The effect of ball milling expressed as the yield of milled wood lignin (MWL) on the structure and molar mass of crude milled wood lignin (MWLc) preparation is studied to better understand the process’ fundamentals and find optimal conditions for MWL isolation (i.e., to obtain the most representative sample with minimal degradation). Softwood (loblolly pine) MWLc preparations with yields of 20–75% have been isolated and characterized based on their molar mass distribution (by Size Exclusion Chromatography (SEC)), hydroxyl groups of different types (³¹P NMR), methoxyl groups (HS-ID GC-MS), and sugar composition (based on methanolysis). Classical MWL purification is not used to access the whole extracted lignin. The results indicate that lignin degradation during ball milling occurs predominantly in the high molar mass fraction and is less pronounced in the low molar mass fraction. This results in a significant decrease in the M_z and M_w of the extracted MWLc with an increase in the yield of MWLc, but has only a very subtle effect on the lignin structure if the yield of MWLc is kept below about 55%. Therefore, no tedious optimization of process variables is necessary to achieve the required MWLc yield in this range for structural studies of softwood MWL. The sugar composition shows higher amounts of pectin components in MWLs of low yields and higher amounts of glucan and mannan in high-yield MWLs, confirming that lignin extraction starts from the middle lamella in the earlier stages of MWL isolation, followed by lignin extraction from the secondary wall region.

Lignin, a complex and heterogeneous biopolymer derived from wood, holds immense potential as a sustainable feedstock for various industrial applications. Kraft pulping, a widely employed process in the paper industry, generates black liquor containing lignin along with other organic and inorganic compounds. The precipitation of lignin from black liquor offers an opportunity for valorization, contributing to the circular economy and reducing the environmental impact. Although the precipitation process of softwood lignin is extensively documented and outlined in the existing literature, the identical process originating from hardwood-derived black liquor poses challenges attributed to the distinct composition of hardwood. This study systematically investigates the individual and combined influences of temperature and pH on the precipitation of hardwood lignin from kraft black liquor, utilizing a factorial design to evaluate lignin functional characteristics. The characterization of the precipitated lignin was performed using various analytical techniques, mainly NIR, elemental composition, UV-VIS, and calorimetry. The results reveal remarkable interactions between temperature and pH, indicating their synergistic effects on lignin precipitation. The optimal conditions for hardwood lignin precipitation were identified and successfully upscaled during piloting experiments conducted under industrial conditions. This research provides valuable insights into the fundamental factors governing hardwood lignin precipitation from kraft black liquor and offers a basis for the development of efficient and sustainable lignin recovery processes in the pulp and paper industry.

Ammoxidation of pine kraft lignin in aqueous 5 wt % ammonia affords a novel type of phenol substitute that significantly accelerates resole synthesis and curing as demonstrated for 40 wt % phenol replacement. Compared to non-ammoxidized lignin, which already shortens significantly the cooking time required to reach a resole viscosity of 1000 Pa·s (250 vs. 150 s) and reduces the typical curing B-time by about 25% at 100 °C, the use of ammoxidized lignin has an even more pronounced impact in this respect. Activation of lignin by Fenton-type oxidation prior to ammoxidation further boosts both synthesis and curing of the resole. This is presumably due to the intermediary formation of polyvalent cross-linkers like N,N,N-tris (methylol) trimethylene triamine triggered by saponification of a larger fraction of nitrogenous moieties present in such a treated lignin (ammonium salts, amide-type nitrogen, urea) and reaction of the released ammonia with formaldehyde. Except for the fact that phenol replacement by ammoxidized lignin results in a somewhat less brittle cured adhesive polymer and higher elastic modulus, the aforementioned acceleration in curing could no longer be observed in the presence of wood, where a significantly delayed wood-adhesive bond formation was observed for the lignin-containing adhesives as evident from the automated bonding evaluation system.