Technical Guide to Selecting Wetting Agents for Coatings and Inks

Technical Guide to Selecting Wetting Agents for Coatings and Inks

First, distinguish between the two types of wetting agents:

(1) Pigment wetting agents (also known as grinding wetting aids) are designed for pigment powders. Added during the grinding process, they displace air adsorbed on the pigment surface, thereby enhancing grinding efficiency and color development; they are typically used in conjunction with dispersants.

(2) Substrate wetting agents act at the solid-liquid interface between the coating/ink and the substrate. They specifically address issues such as cratering, edge retraction, poor spreading, and substrate wetting failure, and are usually added during the paint/ink blending stage.

The following section focuses on the categories of substrate wetting agents most widely used in formulations.

I. Selection Based on the System

1. Solvent-based systems encompass two major application categories: solvent-based gravure inks and solvent-based industrial coatings.

These systems inherently possess low surface tension; the primary requirement is to eliminate defects such as "fish-eyes" and craters (pinholes) that occur on plastics or substrates contaminated with mold release agents.

(1) Non-silicone specialty acrylic substrate wetting agents (e.g., RK-8030): Specialty acrylic-based wetting additives that do not contain silicone structures.

✅ These additives completely avoid silicone-related contamination issues and do not interfere with lamination performance or inter-layer adhesion; they are particularly well-suited for multi-layer overprinting and laminating-grade gravure inks.

⚠️ On substrates heavily contaminated with silicone oil release agents, the effectiveness of these additives in wetting and preventing craters is significantly lower than that of silicone-based wetting agents.

(2) Low-molecular-weight polyether-modified siloxanes: Small-molecule silicone surfactants (e.g., RK-8014XD, RK-8026).

✅ These additives offer outstanding wetting performance, optimizing substrate spreading and effectively preventing craters; they exhibit better system compatibility than similar products with high silicone content.

⚠️ When used in laminating-grade gravure inks, the dosage of these additives must be strictly controlled to prevent silicone migration, which could lead to delamination between layers.

Recommended dosage: 0.03%–0.2%

2. Application systems based on epoxy resin as the primary film-forming agent encompass the two mainstream categories of epoxy coatings and epoxy inks.

Epoxy systems are inherently highly polar; while ensuring adequate substrate wetting, it is crucial to prioritize the adhesion between the coating and the substrate.

For formulation selection, non-silicone wetting agents (such as RK-8030) are preferred; if silicone-based wetting agents are used, small-scale trials must be conducted with caution, as excessive addition can easily compromise inter-coat adhesion.

3. This category encompasses UV-curing inks and coatings, specifically covering three major product types: screen-printing UV inks, offset UV inks, and UV varnishes.

Key system requirements include zero residual solvent, no inhibition of the curing process, rapid curing speeds, and effective wetting within a relatively narrow processing window.

(1) Specialty silicone-free substrate wetting agent (RK-8030) developed for UV systems

✅ Completely eliminates the risk of silicone migration; the preferred wetting agent for overprinting processes and laminating UV inks.

(2) Modified siloxane-based substrate wetting agent (RK-8065) developed for UV-curing systems

✅ Effectively lowers system surface tension, ensuring excellent wetting on various plastic substrates while efficiently preventing defects such as fish-eyes and pinholes.

⚠️ Standard water-based or solvent-based silicone wetting agents must not be used in these systems, as they can trigger polymerization inhibition and interfere with subsequent overprinting performance.

Recommended dosage: 0.05% – 0.3%

II. Comparison of Wetting Agent Types

III. Key performance indicators for the formulation selection stage (all items must be verified through laboratory-scale trials):

1. Inspection for visible paint film defects: Focus on visually checking for four typical surface defects—craters, fish-eyes, edge retraction, and pinholes.

2. Dynamic wetting performance testing: Simulating high-speed spraying or printing conditions; this is a critical assessment of dynamic surface tension for UV ink formulations;

3.Storage stability assessment: Conducting accelerated thermal aging and high/low-temperature cycling tests to observe the system for anomalies such as precipitation or phase separation;

4. Verification of side effects and additive compatibility: Since many silicone-based additives tend to stabilize foam, they must be evaluated in conjunction with appropriate defoamers; for inks, the focus is on verifying lamination bond strength and overprinting performance, while for coatings, the emphasis is on recoat adhesion;

5. For export products, priority must be given to verifying APEO and VOC levels, as well as substances restricted under REACH regulations, to ensure compliance with environmental standards.

IV. Key Core Risks to Avoid and Practical Pitfall Prevention Points in the Full Process of Wetting Agent Selection

1.The functions of substrate wetting agents and pigment dispersants are clearly defined and cannot be substituted for each other: substrate wetting additives cannot replace pigment wetting and dispersing agents used in the grinding process, as their corresponding addition stages are entirely different.

2. The dynamic surface tension value is higher than the static surface tension: For high-speed printing and high-speed spraying operations, the dynamic surface tension index should be prioritized for assessment, while the static surface tension is only applicable to low-speed construction scenarios.

3.Synergistic risks arise from the stacking of silicon-based additives: When both organic silicone leveling agents and organic silicone wetting agents are incorporated into the formulation simultaneously, the cumulative content of silicon elements in the system increases, significantly raising the probability of silicon components migrating to the surface.

4. The additive dosage must be strictly controlled and not exceed the limit: When the dosage surpasses the critical saturation concentration, the additive will accumulate on the coating surface, directly leading to reduced adhesion and film failure. It is recommended to conduct gradient tests using three concentration levels: 0.05%, 0.15%, and 0.3%.

5. Prioritize meeting the standards in substrate pretreatment: Wetting agents serve only as auxiliary backup additives. Severe oil contamination on the substrate surface cannot be eliminated solely by wetting agents; a dedicated substrate degreasing process must be implemented alongside for complete pretreatment.

V. Quick Selection Guide

- For single-coat systems applied to low-surface-energy plastic substrates (PP/PE) that do not require lamination, polyether-modified silicone wetting agents are recommended.

- For solvent-based gravure systems requiring lamination, non-silicone acrylic wetting agents should be prioritized.

- For UV ink systems utilizing overprinting processes, non-silicone wetting agents specifically designed for UV systems are preferred; caution should be exercised when considering additives with high silicone content.

- For water-based ink and inkjet systems requiring both lamination and overprinting, non-silicone wetting agents featuring an acetylenic diol-based Gemini structure are the preferred choice.