Water dispersible granules

Definition and interests

Water-dispersible granules (WG) are obtained by blending and agglomerating a ground solid active ingredient with surfactants and other formulation ingredients, using water (or water + surfactants) as the agglomerating agent. A drying step is necessary to reduce moisture to 1-2%. The shape, size, and performance of the granules vary depending on the manufacturing process used to produce them.

General Method of Preparation

Requirements for the active ingredient

Solid actives are better suited to granulation. Nevertheless, it is possible to formula­te liquid actives as water-dispersible granules by absorbing them onto an inert sup­port, usually micronised silica such as TIXOSIL.

A high melting point of active ingredient(s) is required, particularly for spray-dried granules (MP > 150°C), also for reaching a proper storage stability at 50°C.

Especially for spray-dried granules, it is very important to assess the thermal stability of the active ingredient as a pure material and in slurry (chemical stability, explosion risk, determination of optimal spray-drying conditions, etc.). Final suspension values of WG depend on their fineness; to obtain optimum results, the particle size must be between 1 and 10 μm. A milling process is necessary to achieve the desired particle size of the actives. Wet-milling is generally used for spray-dried formulations. Dry milling (by jet mill) is generally carried out on the active ingredients, mixed with the other ingredients and inerts needed for the granulation processes.

Effect of surfactants

Surfactants play several roles during the preparation of the formulation and condition the final performances of the granules:

• help the milling of the a.i. by adsorption onto the solid parti­cles (for spray-dried formulation)
• help the blending and the extrusion by reducing the friction forces (binding and lubricant effect)
• provide wettability to the particles of the active ingredient
• provide stabilisation by giving cohesion to the system

Wetting agent

Wetting agents decrease the interfacial tension between the solid active ingredient (which is mostly hydrophobic) and water. The choice of a wetting agent must be made with the active's hydrophobic character and the type of dispersing agent used in the formulation in mind. This is the wetting/dispersing system that gives the granules their performance properties; it must be optimised for each formulation based on the granulation process.

Dispersing agent

Dispersing agents alter the potential for interactions by adsorbing onto the surface of the active ingredient particle, preventing reagglomeration or flocculation (steric and electrostatic stabilisation). The lipophilic part is adsorbed onto the organic particles, and the hydrophilic part provides the particles with a good affinity for the aqueous medium.

Binding agent

The binding agent must have a long polymeric chain in order to create cohesive for­ces between solid particles. Polymeric dispersing agents generally also act as bin­ding agents.

Disintegrating agent

This type of additive can sometimes be useful for improving dispersibility. Clays are generally selected for this purpose.

Fillers

Sometimes, to respect a specific active content, a certain quantity of an inert charge can be used to adjust the composition and to improve its performan­ces. Fillers can be water-soluble (for extrusion pan granulation and fluid bed) or not water-soluble (for all processes) and can be used alone or in combination to meet formulation requirements.

Selection of components

Dispersing and binding agents

The most efficient dispersing agents are synthetic polymers; these products also act as binding agents.

Wetting agents

The most effective surfactants to use as wetting agents in WG are anionic, while non-ionic surfactants are seldom used.

Compatibility agents

When the WG formulation has to be mixed together with some other formulation, like EC, or when tank mix compatibility is needed between two active ingredients, we recommend incorporating a compatibility agent into the WG formulation. These products are also efficient in improving the dispersability of the granules.

Disintegrating agents and fillers

Inert mineral compounds such as clays, bentonites, diatomaceous earth, colloidal silica, kaolin, etc., which swell by water absorption, improve in many cases WG's dispersibility. Starch (corn, potatoes, etc.) can be very efficient, especially because they have aqueous swelling properties and also binding capabilities. These inerts can also be used as fillers in order to adjust the active content in the formulation. In some cases, water-soluble salts, such as potassium phosphate, sodium and am­monium sulphates, sodium citrate, or urea can be used. These fillers help to improve dispersibility by creating porosity in the granules. They are mainly recommended for extrusion, pan granulation and fluid bed granulation.

Antifoams

The choice of an antifoam depends on the process used. It can be used to limit foam during the process (spray drying technique) and to limit foam during dilution in the tank or application on crops.

Manufacturing Processes of WG

Spray drying

Three steps to the working process:

• preparation of a slurry
• spray-drying of the slurry
• drying of the resulting granules

General composition

Active ingredient(s)     40 to 80 %

Dispersing agent          4 to 10 %

Wetting agent               2 to 4 %

Lignosulfonate salt       0 to 10 %

Antifoam, stabiliser       0 to 10 %

Water                            up to 100 %

Preparation of the slurry

The slurry is prepared by blending water, active ingredient(s), wetting and dispersing agents with other components. This is then homogenised by strong, suitable mechanical stirring or a high shear mixer.

Such dispersion, if necessary, can be adjusted with water and milled with wet equipment in order to obtain a slurry with the following properties:

• dry extract of the slurry: between 50 and 65%
• average particle size: 1 to 5 μm (particles should not exceed 10 μm)
• viscosity (Brookfield 20 rpm, at 25°C): less than 1000 mPa.s
• The dynamic rheology of the slurry should have a pseudo-plastic behaviour.

Spray drying procedure

The slurry is sprayed through a nozzle (bi-fluid nozzle, pressure nozzle) in large dro­plets (100 to 500 μm). The conditions involved in spray drying (inlet temperature, slurry flow rate, inlet air flow rate, nozzle type, etc.) must be adjusted for each piece of equipment to achieve the desired particle residence time in the spray drier. Spray-dried granules are perfectly spherical (sizes range between 150 and 400 μm) and hollow (see picture). This hollow shape is the result of a migration of the different components of the formulation during water evaporation. Granules have relatively poor resistance to attrition due to their structure. The moisture after spray drying normally varies between 8 and 12%.

Extrusion

Granules are directly obtained from the ground dry mixture of the active, bin­ding/dispersing agent, wetting agent, fillers and other additives mixed with water. Two types of equipment are generally used to wet the ground powder: a low shear, kneading-type mixer or a high shear, high-intensity mixer. The water content must be adjusted to achieve the desired rheology of the mixture. 

Fluid bed

An aqueous solution of the wetting agent is sprayed directly onto a fluidised bed of the blended and milled formulation powder. Hot air is used to fluidise the bed, so that the granules are dried as they agglomerate. The amount of water, the spray rate, and the air temperature should be adjusted to achieve optimal performance. The granules are irregular in shape and in size (between 0.5 and 2 mm). This pro­cess is more suitable for small-scale productions