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How To Develop A Wettable Powder (WP) Formulation
The development of Wettable Powder formulation technology marked the advent of modern agricultural delivery systems. Prior to this time, agricultural products were delivered primarily as "dust" formulations; that is, active ingredients were reduced in concentration in the presence of clay diluents to a target assay, processed to a small particle size using high impact equipment, and broadcast applied as dry powders. The major advantage to the dust formulation was its ability to accommodate both solid and liquid active ingredients.
The adverse implications of the above dust applications lie in the areas of uniform distribution of active ingredient and unnecessary exposure to toxic chemicals. Wettable Powders were therefore the first delivery system developed for agricultural chemicals to address these concerns. Initially, the dust formulations were merely dispersed into water prior to application. However, it was observed that if the dusts could be dispersed into water, they were found to demonstrate unacceptable physical stability: poor suspension in water as a function of time and hardpack sediment formation upon standing. Both of which lead to inconsistent distribution of active ingredient during the course of application.
As a result, a branch of physical chemistry, directed toward improved physical perfor- mance of Wettable Powder formulations upon dilution, came into prominence in the agricultural products market. It proved not only necessary to synthesize molecules to improve the wetting of the powders into water ("wetting agents") but also molecules to improve their physical suspension once they had been wetted into water ("dispersants").
Wettable Powder Development: The basic Wettable Powder formulation contains the following components:
Clay or Silica Diluent
Technical: As with the dust formulations, Wettable Powder formulations can accommodate both liquid and solid technicals; however, their requirements, in terms of diluent and processing, are vastly different. The upper concentration limit of solid technicals is related to the melting point of the technical and the heat generated during processing: once the technical begins to be affected by the processing equipment (usually manifesting itself as thermoplastic), you have reached the upper formulation concentration. Obviously, the higher the melting point of the technical, the higher the concentration it can be formulated at as a Wettable Powder. Also, processing a Wettable Powder using equipment that generates less heat should allow the formulator to increase the product active ingredient concentration.
With liquid technicals, the upper concentration limit is dictated by the amount of diluent powder necessary to isolate the individual liquid droplets. This quantity is a function of the particle size of the diluent: the smaller the particle size, the less %W/W of the diluent that is required to encircle the liquid.
"Impregnating a Wettable Powder" is a term often used to describe the dispersion of a liquid technical into a diluent carrier; however, in many cases this is a misnomer since the liquid never enters the carrier cavities. Rather, the liquid can be viewed under the microscope as liquid spheres surrounded by diluent particles. The exceptions, technicals that can be impregnated, are those which will "wet" onto the diluent surface. Basically, the former technicals can be described as hydrophobic (water insoluble) in character and the latter as hydrophilic (water-soluble).
Wetting Agent: The Wetting Agent functions at the water/powder interface and aids in the incorporation of the Wettable Powder to the water phase at the time of application. Wetting agents are surfactants, both nonionic and anionic, which orient their water soluble "end" into the water phase and their oil soluble "end" at the organic technical surface; thus serving to reduce the surface tension between the solid powder surface and water surface. Gravitational forces then allow the powder to move through the water phase, as a result of density differences, preventing the accumulation of organic powder on the water surface with the resultant fuming of airborne dust particles.
Since Wetting Agents function mainly at the initial incorporation of the Wettable Powder to the water phase and minimally once the powder has been added to the water (except to aid in the disintegration of powder agglomerates), its %W/W requirement in the formulation is low: 1-3%. To more efficiently distribute the Wetting Agent into the Wettable Powder formulation, spray-dried mixtures of Wetting Agent and Dispersant are commercially available. However, since the Wetting Agent and Dispersant are processed at established ratios, it is possible to add an excess of one to meet the requirement of the other.
Fine particle size powders are the preferred Wetting Agent physical form. This allows for the efficient distribution throughout the Wettable Powder formulation.
Dispersant: Once the Wettable Powder has been added to the water phase (with the aid of the Wetting Agent), the dispersant now functions to isolate the individual particles. Dispersants are, for the most part, anionic surfactants which function at the water/solid interface in water and, by means of surface charge, prevent the individual particles from forming stable agglomerates.
Dispersant performance is affected by:
concentration: Since Dispersants partition between the water phase and the solid surface, there must be sufficient Dispersant present to accommo-date the desired equilibrium concentration at the interface.
temperature: Dispersants are surfactants; as such, their solubility in water (or,partitioning at the water/solid interface) is affected by change in temperature. Therefore, physical suspension performance at one temperature may not speak for performance at another temperature.
Water Hardness: The presence of salts in water inhibit the solubility of Dispersants into the water phase. Therefore, it may be necessary to either determine the %W/W Dispersant required to accommodate a range of water hardnesses or select the specific Dispersant composition which performs optimally in the desired water hardness.
Clay or Silica Diluent: The role of the clay or silica diluent, whether the technical is solid or liquid, is to reduce the active ingredient assay to a targeted concentration and to isolate the individual organic particles. This isolation serves two purposes:
It minimizes the impact of processing heat of shear upon the technical and other formulation components;
It facilitates technical dispersion upon addition to water; In general, the smaller the particle size of the diluent, the higher the concentration that a technical can be formulated. However, with reduced diluent particle size comes reduced formulation bulk density that may impact commercial packaging and delivery requirement. In other words, when considering reduction of the diluent particle size to improve formulation physical suspension upon dilution, it must also be determined whether the same weight of formulated wettable powder will still fit into the current commercial container and whether the same unit dose applicators (scoops) are still appropriate.
In order to facilitate dispersion in water, it is desired that the silica be hydrophilic. Al- though higher concentrations of organic liquid technicals (through true impregnation) can be formulated using hydrophobic silicas, it is extremely difficult to wet and disperse these formulated powders into water Isolation of the individual technical particles is extremely important with those low melting technicals that are solid under normal storage conditions but are melted and processed as liquids. Once processed as wettable powder, the liquid technicals will revert to their preferred physical state, as solids, upon storage. This change in physical state will significantly affect physical performance at the time of application. The wettable powder, which at the time of production was free flowing and readily dispersed in water to its primary particle size, will have formed physically stable agglomerates that will adversely affect flowability and physical suspension upon dilution.
Other excipients may be added to the formulation to address concerns associated with chemical stability, odor, surface adhesion, etc.
To develop a Wettable Powder involves a series of steps:
Establishment of performance criteria
Selection of formulation inerts
Establishment of test procedures
Determination of a development methodology
Establishment of performance criteria: It is important to establish upfront how the formulation is expected to perform since this may dictate choice/concentration of inerts.
Formulation concentration: dictates formulation composition. Assuming that the optimum concentrations of the wetting agent and dispersant have beenexperimentally determined, the %W/W of clay or silica in the formulation is adjusted to accommodate changes in technical concentration.
temperature:Chemical/Physical stability: is important to the applicator since efficacy is a function of the quantity of active ingredient added to the spray tank and its uniform distribution in the water phase over the time frame of application.
Formulation flowability: is a function of crystalline structure and bulk density. The more rounded the crystals, the greater the ease of flowability. Also, the less dense the formulation, or the more air is maintained in the powder,the better a wettable powder flows. Incorporation of desiccants, or hydro-phobic silicas, will improve flowability by reducing surface tensionbetween individual particles.
Formulation bulk density: dictates packaging container size and unit-dose applicator, where applicable. However, it must be realized that processing equipment will artificially deflate formulation bulk density through air incorporation. Therefore, it is necessary that packaging accommodatethe formulation volume both at the time of production and upon storage. Changes in formulation bulk density, directly attributable to formula accommodation of varying technical assay, should be kept to a minimum.
Physical suspension upon dilution: is a function of small particle size and choice/ concentration of dispersant. Physical suspension may be impacted by water hardness and water temperature as well as the presence of another formulation: WP, SC, EC.
Dust: is a function of wettable powder fine particle size and may be minimized by the addition of a non-volatile liquid to the wettable powder formulation;for example, an oil. However, controlling dust may be at the expense of flowability, bulk density, and physical suspension upon dilution.
Selection of formulation inerts: Formulation inerts should be selected on a cost/ performance basis: if you don't get the performance, then it doesn't matter what the cost.Where possible, formulation components should be dry powders since liquids may be problematic both in their uniform distribution and stability within the formulation.
One of the major issues associated with clay/silica diluent selection relates to chemical degradation of the active ingredient and appears to be a problem mainly with technicals processed as liquids or wettable powder formulations which contain a liquid inert which solubilizes the active ingredient. Although in its simplest terms the degradation process is associated with diluent pH, this may not be accurate since chemical degradation may occur in the absence of water. Also, carriers of the same pH may demonstrate diametrically opposite chemical stability profiles. Rather it appears that chemical degradation may be related to the specific composition of the carriers or specific sites on the carrier surface.
Where chemical stability issues arise during processing, this may be a function of creating additional degradation sites at the time of milling by fracturing the carrier crystals and forcing the technical into intimate contact with these sites. The more severe the milling operation (this is, hammermill ---> airmill), the greater the chemical degradation.
One factor that is often overlooked during formulation development is the interaction be- tween wetting agent/ dispersant and technical that results in stable agglomerate formation. There are solid technicals that demonstrate variable water solubility as a function of pH. It is necessary to determine whether choice of wetting agent/dispersant could contribute to the establishment of such an environment. Although not an issue at the time of production, the change in solubility could affect formulation performance under storage conditions which would encourage increased technical solubility followed by decreased technical solubility; specifically, a hot Midwestern United States summer followed by a cold Midwestern winter.
The agglomerate formation issue can only be exacerbated when the wettable powder is not treated as an end-use product but rather serves as an intermediate in the processing of a Water Dispersible Granule (WDG) formulation where additional water is intentionally added and removed.
Dispersants should be selected based upon physical suspension stability over a range of water hardnesses and temperatures. Effective dispersant weight percentages in a formulation may range from 5 - 7 %W/W.
Wetting agents serve to efficiently distribute the water throughout the Wettable Powder upon addition to the spray tank and are effective at formulation weight percentages of 1 - 3 %W/W.
Formulation processing :
Traditionally, Wettable Powder processing begins with the addition of formulation components to a blender, with the most widely used being a ribbon blender. However, choice of blender will affect ultimate formulation performance, especially with liquid technicals, since they dictate the degree to which agglomerates are broken down to their primary particles. The amount of shear the equipment places upon the formulation in combination with the resistance to shear the formulation places upon the equipment will determine the efficiency to which the formulation components are mixed. In other words, in order to adequately mix the formulation components, the shear energy of the equipment must exceed the attractive forces holding the primary particles together. In general, the smaller the primary particle size of any formulation component, the more energy required to fully deagglomerate.
Optimally, 95% of the mixing should be accomplished in the blender and the remaining 5% upon passage through a hammermill. Conversely, the more that blending takes place in the hammermill, the more inconsistent the mixture that will be processed.
There is a volume range over which a blender operates (mixes) most efficiently; formulating either above or below this volume may result in inadequate mixing.
With viscous liquid technicals, the resistance to the shear of the mixer is reduced primarily by warming the technical to reduce its viscosity. However, as the warm technical comes in contact with the diluent carrier, the heat of the liquid is rapidly dissipated. The result of this heat loss being that the particle size of technical can not be reduced to the extent obtained upon initial addition to the mixer: the technical is demonstrating increasing resistance to the shear of the mixer as the temperature decreases.
It is possible to maintain the resistance of the technical to the shear of the mixer low by use of a heat-jacketed blender. Processing in this manner, the technical will be low in viscosity throughout the dispersion process. However, if the active ingredient can be chemically degraded by the diluent carrier, the effect will only be exaggerated by use of the heat-jacketed blender.
With liquid technicals, if optimum dispersion is accomplished in the blender and a small particle size diluent carrier is used, it should not be necessary to continue processing beyond hammermilling/ Blend #2 Stage. The role of the hammermill should be to complete the blending operation by means of dispersing the agglomerates which were too small to be separated in the blender. However, if a large particle size diluent carrier is used, which requires actual particle size reduction in order to accomplish the desired physical suspension performance, it may have the undesired effect of smearing the liquid technical onto the solid formulation components surfaces causing agglomeration and/or reduced physical/chemical stability on aging. Choice of hammermill screen design (herringbone, pin hole) and screen size opening dictate residence time in the ham- mermill: too small leads to extended residence time and technical smearing; too large leads to minimum residence time and poor deagglomeration.
With solid technicals, the goal is to minimize the impact of processing heat upon the technical surface. Therefore, the higher the melting point of the technical, the higher active ingredient concentration it can be formulated to. As with liquid technicals, residence time in the hammermill will affect formulation performance. However, the hammermill can be used as part of a stepwise processing operation with low melting solid technicals to deagglomerate and perform minimal particle size reduction. Here, actual milling is performed in the airmill that rapidly dissipates heat upon generation.
This does not mean that the airmill will not affect Wettable Powder performance: the heat generated at the moment of impact inside the airmill may be sufficient enough to allow the incorporation of formulation components into the softened technical surface. Adverse performance attributable to processing equipment can be compensated for by choice of smaller particle size diluent carrier and/or reduced formulation concentration.
Use of a Blender #2 after the hammermill serves two functions:
Uniformly blends the deagglomerated Wettable Powder either prior to pack-off or airmilling
If at least twice the size of Blender #1, it converts the manufacturing process from a batch operation to a continuous process
As stated above, the role of the airmill is to reduce the primary particle size where im- proved physical suspension is the desired performance criteria being addressed. However, it should be noted that along with the improved physical suspension comes reduced bulk density due to air incorporation.
As with Blender #2, use of Blender #3 after the airmill allows for the uniform mixing of the Wettable Powder prior to pack-off.
There is pack-off equipment that will remove incorporated air from the Wettable Pow- der prior to packaging by means of a vacuum placed at the point of discharge. This negates the need for oversized commercial containers at the time of commercial packaging.
Establishment of test procedures: Aside from those related to product registration, it is important that the tests established in the laboratory ultimately speak for the performance of the formulation under actual field conditions. Test procedures fall into three (3) basic categories:
In very few regions of the world will a formulation experience a constant temperature environment with no variation. Therefore, extended storage at one temperature, for instance 50°C, may not be predictive of the formulation's long term physical stability. In the real world, most formulations will experience cyclic temperature conditions: cold winter, moderate spring and fall, and hot summers. In order to under-stand product physical limitations, it is necessary to evaluate the formulation performance under both static and dynamic storage conditions.
Tied into formulation storage conditions are screening test methods for easily distingui- shing the obviously good from the obviously bad formulations. These methods could include, but not be limited to, physical suspension in a series of water hardnesses and chemical stability.
Formulations that pass the screening methods hurdle can now be evaluated in terms of their interaction with application equipment. Here, concerns fall into the areas of equipment shear, equipment hose and seal compatibility, application dilution range, and tank mix compatibility with other formulations.
Determination of a developmental methodology: There are two approaches that can be taken to expeditiously develop a Wettable Powder where the active ingredient concentration has been established:
Select a dispersant product and then determine its requirement in the formulation
Establish an dispersant requirement and then determine which dispersant meets that requirement
With the first approach, the development methodology becomes one of blending three components where one of the components is the dispersant of interest, another is the wetting agent, and the last component would the diluent carrier.
As stated above, there are minimum concentration requirements for dispersant (2 - 7 %W/W) and wetting agent (1 - 3 %W/W) in a Wettable Powder which are dependent upon the respective chemistries involved and the properties of the technical and diluent carrier. The actual requirement can be determined by means of Experimental Design Approaches; specifically, Technical Bulletin 99-04: "Experimental Design: Formulation Development."With the second approach, the basic dispersant requirement has been established and each dispersant is substituted directly into the formula.
Whereas the first approach allows for the evaluation of individual component interaction, the second approach does not. However, the first approach may require a significantly greater time/resource commitment due to the number of sample preparations required, the second approach may be accomplished in a relatively short time frame if the "right" dispersant is identified upfront.
It is important to realize that an Experimental Design approach does not guarantee the successful development of a Wettable Powder formulation. It still falls upon the formulation chemist to select the excipients to be evaluated, to determine the impact of the processing equipment upon the formulation, and whether the formulation performance is impacted by the application equipment.
What experimental design does do is to provide the formulation chemist with a "road map" to monitor whether the changes in formulation inerts, processing equipment, and application equipment are taking him/her closer to or farther from their ultimate destination: a commercial product.
A WORD OF PRECAUTION : A few preliminary trials may be necessary for selecting the best combination of our products, for any given set of toxicant and fillers. It should also be noted that fillers are generally notorious for changing the suspensibility of formulations and minor adjustments in ratios of the surfactants may be necessary.
A slight variation in the ratios may be required, depending upon the purity of the technical. In addition to selecting the best surfactants, the formulator required to pay attention to the uniformity of distribution of all constituents and fineness in milling (grinding)
The wetting agents supplied are slightly hygroscopic and this should be borne in mind by the formulator and lump formation may occur if the dispersing agent is pulverised alone. It is recommended that surfactants should be pulverised along with other diluents to avoid lump formation.
Storage: As indicated the products tend to absorb moisture from the atmosphere. The products must be stored in dry and cool place, tightly packed. The shelf life under storage conditions mentioned above is 24 months.
Technical service: Jeevan Chemicals Private Limited offers technical support with research and development. Technical service team are ready to work with the customers helping to develop products to meet the requirements of suspensibility. We welcome customers specific enquiries. Samples of all the above products are available along with Product Data Sheets, MSDS and formulation guide recipes.
OUR GENERAL RECOMMENDATIONS FOR WETTABLE POWDER AND WATER DISPERSIBLE GRANULES FORMULATIONS
|Formulation||General Recommendation||Total Dosage (%)|
|Acephate||JEEMOL AP||0.5 - 1|
4 - 5
|Metribuzin||JEEMOL DW||4 - 5|
|Metamitron||JEEMOL DW||4 - 5|
|Carbaryl||JEEMOL DW||4 - 6|
|Carbendazim||JEEMOL DW||4 - 6|
|Carbendazim + Mancozeb||JEEMOL DW||3 - 5|
|Chlorothalonil||JEEMOL DW||4 - 6|
|Diazinon||JEEMOL DW||4 - 5|
|Diflubenzuron||JEEMOL DW||4 - 5|
|Dimlin||JEEMOL DW||4 - 5|
|Dodine||Jeemol NI||5 - 6|
|Formothion||JEEMOL DW||5 - 6|
|Lindane||JEEMOL DW||4 - 5|
|Metaxuron||JEEMOL DW||4 - 5|
|Sulphur||JEEMOL DW||5 - 6|
|Topsin M||JEEMOL DN + JEEMOL R||3 + 2|
|Malathion||JEEMOL R( M )||1 - 2|
|Thiodicarb||JEEMOL DW||5 - 7|
|Ipridion||JEEMOL DW||6 - 8|
|Oxydiargyl||JEEMOL DW||7 - 9|
|Ethapon||JEEMOL DW||6 - 8|
|Metalaxyl + Mancozeb||JEEMOL DW||3 - 4|
|Metalaxyl||JEEMOL DW||4 - 6|
|Mancozeb||JEEMOL DN||2 - 3|
|Endosulfan||JEEMOL EN + JEEMOL DW||5 + 5|
|Captafol||JEEMOL DW||4 - 5|
|Thiophenate methyl||JEEMOL DW||5 - 6|
|Permethrin||JEEMOL DW||4 - 5|
|Alphamethrin||JEEMOL DW||3 - 4|
|Deltamethrin||JEEMOL DW||8 - 10|
|Acetamiprid SP||JEEMOL AP||2 - 3|
|Metsulfuron Methyl||JEEMOL DW||6 - 8|
|Microbutanil||JEEMOL DW||6 - 7|
|Sulphur WDG||JEEMOL D425 + JEEMOL DS||8 + 2|
|Carbendazim WDG||JEEMOL D425 + JEEMOL DS||8 + 2|
|Mancozeb WDG||JEEMOL D425 + JEEMOL DS||8 + 2|
|Sulfonyl Urea WDG||JEEMOL D425 + JEEMOL DS
Adjuvant - JEEMOL DSU OR JEEMOL DT 90
|0.1 % Tank Mix|
|Atrazine WDG||JEEMOL D151 / SC 215+ JEEMOL DS||6 + 2|
|Ametryn WDG||JEEMOL D151 / SC 215+ JEEMOL DS||6 + 2|
|Thiomethxam WDG||JEEMOL D151 / SC 215+ JEEMOL DML||6 + 2|