Plastic From Milk: What Polymers Are
- Chemistry
- Ages 11–14 · Grades 6–8
- Hands-on experiment
What you’ll find out
- A polymer is a very long molecule made of many small repeating units called monomers.
- Natural polymers include starch, cellulose, proteins and rubber; most plastics are made from oil and gas.
- The properties that make plastic useful also make it last, which shapes how it should be designed.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Long chains
Many of the most useful materials in the world are made of chain-shaped molecules. A polymer is a very long molecule built by linking thousands of small repeating units, called monomers, end to end, like paper clips hooked into a chain. The length of the chains gives polymers their properties. Long chains tangle together like cooked spaghetti, so the material can be tough and flexible instead of crumbly. Nature is full of polymers. Starch in bread and potatoes is a chain of sugar units; cotton and wood are mostly cellulose, a different chain of sugar; the proteins in hair, muscle and milk are chains too; and rubber comes from tree sap. Most plastics, such as polyethylene and nylon, are made from oil and natural gas.
From milk, and from oil
Before plastics were made from oil, one was made from milk. Milk contains a protein called casein. Warm some milk, stir in vinegar, and the acid makes the casein come out as white, rubbery lumps called curds. Strain and press the curds and they can be moulded into shapes that harden as they dry. In the early twentieth century, hardened casein was sold as a plastic called galalith and used for buttons, combs and jewellery. Oil-based plastics mostly replaced it because they are cheaper and can be made almost any way: stretchy, rigid, heat-proof or clear. That is also the problem. The chains that make plastic last also make it last for centuries in the environment. Chemists now design plastics that are easier to recycle or break down.
Curriculum links: NGSS MS-PS1-3