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A sticking point for engineering graduates?

11 April 2011

Research has shown that not all colleges include engineering adhesives in their curriculum and, as a result, graduates leave without fully understanding the technology. We asked Henkel's Colin Chapman to provide some very basic considerations in the form of a Q and A session

Q: What are the benefits of using an adhesive?
A:  There are many, depending upon the specific application. For example, by replacing mechanical fastening methods with an adhesive, lighter weight components can be created – resulting in cost and energy savings. Extensive testing reveals that adhesives often create a stronger joint than mechanical processes – such as rivets or spot welds. In addition, they produce an aesthetically acceptable joint, and work on a diversity of substrates including, in many applications, dissimilar materials.
Q:  Is there a ‘universal’ glue for all applications?
A:  There are general purpose adhesives, but for the best results, the appropriate adhesive technology should be used for each application.
Q:  So, what are those different types of adhesives?
A: In truth, each technology would require a whole article to describe the uses and advantages. However, a summary provides a useful overview.
Anaerobic curing adhesives are used for threadlocking, threadsealing, retaining and gasketing applications. They provide high compression and shear strengths, good thermal stability and resist vibrations and permanent dynamic stress. The cure process is initiated with the presence of metal ions and absence of oxygen. In practice, oxygen is displaced from the joint as the mating surfaces are brought together.
Light-curing acrylics combine physical toughness and excellent adhesion to a wide selection of substrates, including heavily plasticised PVC. One of their fundamental benefits is the ability to cure-on-demand. That’s because curing will not begin until the adhesive is exposed to the appropriate light source. Another major benefit is the ability to produce clear bond lines when cured in thin sections.
Cyanoacrylate adhesives, also known as Super Glues or Instant Adhesives, cure at room temperature and form strong thermoplastic bonds between surfaces. Normally, residual moisture on the surfaces will trigger curing, although in some cases an activator is needed. These adhesives are often usable in a few minutes and achieve full bond strength in 24 hours. Primers will improve adhesion on difficult-to-bond substrates such as polyethylene and polypropylene.
Epoxies are single part, heat cure or dual part, room temperature curing, structural adhesives used in the construction of aircraft, boats, cars, bicycles, golf clubs, and other applications demanding high strength bonds. They are used on a range of substrates, including metals, glass, ceramics, and wood – and they readily bond dissimilar materials.
Toughened Acrylics produce exceptional bond strength and are frequently used in applications that involve the bonding of magnets, ceramics, and metal to plastic or ceramics. They are often the choice for replacing spot welds and rivets on sheet metalwork where good appearance is important - and where the stress load needs to be spread over a wide area.
Polyurethanes are renowned for their outstanding flexibility, gap filling properties and the production of very reliable seals. They are used to bond windscreens and windows into car bodies and are frequently specified in the manufacture of items such as septic tanks and shower screens.
Q:  Some technologies involve two part products. How important is it to get the mix ratio right?
A:  An improper mix is the most common reason for joint failure. Increasingly, products are being supplied with mixer nozzles that allow the right proportion of each part to be applied.
Q: Just what is meant by “cure times”?
A:  This indicates the time required to reach the specified performance properties of the adhesive. In reality, this period depends on several factors, including ambient temperature, humidity, bond gap and substrates.
While we’re talking of times, it’s also worth mentioning ‘handling strength’. Product information will state how long should elapse before a bonded assembly can be handled, including being removed from clamps or similar holding devices. There is then a further period before full curing is achieved.
Q: How much adhesive is needed on a joint?
A: Just enough to fill the bondline. Use of excess material will be wasteful and lead to migration. Automatic application equipment can control the use of the product on-line.
Q: If adhesives are that good, can a joint be ‘un-bonded’, if necessary?
A: Generally, joints cannot be ‘un-bonded’; however, in the case of threadlocking, threadsealing, retaining and gasketing applications, anaerobic products are available in varying strengths that make dismantling possible. Normally, hand tools can be used for this process.
Q: Where’s the best place to get help with learning more about adhesives?
A: The helpdesk of an adhesives supplier can provide further insights into the various adhesive technologies. More importantly, by discussing the materials involved, the stresses to be met, the environmental considerations and other relevant factors, these specialists can readily point out the most appropriate technology.

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