Acetylenic Diol Additives for Waterborne Industrial Coatings: A Wetting and Crater-Control Selection Guide for Cold-Rolled Steel

Why Are Craters Prone to Form on Cold-Rolled Steel?

Cold-rolled steel surfaces retain rust-preventive oil and oxide scale, and are inherently low in surface energy. Waterborne industrial coatings have high surface tension; if dynamic wetting cannot keep up, craters and fisheyes appear after roll coating or spray application. Surfynol 104, a classic acetylenic diol surfactant, solves this problem by lowering dynamic surface tension. When selecting a replacement, one should not look only at static surface tension, but also at dynamic wetting speed, foam-control performance and compatibility with the resin system.

Key Performance Indicators of Acetylenic Diol Surfactant FS-204

• Dynamic surface tension reduction: the coating process is completed in milliseconds; insufficient dynamic wetting leads to craters.
• Foam control: acetylenic diols are inherently low-foaming, but other surfactants in the formulation may stabilize foam, so a defoamer is often required.
• Compatibility: avoiding loss of recoat adhesion, floating and flooding.
• Environmental compliance: APEO-free, in line with regulatory trends for industrial coatings.

Recommended Replacement Options: Toynol® Acetylenic Diol Series

1. High Dynamic Wetting Type: Superwet-320 / Superwet-360

Superwet-320 is an acetylenic diol polyether nonionic/gemini wetting agent. A 0.1% aqueous solution provides a static surface tension as low as 26.2 dyn/cm. Dosage: 0.3%–0.5%. It is particularly suited to wetting metal substrates and effectively resolves craters and fisheyes on cold-rolled steel. Superwet-360 is also based on acetylenic diol polyether, offering fast dynamic wetting to prevent craters. Dosage: 0.3%–0.5%.

2. Balanced Foam-Control Type: FS-620 / FS-640 / FS-660

FS-620 has a static surface tension of 33.2 dyn/cm and a dynamic surface tension of 34.3 dyn/cm, combining foam inhibition and defoaming. Dosage: 0.3%–0.5%. FS-640 achieves a dynamic surface tension as low as 35.1 dyn/cm and offers good compatibility with a wide range of resin emulsions. FS-660 is designed for wetting metal substrates, resolving craters, fisheyes and poor leveling; suitable for primers and topcoats.

3. Defoamer Matching: Foamic-024 / DF-110B

Craters are sometimes caused by incompatible defoamers. Foamic-024 contains no silicone, mineral oil or APEO, and does not cause craters or pinholes. Dosage: 0.2%–1.0%. It is suitable for waterborne industrial coatings with stringent surface appearance requirements. DF-110B is a gemini nonionic defoamer based on acetylenic diol, offering excellent compatibility and specifically designed to avoid craters caused by conventional defoamers. Dosage: 0.1%–0.8%.

Application Recommendations for Cold-Rolled Steel

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