Why does Saran Wrap become charged?

1. Aug 14, 2014

iScience

Hi

I'm trying determine the electric charge configuration of my saran wrap.

When it's wrapped up like this, I feel no electric field around it.

But when I pull it I feel an E-field around the sheet i just pulled.

At first I thought that the sheet was just polarized with net 0 charge but I've determined that this isn't true.

Could someone draw/explain to me the charge configuration of pulled vs non-pulled saran wrap?

2. Aug 14, 2014

Pythagorean

I think a confounding factor is that saran wrap has adhesives in it. Residues, resin, powders, etc. That allow it to ashere to itself:

http://www.dow.com/productsafety/finder/saran.htm

it does act like a capacitor and can acquire charge easily, as well, and it probably has minimal charge on it when its manufactured and packaged (as part of a clean manufacturing process), but separating adhesive forces from electromagnetic ones is not trivial to me.

3. Aug 14, 2014

collinsmark

What a wonderful question. I love it!

For some reason or another, it seems that the sheet pulled off from the roll has a different electron affinity than the rest of the roll. How could this be?

I speculate that stretching has something to do with it.

Plastic is made up of long, stretchy molecules. When you pull off a sheet of wrap, two things happen:
• The sheet of wrap that you are pulling is stretched, at least temporarily.
• The part of the sheet that is touching the roll is stretched, thus it moves (rubs) against the rest of roll until separation occurs.

So the question is, does the stretching of the plastic change its electron affinity?

Experimentation might help here. iScience, you seem to have some method of measuring electric fields. Try measuring the field around a sheet of wrap that was pulled off with a lot of stretching (like you would normally do). Next, pull off a sheet as gently as possible, in attempt to minimize the stretching when separating from the roll. Is there a difference in the resulting electric field around the sheets?

[Edit: if my speculation is correct, the sheet that was temporarily stretched more (when pulled off the roll) will cling better.]

4. Aug 14, 2014

davenn

Keep in mind this effect happens with all rolls of tape :smile"

and its the same process when walking over a carpet then getting a zap when touching a door knob
This static charge generation is called tribocharging. when the 2 surfaces that are in contact become separated
some electrons are moved and this causes an imbalance of charge

cheers
Dave

Last edited: Aug 14, 2014
5. Aug 14, 2014

davenn

No, I think you will find that is true. there is an initial 0 net charge throughout the material on the roll
Its the act of separating the sheet that generates a charge imbalance as I described in my previous post

Dave

6. Aug 14, 2014

mishima

I found a reference to this in the first edition of Chabay, Matter and Interactions a while ago. The chemists speculate that the polymer strands actually get torn apart at weak bonds, leaving charged "pieces" of molecule left over. It is apparently a topic of current research (or was whenever that book was written).