by Melissa Jaramillo and Julie Snyder
Plastic -- How many things can you think of that are plastic? The jug that holds our milk is often plastic. Some of your toys, such as your doll or toy figure may be plastic. We may drink from plastic cups, talk on our plastic-coated phone, and find many more examples of plastic around us!
Today, plastics are most often made from synthetic byproducts of petroleum. Long before that however "natural" plastics were produced from plant and animal products! How would you like to make your very own plastic? You can! Let's make a plastic from the protein, casein. Hmmm....Where do you think that we can find that? What do we keep in our refrigerator that is white and comes from an animal that makes the sound "moooooo?" Hey! That's right! Cow's milk is loaded with the polymer, casein!
Casein is the substance that makes yummy custards, cheese, and in our case --- plastic! Without this polymer, cheese would come unglued! (Glue would come unglued too, since casein from milk provides its sticking power.)
But what is a polymer anyway? The name "polymer" is made up of the Greek root words of "poly" (meaning "many"), and "mer," (which means "part"). Put that together and it translates to the literal meaning of "many parts"! That is exactly what polymers are! They are long chains of the same small molecule repeated over and over again. These smaller units are known as monomers (mono means "one" or "single"). Chemical bonds are the strong forces holding those smaller units together. Casein, the substance we are going to use for our plastic, is comprised of small amino acids with the order repeated over and over again. What does that make? That's right! A polymer!
Here's another example of a polymer chain. Have you ever made a garland from construction paper? Take a sheet of construction paper and cut into strips. Glue one strip of paper into a loop, then slip another through the loop and glue it. Soon you have a long strand of loops. Amino acids in casein are strung together in much the same manner. Now imagine if you tossed several garlands into a box, they may become tangled and form a single mass. Casein strands also become tangled. It is the way they get tangled up that give plastic its strength.
Most proteins are pretty big molecules! For proteins to stay in solution -- like in milk -- they have a lot of water loving handles (called hydrophillic moieties - Yikes what a name!). Whenever we begin to change the nature of a protein -- perhaps by radically changing the acidity (or pH) or using heat to alter the protein's shape it damages the handles. When enough of these handles are hurt they can no longer hold the protein in the solution form and it separates. Imagine a caterpillar holding onto a wall. The wall gets hot and it's legs begin to slip. When enough let go, the caterpillar falls.
In our activity, we add both heat and acid to encourage the casein to fall out of the milk (precipitate). Making milk plastic is fun and easy, but because of the heat, an adult's help is required.
You now have a plastic that can be sculpted or pressed into a mold. Perhaps you want to shape yours into a dinosaur or a giant heart! Allow your finished product to air dry on waxed paper for 3-4 days. Paint and finish with a lacquer if desired.