Stuctural adhesives are now often the norm for applications where durability is key, says Chris Hall, advanced technologist at 3M UK
Structural adhesives have some inherent performance advantages over mechanical or fusion fastening.
They reduce component weight and increase durability while offering greater design latitude, with less machining typically required – and often more cheaply than using other fixing media. They also offer aesthetic advantages, with cleaner lines and no protruding nails, rivets or weld marks.
Formulations now exist for substrates from steel, aluminium and copper, to low surface energy plastics, rubber, glass, wood and masonry.
They are now often the norm in applications like car manufacture where their durability ensures consistent performance long after mechanical fixings start to loosen or fail.
Structural adhesives are designed to join two surfaces forming part of a load-bearing structure, with typically at least 1000psi overlap shear strength.
Most comprise two separate substances that start to cure at room temperature when mixed in a static mixer or applicator nozzle.
One-part adhesives, meanwhile, require heat curing to reach full bond strength. Able to offer very strong bonding, they can lack two-part adhesives’ flexibility and toughness. The heat curing cost can also make them less attractive.
Epoxy adhesives provide the highest strength and temperature resistance. They are also excellent for void-filling to enhance rigidity and reduce noise in applications like turbine blade manufacture.
Acrylic adhesives are available for the widest variety of substrates including hard-to-bond plastics and oily metals. High-strength bonding is achievable without the surface preparation needed for epoxies and urethanes.
‘Bead-on-bead’ adhesives are two-part acrylic products with each element applied to one substrate. The two are then pushed together, completely eliminating mixing.
Urethane adhesives are typically lower-cost but highly impact-resistant two-part products, curing quickly to an elastic bond in applications requiring flexibility between dissimilar materials.
When specifying, several key timings should be considered. For two-part adhesives, the first is ‘work life’ – the time it can remain in the nozzle before this needs changing due to adhesive hardening.
The ‘open time’ before the adhesive sets also differs. Handling strength is achievable within 30 minutes for some acrylics, while epoxies may take up to six hours. Time to full cure can range from under a day for fast-curing acrylics, to a month at room temperature for some urethanes.
Apart from urethane products, many structural adhesives contain solvents, whose impact on application and substrates should be considered at specification stage.
Solvent-based adhesives require greater outlay on PPE and air extraction systems to ensure safety in the application area. They can also affect some plastic substrates’ chemical composition, resulting in stress cracking.
Both acrylic and epoxy adhesives can emit strong odours that can become harmful if operators are continuously exposed.
Final product choice generally depends on required durability, flexibility, creep resistance, and heat and environmental resistance.
The lowest-cost adhesive consistent with required performance should always be specified. Failure to select a strong enough adhesive can shorten product life, reducing customer confidence with the risk of losing future orders.
Over specifying – assuming an adhesive which appears stronger is necessarily better – can be equally damaging. Specifying too rigid a product that cannot account for thermal expansion may create stress points, greatly increasing product failure potential.
Heat extremes – usually above 150°C – can cause softening and loss of mechanical properties required to maintain sufficient bonding. Where the finished product may be exposed to high temperatures, one option is a single-part system with heat curing, which by definition should perform better in hot conditions. Additional heating must be factored into cost considerations.
Heat can also be used in high throughput applications requiring further processing afterwards. Curing adhesives with long ‘open times’ can be accelerated by using inducted heat to reach handling strength more quickly, with heat from subsequent processes helping to reach full cure.
Environmental resistance refers to the ability to withstand the effect of agents like seawater, which can corrode metal in marine applications. Unless a product is specified which can resist this, further processes, such as secondary priming, may be required.
Viscosity, which governs dispensing speed, can also affect selection. High throughput applications may require a more rapid adhesive flow, meaning a less viscous adhesive.
Optimising performance requires correct surface preparation to create the integrity and uniformity needed to make it most responsive to the adhesive.
Virtually all substrates harbour contamination – mould release agents, oxides or simply dust particles – which could hamper bond effectiveness. They should be cleaned with a solvent cleaner and thoroughly dried before application.
Some surfaces, like decorative anodysed aluminium, can develop weak points through the production process. Here, or wherever the substrate may benefit from further pre-treatment, abrading to increase practical surface area or chemically priming can be beneficial.
Getting the advice of a specialist supplier will help ensure correct product selection. Once the adhesive type and application method is decided, training operatives in effective preparation, handling and usage will ensure delivery to expectations.
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