Materials and working their properties 6 Polymers

Learning objectives By the end of this lesson, students should be able to: understand the different classifications of polymers explain the properties and uses of a variety of thermoforming polymers explain the properties and uses of a variety of thermosetting polymers understand how additives can alter the mechanical and physical properties of polymers.

General characteristics of polymers Polymers are mainly produced from crude oil. Most thermoforming polymers are recyclable. Most thermosetting polymers are not recyclable. Generally, polymers have good resistance to corrosion/degradation. Polymers can be relatively easily moulded into shape. Polymers are self coloured. In sheet form, polymers have a flat, smooth and shiny surface.

Classification of polymers Metals are classified into two groups: thermoforming and thermosetting polymers. Thermoforming polymers can be softened with the use of heat and moulded into shape. Thermosetting polymers once moulded into shape, cannot be remoulded with the use of heat.

Types and uses of thermoforming polymers (i) Acrylic (PMMA) – has a hard, shiny and flat surface, but will scratch easily. It can be transparent, translucent or opaque. Used for illuminated shop signs, bath tubs and shower trays. High impact polystyrene (HIPS) – is tough, easily moulded and durable. It is used for yoghurt pots, children's toys and fridge liners. High density polythene (HDPE) – is hard, stiff and resistant to chemicals. It is used for washing up bowls, buckets and crates.

Types and uses of thermoforming polymers (ii) Polypropylene (PP) is tough, durable, good heat and chemical resistance. It is used for children’s toys, DVD/CD cases and medical equipment. Polyvinyl chloride (PVC) is hard, tough, good chemical and weather resistance. It has a low cost due to high volume production. It is used for pipes, gutters and window frames. Polyethylene terephthate (PET) – is tough, durable, food safe, easily moulded. It is used for drinks bottles and food packaging.

Types and uses of thermosetting polymers (i) Epoxy resin – is easily moulded because it is in a two-part liquid form. When mixed, the resin sets hard and has good insulating properties. It is used as an adhesive and for casing electrical components. Melamine formaldehyde – is stiff and hardwearing with good resistance to heat and staining. It is used for kitchen work surfaces. Phenol formaldehyde – is hard, good electrical and heat insulator. It is used for electrical fittings and pan handles.

Types and uses of thermosetting polymers (ii) Polyester resin – is easily moulded because it is in a two-part liquid form. When mixed, the resin sets hard but is brittle. It is used to bind together the glass fibres when producing glass-fibre reinforced polymer (GRP) boat hulls and car body panels, Urea formaldehyde (UF) – is stiff, hard and an excellent electrical insulator. It is used extensively for electrical fittings.

Polymer additives The mechanical and physical properties of polymers can be enhanced by the addition of specific additives. Plasticisers make polymers such as PVC soft and pliable. Pigments can be added to polymers to change their colour. Stabilisers can be added to polymers such as PVC to help them withstand degradation due to ultra violet light. Fillers can be added to polymers to increase their bulk, which improves their resistance to impact.

Homework Complete the table: examine polymer-based products from around your home. Look very carefully at the product – it is likely that it will have the name of the polymer moulded into the product by its initials (e.g. a polymer product made from Polypropylene will have the letters ‘PP’ moulded into it somewhere). Polymer product Thermoforming or thermosetting? Material Why has this material been used?