Halogen and phosphorous-free flame retardancy of ether-type polyurethane foams via polymer-ligand networks

Extensive use of polyurethane foam (PUF) in households and public sectors poses significant fire hazards due to its intrinsically high flammability. Traditionally, halogenated and organophosphate flame retardants are widely used as flame-retardant additives for PUF, which are increasingly restricted due to concerns on their persistence and toxicity in the environment. There is a pressing need to develop effective and eco-friendly flame-retarding treatment for PUF. Herein, halogen and phosphorus-free flame-retarding system for ether-type PUF using tannic acid (TA)-based polymer-ligand networks coordinated with divalent metal ions (Ca²⁺, Mg²⁺, Zn²⁺, and Cu²⁺) are reported. During the combustion, PUF–TA–Me²⁺ networks form graphitized, thermally stable char layers acting as physical barriers against heat and oxygen, effectively retard PUF combustion. Cone calorimetry and thermogravimetry showed that the polymer-ligand network incorporated with Cu²⁺ (PUF–TA–Cu²⁺) exhibits the lowest peak heat release rate (171.3 kW m⁻²), representing a reduction of 65% compared to pristine PUF with a substantial char yield of 30 wt% at 650 °C. Infrared spectroscopy and confocal Raman micro-spectroscopy confirmed the presence of a dense, thermally stable, and graphitized char layer on PUF–TA–Cu²⁺. Mechanical test results showed that all the PUF–TA–Me²⁺ networks exhibited a higher tensile strength (3–6 folds) and elongation at break (1–1.25 folds) compared to pristine PUF having no flame retardancy. This study provides the first unambiguous evidence that TA diffuses into and through PUF fibrils, functioning as a bridging ligand forming PUF–organic ligand (TA)–Me²⁺ networks with varying degrees of flame retardancy.