Technology

Until Aspen Aerogels’ breakthroughs in drying time and material form, aerogels were limited to the realms of intellectual curiosity and research.

Kistler formed the first aerogels in 1931 by replacing the liquid phase in a gel with air. These first aerogels used silica as the solid phase since gels can be formed via polymerization of silicic acid, which is readily generated by acidic neutralization of sodium silicate in water. In this reaction, polysilicic acid rapidly forms a solid network structure that entrains the water to form a nanoporous gel structure or “hydrogel.” This silica hydrogel is then repeatedly rinsed with volumes of fresh anhydrous methanol to remove virtually all traces of water. The contents of the gel are brought past the critical point of methanol in a high temperature autoclave to a supercritical condition. The system is then slowly depressurized while maintaining a temperature that prevents recondensation of methanol within the porous silica gel structure.

Kistler demonstrated that the gel’s pore structure was maintained by drying the gels under supercritical conditions of methanol. He argued that the surface tension between water and the gel structure caused collapse of pores during drying at atmospheric pressure. As he stated in his 1931 paper in Nature, “no evaporation of liquid can occur [under supercritical conditions] and consequently no contraction of the gel can be brought about by capillary forces at its surface.” The inverse relationship between capillary force and pore size makes pore collapse (with a great deal of associated shrinkage) during solvent evaporation overwhelming for materials with pore sizes on the scale of nanometers (10-9 meters).

Removal of entrained solvent under supercritical conditions brings the liquid-vapor surface tension within the pore structures to near zero, and thus strong capillary forces that would collapse the pores are avoided. The result is a dried gel material that has suffered the least amount of shrinkage, retains the highest porosity, and has the highest pore volume and surface area practically attainable. Because the pores are filled with air after drying, they are called “aerogels”. Drying time for aerogels can take many days using these methods.

Most aerogels have been produced as stiff, brittle monoliths from silica gels by carefully removing the solvent via a supercritical fluid extraction method, leaving behind only the network structure of interconnected silica material. The volume previously occupied by the solvent is then occupied by air, typically comprising as much as 99.8% of the volume of the remaining solid. In other words, aerogels have densities as low as 1/500th of the base material. Due to this unique structure, aerogels provide from two to tens times more thermal and acoustical insulating capacity than existing insulations per unit of material.

Despite these advantages, and the application of supercritical drying, processing times for aerogels remained long and arduous, and costs were prohibitive. In addition, monoliths and later forms such as beads and granules were hard to handle, and required different installation procedures than the flexible batting materials that dominate the insulation landscape.

In the mid to late 1990’s, Aspen Systems perfected proprietary changes in the formulation, processing and drying of aerogels, reducing drying time to a few hours. Besides a cost effective drying time, the aerogels were also isolated in the form of thin, flexible blankets. The blankets were much more robust than the monoliths and beads, and could also be easily installed like any other flexible batting. These breakthroughs led to the formation of Aspen Aerogels, Inc. in 2001, and the commercialization of aerogel technology for broad use.

Aspen Aerogels has created a variety of organic and inorganic/organic hybrid aerogel formulations including those made from polydimethylsiloxane/silica, cellulose polyurethane, polyimide, polymethylmethacrylate/silica, and polybutadiene rubber. These materials show enhanced physical and mechanical properties relative to pure silica aerogel.

We presently manufacture silica aerogels in volume in the form of thin flexible blankets reinforced with fibers, with properties as shown below. For more specific data, please check the Product Data Sheets in our Literature Center.

In addition, a libraries worth of additional information about the properties of aerogels, and about aerogels in general can be found at http://eande.lbl.gov/ECS/aerogels/satoc.htm.