A Novel, Simplified Scheme for Plastics Identification



A Novel, Simplified Scheme for Plastics Identification

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 has the plastics recycling code number 2. 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 six 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: six kinds of plastic containers with different recycle codes, scissors, two cups or beakers, two stirring rods, room temperature water, 70% isopropyl alcohol, graduated cylinder, acetone, 2 plastic pipets, hot plate with a 400 mL beaker of boiling water, and tongs.

1. Record the color and shape of each piece of plastic.

2. Tap Water Test: Place all six pieces of plastic in a beaker of room temperature tap water. Stir vigorously with a stirring rod to dislodge any bubbles from the plastic pieces. Observe and record which pieces sink and which float.

3. 70% Alcohol Test: Measure 40 mL of isopropyl alcohol and pour it into a 150 mL beaker. Remove the pieces of plastic that floated from the water. Add them to a beaker containing the alcohol. Stir. Do the pieces sink or float in the 70% alcohol? Record your observations.

4. Using a wash bottle, add a squirt of water to the beaker containing alcohol. Stir. Do any of the pieces begin to float? 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 observations.

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

6. Set aside all of the plastics that were in the alcohol. These should all be identified and you are finished with them.

7. Boiling Water Test: Place one of the four pieces that sank in tap water in the beaker of boiling water for a minimum of 30 s. Using tongs, remove the piece and test its flexibility, noting their size and color. Record your observations.

8. Repeat the boiling water test for the remaining plastics. Record your observations. If you can positively identify any of the plastics, record its identity in your data table.

7. Acetone Test: Place the final two plastic samples in a small amount of acetone for one minute. Remove the plastic with tongs. Record your observations

.

8. 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.

Questions

Answer each question in complete sentences.

1. How does the density of water compare to the density of the rubbing alcohol?

2. What happens to the density of the alcohol as you add squirts of water from the wash bottle?

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

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

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

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

Related Information from the World Wide Web

1. Royte, Elizabeth. Corn Plastic to the Rescue. Smithsonian Magazine, August 2006. (accessed Dec 2009).

2. Automated recycling. (accessed Dec 2009).

|Table 1. Selected Physical Properties of Common Plastics |

|Number |Plastic Type and Composition |Density, g/mL |Glass Transition |Melting Temperature, °C|

| | | |Temperature, °C | |

|1 |PET or PETE |1.38–1.39 |60–85 |250–265 |

|2 |HDPE |0.95–0.97 |~ –125 |~138 |

|3 |PVC |1.16–1.35 |81–98 |200–300 |

|4 |LDPE |0.92–0.94 |–128 to –30 |~138 |

|5 |PP |0.90–0.91 |–8 |174–177 |

|6 |PS |1.05–1.07 |80–100 |240 |

|7 |PLA |1.25–1.26 |50–80 |173–178 |

Flowchart

[pic]

|Plastics Identification Data Table |

|Letter Label|Sample’s Color |Sample’s Shape |Floats in Water?|Sinkers— |Sinkers— |Floaters— |Plastic Type |

| | | |(Yes/No) |Boiling Water Results |Acetone Test Results |Alcohol Test Results | |

| | | | |(Note color/shape changes.) |(Note texture changes.) |(Note number of squirts.) | |

|A | | | | | | | |

|B | | | | | | | |

|C | | | | | | | |

|D | | | | | | | |

|F | | | | | | | |

|G | | | | | | | |

Instructor Information

A Novel, Simplified Scheme for Plastics Identification

JCE Classroom Activity #104

This document provides supporting information and materials for the Classroom Activity article (titled above) published in the February 2010 issue of the Journal of Chemical Education. The article provides background content and notes for instructors, and should be used in conjunction with the supporting information and materials shared in this document. The article (DOI: 10.1021/ed800055p) can be found online at .

Answers to Questions

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

PP floats first because its density is less than HDPE or LDPE. The instructor may wish to share Table 1 from the Instructor Information section with students.

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

None of the four melted in boiling water. All four did soften and become more flexible.

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

It is a physical change. When acetone is in contact with PS, it softens the plastic and the polymer chains are rearranged, but it does not produce a new substance. The softened piece will harden in the air as the acetone evaporates.

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

The 70% alcohol solution density is equal to or less than 0.89 g/mL because PP, HDPE, and LDPE sink in this solution. In order for LDPE to float and HDPE to sink, the alcohol–water solution must be about 0.945 g/mL.

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Container made using PETE with a recycle code of 1. (Photo credit: Hal Harris)

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