# Preparation and Properties of Polyurethane Composite Foams with Silica-Based Fillers
###### Abstract
In particular, an approach in the form of a composite material using minerals, ceramics, or metals as fillers is attracting much attention from many researchers [19; 20; 21]. Traditionally, polymer composite materials have been studied for a long time as a method to improve the mechanical and thermal properties of polymers themselves. The addition of a small amount of filler to the polymer matrix leads to the improvement of the physical properties of the polymer matrix due to the characteristics of the filler itself, and, in general, the mechanical and thermal properties are greatly improved [20]. This is because the mechanical/thermal properties of inorganic materials used as fillers are relatively superior to those of polymer materials. In the case of a polymer foam, similar effects such as the improvement of mechanical and thermal properties can be expected if a filler-added polymer foam composite is prepared. Since the added filler affects the cell structure in the foam, it can be expected to increase the properties of the foam, particularly the heat insulation effect, through changes in the cell structure as well as the properties of the filler itself. In fact, as a result of analyzing various physical properties of the polyurethane foam composite material produced in several previous studies, further improvement in the insulation effect and the mechanical properties was observed [22]. Therefore, in order to further improve the thermal insulation properties of the polyurethane foam, it is necessary to select and add an appropriate filler and control the cell structure of the foam to form a uniform cell structure of a small size. Silica aerogel is an ideal porous material with a pearl necklace-like three-dimensional structure having interstitial mesopores. Such structural feature makes it possible to use it as an insulator because it shows low density and low thermal conductivity due to high porosity with 90% or more of air [23; 24; 25; 26]. Silica aerogel as an insulating material has a thermal conductivity of 20 mW/mK or less, and has even smaller thermal conductivity than other conventional insulating materials. In addition, due to its high surface area, high porosity, and low density, research is being actively conducted to utilize it in various fields such as insulation or sound-absorbing materials for buildings, liquefied natural gas (LNG) carriers, and refrigeration equipment. However, silica aerogel is brittle and sensitive to humidity, which limits the use of silica aerogel alone as an insulator or sound absorbing material. In this study, in order to improve the properties and performance of polyurethane foam, silica-based filler is added to prepare a polyurethane foam composite. As mentioned above, silica aerogel is a material with a very high porosity, and due to its superior thermal insulation effect, the addition of silica aerogels into polyurethane foams can be expected to improve their thermal insulation performance [27; 28; 29]. The surface of silica aerogels added into the composite foam is modified to interact with polyurethane matrix and improve the dispersion of the silica aerogel in the composite foam. In addition to silica aerogels, in this study, two different types of silica fillers were added to prepare polyurethane composite foam for comparison. First, silica nanoparticles of a size similar to that of silica aerogel are added as a filler. In this case, as in the case of aerogel, the surface of silica nanoparticles is treated to induce the improvement of interfacial adhesion between the polymer matrix and filler. The second method involves making a silica-polyurethane foam composite through a sol-gel method. Polysiloxane functional groups obtained through a sol-gel reaction with polyol is introduced into a polyurethane reaction and foaming process to prepare a silica-polyurethane composite foam [30; 31].