Antifoam efficiency

The parameter Cm can provide important data to be used in estimating the antifoam efficiency, in determining the minimum (threshold) surfactant concentration in the purification of solutions by foam flotation [53], in analysing the causes of foam formation in extraction systems and in establishing the optimum conditions of effective extraction [68]. 

Three characteristics are used in the estimation of antifoam efficiency i) ability to inhibit foam formation (the time needed to produce the foam and the decrease in its stability) ii) duration of foam inhibition action iii) defoaming efficiency at introducing antifoaming agent in the foam. 

The increase in foam stability with the decrease in antifoam efficiency can be expressed as the difference between the relative foam stability at the end and at the beginning of the test... 

There lacks a unified concept about the defoaming mechanism. The major physicochemical properties of an antifoam determining its efficiency comprise its low solubility, high surface activity, ability to spread on the surface of the foaming solution and form a polymolecular film, etc. The influence of these properties on antifoam efficiency implies the existence of a number of mechanisms of defoaming. 

A typical antifoam consists of an oil (polydimethylsiloxane or hydrocarbon), dispersed hydro-phobic solid particles (e.g., hydrophobized silica), or a mixture of both. The oil-solid mixtures are often called antifoam compounds. The weight concentration of the solid particles in compounds is around several percent (typically between 2 and 8). A strong synergistic effect between the oil and the solid particles is observed in the compounds — in most cases, the latter are much more efficient than either of their individual components, if taken separately. The compounds are used at a concentration below 0.1 wt%, whereas the oils are used at higher concentrations (up to several percent) due to their lower antifoam efficiency. The antifoams are usually preemulsified in the form of oil drops or mixed oil-solid globules of micrometer size. 

The experiments show, however, that many oils with positive B and E coefficients might have low antifoam efficiency. In these cases, the stability of the asymmetric oil-water-air films is very high, and the formation of unstable oil bridges becomes impossible for kinetic reasons (factor 2 is decisive). The repulsive interaction that should be overcome for effectuation of the antifoam globules entry on the solution surface is usually termed the entry barrier. A recently... 

Table I. Antifoaming Efficiency of Different Types of Antifoaming Agents Measured by Foam Stability Tests...

Ride of the Pseudoemulsion Film in Antifoaming Action. The antifoaming efficiency strongly depends on the stability of the pseudoemulsion film. It must rupture in order for the drop, lens, or particle to enter the... 

Numerous substances were characterised and tested in their defoaming capability by Ross and others A correlation of about 90% was achieved which can be viewed as success or disappointment, depending on the standpoint of the observer. Some substances such as phenol and pyridine do not spread on surfactant solutions but can well destroy foam, other exceptions are petroleum ether and olive oil which spread rapidly but have a very poor antifoaming efficiency ° ... 

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. 

FIGURE 4.72 Comparison of semi-initial spreading coefficient, S, and antifoam efficiency for PDMS-hydrophobed silica antifoam in SDS solution. (After Kulkarni, R.D., Goddard, E.D., Croatica Chem. Acta, 50, 163, 1977.)... 

The determination of the values of / and Eg and their comparison with the antifoam efficiency of different oils is a difficult task because one needs to know the dependence of the disjoining pressure, Has, on the thickness of the asymmetric film (for h varying from infinity to h )- The most thorough... 

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