Green valorization of rice husk agro-residues in the design of eco-friendly HPMC-based thermo-expanded foams for sustainable packaging.

Biodegradable alternatives to petroleum-derived plastics are needed to mitigate the environmental impacts associated with plastic packaging waste. This study aimed to develop and characterize biodegradable foams based on hydroxypropyl methylcellulose (HPMC) reinforced with rice husk fibers (RHF), an agro-industrial residue, for potential use in disposable packaging. Foams containing 1%, 2%, and 3% (w/v) RHF were produced and characterized by contact angle, water vapor transmission rate (WVTR), Fourier-transform infrared spectroscopy (FTIR), mechanical testing, scanning electron microscopy (SEM), thermogravimetric analysis (TGA/DTG), biodegradation, and substrate toxicity assays. The foam containing 2% RHF exhibited the most balanced overall performance. The foams presented a hardness of 23.2 ± 4.7 N and contact angles ranging from 84.6 ± 5.8° to 66.8 ± 3.3° after 180 s, with both properties comparable to or superior to those reported for starch-based biodegradable foams and disposable expanded polystyrene packaging. The WVTR was 13.94 ± 0.76 g/m2.h, substantially lower than that reported for starch-based foams, indicating improved moisture barrier performance. Almost complete biodegradation was observed after 30 days in soil, while a seed germination rate of 87.5% indicated no relevant phytotoxic effects under the evaluated conditions. SEM analysis confirmed a porous cellular structure potentially suitable for insulation-related applications. Overall, HPMC-RHF foams combined suitable mechanical performance, favorable surface wettability, improved moisture barrier properties, rapid biodegradation, and environmental safety. The use of RHF also contributes to agro-industrial residue valorization without directly competing with food resources, highlighting the potential of these foams as sustainable alternatives for disposable packaging.