What makes plastic wrap (aka cling wrap) stick to stuff, especially itself? Why does it seem to stick to itself more on dry days, but to other stuff better when they’ve been moistened?
From KnowWhat.com, How does Saran wrap work? (by C. Susan Brown, Ph.D.)
click on the link to read the full article
The posting from Brown via Winkelried sounds very wrong to me.
First of all, “Saran” is a registered trademark of Dow Chemical Company for several resins that typically include vinyldene chloride and some other monomer. None of the products they now sell under the trademark are made of polyethylene.
Whatever Brown means by “the properties of static electricity”, Saran is not clingy because of a gross electric charge, i. e. because of Coulombic attraction. I am pretty sure it is clingy because of van der Waal’s interaction, like many other polymer films are. This is an effect that lets submolecular regions of charge in two different materials reorganize themselves a little to present as many opposites as possible immediately across the interface between two objects. van der Waal’s forces are mostly what holds objects together if they are made of many tiny molecules (though some things, for example the rubber in an automobile tire, are really sort of one giant molecule).
Brown’s explanation goes on to build on the idea of vacuums and Saran’s inability to stick to porous materials (though maybe the “…” in the quoted passage makes more sense of this transition). I think Saran does not like to stick to things whose surfaces are rough on the scale of molecular contact because the Saran is too stiff to follow the roughness and create lots of surface contact. For example, Saran would not stick to a ground glass surface well, because of its roughness, not because it is porous (which it isn’t).
I’d sure like to hear a more detailed explanation supporting Brown’s answer.
Napier, this site says
I haven’t been able to find anything on why it sticks.
So it does, donkeyoatey, so it does. My statement to the effect that Saran isn’t polyethylene isn’t true, or at least appears inconsistent with what the site says.
How is “saranbrands” related to Dow? The Dow website makes “Saran” look very much like their food wrap business, saying things like: “Saran polyvinylidene chloride (PVDC) makes it possible for your products to reach every part of it [meaning “the world market” - Napier]. Saran locks in flavor, keeps in moisture, prolongs shelf life, seals out oxygen, and showcases quality.” and “Take Another Look At Saran! In this issue of Packed with Performance, a newsletter of the Dow Food and Specialty Packaging Group, you’ll learn about the latest advances in Saran resins for blown and cast film applications. New multi-layer die technology promises to make it easier for you to unlock the performance of Saran than ever before.”
However, though the website you point out talks about a polyethylene based Saran Wrap, I find no mention of polyethylene on Dow’s Saran sites. What makes it weirder still is that Dow is pretty sloppy with the Saran trademark, using it (against conventional legal wisdom) as if it describes what the substance is rather than describing the source of the product (eg “Saran locks in flavor”). For years, “Saran” has been another name for “PVDC” in industry. So if they apply the trademark to a product that isn’t based on PVDC, it seems like they would just be confusing their own customers for nothing.
I am mystified for now regarding what “Saran” does and doesn’t include.
Now this is too interesting a subject to be sitting at the bottom of page 3…
I found the following at About.com
That seems to support Napiers claim that Saran == PVDC. But it does not mention the ‘sticky’ properties of the film.
My guess is that Saran Wrap is a layered film, with a polyethylene surface to creat the stickiness, and an internal layer of Saran[sup]TM[/sup] to act as a barrier. But the last words haven’t been said yet!
Back to the Teeming Millions!
Trouble is, if somebody called “saranbrands” supports a website saying what they sell is polyethylene, it is pretty hard to believe about.com is right to contradict them. The quote you provide seems to be about one product sold under the Saran tradename, but could hardly be relied upon to rule out all others. After all, any boob at saranbrands could print their tradename on whatever he wanted, sell some, and there you’d have it.
Usually polyethylene isn’t very sticky. In fact it is more often used as a non-stick surface. Tupperware containers are made of polyethylene - you know they aren’t at all tacky. I mean, sticky.