High-density thermoplastic foam

Tbe term structural foam was originally coined by Union Carbide to describe an injection moulded thermoplastic cellular material with a core of relatively low density and a high-density skin. The term has also been used to describe rigid foams that are load bearing. Today it is commonly taken to imply both of the above requirements, i.e. it should be load bearing and with a core of lower density than the skin. In this section the broader load-bearing definition will be used. Whilst structural foams are frequently made from polymers other than polystyrene, this polymer is strongly associated with such products and it is convenient to deal with the topic here. 

Poly(ethylene), high density 1953 1955 Thermoplastics, foams ... 

The relationship between chemical structures and their physical performance is one of the central topics of polymer physics. lUPAC has recommended a whole set of names to describe the detailed chemical structures of polymer chains and their derivatives. However, in our daily communication, people prefer to use the popular names of polymers reflecting their characteristic physical performances, such as high-density polyethylene (HOPE), foamed polystyrene, thermoplastic elastomers, liquid crystal polymers, conductive polymers, and polyelectrolyte. Such terminology allows us to comprehend quickly the basic characteristics of chemical structures responsible for their specific physical properties. 

Polystyrene. Polystyrene [9003-53-6] is a thermoplastic prepared by the polymerization of styrene, primarily the suspension or bulk processes. Polystyrene is a linear polymer that is atactic, amorphous, inert to acids and alkahes, but attacked by aromatic solvents and chlorinated hydrocarbons such as dry cleaning fluids. It is clear but yellows and crazes on outdoor exposure when attacked by uv light. It is britde and does not accept plasticizers, though mbber can be compounded with it to raise the impact strength, ie, high impact polystyrene (HIPS). Its principal use in building products is as a foamed plastic (see Eoamed plastics). The foams are used for interior trim, door and window frames, cabinetry, and, in the low density expanded form, for insulation (see Styrene plastics). 

Cavitation in the rubber particles of PS/high-impact PS (HIPS) was also identified as a heterogeneous nucleation site, using batch-foam processing [15, 16]. The experimentally observed cell densities as a function of the temperature, the rubber (HIPS) concentration, the rubber particle size, and saturation pressure were found to be in good agreement with the proposed nucleation model. Similar nucleation mechanisms of elastomeric particles were claimed for acrylic and di-olefinic latex particles in various thermoplastics [17, 18]. 

A major disadvantage of composites of wood with thermoplastics materials is a relatively high specific gravity compared with those of many natural wood products. A PVC-wood composite, for example, has a specific gravity of about 1.3 g/cc. The manufacture of cellular PVC-based wood composites was studied and the properties that were achieved as the foam density was reduced were examined. Overall, even with densities as low as 0.6 g/cc, the physical properties should be adequate for many wood replacement applications. The composites also exhibited the aesthetics of wood and economics that were favourable compared with those of both rigid and cellular PVC. 6 refs. 

Some special types of foams are (1) structural foams (2) syntactic foams and multifoams and (3) reinforced foams. Structural foams (Figure 2.58c and d), which possess full-density skins and cellular cores, are similar to structural sandwich constructions or to human bones, which have solid surfaces but cellular cores. Structural foams may be manufactured by high pressure processes or by low-pressure processes (Figure 2.61). The first one may provide denser, smoother skins with greater fidelity to fine detail in the mold than maybe true of low-pressure processes. Fine wood detail, for example, is used for simulated wood furniture and simulated wood beams. Surfaces made by low-pressure processes may, however, show swirl or other textures, not necessarily detracting from their usefulness. Almost any thermoplastic or thermosetting polymer can be formulated into a structural foam. 

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