 | |
| Identifiers | |
|---|---|
| ChemSpider |
|
| Properties | |
| (C20H12N4)n | |
| Molar mass | Variable 308.34 (repeating unit) |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Polybenzimidazole (PBI, short forpoly[2,2’-(m-phenylen)-5,5’-bisbenzimidazole]) fiber is asynthetic fiber with a very high decomposition temperature. It does not exhibit amelting point, it has exceptional thermal and chemical stability, and it does not readily ignite.[1] It was first discovered in 1961, by American polymer chemistCarl Shipp Marvel in the pursuit of new materials with superior stability, retention ofstiffness, and toughness at elevated temperature. Due to its high stability, polybenzimidazole is used to fabricate high-performance protective apparel such asfirefighter's gear,astronaut space suits, high temperature protective gloves, welders’ apparel and aircraft wall fabrics. Polybenzimidazole has been applied as amembrane in fuel cells.
Brinker and Robinson first reported aliphatic polybenzimidazoles in 1949.[2] However the discovery of aromatic polybenzimidazole, which shows excellent physical and chemical properties, was generally credited toCarl Shipp Marvel in the 1950s.[3] The Material Laboratory ofWright Patterson Air Force Base approached Marvel. They were looking for materials suitable fordrogue 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 theamide linkage proved to be weak link for the aim of maximalthermal stability of the polymer, whereas Marvel's research focused oncondensation polymers with aromatic andheteroaromaticrepeating units. This progressively led to the discovery of polybenzimidazole.
Its development history can be summarized in the following list:[4]
PBI are usually yellow to brown solid infusible up to 400 °C (752 °F) or higher.[7] Thesolubility of PBI is controversial, because while most of the linear PBI are partly or entirely dissolved in strong protonic acids (for instance,sulfuric acid ormethanesulfonic 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.[8] to be partially soluble informic acid, but completely soluble indimethyl sulfoxide anddimethylacetamide, whereas Varma and Veena[9] reported the same polymer type to dissolve completely informic acid, yet only partially indimethyl sulfoxide ordimethylacetamide.
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 itsaromatic derivatives can withstand temperatures in excess of about 500 °C (932 °F) without softening and degrading. The polymer synthesized fromisophthalic acid and3,3'-Diaminobenzidine is not melted by exposure to a temperature of 770 °C (1,420 °F) and loses only 30% of its weight after exposure to high temperature up to 900 °C (1,650 °F) for several hours.[10]
A property of a material needed to be considered before putting it into application isflammability, 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 aslimiting 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 offlammability. Data shows that PBI is a highly flame resistant material compared to common polymers.[11]
PBI'smoisture 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%).[12]
The preparation of PBI(IV) can be achieved bycondensation 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 270 °C (518 °F) for 1.5 h to form the PBI prepolymer and later the prepolymer was heated at 360 °C (680 °F) 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,[13] 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 (392 °F) and under the pressure of 2.1–4.2 MPa (300–610 psi).[14] 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.[14]
While changing the tetramine and acid, a number of different aromatic poly benzimidazoles have been synthesized. The following table (Table 1)[15] 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 |
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.
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.
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,[16] aluminized crash rescue gear, industrial worker's apparel, and suits for racing car drivers.[17]
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 1,300 °F (704 °C), exceeding Nomex/Kevlar blends (Nomex being at 700 °F (371 °C) and Kevlar at 1,100 °F (593 °C)), thus offering superior break-open and thermal protection.
PBI has been used as the membranes for various separation purposes. Traditionally, PBI was used semi-permeable membranes for electrodialysis, reverse osmosis or ultrafiltration.[18] PBI has also been used for gas separations.[19][20] 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[18] 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.[21]
Molded PBIresin has compressivestrength 58 ksi (400 MPa) and a tensile strength of 23 ksi (160 MPa), a flexural strength of 32 ksi (220 MPa), a ductile compressive failure mode and thedensity of 1.3 g/cm3.[22] The PBI resin comprises a recurring structural unit represented by the following figure.
According to the Composite Materials Research Group at theUniversity of Wyoming, PBI resin parts maintain significanttensile properties andcompressive strength to 700 °F (371 °C). PBI resin parts are also potential materials for the chemical process and oil recovery industries which have demands ofthermal stability andchemical 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 electricalinsulator.[17]
Polybenzimidazole is able to be complexed bystrong acids because of its basic character. Complexation byphosphoric acid makes it a protonconductive material.[23] 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 (302 °F). However, gel PBI membranes made in the PPA Process show good stability for greater than 17,000 hours at 160 °C (320 °F).[24] Application in directmethanolfuel 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 withphosphoric acid was utilized as a high temperaturefuel cell electrolyte.[25] The doped PBI high temperaturefuel cell electrolyte has several advantages. The elevated temperature increases the kinetic rates of the fuel cell reactions. It also can reduce the problem of thecatalyst poisoning by adsorbedcarbon monoxide and it minimizes problems due to electrode flooding.[23] PBI/H3PO4 is conductive even in low relative humidity and it allows less crossover of the methanol at the same time.[26] These contribute PBI/H3PO4 to be superior to some traditional polymer electrolytes such asNafion. Additionally, PBI/H3PO4 maintains good mechanical strength and toughness.[26] Its modulus is three order of magnitudes greater than that ofNafion.[27] This means that the thinner films can be used, thus reducing ohmic loss.
Previously, onlyasbestos could perform well in high-temperature gloves for uses such as foundries,aluminium extrusion, and metal treatment. However, trials have been performed which show that PBI adequately functions as an asbestos replacement. Moreover, a safety garment manufacturer reported that gloves containing PBI outlastedasbestos by two to nine times with an effective cost.[28] Gloves containing PBI fibers are softer and more supple than those made ofasbestos, offering the worker greater mobility and comfort, even if thefabric becomes charred.[dubious –discuss] Further, PBI fiber avoids thechronic toxicity problems associated withasbestos because it is processed on standardtextile and glove fabricating equipment.[29] PBI can also be a good substitute for asbestos in several areas of glass manufacturing.
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.[30] 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.
{{cite book}}:|first= has generic name (help)CS1 maint: multiple names: authors list (link)The chemical formula of poly[2,2’-(m-phenylen)-5,5’ bibenzimidazol] (PBI) is believed to be:([NH-C=CH-C=CH-CH=C-N=C-]2-[C=CH-C=CH-CH=CH-])n OR (C20N4H12)n ofMolar mass 308.336 ± 0.018 g/mol.[citation needed]
| Chemical Resistance | Grade |
|---|---|
| Acids - concentrated | Poor |
| Acids - dilute | Fair-Poor |
| Alcohols | Good |
| Alkalis | Good-Poor |
| Aromatic hydrocarbons | Good |
| Greases and Oils | Good |
| Halogenated Hydrocarbons | Good |
| Ketones | Good |
It is dyeable to dark shades with basic dyes following caustic pretreatment and resistant to most chemicals.
| Electrical | Properties |
|---|---|
| Dielectric constant @ 1 MHz | 3.2 |
| Dielectric strength | 21 kV·mm−1 |
| Volume resistivity | 8x1014 Ω·cm |
Features low electrical conductivity and low static electricity buildup.
| Mechanical | Properties |
|---|---|
| Coefficient of friction | 0.19-0.27 |
| Compressive modulus | 6.2 GPa |
| Compressive strength | 400 MPa |
| Elongation at break | 3% |
| Hardness - Rockwell | K115 |
| Izod impact strength | 590 J·m−1 unnotched |
| Poisson's ratio | 0.34 |
| Tensile modulus | 5.9 GPa |
| Tensile strength | 160 MPa |
Features abrasion resistance.
| Physical | Properties |
|---|---|
| Char Yield (underpyrolysis) | High |
| Density | 1.3 g/cm3 |
| Flammability | Does not burn |
| Limiting oxygen index | 58% |
| Radiation resistance | Good |
| Water absorption - over 24 hours | 0.4% |
Additional features: will not ignite or smolder (burn slowly without flame),mildew- and age-resistant, resistant to sparks and welding spatter.
| Thermal | Properties | Grade |
|---|---|---|
| Coefficient of thermal expansion | 23×10−6·K−1 | Low |
| Heat-deflection temperature - 0.45 MPa | 435 °C (815 °F) | High |
| Thermal conductivity @ 23 °C (73 °F) | 0.41 W·m−1·K−1 | Low |
| Upper working temperature | 260–400 °C (500–752 °F) | High |
Other features: continuous temperature: 540 °C (1,004 °F), does not melt but degrades around the temperature: 760 °C (1,400 °F) under pyrolysis, retains fiber integrity and suppleness up to 540 °C (1,004 °F).