Foam stabilisation

Formulations for one-shot polyether systems are similar to those used for flexible foams and contain polyether, isocyanate, catalyst, surfactant and water. Trichloroethyl phosphate is also often used as a flame retardant. As with polyesters, diphenylmethane di-isocyanate is usually preferred to TDI because of its lower volatility. Tertiary amines and organo-tin catalysts are used as with the flexible foams but not necessarily in combination. Silicone oil surfactants are again found to be good foam stabilisers. Volatile liquids such as trichlorofluoro-methane have been widely used as supplementary blowing agents and give products of low density and of very low thermal conductivity. 

The foaming propensity of surfactants generally reaches a maximum at the critical micelle concentration, beyond which there appears to be little further contribution to foam density. Foam stabilisers are also added in some cases. The two important steps in the foam treatment of textile materials are generating the foam and applying it to the substrate ... 

The main function of the foam stabilising agent is to reinforce the intercellular film wall by contributing rheological characteristics of viscoelasticity. The increased viscosity may also assist handling. The aim, as so often with auxiliaries, is to achieve an optimum balance. If the bubbles are too thin and wet too quickly they will collapse prematurely, whilst too stable a film could hinder uniform application. Examples of products used as foam stabilisers include thickening agents such as the polysaccharides, hydroxyethylcellulose, methylcellulose,... 

Industrially, silicone surfactants are used in a variety of processes including foam, textile, concrete and thermoplastic production, and applications include use as foam stabilisers, defoamers, emulsifiers, dispersants, wetters, adhesives, lubricants and release agents [1]. The ability of silicone surfactants to also function in organic media creates a unique niche for their use, such as in polyurethane foam manufacture and as additives to paints and oil-based formulations, whilst the ability to lower surface tension in aqueous solutions provides useful superwetting properties. The low biological risk associated with these compounds has also led to their use in cosmetics and personal care products [2]. 

Oligoorganosiloxanes with alkoxyoxyalkylene fragments in their branches have recently been widely used as surface active agents, which regulate foam forming and foam stabilising in the production of various foamed polyurethanes. 

Dialkyl tetralins in LAB feedstocks are readily sulphonated and act as hydrotropes. High DAT levels give surfactants with high solubility and low viscosity. This effect is very significant in formulations. For example the salt curve of 15% active H2P LAS with 3% cocodi-ethanolamide (a common thickener and foam stabiliser) can give a maximum viscosity of 600 cPs with a high DAT LAB, but over 1300 cPs with a low DAT LAB. 

The chemical differences between low active and super amides dictate the fields of application. The low active products are liquids used where high purity is not required and where the amine soap helps disperse other ingredients. Super amides are produced in twice the quantity and used in solid products, shampoos and light duty detergents as foam stabilisers. They are generally waxy solids. 

The kinetics of foam collapse, i.e. the process of gas and liquid separation, is characterised by the rate of reduction of foam volume with time or by the rate of decline in its height, if the cross-sectional area is equal along the whole foam column. The stability of the foam as a whole can be characterised quantitatively at any moment by the reciprocal quantity of the rate of foam column destruction. Most often, however, the estimation of the stability of the foam column, is expressed by an integral characteristic time of decay of the whole foam column or a part of it. The relation between the internal foam collapse and the destruction of the foam column is discussed in Section 6.5. Sometimes foam stability is considered in terms of foaming ability of the solution. In general the latter characteristic involves the easiness of foam formation, foam volume and stability. Such an interpretation, however, makes this characteristic rather indefinite. For example, Abramson [12] indicates that for the estimation of the foaming ability of surfactants it is necessary to know the quantity and stability of the foam obtained from a particular surfactant as well as the conditions under which the surfactant acts as a foam stabiliser. That is why it has been repeatedly emphasised that foaming ability... 

There exist also other kinetic factors involved in the stabilising action of surfactants. For example, a relation between the lifetime of foams stabilised with various ionic surfactants and the electroosmotic pressure pti [33] the higher pel the more stable the foam is. The... 

Another point of view concerning foam stability appeared in relation to the development of the general theory of stability of colloid systems (DLVO-theory). It has already been noted that this theory was verified for the first time with foam films [35]. This gave rise to the concept of foam stabilisation on the account of the electrostatic component of disjoining pressure [e.g. 24, 32, 36],... 

Cbi / high foam stability makes it possible to use this concentration as a physicochemical characteristic of surfactants that can serve as an estimate of their foam stabilising ability. 

In order to develop efficient techniques for the preparation and application of foams in industry, agriculture, firefighting, etc., it is necessary to know the physicochemical parameters of surfactants and their relationship with the foam stabilising ability of the surfactant solutions. Usually the criterion of the surfactant foaming ability is the adsorption of these compounds at the solution/air interface and the related to it properties, such as decrease in surface tension, adsorption work, maximum adsorption T. [13,39,43]. CMC is often used as a characteristic of a foaming agent (if micellisation is possible in the surfactant solution). Parameters related to foam stability, such as foam lifetime and foam column height, are also employed [12,13,39],... 

The physicochemical characteristics of the foam films and the foam are in good agreement. Thus, Xr for foams stabilised by the members of the homologous series of oxyethylated dodecyl alcohol does not change after n > 10, which is in accordance with the Cw(n) dependence. The effect of the hydrophilic-lipophilic balance on the foaming ability of surfactants will be treated in Section 7.9. 

It seems promising to estimate the stabilising ability of surfactant mixtures analysing their phase diagrams [76,77]. The comparison between the surfactant state in the bulk phase and the foam stabilising ability allows not only to choose the most suitable foaming agents but also to clarify the main reasons for foam stability. 

Foam stabilising properties of surfactants determined by Foam Pressure... 

These two examples with the homologous series of alkylsulphonates and alkylsulphates indicate the undoubted advantages of Foam Pressure Drop Technique for determining the foam stabilising properties of surfactants. This technique allows to distinguish small differences in the foam stabilising ability of surfactants. 

Though not quantitative, the comparison between the two techniques provides information about the effect of the pressure in the foam liquid phase as well as the effect of the foam film type. The advantages of the Foam Pressure Drop Technique for estimating the foam stabilising ability of the surfactants is indisputable. 

For this group of antifoams an effective way of application is in the form of foams [7]. For example, a foam stabilised by cationic surfactants is introduced in the foam to be destructed, obtained from an anionic surfactant solution. The cationic surfactant solution usually contains a considerable amount of calcium or aluminium salts. The large consumption of substances should be considered as a disadvantage of this group of antifoams. Furthermore,... 

A special case of defoaming is observed when a soap solution (sodium oleate) is added to a foam, stabilised by saponin. It is know that addition of soap to a saponin solution leads to a decrease in foam volume and stability [e.g. 2]. Some saponin + soap mixtures do not produce a foam at all. It is not possible to explain foam inhibition in these systems with the adsorption displacement, since foam inhibition occurs when soap is added to saponin solution as well as when saponin is added to the soap solution. 

Certain antifoams, for example, methyl or ethyl alcohols, when introduced in the foaming solution, affect indirectly the foam stabilising properties of the adsorption layers because they change either the surfactant solubility or its CMC. It has been established [39] that addition of antifoams (2-ethylhexanoI and tributyl phosphate) increases CMC. Schick and Fowkes [40] have observed a certain change in CMC of NaDoS when tetradecanol is added. If the compounds added improve the foaming ability (such as dodecylglycol ether and (3-hydroxyethyl laurylamide), CMC decreases. 

At low surfactant concentrations (for example, the concentrations that are reached at the waste water foam purification from surfactant pollutants) the foaming ability is usually limited by the minimum surfactant concentrations necessary for formation of stable foam films. This concentration is close to the concentration of black spot formation in microscopic films Cbi and black films Cfbu being an important characteristic of the foam stabilising ability of surfactants. The values of Cm for some surfactants are given in Table 3.1 and the dependence of Cm on various factors is considered in Section 3.4. 

The low molecular protein, the antibiotic nisin, was successfully concentrated in industrial scale from a culture liquid in a high expansion ratio foam, stabilised with the foaming agent Tween-80. In the absence of a special foaming agent no stable foam was produced from the culture liquid. Nisin could be extracted by adding Tween-80, though the... 

Emulsifiers, as a consequence of their function, have a significant role in texture control. This involved emulsification of fat and water, aeration control and of foam stabilisation in the dairy based industries. In bakery processes, emulsifiers are used as dough strengtheners, as crumb softeners and as aerating agents. 

---