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5.3 Handling epoxy This section explains the fundamentals of epoxy curing and the steps for proper dispensing, mixing, and adding fillers to assure that every batch of epoxy cures to a useful high-strength solid. 5.3.1 Understanding 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 (Figure 5-3). Figure 5-3 As it cures, mixed epoxy passes from a liquid state, through a gel state, to a solid state. Cure time is shorter when the epoxy is warmer. Cure time is longer when the epoxy is cooler. 1. Liquid—Open time Open time (also working time or wet lay-up time) is the portion of the cure time, after mix- ing, that the resin/hardener mixture remains a liquid and is workable and suitable for appli- cation. 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 stage it progresses from a soft gel consistency to the firmness of hard rubber. You will be able to dent it with your thumb nail. The mixture will become tack free about midway through the initial cure phase. While it is still tacky, 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 dimin- ishes as the mixture approaches the final cure phase. 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 most 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 re-coating to achieve a good mechanical, sec- ondary bond. See Surface Preparation 5.4.1. 5.3.2 Understanding and controlling 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 mix- ture’s open time and overall cure time—hardener cure speed and epoxy temperature. Using WEST SYSTEM® Epoxy 34 5PDF Image | Vacuum Bagging Techniques 002150
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