Polymerization

Using the New Equipment:  Making Plastic

Now we know how to separate protein from milk, we can use the material to create plastic.  

Directions:

1. Repeat the process of denaturing milk protein by heating and adding vinegar.  Set aside portions of the protein (casein) so that additional trials can be made by adding borax, only this time with little or no addition of water.

2. Design an experiment that adds different amounts of borax to at least three different potions of casein. Each mixture will need to be kneaded with a spoon or by hand so that the borax works with the casein to make a polymer. This particular polymer, when molded and then dried or heated, forms a basic plastic. Many buttons are made from casein plastic.

3. 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.

4. Test the rigidity and hardness of each mixture and resulting polymer to determine the best applications for each. Making plastic is an interesting and entertaining process, but it has significance for applied chemists. Finding the right formula for the making of glue or plastic can mean the difference in whether an industry successfully innovates its products for sale and distribution.  Chemical engineers work on the details of how to develop these kinds of materials by testing their products for durability, hardness or flexibility, preservation of details, and other physical characteristics. To see a potential system for performing this kind of engineering task and reporting the results, see Stephanie Kwolek. The Kwolek unit focuses on the way chemical engineers formulate, test and market their discoveries.

Back ground information: Polymerization

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 asunder into short molecules, they can now by 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 aticle on the history and use of casein if found at  http://www.plastiquarian.com/index.php?id=60F.  

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.