Assembly Considerations
With so many different families of adhesives in the market, and each family having multitude of grades available from different suppliers, selecting an adhesive is a daunting task. A systematic approach to the selection process is important in order to weed through different options and to identify the most appropriate and cost-effective adhesive system for your application. One of the first steps is to identify all the systems available for bonding substrates. For example, a large selection will be available for bonding polycarbonate to polycarbonate, followed by the availability for bonding polycarbonate to ABS and the fewest available choices for bonding polycarbonate to metal. Some adhesive systems will allow the use of primers that can improve adhesion to metals. Make an accurate assessment of the service environment that the assembly may be subjected to. Adhesives are polymeric materials which are performance sensitive to temperature, humidity, and other environmental factors. An adhesive system that provides adequate joint strength at one end of the operating temperature range may be too brittle or too pliable at the other. Presence of moisture in the environment can affect the performance of some adhesive systems negatively. The combined effect of high temperatures and high humidity may be even more detrimental. Decide on what should be the expected service life of the assembly. Effects of aging on the adhesive system can differ significantly from one adhesive system to another. Exposure to sunlight can accelerate the aging effect due to ultraviolet light. Wetting ability, which depends on the difference between the surface energy of the substrate and the adhesive, is one of the most important criteria for adhesive selection. For a proper bond, the surface energy of the substrate must exceed that of the adhesive by at least 10 dyn/cm. This becomes more challenging when different substrates are involved. When different substrate materials are involved, differential adhesion is not the only concern; differential expansion or contraction will be a major concern if the assembly is subjected to thermal cycling. The difference in the coefficient of linear thermal expansion (CLTE) is significant between some combinations, especially between metal and plastics. Thermal cycling stresses can lead to premature joint failure. Adhesives that are more elastic will be able to accommodate these stresses. Anticipate the external stresses that the assembly may encounter. Rate, frequency, duration, and the direction of loading are all important factors that can affect the joint, as well as the substrates. Adhesives usually work best in shear. However, the direction of loading can cause bending, cleavage, or peel stresses in the joint. Some adhesives can cope with a particular type of stress better than others. Another important consideration is the chemical compatibility of adhesives with the substrate materials. Solvent-based adhesives can attack the substrates, causing crazing or cracking. If the substrates have applied or molded-in stress, failure is accelerated. Solvents trapped inside a joint can keep working on the substrates for a long time. Injection molded plastic parts may warp due to some design features or the processing conditions. Some reactive-polymer-adhesive systems have good gap-filing qualities and may be used to accommodate a limited amount of warpage or surface imperfections. Monomeric adhesives are thin and do not have any gap-filling ability. Before deciding between two competing adhesive systems, consider the total cost of assembly including cure times, etc., not just the cost of adhesive. For a more detailed list of adhesives suitable for Bayer engineering thermoplastics, consult Bayer's publication "Joining Techniques—A Design Guide". |