Dispersibility
The dispersibility is a process-technical properties of pigments (and other commodities). It says how easily a pigment can be incorporated into the paint system.
theory
When dispersing pigments, which are mostly present as coarse particles, shear forces are applied that break them up. For good division of the agglomerates into aggregates and primary particles , good wetting of the surface and good stabilization of the (smaller) particles obtained must be given at the same time .
A good dispersion of the pigments leads to a stable distribution of fine particles in the paint. Due to the general properties of pigments, the color strength of the pigment increases during the dispersion process and the degree of gloss of the system increases.
The dispersibility states how fast the process of dispersion takes place. The dispersibility of a pigment is a property that is significantly influenced by the coating system. The decisive factors are the type of binder , hardener, solvent and, last but not least, the wetting and dispersing agents that are intended to ensure good wetting and stabilization.
Since the dispersibility is directly related to the production time, the dispersibility is also an important parameter for the economic efficiency of pigments.
exam
The dispersibility is a variable derived from measurements of the degree of dispersion. As a rule, the time until a certain degree of dispersion is reached is recorded or the ratio of two measurements is defined as dispersibility.
Indirect test methods
Since the indirect method is a relatively quick and sufficiently precise method, it is mostly used to determine the dispersibility. Since these are methods for measuring paint properties, they can be used for all paint systems, including powder paints.
Color strength development
The color strength of pigments increases as the particle size decreases. To determine the dispersibility, samples of the lacquer or millbase are taken with increasing dispersion time and colorations are produced. A so-called dispersion curve can now be drawn up from the development of the color strength, which has a limit value. The following values can now be determined as a measure of the dispersibility:
- Time to reach 90% of the limit value
- Ratio of color strengths with different dispersion times (given in%)
Gloss development
Coarser particles cause the surface of the finished coating to become matt. Conversely, conclusions can be drawn about the dispersion from the development of the gloss . The measure for the dispersibility is then the fall below a system-specific degree of gloss.
With this method, the dispersibility of colorless raw materials, e.g. B. fillers are determined.
Direct test procedures
The direct test methods are less suitable for testing dispersibility because they are either too complex or not sufficiently accurate.
Particle size distribution
The measurement of the particle size can be done with various methods such as e.g. B. sedimentation analysis or microscopic examination can be carried out. These methods are unusual for checking dispersion in production and laboratories, as they are too complex. They are even completely unsuitable for powder coatings.
Grindometer test
The grindometer test is one of the most widely used methods of measuring dispersion. Essentially, the paint or the ground material is applied to a wedge-shaped recess and the protruding material is wiped off. The size of the largest still visible agglomerate is determined by means of a scale. A sufficient degree of dispersion is determined by an upper limit for the largest agglomerate. The dispersibility can be determined by the time it takes to reach this limit, but the inaccuracy of the test speaks against it.
special cases
Powder coatings
In contrast to liquid paints, pigments in powder coatings are dispersed in the extruder . In this case, it is not possible to simply extend the dispersion time, so that a determination of the dispersibility is only possible by comparing several extrusion processes. Since this is very complex, the determination of the dispersibility in the powder coating sector is unusual, although it is a very important property of pigments for powder coatings for exactly the same reason.
Over-dispersion
With some pigments (e.g. some azo pigments ) too much dispersion or too high a temperature leads to partial dissolution of the pigment in the surrounding medium. Very small agglomerates are completely dissolved and crystallize on coarser particles. These are aggregates and not re- dissolvable flocculates. These cannot be comminuted again by further dispersing. Since the particle size has increased on average, the degree of dispersion falls again irreversibly . This effect is also known colloquially as dead grinding . When testing the dispersibility, this range must not be reached under any circumstances.
Individual evidence
- ↑ B. Müller, U. Poth; Paint formulation and formulation; Vincentz Publishing House; Hanover; 2005; ISBN 3-87870-170-5 .
- ↑ B. Müller; U. Poth; Paint formulation and formulation; Vincentz Publishing House; Hanover; 2005; ISBN 3-87870-170-5
- ↑ G. Meichsner, T. Mezger, J. Schröder, U. Zorll; Measure and control paint properties; Vincentz Publishing House; Hanover; 2003; ISBN 3-87870-739-8 .
- ↑ a b F. Tragor; Organic Pigments for Coatings; Eurocoat; Barcelona; 2006.
- ↑ W. Herbst, K. Hunger; Industrial organic pigments; 2nd edn .; Wiley-VCH; Weinheim; 1997
- ↑ Archived copy ( Memento of the original dated May 11, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. .
- ↑ A. Goldschmidt, H.-J. Streitberger; BASF Painting Technology Handbook; Vincentz Publishing House; Hanover; 2002; ISBN 3-87870-324-4 .
