Hot-drawing ionic liquid-spun lignin–poly(vinyl alcohol) fibres increases strength and polymer alignment

Abstract

Lignin is an attractive raw material for low-cost sustainable carbon fibres, however, the resulting mechanical properties require improvement before they can be implemented in composite applications. The mechanical properties of conventional polyacrylonitrile-derived carbon fibres depend critically on the molecular alignment induced in the polymer fibres by fibre drawing and on retention of the alignment during subsequent thermal treatments. In this study, alignment was induced in high lignin content fibres wet-spun from a low-cost ionic liquid water mixture by employing similar hot-drawing methods. 75/25 wt/wt% lignin–poly(vinyl alcohol) (lignin–PVA) fibres were continuously wet-spun from a 60/40 wt/wt% N,N-dimethylbutylammonium hydrogen sulfate, [DMBA][HSO₄] water mixture, using deionised water used as the coagulant. Hot-drawn fibres with high draw ratios of up to 20 were generated at 180 °C. By careful selection of the initial extrusion diameter and the subsequent draw ratio, the influence of fibre diameter and draw ratio was systematically distinguished. The draw ratio was found to dominate the mechanical properties of the ductile precursor fibres, while the fibre diameter was more significant after stabilisation. The precursor fibres that experienced the highest draw ratios had tensile strengths of 235–249 MPa (up to four times higher than the undrawn lignin–PVA fibres) and tensile modulus of 7.5–8.2 GPa, while the fibre diameter was reduced from 64–106 μm to 15–23 μm. Wide-Angle X-ray Scattering (WAXS) studies showed that hot-drawing induced orientation and crystallisation of PVA at high draw ratios. The crystallisation and orientation of PVA was lost during the slow oxidative stabilisation at 250 °C, associated with a plateau at around 110 MPa tensile strength and 4 GPa tensile modulus for the stabilised lignin–PVA fibres, regardless of draw ratio. Improvements to the stabilisation aimed at retaining alignment are proposed.