Weed Technology. 2000. Volume 14:773–784
Symposium
Adjuvants—Terminology, Classification, and Chemistry1
JAMES L. HAZEN2
Abstract: An adjuvant is a material added to a tank mix to aid or modify the action of an agrichemical, or the physical properties of the mixture. For the most recent decade, responsible researchers, suppliers, and trade organizations have made significant progress to identify, understand, and
standardize adjuvants. Adjuvant components, their purpose in application, terminology, and now,
even tests that establish minimum performance expectations are under development. Certification of
voluntary adjuvant composition and performance standards will soon be available. The adjuvant
industry has grown into respectability. This paper will attempt to serve as a useful reference for
current terminology and definitions accepted by weed science, and also provide a brief discussion
of the chemistry favored in current adjuvant compositions.
Additional index words: Surfactant, crop oil, activator, wetting agent.
Abbreviations: ASTM, American Society for Testing and Materials; CMC, critical micelle concentration; CPDA, Chemical Producers and Distributors Association; DST, dynamic surface tension;
EST, equilibrium surface tension; HLB, hydrophile–lipophile balance; WSSA, Weed Science Society
of America.
INTRODUCTION
the 1982 Adjuvants for Herbicides monograph published
by the Weed Science Society of America (WSSA), but
to build upon it.
Adjuvants emerged from the humble, honest purpose
of helping other materials to be more effective. The story
goes that the first agricultural adjuvant was a soap solution (Gillette 1888, 1890) used to increase the toxicity
of arsenical formulations on weeds. Animal oil soaps
were common adjuvants in use before 1900 (Gillette
1889), as were kerosene emulsions. Sugars and glue
were considered as stickers and many other materials
followed as adjuvant research continued (McWhorter
1982). Since adjuvant research began, it is probable that
thousands of different materials and combinations have
been tried for nearly as many purposes. A typical adjuvant user has little concept of what secret components
are in the jugs, sold under many catchy names. To understand more, one must first establish descriptive adjuvant terminology. This terminology will lead to more
concise expectations when applied to classification of adjuvants by purpose. Subsequently, it will simplify a brief
look at the chemistry of those adjuvants. Perhaps this
discussion and added knowledge will help weed scientists participate more effectively in the optimization of
agrichemical performance. The intent is not to supplant
ADJUVANT TERMINOLOGY
Definition: ADJUVANT—A material added to a tank mix
to aid or modify the action of an agrichemical, or the
physical characteristics of the mixture. American Society for Testing and Materials (ASTM) Designation E
1519-95.
Definition: ADJUVANT—Any substance in a herbicide
formulation or added to the spray tank to modify herbicidal activity or application characteristics. Weed Science Society of America (WSSA) Herbicide Handbook—7th edition, 1994.
Definition: ADJUVANT—An adjuvant is something
which is added to a spray solution to increase the effectiveness of the active ingredient. They may be packaged
and formulated with the herbicide product or they may
be added to the spray solution as a tank mix. Iowa State
University, Website (www.weeds.iastate.edu)—B. Pringnitz, April 28, 1998.
The definitions offered above were crafted by groups
of varied size and background. The first, from the American Society for Testing and Materials, was developed
as a voluntary concensus standard, drafted by a group
1
Received for publication April 10, 2000, and in revised form August 1,
2000.
2
Global R&D Manager, Agricultural Specialties, Witco–a Crompton business, Dublin, OH. E-mail: [email protected].
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HAZEN: ADJUVANTS—TERMINOLOGY, CLASSIFICATION, AND CHEMISTRY
of 70 adjuvant formulators and users and approved by
an organization of 35,000. The second definition, from
WSSA, was developed by a subcommittee of possibly
10 weed scientists, approved by a terminology committee, and subsequently approved by a Board of Directors.
The final definition, from a University Extension Office,
has an unknown origin, but represents a consultant view,
contrasted with the producer or user definitions. Each
definition manages to convey a functional, albeit biased,
understanding of the word adjuvant.
Terminology is a truly challenging endeavor. As for a
dictionary, a lexicographer takes a best shot at describing
the word in the context of current use and understanding.
With time, the use and meaning of a word will change
either through misuse or a greater understanding of the
concept itself. Such definitions are most accurately developed and maintained by those responsible for the subject of the definition itself. Definitions must be living
descriptors. Any set of definitions that is not occasionally
reviewed for contextual accuracy will soon fail to meet
the needs of the associated industry and authors.
In the present case, this would be the adjuvant manufacturers that are most qualified to define associated
terms. They know what is in the adjuvant, what it can
accomplish, and why they chose to market such a product. However, it is entirely acceptable for a different interest group to modify a set of definitions to relate a
specific focus for a set of terms. For example, active
ingredient means one thing to a herbicide producer or
user, whereas for the adjuvant manufacturer, the same
term represents the amount of surfactant components in
a particular adjuvant recipe that contribute the desired
effect.
Definition: ACTIVE INGREDIENT (ai)—the chemical in
a herbicide formulation primarily responsible for its
phytotoxicity and which is identified as the active ingredient on the product label. WSSA Herbicide Handbook—7th edition, 1994.
Definition: ACTIVE INGREDIENT—a component of the
formulation that produces a specific effect for which the
formulation is designed. ASTM E 1519-95.
Different adjuvant effects will cause the actual active
ingredient component concentrations to change. It is that
material that causes the intended adjuvant effect only;
all other components become inerts for that specified
purpose. What is active for one type of adjuvant may
become an inert in the formulation of a different-purpose
adjuvant. We must maintain clarity that an adjuvant active ingredient is NOT a herbicide active ingredient.
In the early years of adjuvants, compositions and per774
formance claims varied widely and frequently. Small local or regional adjuvant formulators would purvey
blends of readily available surfactants and oils into the
local herbicide markets. Often, there was frequent compositional change, contributing to unexpected performance (year to year or even seasonal) of the co-applied
herbicides. Certainly a profit motive further clouded the
relation of quality and performance to cost and value. In
theory, an adjuvant is worth the amount of additional
herbicide required to achieve a desired level of effect. A
herbicide may have a cost of .$100/kg and typical adjuvants are priced at 1 to 3% of the herbicide cost. It
makes good economic sense when $2 of adjuvant can
support a 20 to 50% reduction in herbicide use. High in
performance value, interest in adjuvants grew quickly.
In time, most herbicide labels required the use of some
type of adjuvant. With demand high and adjuvant standards low, it is not surprising that a somewhat checkered
supply industry grew to enjoy a profitable position and
thus was born the ‘‘Age of Snake-Oil Adjuvants.’’
For the most recent decade, responsible researchers
(Foy 1998; Holloway 1995; Rogiers 1995), producers
(Green 1995; Reeves 1992; Underwood 1992), and trade
organizations (McCarthy 1992; Stickle 1992, 1995) have
made significant progress to identify, understand, and
standardize adjuvants (Roberts 1992). Adjuvant components, their purpose in application, terminology (Ahrens
1994; ASTM 1999), and now tests are under development to establish standard performance attributes for
each type of adjuvant. Through collaboration, ASTM
terminology and test procedures and CPDA (Chemical
Producers and Distributors Association) certification of
voluntary adjuvant composition and performance standards will soon be available (Stickle 1998). The adjuvant
industry has grown into respectability.
Adjuvants typically may be comprised of surfactants,
oils, solvents, polymers, salts, diluents, humectants, and
water. There are two very basic types of adjuvants: (1)
those that modify the physical characteristics of the
spray mixture, and (2) those that enhance the biological
efficacy of the crop production chemical (ASTM 1999).
The first adjuvant type is a tank-mix modifier or utility
adjuvant. Each adjuvant composition has a specific purpose in a tank-mix situation; pH adjustment or buffering,
compatibilization with fertilizer or other pesticide formulations, spray drift reduction, deposition aid, or defoaming agent, for examples.
The second type of adjuvant, activators, are the most
difficult to validate for performance. There are many interacting contributors to enhancing the efficacy of a herVolume 14, Issue 4 (October–December) 2000
WEED TECHNOLOGY
Figure 1. Contact angle, u, is measured as the tangent from the point of
droplet contact with the substrate. A lower contact angle results when the
substrate surface free energy is reduced by a surfactant. Thus, the contact
angle decreases as the droplet flattens on the substrate, indicating better wetting.
bicide; even the improved spray solution physical characteristics by utility adjuvants can favorably influence
efficacy. In the next section, adjuvant classifications will
be related to the intended primary effect. Here, terminology is the key to understanding the adjuvant composition relative to purpose.
ADJUVANT CLASSIFICATION
Utility adjuvants are the topic of another paper being
presented at this symposium. This paper will take the
risk of venturing into a tenuously defined region of adjuvants—bioefficacy enhancing or ‘‘activator’’ adjuvant
compositions.
Wetter–Spreader Adjuvant. This is the most straightforward type of adjuvant. Wetting and spreading are influenced by surface-active agents (surfactants) that lower
the free surface energy of the substrate being wetted.
Wetting allows a droplet to demonstrate a lower contact
angle on a substrate (Figure 1). This means that the droplet becomes less spherical and tends to flatten at the point
of substrate contact. Under certain conditions, wetting
further allows a droplet of given volume to cover a larger
surface area on the target substrate than an equivalent
volume droplet without wetting agent. This enhancement
of target coverage is called spreading.
Because water is the most typical carrier for crop production sprays, discussion in this paper will be based on
aqueous systems. (Similar properties may be understood,
with inherent differences, for oil-carrier systems.) Wetting agents reduce the internal surface tension within a
spray droplet. Surface tension is an inwardly directed
force within a water droplet that causes it to form a relatively spherical shape. This spherical droplet form tends
to prevent the droplet from contacting a hydrophobic
surface with the desired large contact area. The proper
surfactant and concentration will reduce droplet EST
(equilibrium surface tension) and allow the droplet to lie
flat in a relatively thin layer on the waxy leaf surface.
Volume 14, Issue 4 (October–December) 2000
Further, when the tension is reduced to a very low level,
the droplet may begin to spread (Brumbaugh et al. 1995;
Hartley and Graham-Bryce 1980a, 1980b; Lo and Hopkinson 1995) further from the point of deposition, approaching an infinitely thin liquid layer over the surface.
This allows the crop production chemical to be distributed more broadly over the target surface. Of course, this
thin film tends to dry rapidly, leaving the other spray
solution components spread over the once-filmed region.
Spreading is not always the best effect for a particular
spray composition. A higher-volume droplet is often
more capable of influencing a positive osmotic drive for
absorption/penetration/uptake. Further, the surfactant
may have cuticular solubilization capability in concentrated form that is lost when the adjuvant concentration
per unit leaf surface is reduced.
Wetting is seen as the first requirement for adjuvancy.
The concentration of surfactant applied may be critical
to the functionality of the adjuvant. When a surfactant
concentration is above the CMC (critical micelle concentration), the surfactant forms aggregates called micelles and generally the minimum EST is achieved.
Spreading and solubilization are favorably affected by
surfactant concentrations above the CMC. Spray droplets
must contact a target surface to begin the activation process. A lowered DST (dynamic surface tension) is also
important to the deposition process; if velocity and DST
are high, the spray droplet may rebound from the target
and be lost to the environment, decreasing application
efficiency (Friloux and Berger 1996). An excellent review of wetting as related to agriculture was recently
authored (Pallas 1997).
Sticker Adjuvants. Stickers were offered to help keep
solid crop production chemical deposits on the target
surface after droplet dry-down. Wind, physical contact,
and rain may dislodge normal deposits from the leaf.
Stickers are nonevaporating materials that have a viscous
nature, allowing them to adhere, along with the pesticide
deposits, for a longer interval of activity. If the sticker
is not readily water soluble, the deposit may also resist
rain wash-off. Many crop production formulations already contain polymers or polymeric surfactants that inherently resist dislodging of the chemical deposit. Stickers are most commonly used with wettable powder and
water-dispersible granule formulations. Certain oils may
function as stickers when their volatility is very low.
Humectants. Humectants are a variation on stickers in
that they slow the droplet dry-down rate and, by remaining liquid, tend to retain a deposit on the target
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HAZEN: ADJUVANTS—TERMINOLOGY, CLASSIFICATION, AND CHEMISTRY
surface. However, humectants have a more important effect in that they tend to keep the herbicide deposit in a
liquid, more bioavailable form. When a droplet dries, the
active ingredient tends to crystallize. The crystal form
of an active ingredient is the least available form for
absorption and uptake. Humectants may have inherent
liquidity or, like salts, they may draw moisture from the
atmosphere to maintain a higher humidity level near the
spray deposit. Matsumoto et al. (1992) discuss the effect
of humectants on uptake.
Penetration Agents. Here is where the concept of bioefficacy enhancement by adjuvants really gets going. A
penetration agent is a material that for any one of possible characteristics assists the movement of the crop
production chemical from the target surface through the
natural barriers to uptake. Cuticular waxes may be softened, plasticized, or dissolved (Manthey and Nalewaja
1992), or the stomata infiltrated (Stevens et al. 1991,
1992; Gaskin 1995), allowing diffusion movement of the
herbicide to the more hydrophilic structures beneath.
This is basically the process covered by the WSSA definition of Absorption: The process by which a herbicide
passes from one system into another. . . (Ahrens 1994).
There is no present consensus that this absorption
alone constitutes a definition of penetration. Beyond this
point, enhanced efficacy may result from the greatly increased bioavailability of the herbicide or some type of
more active transport assistance. There is a growing
opinion that a Translocation Agent may operate from
this point onward, assisting with movement into the appropriate vascular system and onward to sites permitting
herbicidal activity (Field et al. 1995). Whether the adjuvant itself moves with the herbicide or simply aids
transport into the vascular system remains to be verified.
Herbicide Modifiers. WSSA specifically excludes several types of material from classification as activator adjuvants. This definition of a herbicide modifier is flawed
in that a physiological mechanism does not sufficiently
differentiate from other activator adjuvant effects. Such
modifiers probably have chemical or physical attributes
leading to the desired physiological effect. Judging by
the simple arguments presented, it may be necessary to
revisit these physiological-only definitions.
Definition: HERBICIDE MODIFIER—A chemical substance used with herbicides to change their herbicidal
properties by a physiological mechanism. They include
safeners, synergists, extenders, etc., but do not include
compounds such as surfactants that may modify herbicidal activity by chemical or physical mechanisms.
WSSA Herbicide Handbook—7th edition, 1994.
776
Definition: EXTENDER—A chemical that increases the
longevity of a herbicide in soil. WSSA Herbicide Handbook—7th edition, 1994.
The term ‘‘extender’’ grew from the use of an alternative, microbial-preferred substrate co-applied to the
soil with the herbicide. The soil microbes preferentially
metabolized the alternative substrate, thus sparing or extending the activity of the herbicide. Additionally, a soil
extender may physically reduce herbicide movement
away from the soil target zone. An antimicrobial agent
might inhibit the soil degradation of the herbicide. A
polymer might reduce volatility of the herbicide to keep
it in the soil. These chemical or physical extenders contrast with the original metabolic extender.
Definition: SAFENER—A substance that reduces toxicity
of herbicides to crop plants by a physiological mechanism. WSSA Herbicide Handbook—7th edition, 1994.
Definition: SYNERGIST—For herbicides; a nonherbicidal compound used to increase the phytotoxicity of a
herbicide by a physiological mechanism. WSSA Herbicide Handbook—7th edition, 1994.
Safeners usually accelerate metabolism of the herbicide in the crop plant but not the weed. Sometimes, crop
seed treatment may achieve the desired selectivity over
weeds. In contrast, synergists inhibit the metabolism of
the herbicide in the weed species, thus increasing the
activity of the herbicide.
ADJUVANT CHEMISTRY
Definition: SURFACTANT—A material that improves
the emulsifying, dispersing, spreading, wetting, or other
properties of a liquid by modifying its surface characteristics. WSSA Herbicide Handbook—7th edition,
1994.
Definition: SURFACTANT—A material that when added
to a liquid medium modifies the properties of the medium
at a surface or interface. ASTM E 1519-95.
A discussion of adjuvant chemistry must properly begin with surfactants, those amphipathic (dual-characteristic) molecules having the ability to orient at interfaces
to modify surface properties, such as interfacial tension.
Surfactants may be nonionic or ionic (anionic, cationic,
or amphoteric). Pure surface chemistry is best discussed
by the experts; please refer to Rosen (1989a, 1989b) and
Schick (1966) for a great introduction. This discussion
will stay close to the surfactant principles behind adjuvant applications.
Surfactants are generally classed in four groups on the
basis of their ability to ionize in aqueous solution. First
are nonionic surfactants, which do not ionize (Figure 2).
Volume 14, Issue 4 (October–December) 2000
WEED TECHNOLOGY
Figure 3. Anionic surfactant examples.
taining the lipophilic segment forms exclusively a positive ion (cation) when placed in aqueous solution.
ASTM E 1519-95.
Definition: AMPHOTERIC SURFACTANT—A surfaceactive agent capable of forming, in aqueous solution,
either surface-active anions or surface-active cations depending on the pH. ASTM E 1519-95.
Nonionic surfactants do not ionize in solution. Anionic or cationic surfactants bear a charge in solution, and
amphoteric surfactants have positive and negative chargFigure 2. Nonionic surfactant examples.
Next, there are ionic surfactants, further classed as anionic (Figure 3), cationic, and amphoteric (Figure 4).
Definition: NONIONIC SURFACTANT—A surface-active agent having no ionizable polar end groups but
comprised of hydrophilic and lipophilic segments.
ASTM E 1519-95.
Definition: ANIONIC SURFACTANT—A surface-active
agent in which the active portion of the molecule containing the lipophilic segment forms exclusively a negative ion (anion) when placed in aqueous solution.
ASTM E 1519-95.
Definition: CATIONIC SURFACTANT—A surface-active
agent in which the active portion of the molecule conVolume 14, Issue 4 (October–December) 2000
Figure 4. Cationic and amphoteric surfactant examples.
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HAZEN: ADJUVANTS—TERMINOLOGY, CLASSIFICATION, AND CHEMISTRY
Figure 5. Surfactant hydrophobe and hydrophile.
es and may be anionic or nonionic, depending on pH of
the solution. All types of surfactants may lower surface
tension to cause wetting, but issues of compatibility with
ionic herbicides may preclude the use of one or another
type.
To be effective, a surfactant must be selected for affinity to the surfaces to be modified. For our purpose
with aqueous systems, the hydrophilic portion of the
molecule will associate with the water phase and the
lipophilic portion must have strong affinity for the nonaqueous substrate (Figure 5). The subject substrate might
be the pesticidal crystal, an oil, or a solvent/pesticide
solution. Selecting the ideal surfactant (or blend) is more
often empirical than intuitive. Certainly as the spray solutions become more complex, an expanded range of
surfactants may be required. Experimental design may
be helpful, but an experienced formulation chemist can
easily beat the computer-aided effort to success.
Beyond the lipophilic affinity of a surfactant for a
given substrate, the ability of a surfactant to perform
optimally at an interface depends on the relation of the
hydrophilic portion of the molecule to the lipophilic portion. The common means of relating this correlation is
the term HLB, an abbreviation for hydrophile–lipophile
778
balance (Griffin 1954). For nonionic surfactants, this is
an easy calculation; the percentage of the molecule (molecular weight) that is hydrophilic is divided by 5 to
yield a number between 0 and 20. Theoretically the actual number cannot reach either end or there would be
no corresponding lipophile or hydrophile. In any case,
surfactants can approach the full range by having an extreme affinity for water or lipid. This range of surfactant
character permits a range of performance attributes:
emulsification, wetting, detergency, and solubilization.
Low HLB surfactants permit formation of water-in-oil
(invert emulsions) systems. Mid- to upper-range HLB
surfactants may be wetting agents or form oil-in-water
emulsions. Upper-range HLB surfactants are used as detergents or solubilizers. To be water soluble, a surfactant
would typically need to be HLB 12 or higher (. 60%
hydrophile). One may recognize that all of these properties relate to adjuvancy and pesticide formulation.
The lipophile in common surfactants may be one of
many sources: fatty alcohols, alkylphenols, vegetable
oils or triglycerides, fatty amines, sugar esters, glucosides, alkylbenzenes, or organosiloxanes. The most common hydrophile is polyethylene oxide. Polypropylene
oxide, other polyglycols, and mixtures of oxides are also
used as base hydrophile for nonionic surfactants.
Anionic surfactants are often derivatized nonionic surfactants, meaning that they have been modified via phosphation or sulfonation. Such sulfonate or phosphate
functional groups can be assigned a contributing HLB
value, allowing a loose continuation of the HLB concept
for ionic surfactants. Anionic surfactants are important
for compatibility and dispersion properties, as well as
often being superior wetting agents. Anionic materials
may bind adversely with cationic herbicides.
Cationic surfactants are traditionally quaternary ammonium compounds, often having antimicrobial properties on their own. Cationic surfactants are usually highly toxic to aquatic organisms. Alkylamine ethoxylates
have mild cationic character and are widely used as adjuvants and emulsifiers. The higher the HLB, the more
nonionic they become. Cationic materials have useful
substantive properties, as they bind to cellulosic materials. The mildly cationic tallowamine ethoxylates, frequently used in herbicide formulations, are most typical
of this surfactant class.
Amphoteric surfactants, widely used in personal care
for their mild, nonirritating properties, have found little
use in agricultural formulations to date. Specific screening as herbicide adjuvants demonstrated little favorable
effect. Amphoterics are very compatible in high electroVolume 14, Issue 4 (October–December) 2000
WEED TECHNOLOGY
lyte (salt) systems, where they build viscosity for products like shampoo.
Wetting Agents. Definition: WETTING AGENT—A substance that serves to reduce the interfacial tensions and
causes spray solutions or suspensions to make better
contact with treated surfaces (see surfactant). WSSA
Herbicide Handbook—7th edition, 1994.
Typical wetting agents are comprised of a nonionic
surfactant diluted with water, alcohol, or glycols. The
most common wetting agents are the alkylphenol ethoxylate (nonylphenol or octylphenol) surfactants. Alcohol
ethoxylates and variations, such as isotridecanol or trimethylnonanol ethoxylate, are frequently used. These
adjuvant products are often referred to as 90:10 or 80:
20 surfactants. The first number is supposed to represent
the amount of surfactant active in the blend, but there
has been a history of other components being included
with the surfactant when counting active ingredients.
The other components may have some favorable effect
on the adjuvant performance, but should not be considered as active in a wetting agent composition. These additional components listed above may inhibit gel formation on addition of the adjuvant to cold water or may
act as humectants.
Spreading Agents. Definition: SPREADER—A material
that increases the area that a droplet of a given volume
of spray mixture will cover on a target. ASTM E 151995.
Spreading agents are considered to be those wetting
agents that allow a droplet to spread beyond its initial
diameter of contact. Many organic surfactants allow a
spread diameter increase of two to three times. Typically
the alcohol ethoxylates (Stock et al. 1992) such as tridecanol 1 6 mol ethylene oxide will spread well. Also,
the common anionic surfactant, sodium dioctylsulfosuccinate, had been used extensively for growth regulator coverage. Concentration is the key secondary determinant for spreading.
A most phenomenal demonstration of spreading may
be accomplished with the use of certain trisiloxane
ethoxylate organosiloxane derivatives (Figure 2). Trisiloxane alkoxylates have the ability to cause superspreading, that is, spreading far exceeding the capability of
traditional organic surfactants. The superspreading phenomenon is rapid and succeeds in dramatically increasing coverage (10 to 100 times droplet diameter) from a
herbicidal spray droplet. There is evidence that these surfactants also permit herbicide uptake via stomatal infiltration or flooding (Stevens et al. 1991) and via penetraVolume 14, Issue 4 (October–December) 2000
Figure 6. Sticker component examples.
tion of surface structure defects. The disadvantage is that
some materials in the spray solution may disrupt the
mechanism of superspreading while retaining the positive effect on absorption uptake. In other cases, herbicide
uptake is still enhanced from nonsuperspreading concentrations of such silicone copolymer surfactants. Previously, certain modified organosilicones have been shown
to provide spreading functionality in oil-based systems
(Policello et al. 1995).
Sticking Agents. Definition: STICKER—A material that
assists the spray deposit to adhere or stick to the target
and may be measured in terms of resistance to time,
wind, water, mechanical action, or chemical action.
ASTM E 1519-95.
Stickers may be heavy petroleum fractions, water-soluble polymers, acrylic latex, epoxidized seed oils (similar to boiled linseed oil, which dries on exposure to air),
or alkylphenol condensates called resins (Figure 6).
These materials are dispersed in a dilute adjuvant mix779
HAZEN: ADJUVANTS—TERMINOLOGY, CLASSIFICATION, AND CHEMISTRY
Figure 7. Humectant examples.
Figure 8. Petroleum oil examples.
ture, affording a low-viscosity product for easier handling. Some high-molecular-weight surfactants such as
ethylene oxide/propylene oxide block copolymers have
a natural tendency to adhere to a surface. The degree of
tackiness and resistance to rain wash-off will vary according to the water solubility of the sticker and its concentration relative to the herbicide deposit. Sticker components have often been blended with wetting agents to
permit better target coverage of the pesticide residue before dry-down, which activates the sticker. The term
spreader-sticker has recently fallen from favor, as it
seemingly is an oxymoron term in that sticking is the
opposite of spreading.
Humectants. Definition: HUMECTANT—A material
that increases the equilibrium water content and increases the drying time of an aqueous spray deposit. ASTM
E 15159-95.
Typical humectants are those water-soluble materials
that do not readily dry down after the aqueous portion
of the spray evaporates, because they draw moisture
from the environment. Most common humectants are
glycerin, propylene glycol, diethylene glycol, polyethylene glycol, urea, and to some extent, diammonium sulfate. Such liquid materials are often used as diluents for
wetting agents (Figure 7). The salts are often added to
the spray tank as secondary herbicide activators. Crop
oil concentrates may provide humectancy, as the paraffinic or vegetable oil also resists evaporation. Humectants are most effective when they are low-volatility solvents for the active ingredient being protected. Humec780
tants are an important component in suspension concentrate herbicide formulations.
Penetration Agents. Definition: PENETRANT—A material that enhances the ability of an agrichemical to
enter a substrate or penetrate a surface. ASTM E 151995.
This is the most varied and secretive category of all
activator adjuvants. Extensive/expensive research programs have been designed by basic producers of herbicides and adjuvant formulators to identify specific chemistry favoring superior performance. Many such materials have been discovered since the first ‘‘designer adjuvant’’ (DASHt Adjuvant, a registered trademark of
BASF Corp.) composition and application was patented
in 1989 (U.S. 4,834,908 1989) specifically formulated to
optimize the performance of sethoxydim herbicide. More
often, such a performance advantage is maintained as a
proprietary secret rather than disclosing the chemistry.
Crop oil concentrates. The oldest products in the penetration category are those based upon paraffinic (Chambers 1996; Manthey et al. 1989) and vegetable-derived
oils (Adamczewski and Praczyk 1995; Killick et al.
1995; Nalewaja 1995) (Figures 8 and 9 and reference
Table 1).
Definition: PARAFFINIC OIL—A petroleum oil (derived
from paraffin crude oil) whose paraffinic carbon type
content is typically greater than 60%. ASTM D 2140.
Definition: VEGETABLE OIL—Oil extracted from seeds;
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WEED TECHNOLOGY
Table 1. Composition of oil-based adjuvants.
Adjuvant terma
Dormant oil
Crop oil
Crop oil concentrate
Vegetable oil concentrate
Modified vegetable oil concentrate
Oil component
Emulsifier
component
(%)
Paraffinic (phytobland)
Paraffinic
Paraffinic
Vegetable
Alkylated vegetable oil
2–5
5
5–20
5–20
5–20
a
Standard terminology relating to agricultural tank mix adjuvants taken
from the American Society for Testing and Materials, Designation E 1519.
These terms are the only published terms that accurately represent the current
industry interpretations and product compositions.
Figure 9. Vegetable oil examples.
typically those of corn, cotton, peanut, rapeseed, sunflower, canola, or soybean. ASTM E 1519-95.
Paraffinic or mineral oil adjuvant products have three
basic compositions, differentiated by the emulsifying
surfactant content.
Definition: DORMANT OIL—A horticultural spray oil
applied during the dormant phase of the targeted plant.
ASTM E 1519-95.
There are ‘‘quick-break’’ or dormant oils that use a
very low amount (2 to 5%) of emulsifier for dispersion
into the spray tank. The preferred oil for this application
is defined below as ‘‘phytobland oil’’. This spray is desired to separate quickly after application to allow a rapid dry-down, leaving the oil to suffocate insects or fungal spores.
Definition: CROP OIL CONCENTRATE—An emulsifiable petroleum oil-based product containing 5 to 20%
w/w surfactant and a minimum of 80% w/w phytobland
oil. ASTM E 1519-95.
Definition: PHYTOBLAND OIL—A highly refined paraffinic material with a minimum unsulfonated residue of
92% v/v. ASTM D 483.
Then there is a premium crop oil concentrate, typically having 17% of an emulsifier (typically sorbitan ester ethoxylates, alkylphenol ethoxylates, and other components such as polyethylene glycol esters), some of
which are also known to enhance bioefficacy.
Definition: CROP OIL (EMULSIFIABLE)—An emulsifiable petroleum oil-based product containing up to 5%
w/w surfactant and the remainder of a phytobland oil.
ASTM E 1519-95.
Between these is a low-cost crop oil adjuvant having
Volume 14, Issue 4 (October–December) 2000
about 5% emulsifier for minimal stability in the spray
tank.
Definition: VEGETABLE OIL CONCENTRATE—An
emulsifiable vegetable oil product containing 5 to 20%
w/w surfactant and a minimum of 80% w/w vegetable
oil. ASTM E 1519-95.
There are some vegetable oil concentrates used in the
same manner as the crop oil concentrates, typically based
upon canola or soybean oil, using 5 to 10% emulsifier
for dispersion. Table 2 presents the typical relative fatty
acid compositions for selected seed oils. The fatty acid
composition can affect adjuvant performance on specific
weed targets.
Definition: MODIFIED VEGETABLE OIL—An oil, extracted from seeds, that has been chemically modified
(for example, methylated). ASTM E 1519-95.
Definition: MODIFIED VEGETABLE OIL CONCENTRATE—An emulsifiable, chemically modified vegetable
oil product containing 5 to 20% w/w surfactant and the
remainder chemically modified vegetable oil. ASTM E
1519-95.
Some of the best vegetable-based products are those
modified (derivatized) to methyl and other lower alkyl
esters such as methylated soybean oil, methyl sunflowerate, or ethyl canolate. These methyl or ethyl esters are
typically blended with 5 to 15% nonionic or blended
surfactants for emulsification. All such oil adjuvants are
considered to have their effect via disruption or softening
of the cuticular waxes.
Designer activator adjuvants. Generic penetrator adjuvants (Bayer and Lumb 1973) often have several components because the barriers to penetration are complex.
Aspects of wetting, wax solubilization, and active transport across membranes are all important. For these attributes, appropriate adjuvants might include multiple
solvents, buffers, and superior wetting agents matched
to the herbicide and key target species. Surfactants for
such adjuvants are generally the same as previously de781
782
a
Castor oil contains 88 to 91% ricinoleic [C18:1(OH)]. Data adapted from Witco chromatographic data and from various literature sources. Variations in oil composition typically change adjuvant
effects.
b
Tr, trace.
—
0–2
0–1
0–3
—
—
—
1–3
—
—
—
—
—
2–10
0–1
34–56
34–55
1–2
1–2
17–38
50–60
5–8
30–50
23–44
63–86
10–18
39–66
22–34
5–9
—
—
—
2–4
—
—
Coconut
Corn
Cottonseed
Olive
Palm kernal
Peanut
Soybean
Canola
4–10
—
—
—
3–7
—
—
44–51
—
—
0–1
45–52
—
—
13–18
0–2
0–3
0–2
14–19
Trb
Tr
7–10
8–10
17–23
7–20
6–9
6–11
7–11
1–4
1–4
1–3
1–3
1–3
3–6
2–5
—
—
—
0–1
1–2
5–10
1–3
—
1–2
—
1
0–1
1–2
0–4
C18:2
C10
C8
Source
Table 2. Fatty acid composition of vegetable oils.a
C12
C14
C16
C18
C18
C16:1
C18:1
C18:3
C18:x
HAZEN: ADJUVANTS—TERMINOLOGY, CLASSIFICATION, AND CHEMISTRY
scribed, although there is extensive adjuvant research in
progress with new surfactant chemistries. It is important
to remember that grasses and broadleaf weed targets
have varied requirements for contact and control. Often,
the adjuvant developer must allow a slight weakness on
one weed type or species to maximize the herbicidal effect for control of another species. One cannot forget that
growth-stage, environmental, seasonal, and regional differences will also require different adjuvant compositions. The quest for adjuvant performance superiority
and understanding is ongoing and never-ending.
SUMMARY
Spray-tank adjuvant terminology is covered by the
ASTM E-1519 standard. Adjuvant test methods are developed by ASTM Committee E 35.22 on Pesticide Formulations and Application Systems. All ASTM standards are developed by experts active in the industry;
for this topic they are the producers and users of adjuvants. Standards under development are subject to the
opinions of all interested parties during the approval process. Further, all standards are reviewed every 5 yr and
edited or revised for compliance with current practice
and interpretations. ASTM definitions and test procedures will be cited as criteria for voluntary adjuvant standardization under the auspices of CPDA. The CPDA adjuvant certification program will become active in 2000.
Surfactant shorthand terms and herbicide research terms
are covered in the WSSA herbicide handbook (Ahrens
1994). There are many review publications (Chow and
Grant 1992; Combellack 1995; Foy et al. 1992; Green
and Hazen 1998) and specific topic publications (Bayer
and Lumb 1973; Dexter et al. 1998; Hartley and Graham-Bryce 1980; Hodgson 1982; McCarthy 1992; Stevens et al. 1992; Tinsworth 1992) on the subject of adjuvants and herbicide activation.
I wish that this short summary of adjuvant terminology, classification, and chemistry might enable more
weed science researchers to understand and participate
in the optimization of herbicide efficacy. Additional understanding of how and why adjuvants work would benefit our industry. Consider involving researchers from
other adjuvant-related disciplines in your future research
programs. The result may be surprising. The future of
our industry may well depend upon it!
ACKNOWLEDGMENTS
I recognize Dr. Jerry M. Green of DuPont Crop Protection for his ‘‘persistence of vision’’ that led this adVolume 14, Issue 4 (October–December) 2000
WEED TECHNOLOGY
juvant symposium from ‘‘need to reality’’. I also gratefully acknowledge Dr. Alan Stern of Witco Agricultural
Specialties for his patient construction of the structures
presented.
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