Aromatic tetraamines

V.V. Korshak, A.L. Rusanov, Z.B. Shifrina, Y.N. Margalitadze, S.R. Fidler, B.R. Livshits u. T. V. Lekae, Dokl. Akad. Nauk SSSR 289,367 New-Type Aromatic Tetraamines and Oligoimides Based on Them". 

These plastics, also known as pyrrones, are experimental materials prepared from aromatic dianhydrides and aromatic tetraamines. The polymer syntheses provide soluble prepolymers that are converted to the pyrrone structures by thermal or chemical dehydration. The precursors can be used to cast films or coatings, or can be molded under very high pressures into filled or unfilled forms. The pyrrones combine some of the best properties of the polybenzimidazoles and polyimides. The pyrrone films are exceptionally radiation resistant and retain their strength properties after 10,000 megarads of 1-MeV electrons. 

Polybenzimidazoles are synthesized by the reactions of aromatic diacids and aromatic tetraamines (Figure 1.36). Polybenzimidazoles are mostly used as fibers in parachutes, for reentry vehicles, and so on. Composites of polybenzimidazoles with fibers have excellent basic strength and high-temperatme performance. These composites have been extensively used in nose fairings, aircraft leading edges, reentry nose cones, radomes, and deicer ducts. 

A process that involves the solution polymerization of an inorganic acid salt of an aromatic tetraamine and a dicarboxylic acid or a derivative thereof with heat in polyCphosphoric acid) [18] has been described. 

Although in certain respects the polybenzimidazoles offer a virtually unique combination of properties, they have not enjoyed the success of other High-temperature adhesives. A major reason for this has undoubtedly been due to the monomeric materials required, most notably aromatic tetraamines, being both costly and difficult to obtain in the required purity. In addition, doubts concerning carcinogenic activity have also been expressed (see Health and safety), which, together with the adverse processability mentioned above has severely restricted their acceptance. For these reasons, commercial availability has to date been somewhat limited. 

To conclude this brief overview of the polybenzimidazole adhesives, it is quite clear from the number of test results that PBI cannot compete with polyimides. The major obstacle to the development of Imidite 850 has been its poor thermal-oxidative stability at temperatures greater than 250°C. In addition, the dependence on the use of aromatic tetraamines has introduced two other negative parameters, that is, the cost of these co-monomers and their carcinogenic properties. Furthermore, arsenic derivatives have been forbidden in adhesive composition and unsuccessfully replaced by boron powder which is not as good at preventing air oxidation of joined stainless steel alloys. Table 11 provides some key data to... 

Polybenzimidazoles are synthesized from aromatic tetraamines (bis-o-diamines) and dicarboxylates (acids, esters or amides). Generalized monomers are shown in FIGURE 1 and polybenzimidazoles from those monomers are shown in FIGURE 2. In either figure, R(l) and R(2) can be aromatic or aliphatic and may contain other groups as ether, ketone, sulfone, etc. The structures shown do not exhaust all possible benzimidazole polymers. 

Polybenzimidazoles became commercially available in about 1965 and are prepared by reaction of aromatic tetraamines and dicarboxylic compounds. As with polyimides, emphasis has been on fully aromatic polyimidazoles since these show outstanding thermal stability. The polymer which has been preferred for commercial development is that based on 3,3 -diaminobenzidine and diphenyl isophthalate and the reaction between these compounds will illustrate the general method of preparation of polybenzimidazoles ... 

Polybenzimidazoles may be regarded as derivatives of aromatic polyamides and are thus conveniently considered in this chapter. They are prepared by reaction of aromatic tetraamines and dicarboxylic compounds. 

FIGURE 8.7 Polycondensation of aromatic tetraamines with aromatic dialdehydes. (Reprinted with modification from Neuse, E.W. and Loonat, M.S., Macromolecules, 16,128, 1983. With permission.)... 

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