Abstract
Ultraviolet (UV)-induced cationic frontal polymerization has emerged as a novel technique that allows rapid curing of various epoxy monomers upon UV irradiation within a few seconds. In the presence of a diaryliodonium salt photoinitiator together with a thermal radical initiator, the cationic ring opening polymerization of an epoxide monomer is auto-accelerated in the form of a self-propagating front upon UV irradiation. This hot propagating front generates the required enthalpy to sustain curing reaction throughout the resin formulation without further need for UV irradiation. This unique reaction pathway makes the cationic frontal polymerization a promising route towards the efficient curing of epoxy-based thermosetting resins and related composite structures. This review represents a comprehensive overview of the mechanism and progress of UV-induced cationic frontal polymerization of epoxy monomers that have been reported so far in literature. At the same time, this review covers important aspects on the frontal polymerization of various epoxide monomers involving the chemistry of the initiators, the effect of appropriate sensitizers, diluents and fillers.
Highlights
In the past few decades, polymer resins cured via frontal polymerization technique have been the focus of research for applications such as nanocomposites, functionally graded polymers, sensory materials, hydrogels and fibre-reinforced composites [1,2,3,4,5,6,7]
The results revealed that most of the diphenyliodonium salts absorb at the region of the small extinction coefficient tail of major absorption band, i.e., 237–238 nm [19]
The results showed a three times lower minimal layer thickness of the cured formulation containing including diphenyliodonium tetrakis(perfluorot-butyloxy)aluminate (I-Al) compared to I-Sb at 1 mol % with increasing concentration of thermal radical initiator [40]
Summary
In the past few decades, polymer resins cured via frontal polymerization technique have been the focus of research for applications such as nanocomposites, functionally graded polymers, sensory materials, hydrogels and fibre-reinforced composites [1,2,3,4,5,6,7]. Monomers that undergo frontal polymerization are able to propagate directionally in a confined reaction zone over a period of time, requiring a localized stimulus [8]. The first successful frontal polymerization was reported by Russian scientists in 1972 using methyl methacrylate as the monomer and benzoyl peroxide as thermal initiator [9]. Later, it was successfully implemented at a pilot production plant in Chernogolovka and to a polymeric material synthesis factory in Dzerzhinsk in Russia [10,11,12]. Frontal polymerization requires an external stimulus to trigger the reacting front. Frontal polymerization is classified into three categories namely thermal frontal polymerization, photo-frontal polymerization and isothermal polymerization [8]
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