Plastics are materials that consist of synthetic polymers (artificially produced by man and not found in nature) or modified natural polymers with modifying agents, such as powder or fibrous fillers, thermal stabilizers, UV radiation stabilizers, flame retardants, antistatic agents, foaming agents, dyes, etc. The term "plastics" occurs alongside commonly used terms such as plastic or “plastyk” derived from the Greek word "plastikos", meaning "mouldable", and the word "plastos" meaning "formed".
This refers to the susceptibility of the material to forming or its plasticity during manufacture, which enables casting, extrusion or injection moulding in a wide variety of forms: films, fibres, plates, tubes, bottles, boxes etc. The most correct term for all materials containing polymer as the main component, without distinguishing whether it is of natural or artificial origin, is "polymer plastics".
Polymer plastics are a separate class of materials, apart from ceramics, metals and their alloys and wood, which due to the construction of the cellulose molecules are also considered to be natural polymer materials.
There are two major categories of plastics: thermoplastics and hardening polymers. Thermoplastics appear in the form of solids, and they can pass through multiple changes into the plastic or liquid states when heated and then solidifying upon cooling. This allows the manufacture of products using methods like casting: heating up and melting the material, shaping the product, cooling and solidifying the material. The solidification of thermoplastics is a physical process, and products made of this type of plastic, when heated above a certain temperature, become very soft or liquid. In contrast, hardening plastics may appear in the form of solids or viscous liquids (resins). In this case, after cold-forming (liquid resins) or melting, the process of hardening (cross-linking) takes place during the formation and upon passing into the solid state. The final product remains hard and, in contrast to thermoplastics, cannot be re-melted and can never become soft. Hardening only takes place under the sole influence of temperature (thermosetting plastics) or by adding an additional component, i.e. the 'hardener' (chemically hardened plastics).
The advantages of polymeric plastics are: low density, corrosion resistance and easy processing (low cost for large volumes of the finished product compared to other groups of materials). The latter determined the great growth of the polymer plastic industry in the 20th century and their current universality in the everyday life of man.
The disadvantages of polymeric plastics are: low resistance to high temperatures, relatively high flammability and poorer mechanical properties (e.g. hardness, creep susceptibility) in comparison, for example, to metals and ceramics. Although most mechanical properties of polymeric plastics are less than those of metals, the fibres of some polymeric plastics may have tensile strength comparable to high-quality steel. Similarly, polymer composites are characterized by very high resistance.