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Concentrated Aqueous Emulsion (EW) Formulation

How To Develop A Concentrated Aqueous Emulsion (EW) Formulation

Concentrated Aqueous Emulsions (EW) are the dispersion of a water insoluble organic liquid into water. The organic liquid can be either a liquid technical or a solid technical dissolved in an appropriate water insoluble solvent in which the solid technical remains in solution under all temperature conditions.

Concentrated Aqueous Emulsions are a relatively recent formulation option to Emulsifiable Concentrate (EC) formulations where the presence of organic solvents has proven to be a concern both in terms of the flash point associated with the solvent and the potential for phytotoxicity at the time of application

Both the Emulsifiable Concentrate and Concentrated Aqueous Emulsion are physically stabilized by choice and concentration of emulsifier at the solvent/water interface, sterically by means of high molecular weight EO/PO block copolymer partitioning into the water phase, or a combination of both. The difference between the Concentrated Aqueous Emulsion and the Emulsifiable Concentrate emulsion lies in the means in which the emulsion is formed:

  • The Concentrated Aqueous Emulsion forms an emulsion mechanically at the time of processing;

  • The Emulsifiable Concentrate thermodynamically forms an emulsion upon addition to water at the time of application;


Concentrated Aqueous Emulsion formulations are also related to Microemulsion (ME) formulations in that both contain water as the continuous phase. However, there are significant differences between the formulation technologies:

  1. Optical Clarity = the Microemulsion is clear while the Concentrated Aqueous Emulsion is milky in appearance;

  2. Emulsifier Requirement = the Microemulsion requires approximately equal weights of emulsifier and organic liquid in order to reduce the particle size of the organic phase to optical clarity. The Concentrated Aqueous Emulsion can be formulated using 3-10 %W/W emulsifier at all active ingredient concentrations;

  3. Formulation Concentration = Since the Microemulsion requires equal weights of emulsifier and organic liquid, its upper active ingredient concentration is limited to approximately 2 lb/gal (or 240 gm/L). The Concentrated Aqueous Emulsion can be formulated to active ingredient concentrations approaching 6 lb/gal (or 720 gm/L) depending upon technical, the choice of formulation inerts and processing approach;


Concentrated Aqueous Emulsions can best be viewed as a natural technological progression beyond Emulsifiable Concentrates adapting Suspension Concentrate (SC) techno- logy: a Suspension Concentrate is the dispersion of a solid technical in water, a Concentrated Aqueous Emulsion is the dispersion of a liquid technical in water. Where particle size reduction of a solid technical is accomplished either through wet milling (using an Attritor® or Dyno-Mill®) or dry milling (using a hammermill or airmill), particle size reduction of a liquid technical is accomplished with a high shear either in-tank (using a Polytron® homogenizer) or in-line (using an Arde-Decon® homogenizer).

Viewed in this manner, Concentrated Aqueous Emulsions share three (3) basic requirements with Suspension Concentrate Formulations:

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

  2. Suspension System: required to prevent the technical droplets from agglome- rating 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 in a significant viscosity increase during particle size reduction. This viscosity increase can be directly attributed to the organic liquid 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 homogenized technical to agglomerate. As a result, the dependence upon surfactants to formulate low viscosity Concentrated Aqueous Emulsions 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, molecular weight and HLB (ratio of hydrophile to lipophile), formulation physical performance (stability as a function of temperature and variable viscosity) is dictated. 

As with Suspension Concentrate performance upon spray dilution, anionic surfactants are incorporated into Concentrated Aqueous Emulsion formulations in order to prevent agglomeration. These surfactants serve to disperse and stabilize the organic oil droplets in water 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 an oily sediment when packaged in the commercial container is key to establishing formulation shelf life.
Unlike Suspension Concentrate formulations whose suspension system 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


Concentrated Aqueous Emulsion find application mainly with polyhydroxycellulose thickeners. These polyhydroxycellulose thickeners are high molecular weight polymers, which, through controlled incompatibility with water, build viscosity within the continuous phase. In addition, these thickeners compliment the function of the surfactants to sterically stabilize the oil droplets in the formulation concentrate.

The 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, an aqueous formulation that uses polyhydroxycellulose as the suspension system will demonstrate variable viscosity upon storage as a function of temperature: as temperature increases, viscosity decreases. This may result in a formulation pourability issue at reduced temperatures and/or oil separation 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 performing the function of a surfactant that has application at reduced temperatures. This conclusion is supported by the compositional 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:

  1. Liquid Technical

  2. Antifoam

  3. Bactericide

  4. Surfactant

  5. Viscosity Modifier System

  6. Freeze/Thaw Additive

  7. Water Diluent


Liquid Technical: Unlike Dust and Wettable Powder formulations, where the technical can demonstrate unspecified physical properties, Concentrated Aqueous Emulsion formulations are premised upon two factors:


  1. The technical, or technical solution, must be a liquid under all storage and processing conditions

  2. The technical, or technical solution, must demonstrate minimum water soluble under all storage and processing conditions

It is necessary for the technical to remain completely liquid during processing and under all storage conditions. Formation of technical crystals will permanently incorporate formulation surfactants, depleting their concentration at the liquid/liquid interface and preventing their dynamic partitioning between phases to accommodate changes in temperature.

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 oil sediment that may not resolubilize under the best of temperature conditions in a reasonable time frame. 

The upper active ingredient concentration limit for technicals is related to minimizing available void space between suspended liquid spheres and the specific gravity of the technical; the higher the technical specific gravity, the greater the formulation concentration.

Antifoamer:
 The composition of the Concentrated Aqueous Emulsion 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.

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 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, depending upon their chemical composition, function either traditionally as a bridge between the two phases (by means of mutual solubility in both the polar and non-polar phases) or by sterically hindering oil droplet agglomeration in combination with the anionic surfactant by functioning only in the aqueous phase. This second class of nonionic surfactants are from the same chemical class which has found utility in Suspension Concentrate formulations; namely, EO/PO block co- polymers. 

Both classes of nonionics perform differently: the traditional emulsifiers are extremely sensitive to temperature change and are dependent upon anionic surfactant and viscosity modifier system to prevent oil droplet agglomeration. However, since the EO/PO block copolymers do not function at the liquid/liquid interface, they are less sensitive to temperature change and supplement the oil droplet stabilization attributed to the anionic surfactant and viscosity modifier system. 

Therefore, the most efficient surfactants are those which wet and disperse the technical at formulation concentrations approaching 4 lb/gal (or 480 gm/L) over a wide temperature range and at formulation weights of 3-10 %W/W. Synperonic PE/P105 and Atlas G5000 are ethylene oxide/propylene oxide block co-polymer surfactants that have been found to have excellent application in Concentrated Aqueous Emulsion formulations.

It is possible to formulate Concentrated Aqueous Emulsions to active ingredient concentrations in excess of 4 lb/gal by combining both of the above nonionic classes. Specifically, Atlox® MBA 11/6 can be mixed with the liquid technical and added, under agitation, to water containing Synperonic® PE/L64. The resultant O/W emulsion has been processed to contain 70 %W/W organic liquid.

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 sulfonates) can be used in Concentrated Aqueous Emulsion formulations as well as phosphate esters and their partially neutralized salts.

Viscosity Modifier System: As stated above, it is extremely important to prevent the dispersed oil droplets in the Concentrated Aqueous Emulsion from agglomerating 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 hydroxyethylcellulose (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 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 xanthan gum in the formulation which prevent oil sediment/bleed layer formation at elevated temperatures.

Freeze/Thaw Additive: Organic liquid dispersions in water may change rheological properties upon freezing and subsequent thawing. Physical deterioration of the Concentrated Aqueous Emulsion is first and foremost a function of formulation active ingredient concentra- tion: 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 Concentrated Aqueous Emulsion 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 establish up front 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 formula for other components. Therefore it is necessary to select the most efficient wetting agent, dispersant, viscosity modifier. The lower the active ingredient concen-tration in the formulation, the more "forgiving" the physical performance 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


    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 disperse and physically stabilize the liquid technical into the water phase.

  • 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 affecting technical water solubility

    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 surfactant system. Water hardness and water temperature as well as the presence of a WP, SC, or EC formulation may impact physical suspension:

  • Formulation Tank Mix Compatibility: In general, Concentrated Aqueous Emulsions tank mix very well with another EW or SC, and problematically with EC or WP 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:
    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 concentration - xantham gum is compositionally equivalent to wallpaper paste. Allowed to dry in the commercial container, the Concentrated Aqueous Emulsion will form an oily film that resists water penetration.

    3. Sediment Formation - With the small particle size characteristic of Concentrated Aqueous Emulsions and inadequate viscosity modifier concentra- tion, technical will settle out of suspension with minimum void space between particles and ultimately forming a coalesced oil layer. The result of this physi-cal separation of organic/inorganic phases is to preclude complete removal of the technical from the container.

      The effect of the above formulation properties upon container rinsing is premised upon proper selection of container composition and/or barrier treat-ment


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 three reasons:

  1. exposure to dusts is minimized during handling

  2. materials transfer can be monitored either by weight or volume

  3. ease of dispersing two mutually miscible liquids

Proper surfactant selection is key to formulation viscosity performance across the temperature range of interest. The formulation should, for example, demonstrate emulsion stability as a concentrate (in the presence 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 with the aid of xantham gum which serves to sterically isolate the individual oil droplets. As the active ingredient concentration increases, effective surfactant partitioning at the oil/water interface is critical to preventing oil agglomeration. However, surfactants that perform effectively 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 surfactants for high concentration EW formulations.
From Uniqema, both Synperonic® PE/P105 and Atlas® G-5000 have demonstrated ap- plication in Concentrated Aqueous Emulsions. 

Tank mix compatibility of Concentrated Aqueous Emulsion 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.

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 xantham gum prior to addition to water, provides the additional service of promoting uniform distribution (hydration) of thickener throughout the water phase.

Formulation processing: In order to obtain both the required formulation physical stability and application efficacy, it is necessary to process the Concentrated Aqueous Emulsion to micron particle size. 

This is accomplished by formulation order of addition and processing equipment.

Formulation Order of Addition:

  1. Water Diluent

  2. Defoamer

  3. Bactericide

  4. Surfactants

  5. Propylene Glycol/Xantham Gum(Mix until uniform)

  6. Liquid Technical

 

By building viscosity within the formulation before liquid technical addition (under high shear agitation), the formulation demonstrates resistance to the flow of the liquid technical in water during mixing. This resistance manifests itself in reduced oil droplet particle size that is instantaneously stabilized by the presence of surfactants and xantham gum.

-Processing Equipment: It is extremely difficult to reduce the Concentrated Aqueous Emulsion particle size using standard EC mixing equipment (for example, hurricane-type and Cowles-type mixers). There is no "incentive" to uniformly distribute the two mutually incompatible liquids except by means of high viscosity within the aqueous phase. This would prove prohibitive since the processing shear plus the high formulation viscosity would equal heat formation that could adversely affect formulation performance.

Therefore it is necessary to use processing equipment which, by their very structure, offer resistance to flow through the mixing heads. Commercially, this process can be accom-plished either through in-tank homogenizer (for example, a Polytron® or Silverson® Mixer) or using an in-line recirculating homogenizer (for example, an Arde-Decon®). Both of which must be monitored and controlled for heat generation.

Alternatively, the shear requirement necessary to process the Concentrated Aqueous Emulsion micron particle size can be reduced significantly by selection of a traditional emul-sifier which functions at the liquid/liquid interface and combining it with the EO/PO block co-polymer. However, focus must be placed upon proper selection of the emulsifier since its performance may be affected by temperature change. Also, it is still necessary to include xantham gum into the formulation where the specific gravities of the two phases are not the same.

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 Separation 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 Separation Upon Dilution
  Chemical Stability 0,1, 3, 6 months 3 C, 23 C, 50 C No Loss Of A.I.
Application Dispersion Using Formulations Stored Above Minimum Separation 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.

Determination of a development methodology: 
Concentrated Aqueous Emulsions are the dispersion of an insoluble liquid technical, with a median particle size distribution range of <1X to >20X, in water.

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

  • Surfactant wetting and dispersion

  • Viscosity modifier system

  • Both of which lend them well to Experimental Design Development. "Formulation Development Experimental Design" for suggested approaches.

    It is important to realize that an Experimental Design approach does not guarantee the successful development of a Suspension Concentrate formulation. It still falls upon the formula- tion chemist to select the excipients to be evaluated, to determine the impact of the processing equipment upon the formulation, and whether the application equipment impacts the formulation performance.

    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.