Epoxy resin is an organic compound made up of chains of carbon that are linked to other elements, such as hydrogen, nitrogen, or oxygen.
When curing, a chemical reaction causes the epoxy to harden into a durable but flexible material that can be used in a variety of applications. This includes for molded products, metal coatings, and in construction.
However, epoxy often requires the introduction of a catalyst — known as a hardener — for proper curing.
The epoxy and hardener are packaged separately and mixed at the time of application. Manufacturer instructions will indicate the appropriate ratio of epoxy to hardener, which is measured by weight or volume.
It’s important to note that most products allow for a 5% margin of error when mixing. Altering the recommended mix ratio will affect pot life, cure speed, hardness, and adhesion.
Many two-part epoxies are liquids that can be mixed by hand. In this case, the measured epoxy should be dispensed into a mixing vessel and allowed to settle. The hardener is then added and the solution must be combined with a stirring utensil.
Two-part products are most often dispensed by measuring each part with a scale or graduated measuring cup. Others are packaged in containers with calibrated pumps that are designed to dispense the correct one-unit volume of each part with a single pump.
A steady folding motion will combine the two parts, producing an exothermic reaction and causing the mixing vessel to feel warm. The epoxy will become thicker and more viscous when combined. Complete mixing by hand may take several minutes.
The mixed epoxy will have a limited pot life, so assemblers should only prepare a quantity that can be used in a reasonable amount of time. Pot life varies from minutes to hours, and ambient temperature and humidity can affect the pot life of epoxy products.
With hand mixing, air bubbles might become trapped in the solution. Allowing the mixture to settle briefly can bring some bubbles to the surface where they can be released. Once the epoxy is poured, applying heat can also release air bubbles before curing. However, epoxy is typically flammable so always follow safety guidelines for each product.
The easiest way to ensure a precise mix for two-part epoxy products is to use pre-measured, dual-chamber cartridges. Each part is contained in a chamber with both of the chambers attached to a static mix nozzle. The nozzle incorporates a flow path ensuring an adequate number of stirs to combine the epoxy and hardener while passing through.
For example, a nozzle with 24 elements stirs the two parts 24 times to combine them before they’re forced out of the nozzle tip. The number of stirs will vary by product, so it’s important to only use dedicated nozzles for dual-chamber formulas.
Cartridges come in a variety of sizes. Consider the final product volume, pot life, and ergonomics of the job. When using cartridges that contain more than 600ml of epoxy, the use of a pneumatic gun is recommended.
For large batches of epoxy, several manufacturers offer high-volume equipment, such as metered displacement pumps that dispense a gallon or more of epoxy per minute. Large batches will cure faster than small batches, so pot life must be considered.
Static mixers combine epoxy inside high-pressure tubing. Resin and hardener are forced through a series of baffles that tumble the epoxy hundreds of times before it exits the mixer fully combined. A variety of motor-driven mixers are also available.