High-Pressure Microfluidization as a Green Tool for Optimizing the Mechanical Performance of All-Cellulose Composites

We herein report the production of environmentally inspired all-cellulose composites in response to the ever-growing concern on the extensive usage of nonbiodegradable materials derived from nonrenewable resources. Hydroxypropyl methylcellulose (HPMC) was used as a film-forming matrix, while microcrystalline cellulose (MCC) was added as a reinforcement filler. Because the efficiency of fillers in transferring mechanical strength to polymer matrixes relies upon the dispersion level of the former within the latter, this contribution set out to improve the homogeneity of the composite films through a green, solvent-free approach. Indeed, as-received MCC actually decreased the tensile strength, Young's modulus, and elongation at break of HPMC films in ca. 80%, 33%, and 90%, respectively. High-pressure microfluidization was demonstrated to break MCC particles down, not to play a role on cellulose crystallinity, and to expose surface groups and/or create mechanoradicals, as suggested by a combination of spectroscopic, structural, morphological, and rheological techniques, capable of interacting with HPMC and increasing MCC colloidal stability, thus lessening particle aggregation and improving its dispersion within film matrix. A central composite design guided the optimization of the filler–matrix interaction, which presented a quadratic behavior; that is, overprocessing led to impaired mechanical properties. Seven cycles was the most cost-effective processing condition leading to the strongest, stiffest composites. This study sheds light on the effect of high-pressure microfluidization on cellulosic particles and on the impact of these in all-cellulose composites.