Which material is better, carbon fiber, glass fiber or aramid?
Significant differences in the three fibers can be seen when the densities of the three materials are compared. If you make 3 samples of the exact same size and weight, it quickly becomes apparent that Kevlar® fiber is much lighter, followed by carbon fiber, and E-glass fiber being the heaviest.
Therefore, for the same weight of composite material, carbon fiber or Kevlar® can achieve higher strength. In other words, any structure requiring a given strength made with carbon fiber or Kevlar® composites will be smaller or thinner than a structure made with fiberglass.
When samples were made and tested, it was found that the fiberglass composite was almost twice as heavy as Kevlar® or carbon fiber laminates. This means that using Kevlar® or carbon fiber can save a lot of weight.
Young's modulus is a measure of the stiffness of an elastic material and is a way of describing a material. It is defined as the ratio of uniaxial (in one direction) stress to uniaxial strain (deformation in the same direction). Young's modulus = stress/strain, which means that a material with a high Young's modulus is harder than a material with a lower Young's modulus.
Carbon fiber, Kevlar® and fiberglass vary widely in stiffness. Carbon fiber is about twice as stiff as aramid fiber and five times stiffer than glass fiber. The downside to carbon fiber's superior stiffness is that it tends to be more brittle. When it fails, it tends not to exhibit much strain or deformation.
3. Flammability and thermal degradation
Both Kevlar® and carbon fiber are resistant to high temperatures, and neither has a melting point. Both materials have been used in protective clothing and fire-resistant fabrics. Fiberglass will eventually melt but is also highly resistant to high temperatures. Of course, the use of frosted fiberglass in buildings also improves fire resistance.
Carbon fiber and Kevlar® are used to make protective firefighting or welding blankets or clothing. Kevlar gloves are commonly used in the meat industry to protect hands when using knives. Since fibers are rarely used alone, the heat resistance of the matrix (usually epoxy) is also important. Epoxy resins soften rapidly when exposed to heat.
4. Conductivity
Carbon fiber conducts electricity, but Kevlar® and fiberglass do not. Kevlar® is used for guy wires in transmission towers. Although it does not conduct electricity, it absorbs water, and water does conduct electricity. Therefore, in such applications, a waterproof coating must be applied on top of Kevlar.
Because carbon fiber conducts electricity, galvanic corrosion becomes a problem when it comes into contact with other metal parts.
5. UV degradation
Aramid fibers will degrade in sunlight and high UV environments. Carbon or glass fibers are not very sensitive to UV radiation. However, some commonly used substrates such as epoxy resin will whitish and lose strength if left in sunlight. Polyester and vinyl ester resins are more resistant to UV rays but weaker than epoxy resin.
6. Anti-fatigue
If a part is repeatedly bent and straightened, it will eventually fail due to fatigue. Carbon fiber is somewhat sensitive to fatigue and tends to fail catastrophically, whereas Kevlar® is more resistant to fatigue. Fiberglass is somewhere in between.
7. Wear resistance
Kevlar® is very abrasion resistant, which makes it difficult to cut. One of the common uses of Kevlar® is as protective gloves in areas where hands may be cut by glass or where sharp blades are used. Carbon fiber and fiberglass are less resistant.
8. Chemical resistance
Aramid fibers are sensitive to strong acids, strong bases and certain oxidizing agents such as sodium hypochlorite, which can cause fiber degradation. Common chlorine bleaches (such as Clorox®) and hydrogen peroxide cannot be used with Kevlar®, oxygen bleaches (such as sodium perborate) can be used without damaging aramid fibers.
Carbon fibers are very stable and insensitive to chemical degradation. However, the epoxy matrix will degrade.
9. Matrix bonding performance
For carbon fiber, Kevlar®, and glass to perform optimally, they must be held in place in a matrix, usually an epoxy. Therefore, the ability of epoxy resins to bond together with various fibers is critical.
Both carbon and glass fibers stick to epoxy easily, but the aramid fiber-epoxy bond is not as strong as desired, and this reduced adhesion allows water penetration to occur. As a result, aramid fibers tend to absorb water, which, combined with less than ideal adhesion to epoxy, means that if the surface of the Kevlar® composite is damaged and water can get in, then Kevlar® may absorb moisture along the fibers, and weaken the composite material.
