Kevlar

Kevlar is a synthetic (man-made) substance that is extremely strong. Kevlar is highly versatile, as it is incorporated into bulletproof jackets, bulletproof masks, army tanks, bullet heads, personal fighting armor, combat helmets, etc. Kevlar is 5 times stronger than steel, and under water it is 20 times stronger than steel. Kevlar's spider web-like weaving structure is responsible for its immense strength. 

A sheet of Kevlar. 

Kevlar integrated into a bulletproof vest. 

Kevlar is the commercial name for poly(p-phenylene terephtalamide). Kevlar is a polymer, meaning that it is made  from monomer chains. Each Kevlar segment or monomer contains 14 carbon atoms, 2 nitrogen atoms, 2 oxygen atoms, and 10 hydrogen atoms. The arrangement of polymer chains in Kevlar contributes to its flexibility, strength, and rigidity. Think of Kevlar as a grid of parallel molecules, similar to bendy straws that are stacked parallel to each other in their box. This orderly, untangled arrangement of molecules in Kevlar represents its crystalline structure. Kevlar is a polyaromatic amide. Aromatic refers to compounds with benzene rings (C6H6). Amides refer to a group of carbon, nitrogen, oxygen and hydrogen atoms.

Structure of Kevlar.

There is major hydrogen bonding that occurs in Kevlar. The hydrogen bonding that holds Kevlar's polymer chains occurs between polar amide groups on adjacent chains. In other words, there is a lot of hydrogen bonding between nitrogen and hydrogen atoms. Hydrogen bonding plays the most prominent role out of all the intermolecular forces in Kevlar.

Hydrogen bonding in Kevlar. 

A colored diagram of H-bonding in Kevlar. The individual polymer strands are held together by h-bonds that form between polar amide groups on adjacent chains.

In chemistry, kinetics describes the rate of a chemical reaction. Likewise, thermochemistry describes the energy associated with chemical reactions. In essence, it describes what is possible and what isn't. Thermochemistry focuses more on the amount of energy in the reactants/products and the overall energy change in the system. Back to Kevlar - remember, Kevlar is a man-made substance. Therefore, Kevlar had to be originally synthesized by someone. In fact, Kevlar was first synthesized in 1964 by Stephanie Kwolek at the Dupont laboratories in Wilmington, Delaware. There are 2 main steps in the synthesis of Kevlar. The first step is producing the basic plastic. This plastic is called poly-para-phenylene terephthalamide. The second step focuses on turning the product into strong-structured fibers. As mentioned before, Kevlar is synthesized from the monomers 1,4-phenyl-diamine (para-phenylenediamine) and terephthaloyl chloride. This produces a polymeric aromatic amide with alternating benzene rings and amide groups. Refer to the pictures below for a conceptual understanding of each step. When the amides are made, the polymer strands are aligned randomly. To make the actual material, the polymers are dissolved and spun, shaping the long polymer chains into the fiber. 

Synthesis of Kevlar. The reactants are the monomers 1,4-phenyl-diamine (para-phenylenediamine) and terephthaloyl chloride, respectively.

Some additional properties of Kevlar:

1. Kevlar is insoluble in water. 

2. Kevlar's molar mass is about 238.241 g/mol.

3. Hydrochloric acid (HCl) is a byproduct of the chemical reaction that makes Kevlar.

4. Kevlar is very heat resistant and decomposes above 675K or 402°C without melting.


By: Max Lauring