Polymer Properties and Applications

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. This process is entirely reversible: thermoplastics can be melted and remoulded repeatedly. 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 the material can be remelted, thermoplastics are fundamentally recyclable, a critical advantage for a circular economy. Australia's kerbside recycling systems accept thermoplastics with resin identification codes 1 (PET), 2 (HDPE), 4 (LDPE), and 5 (PP) at most councils.

Thermosets are polymers in which the chains are permanently connected to each other by covalent cross-linkschemical 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.

The thermoplastic/thermoset distinction is the most important factor in polymer recyclability. Thermoplastics are recyclable because they can be remelted. Australia uses a plastic identification system based on resin codes 1–7, printed inside the recycling symbol on packaging: 1 = PET, 2 = HDPE, 3 = PVC, 4 = LDPE, 5 = PP, 6 = PS, 7 = other. Most kerbside programs accept codes 1, 2, and sometimes 5. Codes 3, 4, 6, and 7 are less commonly collected because their markets are smaller.

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.