Aramid fibers are a class of synthetic fibers which are heat resistant and / or exhibit good mechanical properties. They are used in aerospace and military applications, for example in the manufacture of bulletproof vests as well as as a substitute for asbestos. The word aramid comes from the contraction of aromatic polyamide. The component chains are strongly oriented in the direction of fiber strength, so that the forces of molecular interactions can be harnessed for thermal and / or mechanical resistance.

Poly-para-phenylene terephthalamide is a polymer made up of aromatic rings (benzene) separated by amide groups. It belongs to the group of aramid fibers. Poly-para-phenylene terephthalamide has been discovered and marketed under the trade name Kevlar.

Poly-para-phenylene terephthalamide was discovered in 1965 by Stéphanie Kwolek and Herbert Blades, researchers at DuPont. Today the patent has expired, hence the appearance of competing products such as Twaron produced by Teijin.

Poly-para-phenylene terephthalamide is a synthetic fiber which has exceptional qualities of tensile strength and elongation. Only the spider web (three times more resistant), and carbon nanotubes (100 times more resistant and 6 times lighter) surpass it.

There are several grades of Kevlar: Kevlar, Kevlar 52, and Kevlar 49. Basic Kevlar is mostly used for reinforcing tires and other rubbers. Kevlar 29 is mainly used in industrial applications such as cables, or to replace asbestos, brake lining, for reinforcing the hull of a boat, or for the manufacture of bulletproof vests. Kevlar 49 is the grade with the highest tensile strength of all aramids and is used in certain plastics, for shell reinforcement, and for the manufacture of certain aircraft parts and bicycle frames.

His qualities :

• Good specific tensile strength.
• Low density (1.45).
• Zero thermal expansion.
• Absorption of vibrations, damping.
• Excellent resistance to impact and fatigue.
• Good chemical behavior towards fuels.

Her downsides :

• Poor resistance to UV rays.
• Low pressure resistance.
• Significant moisture uptake (4%), baking before impregnation.
• Loss of ballistic resistance when wet.
• Low adhesion with impregnation resins.
• Difficult machining.
• Poor fire resistance (decomposition at 400 ° C)

Poly-para-phenylene terephthalamide has various valuable properties, such as resistance to heat, elongation or shear. It is therefore used in many fields where these characteristics are sought after:

• Bulletproof vest, helmets (associated with one or more other fibers such as glass or carbon fiber) (weaving + resistance).
• Clothing reinforcement more or less important depending on the person.
• Boat sails (resistance to elongation and alkaline solutions).
• Aeronautics, aerospace (ship hulls, airplane wings, etc.).
• Sports equipment (speed skating, snowboarding, skiing, canoeing, fencing, snowshoes or rope, Basque pelota, rowing (sport) ...).
• Tires (shear strength).
• Juggling: Fire jugglers use poly-para-phenylene terephthalamide on their equipment, eg clubs, devil's sticks, bolas, etc. Poly-para-phenylene terephthalamide, soaked in petroleum (dearomatized if it is also used for spitting) is resistant and can be ignited many times before being replaced. Some jugglers use poly-para-phenylene terephthalamide gloves to juggle balls (a rare practice). Sold by the meter, it is wrapped around the material reserved for this use and it is fixed with screws.
• Building: Poly-para-penylene is used for high security buildings (Pentagon in the USA).