Supercritical CO2 solvent exchange and removal of water from dispersed cellulose nanofiber/polylactic acid composites without an alcohol exchange step

We describe a method to form bio-based composites comprised of highly disperse cellulose nanofibers (CNFs) distributed in a polylactic acid (PLA) matrix by using liquid CO₂ (L-CO₂) solvent exchange followed by supercritical CO₂ (SC-CO₂) venting. A slurry of CNF is pre-mixed with PLA prior to SC-CO₂ drying, allowing for contact dewatering to occur when the CNFs adsorb onto PLA particles. This eliminates the need for an alcohol exchange step as the L-CO₂ meniscus pushes water out of the CNF hydrogel. Venting from SC-CO₂ was best at removing water from the CNF/PLA slurries as this retains the fibrous morphology which allows for water to be removed from inside the CNF network. In contrast, aggregation of the CNFs occurs when venting from L-CO₂, trapping water inside the CNF network, leading to a lower amount of water being removed. Using our approach, we remove 97% of the water after the first cycle from a 2 wt% CNF/PLA mixture and 92% of the water after the first cycle from 5 and 10 wt% CNF/PLA mixtures. After three cycles, the CNF/PLA samples were approximately 99% dried.