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.