By Eman Abdallah Kamel
Eman is a writer and textile engineer. She received her bachelor’s degree in textile science from the Faculty of Applied Arts, Egypt.
Kevlar
Kevlar is a synthetic fiber consisting of a strong, lightweight semi-aramid material bonded to other aramids, such as Nomex and Technora. The Kevlar material is widely used in military ballistics and body armor applications. It is five times stronger than steel and can stop a 9mm pistol bullet traveling at 1,200 feet per second with just 20 layers.
Its color is naturally yellow. It is available as a dry fabric and a pre-strengthening material.
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.
A Brief History
Polish-American chemist Stephanie Kwolek invented Kevlar in 1964 at DuPont’s Wilmington, Delaware, laboratories.
In 1971, modern Kevlar was introduced. During the 1980s, Iranian-American scientist Dr. Jacob Lahijani of DuPont invented Kevlar 149.
DuPont is the world’s largest producer of para-aramid. It is currently produced in three countries: Northern Ireland, the United States, and Japan.
Production
Kevlar is a polyamide, a synthetic polymer in which the amide groups are separated by para-phenylene groups, meaning that the amide groups are linked together on opposite sides of the phenyl group.
What are Polymers?
Polymers are any natural or synthetic materials consisting of very large molecules, called macromolecules, which are multiples of simpler chemical units called monomers.
Kevlar is manufactured through a process of repeating amides over and over again.
Amides are chemical compounds in which a carbon-based acid replaces one of the hydrogen atoms in NH₃.
Polyamide is formed by reacting an ammonia-like chemical with an organic acid. During a condensation reaction, two materials are fused into one.
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 byproduct.
The wet spinning process turns basic aramid into long, thin, and stiff fibres. During this process, a hot and highly viscous solution of poly-para-phenylene terephthalamide is pumped through an orifice with several hundred holes.
After the fibres are 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 Kevlar fibers consists of a combination of strong and weak electron system bonds in both the linear and lateral directions of the polymer chains. If the fibers are bent into a loop, they become twisted internally.
Kevlar Forms
Kevlar is available in many short forms, including staple, floc, and pulp.
- A Kevlar staple is a short, finely cut fiber, ¼ inch or longer. It is used to make spun yarn.
- Kevlar floc is a short, finely cut fiber. It is shorter than the Kevlar staple, down to 1 mm in length. This form is used as a reinforcement in a variety of resin systems.
- Kevlar pulp is a highly fibrillated fiber. It is suitable for dispersing into a wide variety of 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 times the strength of steel.
- Electrical conductivity is low.
- The tensile modulus is higher than that of steel wire.
- Kevlar does not shrink when exposed to hot air or hot water.
- Increasing the temperature reduces the modulus, tensile strength, and fracture elongation of the Kevlar filament.
2. Chemical Properties
- Acid and Base 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: Kevlar absorbs moisture at a different rate, depending on the relative humidity. 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 in the presence of sufficient water (pH 7) to form a saturated vapour. The strength loss results are determined by comparing the strength data measured at room temperature for the control and the exposed filaments.
- Decomposition: 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).
- 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.
What is Moisture Recovery?
A process of moisture recovery is the release of atmospheric moisture from fibers until they reach a balanced moisture content at a specific temperature and humidity level.
Types of Kevlar
Kevlar 29:
This type of Kevlar is used in ballistic applications, cords, cables, and 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 kind of yarn is distinguished by its high performance, high durability, and light weight. 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 highly resistant, high-strength, and soft. It is used in the jackets and helmets of both military and law enforcement officers.
Risks of Kevlar Fiber on the Environment and Humans
During the spinning process, sulfuric acid is used. Sulfuric acid is highly toxic to animals and plants. Correct use of this chemical compound is essential to avoid significant risks.
Kevlar is a recyclable fiber. Some companies 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.
During Kevlar manufacturing, fiber dust is generated. 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 may cause lung fibrosis.
Did You Know?
Researchers at the Royal Melbourne Institute of Fashion and Textile Technology have discovered a blend of wool and Kevlar used in body armor. The resulted textile was lighter, cheaper, and worked better in wet conditions than Kevlar alone.
Wool
Wool is a natural fiber. It can easily absorb up to 30% of its weight in moisture without feeling damp.
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 multiseasonal fabric.
Features of the 50% Kevlar/50% Wool Fabric
Researchers found that the Kevlar/wool blend possessed better moisture management properties and improved the properties of the Kevlar fabric.
- Fabrics are lighter, more durable, and more 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.
- The fabrics are machine-washable. And it can be tumble-dried 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 stable Lanaset (acid) dyes.
- This blend has a lower production cost as a result of reducing waste. And simpler processes in dyeing, printing, and finishing.
- This textile blend is more environmentally friendly than Kevlar alone.
Sources
- dupont.com… Kevlar Technical Guide.pdf. Kevlar-aramid fiber technical guide.
- core.ac.UK... The effect of environmental exposure on the aramid Kevlar fiber.
- Development of flame-resistant combat uniform fabrics made from long-staple wool and aramid blend yarn. apps.dtic.mil. pdf
- SAGE Journals: Your gateway to world-class journal research. Thermal comfort properties of Kevlar and Kevlar/wool fabrics.
©Eman Abdallah Kamel, 2022
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