Sheet production polystyrene foam

The production of foamed films and sheets from polypropylene and polystyrene is discussed, with particular reference to packaging applications. Advantages of foamed materials for this application are examined, and the chemical and physical foaming processes are described. Extrusion technology for film and sheet by chemical and physical foaming processes is discussed, and recent developments in the coextrusion of multilayer packaging trays for the food industry are considered. 

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. 

More recently Pactiv, formerly Tenneco Packaging, has manufactured foamed polystyrene sheet and boardstock. A tandem extrusion line is used to produce polystyrene foam insulation products. 

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%. 

Major applications for styrene plastics are summarized in Table III (23). The packaging and serviceware (disposables) markets predominate, and account for approximately 50% of the total. One of the most rapidly growing portions of these markets is in low-density (usually 1-10 Ib/ft ) polystyrene foams, either in the form of extruded foam sheet or expanded polystyrene beads (EPS). Projections indicate that production of these foams will be greater than 2000 metric tons (24). 

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]. 

Low density polyethylene foam products (thin sheets, planks, rounds, tubes) in the range of 32-160 kg/m (2-10 Ib/ft ) have been prepared by an extrusion technique using various gaseous fluorocarbon blowing agents (97,98). The techniques are similar to those described earlier for producing extruded polystyrene foam planks and foam sheets. 

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.)... 

Polystyrene (PS), a highly rigid and surface-hardened thermoplastic, is glass clear and almost colorless. Its typical slight yellow tinge is easy to compensate for by adding transparent blue colorants to adjust the color. Polystyrene softens between 80 and 100°C. It is processed between about 170 and 280°C, up to a maximum of 300°C, without color change, by any of the methods which are recommended for thermoplastics. The list of products includes extrusion made sheets, profiles, and films, which are often foamed. 

Theres a lot of competition between PS and the other five big thermoplastics LDPE, LLDPE, HDPE, PP, and PVC. Polystyrene continues to lose marker share, but it seems to have a permanent place in some applications, particularly molded foams (for carryout food containers), some extrusions, and sheet and film applications. About half the polystyrene ends up in packaging, 17% in electrical/electronics applications, 13% in construction, building products and furniture, and 7% in medical applications. 

Thermoplastic Foam Extrusion. Foamed plastics find applications as rigid profiles, pipe, sheet, packaging material, and thermal insulation. Polystyrene (PS) finds the widest application in foamed products while poly(vinyl chloride) PVC, low-density polyethylene (LDPE), and acrylonitrile-butadiene-styrene (ABS) (21) are also used in large quantities. All common thermoplastics can be foamed by various techniques as described hereafter. 

Foams are available as rigid sheets or slabs (which are used in the majority of roofing systems), as beads and granules (used in cavity wall insulation), and also as spray and pour-in applications. The market is dominated by polyurethane (PU) foams, in particular polyisocyanurate products, expanded polystyrene (PS), and extruded polystyrene. 

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