Enhancement of Antifoam Effectiveness

Similar observations of PDMS-hydrophobed silica antifoam deactivation on inorganic carrier materials have been reported in a pharmaceutical context. The antifoam performance of antiflatulent preparations with PDMS-hydrophobed silica mixed with basic antacid carriers, such as aluminum hydroxide and magnesium carbonate, has been shown to deactivate after the ingredients were either granulated or compressed into tablets [81, 82]. Analysis of the molecular weight distribution of 

PDMS-hydrophobed silica antifoams, after storage in soda ash, revealed no change in molecular weight. This suggests that molecular cleavage at the Si-O bond in PDMS to produce lower molecular weight fragments of lower viscosity cannot provide an explanation for the phenomenon [79], 

By contrast, absorption of PDMS-hydrophobed silica antifoam of viscosity 3 X 1(P mPa s onto organic carriers produced no deactivation of the antifoam when the resultant granules were stored separately from detergent powders [75]. A comparison of the antifoam effectiveness obtained with various inorganic and organic carriers after separate storage is given in Table 8.1. The relative stability of 

Effect of Carrier Type on Stability of High-Viscosity PDMS-Hydrophobed Silica Antifoam before Incorporation in Detergent Powder (Antifoam 

Carrier (microns) Porosity Porosity Porosity (weeks) units)  

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There are a number of problems with this mechanism. The first problem clearly concerns the requirement that the oil spread at the air-water surface. We have already established that this is not a necessary property of antifoam oils. A second problem is conceptual—depletion of surfactant in the foam film by adsorption on particles will give rise to an increase in surface tension, which will in turn produce a Marangoni flow in the direction of the foam film. This will tend to enhance film stability. Finally it is clear that an aspect of the mechanism is an intrinsic tendency for the particle to be removed from the oil phase to the aqueous phase. However, it has been shown [43, 71] that addition of liquid paraffin to a dispersion of hydrophobed silica in sodium alkylbenzene sulfonate solution produces an enhancement of antifoam effectiveness. The duplicated results are given in Table 4.9 where the antifoam effectiveness is indicated by the ratio F (given by Equation 4.71). This is clearly not consistent with a process of removal of particles from oil having a central role in the... 

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