This Week's Experiment - #269 Static Cling?
This week's experiment comes from a question sent to me by list member
Charlotte 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 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 static 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 past 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?
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
To start receiving the Experiment of the Week, just
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