US Patent 11959196 Phosphonated PBI fiber

A fiber is made with a polybenzimidazole (PBI) polymer with a phosphoric acid pick-up (APU) in the range of 1-25% (PBI-p fiber). The PBI-p fiber may have a LOI ≥50% and/or an initial thermal decomposition temperature in air of ≥555° C. A method of making a phosphonated polybenzimidazole fiber comprises the steps of: spinning an untreated PBI resin into a PBI fiber; treating the PBI fiber with phosphoric acid, and thereby obtaining a PBI fiber with 1-25 wt. % phosphoric acid APU.