Editing Emulsion polymerization (section)

==Components==

===Monomers===
Typical [[monomer]]s are those that undergo radical polymerization, are liquid or gaseous at reaction conditions, and are poorly [[soluble]] in water. Solid monomers are difficult to disperse in water. If monomer solubility is too high, particle formation may not occur and the reaction kinetics reduce to that of solution polymerization.

[[Ethene]] and other simple [[olefins]] must be polymerized at very high pressures (up to 800 bar).

===Comonomers===
[[Copolymerization]] is common in emulsion polymerization. The same rules and [[comonomer]] pairs that exist in [[radical polymerization]] operate in emulsion polymerization. However, copolymerization kinetics are greatly influenced by the [[aqueous]] [[solubility]] of the monomers. Monomers with greater aqueous solubility will tend to [[liquid-liquid extraction|partition]] in the aqueous phase and not in the polymer particle. They will not get incorporated as readily in the polymer chain as monomers with lower aqueous solubility. This can be avoided by a programmed addition of monomer using a semi-batch process.

Ethene and other alkenes are used as minor comonomers in emulsion polymerization, notably in [[vinyl acetate]] copolymers.

Small amounts of [[acrylic acid]] or other [[ionizable]] monomers are sometimes used to confer colloidal stability to a dispersion.

===Initiators===
Both [[Heat|thermal]] and [[redox]] generation of free radicals have been used in emulsion polymerization. [[Persulfate]] salts are commonly used in both [[initiation (chemistry)|initiation]] modes. The persulfate ion readily breaks up into sulfate radical ions above about 50&nbsp;°C, providing a thermal source of initiation. Redox initiation takes place when an [[oxidant]] such as a persulfate salt, a [[reducing agent]] such as glucose, [[Rongalite]], or [[sulfite]], and a redox catalyst such as an iron compound are all included in the polymerization recipe. Redox recipes are not limited by temperature and are used for polymerizations that take place below 50&nbsp;°C.

Although organic [[peroxides]] and [[hydroperoxides]] are used in emulsion polymerization, initiators are usually water [[soluble]] and [[liquid-liquid extraction|partition]] into the water phase. This enables the particle generation behavior described in the theory section. In redox initiation, either the oxidant or the reducing agent (or both) must be water-soluble, but one component can be water-insoluble.

===Surfactants===
Selection of the correct [[surfactant]] is critical to the development of any emulsion polymerization process. The surfactant must enable a fast rate of polymerization, minimize [[coagulum]] or [[fouling]] in the reactor and other process equipment, prevent an unacceptably high viscosity during polymerization (which leads to poor heat transfer), and maintain or even improve properties in the final product such as [[tensile strength]], [[gloss (material appearance)|gloss]], and water absorption.

[[Anionic]], [[nonionic]], and [[cationic]] surfactants have been used, although anionic surfactants are by far most prevalent. Surfactants with a low [[critical micelle concentration]] (CMC) are favored; the polymerization rate shows a dramatic increase when the surfactant level is above the CMC, and minimization of the surfactant is preferred for economic reasons and the (usually) adverse effect of surfactant on the physical properties of the resulting polymer. Mixtures of surfactants are often used, including mixtures of anionic with nonionic surfactants. Mixtures of cationic and anionic surfactants form insoluble salts and are not useful.

Examples of surfactants commonly used in emulsion polymerization include [[fatty acids]], [[sodium lauryl sulfate]], and [[alpha-olefin sulfonate]].

===Non-surfactant stabilizers===
Some grades of [[polyvinyl alcohol]] and other water-soluble polymers can promote emulsion polymerization even though they do not typically form micelles and do not act as surfactants (for example, they do not lower [[surface tension]]). It is believed that growing polymer chains graft onto these water-soluble polymers, which stabilize the resulting particles.<ref>{{cite journal|last1=Kim|first1=Noma|last2=Sudol|first2=E. David|last3=Dimonie|first3=Victoria L.|last4=El-Aasser|first4=Mohamed S.|title=Grafting of PVA in Miniemulsion Copolymerizations ofn-Butyl Acrylate and Methyl Methacrylate Using Water-Soluble, Partially Water-Soluble, and Oil-Soluble Initiators|journal=Macromolecules|volume=37|issue=9|year=2004|pages=3180–3187|doi=10.1021/ma035153w|bibcode=2004MaMol..37.3180K }}</ref>

Dispersions prepared with such stabilizers typically exhibit excellent colloidal stability (for example, dry powders may be mixed into the dispersion without causing coagulation). However, they often result in products that are very water sensitive due to the presence of the [[water-soluble polymer]].

===Other ingredients===
Other ingredients found in emulsion polymerization include [[Chain Transfer|chain transfer agents]], [[buffering agent]]s, and inert [[salts]]. [[Preservatives]] are added to products sold as liquid dispersions to retard bacterial growth. These are usually added after polymerization, however.