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Choosing a structural adhesive
There is no such thing as a perfect adhesive, but it’s important to select the most appropriate for your application. Here Peter Swanson of Intertronics shares some tips for choosing an adhesive.
Forward planning is crucial, whether baking a cake, building a house or going travelling. When preparing to manufacture a new product, it is essential to think ahead to ensure you have specified the materials, processes and resources needed when production starts. The specification of an adhesive should not be left to the production team – a holistic consideration during the design stage pays off later in efficiencies and savings. These considerations should include functionality, processability and commercial factors.
In an ideal world, all surfaces would be bonded by one perfect adhesive. However, a single adhesive that cures instantly on demand, can operate at any temperature, resists all solvents, sticks to everything and is completely and instantly reworkable is not possible. Compromise is almost inevitable. But how can design engineers ensure they get as close as possible to the perfect adhesive?
Successful bonding can be a matter of choosing substrates and adhesives wisely. Normally, an adherent force can only be established when the molecules in the substrate are closer than five ångströms (5x10–7mm) to the molecules in the adhesive. For this to occur, wetting of the adhesive to the substrate must be achieved. If the adhesive behaves like water on a greasy plate and forms little balls rather than a homogenous film, wetting, and therefore adhesion, will not occur.
For wetting to be achieved, the adhesive must have a lower surface energy than the substrate. For example, stainless steel has a much higher surface energy than most adhesives, so is usually readily bondable. On the other hand, polytetrafluoroethylene (PTFE) is a substrate with an extremely low surface energy, which makes it almost impossible to bond. Choosing bondable surfaces is an important part of the design process. If UV light can pass through the substrates, light curing adhesives are often a great choice. They offer very fast cures and multiple process advantages, including no mixing requirement. Because the processing cost may be a significant portion of the overall part cost, choosing process-friendly substrates may bring commercial benefits.
Even if the selection of adhesive and substrates results in a good adhesive bond, the subsequent environment needs to be considered. The adhesive must be resistant to any temperature extremes, solvents, other chemicals, weather, stress and vibration that befalls it during the lifetime of the assembly.
If the coefficient of thermal expansion (CTE) is different for each substrate, the adhesive may need to be tough or flexible to withstand the different amounts of thermal expansion. For example, the CTE is 1 ppm/°C for stainless steel and 6ppm/°C for glass. Depending on the size of the assembly and the temperature excursions, there may be significant stress imposed on the bondline, which the adhesive will need to cope with.
Although the priority is to choose an adhesive that will effectively bond together the required substrates, design engineers must also think about the requirements of a production environment. For example, an adhesive that takes a long time to cure may not be suitable for a high production volume industry.
If an adhesive will be applied to a vertical surface, the more viscous the better, whereas if it is for filling in gaps, a runnier adhesive is more suitable. Some adhesives are viscous under normal conditions but become runny when shaken, agitated or otherwise stressed, known as thixotropy. Highly thixotropic adhesives can make dispensing easier, but a combination of viscosity and thixotropy should inform adhesive choice.
Adhesives are commonly specified during product development, often based on functional requirements. When production is upscaled, the chosen adhesive can be less than optimal – application and curing may be difficult, unreliable, challenging to automate, or take too much time. The intended production quantities for the product should therefore influence adhesive choice.
It’s rarely possible to find an adhesive that ticks all boxes. To make it easier to prioritise the properties needed, do not request more strength or heat resistance than is really needed, because this may exclude adhesives that are equally optimal, at a lower cost and with simpler production capabilities. Ideally, the substrate fails before the adhesive, so a low strength substrate does not need a high strength adhesive.
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