Molecular-micron multiscale toughening and flame retarding for polyurethane foams

Phosphorus-based multiscale energy dissipation networks spanning molecular and micron scales enable PUF with outstanding toughness and fire safety. • A molecular-micron multiscale toughening and flame retarding method is proposed for PUF. • Phosphorus-based multiscale networks (PMN) are fabricated during foaming. • The resulting PUF show excellent flame-retardant and toughing performances. • The toughening and flame-retardant mechanism is discussed in detail. Polyurethane foams (PUF), widely used in daily life, has become one of the main triggers of urban fires. The incorporation of flame retardants is the most effective way to reduce the fire risk, but always severely deteriorates the mechanical properties of materials (e.g., fatigue resistance, malleability). Herein, we demonstrate a facile strategy for fabricating phosphorus-based multiscale energy dissipation networks (PMNs) to engineer PUF with superior toughness and fire safety. By exploiting the polarity/reactivity nature of phosphorus-based flame-retardant molecules (PMs), the interpenetrating PMNs are fabricated in-situ during foaming, which consist of molecular scale pendants for plasticization and micron-scale particles with compatible interphase/reinforcing mechanisms. Notably, the unique PMNs synergistically facilitate the redistribution and dissipation of external stress, enabling PUF the outstanding capability to sustain large deformation. The corresponding fracture strength, elongation at break (59%), and toughness (92%) are simultaneously increased, avoiding the fragility caused by conventional crosslinking networks or rigid fillers. In addition, the flame retardant disparity and mechanism of three PMs with tiny chemical differences were investigated. More phosphorus species with radical scavenging abilities, given by hydroxymethyl diphenylphosphine oxide (DPM), endow PUF the ability to rapidly self-extinguish with low addition of 1.8 wt%, which is challenging for the others. The synergistic PMNs, spanning molecular and micron scales, provide new inspiration for solving the contradiction of flame retardancy & mechanical performances and strength & toughness for materials, showing obvious superiority for practical applications.