Crosslinking characteristics of aziridine crosslinkers in polyurethane-based clearcoats for automotive applications

Crosslinking systems play a vital role in the coating industry by significantly improving the wear resistance, chemical resistance, and toughness of the coatings. In particular, polyfunctional aziridines, known for their distinctive nitrogen-containing three-membered ring structures, serve as versatile crosslinkers in automotive coatings and adhesives. These crosslinkers can effectively enhance both the physical and chemical properties of polyurethane coatings. This study focuses on investigating the impact of two different trifunctional aziridines on the thermal curing characteristics of 2K clearcoats containing free-isocyanate crosslinker and polyacrylic resin. The analysis confirmed the ring-opening reaction of the aziridine crosslinker with the carboxyl groups in the resin at room temperature, as evidenced by changes in the –NH stretching frequency in the Fourier transform infrared spectra. To assess the real-time curing performance of aziridines during thermal curing at 80 °C, rheological storage modulus and curing behavior of the formulated 2K clearcoats were measured using a rotational rheometer and a rigid-body pendulum tester. Additionally, nano-indentation and nano-scratch tests were conducted to quantitatively evaluate the surface hardness and scratch resistance of the cured clearcoat films. By examining the relationship between the curing dynamics and the final mechanical properties, this study offers insights into optimizing the amount of aziridines required to enhance the properties of clearcoat films.