“Aerogel is a synthetic porous material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and thermal conductivity. It is nicknamed frozen smoke, solid smoke, solid air or blue smoke owing to its translucent nature and the way light scatters in the material; however, it feels like expanded polystyrene (styrofoam) to the touch.
Aerogel was first created by Samuel Stephens Kistler in 1931, as a result of a bet with Charles Learned over who could replace the liquid in "jellies" with gas without causing shrinkage.
Aerogels are produced by extracting the liquid component of a gel through supercritical drying. This allows the liquid to be slowly drawn off without causing the solid matrix in the gel to collapse from capillary action, as would happen with conventional evaporation. The first aerogels were produced from silica gels. Kistler's later work involved aerogels based on alumina, chromia and tin dioxide. Carbon aerogels were first developed in the late 1980s.”
(Wikipedia, Aerogels, 7/27/2012)
“The formation of aerogels, in general, involves two major steps; the first is the formation of a sol-gel like material, and the second is drying of the sol-gel like material to form the aerogel. In the past, the sol-gel like materials were made by an aqueous condensation of sodium silicate or a similar material. While this process works relatively well, the reaction forms salts, within the gel, that need to be removed by an expensive ion exchange technology and repetitive washing. This common technique thereby renders this production process time consuming, expensive and laborious.
With the recent development of sol-gel chemistry over the last few decades, a vast majority of silica aerogels prepared today utilize silicon alkoxide precursors. The most common silicon alkoxide precursors are tetramethyl orthosilicate (tetramethoxysilane (TMOS) Si(OCH.sub.3).sub.4) and tetraethyl orthosilicate (tetraethoxysilane (TEOS) Si(OCH.sub.2CH.sub.3).sub.4). Less common aerogels can be produced from other metal alkoxides, such as tertapropoxyzirconium. However, many other alkoxides, containing various organic functional groups, can be used to impart different properties to the gel. An alkoxide-based sol-gel chemistry generally avoids the formation of undesirable salt byproducts and allows a much greater degree of control over the final product. The balanced chemical equation, for the formation of a silica gel from TEOS by a conventional method, is: Si(OCH.sub.2CH.sub.3).sub.4(l)+2H.sub.2O(l)SiO.sub.2(s)+4HOCH.sub.2CH.sub- .3(l)
For many applications of aerogels, the areogels are used in a granular form. Since aerogels provide superior insulation capabilities, compared to a variety of other materials, the aerogel granules are commonly used to fill cavities, containers, bags, panels, vessels, etc. (all of which are hereinafter collectively referred to as a "panel, container or structure"), for a variety of insulating applications. A key advantage for silica based aerogels is that they can be produced with a very low index of refraction while still having a high light transmission through the particles. The combination of the high light transmission and excellent insulation values of the silica based aerogels make them particularly suited for the needs of the fenestration industry.
For some applications, hydrophobing agents are added to the sol-gel formulation, or the sol-gel is soaked in a solution composing an organic solvent and hydrophobing agent. The hydrophobing agent then chemically attaches to both the interior and the external surfaces of the gel particles. Once these hydrophobing agent modified aerogels are dried, the aerogel then becomes hydrophobic. Known processes for producing hydrophobic aerogels are described, for example, in U.S. Pat. Nos. 5,496,527 and 5,830,387.
Unfortunately, low density silica based granular aerogels tend to be very delicate and friable. The handling procedure for such aerogels, typically required to fill a panel, container or structure with the aerogel particles, often results in some breakage or fracturing of the aerogel particles. Such breakage and/or fracturing of the aerogels, in turn, results in the development of aerogel fines or dust in the end product. These fines, if allowed to freely build up within the filled panel, container or structure, can result in a significant end product defect. The dust and/or fines can eventually settle or stratify thereby creating undesirable degradation of the aerogel properties, such as local differences in light transmission, air gaps, loss of insulation, and a cosmetically unappealing end product appearance. The dust also tends to be difficult to remove from a surface in the event that the aerogel product either spills or leaks out of the filled panel, container or structure.
It is also to be appreciated that as the aerogel flows into a panel, container or structure, a significant amount of static charge can be generated. This charge tends to impede the desired close, tight and uniform packing of the aerogel within the panel(s), container(s) or structure(s) and can cause aerogel particles to collect or build-up on surfaces, clothing and/or skin of the packaging personnel. Moreover, it is to be appreciated that high levels of static also can create a serious risk of either a fire or an explosion. Special aerogel filling procedures, such as those described in U.S. Pat. No. 7,621,299 which involve vibration, static charge dissipation and tapping, are often utilized in an attempt to facilitate tight, close packing of the panel, container or structure, with the aerogel granules. Notwithstanding such special aerogel filling procedures, further settling of the filled aerogel granules, during subsequent product handling, shipping and/or installation of the panel, container or structure, often occurs.
[Keller et al, US Patent 8,217,087 (7/10/2012)]
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Roger D. Corneliussen
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Copyright 2012 by Roger D. Corneliussen.
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* Date of latest addition; date of first entry is 7/10/2012.