Lightweight but high-strength nanocellulose foams via ambient pressure drying for highly efficient sound and thermal insulation.

Porous foams have been identified as having broad application prospects in a number of fields. However, the existing foams have problems e.g., poor sound absorption performance, low structural strength, and bad degradability. Herein, a lightweight but high-strength biomass foam was reported by drying at ambient pressure, which was composed of cellulose nanofibrils (CNFs) immersed in ethanol/Fe3+ bath. By adjusting the solid contents of CNFs, the compressive strength of the foam was improved, allowing it to support forces up to 4500 times its weight. The macroporous-doped gradient structure was demonstrated to enhance the foam's ability to absorb sound waves. For a 10 mm sample, the average sound absorption coefficient reached 93.4% above 2000 Hz. The macroporous structure inside the foam was conducive to acoustic coupling, while the small pores were effective in the obstruction of sound wave propagation. Moreover, an increase in the solid contents of CNFs was associated with an enhancement in thermal insulation performance of the foam. This work will provide a feasible strategy for promoting the advancement of nanocellulose foams as sophisticated acoustic building materials.