To address the issue of conventional polyurethane coatings being prone to damage and lacking self-healing capabilities, researchers developed self-healing polyurethane coatings containing 5 wt% and 10 wt% healing agents via the Diels–Alder (DA) cycloaddition mechanism. The results indicate that the incorporation of healing agents increases coating hardness by 3%–12% and achieves scratch healing efficiencies of 85.6%–93.6% within 30 minutes at 120 °C, significantly extending the service life of the coatings. This study provides an innovative solution for surface protection of engineering materials.
In the field of engineering materials, the repair of mechanical damage in coating materials has long been a major challenge. Although traditional polyurethane coatings exhibit excellent weather resistance and adhesion, their protective performance deteriorates rapidly once scratches or cracks occur. Inspired by biological self-healing mechanisms, scientists have begun exploring self-healing materials based on dynamic covalent bonds, with the Diels–Alder (DA) reaction gaining significant attention due to its mild reaction conditions and favorable reversibility. However, existing research has primarily focused on linear polyurethane systems, leaving a gap in the study of self-healing properties in cross-linked polyurethane powder coatings.
To break through this technical barrier, domestic researchers innovatively introduced two DA healing agents—furan-maleic anhydride and furan-bismaleimide—into a hydroxylated polyester resin system, developing a polyurethane powder coating with excellent self-healing properties. The study used ¹H NMR to confirm the structure of the healing agents, differential scanning calorimetry (DSC) to verify the reversibility of the DA/retro-DA reactions, and nanoindentation techniques along with surface profilometry to systematically evaluate the mechanical properties and surface characteristics of the coatings.
In terms of key experimental techniques, the research team first synthesized hydroxyl-containing DA healing agents using a two-step method. Subsequently, polyurethane powders containing 5 wt% and 10 wt% healing agents were prepared via melt blending, and applied onto steel substrates using electrostatic spraying. By comparing with control groups without healing agents, the influence of healing agent concentration on material properties was systematically investigated.
1.NMR Analysis Confirms Healing Agent Structure
1 H NMR spectra showed that amine-inserted furan-maleic anhydride (HA-1) exhibited characteristic DA ring peaks at δ = 3.07 ppm and 5.78 ppm, while the furan-bismaleimide adduct (HA-2) displayed a typical DA bond proton signal at δ = 4.69 ppm, confirming the successful synthesis of the healing agents.
2.DSC Reveals Thermally Reversible Characteristics
DSC curves indicated that samples containing healing agents exhibited endothermic peaks for the DA reaction at 75 °C and characteristic peaks for the retro-DA reaction in the range of 110–160 °C. The peak area increased with higher healing agent content, demonstrating excellent thermal reversibility.
3.Nanoindentation Tests Show Hardness Improvement
Depth-sensitive nanoindentation tests revealed that the addition of 5 wt% and 10 wt% healing agents increased coating hardness by 3% and 12%, respectively. A hardness value of 0.227 GPa was maintained even at a depth of 8500 nm, attributed to the cross-linked network formed between the healing agents and the polyurethane matrix.
4.Surface Morphology Analysis
Surface roughness tests showed that pure polyurethane coatings reduced the substrate Rz value by 86%, while coatings with healing agents exhibited a slight increase in roughness due to the presence of larger particles. FESEM images visually illustrated changes in surface texture resulting from the healing agent particles.
5.Breakthrough in Scratch Healing Efficiency
Optical microscopy observations demonstrated that coatings containing 10 wt% healing agent, after heat treatment at 120 °C for 30 minutes, exhibited a reduction in scratch width from 141 μm to 9 μm, achieving a healing efficiency of 93.6%. This performance is significantly superior to that reported in existing literature for linear polyurethane systems.
Published in Next Materials, this study offers multiple innovations: First, the developed DA-modified polyurethane powder coatings combine good mechanical properties with self-healing capability, achieving a hardness improvement of up to 12%. Second, the use of electrostatic spraying technology ensures uniform dispersion of healing agents within the cross-linked network, overcoming the positioning inaccuracy typical of traditional microcapsule techniques. Most importantly, these coatings achieve high healing efficiency at a relatively low temperature (120 °C), offering greater industrial applicability compared to the 145 °C healing temperature reported in existing literature. The study not only provides a new approach to extending the service life of engineering coatings but also establishes a theoretical framework for the molecular design of functional coatings through its quantitative analysis of the “healing agent concentration–performance” relationship. Future optimization of the hydroxyl content in healing agents and the ratio of uretdione cross-linkers is expected to further push the performance limits of self-healing coatings.
Post time: Sep-15-2025