WATER DISPERSIBLE GRANULES (WG)
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.
The general composition of a WG formulation is the following:
Active ingredient(s) 50 to 90%
Dispersing/Binding agents 5 to 20%
Wetting agents 1 to 4%
Antifoam, stabilizer, buffer 0 to 4%
Fillers, disintegrating agents up to 100%
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
- provide wettability to the particles of the active ingredient
- provide stabilization by giving cohesion to the system
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 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.
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.
This type of additive can be some time useful for improving dispersibility. Clays are generally selected for this purpose.
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.
The most effective surfactants to use as wetting agents in WG are anionic while non-ionic are seldom used.
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 to incorporate into the WG formulation a compatibility agent. 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.
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
Processes Advantages Potential inconveniences.
Spray drying Good dispersibility Low density
No tendency to dust Medium resistance to attrition
Extrusion High density Easiness of dispersion
Good resistance to attrition
Fluid bed granulation Good dispersibility Quite dusty
Good resistance to attrition
The working process is a three steps:
preparation of a slurry
spray-drying of the slurry
drying of the resulted granules
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 with water active ingredient(s), wetting and dispersing agents and the other components. Then homogenize this by strong suitable mechanical stirring or high shear mixer.
Such dispersion if necessary shall be adjusted with water and milled in 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 for 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%.
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.
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