|MadSci Network: Physics|
Katie, The answer to your question lies in the US Patent Office. The manufacturers of Saran Wrap and cling wrap have discovered through their research that certain additives to the principal polymer confer "clingy" behavior. To protect their discoveries, they patent it. In a patent, the invertor must describe exactly how to make [or reproduce] his/her discovery. If you read enough patents, you'll learn what the additive is. One place to start your search is at http://www.patents.ibm.com/ibm.html Unless tou got very lucky, the patent search could easily involve reading hundreds of 10 or 20 page long documents. Fortunately for you, I have read them already. For Saran Wrap[Dow Chemical recently sold this business to SC Johnson} the main polymer is poly vinylidene chloride [PVdC] which has very high barrier properties against air and water vapor, which makes it commercially useful in the food packaging industry. Cling wrap, on the other hand is made by many producers under many brand names and the main polymer is low density polyethylene [LDPE]. It is not as good a barrier as Saran, but is much cheaper and is relatively easy to fabricate which makes it a commercial contender as well. Both PVdC and LDPE are relatively high molecular weight polymers which is needed to confer mechanical strength and barrier properties. Neither film, however, clings to itself and you would have to seal the package with string, sticky tape or rubber bands to keep it together.This is not particularly appealing, especially if you also have a roll of aluminum foil handy. While foil doesn't 'stick' to itself, it has the property of 'dead fold', i.e. the package stays wrapped without string etc. Scientists had to find some way to make PVdC and LDPE cling to itself and discovered that certain other LOW molecular weight polymers that weren't quite compatable with the host would do the trick. The two most commonly used today are polyisobutylene and poly[ethylene-vinylacetate] or PIB and EVA, for short. Unlike magnetism, in which like poles repel each other, in adhesive chemistry like materials attract each other. When the low molecular weight additives are put into a high molecular weight host of different composition, they tend to not mix [they form seperate phases]. When the film of PVdC or LDPE contacts itself, the PIB or EVA in the one film seeks out and associates with PIB or EVA in the second film surface. Since the additives are low in molecular weight, they are much more mobile than the host polymer. Higher mobility leads directly to the rapid development of an association that is perceived as 'cling' The chemistry and physics can get pretty detailed after this. Let me know if you need more or need help with the polymer structures. Ken
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