2019/03 | ARTICLE
Dispersing under vacuum expansion creates paints and lacquers with high colour strength. Less titanium dioxide is necessary because of more efficient dispersion by the new technology. By Hans-Joachim Jacob, Ystral
Titanium dioxide (TiO2) is one of the main cost drivers when producing paints and lacquers. Currently around seven million tonnes of TiO2 are produced per year. The global market price per ton has increased by around 50 percent on average over the last few years - for high qualities it almost doubled. A new technology combines inline dispersing technology together with a vacuum expansion method in order to use TiO2 more efficiently.
TiO2, with around 60%, the most important pigment in the production of lacquers, paints and printing inks. It is the basis of white and even coloured tones. It is necessary because colour pigments, only become visible if the unabsorbed spectrum is reflected, for example by TiO2. This is also a large cost block. It has been calculated that roughly 25-40% of the production costs, depending on the application, is due to TiO2. Europeans were previously the largest consumers of TiO2, but this has changed. Currently Asia, primarily China, uses the most material. A single factory there produces as much emulsion paint as all of Germany’s paint producers together.
TiO2 is very fine. The individual particles of this pigment are only around 200-400 nano-meters in size. Their most important property: They bend, diffract and reflect – in short: they scatter the incident light. The wave character of light makes each individual particle scatter the light in a sphere, which is roughly two to three times as large as the particle itself. If the distance between two particles is small enough to make the active spheres overlap, the resulting scattering of light of both particles together is barely higher than that of a single particle. Conclusion: too small particle spaces reduce the light scattering effect.
TiO2 particles agglomerate, they stick together. Additionally, most water-based lacquers and paints have a pH-value close to the isoelectric point of titanium dioxide - increasing the tendency even further. Desagglomeration and subsequent stabilisation through dispersion additives is necessary. The individual particles have to be homogenised at optimum distance from each other in order to generate the maximum light scattering.
The method of dispersion under vacuum expansion aims to separate particles, destroy agglomerates and prevent reagglomeration. Dispersion additives are required in order to keep the dispersion stable, even when the dispersion aggregate is switched off again. The requirements for an optimum dispersion of titanium dioxide are modern dis-persion technology together with optimum dispersion additives and feeding the sufficient quantity at the right time by the same machine.
High speed dissolvers (HSD) have been used previously in lacquer and paint manufacturing, but they only achieve a relatively low shear gradient. High viscosities are needed, in order to generate a shear effect. These are not necessary, if processed with the “Conti-TDS” developed by ystral. It delivers a thousand-times higher shear rate. Wetting the finest powder with high viscous liquids - as in the dissolver - is illogical and difficult. The new technology solves this problem. It requieres 70-90 percent less energy for a proper dispersion and, specifically for titanium dioxide dispersion, up to 95 percent less time.
The machine is installed outside the process vessel. As a re-sult, it works independently of the vessel size and filling level. It can start feeding powder even at low initial liquid levels. This creates optimal dispersion conditions form the very start. Dispersion agents are not completely added at the start, but gradually during the powder feed.
The dissolver not only disperses. It also circulates the highly viscous mass in the vessel. Only a fraction of its power is available for dispersion in a large volume. With the new method, almost the entire power serves the dispersion process. The dispersion zone itself has a volume of less than one litre. As a result, the specific power used for dispersion is around 10.000 times higher. The machine generates a high vacuum in its dispersion zone, which inducts the powder into the liquid without dust and losses.
MORE AIR THAN EXPECTED
TiO2 contains, like any other powder, a high amount of air. In fact, more than 75% of its bulk volume is air. The new technology uses this air. The powder particles touch each other under atmospheric pressure. The generated vacuum expands the air and widens the spaces between the particles. On its way into the zone of maximum vacuum, the air expands even further. This is how the liquid reaches all the spaces that are formed and wet each individual particle completely.
Powder and liquid are not simply mixed together, but the mechanical dispersion takes places with maximum turbulence and maximum vacuum exactly in the shearing field of the dispersion zone. This is the reason, why the vacuum expansion method does not need a wetting agent to wet the powder. Consequently, almost no foam is created, and the system needs less defoamer.
The excess air from the powder is separated from the dispersion by the centrifugal effect of the machine and pumped together with the liquid into the vessel. Large bubbles form and the air escapes easily. Since the wetting takes place outside the vessel, no dust is accumulated above the liquid – for example on the inside of the vessel or on its lid.
MANUFACTURING TINTING PASTES (COLOURANTS)
Titanium dioxide pigment pastes are the most frequently required pigment preparations for inplant and point-of-sale tinting systems. The highly concentrated pastes have a TiO2 content of 60-80 per-cent. They are currently made with a dissolver and milled in a mill to best possible fineness. The dissolver alone is not able to completely des-agglomerate the pigment.
The new technology is not only faster compared to the current process, but it also succeeds without a mill. Fine particle size distribution is achieved directly after adding the powder, unlike anything previously achieved even with an extra mill.
Traditionally the quality of the end product depends greatly on the quality of the TiO2. Experience with the new Conti-TDS has shown that it achieves a better result even with standard or cheaper TiO2 grades. Most dissolver recipes contain lots of wetting agent, but not enough dispersing additives. The new technology does not need any wetting agent at all, but more dispersing agent. Compared with the previous production, a smaller particle size is created along with a narrower particle size distribution. The specific pigment surfaces in need to be stabilized by the dispersing agent, is enlarged. 20 percent more surface means 20 percent more dispersing additive is needed for stabilization.
Fig. 6 (download pdf to see the figures) shows the situation during production of an unaltered traditional dissolver recipe. The entire dispersing additive has been size distribution is smaller, and then larger again as dispersion continues. This is an indicator for not sufficient amounts of dispersing additive.
Fig. 7 (download pdf to see the figures) shows the result with the adjusted dispersing additive quantity and optimised feeding directly into the process. In this way the effect of the TiO2 can be used more efficiently.
Emulsion paints for interior and exterior walls contain fillers such as calcium carbonate, talcum or clay as well as TiO2. In the dissolver process the viscosity, of the basic liquid, is usually increased by adding a thickener at the start. The TiO2 and all fillers are added over the surface. The powders are stirred into the liquid. This method leads to considerable fluctuations in quality. Quality checks and readjustments must be made consistently.
This is not the case with the new technology. In principle, the same recipe and the same sequence could be used. This allows a constant quality to be achieved. If the process sequence is changed, but the recipe is the same, further quality improvements and time savings of 35-70 percent are possible. A significant step is to delay the addition of the thickening agent until the end of the process or even to delay it into the subsequent let down process. As a result, the liquid is less viscous during dispersion. An optimised recipe reduces raw material costs above all.
Organic polymer based thickening agents are not shear stable. A large part of the thickening effect is irreversibly destroyed in the dissolver over the long processing time. Not with the new process. Thus the amount of thickener can be reduced by 20 percent with constant quality and viscosity.
In order to optimise the dispersion for TiO2, the process should only disperse TiO2 at the start under optimum conditions with just a fraction of the liquid. The best possible particle size distribution is achieved early in the process. The particles of the remaining fillers are up to 20 times larger than TiO2 particles. A combined dispersion prevents optimum dispersion of TiO2 and results in crowding of titanium dioxide. The smaller titanium dioxide particles become concentrated in the gaps between the much larger fillers. They are not homogeneous and not optimally distributed. Therefore, dispersing the titanium dioxide in part of the liquid before adding the fillers is advantageous.
HIDING POWER AND COLOUR STRENGTH
The dissolver only functions at an optimum filling level. The new technology works independently of the filling level. It can start with small amounts and the TiO2 is optimally dispersed as a result. The other liquids, the filler and finally the thickening agents are added after the dispersion. No need for further dispersion afterwards. Users, who have optimised their process in this way achieve both: better hiding power and higher colour strength. The previous quality can be achieved by reducing the proportion of TiO2, on average by 8 percent. In addition, less thickening agents, wetting agents and defoamers are needed.
In formulations with very fine composite or hybrid pigments or fillers, the additional savings in TiO2, due to better dispersion, are limited.
TIO2 SLURRY AND INTERMEDIATES
One industry trend moves away from co-grind production and uses slurries instead. The advantage is to disperse all powder components individually in optimum conditions, instead of adding all components at the same time with the same conditions and compromising quality. Paint producers usually produce the slurries in-house in order to be independent from one supplier and keeping sensitive recipes in-house as well. They then keep slurries in stock and the finished products are mixed from liquid slurries.
Even if the previous co-grind production is retained, it is recommended to use at least TiO2 as slurry.
A slurry does not have to comprise a substance with just one single powder. If it contains several powders, these are called intermediates. For an TiO2 intermediate it is positive to combine the TiO2 with a spacer. It should reinforce the effect of TiO2. A typical example is a very fine clay. It ensures the individual TiO2 particles to orient, dis-tribute optimally and stabilise.
Magazine: European Coatings Journal
Author: Dr. Hans-Joachim Jacob
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