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8 SUMMARY OF DISCUSSION: CHEMISTRY The adhesive chemistry discussion focused on aerospace applications of structural adhesives for use in severe environments as implied by the needs of the Army, Air Force, Navy, and NASA. It was recognized that each of the services has requirements for adhesives for use in unique environments besides structural applications. Limited time, however, restricted the discussion primarily to structural adhesives. Severe environments include use under various combinations of temperature, time, stress, moisture, chemical and electrical action, and irradiation. Although the discussion centered on the chemistry of adhesives, it was well recognized that there are many important parameters to be considered in a bonded joint. These include items such as adherend, surface treatment, interface, primer, carrier, filler, adhesive tape character, cure conditions, joint design, stress loads, environment, and failure mode. Thus, a comprehensive fundamental program on adhesives must encompass chemistry, mechanics, and interfaces. The requirements for adhesives for use in severe environments were categorized with regard to maximum use temperatures and use times at those temperatures as shown in the following table: Use Temperature, Â°C (Â°F) 538 to 760 (1000 to 1400) 288 to 371 177 to 232 23 to 93 (550 to (350 to (73 to -240 to -54 (-400 to 700) 450) 200) -65) Use Time Seconds to minutes Hundreds of hours Thousands of hours Tens of thousands of hours Seconds to thousands of hours Obviously, other environmental factors are involved, but these are difficult to define since each application has its own specific requirements. 538 TO 760Â°C (1000 TO 1400Â°F) ADHESIVES Adhesives performing in the 538 to 760Â°C temperature range are needed for use in missiles and advanced weapon systems. Two families of aromatic heterocyclic polymers, PBIs and polyquinoxalines (PQs), have demonstrated 27
28 usable strength in the 538 to 760Â°C range. PQs are unavailable and must be custom synthesized. FBI is commercially available in limited quantities. PQ can be processed at high temperature (e.g., 371 to 399Â°C under 200-psi pressure) with low volatile content, whereas considerable volatiles are evolved during the fabrication of bonded areas with FBI. The adhesive chemistry group recommends that other known high- temperature polymers be evaluated as adhesives for use in the 538 to 760Â°C temperature range. In addition, research should be directed toward the development of new synthetic routes leading to PQ, FBI, and other ultra-high- temperature polymers that alleviate the evolution of condensation volatiles and tenaciously held residual solvent. 288 TO 371Â°C (550 TO 700Â°F) ADHESIVES Adhesives performing in the 288 to 371Â°C temperature range are needed for structural use in advanced aircraft and space vehicles. Several different families of aromatic heterocyclic polymers such as PQs, PBIs, Pis, and PPQs have been evaluated for use in this temperature range. PQs have displayed good adhesive strength retention at 371Â°C after 50 hours at 371Â°C in air, whereas PPQs exhibited good performance at 316Â°C after 500 hours at 316Â°C in air. Commercial PI adhesives are available, but large quantities of volatiles are evolved during fabrication. There are several problems associated with these experimental linear amorphous polymers. High temperatures and high pressures (temperatures >316Â°C and pressures >100 psi) are required in processing. Many of these thermoplastics creep under load at elevated temperatures and, when under stress, exhibit solvent sensitivity. Most of these polymers are made using toxic monomers (e.g., aromatic amines). Research should be directed toward improving the processability of these polymers while retaining their good adhesive properties at elevated temperatures. Research should also be undertaken to develop new synthetic routes that circumvent volatile evolution and the use of toxic monomers. Other known polymers should be evaluated as adhesives for use in the 288 to 371Â°C temperature range. 177 TO 232Â°C (350 TO 450Â°F) ADHESIVES Adhesives performing in the 177 to 232Â°C range are needed primarily for structural use in military airplanes, high-speed advanced commercial aircraft, conventional commercial aircraft (engine area), and advanced helicopters. Several different families of polymers such as PQs, PBIs, Pis, PPQs, addition-type Pis, acetylene-terminated phenylquinoxalines (ATPQs), and other acetylene-terminated polymers (ATPs) have shown promising performance as adhesives in the 177 to 232Â°C range. A linear amorphous PI has shown no loss in lap shear strength at 232Â°C after aging for 30,000 hours at 232Â°C in air. The linear amorphous thermoplastic polymers (e.g., PQ, PI, and PPQ) require processing temperatures about 100Â°C higher than their glass transition temperatures. The addition-type Pis as well as the ATPQs and ATPs are generally brittle.
29 The adhesive chemistry group recommends that research be directed toward toughening the addition-type Pis, ATPQs, and ATPs either by incorporating toughening phases within the polymer chain via monomers or by the addition of toughening agents (elastomeric materials) that will perform in the 177 to 232Â°C range. This may necessitate the synthesis of specific toughening agents (e.g., high-temperature elastomers). In addition, research should be undertaken to determine the mechanism of toughening of these high-temperature adhesives. 23 TO 93Â°C (73 TO 200Â°F) ADHESIVES The most popular structural adhesives for the 23 to 93Â°C range are epoxies, particularly rubber-toughened (RT) epoxies. In general, these materials perform well. However, there are specific applications where debonding occurs. For example, debonding of the sandwich-constructed Army field shelters is a major problem in hot and moist or extremely cold climates. A variety of epoxies are used as RT curing resins for field and depot repair of damaged components. Problems associated with epoxies include moisture sensitivity, long cure time, and limited shelf life. The adhesive chemistry group recommends research to develop adhesive systems (epoxy replacements such as ATPs or cyanates) that alleviate the shortcomings of conventional epoxy adhesives. In addition, research should also be undertaken to develop a better RT curing resin with shorter cure time, longer out time, and better mechanical properties than current RT curing systems. -240 TO -54Â°C (-400 TO -65Â°F) ADHESIVES The need for adhesives for use in a cold environment is recognized, however, the state of the art, problems, and needs for adhesives for use in cold conditions was beyond the scope of the workshop. GENERAL RECOMMENDATIONS It is recommended that research be undertaken in the following areas: 1. Synthetic research should investigate polymer-forming reactions that eliminate evolution of volatiles and provide moisture-resistant polymers with a favorable combination of adhesive properties. This pertains to adhesives for all of the temperature-use ranges. 2. Studies on a model system (e.g., ATPs) should be conducted to generate fundamental information on the relationships among structure, cure, physical properties, and mechanical properties. 3. Basic synthetic work should be directed toward the development of water-based adhesive systems, especially for high-temperature polymers, to eliminate tenaciously held and toxic solvents.