What is Kevlar Fiber?
Kevlar® is the most important high-performance aramid fiber based on the poly (I, 4-phenylene terepthalamide). DuPont introduced (1965) the first such type of fiber under the trade name Kevlar. Kevlar has very high strength, high Young’s modulus, high tenacity, low creep and low elongation at break. It is prepared by the reaction between p-phenylenediamine and terephthaloyl chloride. The polymer is dissolved in sulphuric acid and spun by using dry jet wet spinning technique. The fiber is heat treated for crystallization and further orientation.
Kevlar (poly-paraphenylene terephthalamide [PPTA]) is the trademark for a light, strong para-aramid synthetic fiber. It is spun into fabric sheets used as reinforcement in composite materials when coated with elastomers. This advanced material is lightweight yet has high strength and can provide protection on extreme impact. A Kevlar fabric swatch is shown in Figure 1.
Kevlar is synthesised in solution from the monomers 1,4-phenylene diamine and terephthaloyl chloride in a condensation reaction and yields hydrochloric acid as a by-product.
Kevlar production is expensive because of the difficulties arising from using concentrated sulfuric acid needed to keep the water-insoluble polymer in solution during its synthesis and spinning.
Kevlar consists of long molecular chains produced from PPTA. There are many interchain bonds which make the material extremely strong. Kevlar has relatively rigid molecules which tend to form mostly plain sheet-like structures.
This polymer has very good resistance to high temperature and can maintain its strength and resilience at a cryogenic temperature of –196°C. At higher temperatures, the tensile strength is reduced by 1-20% upon long duration; the strength reduction is ~50% if it is exposed at 260°C for 3 days.
Aramid (aromatic polyamide) long-chain continuous filament yarn exhibits very high tenacity and the lowest elongation at break of textile yarns. It is used in very high-strength rubber-coating applications such as radial tyres, as well as for inflatable devices for military and mechanical applications. There are three grades of Kevlar used as reinforcement in these products. Kevlar29 is lightweight and is suitable for bulletresistant undervests. Kevlar49 has very high tenacity and is used as reinforced fiber in elastomer composites for automobiles, boats, aircraft, and aerospace vehicles. Rubber coated Kevlar can be used instead of traditional steel components in the manufacture of tyres because of its strength-to-weight ratio. This causes significant changes to its properties: very lightweight, lower rolling resistance and better fuel consumption.
This material is well known as a component of vests that protect against bullets and sharp objects. It can be thinly coated with rubber for such important applications. Rubber-coated Kevlar is used as an inner lining for some bicycle tyres to prevent punctures. Elastomer-coated Kevlar can be used for safety clothing for motorcyclists (especially in areas featuring padding on highly abrasive surfaces).
Upon rubber coating, it can be used on the vamp area of rubber footwear to prevent chainsaw injuries. The rubber-coated Kevlar fabric can be utilized as a reinforcing material for making rubber bellows, expansion joints and for other engineering applications. This high-strength coating is used to reinforce rubber hoses used in high-temperature applications. Kevlar in cord form can be used as an outer covering of rubber braided hoses, and on coating the braided outer, it provides high protection against cuts from the sharp objects and can protect the hose from damage if it is dragged on highly abrasive surfaces.
Aramid fibers composites coated with polymers can be widely used in combination with carbon and glass fiber in military and other technical uses. Its major disadvantage of low elongation occurs if the aramid is used in several layers.
Ultra lightweight car tyres have been developed by Dunlop using Kevlar (DuPont) fibers which are claimed to be five times stronger than steel. Replacing the conventional steel components (usually implemented for producing tyres on rubber coatings in sequence to reduce fuel consumption) improved the performance of the tyres.
Properties of Kevlar Fiber
Kevlar® fiber is famous for its exceptional strength-to-weight ratio, thermal stability, and cut resistance, making it one of the most important fibers in technical textiles. Typical properties of Kevlar fiber are given below.
- High tensile strength at low weight.
- Excellent dimensional stability.
- Low elongation at break.
- High modulus and toughness.
- Flame-resistant, self-extinguishing.
- High chemical resistance.
- Low electrical conductivity.
- Low thermal shrinkage.
- High cut resistance.
- Degrades under UV light.
Types of Kevlar Fiber
These are the different grades or categories developed for various uses, based on strength, modulus, and thermal resistance. Common types include:
- Kevlar 29: Standard grade, used for cables (such as those used to support bridges), parachutes, reinforced tapes and hot-air balloons.
- Kevlar 49: High modulus type, used (in both filament and staple form) for plastic reinforcement for aerospace applications, boat hulls, and other related construction.
- Kevlar 149: Ultra-high modulus, excellent stiffness for advanced composites.
- Kevlar KM2: Improved toughness for military armor and helmets.
- Kevlar AP & XP: Newer, lightweight ballistic grades with higher performance.
Uses of Kevlar Fiber
Kevlar has a high modulus, strength to weight ratio greater than other synthetic fibers, less extension as compared with polyester and nylon low corrosion, and creep. These are used in radial tyres, conveyor belts, composites, ropes, cables, ballistics and friction products. Kevlar is used in this type of sports equipment because of its light weight and strength.
Kevlar® is used to make bulletproof vest and puncture-resistant bicycle tires. Blends of Nomex® and Kevlar® are used to make fireproof clothing and fibers. Kevlar is often used to strengthen radial tires because it is lightweight and five times stronger than steel. Kevlar helps make high-performance skis and ski boots smoother, stiffer, and more flexible and improves vibration damping. The qualities expected for high performance skis are speed, stability, and good maneuverability. Kevlar tubing allows ski poles to be lighter, stronger, and more stable. Kevlar provide lightweight protection for racers for gloves used in slalom competitions.
Kevlar has become a popular choice for both equipment manufacturers and customers in the search for lighter, and stronger, sporting goods. Even appealing to athletes, outdoor enthusiasts and anyone else looking for better performance in sports goods.
Typical uses of Kevlar fiber are:
Motorcycle suits from woven Kevlar, which is blended with Cordura (also a polyamide derivative) and Lycra to allow for protection and flexibility. Often, this fabric is placed on a garment where areas of the body are most vulnerable during an accident; elbows, knees, shoulders and spine. Chainsaw chaps (specialized over-trousers similar in style to those worn by cowboys) are made up of six layers of fabric to form a lightweight, flexible sandwich. The outer layer is Cordura, next to a tough nylon shell, then woven Kevlar, two layers of Kevlar felt and the inner layer is Cordura. The Kevlar fibers catch in a moving chainsaw and clog the chains to jam the machinery; this prevents severe damage to the body e.g. legs being severed! Kevlar is also beginning to appear in the leisure and clothing industries.
Conclusion
Kevlar is a remarkable high-performance synthetic fiber characterized by its exceptional strength, lightweight nature, and excellent thermal and chemical resistance, making it indispensable in defense, aerospace, industrial, and sports applications. Its continuous development ensures new variations with even greater performance for future innovations.
References
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[2] Ashford, B. (2016). Fibers to Fabrics.
[3] Kyulavska, M., Toncheva-Moncheva, N., & Rydz, J. (2017). Biobased polyamide ecomaterials and their susceptibility to biodegradation. In Springer eBooks (pp. 1–34). https://doi.org/10.1007/978-3-319-48281-1_126-1
[4] Jindal, A. J. R. (2023). Textile raw materials. Abhishek Publications.
[5] Ahmad, S., Rasheed, A., & Nawab, Y. (2020). Fibers for technical textiles. In Topics in mining, metallurgy and materials engineering. https://doi.org/10.1007/978-3-030-49224-3
