How to choose dispersant for coating


In our coating production process, pigment dispersion is a very important production link, which is directly related to the storage, construction, appearance and film performance of the coating, so the reasonable selection of dispersant is a very important production link. However, dispersion of the coating paste is not only related to dispersant, but also to formulation of the coating formula and selection of the raw materials. The dispersant, as its name suggests, is to disperse all kinds of powders in the solvent reasonably, and make all kinds of solids suspend in the solvent (or dispersion) stably through certain charge repulsion principle or polymer potential resistance effect.


1. Selection of resin


Resin, especially grinding resin, plays a key role in the preparation of color paste.


1) participate in dispersion and anchoring of pigments


2) participate in maintaining the stability of dispersed and isolated pigment particles


The above functions of the resin can be seen through some experiments, such as long oil alkyd resin, polyamide resin, amino resin, aldehyde ketone resin, low relative quality hydroxy acrylic resin, all of which show good wetting ability for pigments, while low hydroxy value acrylic resin, thermoplastic acrylic resin, high relative quality polyester resin, high relative quality saturated polyester resin, ethylene resin Alkenyl copolymer resin and polyolefin resin show poor wettability to pigments. The same pigment, in different resin system to get different hues. Almost all carbon black, organic pigment and transparent iron oxide change their own color phase with different resin systems, especially scattering color phase. Therefore, the proper dispersant is not only used to disperse and stabilize the pigment, but also to adjust the pigment to achieve the correct color, such as blackness, transparency, color light at 45 ° and so on. Therefore, the coordination of dispersant and resin includes:


- Compatibility (sample test, check compatibility after solvent removal)


- viscosity reduction behavior of dispersant in the resin system for determining pigment (detection by rotary viscometer)


- dispersing agent in the resin system to determine the color development behavior of pigments (scraping colorimetry)


- storage stability (flow plate method)


When the resin system changes, the above performance of dispersant will change accordingly. This change requires application testing to determine.


Generally, it is not easy to summarize a simple application principle. For the resin system with pigment factor, the selection of dispersant becomes too many parameters. Therefore, the properties of pigments and fillers must be considered at the same time.






2. Carbon black and organic pigments


As mentioned above, there are many kinds and varieties of industrial pigments. Pigment industry divides them into organic pigment and inorganic pigment. In the coating industry, transparent iron oxide and carbon black are often considered as hard to disperse pigments together with organic pigments.


We have further distinguished the hard disperse pigments.


The distinguishing principle is to see the strength of the hydrogen bond.


In the experiment, we clearly see the dispersion results as follows:


In the fixed resin system, if a dispersant can perform well on carbon black, it can often stabilize phthalocyanine pigments at the same time, and it must show weak dispersion performance on DPP red and other organic pigments.


On the other hand, if a dispersant can disperse and stabilize DPP red, organic violet and other pigments well, it is usually used to disperse carbon black to get the undesirable brown red phase, and the viscosity reduction ability of phthalocyanine pigments is also insufficient.


This kind of phenomenon is applicable to almost all dispersing resins and all dispersants. There are very few dispersants which can show excellent properties for the two kinds of hard to disperse pigments at the same time. This is always a good one, and the other a little worse.


I think it comes from the number and strength of hydrogen bond structure of pigment itself.


For carbon black, phthalocyanine blue and other pigments, the most important interaction between pigments is not dominated by hydrogen bond, but other forces, such as the interaction between carbon black layers, the interaction of phthalocyanine structure, and the interaction of halogen. However, the polar groups in their surface treatment have structural properties relative to the pigment itself.


The organic red and permanent violet pigments represented by DPP have strong hydrogen bond in their own design, which improves the performance of pigments and directly affects the effect of dispersants on pigments. The polar groups at the interface participate in the hydrogen bonding of the pigment itself. This point is beyond doubt after the post-treatment of pigmentation.


According to this, it can be explained that it is not easy to get the best effect in two kinds of pigments with different internal effects by using one dispersant. According to this theory, we can also judge which side the pigment belongs to by its structure. For example, isoindolinone pigments should belong to carbon black phthalocyanine system. Toluidine red should tend to the latter.


In the actual selection of dispersants, for the first kind of pigments which are difficult to disperse, the best results are obtained in the resin system which is easy to be compatible. However, if the compatibility of the resin is not good, such as thermoplastic acrylic acid, a new polyacrylate type dispersant should be used instead. For the second kind of pigments with strong hydrogen bond, PU, polyester and polyacrylate with high polarity, it has a good effect. Only in the system with poor compatibility, PU and polyester with high polarity are affected. At this time, the modified polyacrylate dispersant should be used.


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