FIBERGLASSING FOR MODELERS
(Part 2)

By Cecil Collum

Epoxy Resins

In Part One we saw an overview of Polyester Resins. Part Two is primarily devoted to Epoxy Resins which we, as modelers, use most frequently; particularly for repairing damage to our airplanes. But first, Let’s begin this section by reviewing Safety.

Many years ago a chemist friend told me that if a chemical has a sharp odor, it’s probably dangerous to our health, so let’s all use a respirator and safety glasses while working with fiberglass and the associated resins. For that matter, we should wear a respirator and safety glasses while cutting, sanding, or grinding these resins and fiberglass.

Polyester resins will set and cure with nothing added over a year or two but Epoxy resins are "hardened" resins, which means that they require a catalyst to start the chemical reaction that cures them. Hardened resins also require the addition of a very specific ratio of catalyst to resin and we cannot deviate from this ratio! We must have the correct ratio of resin molecules to hardener molecules although a slight tolerance is built into the equation since we cannot measure to the molecular level. This is one of those places where we must read the directions PRIOR to mixing as not all systems use the same ratios. Most of the epoxy used in modeling use the same 1 to 1 ratio but there are systems out there that use a different ratio (West Systems epoxy uses a different ratio). If we mismeasure and the ratio is way off, we will have a real mess on our hands! (Please don’t ask how I know this to be true!) Always mix the proper ratio for your system and always mix thoroughly.

How do we know when the mixing is correct? Personally, I mix until the color changes, indicating the mixture is complete. One more thing, systems cannot be mixed together. One manufacturer’s product may vary from another to the extent that mixing the two together may cause a gigantic mess! However, once one system cures, another manufacturer’s system may be applied over the first lamination as long as epoxy is applied over epoxy.

Do not over coat any MEKP catalyzed resin with an epoxy; in other words, don’t apply epoxy over polyester resins. Just as with Polyester resins, if you mix a quantity and leave it in the container, it will gel faster than expected due to trapped heat. Spreading the mix into a thin layer will lengthen the working time. Both Polyester and Epoxy systems generate heat but epoxies can actually melt the plastic container they are mixed in.

Epoxies look similar to polyesters when cured but they do have very different properties:

1. Epoxies are not UV stable, as a general rule.
2. Epoxies will soften when exposed to excessive heat, polyesters won’t.
3. Epoxies resist water intrusion and breakdown better than polyesters.
4. Epoxies are stronger and more flexible than polyesters.

It is very important to remember that polyesters will cure (polymerize) completely given enough time. Epoxies have a cross-linked reaction and unless you mix molecule-for-molecule, will never completely cross-link. Mixing correctly is extremely critical to obtain the properties of the epoxy system you are using.

Polyesters are "wicking" resins, meaning they will continue moving along the fibers in a material while Epoxies are "gap-filling" resins. Epoxies will wick effectively but will also tend to fill in minute gaps in materials. Epoxies are stronger, tougher resins than Polyesters and make a better wood glue if used correctly ... and are better for waterproofing.

When epoxy cures, a water soluble film forms on the surface. This film usually forms in the presence of carbon dioxide and water vapor so one would expect more of this on damp, cloudy days. If recoating with epoxy, this film does not affect the recoat and need not be removed. If you are planning to paint this surface, it is necessary to remove this film and sand the surface. Since epoxy is not UV stable, it must be protected from the sun and painting is a good option.

Some people like to use a gel coat for the finish because of it’s durability and ease of repair. However, the gel coat must be catalyzed with MEKP and when applying it over epoxy the cure time of the gel coat is unpredictable and you may end up with a sticky mess.

Materials Used With Resins

Up to this point we have discussed resins. Now it is time to discuss the materials used with the resins.

The materials used in any laminate contribute most of the strength to that laminate. The resin binds the materials together and transfers the load from fiber to fiber or to the core and outer skins. Fiberglass, graphite, Kevlar, and even cotton cloth have all been used for laminating materials but fiberglass is by far the most common material and we will discuss it first. Fiberglass materials are made from glass fibers. No big surprise here!

1. Chopped Strand Mat is a material with short glass strands laid down in a random pattern and held together by a resin-soluble binder. Easily wet out and able to conform to almost any shape, CSM is used as the base layer for many laminates. It is also the most watertight and adhesive of the glass materials. CSM is sold in various widths and lengths, the most common being ¾ oz, 1-1/2 oz., and 2 ounces per square foot.

2. Woven Roving is a coarse plain weave material made of large flat bundles of glass fibers called "rovings". The bundles are woven at 90 degrees to each other, which imparts the highest strength in these directions. WR doesn’t bond well to other layers and is best used as a binder. This material is of little use to the modeler but one we need to be aware of.

3. Fiberglass cloth of various weaves have exceptionally high tensile strength but provide little rigidity. Cloths come in various weights from ¼ ounce to 10 ounces per square yard. Weights of ½ to ¾ ounces per square yard are the most common used for glassing a model with epoxy or polyester resins. One may also use water-based polyurethane to adhere the cloth to the plane’s structure. When used to reinforce a wing joint, one may use either epoxy resin or CA glue to adhere the one or two ounce tape to the wing.

4. Dual-Bias Mat/Roving is a specialty material composed of a lightweight mat, usually ½ oz and two layers of rovings at plus and minus 45 degrees, all stitched together. This material reduces lay-up time, conforms better than woven roving and can be used with any resin system since the fibers are stitched together. This material has little use in modeling in the smaller planes.

5. Carbon/Graphite has an exceptional stiffness to weight ratio but does not point load well and may shatter when impacted. Combining this material with Kevlar adds toughness. Has been used to good effect on wings, particularly those made of foam and may be used with any resin system.

6. Aramid(Kevlar 49) is a lightweight but extremely tough fabric that is difficult to cut and almost impossible to sand. Best use of this system is ballistic armor.

More next month...