MYP Design Polymers and Plastics Notes

Heat Is the Test That Splits the Polymer Family

1. A polymer is a long chain built from the same small molecule repeated thousands of times, usually made from crude oil.
2. Put a polymer in an oven and it does one of two things, and which one it does decides everything else about it.
3. A thermoplastic goes soft and rubbery, can be pushed into a new shape, and hardens again when it cools.
4. Its chains are tangled together but not tied, so heat lets them slide past each other like cooked spaghetti.
5. A thermosetting plastic does not soften at all: heat it enough and it chars, cracks or burns.
6. Its chains are joined by cross links, chemical bridges that form once during curing and never let go.
   1. Curing is one-way, the same way a cooked egg cannot go back to raw.

Five Thermoplastics You Will Meet in the Workshop

1. Acrylic, also sold as PMMA or Perspex, is glass clear, takes a polished edge and laser cuts beautifully, but it cracks if you drill too close to an edge.
   1. 3 mm acrylic softens around 160 degrees Celsius, which is why a strip heater can fold it into a phone stand in about four minutes.
2. HIPS, high impact polystyrene, is cheap, opaque and tough, and it is the standard sheet for vacuum forming in schools.
3. PP, polypropylene, survives being bent back and forth thousands of times, which is why it makes the living hinge on a shampoo bottle lid.
4. PET is clear, strong and holds pressure, so it becomes fizzy drink bottles, and it is the most widely recycled polymer there is.
5. ABS is tough, slightly flexible and takes a screw thread, which is why it forms 3D printer filament, plug sockets and protective cases.

Thermosets Take the Jobs That Involve Heat or Chemicals

1. Polyester resin arrives as a syrup, sets solid when a catalyst is stirred in, and is what holds glass fibre in a boat hull or a pond liner.
2. Epoxy resin comes as two parts you mix in a set ratio, cures harder and sticks to more surfaces than polyester, and is the glue in Araldite.
3. Urea formaldehyde makes pale electrical fittings, and melamine gives worktops and picnic plates a hard scratch-resistant face.
4. All of them stay rigid at temperatures that would slump a thermoplastic, which is why a saucepan handle is a thermoset and a drinks bottle is not.
5. The price is permanence: a cured thermoset cannot be melted down, so it is ground into filler or sent to landfill.

Recycling Codes Tell You What Happens Next

1. Codes 1 and 2 are collected almost everywhere, while 3, 6 and 7 are frequently rejected and burned or buried instead.
2. Mixing polymers in one part wrecks recycling, so a bottle of PET with a PP cap and a PVC label has to be pulled apart first.
3. Designing a product from a single polymer, or making parts easy to separate by hand, is a real sustainability decision you can defend.
4. The triangle of arrows moulded into a plastic part carries a number from 1 to 7 that names the polymer.

School Workshops Run on Thermoplastics for Three Reasons

1. Thermoplastics reheat, so a bad bend on a strip heater can be flattened and folded again instead of thrown away.
2. They soften at temperatures a strip heater, oven or vacuum former reaches easily, roughly 120 to 180 degrees Celsius.
3. They arrive as flat sheet in useful sizes and thicknesses, which suits laser cutting and line bending in a single lesson.
4. Thermosets need mixing, measuring, moulds and curing time, and a mistake costs you the material and the mould together.
5. Sheet thermoplastic also scales honestly: the same acrylic you fold by hand is injection moulded in a factory, a process a sibling article covers.

Choosing a Polymer Means Naming the Grade, Not the Colour

1. Start from the job: does the part need to be clear, tough, food safe, flexible, or stable in heat?
2. Clear and decorative points to acrylic, impact and cheap points to HIPS, repeated flexing points to PP.
3. Then fix the thickness, because 2 mm acrylic bends under a hand load where 5 mm stays rigid.
4. Polymers are light and waterproof but soften in sunlight over years, so an outdoor part needs a UV stable grade or a shaded position.

Telling the Two Types Apart Safely

1. Check the label or the moulded recycling code first, since that answers it without any risk.
2. If there is no mark, ask what the product does: anything that must survive a flame, an element or a hot pan is almost certainly a thermoset.
3. Never test by burning a sample, because several polymers give off genuinely toxic smoke.