How to formulate adjuvants for pesticide suspension concentrates (SC)? A complete dispersion and stabilization solution
1. Why are pesticide SC formulations difficult to stabilize?
Pesticide suspension concentrate (SC) is a liquid formulation in which water-insoluble solid technical active ingredients are milled to micron-sized particles and suspended in an aqueous phase. It is an important waterborne formulation type for pesticides. Its stability challenges arise from four closely linked contradictions:
- The technical active ingredient is difficult to wet: most solid technicals are strongly hydrophobic, floating on the water surface and failing to disperse during dosing; the premix stage fails;
- The finer the particles, the harder they are to stabilize: milling reduces particle size, dramatically increases the specific surface area, and strengthens van der Waals attraction between particles, leading to re-agglomeration (coarsening) and coalescence/growth. Fineness is a core indicator for SC, but “grinding finer” and “keeping stable” are inherently in conflict;
- Particles always sediment: the density of the technical active ingredient is generally higher than water, so particles settle slowly during static storage; in severe cases, a hard cake forms at the bottom that cannot be redispersed by shaking or poured out;
- Storage makes things worse: Ostwald ripening (small particles dissolve, large particles grow), water separation, viscosity drift, and particle coarsening after hot storage. Chinese pesticide formulation standards have explicit requirements for hot storage stability (e.g., storage at 54 °C); formulations that fail cannot be marketed.
In short: the four major SC challenges—wetting, dispersion, sedimentation, and ripening—correspond to different parts of the adjuvant system. If any one part is missing, the stability test will reveal it.
2. Complete technical route for SC dispersion and stabilization
2.1 Step 1: Wetting—getting the technical active ingredient “in”
If hydrophobic technical powder cannot enter water, everything else is in vain. The role of the wetting agent is to reduce the solid-liquid interfacial tension so that water can quickly replace the air on the particle surface. For waterborne systems, Toynol® acetylenic diol wetting agents FS-204 or the Superwet series can be evaluated: acetylenic diols wet quickly and give low foam, making them suitable for SC premix dosing.
2.2 Step 2: Dispersion—keeping particles “separated” and preventing coarsening
During milling, the dispersant continuously adsorbs onto freshly exposed particle surfaces and maintains the dispersed state through steric hindrance / electrostatic repulsion. SC dispersant selection is based on three dimensions:
| Selection dimension | Key consideration | Notes |
|---|---|---|
| Anchoring capability | Binding force with the technical surface | Polarity varies greatly among different technicals; screening must be done with the actual technical material |
| Steric effect | Molecular weight and chain structure | Polymeric dispersants provide stronger steric hindrance (corresponding to the Toynol® DS series direction) |
| Ionic type matching | Compatibility with the system’s charge environment | Avoid charge conflicts with the technical or other adjuvants |
Note: dispersant grades cannot be copied directly—organic technicals and inorganic technicals behave completely differently in dispersant adsorption, so small-scale screening must be performed with the actual technical system.
2.3 Step 3: Suspension stabilization—keeping particles “from settling”
Even well-dispersed particles still settle. A suspension stabilizer is needed to build a spatial network and give the slurry suitable rheological properties to “hold” the particles. The Toynol® AN-12 and AN-13 suspension stabilizers address this direction. The dosage of suspension stabilizer should be restrained: excessive thickening can affect milling efficiency, filling, and pourability.
2.4 Step 4: Inhibiting Ostwald ripening—making particle growth “slower”
Ostwald ripening is the main mechanism behind particle coarsening of SC during hot storage: small particles dissolve and large particles grow. Measures to inhibit it include: controlling a narrow initial particle size distribution (avoiding over-fine particles), ensuring full dispersant coverage on particle surfaces, and reducing the solubility of the technical active ingredient in the medium (e.g., by adjusting the formulation). This step relies on the combination of process and adjuvants; a single adjuvant cannot solve it alone.
In short: wetting controls “getting in”, dispersion controls “separating”, suspension controls “not settling”, and process controls “not growing”—the four steps are interlinked.
3. Troubleshooting table for common SC quality problems and adjuvant responses
| Quality phenomenon | Possible cause | Investigation direction | Adjuvant response |
|---|---|---|---|
| Floating powder / does not disperse during dosing | Insufficient wetting | Observe wetting in premix stage | Add wetting agent (FS-204/Superwet direction) |
| Re-agglomeration / large particle size after milling | Insufficient or mismatched dispersant | Measure particle size distribution (D50/D90) | Adjust DS series dispersant dosage/grade |
| Water separation / settling during storage | Insufficient suspension stabilization | Measure water separation rate and sedimentation rate | Add AN-12/AN-13 suspension stabilizer |
| Hard caking at bottom | Sedimentation + particle growth combined | Check particle size + sedimentation | Supplement both dispersion and suspension stabilization |
| Particle coarsening after hot storage | Ostwald ripening | Compare particle size before and after hot storage | Control initial particle size distribution + ensure full dispersant coverage |
| Difficult filling/pouring | Excessive thickening | Measure viscosity and rheology | Reduce suspension stabilizer dosage |
| Foaming of formulation | Surfactant-stabilized foam | Stirring foam test | Switch to a low-foam wetting agent / add DF series defoamer |
4. Common pitfalls in SC adjuvant systems
- Only adding dispersant without preventing sedimentation: water separation and caking still occur during storage; a suspension stabilizer must also be added;
- Only adding suspension stabilizer without proper dispersion: particle agglomerates are not broken down; thickening just turns one “lump” into a “viscous lump”, and fineness does not meet the target;
- Overdosing suspension stabilizer: excessive viscosity build-up affects milling, filling, and pourability; dosage must be balanced with the dispersant;
- Copying grades directly: different technical active ingredients differ greatly in dispersant adsorption behavior; validation must be based on the actual technical system;
- Checking only appearance in hot storage tests: looking only for “layering or not” is not enough. Particle size change, redispersibility, and suspension rate must also be measured to confirm true stability.
Frequently Asked Questions (FAQ)
Q1: The technical active ingredient always floats on the water surface and cannot be dispersed during SC dosing. What should I do?
A: The technical is strongly hydrophobic and insufficiently wetted. Add a wetting agent in the premix stage to reduce solid-liquid interfacial tension, allowing water to quickly displace the air on the particle surface. Toynol® FS-204/Superwet series acetylenic diol wetting agents can be evaluated, and the improvement in wetting during dosing should be observed.
Q2: The particle size is still large after milling the SC formulation. What is the cause?
A: The likely cause is insufficient or mismatched dispersant, causing particles to repeatedly agglomerate during milling. Use a laser particle size analyzer to measure D50/D90 to locate the problem, then screen dispersants (e.g., Toynol® DS series) against the actual technical system and run a dosage gradient.
Q3: The SC cakes at the bottom and cannot be redispersed after long storage. What should I do?
A: Caking is the combined result of sedimentation and particle growth: first measure particle size to confirm whether coarsening has occurred (supplement dispersant or evaluate milling), then check the suspension stabilization system (add AN-12/AN-13 direction). In severe cases, the storage stability design has failed; reformulate and optimize based on hot storage and static storage tests.
Q4: Why does the particle size of SC become coarser after hot storage?
A: The main mechanism is Ostwald ripening—small particles dissolve and large particles grow. Countermeasures: control the initial particle size distribution so it is not too broad, ensure full dispersant coverage on particle surfaces, and reduce the solubility of the technical active ingredient in the medium. Verify the effect by comparing particle size before and after hot storage (e.g., 54 °C).
Q5: Which adjuvants are recommended for SC formulations?
A: For waterborne SC systems, Toynol® DS series dispersants + AN-12/AN-13 suspension stabilizers + FS-204/Superwet wetting agents can be evaluated; when severe foaming occurs, supplement with DF series defoamers. Specific grades and dosages must be verified by small-scale tests with the actual technical system, and the TDS should be followed.
Core conclusions
- The four major SC stability challenges—wetting, dispersion, sedimentation, and ripening—correspond to four adjuvant system components: wetting agent, dispersant, suspension stabilizer, and process control
- Complete route: wetting (FS-204/Superwet) → dispersion (DS series) → suspension stabilization (AN-12/AN-13) → foam control if needed (DF series)
- Quality troubleshooting is based on four data sets: wettability (premix), particle size distribution (D50/D90), water separation/sedimentation rate (static storage + hot storage), and redispersibility
- Do not copy grades directly: run gradient small-scale tests with the actual technical system, and follow the TDS and actual formulation test results.
