Jeevan Chemicals Private Limited
Creating your advantage through Innovation
Creating your advantage through Innovation
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:
Surfactant System: required to physically stabilize the organic solid in water;
Suspension System: required to prevent the technical particles from settling in the container upon storage;
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:
Matching the density of the water solution to that of the technical
Use of "swelling" clays
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:
Product Label language ("Do not store below 32 F")
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: Unlike powder seed coating formulations, where the
technical can demon-strate unspecified physical properties, aqueous seed coating
formulations are premised upon two factors:
The technical must be a solid under all storage and processing conditions
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:
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;
It must adhere the agricultural chemical to the seed surface under all
storage and application abrasion conditions;
Binders, or stickers, fall into two categories:
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);
Liquid water-dispersible latexes, such as polyvinyl acetate (PVAc),
which contain at most 55% polymer solids;
Binder system selection should be based upon:
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:
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:
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:
It is important to establish up front how the formulation is expected to
perform since this may dictate choice/concentration of inerts.
Sediment formation: is a function of four factors:
inadequate viscosity modifier in the formulation
heat adversely affecting xanthan gum hydration
heat adversely affecting surfactant partitioning at the technical surface/water interface
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:
Formulation Viscosity - the lower the viscosity, the less material will remain in the container upon pouring;
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;
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:
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:
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.
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:
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:
Screening methods
Storage conditions
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:
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.