Dual-curable isocyanate crosslinking agents blocked by methacrylate-functionalized pyrazoles with lower curing temperature

Abstract

To develop a polyurethane-based automotive clearcoat with a lower curing temperature, methacrylatefunctionalized pyrazole derivatives were newly designed and synthesized as isocyanate blocking agents and their chemical structures were identified by 1H NMR. These novel blocking agents were used to block the functional groups of hexamethylene diisocyanate (HDI) trimer (commercially known as Desmodur N3300), and density functional theory simulations were performed to predict the activation energy of the deblocking process and to determine the H-N distance in the pyrazole molecules. The curing behavior of the blocked isocyanate with polyol resin at 110, 120, and 130 degrees C was investigated using an oscillatory rheometer and a rigid-body pendulum tester. In addition, their results were compared with those for a commercially available 3,5-dimethyl pyrazole-blocked HDI trimer (Desmodur PL350). The thermal properties of the clearcoat films cured at 130 degrees C were investigated using thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis, and the surface mechanical properties of the cured films were evaluated using a nano-indentation tester. The novel pyrazole blocking agents contributed to lowering the deblocking temperature and enhancing the mechanical properties via supplementary radical polymerization of deblocked methacrylate-functionalized pyrazoles, resulting in interpenetrating crosslinked polymer networks. This study provides a new concept of isocyanate blocking agents with lower deblocking temperatures and higher crosslinking densities, resulting in blocked isocyanates that can be used in various coating materials.