Hardener Selection Controls Cure Time
An epoxy’s open time and overall cure time govern much of the activity of building and repairing with epoxy, and hardener selection is crucial to determining open time and cure time. Open time dictates the number of minutes available for mixing, application, smoothing, shaping, assembly, and clamping. Epoxy cure time dictates how long you must wait before removing clamps, or before you can sand or go on to the next step in your project. Two factors determine an epoxy mixture’s open time and overall cure time—hardener cure speed and epoxy temperature.
| HARDENER | USES Resin/Hardener |
||||
|---|---|---|---|---|---|
| HARDENER TEMPERATURE RANGE (°F) | POT LIFE 100g cupful |
WORKING TIME thin film |
CURE TO SOLID thin film |
||
| General bonding, applying fabric, barrier coating |   |
9-12 min |
60-70 min |
6-8 hours |
|
| General bonding, applying fabric, barrier coating |  |
20-25 min |
90-110 min |
10-15 hours |
|
| General bonding, applying fabric, barrier coating |  |
40-50 min |
3-4 hours |
20-24 hours |
|
| Applying fabric, clear finish, general bonding, barrier coating |  |
20-26 min |
100-120 min |
10-15 hours |
*Epoxy cures faster in warmer temperatures and in thicker applications. Epoxy cures slower in cooler temperatures and in thinner applications.
Hardener Cure Speed
Each WEST SYSTEM Epoxy hardener has an ideal temperature cure range. At any given temperature, each epoxy resin/hardener combination will go through the same cure stages but at different rates of speed. Base your hardener selection on the amount of working time you’ll need for the job you’ll be doing at the temperature conditions you’ll be working under. The hardener selection guide and epoxy hardener container labels describe hardener pot lives and cure times.
The data we provide pot life is based on 100 grams or 4 pump strokes each of resin and hardener using our 300 Mini Pumps. If you are using a larger amount of epoxy or a different size of mixing pot the length of pot life can vary. To increase the pot life of a cup of mixed epoxy, spread it over a larger surface area, work in a cooler environment, or mix multiple, smaller batches as you go.
Epoxy Temperature
The warmer the temperature of curing epoxy, the faster it cures. The temperature of curing epoxy is determined by 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 temperatures 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. Hardener selection should take into consideration working temperatures in your shop.
Exothermic Heat
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 even more heat. The mixing container’s shape and the mixed quantity have a great effect on this exothermic reaction. A contained mass of curing epoxy (8 fl oz or more) 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.
The mechanism that causes heat buildup when epoxy is contained in a thick mass is also why epoxy takes longer to cure when it is applied as a thin film. The film does not build heat, so the temperature through the thickness of the epoxy film is pretty close to the ambient temperature.
Adapt Hardener Selection to Working Temperatures
In warm working conditions select a slower hardener to gain open time. 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 exothermic heat to dissipate and extending open time. After thorough mixing of epoxy resin and hardener, the sooner the mixture is transferred or applied, the more of its open time will be available for coating, lay-up or assembly.
In cool working conditions select a fastener 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 another heat source to warm the resin and hardener before mixing or after the epoxy is applied. At room temperature, supplemental heat is useful when you want the epoxy to cure faster. But do NOT use unvented kerosene or propane heaters because these can inhibit the cure of epoxy and contaminate epoxy surfaces with unburned hydrocarbons.
WARNING! Curing epoxy generates heat. Do not fill voids or cast layers of epoxy thicker than ½”—thinner if enclosed by foam or other insulating material Several inches of mixed epoxy in a confined mass such as a mixing cup will generate enough heat to melt a plastic cup, burn your skin or ignite combustible materials if left to stand for its full pot life. For this reason, do not use foam or glass mixing containers or pour epoxy into confined spaces. If a pot of mixed epoxy begins to heat up quickly (exotherm), quickly move it outdoors. Avoid breathing the fumes. Do not dispose of the mixture until the reaction is complete and has cooled.
Read about the effects of cool temperatures on epoxy adhesion at Epoxyworks.