WO2008053033A2

WO2008053033A2 - Lubricating oil composition comprising hydroxy-containing poly (meth) acrylate and metal dithiophosphate

Info

Publication number: WO2008053033A2
Application number: PCT/EP2007/061801
Authority: WO
Inventors: Ryuji Maruyama; Eiji Nagatomi; Tetsuo Wakizono
Original assignee: Showa Shell Sekiyu K.K.
Priority date: 2006-11-01
Filing date: 2007-11-01
Publication date: 2008-05-08
Also published as: CN101535455A; WO2008053033A3; US20100093576A1; EP2066773A2

Abstract

As transmissions have become more compact and are run under conditions of higher speeds and higher loads, the conditions of use for lubricating oils have become more severe. To this end, the invention provides a lubricating oil composition comprising a metal dithiophosphate, regarded in the prior art as having low extreme-pressure characteristics, and a poly (meth) acrylate containing a polar hydroxyl group in a base oil. With this lubricating oil composition, the anti-seizing characteristics are good and the extreme-pressure characteristics are the same as or better than with sulphur/phosphorus type extreme-pressure agents. Also, the fatigue characteristics are low, oxidation stability is high and long life can be achieved.

Description

LUBRICATING OIL COMPOSITION

This invention provides lubricating oil compositions and in particular transmission oils.

Since the transmissions in automotive vehicles are used under conditions of high speed and high load, the transmission oils that are used therein are required to have superior anti-seizing characteristics and anti-wear characteristics. Improvements in fuel consumption are also required. The usual methods of improving fuel consumption are to reduce viscosity and to reduce friction by adding friction modifiers, but reducing friction by adding friction modifiers gives rise to malfunctions in the synchronising mechanisms in the case of manual transmissions, so that the method of improving fuel consumption by reducing viscosity has become the main one.

However, reducing the viscosity of a lubricating oil gives rise to problems whereby the seizing characteristics are reduced and damage occurs in the machinery. Therefore, in order to increase their anti- seizing characteristics, transmission oils in automotive vehicles make use of extra amounts of extreme-pressure additives such as sulphur-type additives as typified by sulphurised olefins and sulphurised esters or phosphorus- type additives such as phosphate esters or amine salts of phosphate esters. See Japanese Laid-open Patent 2004- 262980, Japanese Laid-open Patent 10-259393 (1998) , Japanese Laid-open Patent 10-316987 (1998) .

In recent years, because of a sudden increase in the demand for high-speed transport used on high-speed roads, the proportion of high-speed running by automotive vehicles has increased, and at the same time there has been a considerable rise in temperatures within engines for transmission oils used in manual gearboxes and reduction gears, as transmission gear units have been made more compact in response to measures to achieve higher engine outputs and improved aerodynamics.

To deal with this, it has been necessary to add extra amounts of extreme-pressure agents such as the above mentioned sulphurised olefins and phosphate esters, but there has been a strong tendency thereby for deficiencies in oxidation stability and corrosive wear to be promoted at high temperatures (normally 150⁰C and above) . Problems with corrosion of metals, especially copper, and sludge arise and as a result the phenomenon of wear and seizure being promoted on gear-tooth surfaces occurs. Furthermore, in the case of manual transmissions, lowering of the friction coefficient or abnormal wear occurs in the synchroniser rings and gear cones within the synchromesh mechanisms, so that the synchronisation does not work well and problems develop such as increased effort needed to shift gears or even being unable to change gear .

Most recently, in order to resolve the above mentioned shift/synchronisation problems, there have been proposals to move from the above mentioned sulphur/phosphorus type extreme-pressure agents to the metallic detergent/zinc dithiophosphate type. But problems to do with not being able to achieve sufficient extreme-pressure performance have been pointed out. This invention is intended to obtain a lubricating oil composition whereby, even under high-speed and high- load operating conditions due to the more compact size of transmissions, no anti-seizing performance is lost, and yet there are adequate extreme-pressure qualities, fatigue characteristics are low, oxidation stability is high and longer life can be achieved.

In order to resolve the above mentioned problems, this invention provides a lubricating oil composition by incorporating into a mineral oil and/or synthetic oil base oil a poly (meth) acrylate which contains a polar hydroxyl group, together with a metal dithiophosphate, hitherto regarded as having low extreme-pressure characteristics.

According to this invention, it is possible to obtain a lubricating oil composition wherein the anti- seizing characteristics are good, the extreme-pressure characteristics are the same as or better than with sulphur/phosphorus type extreme-pressure agents, the fatigue characteristics are low, oxidation stability is high and long life can be achieved, and it is thus possible to obtain satisfactory lubrication performance even under conditions of high-speed and high-load operation where transmissions have been made more compact .

There are no special restrictions on the base oil in the lubricating oil composition of this invention, and mineral oil type base oils and/or synthetic type base oils as used in normal lubricating oils may be used.

These mineral oil base oils and synthetic base oils may be used by mixing, in any proportions, suitable combinations of mineral oil base oils, combinations of synthetic base oils, or mineral oil base oils and synthetic base oils together.

There are no special restrictions on the viscosity index of the lubricating oil base oil, but the value thereof is preferably be at least 90 (according to ASTM D2270) so as to obtain superior viscosity characteristics from low to high temperatures, or more preferably at least 100 and most preferably at least 110.

There are no special restrictions on the upper limit of the viscosity index, and it is possible for it to be around 135 to 180 as in mineral oils such as the normal paraffin, slack wax and GTL wax type or isoparaffin type in which these are isomerised, or around 150 to 225 as in complex ester type base oils or HVI-PAO type base oils. If the viscosity index of the lubricating oil base oil is less than 90, the amount of poly (meth) acrylate, which is added as a viscosity index improver, increases, and since poly (meth) acrylates of large molecular weight are used, the shear stability deteriorates, which is undesirable.

As specific illustration of mineral oil type base oils mention may be made of those refined by subjecting lubricating oil fractions obtained by vacuum distillation of atmospheric residues obtained by atmospheric distillation of crude oil to treatments such as solvent deasphalting, solvent extraction of aromatics, hydrocracking, solvent dewaxing and hydrorefining, or wax- isomerised mineral oils.

There are no special restrictions on lubricating oil base oils different from the above mentioned mineral oil base oils, and it is possible to use the synthetic oils used in the prior art. They may be selected or designated according, for instance, to application and may be used in any proportions with mineral oils. For example, mention may be made of poly-α-olefins, α-olefin copolymers, polybutenes, polyol esters, dibasic esters, polyhydric alcohol esters, polyoxyalkylene glycols, polyoxyalkylene glycol esters, polyoxyalkylene glycol ethers, and cycloalkane compounds.

Further examples are lubricating oil base oils manufactured by the method of isomerising GTL waxes (gas- to-liquid waxes) manufactured from natural gas by the Fischer-Tropsch process and so on.

The upper limit of the kinetic viscosity of the base oil at 100⁰C (according to ASTM D445) is preferably 8 mm²/s and more preferably 6 mm²/s. The lower limit of the kinetic viscosity of the base oil at 100⁰C is preferably 2 mm²/s and more preferably 3 mm²/s. If the kinetic viscosity at 100⁰C is less than 2 mm²/s,_, there will be inadequate formation of an oil film at the lubricating points, so that lubrication will deteriorate and there will be concern over seizing and wear of tooth surfaces. The evaporation losses of the base oil will also increase, which is not desirable.

A metal dithiophosphate is incorporated in the base oil in this invention. Zinc dialkyl dithiophosphates and or molybdenum dialkyl dithiophosphates and so on may be used for the metal dithiophosphate. Normally, zinc dialkyl dithiophosphates or molybdenum dialkyl dithiophosphates having, as the alkyl groups therein, primary or secondary alkyl groups having from 3 to 22 carbons or alkylaryl groups substituted by alkyl groups of from 3 to 18 carbons are used.

These zinc dialkyl dithiophosphates and molybdenum dialkyl dithiophosphates may be used alone or in combinations of two or more thereof, but particularly preferred, as they increase the wear resistance, are those in which the main constituent is a zinc dialkyl dithiophosphate with a secondary alkyl group. As specific examples of the above mentioned zinc dialkyl dithiophosphates, mention may be made of zinc dipropyl dithiophosphate, zinc dibutyl dithiophosphate, zinc dipentyl dithiophosphate, zinc dihexyl dithiophosphate, zinc diisopentyl dithiophosphate, zinc diethylhexyl dithiophosphate, zinc dioctyl dithiophosphate, zinc dinonyl dithiophosphate, zinc didecyl dithiophosphate, zinc didodecyl dithiophosphate, zinc dipropylphenyl dithiophosphate, zinc dipentylphenyl dithiophosphate, zinc dipropylmethylphenyl dithiophosphate, zinc dinonylphenyl dithiophosphate and zinc didodecylphenyl dithiophosphate.

As specific examples of the above mentioned molybdenum dialkyl dithiophosphates, mention may be made of molybdenum dipropyl dithiophosphate, molybdenum dibutyl dithiophosphate, molybdenum dipentyl dithiophosphate, molybdenum dihexyl dithiophosphate, molybdenum diisopentyl dithiophosphate, molybdenum diethylhexyl dithiophosphate, molybdenum dioctyl dithiophosphate, molybdenum dinonyl dithiophosphate, molybdenum didecyl dithiophosphate, molybdenum didodecyl dithiophosphate, molybdenum dipropylphenyl dithiophosphate, molybdenum dipentylphenyl dithiophosphate, molybdenum dipropylmethylphenyl dithiophosphate, molybdenum dinonylphenyl dithiophosphate and molybdenum didodecylphenyl dithiophosphate.

The zinc or molybdenum content in the lubricating oil composition in the case of blends with these zinc dialkyl dithiophosphates and/or molybdenum dialkyl dithiophosphates is approximately from 0.02 to 1.0 wt.%, preferably from 0.04 to 0.5 wt.%, and more preferably from 0.08 to 0.4 wt.%. A poly (meth) acrylate having a hydroxyl group is incorporated in the lubricating oil composition together with the above mentioned metal dithiophosphate . This poly (meth) acrylate having a hydroxyl group is a co- polymer, and is a co-polymer which has as its essential constituent monomers an alkyl (meth) acrylate having an alkyl group of from 1 to 20 carbons and a vinyl monomer containing a hydroxyl group.

As specific examples of the above mentioned alkyl (meth) acrylate (a) having an alkyl group of 1 to 20 carbons, mention may be made of:

(al) alkyl (meth) acrylates having an alkyl group of 1 to 4 carbons, for example: methyl (meth) acrylate, ethyl (meth) acrylate, n- or iso-propyl (meth) acrylate and n-, iso- or sec-butyl (meth) acrylate;

(a2) alkyl (meth) acrylates having an alkyl group of 8 to 20 carbons, for example: n-octyl (meth) acrylate, 2- ethylhexyl (meth) acrylate, n-decyl (meth) acrylate, n- isodecyl (meth) acrylate, n-undecyl (meth) acrylate, n- dodecyl (meth) acrylate, 2-methylundecyl (meth) acrylate, n-tridecyl (meth) acrylate, 2-methyldodecyl (meth) acrylate, n-tetradecyl (meth) acrylate, 2- methyltridecyl (meth) acrylate, n-pentadecyl (meth) acrylate, 2 -methyltetradecyl (meth) acrylate, n- hexadecyl (meth) acrylate and n-octadecyl (meth) acrylate, n-eicosyl (meth) acrylate, n-docosyl (meth) acrylate, (meth) acrylate of Dobanol 23 [mixture of oxoalcohols of 12 carbons/13 carbons manufactured by Mitsubishi Kasei Corp. Ltd], and (meth) acrylate of Dobanol 45 [mixture of oxoalcohols of 13 carbons/14 carbons manufactured by Mitsubishi Kasei Corp.]; (a3) alkyl (meth) acrylates having an alkyl group of 5 to 7 carbons, for example: n-pentyl (meth) acrylate and n- hexyl (meth) acrylate .

Of the above mentioned (al) to (a3), the preferred substances are those belonging to (al) and (a2) , and more preferably the substances of (a2) . Also, within (al) , those preferred from the standpoint of the viscosity index are those with alkyl groups of 1 to 2 carbons. Within the above mentioned (a2), those preferred from the standpoint of solubility in the base oil and low- temperature characteristics are those with alkyl groups of from 10 to 20 carbons, and more preferably from 12 to 14 carbons .

The above mentioned vinyl monomer (b) containing a hydroxl group and forming a co-polymer with the alkyl (meth) acrylate having an alkyl group of from 1 to 20 carbons is a vinyl monomer containing in its molecules one or more than one hydroxyl group (preferably one or two) . As specific examples, mention may be made of: (bl) Hydroxyalkyl (2 - 6 carbons) (meth) acrylates, for example: 2-hydroxyethyl (meth) acrylate, 2- or 3- hydroxypropyl (meth) acrylate, 2-hydroxybutyl (meth) acrylate and 1 -methyl -2-hydroxyethyl (meth) acrylate ; (b2) Mono- or di-hydroxyalkyl (1 - 4 carbons) substituted (meth) acrylamides, for example: N,N-dihydroxymethyl (meth) acrylamide, N,N-dihydroxypropyl (meth) acrylamide and N, N-di-2-hydroxybutyl (meth) acrylamide; (b3) Vinyl alcohols (formed by hydrolysis of vinyl acetate units) ;

(b4) Alkenols of from 3 to 12 carbons, for example: (meth)allyl alcohol, crotyl alcohol, isocrotyl alcohol, 1-octenol and 1-undecenol ; (b5) Alkene diols of from 4 to 12 carbons, for example: l-buten-3-ol, 2-buten-l-ol and 2-buten-l, 4-diol ; (b6) Hydroxyalkyl (from 1 to 6 carbons) alkenyl (3 - 10 carbons) ethers, for example: 2-hydroxyethylpropenyl ether;

(b7) Aromatic monomers containing a hydroxyl group, for example: o- , m- or p-hydroxystyrene;

(b8) Polyhydric (from trihydric to octahydric) alcohols, for example: alkane polyols, intramolecular or intermolecular dehydrates thereof, alkenyl (3 - 10 carbons) ethers of sugars (e.g. glycerine, pentaerythritol, sorbitol, sorbitan, diglycerine, sucrose) or (meth) acrylates of sugars (e.g. sucrose (meth) acryl ether) ; (b9) polyoxyakylene chains and vinyl monomers containing hydroxyl groups, for example: mono (meth)'acrylates or mono (meth) allyl ethers of polyoxyalkylene glycols (alkylene group of from 2 to 4 carbons, degree of polymerisation from 2 to 50) or polyoxyalkylene polyols {polyoxyalkylene ethers (alkyl groups of from 2 to 4 carbons, degree of polymerisation from 2 to 100) of the above mentioned trihydric to octahydric alcohols} {e.g. polyethylene glycol (degree of polymerisation from 2 to 9) mono (meth) acrylates, polypropylene glycol (degree of polymerisation from 2 to 30) mono (meth) allyl ether}.

Of the above mentioned (bl) to (b9) , from the standpoint of effect of improving the viscosity index the preferred type is (bl) , and 2 -hydroxy-ethyl methacrylate in particular. The respective proportions in the monomer constituting the above mentioned poly (meth) acrylate copolymer containing a hydroxyl group are, from the standpoint of the viscosity index, preferably as follows. The lower limit of the above mentioned constituent (a) is preferably 50 wt . % and more preferably 75 wt.%, and the upper limit is preferably 95 wt.% and more preferably 85 wt.%. The lower limit of the above mentioned (al) is preferably 0 wt . % and more preferably 1 wt.%, and the upper limit is preferably 20 wt.% and more preferably 10 wt.%.

The lower limit of the above mentioned (a2) is preferably 50 wt.% and more preferably 70 wt.%, and the upper limit is preferably 95 wt.% and more preferably 90 wt.%.

The lower limit of the above mentioned (b) is preferably 5 'wt.% and more preferably 7 wt.%, but particularly preferable is 11 wt.% , and the upper limit is preferably 50 wt.% and more preferably 30 wt.%, but particularly preferable is 15 wt.%.

The lower limit of the sum of the above mentioned (a) + (b) is preferably 55 wt.% and more preferably 82 wt.%, and the upper limit is preferably 100 wt.%.

Other monomers may be co-polymerised together with the above mentioned (a) and (b) in the above mentioned poly (meth) acrylate co-polymer containing a hydroxyl group. Such monomers include monomers containing nitrogen atoms (c) . Specific examples include:

(cl) Monomers containing a nitro group, for example 4- nitrostyrene;

(c2) Vinyl monomers containing primary ~ tertiary amines, such as : (c2-l) Vinyl ^' monomers containing primary amines, for example: 3-6-carbon alkenyl amine [ (meth) allylamine, crotylamine, and so on] , aminoalkyl (2 - 6 carbons) (meth) acrylates [aminoethyl (meth) acrylate and so on] ; (c2-2) Vinyl monomers containing secondary amines, for example: alkyl (1 - 6 carbons) aminoalkyl (2 - 6 carbons) (meth) acrylates [t-butylaminoethyl methacrylate, methylaminoethyl (meth) acrylate, and so on], diphenylamine (meth) acrylamides [4-diphenylamine

(meth) acrylamide, 2 -diphenylamine (meth) acrylamide, and so on] , and dialkenylamines with 6 - 12 'carbons

[di (meth) allyl amine and so on] ;

(c2-3) Vinyl monomers containing tertiary amines, for example: dialkyl (1 - 4 carbons) aminoalkyl (2 - 6 carbons) (meth) acrylates [dimethylaminoethyl (meth) acrylate, diethylaminoethyl (meth),acrylate, and so on] , dialkyl (1 - 4 carbons) aminoalkyl (2 - 6 carbons) (meth) acrylamides [dimethylaminoethyl (meth) acrylamide, diethylaminoethyl (meth) acrylamide, dimethylaminopropyl

(meth) acrylamide, and so on], and aromatic vinyl monomers containing tertiary amino groups [N, N- dimethylaminostyrene and so on] ; (c2-4) Vinyl monomers containing complex rings containing nitrogen [morpholinoethyl (meth) acrylate, 4- vinylpyridine, 2-vinylpyridine, N-vinylpyrrole, N- vinylpyrrolidone, N-vinylthiopyrrolidone, and so on]; (c3) Amphoteric vinyl monomers, for example: N- (meth) acryloyloxy (or amino) alkyl (1 - 10 carbons) N,N- dialkyl (1 - 5 carbons) ammonium-N-alkyl (1 - 5 carbons) carboxylates (or sulphates), e.g. N-

(meth) acryloyloxyethyl N, N-dimethylammonium N-methyl carboxylate, and N- (meth) acryloyloxyethyl N₇N- dimethylammonium propyl sulphate; (c4) monomers containing nitrile groups, for example: acrylonitrile . Such monomers also include aliphatic hydrocarbon vinyl monomers (d) , for example: alkenes of 2 ~ 20 carbons [ethylene, propylene, butene, isobutylene, pentene, heptene, diisobutylene, octene, dodecene, octadecene, and so on] and alkadienes of 4 ~ 12 carbons [butadiene, isoprene, 1, 4-pentadiene, 1, 6-heptadiene, 1 , 7-octadiene, and so on] .

Further, there are alicyclic hydrocarbon vinyl monomers (e) , for example: cyclohexene, (di) cyclopentadiene, pinene, limonene, indene, vinylcyclohexene and ethylidenebicycloheptene .

There are also aromatic hydrocarbon vinyl monomers (f) , for example: styrene, α-methylstyrene, vinyltoluene, 2 , 4-dimethylstyrene, 4 -ethylstyrene, 4-isopropylstyrene, 4-butylstyrene, 4 -phenylstyrene, 4 -cyclohexylstyrene, 4- benzylstyrene , 4-crotylbenzene and 2-vinylnaphthalene .

And there are vinyl esters, vinyl ethers, vinyl ketones (g) , for example: vinyl esters of saturated fatty acids of 2 - 12 carbons [vinyl acetate, vinyl propionate, vinyl butyrate, vinyl octanoate, and so on] , vinyl ethers of alkyls, aryls or alkoxyalkyls of 1 ~ 12 carbons [methylvinyl ether, ethylvinyl ether, propylvinyl ether, butylvinyl ether, 2-ethylhexylvinyl ether, phenylvinyl ether, vinyl 2-methoxyethyl ether, vinyl 2-butoxyethyl ether, and so on] , and vinyl ketones of alkyls or aryls of 1 ~ 8 carbons [methylvinylketone, ethylvinylketone, phenylvinylketone] .

Further,, there are esters of unsaturated polycarboxylic acids (h) . For example mention may be made of alkyl, cycloalkyl or aralkyl esters of unsaturated polycarboxylic acids, and these include alkyl diesters of unsaturated dicarboxylic acids [maleic acid, fumaric acid, itaconic acid, and so on] with 1-8 carbons [dimethyl maleate, dimethyl fumarate, diethyl maleate, dioctyl maleate] .

It is also possible to use vinyl monomers containing polyoxyalkylene chains (those not containing hydroxyl groups) (i) , for example: mono (meth) acrylates of monoalkyl (1- 18 carbons) ethers of polyoxyalkylene glycols (alkylene groups of 2 ~ 4 carbons, degree of polymerisation 2 - 50) or polyoxyalkylene polyols [polyoxyalkylene ethers of the above mentioned trihydric to octahydric alcohols (alkyl groups of 2 ~ 4 carbons, degree of polymerisation 2 ~ 100)] [for example, methoxypolyethylene glycol (molecular weight 110 ~ 310) (meth) acrylate and lauryl alcohol ethylene oxide adduct (2 ~ 30 mol) (meth) acrylate] . It is possible to use vinyl monomers containing carboxyl groups (j), for example: vinyl monomers containing monocarboxyl groups such as unsaturated monocarboxylic acids [ (meth) acrylic acid, α-methyl (meth) acrylic acid, crotonic acid, cinnamic acid, and so on] , monoalkyl (1 - 8 carbons) esters of unsaturated dicarboxylic acids [monoalkyl esters of maleic acid, monoalkyl esters of fumaric acid, monoalkyl esters of itaconic acid, and so on] ; it is also possible to use copolymers of two or more vinyl monomers containing carboxyl groups, for example maleic, acid, fumaric acid, itaconic acid and citraconic acid.

Of the above mentioned additional monomers (c) , (d) , (e) , (f) , (g) , (h) , (i) and (j), those preferred are (c) , and of (c) it is possible to use two or more. Of the above mentioned (c) , those preferred are (c2) , and even more preferred are dimethylaminoethyl (meth) acrylate and diethylaminoethyl (meth) acrylate . The lower limit of the weight average molecular weight of the poly (meth) acrylate containing a hydroxyl group is preferably 5,000 and more preferably 8,000, while 10,000 is especially preferred. The upper limit is preferably 50,000 and more preferably 40,000, while

35,000 is especially preferred and 30,000 is even more especially preferred.

If the weight average molecular weight is within the above mentioned range, good shear stability can be imparted. This weight average molecular weight is according to gel permeation chromatography, and is obtained by conversion to polystyrene.

The above mentioned weight average molecular weights may be adjusted on the basis of the temperature at the time of polymerisation, monomer density (solvent concentration) , amount of catalyst or amount of chain transfer agent.

The polydispersity (Mw/Mn) of this poly (meth) acrylate containing a hydroxyl group is preferably 1.0 ~ 2.5, and more preferably 1.2 - 2.0, but especially preferred is 1.5 - 1.7. With a small polydispersity, the shear stability is good. Mn is obtained in the same way as Mw.

Also, the lower limit of the solubility parameter thereof is preferably 8.6, and more preferably 9.2, but especially preferred is 9.3, while the upper limit is preferably 11, and more preferably 10.5, but especially preferred is 9.7. This solubility parameter value was calculated by the method of Fedors (Polym. Eng. Sci. 14 (2) , 152 (1974) ) .

Further, the HLB of the poly (meth) acrylate containing a ^'hydroxyl group is preferably 0.5 - 7. If the HLB is within this range, the demulsification characteristics are especially good. More preferable is a HLB of 1 ~ 6.5, and especially preferred is 1.5 ~ 6. This HLB value is calculated by Oda ' s method, which is based on organic and inorganic concepts ("New Introduction to Surfactants", published by Sanyo Kasei Kogyo Co. Ltd., page 128) .

The hydroxyl number of the poly (meth) acrylate containing a hydroxyl group, used as an additive, is 10 ~ 100, preferably 20 ~ 50, and more preferably 25 ~ 35. The measurement of the hydroxyl number is the value obtained by measuring in accordance with JIS K3342 (1961) . It shows the amount of hydroxyl group in the additive.

It is also possible to add the normal type of poly (meth) acrylates of the prior art which do not contain a hydroxyl group at the same time as the poly (meth) acrylate containing a hydroxyl group. The amount thereof is encompassed by the ratio poly (meth) acrylate containing a hydroxyl group : poly (meth) acrylate not containing a hydroxyl group being 100 :0 - 40:60.

It is also possible to add, as required, suitable amounts of other additives such as rust preventatives, detergents, dispersants, anti-oxidants, extreme-pressure agents, oiliness agents, friction modifiers, pour-point depressants and defoaming agents. Examples

The following test materials were prepared to produce Examples of Embodiment 1 to 6 and Comparative Examples 1 to 5. (1) Base oils

(1-1) Base Oil A: the oil type was a mixture of Group I, Group II and Group III oils (characteristics: kinetic viscosity at 40⁰C, 18.7 mm²/s; kinetic viscosity at 100⁰C, 4.1 mm²/s; viscosity index, 122);

(1-2) Base Oil B: the oil type was a mixture of Group I, Group II and 'Group V oils (characteristics: kinetic viscosity at 40⁰C, 16.5 mm²/s; kinetic viscosity at 100⁰C, 3.6 mm²/s; viscosity index, 96).

(2) Poly (meth) acrylate (PMA) containing a hydroxyl group: weight average molecular weight (Mw), 16,000; hydroxyl number 30 (Aclube V1070 manufactured by Sanyo Kasei Kogyo Co. Ltd.) .

(3) Poly (meth) acrylates (PMA) of the conventional type not containing a hydroxyl group

(3-1) Conventional PMA (I) : weight average molecular weight, 15,000 (Viscoplex 0-050 manufactured by Degussa); (3-2) Conventional PMA (II) : weight average molecular weight, 35,000 (Aclube 811 manufactured by Sanyo Kasei Kogyo Co . Ltd. ) ;

(3-3) Conventional PMA (III) : weight average molecular weight, 75,000 (Viscoplex 0-291 manufactured by Degussa) . (4) Zinc dialkyl dithiophosphates

(4-1) ZnDTP (I) : Primary-type zinc diethylhexyl dithiophosphate ;

(4-2) ZnDTP (II) : Secondary-type zinc dialkyl dithiophosphate with 3 - 6 carbons. (5) Phenolic , anti -oxidant : manufactured by Ciba Specialty Chemicals, Irganox-L135 , benzenepropanoic acid, 3,5- bis (1, 1-dimethyl-ethyl) -4-hydroxy, C7-C9 side chain alkyl ester . (6) Sulphurised fat/oil: sulphurised lard oil (7) Phosphate ester: alkylated triphenyl phosphate

(8) Packaged additives: detergent + dispersant + antioxidant . The lubricating oil compositions for Examples 1 to 6 and Comparative Examples 1 to 4 that were used to compare therewith were obtained by using the above mentioned test materials and thoroughly mixing them on the basis of the compositions shown in Tables 1 and 2. A commercial transmission oil was used for Comparative Example 5. Tests

The following tests were carried out in order to compare the performances of the examples and the comparative examples.

Tests to measure characteristics and composition

1. 40⁰C kinetic viscosity (ASTM D445; 'units: mm²/s)

2. 100⁰C kinetic viscosity (ASTM D445; units: mm²/s)

3. Viscosity index (VI; ASTM D2270) 4. Phosphorus content (units: % by weight) 5. Sulphur content (units: % by weight) Falex seizure test

Test conditions: The test pieces were set up in the test rig and an oil bath containing 60 ml of test oil was heated by means of a heater until the oil temperature reached 100⁰C.

When the oil temperature had reached 100⁰C, the test rig drive motor was actuated and the test began with a load on the block of 136.08 kg (300 pounds) . After running for 2 minutes at 136.08 kg (300 pounds), the load was increased to 226.8 kg (500 pounds), running for 1 minute. Thereafter, the load was increased by 136.08 kg (300 pounds) a time, running for 1 minute, this being repeated until there was an occurrence of seizing. Test pieces: (1) Falex #8 test pin

(2) Falex Xee block [standard]

Evaluation: Assessment of the occurrence of seizing was based on the occurrence of a sudden rise in friction torque and the abnormal screeching noise, or on rupture of the pin.

The heater was stopped when the temperature reached 100⁰C, and the test was continued in that state. Higher values show superior anti-seizing properties.

ISOT test

Test specimens on which an oxidation test had been carried out under conditions of 135 + 0.5⁰C x 96 hr in accordance with JIS K2514 were compared with new oil, and the rate of change in kinetic viscosity at 4O⁰C (increase rate) and rate of change in kinetic viscosity at 100⁰C (increase rate) were measured. A small increase rate is best and the¹pass value was set at 8 or below. Sonic test Under the test conditions specified in JASO M347-95, a 30 ml test sample was exposed to ultrasonic waves for 1 hour, and the rate of change in kinetic viscosity at 40⁰C (reduction rate) and rate of change in kinetic viscosity at 100⁰C (reduction rate) were measured with new oil. A small reduction rate is best and the pass value was set at 7 or below. FZG pitting test

In accordance with DIN 51354. Type: PT-C gears. Performed with load: 9 stages, oil temperature: 120⁰C, speed: 1440 revolutions/min.

The method of assessing fatigue life was to stop the test rig and carry out periodic inspections of the tooth surfaces, and to reckon it as the time (hr) when 1 mm² of pitting had occurred. The inspection intervals were every 8 hours in the initial 24 hours and then every 2 - 4 hours . Results

The results of the above mentioned tests are shown in Tables 1 and 2. Discussion In the case of Examples 1 - 6, good results were obtained for anti-seizing properties on the basis that all were 907.2 kg (2000 pounds) or higher in the Falex seizing test. Long fatigue lives of 42 ~ 75 hours were obtained, too, in the FZG pitting tests. Good results at a steady level were also obtained in the ISOT and Sonic tests .

Comparative Examples 1, 2 and 4 used zinc dipropylphenyl dithiophosphate, but used a conventional type of PMA which did not contain a hydroxyl group. Although Comparative Example 1 gave good results in the ISOT and Sonic tests, it was inferior as regards anti- seizing properties to the examples of embodiment, scoring 567 kg (= 1250 pounds) in the Falex seizing test. Comparative Examples 2 and 4 gave good results in the ISOT test, but were inferior as regards 'anti-seizing properties to the examples of embodiment in the Falex seizing test.

Comparative Example 3 contained a PMA containing a hydroxyl group and a conventional type of PMA not containing a hydroxyl group, but it did ,not incorporate ZnDTP. Instead, it contained an SP type additive, and a reasonable enough result for anti-seizing properties was obtained with 793.8 kg (= 1750 pounds) in the Falex seizing test, but a satisfactory result was not achieved as regards oxidation stability in the ISOT test.

Also, in the FZG pitting test, Comparative Example 1 scored 42 hours but in the case of Comparative Examples 2 and 4 the fatigue life was short, at 30 ~ 32 hours. Comparative Example 3 had bad results in the ISOT test and so the FZG pitting test was omitted.

The commercial transmission oil of Comparative Example 5 had poor results in the Sonic test, and did not give good results either in the Falex seizing test and FZG pitting test.

Table 1

Table 1 (continued)

K* K*

Table 2

K*

Table 2 (continued)

Claims

C L A I M S

1. Lubricating oil composition comprising a base oil, a poly (meth) acrylate containing a hydroxyl group and a metal dithiophosphate .

2. Lubricating oil composition according to Claim 1 wherein the weight average molecular weight of the above mentioned poly (meth) acrylate containing .a hydroxyl group is from 5,000 to 50,000.

3. Lubricating oil composition according to Claim 1 or 2 comprising a further poly (meth) acrylate which does not contain a hydroxyl group.

4. Lubricating oil composition according to Claim 3 wherein the weight average molecular weight of the above mentioned poly (meth) acrylate which does not contain a hydroxyl group is from 20,000 to 100,000.

5. Lubricating oil composition according to Claim 3 or 4 wherein the weight ratio of the above mentioned poly (meth) acrylate containing a hydroxyl group and the above mentioned poly (meth) acrylate which does not contain a hydroxyl group is in the range of 100:0 to 40:60.

6. Lubricating oil composition according to any of

Claims 1 to 5 further comprising a sulphurised fat or oil and a phosphate ester.

7. Lubricating oil composition according to any of Claims 1 to 6 wherein the above mentioned base oil has a kinetic viscosity at 100⁰C of at least 2 mm²/s and a viscosity index of at least 90.

8. Lubricating oil composition according to any of Claims 1 to 7 wherein the above mentioned metal dithiophosphate is a zinc dithiophosphate.

9. Use of the lubricating oil composition according to any one of the preceding Claims 1 to 8 as a transmission oil.

10. Method of improving anti-seizing properties by using the lubricating oil composition according to any one of the preceding Claims 1 to 8.