page_title: Polybenzimidazole fiber
section_title: Synthesis
hierarchical_section_title: Polybenzimidazole fiber / Synthesis
caption_attribution_description: English: The PBI fiber is made in a serial of steps following polymerization to get PBI staple form for direct usage.
context_page_description: 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.
context_section_description: 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 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°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 (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.
caption: The PBI fiber is made in a serial of steps following polymerization to get PBI staple form for direct usage.
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