Structured solid foams

The stiffness ratios (i.e. stiffness of the foam sandwich beam relative to the original solid beam) are also given in Fig. 2.21. In both cases the values given are independent of the original solid material or its dimensions, so this provides a good design chart. The design of solid/foam sandwich structures is also considered in Chapter 3 in the laminate analysis. 

Foams have limited use for these purposes. Rigid cellular PVC is good as a thermal barrier but not for structural parts. Doors and frames of structural molded foam, eg, foamed high impact polystyrene, can be made by injection molding, with recesses for hinges, striker plates, and miter corners. Solid polystyrene and structural foam-molded polyurethane have been molded for door frames. 

When you pour out a glass of fizzy drink, the frothy part at the top of the drink is a gas/liquid mixture called a foam. The gas, carbon dioxide, has formed tiny bubbles in the liquid but has not dissolved in it. If left to stand, foams like this one collapse as the tiny bubbles join together to form bigger bubbles which then escape. It is possible to form solid foams where the gases are trapped in a solid structure. This happens in foam rubber and bread (Figure 2.38). 

Table 1.1 Types of dispersions. Porous solids have a bicontinuous structure while in a solid foam the gas phase is clearly dispersed.

Foams have a large variety of applications. Solid foams are widely used as insulating materials. Due to the presence of air bubbles they have a low thermal conductivity. Polyurethane foams and Styrofoam are examples. Styrofoam is also used as a packing material. The light weight of polymer foams makes them attractive as filling materials to stabilize otherwise hollow structures. A natural solid foam is pumice stone. Metal foams are used in the automotive and aerospace industry as light and stable materials [567], Ceramic foams are developed for electronic applications as piezoelectric transducers and low dielectric constant substrates [568],... 

To make a solid foam we start from a liquid foam and induce solidification. This can be achieved by a chemical polymerization (Styrofoam), by lowering the temperature (pumice stone or a souffle), or by increasing the temperature to induce a structural transition (baking of bread). Porous solids can appear as solid foams because of their low density and their high content of gas. The difference between the two is that in a porous solid we have a bicontinuous structure while the individual cavities in a foam are closed. This is an important difference because porous solids tend to adsorb liquids due to capillary effects and then completely change their properties. 

Here you find the pastes. Hazelnut paste is a dispersion of particles in a thick emulsion of two liquids, as is peanut butter. Jam is thickened by natural polymers. Soft cheese, butter and margarine are in the refrigerator these are complicated structures of fat crystals, oil, water and many other components. All these pastes have a yield stress that is low enough to let them be spread by a knife, but not so low that they run off bread. Users do find the cold butter a bit stiff and the jam a bit thin. As a developer you might want to improve these things. Bread - a solid foam - is a surprising structure when looked at it closely. Fresh bread is often too soft to cut easily. 

Besides ceramic monoliths, metallic monoliths are available (64). In comparison with ceramic monoliths, metallic monoliths can be produced in more advanced structures, for example, to create turbulence in the flow in the channels (65). Several structured catalyst supports, such as solid foams or Sulzer packings, are usually made from metal. The surface area of the metal itself will be usually too low for practical applications. 

The perceived crispiness of chips and many cereal-based products is the key determinant characterising product quality. Most crispy products are characterized by a cellular, lamellar or puffed structure often described as solid foam. The crispiness of a product is closely related to its structure and the extent of porosity. However, sensory evaluation of crispiness is difficult to correlate with instrumental parameters mainly because crispiness is not a clearly defined attribute (Roudaut et al. 2002). 

A clearer understanding of the relationship between foam structure and mechanical properties of solid foams has been developed by Gibson and Ashby (1988). They related the mechanical properties (e.g., strength, modulus, yield stress, fracture toughness) of idealised cellular solids to their relative density. This work considered the cell walls of solid foams as a three-dimensional network of beams (Figure 20.18) and treated their deformation in terms of classical solid mechanics, with strength and modulus related to beam thickness and length by the equations ... 

A simple but often very successful approach in imaging of solid materials is the use of liquids as contrast agents, because they can be investigated by standard techniques. Porous structures like foams, internal cracks, voids or other spatial features can easily be analyzed in this way [116]. Furthermore, the local separation of liquids in pastes depending on external forces has been demonstrated in extrusion processes of PTFE/water pastes [94]. This kind of investigation is based on the assumption that the interaction of the... 

The cells of the structure can be either closed or open. The former type is like a foam, in that the matrix can geometrically be compared with the continuous phase of the foam, which consists of thin lamellae and Plateau borders. Here we have thicker lamellae or walls, and beams (struts, ribs) where two lamellae meet if the cells are filled with gas we can call the system a solid foam. Open cells occur when the lamellae contain holes now we speak of a sponge. Some types of sponge structures merely consist of beams. Most plant tissues (see, e.g., Figure 9.4) consist of closed cells that are mainly filled with an aqueous liquid. 

Generally, Eq. (17.21) is well obeyed, but for most solid foams the exponent is smaller than three, sometimes even as small as two, because even for a closed-cell structure most of the matrix material may be in the beams rather than in the walls. The theory can be extended for other rheological parameters, such as the yield stress, but these relations are mostly not well obeyed. As a general rule, however, one may state that for any rheological parameter Z... 

A foam is a dispersion of gas bubbles in a relatively small volume of a liquid or solid continuous phase. Liquid foams consist of gas bubbles separated by thin liquid films. It is not possible to make a foam from pure water the bubbles disappear as soon as they are created. However, if surface active molecules, such as soap, emulsifiers or certain proteins, are present they adsorb to the gas-liquid interfaces and stabilize the bubbles. Solid foams, e.g. bread, sponge cake or lava, have solid walls between the gas bubbles. Liquid foams have unusual macroscopic properties that arise from the physical chemistry of bubble interfaces and the structure formed by the packing of the gas bubbles. For small, gentle deformations they behave like an elastic solid and, when deformed more, they can flow like a liquid. When the pressure or temperature is changed, their volume changes approximately according to the ideal gas law (PF/r= constant). Thus, foams exhibit features of all three fundamental states of matter. In ice cream, the gas phase volume is relatively low for a foam (about 50%), so the bubbles do not come into contact, and therefore are spherical. Some foams, for example bubble bath. 

A.M. Kraynik, 2003, Foam Structure From Soap Froth to Solid Foams , Materials Research Society Bulletin 28 275. 

Cavity insulation applications, where a solid foam is produced in situ inside the cavity walls, without dismantling the walls are done hy direct injection of the liquid mixture of components and catalyst together into the cavity. This method is simple enough to carryout, but has the risks of carrying toxic chemicals and remnants of monomer with the foam structure, that can later migrate slowly indoors. 

Figure 15.14 Riesterer, J.L., Gilliss, S.R. CBC. Figure 15.17 Courtesy of Andrew M. Kraynik, from Kraynik, A.M. (2003) Foam structure From soap froth to solid foams , MRS Bulletin April 275. By permission of the Materials Research Society. (Redrawn)... 

Foams were proved to be highly suitable as catalytic carrier when low pressure drop is mandatory. In comparison to monoliths, they allow radial mixing of the fluid combined with enhanced heat transfer properties because of the solid continuous phase of the foam structure. Catalytic foams are successfully used for partial oxidation of hydrocarbons, catalytic combustion, and removal of soot from diesel engines [14]. The integration of foam catalysts in combination with microstructured devices was reported by Yu et al. [15]. The authors used metal foams as catalyst support for a microstructured methanol reformer and studied the influence of the foam material on the catalytic selectivity and activity. Moritz et al. [16] constructed a ceramic MSR with an inserted SiC-foam. The electric conductive material can be used as internal heating elements and allows a very rapid heating up to temperatures of 800-1000°C. In addition, heat conductivity of metal or SiC foams avoids axial and radial temperature profiles facilitating isothermal reactor operation. 

In all materials (plastics, metals, wood, etc.) elementary mechanical theory demonstrates that some shapes resist deformation from external loads. This phenomenon stems from the basic physical fact that deformation in beam or sheet sections depends upon the mathematical product of the modulus of elasticity (E) and the moment of inertia (I), commonly expressed as El (Chapter 3, Stress-strain behavior). It is applied to all types of constructions such as solids, foams, and sandwich structures. In many applications plastics can lend themselves in the form of a sophisticated lightweight stiff structure and the requirements are such that the structure must be of plastics. In other instances, the economics of fabrication and erection of a plastics lightweight structure and the intrinsic appearance and other desirable properties make it preferable to other materials. 

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