Joining forces with epoxies
01 June 2012
Epoxy adhesives can be designed into projects to provide cost economies and reduce component weight. By its very nature, structural bonding needs to be safe and secure. There can be no room for error or mistakes. And when it comes to high strength structural bonding, epoxy resins are the undisputed leader. We take a closer look at this bonding technology and examine some applications.
A favourite of the aerospace industry, Loctite Hysol two-part epoxies also have a role to play in everyday industrial application
Epoxy resins can provide a number of distinct advantages over other adhesive technologies, including resistance to chemicals, exceptional fatigue and thermal shock resistance, the capability of joining dissimilar materials, corrosion resistance and the ability to be used in high temperature environments. Alongside these advantages is the fact that adhesives are lighter – and often less costly – compared with conventional fastening methods. So, just what do design engineers need to bear in mind when they are considering epoxy adhesives as a bonding technique?
“When it comes to design considerations, less critical tolerances can be specified because of the gap filling capability of epoxies,” explains Henkel Loctite Adhesives’ application engineering manager, Mike North. “In fact, careful joint design and proper surface pre-treatments are vital if full advantage is to be taken of the versatility and performance potential of epoxy adhesives.”
To help design engineers determine the suitability of various products, Henkel Loctite offers a computer program known as JointCalc. This allows engineers to understand where the main stresses are localised, to change the joint data to minimise peeling effect and to improve shear strength by switching adhesive. It also provides an estimation of safety factors – an essential early stage design consideration.
“As with all adhesive bonding the joints should be designed to avoid peel and cleavage loads,” says Mr North. “JointCalc helps identify a potential failure criterion, and because of the numerical model used for the stress calculation, the dependability of the results is much higher than that taken from equations based on the average stress over a bonded area.” Indeed, this program can be used as a ‘go - no go’ gauge for selecting epoxies, which, broadly speaking, can be categorised as general purpose, five-minute pot life products, toughened, metal filled and high temperature.
General purpose is really self-explanatory. These are multi-purpose adhesives that come in a variety of viscosities and are used for general bonding requirements such as metals, ceramics, glass and most plastics. Five-minute epoxies are ideal of multi-purpose use or metal repair where a rapid cure is required.
Toughened epoxies offer exceptional strength and impact resistance and, among other uses, are excellent for bonding high performance composites like GRP and CFRP. Metal filled epoxies are designed for repairing worn or damaged parts, while High Temperature products can operate within a range from -50 to +180oC, and provide shear strengths up to 35N/mm2.
In most of these categories, manufacturers provide both one- and two-part products. Single component epoxies have the hardener already included into the adhesive, and remains inactive in the formulation until the adhesive is heated. Although the heating method is not critical, air-circulating ovens provide cost-effective curing, while induction heating remains a popular alternative because of its rapid curing speed.
Two-part epoxy formulations comprise a resin and a hardener, with curing taking place when they are mixed together. The specific mix ratio is dependent on the particular application. Two-part epoxies are available in pre-measured twin-cartridge packaging - an arrangement that helps avoid the possibility of an ‘incorrect mix’, which is the main reason for epoxy adhesive failure.
Aerospace and Formula One
The Airbus A380 provides many examples of the use of epoxy products - both to give strength, but also to limit overall weight. Loctite’s Hysol product line was used in the construction of this aircraft wherever parts need to be bonded together, including engine nacelles, flaps, slats, ailerons, landing gear doors and internal fittings. Hysol products are now used in over 5,000 aerospace specifications, from conventional aircraft to the Space Shuttle.
Formula One racing cars are constructed extensively from composite materials, and epoxies are ideal for bonding these materials. As partners of Team McLaren Mercedes, Henkel supplies adhesive products that are used in both the assembly of the car and the construction of the composite materials themselves, providing stiffness and rigidity without adding weight.
Specialist epoxy pastes and films are used to bond composite structures such as the outer carbon fibre layer to underlying honeycomb elements, and to enhance composite structures such as the spars that are bonded to the skins of the front wing to add strength.
Aerospace and high-speed automotive applications apart, epoxies also have a role to play in everyday industrial applications, where they are used with the same degree of confidence. For example, Yorkshire based, Pace Cycles chose Loctite Hysol 3425 for joining magnesium components to composite materials on its super-lightweight bicycle forks. When bonding the forged aluminium crown to the carbon fibre fork leg, the adhesive is applied through pre-drilled holes, allowing it to wick into the joint.
DMM International, which produces a range of mountaineering equipment, including several types of ice axes, is using Hysol 9450 to bond the axe heads to their shafts – a critical joint that must tolerate high impacts and often very low temperatures. The substrates involved are steel and aluminium and the gap that requires filling is up to 0.15mm. The epoxy not only provides a secure bond, but also an hermetically sealed the joint.
Stick to the right adhesive
So, epoxy resins are ideal for a whole variety of structural bonding applications. They can be used to bond most substrates including metals, glass, ceramics, wood and plastics. They cure rapidly at room or raised temperatures and are ideal for automated processes to replace welding and riveting with a corrosion-free, reduced stress joint. As with all bonding technologies, the technical specialists at your adhesives supplier can provide more in-depth details and answer your specific questions.
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