Background

The majority of plastic waste is composed of six common polymers:

According to the U.S. Environmental Protection Agency, in 2017, the U.S. generated 258 million tons of municipal solid waste. It contained tens of millions tons of plastics, including over 12 million tons of containers and packaging. Of all these plastics, only 9% is recycled. This rate is well below those for other materials such as newspapers (67%), aluminum cans (55%), and glass containers (34%). If you include other types of waste, including industrial, medical, hazardous, and agricultural waste, this amount of waste generated by the U.S. in 2017 was 8.4 billion tons.

Compliance in labeling plastic is voluntary, and not all plastics have an identification code symbol. Without code numbers, plastics are difficult to separate by appearance.

Prelab Questions

1. How are polymers destroyed?
2. How do we recycle polymers?
3. Which types of items are easy to recycle in your area, and which are more difficult?
4. Why must plastics get sorted for recycling?
5. How can the properties of polymers be used to sort them?
6. If something cannot be destroyed, how can it be reused?

Objective

Identify as many of the Big Six Plastics through a series of tests. Then, identify which of the Big Six Plastics is the most durable and use it to create a compost bin.

Materials

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Wash your hands thoroughly before leaving the lab.
• Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
• Wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
• Part 2: Take great care when using an open flame. Long hair must be tied back, and loose sleeves should be secured. Pieces of hot molten plastic can cause burns if dropped onto your skin or clothing.
• Part 3: To be completed as a teacher-demo: Toxic fumes are produced during the ignition test, so it is imperative that this part of the investigation be done in a fume hood. If a fume hood is not available, do not do this part of the investigation.
• Part 4: To be completed as a teacher-demo: Toxic fumes are produced during the copper-wire test, so this part of the investigation MUST be done in a fume hood. If a fume hood is not available, do not do this part of the investigation.

Procedure

Part 1. Density Tests

The relative density of each plastic will be measured by checking to see whether the samples float or sink in three liquids with differing densities.

1. Obtain three test tubes and label them with the identity and density of each solution. Put about 5 mL of liquid in its respective test tube.
2. Obtain six small beakers and label them with the six plastics (see the Table in the Background section above). Place two narrow strips of each plastic in its respective beaker. Cut one strip of each plastic into three small pieces. Keep the pieces in the labeled beakers for now.
3. Place one small piece of PET into each of the three density test tubes. Push each piece under the liquid surface with a glass stirring rod. If it floats, the density is less than the test liquid. If it sinks, the density is greater than the test liquid. Record observations in your data table.
4. Remove PET from the test tubes with a pair of tweezers.
5. Test the next five plastics following Steps 3 and 4, one plastic at a time.

Part 2. Melt Test

The Big Six plastics melt reversibly, so when they are cooled they harden and regain their original properties. If a plastic sample does not melt, it is a thermosetting plastic, which does not melt cleanly and reversibly but tends to char.

1. Place a small sample of each plastic, one at a time, on the end of a metal spatula and hold the end of the spatula over a light blue burner flame.
2. Heat slowly and observe the plastic as it warms and melts. DO NOT heat the sample so strongly that the plastic catches on fire. Record observations in the data table below.
3. Cool the sample and examine its appearance. Bend it to observe its flexibility. Record observations in the data table below. You may use the melted and cooled plastic for the next tests.

Part 3. Ignition Test (completed as a Teacher Demo)

All common plastics burn (some only if held directly in a flame), but they do so with slightly different characteristics and different noxious fumes. The vapors from the burning plastic may have different properties depending on the type of plastic.

1. Place a Bunsen burner and a large beaker of water in a fume hood. Light the burner and adjust it to a small flame.
2. Hold one end of a small strip of plastic with a pair of tongs and place it directly in the flame. Observe the flame color. Is a lot of smoke or visible vapor given off? Does the plastic continue to burn after it is removed from the flame? Record observations in the data table below.
3. Test the vapors given off for acidic properties by holding a piece of wet litmus paper in the vapors above the burning plastic. If the paper turns red, acidic fumes are being formed as the plastic burns; blue means base. Record your observations in the data table below.
4. Extinguish the burning plastic by dropping it into the beaker of water. Repeat the ignition test for the other plastics.

Part 4. Copper-Wire Test (completed as a Teacher Demo)

To determine whether a halogen, such as chlorine, is part of the plastic, you will use a test called the copper-wire test.

1. Push the end of a 6-inch length of copper wire into a small cork.
2. Use the cork as a handle and heat the free end of the wire in a flame until the flame has no green color.
3. Touch the hot copper wire to the plastic you are testing and then return the wire to the flame. (The hot wire should pick up a tiny bit of plastic.) When the tip of the wire is put in the flame, watch for a slight flash of bright flame. This indicates that you have correctly picked up a little bit of plastic on the wire.
4. Watch for the appearance of a green flame or green color in the flame when the plastic is heated. The green color indicates the presence of a halogen, such as chlorine, in the plastic.
5. Test each plastic sample and record observations.

Analysis

Parts 1-4

1. Rank the six plastics from least dense to most dense.
2. What differences did you observe between the plastics in the melt test?
3. What differences did you observe in the ignition test? Did one plastic stand out? If so, describe this result.
4. What differences did you observe in the copper-wire test? Did you correctly predict which plastic(s) would give a positive halogen test?
5. Devise a scheme/flow chart to identify the six plastics based on the tests you performed (for your reference an example of a scheme/flow chart is shown for a broken lamp). Find the minimum number of tests that will correctly identify each plastic. Draw a flow chart or outline your scheme. When you have finished complete Part 5: Identifying Unknown Plastics.

Procedure

Part 5. Identifying Unknown Plastics

1. You will be given two unknown plastic samples.
2. Analyze your results from the four tests and use the data to devise a method for identifying the six plastics that you tested.
3. Create a data table below for the necessary information in order to identify the unknown plastic samples.
4. Complete any of the tests from Parts 1-4 in order to identify the unknown plastic samples. Record all results in the data table created for the unknown plastics.
5. Use your classification scheme to help you to identify the two unknown plastic samples.
6. Write a statement that identifies both of the unknown plastic samples, use data to justify your response.

Data Table: Identifying Unknown Plastics

Create a data table in the space below

Part 6. Designing a Compost Bin

*This can be completed with physical materials, or hypothetically, as a class discussion.

1. Select a type of plastic that you think will be best suited to contain a compost pile.
2. Select what you will place inside your compost bin. You want to have a mixture that has 50-60% water by weight (should feel like a wrung out sponge to the touch) and a mixture of that has a carbon to nitrogen ratio of 30:1. Carbon rich materials include wood chips, newspaper shreds, and brown leaves. Nitrogen rich materials include food scraps that do NOT contain fats, coffee grounds, and green leaves or grass clippings. Make sure that your particles sizes are not larger than 1-2 cm.
3. Cover your compost bin with a piece of nylon secured with a rubber band.
4. Label your bin with your name and date, and make a mark on the outer or inner edge of the level of the materials.
5. Place the compost bin in an area that gets a lot of sunlight for four to six weeks. Each week, mark the level of the compost and make observations about the changes.

Post-Lab Questions

1. What characteristics of a plastic would you want as the container for a compost bin? Which plastic tested best fits your needs?
2. Were all four of the tests necessary for identifying the six plastics, or could you have focused on fewer tests?
3. Suppose you added two additional plastics to your scheme: polymethylmethacrylate (density 1.18–1.20 g/mL) and poly-4-methyl-1-pentene (density 0.83 g/mL). Where would they fit into your scheme?
4. Why are plastic recyclers concerned about identifying the different polymers and not mixing them together?
5. Currently, polyethylene terephthalate (PET) is the most valuable waste plastic. Suggest a way to separate it on a commercial scale from other waste plastics.
6. Since waste plastic is mostly hydrocarbons, some suggest waste plastic could be used as fuel. Consider your observations in this investigation. Do you think this is a reasonable suggestion? Would some plastics be more dangerous to burn than others? Defend your answer.
7. Mater-Bi is an example of a new biodegradable polymer that has come to market in the last decade. Use the internet to answers these questions. Cite your sources.
   1. Identify two other commercially available biodegradable polymers.
   2. Besides their use in the collection of waste for community composting, what other applications do biodegradable polymers have? (One example: Have you ever had dissolving stitches to close a wound?)
8. Research modern recycling sorting machines and describe how they use the physical properties of materials to sort different types of recyclable materials.