Polybenzimidazole adhesives

Processing is normally carried out in a preheated press at 370 °C with pressure maintained at 0.03 MPa for 30 s. The pressure is then increased to 0.6—1.4 MPa, and the glue line temperature is maintained at 370 °C for 3 h. The temperature is then reduced to 260 °C or less, and the assembly is removed from the press. Autoclave techniques can also be employed. For improved mechanical properties, postcuring in an inert atmosphere (nitrogen, helium, or vacuum oven) is recommended. The desirable conditions are 24 h each at 316 °C, 345 °C, 370 C, and 400 °C followed by 8 h at 427 °C in air to achieve maximum properties. Obviously, these literature recommendations should be checked against manufacturers recommendations, but they provide a starting point for PBI processing. 

PBI adhesives have good resistance to salt spray, 100% humidity, aromatic fuels, hydrocarbons, and hydraulic oils. About 30% loss of strength occurs after exposure to boiUng water for 2 h. Electrical properties are fairly constant throughout the temperature range, up to 200 °C. Thermal stability at high temperatures for short periods is satisfactory, such as exposure at 540 °C for 10 min or at 260 °C for 1000 h. The useful service-temperature range as adhesives 

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Similarly, Polybenzimidazole adhesives also offer excellent high-temperature capabilities for short-term use, with the ability to retain 50% of room temperature strength at 450 °C. Unfortunately, as with the epoxy-phenolics, this capability is not maintained under long-term, high-temperature conditions because of the susceptibility of Polybenzimida-zoles to oxidative degradation at temperatures in excess of 250 °C. 

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... 

Iqbal HMS, Bhowmik S, Benedictus R (2014) Process optimization of solvent based polybenzimidazole adhesive for aerospace applications. Int J Adhes Adhes 48 188-193... 

Suitable for alloys to be bonded with polybenzimidazole adhesives. Bond within 10 min of treatment. A8TM D 2651... 

Heat-resistant polyaromatic adhesives also have shown promising low-temperature properties. The shear strength of a polybenzimidazole adhesive on stainless-steel substrates is 5,690 Ib/in at a test temperature of 23°F, and polyimide adhesives have exhibited shear strength of 4,100 Ib/in at -320°F. These unique properties show the applicability of polyaromatic adhesives on structures seeing both very high and low temperatures. 

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. 

If one amino group in o-phenylenediamine is converted to an amide group by formic acid, the intermediate benzimidazole is formed. This reaction, conducted with a wide range of reactants, produces resins (polybenzimidazoles) used as high-temperature adhesives for laminates in the aerospace industry. Heat insulation is made by including tiny bubbles of silica and all... 

Much attention has been paid to the synthesis of fluorine-containing condensation polymers because of their unique properties (43) and different classes of polymers including polyethers, polyesters, polycarbonates, polyamides, polyurethanes, polyimides, polybenzimidazoles, and epoxy prepolymers containing pendent or backbone-incorporated bis-trifluoromethyl groups have been developed. These polymers exhibit promise as film formers, gas separation membranes, seals, soluble polymers, coatings, adhesives, and in other high temperature applications (103,104). Such polymers show increased solubility, glass-transition temperature, flame resistance, thermal stability, oxidation and environmental stability, decreased color, crystallinity, dielectric constant, and water absorption. 

Epoxy Coreactants. One of the most successful epoxy coreactant systems developed thus far is an epoxy-phenolic alloy. The excellent thermal stability of the phenolic resins is coupled with the valuable adhesion properties of epoxies to provide an adhesive capable of 371°C short-term operation and continuous use at 175°C. The heat resistance and thermal-aging properties of an epoxy phenolic adhesive are compared with those of other high-temperature adhesives in Fig. 15.5. Epoxy-phenolic adhesives are generally preferred over other high-temperature adhesives, such as the polyimides and polybenzimidazoles, because of their lower cost and ease of processing. 

As a result, extensive research has been underway for over two decades in search of structural adhesives which have the requisite balance of processability, properties and cost effectiveness. The most promising materials have been derived from polymers containing various aromatic and heteroaromatic rings including, among others, various polyimides, polybenzimidazoles and polyquinoxalines. 

One of the first all-aromatic heterocyclic polymers to undergo development as a high temperature adhesive was a polybenzimidazole (PBI). In 1961, Vogel and Marvel (1)... 

The most important resins available for use as adhesives in high-temperature structural applications are polyimides (Pis) and polybenzimidazoles (PBIs), both of which are described later (see Sections 5.35 and 5.33). These resins are supplied as prepolymers containing open heterocyclic rings, which are soluble and fusible. At elevated temperatures, the prepolymers undergo condensation reactions leading to ring closure and the formation of insoluble and infusible cured resins. 

Figure 5.5 Performance of high-temperature adhesives (polybenzimidazole and polyimide) at 371°C. ...

The polyaromatic resins, polyimide and polybenzimidazole, offer greater thermal resistance than any other commercially available adhesive. The rigidity of their molecular chains decreases the possibility of chain scission caused by thermally agitated chemical bonds. The aromaticity of the... 

The traditional Structural adhesives capable of operating at temperatures in excess of 150 °C for both short- and long-term applications can be divided into three classes, namely (1) phenolics, (2) polybenzimidazoles, (3) condensation polyimides (see Phenolic adhesives single-stage resoles. Phenolic adhesives two-stage novolacs, Polyhenzimida-zoles and Polyimide adhesives). 

A serious limitation to the use of organic polymers in general and of adhesives, in particular, is their poor resistance to thermal degradation. Considerable effort has been put into the development of High-temperature adhesives and examples of the materials that have been produced are described in articles on Polybenzimidazoles, Polyether ether ketone, Polyimide adhesives and Polyphenylquinoxalines. Some of the general principles used in the search for enhanced thermal stability are discussed in this article. 

Initially developed in the late 1950s and early 1960s, polybenzimidazoles are prepared by reaction of tetrafunctional aromatic amines with aromatic esters (Fig. 1). They find application as High-temperature adhesives. 

Since high-molecular-weight polybenzimidazoles exhibit, depending upon structure, fair to high levels of intractability, adhesive formulations based on them generally employ the... 

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. 

Further discussion on the general theme of high-temperature polymers and adhesives can be found in the articles entitled Polyphenylquinoxalines, Polybenzimidazoles and Polyether ether ketones. (See also article on High-temperature adhesives.)... 

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