GB1577715A

GB1577715A - Hydraulic fluids

Info

Publication number: GB1577715A
Application number: GB48009/75A
Authority: GB
Original assignee: Castrol Ltd
Current assignee: Castrol Ltd
Priority date: 1975-11-21
Filing date: 1975-11-21
Publication date: 1980-10-29
Also published as: DK154024C; NZ182641A; NO763957L; NL174467B; SE7612899L; FR2332281A1; IE44233L; FR2332281B1; IT1121743B; SE431987B; CH629250A5; DK523976A; AU506008B2; DK154024B; DE2652719C2; JPS6019318B2; NO155623C; IE44233B1; ZA766833B; NL7612915A

Abstract

Hydraulic fluids are described which contain at least one silane derivative of the formula I <IMAGE> and at least one further component. In the silane derivatives, the substituents are as defined in Patent Claim 1. The silanes used in the hydraulic fluids do not cause natural rubber or synthetic rubber to swell even during prolonged contact and are therefore suitable for producing hydraulic systems which must meet extreme safety demands, for example in braking systems.

Description

(54) HYDRAULIC FLUIDS (71) We, CASTROL LIMITED, a British Company, of Burmah House, Pipers Way, Swindon, Wiltshire, SN3 1RE, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to hydraulic fluids.

Hydraulic fluids based on glycol ethers have been used in, for example, vehicle brake and clutch systems for many years and still remain the most commonly used type of fluid. However, specifications of inquired quality standards laid down by hydraulic systems manufacturers and non-commercial organisations such as the Society of Automotive Engineers and the U.S. Department of Transportation have become progressively more severe. In particular, a need has arisen for fluids having higher boiling points and, more importantly, higher vapour lock temperatures both for the fluid as formulated by the manufacturers and also for the fluid in the presence of 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 tum 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 dangerous extent. When subjected to heat, e.g. generated by heavy braking, the fluid may boil or vaporise to a sufficient 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 insensitive to the effect of atmospheric moisture, but are more expensive than glycol ether based fluids and have certain technical disadvantages, e.g. their viscosity properties are inferior to those of glycol ether based fluids. Consequently, use of these low hygroscopicity fluids has been mainly limited to where the 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 of 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 highlly detrimental to the natural and styrene/butadiene rubbers used in the construction of hydraulic brake and clutch systems. This results in excessive swelling of the latter seals which can lead to a serious malfunction of the brake or clutch system. Conversely, 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 detrimental effect on the nitrile and chioroprene rubber gaskets used in power-seeering systems and shock absorbers which can also lead to malfunctioning. In the case of vehicle operation the characteristic of reliability in operation, which is generally desirable in all mechanical devices, is increased in importance to an absolutely essential requirement by virtue of safety considerations. The need has therefore arisen for a fluid which can be used satisfactorily in a central system controlling the operation of a number of different items of equipment.

We have now found that certain silicon compounds 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.

According to one aspect of the present invention, there is provided a hydraulic fluid comprising from 0.5 to 99% by weight of one or more silane derivatives having the general formula

wherein: (a) R is a group of the formula R4-(OR')-OR6-; (b) each of R1 and R2 is independently alkyl, preferably containing from 1 to 18 carbon atoms, more preferably methyl; alkenyl, preferably containing from 1 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; a group of the formula -OR3; or a group of the formula R4-O(R5)m-OR6-; (c) R@ is a group of the formula R4-(OR')- or a group of the formula

and each R3 may be the same as or different from any other group R3; (d) R4 is alkyl, preferably containing from 1 to 18 carbon atoms; alkenyl, preferably containing from 1 to 18 carbon atoms; alkyl, preferably phenyl; alkaryl, preferably alkyl substituted phenyl in which the alkyl substituent contains from 1 to 12 carbon atoms; or aralkyl, preferably benzyl; and each R4 may be the same as or different from any other group R4; (e) R5 is an alkylene group, preferably containing from 1 to 15, more preferably 1 to 4, carbon atoms, especially ethylene or propylene; and each R5 may be the same as or different from any other group R5; (f) R is an alkylene group, preferably containing from 1 to 15, more preferably 1 to 6, carbon atoms especially propylene; and each R6 may be the same as or different from any other group R6; (g) m is zero or an integer, preferably zero or an integer of from 1 to 4; and each m may be the same as or different from any other m; (h) each of R' and R9 is independently alkyl, preferably containing from 1 to 18 carbon atoms, more preferably methyl; alkenyl, preferably containing from 1 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; 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 (i) R9 is a group of the formula R4-(OR5)- and each R9 may be the same as or different from any other group R9.

It is preferred that the above-defined silane derivatives of formula I contain no more than 2 silicon atoms.

Certain of the compounds of formula I are novel and those compounds are claimed per se in our co-pending Application No. 7 927 517 (Serial No. 1 577 716).

Specific Examples of silanes of the formula I are those wherein R8 is a group of the formula R4(OR5)p, wherein p is an integer with a value of at least 1, and those wherein R is a group of the formula R4-(OR5)p-OR6 wherein R4, R5, R6 and p are each as defined above.

In the case of silane derivatives for use in hydraulic brake and clutch systems it is preferable for any terminal alkyl groups present to be relatively short chain alkyl groups, e.g. containing from 1 to 4, more preferably 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 conflicting, for each of the various seal and gasket materials. In this case some, or all, of the terminal alkyl groups may be longer chain alkyl groups, e.g. up to 6, or even 8, carbon atoms. Furthermore, 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 in order 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 invention may be readily prepared from appropriate haloalkyl silanes using well-known techniques.

The silane derivatives of the invention may be used in 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 (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 fluid, for example from 75 % or 80% to 99% by weight, based on the total weight of the hydraulic fluid. The remainder of the hydraulic fluid may be composed of conventional hydraulic fluid additives and/or small quantities of other hydraulic fluid base-stocks.

Wl1en used as a component of a blend of base stocks the total blend of base stocks will likewise constitute the bulk of the hydraulic fluid. 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% silane 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 derivatives, 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 or a blend of base stocks the resulting hydraulic fluids may contain conventional hydraulic fluid additives in like manner as when the base stock substantially consists of the silane derivatives. Similarly, when used as an additive the silane derivatives may, if desired, be used in conjunction with conventional hydraulic fluid additives.

Conventional additives are normally employed in small amounts such as 0.05% to 10%, for example, 0.1 to 2% by weight.

Base stocks with which the silane derivatives may be blended, or with which they may be used as additives, include hydrocarbon oils, e.g. mineral oil, polyoxyalkylene glycols and ethers thereof, alkyl and polyoxyalkylene glycol ether esters of mono-, di or poly carboxylic acids or boric acid, formals, acetals, phosphate esters, silicones, monocarboxylic acid esters of di- or polyalcohols and similar fluids well known in the art.

Particularly suitable for use in conjunction with the compounds of formula I in hydraulic fluids are compounds of the general formula

wherein Rl' is alkyl, preferably containing from 1 to 18 carbon atoms, or aryl, preferably phenyl; R'2 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 1 to 12 carbon atoms, or aralkyl, preferably benzyl, or a group of the formula R4,(OR5); 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 1 to 12 carbon atoms, or aralkyl, preferably benzyl, or a group of the formula R4-(OR5)-O-; and R4, R0 and m are as hereinbefole defined.

In a particular aspect of the invention there is provided a hydraulic,fluid consisting essentially of a combination 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 of the general formula II, III or IV as defined above, in a glycol ether base stock.

In a further particular aspect of 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 of precise composition it is highly desirable that the hydraulic fluids of the present invention have a kinematic viscosity at --40"C. of not more than 5,000 cSt, especially not more than 2,000 cSt.

In a further aspect of the present invention there is provided a hydraulic system for transrnirting power by hydraulic means which system contains as the functional fluid, a hydraulic fluid as hereinbefore described.

In yet another aspect 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 reference to the following Examples in which Examples 1, 2 and 3 are Examples of methods for the preparation of compounds of Formula I, and Examples 4 onwards are Examples of fluids according to the invention.

Example 1.

Tris(methoxy)-3-chloropropyl silane (794 g; 4 moles) and methyl triglycol (2296 g; 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 heating continued at a bottom temperature of 200"C. until 317.4 g (calc. 384) had been obtained.

Sodium (101.2 g; 4.4 g atom) was dissolved in methyl triglycol (1000 g; 6.1 mole) and the resultant slurry added to the reaction mixture which was maintained at 1000 C. for 3 hours. The product was filtered, stripped under 0.1 mm Hg at a bottom temperature of 1800C., and refiltered.

2660 g (92%) of a dark liquid was obtained containing 4.20% Si, (calc. 3.88% Si for tris(methyltriglycol)-3-methyltriglycolpropyl silane). The product had a boiling point of 320"C. and a viscosity at 400 C. of 2306 cS. Good results were obtained with rubber swell tests on SBR G9 (4.2%) and Natural R32 (1.0%) rubbers, as used in Girling brake systems.

Example 2.

Tris (methyldiglycol)-3-methyldiglycolpropyl silane was prepared in a similar manner from tris(methoxy)-3-chloropropyl silane (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 400C. of 1051 cS, a boiling point of 310 C. and gave good results in rubber swell test: SBR G9 8.90/c and Natural R32 2.0%.

Example 3.

Tris (tridecanoxy)-3-tridecanoxypropyl silane was prepared 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).

Example 4.

A blend of the product of Example 3 (50 /C) in a DTD 585 mineral oil (50%) was prepared. The blend had a -400C. viscosity of 2313 cS. The vapour lock temperature after heating with 0.5% H20 at 1000C. for 72 hours was 22S"C.

Examples 5 to 26.

Hydraulic fluids in accordance with the invention were prepared and were subjected to one or more of the following tests:- (a) Kinematic viscosities at --40"C., in centistokes (cS), were measured in the manner set forth in the SAE J 1703 f specification.

(b) Rubber swell properties were evaluated for styrene/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 1200C. 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 substantially 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 Alarkey Vapour Lock Test carried out in the apparatus and in the manner as laid down in the SAE XJ 1705 Information Report for silicone brake fluids. Vapour lock temperatures for non-glycol ether based fluids were determined by the Gilpin Vapour Lock Test as specified in SAE Paper 710 253 entitled " Operating performance of motor vehicle braking systems as affected by fluid water content", the Gilpin vapour-lock temperature being taken to be the temperature corresponding with the appearance of 3 ml of vapour.

(d) Hydrolytic stabilities were determined by heating the test fluid + 10% water in a sealed ampoule for 24 hours at 1000C 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 or separate 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 &commat; -40 F., 3.5 cS &commat; 1000F., and 1.31 cS &commat; 2100F., Pour point > -700F., Boiling point 248"C., Flash point 208"C. aniline pt. 76"C.

Ester D - Ditridecyl dodecanedioate Refrigerant Oil B - Commercially available refrigerant oil manufactured by British Petro leum under the trade mark ZER ICE S 53 and believed to be a mix ture of alkylated benzenes.

Refrigerant Oil C - A blend of naphthenic mineral oils: relative density 0.920, Viscosity at 100cF. 63.0 cS, Viscosity at 210 F. 6.1 cS, Open Flash point 1830C., pour point -360C.

Silicone Fluid - Commercially available silicone brake fluid supplied by Dow Corn ing under the designation Q2-1062.

E 555 - Commercially available ethylene/ propylene glycol ether supplied by Dow Chemical Company having a molecular weight of about 243 and wherein the terminal ether alkyl groups are believed to be predomi nantly methyl but with a proportion being ethyl.

A 79 Nitrile Rubber - Commercially available nitrile rub ber - as used in Girling brake systems.

ETG - Triethylene glycol mono ethyl ether.

TABLE 1

Rubber Swell % Silane SAE SBR Cup Viscosity Example Derivative 120 C. 70 Hrs. at -40 Markey Vapour Lock No. Silane Derivative Tested in MTG (% Increase) (cS) Temp. ( C.) 5 Tris (butylmonogly col)-3-butyl-monoglycol 15* 20.3 371.2 140.5 propyl silane (slightly cloudy) 6 Tris (DPM) (3-DPM propyl) silane 40 13.2 502.7 141 7 Bis (MTG) (3-MTG propyl) methyl silane 30 6.8 621 152.5 8 MTG-dimethyl-(MTG methyl) silane 20 10.9 252.7 147 9 Tris (MTG) (3-MTG propyl) silane 80 4.2 1360 187 10 Tris (MTG (3-MTG propyl) silane 50 5.8 718 158 11 Tris (MTG) (3-MTG propyl) silane 5 12.4 150 12 Tris (MDG) (3-MDG propyl) silane 60 8.2 462 179 *In ETG. (Compound not soluble in MTG).

TABLE 2

Rubber Swell Gilpin A79 Nitrile Vapour Rubber Viscosity Lock Example Silane Derivative 120 C. 70 Hrs. at -40 Temp.

No. Silane Derivative Tested Base Oil % in Base Oil (% Increase) (cS) ( C) 13 Tris (butylmonoglycol)-3-butyl mono- Mineral 20 16.0 130 190 glycolpropyl silane Oil A 14 Tris (butyl monoglycol)-3-butyl mono- Refrigerant 1 -2.0 280 glycolpropyl silane Oil B 15 Tris (DPM) (3DPM propyl) silane Tributyl 20 15.25* 519.5 126 Phosphate 16 Tris (tridecanoxy) (3-tridecanoxypropyl) Mineral 20 11.14 343 226 silane Oil A 17 Tris (tridecanoxy) (3-tridecanoxypropyl) Mineral 60 12.4 3930 213 silane Oil A 18 Tris (DPM) (3-DPM-2-methylpropyl) Refrigerant 1 8.8 280 silane Oil C 19 Tris (DPM) (3-DPM-2-methylpropyl) Ester D 15 3.7 silane 20 Tris (DPM) (3-DPM-2-methylpropyl) Mineral 15 12.1 215 silane Oil A 21 Tris (2-ethylhexanoxy)-3-(2-ethyl- Silicone 20 324 232 hexanoxy) propyl silane Fluid *Test carried out using Ethylene/Propylene Rubber ring seals (as used in the aviation industry). TABLE 3

Example No. Test Fluid Hydrolytic Stability 22 Bis (E555)-dimethyl silane - 10% Clear, mobile, Tris (MDG) (3-MDG propyl) silane - no sediment 90% 23 Tris (DPM)-methyl silane - 40% Mobile - not star Tris (DPM) (3-DPM propyl)silane bright, some deposit 60% believed due to impurity of products, otherwise satisfactory 24 Tris (DPM)-methyl silane - 10% No separation, slightly Tri s (2-ethyl hexanoxy)-3-(2ethyl- cloudy, no gelling hexanoxy) propyl silane - 90% 25 Bis (E555)-dimethyl silane - 25% Satisfactory Tris (MDG) (3-MDG propyl) silane 15% Triethyleneglycolmonoethylether-60% 26 Bis (E555)-dimethyl silane - 20% Satisfactory Tris (MDG) (3-MDG propyl)silane 20% Triethylene glycol monoethyl ether 60%

Claims

WHAT WE CLAIM IS:-

1. A hydraulic fluid comprising from 0.5 to 99% by weight of one or more silane derivatives having the general formula:

wherein: (a) R is a group of the formula R4-(OR5)m-OR6-; (b) 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-; (c) 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; (d) R4 is alkyl, alkenyl, aryl, alkaryl or aralkyl and each R4 may be the same as or different from any other group R4; (e) R@ is an alkylene group and each R5 may be the same as or different from any other group R5; (f) R6 is an alkylene group and each R6 may be the same as or different from any other group R6; (g) m is zero or an integer and each m may be the same as or different from any other m; (h) 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 (i) 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.

2. A hydraulic fluid as claimed in claim 1, wherein when Rl and R2 are ethoxy, or methyl, and R3 is ethyl, R is not a group of the formula C4H9O-C2H4-OC3H6and when R1 and R2 are ethoxy, and R3 is ethyl, R is not a group of the formula C2H5-O-C2H4.

3. A hydraulic fluid as claimed in claim 1 or claim 2, wherein R3 is a group of the formula R4-(OR5)0 wherein p is an integer with a value of at least 1.

4. A hydraulic fluid as claimed in any one of claims 1 to 3, wherein R is a group of the formula R4-(OR5)p-OR6, wherein R4, R5 and R6 are each as defined in claim 1, and p is as defined in claim 3.

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

6. A hydraulic fluid as claimed in any one of the preceding claims, wherein: (a) R is a group of the formula R4-(OR5)m-OR6-; (b) 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-; (c) R0 is a group of the formula R4-(OR') or a group of the formula:

and each R0 may be the same as or different from any other group R3; (d) R4 is an alkyl containing from 1 to 18 carbon atoms, alkenyl containing from 1 to 18 carbon atoms, phenyl, alkyl substituted phenyl in which the alkyl substi tuent contains from 1 to 12 carbon atoms or benzyl and each R4 may be the same as or different from any other group R4; (e) 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 RS; (f) R6 is an alkylene group containing from 1 to 15 carbon atoms and each R4 may be the same as or different from any other group R6; (g) 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; (h) each of R' 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 (i) 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.

7. A hydraulic fluid as claimed in any one of the preceding claims, wherein each of R' and R2 is methyl.

8. A hydraulic fluid as claimed in any one of the preceding claims, wherein R@ is alkylene containing from 1 to 4 carbon atoms.

9. A hydraulic fluid as claimed in claim 8, wherein R5 is ethylene or propylene.

10. A hydraulic fluid as claimed in any one of the preceding claims, wherein R6 is alkylene containing from 1 to 6 carbon atoms.

11. A hydraulic fluid as claimed in claim 10, wherein Re is propylene.

12. A hydraulic fluid as claimed in any one of the preceding claims, wherein each of R2 and R8 is methyl.

13. A hydraulic fluid as claimed in any one of the preceding claims wherein the or each silane derivative of Formula I contains a maximum of 2 silicon atoms.

14. A hydraulic fluid as claimed in claim 1 wherein the silane derivative of Formula I is the compound named as the silane derivative tested in any one of Examples 4 to 21.

15. A hydraulic fluid as claimed in any one of claims 1 to 14 comprising from 75 to 99% by weight of the one or more silane derivatives of Formula I.

16. A hydraulic fluid as claimed in any one of claims 1 to 14 comprising from 55 to 75% by weight of the one or more silane derivatives of Formula I.

17. A hydraulic fluid as claimed in any one of claims 1 to 14 comprising from 20 to 55% by weight of the one or more silane derivatives of Formula I.

18. A hydraulic fluid as claimed in any one of claims 1 to 14 comprising from Q5 to 20 %by weight of the one or more silane derivatives of Formula I.

19. A hydraulic fluid as claimed in any one of the preceding claims comprising at least one conventional hydraulic fluid additive and/or base stock as hereinbefore specifically mentioned.

20. A hydraulic fluid as claimed in any one of claims 1 to 19 which also comprises at least one compound having the general formula

wherein R" is an alkyl or aryl group, R12 is an alkyl, aryl, alkaryl or aralkyl group or a group of the formula R4(OR )l", R13 is an alkyl, aryl, alkaryl or aralkyl group or a group of the formula R4(OR)rn-O and R4, Rs and mare as defined in claim 1.

21. A hydraulic fluid as claimed in claim 20 consisting essentially of a combina tion of from 5 to 30% by weight of at least one compound of Formula I and from 5 to 30% by weight of the said compound or compounds of formula II, III or IV as defined in claim 20, in a glycol ether base stock.

22. A hydraulic fluid as claimed in claim 20 consisting essentially of from 10 to 90% by weight of at least one compound of formula I and from 90 to 10% by weight of at least one compound of formula II, III or IV as defined in claim 20.

23. A hydraulic fluid substantially as hereinbefore described in any one of Examples 4 to 26.

24. A hydraulic system for transmitting power by hydraulic means which system -contains a hydraulic fluid as claimed in any one of the preceding claims.

25. A method of operating a hydraulic system which comprises introducing into the system a hydraulic fluid as claimed in any one of claims 1 to 19 and transmitting power by applying pressure to the hydraulic fluid.