Foam control choice

Most thermoplastics and thermosets can be foamed, many of them into either flexible or rigid foams. The choice is controlled by the blowing agent, additives, surfactants, and mechanical handling. Some polymers can be expanded as much as 40 times their original density and still retain a substantial part of their strength. Most commercial foams are expanded to derisities of two to five pounds per cubic foot. (Water is 62 pounds per cubic foot.)... 

The word "surfactant" is a widely used contraction of "surface-active agent," a compound that alters the surface tension of a liquid in which it is dissolved (1). Surfactants impart stability to polymers during the foaming process. They help control cell structure by regulating the size, and to a large degree, the uniformity of the cells. In urethane foams the choice of surfactant is governed by factors such as polyol type and method of foam preparation (13). 

A different reactor can change the regime. A good aim is to look for a reactor which operates in Regime I, i.e. high kio where the chemistry is not restrained by mass transfer. The final choice must obviously take account of requirements for heat transfer, particle suspension, foam control, materials of construction and a feasible size for the full scale equipment. Only in the simplest cases are there sufficient degrees of freedom to allow economic optimization of reactor type. 

Reliance on the reaction of hardness ions with soap or alkyl phosphate ions to form calcium soap or calcium alkyl phosphate precipitates for foam control has an obvious limitation in the case of soft water. This factor has encouraged a search for alternative materials that can also form finely divided hydrophobic particles. Hydrocarbon waxes represent an obvious choice in this context. 

By tire coiTect choice of the metal oxide/carbon ratio in the ingoing burden for the furnace, the alloy which is produced can have a controlled content of carbon, which does not lead to the separation of solid carbides during the reduction reaction. The combination of the carbon electrode, tire gaseous oxides and the foamed slag probably causes tire formation of a plasma region between the electrode aird the slag, and this is responsible for the reduction of elecU ical and audible noise which is found in this operation, in comparison with tire arc melting of scrap iron which is extremely noisy, and which injects unwanted electrical noise into the local electrical distribution network. 

In the production of a hydrophilic polyurethane, the choice of surfactant or emulsifying agent is the design tool of choice to develop a specific type of cell structure. While flow-through is not an important factor for hydrophilic polyurethanes, control of the cell structure in composites based on use provides an opportunity to develop multiples of the surface area in standard reticulated foams. 

Work previously reported has shown very significant reductions in gas mobility when in situ generated foam was used in laboratory core tests. Using unconsolidated core models, investigators measured ten to several hundredfold decreases in gas mobilityThe effectiveness of foam for mobility control is vitally dependent on the choice of the surfactant used. Early tests using 1% ammonium lauryl sulfate were not totally successful in demonstrating foam effectiveness. ... 

This chapter presents information on what foam/entrained air is, its primary causes and effective strategies of control. In addition, it examines the choices of products available based on chemistry and application, and provides troubleshooting options for process engineers, suppliers and papermakers. 

Features Choice tor most closed cell sheet stocks because it provides rapid controlled foam development... 

The number, size, and distribution of holes in 3D foam or the honeycomb-like structures can be easily controlled by the choice of appropriate electrolysis parameters [20, 24, 25]. Also, the size of grains constructing dendrites or cauliflower-like agglomerates formed around holes, as well as the size of channels between small metal grains agglomerates, is controlled by conditions of electrolysis. The cauliflower-like agglomerates and dendrites among holes define the wall width in this structure type. The specific surface area of the 3D foam or the honeycomb-like... 

Optimum insulation is achieved by low-density, small, closed cells. This foam structure is produced by choice of surfactants and by control of the temperature and the balance between rate of polymerization/cure (viscosity = melt strength) versus the rate of gas evolution. (See Table 3.3.)... 

Foaming can often be a serious problem in production and processing of polymer dispersions and latices, and silicone-based control systems have been developed to counter this effect. A range of products is available, giving choice of an appropriate grade to achieve high antifoam efficiency and/or high compatibility with specific products. 

As can be seen, one of the main ways of controlling the final characteristics of PU foams consists of a proper material selection and formulation, that is, in a proper choice and proportion of both isocyanate and polyol(s) components. 

As indicated above, a satisfactory foam is obtained only if these two reactions are in step. Although choice of temperature and reactants permit a measure of control over the reactions, much broader control may be exercised through the use of catalysts. As will be seen, catalysts play a crucial role in commercial operations. 

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