(429d) Formation of “Vespula-Like” Structure By the Foaming of Poly(methyl methacrylate)/ Carbon Dioxide / Tetramethoxysilane Ternary System

In recent years, for the effective energy usage, development of high performance heat insulator has been the subject of research for engineers. Polymeric foams have been used as thermal insulator due to their inherent advantages, such as low density, high mechanical strength, and low thermal conductivity. The use of supercritical carbon dioxide (scCO₂) as an alternate foaming agent has been gathering a great attention. With scCO₂ as a foaming agent, foams that have high cell density and mechanical strength could be obtained. However, scCO₂ also has the weak point that its adiabaticity is comparatively lower than that of other foaming agents. To overcome this, addition of third component was examined. Foam that inside of the cells filled with silica aerogel, or the “vespula-like” structure, is regarded as one of the promising candidate for high performance heat insulator.

In this study, general conditions for the forming of the “vespula-like” structure was investigated. Tetramethoxysilane (TMOS) and Poly(methyl methacrylate) (PMMA) were chosen as a model system because PMMA/TMOS/CO₂ system is known to form the “vespula-like” structure. PMMA/silica aerogel composite with “vespula-like” structure was prepared under various condition and the morphology of those was observed with scanning electron microscope (SEM).

The PMMA/silica aerogel composite foams were prepared by batch foaming process followed by gelation process. In the batch foaming process, PMMA samples and TMOS were placed in high pressure vessel. CO₂ was fed to the pressure vessel at constant temperature and pressure. After saturating PMMA with CO₂ and TMOS, the pressure was released at constant decompression rate. The experimental parameters were saturation temperatures, pressures, and decompression rates. After the foaming process, in order to fill cells with silica aerogel, gelation was conducted under saturated conditions of water or aqueous 3 wt% NH₃ solution. In the cells, TMOS that was separated from PMMA by releasing pressure was hydrolyzed and polymerized by the vapor to obtain the solid silicone earogel.

It was found that the number of cells filled with silica aerogel was increased by foaming at higher temperatures. TMOS likely have a relatively superior solubility at high temperature. Therefore, there may be more TMOS in PMMA at high temperature. At the same temperature, the number of cells filled with silica aerogel was increased by the increase in the saturation pressure followed by faster depressurization rate. It is known that high pressure saturation and fast depressurization rate lead to smaller average cell size. Due to the fact that the cells prepared at these conditions had small volume, it may be easily filled with silica aerogel.

To obtain the foaming structures that most of the cells are filled with silica aerogel, controlling of the foaming conditions, PMMA/silica aerogel compositions are important. Further experiments should be conducted for the full understandings of the foaming structure of this system.