Polybenzimidazole fiber


Polybenzimidazole fiber is a synthetic fiber with a very high decomposition temperature and doesn't exhibit a 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. 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 show excellent physical and chemical properties was generally credited to Carl Shipp Marvel in the 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 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:

General physical properties

PBI are usually yellow to brown solid infusible up to 400 °C or higher. The solubility of PBI is controversial, because while most of the linear PBI are partly or entirely dissolved in strong protonic acids, contradictory observations of solubities 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

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 can withstand temperatures in excess of about 500°C without softening and degrading. The polymer synthesized from isophthalic acid and 3,3'-diaminobenzidine is not melted by exposure to a temperature of 770°C and loses only 30% of its weight after exposure to high temperature up to 900°C 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, 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 show that PBI are 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 compares favorably with cotton.

Synthesis

The preparation of PBI can be achieved by condensation reaction of diphenyl isophthalate and 3,3’,4,4’-tetraaminodiphenyl . The spontaneous cyclization of the intermediately formed animo-amide to PBI 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°C for 1.5 h to form the PBI prepolymer and later the prepolymer was heated at 360°C 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 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 that there are still some liquids which 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 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 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 rescued 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. Recognized as the leading brand of high-performance protective fabrics, PBI outer shells are selected when fire departments seek increased thermal protection and durability. The high decomposition temperature at which PBI starts to degrade is 1300 °F, far exceeding Nomex/Kevlar blends, thus offering superior break-open and thermal protection. That's the reason why more firefighters in North America are protected by PBI fabrics than any other fabrics and they hold leading market shares in North-America, Great Britain, Europe, Australia, New Zealand and the Asia-Pacific region.

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 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, covlant 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

PBI resin is molded via a sintering process that was jointly developed by Hoechst Celanese and Alpha Precision Plastics, Inc.. 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 density of 1.3 g/cm3. Moreover, its thermal and electrical properties also make it a well known 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, PBI resin parts maintain significant tensile properties and compressive strength to. PBI resin 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, PBI resin has been successfully applied in demanding sealing, for instance, valve seats, stem seals, hydraulic seals and backup rings. In the aerospace industry, PBI resin has high strength and short term high temperature resistance advantages. In the industrial sector, PBI resin's high dimensional stability as well as retention of electrical properties at high temperature make it used as a thermal and electrical insulator.

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°C. However, gel PBI membranes made in the PPA Process show good stability for greater than 17,000 hours at 160°C. 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 a high temperature fuel cell electrolyte. The doped PBI high temperature fuel cell electrolyte has several advantages. The elevated 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 mechanical strength and toughness. Its modulus is three order of magnitudes greater than that of Nafion. This means that the thinner films can be used, thus reducing ohmic loss.

Asbestos replacement

Previously, only asbestos can perform well 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 an 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.

Appendix of properties

PBI fiber characteristics

The chemical formula of poly is believed to be:
n OR n of Molar mass 308.336 ± 0.018 g/mol.

Chemical resistance

It is dyeable to dark shades with basic dyes following caustic pretreatment and resistant to most chemicals.

Electrical properties

Features low electrical conductivity and low static electricity buildup.

Mechanical properties

Features abrasion resistance.

Physical Properties

Additional features: will not ignite or smolder, mildew- and age-resistant, resistant to sparks and welding spatter.

Thermal Properties

Other features: continuous temperature:, does not melt but degrades around the temperature: under pyrolysis, retains fiber integrity and suppleness up to.