Surfactants and Adherent Silicon Resins - New Protective Agents for Natural Stone
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SURFACTANTS AND ADHERENT SILICON RESINS NEW PROTECTIVE AGENTS FOR NATURAL STONE ROLF SNETHLAGE* AND EBERHARD WENDLER** *Bavarian State Conservation Office, Central Laboratory, Luisenstrafe 37, D-8000 Munich 2, Federal Republic of Germany **Geological Institute of the Ludwig-Maximilians-University, Luisenstrape 37, D-8000 Munich 2, Federal Republic of Germany ABSTRACT The present paper reports investigations on the application of surfactants in the stone conservation. It is found that alkyl ammonium chlorides reduce the hygric dilatation of clay-rich sandstones to half of the original value. Polysiloxane films can be tightly bound to clay mineral basal planes using aminoalkyl silane as primer. The water uptake as well as the hygric swelling are markedly reduced. Well defined mixtures of ethanolic alkyl silane and aqueous aminoalkyl silane solutions form stable microemulsions. They can be used as commercial conservative products which are compatable with the environment. INTRODUCTION Silicon organic chemicals have been used for the conservation of natural stones for more than twenty years. The basic idea for the application of ethyl silicate (i.e. its reaction product Si02-gel) as consolidant is based on the empirical observation that the weathering of a stone can be described in terms of a loss of strength of the stone structure. Stone conservation therefore should return this strength to the level of the unweathered stone by applying a consolidant like ethyl silicate. On the other hand it is known that the presence of water is responsible for all weathering processes. With this in mind it is concluded that to preserve stones from further decay hydrophobing chemicals such as alkyl trialkoxy silanes must be applied which are able to protect the stones against water by forming a polysiloxane network. Both approaches, however, are based on overall, macroscopic knowledge; they do not consider the essential intergranular interactions which define the cohesion and the strength of the grain structure of natural stone. The goal of the present paper is to show that these intergranular interactions can be influenced by surfactants, and that the effect of organosilicon compounds can be improved by adding surfactants. The investigations are carried out on sandstones, but will be extended to limestones and marbles in the next future. THE ROLE OF SURFACTANTS IN THE GRAIN STRUCTURE The mineral grains of sandstones - quartz, feldspar, clays - have a negative surface potential. The cohesion of the grains is caused by - water films - electrostatic forces - van der Waals forces. Mat. Res. Soc. Symp. Proc. Vol. 185. ,1991 Materials Research Society
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The strength of the cohesion is influenced by the amount and the type of binding cations which balance the negative surface charge. To influence the grain-grain interaction, an exchange of the binding cations against cationic surfactants is necessary. It is known [1] that alkylammonium ions are able to displace the binding cations like Na+, Ca +, Mg2+ and, to a limited extent even K-.
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