flyingllama-ga:
The reason why potato chip bags shrink in the microwave, has to do
with the material that the potato chip bag is made of.
When potato chip manufacturers select a material to package their
products in, they look for a material that is relatively light-weight
(after all, they are selling you chips, not bags, and shipping costs
money), relatively impermeable to air (so that the chips stay 'fresh'
longer), relatively inert (so that it doesn't affect the taste or
safety of the chips) and easy to work with in high-speed packaging
machines.
Polymer resins such as polyethylene terephthalate (PET) fit these
requirements well. When melted and squeezed under high pressure into
thin sheets (sometimes as thin as 5 microns), these plastics can be
further modified by, for example, coating them with a thin layer of
aluminum, to result in a thin, flexible, air-impermeable,
heat-workable, packaging material. Modern packaging materials
typically consist of several layers of material, with each layer
serving a different need (strength, permeability, visual appeal, among
others). Product labelling and graphics can be printed onto the
material while it is still in its flat, continuous sheet format. Rolls
of the material can be easily shipped to the potato chip factory, then
fed into high-speed packaging machines that fold it together, and
heat-seals then cuts the material into individual bags to be filled
with chips before again being heat-sealed.
It is the fact that the bags need to be heat-sealed at a reasonable
temperature to enable the high-speed manufacturing process, that
allows us to play games of "Shrinky Dink" (the term used in the
article you referenced) with empty potato chip bags. Polymers are
simply very long chains of molecules. The natural form of a polymer is
to be randomly curled up onto itself. The high-pressure squeezing I
mentioned earlier, causes the polymer chains to be stretched out to
form the strong, thin sheets. This high pressure squeezing also locks
the chains in their stretched form. However, if the material is heated
up, enough energy is imparted into the chains to allow them to break
free and try to return to their natural state... thus 'shrinking'. As
the chains shrink, they are still bound to each other, and they are
still stacked together in the multiple thin layers that make up the
actual packaging sheet, so the overall shape of the bag tends to
remain the same, only the size changes. Since any of the labelling
that is printed on the surface of the bag is simply a very thin layer
of ink, it whill shrink with the surface too.
It is possible to tweak the chemical composition of the polymers used
in packaging, so that the temperature at which the shrinking occurs
can be controlled. So, some microwavable frozen foods are packaged in
plastic trays made of PET too, but these trays are engineered to
resist shrinking at the temperatures that are reached during the
microwave-cooking of the meal. Other materials are engineered to
shrink at much lower temperatures, so that they can be used to
shrink-wrap goods such as CDs or DVDs, for example, without requiring
temperatures so high that the product itself would melt!
For more information about how polymers work, I would suggest you
check out the following online lesson. A word of warning, it is a VERY
comprehensive lesson about polymers in general, not just how polymers
react to heat:
http://www.makingthemodernworld.org.uk/learning_modules/chemistry/03.TU.02/
You can learn more about plastics in general and how they are used in
food packaging as well as other industries, from the following sites:
American Plastics Council
http://www.americanplasticscouncil.org
http://www.plasticsinfo.org
Dupont Packaging Division
http://www2.dupont.com/Packaging/en_US/index.html
Regards,
aht-ga
Google Answers Researcher |
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