The principle of action of dispersants and selection methods are explained
What is a dispersant – the surfactant that can make the solid particles in solid-liquid suspension stable and dispersed in the medium is called a dispersant.
Dispersant principle of action – dispersion is the process of uniform distribution of solid particles in the dispersion, which has a certain degree of stability.
The main manifestations are:
1, adsorption on the surface of solid particles, so that the surface of the cohesive solid particles can be easily wetted.
2、Polymer type dispersant, forming an adsorption layer on the surface of solid particles, increasing the charge on the surface of solid particles and improving the reaction force between the particles that form a three-dimensional obstruction.
3、To make the surface of solid particles form a bimolecular layer structure, the polar end of the outer dispersant has a strong affinity with water, increasing the degree of wetting of solid particles by water. Solid particles are far away from each other due to electrostatic repulsion 4. make the system uniform, suspension performance increases, no precipitation, so that the whole system physical and chemical properties the same
The above-mentioned, the use of dispersants can be stable dispersion of solid particles in the liquid.
Dispersant dispersion mechanism
1
Double electric layer principle – the dispersants used in water-based coatings must be water-soluble, and they are selectively adsorbed to the interface between the powder and water. Currently, they are commonly used as anionic type, they ionize in water to form anions and have certain surface activity, which are adsorbed by the powder surface. After the powder particle surface adsorption dispersant to form a double electric layer, the anion is closely adsorbed by the particle surface, known as surface ions. The ions with opposite charge in the medium are called counter ions. They are adsorbed by the surface ions through electrostatic adsorption, and a portion of the counter ions are more tightly bound to the particles and surface ions, and they are called bound counter ions. They become a moving whole in the medium with a negative charge, and another part of the counter ions are surrounded by them, they are called free counter ions and form a diffusion layer. Thus a double electric layer is formed between the surface ions and counter ions.
2
Kinetic potential – the negative charge carried by the particle and the positive charge carried by the diffuse layer form a double layer, called kinetic potential . Thermodynamic potential: the double electric layer formed between all anions and cations, the corresponding potential.
The role of dispersion is the kinetic potential rather than the thermodynamic potential, the kinetic potential charge imbalance, there is a charge rejection phenomenon, while the thermodynamic potential belongs to the charge balance phenomenon. If the medium increases the concentration of counter ions, and the free counter ions in the diffusion layer will be forced into the bound counter ion layer due to electrostatic repulsion, so that the double electric layer is compressed, the kinetic potential decreases, when all the free counter ions become bound counter ions, the kinetic potential is zero, called the isoelectric point. There is no charge repulsion and the system is not stable for flocculation to occur.
3
The formation of a stable dispersion system, in addition to the use of electrostatic repulsion, that is, the negative charges adsorbed on the surface of the particles repel each other to prevent particle-to-particle adsorption/aggregation and the final formation of large particles and delamination/sedimentation, but also the use of the theory of spatial resistance effect, that is, when the particles have adsorbed negative charges close to each other This type of surfactant with spatial reluctance is generally a non-ionic surfactant. A highly stable dispersion system can be formed by the flexible use of electrostatic repulsion in combination with the theory of spatial resistance.
The polymer adsorption layer has a certain thickness, which can effectively block the mutual adsorption of particles, mainly relying on the solventized layer of the polymer. When the adsorption layer on the powder surface reaches 8-9nm, the repulsive force between them can protect the particles from flocculation. Therefore, polymeric dispersants are better than ordinary surfactants.
The process of dispersant action
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 condition for the dispersion of solid particles, if the solid particles are to be evenly dispersed in the medium, each solid particle or particle group must be wetted sufficiently.
2. ion cluster dispersion or fragmentation This process to make the particle cluster dispersion or fragmentation, involving the particle cluster and the internal solid-solid interface separation problem. Different types of surfactants play different roles in the dispersion or fragmentation of particle clusters.
a. Generally, when water is used as the medium, the solid surface tends to have a negative charge. For anionic surfactants, although also negatively charged, but in the conditions of the solid surface potential is not very strong anionic surfactants can overcome electrostatic repulsion through van der Waals forces or through the mosaic way and be adsorbed on the surface of the gap, so that the surface with the same charge and the repulsive force is enhanced, as well as permeable water to generate osmotic pressure combined to reduce the bond between particles, reducing the solid particles or particle clusters required for fragmentation The mechanical work required to break up the solid particles or particle clusters is reduced, so that the particle clusters are broken up or the particles are broken up into smaller crystals and gradually dispersed in the liquid medium.
b. Non-ionic surfactants are also adsorbed on the gap wall by van der Waals forces, and the presence of non-ionic surfactants cannot produce point repulsion but can produce entropic repulsion and osmotic hydration, which reduces the bond strength between microcracks in the particle cluster and facilitates particle cluster fragmentation. c. Cationic surfactants can be adsorbed on the gap wall by electrostatic attraction, but the adsorption state is different from that of anionic surfactants and nonionic surfactants.
3. prevent the re-agglomeration of solid particles Once the solid particles are dispersed in the liquid, a uniform dispersion system is obtained, but the stability or not yo ah depends on whether the respective dispersed solid particles can re-agglomerate to form cohesions.
II. Dispersion stabilization of surfactant in aqueous medium
1. Dispersing effect on non-polar solid particles After surfactant is added to the suspension, because surfactant can reduce the surface tension of water, and the hydrophobic bond of surfactant can be adsorbed on the surface of non-polar solid particles through van der Waals force, the hydrophilic group reaches into water to improve the hydrophilicity of its surface, so that the wettability of non-polar solid particles is improved.
2. Dispersing and stabilizing effect on charged masses
When the ionic surfactant has the same charge as the surface of the plasmid, it is not easy for the ionic surfactant to be adsorbed on the surface of the plasmid with a point due to electrostatic repulsion; however, if the van der Waals force between the ionic surfactant and the plasmid is strong enough to overcome the electrostatic repulsion, the ionic surfactant can be adsorbed on the surface of the plasmid through characteristic adsorption, which will The absolute value of the zeta potential on the surface of the plasmas increases, making the plasmas with dots more stable in water.
If the ionic surfactant used has the opposite charge to the surface of the plasmid, the surface charge of the plasmid will be neutralized at a low surfactant concentration, so that the electrostatic repulsion is eliminated and flocculation may occur; however, when the surfactant concentration is high, the solid particles will be charged again after a second layer of surfactant ions is adsorbed on the electrically neutralized particles. With the charge, the solid particles are re-dispersed again due to the repulsive force of electrostatic force.
Dispersion and stabilization of surfactants in organic media
The dispersion of masses in organic media is mainly achieved by the entropic repulsive force generated by the spatial potential resistance. For the non-polar masses, they are stably dispersed in the organic medium by overcoming the van der Waals forces between the masses. The surface treatment of organic pigments can be achieved in several ways.
1. surface treatment of organic pigments with organic amines
2. surface treatment of organic pigments with pigment derivatives.
How to choose dispersant for coating
In our coating production process, pigment dispersion is a very important part of the production process, which is directly related to the storage, construction, appearance and performance of the paint film, so a reasonable selection of dispersant is a very important part of the production process. However, the good or bad dispersion of the paint paste is not only related to the dispersant, but also to the formulation of the paint and the choice of raw materials. Dispersant, as the name suggests, is to reasonably disperse various powders in the solvent, so that various solids can be suspended in the solvent (or dispersion) very stably through a certain charge repulsion principle or polymeric bit resistance effect.
1、For the selection of resin
Resin, especially the resin for grinding, plays a key role in the preparation of color paste.
1) Participate in the dispersion and anchoring of pigments
2) Participate in maintaining the stability of the pigment particles that have been dispersed and isolated
The above role of resin can be seen through some experiments, such as long oil alkyd resin, polyamide resin, amino resin, aldehyde ketone resin, low relative mass hydroxy acrylic resin, all show good wetting ability to pigment, while low hydroxyl value acrylic resin, thermoplastic acrylic resin, high relative mass polyester resin, high relative mass saturated polyester resin, vinyl copolymer resin, polyolefin resin, etc. All of them show poor wettability to pigments. The same pigment is obtained in different resin systems with different color phases. Almost all carbon black and organic pigments and transparent iron oxide change their hue, especially the scattering hue, with different resin systems. Therefore, choosing the right dispersant is not only used to disperse and stabilize the pigment, but also to adjust the pigment to achieve the correct color we need, such as blackness, transparency, color light at 45°, etc. Therefore, the compatibility of dispersant and resin includes.
-Compatibility (sampling and testing, checking compatibility after removing solvent)
-Viscosity reduction behavior of the dispersant in the resin system to determine the pigment (rotational viscometer test)
-The color spreading behavior of the dispersant in the resin system to determine the pigment (scraping colorimetric)
-Storage stability (flow plate method)
When the resin system is changed, the performance of the dispersant will be changed accordingly.
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