Preserving metals
Left untreated, most metals corrode (rust) when exposed to air and moisture. Three common ways to protect metal are:
| Method | How it works |
|---|
| Painting | A protective coating is applied over the metal, sealing it away from air and moisture |
| Galvanising | Steel is coated with a layer of zinc, which corrodes in place of the steel and so protects it from rust |
| Electroplating | An electric current is used to deposit a thin layer of another metal (e.g. chromium or zinc) onto the surface of a metal object |
Galvanising vs electroplating: both coat a metal with zinc, but galvanising usually dips the object in molten zinc, while electroplating uses an electric current to deposit a much thinner, more even layer of metal — often used for a decorative or corrosion-resistant finish.
Preserving food
Food preservation methods work by removing the moisture, oxygen or conditions that bacteria and other organisms need to grow and spoil food:
- Storing grain — kept in dry, sealed, pest-proof silos or containers so moisture and insects cannot spoil it.
- Pickling — food is preserved in an acidic liquid (vinegar) or a strong salt solution (brine), which stops the growth of the bacteria that cause spoiling.
- Drying and/or salting — removing the moisture that bacteria need to survive and multiply, e.g. biltong, dried fruit, and salted fish.
All food preservation methods share one goal: stop bacteria from growing, by removing water, adding acid/salt, or sealing out air.
Indigenous technology: these aren't modern inventions — they're indigenous knowledge refined over centuries, long before refrigeration existed. Many rural South African communities still store grain in sealed underground pits or clay silos to keep it dry and pest-free, and still sun-dry meat (biltong), fruit and fish using techniques passed down through generations.
Properties of plastics
Plastics are used almost everywhere because of a useful combination of properties:
- Lightweight — much lower density than metal or glass, reducing transport costs and weight.
- Corrosion and moisture resistant — plastics don't rust and are unaffected by water, unlike most metals.
- Electrical and thermal insulator — plastics do not conduct electricity or heat well, which is why they're used for cable insulation and appliance handles.
- Easily moulded — most plastics are thermoplastic: they soften when heated and can be shaped, then re-melted and reshaped again, which is exactly what makes recycling possible.
- Poor biodegradability — plastic can take hundreds of years to break down naturally, which is the main reason reducing, reusing and recycling plastic matters so much.
Types of plastics and recycling codes
Plastic products are marked with a small triangle made of three arrows, containing a number from 1 to 7. This is the plastic identification code, and it tells recyclers exactly what type of plastic the item is made from, so it can be sorted correctly.
| Symbol | Code | Plastic | Common uses |
|---|
| 1 — PET | Polyethylene terephthalate | Cooldrink and water bottles |
| 2 — HDPE | High-density polyethylene | Milk bottles, shampoo bottles |
| 3 — PVC | Polyvinyl chloride | Pipes, hoses, cable insulation |
| 4 — LDPE | Low-density polyethylene | Plastic shopping bags, cling wrap |
| 5 — PP | Polypropylene | Yoghurt tubs, bottle caps |
| 6 — PS | Polystyrene | Disposable cups, foam packaging |
| 7 — Other | Mixed / other plastics | Multi-layer packaging, some bottles |
Reduce, reuse, recycle
This well-known principle guides how we should deal with waste, in order of preference:
- Reduce — use less plastic in the first place, e.g. avoiding unnecessary packaging.
- Reuse — use an item again for the same or a different purpose, e.g. refilling a bottle.
- Recycle — process waste plastic so the material can be made into something new.
Case studies: plastics recycling in action
Remanufacturing waste plastic into pellets: collected plastic waste is sorted by code, cleaned, shredded into small flakes, and melted down. The melted plastic is then extruded and cut into small uniform pellets, which are easy to store, transport and sell to manufacturers as a raw material.
Moulding recycled pellets into new products: these recycled pellets are fed into a moulding machine, heated until soft, and forced into a mould under pressure to form a brand-new product — for example, park benches, plastic crates, or new bottles — completing the recycling loop.
Why sorting by code matters: different plastic types melt at different temperatures and have different properties, so mixing codes together during recycling produces a weak, unreliable final material. Correct sorting is essential for good-quality recycled products.
Systems diagram: the plastics recycling project
Like any process, plastics recycling can be broken down into input → process → output:
Input
Collected waste plastic, sorted by identification code
→
Process
Cleaned, shredded into flakes, melted, then extruded and cut into uniform pellets
→
Output
Plastic pellets sold as raw material to manufacturers, who mould them into new products