Kevlar Aramid properties
Kevlar fiber is a synthetic aromatic polyamide (aramid), known for its exceptional mechanical strength, low density, and high thermal stability. Developed by DuPont™, it belongs to the para-aramid family and is marketed in different grades – mainly Kevlar 29 and Kevlar 49 – to meet specific requirements in sectors such as aerospace, military, automotive, and industrial.
Chemical composition
Section titled “Chemical composition”Kevlar is poly(p-phenylene terephthalamide) (PPTA), a polymer formed by repeating terephthalamide units linked in para position to aromatic rings. The molecular chains are oriented longitudinally during the spinning process, generating a strong network of intermolecular hydrogen bonds responsible for the high stiffness and strength of the fiber. Its molecular formula is ([-CO\text{–}C_6H_4\text{–}CO\text{–}NH\text{–}C_6H_4\text{–}NH{-}]_n).
Mechanical properties
Section titled “Mechanical properties”Kevlar fiber presents a unique combination of high tensile strength and low weight. The most representative values for commercial grades are shown in the following table.
| Property | Kevlar 29 | Kevlar 49 |
|---|---|---|
| Tensile strength (MPa) | 2 860 | 3 650 |
| Tensile strength (10³ psi) | 410 | 530 |
| Tensile modulus (GPa) | 64 | 124 |
| Tensile modulus (10⁶ psi) | 9,3 | 18 |
| Elongation at break (%) | 3,5 | 2,5 |
| Density (kg/m³) | 1 440 | 1 440 |
| Density (lb/in³) | 0,052 | 0,052 |
Mechanical properties by treatment condition
Section titled “Mechanical properties by treatment condition”Kevlar is supplied in two main treatment conditions that define its modulus and elongation. Kevlar 29, with standard modulus, is used in ballistic and industrial applications where energy absorption is valued (elongation ≈ 3,5 %). Kevlar 49, with high modulus treatment, reaches a modulus of 124 GPa and an elongation of 2,5 %, being ideal for composite reinforcement in aeronautical and marine structures. Both maintain the same density of 1 440 kg/m³.
Physical properties
Section titled “Physical properties”Density is 1,44 g/cm³ (1 440 kg/m³), which gives it a specific strength much higher than that of steel. The fiber has a characteristic golden yellow color. Moisture absorption under ambient conditions is moderate, with an equilibrium moisture content of 4 % to 5 %. The fiber is not electrically conductive and exhibits excellent abrasion resistance.
Thermal properties
Section titled “Thermal properties”The decomposition temperature of Kevlar exceeds 500 °C (932 °F) without melting, and the fiber maintains its structural integrity up to approximately 160 °C in continuous service. The coefficient of linear thermal expansion is anisotropic: −2 × 10⁻⁶ K⁻¹ in the longitudinal direction and 5 × 10⁻⁵ K⁻¹ in the transverse direction. Thermal conductivity is low, around 0,04 W/(m·K), and the specific heat capacity is approximately 1,42 kJ/(kg·K).
Electrical properties
Section titled “Electrical properties”Kevlar is an excellent electrical insulator, with a typical volume resistivity greater than 10¹⁵ Ω·cm. The dielectric strength of aramid fabrics is around 20 kV/mm, and the dielectric constant at 1 MHz is close to 3,5 with a very low dissipation factor. These properties make it suitable for fiber optic cables and dielectric reinforcements.
Chemical resistance
Section titled “Chemical resistance”Kevlar shows good resistance to most organic solvents, lubricants, fuels, and saline solutions. It is not attacked by dilute acids and bases at room temperature. However, concentrated acids and bases at high temperature can degrade the amide structure. Likewise, prolonged exposure to ultraviolet radiation causes a gradual loss of mechanical strength, so it is recommended to protect exposed surfaces in outdoor applications.
Applications by industry
Section titled “Applications by industry”The main applications of Kevlar and aramids are grouped by sector:
- Defense and security: bulletproof vests, helmets, lightweight armor.
- Aerospace: fuselage panels, fairings, aircraft components.
- Naval: boat hulls, sails, high-strength ropes.
- Automotive: belts, hoses, clutch linings and brake pads, protective clothing for motorcyclists.
- Industrial: gaskets for high pressures and temperatures, reinforcement of electromechanical and fiber optic cables, adhesive coatings and sealants.
- Sports: tennis rackets, bicycles, skis, and other high-performance items.
Comparison with similar materials
Section titled “Comparison with similar materials”The following table contrasts the most relevant mechanical properties of Kevlar against other common reinforcement fibers.
| Fiber | Density (kg/m³) | Tensile strength (MPa) | Tensile modulus (GPa) | Elongation (%) |
|---|---|---|---|---|
| Kevlar 29 | 1 440 | 2 860 | 64 | 3,5 |
| Kevlar 49 | 1 440 | 3 650 | 124 | 2,5 |
| E‑Glass | 2 630 | 3 500 | 72,5 | 4,9 |
| S‑Glass | 2 490 | 4 600 | 88 | 5,5 |
| Carbon AS‑4 (PAN) | 1 800 | 4 000 | 245 | 1,6 |
| Carbon IM‑7 (PAN) | 1 744 | 4 900 | 317 | 1,7 |
Kevlar stands out for its very high specific strength (strength/density), comparable or superior to glasses and some carbons, although its modulus is lower than high-modulus carbon fibers.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the tensile strength of Kevlar 29?
Section titled “What is the tensile strength of Kevlar 29?”Kevlar 29 has a tensile strength of 2 860 MPa (410 ksi) and an elongation at break of 3,5 %.
What differentiates Kevlar 49 from Kevlar 29?
Section titled “What differentiates Kevlar 49 from Kevlar 29?”Kevlar 49 presents a tensile modulus of 124 GPa compared to 64 GPa for Kevlar 29, and a lower elongation (2,5 % vs 3,5 %). This makes it preferred for structural reinforcement in composites.
At what temperature does Kevlar decompose?
Section titled “At what temperature does Kevlar decompose?”The decomposition temperature exceeds 500 °C, without the fiber melting, and it can work in continuous service up to about 160 °C.
Is Kevlar resistant to ballistic impacts?
Section titled “Is Kevlar resistant to ballistic impacts?”Yes; Kevlar 29 absorbs impact energy through a tensile strength of 2 860 MPa and an elongation of 3,5 %, dissipating the kinetic energy of the projectile.
How does UV radiation affect Kevlar?
Section titled “How does UV radiation affect Kevlar?”Prolonged exposure to UV rays causes degradation that reduces tensile strength. It is recommended to protect the fiber with coatings when used outdoors.
What is the density of Kevlar?
Section titled “What is the density of Kevlar?”The density of Kevlar 29 and 49 grades is 1 440 kg/m³ (0,052 lb/in³), approximately 55 % of the density of E-glass (2 630 kg/m³).
References
Section titled “References”- azom.com: https://www.azom.com/article.aspx?ArticleID=1384
- engineeringtoolbox.com: https://www.engineeringtoolbox.com/polymer-composite-fibers-d_1226.html
- steelnumber.com: https://www.steelnumber.com/en/steel_composition_eu.php?name_id=642