Polymer Identification

Classic chemistry experiments

12.

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Identifying polymers

Topic Polymers.

Timing 45 min.

Description The students place samples of plastic into solutions of known density to identify the polymers.

Apparatus and equipment (per group) ▼ Six test-tubes ▼ Test-tube rack ▼ Glass rod ▼ Samples of seven different plastics.

Chemicals (per group) ▼ Solutions 1–6 (see below for preparation) Samples of:

Polymer

–3

Density range/g cm

EPS – expanded polystyrene

0.02–0.06

PP – polypropylene

0.89–0.91

LDPE – low density polyethylene

0.91–0.93

HDPE – high density polyethylene

0.94–0.96

PS – polystyrene

1.04–1.11

PVC – polyvinyl chloride

1.20–1.55

PET – polyethylene terephthalate

1.38–1.40

NB The names commonly used in industry are given here.

Sources of polymers The following product types can be made from the named polymers: HDPE – plastic bottles for milk, fruit juices, household cleaners and chemicals. Motor oil containers, some carrier bags and most aerosol caps. LDPE – Jif lemon juice container. Some squeezy containers for sauces, cosmetics and plastic films – ie shrink wrap, sacks, freezer bags, carrier bags that are not crinkly, disposable pipettes, some aerosol caps, some plant pots and ink-tubes in ball-point pens. PVC – plastic bottles for mineral water, fruit squash, cooking oil and shampoo. Sandwich and cake packs, food packaging trays, DIY blister packs, baby care product containers, cling film, ring-binder covers, records and watch straps.

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Classic chemistry experiments

PS – yoghurt pots, margarine tubs, clear egg boxes, food packaging trays, plastic cutlery and cups, clear plastic glasses, ball-point pen cases, cassette boxes and plastic coathangers EPS – fast food packaging, meat packaging trays and egg boxes. PP – plastic straws, containers for soft cheeses and fats, some margarine tubs, microwaveable food tubs and trays, film bags for crisps, biscuits and snacks, ketchup bottles and bottle caps. PET – most plastic bottles for fizzy drinks, ovenproof food trays and roasting bags, audio and videotape. For different coloured suggestions: Container

Colour

Polymer

Low fat soft cheese tub

Blue

Polypropylene

Lemonade bottle

Colourless and clear

Polyethylene terephthalate

Fabric conditioner bottle

Pink

High density polyethylene

Shampoo bottle

Green

Polyvinyl chloride

Yoghurt pot

White

Polystyrene

Jif lemon juice container

Yellow

Low density polyethylene

Burger box

Gold

Expanded polystyrene

3

Solution

Density

Composition of 1000 cm solution

1

0.79

Pure ethanol (IMS)

2

0.91

471 g (596 cm ) ethanol in 439 cm deionised water

3

0.94

354 g (448 cm ) ethanol in 586 cm deionised water

4

1.00

Deionised water

5

1.15

184 g K2CO3 in 965 cm deionised water

6

1.38

513 g K2CO3 in 866 cm deionised water

3

3

3

3

3 3

A class of 30 students requires approximately 90 cut samples of each polymer. Containers should therefore be easily and quickly cut. Samples should be about 4 mm square and labelled with the name of the container, not the polymer. If all the containers are not different colours, samples could be cut into different shapes. Samples can be recovered at the end of the lesson.

Teaching tips Do not give out the questions (with results) until students have completed the experiment.

Classic chemistry experiments

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Background theory Fingerprint Polymer name

Acronym

Colour

1

2

3

4

5

6

Polyethylene terephthalate

PET

any

S

S

S

S

S

S

Polyvinyl chloride

PVC

any

S

S

S

S

S

–

Polystyrene

PS

any

S

S

S

S

–

–

High density polyethylene

HDPE

any

S

S

S

–

–

–

Low density polyethylene

LDPE

any

S

S

–

–

–

–

Polypropylene

PP

any

S

–

–

–

–

–

Expanded polystyrene

EPS

any

–

–

–

–

–

–

Samples sink if their density is greater than the density of the solution into which they are put. Students can collect waste plastics over several weeks before this lesson. Product containers and their lids are not always made out of the same material so it is important to check their identities prior to the lesson. The identity of polystyrene is –3 confirmed if samples float in a solution of density 1.15 g cm but sink in a solution of –3 density 1.00 g cm . The solutions will need to be made in advance so the plastics can be tested in advance of the lesson. Solutions should be labelled with their densities –3 (given here in g cm at 20 °C). Temperature changes affect densities of solutions so they should be stored together and not in a cold store. Instructions for preparing 3 solutions are as follows for 1 dm of each. A class of 30 students will use 3 approximately 200 cm of each.

Safety Wear eye protection. Ethanol is highly flammable and toxic. Potassium carbonate solid is an irritant, as are concentrated solutions.

Answers 1. To dislodge air bubbles, which may make a ‘sinker’, float. 2. Depends on what is provided.

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Classic chemistry experiments

Identifying polymers Introduction In this experiment solutions with known densities are used to identify the polymers used in everyday materials.

Glass rod

Floaters

Test-tube Samples of plastic containers

Sinkers

What to record Solutions Sample

Colour (or shape)

1

2

3

4

5

6

What to do 1. Fill six test-tubes with solutions 1 to 6 and label each tube. 2. Place a sample of each type of polymer into solution 1. 3. Use a glass rod to stir the contents of the tube. Observe whether the waste plastics float or sink. 4. For samples that sink, write the letter S in column 1 of the results table. 5. Wash the glass rod and dry it on a tissue or paper towel. 6. Repeat the test for solutions 2 to 6. Use a new sample each time.

Safety Wear eye protection. Solutions 1,2 and 3 are highly flammable and toxic. Solutions 5 and 6 are irritants.

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Questions 1. Why were the solutions stirred once the plastics were added? 2. Use the following table to identify the plastics. Fill in the table. Fingerprint Polymer name

Acronym

Colour

1

2

3

4

5

6

Polyethylene terephthalate

PET

Any

S

S

S

S

S

S

Polyvinyl chloride PVC

Any

S

S

S

S

S

–

Polystyrene

PS

Any

S

S

S

S

–

–

High density polyethylene

HDPE

Any

S

S

S

-

–

–

Low density polyethylene

LDPE

Any

S

S

–

-

–

–

Polypropylene

PP

Any

S

–

–

–

–

–

Expanded polystyrene

EPS

Any

–

–

–

–

–

–

Polymer name

Acronym

Polyethylene terephthalate

PET

Polyvinyl chloride

PVC

Polystyrene

PS

High density polyethylene

HDPE

Low density polyethylene

LDPE

Polypropylene

PP

Expanded polystyrene

EPS

Sample