Thermoforming Polymers and Thermosetting Polymers

Thermoforming Plastic

Thermoforming plastics generally make the bendy types of plastic, they aren’t very resistant to heat so can be melted easily. They are easy to recycle by grinding them down into pellets and reforming them:

  • Polythene HDPE (high density): Milk crates, buckets, plates.

  • Polythene LDPE (low density): Food packaging, carrier bags, washing up and shampoo bottles.

  • Polypropylene PP: Syringes, reusable food containers.

  • High impact polystyrene HIPS: Casing on vacuum cleaners, radios etc

  • Nylon: Hinges, combs, clothes.

  • Poly vinyl chloride PVC: Pipes, shoe soles, tablet packaging.

  • Acrylic (Perspex): Baths, machine guards.

  • Styrofoam: Blue board foam used as insulation in walls when building houses.

  • Acrylonitrile-butadiene-styrene (ABS): Lego bricks.

  • Urethane/polyurethane: Foam for upholstery and sponges.

  • Fluroelastomer: Synthetic rubber, engine gaskets, sports watch straps.

Introduction

To put it basically plastics fit into two different types:

Thermoforming plastics (polymers), can be heated and formed repeatedly.

Thermosetting plastics (polymers), once heated, cannot be re melted.

The science bit:

Synthetic plastics all have a starting point of carbon – oil, gas or coal. The process of chemically making plastic is all about linking chains of monomers to create polymers.

Thermosetting plastics are interlinked like a net which, makes them more rigid. Thermoforming plastics are linked together as long chains making them easy to reheat and remould.

Polymers:

Plastics (polymers) come from crude oil. The oil is mined from underground and then refined in a process called fractional distillation separating the oil into chemicals called fractions. These fractions can be split into monomers, when monomers or joined together by chemicals they become polymers – polymerisation. Some fractions are too big to be joined so are heated in a process called cracking, this makes them small enough to be polymerised. Once polymers are chemically formed they can be moulded. They are available in sheet, film, bar, rod and tube.

Just to add to the confusion there are several subtypes in those two sorts of plastic.

Thermoforming Polymers and Thermosetting Polymers, figure 1

Thermosetting Plastic

Thermosetting plastics are more rigid, they are highly resistant to heat which makes them suitable for electrical parts and pan handles. They are often cast into shapes from a liquid mix which hardens and cannot be reformed:

  • Melamine: Camping plates, worktops.

  • Epoxy resin: A two part resin and hardener that sets when mixed, Araldite glue, casting resin.

  • Phenol formaldehyde: Pan handles, bottle tops.

  • Urea formaldehyde: Plug sockets, electrical switches, door handles.

  • Polyester resin: Car bodies, boats.

The world price of oil has a massive knock on effect on the price of polymer items. As oil is a diminishing resource which is controlled by a few countries there is a very competitive market where prices can fluctuate daily.

Other plastics are being developed which have a much better environmental footprint, these will have biodegradable and compostable features to try and minimise its effects on the environment. Biopol is the brand name for PHB (polyhydroxybutyrate), produced by fermenting sugars from sweet potato, pee starch, soya and vegetable oil. It is compostable and degrades in a couple of months.

In your exam you need to try and remember the difference between the two and be able to quote some examples. Help try to figure out which is which by having a look around your house and categorise the plastic you find using the descriptions above. Look closely at bottle labelling, it often tells you what type of plastic it is.