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Emulsion Polymerization

Monday, September 7, 2009 § 1

Mostly, emulsion polymerization is using a free radical polymerization method. They can be carried out using three processes: batch, continuous and semi-continuous. For batch process, all ingredients will be added at one time. While this method can save time usage, it also can be very dangerous. Emulsion polymerization is an exothermic process, thus batch process can generates a lot of heat. A safer process like continuous process seems applicable, but it is a high capital process. So normally, for emulsion polymerization we will use semi-continuous process. A portion of monomer is allowed to react first, and then the balance of monomer will be added in gradually increment. Semi-continuous process has advantages in good distribution of monomer and better control of heat loss.

Typically, emulsion polymerization can be said dispersion of liquid in liquid. Typical emulsion polymerization recipe consist of monomer (40 – 60%), water (60 – 40%), surfactant (1 – 3%), buffer (0 – 0.5%), catalyst (0.1 – 0.2%), preservative or biocide (0.1 – 0.2%) and antifoam (0 – 0.5%). Delay process usually takes 2 – 4 hours where we gradually adding the balance monomer. Holding process can takes 0.5 – 1 hour where we constant the temperature at 70 – 90oC. During the holding process, the conversion of monomer into polymer is completely complete.

Emulsion polymerization mechanism can be divided into four stages. During the first stage, surfactants inside the aqueous solution will stabilize itself to form micelle. Hydrophobic tails will attract each other in the inside of the micelle while hydrophilic head will face at the outside. Surfactants also will dissolve in the monomer. Monomer, in droplets form will dissolve in the micelle. Stage two, also known as nucleation stage where free radicals generate from initiator decomposition. An example is the following equation:
S2O82– SO4 – •

Radicals will diffuse into the micelle and growing monomer-polymer particles. Polymerization will begins and growing monomer becomes oligomer. Oligomer will stabilize itself by diffusing into the micelle. The third stage is the growth stage. Polymer particles will be polymerized rapidly. Monomer droplets will be shrinking. No more micelle left and the monomer will diffuse into the growing particles. The final stage is the concluding stage. During this stage no more monomer droplets because all of the monomer is completely polymerized.

Product from emulsion polymerization can be used to compound paint. Emulsion (later will be called dispersion) is a very important component in the latex paint formulation. Their special characteristics serve as a media for binding other ingredients in the latex paint system. Beside dispersion, water based paint also consist of pigment, extenders, thickener or rheology modifiers, dispersant, coalescing solvent, freeze-thaw stabilizers, preservatives, antifoam and of course water. Unlike dispersion, there are no typical formulations for the paint. Paint with low pigments volume concentration (PVC) is a better paint and has a high gloss compare to the paint with high PVC. Sometimes, to obtain a gloss or semi-gloss paint, we have to use Tioxide as the pigments. Also if the paint not achieve the required mid shear viscosity, we also have to add some more thickener. So from here, what we can say is there are no typical ratios in the paint formulation. We have to try and modified the ingredients to see if there are significant changes in the properties.

The properties of the paint generally can be divided into three: fluid properties, dry film properties and ageing properties. Including in the fluid properties are total solid content, pH, viscosity (low, mid and high shear) and specific gravity. Paint dry film properties including color development, water resistance, alkali resistance, dirt pick up resistance etc. Including in ageing properties is paint storage stability, color change to QUV exposure, resistance towards natural weathering exposure etc. These properties are important for the paint, so that we can know its performance. This information is valuable for us to beat the competitor’s paint.

Recently, there is huge competition in paint industries. Scientist all over the world has conducted various research and development job to improvise and makes a better paint. Better paint doesn’t mean a good color and texture only, but also healthy for the use of human being and environmental friendly. So this is a part of paint chemists’ job to come with a paint that will lead to a sustainable lifestyle of the consumer.

p/s: something I wanna share. I wrote this article by myself. Sometime it's good to share something that u learn... ehe...

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