Thermoplastics
Definition and meaning of Thermoplastics in chemistry.
Thermoplastics are polymers that soften and become moldable when heated. They harden again once they cool down. This heating and cooling cycle can repeat many times without seriously damaging the material's properties.
In more detail
Thermoplastics differ from thermoset plastics in an important way. Thermosets form permanent, irreversible cross links between chains when heated, so they cannot be remelted once set. Thermoplastics, on the other hand, keep their linear or branched chain structure.
This structure allows them to be melted and reshaped again and again. This reversible behavior comes from the type of forces holding the solid polymer together. When a thermoplastic cools, its chains lock into place.
They are held there by weak secondary forces, such as van der Waals forces and hydrogen bonding. Heat disrupts these weaker forces, which lets the chains flow and be shaped into something new. The strong, primary covalent bonds inside each chain stay intact the whole time.
This property makes thermoplastics excellent candidates for recycling. Manufacturers process them using techniques like injection molding, extrusion, and blow molding. Everyday items like plastic bags, food containers, and PVC pipe are all made from different types of thermoplastics.
The material can be melted and reformed repeatedly without breaking its chemical bonds. This is why thermoplastics fit naturally into a recycling stream. Heating a thermoplastic too many times can eventually degrade its properties. The chain backbone itself, though, stays chemically the same.
Key facts
| Field | Physical Chemistry |
|---|---|
| Key property | Softens reversibly when heated, hardens when cooled |
| Chain structure | Linear or branched chains, without cross linking |
| Common examples | Polyethylene (PE), polypropylene (PP), PET, and polystyrene (PS) |
| Compared to | Thermoset plastics, which cross link permanently and cannot be remelted |
Polyethylene terephthalate, known as PET, with the repeating formula (C10H8O4)n, is a thermoplastic used in beverage bottles and polyester fibers. It softens at around 250 to 260 degrees Celsius, letting it be molded into bottles or drawn into fibers. It then hardens as it cools, with no permanent cross linking taking place.
Frequently asked questions
How do thermoplastics differ from thermoset plastics?
Thermoplastics have linear or branched chains and can be melted and reshaped many times. Thermoset plastics form permanent cross links during processing, so they stay rigid and cannot be remelted or recycled the same way.
Why are thermoplastics recyclable?
They can be reheated to their softening point and molded into new shapes without breaking the covalent bonds inside their chains. This lets manufacturers turn recycled thermoplastic waste into new products.
What forces hold a thermoplastic together when it is solid?
Weak secondary forces, such as van der Waals forces and hydrogen bonding, hold the polymer chains together when the material is cool and solid. Heat disrupts these weaker forces, letting the material soften.