GB2247622A - Herbicides - Google Patents

Abstract

A herbicidal composition, comprising a continuous aqueous phase containing a solute, the continuous aqueous phase containing an emulsifier of high HLB value and having a dispersed therein drops of an emulsion comprising a continuous oil phase containing an emulsifier of low HLB value and having dispersed therein droplets of an aqueous solution of a salt of a herbicidal bipyridylium diquaternary cation selected from paraquat and diquat, or mixtures thereof, the said solute comprising magnesium chloride in a concentration sufficient to bring the continuous aqueous phase into substantial osmotic balance with the aqueous solution of the bipyridylium cation, the concentration of the herbicidal bipyridylium diquaternary cation being at least 75 grams per litre of the composition.

Description

HERBICIDAL COMPOSITIONS This invention relates to herbicidal compositions comprising a multiple emulsion containing a bipyridylium diquaternary salt as the active ingredient, and to a method of preparing such compositions.

Various herbicidal bipyridylium diquaternary salts are described in United Kingdom Patent Specifications No 785732, 813531 and 813532. Certain of these compounds are in widespread use in agriculture, and are manufactured for sale in the form of concentrated aqueous solutions. When required for use, these concentrated solutions are diluted with water to form a solution which is then sprayed upon unwanted plants. When used with due care, and in accordance with recommended procedures, the concentrated aqueous solutions present no hazard. However, hazards may develop if adherence to the recommended handling practices is not maintained. Thus it may happen that an operator may decant some of the concentrate into a domestic container such as a beverage bottle for subsequent use in his own garden at home.The hazard associated with such a practice is of course that a child or incautious adult coming upon the bottle may swallow some of the contents, with possible serious consequences.

Various methods have been proposed to reduce the likelihood of accidental swallowing of concentrated solutions of bipyridylium diquaternary salts in the circumstances described above. These include the inclusion of an odourant in the concentrate as a warning (see UK Patents 1406881 and 1570981) and the addition of a gelling agent (see UK Patent 1395502). Another approach has been to include an emetic in the concentrate, so that the concentrate will be regurgitated shortly after its ingestion (see UK Patent 1507407). In addition to methods of formulating bipyridylium herbicides so as to reduce the likelihood of accidental ingestion, a variety of methods has been proposed for formulating bipyridylium herbicides so as to improve their suitability for particular applications.Thus, it has been proposed to formulate an aqueous solution of paraquat as an invert emulsion in oil (see United States Patent 4115098) in order to reduce drift and evaporation of the droplets of spray when the paraquat is applied as a herbicide.

A further way of formulating bipyridylium herbicides so as to reduce the likelihood of accidental ingestion, and to reduce the hazard associated with such ingestion, has been proposed in published European Patent Application No. 276911.

This proposes a method of formulating herbicidal bipyridylium diquaternary salts as a multiple emulsion, whereby a concentrated aqueous solution of the herbicidal bipyridylium diquaternary salt is emulsified in an oil by agitation with a suitable emulsifier, to give an emulsion (the "first emulsion") wherein droplets of the aqueous solution are dispersed through a continuous oil phase.

The first emulsion is then itself dispersed by aqitation into an aqueous solution containing a second emulsifier to give a multiple emulsion (the "second emulsion") which comprises drops of the first emulsion dispersed in a continuous aqueous phase. The drops of the first emulsion themselves contain droplets of the concentrated aqueous solution of the herbicidal bipyridylium salt. On average, the drops of the first emulsion are usually about 30 microns in diameter and the droplets of bipyridylium salt solution usually about 1 micron in diameter. The oil surrounding these droplets of aqueous bipyridylium salt solution acts as a barrier and prevents the bipyridylium salt from passing out into the continuous aqueous phase.

Preferably, the continuous aqueous phase contains a solute to maintain osmotic balance between this phase and the aqueous solution of bipyridylium salt. When required for use as a herbicide, the multiple emulsion is diluted and agitated with water. The emulsion then either breaks up and liberates the herbicidal bipyridylium diquaternary salt into the aqueous. spray solution orewhen the spray solution is applied to- weeds the emulsion breaks up or evaporates on- the surface of the weeds to liberate the bipyridylium herbicide.However, if a quantity of the concentrated multiple emulsion formulation is inadvertently swallowed, as a result for example of transfer of the concentrate to a beverage bottle as mentioned above, the volume of fluid in the gastrointestinal tract and the osmotic differential between this fluid and the solution of the herbicidal bipyridylium diquaternary salt are insufficient in combination to break down the multiple emulsion to any substantial extent. The bipyridylium diquaternary salt accordingly remains substantially contained in the drops of the emulsion, which passes along the gastro-intestinal tract with a substantial reduction in the amount of the bipyridylium diquaternary salt absorbed through the gut wall or into the bloodstream as compared with the absorption which takes place as a result of swallowing an aqueous solution of a bipyridylium diquaternary salt.

As noted above, published European Patent Application No. 276911 discloses the use of a solute in the continuous aqueous phase of the multiple emulsion to maintain osmotic balance between this phase and the aqueous solution of bipyridylium salt. The solutes specifically disclosed are sodium chloride, potassium nitrate, and sucrose. Sodium chloride is said to be preferred because of its cheapness and ready availability. It has now been discovered that by using magnesium chloride as the solute, a substantial improvement in the stability of the multiple emulsions during storage can be obtained.

If desired, a mixture of magnesium chloride with one or more other electrolytes (e.g. sodium chloride) may be used.

Stability during transport and storage is an important factor in the commercial viability of a herbicide formulation, since there will normally be a period of weeks or months, sometimes many months, between the manufacture of a herbicidal formulation and its use by the farmer. During this period, the formulation may be transported for long distances and stored for long periods at varying temperatures, but must remain in a condition wherein it can readily be dispersed in water when required for use.

According to the present invention therefore, there is provided a process of preparing a herbicidal composition comprising as an active ingredient 75 grams per litre or more of a salt of a herbicidal bipyridylium diquaternary cation selected from paraquat and diquat, or mixtures thereof, comprising as a first step agitating an aqueous solution of the herbicidal bipyridylium diquaternary salt with an oil and a first emulsifier having a low HLB value to give a first emulsion comprising droplets of herbicidal bipyridylium diquaternary salt solution dispersed in a continuous oil phase, and as a second step, agitating the first emulsion with water and a second emulsifier having a high HLB value, to give a second emulsion comprising drops of the first emulsion dispersed in a continuous aqueous phase, the continuous aqueous phase containing a solute comprising magnesium chloride in sufficient concentration to bring it into substantial osmotic balance with the aqueous solution of the bipyridylium diquaternary salt. Preferably, the relative osmotic pressures of the continuous aqueous phase and the aqueous solution of the bipyridylium diquaternary salt are such that there is a slight tendency for water to diffuse into the latter from the continuous aqueous phase.

This tends to minimise diffusion out of the droplets and to keep them intact.

Paraquat is the common name of the 1,1'-dimethyl-4, 4'-bipyridylium cation, having the following structure:

Diquat is the common name of the 1,1'-ethylene-2,2'- bipyridylium cations having the following structure:

Salts of paraquat and diquat necessarily contain anions carrying sufficient negative charges to balance the two positive charges on the bipyridylium nucleus.

Since the characteristic herbicidal effect of a bipyridylium quaternary cation is independent of the nature of the associated anion, the choice of the anion is a matter of convenience, depending, for example, on cost.

Preferably the anion is one which gives rise to a salt of convenient water solubility. Examples of anions, which may be mono- or poly-valent, include acetate, benzenesulphonate, benzoate, bromide, butyrate, chloride, citrate, fluorosilicate, fumarate, fluoroborate, iodide, lactate, malate, maleate, methylsulphate, nitrate, propionate, phosphate, salicylate, succinate, sulphate, thiocyanate, tartrate, and p-toluenesulphonate. The salt of the herbicidal bipyridylium cation may be formed from a number of similar anions of mixtures of different ones.A salt having any particular desired anion may be prepared either by direct synthesis from reactants which include the desired anion, or by exchanging the anion of a previously prepared salt of the preferred anion by methods well known in the art, for example by passage of a solution of the previously prepared salt through an ion-exchange resin. For reasons of convenience and economy, paraquat is normally manufactured and sold as paraquat dichloride while diquat is manufactured and sold as diquat dibromide.

Since the characteristic herbicidal activity of a salt of a herbicidal bipyridylium quaternary cation resides in the cation only, it is customary to quote concentrations of active ingredient and rates of application in terms of the amount of bipyridylium quaternary cation used. Application rates and concentrations quoted in this specification therefore relate to the amount of bipyridylium quaternary cation unless otherwise stated.

The expression HLB used above is an abbreviation for the term hydrophilic-lipophilic balance. HLB may be defined in terms of a numerical scale, running from 3 to 18 as described by Griffin, J. Soc. Cosmet. Chem 1, 34 (1949) and 5, 249 (1954). By low HLB we mean a value of less than about 8, for example a value from 3 to 8. By high HLB we mean a value of from about 11 to 18.

The nature of the oil used in the process of the invention is not narrowly critical and a wide variety of oils may be used. The oil may be for example a vegetable oil, for example soya bean oil, castor oil, or sunflower oil. The oil may also be a mineral oil, for example liquid paraffin.

Other oils include white oil, and the oils sold under the trade names Solvesso, Isopar, and Exxsol. The Solvesso oils comprise alkylated benzenes. The Isopar oils (eg. Isopar L and Isopar M) comprise isoparaffinic hydrocarbons. The Exxsol oils comprise de-aromatised paraffinic hydrocarbons. Diesel oil may also be used.

Commercially available diesel oil usually contains additives to lower its freezing point and to prevent wax separation. These do not appear to affect the use of the oil in the present invention. The gas oil from which commercial diesel oil is prepared is also suitable for use in the process of the present invent ion. Mixtures of oils may be used, for example mixtures of Isopar M with diesel oil in the ratio of from 1:10 to 10:1.

Examples of emulsifiers that can be used as the first emulsifier in the process of the invention include an ABA block co-polymer of poly-12-hydroxystearic acid and polyethylene oxide. Such co-polymers are described in, for example, published UK Patent Application No 2002400.

A co-polymer of this kind having a molecular weight of about 5000 is referred to below as Surfactant D.

Another example of an emulsifier is a reaction product of polyisobutylenesuccinic anhydride (PIBSA) and ethanolamine, having the general formula below

and an average molecular weight in the range 400-5000, for example 1200. This polymer is designated below as Surfactant A. Such polymers are described in published UK Patent Application 2156799 (see for example page 3 at lines 9-65 and Examples 2 and 3). Other polymers which may be used include those described at column 5, line 26 to column 6, line 10 of US Patent 4244816. A further example of a first emulsifier is a related polymer of the above formula which has been reacted with one mole of phosphoric acid to yield the mono-phosphate derivative (see Example 5 of UK Patent Application 2156799). This emulsifier is referred to as Surfactant B below.

Other examples of first emulsifiers include the following: "Span" 80 (sorbitan monooleate) Mixtures of "Span" 80 with "Tween" 20 (sorbitan monolaurate condensed with 20 molar proportions of ethylene oxide) "Tween 85" (sorbitan trioleate condensed with 20 molar proportions of ethylene oxide) Lecithin (phosphatidylcholine) "Atlox 1045A (polyoxyethylene sorbitol oleate/laurate) "Span" 80/lecithin mixtures "Arlacel" 83 (sorbitan sesquioleate), optionally mixed with lecithin "Atlas" G 1086 (polyoxyethylene sorbitol hexa-oleate) Particular examples of mixtures of emulsifiers include mixtures of "Arlacel" 83 with lecithin, and mixtures of "Arlacel" 83 together with lecithin and Surfactant D referred to above, and mixtures of "Span" and "Tween" surfactants.

Examples of second emulsifiers include the condensate of p-nonylphenol with propylene oxide and ethylene oxide having the following formula :

wherein x may be in the range from 2 to 50 or more, and y may be in the range from 10 to 200 or more.

The emulsifier referred to as Surfactant C below is a condensate of the last foregoing formula in which x has values ranging from 10.7 to 15.1 with an average value of 12.9 and y has values ranging from 24 to 31 with an average value of 27.5. Other members of this series having different values for y are referred to below by the code names shown in the following table.

Values of y Name 48.0+5.0 Surfactant E 79.4+8.6 Surfactant F 174.2+19.4 Surfactant G Another material useful in preparing the second emulsion is a copolymer of the product referred to above as PIBSA having a molecular weight of about 1000, which is co-polymerised with methoxypolyethylene glycol in a molar ratio of 1:2. This co-polymer is referred to below as Surfactant H. A further material useful as a second emulsifier comprises a modified polyester with an HLB value of 13 to 15 sold under the code number "Hypermer" A109.

Additional second emulsifiers include condensates of para-nonylphenol with ethylene oxide residues. The product having 20 ethylene oxide residues is referred to below as Surfactant I and the product with 35 ethylene oxide residues is referred to below as Surfactant J.

Condensates of C13 to C15 alkanols with from 2 to 50 molar proportions of ethylene oxide may also be used as second emulsifiers. An example of this class containing 30 molar proportions of ethylene oxide is referred to below as Surfactant K. Other examples of second emulsifiers include those emulsifiers sold under the Trade Names "Pluronic" and "Tween" and polyvinyl alcohol.

The "Pluronic" emulsifiers are block copolymers of polyethylene oxide and polypropylene oxide. Particular examples of these include the following: Code No. Percentage of Approximate ethylene oxide Molecular weight F38 80 4800 L92 20 3450 P75 50 4150 F68 80 8350 F88 80 11800 F108 80 14000 P94 40 4600 The "Tween" series of emulsifiers comprises a range of sorbitan esters condensed with various molar proportions of ethylene oxide. These emulsifiers may be used alone or in combination with emulsifiers of the type of, for example, Surfactant C referred to above.

Other emulsifiers that may be used include the following products produced by Rhone-Poulenc: Code No. Chemical composition BSU Ethoxylated polyarylphenol FL Ethoxylated polyarylphenol phosphate neutralised with triethanolamine 3033 Phosphoric ester of ethoxylated aryl-phenol 40884 Sulphated polyarylphenol ethoxylate The agitation of the aqueous solution of the herbicidal bipyridylium diquaternary salt with an oil to form the first emulsion may be carried out for example by high speed stirring (using for example an Elado stirrer, which operates at speeds for example of up to 25,000 revolutions per minute).

The agitation of the first emulsion with the aqueous solution to give the second emulsion may be carried out for example by low-speed stirring using a conventional paddle stirrer at very low speeds. Alternatively, high-speed stirring may be used, but if so, it should be used only for a very short time.

Methods of agitating other than those referred to above may be used to prepare the emulsions according to the invention. Thus, the preparation of the first emulsion may be carried out by pressure homogenisation, or by use of ultra-sonic agitation. The preparation of the second emulsion may be carried out for example. by use of static mixers. The period of agitation required to form the emulsions may vary according to the scale on which the emulsions are prepared, but may readily be ascertained by simple trials. Generally, in the preparation of quantities of the order of 1 to 5 litres, the preparation of the first emulsion by high-speed stirring (eg. using an Ystral Elado stirrer) involves gradually adding the bipyridylium solution with stirring to the oil over a period of about 2 minutes followed by stirring for up to 10 minutes.The preparation of the second emulsion by conventional paddle stirring may take from 30 to 60 minutes. Alternatively, the first emulsion may be added over a period of about 2 to 5 minutes with stirring at 400 rpm. If the second emulsion is prepared by high-speed stirring, this may take of the order of 30 seconds.

In a further aspect, the present invention provides a process of preparing a herbicidal composition as hereinbefore described, which further comprises including in the composition a gelling agent which is soluble in the continuous aqueous phase at a pH of above about 3, but which forms a gel at a pH of about 3 or less.

An example of such a gelling agent is polymethacrylic acid. Preferably the polymethacrylic acid has a molecular weight in the range 10,000 to 25,000 or more. Other gelling agents may be used, for example, bacterial polysaccharides (e.g. xanthan gum), gelatine, gum arabic, alginates, and carrageenans. Block copolymers containing hydrophobic and hydrophilic components may also be used.

Gelling may be enhanced by including further components in the composition, for example aluminium salts, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide, or polyacrylamide.

The function of the gelling agent is to assist in retaining the bipyridylium quaternary salt within the multiple emulsion if a quantity of the emulsion is inadvertently swallowed. In this event, since the pH of the stomach is below 3, the gelling agent will form a gel coating the drops of the first emulsion and encapsulate them so as to reduce leakage of the bipyridylium quaternary salt out of the emulsion, or will immobilise the drops in a gel network.

In a further aspect, the invention provides a process of preparing a herbicidal composition as hereinbefore described, which further comprises including in the composition an emetic.

Preferably, the emetic is an emetic of the class described in UK Patent 1507407. A preferred emetic of this class is 2-amino-6-methyl-5-oxo-4-1-propyl-4, 5- dihydro-s-triazolo[1,5-a]-pyrimidine, which may be incorporated in the composition in the proportion of for example from 0.25 to 2.0 parts by weight per 100 parts of bipyridylium quaternary cation.

According to a further aspect of the invention, there is provided a herbicidal composition, comprising a continuous aqueous phase containing a solute, the continuous aqueous phase containing an emulsifier of high HLB value and having dispersed therein drops of an emulsion comprising a continuous oil phase containing an emulsifier of low HLB value and having dispersed therein droplets of an aqueous solution of a salt of a herbicidal bipyridylium diquaternary cation selected from paraquat and diquat, or mixtures thereof, the said solute comprising magnesium chloride in a concentration sufficient to bring the continuous aqueous phase into substantial osmotic balance with the aqueous solution of the bipyridylium cation, the concentration of the herbicidal bipyridylium diquaternary cation being at least 75 grams per litre of the composition.

More preferably the composition contains at least 100 grams per litre of the herbicidal bipyridylium diquaternary cation. Compositions containing 200 grams or more per litre, for example 300 grams per litre, may be prepared. Especially high concentrations may be prepared by using super-saturated solutions of the-bipyridylium salts. Thus, hot saturated solutions of the bipyridylium salts may be used in preparing the emulsions. As the emulsion cools, the bipyridylium salt solution becomes super-saturated.

Examples of compositions according to the invention include those in which the bipyridylium quaternary salt is paraquat dichloride and those in which the bipyridylium quaternary salt comprises a mixture of equal amounts of paraquat dichloride and diquat dibromide (calculated on bipyridylium cation basis).

The amount of the first (low HLB) emulsifier used in the compositions may be for example from 10 to 100 grams per litre, preferably 40 to 75 grams per litre of the first emulsion. The amount of the second emulsifier may be from 5 to 200 grams per litre, preferably 10 to 50 grams per litre of the continuous aqueous phase.

It will be evident that a composition containing a particular concentration of bipyridylium diquaternary salt may be prepared in different ways, by using different concentrations of aqueous bipyridylium salt solution in combination with different amounts of oil. In the Examples below, the proportions of ingredients have been quoted in terms of volume fractions F1 and F2 where F1 is the ratio of the volume of bipyridylium diquaternary salt solution to the total volume of the first emulsion, and F2 is the ratio of the volume of first emulsion to the total volume of the finished formulation. Preferably, the volume fraction F1, has a value above 0.5, more preferably above 0.6, for example 0.65. The volume fraction F2 is preferably below 0.5 and more preferably below 0.4, for example 0.38.

The compositions of the invention may also comprise ingredients which have previously been proposed to reduce the hazards associated with the possibility of accidental swallowing of herbicidal bipyridylium diquaternary salts.

Thus for example the compositions may contain a colouring agent, for example a blue pigment (eg. the pigment sold under the trade name Monastral Blue). The compositions may also contain an odourant as a warning. Examples of odourants include pyridine base (see UK Patent 1406881), valeric acid, and tetrahydrothiophene. Pyridine base may be included at a concentration of, for example, 5 to 20 grams per litre of the composition.

The following Examples illustrate compositions according to the invention, and their preparation.

EXAMPLE 1 A multiple emulsion formulation according to the invention was prepared as follows: Surfactant D (77 grams per litre of oil) was dissolved in diesel oil. A solution of paraquat chloride in water (400 grams of paraquat cation per litre) was added gradually to the oil/emulsifier over a period of about two minutes, with stirring by an Ystral Elado stirrer at setting 4 (about 5-10,000 rpm) (on a 1 to 10 scale). The stirring speed was then increased to 6-7 (about 15-20,000 rpm) for 10 minutes. The volume fraction F1 of the resulting emulsion was 0.65.

The first emulsion so prepared was stirred into an aqueous solution of magnesium chloride (concentration 1.5 molar) containing 10 grams per litre of surfactant C to give a multiple emulsion with a volume fraction F2 of 0.38. The final emulsion had a paraquat cation concentration of approximately 100 grams per litre.

In order to assess how readily this formulation would disperse in water, a portion (4ml) of the formulation was made up to lOOml with water in a measuring cylinder, the cylinder inverted 10 times and the mixture then left to stand for 24 hours. After this time, the "cream height" was 5ml. When a similar formulation was prepared starting with paraquat dichloride solution containing 368 grams per litre of paraquat cation, and with volume fractions F1 and F2 of 0.54 and 0.5 respectively (final paraquat cation concentration approximately 100 grams per litre, and subjected to the same test of its dispersibility in water, the "cream height" after 24 hours was 13ml.

EXAMPLE 2 This Example illustrates further compositions according to the invention. The procedure described in Example 1 was followed. The proportions of ingredients used were selected to give a concentration of paraquat in the final multiple emulsion composition of approximately 100 grams per litre. Secondary emulsification was by paddle stirring in each case. The proportions in which the oil and aqueous phases were mixed are given by the volume fractions F1 and F2. Thus for example a value of F1 of 0.65 means that the paraquat solution was mixed with oil to form the first emulsion in the ratio of 0.65 parts by volume of aqueous solution to 0.35 parts of oil while a value of 0.38 for F2 means that 0.38 parts by volume of the first emulsion was mixed with 0.62 parts of the continuous aqueous phase to form the final multiple emulsion composition.In cases where the primary emulsion was further diluted with oil (to increase the oil film thickness), the amount of continous aqueous phase was correspondingly decreased to maintain the concentration of paraquat at approximately 100 grams per litre. The compositions prepared are listed in Table 1 below.

TABLE 1 Composition First Second Oil Volume Concentration Notes No. Emulsifier Emulsifier Fractions of MgCl2 F1/F2 1 Surfactant D Surfactant C Diesel 0.65/05 1.5M 77g/l 10g/l 2 " " " " 0.75M 3 " " " " 1.25M 4 " " EX/ES(3:1) " 1.0M a 5 " " Diesel " 1.0M 6 " " " " 2.0M 7 " " Isopar M " 1.75M TABLE 1 (Contd) Composition First Second Oil Volume Concentration Notes No. Emulsifier Emulsifier Fractions of MgCl2 F1/F2 8 " " Isopar M " 2.0M 9 " " EX/EX(3:1) 0.65/0.5 0.75M a 10 " " " " 1.25M a 11 " " " " 1.75M a 12 " " Diesel " 1.5M b 13 " " " 0.65/0.38 1.5M 14 " " " 0.65/0.5 1.5M c TABLE 1 (Contd) Composition First Second Oil Volume Concentration Notes No.Emulsifier Emulsifier Fractions of MgCl2 F1/F2 15 " Surfactant C " 0.65/0.5 1.5M c 50g/l 16 " Surfactant C " " " d 10g/l 17 " " " " " 18 " " " " " a,f 19 " " " 0.65/0.48 1.5M g 20 " " " 0.65/0.43 " h TABLE 1 (Contd) Composition First Second Oil Volume Concentration Notes No. Emulsifier Emulsifier Fractions of MgCl2 F1/F2 21 " Surfactant C Isopar M 0.65/0.5 1.75M 29g/l 22 " " " 0.6/0.5 " 23 " Surfactant C EX/EX/3:1 0.65/0.42 1.75M 10g/l 24 " " " 0.65/0.38 1.5M 25 " " Diesel 0.65/0.5 1.5M i 26 " Surfactant C " " " j 47g/litre TABLE 1 (Contd) Composition First Second Oil Volume Concentration Notes No.Emulsifier Emulsifier Fractions of MgCl2 F1/F2 27 " Surfactant C " 0.54/0.5 " 10g/litre 28 " Surfactant G " 0.65/0.5 1.OM k 10g/litre 29 " Surfactant C " " 1.5M 50g/litre 30 " Surfactant G " " " 10g/litre 31 " Surfactant G " " " 5g/litre TABLE 1 (Contd) Composition First Second Oil Volume Concentration Notes No. Emulsifier Emulsifier Fractions of MgCl2 F1/F2 32 " Surfactant G " " " 5g/litre + Surfactant C 7.6g/litre 33 " Surfactant C " 0.65/0.71 1.M 10g/litre 34 " " " 0.65/0.38 " l 35 " Surfactant C " 0.65/0.5 " 100g/litre 36 " Suractant C " " " m 80g/litre TABLE 1 (Contd) Composition First Second Oil Volume Concentration Notes No.Emulsifier Emulsifier Fractions of MgCl2 F1/F2 37 " Surfactant C " 0.65/0.42 " m 100g/litre 38 " Surfactant C " 0.65/0.5 " m 160g/litre 39 " Surfactant C " 0.65/0.42 " l,m 100g/litre Notes on Table 1 a. EX stands for an oil commercially available under the trade designation EXXSOL D80. Exxsol D80 is an oil comprising straight and branched chain and alicyclic hydrocarbons, containing less than 1% aromatic hydrocarbons. ES stands for an oil commercially available under the trade designation Escaid 100.

This oil has flashpoint, density and viscosity similar to that of Exxsol D80, but contains 25% of aromatic hydrocarbons.

b. The composition contained PEG200 (i.e. polyethylene glycol having a molecular weight of 200+5). (200 grams per litre in the bipyridylium diquaternary salt solution).

c. The primary emulsion was originally made up with a volume fraction F1=0.65, and then diluted to a volume fraction of 0.5 by adding diesel oil. In this example the oil was added to the continuous aqueous phase to form small oil drops.

d. The pH of the continuous aqueous phase was adjusted to 6.4 by adding sodium hydroxide solution.

e. The continuous aqueous phase contained 20 grams per litre of polyvinyl alcohol. (The polyvinyl alcohol had a molecular weight in the range 20,000 to 50,000, and an acetate content of 12X.

f. The pH of the bipyridylium diquaternary salt solution was adjusted to 1 by adding hydrochloric acid.

g. The bipyridylium diquaternary salt solution contained sucrose at a concentration of 0.5 molar.

h. The bipyridylium diquaternary salt solution contained 100 grams per litre of propylene glycol.

i. This formulation contained 50 grams per litre of polyvinyl alcohol.

j. This formulation contained 2 grams per litre of Span 80 and 50 grams per litre of polyvinyl alcohol.

k. The continuous aqueous phase of this formulation also contained magnesium sulphate (0.5M).

1. This formulation contained 1.2 grams per litre of 2-amino-6-me thyl-5-oxo-4-1-propyl-4, 5-dihydro-s- triazolo[1,5-a]pyrimidine.

(m) This formulation contained 1 gram per litre of Kelzan. (Kelzan is a Trade Mark for a thickening agent comprising a bacterial polysaccharide).

Claims (6)

1. A process of preparing a herbicidal composition comprising as an active ingredient 75 grams per litre or more of a salt of a herbicidal bipyridylium diquaternary cation selected from paraquat and diquat, or mixtures thereof, comprising as a first step agitating an aqueous solution of the herbicidal bipyridylium diquaternary salt with an oil and a first emulsifier having a low HLB value to give a first emulsion comprising droplets of herbicidal bipyridylium diquaternary salt solution dispersed in a continuous oil phase, and as a second step, agitating the first emulsion with water and a second emulsifier having a high HLB value, to give a second emulsion comprising drops of the first emulsion dispersed in a continuous aqueous phase, the continous aqueous phase containing a solute comprising magnesium chloride in sufficient concentration to bring it into substantial osmotic balance with the aqueous solution of the bipyridylium diquaternary salt.

2. A process as claimed in Claim 1, which further comprises including in the composition a gelling agent which is soluble in the continuous aqueous phase at a pH of above about 3, but which forms a gel at a pH of about 3 or less.

3. A process as claimed in claim 2 in which the gelling agent is polymethacrylic acid.

4. A process as claimed in claims 1 to 3, which further comprises including in the composition an emetic.

5. A process as claimed in claim 4 wherein the emetic is 2-amino-6-me thyl-5-oxo-4-1-propyl-4, 5-dihydro-s- triazolo[l,5-a]-pyrimidine.

6. A herbicidal composition, comprising a continuous aqueous phase containing a solute, the continuous aqueous phase containing an emulsifier of high HLB value and having a dispersed therein drops of an emulsion comprising a continuous oil phase containing an emulsifier of low HLB value and having dispersed therein droplets of an aqueous solution of a salt of a herbicidal bipyridylium diquaternary cation selected from paraquat and diquat, or mixtures thereof, the said solute comprising magnesium chloride in a concentration sufficient to bring the continuous aqueous phase into substantial osmotic balance with the aqueous solution of the bipyridylium cation, the concentration of the herbicidal bipyridylium diquaternary cation being at least 75 grams per litre of the composition.