Liquid foam process

In industrial practice the foaming processes are sometimes divided into chemical and physical processes. Chemical processes are those where the formation of gas takes place by decomposition of an unstable inorganic or organic compound or by a chemical reaction. Physical processes mean such techniques where the foaming gases are pumped into the polymer or are formed by the evaporation of liquids. 

Figure 12. E ect of succinylation on the foam and protein solubility properties of liquid cyclone processed cottonseed flour in suspensions at various pH values...

These data demonstrate that changes in foam properties of liquid cyclone processed cottonseed flour are inducible by treatment with succinic anhydride. Gel electrophoretic and solubility data show that there are alterations in the physical and chemical properties of proteins, and in certain cases these changes improve foam properties, that is, improve solubility and polypeptide dissociation of proteins at the interface of the foaming solution. Similar results have been reported for succinylated soybean and sunflower seed proteins (44. 46). 

The first application which comes into mind when thinking about liquid foams is in cleaning processes, such a shampoo or shaving foams. More important, however, are liquid foams in mineral froth flotation and, a closely related process, the de-inking of recycled paper. Other uses include fire fighting. In food we often find foams such as whipped cream or egg whites. 

Liquid foams evolve mainly by two processes coarsening and drainage. The first process is slow and becomes effective after many minutes, hours or even days. Drainage under the influence of gravitation is faster and it is usually the main process, which destabilizes foams. 

Figure 8.6 Liquid (liquid), froth (center), and spray polyurethane foaming processes...

The simplest technique for determination of the average by volume expansion ratio (and density) is the direct measurement of the total foam volume and the liquid volume in it (or its mass). Using barbotage methods for foam production the foaming process can be run until the initial solution is completely transformed into foam. Foam expansion ratio and its density are calculated by the formula... 

Stability is the most important property of a bubble-containing product once the desired characteristics for the product have been achieved, the structure must be kept at least until product consumption. Destabilization processes can combine in various ways and at various rates between foam formation and phase separation thus the reference to foam stability must be accompanied by the specific mechanism under observation (Hailing 1981). In general, the following phenomena can be distinguished when a liquid foam is allowed to stand (Prins 1986) ... 

Precipitation foam processes where a polymer phase is formed by polymerization or precipitation from a liquid which is later allowed to escape. 

The continuous loaf of foam made by the continuous pouring of liquid foaming components on a moving conveyor is called a slabstock foam, and a cut-off segment of the slabstock is called block foam or bun foam. The horizontal-conveyer process has been used widely since the beginning of the urethane foam industry. Recently, however, vertical production processes have been developed. 

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). 

Fillers affect foams in several ways they induce nucleation of gas bubbles and polymer crystallization, they destabilize liquid foam, they interfere with rate of chemical reactions and gas generation processes, they influence the viscosity of liquid premixes, they improve mechanical properties of foam, and foam durability. Care must be used in the selection of fillers for foams. They are used only sparingly in spite of the potential gains in foam performance which might be expected. 

These hydroxyl groups attached to the solid fraction can react with diisocyanate in the foaming process and can contribute to the crosslinking density increase in the resultant PU. Unfortunately, the presence of these hydroxyl groups at the surface of a separate solid phase makes the reaction with diisocyanates, at the solid - liquid interface, take place to a small extent only. 

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