Polymer Milk Jug

Chemistry, Biology:	Polymers, Recycling Plastics
Grade Range:	Elementary School, Middle School, High School
Format:	Stage, Hands-on

This demonstration is a thought-provoking example on how we recycle plastics. Students are likely familiar with the idea to recycle, but this will intrigue them as an example of how it can be done.

Materials

• Clean HDPE Containers (#2 Plastics)
• Laboratory Heat Gun

Safety Precautions

Please read the Fire Safety section of the Demonstration Safety Page. Although this demonstration does not use fire, the Heat Gun does get very hot and can burn things.

Demonstration

1. Hold the container in one hand and the heat gun in the other. Turn the heat gun on and slowly wave it back and forth over one side of the container.
2. While heating the container, explain to the audience or students present that the plastic is starting to melt, and the bonds are getting weaker. Point out to them when the side of the container turns clear, and continue to heat it until the clear spot is several inches wide.
3. Turn off the heat gun, and put the mouth of the container to your lips with the now-clear side facing upwards. Blow into the container gently and in a controlled fashion, and watch as the clear side expands and becomes a bubble! If you blow too quickly, the bubble will pop and be less impressive. Continue to blow into the container until the clear plastic all turns foggy again.
4. After a minute, the bubble will be cool enough for students to touch and feel. It is very different from the rest of the container!
5. After making the first bubble, you can make up to three more bubbles by using the other three sides of the container. Don't try to use the bottom of the container; The seam will break open once you start blowing into it.

Why This Works

Polymers have become an essential part of our lives. We encounter polymers throughout our entire day, from the food we eat to the clothes we wear and the toys we play with. Polymers are strings of molecules bonded together in extremely long repeating chains. These chains connect end to end, and can stretch, bend or warp to resist breaking or separating. This demonstration focuses on what we call Thermoplastics, which are plastic polymers that can be altered by heating them.

Our milk jug is a plastic, and plastics are usually numbered from #1 to #7. There are seven types of plastics, and each one has different properties to them. Each type of plastic can be recycled, but when they are recycled they might be transformed from one type to another, depending on what type is needed. Our container is a #2 plastic, which is high density polyethylene or HDPE. HDPE has a rigid polymer, which is why it is stiff and doesn't bend easily. When we heat the wall of the container, we are making the polymer melt, making it less stiff. By blowing air into the container, we then are stretching out the melted polymer, which will stretch as far as it can to resist breaking. Polymers don't like to separate, so the wall will stretch out a LOT before it cools down and re-solidifies. Once cooled, it is now a #4 plastic, a low density polyethylene or LDPE!

Additional Information

• All kinds of things are made with polymers! Some examples are:
  • Leather in footballs, rubber in basketballs, and plastics in golf balls and soccer balls.
  • Nylon and polyester, both of which are often used in clothing.
  • Acrylic, such as in paint or nail polish.
  • Starch in potatoes and corn, carbohydrates in bread, and proteins in meat are all types of polymers!
• This demonstration pairs well with the Vanishing Peanuts.
• This demonstration is a part of the Environmental Show.