Surface Tension And Coating Defects

Static Versus Dynamic Surface Tension

Static surface tension is the equilibrium value, measured after the surfactant has had all the time it needs to reach the interface. Dynamic surface tension is the value at a defined short surface age, which is what a coating actually experiences. On a fast-moving wet film new surface is created continuously, and if surfactant cannot migrate to it quickly enough, the local surface tension stays high and a defect forms. This is why a coating can pass every laboratory test and still crater at line speed.

What The Published Numbers Mean

QK706E publishes both: 38.5 mN/m dynamic and 37.7 mN/m static, measured at 0.1% in water at 25C. The gap between the two is small, which is the point. A Gemini surfactant structure diffuses to a new interface quickly, so the dynamic value stays close to the equilibrium value instead of collapsing at short surface age.

Craters, Pinholes And Fisheyes

All three are the same failure with different geometry. A low-energy contamination spot on the substrate, an air bubble breaking at the surface, or a droplet of incompatible material creates a local surface tension gradient, and the wet film flows away from it. Prevention is a matter of getting the coating surface tension low enough and fast enough that the gradient never becomes large enough to drive flow.

Wetting Agents And Levelling Agents Do Different Jobs

QK706E lowers surface tension and suppresses foam, which addresses substrate wetting and defect prevention. QK702 improves flow and long-wave levelling and raises gloss, which addresses the appearance of a film that is already wetting correctly. Reaching for a levelling agent when the problem is wetting, or the other way round, is a common misdiagnosis.

Addition Point Matters

QK706E should be added before pigment and filler, so it wets the solids and suppresses the foam generated during mixing rather than trying to knock it down afterwards. It should also go in together with other surfactants and polymers so everything dissolves or disperses properly. Adding it last, as a rescue, gives a much weaker result.

The Recoating Constraint

Any additive that lowers surface tension can compromise the next coat. QK702 is specified to reduce surface tension only slightly, and the data sheet states it does not affect recoating or intercoat adhesion and does not cause varnish turbidity or fogging in pigmented colours. That property is worth verifying with an adhesion test in your own system whenever the dosage changes.