Foam-Formed Lupine and Reed Pulp: Anisotropic Bio-Based Foams for Sound Absorption Applications

Bio-based foams, often derived from wood pulp, have recently gained attention for their sound-absorbing properties. To avoid increasing wood demand for these potentially high-volume uses, diversification of raw material sources is needed. Here, an alternative route of soda-pulping and foam-forming of underutilized invasive plants, garden lupine (Lupinus polyphyllus Lindl.) and common reed (Phragmites australis), is presented. The foams of three different bulk densities exhibited distinct pore architectures depending on the raw material: lupine foams consisted of larger pores owing to shorter fibers, whereas the compact structure of the reed foams was caused by drying stresses. X-ray tomography quantified the anisotropy with porosity variations across the thickness, showing that all foams displayed vertical density gradients with a denser bottom layer. While all foams reached high sound absorption coefficients, lupine foams outperformed reed foams. Absorption is density- and orientation-dependent: the top side absorbed better at higher frequencies, while the bottom side performed better at lower frequencies. An improved absorption at high and low frequencies was achieved by increasing the thickness of the foams, matching the absorption of commercial materials. These results establish underutilized invasive plants as viable feedstock for anisotropic sound-absorbing foams and motivate tomography-guided studies to decouple layer-specific acoustic behavior.