Ultraviolet (UV)-curing technology is being developed due to the 5E characteristics of high efficiency, energy saving, environmental friendly, wide adaptability, and economic (Fu et al., 2019, Su et al., 2020, Yu et al., 2018). This technology has been widely used in various fields of electronics, automotive industry, printing and processing, furniture manufacturing, and healthcare, (Fu et al., 2020, Liang et al., 2019b, Ren et al., 2019), which has been utilized evolved from the earliest adhesives to UV-curing coatings and inks (Liu et al., 2015a, Liu et al., 2015b). Among them, UV-curing coatings accounted for approximately 65% of the entire industry, which represented the development direction of environment friendly coatings. However, there are inevitably some drawbacks, such as the oxygen inhibition during the surface curing process, and limits the curing depth and thickness, as well as the irritation and toxicity of raw materials limit their application in the food and drug industries.
The concept of click reaction was first proposed by Sharpless in 2001, as a class of efficient and highly selective chemical methods (Kolb et al., 2001, Resetco et al., 2017). Click chemistry is characterized by simple reaction operation, insensitivity to oxygen and water, high yield, good stereoselectivity, and wide application range (Luo et al., 2011, Yang et al., 2009), which were utilized in the fields of micro ceramic devices, optical components, and coating of biomedical materials, as well as in the preparation of UV-curable adhesives, and other functional polymer materials. Although the thiol-ene click reaction has these advantages, the drawbacks are also obvious, mainly due to the unpleasant taste of thiol compounds. The odor problems of the thiol groups are mainly related to the small molecular weight and content of thiol compounds (Feng et al., 2017, Modjinou et al., 2016, Wang et al., 2016). Previous reported indicated that the adding non reactive small molecule thiol compounds to the UV-curing can resolve the odor problems. This challenge can be solved by increasing the functionality of small molecule thiol compounds, grafting them onto polymer structures, or increasing the molecular weight of thiol compounds(Kim et al., 2018).
Castor oil has attracted much attention due to their low toxicity, biodegradability, and non volatility in the research of coatings (Gao et al., 2012, Paraskar et al., 2020). It not only contains triglycerides and fatty chain structures, but also possesses the hydroxyl groups in the chemical structure. Due to the presence of hydroxyl groups, castor oil can be polymerized with diisochlorate to prepare oil-based polyurethane without any chemical modification. The investigation about the vegetable oil-based photosensitive oligomers have been reported extensively (Liang et al., 2019a, Liang et al., 2019b). Our group also had fabricated a series of oil-based oligomers to develop high performance UV-curing coatings (Hu et al., 2018, Wu et al., 2018, Zhang et al., 2020, Zhu et al., 2021). However, the problem of the serious volume shrinkage in the UV-cured resin leads to warping or deformation of the cured resin, restricting the applications of UV-curing in the field of precision molding.
In this paper, a castor oil-based reactive diluent (COSH) was synthesize through thiol-ene click reaction. Then the COSH was mixed with the cardanol-based PUA (CNPUA) oligomer to prepared bio-based PUA materials (Hu et al., 2019), which to solve the problems of the severe volume shrinkage, and the strong odor of thiol compounds. In addition, two petroleum-based diluents of trimethylolpropane triacrylate (TM) and trimethylolpropane tris (3-mercaptopropionate) (TMSH) were also used to prepare PUA materials, and compared the comparative property with the CNPUA/COSH resins.
