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5 STATE OF THE ART: CHEMISTRY New adhesive materials designed to meet the demands for use in severe environments and for long service life have recently been reviewed [Lee 1984]. The specific environments of concern are extreme high temperatures, extreme low temperatures, extreme high humidity, seawater, and others, including fire and corrosive gases and liquids. EXTREME HIGH TEMPERATURES During the past decade, many high-temperature adhesives have been synthesized for aerospace applications. Adhesives for supersonic cruise aircraft must be durable for tens of thousands of cumulative hours at 232Â°C (500Â°F), while spacecraft adhesives must withstand hundreds of hours at 316Â°C (600Â°F). Missile adhesives must survive for less than 1 minute at temperatures greater than 538Â°C (1000Â°F) [Lee 1984]. Besides thermal stability, high-temperature adhesive materials should exhibit processability at approximately 100Â°C above their Tg. Yet many high-temperature polymers such as polybenzoxazoles (PBOs) or polybenzothiazoles (PBTs) are difficult to process. For durable bonds, adhesives should be tough. However, many high-temperature adhesives [addition-type polyimides, bismaleimides (BMI), and acetylene-terminated resins] are brittle and need to be toughened by elastomers. Several processable, thermally stable polymers have been developed, including polyimides (Pis), polyimidesulfone, polyphenylquinoxalines (PPQs), polyimidesulfides hot melt [St. Glair 1984], and triaryl-s-triazine rings (TSTRs) [Hsu and Philipp 1982]. Some promising polymers, such as TSTR and polybenzimidazoles (PBIs), have not been evaluated as structural adhesives. PBIs are suitable for short-term use between 538 and 760Â°C (1000 to 1400Â°F). 13
14 EXTREME LOW TEMPERATURES Adhesives for cryogenic engines should maintain their mechanical performance between -184 and -196Â°C (-300 to -320Â°F). To meet this need, an experimental composite or a blend of adhesives in a multilayer form has been developed. Apparently work of this type has been carried out at Hughes Aircraft Company, McDonnell Douglas Corporation, or the National Aeronautics and Space Administration-Lewis Research Center, but it is not generally known. For intermediate low temperatures [e.g., -70Â°C (-94Â°F)], several polymers [Schmitt 1983] have been developed for advanced aerospace sealants: cyanosilicones (Product Research Chemical), fluoroalkyl- arylenesiloxanylene (FASIL, by the Air Force Materials Laboratory), phosphonitrilic fluoroelastomer (PNF, by Firestone Rubber and Tire Company), and flexible polyimides. Future research on intermediate low-temperature adhesives might be pursued along the following lines: (1) cyclic phosphazenes and phosphazene elastomers [Singler et al., in press], (2) polysilarylenesiloxanes [Koide and Lenz 1983], and (3) new fluoroelastomers. EXTREME HIGH HUMIDITY Water is a major problem for most adhesive bonds. At present there is no solution for this problem, although some improvements in bond durability have been achieved through the proper selection of a coupling agent or by the application of a corrosion inhibitor, such as nitrilotris-(methylene) phosphonic acid (NTMP) for aluminum] [Hardwick et al. 1984]. Water resis- tance can be improved by the use of fluoropolymers, such as fluoroepoxides [Griffith 1982] or fluoropolyimides. Recently a coordination compound, Volan II, has been shown [Lee 1984] to impart water resistance to a poly- ethylene and aluminum bond. Three years' water immersion produced no separation of the bond. Thus there is hope for a chemical solution to the water problem. SEAWATER Besides bond degradation, the corrosion of metals by salt is another serious problem, and the solution is even more difficult than that for water alone. Researchers developing sonar transducers [R. W. Timme, private communication, August 1983] have encountered this problem with the elastomer-metal bonds. For aluminum, a surface treatment such as anodizing may improve the resistance of the adhesive joint to salt water [Mindford 1980]. OTHER FACTORS Fire is not a major concern with adhesives. In addition to additives, phosphorylation has been used to increase the fire resistance for epoxides and polyimides.
15 For corrosive gases or liquids, no general rule exists. A chemical can be corrosive to a certain type of adhesive and, yet, may be harmless to another. REFERENCES Griffith, J. R. 1982. Epoxy resins containing fluorine. CHEMTECH 12:290. Hardwick, D. A., J. S. Ahearn, and J. D. Venables. 1984. Environmental durability of aluminum adhesive joints protected with hydration inhibitors. J. Mat. Sci. 19:223. Hsu, L., and W. H. Philipp. 1982. Cyclopolymerization of Aromatic Nitriles and Polymers With Ring-Chain Structures Made Thereby. Presentation at the International Union of Pure and Applied Chemistry Macro Conference, University of Massachusetts, Amherst. Koide, N., and R. W. Lenz. 1983. Preparation and properties of poly(silarylene siloxanes). J. Polymer Science 70:91. Lee, L. H. 1984. Recent Developments in Adhesive and Sealant Chemistry. To be published in Adhesive Chemistry-Developments and Trends. New York: Plenum Press. Mindford, J. D. 1980. Effect of surface preparation of stressed aluminum joints in corrosive saltwater exposure. Adhesives Age 23(10):36. Schmitt, G. F., Jr. 1983. Spacecraft, Aircraft, and Missiles: Pushing the Limits of Adhesives, Sealants, and Coatings. Presentation at the Inaugural Symposium for the Case Center for Adhesives, Sealants, and Coatings, Case Western Reserve University, Dayton, Ohio. Singler, R. E., G. L. Hagnauer, and R. W. Sicka. In press. Phosphazene Elastomers: Synthesis-Properties-Applications, Part II. To be published in American Chemical Society Symposium Series. St. Clair, T. L. 1984. Adhesive Development at NASA-Langley. Presentation at the April 29-May 2, 1984, Program Review/Workshop, Center for Adhesion Science, Virginia Polytechnic Institute and State University, Blacksburg, Virginia.