All you want to know about thermoplastics and thermosets

The term, thermoplastics mean that once the polymer is formed, it can be heated and reformed over and over again. The molecules are held together by weak secondary bonding forces that causes the plastic to soften when exposed to light and return back to its original shape, when cooled back down to room temperature. The structure associated with thermoplastics is that of individual molecules that are separate from one another and flow past one another.

The molecules may have low or extremely high molecular weight, and they may be branched or linear in structure, but the essential feature is that of separability and consequent mobility. It should also be noted that these substances also have little cross-linking. Since extrusion, molding or pressing when exposed to heat can shape thermoplastics; it therefore offers versatility and a large range of applications.

This property of plastics allows for easy processing and facilitates recycling; the majority of polymers are therefore thermoplastics.

• Polyethylene: packaging, electrical insulation, milk and water bottles, packaging film, house wrap, agricultural film
• Polypropylene: carpet fibers, car bumpers, microwave containers
• Polyvinyl chloride (PVC): floor and wall coverings, siding, credit cards, car instrument panels.

Thermosets
A thermoset is a polymer that solidifies irreversibly when heated. Similar to the relationship between a raw and a cooked egg, once heated, a thermoset polymer can’t be softened again; the egg can’t reverts back to its original form. During the initial processing, thermosetting resins undergo a chemical reaction that results in an insoluble network. Essentially, the entire heated, finished article becomes one large molecule. For example, the polymer used in making a fiber-reinforced laminate for a golf club undergoes a cross-linking reaction when it is molded at a high temperature.

Any amount of heat does not soften the material to the point where it can be reworked and indeed may serve only to break it down. Hence, thermosetting substances must have sufficient cross-linking of their chains to prevent molecular motion on heating. Thermosets are therefore valued for their durability and strength and are primarily used in vehicles and construction.

• Polyurethanes: mattresses, insulation, toys
• Unsaturated Polyesters: varnishes, furniture, boat hulls
• Epoxies: glues, coating for electrical circuits, helicopter blades.