Making Plastic

Now that dwe know how to separate protein from milk, we can use these materials to create plastic!

Materials Required

• Graduated cylinder, cup and stove from the first two explorations

• Borax

• Mixing cups, spoon

Procedure

1. Repeat the process of denaturing milk protein by heating and adding vinegar.  Set aside portions of the curd protein (casein) so that you can do several experiments.

2. Just like when making glue, we will add borax to our curds. This time, more borax!  Add  1/4 teaspoon and stir.  Observe the consistency. Add an additional 1/4 teaspoon and stir. Again, check the consistency of the mix.  

3. When it becomes clay-like, and it looks like it could be molded, remove to from the cup and knead it.

4. Shape the mixture in a desired form and either heat it on the candle burner in an aluminum baking cup or air dry it for several hours.

Extension

5. Design an experiment that adds different amounts of borax to the same amount of curds. Each mixture will need to be kneaded with a spoon or by hand so that the borax mixes with the milk protein, casein. The borax takes the bits of protein pieces and makes a chain or polymer. 

6. Test the rigidity and hardness of each mixture to determine the best applications for each. 

Question:  After testing different combinations of borax and casein, what is your best recommendation for making a plastic? What would you use this plastic for?

Discussion

Milk contains a complex protein called casein. Casein is a complex molecule made of chains of other molecules. Casein’s atomic weight is approximately 22,000 so each molecule of casein has about 2500 individual atoms! When acid is added to milk, the casein molecule unfolds and partially comes apart into smaller chains of molecules. The process of breaking the molecule into smaller parts is called denaturing a protein.

After proteins are denatured, or literally separated and torn into short molecules, they can be reassembled into longer chains.  Polymerization is a process by which smaller molecules are chained together. 

“Poly” means "many" so it indicates many molecules linked together.  Chemicals like borax help bind protein molecules together. When borax is added to the protein, some of the chains are encouraged to line up to form even longer chains. These longer chains are called polymers. Borax, or sodium borate, contains the element boron. Boron assists in lining up these protein chains.

Chains of molecules tend to act differently than individual molecules.  Just as spaghetti, a starch, is different from rice, another starch, a chain acts differently than individual atoms. Like spaghetti, chains of proteins tend to be more elastic, rubbery, and even glue-like.

When milk protein and borax are mixed, protein polymers are formed.  Depending on how much borax is added, the milk protein can form either a white glue or a rubbery plastic that can be molded.  When dry, the proteins are stuck in position and are solid.  The white glue we typically use for household repairs and carpentry products is made of casein.

Milk plastic is still used occasionally for specialty buttons, but for the most part it has been replaced by modern plastics.  A good article on the history and use of casein if found at  http://www.plastiquarian.com/index.php?id=6  

Polymers can be occur naturally.  Cellulose is a natural polymer made of chains of sugar molecules. In cellulose, each sugar molecule (C6H10O5) is interconnected with hundreds to thousands of other sugar molecules to form the tough polymer that is plant fiber or wood. Early chemists investigated how natural polymers occur and used this information to synthesize polymers. Vulcanization, for example, was one of these important discoveries.  Natural rubber plants produce a sticky plastic-like sap. The sap can make a rubbery-like material but it loses its elasticity and breaks down easily.  In 1843, Charles Goodyear discovered that by adding sulfur to the sap and heating it, a more durable rubber could be made. The added catalyst of the sulfur bonded the molecules of the sap in chains to make a stable polymer. 

We are surrounded by polymers in our daily life. Most familiar to us are plastics. Many of today’s plastics are synthetic polymers, long chains of relatively simple molecules derived from petroleum, coal and natural gas. The molecules are linked together through polymerizing processes and a wide array of plastics can be created.