Application of polycarbonate diol containing hindered urea to polyurethane-based clearcoats for tuning of scratch-healing properties

Abstract

Newly synthesized polycarbonate diols containing hindered urea (PCD-HUs) were employed to modify the scratch-healing properties of conventional two-component clearcoats. To elucidate their contributions to the crosslinking properties of the coatings under conventional thermal curing (150°C), the real-time rheological properties of the clearcoat mixtures were measured using a rotational rheometer in the small-amplitude oscillatory shear mode. A dynamic mechanical analysis was carried out to compare the thermomechanical properties of the cured clearcoat free films. PCD-HU, a key component for the tuning of the scratch-healing, was further identified by analyzing the clearcoat layer cured over a phosphated metal substrate with undercoats such as primer and basecoat as a precoated metal (PCM) sheet. Elastic recovery and scratch profiles of the outermost clearcoat layer in the PCM sheet were acquired through nano-indentation and nano-scratch tests by gradually increasing the quantitative normal force. Finally, the healing patterns of the scratches on various clearcoat layers at a certain normal force were visualized by three-dimensional and one-dimensional cross-cut images using atomic force microscopy under specified temperature and healing time conditions. The surface durability provided by the PCD-HU in the clearcoat layer was confirmed by repeated scratching and healing tests on the same surface positions. Therefore, the coating layer containing HU, which possessed similar surface mechanical properties to those of conventional references, exhibited better scratch-healing performance.