Preparation and characterisation of hydrophobic ultraviolet-curable fluorinated polyurethane acrylate using ultraviolet-light emitting diode photopolymerisation

Abstract

Nowadays, the green aspects of the ultraviolet (UV) curing technology have been extensively explored in order to make the technology more energy-efficient and more environmental-friendly. The elimination of mercury, low energy consumption and low operation cost make the ultraviolet light emitting diode (UV-LED) system seem to be a promising alternative. Nevertheless, the UV-LED system also has several challenges such as limited photoinitiators and surface cure issues. Therefore, it would be necessary to reformulate the coating composition to match the irradiation profile of UV-LED. In this study, UV-curable coating was formulated based on aliphatic urethane acrylate oligomer (BOMAR™ 7432-GB), 2-ethylhexyl acrylate, methyl methacrylate, trimethylolpropane triacrylate and commercial photoinitiators (Chivacure® 300 and Irgacure® 184). The feasibility of the UV-LED system to replace conventional UV-mercury method was first conducted by comparing its curing behaviours with respect to irradiation time. From the Fourier transform infra-red (FTIR) spectra obtained, the C=C absorption peak observed at 1635 cm-1 and 810 cm-1 were found to be decreasing upon UV irradiation indicating the proceeding of photopolymerisation. A non-tacky coating was achieved after 25 minutes of curing and the degree of conversion and gel fraction measurement were 98% and 95%, respectively indicating that the photopolymerisation via UV-LED is superior to UV-mercury. Since UV-LED system showed promising results in parallel with its inherently environmental friendly (mercury-free) nature, the hydrophobic UV-curable coating was prepared with the sufficient curing time of 25 minutes. One of the strategies to create hydrophobic surface is to reduce its surface free energy (SFE) by incorporating heptadecafluorodecyl methacrylate monomer (HDFDMA). The presence of fluorine group in the hydrophobic coating was examined by FTIR and nuclear magnetic resonance analysis. It is well accepted that fluorine-based material can change the surface wettability due to its exceptionally low SFE. The water contact angle of the coating evolved from 88.4° to 121.2° as the HDFDMA content increased which in turn, reducing the calculated SFE from 32.85 mJ/m2 to 9.68 mJ/m2. Scanning electron microscopy and atomic force microscopy confirmed the migration of the fluorine component towards the coating surface as it is the key factor that contributes to the hydrophobicity. However, the transparency of the cured coating dropped markedly with increasing HDFDMA content. Analogously, these results may attain various potential applications such as liquid-repellent coating and anti-fouling coating. In conclusion, the utilisation of UV-LED in this research was a great initiative to develop green aspect for photopolymerisation