Some General Properties of Foams

Before considering the behavior of antifoams, we review the relevant properties of foams. Only a brief summary is given here. It is for the most part only concerned with those aspects that may have relevance for the understanding of antifoam action. For more complete accounts, the reader is referred to the many books [1-9] and reviews on the subject [10-27]. 

This brief review includes definition of the structural features of foams. A summary of the processes occurring in foam films follows with particular emphasis on the factors that determine the stability of those films. Finally, we include an outline of the processes of drainage and diffusion-driven coarsening, which concern the entire body of a foam and not just the constituent parts. 

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Some General Properties of Open Cell PU Foams for Vacuum Insulated Panels... 

Mechanical Properties and Structural Performance. As a result of the manufacturing process, some cellular plastics have an elongated cell shape and thus exhibit anisotropy in mechanical, thermal, and expansion properties (35,36). Efforts are underway to develop manufacturing techniques that reduce such anisotropy and its effects. In general, higher strengths occur for the paraHel-to-rise direction than in the perpendicular-to-rise orientation. Properties of these materials show variabiUty due to specimen form and position in the bulk material and to uncertainty in the axes with respect to direction of foam rise. Expanded and molded bead products exhibit Httie anisotropy. 

Chemical binders are appHed to webs in amounts ranging from about 5 to 60 wt %. In some instances when clays (qv) or other weight additives ate included, add-on levels can approach or even exceed the weight of the web. Waterborne binders ate appHed by spray, saturation, print, and foam methods. A general objective of each method is to apply the binder material in a manner sufficient to interlock the fibers and provide chemical and mechanical properties sufficient for the intended use of the fabfic. 

The stability of foams in constraining media, such as porous media, is much more complicated. Some combination of surface elasticity, surface viscosity and disjoining pressure is still needed, but the specific requirements for an effective foam in porous media remain elusive, partly because little relevant information is available and partly because what information there is appears to be somewhat conflicting. For example, both direct [304] and inverse [305] correlations have been found between surface elasticity and foam stability and performance in porous media. Overall, it is generally found that the effectiveness of foams in porous media is not reliably predicted based on bulk physical properties or on bulk foam measurements. Instead, it tends to be more useful to study the foaming properties in porous media at various laboratory scales micro-, meso-, and macro-scale. 

It is possible to draw some general trends and it is certainly true to say that the differences between ether sulphates are less than the difference between ether sulphate and its corresponding alkyl sulphate. Figure 4.19 shows the foaming properties of a series of alkyl sulphates and their corresponding 2 and 3 mol ether sulphates. 

The strong effect of density and polymer composition on compressive strength and modulus are illustrated in Tables 10.1 and 10.2 [8]. The cell shape or geometry also influences compressive properties [8,22,23,27,28]. In fact, the foam cell structure is controlled in some cases to optimize certain physical properties of rigid cellular polymers. In general, compressive strength and modulus of low-density foams may be expressed as... 

Foamed PDMS elastomers generally play an important role in a number of specific stress absorbing applications. Reports on Polystyrene and Polyurethane foams have tended to dominate the literature in recent years. Such systems generally exhibit high stiffness to weight ratios and some unique mechanical properties that are dependent upon the microstructure of the foam and the properties of the polymer making up the cell walls 10). In general, the desirable material property requirements of foams may be summarised as shown below ... 

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