Choosing and using structural adhesives

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Structural adhesives are high load-bearing and have aesthetic advantages over welds and surfaces penetrated by mechanical fasteners. Unlike heat, there is no risk of distortion and the bonds often spread the load over wider areas. Their greatest advantage, however, is their sheer versatility. They come in an enormous range of chemical and physical properties and cure times, so much so that the biggest problem is choosing the best one for the job.

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Environmental factors

UV light, heat and moisture can radically accelerate or impede the setting speed. This can be an advantage or a disadvantage. How will the bond stand up to high and low temperatures or rain, saltwater or humidity? Is the bond exposed to oil, detergents or polishing?

Handling

A fast setting speed is valuable for productivity, but slow setting speed enables parts to be carefully positioned. While still in the tin, does it have a good shelf life? How permanent should your bond be? Being able to release it in the future may enable replacement of worn components.

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Safety

Always investigate an adhesive’s toxicity to people and the environment (see http://www.adhesives.org/adhesives-sealants/adhesives-sealants-overview/health-safety/industrial-consumer-safety).

Functional qualities

Some bonds need a runny adhesive that flows into thread or narrow gaps while others need to be thick to stay in position or fill uneven surfaces. Most adhesives only work on specific substrates; metal bonding adhesive that works on steel may not work on lead or aluminium. With metal bonding adhesive its ability to conduct or insulate heat and electricity is often important.

Mechanical qualities

Shear, flex and peel strength are each subjects in their own right. Strength will mean different things in different contexts; sudden shock rather than a steady force may find different adhesives ranking differently. Some epoxies are the strongest on paper, but their weaknesses (brittleness and difficulty mixing) could shift your choice to an acrylic, cyanoacrylate or urethane.

In many cases, one all-purpose adhesive will be more convenient than a host of specialised ones. An adhesive with a broad range of applications can be found here http://www.ct1ltd.com/.

In summary, acrylics are excellent on plastics and often on metals but can be susceptible to vibration and temperature. Cyanoacrylates have excellent shear strength but can be brittle. Urethanes are lower strength but have good flexibility and versatility. Epoxies are good at bonding metals and some varieties have additives to absorb vibrations.

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