What is Kevlar? – Kevlar Uses, properties and processing

What is Kevlar? – Kevlar Uses, properties and processing

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The important question that comes into mind is “What is Kevlar in the field?” DuPont introduced the first organic fiber with high strength and aramid fiber with trade name Kevlar 29 in early 1970s. It was originally developed to replace steel in radial tires and known to have tensile strength five times more than steel. The invention enabled to fabricate flexible bulletproof body armor. In 1988, the second generation of Kevlar fiber was developed which offers ballistic resistance against 9mm FMJ.

KEVLAR is commercially available aramid fiber with excellent heat resistance and high stifness related to other aramid variants like Nomex. DuPoint, firstly, deveoped KEVLAR fiber in 1970 for their use in racing tires as a replacement of steel. It is commonly available in grades like K-29, K-49, K-100, K-119, and K-129.

Presently, Kevlar fiber is available in a number of variants:

  • Kevlar K-29 It is a high toughness grade used in industrial applications, such as cables, body/vehicle armor, brake linings.
  • Kevlar K49 – It has a high modulus used in rope and cable products.
  • K100 – The colored version of Kevlar
  • Kevlar K119 – It has higher-elongation, and more fatigue resistant
  • Kevlar K129 – It is a higher tenacity grade utilized in ballistic applications
  • Kevlar AP – It has 15% higher tensile strength than K-29

What is Kevlar

Kevlar is a poly para-phenyleneterephthalamide (PPD-T) commonly known as para-aramid. The structure is composed of benzene rings responsible for high thermal stability along with para substitutions which result in high modulus and strength. The filaments for fibers are extruded through the spinning of the precursor. The extruded rod-like para-aramid structure has high anisotropic properties. The strength and stiffness is

higher in the axial direction and lowers in the transverse direction.

Kevlar structure
KEVLAR Structure

Manufacturing Process

Kevlar is made from a condensation reaction of 1,4- para-phenylenediamine and terephthalic acid. The presence of amine groups on aromatic ring results in a rod-like structure which has high glass transition temperature and low solubility. The chains of polymer are connected to each other via Hydrogen bonding between adjacent polar amide groups explaining what is Kevlar. The fiber structure is consisting of orderly oriented molecules parallel to one another forming crystalline structure.    

Due to high glass transition temperature and poor solubility, These fibers are difficult to process via conventional drawing techniques hence melt spinning is used for their fabrication.  During melt spinning, the PPD-T solution is extruded in a spinneret and drawn through an air gap resulting in the orientation of the liquid crystalline domains in the flow direction. The polymer chains also align in a fiber axis resulting in a high degree of anisotropy in this direction.

Manufacturing reaction of KEVLAR fiber
Condensation reaction for formation of KEVLAR monomer

Melt Spinning Process

Fibers are manufactured by melt spinning of PPD-T solution extruded through a spinneret and then directly solidification upon cooling. The solution is heated to attain required viscosity in induction heated extruders. This melt is passed at high pressure and constant rate from a spinneret and enters an air-cooled stream which solidifies it into a filament form. At the lower end of the spinn ing setup, a guided coverage converts these individual filaments to form a continuous spun yarn. This yarn is then wound onto bobbins or treated further for end-use applications.

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Melt Spinning of KEVLAR
Melt spinning process for Kevlar

Mechanical Properties

Kevlar fibers have tensile strength twice than Nylon 6,6 ranging from about 2.6 to 4.1 GPa. The mechanical properties of different grades are given in the table below:

GradeDensity (g/cm3)Tensile Modulus (GPa)Tensile Strength (GPa)Elongation (%)
291.44833.64
491.441313.6-4.12.8
1491.471863.42.0

The structure of Kevlar has unique kinks bands responsible for the compression buckling of molecules and hence they behave elastically in tension. In compression, they yield at 0.3% to 0.5% strain and shows nonlinear, ductile behavior. The high thermal stability attributed to aromatic rings makes these fibers inherently flame resistant with a decomposition temperature of about 425°C. and are inherently flame resistant. The generic properties of these fibers are:

  • High tenacity
  • High modulus of rigidity (structural rigidity).
  • Low ductility.
  • Low electrical conductivity.
  • Low coefficient of thermal expansion.
  • High toughness (work-to-break).
  • High chemical resistance.
  • Excellent dimensional stability.
  • Flame retardant, self-extinguishing.

Applications of KEVLAR

            The exceptional and unique properties of fiber enable them to apply in a variety of applications. These applications range from premium sports goods and deep-sea umbilical lines and to high-performance structural composites in aircraft components, boat hulls, and high-performance cars.                     

The high tenacity and thermally stable fibers are used for lightweight bulletproof body armor, and also due to weight-saving it can also replace heavier materials in airplanes, for fuel saving. The textile application was explored by Baxter, a textile engineering graduate of Clemson University, who fabricated a Draggin’ Jeans, using 100% Kevlar in denim jeans.

The variety of applications are listed below:

  • Ballistics and defense
  • Composites in Aircraft structural parts
  • Belts and hoses for automotive heating/cooling systems
  • Fiber optic and electromechanical cables
  • Friction products and gaskets
  • Adhesives and sealants
  • Protective apparel in automobile and Aircraft

KEVLAR Strings

In tennis strings, high durability is considered a primary factor for consideration. KEVLAR Grade 16, 17, and 18 are primary grades which are used for making KEVLAR Strings. Polyester has slowly replaced KEVLAR strings due to better strength and durability.

KEVLAR Tires

In fast racing cars and bikes, excessive friction on tires causes burning and overheating. The tire is composed of steel wires wrapped in rubber. These steel wires are insufficient to absorb all heat. That’s why materials challenges come up at high speed. KEVLAR is good heat resistant material with the ability to work in high tension situations. Nowadays racing car tires have KEVLAR as their essential ingredient.

KEVLAR SUIT

Bulletproof vests are the most common application of KEVLAR Suits. Bullets are small metal parts that penetrate the body due to their high speed and design. In KEVLAR suits, all layers are threaded tightly together. These threads absorb the kinetic energy of bullets which lowers the ability of a bullet to penetrate the body. Without sufficient penetration power, bullets become harmless. 

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For US Customers, you can Buy;

KEVLAR Gloves on AMAZON

Heat Proof KEVLAR Sleeves

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2 Comments
  1. Robert Spencer says

    “Without sufficient penetration power, bullets become harmful. ” Huh?

    Perhaps you meant, “harmless”?

    1. hamza says

      Thank you for reading the article and pointing out my mistake. I have corrected it. Ofcourse, without kinetic energy, bullets are harmless.