Dress shoes are still mostly made of leather. Athletic shoes these days are mostly rubber, plastic, and cloth, sometimes with a little leather.
Untreated animal skins absorb water and rot easily. To prevent this, the skins are tanned. Tanning is a process in which skins are treated with chemicals that cause the proteins to link together (called cross-linking). Tanning makes the leather more waterproof, less likely to rot, and more supple.
Tanning is a complex operation, involving steps to remove fats and hair from the skins, adjusting the acidity of the skins, and then allowing the skins to soak in a tanning solution. The tanning solution can use tannins, a kind of acid found in oak bark and other plant tissue, or it can use chromium compounds, which tan the hide faster, and make a leather that is more resistant to shrinkage.
The soles of shoes can be made of leather, but are increasingly made from synthetic materials that offer better traction, durability, and water resistance. Some of the materials used are ethylene vinyl acetate, rubber, thermoplastic rubber, rubber foam, and polyurethane.
Thermoplastic rubber is actually two compounds mixed together. One is styrene, the plastic that model airplanes are made out of. The other is a synthetic rubber called butadiene. The word thermoplastic means that the material can be melted and poured into molds, making it easy to form shoe soles with traction patterns in them. It also means they can be recycled.
Ethylene vinyl acetate is also a mix of two polymers. It is what hot glue sticks are made from. It is also what many foam rubber items are made from.
Ethylene vinyl acetate
The molecules shown on this page are the monomers of the plastic. These monomers are linked together into long chains to form the polymers that give the plastic its solid form, its rubbery texture, and other qualities.