WHAT IS EPOXY RESIN CRYSTALLIZATION?
It can often come as a surprise, but crystallization is more of an inconvenience rather than a problem. By definition, it is referred to as a phase change from a liquid resin to a solid format. In epoxies its exhibited by an orginally clear liquid becoming cloudy, turbid or even solid upon inspection. Crystallization of epoxy resins is completely reversible and does not effect the properties of epoxy resin.
Again, crystallization of epoxy resins is more of an inconvenience rather than a problem. Heating the resin to a temperature above 100°F is usually sufficient for re-melting the crystals. The required amount of time will depend upon degree of crystallization, but typically 1-2 hours will suffice. It is important to be certain that all of the crystals have been melted away and can no longer act as seeds before cooling to room temperature. This can be done by closely examining the container sides, bottom and areas around the caps for any signs of crystallization the could nucleate additional growth. If possible, it is recommended to clean the bottle caps and bottle neck with denatured alcohol after each use in order to prevent seeds from developing. The same applies for spigots, spouts, pumps, piping and valves. Controlling and monitoring shipping and storage temperatures is a good way to prevent crystallization from fluctuations in temperature. Good housekeeping is also a great way of preventing this as well.
SIGNS OF CRYSTALLIZATION
Crystallization appears in the form of cloudiness, free-floating crystals, crystal masses or as a completely solidified. Since the crystals are higher density than the liquid resin, they sink to the bottom of the container. At the onset of crystallization, the clear resin begins to look foggy, cloudy, hazy or turbid to milky white. The white sedimentation continues to build, pack and spread, typically from the bottom of the container to corresponding side-walls. This sandy-like texture will eventually overcome the entire contents of the container. Once solidified, crystallized epoxy resin can be stored indefinitely in this state.
WHY DOES CRYSTALLIZATION OCCUR?
Many plastic resins are super-cooled liquids, including epoxy resins. They are solids in format at room temperature but remain in a liquid state below their freezing temperature. In general, super-cooled liquid resins have a natural tendency to crystallize at low temperatures. Other factors such as extreme cold, fluctuation in ambient temperatures and thermal cycling can cause seed crystal growth and may induce materials to revert back to their natural, solid state.
CAUSES OF CRYSTALLIZATION
Crystallization can be difficult to predict and eliminate entirely. It happens randomly, without warning, and may affect parts of a given production batch. It is normal for a few containers from the same batch to show differing degrees of crystallization deposits. Understanding the factors that contribute towards crystallization helps with knowing how to deal with them.
Temperature – While cold temperatures can reduce the crystal formation/growth by slowing movement within the liquid, it also accelerates crystal formation once seed crystals have been formed. Try to store resin at a constant room temperature.
Thermal Cycles – Temperature cycles of as little as 20-30°C are the most common cause of crystallization. Once the material is warmed, molecular motion is enhanced allowing the liquid epoxy to orient itself around the seed crystals. Subsequent exposure of an oriented material to cold temperatures will then accelerate crystal growth. Once started, the crystallization typically will go to completion resulting in a solid mass. The temperature fluctuations than occur between night and day can start or enhance the crystal growth process.