Polybenzimidazoles, manufacture

PBI is being marketed as a replacement for asbestos and as a high temperature filtration fabric with exceUent textile apparel properties. The synthesis of whoUy aromatic polybenzimidazoles with improved thermal stabUities was reported in 1961 (12). The Non-MetaUic Materials and Manufacturing Technology Division of the U.S. Air Force Materials Laboratory, Wright-Patterson Air Force Base, awarded a contract to the Narmco Research and Development Division of the Whittaker Corp. for development of these materials into high temperature adhesives and laminates. 

Other membrane materials include mainly polyimide, polyacrylonitrile and polybenzimidazole. An overview of commercially available membranes is given in Table 3.2. These membranes are manufactured in procedures usually derived from practical experience by using high-throughput screening, it was shown that optimization is possible [26]. Many other membrane materials are described in the scientific literature and in patents an overview is given by Cuperus and Ebert [27]. 

Electro-insulation materials. The retention of dielectric properties in a high-temperature environment, coupled with good corrosion resistance in contact with certain reactive chemicals, suggests excellent possibilities of polybenzimidazole use in electrical insulation and other dielectric applications at high operating temperatures and/or in aggressive chemical environments. Typical applications, hence, can be foimd in special cable and wire insulation, in the manufacture of circuit boards and radomes for supersonic aircraft, as battery and electrolytic cell separators, and as fuel cell frame structural materials. Some recent publications in the patent and technical report literature may serve to illustrate such applications. 

Use Manufacture of polybenzimidazoles, high-temperature-resistant polymers. 

Polybenzimidazole (PBI) (initially manufactured by Hoechst-Celanese, now PE ME A) is one of the few polymers under consideration for high-temperature operation. The application of PBI [206, 207] and the noncommercial AB-PBI [208] in fuel cells was introduced by Savinell and coworkers. For that, the membrane was immersed in concentrated phosphoric acid to reach the needed proton conductivity. Operation up to 200 °C is reported [209]. A disadvantage of this class of membranes is the acid leaching out during operation, particularly problematic for cells directly fed with liquid fuels. Additionally, the phosphoric acid may adsorb on the platinum surface. A review on membranes for fuel cells operating above 100 °C has been recently published [209]. 

A combination of phosphoric acid and polybenzimidazole (PBI) is currently the most interesting material for HT-PEFC membranes. PBI membranes doped with phosphoric acid can be manufactured in a synthesis process using different methods. The basic difference between them lies in whether doping with phosphoric acid is part of polycondensation, that is, whether it takes place in situ, or whether doping takes place by soaking the PBI foil in phosphoric acid, or whether it is affected via the gas diffusion layer (GDL) or the catalyst. The polycondensation method was developed and patented by BASF, which is currently the only company manufacturing membranes in this way. 

The general process used to synthesise aromatic polybenzimidazoles (PBIs) is presented in Section 4.3.3. More detailed information can be found in previously published books [87,88]. During 1960-1970 a number of publications, comparable to those on polyimides, reported the synthesis and properties of all aromatic and aryl-aliphatic polybenzimidazoles. Most of these polymers were prepared by the two-step process illustrated in Fig. 14 with the reaction of 1,3-benzenedicar-boxylic acid diphenyl ester 27 and [l,l -biphenyl]-3,3, 4,4 -tetramine 11 yielding ultimately PBI 29. All the applications - laminates and filament winding resins, adhesives, fibres and foams - used polymer 29, which was produced in semicommercial quantities by the Whittaker Corporation (Narmco Division) under the generic trade mark Imidite . Currently, forty years later, this polymer is manufactured by Hoechst-Celanese and its only commercial success is in the area of heat resistant fibres and fabrics. It is, however, worth noting the adhesive properties of this polymer and the reasons explaining the major obstacles to the development of PBIs as heat-resistant adhesives. 

Despite the copious evolution of phenol and water during processing, Narmco s commercial adhesive system (Imidite 850) was based on prepolymer 28 mixed with aluminium powder, inorganic arsenic compounds such as arsenic thioarsen-ate or arsenic pentasulphide, and sterically hindered polyphenol antioxidant. Adhesive tapes were manufactured hy hot-melt deposition of this mixture on a 112-type glass cloth carrier used in conjunction with a heat-resistant primer (polybenzimidazole or polyimide). As with all other heat resistant polymers, surface preparation of flat and honeycomb metal surfaces is an important factor in... 

The screen printing process is the most used process to manufacture gas diffusion electrodes for HT- and LT-PEM. This process works in a very efficient way. Here the membrane of a fuel cell, for HT-PEM it s usually a polybenzimidazole and for LT-PEM a Nafion-based membrane, is fixed in its undoped form in a holder. After that a catalyst slurry, what is an electrode ink mixed with catalyst, is getting sprayed on the membrane and afterwards dried. The slurry is usually consisting of binder materials like poly (1,1,2,2-tetrafluoroethylene) and newest more often poly-l,l-difluoroethene, catalyst-loaded support material what is most likely carbon, for example, and dispersing agents like water and alcohols. 

The sandwich construction method mentioned above involves joining metals to other materials by bonding. Further examples include the bonding of brake linings (phenolic adhesives) and compound materials in ski manufacture, where aluminum is bonded to plastics, wood, etc. (phenolic and epoxy adhesives). Highly alloyed steels, beryllium and titanium alloys, and other special metals can be bonded with adhesives (e.g., polyi-mides, polybenzimidazoles) that have comparable high-temperature resistance. 

In the case study carried out by the author, polybenzimidazole (FBI) sheets of service temperature ranging from —260°C to +500° C, tensile strength of 160 MFa, and density of 1.3 g/cm, as reported by the manufacturer, Boedeker Flastics, Inc., and ultrahigh temperature-resistant epoxy adhesive, DURALCO 4703, manufactured by Cotronics Corp. Brooklyn, NY, USA, of service temperature ranging from —260°C to +370°C were used... 

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