Commercial foamed plastics structural foams

Unique Properties of Foamed Plastics Structural Features of Foamed Plastics Leading Commercial Polymers Polyurethane Polystyrene Poly(vinyl chloride)... 

Lactomes may also be polymerized by ring-opening anionic polymerization techniques. While the five-membered ring is not readily cleaved, the smaller rings polymerize easily producing linear polyesters (structure 5.46). These polymers are commercially used as biodegradable plastics and in PU foams. 

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. 

Thermal degradation of foams is not different from that of the solid polymer, except in that the foam structure imparts superior thermal insulation properties, so that the decomposition of the foam will be slower than that of the solid polymer. Almost every plastic can be produced with a foam structure, but only a few are commercially significant. Of these flexible and rigid polyurethane (PU) foams, those which have urethane links in the polymer chain are the most important. The thermal decomposition products of PU will depend on its composition that can be chemically complex due to the wide range of starting materials and combinations, which can be used to produce them and their required properties. Basically, these involve the reaction between isocyanates, such as toluene 2,4- and 2,6-diisocyanate (TDI) or diphenylmethane 4,3-diisocyanate (MDI), and polyols. If the requirement is for greater heat stability and reduced brittleness, then MDI is favored over TDI. 

The relative strength of hollow-sphere foams lies between the theoretical performance of open- and closed-cell foams. The performance of optimized truss structures is similar to that of closed-cell foams and, for the Kagome truss, approaches the behavior of a Hashin-Shtrikman porous material. Honeycombs are the most efficient structures when loaded purely out-of-plane. However, plastic buckling can decrease its performance at low relative densities. Further, since honeycomb is highly anisotropic, any inplane loading results in severely reduced performance. Although the theoretical performance of closed-cell foams far exceeds that of open-cell foams, processing defects result in commercially available material that behaves similar to an open-cell material at low relative densities. Commercially available samples of other types of low-density metallic structures behave nearly as predicted. [17]... 

Mechanical testing on the foam is performed to measure the compressive stress and the resiliency. The results from diese tests are shown in table 2, in which the properties of extruded polystyrene foam (XPS) and commercial starch-based loose-frll foams (Eco-foam and Mater-Bi) together with EPS loose-fill foam (Pelaspan Pac) are added for conq>arison. The values of XPS are obtained from tests on typical XPS retail packaging trays. The table shows that the compressive stress reached with potato starch foam is comparable with that of XPS. Through the cell structure of the potato starch foam (high cell density, very small cells) a good resiliency can be obtained, although pure starch plastics exhibit brittle fracture behavior. This brittle fracture still is present on the microscopic scale of the individual cells but due to the cell density, the foam exhibits resiliency on macroscopic scale. 

Polystyrene is an aromatic polymer made from styrene, an aromatic monomer which is commercially manufactured from petroleum. Polystyrene is commonly injection moulded or extruded while expanded polystyrene is either extruded or moulded in a special process. Solid polystyrene is used in disposable cutlery, plastic models, CD and DVD cases, etc. Foamed polystyrene is mainly used for packing materials, insulation, foam drink cups, etc. Polystyrene foams are good thermal insulators and therefore used as building insulation materials such as in structural insulated panel building systems. They are also used for non-weight-bearing architectural structures. The information on OPF-polystyrene composites is limited. 

Poly(propylene oxide)s of low molecular weight (i.e., from 500 to 3500), often referred to as polypropylene glycols (PPGs), are important commercial materials mainly because of their extensive use in the production of polyurethane foams (see Chapter 1 of Plastics Fabrication and Recycling and Chapter 1). PPCs are less hydrophilic and lower in cost and may be prepared by polymerizing propylene oxide in the presence of propylene glycol as an initiator and sodium hydroxide as a catalyst at about 160°C. The polymers have the general structure... 

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