stabilizing titanium dioxide

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European Coatings Journal

07-08/2004

34

Stabilizing titanium dioxide

Simona Schwarz, Gudrun Petzold, Uwe Wienhold.The characterization of particulate systems in terms ofcharge, size, shape and morphology is fundamental for theoptimization of such processes as stabilization. TiO2dispersions with low particle sizes of about 400 nm andadditional basic surface modification show a good stability.The addition of macromolecules like MSA improves thestability of dispersions.Over the past few years, there has been a rapidly growinginterest in industry in water-based coating materials.Development of such coatings will enable the use of organicsolvents to be reduced. This fact determines the high andconstantly growing potential of these materials forapplication in environmentally friendly technologies.Knowledge of the stability of water-based coatingdispersions is crucial for their practical application. Pigmentsand their properties play an important role in the stability ofsystems, mainly because their properties affect dispersionstability.Pigments and fillers may be titanium oxide, iron oxidepigment, dolomite, chalk, and talc. Titanium dioxide is widelyused as a white pigment in paints, paper, plastics, ceramics,rubber, inks and a variety of other products. Titaniumdioxide used for coating applications is predominantly in therutile form. Rutile TiO2 is modified with thin films of inorganicoxides like alumina, silica or zirconia. TiO2 is easy to wetand to disperse in aqueous media. An important aim is thestabilization of the pigments against flocculation.Systematic investigations of the influence of processingconditions (particle size and particle size distribution, pH,polymer concentration and dispersant stabilisingmechanism) on colloidal processing of TiO2 were carriedout. The goal was to establish a parameter for stabilizationwhich could reliably characterize system stability. Particlecharacterization was the main tool used to evaluate andcontrol the stabilization condition of the powders. The mostimportant factors determining properties of the colloidal TiO2dispersion are conditions of suspensions preparation andstabilization.

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

Dynamic light scattering (UPA), streaming potentialmeasurements (PEL-titration, PCD), REM and "LUMiFuge"were used to investigate the particles. Particlecharacterization methods were used to quantitativelydetermine specific properties of dispersions and to measurecontrol or optimize changes in dispersion states.

Surface charge density and zeta-potentialThe properties of the electrical double layer of the particlesplay an important role in colloids stabilization. The stability ofdispersions can be characterized by electrokineticmeasurements. Zeta-potential is a quantity for the stability ofnet charge at the shear plane. In contrast to other interfacepotentials, the zeta-potential is easily measurable and it isan important experimental parameter characterizing thecharge conditions at the particle surface, the adsorbedlayers and the colloidal stability.Measurements of the electrokinetic (or zeta) potential giveinformation about the existence of acidic or basic moleculargroups on the solid surface and their dissociation constants.They also give information about the interaction between thecomponents of the electrolyte solution and the solid surface.The repulsive forces in a stable dispersion were long agoidentified as being of electrical origin. A surface potentialexists at the interface between the solid particle and thesurrounding liquid due to the presence of a surface charge.The commercial TiO2 were modified with thin films ofdifferent 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 bypolyelectrolyte titration with a particle charge detector (PCD)(Table 1).Knowing the charge density at a certain pH gives a firststatement about a sample's stability. As Table 1 shows, thecharge density at pH 6 was positive for all three RMsamples from Sachtleben and for "KR2100" from Kronos.The best dispersion stability independent of the sign ofcharge was achieved for the samples with high values ofcharge density ("RM120", "KR2044" and "KR2100").

pH-dependenceThe pH-dependence of charge for the different pigmentsmust 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 (Figure1a). The isoelectric point and the shape of the plot provideinformation about the character of the solid surface. Thecurves show a shift of the isoelectric point (the point, wherethe value of the zeta-potential is zero) towards the basicregion depending on the surface charge. Both types ofacidic functional groups exist on the pure TiO2 surface andbasic functional groups are the result of the modification. Aplateau in the basic media is detected for "RM120" and"RM220".The zeta-potential pH profiles are in a good agreement withthe investigations of sedimentation profiles by "LUMiFuge"test (Figure 1b). "RM300" shows a fast destabilization in ashort time of about 50s. The dispersion of "RM120"-particlesis more stable. The supernatant is nearly clear after 180s.To obtain defect free bodies from ceramic slips or smooththin layers from paint slurries, suspensions are required tobe well dispersed stable systems. In any aqueous colloidalsystem the pH is one of the main factors determining the

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stability of the suspensions. Thus, the most frequently usedmethod to obtain well stabilized suspensions is to work atvery high or very low pH values. The best stability conditionsare far from the isoelectric point in the uncharged state.

Particle size, charge and size distributionParticle charge, particle size and particle size distributionare important parameters for a stable dispersion. Adecrease of the mean particle size leads to an increase ofstability of the dispersion.The mean diameter of the bulk population was determinedby photon correlation spectroscopy (PCS; UPA, Grimm) andthe particle size distribution was obtained (Figure 2). Thesmallest particle sizes with values of d50 1,25 µm weredetermined for "RM120". The particle sizes for "RM220" and"RM300" were 6,03 µm and 9,0 µm respectively. All curvesshow a bimodal distribution. "RM120" has a small part ofparticles with sizes higher than 10 µm. Further timedependence investigation for ultrasonic treatment arenecessary.PCS does not give any information about the shape of theTiO2-nanoparticles. Therefore TiO2 particles wereintensively analyzed by electron microscopy (EM) (Figure 3).The EM pictures in Figure 3 show big aggregates of TiO2 inall the samples. To destroy the aggregates ultrasonictreatment is necessary. The main conclusion is that the formof the particles depends on the preparation conditions.Generally, the particles are porous aggregates of sphericalshape. The EM-pictures show that the particle population isnot uniform in size and shape. The increase in particle sizein comparison to the PCS results is caused by aggregationof the smaller particles.The zeta-potential vs. pH behaviour for the four different TiO2dispersions from Kronos was also studied (Figure 4). Theisoelectric point is shifted towards the basic region independence on the surface charge. A plateau over a broadpH range was observed for "KR2100". This means that thesurface of these particles is covered with basic groups. Themost 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 TiO2products from Kronos have smaller particle sizes of around330 nm compared with the larger RM products, which havea d50 of up to 9 µm."KR" TiO2 dispersions at pH 9 haveparticle sizes in the range of 250 nm, which is also about 80nm lower than the sizes of the samples at pH 6. The largerthe particle size the faster the demixing. The differentsurface charge properties for these particles determine thebest result for "KR2100". The EM-picture (Figure 5) shows asmaller particle size than in Figure 3. The EM-pictures are ina good agreement with the results of particle size by PCS.

Adding macromoleculesColloidal dispersions are two-phase systems comprising thedispersed phase and the continuous or dispersion medium.The mixture is homogeneous over an appreciable timeperiod. By contrast, domestic dispersions such as paint andabrasive cleaners have a shelf-life of several months oryears. The properties of the dispersions are determined to agreat extent by the nature of the dispersed phase/dispersionmedium interface. Macromolecules were added to stabilizethe dispersion. Polyelectrolytes are used in many fields inorder to influence the surface properties and, therefore, thestability and coagulation properties of the dispersionsystems.Macromolecules are adsorbed on the surface with formationof trains, segments attached to the surface, and loops, and

tails - segments extending into the liquid phase. Except asspecial cases, the presence of a saturated adsorbed layeralways leads to a total stabilization of the dispersion. Weobserved the best result with the lowest slope for dispersionwith "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 wereobtained for "KR2044"- "KR2100".More detailed investigations on TiO2 dispersion stability inthe presence of various cationic and anionic polymers arecurrently in progress.

AcknowledgementsSpecial thanks to N. Stiehl, M. Oelmann, M. Franke for alarge amount of experimental work. This project is supportedby BMBF01RC0176.

Results at a glanceFor dispersions in water at pH 6 the best stability wasachieved for the particles with the highest charge.The best stability conditions are far from the values at theisoelectric point in the uncharged state.Electron microscopy pictures showed big aggregates of TiO2in all the RM samples, which could be destroyed withultrasonic treatment.Generally, the particles (in the RM samples) are porousaggregates of spherical shape and the particle population isnot uniform in size and shape."KR" TiO2 dispersions at pH 9 have particle sizes which areabout 100nm lower than the sizes of the samples at pH 6.

The authors:> Simona Schwarz, and Gudrun Petzold are at the Instituteof Polymer Research Dresden, Germany. Uwe Wienholdworks at iLF GmbH Magdeburg of Magdeburg, Germany


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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.


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


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


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


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


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Figure 5: EM picture of KR-TiO2 particles.


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


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stabilizing titanium dioxide

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