Solvent‐Free Synthesis of Flame‐Retardant Polyurethane Hot Melt Adhesive via Diels‐Alder Reaction

ABSTRACT

To address the core dilemma prevailing in the modification technologies of conventional polyurethane hot‐melt adhesives (PU HMAs), in which flame retardancy, bonding performance and environmental friendliness cannot be achieved simultaneously, a green solvent‐free strategy was proposed to prepare synergistic flame‐retardant PU HMAs via Diels–Alder (DA) reaction. Two flame‐retardant polyols (Raynol 190A, BY3010) were blended with polycarbonate polyol (PCC) and polypropylene glycol (PPG) to synthesize four PU HMAs (PU‐PCC‐190A, PU‐PPG‐190A, PU‐PCC‐BY, PU‐PPG‐BY). Structural and performance analyses were carried out using FT‐IR, TG‐DSC, bonding strength tests, limiting oxygen index (LOI), and cone calorimetry. Results confirmed that DA bonds and flame‐retardant groups were successfully incorporated, improving thermal stability. The PU‐BY samples showed greatly enhanced bonding properties: shear tensile strength and 5 min initial tack increased markedly due to higher hard‐segment content and stronger hydrogen bonding. The re‐bonding strength is maximally increased by 186.8% and 30.2%, respectively, compared with the blank control group. Notably, PU‐PPG‐BY and PU‐PCC‐BY reached LOI values of 33.1% and 32.7%, with significantly reduced peak heat release rate (HRR) and total heat release (THR). Synergy of phosphorus polyol and DA bonds suppresses molten drip combustion, providing a green route for flame‐retardant PU HMAs for fire‐safe industries.