Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Prof. Charles (Chunbao) Xu, Dr. Mark Anderson

Abstract

Epoxy resin is one of the most versatile thermosetting polymers with diverse applications. Epoxy resins are mainly produced from the reaction of bisphenol-A (BPA) and epichlorohydrin. The consumption of bisphenol-A is facing growing concerns over its carcinogenic effects and its sustainability. Lignin can be a promising renewable substitute of bisphenol-A in the synthesis of epoxy resins.

In this thesis work, a novel method has been developed for the synthesis of bio-based epoxy resins with reduced side reactions, employing de-polymerized lignin from organosolv lignin (DOL), kraft lignin (DKL) and hydrolysis lignin (DHL) under alkaline condition in the presence of a phase transfer catalyst, tetrabutylammonium bromide (TBAB). The preliminary results showed that the reaction temperature, reaction time, NaOH/lignin molar ratio and epichlorohydrin/lignin molar ratio influenced the reaction yield and the molecular weights of the final products. The synthesis process has been optimized by using response surface methodology to produce bio-based epoxy resins with a higher epoxy content and greater yield. Under the optimum conditions using DOL, DOL-based epoxy resins with an epoxy content as high as ~8 was produced at 99% yield. The synthesis using DKL at the optimum conditions led to a DKL-based epoxy resin with an epoxy content of 5.6 at 97% yield. In addition, bio-based epoxy resins derived from hydrolysis lignin (HL) have also been synthesized with lower amount of catalyst using DHL. The DHL-epoxy resin obtained at the optimum conditions has an epoxy content of ~5 and 95% yield.

The curing kinetics of admixtures of the synthesized lignin-based epoxy resins and a conventional BPA-based epoxy resin at various blending ratios were investigated using differential scanning calorimetry (DSC) with two kinds of curing agents, i.e. diaminodiphenyl methane (DDM), diethylenetriamine (DETA). The kinetic parameters were evaluated by the model-free kinetics and the activation energy for the curing reaction was calculated as a function of conversion. The activation energy changed with the progress of curing process and it revealed that the curing of a lignin-based epoxy resin is a complex process with several reactions occurring at the same time. The fluctuation of activation energy depended on the content of the lignin-based epoxy resin in the blended mixture, type of lignin-based epoxy resin and the type of curing agent used.

The synthesized lignin-based epoxy resins were blended with a conventional BPA-based epoxy resin at a blending ratio of 0-100wt.% and used as polymer matrixes for manufacturing fiber reinforced plastics (FRPs). The thermal stability of the obtained lignin-based epoxy resins was also characterized using TGA-FTIR analysis. The mechanical/thermal characterization results indicated that the lignin-based epoxy resins have excellent mechanical/thermal properties and can be used as a substitute for conventional BPA-based epoxy resins at a ratio up to 75 wt% in FRPs without compromising their properties.


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