Hydrophilic groups should be present in the molecular structure of the film former. One can incorporate, for example, a polyol which contains precisely such groups e.g. a polyethylene glycol. An alkyd resin based on this has good water compatibility.
A high degree of water solubility is helped by an appreciable concentration of carboxyl groups. This can be checked by the acid value of the resin. Of practical importance is a reaction with trimellitic anhydride carried out after termination of the alkyd resin synthesis and which leads to this state.
During ring opening, one of the carboxyl groups is esterified with available hydroxyl groups while the remaining hydroxyl groups impart the desired hydrophilic character to the resin. Another formulation starts with dimethylolpropionic acid. Neutralisation of the acid groups must be undertaken in the aqueous phase of the lacquer. usually with amines. The hydrophilic groups must have reacted sufficiently after film formation so that no water solubility remains.