Open- and closed-cell foam

There are two types of foams closed cell foams and open cell (or reticulated) foams. In open foams, air or other fluids are free to circulate. These are used for filters and as skeletons. They are often made by collapsing the walls of closed cell foams. Closed cell foams are much stiffer and stronger than open cell foams because compression is partially resisted by increased air pressure inside the cells. Figure 19.1 shows that the geometry of open and closed cell foams can modeled by Kelvin tetrakaidecahedra. 

Open and closed cell foams modeled by tetrakaidecahedra. 

Key words cellular metals, metallic foam, open- and closed-cell foam, hollow-sphere foam, optimized truss structures, honeycomb. 

The relative strength of hollow-sphere foams lies between the theoretical performance of open- and closed-cell foams. The performance of optimized truss structures is similar to that of closed-cell foams and, for the Kagome truss, approaches the behavior of a Hashin-Shtrikman porous material. Honeycombs are the most efficient structures when loaded purely out-of-plane. However, plastic buckling can decrease its performance at low relative densities. Further, since honeycomb is highly anisotropic, any inplane loading results in severely reduced performance. Although the theoretical performance of closed-cell foams far exceeds that of open-cell foams, processing defects result in commercially available material that behaves similar to an open-cell material at low relative densities. Commercially available samples of other types of low-density metallic structures behave nearly as predicted. [17]... 

Figure 6.27 Relative plastic collapse stress for open- and closed-cell foams.9...

The above analytical model will give a reasonable approximation when designing a syntactic foam material. There are examples of more complicated numerical models available throughout literature if more detailed analysis is desired. We refer the reader to Gibson and Ashby (15) for analytical relationships for elastic properties for both open- and closed-celled foam that are based on the properties of the unfoamed material and relative density of the foam. 

Foamed polymers are characterised by a cellular structure and reduced density compared to solid material. They may be divided into open and closed-cell foams as well as foams with homogeneous cell structures and those with a porous core and compact (unfoamed) outer skin. Foam densities range from as low as approx. 10 kg/m in some polymers to close to that of compact materials. 

Once the surface tension of foams is sufficiently raised, adhesives can more effectively wet and bond to open- and closed-cell foams. In open cell foams, adhesives can spread into the pores of the foam. Although the benefit can be superior adhesion as a result of mechanical bonding, the functional properties of the foam can be adversely affected. Typically, permeability and thermal properties of the foam may be compromised to the point where the structural integrity of the foam is weakened. This is particularly true when introducing solvent-borne adhesives. 

As previously mentioned, the homogeneity of foamed materials and the proportion of open and closed cells can be influenced by additives such as emulsifiers and stabilizers. Emulsifiers (e.g., sodium, potassium, or zinc salts of long-chain fatty acids) cause a uniform distribution of water in the reaction mixture, ensuring homogeneous foaming, while stabilizers (certain silicone oils) prevent a breakdown of the cell structure at the beginning of the reaction and also act as pore regulators. 

In order to establish and evaluate quantitatively the relation between the morphology and the properties of foamed polymers the basic macrostructural parameters must be determined. These parameters include relative number of open and closed cells, volumetric weight or apparent density cell size, shape, wall thickness, cell distribution according to size and shape in a given volume and specific surface area of the foamed plastic material. 

The volumetric weight and the ratio between the number of open and closed cells are the fundamental morphological parameters of foamed plastics. Nevertheless, it has been reported that even for the same volumetric weight and number of open (or closed) cells the strength and thermophysical parameters may be widely different in plastic foams made of the same polymer grade. The differences in cell shapes and sizes are responsible for this fact. 

Open/Closed-Cell Ratio. The ratio of open to closed cells in a foam has important effects on many important properties. Although poor measurement techniques have reduced many studies from quantitative to simply qualitative, and although many foam processes produce only a semicontrollable mixture of open and closed cells, the basic relationships are of major theoretical and practical significance. 

FIGURE 41.7 Cellular solids structures, after Gibson and Ashby [1988]. Left synthetic cellular solids (a) open-cell polyurethane, (b) closed-cell polyethylene, (c) foamed nickel, (d) foamed copper, (e) foamed zirconia, (f) foamed mulllte, (g) foamed glass, (h) polyester foam with both open and closed cells. Right natural cellular solids (a) cork, (b) balsa wood, (c) sponge, (d) cancellous bone, (e) coral, (f) cuttlefish bone, (g) iris leaf, (h) plant stalk. 

Definition Types include flexible, semirigid, and rigid foams, open- or closed-cell foams, and high-... 

The act of foaming a plastic material results in products with a wide range of densities. These materials are often termed cellular plastics. Cellular plastics can exist in two basic structures closed-cell or open-ceU. Closed-cell materials have individual voids or cells that are completely enclosed by plastics, and gas transport takes place by diffusion through the cell walls. In contrast, open-ceU foams have cells that are interconnected, and fluids may pass easily between the cells. The two structures may exist together in a material so that it may be a combination of open and closed cells. 

Processes and ingredients used in the manufacture of rubber latex foams and open- and closed-cell sponge rubber are described. 11 refs. 

Fosm. Phenolic resin foam is a cured system composed of open and closed cells with an overall density of 0.16-0.80 g/cm . Principal applications are in the areas of insulation and sponge-like floral foam. The resins are aqueous resoles catalyzed by NaOH at a formaldehyde/phenol ratio of 2 1. Free phenol and formaldehyde content should be low, although urea may be used as a formaldehyde scavenger. 

Plastic foams are also classified according to their mechanical properties. If the walls of the cell are stiff under stress and relatively inflexible, the foam is called rigid. The foam is flexible if its wall collapses under stress. Both open and closed structure foams can form rigid and flexible foam. 

Both open- and closed-cell backups are used. The closed-cell backup does not permit any water to contact the sealant but has been a problem where the backer rod is first compressed into the joint. If the rod is punctured during installation, gas will slowly be released into the uncured sealant, causing surface bubbles. This bubbling can be quite conspicuous if the wall is exposed to sunlight on a hot day before the sealant cures. The solution is to use open-cell foam. Open-cell foam is desirable if a one-part sealant that requires moisture activation is used. Here, the sealant will begin curing from both sides. However, the open-cell foam may in time become saturated with moisture and cause adhesion problems or even frost formation and spalling. Properly vented walls would reduce this problem. There is no ideal solution, and the selection of backup material will depend on the sealant used and job site conditions. 

With low-density foams, the relationships of physical properties to structure are very complex because of the wide variation of cell structures possible. Polyurethane foams are probably the most versatile of all. They are capable of being made rigid or flexible, and with open or closed cells. Foam rigidity is controlled by the degree of cross-linking, generally based on the type and concentration of trifunctional reactants in the formulation. 

Micromechanics theories for closed cell foams are less well advanced for than those for open cell foams. The elastic moduli of the closed-cell Kelvin foam were obtained by Finite Element Analysis (FEA) by Kraynik and co-workers (a. 14), and the high strain compressive response predicted by Mills and Zhu (a. 15). The Young s moduli predicted by the Kraynik model, which assumes the cell faces remain flat, lie above the experimental data (Figure 7), while those predicted by the Mills and Zhu model, which assumes that inplane compressive stresses will buckle faces, lie beneath the data. The experimental data is closer to the Mills and Zhu model at low densities, but closer to the Kraynik theory at high foam densities. 

The benefits of the Sioplas moisture crosslinking process for the manufacture of crosslinked PE foam and the end-uses of such foams are discussed. The process involves grafting of alkoxysilanes onto ethylene homo- or copolymers to provide moisture crosslinkable polymers suitable for the manufacture of flexible, rigid, open-ceUed, closed-cell or very low density foams. 11 refs. 

Closed-cell foams made from metallocene-based polyolefins (MPO) have potential for use in various applications because of their uniform composition and low toxicity. Compressive stress relaxation is used to investigate the behaviour of these foams. In particular, its behaviour is compared with open-cell PU foams, a material MPO foams could possibly replace. The effect of gamma radiation on MPO foam behaviour is also... 

What are the advantages of open-celled foams and how do they differ from closed-cell foams ... 

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