PBI Intro| Structure | Uses | The good the bad and the burny|History|
Polybenzimidazole (PBI) is a burn “proof” fiber used in the protective gear for firefighters, astronauts, and other dangerous professions. This website will focus on its role in the fire service, and how its invention and use has been both beneficial and detrimental to firefighters in the United States and Across the globe.
It looks like this 
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On Azom.com, a site used for material supplies, PBI is described as follows.
An infusible, polybenzimidazole (PBI) is a black high performance polymer with a wholly aromatic molecule of high thermal stability. Semi-fabricated items are made, normally by the polymer manufacturer, by powder sintering processes.
It has excellent high temperature properties – probably on balance the best of any commercially available polymer – and is very expensive. It is strong, stiff, very hard, has a particularly high strength in and recovery from compression and a CTE similar to that of aluminium. It has good chemical resistance resisting many harsh chemical environments better than many other high performance polymers, but is attacked by polar aprotic solvents and, at elevated temperatures, by strong aqueous acids and (to a lesser extent) alkalis. Despite absorbing (slowly) a high percentage of water at saturation, it is stable to hydrolysis and resists high pressure steam.
Because it is a “hard” material it is used in combination with other more plyable materials such as nomex in making firefighter turnout gear. The combination in Globe (my personal favorite and perferred brand) is 40% PBI 60 % Kevlar (for rip protection and binding). (http://www.globefiresuits.com/globe/materials/outer-performance.asp#pbimatrix).