Patent ID: 8434857

Claim:
A printing system for applying a printing fluid to a substrate, comprising a printing fluid applicator and a recirculating printing fluid supply supplying printing fluid to the applicator, wherein the printing fluid comprises water and a water dispersible polyurethane additive of the general formula of (I): wherein Z is the central portion of a monomer unit that is the polymerization product of a diisocyanate; X 1 —Y 1 —X 1 represents one or more soft segments wherein Y 1 represents the central portion of a unit that is the polymerization product of a diamine or diol prepolymer having a molecular weight of greater than 300 Daltons; W is the central portion of one or more units containing an acid group; X 2 —Y 2 —X 2 represents one or more hard segments wherein Y 2 represents the central portion of a unit that is the polymerization product of a C 2 -C 8 diol or diamine having a molecular weight of less than 250 Daltons; and X 1 , V and X 2 can be the same or different and are an —O— or —N— atom; and further wherein the polyurethane additive has a weight average molecular weight of at least 6,000 Daltons and a sufficient number of acid groups to provide an acid number greater than 35, and the one or more X 2 —Y 2 —X 2 hard segments are present at from 1 wt % to less than 13 wt % of the polyurethane additive.