High-temperature adhesives polybenzimidazoles

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

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... 

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. 

Suitable for alloys to be bonded with high-temperature adhesives (e.g., polybenzimidazole). Bond within 10 min of treatment. ASTM D 2651... 

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)... 

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. 

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.)... 

Polybenzimidazoles S J SHAW Structure use as high-temperature adhesive... 

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. 

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. 

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. 

Materials that come close to the ideal ladder structure are thermosetting polyimides and polybenzimidazoles. These are used primarily in high-temperature aerospace applications as composites and adhesives. 

The use of adhesives can be traced back many centuries, while the production of adhesives, on an industrial scale, started about 300 years ago. The birth of modern structural adhesives can be dated from about 1910, with the introduction of the phenol-formaldehyde resins.Table II summarizes the historical development of structural adhesives, with the dates referring to the approximate time period during which each adhesive became commercially available. The introduction dates for the high-temperature polymers (polyimide, polybenzimidazole, and polyquinoxa-line) have been included for reference, although, as previously mentioned, there are presently few commercial products based on these polymers. 

Aromatic polybenzimidazoles were synthesized by H. Vogel and C. S. Marvel in 1951 with anticipation, later justified, that the polymers would have exceptional thermal and oxidative stability. Subsequently, NASA and the Air Force Materials Laboratory (AFML) sponsored considerable work with polybenzimidazoles for aerospace and defense applications as a non-flammable and thermally stable textile fiber and as high temperature matrix resins, adhesives and foams. The route to fiber used solutions of high molecular weight polymer. Structural applications used low temperature melting pre-polymers that were cured (polymerized) in place. Applications of polybenzimidazoles were not implemented in the 60 s and 70 s since the polymers tetraamine precursors were not commercially available. 

Polybenzimidazoles are of interest chiefly because of the maintenance of physical properties at elevated temperatures. Prolonged heating in air at temperatures up to about 250°C leads to little change in properties above 250°C, oxidative degradation results in gradual deterioration. Polybenzimidazoles have been employed in laminates and adhesives for use at high temperatures, mainly in the aerospace field. Wider development of these polymers has been curtailed by their difficult processing requirements. 

Polyimides and Polybenzimidazoles [53]. The highly temperature resistant polyaromatic adhesive resins of the polyimide and polybenzimidazole type are applied as precondensates in the form of solutions or films and are condensed to completion at 230-250°C under a relatively high pressure of 8-10 bar. The precondensates remain stable for only a few hours at room temperature and have to be stored at -18 °C. 

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. 

Several specialised adhesives are available for extreme high-temperature service. These include epoxy phenolic, nitrile phenolic, quinoxaline [1] bismaleimides [2-4], polyimide [5, 6], and polybenzimidazoles [7]. They are used mainly in aerospace applications [8]. 

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