How To Develop A Suspension Concentrate (SC) Formulation

Suspension Concentrate (SC), or Flowable (F), formulations are the dispersion of an insoluble solid into a liquid media. The liquid media can be water or any organic/inorganic solvent in which the solid meets the insolubility criteria, in ppm concentrations, under all temperature conditions. 

Suspension Concentrates were a natural progression in technology beyond Dust and Wettable Powder formulations where minimization of exposure to toxic chemicals to the applicator was the ultimate goal. However, as with any technological advance, the tools necessary to accomplish the task, on a routine basis, were found to be lacking both from raw materials and processing standpoint. The result being that over the past twenty years, dramatic advance have been made both in the understanding of Suspension Concentrate technology and the excipients necessary to process a physically stable formulation.

Suspension Concentrate formulations have three (3) basic requirements which, to that time, had not been a major concern with Dust and Wettable Powder 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;t

  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 in 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. 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 a low viscosity Suspension Concentrate 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 thereby acting as "bridges" between 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 Suspension Concentrate formulations in order to prevent flocculation. These surfactants serve to disperse the organic technical particles by means of surface charge.

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

Each has its benefits and limitations.
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 a 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 which can be easily shaken; it may be a major issue in a 5000-gallon storage tank.
"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 (fertilizer) solutions and may be a major culprit when compatibility issues are identified. Also, a formulation, upon storage, will demonstrate variable viscosity as a function of temperature: as temperature increases, viscosity decreases. This 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, 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 functioning as a surfactant which 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.


Suspension Concentrate Development : The basic Suspension Concentrate formulation contains the following components:

  1. Solid Technical

  2. Antifoam

  3. Bactericide*

  4. Surfactant

  5. Density/Viscosity Modifier System

  6. Freeze/Thaw Additive

  7. Water Diluent

* Required only where hydroxycellulose viscosity modifiers are used


Solid Technical : Unlike Dust and Wettable Powder formulations, where the technical can demonstrate unspecified physical properties, Suspension Concentrate 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 processing 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 a new environment. The resultant equilibration process may result in the formation of technical crystals, which 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. Obviously, the higher the melting point and the specific gravity of the technical, the higher the concentration the technical can be formulated at as a Suspension Concentrate.


Antifoamer : The composition of the Suspension Concentrate 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 and the bulk density of the formulation during packaging. Therefore, anti-foamers are incorporated into the formulation in order to prevent the formation of foam during processing.

Anti-foamers differ from defoamers which 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.

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 or 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 : 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 surfactants 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. Jeemox PE/P105 and Jeemox G5000 are ethylene oxide/propylene oxide block co-polymer surfactants which have been found to have excellent application in Suspension Concentrate formulations.

Anionic surfactants function at the water/solid interface to prevent particulate agglo- meration by adsorption onto the particle surface.Jeemox 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 sulfonates) can be used in Suspension Concentrate formulations as well as phosphate esters and their partially neutralized salts.


Density/Viscosity Modifier System : As stated above, it is extremely important to prevent the dispersed solid particles in the Suspension Concentrate 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 xanthan 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 aqueous formulation are low; ranging from approximately 0.20%W/W for a 4 lb/gal (480 g/L) to 0.50%W/W for a 50 g/L Suspension Concentrate. 

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 xantham 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 xanthan 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 introduction of neutralizing ions do not adversely affect formulation performance.

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 "wetting" 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, formulation 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 xantham gum thickeners at very low rates. However, the hydration of xantham 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 Suspension Concentrate 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.

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.


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). 
To develop a Suspension Concentrate involves a series of steps:


  • Establishment of performance criteria

  • Selection of formulation inerts

  • Formulation processing

  • Establishment of test procedures

  • Determination of a development methodology

Establishment of performance criteria : It is important to first establish how the formulation is expected to perform since this may dictate choice/concentration of inerts.

  • Formulation concentration : dictates formulation composition. As the formula- tion active ingredient concentration increases, there is less room (%W/W) in the formulation for other components. Therefore it is necessary to select the most efficient wetting agent, dispersant, viscosity modifier. The lower the active in- gredient concentration in the formulation, the more "forgiving" the physical per-formance is to surfactant choice.

  • 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 formulation viscosity. There are two formulation components that may impact flowability:

    1. Surfactant choice/concentration

    2. Viscosity modifier concentration

    3. Addition of viscosity modifier is the controlled manner in which to establish formulation flowability and may be affected by temperature. 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).

      Also, as the concentration of surfactant in the aqueous phase approaches 20%W/W, de pending upon surfactant composition, a viscosity increase can de detected which is directly attributable to the surfactant choice.

  • Sediment formation : is a function of four factors:

    1. inadequate viscosity modifier in the formulation

    2. heat adversely affecting xantham gum hydration

    3. cyclic temperature storage of a water soluble technical

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

  • Physical suspension upon dilution : is a function of small particle size and choice/ concentration of dispersant. Water hardness and water temperature as well as the presence of another WP, SC, or EC formulation may impact physical suspension.

  • Formulation Tank Mix Compatibility : In general, SC tank mix very well with another SC or WP, and problematically with EC formulations.

  • Container Rinsing : Government agencies have become increasing rigorous in the enforcement of proper container clean-out 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 equiva- lent to wallpaper paste. Allowed to dry in the commercial container, the Suspension Concentrate will form a film which resists water penetration. Salts and "swelling" clays do not demonstrate to the same degree resistance to water penetration upon drying;

    3. Sediment Formation - With the small particle size characteristic of Suspen-sion Concentrates and inadequate viscosity modifier concentration, technical will settle out of suspension with minimum void space between particles. The result of this close packing of water insoluble technical 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.
    Although formulation components may be either liquids or powders, the preference, if there is one, may be for liquids for two reasons:

    1. exposure to dusts is minimized during handling

    2. 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. 

    For instance, where physical suspension both as a concentrate and upon dilution 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 depleting both the surfactant available to function as a dispersant and collapsing the internal structure generated to support the solid formulation components.

    Proper selection of wetting agent is key to formulation 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 (nonylphenol ethoxylate, alcohol ethoxylate, and 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, surfactants that demonstrate low viscosity at high active ingredient concentration will perform acceptably at low concentration.

    Therefore, it is possible to identify the "best" surfactant for a range of active ingredient concentrations by using the highest active ingredient concentration as the vehicle for surfactant evaluation. From this evaluation it would be determined that ethylene oxide/propylene oxide block copolymers are the most effective (low viscosity) surfactants for high concentration Suspension Concentrates. 

    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.

    Also, as the active ingredient concentration increases, the %W/W surfactant requirement must be determined incrementally since the viscosity increase attributable to the surfactant concentration in the aqueous phase may hinder proper surfactant selection. In other words, given the following formulas:

            Technical                                              50%W/W               70%W/W
            EO/PO block copolymer                      3 %W/W                3%W/W
            Water                                                    47%W/W               27%W/W

  • the second formulation may prove too viscous either because the surfactant concentration in the aqueous phase is too high or the surfactant was not of the "right" EO/PO composition to accommodate the temperature of interest (processing and/or storage). Whereas, with the first formulation, the surfactant concentration in the aqueous phase does not contribute to formulation viscosity and failure is attributable to EO/PO composition.

    Tank mix compatibility of Suspension Concentrate formulations with high electrolyte solutions (fertilizers) may be adversely affected by choice of viscosity modifier. Xanthan gum, which generates increased viscosity through controlled incompatibility with the aqueous phase, will lose water solubility in the presence of water soluble salts. The result being that the xanthan gum will agglomerate within the water phase forming a slimy, gooey mess and providing visual confirmation that mixture is non-homogeneous.

    With salt density modification and "swelling" clay thickeners, agglomeration does not occur and breakdown of formulation internal clay structure is attributed to Suspension Concentrate dilution. Maintained under constant agitation, the formulation gives the appearance of fertilizer compatibility.

    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 Suspension Concentrate 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 Suspension Concentrate 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.

  • Formulation processing : Uncontrolled heat during processing is the death knell for any Suspension Concentrate formulation. Heat affects the melting point of the technical, the water solubility of the technical, and the partitioning of the surfactants between the solid surface and the aqueous media.
    Particle size reduction of Suspension Concentrates can be accomplished either wet or dry:
    Dry Processing:

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

    • 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);

    • 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.]

    Wet Processing:

    • 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:
      1. milling media. The smaller the media diameter, the smaller the Suspension Concentrate particle size.
      2. concentration. The greater the slurry concentration, the smaller the Suspension Concentrate particle size.]

    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 Suspension Concentrate will have significant impact upon formulation performance. Any clay or silica, added to the Suspension Concentrate formulation, is perceived by the surfactants as another surface to be wetted into the aqueous phase. This increased surface area 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. 
    With wet milling the shear of the processing equipment generates sufficient heat at the technical/media interface to momentarily distort the surface of the technical. At this moment of surface softening three interactions are possible:

    1. Two particles of technical reform a stable agglomerate

    2. Surfactant, present at the technical surface, is permanently adsorbed into the technical

    3. 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 hammermill 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 increasing the ultimate particle size of the milled technical can minimize the impact of heat of shear with the hammermill. However, this may adversely affect physical suspension of the Suspension Concentrate upon dilution. 

    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:

    1. The efficiency of mixing equipment

    2. The kinetics of surfactant equilibrium at the solid/liquid interface

    3. 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 con- tinue 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 CAS No 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
    1, 3, 6 months 3°C, 23°C, 50°C, F/T Minimum Sediment Upon Dilution
  Chemical Stability 0, 1, 3, 6months 3°C,23°C,50 No Loss Of A.I.
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 hardness 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.

Determination of a development methodology: Suspension Concentrates are the dispersion of an insoluble technical, with a median particle size distribution range of <1T to >20T, in water. 
Consider the following:

  • If the specific gravity of the water insoluble technical is 1.2 X 100 gm/mL, then a particle 1T3 in volume would weigh approximately 1.2 X 10-8 gm;

  • If the molecular weight of the water insoluble technical is 250, then the 1T3 particle would consist of approximately 2.88 X 1013 molecules of which a fraction are at the surface;

  • If you assume that the solid surface structure may not follow a consistent molecular pattern (due to processing and the presence of impurities, for instance), then the organic solid surface can be viewed as consisting of carbon, hydrogen, oxygen, nitrogen, etc. and not as atrazine (MW = 216), carbaryl (MW = 201), chlorothalonil (MW= 266), etc. A surfactant, with a molecular weight of 2500, can be seen as being too big to 'identify' the specific molecular structure of an active ingredient.

How then does a surfactant distinguish among the different active ingredients?

Since surfactants are, by definition, surface active agents, they are influenced by properties that manifest themselves at the solid surface:

  • technical melting point (as affected by the heat of shear of processing equipment);

  • technical water solubility;

  • crystalline structure;

  • particle size;

  • surface area;

  • surface imperfections;

  • surface contaminants (impurities);

Of these, technical melting point and water solubility are truly character of the active ingredient; the remaining are characteristic of the processing.

With this in mind, it should not be necessary to screen all surfactants for a new active ingredient; rather, optimum surfactant performance may be selected based upon the technical physical properties:

  • high melting point/high water solubility

  • high melting point/low water solubility

  • low melting point/high water solubility

  • low melting point/low water solubility

As noted above, optimum surfactant performance is critical at increasing active ingre- dient concentration: approximately 50 %V/V, or 4 lb/gal (480 gm/L). At concentration less than 50 %V/V, choice of surfactant is less critical and the surfactant that performs acceptably for one of the above sets of technical physical properties should perform acceptably for the others.

Lot to lot (and source to source) variability of technical performance is related to dif- ferences in technical surface characteristics: crystalline structure, particle size, surface area, surface imperfections, and surface contaminants. These characteristics affect not only the quantity of surfactant required to wet the organic technical into water but also the partitioning of the surfactant between the organic surface and the aqueous phase.

Partitioning of the surfactant between the organic surface and aqueous phase dictates formulation viscosity. Therefore, lot to lot technical variability may prove significant where optimum surfactant/technical interaction is critical to formulation performance.

Although quite complex in terms of composition, it is easier to view a formulation in terms of functionality with the two key areas being:

  • Surfactant wetting and dispersion

  • Viscosity modifier system