Kevin Brownsill, head of technical learning and development at Intertronics, discusses four considerations for selecting structural adhesives for performance and production
Structural adhesives can provide strong, load-bearing joints across a range of industrial and engineering applications. However, achieving reliable results requires more than choosing the ‘strongest’ material: process speed, substrate compatibility, production variability, automation requirements, and overall manufacturing costs must also be considered.
Here, we consider four factors engineers should take into account when specifying a structural adhesive for high-performance applications.
LOOK BEYOND THE DATA SHEETS
Polyurethanes (PUs), epoxies, and methyl methacrylates (MMAs) are among the main structural adhesive chemistries, offering a combination of adhesive and cohesive strength. Other potential options include cyanoacrylate adhesives (CAs) and single-part UV-curing acrylic adhesives.
Structural adhesive selector guides and data sheets are useful for narrowing down the options, but they should not be relied on alone. Published properties are often measured under controlled laboratory conditions, which may differ significantly from those of the application. Practical trials and testing are therefore important, as is advice from an adhesives specialist with experience of how materials perform in different applications and conditions.
BOND LINE AND SUBSTRATE REQUIREMENTS
Substrate compatibility is fundamental to achieving a reliable bond. For an adhesive to wet a substrate effectively, it needs to have a lower surface energy than the substrate.
High-performance plastics such as polypropylene and other polyolefins can be challenging to bond because their surface energies are typically low, often between 20 and 40dynes/cm2. Where these plastics are selected for their functional properties, surface preparation such as priming or plasma treatment may be necessary.
The bond line itself also needs consideration. For larger bond areas, the adhesive’s gel time or pot life needs to allow sufficient time for dispensing or assembly before curing begins. Gap-filling requirements should also be assessed. Variations in moulded component dimensions can result in gaps ranging from zero to several millimetres within the same assembly, affecting the amount of adhesive required.
Adhesive rheology is another factor. A thixotropic or low-viscosity adhesive, for example, will behave differently depending on the geometry of the bond line and the method of application.
Bringing an adhesive specialist into the design process early on can help address these factors before the substrate and bond line is finalised. Leaving adhesive selection until later can restrict the available options and make it harder to meet all the application requirements.
CONSIDER THE MANUFACTURING PROCESS
Adhesive selection is as much a manufacturing process decision as a chemistry one, and trade-offs must be factored in. Considering this from the outset allows the adhesive and application process to scale with production.
For instance, while an epoxy may be desirable given its high strength, a manufacturer may have to accept its relatively slow room-temperature cure. Two-part adhesives with faster cure times can begin to gel within the mixing nozzle if they are not dispensed continuously or regularly purged. Single-part UV-curing adhesives, meanwhile, can provide rapid, on-demand curing and reduce the time components need to remain fixtured.
While it may seem like a small factor, the packaging the adhesive is supplied in can have an important impact on the process, and therefore the selection of a material. A 10g container, for example, may be unsuitable where automated dispensing is required.
Manual application can also be supplemented with dispensing equipment, benchtop robots or full Archytas automated work cell systems to increase accuracy, repeatability and throughput.
THINK CAREFULLY ABOUT COST
Does the adhesive you have chosen make commercial sense? It might be technically perfect but expensive enough to damage profitability. Similarly, selecting the cheapest adhesive is not necessarily the most economical approach either.
An inexpensive adhesive may require more processing time, labour or equipment, ultimately increasing the cost of each finished part. The commercial calculation should therefore analyse the overall cost per finished part. including the adhesive, assembly process time and complexity, and the labour cost.
Structural adhesive selection is no straightforward task. Nonetheless, looking beyond the data sheet, balancing process and commercial realities with technical performance, and involving an adhesives specialist can help manufacturers to develop a reliable bonding process that remains practical as production scales.
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