Patent ID: 8637632

Claim:
A method for producing a binder resin, comprising polycondensing a polycondensable monomer at a temperature of from 70° C. to 140° C. by using a polycondensation catalyst, wherein the binder resin has a weight-average molecular weight (M w ) of from 7,200 to 18,500 and a glass transition temperature of from 55° C. to 68.5° C., and by using a polycondensation catalyst, wherein the polycondensation catalyst comprises: at least one compound represented by formula (I) or (II); and at least one compound represented by formula (III) or (IV), a weight ratio of total amount of the compounds represented by formula (I) or (II) used to total amount of the compounds represented by formula (III) or (IV) used is from 5:95 to 95:5, and a total concentration of the polycondensation catalyst is from 9.9 to 0.2 mol % based on a total monomer content: wherein R I represents a C 10 -C 20 straight-chain alkyl group; R 2 represents a monovalent organic group; and number n of substituents R 2 represents an integer of from 0 to 4; R 3 —SO 3 H (II) wherein R 3 represents a C 10 -C 20 straight-chain alkyl group; wherein R 4 represents a C 10 -C 20 branched alkyl group; R 5 represents a monovalent organic group; and number m of substituents R 5 represents an integer of from 0 to 4; and R 6 —SO 3 H (IV) wherein R 6 represents a C 10 -C 20 branched alkyl group.