Dispersion with KREIS-DISSOLVER®
Pigment powder, the basic material for the majority of dispersed products feature a large number of primary particles which adhere to each other thus forming agglomerates, i.e. simple clusters. The decisive objective of a dispersion machine is to be seen in breaking up the agglomerates and in wetting, which means coating, each individual primary particle with the binder film.
The dispersion process
Optimum dispersion and stabilization of pigments and fillers are responsible for the characteristics of paints and coatings. Maximum color strength, brilliance and opacity depend on the particle size achieved for the solids to be dispersed.
Solids, like powders, are divided into three types of particles:
- Primary particles - Consist of a single particle or crystal. They can only be grinded and not be dispersed.
- Aggregates - are combinations of primary particles that are fused together at the surfaces. Aggregates can also only be grinded and not be dispersed.
- Agglomerates - are several primary particles and/or aggregates lying next to each other. In contrast to primary particles and aggregates, agglomerates are dispersible.
The aim of dispersion is therefore to break down agglomerated solids (usually powders) into smaller agglomerates and primary particles and to stabilize these in a medium. This is achieved, for example, by using suitable additives. The more compact the agglomerates and aggregates to be dispersed are, the higher the shear forces of the dissolver acting on them must be. Another important function is the wetting of all primary particles with the binding material film. The degree of dispersion influences the physical and optical characteristics of a product.
This distinguishes the dispersing process from the grinding process, since only physical bonds are broken during dispersion. As soon as aggregates, i.e. chemical compounds, are separated, ten times more energy is required for this. This is then referred to as the grinding process.
For an economic paint and lacquer production, it is essential today to make maximum use of the pigments. With KREIS-DISSOLVER®s in connection with the proven NIEMANN process technology it is possible, e.g., to grind TiO₂- white enamel or flexo printing inks.
Finenesses ˂ 5µm are feasible. After dispersing the mill base, it is going to be completed in a process container. A grinding with bead mill’s is not required. By the maximum grinding of the pigment a reduction of the pigment quantity is possible in case of a given standard colour strength. The consequence is a major cost saving.
Also inorganic pigments can already be dispersed with the KREIS-DISSOLVER® to the final quality (fineness, colour strength etc.).
During dispersion, all parameters of the dissolver must be coordinated so that a doughnut structure forms around the rotating toothed disc. This shape formation signals a maximum energy input into the product to be dispersed.
Important factors here are the diameter of the toothed disc in relation to the diameter of the container, the disc height above the container base, and the filling height of the grinding media. Another important point for an optimum dispersion result is the peripheral speed of the toothed disc. Only at a peripheral speed of at least 20 m/sec a dispersion of the grinding media can take place. At lower speeds, ideally a stirring effect is obtained. Accordingly, no doughnut is formed.
The flow pattern of the doughnut is influenced by the viscosity of the product. If the viscosity is too low, there can’t be achieved sufficient power input. The product batch begins to foam and splash. No doughnut formation takes place. If the viscosity is too high, the batch begins to falter, whereby as a result, not all the pigments reach the toothed disc. In the worst case, the toothed disc will turn into empty space after a certain time.
Variable speed adjustment
The variable speed adjustment allows ideal conditions for a dispersion. In the past, this has been realised exclusively by mechanical drive systems. Today, most of the dissolver drives are equipped with a frequency converter for the variable speed adjustment.
With the variable speed adjustment, it is possible to adjust the speed to the production stage (dosing of liquids and powders, dispersing, dilution) and thus to the product viscosity.
Height adjustable toothed disc
Another important prerequisite is the height adjustment of the toothed disc. At the beginning of the production cycle, the disc turns just above the container bottom i order to start with the necessary reduced binder quantity. The low initial speed prevents the binder from splashing on the container wall. These splashes would, with the filling level rising, lead to fast and uncontrolled formation of pigment encapsulation which would then be excluded from the dispersion process. Because of the changing container filling level the infinite height adjustment of the toothed disc represents a crucial prerequisite, as only then a uniform product circulation and thus perfect dispersion is guaranteed.
Dissolver operation and energy requirement
According to our knowledge, the amount of electrical energy applied for optimum product composition, which is converted into friction heat during dissolver operation, is decisive for the degree of dispersion and the product fineness and modulus. For this reason, high rated drives feature a corresponding current consumption.
However, when calculating the energy requirement it is essential to take into consideration how the power is applied during the production process. Due to the short dispersion time and to the fact that the full drive power is virtually only needed for the dispersion phase itself, the power costs are relatively low. This means that a 110 kW dissolver requires less than 30 kW for a 2000 kg batch during a cycle time of only 15 min. At a basic price of EUR 0,13/kWh this results in approx. EUR 3,90 per 2000 kg of the product, i. e. less than half a Cent per kilogram. Not to mention the increased product quality.
V = Peripheral Speed, d = Dissolver disc Ø in mm, n = Revolutions per min-1
Intensive development over many years, especially in the field of "Speed Control" have led to the increasing capacity and output of our machines. Today a surprisingly large percentage of high quality paint products can be produced on KREIS-DISSOLVER®s without the use of pearl mills. Even when it is particularly difficult to disperse, pigments still require the use of other friction machines, the optimum predispersion is, also for economical reasons, indispensable. Any insufficiently predispersed product not only entails extended cycle times on down-stream systems, but will, in addition, be contaminated and quality impaired by abrasion particles from the grinding tools etc. Thus it can be concluded that a neglected or faulty predispersion can never be compensated for during later processing in other friction systems. Though the quantity of the required energy for a full dispersion cycle is identical with all relevant systems, economy can differ substantially.
A very important prerequisite for the achievement of the dispersion task is a uniform product circulation up to the mixing container wall zones. To the best of our knowledge the design of stirring discs, tooth shapes etc. is not decisive. It is even possible that with completely smooth discs of a correspondingly large diameter an excellent dispersion result can be achieved. In practice, smooth discs have the disadvantage, however, that during pigment addition the contact with the mixed product is not always assured.
High viscosity pastes
The increased quality requirements placed on today's products call for growing production equipment perfection. Even pastes and high viscosity products exceeding 500.000 cP which today are frequently produced in kneaders or planetary paddle mixers, can be produced more economically and with considerably better quality using our KREIS-DISSOLVER®s. Even for the production of high quality Polyester car body putty and glass fibre reinforced special qualities KREIS-DISSOLVER® with viscosity dependent speed control can be used due to its convincing success. Also here the improved dispersion has a positive influence on storage stability.
Use of container wall scrapers
Optimum dispersion results are going to be reached during a laminar flow in the mixing container. Beside other constructive measures container wall scrapers can be helpful during the production of extremely thixotropic products, while mixing dispersed products and with tempered dispersing processes for a better heat transfer (container wall products).
In case of closed systems, e.g. the KREIS-DISSOLVER® in top-mounted design, container wall and bottom scrapers are used often. The scrapers support the cleaning and emptying process.
KREIS-DISSOLVER® with weighing technology
Many production processes with dissolvers are based on too low-viscosity mill base concentrations. The reason often is that too large quantities of binders are weighed in at dosingstations with the mixing containers. After completing the dispersion, the dilution is then also carried out by moving the container to the dosing station, as the transport of weighed binder - which is required for the dilution - possibly causes difficulties.
As a highly concentrated dispersion batch ,however, requires the dilution (this means addition) with a running machine, the dosage has to be carried out in the mixing container right after the dispersion when the dissolver is still running.
The weighing technology mostly is the requirement for automatic production processes. With accurate working weighing systems, raw materials can be dosed directly out of the silo, Big-Bags or manually into the production container. Furthermore, the balance signal is used to control the dispersion parameters.
PC control system KD-BATCH-CONTROL®
KD-BATCH-CONTROL® is a semi- and fully automatic PC/plc-control made by NIEMANN. Whether the production process is semi automatic or fully automatic are based upon the automatisation level of the available liquid and powdery raw materials.
The dispersion parameters are controlled indipendent from the operator by the system. That is a basic requirement for reproducable and high product qualities.
CIP cleaning of KREIS-DISSOLVER®s
For large dissolvers in a closed design, which are operated fully automatic, a CIP cleaning products when changing is essential.
Depending on the application in case of – e.g. water-based dispersion paints or solvent-based industrial coatings - KREIS-DISSOLVER®s can be equipped with cleaning systems.
A distinction is made between linear movable cleaning lances, fixed cleaning heads and motor-driven or fluid-driven cleaning nozzles.