PLASTICS: A COMMON OCCURENCE
Introduction
Plastics are very common in our society today. They are used extensively for
food storage and in this area alone, generate tons of waste for landfills. If one
evaluates the amount of household garbage generated, it could easily be seen
plastic and paper combined accounts for around 70% of the waste.
Plastics can be recycled for use. This cuts down appreciably on the amount of
waste that needs to be buried in the landfills. To make recycling more efficient, a
code was established and stamped on the bottom of most containers to identify
the composition of the plastic. Although more than 60,000 types of plastic are
manufactured, there are only six plastics that are commonly identified by this
code. These six make up about 70% of the plastics used today.
The code is a triangle composed of arrows with a number in the center of the
triangle. It usually appears on the bottom of the container.
Number
Name of Plastic
Abbreviation
1 Polyethylene terephthalate
PET
2 High density polyethylene
HDPE
3 Polyvinyl chloride
PVC
4 Low density polyethylene
LDPE
5 Polypropylene
PP
6 Polystryene
PS
7 Others/mixtures
Most plastics used today are identified by this triangle but their identity can also
be predicted based on their use or appearance. Most soft drink containers are
made of polyethylene terephthalate. Most opaque translucent containers such
as milk containers are high density polyethylene,
Today the properties of the six common plastics will be identified. Four tests will
be run on each of the six and observations will be made to help distinguish
between them. Using these observations, a flow chart will be made which will
then be used to identify unknown plastics samples. A sample data sheet is
included at the end to aid with observations. Do not feel limited though by this
sheet. The more detailed the observations, the easier it will be to construct and
use the flow chart.
Tests
1.
2.
3.
4.
Density test
Melt test
Ignition test
Copper wire test
Safety Notes:
1. Burn all samples in your individual fume hoods, as the burning process will
generate unpleasant and toxic vapors.
2. Care should be taken when melting plastics to avoid burns due to contact
with the hot liquid.
Procedure
I.
Density
Perform the following procedure on a sample of each of the six common
plastics.
1. Gather three test tubes. Label and half fill each test tube with one of the
following:
1:1 95% ethanol: water (Density is 0.94 g/cm3)
Water (Density is 1.00 g/cm3)
10% NaCl (Density is 1.08 g/cm3)
2. In each test tube place a narrow strip of the plastic sample. Be sure the
sample size is small enough so it does not touch the sides of the test tube.
3. Using a stirring rod, push each plastic sample under the surface of the
liquid. Observe whether the sample floats, sinks, or suspends in the
liquid. Record observations.
4. Interpret observations. If the sample floats, the density of the plastic is
less than that of the liquid. If the sample suspends in the liquid, the
density is the same as the liquid. If the sample sinks, the density is
greater than the liquid’s density.
5. Once all six plastic types are tested, rank the plastics in order of their
density, from the least to the greatest.
II. Melt Test
Perform the following procedure on a sample of each of the six common
plastics.
1. Place a sample of the plastic in a metal spatula.
2. Slowly heat the sample over a Bunsen burner in the small hood provided
at each workstation. Do not heat too quickly. The goal is to melt the
sample, not char or ignite it.
3. Record observations of the sample once it has melted and again after it
cools. Look at both the appearance and flexibility.
III. Ignition Test
Using the samples from the melt test, perform the following procedure on
a sample of each of the six common plastics. Be sure the test is performed
in the fume hood.
1. Fill a beaker with water and have available for use. Once samples ignite
and observations are made, the burning samples will be placed in the
beaker to extinguish the flame.
2. Using tongs or forceps, place a piece of the melted plastic directly into
the burner flame and observe. Things to consider: Look at the color of the
flame and its characteristics, the color and amount of smoke or vapor
generated and did the sample continue to burn once it was removed from
the flame?
3. Hold a piece of wet litmus paper in the vapors. Does the color indicate
acidic, or basic properties? Red indicates acidic nature, blue indicates
basic nature.
4. Extinguish the plastic by putting the sample in the beaker of water
prepared in step 1 of this section.
IV. Copper Wire Test
Perform the following procedure on a sample of each of the six common
plastics. . Be sure the test is performed in the fume hood.
1. Obtain a copper wire with a cork attached to one end. The cork will serve
as a handle to hold the wire.
2. Heat the free end of the copper wire in the flame until there is no evidence
of a green color in the flame.
3. Melt a sample of the plastic in a metal spatula as before and then dip the
hot wire into the melted sample so a bit of the sample adheres to the wire.
4. Place the wire with sample back into the flame and observe the color of
the flame as the sample ignites. As the plastic ignites a slight luminous
flash should be evident.
5. If the temporary flash of flame is green in color, chlorine is present.
Using the results of these four tests, devise a separation scheme (flow chart) that
could identify an unknown plastic sample using these test results. The goal is to
use the minimum number of tests possible to identify an unknown plastic. Once
your scheme is complete, test it on the samples brought from home. If
necessary the separation scheme can be fine-tuned.
Now using your separation scheme, test and identify the unknown samples
supplied by your TA.
Postlab Questions
1. Outline the procedure used to separate the six plastics.
2. Suppose you decide to add two more plastics to your separation scheme,
polymethylmethacrylate (density 1.18-1.20 g/cm3) and poly-4-methyl-1pentene (density 0.83 g/cm3). Where would they fit in your separation
procedure?
3. Polyethylene terephthalate (PET) is the most valuable of the waste
plastics. Suggest a method to separate it from the other five waste
plastics.
4. Why is it preferred to identify and separate the six plastics during the
recycling process rather than just mixing them together?
5. Waste plastics consist mostly of hydrocarbons. Could they be used as a
fuel source? Using your data from this experiment, would this be
feasible? Would some be preferred over others? Explain and defend
your opinion.
Unknown #2
Unknown #1
Home Sample #2
Home Sample #1
PS
PP
LDPE
PVC
HDPE
PET
Plastic
0.94g/cm3
1:1 Ethanol:Water
1.00g/cm3
1.08g/cm3
Water
10% NaCl
Observation In
PLASTICS – DATA SHEET
I. Density results
Observation In
Plastic
1:1 Ethanol:Water
Water
10% NaCl
0.94g/cm3
1.00g/cm3
1.08g/cm3
PET
HDPE
PVC
LDPE
PP
PS
Home Sample #1
Home Sample #2
Unknown #1
Unknown #2
Summary of density results
< 0.94g/cm3
<1.00g/cm3
<1.08g/cm3
>1.08g/cm3
Ranking of density results
(lowest)_______ _______ _______ _______ _______ _______ (highest)
II, III, IV Melt Test, Ignition Test, Copper Wire Test
Plastic
Melt Test
Ignition Test
PET
Acidic smoke?yes/no_____
HDPE
Acidic smoke?yes/no_____
PVC
Acidic smoke?yes/no_____
LDPE
Acidic smoke?yes/no_____
PP
Acidic smoke?yes/no_____
PS
Home
Acidic smoke?yes/no_____
Copper Wire Test
Sample
#1
Home
Acidic smoke?yes/no_____
Sample
#2
Acidic smoke?yes/no_____
Acidic smoke?yes/no_____
Acidic smoke?yes/no_____
Unknown
#1
Acidic smoke?yes/no_____
Unknown
#2
Acidic smoke?yes/no_____
Identification of Unknown Samples
Home sample #1: Identity__________ Source____________
Home sample #2: Identity___________
Source____________
Unknown 1 number______ Identity__________
Unknown 2 number ______Identity__________