Nanoparticle-Production in stirred media mills

Transcription

1 Production Grinding & Dispersing 1 Research & Development Application Nanoparticle-Production in stirred media mills Sandra Breitung-Faes, Arno Kwade Institute for Particle Technology, Technical University of Braunschweig Volkmaroder Straße 5, D Braunschweig, Germany Tel.: , s.breitung@tu-bs.de ABSTRACT The possibility of grinding Al 2 O 3 down to 10 nm in aqueous suspensions could be shown recently. Therefore steric and electrostatic stabilization mechanisms were adopted to avoid agglomeration processes during grinding process [1, 2, 3]. Based on these results the effects of changing the fluid from water to ethanol and the stabilization mode are investigated. Ethanol is a less polaric fluid than water, but it is applicable as well for electrostatic stabilization as for steric stabilization. As grinding media native grinding media are used, so no strange particles or ions could arise. Especially, the influence of the stabilisation mode on the grinding result and the grinding media wear in an organic fluid are observed and compared with the results achieved with aqueous suspensions Introduction The production of Al 2 O 3 -particles with sizes down to 10 nanometers by grinding in a stirred media mill with standard disc agitator could be shown recently in a joined project with the Institute of Particle Technology Erlangen- Nuremberg [1, 2, 3]. However, this process is not very economic due to high specific energy consumption as well as high grinding media wear which leads to high operation costs and non acceptable product contamination. The grinding-process of fused corundum in a stirred media mill could be improved due to the adoption of native grinding media and a modified electrostatic stabilization in an aqueous suspension. This leads to a reduction of the costs resulting from the grinding media wear of more than 80 % compared to yttrium-stabilized zirconium oxide grinding media and to a stabilization using nitric acid [2]. Particles with sizes below 100 nm dispose of high attractive surface forces the so called van der Waals-forces. These forces lead to agglomeration of the particles during grinding which is very counterproductively, because it is very difficult to break such agglomerates. Therefore, a good stabilization during this process is essential. Mainly there are two ways to stabilize a suspension: electrostatic or steric stabilization. For electrostatic stabilization potential determined ions are given to the suspensions, which build an electrical double-layer around the particles. These layers push-off each other. These repulsive forces act contrary to the attractive van der Waals-forces. In a well stabilized suspension the repulsive forces have to be higher than the attractive forces. Advantages of this stabilization mode are the fast diffusion of ions and no product-contamination. Moreover, the process is easy to control, because the zeta-potential can be measured and directly be connected with the degree of the repulsive forces [3]. In case of steric stabilization molecules are added to the suspension. These molecules are normally built out of two parts: a stabilizing and an anchor segment. The stabilizing segment should be well soluble in the fluid whereas the anchor segment should build a strong connection to the particles surface 2

2 by adsorption or chemisorption. This leads to molecule layers around the particles, which should act as a kind of spacer between the particles. The distance between two particles has to be higher than the distance where the attractive forces act. Here both types of stabilization are used. First the experiments are carried out in an aqueous environment, and secondly, based on the achieved knowledge transferred to ethanol. Experimental Setup Material Grinding material The fused alumina with the trade name Martoxid MZS1 from the company Martinswerk was used for the grinding experiments carried out in water and ethanol. The median particle size of the material is located between 1.5 and 1.9 µm. The fused corundum has a purity of 99.8 %. Stabilizator for electrostatic stabilization: Electrostatic stabilization in aqueous suspensions was carried out with nitric acid as potential determined ion donator. The ph-value was adjusted at ph 5 for the whole experiments because the zeta-potentials reached high values at this ph range. Stabilisator for steric stabilization: As dispersing agent the polymer DAPRAL GE 202 (Akzo Nobel Chemicals, Italy) an alternate maleic anhydride α-olefin copolymer with lipophobic poly(ethylene glycol) and lipophilic polyolefin side chains, was used. Due to its unique structure that incorporates both lipophilic and lipophobic side chains this polymer adsorbed on oxide surfaces from various solvents of different polarity in various conformations. The structural formula of the used polymer is shown in Fig. 1. In this study DAPRAL GE 202 with a kinimatic viscosity of 22 cm 2 /s measured at 40 C was used. The synthesis of this type of polymer is described elsewhere [5]. The molecular weight of DAPRAL GE 202 is approximately g/mol. Fig. 1: Structure of the polymer DAPRAL GE 202 Grinding media: As grinding media Al 2 O 3 -grinding media with a purity of 99.6 % are used. They are produced with a sol-gel-process, which leads to a very consistent structure. The mean crystal size is about 200 nm. Moreover, they have the advantage that no contamination of the product occurs by the wear particles. The grinding media have a median size of 420 µm measured by image analysis. Grinding Process The milling experiments were performed in a commercial laboratory mill (LabStar, Netzsch, Germany). As shown in Fig. 2 the experiments were carried out in circular mode. The measurement equipment for the conductivity and the ph-value as well as the dosaging device of the stabilizing agents are located in the stirred vessel,. The mill is equipped with a centrifugal separating system for the grinding media. This allows the usage of small grinding media down to 0.1 mm. For wear protection reasons the grinding chamber and the stirrer consist out of polyurethane. Due to bad heat transfer of the polyurethane coating the experiments were carried out with moderate stirrer tip speeds of 9 m/s. 3 4

3 Fig. 2: Experimental setup Analytical methods The particle size and the zeta-potential were measured with the DT1200 (Dispersion Technology), which uses ultrasonic spectroscopy. Besides the ultrasonic spectroscopy a sedimentation method was used for particle size characterisation. The CPS disc centrifuge possesses particle size measuring range of 50 µm down to 5 nm. The advantages of the disc centrifuge are the very small sample volumes and the relatively quick measurement. The disc centrifuge was used for the observation of the process because for the dosage of the steric stabilizator the current particle size distribution in the process has to be known. After each experiment the rheological properties of the suspension was characterized by a Rheometer (Bohlin). The shear rate was given and the shear stress measured. Therewith, the dynamic viscosity for a given shear rate could be detected. 5 Results Grinding result The grinding processes were carried out with a stirrer tip speed of 9 m/s and a solid concentration of 20 %. After each experiment the particle size was analysed with the ultrasonic spectrometer and the sedimentation disc centrifuge. The comparison of these both measurement methods gives insights to the real grinding process and the agglomeration degree of the particles after leaving the milling process. The ultrasonic spectrometer detects mainly the primary particle size of the grinding product. Only strong agglomerates which move as one particle in the ultrasonic spectrometer are detected as agglomerate. So the real grinding process could be observed. The measurements with the disc centrifuge show the degree of agglomeration due to the difference in particle size compared with the ultrasonic spectrometer. In the following three types of suspension systems were adopted to the grinding process. 1. Aqueous suspension with electrostatic stabilization 2. Aqueous suspension with steric stabilization 3. Ethanol suspension with steric stabilization The grinding results are shown in Fig. 3. The median particle size is plotted versus the specific energy input. The specific Energy is determined by E m, V () t = m F () t E 1 + Δm 2 = t ( P( τ ) P0 ) 0 m F Δm dτ Eq. 1

4 Fig. 3: Comparison of the grinding results at different stabilization mechanism in aqueous and ethanol suspensions Fig. 4: rheological behaviour of the different suspensions during the grinding process The values for the particle sizes differ as function of stabilization mode and particle size measurement method. The aqueous suspensions show a good grinding progress regarding the primary particle size. But they show also agglomeration at the end of the grinding process, whereas the agglomeration sets in earlier for the steric stabilized suspension. The primary particles milled in ethanol compared to the aqueous ones are coarser, but nearly no agglomeration processes could be detected. This could be seen in the similar results by the particle size measurements with the DT 1200 and the CPS disc centrifuge. Fig. 4 shows the rheological behaviour of the different suspensions during the grinding process. Here the shear stress is plotted versus the shear rate. 7 For the aqueous suspensions a direct connection between the agglomeration degree and the rheological behaviour could be observed. In the moment where the first agglomerates could be detected in the disc centrifuge the shear stress increases if higher shear rates, like in the grinding chamber are acting. The behaviour of the ethanol made suspension shows a completely other behaviour: First the zero - shear rate relating to the grinding time shows higher values than the aqueous suspensions. Moreover, the increase of the shear stress during the grinding process could not be ascribed to the starting agglomeration process as the results from the particle size analysis show. This is an indicator for the different behaviour of the DAPRAL GE 202 in the different suspension. Rheological investigation for two component systems 8

5 water/dapral GE 202 or ethanol/dapral GE 202 clearly show that the viscosity of the water-dapral GE 202 mix increases with increasing DAPRAL GE 202 concentration whereas an increasing DAPRAL GE 202 concentration does not influence the viscosity of the ethanol-dapral GE 202 mix. It seems as if the DAPRAL GE 202 gets or looses protons so charges in the molecule occur. The rheological behaviour is not the reason for the good stability and the worse grinding result of the ethanol suspension because in the ranges of high shear stresses like in the grinding chamber no differences between aqueous and ethanol suspensions occur. Therefore, the molecular size of DAPRAL GE 202 in water and ethanol was measured, again with the disc centrifuge. The results show particle size distributions in the range of 50 nm to 80 nm in deionised water and 70 nm to 200 nm in ethanol. Maltesh et. al. [6] postulated a size of the DAPRAL GE 202 in water of about 70 nm which is in the same range as measured here. This fact shows that the molecule unfolds itself much better in ethanol than in water because water has a more polaric character than ethanol. If these results are taken into a particular system it gets obvious why the grinding result is worse and the suspension stability better in ethanol. The carboxyl of the DAPRAL GE 202 is the anchor segment of the molecule. The ethoxyl and the alkyl chains should build the stabilising segments. In water ions are built and maybe watery bridges which lead to worse unfold of the polar ethoxyl group. The alkyl group is building a clew because of its totally non polaric character. In ethanol no watery bridges and fewer ions are build so the ethoxyl group unfold much better. Due to its less polaric character the clew building of the alkyl group might be less significant. This means: The polymer layer around the particles suspended in ethanol has a greater thickness than in water which leads to a better stabilisation of the particles due to greater distances between them. a worse grinding result because more energy is lost by damping effects of the polymer layer fewer shear stress increasing during the grinding process because more polymer is adsorbed at the particle surface and less polymer is in the suspension. It seems that the shear stress increasing in the aqueous suspension originates on the one hand from the stabilized particles but on the other hand from the DAPRAL GE 202 which is free soluted in the water. Grinding media wear Besides the grinding result the grinding media wear is an important subject for the quality of a grinding process. Here native grinding media are used so no product contamination takes place. Fig. 5 shows the product relating grinding media wear versus the median particle size of the product. The product relating grinding media wear is defined as: Δm m p Δm = Δm + m Feed Eq. 2 The comparison of the results achieved in aqueous suspension show nearly no differences although the rheological behaviour in the low shear stress area is so different. In the grinding chamber high shear stresses are acting and if the rheological curves are extrapolated to higher shear stresses is gets obvious that no differences could be anticipated. In aqueous suspensions the grinding media wear is not influenced by the stabilization mechanism. This could also be seen if the results are compared with the experiments done in ethanol with steric stabilisation. 9 10

6 Fig. 5: product relating grinding media wear relating to the median particle size Conclusion Electrostatic stabilisation in aqueous suspension is well adoptable in grinding processes of ceramics due to its easy controlling and adjusting. Beside the electrostatic stabilisation steric stabilisation in aqueous suspension was investigated. To transfer the acknowledgement of these experiments onto experiments carried out with different organic solvents, an oligomer was chosen which contains hydrophilic as well as hydrophobic chains: DAPRAL GE 202. The comparison of the two stabilisation mechanisms in aqueous suspensions shows a similar behaviour for both types of stabilization. The dynamic viscosity or the shear stress is increasing with the degree of 11 agglomeration during the milling. Both stabilization mechanisms are not able to avoid agglomeration processes during the milling experiment, but with the electrostatic stabilisation the agglomeration sets in later. The achieved primary particle size is nearly the same although the rheological data show differences. The reason for this might be the difference in the shear rates. For the measurement of the rheological behaviour comparative low shear rate are used. The shear rates in the grinding chamber are located at higher values. If the curves from the measurement device are extrapolated to higher shear rates the shear stresses are similar. First experiments with steric stabilization in a suspension made of ethanol and aluminium oxide were carried out. The results show more or less great differences if the solvent medium is changed although ethanol is also characterised through a polaric character. It could be shown that the unfold of the DAPRAL GE 202 in ethanol is better than in water. Therewith, a better stabilization occurs during the grinding process. This could be seen if the particle size measurement results from the ultrasonic spectrometer are compared with the results from the disc centrifuge because the ultrasonic spectrometer measures primary particle sizes whereas the centrifuge leads to agglomerate sizes. In ethanol the values of both devices are nearly the same, which shows the low agglomeration degree of the suspension. All in all the grinding result is worse compared to aqueous suspensions. This leads to the assumption that energy is lost due to the disintegration of the oligomer layer around each particle. Acknowledgement We like to thank the Institute of Particle Technology Erlangen-Nuremberg for scientific discussions and support. We further acknowledge financial support from the German Research Foundation (DFG). 12

7 Symbol Index E(t) [J]: energy input during the comminution time t P(τ) [W]: power at the time τ P 0 [W]: no-load power m F [kg]: feed material Δm [kg]: grinding media wear References [ 1 ] Mende, S.; Stenger, F.; Peukert, W. and Schwedes, J.: Mechanical production and stabilization of submicron particles in stirred media mills, Powder Technology 2003, 132(1):64 73 [ 2 ] Stenger, F.; Mende, S.; Peukert, W. and Schwedes, J. Nanomilling in stirred media mills, Chem. Eng. Sci.2005, 60 (16), [ 3 ] Breitung, S.; Kwade, A.; Sommer, M.; Peukert, W. Nanomilling in aqueous and organic suspensions with stirred media mills. 4. Colloquium "Grinding and Dispersing with stirred media mills" 2005, [ 4 ] Breitung, S.; Kwade, A. Einsatz unterschiedlicher Rührwerkskugelmühlen und Stabilisatoren für die Erzeugung von Nanopartikeln. Produktgestaltung in der Partikeltechnologie 2006, Band 3, 21-35, [ 5 ] Tanchuk, Y.V. Kataliz i neftekhimiya, 1(3), [ 6 ] Maltesh, C.; Xu, Q.; Somasundara, P.; Benton, W. J.; Nguyen, H. Aggregation Behavior of and Surface Tension Reduction by Comblike Amphiphilic Polymers; Langmuir 1992, 8,