Sustainable Lignocellulosic Composites Derived from Recycled Paper and Cardboard for Building Applications

The valorization of post-consumer waste materials is an important strategy for reducing environmental impact and supporting circular material use. In this study, lightweight sandwich composites were developed using recycled paper and cardboard as core materials, producing sustainable panels for thermal and acoustic insulation. Core panels were manufactured from 100% paper, 100% cardboard, and a 50–50% paper–cardboard mixture. Environmentally friendly foaming agents were added to increase porosity and reduce density. The cores were subsequently combined with 3 mm medium-density fiberboard (MDF), 1 mm oak veneer, and date palm midrib fibers to provide different surface characteristics. The resulting sandwich composites were evaluated through standardized measurements of thermal conductivity and sound absorption coefficients. Microstructural characteristics were investigated using stereomicroscopy and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDX). The results indicate that both the core composition and the type of face layer influence their performance. Whilst composites with cardboard-rich cores had higher porosity and better thermal insulation, introducing perforations and increasing the panel thickness improved sound absorption. The findings demonstrate that recycled paper and cardboard can be effectively used as sustainable raw materials to produce lightweight sandwich composites, tested at material scale, for non-structural interior insulation/acoustic panels.