Views:5 Author:Site Editor Publish Time: 2017-02-05 Origin:Site
Surfactants which stabilize the solid particles in the solid-liquid suspension in the medium are called dispersants. Dispersion is the process of uniformly distributing the solid particles in the dispersion, and the dispersion has a certain stability.
1. adsorbed on the surface of solid particles, so that the surface of condensed solid particles easy to wet.
2. Polymer type of dispersant, the surface of the solid particles to form the adsorption layer, so that the surface charge of solid particles increased, the formation of three-dimensional obstacles to improve the reaction between the particles.
3. The surface of the solid particles to form bilayer structure, the outer end of the polar dispersant and water has a strong affinity to increase the degree of solid particles were water wetting. Static particles between the particles due to electrostatic repulsion
4. To make the system uniform, suspension performance increased, no precipitation, so that the physical and chemical properties of the entire system the same as described above, the use of a dispersant enables the stable dispersion of solid particles in the liquid.
The dispersant is selected
In our paint production process, pigment dispersion is a very important production links, it is directly related to the paint storage, construction, appearance and performance of the film, so a reasonable choice of dispersant is a very important production processes. However, the dispersion of coating paste is not only a good relationship with the dispersant, and the formulation of paint formulations and the choice of raw materials are related. Dispersants by name suggests, is a variety of powder reasonably dispersed in the solvent, through a certain charge rejection principle or polymer steric effect, so that a variety of solid is very stable suspension in the solvent (or dispersion).
Double layer principle
Dispersants used in aqueous coatings must be water soluble, and they are selectively adsorbed onto the interface of the powder and water. Now commonly used is anionic, they form anion in the water ionization, and has a surface activity, the powder surface adsorption. Powder particles adsorbed on the surface after the formation of double-layer dispersant, the anion is closely adsorbed by the particle surface, known as surface ions. In the medium with the opposite charge of ions called counter ions. They are the surface ions through the electrostatic adsorption, a part of the counter ions and particles and surface ions are more closely binding, they are said to bind the counter-ion. They become a moving whole in the medium, with a negative charge, the other part of the counter-ion is surrounded by, they are called free counter ions, the formation of a diffusion layer. This creates an electrical double layer between the surface and counter ions.
Electrokinetic potential: the negative charged by the particles and the positive layer formed by the proliferation of electric double layer, called the mobile potential. Thermoelectric potential: all anions and cations formed between the electric double layer, the corresponding potential.
From the action of the potential is not a thermodynamic potential, the potential charge is not balanced, there is charge rejection phenomenon, and the thermoelectric potential is a charge balance phenomenon. If the concentration of counter ions increases in the medium and the free counter ions in the diffusion layer are forced into the bound counterion layer due to the electrostatic repulsion, the electric double layer is compressed and the electrokinetic potential decreases and when all the free counter ions become bound After the counter ion, the electrokinetic potential is zero, called the isoelectric point. No charge rejection, the system does not have the stability of flocculation.
The formation of a stable dispersion system, in addition to the use of electrostatic repulsion, that negative charges adsorbed on the particle surfaces repel each other to prevent the adsorption / aggregation between the particles and particles and finally to form large particles and to delaminate / The theory of steric hindrance is that when the negatively charged particles are close to each other, they are staggered with each other. Such sterically hindered surfactants are generally nonionic surfactants. Flexible use of electrostatic exclusion with the theory of steric hindrance, both can constitute a highly stable dispersion system.
Polymer adsorption layer has a certain thickness, can effectively block the mutual adsorption of particles, mainly rely on the polymer solvation layer, when the powder surface adsorption layer of 8-9nm, the repulsion between them can protect the particles will not Flocculation. So the polymer dispersant than ordinary surfactant.
The role of process:
一、 Solid particle dispersion process Solid particles in the medium dispersion process is generally divided into three stages.
1, the wetting of solid particles is the most basic conditions for the dispersion of solid particles, if the solid particles to be uniformly dispersed in the medium, we must first make each solid particles or particles, can be cut to fully moist
2. Dispersions or fragmentation of ions In this process to make the particles dispersed or fragmented, involving the particle group and the internal solid interface separation problem. Different types of surfactants play a role in the dispersion or fragmentation of the particles.
A. Generally, when water is used as a medium, the solid surface tends to be negatively charged. Anionic surfactants can also be adsorbed on the surface of the slit by van der Waals force, or by inlaying the surface, because the anionic surface active agent is negatively charged, but the surface potential is not very strong. And the permeation of osmotic pressure to produce a common role to reduce the adhesion between the particles, reducing the solid particles or particles required to break the mechanical work, so that the particles are broken or the particle fragmentation Into a smaller crystal, and gradually dispersed in the liquid medium.
B. Nonionic surfactants are also adsorbed on the wall by the van der Waals force. Nonionic surfactants can not produce point repulsive force but can produce entropy repulsion and permeation hydration force, and make the bond between microcracks in the particle clusters And the strength is decreased to facilitate the fragmentation of the particles
C. Cationic surfactants can be adsorbed to the wall of the gap by electrostatic attraction, but the adsorption state is different from that of anionic surfactants and nonionic surfactants.
3. Prevent the re-aggregation of solid particles Once the solid particles dispersed in the liquid, resulting in a uniform dispersion, but the stability of yo ah depends on whether the dispersion of solid particles can be re-aggregated to form aggregates.
二、 Dispersion - stabilization of Surfactants in Aqueous Media
1. Dispersion of non-polar solid particles After the surfactant is added to the suspension, the surface tension of the water can be reduced by the surfactant, and the hydrophobic bond of the surfactant can be adsorbed on the surface of the non-polar solid particles by van der Waals force, The hydrophilicity of the non-polar solid particles is improved by the hydrophilic groups extending into the water to improve the hydrophilicity of the surface.
2. Dispersion stabilization of charged particles
A. Ionic surfactants have the same charge on the surface of fish particles. When ionic surfactants have the same charge as the particle surface, the ionic surfactants can not be easily adsorbed on the surface of the dotted surface due to the electrostatic repulsion. Ionic surfactants and particles between the van der Waals force is strong, can overcome the electrostatic repulsion of ionic surfactants can be adsorbed by the characteristics of adsorption on the particle surface, this will make the particle surface zeta potential of the absolute value increases, so that Dot particles are more stable in water.
B. Ionic surfactants and the surface of the particles with the opposite charge If the use of ionic surfactants and charged between the opposite particles in the surfactant concentration is low, the particle surface charge will be neutralized to eliminate the electrostatic repulsion, Flocculation may occur; however, when the concentration of the surfactant is high, the second layer of the surfactant ion is reabsorbed on the particles which are electrically neutralized, and the solid particles are again charged, and due to the repulsive force of the static electricity, The solid particles are re-dispersed.
三、 The dispersion of the surfactant in the organic medium The dispersion of the particle in the organic medium is mainly achieved by the entropy repulsion produced by the steric hindrance. For the non-polar particles, in order to overcome the van der Waals force between the particles and stable dispersion in organic media. The surface treatment of organic pigments can be achieved in several ways.
1. The organic pigments are surface-treated with organic amines
2. The use of pigment derivatives for organic pigments surface treatment.