Kevlar

Location:

Concepts Shown:

The structure and properties of Kevlar

Equipment:

Sample of Kevlar fabric from Prof. Laine and the straw models to demonstrate Kevlar fibrillation and kinking.

Procedure:

The instructor should drape the Kevlar fabric over him/herself and tell the students that this weave is the same material used in bullet proof vets. He/she should then explain how the fabric behaves mechanically. This explanation of properties can be supplemented by the straw models, which demonstrate the process fibrillation and kinking. Additionally, the students can be shown the discoloration of the fabric as a result of photodegradation.

Science:

Chemically, the Kevlar fiber is poly (p-phenyleneterephthalamide). The aromatic rings impart the rigid rod-like characteristics of Kevlar. These chains are highly oriented and extended along the fiber axis resulting in high longitudinal modulus. Kevlar has a highly crystalline structure. Strong covalent bonding in the fiber direction and weak hydrogen bonding in the transverse direction result in highly anisotropic properties of Kevlar fiber. In tensile loading, the load is carried by the strong covalent bonds, while in compressive loading, weak hydrogen bonding and van der Waals bonds come to play, which lead to rather easy local yielding, buckling and kinking of the fiber. Kevlar fiber being an organic fiber undergoes photodegradation when exposed to light (both visible as well as ultraviolet), which shows up as a discoloration and loss in mechanical properties. [eq]. Kevlar aramid fibers provide an impressive array of properties and applications. The fibers are available in three types, each type meant for specific applications. a) Kevlar - meant mainly for use as rubber reinforcement for tires, and in general for mechanical rubber goods. b) Kevlar 29 - meant for ropes, cables, coated fabrics and ballistic protection fabrics (bullet proof vests). c) Kevlar 49 - meant for reinforcement of epoxy, polyester, and other resins for use in aerospace, marine, automotive, and sports industries. [eq]. Each Kevlar fiber has a number of highly aligned fibrils. The fibrils form a crystalline array, although bonding between them is rather weak. A bullet hitting a Kevlar vest has to shred the fibrils as it attempts to penetrate the vest. However in the process of this fibrillation, the projectile loses energy and does not hit the victim wearing the vest. Remarks: This is an inexpensive (fabric is already available) yet very effective method of introducing the properties of Kevlar.

References:

Author:
Rahul Pinto
Credits:


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