Toward Intrinsically Flame‐Retardant Polyurethane via Molecular Design of Polyester Polyol

ABSTRACT Given the defects existing in traditional additive flame retardants, it is of great research significance to endow polyurethane (PU) with intrinsic flame retardancy through molecular structure design. The polyester polyol (PAHI) is first synthesized by esterification with 1, 6‐adipic acid, 1, 6‐hexanediol, and itaconic acid, and then the itaconic acid reacts with 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) to form a polyester polyol (PAHI‐DOPO). The PAHI‐DOPO monomer is used as part of PU flexible section to be cured with diphenylmethane diisocyanate (MDI) to obtain PU with an increased LOI value of 30.4% and flame‐retardant grades reaching the V‐0 level. Moreover, a small amount of PAHI‐DOPO can significantly enhance the mechanical properties of PU, resulting in a strength increase from 15.1 to 18.2 MPa and an improvement in toughness from 753% to 815%. Compared with direct addition of equal amount of DOPO as flame retardant, the transparency and mechanical properties of PU are significantly improved. Interestingly, the intrinsic flame‐retardant approach effectively avoids the sharp decrease in mechanical properties that occurs when the flame retardant migrates from the samples upon immersion in water. Thus, this synthetic flame‐retardant polyester polyol offers an efficient, durable, and environmentally friendly approach to enhancing the flame‐retardancy of PU.