The Basics of Pigment Dispersion

HSD · 发表于 2008-4-12 19:04:47

The Basics of Pigment DispersionIntroduction When dispersing pigments, in particular organic pigments, one frequently encounters problems such as flocculation, insufficient color or transparency, poor rheological qualities or stability. This issue deals with possible causes and the elimination of these problems by dmaking the right selection of Raw Materials and Dispersion Equipment.
What is “Good Dispersion”?Purely theoretically speaking this means that every pigment particle is completely deflocculated – i.e. enveloped by a layer of binder or solvent. However, in practice, this is difficult to achieve. It seems to be more important to keep the percentage of oversized pigment agglomerate as low as possible.
To what extent does the Pigment Dispersion influence the Paint Properties?
Many important paint properties are influenced by the quality of pigment dispersion:
·
Gloss
·
Weather and chemical resistance
·
Shelf life
·
Colour, tinting strength and transparency
·
Hiding power

Gloss: If a pigment is optimally dispersed, this directly improves the degree of gloss. In principle, gloss is determined by the properties of the binder. The necessary addition of pigments can only detrimentally influence the gloss: if a pigment is not optimally dispersed, then certain pigment agglomerates will detrimentally affect the smoothness of the surface and therefore through scattering of the light will reduce the apparent gloss.

Resistance properties: The resistance to the exposure to weather and chemicals will also be predominantly determined by the choice of binder. Here again insufficiently dispersed pigment particles reduce the resistance. Projecting pigment particles provide points for attack by chemicals and the effects of weather. An optimally dispersed pigment on the other hand – if correctly chosen
- only improves the system, such as for example: better weather resistance through reflecting (Ti0 2 ) or
absorption of the UV radiation
(Oxide Red). A suitable pigment – or an extender – if optimally dispersed can also improve the chemical resistance of a binder.

Shelf life: The shelf life of paint system depends very much on the quality of the pigment dispersion. The detrimental effect through later settlement when there is an excessive percentage of a coarse pigment particle is easily understood. Such stability problems are caused either by the so-called “post wetting ” or by the opposite, reflocculation of the pigment. Post –wetting may be induce, if the pigment has not been fully reflocculated right at the start – through complete dispersion. The Reason for reflocculation of previously correctly dispersed pigment dispersion is insufficient stability of the pigment paste. This depends very much on the stabilizing properties of the dispersion binder or the ratio of binder/solvent during the dispersion process. Solvents generally have outstanding wetting properties, whereas binders have a positive effect on stability. The problem is obtaining an ideal combination of wetting (=solvent) and stability (= binder)

Color, Intensity, transparency and Hiding Power: All these properties are very much dependent on the available pigment surface or the “optimum dispersion”. Complete dispersion of the pigment leads to increased hiding powering in the case of covering pigments, and to greater intensity of color (without flocculation ) of toner colors, and, in the case of
transparent pigments, to a better transparency. In all pigments it leads to a complete development of color tone and “purity ” of the color. The pigments such as carbazol violet or phthalo blue are very much determined by the quality of the pigment dispersion.

How is the quality of a Dispersion determined?There are four – more or less – simple methods for the determination of the quality of pigment dispersion:
·
Grindometer
·
Electron microscope
·
Coulter counter
·
Tinting strength

Grindometer:
The grindometer is certainly the simplest – and most used – method.
It has one great advantage speed. Regularly this method does not provide reliable information on the actual quality of the pigment dispersion, it only indicates the size of the largest pigment agglomerates and provides no information whatsoever about the particle size of distribution.
Electron microscope:
The electron microscope is a very accurate – but expensive and time-consuming – method. It is not suitable at all for day-to-day practical use.
Coulter Counter: The Coulter Counter is also a very good method 2 It provides a very good idea of the particle size distribution in pigment dispersion and, compared with the electron microscope method, is relatively easy to perform.
Measurement of Tinting Strength:Measurement of the tinting strength is a very simple but very important method. This method makes it easy to determine whether the pigment is optimally dispersed or not. This method is predominantly used in the case of the examples (“phthalo blue”) and (“carbon black toner”).

What are the factors, which influence the Quality of a Dispersion?
All constituent part of a pigment dispersion have a significant effect on the progress of dispersion. However, the dispersion machine used also plays a very important part. Here three variables need to be taken into consideration:
·
The pigment as such
·
The vehicle (+ solvent and additive)
·
The dispersion machine
The PigmentThe selection of the pigment as such is of significance. Even chemically identical pigments may produce significant differences during the dispersion process. In principle, there are virtually no completely identical pigments, because a series of properties would need to be identical in such circumstances. The most important ones are:
·
Color
·
Color intensity
·
Rheological behavior
·
Wettability
·
Light and weather resistance
·
Compatibility with other pigments (flocculation)
·
Solvent resistance
The BinderThe properties of a pigment dispersion are also influenced very much by the selection of the dispersion binder. In daily practice, it is frequently assumed that the safest and simplest method is the use of the same binder on which the actual paint is based for the dispersion of the pigment as well. Regrettably, this is not always the right solution, certainly not for “critical” pigments or binders with poor wetting properties.
The Dispersion Units
The dispersion machinery used is also very important. The trend today is towards dispersion machines with the highest possible out put and with a small space and energy requirement. For many normal pigments, i.e. pigment, which are easy to wet, such modern dispersing machines are usually completely sufficient. Problems only arise where it is necessary to disperse indispensable organic pigment, which are usually expensive-especially as these frequently involve small batches only.
Dispersion energy means the energy available for dispersion, which does not necessarily mean the same thing as the energy consumption of the machinery. In practice this means that pigments which are difficult to wet, such as, for example, carbazol violet, black, phthalo blue, etc.., can only develop fully their color and intensity if the dispersion machinery can provide the necessary dispersion energy.

Following are the grinding equipments according to their dispersion energy. Stirrer < Dispersion < Sandmill < Perlmill < Ballmill < Attritor < 3-Rollermill
What Practical Solutions are Available?
For the in-house production of a good pigment dispersion, the correct choice of the three variables pigments, binders and dispersion machinery are of extra-ordinary significance, as mentioned above. Of course it is possible to use alternative routes, such as the use of so-called “pretreated pigments”, flushed pigment pastes of finished pastes. But here again one frequently needs to accept advantages and disadvantages at the same time.
When using pretreated pigments for example, it is necessary to take into consideration up to 25% of the pigment has been produced from a combination of wetting agents. In other words: reduced color intensity if the pigment is optimally dispersed.
Flush pigments may effectively solve the problem. But here limitations apply as well. There are normally only pigment flushes of pigments, which are produced, in an aqueous phase. There are also only a few pigments in an industrial binder. Last-but not least: the finished pastes. Powerful industrial companies have long recognized the advantages of using semi-finish products – or out –processing. But again and again one hears objections as well: the use of finished pastes is rather expensive. For large batches of easily dispersed pigments, this may certainly apply. But for difficult pigments in the genuineness of the word – and also for batches, which are simply not economical in a well-equipped company, the use of finished pastes is most certainly one of the most economical alternatives.
The major advantages of the use of finished pastes – and also of other semi-finished products – are frequently overlooked. For example, the storage of raw material is very much simpler and less complicated. Pastes can be produced predominantly in handy packs, at short notice. The number of production formulae in use is reduced: this reduces the risks of faulty batches. If the semi-finished products are included in the raw material inspection, a high level of reproducibility is obtained, so that even the final shading takes less time and costs less money.
When comparing the costs of in-house production and buying out of finished pastes, it is important to take into consideration all-actual costs at their real level.
Make your pick! You now have many ways to solve the dispersion issues of difficult- and expensive- pigments. Never forget the basic principles in the first place!

[ 本帖最后由 cock 于 2008-4-12 19:06 编辑 ]