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Re: Saran Wrap



Saran wrap is, I believe, poly(vinylidenechloride),
not poly(ethylene). If PVDC behaves like PVC, would
this explain the effect? John Barrere
--- Wolfgang Rueckner <rueckner@FAS.HARVARD.EDU>
wrote:
I have an electrostatics question for the group:

I have always answered the question "why doesn't
Saran Wrap seem to
stick to polyethylene or Styrofoam containers, but
does stick very
well to glass?" by invoking the triboelectric series
and noting that
polyethylene and Styrofoam are close to Saran Wrap
-- in fact, Saran
Wrap is some kind of polyethylene. Glass, on the
other hand, tends
to lose electrons more readily while Saran Wrap
attracts them, and
these two substances are consequently much farther
apart in the
triboelectric series.

As I understand it, the answer has to do with
electrostatic
attraction -- the van der Waals force between two
surfaces --
positively charged particles on one surface tend to
be attracted to
negatively charged particles on the other. But why
does Saran Wrap
cling to itself? That has always bugged me and
suggests there's more
to this explanation.

The following is from
http://www.edgerton.org/kidscorner/staticcling.html
. "So if it is
not static, then what makes it cling? .... most
kitchen plastic wrap
is made of low density polyethylene, also know 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."

It seems to me that adding another chemical doesn't
really change the
fundamental argument or answer the question any
better. What am I
missing? Thanks in advance -- Wolfgang


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