How long will my adhesive bond last?
This is a difficult question, engineering or structural adhesives are designed to be permanent i.e. last the useful lifetime of the bonded parts. It’s impossible to say “in seventeen year, five months and three days time your adhesive bond is scheduled to fall apart” the adhesive bond should last indefinitely.
There are some exceptions to this. Certain types of adhesive are designed to be temporary so will break down as and when they are supposed to 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, it will be rapidly corroded giving rise to substrate failure in the bonded 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, rather than “taking a chance” and hoping the adhesive lasts, adhesive manufacturers recommend that the user check adhesive suitability and lifespan on the parts. What are you supposed to do, wait until the 17 years 5 months etc. is up? 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, combining 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.