Lignins inside and outside the cell wall: Inherent and modified thermoplasticity

Doctoral thesis, 2025

Lignins are a group of irregular polyphenolic macromolecules found in vascular plants which, together with cellulose and hemicellulose, form highly robust materials. In the pulping process, individual wood cells are separated either by thermomechanical shearing or by chemically removing the lignified material that binds them together. Thus, from an industrial perspective there are three categories of lignin: native lignin in biomass, residual lignin in pulp and technical lignin extracted from biomass during pulping. This thesis concerns itself with all three categories of lignin, with the aim of understanding how the varying chemical structures of these lignins affect their properties – both inherent, and when modified for improved processability. It is especially the thermomechanical properties of these lignins that are in focus, as one of their potential uses is as thermoplastic components in biomass, in blends with synthetic polymers, or on their own. However, the glass transition temperature (Tg) of lignins is high, and their flow properties are poor due to the aromatic backbone, hindering thermal processing. But if the thermoplastic potential was realized, reliance on fossil-based thermoplastics could be overcome.

To investigate the thermomechanical properties, a powder sample holder for dynamic mechanical analysis (DMA) was employed, which allowed the determination of the Tg of isolated lignin as well as the Tg of in situ lignin in pulp and milled wood. By constructing Flory-Fox plots, the Tg could be compared beyond the effects of molar mass for the isolated lignins. Native structures appear beneficial for lower processing temperatures: residual lignin in softwood kraft pulp and softwood kraft lignin were projected to have a higher Tg at a given molar mass than native Norway spruce lignin. Upon modification with either external plasticizers or quantitative esterification (C2-4), the response was uniform: external plasticizers were more efficient in reducing the Tg on a weight-addition basis for all lignins; however, the solubility of the plasticizers in the different lignins varied, with generally better compatibility with small flexible aprotic compounds. Additionally, plasticization was found to homogenize the physical properties of compositionally heterogenous lignins as well as increase coalescence of the otherwise brittle lignin materials.

Lastly, the external plasticization of softwood kraft pulp was investigated by probing which components the plasticizers were interacting with and their effect on the thermoformability of the cell wall. By employing optical photothermal infrared spectroscopy (O-PTIR) and solid-state NMR, plasticizers were found to be distributed throughout the fibers and interacting with all the major components – cellulose, hemicellulose and lignin. With DMA, the Tg of lignin was found to have dropped from around 230 °C to 120 °C. The plasticized pulp was hot pressed above and below this temperature, and the cell wall organization was investigated using X-ray scattering. Cellulose elements were found to aggregate; however, this was achieved without a reduction in crystallinity only in the presence of plasticizers and at higher temperatures. This indicates that the displacement mechanism for cellulose units during hot-pressing was more plastic when operating above the Tg of lignin.