Sustainable and Degradable Epoxy Resins Containing Multifunctional Biobased Components

Epoxy resins were synthesized from a variety of biorenewable feedstocks, including epoxidized vanillic acid (EVA) and epoxidized 4-hydroxy benzoic acid (E4HBA), derived from lignin, epoxidized salicylic acid (ESA, a plant-based phenolic acid), and epoxidized soybean oil (ESO). The epoxy monomers were cured with an anhydride curing agent, and the curing behavior was studied using Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry to ensure high conversion of functional groups to form the epoxy network. Evolution of viscoelastic behavior of the EVA-based epoxy resin was monitored through in situ rheology and compared to a conventional epoxy resin derived from the diglycidyl ether of bisphenol A (DGEBA). The EVA-based epoxy resin exhibited faster curing kinetics as compared to the DGEBA-based epoxy resin, and the storage (G’) and loss (G”) moduli of the EVA-based epoxy resin were higher than that of the DGEBA-based epoxy resin after gelation was achieved. The complex viscosity of the EVA-based epoxy resin was consistently higher than that of the DGEBA-based epoxy resin throughout the curing process. Both resins exhibited similar volume change during curing. The resulting EVA-based epoxy showed promising thermal and mechanical properties and can serve as a suitable replacement for the conventional DGEBA-based epoxy resin in applications.

The accelerated hydrolytic degradation behaviors of the epoxy resins were monitored in basic solution at 80 °C. All biobased epoxy resins underwent rapid degradation in a basic solution as compared to the conventional DGEBA-based epoxy resin. ESO- and ESA-based epoxy resins exhibited the fastest degradation rates, whereas E4HBA- and EVA-based epoxy resins exhibited more moderate degradation rates. The degradation profiles, observed as the mass loss as a function of exposure time in the basic solution, showed good agreement with predictions from a solid-state kinetic model. Mass spectrometry and scanning electron microscopy analyses confirmed the epoxy resins underwent hydrolytic degradation, through a surface erosion mechanism in basic solutions. The impacts of various factors on the degradation rate were explored. including differences in the epoxy monomer structures; crosslink and ester densities, degree of hydrophilicity, and glass transition temperature of the resin; as well as solubility of degradation products.

The accelerated hydrolytic degradation behaviors of EVA- and ESO-based epoxy resins were also investigated in acidic solutions. The epoxy resins exhibited sigmoidal degradation kinetics in acidic solutions, consistent with bulk erosion mechanisms observed in linear polyesters. A solid-state reaction order model with autocatalysis was utilized to predict the mass fraction remaining as a function of exposure time in acidic solution and the data and model were in good agreement. Mass spectrometry and FTIR analyses confirmed the degradation mechanism as cleavage of ester groups in the crosslinked structures. The influences of solvent composition and temperature on degradation kinetics were also explored.

These combined results demonstrate biobased, ester-containing epoxy resins undergo rapid hydrolysis in both basic and acidic solutions, providing a route for end-of-life management of thermoset waste.