Oil Dispersion Formulation Agrochemical: OD and EW Guide

Oil Dispersion Formulation Agrochemical Basics: What OD and EW Are

OD and EW look like neighbouring products, yet they sit on opposite sides of the oil-water divide. An oil dispersion carries solid active particles in an oil, while an emulsion in water carries oil droplets in an aqueous phase. That single difference decides which actives fit, how the product behaves on dilution and which failures to expect. The specifications manual and the CropLife monograph define both precisely.

The FAO/WHO specifications manual defines an oil dispersion as a stable suspension of active ingredients in an organic fluid, intended for dilution with water before use. It defines an emulsion, oil in water, as a stable emulsion of active ingredients in an aqueous phase. In an EW the active is normally a liquid forming the dispersed oil phase, or a solid or liquid dissolved in a water-immiscible solvent. The EW guideline applies to aqueous macro-emulsions only, not to suspo-emulsions, water-in-oil emulsions or micro-emulsions.

With the definitions fixed, the next question is how an oil dispersion is built.

Designing an Oil Dispersion: Carrier Oil, Dispersant, Rheology Modifier and Emulsifier

An oil dispersion has five jobs to do: carry the particles, keep them apart, stop them settling, emulsify on dilution and stay homogeneous through storage. Each job maps to a class of ingredient, and the classes interact. A supplier technical page states that choosing the right oil and surfactant system, including a non-aqueous dispersant, is the key to a good result. A formulator therefore designs the oil, the dispersant and the thickener as one system.

The process follows the same logic as a suspension concentrate. One supplier formulation guide mixes the dispersant into the oil, adds the active, disperses it under high shear and wet-grinds the premix until finely dispersed. The remaining ingredients go in afterwards. An open-access study of a botanical oil dispersion milled its premix in a bead mill with zirconium beads until the median particle size was 5 micrometres or less. Our guide to wet milling for suspension concentrates covers the milling principles that carry over.

Oil dispersions have clear advantages and clear limits. A supplier page reports that they suit hydrolytically unstable actives such as sulfonylureas. The oil phase acts as a built-in adjuvant, and no preservative is needed because the product contains no water. It also lists more sedimentation than water-based systems and difficult formulation as limitations. The botanical study's authors conclude, against a soluble concentrate reported in earlier work, that the oil dispersion improved spray uniformity and leaf coverage.

An emulsion in water is a different balancing act, because the dispersed phase is liquid.

Emulsion in Water (EW) Design: Solvent, Emulsifier Pair and Droplet Stability

An EW packs the oil phase as droplets in water, so the formulator controls droplet size, the interfacial film and the solvent together. The active is dissolved in a water-immiscible solvent, and an emulsifier system holds the droplets apart. Because the pack already contains an emulsion, stability must last through storage, not only through dilution. The design choices are the solvent, the emulsifier balance and the continuous phase additives.

The supplier guide shows the structure with a fungicide example. It combines an emulsifier made of anionic and non-ionic surfactants, a water-immiscible solvent for the active, propylene glycol as an antifreeze and water as the aqueous phase. The order of addition is simple: mix emulsifier and solvent, then add the active, the glycol and the water and homogenise. An open-access review of pesticide emulsions reports that surfactants with a hydrophilic-lipophilic balance of about 10 to 16 give good oil-in-water agricultural formulations.

The review defines Ostwald ripening as the net transport of oil from smaller droplets to larger ones through the continuous phase. It adds that an oil that is insoluble in the continuous phase gives a kinetic barrier to ripening. Solvent choice is therefore a stability decision as well as a solubility decision. Our guides to emulsion science and to creaming, sedimentation and coalescence explain these mechanisms in depth.

Whichever type you choose, the specification clauses decide whether the product passes.

Stability Testing for OD and EW: Dispersion, Emulsion and Storage Clauses

The FAO/WHO manual gives a template specification for each type, and the templates share a skeleton. They cover pourability, behaviour on dilution, foam and two storage tests. They differ in what happens after dilution: an oil dispersion is checked for cream, free oil and sediment, while an emulsion is checked for cream and free oil. Knowing the clauses early lets a formulator design to the test, not discover it at registration.

The manual explains the logic for oil dispersions. Pourability confirms the product leaves its container, dispersion stability, wet sieve and foam confirm sprayability, and storage at elevated temperature confirms the absence of crystal growth. The manual also says laser diffraction particle size is not an ideal criterion for liquid formulations. Wet sieve plus suspensibility or dispersion stability should be used instead. The 0 °C test is not normally an essential clause for an oil dispersion.

Development studies add measurements beyond the template. The botanical oil dispersion study followed stability by multiple light scattering and measured suspensibility after hot storage. It judged wetting by static surface tension and dynamic contact angle, and spray deposition by droplet density and leaf coverage. Those tests tie the bench formulation to field behaviour, but they supplement the specification clauses rather than replace them.

The tests exist because particular failures keep recurring.

OD and EW Failure Modes: Sedimentation, Crystal Growth, Free Oil and Foam

Most failures of these two systems trace back to the physical state of the dispersed phase. Solid particles in oil settle, grow or clump, while liquid droplets in water cream, merge or ripen. Each failure has a signature the test clauses are designed to catch. Reading the signature points to the fix.

Crystal growth deserves special attention in oil dispersions. The manual states that elevated-temperature storage exists to ensure the absence of crystal growth upon storage, and the wet sieve then detects the crystals that formed. Solubility of the active in the carrier oil is therefore a measurement to make early, not an assumption. Our agrochemical technology and formulations guide places OD and EW among the other formulation types.

Knowing the failure modes makes the choice between formulation types more concrete.

Choosing Between OD, EW, SC and EC: Active Chemistry and Regulatory Fit

Formulation type follows the active, not the other way round. The active's state, its sensitivity to water and its solubility in practical solvents narrow the choice quickly. Commercial factors then decide among the remaining options, including solvent regulation, packaging and the registration route. A short decision framework prevents a long development detour.

1. Liquid active or one that dissolves well in a safe solvent — EC or EW; the EW holds the solution as droplets in water.
2. Solid, stable in water — SC or a water-dispersible granule, with water as the fluid.
3. Solid, hydrolytically unstable — OD, because the oil phase excludes water and needs no preservative.
4. Solid that must be loaded with other oil-soluble actives — OD, since the definition allows other dissolved actives in the oil.
5. Active that needs better leaf contact — OD, where the oil phase acts as a built-in adjuvant according to supplier literature.
6. Solvent load is a concern — EW rather than EC, because water carries part of the volume.

Registration then follows the formulation. In India, the CIB&RC publishes a list of pesticide formulations registered under the Insecticides Act, 1968. That list is the first place to check whether a given active and formulation type is already registered. Specification templates from FAO and WHO then give the physical clauses buyers expect. Dose rates, loadings and use claims are outside this guide by design, because registration and field data set them for each product.

The botanical oil dispersion study also reminds formulators that the application method matters. That study designed its product for unmanned aerial spraying and screened wetting and deposition accordingly. A consultant brings these constraints into the brief at the start.

Turning that choice into a robust product is the work of a formulation partner.

Working With an Agrochemical Formulation Consultant

OD and EW development combines colloid science, solvent and oil selection, milling or emulsification engineering and regulatory knowledge. An agrochemical formulation consultant measures the active's solubility and water stability, screens oils and surfactant systems, and designs the test plan around the FAO/WHO clauses. Agrochemical product development services extend that into pilot batches, stability data packages and technical documents for registration. A contract formulation agrochemical manufacturer then runs the validated process at scale.

1. Characterise the active — physical form, solubility in oils and solvents, and stability in water.
2. Pick the formulation type — OD, EW, SC or EC, using the decision framework above.
3. Screen the carrier and surfactants — oil, dispersant, emulsifier and rheology modifier as one system.
4. Mill or emulsify — set the particle size or droplet size target and the equipment.
5. Run the specification tests — dispersion or emulsion stability, wet sieve, foam and both storage tests.
6. Pilot and scale — confirm repeatability across batches.
7. Transfer to manufacture — write the process, quality limits and release tests for the contract partner.

Our agro chemicals range shows where formulation expertise supports crop protection products. Global Formulation provides formulation screening, stability programme design, specification support and contract manufacturing transfer for agrochemical manufacturers. Active loadings and surfactant ratios stay outside this guide by design, because they belong to each client's product.