Extruded Polystyrene Foam
Extruded PS foam is produced using extrusion, rather molding. The PS resin is melted in an extruder, the blowing agent and a nucleator mixed in, and the blend extruded. As with molded foam, the blowing agent creates the cells as it vaporizes. The purpose of the nucleator is to aid in obtaining the desired cell size and uniformity by providing sites for bubble growth. Talc, citric acid, and blends of citric acid with sodium bicarbonate are often used as nucleators.
The blowing agent is usually a hydrocarbon or hydrocarbon blend, which is injected into the melt as a liquid, or sometimes as a pressurized gas. In recent years, use of carbon dioxide as a blowing agent, either in place of or blended with hydrocarbons, has increased substantially. Chlorofluorocarbons (CFCs) were common blowing agents in the past, but are no longer allowed dye to their ozone depleting activity; hydrochlorofluorocarbons (HCFCs), which arte less active in ozone depletion, cannot be used as blowing agents in the U.S., and are also being phased out worldwide.
The melt is kept under pressure until it leave~ the die, preventing vaporization of the blowing agent. When the melt exits the die, the pressure is released and the blowing agent immediately vaporizes, expanding the melt. If the melt strength of the polymer is not high enough, this abrupt expansion will fracture the melt. It must be able to withstand the pressure exerted by the blowing agent, forming a uniform network of cells. For polystyrene, this requires that the melt be cooled after the blowing agent has been introduced and before the pressure is released. Two methods are commonly used for cooling the melt to the required temperature. One method is to incorporate a cooling zone in the extruder, after the blowing agent has been blended in. The more common approach is to use a two-extruder system~ or tandem system. The polymer is melted and the blowing agent and nucleator added in the first extruder. The melt is then introduced, still under pressure, to the second extruder, where it is cooled and then released through the die.
The foam can be produced as a flat sheet, using a slit die as is done for cast film. Most often, however, a tubular film is produced, using an annular die. In this process, a compressed air ring around the circumference of the die forms a thin skin on the foam surface as it exits the die and begins to foam. The tube of foamed PS passes over a water-cooled mandrel, which provides additional cooling. Next the foam is slit, flattened, and wound into rolls.
The amount of blowing agent introduced is the primary determinant of the final density of the foam. The size and number of cells are controlled by the amount of nucleating agent. Extruded PS foams used for packaging typically have densities of 64-96 kg/m3 (4-6 lb/ft3).
As mentioned previously, the sheet is commonly thermoformed, often using matched molds, to produce the desired package shapes. Matched mold forming is used to minimize the distortion that would otherwise likely result when the sheet is heated. For best results, the sheet should be aged for 3 t0 5 days before thermoforming, so the gas pressure in the cells can equilibrate. The scrap produced during thermoforming can be ground and densified in an extruder for recycling. Used foams can also be recycled.