Question

Plastics Identification Data Table
Letter Label	Sample’s Color	Sample’s Shape	Floats in Water? (Yes/No)	Sinkers— Boiling Water Results (Note color/shape changes.)	Sinkers— Acetone Test Results (Note texture changes.)	Floaters— Alcohol Test Results (Note number of squirts.)	Plastic Type
A
B
C
D
E
F
G

Questions

Why does PP float before HDPE and LDPE in the alcohol–water solution?

Did any of the four plastics melt in boiling water? (Hint: How does melting differ from softening in plastics?)

When PS reacts with acetone, is this a physical or chemical change? Explain.

What is the approximate density of the alcohol–water solution that was diluted with water to help identify LDPE from HDPE?

Looking at plastic containers like milk jugs, deli boxes, detergent bottles, soda bottles, and pill vials makes a person wonder whether they are really all that different from one another. The numbers and letters in the recycle codes seem to indicate that the plastics aren’t the same, because these codes all refer to different kinds of plastics. For example, HDPE (high-density polyethylene) has the plastics recycling code number 2, while number 3 is used for PVC or V (polyvinyl chloride). An example of the polyethylene terephthalate (PETE) recycle code is shown at right. In this Activity, you will follow a flowchart to determine the identity of all seven recyclable plastics, without their labels; the first separation involves comparing the densities of plastic pieces to water.

A recycling plant may use manual separation of containers by having people visually recognize and remove containers as they travel along a conveyor belt. All of one kind of plastic will be collected at one location in the plant. Automated sorting may use whole containers or cut-up containers, called flake. Flake sorting involves putting the flakes into large containers of water to separate the “floaters” from the “sinkers”. Whole container sorting uses electromagnetic radiation such as visible, near infrared, and X-ray systems. Optical sorting uses visible light to sort containers by color. Containers exposed to near infrared light absorb specific wavelengths and transmit others based on crystallinity of the plastics. Sensors detect the change in wavelength for each container and initiate the sorting by using jets of air to blast a container to a specific location. X-ray systems detect chlorine atoms in PVC. Recycle plants may sort plastics by using a combination of the systems described above or other technologies that are currently being developed.

Try This

You will need: seven kinds of plastic containers with different recycle codes, scissors, two cups or beakers, two stirring rods or wooden craft sticks, room temperature water, 70% isopropyl alcohol, graduated cylinder, acetone, 2 plastic pipets, hot plate with a 400 mL beaker of boiling water (student groups may share the boiling water), tongs, and forceps.

Cut a small, flat piece (1.5 cm ´ 1.5 cm) from each container (seven total) and start a data table to record the color and shape of each piece as well as the permanent marker letter label (A–G) your instructor previously marked on each container. If two containers are the same color, cut different shapes (triangle, rectangle, circle, etc.) to help with piece identification. Now you are ready to follow the flowchart. Record your observations in the data table as you go along.

Tap Water Test: Place all seven pieces of plastic in a beaker or cup of room temperature tap water. Stir vigorously with a stirring rod or wooden craft stick to dislodge any bubbles from the plastic pieces. Bubbles tend to adhere to plastics. How would this change the apparent density? Observe and record which pieces sink and which float.

70% Alcohol Test: Remove the pieces that float from the water. Add them to a beaker or cup containing about 20 mL of 70% isopropyl alcohol. Stir. Do the pieces sink or float in the 70% alcohol? How does the density of alcohol compare to water?

Using a plastic pipet, add a squirt of tap water to the beaker containing alcohol. Stir. Do any of the pieces begin to float? When adding water to 70% alcohol, how does the density of the solution change? Keep adding squirts of water and stir until one piece floats. According to the flowchart, what is the identity of this plastic? Remove the piece and record your results.

Add more squirts of water until a second piece floats. Remove the piece and record the identity of the second plastic. You can now identify the third piece since it is still sinking.

Boiling Water Test: Place the four pieces that sank in tap water in the beaker of boiling water for a minimum of 30 s. Using tongs, remove them one at a time and test their flexibility, noting their size and color. Record your results. Which plastics can you now identify using the flowchart?

Acetone Test: Place the final two plastic samples in a small amount of acetone for one minute. Record your results.

Check with your instructor to see whether you correctly identified the seven plastics. You are now ready for an unknown. If your unknown floats in the tap water test, you will need to use the known pieces for PP, HDPE, and LDPE along with your unknown to help you with the identification in the 70% alcohol test.

More Things To Try

You might want to explore plastics coded as 7, “other”, or “mixed” with this approach (see the figure at right). The results may be inconclusive because two or more polymers may be used to make these containers, or it might be 95% of one plastic and 5% another so that it will test as one component. Some containers have many additives to give the container a softer texture; some have several coloring agents. These additives will alter the density of the plastic. Another experiment to try is to test whether black microwavable PET dinner trays react like the transparent PET bottles.