Milk Casein Plastic Project

Learn how to make the well known polymer called plastic using kitchen ingredients! This experiment yields a ball of soft “dough.” Use your imagination to shape into whatever you’d like, then leave it to dry for a few days and it will harden like plastic.

What You Need:

What You Do:

  1. Make a stack of 5 or 6 layers of paper towels.
  2. Pour 1 cup of milk into your glass measuring cup.
  3. With an adult’s help, microwave the milk for 1 minute. You want it to be heated through but not boiling, heat for another 20-30 seconds if needed.
  4. Use a potholder to remove the cup. Add 4 teaspoons of vinegar to the milk and stir gently. There should be solids floating around in the cup. (Warning: it’s a bit stinky!)
  5. Use your spoon to scoop out as many of the solids as you can onto the stack of paper towels.
  6. Once you have removed most of the solids, carefully strain the liquid through a fine mesh strainer into the sink or a bowl.
  7. Dump the solids left in the strainer onto the stack of paper towels and gently pat them dry (use more paper towels if necessary to soak up as much moisture from the solids as you can).
  8. When the solids are fairly dry, gather them together and knead them in your hands or on a table into a smooth ball.
  9. Once it is smooth, you can mold the pliable dough into shapes. Try rolling it flat and using cookie cutters to shape it, press it into play dough molds, form it into beads (use a toothpick to make holes), or just shape it with your fingers.
  10. Set your shapes on a sheet of waxed paper and leave them out to dry for a few days until they become hard like plastic. (Note: keep in mind that the shapes will warp slightly and possibly even crack as they dry, so don’t expect your shapes to be perfect when they’ve hardened.)
  11. Remove any remaining solids from your dishes with a paper towel before washing with soap and warm water.

What Happened:

When you mixed vinegar with milk and heated it, the milk curdled or separated into curds (the solids) and whey (the liquid). Curds are the fat and casein (protein) from the milk. They form together in chains known as polymers. All plastics are polymers. The casein chains you formed in this experiment will harden into a type of plastic. In the early 1900’s, casein plastic like this was used to make many plastic objects including buttons and jewelry.

You could also try this experiment with different types of milk (we got an interesting rubbery substance with skim milk) and different amounts of vinegar to see what kind of effect it has on the polymer that results.

For Valentine’s day, we made the plastic dough into heart shapes and beads! You may also like to try adding food coloring as you knead the dough (please get an adult’s permission and wear disposable gloves to protect your hands from the dye) or try painting the plastic after it hardens.