Comprehensive Insight into Foams Made of Thermomechanical Pulp Fibers and Cellulose Nanofibrils via Microwave Radiation

The lack of sustainable and scalable processes to produce low-density foams made solely of renewable materials has long been a challenge. This study investigates a novel approach to produce all-renewable foams from thermomechanical pulp fibers (TMP) and cellulose nanofibrils (CNF) using microwave radiation. The foams were produced without foaming agents or chemical reinforcing agents, and CNF was the only binder in the foam network. The foams were produced in two orientation modes simply by controlling the design of the mold in which the desired shape was formed prior to drying. This resulted in foam samples with fibers aligned horizontally (XY-foam) and vertically (Z-foam) due to the natural tendency of elongated fibers to lay horizontally in the plane upon loading the sample. The compressive moduli of the foams were strongly dependent on the orientation of the fibers (0.85 MPa versus 3.72 MPa at 0.10 g/cm3). XY-foams exhibited a thickness recovery of up to 85% within 1 min after compression and almost full recovery after soaking in water and redrying. The thermal conductivity was as low as 0.046 W/m.K. The foams also had the potential to be used in sound absorption applications. The new chemical-free process to form all-renewable fiber-based foams can be scaled up much easier than other foaming technologies.