Some uses of Kevlar.
Kevlar’s most famous use is in bullet-proof vests. This is because of the fact that it has a high cut resistance.
Underwater, because underwater Kevlar is 20 times stronger than steel.
These Kevlar cables are supporting a sonar facility that the U.S. Navy uses to find out how much noise submarines make.
Kevlar is used to make windsurfing sails that can withstand the force of 60 mph winds and don't rip easily are.
KEVLAR is a long chain-like molecule known as a polymer, which consists of repeating units called monomers.
A Kevlar fiber is an array of molecules oriented parallel to each other like a package of uncooked spaghetti. This orderly, untangled arrangement of molecules is described as a crystalline structure. Crystallinity is obtained by a manufacturing process known as spinning, which involves extruding the molten polymer solution through small holes. The crystallinity of the Kevlar polymer strands contributes significantly to Kevlar's unique strength and rigidity.
Kevlar is a polyaromatic amide. That is, it contains aromatic and amide groups. An aramid is a manufactured fiber related in chemical composition to the nylon family, yet its properties vary greatly from nylon. Other polymers with a high breaking strength often contain one or both of these molecular groups.
The individual polymer strands of Kevlar are held together by hydrogen bonds that form between the polar amide groups on adjacent chains.
The aromatic components of Kevlar polymers have a radial (spoke-like) orientation, which gives a high degree of symmetry and regularity to the internal structure of the fibers. This crystalline-like regularity is the largest contributing factor in the strength of Kevlar.
The following is a XANES images confirm that the aromatic components of Kevlar have a radial (spoke-like) orientation, which allows for a high degree of symmetry and order.
Some Relevant Formulas
Strain=extension /original length
Stress=force/area
Young’s Modulus= stress/strain
Physical Properties
Tensile Strength (maximum stress material can experience):3.6 GPa
Strain: 94240 million billion
Yield Stress (stress at which slip first occurs): 1240 MPa
Young’s Modulus: 76 GPa
Density: 1380 kg m-3
A stress: strain graph is always a straight line
Kevlar does also have a few disadvantages. The fibres themselves absorb moisture, making Kevlar composites more sensitive to the environment. Although tensile strength and modulus are high, compressive properties are relatively poor. Kevlar is also very difficult to cut. Special scissors are needed for cutting dry fabric and special drill bits for drilling cured laminates (1). But despite its disadvantages, Kevlar’s unique properties have helped it to become one of the most important man-made organic fibres ever developed.
Sources:
- Advancing Physics AS School Book
