## 3 Results and Discussion  
### Properties of Coating Color  
As shown in Figure 2, the pH of the coating color remained largely unchanged across different zirconium contents despite the addition of crosslinkers ranging from 0.5 to 0.9 parts. The pH of all coating colors was consistently maintained at approximately 9. This indicates that crosslinkers from the polyamine and KZC series exhibit low sensitivity to pH variations [38]. Figure 3 compares the water retention values of the coating colors at different zirconium contents of the crosslinkers. Among the KZC series, the crosslinker KZC7, with a 7% zirconium content, exhibited a water-holding capacity comparable to that of PBC from the polyamine series. As the zirconium content increased, the crosslinkers KZC10 and KZC20 demonstrated slight improvements in water retention compared to PBC and KZC7. Figure 2: pH of coating colors mixed with KZC series at different zirconium contents. The KZC series crosslinkers were added at 0.5 part and 0.9 part, assuming a 100% solution regardless of the actual solid contents of 7%, 10%, and 20%, meaning that much less of the KZC crosslinkers were used in the coating color formulation compared to the polyamine-based PBC. Considering this, the KZC series crosslinkers can be regarded as having better water holding ability than the polyamine series crosslinker. Specifically, the water retention ability of the KZC series showed noticeable enhancements as more crosslinkers were added. This suggests that higher zirconium contents in the KZC series contribute to improved water retention in the coating colors, likely due to zirconium's covalent bonds with carboxyl groups and the weaker hydrogen bonds with hydroxyl groups of binders (refer to Figure 4). Thus, the water sensitivity can be decreased by crosslinking soluble binders with the crosslinkers or by forming an insoluble net around the binders [10; 11]. This observation underscores the importance of optimizing the zirconium content in KZC-based crosslinkers to achieve the desired performance characteristics in pigment coatings. Figure 5 illustrates the viscosity of coating colors with varying zirconium contents of crosslinkers, highlighting its crucial role in the coating process by affecting application uniformity, coating weight control, flow and leveling, drying behavior, equipment compatibility, and ultimately, the quality of the finished product [2; 39]. At a low concentration of 0.5 parts, the polyamine-based PBC demonstrated lower low shear viscosity compared to the zirconium-based KZC series, indicating that crosslinker type plays a pivotal role in viscosity at lower concentrations. Interestingly, within the KZC series, low shear viscosity remained relatively stable across a zirconium content range of 7% to 20%, highlighting an insensitivity to zirconium content variations within this range. However, at an increased concentration of 0.9 parts, a sharp rise in low shear viscosity was observed for all crosslinkers. KZC crosslinkers with zirconium contents above 10% exhibited slightly higher viscosities than PBC and KZC7. The zirconium-based KZC series is much more sensitive than the polyamine-based PBC due to the inherent chemical and physical properties of zirconium compounds, which tend to form stronger and more numerous crosslinks within the polymer matrix. This increased crosslink density results in a more pronounced increase in viscosity with the addition of zirconium content compared to the polyamine-based PBC. Figure 4: Reactions of KZC with carboxy group and hydroxyl group in binders: (**a**) covalent bond with carboxyl group in red circle; (**b**) hydrogen bond with hydroxyl group in red circle. Figure 3: Dewatering amount of coating colors mixed with KZC series at different zirconium contents. Moreover, the KZC series crosslinkers were added at 0.5 part and 0.9 part, assuming a 100% solution, regardless of the actual solid contents (7%, 10%, and 20%).