BASIC EPOXY CHEMISTRY


West System Products publishes a full run down on how to use their products. This is a small sample of one of their sections. For the complete set of instructions contact your local supplier for West System Products or call: Gougeon Brothers, Inc. (517) 684-7286 0r fax: 584-1374. They have a web site now too. And some of this stuff is on their site. Since they have had several revised versions, this one here may be a bit different than the one they have on their site. (I must admit, I haven't read their web site version.) Anyway, when doing hull repairs, we strongly recommend considering their complete line of products.


  Mixing the epoxy resin and hardener together begins a chemical reaction that gradually changes the combined ingredients from a liquid to a solid. Careful measuring and thorough mixing are essential for the reaction to occur. Whether the resin/hardener mixture is applied as a coating or modified with fillers or additives, the chemical transition to an epoxy solid continues to take place. This transition period is generally labeled the cure time and can be divided into three phases-pot life, initial cure and final cure. The speed of the reaction (and the length of these phases and the total cure time) varies relative to the ambient temperature.

1. Pot life and wet lay-up time
"Pot life" or working life is the phase of the cure time, after thorough mixing, that the resin/hardener remains in a liquid state and is workable or suitable for application. "Wet lay-up time" or assembly time is that portion of the pot life between application of the mixed epoxy to the surface and the last opportunity to apply clamping pressure to the assembly before the epoxy has cured too much for a dependable bond. Since the mixed epoxy will continue to cure whether it's applied to the surface or left in the mixing pot, the sooner you apply the mixture, the more of the mixture's useful pot life will be available for assembly time.

2. Initial cure phase
The pot life is over when the mixture passes into an initial or partial cure phase (sometimes called the green stage) and has reached a gel state. At this Point the epoxy will feel tack free and the reaction appears to be complete. It is hard enough to be shaped with files or planes, but too soft to dry sand. You will still be able to dent it with your thumb nail. It may still be bonded to or re-coated at this point, without sanding.

3. Final cure phase
In the final cure phase, the epoxy mixture will have cured to a solid state and will allow dry sanding and shaping, and must be sanded before re-coating. You should not be able to dent it with your thumbnail. At this Point the epoxy will have reached about 90% of its ultimate strength, so clamps can be removed. The epoxy will continue to cure over the next several days at room temperature conditions.

HANDLING EPOXY

Dispensing
Most problems related to curing of the epoxy can be traced to the wrong ratio of resin and hardener. To simplify metering, Gougeon Brothers, Inc. recommends using calibrated WEST SYSTEM Mini Pumps to dispense the resin and hardener.
Dispense WEST SYSTEM resin and hardener into a clean plastic, metal or paper container. Don't use glass or foam containers because of the potential danger from exothermic heat build-up.

301 Mini Pumps and 303 Special Ratio Mini Pumps will deliver the proper working ratio with one full pump stroke of Resin for each one full pump stroke of hardener.
Before you use the first mixture on a project, verify the proper ratio according to the instructions that come with the pumps. Re-check the ratio anytime you experience problems with curing.

Weight/Volume measure
To measure 105 Resin and 205 or 206 Hardener by weight or volume, combine five parts resin with one part hardener.
To measure 105 Resin and 207 or 209 Hardener by weight or volume, combine three parts resin with one part hardener.

MIXING

Mixing epoxy with error-free results involves three separate steps:

1. Dispense the proper proportions of the resin and hardener into a mixing pot. Begin with a small batch if you are unfamiliar with the pot life or coverage of the epoxy.

2. Stir the two ingredients together thoroughly with a wooden mixing stick (1 to 2 minutes is recommended). Scrape the sides and bottom of the pot as you mix. Use the flat end of the mixing stick to reach the inside corner of the pot.

3. Thoroughly stir in additives, such as pigments and fillers, if required.
If you are going to be using the mixture out of a roller pan, mix it thoroughly in a mixing pot before transferring it to the roller pan. Do not use a Power mixer unless you thoroughly scrape the sides and corners of the mixing pot while mixing.

CAUTION! Heat is generated by the chemical reaction that cures epoxy. A plastic mixing cup full of mixed epoxy will generate enough heat to melt the cup, if left to stand for its full pot life. If a pot of mixed epoxy begins to exotherm (heat up), quickly move it outdoors. Avoid breathing the fumes. Do not dispose of the mixture until the reaction is complete and has cooled.

Controlling cure time
Several factors affect the length of pot life, wet lay-up time and overall cure time of an epoxy mixture:

1. Type of hardener
Each resin/hardener combination will go through the same cure phases, but at different times. The catalog section lists the hardeners with their pot lives and cure times. Choose the hardener that gives you adequate working time for the job you are doing at the temperature and conditions you are working under. Pot life can also be manipulated by mixing 205 and 206 Slow Hardener. It is critical, however, that the proper resin-to-hardener ratio for the epoxy you are using is maintained. Do not mix 205 or 206 (5-to-1 ratio) Hardeners with 207 or 209 (3-to-1 ratio) Hardeners.

2. Mixed quantity
Mixing resin and hardener together creates an exothermic (heat producing) reaction. The greater the quantity, the more heat generated, the shorter the pot life and cure time. Smaller batches of epoxy generate less heat than larger batches and have longer pot lives and cure times. Similarly, a thicker joint or layer of epoxy will cure sooner than a thin layer.

3. Temperature
An industrial hot air gun, hair dryer or heat lamp can be used to heat the applied epoxy and shorten its cure time, or a fan can be used to draw heat from the surface and extend the epoxy's cure time. Never heat the epoxy over 120"F (49"C). Be aware that heating epoxy that has been applied to a porous material (wood) may cause "out-gassing" (air expands and passes from the material, forming bubbles in the epoxy coating). This would only be a concern if you desire a clear finish.

4. Container shape
Heat generated by a given quantity of resin/hardener mixture can also be dissipated by pouring the mixture into a container with greater surface area (a roller pan, for example), thereby extending the pot life.
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 mixture.

Adding fillers and additives

Throughout this manual, we may refer to epoxy or resin/hardener mixture, meaning mixed resin and hardener without fillers added; and thickened mixture, meaning resin/hardener with either high-density or low-density fillers added. Fillers are used to thicken the basic resin/hardener mixture for specific applications. Each filler possesses a unique set of physical characteristics, but the fillers can be generally categorized as either high-density or low-density.

Mixing The thickness of a mixture required for a particular job is controlled by the amount of filler added. Figure 2 gives you a general guide to the differences between un-thickened epoxy and the three most commonly used consistencies.

Always add fillers in a two-step process:

1 . Mix the desired quantity of resin and hardener thoroughly before adding fillers. Begin with a small batch.

2. Stir in small handfuls or scoops of the appropriate filler until the desired consistency is reached. Be sure all of the filler is thoroughly blended before the mixture is applied.

Additives
Although additives are blended with the mixed epoxy in the same two-step process as fillers, they are not designed to thicken the epoxy. Additives are used in smaller quantities to give the epoxy additional physical properties when used as a coating.
A complete description of fillers and additives and a buying guide for determining approximate epoxy-to-filler quantities can be found in the catalog section beginning on page 6.

Cleanup
Remove excess or spilled resin and mixed epoxy with 850 Cleaning Solvent or acetone. First scrape up as much excess or spilled material as possible with a squeegee, sharpened mixing stick or a putty knife. Wipe up residue with a dampened clean rag or paper towel. Use 855 Cleaning Solution or vinegar if a solvent is unavailable. Use hot water to remove spilled hardener.

If you get resin, hardener or uncured epoxy on your skin, wash with waterless skin cleaner followed by soap and water.

WARNING! DO NOT USE SOLVENTS DIRECTLY ON YOUR SKIN. READ DIRECTIONS AND PRECAUTIONS ON SOLVENT CONTAINERS BEFORE USING.


The following procedures are basic operations that you will use over and over regardless of the type of structure or material you are working with.

Surface preparation

Whether you are bonding, laminating, filleting, fairing or applying fabrics, the success of the application depends not only on the strength of the epoxy, but also on how well the epoxy adheres to the surface to which it is being applied. That is why the following three steps of surface preparation are a critical part of any epoxy operation:

1. Cleaning
Surfaces must be free of any contaminants such as grease, oil, wax or mold release. Clean contaminated surfaces with a silicone and wax remover such as DuPont Prep-SolTM3919S. WEST SYSTEM 850 Solvent or acetone works well on many contaminants. Wipe the surface with plain white paper towels before the solvent dries. Clean surfaces before sanding to avoid sanding the contaminant into the surface. CAUTION! Follow all safety precautions when working with solvents.

2. Drying
All bonding surfaces must be as dry as possible for good adhesion. If necessary, accelerate drying by warming the bonding surface with hot air guns, hair dryers or heat lamps. Use fans to move the air in confined or enclosed spaces. Watch for condensation when working outdoors or whenever the temperature of the work environment changes.

3. Sanding
Sand hardwoods and non-porous surfaces thoroughly to obtain an abraded surface. 80-grit aluminum oxide paper will provide a good texture for the epoxy to "key" into. Be sure the surface to be bonded is solid. Remove any flaking, chalking, blistering, or old coating before sanding. Remove all dust after sanding.

Removing amine blush
Amine blush is a by-product of the epoxy curing process that appears as a wax-like film on epoxy surfaces during the final cure phase. The blush is water soluble and can easily be removed, but can clog sandpaper and inhibit subsequent bonding if not removed. Wash the surface with clean water and an abrasive pad. We use and recommend 3-M ScotchbriteTM 7447 general purpose hand pads. Dry the surface with plain white paper towels to remove the dissolved blush before it dries on the surface. After washing with the abrasive pad, the surface should appear dull. Sand any remaining glossy areas with 80-grit sandpaper.

When to sand
If you can make an impression in the epoxy with your thumbnail, it is not hard enough to sand, and can still be re-coated without sanding. If there is any doubt or if the surface feels waxy, allow the epoxy to cure fully, then wash and sand.


Bonding
This section refers to two types of bonding. Single-step bonding is occasionally used when joints have minimal loads and excess absorption into Porous surfaces is not a problem. Two-step bonding is the preferred method for most situations because it promotes maximum epoxy penetration into the bonding surface and prevents resin-starved joints.

Two-step bonding
Before mixing epoxy, check all parts to be bonded for proper fit and surface preparation (Section 4.1), gather all the clamps and tools necessary for the operation, and cover any areas that need protection from spills.

1. Wet-out
Apply a straight resin/hardener mixture (without fillers) to the surfaces to be joined (Figure 3). This is called wetting-out the surface. The resin/hardener mixture may be applied with a disposable brush for small or tight areas, or a foam roller for larger areas. A large horizontal area can also be wet out by spreading the resin/hardener mixture evenly over the surface with a plastic squeegee. You may immediately proceed with step two.

2. Applying thickened epoxy
Modify the resin /hardener mixture by stirring in the appropriate filler until it becomes thick enough to bridge any gaps between the mating surfaces and to Prevent "resin-starved" joints. Apply an even coat of the thickened mixture to one of the surfaces to be joined (Figure 4.)

Thickened epoxy can be applied immediately over the wet-out surface or any time before the wet-out reaches its final cure. For most small bonding operations, add the filler to the resin/hardener mixture remaining in the batch that was used for the wet-out. Mix enough resin/hardener for both steps. Add the filler quickly after the surface is wet out and allow for a shorter working life of the mixture. Fully cured epoxy surfaces that have been washed and sanded do not need to be wet out.
Apply enough of the mixture so that a small amount will squeeze out when the surfaces are joined together with a force equivalent to a firm hand grip.

Single-step bonding
Single-step bonding is applying the thickened epoxy directly to the component without first wetting out with resin/hardener only. We recommend that you thicken the epoxy no more than is necessary to bridge gaps in the joint (the thinner the mixture, the more it can penetrate the surface) and that you do not use this method for highly-loaded joints or for bonding end grain or other porous surfaces.

Clamping
When the parts being bonded are properly positioned, attach clamps as necessary to hold the components in place. Use just enough clamping pressure to squeeze a small amount of the epoxy mixture from the joint, indicating that the epoxy is making good contact with both mating surfaces (Figure 5). Avoid squeezing all of the epoxy mixture out of the joint by using too much clamping pressure.

epoxy-fig5.jpg (6975 bytes)


Any method of clamping is suitable as long as the parts to be joined are held so that movement will not occur. Methods of clamping include spring clamps, "C" clamps and adjustable bar clamps, heavy rubber bands cut from inner tubes, nylon-reinforced packaging tape, and heavy weights. When placing clamps near epoxy-covered areas, use polyethylene sheeting or Peel Ply under the clamps so they don't inadvertently bond to the surface. Staples, nails and drywall screws are often used where conventional clamps will not work and are removed after the epoxy cures. Any fasteners that need to be left in should be of a non-corroding alloy such as bronze.
Shape or remove any excess adhesive that squeezes out of the joint as soon as the joint is secured with clamps. A wood mixing stick with one end sanded to a chisel edge is an ideal tool for removing the excess (Figure 6).

epoxy-fig6.jpg (11533 bytes)


Bonding with fillets

A fillet (fil'it) is a cove-shaped application of thickened epoxy that bridges an inside corner joint. It is excellent for bonding parts because it increases the surface area of the bond and serves as a structural brace. All joints that will be covered with fiberglass cloth will require a fillet to support the cloth at the inside corner of the joint.

The procedure for bonding with fillets is the same as normal bonding except that instead of removing the squeezed-out thickened epoxy after the components are clamped in position, you shape it into a fillet. For larger fillets, add thickened mixture to the joint as soon as the bonding operation is complete, before the bonding mixture is fully cured, or any time after the final cure and sanding of exposed epoxy in the fillet area.

1. Mix the resin/hardener/filler to a non-sagging peanut butter consistency.
2. Apply the fillet mixture along the joint line with the rounded mixing stick, using enough mixture to create the desired size of fillet. For larger or multiple fillets, empty caulking gun
cartridges or disposable cake decorating bags can be used. Cut the plastic tip to lay a bead of thickened epoxy large enough for the desired fillet size. Heavy duty, seal-able food storage bags with one corner cut off may also be used.

3. Shape and smooth the fillet by drawing a rounded filleting tool (mixing stick) along the joint, dragging excess material ahead of the tool and leaving a smooth cove-shaped fillet bordered on each side by a clean margin. Some excess filleting material will remain outside of the margin (Figure 7). Use the excess material to re-fill any voids. Smooth the fillet until you are satisfied with its appearance. A mixing stick will leave a fillet with about a 3/8" radius. For larger fillets, an 808 Plastic Squeegee, cut to shape or bent to the desired radius, works well.

epoxy-fig7.jpg (8170 bytes)


4. Clean up the remaining excess material outside of the margin by using a sharpened mixing stick or a putty knife (Figure 8). Fiberglass cloth or tape may be applied over the fillet area before the fillet has cured (or after the fillet is cured and sanded).

epoxy-fig8.jpg (9465 bytes)


5. Sand smooth with 80-grit sandpaper after the fillet has fully cured. Wipe the surface clean of any dust and apply several coats of resin/hardener over the entire fillet area before final finishing.

Laminating

The term laminating refers to the process of bonding numbers of relatively thin sheets, like plywood, veneers, fabrics or core material. The laminate may be any number of layers of the same material or combinations of different materials. Methods of epoxy application and clamping will differ depending on what you are laminating.
Because of large surface areas and limitations of wet lay-up time, a roller is the most common application method. A faster method for large surfaces is to simply pour the resin/hardener mixture onto the middle of the panel and spread the mixture evenly over the surface with a plastic squeegee. Apply thickened mixtures with a notched squeegee. Staples or screws are the most common method of clamping when you fasten to a solid substrate.
An even distribution of weights will work when you are laminating over a base that will not hold staples or screws, such as a foam or honeycomb core material.
The ideal clamping method is vacuum bagging, which, through the use of a vacuum pump and plastic sheeting, applies perfectly even pressure over all areas of the panel regardless of the size, shape or number of layers.
For more information about vacuum bagging, refer to 002-150 ADVANCED VACUUM BAGGING TECHNIQUES.

See part two of this set on epoxy

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