Accelerated hydrolytic degradation of ester-containing biobased epoxy resins

Abstract

The accelerated hydrolytic degradation of biobased epoxy resins containing ester linkages was investigated. Epoxidized biobased molecules were utilized as sustainable replacements for the diglycidyl ether of bisphenol A (DGEBA) as an epoxy monomer, including epoxidized vanillic acid (EVA, derived from lignin), epoxidized plant-based phenolic acids (epoxidized salicylic acid, ESA, and 4-hydroxybenzoic acid, E4HBA), and epoxidized soybean oil (ESO). All biobased epoxy monomers contain esters (3 per molecule for ESO and 1 per molecule for EVA, ESA and E4HBA), in contrast to DGEBA (containing no esters). The epoxidized molecules were cured through reaction with an anhydride curing agent. Epoxy resins derived from EVA, ESA, and E4HBA exhibited comparable glass transition temperatures to that of the DGEBA-based epoxy resin. 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. Variations in degradation rate are attributed to differences in epoxide content, monomer structure, degree of hydrophilicity, crosslink density, and proximity to glass transition temperature. The degradation profiles, 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.