Adhesives have been known to mankind for thousands of years. The earliest adhesives were based on proteins gained from boiling down bones, albumin, etc. These adhesives were often stronger than the wood they bonded together. Today’s adhesives are based on numerous chemistries. They are used to bond many different substrates. Adhesives are either low molecular weight reactive liquids (structural adhesives) or they are (tackified) viscoelastic polymers (pressure sensitive adhesives). In the first case the adhesive is applied as a liquid to the substrate, and then chemically crosslinked (cured), and in the second case, it is applied with the right amount of pressure and without any post cure.
Adhesives are used for numerous applications in almost all industries and have replaced traditional joining methods, such as bolts, rivets, screws, nuts, welds, and solder joints.1 Important benefits of adhesives over mechanical fasteners are reduced material, labor, and machine costs; faster assembly; ease of use; and easier disassembly of parts, if needed.
Adhesives have many design and performance advantages over traditional mechanical fasteners; unlike mechanical fasteners, adhesives provide joints that have smoother contours and are more aesthetically pleasing. Adhesive joints also offer a better strength-to-weight ratio than mechanical fasteners, and provide a larger stress-bearing area with a more uniform stress distribution, leading to lighter and stronger assemblies. Other important performance benefits are improved impact resistance, lower residual stress, better sealing capability, and much reduced galvanic corrosion.
Adhesives are able to join any combination of materials regardless of mismatch in physical and mechanical properties such as coefficient of thermal expansion, elastic modulus, and interfacial energy. They are also well suited for joining thin and/or small parts.
Despite the many benefits, adhesive joints have some limitations. For example, their operational temperature range is (much) smaller than that of metal fasteners and the mechanical and physical properties below the glass transition temperature and below the brittle point are markedly different from those above these temperatures. Another major drawback of (thermoplastic) adhesives is the comparatively low creep and fatigue resistance. Particularity at elevated temperatures and cyclic loads, adhesive joints creep at much higher rates than mechanical fastened joints. Usually, creep rates increase exponentially with temperature and applied stress. In the case of a thermosetting adhesives, the glass transition temperature should be about 50°C higher than the expected service temperature of the assembly.
For very demanding applications where very high and predictable bond reliability is required over a long period of time, adhesive joints are often combined with mechanical fasteners. However, welding or installing mechanical fasteners is usually more time consuming and more expensive.
1Source: TE Preferred Fiber Market Report (2016) and Lenzing Global Fiber Market in 2016