저밀도 실리카 에어로젤은 두 단계 솔-젤공정에서 습윤젤을 제조하고 이를 초임계 유체를 이용한 건조과정을 거쳐서 얻어진다. 첫 번째 솔-젤공정 단계에서는 상호 불용성인 TMOS와 물에 알코올과 산성 촉매를 첨가하여 가수분해 반응을 시킨 후 알코올을 증류법으로 제거한다. 두 번째 단계에서는 비알코올성 희석용매와 염기성 촉매를 첨가하여 중합반응을 시켜서 습윤젤을 제조한다. 본 연구에서는 알코올을 첨가하지 않고 TMOS, 물 그리고 HCl 혼합용액이 균일한 단일상을 이루는 상거동 영역을 찾아내어 직접 가수분해시킨 후 알코올의 제거없이 직접 중합하는 단순화된 두 단계 솔-젤공정을 개발하였다. 단순화된 공정으로 제조된 습윤젤을 직접 초임계 이산화탄소를 이용하여 40℃, 8MPa에서 건조하여 실리카 에어로젤을 얻어내었다. 또한 초임계 이산화탄소 건조로 얻어낸 친수성 에어로젤을 180℃에서 메톡시화 반응으로 표면을 소수성으로 안정화시켰다. 이 결과 밀도 0.03 g/㎤의 투명한 소수성의 실리카 에어로젤을 제조할 수 있었다.

Ultra-low density silica aerogel is synthesized through a two-step sol-gel processing and supercritical fluid drying. However, in existing two-step sol-gel processing, a wet gel is obtained by complicated steps : addition of alcohol as a cosolvent to the mutually immiscible TMOS and water solution; hydrolysis of under acidic solution(i.e., HCI) : separation of alcohol by distillation; dilution of the mother liquor by a diluent such as acetone; and polymerisation to obtain wet gel. In the present study, a new simplified two-step sol-gel processing is proposed. First, through a phase behavior study for the ternary solution containing TMOS, water and acid, a condition which becomes a homogeneous single phase was searched. With this solution, hydrolysis is performed. Thus, the separation step of alcohol can be eliminated without adding alcohol. The wet gel which was obtained by the new process was directly dried to obtain the aerogel by SC CO₂ at 40℃ and 8 MPa. In general, the aerogel obtained by the low-temperature supercritical carbon dioxide drying is extremely hydrophilic and easily destructed by moisture. However, by a methoxylation of the surface of the aerogel with methanol at 180℃, hydrophobic aerogel can be made. For most cases, the density of the aerogels obtained in this work stays below 0.03 g/cm³.

Silica Aerosol Supercritical Drying Sol-Gel Surface Modification

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