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Emulsion polymerization was developed by The Goodyear Tire & Rubber Company in the 1920s. The emulsion-polymerization process results in a latex particle, which is a dispersion of polymer in water. Waterborne coatings that primarily use emulsion polymers are the largest type of coating technology used on a global basis and are expected to continue to grow as a percent of the total coatings market.

In emulsion polymerization, monomers are first dispersed in the aqueous phase. Initiator radicals are generated in the aqueous phase and migrate into the soap micelles that are swollen with monomer molecules. As the polymerization proceeds, more monomers migrate into the micelle to enable the polymerization to continue.

Since only one free radical is present in the micelle prior to termination, very high molecular weights are possible., on the order of 1,000,000 or higher. Unlike solution polymers, the viscosity of latexes are governed by the viscosity of the medium the particles are dispersed in (continuous medium). Chain transfer agents are added to control the molecular weight. The resultant emulsion particle is an oil in water emulsion. monomer in the aqueous phase.

A less commonly used emulsion technique called the inverse emulsion-polymerization process involves dispersing an aqueous solution of monomer in the nonaqueous phase.

Emulsion polymerization can occur using a batch process, semi-continuous process or continuous process. Commercial latex polymers are made using a semi-continuous or continuous process rather than a simple batch process because the heat evolved in a simple emulsion batch process would be uncontrollable in a large reaction vessel. In the semi-continuous batch process, monomers and initiators are added in proportions and at a controlled rate so that rapid polymerization occurs. In this method, the monomer concentration is low, also called under-starved monomer conditions, to facilitate temperature control. It is also common to start the polymerization using a seed latex.

In the continuous process, the reaction system is continuously fed to, and removed from, a suitable reactor at rates such that the total volume of the system undergoing reaction at any instant is constant.

Mini-emulsions are produced by dispersing monomers in water by means of vigorous mechanical agitation or homogenization using a mixed emulsifier system, including a classical emulsifier and a water-insoluble co-surfactant, such as a long-chain fatty alcohol or alkane (e.g. cetyl alcohol or hexadecane). The final polymer particles have almost the same size as the initial monomer droplets. Their particle size distribution is broader than those obtained by conventional means.4.

 

Table I. Raw Material Selection for Emulsion Polymerization

Material	Example(s)	Considerations and Comments
Monomers	– Methyl methacrylate – Butyl methacrylate – Styrene – Vinyl esters – Methacrylic acid – Acrylic acid – Hydroxyethyl acrylate – Hydroxyethyl methacrylate – Vinyl acetate	– Monomers that do not react with water – Monomers that undergo free-radical chain polymerization – Monomers with limited water solubility – Carboxyl functional monomers used in low    levels for particle stability – Hydroxyl functional monomers for reactivity with crosslinker – Monomer compatibility and rate of polymerization
Initiators	– Sodium, ammonium or potassium persulfate – Azobisisobutryl nitrile (AIBN) – t-butyl hydroperoxide (TBHP)	– Must be water soluble for REDOX type initiators (persulfates) – Thermal initiation for free-radical types (e.g. AIBN, TBHP)
Surfactant	– Anionic and nonionic types – Anionic types – Sodium lauryl sulfonate – Nonionic types – Nonylphenol ethoxylates	– Limited water solubility – Toxicity issues with nonylphenol ethoxylates
Other ingredients	– Water – Buffer (e.g. sodium bicarbonate) – Amine – Coalescent	– Normally deionized water to minimize variability – Buffer to regulate pH – pH control, stability – Aids film formation

 

In micro-emulsion polymerization, the initial system consists of monomer droplets varying from 10 to 100 nm dispersed in water with the aid of a classical emulsifier, such as sodium dodecyl sulfate (SLS), and a co-surfactant, such as a low molar mass alcohol (pentanol or hexanol). Micro-emulsions are thermodynamically stable and optically a one-phase solution. Reaction rates are normally very fast. The micro-emulsion polymerization process produces latex particles that are less than 50 nm. The average micro-emulsion particle contains only one polymer molecule with average molecular weight exceeding one million.3

Table II – Characteristics of Various Waterborne Resins 1

Resin Type	Major Characteristic(s)
Latex	Particle size of  5 nm – 10 microns, opaque liquid
Water Reducible	Reduction with water results in irregular viscosity vs. solids relationship
Polyurethane Dispersion (PUD)	– Particle size of 1-20 microns – Lower Minimum Film   Forming temperature in relation to dry film Tg – Lower cosolvent demand
Miniemulsion	– 50-500 nm particle size
Microemulsion	– Particle size of 10-100 nm, clear liquid
Core-Shell (Low Tg shell with high Tg inner core)	– Reduced blocking – Lower coalescent demand

 

Additional information on waterborne coatings can be found in the following Prospector articles:

• Fundamentals of Waterborne Resin Technology
• Improving Performance in Ambient Cured Latex Paints
• Dispersing and Wetting Hydrophobic Pigments and Fillers in Water-based Paints to Avoid Pigment Floating and Flooding

1. Lewarchik, R. Fundamentals of Waterborne Resin Technology. 2015 Sept 18. Overland Park (KS): UL Prospector; [accessed 2016 Jun 8]. https://knowledge.ulprospector.com/3069/pc-fundamentals-waterborne-resin-technology/
2. Nanostructured, Nonuniform and Core-Shell Polyurethane Dispersions. 2005 Oct 1. Troy (MI): PCI Magazine; [accessed 2016 Jun 8]. http://www.pcimag.com/articles/83072-nanostructured-nonuniform-and-core-shell-polyurethane-dispersions
3. Micro-Emulsion. c2016. Derby (UK): Enviroquest; [accessed 2016 Jun 8]. http://www.enviroquestgpt.co.uk/content/micro-emulsion.html
4. Poehlein, GW. Encyclopedia of Polymer Science and Engineering, 2nd Edn., Mark, HF, et al., Eds. 1986. Vol. 6: 1

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