PARA – ARAMID FIBER (KEVLAR)

             

In this group fall the para – aramid fibers based upon poly (paraphenylene terephthalamide) (PPT), notably Kevlar (dupont) and Twaron (Enka) and the polybenzobisthiazole (PBZT) and polybenzobisoxazole (PBZO) fiber which possesses para – linked heterocyclic structures free from amide groups, and therefore fall outside the aramid category. These are all solution spun using an air – gap wet spinning technique from nematic liquid crystalline solutions.

Production

Kevlar is synthesized from the monomers 1,4-phenyl-diamine (para-phenylenediamine) and terephthaloyl chloride. The result is a polymeric aromatic amide (aramid) with alternating benzene rings and amide groups. When they are produced, these polymer strands are aligned randomly. To make Kevlar, they are dissolved and spun, causing the polymer chains to orient in the direction of the fiber.

Kevlar has a high price at least partly because of the difficulties caused by the use of concentrated sulfuric acid in its manufacture. These harsh conditions are needed to keep the highly insoluble polymer in solution during synthesis and spinning.The chemical synthesis of kevlar from 1,4-phenyl-diamine (para-phenylenediamine) and terephthaloyl chloride.

Para – Aramid Fibers Properties

ð  Density                        :           1.45 g/cc

ð  Melting Temperature   :           500°C

ð  Solvent                        :           H₂SO₄

ð  Tenacity (g/d)              :           Dry :- 8 – 22  ;  Wet :- 7 – 21

ð  Elongation (%)            :           8 – 20

ð  Modulus (g/d)             :           120 – 130

ð  Recovery (%)              :           100

ð  Moisture Uptake (%)  :           3.0 %

ð  At elevated temperatures, the aromatic polyamides have greater mechanical properties.

ð  Kevlar fibe is one of the strongest of man – made fibers.

ð  Dimensional stability of Kevlar is excellent. There is negligible change in length in hot water or in air at temperature as high as 160°C.

ð  Kevlar is susceptible to photo – degradation.

ð  Kevlar is unaffected by most organic solvents but it is attacked by strong acids and bases at high concentration or at high temperatures.

Applications of Para – Aramid Fibers

ð  They are being used in some designs of helicopters and light weight fighter planes.

ð  Its lower creep and excellent creep rupture characteristics are important in the use of aramids as filament wound, pressure vessel reinforcement.

ð  Protective gloves, aprons, vests, helmets, jackets, boots etc. are developed utilizing the cut and penetration resistance of these high performance fibers.

ð  Aramid fibers are providing improved damage tolerance in off – shore power boats, base boats, ocean racing yachts, small sail boats, canoes and kayaks.

ð  In sporting goods aramid fibers are applied such as golf shafts, archery bows, paddles, tennis rackets, fishing rods, track and field equipment.

ð  Aramid fibers are also used to manufacture ropes and cables with high strength and low stretch, such as wire ropes and electromechanical cables.

High strength and modulus of aramid fibers has led to their use in a number of rubber (tires, v – belts, hose etc.), plastic (epoxy, polyester,PVC etc.) and elastomer (neoprene, urethane etc.) reinforcement applications.