Lignin epoxy resins: synthesis and evaluation as coatings and composites

Abstract

Epoxy resins are one of the most relevant and widely used thermosets in the market covering a wide range of applications. Industries are being forced to quickly transition towards sustainable alternatives due to the pressure from emerging environmental concerns, the depletion of petrochemical supplies, and compliance with environmental legislation. Since the primary component of epoxy resins is derived from petroleum (Bisphenol A (BPA)), companies like Westlake Epoxy have joined the search for innovative, environmentally friendly solutions. Many bio-based substitutes have surfaced in recent years, in efforts to eliminate or reduce the quantity of BPA in epoxy resins. Attention has been focused on lignin biomass as a viable feedstock in the manufacturing of these thermosets because of its large production volume and some structural characteristics.

This study addresses the synthesis of novel lignin-based epoxy resins and the evaluation of their potential application in the field of coatings and composites. For this research, different sources of technical Kraft lignin were employed. Since technical lignin is not very reactive due to the large molecular weight polymers, a Confidential Fractionation process has been developed. For its synthesis, both small- and large-scale glycidation processes were successfully implemented. To understand the chemical structure of lignin and its corresponding resins, different analytical techniques were used such as Gas Chromatography (GC), titrations, Nuclear Magnetic Resonance (NMR), Fourier-Transform Infrared Spectroscopy (FT-IR) and Gel Permeation Chromatography (GPC). Additionally, for the development and characterisation of coatings and composites, a variety of material testing techniques were employed, namely Differential Scanning Calorimetry (DSC), Dynamical Mechanical Analysis (DMA), Thermogravimetric Analysis (TGA), Interlaminar Shear strength (ILSS), Impact testing, Pendulum hardness, among many others.

Experimental results showed an average fractionation yield of 50-60% of technical Kraft lignin using the Confidential Fractionation process. Compositional differences in the fractionated lignin substrates studied via NMR analysis, predict some differences in the reactivity of these substrates towards glycidation, which is supported by different Epoxy Group Content (EGC) of the corresponding epoxy resins. The EGC of the lignin-based epoxy resin is significantly lower than that of the reference resin due to the presence of bulky moieties that hinder the reactivity of active sites. While lignin-based coatings exhibit comparable performance in hardness and direct impact resistance, however, its high viscosity and stiffness results detrimental in other areas. On the other hand, this high viscosity is a major challenge in the processes of prepreg laminates leading to poor adhesion of the fibres to the matrix, which has a negative effect when it comes to mechanical performance. However, adding lignin to epoxy resins has proved to improve the thermal stability of these materials.