Structural adhesives: A joined-up approach
October 1st 2010 With greater use being made of structural adhesives,Chris Hall, advanced technologist, Industrial Adhesives & Tapes Division, 3M United Kingdom, explains a bit more about the different products
Structural adhesives afford 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. 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, require heat curing to reach full bond strength.
Able to offer very strong bonding, they can lack the flexibility and toughness of two-part adhesives. Heat curing costs can also make them less attractive.
Epoxy adhesives provide the highest strength and temperature resistance. They are also suitable 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 then 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.
A question of time
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 has a differing 'open time' the time available to bring 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; epoxies may take up to 6h. 'Time to full cure' ranges from less than a day for fast-curing acrylics, to a month at room temperature for some urethanes.
The impact of solvents
Apart from urethane products, many structural adhesives contain solvents, whose impact on application and substrates should be considered at specification stage. Solventbased 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.
Key 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 with the risk of losing future orders.
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, 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 need a more rapid adhesive flow, meaning a less viscous adhesive.
Surface preparation
Correct surface preparation will create the integrity and uniformity making it most responsive to the adhesive. All substrates 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 pretreatment, abrading to increase practical surface area or chemically priming can be beneficial. Additional processes before or after application can affect cost, and should be considered alongside adhesive costs when specifying.
Consulting a specialist supplier will help ensure correct product selection. Once the adhesive type and application method is decided, training in effective preparation, handling and use will ensure efficient delivery. More articles from 3M UK Plc: | 
