Polybenzimidazole

Polybenzimidazole (PBI) is a high-temperature, aromatic polymer characterized by benzimidazole rings linked by para-phenylene spacers. This rigid backbone imparts exceptional thermal stability and flame resistance, with decomposition temperatures well above typical engineering polymers and high char yields.

PBI is generally insoluble in common organic solvents and is processed by dissolving in strong protonic acids, such as polyphosphoric acid or concentrated sulfuric acid, to form cast films or fibers that are later cyclized and hardened. The resulting materials retain mechanical integrity at elevated temperatures and show resistance to many chemical environments.

Synthesis involves the polycondensation of aromatic diamines and dicarboxylic acids under dehydrating conditions, followed by cyclodehydration to form benzimidazole rings; polyphosphoric acid is commonly used as a solvent/catalyst in the cyclization step. The process yields high-molecular-weight polymers suitable for high-performance applications.

Applications include protective clothing and heat barriers for firefighters, insulation and fire-resistant coatings, high-temperature filtration media, and membranes for gas separation or proton exchange in fuel cells when doped with phosphoric acid. PBI fibers and molded parts are used in aerospace, electronics, and chemical-processing equipment because of their stability at temperatures beyond 200–300 C.

Commercially, PBI is available in resin, film, and fiber forms under several trade names, most notably Celazole. Its combination of thermal robustness, flame retardance, and chemical resistance makes it a benchmark material among high-temperature polymers, despite processing challenges and cost.