Thermal transport and chemical effects of fillers on free‐radical frontal polymerization

Abstract

Frontal polymerization (FP) is a process in which a front propagates in a localized reaction zone, converting monomer into polymer through the coupling of thermal diffusion with the Arrhenius kinetics of an exothermic reaction. Fillers are added to control the rheological properties of the formulation and to enhance the mechanical properties of the product. However, the thermal and chemical effects of these fillers on the front propagation have not been thoroughly explored. Herein we report the thermal and chemical effects of fillers on free‐radical frontal polymerization. It was found that fillers with high thermal diffusivities, such as milled carbon fiber and boron nitride increased the front velocity. Despite their high thermal diffusivities, fillers such as aluminum and alumina decreased the front velocity. This is likely due to the radical‐scavenging ability of aluminum oxide, which was explored with clay minerals. It was found that the presence of water within clay fillers can also decrease the front velocity. To probe the chemical effects, acid‐activated clay minerals were utilized. The results demonstrate that some fillers can increase front velocity through their high thermal diffusivities while others decrease it by acting as radical scavengers.

Front velocity dependence on vinyl ether and initiator concentration in radical‐induced cationic frontal polymerization of epoxies

“…29 Front-quenching can occur due to buoyancy-driven convection but this can be avoided by increasing the viscosity through addition of fillers such as fumed silica. [30][31][32] Addition of fumed silica as an inorganic filler also allows for horizontal frontal polymerization. 33 One class of photoinitiators for cationic polymerization are diaryliodonium salt photoacid generators, first discovered by Crivello et al in 1977.…”

“…Typically, most monomers capable of supporting fronts proceed by free‐radical mechanisms due to their high rate of reactivity and exothermicity 29 . Front‐quenching can occur due to buoyancy‐driven convection but this can be avoided by increasing the viscosity through addition of fillers such as fumed silica 30–32 . Addition of fumed silica as an inorganic filler also allows for horizontal frontal polymerization 33 …”

Charge transfer complexes as dual thermal/photo initiators for free‐radical frontal polymerization

“…Photopolymerization is a rapid process that uses less energy than thermal curing but is limited when it comes to the curing of thick and/or filled materials 1–3 . Frontal polymerization (FP) is a process that can combine the in‐depth cure and curing of highly filled systems 4,5 offered by bulk polymerization with the high speed of photopolymerization 6 . Dual‐cure initiators that can serve as both photo and thermal initiators for polymerization are of interest in the search for more energy‐efficient methods of polymerization 7–10 …”

Effect of resin staging on frontal polymerization of dicyclopentadiene

“…The result is a propagating reaction wave that rapidly transforms the monomer into polymer. FP has been previously demonstrated for various resin chemistries, including hydrogels, 7–9 acrylates, 10–12 polyurethanes, 13 epoxies, 14,15 thiolenes, 16 and cyclic olefins 17,18 . Development of FP resin chemistries requires the material to have a high energy density and very high rate of reaction at the front temperature but be stable and exhibit low rate of reaction at initial resin temperature 6 .…”