Seed Coating Formulation

How To Develop An Aqueous Based Seed Coating Formulation

Seed coating formulations can be viewed as modified suspension concentrate (SC), or flowable (F), formulations with additional excipients added for adhesion to seed surface and dye as an indication of toxic seed treatment either built-in to the suspension concentrate or added to the suspension concentrate at the time of application. Seed coating formulations, in which the adhesive and indicator are built-in, require significantly more development effort than those in which the individual components must demonstrate only short-term physical compatibility in an application tank.

In addition, liquid technicals can also be formulated as Seed Treater formulations; however, these are usually processed as a capsule suspension (CS) in order to minimize phytotoxicity to the seed.Seed Treaters serve two purposes:

  • They allow for minimum application of toxic at the time of planting; and

  • They allow for the application of crop protection chemicals where minimum planting acreage suggests separate application is cost prohibitive.

Seed coatings were a natural progression in technology from the Shaker Box Applicator in which agricultural chemicals, formulated dusts or wettable powders, were added to a box containing a known volume of seeds at the time of planting and literally shaken to distribute the chemical among the seeds. Inherent to the Shaker Box Application were applicator exposure to air-borne dust and non-uniform distribution of toxic over the seed surface. Aqueous seed coatings address both concerns by dispersing the active ingredient in water as a concentrate and adjusting the A.I. concentration with water to uniformly coat the seed at the time of application. 

Built-in seed coating formulations have three (3) basic requirements which they share with suspension concentrate formulations:

  1. Surfactant System: required to physically stabilize the organic solid in water;

  2. Suspension System: required to prevent the technical particles from settling in the container upon storage;

  3. Freeze/Thaw Stabilizer: required to prevent physical deterioration of the formulation upon being allowed to freeze and then thaw prior to application;

Surfactant System: The physical incompatibility between the organic technical and water manifests itself as a significant viscosity increase during particle size reduction. This viscosity increase can be directly attributed to the organic solid trying to agglomerate within the water phase while at the same time being sheared by the processing equipment. As you would expect, the higher the technical concentration in the formulation, the greater the tendency of the milled technical to agglomerate. As a result, the dependence upon surfactants to formulate low viscosity seed coatings increases as the formulation concentration increases..

Surfactants, by their chemical composition, are single molecules that demonstrate varying degrees of solubility in both polar and non-polar solvents. Consequently, they act as "bridges" between the two phases. By means of surfactant hydrophobe composition and HLB (ratio of hydrophile to lipophile), formulation physical performance (stability as a function of temperature and variable viscosity) is dictated. 

As with wettable powder performance upon spray dilution, anionic surfactants are incorporated into seed coating formulations in order to prevent flocculation both as a concentrate and upon dilution at the time of application. Anionic surfactants serve to disperse the organic technical particles by means of surface charge at the solid surface/water interface.

Suspension System: Since most technicals have a density greater than 1.000 gm/mL, gravitational forces will cause the movement of the technical through the water phase (conversely, in extremely rare situations, technicals with a density less than 1.000 gm/mL will move to the water surface). Preventing the individual technical particles from forming a hard pack sediment when packaged in the commercial container is key to establishing formulation shelf life.
Suspension systems can take any of three forms:

  1. Matching the density of the water solution to that of the technical

  2. Use of "swelling" clays

  3. Use of polyhydroxycellulose

However, with significant interaction between the first two forms listed and formulation excipients, most seed coatings are developed using polyhydroxycellulose.

Use of high electrolyte solutions, to match density of dispersed technical, does not allow for the processing of high concentration formulations since the presence of salt inhibits the effective partitioning of surfactants between the water phase and technical surface. Also, the electrolyte solution density may not match the technical density over a range of storage temperatures; with the effect that a bleedlayer may form at some point in time. This bleedlayer formation may be indicative of either sediment formation or non-uniform distribution of active ingredient throughout the slurry. Although not a serious problem with a small 1-gallon commercial container that can be easily shaken; it may be a major issue in a 5000-gallon storage tank prior to commercial packaging. 

"Swelling" clays thicken by means of generating a structure within the water phase based upon surface charge. Where surfactant salts are used as anionics, these dispersants may adversely affect clay viscosity builders by neutralizing surface charge, causing the internal structure to collapse.

Polyhydroxycellulose thickeners are used at a relatively small %W/W concentration in the formulation. However, their water dispersibility is adversely affected by the presence of high electrolyte concentrations. Also, a formulation containing polyhydroxycellulose will demonstrate variable viscosity as a function of temperature: as temperature increases, viscosity decreases which may result in a formulation pourability issue at reduced temperatures and sediment formation upon extended storage at elevated temperatures.

Freeze/Thaw Stabilizers: There are two ways to address freeze/thaw stability issues:

  1. Product Label language ("Do not store below 32 F")

  2. Addition of freeze/thaw stabilizers

Traditionally, with suspension concentrate formulations, freeze/thaw stabilizers have taken the form of propylene glycol addition to the aqueous formulation with the presumption that it functions as a freeze point suppressant. However, there are two points to consider:

First; water must contain approximately 15%W/W propylene glycol in order to suppress the freezing point of water 9 F; 
Second; formulations are more sensitive to freeze/thaw stability as the active ingredient concentration increases (or, as the ratio of propylene glycol to water increases).

The implication of the above observations may be that freeze/thaw stability of aqueous formulations is not related to freeze point suppression. Rather, freeze/thaw stability may be related to the propylene glycol performing the function of a surfactant that has application at reduced temperatures. This conclusion is supported by the partitioning requirement of ethylene oxide/ propylene oxide (EO/PO) block co-polymer surfactants to wet a solid surface as a function of temperature: as temperature decreases, surfactant composition requires higher propylene oxide content.
Concentrated Aqueous Emulsion Development: The basic Concentrated Aqueous Emulsion formulation contains the following components:

With seed coating formulations there is the added possible physical instability of the binder system to freeze/thaw cycling which must be considered.

Aqueous Seed Treater Development:
The basic seed coating formulation contains the following components at the time of application either built-in or upon addition to the application tank:

  • Solid Technical
  • Antifoam
  • Bactericide*
  • Surfactant
  • Density/Viscosity Modifier System
  • Freeze/Thaw Additive
  • Binder System
  • Indicator Dye
  • Water Diluent
  • Solid Diluent
    *Required only where hydroxycellulose viscosity modifiers are used

Solid Technical: Unlike powder seed coating formulations, where the technical can demon-strate unspecified physical properties, aqueous seed coating formulations are premised upon two factors:

  1. The technical must be a solid under all storage and processing conditions

  2. The technical must demonstrate minimum water soluble under all storage and pro-cessing conditions

It is not necessary for the technical to become completely liquid, either during processing or under storage, to adversely affect formulation performance. Softening of the surface during processing will permanently incorporate formulation surfactants, depleting their concentration at the solid/liquid interface and preventing their change in partitioning between phases to accommodate changes in temperature.

Exposing the formulation to cyclic temperatures, which allow the technical surface to alternate between tacky and non-tacky, may result in stable agglomerate formation. 

Technical water solubility/insolubility is not critical to formulation performance; changes in technical water solubility, as a function of temperature, is critical to formulation performance. Where technical solubility into water is equilibrated either through high shear or elevated temperature, removing the shear or reducing the temperature will result in a supersaturated solution that must now equilibrate to new environment. The resultant equilibration process may result in the formation of technical crystals that may not resolubilize under the best of temperature conditions in a reasonable time frame. 

Aside from minimizing available void space between solid particles, the upper active ingredient concentration limit for 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 a dramatic viscosity increase), the upper formulation concentration has been reached. The higher the melting point and the specific gravity of the technical, the higher the concentration the technical can be formulated at as a seed coating.

Antifoamer :
 The composition of the seed coating formulation, organic/ inorganic technical, surfactant, and water, promotes the generation of foam in the presence of high shear equipment. Foam may adversely affect the efficiency of processing equipment, the bulk density of the formulation during packaging, and uniform film formation during seed treatment. Therefore, anti-foamers are incorporated into the formulation in order to prevent the formation of foam.
Anti-foamers differ from defoamers that eliminate foam once it has formed.

Bactericide : Where hydroxycellulose is used as the viscosity modifier, a perfect medium has been introduced into the formulation for bacteria growth. Aside from the aesthetic of the black bacteria colony formation and associated odor, there is a major concern that the viscosity building structure has been affected. In addition, with some technicals, the bacteria may actually be found to chemically degrade the active ingredient.
Finally, if the bacteria colonies that form are physically stable enough, they may actually be found to block in-line screens during the application process and adversely affect the uniform distribution and adhesion of the agricultural chemical to the seed surface.

Bactericides are therefore added at low concentrations to prevent the formation of bac- teria colonies. It is important to realize that some bactericides also demonstrate pesticidal activity and are EPA registered. The only difference between the bactericide being considered a formulation inert and a formulation active ingredient is its concentration in the formulation. Therefore it is important to consult product literature and technical representatives for proper handling of the bactericide.

Surfactants : Surfactants function both in the built-in seed coating formulation, to 'wet' the technical into the water phase, and at the seed surface in order to aid in the uniform distribution of the active ingredient over the seed surface. 

In terms aqueous formulation performance, surfactants can be divided into two classes: nonionic surfactants which serve to wet the organic technical into water and anionic surfantants which serve to uniformly disperse the organic technical into water; both as a concentrate and upon dilution. 

When the technical and water are mixed, their mutual incompatibility, in the absence of surfactants, results in a significant viscosity increase in the presence of shear. Nonionic surfactants, by their very chemical composition, function as a bridge between the two phases by partitioning between the aqueous and non-aqueous phases. The degree to which a nonionic surfactant reduces the resistance of one phase to the other is an indication of the surfactant efficiency and can be measured (in the absence of viscosity modifier) using a viscometer; the lower the viscosity, the more efficient the surfactant wets the technical surface. 

However, since nonionic surfactant partitioning (solubility) between aqueous and non-aqueous phases is affected by temperature, the optimum wetting surfactant at one temperature may not be the optimum wetting surfactant at another temperature.

As the formulation active ingredient concentration increases, the quantity of nonionic surfactant required to wet the technical into water also increases. Choice of surfactant also becomes critical with increasing concentration since the space available in the formulation for partitioning (between increasing solid surface area and decreasing aqueous media) becomes less and less.

Therefore, the most efficient surfactants are those which wet the technical (measured as low viscosity) at high formulation concentrations, over a wide temperature range and at low formulation weights. Synperonic PE/P105 and Atlas G5000 are ethylene oxide/propylene oxide block co-polymer surfactants that have been found to have excellent application in seed coating formulations.

Anionic surfactants function at the water/solid interface to prevent particulate agglomeration by adsorption onto the particle surface. Atlox 4913 has been found to perform as a very effective dispersant for aqueous based formulations. In addition, traditional Wettable Powder dispersants (sodium lignosulfonates and sodium naphthelene condensate sulfonates) can be used in seed coating formulations, although impact upon seed coating color may be a factor, as well as phosphate esters and their partially neutralized salts. 

Uniform distribution of the active ingredient over the seed surface, by proper surfactant system selection, is the primary determinant of coating adhesion on to the seed.

Density/Viscosity Modifier System: As stated above, it is extremely important to prevent the dispersed solid particles in the aqueous seed coating from forming a hard pack sediment upon storage. Since uniform distribution of active ingredient upon application begins with uniform distribution in the commercial container, any deterioration of packaged product quality may ultimately adversely affect application efficacy.

In order to maintain uniform distribution of active ingredient in the commercial container, use of viscosity modifiers, for example xantham gum, have proven quite effective. xanthan gum thickeners generate a supporting structure within the aqueous phase through controlled incompatibility with the water phase. Their effective concentrations in the built-in aqueous formulation are low; ranging from approximately 0.20%W/W for a 4 lb/gal (480 g/L) to 0.35%W/W for a 2 lb/gal (240 g/L) seed coating. 

Xanthan gums affect formulation viscosity as a function of temperature: as the temperature increases, viscosity decreases. Therefore, it is necessary to determine the concentration of xanthan gum in the formulation which prevent sediment/bleed layer formation at elevated temperatures.

"Swelling" clays can be utilized to generate a supporting structure in water; however, attention must be given to the use of dispersant surfactants that are salts. The introduction of ions, in the form of surfantants, may neutralize the clay surface charge and thus cause the suspension system to collapse.

Clays may also be used in combination with xantham gum to theoretically improve technical suspension at a lower formulation viscosity through synergistic interaction of the two suspension aids. As with application of swelling clays, care must be taken that the introductions of neutralizing ions do not adversely affect formulation performance. However, in aqueous seed coating formulations, clay interaction with dye solution may be used to preclude an unfavorable interaction (viscosity increase) with the binder system.

Use of salt addition to the water phase to match the density of the aqueous media to that of the solid particulate to be suspended, may adversely impact the performance of the surfactant. The ultimate effect of the salt/surfactant interaction is to limit the upper concentration of active ingredient to which the technical can be formulated and to introduce formulation physical stability concerns under standard storage conditions.

Since the density of the aqueous solution will change as a function of temperature, for- mulation separation may occur upon extended storage at either elevated or reduced temperatures. Therefore, the use of salt solution density modification is sometime supplemented by the use of traditional xanthan gum thickeners at very low rates. However, the hydration of xanthan gum may be inhibited in the presence of high salt concentrations.

Freeze/Thaw Additive: Organic solid dispersions in water may change rheological properties upon freezing and subsequent thawing. Physical deterioration of the aqueous seed coating is first and foremost a function of formulation active ingredient concentration: the higher the concentration, the greater the susceptibility of the formulation to failure after freeze/thaw cycling. Secondly, poor freeze/thaw stability is a function of binder system composition. It is strongly suggested that binders, or sticker solutions, be freeze/thaw stable as a criteria for selection. 

In order to address aqueous rheological deterioration at reduced temperatures, propylene glycol has been traditionally included in the formulation composition as a freeze point suppressant. Alternatively, urea may be included as a formulation excipient to facilitate freeze/thaw stability. However, since both urea and propylene glycol demonstrate water solubility, they may adversely affect surfactant partitioning and related performance.

Alternatively, it is possible to introduce freeze/thaw stability to the formulation while at the same time providing desired dispersant functionality by use of partially neutralized phosphate ester surfactants.

Binder System: Whereas the surfactant system is critical to aqueous seed coating wetting performance both as a concentrate and upon application, the binder system can significantly improve abrasion resistance of the coating on the seed surface. The effective binder meets the following performance criteria:

  1. It must be chemically/physically non-reactive with the other formulation components under all storage and application temperature conditions. This is especially true when the active ingredient is a liquid which has been encapsulated by means of interfacial polymerization;

  2. It must adhere the agricultural chemical to the seed surface under all storage and application abrasion conditions; 
    Binders, or stickers, fall into two categories:

    1. Solid water-soluble polymers, such as polyvinyl alcohol (PVA), which are considered to be 100%W/W polymer solids (unless predispersed in water at a known concen tration);

    2. Liquid water-dispersible latexes, such as polyvinyl acetate (PVAc), which contain at most 55% polymer solids;

      Binder system selection should be based upon:

    low viscosity in order to facilitate mixing and product transfer; small particle size in order to facilitate physical stability and film forming properties; binder physical stability under all expected storage conditions; for example, cold (including freeze/thaw) and elevated temperatures; non-reactivity with other formulation components; adhesion to seed surface with zero dust formation under abrasive conditions; no adverse effect upon seed germination upon planting;

Seeds treated with seed coating formulations containing hygroscopic, water-soluble stickers may lose coating integrity when exposed to relatively high moisture conditions thus resulting in possible toxic exposure and seed agglomeration at the time of planting.

With latex emulsion polymers, the choices of stickers range from those forming hard, brittle films to those forming soft, flexible films.

Optimally, the sticker should demonstrate the following coating properties:

    fast drying film-forming at temperatures approaching 32°F (0°C) hard flexible abrasive resistant color fast

To accomplish the above may involve blending compositionally different latex emulsions.

Indicator Dye: It is important that an applicator 'know' when he/she is being exposed to potentially harmful chemicals; therefore, a dye or pigment is added to the seed coating formulation in order to distinguish treated from untreated seeds. However, the dye does not perform innocuously in the formulation.

Since water is the continuous phase in aqueous seed coatings, the dyes used are water soluble to approximately 10,000ppm. Therefore, formulation concentrations in excess of 1%W/W dye solids will not contribute to color intensity and may adversely affect formulation performance.

In order to minimize exposure to airborne dye powder (some of which are considered carcinogenic), dyes are commercially available as 20-40%W/W solutions in either propylene glycol/water (20% dye) or acetic acid/hydrochloric acid (40%). However, there are concerns associated with both the dye itself and the solvent solution:

    Dye = since the dye, in order to be water soluble, is usually a(n amine) salt of an organic xanthene, it will neutralize any clay present in the formulation as part of the viscosity modifier system. Ultimately, this interaction may affect the performance of both dye and clay in the formulation. 
    There is a specific dye/polymer interaction, dependent upon polymer composi- tion, which may range from variable coating color to uncontrolled viscosity increase to formulation gel formation. In other words, each latex polymer will 'wet' the dye solids differently possibly resulting in variable dried Seed Treater coating color.

    This first dye/clay interaction may be allowed to occur intentionally in order to mini- mize the dye/polymer interaction. Solvent system = with the 20%W/W dye solution, the presence of propylene glycol and water do not impact performance since they are components of the basic formulation.

    However, the presence of acid in the 40%W/W dye solution may affect the polymerization of the latex resin during coating formation acting as a film plasticizer.

Water Diluent: 
Water composition may impact formulation performance either upon production or upon extended storage. As noted above, the presence of dissolved salts may adversely affect surfactant partitioning; the presence of suspended solids may preferentially adsorb dissolved/dispersed surfactant. Both may result in formulation physical deterioration (phase separation or viscosity increase).

Solid Diluent:
 There appears to be a minimum coating solids requirement in order for a seed coating to demonstrate maximum abrasion resistance. In the past, with active ingredients which demonstrated efficacy at relatively high concentrations on the seed surface, this was not a a major issue. However, with the advent of active ingredients which demonstrate efficacy at low application rates, it is necessary to supplement the solids associated with the active ingredient with the addition of a solid diluent.

Choice and concentration of diluent will impact seed coating integrity. The smaller the particle size of the diluent, the more uniform the coating continuity on the seed surface. As the solid diluent concentration in the seed coating formulation increases, so too does the adhesive polymer requirement within the formulation. It is therefore necessary to determine the "optimum" polymer and diluent concentrations using experimental design.

To develop a seed coating formulation involves a series of steps:

    Establishment of formulation and end-use performance criteria Selection of formulation inerts Formulation processing Determination of a development methodology

Establishment of formulation and end-use performance criteria:

It is important to establish up front how the formulation is expected to perform since this may dictate choice/concentration of inerts.

    Formulation concentration: For a built-in seed coating formulation, active ingredient concentration dictates formulation composition. As the formulation active ingredient concentration increases, there is less room (%W/W) in the formula for other components. Therefore it is necessary to select the most efficient wetting agent, dispersant, viscosity modifier, sticker and dye which are mutually non-reactive. The lower the active ingredient concentration in the formulation, the more "forgiving" the physical performance is to inert selection but not to potential interactions among the formulation excipients.
    Also, as the seed coating active ingredient increases, the greater the versatility of the formulation to accommodate a range of seed sizes and coating A.I. concentrations is accom- plished.

    This first dye/clay interaction may be allowed to occur intentionally in order to mini- mize the dye/polymer interaction. Chemical/Physical stability:is important to the applicator since efficacy is a function of the quantity of active ingredient and its uniform distribution over the seed surface. Treated seeds must retain coating integrity from the time of application to the time of planting. In addition, the seed coating must release the formulated active ingredient at a rate sufficient to control the targeted organism while at the same time not adversely affecting seed germination. 

    -Formulation flowability: is the main contributant to active ingredient distribution over the seed surface aside from seed coating formulation concentration and surfactant system composition. 

    Addition of viscosity modifier is the controlled manner in which to establish formulation flowability and may be affected by temperature: formulation viscosity increases as temperature decreases. The surfactant effect upon flowability relates to its efficiency to wet the solid technical into the water phase: the more efficient the surfactant, the lower the formulation viscosity (in the absence of viscosity modifier) as well as onto the seed surface at the time of application.
  • Sediment formation: is a function of four factors:

    1. inadequate viscosity modifier in the formulation

    2. heat adversely affecting xanthan gum hydration

    3. heat adversely affecting surfactant partitioning at the technical surface/water interface

    4. reactivity among formulation components

  • Physical suspension upon dilution: where the targeted seed coating assay and uniform coating can best be met by reduction of seed coating formulation assay with water, it is important that the active ingredient be uniformly distributed in the dilution over the application time frame.

  • Container Rinsing: Government agencies have become increasing rigorous in the enforcement of proper container cleanout prior to disposal. The following formulation properties have been found to impact effective container rinsing:

    1. Formulation Viscosity - the lower the viscosity, the less material will remain in the container upon pouring;

    2. Viscosity Modifier Composition - xantham gum is compositionally equivalent to wallpaper paste. Allowed to dry in the commercial container, the aqueous seed coating formulations will form a film that resists water penetration. Salts and "swelling" clays do not demonstrate the same degree of resistance to water penetration upon drying;

    3. Sediment Formation - With the small particle size characteristic of suspension concentrate formulations and inadequate viscosity modifier con-centration, technical will settle with variable particle size latex emulsions out of suspension with minimum void space between particles. The result of this close packing of water insoluble technical and organic polymer is to allow little room for water penetration and resuspension with agitation (shaking).

      The effect of the above formulation properties upon container rinsing is premised upon proper selection of container composition and/or barrier treatment.

Selection of formulation inerts:
 Formulation inerts should be selected first and foremost on a cost/performance basis: if you don't get the performance, then it doesn't matter what the cost.

Second, where governmental regulations and the product registration process encourage the use of formulation inerts from those exempt from tolerance from residue in crops, commercialization of seed coating formulations may be expedited by first evaluating excipients so listed. 

Although formulation components may be either liquids or powders, the preference, if there is one, may be for liquids for three reasons:

    exposure to dusts is minimized during handling materials transfer can be monitored either by weight or volume

As noted above, one of the key factors associated with formulation inert selection is the possibility of unacceptable raw materials interactions which affects physical performance. Aqueous seed coating formulations are relatively complex, compared to suspension concentrate formulations, with the potential for significant interactions between technical and surfactant, surfactant and viscosity modifier system, dye and viscosity modifier system, and dye solution and latex emulsion. For instance, where physical suspension, both as a concentrate and upon dilution at the time of seed treatment, is a critical performance criteria, choice/concentration of both anionic surfactant and viscosity modifier systems must be scrutinized for possible interaction. Their selection could have the unwanted effect of collapsing the internal structure generated to support the solid formulation components as a concentrate and depleting the surfactant available to function as a dispersant upon dilution.

Proper selection of wetting agent is key to formulation physical stability and viscosity performance across the temperature range of interest. The formulation should, for example, demonstrate low viscosity (in the absence of viscosity modifier) over the temperature range of 1°C to 50°C. At low formulation active ingredient concentration, any surfactant class (nonyl- phenol ethoxylate, alcohol ethoxylate, ethylene oxide/propylene oxide block co-polymer) will achieve the intended goal. As the active ingredient concentration increases, effective surfactant partitioning at the solid/ water interface is critical to maintaining low viscosity. However, ethylene oxide/propylene oxide block copolymers surfactants, which demonstrate low viscosity at high active ingredient concentration, will perform acceptably at low concentration as well.

Interestingly, even within the ethylene oxide/propylene oxide block copolymer class of surfactants, as the active ingredient concentration increases, the selection of surfactant becomes highly specific to the temperature at which the formulation is processed (or stored long term). As the temperature increases, formulation requirement shifts from high propylene oxide (to ethylene oxide ratio) to high ethylene oxide (to propylene oxide ratio) surfactant composition. In this case it may be possible to blend a high ethylene oxide with a high propylene oxide EO/PO block copolymer surfactant to accommodate a wide temperature range.

As stated above, assuming latex emulsion freeze/thaw stability, seed coating freeze/thaw stability is determined by the sum total contribution of propylene glycol, water soluble salt, and/ or anionic surfactant in the Seed Treater formulation. With low active ingredient concentration, freeze/thaw stability can be formulated with the addition of propylene glycol to water alone. As active ingredient concentration in the seed coating formulation increases, the quantity of propylene glycol added to the water to maintain freeze/thaw stability is prohibitive and may adversely affect surfactant performance. In this case it is more beneficial to use anionic surfactants in combination with propylene glycol which can then find application at significantly lower weights (%W/W).

As stated above, freeze/thaw stability is determined by the sum total contribution of propylene glycol, water soluble salt, and/or anionic surfactant in the Concentrated Aqueous Emulsion formulation. With low active ingredient concentration, freeze/thaw stability can be formulated with the addition of propylene glycol to water alone. As active ingredient concentration in the Concentrated Aqueous Emulsion formulation increases, the quantity of propylene glycol added to the water to maintain freeze/thaw stability is prohibitive and may adversely affect surfactant performance. In this case it is more beneficial to use anionic surfactants in combination with propylene glycol which can then find application at significantly lower weights (%W/W).

Propylene glycol, by mixing with the xanthan gum prior to addition to water, provides the additional service of promoting uniform distribution (hydration) of thickener throughout the water phase. With the dye solution/latex interaction being responsible for significant physical performance issues with seed coating formulations, it is best to first identify those dye solution which are exempt from tolerance in crop residue and then evaluate latex emulsion which do not result in physical stability issues.

Seed coating formulations, which were found to be physically stable under all potential storage temperature conditions and time frames, can now be applied to seeds of interest and evaluated for chemical stability, coating integrity and phytotoxicity.

Formulation processing: Where a crop protection chemical is commercialized as both suspension concentrate and Seed Treater formulations, it may be possible to develop a suspension concentrate formu- lation which can subsequently be transformed to a seed coating formulation by addition of latex emulsion and dye indicator.

Alternatively, where seed coating is the only targeted formulation for an active ingredient, it is best to process in the absence of latex polymer and dye solution in order to minimize equipment cleanup. Latex polymer and dye solution can be post-added in the slurry holding tank under agitation.

Uncontrolled heat during dry or wet processing is the death knell for any suspension concentrate/seed coating formulation where heat affects the melting point of the technical. In addition, during wet milling, both the water solubility of the technical and the partitioning of the surfactants between the solid surface and the aqueous media may also be affected by uncontrolled heat during processing.


  • Dry Processing:

    • a high melting technical is passed through a hammermill and/or airmill until the desired particle size distribution is obtained and collected in an appropriate container;

    • a dispersion is made of formula weights of water, surfactants, biocide, defoamer (where appropriate, salt is also added for density modification as well as "swelling" clay) in a mix tank equipped with high shear agitation;

    • the proportional weight of milled technical is added to the above dispersion and mixed until uniform;

    • the proportional weight of xanthan gum/propylene glycol pre-slurry (where appropriate) is added to the dispersed technical slurry under agitation and mixed until uniform;
      [NOTE: With technical particle size distribution determined by air flow velocity, the airmill can process significantly smaller particle size than the hammermill.]

    • the proportional weight of dye solution is added to the mix tank and agitated until uniform in order to minimize potential interaction with the latex emulsion;

    • the proportional weight of latex polymer is added to the mix tank and agitated until uniform;

  • Wet Process

    • A slurry is made of formula weights of water, surfactants, biocide, defoamer, density/viscosity modifier system and mixed until uniform;

    • the proportional weight of crystalline technical is added to the dispersed slurry and mixed until uniform;

    • the uniform slurry mixture is then passed through appropriate particle size reduction equipment (Attritor or Dyno-Mill ) until the desired particle size is achieved; [NOTE: Particle size distribution is a function of:

        milling media. The smaller the media diameter, the smaller the slurry particle size. concentration. The greater the slurry concentration, the smaller the particle size.]
  • the milled slurry is then transferred to a holding tank, the proportional weight of dye solution is added and mixed until uniform in order to minimize potential interaction with the latex emulsion;

  • the proportional weight of latex polymer is added to the holding tank and mixed until uniform;


With both wet and dry milling, technical melting point must be considered during pro- cessing. Unlike wettable powder processing, where a clay or silica milling aid can be included in the formulation to minimize the effect of heat upon the technical surface, dry technical processing for aqueous seed coating will have significant impact upon formulation performance. Any clay or silica, added to the seed coating formulation, is perceived by the surfactants as another surface to be wetted into the aqueous phase. Its presence may deplete wetting agent (resulting in viscosity increase) or dispersant (resulting in flocculation) at room temperature or upon extended storage at reduced or elevated temperatures.

Addition of milling aid to the dry technical processing therefore limits the concentration to which the active ingredient can be formulated: the lower the technical melting point (or the more milling aid added), the lower the active ingredient concentration. 

    Two particles of technical reform a stable agglomerate Surfactant, present at the technical surface, is permanently adsorbed into the technical Combination of #1 and #2 thereby resulting in a dramatic and significant viscosity increase.

As with dry technical processing, the lower the technical melting point, the greater the impact of processing shear during wet milling. By reducing the active ingredient concentration (and increasing the water content) in the formulation, the probability of the above interactions occurring is also reduced. 

Not all processing equipment generates the same magnitude of heat of shear: a hammer- mill generates more heat than an airmill, a Dyna-Mill generates more heat than an Attritor . Therefore, it is possible to process lower melting technicals (to a given active ingredient concentration) with the airmill than the hammermill, and the Attritor than the Dyna-Mill . 

It should be noted that the impact of heat of shear generated during processing with the hammermill can be minimized by increasing the ultimate particle size of the milled technical. However, this may adversely affect physical suspension of the seed coating both as a concentrate and upon dilution at the time of application.

The kinetic of surfactant partitioning between the technical surface and water phase is significantly different for dry and wet milled technical. With wet milled technical, surfactant is immediately available to the solid surface at the time of cleaving and effective partitioning equilibrium is characteristic of processing: time and temperature.

With dry milled technical, it is necessary to first mechanically deagglomerate to primary technical particles in the water phase and then allow surfactant to partition to the solid/liquid interface. The problem is: How do you efficiently and effectively deagglomerate the technical mechanically to its primary particle size?
There are three factors that dictate the extent to which the milled technical has been stabilized at its primary particle size:

    The efficiency of mixing equipment The kinetics of surfactant equilibrium at the solid/liquid interface Time

Most mixing equipment will not return technical agglomerates to their primary particle size; deagglomeration is dependent upon the kinetics of the surfactant partitioning to the solid surface being relatively fast. Therefore, upon extended storage (time), the technical may continue to deagglomerate (kinetics of surfactant surface equilibrium) to the point that excessive surface has been exposed to accommodate the surfactant present. The formulation will then make the equilibrium adjustment that may result in a significant viscosity increase under some temperature storage conditions.

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:

    1. Screening methods

    2. Storage conditions

    3. Application methods

Category Test Testing Interval
Storage Temperature Criteria of Performance
Screening Viscosity Initial 23°C Low Viscosity
  Dispersion Initial 23°C Minimum Sediment  Upon Dilution
Storage Viscosity 1, 3, 6 months 3 C, 23 C, 50 C, F/T Low Viscosity
  Dispersion 0,1, 3, 6 months 3 C, 23 C, 50 C, F/T Minimum Sediment  Upon Dilution
Application Dispersion Using Formulations Stored Above Minimum Sediment  Upon Dilution
  Compatibility With Other Formulations Using Formulations Stored Above Uniform Dispersion


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 understand 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 distinguishing the obviously good from the obviously bad formulations. These methods could include, but not be limited to, formulation viscosity and physical suspension (dispersion) upon dilution. Formulations should demonstrate low viscosity and physical suspension upon dilution in various water hardnesses after initial processing.

Those formulations that demonstrate acceptable performance are then placed in storage at cold temperatures, room temperature, and elevated temperatures in order to determine the 'versatility' of the acceptable performance. Those formulation which demonstrate increased viscosity and/or poor physical suspension in various water hardnesses are rejected.

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.

Treated Seed Testing:

Category Test Testing Interval
Storage Temperature Criteria of Performance
Screening Coating Adhesion Initial 23°C Zero Dust
  Germination Initial 23°C 100% Germination
Storage Coating Adhesion 0, 1, 3, 6 months 3°C, 23°C, 40° C Low Viscosity
  Germination 0,1, 3, 6 months 3° C, 23°C, 40° C 100% Germination

Seed coating formulations, which demonstrate acceptable physical stability, must also demonstrate acceptable coating performance, in terms of:

  • uniform distribution over seed coating
  • zero dust formation during application
  • no seed phytotoxicity

Although the seed coating should ultimately be tested on the specific seed(s) of interest, corn or sorghum provides an excellent screening matrix due to their smooth, waxy surface.