Adhesives load-bearing capabilities

One of the most important requirements of a structural adhesive bond is durability that is, the ability to retain a significant portion of its load bearing capability for long periods of time under the wide variety of environmental conditions which are likely to be encountered during service life.CjJ Unfortunately, the poor durability of metal/adhesive bonds in wet, hostile environments has proven to be the major obstacle to widespread development and practical usage within many industries. 

Table 1 provides the approximate load-bearing capabilities of various adhesive types. Because the load-beanng capabilities of an adhesive are dependent upon the adherend material, the loading rate, temperature, and design of the adhesive joint, wide ranges of performance are listed. 

Most of the adhesive families have either a thermoset or thermoplastic base. This is also the primary and the most traditional way of categorising adhesives, although within some adhesive families, such as polyurethanes, both thermoset and thermoplastic adhesives may be found. Thermoset adhesives form bonds that are essentially infusible and insoluble after curing and they typically have a much higher load-bearing capability than thermoplastic adhesives. Thermoplastic adhesives are fusible, soluble, soften when heated and their creep resistance is lower than that of the thermoset adhesives. The most common thermoset adhesives are epoxies, phenolics and polyurethanes, while the most widely used thermoplastic adhesives include acrylics (including anaerobics, hot melts and cyanoacrylates) and thermoplastic polyurethanes. A brief description of these adhesives (both thermoset and thermoplastic) is given below from reference 5.20 and 5.28. 

If we consider the load bearing capabilities of a high strength structural adhesive under all these conditions, it is clear that the nature of the joint is fundamental in achieving satisfactory performance (Fig. 4). 

Figure 42 Excess adhesive outside the joint improves the load bearing capability.

These compounds are the next stop up in strength and general service in hot-melt adhesives. These so-called high-performance hot melts are used to assemble products made from glass, hardboard, wood, fabric, foam, leather, hard rubber, and some metals. Service temperatures range from 0°C to about 82°C. A number of formulations are available that can be used at >93 °C. Some are capable of being used in non-load-bearing applications at>149°C. ... 

---