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Understanding Epoxy Chemistry and the Stages of Cure
Whatever techniques are used, a fundamental understanding of epoxy chemistry, as well as careful assembly and application planning, will allow you to take full advantage of epoxy’s open time and cure time.
In this article, we will explain the various epoxy cure stages to ensure a successful application.
Understanding epoxy chemistry is essential for using epoxy safely and effectively. When epoxy resin and hardener are mixed, a chemical reaction occurs, converting the liquid ingredients into solids. Cure time is the time required for the chemical reaction from liquid to solid to occur. As it cures, a liquid epoxy transforms into a gel, and then into a solid. During cure, mixed epoxy transitions through three states: liquid, gel, and solid. The following epoxy cure stages are described in detail below.
Stage 1: Liquid
Open time (also known as working time or wet lay-up time) in epoxy chemistry refers to the period after the resin and hardener have been mixed and the reaction has begun during which the mixture remains liquid and suitable for the application. Assembly and clamping should be done during the open period to ensure a reliable bond.
Stage 2: Gel
When the epoxy mixture begins to gel (also known as the “green stage”), it becomes unusable. As the epoxy progresses from a tacky, gel consistency to a dense rubber consistency, you will be able to dent it with your thumbnail. New epoxy applications continue to chemically link to the partially cured epoxy mixture until the epoxy mixture completely cures. This means that the surface can be bonded to or recoated without the need for additional preparation. This ability diminishes as a mixture approaches its final cure point.
Stage 3: Solid
A chemical reaction has occurred between the epoxy and the catalyst. After it has fully cured, your thumbnail should be unable to dent, indicating that the mixture can be shaped and dry-sanded. At this point in epoxy chemistry, approximately 90% of the product’s ultimate strength has been achieved, and a room-temperature cure will continue for several days.
Post Epoxy Curing
A new epoxy application will no longer chemically adhere to it. To achieve a successful mechanical, secondary bond, the epoxy surface must be properly prepared and sanded before re-coating. After the epoxy has solidified, low-temperature heat can be used to improve thermal performance and reduce prints through fabrics.
How Epoxy Cure Time Works
The open time and cure time govern many of the processes involved in the construction and preparation of epoxy. The materials are mixed, applied, smoothed, shaped, assembled, and clamped during the open time. It is critical to understand how long you must wait for your project to cure before proceeding to the next step. Two factors influence the open time and overall cure time of an epoxy mixture: hardener cure speed and epoxy temperature.
Hardening Speed of Epoxy
The epoxy hardener cure temperature range varies. Each resin/hardener combination will go through the same curing stages regardless of temperature but at a different rate. Choose a hardener that allows you enough working time for the job you’re doing and the conditions you’re working in. The product guide and container labels describe the hardener pot life and cure time.
The pot life of epoxy is a measurement of how quickly the epoxy cures when mixed with various hardeners. It describes how long a specific mass of resin and hardener remains a liquid at a given temperature. The pot life of an epoxy hardener is much shorter than its open time because it measures how quickly a mass of epoxy cures rather than just a thin film.
Epoxy Temperature Explained
Epoxy generally cures faster at higher temperatures. Heat accelerates the chemical reaction between epoxy components. The temperature of the curing epoxy is determined by the exothermic heat generated by the cure of epoxy.
The ambient temperature, which is the temperature of the air or material with which it comes into contact, heats up epoxy. Unless epoxy is applied to a different temperature surface, air temperatures are usually the ambient temperatures. For the epoxy to cure faster, the air temperature should be higher.
Exothermic heat is produced during the chemical reactions that cure epoxy. When epoxy surfaces are thick or exposed, they generate heat. Thicker masses retain more heat, resulting in a faster reaction and more heat. The shape of the mixing container and the amount of mixture have a large impact on this exothermic reaction.
When curing epoxy is added to a plastic mixing cup, it can generate enough heat to melt the cup and burn your skin. Chemical burns could occur if the epoxy is strong enough to melt the cup. A thin layer of the same amount, on the other hand, dissipates exothermic heat, so the cure time of the epoxy is determined by the ambient temperature. Curing epoxy takes longer when the layer is thinner because exothermic heat is less effective.
How To Control Epoxy Cure Times
If the weather is warm, use a slower epoxy hardener. Smaller batches are easier to handle and use, or you can pour the epoxy mixture into a larger container (such as a roller pan), extending the open time and dissipating the exothermic heat.
The earlier mixtures that can be coated, laid up, or assembled are applied, the longer their useful open time (after thorough mixing). Use a faster hardener or supplemental heat if the epoxy temperature falls below the hardener’s minimum recommended application temperature. Using a hot air gun, heat lamp, or another heat source, warm the resin and hardener before mixing or after the epoxy is applied. By adding supplemental heat to the room temperature, a quick cure can be achieved.
Epoxy Safety Precautions
Heat is produced during the epoxy curing process. Fill voids with epoxy that is no more than 1/2 inch thick. Even after standing for several hours, several inches of mixed epoxy (like a mixing cup) will generate enough heat to burn your skin, melt plastic ups, or ignite combustible materials in a confined mass. This is why you should avoid mixing in foam or glass containers, as well as pouring into confined spaces.
If exothermic epoxy begins to heat up, it should be moved outside. Take care not to inhale the fumes. Do not discard the mixture after the reaction has been completed and cooled.
Outgassing
Heat reduces the viscosity of uncured epoxy, making it easier to run and sag on vertical surfaces. Bubbles may form in epoxy coatings applied to porous substrates due to heat (concrete, softwood, or low-density core materials). To avoid outgassing, wait until the epoxy coating has gelled before warming it. Mixed epoxy should never be heated above 49° C.
Conclusion
Whatever techniques are used to control the cure time of epoxy, a fundamental understanding of epoxy chemistry, as well as careful assembly and application planning, will allow you to make full use of the open time and cure time of epoxy.
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