Kevlar Fiber: Composition, Production Process, and Applications

By Eman Abdallah Kamel

Kevlar is an aramid fibre with remarkable properties. In this article, you will learn about its definition, composition, spinning process, and applications.

Introduction

Kevlar® is an aromatic polyamide (aramid) fiber known for its high strength, high modulus, toughness, and thermal stability. Its unique chemical structure distinguishes it from other synthetic fibers, making it suitable for demanding industrial and advanced-technology applications.

What is an aramid fiber?

An aramid fiber is a synthetic polyamide fiber in which at least 85% of the amide linkages are directly attached to two aromatic rings.

It has about five times the tensile strength of steel and was initially developed to replace steel in radial tyres. It is now widely used in advanced composites and other applications.

In 1965, DuPont scientists discovered that poly-p-benzamide could form liquid-crystalline solutions due to its repetitive, rod-like molecular structure. The key feature was the para orientation of the benzene rings, leading to the development of Kevlar®.

Chemical synthesis of Kevlar from 1,4-phenylenediamine (para-phenylenediamine) and terephthaloyl chloride. Image source: Wikimedia.org

Composition

Chemically, Kevlar is poly-para-phenylene terephthalamide (PPD-T) and is more precisely classified as a para-aramid; it consists of aromatic units substituted in the para position.

Did You Know?

While aramids belong to the nylon family, common nylon types—such as nylon 6,6—lack the superior structural properties of aramids, making the distinction between para-aramids and Kevlar fibers crucial. Unlike the linear compounds used to manufacture standard nylons, aramid fibers like Nomex and Kevlar are cyclic compounds based on a benzene structure.

The aramid ring imparts thermal stability to Kevlar, while the para structure provides high strength and a high modulus of elasticity. Kevlar filaments are produced by extruding the raw material through a spinneret. The rod-like shape of the para-aramid molecules and the extrusion process render Kevlar fibers anisotropic, meaning they are stronger and stiffer in the axial direction than in the transverse direction.

It is made by

A condensation reaction of PPD and terephthaloyl chloride. It contains aromatic and amide groups, forming rigid rod-like polymers.
The rigid structure gives a high glass transition temperature and poor solubility.
Conventional drawing is difficult; instead, Kevlar is melt-spun from liquid crystalline solutions, with molecules arranged in parallel in a crystalline structure.

Remember

Kevlar is made by spinning, where molten polymer is pushed through small holes. During this process, the polymer chains align in the fiber direction, giving Kevlar high strength and stiffness. It has anisotropic properties, meaning it is stronger along the fiber length. This structure results in poor shear and compressive properties in aramid-based composites because hydrogen bonds form between the polar amide groups on adjacent chains, binding the individual Kevlar polymer chains together.

Process

Spinnerets

Spinnerets contain many tiny holes that are sensitive to impurities and corrosion. The feed must be carefully filtered, and corrosion-resistant metals may be required. Regular cleaning and maintenance are essential to prevent clogging. As filaments emerge from the spinneret, the liquid polymer becomes rubbery and then solidifies. This extrusion and solidification process is called spinning; it differs from textile spinning, which twists staple fibers into yarn. The four methods are wet, dry, melt, and gel spinning.

Wet spinning: Wet spinning involves extruding a spinning solution through spinnerets into a coagulation bath containing a precipitant. The fibers form as the solution precipitates in the bath. This process can produce acrylic, rayon, aramid, modacrylic, and spandex.

Did You Know?

Wet spinning is the oldest spinning method, used for polymers dissolved in a solvent. The spinneret is immersed in a chemical bath, where the emerging filaments precipitate and solidify. Polymers are dissolved in solvents such as DMF, DMAc, acetone, or weak inorganic solutions such as zinc chloride or aqueous sodium thiocyanate.

Dry spinning: A polymer solution is extruded, and the solvent is evaporated by a stream of air or inert gas, causing the fibers to solidify. Unlike wet spinning, no precipitating liquid is used, so drying is unnecessary, and solvent recovery is easier.

The dry process involves the following steps:

Dissolving the polymer in an organic solvent.
Adding additives and filter to form a viscous polymer solution called dope.
Extruding the dope through spinnerets as filaments.
Heating the filaments in a gas/vapour zone.
The solvent evaporates, leaving solidified filaments.
Further treatment is applied to the filaments.

Melt spinning: This process uses heat to melt a polymer to a viscosity suitable for extrusion. It is suitable for polymers that can withstand extrusion temperatures without degradation. Polymer chips are increasingly melted and immediately extruded using an electrically heated screw extruder. On the other hand, the molten polymer is processed under nitrogen and metered through a gear pump and filter. It is then extruded through a spinneret into cooler air, which solidifies the filaments. Lubricants and finishing oils are applied during spinning.

Did You Know?

Melt-spun fibers can have various cross-sectional shapes, including round, trilobal, pentagonal, octagonal, and hollow. Trilobal fibers enhance light reflection and sparkle, while pentagonal and hollow fibers reduce visible soil in carpets. Octagonal fibers provide a glitter-free appearance. Hollow fibers trap air, improving insulation and loft, similar to or better than down.

Gel spinning: It produces high-strength fibers by extruding a polymer in a partially liquid-crystalline state. The polymer chains remain interconnected, creating strong inter-chain forces. During extrusion, shear forces align the chains along the fiber axis, producing highly oriented filaments with enhanced tensile strength.

Did You Know?

Gel spinning, also called dry-wet spinning, involves filaments passing through air before further cooling in a liquid bath. It is used to produce high-strength polyethylene and aramid fibers.

Applications

1. Kevlar® is a high-performance fibre widely used to improve safety and efficiency. Its unique properties provide:

Low weight
High strength
Corrosion resistance

2. It is used in many applications, including

Aircraft components,
Suspension bridges and cables,
Fiber-optic cables,
Military and law enforcement body armor.

3. Since the 1970s, Kevlar® has been used in military body armor and flak jackets to protect against ballistic projectiles and explosive fragmentation.

4. Kevlar® armor is lightweight and comfortable, improving mobility and reducing fatigue.

5. Kevlar® is inherently flame-resistant, providing thermal protection from fire.

6. DuPont™ Kevlar® fibre improves the safety, performance, and durability of automotive components across various vehicles.

7. Kevlar fibers are available as filament yarn, short fibre, pulp, and engineered elastomer, making them suitable for reinforcement, friction, sealing, and textile applications.

Sources

About the author: Eman is a textile engineer and writer. She received her bachelor’s degree in textile science from Egypt’s Faculty of Applied Arts.

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