Lubricating oil composition, flash point improver, and flash point improving method

Abstract

Provided is a lubricating oil composition, containing: a lubricant base oil (X); a diphenylamine-based antioxidant (A); a naphthylamine-based antioxidant (B); and a phosphorus-containing phenol-based antioxidant (C), in which the lubricant base oil (X) satisfies requirements (1) to (4) below: —requirement (1): having a viscosity index of 120 or more, —requirement (2): having a % C P according to ring analysis (n-d-M method) of 85 or more, —requirement (3): having a density at 15° C. of 0.835 g/cm 3 or less, —requirement (4): having a kinematic viscosity at 40° C. of 28.8 mm 2 /s or more, a content ratio [(B)/(A)] of the naphthylamine-based antioxidant (B) to the diphenylamine-based antioxidant (A) is 2.0 or more in mass ratio, and a content ratio [(C)/(B)] of the phosphorus-containing phenol-based antioxidant (C) to the naphthylamine-based antioxidant (B) is 0.15 or less in mass ratio.

1 : A lubricating oil composition, comprising:
 a lubricant base oil;   a diphenylamine-based antioxidant;   a naphthylamine-based antioxidant; and   a phosphorus-containing phenol-based antioxidant,   wherein the lubricant base oil has a viscosity index of 120 or more, a % C P  according to ring analysis (n-d-M method) of 85 or more, a density at 15° C. of 0.835 g/cm 3  or less, and a kinematic viscosity at 40° C. of 28.8 mm 2 /s or more, a content ratio of the naphthylamine-based antioxidant to the diphenylamine-based antioxidant is 2.0 or more in mass ratio, and a content ratio of the phosphorus-containing phenol-based antioxidant to the naphthylamine-based antioxidant is 0.15 or less in mass ratio.   
     
     
         2 : The lubricating oil composition according to  claim 1 , wherein the diphenylamine-based antioxidant comprises a compound of formula (a1), 
       
         
           
           
               
               
           
         
       
       where R a1  and R a2  are each independently an alkyl group having 1 to 30 carbon atoms, and na1 and na2 are each independently an integer of 1 to 5. 
     
     
         3 : The lubricating oil composition according to  claim 1 , wherein the naphthylamine-based antioxidant comprises a compound of formula (b1), 
       
         
           
           
               
               
           
         
       
       where R b1  is an alkyl group having 1 to 30 carbon atoms, and nb1 is an integer of 1 to 5. 
     
     
         4 : The lubricating oil composition according to  claim 1 , wherein the phosphorus-containing phenol-based antioxidant comprises a compound of formula (c1), 
       
         
           
           
               
               
           
         
       
       where R c1 , R c2 , R c3 , and R c4  are each independently a hydrogen atom or an alkyl group having 1 to 30 carbon atoms, and R c5  is an alkylene group having 1 to 5 carbon atoms. 
     
     
         5 : The lubricating oil composition according to  claim 1 , further comprising:
 a phosphorus-free phenol-based antioxidant.   
     
     
         6 : The lubricating oil composition according to  claim 5 , wherein the phosphorus-free phenol-based antioxidant comprises a compound of formula (d1), 
       
         
           
           
               
               
           
         
       
       where R d1  is an alkylene group having 1 to 5 carbon atoms, and R d2  is an alkyl group having 1 to 25 carbon atoms. 
     
     
         7 : The lubricating oil composition according to  claim 1 , wherein a total content of the diphenylamine-based antioxidant, the naphthylamine-based antioxidant, and the phosphorus-containing phenol-based antioxidant is 0.30% by mass or more based on a total amount of the lubricating oil composition. 
     
     
         8 : A flash point improver for a lubricant base oil, comprising:
 a diphenylamine-based antioxidant;   a naphthylamine-based antioxidant; and   a phosphorus-containing phenol-based antioxidant,   wherein a content ratio of the naphthylamine-based antioxidant to the diphenylamine-based antioxidant is 2.0 or more in mass ratio, and a content ratio of the phosphorus-containing phenol-based antioxidant to the naphthylamine-based antioxidant is 0.15 or less in mass ratio.   
     
     
         9 : The flash point improver for a lubricant base oil according to  claim 8 , further comprising:
 a phosphorus-free phenol-based antioxidant.   
     
     
         10 : The flash point improver according to  claim 8 , wherein the lubricant base oil has a viscosity index of 120 or more, a % C P  according to ring analysis (n-d-M method) of 85 or more, a density at 15° C. of 0.835 g/cm 3  or less, and a kinematic viscosity at 40° C. of 28.8 mm 2 /s or more. 
     
     
         11 : A method for improving a flash point of a lubricant base oil, comprising:
 blending the flash point improver of  claim 8  into a lubricant base oil having a viscosity index of 120 or more, a % C P  according to ring analysis (n-d-M method) of 85 or more, a density at 15° C. of 0.835 g/cm 3  or less, and a kinematic viscosity at 40° C. of 28.8 mm 2 /s or more.   
     
     
         12 : The lubricating oil composition according to  claim 2 , wherein the naphthylamine-based antioxidant comprises a compound of formula (b1), 
       
         
           
           
               
               
           
         
       
       where R b1  is an alkyl group having 1 to 30 carbon atoms, and nb1 is an integer of 1 to 5. 
     
     
         13 : The lubricating oil composition according to  claim 2 , wherein the phosphorus-containing phenol-based antioxidant comprises a compound of formula (c1), 
       
         
           
           
               
               
           
         
       
       where R c1 , R c2 , R c3 , and R c4  are each independently a hydrogen atom or an alkyl group having 1 to 30 carbon atoms, and R c5  is an alkylene group having 1 to 5 carbon atoms. 
     
     
         14 : The lubricating oil composition according to  claim 2 , further comprising:
 a phosphorus-free phenol-based antioxidant.   
     
     
         15 : The lubricating oil composition according to  claim 14 , wherein the phosphorus-free phenol-based antioxidant comprises a compound of formula (d1), 
       
         
           
           
               
               
           
         
       
       where R d1  is an alkylene group having 1 to 5 carbon atoms, and R d2  is an alkyl group having 1 to 25 carbon atoms. 
     
     
         16 : The lubricating oil composition according to  claim 2 , wherein a total content of the diphenylamine-based antioxidant, the naphthylamine-based antioxidant, and the phosphorus-containing phenol-based antioxidant is 0.30% by mass or more based on a total amount of the lubricating oil composition. 
     
     
         17 : The lubricating oil composition according to  claim 3 , wherein the phosphorus-containing phenol-based antioxidant comprises a compound of formula (c1), 
       
         
           
           
               
               
           
         
       
       where R c1 , R c2 , R c3 , and R c4  are each independently a hydrogen atom or an alkyl group having 1 to 30 carbon atoms, and R c5  is an alkylene group having 1 to 5 carbon atoms. 
     
     
         18 : The lubricating oil composition according to  claim 3 , further comprising:
 a phosphorus-free phenol-based antioxidant.   
     
     
         19 : The lubricating oil composition according to  claim 18 , wherein the phosphorus-free phenol-based antioxidant comprises a compound of formula (d1), 
       
         
           
           
               
               
           
         
       
       where R d1  is an alkylene group having 1 to 5 carbon atoms, and R d2  is an alkyl group having 1 to 25 carbon atoms. 
     
     
         20 : The lubricating oil composition according to  claim 3 , wherein a total content of the diphenylamine-based antioxidant, the naphthylamine-based antioxidant, and the phosphorus-containing phenol-based antioxidant is 0.30% by mass or more based on a total amount of the lubricating oil composition.