This is a difficult question. Permanent engineering or structural adhesive bonds should last the useful lifetime of the parts. It’s impossible to say, “in seventeen years, five months, and three days’ time, your adhesive bond will fall apart.” But will the adhesive bond last indefinitely?
There are some exceptions to this. Certain types of adhesive are temporary. They will break down under the correct conditions. The other exception is when a permanent adhesive is put into very stressful conditions. Such as temperature extremes, exposure to harsh chemicals, or a combination of the two. If the adhesive joint is unduly loaded and becomes fatigued, it can also lead to long term failure. Harsh chemicals include strong acids and alkalis, and polar solvents, which can have a damaging effect on adhesives.
It is important to consider that it is not only the adhesive bond that can be attacked but also the substrate material underneath. A good example of this is mild steel. In a salt spray environment, mild steel will rapidly corrode causing substrate failure in the bond area. In a situation like this, it is a good idea to use an adhesive which will help protect and increase the lifespan of the substrate underneath.
Before committing to an adhesive and bonding the final production assemblies, adhesive manufacturers recommend that users check the adhesive suitability and lifespan on the parts. What are you supposed to do, wait 17 years and 5 months etc.? No, there is a much easier way of checking long-term durability: accelerated aging. This is aging carried out under strictly controlled extreme conditions – often done in a humidity chamber or salt spray cabinet. It combines heat and chemical attack (remember the two factors combined really puts the adhesive through its paces). Some adhesive manufacturers or private testing laboratories can help carry out these sorts of tests for customers. Don’t forget though, every substrate, joint configuration, and environmental condition varies between application and customer. So it is vital to design a testing regime that is relevant to the application.
For more help and advice about accelerated aging tests and adhesive lifespans, please contact Permabond.
