Emulsion addition method

Of the various methods of latex preparation known and practiced, a variant of the emulsion-addition method was chosen for further investigation because the reaction temperature is easy to control and coagulum... 

Chen, H Wu, J.-C. and Chen, H.-Y. (1995) Preparation of ethylcellulose microcapsules containing theophylline by using emulsion non-solvent addition method. Journal of Microencapsulation, 12, 137-147. 

Two principal approaches for the demulsification of the loaded emulsion are chemical and physical treatments. Chemical treatment involves the addition of a demulsifier to the emulsion. This method seems to be very effective. However, the added demulsifier will change the properties of the membrane phase and thus inhibits its reuse. In addition, the recovery of the demulsifier by distillation is rather expensive. Therefore, chemical treatment is usually not suitable for breaking emulsion liquid membrane, although few examples of chemical demulsification have been reported for certain liquid membrane systems [88]. 

Ultrasound-assisted emulsification in aqueous samples is the basis for the so-called liquid membrane process (LMP). This has been used mostly for the concentration and separation of metallic elements or other species such as weak acids and bases, hydrocarbons, gas mixtures and biologically important compounds such as amino acids [61-64]. LMP has aroused much interest as an alternative to conventional LLE. An LMP involves the previous preparation of the emulsion and its addition to the aqueous liquid sample. In this way, the continuous phase acts as a membrane between both the aqueous phases viz. those constituting the droplets and the sample). The separation principle is the diffusion of the target analytes from the sample to the droplets of the dispersed phase through the continuous phase. In comparison to conventional LLE, the emulsion-based method always affords easier, faster extraction and separation of the extract — which is sometimes mandatory in order to remove interferences from the organic solvents prior to detection. The formation and destruction of o/w or w/o emulsions by sonication have proved an effective method for extracting target species. 

Chen, H. Wu, J.C. Chen, H.Y. Preparation of ethyl cellulose microcapsules containing teophylline by using emulsion non-solvent addition method. J. Microencapsulation 1995, 12 (2), 137-147. 

The choice of method for introducing the monomer to the reaction vessel can have significant effects upon the particle size and panicle size distribution of the latex produced [e.g. 56]. Consider the effects of monomer addition and emulsion addition processes when used vdth the same formulation. The surfactant concentration in the seed stage will be higher fw the monomer addition process because all of the surfactant is used in the seed stage, whoeas only pan can be used for the emulsion addition process since some surfactant is required to prepare... 

Supercritical fluid extraction of emulsions (SFEE) is a combination of the conventional emulsion precipitation process with the SAS processes. This process emerges as a solution for several of the drawbacks of each separated technology. Conventional emulsion precipitation methods involve large quantities of organic solvents and their removal requires additional separation techniques that commonly need high temperatures and high cost. On the other hand, as shown in Table 24.1 by SAS process, often it is not possible to obtain particle sizes within the nanometric scale and the products can present agglomeration problems. 

Jelvehgari M, Atapour F, Nokhodchi A. Micromeritics and release behaviours of cellulose acetate butyrate microspheres containing theophylline prepared by emulsion solvent evaporation and emulsion non-solvent addition method. Archives ofPharmacal Research. July 2009 32(7) 1019-1028. PubMed PMID 19641883. 

Lu et al. [95] have also prepared ER fluid based on PANI-MMT nanocomposite with small particles diameter about 100-200 nm by an emulsion intercalation method. No additional surfactant is used and the aniline monomer remains excess amount in their intercalation reaction, and thus, the positive charged aniline monomer can well replace the absorbed sodium ions in interlayer of MMT. In order to further reduce the conductivity of the PANI-MMT particles prepared by the chemical oxidation of aniline in the presence of acidic dopant, the PANI-MMT particles must be immersed in NH aqueous solution. This immersed time for controlling conductivity of this nanocomposite is longer (several hours) compared with immersed time (only several minutes) of pure PANI. This may be related to the protection function of MMT layer to PANI macromolecular. The yield stress of PANI-MMT ER fluid is 7.19 kPa at 3 kV/mm, which is much higher than that of pure MMT as well as that of the mixture of PANI with clay (PANI+MMT). 

Chujo K, Harada Y, Tokuhara S, Tanaka, K (1969) The Effects of Various Monomer Addition Methods on the Emulsion Copolymerization of Vinyl Acetate and Butyl Acrylate. J. Polym. Sci. Part C. 27 321-332. 

Hybrid (or composite) latexes (169) are essentially a combination of the artificial latex and emulsion polymerisation methods (68, 167). A water-insoluble species (such as polymer) may be dissolved in monomer and dispersed in water in the same marmer as the artificial latexes. However, rather than removing the monomeric solvent, it is polymerised in the droplets by the addition of initiator. The monomer-swollen polymer particles capture radicals and polymerise to form a polymeric blend or structured domains. In this maimer, polystyrene particles with styrene-butadiene mbber (SBR) inclusions have been prepared for impact modification applications. 

Methacrylate/acrylate latex IPNs were synthesized by a modified emulsion polymerization technique.To 250 ml deionized, deaerated, stirred water at 60 C, 50 ml 10% (w/v) lauryl sodium sulfate solution was added, followed by 5 ml 5% (w/v) potassium persulfate solution. The calculated quantity of monomer I containing 0.4% tetraethylene glycol dimethacrylate (TEGDM) as a crosslinking agent was added at a rate of approximately 2 ml/min. When the first monomer was completely added, a minimum time of 1 hr was allowed for completion of the polymerization. Then a second 5 ml of the potassium persulfate solution was added followed by monomer II, which also contained 0.4% TEGDM. The same reaction conditions as above were followed. The combined amounts of networks I and II were 30% solids in the final latex. This method is referred to as the dropwise addition method. 

Addition polymerizations proceed either by free-radical or by ionic mechanisms and can be carried out either in bulk solution, i.e., on the neat monomer, or in suspension or emulsion. Each method has its own advantages and disadvantages. The choice of method of polymerization also depends to a very great extent both on the nature of the monomer and on the product desired. Polycondensations or step-reactions proceed according to the mechanism demanded by the reactive functional groups. Some common step-reactions are esterification, amidification, and urethane formation, as well as ring-opening or transesterification. 

Emulsion polymerization is becoming a preferred method for production of many different commercial materials, including paints, adhesives, and synthetic rubbers. Due to environmental concerns identified, under the Montreal Protocol [61], pertaining to solvent waste disposal and solvent costs, aqueous-based emulsion polymerization methods are being adopted in preference to organic solvent-base methods. In addition to the environmental... 

This chapter serves as an introduction to the origin of nonuniform latex particles. First, a brief discussion of the seeded emulsion polymerization technique that has been widely used to prepare composite polymer particles with a variety of morphological structures is given. This is followed by the illustration of the effects of important factors such as initiators, monomer addition methods, polymer molecular weight, volume ratio of the second-stage monomer to the seed polymer, and polymerization temperature that affect the morphological structures of latex particles. The development of morphological structures of nonuniform latex particles will then be covered at the end of this chapter. 

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