Pestic. Sci. 1990, 29, 4 3 7 4 9

Trends in the Formulation of Pesticides -An Overview*

David Seaman

ICI Agrochemicals, Yalding, Kent ME18 6HN, UK

(Manuscript received 18 January 1990; accepted 20 January 1990)

ABSTRACT

The customer increasingly requires safer and more convenient pesticide formulations. Emulsifiable concentrates and wettable powders are nowadays viewed less favourably by farmers and registration authorities. Suspension concentrates are now common and water-dispersible granules and emulsions in water are receiving increasing attention. Capsule suspensions offer both safer and more effective performance in favourable cases. For convenience the farmer requires multi-component products, either as mixtures of active ingredients, which might be as suspoemulsions, or with built-in enhancing surfactants and oils. Seed treatments are now required as aqueous suspensions and the seed is increasingly of interest as an eficient carrier for pesticides. Research into biological control agents sets the difJicult challenge of formulating these products as viable organisms. Three examples of developments are described. These are the applications of polymeric surfactants for suspension concentrates and suspoemubions, studies of enhancement of pesticide uptake with surfactants and developments in microencapsulation.

1 OBJECTIVES AND CONSTRAINTS

The principal objective of the formulation chemist is to develop a formulation which delivers the necessary biological performance and which has a lengthy shelf life. The material he works on is the pesticide the synthetic chemist provides, which has been optimised primarily for biological performance at this early stage.

During the evaluation process, when analogues from a chemical series are under test, the formulation chemist has some influence in the choice of the chemical for

* Based on a paper given at the symposium ‘Formulations of Pesticides and Pharmaceuticals II- Pesticide Formulations’, organised by the Physicochemical and Biophysical Panel of the Pesticides Group of the Society of Chemical Industry and held in London on 31st October, 1989.

437

Pestic. Sci. 0031-613X/90/$03.50 0 1990 SCI. Printed in Great Britain

438 D. Seaman

development but the final selection is based on the best balance of properties for biological performance, toxicology, environmental impact and cost.

The physical and chemical properties of the selected pesticide determine, to a large extent, the preferred formulation in the simple case where a product containing just the one pesticide is required. The most common formulations are still soluble concentrates (SL) for water-soluble chemicals, emulsifiable concentrates (EC) for solvent-soluble chemicals and wettable powders (WP) and suspension concentrates (SC) for insoluble solids.

The biological performance of a pesticide is frequently affected by the choice of formulation type. A formulation which presents the chemical in a solution, as in SL and EC formulations, is commonly more biologically active (and morephytotoxic) than WP or SC formulations. There are also cases where an SC is more active than a WP as a result of smaller particle size in the SC making the pesticide more available.

As well as considering the physicochemical and biological properties of the pesticide in devising his formulation, the formulation chemist must also provide a product which is convenient for the farmer, safe in use and effective at an acceptable cost.

In a business which is becoming progressively more competitive, the formulation can give a competitive edge in the eyes of the farmer. He increasingly requires a product which is quick and simple to use. He requires foolproof complete products and less tank-mixing.

In a world which is becoming more safety and environmentally conscious, both the farmer and the registration authorities are pressing for safer formulations. From an environmental point of view, the registration authorities are also demanding formulations which are foolproof and complete, in order to ensure that pesticides are used efficiently.

The requirement for increasing cost effectiveness brings the formulation chemist into the centre of the development process. In the synthetic process for the pesticide, the cost of a pesticide can be increased substantially if the formulation chemist insists on a demanding specification. He has to work with the synthetic chemist to reach a satisfactory compromise with regard to the form and composition of the technical pesticide. In extreme cases this consideration can drive the choice of formulation type.

The formulation chemist is also expected wherever possible to use available formulation plant and equipment. New plant, and even plant modifications, can involve high capital costs and take time to build. New processes require lengthy development work.

The pressures and constraints described above have always existed, but every one has intensified. The formulation chemist is expected to meet the highest standards under all these pressures.

2 REVIEW OF TRENDS

Table 1 displays formulations sold in France in 1989. The ‘old’ formulations, EC, WP and SL, remained more than 50% of the total but suspension concentrates (SC)

-. 3 P k TA

BLE

1

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Form

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% i

( %)

(%I

( %)

(%I

Emul

sifia

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once

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19

10

2 7

All

type

s 40

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27

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7

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9 g.

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once

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16

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s G

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440 D. Seaman

were now very significant. Capsule suspensions (CS), waterdispersible granules (WG), emulsions in water (EW) and suspoemulsions (SE) appeared and are likely to increase in the future at the expense of the other formulations. It should also be noted that mixtures which contain two or more pesticide components in the formulation represented an increased proportion of products on sale. (Suspoemulsions are mixtures of solid pesticides as suspensions and water- immiscible liquids as emulsions.)

The reasons for these trends are as follows. Powders are dusty and therefore are less safe, not easy to measure and bulky. EC formulations can be more dermally toxic and irritant; a number of solvents used have some undesirable toxicological characteristics and many are flammable. Also there is an anti-EC, anti-solvent bandwagon which is not wholly logical but which makes ECs less acceptable. Nevertheless it should be realised that most EC formulations are safe and satisfactory, as are many solvents.

SC formulations are favoured over WPs as they are liquid, less bulky and easier to measure. The containers are easy to wash and the product has good dispersing and suspensibility properties. However, SCs cannot be made for pesticides with a moderate solubility in water or which are unstable in an aqueous environment.

WGs are an improvement on both WP and SC formulations as they are free- flowing, without dust, easy to measure and offer easy pack disposal. They have the disadvantage of high capital plant costs and have limited applicability for low- melting solids and liquids.

EWs are a response to solvent, cost or toxicity problems, in particular for dilute formulations. They are now finding a few applications. EWs can reduce dermal irritation and toxicity over ECs. They have no or only low flammability. EWs can still require some solvent in compositions containing viscous liquids or low melting solids.

Surfactants and oils can enhance the biological efficacy of pesticides. There are already many examples of formulations with built-in surfactants but both surfactants and oils may occur as tank additives. For ease of use, and more foolproof formulations, there is a greater need for these adjuvants to be built into the formulation. In fact, some registration authorities now insist that this is so.

There is an increasing interest in controlled-release formulations, either as microencapsulated products, coated granules or polymer matrix systems. There are examples of these in the market place, in particular of microencapsulated products (CS). The benefits achieved are to extend the persistence of biological activity, reduce phytotoxicity, reduce pesticide levels in the environment, and reduce mammalian toxicity.

Other trends to note are the move of seed treatments to suspension (FS) from dustable powders (DS) and water-dispersible powder (WS) formulations for reasons similar to those for the change to SC from WP formulations.

The seed is under closer examination as a carrier for pesticides. With seed coating and pelleting, most progress is being made with high-value seeds. The future of these techniques for cereal seeds is not yet clear. They require new processes and capital plant. The benefit achieved has to justify the extra expense and the justification will be based on better distribution and adhesion to the seed. To ascertain whether this

Trends in the formulation of pesticides-An overview 441

shows up in better biological performance and better user characteristics, sufficient to pay for the processing costs or not, requires costly development programmes.

A major trend for the late 1990s and the next century will be the introduction of significant numbers of products from biotechnology work currently under way. Likely products are:

Microbial insecticides -bacteria -fungi -viruses -nematodes Microbial fungicides and bactericides Mycoherbicides -fungi

Many of the products will be based on live organisms. The viability of these organisms will have to be maintained at acceptable levels during the formulation process and shelf life of these products. When used, the organisms must revive from their dormant state in order to work. The challenge in formulating these products is considerable and a fundamental understanding of processes causing loss of viability is necessary for progress to be made in this difficult area.

To summarise, the trends in the formulation of pesticides are:

(a) From EC to EW formulations to avoid solvents, and to reduce flammability and dermal toxicity.

(b) From WP to SC formulations to provide nondusty, less bulky, easier-to- measure products.

(c) From WP and SC to WG formulations to provide nondusty, less bulky, easier-to-measure products with easier pack disposal.

(d) From singlecomponent to multicomponent formulations, particularly as SC and SE formulations, for easier use by the farmer and to avoid tank mixing.

(e) From tank-mixed adjuvants, such as surfactants and oils to enhance activity, towards complete formulations with built-in adjuvants.

(f) Increasing number of controlled-release formulations to optimise availability of the pesticide, thereby optimising biological effect, and reducing phytotoxicity and mammalian toxicity.

(g) Research into biological control agents requiring formulation as viable organisms.

3 EXAMPLES OF RECENT DEVELOPMENTS

To exemplify the progress in formulation chemistry, three examples are taken from current work at ICI.

1. Polymeric surfactants for SC and SE formulations 2. Enhancement of biological activity with surfactants 3. Developments in microencapsulation

442 D. Seaman

3.1 Polymeric dispersants for SC and SE formulations

Many SC formulations are stabilised with anionic dispersants, e.g. naphthalene sulphonate/formaldehyde condensates, lignosulphonates or tristyryl ethoxy- phosphates. These are adequate for many formulations where they are adsorbed sufficiently by the disperse phase. However, there are a number of situations where conventional dispersing agents have limitations, e.g.:

(a) where these dispersants are poorly adsorbed on the pesticides; (b) for multicomponent SCs, where hetero-interactions (flocculation) occur; (c) for suspoemulsions where dispersant and emulsifier compete for the solid

Figures 1 and 2, reported from work by Dr Tadros,’ demonstrate the beneficial properties of ‘comb’ polymeric surfactants which have a ‘backbone’ of polymethyl methacrylate/methacrylic acid and polyoxyethylene ‘teeth’. The total molecular weight is in the region of 20-30 OOOand the molecular weight of the polyoxyethylene chains is 750.

Figure 1 shows the adsorption isotherm (obtained at room temperature, 20 (f 2)”C) of the ‘comb’ stabiliser on ethirimol. The isotherm is of the high-afinity type and the adsorption reached a plateau value of 20 mg m-’. No desorption was detected when the centrifuged sediment was redispersed into water. The results clearly show that the ‘comb’ stabiliser is firmly attached to the particles (high- affinity isotherm) and no desorption occurs on particle-particle collision.

Figure 2 shows the viscosity as a function of volume fraction of ethirimol particles. The results are typical for concentrated suspensions, showing a rapid increase in viscosity above a critical volume fraction of the dispersed phase. When the volume fraction of the dispersion reaches the socalled packing fraction, the viscosity reaches infinity.

These results, using macromolecular surfactants which are strongly anchored to the particle surface and having stabilising chains (PEO) that are well solvated in water, show that highly concentrated suspensions approaching the theoretical

disperse phase.

c2 (mg lit&)

Fig. 1. Adsorption isotherm of the ‘comb’stabiliser on ethirimol. r is the adsorption of the surfactant; Cz is the concentration of surfactant in equilibrium in solution.

Trends in the formulation of pesticides- An overview 443

9

Fig. 2. Viscosity/volume-fraction curves for ethirimol suspensions stabilised using the ‘comb’ surfactant. q is the Casson viscosity of the suspension; 4 is the volume fraction of the disperse phase. ( x )

Experimental and (0) theoretical results.

packing fraction can be produced. These dispersants are strongly and irreversibly adsorbed and exhibit good stabilisation of particles. Being of high molecular weight they diffuse slowly so it can help to include a conventional non-ionic or anionic dispersant at the milling stage. The polymeric surfactant, being more strongly adsorbed, will replace conventional dispersant within a short time. Such strong adsorption limits the hetero-interaction effect which otherwise can lead to thickening of SC formulations; it also limits a number of effects in SE formulations, i.e. phase transfer of solid into emulsion phase leading to crystallisation, or heteroflocculation of solid and liquid disperse phase, which can cause gelling and generation of oversize particles resulting in sieve blockage in sprayers and nozzles.

Another advantage is that highly concentrated suspensions can be obtained, because there is no long-range interaction, as found with anionics, which leads to highly viscous systems at high disperse-phase volumes.

3.2 Enhancing biological activity with surfactants

Many systemic pesticides exhibit enhanced biological activity in the presence of surfactants. Some examples from current ICI work are shown in Table 2.

Over a number of years, collaborative work with Dr N. H. Anderson (for retention studies) and Dr P. Holloway (for uptake) of Long Ashton Research Station has been undertaken to increase our understanding of surfactant processes in enhancing biological activity. The work examined a limited range of non-ionic surfactants for their effects on retention of the spray and uptake of the pesticide using outdoor-grown wheat crops. In statistically designed experiments Anderson et aL2 surprisingly found no evidence for enhanced spray retention in the presence of

444 D. Seaman

TABLE 2 Examples of Pesticides Whose Field Effectiveness is Influenced by Surfactants

Pesticide Biological Form Solubility Type of Formulation activity in water surfactant type

Paraquat Herbicide Salt Very high Non-ionic/ SL

(mg litre- ’)

cationic or non-ionic/ anionic

Ethirimol Fungicide Solid 200 Non-ionic sc Diclobutrazol Fungicide Solid 9 Non-ionic sc Fomesafen Herbicide Salt High Non-ionic or SL

Flutriafol Fungicide Solid 100 Non-ionic sc

Fluazifopbutyl Herbicide Liquid 2 Non-ionic EC

non-ionic/ anionic

TABLE 3 Properties of Surfactants Added to Diclobutrazol or Ethirimol

Surfactant”

Form

A7

Viscous

Pour point (“C) Cloud point (“C) HLB Surface tension (m N m-’)

1 g litre-’ 0 2 g litre-’

Deposit area * (mm2)

liquid 21

45-49 12.2

28.6

3.5

A l l

Soft paste

27 84-89 13.9

A20

Hard wax

37 100

16.2

32.8

1.4

41.3

1.0

~~

NP8 T20

Viscous liquid

0 Immiscible

10.2

29.4

8.3

Viscous liquid

0 3&34 12.3

30.0 31.0 2.7

Viscous liquid

16.7

34.7 37.9 0 8

“ ‘Synperonic’ A7, A1 1 and A20 (C, alcohol condensed with 7 , l l and 20moles ethylene oxide respectively); ‘Synperonics’ NP5 and NP8 (nonyl phenol condensed with 5 and 8 moles ethylene oxide respectively) ‘Tween 20’ (sorbitan monolaurate condensed with 20 moles ethylene oxide). bThe deposit area is for a 0 . 2 4 drop placed on a wheat leaf.

surfactants. These authors also studied retention and distribution at different growth stages. Whilst horizontal leaves retained spray better than vertical ones, there were no significant differences between surfactant treatments. Holloway (to be published) examined the uptake of the fungicides diclobutrazol and ethirimol into outdoor-grown wheat. The physical properties of these compounds are as follows:

Diclobutrazol Ethirimol Mol. wt 3 28 209 Melting point (“C) 147-9 159-60 Octanol/water partition coefficient (log P ) 3.8 2.2

Water solubility (mg litre-’) 9 120

Trends in the formulation of pesticides- An overview 445

TABLE 4 The Effect of the Surfactants in Table 3 on Uptake in Wheat: Percentage 10 Days after

Treatment

Surfactant A7 A l l A20 N P 5 NP8 T20 Pesticide Growth stage

Diclobutrazol 35-1 13 11 7 7 6 6 41-5 9 8 5 6 6 Suspect

Ethirimol 35-7 49 78 74 15 29 41 41-5 36 70 83 14 23 32

"Aqueous fungicide suspensions were used at 05 g litre-' with test surfactants at 1.0 g litre-'. Uptake is that of radio-label from the 14C-labelled active ingredient.

_... ...' .... , . .' ....

I I I 0 2 4 6 8 10 12

Time after application (days)

Fig.3.Uptakeofethirimolinto wheat. (-)NP5, (---)NP8, (......) T20, (-.-.)A7, (-x-)A20and (- 0-) A l l .

Thus, ethirimol is an example of a moderately soluble, polar pesticide, whereas diclobutrazol is less water-soluble and more lipid-soluble. Both show enhanced biological activity in the presence of surfactant.

Results on uptake are given in Table 4 and the rate of uptake for ethirimol is shown in Fig. 3. The aliphatic alcohol series surfactants enhanced uptake more than the nonyl phenol series. Optimum uptake was obtained with A7 for diclobutrazol and with A l l and A20 for ethirimol. This parallels observations on biological activity for these chemicals.

The mode of action of surfactants in enhancing uptake may be due to:

- solubilisation -hygroscopic water retention -co-penetration

2 0 251 15

5

0 7 A l l A 2 0 T20 NP5 NP8 A7

Surfactant

(a)

D . Seaman

1 A20 T20 NP5 NP8 Surfactant

Fig. 4. Solubilisation of (a) diclobutrazol and (b) ethirimol. (W) 1 %, (0) 10% and (ma) 20%.

-modification of cuticle permeability by the surfactant --other interactions in ‘dried down’ deposits.

Solubilisation measurements are shown in Fig. 4 for diclobutrazol and ethirimol, solubilisation being reported as the ratio of the solubility in the surfactant solution to that in water. This figure shows dramatic increases in solubilised pesticide, particularly for diclobutrazol, and probably explains why surfactants enhance uptake and biological activity in the first place. However, there is no direct correlation between solubilisation and biological activity for the two fungicides and the surfactants studied here; the differences are probably due to complex pesticide, surfactant and cuticle effects which require further study.

3.3 Developments in microencapsulation

A number of microencapsulated products are now on the market. These are made by an interfacial polymerisation process3 whereby one monomer (or monomers) is present in emulsion droplets of the pesticide and the other monomer (or monomers) is present in the continuous aqueous phase. They react to form a polymer skin which controls the release of pesticide.

A particularly interesting example has come fromjoint work by G. J. Marrs and colleagues (pers. comm.) who have microencapsulated a pheromone in order to obtain the necessary emission of the required concentration of vapour over 10-14 days. In order to reduce degradation of both pheromone and polymer, they included an oil-soluble dye and an oil-soluble anti-oxidant (see Table 5).

Scher4 has introduced an in-situ interfacial condensation polymerisation microencapsulation process which uses only isocyanate monomers, such as polymethylene-polyphenyl-isocyanate (PAPI) and toluene di-isocyanate (TDI), in the disperse phase. The emulsion is heated in order to hydrolyse the isocyanate monomers to form amines, which then react with unhydrolysed isocyanate to form a polyurea microcapsule wall, as shown in Fig. 5. This process produces a smooth,

Trends in the formulation of pesticides-An overview 447

TABLE 5 Persistence of a Microencapsulated Pheromone

Persistence of Z-9-tetradecenylacetate ~~

Stabiliser Half-life (days)

None 0.6 Oil-soluble black dye 2.7 Hindered amine oil-soluble

anti-oxidant 2.3 Dye and anti-oxidant 7.0

Isocyanate monomers

N c o

(PAPI) ( T W

Wall forming reaction 0 II

Isocyanate Carbamic Acid Amine -NCO + HZO - - +-N--C-OH+H,O- - +-NH, + CO,f

H O H I I1 I

-NCO + -NHz _ - _ _ + -N-C-N-

Isocyanate Amine Polyurea

Fig. 5. Reactions involved in the in-situ interfacial condensation polymerisation microencapsulation process.

very thin and efficient membrane, with no monomer residue in the aqueous phase and enables high pesticide loadings to be achieved (Fig. 6).

Two examples of products based on this microencapsulation process are given below.

3.3.1 Fonofos CS as an insecticidal F S seed treatment Microencapsulation reduces the oral toxicity 100 times compared with technical fonofos, and dermal toxicity 10 times. Phytotoxicity on stored seed is also reduced. It is compatible with other water-based seed treatments and gives a good adherent seed coating.

3.3.2 EPTC CS as a selectiue herbicidal SC for maize Microencapsulation reduces volatility and thereby reduces field dose. The chemical

448 D. Seaman

Fig. 6. Pesticide microcapsules (10-40 pm) produced by in-situ interfacial condensation. Note: (1) Smooth membrane-like outer wall surface. (2) Pesticide release rate controlled by rate of diffusion

through wall.

can be sprayed from the air, since the volatility of EPTC is reduced. Seed bed incorporation can be delayed, allowing better use of equipment. Skin and eye irritation are reduced significantly compared with the EC formulation.

4 CONCLUSION

The requirement for safer, easier to use and more efficient formulations leads to a preference for WG, EW, SC and SE, CS and FS formulations with built-in enhancing adjuvants where required. This in turn leads to a requirement for improved composition, improved processes and the need for scientific understanding. Biological control agents provide a problem in another dimension with the need to maintain viability of the component organisms. The formulation chemist thus enjoys a more demanding and significant role in the development of pesticidal products.

ACKNOWLEDGEMENTS

The author would like to thank his colleagues Dr Tharwat Tadros and Gordon Marrs of ICI Agrochemicals, Jealott’s Hill Research Station, UK and Dr Herb Sher, ICI Americas, Richmond, USA for providing some examples of their work incorporated in the paper. He would also like to thank Dr Peter Holloway of Long Ashton Research Station, Bristol, UK for providing the uptake data.

Trends in the formulation of pesticides- An overview 449

REFERENCES

1. Tadros, T. F., Proc. 2nd World Surfactants Congr. 24-27 May 1988, Section D, ASPA,

2. Anderson, N. H., Hall, D. J. & Seaman, D., Asp. Appl. Biol., 14 (1987) 233-43. 3. Graber, G., L’actualiti Chimique, March 1988, pp. 47-51. 4: Sher, H. B., IUPAC Pesticide Chemistry-Human Welfare and the Environment, ed. J.

Miyamote & P. C . Kearney, Vol. 4. Pergamon Press, Oxford, 1983, pp. 295-300.

Paris, pp. 271-83.