Engineering Question #3445

Jenny, a 16 year old female from Toronto asks on May 13, 2006,

What do paper and plastic bags degrade to? How much does it cost to make a paper and plastic bag?

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The answer

Barry Shell answered on May 14, 2006

Short answer: Paper and plastic ultimately degrade to carbon and water in one form or another. Depending on the type, quality, quantity, and whether you are purchasing wholesale or retail, they cost anywhere from a fraction of a cent (US dollars) to 15 or 20 cents each.

Plastic and paper are both made of organic polymers. Organic means "made mostly of carbon" in this sense. Polymers are big molecules that consist of repeating units. For plastic a unit might be called "ethylene" or "styrene" so the polymer version is called "polyethylene" or "polystyrene". The word "poly" means "many". So polyethylene is a kind of plastic made of long molecule chains of many ethylene units. The long polymer chains are linked together with other chains and these links are accomplished with other chemicals called plasticizers. The different kinds of links give the plastic bag it's strechiness, or resistance to tearing. Depending on the kind of polymer and the kind of plasticizer and other things, the plastic can have different qualities, including its ability to degrade.

Plastics degrade at different rates due to different things: sunlight, physical action, and even digestion by bacteria and other microbes. In general it takes a pretty long time, but EVENTUALLY plastic will degrade. Unfortunately it might take a very long time. Check out this movie about plastic in the ocean.

Paper is also made of a polymer with the single unit being glucose (a type of sugar). The polymer is a long chain of sugar molecules and it is called cellulose. Cellulose is the single most abundant organic molecule on earth because virtually all plant material is made of cellulose, this includes wood, cotton, bamboo, paper and a lot of other things. Because cellulose is a natural polymer that evolved naturally and has been on earth for hundreds of millions of years, there are also hundreds of millions of animals and organisms that can degrade cellulose. These include everything from cows to bacteria and fungi. The same cannot be said for plastic polymers which, if you think about it, have only been around for about 40 years. Some have only been here for 10 years. Nature has not created very many organisms that know how to digest and degrade plastic yet--but She will. It just may take a few thousand years.

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