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Review on UV-Induced Cationic Frontal Polymerization of Epoxy Monomers

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Review on UV-Induced Cationic Frontal Polymerization of Epoxy Monomers polymers Review Review on UV-Induced Cationic Frontal Polymerization of Epoxy Monomers Muhammad Salman Malik 1 , Sandra Schlögl 1 , Markus Wolfahrt 1 and Marco Sangermano 2,* 1 Polymer Competence Center Leoben GmbH, Rossegerstrasse 12, 8700 Leoben, Austria; [email protected] (M.S.M.); [email protected] (S.S.); [email protected] (M.W.) 2 Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, C.so Duca degli Abruzzi 24, I-10129 Torino, Italy * Correspondence: [email protected] Received: 28 August 2020; Accepted: 18 September 2020; Published: 20 September 2020 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. Keywords: photoinitiator; thermal initiator; UV light; cationic polymerization; epoxy monomers; frontal polymerization 1. Introduction 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 [1–7]. By definition, 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]. Once triggered, the polymerization front propagates in a wave-like fashion to all portions of the sample using the enthalpy of polymerization, without further need for an external stimulus. 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 [10–12]. Frontal polymerization requires an external stimulus to trigger the reacting front. Based on different stimuli, frontal polymerization is classified into three categories namely thermal frontal polymerization, photo-frontal polymerization and isothermal polymerization [8]. In particular, ultraviolet (UV)-induced frontal polymerization has attracted a great number of industrial sectors for various applications due to its low operational costs, ambient temperature processing, Polymers 2020, 12, 2146; doi:10.3390/polym12092146 www.mdpi.com/journal/polymers Polymers 2020, 12, 2146 2 of 34 solvent-lessPolymers 2020, formulation 12, x FOR PEER and REVIEW thefreedom to have tuneable final properties of the cured polymer2 of [ 1333]. UV-curing is very well known for applications such as printing inks and adhesives, varnishes, protective coatings,varnishes, which protective have been coatings, implemented which have on abeen vast im scaleplemented thanks toon rapid a vast curing scale thanks[13–17] .to Photoinduced rapid curing polymerizations[13–17]. Photoinduced proceed bypolymerizations a chain reaction proceed mechanism by a involving chain reaction the propagation mechanism of aninvolving active centre the bypropagation the interaction of an with active a monomer. centre by the The interaction active centre with can a monomer. be a radical, The a cation active orcentre more can rarely be a an radical, anion. a cationWhile or photoinducedmore rarely an radicalanion. chain growth polymerization has been widely investigated and exploitedWhile from photoinduced an industrial radical point ofchain view, growth it was notpolyme untilrization the discovery has been of oniumwidely salt investigated photoinitiators and byexploited James V. from Crivello an industrial in the 1970s point that of theview, first it cationicwas not photoinduceduntil the discovery polymerization of onium salt was ph reportedotoinitiators [18]. Typically,by James onium V. Crivello salts, eitherin the triarylsulfonium1970s that the first or cationic diaryliodonium photoinduced salts, arepolymerization considered as was a photo-acid reported generator[18]. Typically, under UV-irradiation.onium salts, either The triarylsulfonium strong photo-acid or generateddiaryliodonium is able salts, to promote are considered cationic as ring- a openingphoto-acid polymerization generator under of an UV-irr epoxyadiation. resin (reaction The strong reported photo-acid in Scheme generated1)[ 18 is– 20able]. These to promote highly reactivecationic cationsring-opening attack poly adjacentmerization epoxide of an groups epoxy resin forming (reaction polyether reported in in the Scheme ring opening1) [18–20]. cationic These polymerizationhighly reactive (seecations reaction attack Scheme adjacent1)[ epoxide21,22]. grou Theps Lewis forming acid polyether generated in in the cationic ring opening polymerization cationic sustainspolymerization its reactivity (see reaction with the Scheme epoxy 1) monomers [21,22]. The for Lewis a long acid time generated without in cationic further polymerization UV irradiation needed,sustains as its opposed reactivity to freewith radical the epoxy polymerization monomers wherefor a long thecuring time without reaction further stops upon UV irradiation removal of UVneeded, irradiation as opposed [13]. Thisto free so-called radical “darkpolymerization curing” o whffersere the the advantage curing reaction of curing stops residual upon removal monomer of andUV thermalirradiation post [13]. treatment This so-called for complete “dark conversioncuring” offers in cationicthe advantage polymerization of curing residual [23]. Furthermore, monomer cationicand thermal photopolymerizable post treatment monomersfor complete such conversion as epoxies in are cationic less toxic polymerization and irritating [23]. in comparisonFurthermore, to frequentlycationic photopolymerizable used acrylates in free monomers radical photopolymerization. such as epoxies are less Epoxide toxic and monomers irritating curedin comparison via cationic to routefrequently have lowerused acrylates volumetric in free shrinkage radical andphotopol also provideymerization. better Epoxide matrix monomers adhesion ascured opposed via cationic to free radicalroute photopolymerizedhave lower volumetric monomers shrinkage [17 ,24and–26 also]. Therefore, provide better most ofmatrix the early adhesion investigations as opposed have to beenfree focusedradical onphotopolymerized UV curing of epoxide monomers monomers [17,24–26]. for inks, Ther coatingsefore, andmost adhesives of the early where investigations they provide have high polymerizationbeen focused on rates, UV insensitivity curing of epoxide to oxygen monomers and low energyfor inks, requirement coatings and [27 –adhesives31]. where they provide high polymerization rates, insensitivity to oxygen and low energy requirement [27–31]. SchemeScheme 1. 1.Reaction Reaction schemescheme forfor cationiccationic ringring openingopening frontalfrontal polymerization of oxiranes. (1) (1) Photo- Photo- inducedinduced cleavage cleavage of of iodonium iodonium saltsalt fromfrom aa singletsinglet (S1)(S1) andand/or/or triplettriplet (T1) excited state, that that generates generates cations,cations, radical radical cations cations and and free free radicals radicals ultimatelyultimately producingproducing a superacid; (2) Protonation of of oxirane oxirane monomer;monomer; (3)(3) Exothermic re reactionaction of of protonated protonated monomer monomer with with oxiranes oxiranes resulting resulting in a polymer in a polymer chain chaingrowth. growth. However,However, the the main main limitation limitation of UV-induced of UV-induced crosslinking crosslinking reaction reaction is related is to related the low to penetration the low depthpenetration of UV light,depth which of limitsUV polymerizationlight, which techniques,limits polymerization even cationic photopolymerization,techniques, even cationic to thin filmsphotopolymerization, and adhesive applications to thin [ 31films,32]. Theand firstadhesive report onapplications photoactivated [31,32]. cationic The polymerizations first report on to achievephotoactivated self-sustained cationic frontal polymerizations polymerizations to achieve was reported self-sustained by Crivello frontal et al. [polymerizations33], who showed was that thereported local application by Crivello of heatet al. to [33], an irradiated who showed monomer that the sample local results application in polymerization of heat to an that irradiated occurs as amonomer front, propagating sample results rapidly in throughoutpolymerization the entire that oc reactioncurs as massa front, [33 –propagating35].
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