Molded expanded polystyrene foam

Other styrenic polymer foams were developed in the mid-1950s through the early 1960s. Examples are molded expanded polystyrene foam (MEPS), extruded polystyrene foam sheet, and expanded polystyrene loose-fill packaging material. 

The most important use of polystyrene is in packaging. Molded polystyrene is used in items such as automobile interior parts, furniture, and home appliances. Packaging uses plus specialized food uses such as containers for carryout food are growth areas. Expanded polystyrene foams, which are produced by polymerizing styrene with a volatile solvent such as pentane, have low densities. They are used extensively in insulation and flotation (lifejackets). 

MIL-P-19644C Plastic Molding Material (Polystyrene Foam, Expanded Bead), 10 July 1970, 15 pp (FSC 9330) (OS). Int amend 1 was issued 1 AprU 91, 3 pp (SA)... 

Polystyrene, as mentioned, is the most common packaging foam. Polystyrene foam is the material of choice if it can perform acceptably, since it is typically the least expensive packaging foam available. It is used extensively for containers as well as for cushioning material, in molded shapes and in extruded form. Molded shapes are commonly termed expanded polystyrene foam (EPS), while the extruded material is called simply extruded PS foam. The term styrofoam is often incorrectly used for these materials, but Styrofoam is a Dow Chemical Company trade-marked extruded polystyrene foam used primarily for building insulation and not found in packaging. 

PS is a stiff, brittle material with relatively poor barrier properties. Crystal PS has excellent transparency. In its foamed form, PS has very good cushioning abilities, as well as insulation properties. Cost is relatively low, especially in molded expanded polystyrene (EPS) and extruded foam PS, where a small amount of mass can yield a large volume. PS is readily shaped by injection molding, extrusion, or thermoforming, in either foamed or unfoamed form. 

Expandable polystyrene is the usual name for the beads used to make molded polystyrene foam parts. Parts made with these beads are usually made in a two-step process. The first step consists of pre-expansion of the beads by heat. After this step, the beads are stored for 6-12 h to allow them to reach equilibrium. The beads are then conveyed to the mold, where they expand to the final dimensions. Steam is the preferred mode of heating, either by introducing live steam through perforations in the mold or by the means of steam probes, which are withdrawn as the beads are expanding. Expandable polystyrene foam can be distinguished from expanded polystyrene foam by the round bead outlines on the surface of the part. The expanded foam has a smooth surface. 

Figure 2.13 Coffee cups, molded from expandable polystyrene foam. design56/Shutterstock.com.

There are five basic types of polystyrene foams produced in a wide range of densities and employed in a wide variety of apphcations (/) extmded polystyrene board (2) extmded polystyrene sheet (2) expanded bead mol ding (4) injection molded stmctural foam and (5) expanded polystyrene loose-fiU packaging. 

Roofiag panels have been made from polyisocyanurate foams, both foam- and felt-reiaforced with glass fiber. PhenoHc resias are used especially for decorative laminates for paneling. The substrate may be fiberboard or a core of expanded polystyrene beads. In one case the beads are coated with phenoHc resia, then expanded ia a mold to form a stmctural foam panel. 

While unaffected by water, styrofoam is dissolved by many organic solvents and is unsuitable for high-temperature applications because its heat-distortion temperature is around 77°C. Molded styrofoam objects are produced commercially from expandable polystyrene beads, but this process does not appear attractive for laboratory applications because polyurethane foams are much easier to foam in place. However, extruded polystyrene foam is available in slabs and boards which may be sawed, carved, or sanded into desired shapes and may be cemented. It is generally undesirable to join expanded polystyrene parts with cements that contain solvents which will dissolve the plastic and thus cause collapse of the cellular structure. This excludes from use a large number of cements which contain volatile aromatic hydrocarbons, ketones, or esters. Some suitable cements are room-temperature-vulcanizing silicone rubber (see below) and solvent-free epoxy cements. When a strong bond is not necessary, polyvinyl-acetate emulsion (Elmer s Glue-All) will work. 

Tn 1967 the total sales of polystyrene foam in the United States were about 193 million pounds (78). Foams molded or extruded from expandable polystyrene amounted to 145 million pounds. The recent growth rate of polystyrene foam in the United States is depicted in Figure 1. The lower section represents expandable polystyrene. 

Mixed Polymers. A mixture of 5 parts of polyethylene and 95 parts of expandable polystyrene is extruded and converted to quenched pellets. The pellets are expanded and converted into molded foams with improved resistance to solvents and water vapor transmission (29). There is a polyethylene skin on the surface. 

The production and application in 1967 of 193 million pounds of polystyrene foam reflects considerable scientific, engineering, and commercial activity. The continuing growth of expandable polystyrene in this field is attributed to its ability to be steam molded economically into a variety of useful items. 

Foam molding operations are those in which a liquid mixture of foam components is used. It is poured into a mold cavity to form a cellular shaped product. The molded product is later removed after setting or curing. As reviewed in the case of expandable polystyrene beads the preexpanded or virgin beads are poured into a mold and heated to form the desired object. In this case, liquids are not used, although the free-flowing beads might be considered a fluid. 

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