Papers by Author: Anja Caspari

Abstract: There is evidence that the fiber-matrix interaction in fiber-reinforced cementitious composites is determined by the wettability and roughness of the fibers. Due to the high ionic strength in the hydrated cement, also the surface charge or acid/base behavior of the fibers is assumed to play a role. To create fibers with alkaline and acidic surface functionalities, water-insoluble poly (vinyl alcohol) fibers were permanently modified by adsorption of various polyelectrolytes. X-ray photoelectron spectroscopy, zeta potential, and contact angle measurements revealed acidic, alkaline or amphoteric fiber surfaces with advancing water contact angles between 34° and 58°. In a first step to study the interaction with cementitious materials, the interaction of these fibers with pore solution (the liquid phase of hydrated cement) and the adsorption of calcium ions on the fiber surface were investigated. The work will be continued by studying the fiber-matrix interaction in cementitious matrices and its influence on the composite strength.

Abstract: It is well known that colloid-chemical aspects, such as agglomeration processes, wetting and adsorption phenomena, have a decisive influence on the separation behaviour and coating quality of a composite plating. The following processing steps for electrocodeposition have to be considered: preparation of a stable dispersion of the particles in the electrolytic bath, transportation of the particles to the metal surface, adhesion of the particles onto the surface, incorporation of the particles in the metal matrix. Celis [1,2] and Hyashi [3] could show that ion adsorption onto the particle surface is very important for electrophoretic mobility and layer quality. On the other site, Fransaer and others [2,4] showed that surface free energy plays an important role for incorporation of particles in a metal matrix. They could demonstrate that hydrophilic particles do not make contact with the electrode, probably due to repulsive hydration forces. Hydrophobic particles make contact with the electrode, due to an attractive hydrophobic force. Hence it is important to have a method for estimating the hydrophilic/ hydrophobic surface properties of such particles to select a suitable surface modification strategy. A direct way to measure the surface free energy of solid particles is not available so far. Therefore, it is generally accepted to use the phenomenon of capillary penetration of liquids into porous media to determine the wetting properties of particles by measuring the penetration velocity of well-defined liquids in a powder packing. The kinetics of penetration correlates mainly to the geometric structure of the powder packing and the wettability of the particles. By using the equation-of-state approach for solid-liquid interfacial tensions the solid surface free energy of the particles can be determined [5]. In this paper, we show the usefulness of capillary penetration experiments and discuss some parameters that should be considered for the interpretation of the data. Ion adsorption processes, on the other hand, can be described by electrokinetic measurements [6,7].

Abstract: The process of electrophoretic deposition depends strongly on the electrokinetic properties and with it the surface properties of the material that will be processed. Different additives, conditioners but also the suspending liquid influence the surface of the applied material by adsorption. Electrokinetic investigations reflect changes in properties at the outermost solid surface very sensitive. Streaming potential measurements are especially suited for studying such changes of surface chemistry at solids with different shapes. Two approaches are applicable: 1. The adsorption process was done before measuring. The result of this process should be shown. In this case it will be interesting to see differences in the functionality of the solid surface. The zeta potential will be measured versus different pH value. 2. The adsorption process will be studied directly. The zeta potential will be determined versus the concentration of the adsorptive. The second approach can be used for investigation of adsorption of multicomponent mixtures. Competing adsorption processes are detectable.