Controlling Lignin Solubility in Nonpolar Lubricating Oil via Multiple Fatty Chains Modification Strategy

In this study, we developed a multiple fatty chains modification strategy to utilize lignin effectively. First, different biomass materials (hardwood, softwood and herbaceous) were used to extract and modify lignin with uniform molecular weight and aliphatic hydroxyl alkylation under mild conditions using dodecyl aldehyde as an inhibitory polycondensation agent. Palmitoyl chloride was then used to tether the long fatty chain branches to the phenolic hydroxyl groups. Nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), and Fourier-transform infrared spectroscopy (FTIR) were used to analyze the structural changes during lignin modification. The experimental results demonstrated that the contact angle of lignin increased from 74.2 to 114° after the modification, and the modified lignin exhibited complete solubility in nonpolar lubricating oil over a long-term. In addition, the COSMO-RS results indicated that the influence of alkylation treatment on solubility was greater than that of the molecular weight. Modified lignin improved the tribological properties of nonpolar lubricants by reducing the friction coefficient and wear volume by 10.6 and 81.3%, respectively. These findings are expected to contribute significantly to the development of lignin based value-added products.