Structural-Foam Types

Chapter 3 on Thermoplastic Foams discusses structural foams in general and considers the properties of individual structural-foam types. This discussion is concerned solely with methods of manufacturing these foams. 

In this case the designer has freedom of choice of both form and dimension as well as in the selection of the materials. Given this freedom, it would be desirable to examine several of the alternatives to see which would provide the best seating at the lowest cost. Obviously, there is no point in doing all of the possibilities so a selection should be made on the basis of anticipated use as well as style requirements. Three types will be analyzed. They are the single curve sheet cantilever mounted from the back, the molded pan supported on four legs, and the structural foam molding which is front supported. 

The following review concerns structural foams (SFs). Review Chapter 8, FOAMING regarding the different TP and TS types of foam available. Most of the foamed plastics... 

To demonstrate the use of such a comparative cost analysis, the production of a panel was analyzed according to different processes (Fig. 9-6). In these case studies the following conditions existed (1) the panels measured 61 x 91 cm (24 x 36 in.) with the wall thickness dictated by the process and part requirements so that the weights of the panels differed (2) production was at a level of 40,000/yr. (3) the plastics for all panels were of the same type, except that different grades had to be used, based on the process requirements, so that costs changed (4) each panel received one coat of paint, except that the structural foam also had a primer coating and (5) costs were allocated as needed to those processes that required trimming and other secondary operations. 

The content of the gas is called quality therefore a 70 quality contains 70% gas. Recently, foams with 95% gas have been examined. For such foam types, only foam prepared from 2% of an anionic surfactant with plain water had uniform, fine-bubble structure [782]. 

Polyurethane rubber (PUR). Not only in the thermosets (and the thermoplastics), but also in the field of synthetic elastomers polyurethanes have found a position, namely as a softer type. It is, again, formed from two components and is, with a blowing agent, processed into a foam. Polyether mattresses belong to this category, but also microcellular structural foams, used in bumpers, head- and arm-rests in motorcars, etc. 

In order to gain a more profound knowledge, the first problem must be a wider use of the concepts of polymer physics and physicochemistry. This would enable an evaluation of the specific polymeric features of plastic foam morphology, primarily the different structures and types of the submolecular organization of cell walls and struts, and thus allow to understand their effect on the microstructure of the foam density and the degree of orientation of foam cells. 

This chapter will discuss all types of thermoplastic foams, including rigid, semi-rigid and structural foams. 

Chapter 3 and all subsequent chapters were prepared by the editor, A.H. Landrock. Chapter 3 covers all types of thermoplastic foams, including rigid, semi-rigid, and structural foams. Chapter 4 briefly discusses elastomeric foams. Chapter 5 discusses a number of miscellaneous and specialty foams, many of which were also covered in Chapter 2. 

Many solid adsorbents liberate gas as a result of desorption of volatile liquids under the influence of heat. Typical adsorbents with microporous structures such as activated carbons, or precipitated silicas and renewable resources have been used as a coblowing agent in producing low-density extruded polystyrene foam boards. Incorporation of corn cobs or other renewable vegetable matter containing about 10% water together with a primary PBA into polystyrene in the extrusion process produced a low-density polystyrene foam board with bimodal cellular structures. This type of foam with bimodal cell structures has about 10-15% lower K-factor than similar foams without bimodal cellular structures. Similar results were obtained with a precipitated silica for producing a low-density extruded polystyrene foam with bimodal cellular structures. ... 

FIGURE 2.58 Schematic representations of section through different types of cellular polymer, (a) Low-density open-cell foam, (b) High-density closed-cell foam, (c) Single-component structural foam with cellular core and integral solid skin, (d) Multicomponent structural foam, (e) Fiber-reinforced closed-cell foam, (f) Syntactic foam. 

A variety of foams can be produced from various types of polyethylenes and cross-linked systems having a very wide range of physical properties, and foams can be tailor-made to a specific application. Polypropylene has a higher thermostability than polyethylene. The production volume of polyolefin foams is not as high as that of polystyrene, polyurethane, or PVC foams. This is due to the higher cost of production and some technical difficulties in the production of polyolefin foams. The structural foam injection molding process, described previously for polystyrene, is also used for polyethylene and polypropylene structural foams (see Figure 2.61). 

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. 

A plentitude of different flow field structures have been described in the literature starting from simple arrays of fence posts via arrays of straight parallel chaimels to complex serpentine type stmctures. In addition foam type structures, expanded metals and woven designs have been proposed as well. Under ordinary circumstances, gas flow will be maintained in the channels of the flow field while transport to the reaction site is by diffusion through the porous gas diffusion media next to the catalyst layer. 

---