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.
Both wetting agents and dispersants must be suited to the medium and the resin to avoid flaws, comply with regulatory, and safety demands all while being low foaming and effective at low dosages.
Pigment wetting in water-based systems is a challenge, mainly because of the high surface tension of the water compared with other media. This high surface energy creates a great energetic difference between the pigment/water interface, hampering the wettability of aggregates and particle clumps and their incorporation into the grinding load. A wetting surfactant reduces the surface tension of the water and, consequently, reduces the energy difference between the pigment/water interface—helping water penetrate the pigment particle clumps and aiding the dispersion process.
Each load and pigment requires a surfactant with a specific HLB, which will maximize its properties in the paint. The higher the concentration of pigments and loads in paint, the more important is the use of wetting agents and dispersants.
Clumps and aggregates are mechanically broken into smaller particles during grinding. During this process, the viscosity of the system is increased by the formation of new high-energy surfaces, which tend to come together and flocculate. With the presence of dispersing agents in the medium, this process is significantly enabled because the additives are also adsorbed on the newly formed surfaces, minimizing interaction between particles and ensuring a constant viscosity of the load concentrate throughout the process.
Wetting and dispersion surfactants play a key role in the dispersion process. Selecting a suitable wetting agent and dispersion agent at this stage is key to maximize the efficiency of the process as far as the time and energy required are concerned, deliver an optimized pigment particle size and enable greater pigment contents to be incorporated, in order to ensure maximum color development in the paint. The choice of suitable additives, as well as, the dosages to be used will depend on the types of pigments, loading, and respective amounts to be dispersed, in addition to compatibility with the medium and the resin.
Dispersion agents maintain the dispersion properties over time and prevent the re-agglomeration of pigment particles. The chemical structure of the dispersion additive will determine the stabilization mechanism of the pigment particles: electrostatic, steric, or electro-steric repulsion. The proper choice of dispersant, as well as, the dosage to be used is key to avoid problems such as pigment sedimentation and flocculation, changes in viscosity, and variations in color development over time.
EMULSION POLYMERIZATION PROCESS
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.
Learn more about the emulsion polymerization process.
Article written by Bruno Soares Dario, Technical Services & Development for Coatings at Oxiteno