Impact of Different Substituents on Thermal Curing and Flame‐Retardant Behaviors of 2,2′‐Methylenebis(4‐nitrophenol)‐Based (Poly)benzoxazines

Abstract

Polymerization of benzoxazine at low temperatures is important because the high‐temperature polymerization restricts its utilization in a wide range of applications, though the polybenzoxazines possess excellent properties. The effect of nitro functionality and various substituents on the thermal curing of benzoxazine and the flame‐retardant behavior are mainly focused in this study. In the present work, 2,2′‐methylenebis(4‐nitrophenol) (NBP) was synthesized from 4‐nitrophenol and is used for the synthesis of benzoxazine derivatives with five structurally different aromatic amines. As the synthesized benzoxazine contains nitro‐groups in the phenolic part and various substituted functional groups in different positions of the amine part, the curing temperatures significantly varied among each other. The effect of different substituents on the thermal ring‐opening polymerization (ROP) of benzoxazine was studied with exotherms of the DSC thermogram. The 4‐fluoroaniline and 2‐aminobenzotrifluoride‐based benzoxazines show the highest and lowest curing temperatures of 272 and 189 °C, respectively, which is based on their substituents and their position that can act as an internal catalyst. The resulting polybenzoxazines possess higher char yield (49% ≤ 63%) and higher values of LOI (37 ≤ 43) including excellent flame retardant (UL94‐V0) behaviors suggesting that the polybenzoxazine can be used as coatings for high‐performance industrial applications.