How Particle Size Affects SC Pesticide Performance

Particle size is one of the key physical variables in a suspension concentrate, or SC, because the active ingredient remains as fine solid particles rather than being completely dissolved in the liquid phase.

The size and distribution of those particles can influence sedimentation, suspension stability, redispersibility, dilution behavior, sprayability and biological performance.

However, smaller particles are not automatically better.

Very coarse particles can settle rapidly or create undesirable residue, while excessively fine particles increase total surface area and particle interactions. A good SC formulation therefore requires a controlled particle-size distribution that works together with the dispersant system, rheology, active ingredient properties and storage conditions.

Particle size should be treated as one part of SC formulation quality—not as a single number that determines whether an SC product is good or bad.

Why Does Particle Size Matter in an SC Formulation?

An SC is a multiphase liquid formulation.

Fine solid particles of the active ingredient are dispersed through a continuous liquid phase, usually together with:

• dispersants;
• wetting agents;
• rheology modifiers;
• antifoaming agents;
• preservatives;
• other formulation aids.

Because these particles remain physically separate from the liquid phase, they can move, settle, collide, agglomerate or change during storage.

Particle size affects several of these processes.

Larger particles may respond more strongly to gravity and settle more rapidly under otherwise similar conditions. Smaller particles generally have a larger total surface area relative to their mass and interact more strongly with surrounding formulants and other particles.

This is why SC formulation development is not simply about grinding the active ingredient as finely as possible.

The objective is to create a stable particle-size distribution that remains suitable through manufacturing, storage, dilution and application.

How Does Particle Size Affect Suspension Stability?

Particle size is closely related to SC suspension stability, but it does not act alone.

The broader stability of an SC also depends on:

• particle density;
• continuous-phase density;
• viscosity;
• rheological structure;
• dispersant performance;
• particle interactions;
• temperature;
• storage time.

Particle Size and Sedimentation

Under comparable conditions, larger suspended particles generally tend to settle more readily than smaller ones.

Reducing particle size can therefore help reduce rapid sedimentation.

However, this does not mean small particles remain permanently suspended.

Even a finely milled SC can develop sediment if the formulation has poor rheology or insufficient stabilization.

The important questions are:

• how quickly particles settle;
• whether the sediment remains soft;
• whether the product can be adequately redispersed;
• whether the formulation remains within specification.

A controlled particle size is therefore one part of sedimentation management rather than a complete solution.

For a broader explanation of this physical system, see Why Suspension Stability Matters in SC Pesticide Formulations.

Caking and Redispersibility

Sedimentation becomes more serious when particles form a compact layer at the bottom of the container.

A soft sediment that can be readily returned to suspension is very different from a hard cake.

Particle size can influence this process because the surface area and interaction behavior of particles change as the formulation becomes finer.

Poorly controlled particles can also agglomerate, effectively creating larger structures even if the formulation was originally milled to a fine size.

This is why redispersibility is as important as initial particle fineness.

The SC needs to maintain a particle system that remains physically manageable after storage.

Is Smaller Particle Size Always Better?

No.

This is one of the most common oversimplifications in discussions about SC quality.

A finer particle distribution can provide advantages, but continuously reducing particle size can also create new formulation challenges.

When particles become very fine, their total surface area increases.

That means the formulation may require more effective surface stabilization from dispersants and other formulation components.

Very fine particles can also affect:

• rheology;
• viscosity;
• agglomeration tendency;
• crystal-growth behavior;
• interaction with surfactants.

Therefore, the formulation objective is not:

Make the particle size as small as technically possible.

It is:

Develop a particle-size distribution that remains stable and performs consistently throughout the product lifecycle.

How Does Particle Size Affect SC Spray Performance?

The importance of particle size does not end inside the original pesticide container.

An SC is normally diluted with water before spray application, so the particles must also behave appropriately in the diluted suspension.

Dispersion After Dilution

When an SC enters the spray tank, the concentrated particle system must disperse through a much larger volume of water.

Particle size and surface stabilization can affect how easily this happens.

A well-designed formulation should disperse sufficiently under the conditions specified for the product.

If coarse particles, agglomerates or unstable material remain, the spray mixture may become less uniform.

Water quality, agitation and formulation chemistry also influence this process, so particle size should not be considered independently.

Coarse Particles and Wet-Sieve Residue

One reason coarse particles matter is their potential effect on spray preparation and equipment.

Wet-sieve testing is commonly used to detect an unacceptable amount of oversized material or coarse residue in formulations intended to form suspensions.

This is particularly relevant because excessively large particles or agglomerates may contribute to:

• poor dispersion;
• uneven suspension;
• screen or nozzle problems;
• undesirable spray residue.

However, wet sieve does not tell you the complete particle-size distribution.

It mainly helps identify coarse material above the relevant sieve size.

Particle Size Is Not Spray Droplet Size

These two terms should not be confused.

SC particle size refers to the size of the solid active ingredient particles dispersed in the formulation.

Spray droplet size refers to the size of the liquid droplets produced by the spray nozzle during application.

A spray droplet can contain many much smaller SC particles.

The two measurements describe completely different physical objects and affect different parts of pesticide performance.

Particle size is primarily a formulation property.

Droplet size is primarily an application property influenced by factors such as:

• nozzle type;
• pressure;
• spray solution;
• application equipment.

Using these terms interchangeably can lead to incorrect conclusions about SC performance.

Can Smaller Particles Improve Pesticide Efficacy?

They can influence efficacy in some formulations, but smaller particle size does not guarantee stronger biological performance.

Reducing particle size increases surface area.

Depending on the active ingredient and target, this may influence:

• dispersion;
• surface contact;
• deposition;
• dissolution behavior;
• biological availability.

But efficacy is determined by far more than particle size.

It also depends on:

• active ingredient properties;
• concentration;
• formulation composition;
• target pest, disease or weed;
• application rate;
• spray coverage;
• environmental conditions;
• uptake characteristics.

Two SC products with different particle-size distributions may therefore perform differently, but it would be incorrect to assume that the product with the lowest Dv50 must always be more effective.

A more useful formulation principle is:

Optimize particle size for balanced formulation and biological performance rather than minimize particle size as an isolated target.

How Are Particle Size and Crystal Growth Connected?

Particle size measured immediately after manufacturing does not necessarily remain unchanged throughout storage.

This is important.

Some active ingredients have limited but non-zero solubility in the continuous phase.

During storage, material can dissolve from smaller particles and contribute to the growth of larger crystals under suitable conditions.

This general phenomenon is often associated with Ostwald ripening.

The practical consequence is that an SC initially produced with a fine particle distribution may gradually develop larger crystals.

This can contribute to:

• changing particle-size distribution;
• faster sedimentation;
• wet-sieve residue;
• poor redispersibility;
• storage instability.

Particle growth is influenced by the active ingredient, temperature and formulation environment.

For this reason, SC particle size should not be evaluated only immediately after milling.

Storage stability matters as well.

Pomais explains the broader relationship between formulation aging and physical stability in How Pesticide Formulation Affects Shelf Life.

How Is Particle Size Controlled During SC Manufacturing?

Particle size is normally established during the dispersion and milling stages of SC production.

The exact industrial process varies by active ingredient and formulation, but the general principle involves reducing solid particles while preventing them from forming unstable aggregates.

Pre-Dispersion

Before fine milling, the technical active ingredient is incorporated into the liquid system with appropriate dispersing and wetting components.

The objective is to create a sufficiently uniform mill base.

Poor initial wetting can make later particle-size reduction more difficult.

Wet Milling or Bead Milling

SC formulations commonly use wet-milling technologies such as bead milling to reduce particle size.

During milling, mechanical energy breaks larger particles into smaller ones.

Process control matters because milling can influence:

• particle-size distribution;
• temperature;
• viscosity;
• particle surface area;
• formulation stability.

Excessive milling is not automatically an advantage.

The target is the required particle distribution for that formulation.

Role of Dispersants

Freshly created particle surfaces can interact strongly with one another.

Dispersants help stabilize these surfaces and reduce uncontrolled aggregation or flocculation.

This explains why particle-size reduction and dispersant selection must be developed together.

Milling alone does not create a stable SC.

After the desired dispersion is established, the formulation may also require rheology adjustment to balance settling resistance with pourability and practical handling.

How Is Particle Size Evaluated in SC Pesticides?

Particle-size evaluation can involve more than one method.

Different tests answer different questions.

Laser Diffraction

Laser diffraction is widely used to measure particle-size distribution.

Instead of producing only one particle-size number, it provides a distribution profile.

Common values include:

Dv10
Approximately 10% of the measured particle volume is below this size.

Dv50
Approximately 50% of the measured particle volume is below this size. This is often described as the volume median particle diameter.

Dv90
Approximately 90% of the measured particle volume is below this size.

Looking at these values together provides more information than Dv50 alone.

For example, two SC products can have similar Dv50 values while having very different coarse tails represented by Dv90.

That difference may matter for formulation behavior.

Wet Sieve Testing

Wet-sieve testing serves a different purpose.

It evaluates the amount of material retained on a specified sieve after the formulation is dispersed according to the relevant method.

It is particularly useful for identifying:

• coarse particles;
• large crystals;
• agglomerated material;
• oversized residue.

Therefore:

Wet sieve is not the same as particle-size distribution analysis.

Laser diffraction provides information about the particle-size distribution.

Wet sieve primarily identifies unacceptable coarse fractions above the sieve threshold used for that product specification.

Both can be useful, but they answer different questions.

Is There an Ideal Particle Size for SC Pesticides?

There is no single universal particle size that is ideal for every SC pesticide.

Micron-scale particles are common in SC technology, but an arbitrary statement such as:

“All good SC formulations should have particles between X and Y microns”

is too simplistic.

The appropriate distribution depends on factors such as:

• active ingredient crystal properties;
• solubility;
• concentration;
• density;
• dispersant system;
• rheology;
• biological target;
• storage conditions.

This is also why SC quality specifications commonly rely on multiple formulation-performance parameters instead of using one universal particle-size limit.

A formulation should be evaluated against its own applicable product specification and stability data.

Why Is Dv50 Alone Not Enough?

Dv50 is useful, but it does not describe the whole particle population.

Consider two hypothetical formulations:

SC A:
relatively narrow particle distribution.

SC B:
similar Dv50 but a much larger coarse-particle fraction.

If only Dv50 is compared, they may appear similar.

But their Dv90, wet-sieve results and storage behavior may be very different.

Likewise, a very low Dv50 does not prove:

• good suspensibility;
• good redispersibility;
• acceptable viscosity;
• no crystal growth;
• good storage stability.

For technical evaluation, the more useful question is:

How does the complete particle-size distribution interact with the rest of the SC system?

What Should Be Checked Alongside Particle Size?

Particle size should always be interpreted with other SC quality parameters.

This is why particle size should not be used as a standalone purchasing or QC decision.

For example, an SC may show a fine initial Dv50 but still develop crystal growth during storage.

Another product may have a slightly larger median particle size but maintain better overall suspension behavior.

The more meaningful comparison is system performance.

A commercial formulation such as POMAIS Atrazine 50% + Mesotrione 5% SC illustrates this approach: particle-size governance is considered alongside rheology, suspensibility, screening and storage stability rather than being treated as the only measure of SC quality.

Does a Fine Particle Size Guarantee Good SC Quality?

No.

Good SC quality depends on the interaction of multiple formulation variables.

A technically sound suspension concentrate needs an appropriate combination of:

• particle-size distribution;
• dispersant system;
• rheology;
• viscosity;
• suspensibility;
• redispersibility;
• storage stability;
• dilution performance.

Particle size is important because it affects several of these properties.

But it cannot compensate for poor dispersant selection, weak rheology or unstable crystal behavior.

This explains why two products containing the same active ingredient at the same concentration can still show different physical performance.

The formulation around the active ingredient matters.

Frequently Asked Questions

What is the typical particle size of an SC pesticide?

SC formulations commonly contain micron-scale active ingredient particles, but there is no universal optimum particle-size range for every SC. The appropriate distribution depends on the active ingredient and complete formulation system.

Is smaller particle size always better in an SC formulation?

No. Smaller particles can reduce some sedimentation problems and increase surface area, but extremely fine particles can also increase surface interactions, alter rheology and create additional formulation-stability challenges.

Does particle size affect pesticide efficacy?

It can. Particle size may influence dispersion, surface area, deposition and biological availability, but efficacy also depends on the active ingredient, formulation, target organism and application conditions. Smaller particles do not automatically mean better control.

What does Dv50 mean in pesticide particle-size analysis?

Dv50 is the particle diameter below which approximately 50% of the measured particle volume falls. It represents the volume median diameter, but it should be interpreted together with values such as Dv10 and Dv90.

Is wet sieve the same as particle-size analysis?

No. Wet sieve mainly evaluates the amount of oversized material retained on a specified sieve. Particle-size analysis such as laser diffraction measures the broader distribution of particle sizes.

Why Particle Size Should Be Evaluated as Part of the Complete SC System

Particle size has a major influence on how a suspension concentrate behaves, but the correct formulation objective is not simply to produce the smallest particles possible.

A useful SC needs a controlled particle distribution that supports:

• manageable sedimentation;
• adequate redispersibility;
• stable storage;
• effective dispersion after dilution;
• acceptable sprayability;
• consistent formulation performance.

The most important technical principle is therefore:

Particle size affects SC performance, but particle size alone does not define SC quality.

A reliable evaluation should combine particle-size distribution with suspensibility, wet-sieve results, rheology, redispersibility and storage stability.

This system-level approach provides a more meaningful picture than comparing Dv50 values alone.

For commercial crop-protection programs requiring suspension concentrate products, POMAIS provides a broad range of pesticide formulations with SC options developed for different active ingredients, crops and registered market requirements.