Emulsifier (emulsion polymerization) 

  These materials perform many important functions in emulsion polymerizations such as (i) reducing the interfacial tension between the monomer phase and the water phase so that, with agitation, the monomer is dispersed (or emulsified) in the water phase. (ii) Micelle generating substances. If these substances are used above the critical micelle concentration (CMC), they will form micelles which are ordered clusters of emulsifier molecules, with the oil-soluble part of the molecule oriented toward the center of the cluster and the water-soluble part of the molecule toward the water. (iii) Stabilizing the monomer droplets in an emulsion form. (iv) Serving to solubilize the monomer within emulsifier micelles. (v) Stabilizing the growing latex particles. (vi) Also, stabilizing the particles of the final latex. (vii) Acting to solubilize the polymer. (viii) Serving as the site for the nucleation of particles. (ix) Acting as chain transfer agents or retarders.

Emulsifiers (also referred to as surfactant, soap, dispersing agent, and detergents) are surfaceactive agents. These materials consist of a long-chain hydrophobic (oil-soluble) group (dodecyl, hexadecyl or alkyl-benzene) and a hydrophilic (water-soluble) head group. They are usually classified according to the nature of this head group. This group may be anionic, cationic, zwitterionic or non-ionic [30]. Anionic emulsifiers having negatively charged hydrophilic head group are the sodium, potassium and ammonium salts of higher fatty acids, and sulfonated derivatives of of aliphatic, arylaliphatic, or naphtenic compounds. Sodium lauryl (dodecyl) sulfate, [C₁₂H₂₅OSO₃ Na+], sodium dodecyl benzene sulfonate, [C₁₂H₂₅C₆H₄SO₃ Na+] and sodium dioctyl sulfosuccinate, [(C₁₈H₇COOCH₂)2SO₃-Na+] are commonly used in emulsion polymerizations as anionic emulsifiers. Quaternary salts such as acetyl dimethyl benzyl ammonium chloride and hexadecyl trimethyl ammonium bromide may be given examples for cationic emulsifiers. Zwitterionic (amphoteric) emulsifiers can show cationic or anionic properties depending on pH of the medium. They are mainly alkylamino or alkylimino propionic acids. Non-ionic emulsifiers carry no charge unlike ionic emulsifiers. The most used type of these emulsifiers is that with a head group of ethylene oxide (EO) units.

  Polyoxyethylenated alkylphenols, polyoxyethylenated straight-chain alcohols and polyoxyethylenated polyoxypropylene glycols (i.e., block copolymers formed from ethylene oxide and propylene oxide) are the most commonly three classes of non ionic emulsifiers used for emulsion polymerization formulations. Polyoxyethylenated alkylphenol type of emulsifiers includes two main members: nonylphenol polyoxyethylene glycol, [C₉H₁₇C₆H₄O-(CH₂CH₂-O)nH],and octylphenol polyoxyethylene glycol, [C₈H₁₅C₆H₄O-(CH₂CH₂-O)nH]. The number of EO units, (n), may be diversified from a few toabout 100 (typically from 1 to 70 EO units), which characterize the distribution of polyEO chain lengths for each specific emulsifier. A typical example for polyoxyethylenated polyoxypropylene glycols is polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer, [H-(OCH₂CH₂)a-(OCH₃CH-CH₂)b-(OCH₂CH₂)a -OH], in which the polyEO portion constitutes between 10 and 80% of the copolymer.

  In general, the anionic emulsifiers are extensively preferred in many emulsion polymerization systems. They serve as strong particle generators and stabilize the latex particles via electrostatic repulsion mechanism. But latexes stabilized with this type of emulsifiers are often unstable upon addition of electrolytes and in freeze-thaw cycles.

  Furthermore, these emulsifiers have limited stabilizing effectiveness at high solids (e.g.,> 40%) and present high water sensitivity. To overcome these problems, non-ionic emulsifiers can be used to nucleate and stabilize the particles in the course of emulsion polymerization. In this case, it is the steric stabilization mechanism that protects the interactive particles from coagulation. In addition, the use of non-ionic types improves the stability of latex product against electrolytes, freeze-thaw cycles, water and high shear rates. As a result of them, in many emulsion polymerization recipes (particularly in industry), mixtures of anionic and non-ionic emulsifiers have been widely used together in a synergistic manner to control the particle size and to impart enhanced colloidal stability. The cationic and zwitterionic emulsifiers are used infrequently in emulsion polymerization applications. Besides all these types of emulsifiers, polymeric and reactive emulsifiers can be used in emulsion polymerizations. Polymeric emulsifiers are often non ionic water-soluble polymers such as poly(vinyl alcohol), hydroxyethyl cellulose and poly(vinyl pyrrolidone), and called sometimes as a “protective colloid”.

  They are used to increase the particle stability in latexes against coagulation. Reactive emulsifiers (“surfmers”), which have polymerizable reactive group, can copolymerize with the main monomer and be covalently anchored onto the surface of latex particles. When these compounds used in emulsion polymerizations, the emulsifier migration is reduced. Furthermore, surfmers improve the water resistance and surface adhesion as well as resistance against electrolytes and freeze-thaw cycles in comparison to conventional emulsifiers. Surfmers can be anionic with sulfate or sulfonate head groups (sodium dodecyl allyl sulfosuccinate), cationic (alkyl maleate trimethylamino ethyl bromide), or non-ionic (functionalized poly(ethylene oxide)-poly(butylenes oxide)copolymer). The reactive groups can be in different types, for example, allylics, acrylamides, (meth)acrylates, styrenics, or maleates

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