Silika aerojellerin hidroksil-son gruplu-poli(dimetilsilokzan) ile kompozitlerinin geliştirilmesi

Vacuum insulation panels (VIPs) with typical thermal conductivity values of 3 to 5 mW/mK are emerging as excellent systems nowadays for effective thermal insulation in buildings and household applications. The achievement of such low thermal conductivities in VIPs relies on the suppression of the gaseous convection by applying vacuum. A VIP structure is composed of a core insulation material and an envelope film covering the core material. Among different materials, fumed silica and glass fiber are the most commonly utilized core materials owing to their appreciably low thermal conductivity values, especially under vacuum conditions. However, these materials are not transparent and thus cannot be used in the development of transparent vacuum insulation panels. The idea of transparent VIPs has been recently established in order to replace the conventional window glazing. Since then, research about transparent core materials and barrier films that can be utilized in the development of transparent vacuum insulation panels has been gaining increasing interest. Silica aerogels appear as the most promising nanostructured materials to be implemented as filler materials in transparent vacuum insulation panels due to their transparency in addition to extremely low thermal conductivity. One drawback of silica aerogels is their poor mechanical properties which makes their utilization as monolithic and crack-free materials challenging. This problem can be solved by reinforcing aerogels with polymers which results in improved resilience that would allow for practical utilization. In this study, monolithic composites of silica aerogels with hydroxyl-terminated poly(dimethylsiloxane) (PDMS(OH)) were developed. The first route that was followed for the synthesis of the composites was the modification of the conventional two-step sol-gel process. The incorporation of the polymer in the synthesis was performed at different stages of the sol-gel process. Additionally, different co-solvents such as THF and toluene were used. The effects of several processing parameters such as polymer amount, type of co-solvent and the polymer addition step, on the properties of the composites were investigated. The composites synthesized with this route were obtained as opaque materials since PDMS(OH) was not soluble in the sol mixture. Utilization of THF and toluene as the co-solvents avoided the solubility problem, however the mechanical durability of the aerogel composites was very low which resulted in very high volumetric shrinkage during the supercritical drying. Reactive supercritical deposition technique was employed as the second route and the composites of silica aerogels with PDMS(OH) were developed by the deposition of the polymer from supercritical CO2. The technique is composed of two stages; the first stage includes the dissolution of PDMS(OH) in supercritical CO2 that results in a single phase binary mixture of PDMS(OH)-CO2 and the second stage is the exposure of the silica aerogel samples to the single phase binary mixture. Initially, the demixing pressures of PDMS(OH)-CO2 binary mixtures at various compositions were measured up to 24 MPa to determine the single phase region of the binary mixture. The demixing pressures were observed to decrease with increasing polymer content of the binary mixture. Subsequently, deposition experiments were performed and monolithic aerogel composites were obtained. The deposited samples were characterized by ATR-FTIR and BET analysis. It was revealed that during the course of the deposition, the polymer molecules react with the surface –OH groups of the aerogel. The effects of various parameters such as polymer concentration, deposition temperature, deposition time and polymer molecular weight on the properties of composites were investigated. The polymer uptake of the deposited aerogels increased with increasing PDMS(OH) concentration, deposition time and deposition temperature. It was found that the transparency of the aerogels can be controlled by the amount of the polymer loaded to the samples. It was also demonstrated that the deposition resulted in the coating of silica aerogel surface with a thin layer (~1-2 nm) of polymer molecules. According to the thermal conductivity model simulations, such a thin coating layer did not cause a noticeable increase in the thermal conductivity of the composites. Moreover, compression tests revealed a threefold improvement in the mechanical strength of the composites when compared to native silica aerogels. Hereby, this work presents silica aerogel-PDMS(OH) composite materials as novel candidates to be used as core insulation materials in transparent VIPs.