When adhesives exotherm….
What is an exothermic reaction?
When chemicals react with each other they often give out heat during the reaction. This is known as an exotherm and it is something that is often witnessed when mixing adhesives (e.g. resin and hardener systems such as two-part epoxy adhesives) or sometimes when curing too much adhesive in one go (such as placing a bulk quantity of heat cure epoxy in the oven).
My first experience of an exothermic reaction was quite a shock to the system! As a youngster earning my stripes in the QC lab, I was a bit over enthusiastic with a clean-up operation and was curing off old adhesive samples in the fume cabinet. Anaerobic adhesives remain liquid unless in a tightly fitting joint between metal parts. But not with me around… Add cumene hydroperoxide. Nope not much happening. Add quite a bit more. Get bored. Wander off. 10 minutes later wonder what that crackling noise is…. Uh oh!!!! The container was melting and a bubbling sticky fuming mess like a cow pat is spewing all over the fume cupboard. After a good telling off from Health and Safety, a stint doing job secondment in production and learning the ropes in R&D, it became apparent that not only I was a liability, but that I had added far too much rocket fuel to my concoction.
Around that time in QC I also became familiar with an exotherm test that was conducted on two-part epoxy adhesives (but only on the slow curing grades). 100ml of adhesive is thoroughly mixed, thermocouple wires inserted and then a temperature logger measures the time vs temperature so you can check a product’s exotherm. Other faster curing products such as certain MMA structural acrylic adhesives offer quite spectacular exothermic reactions – dispense a whole cartridge into a cup and put it in the fume cupboard and watch an impressive mushroom bubble up and shrink back down into the cup.
Single-part epoxies require heat to cure the adhesive but they can still become exothermic – if you cure too much in one go it can overheat, bubble and char and give an undesirable finish.
Even if an exotherm isn’t as dramatic as these examples, it could still be sufficient to damage substrate materials, entrap air bubbles, shrink excessively, affect aesthetic appearance of parts or damage sensitive electronic components.
How to avoid an unwanted exotherm
Of course a smouldering dung heap mess or an expanding mushroom are not on the wish list for engineers looking to use adhesives so there are some top tips for minimising exotherm:
- Be cautious of the amount of adhesive you mix; high volumes of material are much more likely to exotherm. If you do need to mix up a lot in one go, use a slower curing, less reactive product.
- For heat cure epoxies, exercise caution if using product for high volume potting and cure material at the lowest temperature listed on the cure schedule.
- Be extra careful when working in a hot environment as reactions will be faster and more vigorous.
- Keep single part epoxy in the fridge (especially bulk material).
- Ensure you are familiar with the adhesive’s curing instruction available on the technical datasheet (TDS) and the health and safety precautions on the Safety Data Sheet (SDS).
- If using an oven for curing heat-cure epoxies, this should be set up in a well-ventilated area possibly with extractors above the oven. This is good practice regardless of exotherm as often metal parts have oil contamination which can burn off in the oven. When opening an oven stand back so as not to inhale any fumes.
- Don’t mix unknown chemicals!
What to do in the event of an exotherm
If safe to do so, quickly move the container or part into a fume cupboard, under an extractor or outdoors to an open space. Avoid breathing the fumes. Use oven gloves for handling hot containers.
For questions relating to adhesive exotherms or anything else to do with adhesives for that matter, please contact our technical team for further assistance.