chain terminal alkyl groups, e.g. containing up to 16, or even 18, carbon atoms, may be necessary incrder to effect oil solubility. The terminal alkyl groups may be straight or branched chain but for oil solubility, particularly in mineral oil, branched chain alkyl groups are preferred.
The silane derivatives of the present inventlon may be readily prepared from appropriate haloalkyl silanes using well-known techniques.
The silane derivatives of the invention may be used ih hydraulic fluids as an additive, as a base stock or as a component of a blend of base stocks. The proportions employed may therefore vary over a very wide range e.g. from 0.5 to 99~ by weight based on the total weight of the hydraulic ~ fluid. When used as a base stock the silane derivatives ; will constitute the bulk of the hydraulic fluld, ~or example from 75% or 80% to 99~ by weight, based on the total weight of the hydraulic fluid. ~he remainder of the hydraulic fluid may be composed of conventional hydraulic fluld additives and/or small quantities of ot~ hydraulic fluid base-stocks.
When used as a component of a blend of base stocks ~.~f33S9;~

the total blend of base stocks wlll llkewise constitute the bulk of the hydraulic fluld. In this case, the base stocks may be predominantly one or more silane derivatives blended with a lesser quantity of one or more other base stocks so as to modify the properties of the silane derivatives. Thus, the hydraulic fluid may contain, for example 55% to 75% by weight of one or more silane derivatives based on the total weight of the hydraulic fluid. Alternatively, one or more other base stocks may be modified by blending with a lesser quantity of silane derivatives so that the hydraulic fluid contains, e.g. from 20% to 40% by weight silane derivative.
In addition, a compromise between the properties of the silane derivatives and the other fluids may be effected by blending in approximately equal quantities to provide fluids containing from 40% to 55% silané derivative.
When used to suppress the sensitivity of hydraulic fluids, and in particular the boiling point and vapour lock temperatures of the fluids, to water the silane derivatives ; are preferably used in amounts in the range of 20% to 55%, more preferably 20% to 40%. Alternatively but less preferably, an improvement can also be obtained using lower amounts of the silane ~erivatives, e.g. from 0.5% to 15% or 20% by weight based on the total weight of the hydraulic fluids. The bulk of such fluids will be constituted by one or more base stocks such as hereinafter described.
When the silane derivatives are used as a component ` of a blend of base stocks the resulting hydraulic fluids may -~ contain conventional hydraulic fluid additives in llke manner as when the base stock substantially consists o~ the silane derivatives. Similarly, when used as an additive the silane derivatives may, i~ desired, be used in conjunction with conventional hydraulic fluid additives.

.
.
.... .;

35~33 Co~ventional additives are normally employed in small amounts such as 0.05% to 10%, for example, 0.1 to 2%
by welght.
Base stocks with which the silane derivatives may s be blended, or with which they may be used as additives, include mineral oil, polyoxyalkylene ylycols and ethers thereof, alkyl and polyoxyalkylene glycol ether esters o~ mono-, di or poly carboxylic acids or boric aci~, formals, acetals, phosphate esters, silicones, monocarboxylic acid esters of di- or polyalcohols and similar fluids well known in the art.
In a particular aspect of the invention there is provided a hydraulic fluid consisting essentially of a com-bination of from 5 to 30~ by weight of at least one silane derivative of Formula I as hereinbefore defined and from 5 to 30% by weight of at least one compound having the general formula:-OR12 Rll ORlZ oR12 _ Si - OR12or Rll _ Si - 0R12 or Rll _ Si - 0 - Si -oR12 oR12 l13 R13 ; 20 II III IV
- wherein Rll is alkyl, preferably containing from l to 18 carbon atoms, or~aryl, preferably phenyl; Rl2 is alkyl, preferably containing from l to 18 carbon atoms, aryl, preferably phenyl, alkaryl, preferably alkyl substituted phenyl in which the alkyl substituent contains from 1 to 12 carbon atoms, or aralkyl, preferably benzyl, or a group of the formula R4-(oR5)m-; R13 is alkyl, preferably containing from 1 to 18 carbon atoms, aryl, preferably phenyl, alkaryl, preferably alkyl substituted phenyl in which the alkyl substituent contains from l to 12 carbon atoms, or aralkyl, preferably benzyl, or a group of the formula R4-(oRS)m-o-; and R4, R5 and m are as hereinbefore defined, in a glycol ether base stock.

: .... ~ ,. .

lU513S93 In a further particular aspect o~ the invention there is provided a hydraulic fluid consisting essentially of from 10 to 90% by weight of at least one silane derivative of Formula I as hereinbefore defined and from 90 to 10%
by weight of at least one compound of Formula II, III or IV
as hereinbefore defined.
Regardless o~ precise composition it is highly desirable that the hydraulic fluids of the present lnvention have a kinematic viscosity at -40C of not more than 5,000 cSt, especially not more than 2,000 cSt. It is also desirable that the hydraulic fluids have a boiling point of at least 260C.
In a further aspect of the present invention there is provided a hydraulic system for transmitting power by hydraulic means which system contains as the functional fluid, a hydraulic fluid as hereinbefore described.
In yet another aspec~ of the present invention there is provided a method of operating a hydraulic system which comprises introducing into the hydraulic system a hydraulic fluid as hereinbefore described and transmitting power by applying pressure to the hydraulic fluid.
The present invention will now be illustrated with refere~ce to the following Exam~les:-Example 1 Tris(methoxy)-3-chloropropyl silane (794 g; 4 moles) and methyl ~-riglycol (2296g; 14 moles) were heated under nitrogen cover in a glass flask fitted with a 1 ft packed column until 188.1 g of MeOH had been removed from the mixture.
The column was removed and heatin~ continued at a bottom temperature of 200C until 317.4 g (calc. 384) had been obtained.
Sodium (101.2 g; 4.4 g atom) was dis~olved in methyl triglycol (1000 g, 6.1 mole) and the resultant slurry added to the reaction mixture which was maintained a-t 100C

9.

~J83593 for 3 hours. The product was ~lltered, stripped under 0.1 mm Hg at a bottom temperature of 180C, and reflltered.
2660 g (92%) of a dark liquld was obtained con-talnlng 4.20% Si~ (calc. 3.88% Si for tris (methyltriglycol)-

3-methyltriglycolpropyl silane). The product had a boiling point of 320C and a vi~cosity at -40C of 2306 cS. Good '~ results were obtained with rubber swell tests on SBR G9 ; (4~2%) and Natural R32 (-1.0%).
Example 2 Tris (methyldiglycol)-3-methyldiglycolpropyl silane was prepared in similar manner from tris(methoxy)-3-chloro-propyl sllane (198.5 g; 1 mole), methyl diglycol (396 g;
3.3 mole) and sodium (25 g; 1.1 g atom) in methyl diglycol (400 g; 3.4 mole).
77 g (calc. 96 g) of MeOH was collected in the first stage. 409 g (75%) of product was obtained containing 6.092% Si (calc. 5.12) and 0.2% chlorine. The product had a viscosity at -40C of 1051 cS, a boiling point of 310C. and ; gave good results,in rubber swell tests: SBR G9 8.9~ and Natural R32 2.0~.
Example 3 Tris (tridecanoxy)-4-trid~canoxypropyl silane was prepaxed in similar manner from 3-chloropropyl-trimethoxy silane (397 g; 2 mole)and tridecanol (1400 g; 7 mole) and p-toluene sulphonic acid (0.2 g) and sodium (50.6 g;
2.2 g atom) in tridecanol (800 ml, 4 mole). 168 g (calc.
192) methanol was obtained. 711 g (82%) of product was obtained containing 3.46% Si (calc. 3.23).
A blend of this product (50%) in a DTD 585 mineral 30 oil (50~) had a -40C viscos:lty of 2313 cS. The vapour lock - temperature after heating with 0.5~ H20 at 100C for 72 hours was 225C.

,~ 10.

` 1()~35~3 Exampl e ~ 4 to 2 6 Hydraullc flulds ln accordance with the invention were subjected to one or more o~ the following tests:-(a~ Kinematic viscoslties at -40C, in centistokes (CS), were measured ln the manner set forth ln the SAE J 1703c specification.
(b) Rubber swell properties were evaluated for s~yrene/butadiene (SBR), nitrile and ethylene/propylene rubbers. In the case of SBR, the test was carried out using standard SAE SBR cups in the manner set forth in the FMVSS 116 DOT 3/4 specification. Rubber swell properties with respect to nitrile rubber were determined by measuring the increase in volume of a 2.54 cm square, 2 mm thick nitrile rubber specimen in 50 mls of test fluid at 120C for 70 hours. The test with respect to ethylene/propylene rubber was carried out in the same manner as for nitrile rubber but using ethylene/propylene rubber ring seals (as used in the aviation industry~.
(c) Vapour lock temperatures were determined after prior subjection of the fluids to a Humidity Test s~bstantially as described in the FMVSS 116 specification, but without a reference fluid. In the case of fluids comprising a glycol ether base stock, the vapour lock temperatures were determined by the Markey Vapour Lock Test carried out in the apparatus and in the manner as laid down in the SAE J 1705 specification for sillcone brake fluids. Vapour lock temperature~ for non-glycol ether based fluids were determined by the Gilpin Vapour Lock Test as specified ln SAE Paper 710 253 entitled "Operatlng Performance of motor vehlcle ..

11 .

.. . .
' . , `` 9 ~835~3 brakin~ systems as affected by fluid w~ter con-tent", the Gilpin vapour-lock temperature being taken to be the temperature corresponding with the appearance of 3 ml of bubbles.
(d) Hydrolyticstabilities were determined by heating the test fluid + 10% water in a sealed ampoule for 24 hours at 100C and thereafter, on cooling, noting any gelling or separation. (Silanes of Formulae II-IV alone or in admixture with glycol ether base stocks normally gel under these conditions.) Details of the fluids tested and of the results obtained are given in Tables 1-3.
The abbreviations and commercial products referred to in Tables 1-3 are as follows:-Butyl Monoglycol - Ethylene glycol mono butyl ether D P M - Dipropylene glycol mono methyl ether M T G - Triethylene glycol mono methyl ether M D G - Diethylene glycol mono methyl ether Mineral Oil A _ Naphthenic mineral oil having a viscosity of ~ 130 cS @ -40F., 3.5 cS
@ 100F., and 1.31 cS
@ 210~F.j pour point -700F., Boiling point 248 C. Flash point 208~C.
aniline pt. 76~C.
Mineral Oil D ~ Ditridecyl dodecanedioate Refrigerant Oil B - Commercially available - refrigerant oil manufactured ; by Briti~h Petroleum under the trade mark ZERICE S 53 and believed to be a mixture of alkylated benzenes.
12.

`: ~O~ig3 ,..~
Ra~rigerant Oil C - ~ blend o naphthenic -: mlneral oils:relakive density 0.920, Viscosity at 100uF. 63.0 cS, Viscosity at 210VF.
6.1 cS, Open Flash point 183~C., pour point -36~C.
Silicone Fluid - Commercially available silicone brake fluid supplied by Dow Corning under the designation 02-1062.
: E 555 - Commercially available ethylene/propylene glycol ether supplied by Dow Chemical Company having :- a molecul~r weight of about 243 and wherein the terminal ether alkyl . groups are believed to : be predominantly methyl but with a proportion being ethyl.
A 79 Nitrile Rubber - Commercially available nitrile rubber - as ~LJ used in Girling~brake systems.
r~ ema~-~

.

: ` ' , ~
~ ' ~`?

: 13.

i~ .. . ~ . .. ....

~33~ 3 :~

¦ ~ ~ I ~ IN ~ IN D la~ ¦ O IN
~ _ _ ~ ~ ~ oo a~ ~ 03 ~ ~o ~ _~
m ~ o ~ o ~ ~ o ~ u~ ~ ~ a:
H ~ ~ ~ ~ H ~;;

r~ h 0 ~ ~ ~ . ~1 .~

u h h ô ~ ô ~ o o o R ~ f ~

rn ~ ~ ~ ~ r~ ~3 ~ r~ ~ .

. M F~ r~ ~ ~ r-~ rh c-~ r~ r~l ~ ~ ~ ,.~ ~, ~ a) o~ () , ~
~- ~ - - -10~$g3 ~ Q~ r~
o~ ~ t ~o s~ ~ ~ o ~ .~ .
. _ , Xa ~'~ ~ O O N d- CO ~~ 1--l i ~
,~ O HI--I ~J1--I ~ ~1 CO ~ ~ _ H _ _ . _ .

~ H . .

~1 ~ ~ ~ ~10~
r ~ R
., ._ __ , a R a ~ R ~ ~ ~ L i ~

~ ~ I .1 0 -- o ,, hH ~ tl ~1 .1 N ~ O
~ _ ___ ~ __ ___ (~D~
~3 ~LI r; r~ r-l rl r -I r I N N E-i ,: , ' TAB~. S
~ .
EXAMP~E TI~ST FLUID ~YDRO~YTIC S'rABI~ITY

-- . .~ . I ~__ , 22 ~i3 (E555~-dimethyl silane - 10 Clear, mobile, (MDG)(3-MDG propyl)silane no sediment . ..... , .......... . ~ .
23 Tris 5DPM)-methyl silane - 40~ Mobile - not star Tris (DP~X3-DPM propyl)silane bright, some deposit - 60% believed due to irnpurity of products, : otherwise s~t~sfactory 24 ~''Tri~ ~DPM)-me'~hyl silane - 10% No separation, slightl~
Tris (2-ethylhexanoxy)-3-(2- cloudy, no gelling : ethylhexanoxy) propyl silane - 90%
- 25--- Bis ~E555)-dimethyl silane - ?~ ~ Satisfactory Tris (MDG)(3 MDG propyl~silane : Triethyleneglycolmonoethylether - 60%
26 Bis (E555)-dimethyl silane - 20 ~ Satisfactory ~
: . Tris (MDG)(3-MDG propyl)silane ~ Triethylene glycol monoethyl : ether - 60%
I_ .':
15.

:`
. ' , - , , , , -, .. . .

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A silane derivative having the general formula:- I

wherein:

(a) each of R1 and R2 is independently alkyl, alkenyl, aryl, alkaryl or aralkyl, a group of the formula -OR3 or a group of the formula R4-(OR5)m-OR6-;
(b) R3 is a group of the formula R4-(OR5)m- or a group of the formula:- and each R3 may be the same as or different from any other group R3;
(c) R4 is alkyl, alkenyl, aryl, alkaryl or aralkyl and each R4 may be the same as or different from any other group R4;
(d) R5 is an alkylene group and each R5 may be the same as or different from any other group R5;
(e) R6 is an alkylene group and each R6 may be the same as or different from any other group R6;
(f) m is zero or an integer and each m may be the same as or different from any other m;
(g) each of R7 and R8 is independently alkyl, alkenyl, aryl, alkaryl or aralkyl, a group of the formula 17.

R4 - (OR5)m - OR6 or a group of the formula -OR9 provided that only one of R7 and R8 may be a group of the formula -OR9, and (h) R9 is a group of the formula R4 - (OR5)m- and each R9 may be the same as or different from any other group R9;
provided that when R1 and R2 are ethoxy or methyl, and R3 is ethyl; R4 - (OR5)m - OR6 - is not a group of the formula C4H90-C2H4-OC3H6, and when R1 and R2 are ethoxy and R3 is ethyl, R4 - (OR5)m - OR6 - is not a group of the formula C2H5-0-C2H4.

2. A compound as claimed in claim 1 wherein:
(a) each of R1 and R2 is independently alkyl containing from 1 to 18 carbon atoms, alkenyl containing from 1 to 18 carbon atoms, phenyl, alkyl substituted phenyl in which the alkyl substituent contains from 1 to 12 carbon atoms or benzyl, a group of the formula -OR3 or a group of the formula R4 - (OR5)m - OR6-;
(b) R3 is a group of the formula R4 - (OR5)m - or a group of the formula:

and each R3 may be the same as or different from any other group R3;
(c) R4 is alkyl containing from 1 to 18 carbon atoms, alkenyl contain-ing from 1 to 18 carbon atoms, phenyl, alkyl substituted phenyl in which the alkyl substituent contains from 1 to 12 carbon atoms or benzyl and each R4 may be the same as or different from any other group R4;
(d) R5 is an alkylene group containing from 1 to 15 carbon atoms and each R5 may he the same as or different from any other group R5;
(e) R6 is an alkylene group containing from 1 to 15 carbon atoms and each R6 may be the same as or different from any other group R6;
(f) m is zero or an integer of from 1 to 4 and each m may be the same as or different from any other m;
(g) each of R7 and R8 is independently alkyl containing from 1 to 18 carbon atoms, alkenyl containing from 1 to 18 carbon atoms, phenyl, alkyl substituted phenyl in which the alkyl substituent contains from 1 to 12 carbon atoms or benzyl, a group of the formula R4 - (OR5)m - OR6-, a group of the formula -OR9 provided that only one of R7 and R8 may be a group of the formula -OR9; and (h) R9 is a group of the formula R4- (OR5)m - and each R9 may be the same as or different from any other group R9.

3. A compound as claimed in claim 1 or claim 2 wherein R3 is a group of the formula R4 - (OR5)p, wherein p is an integer with a value of at least 1.

4. A compound as claimed in claim 1 or 2 wherein m is an integer of at least 1.

5. A compound as claimed in claim 1 or claim 2 wherein each m is an integer with a value of at least 1.

6. A compound as claimed in claim 2 wherein each of R1 and R2 is methyl.

7. A compound as claimed in claim 2 wherein R5 is alkylene containing from 1 to 4 carbon atoms.

8. A compound as claimed in claim 7 wherein R5 is ethylene or propy-lene.

9. A compound as claimed in claim 7 wherein R6 is alkylene containing from 1 to 6 carbon atoms.

10. A compound as claimed in claim 9 wherein R6 is propylene.

11. A compound as claimed in claim 2 wherein each of R7 and R8 is methyl.

12. A compound as claimed in claim 2 which contains a maximum of 2 silicon atoms.

13. Tris(triethyleneglycolmonomethylether)-3-triethyleneglycolmono-methyletherpropyl silane.

14. Tris(diethyleneglycolmonomethylether)-3-diethyleneglycolmono-methyletherpropyl silane.

15. Tris(tridecanoxy)-3-tridecanoxypropyl silane.

16. Tris(ethyleneglycolmonobutylether)-3-ethylene-glycolmonobutyl-etherpropyl silane.

17. Tris(dipropyleneglycolmonomethylether)-3-dipropyleneglycolmono-methyletherpropyl silane.

18. Bis(triethyleneglycolmonomethylether)-3-triethyleneglycolmono-methyletherpropyl-methyl silane.

19. Triethyleneglycolmonomethylether-dimethyl-triethyleneglycolmono-methylethermethyl silane.

20. Tris(2-ethylhexanoxy)-3-(2-ethylhexanoxy) propyl silane.

21. Tris(dipropyleneglycolmonomethylether)-3-dipropyleneglycolmono-methylether-2-methylpropyl silane.

22. A hydraulic fluid comprising from 0.5 to 99% of at least one com-pound having the general formula:

Ia wherein:

(a) each of R1 and R2 is independently alkyl, alkenyl, aryl, alkaryl or aralkyl, a group of the formula -OR3 or a group of the formula R4 - (OR5)m - OR6 -;
(b) R3 is a group of the formula R4 - (OR5) - or a group of the formula:

and each R3 may be the same as or different from any other group R3;
(c) R4 is alkyl, alkenyl, aryl, alkaryl or aralkyl and each R4 may be the same as or different from any other group R4;
(d) R5 is an alkylene group and each R5 may be the same as or different from any other group R5;
(e) R6 is an alkylene group and each R6 may be the same as or different from any other group R6;
(f) m is zero or an integer and each m may be the same as or different from any other m;
(g) each of R7 and R8 is independently alkyl, alkenyl, aryl, alkaryl or aralkyl, a group of the formula R4 - (OR5)m - OR6 - or a group of the formula -OR9 provided that only one of R7 and R8 may be a group of the formula -OR9; and (h) R9 is a group of the formula R4 - (OR5)m - and each R9 may be the same as or different from any other group R9.

23. A hydraulic fluid as claimed in claim 22, wherein when R1 and R2 are ethoxy or methyl, and R3 is ethyl, R4 - (OR5)m - OR6 - is not a group of the formula C4H9O-C2H4-OC3H6- and when R1 and R2 are ethoxy, and R3 is ethyl, R4 - (OR5)m - OR6 - is not a group of the formula C2H5-0-C2H4.

24. A hydraulic fluid as claimed in claim 22 wherein:
(a) each of R1 and R2 is independently alkyl containing from 1 to 18 carbon atoms, alkenyl containing from 1 to 18 carbon atoms, phenyl, alkyl substituted phenyl in which the alkyl substituent contains from 1 to 12 carbon atoms or benzyl, a group Or the formula -OR3 or a group of the formula R4 - (OR5)m - OR6-;
(b) R3 is a group of the formula R4 - (OR5)m- or a group of the formula:

and each R3 may be the same as or different from any other group R3;
(c) R4 is alkyl containing from 1 to 18 carbon atoms, alkenyl containing from 1 to 18 carbon atoms, phenyl, alkyl substituted phenyl in which the alkyl substituent contains from 1 to 12 carbon atoms or benzyl and each R4 may be the same as or different from any other group R4;
(d) R5 is an alkylene group containing from 1 to 15 carbon atoms and each R5 may be the same as or different from any other group R5;
(e) R6 is an alkylene group containing from 1 to 15 carbon atoms and each R6 may be the same as or different from any other group R6;
(f) m is zero or an integer of from 1 to 4 and each m may be the same as or different from any other m;
(g) each of R7 and R8 is independently alkyl containing from 1 to 18 carbon atoms, alkenyl containing from 1 to 18 carbon atoms, phenyl, alkyl substituted phenyl in which the alkyl substituent contains from 1 to 12 carbon atoms or benzyl, a group of the formula R4 - (OR5)m - OR6-or a group of the formula - OR9 provided that only one of R7 and R8 may be a group of the formula -OR9; and (h) R9 is a group of the formula R5 - (OR5)m - and each R9 may be the same as or different from any other group R9.

25. A hydraulic fluid as claimed in claim 22 comprising at least one conventional hydraulic fluid additive and/or base stock.

26. A hydraulic fluid as claimed in claim 22 consisting essentially of a combination of from 5 to 30% by weight of at least one compound of Formula Ia as claimed in claim 22 and from 5 to 30% by weight of at least one compound having the general formula:

or or II III IV
II III IV
wherein R11 is an alkyl or aryl group, R12 is an alkyl, aryl, alkaryl or aralkyl group or a group of the formula R4-(OR5)m-, R13 is an alkyl, aryl, alkaryl or aralkyl group or a group of the formula R4-(OR5)m-0-, R4 is alkyl, alkenyl, aryl, alkaryl or aralkyl, R5 is alkylene and m is zero or an integer, in a glycol ether base stock.

27. A hydraulic fluid as claimed in claim 22 consisting essentially of from 10 to 90% by weight of at least one compound of Formula Ia as defined in claim 22 and from 90 to 10% by weight of at least one compound of formula II, III or IV as defined in claim 26.

28. A hydraulic fluid as claimed in claim 22 comprising at least one compound of Formula Ia as defined in claim 22 and at least one compound of Formula II, III or IV as defined in claim 26.