Aspartate-based polyurea coatings: Ambient cure process and inevitable transformation of urea groups into hydantoin cycles in polyurea networks and their impact on film properties

Polyurea protective coatings have gained large interest from research and application in coating and gel polyelectrolyte areas due to their durability, excellent thermal, and mechanical stability, and the possibility of cure without the use of any catalyst or solvent. In this study, we address a chemical mechanism of the aspartate amine–isocyanate cure that involves an unexpected transition of urea bonds into hydantoin bonds. Three aspartic esters (ASPEs) with varying chemical structures were used to study model high-solid (≥60 wt.-% solid content) polyurea coatings. Two linear aliphatic ASPEs with flexible backbones that were prepared using the aza-Michael addition, and a commercial cycloaliphatic aspartic ester (Desmophen® NH 1420) were used as polyurea-based network precursors and were crosslinked with the plant-based tri-isocyanate Desmodur® eco N 7300. The coatings were cured under ambient conditions, and the effect of the molecular structure, chain length of ASPE, cure time, and the simultaneous chemical crosslinking and solvent evaporation on the coating properties were thoroughly investigated. The chemical changes that occurred throughout the film depths were analyzed by FTIR and Raman confocal microscopy. The study revealed the inherent transformation of urea bonds to substituted hydantoin rings. The intensity of transformation depended on the mobility of the polyurea network chain segments in ASPEs. The thermal and mechanical properties characterized by DSC, TGA, uniaxial tensile test, and microindentation hardness test showed the softening effect of the hydantoin group on the networks effective within weeks after the NCO primary cure.