Reversible polymer networks based on the dynamic hindered urea bond for scratch healing in automotive clearcoats

A series of dynamic polyacrylate urethane urea networks containing hindered urea (HU) groups (C-HU-ethylene glycol, C-HU-EG; C-HU-tetra(ethylene glycol), C-HU-TEG; C-HU-poly(ethylene glycol), C-HU-PEG) were successfully synthesized for use in automotive clearcoats. The material properties of the clearcoats were controlled using different HU adducts and by adjusting their contents in the polymer networks. The indentation hardness (H-IT), modulus (E-IT), thermal stability (T-d), and glass-transition temperature (T-g) of the polymer networks were characterized by nano-indentation tests, thermogravimetric analysis, dynamic mechanical analysis, and differential scanning calorimetry, respectively. The scratch-resistance and healing performances of the polymer networks were evaluated quantitatively using a micro-scratch tester in conjunction with optical microscopy. The results reveal that the C-HU-TEG polymer networks not only exhibited physical properties most similar to those of the commercial clearcoat (T30) but also demonstrated the best self-healing performance. The balanced chemical structure of the HU-TEG adduct between the flexible unit (TEG) and the rigid unit (IPDI, urea, and urethane bond) played an important role of increasing scratch-healing performance while maintaining the clearcoat material properties.