Polymer Origins, Types and Properties

From small molecules to useful materials

A polymer is a large molecule made from many repeating units. The small molecules used to build it are called monomers. During polymerisation, monomers join chemically to form long polymer molecules. Think of individual beads being connected into a long necklace: joining changes separate small units into a much larger structure.

Polythene, also called polyethylene, is an example. Its starting monomer is ethene; many ethene molecules join to form polythene. The resulting material has different properties from its starting substance. This is a chemical process, not simply melting or mixing ingredients.

Polymers can have natural or synthetic origins. Natural polymers occur in living things: cellulose is found in plants, while wool and silk contain proteins. Natural resources such as cellulose can provide the starting material for plastics. Synthetic polymers are manufactured through chemical processes, commonly using raw materials obtained from crude oil. Polythene, PVC and nylon are examples.

‘Natural’ describes the origin, rather than guaranteeing that a material is suitable for a particular product. Designers still need to consider its properties. Also, not every natural polymer is a plastic: wool is a polymer material, but it is not a moulded plastic casing.

Two families with different responses to heat

Thermoforming plastics, also called thermoplastics, soften when heated. They can be shaped and then cooled so that they retain the new shape. Heating can soften them again, allowing repeated reshaping without the shaping cycle itself causing a chemical change. This makes remelting possible for recycling.

Thermosetting plastics set permanently during their initial forming or curing process. Once cured, they cannot be softened and remoulded by reheating. They therefore cannot be recycled by the same melt-and-remould route as thermoplastics.

Curing is not identical for every thermoset. Epoxy resin, for example, is supplied as resin and hardener: mixing the two starts the process that produces the set material. The important distinction is between a thermoplastic’s reversible softening and a thermoset’s permanent setting.

Reading a plastic’s properties

A material’s physical properties include how it transfers heat and electricity. Most common plastics are poor conductors of both, making them useful insulators.

Low thermal conductivity means that heat travels through the material slowly. Heat resistance means that the material can withstand elevated temperatures without losing its useful properties. These are not interchangeable: a material may slow heat transfer but still soften when it becomes hot.

Electrical insulation means resistance to the passage of current. A plug casing needs to insulate the user from the conducting parts inside it; the casing and the internal metal contacts therefore need different properties.

Mechanical properties describe how materials respond to forces:

• Tensile strength is resistance to pulling forces. It matters when a component is stretched or loaded in tension.
• Toughness is the ability to withstand impact without breaking.
• Hardness is resistance to scratching, indentation and surface wear. A hard material is not necessarily tough: acrylic can be hard yet brittle.
• Plasticity is the ability to change shape permanently without breaking. Acrylic and polythene have useful plasticity when heated.

These properties must be considered at the temperature at which the product will be made and used. A plastic that is stiff at room temperature may become easy to shape when heated.

Selecting common thermoforming plastics

A useful material choice connects a property to a product requirement, rather than simply calling the material ‘strong’ or ‘good’.

Polystyrene is another thermoforming polymer. It is inexpensive and available as sheet or foam, with opaque coloured and transparent forms. Polystyrene sheet is used for vacuum forming in schools: the ability to soften and take a new shape is central to this use.

Polystyrene foams: EPS and Styrofoam

Foaming changes how a plastic behaves by creating a structure containing many gas-filled cells. Polystyrene foams are very light and provide thermal insulation because the trapped gas slows heat transfer.

Expanded polystyrene (EPS) is made from beads that expand and are fused together in a mould. It is the familiar beaded foam used for protective packaging and insulation boards. It is inexpensive, lightweight and can be moulded into packaging shapes or cut from blocks into boards.

EPS has limitations: it is combustible, and the small spaces between its fused beads can allow water uptake during prolonged wet exposure. Standard EPS also has lower compressive strength than XPS, although its performance varies with grade. It is useful where low cost, low weight and insulation matter, but persistent moisture or heavy loading can make another material more suitable.

Styrofoam is a trade name for extruded polystyrene (XPS), rather than a technical name for EPS. XPS is made by extruding a foamed polystyrene mixture through a shaped opening to produce a continuous board. Its uniform closed-cell structure gives it good thermal insulation, greater resistance to water absorption and higher compressive strength than standard EPS.

These properties make XPS useful for insulation in damp or load-bearing situations. Its disadvantages include higher cost and a smooth surface that can make adhesive bonding more difficult. The choice is therefore a trade-off: EPS offers economical insulation, while XPS offers better moisture and load resistance where those properties are needed.

Selecting common thermosetting plastics

Thermosets are useful where a permanently set material and particular resistance or insulation properties are needed.

For these materials, permanent setting is useful when the finished shape or bond must remain stable. It is a limitation if the designer needs a product that can later be reshaped by heating.

Making a justified choice

Start with the product’s job, then identify the property needed. For an outdoor window frame, weather resistance makes rigid PVC a suitable choice. For a plug casing, electrical insulation makes urea formaldehyde suitable. For a kitchen worktop surface, melamine’s resistance to heat, moisture, scratches and stains addresses several demands at once.

A choice also involves trade-offs. Acrylic offers useful heated plasticity and weather resistance, but scratching and brittleness may limit its suitability. Polycarbonate is a better candidate where impact resistance is the priority, despite its lower hardness. Thermoplastics can be reheated for reshaping, but that same response to heat must be considered during use. Thermosets retain their permanently set form, but cannot be remoulded in this way.

Finally, consider the material’s supplied form. Polymers are available as sheet, film, bar, rod and tube. The appropriate form depends on the component and how it will be made; a thin film and a rigid sheet do not serve the same purpose.

Origins and formation

• Natural polymers: cellulose and proteins, including those in wool and silk.
• Synthetic polymers: manufactured chemically, commonly from crude-oil-derived raw materials.
• Polymerisation: monomers join chemically to form . Ethene forms polythene.