a process called vulcanization

The outer soles of most shoes are made of rubber that gone through a process called vulcanization.This technique adds sulfur to superheated raw rubber to create a material that both elastic and sturdy.Until recently, sneakers used natural rubber for this process.But today, most outer soles are made with a synthetic blend of natural rubber and byproducts from coal and oil.Producing these materials accounts for 20% of a sneaker carbon footprint.