## 3 Results
In addition, the thickener acts to increase the uniformity when mixed with water [30]. The thickener imparts adhesive force by hydrogen bonding. However, when the thickener is added, the fluidity is reduced, meaning that the resin does not permeate and diffuse into the micropores on the surface of the adherent, thereby decreasing the physical adhesion. However, if the viscosity is optimized, good hydrogen bonds are formed, which increases the adhesion [31]. Figure 7a shows the compressive strengths and densities at 45, 60, and 80 PPI when the thickener was added. Overall, the densities show a trend similar to that of the compressive strength. At 45 PPI, the compressive strength increased from 0.705 MPa to 1.620 MPa. The value of the compressive strength at 45 PPI is about 1.62 MPa; at 60 PPI it is approximately 2.26 MPa and at 80 PPI it is about 2.32 MPa. Hence, it could be confirmed that the addition of a thickener effectively improved the compressive strength of the reticulated porous ZTA. Figure 7b shows the compressive strengths and densities at 45, 60, and 80 PPI. In the case of fine alumina, because it has sufficiently high viscosity, if a thickener is added, the slurry cannot be uniformly coated onto the strut walls of the polyurethane foam due to the excessively high viscosity, thereby reducing the compressive strength. Therefore,  
Figure 5: (**a**) Viscosities at a shear rate of 0.01 s\({}^{-1}\) of the ZTA slurries, (**b**) compressive strengths of reticulated porous ZTA (**c**) densities and linear shrinkage rates, and (**d**) X-ray diffraction patterns of reticulated porous ZTA with different amounts of zirconia added. the fine alumina was used in an attempt to improve compressive strength by increasing the PPI of the polyurethane foam. However, for the polyurethane foams at 60 and 80 PPI despite the fact that the thickener was not added, the ZTA slurry was not uniformly coated deep inside the polyurethane foam due to the high viscosity of the ZTA slurry, causing the compressive strength to be decreased. Overall, the densities show a trend similar to that of the compressive strength. The slurry prepared with coarse alumina has relatively low viscosity compared to that prepared from fine alumina, leading to an increase in the compressive strength when the density of the polyurethane foam strut walls increases. In contrast, fine alumina negatively affects the compressive strength due to the non-uniform coating on the strut walls if the density of the polyurethane foam strut walls is increased due to the high viscosity of the slurry. Figure 6: Typical BSE images of the reticulated porous ZTA specimens, zirconia amounts when using coarse alumina as a matrix: (**a**) at 10 wt. %, and (**b**) at 40 wt. %; zirconia amounts when using fine alumina as a matrix: (**c**) 10 wt. %, and (**d**) 40 wt. %. Figure 7: (**a**) Compressive strengths and densities with pore densities of 45, 60, and 80 PPI with a thickener (using a coarse particle size for the matrix), and (**b**) compressive strengths and densities with pore densities of 45, 60, and 80 PPI (using a fine particle size for the matrix). To confirm the effect of the particle size, using coarse alumina as a matrix, zirconia at 40 wt. % and the ZTA slurry with the thickener added were used to control the particle size. The particle size distribution of ZTA according to the ball-milling time is shown in Figure 8a. The particle size is a process variable used to obtain the optimal composition of the reticulated porous ZTA. As the ball-milling time increases, the particle size distribution decreases to 1.360 \(\upmu\)m, 1.218 \(\upmu\)m, 1.165 \(\upmu\)m, and 1.031 \(\upmu\)m, relative to the aforementioned particle sizes. Figure 8a shows the viscosities as a function of the shear rate of the ZTA slurries according to the ball-milling time. As the particle size decreases due to ball-milling, the specific surface area increases, and the area where the PVA becomes attached to the particles increases. Accordingly, the viscosity increases as the ball-milling time increases.