This group covers a large and increasing range, from the natural materials such as rubber to the various man-made materials which have been comparatively recently developed and are already in widespread use, such as nylon, terylene and P.V.C. A plastics material could be simply defined as a material based on organic compounds, of which carbon is a main constituent, and which, on the application of sufficient heat and temperature, can be made to flow and take up a desired shape. If the temperature and pressure are relieved, the new shape is retained. As a result of the organic nature of a plastics material, it cannot usually withstand very high temperatures, and will often begin to decompose at about 200°C or so. Plastics materials can be broadly divided into two groups: (i) thermosoftening or 'thermoplastic' materials and (ii) thermohardening or 'thermosetting' materials. Thermoplastic materials can be softened and resoftened indefinitely by the application of the required heat and pressure. An object made of a thermoplastic material can be distorted by being heated to below its decomposition temperature and distorting pressure then being applied.