Kraft lignin (KL) is a type of technical lignin, <i>i.e., </i>it is obtained as a by-product from pulp manufacturing via the kraft process. KL is a promising resource to substitute or partially replace fossil-based chemical and materials due to its abundance, renewability, aromaticity, and functionalities.  Hence, the global revenue of lignin-based materials is projected to considerably increase in the next decade. However, given lignin’s complexity and heterogeneity more research and development are needed to make its utilization economically feasible and reach the quality demands of the commercial lignin market. The present investigation evaluated lignins precipitated from black liquors obtained from sweetgum and pine at different levels of the kraft process and at different pHs (9.5 and 2.5). Recovery yield, composition, and molecular weight distribution were determined. The information gathered in the current work is crucial for lignin post-processing and high-value applications, thus it is expected to greatly amplify its utilization. Additionally, most of the work on lignin has been performed on softwoods, while much less has been published regarding hardwoods. It is well known that lignin properties depend on the feedstock from which they are obtained, the extraction method employed, and downstream purification processes, which motivated the present investigation. Overall, the recovery of pine lignins is superior to the recovery of sweetgum lignins with respect to yield and purity. Also, the molecular weight (MW) of the former is higher than the latter. Finally, the MW of lignins from both feedstocks followed gel degradation theory, <i>i.e.,</i> at the onset of the kraft pulping process low molecular weight-lignins were obtained, and as pulping progressed, the molecular weight peaked and subsequently decreased.

Insights into the composition and properties of pine and sweetgum kraft lignin fractions: Potential feedstocks for renewable materials

ACS Spring 2022

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