Year 9 Science · Unit 2 · Lesson 17
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Learning Goals
1. Complete the table to compare the two kinds of polymer.
| Feature | Thermoplastic | Thermoset |
|---|---|---|
| Can it be remelted? | ||
| What holds the chains together? | ||
| An example | ||
| Can it be recycled into the same polymer? |
Order the steps
These steps in recycling PET bottles are jumbled. Number them 1 to 6 in the correct order.
| Order | Step |
|---|---|
| Bottles are sorted by polymer type, so the PET (code 1) is kept together. | |
| The flakes are melted and made into pellets. | |
| A shopper puts an empty PET bottle in the recycling bin. | |
| The pellets are made into a new product, such as fleece fabric or a new bottle. | |
| A truck takes the bottles to a materials recovery facility. | |
| The PET bottles are washed and shredded into small flakes. |
2. Explain why a PET bottle can be recycled into fleece fabric, but a car tyre made of vulcanised rubber cannot be melted down and made into a new tyre. Refer to the structure of each polymer.
Scenario: sorting flakes by floating them
A recycling plant receives a load of mixed plastic flakes. It sorts them in tanks of liquid: a flake floats if it is less dense than the liquid, and sinks if it is denser. Water has a density of 1.00 g/cm³.
| Code | Plastic | Density (g/cm³) |
|---|---|---|
| 1 | PET | 1.38–1.39 |
| 2 | HDPE | 0.95–0.96 |
| 3 | PVC (rigid) | 1.36–1.40 |
| 4 | LDPE | 0.92–0.94 |
| 5 | PP | 0.90–0.91 |
| 6 | PS (solid) | 1.05–1.07 |
Source: American Plastics Council, Hands On Plastics teacher notes (2004). Each plastic has a range because samples vary in how they are made.
3. The flakes are first put into water. Which plastics float and which sink? Use Table 1 to explain.
4. The floating flakes are skimmed off. The plant has two more tanks, with liquids of density 0.915 g/cm³ and 0.945 g/cm³. Describe how it could use them to separate the three floating plastics from each other.
5. (a) Solid PS sinks in water, but expanded PS foam, used for packaging, floats. Use density to explain why. (b) Use Table 1 to explain why no liquid could cleanly separate PET flakes from rigid PVC flakes.
6. A café wants reusable cups that hold drinks up to 100 °C, survive a dishwasher, and can be recycled when they wear out. The choices are polypropylene (a thermoplastic that softens at about 160 °C), LDPE (a thermoplastic that softens at about 110 °C) and melamine-formaldehyde (a thermoset). Evaluate each choice and recommend one.