A Statistical Design of Experiments: Analysis of the Aerogel Production Process

The unique characteristics of silica aerogels allow them to be used in a wide variety of applications. They are extremely porous and have a low density, low thermal conductivity, large surface area and are relatively translucent. They can be used in a wide variety of applications such as in window systems, acoustic devices and in insulation. Making a large monolithic aerogel requires the use of a supercritical extraction process. At Union, we use the patented Rapid Supercritical Extraction (RSCE) method which uses a hydraulic hot press and confined metal mold, and have produced a 10 cm x 11 cm x 1.5 cm aerogel in a little over 10 hours. Various studies have been completed to understand what factors lead to the formation of cracks in the silica aerogel monoliths under some processing conditions. In this work, we statistically analyzed the factors that have the largest effect on the quality of the aerogel, i.e., the amount of cracking in each sample. The goal was to minimize cracking, thus maximize the quality. An initial screening design studied the effects of the heating rate, cooling rate, maximum temperature, sealing force, amount of grease, catalyst gel time, and force release rate, and found that the heating and cooling rates were the most important factors. A low heating rate and high cooling rate were found to best prevent crack formation. From this, a second screening was set up to look more closely at the cooling rate (how fast) and the length of the dwell time used at high temperature and pressure before the actual supercritical extraction step. In the end, we developed a set of parameters to produce a crack free aerogel that included a heating rate of 2 °F/min, cooling rate of 30 °F/min, and a dwell time of 60 min. Using these settings, it only takes a little over 6 hours in the hot press to produce an aerogel which is a 40% reduction in production time.