# Preparation and Properties of Polyurethane Composite Foams with Silica-Based Fillers
###### Abstract  
Polyurethane composite foams were prepared by adding three different types of silica materials as a filler to improve the mechanical and thermal insulation properties. The first type of filler consists of silica aerogels with high-volume pores, with the expectation of improving the thermal insulation of PU foams because silica aerogel itself has superior thermal insulation properties. Silica nanoparticle is used for the second type that has a size very similar to the pore size of silica aerogels for comparison. The last type to produce polyurethane composite foam uses a sol-gel reaction to produce polysiloxane that reacts with polyids during the urethane reaction and forming process. In particular, in the case of silica aerogels and nanoparticles, their surfaces are modified with APTES and then polymeric methylene diphenylene diisocyanate (PMDI) to increase the interaction between the polymer matrix and inorganic fillers. The polyurethane foam structure was successfully produced in all cases of composite foams. As expected, the mechanical properties and the thermal insulation effect were enhanced by the addition of silica fillers, but found to be closely related to the cell structure of polyurethane foams. The addition of small amounts of inorganic fillers improves the mechanical and thermal properties, but the higher the amount of filler, the worse they are due to the agglomeration of fillers on the cell walls. The dispersion of added inorganic fillers within the foam cells should be controlled effectively. Surface-modified silica fillers exhibit better enhancement of mechanical and thermal insulation properties. polyurethane composite foam; fillers; silica aerogels; silica nanoparticles; sol-gel reaction; thermal insulation; surface modification +
Footnote †: journal: Applied Sciences  
through urethane bonding between a polyol corresponding to a soft segment and a diisocyanate of a hard segment [9]. The hard-segment domains serve as physical crosslinks for the rubbery soft-segment matrix, imparting high elasticity of the material. The soft segment generally consists of polyether or polyester polyol, while the hard segment is formed by the extension of a diisocyanate with a chain extender of a low molecular weight polyol or amine. Polyurethane foams are formed by the reaction of urethane bonding in the presence of a blowing agent and an amine as a catalyst. Polyurethane foam is largely divided into soft and rigid foam, and rigid polyurethane foam is the most used as an insulator [10]. It is applied not only to buildings, but also to home appliances such as refrigerators, storage tanks for transport, and pipes. The insulation effect in foam structures can be divided into four major contributions: (1) heat conduction through the solid phase, (2) heat conduction through the gas-filled interior, (3) convection within the cell, and (4) radiation through the cell walls [11; 12; 13]. The individual contributions add up to give overall thermal insulation properties. If the cell size in the polymer foam is smaller than 4 mm, the contribution to convection becomes negligible, and the heat conduction of the polyurethane foam is ultimately determined by the foam agent, the solid polymer, and the cell structure in the foam. Therefore, when the foaming agent and the solid polymer are fixed, the thermal insulation property of the polyurethane foam is determined by the cell structure, and the smaller and more uniform the cell size, the lower the thermal conductivity and the better the insulation effect. Since the most important characteristic of polyurethane foam is the insulation effect, many studies are still being conducted to improve its insulation performance [14; 15; 16; 17; 18]. As mentioned above, since the insulation effect is determined by several factors, many efforts have been made to enhance the insulation property from various viewpoints and methods.