A lower-MFFT latex made by extra hydrophilic emulsifier can spot with water. Record water resistance with the coalescent curve, not instead of it.
Two ways to make a film
Coalescent plasticizes the particle for a short time and mostly leaves. Emulsifier plasticizes the surface and mostly stays. Formulators who cut coalescent for VOC and then add a more hydrophilic stabilizer often trade solvent for water spots.
Esteem emulsifiers are chosen in the reactor for stability. The paint lab still owns the film tests. Send those results back with the sample request so the next grade is less hydrophilic or better matched, not simply “stronger.”
What to hold constant
Pigment volume concentration, thickener, and substrate wetter all change gloss and film. An emulsifier trial that also changes the substrate wetter cannot be read. Hold the wetter, which is its own article, and move one polymerization emulsifier.
Low-temperature touch-up and early rain are the practical tests for exterior latex. A 40 °C oven film will not show them.
| Lever | Leaves the film? | What it changes |
|---|---|---|
| Coalescent | Mostly yes | MFFT, open time |
| Emulsifier | Mostly no | Stability and water sensitivity |
| Substrate wetter | Mostly no | Craters, not MFFT |
| Reactive monomer | Polymerized | Adhesion, crosslink |
Particles have to deform
A latex dries by water leaving, particles packing, and then the polymer flowing across the boundary. If the glass transition is above the drying temperature, the particles do not flow and the film mud-cracks. A coalescent is a temporary plasticiser that swells the particle during drying and later evaporates. It is not a surfactant, but it competes with surfactant for how soft the surface is.
Emulsifiers left from polymerisation sit in the serum and at the particle surface. A high-EO nonionic can lower minimum film-formation temperature slightly and, after drying, leave a hydrophilic network. That network is why some films pass a crack test and fail an early water test.
Coalescent distribution
The coalescent has to partition into the particle, not stay dissolved in the water with the surfactant. A very hydrophilic surfactant system can hold coalescent in the water phase so the particle never softens. The film then cracks even though the formula contains the “right” coalescent level on paper.
Slow and fast coalescents are chosen for open time and hardness development. Surfactant that migrates to the surface can also change gloss and block. If the complaint is block, look at surfactant and coalescent together, not only at the resin glass transition.
What to measure
Measure minimum film-formation temperature of the latex with and without the coalescent, then water resistance of the film after a defined dry. A film that is continuous at room temperature can still blush if the surfactant has not left the hydrophilic path.
Name the resin glass transition, the drying temperature of the customer’s line, and whether early rain or early block is the failure. Those conditions decide coalescent level more accurately than a generic percentage of latex solids.
How to Specify with Esteem
- Report MFFT with and without coalescent.
- State the coalescent type and level you are allowed to use.
- Include a 24-hour water-spot or wet-scrub note.
- Keep pigment volume concentration constant when comparing emulsifiers.
- Do not lower coalescent and emulsifier hydrophile in the same trial.
See the article on emulsifiers, gloss, and film formation, and paint and coating chemicals.