2. EPS Geofoam Manufacturing

Expandable polystyrene is the raw material or resin used for the molding of expanded polystyrene, EPS, geofoam. Expandable polystyrene is a plastic/polymeric material with a chemical composition of C8H8 (Ravve, 2000). Polymers are long chain molecules in which one to three basic units, monomers, are repeatedly linked over and over in a process called polymerization.

Polymerization of styrene monomer used to be followed by impregnation of the polymerized polystyrene beads with a blowing agent. Today almost all process carries out polymerization and impregnation in a one step process as shown in figure 2-1. The reaction occurs in a single reactor designed to control the temperature and pressure of the reaction. Styrene monomer and water is charged to the reaction kettle equipped with an agitator. Various chemicals are added to effect suspension of the monomer in water and to control the polymerized bead growth, molecular weight and other parameters necessary to produce the desired product. In the second phase of the process, the blowing agent is added under pressure and impregnates the soft polysty-rene beads. The total batch cycle takes a little less than ten hours. When completed the entire batch is dumped to de-water the system. The beads are then dried. The beads are screened to obtain different bead sizes. They will be in different grades each has its own specification and use. Some are good for leak resistant containers (Huntsman, 1999d), some are for impact absorption packaging applications (Hunts-man, 2000), some are for general block and shape molding operations (Huntsman, 1999b), etc. They are packaged and transferred to the geofoam block manufacturer.

Figure 2-1 Expandable Polystyrene Beads Manufacturing Process

Figure 2-2 shows the process of manufacturing geofoam from the expandable polystyrene beads. Five stages can be seen in the figure. First the beads are fed to a vertical tank containing an agitator and a controlled steam input. The final material density is determined at this stage. Density adjustment is carried out by controlling the length of the time the beads remain in the expander and/or the pressure in the ex-pander. Second, the expanded beads, called prepuffs, are stored in the open air for few hours as a drying stage. Third, the prepuffs are stored in large silos until trans-ferred for the following step. During storage they are allowed to reach an ambient temperature. This process takes as long as three days or as short as few hours. This process is called the stabilization process, as condensation of the blowing agent and the surrounding water vapor occurs. The prepuffs then are poured in a mold of vari-ous sizes, depending on the manufacturer. A 0.6m X 1.2m X 2.4m is a common mold size. Steam is injected from the walls of the mould through longitudinal tiny slots where fusion takes place. Approximately 5% of recycled expanded polystyrene (re-grind) is shredded and mixed with the prepuffs. Additional expansion and fusion takes place. The molded block is then pushed out where it is ultimately taken to a storage place to dry.

Figure 2-2 EPS Geofoam Manufacturing Process

The basic EPS product is white, although it can be colored otherwise. About 140 geofoam manufacturer plants are distributed throughout the United States (Ne-gussey, 1998). Material prices vary depending on the type and density of geofoam, as well as on the job, size and location.