Technical Primer
Epoxy Crystallization
All epoxy resins are susceptible to a natural occurrence called crystallization. This phenomenon is similar to what happens to honey left undisturbed for long periods. Crystallization starts out slowly, with single tiny seed crystals forming at places called nucleation sites within the material. From these seed crystals, other crystals start forming and moving outward.
In epoxies, crystallization shows up as cloudiness, free floating crystals, clumps of crystal masses or in extreme cases the liquid can turn to a hard, brittle solid. Crystallization can occur in virtually all resins and hardeners.
The liquid polymeric materials that are common to all epoxy formulations are classified as super cooled liquids at room temperature. Because of their composition, they remain liquid at room temperature because the rate of crystallization, or solidification, takes place at a relatively slow rate. If certain conditions occur that allow the formation of small seed crystals in the material, then the rate of crystallization increases.
Because of the complexity of crystal formation in nature, crystallization can be very difficult to predict and control when working with epoxy adhesives.
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Extreme Cold
While cold storing an epoxy material can slow down the rate of crystal formation by slowing down the movement of the molecules, extreme cold storage even for short times can cause rapid crystal formations if small seed crystals are present.
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Temperature Cycling
Temperature cycling is perhaps the most frequent cause of most crystallization. Warming a material slightly allows the molecules to move around and contact seed crystals. The amount of temperature cycling required to induce crystallization is relatively small. A temperature swing of only 20oC can begin the crystallization process. As a result, temperature during shipping and warehouse storage must be taken into consideration. When epoxy materials are shipped by air, the combination of extreme cold in the cargo portion of a plane and exposure to multiple warm to cold temperature cycles can accelerate crystallization greatly.
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Accommodating Crystallization in Epoxies
Crystallization can be very frustrating to deal with in epoxy adhesives, because it cannot be attributed to one single factor, and its occurrence can be arbitrary. It is possible to have a shipment with hundreds of small containers of epoxy, and discover only a few of them randomly placed throughout the box will have varying degrees of crystallization.
Fortunately, two part epoxy systems can be treated easily to revert them back to a liquid state so they are suitable for use.
Simply heating the epoxy to 50oC for a short time is enough to revert it back to a liquid state. It is important to heat the epoxy long enough to make sure that all of the seed crystals have been liquefied. Once heating is complete, the material should be allowed to return to room temperature before mixing, or the excess heat will begin the crosslinking reaction more quickly when the two parts are mixed.
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