The Effect of Sulphuric Acid on the Thermostabilisation of Lignin/Biopolyamide 1010 Precursor Fibres for Carbon Fibre Production

Sustainable precursor fibres for low-cost carbon fibre production are being extensively explored, particularly those based on lignin that are melt-blended with other thermoplastic polymers. Being prone to melting during melt processing is one of the main issues of using lignin to replace petroleum-based polyacrylonitrile (PAN) as precursor fibres. A time-consuming thermostabilisation process is normally needed to convert the thermoplastic blend’s polymer molecular chains to thermally stable structures prior to carbonisation. In order to accelerate the thermostabilisation process to facilitate industrial carbon fibre manufacturing, promising suitable surface treatments of lignin-based precursor fibres have emerged. In this work, sulphuric acid was used as a surface treatment agent to sensitise the crosslinking chemistry between interspersed components during the thermal stabilisation of lignin/polyamide blend precursor fibres. The impact of sulphuric acid concentration and the accelerated thermostabilisation conditions on the chemical structure and mechanical properties of precursor fibres were investigated. The surface-treated precursor fibres were directly stabilised at a high temperature, i.e., 200 °C, without a slow temperature elevation stage and the fusion of fibres using sulphuric acid with a concentration as low as 0.5 M. This was attributed to the improved condensation and crosslinking process via the sulphonation of lignin and to some extent PA1010. The isothermal time (<5 h) and final temperature (<240 °C) of the thermostabilisation process had significant effects on the final properties of surface-treated precursor fibres. The mechanical properties of thermostabilised precursor fibres were also investigated. The tensile modulus of the thermally stabilised fibres derived from surface-treated precursor fibres was increased by up to 50% using an optimised stabilisation process; however, tensile strength was decreased due to crosslinking.