Stabilizing titanium dioxide - European Coatings

Quelle/Publication: European Coatings Journal 07-08/2004 Ausgabe/Issue: 34 Seite/Page:

Stabilizing titanium dioxide Simona Schwarz, Gudrun Petzold, Uwe Wienhold. The characterization of particulate systems in terms of charge, size, shape and morphology is fundamental for the optimization of such processes as stabilization. TiO2 dispersions with low particle sizes of about 400 nm and additional basic surface modification show a good stability. The addition of macromolecules like MSA improves the stability of dispersions. Over the past few years, there has been a rapidly growing interest in industry in water-based coating materials. Development of such coatings will enable the use of organic solvents to be reduced. This fact determines the high and constantly growing potential of these materials for application in environmentally friendly technologies. Knowledge of the stability of water-based coating dispersions is crucial for their practical application. Pigments and their properties play an important role in the stability of systems, mainly because their properties affect dispersion stability. Pigments and fillers may be titanium oxide, iron oxide pigment, dolomite, chalk, and talc. Titanium dioxide is widely used as a white pigment in paints, paper, plastics, ceramics, rubber, inks and a variety of other products. Titanium dioxide used for coating applications is predominantly in the rutile form. Rutile TiO2 is modified with thin films of inorganic oxides like alumina, silica or zirconia. TiO2 is easy to wet and to disperse in aqueous media. An important aim is the stabilization of the pigments against flocculation. Systematic investigations of the influence of processing conditions (particle size and particle size distribution, pH, polymer concentration and dispersant stabilising mechanism) on colloidal processing of TiO2 were carried out. The goal was to establish a parameter for stabilization which could reliably characterize system stability. Particle characterization was the main tool used to evaluate and control the stabilization condition of the powders. The most important factors determining properties of the colloidal TiO2 dispersion are conditions of suspensions preparation and stabilization.

Surface charge density and zeta-potential The properties of the electrical double layer of the particles play an important role in colloids stabilization. The stability of dispersions can be characterized by electrokinetic measurements. Zeta-potential is a quantity for the stability of net charge at the shear plane. In contrast to other interface potentials, the zeta-potential is easily measurable and it is an important experimental parameter characterizing the charge conditions at the particle surface, the adsorbed layers and the colloidal stability. Measurements of the electrokinetic (or zeta) potential give information about the existence of acidic or basic molecular groups on the solid surface and their dissociation constants. They also give information about the interaction between the components of the electrolyte solution and the solid surface. The repulsive forces in a stable dispersion were long ago identified as being of electrical origin. A surface potential exists at the interface between the solid particle and the surrounding liquid due to the presence of a surface charge. The commercial TiO2 were modified with thin films of different inorganic oxides such as alumina, silica or zirconia. Measurement of the surface charge density (sign and value) and zeta potential in water at pH 6 were carried out by polyelectrolyte titration with a particle charge detector (PCD) (Table 1). Knowing the charge density at a certain pH gives a first statement about a sample's stability. As Table 1 shows, the charge density at pH 6 was positive for all three RM samples from Sachtleben and for "KR2100" from Kronos. The best dispersion stability independent of the sign of charge was achieved for the samples with high values of charge density ("RM120", "KR2044" and "KR2100").

Titanium dioxides and equipment Seven different commercial titanium dioxides from two different companies (RM-products from Sachtleben Chemie GmbH; Kr-products from Kronos Titan, Inc.) with different surface properties, particle size and size distribution were used. Three commercial dispersants were used to disperse aqueous TiO2 suspensions: Polystabil AN (PSAN), Polystabil W (PSW) (Stockhausen.degussa) and a commercial ethylene maleic anhydride (MSA) copolymer. Deionised, Milli Q-Plus-water was used in the preparation of all solutions and suspensions. Zeta-potential was measured as streaming potential by particle charge detector "PCDO3pH", Mütek. 1g/l TiO2 was suspended with an ultrasonic treatment for 15 min. at 22 °C. The stability was measured with a special centrifuge "LUMiFuge". This is a microprocessor controlled, analytical centrifuge for rapid classification of stability and separation of evenly concentrated dispersions. It records the kinetics of transmission changes for 8 samples simultaneously, like a time lapse motion picture, up to 25 000 times faster than tests at gravity by naked eye. The centrifugation at 12xg 1200 xg results in an accelerated migration of the particles.

pH-dependence The pH-dependence of charge for the different pigments must be known for applications at different pH values. To analyze acid-base properties zeta-potential versus pH, plots was recorded for 3 different TiO2 dispersions (Figure 1a). The isoelectric point and the shape of the plot provide information about the character of the solid surface. The curves show a shift of the isoelectric point (the point, where the value of the zeta-potential is zero) towards the basic region depending on the surface charge. Both types of acidic functional groups exist on the pure TiO2 surface and basic functional groups are the result of the modification. A plateau in the basic media is detected for "RM120" and "RM220". The zeta-potential pH profiles are in a good agreement with the investigations of sedimentation profiles by "LUMiFuge" test (Figure 1b). "RM300" shows a fast destabilization in a short time of about 50s. The dispersion of "RM120"-particles is more stable. The supernatant is nearly clear after 180s. To obtain defect free bodies from ceramic slips or smooth thin layers from paint slurries, suspensions are required to be well dispersed stable systems. In any aqueous colloidal system the pH is one of the main factors determining the

Dynamic light scattering (UPA), streaming potential measurements (PEL-titration, PCD), REM and "LUMiFuge" were used to investigate the particles. Particle characterization methods were used to quantitatively determine specific properties of dispersions and to measure control or optimize changes in dispersion states.

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stability of the suspensions. Thus, the most frequently used method to obtain well stabilized suspensions is to work at very high or very low pH values. The best stability conditions are far from the isoelectric point in the uncharged state. Particle size, charge and size distribution Particle charge, particle size and particle size distribution are important parameters for a stable dispersion. A decrease of the mean particle size leads to an increase of stability of the dispersion. The mean diameter of the bulk population was determined by photon correlation spectroscopy (PCS; UPA, Grimm) and the particle size distribution was obtained (Figure 2). The smallest particle sizes with values of d50 1,25 µm were determined for "RM120". The particle sizes for "RM220" and "RM300" were 6,03 µm and 9,0 µm respectively. All curves show a bimodal distribution. "RM120" has a small part of particles with sizes higher than 10 µm. Further time dependence investigation for ultrasonic treatment are necessary. PCS does not give any information about the shape of the Therefore TiO2 particles were TiO2-nanoparticles. intensively analyzed by electron microscopy (EM) (Figure 3). The EM pictures in Figure 3 show big aggregates of TiO2 in all the samples. To destroy the aggregates ultrasonic treatment is necessary. The main conclusion is that the form of the particles depends on the preparation conditions. Generally, the particles are porous aggregates of spherical shape. The EM-pictures show that the particle population is not uniform in size and shape. The increase in particle size in comparison to the PCS results is caused by aggregation of the smaller particles. The zeta-potential vs. pH behaviour for the four different TiO2 dispersions from Kronos was also studied (Figure 4). The isoelectric point is shifted towards the basic region in dependence on the surface charge. A plateau over a broad pH range was observed for "KR2100". This means that the surface of these particles is covered with basic groups. The most stable dispersion was found for "KR2100". Nevertheless, the dispersion of "KR2044", "KR2047" and "KR2100" are more stable than "RM120", "RM220" and "RM300". The slope is much lower than in Figure 2b. The particle size distribution shows that the four TiO2 products from Kronos have smaller particle sizes of around 330 nm compared with the larger RM products, which have a d50 of up to 9 µm."KR" TiO2 dispersions at pH 9 have particle sizes in the range of 250 nm, which is also about 80 nm lower than the sizes of the samples at pH 6. The larger the particle size the faster the demixing. The different surface charge properties for these particles determine the best result for "KR2100". The EM-picture (Figure 5) shows a smaller particle size than in Figure 3. The EM-pictures are in a good agreement with the results of particle size by PCS.

tails - segments extending into the liquid phase. Except as special cases, the presence of a saturated adsorbed layer always leads to a total stabilization of the dispersion. We observed the best result with the lowest slope for dispersion with "KR2043" dispersed at pH 10 with addition of MSA. This dispersion is more stable than the dispersion of pure "KR2043" in water at pH6 (Figure 6). Similar results were obtained for "KR2044"- "KR2100". More detailed investigations on TiO2 dispersion stability in the presence of various cationic and anionic polymers are currently in progress. Acknowledgements Special thanks to N. Stiehl, M. Oelmann, M. Franke for a large amount of experimental work. This project is supported by BMBF01RC0176. Results at a glance For dispersions in water at pH 6 the best stability was achieved for the particles with the highest charge. The best stability conditions are far from the values at the isoelectric point in the uncharged state. Electron microscopy pictures showed big aggregates of TiO2 in all the RM samples, which could be destroyed with ultrasonic treatment. Generally, the particles (in the RM samples) are porous aggregates of spherical shape and the particle population is not uniform in size and shape. "KR" TiO2 dispersions at pH 9 have particle sizes which are about 100nm lower than the sizes of the samples at pH 6. The authors: > Simona Schwarz, and Gudrun Petzold are at the Institute of Polymer Research Dresden, Germany. Uwe Wienhold works at iLF GmbH Magdeburg of Magdeburg, Germany

Adding macromolecules Colloidal dispersions are two-phase systems comprising the dispersed phase and the continuous or dispersion medium. The mixture is homogeneous over an appreciable time period. By contrast, domestic dispersions such as paint and abrasive cleaners have a shelf-life of several months or years. The properties of the dispersions are determined to a great extent by the nature of the dispersed phase/dispersion medium interface. Macromolecules were added to stabilize the dispersion. Polyelectrolytes are used in many fields in order to influence the surface properties and, therefore, the stability and coagulation properties of the dispersion systems. Macromolecules are adsorbed on the surface with formation of trains, segments attached to the surface, and loops, and



Figure 1a: Streaming potential of TiO2 dispersion in dependence on pH.

Figure 1b: Sedimentation profiles of TiO2 dispersion.

Figure 2: particle size distribution of TiO2 dispersion in water at pH 6.



Figure 3a: EM picture of RM- TiO2 particles.



Figure 3b: EM picture of RM- TiO2 particles.



Figure 3c: EM picture of RM- TiO2 particles.



Figure 4: Streaming potential of TiO2 dispersion in dependence on pH.



Figure 5: EM picture of KR-TiO2 particles.



Figure 6: Sedimentation profiles of TiO2 dispersion with different stabilizers.



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