The structure of pigments and the role of dispersants

2022-03-25   Pageview:132

Pigment morphology and particle size

In coatings, pigments are mostly used as suspensions.

The particle size distribution of pigments is very wide, generally from 0.05μm to 1mm. However, studies have shown that when the particle size of the pigment is between 0.05 μm and 0.5 μm, it has good tinting strength, gloss, hiding power and weather resistance.

The dispersion process of the pigment consists of three steps of wetting, dispersion and stabilization: During the wetting process, the air and water vapor on the surface of the pigment are replaced by the resin solution. The solid/gas phase (pigment/air) is converted into a solid/liquid phase (pigment/resin solution). Dispersants, especially low-molecular-weight wetting and dispersing agents, can speed up the wetting process. During the dispersion process, mainly relying on the action of mechanical energy (impact and shear force), the agglomerated state of the pigment is broken into smaller particles and becomes a dispersed state (uniform distribution). The final third step is the stabilization process, where dispersants are used to keep the pigment dispersion stable and prevent runaway flocculation. And according to the type and molecular structure of the binder adsorbed on the surface of the pigment, the suspension is promoted to obtain a stable state. Dispersants, especially polymer dispersants, play a significant role in the stabilization of pigment particles.

Why use a dispersant

Dispersants play a very important role in coating production. The stability of the dispersion system can avoid many coating problems and film defects. If the formula is reasonable, adding an appropriate amount of dispersant can effectively reduce the cost and improve the performance of the coating. All in all, dispersants have the following functions:

Improve gloss and increase leveling effect

Experiments have shown that with the same formula, using different dispersants, the gloss of the paint film obtained will be significantly different. If the dispersant used is inappropriate, the color will become coarse after flocculation, and its effect on gloss is similar to that of matting wax powder in coatings, which will naturally affect the gloss.

Prevent floating color

Preventing floating and blooming is one of the well-known functions of dispersants. By choosing a suitable dispersant, whether it is the floating color in the tank or the floating color of the paint film, it can be improved or eliminated. Some dispersants are controlled flocculation type dispersants. The molecular chain contains some carboxyl groups, which have affinity with the pigment and also form hydrogen bonds with each other, so as to stabilize the pigment particles and achieve the effect of preventing floating color and blooming.

Improve tinting strength

The pigment is dispersed and stabilized well, and its tinting strength will be significantly enhanced.

Reduce viscosity, increase pigment loading

With a suitable dispersant, the viscosity of the resulting color paste will be significantly reduced. This increases pigment loading and improves production efficiency.

Prevent thickening and increase storage stability

If an unsuitable dispersant is used, the color paste will be thickened or the color will change due to flocculation, which is not what everyone wants. In addition to the resin system, the dispersant is an important factor affecting the storage stability of the coating. Poorly dispersed colorants will experience coarsening, sedimentation and decrease in coloring strength.

Improve grinding efficiency and reduce production costs

The wetting function of the wetting and dispersing agent enables the air and water on the pigment surface to be replaced more rapidly by the liquid of the grinding medium, while maintaining and controlling the separation of primary pigment particles achieved during the dispersion stage. These all improve grinding efficiency and reduce energy consumption and labor costs.

To make the pigments and fillers work in the coating system, the application and reasonable selection of dispersants are necessary. Dispersants have many roles, and the role of dispersants listed above may not be comprehensive or accurate. From the comprehensive results, reasonable screening of dispersants and addition of appropriate amounts can save costs and greatly improve the performance of coatings.

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