Last Updated: August 21, 2019

FOC-kbarkerDuring a recent visit to a customer’s production facility, I was shown two containers of crystallized epoxy: one looked milky, one was nearly solid. My customer was understandably concerned and feared the epoxy had “gone bad.”

It may sound strange, but I was glad this happened! This situation gave me the opportunity to educate my customer about crystallization in epoxies:

  • How this happens.
  • Why the epoxy is perfectly usable (it has not “gone bad”).
  • How they can easily return the epoxy back to its clear, liquid form.

Crystallization in epoxies is such a common occurrence that I want to educate all our customers. Here are some of the questions our customers ask us. I hope you find the following answers and advice helpful!

Crystallization in Epoxies Bar Fiber Optic Center

“How can I identify crystallization in epoxies?”

If you put a glass of water in the freezer for a few minutes, you’ll see ice crystals begin to form as the water changes to ice (its solid state). Similarly, crystals often form in epoxies. Crystallization in epoxies can look like salt granules in the otherwise clear liquid. The epoxy may look foggy and milky, or it may have solidified.

“Why does this happen?”

Simply put, the liquid resin is fighting to return to its natural state, which is a solid form. Crystallization in epoxies is so common that major manufacturers have written datasheets on the subject. According to these technical bulletins, extreme temperature cycles may kick-start the creation of crystals. For example, let’s say a container of epoxy is air-shipped to Mexico City. The cold temperatures in the high-altitude airplane are followed by hot temperatures in Mexico’s capital city. Temperature cycles are just one of many potential causes. Please keep this in mind: Crystallization is not in anyone’s control and, therefore, it’s difficult to avoid.

“Once the epoxy has crystallized, is it usable?”

Rest assured, the material has not “gone bad.” Once you return the epoxy to its clear liquid state, it is perfectly usable and still has the same material properties. As noted earlier, this is quite similar to water changing to ice, then back to water (liquid à solid à liquid).

“How do I return the material to its clear liquid state?”

While you can’t stop crystallization from happening, you can spot it and remedy it – right then and there. To avoid production delays, check your entire stock and heat treat all crystallized epoxy at the same time.

Follow this industry-standard heat treatment procedure:

  • Crystals can be eliminated by increasing the temperature of the liquid resin to about 50°C (about 122°F) until all signs of the crystals are gone. We suggest checking the material at 10 minutes, at 20 minutes, at 30 minutes, and at 1 hour. Typically, it takes only 20-40 minutes to eliminate crystals and fully return the epoxy to a clear liquid, but it could take several hours. (Note that this low-temperature heat treatment does not damage the epoxy or impact its material properties. Also, the epoxy won’t cure, because you have not yet mixed the material.)
  • Allow the resin to cool to room temperature, then proceed to use normally. It can be stored in its original packaging for later use.

“Should we check for crystallization? Do we need to adjust our inspection process?”

Yes, here at Fiber Optic Center, we strongly encourage all our customers to actively look for crystallization in epoxy, since it’s a common occurrence. To avoid production delays, we recommend incorporating the following steps into your process:

  1. Inspect upon receipt – When you receive the material, conduct an initial inspection. As noted above, if you discover crystallization, check your entire stock and heat treat all crystallized epoxy at the same time.
  2. Store properly – Ensure your epoxy is stored according to the datasheet’s recommended storage temperatures. (Note that crystallization can occur while properly stored.)
  3. Inspect prior to use – Train your production line employees to inspect the epoxy prior to use. If crystallization is found, you’ll need to heat treat that epoxy before use.

“Does Fiber Optic Center accept returns of crystallized epoxies?”

It’s important to remember that there’s nothing wrong with the material. Since crystallized epoxy is not defective, we do not accept returns. Of course, if you encounter a different issue, we’ll be glad to look into it.

Finally, a word of encouragement…

Crystallization in epoxies is common, and it’s likely that you’ll discover a crystallized container. When you do, please don’t be alarmed. Simply follow the heat treatment procedure noted above. This is a fast and easy remedy. In fact, one of my customers commented that everything should be this easy to fix!

Additional resources from the FOC team include:

Follow Kelly @EpoxyExprtsFOC


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