What material is used for potato chip bags?

Why do potato chip bags shrink in the microwave?

• Why do potato chip bags shrink in the microwave? What is the semi/full scientific explanation? Some reference... http://media.www.studlife.com/media/storage/paper337/news/2001/09/28/Cadenza/Kitchen.Appliance.Delinquency-103511.shtml?sourcedomain=www.studlife.com&MIIHost=media.collegepublisher.com Thanks!

• Answer:

  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