# Preparation and Properties of Polyurethane Composite Foams with Silica-Based Fillers
## 3 Results and Discussion  
In this study, an effort was made to chemically bond the silica materials added as fillers to the polyurethane chains when preparing the polyurethane composite foam. This is expected to help the silica fillers to be well dispersed in the foam and to help improve the properties and performance of composite foams through synergetic effects of polymer matrix and filer. For this purpose, in the case of silica aerogels and nanoparticles, APTES, a silane coupling agent, is first introduced to the surface through a hydrolysis reaction, and then APTES attached to the surface of silica aerogels and silica nanoparticles is reacted with PMDI to produce silica aerogels and silica nanoparticles chemically modified with PMDI. These surface-modified silica aerogels and nanoparticles are mixed with PMDI and then reacted with a polyol to finally form a composite foam via foaming process. Figure 1 shows the surface modification results of silica aerogels and silica nanoparticles. Figure 1a,b correspond to the FT-IR spectra before and after modification of silica aerogels and silica nanoparticles with APTES and PMDI. As a result of the modification with APTES, it can be seen that the -NH\({}_{2}\) characteristic peak at the end of APTES is very weak yet clearly present. After reaction with PMDI, the corresponding characteristic peaks are clearly observed with -C=O peak at 1720 cm\({}^{-1}\), characteristic peak of the benzene ring at 1660 cm\({}^{-1}\), and -N-H stretching motion near 1540 cm\({}^{-1}\). The same results are observed in the case of silica nanoparticles, and all these results indicate that PMDI was successfully bonded to the silica surface. This result also indirectly indicates that APTES was successfully attached to the silica surface by chemical bonding. The TGA results in Figure 2a show the APTES modification result more directly. A greater weight loss can be observed in the silica aerogel modified with ATPES, which is due to the thermal decomposition of APTES adsorbed on the surface of the silica aerogel. Figure 2a also shows the effect of reaction temperature when APTES reacts on the silica aerogel surface, and it can be seen that a relatively large amount reacts on the silica surface when reacted at 80 \({}^{\circ}\)C. Based on these results, the ATPES-silica reaction was then performed at 80 \({}^{\circ}\)C. Figure 2b-d present the SEM micrographs showing the structure of silica aerogel before and after modification. It can be observed that there is little change in the appearance of the silica aerogel through the modification. Although not included here, the same results were obtained for silica nanoparticles. As described in the experimental section, silica aerogels and silica nanoparticles surface-modified with PMDI were mixed with PMDI and then reacted with polyol to prepare a polyurethane composite foam. Even with the addition of silica aerogels and nanoparticles, it was successfully foamed to produce a desired composite foam. Figure 3 presents the SEM micrographs showing the internal structure of a polyurethane composite foam to which silica aerogel is added. For comparison, polyurethane foam without filler and with unmodified silica aerogel were also prepared, and the results are included. As shown in Figure 3, a typical polyurethane foam structure is produced well in all cases. A foamed structure having pores of relatively uniform size was formed regardless of whether or not the surface of silica aerogels was modified. A closer examination reveals that when a small amount of silica aerogel is added, the pore size becomes smaller than when no silica aerogel is added. Furthermore, in the case of the modified silica aerogel, it can be observed that the pore size is slightly smaller than that of the unmodified silica aerogel. However, the excessive addition of silica aerogels was found to lead to an even worse result of the pore size becoming rather large and non-uniform, as shown in Figure 3c,g.