Preparation and properties of potassium-vermiculite films
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Preparation and properties of potassium-vermiculite films Caroline Minker-Villemin, Paul Bowen, Jacques Lemaˆıtre, and Terry A. Ringa) Powder Technology Laboratory (LTP), Materials Department, Swiss Federal Institute of Technology (EPFL), 1015 Lausanne, Switzerland (Received 11 June 1996; accepted 25 April 1997)
Flexible films of vermiculite have been prepared from aqueous suspensions after swelling by cation exchange and mechanical delamination. Two different swelling cations, lithium and butylammonium, have been investigated. The degree of swelling and delamination during the suspension preparation was characterized by the percentage of water reabsorbed by dried clays and the adsorption of methylene blue. The vermiculite saturated with lithium ions is more easily delaminated but contains more water than those saturated with butylammonium. Good quality coherent flexible films could be prepared from both the lithium and butylammonium exchanged vermiculites but the high percentage of water found in the films has a detrimental effect on their dielectric properties. To reduce the amount of water in the exchanged vermiculites a second ion exchange with potassium, a less hydratable cation, was investigated. Films prepared after exchange with potassium showed significant improvements in their dielectric properties, with a dielectric constant e around 10 and a dissipation factor tan d around 0.06 at 25 ±C and a frequency of 1 kHz.
I. INTRODUCTION
Vermiculite is a layered sheet silicate of the phyllosilicate family and originates from the alteration of mica. The potassium interlayer cation which maintains the charge neutrality of the sheets in mica is replaced by a hydrated cation, normally Mg in the case of vermiculite.1 The physical properties of micas and vermiculite which render these materials interesting are their low thermal conductivity, high thermal and chemical stability, and particularly their low electrical conductivity. Sheet micas have been used in electrical equipment as an electrical insulator for many years2 and although in many applications they have been replaced by polymeric films, their high thermal stability still render them useful for certain applications. One of the limitations of sheet micas is their fragility and consequent low flexibility which limits their application for certain geometries. The fact that the interlayer cation in vermiculite is hydrated allows it to be swollen by a treatment in aqueous salt solutions,3,4 and when followed by delamination flexible films can be produced.5 The production of flexible mica films is rendered more difficult because they do not swell and delamination by grinding is difficult although some high surface area micas have become available more recently.6 The vermiculite films
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Current address: Department of Chemical and Fuels Engineering, University of Utah, 3290 Merrill Engineering, Salt Lake City, Utah 84112.
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http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 1, Jan 1998
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