Reasons for the yellowness of photocured samples by the benzophenone/1,3-benzodioxole photoinitiating system

The benzophenone (BP)/1,3-benzodioxole (BDO) bimolecular photoinitiating system was developed in recent years. However, the obvious yellowness of the samples cured by the BP-BDO photoinitiating system would limit its wide application in the photocuring field. To study the reasons for the yellowness of the cured samples, the photoreaction products of the mixture of BP and BDO were analyzed by infrared spectroscopy (IR) and GC-MS. Electron spin resonance (ESR) spectroscopy was also used to gain insight into the formation of the initiating radicals with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) as a spin trap. The results of IR and GC-MS indicated that after the UV exposure of the BP-BDO mixture, four other main photoreaction products (ingredient 1-4) were generated via the interaction of excited BP and BDO. From the ESR hyperfine spectrum, only phenyl radical could be observed in the absence of air, which was significant to initiate the photopolymerization of HDDA and to form the three dimensional network. However, a minor amount of hydroxyl radical could also be generated besides the phenyl radical in the presence of air. The generation of hydroxyl radical in the photoreaction system is attributed to the formation of the phenol compound ingredient 1, which greatly contributed to the obvious yellowness of cured samples photoinduced by BP-BDO in practical applications.