Solids based on organic macromolecules which are either synthetically produced or modified natural products. They are divided into thermoplastics that can be melted and cast or moulded, and thermosets that can only be cast in the monomeric state, are cured by polymerisation and are then no longer meltable. Plastics in their pure form are good thermal and electrical insulators. Their specific weight is between 0.9 g/cm3 and 1.5 g/cm3, except when foamed. Usually, they are combustible. The hardness, stiffness and strength are smaller than those of metallic construction materials. A number of copolymers can have rubber-like elastic properties.
Plastics can be modified in many ways by additives and fillers so that, for example, they can be made electroconductive or reinforced with high-performance fibres to achieve stiffness values that are superior to those of steel.