Surfactant molecules are composed of polar and nonpolar ends. The polar end, also called the hydrophilic end, is water-attracting. The nonpolar end, also known as the hydrophobic end, is water-repelling. Surfactants have applications in a number of different industries, such as food, cosmetics, pharmaceuticals, agrochemicals, and paints, and it is this association of two polar ends in the same molecule that confers most of the products’ features.
They are essential components for the formulation of paints. Surfactants are added during the latex polymerization process and, in the final paint product, during the process of pigment dispersion and loading. In both processes, they act as system stabilizers. Surfactants can also be used as additives at the end of the entire process to enhance specific properties. One method for determining the correct surfactant to use the surfactant’s hydrophilic-lipophilic balance (HLB) value—the correct surfactant has major impact on the final performance of the coating.
In most systems, a mixture of surfactants is more efficient during emulsification rather than a single product. When mixing surfactants, it is possible to adapt the proportion of each one to reach the exact HLB value of the substrate to be emulsified. The mixture also enables small adjustments to be made to optimize the emulsion, which is not possible when using a single compound. Such mixtures are usually made up of surfactants with different affinities – that is to say, one with a greater hydrophilic profile and the other with a greater lipophilic (hydrophobic) profile.
In paints, mainly water-based ones, there are extraordinarily complex systems that contain several components which, under normal conditions, would not form a homogeneous solution. So, several surfactants are used to stabilize and deliver the properties of each type of paint.
The pigment dispersion process can be divided into three stages: wetting, dispersion/grinding, and stabilization. In wetting, it is important to use an additive that can reduce the surface tension of the water, enabling it to penetrate pigment pores thereby wetting of the pigments to be dispersed.
In the dispersion/grinding stage, the pigment clumps are mechanically separated into smaller particles. The wetting and dispersing surfactants act synergistically, to ensure that the wetting of the smaller particles created happens faster and the particles do not clump again, so reducing process time and, consequently, energy spent. The proper choice of a wetting and dispersion surfactant reduces the interaction between the pigment particles, decreases the viscosity of the medium, allows greater incorporation of the pigment content, and ensures the stabilization of the dispersed particles over time. As the pigments are colored through the absorption and scattering of light on the surface, the size of the pigment particles, in addition to the amount loaded, directly impacts the color created.
Article written by Bruno Soares Dario, Technical Services & Development for Coatings at Oxiteno