Foamed or cellular plastics are defined as plastics whose apparent density is decreased substantially by the presence of numerous cells dispersed throughout their mass. The terms foamed plastic, cel-lular plastic, expanded plastic and plastic foam are used interchangeably. These materials have been used widely since the 1940s, due largely to their desirable properties that include a high strength-to-weight ratio and good insulating and cushioning properties. A wide range of plastic polymers can be foamed, including LDPE, HDPE, PP, EVA, PS and polyurethane. The market is dominated by the latter two polymers, with PS being used in preference to polyurethane for food contact applications.
Plastic foams are classified as either flexible or rigid, and may have an open or closed cell struc-ture; in the former, the cells are interconnected, whereas in the latter most cells are closed and separate. Cell formation is initiated by foaming agents. Physical foaming agents are compounds that
change their physical state during cell growth and the most important are volatile liquids (typically aliphatic HCs and CO2) with boiling points below 110°C at atmospheric pressure. Chemical foam-ing agents decompose under heat to at least one gaseous decomposition product, commonly N2.
PS foams can be produced by two processes: injection molding and extrusion. With injection molded foam, machines similar to normal injection molding machines are used, except that steam is injected to heat the beads that contain a foaming agent. Extruded PS foam is produced by free expansion of hot PS, blowing agents and additives through the slit orifice of a high L/D ratio extruder to about 40 times the pre-extrusion volume. The amount and type of blowing agent control the density of the foam produced. The major food packaging uses for extruded PS foam sheet are egg cartons and meat and poultry trays.
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