Simultaneous synthesis of cellulose nanocrystals and a lignin-silica biofiller from rice husk: Application for elastomeric compounds

A biorefinery process was set up to simultaneously produce cellulose nanocrystals (CNCs) and an inorganic-organic material composed by silica and lignin starting from rice husk, a largely available agricultural side product. The opportunity to synthesize this new material, combining lignin and silica at the microscopic level, ensued naturally as the two substances were extracted simultaneously during the process. The main parameters influencing the yield, the composition, and the properties of the lignin-silica material (LSM) were optimized. The overall yield of the process was 35% (12% CNCs and 23% LSM). The sub-micrometric irregular particles constituting the LSM were approximately equally composed by lignin and silica. Larger batches were produced to assess the possibility to use the LSM as a reinforcing biofiller in elastomeric composites based on natural rubber. The lignin-silica material reinforced natural rubber (Ultimate elongation +17%, ultimate strength +198%). The overall effect was rationalized in terms of individual contributions of the two constituents: silica was supposed to contribute largely to the reinforcement, whereas lignin seemed to protect natural rubber from degradation, conveniently enhancing the elongation at break and lowering the total density of the biofiller.