Significance With the growing emphasis on green environmental protection concepts in society, waterborne polyurethane (WPU) coatings are widely used in various fields such as automobiles, construction, shipping, and wood products due to their environmental benefits. Using water as the solvent, WPU coatings have lower emissions of volatile organic compounds (VOCs) than traditional organic solvent-based coatings, meeting current environmental protection regulations. However, although WPU coatings have obvious advantages in terms of environmental protection, their performance in mechanical properties, solvent resistance and coating effect still needs to be improved. These performance deficiencies to some extent limit the further application and development of WPU coatings. To address these issues, researchers are seeking new materials and technologies in an effort to enhance the overall performance of WPU coatings. Solketal, as a bio-based material, possesses advantages such as low odor, non-toxicity, no irritation to the human body, and being of biological origin. It is regarded as an ideal base material and solvent for WPU coatings. The introduction of solketal can not only improve the environmental performance of WPU coatings but also effectively enhance their mechanical strength and coating properties, enabling them to possess more outstanding comprehensive performance while meeting environmental protection requirements. Therefore, the application research of solketal in WPU coatings is of great significance for promoting the green development of the coatings industry.

Progress Currently, when solketal is used as a base material for WPU coatings, it primarily participates in the preparation of the polyurethane matrix through hydrolysis to release active hydroxyl groups (Fig.2). Olson et al. first prepared functionalized solketal, which was then hydrolyzed and incorporated into the polyurethane matrix. The resulting WPU varnish exhibited excellent performance in coating tests. Bigwood et al. utilized solketal as a polyol to prepare polyurethane, which facilitated the formation of a cross-linked network structure, thereby enhancing the mechanical properties, solvent resistance, and thermal stability of the polyurethane. These studies demonstrate that solketal can not only enhance the mechanical properties of the coatings through chemical bonding but also optimize the coating process, thereby comprehensively improving the overall performance of WPU coatings. Furthermore, the application of solketal in the WPU systems is not limited to chemical modification, and it can also serve as a functional additive to improve the coating performance. For instance, Zhang Peng employed solketal as a slow-evaporating agent in the preparation of WPU coatings. When the resulting WPU was used in combination with defoamer NXZ and leveling agents, it can completely eliminate coating defects caused by bubbles, ensuring the coating quality. Wang Rongfang et al. used solketal in combination with various solvents as the solvent for WPU topcoats, where solketal played the role of smoothing the paint film surface and improving the gloss of the paint layer in the system. These application examples further demonstrate that solketal can synergistically function at both the molecular structure regulation and process optimization levels, enabling WPU coatings to maintain their eco-friendly characteristics while achieving outstanding mechanical strength and coating performance.

Prospects The use of solketal has opened up a new pathway for enhancing the performance of WPU coatings, demonstrating broad application potential. With further research, its applications in WPU coatings will continue to expand, and it is expected to achieve both environmental sustainability and performance improvements in more fields. Additionally, the bio-based nature of solketal aligns with the trend of sustainable development, strongly driving the green transformation of the coatings industry. Looking ahead, against the backdrop of increasingly stringent environmental regulations and continuous technological advancements, solketal is expected to play a significant role in the coatings and adhesives industries, particularly in enhancing product performance and reducing environmental impact.