Which type of plastic cannot be remelted once cured?

Thermoset plastics are a specific category of polymers that undergo a chemical change when they are cured through heat or a chemical reaction. This curing process creates a three-dimensional network of molecular bonds, making the material rigid and unable to be remelted or reformed. Once a thermosetting plastic has been set, it retains its shape and strength even at elevated temperatures, but it cannot be reprocessed or reshaped by heat application.

This characteristic contrasts with thermoplastics, which can be melted and reformed multiple times without significant degradation of their properties. Elastomers, while flexible and stretchable, also fall under the category of thermosetting materials once cured and behave similarly in terms of remelting. Composites are materials made from two or more constituent materials, and their ability to be remelted depends on the types of plastic used within them. However, thermoset plastics specifically are known for their permanent hardness and structural integrity post-curing, emphasizing the reason why this type is the correct answer to the question.