What are Synthetic Emulsion Polymers?

Emulsion polymerization is a unique process that involves emulsification of hydrophobic monomers by oil-in-water emulsifier, followed by reaction initiation with either a water-soluble initiator (e.g., potassium persulfate (K2S2O8)) or an oil-soluble initiator (e.g., 2,2-azobisisobutyronitrile (AIBN)). This is done in the presence of a stabilizer, which may be ionic, nonionic, or a protective colloid, to disperse the hydrophobic monomer through the aqueous solution. Typical polymerization monomers involve vinyl monomers of the structure (CH2=CH-) and these emulsion polymers find widespread applications, such as synthetic rubbers, thermoplastics, coatings, adhesives, binders, rheological modifiers, and plastic pigments. Emulsion polymerization is complex as nucleation, growth, and stabilization of polymer particles are governed by free radical polymerization mechanisms alongside various colloidal phenomena. Compared to other techniques, emulsion polymerization allows for the increasing molecular weight of the formed latexes by lowering the polymerization rate, either through reduced initiator concentration or lower reaction temperatures. The techniques include (1) conventional emulsion polymerization, where a hydrophobic monomer emulsified in water undergoes polymerization with a water-soluble initiator; (2) inverse emulsion polymerization, using low-polarity organic solvents as a polymerization medium; (3) mini-emulsion polymerization, characterized by much smaller monomer droplets, surfactant concentrations below critical micelle concentration (CMC), and the use of water-insoluble co-stabilizers like hexadecane to prevent Ostwald ripening; and (4) microemulsion polymerization, involving very small monomer droplets, characterized by surfactant concentrations above CMC and the use of water-soluble initiators. Miniemulsion, microemulsion, and conventional emulsion polymerizations exhibit different particle nucleation and growth mechanisms.

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There is a separate monomer phase in interval I. The particle number increases with time in interval I and particle nucleation occurs in interval I. At the end of this stage, most of the surfactants are exhausted (i.e., micelles are exhausted). About one of every 10^2-10^3 micelles can successfully convert into latex particles. The particle nucleation process is highly affected by surfactant concentration, which in turn affects particle size and particle size distribution of the latex. Lower surfactant concentrations result in shorter nucleation periods and narrower particle size distributions. During interval II (particle growth stage), the polymerization continues as polymer particles increase in size until monomer droplets are exhausted, acting as reservoirs to supply growing particles with monomer and surfactant species. In interval III, the polymer size increases as latex particles become monomer-starved, and the concentration of monomer in the reaction loci continues to decrease until the end of polymerization.

2.1. Initiators

Initiators generate free radicals through thermal decomposition or redox reactions. They may include (1) water-soluble initiators like 2,2-Azobis(2-amidinopropane) dihydrochloride, K2S2O8, APS (Ammonium persulfate), and H2O2 (hydrogen peroxide); (2) partially water-soluble peroxides like t-butyl hydroperoxide and succinic acid peroxide, and azo compounds like 4,4-azobis(4-cyanopentanoic acid); and (3) redox systems, such as persulfate with ferrous ion, cumyl hydroperoxide, or hydrogen peroxide with ferrous, sulfite, or bisulfite ion. Surface active initiators, such as bis[2-(4'-sulfophenyl)alkyl]-2,2'-azodiisobutyrate ammonium salts and 2,2'-azobis(N-2'-methylpropanoyl-2-amino-alkyl-1-sulfonate), can initiate emulsion polymerization without stabilizers.

2.2. Surfactants

Surfactants reduce the interfacial tension between the monomer and aqueous phase, stabilize the latex, and generate micelles where monomers emulsify and nucleation proceeds. These surfactants increase particle number and decrease particle size. They can be (1) anionic, such as fatty acid soaps (sodium or potassium stearate, laurate, palmitate), sulfates, and sulfonates; (2) nonionic, such as poly (ethylene oxide), poly (vinyl alcohol), and hydroxyethyl cellulose; and (3) cationic, such as dodecylammonium chloride and cetyltrimethylammonium bromide. For ionic surfactants, micelles form only above the Krafft point, and for nonionic surfactants, micelles form below the cloud point. Polymerizable surfactants (surfactants with active double bonds) are used to produce latexes with chemically bound surface-active groups. These surfactants consist of a hydrophobic tail and hydrophilic head group, along with polymerized vinyl groups in their molecular structure, imparting unique physicochemical properties. They allow the development of hybrid nano-sized reactions and templating media, synthesizing inorganic/organic nanocomposites, and enhancing oil recovery. The first synthesis of vinyl monomers serving as emulsifying agents was reported by Freedman et al. Polymerizable surfactants, including anionic, cationic, and nonionic variants, are promising for coatings, adhesives, and enhanced oil recovery.

2.3. Dispersion medium

Water is the most frequently used dispersion medium in emulsion polymerization due to its cost-effectiveness and environmental friendliness. It acts as the medium for transferring the monomer from droplets to particles and as a solvent for emulsifiers, initiators, and other ingredients.

2.4. Monomer

Emulsion polymerization requires free radical polymerizable monomers, typically vinyl monomers like acrylamide, acrylic acid, butadiene, styrene, acrylonitrile, acrylate and methacrylate esters, vinyl acetate, and vinyl chloride. There are three categories of monomers based on their solubility in aqueous phase: (1) highly soluble monomers like acrylonitrile, (2) moderately soluble monomers like methyl methacrylate, and (3) insoluble monomers like butadiene and styrene.

2.5. Other constituents

Other components used in emulsion polymerization medium include deionized water, anti-freeze additives, sequestering agents, buffers, and chain transfer agents. Anti-freeze additives, such as inorganic electrolytes, ethylene glycol, glycerol, methanol, and monoalkyl ethers of ethylene glycol, allow polymerization at temperatures below 0°C. Sequestering agents, such as ethylene diamine tetra acetic acid or its alkali metal salts, are used to solubilize the initiator system or deactivate traces of hardness elements (Ca²⁺, Mg²⁺ ions). Buffers, like phosphate or citrate salts, stabilize the latex pH. Chain transfer agents, such as mercaptans, lower molecular weight.

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