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Polybenzimidazole (Poly-[2,2’-(m-phenylen)-5,5’-bisbenzimidazole], short for PBI) fiber is a synthetic fiber with a very high melting point. It has exceptional thermal and chemical stability and does not readily ignite. It was first discovered by American polymer chemists Carl Shipp Marvel in the pursuit of new materials with superior stability, retention of stiffness, toughness at elevated temperature. Due to its high stability, Polybenzimidazole is used to fabricate high-performance protective apparel such as firefighter’s gear, astronaut space suits, high temperature protective gloves, welders’ apparel and aircraft wall fabrics. In recently years, polybenzimidazole found its application as membrane in fule cells.

Discovery
Polybenzimidazole were discovered by Carl ship Marvel in 1950s. The materials Laboratory of Wright Patterson Air Force Base approached Marvel. They were looking for materials suitable for drogue chutes which could tolerate short-time mechanical strength. However, the thermal resistance of all known filaments at that time was completely inadequate. The original search concentrated on aromatic condensation polymers but the amide linkage proved to be weak link for the aim of maximal thermal stability of the polymer. While Marvel’s research focused on condensation polymers with aromatic and heteroaromatic repeating unit. This progressively led to the discovery of polybenzimidazole.

Development
•	1961-Polybenzimidazoles were developed by H. Vogel and C.S. Marvel with anticipation that the polymers would have exceptional thermal and oxidative stability.

•	1963-NASA and the Air Force Materials Lab sponsored considerable work with polybenimidazoles for aerospace and defense applications as a non-flammable and thermally stable textile fiber.

•	1969-The United States Air Force selected polybenzimidazole (PBI) for its superior thermal protective performance after a fire aboard an Apollo spacecraft killed three astronauts in 1967.

•	1970’s-NASA continued to use PBI as part of the astronauts’clothing on Apollo, Skylab and numerous space shuttle flights.

•	1980’s-PBI was introduced to the fire service, and through Project Fires an outer shell for turnout gear was developed. PBI Gold® fabric was born, consisting of 40% PBI/60% Para-aramid.

•	1983-A unique production plant goes on-line and PBI fibers become commercially available. •	1990’s-Short cut PBI fibers are introduced for use in automotive braking systems. PBI staple fiber enters the aircraft market for seat fire blocking layers.

•	1992-Lightweight PBI fabrics are developed for flame resistant workwear for electric utility and petrochemical applications.

•	1994-PBI Gold fabric is engineered in black and was specified by FDNY.

•	2001-After the terrorist attacks on 9/11 many of the 343 Fire Fighters killed were only identifiable by their PBI Turnout Gear.

•	2003-PBI Matrix® was commercialized and introduced as the next generation PBI for firefighter turnout gear

Properties
Imidazole derivatives are known to be stable compounds. Many of them are resistant to the most drastic treatments with acids and bases and not easily oxidized. The high melting point and high stability at over 400 degree suggest a polymer with benzimidazoles as repeating unit may also show high heat stability. Polybenzimidazole and its aromatic derivatives can withstand temperatures in excess of about 500 dgree without softening and degrading. The polymer synthesized from isophthalic acid and 3,3’-di-aminobenzidine is not melted by exposure to a temperature of 770 degree and loses only 30% of its weight after exposure to high temperature up to 900 degree for several hours. This proves a high thermal stability for PBI Testing regarding limiting oxygen index(LOI), i.e., the minimum oxygen concentration at which a given sample can be induced to burn permit a comparison of flammability. Data shows that PBI are highly flame resistant material compared to common polymers. Due to the high melting temperature for polybenzimidazole, solutions can be prepared from which shaped form may be formed like fibers, film and other shape of wide utility. Polybenzimidazole is soluable in concentrated sulfuric acid, formic acid, dimethyl sulfoxide, N,N-dimethylformamide forming stable solutions which are suited for casting of films and spinning of fibers. Now in industry N,N-dimethylacetamide(DMAC) was used to make the solution of PBI. Another particular property of PBI, the remarkable moisture regain is useful in protective clothing which make the clothing quite comfortable to wear in sharp contrast to other synthetic polymers. The moisture regain ability of PBI compares favorably with cotton which regains 13% of moisture with a 16% of cotton.

Synthesis The preparation of PBI(IV) can be achieved by condensation reaction of 3,3’,4,4’-tetraaminodiphenyl(I) and diphenyl isophthalate(II). The spontaneous cyclization of the intermediately formed animo-amide (III) to PBI (IV) provided a much more stable amide linkage. While changing the tetramine and acid, a number of different aromatic poly benzimidazole have been synthesized. The following table lists out some of the combination possibilities that have been synthesized in the literature. Some of the combination have actually been translated into fibers on a small scale. However, the only significant progress have been made to date is PBI. Application Before 1980s, major applications of PBI are for fire-blocking, thermal protective apparel and reverse osmosis membranes. Its applications became various by 1990s with the fact that the molded PBI parts and microporous membranes were developed. Protective apparel PBI filaments were fabricated into protective gears like firefighter’s gear, astronauts suits. PBI filaments are dry spun from dimethylacetamide containing lithium chloride. After washing and drying the resulting yarn is golden brown. PBI fiber is an excellent candidate for applications in severe environments due to its combination of thermal, chemical and textile properties. Flame and thermal resistance are the critical properties of protective apparel. This kind of apparel applications includes firefighter’s protective apparel, astronaut’s suits, aluminized crash rescued gear, industrial worker’s apparel, and suits for racing car drivers. Semipermeable membranes A comparison of the transport characteristics of PBI membrane with cellulose acetate film-currently one of the standards for reverse osmosis membranes- gave promising resulting. PBI can even retained (retain) its ability at higher temperatures where acetate membranes become completely impermeable.

Molded PBI resin Molded PBI resin is an excellent candidate for high strength, low weight material. Since it has the highest compressive strength, 58 ksi, of any available, unfilled resin and other mechanical properties such as a tensile strength of 23 ksi, a flexural strength of 32 ksi, a ductile compressive failure mode and the relatively low density of 1.3 g/cm3. Moreover, its thermal and electrical properties also make it a best know thermoplastic resin.

Fuel cell electrolyte Polybenzimidazole is able to be complexed by strongn acids because of its basic character. Complexation by phosphoric acid makes it a proton conductive material. This renders the possible application to high temperature fuel cells. Cell performance test show a good stability in performance for 200 h runs at 150 dgree (degree). Application in direct methanol fuel cells may be also of interest because of a better selectivity water/methanol compared to existing membranes.

Asbestos Replacement Previously, only asbestos can well perform in temperature gloves such as for foundries, aluminum extrusion, and metal treatment, while PBI trials have been developed and show adequately functions as asbestos. Moreover, a safety garment manufacturer reported that gloves containing PBI outlasted asbestos two to nine times with a effective cost. PBI also can also be a good substitute for asbestos in several areas of glass manufacturing.

Flue gas filtration PBI’s chemical, thermal and physical properties demonstrate that it can be a promising material as a flue gas filter fabric for coal fired boilers. Few fabrics can survive in the acidic and high temperature environment encountered in coal fired boiler flue gas. The filter bags also must be able to bear the abrasion from the periodic cleaning to remove accumulated dust. PBI fabric has a good abrasion resistance property. The acid and abrasion resistance and thermal stability properties make PBI a competitor for this application.