Eco‐Friendly, Cost‐Effective Halogen‐Free Phosphorus‐Nitrogen Flame Retardant for Fire‐Safe Silicone Rubber With Excellent Properties

Silicone rubber (SR), a room‐temperature‐vulcanizable elastomer valued for outstanding thermal stability, chemical resistance, and mechanical resilience, is nevertheless limited in fire‐sensitive applications due to its inherent flammability. In this work, a cost‐effective, halogen‐free phosphorus‐nitrogen flame retardant, urea‐1‐hydroxyethylidene‐1,1‐diphosphonate (UHP), was synthesized from readily available urea and 1‐hydroxyethylidene diphosphonic acid and subsequently incorporated into SR. The resulting SR/UHP composites show substantially enhanced flame retardancy while maintaining good elasticity and acceptable mechanical performance. UHP's well‐defined P‐N‐containing structure enables pronounced phosphorus‐nitrogen synergism and promotes efficient char formation within the silicone matrix. At 20 phr, the SR/UHP composite reaches a limiting oxygen index of 27.6% and achieves a UL‐94 V‐0 rating. Cone calorimetry indicates a reduced peak heat release rate and total heat release, together with increased char residue and suppressed smoke generation. TG‐FTIR, complemented by FTIR, XPS, and SEM analyses of the combustion residues, supports a dual‐phase mechanism in which gas‐phase radical inhibition is coupled with condensed‐phase catalytic charring to form a compact carbonaceous‐siliceous protective layer. UHP offers a structurally simple, cost‐effective option for imparting efficient, halogen‐free flame retardancy to silicone rubber and provides a practical route towards greener flame‐retardant systems for silicone‐based elastomers and other polymers.