Viscosity index improver, refrigerator oil, and working fluid composition for refrigerators

A working fluid composition for refrigerating machines, comprising a refrigerating machine oil and a refrigerant,

• wherein the refrigerating machine oil comprises:
• a lubricant base oil;
• and
• a viscosity index improver,
• wherein the lubricant base oil is an oxygen atom-containing base oil (B),
• wherein the viscosity index improver comprises:
• a copolymer (A) that contains, as essential constituent monomers, a fluorine atom- containing monomer (a) represented by the following formula (1) and a monomer (b) represented by the following formula (2):
• wherein R1 represents a hydrogen atom or a methyl group;
• R2 is a C2-C4 alkylene group;
• p is an integer of 0 or 1;
• q is an integer from 0 to 20, each R2 may be the same as or different from each other when q is 2 or greater;
• and Y is a monovalent group in which one or more or all of hydrogen atoms in a C1-C40 hydrocarbon group are replaced by fluorine atoms, and
• wherein R3 represents a hydrogen atom or a methyl group;
• —X1— is a group represented by —O— or —NH—;
• R4 is a C2-C4 alkylene group;
• r is an integer of 0 or 1;
• s is an integer from 0 to 20, and each R4 may be the same as or different from each other when s is 2 or greater;
• and R5 is a C1-C40 hydrocarbon group,
• wherein the copolymer (A) has a solubility parameter of 8.1 to 10.0 (cal/cm3)1/2,
• a mass ratio (b/a) of the monomer (b) to the monomer (a) in the monomers constituting the copolymer (A) is 0.01 to 42, and
• the content of the copolymer (A) is 1 to 20 mass % based on the total amount of the refrigerating machine oil.