Viscosity index improver, lubricant oil additive, and lubricant oil composition

Abstract

A viscosity index improver of the present invention contains a star polymer obtained by a reaction of divinylbenzene with alkyl methacrylate containing stearyl methacrylate, the star polymer having a core part and an arm part, wherein the core part is derived from the divinylbenzene and the arm part is a polymer chain of the alkyl methacrylate. A lubricant oil additive and a lubricant oil composition of the present invention contain the above viscosity index improver.

1 . A viscosity index improver comprising a star polymer obtainable by a reaction of divinylbenzene with alkyl methacrylate containing stearyl methacrylate, the star polymer having a core part and an arm part, wherein the core part is derived from the divinylbenzene and the arm part is a polymer chain of the alkyl methacrylate. 
     
     
         2 . The viscosity index improver according to  claim 1 , wherein a proportion of stearyl methacrylate in the alkyl methacrylate is 40 to 100% by mass based on the total amount of the alkyl methacrylate. 
     
     
         3 . The viscosity index improver according to  claim 1 , wherein a proportion of methyl methacrylate in the alkyl methacrylate is 0 to 50% by mass based on the total amount of the alkyl methacrylate. 
     
     
         4 . The viscosity index improver according to  claim 1 , wherein a proportion of alkyl methacrylate having an alkyl group having 10 to 16 carbon atoms in the alkyl methacrylate is 0 to 40% by mass based on the total amount of the alkyl methacrylate. 
     
     
         5 . The viscosity index improver according to  claim 1 , wherein the arm part has a number average molecular weight of 10,000 to 60,000. 
     
     
         6 . The viscosity index improver according to  claim 1 , wherein the star polymer has a number average molecular weight of 50,000 to 1,000,000. 
     
     
         7 . A lubricant oil additive comprising the viscosity index improver according to  claim 1 . 
     
     
         8 . A lubricant oil composition comprising a lubricating base oil and the viscosity index improver according to  claim 1 . 
     
     
         9 . The lubricant oil composition according to  claim 8 , wherein the lubricant oil composition has a HTHS viscosity of 1.4 mPa·s or greater at 150° C. and a HTHS viscosity of less than 5.4 mPa·s at 100° C.