PNNL

Abstract

Chemical depolymerized fully cured epoxy resin with 20% reactive modifier via solvent-assisted solvolysis process into low molecular weight recyclable oligomers (RO) was successfully performed at 240 oC in a pressure vessel at 650 psi for 4 h. The thermoset epoxy resin was depolymerized into transparent brown viscous fluid with a higher viscosity than the uncured epoxy resin with approximately 93% yield. Different concentrations of the RO were homogenously mixed with the pure epoxy resin and their curing kinetics, viscosity, FTIR, mechanical properties, DMA, and crosslink density were investigated. The curing kinetics of the pure reactive modified epoxy resin (bassline) and its mixtures with RO of different concentrations were investigated under both isothermal and non-isothermal conditions using small amplitude oscillatory shear flow. The elastic and viscous moduli (G' and G?), complex viscosity (?*), and tan ? were evaluated at different curing times and temperatures. The G', G?, and ?* increased dramatically, while tan ??decreased strongly by several orders of magnitude at the gel point. The zero-share viscosity (?o) was determined from the angular frequency-dependent of ?* based on the Cross model for different blend compositions in the liquid state before curing. The ?o of the epoxy resin/RO blends increased gradually with increasing the content of RO in the blend. The composition dependence of ?o showed a positive deviation from the linear mixing rule and was well described by Lecyar model. The apparent activation energy of curing (Ea) was evaluated from the slope of the linear relationship between gel time and the inverse of absolute temperature according to the Arrhenius equation. The Ea for all epoxy/RO blends was found to be 46 ±2 kJ/mol regardless of the RO different concentrations. Although the RO showed dramatic influence on the mechanical, thermomechanical, and Tg, it has no significant impact on the curing kinetics and Ea. For all blends up to 40 wt.% RO, only one tan ? peak systematically shifted to lower temperatures with increasing the content of the RO was observed in the DMA measurements indicating that the epoxy resin and the RO are miscible with up to 40 wt.% RO. Significant increases in the modulus by 40 and 20% were observed for epoxy/RO blends with 15 and 25 wt.% RO, respectively. A dramatic increase in the elongation by 700% was observed for the blend with 40 wt.% RO. The reported data in this manuscript proves the successful chemical depolymerization of thermoset epoxy resin and the utilization of the RO in making new materials with comparable mechanical, thermomechanical, and rheological properties to the pristine thermoset.