Plastics are divided into thermoplastics and thermoset plastics.

Thermoplastics can be heated and shaped many times.

Thermoplastics pellets soften when heated and become more fluid as additional heat is applied. The curing process is completely reversible as no chemical bonding takes place. This characteristic allows thermoplastics to be remolded and recycled without negatively affecting the material’s physical properties.

Thermoset plastics can only be heated and shaped once.

Thermoset materials are usually liquid or malleable prior to curing and designed to be molded into their end form, or used as adhesives. Others are solids like that of the molding compound used in semiconductors and integrated circuits (IC

Thermoset plastics contain polymers that cross-link together during the curing process to form an irreversible chemical bond. The cross-linking process eliminates the risk of the product remelting when heat is applied, making thermosets ideal for high-heat applications such as electronics and appliances.

These polymers are highly cross-linked, which means the molecules have extremely strong chemical bonds. Once you’ve initially heated them up to set them into the shape you want – hence the name “thermoset” – they’re irreversibly bound. That means they’re much stronger than thermoplastics, but also that heating them up again won’t break down the bonds in a way that enables you to usefully reshape them, as with thermoplastics. It’ll just char them and crack them instead.

Thermoset plastics that can be remoulded and recycled several times over when heated to about 100ºC are possible, claim researchers in The Netherlands. 

The self-healing polymers make use of the Diels-Alder and Retro-Diels-Alder reactions between thermosetting polyketones and bis-maleimide, allowing the strong covalent bonds of the thermoset materials to be broken and reformed.

Examples of some thermoplastics.

Name Properties Principal uses
Polyamide (Nylon) Creamy colour, tough, fairly hard, resists wear, self-lubricating, good resistance to chemicals and machines Bearings, gear wheels, casings for power tools, hinges for small cupboards, curtain rail fittings and clothing
Polymethyl methacrylate (Acrylic) Stiff, hard but scratches easily, durable, brittle in small sections, good electrical insulator, machines and polishes well Signs, covers of storage boxes, aircraft canopies and windows, covers for car lights, wash basins and baths
Polypropylene Light, hard but scratches easily, tough, good resistance to chemicals, resists work fatigue Medical equipment, laboratory equipment, containers with built-in hinges, ‘plastic’ seats, string, rope, kitchen equipment
Polystyrene Light, hard, stiff, transparent, brittle, with good water resistance Toys, especially model kits, packaging, ‘plastic’ boxes and containers
Low density polythene (LDPE) Tough, good resistance to chemicals, flexible, fairly soft, good electrical insulator Packaging, especially bottles, toys, packaging film and bags
High density polythene (HDPE) Hard, stiff, able to be sterilised Plastic bottles, tubing, household equipment

Properties and uses of the thermoset plastics.

Name Properties Principal uses
Epoxy resin Good electrical insulator, hard, brittle unless reinforced, resists chemicals well Casting and encapsulation, adhesives, bonding of other materials
Melamine formaldehyde Stiff, hard, strong, resists some chemicals and stains Laminates for work surfaces, electrical insulation, tableware
Polyester resin Stiff, hard, brittle unless laminated, good electrical insulator, resists chemicals well Casting and encapsulation, bonding of other materials
Urea formaldehyde Stiff, hard, strong, brittle, good electrical insulator Electrical fittings, handles and control knobs, adhesives

Find out more about the above plastics and many others here .

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