The transition period of an epoxy mixture from a liquid to a solid is called the cure time. Getting the cure time right is the key to getting a resin/hardener mix that will perform well in each application.
The curing process of PRO-SET® epoxy can be divided into three phases: working time, also called open time or wet lay-up time, (liquid state), initial cure (gel state) and final cure (solid state). The speed of the reaction, the length of these phases and the total cure time vary relative to the ambient temperature.
1. Working time
Working time is the assembly time of mixed epoxy. It is the portion of the cure time, after mixing, that the epoxy will remain in a liquid state and be workable. The end of the working time marks the last opportunity to apply clamping pressure to an assembly and obtain a dependable bond.
2. Initial cure phase
The working time is over when the mixture passes into an initial cure phase and has reached a gel state. It may be hard enough to be shaped with files or planes, but too soft to dry sand. Post-cure heating may begin once the mixture has reached an initial cure.
3. Final cure phase
In the final cure phase the epoxy mixture has cured to a solid state, and if not post-cured, will continue to cure over the next couple of weeks at room temperature. Post-curing at elevated temperatures will shorten the final cure phase of PRO-SET epoxies, and is necessary for components requiring the best thermal properties.
Controlling cure time
Several factors affect cure time and can be manipulated to extend the length of the cure time and working time to suit your requirements.
1. Type of hardener
Each resin/hardener combination will go through the same cure phases, but at different rates. Choose the hardener that gives you adequate working time for the job you are doing at the temperature and conditions you are working under. PRO-SET Hardeners may also be mixed with each other to provide a custom blend with an intermediate cure time.
2. Mixed quantity
Mixing resin and hardener together creates an exothermic (heat producing) reaction. A larger quantity of mixed epoxy will generate more heat and yield a shorter working time and overall cure time. Smaller batches of epoxy generate less heat than larger batches and have longer cure times. Therefore, a thicker joint, thicker laminate or layer of epoxy will cure quicker than a thin layer.
3. Container shape
Heat generated by a given quantity of epoxy can be dissipated by pouring the mixture into a container with greater surface area (a roller pan, for example), thereby extending the working time. Since the mixed epoxy will cure at a faster rate while it’s in the mixing pot, the sooner the mixture is transferred or applied, the more of the mixture’s working time will be available for assembly.
Heat can be applied to or removed from the epoxy to shorten or extend working and cure times. This can be especially beneficial when assembling very large or complicated components that require maximum working time and minimum final cure time. Be sure you fully understand the effects of heating and cooling on the mould before implementing these techniques.
The importance of temperature when curing epoxy
Before mixing, moderate heat can be applied to the resin and hardener to shorten the epoxy’s working time. Conversely, a cooler box can be used to draw heat from a roller pan to extend working time. For larger operations, impregnating machines with water-cooled rollers are available to extend working time.
After the epoxy is applied, a fan can be used to draw heat from the lay-up or application and extend the epoxy’s working time. The tooling itself can be designed to both extend working time and shorten cure time. It is possible to build tooling with tubing embedded.
During the lay-up, cool water pumped through the mould draws heat from the lamination, extending the working time. When the lay-up is complete, hot water or steam pumped through the mould will speed the cure of the laminate.
Moderate heat (hot air gun or heat lamp) applied to the assembly will shorten the epoxy’s cure time. Heat can be applied as soon as the assembly is completed, but most often heat should be applied after the epoxy has reached its initial cure. Heating epoxy that has not reached its initial cure will lower its viscosity, causing the epoxy to run or sag on vertical surfaces.
In some processing procedures, heating too soon can lower the resin content of the laminate to unacceptable levels. In addition, heating parts that contain porous materials (wood or low density core material) can cause the substrate to ‘outgas’.
When air in the porous material expands and passes through the curing epoxy, it can leave bubbles or pinholes in the cured epoxy. 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 the working life of the epoxy mixture.
Post-curing: to improve the strength of the cured epoxy
Resin/hardener combinations reach an excellent degree of cure for most applications with only a room temperature cure. Resin/hardener combinations with LAM-251-HT Hardener require an elevated temperature post-cure to achieve optimal physical properties.
Post-curing is the controlled heating of an epoxy laminate—after it has reached or passed its initial cure stage—to improve the physical strength and thermal properties of the cured epoxy. Each PRO-SET resin/hardener combination has potential maximum cured properties that can only be achieved by post-curing the laminate above a minimum target temperature.
For each resin/hardener combination there is a range of target temperatures (above the minimum target temperature) that will allow the laminate to reach 100% of its potential cured properties. Each target temperature within the range has a corresponding minimum hold time. Higher target temperatures require shorter hold times—lower target temperatures require longer hold times.
Maximum properties will not be reached if the actual post-cure temperature is below the minimum temperature in the range. However, even with post-cure temperatures below the minimum target temperature, most resin/hardener combinations will achieve increased properties.
Determine the post cure temperature for a resin/hardener combination by the desired physical properties of the component, or by the limits of the equipment to reach or hold a target temperature. Thermal shock can induce flaws in the laminate. To avoid this, increase the temperature slowly and do not exceed the maximum target temperature.