Patent ID: 7759411

Claim:
An ink composition comprising at least a pigment, a polymer which encapsulates the pigment to make the pigment dispersible in the ink composition, a multivalent metal ion and water, wherein the polymer is a vinyl polymer obtained by polymerizing: A) at least one monomer selected from the group consisting of: monomer A1 represented by formula (I): wherein R 1 represents a hydrogen atom or a methyl group, R 2 represents a hydrogen atom, an alkyl group having 1 to 20 carbon atoms or an alkylphenyl group having 1 to 9 carbon atoms, and o represents a number of 1 to 30; monomer A2 represented by formula (II): wherein R 1 , R 2 and o have the same meanings as defined in formula (I), p represents a number of 1 to 30, the form of addition of oxyethylene groups and oxypropylene groups in brackets may be either block addition or random addition; and monomer A3 represented by formula (III): wherein R 1 , R 2 and o have the same meanings as defined in formula (I); B) a monomer having a salt-forming group; C) a macromer having a number average molecular weight of 500 to 500,000, wherein the macromer is a styrenic macromer having a polymerizable functional group at one end thereof; and D) a monomer copolymerizable with the monomers A), B) and C), and wherein the multivalent metal ion is calcium and is present in an amount of from 5 to 100 ppm based on the total amount of the ink composition.