Marine lubricating composition
BACKGROUND OF THE INVENTION 
     This invention relates to an improved lubricating oil composition which is particularly useful as a lubricant in marine applications and particularly as a marine cylinder lubricant (MDCL). More particularly, this invention relates to a finished lubricant formulation exhibiting improved ring wear and liner wear performance which employ certain amounts of borated dispersants and zinc compounds. 
     The present invention is based upon the discovery that the incorporation of certain amounts of borated dispersants such as polyisobutenyl succinic anhydride-polyamine borated derivatives into a lubricating oil composition in combination with certain amounts of zinc dialkyl dithiophosphate antiwear additives in MDCL formulations give surprising wear results particularly in cylinder liner and piston ring wear. Combinations of borated dispersants and zinc antiwear additives are known in lubricants for automobile engines, but have not been described in compositions suitable for marine diesels lubricants. GB Pat. No. 1054310 describes nitrogen- and boron-containing compositions comprising borated dispersants such as polyisobutenyl succinic anhydride - polyamine borated derivatives and their use in gasoline and diesel engines including use as MDCL. The use of such borated dispersants in combination with zinc dialkyl dithiophosphate antiwear additives in general is disclosed, but there is no disclosure of a combination of borated dispersant and zinc antiwear additive in a formulation having the viscosity and total base number appropriate for MDCL. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided an improved lubricating oil composition for use in marine applications comprising a major amount of an oil of lubricating viscosity and: 
     (a) at least 0.5 wt % of a borated ashless dispersant; 
     (b) at least 12.1 wt % of one or more overbased metal compound as a detergent additive or a mixture thereof with neutral metal detergent additives, said overbased metal compound may be selected from soluble calcium, magnesium and barium sulphonates, phenates and sulphurized phenates, but overbased calcium sulphonates and phenates are preferred; and 
     (c) at least 0.1 wt % of a zinc dialkyl dithiophosphate as an anti-wear additive, wherein the boron content of the composition is from 0.01 to 0.016 wt %, the zinc content of the composition is from 0.02 to 0.23 wt %, and the total base number (ASTM 2896) or TBN is at least 60, more usually at least 70. 
     It has been found that using the amounts of boron and zinc indicated provide a marine diesel cylinder lubricating oil composition exhibiting greatly improved wear performance and cleanliness. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The lubricating oil compositions of this invention are used as MDCL lubricants for marine diesel engines. Thus, the compositions of the present invention achieve, through the use of the combination of dispersant and zinc antiwear additive, the highly desirable objective of providing a finished lubricating oil satisfying the relevant qualification tests and standards for marine diesel cylinder lubricating oil compositions. 
     The finished lubricating oil prepared as described above will preferably contain the following active ingredient percentages by weight: 1.5-5 percent by weight of the dispersant 12.25-30 weight per cent of overbased metal detergent and 0.1-0.5 weight per cent of zinc dihydrocarbyl dithiophosphate. The actual components of the finished oil may be added as additives or additive mixture in a diluent. 
     There may also be present in a finished oil small but effective amounts of other special purpose additives and these include anti-oxidants, anti-foamants, and rust inhibitors. These are additives whose functions are not directed to provide improvements in wear. 
     The preferred dispersants are the borated ashless polyalkenyl succinimide dispersants where the alkenyl group of the succinic acid or anhydride is derived from a polymer of a C 3  or C 4  monoolefin, especially a polyisobutylene wherein the polyisobutenyl group has a number average molecular weight (Mn) of from 700 to 5,000 more preferably from 900 to 2,500. Such dispersants preferably have at least 1, preferably 1 to 2, more preferably 1.1 to 1.8, succinic groups for each polyisobutenyl group. 
     Suitable polyamines for reaction with the aforesaid succinic acids or anhydrides to provide the succinimide are those polyalkyleneamines represented by the formula 
     
         NH.sub.2 (CH.sub.2).sub.n --(NH(CH.sub.2).sub.n).sub.m --NH.sub.2 
    
     wherein n is 2 to 3 and m is 0 to 10. Illustrative are ethylene diamine, diethylene trimaine, triethylene tetramine, tetraethylene pentamine, tetrapropylene pentamine, pentaethylene hexamine and the like, as well as the commercially available mixtures of such polyamines. The amines are reacted with the alkenyl succinic acid or anhydride in conventional ratios of about 1:1 to 10:1 moles of alkenyl succinic acid or anhydride to polyamine, and preferably in a ratio of about 2:1. 
     The borated alkenyl succinimide dispersants are also well known in the art as disclosed in U.S. Pat. No. 3,254,025. These derivatives are provided by treating the alkenyl succinimide with a boron compound selected from the group consisting of boron oxides, boron halides, boron acids and esters thereof, in an amount to provide from about 0.1 atomic proportion of boron to about 10 atomic proportions of boron for each atomic proportion of nitrogen in the dispersant. 
     The borated product will generally contain 0.1 to 2.0, preferably 0.2 to 0.8 weight per cent boron based upon the total weight of the borated dispersant. Boron is considered to be present as dehydrated boric acid polymers attaching as the metaborate salt of the imide. The boration reaction is readily carried out adding from about 1 to 3 weight per cent based on the weight of dispersant, of said boron compound, preferably boric acid, to the dispersant as a slurry in mineral oil and heating with stirring from 135° C. to 165° C. for 1 to 5 hours followed by nitrogen stripping filtration of the product. 
     The metal detergent additives suitable in the diesel oil formulations of the present invention are known in the art and include one or more members selected from the group consisting of overbased oil-soluble calcium, magnesium and barium phenates, sulphurised phenates, and sulphonates, especially the overbased calcium sulphurised phenates obtained from C 9  or C 12  alkyl phenols and sulphonates of C 16  -C 50  alkyl substituted benzene or toluene sulphonic acids which have a total base number of from 200 to 500. 
     These overbased materials may be used as the sole metal detergent additive or in combination with the same additives in the neutral form but the overall metal detergent additive combination should have the same basicity as represented by the foregoing total base number. Preferably they are present in amounts of from 12.5 to 15 wt % with the aforementioned mixture of overbased calcium sulphurised phenate and calcium sulphonate being especially useful. The weight ratio of sulphonate to phenate is desirably from 1:1 to 15:1, preferably from 5:1 to 15:1, typically from 10:1 to 12:1. 
     The anti-wear additives useful are the oil-soluble zinc dihydrocarbyldithiophosphate having a total of at least 5 carbon atoms, the alkyl group being preferably C 3  -C 9 , typically used in amounts of 0.1-0.6 wt %. 
     While a wide variety of lubricating oil base stocks may be used in preparing the composition of this invention, most typically mineral oils having a viscosity of about 2-40 centistokes (ASTM-D-445) at 99° C. are employed preferably to give a finished lubricant with a viscosity meeting the requirements of SAE 50. 
     The invention is further illustrated by the following examples which are not to be considered as limitative of its scope. Percentages are by weight except where otherwise indicated. 
     EXAMPLES 
     MDCL formulations were prepared from solution concentrates prepared by blending dispersant, metal additives, and zinc antiwear additive, at 65° C. to form a homogeneous solution prior to diluting the concentrate to provide the finished lubricating oil. 
     Lubricating oil formulations of this invention were evaluated in the Abingdon B-1 engine test - details of which are given below: 
     
         ______________________________________                                    
B-1 Engine Test Conditions                                                
Test Conditions (Total Test Duration is 113 hours)                        
______________________________________                                    
1.  BREAK-IN PERIOD                                                       
    (13 Hours)                                                            
    Fuel 0-1 hrs      1% Sulphur Gas Oil                                  
    1-13 hrs          Residual Fuel                                       
    Engine Speed      350 rpm                                             
2.  TEST PERIOD (100 hours)                                               
    Fuel              Test Fuel Residual                                  
    Fuel Flow Rate    120 = 1 Sec/Lb                                      
    Cylinder Oil Feed Rate                                                
                      6 g/hr                                              
    Engine Speed      350 rpm                                             
    Engine Brake Load Adjusted to Maintain Correct                        
                      Fuel Flow Rate (Approx 66                           
                      66 bhp)                                             
    IMEP              169 PsI (11.9 kg/cm.sup.2)                          
    BMEP              139 PsI (9.8 kg/cm.sup.2)                           
    Scavenge Air Pressure                                                 
                      8.0 PsI (0.6 kg/cm.sup.2)                           
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         ______________________________________                                    
Mechanical Details of B-1 Engine                                          
______________________________________                                    
Cylinder Bore     8.0 in (203 mm)                                         
Stroke            10.75 in (273 mm)                                       
Compression Ratio 12.5:1                                                  
Maximum Cylinder Pressure                                                 
                  1900 lb/in.sup.2 (134 kg/cm.sup.2)                      
Allowable                                                                 
Scavenge System   Cylinder Port, Exhaust                                  
                  Valves In Head, End Scavenge                            
Operating Cycle   2-stroke                                                
Running speed maximum                                                     
                  450 rev/min                                             
Mean indicated pressure                                                   
                  228 lb/in.sup.2 (16 kg/cm.sup.2)                        
maximum                                                                   
Combustion chamber                                                        
                  Open without swirl aids                                 
Fuel injection    Central single fuel valve                               
Fuel injection rate                                                       
                  176 mm.sup.3 /Degree                                    
Scavenge Air      Positive displacement                                   
                  compressor separately driven                            
Running line      Crosshead with diaphragm and                            
                  piston rod                                              
Cylinder lubrication                                                      
                  4 quills in way of lower                                
                  stroke; multi-point                                     
                  separately driven                                       
Piston cooling    Solid oil by hollow piston                              
                  rod                                                     
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     The lubricating Oil Test Formulation detailed below were evaluated for diesel engine performance in the Abingdon B-1 engine. 
     
         ______________________________________                                    
Test Formulations, weight per cent                                        
                                     Component                            
                                     (wt %)                               
Component       A       B      C     D                                    
______________________________________                                    
Non-borated dispersant                                                    
                3.92    3.92   0     0                                    
Borated dispersant                                                        
                0       0      3.92  3.92                                 
Detergent       23.38   23.38  23.38 23.38                                
Antiwear        0       0.28   0.28  0.56                                 
Base Oil        72.70   72.42  72.42 72.14                                
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     The borated dispersant was an oil solution containing 51.5 wt % of a borated polyisobutenyl succinic anhydride-polyamine reaction product containing 1.27 succinic groups per polyisobutenyl group in which the polyisobutenyl group had a number average molecular weight (Mn)=900 and a boron content of 7.7 wt %. The non-borated dispersant was similar but contained no boron. 
     The detergent was a mixture of an overbased calcium sulphonate additive and an overbased calcium sulphurised phenate additive. The overbased calcium sulphonate additive comprised 21% of the formulation and was an oil solution comprising 54% of active ingredient and having a TBN of 300. The overbased calcium phenate additive comprised 2.38% of the formulation and was an oil solution comprising 67% of active ingredient and having a TBN of 250. The antiwear additive was an oil solution comprising 90wt % of zinc diisooctyl dithiophosphate. 
     Formulation A and B contained no boron, A also contained no zinc. The boron content of formulation C was 0.014 wt % and the zinc content was 0.022 wt %. The boron content of formulation D was 0.014 wt % and the zinc content was 0.044 wt %. Formulation C was an example of the invention. 
     The results obtained in the Abingdon B-1 engine test are set out below: 
     
         ______________________________________                                    
            Total ringwear                                                
                        Average liner                                     
Formulation gm/1000 hrs wear mm/1000 hrs                                  
______________________________________                                    
Reference oil                                                             
            125.4       0.68                                              
A           128.7       0.85                                              
B           129.4       0.87                                              
 C*         100.1       0.68                                              
D           122.4       0.87                                              
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 *average of three repeats                                                
 
    
     As compared with comparative formulations A and B, formulation C gives a significant reduction in both ring and liner wear, whereas formulation D shows that increased zinc content results in a loss in performance benefits.