Eman is a writer and textile engineer. She obtained her bachelor's degree in textile sciences from the Faculty of Applied Arts, Egypt.
Kevlar is a synthetic fiber made up of a strong and lightweight semi-aramid material linked to other aramids such as Nomex and Technora. Kevlar is usually used as fabrics, textiles, or as a major part of composites.
Kevlar is often used for military ballistic and body armor applications. It's five times stronger weight than steel, and just 20 layers can stop a 9mm pistol bullet traveling at 1,200 feet per second. It is naturally yellow in color, available as dry fabric and pre-stiffeners.
There are two types of aramid fibers used in the aerospace industry. Kevlar 29 has low hardness, and kevlar 49 has high hardness.
Aramid fiber bundles are not measured by the number of fibers such as carbon or glass fibers but by weight.
In 1964, Kevlar was first synthesized by Polish-American chemist Stephanie Kwolek at DuPont Laboratories in Wilmington, Delaware, United States. In 1971, modern Kevlar was introduced. During the 1980s, Iranian-American scientist Dr. Jacob Lahijani of Dupont invented Kevlar 149.
DuPont is the largest producer of Para-Aramid in the world. It is currently produced in three countries, Northern Ireland, the United States, and Japan.
Kevlar Manufacturing Process
Production of Kevlar
Kevlar is a polyamide, which is a type of synthetic polymer, in which the amide groups are separated by para phenylene groups, which means that the amide groups are linked to each other on opposite sides of the phenyl group.
Kevlar is synthesized in a solution of the monomers 1,4-phenylenediamine and terephthaloyl chloride in a condensation reaction that produces hydrochloric acid as a by-product.
Through a process of repeating amides over and over again, a polyamide such as Kevlar is synthesized. Amides are chemical compounds in which carbon-based acid replaces one of the hydrogen atoms in NH3. Polyamide is formed by the reaction of an ammonia-like chemical with an organic acid. Thus, two materials are fused together into one through a process called a condensation reaction.
The wet spinning process is used to convert basic aramid into long, thin, and rigid fibers. During this process, the concentrated, hot, and extremely viscous solution of poly-para-phenylene terephthalamide is pumped through an opening that basically resembles a shower head with several hundreds of holes. After the fibers have been processed, they are cut lengthwise and woven.
The production of Kevlar is expensive due to the difficulties arising from the use of concentrated sulfuric acid. It is essential to keep the water-insoluble polymer in solution during its manufacture.
Did You Know?
The yellow color of a Kevlar fiber is formed by a set of strong and weak electron system bonds in both linear and lateral directions of the polymer chains. If the fibers are bent into a loop, they become twisted internally.
Kevlar is available in many short forms, including staple, floc, and pulp.
- Kevlar staple is made of precision-cut, short fibers, ¼ inch or longer. It is used to make spun yarns.
- Kevlar floc refers to short, finely cut fibers, shorter than the staple, down to 1 mm in length. It can be used as reinforcement in a variety of resin systems.
- Kevlar pulp is a highly fibrillated form of fiber that can be dispersed into many different matrix systems. It is also unbreakable, available in wet form, and enhances the performance of elastomers, thermoplastics, and thermosetting resins.
Properties of Kevlar
1- Physical Properties
- The absorption of kinetic energy is high.
- Low brake elongation.
- The breaking tenacity is high.
- Nearly 4-5 time strength of steel.
- Electrical conductivity is low.
- Tensile modulus is higher than steel wire.
- Kevlar does not shrink when exposed to hot air or hot water.
2- Chemical Properties
- Acid and Basis Effect: Kevlar degrades when exposed to strong acids and bases. At a neutral pH (pH 7), filament stability remains virtually unchanged after exposure at 149 °F (65 °C) for more than 200 days.
- Moisture Effect: Relative humidity has a significant effect on the rate of moisture absorption by Kevlar and the level of equilibrium reached. The higher the relative humidity level, the faster the Kevlar absorbs moisture during the initial phase of moisture acquisition and the higher the final equilibrium level.
- Hydrolysis Resistance: The resistance of Kevlar to hydrolysis in saturated steam is measured in a closed tube test. A Kevlar strand (1500 denier) is kept in a strand at 280 °F (138 °C) for various periods of time in the presence of sufficient water (pH 7) to form a saturated vapor. The strength loss results are determined by comparing the strength data measured at room temperature for the control and the exposed filaments.
- Melting: Kevlar decomposes at relatively high temperatures (800 °F to 900 °F (427 °C-482 °C) in air and about 1000 °F (538 °C) in nitrogen).
- Temperature: Increasing the temperature reduces the modulus, tensile strength, and fracture elongation of the Kevlar filament.
- Burning: Kevlar is flame resistant but can be ignited. Combustion usually stops when the ignition source is removed; however, pulp, once ignited, may continue to burn.
- Ultraviolet Effect: Kevlar is sensitive to UV rays. Unprotected strands turn from yellow to brown after long exposure. Prolonged exposure to UV rays can also cause a loss of mechanical properties, depending on the wavelength, exposure time, radiation intensity, and product geometry.
Types of Kevlar
- Kevlar 29: This type of Kevlar is used in ballistic applications, cords, and cables, protective clothing such as cut-resistant gloves, helmets, shields, and rubber reinforcements in tires and automotive hoses.
- Kevlar AP: These fibers provide advanced performance, value, and increased design flexibility in many applications. It also has a 15% higher tensile strength than K29.
- Kevlar 49: This type is used in textile processing, reinforcing plastics, ropes, optical cables, and marine sporting goods.
- Kevlar 100: This colored version of Kevlar is used in ropes, cables, tapes, strappings, gloves, and sporting goods.
- Kevlar 119: This type of Kevlar has a higher elongation. It is also elastic and fatigue resistant; it is used in mechanical rubber products, such as tires, car belts, and hoses.
- Kevlar 129: This type of yarn is characterized by its lightweight, high performance, and high durability. It is usually used in speedway equipment, ropes, cables, and high-pressure hoses used in the oil and gas industry.
- Kevlar KM2: This type is woven into a fabric that meets performance requirements for high-performance helmets, military jackets, and shrapnel liners.
- Kevlar KM2 Plus: These fibers are characterized by high resistance, high strength, and softer fibers; it is used in vests and helmets for both military and law enforcement officers.
Risks of Kevlar Fiber on the Environment and Humans
The Environment: Sulfuric acid is one of the main substances used in the production of Kevlar. Sulfuric acid is used to keep Kevlar in solution during the spinning process. Sulfuric acid is very toxic to animals and plants. If disposed of or used improperly, it can cause great harm. Kevlar is 100% recyclable. There are several companies that specialize in recycling Kevlar. During the recycling process, Kevlar is cut into 3-6 mm fibers and shaped into a new product. Once Kevlar is manufactured, the environmental impact is minimal, as long as it is disposed of properly.
Humans: Kevlar dust resulting from the processing may be inhaled into the nose and throat. Working without protection in dusty conditions may cause upper respiratory irritation and cold-like symptoms. Based on animal tests, inhaling Kevlar dust at too high a concentration repeatedly over long periods of time may cause lung fibrosis.
Did You Know?
Researchers at the Royal Melbourne Institute of Technology for Fashion and Textiles have discovered a blend of wool and Kevlar, used in body armor, that was lighter, cheaper, and worked better in wet conditions than Kevlar alone.
Wool is a natural fiber that has unique properties. Wool fiber can easily absorb up to 30% of its weight in moisture without feeling that moisture. Fleece is a temperature regulator because it can protect the body in both cold and warm conditions. Wool absorbs moisture from atmospheres of greater moisture and releases it into the driest environment because it creates a balance in humidity conditions. This property makes wool a multi-seasonal fabric. Wool fibers also are very flexible as well as non-flammable.
Bullet-Resistant Wool - RMIT University
Features of the 50% Kevlar/50% Wool Fabric
Researchers found that Kevlar/wool blend possessed better moisture management properties and improved properties of Kevlar fabric.
- Fabrics are lighter, more durable, and comfortable due to their lightweight and smooth surfaces.
- The wet Kevlar loses about 20% of its potency, so it requires expensive water resistance. Blending wool with Kevlar lowered the cost of using Kevlar alone.
- The fabrics have very high colorfastness properties.
- Fabrics are machine washable and can be dried with a dryer without the need for any topical chemical application to the fabric.
- All required color standards can be achieved by coloring the wool component only with the most advanced and most stable Lanaset (acid) dyes.
- This blend has a lower cost of production by increasing production productivity as a result of reducing waste and through simpler processes in dyeing, printing, and finishing.
- This textile blend is more environmentally friendly than Kevlar alone.
- https://www.dupont.com/content/dam/dupont/amer/us/en/safety/public/documents/en/Kevlar_Technical_Guide_0319.pdf. Kevlar aramid fiber technical guide.
- https://core.ac.uk/download/pdf/151638791.pdf. The effect of environmental exposure on aramid Kevlar fiber.
- Development of flame-resistant combat uniform fabrics made from long-staple wool and aramid blend yarn. https://apps.dtic.mil/sti/pdfs/ADA578994.pdf
- SAGE Journals: Your gateway to world-class journal research. Thermal comfort properties of Kevlar and Kevlar/wool fabrics.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2021 Eman Abdallah Kamel
Eman Abdallah Kamel (author) from Egypt on March 27, 2021:
Thanks, Devika Primić, for reading the article and the comment. Wool is a natural fiber and has distinctive properties. Fleece is so interesting.
Eman Abdallah Kamel (author) from Egypt on March 27, 2021:
Thank you very much, Ravi Rajan, for reading the article and the comment.
Ravi Rajan from Mumbai on March 27, 2021:
Wow Emmy I learned something new today. I had also known that Kevlar was some sort of plastic used for making bulletproof jackets but your hub has really enlightened me about its various forms, usage, and properties. Thanks for sharing.
Devika Primić from Dubrovnik, Croatia on March 27, 2021:
Emmy ali you have put together a well-informed hub. Many details about the wool and most interesting about its performances.
Eman Abdallah Kamel (author) from Egypt on March 25, 2021:
It's my pleasure, Linda. Despite the many important uses of Kevlar. It is like other synthetic fibers that are harmful to the environment. The research of blending natural wool with Kevlar to improve its efficiency and reduce environmental damage is interesting.
Linda Crampton from British Columbia, Canada on March 24, 2021:
I knew Kevlar was strong, but you’ve included so many details that I didn’t know in your article. Like all your articles, this one is very informative. Thank you for sharing the information.
Eman Abdallah Kamel (author) from Egypt on March 23, 2021:
Thank you, Linda, for your feedback. I agree with you that there are many types of Kevlar and natural wool improves the performance of Kevlar.
Linda Chechar from Arizona on March 23, 2021:
These fabrics! So much the Kevlar as so many kinds of wool, fabric is military strength fibers.