This experiment comes from a question sent to me by a subscriber and her daughter. They wanted to know why plastic cling wrap clings. I thought I knew the answer until I began experimenting. That lead to more research and all sorts of interesting things. To follow along, you will need:
- some plastic cling wrap. (Saran Wrap, Glad Wrap, etc.)
- hot water
- a metal pan or bowl
- a window pane or mirror
My first thought was static electricity. Part of this was because of the plastic "static stickers" made to stick on windows and partly the word cling made me think of static. Thinking about it for a minute made me have doubts.
Take a piece of plastic wrap and fold it in half. Does it stick to itself? Yes. What difference does that make? In some ways, static charges are like magnetic charges. With both, two things with the same charge will repel, they push away from each other. If the plastic had a static charge, then it should not stick to itself. In fact, it should push away from itself.
Second, as we have seen in other experiments, static experiments do not work well when things are humid. Even a tiny bit of moisture makes surfaces conduct electricity, letting the static charge flow away. Turn on the hot water and let it run in the sink for a few seconds. Hold the piece of plastic wrap in the steam and then see whether it will stick to itself, or to a window or mirror. Did it stick? Yes. Again, not at all what we would expect from static.
Not convinced yet? Get a clean, metal cookie sheet or stainless steel mixing bowl. Since metal conducts electricity, there is no way that something would stick to it with static cling. Start with a fresh piece of cling wrap and try it. Did it stick? Yes. OK, convinced now? I was, but I also tried several other tests (attracting small bits of paper, using a versorium from experiment #63, and an FET static detector) and they all came up negative for static.
So if it is not static, then what makes it cling? It took quite a bit of digging, but I found that most kitchen plastic wrap is made of low density polyethylene, also known as LDPE. This plastic makes a good barrier to water and air, but it does not stick to itself. To make it cling, they add another chemical, such as polyisobutylene or poly[ethylene-vinylacetate]. These chemicals do not mix totally with the LDPE. Instead, they act like the adhesive on tape, only not as sticky, and it only sticks to certain things. Want to see? Try sticking it to paper, different plastics, wood, and other substances. Then you will know why the plastic wrap will stick easily to some containers, but not to others. If it will not stick, then you just have to make room to eat that last piece of pie, right?