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 degrees C, on lignin nanosizing. Our results showed that low temperature (0 degrees C) significantly favored the size reduction of lignin where we achieved a median particle size of-100 nm after 12 grinding passes. At 70 degrees 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 degrees 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 degrees 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, how-ever, samples generated at 0 degrees C exhibited significantly fewer inter-unit linkages than those generated at 70 degrees C. Our study thus shows that varying the grinding temperature facilitates the customization of LMNP micromor-phology. Furthermore, the proposed method for LMNP manufacturing is simple and eco-friendly, and presents a viable approach for lignin valorization.

Automated summary

Mechanical approach, ultrafine friction grinding (UFG), is used to produce green lignin micro-and nano-particles (LMNPs) with controlled size and micromorphology, without organic solvents. Results show that low temperature (0℃) favors size reduction of LMNPs, while high temperature (70℃) does not further decrease particle size. Varying the grinding temperature can customize the micromorphology of LMNPs. This eco-friendly method presents a viable approach for lignin valorization.