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Structural Adhesives

What Are Structural Adhesives

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Carbon Fiber Hood

Carbon fiber hood assembled with a structural adhesive.

spe.automotive via Flickr (Creative Commons)

There was a time was when paper kites were held together by a flour and water adhesive mixture. When you are next seven miles above the earth's surface in an airplane (and a lot higher than a kite), you might like to consider that the airplane you are in is held together by adhesive - structural adhesive. Obviously not flour and water, we have moved on a bit since then! Yes, there are rivets, bolts, nuts and welds too, but adhesives are playing an increasing role in aerospace and many other application areas as they offer significant advantages over other fastenings.

Why Use Structural Adhesives?

There are many advantages to using adhesives for structural fastening, though many people do have a problem of perception, believing that mechanical or fusion (weld) fastening is much more secure:

  • Reduced weight
  • High durability
  • Vibration damping
  • Greater design flexibility
  • Lower investment in tooling
  • Less machining
  • Lower level of labor skills
  • Cleaner visual lines
  • No protruding fixings
  • Invisible fixing
  • Non-corroding
  • Often less costly

Categories of Structural Adhesive

Firstly, we have reactive and non-reactive. Reactive curing covers solvent evaporation and catalyst triggered. Typically the former is a single component adhesive and the latter two or more components.

Some adhesives are designed to react to light (UV). These may cure in as little as one second and offer high bond strength. Other adhesives use heat (infrared or ambient temperature) to cure. Others, such as cyanoacrylates are cured by moisture. These offer control whilst an assembly is fabricated then curing is triggered.

Multi-component adhesives have two or more chemicals which react with each other on mixing. Full bond strength only develops on final curing. The multi-component resins can be either solvent-based or solvent-less. Those which are solvent based dry during the curing process, much as paint dries.

Chemical compositions range through Urethane, Epoxy and Acrylic based on specific families of chemical structure.

Methyl methacrylate (MMA) adhesives cure differently to regular two-part formulations. The catalyst/base ratio can range between 50/50 to just 3%, and enables cure time to be adjusted. So, they have a 'slow start' - induction period - with a longer worklife. Then, cure accelerates.

Non-reactive adhesives span 'hot melt' (the simple DIY thermal 'glue gun') and contact adhesives. For hot melt adhesives, the range of base formulations is extensive, including EVA (ethyl vinyl acetate), polyolefins (e.g polyethylene), polyamides and polyesters, polyurethanes, polycarbonates and silicone rubbers.

Applications:

The range of applications for structural adhesives is vast. Adhesives have to be chosen carefully for the environment they are to be used in. Most adhesives do not corrode under normal circumstances, unlike nails and other steel fixtures, though they can degrade in UV for example unless a protective chemical is included in the formulation. Clearly, a water soluble structural adhesive such as PVA used for structural wood joints would not be weatherproof.

We mentioned aerospace applications - the ability to provide a continuous load bearing joint without the local stresses that a riveted component might experience is important for fatigue reduction and weight saving in aircraft; however, a small number of rivets may still be used to hold the joint for setting. Automobiles and carbon fiber bicycles - in fact almost any application where composites are used - utilize advanced structural adhesives.

So called 'superglue' (cyanoacrylate) formulations are now being used for wound closure in medical applications. We might not initially think of it as a structural adhesive, but that is what it is. It has to hold the wound together and is subject to shear stress and strain. Quick setting and tough, it also provides a continuous wound seal as compared with stitches which are localized (like rivets)! This continuous seal reduces the risk of infection and the healed scar is much less prominent.

Builders use adhesives which have the street name of 'liquid nails'. Key parameters for these adhesives is 'grab time' - how long in position before a load bearing bond is formed, and the ability to adjust the fixture's position before the final cure is complete. Some contact adhesives have a 'memory' and position adjustment is therefore difficult once the contact has been made.

The Future

Structural adhesives will stick with us indefinitely. Research is developing new formulations and refinements on a continuous basis. Product diversity is extensive -the display shelving in the larger DIY depots now offer as many as 100 different structural adhesives.

Besides diversifying, the market for structural adhesives is expanding geographically as the high growth BRIC economies increase their usage of structural adhesives in construction, aerospace and other industry sectors.

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