Modeling the dissolution of thermosetting polymers and composites via solvent assisted exchange reactions

Abstract

Thermosetting polymers and composites featuring chemical cross-linking are usually regarded as unrecyclable. Recently, novel thermosets containing dynamic chemical bonds or labile bonds have been developed to achieve chemical recycling under mild conditions via solvent assisted exchange or cleavage reactions. This concept has been extended to recycle conventional thermosets and composites using a proper catalyst/solvent system by selective bond cleavage. In this paper, we develop a modeling framework to probe into the complicated reaction-diffusion coupling and reveal the effect of reinforcement, such as carbon fiber, on the dissolution process of carbon fiber reinforced epoxy (CFRE). A model involving chemical species concentration dependent diffusion and reaction rates is developed to inquire into the evolution of different functional group concentrations and network structure via solvent assisted exchange reactions. We further apply the reaction-diffusion based model to a multiphase structure to simulate the CFRE dissolution process. The model is experimentally validated by the mass evolution of a virgin cured epoxy-anhydride resin matrix in an alcohol/catalyst solution proceeding with transesterification. The established model can predict the influence of dissolution processing parameters including catalyst concentration and solvent diffusivity. Parametric studies are used to further assess the structural parameters of fiber loading on the dissolution rate. This work provides a deep understanding on the dissolution of thermosets and their fiber-reinforced composites and can guide the future engineering application of sustainable recycling approaches of thermosets and composites.