More choosing to stick together
November 1st 2011

Structural adhesives are seeing increased use, reports Chris Hall, advanced technologist, Industrial Adhesives & Tapes Division, 3M United Kingdom. Here, he explains some of the benefits they offer over mechanical or fusion fastening

Structural adhesives reduce component weight, increasing durability while offering greater design latitude, with less machining typically required – often more cheaply than other fixing media. They also offer aesthetic advantages, with cleaner lines and no protruding nails, rivets or weld marks, while their durability frequently ensures consistent performance long after mechanical fixings start to loosen or fail.

What are structural adhesives Structural adhesives are designed to join two surfaces forming part of a load-bearing structure, with typically at least 1000psi overlap shear strength.Most are two-part products, comprising two separate substances which start to cure at room temperature when mixed in a static mixer or applicator nozzle.

One-part adhesives, meanwhile, need heat curing to reach full bond strength. Able to offer very strong bonding, they can lack twopart adhesives' flexibility and toughness. Heat curing costs can make them less attractive.

Epoxy adhesives provide the highest strength and temperature resistance. They are excellent for void-filling to enhance rigidity and reduce noise.

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 and the two pushed together.

Urethane adhesives are typically lowercost 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. Each adhesive also has a differing open time – the time available to bring the surfaces together before it sets – a time to handling strength and a time to full cure. Usually measured as 50psi, handling strength is achievable within 30min for some acrylics, while epoxies may take up to 6h. Time to full cure can be less than a day for fast-curing acrylics, to a month at room temperature for some urethanes.

Impact of solvents 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 application area safety. They can also affect some plastic substrates' chemical composition, resulting in stress cracking. Both acrylic and epoxy adhesives can emit strong odours which can become harmful if operators are continuously exposed.

Factors when specifying Specification 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. Not selecting a strong enough adhesive can shorten product life, reducing customer confidence. Overspecifying – assuming an adhesive which appears stronger is necessarily better – can be equally damaging. Too rigid a product which cannot account for thermal expansion may create stress points, increasing failure potential. Creep can also affect components subject to heavy loads over extended periods.

Taking the heat 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.

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 more rapid adhesive flow, meaning a less viscous adhesive.

Surface preparation Correct surface preparation will create integrity and uniformity making it most responsive to the adhesive.All substrates should be cleaned with a solvent cleaner and thoroughly dried.

Some surfaces, such as decorative anodised aluminium, can develop weak points through the production process.Here, or wherever the substrate may benefit from further pretreatment, abrading to increase practical surface area or chemically priming can be beneficial.

Additional processes should be considered alongside adhesive costs when specifying.

Consulting a specialist supplier will help ensure correct product selection, and training operatives in effective preparation, handling and use will ensure delivery to expectations.

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