Turn Milk into Plastic
IntroductionHave you ever heard that plastic can be made out of milk? If this sounds like something made-up to you, you may be surprised to learn that from the early 1900s until about 1945, milk was commonly used to make many different plastic ornaments, including buttons, decorative buckles, beads and other jewelry, fountain pens, the backings for hand-held mirrors, and fancy comb and brush sets. Milk plastic (usually called casein plastic) was even used to make jewelry for Queen Mary of England! In this activity you will make your own casein plastic out of hot milk and vinegar.
- Milk (1 cup)
- White vinegar (4 teaspoons)
- Measuring cup
- Measuring spoons
- Optional: Thermos
- Mug or other heat-resistant cup large enough to hold at least 1 cup of milk
- Paper towels
- Optional: Cookie cutters, glitter, food coloring, markers
- Stovetop oven and pan or microwave and microwaveable container
- Heat 1 cup of milk in a pan or stovetop until the milk is steaming. Alternatively, you can microwave the milk in a microwaveable container by warming it at 50% power for 5 minutes. It should be about the same temperature as you would want milk to be for making hot cocoa. Heat for more time if needed.
- If you cannot do the rest of the activity right away, store the hot milk in a thermos until it is needed.
- Add 4 teaspoons (tsp.) of white vinegar to a mug or other heat-resistant cup.
- Add the 1 cup of hot milk to the mug. You should see the milk form white clumps (curds).Why do you think the milk forms curds when it is added to the vinegar? What do you think they are made of?
- Mix the mug slowly with a spoon for a few seconds.What happens when the milk and vinegar are mixed together? Why do you think this is?
- Stack four layers of paper towels on a hard surface that is safe to get damp.
- Once the milk and vinegar mixture has cooled a bit, use a spoon to scoop out the curds. You can do this by tilting the spoon against the inside of the mug to let excess liquid drain out while retaining the curds in the spoon. Collect as many curds as you can in this way and put them on top of the paper towel stack.
- Fold the edges of the paper towel stack over the curds and press down on them to absorb excess liquid from the curds. Use extra paper towels if needed to soak up the rest of the extra liquid.
- Knead all of the curds together in a ball of dough. This is the casein plastic.How do the kneaded curds feel and look differently than the curds did originally?
- If you want to make the casein plastic into something, you can color, shape, or mold it now (within an hour of making the plastic dough) and leave it to dry on paper towels for at least 48 hours. Once it has dried, the casein plastic will be hard. Tip: To shape the plastic, the dough must be kneaded well. Molds and cookie cutters work well, or, with more patience, the dough can be sculpted. Food coloring, glitter, or other decorative bits can be added to the wet casein plastic dough, and dried casein plastic can be painted or colored with markers.
- To avoid clogging the sink discard any unused curds in the trash— do not pour them down the sink.
When you added the hot milk to the vinegar, small, white chunks should have become visible in the mixture. This is because adding an acid, such as vinegar, to the milk changes the pH of the milk and makes the casein molecules unfold and reorganize into a long chain, curdling the milk. The white chunks are curds. You should have been able to use a spoon to separate the curds from most of the liquid. Additional drying of the curds with the paper towels should have made the curds ready to knead in to a ball and use as casein plastic, which can be molded and decorated.
Plastics are a group of materials that can look or feel different, but can all be molded into many shapes. The similarities and differences between different plastic products come down to the molecules they are made of. Plastics are all similar because they are all made up of molecules that are repeated over and over again in a chain, called a polymer. Polymers can be chains of one type of molecule, or chains of different types of molecules linked together in a regular pattern. In a polymer, a single repeat of the pattern of molecules is called a monomer (even if the polymer is made up of only one type of molecule).
Milk contains many molecules of a protein called casein. When milk is heated and combined with an acid, such as vinegar, the casein molecules unfold and reorganize into a long chain. Each casein molecule is a monomer and the chain of casein monomers is a polymer. The polymer can be scooped up and molded, which is why plastic made from milk is called casein plastic.
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For Further Exploration
- How does the amount of vinegar used affect the yield of casein plastic? To find out you can repeat this activity, but in addition to testing 4 tsp. of white vinegar with 1 cup of hot milk, try also testing 1 tsp, 2 tsp, or 8 tsp of white vinegar, each with 1 cup of hot milk. To collect more of the curds and get a better idea of the yield of the casein plastic, instead of scooping out the curds with a spoon, you can pour the vinegar and milk mixture through a piece of cotton cloth (such as an old T-shirt) secured with rubber bands on top of a cup.
- In addition to vinegar, there are a lot of other acids that we encounter in the kitchen all the time, such as lemon juice, orange juice, soda pop, and tomato juice. Do some of these common acids work better than others to make casein plastic?
- You used hot milk in this activity that was not a specific temperature, but using hotter or colder milk might affect the casein plastic reaction. Design an experiment to investigate this. How does the temperature of the milk affect how much casein plastic you can produce?
- Blog Post: Science in a Flash: Turn Milk into Plastic!