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Polymer Fiber Orientation Activity

   
 

Introduction: Have you ever played tug-of-war with a plastic bag? In this activity you will explore how the orientation of fibers affects the strength of materials such as paper towels and plastic grocery bags.

Materials: Before you begin, you will need to gather the following materials:

  • one paper towel section -- be sure it is the kind that comes from a roll.
  • one plastic grocery bag -- the kind you get at the grocery store when you are asked if you want your food packed in "Paper or Plastic?"
  • a pair of scissors
Action: In this experiment you will simply try to tear apart a paper towel and a plastic bag several different ways. How a material tears can help you to determine the orientation of fibers within this material.

Try the experiment this way:

1. With a pen or pencil write the word "TOP" on any one side of the paper towel square. Then, using both hands, grab the paper towel at this edge and quickly rip it apart like this:

Look at the torn edges. How would you describe what they look like?

2. Take one of the torn halves and turn it sideways so that you can form a rip perpendicular to the first tear. Again, with both hands go ahead and quickly rip the paper towel apart like this:

  • What do these new torn edges look like?
  • How do they compare to the first tear?
  • Which way was it easiest to rip the paper towel?

Make a guess: Which of the pictures below best shows the way you think the fibers are oriented in your paper towel?

Usually a material will rip apart more easily in the same direction that the fibers run. It takes less strength to separate fibers from each other than to break the fibers apart. You may have experienced another example of this idea when you have tried to cut a piece of meat. Often you can separate the fibers of meat with a fork, but it requires a knife to cut across the fibers. What other examples can you think of?

Action: See if you can discover if the plastic bag tears easier in the up and down direction or in the side to side direction.

  • How is this related to the orientation of the polymer fibers?
  • Why would this be important in the design of a plastic bag?

1. Hold the plastic bag up to the light. Can you see lines running in one direction. Based on what you see, predict if the bag will tear more easily and in a straight line, in the up and down direction or from side to side.

2. Lay the plastic bag out flat on a table and with a pen or pencil write TOP on the upper edge of the bag. (Usually, this is between the handles.)

3. With a pair of scissors cut out a square of plastic that includes the word TOP. Then, with the scissors make a little cut on the TOP edge and on one side edge like this:

STOP and make a prediction: Will the bag tear more easily in the up and down direction, or from side to side?

4. Now, just as you did with the paper towel, quickly rip the plastic square at the little cut on the top edge. Then, rip the plastic bag square from the little cut on the side edge like this:
  • What did you notice about the torn edges?
  • How do the two torn edges compare? Are they straight tears? Are the edges smooth?
  • Which direction was the plastic easiest to rip apart? Was your prediction correct?
When you have a plastic bag full of groceries, what advantage would there be to having the polymer fibers oriented up and down? If you had a heavy bag of groceries, what do you guess would happen if the fibers were oriented from side to side? If you were to play tug-of-war with a plastic bag, which way would you want to hold the bag so that it does not break in the middle of the game?

Try an experiment like this one, but using different materials.


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