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 ( 1 H 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.