Foamed-cell collapse

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 comparison of the experimentally determined rate of decrease in the number of foam cells in a foam from an aqueous NaDoS solution, with that calculated from the equation for the diffusion bubble expansion, supports the conclusion that coalescence contributes significantly to the internal foam collapse. This has been evidenced by New [28], Film rupture in the foam was registered directly by a camera. 

The structure of a foam, which deforms in a specific manner also influences physical properties. Upon initial compression there is a deformation of the structure that requires an increase in the amount of force applied. Once the foam has been deformed significantly the sides of the cell walls buckle leading to cell collapse and the production of elliptically deformed structures. A picture of such cells deformed under extreme compression is shown in Figure 14. During this phase of the compression the rate of increase in the force applied is significantly decreased. 

It is clear from Figure 12 that open-cell foams can be obtained only by a good balance between two reactions, i.e., polyurethane formation and gas generation. When gas generation is too fast, the rising foam may collapse like beer bubbles, because the foam is not stable. 

Most thermoplastic foams can be solvent cemented. However, some solvent cements will collapse thermoplastic foams. The best way to determine if such a problem exists is to try it. In cases where the foam collapses due to softening of the foam cell walls it is desirable to use water-based adhesives based on SBR or polyvinyl acetate, or 100%-solids adhesives. In general, the relatively amorphous thermoplastics, such as the cellulosics, polycarbonate, and polystyrene are easier to solvent cement than the crystalline materials, but there are exceptions. 

Polyester- and polyether-based rigid urethane foams generally require a surfactant, whether expanded with COj from the water-isocyanate reaction, or with an inert blowing agent such as fluorocarbon. Without surfactant the foam may collapse or have a coarse cell structure. Castor-oil-based systems generally do not require surfactants, but better results will be obtained if they are used (20). 

A number of fundamental works are dedicated to the investigation of foam stability [6, 17] but the interest of the investigators in this problem is persistent, which is evidenced by recent review papers, e.g. [20]. In this section data are given which can be of interest for the practical application of foams. First of all, several processes take place simultaneously in real foams, which lead to foam collapse. The main processes are redistribution of the disperse medium in differently high foam column layers and the change of the mean radius of the foam cells [12]. 

The result of these processes is a gradual decrease in the foam column height (H) in a layer-wise foam collapse or a avalanche -like decay of the foam volume when reaching a critical size of the polyhedral foam cells. 

Phenylene oxide-based resins (Noryl ) epoxy, polyisocyanate, polyvinyl butyral, nitrile rubber, neoprene rubber, polyurethane rubber, polyvinyUdene chloride, and acrylic. Polyethylene-nitrile rubber, polyisobutylene rubber, flexible epoxy, nitrile-phenolic, and water-based (emulsion) adhesives. Polystyrene for these foams (expanded polystyrene (EPS)), aromatic solvent adhesives (e.g., toluol) can cause collapse of the foam cell walls. For this reason, it is advisable to use either 100% solids adhesives or water-based adhesives based on SBR or polyvinyl acetate. Specific adhesives recommended include urea-formaldehyde, epoxy, polyester-isocyanate, polyvinyl acetate, vinyl chloride-vinyl acetate copolymer, and reclaim rubber. Polystyrene foam can be bonded satisfactorily with any of the following general adhesive types ... 

Cell collapse n. A defeat in foamed plastics characterized by slumping and cratered surfaces, with the internal cells resembling a stack of leaflets when viewed in cross section under a microscope. The condition is caused by tearing of the cell walls, weakened by plasticization or other mechanism. 

Surfactants are used to control cell development. Both cell size and shape are affected to the extent that, without surfactant additives, large irregular cells may develop or the foam may collapse altogether. The most widely used type of surfactant for rigid-closed cell PU foams is based on silicone copolymers, although organic types, such as sulfonated castor oil and amine esters of fatty acids, are also used. 

Under normal processing conditions the surface of the moulded part always has a swirl pattern. This occurs on mould filling and is due to the collapse of the foam cells as they come into contact with the cold mould surface. 

Amine catalysts are primarily used to catalyze the isocyanate-water reaction ( blowing catalyst ), while tin or other metal catalysts are used to regulate the rate of the isocyanate polyol reaction ( gelling catalyst ). Surfactants are used up to 2 pph (parts per hundred) to regulate the cell size. Higher amounts of the surfactant produce thinner cell walls and smaller cells. An excessive amount would cause collapse of the foam as the walls and ribs of the foam cells could not support the pressure of the gas. 

Foaming by discharging the melt into the atmosphere, on the other hand, produced closed cells with a mean cell diameter of ca. 500 LLm. The foam cells were polyhedral in shape and quite uniform in size, which is comparable to a commercial foam with a mean cell size of ca. 1.0 mm. But, the DDC foams contained slightly collapsed cellular structures. Seemingly, this foaming process created a more rapid and uniform cooling of the melt, causing a rapid build-up of viscosity, which substantially stabilized bubbles. 

Cell Collapse n A defeat in foamed plastics characterized by slumping and cratered surfaces, with the internal... 

Low-density thermoplastic foam cannot be heated to a forming temperature appropriate for the plastic without excessive cell collapse and poor product quality. Inadequate heating yields low secondary expansion and products that do not replicate the mold cavities with just vacuum. Pressure forming will collapse the foam cell structure. As a result, foam sheet is usually heated in roll-fed machines on traditional pin-chain rails. The ovens are usually extended in length and have heaters that gradually heat the sheet to temperatures where the increasing internal gas... 

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