Kinetic and Chemical Effects of Clays and Other Fillers in the Preparation of Epoxy–Vinyl Ether Composites Using Radical-Induced Cationic Frontal Polymerization

Abstract

Addition of fillers to formulations can generate composites with improved mechanical properties and lower the overall cost through a reduction of chemicals needed. In this study, fillers were added to resin systems consisting of epoxies and vinyl ethers that frontally polymerized through a radical-induced cationic frontal polymerization (RICFP) mechanism. Different clays, along with inert fumed silica, were added to increase the viscosity and reduce the convection, results of which did not follow many trends present in free-radical frontal polymerization. The clays were found to reduce the front velocity of RICFP systems overall compared to systems with only fumed silica. It is hypothesized that chemical effects and water content produce this reduction when clays are added to the cationic system. Mechanical and thermal properties of composites were studied, along with filler dispersion in the cured material. Drying the clays in an oven increased the front velocity. Comparing thermally insulating wood flour to thermally conducting carbon fibers, we observed that the carbon fibers resulted in an increase in front velocity, while the wood flour reduced the front velocity. Finally, it was shown that acid-treated montmorillonite K10 polymerizes RICFP systems containing vinyl ether even in the absence of an initiator, resulting in a short pot life.

Redox cationic frontal polymerization: a new strategy towards fast and efficient curing of defect-free fiber reinforced polymer composites

“…Fillers or fibers have a positive impact on the propagation of frontally polymerizable resins, which might explain the distinctive increase in the T g from neat resin to the composite. 2,52 The successful RCFP curing of high thickness CFRC showed that it is possible to cure such components under low oven heating and at a much shorter curing duration than conventional anhydride cured resins.…”

“…Radical induced cationic frontal polymerization (RICFP) has gained a lot of attention amongst researchers in the past few years, as a rapid curing substitute for various cyclic ether monomers. 1–7 The well-known cationic frontal polymerization involves generation of radicals and cations when diaryliodonium salts are photo or thermal cleaved, when subjected to ultraviolet light or heat. A strong Brønsted acid is produced that protonates cyclic ether monomers forming highly reactive cations that are able to react with adjacent cyclic ether groups forming polyether in a ring opening cationic polymerization.…”

Free-Standing 3D Printing of Epoxy-Vinyl Ether Structures Using Radical-Induced Cationic Frontal Polymerization

“…This is likely due to the differences in structure of the TEGDVE versus BADGE, where the former is a structurally linear monomer with ether linkages that facilitate bending compared to the aromatic rings of BADGE, which provide rigidity. Polymers containing TEGDVE with epoxy and produced with RICFP have been previously reported to be flexible. , To maintain reactivity but improve the rigidity of the polymer, CE was added, which contains two cyclohexane rings for increased rigidity unlike TEGDVE. The final resin composition of 3:1:1 BADGE:CE:TEGDVE (wt:wt:wt) with 1 phr IOC-8 and 1 phr Luperox 231 was chosen as the optimal balance of reactivity and physical properties while supporting a front at the printing diameter to allow for the front-driven printing.…”

“…The effects of resin composition, initiator concentration, filler type, and loading on front kinetics were investigated. Carbon nanofibers (CNFs) and milled carbon fibers (MCFs) were compared as fillers to tune front kinetics, and CNFs were investigated to manipulate mechanical properties, while fumed silica (FS) was used to further tailor viscosity, as FS is inert in RICFP. , CNFs are smaller than carbon fibers and have a higher specific surface area, which could assist in improving physical properties. They are larger than CNTs, less thermally conductive, and differ in their physical structure, being cylinders composed of stacked layers versus hollow CNTs; , CNFs have the advantage of being less expensive than CNTs while still possessing beneficial thermal properties for FP.…”

“…During RICFP, if thermally initiated, the radical initiator decomposes to produce radicals that reduce the superacid generator, which generates a superacid based on the counterion after decomposition steps. The superacid can then initiate cationic polymerization, and heat from propagation generates radicals from the radical initiator, looping the process. , If initiated by ultraviolet (UV) light, the superacid generator instead decomposes and generates the superacid from excitation by the UV, which then initiates polymerization, and the heat generated by propagation cleaves the radical initiator to promote superacid generator decomposition . A simplified mechanism of the RICFP process is shown in Figure S1.…”