Achieving the very best results from engineering adhesives means applying the correct technology; make the wrong choice and your joint is likely to fail, writes Colin Chapman. So, with that in mind, it is useful to understand a little about the types of adhesive that are available and their applications.
Cyanoacrylates - so called ‘super glues’, cyanoacrylates (CAs) are high-strength, single part products available in a range of viscosities that are generally used for bonding a variety of small part substrates. Their rapid cure, activated where microscopic surface moisture exists, means they are perfect for high-volume, automated production applications
More recent developments include toughened, high peel strength, flexible and UV cure versions, which bond instantly when exposed to an appropriate light source, reaching 60% of their final strength within just five seconds. Now more user-friendly visible light lamps can be employed with the appropriate grade of adhesive.
Anaerobics - these harden when air is excluded from the joint and are ideal for threadlocking, threadsealing, retaining and gasketing applications. Retaining anaerobics enable cylindrical components, such as gears and bearings, to be effectively bonded on shafts. They also prevent fretting and corrosion. Although anaerobics (like most adhesives) work best on clean surfaces, oil-tolerant retaining and threadlocking products are now available for use on applications where degreasing is not always possible.
The ability of anaerobic adhesive to fill every microscopic irregularity in a substrate’s surface means they are also ideal for gasketing applications. Another benefit is the ease with which production processes can be automated. Not only does this speed up throughput, but also means greater accuracy in adhesive application.
UV curing acrylics - these single-component, solvent-free liquids turn to thermoset plastics when exposed to UV light. They can be readily used on a variety of materials, achieving a fixture time of around 2 to 60 seconds. Their ability to cure-on-demand means that parts can be positioned and, if necessary, repositioned prior to the curing process being started. Some adhesives in this category employ a secondary cure mechanism involving heat or chemical activators, to cure completely any product that is applied in areas that are shadowed from the light source.
One of the biggest growth areas for UV acrylics is the manufacture of medical devices, where the short cure times helps accommodate demanding, high volume production schedules.
Epoxies - these are structural adhesives that bond well to a wide variety of substrates, emit no by-products, fill large volumes and gaps, and shrink minimally when cured. Epoxies are almost certainly the top choice where ultimate strength is necessary, particularly when gaps are present. Although single part, heat-curing epoxies are available, most adhesives in this group consist of two part components - a resin and a hardener, which, when mixed, activate the adhesive’s cure mechanism.
Toughened acrylics - these two-part adhesives are frequently used to bond sheet metalwork in place of blind rivets or spot welds – especially when the appearance of the finished product is crucial and where there is a necessity to spread the stress load over a wide area. Certain toughened acrylics can be employed on ‘as received' steel and will withstand paint bake cycles. Handling strength is normally possible in less than 30 minutes at room temperature.
Polyurethanes - although the primary application of polyurethanes is the bonding of direct glazing windscreens, other applications include use in the manufacture of items such as shower screens and septic tanks. They generate flexible bonds and secure seals with little likelihood of leaks and seepage.
Adhesives can offer improved performance over mechanical methods, processes can be automated, finished products can be lighter, appearance can be enhanced, and a seal can be achieved. All of which means designers have good reason for specifying them.
Colin Chapman is with Henkel Loctite