What type of resin cannot be re-formed once cured?

Thermoset resins are characterized by their chemical structure, which undergoes a significant transformation during the curing process. When thermoset resins are mixed with a hardener or catalyst, they undergo a reaction that leads to a cross-linking of their polymer chains. This cross-linking results in a rigid, inflexible material that cannot be re-shaped or re-formed by heating. Once cured, the structure of thermoset resins is chemically altered in such a way that it remains stable under heat and stress over time.

In contrast, thermoplastic resins can be reshaped when heated without undergoing a chemical change, allowing them to be re-molded multiple times. Elastomers are similar to thermoplastics but are designed to give rubber-like properties; they also can be stretched and reformed. Composites refer to materials made from two or more constituent materials with significantly different physical or chemical properties. While composites often utilize thermosets, they can also incorporate thermoplastics and elastomers, which can be re-formed when heated.

Therefore, thermoset resins are uniquely identifiable by their inability to return to a plastic state after they have cured, making them the correct answer in this context.