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1. Tensile strength
Tensile strength is the maximum stress a material can withstand before stretching. Some non-brittle materials deform before rupture, but Kevlar® (aramid) fibers, carbon fibers, and E-glass fibers are fragile and rupture with little deformation. Tensile strength is measured as force per unit area (Pa or Pascals).

2. Density and Strength-to-Weight Ratio
When comparing the densities of the three materials, significant differences in the three fibers can be seen. If three samples of exactly the same size and weight are made, it quickly becomes apparent that Kevlar® fibers are much lighter, with carbon fibers a close second and E-glass fibers the heaviest.

3. Young’s Modulus
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 materials with a high Young’s modulus are stiffer than those with a low Young’s modulus.
The stiffness of carbon fiber, Kevlar®, and glass fiber varies greatly. Carbon fiber is about twice as stiff as aramid fibers and five times stiffer than glass fibers. The downside of carbon fiber’s excellent stiffness is that it tends to be more brittle. When it fails, it tends not to exhibit much strain or deformation.

4. 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, frosted glass fibers used in buildings can also increase fire resistance.
Carbon fiber and Kevlar® are used to make protective firefighting or welding blankets or clothing. kevlar gloves are often used in the meat industry to protect hands when using knives. Since the fibers are rarely used on their own, the heat resistance of the matrix (usually epoxy) is also important. When heated, epoxy resin softens rapidly.

5. Electrical Conductivity
Carbon fiber conducts electricity, but Kevlar® and fiberglass do not.Kevlar® is used for pulling wires in transmission towers. Although it does not conduct electricity, it absorbs water and water does conduct electricity. Therefore, a waterproof coating must be applied to Kevlar in such applications.

6. 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 common matrices such as epoxy resins are retained in sunlight where it will whiten and lose strength. Polyester and vinyl ester resins are more resistant to UV, but weaker than epoxy resins.

7. Fatigue resistance
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.

8. Abrasion resistance
Kevlar® is highly resistant to abrasion, which makes it difficult to cut, and one of the common uses of Kevlar® is as protective gloves for areas where hands may be cut by glass or where sharp blades are used. Carbon and glass fibers are less resistant.

9. Chemical resistance
Aramid fibers are sensitive to strong acids, bases and certain oxidizing agents (e.g., sodium hypochlorite), which can cause fiber degradation. Ordinary chlorine bleach (e.g. Clorox®) and hydrogen peroxide cannot be used with Kevlar®. Oxygen bleach (e.g. sodium perborate) can be used without damaging aramid fibers.

10. Body bonding properties
In order for carbon fibers, Kevlar® and glass to perform optimally, they must be held in place in the matrix (usually an epoxy resin). Therefore, the ability of the epoxy to bond to the various fibers is critical.
Both carbon and glass fibers can easily stick to epoxy, but the aramid fiber-epoxy bond is not as strong as desired, and this reduced adhesion allows water penetration to occur. As a result, the ease with which aramid fibers can absorb water, combined with the undesirable adhesion to epoxy, means that if the surface of the kevlar® composite is damaged and water may enter, then Kevlar® may absorb water along the fibers and weaken the composite.

11. Color and weave
Aramid is light gold in its natural state, it can be colored and now comes in many nice shades. Fiberglass also comes in colored versions. Carbon fiber is always black and can be blended with colored aramid, but it cannot be colored itself.