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Handling Epoxy
This section explains the basic 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.
Understanding cure stages
Mixing epoxy resin and curing, agent 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. Liquid-Open time (also "working time" or "wet lay-up time") is the portion of the cure time, after mixing, that the resin/curing agent 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.
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. 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.
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 sanded before re-coating
to achieve a good mechanical, secondary bond.
Understanding and controlling cure time
Curing Agent speed
Each curing agent has an ideal temperature cure range. At any given temperature, each resin/curing agent 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. Pot life is a term used to compare the usable life of different hardeners. It is the amount of time 100 grams at 72 F remains a liquid. 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 open time.
Curing temperature
The warmer the temperature, the faster the epoxy
will cure. The cure temperature is determined by the ambient temperature
plus the exothermic heat generated during 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 it is warmer. Exothermic heat is produced by the
chemical reaction during cure. The amount of heat produced
depends on the thickness and surface area of mixed epoxy.
In a thicker mass, more heat is retained, causing a faster reaction and
more heat. The mixing container shape and mixed quantity have a great affect
on this exothermic reaction. A contained mass of curing epoxy (1 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 curing agent, or mix smaller
batches that can be used up quickly, or quickly pour the epoxy mixture
into a container with greater surface area (a flat 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 curing agent, or use supplemental
heat to raise the epoxy temperature above the curing agents minimum recommended
application temperature. Use a hot air gun, heat lamp or other heat source
to warm the resin and curing agent before mixing or after the epoxy is
applied. At room temperature, supplemental heat is useful when a quicker
cure is desired.
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 (soft wood 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 120F(49C).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.
Dispensing and mixing
Careful measuring of epoxy resin and curing agent
and thorough mixing are essential for a proper cure. Whether the resin/curing
agent mixture is applied as a coating or modified with fillers, observing
the following procedures will assure a controlled and thorough chemical
transition to a high-strength epoxy solid. Dispense the proper proportions
of resin and curing agent into a clean plastic, metal or wax-free paper
container. Don't use glass or foam containers because of the potential
danger from exothermic heat build-up. DO NOT
attempt to alter the cure time by altering the ratio.
An accurate ratio is essential for a proper cure and full development of
physical properties.
Dispensing with Mini pumps or Dual Syringes
Most problems related to curing of the epoxy can be traced
to the wrong ratio of resin and hardener. To simplify metering, we recommend
using calibrated Anchor Bond Mini Pumps or Dual Syringes to dispense the
resin and curing agent. Mini Pumps are calibrated to deliver the proper
working ratio of resin to curing agent. Pump one full pump stroke of resin
for each one full pump stroke
of curing agent. Depress each pump head fully and allow
the head to come completely back to the top before beginning the next stroke.
Partial strokes will give the wrong ratio. Read the pump instructions before
using pumps. Before you use the first mixture on a project, verify the
proper ratio according to the instructions that come with the pumps.
First Time Users
If this is the first time you have used Anchor Bond epoxy, begin with a small test batch to get the feel for the mixing and curing process, before applying the mixture to your project. This will demonstrate the hardener's open time for the temperature you are working in and assure you that the resin/curing agent ratio is metered properly. Mix small batches until you are confident of the mixture's handling characteristics.
Mixing
Stir the two ingredients together thoroughly. To assure
thorough mixing, scrape the sides and bottom of the cup as you mix. Use
the flat end of the mixing stick to reach the inside corner of the cup.
If you are going to be using the mixture for coating, quickly pour it into
a wide dish or pan to extend the open time.
Warning! Curing epoxy generates heat. Do not fill or
cast layers of epoxy thicker than 1/4", thinner if enclosed by foam or
other insulated material. One inch of mixed epoxy in a plastic mixing cup
will generate enough heat to melt the cup if left to stand for its full
pot life. For this reason do not use foam or glass mixing containers. 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, as it might cause a fire.
Adding Fillers
After selecting an appropriate filler for your job, use it to modify the epoxy mixture to the desired consistency. The thickness of a mixture required for a particular job is controlled by the amount of filler added. There is no strict formula or measuring involved-use your eye to judge what consistency will work best.
Always add fillers in a two-step process:
1. Mix the desired quantity of resin and curing agent thoroughly before adding fillers. Begin with a small batch to allow room for the filler.
2. Blend in small amounts of the appropriate filler until the desired consistency is reached.
3. For thick mixtures, don't fill the mixing cup more than 1/3 full of epoxy before adding filler.
Anchor Balloons are intended as a fairing and cosmetic filler. They sand and shape very easily. For fairing add as many Anchor Balloons as you can blend in smoothly for easy sanding, usually the thicker the better.
Anchor Fibers are intended for use in high strength reinforcing applications, where fiberglass cloth is inappropriate or impractical to use.
Anchor Thickening is a fumed silica, intended for use in adjusting the viscosity (thickness). It produces a stronger finished product than Anchor Balloons, but is also much harder to sand. Be sure all of the fillers are thoroughly blended before the mixture is applied. For maximum strength, add only enough filler to completely bridge gaps between surfaces without sagging or running out of the joint or gap. A small amount should squeeze out of joints when clamped. All Anchor Bond fillers can be combined with each other to suit specific applications and construction techniques.
Basic techniques
The following basic techniques are common to most repair and building projects, regardless of the type of structure or material you are working with.
Surface preparation
Whether you are bonding, 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. Unless you are bonding to partially cured epoxy, the strength of the bond relies on the epoxy's ability to mechanically "key" into the surface. That is why the following three steps of surface preparation are a critical part of any secondary bonding operation. For good adhesion, bonding surfaces should be:
Clean
Bonding surfaces must be free of any contaminants such as grease, oil, wax or mold release. Clean contaminated surfaces with lacquer thinner, alcohol, acetone, MEK or brake cleaner. Be sure to read, understand and follow the precautions on the container. Wipe the surface with paper towels before the solvent dries. Clean surfaces before sanding to avoid sanding the contaminant into the surface.
Dry
All bonding surfaces must be as dry as possible for good adhesion. If necessary, accelerate drying by warning the bonding surface with hot air guns, hair dryers or heat lamps. Use fans to move the air in confined or enclosed spaces.
Sanded
Sand smooth non-porous surfaces thoroughly. 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, by using a air gun, or tack cloth. Epoxy surfaces that have not fully cured may be bonded to or coated with epoxy, without washing or sanding. Before applying coatings other than epoxy, allow epoxy surfaces to cure fully.
Oily Wood
Wipe with acetone 15 minutes before coating. Solvent dries the oil at the surface and allows epoxy to penetrate. Be sure solvent has evaporated before coating.
Porous woods
No special preparation needed. If surface is burnished, sand with 80-grit paper to open pores.
Metal
Remove contamination, sand or grind to bright metal, coat with epoxy. Recoat or bond after first coat gels, if desired.
Polyester (fiberglass)
Clean with a silicone and wax remover. Sand with 80-grit
paper to a dull finish.
Plastic
Adhesion varies. If a plastic is impervious to solvents such as acetone, epoxy generally will not bond to it. Plastics such as: polyethylene, polypropylene,nylon, Teflon, Plexiglas and polycarbonate fall into this category.
Hard, rigid plastics
Plastics such as PVC, ABS and polystyrene provide better adhesion with good surface preparation and adequate bonding area. After sanding, flame oxidizing (by quickly passing propane torch over the surface without melting the plastic) can improve bonding in some plastics. Its a good idea to conduct an adhesion test on a plastic that you are uncertain about.
Bonding (gluing)
This section refers to two types of bonding. 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. Single-step bonding can be used when joints have minimal loads and excess absorption into porous surfaces is not a problem. Before mixing epoxy, check all parts to be bonded for proper fit and surface preparation. Gather all the clamps and tools necessary for the operation, and cover any areas that need protection from spills.
Two-step bonding
1. Wet-out bonding surfaces by applying a
straight resin/curing agent mixture (without fillers)
to the surfaces to be joined. Wet-out small or tight areas
with a disposable brush. Wet-out larger areas with a foam roller or by
spreading the resin/curing agent mixture evenly over the surface with a
plastic squeegee. You may proceed with step two immediately or any time
before the wet-out coat reaches the final cure stage.
2. Apply thickened epoxy to one bonding surface. Modify the resin/curing agent 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 enough of the mixture to one of the surfaces so that a small amount will squeeze out when the surfaces are joined together with a force equivalent to, a firm hand grip. 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/curing agent mixture remaining in the batch that was used for the wetout. Mix enough resin/curing agent for both steps. Add the filler quickly after the surface is wet out and allow for a shorter working, life of the mixture.
3. Clamp components. 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. Avoid using too much clamping pressure, which can squeeze all of the epoxy mixture out of the joint.
4. Remove or shape excess adhesive that squeezes out of the joint as soon as the joint is secured with clamps. A wooden mixing stick with one end sanded to a chisel edge is an ideal tool for removing the excess.
Single-step bonding
In single-step bonding, apply the thickened epoxy directly to the component without first wetting out with resin/curing agent. 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.
Laminating
The term "laminating" refers to the process of bonding numbers of relatively thin sheets, like plywood, veneers, fabrics or core material to create a composite. A composite 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, roller application is the most common method for applying epoxy. A faster method for large surfaces is to simply pour the resin/curing agent mixture onto the middle of the panel and spread the mixture evenly over the surface with a plastic spreader. An even distribution of weights will work when you are laminating a solid material to a base that will not hold staples, such as a foam or honeycomb core material. Vacuum bagging is also a method for laminating a wide range of materials. Through the use of a vacuum pump and plastic sheeting, the atmosphere is used to apply even clamping pressure over all areas of a panel regardless of the size, shape or number of layers.
Clamping
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, heavy rubber bands, nylon-reinforced packaging tape, applying weights, and vacuum bagging. When placing clamps near epoxy-covered areas, use polyethylene sheeting or wax paper under the clamps so they don't inadvertently bond to the surface. In some cases the thickened epoxy or gravity will hold parts in position without clamps.
Bonding with fillets
A fillet 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.Bond parts as described above.
2. Shape and smooth the squeezed-out thick epoxy into a 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. 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.
3. Clean up the remaining excess material outside of the margin by using a sharpened mixing stick or a putty knife. Fiberglass cloth or tape may be applied over the fillet area before the fillet has cured (or after the fillet is cured and sanded).
4. Sand smooth with 80-grit sandpaper after the fillet has fully cured. Wipe the surface clean of any dust.
Fairing
1. Fairing refers to the filling and shaping of low areas so they blend with the surrounding surfaces and appear "fair" to the eye and touch. After major structural assembly has been completed, final fairing can be easily accomplished with Anchor Bond epoxy and low-density fillers.
2. Prepare the surface as you would for bonding. Sand smooth any bumps or ridges on the surface and remove all dust from the area to be faired.
3. Wet out porous surfaces with un-thickened epoxy.
4. Mix resin/curing agent and Anchor Balloons to a peanut butter consistency.
5. Trowel on the thickened epoxy mixture with a plastic squeegee, working it into all voids and depressions. Smooth the mixture to the desired shape, leaving the mixture slightly higher than the surrounding area. Remove any excess thickened epoxy before it cures.
6. Allow the final thickened epoxy application to cure thoroughly.
7. Sand the fairing material to blend with the surrounding contour. Begin with 50-grit sandpaper if you have a lot of fairing material to remove. Use 80-grit paper on the appropriate sanding block when you are close to the final contour. CAUTION! Don't forget your dust mask. Remove the sanding dust and fill any remaining voids following the same procedure.
Applying woven cloth & tape
Fiberglass cloth is usually applied after fairing
and shaping are completed, and before the final coating operation. it is
also applied in multiple layers (laminated) and in combination with
other materials to build composite parts. Fiberglass cloth may be applied
to surfaces by either of two methods. The "dry" method refers to applying
the cloth over a dry surface. The "wet" method refers to applying
the cloth to an epoxy-coated surface often after the wet-out coat becomes
tacky, which helps it cling to vertical or overhead surfaces. Since
this method makes it more difficult to position the cloth,
the dry method is the preferred method especially with thinner cloth.
Dry method
1. Prepare the surface as you would for bonding.
2. Position the cloth over the surface and cut it several inches larger on all sides. If the surface area you are covering is larger than the cloth size, allow multiple pieces to overlap. On sloped or vertical surfaces, hold the cloth in place with masking or duct tape, or with CA.
3. Mix a small quantity of epoxy.
4. Pour a small pool of resin/curing agent near the center of the cloth.
5. Spread the epoxy over the cloth surface with a plastic spreader, working the epoxy gently from the pool into the dry areas. Use a foam roller or brush to wet out fabric on vertical surfaces. Properly wet out fabric is transparent. White areas indicate dry fabric. If you are applying the cloth over a porous surface, be sure to leave enough epoxy to be absorbed by both the cloth and the surface below it. Try to limit the amount of squeegeeing you do. The more you "work" the wet surface, the more minute air bubbles are placed in suspension in the epoxy. This is especially important if you plan to use a clear finish (see below). You may use a roller or brush to apply epoxy to horizontal as well as vertical surfaces.
6. Smooth wrinkles and position the cloth as you work your way to the edges. Check for dry areas (especially over porous surfaces) and re-wet them as necessary before proceeding to the next step. If you have to cut a pleat or notch in the cloth to lay it flat on a compound curve or corner, make the cut with a pair of sharp scissors and overlap the edges.
7. Note: For clear wood finishes, an alternative wet out method is to lay the epoxy onto the fabric with a short-bristled brush. Dip the brush in the epoxy and lay the epoxy on the surface in a light even stroke. Don't force the epoxy into the cloth, which may trap air in the fabric and show through the clear finish. Apply enough epoxy to saturate the fabric and the wood below. After several minutes, lay on additional epoxy to dry (white) areas.
8. Squeegee away excess epoxy before the first batch begins to gel. Drag the squeegee over the fabric, using even-pressured, overlapping strokes. Use enough pressure to remove excess epoxy that would allow the cloth to float off the surface, but not enough pressure to create dry spots. Excess epoxy appears as a shiny area, while a properly wet-out surface appears evenly transparent, with a smooth, cloth texture. Later coats of epoxy will fill the weave of the cloth. Trim the excess and overlapped cloth after the epoxy has reached its initial cure. The cloth will cut easily with a sharp utility knife. Coat the surface to fill the weave before the wet-out reaches its final cure stage. Follow the procedures for final coating below. It could take two or three coats to completely fill the weave of the cloth and to allow for a final sanding that will not affect the cloth.
Wet method
An alternative is to apply the fabric or tape to a surface coated with wet epoxy.As mentioned, this is not the preferred method, especially with large pieces of cloth, because of the difficulty removing wrinkles or adjusting the position of the cloth as it is being wet out.However, you may come across situations when this method may be useful or necessary,
1. Prepare the surface for bonding.
2. Pre-fit and trim the cloth to size. Roll the cloth neatly so that it may be conveniently rolled back into position later.
3. Roll a heavy coat of epoxy on the surface
4. Unroll the glass cloth over the wet epoxy and position it. Surface tension will hold most cloth in position. If you are applying the cloth vertically or overhead, you may: want to wait until the epoxy becomes tacky. Work out wrinkles by lifting the edge of the cloth and smoothing from the center with your gloved hand or a squeegee.
5. Apply a second coat of epoxy with a foam roller, or brush. Apply enough epoxy to thoroughly wet out the cloth.
6. Follow steps 6, 7, and 8 under the dry method to finish the procedure.
7. Any remaining irregularities or transitions between cloth and substrate can be faired by using an epoxy/filler fairing compound.
8. Note: A third alternative,
a variation of both methods, is to apply the fabric after a wet out coat
has reached an initial cure. Follow the first three steps of the Wet Method,
but wait until the epoxy cures tacky to the touch before positioning the
fabric and continuing with Step 3 of the Dry Method. Apply the fabric before
the first coat reaches its final cure phase.
Final surface preparation
Proper finishing techniques will not only add beauty to your efforts, but will also protect your work from ultraviolet light which will- break down the epoxy over time. The most common methods of finishing are painting or film coating.These coating systems protect the epoxy from ultraviolet light and require proper preparation of the surface before application. Preparation of the final finish is just as important as it is for re-coating with epoxy. The surface must first be clean, dry and sanded. Allow the final epoxy coat to cure thoroughly.
1. Sand to a smooth finish. If there are runs or sags, begin sanding with 80 grit paper to remove the highest areas. Sand until the surface feels and looks fair. Complete sanding with the appropriate grit for the type of coating to be applied. Paint adhesion relies on the mechanical grip of the paint keying into the sanding scratches in the epoxy's surface. If a high-build or filling primer is to be applied, 100 grit is usually sufficient. 120-180 grit may be inadequate for primers and high-solids coatings. Finishing with 220-400 grit paper will result in a high-gloss finish for most paints. Grits finer than this may not provide enough tooth for good adhesion. Wet sanding is preferred by many people because it reduces sanding dust.
2. After you are satisfied with the
texture and fairness of the surface, rinse the surface with fresh water.
Rinse water should sheet evenly without beading or fish-eyeing. If rinse
water beads
up (a sign of contamination), wipe the area with solvent and
dry with a paper towel, then wet sand again until beading is eliminated.
Proceed with your final coating after the surface has dried thoroughly.
To reduce the possibility of contamination, it is a good idea to begin
coating within 24 hours of the final sanding. Follow all of the instructions
from the coating system's manufacturer. It may be a good idea to make a
test panel to evaluate the degree of surface preparation required
and the compatibility of the finish system.
ANCHOR SEAL, INC.
16 Riverside Avenue
Danvers, MA 01323-3281
www.anchorseal.com
ORDERING
Phone (800) 669-5217
Fax (978) 774-0638
Email: Anchorseal@AOL.com
TECHNICAL SERVICE
(978) 774-5217