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September 13, 2016 3 min read

If you've used epoxy before, you probably know that mixed resin and hardener can get hot in a hurry. Case in point: Figure 1 - that cup has seen its last day.

This mixed epoxy, along with its 221 °F reading, is the aftermath of what is known as an exothermic reaction. To put it in plain terms: an exothermic reaction is a freight train of heat that sends your mixed epoxy into oblivion. But how does this happen and how can it be prevented?

Figure 1: Exothermically reacted epoxy with temperature for reference.

The driving variable that affects the curing time of epoxy products is temperature. All of the information relating to cure and set times on our product labels is based on a standard temperature of 77 °F (25 °C). Assuming your local environment will be hotter or cooler than 77 °F, a great rule of thumb to keep in mind is that for every additional 18 °F (10 °C) above 77 °F, your cure time will be cut in half. Conversely, for every 18 °F cooler, be prepared for your cure time to double.

The byproduct of an epoxy curing reaction is heat. The fewer opportunities this heat has to escape, the faster this reaction will occur. Looking back at Figure 1 - and thinking about our rule of 18 °F - if the temperature of your mixed epoxy has risen 18 °F after 5 minutes in a cup, its cure time will be halved. Two and a half minutes later, and another 18 °F, now your cure time is halved again. This doesn't have to go on for very long before your cup is smoking, possibly melting, and you're left wondering what the heck happened!

Rotfix, one of the products in our EndRot line, is a great example of how heat effects epoxy. While we do offer a helpful mixing bottle with our 24 Oz. kit, folks often get themselves in trouble by mixing too much product in the bottle and leaving it for several minutes before getting down to work. By the time they are ready to begin, the product has reacted and become very hot! We strongly suggest starting with small, graduated medical cups for RotFix applications since a little product goes a long way, and gets hot fairly fast. This also helps users become more familiar with how our products function.

Preventing exothermic reaction comes down to three things:

  1. Organization
  2. Surface Preparation
  3. Execution

1. Organization

Figure 2: Measuring, mixing and application tools you may need.

Preparing for your project before jumping in will really make a big difference! If you're laminating fiberglass or carbon fiber, this means getting your fabrics cut to size and staged, and then calculating a rough estimate of how much resin you will need (this will come with experience and time using the products). You'll also need measuring, mixing and application tools, like those seen in Figure 2. Getting organized before starting your project will mean spending less time finding your tools and more time getting the epoxy mixed and applied.

2. Surface Area

Figure 3: Great (and inexpensive!) tray to use for expanding surface area.

As discussed in our intro, the reason that mixed epoxy gets so hot in a cup is that the heat being generated has nowhere to go. Preventing exothermic reaction, depending on the application, is usually as easy as pouring the mixture out into a pan. This spreads the epoxy out, thereby increasing its surface area, and allows the mixture to release heat. As seen in Figure 3, paint trays and other disposable baking pans (consider going to your local Dollar Store for these) are great for giving your epoxy, and you, a chance to breathe during the application process.

3. Execution

When applying epoxy, don't jump into a project that you aren't comfortable doing. It's all about starting small and then working up to bigger jobs as you gain more familiarity with the product. Mixing small batches, or however much can be used in a reasonable amount of time, is still something that we continue to do even though we have been using this stuff for many years. It will allow for the flexibility of working with multiple products when your project demands it, you'll eliminate wasted material and save money, and you also won't burn your hand!

We hope this post has helped to clarify how heat affects your epoxy projects. For more information, check out our video below on exothermic reaction:

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