Effect of Urethane Crosslinking by Blocked Isocyanates with Pyrazole-Based Blocking Agents on Rheological and Mechanical Performance of Clearcoats

Abstract

A novel blocked isocyanate crosslinker was synthesized, and its applicability was investigated for the low-temperature curing of automotive clearcoats. Various pyrazole derivatives were prepared as blocking agents in isocyanate crosslinkers, which strongly affect the deblocking and curing properties of the urethane-bonded coating systems. The thermal curing properties of clearcoat samples containing a pyrazole-based blocked isocyanate crosslinker and polyol resin were characterized under two different temperature conditions (120 and 150 degrees C). The decrease in the amount of hydroxyl groups in the polyol before and after curing was expressed by the change in OH stretching frequency in the Fourier transform infrared (FT-IR) spectra. The real-time rheological storage moduli of the bulk clearcoat mixtures were measured via a rotational rheometer to determine the effect of pyrazole-based blocking agents on the curing dynamics. In addition, a rigid-body pendulum tester (RPT) was employed to investigate the curing behavior in the thin film form. The nano-indentation and the nano-scratch tests were conducted to examine the surface hardness and scratch resistance characteristics of the cured clearcoat films. The results show that a low-temperature curing system of clearcoats can be realized by tuning the curing temperature and reactivity of isocyanate crosslinkers blocked with pyrazole-based substituents.