In the preparation of PVC ink, the fineness of pigment particles directly affects the saturation and gloss of printed colors. Optimizing the dispersion process can effectively improve the uniformity and stability of pigment particles, thereby enhancing the ink's printing performance. Specifically, optimizing the dispersion process requires comprehensive consideration from multiple dimensions, including equipment selection, dispersant application, grinding parameter control, and process flow design.
The three-roll mill is the core equipment in the PVC ink dispersion process. It refines pigment particles through the shearing and grinding forces between the rollers. During the grinding process, the pigment and binder are initially dispersed in the gap between the feed roller and the intermediate roller, and then further refined in the gap between the intermediate roller and the discharge roller. By adjusting the roller spacing, the final particle size of the pigment particles can be controlled, typically at the micron level to ensure uniform dispersion. The advantage of the three-roll mill is its ability to achieve sub-micron level fine dispersion while avoiding impurities that may be introduced by high-speed stirring or ball milling processes, thus ensuring the purity and color reproduction capability of the ink.
The selection and application of dispersants are crucial for the refinement of pigment particles. Dispersants prevent particle re-aggregation by adsorbing onto the pigment surface and creating electrostatic repulsion or steric hindrance. In PVC inks, nonionic dispersants are widely used due to their good compatibility with PVC resin; their molecular chains can extend into the binder, forming a stable dispersion system. Furthermore, the amount of dispersant added must be precisely controlled according to the pigment type and ink formulation. Excessive amounts may increase ink viscosity, affecting printability; insufficient amounts will fail to effectively prevent particle agglomeration, resulting in substandard fineness.
Controlling grinding process parameters is crucial for optimizing fineness. Grinding temperature must be maintained within a suitable range; excessively high temperatures may cause binder denaturation, while excessively low temperatures will affect dispersion efficiency. Grinding time needs to be adjusted according to the pigment type and initial particle size, typically requiring multiple grinding cycles to achieve the target fineness. Simultaneously, the filling amount and rotation speed of the grinding media must be matched to avoid uneven dispersion due to too much or too little media. For example, in ball milling, optimizing media size and filling rate can significantly improve grinding efficiency, but care must be taken to prevent impurities from media wear from contaminating the ink.
The process flow design must balance efficiency and quality. The pre-dispersion stage uses high-speed stirring to initially mix the pigments and binders, forming a uniform slurry that lays the foundation for subsequent grinding. The grinding stage requires selecting appropriate equipment and parameters based on the pigment characteristics. For pigments that are difficult to disperse, multi-stage grinding or composite dispersion processes can be used. The post-treatment stage removes air bubbles and coarse particles from the ink through filtration and degassing, further improving printability. For example, in the preparation of PVC film inks, optimizing the number of grinding passes and filtration precision can significantly reduce plate clogging during printing and improve pattern clarity.
The fineness of pigment particles affects the saturation of printed colors on multiple levels. Ink with insufficient fineness is prone to a grainy appearance during printing, leading to dull colors and reduced gloss; while excessive grinding may damage the pigment crystal structure, affecting color stability. Therefore, the optimal grinding particle size needs to be determined experimentally, typically 1-2 times the original pigment particle size. Furthermore, uniformity of fineness is equally important; an excessively wide particle size distribution can lead to inconsistent ink layer thickness during printing, affecting color consistency.
Optimization of the dispersion process also needs to consider the final application scenario of the PVC ink. For example, in packaging printing requiring high gloss, a mirror effect must be achieved by controlling the pigment particle size and dispersion state; for outdoor applications, both fineness and weather resistance must be considered to prevent color fading under ultraviolet radiation due to poor pigment dispersion. By specifically adjusting the dispersion process, the differentiated performance requirements of PVC ink in different fields can be met.
