Foamed cell closed

Fig. 25.8. Poly meric foams, showing the polyhedral cells. Some foams hove closed cells, others hove cells which ore open.

Wood, then, is a foamed fibrous composite. Both the foam cells and the cellulose fibres in the cell wall are aligned predominantly along the grain of the wood (i.e. parallel to the axis of the trunk). Not surprisingly, wood is mechanically very anisotropic the properties along the grain are quite different from those across it. But if all woods are made of the same stuff, why do the properties range so widely from one sort of wood to another The differences between woods are primarily due to the differences in their relative densities (see Table 26.1). This we now examine more closely. 

Rigid polyurethane foams are mostly based on polyether alcohol and are highly cross-linked. Rigid foams are many times blown by halogenated alkanes like trichlorofluoromethane. These foams have closed cell structures and are used for thermal insulation. Semi-rigid foams are used in car crashpads and packaging. 

The absorption and the permeability to water or moisture are low for those foams with closed cells and their hydrolysis behaviour is generally fair. 

This rigid linear foam with closed cells was developed by ATC Chemicals Company to be used as the core in lightweight structural sandwich composites particularly intended for boatbuilding. SAN foams have ... 

This low-density foam with closed cells is associated with ... 

An alternative means of generating a polyimide foam with pore sizes in the nanometer regime has been developed [80-90]. This approach involves the use of block copolymers composed of a high temperature, high Tg polymer and a second component which can undergo clean thermal decomposition with the evolution of gaseous by-products to foam a closed-cell, porous structure (Fig. 7). 

A collection of polyolefin foams with closed cell structure and different chemical compositions and densities was studied by using SEM, DSC and dynamic mechanical analysis. Deformation mechanisms were also studied. 26 refs. 

When a foam contains closed cells, we can apply Kerner s formula with a = 0,... 

The conclusion is that the first foam has closed cells, the third one is completely open-celled and the second one contains partly open, partly closed cells. 

The so-called bi-liquid foams described by Sebba and Vincent [17-20] will not be discussed since they are not true colloidal emulsions, but resemble conventional foams more closely, possessing polyhedral cells of centimetre dimensions. 

The pentagonal dodecahedron, however, is not entirely space-filling, i.e. a close-packed array of such figures has a number of interstitial voids. On the other hand, Kelvin s tetrakaidecahedron and the P-tetrakaidecahedron are. The latter requires 4% more surface area, so a system of such figures would spontaneously rearrange to the more stable array of Kelvin cells. Thus, it would seem that Kelvin s tetrakaidecahedron is the ideal candidate nevertheless, this is not observed in real systems Pentagonal faces are shown on foam cells. These... 

In a subsequent theoretical study, Stamenovic [60] obtained an expression for the shear modulus independent of foam geometry or deformation model. The value of G was reported to depend only on the capillary pressure, which is the difference between the gas pressure in the foam cells and the external pressure, again for the case of <)> ca 1. Budiansky et al. [61] employed a foam model consisting of 3D dodecahedral cells, and found that the ratio of shear modulus to capillary pressure was close to that obtained by Princen, but within the experimental limits given by Stamenovic and Wilson. 

The resulting polymer foam composition is substantially of the closed cell type. This is evidenced by the fact that for equivalent densities, foams of EVA and acid copolymer are found to exhibit lower helium densities than foams of EVA alone. This is an indication that more of the cells in the EVA/acid copolymer foam are closed. Particularly for an acid copolymer content in the range of about 3-15%, the acid copolymer has been observed to be uniformly dispersed within the EVA in micron-sized particles when analyzed by transmission electron microscopy. 

Flexible Polyurethane. 1 hcse foams are produced from long-chain, lightly branched polyols reacting wilh a diisocyanale. usually toluene diisocyanale (TDI), to form an open-celled structure wilh free air flow during flexure. During manufacture these foams arc closely controlled for proper density, ranging from 17 to 80 kg/ml (0.8-5 Ibs/ft ). to achieve the desired physical properties and cost. 

Foamed plastics can be classified in different ways, for instance by their nature (flexible vs. rigid), chemical composition of the matrix, density, cell size, cell structure (open-celled vs. closed-celled), processing method, and dimensions. It is the aimed combination of these properties that determines the final application of the cellular polymer. As an example, open-celled ultra-low density foams are highly desirable for acoustical insulation, while rigid foams with closed-cells and elevated densities are preferred as load-carrying core materials in composite materials. 

Certain grades of ETFE are used for extruded foams with void contents from 20 to 50%. The closed foam cells are 0.001 to 0.003 in. (0.02 to 0.08 mm) in diameter. Special grades of ETFE processed in gas-injection foaming process may have void contents up to 70%. Foamed ETFE is used in electrical applications, mainly in cables, because it exhibits lower apparent dielectric constant and dissipation factor and reduces cable weight. 

The foam scrubbing technique is effective because it brings the hazardous material into close contact with the foam by getting it into the bubbles. This is different from using a foam blanket as a cover for spills (see Chapter 3). With the large internal surface area of the foam available for absorption or mass transfer, an equilibrium concentration between the contaminated air inside the bubble and the foam cell wall liquid can be developed rapidly. Unabsorbed gas that is still in the foam bubbles when they collapse is released. This results in the slower release of a smaller quantity of hazardous material, which should result in a reduced hazard zone downfield. 

The volume and shape of Plateau borders depend on the expansion ratio of the foam. In a spherical monodisperse foam with close packing of bubbles all air/liquid interfaces are spherical and the liquid volume which belongs to one cell can be derived from the difference between the volumes of the corresponding polyhedron (for example, a dodecahedron) and the inscribed in it sphere, having in mind the co-ordination number of the foam cell. 

The edges of this dodecahedron sized a - 8.5 cm. When the volume of the rubber balloon at inflation became bigger than the volume of the sphere inscribed in the dodecahedron, the balloon was deformed by the dedecahedron faces and took a shape close to the respective shape of a bubble in a monodisperse dodecahedral foam with a definite expansion ratio. The expansion ratio of the foam was determined by the volume of liquid (surfactant solution or black ink in the presence of sodium dodecylsulphate) poured into the dodecahedron. An electric bulb fixed in the centre of the balloon was used to take pictures of the model of the foam cell obtained. The film shape and the projection of the borders and vertexes on the dodecahedron face are clearly seen in Fig. 1.10. 

In a polyhedral foam the liquid is distributed between films and borders and for that reason the structure coefficient B depends not only on foam expansion ratio but also on the liquid distribution between the elements of the liquid phase (borders and films). Manegold [5] has obtained B = 1.5 for a cubic model of foam cells, assuming that from the six films (cube faces) only four contribute to the conductivity. He has also obtained an experimental value for B close to the calculated one, studying a foam from a 2% solution of Nekal BX. Bikerman [7] has discussed another flat cell model in which a raw of cubes (bubbles) is shifted to 1/2 of the edge length and the value obtained was B = 2.25. A more detailed analysis of this model [45,46] gives value for B = 1.5, just as in Manegold s model. 

Moisture absorptfon of plastic foams is closely connected with cell properties, including thermal insulation properties. Investigation of heat insulation of plastic foams containing a liquid phase helps to elucidate the mechanisms of moisture and mass transfer as weU as of heat transfer in gas-filled materials. 

This survey deals with the fundamental morphological parameters of foamed polymers including size, shape and number of cells, closeness of cells, cellular structure anisotropy, cell size distribution, surface area etc. The methods of measurement and calculation of these parameters are discussed. Attempts are made to evaluate the effect and the contribution of each of these parameters to the main physical properties of foamed polymers namely apparent density, strength and thermoconductivity. The cellular structure of foamed polymers is considered as a particular case of porous statistical systems. Future trends and tasks in the study of the morphology and cellular structure-properties relations are discussed. 

Surfactants. A surfactant is a major raw material for polyurethane foams. Surfactants play an important role in obtaining required cell structures, e.g., fine cells, coarse cells, closed cells, and open cells, and these cell structures then influence foam properties. 

Graft Polyol Technology. Graft polyols (or polymer polyols) are prepared by grafting both acrylonitrile and styrene monomer or acrylonitrile alone to conventional polyether polyols. Graft polyols provide increased load-bearing ability as well as cell-opening, which prevent or minimize the formation of closed-cell foams, because closed-cell flexible foams readily shrink. 

In contrast, when polyurethane formation is too fast, the resulting foam cells are stable, cell membranes are not broken during foam rise, and closed cells are formed. The closed cells are the cause of shrunk foams, because the internal gas pressure becomes lower than the atmospheric pressure. 

Uses a neoprene foam of closed-cell construction as a component. Density is 10 Ib/ft 3 (160 kg/m ) minimum. 

Foamed adhesive tapes Adhesive tapes without a substrate, with the total system consisting of a pressure-sensitive adhesive polymer with two-sided adhesive properties available in foamed and closed-cell structure. They are not to be confused with one or two-side adhesive tapes with foam-structure substrates. In... 

The most Important distinction is between closed- and open-cell foams. In closed-cell (unicellular) foams, each gas bubble is separated from the others by thin walls of polymer these foams are optimal for flotation applications, structural rigidity, and thermal insulation. In open-cell foams, the cells are all interconnecting, and fluids and especially air can flow freely through the foam structure these are optimal for sponge products and for soft flexible materials. In the extreme case, when the last few remaining cell walls (windows) have been chemically dissolved out of an open-cell foam, it is sometimes called "reticulated."... 

Mechanical properties of solid polymers generally become considerably "softer" in the corresponding foams. This is particularly true in open-cell foams. In closed-cell foams, properties per unit area or unit volume are also generally somewhat softer but properties per unit weight in the expanded polymer may be considerably harder and stronger than in the solid polymer because of the principles of sandwich structures. 

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