aircraft and Formula 1 cars. Flexibility, toughness, fluidity and thixotropic qualities can all be varied according to customer specification and need. Two-part epoxy products lend themselves well to bonding metal, wood and low surface energy plastics, although surfaces must always be thoroughly cleaned and dried before application. Open time – the window of time available to bring the surfaces together can vary between 30 seconds and several hours. Full cure times can be anything between one and three days, but some rapid products have a ‘time to handling’ as short as three minutes, although once again a few hours is more common. This can be a disadvantage in high throughput applications, particularly given the typically higher costs of two-part epoxy adhesives compared with PVA. Required surface preparation can also be an issue – many substrates will require abrasion or priming to ensure they are sufficiently receptive to the adhesive and that the required strength, performance and durability will be achieved. However, some two-part epoxy products are oil-compatible, meaning they can be applied to oily surfaces as long as the type and nature of the oil has been identified and adequate testing has taken place. Where lower shear performance is acceptable an effective
alternative may be two-part polyurethane products, which can be re-combined using a static mixer, by hand or with automated processes. These products generally offer excellent flexibility and impact performance and reach handling strength very rapidly compared with epoxy products (although time to full cure may approach 30 days). They can be ideal for bonding to powder coated products, and can offer good durability and non- sag capability. Care should be taken when bonding to metal substrates with polyurethanes, however, as priming or complex surface preparation is likely to be required.
Meeting the most demanding bonding criteria For highly demanding applications requiring a structural
strength bond, acrylic adhesives deliver a rapid result at reasonable cost. Acrylic adhesives are structural in performance and reach handling strength very quickly, typically in less than an hour. Full cure usually happens within 24 hours. They also require less surface preparation than epoxy products, may cope well with oily surfaces, and need minimal mixing. While most are two-part products that require mixing, acrylic ‘bead on bead’ products also exist with each part applied to the substrates, which are then pushed together. Acrylics are ideal for metals, plastic and composite substrates, while highly modified products are available for low surface energy plastics. Acrylic products typically emit a strong and unpleasant odour
(though some low odour products are now available), while the lower cost compared with epoxy adhesives is reflected in their lower flexibility compared with these alternatives. Where the requirement is more for fixing than for bonding, hot melt adhesives offer a versatile and cost-effective solution. Generally designed for non-critical and temporary fixing applications, hot melt products are non-curing systems which generally reach handling strength quickly after being dispensed. Suitable for
126 Fastener + Fixing Magazine • Issue 68 March 2011
almost all substrates, minimal surface preparation is usually required. Hot melt products are dispensed through a heating gun, requiring a pneumatic supply, with a typical application temperature of around 180ºC, although ‘low melt’ products exist which are applied at around 120ºC. The costs of heating and energy must be factored into the overall specification costs.
Steps to specification Whatever the application,
a few simple steps should be followed in specification. The product most appropriate to the job should always be selected, remembering that too strong an adhesive can create almost as many problems as one that is not strong enough. Overspecification is also likely to add considerably to cost. The characteristics and requirements of the substrates to be
bonded will always play a key role in the specification decision. When calculating cost, the specifier should always look well beyond the purchase price of the adhesive and also factor in the costs of additional surface preparation, cleaning, operator training, adhesive heating and energy requirements, and any necessary PPE. PPE will be needed if epoxy, polyurethane or acrylic adhesives are specified. Consideration must also be given to the required production
throughput – and so the time to handling strength and to full cure of potential adhesive solutions - and how soon each component will need to undergo further processing after application of the adhesive. Consulting an adhesive specialist, who can advise on the
correct product and how to get the best out of it, from the early design stages, will help ensure optimum product selection and ultimately the effectiveness of the process.
About 3M 3M captures the spark of new ideas and transforms
them into thousands of ingenious products. Its culture of creative collaboration inspires a never-ending stream of powerful technologies that make life better. The US$27 billion diversified technology company
has, since 1902, been creating innovative products that help make the world healthier, safer and more productive. Well known 3M brands include Scotch® Post-it®, Scotchgard™, Thinsulate™ and
Scotch-Brite™. 3M employs some 80,000 people worldwide and
has operations in more than 65 countries. The UK and Ireland is home to one of the largest 3M subsidiaries outside the USA, employing more than 3,500 people across 19 locations, including 10 manufacturing sites.