These plastics are made by reacting an organic acid having at least two acid (-COOH) groups with an alcohol having at least two alcohol (-OH) groups. Polyester molecules have repeating units with the following general formula:
Light-sensitive plastics are used in dentistry. The material is mixed, applied to a dental cavity and carved to shape. It is then subjected to an intense beam of light or ultraviolet radiation, which causes it to harden.
Thermoplastic binders are used, in combination with metal dusts, to produce a molding feedstock that can be formed into intricate shapes by techniques such as injection molding, vacuum forming and extrusion, at low temperatures and pressures. The thermoplastic binder is removed by converting it to liquid or gas, then the article is finished at high temperature.
In the 1880s celluloid, one of the first plastics, began to replace ivory for the manufacture of billiard balls. Celluloid was derived from guncotton, so there were alarming incidences of balls causing explosions and fires.
Ri and R2 represent organic groups which depend on the particular polyester. In Dacron, a common saturated polyester used to make synthetic fibers, Ri represents a benzene ring and R2 represents a -CH2-CH2- group. Polyesters are also used to make high-quality plastic films and PET bottles.