Properties of Green Foam-Type Composites Made from Recycled Paper and Cardboard

This study developed sustainable foam-type composites from recycled paper (P), corrugated cardboard (C), and their 1:1 mixture (PC) for use in thermal and acoustic insulation. The materials were produced by water-assisted defibration, gas foaming with sodium bicarbonate and yeast, and oven curing, resulting in lightweight porous panels without synthetic binders. The composites exhibited distinct density and porosity profiles that influenced moisture behavior and stability. Cardboard-based panels absorbed the most water and swelled the most, while paper-based panels were more resistant. Despite these differences, all materials showed uniformly low thermal conductivity, confirming their strong insulation capability. Acoustic performance was enhanced by perforation and multilayer assembly. Cardboard panels with a triple-layer perforated design achieved the highest sound absorption, while mixed paper–cardboard composites provided balanced broadband performance. Microscopy revealed that fiber morphology—coarse in cardboard, fine in paper, and interlaced in mixtures—shaped the porous structure and bonding. Mechanical tests indicated comparable stiffness and strength across all types, with cardboard showing the strongest internal bonding. Overall, the results demonstrate that fiber structure and porosity govern material performance. These foam composites combine effective thermal insulation, competitive sound absorption, and sufficient mechanical strength, positioning them as biodegradable, low-cost alternatives for sustainable construction and acoustic applications.