Balancing flame retardancy and high toughness in solvent-free reactive polyurethane films via P/Si synergistic strategy

Reactive polyurethane (RPU) films in a solvent-free medium exhibit exceptional stability and mechanical properties due to their highly cross-linked structure. Achieving inherent fire resistance, waterproofing, and high mechanical performance in RPUs is challenging. Here, we prepared an intrinsic flame retardant, waterproof, and flexible RPU using a P/Si synergistic system with hydroxylated ammonium polyphosphate aAPP/Si-RPU via in-situ polyaddition of NCO-double capped prepolymer and polyol components, including polydimethylsiloxane (PDMS) and aAPP. The aAPP was synthesized through cation exchange between diethylene glycol amine (DGA) and ammonium polyphosphate (APP). Compared to pure RPU, aAPP/Si 15% -RPU showed a 60.3 % reduction in peak heat release rate (pk-HRR) and a 14.1 % reduction in total heat release (THR), with the limiting oxygen index (LOI) increasing from 18.2 % to 27.8 %. Mechanical properties and hydrophobicity improved, with tensile strength at 31.9 ± 1.9 MPa, elongation at break at 522.7 ± 20.3 %, and a water contact angle of 127.1°. The balance of properties was due to ordered microphase separation from the organic–inorganic hybrid structure of aAPP and PDMS, where aAPP particles enhanced microstructure and crosslinked interactions, and the Si-O bond in PDMS provided flexibility and water resistance. This study presents a strategy to resolve the conflict between rigidity and toughness, offering design ideas for intrinsic flame retardant polyurethane.