Selection of wetting and dispersing agents for dispersing organic pigments and carbon black

The choice of wetting dispersants for dispersing organic pigments and carbon blacks has a long history of using pigments in coatings, most of which were inorganic minerals and belong to inorganic pigments. Organic pigments are gradually popularized on the basis of the development of organic synthesis technology, so the history of these pigments is short, but its variety is more numerous than inorganic pigments with the continuous development of synthetic technology. More than that, organic pigments are increasingly used in the coatings industry due to their brighter colors, brighter hues and lower toxicity

. In particular, the emergence of high-performance organic pigments makes it possible to use a variety of high-performance coatings. Carbon black has excellent light, chemical, high temperature and other properties, so its application is also very wide, covering coatings, inks, plastics, sealants and other industries. Although the color variation of carbon black is not as rich as other pigments, it occupies the irreplaceable position of other colors in aesthetic application. Generally speaking, compared with inorganic pigments, organic pigments have small particle size, longer than the surface, low polar characteristics, and carbon black although from a chemical point of view is inorganic, but from the point of view of the coating industry, it is very similar to the use of organic pigments, so when selecting dispersants, these two pigments can usually use the same dispersant additives. At the same time, due to the above characteristics of these two types of pigments, they are difficult to get good dispersion compared to inorance pigments. Therefore, it is necessary to carefully analyze and consider when selecting wetting dispersants, in addition to the indispensable laboratory screening, in order to design the best dispersion and coating formula, so as to obtain the best results of the coating.
1. Characteristics of organic pigments and carbon black pigments
types of pigments can be roughly divided into organic and inorganic two categories. Inorganic pigments are mostly metal oxides, organic pigments can be divided into alumina and poly-rings two categories. Azos include monoazole nitrogen, double azo, special azo and metal complexes, and polycyclic classes include ketones, -series, pyridine and cocoonates, clickazole, pyridine, quinoxone, isoproid ketones and other types. As can be seen from above, the biggest difference between inorganic and organic pigments is the difference in surface polarity of pigments. Inorganic pigment polarity is high, and organic pigment composition is mainly hydrocarbon elements, so the polarity is low. In addition, many organic pigments have much smaller particle sizes than inorganic pigments and correspondingly much larger than surfaces, but some organic pigments have larger particle sizes and smaller surfaces similar to inorganic pigments.
Some kinds of carbon black have similar particle size (50 to 100nm) to transparent organic pigments, but there are also some carbon black particle size can be as low as a dozen nanometers, far less than many organic pigments. Carbon black particles of different particle sizes have different sizes than the surface, forming a series of products with high pigment, medium pigment, ordinary pigment, up to low pigment. This series of different particle sizes of carbon black, not only the amount of dispersant required varies with the surface, but also may need to use different dispersant varieties to achieve the desired dispersion state. Another characteristic of carbon black is that many varieties have fairly low pH (2.5 to 4.5), which also has a great influence on the selection of dispersants. In addition, since most water-based resin systems are suitable for alkaline working environments, it is important to pay attention to the pH of carbon black slurry in water-based systems, and usually adjust their pH to a range similar to that of the paint resin system before adding them to the resin system.
2. Dispersing organic pigments and carbon black
using high molecular weight wetting dispersants is classified 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 history of dispersants, the original dispersant products were low molecular weight polymers, which were well used to disperse inormeric pigments that were widely used at the time. With the development of the coatings industry and the use of organic pigments, chemists have invented polymer dispersants for organic pigments that are more difficult to disperse than inorganic pigments. Compared to earlier dispersants, high molecular weight dispersants have higher dispersion stability, a wider range of applications, and fewer negative effects on coating properties.
for the dispersant to be effective, the adsorption capacity of the dispersant on the pigment surface is very important, and this process is very relevant to the surface properties of the pigment. Inorgeable pigments have ion-type structures and high surface polarity, so the adsorption of dispersants is relatively easy. Organic pigments and carbon black are the crystallization of non-polar molecules, so there is a non-polar surface, low molecular weight dispersant adsorption on such pigment surface is quite difficult, the result is that such dispersants on organic pigments and carbon black flocculation and stabilization effect is not enough, this problem until the emergence of polymer-type polymer-type wetting dispersants can be perfectly resolved (Figure 1).
Figure 1 Comparison of low molecular weight with high molecular weight polymer-type wetting dispersants
Compared with low molecular weight dispersants, polymer mass dispersants are characterized by resin properties due to their much larger molecular weight. More importantly, the molecular structure of these dispersants has a large number of pigment affinity groups, so that a solid and long-lasting adsorption layer can be formed on the organic pigment, on the other hand, in the polymer compatible chain segment and the system compatible with the good conditions, solventized polymer chain segment of the spatial shielding action can stabilize the dispersion of pigments. Therefore, as with other dispersant products, it is only possible to obtain optimal stabilization of pigments if it is compatible with the surrounding resin solution and the polymer chain segment is sufficiently stretched. If there is a problem with compatibness, then the polymer chain shrinks and loses the spatial shielding and the stabilization it produces. Polymer dispersant molecular structure not only has a large number of pigment affinity groups, but also these pigment affinity groups can also be designed to include a variety of different types, so that polymer-like wetting dispersants can be applied to a variety of different types of organic pigments, with a wide range of versatility, which provides great convenience for the production process of coatings. Often in the same coating system, just one wetting dispersant can disperse almost all kinds of organic pigments and carbon black, and even cover inorganic pigment fillers, although high molecular weight wetting dispersants were originally developed for the dispersion of organic pigments.
3. Relationship between molecular weight and dispersion properties and compatibleness
the molecular weight of polymer-type wetting dispersants must be within a reasonable range, and if the molecular weight is too low, additives are more similar to surfactants, which have poor stability in pigment dispersion. But if the molecular weight is too high, it may also occur sticky too high, long chains entangled with each other and other problems, is not conducive to pigment dispersion.
is well known that dispersants have two basic components, namely pigment affinity groups and system-compatible chain segments. Thus, in general, the increase in molecular weight corresponds to the longer chain segment of the pigment affinity and/or the system-compatible chain segment, which usually means more pigment affinity groups and therefore the adsorption of pigment particles is stronger and longer lasting; Therefore, it is not difficult to understand that within a reasonable molecular weight range, there is a common phenomenon that the dispersion stability of dispersants to organic pigments and carbon black increases with the increase of molecular weight, but the possible problem is that the compatibness of dispersants and dispersion systems decreases with the increase of molecular weight. Scott's Scott equation, based on the Flory-Huggins lattic model theory, can quantify this phenomenon.
the above phenomena can be clearly observed in the same series of dispersant products with the same chemical composition and similar molecular structure (Figure 2), the DIS-160 series in the figure is BYK's polyurethane wetting dispersant series. However, for dispersants with different chemical compositions and molecular structures, it is difficult to judge their compatible properties solely on the basis of their molecular weight, because the compatibleness of dispersants also depends to a large extent on the interaction between dispersants and dispersion systems.
2 The relationship between molecular weight and dispersion properties and compatibness
4. The new variety of high molecular weight wetting dispersants - CPT polymer
commonly used to disperse organic pigments and carbon black polymer wetting dispersants from the chemical structure classification, mainly polyurethane, acrylic, high branch polyamine and other types. Acrylic-type dispersants have different synthesis techniques at different times, which can be roughly divided into two categories, namely random free-form polymerization technology and controlled polymerization technology (CPT, or Controlled Polymerization Technologies).
Controlled polymerization technology, also known as active polymerization technology, began in the 1950s when anion polymerization technology was discovered, but it was not until the 1980s that the discovery of one of the controlled polymerization technologies, the base group transfer polymerization method, opened the door to the industrial application of this technology. In the 1990s, the emergence of various forms of active free-form polymerization technology has brought the development of controlled polymerization technology to a new level. Controlled aggregation is becoming easier to achieve, manufacturing costs are significantly reduced, and applications are becoming more widespread. Therefore, controlled polymerization technology includes different types of polymerization technology found at different times, which can only be applied to the polymerization of acrylic monos.
When polymer polymerization is carried out using random free-form polymerization method, the reaction of each polymer chain cannot be guaranteed to be carried out at the same time and kept in sync, so the reactions of each polymer chain are first and then, some polymer chains have lost their activity in the reaction process, while others polymer chains are also active. The resulting polymer product is a random co-polymer, and the positions of various monomers in the polymer are randomly distributed. Therefore, the method can not locate special function groups or polarity in the polymer chain, nor can it obtain structured polymers, such as segment copolymers or gradient copolymers. The molecular weight of each molecule varies greatly, the molecular weight distribution is very wide, and the coefficient of multi-dispersion is generally greater than 2.
, controlled aggregation technology allows the polymerization reaction of a single body to be performed in a uniform and synchronized manner. Since there is no side effect, all copolymers have the same reaction process, resulting in polymers with a narrow molecular weight distribution, as shown in Figure 3.
Figure 3 Compared to the molecular weight distribution of random copolymers (FRPs) and controlled polymers (CPT
the molecular weight distribution of polymers synthesized by controlled polymerization technology is narrow, which is very beneficial to reduce the viscosity of the slurry and improve the compatible range of dispersants. What's more, many CPT wetting dispersants have high dispersion properties, especially for hard-to-disperse pigments such as dispersed organic pigments and carbon black. The figure below shows the blackness of different types of dispersants dispersing the FW200 in solvent-based 2KPU systems.
4 Comparison of the appearance of polyurethane dispersants with CPT dispersants dispersing high-pigment carbon black
controlled polymerization technology is limited to acrylic polymers, which complements traditional polymerization techniques, a new synthetic technology that uses exactly the same raw materials for higher-performance wetting dispersant products, thus meeting the formulation requirements of coating manufacturers for a growing number of different use cases. There is also a special type of pigment derivatives can be used as a compound for the above-mentioned various wetting dispersants to help improve the dispersion performance of organic pigments and carbon blacks, the role of these additives is usually shown to reduce the viscosity of abrasives, as well as improve the appearance of paint film performance, such as transparency, gloss, fog shadow and so on.
the dispersion of
organic pigments and carbon black pigments is more challenging than inorganic pigments, and it is necessary to use a high molecular weight type of wetting dispersant, while also ensuring good compatible between the dispersant and the base material system. The latest polymeric wetting dispersant synthesis technology is applied to the controlled polymerization technology of acrylic monobolic polymerization, which can use the same monosome raw materials and synthesize new dispersant products with better performance, and then improve the properties of coatings to meet the needs of the market.
recommended for wetting dispersants commonly used to disperse organic pigments and carbon black:
solvent-based systems
BYK-9076, DISPERBYK-2013, 2055, 2001, 2025, 2150, 2000,
water-
DISPERBYK-190, 2012, 2015, 2055, etc
.