(688g) Fractionation of Kraft Lignin for Production of Alkyd Resin for Bio-Based Coatings

Lignin, a naturally occurring renewable, bio-based, polyphenolic material with high content of phenolic hydroxyl (OH) groups, is a promising feedstock for producing a variety of value-added, sustainable materials. Kraft lignin, a major by-stream of kraft pulping, serves as the predominant source of commercially available lignin. However, kraft lignin at present is mostly burned to generate bioenergy for the pulp mill operations. Alternatively, by exploiting lignin’s natural chemical characteristics, kraft lignin can be upgraded to bioproducts of higher value such as alkyd resin for coatings. Petroleum-derived polyacids (mainly, phthalic anhydride) and polyols are widely used as the building block chemicals for production of alkyd resin. Therefore, kraft lignin, if valorized to alkyd resin, could act as a bio-based, renewable substitute of fossil-based chemicals in the value chain of alkyd resins and thus, aid the decarbonization of paints and coatings industry. However, the literature is quite scarce when it comes to application of kraft lignin as a resource for green manufacturing of alkyd resins at laboratory or large scale. The diverse structure of kraft lignin which is featured by high molecular weight, polydispersity, and various functional groups imposes a great challenge to its industrial valorization to coatings. To overcome this heterogeneity of lignin, solvent fractionation methods can be applied to separate kraft lignin into different components. Some of these fractions could have significantly lower molecular weights and more uniform physicochemical properties than the parent lignin without undergoing any chemical modification of the structure. Solvent fractionation helps retain lignin’s polymeric structure and increase the economic and sustainability potential of the process by avoiding severe conditions, expensive and toxic chemicals, complicated and cost-intensive separation steps as often used in lignin depolymerization or chemical functionalization technologies.

In this work, softwood kraft lignin was mixed in aqueous ethanol solutions, prepared at different ratios, at 25°C (or room temperature) for 2 hours to generate a soluble and an insoluble fraction of lignin. The soluble fraction was further dried by rotary evaporation and freeze-drying while the insoluble fraction was filtered, washed, and freeze-dried before analysis. The best combination of yields and molecular weight of soluble lignin fraction (16-36% yield, 1750-1900 Da) could be attained using 50-80 vol% ethanol as the fractionation solvent. Therefore, these conditions were further employed at pilot scale to demonstrate fractionation of industrial kraft lignin. The soluble fraction, characterized as a low molecular weight (MW <2200 Da), homogeneous material with relatively high/increased concentration of phenolic OH and carboxylic acid groups and low ash content, was subsequently used as a polyacid/polyol substrate in preparation of alkyd resin formulation. The above properties of the soluble lignin fraction helped in achieving the target properties of an alkyd resin formulation for surface coating of metal as compared to an industrial standard. Various analytical tests were performed to evaluate the feasibility of using the soluble lignin material in alkyd resin formulation including solubility tests at different pH levels and release of sulfur compounds and volatile organic carbons (VOCs) during the coating preparation process by thermal desorption and gas chromatography method. To fully valorize all kraft lignin fractions, the insoluble fraction of this process (MW > 6000 Da), could be utilized by alkali O₂-oxidation to make a lignin-based dispersant for special carbon black. Overall, this study presents a new, simple strategy for developing a cost-effective, efficient, green route for upgrading of kraft lignin into value-added intermediates to produce bio-based alkyd resins with demonstrated potential application at large scale.