Thermoforming and Thermosetting Polymers

What polymers are and where they come from

A polymer is a material made from very large molecules containing many repeating units. Familiar plastics are polymers, but polymers also occur naturally: wool, silk, cellulose and proteins are examples. Synthetic polymers are manufactured through chemical processes, commonly using raw materials obtained from crude oil.

The source of a polymer and its behaviour when heated are two different ways of describing it. Natural or synthetic tells you about its origin; thermoforming or thermosetting tells you whether its shape can be changed again after it has been formed.

Two different responses to heat

Thermoforming polymers, also called thermoplastics, soften when heated. They can then be shaped and allowed to cool, when they harden and retain their new shape. Heating and cooling can be repeated without the permanent chemical change associated with setting a thermoset. This makes thermoplastics suitable for recycling by melting and remoulding.

A thermosetting polymer sets permanently during a process called curing. Depending on the material, curing may involve heat or mixing a resin with a hardener. The material undergoes a chemical change and cannot then be softened and remoulded like a thermoplastic. Excessive reheating damages it rather than making it ready to reshape. Thermosets therefore cannot be recycled by melting and remoulding.

Heat resistance does not mean that a polymer remains unaffected at every temperature. For example, polypropylene has useful heat resistance in service but is still a thermoplastic that can be softened for forming.

Understanding the properties used to choose a polymer

A designer needs more than the label ‘plastic’. Different polymers suit different loads, temperatures and surroundings.

• Weight: lightweight polymers can make products easier to carry. Comparing pieces of equal volume helps show differences between materials rather than simply differences in size.
• Hardness: a hard surface resists scratches and indentation, which matters for surfaces that receive frequent contact.
• Elasticity: an elastic material returns to its original shape after being stretched or bent and released. Being easy to bend does not, by itself, prove that a material is elastic.
• Conductivity and insulation: electrical insulation restricts the passage of electric current; thermal insulation restricts heat transfer. These are different properties, so specify which one a product needs.

Stiffness is also useful when comparing polymers: it describes resistance to bending or changing shape under a load. It is not the same as strength. Chemical resistance, weather resistance, surface appearance and cost can also affect the final choice.

The named thermoplastics

The following polymers can all be reheated and reshaped, but they do not have identical properties.

Notice that a polymer name does not always imply one fixed level of flexibility. PVC can be supplied in rigid and flexible forms. Likewise, the properties of HDPE should not automatically be assumed to describe every form of polythene.

The named thermosetting polymers

Thermosets are useful where a product needs a permanently set shape, a durable surface or resistance to heat. Their individual properties still need to match the application.

For epoxy resin, mixing the two parts starts the permanent setting process. It is not simply a liquid thermoplastic that will become liquid again on reheating.

Stock forms and material selection

Polymers are sold as sheet, film, bar, rod and tube. These describe the supplied shape, not a different polymer family.

Sheet is a flat form suitable for cutting or forming; film is very thin and suits wrapping. Bar is solid stock, commonly with a square or rectangular cross-section; rod is solid stock with a circular cross-section; tube is hollow. For example, polystyrene sheet can be chosen for vacuum forming, while polypropylene film can be chosen for food wrapping.

A useful material choice brings together the polymer, its properties and its stock form. For a kitchen-worktop surface, MF's resistance to heat, moisture, scratching and staining gives several relevant reasons for choosing it. For a bucket, lightweight, tough and chemically resistant HDPE is a better match. Neither material is universally ‘best’: each suits a different set of requirements.

Heating behaviour

• Thermoforming / thermoplastic: softens on heating, hardens on cooling; can be reheated and reshaped. Suitable for recycling by melting and remoulding.
• Thermosetting: cures permanently; cannot be melted and remoulded afterwards.
• Origins: natural resources include wool, silk and cellulose; synthetic polymers are commonly made from crude-oil-derived resources.

Thermoplastics to recall