Environmentally-Benign Halloysite Nanotube Multilayer Assembly Significantly Reduces Polyurethane Flammability

In an effort to develop a more environmentally benign flame retardant for polyurethane foam (PUF), layers of halloysite clay nanotubes (HNT) stabilized by branched polyethylenimine (BPEI) or poly(acrylic acid) (PAA) are deposited from aqueous suspensions to create multilayered nanocomposite coatings. PUF is very flammable and widely used in upholstered furniture throughout the world. Foam treated with five BPEI-HNT/PAA-HNT bilayers, deposited using layer-by-layer assembly, is rendered self-extinguishing in open flame testing. Cone calorimetry reveals that this coating reduces the peak heat release rate (pkHRR) by 62%. Due to the tubular morphology of HNT, small volatile gasses given off during combustion are trapped, so total smoke release (TSR) is reduced by 60%. Infrared spectroscopy suggests this multilayer film survives during combustion, forming an HNT-rich barrier that prevents mass and energy transfer during open flame testing and calorimetry. The significant reductions in pkHRR and TSR, along with the self-extinguishing behavior, indicate that these halloysite-based multilayer films have the potential to greatly improve PUF fire safety. The low cost and natural abundance of HNT makes this technology especially amenable to widespread use.