Emulsion polymerization creates polymers in an aqueous solution, commonly used for water-based paints, adhesives, and coatings. The process involves mixing small monomer chains with initiators, modifiers, and emulsifiers to create a stable emulsion. The length of the polymer chain affects the properties of the solution and dry film. The end products are sold as water-based solutions, which are environmentally safe. Synthetic rubber chemicals can also be produced using emulsion polymerization systems. Additional washing and drying steps are required to remove initiators or other chemicals.
Emulsion polymerization is a process for creating polymers, or linked groups of smaller chemical chains called monomers, in an aqueous solution. The process is commonly used to create water-based paints, adhesives and coatings where the water stays with the polymer and sold as a liquid product. An emulsion is a stable mixture of very small droplets of the monomer or polymer suspended in the aqueous phase. An example of a stable emulsion is liquid hand lotion, which contains various chemicals mixed in water but do not react with it.
Many plastics and some rubber compounds are polymerized molecules. Small monomer chains are mixed with various chemicals that initiate the reaction, control the size of the polymer chains that form, and keep the mixture stable. The chemicals that control or initiate the reaction are initiators, those that control the polymer chains are modifiers, and emulsifiers keep the mixture as a stable emulsion.
An emulsion polymerization reaction occurs in water and the monomer can have little or no solubility in water, meaning it stays separate and does not dissolve. When the chemicals are well mixed, the small droplets of monomer are distributed uniformly throughout the aqueous phase and remain so due to the action of the emulsifier. Very small droplets of monomer, known as micelles, begin to form and are surrounded by the initiator and modifier molecules. Each monomer chain begins to link with the others within the micelle, until the monomer disappears.
The length of the polymer chain will affect both the properties of the aqueous solution and any dry film created by it. Manufacturers control these properties by varying the amounts of initiator and modifiers as well as the temperatures during the reaction. The goal of an emulsion cure is to create a paint or adhesive that retains its properties over time and is easy to handle. Overreaction can create very thick and viscous emulsions which may have industrial uses but are not appreciated by consumers.
Most end products are sold as water-based solutions. Concern about the environmental and health risks of solvents has led to a growth in emulsion polymerization products in the 21st century. Once applied as a coating, the polymer dries and cures and the water evaporates as an environmentally safe vapor. The reacted polymer can also be spray dried or in some cases filtered out of water, but this is less common. Other types of chemical reactions can produce a dry product more easily than from an aqueous emulsion.
Synthetic or artificial rubber chemicals can also be produced in emulsion polymerization systems. Styrene-butadiene and nitrile rubber are two common synthetic rubbers produced by emulsion chemistry. Processing these products into a dry solid requires additional washing and drying steps to remove initiators or other chemicals. These materials can eventually degrade the rubber if they are not removed.
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