Polybenzimidazoles

General structural element of polybenzimidazoles

Benzimidazole units are formed in the main chain during production .

Polybenzimidazoles ( abbreviation PBI ) are aromatic polymers with benzimidazole units in the main chains. Polybenzimidazoles have a very high glass transition temperature and are not flammable.

history

Polybenzimidazole was synthesized by Carl S. Marvel in the late 1950s while doing research for the US Air Force . In 1961 Carl Marvel and Herward Vogel developed PBI further and in 1962 applied for a patent on polybenzimidazole and its production via the University of Illinois, which was granted in 1965.

In 1963, NASA and Air Force continued to fund the development of PBI fibers, as they are non-flammable (according to ISO 13943: 2017: Fire Safety-Vocabulary) and so temperature-stable that they can be used in space and military applications. To this end, NASA signed a contract with Celanese Corp., New York, for the production of PBI fibers and products made from them. The production line opened in 1983 at a facility in Rock Hill, South Carolina. Shortly after its market launch in 1983, PBI was used as the outer fabric of fire service suits and for protective hoods. Celanese Corp. sold fiber and polymer manufacturing to PBI Performance Products Inc. in 2005, which belongs to the InterTech Group and is still the only manufacturer of PBI fibers and polymers worldwide.

Manufacturing

PBI is produced in a polycondensation, for example, from 3,3'-diaminobenzidine and diphenyl isophatalate. The reaction takes place in two steps: The first step is formed by heating to 300 ° C an even-soluble polyamide . After processing z. B. in fibers takes place in the second step at 350 to 400 ° C, a further condensation with the formation of the no longer soluble polybenzimidazole, which is included in the aromatic polyimides. The glass temperature is 425 ° C.

properties

Residual strength of various fibers

Polybenzimidazole is a polymer that consists entirely of aromatic monomer units and has a very high melting point. PBI is firm, hard, pressure-resistant and quickly recovers from the effects of high pressure. Polybenzimidazole has good resistance under highly chemically stressful conditions, but is attacked by polar aprotic solvents and, at higher temperatures, by strong, water-based acids and, to a lesser extent, by alkalis. Although polybenzimidazole still absorbs a high percentage of water even at the saturation point, it is stable to hydrolysis . The thermal expansion value is similar to that of aluminum .

The PBI fibers have a fineness-related dry tensile strength of 24 cN / tex and a dry elongation at break of 27% as well as an initial modulus (E modulus) of 400 cN / tex. The moisture absorption is 15%, which contributes to the good wearing comfort of the items of clothing made from these fibers. The high thermal stability and flame retardancy of the PBI fibers (LOI> 41) should be emphasized. The fiber does not burn in air, does not melt and retains strength and flexibility without embrittlement even in extreme heat and develops little smoke or toxic gases. After an exposure time of 60 minutes at 300 ° C, PBI fibers have 100% of their original strength. The PBI staple fibers are produced in a fineness of 1.7 dtex and with standard cutting lengths of 3, 6, 38, 50, 76 and 100 mm.

Applications

In addition to the use of PBI fibers in fire protection clothing, the material is also used as a high-temperature membrane for polymer electrolyte fuel cells (PEFC). It serves as a matrix for phosphoric acid , which ensures proton conduction.

Individual evidence

1. ↑ U.S. Patent Polybenzimidazoles And Their Preparation
2. ↑ NASA: Polymer Fabric Protects Firefighters, Military, and Civilians (accessed May 18, 2020).
3. ↑ PBI: [1] (accessed on May 18, 2020)
4. ↑ Bernd Tieke: Makromolekulare Chemie , 3rd edition, Wiley-VCH, Weinheim, 2014, p. 35.
5. ^ Karl Oberbach (Ed.): Saechtling plastic pocket book. Carl Hanser Verlag, Munich / Vienna 2004, ISBN 3-446-22670-2 , pp. 516/523.
6. ↑ [2] Properties of PBI staple fibers.
7. ↑ Walter Loy: Chemical fibers for technical textile products. 2nd, fundamental revised and expanded edition. Deutscher Fachverlag, Frankfurt am Main 2008, ISBN 978-3-86641-197-5 , p. 110.
8. ↑ PBI protective clothing