By lubricant base stocks is meant lubricants such as mineral and synthetic base oils chosen from Group I, Group II, Group III, Group IV and Group V base stocks as classified by the American Petroleum Institute, as well as natural and synthetic esters including polyol esters, more particularly, but not limited to automotive oils, circulatory oils, gear oils, greases, hydraulic fluids, turbine fluids and metal working fluids.

By lubricant formulations, is meant a lubricant base stock as described above, plus additional additives including but not limited to antioxidants, rust inhibitors, metal passivators, ashless dispersants, antiwear additives, extreme pressure additives and detergents, viscosity index improvers and antifoam agents.

According to the present invention a stabilising composition for lubricant base stocks and lubricant compositions comprises: (a) at least one aromatic aminic antioxidant having the general formula: wherein Ri and R2 are the same or different and may be H, an alkyl group with 4 to 9 carbon atoms or an aralkyl group with 8 carbon atoms and m and n are 0 to 2 but m + n is not greater than 3, either alone or optionally blended with at least one hindered phenolic antioxidant selected from those corresponding to one of the following general formulae: wherein R3, R4, Rs and R6 are the same or different and may be an alkyl group with 1 to 4 carbon atoms and X is-S-or-CH2-or-CH2CH2-S-CH2CH2-or- -CH2-S-CH2-or-CH2CH2C02CH2CH2-S-CH2CH2C02CH2CH2- III wherein R7 is an alkyl group with 6 to 20 carbon atoms; and with (b) at least one neutral organo phosphate or phosphite optionally mixed with at least one acid organo phosphate or phosphite selected from those corresponding to the following general formulae: (i) neutral triaryl phosphates: v wherein Rs, Rg and Rio are the same or different and may be H, CH3, isopropyl or tertiary butyl and m is 0 to 2; (ii) neutral trialkyl phosphates: wherein Rll, R12 and Rig are all the same and are alkyl groups with 4 to 12 carbon atoms; (iii) neutral alkyl aryl phosphates: VII wherein Rl4 is different from Ris and Ri6 and R14 is an alkyl group with 4 to 12 carbon atoms and Ris is phenyl or substituted phenyl with 7 to 10 carbon atoms and R16 may be an alkyl group with 4 to 12 carbon atoms or phenyl or substituted phenyl with 7 to 10 carbon atoms ; (iv) alkyl acid phosphates: wherein Rl7 is an alkyl group with 5 to 12 carbon atoms and m is 1 or 2 and n is 1 or 2 but m+n is not greater than 3 ; (v) neutral triaryl phosphites: wherein R18, R19, R20 are the same or different and are H or alkyl with 1 to 9 carbon atoms; (vi) neutral trialkyl phosphites: wherein R2i, R22 and R23 are the same or different and are alkyl groups with 1 to 18 carbon atoms.

(vii) neutral alkyl aryl phosphites: wherein R24 and R25 are the same or different and are alkyl groups with 1 to 12 carbon atoms or R24 is phenyl or substituted phenyl where R26 is alkyl with 1 to 9 carbon atoms.

(viii) alkyl acid phosphites: (HOtn P (OR27) n XII wherein R27 may be an alkyl group with 4 to 18 carbon atoms, phenyl or substituted phenyl with 7 to 10 carbon atoms and m is 1 or 2, n is 1 or 2 but m + n is not greater than 3, and wherein the antioxidants and the phosphate (s) and/or phosphite (s) are utilised in a weight ratio from 10: 1 to 1: 10, preferably 1: 1 to 1: 5.

In one embodiment of the invention, the stabilising composition is prepared by heating the antioxidant (s) and the phosphate (s) and/or phosphite (s) for 0.2 to 6 hours at a temperature range of 40°C to 150°C. The treat rates of the antioxidant (s) and the phosphate (s) and/or phosphite (s) in the lubricant base stocks and lubricant compositions preferably are within the following weight ranges: (a) aromatic amine antioxidants-0.1 to 5% ; (b) hindered phenol antioxidants-0. 1 to 5% ; (c) neutral aryl phosphates-0. 2 to 10% ; (d) neutral alkyl phosphates-0.2 to 10% ; (e) neutral alkyl aryl phosphates-0.2 to 10% ; (f) alkyl acid phosphates-0. 01 to 5% ; (g) neutral aryl phosphites-0.2 to 10% ; (h) neutral alkyl phosphites-0. 2 to 10% ; (i) neutral alkyl aryl phosphites-0.2 to 10% ; (j) alkyl acid phosphites-0.01 to 5%; Preferably, the amine and hindered phenol antioxidants are used at 0.5 to 2.0% each.

The neutral alkyl, aryl and alkyl aryl phosphates and phosphites are used at 0.5 to 4%.

The acid phosphates and phosphites at 0.025 to 0.05% (when used together with the neutral phosphates).

Examples of formula I compounds are 4-tert-butyl, 4'-tert-octyl diphenylamine ; 4,4'- di-tert-octyl diphenyl amine and 4,4'-di-iso-nonyl diphenylamine.

Examples of formula II compounds are 4,4'-methylene bis (2,6 tertiary butyl phenol) ; 2,2' thiodiethylene bis [3- (3, 5-di-tert-butyl-4-hydroxy-phenyl) propionate]; 4,4'-thiobis (2-t-butyl-5-methyl phenol) ; 6,6'-thiobis (2-methyl-4-t-butylphenol).

Examples of formula III compounds are C7 9 alkyl ester of 3, 5-di-tert-butyl-4-hydroxy benzene-3-propionic acid ; C14-16 alkyl ester of 3,5-di-tert-butyl-4-hydroxybenzene-3- propionic acid.

An Example of formula IV compounds is pentaerythritol tetra ester of 3,5 di-tert-butyl- 4-hydroxy benzene-3-propionic acid. Examples of formula V compounds are tri-dimethyl phenyl phosphate; tri-isopropyl phenyl phosphate ; mono-phenyl-di (-tert-butyl phenyl) phosphate.

Examples of formula VI compounds are tributyl phosphate; trioctyl phosphate.

Examples of formula VII compounds are octyldiphenyl phosphate; dibutyl-mono-phenyl phosphate.

Examples of formula VIII compounds are dipentyl hydrogen phosphate ; di Cio-i2 alkyl- hydrogen phosphate; Cl0 l2 alkyl-dihydrogen phosphate.

An Example of formula IX is tris nonyl phenyl phosphite.

Examples of formula X are trilauryl phosphite; trioctyl phosphite.

Examples of formula XI are decyl diphenyl phosphite; didecyl monophenyl phosphite.

Examples of formula XII are dibutyl hydrogen phosphite; dioctyl hydrogen phosphite ; cetyl stearyl hydrogen phosphite.

Using stabilising compositions of the invention, lubricant base stocks and lubricant formulations can have varying levels of stability dependent on: (i) proper choice of the antioxidant (s) and phosphate (s) and/or phosphite (s) to provide for the best performance, in terms of stability, within particular lubricant base stocks or particular lubricant formulations; or (ii) by changing the ratio of the antioxidant to the phosphate (s) and/or phosphite (s) ; or (iii) keeping the ratio of the antioxidant (s) to the phosphate (s) and/or phosphite (s) constant while altering or decreasing the treat rate of the antioxidant (s) and phosphate (s) and/or phosphite (s).

(iv) changing the ratio of neutral phosphate to acid phosphate and neutral phosphite to acid phosphite.

In other words, the components of the mixtures of the invention, either individually or in combination, can be tailored to provide the required performance characteristics in terms of stability, in a particular base stock and/or lubricant formulation.

Embodiments of the invention will now be described by way of example with reference to the Table set out below.

The Table shows the relative oxidation stability results obtained by adding certain antioxidant (s) and phosphate (s) and/or phosphite (s) mixtures to Group III base stocks.

The relative oxidation stabilities of the lubricant base stock is determined by the rotating bomb oxidation stability test (RBOT), IP test method number 229. The oxidation stability is defined as the number of minutes required to effect a reduction in oxygen pressure of 25.4 PSI or 1.75 bar when the oil and additives are heated at 150°C in an oxygen pressurised vessel.

The results in the Table show that there is a synergistic effect (in terms of stability) when a blend of additives according to the invention is added to Group III lubricant base stocks.

ASTM IP 229 ROTARY BOME OXIDATION TEST DATA BASE OIL Conc ANTIOXIDANT Conc PHOSPHATE ESTER RBOTLIFE (%) (%) (mins) Group III Mineral oil 25 @ 1.0 Neutral Tri Aryl Phosphate (Isopropylated phenyl phosphate) 40 @ 1.0 Neutral Tri Aryl Phosphate (Tertiary Butylated phenyl phosphate) 22 1.0 Neutral Tri Alkyl Phosphate ("Tri Octyl phosphate)) 17 @ 1.0 Alkyl Aryl Phosphate (Ootyl diphenyl phosphate) 30 @ 1.0 Alkyl Aryl Phosphate (Iso deoyl diphenyl phosp9hate) 25 @ 1.0 Alkyl Aryl Phosphate (C10-C12) 18 @ 0.05 Alkyl Aryl Phosphate (C10-C12) 27 1.0 95:5 Neutral Tri Aryl Phosphate (Isopropylated phenyl phosphate) 20 + Acid Alkyl phosphate (C10-C12) @ 0.5 Butyl/Octyl diphenylamine 1088 0.5 Butyl/Octyl diphenylamine 1.0 Neutral Tri Aryl Phosphate (Isopropylated phenyl phosphate) 1250 @ 0.5 Butyl/Octyl diphenylamine 1.0 Neutral Tri Aryl Phosphate (Tertiary Butylated phenyl phosphate) 1430 0.5 Butyl/Octyl diphenylamine 1.0 Neutral Tri Alkyl Phosphate (Tri Octyl phosphate) 1795 @ 0.5 Butyl/Octyl diphenylamine 1.0 Acid Alkyl phosphate (C10-C12) 880 0.5 Butyl/Octyl diphenylamine 1.0 3:1 Neutral Tri Aryl Phosphate (Isopropylated phenyl phosphate) + 1415 Acid Alkyl phosphate (C10-C12) @ 0.5 Butyl/Octyl diphenylamine 1.0 9:1 Neutral Tri Aryl Phosphate (Isopropylated phenyl phosphate) + 1817 Acid Alkyl phosphate (C10-C12) 0.5 Butyl/Octyl diphenylamine 1.0 95:5 Neutral Tri Aryl Phosphate (Isopropylated phenyl phosphate) 2068 + Acid Alkyl phosphate (C10-C12) @ 0.5 Butyl/Octyl diphenylamine 1.0 95:5 Alkyl Aryl Phosphate (Octyl dipheuyl phosphate) + Acid 1967 Alkyl phosphate (C10-C12) 0.5 Butyl/Octyl diphenylamine 1.0 95:5 Alkyl Aryl Phosphate (Iso decyl dipheuyl phosphate) + Acid 1915 Alkyl phosphate (C10-C12) @ 0.5 Di Nonyl diphenylamine 640 @ 0.5 Di Nonyl diphenylamine 1.0 Neutral (Isopropylated phenyl phosphate) + Acid Alkyl phosphate 833 (C10-C12) 95:5 @ 0.5 1:4 High molecular weight phenolic (Form 4) + Butyl / Octyl diphenylamine 730 @ 0.5 1:4 High molecular weight phenolic (Form 4) + Butyl / Octyl diphenylamine 1.0 95:5 Neutral Tri Aryl Phosphate (Isopropylated phenyl phosphate) 1537 Tri Aryl Phosphate + Acid Alkyl phosphate (C10-C12) @ 0.5 15:15:70 Sulphur containing phenolic (Form 2) + High molecular weight 372 phenolic (Form 4) + Bulyl / Octyl diphenylamine 0.5 15:15:70 Sulphur containing phenolic (Form 2) + High molecular weight 1.0 95:5 Neutral Tri Aryl Phosphate (Isopropylated phenyl phosphate) 646 phenolic (Form 4) + Bulyl / Octyl diphenylamine + Acid Alkyl phosphate (C10-C12) @ 0.5 1:4 Sulphur containing phenolic (Form 2) + Octyl/ Styryl diphenylamine 355 0.5 1:4 Sulphur containing phenolic (Form 2) + Octyl/ Styryl diphenylamine 1.0 95:5 Neutral Tri ArylPhosphate (Isopropylated Phenyl Phosphate) 908 + Acid Alkyl phosphate (C10-C12) 0.5 1:4 Sulphur containing phenolic (Form 2) + Butyl / Octyl diphenylamine 365 0.5 1:4 Sulphur containing phenolic (Form 2) + Butyl / Octyl diphenylamine 1.0 95:5 Neutral Tri Aryl Phosphate (Isopropylated phenyl phosphate) 755 + Acid Alkyl phosphate (C10-C12)