Chemical Binder-Free and Oven-Dried Lignocellulose/Clay Composite Foams: Flame Resistance, Thermal Insulation, and Recyclability

Global efforts to reduce carbon emissions and improve thermal comfort demand sustainable and safe-to-use insulative materials. This study advances a new type of chemical binder-free lignocellulose/clay composite foam as a sustainable alternative to the currently used synthetic and glass/mineral counterparts. Pressurized disk milling unraveled submicron "hairy" fibrillation on the surface of refiner mechanical pulp (RMP). Such fibrillated fibers were then subjected to a foam laying process, with kaolinite being incorporated as an efficient and cost-effective fire retardant. Upon oven drying, the foams display suitable structural and mechanical robustness. Through mild fibrillation treatment of the RMP, a clay retention of up to 2-fold by weight was achieved without compromising the properties of the foam, removing the need for the addition of chemical binders. The foam density, mechanical and thermal properties, and flame resistance were systematically investigated with respect to the relative fiber loading as well as surfactant and clay addition. A low thermal conductivity (43.7 ± 0.7 mW/(m·K)) and high flame resistance (limiting oxygen index of ∼43%) were demonstrated for hybrid foams of apparent density of 136 ± 1 kg/m3 that also displayed good compressive strength (Young's modulus of 0.805 ± 0.158 MPa and compressive stress of 0.126 ± 0.008 MPa at 25% strain). Remarkably, owing to the absence of chemical binding, facile recyclability was demonstrated over three cycles, with no significant reduction in performance. Overall, this work proposes a readily scalable technology toward safe-to-use, recyclable lignocellulose/clay composite foams for building insulation.