Teacher Notes

Recycling Plastics by Density

Student Laboratory Kit

Materials Included In Kit

Corn oil, 500 mL†
Ethyl alcohol 250 mL†‡
Glycerin, 500 mL†
Sodium chloride solution, 10%, 500 mL†
HDPE #2: Plastic sheet, 3" x 3"*
LDPE #4: Round bottles, 4-oz, 2*
PETE #1: Soda bottle, 1-L*
PP #5: Medicine cups, 4
PS #6: Disposable Petri dishes, 2*
PVC #3: Black plastic disks, 2*
Weighing dishes, 6
*“Unkown” plastic samples
Liquids for density determination
See Prelab Preparation.

Additional Materials Required

Water, distilled or deionized, 1 L†
Beakers or cups, 150-mL, 5 (may be shared)
Forceps
Metric ruler*
Paper towels
Rinse beaker, 400-mL
Scalpel or file (see Lab Hints)
Scissors, heavy-duty*
Wash bottle
*See Prelab Preparation.
Liquid for density determination

Prelab Preparation

Unknown Plastic Samples, A–F: Assign each type of plastic a random letter code A–F, and record the code in a notebook or file for this experiment. Use heavy-duty scissors and a metric ruler to cut the plastic samples supplied with the kit into 1-cm squares. (Remember to keep each type of plastic separate.) Label six large weighing dishes A–F and place the cut up plastic squares into their respective weighing dishes.

Ethyl Alcohol, 35%: This is a mass percent solution (also called a w/w solution). Weigh 175 grams of denatured, absolute ethyl alcohol into a flask or bottle. Add distilled or deionized water to obtain a final mass of 500 g of solution. Stopper the flask and mix the solution thoroughly. Keep the solution tightly covered to prevent evaporation of the alcohol.

Safety Precautions

Ethyl alcohol contains a denaturant and is toxic by ingestion. It is a flammable liquid and a dangerous fire risk—keep away from heat, flames and other sources of ignition. Do not use a Bunsen burner in the lab when working with flammable solvents. Please observe all normal laboratory safety guidelines. Wear goggles or safety glasses whenever working with chemicals, heat or glassware in the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information. Remind students to wash their hands thoroughly with soap and water before leaving the lab.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Corn oil, glycerin and the sodium chloride solution may be saved for future use. The ethyl alcohol solution may be disposed of by rinsing down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b. (Evaporation of ethyl alcohol during the course of the experiment will change the density of the solution and may alter the results of the experiment.)

Lab Hints

  • For best results, schedule at least two 50-minute lab periods for completion of this activity. The experimental work itself does not take that much time, but students will need to stop and think when they reach various “decision trees” in the Procedure. Remind students to refer to their completed flow charts before deciding what their next steps should be.
  • The Prelab Assignment is an essential part of this experiment. Students need the conceptual understanding and the completed flow chart to follow the procedure and to identify the plastics based on their densities and recycling codes.
  • Rigid, brittle plastics such as polystyrene (PS) may be difficult to cut with heavy-duty scissors. We found that the best way to cut the PS Petri dish was to score the flat parts of the dish using a scalpel or file. The PS will then break evenly along the scored line.
  • The densities of the liquids given in the Prelab Assignment were measured under typical experimental conditions at actual “room temperature” (21–22 °C). The values, therefore, may not be identical to literature values, which are for 20 or 25 °C. The density of the ethyl alcohol solution is very sensitive to both the concentration of water and the presence of denaturants in “absolute” alcohol.
  • The densities of LDPE and HDPE are very similar and may also depend on the presence of additives or fillers. In order to distinguish these two plastics, the concentration and the density of the ethyl alcohol solution must be within a narrow and precise range. We used a 35% concentration of ethyl alcohol to obtain a density of 0.94 g/mL under our laboratory conditions (22 °C).
  • In the experiment as written, students are asked to identify “unknown” plastics based on their densities. Alternatively, the experiment may be carried out as a confirmation lab, where students determine the densities of known plastic samples. Encourage students to bring in a variety of plastics with different recycling codes. Table 1 in the Background section gives typical consumer items or examples for each type of plastic used in this activity.

Teacher Tips

  • Have students research and prepare poster boards on the structure, composition, preparation, properties, and uses of the most common plastics (recycling codes #1–6). Each student group could be responsible for one polymer.
  • Conduct a mini-research seminar on the preparation and uses of so-called biodegradable plastics. Plastics that degrade in the environment may be obtained from conventional, petrochemical-based plastics by inserting reactive groups into the structure of a polymer. Alternatively, biodegradable plastics may be synthesized from plant-based chemicals. Polylactic acid or PLA is a biodegradable polymer derived from lactic acid. It is made from 100% renewable resources such as corn, sugar, beets, and wheat. PLA is a promising candidate to replace conventional plastics in packaging. It is clear and glossy, like polystyrene films, and is resistant to moisture and grease. PLA may even replace PETE in soft-drink bottles.

Correlation to Next Generation Science Standards (NGSS)

Science & Engineering Practices

Developing and using models

Disciplinary Core Ideas

MS-PS1.A: Structure and Properties of Matter
HS-PS1.A: Structure and Properties of Matter

Crosscutting Concepts

Scale, proportion, and quantity
Structure and function

Performance Expectations

MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended structures.
MS-PS1-3. Gather and make sense of information to describe that synthetic materials come from natural resources and impact society.

Answers to Prelab Questions

The following flow chart describes the order in which the density tests will be done. All plastics will be placed in water. If a plastic floats in water, it will be tested next in ethyl alcohol solution. If it sinks in water, it will be tested next in sodium chloride solution. The “path” for each plastic will be different, based on whether it floats or sinks in a given liquid.

  1. Fill in the missing density ranges (circled) in the flow chart.

    See chart for answers.

  2. For each density range, identify the recycling code(s) (#1–6) for the corresponding plastic(s) (see Table 1).

    See chart.

  3. What are the two plastics that cannot be distinguished using this method?

    PETE (#1) and PVC (#3) cannot be distinguished using this method. Both PETE and PVC are more dense than all of the test liquids used in this experiment.

    {12554_Answers_Figure_2_Identification of plastics based on density}

Sample Data

{12554_Data_Table_2}

Answers to Questions

  1. Determine the density range and identify the recycling code #1–6 for each unknown plastic. Enter the results in the data table.

    See the Sample Data section.

    1. Why were the unknown plastics cut into uniform size and shape pieces before measuring their densities?

      Density is a characteristic physical property of a substance. The “apparent density” of an object, however, may depend on its shape and size if the object is hollow or traps air (like a boat). Similarly, gas bubbles may adhere to the surface of an object and prevent it from sinking. Note: Students may recall building clay boats in a physical science classroom. Clay is more dense than water. However, clay molded into the shape of a boat will float if it displaces a volume of water whose mass is greater than the mass of the clay.

    2. Why was it necessary to rinse the unknown plastics with water and pat them dry before testing in each new liquid?

      It is important not to contaminate the different liquids. The density of a mixture depends on its composition. Mixing water with another liquid will change the density of the liquid. The density of the 35% ethyl alcohol solution will increase if water is added, because water is more dense than ethyl alcohol, In contrast, the density of the sodium chloride solution will decrease if water is added.

  2. The density of a pure substance is a characteristic physical property. Why is the density of a plastic such as LDPE always stated as a range (0.90–0.94 g/mL)?

    The density of a plastic depends on the average molar mass of the polymer molecules, on the amount of branching in the polymer “chains,” and on the use of fillers or additives that are added to improve the processing characteristics of the polymer.

  3. Explain why if a sample sank in water and floated in the 10% sodium chloride solution, it did not need to be tested in other liquids.

    All plastics were first tested in water. If the sample sank in water, it was then tested in 10% sodium chloride solution. Since the object sank in water, its density must be greater than 1.0 g/mL. If the object then floated in sodium chloride solution, its density must be less than 1.06 g/mL. The density range of the plastic is therefore 1.0 < d < 1.06 g/mL. No further information about its density can be obtained by testing it with the other liquids used in this experiment.

  4. Which two plastics (#1–6) cannot be identified using the liquids in this experiment? What is the density of a “Liquid #4” that could be used to determine the densities of these two plastics more precisely?

    PETE (plastic #1) and PVC (plastic #3) cannot be distinguished using the different density liquids in this experiment. Using a “Liquid #4” with a density of 1.35–1.40 g/mL would make it possible to determine the densities of PETE and PVC more precisely. Note: Corn syrup (d = 1.39 g/mL) or 36% calcium chloride solution (d = 1.35 g/mL) would be appropriate choices.

  5. What are the advantages and disadvantages of the two principal methods of recycling plastics?

    Energy recovery involves burning plastics to generate energy. The value of the energy obtained in this way is generally less than the replacement value of the plastics. This is therefore a “fall-back position” because it doesn’t recoup the material or energy investment that is needed to make more plastic. Mechanical recycling involves sorting and separating plastics to make recycled plastic. Using recycled plastic instead of making new plastic conserves or saves precious resources. This process, however, generally requires a lot of energy and labor. As a result, the recycled plastic may cost more but be lower in quality than virgin plastic. Using recycled plastic requires a commitment to something other than just lower cost!

  6. (Optional) Conduct a household “audit” to find 15–20 different plastic products. Write down the name or source of each product, and identify the recycling code of the plastic.
    {12554_Answers_Figure_3}

References

This activity was adapted from Flinn ChemTopic Labs, Vol. 21, Polymers; Cesa, I., Editor; Flinn Scientific Inc.: Batavia IL (2006).

Student Pages

Recycling Plastics by Density

Introduction

The United States produces almost 100 billion pounds of plastics every year. In 2005, only about 4% of this amount was recycled—considerably less than the recycling rate for either paper (45%) or glass (23%). Recycling plastics is a special challenge because of the number of different plastics in everyday use. Let’s investigate the composition and properties of plastics and their recycling codes.

Concepts

  • Plastics and polymers
  • Recycling code
  • Physical properties
  • Density

Background

Most plastics are obtained from byproducts of petroleum refining and natural gas processing, such as ethylene (CH2=CH2), propylene (CH3CH=CH2), and styrene (C6H5CH=CH2). Manufacturing plastics consumes nonrenewable resources and requires large investments of energy. Disposing of plastics is also an environmental concern, because most plastics are non-biodegradable. It has been estimated that it would take more than five years for a discarded plastic milk jug to decompose when exposed to environmental conditions.

There are two main methods for recycling plastics. Energy recovery involves burning plastics in an incinerator to recover their energy value. The advantage of this method is that it is cheap and does not require separating different types of plastics. The disadvantage is that it does not return much dollar value for the recycled plastics. Mechanical recycling involves sorting and separating mixed plastics to make recycled plastics. Plastics are sent to a reclaiming facility, where they are washed and cleaned and shredded to plastic flakes. The shredded plastics may be placed in a cyclone device or in a flotation tank to separate plastics that have different densities. Plastics that are more dense than water will sink, while those that are less dense than water will float. Recycled plastics are used to make carpeting and textiles, clothing, auto parts, tennis balls, park benches, patio furniture, toys, etc.

In 1988, the plastics industry in the United States adopted a voluntary labeling system for different plastics. The goal was to make it easier to sort and separate different types of plastic and to increase the amount of plastic that is recycled. Different types of plastic may be distinguished based on color, density, UV absorption, x-ray fluorescence, etc. The recycling code for a plastic consists of a triangle made of arrows and a number inside the triangle. The number refers to the chemical composition of the polymer. The use of recycling codes has increased the rate of recycling for many household items. Virtually all plastic bottles are now made from either #1 PETE or #2 HDPE, for example, and their recycling rate has improved to 40%. The identity, composition, density, and uses for seven plastics and their recycling codes are shown in Table 1.

{12554_Background_Table_1_Recycling Codes for Plastics}

*This range is for rigid PVC, which normally contains fillers. Plasticizers are added to make flexible PVC, which has a lower density.

Experiment Overview

The purpose of this experiment is to identify unknown plastics based on their densities. The density range for each plastic will be determined by testing whether it floats or sinks in a series of liquids having different densities. All of the plastics will be tested first in water (d = 1.0 g/mL). A plastic that floats in water will then be tested in 35% ethyl alcohol (d = 0.94 g/mL), and in corn oil (d = 0.91 g/mL), if necessary, until the plastic sinks. A plastic that sinks in water will be tested in 10% sodium chloride solution (d = 1.07 g/mL) and in glycerin (d = 1.25 g/ml), if necessary, until the plastic floats. See the flow chart in the Prelab Assignment.

Materials

Corn oil, 90 mL*
Ethyl alcohol in water, 35% w/w, 90 mL*
Glycerin, 90 mL*
Sodium chloride solution, 10%, 90 mL*
Water, distilled or deionized (DI), 90 mL*
Beakers or cups, 150-mL, 5
Forceps
Metric ruler
Paper towels
Scissors, heavy-duty
“Unknown” plastic samples, cut into uniform 1-cm square pieces, labeled A–F (e.g., plastic containers, bottles, bags, toys, cards)
Wash bottle
*Liquids for density determination (one set may be shared)

Prelab Questions

The flow chart in Figure 1 describes the order in which the density tests will be done. All plastics will be placed in water. If a plastic floats in water, it will be tested next in ethyl alcohol solution. If it sinks in water, it will be tested next in sodium chloride solution. The “path” for each plastic will be different, based on whether it floats or sinks in a given liquid.

{12554_PreLab_Figure_1_Flow chart for the identification of plastics based on density}
  1. Fill in the missing density ranges (circled) in the flow chart.
  2. For each density range, identify the recycling code(s) (1–6) for the corresponding plastic(s) (see Table 1 in the Background section).
  3. What are the two plastics that cannot be distinguished using this method? Explain.
Example: If a plastic sinks in water, but floats in 10% sodium chloride solution, the density of the plastic is equal to 1.0–1.06 g/mL, corresponding to polystyrene (PS, recycling code #6).

Safety Precautions

Ethyl alcohol solution contains a denaturant and is toxic by ingestion. Please observe all normal laboratory safety guidelines. Wear goggles or safety glasses whenever working with chemicals, heat or glassware in the laboratory. Wash hands thoroughly with soap and water before leaving the lab.

Procedure

  1. Obtain “unknown” plastics from the teacher. Place each plastic on a paper towel or sheet of paper and label the towel with the unknown letter code (e.g., A, B, C).
  2. (Optional) Cut the plastic into 1-cm square pieces, if needed. (The plastics may already have been cut to the desired size by the teacher.)
  3. Label five beakers or cups with the names of the liquids to be used in the density tests (e.g., corn oil, DI water).
  4. Pour about 90 mL of the appropriate liquid into each labeled beaker. (If using cups, fill cups about two-thirds full.)
  5. Using forceps, gently push the first unknown plastic sample down to the bottom of the DI water. Release the sample and let the plastic “sink or swim.” Wait about one minute, then record in the data table whether the plastic floats or sinks in water.
  6. Remove the sample from the DI water and pat dry on a paper towel. If the plastic floated in water, go to step 7. If the plastic sank in water, go to step 8. (See the Prelab Assignment.)
  7. If the plastic floated in water, test it in 35% ethyl alcohol solution. Write “ethyl alcohol” in the Liquid #2 column in the data table and determine whether the plastic floats or sinks. Record the observation in the data table.
  8. If the plastic sank in water, test it in 10% sodium chloride solution. Write “sodium chloride” in the Liquid #2 column in the data table and determine whether the plastic floats or sinks. Record the result in the data table.
  9. Remove the sample from the liquid (step 7 or 8) and rinse with distilled water from a wash bottle. Pat dry on a paper towel.
Consult the flow chart in the Prelab Assignment:
  1. Determine whether the plastic must be tested in a third liquid in order to determine its density range. If the plastic does not need to be tested further, write N/A in the “Liquid #3” column in the data table and record the density range.
  2. If the plastic must be tested in a third liquid (for example, the sample floated in ethyl alcohol), write the name of the appropriate liquid in the “Liquid #3” column in the data table. Determine whether the plastic floats or sinks in Liquid #3 and record the observation in the data table.
  3. Repeat steps 1–11 with each unknown plastic sample. Consult the flow chart frequently to check the appropriate liquids for testing each plastic, and record all observations in the data table. Rinse each plastic with distilled water and pat dry on a paper towel before testing the sample in a different liquid.

Student Worksheet PDF

12554_Student1.pdf

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