Epoxy's cure stages
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
As it cures, mixed epoxy pass from a liquid state, through a gel state, to a solid state.
1. Liquid-Open time 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 assure a dependable bond.
2. Gel-Initial cure 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 no longer feel tacky. During this do not disturb
stage it progresses from a soft gel consistency to the firmness of hard rubber. You will
be able to dent it with your thumbnail.
Because the mixture is only partially cured, a new
application of epoxy will still chemically link with it, so the surface may still be
bonded to or re-coated without special preparation. However, this ability diminishes as the
mixture approaches final cure.
3. Solid-Final cure The epoxy mixture has cured to a solid state and can be dry sanded and
shaped. You should not be able to dent it with your thumbnail. At this point the epoxy has
reached about 90% of its ultimate strength, so clamps can be removed. It will continue to
cure over the next several days at room temperature.
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. See Surface Preparation
Understanding cure time
Open time and 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 in 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. The product
guide and container labels describe hardener 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. (A 100g-mass mixture in a standard container,
at 72°F). 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
The warmer the temperature of curing epoxy, the faster it
cures (Figure 1). 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 container's shape and the mixed quantity have a great
affect 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.
Controlling cure time
In warm conditions use a slower
hardener, if possible. 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. 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, lay-up 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.
For detailed information on working with epoxy
at low temperatures, refer to 002-915 Cold
Temperature Bonding and Coating with Epoxy.
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 (softwood or
low-density core material) may cause the substrate to "out-gas" and form bubbles
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 120°F (49°C).
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.