To achieve an optimum result using wetting and dispersing additives, a few hints and suggestions should be observed when using these products.
Additives are frequently added to the coating during the finalization phase. However, this procedure is not suitable for wetting and dispersing additives. As these additives are needed to accelerate pigment wetting and pigment dispersion, they must be added to the millbase and ground with it. Only in this way can they be fully effective. If, in exceptional circumstances (e.g. for batch correction purposes), it is necessary to add the additives later on, they must be incorporated using as high as possible shear forces; it is preferable to run the system through the dispersing unit again. Nevertheless, you will find in the majority of cases that wetting and dispersing agents which have been added in this way are less effective and require greater doses.
How much additive do you need to add? The correct dosage is key to the effect. Since the additive is designed to attach to the pigment surface, the required dosage of additive depends upon the amount of pigment surface present. Apart from a few exceptions, calculation formulas which link the additive dosage with, for example, the BET surface of the pigments or the oil number, are not particularly reliable and should be used only for specific pigment types. In practice, you would tend to base the dosage on the recommendations of the additive supplier and then develop a series of laboratory tests in order to optimize the dosage for your needs. You can use, for example, gloss and haze values of the coatings and the ∆E of the rub-up test as test criteria.
In the case of classic wetting and dispersing additives based on low-molecular weight polymers, a dosage of 0.5-2% for inorganic pigments and 1-5% for organic pigments are standard (additive delivery form based upon pigment weight). Typical additive dosages for polymeric wetting and dispersing additives are 1-10% (inorganic pigments) and 10-30% (organic pigments). In the case of very fine-particle pigments (e.g. some carbon blacks), greater dosages of up to 80 or 100% are required for very high-quality formulations. As these pigments can be found only in small quantities in the formulation, the additive dosage refers to the entire formulation, but is still not excessively high. A greater dosage will not have any negative effects on the coating film properties as the polymeric additives have a character that is similar to that of binder.
We should emphasize once again that all pigments must be stabilized in a coating formulation. Stabilization is also essential for allegedly “simple” pigments, such as titanium dioxide; otherwise, when mixed with other (well-stabilized) pigments there will be inevitable floating issues.
Whenever just one pigment is being dispersed, all the parameters (additive dosage and grinding conditions) can be optimized. The end result is the best possible grind quality. However, in practice, a co-grinding of all pigments is preferred to the more time-consuming single grinding process. Compromises in regard to grinding parameters are therefore necessary and the result cannot always be compared with the single grinding process. At least when working out a new formulation in the laboratory, you should always take into consideration the individual ground pigments. This way you can identify any “difficult” pigments. The subsequent procedure should be determined on an individual basis, e.g. problematic pigments can be exchanged for other pigments, they can be ground separately, or can even be added as pigment concentrates.
Virtually all of today’s standard pigments have been provided with surface post-treatments of varied chemical nature. The pigment manufacturer applies these during the production of the pigments. The purpose of the surface treatment is to facilitate and improve the wettability, dispersion and stabilization of the pigments. In the case of wetting and dispersing additives, this means that they do not come into contact with the actual pigment, they are in contact with the chemical surface treatment on the pigment. Therefore, in practice, if there are unsatisfactory grinding results with a specific pigment, the same pigment should also be considered with other surface treatments.
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