Coating Dispersion Instability? A Comprehensive Guide to Dispersant Selection and Solutions

Analysis of Dispersion Instability in Printing and Coating Processes

In gravure, flexographic, roller coating and other printing and coating processes, “dispersion instability” is one of the most troublesome pain points for engineers. Typical symptoms include flocculation and re-agglomeration of pastes, abnormal viscosity increase, particles appearing on the coated film, loss of gloss, and even screen clogging. This often originates from re-agglomeration of pigment particles due to van der Waals forces and destruction of the dispersion system during dynamic processes (such as high-speed mixing or coating shear). To solve this problem, it is essential to select an appropriate dispersant and combine it with wetting agents and defoamers as a integrated solution.

Core Strategy: Logic for Dispersant Selection

To resolve dispersion instability in coating, the key is to choose a dispersant that provides a dual stabilization mechanism of strong steric hindrance and electrostatic repulsion.

High-Performance General-Purpose Dispersant: Toynol® ADS-20

ADS-20 (a blend of styrene-maleic anhydride copolymer with acetylenic diols) is a versatile nonionic dispersant. Its core advantage lies in high efficiency — for example, in a ceria polishing powder system, adding 5.7% ADS-20 can grind the fineness to ≤2.5 μm within 30 minutes. This is highly valuable for printing ink paste preparation where rapid achievement of target fineness is required.

For Difficult-to-Disperse Pigments: Toynol® DS-195 Series

For pigments such as transparent iron oxide, carbon black, and titanium dioxide that are difficult to disperse and prone to re-agglomeration, it is recommended to prioritize DS-195H (solid content 39-44%) or DS-195L (solid content 38-42%). This series of dispersants utilizes a polymer structure to provide strong steric stabilization, effectively preventing secondary agglomeration of pigments during storage or under coating shear. This ensures long-term stability of the color paste and is suitable for high-concentration pastes and automotive coatings.

Auxiliary Tools: Wetting and Foam Control to Address Application Instability

Dispersion instability is not only caused by pigment agglomeration, but also often by poor dynamic wetting and microfoam leading to surface defects.

Solving Uneven Wetting at Coating Edges: Toynol® FS-204 Series

In high-speed gravure and roller coating processes, inks need to spread rapidly. FS-204, FS-204BC, and FS-204DPM (acetylenic diol-based wetting agents) can rapidly reduce dynamic surface tension, ensuring uniform wetting of the ink on low-energy substrates (e.g., plastic films, metal foils) and avoiding dispersion failure caused by wetting differences (e.g., cratering).

Suppressing Microfoam in Coating: Toynol® Superwet-320 and Foamic 024

Bubbles generated during high-speed coating can disrupt coating continuity, causing dispersed particles to accumulate at bubble burst sites. Superwet-320 (static surface tension as low as 26.2 dyn/cm in 0.1% aqueous solution) combines wetting and foam control functions to inhibit bubble generation. If foam has already significantly affected dispersion uniformity, it is recommended to combine with Foamic 024 (a silicone-free and mineral-oil-free nonionic organic polymer defoamer) at a dosage of 0.2%-1.0% to safely eliminate pinholes and crater defects without affecting the leveling of the dispersion system.

Summary and Application Recommendations

To solve dispersion instability in printing and coating processes, a combined strategy of “strong dispersion + dynamic wetting + mild defoaming” is recommended:

• For general-purpose color pastes, pre-disperse with ADS-20 or DS-195L first.
• During the application stage (ink adjustment, coating), add FS-204 to improve dynamic wetting, and if necessary, supplement with Foamic 024 or Superwet-320 to control microfoam.

In practice, the addition sequence and dosage of additives should be adjusted according to the specific resin system (e.g., acrylic, polyurethane) and coating speed. A dose-response study is recommended.