Expt 069 -- Plastics

Samples of plastic are distinguished on the basis of floating or sinking in solvents of known density.

Background

• A tremendous variety of plastics have become commonplace in contemporary society. Unknown in 1950, plastics represent an enormous portion of the packaging industry as we approach 2000.
• The plastics are tailored to provide properties related to their use. The compositions vary accordingly. Some plastics containers may be layered for special properties. One layer may be less permeable to oxygen, and another layer may be a good water barrier. These layered plastics may not be recycled because different types of polymers are heat sealed together.
• It turns out the each composition has a range of densities. Thus, measuring the density of a scrap of plastic gives insight about the plastic's composition. Consider the following values:

  Recycle #	Name	Abbrev	Density g/mL
  5	Polypropylene	PP	0.90-0.91
  4	Low density polyethylene	LDPE	0.92-0.94
  2	High density Polyethylene	HDPE	0.95-0.97
  6	Polystyrene	PS	1.05-1.07
  3	Polyvinyl chloride	V	1.18-1.65
  1	Polyethylene terephthalate	PETE	1.38-1.39

• The recycling numbers are being placed on plastics to facilitate the recovery of this resource for reuse.
• The densities suggest something very straightforward. For example, polypropylene, low density polyethylene, and high density polyethylene are expected to float on water while polystyrene, polyvinyl chloride, and polyethylene terephthalate are expected to sink under water.

Safety

The ethanol is flammable. Keep open flames and spark sources away from the solvents. Know the location of the fire equipment; review use of that equipment before beginning work. Wear goggles and apron. Wash spills with water. Avoid ingesting the chemicals. Wash hands after the experiment.

Procedure

1. Using a plastic transfer pipets, fill disposable culture tubes, one each with the density solvents available.

   !!!Click here to See Picture.

2. Place small samples of the colored plastics, one of each color, into each tube. Surface tensions will cause all of the plastics to float until each is "wetted" and submerged.

   !!!Click here to See Movie.

3. Use a toothpick to poke at the plastics until they are submerged. Be sure no bubbles are trapped by the plastic chips. Note and record whether the plastic floats on a solvent by solvent basis.
4. !!!Click here to See Movie.
5. Repeat for each plastic sample.
6. !!!Click here to See Movie.
7. Note and record which plastics float, and which sink on a solvent by solvent basis.
8. !!!Click here to See Movie.
9. !!!Click here to See Picture.

Questions

1. An unknown plastic floats on water but sinks under 10% NaCl. Suggest the composition of this plastic.
2. The same principles used here are used by gas station attendants to check antifreeze. Explain what is done and the basis for the procedure.
3. A cocktail called a Pousse-Cafe is prepared by layering liqueurs one atop another. Explain the principles which permit this elegant after-dinner drink to be prepared.
4. Note two problems that large recycling plants may encounter in trying to separate plastic chips by density.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 069 Plastics

Watch the movies and answer the questions.

Note and record which plastics float, and which sink on a solvent by solvent basis. You may use the color for an identification of each plastic.

Identify each of the colored plastics from the density measured.

Plastic	Color
5, PP, density 0.90-0.91 g/mL
4, LDPE, density 0.92-0.94 g/mL
2, HDPE, density 0.95-0.97 g/mL
6, PS, density 1.05-1.07 g/mL
3, V, density 1.18-1.30 g/mL
1, PETE, density 1.38-1.39 g/mL

Curriculum-

Use when characteristic properties or density are discussed. This is a good applied chemistry activity.

Safety-

The ethanol is flammable. Keep open flames and spark sources away from the solvents. Know the location of the fire safety equipment; review use of that equipment before beginning work. Wear goggles and apron. Wash spills with water. Avoid ingesting the chemicals. Wash hands after the experiment.

Time-

Teacher Preparation: 15 minutes

Class Time: 40 minutes

Materials-

• The choice of solvents will depend upon the plastics actually used. These solvents worked well for the plastics used in this experiment.

  solvent	density g/mL
  sat MgCl2	1.34
  10% NaCl in water	1.08
  water	1.00
  1:1 ethanol:water	0.94
  4:3 ethanol: water	0.92

• Saturated MgCl₂ -- place 20 g MgCl₂•6H₂O in a 100-mL beaker, add 50 mL H₂O, and stir daily for one week.
• 10% NaCl in water -- place 5 g NaCl in 45 mL of water.
• 1:1 ethanol:water -- add 25 mL 95% ethanol to 25 mL of water.
• 4:3 ethanol: water -- add 36 mL of 95% ethanol to 27 mL of water.
• Unknowns -- find plastics whose identities are certain and that are colored uniquely. Use these as unknowns. When plastics are labeled with their recycling numbers, as more and more products are, identification is automatic.
• Pieces of plastic were cut to very small sizes, approximately squares 2 mm on an edge.

  Other Possible Plastics	Density	Possible Source
  poly-4-methyl-1-pentene	0.83	clear plastic labware
  Nylon	1.14-1.16	plastic machine parts
  polymethymethacrylate	1.18-1.20	Plexiglas
  Teflon	2.2	plumbers joint tape

• 5 culture tubes for testing
• 5 Transfer Pipets for filling tubes
• 1 well plate to support culture tubes (act as a test tube rack)

Disposal-

All of the liquids may be disposed of safely at the sink. All of the solids may be discard with ordinary solid trash.

Lab Hints-

• By selecting suitable sources for plastics, the pieces used in the experiment can be color coded.
• In preparing the visuals for this experiment, the following sources were used:
  • 5, PP, density 0.90-0.91 g/mL, white plastic from a quart size yogurt container.
  • 4, LDPE, density 0.92-0.94 g/mL, yellow plastic from a squeezable mustard jar.
  • 2, HDPE, density 0.95-0.97 g/mL, blue plastic from a Miracle White^® bottle.
  • 6, PS, density 1.05-1.07 g/mL, red plastic from a plastic party plate.
  • 3, V, density 1.18-1.30 g/mL, gold plastic from an old credit card.
  • 1, PETE, density 1.38-1.39 g/mL, green plastic from the clear part of a Mountain Dew¨ soda bottle.

Data Table-

solvent	density g/mL	float	sink
sat MgCl2	1.34	2,3,4,5,6	1
10% NaCl in water	1.08	2,4,5,6	1,3
water	1.00	2,4,5	1,3,6
1:1 ethanol:water	0.94	4,5	1,2,3,6
4:3 ethanol: water	0.92	5	1,2,3,4,6

Answers-

Q1. An unknown plastic floats on water but sinks under 10% NaCl. Suggest the composition of this plastic.

A1. Polystyrene floats on water but sinks under 10% NaCl.

Q2. The same principles used here are used by gas station attendants to check antifreeze. Explain what is done and the basis for the procedure.

A2. A small sample of fluid is drawn into a plastic tube containing colored plastic beads of known density. By noting which colors float and which sink, the attendant is able to determine the approximate density of the fluid which suggests a concentration of ethylene glycol (or propylene glycol) for the antifreeze. The ethylene glycol concentration determines the temperature to which protection is afforded.

Q3. A cocktail called a Pousse-Cafe is prepared by layering liqueurs one atop another. Explain the principles which permit this elegant after-dinner drink to be prepared.

A3. Liqueurs contain varying amounts of sugar and alcohol and, as such, have different densities. The only rule is that the more dense liqueurs go in first, and the lass dense ones are floated atop them. Cognacs are among the least dense, so they usually end up on the very top.

Reference-

Many helpful suggestions from Robert Silberman, Cortland, NY, are gratefully acknowledged.

Key Words 1-

characteristic property, property, density, plastic, applied chemistry

Elements-

C Cl