Foam Breakdown by Antifoams

The mechanical ways of foam breakdown are realised by various rotating devices (mixers, disks, centrifuges, propellers, etc.) or by employing stream of air, vapour or liquid or by running the foam through cyclones and narrow gaps [1,2]. In all these cases the liquid outflow from the foam is accelerated which creates critical deformations and stresses in films and other foam elements. A convenient classification of the mechanical means of foam breakdown and the methods designing such mechanical devices are considered in [3]. The 

The physical means of defoaming involve foam breakdown by thermal treatment (heating, processing with overheated vapour, freezing), by acoustic (mainly ultrasound) waves, vibrations, a-particles irradiation, creation of high capillary pressure in the foam, etc. 

The most widely used in foam control are special materials called antifoams. They could be fats, oils, waxes, fatty alcohols and acids, ethers, organic compounds containing nitrogen and sulphur (alkylamines and amides, sulphides, thioethers, etc.), phosphorus organic compounds (in particular, tributylphosphate), silicon-organic compounds (silicon oils) and some non-ionic surfactants [e.g. 4,5]. 

As a rule the choice of antifoams is empirical and is imposed by the manufacture processes and conditions. However, the mechanism of action of antifoams remains unclear. 

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A number of authors investigated the process of foam breakdown by antifoams introduced in the foaming system or the foam such as O/W emulsion and solutions in water-soluble organic solvents. Under such defoaming conditions the mechanism of antifoaming action becomes more complex and a destabilisation of the films is possible to occur by an entirely different (heterogeneous) mechanism (see below). 

In contrast to the pure heterogeneous defoaming by hydrocarbons, the foam breakdown by typical diphilic antifoams occurs as a result of homogenous and heterogeneous... 

The antifoams were studied on a foam from 1% commercial sulphonol solution. The rate of foam breakdown, caused by the antifoam sprayed, is determined by the foam volume destroyed for 1 min and by the lifetime of one half of the foam column. As tests showed the change in the content of the foaming composition affects the activity of the antifoaming agent but the relative foam stability almost does not change. 

It has been established [55] that hydrocarbon (decane, benzene) promoted foam breakdown, occurring with formation of unstable films (barrierless rupture), is only possible at very low surfactant concentrations (less than 0.003-0.004% saponin and OP-10). At higher surfactant concentrations the defoaming ability of the antifoam results from the lowering of the energy barrier of film rupture. The latter is determined by the properties of the adsorption layers and other film parameters. 

The main subject in this paragraph is foam breakdown mechanisms by antifoam particles. Various mechanisms proposed for the action of antifoams are described in many superior reviews [112,180-184]. A number of investigations about the breakdown of foams by dispersed insoluble oils [185-202], hydrophobic solid particles [203-214], and oils and particles in combination [215-220] were reported as antifoams. The mixed-type antifoams exhibit excellent defoaming performance thus they are widely used in many industries. Several theories have been presented in the literatures on the mechanisms of defoaming through the use of three types of antifoams [216,217]. A... 

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