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Polybenzimidazole (PBI,short for Poly-[2,2’-(m-phenylen)-5,5’-bisbenzimidazole]) 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 chemist 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 fuel cells.

Discovery
Brinker and Robinson invented the first aliphatic polybenzimidazoles in 1949. However the discovery of aromatic polybenzimidazole which show excellent physical and chemical properties was generally credited to Carl Shipp Marvel in 1950s. The materials Laboratory of Wright Patterson Air Force Base approached Marvel. They were looking for materials suitable for drogue parachutes 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
Its developing history can be summarized in the following list:
 * 1961 Polybenzimidazole were developed by H. Vogel and C.S. Marvel with anticipation that the polymers would have exceptional thermal and oxidative stability.
 * 1963 Subsequently, NASA and the Air Force Materials Lab sponsored considerable work with polybenimidazole for aerospace and defense applications as a non-flammable and thermally stable textile fiber.
 * 1969 The United States Air Force selects polybenzimidazole (PBI) for its superior thermal protective performance after a fire aboard an Apollo spacecraft killed three astronauts in 1967.
 * 1970’s NASA continues to use PBI as part of the astronauts’ clothing on Apollo, Skylab and numerous space shuttle flights.
 * When the Sky lab fell to the earth, the part that survived the re-entry was coated in PBI and thus did not burn up.
 * 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. Previous to this there had been combinations of Nomex, Leather, and Kevlar materials.
 * Leather and Rubber fire gear left “vital” areas exposed.
 * PBI gear is a full suit and looks like a snow suit (suspender pants) with a winter coat.
 * 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

General Physical Properties
For the physical properties, PBI are usually yellow to brown solid infusible up to 400 °C or higher. . The solubility of PBI is controversial. Because although most of the linear PBI are partly or entirely dissolved in strong protonic acids for instance, sulfuric acid or methanesulfonic acid, contradictory observations of solubities have been recorded among such weaker acids as formic acid, and in non-acidic media, such as the aprotic amide-type solvents and dimethyl sulfoxide. For example, one type pf PBI prepared in phosphoric acid was found by Iwakura et al. to be partially soluble in formic acid, but completely soluble in dimethyl sulfoxide and dimethylacetamide, whereas Varma and Veena reported the same polymer type to dissolve completely in formic acid, yet only partially in dimethyl sulfoxide or dimethylacetamide.

Thermal Stability
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 suggests a polymer with benzimidazole 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'-diaminobenzidine 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.

Flame Resistance
A property of a material needed to be considered before putting it into application is flammability, which demonstrates how easily one material can ignite and combust under the realistic operating conditions. This may affect its application in vary areas, such as in construction, plant design, and interior decoration. The quantitative assessment of flammability is based on 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.

Moisture Regain
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)（Figure 1). The spontaneous cyclization of the intermediately formed animo-amide (III) to PBI (IV) provided a much more stable amide linkage. This synthetic method was first used in the lab and later developed into a two step process. In a typical condition, starting materials were heated at 270 degree for 1.5 h to form the PBI prepolymer and later the prepolymer was heated at 360 degree for another 1 h to form the final commercial-grade product. The reason for the second step is due to the formation of the by-product phenol and water in the first step creating voluminous foam, which leads to the volume expansion of several times of the original. This is the issue must be considered by the industrial manufacturers.  This foam can be reduced by conducting the polycondensation  at a high temperature around 200°C and under the pressure of 2.1-4.2 MPa. . The foam can also be controlled by adding high boiling point liquids such as diphenylether or cetane to the polycondesation. The boiling point can make the liquid stay in the first stage of polycondesation but evaporate in the second stage of solid condensation. The disadvantage of this method is there are still some liquid remain in PBI and it is hard to remove them completely. .

While changing the tetramine and acid, a number of different aromatic poly benzimidazoles have been synthesized. The following table (Table 1) 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.

The most common form of PBI used in industry is the fiber form. The fiber process following polymerization is shown in the figure. The polymer is made into solution using dimethylacetamide as solvent. The solution is filtered and converted into fiber using a high temperature dry-spinning process. The fiber is subsequently drawn at elevated temperature to get desired mechanical properties. It is then sulfonated and made into staple using conventional crimping and cutting techniques.

Applications
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
The properties such as thermal stability, flame resistant, and moisture regain of PBI and its conventional textile processing character enable it to be processed on conventional staple fiber textile equipment. These characters lead to one of the most important applications of PBI is for 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.

The problem of PBI protective apparel is that it keeps heat out but it also keeps the heat in, too. Thus, by the time the firefighters feel heat and pain, it is too late and they will get burnt or killed since it is hard for the heat to penetrate the suit and be released into the air. There are still some major fire departments have not yet switch from old fire gear to PBI due to the fear of what PBI can cause. One example is the Chicago Fire Department still replies on the the old type of rubber or leather coats.

PBI Membranes
PBI has been used as the membranes for various separation purposes. Traditionally, PBI was used semi-permeable membranes for electrodialysis, reverse osmosis or ultraﬁltration. Recently PBI is also used for gas separations. due to its close chain packing since PBI has rigidity structure and strong hydrogen bonding. PBI membranes are dense, with very low gas permeability.To be proton conductive, PBI usually is doped with acid. The higher level of the acid doping, the more conductive PBI is. But one problem raised is the mechanical strength of PBI decreases The same time. The optimum doping level is thus a compromise between these two effects. Thus, multiple methods such as ionic cross-linking, covlant cross-linking and composite membranes have been researched to optimize the doping level at which PBI has a improved conductivity without sacriﬁcing mechanical strength. Kerres et al. recently have recently synthesized sulphonated partially fluorinated arylene main chain polymer. Their blend membranes with PBI demonstrate high level acid-doping levels with thermal and extended stability, high proton conductivities, less acid swelling, reasonable mechanical strength.

Molded PBI Resin
PBI resin, trademarked Celazole®; is molded via a proprietary sintering process that was jointly developed by Hoechst Celanese (North Carolina, USA) and Alpha Precision Plastics, Inc. (Houston, Texas, USA). 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. The PBI resin comprises a recurring structural unit represented by the following figure.

According to the Composite Materials Research Group at the University of Wyoming, Celazole parts maintain significant tensile properties and compressive strength to 700°F (371°C). Celazole parts are also potential materials for the chemical process and oil recovery industries which have demands of thermal stability and chemical resistance. In these areas, Celazole has been successfully applied in demanding sealing, for instance, valve seats, stem seals, hydraulic seals and backup rings. In the aerospace industry, Celazole has high strength and short term high temperature resistance advantages. In the industrial sector, Celazole's high dimensional stability as well as retention of electrical properties at high temperature make it used as a thermal and electrical insulator. . More details of molded PBI resin can be find in Thermech website.

Fuel Cell Electrolyte
Polybenzimidazole is able to be complexed by strong 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 degree. Application in direct methanol fuel cells may be also of interest because of a better selectivity water/methanol compared to existing membranes. Wainright, Wang et. al reported that PBI doped with phosphoric acid was utilized as an high temperature fuel cell electrolyte. The doped PBI high temperature fuel cell electrolyte has several advantages. The evevated temperature increases the kinetic rates of the fuel cell reactions. It also can reduce the problem of the catalyst poisoning by adsorbed carbon monoxide and it minimizes problems due to electrode flooding. PBI/H3PO4 is conductive even in low relative humidity and it allows less crossover of the methanol at the same time. These contribute PBI/H3PO4 to be superior to some traditional polymer electrolytes such as Nafion. Additionally, PBI/H3PO4 maintains good mechanica strength and toughness. Its modulus is three order of magnitude greater than that of Nafion. This means that the thinner films can be used, thus reducing ohmic loss.

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. Gloves containing PBI fibers are softer and more supple than those made of asbestos, offering,the worker greater mobility, and comfort even if the fabric becomes charred. Besides, PBI fiber avoids the chronic toxicity problems associated with asbestos because it processes on standard textile and glove fabricating equipment. 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.