Lubricant composition containing 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

[0001] This invention relates to lubricant compositions, especially for the lubrication of internal combustion engines, having improved antioxidant performance.

[0002] 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.

[0003] 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-dithioatederived alcohol borates as antioxidant and antiwear additive for lubricants and fuels.

[0004] 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.

[0005] Accordingly, in one aspect, the present invention provides an additive combination comprising:

(a) an aromatic amine antioxidant, and

(b) 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.

[0006] Preferably the weight ratio of boron to nitrogen contained in the aromatic amine antioxidant is from 1:1 to 1:250, more preferably from 1:10 to 1:200, most preferably from 1:20 to 1:100, and especially from 1:40 to 1:80. 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).

[0007] In a second aspect, the present invention provides the use of boron or a boron-containing compound to enhance the antioxidant performance of an aromatic amine antioxidant in a lubricant composition.

[0008] The aromatic amine antioxidant may be any aromatic amine compound suitable for use as an antioxidant in lubricant compositions. 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 R¹ is a phenyl, C₇-C₁₈ alkylphenyl, C₇-C₁₈ alkoxyphenyl or naphthyl group;

R² is a phenyl, C₇-C₁₈ alkylphenol, C₇-C₁₈ alkoxyphenyl, naphthyl, C₁-C₁₈ alkyl, C₇-C₁₈ phenylalkyl, or C₅-C₁₈ cycloalkyl group; and

R³ is hydrogen or a C₁-C₁₂ alkyl, benzyl, allyl or phenyl group.

[0009] Preferably both R¹ and R² are aryl groups; that is they are preferably each selected from the list of groups suitable for R¹, as specified above. Preferably R³ is hydrogen. Thus it is preferred that the aromatic antioxidant is a diarylamine compound.

[0010] Examples of suitable aromatic antioxidants include phenylamine, diphenylamine, N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine, di-[4-1,3-dimethylbutylphenyl]-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.

[0011] The boron is preferably present in the lubricant composition according to the invention in the form of a borated compound, especially a borated dispersant. The dispersant may be any suitable ashless or metal-containing compound suitable as a lubricant dispersant that is capable of being borated. Preferably however it is an ashless alkenyl-succinic acid or anhydride, or alkenyl-succinimide dispersant. Most preferred are the polyisobutene-succinic acid or anhydride (PIBSA) dispersants, 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.

[0012] 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.

[0013] 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.

[0014] Alternative dispersants are ethylene butene dispersants. Ethylene butene dispersants may be prepared from an unsaturated ethylene-butene copolymer (Mn = 1500 - 5,000, typically 3,000 - 3,900, Mw/Mn = 1.5 - 3.5, ethylene content = 20 - 55%) with olefin groups about 50% vinylidene and the remainder largely trisubstituted. The polymer is prepared via Ziegler/Natta polymerization using a bridged zirconecene and methyalumoxane co-catalyst. Polymeric olefin may be converted to a 2-chloro-4-methylphenyl (cmp) ester via the Koch reaction (BF3- catalyzed carbonylation in a continuous reactor). The ester may be aminated with polyamine. Ethylene butene and other copolymer dispersants are described, for example, in International patent publication number WO95/29976, the disclosure of which is incorporated herein by reference.

[0015] In addition to borated dispersants, other sources of boron which may provide or contribute to the total boron concentration include borated viscosity index improvers, borated detergents, alkyl borates and esters of borate acids.

[0016] 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 95.5 wt.%, more preferably from 80 to 98 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.

[0017] 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.

[0018] The amount of boron-containing compound, for example a borated dispersant, 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.

[0019] 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.

[0020] 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.

[0021] 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 C₅ to C₁₂ monocarboxylic acids and polyols and polyol ethers.

[0022] Silicon-based oils (such as the polyalkyl-, polyaryl-, polyalkoxy-, or polyaryloxy-siloxane 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] The invention is illustrated by the following Example :

Example 1

[0027] 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.

[0028] Comparative formulations were also blended containing no diphenylamine antioxidant.

[0029] The antioxidant performance of each of the formulations was determined using two test methods :

[0030] 

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.

[0031] 

TABLE 1

Component (wt%)	Formulation No.
	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)1,3	3	223	704	4	225	702
Wt.% N in diphenylamine2,3	0.044	0.045	0.047	-	-	-
Wt. ratio boron to nitrogen in diphenylamine	1:1.5x105	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

Notes 1 - Parts per million boron measured by Inductively Coupled Plasma Emission Spectroscopy (ASTM D 5185)

2 - Wt.% nitrogen measured according to the Kjeldahl method (ASTM D 3228)

3 - The amounts of boron and nitrogen are based on the total weight of the engine oil formulation

[0032] 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

1. An additive combination comprising:

(a) an aromatic amine antioxidant, and

(b) 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.

2. An additive combination according to claim 1 wherein the aromatic amine antioxidant is selected from phenylamine, diphenylamine, phenylnaphthylamine, and mixtures thereof, which compounds may be substituted or unsubstituted.

3. An additive combination according to claim 1 wherein the aromatic amine antioxidant is selected from alkyl substituted phenylamine, alkyl substituted diphenylamine, alkyl substituted phenylalphanaphthylamine, and mixtures thereof.

4. An additive combination according to any preceding claim wherein the boron-containing compound is a dispersant.

5. An additive combination according to claim 5 wherein the dispersant is a polyisobutene succinic acid, a polyisobutene succinic anhydride, a polyisobutene succinic acid - polyamine, a polyisobutene succinic anhydride - polyamine, or a derivative thereof.

6. A lubricant composition according to any of claims 1 to 4 which additionally comprises an ethylene butene dispersant.

7. A lubricant composition comprising a major amount of a lubricating oil basestock and a minor amount of the additive combination according to any preceding claim.

8. Use of boron or a boron-containing compound to enhance the antioxidant performance of an aromatic amine antioxidant in a lubricant composition.