## 3 Results and Discussion
### Printing Properties of Coated Paper  
Figure 6 illustrates the wet rub resistance of coated paper using coating colors with varying zirconium contents of crosslinkers and the polyamine-based crosslinker, highlighting the critical role of crosslinkers in reducing the water sensitivity of the coating layer. This water sensitivity is evaluated through wet rub and wet pick resistance tests. At a concentration of 0.5 parts, KZC7 demonstrated good wet rub resistance, while PBC, KZC10, and KZC20 exhibited comparable performance. Increasing the concentration to 0.9 parts resulted in enhanced wet rub resistance for all crosslinkers, indicating that higher zirconium contents contribute to improved water resistance in the coating layer, comparable to PBC. The data suggest that the zirconium-based crosslinkers, such as the KZC series, effectively enhance the coating layer's wet rub resistance. While KZC7 showed good performance at lower concentrations, all crosslinkers, including PBC, KZC10, and KZC20, benefitted from increased concentrations, showcasing improved wet rub resistance. This trend underscores the importance of zirconium content in the formulation, as higher zirconium levels lead to a more robust and water-resistant coating layer. Figure 5: Viscosity of coating color mixed with different crosslinkers: (**a**) low shear viscosity; (**b**) high shear viscosity. Figure 7 presents images of the coated paper after the wet pick test, revealing the performance of different crosslinkers and emphasizing the effects of zirconium content on wet pick resistance. At a concentration of 0.5 parts, all crosslinkers exhibited poor wet pick resistance, with the KZC series performing particularly poorly. However, at a higher concentration of 0.9 parts, the KZC series demonstrated superior wet pick resistance compared to the polyamine-based crosslinker PBC. Notably, KZC7, which has the lowest zirconium content among the KZC series, showed the best wet pick resistance at this higher concentration. This suggests that KZC7's unique formulation might allow for more efficient crosslinking or better film formation, enhancing the coating layer's durability and resistance to wet pick more effectively than KZC10, KZC20, and PBC. Conversely, increasing the zirconium content beyond 10% within the KZC series, even at a concentration of 0.9 parts, may lead to decreased wet pick resistance. This could be due to potential issues such as increased brittleness or suboptimal crosslinking density, which can negatively impact the coating's flexibility and adhesion. Thus, while higher concentrations of zirconium-based crosslinkers generally improve wet pick resistance, an optimal range of zirconium content maximizes this benefit. Understanding these relationships is crucial for selecting the appropriate crosslinker type and concentration to achieve the desired wet pick resistance for specific coated paper applications. Figure 8 displays images of the dry pick resistance of coated paper treated with coating colors containing crosslinkers with different zirconium contents and a polyamine-based crosslinker, emphasizing the impact of zirconium content on this property. The results indicate that all crosslinkers exhibited excellent dry pick resistance, regardless of their type and zirconium content, suggesting that the crosslinkers effectively contribute to the dry pick resistance of the coating layer. Notably, there were no significant differences among  
Figure 6: Wet-rub resistance of the coated paper. Figure 7: Images demonstrating different wet pick resistance after the wet pick test of the coated paper: (**a**) wet pick images at 0.5 part (indicating lower crosslinker concentration); (**b**) wet pick images at 0.9 part (indicating higher crosslinker concentration). the various crosslinkers, including the zirconium-based KZC series and the polyamine-based PBC. This indicates that the type and zirconium content of the crosslinker do not significantly affect the dry pick resistance of the coated paper.