Plastics Identification
Introduction:
A tremendous variety of plastics have become commonplace in contemporary
society. Unknown in 1950, plastics represent an enormous portion of the packaging
industry in the 21st century. The plastics are tailored to provide properties
related to their use. The compositions vary accordingly.
Unlike many discarded materials, most plastics in common use do not
biodegrade. Instead of biodegrading, plastics will photodegrade.
Photodegradation is a process whereby sunlight breaks plastics into progressively
smaller pieces, all of which are still plastic polymers. In fact, the degradation
eventually yields individual molecules of plastic, but these are still too tough for
most anything—even such indiscriminate consumers as bacteria—to digest.
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. 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. Consider the following values:
Table 1
Name
Abbrev
Density
g/mL
Polypropylene
PP
0.90-0.91
Low density
polyethylene
LDPE
0.92-0.94
High density
Polyethylene
HDPE
0.95-0.97
Polystyrene
PS
1.05-1.07
Polyvinyl chloride
V
1.18-1.65
Polyethylene
terephthalate
PETE
1.38-1.39
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Objective:
Samples of plastic can be distinguished on the basis of floating or sinking in
solvents of known density.
Materials:
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5 test tubes per lab group
Test tube rack (one per lab group)
Masking tape & markers for labeling test tubes
Colored Plastic samples
5 types of solvent
Toothpicks
5 plastic disposable transfer pipettes per lab group
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. Label 5 test tubes with the name of each solvent being used. See table 3 to
properly label each test tube with the name of the solvent.
2. Use a different disposable plastic transfer pipettes to fill each labeled test
tube 2/3’s full with the density solvents.
3. Place small samples of the colored plastics, one of each color, into each tube.
HINT: Each colored plastic sample has been numbered. Refer to Table
4.
4. Surface tensions will cause all of the plastics to float until each is "wetted"
and submerged. Use a toothpick to poke at the plastics until they are
submerged. Be sure no bubbles are trapped by the plastic chips.
5. Note and RECORD in table 3 which plastics FLOAT, and which SINK on a
solvent by solvent basis.
6. Repeat for each plastic sample.
7. After all plastics have been tested and recorded in data table 3, determine
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Results:
Table 2
Color of Plastic Sample
Green
Blue
Gold
Yellow
White
Red
Number
1
2
3
4
5
6
Table 3
Test
Tube
Number
solvent
density g/mL
1
sat MgCl2
1.34
2
10% NaCl in water
1.08
3
water
1.00
4
1:1 ethanol:water
0.94
5
4:3 ethanol: water
0.92
3
Float
(plastic #)
Sink
(plastic #)
Table 4
Plastic Type
Density of Plastic
PP
0.90-0.91 g/ml
LDPE
0.92-0.94 g/mL
HDPE
0.95-0.97 g/mL
PS
1.05-1.07 g/mL
V
1.18-1.30 g/mL
PETE
1.38-1.39 g/mL
Color
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.
Image: http://www.mindfully.org/Plastic/Plasticizers/Out-Of-DietPG5nov03.htm
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