How Dispersant Chemistry Controls Particle Stability in Agrochemical Formulations

Learn how dispersant chemistry controls particle stability, aggregation, sedimentation and redispersibility in suspension concentrate agrochemical formulations.

In Agrochemical formulations, achieving uniform dispersion of an active ingredient is only the beginning. The more difficult challenge is maintaining that dispersion during milling, storage, dilution and application.

This is particularly important in suspension concentrates (SCs), where finely milled solid active ingredients are dispersed in a liquid phase. Without adequate stabilization, particles can interact, agglomerate and settle, resulting in caking, increased viscosity and poor redispersibility. Changes in particle size during storage can further affect formulation consistency.

This is why dispersant selection is not simply a formulation additive decision. It is a particle stabilization strategy.

What Does a Dispersant Control?

Dispersants are designed to adsorb onto the surface of active ingredient particles and modify the interactions between them. By influencing the particle surface, they can reduce the tendency of particles to approach, aggregate or form a compact sediment. The basic mechanism can be represented as:

Active ingredient surface → dispersant adsorption → particle stabilization → improved dispersion

However, dispersant performance depends strongly on the chemistry of both the dispersant and the active ingredient. Surface polarity, particle characteristics, formulation medium, pH, ionic strength and dispersant molecular structure can all influence adsorption and stabilization.

Therefore, a dispersant that performs well with one active ingredient may not provide the same stability with another.

Electrostatic and Steric Stabilization

Particle stability can be achieved through different mechanisms.

Electrostatic stabilization creates repulsive forces between particles carrying similar charges. This can reduce particle aggregation, but its effectiveness may change with pH, electrolyte concentration and ionic strength.

Steric stabilization works differently. Polymeric or appropriately structured dispersants adsorb onto the particle surface and create a physical barrier between particles. This barrier reduces close particle contact and helps limit aggregation.

In many formulations, electrostatic and steric effects can contribute together to provide more robust stabilization. 

Why Dispersant Chemistry Matters During Milling

Dispersant performance becomes critical during wet milling, where particle size is reduced to achieve the required formulation characteristics.

Milling continuously generates fresh active ingredient surfaces. If these surfaces are not adequately stabilized, newly formed particles can re-agglomerate. This can reduce milling efficiency and make it difficult to maintain the desired particle size distribution.

Effective formulation design therefore connects:
Wetting → Milling → Particle stabilization → Storage stability

The objective is not simply to achieve a small particle size. It is to prevent that particle size from changing significantly during storage and subsequent handling.

What Causes Particle Instability During Storage?

A formulation can appear stable immediately after production but develop instability over time.

Agglomeration can increase effective particle size as particles associate with each other. Sedimentation and caking can make the formulation difficult to redisperse. In some systems, Ostwald ripening can also contribute to particle growth, where material from smaller particles is transferred to larger particles. These mechanisms can influence viscosity, sedimentation behaviour, redispersibility and ultimately application consistency.

How Should Dispersant Performance Be Evaluated?

Dispersant selection should be based on more than initial dispersion quality. Formulators should consider:

  • Particle size distribution
  • Sedimentation behaviour
  • Redispersibility
  • Viscosity and rheology
  • Storage stability
  • Dilution stability
  • Resistance to particle growth or agglomeration

Testing these parameters together provides a better understanding of long term formulation stability.

Designing the Right Dispersant System

There is no universal dispersant for every agrochemical active ingredient. The most effective approach is to match dispersant chemistry with active ingredient properties and formulation conditions.

At Dai ichi, our specialty surfactant expertise supports formulation development by considering these interactions together. The focus is not only on achieving initial dispersion, but on maintaining particle stability throughout processing, storage and application.

Dai-ichi offers dispersants based on various chemistries. It offers anionic dispersants based on sulfation and phosphorylation chemistries which provide electrostatic stabilisation, and nonionic dispersants based on high molecular weight alkoxylates and graft copolymers which provide steric stabilisation. 

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