Mechanism and Catalysis of Oxidative Degradation of Fiber-Reinforced Epoxy Composites

Abstract

Carbon fiber-reinforced polymer (CFRP) materials are widely used in aerospace and recreational equipment, but there is no efficient procedure for their end-of-life recycling. Ongoing work in the chemistry and engineering communities emphasizes recovering carbon fibers from such waste streams by dissolving or destroying the polymer binding. By contrast, our goal is to depolymerize amine-cured epoxy CFRP composites catalytically, thus enabling not only isolation of high-value carbon fibers, but simultaneously opening an approach to recovery of small molecule monomers that can be used to regenerate precursors to new composite resin. To do so will require understanding of the molecular mechanism(s) of such degradation sequences. Prior work has shown the utility of hydrogen peroxide as a reagent to affect epoxy matrix decomposition [1]. Herein we describe the chemical transformations involved in that sequence: the reaction proceeds by oxygen atom transfer to the polymer’s linking aniline group, forming an N-oxide intermediate. The polymer is then cleaved by an elimination and hydrolysis sequence. We find that elimination is the slower step. Scandium trichloride is an efficient catalyst for this step, reducing reaction time in homogeneous model systems and neat cured matrix blocks. The conditions can be applied to composed composite materials, from which pristine carbon fibers can be recovered.

A structural chemistry look at composites recycling

“…41,42 We have previously reported the formation of imines from N-oxides via NMR spectroscopy in our degradation studies of small molecule analogues of amine-cured epoxies. 43 Through this molecular study, we argue that other successful reports of amine-cured epoxy resin degradation involving oxidants like hydrogen peroxide misattribute the mechanism to a series of hydroxyl radical-based processes. [44][45][46] Identifying the most-likely mechanism occurring during these degradation reactions is key to build on this knowledge as the community develops an optimal process for depolymerizing amine-cured epoxies.…”

Recovery of Industrial Wastes as Fillers in the Epoxy Thermosets for Building Application

“…The added functionality of the phenolic resin increases the ability of the resin to crosslink, creating a stronger polymer network with high resistivities. The high chemical and solvent resistivities and temperature compatibility of epoxy phenolic resins are most useful when used in high-performance applications and in corrosion resistance [59,60]. Samples with NS and the ER3 resin also exhibited high chemical resistance, and they can be used in a chemically aggressive environment to avoid a possible release of contaminants from the material to the environment, thus, ensuring appropriate environmental protection.…”

Measuring impacts of microplastic treatments via image recognition on immobilised particles below 100 μm

“…deemed effective for an epoxide matrix decomposition in an overnight treatment [54]. When epoxy resin particles are targeted within environmental MP studies and peroxide digestion protocols are applied, we recommend for future studies to note that numbers are likely underrepresenting the actual environmental load for these polymers.…”