Abstract
The present study aims to design high performance functionalized lignin-epoxy composites. In this work, the triethylammonium hydrogen sulphate ionic liquid (IL) was introduced onto the surface of lignin micro/nanoparticles while it was extracted from biomass, to prepare a highly functional and reinforcing IL-Lignin filler in an epoxy matrix. A bio-composite of IL-Lignin epoxy was prepared from IL-Lignin and a fast curing epoxy pre-polymer hardened with an anhydride-based curing agent using a simple one-step method. The thermal degradation mechanisms and the influence of the IL-Lignin on the thermal stability and cure kinetics of IL-Lignin epoxy networks have been investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Fourier Transformation Infrared spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) analysis confirm that the ammonium-based IL coupled with lignin is an effective promoter for crosslinking of the epoxy pre-polymer resulting in higher and faster degrees of conversion. Nuclear magnetic resonance spectroscopy reveals the characteristics of IL extracted lignin. Using atomic force microscopy (AFM) and dynamic light scattering (DLS), topographical features of the IL-Lignin surfaces and particle size of lignin at nanoscale were investigated and supporting evidence inferred underpinning the improved mechanical properties. The addition of 2 wt% IL-Lignin yielded an 80% increase in flexural strength (99.10 MPa), a 57% increase in flexural modulus (2.84 GPa), a 52% increase in tensile strength (40.90 MPa) and a 23% increase in toughness (86.08 kJ/m3), compared to the neat epoxy matrix. There was, however, a softening effect on the glass transition temperature due to the addition of IL-Lignin. This particular system of composite matrix is prone to controlled breakdown at the end of its lifecycle even after post cure due to biodegradable lignin linkages.
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