Abstract:
A two-stroke or four-stroke marine engine lubricating oil composition comprising an oil of lubricating viscosity in a major amount and (A) additives in respective minor amounts; and (B) a polymethacrylate viscosity modifier. Preferably, brightstock is completely or substantially absent from the composition.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to the lubrication of 2-stroke and 4-stroke marine diesel internal combustion engines, the former usually being referred to as cross-head engines and the latter as trunk piston engines. Respective lubricants therefor are usually known as marine diesel cylinder lubricants (“MDCL&#39;s”) and trunk piston engine oils (“TPEO&#39;s”). 
       BACKGROUND OF THE INVENTION 
       [0002]    Cross-head engines are slow engines with a high to very high power range. They include two separately-lubricated parts: the piston/cylinder assembly lubricated with total-loss lubrication by a highly viscous oil (an MDCL); and the crankshaft lubricated by a less viscous lubricant, usually referred to as a system oil. 
         [0003]    Trunk piston engines may be used in marine, power-generation and rail traction applications and have a higher speed than cross-head engines. A single lubricant (TPEO) is used for crankcase and cylinder lubrication. All major moving parts of the engine, i.e. the main and big end bearings, camshaft and valve gear, are lubricated by means of a pumped circulation system. The cylinder liners are lubricated partially by splash lubrication and partially by oil from the circulation systems that finds its way to the cylinder wall through holes in the piston skirt via the connecting rod and gudgeon pin. 
         [0004]    It is known in the art to include brightstock in MDCL&#39;s and TPEO&#39;s, brightstock being a high viscosity oil that is highly refined and dewaxed and that is produced from residual stocks or bottoms. It may, for example, have a kinematic viscosity at 100° C. of greater than 25, usually greater than 30, mm 2 s −1 , such as a solvent-extracted, de-asphalted product from vacuum residuum generally having a kinematic viscosity at 100° C. of 28-36 mm 2 s −1 . 
         [0005]    Brightstock is however expensive and art describes ways of replacing it. WO 99/64543 describes MDCL&#39;s formulated without brightstock and US 2008/0287329 describes a TPEO containing little or no brightstock. 
         [0006]    A problem in the art is to formulate brightstock-free MDCL&#39;s and TPEO&#39;s at reduced cost and at the same time provide improved antiwear properties. 
       SUMMARY OF THE INVENTION 
       [0007]    It is now found that the use of polymethacrylate in an MDCL or a TPEO enables the above problem to be overcome. 
         [0008]    Thus, the present invention provides a two-stroke or four-stroke marine engine lubricating oil composition comprising an oil of lubricating viscosity in a major amount and
       (A) additives, in respective minor amounts; and   (B) a viscosity modifier in the form of a polymethacrylate in an amount in the range of 0.05-6 mass %,
 
wherein the composition includes less than 0.5 mass %, preferably less than 0.1 mass %, of brightstock; preferably brightstock is completely or substantially absent from the composition.
       
 
         [0011]    In further aspects the present invention comprises: 
         [0012]    The use of a viscosity modifier (B) to improve the anti-wear properties of a marine diesel cylinder lubricant of a trunk piston engine oil which includes less than 0.5 mass %, preferably less than 0.1 mass %, of brightstock; preferably brightstock is absent or is substantially absent from the marine diesel cylinder lubricant or the trunk piston engine oil; 
         [0013]    A method of lubricating a cross-head marine diesel engine comprising supplying the composition to the piston/cylinder assembly of the engine; and 
         [0014]    A method of lubricating a trunk piston marine diesel engine comprising supplying the composition to the engine. 
         [0015]    In this specification, the following words and expressions, if and when used, have the meanings ascribed below: 
         [0016]    “active ingredients” or “(a.i.)” refers to additive material that is not diluent or solvent; 
         [0017]    “comprising” or any cognate word specifies the presence of stated features, steps, or integers or components, but does not preclude the presence or addition of one or more other features, steps, integers, components or groups thereof; the expressions “consists of” or “consists essentially of” or cognates may be embraced within “comprises” or cognates, wherein “consists essentially of” permits inclusion of substances not materially affecting the characteristics of the composition to which it applies; 
         [0018]    “major amount” means 40 or 50 mass % or more of a composition; 
         [0019]    “minor amount” means less than 50 mass % of a composition; 
         [0020]    “TBN” means total base number as measured by ASTM D2896. 
         [0000]    Furthermore in this specification, if and when used: 
         [0021]    “calcium content” is as measured by ASTM 4951; 
         [0022]    “phosphorus content” is as measured by ASTM D5185; 
         [0023]    “sulphated ash content” is as measured by ASTM D874; 
         [0024]    “sulphur content” is as measured by ASTM D2622; 
         [0025]    “KV100” means kinematic viscosity at 100° C. as measured by ASTM D445. 
         [0026]    Also, it will be understood that various components used, essential as well as optimal and customary, may react under conditions of formulation, storage or use and that the invention also provides the product obtainable or obtained as a result of any such reaction. 
         [0027]    Further, it is understood that any upper and lower quantity, range and ratio limits set forth herein may be independently combined. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The features of the invention will now be discussed in more detail below. 
       Oil of Lubricating Viscosity 
       [0029]    The lubricant composition contains a major proportion of an oil of lubricating viscosity. Such lubricating oils may range in viscosity from light distillate mineral oils to heavy lubricating oils. Generally, the viscosity of the oil ranges from 2 to 40, such as 3 to 15, mm 2 /sec, as measured at 100° C., and a viscosity index of 80 to 100, such as 90 to 95. The lubricating oil may comprise greater than 60, typically greater than 70. mass % of the composition. 
         [0030]    Natural oils include animal oils and vegetable oils (e.g., castor oil, lard oil); liquid petroleum oils and hydrorefined, solvent-treated or acid-treated mineral oils of the paraffinic, naphthenic and mixed paraffinic-naphthenic types. Oils of lubricating viscosity derived from coal or shale also serve as useful base oils. 
         [0031]    Synthetic lubricating oils include hydrocarbon oils and halo-substituted hydrocarbon oils such as polymerized and interpolymerized olefins (e.g., polybutylenes, polypropylenes, propylene-isobutylene copolymers, chlorinated polybutylenes, poly(1-hexenes), poly(1-octenes), poly(1-decenes)); alkybenzenes (e.g., dodecylbenzenes, tetradecylbenzenes, dinonylbenzenes, di(2-ethylhexyl)benzenes); polyphenyls (e.g., biphenyls, terphenyls, alkylated polyphenols); and alkylated diphenyl ethers and alkylated diphenyl sulphides and derivative, analogues and homologues thereof 
         [0032]    Alkylene oxide polymers and interpolymers and derivatives thereof where the terminal hydroxyl groups have been modified by esterification, etherification, etc., constitute another class of known synthetic lubricating oils. These are exemplified by polyoxyalkylene polymers prepared by polymerization of ethylene oxide or propylene oxide, and the alkyl and aryl ethers of polyoxyalkylene polymers (e.g., methyl-polyiso-propylene glycol ether having a molecular weight of 1000 or diphenyl ether of poly-ethylene glycol having a molecular weight of 1000 to 1500); and mono- and polycarboxylic esters thereof, for example, the acetic acid esters, mixed C 3 -C 8  fatty acid esters and C 13  oxo acid diester of tetraethylene glycol. 
         [0033]    Another suitable class of synthetic lubricating oils comprises the esters of dicarboxylic acids (e.g., phthalic acid, succinic acid, alkyl succinic acids and alkenyl succinic acids, maleic acid, azelaic acid, suberic acid, sebacic acid, fumaric acid, adipic acid, linoleic acid dimer, malonic acid, alkylmalonic acids, alkenyl malonic acids) with a variety of alcohols (e.g., butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol, diethylene glycol monoether, propylene glycol). Specific examples of such esters includes dibutyl adipate, di(2-ethylhexyl) sebacate, di-n-hexyl fumarate, dioctyl sebacate, diisooctyl azelate, diisodecyl azelate, dioctyl phthalate, didecyl phthalate, dieicosyl sebacate, the 2-ethylhexyl diester of linoleic acid dimer, and the complex ester formed by reacting one mole of sebacic acid with two moles of tetraethylene glycol and two moles of 2-ethylhexanoic acid. 
         [0034]    Esters useful as synthetic oils also include those made from C 5  to C 12  monocarboxylic acids and polyols and polyol esters such as neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol and tripentaerythritol. 
         [0035]    Silicon-based oils such as the polyalkyl-, polyaryl-, polyalkoxy- or polyaryloxysilicone oils and silicate oils comprise another useful class of synthetic lubricants; such oils include tetraethyl silicate, tetraisopropyl silicate, tetra-(2-ethylhexyl)silicate, tetra-(4-methyl-2-ethylhexyl)silicate, tetra-(p-tert-butyl-phenyl) silicate, hexa-(4-methyl-2-ethylhexyl)disiloxane, poly(methyl)siloxanes and poly(methylphenyl)siloxanes. Other synthetic lubricating oils include liquid esters of phosphorus-containing acids (e.g., tricresyl phosphate, trioctyl phosphate, diethyl ester of decylphosphonic acid) and polymeric tetrahydrofurans. 
         [0036]    Unrefined, refined and re-refined oils can be used in lubricants of the present invention. Unrefined oils are those obtained directly from a natural or synthetic source without further purification treatment. For example, a shale oil obtained directly from retorting operations; petroleum oil obtained directly from distillation; or ester oil obtained directly from esterification and used without further treatment are unrefined oils. 
       Marine Diesel Cylinder Lubricant (“MDCL”) 
       [0037]    An MDCL may employ 10-35, preferably 13-30, most preferably 16-24, mass % of a concentrate or additive package, the remainder being base stock. It preferably includes at least 50, more preferably at least 60, even more preferably at least 70, mass % of oil of lubricating viscosity based on the total mass of MDCL. Preferably, the MDCL has a compositional TBN (using ASTM D2896) of 40-100, such as 50-60. 
         [0038]    The following may be mentioned as examples of typical proportions of additives in an MDCL. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Mass % a.i. 
                 Mass % a.i. 
               
               
                   
                 Additive 
                 (Broad) 
                 (Preferred) 
               
               
                   
                   
               
             
             
               
                   
                 detergent(s) 
                  1-20 
                  3-15 
               
               
                   
                 dispersant(s) 
                 0.5-5   
                 1-3 
               
               
                   
                 anti-wear agent(s) 
                 0.1-1.5 
                 0.5-1.3 
               
               
                   
                 pour point dispersant 
                 0.03-1.15 
                 0.05-0.1  
               
               
                   
                 base stock 
                 balance 
                 balance 
               
               
                   
                   
               
             
          
         
       
     
       Trunk Piston Engine Oil (“TPEO”) 
       [0039]    A TPEO may employ 7-35, preferably 10-28, more preferably 12-24, mass % of a concentrate or additives package, the remainder being base stock. Preferably, the TPEO has a compositional TBN (using D2896) of 20-60, such as 25-55. 
         [0040]    The following may be mentioned as typical proportions of additives in a TPEO. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Mass % a.i. 
                 Mass % a.i. 
               
               
                   
                 Additive 
                 (Broad) 
                 (Preferred) 
               
               
                   
                   
               
             
             
               
                   
                 detergent(s) 
                 0.5-12  
                 2-8 
               
               
                   
                 dispersant(s) 
                 0.5-5   
                 1-3 
               
               
                   
                 anti-wear agent(s) 
                 0.1-1.5 
                 0.5-1.3 
               
               
                   
                 oxidation inhibitor 
                 0.2-2   
                 0.5-1.5 
               
               
                   
                 rust inhibitor 
                 0.03-0.15 
                 0.05-0.1  
               
               
                   
                 pour point dispersant 
                 0.03-1.15 
                 0.05-0.1  
               
               
                   
                 base stock 
                 balance 
                 balance 
               
               
                   
                   
               
             
          
         
       
     
         [0041]    When a plurality of additives is employed it may be desirable, although not essential, to prepare one or more additive packages comprising the additives, whereby several additives can be added simultaneously to the base oil to form the lubricating oil composition. Dissolution of the additive package(s) into the lubricating oil may be facilitated by solvents and by mixing accompanied with mild heating, but this is not essential. The additive package(s) will typically be formulated to contain the additive(s) in proper amounts to provide the desired concentration, and/or to carry out the intended function, in the final formulation when the additive package(s) is/are combined with a predetermined amount of base lubricant. Thus, compounds in accordance with the present invention may be admixed with small amounts of base oil or other compatible solvents together with other desirable additives to form additive packages containing active ingredients. 
         [0042]    More detailed description of additive components is given below. 
       Detergents 
       [0043]    A detergent is an additive that reduces formation of deposits, for example, high-temperature varnish and lacquer deposits, in engines; it has acid-neutralising properties and is capable of keeping finely divided solids in suspension. It is based on metal “soaps”, that is, metal salts of acidic organic compounds, sometimes referred to as surfactants. 
         [0044]    A detergent comprises a polar head with a long hydrophobic tail. Large amounts of a metal base are included by reacting an excess of a metal compound, such as an oxide or hydroxide, with an acidic gas such as carbon dioxide to give an overbased detergent which comprises neutralised detergent as the outer layer of a metal base (e.g. carbonate) micelle. 
         [0045]    The detergent is preferably an alkali metal or alkaline earth metal additive such as an overbased oil-soluble or oil-dispersible calcium, magnesium, sodium or barium salt of a surfactant selected from phenol, sulphonic acid, carboxylic acid, salicylic acid and naphthenic acid, wherein the overbasing is provided by an oil-insoluble salt of the metal, e.g. carbonate, basic carbonate, acetate, formate, hydroxide or oxalate, which is stabilised by the oil-soluble salt of the surfactant. The metal of the oil-soluble surfactant salt may be the same or different from that of the metal of the oil-insoluble salt. Preferably the metal, whether the metal of the oil-soluble or oil-insoluble salt, is calcium. 
         [0046]    The TBN of the detergent may be low, i.e. less than 50 mg KOH/g, medium, i.e. 50-150 mg KOH/g, or high, i.e. over 150 mg KOH/g, as determined by ASTM D2896. Preferably the TBN is medium or high, i.e. more than 50 TBN. More preferably, the TBN is at least 60, more preferably at least 100, more preferably at least 150, and up to 500, such as up to 350 mg KOH/g, as determined by ASTM D2896. 
       Anti-Oxidants 
       [0047]    The trunk piston diesel engine lubricant composition may include at least one anti-oxidant. The anti-oxidant may be aminic or phenolic. As examples of amines there may be mentioned secondary aromatic amines such as diarylamines, for example diphenylamines wherein each phenyl group is alkyl-substituted with an alkyl group having 4 to 9 carbon atoms. As examples of anti-oxidants there may be mentioned hindered phenols, including mono-phenols and bis-phenols. 
         [0048]    Preferably, the anti-oxidant, if present, is provided in the composition in an amount of up to 3 mass %, based on the total amount of the lubricant composition. 
         [0049]    Other additives such as pour point depressants, anti-foamants, metal rust inhibitors, pour point depressants and/or demulsifiers may be provided, if necessary. 
         [0050]    The terms ‘oil-soluble’ or ‘oil-dispersable’ as used herein do not necessarily indicate that the compounds or additives are soluble, dissolvable, miscible or capable of being suspended in the oil in all proportions. These do mean, however, that they are, for instance, soluble or stably dispersible in oil to an extent sufficient to exert their intended effect in the environment in which the oil is employed. Moreover, the additional incorporation of other additives may also permit incorporation of higher levels of a particular additive, if desired. 
         [0051]    The lubricant compositions of this invention comprise defined individual (i.e. separate) components that may or may not remain the same chemically before and after mixing. 
         [0052]    It may be desirable, although not essential, to prepare one or more additive packages or concentrates comprising the additives, whereby the additives can be added simultaneously to the oil of lubricating viscosity to form the lubricating oil composition. Dissolution of the additive package(s) into the lubricating oil may be facilitated by solvents and by mixing accompanied with mild heating, but this is not essential. The additive package(s) will typically be formulated to contain the additive(s) in proper amounts to provide the desired concentration, and/or to carry out the intended function in the final formulation when the additive package(s) is/are combined with a predetermined amount of base lubricant. 
         [0053]    Thus, the additives may be admixed with small amounts of base oil or other compatible solvents together with other desirable additives to form additive packages containing active ingredients in an amount, based on the additive package, of, for example, from 2.5 to 90, preferably from 5 to 75, most preferably from 8 to 60, mass % of additives in the appropriate proportions, the remainder being base oil. 
         [0054]    The final formulations may typically contain about 5 to 40 mass % of the additive packages(s), the remainder being base oil. 
       Viscosity Modifier 
       [0055]    In this invention, as stated above, a viscosity modifier (B) is additionally provided. Examples of ranges in the composition include 0.1-6, 0.1-5, 0.1-4, 0.1-2.5, and lower limits of 0.5 and 1 mass %. 
         [0056]    The polymethacrylate-based viscosity index improvers which may be used in the present invention are any type of non-dispersion type or dispersion type polymethacrylate compounds which are used as viscosity modifiers for a lubricating oil. 
         [0057]    The non-dispersion type polymethacrylate-based viscosity index improver may be a polymer of a compound represented by the formula 
         [0000]      CH 2 ═C(CH 3 )COOR 1   (I)
 
         [0058]    In formula (1) R 1  is a straight chain or branched alkyl group such as methyl, ethyl, propyl, butyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, and octadecyl groups. 
         [0059]    Specific examples of the dispersion type polymethacrylate-based viscosity index improver are copolymers obtained by copolymerizing one or more monomers selected from compounds represented by formula (1) with one or more nitrogen-containing monomers selected from compounds represented by formulae (2) and (3) 
         [0000]      CH 2 ═C(R 2 )COO—(R 3 ) a —X 1   (2)
 
         [0000]      CH 2 ═C(R 4 )X 2   (3)
 
         [0060]    In formulae (2) and (3) R 2  and R 4  are each independently hydrogen or methyl. R 3  is a straight chain or branched alkylene group having 1 to 18 carbon atoms, such as ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene, decylene, undecylene, dodecylene, tridecylene, tetradecylene, pentadecylene, hexadecylene, heptadecylene, and octadecylene groups; a is 0 or 1; X 1  and X 2  are each independently an amino-or heterocyclic-residue having 1 or 2 nitrogen atoms and 0 to 2 oxygen atoms. Specific examples of X 1  and X 2  are dimethylamino, diethylamino, dipropylamino, dibutylamino, anilino, toluidino, xylidino, acetylamino, benzoilamino, morpholino, pryrolyl, pyridyl, methylpyridyl, pyrolidinyl, piperidinyl, quinonyl, pyrrolidonyl, pyrrolidono, imidazolino, and pyrazino groups. 
         [0061]    Specific examples of the nitrogen-containing monomers represented by formula (2) and (3) are dimethylaminomethylmethacrylate, diethylaminomethylmethacrylate, dimethylaminoethylmethacrylate, diethylaminoethylmethacrylate, 2-methyl-5-vinylpyridine, morpholinomethylmethacrylate, morpholinoethylmethacrylate, N-vinylpyrrolidone, and mixtures thereof. 
         [0062]    The lower limit of the weight-average molecular weight of the polymethacrylate-based viscosity modifier, which is effective in improving the performance of an engine oil, is preferably 180,000, more preferably 190,000. The upper limit is preferably 500,000, more preferably 400,000. 
         [0063]    An engine oil composition according to the present invention may contain the polymethacrylate-based viscosity modifier in such an amount that the composition has a kinematic viscosity at 100° C. of 4.0 to 9.3 mm 2 /s. 
       EXAMPLES 
       [0064]    The present invention is illustrated by, but in no way limited to, the following examples. 
       MDCL&#39;s 
       [0065]    A set of MDCL&#39;s was formulated, each containing 20.89 mass % of the same additives in the proportions and having a TBN of about 70. The set comprised a control consisting of additive and base oil; a reference consisting of additives, base oil and brightstock; and an inventive MDCL consisting of additives, base oil and viscosity modifier. The additives were additives known in the art and used in proportions known in the art for conferring MDCL properties. The viscosity modifier was a polymethcylate (PMA) supplied by Rohmax. The brightstock was a Group I bright stock with a kinematic viscosity of &gt;20 cSt at 100° C. The base oil was a Group 1 base oil. 
       TPEO&#39;s 
       [0066]    A set of TPEO&#39;s was formulated, each containing 16 mass % of the same additives in the same proportions and having a TBN of about 40. The set comprised a control consisting of additives and base oil; a reference consisting of additives, base oil and bright stock; and an inventive MDCL consisting of additives, base oil and viscosity modifier. The additives were additives known in the art and used in proportions known in the art for conferring TPEO properties. The viscosity modifier, brightstock and base oil were used in the MDCL&#39;s. 
       Testing &amp; Results 
       [0067]    Samples of the above formulations were tested using a PCS Instruments high frequency reciprocating rig (HFRR) on a standard protocol comprising the following conditions:
       120 minutes   20 Hz reciprocation of 1 mm stroke length   200 g load using standard equipment manufacturer supplied steel substrates.       
 
         [0071]    Each test was repeated two further times and the recorded wear measurement was the average of these values. 
         [0072]    The HFRR data for the compositions are summarized in the table below. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Result 
               
               
                   
                 Additive 
                 Base oil 
                 Brightstock 
                 PMA 
                 (wear 
               
               
                 Ex 
                 (mass %) 
                 (mass %) 
                 (mass %) 
                 (mass %) 
                 vol m 3 ) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 TPEO 
                   
                   
                   
                   
                   
               
               
                 Control 
                 16 
                 84 
                 — 
                 — 
                 5,584 
               
               
                 Reference 1 
                 16 
                 75.5 
                 8.5 
                 — 
                 8,279 
               
               
                   
                 16 
                 83.37 
                 — 
                 0.63 
                 4,646 
               
               
                 MDCL 
               
               
                 Control 
                 20.89 
                 79.11 
                 — 
                 — 
                 33,960 
               
               
                 Reference 2 
                 20.89 
                 58.89 
                 20.22 
                 — 
                 3,940 
               
               
                   
                 20.89 
                 77.61 
                 — 
                 1.5  
                 6,792 
               
               
                   
               
             
          
         
       
     
         [0073]    The above results show that the use of polymethacrylate advantageously reduces the wear scar volume as compared to the control and reference samples for TPEO oils. For MDCL it is clearly advantageous to include the polymethacrylate versus using no brightstock at all.