Browsing by Author "Stieger, Gregor"

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    Covalent and heterosupramolecular interaction of ceramic particles
    (2002) Stieger, Gregor; Aldinger, Fritz (Prof. Dr.)
    New concepts of particle interaction for the processing of ceramic powders are developed. They are based on chemical reactions either by heterosupramolecular or covalent interaction of proper reactants. For this the particles are functionalized so that they are able to undergo defined reactions with each other. A commercially available beta-cyclodextrin derivative, 3-chlor-5-sodium-hydroxyl-trianzinyl-beta-cyclodextrin, is covalently bound to the Si3N4 surface in an one step reaction. An effortful derivatisation of the cyclodextrin is not required. The triazinyl ring is not bound to a defined position at the cyclodextrin torus (in average 2.8 triazinyl groups per cyclodextrin). In the next covalent approach mono-6-amino-?-cyclodextrin is bound to epoxy-functionalized Si3N4. This route provides a regioselectivity and a spacer between the cyclodextrin torus and the surface ensuring flexibility. The ad- and desorption behavior is investigated by rotation angle measurements, the adsorbed amount is low: approximatively 25% of the total mass is adsorbed. This is in contrast to 9% for the non-modified Si3N4 revealing that mono-6-amino-beta-cyclodextrin binds to the epoxy group. Washing desorbs the loosely bound parts, 17% are still remaining. Because of the seven amino groups of heptakis-6-amino-?-cyclodextrin this molecule binds more easily to epoxy-functionalized Si3N4 than mono-6-amino-beta-cyclodextrin with one amino group. This is exhibited by the intensive DRIFT-signals. The cyclodextrinfunctionalized Si3N4 could heterosupramolecularly be crosslinked by a guest polymer bearing tert-butyl-anilide as an anchor for the cyclodextrin in the side chain. This is interesting for the consolidation step in plastic forming. Al2O3 powder could be cyclodextrin-functionalized in one step by the deposition of a polymeric beta-cyclodextrin derivative. A two layer system consisting of a cyclodextrin polymer and a guest polymer was built up on Al2O3 powder particles. Due to the enlarged steric barrier the dispersibility of the Al2O3 is improved which is interesting for ceramic powder processing. A covalent interaction between two ceramic particles is also achieved by functionalizing Si3N4 with two different commercially available silanes which can react with each other with their non silyl termini thus forming a molecular bridge between particles. 1-triethoxysilyl-3-isocyanato-propane is bonded to Si3N4 and 1-trimethoxysilyl-3-amino-propane reacts with Si3N4. The reaction is indicated in diluted suspension by the formation of agglomerates (50-500 micrometer) and sedimentation. Stable highly concentrated suspensions (20-37.6 vol%) could be prepared with a maximum solid loading of phimax=0.39, too, gelling at the critical volume fraction phicrit=0.35. The data of the relative viscosity etarel in dependence of the volume fraction phi can be fitted with the Mooney equation and the moduli G'(tau) and G''(tau) were measured. The viscosity of the 20-28 vol% gels is lower and G' and G'' have an additional cross over compared to suspensions of the iso-cyanatofunctionalized Si3N4 and the aminofunctionalized Si3N4. Concluding, Si3N4 powder could successfully be crosslinked by using low cost silanes. This technique is a promising possibility for the consolidation step in plastic forming.