Hydroxypropyl-modified and organosolv lignin/bio-based polyamide blend filaments as carbon fibre precursors’

Hydroxypropyl-modified lignin (TcC) and organosolv lignin (TcA) were melt blended with two types of bio-based polyamides (PA) (PA1010 and PA1012) before being melt spun into filaments. With a lignin/bio-PA ratio 50/50 wt%, the filaments could be continuously produced and spooled having tensile strengths ≥ 20 MPa and moduli ≥  500 MPa. The influence of each polyamide blended with each lignin on structural (FTIR), thermal (DSC, TGA, DMA), mechanical, rheological and morphological properties of the resultant extrudates and/or filaments was studied. The melting point of each polyamide was reduced in the presence of TcA and TcC lignin, and shifts in the glass transition temperature (Tg) and FTIR characteristic peaks occurred, which suggests that the selected polyamides and lignins are compatible with each other. The improved lignin/polyamide compatibilities were further supported by Pukanszky interfacial adhesion modelling. Despite the evidence for strong interactions, heterogeneous morphologies were observed in the resulting blends and scanning electron microscopy was used to determine dispersed lignin domain sizes which decreased when stronger lignin/polyamide interactions resulted. As a possible indicator of carbonization efficiencies, blends were subjected to a simulated stabilization/cross-linking process and subsequent TGA char residues were higher than those theoretically calculated at 880 °C, indicating that the bio-polyamide acted as a char-promoting agent for the lignin besides being a good blending partner. Overall, this study has indicated that the developed lignin/bio-polyamide filaments have potential as melt-extrudable precursors for carbon fibre production.