1 Selection of resin
Resins, especially grinding resins, play a key role in the preparation of color pastes.
1) Participates in the dispersion and anchoring of pigments
2) Participate in keeping the pigment particles already dispersed and isolated stable
The above role of resin can be seen through some experiments, such as long oil alkyd resin, polyamide resin, amino resin, aldehyde ketone resin, low relative molecular mass hydroxy acrylic resin, all show good wetting ability for pigment, while low hydroxyl value acrylic resin, thermoplastic acrylic resin, high relative molecular mass polyester resin, high relative molecular mass saturated polyester resin, vinyl copolymer resin, polyolefin resin, etc. have good wetting ability for pigment. Polyolefin resins, etc. all show poor wettability to pigments. The same pigment is obtained in different resin systems with different color phases. Almost all carbon black and organic pigments and transparent iron oxide change their hue, especially the scattering hue, with different resin systems. Therefore, choosing the right dispersant is not only used to disperse and stabilize the pigment, but also to adjust the pigment to achieve the correct color we need, such as blackness, transparency, color light at 45°, etc. Therefore, the compatibility of dispersant and resin includes:
-Compatibility (sampling and testing, checking compatibility after removing solvent)
-Viscosity reduction behavior of the dispersant in the resin system to determine the pigment (rotational viscometer test)
-The color spreading behavior of the dispersant in the resin system to determine the pigment (smear colorimetric)
-Storage stability (flow plate method)
When the resin system changes, the performance of the dispersant will change accordingly. Such changes need to be determined by applying tests.
In general, it is not easy to summarize a simple application principle. For resin systems with pigmentation, the choice of dispersant becomes too parameterized. Therefore, the properties of the pigment filler must also be considered.
2 Selection of pigment fillers
Carbon black and organic pigments
As mentioned above, there are many different types and varieties of pigments for industrial use. The pigment industry divides them into organic and inorganic pigments. In the coating industry, transparent iron oxide and carbon black are often considered as hard-to-disperse pigments together with organic pigments.
Based on the results of the author’s research, we have further differentiated the hard-to-disperse pigments.
The principle of differentiation is the strength of the hydrogen bond.
In our experiments, we clearly see such dispersion results:
In a fixed resin system, if a dispersant can perform well for carbon black, it often stabilizes phthalocyanine pigments at the same time, and, inevitably, shows weak dispersion for other organic pigments such as DPP red.
On the contrary, if a dispersant can well disperse and stabilize DPP red, organic violet and other pigments, it is usually used to disperse carbon black to get the brownish red phase which is not liked, and the viscosity reduction ability of phthalocyanine pigments is also insufficient.
This kind of phenomenon is applicable to all dispersants in almost all dispersing resins. Very few dispersants can show extremely good performance for both of the above two categories of difficult to disperse pigments. It is always the case that this category is very good and the other category is slightly worse.
I think this comes from the amount and strength of the hydrogen bonding structure of the pigment itself.
Carbon black, phthalocyanine blue and other pigments, the main interaction force between the pigments is not dominated by hydrogen bonding, but other forces, such as the coupling between carbon black lamellar molecules, the coupling of phthalocyanine structure, the role of halogen. And the polar groups they carry in their surface treatment have independence with respect to the structure of the pigment itself.
The organic red and permanent violet pigments represented by DPP have a strong hydrogen bond in their own design, which improves the performance of the pigment and directly affects the effect of dispersant on the pigment. The polar groups at their interfaces are involved in the hydrogen bonding of the pigments themselves. This is confirmed by the pigmentation post-processing.
Accordingly, it can be explained that it is not easy to use a single structure of dispersant to get the best results in two different types of pigments with different intrinsic effects at the same time. According to this theory, we can also determine which side the pigment should belong to by its structure. For example, isoindolinone pigments should belong to the carbon black-phthalocyanine category. And toluidine red should be inclined to the latter.
In the actual dispersant selection, for the first category of difficult to disperse pigments, the best results are obtained in resin systems that are easily compatible. However, if the resin is not compatible, such as thermoplastic acrylic acid, then a new polyacrylate type dispersant is needed. For the second category of pigments with strong hydrogen bonds, highly polar PU, polyester and polyacrylate, all have good results. Only in poorly compatible systems, highly polar PU and polyester are limited. In this case, it is necessary to change to modified polyacrylate dispersant.
● Blackness of carbon black
The blackness of carbon black is an extremely important topic when studying dispersants and is most often discussed.
The practice so far shows that the detection by instruments is not as accurate as visual inspection; the blackness varies under different light; the blackness also varies under different angles; different dispersants will choose different carbon blacks to give different blackness; the carbon black masterbatch with high blackness does not necessarily have higher coloring power.
All this is easy to explain. It is because of the transparent lamellar structure of carbon black + the ability of carbon black to absorb light. Its transparent lamellar structure with a particle size of <1μm and the orientation of the lamellar arrangement will inevitably lead to light transmission, reflection, refraction and scattering. And these derived color light will be conditionally absorbed partly by carbon black, and another part will continue its journey. This is a complex and full of variables superposition effect. Therefore, there is no such thing as 100% black, that is, there is no blackest, only blacker compared to the pair.
Although it can be understood, it is extremely difficult to control. Additive companies have never stopped working on the subject of improving the blackness of a particular carbon black in a particular system. The first thing to be tested before any new dispersant is launched is the performance of the dispersed carbon black in the target system. So far, we have no exact theory on the relationship between the blackness of carbon black and dispersant selection. We can only continue the actual measurement comparison by experimenting with reference samples and then correcting or replacing the structure.
Titanium white
At first, everyone thought that titanium dioxide was so easy to disperse that it could be used with or without dispersants.
However, when compounded with other difficult to disperse pigments, titanium dioxide can become involved in floating colors;
In the preparation of high grade pure whites, titanium dioxide can have a hazy appearance;
In the special requirements of products, titanium white needs excellent coverage and whiteness, and does not allow yellowing at high temperatures; many general industrial applications do not want to use expensive high-grade titanium white, or even use titanium white substitute pigments; the above problems have caused the additives industry to pay attention to the dispersion of titanium white.
According to its surface structure and treatment, experiments have been conducted to disperse titanium dioxide using.
-Conventional wetting and dispersing agents, including organic carboxylic acids with acid value AV, ammonia value AMV, amine salts of phosphoric acid and flocculation control wetting and dispersing agents to inhibit their floating color
-Organic phosphate esters
-Special PU polymeric dispersants
-Specialized vinyl polymeric titanium dioxide dispersants
-Polymeric Surfactant (e.g. A6226) which is widely effective in water-based applications
The above products have been developed and recommended by various professional additives companies.
Among them, wetting and dispersing agents are the universal choice. It has a wide range of adaptability to the system, but cannot be adapted to special requirements.
Organophosphate esters are often recommended to prepare high-level pure white to remove haze shadows, while high relative molecular mass dispersants are considered to control floating colors, as well as the ability to control their whiteness. New dispersant technology in aqueous systems has helped factory-made benchmark whites to be able to accept commercially available masterbatches. As a result, the use of dispersants for titanium whites is now common knowledge.
● Transparent Iron Oxide
Transparent iron oxide has a nanometer particle size and an amphoteric surface. It seems to disperse easily at low pigment concentrations, with low paste viscosity, but transparency is not easily obtained at its best; once the critical pigment concentration is slightly exceeded, the paste immediately becomes too thick to stir, resulting in loss of efficiency in the sander.
The transparency of iron oxide, somewhat like the blackness of carbon black, always seems to be able to continue to improve its transparency. Our experimental results show that a sample which we already consider to be of good transparency may still have a heavy haze shadow when viewed at 45°.
So, what is the best to use? This question is another elusive quandary.
Each auxiliary company is equally giving its own solution. This can be seen in the publicly available recommendation information.
Add to this the selectivity due to differences in resin systems, and there is more than one recommended solution.
For example, there are methods using phosphate esters; there are methods using polymeric dispersants with acidic end group wetting agents, separately designed polymeric dispersants, etc. They show adaptability in different systems for the same transparent iron oxide pigment.
● Matting powder
Matting powder itself is actually not difficult to disperse. It is pre-micronized at the time of production. Some of them have a wax treatment on the surface, some do not, and have polar hydroxyl groups. However, the problem of dispersion of matting powder comes from the application requirements.
Some require a matte coating that can be adapted to multiple application methods with a single formulation, such as a consistent gloss for airbrushing.
Some require that the matting uniformity is not affected in high temperature and high humidity environments.
Some require minimal settling of matting powder under low viscosity conditions.
Some require the highest transparency.
Some require excellent friction resistance, and the introduction of hard quartz powder, so they need to be dispersed together, etc.
This has led to a consequent change in dispersants. From traditional wetting and dispersing agents, to special polymer PU dispersants, to phosphate esters, to amine salts of phosphate esters, to other special polymers, all have been used to disperse matting powder. So which one is the best? As mentioned before, it depends on what you require. You cannot expect one dispersant to solve all the above requirements at the same time.
In principle, the wetting agent can improve the flow ability of the final system; the high relative molecular mass dispersant can prevent settling and control the movement of matting powder in the wet film and make it easier to orient and get uniform matting.
● Metallic glitter pigments such as aluminum powder and pearl powder
The common solution is wetting agent.
It is also possible to disperse them with polymeric dispersant compatible with resin. Also control their movement
The solution is a wetting agent. There are examples of successful formulations of these.
Nanoscale titanium dioxide and other nanodispersions
In this case, the best stabilization can be obtained if the polymer PU is compatible.
Otherwise, an acrylic-based dispersant is required.
● Determination of a main dispersant
In general, AFCONA recommends the following method to select a suitable primary dispersant for a defined resin and solvent system.
First, disperse four pigments: highly pigmented carbon black, titanium white, DPP red, and common iron oxide red.
Evaluate whether the dispersant has any difficulties in the preparation of these four conventional masterbatches, e.g. whether the viscosity reduction behavior is sufficient.
Evaluate the color spreading strength
Evaluate the storage stability (flow plate and thermal storage).
If a dispersant can show good dispersion of the above four pigments in this particular system
in this particular system, then it is basically capable of handling the requirements of various other pigments. It can be chosen as the main dispersant for this system. Of course, special pigments, such as iron permeable, may still be an exception.
This method can also be used to evaluate the combined performance of two different dispersants to find the right type of pigment to be treated.
