Polybenzimidazole (PBI, short for poly[2,2’-(m-phenylen)-5,5’-bisbenzimidazole]) fiber is a synthetic fiber with a very high decomposition temperature. It does not exhibit a melting point, does not readily ignite, and has exceptional thermal and chemical stability. It was first discovered in 1961, by American polymer chemist Carl Shipp Marvel in the pursuit of new materials with superior stability, retention of stiffness, and 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. Polybenzimidazole has been applied as a membrane in fuel cells.
History
Discovery Brinker and Robinson first reported aliphatic polybenzimidazoles in 1949. However the discovery of aromatic polybenzimidazole, which shows excellent physical and chemical properties, was generally credited to Carl Shipp Marvel in the 1950s. The Material Laboratory of Wright Patterson Air Force Base approached Marvel. They were looking for materials suitable for drogue parachutes which could tolerate short-time mechanical stress. However, the thermal resistance of all known filaments at that time was 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, whereas Marvel's research focused on condensation polymers with aromatic and heteroaromatic repeating units. This progressively led to the discovery of polybenzimidazole.
Development
Its development history can be summarized in the following list: In 1961, polybenzimidazole was developed by H. Vogel and C.S. Marvel with anticipation that the polymers would have exceptional thermal and oxidative stability. Subsequently, in 1963, NASA and the Air Force Materials Lab sponsored considerable work with polybenzimidazole for aerospace and defense applications as a non-flammable and thermally stable textile fiber. In 1969, the United States Air Force selected polybenzimidazole (PBI) for its superior thermal protective performance after a 1967 fire aboard the Apollo 1 spacecraft killed three astronauts. In the early 1970s USAF laboratories experimented with polybenzimidazole fibers for protective clothing to reduce aircrew deaths from fires.Statement of Hon. Grant L. Hansen, Assistant Secretary of the Air Force (Research and Development), Department of Defense Appropriations for Fiscal Year 1972, p. 612. In the 1970s, NASA continued to use PBI as part of the astronauts’ clothing on Apollo, Skylab and numerous space shuttle flights. When Skylab fell to Earth, the part that survived the re-entry was coated in PBI and thus did not burn up. 1980s – 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, combinations of Nomex, leather, and Kevlar materials were used in the US. 1983 – A unique production plant goes on-line and PBI fibers become commercially available. 1990s – 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 the FDNY. 2001 – After the terrorist attacks on September 11, many of the 343 fire fighters killed were only identifiable by their TenCate PBI Turnout Gear. 2003 – PBI Matrix was commercialized and introduced as the next-generation PBI for firefighter turnout gear.
Properties
General physical properties PBI are usually yellow to brown solid infusible up to or higher. The solubility of PBI is controversial, because while most of the linear PBI are partly or entirely dissolved in strong protonic acids (for instance, sulfuric acid or methanesulfonic acid), contradictory observations of solubilities have been recorded among weaker acids like formic acid, and in non-acidic media, such as the aprotic amide-type solvents and dimethyl sulfoxide. For example, one type of 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 decomposition temperature and high stability at over 400 °C suggests a polymer with benzimidazole as the repeating unit may also show high heat stability. Polybenzimidazole and its aromatic derivatives can withstand temperatures in excess of about without softening and degrading. The polymer synthesized from isophthalic acid and 3,3'-Diaminobenzidine is not melted by exposure to a temperature of and loses only 30% of its weight after exposure to high temperature up to for several hours.
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 varied areas, such as in construction, plant design, and interior decoration. A number of quantitative assessments of flammability exist, such as limiting oxygen index (LOI), i.e., the minimum oxygen concentration at which a given sample can be induced to burn in a candle like configuration. These permit estimation of a 'ranking' comparison of flammability. Data shows that PBI is a highly flame resistant material compared to common polymers.
Moisture regain PBI's moisture regain is useful in protective clothing; this makes the clothing comfortable to wear, in sharp contrast to other synthetic polymers. The moisture regain ability of PBI (13%) compares favorably with cotton (16%).
Synthesis The preparation of PBI(IV) can be achieved by condensation reaction of diphenyl isophthalate (I) and 3,3’,4,4’-tetraaminodiphenyl (II) (Figure 1). The spontaneous cyclization of the intermediately formed amino-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 synthesis, starting materials were heated at for 1.5 h to form the PBI prepolymer and later the prepolymer was heated at 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 that must be considered by the industrial manufacturers. This foam can be reduced by conducting the polycondensation at a high temperature around and under the pressure of . The foam can also be controlled by adding high boiling point liquids such as diphenylether or cetane to the polycondensation. The boiling point can make the liquid stay in the first stage of polycondensation but evaporate in the second stage of solid condensation. The disadvantage of this method is that there are still some liquids which remain in the 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 some of the combination possibilities that have been synthesized in the literature. Some of the combinations have actually been translated into fibers on a small scale. However, the only significant progress that has been made to date is with PBI.
R(Tetramine) R'(acid)) Benzene Benzene Diphenyl Diphenyl Diphenylether Diphenylether Diphenylsulfone Naphthalene Naphthalene Pyridine Pyridine Anthraquinone Anthraquinone Ferrocene Anthracene+ Table 1. Other monomers which form derivatives of polybenzimidazole 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 the 1980s, the major applications of PBI were fire-blocking, thermal protective apparel, and reverse osmosis membranes. Its applications became various by the 1990s when molded PBI parts and microporous membranes were developed.
Protective apparel The thermal stability, flame resistance, and moisture regain of PBI and its conventional textile processing character enable it to be processed on conventional staple fiber textile equipment. These characteristics lead to one of the most important applications of PBI: protective apparel. PBI filaments were fabricated into protective clothing like firefighters' gear and 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 rescue gear, industrial worker's apparel, and suits for racing car drivers.
PBI-blended fabrics have been the preferred choice of active fire departments across the Americas and around the world for over 30 years. From New York, San Diego, San Francisco, Philadelphia, Seattle, Nashville to São Paulo, Belin, Hong Kong and many more. The high decomposition temperature at which PBI starts to degrade is , exceeding Nomex/Kevlar blends (Nomex being at and Kevlar at ), thus offering superior break-open and thermal protection.
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. PBI has also been 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 at the same time. The optimum doping level is thus a compromise between these two effects. Thus, multiple methods such as ionic cross-linking, covalent cross-linking and composite membranes have been researched to optimize the doping level at which PBI has an improved conductivity without sacrificing mechanical strength. Sulfonated partially fluorinated arylene main chain polymer exhibit good thermal and extended stability, high proton conductivities, less acid swelling, reasonable mechanical strength.
Molded PBI resin Molded PBI resin has compressive strength and a tensile strength of , a flexural strength of , a ductile compressive failure mode and the density of 1.3 g/cm3. The PBI resin comprises a recurring structural unit represented by the following figure.