Choosing adhesives - how not to get stuck
01 September 2008
Nicky Bradley assures DPA readers that adhesive specification is no ‘black art’. Just be sure to ask the right questions at the beginning of the design process
With cost, weight saving and speed of manufacture all having even greater impact in just about every fabrication procedure you care to mention, adhesives are playing an ever increasing role in many bonding applications. From automotive body in white, to on- and off-site construction cladding, mechanical fasteners are starting to see their traditional role usurped as adhesives prove themselves to be a more than capable replacement.
The time, cost and weight savings achievable by utilising adhesives are well documented, as many of the lengthy and sometimes part-destructive preparation and application steps needed by mechanical fasteners are removed. In many cases, adhesives allow bonded components to become one homogenous structure as opposed to two joined together by a few rivets or welds. Although the specification process may seem lengthy, the benefits on offer far outweigh the time taken to research at the design stage. One must also take into account the preparation needed to qualify and perform mechanical joins such as hole and fastener size optimisation, spacing and measurement arrays and subsequent testing and analysis.
The traditional issues that many people have had with adhesives fall into two main areas; first, having faith that adhesives are as capable as the mechanical fasteners they will ultimately replace and second, knowing which adhesive to use for which job. The first issue can be addressed by some of the truly astounding performance figures that many adhesives manufacturers can provide, in some cases far in excess of their mechanical peers; however the second issue requires a bit more explanation. Nicky Bradley takes us through some of the steps that designers and manufacturers need to consider prior to specifying the right adhesive for the job.
"The first thing you must consider in any adhesive application is the substrates being bonded," Ms Bradley begins. "But rather than just metal or plastic, we must consider the specifics of the material, especially the surface finish. For example, steel can be stainless or mild, painted or powder coated, and certain plastics exhibit lower surface energy than others, so it is plain to see that this is a logical first step." An example here would be in any sheet steel application where steel stock is treated in one way or another either to preserve the surface or enhance the aesthetics of the material.
The next thing to consider is the type of bond required. "Engineers need to understand what the application is and what the bond is being designed to achieve in functional terms." Ms Bradley continues. "We must take into account the stresses and strains associated with the loads and weights being applied to the bond. This is also not just one off; we must also think about the life of the bond and any cyclical stresses it may encounter." In furniture manufacturing, bonds can take on a variety of guises including butt joints or lap joints; both offer their own functional attributes and require different approaches to bond design.
"The next stage is to assess the application process that will ultimately be used in the manufacturing run. This can have a huge impact on which formulation is recommended. Automated processes normally require quicker-curing adhesives, or at least those that can provide a handling-strength bond within a short interval. Manually applied adhesives, on the other hand, do not normally require such a fast action, but this does depend on how quickly assemblies are passed from one manufacturing station to another."
The working environment of the finished component will also have a large bearing on which adhesive is to be used. Temperature maxima and minima must be considered, as well as temperature cycling; any solvents or chemicals that may be present and, of course, any moisture or precipitation that the bond is like to encounter. These issues can be compounded when more than one environmental element is acting on the bond. Ageing of adhesives and the long-term affect of UV on the bond are also important considerations.
The final step is the preparation of the joint, which requires an assessment of the bond’s functional requirements and the process steps available (which must also include any health and safety considerations). "Primers are the first and most obvious route,” says Ms Bradley, “but there are also solvents that can be used to degrease metal or key plastic surfaces. If a factory is not equipped to deal with primers or solvents, due to health and safety issues, then we must think about alternatives. Many metal stock forms come pre-treated with grease, so bond preparation is very important, whereas plastics exhibit all sorts of surface properties, so keying surfaces to enhance the bond interface plays a large role.”
Two-part acrylic-based adhesives can bond many low-surface-energy plastics, including many grades of polypropylene and polyethylene, without special surface preparation. Not only does this remove the risks associated with some primers, it also removes a complete production step, often speeding up the manufacturing process. Alternatives, such as formulations that cure with moisture, combine the physical properties of a urethane adhesive with the application qualities of a hot melt adhesive. This gives a quick functional bond for rapid handling, but unlike a hot melt, it builds up a structural strength and resistance to temperature.
"If all of these questions have been answered, the chances are that we can narrow down the adhesive to specific types and even formulations. But in certain circumstances, specialist applications or specialist bonds might require more investigation; this is where laboratories such as 3M’s come into play," says Ms Bradley.
"It is fair to say that if a bond fails, it is rarely the fault of the adhesive; it is normally to do with the application process or the preparation of the substrate. Many customers ask us 'what went wrong?' and through iterative and holistic research methods we are able to narrow down any potential causes.”
As well as these reactive issues, 3M is also involved in long-term testing for future applications, a process which may involve life testing, thermal cycling, thermal shocks, salt spray testing and moisture ingress.
Nicky Bradley is a technical specialist with 3M
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