Dynamic polymer networks containing photothermal materials have been reported to demonstrate highly efficient intrinsic self-healing under irradiation. In particular, organic near-infrared-absorbing ionic salts, such as diimmonium dyes, function as transparent polymer heaters and can enhance the self-healing properties of clearcoats. In this study, we designed a self-healing automotive clearcoat with a reversible polymer network based on acryl polyol (AP) and dynamic hindered urea (HU) bonds and introduced N-butyl-substituted diimmonium borate dye (DID) as a photothermal dye. To optimize the self-healing efficiency of the clearcoat and its transparency in the visible light region, the effects of the presence or absence of dynamic HU bonds and the concentration of the photothermal dye were precisely investigated. For a polymer system containing HU with 0.1 wt % DID (AP/HU-DID_0.1), the transparent automotive clearcoat was heated to ∼70 °C under focused sunlight irradiation and exhibited excellent (∼100% healing efficiency) and fast (<30 s) scratch-healing performance compared with a commercial automotive clearcoat. In addition, this photothermal effect-based self-healing clearcoat exhibited outstanding transparency (over 95%) and has a strong advantage with respect to energy consumption because it enables faster and more localized healing compared with thermal healing processes that require heating the entire product.
We conducted a detailed investigation of the influence of the material properties of dynamic polymer network coatings on their self-healing and damage-reporting performance. A series of reversible polyacrylate urethane networks containing the damage-reporting diarylbibenzofuranone unit were synthesized, and their material properties (e.g., indentation modulus, hardness modulus, and glass-transition temperature) were measured conducting nanoindentation and differential scanning calorimetry experiments. The damage-reporting and self-healing performances of the dynamic polymer network coatings exhibited opposite tendencies with respect to the material properties of the polymer network coatings. Soft polymer network coatings with low glass-transition temperature (~10 °C) and indentation hardness (20 MPa) exhibited better self-healing performance (almost 100%) but two times worse damage-reporting properties than hard polymer network coatings with high glass-transition temperature (35~50 °C) and indentation hardness (150~200 MPa). These features of the dynamic polymer network coatings are unique; they are not observed in elastomers, films, and hydrogels, whereby the polymer networks are bound to the substrate surface. Evidence indicates that controlling the polymer’s physical properties is a key factor in designing high-performance self-healing and damage-reporting polymer coatings based on mechanophores.
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