The National Science Education Standards for Content indicate that students need to develop the abilities necessary to do scientific inquiry. This section challenges students with a variety of science questions based on industrial testing. Rather than follow a pre-set lesson plan, students are encouraged to develop simple procedures to test various plastics, compare and contrast results, and apply the results to everyday applications.

- Tensile Strength Testing
- Falling Dart and Drop Dart Tests
- Abrasion Resistance

To print out this document in its entirety click here now.
(This file is a PDF file and requires the free Adobe Acrobat Reader).

Tensile Strength Testing

Suggested materials

When instructed to cut out plastic bars, you may also use the template we provide below. All samples should be cut in the same direction unless otherwise noted (see Activity B).

Click here to print out a life size tensile bar template

Using the suggestions that follow, have the students develop their own procedure to test tensile strength. Students' results can be either quantitative (producing numeric values as their results) OR qualitative (producing descriptive text as their results).

Activity A

Have your students cut strips from a variety of different plastics. They could use different brands of freezer bags, food wrap, microwave wrap, trash bags, grocery bags, or any other type of similar plastic. Your students will be testing the strength needed to break each of these strips of plastic. Your students could compare the strength needed to break different brands of trash bags or grocery bags.

Have your students clamp down one end of one of their plastic strip samples.

Give your students a variety of weights that they can attach to the other end of the plastic strip. Have your students develop a test to determine the weight needed to break each of their plastic strips.

Activity B

Hold a plastic bag up to a light and note the grain of the material. The lines you see indicate the general direction of the polymer chains of the sheet of plastic. The direction in which these chains are lined up is also known as the anisotropic nature of the material, or the direction of extrusion.

Have your students develop a procedure which tests to see if there is a difference in the tensile strength of a material when they pull with the grain and against the grain.

When pulling the sample against the grain very slowly, did you notice how the material thinned? What you are seeing is called "necking". The molecules reorient from across the sample to the direction you are pulling.

If you continue to slowly pull, the strain you feel will increase as the molecules are reconfigured.

Activity C

Tensile strength is also affected by how fast the sample is pulled. Have the students develop a simple procedure to test the difference in tensile strength at different pulling speeds. Ask your students to record the grain of each of the materials they compare.

Activity D

Some plastic strips will stretch further before breaking than others. How far they stretch depends on the material itself (i.e. what type of plastic), the direction of the material's grain, the size of the sample, and the speed of the pull.

Have your students develop a procedure to test the maximum elongation of various plastic strips.

As we said earlier, there are four factors in determining how much a plastic strip stretches. Make sure your students keep three of these four variables constant (e.g. the speed of the pull) when performing this activity.

Falling Dart and Drop Dart Tests

Suggested materials

• Large coffee can
• Wide rubber bands
• Meter stick or tape measure
• Darts: various sizes of weighted balls, smooth weights, or bolts with smooth and rounded tops
• Various plastic film samples (e.g. trash bags, food wrap, microwave wrap, grocery bags, dry cleaning bags)

Two industrial tests that are used to determine a material's impact strength are the falling dart and drop dart tests.

To determine the force of impact, multiply the mass of the dart by the height of the fall. Note: notice how the coffee can has a plastic film sample secured over the opening with a strong elastic band.

Impact calculation:
mass x distance = force

Activity A

Ask your students to develop a Falling Dart Test (where the weight of the dart changes while the height remains fixed). Ask them to determine the impact force needed to break a variety of plastic film samples.

Activity B

Ask your students to develop a Drop Dart Test (where the height of the fall changes while the weight of the dart remains fixed). Ask them to determine the impact force needed to break a variety of plastic film samples.

Activity C

Ask your students if they think there is a difference in impact force needed to break a sample if the same spot is impacted every time or if instead a new sample plastic film is tested each time. Ask your students to develop a test to verify their hypothesis.

Abrasion Resistance

Suggested materials

• Abradants: various types of sandpaper (e.g. fine, medium and coarse) and a gritty, abrasive bathroom cleanser
• Various plastic samples that will resist some scratching (e.g. different types of plastic floor tile samples, plastic fabric samples such as nylon and polyester, and plastic countertop samples)
• Various weights
• Stopwatch (optional)
• Balance (+/- 0.001 g)
• Calipers (+/- 0.001 in or cm)

Many plastic products, such as floor tile, countertops, clothing and carpet, are designed to be abrasion resistant.

Abrasion is caused by three main factors: the type of abradant, the surface area, and the force of abrasion.

In simple terms, a small child with sandpaper on her shoes will cause less abrasion on a floor than an adult. The child has smaller soles on the shoes of her feet (surface area) and weighs less than the adult (force).

Likewise, fine sandpaper will cause less abrasion than coarse grit sandpaper (type of abradant).

Activity A

Ask your students to develop an abrasion test that determines the abrasion resistance of three different brands of plastic floor tiles.

Activity B

Using one type of sandpaper (fine, medium or coarse), ask your students to develop a test to determine the effect of varied load weights on the abrasion resistance of one of your plastic samples.

Activity C

Ask your students to develop a test to determine the number of strokes or time span that will result in abrasion damage to one of your plastic samples, keeping the abradant (type of sandpaper or abrasive cleanser) and the load weight constant.

Activity D

Ask your students if they think samples change in mass or thickness after abrasion. Now ask your students to test their hypothesis.

As students complete their testing projects, students are encouraged to create in-class displays and presentations of their procedures and results to share with other students. Photos of completed displays and copies of procedures may be submitted for potential display on this website.