In autumn and winter when the temperature is low, many customers often encounter such problems when using epoxy resin adhesive: epoxy resin adhesive sometimes becomes turbid and unclear, and partially lumped, in the same state as rice grains and fish seeds, and even becomes solid, which brings inconvenience to the construction of FRP.
In fact, this is a manifestation of the crystallization of epoxy resin. So, what is the crystallization process? Epoxy resin crystallized, and how to solve? Next, we give a brief explanation about the crystallization related problems.
What is the crystallization process?
Under normal conditions, liquid epoxy resin is a colorless transparent liquid state with fluidity. Generally, crystallization is easy to occur in the alternating period of the season and the region where the temperature difference between day and night is too large. In addition, the epoxy resin formulation in summer is easy to crystallize when it is stored in winter. Here are two processes of crystallization:
1. When the color of liquid epoxy resin becomes grey, free-floating crystals, crystal clusters or a fully solidified solid appear, and transparent resins become blurred, cloudy or opaque gradually, which is a sign of the early crystallization of epoxy resin.
2. More and more white crystals will accumulate at the bottom of the container. These sand crystals will continue to accumulate until all the contents are solidified into crystals. This process is actually similar to the crystallization of common honey.
These crystal structures are reversible and can be restored to liquid state at a certain temperature without affecting the properties of the products. It's the same thing that water's nature doesn't change in the course of repeated freezing and melting.
Why does crystallization occur?
There are many reasons leading to the crystallization of epoxy resin, mainly including the following factors:
a. high purity: crystals are high purity, and so is epoxy resin. The higher the purity is, the narrower the molecular distribution is, the easier the crystallization happens.
b. Low viscosity: The lower the viscosity is, the faster the crystallization rate is.
c. Impurities: Solid impurities are usually seed of crystal growth. Adding some precipitating pigments and fillers can easily lead to crystallization. Comparatively speaking, larger particle size and higher addition amount are not easy to induce crystallization.
d. Temperature: Such a narrow period of temperature change at 20-30 C is a common cause of crystallization, and the fluctuation of day and night temperature will start or accelerate the crystallization rate. In addition, crystallization will be accelerated at ultra-low temperature (below 5 C).
e. Moisture: The higher the moisture content is, the easier crystallization will occur.
All resins and curing agents have the possibility of crystallization, which is the transformation of substances from liquid to solid crystalline state. The crystallization principle of epoxy resin is similar to that of other substances. High purity, low viscosity, impurities, extremely cold environment, cold and hot cycles will both increase the possibility of crystallization.
Epoxy resin is sensitive to temperature. When extreme cold environment or temperature fluctuation is large, it is very easy to change from liquid to solid, that is, happening the crystallization of epoxy resin.
How to avoid crystallization?
1. Reduce the purity of epoxy resin, such as adding other types of epoxy resin.
2. Use low crystallization tendency or non-crystallization epoxy resin.
3. Storage conditions: Storage temperature is higher than 40 degrees Celsius to prevent crystallization. Generally, the crystallization risk can be reduced if the temperature is higher than 25 C, but excessive temperature fluctuation should be avoided. For the requirement of humidity, the relative humidity should be less than 50%. At the same time, the sealing of packaging should be strictly required to avoid water absorption leading to crystallization.
The epoxy resin glue is crystallized. What should we do?
1. Water bath heating method:
When putting the epoxy resin in the water, isolating from the water, the water temperature of epoxy resin should be between 50 and 60 degrees centigrade, and the colloid should be agitated unidirectionally at the same time. To ensure that all crystals are completely melted, any tiny unmelted crystal can be used as a seed to initiate recrystallization in a few days.
Therefore, the mixing time should not be too short to ensure that the crystals on the wall and bottom of the container melt and the heat distribution is uniform. Mixing uniformly until crystallization completely disappears before construction.
Note: The heated epoxy resin whiskers are cooled to room temperature, otherwise explosion will occur easily.
2. Heating equipment for heating:
The epoxy resin glue is arranged in a circular shape with 1 m wide area of discharging heating device in the middle. The outer part of the epoxy resin is enclosed with plastic cloth or strip cloth to form a closed space, which is heated continuously until the epoxy resin restores its initial state.
3. Solar lamp heating:
The epoxy resin is baked with a solar lamp, stirred while heated until the crystallization is completely melted, and the colloid can continue to be used after its initial state is restored.
4. Other reasonable heating measures can also solve the problem of crystallization of epoxy resin.
If the crystallization occurs again, it is heated again and the melting process is repeated. The melted epoxy resin can be remained for several hours at the environment of 50℃, and it can melt epoxy resin again.. The melted liquid epoxy resin can still be used, but it is not suitable for heating treatment when the curing agent has been added.