Foamed cell size

Numbers without circles = largest foam cell size (pm) fEstimated... 

Figure 2. Results for emulsion and foam cell size in PS foam RSM experiment.

Foam cell size and size distribution are important variables when studying foams. Other surfactant properties crucial to the success of an enhanced oil recovery process include critical... 

The characterization of the plastic foam cell size by a linear dimension (often referred to as celt diameter) is simple, convenient and now commonly applied. However, a single linear dimension can unambiguously describe only geometrically regular cells, i.e. spheres and ellipsoids. In all the other cases studied it remains to be defined what is meant by linear dimension. 

As with cell shapes of a real foam, cell sizes in this material can also be characterized only by nominal (effective) values. The actual effective values depend, first, on the observation method (whether direct — macroscopic, or indirect — adsorption, volumetric, picnometric, etc.). Secondly, they depend on the particular simplified model of the structure and cell shape and thirdly on the method of processing the measured data. 

To calculate the foam cell size distribution function we consider, following Mihira an isolated cell foam structure model (Fig. 24). Let r be a true cell radius, and s the radius of sectional circles on the cut surface X, f and s their mean values, of and of their mean square deviations, and f(r) and f(s) their distribution functions. We will denote by x the depth of a cell dissected by the plane X (Fig. 24) and calculate the probability P(r,x) of cells having a radius in the range from r to (r + dr) and a depth from x to (x + dx). The probability P(r) for the cells dissected by the plane X to have a radius r is ... 

Foam cell size and shape, foam cell size distribution (if aerosol-type product)... 

Postulating that the rubber particles are stretched to membranes all having the same thickness, the foam cell size can be expressed as ... 

Ki. and S (i.e., COD) have also been employed in fracture toughness determination of foams using expressions appropriate for plastics [196.197]. The studies have included the effects of specimen geometry (e.g.. ajW), loading rate, and foam cell size [196]. and foam density [197] on the properties. 

Foams 13 Internal vs. external foaming agents, foam cell size consistency Easier-to-control foaming agents, finer cell sizes possible... 

What factors and additives improve foam quality or control foam cell size ... 

Foaming introduces unique factors to monitor, since it is a process in which part quality is determined heavily by radical internal changes in the part. Various factors and additives can influence foam cell size, consistency, or quality. Other economic factors determine whether foaming may even be appropriate for an application. 

Generally, pure polymers have low thermal conductivities, ranging from 0.1 to 0.6W/(m K), as listed in Table 10.1. Foaming polymers may further enhance this low thermal conductivity. Polymer foams with lower density have more air and thus have lower thermal cmiductivity. The cell size of foamed polymers may also have an effect on thermal cOTiductivity. Smaller foam cell size tends to yield lower thermal cmiductivity. Most foamed polymers have thermal cOTiductivity values in the order of 10 W/(m K), which is about 10 times less than the same polymers. Table 10.2 is the list of thermal conductivities for common commercial foamed polymers. 

Lee et al. (99) investigated the sound absorption characteristics of PE qCjqT foams, particularly, the effect of both cell size and foam dimensions on the absorption coefficients. It was observed that larger foam cell sizes exhibited a significant increase in absorption, which was dependent on the frequency of the incoming acoustic energy. 

A special application of the coextrusion technique is the production of polymeric foamed materials with a high strength-to-weight ratio and good acoustic and thermal insulation. Control of the foam cell size is very critical in obtaining these key performance properties. The control of cell size in the thin layers by the microlayer coextrusion technology makes the creation of microcellular foam possible [8]. 

Polymer foams are usually produced with the conventional foam extrusion process, either as rigid foams or flexible foams. A key performance property is thermal insulation, which critically depends on the foam cell size and the thickness of the polymer walls between the pores. Control of the foam cell size can be difficult due to the low solubility of blowing agents and the gas created in the polymer and inhomogeneous nucleation. In particular, the production of polymer foams with cell sizes in the micron and submicron range is a current challenge. 

Figure 2.13. Foam stability as observed against a wetted wall, where the foam cell size is determined by image analysis by using the following. Transformation steps (a) raw image (b) process binary image (c) measured grey-scale image (d) bubble size distribution (from ref. (11)), reproduced with permission...

The main driver for fluoroplastic foams has been the insulation for data transmission cables. An example is coaxial cables that have relatively thick insulation. Its low dielectric constant and dissipation factor are desirable electrical properties. Air has the ideal dielectric constant (1.0). The ideal dissipation factor for data-cable insulation is zero. Perfluoropolymers have low dielectric constant and dissipation factor values (Table 11.2, see Ch. 6 for additional data). Foaming perfluorinated fluoropolymers further reduces the dielectric constants toward 1.0 and moves the dissipation factors closer to zero because the resin is replaced with air-filled cells in the insulation. The decrease in the dielectric constant is proportional for example, FEP insulation with 60% void content had a dielectric constant of More uniform foam cell size and smaller cells yield foams with the best electrical properties. 

Figure 11.2. Effect of a second salt (calcium tetraborate) on the foam cell size of FEP. ...

Table 11.6. Effect of Wire Speed on the FEP Foam Cell Size i...

A fixed center or an adjustable crosshead could be used for the extrusion of foam onto the conductor (Fig. 11.5). More information about wire extrusion and crosshead die design can be found in Sec. 8.3. Operating conditions for FEP and PFA foaming have been given in Fig. 11.6(a). A boron nitride concentrate was blended at a 1 9 ratio with virgin resin. The concentration of boron nitride depends on the desired void content and foam cell size. For example, when the concentrate contained 2% boron nitride, FEP wire insulation had a void content of 60% and a dielectric constant of 1.3. 

The high nucleation efficiency of nanoparticles has been shown to be particularly advantageous for manufacturing microcellular foam (cell size <10 pm, cell density >10 cells/ctf) (Martini-Vvedensky and Waldman et al, 1982). The nucleation efficiency of the nanoparticles is dependent on the particle geometry, aspect ratio, dispersion, concentration, and particle surface treatment. These are discussed in detail in this section. The resulting changes in foam structure (bubble density, bubble size, and size distribution) and matrix properties have profound influence on the foam mechanical properties. 

Research Module 3 Further development of the extrusion-assisted rotational foam molding technology based on the acquired fundamental understanding of the process, which includes feasibility investigation to decrease the average foam cell size to the sub 100 micron level with a view of creating a new class of rotationally molded microcellular foams. 

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