Why Do Plastic Bags Shrink When Burned?

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Why Do Plastic Bags Shrink When Burned? Rating: -70 from 454 votes Views: 2,367

Submitted By: Noah on 2/6/2007. (

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Why do Plastic Grocery Bags shrink when burning?

Everyone has seen a Plastic Grocery Bag; most of those people have also wondered why the bag shrinks when it burns.

To understand this concept, we must first look at how this plastic is created. Plastic bags start out as a simple molecular chain of polyethylene (as pictured above). This molecular chain is created through a free radical polymerization process which primarily results in creating Low Density Polyethylene (LPDE). LDPE has low tensile properties and is very ductile making it a good candidate for inexpensive products such as Plastic Grocery Bags. The free radical polymerization process is also the least expensive and least technical for creating polyethylene.

Once the LDPE is created, it is usually in pellet form. These pellets are then fed into a machine that re-'melts' the plastic. With the plastic in a molten state, it is then forced through a die with extreme pressures and high temperatures, stressing the plastic molecules and aligning them in the direction of flow through the die. The result is a very thin film that is then cut and pressed into what we know as a Plastic Grocery Bag.

Now, you as a consumer, have numerous plastic bags and would like to get rid of them. It's preferred that you take them to your nearest grocery store and recycle them, but it's not a perfect world and we all like to see things burn! You apply fire (and as we all know, fire has heat) to the Plastic Grocery Bag and your thought process begins.

You are seeing the LDPE molecules relax. The film is very thin and does not take long to heat up from the flame. Once the film comes up to a certain temperature (in this case, around 105 degrees Celsius), the film begins to shrink. This is in fact the molecules going from a stressed state (due to the pressure from the die) to a more natural, relaxed state. The applied heat allows the plastic molecules to move around again.

Tags: burn burning plastic plastic bags polyethylene recycle recycling

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