Enhanced Fire Protection: Improved Efficacy of <scp>ABPBI</scp> Over Aramids as the Outer Layer in Firefighter Suits

ABSTRACT The outer shell of firefighter protective clothing is typically made from high‐performance, inherently flame‐retardant fibers such as aramids, polybenzimidazoles, polyamide‐imides, and polyimides. Among these, Poly[2,2′‐(m‐phenylene)‐5,5′‐bisbenzimidazole] (PBI) is valued for its non‐flammability and low smoke emission. Poly(2,5‐benzimidazole) (ABPBI), synthesized from less carcinogenic monomers, offers even higher thermal and flame resistance, with an exceptionally high Limiting Oxygen Index (LOI) of ∼72, compared to PBI (∼38.3) and aramids (∼25). Despite this, studies on ABPBI's potential in the field of fire‐retardant textiles remain limited. Density Functional Theory (DFT) calculations, including HOMO–LUMO analysis, bond dissociation energy (BDE), and Mayer bond order analysis, were performed to provide molecular‐level insight into the thermal stability of both polymers. This study evaluates the performance of ABPBI fabric for use as an outer shell in firefighter clothing, benchmarking it against a conventional aramid fabric. Twill‐woven ABPBI and aramid fabrics (2/1, 260 gsm) were characterized and subjected to vertical flammability tests and radiant and convective heat exposures. Both fabrics exhibited no after‐flame or after‐glow and exhibited &lt; 5% char residue. However, under convective heat (84 kW m −2 for 30 s), the aramid fabric exhibited “break‐open” behavior due to brittleness, whereas ABPBI retained its flexibility and dimensional stability. Dynamic Mechanical Analysis of heat‐exposed ABPBI revealed a tenfold decrease in storage modulus, linked to a 26.2% oxygen loss and 9.3% carbon increase (EDS analysis), however aramid could not be tested. Exposure to a radiant heat flux of 10 kW m −2 for 180 s did not cause any noticeable loss in tensile strength of either the aramid or ABPBI fabric. The entire ensemble of firefighter clothing with ABPBI outer shell exhibited a Thermal Protective Performance of 60 cal cm −2 , and aramid shell based ensemble exhibited 44 cal cm −2 , both exceeding the minimum requirement of 35 cal cm −2 . The combined computational and experimental results confirm the superior thermal stability, char‐forming ability, and protective performance of ABPBI, supporting its potential as an advanced outer‐shell material for firefighter protective clothing.