Generally speaking, compared to inorganic pigments, organic pigments have the characteristics of smaller particle size, larger surface area, and lower polarity; while carbon black is an inorganic substance from a chemical point of view, but from the perspective of the coating industry, it However, the performance of organic pigments is very similar, so when selecting a dispersant, these two types of pigments can usually use the same dispersing aid. At the same time, due to the above characteristics of these two types of pigments, it is difficult to obtain good dispersion compared to inorganic pigments. Therefore, careful analysis and comprehensive consideration must be made in the selection of wetting and dispersing agents. In addition, multi-screening in the laboratory must be indispensable in order to design the optimal dispersion and coating formula, so as to obtain the most ideal coating effect.
 
1. Features of organic pigments and carbon black pigments
The types of pigments can be roughly divided into two categories, organic and inorganic. Inorganic pigments are mostly metal oxides, and organic pigments can be divided into two categories: azo and polycyclic. Azos include monoazo, disazo, special azo and metal complexes, etc. Polycyclics include quinacridone, actinide, pyrrolopyrrole dione, carbazole, phthalocyanine, quinophthalone, Isoindolinone and other species. It can be seen from the above that the biggest difference between inorganic and organic pigments is the difference in the surface polarity of the pigments. Inorganic pigments have higher polarity, while organic pigments are dominated by hydrocarbons, so they have lower polarity. In addition, many organic pigments have a much smaller particle size and a correspondingly larger specific surface area than inorganic pigments, but some organic pigments have larger particle sizes and smaller specific surfaces similar to inorganic pigments.

Some types of carbon black have a particle size similar to that of transparent organic pigments (50 ~ 100nm), but there are also carbon blacks with particle sizes as low as a dozen nanometers, which is much smaller than many organic pigments. Carbon black particles of different particle sizes have different specific surfaces, forming a series of products with high pigment, medium pigment, ordinary pigment, and even low pigment. In this series of carbon blacks with different particle sizes, not only the amount of dispersant required varies with the specific surface, but also different dispersant varieties may be required to achieve the desired dispersion state. Another feature of carbon black is that many varieties have a relatively low pH (2.5 ~ 4.5), which also has a great influence on the choice of dispersant. In addition, since most waterborne resin systems are suitable for alkaline working environments, the pH value of carbon black pulp must be paid attention to in waterborne systems. Usually, they need to be adjusted before adding them to the resin system. To the range close to the paint resin system.

2.Use high molecular weight wetting and dispersing agent to disperse organic pigment and carbon black
According to the molecular weight of dispersants, dispersants can be divided into two categories: high molecular weight and low molecular weight. Looking back at the development history of dispersants, the first dispersant products that appeared were low-molecular-weight polymers, which could be well used to disperse inorganic pigments that were widely used at the time. With the development of the coatings industry and the use of organic pigments, chemists have invented high molecular weight dispersants for organic pigments that are more difficult to disperse than inorganic pigments. Compared with earlier dispersants, high molecular weight dispersants have higher dispersion stability performance, a wider range of applications, and less negative impact on coating performance.

For the dispersant to be effective, the ability of the dispersant to adsorb on the surface of the pigment is very important, and this process is closely related to the surface properties of the pigment. Inorganic pigments have an ionic structure and a high surface polarity, making it easier to adsorb dispersants. Organic pigments and carbon black are crystals composed of non-polar molecules, so they have non-polar surfaces. It is very difficult for low-molecular-weight dispersants to adsorb on such pigment surfaces. And the deflocculation and stabilization of carbon black is not enough. This problem was not solved until the appearance of high molecular weight polymer wetting and dispersing agents.

Compared with low-molecular-weight dispersants, high-molecular-weight dispersants are characterized by their resin characteristics due to their much larger molecular weights. What's more important is that this type of dispersant has a large number of pigment affinity groups in the molecular structure, so it can form a strong and durable adsorption layer on organic pigments; on the other hand, in polymer compatible segments and systems In the case of good compatibility, the spatial shielding effect of the solvated polymer segment can stabilize the dispersion of the pigment. Therefore, like other dispersant products, only when it is compatible with the surrounding resin solution and the polymer segment can fully stretch, it is possible to obtain the best stabilization effect on the pigment. If there is a problem with compatibility, the polymer segment shrinks and loses its spatial shielding and the resulting stabilizing effect.

The molecular structure of the polymer dispersant not only has a large number of pigment affinity groups, but these pigment affinity groups can also be designed to include a variety of different types, so that the polymer wetting and dispersing agent can be applied to various Different types of organic pigments have a wide range of versatility, which provides great convenience for the production process of coatings. Often in the same coating system, only one wetting and dispersing agent can disperse almost all kinds of organic pigments and carbon blacks, and even inorganic pigments and fillers can be covered, although high molecular weight wetting and dispersing agents were originally organic pigments. Decentralized and developed.