Water dispersible granules

Definition and interests

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

General Method of Preparation

Requirements for the active ingredient

Solid actives are more suitable for granulation. Nevertheless, it is possible to formula­te liquid actives as water dispersible granules by absorbing them onto an inert sup­port, usually micronized 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 concerning spray-dried granules, it is very important to check the thermal stability of the active ingredient as pure material and when in the slurry (chemical stability, risk of explosion, determination of optimum spray drying conditions, etc.). Final suspension values of WG depend on their fineness; to obtain optimum results the particle size must be between 1 to 10 μm. A milling process is necessary in order to get the right 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 together 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 stabilization by giving cohesion to the system

Wetting agent

Wetting agents decrease the interfacial tension between the solid active ingredient (which is mostly hydrophobe) and water. The choice of a wetting agent has to be made taking into account the hydrophobic character of the active, as well as the type of dispersing agent used in the formulation. This is the wetting/dispersing system which gives the performance properties to the granules: it has to be optimized for each formulation according to the process used to granulate.

Dispersing agent

Dispersing agents change the potential of interactions by adsorption on the surface of the active ingredient particle, avoiding re agglomeration or flocculation (steric and electrostatic stabilization). The lipophilic part is adsorbed onto the organic particles and the hydrophilic part provides to 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 be some time 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, fluid bed) or not water soluble (all processes) and can be used alone or in combination to meet the formula­tion requirements.

Selection of components

Dispersing and binding agents

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

Wetting agents

The most effective surfactants to use as wetting agents in WG are anionic while non-ionic 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 efficient also to improve the dispersability of the granules.

Disintegrating agents and fillers

Inert mineral compound such as clays, bentonites, diatomaceous, 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 orurea can be used. These fillers are susceptible to help 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 resulted 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, stabilizer       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 homogenized by strong suitable mechanical stirring or 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

• 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 the spray drying (inlet temperature, slurry flow rate, inlet air flow rate, nozzle type etc.) must be adjusted depending on the equipment, in order to get the right residential time of the particles inside 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 a relatively poor resistance to attrition because of their structures. 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 has to be adjusted in order to obtain the right rheology of the mixture. 

Fluid bed

An aqueous solution of the wetting agent is sprayed directly onto a fluidized bed of the blended and milled formulation powder. Hot air is used to fluidize the bed, so that the granules are being dried as they agglomerate. The amount of water, the flow rate of the spraying and the air temperature should be adjusted to obtain the optimum 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