How do I electrically charge a sheet of plastic strongly enough that it firmly sticks to the wall?

Question

electrostatic whiteboards

I have a bunch of these special dry erase sheets. Someone gave them to me as a gift about a year ago; I see they are still available for purchase on the Internet — you can find them if you look for something like «static cling dry erase sheet» or «electrostatic whiteboard»; they are available from different companies and, as far as I can tell, there is no single standard name for them. I am going to call one such an «electrostatic whiteboard» further in this question.

So, an electrostatic whiteboard is a flexible rectangular sheet, about 0.5 mm thick and about 1 m² in surface. Since it is light, transparent and warm to the touch, I assume it is made out of some kind of a plastic. The magic of it is that, put against a wall, it sticks with enough strength to hold its own weight and then some, but remains easy to peel off, stick again and so on, without any visible changes to either the wall or the electrostatic whiteboard.

Sadly, after about a year of being stuck to a wall and serving me faithfully, my electrostatic whiteboards slid down one after another and lost their stickiness. Even though they are not that expensive, I loathe to buy new ones since it is not environmentally conscious. Rather, I should like to restore the electrostatic whiteboards I already have to their former glory.

my theory

My understanding is that electrostatic whiteboards come strongly charged with static electricity. They are made of a strong electrical insulator, so it takes a long time for local surplus or deficit of electrons to «escape» to the surface and dissipate. The wall, meanwhile, is covered with paint that is, I guess, dielectric — so, upon coming near it, the electrically charged electrostatic whiteboard polarizes the surface of the wall into carrying the opposite charge and sticks to it thanks to the electrostatic force. However, over time electrons move through the material of the electrostatic whiteboard and it loses its charge. No charge — no stickiness.

my question

So, how do they charge electrostatic whiteboards at the factory?

I tried rubbing mine against the carpet but it does not help. I tried rubbing a thin polyethylene bag against the carpet and it does stick to the wall, but only weakly — it falls down under its own tiny weight within a minute. So, rubbing sheets of plastic against the carpet does not seem to be an efficient way to electrically charge them.

Should I take a more fluffy carpet? Should I rub more intensely, or for longer? Do I need to charge both sides of a sheet? Should I apply strong enough electricity that the electric charge soaks through the whole depth of the sheet despite its very low electron mobility?

Is my theory overall right or wrong?

Answer 1

Despite its name, a "static cling" or "electrostatic" whiteboard sticks to a wall because of molecular adhesion and air pressure, not static electricity, so trying to recharge them will not revive them. What might help is carefully cleaning both the wall and the wall-facing side of the whiteboard.

This is because when you place the whiteboard against a wall, areas of the sheet that touch the wall "stick" via intermolecular forces. These are very short-range, but the area of the sheet is large and these weak molecular forces are very strongly reinforced by air pressure. Sea level air pressure over just one square centimetre is sufficient to support a kilogram weight, so it doesn't take much direct contact area for air pressure and friction to hold the whiteboard sheet in place. Any dirt, dust, film, wrinkles, roughness, distortion, … that interferes with direct wall-whiteboard contact will reduce stickiness.

Static electricity is not, however, completely irrelevant. One form of molecular adhesion is caused by the different electron affinities of different materials. When you peel the sheet away from the wall, you are likely to peel away some electrons or leave them behind, charging the sheet relative to the wall. This contact electrification may cause some temporary electrostatic forces, but any macroscopic electrostatic charge should not last long, since ions in the air move to neutralize the charge. This is why if you rub a balloon in your hair and stick it to a wall, it will typically fall down within a few hours.

Static cling decal companies explain this much better than any static cling whiteboard vendor I can find:

• The Science of Static Cling Decals
• An Introductory Guide For Static Cling Labels

One cling film sheet manufacturer says that their product becomes less sticky with time because "the effect of the plasticizers relax". (I am not exactly sure what this means, but my guess is that the sheet becomes less flexible so it can't mould itself to the surface to make good contact.) So you may be out of luck if your whiteboards have aged beyond their expected lifetime.