Fiberglass: Exotic Materials


Compressive behavior of Kevlar is quite different from fiberglass. Where a compressive failure in a fiberglass laminate is abrupt and catastrophic, but at a high load, in a Kevlar laminate, a compressive failure resembles that of a ductile metal; in other words, it tends to bend or dent. While this may seem to be an asset, such compressive failures in Kevlar laminates occur at relatively low stress levels, thereby causing resin failure through crazing and delamination. This weakness can be overcome, or at least somewhat compensated for, in a couple of ways. First, the Kevlar layer can be covered with another more suitable material, such as fiberglass. However, in a sheathing situation where strength is secondary, this would be costly and add unnecessary weight, although localized reinforcing, such as at seams could be handled in this manner. Second, there are "hybrid" fabrics available consisting of fiberglass and/or carbon fiber to compensate for the weaknesses of Kevlar, but these would serve little purpose in sheathings also.

Still there are other problems with Kevlar. The material is sensitive to ultraviolet light and should not be used in direct sunlight without the surface being protected by resin containing pigments, or by other protective coatings such as paint or UV-inhibited coatings. Unlike fiberglass and other sheathing materials, Kevlar does not become translucent when wetted out with resin; it retains its yellow cast. This can not only be unattractive (making it impractical for clear coatings), but it makes it difficult (at least for the amateur) to tell if the material is properly and totally saturated with resin.

Other handling difficulties present themselves. Because of its strength, Kevlar is very difficult to cut, both in bare fabric and in cured laminate form. Carbide cutting tools are a virtual necessity, and sanding or fairing against a Kevlar surface is virtually impractical; the material just fuzzes up when abraded or sanded. In the raw state, Kevlar fabric should not be folded so that hard creases form; this can lead to abrasions that damage the fibers, causing loss of strength. Basically, Kevlar is a fabric that was never intended to be bonded to itself. This means that bonding problems can occur between layers of Kevlar, and if such a laminate is desired, alternate between with a thin layer of fiberglass mat. Because fiberglass and Kevlar have about the same 3% tensile elongation, they are compatible with one another in this setting. Since Kevlar is made without finish agents, it can be used with many different types of resins including polyester, vinylester, and epoxy. However, to realize its high impact resistance and other physical properties in a laminate, vinylester and epoxy resins are usually advised. Even then, tear and peel strength are not as good as with Vectra when each is used alone in a sheathing situation over a wood substrate.

Considering all aspects of Kevlar, the benefits seem to be greatly overshadowed by the negative aspects, at least for sheathing work. Cost of the fabric is many times higher than fiberglass fabrics that would be comparably used, although availability is becoming more common since the material has many other uses.

The fabric is ordinarily called "Kevlar 49" and is available in several weights per square yard, usually in 38" and 50" widths. While localized reinforcement may be a practical use for Kevlar as long as it is clad with a more durable material such as fiberglass, Kevlar has little to recommend it as a general sheathing material.

Carbon Fiber

Carbon fibers are commonly available in the pure fiber form called "tows". These fibers are often bonded with epoxy for localized reinforcement purposes where additional stiffening and strengthening is required. In this application, they have many uses and perform splendidly if applied properly.

Carbon fiber is not quite as strong in specific tensile strength as Kevlar, but it is considerably stronger in compressive strength. However, carbon fiber is very weak in impact strength. Hence, the two materials are sometimes used together in "hybrid" form to compensate for each material's weaknesses. As with Kevlar, however, carbon fiber cannot stand up to abuse without the help of other materials such as fiberglass.

Besides being weak in impact strength, carbon fiber is weak in sheer strength and must therefore be shielded from abrasion. This quality alone rules it out for most sheathing applications. While carbon fibers can be woven either individually or in hybrid form that could conceivably be used for sheathing, the fibers will lose a certain degree of strength when woven, just as all fibers will. Carbon fibers must be kept straight with no wrinkles or kinks if maximum strength is to be realized. Thus, considering the high premium paid for carbon fiber (it's even more costly than Kevlar), it seems a waste to use the material in any woven form that takes the fibers out of a straight orientation.

There is much disagreement as to the resins that are suitable for use with carbon fiber. Technically, polyester, vinylester, and epoxy resins will all wet out the material. However, some feel that in order to realize optimum strength properties, more flexible resins such as vinylester and epoxy are necessary. From a pure bonding standpoint when applying to a substrate material, epoxy would be the better choice.

Carbon fiber materials are coal black and will appear this way when wetted out with resin, making them impractical for clear coatings. There is some danger to breathing carbon in the air. Unattached fibers can become airborn easily, so a filter mask should be worn when cutting and fabricating with the material. Metals in contact with carbon (graphite) fiber in salt water are highly susceptible to corrosion; such contact must be avoided. In view of the preceding, carbon fiber has even less to recommend it as a sheathing material than Kevlar.