The essential difference is that thermoplastics remain permanently fusible so that they will soften and eventually melt when heat is applied, whereas cured thermoset polymers do not soften, and will only char and break down at high temperatures. This allows thermoplastic materials to be reclaimed and recycled.
The reason for this is that thermoplastics have relatively weak forces of attraction between the chains, which are overcome when the material is heated, unlike thermosets, where the cross-linking of the molecules is by strong chemical bonds. Effectively the thermoset is one large molecule, with no crystalline structure.
Compared with thermoplastics, thermosets are generally harder, more rigid and more brittle, and their mechanical properties are not heat sensitive. They are also less soluble in organic solvents.
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