If you like action films, you might watch the hero battle all the villains, and remain unhurt even though the villains are firing hundreds of bullets at him. He's wearing a bullet-proof jacket, but how does that work?
The power of cross-linking
Here's something to try at home, using some drinking straws, some gauze and a working toy gun.
Take a drinking straw and bend it to make a square frame. Now stick a piece of gauze over it. Make three such arrangements and label them 1, 2, 3. Leave frame 1 unchanged. Stick another piece of gauze over frame 2. In frame 3, remove all the horizontal threads from the gauze, leaving only the vertical threads (So its structure goes from ### to //////). Now we're ready to start our experiment.
Take your toy gun (or a catapult) and fire it at the three frames. What did you notice? Did the bullet shoot through the gauze, leaving a hole? Did it pass through frame #2 at all? Did it seem to pass more easily through frame #3 than the others?
You just saw an illustration of the power of cross-linking.
How jackets are made bulletproof
Bullet-proof jackets are made of a material called Kevlar. Kevlar belongs to a class of materials known as polymers. Polymers work just like gauze, only at a more microscopic level.
The basic unit of a polymer is a chain. Each chain is in turn made of millions of units called 'monomers', linked to each other by chemical bonds. In Kevlar, the monomers are not only linked within their own chains, but also cross-link to monomers in other chains.
The more the cross-linking, the harder it is to separate the chains from each other. This is what gives Kevlar its toughness and strength, which is five times that of steel. When you make a thick layer of Kevlar, it can withstand a bullet at great speed.
The more the cross-linking, the harder it is to separate the chains of Kevlar from each other, which gives it its toughness.
The uses of Kevlar
A lot of sporting equipment like gloves, helmets, pads etc is made of Kevlar to withstand physical stress. It is even used to make puncture-resistant tyres. As Kevlar can also withstand high temperatures, it is used to make protective clothing for firefighters.