Polymer Properties and Applications
You'll be able to classify polymers as thermoplastic or thermoset based on properties.
In 1938, Roy Plunkett at DuPont accidentally created Teflon, a fluoropolymer so chemically inert that nothing sticks to it, with a melting point of 327 °C that no ordinary thermoplastic could match.
Printable Worksheets
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Q1 · A plastic shopping bag is flexible and stretchy, but a plastic chair is rigid and stiff, if both are made of polymers, what do you think causes such different physical properties?
Q2 · Why do you think knowing the exact properties of a polymer (e.g. flexibility, melting point, strength) would be important before choosing it for a specific application like food packaging or a car part?
● Know
- The difference between thermoplastics and thermosets
- How cross-linking affects polymer properties
- How HDPE and LDPE differ in structure and application
● Understand
- Why cross-linked polymers cannot be melted and reshaped
- Why chain packing (HDPE vs LDPE) determines flexibility
- Why different polymer structures are suited to different applications
● Can do
- Classify polymers as thermoplastic or thermoset based on properties
- Explain how cross-linking causes rigidity
- Match polymer types to appropriate applications based on their properties
Drop a PET water bottle into a bin and it can be shredded, remelted at 260 °C, and re-extruded into fibres for a fleece jacket, the same atoms, rearranged, finding a new use. Thermoplastics are polymers whose chains are held together only by weak intermolecular forces (van der Waals forces and, for polar polymers, dipole–dipole attractions). Because no covalent bonds connect adjacent chains, heating the polymer to above its melting point provides enough energy to overcome these weak forces, allowing the chains to slide past each other, the plastic becomes fluid and can be moulded. On cooling, the chains freeze in their new positions. Thermoplastics can be softened and remoulded in mechanical recycling, although repeated heating and processing can degrade some polymers and affect their properties. Common thermoplastics: polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), and polyethylene terephthalate (PET, used in drink bottles).
The processing methods for thermoplastics exploit their melt behaviour: injection moulding (for car parts, toys, containers), extrusion (for pipes, films, fibres), and blow moulding (for bottles). Because thermoplastics can soften and flow when heated, they can often be mechanically reprocessed. Whether an item is recycled into a useful product depends on collection, sorting, material condition, process quality and available markets. Local kerbside acceptance varies; check the relevant council or scheme guidance.
A PET water bottle (thermoplastic, resin code 1) collected via NSW Return and Earn is shredded into flakes, melted at 260 °C, filtered, and extruded into pellets. The pellets are injection-moulded into preforms, which are then stretch-blow-moulded into new bottles. Repeated melt processing can cause PET chain degradation, reducing average molecular weight and intrinsic viscosity and potentially affecting material properties. The extent depends on factors such as drying, temperature, residence time, contamination and process controls; there is no single cycle limit or fixed strength loss that applies to every PET stream.
Pact Group's Smithfield (NSW) plant recycles over 50,000 tonnes of thermoplastic per year, PET, HDPE, and PP from NSW kerbside and Return and Earn streams. Their thermoplastic structure allows remelting, while successful recycling also depends on the quality and cleanliness of collected material and the available reprocessing route. Cross-linked thermosets generally cannot be remelted into their original shape, though some can be ground for use as filler or processed through other recovery methods.
Thermosets are polymers in which the chains are permanently connected to each other by covalent cross-links chemical bonds that form during the curing (setting) process. Once cured, the polymer is a single giant interconnected network extending throughout the entire material. These covalent cross-links cannot be broken by heating (short of destroying the polymer chemically). When a thermoset is heated, the cross-links prevent chain movement, the material simply degrades (chars) rather than melting. Thermosets cannot be remoulded once set.
The covalent network structure gives thermosets excellent properties: high hardness, dimensional stability (they don't creep or deform under load at high temperature), and excellent chemical resistance. Common thermosets: Bakelite (phenol formaldehyde resin, used in electrical fittings since 1907), epoxy resins (structural adhesives, circuit boards), melamine formaldehyde (kitchen bench tops, Formica laminates), and vulcanised rubber (car tyres). Each of these materials owes its properties to the covalent cross-linked network, and each cannot be recycled back into the same polymer.
Epoxy resin adhesive: Part A (epoxy monomer) + Part B (amine hardener) react to form a 3D cross-linked network. Once cured, the adhesive has shear strength >20 MPa, it cannot be softened or dissolved. Engineers at Sydney Metro used epoxy resin to bond aluminium cladding panels to tunnel walls, where remeltability would be dangerous in a fire.
Huntsman Corporation's Port Botany (Sydney) facility manufactures epoxy resins used in wind turbine blades, marine composites, and electrical insulators across Australia. The thermoset nature of cured epoxy is essential for wind turbines, the blades experience temperatures from −10 °C to +60 °C and cannot soften or creep over 25 years of operation.
The thermoplastic/thermoset distinction is the most important factor in polymer recyclability. Thermoplastics can be remelted, but that alone does not ensure an item will be collected or recycled. Australia uses resin identification codes 1–7 to identify the main plastic type: 1 = PET, 2 = HDPE, 3 = PVC, 4 = LDPE, 5 = PP, 6 = PS, 7 = other. Local collection rules and recycling options vary; check the relevant council or scheme guidance rather than assuming a code guarantees kerbside acceptance.
Thermosets cannot be conventionally recycled into the same polymer. They must either be landfilled, incinerated for energy recovery, or ground into powder and used as filler in concrete or road base, a downcycling process. This is a significant waste challenge: every fibre-reinforced epoxy wind turbine blade (50+ tonnes) and every car tyre is a thermoset destined for downcycling at end of life. Research at UNSW and the University of Queensland is investigating chemical depolymerisation of thermosets to recover the original monomers, but this remains expensive and not yet commercial at scale.
Car tyre (thermoset vulcanised rubber): cannot be remelted. End-of-life options in NSW: retreading (extends life), granulation into rubber crumb for playgrounds and running tracks, pyrolysis into fuel oil, or landfill (banned in NSW since 2014 for whole tyres). Over 55 million waste tyres are generated in Australia per year, the thermoset structure is the entire reason disposal is challenging.
Hunter Valley manufacturer ResourceCo processes waste tyres using pyrolysis, heating in the absence of oxygen, to produce fuel oil, carbon black, and steel wire. NSW Environment Protection Authority grants a resource recovery order for this process, recognising it as a higher-value alternative to landfill, but it recovers energy rather than recycling the polymer, a direct consequence of the thermoset structure.
Thermoplastics can be recycled because they can be . Their chains are held together only by weak forces, not covalent cross-links. Australia labels plastics with resin numbered 1 to 7. Most kerbside programs accept codes 1, 2, and sometimes . cannot be remelted, so they are landfilled, incinerated, or downcycled as filler.
At the start of this lesson, you heard that a PET bottle can be remelted and reshaped when collected and processed appropriately, while a cured epoxy resin cannot be remelted into its original shape. Repeated processing can degrade PET, and end-of-life options vary. Cross-linking explains why thermosets do not flow on heating; collection, material quality and processing also affect whether a thermoplastic is recycled.
Now that you've worked through the lesson, how has your thinking about polymer structure changed? Can you now explain why cross-linking keeps a thermoset from softening and flowing when heated, and give an example of a thermoplastic and a thermoset you encounter in everyday life?
Q1. Explain the difference between a thermoplastic and a thermoset polymer. Give one example of each and state one application for each.
Q2. Using your knowledge of polymer structure, explain why HDPE is used for milk bottles while LDPE is used for plastic shopping bags.
Q3. A design engineer needs to choose a polymer for a car dashboard that must withstand high temperatures without deforming. Evaluate whether a thermoplastic or a thermoset would be more suitable. Justify your choice.
Model answers (click to reveal)
SAQ 1 (2 marks)
Model answer: A thermoplastic is a polymer whose chains are held together only by weak intermolecular forces, so it softens when heated and can be remelted and reshaped. Reprocessing can degrade the polymer, so the number of useful cycles depends on the material and processing conditions. A thermoset is a polymer whose chains are joined by strong covalent cross-links that form when it sets, so it cannot be remelted and simply chars if heated strongly. An example of a thermoplastic is polyethylene terephthalate (PET), used to make drink bottles, and an example of a thermoset is Bakelite (or epoxy resin), used to make electrical fittings.
SAQ 2 (3 marks)
Model answer: HDPE is made of straight, unbranched chains that pack closely and tightly together, which makes the material dense, rigid and strong. This rigidity is needed for a milk bottle so it holds its shape and stands up on its own when full. LDPE is made of branched chains that cannot pack closely, leaving the chains loosely arranged so they slide past one another easily. This makes LDPE soft, flexible and stretchy, which is exactly what is needed for a thin shopping bag that must bend and stretch without tearing. The difference in chain packing therefore explains why each polymer is matched to its application.
SAQ 3 (3 marks)
Model answer: A thermoset would be the more suitable choice for the car dashboard. Because a thermoset has a covalent cross-linked network connecting all of its chains, the chains cannot slide past each other when heated, so the material does not soften, creep or deform and keeps its shape and dimensional stability at the high temperatures reached inside a parked car. A thermoplastic, in contrast, is held together only by weak intermolecular forces, so it would soften, sag or warp on a hot day, which would make the dashboard fail. Therefore, weighing the two options, the heat resistance and dimensional stability of a thermoset make it the better material for this application.