Figure 15 The schematic diagram of photocuring reaction of fluorosilicone coatings.[68]
About reactive diluents, there are some drawbacks of petroleum-based reactive diluents such as non-renewable feedstock, environmental pollution, skin irritation and poor adhesion in practical application.[75-76] Therefore, it is a great trend in field of photopolymerization to develop more environmentally friendly and healthy bio-based reactive diluents to substitute petroleum-based one. Yang et al.[77] synthesized a polyfunctional and renewable reactive diluent tung oil-based methacrylate (TDMM) by amidation, thiol-ene click reaction and esterification, TDMM and acrylated epoxidized soybean oil (AESO) at different proportions to formulate a “green + green” UV-LED curable systems with high bio-content, the chemical structures of AESO and PIs, synthesis route of TDMM are exhibited in Figure 16 and Figure 17, respectively. TDMM/AESO systems show higher Tg but lower thermal stability, better modulus and tensile strength than neat AESO. In addition, the UV-LED cured coatings exhibited excellent physical and chemical properties. In a word, TDMM shows to be an effective reactive diluent, and considered to have good potential in the UV-LED curable coatings.
Figure 16 Chemical structures of AESO and PIs.[77]
Figure 17 Synthesis route of tung-oil-based acrylate (TDMM).[77]
As plant phenols from natural origin, cardanol and eugenol are considered renewable raw material for alternatives to fossil feedstock,[78] and have attracted a lot of attention because their structural versatility, such as C=C bonds, hydroxyl groups, and benzene rings in their structure. Feng et al.[70] synthesized two allyl monomers (F-CA and F-EU) by cardanol and eugenol underwent esterification reaction with tetrafluoroterephthalic acid, next, a series of biobased polymer networks were obtained via photoclick thiol–ene reaction and thermal treatment between allyl monomers and thiols, the schematic diagram of the preparation of the cardanol-based and eugenol-based thiol−ene polymer networks is displayed in Figure 18. All the polymer networks exhibited high thermal stability (up to 283 °C), excellent solvent resistance and gel content of over 91.36%. Furthermore, the biobased polymer networks displayed a good degradability in alkaline solution on account of the presence of ester bonds and electron-deficient aromatic ring system. The multifunctional biobased thiol–ene polymer networks may have practical applications in many areas such as coating, composites, adhesives, etc.