How to choose a dispersant?
I. Selection of main resin for dispersion
Resin, especially the resin for grinding, plays a key role in the preparation of colour paste, and the role of the main resin is as follows:
(1) To disperse and anchor the pigment.
(2) Stabilising the pigment particles which have been dispersed and segregated.
The above functions of main resin can be seen through some experiments, for example, long oil alkyd resin, polyamide resin, amino resin, aldehydes and ketones resin, low molecular weight hydroxyl acrylic resin show good wetting ability to pigment, while low hydroxyl acrylic resin, thermoplastic acrylic resin, high molecular weight polyester resin, high molecular weight saturated polyester resin, vinyl copolymerisation resin and polyolefin resin show poor wetting ability to pigment. pigments are poor in wettability. The same pigment has different colour phases in different resin systems. Almost all carbon blacks, organic pigments and transparent iron oxides change their colour phases, especially scattering colour phases, with different resin systems. Therefore, choosing the right dispersant in the determined main resin system is not only used to disperse and stabilise the pigment, but also used to adjust the pigment to reach the correct colour we need, such as darkness, transparency, colour light at 45°, etc. Therefore, the reasonable matching between dispersant and resin includes
①Compatibility (sample test, check the compatibility after removing the solvent)
② The viscosity reduction behaviour of the dispersant in the resin system to determine the pigment (rotational viscometer test).
③Colour spreading behaviour of the dispersant in the resin system to the definite pigment (scraping and colour comparison).
④Storage stability (flow plate method)
When the resin system changes, the above performance of dispersant will change accordingly. Such changes need to be determined by application tests.
Generally, it is not easy to summarise a simple application principle. For the resin system plus the pigment factor, the choice of dispersant becomes too many parameters. Therefore, it is necessary to consider not only the combination of the host resin and the dispersant, but also the nature of the pigment and filler.
Second, for the choice of pigment filler
Carbon black and organic pigments
As mentioned earlier, there are many different types and varieties of industrial pigments. The pigment industry classifies them into organic pigments and inorganic pigments. In the paint industry, transparent iron oxide and carbon black are often considered as difficult-to-dispersed pigments together with organic pigments.
We have made a further distinction between difficult-to-dispersed pigments by looking at the strength of their hydrogen bonds.
In our experiments, we clearly see such dispersion results.
In a fixed resin system, if a dispersant performs well with carbon black, it will often stabilise phthalocyanine pigments at the same time and, inevitably, show weak dispersing properties with other organic pigments such as DPP red. On the other hand, if a dispersant can disperse and stabilise DPP red, organic violet and other pigments very well, it is usually used to disperse carbon black to get the disliked brownish-red phase, and the viscosity-reducing ability of phthalocyanine pigments is not enough. This kind of phenomenon applies to almost all dispersing resins and all dispersants. There are very few dispersants that can show extremely good performance for both of the above two categories of difficult-to-dispersed pigments at the same time. It is always the case that one group is very good and the other group is slightly poor.
The main reason is the number 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 effect between carbon black layer molecules, the coupling effect of phthalocyanine structure, the role of halogens. And the polar groups carried in their surface treatment have independence relative to the structure of the pigment itself.
Organic red and violet pigments, represented by DPP, have strong hydrogen bonding in the design of the pigment itself, which improves the performance of the pigment and directly affects the effect of the dispersant on the pigment, and the polar groups at the interface are involved in the hydrogen bonding of the pigment itself. This is confirmed by the post-processing of the pigment.
Accordingly, it is not easy to achieve optimum results with two different types of pigments with different intrinsic effects at the same time using a single structure of dispersant. Based on this theory, it is also possible to determine which side of the pigment a pigment belongs to by its structure. For example, an isoindolinone pigment should belong to the carbon black-phthalocyanine group. Toluidine red should be inclined to the latter.
In practical dispersant selection, the best results for the first group of difficult-to-dispersed pigments are obtained with easily compatible resin systems. However, if resin compatibility is poor, e.g. thermoplastic acrylics, a change to a new polyacrylate dispersant is required. For the second category of strongly hydrogen bonded pigments, highly polar PU, polyester and polyacrylate, good results can be obtained. Only in the system of poor compatibility, high polarity 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 discussing dispersants and is most often discussed.
Practice so far shows that the instrument detection is not as accurate as visual inspection; under different light, the blackness varies; under different angles, the blackness varies; different dispersants will choose different carbon blacks to give different blackness; carbon black masterbatch with high blackness does not necessarily improve the colouring power.
All this is easy to explain. Because of the transparent lamellar structure of carbon black + the ability of carbon black to absorb light. The transparent lamellar structure in
3. Titanium dioxide
At first, everyone thought that titanium dioxide was so easy to disperse that it could be used with or without dispersants. However, when compounding with other difficult-to-dispersed pigments, titanium dioxide will be involved in floating colour; when preparing high-level pure white, titanium dioxide will have haze; in special requirement products, titanium dioxide needs to have excellent covering and whiteness, and it is not allowed to be yellowed under high temperature; many common industrial occasions are reluctant to use expensive high-level titanium dioxide, or even use titanium dioxide as a substitute pigment; the above problems have aroused the auxiliary industry to pay attention to the dispersion of titanium dioxide.
4. Transparent iron oxide
The particle size of transparent iron oxide is in the level of nanometer, and its surface is amphoteric, it seems to be easily dispersed when the pigment concentration is low, and the viscosity of the colour paste is very low, but the transparency is not easy to get the best; and once it slightly exceeds the critical concentration of the pigment, the colour paste immediately thickens to the point that it can not be stirred, which results in the loss of efficiency of the sand mill.
The transparency of iron oxide, somewhat like the blackness of carbon black, always seems to continue to improve. Our experiments have shown that a sample that we already consider to have good transparency may still have a heavy haze when viewed at 45°;
So, what is best to use? This question is another elusive quandary.
The additive companies are likewise giving their own programmes. This can be seen in the published recommendations.
Together with the selectivity caused by differences in resin systems, there is more than one recommended solution.
5. Matting Powder
Matting powder itself is not difficult to disperse. It is pre-micronised at the time of manufacture. Some have a surface wax treatment and some do not, with polar hydroxyl groups. However, the problem of dispersion of matting powders arises from the requirements of the application.
Some require matte coatings to be able to be adapted to a variety of application methods with a single formulation, e.g. spray brushing for consistent gloss;
Some require that the uniformity of matting is not affected under high temperature and high humidity conditions;
There are requirements for low viscosity conditions, matting powder has the smallest settlement;
Some require the highest transparency;
Some require excellent friction resistance, and the introduction of hard quartz powder, so you need to be dispersed together, and so on.
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 powders. So which one is best? As mentioned earlier, it depends on how you require it. You can’t expect one dispersant to solve all of the above requirements at the same time.
In principle, a wetting agent improves the flow ability of the final system; a high relative molecular mass dispersant prevents settling and controls the movement of the matting powder through the wet film so that it can be more easily orientated to give a uniform matting.
6. Metallic glitter pigments such as aluminium powder and pearlescent powder
A common solution is a wetting agent.
They can also be dispersed with polymer dispersants compatible with the resin. Also control their movement. There are examples of successful formulations for these.
7. Nanoscale titanium dioxide and other nano-dispersions
On this occasion, polymer PUs give the best stabilising effect if they are compatible. Otherwise, an acrylic-based dispersant is required.
8. Determination of a main dispersant
Generally, in a defined resin and solvent system, MANTOS recommends this method for selecting a suitable main dispersant:
Firstly, dispersing four pigments: highly pigmented carbon black, titanium dioxide, DPP red and ordinary iron oxide red.
① Evaluate whether the dispersant has any difficulties in the preparation of these four regular masterbatches, e.g. whether the viscosity reduction behaviour is sufficient.
② Evaluate the strength of colour spreading.
(iii) Evaluate the storage stability (flow plate and thermal storage).
If a dispersant can show good dispersing ability for the above four pigments in this particular system, then it is basically competent for various other pigments. It can then be chosen as the main dispersant for the system. Of course, exceptions may still be made for special pigments such as transparent iron oxide pigments.
This method can also be used to evaluate the combined performance of two different dispersants to find the right type of pigment for the application.