The blocked polyisocyanate is a polyisocyanate whose isocyanate group has been blocked with a blocking agent. Although the blocked polyisocyanate is a stable compound at normal temperature, the blocking agent dissociates by heating to regenerate isocyanate groups. The isocyanate group is highly reactive, and reacts with a so-called active hydrogen. By utilizing this function, the blocked polyisocyanate is applied to various uses such as coatings, adhesives, sealants and surface treating agents.
The reduction of the discharge of global environmental burden substances has recently become an important subject. Attempts to make blocked polyisocyanates into aqueous compositions have therefore been made. Additionally, melamine compounds, which are compounds applicable to aqueous compositions, have apprehension of generating formalin; and expectations of blocked polyisocyanates have been more raised (Patent Document 1).
However, although a blocked polyisocyanate, particularly a blocked polyisocyanate obtained by using as a precursor a polyisocyanate derived from an aliphatic diisocyanate monomer, is excellent in yellowing resistance, the blocked polyisocyanate needs to be baked at a high temperature. Although some technologies are disclosed for this technical problem (Patent Documents 2, 3 and 4), further improvements in low-temperature curability are demanded.
In order to self-disperse and solubilize a blocked polyisocyanate in water, the blocked polyisocyanate needs to be imparted with a hydrophilic group. However, conventional technologies have some problems. For example, in the case where a nonionic hydrophilic group is imparted, the hydrophilic group remains in a substrate even after heating, and the water resistance decreases in many cases. In the case of an ionic hydrophilic group, a neutralizing substance volatilizes from a substrate during heating. Due to the volatilization, the hydrophilicity of the ionic hydrophilic group decreases. The water resistance and the like are therefore excellent as compared with those of the nonionic hydrophilic group. An anionic hydrophilic group as one of the ionic groups is neutralized with a cationic compound containing a hydroxyl group in many cases. The storage stability of a blocked polyisocyanate having an anionic hydrophilic group thus neutralized, in an aqueous solution, is insufficient.