Preventing Active Ingredient Crystallization in SC

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The production of suspension pesticide formulations (SC) is one of the technologically demanding processes, where even minor deviations in the recipe or production conditions can lead to serious issues. One of our clients, a manufacturer of broad-spectrum fungicides, faced repeated crystallization of the active ingredient during storage, which caused clogging of application equipment and complaints from end users. Problem analysis revealed that the key factor was not only the choice of surfactants but also the interaction between dispersing agents, stabilizers, and the viscosity of the system. In this article, we will explore how we approached the solution and what adjustments led to a long-term stable formulation.

Crystallization Issues in Concentrated Suspension Pesticides: When the Active Ingredient Refuses to Stay in Solution

Crystallization of active ingredients in suspension concentrates (SC) is one of the most common and challenging issues in the production of pesticide formulations. It typically manifests as the formation of solid particles or sediments, which clog application equipment, reduce product efficacy, and shorten its shelf life. The cause is often a mismatch between the solubility of the active ingredient and the physico-chemical conditions of the formulation—particularly temperature fluctuations, pH, or interactions with other components such as surfactants, dispersants, or antifreeze additives.

In practice, this phenomenon is encountered, for example, with herbicides based on sulfonylureas or certain fungicides from the triazole group. These substances have limited water solubility and begin to crystallize spontaneously at active ingredient concentrations above 40%, especially if the temperature drops below 10 °C. The problem is further exacerbated during storage in unheated spaces or transportation in winter months. Therefore, it is crucial at the formulation development stage to consider not only chemical stability but also the rheological properties of the suspension and its behavior under various conditions.

Root Cause Analysis: How to Identify the Source of Crystallization in Operational Conditions

The first step in addressing crystallization is a systematic analysis of its causes. In laboratory conditions, a combination of microscopic observation, particle size measurement (e.g., laser diffraction), and thermal analysis (DSC) has proven effective. Microscopy reveals the morphology of the crystals—whether they are needle-like, plate-like, or amorphous structures—which provides insight into their formation mechanism. For example, needle-like crystals often indicate rapid nucleation caused by supercooling, while plate-like structures may suggest interaction with dispersing agents.

In operation, it is crucial to monitor the temperature profile during production and storage. Crystallization is often triggered by sudden temperature changes, such as cooling the mixture after grinding or moving from a warm warehouse to a cold environment. Another critical factor is pH – some active substances (e.g., phenoxyacetic acid derivatives) exhibit minimal solubility within a narrow pH range, typically around 5–6. Here, the use of pH stabilizers, such as citrates or phosphates, helps maintain optimal conditions for solubility. Impurities in raw materials, which can act as nucleation centers for crystallization, also have a significant impact.

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Practical solution: Optimizing formulations and process parameters

Based on the analysis, adjustments to the formulation can be made. One of the most effective approaches is the use of suitable solubilizers and co-solvents, which enhance the solubility of the active substance. For aqueous suspensions, glycols (propylene glycol, dipropylene glycol) or low-volatility alcohols, which lower the freezing point of the mixture, have proven effective. For some substances, the addition of antifreeze additives such as urea or salts of organic acids, which prevent the formation of ice crystals and thus secondary crystallization of the active ingredient, is effective.

Equally important is the optimization of process parameters. During suspension milling, care must be taken to ensure that the particle size of the active substance does not exceed the critical limit (usually below 5 micrometers), which minimizes the risk of agglomeration. It is also advisable to control the cooling rate after milling – too rapid cooling can lead to local supercooling and subsequent crystallization. In some cases, dynamic stirring during storage helps prevent sedimentation and the formation of large crystals. For long-term stability, it is crucial to test the formulation under real conditions, including cyclic temperature tests between -5 °C and 40 °C.

Prevention and Long-Term Stability: How to Keep a Suspension Homogeneous Throughout Its Shelf Life

Preventing crystallization requires a comprehensive approach that includes both the correct selection of raw materials and adherence to technological procedures. The foundation is the use of high-quality dispersing agents, which stabilize the suspension and prevent particle aggregation. For this purpose, polymeric dispersants based on polycarboxylates or lignosulfonates, which create a stable protective layer around the particles, are often chosen. It is also important to adhere to the recommended dosage – too low a concentration of dispersant leads to instability, while an excess can cause increased viscosity or foaming.

For long-term stability, it is essential to perform regular quality checks, including visual assessment, viscosity measurements, and sedimentation tests. In practice, it has proven effective to store suspensions at a constant temperature (ideally 15–25 °C) and protect them from direct sunlight, which can accelerate the degradation of certain components. If crystallization is unavoidable, it can at least be slowed down by adding crystallization inhibitors, such as acrylic acid derivatives or specific surfactants with an amphiphilic structure. In any case, it is crucial to collaborate with a raw material supplier experienced in similar formulations, who can recommend proven solutions.

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The impact of adjuvants on suspension stability: How to select the right excipients

Crystallization of active substances in suspension pesticides is often linked to inappropriate selection or dosing of adjuvants. These auxiliary substances play a key role in maintaining dispersion, influencing surface tension, viscosity, and interactions between particles. For example, wetting agents based on alkyl polyglucosides or organosilicones can significantly reduce the risk of aggregation, but their effectiveness depends on the pH and ionic strength of the medium. In practice, combining wetting agents with dispersing agents, such as polycarboxylates or lignosulfonates, which prevent sedimentation and crystal formation, has proven effective.

When selecting adjuvants, compatibility with the active substance must also be considered. Some surfactants, for instance, may promote micellar solubilization but simultaneously destabilize the suspension at higher concentrations. It is recommended to perform compatibility tests according to standard methods, including stability tests at various temperatures (e.g., 0 °C, 25 °C, and 54 °C) and monitoring changes in viscosity or particle size using laser diffraction. A properly selected adjuvant can extend suspension stability by 30–50% without the need for significant changes to the formulation.

Process Parameters and Their Impact on Crystallization: Temperature, Mixing, and Homogenization Time

Crystallization in suspension pesticides is not just a matter of formulation but also of process conditions. Temperature during production and storage has a fundamental impact on the solubility of active substances—for example, in some herbicides, a temperature drop below 10 °C can trigger rapid crystallization. In operation, it is therefore crucial to maintain a stable temperature regime, ideally between 15–25 °C, and avoid sudden fluctuations. During mixing, it is important to ensure sufficient intensity to prevent local thickening of the suspension but not exceed a critical value that could damage particle structure.

The homogenization time also affects the size and distribution of particles. Too short a time may lead to insufficient dispersion, while too long a time can cause excessive heating of the mixture and accelerate crystallization. In practice, gradual stirring with a controlled increase in speed and subsequent cooling of the mixture has proven effective. The process can be optimized using rheological measurements, which help determine the ideal duration and intensity of stirring for a specific formulation. It is also important to monitor viscosity during production, as changes in viscosity can signal the onset of crystallization.

Monitoring and Validation: How to Verify Suspension Stability Before Market Launch

Before launching a suspension pesticide on the market, it is essential to conduct comprehensive stability validation, which includes both laboratory tests and accelerated aging tests. Standard procedures involve monitoring physical stability at various temperatures (e.g., cycling between 5 °C and 40 °C) and humidity levels, simulating real storage conditions. It is also important to measure particle size and distribution using laser diffraction or microscopy, as changes in these parameters often precede visible crystallization.

For long-term stability prediction, accelerated tests are used, in which samples are exposed to elevated temperatures (e.g., 54 °C for 14 days), and the degree of crystallization, sedimentation, or viscosity change is subsequently evaluated. The results of these tests can be correlated with the actual shelf life of the product using the Arrhenius equation, which allows for estimating stability under normal storage conditions. Validation should also include compatibility tests with packaging materials, as some plastics or metal components may release substances that destabilize the suspension. Only after successfully passing all tests can the product be considered stable and ready for distribution.

The stability of your formulations is our priority

Every agrochemical formulation requires an individual approach – from raw material selection to technological processing. GCG Group supplies not only high-quality surfactants, dispersants, and stabilizers, but also comprehensive technical support, including problem analysis and process optimization. For each raw material, we provide detailed safety data sheets and application recommendations. Contact us – or browse our catalog of over 1,300 products.