| 摘要 |
PROBLEM TO BE SOLVED: To provide a viscosity index improver excellent in viscosity index improving effect and shear stability.SOLUTION: There is provided a viscosity index improver containing a (co)polymer (A1), where (A1) is a (co)polymer satisfying the following (1) to (5). (1) The weight average molecular weight of the (c)polymer is 150000 to less than 200000. (2) The peak area of a region corresponding to one-fourth of the peak top molecular weight in a molecular weight distribution curve of the (co)polymer is 11.0% or less of the total peak area. (3) The ratio Mw/Mp of the (co)polymer is 1.08 or more. (4) The solubility parameter of (co)polymer is 7.3 to 9.5 (cal/cm). (5) A (co)polymer contains a (meth)acrylic acid alkyl ester having a branched alkyl group having 5 to 36 carbon atoms as an essential constituent monomer. |
