Crosslink density and cure window of oligourethane diol melamine high-solids coatings

Oligourethane diols were crosslinked with melamine formaldehyde, and their crosslink density was determined by using an equilibrium swelling method. In formulations containing primary or secondary hydroxylated diol the major part of the crosslinker is consumed by transetherification reactions and self-condensation. The aromatic nature of TDI imparts rigidity while the aliphatic IPDI results in flexibility of the backbone chain of oligomer. The profile of cure schedules has been determined in the form of a cure window which measures the extent of reaction in terms of crosslink density as a function of bake temperature. The experimentally determined properties like tensile strength and MEK rub-resistance have been taken into consideration for fixing the lower and upper limits of XLD in designing the cure windows of individual coating formulations. The baking schedules of coatings have also been expressed in the form of nominal and true cure windows.