What is the primary reason that thermosets cannot be re-melted?

The primary reason that thermosets cannot be re-melted is due to their molecular structure; specifically, they undergo a chemical reaction during the curing process that causes their molecules to cross-link. This cross-linking alters the material permanently, creating a three-dimensional network that provides thermal stability and rigidity. Once this process occurs, the structure cannot be undone simply by applying heat, unlike thermoplastics, which can be re-melted and reshaped.

In contrast, the other options do not accurately capture the fundamental characteristic that prevents thermosets from being re-melted. While it's true that thermosets can be more expensive and that their heat-setting process is essential for forming the final product, these factors do not inherently prevent re-melting. Additionally, the statement about thermosets being less durable is misleading since thermosets are often prized for their durability and resistance to deformation at higher temperatures. Therefore, it is the cross-linking of molecules during curing that fundamentally distinguishes thermosets and accounts for their inability to be re-melted.