| 摘要 |
PROBLEM TO BE SOLVED: To provide a viscosity index improver which realizes excellent shear stability of a lubricant composition and low HTHS viscosity of the lubricant composition and hardly increases low-temperature viscosity of the lubricant composition.SOLUTION: A viscosity index improver contains a copolymer (A) whose essential constituent monomers are a monomer (a) represented by the general formula (1), a monomer (b) represented by the general formula (2), and a (meth)acrylic acid alkyl ester (c) having a C1-4 linear alkyl group. In the viscosity index improver, Ris a polymer residue obtained by removing one H from a hydrocarbon polymer whose essential component is isobutylene and/or 1,2-butylene in the general formula (1), In the viscosity index improver, the absolute value of the difference between the SP value of (A) and the SP value of a base oil is 0.5 to 1.6. |
