Nanoparticles from softwood kraft lignin as bio-derived fillers in polymer-based nanocomposites

Lignin is a naturally occurring aromatic polymer found in the wall of cellular plants. The large availability of lignin on Earth, second only to cellulose and hemicellulose, makes it particularly interesting as valuable renewable source of aromatics for the development of high performance and environmentally friendly polymer-based materials.1-3 Despite such great potential, lignin is still enormously underutilized at industrial scale, as its fate is typically to be burnt as low-cost fuel for energy generation. In this context, the large number of functional groups present in lignin (e.g., aliphatic and aromatic hydroxyls, carbonyls, carboxyls) makes this material particularly attractive as reinforcing filler for the preparation of polymer-based composites, due to the potentially favourable covalent and non-covalent interactions that may arise between such highly-functional filler and the polymeric matrix. To this purpose, different attempts have been made involving the incorporation of lignin particles (generally in the 10-100 μm range) into plastics and rubbers. Typically, it was shown that straightforward addition of lignin into the target matrix yielded little or no effect on the mechanical response of the so-obtained composite. An alternative strategy to achieve enhanced dispersion level of the lignin filler within the polymer matrix is the preparation and use of lignin-based nanoparticles (LNP). Indeed, LNP are expected to provide a more efficient interaction with the polymer matrix compared with the micrometer-sized counterpart owing to their larger surface-to-volume ratio thus ultimately resulting in improved mechanical response of the LNP-based polymer composite material.4-6 In this work, LNP were prepared by ultrasonic treatment of technical softwood kraft lignin to obtain lignin-water dispersions with excellent colloidal stability. A thorough chemical, physical and morphological characterization was carried out on the LNP system and a comparison with the parent untreated material was performed. Such LNP were incorporated into a waterborne thermoplastic polyurethane matrix at different concentrations to yield bio-based nanocomposite materials. The effect bio-filler type (LNP vs. untreated lignin) and concentration on the chemical-physical, thermal and morphological characteristics of the resulting nanocomposites was investigated and the reinforcing effect was discussed based on mechanical tests. The results of this study give a direct demonstration of a viable environmentally friendly approach to obtain waterborne polyurethane-based nanocomposites reinforced with LNP in a straightforward and accessible way and provide clear evidence of the potential of LNP as fully bio-derived fillers for advanced nanocomposite applications. Acknowledgements This work has been performed as part of the ValorPlus Project that has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement no FP7-KBBE-2013-7-613802. References [1] Laurichesse, S.; Avérous, L. Prog. Polym. Sci. 2014, 39, 1266. [2] Griffini, G.; Passoni, V.; Suriano, R.; Levi, M.; Turri, S. ACS Sustainable Chem. Eng. 2015, 3, 1145. [3] Passoni, V.; Scarica, C.; Levi, M.; Turri, S.; Griffini, G. ACS Sustainable Chem. Eng. 2016, 4, 2232. [4] Thakur, V.K.; Thakur, M.K.; Raghavan, P.; Kessler, M.R. ACS Sustainable Chem. Eng. 2014, 2, 1072. [5] Nair, S.S.; Sharma, S.; Pu, Y.; Sun, Q.; Pan, S.; Zhu, J.Y.; Deng, Y.; Ragauskas, A.J. ChemSusChem 2014, 7, 3513. [6] Gilca, I.A.; Popa, V.I.; Crestini, C. Ultrason. Sonochem. 2015, 23, 369.