Selection rule of wetting and dispersing agent
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1. Selection of resin
Resin, especially resin for grinding, plays a key role in preparing color paste.
1) Participates in dispersion and anchoring of pigments
2) Participate in maintaining the stability of pigment particles that have been dispersed and isolated
The above functions of resins can be seen through some experiments, such as long oil alkyd resin, polyamide resin, amino resin, aldehyde ketone resin, low relative molecular weight hydroxy acrylic resin, all show good wetting ability to pigments, while low hydroxyl value acrylic resin, thermoplastic acrylic resin, high relative molecular weight polyester resin, high relative molecular weight saturated polyester resin, vinyl copolymer resin Polyolefin resin and other pigments show poor wettability. The same pigment has different hue in different resin systems. Almost all carbon black, organic pigment and transparent iron oxide change their own color, especially the scattering color, with different resin systems. Therefore, appropriate dispersants should be selected not only to disperse and stabilize the pigment, but also to adjust the pigment to finally achieve the correct color we need, such as blackness, transparency, color light at 45 °, etc. Therefore, the coordination of dispersant and resin includes:
– Compatibility (sampling test, check compatibility after solvent removal)
– The viscosity reduction behavior of dispersant on the determined pigment in the resin system (detected by rotary viscometer)
– Dispersing agent’s effect on the color development behavior of the determined pigment in the resin system (scraping and coating color comparison)
– Storage stability (flow plate method)
When the resin system changes, the above performance of the dispersant will change accordingly. This change needs to be determined by applying tests.
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. Selection of pigment filler
Carbon black and organic pigment
As mentioned above, there are many categories and varieties of industrial pigments. The pigment industry divides them into organic pigments and inorganic pigments. The paint industry often considers transparent iron oxide and carbon black as non dispersible pigments together with organic pigments.
According to the results of the author’s research, we further distinguish the non dispersible pigments.
The distinguishing principle is to see the strength of its hydrogen bond.
In the experiment, we clearly see such dispersion results:
In a 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 other organic pigments such as DPP red.
On the other hand, if a dispersant can well disperse and stabilize pigments such as DPP red and organic violet, it is usually used to disperse carbon black to obtain an undesirable brownish red phase, which is also insufficient for viscosity reduction of phthalocyanine pigments.
This phenomenon is applicable to almost all dispersion resins and dispersants. Few dispersants can simultaneously show excellent performance on the above two types of hard dispersible pigments. This category is always good, while the other category is slightly worse.
I think it comes from the number and strength of hydrogen bond structure of the pigment itself.
For pigments such as carbon black and phthalocyanine blue, the main interaction between pigments is not dominated by hydrogen bond, but other forces, such as the coupling between carbon black layered molecules, phthalocyanine structure and halogen. The polar groups in their surface treatment are independent of the structure of the pigment itself.
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 is absolutely true after the pigment post treatment.
Based on this, it can be explained that it is not easy to get the best effect in two kinds of pigments with different internal functions at the same time by using a single dispersant with one structure. According to this theory, we can also judge which side the pigment belongs to by its structure. For example, isoindolinone pigments should belong to the carbon black phthalocyanine system. Toluidine red should be preferred to the latter.
In the actual selection of dispersants, for the first class of pigments that are difficult to disperse, it has the best results in the easily compatible resin system. However, if the compatibility of the resin is poor, such as thermoplastic acrylic acid, it is necessary to use a new polyacrylate dispersant. For the second kind of strong hydrogen bond pigment category, high polarity PU, polyester, polyacrylate, all can have a good effect. Only in systems with poor compatibility, highly polar PU and polyester are limited. At this time, the modified polyacrylate dispersant should be used.
Blackness of carbon black
The blackness of carbon black is an extremely important topic in the study of dispersants, and is also the most frequently discussed topic.
Up to now, the practice shows that the detection of the instrument is less accurate than that of visual inspection; The blackness varies under different light; At different angles, the blackness also changes; Different dispersants will choose different carbon black to give different blackness; Carbon black masterbatch with high blackness may not also improve its coloring power.
All this is easy to explain. Because the transparent sheet structure of carbon black+the ability of carbon black to absorb light. Its transparent sheet structure is less than 1 μ The film diameter and arrangement orientation under the particle size of m will inevitably lead to the transmission, reflection, refraction and scattering of light. While some of these derived colored light will be conditionally absorbed by carbon black, the other part will continue its journey. This is a complex and variable superposition effect. Therefore, there is no 100% black, that is, there is no blackest, only darker than the one behind.
Although it can be understood, control is extremely difficult. Auxiliaries companies have never stopped to improve the blackness of a certain carbon black in a certain system. Before any new dispersant comes into the market, the first thing to be tested is the performance of dispersing carbon black in the target system. Up to now, we have no exact theory to inquire about the relationship between the blackness of carbon black and the choice of dispersant. Only by comparing the experiment with the reference sample, the structure can be corrected or replaced, and the actual measurement and comparison can continue.
At first, everyone thought that titanium dioxide was very easy to disperse, so that it could be used with or without dispersants. However, when mixed with other hard dispersible pigments, titanium white will participate in floating color; When preparing high grade pure white, titanium white will appear fog shadow; On products with special requirements, titanium white needs excellent coverage and whiteness, and yellowing at high temperature is not allowed; Many ordinary industrial occasions do not want to use expensive high-grade titanium white, or even use titanium white as a substitute pigment; The above problems have aroused the attention of the auxiliary industry to the dispersion of titanium dioxide. According to its surface structure and treatment, experiments have been conducted, and dispersed titanium dioxide can be used as follows:
– Traditional wetting and dispersing agents, including organic carboxylic acid with acid value AV, ammonia value AMV, amine salt of phosphoric acid and flocculant controlling wetting and dispersing agent, are used to inhibit its floating color
– Organic phosphate
– Special PU polymer dispersant
– Special vinyl polymer type titanium dioxide dispersant
– Polymeric Surfactant, which is widely effective in waterborne applications (e.g. A6226)
The above products have been developed and recommended by various professional auxiliary companies.
Among them, wetting and dispersing agent is the general choice. It has extensive adaptability to the system, but can not adapt to special requirements.
Organic phosphate esters are often recommended to prepare high grade pure white to remove haze, while high molecular weight dispersants consider the ability to control floating color and whiteness. The new technology of dispersant in water-based system helps the factory’s self-made reference white to accept the commercially available color masterbatch. Therefore, it is now common knowledge that dispersants are used for titanium dioxide.
transparent iron oxide
The particle size of transparent ferric oxide is at the nanometer level, and its surface is amphoteric. It seems to disperse easily at low pigment concentration, and the viscosity of color paste is very low, but the transparency is not easy to get the best; Once the pigment concentration slightly exceeds the critical value, the color paste immediately becomes too thick to be stirred, resulting in the loss of efficiency of the sander.
It seems that the transparency of iron oxide, like the blackness of carbon black, can always be improved. Our experimental results show that a sample with good transparency that we have already considered may still have a heavy fog shadow when observed at 45 °;
So, what is the best? This question is another uncertain puzzle.
The auxiliary companies are also giving their own plans. This can be seen from the public recommendations. Plus the selectivity caused by different resin systems, there is more than one recommended solution.
For example, there is a method of using phosphate ester; There are methods of using polymer dispersant with acidic end group wetting agent, and there are independently designed polymer dispersants. They show adaptability to the same transparent iron oxide pigment in different systems.
The extinction powder itself is not difficult to disperse. It was micronized in advance during production. Some have surface wax treatment, some do not, with polar hydroxyl. However, the dispersion problem of matting powder comes from the application requirements.
Some coatings that require matt can adapt to a variety of construction methods with one formula, such as consistent gloss for spraying and brushing; Some require that the extinction uniformity will not be affected under the environment of high temperature and high humidity; Some require that under the condition of low viscosity, the extinction powder has the minimum sedimentation; Some require the highest transparency;
Some require excellent friction resistance, while hard quartz powder is introduced, so it needs to be dispersed together. This leads to a consequent change in dispersants. From traditional wetting dispersants, to special polymer PU dispersants, to phosphate esters, to amine salts of phosphate esters, to other special polymers, they are all used to disperse matting powders. So which is the best? As mentioned before, it depends on how you ask. One dispersant cannot be expected to solve all the above requirements at the same time.
In principle, the wetting agent can improve the flowability of the final system; The dispersant with high relative molecular weight can prevent sedimentation and control the movement of the matting powder in the wet film, so it is easier to orient and obtain uniform matting.
Wetting agent is a common solution for metallic flash pigments such as aluminum powder and pearlescent powder. They can also be dispersed with a polymer dispersant compatible with the resin. Control their movements at the same time. These are examples of successful recipes.
Nanometer titanium dioxide and other nano dispersions In this case, if the polymer PU can be compatible, the best stability effect can be obtained. Otherwise, acrylic dispersant is required.
Determine a main dispersant
Generally, in a determined resin and solvent system, AFCONA recommends this method to screen a suitable main dispersant.
First of all, disperse four pigments: high pigment carbon black, titanium white, DPP red and ordinary iron oxide red. Evaluate whether the dispersant is difficult to prepare the four conventional color masterbatch, such as whether the viscosity reduction behavior is sufficient.
Evaluate the color development intensity
Evaluate storage stability (flow plate and hot storage). If a dispersant can show good dispersion ability for the above four pigments in this specific system, it can basically meet the requirements of other pigments. It can be selected as the main dispersant of this system. Of course, special pigments such as penetrating iron may still be exceptional.
This method can also be used to evaluate the comprehensive performance of two different dispersants, and find the type of pigment suitable for its treatment.