Abstract

The article highlights the impact of the mechanochemical stress phenomenon occurring during the air jet milling on the final physicochemical properties of WG pesticide formulations.

WG (WDG) is a solid pesticide formulation containing granules which when applied are disintegrated and dispersed in water. Production process for the WG formulations involves a fine grinding step, followed by granulation and subsequent drying. For the application of the resulting composition, the granules must be dispersed in the water of the spray tank and completely disintegrated to the primary micro-particles obtained after fine grinding. Then obtained aqueous suspension should be sprayed in the field.

The ability of WG granules to disintegrate rapidly in the spray tank forming a stable aqueous suspension of fine particles of the biologically active ingredients is a major requirement for qualified WG products. The fine grinding as the first step of the production process defines the further granulometric distribution of A.I. particles in the spray tank.

JET MILLING

Jet mills are high intensity grinding mills. They are used to produce particles smaller than 10 µm. In the jet mill, grinding gas, usually compressed air, is fed into the grinding chamber through special nozzles and is accelerated so much that particles are carried away. The particles collide repeatedly with each other and are thus crushed. The fineness of the product and the granulometric distribution of the ground particles are affected by the mechanical parameters of jet mill: feed rate, grinding air flow rate, the retention time in the grinding chamber, the grinding air pressure, and the setup and frequency of the classifier.

In air jet mill, the main breakage mechanisms of the particles are:

1. destructive breakage due to impact and
2. abrasion because of attrition between particles.

High grinding pressure results in destructive breakage, while the abrasion grinding mechanism prevails at the lower grinding pressure. The abrasion breakage mechanism produces more rounded particles with less sharp edges. The mechanism of abrasion breakage in the jet mill depends on the particle concentration in the grinding chamber and is characterized by an increase in the number of fine particles that led to the formation of a new surface area with an increased retention time.

MECHANOCHEMICAL STRESS

Although fine grinding in the air jet mill is aimed at reducing the size of the particles, another phenomenon also occurs because of the amount of energy transmitted to the particulate matter. Some of this energy is absorbed by the material. This leads to structural changes in the particulate material because of the loss of crystal network regularity - amorphization and the formation of active surfaces. Such active surfaces cause an increase in reactivity in the near-surface region where solid particles come into contact. These chemical and physicochemical effects in solids, caused by mechanical forces, are called mechanochemical effects.

The finest particles under mechanochemical stress cause surface abnormalities, such as increased leaching due to amorphization and increased reactivity, lower thermal decomposition temperature and decreased sintering temperature (the temperature at which particles can be associated into one large solid particle under pressure and heat without melting the material). Thus, part of the particulates after the fine grinding process becomes more sensitive to further granulation parameters, such as increased temperature, particulate wetting, and shear stress.

FURTHER QUALITY PROBLEMS

The main problem with increased surface energy on the smallest particles is the interaction between them, resulting in larger particles. This is true for particles smaller than 10 µm. There are three stages of interaction between the smallest material particles that result in larger particles:

• Adherence — the particles coat the apparatus and grinding bodies.
• Aggregation — the particles are associated by weak Van Der Waals type of adhesion. This interaction is reversible.
• Agglomeration - the particles interact with each other through a very compact irreversible interaction, in which chemical bonding can also play a role. Agglomeration is harmful to both the activity and the quality of the product.

The interaction between fine particles, resulting in larger particles, creates a so-called grinding limit, where the results of the grinding are determined by two opposite mechanisms: particles breakage and agglomeration of the resulting particle fragments. These competing processes result in a particle-sized plateau, whereas the power supply of the fine grinding increases, the particle size no longer decreases. The agglomeration and therefore the final size of the agglomerates are controlled by the addition of stabilizers and the applied shear stress.

During the subsequent production stages, such as granulation and drying, the agglomeration between the crushed fine particles is increased by the above-mentioned surface abnormalities. This results in loss of ability to further disintegrate in the spray tank. In addition, amorphous areas formed on the surface of fine particulates make the particles very sensitive to further agglomeration because of "caking" during subsequent storage. Normally, stabilizers that are added to the solid premix before grinding contain water-soluble salts. When storing a granulated product, the difference in the air relative humidity results in inert additive particles being dissolved in condensed moisture at higher humidity and crystallized because of evaporation at lower humidity. Under the pressure on the particles due to the weight of the package, this causes the formation of solid bridges between A.I. particles. This agglomeration of particles inside the granules slows their disintegration and dispersion in the spray tank because of more time and higher temperature, necessary for dissolving solid salt bridges.

Sometimes the manufacturer of active substances supplies the ground material with a wide particle size distribution. The fine grinding of the formulation at the first stage of WG production, when it contains preliminary ground active substance, can cause "over-milling". This further increases the adverse effects of mechanochemical stress on A.I. particles.

CONCLUSION

So, we see that mechanochemical stress in the air jet milling may further have a very serious negative effect on the basic properties of the final WG formulation – its ability to rapidly disintegrate granules in the spray tank and to produce a stable suspension of fine A.I. particulates in the aqueous tank mix.

Thus, a strong focus on the design and operating parameters of the fine grinding process in air jet mills must be a key part of the WG formulation development.