Use of a lubricant composition containing an antioxidant additive combination

Abstract: An additive combination for a lubricant composition which comprises an aromatic amine antioxidant and a boron-containing compound wherein the additive combination contains by weight at least one part boron for every 250 parts nitrogen present in the aromatic amine antioxidant. The relatively high level of boron improves the antioxidant performance of the lubricant composition.

Description: This invention relates to lubricant compositions, especially for the lubrication of internal combustion engines, having improved antioxidant performance. Engine oil lubricants contain a variety of performance enhancing additives including antioxidants and dispersants. The antioxidants, such as diphenylamines and hindered phenols, are present to prevent the oil from oxidising. They function by trapping radicals formed during oil usage, thus interrupting the chain reaction which leads to acid build-up and hence sludge formation. The dispersants, which typically consist of a polar head, such as a polyamine, attached to an inert, oil-soluble body, such as polyisobutene, function through the polar head binding to insoluble degradation products, thus suspending them in the oil. The polyamine employed for this purpose, however, is often aggressive to the fluorocarbon elastomer seals used in the engine. To alleviate this problem the polyamine is usually capped by boron; the more aggressive the polyamine the higher the required boron level of the dispersant. With the advent of engine design changes that result in increased thermal stress being placed on the lubricant, there is a need for improved antioxidant performance. There are numerous patents describing lubricant additives with improved antioxidant performance. For example GB-A-1271556 describes an antioxidant additive which is a mixture of (a) the reaction product of a boron compound with a long chain hydrocarbyl carboxylic acid or anhydride thereof with a primary or secondary amine, and (b) a polycyclic phenolic compound. US-A-5354484 describes a lubricant additive which provides high temperature stability comprising a mixture of (i) an amine salt of a substituted phosphoric acid and (ii) an amine-substituted hydrocarbyl succinic acid compound. EP-A-471120 describes the use of hindered phenolic/phosphoro-dithioate-derived alcohol borates as antioxidant and antiwear additive for lubricants and fuels. Moreover, several prior art references indicate the use of a combination of a boron compound and an aromatic amine in lubricant compositions. Examples of such references include EP-A-0678569, EP-A-0673991, US-A-4657686, US-A-4689162, EP-A-0620267, EP-A-0447916 and JP-A-07126681. Of these, EP-A-0620267 in particular describes the use of a combination of a borated polyalkylene succinimide and an aryl amine. However, none of these prior art references specify the relative amounts of boron to the aminic nitrogen in the final additive composition. The criticality of the degree of boration has hitherto not been recognised, especially with respect to maximising the oxidation stability of the lubricants, particularly at relatively high temperatures, e.g. above 200°C, such as those encountered in internal combustion engines. We have found surprisingly that when a lubricant is formulated containing an aromatic amine antioxidant, the antioxidant performance can be improved by increasing the level of boron contained in the lubricant. The present invention provides the use of a boron-containing compound to enhance the antioxidant performance in an internal combustion engine of an aromatic amine antioxidant in a lubricant composition in which: (a) the aromatic amine antioxidant is selected from phenylamine, diphenylamine, phenylnaphthylamine, and mixtures thereof, which compounds may be substituted or unsubstituted, and (b) the boron-containing compound is a borated ashless dispersant, wherein the dispersant is a polyisobutene succinic acid, a polyisobutene succinic anhydride, a polyisobutene succinic acid - polyamine or a polyisobutene succinic anhydride - polyamine, wherein the additive combination contains by weight at least one part boron for every 250 parts nitrogen present in the aromatic amine antioxidant. Boron content is measured according to Inductively Coupled Plasma Emission Spectroscopy (ASTM D 5185). Nitrogen content is measured according to the Kjeldahl method (ASTM D 3228). By "aromatic amine" is meant an amine compound wherein at least one nitrogen atom is bonded directly to at least one aryl moiety. Generally the amine compound is of the formula: where R 1 is a phenyl, C 7 -C 18 alkylphenyl or C 7 -C 18 alkoxyphenyl group; R 2 is a phenyl, C 7 -C 18 alkylphenol, C 7 -C 18 alkoxyphenyl, naphthyl, C 1 -C 18 alkyl, C 7 -C 18 phenylalkyl, or C 5 -C 18 cycloalkyl group; and R 3 is hydrogen or a C 1 -C 12 alkyl, benzyl, allyl or phenyl group. Preferably both R 1 and R 2 are aryl groups. Preferably R 3 is hydrogen. Thus it is preferred that the aromatic antioxidant is a diarylamine compound. Examples of suitable aromatic antioxidants include phenylamine, diphenyl-amine, N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine, di-[4-1,3-dimethylbutyl-phenyl]-amine, di-[4-(1,1,3,3-tetramethyl butyl)-phenyl]-amine, N-allyldiphenylamine, 4-isopropoxydiphenyl-amine and di-4-methoxyphenylamine. These and similar compounds are described in EP-A-356677, which disclosure is incorporated herein by reference. The compounds can be prepared using conventional methods and/or are commercially available, for example from Ciba-Geigy Ltd. The ashless dispersant is most preferably a borated polyisobutene-succinic acid or anhydride (PIBSA) dispersant, especially the PIBSA - polyamine (PIBSA-PAM) compounds formed by the reaction of the PIBSA compound with an amine such as polyethylene amine, polyoxyethylene or a polyol amine. The dispersants may be borated using conventional techniques, for example by reacting the dispersant with a boron compound selected from, for example, boron oxide, boron halides, boron acids and esters of boron acids. Suitable borated dispersants and their methods of manufacture are described for example in EP-A-351964 and US-A-5354484, the disclosures of which are incorporated herein by reference. In another aspect the present invention provides a lubricant composition comprising a major amount of lubricating oil basestock and a minor amount of the boron/aromatic amine antioxidant combination as described above. By 'major amount' is meant greater than 50 wt.%, preferably from more than 50 to 98 wt.%, more preferably from 80 to 95.5 wt.%, the balance being made up of the boron/aromatic amine antioxidant combination and, optionally, other lubricant additives. By' minor amount' is meant an amount that is sufficient to improve the antioxidant performance of the lubricant composition. The amount of aromatic amine antioxidant contained in the lubricant composition is preferably from 0.01 to 5 wt.%, more preferably from 0.05 to 2 wt.%, and most preferably from 0.2 to 0.8 wt.% based on the total weight of the composition. The amount of ashless borated dispersant referred to above, is generally in the range from 0.5 to 15 wt.%, preferably from 1 to 10 wt.%, more preferably from 2 to 8 wt.%, based on the total weight of the composition. The lubricating oil basestock can be derived from natural lubricating oils, synthetic lubricating oils, or mixtures thereof. Suitable lubricating oil basestocks include basestocks obtained by isomerization of synthetic wax and slack wax, as well as hydrocrackate basestocks produced by hydrocracking (rather than solvent extracting) the aromatic and polar components of the crude. In general, the lubricating oil basestock will have a kinematic viscosity ranging from about 2 to about 1,000 cSt at 40°C. Natural lubricating oils include animal oils, vegetable oils (e.g., castor oils and lard oil), petroleum oils, mineral oils, and oils derived from coal or shale. Synthetic oils include hydrocarbon oils and halo-substituted hydrocarbon oils such as polymerized and interpolymerized olefins, alkylbenzenes, polyphenyls, alkylated diphenyl ethers, alkylated diphenyl ethers, alkylated diphenyl sulfides, as well as their derivatives, analogs, and homologs thereof, and the like. Synthetic lubricating oils also include alkylene oxide polymers, interpolymers, copolymers and derivatives thereof wherein the terminal hydroxyl groups have been modified by esterification, etherification, etc. Another suitable class of synthetic lubricating oils comprises the esters of dicarboxylic acids with a variety of alcohols. Esters useful as synthetic oils also include those made from C 5 to C 12 monocarboxylic acids and polyols and polyol ethers. Silicon-based oils (such as the polyalkyl-, polyaryl-, polyalkoxy-, or polyaryloxysiloxane oils and silicate oils) comprise another useful class of synthetic lubricating oils. Other synthetic lubricating oils include liquid esters of phosphorus-containing acids, polymeric tetrahydro-furans, polyalphaolefins, and the like. The lubricating oil may be derived from unrefined, refined, rerefined oils, or mixtures thereof. Unrefined oils are obtained directly from a natural source or synthetic source (e.g., coal, shale, or tar sands bitumen) without further purification or treatment. Examples of unrefined oils include a shale oil obtained directly from a retorting operation, a petroleum oil obtained directly from distillation, or an ester oil obtained directly from an esterification process, each of which is then used without further treatment. Refined oils are similar to the unrefined oils except that refined oils have been treated in one or more purification steps to improve one or more properties. Suitable purification techniques include distillation, hydro-treating, dewaxing, solvent extraction, acid or base extraction, filtration, and percolation, all of which are known to those skilled in the art. Rerefined oils are obtained by treating refined oils in processes similar to those used to obtain the refined oils. These rerefined oils are also known as reclaimed or reprocessed oils and often are additionally processed by techniques for removal of spent additives and oil breakdown products. The lubricant composition may also contain other additives such as detergents, dispersants, friction modifiers, antiwear agents, extreme pressure agents, other antioxidants, rust and corrosion inhibitors, pour point depressants, viscosity index improvers, antifoam agents, demulsifiers, and hydrolytic stabilisers. Such additives are well known in the art, and are described, for example, in 'Lubricants and Related Products' by Dieter Klamann, Verlag Chemie, Weinheim, Germany 1984. The lubricant composition according to the invention may be used in any suitable lubricating application which requires stability against lubricant oxidation. However, it is especially useful as an engine oil, for example in an internal combustion engine. The invention is illustrated by the following Example : Example 1 Engine oil formulations were blended as shown in Table 1 below. Each formulation contained mineral oil basestock, a conventional engine oil addpack, a conventional viscosity index (VI) improver, Irganox L57 - an alkyl-substituted diphenylamine antioxidant available from Ciba-Geigy Ltd., and a PIBSA-PAM dispersant available from Exxon Chemical Ltd. The boration level of the dispersant was varied from zero through to heavily borated. Comparative formulations were also blended containing no diphenylamine antioxidant. The antioxidant performance of each of the formulations was determined using two test methods: TEST 1: Differential Scanning Calorimetry (DSC) Degradation Temperature. This measures the temperature at which the lubricant starts to degrade - the higher the temperature, the better the antioxidant performance. A sample of the lubricant was monitored at a pressure of 100 psi oxygen, starting at ambient temperature and increasing at 5°C/min. until degradation started to occur. TEST 2 : Differential Scanning Calorimetry (DSC) Induction Time. This measures the length of time a lubricant can withstand degradation when exposed to a temperature of 210°C. The longer the time, the better the antioxidant performance. A sample of the lubricant was monitored at a pressure of 100 psi oxygen and a temperature of 210°C, and the time at which degradation started to occur was recorded. Formulation No. Component (wt%) 1 2 3 4 5 6 Base oil 78 78 78 79 79 79 Addpack 6.5 6.5 6.5 6.5 6.5 6.5 VI improver 8.5 8.5 8.5 8.5 8.5 8.5 PIBSA-PAM (non-borated) 6 - - 6 - - PIBSA-PAM (borated) - 6 - - 6 - PIBSA-PAM (heavily borated) - - 6 - - 6 Diphenylamine antioxidant 1 1 1 - - - Amount boron (ppm) , 3 223 704 4 225 702 Wt.% N in diphenylamine , 0.044 0.045 0.047 - - - Wt. ratio boron to nitrogen in diphenylamine 1:1.5x10 5 1:324 1:67 N/A N/A N/A DSC Degradation Temperature (°C) 246 251 258 229 228 229 DSC Induction Time @ 210°C (min.) 29.3 42.1 59.9 7.1 7.7 7.9 The results show that with no aromatic amine antioxidant present the antioxidant performance is approximately constant regardless of the amount of boron present. However, when an aromatic amine antioxidant is present, the antioxidant performance is improved, and the degree of improvement depends upon the boron treat level, the higher the amount of boron, the better the antioxidant performance. Thus the results show a synergistic interaction exists between the aromatic amine antioxidant and high boron levels, resulting in greater lubricant oxidation control.

Claims: The use of a boron-containing compound to enhance the antioxidant performance in an internal combustion engine of an aromatic amine antioxidant in a lubricant composition in which: (a) the aromatic amine antioxidant is selected from phenylamine, diphenylamine, phenylnaphthylamine, and mixtures thereof, which compounds may be substituted or unsubstituted, and (b) the boron-containing compound is a borated ashless dispersant, wherein the dispersant is a polyisobutene succinic acid, a polyisobutene succinic anhydride, a polyisobutene succinic acid - polyamine or a polyisobutene succinic anhydride - polyamine, wherein the additive combination contains by weight at least one part boron for every 250 parts nitrogen present in the aromatic amine antioxidant. The use according to claim 1 wherein the aromatic amine antioxidant is selected from alkyl substituted phenylamine, alkyl substituted diphenylamine, alkyl substituted phenylalphanaphthylamine, and mixtures thereof.

Inventors: HOLT DAVID GARY LAWTON, HOLT, DAVID GARY LAWTON, Holt, David Gary Lawton, Esso Petroleum Co., Ltd.
Applicants: EXXONMOBIL RES & ENG CO, EXXONMOBIL RESEARCH AND ENGINEERING COMPANY
Application year: 1996
Main classification: C10M133
IPC codes: C10M133, C10M141