Experiment 5

Plastics the Second Time Around

Physical Properties of Polymers

Objective: The objective of this experiment is to test and compare the physical properties of thermoplastic polymers.

Review of Scientific Principles:

Plastics are long chain molecules. Depending upon the monomers, the plastic will have different physical and chemical properties. Chemical properties are difficult to test for and usually call for the destruction of the plastic through incineration. Burning plastics can give off toxic fumes. This is one of the reasons firemen wear a self contained breathing apparatus when entering a burning building.

It is easier and safer to check the physical properties. Different plastics look, feel, and behave differently. Some are clear and colorless, while others are opaque. Some feel soft, while others feel slimy, slippery, or tacky. Some are more rigid than others. Each plastic has a unique density. Each plastic has a temperature at which it softens and/or melts as we saw in the last laboratory experiment.

The densities (in g/ml) of the plastics you will be checking are:

HDPE 0.952 to 0.965

LDPE 0.917 to 0.940

PET 1.29 to 1.4

PP 0.900 to 0.910

PS (in solid form) 1.04 to 1.05

PS (in foam form) variable but always less than 1

PVC (rigid) 1.30 to 1.58

PVC (flexible) 1.16 to 1.35

The melting or softening point we discussed in the last investigation is important when recycling, because when a plastic is softened or melted, it will adhere (stick) to itself. Generally when two or more plastics are softened or melted, the plastics will not adhere to one another. This is one of the reasons why recycled plastics must be sorted. This is an expensive process that adds to the costs of recycling.

Time: To perform this experiment and answer the questions will require 35-40 minutes.

Materials and Supplies:

• Samples of the following plastics:
• HDPE (high density polyethylene),
• LDPE (low density polyethylene),
• PET (polyethylene terephthalate),
• PP (polypropylene),
• PS [in solid form] (polystyrene),
• PS [in foam form], and
• PVC (polyvinyl chloride);
• ethanol/water solutions of various concentrations:
• 52% ethanol (density = 0.911),
• 38% ethanol (density = 0.9408), and
• 24% ethanol (density = 0.9549)
• calcium chloride/water solutions of various concentrations:
• 6% CaCl₂ (density = 1.0505),
• 32% CaCl₂ (density = 1.3059), and
• 40% CaCl₂ (density = 1.3982)
• 250 ml beaker

Procedure:

1. Obtain a sample of each type of plastic, noting the letter on each piece. The letters are used to reference each sample.
2. Examine each sample and write a visual description in the proper location in the data table.
   a) Is the sample clear? Is the sample opaque? Does it have color?
   b) In the data table describe how the sample feels. Is the sample smooth or rough? Does it have a pattern?

   c) Flex each sample through an angle of 10^o to 30^o. Note in the data table how easy it was to flex the sample. Is it flexible or rigid? You might want to compare the various samples.

3. Pour 50 ml of the 40% CaCl₂ solution into a 150 ml beaker. Place each of the plastic samples in the solution. Note which samples sink (S) or float (F) in the DATA TABLE.
4. Return the solution to the appropriate container and dry out the beaker.
5. Dry off your samples.
6. Repeat Step #3-5 with:
   32% calcium chloride,
   6% calcium chloride,
   24% ethanol,
   38% ethanol, and
   52% ethanol
7. Return all plastics to the recycling box after usage.

Video Clip

Data Table:

Questions:

1. From the data given in the REVIEW OF SCIENTIFIC PRINCIPLES section identify each of the plastics by proper recycling number and proper name.
   Sample A
   Sample B
   Sample C
   Sample D
   Sample E
   Sample F
   Sample G
2. When testing for the density of the plastic samples, why did some of the samples stick out of the solution more than other samples?
3. If you were given two plastic samples, how would you identify them?
4. Which of the polymer (plastics) would be used as a material in making each of the following?
   Use letters and names to identify each polymer.

   
   i) A covering to go around a sandwich?
   ii) A replacement for a picture window ?
   iii) As a covering for a plastic bowl?
   iv) As a replacement for the lead sinkers used in fishing?
   v) As a clip board to write on?

5. Which polymer was most flexible?
6. Which polymer was most transparent?

Teacher Notes:

Time : This lab is expected to take 40 minutes.

Materials and Supplies:

• Samples of the following plastics:
• HDPE (high density polyethylene)
• LDPE (low density polyethylene)
• PET (polyethylene terephthalate)
• PP (polypropylene)
• PS [in solid form] (polystyrene)
• PS [in foam form]
• PVC (polyvinyl chloride)

• Three different ethanol solutions of various concentrations are used. 50 ml of each solution is needed for every group.
• 52% (density = 0.911) - to 619.1 ml of 95% ethanol add distilled water to make 1000.0 ml
• 38% (density = 0.941) - to 467.2 ml of 95% ethanol add distilled water to make 1000.0 ml
• 24% (density = 0.965) - to 302.7 ml of 95% ethanol add distilled water to make 1000.0 ml
• Three different calcium chloride solutions of various concentrations are used. 50 ml of each solution is needed for every group.
• Caution: The hydration of CaCl₂ is an exothermic reaction. The solutions should be made ahead of time to allow the solution to cool. One day in advance is recommended.
• 6% (density = 1.0505) - 6 g of CaCl₂ (anhyd.) plus 94 ml distilled water or7.95 g of CaCl₂*2 H₂O plus 92.05 ml of distilled water.
• 32% (density = 1.306) - 32 g of CaCl₂ (anhyd.) plus 68 ml of distilled water or 42.39 g of CaCl₂*2 H₂O plus 58.61 ml of distilled water.
• 40% (density = 1.398) - 40 g of CaCl₂ (anhyd.) plus 60 ml of distilled water or 52.99 g of CaCl₂*2 H₂O plus 47.06 ml of distilled water.

• 250 ml beaker

Procedure:

1. Prepare the solutions given above and ask the students to return them to the original containers after use. An alternative procedure might be to provide a number of beakers each containing one solution. Identify the beakers so the students can simply take these beakers to their desk for usage and exchange beakers among lab groups after their use.
2. Using the polymers brought by the students in the previous lab, cut the polymers into 2 cm by 4 cm pieces. With an indelible marker, write a letter A-G on each type of polymer. These letters provide a technique for identifying and talking about the polymers used. A polymer can be identified using the recycling code on the bottom of the plastic. The recycling codes are: 1 = PET, 2 = HDPE, 3 = PVC, 4 = LDPE, 5 = PP, and 6 = PS. Students should be familiar with the code numbers found in the 3 bent arrow recycling symbol. At this time you should not identify any of the polymers used in this experiment nor their recycling numbers.
3. Students should get one sample of each type of polymer to use in this experiment. Should the indelible ink be leached from a polymer, a fresh identification marking should be reapplied.
4. When the experiment is finished, students are to place their polymers in the recycling box.

• Students will obtain good results with the densities except for the PET and PVC samples. These two polymers are hard to distinguish using their densities.
• Students will be able to identify Styrofoam from its appearance. After the lab, ask the class about the appearance, color, feel, and rigidity of the samples. These are extrinsic, physical properties. Density, melting, and softening point are intrinsic physical properties. Depending upon the placement of this lab in the school year, these terms might be new or a review for the students.
• Review the manufacturer's recycling code for the plastics with the students.

Answers to Questions:

1. Student answers.
2. Differences in density.
3. Using density or physical appearance.

4. i) HDPE and/or LDPE
   ii) PP and/or PS rigid
   iii) PET, HDPE, and/or LDPE
   iv) PVC
   v) PS (rigid), PP, and/or PVC
5. LDPE and/or PET
6. PP, PS (rigid), and/or PET