Re: Hi, I really need to know the way SARAN WRAP/CLING WRAP works, thank you

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