Which of the following is NOT a characteristic of thermoplastic materials?

Thermoplastic materials are characterized by their ability to be reshaped and remoulded when heated, which allows them to become flexible and take on new forms. This property is primarily due to their molecular structure, which consists of long polymer chains that can slide past one another when heat is applied.

The ability to be reshaped upon heating and their general flexibility are quintessential traits of thermoplastics. Therefore, the option identifying a rigid structure does not align with the characteristics typically associated with these materials. Unlike thermoset materials, which harden permanently after being set, thermoplastics can return to a pliable state upon heating, emphasizing their versatility in manufacturing processes.

Thus, while they can exhibit rigidity in certain applications or conditions, a rigid structure is not an inherent characteristic of thermoplastics as a class. Instead, they are more correctly described by their capacity for remoulding and flexibility, making the identification of a rigid structure as a non-characteristic correct.