CHEMISTRY AND TECHNOLOGY 0F
`
`AGROCHEMICAL
`
`FORMULATIONS
`
`Edited by
`DA. Knowles
`
`1
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`TIDE 1024
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`

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`Chemistry and Technology of Agrochemical Formulations
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`2
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`

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`Chemistry and Technology of
`Agrochemical Formulations
`
`Edited by
`
`D. A. Knowles
`
`FORM-AK Formulation Consultancy Services,
`Tonbridge, Kent, UK
`
`'93
`
`Springer Science+Business Media, B.V.
`
`3
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`

`

`A C.I.P catalogue record for this book is available from the Library of Congress
`
`ISBN 978-94-011-4956-3 (eBook)
`ISBN 978—94—010-6080-6
`DOI l0.l007/978-94-0l l-4956-3
`
`Printed on acid—free paper
`
`All Rights Reserved
`© 1998 Springer Science+Business Media Dordrecht
`Originally published by Kluwer Academic Publishers in 1998
`Softcover reprint of the hardcover 1st edition 1998
`No part of the material protected by this copyright notice may be reproduced or
`utilized in any form or by any means, electronic or mechanical,
`including photocopying, recording or by any information storage and
`retrieval system, without written permission from the copyright owner.
`
`4
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`

`

`3 Formulation of agrochemicals
`D. A. KNOWLES
`
`3.1 Introduction
`
`Farmers and growers in all the main agricultural areas of the world rely very
`substantially upon crop protection chemicals to help them meet the ever-
`increasing demand for food and other materials such as natural fibres. The
`consumer continues to seek higher quality and greater variety of produce.
`Simple dusting powders and spray oil formulations have been used for
`many years to protect growing crops from weeds, pests and diseases.
`However, since the 1940s the chemical industry has endeavoured to satisfy
`the demands of farmers and growers for increased crop yield and quality by
`the continuous development and introduction of crop protection chemicals
`into the international market place. Today, there is an effective herbicide,
`insecticide or fungicide to combat almost every significant problem faced by
`the modern farmer and grower.
`This development has led to a need for a wide range of product formula-
`tions, additives and process technology to accommodate the variety of
`physical and chemical properties of the pesticide active ingredients. For
`example, water-soluble active ingredients may be prepared as aqueous
`solutions or powder formulations, whereas oily liquid active ingredients are
`usually formulated as hydrocarbon solvent—based emulsifiable concen-
`trates. Active ingredients which have very low solubility in either water or
`hydrocarbon oils may be formulated as suspensions, powders or water-
`dispersible granules [1].
`In the 19805 and 19903, pressure from government authorities and the
`consumer highlighted a need for products and formulations which are safer
`and more convenient to use, more effective at much lower application rates,
`less toxic to non—target species and more environmentally friendly.
`By far the most important method of application of agrochemicals is by
`spraying, usually with water but occasionally with oils as the principal
`carrier. Formulations are also made for direct application to the soil or for
`treating seeds before planting, and for protecting stored crops from various
`pests and diseases (fungi, insects or rodents), which in some countries could
`destroy as much as 30—40% of the harvest.
`Pesticidal active ingredients encompass a broad range of chemicals, each
`with its unique chemical and physical properties and mode of action. The
`main categories of pesticides are herbicides, insecticides, fungicides, plant
`
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`42
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`CHEMISTRY AND TECHNOLOGY OF AGROCHEMICAL FORMULATIONS
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`growth regulators, molluscicides and rodenticides. A great deal of research
`work has been carried out into understanding the modes of action and
`physiological effects of active ingredients and the influence of formulation
`type on the biological performance of the pesticide [2].
`The successful use of any active ingredient depends on its correct formu—
`lation into a preparation which can be applied for crop protection safely and
`with low risk to those applying the material, to non-target species and to the
`environment in general. The earliest pesticide formulations were based on
`simple dusts, powders, granules, aqueous solutions and mineral oil-in-water
`emulsions. In recent times, particularly during the period from 1970 on-
`wards, there has been a rapid development of more sophisticated formula-
`tions based on the availability of more powerful surfactants and other
`additives, and a much better understanding of the principles of colloid and
`surface chemistry to improve formulation stability and biological activity.
`Processing technology has also developed over this period to give much
`smaller particle size for better stability and activity for water— and solvent-
`insoluble active ingredients.
`The main objectives of formulation can be summarized as follows: to
`provide the user with a convenient, safe product which will not deteriorate
`over a period of time, and to obtain the maximum activity inherent in the
`active ingredient.
`The formulation chemist needs to take into account a number of interact-
`
`ing factors in the choice of the specific formulation type for each active
`ingredient. The main factors which need to be taken into account are
`
`physico—chemical properties;
`biological activity and mode of action;
`method of application;
`safety in use;
`formulation costs;
`market preference.
`
`Once these parameters have been determined, proper selection can be
`made of the final formulation type and the use of inert ingredients, includ-
`ing surfactants and other additives, to produce a stable formulation with at
`least a 2—year shelf life during storage under varying climatic conditions.
`The most common formulations are still soluble concentrates for water-
`
`soluble chemicals, emulsifiable concentrates for oil-soluble chemicals, and
`
`wettable powders and suspension concentrates for insoluble solids. Gran-
`ules and seed treatments for direct application have also been produced for
`many years. In recent years the number of formulation types has increased
`enormously to meet the needs of operator and environmental safety or to
`improve the activity and persistence of the active ingredient. An interna-
`tional coding system was therefore devised by GIFAP in 1984 (in 1996
`GIFAP was renamed GCPF — Global Crop Protection Federation, based in
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`FORMULATION OF AGROCHEMICALS
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`43
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`Table 3.1 Major types of pesticide formulations
`
`Formulation type
`
`Granules
`Solution concentrates
`Emulsifiable concentrates
`Wettable powders
`Suspension concentrates
`O/W emulsions
`Suspoemulsions
`Microemulsions
`Water-dispersible granules
`Microcapsules
`Seed treatments
`
`Code
`
`GR
`SL
`EC
`WP
`SC
`EW
`SE
`ME
`WG
`CS
`DS, WS, LS, FS
`
`Brussels, Belgium). The major types of formulations and international
`codes are shown in Table 3.1.
`The most common formulations are those which are made for dilution
`
`into water in a spray tank. In these cases the choice of formulation additives
`is very important to ensure that the product mixes and dilutes easily. Some-
`times products may be mixed together in the spray tank or may be mixed
`with spray adjuvants to enhance biological activity. Products such as gran-
`ules or seed treatments are usually applied undiluted to the soil or to the
`seed respectively. A few products are formulated to be diluted and sprayed
`in oils, and there are many minor formulations such as baits, pellets, smokes
`and aerosols for special purposes.
`
`3.2 Conventional formulations
`
`3.2.1 Granules (GR)
`
`Granular formulations are used for direct broadcasting to the field, often as
`pre-emergence herbicides or as soil insecticides. The active ingredient con—
`centration is usually between 1 and 40% and the granule mesh size is
`generally between 250 and 1000 um. The granules should be non-caking,
`non-dusty, free flowing and should disintegrate in the soil to release the
`active ingredient.
`Granules are usually made either by coating a fine powder onto a
`substrate, e.g. sand, using a sticker such as PVP solution, or by solvent
`impregnation onto an absorbent carrier. Resins or polymers may be
`sprayed onto the granules to control release rates. Absorbent carriers may
`be mineral or vegetable, as shown in Table 3.2.
`The absorptive capacity of the carrier is an important parameter and is a
`function of the crystalline structure and the available surface area of the
`carrier particles. Some typical absorptive capacities are shown in Table 3.3.
`
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`CHEMISTRY AND TECHNOLOGY OF AGROCHEMICAL FORMULATIONS
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`Table 3.2 Classification of carriers
`
`Class
`
`Examples
`
`Silicate clays Attapulgite, montmorillonite, kaolin, talc, mica, vermiculite
`Carbonates
`Calcite, dolomite
`Synthetics
`Calcium silicate, precipitated silica, fumed silica
`Botanicals
`Corn cob grits, ground grains, rice hulls, soybean, walnut shell, coconut shell
`Others
`Pumice
`
`Table 3.3 Absorptive capacities of granule carriers
`
`
`
`Carrier Oil absorption (g/100 g)
`Silica
`200
`Attapulgite
`100
`Montmorillonite
`23—70
`Kaolin
`20—54
`Talc
`20—40
`Calcium carbonate
`5~18
`Corn cob grits
`60—80
`Walnut shell
`20—40
`
`3.2.2 Solution concentrates (SL)
`
`The simplest of all formulations to make is the solution concentrate, an
`aqueous solution of the active ingredient which merely requires dilution in
`the spray tank. The number of pesticides which can be formulated in this
`way is limited by solubility and hydrolytic stability. Some solution concen-
`trate formulations contain a surfactant, usually a non-ionic ethylene oxide
`condensate, to assist wetting onto the leaf surface. Solution concentrate
`formulations are usually very stable and therefore present few storage
`problems. Some problems do occur occasionally, such as precipitation dur-
`ing dilution and corrosion of metal containers or spray applicators. A
`typical solution concentrate formulation (per cent by weight) is shown
`below:
`
`Active ingredient
`Wetting agent
`Antifreeze
`Water
`Water-miscible solvent
`
`}
`
`20_50%
`3_10%
`5_10%
`o
`to 100 /°
`
`Nonylphenol or tallow amine ethoxylates are often used as tank mix wetters
`for solution concentrate formulations. Alternatively, the wetting agent may
`be built into the formulation to ensure that the correct rate of wetting agent
`is applied to optimize biological activity. This is often the case, for example,
`with paraquat and glyphosate formulations. A considerable amount of
`
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`FORMULATION OF AGROCHEMICALS
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`45
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`work is being carried out on new surfactant wetting agents for glyphosate
`formulations [3]. In some cases preservatives may be nessessary to prevent
`mould growth or bacterial spoilage during long-term storage.
`
`3.2.3 Emulsifiable concentrates (EC)
`
`Emulsifiable concentrate formulations have been very popular for many
`years and represent the largest volume of all pesticide formulations in terms
`of consumption. Emulsifiable concentrates are made from oily active ingre-
`dients or from low—melting, waxy, solid active ingredients which are soluble
`in non-polar hydrocarbon solvents, such as xylene, Cg—C10 solvents, solvent
`naphtha, odourless kerosene or other proprietory hydrocarbon solvents.
`Surfactant emulsifiers are added to these formulations to ensure spontane-
`ous emulsification with good emulsion stability properties in the spray tank.
`Careful selection of a ‘balanced pair’ emulsifier blend is frequently neces-
`sary to ensure that emulsion dilution stability is maintained over widely
`differing climatic conditions and degrees of water hardness. Emulsion
`droplets of 0.1—5 mm are produced when the formulation is mixed with
`water.
`
`The formulation of emulsiflable concentrates has been greatly facilitated
`by the commercial development over the last 20 years of non-ionic emulsi-
`fying agents in which the hydrophilic portion of the molecule consists of a
`polyethylene oxide chain. The non—ionic surfactant which is commonly used
`is a nonylphenol hydrophobic chain condensed with 12 or more moles of
`ethylene oxide. The other component of the balanced pair is generally an
`anionic surfactant such as the oil-soluble calcium salt of dodecylbenzene
`sulphonic acid. Recently, however, nonylphenol ethoxylates have been sus-
`pected of having endocrine modulating properties from metabolites in ef-
`fluents or by leaching into ground drinking water. Because of this potential
`toxic effect, alternative ethylene oxide condensates based on aliphatic
`hydrophobes are being investigated.
`The total concentration of the emulsifier blend is usually 5—10% of the
`formulation. There are no definite rules to determine the ratio of anionic to
`
`non-ionic surfactant in the mixed emulsifiers, but guidance can be obtained
`from the HLB (hydrophile—lipophile balance) system: the higher the HLB,
`the more hydrophilic (water-soluble) is the surfactant. The HLB range 8—18
`will normally provide good oil-in-water emulsions. The optimum ratio of
`anionic to non-ionic surfactants is determined experimentally to give spon-
`taneous emulsification in water, and to give a stable emulsion with very
`little creaming and no oil droplet coalescence.
`Emulsifiable concentrates are limited in the number of active ingredients
`for which they are suitable. Many pesticides are not soluble enough to be
`supplied economically in this form. However, it may be possible to boost
`the solubility of the active ingredient by the addition of a more polar solvent
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`CHEMISTRY AND TECHNOLOGY OF AGROCHEMICAL FORMULATIONS
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`without increasing the risk of crystallization in the spray tank. A typical
`emulsifiable concentrate formulation (per cent by weight)
`is shown
`below:
`
`Active ingredient
`Emulsifier blend
`Solvent
`}
`Cosolvent
`
`20—70%
`5—10%
`o
`to 100 A)
`
`The presence of solvents and emulsifiers in emulsion concentrate formu-
`lations can sometimes give enhanced biological efficacy compared with
`other formulations. Many insecticides, e.g. organophosphorous compounds
`and pyrethroids, are oil-soluble active ingredients and are readily formu-
`lated as emulsifiable concentrates, and a few active ingredients need to be
`formulated with solvents for optimum biological activity.
`Health, safety and environmental pressures on the use of petroleum-
`based solvents generally are influencing a move away from these solvent-
`based formulations. However,
`it seems unlikely that solvents can be
`replaced entirely for some products, and safer high-flash-point solvents are
`being introduced along with new ideas for packaging to reduce physical
`contact between the product and the operator.
`
`3.2.4 Wettable powders (WP)
`
`Wettable powder formulations of pesticides have been known for many
`years and are usually made from solid active ingredients with high melting
`points which are suitable for dry grinding through a mechanical grinder,
`such as a hammer— or pin-type mill, or by air milling with a fluid energy
`micronizer. Air milling gives much finer particles than mechanical milling
`and can also be more suitable for active ingredients with lower melting
`points. However, care must be taken to prevent, suppress or contain dust
`explosions which may occur if a source of ignition, such as static energy, is
`present in either type of mill.
`Wettable powders usually contain dry surfactants as powder wetting and
`dispersing agents and inert carriers or fillers. They frequently contain more
`than 50% active ingredient and the upper limit is usually determined by the
`amount of inert material, such as silica, required to prevent the active
`ingredient particles fusing together during processing in the dry grinding
`mills. An inert filler such as kaolin or talc is also needed to prevent the
`formulated product from caking or aggregating during storage.
`Wettable powders have a high proportion of particles less than 5 um and
`all the particles should pass through a 44 um screen. Ideally, the amount of
`surfactants should be sufficient to allow the spray droplets to wet and
`spread over the target surface, but the particles should not be easily washed
`off by rain. Powder formulations contain a wetting agent to lower the
`
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`FORMULATION OF AGROCHEMICALS
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`47
`
`interfacial tension between the solid particles and water and ensure that the
`powder wets and mixes with water in the spray tank easily. A dispersing
`agent is also necessary to prevent the particles in the spray tank from
`flocculating or aggregating together, and to ensure that the particles remain
`suspended during the spraying operation. The types of wetting agents com-
`monly used are
`
`sodium dodecylbenzene sulphonate;
`sodium lauryl sulphate;
`sodium dioctyl sulphosuccinate;
`aliphatic alcohol ethoxylates;
`nonylphenol ethoxylates.
`
`The comments on nonylphenol ethoxylates mentioned previously for
`emulsifiable concentrates also apply to wettable powders.
`The following dispersing agents are often used in wettable powder
`formulations:
`
`0 sodium lignosulphonates;
`0 sodium naphthalene sulphonate formaldehyde condensates.
`
`A typical wettable powder formulation (per cent by weight) is shown below:
`
`Active ingredient
`Wetting agent
`Dispersing agent
`Inert filler/carrier
`
`25—80%
`1—3%
`2—5%
`to 100%
`
`Wettable powders can also be made from liquid pesticides by using absor-
`bent fillers such as diatomaceous earth or high-surface-area synthetic silica.
`However, in this case the active ingredient concentration is usually limited
`to 40%. Many pesticides, especially herbicides and fungicides are formu-
`lated as wettable powders. However, due to their low-technology image
`arising from their dustiness, which creates toxic hazards on handling, they
`are now being superseded by suspension concentrates or water-dispersible
`granules.
`
`3.2.5 Suspension concentrates (SC)
`
`Suspension concentrate technology has been increasingly applied to the
`formulation of many solid crystalline pesticides since the early 1970s. Pesti-
`cide particles may be suspended in an oil phase, but it is much more usual
`for suspension concentrates to be dispersions in water. Considerable atten-
`tion has been given in recent years to the production of aqueous suspension
`concentrates by wet grinding processes such as bead milling. The use of
`surfactants as wetting and dispersing agents has also led to a great deal of
`research on the colloidal and surface chemistry aspects of dispersion and
`stabilization of solid—liquid dispersions [4].
`
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`CHEMISTRY AND TECHNOLOGY OF AGROCHEMICAL FORMULATIONS
`
`Water-based suspension concentrate formulations offer many advan-
`tages, such as high concentration of insoluble active ingredients, ease of
`handling and application, safety to the operator and environment, and
`relatively low cost. They also enable water-soluble adjuvants to be built into
`the formulation to give enhanced biological activity. Farmers generally
`prefer suspension concentrates to wettable powders because they are non—
`dusty and easy to measure and pour into the spray tank. However, there are
`some disadvantages, notably the need to produce formulations which do
`not separate badly on storage, and also to protect the product from freezing,
`which may cause aggregation of the particles.
`In most cases, suspension concentrates are made by dispersing the active
`ingredient powder in an aqueous solution of a wetting and/or dispersing
`agent using a high-shear mixer to give a concentrated premix, followed by
`a wet grinding process in a bead mill to give a particle size distribution in the
`range 0.1—5 um. The wetting/dispersing agent aids the wetting of the powder
`into water and the breaking of aggregates, agglomerates and single crystals
`into smaller particles. In addition, the surfactant which becomes adsorbed
`onto the freshly formed particle surface during the grinding process should
`prevent reaggregation of the small particles and should ensure colloidal
`stability of the dispersion. Typical wetting/dispersing agents used in suspen-
`sion concentrate formulations are:
`
`0 sodium lignosulphonates;
`0 sodium naphthalene sulphonate formaldehyde condensates;
`0 aliphatic alcohol ethoxylates;
`0 tristyrylphenol ethoxylates and esters;
`0 ethylene oxide-propylene oxide block copolymers.
`
`Also available are polymeric surfactants which adsorb strongly on particle
`surfaces and may give considerably improved stabilization of suspension
`concentrates for long-term storage [5]. A typical suspension concentrate
`formulation (per cent by weight) is shown below:
`
`Active ingredient
`Wetting/dispersing agent
`Propylene glycol antifreeze
`Anti-settling agent
`Water
`
`20—50%
`2—5%
`5-10%
`0.2%—2%
`to 100%
`
`The anti-settling agent is added to increase Viscosity and build up a three—
`dimensional network structure to prevent separation of particles during
`long-term storage. The anti—settling agent is usually a swelling clay such as
`bentonite (sodium montmorillonite) and may be mixed with water-soluble
`polymers to give synergistic rheological effects. The water—soluble polymers
`are often cellulose derivatives, natural gums or other types of poly-
`saccharides, such as xanthan gum, and they are generally susceptible to
`
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`FORMULATION 0F AGROCHEMICALS
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`49
`
`microbial attack. For this reason, preservatives are usually added to suspen—
`sion concentrate formulations to prevent degradation of the anti—settling
`agent so that long-term stability of the product is not impaired. A great deal
`of research has been carried out using rheological techniques to measure
`the forces acting between particles and polymers to enable storage stability
`to be predicted. However, it is still necessary to carry out long-term storage
`tests over a range of temperatures to ensure that the particles do not
`aggregate or separate irreversibly under normal storage conditions in the
`sales pack [6].
`Many crystalline solid active ingredients are now available as suspension
`concentrates. However, there is increasing pressure, especially in Western
`Europe and the USA, to enforce stringent pack rinsing and disposal regu-
`lations, which may have a serious impact on the future of suspension con-
`centrates and their packaging.
`
`3.2.6 Seed treatments (DS, WS, LS, FS)
`
`Although most pesticide formulations are applied by spraying onto crops or
`weeds, significant quantities of fungicide and insecticide products are ap-
`plied directly onto seeds prior to planting into the soil. It is estimated that
`the market value of seed treatment formulations currently represents about
`3—3.5% of the total market for agrochemical products, and approximately
`50% of seed treatment formulations are applied to seeds in Europe.
`Fungicides dominate the seed treatment market with about a 70% share.
`The most important seed treatment applications are on small-grain cereal
`seeds, which comprise over 50% of the world market and over 60% of the
`European market.
`Products for seed treatment fall into four categories:
`
`0 powder for dry seed treatment (DS);
`0 water-slurryable powder for seed treatment (WS);
`0 non-aqueous solution for seed treatment (LS);
`0 flowable suspension for seed treatment (F8).
`
`The choice of formulation type is usually governed by the physico-chemical
`properties of the active ingredients, the type of application equipment
`available or market preference. Powder formulations (D8) are dusty and
`have poor retention on seed. Water-slurryable formulations (WS) are still
`used to a certain extent, particularly in France. Solvent—based formulations
`(LS) are gradually being phased out because of operator handling safety
`problems. Water-based flowable suspensions (F8) are more environmen—
`tally friendly than powders or solutions, have good retention on seed and
`are now becoming more popular.
`The technology for producing flowable suspensions is similar to that for
`producing suspension concentrates, and the surfactants used are also
`
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`CHEMISTRY AND TECHNOLOGY OF AGROCHEMICAL FORMULATIONS
`
`similar to those used for suspension concentrate formulations. Extra thick—
`eners and anti—settling agents are usually added to prevent separation of the
`dispersed phase because these products are generally applied directly to the
`seed without dilution. Seed treatment formulations can be applied to seeds
`in simple rotating mixing bowls, auger mixers such as the Plantector, or
`sprayed into rotating bowls such as the Rotostat or the Centaur [7]. High-
`value seeds such as vegetable and horticultural seeds are sometimes coated
`with polymers to prevent loss of the seed treatment chemical. They may
`also be pelleted with clays and polymers to produce a spherical seed pellet
`which is easy to handle and plant.
`Because seed treatments are applied directly to the seed, there is very
`little wastage of active ingredient. Seed treatments are, therefore, seen as a
`very efficient means of targeting pesticides to crops and are regarded as an
`environmentally safe way of applying pesticides. They may become more
`important in the future with the introduction of transgenic crops and an
`increasing need to protect such high-value seeds with fungicides and
`insecticides.
`
`3.3 New-generation formulations
`
`3.3.1 General trends
`
`Over the last few years there has been increasing pressure from government
`and regulatory authorities to develop formulations which have less impact
`on the environment generally [8]. The main issues which are being
`addressed are
`
`safety in manufacture and use;
`convenience for the user;
`
`ease of pack disposal or reuse;
`reduction of the amount of pesticide applied;
`reduction of waste and effluent of all kinds.
`
`The current trends in the development of pesticide formulations are
`
`0 to use safer solvents or to eliminate solvents wherever possible and use
`aqueous emulsions;
`0 to replace wettable powders by aqueous suspension concentrates or
`water-dispersible granules;
`0 to develop multiple active ingredient formulations;
`0 to build in bioenhancing surfactant wetters;
`0 to control release rate and targeting of pesticides by encapsulation
`techniques and seed treatment;
`0 to develop novel formulations such as tablets or gels;
`
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`FORMULATION OF AGROCHEMICALS
`
`51
`
`0 to develop more effective spray adjuvants to enhance biological activity
`and reduce pesticide dosage.
`
`These complex requirements are being met by technical advances in
`surfactants and other
`formulation additives, particularly blends of
`surfactants, more powerful dispersing agents and a better understanding of
`the principles of colloid and surface chemistry and rheology [9]. The ideal
`product would seem to be one which is free from volatile solvents, gives
`no operator exposure hazard, has the maximum biological activity at the
`lowest dose level, and produces the minimum of pack disposal problems.
`Water-dispersible granules or wettable powders in water-soluble sachets,
`which can be added directly to a spray tank, go a long way towards meeting
`these requirements, and development work is being carried out on these
`options by all the major agrochemical companies. However, it will never be
`possible to formulate all active ingredients this way, and so other options
`are being evaluated extensively, along with ideas for packaging and closed-
`transfer spray application systems. Aqueous-based formulations will be a
`necessary and safe alternative to water-dispersible granule formulations,
`and these options include (in addition to suspension concentrates which
`have been already discussed):
`
`- suspoemulsions;
`0 O/W emulsions or concentrated emulsions;
`0 Microemulsions;
`
`0 microencapsulation.
`
`Other possibilities involving specialized packaging are gels and effer-
`vescent tablets. The new-generation formulations are discussed in more
`detail in separate chapters in this book. Only brief summaries are given
`here [10].
`
`3.3.2 Oil-in-water emulsions (EW)
`
`Oil-in-water emulsions are now receiving considerable attention because of
`the need to reduce or eliminate volatile organic solvents for safer handling.
`Because they are water based, oil-in-water emulsions can have significant
`advantages over emulsifiable concentrates in terms of cost and safety
`in manufacture, transportation and use. However, they require careful
`selection of surfactant emulsifiers to prevent flocculation, creaming and
`coalescence of the oil droplets, as shown diagrammatically in Figure 3.1.
`Non-ionic surfactants and polymeric surfactants are now being used to
`produce stable emulsions. In the case of non-ionic surfactants it is some-
`times useful to combine a low and a high HLB surfactant to give an average
`HLB of 11—16 for optimum emulsion stability [6].
`Droplet size is also a good indicator of stability and should be below 2 pm.
`The emulsions are usually thickened with polysaccharides such as xanthan
`
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`CHEMISTRY AND TECHNOLOGY OF AGROCHEMICAL FORMULATIONS
`
`crea in
`
`sedimentation
`
`(lo cu ation
`
`
`
`phase
`inv rsion
`
`
`
`.
`coalescence .
`ripening
`
`Figure 3.1 O/W emulsion stability problems. (From Morpeth, F.F., Preservation of Surfactant
`Formulations, Blackie Academic and Professional, London, 1995.)
`
`gum to prevent separation of the oil droplets. Sometimes polymers such as
`polyvinyl alcohol are used as both emulsifier and thickener/stabilizer.
`
`3.3.3 Suspoemulsions (SE)
`
`Mixed formulations are becoming more popular because of their conveni-
`ence, to ensure that the farmer applies the correct amount of each compo-
`nent pesticide and to overcome problems of tank mix incompatibility. If one
`active ingredient is a solid and the other is a liquid, it is necessary to produce
`a suspoemulsion formulation which consists of three phases:
`
`0 liquid oil droplets;
`0 solid dispersed particles;
`0 continuous phase, usually water.
`
`Suspoemulsions can, therefore, be considered to be mixtures of suspen-
`sion concentrates and oil-in-water emulsions with added surfactants to
`
`prevent flocculation and thickeners to prevent separation of the dispersed
`phases. Surfactants used as dispersing agents for the solid phase are similar
`to those already mentioned for suspension concentrates. Emulsifiers for the
`oily liquid phase are similar to those used for oil-in-water emulsions. As
`these formulations are aqueous based and generally thickened with
`polysaccharides, it is necessary to add a preservative to prevent degradation
`of the thickener. Some problems of heteroflocculation between the solid
`
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`FORMULATION OF AGROCHEMICALS
`
`53
`
`particles and oil droplets can occur, and extensive storage testing of these
`formulations is necessary [11].
`
`3.3.4 Microemulsions (ME)
`
`Microemulsions are thermodynamically stable, transparent emulsions and
`are stable over a wide temperature range. They have a very fine droplet size
`of less than 0.1 um and consist of three components:
`
`0 oily liquid or solid dissolved in organic solvent;
`0 water;
`0 surfactant/cosurfactant.
`
`These components form a single phase containing relatively large ‘swollen
`micelles’ in which the non-aqueous phase of the active ingredient and
`solvent are dissolved or solubilized.
`
`In the preparation of microemulsions, two different types of surfactants
`are needed: one water soluble and one oil soluble. The water—soluble
`
`surfactant is usually anionic or non-ionic with a very high HLB value, and
`the hydrophobic part of
`the molecule should match the oil. The
`cosurfactant should be oil soluble and should have a very low HLB value,
`such as hexanol. The total concentration of surfactants for a microemulsion
`
`can be as high as 10—30%, compared with about 5% for an O/W emulsion
`[6]. Microemulsions have relatively low active ingredient concentrations,
`but may have enhanced biological activity.
`
`3.3.5 Controlled-release formulations
`
`The application of controlled release technology has been slow to reach
`commercialization despite interesting research and development work by
`the major agrochemical companies over the last 10—20 years. Controlled-
`release formulations can have a number of advantages over conventional
`formulations: they
`
`have longer residual biological activity;
`may reduce mammalian toxicity;
`control or reduce evaporation of pesticide;
`may reduce phytotoxicity to the crop;
`improve compatibility in the spray tank;
`reduce fish toxicity;
`reduce groundwater leaching;
`reduce solvent usage in the formulation;
`may reduce the pesticide application rate.
`
`Controlled-release pesticide formulations can be divided into four main
`types:
`
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`54
`
`CHEMISTRY AND TECHNOLOGY OF AGROCHEMICAL FORMULATIONS
`
`coated pesticide granules;
`matrix systems containing physically trapped pesticides;
`polymer systems containing covalently bound pesticides;
`polymer membrane—pesticide reservoir systems, e.g. microencapsulation.
`
`The polymer membrane, or microencapsulation, technique has become
`popular in recent years. A well—known method of microencapsulation
`uses the principle of interfacial polymerization. In this process the active
`ingredient, usually a liquid or low-melting waxy solid, is dissolved in an
`aromatic solvent, such as the C9 and C10 solvents used for emulsifiable
`concentrates. An oil-