Inhibitors, foam

Foam inhibitors are always insoluble materials, usually liquids, but some solids such as waxes, gums, or soaps are used. Their foam-inhibiting action arises from their ability to spread spontaneously over the surface of the foamy liquid. A single drop of the spreading liquid, once it has arrived at the surface of the liquid film, acts effectively is a venturi pump, ejecting on every side all the liquid lying 

The major requirement of a foam inhibitor is cost-effectiveness accordingly, some useful characteristics are low volatility (to prevent stripping from the system before it is dispersed and does its work), ease of dispersion and strong spreading power, and surface attraction origination. Also important are effects on product quality, downstream units, and on the environment and health. Several common types are discussed elsewhere (36, 216, 319, 339). Often, a foam inhibitor does not use a single compound, but a combination also including a carrier (usually a hydrocarbon oil or water, which supports the release and spread of the primary defoamer) a secondary defoamer an emulsifier, which enhances the speed of dispersion and a stabilizer, which enhances the inhibitor s stability. 

Foaming commonly occurs in some systems, while it seldom occurs in others. Some guidelines which may assist in identifying the potential of foam formation are presented below. 

In the above type of application, the presence of compounds such liquid hydrocarbons and organic acids often promotes or induces foaming. 

Corrosion inhibitors are often surface-active agents and generally severe foamers. For example, corrosion inhibitors ii jected into 

Two main types of inhibition may be distinguished Antifoamers that are added to prevent foam formation and deformers that are added to eliminate an existing foam. For example, alcohols such as octanol are effective as defoamers but ineffec- 

Chemical Inhibitors that Both Lower Viscosity and Increase Drainage 

Chemicals that reduce the bulk viscosity and increase drainage can cause a decrease in foam stability. The same applies to materials that reduce surface viscosity and elasticity (swamping the surface layer with excess compound of lower viscosity). 

It has been suggested that a spreading film of antifoam may simply displace the stabilising surfactant monolayer. As the oil lens spreads and expands on the surface, the tension will be gradually reduced to a lower uniform value. This will eliminate the stabilising effect of the interfacial tension gradients, i.e. elimination of surface elasticity. 

Reduction of surface viscosity and elasticity may be achieved by low molecular weight surfactants. This will reduce the coherence of the layer, e.g. by addition of small amounts of nonionic surfactants. These effects depend on the molecular structure of the added surfactant. Other materials, which are not surface active, can also destabilise the film by acting as cosolvents that reduce the surfactant concentration in the liquid layer. Unfortunately, these non-surface-active materials, such as methanol or ethanol, need to be added in large quantities ( 10%). 

---

Foaming extrusion Foaming-in-place beads Foaming power Foam inhibitors... 

Foam Inhibitors. Methyl sihcone polymers of 300-1000 mm /s(= cSt)) at 40°C are effective additives at only 3—150 ppm for defoaming oils in internal combustion engines, turbines, gears, and aircraft appHcations. Without these additives, severe churning and mixing of oil with air may sometimes cause foam to overflow from the lubrication system or interfere with normal oil circulation. Because sihcone oil is not completely soluble in oil, it forms a dispersion of minute droplets of low surface tension that aid in breaking foam bubbles. 

When a-sulfo fatty acid esters are used as the major active component in detergents they can cause problems because of their foaming properties. In European horizontal drum-type automatic washers they produce too much foam, and in the rinse cycle of the American and Japanese pulsator-type washers the foam cannot be completely rinsed out [38]. The problem of inefficient rinsing can be solved by the addition of soap [63] or sulfonated unsaturated fatty acid esters [64]. For European applications special foam inhibitors are needed. 

A textured metallocene polyethylene foam sheet suitable for use in a floor covering is made using a highly coactivated azodicarbonamide package which blows the metallocene polyethylene effectively. The preferred coactivators are zinc oxide and urea. The textured surface of metallocene polyethylene foam is formed by a chemical embossing process which utilises a liquid triazole having an alkyl moiety as a foam-expansion inhibitor. The triazole is dissolved in a non-polar solvent to form the foam inhibitor. The preferred inhibitor is a hydrocarbon which may be halogenated. 

BP Research Centre at Sunhury investigated the foaming characteristics of the crude and developed, with others, a novel foam inhibitor that effectively prevented foam generation in the separators... 

To avoid reliance on foam inhibitor addition, a plant iumution assessment was conducted to determine the maximum crude feed rate that could be handled. This was found to be only 40% of the design rate, which is equivalent to a crude resistance time of 8.0 and 11.5 minutes in the HP and LP separators, respectively. To install additional vessels to compensate for this would have been both difficult and costly. 

Bendure indicates 10 ways to increase foam stability (1) increase bulk liquid viscosity, (2) increase surface viscosity, (3) maintain thick walls (higher liquid-to-gas ratio), (4) reduce liquid surface tension, (5) increase surface elasticity, (6) increase surface concentration, (7) reduce surfactant-adsorption rate, (8) prevent liquid evaporation, (9) avoid mechanical stresses, and (10) eliminate foam inhibitors. Obviously, the reverse of each of these actions, when possible, is a way to control and break foam. 

The prevention of foaming and the destruction of existing foams is often a matter of practical importance for example, polyamides and silicones find use as foam inhibitors in water boilers. Antifoaming agents act against the various factors which promote foam stability (described above) and, therefore, a number of mechanisms may be operative. 

Although many factors, such as film thickness and adsorption behaviour, have to be taken into account, the ability of a surfactant to reduce surface tension and contribute to surface elasticity are among the most important features of foam stabilization (see Section 5.4.2). The relation between Marangoni surface elasticity and foam stability [201,204,305,443] partially explains why some surfactants will act to promote foaming while others reduce foam stability (foam breakers or defoamers), and still others prevent foam formation in the first place (foam preventatives, foam inhibitors). Continued research into the dynamic physical properties of thin-liquid films and bubble surfaces is necessary to more fully understand foaming behaviour. Schramm et al. [306] discuss some of the factors that must be considered in the selection of practical foam-forming surfactants for industrial processes. 

Not all foams are wanted though. Foams, other than flotation froths, are generally not wanted in the process industries where they tend to interfere with process unit operations and may cause upsets. Some agents will act to reduce the foam stability of a system (termed foam-breakers or defoamers) while others can prevent foam formation in the first place (foam preventatives, foam inhibitors). There are many such agents and Kerner [327] describes several hundred different formulations for foam... 

Any substance that acts to reduce the stability of a foam it can also act to prevent foam formation. Terms such as antifoamer or foam inhibitor specify the prevention of foaming, and terms such as defoamer or foam breaker specify the reduction or elimination of foam stability. Example Poly(dimethylsiloxane)s, (CH3)3SiO[(CH3)2SiO]xR, where R represents any of a number of organic functional groups. Antifoamers can act by any of a number of mechanisms. 

In Figure 2 the clouding points of different oils are shown in dependence of pressure at 50°C. For simplicity the oils of different producers and applications are termed by the capitals A-F. The main characteristics of the oils are presented in Table 1. In addition to the different composition of their hydrocarbon content the oils contain a variety of additives, e.g. aging inhibitors, anticorrosives, detergents, pourpoint depressants, foam inhibitors and others. 

Since antifoams can be either foam breakers or foam inhibitors [8,9], this should be taken into account in the estimation of their foam breakdown ability. For example, addition of one drop of buthanol or siloxane antifoams breaks down a foam from a saponin solution. However, further shaking of the solution containing the buthanol drop leads to intensive foaming and a large volume of a stable foam is formed. On the contrary, shaking the solution containing siloxane antifoam does not lead to formation of a foam [8]. 

J.V.Robinson W.W.Woods, A General Method of Selecting Foam Inhibitors, Technical Note 1205, NACA, Washington,DC (1946) 5)Kirk Othmer 5(1950), 715-17... 

CNC Antifoam 1 -A is supplied in dispersible fluid form which minimizes the danger of spot formation on fabric passing through the bath. Low concentrations of CNC Antifoam 1-A minimizes contamination of the finishing bath. CNC Antifoam 1-A is an efficient foam inhibitor of textile wet processing applications. 

On the one hand they are used as "external" foam inhibitors by adding to foam-producing processes, on the other hand they are also employed as internal foam inhibitors by incorporating them in a non-foaming finishing formulation. 

Use Silicone oil base, foam inhibitor in lubricating oils. 

---