Mechanical film, emulsions

Surface-active agents. Surface-active agents such as emulsifiers and surfactants play a very significant role in the stability of emulsions. They greatly extend the time of coalescence, and thus they stabilize the emulsions. Mechanisms by which the surface-active agents stabilize the emulsion are discussed in detail by Becher (19) and Coskuner 14). They form mechanically strong films at the oil-water interface that act as barriers to coalescence. The emulsion droplets are sterically stabilized by the asphaltene and resin fractions of the crude oil, and these can reduce interfacial tension in some systems even at very low concentrations (i7, 20). In situ emulsifiers are formed from the asphaltic and resinous materials found in crude oils combined with ions in the brine and insoluble dispersed fines that exist in the oil-brine system. Certain oil-soluble organic acids such as naphthenic, fatty, and aromatic acids contribute to emulsification 21). 

The authors of this review (46) have used BSA along with monomeric emulsifiers, both in the inner and the outer interfaces (in low concentrations of up to 0.2 wt %), and found significant improvement both in the stability and in the release of markers as compared to the use of the protein in the external phase only (Fig. 13). It was postulated that while the BSA has no stability effect at the inner phase it has strong effect on the release of the markers (mechanical film barrier). On the other hand, BSA together with small amounts of monomeric emulsifiers (or hydrocolloids) serve as good steric stabilizers, improve stability and shelf-life, and slow down the release of the markers. The BSA plays, therefore, a double role in the emulsions as film former and barrier to the release of small molecules at the internal interface, and as steric stabilizer at the external interface. The release mechanisms involving reverse micellar trans-... 

In general, where electrical surface charge is an important determinant of stability it tends to be easier to formulate a stable O/W emulsion than a W/O emulsion because the EDL thickness is much greater in water than in oil. W/O emulsions tend to be stabilized by other mechanisms, including steric and mechanical film stabilization. 

The results are reported of a study of the mechanism of stabilisation during vinyl acetate-ethylene emulsion copolymerisation using various colloidal stabilisers. These stabilisers included PVAl, aUcylphenol ethoxylate and a diisocyanate chain extended polyethylene glycol. The effects of these stabilisers on emulsion characteristics, film properties and applications behaviour are discussed. 5 refs. (217th ACS National Meeting, Anaheim, Calif., 21-25 March, 1999)... 

Comparing an uncrosslinked emulsion polymer film and a highly cross-linked automotive top coat at temperatures well below their respective glass transition temperatures as an example, both polymeric films have tensile modulus values of approximately 2 x 10 dyncm [8]. Nevertheless, these two polymer systems show very different mechanical properties in the rubbery plateau region. The tensile modulus values are approximately (5-10) x 10 and (2-8) X 10 dyncm for the emulsion polymer and automotive top coat, respectively. Thus, one can certainly improve the physical properties of this latex... 

Multiple emulsion stability was significantly improved in the presence of amphiphilic proteins because of two important factors. First, the multianchoring flexible macromolecules act to improve the steric stabilization by forming a thick multilayered coating on the droplets. Second, the proteins used in the inner phase create a mechanical film barrier that prevents uncontrolled release of the entrapped ingredients. 

Transparent exfoliated-intercalated PEA/bentonite nanocomposites are prepared by in situ emulsion polymerization in aqueous dispersions containing bentonite, following directly casting the resulting emulsion into film. XRD and TEM reveal that disorderedly exfoliated silicate layers and intercalated silicate layers coexist in the PEA matrix. Thermal stability, mechanical properties and barrier properties of the obtained materials were greatly improved. Because relatively small amounts (<10%) of nanometer-size clay particles can provide large... 

An important aspect of the stabilization of emulsions by adsorbed films is that of the role played by the film in resisting the coalescence of two droplets of inner phase. Such coalescence involves a local mechanical compression at the point of encounter that would be resisted (much as in the approach of two boundary lubricated surfaces discussed in Section XII-7B) and then, if coalescence is to occur, the discharge from the surface region of some of the surfactant material. 

There appear to be two stages in the collapse of emulsions flocculation, in which some clustering of emulsion droplets takes place, and coalescence, in which the number of distinct droplets decreases (see Refs. 31-33). Coalescence rates very likely depend primarily on the film-film surface chemical repulsion and on the degree of irreversibility of film desorption, as discussed. However, if emulsions are centrifuged, a compressed polyhedral structure similar to that of foams results [32-34]—see Section XIV-8—and coalescence may now take on mechanisms more related to those operative in the thinning of foams. 

Many different combinations of surfactant and protective coUoid are used in emulsion polymerizations of vinyl acetate as stabilizers. The properties of the emulsion and the polymeric film depend to a large extent on the identity and quantity of the stabilizers. The choice of stabilizer affects the mean and distribution of particle size which affects the rheology and film formation. The stabilizer system also impacts the stabiUty of the emulsion to mechanical shear, temperature change, and compounding. Characteristics of the coalesced resin affected by the stabilizer include tack, smoothness, opacity, water resistance, and film strength (41,42). 

Before determining the degree of stabiUty of an emulsion and the reason for this stabiUty, the mechanisms of its destabilization should be considered. When an emulsion starts to separate, an oil layer appears on top, and an aqueous layer appears on the bottom. This separation is the final state of the destabilization of the emulsion the initial two processes are called flocculation and coalescence (Fig. 5). In flocculation, two droplets become attached to each other but are stiU separated by a thin film of the Hquid. When more droplets are added, an aggregate is formed, ia which the iadividual droplets cluster but retain the thin Hquid films between them, as ia Figure 5a. The emulsifier molecules remain at the surface of the iadividual droplets duiing this process, as iadicated ia Figure 6. 

Stockpiles of milled peat are prevented from self heating and ignition by sprayed application of bitumen emulsion to form a 2-2.5 mm protective permeable film [1], The mechanism of self heating and ignition first involves aerobic microbiological processes, then chemical transformation of iron-containing minerals in the peat into pyrophoric iron compounds which later ignite the peat mound [2],... 

Emulsion homopolymers and copolymers (latexes) are widely used in architectural interior and exterior paints, adhesives, and textile industries [2-4]. Colloidal stabilizators in the emulsion polymerization strongly affect not only the colloidal properties of latexes but also the film and mechanical properties, in general. Additionally, the properties of polymer/copolymer latexes depend on the copolymer composition, polymer morphology, initiator, polymerization medium and colloidal characteristics of copolymer particles. 

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