Lubricating oil composition

Abstract

A lubricating oil composition which is excellent in wear resistance, despite its low phosphorus content, low sulfur content and low sulfuric acid ash content, and which exhibits excellent friction reducing effect even when used for a DLC-treated sliding part, is provided by compounding a specific sulfur-containing compound and a specific polar group-containing compound in a base oil.

1 . A lubricating oil composition, comprising:
 (A) a base oil;   (B) at least one selected from the group consisting of a sulfur-comprising compound of formula (I) and formula (II):   
       
         
           
           
               
               
           
         
         
           wherein: 
           R 1  to R 12  are each independently is a hydrogen atom; a hydrocarbon group selected from the group consisting of an alkyl group, a cycloalkyl group, an alkenyl group, a cycloalkenyl group, and an aryl group; or a hetero atom-comprising group comprising an atom selected from an oxygen atom, a nitrogen atom, and a sulfur atom, which is contained a hydrocarbon group selected from the group consisting of an alkyl group, a cycloalkyl group, an alkenyl group, a cycloalkenyl group, and an aryl group; 
           each Y is independently a divalent group selected from the group consisting of —O—, —S—, —SO—, —SO 2 —, —(C═O)O—, —(C═O)NH—, —O(C═O)NH—, —C(═O)—, —N(H)—, —NHCONH—, —N═N—, —NH—C(═NH)—NH—, —S—C(═O)—, —NH—C(═S)—, and —NH—C(═S)—NH—; 
           x is an integer from 1 to 3; and 
           each n is independently an integer from 1 to 5; and 
         
         (C) a polar group-comprising compound comprising at least one polar group selected from the group consisting of an amino group, an amide group, and a hydroxyl group, and which comprises a C 3  to C 24  alkyl group. 
       
     
     
         2 . The composition of  claim 1 , wherein compound (C) is at least one selected from the group consisting of a glycerol partial ester of a fatty acid, a glycerol monoether compound, an amine compound, and an amide compound. 
     
     
         3 . The composition of  claim 1 , wherein compound (C) is a glycerol monoester of a fatty acid represented by the general formula (III) or (IV): 
       
         
           
           
               
               
           
         
         wherein each R 13  is independently a C 3  to C 24  alkyl group, 
       
       or
 a glycerol monoether compound of formula (V) or (VI): 
 
       
         
           
           
               
               
           
         
         wherein each R 14  is independently a C 3  to C 24  alkyl group. 
       
     
     
         4 . The composition of  claim 1 , wherein the compound (C) is an amine compound of formula (VII): 
       
         
           
           
               
               
           
         
         wherein R 15  is a C 3  to C 24  alkyl group and each R 16  is independently a hydrogen atom or a group comprising a hydroxyl group substituted for a terminal hydrogen atom of a straight chained C 2  to C 4  alkyl group, 
       
       or
 an amide compound of formula (VIII): 
 
       
         
           
           
               
               
           
         
         wherein is a C 3  to C 24  alkyl group, and each R 18  is independently a hydrogen atom or a group comprising a hydroxyl group substituted for a terminal hydrogen atom of a straight chained C 2  to C 4  alkyl group. 
       
     
     
         5 . The composition of  claim 1 , comprising a phosphorus content of 0.5% by mass or less and a sulfuric acid ash content of 0.6% by mass or less, each based on a total mass of the composition. 
     
     
         6 . The composition of  claim 1 , comprising a phosphorus content of 0% by mass and a sulfuric acid ash content of 0.1% by mass or less, each based on a total mass of the composition. 
     
     
         7 . The composition of  claim 1 , being adapted for a sliding part which is treated with diamond-like carbon (DLC). 
     
     
         8 . The composition of  claim 1 , wherein the base oil (A) has a kinematic viscosity at 100° C. of 2 to 30 mm 2 /s. 
     
     
         9 . The composition of  claim 1 , wherein the base oil (A) has a viscosity index of 70 or more. 
     
     
         10 . The composition of  claim 1 , comprising 0.01% to 5.0% by mass of the compound (B), based on a total mass of the composition. 
     
     
         11 . The composition of  claim 1 , comprising 0.1% to 2.0% by mass of the compound (B), based on a total mass of the composition. 
     
     
         12 . The composition of  claim 1 , comprising 0.01% to 5.0% by mass of the compound (C), based on a total mass of the composition. 
     
     
         13 . The composition of  claim 1 , comprising 0.1% to 2.0% by mass of the compound (C), based on a total mass of the composition. 
     
     
         14 . The composition of  claim 1 , wherein the compound (B) of formula (I) is at least one selected from the group consisting bis(methoxycarbonylmethyl)disulfide, bis(ethoxycarbonylmethyl)disulfide, bis(n-propoxycarbonylmethyl)disulfide, bis(isopropoxycarbonylmethyl)disulfide, bis(n-butoxycarbonylmethyl)disulfide, bis(n-octoxycarbonylmethyl)disulfide, bis(n-dodecyloxycarbonylmethyl)disulfide, bis(cyclopropoxycarbonylmethyl)disulfide, 1,1-bis(2-methoxycarbonylethyl)disulfide, 1,1-bis(3-methoxycarbonyl-n-propyl)disulfide, 1,1-bis(4-methoxycarbonyl-n-butyl)disulfide, 1,1-bis(2-ethoxycarbonylethyl)disulfide, 1,1-bis(2-n-propoxycarbonylethyl)disulfide, 1,1-bis(2-isopropoxycarbonylethyl)disulfide, and 1,1-bis(2-cyclopropoxycarbonylethyl)disulfide. 
     
     
         15 . The composition of  claim 1 , wherein the compound (B) of formula (II) is at least one selected from the group consisting of tetramethyl dithiomalate, tetraethyl dithiomalate, tetra-1-propyl dithiomalate, tetra-2-propyl dithiomalate, tetra-1-butyl dithiomalate, tetra-2-butyl dithiomalate, tetraisobutyl dithiomalate, tetra-1-hexyl dithiomalate, tetra-1-octyl dithiomalate, tetra-1-(2-ethyl)hexyl dithiomalate, tetra-1-(3,5,5-trimethyl)hexyl dithiomalate, tetra-1-decyl dithiomalate, tetra-1-dodecyl dithiomalate, tetra-1-hexadecyl dithiomalate, tetra-1-octadecyl dithiomalate, tetrabenzyl dithiomalate, tetra-α-(methyl)benzyl dithiomalate, tetra-α,α-dimethylbenzyl dithiomalate, tetra-1-(2-methoxy)ethyl dithiomalate, tetra-1-(2-ethoxy)ethyl dithiomalate, tetra-1-(2-butoxy)ethyl dithiomalate, tetra-1-(2-ethoxy)ethyl dithiomalate, tetra-1-(2-butoxybutoxy)ethyl dithiomalate and tetra-1-(2-phenoxy)ethyl dithiomalate.