Preparation of Foams

Foams (or foam solutions) can be prepared from two overall processes termed condensation and dispersion. In condensation (of a gas), foam is generated from a liquid supersaturated with a gas. A beer in a can is a typical example of this. When the can is opened, the pressure is reduced and less gas (carbon dioxide) can be contained in the hquid and therefore comes out as bubbles and produces foam. Heating can 

If a gas and a liquid are mixed together in a container and then shaken, a foam will be formed. A foam structure can always be formed in a liquid if bubbles of gas are injected faster than the liquid between bubbles can drain away. Even though the bubbles coalesce as soon as the liquid between them has drained away, a temporary dispersion is formed. An example would be the foam formed when bubbles are vigorously blown into a viscous oil. Such a foam comprising spherical, well-separated bubbles is referred to as a wet foam or Kugelschaum. Here, the distinction of whether one should consider this to be a foam or not relates to stability. 

But it is complicated by the fact that, as for other types of colloidal dispersions, no foams are thermodynamically stable. Eventually they all collapse. 

In pure liquids, gas bubbles will rise up and separate, more or less according to Stokes law. When two or more bubbles come together coalescence occurs very rapidly, without detectable flattening of the interface between them, that is, there is no thin film persistence. It is the adsorption of surfactant, at the gas-liquid 

1) Wet foams in which the liquid lamellae have thicknesses on the same scale as the bubble sizes are sometimes referred to as gas emulsions. Typically in these cases, the gas bubbles have spherical rather than polyhedral shape. 

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 is among the most important features of foam stabilization (see Section 5.4.3). The relation between Marangoni surface elasticity and foam stability [40-43] 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 /. [44] discuss some of the factors that must be considered in the selection of practical foam-forming surfactants for industrial processes. Sanders [45] provides a number of surfactant choices and formulation approached for the preparation of non-aqueous foam and non-aqueous aerosol foams. 

1) Wet foams in which the liquid lamellae have Typically, in these cases the gas bubbles have 

In pure liquids, gas bubbles will rise up and separate, more or less according to Stokes law. When two or more bubbles come together coalescence occurs very rapidly, without detectable flattening of the interface between them, i.e., there is no thin-film persistence. It is the adsorption of surfactant, at the gas-liquid interface, that promotes thin-film stability between the bubbles and lends a certain persistence to the foam structure. Here, when two bubbles of gas approach, the liquid film thins down to a persistent lamella instead of rupturing at the point of closest approach. In carefully controlled environments, it has been possible to make surfactant-stabilized, static, bubbles, and films with lifetimes on the order of months to years [45], 

The actual generation of a foam can be accomplished by a variety of methods, from some that are extremely simple to some that are quite complex (see also Section 2.6.2). Some examples include  

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Perfluoroalkyl group-containing surfactants like fluoroalkylsuccinamic surfactant and fluoroalkylthio derivatives from the reaction of bis-2-hydroxyethyl-aminomethylphosphonic acid ester with maleic anhydride were especially useful for the preparation of foams for extinguishing burning hydrocarbon liquids. A surface tension of 19.8 mN/m in a 0.1% aqueous solution was observed [88. ... 

Patent Number US 5324753 A 19940628 PROCESS FOR THE PREPARATION OF FOAMED PROPYLENE POLYMER ARTICLES... 

Since water is usually used in the preparation of foaming solutions and its density is approximately 1000 times higher than that of air, for expansion ratio n < 1000, foam density can be expressed as follows... 

The preparation of foams of PCHE and copolymers has also been claimed [79]. A combination of a low-boiling hydrocarbon (butane) foaming agent and a higher boiling hydrocarbon plasticizer (toluene) were used to prepare foams. The increased heat distortion temperature of PCHE has been proposed to lead to utility in insulating foams for hot water pipes and similar applications. In addition, the superior weatherability of this material would allow use in applications in which the product was subjected to UV exposure. 

Among promising oligomeric foams one should also mention 2-pyranyl foams for a discussion of the chemistry, technology and properties of these materials see Finally, several works concerned with the preparation of foams from cumaron-indene and aniline-formaldehyde oligomers have been reported ... 

The particular case of polymer-oligomeric compositions creates new technological possibilities such as the preparation of foamed articles directly at the place of utilization by foaming solutions or pastes at room temperature or sl tly elevated temperatures. It als) leads to a considerable reduction of enei consumption as cOTii ed to traditional processing methods (extrusion, casting and pressing). 

This section is concerned with the question of the general technological strategy in the development of new, and improvement of existing, methods for the preparation of foamed materials. 

Catalysts mentioned in this section are also utilised singly or at times in combination for the preparation of foams, sealants, coatings, adhesives and certain types of elastomers. 

The preceding sections have shown the effect of changes in structure of isocyanate and hydroxyl compound, solvent and temperature on the isocyanate/hydroxyl reaction, and have indicated the role of basic catalysts. The largest commercial polyurethane processes utilize catalysed reactions, especially the preparation of foams. For this reason the catalysis of the isocyanate/hydroxyl reaction has been the object of extensive research. 

Therefore, the preparation of foams with a high percentage of closed cells or high-density foams is difficult. These foams are quite different from other isocyanate-based foams, e.g., urethane foams, isocyanurate foams and oxazolidone foams. 

Sample Preparation of Foam and Water Samples and Humic Substances Isolation. All foam and water samples were filtered through 0.45- Lim silver filter using stainless-steel filtration units. Silver filtration of Como Creek and Suwannee River foam samples resulted in build up of a brown extract on the filter paper, which was readily solubilized in 0.1 N sodium hydroxide. This extract was refiltered through silver filters as a sodium hydroxide solution. This fraction was believed to be colloidal in nature and was treated as a separate humic fraction, called the "foam-extract" fraction. A part of the filtered foam was freeze dried directly and considered "raw" foam. Fulvic and humic acids were isolated from foam and stream-water samples via the XAD-8 adsorption technique developed by Thurman and Malcolm (77), freeze dried, and weighed. To obtain a sufficient mass of humic substances, each entire sample was used for one extraction. As multiple samples were not extracted, calculation of the error associated with humic substances isolation cannot be made, and the contributions of humic substances to the DOC content must be regarded as estimates. 

Details are also given for preparation of WMA mixtures with WMA additives added to the binder, added to the mixture, with a wet fraction of aggregate or preparation of foamed asphalt mixtures. 

As is discussed later, the relative rates at which various isocyanate reactions proceed is a matter of great practical significance, particularly in the preparation of foams. In general, the order of reactivity of active hydrogen compounds with isocyanates in uncatalysed systems is as follows (most reactive first) ... 

As mentioned in Section 8.4.3.1., the majority of hydroxy groups in a polyether triol are secondary groups and are comparatively unreactive towards isocyanates. It is therefore necessary to select a catalyst which favours the formation of urethane links relatively more than the formation of gas by the reaction of isocyanate and water. Tin compounds (e.g., stannous octoate and dibutyltin dilaurate) are particularly effective in this respect (cf.. Table 14.3) and are very widely used. In addition to the primary isocyanate-polyol and isocyanate-water reactions, several secondary reactions occur during the preparation of foam. As shown in Section 14.4, the final product may contain allophanate, biuret, isocyanurate and uretidione links. It will be appreciated that in a polymeric system, which is based on a diisocyanate, all of these links (except uretidione) represent points of branching or cross-linking. These secondary reactions are particularly favoured by tertiary amines (e.g., triethylenediamine and 4-dimethylaminopyridine) and these catalysts therefore contribute to the final cross-linking of the foam and hence to the achievement of, for example, a low compression set. Mixtures of tin compounds and tertiary amines are more... 

The most important application of styrene-butadiene rubbers is in car tyres but there is also widespread use in mechanical and industrial goods. Latices find use in paper making for improving strength, in carpet manufacture as a binder for non-woven types and in the preparation of foam sponge rubber. 

In a literature survey of randomized controlled trials, meta-analyses, and observational studies using survival analysis for long-term outcomes, foam was more effective than liquid for ultrasound-guided sclerotherapy [87 ]. The two types of sclerosants are equally effective for sclerotherapy of small veins, but little else is known, according to this study, about the optimal preparation of foam sclerosants and the best technique for administering foam. 

For the preparation of foams, the R component is a poly ether or polyester with reactive end groups of hydroxyl and carboxyl. The reaction is... 

However, we will begin the background to this chapter by reviewing the preparation of foams. Initially, it is important to stress that a foam cannot be produced in a pure liquid, unless a surface-active material is present. In considering surface-active foaming materials in an aqueous environment, we must include particles, polymers, specific adsorbed cations or anions from inorganic salts, etc., and many of these substances can often cause foaming at extremely low concentrations (as... 

Figure 3. Cell configuration for the preparation of foam structure with graded pore size. Reprinted with permission from or J. Mater. Res., 18 (2008) 163. Copyright (2008) Materials Research Society of Korea.

The preparation of foams implies the presence of porogens. Two methods are used for this purpose. The first one involves the in situ generation of carbon dioxide by a controlled reaction of water with an excess of diisocyanate in an initial phase. It is a useful solution but expensive due to the cost of isocyanates it is, however, the method chosen for the production of the flexible foams. It is sometimes also used for rigid foams... 

Emulsion polymerisation is used at present in the manufacture of plastics dispersions. In this process, the insoluble monomer is emulsified in water. Emulsifiers are also required for the preparation of foamed materials [43]. 

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