Extruded polystyrene

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

Polyurethane, PVC, and extruded polystyrene provide the bulk of the cellular plastics used for low and cryogenic temperature appHcations. In some cases, eg, the insulation of Hquid hydrogen tanks on space systems, foams have been reinforced with continuous glass fibers throughout the matrix. This improves strength without affecting thermal performance significantly. 

The main reason for extruding polystyrene is to prepare high-impact polystyrene sheet. Such sheet can be formed without difficulty by vacuum forming techniques. In principle the process consists of clamping the sheet above the mould, heating it so that it softens and becomes rubbery and then applying a vacuum to draw out the air between the mould and the sheet so that the sheet takes up the contours of the mould. 

Extruded polystyrene sheet - Alternatives currently include HCFC-22, hydrocarbons, injected carbon dioxide, and HFC-152a. In the long term, these same alternatives (except for HCFC-22) will be used, along with possible use of atmospheric gases. 

Extruded polystyrene boardstock - HCFC-22 and -142b and injected carbon dioxide are the current alternatives. Long-term alternatives will be HFCs and injected carbon dioxide. 

Much has been written lately about CFCs. This gas is used in the manufacture of a range of plastic insulants, including polyurethane, phenolic, polyisocyanurates and extruded polystyrenes. 

In modem PMR construction, thermal insulation that is unaffected by water or that can be kept dry in some manner is required. Extruded polystyrene (XEPS) foam insulation boards are commonly employed (see Insulation, THERMAL). They are placed on top of the waterproofing roof membrane, which is next to the deck. The insulation should not be adhered to the membrane. Ballast at the rate of >48.8 kg/m2 (1000 lb/100 ft2) holds the insulation in place and offers protection from the sun. The insulation joints are open and drainage must be provided. Various other materials, eg, patio blocks and concrete slabs, are also used as surfacings and ballast. The extra weight imposes more exacting requirements on construction. 

Extruded polystyrene board was first introduced in ihe early 1040s by Dow Chemical Co. with the tradename Styrofoam. The Styrofoam process consists of the extrusion of a mixture of polystyrene and volatile liquid blowing agent expanded through a die to form boards in various sizes. The conlinuous hoards are then passed through Ihe finishing equipment for further sizing. 

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. 

Fig. 3. Aging effect on thermal conductivity of cellular plastics A, extruded polystyrene B, unfaced polyurethane C, unfaced phenolic and D, polyurethane...

Fig. 4. Thermal conductivity of cellular plastics A, extruded polystyrene, 32 kg/m3 B, polyurethane, 32 kg/m3 and C, PVC foam, 32 kg/m3.

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. 

Daigneault LE, Gendron R (2001) Blends of C02 and 2-ethyl hexanol as replacement foaming agents for extruded polystyrene. J Cell Plast 37 262-272... 

The values of ul calculated using Eq. (7) are very similar (differ in less than 3%) to the values of velocity at the axis z/j(0) for beds of sawdust and expanded polystyrene. Consequently, the velocity profile in the spout zone is almost flat in beds of these solids. Nevertheless, for beds of glass beads and extruded polystyrene the velocity profiles in the spout zone are more pronounced. 

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. 

Table 10.1 Physical properties of commercial extruded polystyrene foams ASTM C578 - Standard for Rigid, Cellular Polystyrene Thermal Insulation... 

Figure 10.1 Compressive strength at yield vs density for extruded polystyrene foam. Unpublished data of J. M. Corbett. To convert kg/m3 to Ib/ft3, multiply by 0.0624...

In plastic roof insulations under controlled thermal gradients, the moisture gains found in polyurethane are greater than those of bead polystyrene, and much greater than those of extruded polystyrene [9]. 

We conclude from findings on moisture absorption and freeze-thaw resistance of various insulations, and the effect of moisture on thermal performance, that in protected-membrane roofing applications the order of resisting moisture pickup is extruded polystyrene > polyurethane > molded polystyrene [57]. Water absorption values for insulation in use for 5 years were 0.2vol% for extruded polystyrene, 5vol% for polyurethane without skins, and 8-30 vol%... 

Owens Corning manufactures a similar extruded polystyrene foam under the trade name FoamulaR. A single-screw tandem extrusion line is used to produce foam boardstock in a vacuum chamber connected to a barometric leg that acts as a vacuum seal [78]. Owens Corning also licenses their process to a number of international parties. 

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