Mechanistic study of char formation in lignin and a lignin/biopolyamide blend for value-added applications

This study investigates the mechanism of charring of a hydroxypropyl-modified lignin (TcC) and its 50:50 wt% blend with a bio-based polyamide (PA1010). Potential applications are in carbon fibre and activated carbon production. Thermogravimetric analysis (TGA) coupled with Fourier transform infrared (FTIR) spectroscopy revealed that the blend’s thermal stability up to 500°C was lower than expected based on the TGA profiles of the individual components. However, above 500°C, the blend exhibited improved thermal stability. Isothermal pyrolysis was conducted at temperatures between 300°C to 800°C in 50°C intervals. Chars were characterized using FTIR, scanning electron microscopy (SEM), and porosity measurements. There is no evidence of covalent bond formation between the two degrading polymers in the blend. However the melting of PA1010, which surrounds the lignin particles, at 180°C and the relatively high thermal stability of the molten PA1010 up to 400°C, leads to delayed but extended initial thermal dehydration and decarboxylation of the lignin. This results in enhanced aromatization and increased thermal stability of the lignin above 500°C, contributing to enhanced char formation (20.8 % compared to a theoretical value of 17.5 %, calculated form the averaged sum of the chars from its components). These findings indicate the suitability of the blend for carbon fibre formation. However, the reduced porosity of the blend’s char (0.5 %), compared to that of lignin alone (4.7 %), indicates that the blend is not suitable for producing an activated carbon. This latter aspect will be discussed in a forthcoming publication. • Lignin/PA1010 blend less stable than lignin, but produces more char than expected. • No chemical, only physical interactions between lignin and PA1010 during pyrolysis. • Molten PA1010 encapsulates lignin, promoting aromatisation and char formation. • Blend a suitable precursor for carbon fibre, but for high-porosity activated carbon.