Mixing epoxy resin and hardener begins a chemical reaction that transforms the combined liquid ingredients to a solid. The time it takes for this transformation is the cure time. As it cures, the epoxy passes from the liquid state , through a gel state, before it reaches a solid state.
Open time (also working time or wet lay-up time) is the portion of the cure time, after mixing, that the resin/hardener mixture remains a liquid and is workable and suitable for application. All assembly and clamping should take place during the open time to assume a dependable bond.
The mixture passes into an initial cure phase ( also called the green stage) when it begins to gel, or “kick-off”. The epoxy is no longer workable and will progress from a tacky, gel consistency to the firmness of hard rubber. You will be able to dent it with your thumbnail.
The mixture will become tack free about midway through the initial cure phase. While it is still tacky ( about like masking tape), a new application of epoxy will still chemically bond to or recoat the surface without special preparation. However this ability diminishes as the mixture approaches the final cure phase.
The epoxy mixture has cured to a solid state and can be dry sanded. You will no longer be able to dent it with your thumbnail. At this point the epoxy has reached most of its ultimate strength, so clamps can be removed. A new application of epoxy will no longer chemically link to it, so the surface of the epoxy must be properly prepared and sanded before recoating to achieve a good mechanical, secondary bond.
Controlling Cure Time
Open time and overall cure time govern much of the activity of building and repairing with epoxy. Open time dictates the time available for mixing, application, smoothing, shaping, assembly and clamping. Cure time dictates how long you must wait before removing clamps, or before you can sand or go on to the next step of the project. Two factors determine an epoxy mixture’s open time and overall cure time – hardener cure speed and epoxy temperature.
Each hardener has an ideal temperature cure range. At any given temperature, each resin/hardener combination will go through the same cure stages, but at different rates. Select the hardener that gives you adequate working time for the job you are doing at the temperature and conditions you are working under. Check data sheets for pot lives and cure times.
Pot Life is a term used to compare the cure speeds of different hardeners. It is the amount of time a specific mass of mixed resin and hardener remains a liquid at a specific temperature. Because pot life is a measure of the cure speed of a specific contained mass (volume) of epoxy rather than a thin film, a hardener’s pot life is much shorter than its open time.
The warmer the temperature of the curing epoxy, the faster it cures. The temperature of curing epoxy is determined by the ambient temperature plus the exothermic heat generated by its cure.
Ambient temperature is the temperature of the air or material in contact with the epoxy. Air temperature is most often the ambient temperature unless the epoxy is applied to a surface with a different temperature. Generally, epoxy cures faster when the air temperature is warmer.
Exothermic heat is produced by the chemical reaction that cures epoxy. The amount of heat produced depends on the thickness or exposed surface area of mixed epoxy. In a thicker mass, more heat is retained, causing a faster reaction and more heat. The mixing containers shape and the mixed quantity have a great affect on this exothermic reaction. A contained mass of curing epoxy in a plastic mixing cup can quickly generate enough heat to melt the cup and burn your skin. However, if the same quantity is spread into a thin layer, exothermic heat is dissipated, and the epoxy’s cure time is determined by the ambient temperature. The thinner the layer of curing epoxy, the less it is affected by exothermic heat, and the slower it cures.
Adapting to warm and cool temperatures
In warm conditions, gain open time by using a slower hardener. Mix smaller batches that can be used up quickly, or pour the epoxy mixture into a container with greater surface area (a roller pan, for example), thereby allowing the exothermic heat to dissipate and extending open time. The sooner the mixture is transferred or applied (after thorough mixing), the more of the mixture’s useful open time will be available for coating, laminating or assembly.
In cool conditions use a faster hardener, or use supplemental heat to raise the epoxy temperature above the hardener’s minimum recommended application temperature. Use a hot air gun, heat lamp or other heat source to warm the resin and hardener before mixing or after the epoxy is applied. At room temperature, supplemental heat is useful when a quicker cure is desired.
NOTE: Unvented kerosene or propane heaters can inhibit the cure of epoxy and contaminate epoxy surfaces with unburned hydrocarbons.
CAUTION: Heating epoxy that has not gelled will lower its viscosity, allowing the epoxy to run or sag more easily on vertical surfaces. In addition, heating epoxy applied to a porous substrate (soft wood or low density core material) may cause the substrate to “out-gas” and form bubbles or pinholes in the epoxy coating. To avoid out-gassing, wait until the epoxy coating has gelled before warming it. Never heat mixed epoxy in a liquid state over 49oC. Regardless of what steps are taken to control the cure time, thorough planning of the application and assembly will allow you to make maximum use of epoxy’s open time and cure time.