Properties of sandwich boards with a core made of bio-composite particleboard containing wood particles and walnut shells

The aim of the research was to investigate the possibility of producing bio-composite particleboard with a density reduced to 500–550 kg/m3, containing 25% and 50% of walnut shells. In addition, the study also concerned the possibility of using these materials in sandwich systems. Based on the results, it was found that partial replacement of wood particles with ground shells leads to a significant reduction in the strength of the boards bonded with urea-formaldehyde (UF) resin. However, the implementation of a hybrid gluing method consisting of gluing wood particles with UF resin and walnut shells with 4,4′-methylenediphenyl isocyanate (pMDI) caused a significant improvement in the strength of the boards, especially for the variant with the highest shells content. Despite that, the manufactured materials still do not meet the requirements for furniture boards. The next step of the research has shown that these boards can perform well as a core layer in the sandwich boards covered with high-strength HDF boards. Moreover, it was found that increasing the share of walnut shells positively affected the dimensional stability of the resultant boards (thickness swelling and water absorption). However, substitution of wood with shells accelerated the ignition and flameout times of the boards. It increased the heat release without significantly affecting the percentage loss of the boards’ mass during exposure to fire.