Vapor lock resistant hydraulic fluids

Abstract

A composition suitable for use as a hydraulic fluid comprises:(a) a lubricating oil preferably in an amount of from 20 to 80% by weight;(b) a minor amount of oil-soluble borate ester;(c) an amine which prevents deposition of oil-insoluble hydrolysis products of the borate ester; and optionally(d) an oil soluble orthoester of the formula: ##STR1## wherein R.sup.7, R.sup.8, R.sup.9 and R.sup.10 are specified groups.

Description

This invention relates to hydraulic fluids and particularly to mineral oil hydraulic fluids.

In hydraulic systems for which mineral oil or certain other fluids are specified as the operative fluid problems arise due to the presence of water. This water may be present as the result of condensation or inadequate drying prior to the filling of the system or subsequently, by diffusing through worn seals or flexible hoses. In either case the vapour lock temperature of the fluid can be reduced to the boiling point of water even by the presence of very minor amounts of water, e.g. of the order of 0.5%. This constitutes a serious deficiency with particularly dangerous consequences in those systems which are brake systems or central hydraulic systems including brake systems.

According to the present invention there is provided a composition suitable for use as a hydraulic fluid which comprises:

(a) a lubricating oil, preferably in an amount of from 20 to 80% by weight based on the total weight of the composition;

(b) a minor amount of oil-soluble borate ester, preferably of the general formula: ##STR2## wherein: (i) R.sup.1, R.sup.2 and R.sup.3 are the same or different and each is an aryl group or a straight or branched chain alkyl group or alicyclic group, preferably containing from 4 to 20 carbon atoms, or is a group of the formula --(R.sup.4 O).sub.n R.sup.5 wherein each R.sup.4 is the same or different and each is an alkylene group, preferably ethylene, propylene or butylene, R.sup.5 is an alkyl group, preferably containing from 1 to 18, more preferably 1 to 4, carbon atoms, or an aryl group, and n is an integer, preferably of from 1 to 10, more preferably of from 2 to 4;

or (ii) R.sup.1 and R.sup.2 are the same or different and as defined above and R.sup.3 is a group of the general formula: ##STR3## wherein R.sup.1 and R.sup.2 are as defined above and R.sup.6 is an alkylene group, preferably containing at least 4, more preferably from 4 to 20, carbon atoms, or is an oxyalkylene radical of the formula --R.sup.4 --OR.sup.4).sub.n wherein R.sup.4 and n are as defined above; or (iii) R.sup.1 is as defined above and R.sup.2 and R.sup.3 together form the group ##STR4## wherein R.sup.1 is as defined above;

(c) an amine which prevents deposition of oil-insoluble hydrolysis products of the borate ester; and optionally

(d) an oil soluble orthoester of the formula: ##STR5## wherein R.sup.7 is hydrogen or an alkyl group, preferably methyl or an aryl, alkaryl, or aralkyl group; R.sup.8, R.sup.9 and R.sup.10 are the same or different, preferably the same, and each is a straight or branched chain alkyl group, preferably containing from 4 to 20 carbon atoms, an aryl, alkaryl, or aralkyl group, or the group --(R.sup.11 --O).sub.m R.sup.12 wherein each R.sup.11 is the same or different and each is an alkylene group, preferably ethylene or propylene; R.sup.12 is an alkyl group, preferably containing from 1 to 20 carbon atoms, or an aryl, alkaryl or aralkyl group; and m is an integer of from 2 to 10.

It is required that the borate ester used in the composition of the invention should be oil-soluble and, in the case of trialkyl borates, oil solubility may be provided by selecting an ester made from straight chain alcohols containing less than 12 carbon atoms or from branched chain alcohols containing up to 24 carbon atoms. In the case of borate esters of the type derived from di- and polyoxyalkylene glycol ethers, those derived from di- and polyoxyethylene glycol ethers are generally insoluble unless at least one of the terminal ether groups is sufficient to solubilise the ester. Alternatively oil-solubility for this latter type of ester may be achieved by incorporating polyoxypropylene or higher polyoxyalkylene radicals into the molecule.

Examples of particularly useful borate esters include

tris (dipropylene glycol monomethyl ether) borate tris (ethylene glycol monobutyl ether) borate tris (triethylene glycol monobutyl ether) borate tris (tripropylene glycol monomethyl ether) borate tri n-decyl borate tri (isotridecyl) borate tri (2-ethyl hexyl) borate tri (3,3-dimethyl butyl) borate

The amine used in the present invention should have a reasonably low vapour pressure consistent with providing a vapour lock temperature in excess of 120.degree. C. The amine used will also depend upon the borate ester used. A simple test for determining whether a particular amine is suitable for preventing deposition of oil-insoluble hydrolysis products of a particular borate ester consists of dissolving the amine and borate in the selected lubricating oil, (in the amounts intended in the final hydraulic fluid), sealing the resulting fluid together with 0.5% by weight of water in a clear glass ampoule and heating at 100.degree. C. for 24 hours and cooling. In this so called "ampoule" test, if the resulting solution is clear and bright then the combination is satisfactory.

Many amines have been found to be suitable, including primary, secondary and tertiary amines, especially those containing a total of at least 5 carbon atoms. Amines which have been found to be particularly useful with a wide range of borates include Primene 81 R and Primene JMT which are commercially available primary amines with two methyl groups on the alpha carbon atom.

Other amines which may be useful are Mannich bases formed by condensation of an amine and formaldehyde with a phenol previously alkylated with di or polyisobutylene; polyisobutenyl succinimides derived from di or polyamines; or amides derived from di or polyalkyl polyamines and polyisbutenyl substituted monocarboxylic acids.

The amounts of components (b), (c) and (d) (when present) may each vary over a wide range. However, it is preferred to use from 1 to 50%, more preferably from 1 to 20%, particularly from 5 to 10% by weight of the borate ester; from 1 to 50%, more preferably from 5 to 30%, particularly from 10 to 20% by weight of the orthoester; and from 0.5 to 20%, more particularly from 1 to 10%, by weight of amine, the percentages being based on the total weight of the composition.

In compositions containing orthoester, the ratio of orthoester to borate may vary, for example, from 10:1 to 1:10 by weight but in general more orthoester than borate will be used, the preferred ratio varying from 5:1 to 2:1 by weight.

The ratio of borate to amine will depend upon the nature of the two particular compounds used but it is preferred to use from 5:1 to 1:1 by weight.

The lubricating oil used as base fluid in the compositions of the invention is preferably a mineral oil but may also be a synthetic hydrocarbon oil, a synthetic carboxylic acid ester or mixture thereof, a siloxane or phosphate ester or other well known synthetic lubricant.

The invention will now be illustrated by the following Examples:

Example 1

______________________________________Mineral oil blend 85%Tris(tripropylene glycol 10%monomethyl ether)boratetridecyl borate 3%Primene JMT 2%______________________________________

This composition has the following physical characteristics: Boiling point 257.degree. C., Viscosity at -40.degree. C. 1328cS, Viscosity index 218. The composition was subjected to the Markey vapour-lock test in the dry state and after contamination with varying amounts of water, with the following results:

______________________________________Water present (wt. %) dry 0.2% 0.5% 0.75%Vapour lock temperature 253 238 178 140(.degree.C.)______________________________________

The mineral oil alone with 0.5% water had a vapour lock temperature of 101.degree. C.

Examples 2 to 32

Further blends comprising various combinations of borate ester and amine inmineral oil were formulated and subjected to the Gilpin vapour-lock test. Details of these blends and of the results obtained are given in Table 1.

The base fluid used in each case was a naphthenic mineral oil having the following characteristics:

______________________________________Viscosity 130cS at -40.degree. C., 3.5cS at 100.degree. F. and 1.31cS at 210.degree. F.Pour point <-70.degree. F.Boiling Point 248.degree. C.Flash Point (closed) 208.degree. C.Aniline Point 76.degree. C.______________________________________

The Gilpin vapour-lock test was conducted in a Gilpin apparatus and by the Gilpin method as described in S.A.E. Paper 710 253 entitled "Operating performance of motor vehicle braking systems as affected by fluid water content." The Gilpin vapour-lock temperature (VLT) was taken to be the temperature which corresponded with the appearance of 3 ml of bubbles.

Examples 33 to 47

Gilpin vapour lock temperatures were evaluated (in the manner described above) for a range of compositions containing different combinations of borate ester and lubricating oil base fluid, Primene JMT being used as theamine component in each case. Details of these compositions and of the results obtained are given in Table 2.

Example 48

______________________________________Tris - (tridecyl)orthoformate 10%Tris(dipropylene glycol monomethyl- 10%ether)boratePrimene JMT 5%Mineral Oil 75%______________________________________

This blend conformed with the base oil requirements of Specification DTD 585. The Gilpin vapour-lock temperature was 177.degree. C. after heating at 100.degree. C. for 24 hours with 0.5% water in a sealed glass ampoule.

Example 49

______________________________________Tris(tridecyl)orthoformate 20%Tris(dipropylene glycol monomethyl 5%ether)boratePrimene JMT 5%Mineral Oil 70%______________________________________

This blend also conformed to the base oil requirements to DTD 585 Specification and the Gilpin (3 ml) wet vapour-lock temperature was 203.degree. C.

Example 50

______________________________________Tris(tridecyl)orthoformate 20%Tris(dipropylene glycol monomethyl) 5%boratePrimene JMT 3%Mineral Oil 72%______________________________________

This blend also conformed to the base oil requirements of the DTD 585 Specification and the Gilpin (3 ml) wet vapour-lock temperature was 206.degree. C.

Example 51

______________________________________Tris(tridecyl)orthoformate 20%Tris(dipropylene glycol monomethyl 5%ether)borateTris(tridecyl)borate 2%Primene JMT 3%Mineral Oil 70%______________________________________

This blend conformed to the base oil requirements of the DTD 858 Specification and has a Gilpin (3 ml) wet vapour-lock temperature of 205.5.degree. C.

Examples 52 to 81

Further blends containing orthoester were formulated from a range of different orthoesters and borate esters. In each case Primene JMT was usedas the amine component and the base fluid was the naphthenic mineral oil used in Examples 2 to 32. Samples of these blends were subjected to the Gilpin (3 ml) vapour-lock test (i) after reaction with 0.5% water at 100.degree. C. for 24 hours and (ii) after subjection to a humidity test at a Relative Humidity (RH) of 80% and temperature of 22.degree. C. substantially as described in the FMVSS 113 Specification but extended to a 5 day period and without a reference fluid. Also, the Rubber Swell properties of the test fluids 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 fluid at 120.degree. C. for 3 days.

Details of these blends and of the results obtained are given in Table 3.

The abbreviations and commercial products referred to in Tables 1 to 3 are as follows:

______________________________________DPM dipropylene glycol monomethyl etherTPM tripropylene glycol monomethyl etherPPG polypropylene glycolPrimene 81 R and commercially available primaryPrimene JMT amines with two methyl groups on the alpha carbon atomLubrizol 894 and commercially available poly-Hitec E 638 isobutenyl succinimides of(Edwin Cooper) polyalkylene polyamines.Empilan KS 3 commercially available mixture of triethyleneglycol mono ethers of C.sub.9 to C.sub.11 alcohols.Empilan KB 2 commerciall available mixture of diethylene glycol mono ethers of C.sub.12 to C.sub.14 alcohols.Burning Oil a paraffinic heavy kerosene having a flash point of 260.degree. F., a specific gravity of 0.82 and viscosities at 100.degree. F. and 210.degree. F. of 4.5cS and 1.6cS respectively.Refrigerant Oil A a blend of naphthenic mineral oils having a specific gravity of 0.892, viscosity at 100.degree. F. of 48cS, flash point of 360.degree. F. and a pour point of -30.degree. F.Refrigerant Oil B a blend of naphthenic mineral oils having a specific gravity of 0.983, flash point of 330.degree. F. pour point of -30.degree. F. and viscosities at 100.degree. F. and 210.degree. F. of 53.4cS and 5.36cS respect- ively.Refrigerant Oil C a commercially available refrigerant oil manufactured by British Petroleum under the trademark ZERICE 353 and believed to be a mixture of alkylated benzenes.Silicone Fluid an experimental silicone brake fluid supplied by Union Carbide Corporation.______________________________________

The vapour-lock test results set out in the foregoing Examples and in Tables 1 to 3 show that fluids in accordance with the invention retain unexpectedly high vapour-lock temperatures even in the presence of water. Furthermore, the rubber swell test results set out in Table 3 show that fluids in accordance with the invention may be blended so as to provide fluids having rubber swell properties acceptable in commercial hydraulic systems.

TABLE I__________________________________________________________________________BORATE ESTER AMINEExample wt. wt.No % % GILPIN VLT (.degree.C.)__________________________________________________________________________2 TETRA(DPM)PYROBORATE 10 PRIMENE 81 R 2 1813 TETRA(TPM)PYROBORATE 10 LUBRIZOL 894 20 1784 TRIS n-BUTYL BORATE 15 2-ETHYL HEXYLAMINE 2 1335 TETRA(PENT 2YL) 10 PRIMENE JMT 5 170 PYROBORATE6 TRIS(DPM)METABORATE 10 PRIMENE 81R 10 1867 TRIS(n-OCTYL) 10 PRIMENE JMT 10 155 METABORATE8 TRIS(BUTYL MONOGLYCOL) 10 PRIMENE JMT 5 167 METABORATE9 TRIHEXYLENE GLYCOL 10 PRIMENE JMT 2 143 BISBORATE10 TRIS(OLEYL)BORATE 15 PRIMENE JMT 10 12011 TRIS(m-TOLYL)BORATE 10 PRIMENE JMT 2 22212 TRIS(p-TOLYL)BORATE 10 PRIMENE JMT 2 20513 TRIS(o-TOLYL)BORATE 10 PRIMENE JMT 2 20614 TRIS(p-tert.BUTYL 10 PRIMENE JMT 10 177 PHENYL)BORATE15 TRIS(BENZYL)BORATE 10 PRIMENE JMT 5 15116 TRIS(n-OCTYL)BORATE 15 DIHEXYLAMINE 2 13917 TRIS(PENT 2YL) 15 PRIMENE 81R 5 144 BORATE18 TRIS(2 NAPHTHYL) 10 PRIMENE JMT 10 218 BORATE19 HEXYLENE GLYCOL BIS 10 PRIMENE JMT 2 167 DPM BIS BORATE20 PPG 1200 BIS ISO- 15 PRIMENE JMT 10 138 TRIDECYL BIS BORATE21 TRIS(OCT 2YL)BORATE 15 PRIMENE JMT 7 17922 TRIS(ISODECYL)BORATE 15 PRIMENE JMT 10 13523 TRIS(ISOTRIDECYL) 15 PRIMENE JMT 10 131 BORATE24 TRIS(ISO-OCTADECYL) 20 PRIMENE JMT 20 126 BORATE25 TRIS(3 METHYL 15 TRIOCTYLAMINE 20 154 1 BUTYL)BORATE26 TRIS(3 METHYL PENT 15 PRIMENE JMT 5 171 3YL)BORATE27 TRIS(2 METHYL 15 PRIMENE JMT 5 171 CYCLOHEXYL)BORATE28 TRIS(BUTYL MONO- 15 DIALLYLAMINE 5 169 GLYCOL)BORATE29 TRIS(BUTYL TRIGLYCOL) 15 n-HEXYLAMINE 2 184 BORATE30 TRIS(HEXYL-DIGLYCOL) 15 PRIMENE 81R 2 14531 TRIS(DPM)BORATE 10 HITEC E 638 20 18632 TRIS(TPM)BORATE 10 PRIMENE JMT 10 181__________________________________________________________________________ TABLE 2__________________________________________________________________________BORATE ESTERExample wt PRIMENE GILPINNo % JMT (wt %) BASE FLUID VLT (.degree.C.)__________________________________________________________________________33 TRIS(n-BUTYL)BORATE 15 5 BURNING OIL 14234 TRIS HEXYLENE GLYCOL 10 5 BURNING OIL 157 BIS BORATE35 TRIS (BUTOXYETHOXY 15 5 BURNING OIL 166 PROPYL)BORATE36 TRIS(2 METHYL CYCLO- 15 5 DI(ISO-OCTYL) 220 HEXYL)BORATE ADIPATE37 TRIS(BUTYL MONO 15 2 REFRIGERANT OIL A 185 GLYCOL)BORATE38 TRIS(3 METHYL 1 15 5 REFRIGERANT OIL B 153 BUTYL)BORATE39 TRIS(o-BUTYL) 15 5 REFRIGERANT OIL C 236 BORATE/TRIS(O-TOLYL) BORATE40 TRIS(n-BUTYL)BORATE 15 5 SILICONE FLUID 13341 TRIS(BUTYL MONO- 15 5 SILICONE FLUID 188 GLYCOL)BORATE42 TRIS(3 METHYL 1 BUTYL) 15 5 PENTAERYTHRITOL- 180 BORATE HEPTANOATE43 TRIS(o-TOLYL)BORATE 10 5 TRIMETHYLOL PROPANE 25644 TRIS(BUTYL TRIGLYCOL) 15 5 DI (TRIDECYL) 223 BORATE DODECANE DIOATE45 TRIS(DPM)BORATE 10 5 DI(2-ETHYLHEXYL) 218 DODECANE DIOATE46 TRIS(PHENYL GLYCOL ETHER) 10 2 TRIBUTYL PHOSPHATE 205 BORATE47 TRIS(EMPILAN KS 3) 10 2 TRI BUTYL PHOSPHATE 195 BORATE__________________________________________________________________________ TABLE 3__________________________________________________________________________ORTHOESTER BORATE ESTERExample wt wtNo Name % NAME %__________________________________________________________________________52 TRIS(BUTYL)ORTHOFORMATE 25 TRIS(n-DECYL)ORTHOBORATE 1253 TRIS(BUTYL)ORTHOFORMATE 15 TRIS(n-OCTYL)ORTHOBORATE 554 TRIS(2 ETHYL HEXYL) 28 TRIS(n-BUTYL)ORTHOBORATE 7 ORTHOFORMATE55 TRIS(2 ETHYL HEXYL) 25 TRIS(TRIDECYL)ORTHOBORATE 20 ORTHOFORMATE56 TRIS(BUTYL TRIGLYCOL) 30 TRIS(ISO DECYL)ORTHOBORATE 11 ORTHOFORMATE57 TRIS(DOWANOL DPM) 15 TRIS(METHYL BUTYL) 13 ORTHOFORMATE ORTHOBORATE58 TRIS(TRIDECYL) 30 TRIS(PENT-2-YL) 15 ORTHOFORMATE ORTHOBORATE59 TRIS(TRIDECYL) 15 TRIS(2-OCTYL)ORTHOBORATE 13 ORTHOACETATE60 TRIS(TRIDECYL) 25 TRIS(OLEYL)ORTHOBORATE 15 ORTHOFORMATE61 TRIS(OLEYL) ORTHOFORMATE 15 TRIS(p-TOLYL)ORTHOBORATE 362 TRIS(DPM)ORTHOACETATE 10 TRIS(BUTYL MONOGLYCOL) 663 TRIS(BUTYL TRIGLYCOL) 30 TRIS(BUTYL TRIGLYCOL) 17 ORTHOFORMATE ORTHOBORATE64 TRIS(DPM)ORTHOFORMATE 15 TRIS(HEXYL DIGLYCOL) 4 ORTHOBORATE65 TRIS(OLEYL)ORTHOFORMATE 20 TRIS(DPM)ORTHOBORATE 566 TRIS(BENZYL)ORTHOFORMATE 20 TRIS(BUTYL MONOGLYCOL) 5 ORTHOBORATE67 TRIS(ISO OCTADECYL) 25 TETRA(TPM)PYBORATE 4 ORTHOFORMATE68 TRIS(DPM)ORTHOACETATE 17 TRIS(DPM)METABORATE 469 TRIS(ALLYL)ORTHOFORMATE 19 TRIS(BENZYL)ORTHOBORATE 770 TRIS(BUTYL MONOGLYCOL) 25 TRIS(METHYL CYCLOHEXYL) 10 ORTHOACETATE ORTHOBORATE71 TRIS(TRIDECYL)ORTHO- 15 TRIS(TPM)ORTHOBORATE 2 FORMATE72 TRIS(BUTYL MONOGLYCOL) 30 TETRA(DPM)PYROBORATE 3 ORTHOACETATE73 TRIS(ISO OCTADECYL) 27 HEXAMETHYLENE GLYCOL BIS 9 ORTHOFORMATE DPM. BIS BORATE74 TRIS(2 ETHYL HEXYL) 22 TETRA(BUTYL MONOGLYCOL) 5 ORTHOFORMATE PYROBORATE75 TRIS(EMPILAN KB2) 14 TRIS(EMPILAN KB2) 5 ORTHOFORMATE ORTHOBORATE76 TRIS(DPM)ORTHOACETATE 23 TRIS(ISOOCTADECYL) 6 ORTHOBORATE77 TRIS(BUTYL MONOGLYCOL) 21 TRIS(p-tert.BUTYL 8 ORTHOACETATE PHENYL)ORTHOBORATE78 TRIS(BUTYL TRIGLYCOL) 24 TRIS(2 NAPHTHYL) 9 ORTHOFORMATE ORTHOBORATE79 TRIS(DPM)ORTHOFORMATE 14 PPG 1200 BIS TRIDECYL 3 BIS BORATE80 TRIS(3 METHYL PENTYL) 15 TRIS(TPM)ORTHOBORATE 4 ORTHOFORMATE81 TRIS(TRIDECYL)ORTHO- 15 TRIS(DPM)ORTHOBORATE 2 VALERATE__________________________________________________________________________ GILPIN VLT GILPIN VLT AFTER 5 DAY NITRILE RUBBER AFTER REACTION HUMIDITY TESTExample AMINE VOLUME SWELL (%) WITH 0.5% WATER 80% RH 22.degree. C.No NAME wt % (3 days at 120.degree. C.) (.degree.C.) (.degree.C.)__________________________________________________________________________52 PRIMENE JMT 5 1.55 144 12853 PRIMENE JMT 6 3.5 134 14854 PRIMENE 81R 2 0.7 161 15455 PRIMENE JMT 10 0.9 198 20356 PRIMENE JMT 10 12.4 195 13857 PRIMENE JMT 10 5.1 172 13058 PRIMENE JMT 12 -1.6 164 12059 PRIMENE JMT 8 3.4 184 11560 PRIMENE JMT 10 3.5 185 13361 PRIMENE JMT 3 11.3 -- 22162 PRIMENE JMT 2 6.1 152 12063 PRIMENE 81R 3 19.8 168 13764 PRIMENE UMT 1 8.8 174 21165 PRIMENE JMT 3 2.5 129 15666 PRIMENE JMT 5 32.9 199 19667 PRIMENE JMT 2 -0.2 154 24968 PRIMENE JMT 10 13.3 171 17569 PRIMENE JMT 4 17.1 120 11070 PRIMENE JMT 5 7.8 193 15571 PRIMENE JMT 2 0.3 147 24272 PRIMENE JMT 6 9.7 197 20273 PRIMENE JMT 2 0.2 214 21874 PRIMENE JMT 5 4.5 178 14475 PRIMENE JMT 3 2.9 126 13376 PRIMENE JMT 3 8.0 179 20777 PRIMENE JMT 8 23.1 205 14978 PRIMENE JMT 9 35.6 204 17279 PRIMENE JMT 5 6.0 166 21280 PRIMENE JMT 3 11.3 112.5 12381 PRIMENE JMT 2 1.9 127* 149__________________________________________________________________________*With 0.25% Water

Claims

1. A composition of matter suitable for use as a hydraulic fluid which comprises

(a) at least 20%, by weight, of a lubricating oil which is a member selected from the group consisting of a mineral oil, a synthetic hydrocarbon oil, a synthetic carboxylic acid ester, a siloxane or a phosphate ester;

(b) 1 to 50%, by weight, of oil-soluble borate ester which is a compound, or mixture of compounds, having the formula: ##STR6## wherein: (i) R.sup.1, R.sup.2 and R.sup.3 are the same or different and each is an aryl group or a straight or branched chain alkyl group or alicyclic group containing from 4 to 20 carbon atoms, or is a group of the formula --R.sup.4 O).sub.n R.sup.5 wherein each R.sup.4 is the same or different and each is an alkylene group selected from ethylene, propylene or butylene, R.sup.5 is an alkyl group containing from 1 to 18 carbon atoms, or an aryl group, and n is an integer, from 1 to 10; or

(ii) R.sup.1 and R.sup.2 are the same or different and as defined above and R.sup.3 is a group of the general formula: ##STR7## wherein R.sup.1 and R.sup.2 are as defined above and R.sup.6 is an alkylene group containing from 4 to 20 carbon atoms, or is an oxyalkylene radical of the formula --R.sup.4 --OR.sup.4).sub.n wherein R.sup.4 and n are as defined above; or

(iii) R.sup.1 is as defined above and R.sup.2 and R.sup.3 together form the group ##STR8## wherein R.sub.1 is as defined above; (c) 0.5 to 20%, by weight, of an amine which prevents deposition of oil insoluble hydrolysis products of the borate ester; and

(d) 1 to 50%, by weight, of an oil soluble orthoester of the formula: ##STR9## wherein R.sup.7 is hydrogen or lower alkyl; R.sup.8, R.sup.9 and R.sup.10 are the same or different and each is a straight or branched chain alkyl containing from 4 to 20 carbon atoms, benzyl or the group --(R.sup.11 --O).sub.m --R.sup.12 where R.sup.11 is a lower alkylene group and R.sup.12 is an alkyl group containing 1 to 20 carbon atoms; and m is an integer from 2 to 10.

2. A composition as claimed in claim 1 wherein R.sup.7 is a hydrogen atom or a methyl group.

3. A composition as claimed in claim 1 which contains from 5 to 30% by weight of orthoester based on the total weight of the composition.

4. A composition as claimed in claim 1 wherein the ratio of orthoester to borate ester is in the range from 5:1 to 2:1 by weight.

5. A composition as claimed in claim 1 containing from 1 to 20% by weight of borate ester based on the total weight of the composition.

6. A composition as claimed in claim 1 containing from 1 to 10% by weight of amine based on the total weight of the composition.

7. A composition as claimed in claim 1 wherein the ratio of borate ester to amine is from 5:1 to 1:1 by weight.