Polystyrene foam sheets

FOAAffiD PLASTICS] (Vol 11) Extruded polystyrene foam sheet... 

The clean, durable, nondust-forrning character of polyethylene foam has led to its acceptance in packaging missile parts (230). Polyethylene foam sheet has also displaced polystyrene foam sheet for packaging glass bottles and containers because of its greater resUiency and tear resistance. 

Eamination ofTrycite Plastic Films to Polystyrene Foam Sheet, Bulletin No. 500-898-79, The Dow Chemical Co., Midland, Mich., 1979. 

Extruded polystyrene foam sheet is primarily produced in a single screw tandem extrusion line. Primary application of foam sheet is as a packaging material in items such as disposable dishes and food containers, trays for meal, poultry and produce products, and egg canons. 

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. 

Low-density polystyrene foam sheet was first produced by the extrusion of expandable polystyrene beads or pellets containing pentane as blowing agent [81,82]. Currently, polystyrene foam is extruded in a single-screw tandem line or in a twin-screw extruder. 

Two of the most important food packaging materials using polystyrene resins are oriented polystyrene sheet (OPS) and polystyrene foam sheet. Together, OPS and polystyrene foam sheet packaging account for nearly 40 % of the polystyrene resins used in food packaging applications in North America. 

Polystyrene foam sheet can be made by a variety of extrusion processes (single screw, twin screw) but is most commonly made using a tandem extrusion process as shown in Figure 11.6. This process uses two extruders in series, which effectively optimize the melting, mixing, and cooling unit operations. The diameters of the extruders currently used in this process are usually 115 mm for the primary extruder and 150 mm for the secondary extruder, with an output capability of 400 kg/h. 

Figure 11.6 A tandem extrusion process for the manufacture of polystyrene foam sheet. (1) Primary extruder (2) blowing agent addition system (3) screen changer (4) secondary extruder (5) annular die (6) cooling mandrel (7) S-wrap (8) winders...

Table 11.4 Polystyrene foam sheet tandem extrusion conditions using n-butane...

Figure 11.8 Polystyrene foam sheet surface and center temperature versus thermoforming cycle time (3s). Sheet index indicates sheet movement from oven to forming station [13]...

The cost of manufacturing thermoformed, polystyrene foam sheet parts is less dependent on raw material cost than other extrusion processes. This is largely due to the combined effects of additional energy costs required to operate two extruders, heat removal requirements in the secondary extruder, cost of pelletizing (densifying) regrind and the relatively low output of the process for the equipment scale and cost. Typical cost factors for the manufacture of thermoformed polystyrene foam sheet products include raw materials 35%, labor 27%, sales and administration 16%, depreciation 8%, utilities 7% and other 7%. 

Figure 11.9 Polystyrene foam sheet density versus blowing agent concentration...

Figure 11.11 Various polystyrene foam sheet food packaging applications...

The use of polystyrene foam sheet has grown significantly since its introduction in the mid-1960s. In 1998, approximately 338000 metric tons of polystyrene foam sheet were produced (Table 11.1). Most of the current use is in disposable thermoformed food packaging such as egg cartons, meat, poultry and produce trays, fast food containers, dinnerware (bowls and plates) and bottle labels (Figure 11.11). 

Effective nucleating agents for extruding polystyrene foam sheet include the following (47) ... 

The expandable polystyrene beads may be used in a tabular blow-extrusion process (Figure 2.59) to produce polystyrene foam sheet, which can subsequently be formed into containers, such as egg cartons and cold-drink cups, by thermoforming techniques. 

FIGURE 2.59 Tubular blow extrusion for production of low-density polystyrene foam sheet. 

In Table 1.1, the critical properties of some compounds which are commonly used as supercritical fluids are shown. Of these, carbon dioxide and water are the most frequently used in a wide range of applications. The production of polyethylene in supercritical propane is described in a loop reactor [13]. Supercritical ethylene and propylene are also apphed, where they usually act both as a solvent and as the reacting monomer. In the field of polymer processing, the Dow Chemical Company has developed a process in which carbon dioxide is used to replace chlorofluorocarbon as the blowing agent in the manufacture of polystyrene foam sheet [14, 15]. 

Carbon Dioxide as a Blowing Agent for the Polystyrene Foam Sheet Packaging Market... 

Japan Polystyrene Foamed Sheet Industry Association Token Bldg. 

Prefabricated polystyrene foam sheets are used mainly for thermal insulation and waterproofing in basements. Polyethylene foam is used, e.g. for pipe insulation, and is fixed with an adhesive or tape. Urea-formaldehyde foam was used for improving thermal insulation in old houses, but uncontrollable formaldehyde emissions have caused its use to be minimised. Polyurethane foam is used for thermal insulation as prefabricated foam forms, e.g. on pipes and as sheets on roofs or walls. In pipe insulation, adhesives may be used in fixing. Uncured diphenylmethane diisocyanate-based (MDI) polyurethane foam is sprayed on site (e.g. for window frames) or for technical applications (e.g. on large boilers). When spraying foam on boilers, good ventilation and personal protection is needed. Tools used in thermal insulation include knives, saws, scissors, jointing tools, etc. 

This process from Dow Chemical for manufacturing polystyrene foam sheets uses 100% carbon dioxide (CO2) as a blowing agent, eliminating... 

FIGURE 14.34 Production of low-density polystyrene foam sheet from tubular film die. (Data from Collins, F. H., SPE J., 16, 705, July 1960.)... 

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