Polystyrene was first produced commercially in the 1930s and the ready availability of styrene feedstock has helped it to grow. Styrene will polymerise spontaneously on heating in an oxygen-free atmosphere but catalysts are added to ensure complete polymerisation at lower temperatures. Processes have been designed to aid heat transfer from the exothermic reaction, which can lead to low molecular weight polymers being formed if not controlled.
Two main types of polymers are produced: crystal which is a clear, amorphous resin with good stiffness and electrical properties; and impact which contains varying levels of polybutadiene to improve toughness and impact resistance.
There are three types of processes generally used - suspension, solution and mass (bulk) polymerisation. The advantages of the solution route, which can be continuous or batch operation, are low residual monomer content and high purity polymers. The suspension route produces polymers of different molecular weights and can make specialist crystal and high impact grades of polystyrene. The main advantages of the mass process are the clarity and excellent colour of the resins produced.
A recent development by Dow Chemical and Idemitsu is the use of metallocene catalysts to produce syndiotactic PS. However, these resins are like engineering plastics with a highly regulated stereo structure and a crystalline nature.