Sometimes we can take a polymer and add enough binding agent that it can become a hard plastic. In this exploration, students take warm milk, add vinegar to remove a different protein, casein. Depending on the amount of borax added, the material becomes hard.
Procedure:
1. Prepare some warm skim milk on a hot plate or in the microwave. Pour out 10 ml of warm milk into a cup. Add 10 ml of vinegar. Stir.
2. Use a spoon to remove the curdled part of the milk. This is the protein part, casein.
3, strain the casein on a paper towel to remove most of the excess water (whey).
4. Note the consistency of the casein and record the observations.
5. Add borax. Depending on how much is added, it will determine how polymerized the casein will become. The class should agree on what each group's proportion should be so that a resultant range of possibilities are the outcome. Alternatively, if there is enough allotted time, each group could make enough samples to test the differences. .5 ml of borax may only create white glue. 1 ml may create flubber. 2 ml may harden it to a point where it can dry and form a plastic;
Casein has been used throughout human history as a glue and as a plastic. White glue has traditionally been made from casein. Before some modern plastics were invented, casein was used for the manufacture of small items like buttons.