Epoxy Resin Crystallization

What is epoxy crystallization?

Crystallization of epoxy resins is a common issue in the modern electronics market. It is a phase change from liquid state to the natural solid state of epoxy. Epoxy resin is a “super-cooled” liquid, meaning it stays liquid even below its freezing point. Super-cooled liquids naturally tend to form crystals at lower temperatures if enough “seeds” are formed. These tiny seeds grow over time and can eventually prevent the product from performing properly.

Epoxy crystallization is more common in two-part systems (usually the resin side, labeled as Part A). But it can also occur in one-part heat cure epoxy. In rare cases, crystallization happens in curing agents too (usually labeled as Part B).

Crystallization cannot be avoided completely because it is naturally occurring, but the effects can easily be undone with modest amounts of heat. So, while it can be annoying, crystallization is not a difficult issue to overcome.

How to recognize epoxy crystallization

Generally, crystals appear as small floating lumps, and since they are heavier than the liquid resin, they precipitate to the bottoms of containers. Cloudy, hazy, and foggy resin usually indicate the initiation of crystallization. Small crystals will grow and spread usually from the bottom and side walls of containers, and if left undetected and untreated for extended periods, they can form a single solid mass.

It is important to note that if crystallization occurs, the resin is not defective and still can be used. It only has to be reconstituted for it to function properly.

Causes of epoxy crystallization and how to avoid them

Crystallization is unpredictable and difficult to avoid completely. Temperature fluctuations, lengthy exposure to extreme cold, high moisture content, high purity of resin, low viscosity, and product contamination are all factors that contribute to epoxy crystallization. The following simple tips will reduce the chances of crystallization:

  1. Clean container lids and edges thoroughly after use.
  2. Avoid contamination of resin by using only clean hardware and tools.
  3. Keep temperatures above 25 °C during storage and transportation.

How to reconstitute a crystallized epoxy

Before using an epoxy resin kit, it should be inspected for crystallization. If crystallization has occurred, reconstitute the product by warming it to between 55 °C and 65 °C, until it becomes fully liquified. The resin must be thoroughly stirred and scraped from the sides and bottom of the container to ensure even heat circulation and melting of all crystals. The time needed depends on the size of container, amount of product and the state of crystallization. It may take only a few minutes to reconstitute a small kit with a small amount of crystals, but a big drum with a severe crystallization may require overnight heating. After all crystals have been liquified, the product should be left to cool to room temperature before mixing, in order to prevent flash curing. Once cooled, the resin will function as if crystallization had never occurred.

It is important to note that this method will not work for one-part heat cure epoxy systems, where heating of material will initiate curing.

This simple and inexpensive procedure is usually all that is needed to control crystallization, which can be aggravating for unprepared users, especially when it occurs unexpectedly. 

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