Effect of processing temperature on nanolignin quality during ultrafine friction grinding.

Nanoparticles are a new frontier to valorize underutilized and abundant lignin generated by biorefineries. Typical chemical processes to synthesize lignin micro- and nano-particles (LMNPs) use large quantity of organic solvents and therefore, could adversely impact the environment. Hence, we have employed an eco-friendly mechanical approach, i.e. , ultrafine friction grinding (UFG), to produce "green" LMNPs with controlled size and micromorphology. Specifically, we investigated the effect of processing temperature, at 0, 25 and 70 °C, on lignin nanosizing. Our results showed that low temperature (0 °C) significantly favored the size reduction of lignin where we achieved a median particle size of ∼100 nm after 12 grinding passes. At 70 °C, LMNPs reached a size equilibrium of ∼160 nm within four passes but did not decrease in size upon subsequent grinding. Moreover, analysis of the LMNPs by SEM showed that samples produced at 0 °C mostly had irregular flat shapes with sharp angles, such as stars and polyhedrons, with a glabrous surface, whereas, samples produced at 25 and 70 °C featured spheres and rods with nanoscopic pores on the surface. Physico-chemical properties of LMNPs, such as molecular weight and thermal degradation, did not change significantly compared to the original lignin, however, samples generated at 0 °C exhibited significantly fewer inter-unit linkages than those generated at 70 °C. Our study thus shows that varying the grinding temperature facilitates the customization of LMNP micromorphology. Furthermore, the proposed method for LMNP manufacturing is simple and eco-friendly, and presents a viable approach for lignin valorization. [Display omitted] • Modulating grinding temperature facilitates efficient lignin nanosizing. • At 70 °C very few grinding passes were required to achieve size equilibrium. • The smallest particle size of 100 nm was achieved by processing lignin at 0 °C. • Lignin formed flat polyhedral or spherical nanoparticles at different temperatures. • Grinding causes minimal physico-chemical changes in lignin especially at 70 °C. [ABSTRACT FROM AUTHOR]