Salvaged thermoplastic can be sorted, cleaned, shredded, melted and molded again, and the order matters: identify and separate each plastic first, because mixed plastics give a weak, brittle product and PVC gives off corrosive gas when heated. Thermosets such as Bakelite cannot be remelted and can only be ground for filler. Each remelt weakens the plastic, so the carrier’s rule of thumb is to blend in fresh material or filler after 5-7 cycles.
Part of Plastics and Polymers
Plastic recycling is the identification, sorting, cleaning and remelting of used thermoplastic into new products without chemical synthesis.
A Collapsed Civilisation Leaves A Large Stock To Reuse
Bottles, jugs, crates, drums, pipe, sheeting and cable insulation survive for decades, and most of it is thermoplastic. Learning to identify, sort and reprocess it extends your material supply without making new plastic, which is why recycling is the practical heart of this section. A settlement is far more likely to be remelting plastic than synthesising it: see Petroleum Cracking.
The Chain Of Steps
- Identify and sort by type. Sorting and Identification. Do this before any heating.
- Clean thoroughly. Dirt, food residue, glue and labels contaminate the melt and leave streaks and weak points.
- Shred into small pieces, 5-10 mm per Plastics Manufacturing, so that melting is even.
- Melt at the correct temperature for the type: Remelting.
- Shape by extrusion or molding, as with new material.
What Is Worth Salvaging, And What Is Not
| Salvage | Verdict |
|---|---|
| Polyethylene and polypropylene (jugs, crates, drums, caps) | Best: easy to melt, forgiving, floats in water |
| Polystyrene (rigid clear or foam packaging) | Usable but brittle; foam needs compaction |
| PET (drink bottles) | Harder to reprocess; keep separate |
| PVC (pipe, conduit, window frames) | Reuse whole as pipe or conduit; do not melt in quantity |
| Bakelite and other thermosets | Reuse whole parts; grind scrap only as filler |
| Anything that carried fuel, solvent or chemicals | Reject for water or food use |
| Unknown plastic | Treat as unidentified: do not heat |
The corpus applies the same limit to pipe in Salvaged Modern Pipe: reject anything that carried fuel or chemicals. Salvaged plastic can also contain additives such as plasticisers, stabilisers and flame retardants that you cannot see. Do not make drinking vessels or water pipe from plastic whose history you do not know.
Worked Example: How Many Jugs Make A Pipeline
HDPE pipe with a 25 mm outside diameter and a 2.5 mm wall has an inside diameter of 20 mm. Its wall area is π/4 × (25² - 20²) = 0.7854 × 225 = 176.7 mm². One metre has a volume of 176.7 mm² × 1,000 mm = 176,700 mm³ = 176.7 cm³.
With an assumed density of 0.95 g/cm³ (a typical figure, not a corpus number) one metre weighs 176.7 × 0.95 = 168 g. A 100 m line needs 100 × 168 g = 16.8 kg. If a salvaged jug weighs about 60 g (an assumed figure), that is 16,800 / 60 = 280 jugs, before allowing for waste in cleaning, trimming and start-up. The point is the scale: a pipeline is hundreds of containers, so sorting must be efficient.
Quality Falls With Each Cycle
Each cycle slightly degrades the plastic, because heat and shear break the chains. After 5-7 cycles, mix in fresh material or filler to hold strength. The 5-7 figure is the carrier’s, unsourced, so test a sample of your own material (see Safety and Quality). Keep regrind from gates and runners in the same type batch rather than letting it wander.
Common Mistakes
- Mixing types to save sorting — weak, brittle product.
- Including PVC with polyethylene — fume, corrosion of equipment, black specks in the product.
- Skipping cleaning — contaminants burn and stain the melt.
- Overheating to hurry — degradation and fume.
Fume from heating unidentified plastic
Heated PVC releases hydrogen chloride, heated PTFE gives an acutely toxic fume, and nitrogen-bearing plastics can release hydrogen cyanide. You must identify a plastic before you heat it. Work outdoors or with strong ventilation. Hot melt sticks to skin and burns deeply. See Fume Safety.
What’s Next
- Learn next: Sorting and Identification — the step that makes the rest safe
- Combine with: Pipes and Containers — what to make from the regrind