# Why do Plastic Layers Stick Together?

Q:
I am working on a project to develop a product to assist in the opening of plastic bags (particularly for the elderly and physically disabled). As part of this I need to investigate WHY the layers of plastic bags (disposable carrier bags) have a tendency to stick together.I have done some background research and think it may be due to static electricity or 'layer to layer surface adhesion'. I am not entirely sure however as my area of study is design and so I dont know a great deal about forces and the properties of materials etc.Could you please explain what 'layer to layer surface adhesion' actually is. And do you know why thin layers of plastic tend to stick together? Are there any other reasons?Thank you, any help is greatly appreciated!
- Michael Dawson-Bowman (age 20)
Scotland
A:

Static electricity is simple enough. Things can pick up a little electrical charge, usual by rubbing against some other surface, and if they pick up opposite charges they'll attract. This doesn't sound like a particularly likely reason for similar pieces of plastc to stick, since they'd typically pick up the same sign of charge, and thus repel. If somehow the plastic picked up patches of positive and of negative charge, then oppositely charged patches would find each other and stick. One way to test whether this sort of effect is important is to get a little hand-held piezo-electric ion gun and see whether spritzing ions at the plastic makes much difference for the stickiness.

"Surface-to-surface adhesion" is just a fancy name for things sticking together, so it doesn't tell you much beyond what you were asking. The main mechanism is probably vanderWaals forces between the surfaces. These are the sort of forces that tend to make all sorts of different materials stick to themselves and each other. They involve the electric charges in the material doing a sort of coordinated dance, so that even though both pieces are neutral the positives in one tend to be close to the negatives in the other at any time. This generic mechanism is sure to be present.

I just grabbed one of those bags that tends to stick together off the roll at the store and rubbed it on my robe to give it some static charge. It now sticks to me like crazy, but once I shook it off the bag tends to stay open. The similarly charged sides do repel each other.

This may be a ridiculously complicated way to do things, but maybe using one of those piezo guns (they used to be sold to remove static from vinyl records) could be used to charge up the plastic sheets and help separate them. Or any little device that spits out ions might work.

Mike W.

(published on 01/10/2015)

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