Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
8. Chemistry in the World
What is plastic made of?
Plastic is long chains of small molecules called monomers that are joined together. The long chains are called polymers.
Your hair, your fingernails, and the tendons in your arms and legs are made of polymers. In those polymers (called proteins) the building blocks are called amino acids. They are chained together to form strong ropes of material to be used where strength is important.
You can make your own protein plastic from milk. Add some vinegar to nonfat milk, and it will curdle into white clumps of casein protein. You can compress this into a mold and let it dry into a hard plastic. Umbrella handles used to be made this way.