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How do thermoplastic polymers differ from thermosetting polymers?

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Diagram illustrating a homopolymer arrangement using polyvinyl chloride as an example, showing a chain of identical repeating vinyl chloride units with the chemical structure highlighted. — polymer Industrial polymers are synthesized from simple compounds joined together to form long chains. For example, polyvinyl chloride is an industrial homopolymer synthesized from repeating units of vinyl chloride.

Thermoplastic and thermosetting polymers differ primarily in their molecular structure and their response to heat, which dictates how they are processed and recycled.1

Thermoplastic Polymers

Thermoplastics consist of individual, separate molecules that can flow past one another. Because these molecules are not chemically bonded into a single network, the material can be repeatedly heated, molded, and reshaped. When heated above their melting temperature or glass transition temperature, they soften and become pliable, allowing them to be extruded or molded into new forms upon cooling. Common examples include polyethylene and polystyrene.12

Thermosetting Polymers

Thermosets undergo a chemical reaction during their initial processing that creates a permanent, infusible, and insoluble network. This process, known as cross-linking, effectively turns the entire object into one large molecule. Consequently, thermosets cannot be reprocessed or remelted; applying heat to a finished thermoset will not soften it for reshaping and may instead cause the material to break down. Examples include epoxy and phenol formaldehyde.123