What is an Exothermic Reaction & How to Prevent It
An exothermic reaction is a chemical reaction that results in the release of energy in the form of heat, light, or sound.
Epoxy resins are either organic or synthetic materials commonly used in adhesives and coatings. They are made up of long chains of molecules and their structure allows them to harden when they encounter other materials. For example, once the resin is mixed with a separate hardener, they will begin to develop new chemical bonds, which will, in turn, produce the epoxy resin exothermic reaction, releasing the heat once the mixture itself begins to catalyse, causing the epoxy resin to ‘cure’ or harden.
This is what makes epoxy resins ideal for applications where a strong bond is required, whether that be for industrial construction or engineering, or in your unique art projects.
But while exothermic reactions have many useful applications, they can also be dangerous.
If the exothermic reaction is not carefully controlled, users may notice their epoxy resin start to smoke, their mixing cups melt, or even more dangerous repercussions from the build-up of heat. The phrase ‘exothermic runaway’ is used when the epoxy mixture becomes excessively hot and starts to bubble, smoke or crack. Given the process described above, some exothermic reactions will be required to help the mixture solidify, but if temperature and volume are not controlled, the amount of heat can build to disastrous levels.
What Causes Epoxy to Overheat?
Several factors can cause the epoxy to overheat.
1. More than 10% ink/pigment/acrylic paint to Resin ratio used *Making note that some acrylic paints or pigment additives may not be compatible and may accelerate curing due to high water content 2. Incorrect ratios, measuring is to be precise. Measuring cups may not be calibrated correctly - not all cups are created equal and can play about with the ratio. When measuring small amounts of resin/hardener, this can also play about with the ratio, as there is very little room for error (as the cups are not calibrated correctly). You may find mixing larger amounts assists with this. 3. Room being too warm, 22-25 degrees is ideal 4. Leaving mixed Resin in a confined space (eg cup, jug) will accelerate curing and could encounter aggressive exothermic reaction
Before application, be sure to review the recommended ‘safe’ pouring depth for your epoxy resin when at room temperature. Should you exceed this depth limit, then the transfer of heat during the chemical reaction will prove difficult. Scientifically speaking, the amount of thermal mass will be greater than the thermal transfer rate in the generation of heat; in other words, the thicker the layer of epoxy, the longer it will take to cure, and the more heat will be generated in a snowball effect, potentially causing disaster for your project, and even putting your surrounding environment (as well as people in close vicinity) at risk.
Previous Layers Haven’t Cooled Before Further Application
To avoid overly thick layers being applied at one time, it makes sense to apply several layers to your projects. When doing so, ensure the previous layer has properly cooled before pouring the next. Heat will always rise, after all, and the newest layer could potentially overheat if the previous is still generating additional heat.
As we have discussed, temperature plays an instrumental role in the curing process of epoxy resin. It is therefore important to control the ambient temperature of your surroundings, as this will influence the heat levels within your mixture. Experts agree that a consistent temperature range of 18°C to 27°C is perfect for optimal curing.
Inadequate Transfer of Heat
If the materials or moulds you are using have insulating properties that greatly retain heat, this will also occur with your epoxy temperature, causing it to significantly increase.
Warming Before Pouring
Again, we must highlight the role temperature plays in mixing epoxy resin for it to cure. If you are operating in a colder environment, and you feel you are required to warm your epoxy so it can properly mix, then ensure you adequately spread the epoxy across the surface of your project, rather than letting it spend too much time sitting within the mixing container. This is because warming the epoxy will have accelerated the specific chemical reaction described above, leading to it potentially curing within the container, and proving unsuitable for further use.
How to Prevent an Exotherm From Occurring
While an exotherm may sound concerning, there are a few simple measures you can take to prevent an unexpected reaction from occurring, such as:
Only Mix the Amount of Epoxy You Require
As mentioned above, epoxy that sits in greater masses will continue to exponentially exotherm and produce heat. It is therefore important to only mix the amount of epoxy you plan to apply within your project. Spread it out evenly over the surface of your object or item, and work in small batches. This allows you to minimise the amount of time the mixture spends in the mixing cup (especially whilst the chemical reaction is occurring and overheating the cup itself), as well as avoiding any potential waste.
Pour in Thin Layers
Be conscious of your maximum depth before pouring. When you are new to pouring epoxy resin, it is always better to be safe than sorry, especially when working with higher volumes of the mixture. For example, if you are pouring around 3 litres, pour a thinner layer to enable a better transfer of heat. As a general rule of thumb, if you are selecting an epoxy for casting purposes, the shorter the gel time, the lower the amount of mixture you will be able to pour at one time.
Maintain a Cooler Ambient Temperature
We have spoken a bit about the optimal temperature of your curing environment, but if you are concerned you have poured a bit too deep, or the exotherm is generating too much heat, you can try to cool the workspace further to slow it down. This could merely be opening a door, or window or setting up a fan to blow cool air over your project, but remember that with added airflow there may also be dust and other fine particles that may get caught in your resin.
Perform Test Runs
If you are new to working with epoxy resin, begin with a few smaller, practice mixes to first get a better understanding of the chemical reactions involved, especially at the different depths when pouring. Epoxy resin systems will vary amongst brands, so this also helps you to find a product you are comfortable using moving forward.