Detailed explanation of the functioning principle and selection method of dispersant
What is a dispersant – can make the solid-liquid suspension in the solid particles stably dispersed in the medium of the surfactant is called dispersant.
The role of the principle of dispersant – dispersion is the solid particles uniformly distributed in the process of dispersing liquid, dispersing liquid has a certain degree of stability.
The main performance:
1, adsorbed on the surface of solid particles, so that the surface of the cohesive solid particles are easy to wet.
2, polymer-type dispersant, the formation of adsorption layer on the surface of solid particles, so that the charge on the surface of the solid particles increased to improve the formation of three-dimensional obstruction of the reaction force between the particles.
3, so that the surface of solid particles to form a bilayer structure, the outer dispersant polar end and water have a strong affinity, increasing the degree of solid particles are wetted by water. Solid particles are far away from each other due to electrostatic repulsion 4. Make the system uniform, increase the suspension performance, no precipitation, so that the whole system has the same physical and chemical properties
As mentioned above, the use of dispersant can stabilize the dispersion of solid particles in the liquid.
Dispersant dispersing mechanism
1
Bilayer principle – water-based coatings using dispersants must be water-soluble, they are selected to adsorb to the interface between the powder and water. Currently commonly used is anionic type, they ionize in water to form anions, and has a certain surface activity, adsorbed by the powder surface. Powder particle surface adsorption of dispersant after the formation of a double electric layer, the anion is closely adsorbed by the particle surface, known as surface ions. Ions with opposite charge in the medium are called counterions. They are surface ions through electrostatic adsorption, a part of the counter-ion and the particle and surface ions combined more closely, they are called bound counter-ion. They become a moving whole in the medium with a negative charge, and the other part of the counterions are surrounded by them, they are called free counterions, forming a diffusion layer. In this way between the surface ions and counterions form a double electric layer.
2
Kinetic potential – The negative charge carried by the particles and the positive charge carried by the diffusion layer form a double electric layer called kinetic potential . Thermal potential: the double electric layer formed between all anions and cations, the corresponding potential.
Play a dispersive role is the kinetic potential rather than thermal potential, kinetic potential charge imbalance, there is charge rejection phenomenon, while the thermal potential belongs to the charge balance phenomenon. If the medium increases the concentration of counterions, and the diffusion layer of free counterions will be forced to enter the bound counterion layer due to electrostatic repulsion, so that the double layer is compressed, the kinetic potential decreases, when all the free counterions into bound counterions, the kinetic potential is zero, called the isoelectric point. There is no charge repulsion, the system is not stable flocculation occurs
3
Positional resistance effect – the formation of a stable dispersion system, in addition to the use of electrostatic repulsion, that is, adsorbed on the particle surface of the negative charge repel each other, in order to prevent the adsorption/aggregation of particles and particles and finally form large particles and stratification/sedimentation in addition to the use of spatial positional resistance effect of the theory, that is, in the adsorption of negatively charged particles close to each other, so that they slide each other stagger In addition to utilizing the theory of spatial barrier effect, it is also necessary to utilize the theory of spatial barrier effect, i.e., when the adsorbed negatively charged particles are close to each other, to make them slide away from each other. Flexible use of electrostatic repulsion with the theory of spatial resistance can constitute a highly stable dispersion system.
Polymer adsorption layer has a certain thickness, can effectively block the mutual adsorption of particles, mainly relying on the solvent layer of polymers, when the powder surface adsorption layer of 8-9nm, the repulsive force between them can protect the particles from flocculation. So polymer dispersant is better than common surfactant.
The process of dispersant
I. Solid particle dispersion process The dispersion process of solid particles in the medium is generally divided into three stages.
1. wetting of solid particles Wetting is the most basic conditions of solid particle dispersion, if the solid particles are uniformly dispersed in the medium, first of all, each solid particle or particle group, can be as of fully wet. 2.
2. Ion group dispersion or fragmentation This process to make the particle group dispersion or fragmentation, involving the particle group and the internal solid-solid interface separation problem. The type of surfactant plays a different role in the dispersion or fragmentation of the ionomer.
a. Generally, solid surfaces tend to be negatively charged when water is the medium. For anionic surfactant although also negatively charged, but in the solid surface potential is not very strong conditions anionic surfactant can overcome the electrostatic repulsion through the van der Waals force or through the mosaic way and is adsorbed in the gap surface, so that the surface due to the same kind of charge and the repulsive force to enhance the osmotic pressure and osmosis of water together to make the bond between the particles to reduce the degree of reduction of the solid particles or particle clusters required to break up the mechanical work, so that the particle clusters are broken up. Mechanical work, so that the particle group is broken or make the particles broken into smaller crystals, and gradually dispersed in the liquid medium.
b. Nonionic surfactants are also adsorbed in the crevices by van der Waals forces.
