In the pigment production process, no matter how fine the pigment powder is ground, there will always be some aggregated and flocculated particles. In the process of transportation and storage, the pigment will be further flocculated into large particles due to extrusion and moisture, and the finer the pigment, the larger the surface area and higher the surface energy, the easier it is to flocculate together. If treated with appropriate surfactants, these flocculated large particles are easily dispersed during use, and the dispersion mechanism is mainly as follows:
1. Wetting
The dispersion of inorganic pigment powder in liquid mainly goes through the following three stages:
① For the wetting of the powder, the liquid should not only wet the surface of the powder, but also replace the air and moisture between the powder particles;
② After passing through the wet powder and displacing the air and moisture between the particles, the flocs and aggregates in the pigment powder are destroyed;
③ The wetted and destroyed flocs and aggregate powders maintain a stable dispersion state in the liquid. That is to say, dispersion is a process of wetting-dispersing-keeping the dispersion stable.
Under normal circumstances, inorganic pigments are rarely dried before use, and the surface of the pigment is not only mixed with air, but also absorbs a layer of water film. The amount of water usually adsorbed on the surface of the pigment is equivalent to the amount of water required to form a monomolecular film on the solid surface. For example, the surface area per gram of TiO2 is 10m2, the thickness of the water molecule adsorption layer is 10×10-10m, and the amount of water required by the monomolecular film is about 0.3% of the weight of the pigment, so the moisture content in the pigment is also one of the main factors affecting its dispersion performance. one. Whether the solid is wetted or not can be judged according to its contact angle. A contact angle of 0° means that it is completely wet, and the liquid is completely spread on the surface of the solid; a contact angle of 180° means that it is not wet at all, and the liquid adheres to the surface in the form of water droplets. solid surface.
Whether a solid can be well wetted in a liquid can be judged not only by the size of the contact angle, but also by measuring the size of its heat of wetting. Generally, hydrophilic powders (such as TiO2) have a large heat of wetting in polar liquids, and in non-polar liquids The heat of wetting in polar liquids is small, while the heat of wetting of hydrophobic powders in polar and non-polar liquids is roughly constant.
The settling speed and settling volume of solid powder in liquid can also judge the degree of wetness. A solid with high polarity such as TiO2 has a small settling volume in a highly polar solution, and a small solid in a low polar solution. is large; non-polar solid powders generally have large sedimentation volumes. After the addition of surfactant treatment, because the surfactant molecules are strongly oriented and adsorbed on the surface of the solid, it helps to reduce the surface tension of the liquid and improve its wetting and dispersing properties.
2. Electric repulsion (ξ potential)
The dispersion and dispersion stability of inorganic pigments in aqueous solution are mainly determined by their electrical repulsion in water, that is, the ξ potential.
Electric repulsion is the use of charge repulsion to maintain dispersion stability.
Surfactants can ionize a large number of negatively charged (or positively charged) ions in aqueous solution, which are firmly adsorbed on the surface of pigment particles, so that these particles have the same charge, and other ions with opposite charges freely diffuse into the liquid medium. Around, a diffusion layer (electric double layer) of charged ions is formed. The potential difference between the two layers of ions from the solid surface to the farthest point of the diffusion layer (that is, where the opposite charge is 0) is called the ξ potential. The electrostatic repulsion between particles comes from this, and these particles with the same charge will repel each other once they come into contact, so as to maintain the stability of the dispersed system, which is the famous D.L.V.O. theory.
In the case of electric repulsion, the surfactant must have high ionization performance, and anionic surfactants and some inorganic dielectrics are usually used, such as: tripotassium polyphosphate, potassium pyrophosphate, sodium polyphosphate, alkyl aryl sulfonate Sodium Naphthalene Sulfonate, Sodium Methylene Naphthalene Sulfonate, Sodium Polycarboxylate, etc.
3. Steric hindrance effect (or entropy effect)
When the pigment is dispersed in a non-aqueous medium, the possibility of the above-mentioned ionic reaction is greatly eliminated, and the non-ionic surfactant is not ionized in water. In this case, the effect of the surfactant is called the steric hindrance effect or entropy effect. Because the surfactant can be directional adsorbed on the surface of the pigment particles to form a monomolecular adsorption layer, this directional buffer layer can prevent the aggregation of the particles, thereby maintaining the stability of the dispersion system (also known as protective colloid or micelle).
Surfactant molecular groups on the surface of the pigment, as the concentration of the surfactant increases, its entropy will decrease and its movement will be restricted. The closer and more compressed the pigment particles are, the further their entropy will decrease, which is beneficial to the stability of the dispersion system.
