Final answer:
The conversion to a hard, rigid material is characteristic of thermosetting polymers, which harden permanently after molding and decompose upon reheating.
Step-by-step explanation:
The transformation of a resin material from a soft, pliable state to a hard, rigid material is characteristic of thermosetting polymers. Thermosets soften when heated and can be molded; however, once they are set, they harden permanently and will not soften again upon reheating, as they decompose instead. This change is a result of a polymerization reaction where the material undergoes chemical changes to form a hardened network structure. Unlike thermoplastics, which can be remelted and reformed, thermosets maintain their form even when subjected to high temperatures after the initial molding process. Common examples of thermosetting polymers include Bakelite and epoxy resins.