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To create a synthetic latex, the main component of water-based paints and adhesives, the leading process used is emulsion polymerization. Monomers (acrylics, methacrylics, vinyls, etc.) are polymerized in a water-based medium using radical primers, where surfactants are required to stabilize the system during polymerization, and the dispersed polymer particles formed.
The batch polymerization process consists of adding all the inputs, such as surfactant solution, monomer mixture and primer solution, to the reactor at the beginning of the process and keep the polymerization under a certain stirring and temperature condition until the monomer is almost totally converted into a polymer.
The beginning of the polymerization (A) consists of drops of emulsified monomers, a fraction of water-soluble monomer, micelles of surfactants swollen by monomers, and water-soluble initiator. In stage B, the particles are developed, the water-soluble monomer molecules are polymerized, and the oligomeric radicals are created. In stage C, the polymerization takes place inside the particles with a concentration of monomers, resulting in a drop in the conversion rate and the completion of the polymerization. Finally, in stage D, the polymer particles from the completion of the polymerization process are dispersed and they are known as latex.
In emulsion polymerization, surfactants promote the emulsification of monomers’ droplets, solubilize monomers inside the micelles, control the number of nucleated particles in Stage A, the stability of the particles along stages C and D, the particle size breakdown, the surface stability and tension of the final latex and the formation of a clot in the reactor.
CHOOSING SURFACTANTS
The choice of surfactants is of the utmost importance to develop an emulsion polymerization process and deliver a latex with appropriate particle size breakdown and stability for each application. The surfactants used in emulsion polymerization must be able to reduce the surface tension of the aqueous phase and adsorption in the dispersion phases.
Anionic and nonionic surfactants are widely used in polymerization in emulsion of regular anionic latexes. Usually, a single surfactant is not enough to create a latex which meets all the requested requirements. Preferably, anionic surfactants boost the mechanical stability of latex through electrostatic stabilization of particles, while nonionic surfactants boost the stability of electrolytes and heating and cooling cycles through steric stabilization.
One of the most critical steps during the manufacture of paints is the dispersion of pigments in the medium, which demands a lot of energy and time. Pigment wetting and dispersion agents are key in this step, as they ensure proper homogeneity, stability, and viscosity, in addition to enabling the incorporation of dies to add value to the end-product.
The OXITIVE® 7000 line of wetting additives and OXITIVE® 8000 dispersion additives have been developed based on these needs and market trends. With multifunctional surfactants, they make it possible to formulate high-performance, competitive, and more sustainable paints with low VOC (volatile organic compounds), APE-free (free from ethoxylated alkylphenol), and with low foaming. Oxiteno, in tune with the industry’s trends and needs, is always researching and developing solutions to meet different applications, to bring the best in terms of chemistry to the market.
Emulsifying agents are surfactants added in the emulsion processes to increase their stability by making them stable and homogeneous. The molecule of an emulsifier has a polar part (water-suitable) and a nonpolar part (polymer-suitable). They are key to the emulsion polymerization process.
The emulsifier OXIMULSION® line features APE-free primary surfactants for emulsion polymerization (for example acrylic, styrenic and vinyl latex) and resin emulsification (for example, alkyd). The main qualities of this line include superior performance compared to surfactants from ethoxylated alkylphenol; enabling high particle control, viscosity, and conversion; low VOC; low toxicity; and high solids content. In short, the OXIMULSION® emulsifiers enable the formulator to have a latex with low toxicity, high stability and performance. These optimizations, such as formulations with a high solids content, are more cost-effective to the entire coating formulation.
Article written by Bruno Soares Dario, Technical Services & Development for Coatings at Oxiteno
