(204f) Comparative Adhesion Force Measurements on Conventional and Novel Nanostructured and Highly Porous Filtration Membranes

Introduction and Objective: Gas filtration of fine particles and, in particular, the complete recleaning of conventional filtration membranes meets with a significant problem. The flow dynamics forces of conventional recleaning methods do not or hardly allow for the recleaning of fine particles. From the point of view of filtration technology, a filtration membrane with a very small adhesion force on the surface only could ensure reliable recleaning operation. Nanostructured, highly porous filtration membranes promise to be successful in reducing the adhesion of dust particles and entire filter cakes [1]. This is achieved by the reduction of the absolute number of contact points on the surface and by minimizing the individual adhesion forces between the dust particle and membrane grain. As these filtration membranes are planned to be operated at high temperatures, only ceramic membrane grains can be used. The work described here is aimed at optimizing the structure of these filtration membranes in terms of minimized adhesion forces. The potential of nanostructured membranes shall be demonstrated by a first comparison of the adhesion forces of particles on these membranes and on conventional hot-gas filtration membranes.

Fundamentals: When assuming van-der-Waals forces, the adhesion forces between dust particles and filtration membrane can be described theoretically by the microscopic model of Hamaker or the macroscopic model of Lifshitz for plate-sphere and sphere-sphere geometries. The bodies are postulated to be ideally smooth and rigid. Accordingly, adhesion force is influenced by the geometries and the material-dependent Hamaker or Lifshitz van-der-Waals constants. Modified models accounting for structured particles or a structured surface in the form of semi-spherical roughnesses have already been discussed [2] and extended [3]. Roughness elevations of 10 to 200 nm in diameter cause a reduction of the adhesion force due to the increased contact distance between the adhesion partners. Direct relation to the number of contact points of a particle adhering to a structured surface, however, is still lacking.

Material and Methods: The membranes are produced by filtrating gas-borne nanoparticles from a sprayed nanoparticle suspension onto porous substrates. Such a substrate may be a conventional hot-gas filtration membrane. Using this method, very homogeneous, nanostructured membranes with porosities of up to 90% and layer thicknesses between 10 and 50 µm can be obtained, depending on the process conditions. On the laboratory scale, the nanostructured filtration membrane with a mean grain size of γ-Al2O3 nanoparticles of about 180 nm was produced at a filtration velocity of 7.5 cm/s. To measure the adhesion force of the fragile structure, a cyanacrylate vapor flow was passed through the membrane. This vapor condenses in the gaps, i.e. at the weak points of the membrane, and solidifies in humid air. Adhesion forces on the nanostructured membrane surface (porosity larger than 80%, mean grain size 180 nm, surface material: Cyanacrylate) were measured and compared with those of a conventional hot-gas filtration membrane (porosity about 40%, mean grain size about 10 ? 40 µm, surface material: Mullite).

To measure the adhesion forces, the centrifugation method was applied. The adhesion force is defined as the force separating individual particles at room temperature. Onto the membrane surfaces (dimensions 10.8 x 10.8 x 3.6 mm, laser-cut), monodisperse, spherical model dust particles (material: Melamine resin with a black polypyrrol coating, particle diameter: 9.7 µm, variation coefficient: 1.94%) were deposited in an aqueous solution. In this way, a large number of individual particles could be positioned on the area studied (in total 36 mm2 each). Measurements were performed after the removal and drying of the samples in a desiccator in air. Prior to centrifugation, the initial particle number was counted under a light microscope. Preliminary studies have shown that an initial particle number of 800 is statistically sufficient. The particle-loaded membrane surfaces were subjected to a centrifugal force that increased in steps, with each centrifugation step lasting 5 min. The particles remaining on the surface were counted after each centrifugation step. Correlation of centrifugal forces with the remaining particles yields the separation force distribution. From this, the mean adhesion force of the particles can be defined for 50% centrifuged particles. Adhesion force distributions vary depending on the membrane structure and the homogeneity of the surface. A measure of the width of adhesion force distribution is the interquartil range. It is defined to be the separation force required for detaching 75% particles minus the separation force needed to detach 25% particles.

Results First adhesion force measurements were carried out using conventional mullite membranes. The influence of the relative air humidity and electric charge were determined, as were local disruptions of the brittle ceramic surfaces due to centrifugal loading. As a result, characteristic separation force curves were obtained that could be distinguished clearly. When conducting the test under certain conditions (for example, at 50% relative air humidity), the van-der-Waals forces predominated and measurements could be reproduced well. For the polypyrrol particles, mean adhesion forces of 181 nN were measured on the mullite membrane. These values by far exceeded the value of 51 nN obtained for the nanostructured filtration membrane. The influence of the material on the van-der-Waals adhesion forces measured was discussed on the basis of the experimentally determined Hamaker constants. The interquartil range on the nanostructured membrane of 169 nN was less than half of the value measured on the conventional hot-gas filtration surface (436 nN). This suggested a nanostructured membrane surface with very homogeneously distributed contact points for the adhering particles, which is also confirmed by scanning electron microscopies: The membrane is smooth with a homogeneous grain size and porosity distribution, whereas the conventional hot-gas filtration membrane exhibits large variations of pore and grain sizes and appears to be very rough due to the non-spherical grains. It was also obvious from the scanning electron microscopies that particles have sunk into the soft membrane structure. This may be due to strong capillary forces during drying or to the attractive van-der-Waals adhesion forces between the membrane surface and the particles (enhanced adhesion force by elastic and plastic deformation, as discussed by Rumpf [4], under the impact of adhesion forces and external forces).

Summary and Outlook To sum up, the study demonstrates that adhesion forces can be reduced considerably by a nanostructured, highly porous filtration membrane as compared to conventional hot-gas filtration membranes. By systematically varying the filtration velocity (in the range of 2.5 ? 10 cm/s) and grain size distribution (mean grain size about 100 -200 nm), various membrane structures shall be manufactured in the future and characterized in terms of grain size distribution, porosity, and porosity distribution. Adhesion force measurements shall provide information on how the structural parameters influence the adhesion forces. It is planned to extend the adhesion force measurements in the future to cover even smaller model dust particles below 5 µm in order to quantify the influence of the particle size on the adhesion forces. Moreover, dry particle deposition will replace the sedimentation method in order to determine the influence of capillary effects on membrane deformation.

Literature [1]: B. Zimmerlin, H. Leibold, H. Seifert: "Performance of Nano-scaled Ceramic Membranes during Fine and Sticky Dust Filtration", in "High Temperature Gas Cleaning", Vol. II, ed. A. Dittler, G. Hemmer, G. Kasper, 1999, pp. 71-82 [2]: H. Rumpf: "Particle Technology", Chapman & Hall, London/New York, 1990 [3]: Y. Rabinovich et al.: "Adhesion between Nanoscale Rough Surfaces, I. Role of Asperity Geometry", Journal of Colloid and Interface Science 232, 2000, pp. 10-16 [4]: H. Rumpf, K. Sommer, K. Steier: "Mechanismen der Haftkraftverstärkung bei der Partikelhaftung durch plastisches Verformen, Sintern und viskoelastisches Fließen", Chemie Ingenieur Technik 48/4, 1976, pp. 300-307