Identifying polymers by density

In this experiment, students place samples of several polymers found as everyday plastics into a range of liquids of known density. They observe whether these samples float or sink, and use this information to identify each polymer using a table of known polymer densities.

This experiment needs some detailed preparation in advance, but the actual class experiment should be straightforward. If all working groups attempt to identify all the polymers in the plastics provided, it could take up to 45 minutes. However, more effective use of time may be possible by allocating a selection of, say, four different plastics to each working group and pooling class results.

Equipment

Apparatus

• Eye protection
• Boiling tubes, x6 (see note 5 below)
• Boiling tube rack
• Glass stirring rod
• Scissors capable of cutting plastic samples
• Paper towels
• Method for labelling boiling tubes

Chemicals

• Access to liquids of known density, 1–6 (see note 6 and additional notes on preparing the solutions below)
• Samples of plastics for testing (see notes on choosing and collecting samples)

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Ethanol (IDA – industrial denatured alcohol, formerly IMS), C₂H₅OH(l), (HIGHLY FLAMMABLE, HARMFUL) – see CLEAPSS Hazcard HC040A.
• Potassium carbonate, anhydrous, K₂CO₃(s), (IRRITANT) – see CLEAPSS Hazcard HC095A. Note that the higher concentration solution of potassium carbonate, being more concentrated than 1.8 M, is also classed as IRRITANT.
• Large, 150 x 25 mm, test tubes.
• Note that ethanol/water solutions are classed as FLAMMABLE at a concentration of 50% ethanol by volume, and may be FLAMMABLE  in certain circumstances even at a concentration of 45% ethanol. Therefore both ethanol solutions used in this experiment should be labelled FLAMMABLE, HARMFUL even though the 45% solution under the conditions used in this particular experiment may not be easily flammable.

Preparing the solutions

Details for preparing the solutions of known density are as follows, for 1 dm³ of each. Each working group will need approximately 15 cm³ of each liquid (so 15 working groups might use 200–250 cm³ in total). If carefully controlled, the solutions should be recoverable afterwards to be stored for reuse.

Liquid	Density in g/cm3	Composition of solution (100cm3)
1	0.79	Ethanol (IDA)
2	0.91	471 g (596 cm3) ethanol in 439 cm3 deionised / distilled water
3	0.94	354 g (448 cm3) ethanol in 586 cm3 deionised / distilled water
4	1.00	Deionised / distilled water
5	1.15	184 g potassium carbonate in 965 cm3 deionised / distilled water
6	1.38	513 g potassium carbonate in 866 cm3 deionised / distilled water (IRRITANT at this concentration)

Choosing and collecting samples for testing

Everyday items will be a useful source of polymers or plastics for testing. Most of these will carry the recycling symbol with an identification number and code, as shown in the image below.

Common examples of objects made using each plastic include:

• Polyethylene terephthalate, PET – most plastic bottles for fizzy drinks, ovenproof food trays and roasting bags, audio and videotape. 
• High density polyethylene, HDPE – plastic bottles for milk, fruit juices, household cleaners and chemicals. Motor oil containers, some carrier bags and most aerosol caps.
• Polyvinyl chloride, 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.
• Low density polyethylene, LDPE – ‘Jif’ lemon juice container. Some squeezy containers for sauces, cosmetics and plastic films (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. 
• Polypropylene, 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. 
• Polystyrene, 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 coat-hangers. 
• Expanded polystyrene, EPS – fast food packaging, meat packaging trays and egg boxes. (No code necessary.)

It may be of help to students if each sample has a different colour of plastic, for example:

Container	Colour	Polymer
Low fat soft cheese tub	Blue or white	Poly(propene)(Polypropylene)
Lemonade bottle	Clear and colourless	Poly(ethyleneterephthalate)
Fabric conditioner bottle	Pink	High density poly(ethene)(Polyethylene)
Shampoo bottle	Green	Poly(chloroethene)(Polyvinylchloride)
Yoghurt pot	White	Poly(phenylethene)(Polystyrene)
‘Jiff’ lemon juice	Green	Low density poly(ethene)(Low density polyethylene)
Burger box	Yellow/Gold	Expanded poly(phenylethene)(Expanded polystyrene)

Each working group will need six samples about 4 mm square of each plastic, one to be added to each of the six solutions. Each plastic can either be provided as a larger flat, thin sheet with scissors to cut the samples, or as ready cut samples.

Sheets or samples, if not identifiable by colour (see table above), will need to be labelled with the name of the container they come from, or with a code letter or number, or even cut into different shapes for identification. They should not bear the name of the plastic/polymer. Also note that 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.

Samples can be recovered at the end of the lesson for subsequent reuse if desired. On the other hand, students can be asked to collect waste plastics in the weeks before this lesson.

The plastic samples should be tested in advance of the lesson to check they float and sink as expected in the different solutions. Solutions should be labelled with their densities (given here in g cm^–3 at 20 °C). Temperature changes affect densities of solutions, so they should be stored together at room temperature and not in a cold store.

Density ranges for different polymers

The densities of the seven polymers are somewhat variable. The table below gives the expected normal range of variation for each:

Polymer	Density range  / g cm-3
EPS, expanded polystryrene	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, polystryrene	1.04-1.11
PVC, polyvinyl chloride (PVC)	1.20-1.55
PET, polyethylene terephthalate	1.38-1.40

Procedure

1. Add about 15 cm³ of each of liquids 1–6 to a different boiling tube in a test tube rack. Label each tube with the relevant solution number .
2. Prepare six samples of each of the plastics to be tested. Each sample should be a square of approximately 4 x 4 mm. You may organise your samples by laying them out in separate piles on a paper towel. Make sure you know which sample is which, either by labelling each with a code letter, or by noting the colour. Record these in a suitable table such as:

	Observations for floating/ sinking
Sample code/ colour	1	2	3	4	5	6	Identity of plastic

1. Add samples of each plastic to each of the six solutions so that each tube has samples of six different plastics.
2. Use a glass rod to stir the contents of each tube washing and drying the glass rod between each tube. Observe whether the plastics float or sink. A sample will sink if its density is greater than the density of the solution.
3. For samples that sink write the letter S in the appropriate rows and columns of the results table. You may wish as well to write the letter F for those that float so that you can check you have noted every sample tested.
4. In discussion with the teacher and the rest of the class you should be able to identify the polymer from which each plastic is made. When you have decided what a sample is made of write the name in the last column of your table.

Teaching notes

Teachers will need to decide whether to use the names commonly used for these plastics as in this table or to use their correct polymer names if the students are familiar with them. This is a good opportunity for such students to become more familiar with different naming systems in everyday use in industry and in formal chemistry:

Common name	Abbreviation	Chemical name for polymer
Polystyrene	PS	Poly(phenylethene)
Expanded polystyrene	EPS	Poly(phenylethene)
Low density polythene or polyethylene	LDPE	Poly(ethene)
High density polythene or polyethylene	HDPE	Poly(ethene)
Polyvinyl chloride	PVC	Poly(chloroethene)
Polypropylene	PP	Poly(propene)
Polyethylene terephthalate	PET	Poly(ethenediyl-1 4-benzenedicarboxylate)

As an example the identity of polystyrene will be confirmed if samples float in a solution of density 1.15 g cm^–3 but sink in a solution of density 1.00 g cm^–3. An ideal table of results will look like (using the recycling number codes in column 1 except for EPS):

	Observations for floating/sinking
Sample code/colour	1	2	3	4	5	6	Identity of plastic
1	S	S	S	S	S	S
2	S	S	S	F	F	F
3	S	S	S	S	S	F
4	S	S	F	F	F	F
5	S	F	F	F	F	F
6	S	S	S	S	F	F
EPS	F	F	F	F	F	F

Air bubbles adhering to samples can give serious problems as they may be difficult to dislodge causing ‘sinkers’ to remain afloat. This is the purpose of stirring with the glass rod. Students can be asked why the stirring is done after they have added the samples to test tubes and given a first stir. The discussion should then result in students checking for any remaining air bubbles and if necessary stirring again to dislodge them.

Once the experiment is complete students can be given the table of polymer density ranges above and asked to identify the samples and write their names in the final column of their results table. Note that materials made of polymers may also contain other substances as fillers, plasticisers and stabilisers, which may make the density of a particular sample fall outside the ranges indicated; note also the wide range of PVC densities in the table.