Crystallization of epoxy resin formulations is not uncommon and in some cases is a reflection of the high-performance nature of these materials. Under the right conditions, any epoxy resin can crystalize. Crystallization may present itself initially as cloudiness but can proceed to suspended crystals, sedimentation, and eventually a solid mass, given enough time under the right environmental conditions.
Similar to other types of crystal growth (i.e. water freezing to become ice) the process of epoxy crystallization is a reversible phase change from liquid to solid. The tendency for an epoxy resin to crystallize depends on a number of factors. These include the purity of the resin, viscosity, additives, moisture, and the storage environment.
Crystallization is hard to predict and can happen randomly. While some resins are more prone to crystallization than others, it is not uncommon to have varying degrees of crystallization even within a given batch of resin. This is why it is important to understand how to minimize the risk of crystallization.
The role of thermal cycling
The most common factor contributing to crystallization is thermal cycling. Temperature cycles of as little as 20-30°C can significantly increase the possibility of crystal growth in epoxy resins. Minimizing temperature fluctuations in storage can dramatically improve long-term stability. Keeping resin containers sealed will also prevent moisture from potentially facilitating crystallization. Container spouts should be wiped clean of any remaining resin after each use. Pumps and spigots should be kept clean and free of dust and debris as these can initiate crystal growth in the resin container.
The ease of identifying whether or not a resin has crystallized depends on the resin in question. Resins that are formulated to be clear, low color and free of additives are by far the easiest to identify.
CCR & CPM
Resins such as CCR and CPM may first turn cloudy as the initial stages of crystallization begin. The CCR and CPM resins should be clear when viewed down through the container opening. Any difficulties seeing the bottom of the container may indicate that the early stages of crystallization have begun. These resins are also the easiest when identifying that the crystallization issue has been resolved as they will return to their clear appearance once the crystallization has been reversed.
CLR, BRT, & ONE
Crystallization is a bit more difficult to identify in these resins. All three utilize additives to improve flow and air release and, in the case of BRT, enhance optical characteristics. These additives give CLR an inherent cloudy appearance and BRT a bluish/purple color that is typical of these formulations. Identifying crystallization in these resins is more difficult since the early stages of crystallization may be masked by the inherent cloudiness or color of the resin. C
Identifying crystallization in CLR and BRT must be done by looking for other indicators such as floating crystals, sedimentation, or large masses of crystal growth. Look in containers for any sign of crystals settled to the bottom, suspended at the air/resin interface, or clinging to the container walls. You may also remove a sample of the resin from the container and look for any grainy appearance or inconsistency in how the product flows or pours.
As mentioned above, crystallization is a reversible process. The crystals can be reverted back to their original liquid state with heat. Heating the resin container to a temperature of approximately 125°F will melt the crystals. Blending the resin may also be necessary to remove all traces of crystals. This mixing will ensure even heating and provide a uniform resin blend.
How to heat resin
The most efficient approach to heating resin is the use of a water bath. This provides the best heat transfer to the crystallized resin and reduces the risk of damaging the container. Where a water bath is not practical, such as in drum or tote sizes, other heating techniques may be necessary.
If other types of heating devices are used, such as drum band heaters, blend the resin continually, and carefully monitor the temperature.
Regardless of the heating method you choose, exposing the resin to temperatures above the recommended 125°F can lead to adverse effects such as discoloration.
Length of heating required
The time required to eliminate all signs of crystallization will vary. Small containers will become crystal-free in a few hours. while larger masses such as drums or totes may require a few days due to slow heat transfer in larger masses.
It is important to ensure that all crystals are melted in the heating process. Any remaining crystals can act as a seed and promote re-crystallization.
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