Separating Plastics by Density
Introduction:
When plastics are recycled it is particularly important that they are separated by type.
The numbers on recyclable plastics help this process along, but on a large scale it is
inefficient to hand sort the items. Instead materials are separated by density. We will
mimic this process using liquids and mixtures of various densities to separate out the
plastics by density.
Recyclable plastics are generally separated into 7 categories, the last of which is a
catchall that we will not investigate.
Learning Objectives:
At the completion of this experiment you will be able to devise a procedure to separate
plastics of different density. Given the density of a solid and a liquid, you will be able
predict whether the solid will sink or float.
Materials:
Containers made of the following plastics:
1 -- PETE (Polyethylene terephthalate)
2 -- HDPE (High-density polyethylene)
3 -- V (Polyvinyl chloride)
4 -- LDPE (Low-density polyethylene)
5 -- PP (Polypropylene)
6 -- PS (Polystyrene)
Solutions of the following densities: 0.9, 1.0, 1.1 and 1.3 grams/cm3.
Hazard Information:
Density
grams/cm3
0.9
1.0
1.1
1.3
Composition
Hazard Information
50% 2-propanol
Water
3.5 M NaCl (aq)
4 M CaCl2
Flammable
Low hazard
Low hazard
Low hazard
Waste:
There is no waste from this experiment. All materials are saved for reuse.
Procedure:
Devise your own method to determine the approximate density of the plastics. You will
be provided an alcohol solution, water and the two salt solutions. Plastics that sink
have a density greater than the solution. Plastics that float are less dense than the
solution or are being held on the very top of the solution by surface tension. Push the
piece of plastic below the surface of the solution before making density observations.
CLEANUP: Screw the caps on the bottles. Rinse the pieces of plastic in water and allow
them to dry.
Siena Green Chemistry Summer Institute Lucas Tucker, Ann Klotz V2 Page 1 Student Name:___________________
Date: ___________________
Procedure:
Describe what your method for determining density of the various plastics.
Siena Green Chemistry Summer Institute Lucas Tucker, Ann Klotz V2 Page 2 Observations
Record your observations You may use words or pictures.
Organize your observation into this table.
Plastic
0.9
3
grams/cm
Number
1.0
3
grams/cm
1.1
3
grams/cm
1.3
3
grams/cm
PETE
Polyethylene
Terephthalate
1 HDPE
High-density
Polyethylene
2 V
Poly Vinyl
Chloride
3 LDPE
Low-density
Polyethylene
4 PP
Polypropylene
5 PS
Polystyrene
6 Siena Green Chemistry Summer Institute Lucas Tucker, Ann Klotz V2 Page 3 Lab Questions
1. Which two plastics would be most difficult to separate by density? Why?
2. Why are the samples submerged rather than placed on top of the solutions?
3. Plastics 2 and 4 are both polyethylene. If these materials are made of the same
polymer what is different about them?
Green Question(s)
4. Why is it important to recycle plastics?
5. What waste is generated in the experiment?
References
Vanderbuilt Student Volunteers for Science. “Density Column Using Recycled Plastics”.
http://www.nclark.net/Density_Column.pdf (6/13/14)
Rob Gardner. “Identifying Plastics by Their Densities”. Recycle: Green Science
Projects for a Sustainable Planet. 2001. 24-27.
Environmental Protection Agency, Office of Solid Waste. “Methods to Manage and
Control Plastic
Wastes”. February 1990. (EPA/530-SW-89-051)
The American Plastics Council
1300 Wilson Blvd. 13th Floor, Arlington VA 22209
Siena Green Chemistry Summer Institute Lucas Tucker, Ann Klotz V2 Page 4 TEACHER INFORMATION
Regents Core
Scientific Inquiry S1.1Use theory and /or models to represent an explain observations.
Scientific Inquiry S3.1 Organize observations in a data table.
Teacher Tips:
Put the solutions in small wide mouth bottles.
Cut the pieces of plastic smaller than the opening of the bottle.
Save the solutions for reuse.
Plastic 3 may be difficult to find in bottles. Pipe material may be cut with a saw into
small pieces.
Preparation
Density
grams/cm3
Composition
0.9
50% 2-propanol
1.1
3.5 M NaCl (aq)
Pour 110 mL of 91% isopropyl alcohol into a
graduated cylinder and dilute to 200 mL
Weigh 41 grams of NaCl into a beaker and
then dilute to 200 mL with deionized water.
118 grams of CaCl2.2H2O into a beaker and
then dilute to 200 mL with deionized water.
1.3
Sample Data
Literature
value for
plastic
density
3
grams/cm
Plastic
Number
Alcohol
Solution
0.9
3
grams/cm
Water
solution
1.0
3
grams/cm
NaCl
Solution
1.1
3
grams/cm
CaCl2
Solution
1.3
3
grams/cm
1 sink sink sink float
1.31 2 sink float float float 0.96 3 sink sink sink sink 1.4 4 sink float float float 0.88 PP
Polypropylene
5 float float float float 0.86 PS
Polystyrene
6 sink sink float float 1.05 Plastic
PETE
Polyethylene
Terephthalate
HDPE
High-density
Polyethylene
V
Poly Vinyl
Chloride
LDPE
Low-density
Polyethylene
Siena Green Chemistry Summer Institute Lucas Tucker, Ann Klotz V2 Page 5 Lab Questions
1. Which two plastics would be most difficult to separate by density? Why?
Plastics 4 and 5 have very similar densities and this will make them the most difficult to
separate.
2. Why are the samples submerged rather than placed on top of the solutions?
When plastics are placed on top of the solution, the surface tension of the solution may
be strong enough to prevent them from sinking even though the density of the plastic is
greater than the density of the solution.
3. Plastics 2 and 4 are both polyethylene. If these materials are made of the same
polymer what is different about them?
The atoms in high-density polyethylene plastics are positioned differently than the
atoms in low-density polyethylene plastics.
Green Question(s)
4. Why is it important to recycle plastics?
Recycling plastics reduces the amount of resources needed to produce the plastic and
prevents them from filling the landfills.
5. What waste is generated in the experiment?
All the materials in this experiment are reused. There is no waste generated in this
experiment.
Which of the 12 principles of Green Chemistry were employed to make this lab
greener? VPrevention It’s better to prevent waste than to treat or clean up waste afterwards.
mAtom Economy Design synthetic methods to maximize the incorporation of all
materials used in the process into the final product.
mLess Hazardous Chemical Syntheses Design synthetic methods to use and
generate substances that minimize toxicity to human health and the environment.
VDesigning Safer Chemicals Design chemical products to affect their desired
function while minimizing their toxicity.
VSafer Solvents and Auxiliaries Minimize the use of auxiliary substances
wherever possible make them innocuous when used.
mDesign for Energy Efficiency Minimize the energy requirements of chemical
processes and conduct synthetic methods at ambient temperature and pressure if
possible.
VUse of Renewable Feedstock’s Use renewable raw material or feedstock rather
whenever practicable.
mReduce Derivatives Minimize or avoid unnecessary derivatization if possible,
which requires additional reagents and generate waste.
mCatalysis Catalytic reagents are superior to stoichiometric reagents.
mDesign for Degradation Design chemical products so they break down into
Siena Green Chemistry Summer Institute Lucas Tucker, Ann Klotz V2 Page 6 innocuous products that do not persist in the environment.
mReal-time Analysis for Pollution Prevention Develop analytical methodologies
needed to allow for real-time, in-process monitoring and control prior to the formation
of hazardous substances.
VInherently Safer Chemistry for Accident Prevention Choose substances and the
form of a substance used in a chemical process to minimize the potential for
chemical accidents, including releases, explosions, and fires.1
Siena Green Chemistry Summer Institute Lucas Tucker, Ann Klotz V2 Page 7