A latex that passes a bottle store and fails a gear pump has a shear problem. A latex that passes the pump and shocks with calcium has an electrolyte problem. Write which one you saw.
Two stabilities
Electrolyte stability is the latex surviving calcium, zinc, or a salty thickener. Shear stability is the latex surviving pumps and filters. Plants often report both as “grit” and then change a defoamer. The fix depends on which stress created the grit.
Esteem’s 60% EO block is the published styrene-acrylate stabilizer in this range. Fatty-alcohol ethoxylates remain co-stabilizers. The anionic emulsifier, if you use a sulfosuccinate or sulfate, stays in the recipe until a designed trial removes it.
Paint-side shocks
Tint bases and zinc-oxide paints add ions the reactor never saw. A binder that looked fine at 50% solids can flocculate at let-down. Run the salt test on the latex and on the finished paint.
Associative thickeners also stress the particle. If viscosity is grainy only after the thickener, include that in the note to Esteem rather than asking only for a “stronger emulsifier.”
| Where grit shows | Likely stress | First data to send |
|---|---|---|
| Reactor | Nucleation / emulsifier | Particle size, grit weight |
| Pump or filter | Shear | Pump type and pressure |
| Tint or filler | Electrolyte | Ion and level |
| After thickener | Surfactant competition | Thickener type |
Electrostatic stability has a salt limit
Anionic emulsifiers put a negative charge on the styrene-acrylate particle. Like charges repel, so the latex stays fluid. Salts compress that double layer. Calcium and zinc from pigments and driers compress it further. Past a threshold the particles stick and the latex grits or bodying occurs in the can.
Nonionic ethoxylates add a hydrated EO layer that still repels when the charge is screened. That is steric stability. The EO chain has to be long enough to matter and short enough not to water-sensitise the film. This balance is the usual reason a styrene-acrylate needs both an anionic and a nonionic, not one “strong” surfactant.
Shear removes a weakly held surfactant
In a pump or a pigment grind, particles collide harder than they do in a still tank. Surfactant that is only loosely adsorbed is wiped into the water. The bare patches coagulate. A surfactant that adsorbs strongly, sometimes a phosphate ester or a reactive emulsifier, stays on the particle.
Grit after pumping is the symptom. Measuring filter residue before and after a defined shear test tells you whether the kettle recipe or the plant transfer is the problem. Raising the surfactant in the let-down does not replace a surfactant that should have been bound during polymerisation.
Colorants and zinc are the usual shocks
Universal colorants carry glycol and extra surfactant. They can swell the particle or bridge particles. Zinc oxide and some biocides add multivalent ions. A latex that passed a sodium chloride test can still fail the tint. Run electrolyte stability with the ions the paint will actually see.
Specify particle size, anionic type, nonionic EO level, and the salt or pigment the customer adds. Those four items predict shear and electrolyte behaviour better than solids and viscosity alone.
How to Specify with Esteem
- Report styrene fraction and solids.
- Say whether grit appeared in the reactor, the pump, or after electrolyte.
- Include any calcium, zinc, or associative-thickener salt in the paint.
- Trial the styrene-acrylate block stabilizer beside the current anionic.
- Do not change monomer and emulsifier on the same batch.
EP-4084 is the Esteem block described for styrene-acrylate as well as vinyl acetate and acrylate. See paint emulsifiers.