Method of lubricating and lubricating compositions thereof

1. A method of lubricating an axle gear comprising supplying to the axle gear a lubricating composition comprising an oil of lubricating viscosity, a thiadiazole-functionalised dispersant and a polysulphide, wherein the polysulphide comprises at least 20 wt % of tri- or higher sulphides, and

wherein the thiadiazole-functionalised dispersant is prepared by heating together ingredients comprising:

(i) a dispersant substrate;

(ii) a thiadiazole compound;

(iii) a dicarboxylic acid of an aromatic compound selected from the group consisting of 1,3 diacids and 1,4 diacids,

(iv) optionally a borating agent; and

(v) optionally a phosphorus acid compound, said heating being sufficient to provide a product of (i), (ii), (iii) and optionally (iv) and optionally (v), which is soluble in an oil of lubricating viscosity.

2. The method of claim 1 , wherein the polysulphide comprises at least 30 wt % of tri- or higher sulphides.

3. The method of claim 1 , wherein at least about 50 wt % of the polysulphide molecules comprise a mixture of tri- and tetrasulphides.

4. The method of claim 1 , wherein the polysulphide contains less than about 30 wt % of a disulphide.

5. The method of claim 1 , wherein the polysulphide provides about 0.5 to about 5 wt % of sulphur to the lubricating composition.

6. The method of claim 1 , wherein the polysulphide is present at about 0.01 to about 10 wt % of the lubricating composition.

7. The method of claim 1 , wherein the thiadiazole-functionalised dispersant is prepared by a method selected from the group consisting of heating, reacting and complexing a thiadiazole compound with a dispersant substrate.

8. The method of claim 7 , wherein the dispersant substrate comprises a succinimide dispersant, a Mannich dispersant, an ester-containing dispersant, a condensation product of a fatty hydrocarbyl monocarboxylic acylating agent with an amine or ammonia, an alkyl amino phenol dispersant, a hydrocarbyl-amine dispersant, a polyether dispersant, a polyetheramine dispersant, a viscosity modifier containing dispersant functionality, or mixtures thereof.

9. The method of claim 8 , wherein the dispersant substrate comprises a succinimide dispersant, an ester-containing dispersant or a Mannich dispersant.

10. The method of claim 8 , wherein the dispersant substrate comprises a succinimide dispersant, and wherein about 0 mol % to less than about 50 mol % of the succinimide dispersant molecules contain a carbocyclic ring.

11. The method of claim 7 , wherein the thiadiazole compound comprises a dimercaptothiadiazole, 2,5-dimercapto-[1,3,4]-thiadiazole, 3,5-dimercapto-[1,2,4]-thiadiazole, 3,4-dimercapto-[1,2,5]-thiadiazole, or 4-5-dimercapto-[1,2,3]-thiadaizole.

12. The method of claim 7 , wherein the weight ratio of dispersant substrate to the thiadiazole compound ranges from greater than about 0.1 to about 9.

13. The method of claim 1 , wherein the thiadiazole-functionalised dispersant is present at about 0.1 wt % to about 8 wt % of the lubricating composition.

14. The method of claim 1 , wherein the thiadiazole-functionalised dispersant is present at about 0.1 wt % to about 8 wt % of the lubricating composition; and wherein and the polysulphide is present at about 0.01 to about 10 wt % of the lubricating composition.

15. The method of claim 1 further comprising a phosphorus-containing acid, salt or ester antiwear agent comprising (i) a non-ionic phosphorus compound; (ii) an amine salt of a phosphorus compound; (iii) an ammonium salt of a phosphorus compound; (iv) a monovalent metal salt of a phosphorus compound; or (v) mixtures of (i), (ii), (iii) or (iv); and wherein the phosphorus-containing acid, salt or ester antiwear agent is present at about 0.05 wt % to about 10 wt % of the lubricating composition.

16. The method of claim 1 , wherein the oil of lubricating viscosity comprises at least one of an API Group II, III, IV base oil, or mixtures thereof.

17. The methods of claim 1 , wherein the dicarboxylic acid of an aromatic compound comprises terephthalic acid.

18. A method of lubricating an axle gear comprising supplying to the axle gear a lubricating composition comprising an oil of lubricating viscosity and a thiadiazole-functionalised dispersant, wherein the dispersant comprises either (i) an ester dispersant or (ii) a succinimide dispersant, wherein about 0 mol % to less than about 50 mol % of the succinimide dispersant molecules contain a carbocyclic ring, and wherein the thiadiazole-functionalised dispersant is prepared by heating together ingredients comprising:

(i) a dispersant substrate;

(ii) a thiadiazole compound;

(iii) a dicarboxylic acid of an aromatic compound selected from the group consisting of 1,3 diacids and 1,4 diacids,

(iv) optionally a borating agent; and

(v) optionally a phosphorus acid compound, said heating being sufficient to provide a product of (i), (ii), (iii) and optionally (iv) and optionally (v), which is soluble in an oil of lubricating viscosity.