TABLE OF CONTENTS Considerable progress has been made in improving the thermal and oxidative stabilities of organic resins at high temperatures. Heat-resistant resins and polymers have been developed as adhesives to meet the needs of the aircraft industry (supersonic aircraft) and space vehicles (missiles, satellites, rockets), where resistance to temperatures approaching 316 C is required throughout the life of bonded assemblies based on metals and reinforced plastic composites. The oxidative stability of organic polymers is improved by the incorporation of aromatic and heterocyclic rings (such as imide, imidazole, and thiazole) into the molecules of the polymer.
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.[6]
The high-temperature adhesives are available in film and tape form. They show better bond strengths above 260 C in air than epoxy-phenolic, although the latter gives better strength retention after exposure to water or other polar liquids at lower temperatures. The major disadvantages are their high cost, generally ten or more times that of epoxy-based adhesives, the difficulty in handling or curing, and the problems involved in the elimination of volatiles during cure in order to obtain a void-free bond. A long and careful series of cure and...
