Basics for working with epoxies

This article provides an introduction to working with epoxies and general principles that apply to casting, coating and laminating systems.

Mix Ratio

The most common causes of curing issues are inaccurate measuring and improper mixing. Measure and mix part A and part B carefully and thoroughly. Modifying the mix ratio, such as adding more hardener will not make the epoxy cure faster, but will result in incomplete curing. Consult your product’s Technical Data Sheet for the correct mix ratios by both volume and mass.

If you are new to mixing resins, begin with a small test batch to get comfortable with how the product mixes and cures. This will provide you with an idea of the system’s working time in your ambient conditions. Use small batches until you are comfortable with the processing characteristics and be aware that larger epoxy batches will react more quickly and have shorter working times.

Epoxy Cure Stages

Mixing resin and hardener starts a chemical reaction that transforms the liquid components first to a gel and then finally into a solid.

• In the liquid phase, the epoxy can flow. All work should be completed in this phase, the working time corresponds to epoxy being in its liquid state.
• As chemical bonds (cross-links) form, heat is released, and the epoxy will heat up and thin before rapidly thickening. Once the mixture begins to gel, the epoxy will no longer flow and should not be manipulated.
• The final stage is solidification. At this point, the epoxy is highly cross-linked and will begin to develop mechanical properties and hardness. Once the epoxy has achieved significant hardness and cooled to ambient conditions, it is ready for finishing.

Epoxy cure stages as a function of time and temperature

There are various processing milestones that occur throughout these cure stages:

• Working Time begins when Part A and Part B are first mixed together and continues until the epoxy begins to thicken. Specified working times are based on application immediately after mixing is complete. Working times can be significantly shorter if resin is left in the mixing container for too long. Up until the working time limit is reached, the epoxy can be manipulated to achieve custom effects. It will self level and allow bubbles to rise to the surface.
• Tacky to Touch is the period where an additional application can be done without the need to abrade the surface of the previous layer. During this period, the project will need to be protected from contaminants that can adhere to the surface. To determine tacky to touch, wear gloves and lightly touch the surface of the casting. No resin will stick to the glove’s surface, but tackiness between the glove and surface will be apparent. The onset of tacky to touch has not been reached if the surface significantly deforms in this process.
• Set to Touch is the point in time immediately after the tacky to touch period, where the surface of the project is tack-free. An additional layer is not recommended without first abrading the surface of the previous layer. Determine if set to touch has been reached using the same method as tacky to touch. There is no observable tackiness between the glove and the surface.
• Demolding Time is the point in time at which the casting has cured sufficiently such that it can be carefully removed from the mold without causing damage to the epoxy. Although the project is solid, it is not fully cured and may sag under its own weight. The project should be supported until it reaches a fully cured state.
• Full Cure is the point in time when the project achieves full mechanical properties.

Processing milestones vary between products, consult your product’s Technical Data Sheet for these milestones.

Epoxy Temperature and Curing

Epoxies will take longer to cure at lower temperatures and will react faster with greater exotherm under warmer conditions. Several factors contribute to temperature:

1. The ambient conditions, or temperature of your work area.
2. The temperature of your resin and hardener, or the area where it is stored.
3. The temperature of the surface to which you will apply the epoxy.
4. The heat generated by the resin system during cure, or exothermic heat.
5. Ability of the curing resin and the mold material to shed heat.

Consult the Technical Data Sheet for your EcoPoxy product for recommended storage and working temperatures.

How Volume affects Exothermic Reactions

The conversion of epoxy resin and hardener from a liquid to a solid involves the formation of cross-links that release heat (exotherm). The amount of heat released depends on the amount of epoxy used; a certain amount of mixed part A and part B will result in the release of a certain amount of heat.

However, if the curing epoxy is unable to shed heat efficiently, the result may be overheating. The ability to shed heat is largely governed by the ratio of the top surface area that is open to air relative to the volume. For the same resin system, thinner applications will have greater surface areas in comparison to overall volume and shed heat more efficiently. Thicker applications cannot shed heat as efficiently and may overheat.

How volume affects exothermic reactions

For example, a coating epoxy poured into a cup at several inches of depth can achieve temperatures that can melt a plastic cup. In a traditional coating application, this resin would be applied thinly enough to be able to shed heat to avoid overheating. This is because a coating application has a much larger top surface area relative to volume.

If a cup of mixed epoxy begins to kick off (enters the initial cure phase and heats up uncontrollably), if possible, quickly move it to a well-ventilated area away from flammable materials. If there is time, the cup can be placed in a water bath to help cool it down. Avoid breathing fumes. Do not dispose of the mixture until cooled.