Mix It Up, Sort It Out
Crash! Smash! The recycling truck is seldom quiet. What happens to all
of your plastic and glass when it gets mixed in those bins? The answer:
science!
GRAD
ES
K-4
Collect samples of plastics:
• PETE from a beverage bottle
• HDPE from a milk bottle
• LDPE from a soft, squeezable bottle
• Polypropylene from a medicine bottle or heat-resistant food container
• Polystyrene (Styrofoam) from a plastic coffee cup
• Polycarbonate from a CD
• Plastic bits of an unknown plastic
Cut each sample into similar-sized bits; for the first four categories you can use a manual hole punch to create “punches.” Flatten styrofoam and do the same. Bits of CD of approximately the same size can be prepared by crushing. (Use eye protection
and do this in advance.)
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You’ll also need these materials:
Copies of student pages
Projector or other media to show images
Eye protection for each student
Graduated cylinders (2.50 ml) for each group
Beakers (5), 200 ml or larger, for each group
Isopropyl alcohol (350 ml per group)
Water (400 ml per group)
Sugar (sucrose, 75 g per group)
Stirring rod or plastic spoon (per group)
Balance (accessible to each group)
Samples of unidentified plastics: HDPE, LDPE, Styrofoam, PETE, PP and polycarbonate per group cut to “hole punch”
size, as above.
• Sample of plastic (unknown) for each group
• Camera for extension activity
(The laboratory activity usually requires one class period. Allocate extra time for the reading and discussion of scientific
researchers, and an additional class period for the extension project.)
Show students plastics you’ve collected and then challenge them to add to your collection for a day. The following day, ask
students if the collection has any value. (There are at least three kinds of answers that might be discussed: economic value,
saving energy and saving the planet.)
Ask students to describe what they know about recycling, and if they can identify products which are made from recycled
plastics in the school. (The easiest examples might be in fencing or landscape materials.)
Tell students they are going to investigate the properties of plastics, and how one of those properties can be used to sort them
as a first step in recycling. Since plastics are mixed when collected curbside, the first challenge is to separate them. One basic
property of matter that can be used in the initial separation phase is density.
Review the basics of density with the students. (Research shows this is a difficult concept for middle schoolers so a discussion
of various materials and their densities is always appropriate. This is not an understanding that will be totally achieved in a
single lesson.)
Review the procedures for the lab and insure that students understand the procedures and safety precautions before you
begin. Coach students as they identify and sort plastics by density.
Ask students to develop a flow chart that shows several steps in the recycling process.
The Reading about Recycling: Plastics selection also includes assessment questions.
Ask students to find products made from recycled plastics at their local building supply store or other source. An example and
photo prompt are provided.
Teacher Pages
Mix It Up, Sort It Out
When plastics are collected for recycling, they are generally mixed and may also be collected with denser glass and metals.
Plastics are generally classified into these categories:
Polyethylene Terephthalate, or PETE is used to make many common household items like beverage bottles,
medicine jars, peanut butter jars, combs, bean bags, and rope. Recycled PETE is used to make fabrics and
carpets.
High-Density Polyethylene, or HDPE is quite stable and suitable for foods and drinks. It can be used to contain
milk, motor oil, shampoos and conditioners, soap bottles, detergents, and bleaches, and to make children’s
toys—especially those that might be chewed. It can be recycled make plastic crates, plastic lumber, fencing,
and more.
Polyvinyl Chloride, or PVC is used for plumbing pipes and tiles. It is not commonly recycled, but when it is, it
can be used to make flooring, mobile home skirting, and other valuable products.
Low-Density Polyethylene, or LDPE is both durable and flexible and used to make plastic cling wrap, sandwich
bags, squeezable bottles, and plastic grocery bags. It is not normally recycled in the standard municipal mix,
but when placed in designated containers (often at the stores) it can be used to make garbage cans, lumber,
furniture, and more.
Polypropylene, or PP is strong and can withstand higher temperatures than most plastics. It’s found in plastic
diapers, food containers, prescription bottles, and plastic coffee cups. Only a few municipal systems currently
recycle it, but when it is reclaimed it can be used to make tools.
Polystyrene (Styrofoam), or PS can be recycled but often ends up in landfills because the recycling processes
don’t account for its low density. It’s found in disposable coffee cups, plastic food boxes, plastic cutlery, packing
foam, and packing peanuts are made from PS. It can be recycled to make license plates, rulers and other
durable plastic objects.
If a plastic is unique and doesn’t fit into another category it is classified in this one. Examples include
polycarbonate, used to make water bottles, compact disks and some medical products. It can be recycled to
make plastic lumber.
The activity does not include PVC because few classrooms would have the cutting instruments necessary to break PVC into
small pieces, but it could be added if it can be cut in advance. Do not use beads with holes for the unknown, as the surface
tension of the water makes calculation of density difficult.
Densities of Plastics
In the recycling process, densities can be used as a first step to separate plastics. A simplified version of this process is
demonstrated in the laboratory. In actual recycling other chemicals may be needed.
PETE (Polyethylene terephthalate) 1.38 g/mL
HDPE (High-density polyethylene) 0.95 g/mL
LDPE (Low-density polyethylene) 0.94 g/mL
PP (Polypropylene) 0.91 g/mL
PS (Polystyrene) 1.06 g/mL
Lexan (Polycarbonate) 1.2 g/mL
PVC (Polyvinyl chloride) 1.16-1.38 g/mL
In the activity provided, students develop solutions and use them to separate various bits of plastic. Then students are
provided an unknown sample of plastic to identify by density.
Solution
Amount alcohol
Amount water
1
100 ml
80 ml
40 ml
40 ml
150 ml
2
3
4
5
Amount sugar
Mass
150 ml
150 ml
75 g
Sample Answers
Sample
PETE
HDPE
LDPE
PS
Lexan
PVC
Float or Sink in solution ____
Sinks in all solution
Sinks in all solutions except alcohol
Floats in solutions 3 and 5
Floats in solutions 2, 3 and 5
Sinks in all solutions
Sinks in all solutions
Density
.91 g/ml
.93 g/ml
1.0 g/ml
.78 g/ml
1.14 g/ml
Sample Flow Chart:
High density
materials (metals)
Collect mixed glass,
plastics and metals
at curbside
Clean and break
materials into small
bits
Medium Density
(glass)
Separate by
chemistry
Plastics
(Separate into
Groups by Density)
Separate by heat
(melting)
This activity asks students to hypothesize and then experiment with one possible way to separate the plastics they put in
the bin outside their home or at school for recycling. While it certainly oversimplifies the issue, it represents a way to get
students to understand one possible way to begin the process based on “floating and sinking.” In the activity, watch that
students don’t simply float plastic bits on their flat side.
The first manufactured plastics were created in the 1860’s. “Parkesine” was an organic material derived from cellulose.
Shortly afterward John Wesley Hyatt invented celluloid (the highly flammable material often used for old movie reels).
Students may enjoy researching not only early forms of plastic but the new polymers that are being made from plant materials today. By 1907, one of the earliest forms of plastic used for utensils and toys, Bakelite, was on its way to becoming
commercially successful.
Between 1950 and the present, the global production of plastic has grown approximately nine percent each year. Most are
made from oil, but #2 and #4 can be made from natural gas and increasingly new forms are made from biological materials
like corn. All indications are that the production and use of plastics will continue to grow, even as we move from fossil fuel
(oil) to other sources of organic carbon. That means that the need to ensure these materials are recycled--for their value
and for the environment--is increasingly important.
While this activity asks students to imagine all the ways that recycling plastic is valuable, there’s really no limit to the list.
We not only save raw materials like oil, but save space in landfills and avoid accumulating plastic “trash” that nature cannot break down. Plastic “lumber” is quickly replacing chemically-treated wood for landscaping, eliminating the need to use
hazardous chemicals for treatment. According to the EPA plastic recycling results in a big energy savings (an estimated 50
to 75 million BTUs of energy per ton of recycled material).
The industry is exploding. In 2009, 479 million pounds of non-bottle rigid plastics were recovered--an increase of 47 percent in two years. In 2010, 9.2 billion pounds of plastics were recycled! But there is still a long way to go. We can still find
discarded plastics in the trash and along the road, and some forms of plastic can’t be easily separated here or in other
countries.
Samples of Recycled Materials
Dr. Vilas Pol is a materials scientist who works at Argonne National Laboratory, IL, He has invented a new way to breathe
new life into used plastic bags, plates, and
Composite fencing is made from recycled bottles. This product ↑is from Veranda™
New Balance™ newSKY running shoes are made from recycled bottles.
Wrist bands are often from recycled materials. These are made by imprintitems.com.
Student Pages
Mix It Up, Sort It Out
Collect Data:
Put on your eye protection as you do the following activity.
Find the mass of five empty beakers. Then create solutions of 70 percent isopropyl alcohol, water and sugar as directed
below. After you’ve made the solutions, find the mass of the solution (subtracting the mass of the beaker.) Divide by the
volume to find the density of the solution.
Solution
Amount alcohol
Amount water
1
100 ml
80 ml
40 ml
40 ml
150 ml
2
3
4
5
Amount sugar
Mass
Density
150 ml
150 ml
75 g
Next obtain samples of plastics from your instructor.
Wearing eye protection put a few of each bits of plastic in each solution. Make sure they aren’t flat--floating on the surface-by stirring them in a bit. Categorize the plastics by whether they float or sink in the solutions.
Sample
Float or Sink in solution
Think Like a Scientist:
Density of Plastics
PETE (Polyethylene terephthalate) 1.38-1.39 g/mL
LDPE (Low-density polyethylene) 0.92-0.94 g/mL
PP (Polypropylene) 0.90-0.91 g/mL
PS (Polystyrene) 1.05-1.07 g/mL
Polycarbonate (Lexan) 1.2 g/mL
HDPE (High-density polyethylene) 0.95-0.96 g/mL
Which Samples Might Be In These Categories?
With Your Group: Discuss how the process you’ve used might be used to separate plastics. Summarize the process here
in words and in a concept map:
Go Farther:
Explore some of the materials that can be made from recycled plastic. Then take a camera and look for products using
those materials in your school or home.
Prepare a communication for your community that shows the advantage of re-using plastics, using your photos as
illustrations.
This bridge in Scotland is being built of a recycled structural composite material made entirely of recycled plastic from bottles. Image source:
Axion International
READ ABOUT A PLASTICS RECYCLER [Hyperlink Biddle Here] and RECYCLING [Here]