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Catalog
FIBERGLASS, EPOXYS & COMPOSITES
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In recent years the term "epoxy" has become a household word. "Epoxy" is a general term for a vast number of specialized resin/hardener system, the same as "aluminum" is a general term for a whole family of specialized metal alloys. Just as the aluminum in the spar of a high performance aircraft is vastly different from the "aluminum" pots and pans in your kitchen, the "epoxy" in your aircraft is vastly different from the hardware store variety. Epoxy is the adhesive matrix that keeps the plies of load-carrying glass cloth together. Epoxy alone is weak and heavy. It is important to use it properly so that the full benefits of its adhesive capability are obtained without unnecessary weight. An "epoxy system" is made up of a resin and a hardener tailored to produce a variety of physical and working properties. The mixing of resin with its hardener causes a chemical reaction called curing, which changes the two liquids into a solid. Different epoxy systems produce a wide variety of solids ranging from extremely hard to very flexible. Epoxy systems
also vary greatly in their working properties, some are very thick, slow pouring liquids and others are like water. Some epoxy systems allow hours of working time and others harden almost as fast as they are mixed. A single type of resin is sometimes used with a variety of hardeners to obtain a number of different characteristics. In short, there is no universal epoxy system, each has its own specific purpose and while it may be the best for one application, it could be the worst possible in another use. The working and
strength characteristics of an epoxy system are dependent on the resin, the hardener and on the amount of each in a given mixture. Epoxy systems are engineered for a specific ratio of resin and hardener. It is quite important that the proper mixture be obtained. An accurate balance or ratio pump must be used to accomplish this. Epoxy resin and hardener are mixed in small batches, usually 6 ounces or less, even in the largest layup. The reason for small batches is that, in large batches, as the hardening reaction progresses,
heat is generated which speeds the reaction, which causes even more heat, which ends up in a fast reaction called an exotherm. An exotherm will cause the cup of epoxy to get hot and begin to thicken rapidly. If this occurs, throw it away and mix a new batch. The small volume batch avoids the exotherm. For a large layup, you will mix many small batches rather than a few large ones. With this method you can spend many hours on a large layup using epoxy that has a working life of only a few minutes. If the epoxy is spread thin as in a layup, it will remain only a few degrees above room temperature. However, in a thick buildup or cup, the low surface area to mass ratio will cause the epoxy to retain its heat, increasing its temperature. This results in a faster cure causing more heat. This unstable reaction is called an exotherm. Exotherm temperatures can easily exceed the maximum allowable for foam (200°) and damage the foam-to-glass
bond. Unwaxed paper cups are used for mixing and measuring resin and hardener. DO NOT use waxed cups; the wax will contaminate your epoxy. Mixing is done by stirring with a stick, being careful not to spill any. If you spill part of an unmixed cup, the ratio of resin and hardener may be inaccurate and it should not be used. Mix each cup for a least one minute. You should spend 80% of your mixing time stirring the cup and 20% scraping the sides to assure complete mixing. Do not mix with a brush. The bristles can soak up the hardener, changing the ratio. Use a tongue depressor or wood stick. The working temperature
has a substantial effect on the pot life and cure time. Very hot conditions will cause the cure to speed up. In cold working conditions the cure will be delayed and if it is cold enough, epoxy may not cure at all. Working temperatures must be between 70° and 100°. A range of 75° to 85° is best. Cold epoxy results in increased time required to do a layup, since it takes longer to "wet" and to squeegee the cloth. A layup at 70° F may
take almost twice the time as at 80° F. On most layups (except for joining foam cores) it is best to have 75° to 85° F room temperature and 90° to 100° F epoxy. Resin and hardener can be kept warmer than room temperature by keeping it in a cabinet with a small light bulb on. DO NOT store your resin or hardener on a cold floor if you plan to use it within the next several hours. If you let your shop get cold between working periods, keep some resin and hardener in the warmest place of your house for use on the next layup. Sometimes epoxy hardener will have solids form in the bottom of the bottle. If this happens do not use the hardener. All of the hardener must be liquid before it is used. To use this hardener it is necessary to heat it up and stir or shake it slightly. To do this heat some water on your kitchen stove in a large pan with the hardener in the pan of water. Move the bottle around to be certain not to melt the plastic container. When the water has just started to boil pull the hardener out and slosh it around. This should dissolve all of the sediment in the bottom. If not put it back in the water for a few minutes and then shake it again. Save your mixing cups, as they can be used as a quality check of your epoxy. After a day or two take a sharp knife point or scribe and scratch at the face of the epoxy in the cured cup. If the epoxy cured properly the scribe will make a white scratch mark. If the epoxy has not cured, the scribe will make a dull ridge, indicating a soft surface. If this occurs the epoxy is not cured, either due to inadequate time or temperatures, or bad mixing or bad epoxy. Temperature is very important in working with epoxy. If you are working in a garage in the winter, the room must be heated up well in advance of the start of your layup. The foam blocks are a very good insulator and so it can take as long as four hours for the foam to warm completely to room temperature. Start heating your work area and all materials well in advance of starting your layup. |
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