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    Emulsion Stability

    Synthetic latex is used extensively in water-based coating and adhesive formulations.  These latices are composed of water insoluble polymers in a micelle that must remain colloidally stable as droplets when stored, subjected to shear, or incorporated into a formulation.  The stability of an emulsion is defined as the systems ability to resist changes overtime to its physical and chemical properties.  Commercially there are two main techniques for stabilizing droplets in latex emulsions: electrostatic stabilization and steric stabilization.  Electrostatic stability arises from ionic groups on the surface of the latex particle repelling each other.  Steric stability occurs when bulky groups at the surface of the latex particles keep the particles from agglomerating to prevent flocculation.  These groups can be surfactants or protective colloids like CELLOSIZE™ Hydroxyethyl Cellulose.

Steric Stabilization

In emulsion polymerization, free radicals are generated by an initiator to begin the polymerization.  These free radicals react with insoluble monomers suspended in water to begin the polymerization reaction.  When hydroxyethyl cellulose (HEC) is present in the reaction solution, free radicals can also be generated on the cellulose backbone of hydroxyethylcellulose.  These reactive radical sites can participate in the emulsion polymerization as well causing the hydroxyethylcellulose to be chemically bonded to the monomers in the reaction to become an integral part of the polymer that is being formed.  When this grafting occurs it provides optimal protective colloid performance and allows for drastically improved emulsion stability.  The figure below illustrates the reaction mechanism described. 
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Due to the hydrophilic (water loving) nature of the hydroxyethyl cellulose grafted to the polymer, water molecules are "bound" onto the surface of the polymer particles.  This "bound" water acts as a emulsifier between "free water" in the emulsion and the hydrophobic polymer phase.  This in turn provides stability to the emulsion.  This effect is illustrated in the diagram below.
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This grafting of hydroxyethyl cellulose to the polymer is not the only reaction occurring during polymerization.  The backbone of the hydroxyethyl cellulose is also being cleaved to some extent resulting in smaller hydroxyethyl cellulose fragments.  The two possible reaction routes will normally occur simultaneously with one route being favored based on the reactivity of the monomer and chemical structure of the hydroxyethyl cellulose.   Hydroxyethylcellulose that is not grafted to the polymer does not provide optimal stabilization to the emulsion.  For this reason it is important to understand which CELLOSIZE™ grade is best suited for your system.  The competitive reactions are illustrated below.

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Grade Selection

As mentioned above, the efficiency of grafting CELLOSIZE™ to a polymer during emulsion polymerization varies based on the reactivity of the monomer toward radical intermediates. Knowing how the monomers in a system will effect HEC allows for the selection of a grade with the optimal chemical structure. When an optimal grade is used it lowers the amount of stabilizer ingredients (both surfactants and protective colloids) providing a reduction in raw material cost and lower water adsorption of the latex.
 

Vinylic Systems

Vinylic monomers like vinyl acetate and vinyl versatate have a relatively low reactivity toward radical intermediates.  This means that HEC radicals generated in systems containing only vinylic monomers will prefer the degradation mechanism.  In vinylic systems it is preferable to use CELLOSIZE™ EP 09 as the protective colloid.  This grade has a different hydroxyethyl substitution pattern that reduces the tendency for cleavage and promotes the grafting reaction.

 

Acrylic Systems

Acrylic monomers like methyl methacyrlate, butyl acrylate, and 2-ethylhexyl acrylate are extremely reactive toward radical intermediates.  These systems favor the grafting mechanism for HEC radicals.  This high degree of grafting can result in latex particle bridging if the reaction conditions are not properly controlled.  This ultimately lends to micelle droplet flocculation.  In these systems it is recommended to use a lower molecular weight grade like CELLOSIZE™ EP 09, QP 300, or QP 4400.  It is also recommended to use much less CELLOSIZE™ than vinylic systems (0.2% maximum) and to add the HEC to the system near the end or after the addition of the monomers. 

Recommended CELLOSIZE™ Grades for Emulsion Polymerization

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