Castor Oil Based Biothiol as a Highly Stable and Self-Initiated Oligomer for Photoinitiator-Free UV Coatings

In this paper, we reported castor oil based biothiol (CO-SH) as a highly stable and self-initiated oligomer for photoinitiator (PI)-free UV coating. CO-SH was synthesized via esterification reaction between castor oil (CO) and 3-mercaptopropionic acid (MPA). The results of both proton nuclear magnetic resonance (1H NMR) and Fourier transform infrared spectroscopy (FTIR) revealed that the targeted oligomer was successfully prepared. The oligomer with the number-average molecular weight of 2040 g mol–1 exhibited a narrow molecular weight distribution (PDI = 1.23). CO-SH could be stored at 80 °C for 7 days with only a light C═C conversion (≈6%); hence, the steric effect of the vegetable oil molecule gave the primary biothiol an excellent temporal stability. Furthermore, real-time FTIR results indicated that excellent photopolymerization profiles were obtained, and the final conversions of double bond and thiol were 100% and 77% in the absence of PI, respectively, showing the rapid self-initiated photoactivity. In addition, the self-initiated photopolymerization mechanism of CO-SH was investigated. Finally, the UV-curable films of CO-SH also showed good physical and chemical properties.