The Production of Expanded Polystyrene by an Extrusion Process

The extrusion of expandable polystyrene by a lay-flat technique was first publicly demonstrated in England on a laboratory scale in 1962. It was apparent that the product would: be useful as a packaging medium if the process was scaled up. The mechanisms of extrusion and expansion had not been fully investigated and a greater understanding of these mechanisms was expected to assist in improving the quality of the product. The process forms the basis for the investigations reported in this thesis and is briefly described as follows.

Granular polystyrene containing pentane, citric acid and sodium bicarbonate is fed into an extruder. The polymer is melted and mixed, and pumped into a circular die. As the melt emerges from the die it expands rapidly to at least ten times its original volume. In order to prevent the expansion causing convolution of the tubular extrudate, the tube is sealed at one end and inflated. It is then passed through a collapsing canopy in order to flatten the bubble formed by inflation. The collapsed bubble is then led away through nip rollers. The edges of the bubble are trimmed off, and the two halves, in the form of flat sheets are reeled up separately.

A small experimental unit was built in 1963, and the conclusions reached as a result of work on this enabled a larger Plant to be constructed. This plant was modified to form the basis of a production unit. Part 1 of the thesis reviews the published work in the field and describes the evolution of the production unit.

A study of the theoretical aspects of the formation of cellular structures was undertaken in order to gain a greater understanding of the production process. This study led to a detailed examination of the cellular sheet manufactured on the plant. Part 2 of the thesis covers this work and contains conclusions on the mechanism of cell formation in extruded expanded polystyrene.

As a result of the work undertaken in parts 1 and 2, a satisfactory product became available for commercial use. An examination of the methods of processing the sheet was made to make it more commercially attractive. Costings were carried out on the existing plant and on a proposed larger unit. Finally recommendations for improving the quality of the sheet, and of obtaining a more economic production unit were put forward. These commercial aspects are covered in Part 3 of the thesis.

Also based at Telcon Plastics Ltd, Farnborough Works, Orpington.