Dispersion (paint)
As dispersing or dispersing refers to a process step in the preparation of formulations as, inter alia, in lacquers , printing inks , plastics or pigment preparations are common. The term is mostly used when incorporating pigments and fillers in such a system. A suspension (a special type of dispersion ) is produced, i.e. a solid phase is distributed in a liquid one. In addition to the uniform distribution in the carrier material, the dispersion also describes the wetting of the substance to be dispersed with the carrier material, as well as the comminution of the substance to be dispersed and the subsequent stabilization.
The property of how easily a material can be incorporated is known as dispersibility . The hardness of the dispersion is a quantifying parameter .
Theory of Dispersion
Pigment particles are usually in the form of agglomerates, i.e. loose clusters of smaller particles (over the “corners” of the primary particles) and aggregates (over the “surfaces” of the primary particles). The aim of the process is the stable division of the agglomerates into aggregate and primary particles. The dispersion comprises the three sub-processes wetting, dividing and stabilizing. In practice, these processes run in parallel, since, for example, the breaking up of agglomerates leads to an increase in the surface area and the additional surface must also be wetted.
Wetting
In the first step, the pigment particles present as agglomerates are wetted by the carrier material. Depending on the coating system, the pigments are wetted by solvents , water or the (dissolved or melted) binder . A distinction is made between the spreading of the liquid phase on the agglomerate surface and the penetration into cavities and pores, whereby air is displaced from the system. Since wetting is often difficult or too slow, wetting agents are used to speed up the process.
Cut up
The respective dispersing unit introduces shear forces into the system in different ways, which ensure that the agglomerates (aggregates and primary particles connected by corners and edges) are divided into aggregates (connected by surfaces) and primary particles. In the ideal case, the pigment particles could be crushed down to the primary grain. In practice, however, the energy introduced is not sufficient for comminution down to primary particles, so that mostly aggregates and sometimes agglomerates are still present.
stabilization
The crushed particles usually have a tendency to agglomerate again. This process is called flocculation , the resulting agglomerations as flocculate . Since this is not desired, dispersants are added to counteract this process. The distinction between these additives and wetting agents is not clear, as many additives available on the market have both properties (wetting and stabilizing). A common definition is the division into low molecular weight, amphiphilic substances (wetting agents) and oligomers or polymers that stabilize against flocculation (dispersants).
The stabilization can take place electrostatically, sterically or electrosterically. With electrostatic stabilization, the surface of the particles to be stabilized is covered in such a way that all particles have charges of the same name and thus repel each other. With steric or entropic stabilization, the particle surface is covered with long-chain molecules. If the particle distance were reduced, the entropy of the system would decrease, which counteracts flocculation. With electrosteric stabilization, the long-chain molecules are additionally charged, so that both effects can be used.
Process flow
Initial weight
Before the start of dispersion, the raw materials used (with a few exceptions) are introduced and mixed or predispersed. The liquid phase is first weighed in and the solid phase is added at the end. In systems where a completion is possible, an optimal viscosity for the comminution of the particles is usually set. This is usually higher than the processing viscosity of the end product, so that this is achieved by reducing the amount of solvent or water weighed in. In the case of particularly viscous mixtures, the phase to be dispersed is usually added gradually.
Predispersion
Predispersion includes all steps that lead to better wetting of the disperse phase. On the one hand, this improves the dispersion result itself (faster or better dispersion), but on the other hand it also improves the cost situation, since time is saved in the energy-intensive dispersion process. In the case of poorly wettable materials, the first step is often simply to let the ground material stand after mixing, colloquially known as soaking . This gives the wetting agents used time to better wet the surface of the solid phase. This is followed by predispersion, which in the case of liquid formulations usually takes place with a dissolver. This also serves to improve wetting, but also leads to an initial breakdown of the agglomerates. In the case of materials that are solid at room temperature, for example powder coatings and plastics, the mixing of the raw materials must already be regarded as part of the predispersion, since the mixing of solids already results in a breakdown. With these materials, however, there is no classic predispersion.
Dispersion
After mixing and predispersing, the millbase is placed on a dispersing unit , i.e. a machine for dispersing. There the ground material is dispersed up to the desired grain size so that it can be further processed. In processes that work with the addition of grinding media, a process step follows in which the grinding media are separated from the grinding stock. In processes in which the dispersion takes place in the melt, a cooling step follows and, as a rule, grinding (powder coating) or granulation (plastic).
Complete
Finally, the material to be dispersed is completed (collared). It refers to the addition of the remaining ingredients of the formulation, the viscosity of the end product being adjusted by adding solvent or water. This is a critical part of the process, as shock phenomena are possible at this point , which can partially cancel out the dispersion result and, in extreme cases, lead to the discarding of the entire batch.
In the case of formulations that are solid at room temperature, such as powder coatings or plastics, it is not possible to complete them.
Influence of the dispersion on the system properties
By making the particles smaller, some properties of pigmented systems are significantly changed. First, the particle size decreases with increasing dispersion time. This is accompanied by a significant increase in the paint surface to be wetted. The amount of wetting agent or binding agent that is necessary for complete wetting accordingly increases with increasing dispersion time. The reduction in size of the particles leads to a decrease in the effect on the roughness of the paint surface, which can be seen in an increase in gloss . The most obvious change can be seen in the increase in the color strength of the pigments contained.
Checking the dispersing effect
Those properties which change the most during the dispersion are also used at the same time for testing the dispersion. The most widely used method for determining the degree of dispersion is the grindometer test, which actually determines the size of the largest particles (oversized particles), but not the actual (average) particle size. The test can be carried out directly on the material to be dispersed. The substance to be tested is applied in a wedge-shaped recess of a metal block and then smoothed out. The particles, which have a larger diameter than the depth of the wedge, form strips and can therefore be evaluated. The depth of the wedge at this point shows the respective particle size, which is also referred to as the fineness or graininess.
Measurements of the progress of the dispersion, which are based on the properties of the resulting lacquer, are the measurement of color strength and gloss. Both methods make use of the fact that the properties of the overall system usually approach a limit value (or at least a target value). The gloss increases during the dispersion process, but the influence of the dispersion decreases as the dispersion time increases. The color strength also increases further during the dispersion and approaches a limit value at which (approximately) it is reached, the pigment is designated as fully dispersed . The disadvantage of these methods, however, is the need to finish and apply the paint for each test process, which makes testing more complex.
Dispersing units
Dispersing units (also dispersing machines or dispersing devices) are devices which are used for the dispersion of solid components (pigments, fillers) in the liquid phase of paints and printing inks. Dispersing units transfer energy either by rubbing the ground material between two surfaces or by exerting impact and shear forces through rapidly rotating disks into the ground material. Depending on the viscosity of the millbase, the dispersibility of the pigments and fillers and the required quality of the coating material, various dispersing units are used.
Dissolvers are common for predispersion and for substances that are easy to disperse (often inorganic pigments ) . Ball mills or agitator ball mills are used for substances that are difficult to disperse (for example organic pigments) in aqueous or solvent-based paint systems . For solid, but meltable carrier materials, kneaders or extruders are used. The latter are particularly common in the manufacture of powder coatings . A classic dispersing unit for printing inks that is rarely used today is the three-roller frame (colloquially three-roller), on which pasty carrier materials are required.
Dissolver
Dissolvers are high-speed stirring disk devices that consist of a stirring tank and a (mostly toothed) disk stirrer. They are used to disperse easily wettable materials or wherever there are no high demands for fineness, for example in emulsion paints . The most important use of dissolvers, however, is predispersion in the processing of difficult-to-disperse pigments and in the production of high-quality paints.
Dissolvers break up agglomerates by shearing in the vicinity of the disk and pressure changes when changing between zones with high pressure and negative pressure. At the edge of the dissolver disk there are often impact devices such as teeth or pins. It is also possible to use several stirring disks. The dispersing result is essentially dependent on the correct setting of the viscosity of the millbase, the coordination of the sizes of the agitator disc and agitator vessel and the peripheral speed of the disc. What is important is the formation of a suction cone reaching down to the pane ( donut effect ).
Agitator mills
Agitator mills are the most important dispersing devices in the manufacture of paints and printing inks. They have largely ousted other dispersing devices due to their versatility and performance potential. Agitator mills usually work continuously and are available as open or closed versions. Depending on the type of grinding media, a distinction is made between pearl mills and sand mills. Agitator mills consist of a cylindrical grinding pot filled to about two thirds with grinding media. In the longitudinal axis of this cylinder runs a shaft on which perforated disks for stirring are mounted parallel to one another. The material to be dispersed is pumped into the grinding cylinder. The agglomerates are sheared between the packings when the shaft rotates. In addition to the classic vertical arrangement, horizontally arranged mills are often used.
Multiple dispersion is possible with multi-chamber mills, which consist of several agitator mills located one behind the other. Advantages are the possibility of continuous production, a fast working method and almost no solvent emissions (with closed versions). The high cleaning effort is a disadvantage.
Sand mills
Sand mills are stirred mills, where Ottawa sand was used as grinding media. For years they were the usual dispersing devices in the paint industry. The sand mills, which were often operated openly, were replaced by the closed bead mills.
Pearl mills
Bead mills are closed agitator mills in which ceramic beads or hard glass beads are used as grinding media. Today they are the most widely used dispersing devices in the paint industry. The higher the density of the pearls, the greater the dispersion intensity. Common grinding media are made of glass , ceramic , zirconium dioxide or steel . Glass beads are inexpensive and less abrasive to the mill. ZrO 2 beads are hard and have a high density. They are available cerstabilized and yttrium stabilized. Particularly stubborn pigments, such as nano zinc oxide, can only be fully dispersed with zircon beads. A disadvantage of using hard beads is increased abrasive wear on the wall of the mill.
The finer the pearls, the more area is available for grinding agglomerates. Therefore, with finer pearls, the desired grain fineness can be achieved faster or greater fineness in the same time. Beads that are too small cannot pull in large agglomerates and therefore need careful pre-dispersion with a dissolver. Similar to the dissolver, the grinding intensity increases with the peripheral speed of the agitator disc ends.
Ball mill
Ball mills are cylindrical containers that can be filled with balls of different sizes (10 mm to 30 mm). The balls are made of metal , steatite , corundum or porcelain and make up half the volume. The agglomerates are crushed by sliding and rolling movements of the balls. The advantages are the ease of use, the low loss of solvent (closed system) and no need for constant monitoring. Disadvantages are the long running time, the discontinuous mode of operation, a high cleaning effort, the low flexibility in terms of batch size and the resulting larger machine park. In addition, there is the great volume of ball mills.
Three-roller mill
The three-roller frame , also three-roller frame , roller frame or rolling mill is an open dispersing device that is also used to disperse pasty materials. It works continuously and consists of feed, center and take-off rollers, with the feed and take-off rollers being pressed against the stationary center roller. This creates gaps with a width of 10 µm to 20 µm, in which the agglomerates are broken up by pressure and shear forces. The rollers usually rotate in a ratio of 1: 3: 9 (feed roller: center roller: take-off roller), as this usually leads to the best and most economical dispersion results.
The material to be dispersed enters the feed gap between the feed and the central roller. It migrates on the central roller into the take-off gap between the central and take-off roller. The take-off roller takes over the material to be dispersed, which is then removed with a scraper. The agglomerates are crushed by squeezing and shearing between the rollers. The rollers are cooled from the inside due to the temperature increase of the material to be dispersed. Because of the high loss of solvent, the three-roller mill can only be used for pasty, i.e. low-solvent systems. Significant advantages of three-roller mills are low solids losses, gentle dispersing effect and good cleaning options. The high solvent losses, the need for constant monitoring of the roller, as well as low throughputs and poor reproducibility have a disadvantage.
Extruder
Extruders are machines that take solid to liquid materials and continuously press the so-called extrudate out of an opening. Common are single-screw extruder having a kneading screw inside. Twin screw extruders and planetary roller extruders are rarer (smaller screws revolve around a central screw). Extruders are used to disperse powder coatings, where the meltable raw materials ( binders , additives ) are melted and the constituents to be dispersed are sheared between the screw (s) or between the screw and the wall.
Kneader
This unit, which is also referred to as a mixer , is also mainly used to produce pasty or dough-like systems, mostly putties. Kneaders consist of kneading tools that move against each other or against a fixed surface. The dispersing effect is achieved through the strong compression and shear of the material.
literature
- G. Bentzing et al .: Pigments and dyes for the paint industry . 2nd edition, expert Verlag, 1992, ISBN 3-8169-0752-0 .
- Smock: textbook of paints and coatings . 2nd edition, Volume 8, S. Hirzel, 2004, ISBN 3-7776-1018-6 .
Individual evidence
- ↑ G. Buxbaum, G. Pfaff: Industrial Inorganic Pigments . 3rd edition, Wiley-VCH, Weinheim 2005, ISBN 3527303634 , page 43.
- ↑ DIN 53 206
- ↑ a b c d A. Goldschmidt, H. Streitberger: BASF Handbook Painting Technology . Vincentz Network, Hannover 2002, ISBN 3878703244 , page 205ff.
- ↑ B. Müller, U. Poth: Paint formulation and paint recipe: The textbook for training and practice . Vincentz Network, Hannover 2006, ISBN 3878701705 , page 59.
- ↑ B. Müller, U. Poth: Paint formulation and paint recipe : The textbook for training and practice. Vincentz Network, Hanover 2006, ISBN 3878701705 , page 50.
- ↑ a b c d e f g h A. Goldschmidt, H. Streitberger: BASF handbook paint technology . Vincentz Network, Hannover 2002, ISBN 3878703244 , page 226ff.
- ↑ DIN 55607: 2007-06 Pigments and fillers - Dispersion of pigments in powder coatings and their colorimetric assessment after application
- ↑ a b c d G. Meichsner, T. Mezger, J. Schröder: Measuring and controlling paint properties . Vincentz Network, Hannover 2003, ISBN 3878707398 , page 191.
- ↑ a b H. Römpp: Römpp Lexikon Lacke und Druckfarben . Thieme, Stuttgart 1998, ISBN 9783137760016 , page 147.
- ↑ a b H. Römpp: Römpp Lexikon Lacke und Druckfarben . Thieme, Stuttgart 1998, ISBN 9783137760016 , page 150f.
- ↑ a b H. Römpp: Römpp Lexikon Lacke und Druckfarben . Thieme, Stuttgart 1998, ISBN 9783137760016 .
- ^ H. Römpp: Römpp Lexikon Lacke und Druckfarben . Thieme, Stuttgart 1998, ISBN 9783137760016 , page 506.
- ^ H. Römpp: Römpp Lexikon Lacke und Druckfarben . Thieme, Stuttgart 1998, ISBN 9783137760016 , page 438.
- ↑ a b c H. Römpp: Römpp Lexikon Lacke und Druckfarben . Thieme, Stuttgart 1998, ISBN 9783137760016 , page 156.
- ^ A. Goldschmidt, H. Streitberger: BASF handbook paint technology . Vincentz Network, Hannover 2002, ISBN 3878703244 , page 234ff.
- ^ H. Römpp: Römpp Lexikon Lacke und Druckfarben. Thieme, Stuttgart 1998, ISBN 9783137760016 , page 214.
- ^ H. Römpp: Römpp Lexikon Lacke und Druckfarben . Thieme, Stuttgart 1998, ISBN 9783137760016 , page 325.