Today, a maintenance engineer is spoilt for choice. For bonding, the adhesives market offers epoxies, acrylics, polyurethanes, modified silanes and silicones. Within each category are countless variations, all optimised for specific application requirements. But when it comes to factory maintenance, it’s not viable or economical to have every type sitting on the shelf in case it may be needed for a one-off repair. That’s according to Colin Chapman of Henkel whose brands include Loctite, Teroson and Bonderite
He says: “There is around a 70 per cent cross-over of applications between the various adhesive types. For common repairs there may be a number of different bonding products that could do the job effectively. So in the factory environment a more rational approach should be adopted.”
For mainstream bonding tasks Henkel recommends factory maintenance engineers should first consider epoxies and instant adhesives.
Rigid bonding and elastic bonding are the performance classes into which most adhesives fall. Whilst there is call in maintenance and repair for elastic bonding to absorb and tolerate dynamic stresses, the large proportion of tasks involves rigid structural bonding. Here we are considering applications such as bonding brackets and hinges onto machinery, magnets onto motors or using adhesive to seal potting terminals to prevent moisture ingress.
For these jobs an epoxy-based adhesive is ideal. Epoxies are good all-rounders, not the best choice in every instance, but a tool box essential nonetheless as they are able to bond most materials. This type of adhesive is synonymous with high strength structural bonding. It is generally slower to cure than other technologies but it also offers the advantage of being able to fill large gaps and provides good chemical resistance.
A typical example of this technology is Loctite Hysol 3430. It provides an optically clear bond line and is therefore a good choice for all manner of applications including bonding glass panels.
Should a flexible bond be necessary to the repair, a silicone or silane modified polymer would be the best choice. The key characteristics of silicones are low modulus, high elongation to failure – up to 600 per cent – and their ability to withstand high temperatures up to 350°C.
Modified silanes will also bond almost any substrate and they cure by reaction with moisture so no primer is needed to achieve a good bond. They are particularly effective where large surface areas are involved.
From an application standpoint, polyurethanes sit somewhere in the middle between rigid and elastic bonding. They too are suitable for bonding a wide range of materials but do require the surface to be pre-treated with primer.
The ability of instant adhesives to bond multiple surfaces quickly has made them prime products for factory maintenance. More correctly termed cyanoacrylates, they cure when confined between surfaces and in terms of cure speed they are unmatched.
Take the general-purpose Loctite 401 for example, it has a fixture time of between three to 10 seconds.
Historically this group of adhesives was only suitable for close fitting surfaces but now formulations are also available that are able to gap fill; this extends their potential in maintenance engineering considerably. Loctite 3090 has this ability and will fill gaps up to 5mm. It cures through the volume and won’t run out of the joint in the absence of perfect face-to-face contact or if there are any surface imperfections.
Needless to say, instant adhesives are available in a variety of types optimised for specific application requirements. For example products are available for bonding porous or acidic substrates, for resisting shock and impact, for providing a flexible bond and for withstanding exposure to high temperatures. There are also low-odour and low-bloom instant adhesives. Indeed, the choice in instants is now as extensive as all other types of adhesives.
The use of anaerobic adhesives for many maintenance applications is of course well established. As a method of threadlocking, specific grades of adhesives prevent self-loosening and resist vibration and for thread sealing they create a high pressure seal. Retaining cylindrical parts and gasketing are also proven applications for this technology.
Although many variants are available for all these applications, the selection of the best product for the job is clear cut by comparison with bonding. By and large adhesives are chosen for their strength based on whether the joint needs to be disassembled in the future or remain permanent.
Navigating your way through the huge variety of products available for maintenance applications can be a time-consuming task but advice is readily available. All good adhesive manufacturers offer technical advice and Loctite has recently gone one stage further by introducing a dedicated website for maintenance and repair.
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