Lesson focus: Why does sortingSeparating waste into different types, such as plastics, metals and paper. matter when recycling plasticA material made from long-chain molecules called polymers, used for bottles, bags and toys.?
Describe mechanical recyclingRecycling by shredding, washing and melting old plastic to make new products. and explain why contaminationWhen an unwanted substance gets mixed in, making something less pure. reduces its effectiveness.
Build on earlier learning
MixturesApply the idea that mixtures contain components with different properties.
Practical
Following materials through recycling
- Sort a set of clean waste items by material.
- Trace where each material goes and what it is made into.
- Calculate how much of a 100 kg batch is recovered at each stage.
Record: A flow diagram with masses at each stage.
You will be successful when you can…
- Sequence mechanical recycling.
- Calculate a maximum target mass.
- Explain contamination effects.
- Distinguish thermoplastics from thermosets.
Words that carry the science
- mechanical recycling
- Physical processing of used material into feedstock for new products.
- sorting
- Separating materials into compatible streams.
- contamination
- Unwanted material that reduces purity or process quality.
- thermoplastic
- A polymer material that can soften when heated and harden on cooling.
- thermoset
- A strongly cross-linked polymer material that does not simply remelt when heated.
Track materials through a real recycling sequence
01Sort before reprocessing
Different polymers can melt, flow and cool differently. Mixed streams may produce weak or inconsistent material, so identification and separation matter. So plastics are sorted into types first. If different types are mixed and melted together, the new plastic can be weak.
02Clean and prepare
Food, labels and residues can discolour material, cause odours or disrupt processing. Clean target material is shredded into a more uniform feedstock. Plastics are also cleaned, because leftover food and dirt spoil the recycled plastic: it can be dirty, smelly or poor quality.
03Recycling has limits and losses
A suitable thermoplasticA plastic that goes soft when heated and hard again when cooled, so it can be reshaped. stream may be remelted, but thermosetA plastic that stays hard when heated again and cannot be melted and reshaped. do not simply remelt. Degradation, contaminationWhen an unwanted substance gets mixed in, making something less pure. and product requirements may limit repeated use.
How recycling works
Mixed recycling is collected, then sorted by what it is made of. Each material is cleaned, shredded or melted and made into something new, which saves raw materials and energy.
Controls
Steel is magnetic, so a magnet pulls it out. Aluminium is not magnetic: a spinning magnet (eddy current) pushes it off the belt. Item numbers are illustrative.
Track the target stream
A 100 kg load contains 70 kg target polymer, 20 kg other polymer and 10 kg food residue.
- Start with total mass: 100 kg.
- sortingSeparating waste into different types, such as plastics, metals and paper. identifies 70 kg as target polymer.
- Other polymer and food residue total 30 kg and are not target feedstock.
- Maximum target mass before washing and processing losses is therefore 70 kg.
How to make the reasoning visible: Do not report 70 kg as final product mass because later losses have not been supplied.
Recycling decision table
Decide why each control measure protects the material stream.
| Issue | Evidence | Action | Reason |
|---|---|---|---|
| Mixed polymer | Different softening behaviour | Sort by polymer | Improve compatibility |
| Food residue | Visible/chemical contamination | Wash | Protect quality |
| Thermoset item | Does not soften and flow | Use another route | Cannot simply remelt |
| Dark/weak flakes | Property test fails | Downgrade or reject | Meet product specification |
Why can one-polymer packaging help recycling?
What is one possible trade-off?
Use the table. Why can a thermoset item not simply be melted and reshaped?
Check your understanding
Answer from memory, then check against the suggested answer.
What is mechanical recycling?
What is contamination?
How does a thermoplastic respond to heating?
Why must plastics be sorted before recycling?
Why should food be washed off packaging before recycling?
Why is a food-stained mixed tray not automatically useful recycling feedstock?
Why might recycled polymer be used for a bench but not immediately for food-contact packaging?
Give one reason why reducing plastic use can be better than recycling it.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Plastics are sorted and cleaned before they are recycled. Answer the parts to explain why.
What does sorting separate?
(1)What can happen if different plastics are melted together?
(1)What does cleaning remove?
(1)What would happen to the recycled plastic if it was not cleaned?
(1)Total for this question: 4 marks
Command word: For each step, say what it does and why it matters.