Epoxy resin has become the resin of choice for wooden boat builders, model builders and advanced composite manufacturers. This issue of the Fiberlay Forum is intended to describe the characteristics of epoxy (in particular the Pro-Glas system) as well as the different applications to which it is commonly used. We go into the use of epoxy for coating and sheathing of wood. We also touch on the use of epoxy as a glue, fairing and filleting compound. Finally, we go into the steps associated with repairing single skin fiberglass laminates. (repairing cored laminates will be the subject of a later forum) For those familiar with the use of polyester resins, we conclude the article with a comparative review of polyester and epoxy. We hope you find this Forum informative and useful.
The following areas are covered in this issue of the Fiberlay Forum:
DESCRIPTION
OF FIBERLAY PRO GLAS EPOXY SYSTEMS
CHARACTERISTICS OF EPOXY
CHART OF PROGLAS EPOXY SYSTEM
SHEATHING WITH PRO GLAS EPOXY
GLUEING, FAIRING, AND FILLETING
TOOLS AND RELATED MATERIAL LIST
WORKING WITH EPOXY RESINS
REPAIRING SINGLE SKIN LAMINATES WITH PRO GLAS EPOXY
REPAIRING WHERE BOTH SIDES ARE ACCESSIBLE
CHARACTERISTICS OF EPOXY VERSUS POLYESTER RESINS
SAFETY AND HANDLING
Pro-Glas epoxy is a high performance epoxy resin which may be combined with three different hardeners for optimal performance in different applications.
The backbone of the Pro-Glas epoxy system is the 1314 Resin. It features 100 percent solids (no Volatile Organic Compounds's), low viscosity (allowing fast and efficient wet out of reinforcement material), relatively clear (good for clear casting applications) and low odor and toxicity. It may be combined with three different hardeners, namely 3102, 3103 and 3143.
The 1314/3102 epoxy is our fast system. The resin /hardener mix ratio is 4:1 (by volume). Gel time is 12-15 minutes at 77 degrees F. This epoxy is excellent for low temperature curing, fast setting even in thin film, chemical resistance and high heat deflection temperature (HDT) with post cure. Use for fiberglass laminating, wood coating and when mixed with fillers as a fast curing fairing or filleting compound. It exhibits low odor and toxicity.
The 1314/3103 epoxy is a slower version of the 1314/3102 system. Resin is mixed with hardener in the ratio of 4:1 (by volume). Gel time is 20-25 minutes at 77 degrees F. It retains all features of the 1314/3102 system with slower gel time. It has the highest HDT and clarity of the 3 systems. Low odor and low to moderate toxicity.
The 1314/3143 epoxy is our slow system. It has a mix ratio of 2 parts resin to 1 part hardener (by volume). Gel time is 25-30 minutes at 77 degrees F. This is our most flexible system (highest flexural modulus). It is ideal for woodwork both as glue (when thickened) and for coating. (Increased bond strength with superior impact, abrasion and chemical resistance). Cures at low temperatures and is resistant to blush and water spotting. Use for laminating, adhesives, coatings and structural compounds. This system exhibits low odor and toxicity.
Al Franklin of Lynnwood, Wa., an experienced builder of over 20 model planes talks about building with epoxy. " When I first built this fuselage myself in polyester it was too heavy at 12 lbs. When I went to Fiberlay, they helped me tremendously to change to the right materials and lay-ups. Final weight in epoxy was a little over 6 lbs."
Once applied in a
thin film, the curing rate is affected by ambient temperature. For each
18-20 degrees F. difference in ambient temperature, you can expect to
half or double the set time of the epoxy. The pot life and cure times
of Pro-Glas systems are given in the following chart.
Liquid Properties of Systems |
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| Resin/Hardener Type | 1314/3102 | 1314/3103 | 1314/3143 |
| Speed of hardener |
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Slow/Flexible |
| Mix ratio, Resin to Hardener by volume |
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| Viscosity, cps mixed |
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| Gel time, minutes at 77 degrees F. |
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| Set time, hours at 77 degrees F. |
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| Recoat time, hours |
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| Recommended minimum working temperature ( degrees F) |
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| Suggested uses: F=Fairing, L=Laminating, A=Adhesive |
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Cured Properties of System |
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| Heat Deflection Temperature (HDT) Note: higher HDT's attainable with post cure. |
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| Flexural Strength, Psi |
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| Tensile Strength, Psi |
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| Flexural Modulus, Psi |
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| Tensile Modulus, Psi |
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| Elongation(percentage) |
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- Remove amine blush with clean water
- Light (or heavy
sanding) as required
- Brush off sanding dust and wipe with damp rag. (Isopropyl alcohol or clean acetone may be used in final wipe down). Do not use recycled acetone as it usually contains contaminants.
Measuring and mixing epoxy is critical to consistent success. The epoxy must be mixed in the ratio indicated for the hardener. (By volume or weight). Mix thoroughly for a full minute scraping the sides and working the material from bottom to top. Plastic pumps are available for those who prefer. (fits quart, 1/2 gal. and gal. containers).
The shelf life of Pro-Glas epoxy is unlimited in closed containers. Some haziness and crystallization will occur if stored below 50 degrees F. for prolonged periods. This has no harmful effect on the epoxy and may be alleviated by placing the closed container in hot tap water until the crystals dissolve.
Cured epoxy suffers from ultra-violet degradation if left exposed for lengthy periods. This may be evidenced by chalkiness on the surface (after say 6 months) although much more damage will result with time. Cured epoxy must be coated with UV fortified varnish, urethanes or paint systems.
For those accustomed
to polyester resin and gel coat, always remember that whereas you can
coat almost any surface with epoxy, such is not the case with polyester.
The unreacted amine in the epoxy will inhibit the MEK peroxide catalyst
resulting in uncured polyester resin/gel coat. There are some epoxies
formulated as "tie" coats. Be wary and
call us if in doubt.
- Surface preparation: make sure the surface is dry and clean. (Oils may prevent the epoxy from bonding to the surface). Treated wood or woods with a high oil content may resist coating and should be treated with penetrating epoxies before laying up cloth.
- The use of a penetrating epoxy such as S-1 epoxy sealer is recommended to seal the surface beforehand.
- Lay out the fiberglass fabric to ensure proper fit and coverage. Remember fiberglass cloth has much better "stretch" on the bias (+/- 45 degrees F. direction).
- Mix the Pro-Glas epoxy as directed (mix thoroughly) and pour into a plastic paint tray or mixing pot.
- Apply the epoxy resin/hardener mixture over the surface using a sponge roller, brush and/or squeegee. You may precoat and lay fiberglass cloth onto wet surface or position fiberglass cloth over dry surface and saturate from above.
- Use squeegee to smooth surface and remove excess epoxy. Squeegee from center to sides of fabric.
- Allow to cure. Trim edges with straight edge exacto or utility knife while epoxy is in the "green" state. (i.e., firm but not fully hardened.)
The initial coat of epoxy may not cover the weave of the cloth (depending on weight and weave of fabric). In anticipation of sanding prior to paint or varnish, additional coats of epoxy will be needed to fill the weave of the fabric. This epoxy may be thickened using fumed silica. Add subsequent coats while previous coats are green to prevent additional surface prep. Remember that amine will inhibit proper bonding between coats so apply each coat while the previous one is in a green state. If this is not possible, just make sure you prep the surface before continuing.
After the final coat has fully cured, prepare the surface for a topcoat of paint or varnish.
A typical fairing/filleting compound would include equal parts by volume resin/hardener to microspheres (glass or phenolic) then add fumed silica up to 20 percent of volume. This may be varied depending on the thickness and desired density. The microspheres will thicken the epoxy but will sag on a vertical surface. Only the thickening agents allow for non-sag mixtures. To make a structural filler or filleting compound add the milled glass fibers first, mixing well to fully wet out the fibers. (Adding glass fibers to a thickened mixture may not allow the glass fibers to be properly "wet" with resin). Then mix the other compounds to complete the recipe.
For those who prefer ready made compounds, Fiberlay carries a full line of fairing and structural epoxy compounds already formulated for you.
A structural glue is made by adding thickening agents (fumed silica or wood flour) to the epoxy to give it body. Before thickening, however, coat each contact surface with liquid epoxy, which has been measured and mixed. Add thickened mixture and apply just enough pressure to hold the parts immobile while curing. Too much clamping pressure will result in a glue-starved joint with resultant weakness. Because epoxies continue to cure and therefore gain physical strength several days after initial setting, it is necessary to allow plenty of time to cure on joints which will come under stress as soon as clamping is removed. Metals and certain plastics may be bonded with Pro-Glas epoxy although surface preparation is critical. Testing on sample materials is recommended.
- Disposable natural
bristle brushes
- Foam roller covers
- Roller frames
- Plastic paint tray
liners
- Metal paint trays
- Laminating rollers
- Latex gloves
- Dust and mist masks
- Plastic and rubber
squeegees
- Masking tape
- Plastic drop cloths
- Graduated mixing
containers, stir sticks
- Acetone/Methyl
ethyl keytonefor cleaning
- Fumed silica -
for thickening
- Glass and phenolic
microspheres - for bulking mixture
- Milled fibers -
add tensile strength
- Fiberglass reinforcements-cloth, roving and mat
Remove the damaged
laminate with a high-speed cutting tool. Avoid reciprocating blades as
the rapid up and down motion causes delamination at the cutting edges.
A cutoff wheel or circular saw works best. High speed and slow feed is
preferable. Carbide tipped blades will outlast conventional high-speed
steel blades. When cutting out the damaged area round the corners of the
repair hole in a circle or oval shape. Avoid square corners.
Examine the existing laminate for type and weight of glass reinforcement.
It is usually preferable to rebuild the part with material the same or
similar to the original design. In advanced composite repair it is important
to also match fiber orientation with the original part.
In order to closely match the strength of the repaired part to the undamaged part it is necessary to spread the load of the new matrix and fibers to the original. We do this by creating the largest bonding area around the repair that is possible or practical. Taper the edge of the repair to accept the patch. In conventional marine type laminates the taper scarf will usually range from 10:1 to 12:1. Advanced composite structures may be 20:1 or more. The flatter the angle the more surface area will be bonded with the new material.
After taper sanding the repair edge keep it meticulously clean. Use only clean solvents and rags.
Plan the repair by cutting out replacement plies as close as possible to the original repair. Each new ply will be slightly larger than the one before. Place a backing plate to cover the hole at the rear of the repair (where back side is accessible) Where the back side is unaccessible see next section. Wet out each ply of material working from small diameter to larger diameter pieces taking care to match fiber orientation as much as possible. When reaching the surface plies you may want to add a sacrificial ply for fairing purposes. Once cured, the area can be sanded fair for gel coat* or paint application. To allow the top coat to blend into the surrounding surface care must be taken to allow for the thickness of the top coat. (up to 20 mils for gel coat or 2-3 mils for linear polyurethane paints)*Do not apply gel coat to an epoxy surface without first testing compatibility.
Where the reverse side of the repair is inaccessible, a backing plate may be positioned as follows:
- A backing plate may be fashioned from stiff cardboard to which is
applied chopped strand mat well saturated with resin.
- Avoid chopped strand mat when using epoxy unless it is suitable for
epoxy saturation. (i.e. Multi purpose mat or stitched mat)
- When using epoxy the stitch bond fabrics are ideal in this application. The backing plate will be cut in the shape of the repair hole but larger (approx. 2" larger all around). This works best if the repair hole is "oval" shaped so that the backing plate can be inserted through the repair hole.
- Reach through the hole with sandpaper and abrade the edges of the backside of the repair. Wipe with clean solvent.
- Run wire through holes in the backing plate with the ends protruding from the wet side.
- Position the backing plate through the hole turning to align with the shape of the repair hole.
- Pull wires taut and secure tourniquet style around wooden blocks until resin cures. Remove wooden blocking and cut off protruding wires.
- Proceed with repair as described above.
Characteristic of Resin |
Epoxy |
Polyester |
| Cost | More expensive | Less expensive |
| Ease of Use | Easy to use although careful measuring and mixing is required. | Easy to use |
| Water permeability | Very good | Not as good over time. |
| Flexural modulus | Very good | Very good |
| Tensile modulus | Very good | Very good |
| Tolerant to ultra violet exposure | Not over long term | Much more than epoxy |
| Use with advanced composites -graphite and aramid fibers | Yes | Yes but may not be practical |
| Repair conventional fiberglass | Yes - Note: see below under top coat with gel coat | Yes |
| Sealing wood | Excellent | Good |
| Sheathing wood surfaces | Excellent | Good |
| Fairing and filleting compounds | Use in epoxy applications-Excellent | Use in polyester applications-Excellent |
| Gluing similar and dissimilar substrates | Excellent except for some plastics | Not as good although works well with vinyl (PVC) |
| Surface curing | Yes | Air inhibited, requires surfacing agent |
| Landscape applications | Too expensive | Has many applications |
| Repairing gel coat surfaces | No | Yes - gel coat |
| Heat deflection temperature | Relatively low | Better than epoxy |
| Top coat with gel coat | No | Yes |
| Top Coat with paint system | Yes | Yes |