Foamed surfaces

CeUular urea—formaldehyde and phenoHc resin foams have been used to some extent in interior sound-absorbing panels and, in Europe, expanded polystyrene has been used in the design of sound-absorbing doors (233). In general, cost, dammabUity, and cleaning difficulties have prevented significant penetration of the acoustical tile market. The low percent of redection of sound waves from plastic foam surfaces has led to their use in anechoic chambers (216). 

A rigid foam sheet product is made by heating at least one surface of a crosslinked PE foam sheet material to a temp, sufficient to cause a slight surface post-expansion of the sheet material and contacting the heated, post expanded foam surface with a cold metal element to form a product with the desired, accurate thickness or having a desired three-dimensional design on the sheet material. USA... 

Zhang, X., Guan, R. F., Wu, D. Q. and Chan, K. Y. Enzyme immobilization on amino-functionalized mesostmetured cellular foam surfaces, characterization and catalytic properties, J. Mol. Catal., B, 2005, 33, 43-50. 

For common thermally thick combustible materials (greater than a few millimeters) the time to ignition is proportional to the product k p c (where k is the thermal conductivity, p is the density, and c is the heat capacity), which represents the thermal inertia of the sample. Thermal inertia characterizes the rate of surface temperature rise of the material when exposed to heat. Low values of thermal inertia lead to a rapid temperature rise for a given applied heat flux and hence, to a rapid ignition.4 Polymeric foams have much lower thermal conductivity and density than the corresponding solid materials, thus the surface temperature of the first heats up more rapidly than that of the latter. Foam surface may reach the ignition temperature 10 times faster than the solid polymer.5... 

In order to generate foam, surfaces of thin liquid films always have to be stabilised by layers of surfactants, polymers or particles. This is why pure liquids never foam. Foaming is always accompanied by an increase in the interfacial area and, hence, its free energy. Thus, in a thermodynamic sense foams are basically unstable and are, therefore, sooner or later destroyed. The lifetime of a foam can span a remarkable range from milliseconds to very long duration. 

Foam dispersity is characterised by the average bubble size, by bubble size distribution or by the specific foam surface e. There are three different specific foam surfaces... 

At such a deformation foam surface energy F5 changes according to the following expression (assuming that surface tension does not change)... 

Equalising Eqs. (1.32) and (1.33) and accounting for Eq. (1.34), an expression is derived which indicates that excess (capillary) pressure in bubbles is determined by both specific foam surface area and solution surface tension but does not depend on the shape of films and bubbles... 

A second group of methods for dispersity determination are based on the dependence between the specific foam surface area and the average excess capillary pressure in foam bubbles. 

The main features of this technique are the absence of contact between the foam and the ambient space (i.e. no foam/gas interface) and constant capillary pressure along the whole foam height. This technique allows to study the kinetics of internal foam destruction at various capillary pressures, i.e. decrease in the specific foam surface area without destruction of the foam column. Thus, the influence of surface foam films on foam lifetime and the character of foam destruction can be estimated. 

Fig. 6.1 depicts the experimental time dependence of foam expansion ratio, surfactant concentration in the flowing out solution as well as the rate of internal foam collapse. Within the whole time interval the rate of diminishing of specific foam surface area for a sulphonol foam is less than that for a NP20 foam, though the difference is not significant. Probably this is related to the fact that in the initial stage of internal foam collapse the rate deF/dx is determined by the gas mass-transfer that does not depend considerably on the surfactant kind. 

There is a qualitative agreement between the results for the rate of decrease in the specific foam surface area estimated by the increase in surfactant concentration during gravitational foam breaking and those from the kinetic studies of foam dispersity changes. 

Accounting for Eq. (6.15) the following expression has been derived for the specific foam surface area evs [5]... 

Separating the process of diffusion transfer (decrease in the specific foam surface area) into stages within which Rm can be assumed constant, Clark and Blackman [29] have derived equation for each stage (at R/Rm < 1)... 

Fig. 6.5. presents the dependence of both the relative number of residual bubbles and the specific foam surface area on the dimensionless time. Calculations reveal as well that the relative change in the specific foam surface area, regardless of the accepted distribution function, is considerably smaller than the relative change in bubbles number. For example, at moment T= 1, e(7)/e(0) = 0.8, while the relative number of the remaining bubbles is 0.4 - 0.5. 

Fig. 6.6. Distribution of the relative specific foam surface area by bubble radius at different moments of...

It is technically difficult to distinguish between diffusion foam collapse and coalescence. That is why the rate of the entire process of internal foam collapse is usually studied. Fig. 6.9 depicts some typical curves for the increase in the average bubble size and the decrease in the specific foam surface. 

When foams are formed by blowing highly soluble gases (carbon dioxide, propylene) the decrease in the specific foam surface area is more rapid. This indicates that for a long period of time the decisive process in foam collapse is the diffusion transfer (see Fig. 6.7). 

It is well know that the direct comparison between the methods of estimation of foam stability (in most of the cases it is determined by the foam lifetime) is not possible. Each of the existing methods involves different parameters, for example, time for destruction of a foam column of a definite height (or part of it), rate of decrease in the specific foam surface, etc. The main reason for the impossibility to make such a comparison is that foam stability is determined at different pressures in the foam liquid phase. This means that the rate of drainage as well as the time of reaching an equilibrium state of the films in the foam is different. Another reason could be attributed to the possibility both foam formation (i.e. foam volume... 

A conclusion has been drawn in [67] that the extinction of the luminous flux (7//o, where, 7o is the intensity of the incident light and 7 is the intensity the light passed through the foam) is a linear function of the specific foam surface area. A similar dependence has been used also for the determination of the specific surface area of emulsions [68]. Later, however, it has been shown [69,70] that the quantity 7//o depends not only on the specific surface area (or dispersity) but also on the liquid content in the foams, i.e. on the foam expansion ratio, that during drainage can increase without changing the dispersity. Since foam expansion ratio and dispersity are determined by the radii of border curvatures and film thicknesses, all the structural elements of the foam will contribute to the optical density of foams. This means that... 

From Eq. (10.58) it is seen that can be defined as the cross point of the VF/Vg (c o) dependence with the abscissa axis (further in the text cl,o will be denoted as C). At large surfactant concentrations when the fraction of the dispersed gas retained in the foam strongly increases, the VF/Vg ratio tends to 1. In this case the whole initial solution is transformed into a foam and further increase in the expansion ratio becomes impossible, so a considerable part of the surfactant remains in the solution, i.e. reGVg =c-cLR cL0- cmn. As indicated by the experiments, at low concentrations ct,o - Cmm there is a region where the VF /Vg (C) dependence becomes linear. The extrapolation of this linear segment gives cmjn. The slope angle of this dependence, equal to VLOn / VtTe(ti -1), can be used to estimate the adsorption, needed for the formation of a stable foam with Pf > 1, in case the value of the specific foam surface area is known. 

Fig. 13. Effect of partial oxygen pressure near the polyurethane foam surface and of the oxygen concentration in an oxidant flow on the conditions of combustion A flaming combustion O smouldering slow oxidation (After Ref. with permission)...

During the time of preequilibration part of the crude oil formed a stable oil in water emulsion (more pronounced for AOS). Prior to extracting the liquid from the double concave meniscus, it was observed that floating lenses of crude oil were spread upon the foam surfaces. 

The pattern can be shaped with traditional carving tools or cut to size with a nichrome hot wire cutter, available from model plane companies.Complex patterns are best cut out as simpler subunits. The pieces are then joined with spray adhesive or rubber cement. These glues will vaporize along with the foam.Surface blemishes in the finished pattern can be filled w/combustible wax filler if needed. 

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