|In this work, the main objective was to produce high value products from kraft lignin with an integrated approach in the pulp and paper industry. Production of vanillin, vanillic acid and bio-based vitrimers represent the value-added products that are mainly focused in this study. Vanillin is commonly used as a flavoring agent in food, pharmaceutical and chemical industries. Recently, vanillic acid is gaining notable interest due to its anti-microbial, anti-bacterial, and chemo-preventive properties. Vanillin and vanillic acid have both been obtained as the main products from oxidation of kraft lignin. Vitrimers represent a new class of plastics that are derived from thermosetting polymers which possess self-healing and easy processibility in a wide temperature range. The recent development of bio-based vitrimers from oxidation kraft lignin has received considerable interest. The oxidative conversion of lignin in an alkaline medium using oxygen results in the formation of vanillin and vanillic acid. The yield of these products depends on the reaction chemistry, time, temperature, oxygen partial pressure, lignin concentration and feedstock. In this work, a novel approach using the retro-aldol reaction chemistry was utilized to enhance the yields of vanillin and vanillic acid. The amount of oxygen charged during the retro-aldol reaction plays a key role in determining the product yield. The conventional oxidation of softwood kraft lignin using oxygen at 140°C for 40 min results in a total of 5.17% by wt. of vanillin and vanillic acid. In contrast, using the new approach in which oxygen is charged for only 20 min during the 40 min reaction improved this yield significantly to 6.95% by wt. The precipitated lignin after the reaction has a maximum carboxyl content of 1.41 mmol/g at 130°C. The improvement in the amount carboxylic acid groups allows for further utilization of the precipitated kraft lignin. One of the main issues encountered in the valorization of kraft lignin is its incompatibility with other components due its branched aromatic structure, high molecular weight, and brittle nature. Therefore, it is crucial to understand the structural changes in lignin during oxidation especially in the operating conditions of current kraft pulping processes. The softwood kraft lignin before and after oxygen and ozone oxidations treated in the LignoForce operating conditions were thoroughly characterized using a variety of conductometric titration and 31P NMR techniques. The progressive reduction in aliphatic and condensed OH units accompanied by the marked improvement in carboxylic OH units are the major structural changes occurring in the softwood kraft lignin. Among the oxidizing agents, the amount of carboxylic OH units formed from ozone oxidation was higher compared to oxygen oxidation under similar conditions. Furthermore, the softwood kraft lignin that is obtained after sequential oxidation treatment from oxygen at 130°C and from ozone at 80°C (OxL-COOH) contained 4.06 mmol/g of carboxylic OH units. The higher amount of carboxylation obtained after sequential oxidation treatment allows for improved compatibility of lignin with other components.
To further expand the scope of carboxylated softwood kraft lignin, bio-based vitrimers were synthesized using polyethylene glycol diglycidyl ether (PEG-epoxy), zinc acetylacetonate Zn(acac)2, and sequentially oxidized kraft lignin (OxL-COOH). All OxL-COOH/PEG-epoxy showed high mechanical strength, thermal stability and self-healing. For the first time, the problems in film formation which are bubble formation and porosity are briefly discussed and strategies to troubleshoot these problems are discussed. In aqueous NaOH solutions, the bio-based vitrimer exhibited swelling which is one of the prime indicators of the dynamic transesterification in the cured network. In 0.1 M NaOH solution, the OxL-COOH/PEG-epoxy system showed a swelling ratio of 26.6% in the initial 10 min which further increased to 52.6% in 20 min at a stoichiometric ratio ‘R’ of 1:1 on mole basis. The vitrimers were formed at three different stoichiometric ratios 1.0, 1.3 and 1.5 with maximum lignin content of 49.5% by wt. In summary, a high lignin content utilization was demonstrated for formation of high value products with lower consumption of oxidizing agents and solvents. Overall, this research established a novel and integrated approach to valorize kraft lignin into diverse products.