Advantages And Disadvantages of Kevlar, Carbon Fibre And Glass Fibre

Reinforcement fibre material properties PK

1 Tensile Strength

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

Stress is the force and strain is the deflection due to stress. The table below shows a comparison of the tensile strength of three commonly used reinforcing fibres: carbon fibre, aramid fibre, glass fibre and epoxy resin. It is worth noting that these figures are for comparison only and they can vary with the manufacturing process, the composition of the epoxy resin, the formulation of the aramid, the precursor fibre of the carbon fibre, etc., and are expressed in MPa.

2. Density and Strength-to-Weight Ratio

When comparing the densities of the three materials, significant differences can be seen between the three fibres. If three samples of exactly the same size and weight are made, it quickly becomes apparent that Kevlar® fibres are much lighter, with carbon fibres coming a close second and E-glass fibres the heaviest. Therefore, for the same weight of composite, higher strength can be obtained with carbon fibre or Kevlar®. In other words, any structure made from carbon fibre or Kevlar® composites that requires a given strength is smaller or thinner than one made from glass fibre. After samples have been made and tested, it will be found that glass fibre composites weigh almost twice as much as Kevlar® or carbon fibre laminates. This means that a lot of weight can be saved by using Kevlar® or carbon fibre. This property is called strength-to-weight ratio.

3. Young's Modulus 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 fibre, Kevlar® and glass fibre varies considerably. Carbon fibre is about twice as stiff as aramid fibre and five times stiffer than glass fibre. The downside of carbon fibre's excellent stiffness is that it tends to be more brittle. When it fails, it tends not to show much strain or deformation.

4. Flammability and Thermal Explanation

Both Kevlar® and carbon fibre are resistant to high temperatures and neither has a melting point. Both materials have been used in protective clothing and fire-resistant fabrics. Glass fibre will eventually melt, but is also highly resistant to high temperatures. Of course, frosted glass fibres used in buildings can also increase fire resistance.

Carbon fibre and Kevlar® are used to make protective firefighting or welding blankets or clothing. kevlar gloves are often used in the meat industry to protect the hands when using knives. The heat resistance of the matrix (usually epoxy) is also important as the fibres are rarely used on their own. When exposed to heat, epoxy resin softens rapidly.

5. Electrical Conductivity, Conductivity

Carbon fibre conducts electricity, but Kevlar® and glass fibre 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.

Because carbon fibre can conduct electricity, galvanic coupling corrosion becomes a problem when it comes into contact with other metal parts.

6. UV degradation

Aramid fibres will degrade in sunlight and high UV environments. Carbon or glass fibres are not very sensitive to UV radiation. However, some commonly used matrices such as epoxy resins are retained in sunlight where it will whiten and lose its 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 fibre is somewhat sensitive to fatigue and tends to fail catastrophically, whereas Kevlar® is more resistant to fatigue. Glass fibre is somewhere in between.

8. Abrasion resistance

Kevlar® is highly resistant to abrasion, which makes it difficult to cut. 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 fibres are less resistant.

9. Chemical resistance 

Aramid fibres are sensitive to strong acids, alkalis and certain oxidising agents (e.g. sodium hypochlorite), which can cause fibre degradation. Common chlorine bleaches (e.g. Clorox®) and hydrogen peroxide cannot be used with Kevlar®. Oxygen bleaches (e.g. sodium perborate) can be used without damaging aramid fibres.

Carbon fibres are very stable and insensitive to chemical degradation. However, the epoxy matrix will degrade.

10. Body bonding properties 

In order for carbon fibres, Kevlar® and glass to perform optimally, they must be held in place in the matrix (usually epoxy resin). The ability of the epoxy resin to bond to the various fibres is therefore critical.

Both carbon and glass fibres can easily stick to epoxy, but the aramid fibre-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 fibres can absorb water, coupled 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 fibres and weaken the composite.

11. Colour and weave

Aramid is light gold in its natural state, it can be coloured and now comes in many nice shades. Glass fibre is also available in coloured versions. Carbon fibre is always black and can be blended with coloured aramid, but it cannot be coloured itself.