The dispersal of pigment
The dispersing process of pigment consists of three steps: wetting, dispersing and stabilizing. During wetting, the air and water vapor on the pigment surface are replaced by the resin solution, and the solid/gas phase (pigment/air) is converted into the solid/liquid phase (pigment/resin solution). Dispersants especially low molecular wetting dispersants can accelerate the wetting process. In the process of dispersal, the agglomeration state of pigment is broken into smaller particles and becomes dispersive state (uniform distribution) mainly by the action of mechanical energy (impact and shear force). The third step is the process of stabilization. Dispersants are used to keep the dispersive state of pigments stable, prevent runaway flocculation, and promote the suspension to obtain a stable state according to the type and molecular structure of adhesives adsorbed on the pigment surface. Dispersants especially macromolecular dispersants play an important role in stabilizing pigment particles.
The role of dispersants
Dispersants play an important role in coating production. The stability of dispersion system can avoid many coating problems and film defects, if the formula is reasonable, the right amount of dispersant can effectively reduce the cost, improve the performance of the coating.
4.1. Improve luster and leveling effect
Experiments show that the same formula, the use of different dispersants, paint film gloss will have obvious differences. For example, in a black coil coating system, three different dispersants 20 ° gloss of paint film is 69.7%, 81.5% and 77.1% respectively. If the dispersant used is not appropriate, color flocculation thickened, its role in gloss is similar to the extinction powder, will naturally affect gloss.
4.2 prevent floating color hair
Preventing floating hair is one of the well-known dispersants. Choose appropriate dispersant, no matter be the float inside canister color sends a flower or the float color of paint film sends a flower, can get improvement or eliminate. Figure 3 shows the result of floating color in the tank after the blue paint and the white paint are mixed evenly in the same proportion with different dispersants. Some dispersants, such as sn-2004, are dispersants of controlled flocculation. The molecular chain contains some carboxyl groups, which, while affinity with pigments, also form hydrogen bonds with each other, so as to stabilize pigment particles and prevent floating color hair.
4.3. Improve coloring power
Pigment dispersion and stability are better, its coloring power will be significantly strengthened. FIG. 4 shows the results of the black paste dispersed with different dispersants and the coating film mixed with white paint in the same proportion. It can be seen that the difference in coloring power is obvious.
4.4 reduce viscosity and increase pigment loading
With suitable dispersant, the viscosity of the obtained color paste will decrease obviously. This can increase the load of pigment, improve production efficiency. FIG. 5 is the viscosity comparison of red paste dispersed with different dispersants.
4.5 reduce flocculation and increase constructiveness and usability
Many people do finger studies when evaluating dispersants. If the dispersants are not properly matched, there will be obvious color difference between the area where the fingers are grinded and the area where the fingers are not grinded, and the coloring force of the flocculated pigments will decline, which brings difficulties to the color mixing and coating construction. Flocculation can also be observed by flow plate test.
4.6. Prevent roughness and increase storage stability
If the choice of the wrong dispersant, the color slurry back coarse or because of flocculation and color changes, are not expected. In addition to the resin system, dispersants are important factors affecting the storage stability of coatings. Poor dispersion of the color paste will occur coarse, sedimentation and coloring force decline.
4.7. Increase chromaticity and color saturation
The effect of dispersants is most easily overlooked. Some examples show that when dispersing the same pigment with different dispersants, the hue and saturation will be obviously different. Generally speaking, the better the dispersion, the higher the saturation (if it is carbon black, the better the dispersion, the lower the L value, the higher the blackness).
4.8 increase transparency (organic pigment) or hiding power (inorganic pigment)
For aluminum paint, you want the transparency of the color paste as high as possible. And be aimed at plain coloured lacquer, the covering force that hopes chroma again is taller the better. In fact, this is related to the size of pigment particles. Some data show that in addition to refractive index, pigment particle size distribution is another important factor of transparency. As the particle size increases, the ability to scatter light increases until it reaches its maximum value and then begins to decline. This ability to scatter light increases the hiding power of the pigment. When the scattering power is at its strongest, it reaches its maximum. If the particle size continues to increase, the hiding power will decrease. When the particle size of pigment is lower than a certain value, the transparency will increase with the decrease of particle size. Dispersants can not change the properties of pigments, but can control the size distribution of pigments to achieve better color effect.
Figure 6 shows the transparency comparison of red pigment after dispersing with different dispersants.
4.9. Improve grinding efficiency and reduce production costs
The wetting function of wetting dispersants enables the air and water on the pigment surface to be replaced more rapidly by the liquid of grinding medium, and the separation of the primary particles of pigment obtained in the dispersing stage can be maintained and controlled. These have improved the grinding efficiency, reduced energy consumption and labor costs.
4.10. Prevent settlement
Many people take it for granted that anti – sinking must rely on anti – sinking agent, in fact, this is one-sided. If flocculation is produced, the pigment particles become larger and the density increases, and will settle down quickly. The color paste with suitable dispersant can be added with little or no anti-settling agent. The author did 70% of titanium pigment resin paste, its viscosity is very low (91.1 mPa s., shear rate is 10 s – 1), did not join any anti-settling agents, over the last 15 d thermal storage (40 ℃) and room temperature placed 3 months, no any precipitation. The anti – settling effect of dispersants is obviously helpful to the storage stability of coatings.
21
Dec
