Science Buddies: "Ask an Expert"

why don't petroleum-made bags decompose?...or why does it take them so long to decompose?

Hi there,

That's an interesting question! To answer it, you need to understand a couple of things about plastic and about the degradation process itself.

First off, what are plastic bags made of? The common plastic grocery bag is made of a chemical called polyethylene. It's a polymer, which means that it's made up of long chains of a particular chemical building block. There are lots of polymers in nature: for example, cellulose, which makes up the cell walls of plants, is a polymer composed of many sugar molecules strung together. In the case of polyethylene, the building block is a small chemical called ethylene, which is made up of just two carbon atoms connected by a double bond (each carbon atom in ethylene is also bound to a couple of hydrogen atoms). Chemical processes are used to connect the ethylene molecules together in long chains, which can then be used to make tough plastic films such as grocery bags.

As you know, natural polymers like cellulose are "biodegradable," which means that microorganisms like bacteria are able to use special enzymes to digest cellulose and take it apart into its component sugars, which the bacteria can then use for food. So why is polyethylene not biodegradable? A more specific way to ask this question is, why don't common bacterial enzymes digest polyethylene?

I would suggest that you do some reading on enzymes to see if you can find the answer to this question. A Google search is probably a good way to start, and you might also want to check out http://www.chem4kids.com/files/bio_enzymes.html to get started. Let us know what you discover!

Best,

Will Walker

Hello!

Some extra info for you:
Plastics do degrade, however they do so at different rates depending on their composition. Scientists can only estimate how long they take to degrade by conducting respirometry tests that assess how long it takes for them to degrade. The estimates for the degradation of plastic are scattered some say it takes 25 years but the majority of others say it takes 500 years or more.

Just wanted to give add some more information! Although the previous expert covered it thoroughly I believe.It was an excellent question by the way especially because of the current global and national situations about their effect on the environment and on our society.
Also a short article to help you : http://www.slate.com/id/2169287/