Adhesives for Elevated-Temperature Applications
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Adhesives for
ElevatedTemperature Applications
S. Millington and S.J. Shaw Abstract Although adhesives, particularly those based on epoxy resins, are finding increasing use in structural applications, their utilization at elevated temperature (150C) has been limited by their relatively poor thermal and thermo-oxidative stability. As a result, significant effort has been directed in recent years toward the development of polymers exhibiting increased thermal resistance. Although a wealth of research conducted over several decades has resulted in a myriad of polymer types exhibiting, in some cases, impressive high-temperature performance, many systems have demonstrated poor processability. Thus, much emphasis has been placed on developing high-temperature performance while providing processability characteristics that are similar, if not identical, to epoxies. This article considers the various approaches that have been shown to offer such dual capabilities. In addition, the results of various studies undertaken to investigate the effects of elevated temperature on the strength and fatigue resistance of bonded joints are reported. Keywords: adhesives, glass transition, thermal properties.
Introduction Structural adhesive bonding offers many advantages over traditional joining techniques (nuts, bolts, rivets, etc.), including lighter weight, improved fatigue performance, the ability to join dissimilar and thingauge materials, and the potential for lower manufacturing and lifetime costs. Consequently, bonded joints have replaced mechanical fasteners in a wide variety of applications, including aerospace, automotive, household goods, packaging, and machinery. However, the upper temperature limit at which softening occurs for most conventional epoxy-based structural adhesives is on the order of 150C, and this prohibits their use in applications where high resistance to temperature is a requirement. The high-temperature performance of a polymeric adhesive is a result of its molecular composition and structure; over the past 40 to 50 years, significant effort has been directed toward the synthesis of polymers with increased thermal resistance.1,2 Not all of these efforts have 428
been successful, since improved hightemperature performance has often been attained at the expense of increased brittleness and complexity of processing. Despite these problems, commercial adhesives are now available that have upper temperature limits in excess of 300C. This article considers a range of hightemperature adhesive types and examines how their thermal performance can be related to their molecular structure. The strategy for the future development of polymers having service temperatures significantly above what has currently been achieved will also be described. Finally, some physical and mechanical data will be presented, with particular emphasis on the effect of temperature on bond strength and fatigue resistance.
Chemistry of High-Temperature Adhesives To achieve acceptable high-temperature performance, an adhesive requires
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