Thermoplastics
Definition and meaning of Thermoplastics in chemistry.
Thermoplastics are polymers that soften and become moldable when heated and harden again upon cooling, a process that can be repeated multiple times without significant degradation of their material properties. Unlike thermoset plastics, which undergo irreversible cross-linking when heated, thermoplastics retain linear or branched polymer chain structures that allow them to be remelted and reshaped.
In more detail
The reversibility of thermoplastic behavior arises from the nature of intermolecular forces holding the solid polymer together. When cooled, polymer chains interact through weak secondary forces such as van der Waals forces and hydrogen bonding. Heat disrupts these secondary interactions, allowing the chains to flow and be molded into new shapes, but the primary covalent bonds within the polymer backbone remain intact. This property makes thermoplastics ideal for recycling and industrial processing through injection molding, extrusion, and blow molding, since the material can be repeatedly melted and reformed without chemical decomposition.
Key facts
| Field | Physical Chemistry |
|---|---|
| Key Property | Reversibly soften when heated, harden when cooled |
| Chain Structure | Linear or branched polymer chains without cross-linking |
| Common Examples | Polyethylene (PE), polypropylene (PP), PET, polystyrene (PS) |
Polyethylene terephthalate (C10H8O4)n, known as PET, is the thermoplastic used in plastic beverage bottles and polyester textile fibers. It softens around 250-260 degrees Celsius, allowing it to be molded into bottles or drawn into fibers, then hardens as it cools without undergoing any permanent cross-linking reaction.
Frequently asked questions
How do thermoplastics differ from thermoset plastics?
Thermoplastics have linear or branched chains and can be melted and reformed multiple times; thermoset plastics undergo irreversible cross-linking during processing and cannot be remelted, making them rigid and non-recyclable.
Why are thermoplastics recyclable?
Because they can be reheated to their melting point and molded into new shapes without breaking chemical bonds in the backbone, they can be processed into new products from recycled waste.