MICROCAPSULES AND MICROENCAPSULATION PROCESS

Abstract: Microcapsules containing a substantially water-insoluble liquid material, such as an anilide herbicide, and optionally also a herbicide antidote, within a porous shell to effect a slow rate of release of said herbicide (and optionally said antidote) through said shell, are produced by a process which comprises (a) providing an organic solution comprising said material and an etherified amino resin prepolymer dissolved therein which from about 50 % to about 98 % of the methylol groups of said prepolymer have been etherified with a C4-C10 alcohol; (b) creating an emulsion of said organic solution in a continuous phase aqueous solution comprising water and a surface-active agent, wherein said emulsion comprises discrete droplets of said organic solution, there being formed thereby an interface between the discrete droplets of organic solution and the surrounding continuous phase aqueous solution; and (c) causing in situ self-condensation and curing of said amino resin prepolymer in the organic phase of said discrete droplets adjacent to said interface by simultaneously heating said emulsion to a temperature between about 20 DEG C to about 100 DEG C, and adding to said emulsion an acidifying agent and maintaining said emulsion at a pH of between about 0 to about 4 for a sufficient period of time to allow substantial completion of in situ condensation of said resin prepolymers to convert the liquid droplets of said organic solution to capsules consisting of solid permeable polymer shells enclosing said liquid material. Also disclosed are the microcapsules formed by the above-described process.

Description: MICROCAPSULES AND MICROENCAPSUIATION PROCESS Cross Reference to Related Applications This application is a continuation-in-part of appli¬ cation Serial No. 540,560, filed June 20, 1990, which is a continuation of application Serial No. 151,048, filed February 1, 1988, now U.S. Patent 4,956,129, which in turn is a continuation-in-part of application Serial No. 595,136, filed March 30, 1984, now abandoned; which in turn is a continua¬ tion-in-part of application Serial No. 499,973, filed June 1, 1983, now abandoned; which in turn is a continuation-in-part of application Serial No. 201,686, filed October 30, 1980, now abandoned. Background of the Invention A. Field of the Invention This invention relates to microcapsules and to a process for their production. In particular, this invention relates to encapsulated droplets of a liquid material which is substantially insoluble in water, where the encapsulating agent is a film formed from a modified amino resin polymer. B. Description of the Prior Art The use of membranes, coatings, and capsules for the controlled release of liquid materials is well known in the art of both agricultural and non-agricultural chemicals. In agriculture, controlled-release techniques have improved the efficiency of herbicides, insecticides, fungicides, bac- tericides, and fertilizers. Non-agricultural uses include encapsulated dyes, inks, pharmaceuticals, flavoring agents, and fragrances. The most common forms of controlled-release mater¬ ials are coated droplets or microcapsules, coated solids in¬ cluding both porous and non-porous particles, and coated ag¬ gregates of solid particles. In some instances, a water- soluble encapsulating film is desired, which releases the en¬ capsulated material when the capsule is placed in contact with water. Other coatings are designed to release the entrapped material when the capsule is ruptured by external force. Still further coatings are porous in nature and re¬ lease the entrapped material to the surrounding medium at a slow rate by diffusion through the pores. In addition to providing controlled release, such coatings also serve to facilitate the dispersion of water-immiscible liquids into water and water-containing media such as wet soil. Droplets encapsulated in this manner are particularly useful in agri¬ culture, where water from irrigation, rain, and water sprays is frequently present. A variety of processes for producing such capsules is known. In one process, the capsules are formed by phase separation from an aqueous solution through the coacervation of a hydrophilic colloid sol. This is described in U.S. Patent Nos. 2,800,457 (Green et al., July 23, 1957) and 2,800,458 (Green, July 23, 1957). An interfacial polymerization process is disclosed in U.S. Patent Nos. 4,046,741 (Scher, September 6, 1977) and 4,140,516 (Scher, February 20, 1979), whereby the film-forming reactants are dissolved in the hydrophobic liquid which is dispersed in water, the reaction occurring at the interface when the phases are placed in contact as an emulsion. A further interfacial polymerization process is described in U.S. Patent No. 3,726,804 (Matsukawa et al. , April 10, 1973) whereby all the film-forming ingredients initially reside in hydrophobic droplets which also contain a low boiling or polar solvent in addition to the material to be encapsulated. Upon heating, the solvent is released into the aqueous phase (the continuous phase of the emulsion) , and the film-forming materials accumulate at the interface and polymerize. Olefin polymerization using a peroxide catalyst is described in Japanese Patent Publication No. 9168/1961, whereby an oil-insoluble polymer is formed at the surfaces of oil drops. British Patent Nos. 952,807 and 965,074 describe a process whereby a solid such as a wax or a thermoplastic resin is melted, dispersed and cooled to form an encapsulating film around liquid droplets. U.S. Patent No. 3,111,407 (Lindquist et al., Novem¬ ber 19, 1963) describes a spray drying method which forms en¬ capsulated droplets at the instant of atomization. These processes vary in terms of equipment expense, energy requirements, ease of controlling the microcapsule size, the need for extra reagents such as catalysts and anti- settling agents, and percent microcapsule phase. It is there¬ fore an object of the present invention to provide a simple, inexpensive method for producing microcapsules of uniform and readily controlled size, which are suitable for use without further treatment. J Other objects of" e nvention will be apparent from the following description. Summary of the Invention It has now been discovered that a liquid material which is substantially insoluble in water can be microencap- sulated within a porous shell by a process which comprises: (a) providing an organic solution comprising said material and an etherified amino resin prepolymer dissolved therein in which from about 50% to about 98% of the methylol groups of said prepolymer have been etherified with a C.-C 10 alcohol; (b) creating an emulsion of said organic solution in a continuous phase aqueous solution comprising water and a surface-active agent, wherein said emulsion comprises discrete droplets of said organic solution dispersed in said continuous phase aqueous solution, there being formed thereby an inter¬ face between the discrete droplets of organic solution and the surrounding continuous phase aqueous solution; and (c) causing in situ self-condensation and curing of said amino resin prepoly ers in the organic phase of said discrete droplets adjacent to said interface by simultaneously heating said emulsion to a temperature between about 20°C to about 100 β C, and adding to said emulsion an acidifying agent and maintaining said emulsion at a pH of between about 0 to about 4 for a sufficient period of time to allow substantial completion of in situ condensation of said resin prepolymers to convert the liquid droplets of said organic solution to capsules consisting of solid permeable polymer shells enclos¬ ing said liquid material. Microcapsules formed by this process are capable of effecting a slow rate of release of the encapsulated liquid by diffusion through the shell to the surrounding medium. The present invention resides in both the process described above and the microcapsules thus formed. Description of the Preferred Embodiments The present invention can be readily adapted to accommodate variations in the materials used, the kind of product desired, and economic factors in general. As the following indicates, both essential and optional features of the process and the product thereof can be varied over a wide range. A. Core Liquid It is essential that the organic solution which forms the interior of the capsules (i.e., the core liquid) be substantially insoluble in water. Preferably, its solubility under ambient conditions is approximately 5000 parts per mil¬ lion (ppm) by weight or less. The organic solution may con¬ sist of a single liquid material or one or more active liquid or solid materials dissolved in an inert solvent which has at most a slight solubility in water. In the latter case, the liquid or solid solute must reside preferentially in the or¬ ganic phase when the two phases are in equilibrium. A wide variety of liquids can be encapsulated by the present process. The most useful liquids are those which do not react with either the prepolymer, the acid used in the self-condensation wall-forming step, or any of the other com¬ ponents in the system. Thus, any nonreactive liquid which will diffuse through the shell membrane is suitable. The liquid can be a single chemical compound or a mixture of two or more compounds. It can diffuse into water, soil, air, or any other surrounding medium. Liquids suitable for encapsulation include chemi¬ cal-biological agents such as herbicides, insecticides, fun¬ gicides, nematocides, bactericides, rodenticides, molluscides, acaricides, larvicides, animal, insect, and bird repellents, plant growth regulators, fertilizers, pheromones, sex lures and attractants, and flavor and odor compositions. The micro¬ capsules of the present invention are particularly well adapted to pesticides, including thiocarbamates, dithiocarba- ates, acetamides, anilides, sulfonamides, triazines, organo- phosphorus compounds, and pyrethroids. The following are examples of such compounds, followed in parentheses by their common names where available: HERBICIDES S-ethyl-N-cyclohexyl-N-ethylthiocarbamate (cycloate) S-ethyl hexahydro-lH-azepine-1-carbothioate (molinate) S-2,3-dichloroallyl di-isopropylthiocarbamate (diallate) S-2,3,3-trichloroallyl di-isopropylthiocarbamate (triallate) S-ethyl dipropylthiocarbamate (EPTC) S-4-chlorobenzyl diethylthiocarbamate (benthiocarb) S-ethyl diisobutylthiocarbamate (butylate) S-benzyl di-εec-butylthiocarbamate S-propyl dipropylthiocarbamate (vernolate) S-propyl butylethylthiocarbamate (pebulate) N,N-diallylchloroacetamide (allidochlor) o-chloro-6'-ethyl-N-(2-methoxy-l-methylethyl)-acetanilide (metolachlor) N-butoxymethyl-α-chloro-2 ' , S r -diethylacetanilide (butachlor) S-(0,0-diisopropyl phosphorodithioate) ester of N-(2-mercaptoethyl)benzenesulfonamide (bensulide) N-benzyl-N-isopropyltrimethylacetamide (butam) 2-chloroallyl diethyldithiocarbamate (CDEC) 2-sec-butyl-4,6-dinitrophenol (dinoseb) 2 ,6-dinitro-N,N-dipropylcumidine (isopropalin) N-(cyclopropylmethyl)-α,α,o,-trifluoro-2,6-dinitro-N-propyl- p-toluidine (profluralin) 2-(1,2-dimethylpropylamino)-4-ethylamino-6-methylthio-l,3,5- triazine (dimetha etryn) 2-ethyl-5-methyl-5-(2-methylbenzyloxy)-1,3-dioxane INSECTICIDES S-tert-butylthiomethyl 0,0-diethyl phosphorodithioate (terbufos) 0,O-diethy1-0-4-methylsulfinylphenyl phosphorothioate (fensulfothion) 0,0-diethyl 0-2-isopropyl-6-methylpyrimidin-4-yl phosphorothioate (diazinon) 0,0-diethyl S-2-ethylthioethyl phosphorodithioate (disulfoton) S-chloromethyl 0,0-diethyl phosphorodithioate (chlormephos) 0-ethyl S,S-dipropyl phosphorodithioate (ethoprophos) 0,0-diethyl S-ethylthiomethyl phosphorodithioate (phorate) 0-{4-bromo-2-chlorophenyl) 0-ethyl S-propyl phosphorodithioate (prophenofos) S-1,2-di(ethoxycarbonyl)ethyl 0 ,O-dimethyl phosphorodithioate ( alathion) 0,0,0',0'-tetraethyl s,S'-methylene di(phosphorodithioate) (ethion) 0-(4-bromo-2,5-dichlorophenyl) 0,O-diethyl phosphorothioate (bromophosethyl) S-4-chlorophenylthiomethyl 0,0-diethyl phosphorodithioate (carbophenothion) 2-chloro-l-(2,4-dichlorophenyl)vinyl diethyl phosphate (chlorphenvinphos) 0-2,5-dichloro-4-(methylthio)phenyl 0,0-diethyl phosphorodithioate (chlorthiophos) 0-4-cyanophenyl 0,0-dimethyl phosphorothioate (cyanophos) 0,O-dimethy1 0-2-methylthioethyl phosphorothioate (demephion) 0,0-diethyl 0-2-ethylthioethyl phosphorothioate (demeton) 0-2,4-dichlorophenyl 0,0-diethyl phosphorothioate (dichlorofenthion) 0-2,4-dichlorophenyl O-ethyl phenylphosphonothioate (EPBP) 0,0-diethyl 0-5-phenylisoxazol-3-yl phosphorothioate (isoxathion) 1,3-di(methoxycarbonyl)-l-propen-2-yl dimethyl phosphate S,S -(l,4-dioxane-2,3-diyl) 0,0,0'0'-tetraethyl di(phosphorodithioate) (dioxathion) 0,O-dimethy1-0-4-nitro-m-tolyl phosphorothioate (fenitrothion) 0,0-dimethyl 0-4-methylthio-m-tolyl phosphorothioate (fenthion) 0-(5-chloro-l-isopropyl-l,2,4-triazol-3-yl) 0,0-diethyl phosphorothioate (isazophos) S-2-isopropylthioethyl 0,0-dimethyl phosphorodithioate (isothioate) 4-(methylthio)phenyl dipropyl phosphate (propaphos) l,2-dibromo-2,2-dichloroethyl dimethyl phosphate (naled) 0,O-diethyl α-cyanobenzylideneamino-oxyphosphonothioate (phoxim) 0,0-diethyl 0-4-nitrophenyl phosphorothioate (parathion) -2-diethylamino-6-methylpyrimidin-4-yl 0,O-diethyl phosphorothioate (pirimiphos-ethyl) 0-2-diethylamino-6-methylpyrimidin-4-yl O,O-dimethy1 phosphorothioate (pirimiphoε-methy1) (E)-0-2-isopropoxycarbonyl-l-methylvinyl O-methyl ethylphosphoramidothioate (propetamphos) 0,0,0',0'-tetraethyldithiopyrophosphate (sulfotep) 0 ,0 ,0',o '-tetramethyl 0,0'-thiodi-p-phenylene diphosphorothioate (temephos) S-2-ethylthioethyl 0,0-dimethyl phosphorodithioate (thiometon) 0,0-diethyl 0-l-phenyl-l,2,4-triazol-3-yl phosphorothioate (triazophos) 0-ethyl 0-2,4,5-trichlorophenyl ethylphosphonothioate (trichloronate) (±)-3-allyl-2-methyl-4-oxocyclopent-2-enyl (±)-cis,trans- chrysanthemate (allethrin) (±)-3-allyl-2-methyl-4-oxocyclopent-2-enyl (±)-trans- chrysanthemate (bioallethrin) 3-phenoxybenzyl (±)-cis,trans-chrysanthemate (phenothrin) pyrethrins 2-(2-butoxyethoxy)ethyl thiocyanate isobornyl thiocyanoacetate (terpinyl thiocyanoacetate) carbon disulfide 2-(4-tert-butylphenoxy)cyclohexyl prop-2-ynyl sulphite (propargite) 4,6-dinitro-6-octylphenyl crotonates (dinocap) ethyl 4,4'-dichlorobenzilate (chlorobenzilate) DEFOLIANTS S,S,S-tributyl phosphorotrithioate tributyl phosphorotrithioite (merphos)' FUNGICIDES copper naphthenates 5-ethoxy-3-trichloromethyl-l , 2 ,4-thiadiazole (etridiazole) 0-ethyl S,S-diphenyl phosphorodithioate (edifenphos) INSECT REPELLENTS -butoxycarbonyl-2,3-dihydro-2,2-dimethylpyran-4-one (butopyronoxyl) N,N-diethyl-m-toluamide (deet) dibutyl phthalate dibutyl succinate 1,5a,6,9 ,9a ,9b-hexahydro-4a(4H)-dibenzofurancarboxaldehyde dipropyl pyridine-2,5-dicarboxylate Of the many different types of core liquids useful in the present composition, pesticides are preferred, and certain classes of pesticides are particularly preferred. One such class is that of substituted thiocarbamates, partic¬ ularly those of the formula in which R is selected from the group consisting of C.-C- alkyl, C.-Cg alkenyl, and C_-C 8 phenylalkyl, and is option¬ ally substituted with up to three groups selected from halo¬ gen and nitro; and R and R are either independently C. -C 6 alkyl or C 5 -C_ cycloalkyl, or conjointly form C -C_ alkylene. The terms "alkyl", "alkenyl", and "alkylene" are intended to include both straight-chain and branched-chain groups, and all carbon atom ranges are intended to be inclusive of the upper and lower limits stated. More preferred thiocarbamates are those in which R 1 is C -C . alkyl and R2 and R3 either independently form C_-C z alkyl or conjointly form hexa ethy- lene. The most preferred are those in which R 1, R2, and R3 are all independently C_-C. alkyl. Thiocarbamates are par¬ ticularly useful as pre-emergence and post-emergence herbicides. Another class of pesticide which is particularly preferred is that of the anilide herbicides, preferably the subclass of these known in the art as o-haloacetanilide, or more specifically α-chloroacetanilide herbicides. This latter group can be represented by the formula in which X is halogen; R 4 is one or more of C.-C- alkyl, C.-C 6 alkoxy or halogen, n is 0 or an integer from 1 to 5; and 5 is C.-C- alkyl, C--C- alkoxyalkyl or pyrazol-1-methyl. Preferably, X is chloro or bromo (most preferably chloro; n is 0, 1, or 2, R. is C χ -C alkyl (most preferably methyl, ethyl or tertiary butyl); and R 5 is C -C- alkoxyalkyl. Some specific compounds of this class include: o-chloro-2'-methyl ,6'-ethyl-N-(2-methoxy-l-methylethyl) acetanilide (metolachlor) N-butoxymethyl-α-chloro-2',6'-diethylacetanilide (butachlor) α-chloro-2',6'-diethyl-N-(methoxymethyl) acetanilide (alachlor) o-chloro-2'-methyl,6'-ethyl-N-(ethoxymethyl) acetanilide (acetochlor) α-chloro-2',6'-dimethyl-N-(lH-pyrazol-l-yl-methyl) acetanilide (metazochlor) α-chloro-2',6'-diethyl-N-(2-propoxyethyl) acetanilide (pretilachlor) a-chloro-2' ,6'-dimethyl-N-(methoxyethyl) acetanilide (dimethachlor) α-chloro-N-isopropyl acetanilide (propachlor) One can broaden the variety of crops on which cer¬ tain pesticides, particularly herbicides, can be effectively used by including an antidote or safener in the composition. The antidote helps to protect the crop from injury by the herbicide, without appreciable effect on the potency of the herbicide against the undesi ed weed species. The antidote thus renders the herbicide more selective in its action. Useful antidotes include dichloroacetamides such as N,N- diallyl-2 ,2-dichloroacetamide, 2,2-dimethyl-3-dichloroacetyl oxazolidine, 2,2-dimethyl-5-phenyl-3-dichloroacetyl oxazoli- dine, 2,2,5-trimethyl-3-dichloroacetyl oxazolidine and 2,2- spirocyclohexyl-3-dichloroacetyl oxazolidine, dioxolanes such as 2-(dichloromethyl)-2-methyl-l,3-dioxolane and 2-(dichloro- methyl)-2-methyl-4-ethyl-l,3-dioxolane, S-thiazolecarboxylic acid, 2-chloro-4-(trifluoromethyl) (phenylmethyl) ester ("flur- azole") , ethanone-2,2-dichloro-l-(1,2,3,4-tetrahydro-l-methyl- 2-isoquinolyl) and other compounds disclosed in U.S. Patent 4,936,901, various substituted aryl cyclopropane carbo- nitriles as disclosed in U.S. Patent 4,859,232, and 1,8- naphthalic anhydride. For maximum effect, the antidote is present in the composition in a non-phytotoxic, antidotally effective amount. By "non-phytotoxic" is meant an amount which causes at most minor injury to the crop. By "antidot¬ ally effective" is meant an amount which substantially de¬ creases the extent of injury caused by the herbicide to the crop. The preferred weight ratio of herbicide to antidote is about 0.1:1 to about 30:1. The most preferred range for this ratio is about 3:1 to about 20:1. The use of this invention for the production of microencapsulated products which contain an anilide, or more particularly an o-haloacetanilide herbicide, especially those mentioned above as being preferred, can provide exemplary re¬ sults with those anilides or α-haloacetanilides which normally require an antidote for application to certain crops. Acetochlor and metolachlor are two examples of such herbicides. By appropriate choice of the amino resin and the process con¬ ditions, microencapsulated compositions of anilides, espe¬ cially o-haloacetanilides may be prepared which, as compared to other compositions of the same herbicide, may contain a smaller amount of the same antidote, may contain a weaker antidote, or may even dispense with the need for an antidote. While not intending to be bound by any theory, it is believed that the microcapsules of this invention have walls which are sufficiently permeable to permit passage, in a controlled manner, of anilide, or particularly, α-haloacetanilide herbi¬ cide in an amount, or at a rate, sufficient to control weeds, but insufficient to cause material damage to certain crops, particularly corn. The utility of many pesticides can also be broad¬ ened by the inclusion in the microcapsule core, of synergists in the pesticide composition. Synergists are compounds which have little or no pesticidal activity of their own, but when combined with a pesticide produce a combination with a potency significantly greater than the additive sum of the potencies of the compounds applied individually. Useful synergists in¬ clude 5-1-[2-(2-ethoxyethoxy)-ethoxy]-ethoxy-1,3-benzodioxole (sesamex), l,4-di-(l,3-benzodioxol-5-yl)-tetrahydrofuro [3,4-c] furan (sesamin) , l-methyl-2-(3,4-methylenedioxyphenyl)ethyl octyl sulphoxide (sulfoxide) , and 5-[2-(2-butoxyethoxy)ethoxy- methyl]-6-propyl-l,3-benzodioxole (piperonyl butoxide) . When included, synergists are present in effective amounts, i.e., at any pesticide-to-synergist ratio at which a synergistic effect is observed. This ratio varies widely from one combi¬ nation to the next. B. Prepolymer Prepolymers suitable to the present invention are partially etherified amino resin prepolymers with a high sol¬ ubility in the organic phase and a low solubility in water. In its non-etherified form, the prepolymer contains a large number of methylol groups, -CH_OH, in its molecular structure. Etherification is the replacement of the hydroxyl hydrogens with alkyl groups, and is achieved by condensation of the prepolymer with an alcohol. When the alkyl groups comprise four carbon atoms or more and they have replaced more than about 50% of the hydroxyl hydrogen atoms on the prepolymer molecule, the prepolymer becomes soluble in the organic phase. Complete etherification is to be avoided, however, since hydroxyl groups are needed for the in situ self-condensation polymerization which occurs in the wall-forming step. There¬ fore, the prepolymers useful in the present invention are those in which from about 50% to about 98% of the hydroxyl hydrogen atoms have been replaced by alkyl groups of 4 to 10 carbon atoms each. In preferred practice, about 70% to about 90% of the groups have been etherified with a C.-C 8 alcohol. Both straight-chain and branched-chain alcohols are useful in the present invention, and all carbon atom ranges quoted herein are to be inclusive of their upper and lower limits. Amino resins are a known class of polymer and are described, for instance, in "50 Years of Amino Coating Resins", Albert J. Kirsch, ed., Winchell Co. (Philadelphia), 1986. These are prepared from an amine-containing compound and formaldehyde. Amino resins generally fall into four sub¬ classes: urea-formaldehyde, melamine-formaldehyde, benzo- guanamine-formaldehyde and glycoluril-formaldehyde. The first two mentioned are preferred in this invention, with urea- formaldehyde prepolymers being most preferred. Etherified amino resin prepolymers are commercially available as solutions in alcohol or in a mixture of alcohol and xylene. The alcohol used as the solvent is normally identical to that used as the etherifying agent. Those in most common use are n-butanol and iso-butanol. The degree of etherification (butylation) in these commercial products ranges between 70% and ~ 90%, " and the solution contains ϊrom 50% to 85% by weight of prepolymer. Minor amounts of free formaldehyde are also frequently present. These solutions are typically sold as cross-linking agents for alkyd resins and used primarily for the formulation of coating and finish¬ ing products such as paints and lacquers. Amino resin prepolymers which have not been ethe¬ rified are also available commercially, either in aqueous so¬ lutions or as water-dissolvable solids, for use as adhesives. These can be etherified by condensation with the desired al¬ cohol in a weakly acidic alcohol solution. The water of con¬ densation is distilled off as an azeotrope with the alcohol until the desired degree of condensation (etherification) has been reached. Some commercially available prepolymers, in addi¬ tion to those mentioned in the Examples, are those sold by American Cyanamid Co. under the trademark CYMEL ® (melamine- formaldehyde, benzoguanamine-formaldehyde and glycoluril- formaldehyde) . The amino resin prepolymers themselves can be pre¬ pared by known techniques, by the reaction between the amine (preferably urea or melamine) and formaldehyde. Urea-formal¬ dehyde prepolymers, for instance, can be prepared by the base- catalyzed reaction between urea and formaldehyde in water at a weight ratio of 0.6 to 1.3 parts formaldehyde to one part urea by weight (1.2:1 to 2.6:1 on a molar basis), at a pH of 7.5 to 11.0 and a temperature of 50°C to 90°C. Etherification is then accomplished as described in the preceding paragraph. The degree of etherification can be monitored by the quantity of water driven off during the distillation. Although the degree of etherification can be varied over a wide range to accommodate the needs of the reaction system, the rate of polymerization in the subsequent wall-forming step decreases as the degree of etherification increases. Too high a degree of etherification, therefore, tends to inhibit the progress of the wall formation. However, the water solubility of the prepolymer also decreases with increasing degree of etherification. Since low water solubility is a desirable feature of the prepolymer, it is best to avoid too low a degree of etherification. Thus, the suitable and preferred ranges are those stated above. The organic solution comprising the core liquid and the etherified prepolymer is most conveniently formed when the latter is predissolved in a solvent, as it is when com¬ mercially sold for the coatings and finishings industry. In the absence of such a^ solvent, there is a high degree of hy¬ drogen bonding between the hydroxyl groups, and the prepolymer is a waxy solid which is difficult to dissolve in the capsule core liquid. Polar organic solvents are particularly useful for preventing the hydrogen bonding and dissolving the prepolymer; examples include alcohols, ketones, esters, and aromatics. When etherifying agents of high chain length are used, aliphatics and other non-polar solvents can also be used. The most useful solvents are the same alcohols used as the etherifying agents, the solution being taken directly from the reaction mixture of the etherification process. The concentration of the prepolymer in the organic phase is not critical to the practice of the invention, but can vary over a wide range depending on the desired capsule wall strength and the desired quantity of core liquid in the finished capsule. It will be most convenient, however, to use an organic phase with a prepolymer concentration of from about 1% to about 70% on a weight basis, preferably from about 5% to about 50%. C. Optional Additives Optional additives include solvents, polymerization catalysts, and wall-modifying agents. Solvents provide a means for controlling the wall- forming reaction. As explained in Section E below, the reac¬ tion occurs when protons come in contact with the urea-formal¬ dehyde prepolymer. The organic phase must be sufficiently hydrophilic to attract protons to the interface from the bulk of the aqueous phase, yet sufficiently hydrophobic to prevent large amounts of protons from crossing the interface and causing polymerization to occur throughout the bulk of the droplet. An appropriately selected solvent added to the or¬ ganic phase can correct the character of the organic phase to achieve these results. Clearly, the need for a solvent and the type of solvent neede —hydrophobic or hydrophilic— depends on the nature of the liquid core material. Aliphatic and alicyclic solvents are examples of hydrophobic solvents, and alcohols and ketones are examples of hydrophilic solvents. The amount of solvent can be varied as needed to achieve the desired results. Catalysts capable of enhancing the wall-forming re¬ action can be placed in either the aqueous or organic phase. Catalysts are generally used when the core material is too hydrophobic, since they serve to attract protons toward the organic phase. Any water-soluble catalyst which has a high affinity for the organic phase and is capable of carrying a proton can be used. Carboxylic and sulfonic acids are par¬ ticularly useful. Examples include orthochlorobenzoic acid, 2-phenyl-2,2-dichloroacetic acid, benzoic acid, salicylic acid, p-toluenesulfonic acid and dodecylbenzene sulfonic acid. The same catalytic effect can be accomplished by dissolving salts of these acids in the aqueous or organic phase and then acidi¬ fying the aqueous phase. The acid form is produced by ion exchange. Wall-modifying agents serve to modify the character of the wall by varying its permeability to the core material. Suitable wall-modifying agents contain a substantial number of hydroxyl or mercapto groups capable of reacting with the methylol groups on the prepolymer. The wall modifier can be used in the organic solution to add multiple linkages to the methylol groups to increase the degree of cross-linking, or to exhaust active sites on the prepolymer to decrease the de¬ gree of cross-linking. Thus, depending on the kind of mod¬ ifier used and the ratio of modifier to prepolymer, the per¬ meability of the wall (and consequently the release rate of the core liquid) can be either increased or decreased. Cas¬ tor oil is one example of a cross-linking agent. The pre¬ ferred cross-linking wall-modifying agent is pentaerythritol tetrakis (mercaptopropionate) sold under the tradename Mercap- tate Q-43 Ester, by Cincinnati Milacron Chemicals. Other poly-functional mercaptan esters of a similar nature can be used. D. Emulsion Formation Once the organic solution is formed, an emulsion is formed by dispersing the organic solution in an aqueous solu¬ tion comprising water and a surface-active agent. The relative quantities of organic and aqueous phases are not critical to the practice of the invention, and can vary over a wide range, limited mostly by convenience and ease of han¬ dling. In practical usage, the organic phase will comprise a maximum of about 55% by volume of the total emulsion and will comprise discrete droplets of organic solution dispersed in the aqueous solution. The surface-active agent can be any of the wide variety of compounds known to be useful for lowering the sur¬ face tension of a fluid interface. Nonionic and anionic types are both useful. Examples of nonionic agents are long chain alkyl and mercaptan polyethoxy alcohols, alkylaryl polyethoxy alcohols, alkylaryl polyether alcohols, alkyl polyether alco¬ hols, polyoxyethylene sorbitan fatty acid esters, polyoxy- ethylene ethers, and polyethylene glycol esters with fatty or rosin acids. Examples of anionic agents are the calcium, amine, alkanolamine, and alkali salts of alkyl and alkylaryl sufonates; vegetable sulfonates; and ethoxylated and propoxy- lated mono- and diethers of phosphoric acid. Blends of surface-active agents are also useful. Preferred surface- active agents are polyethylene glycol ethers of linear alco¬ hols and alkali salts of alkyl and alkylaryl sulfonates. The quantity of surface-active agent is not criti¬ cal to the invention, and can vary over a wide range. For convenience, the agent generally comprises from about 0.1% to about 5.0% by weight of the aqueous phase. The agent can be added before or after the emulsion is formed. In some systems, emulsion stability can be enhanced by adding a protective colloid to the aqueous phase. A pro¬ tective colloid stabilizes a dispersed system against aggre¬ gation, flocculation, and coalescense. Many materials are known to function as protective colloids and are available commercially, including polyvinyl alcohols, alginates, alpha- and gamma-protein, casein, methyl cellulose, carboxymethyl cellulose, gelatin, glues, natural gums, polyacids, and starch. The colloid can be added to the aqueous phase prior to the formation of the emulsion, or to the emulsion itself after it has been formed. Although the colloid is an optional additive, its inclusion in the present system is preferred. Polyvinyl alcohol protective colloids are particularly preferred. Additional compounds which serve as protective col¬ loids are the salts of lignin sulfonate, such as the sodium, potassium, magnesium, calcium or ammonium salts. Among com¬ mercial lignin sulfonates are Treax ® , LTS, LTK and LTM, re¬ spectively, the potassium, magnesium and sodium salts of lignosulfonate (50% aqueous solutions), Scott Paper Co., Forest Chemical Products; Marasperse CR ® and Marasperse CBOS-3 ® , sodium lignosulfonate, American Can Co.; Polyfon O ® , Polyfon T ® , Reax 88B ® , Reax 85B ® , Reax 100M ® , sodium salts of lignin sulfonate and Reax C-21 ® , calcium salt of lignin sul¬ fonate, Westvaco Polychemicals; Orzan S and Orzan A, the sodium and ammonium salts of lignosulfonate, ITT Rayonier, Inc. The actual quantity of colloid is not critical and any amount which is effective in enhancing the stability of the emulsion can be used. It is most convenient to use be¬ tween about 0.1% and about 5.0% colloid by weight in terms of the aqueous phase. The droplet size in the emulsion is not critical to the invention. For greatest utility of the final product, the droplet size will fall in the range of about 0.5 microns to about 4000 microns in diameter. The preferred range for most pesticidal applications is from about 1 micron to about 100 microns in diameter. The emulsion is prepared by the use of any conventional high shear stirring device. Once the de¬ sired droplet size is attained, mild agitation is generally sufficient to prevent droplet growth throughout the balance of the process. E. Wall Formation Once the dispersion and desired droplet size are attained, the system is acidified to a pH of between about 0 and about 4.0, preferably between about 1.0 and about 3.0. This causes the prepolymer to polymerize by self-condensing in situ and form a shell completely enclosing each droplet. Acidification can be accomplished by any suitable means, in¬ cluding adding any acid which is water-soluble, including formic acid, citric acid, hydrochloric acid, εulfuric acid, phosphoric acid, and the like. Acidification can also be achieved by the use of acidic dispersants or surface-active agents, provided that such components are added to the syεtem after the emulsion haε been formed. As the polymer wall becomes more rigid, contact be¬ tween the active groups on the prepolymer becomes increasingly more difficult. Thus, the in situ self-condensation polymer¬ ization reaction is self-terminating and is generally allowed to run to completion. The reaction can be arrested before completion, however, by raising the pH. In this manner, the wall tightness, rigidity, and permeability can be controlled. This can also be accomplished in most cases by a wall modifier as described above. The rate of the in situ self-condensation polymer¬ ization reaction increases with both acidity and temperature depending upon the pH. The reaction can therefore be con¬ ducted anywhere within the range of about 20°C to about 100°C, preferably between about 40°C and about 70°C. The reaction will generally be complete within a few hours, although with high acidity and high temperature, the reaction can be com¬ pleted within minutes. Once the capsules are formed, they can be stored and used as an aqueous dispersion, or filtered and recovered as dried capsules. In either form, the capsules are useful and effective in the εlow release of the core liquid. Dis¬ persions are preferably stabilized by dispersants dissolved in the continuous phase. Since most dispersants are more effective in neutral or baεic εolutions, it iε preferable to raiεe the pH of the diεpersion once the wall has been formed. This is accompliεhed by any water-soluble base. Any conven¬ tional dispersant can be used. Typical diεperεants include lignin sulfonates, polymeric alkylnaphthalene sulfonates, sodium naphthalene sulfonate, poly ethylene bis-naphthalene sulfonate, and sodium-N-methyl-N-(long chain acid) taurates. A unique feature of the process of the invention is that the solid permeable polymer shells enclosing the organic phase droplets are formed by means of condensation of the prepolymer in the organic phase adjacent to the interface formed between the organic phase droplets and the aqueous phase solution. This is a consequence of the prepolymers being dissolved in the organic phase. The advantages of forming the polymer shells on the organic side of the interface are several. The first is that the process itself is more easily controlled than the prior art processes, which involve wall-forming condensation in the aqueous phase. When the condenεation takes place in the aqueous phase, the wall-forming polymer can deposit upon the walls of the container in which the emulsion is present, on the agitator or any other εtructure which may be present, in addition to depositing on the droplets. In contrast, the wall-forming polymer that condenses on the organic side of the interface does not deposit on any of the container walls or other structures. Additionally, when the condensation takes place in the aqueous phase, as in the prior art, a reduced amount of dispersed organic phase must be used inasmuch as if a higher dispersed organic phase content is utilized, the dispersion gets too thick and gels, thus effectively preventing forma¬ tion of the microcapsules. Condensation on the organic side of the interface thus allows higher dispersed organic phase loading to be obtained because a gel is not formed in the aqueous phase. In the examples εet forth herein, in which the or¬ ganic phaεe containε a peεticide, a higher loading of organic phase resultε in a more concentrated peεticide formulation. This enableε εubstantial cost savingε to be achieved in manu¬ facturing, packaging and transportation. The following examples are offered as illuεtrative of both the process and product of the preεent invention, and are intended neither to define nor limit the invention in any manner. EXAMPLE 1 An aqueous εolution was prepared, comprising 2.0% (weight) Gelvatol ® 40-20 and 0.3% Tergitol ® 15-S-7, with a total solution weight of 300 g. Gelvatol ® 40-20 is a poly- vinyl alcohol protective colloid (degree of hydrolyεis 73-77%) , with an average molecular weight of about 3000, ob¬ tained from Monsanto Company, Indian Orchard, Massachusetts. Tergitol ® 15-S-7 is a nonionic surfactant consisting of a polyethylene glycol ether of a linear alcohol, obtained from Union Carbide Chemicals and Plastics Company, New York, New York. In a separate vesεel, 100 g of S-ethyl diisobutyl- thiocarba ate (a herbicide known by the common name "butyl- ate") and 50 g of Beckamine ® 21-625 were blended into a homo¬ geneous solution. Beckamine ® 21-625 is a 70-75% n-butanol solution of a partially butylated ureaformaldehyde prepolymer in which the degree of butylation is approximately 80-90%, obtained from Reichhold Chemicals, Inc., White Plains, New York. The thiocarbamate/prepolymer (organic) solution was added to the aqueous solution and an emulsion was formed by means of a high shear εtirrer, the organic εolution forming the dispersed phase with dropletε ranging in εize from 5 to 40 microns in diameter. While mild agitation was maintained, the pH of the emulsion was adjusted to 2.0 with concentrated hydrochloric acid and the temperature was raised to 50 β C for three hours. The resulting εuεpension was then allowed to cool to room temperature and concentrated aqueous sodium hydroxide was added to raise the pH to 7.0. Observation of the suspension under both a labora¬ tory microscope and an electron microεcope revealed diεcrete, roughly spherical, fully enclosed capsuleε with εmooth- εurfaced outer walls. The capsules were about 5 to 40 microns in diameter and although some were touching each other, none were fused together. EXAMPLE 2 An organic solution was prepared, comprising 162.2 g of 2-methoxy-9-(p-isopropylphenyl)-2,6-dimethylnonane (a known insect maturation inhibitor—see U.S. Patent No. 4,002,769, issued January 11, 1977, to Schwarz et al.) and 48.0 g of Resimene ® X-918. The latter is a 70% n-butanol solution of a partially butylated ureaformaldehyde prepolymer with a degree of butylation of approximately 80-90%, a prod¬ uct of Monsanto Plastics and Resins Company, Newport Beach, California. This εolution waε added to an aqueous solution com¬ prising 168.1 g of water and 1.87 g of Gelvatol ® 40-20 and an emulsion was formed as in Example 1, with droplets ranging in diameter from 1 to 40 microns. To this emulsion was added 20 g of water containing 1.87 g each of the disperεants Lo ar NCO ® and Darvan ® ≠2. The former is a product of Diamond Shamrock Chemical Company, Nopco Division, Morristown, New Jersey, and is a sodium salt of a condensed mononaphthalene sulfonic acid. The latter is a product of R. T. Vanderbilt Company, Inc. , Norwalk, Connecticut, and is comprised of sodium salts of polymerized subεtituted benzoic alkyl sulfonic acids. A 5% hydrochloric acid solution was added to lower the pH of the emulsion to 2.0 and the temperature was raiεed to 50°C with continued εtirring for three hours. The reεulting diεperεion waε then cooled to room temperature and concentrated cauεtic solution waε added to raiεe the pH to 9.0. Microεcopic obεervation of the diεperεion revealed fully formed, diεcrete capεuleε as in Example 1. EXAMPLE 3 The organic εolution for thiε example consisted of 139.9 g of O-ethyl S-phenyl ethylphosphonodithioate (a com¬ mercial insecticide alεo known by the common name "fonofoε") and 39.9 g of Reεimene ® X-918. Thiε εolution was emulsified in an aqueous solution conεiεting of 200 g of water and 2.35 g of Gelvatol ® 40-20 to a droplet εize of 1 to 40 microns, and 35 g of water containing 2.35 g each of the dispersantε Lomar NCO and Darvan #2, aε well aε 2.4 g of p-toluene εul- fonic acid, was added. The temperature was raised to 60°C and εtirring was continued for three hours. The dispersion was then allowed to cool to room temperature and the pH was raised to 9.0 with caustic solution. Microscopic observation of the dispersion revealed fully formed, discrete capsules aε in Example 1. EXAMPLE 4 The organic solution for this example consisted of 156 g of HI-SOL ® 4-3 (solvent) and 43.5 g of Beckamine ® 21-625. The former iε a heavy aromatic naphtha, with boiling temperature ranging from 238°C to 286°C, a product of Aεhland Chemical Company, Induεtrial Chemicalε and Solventε Diviεion, Columbus, Ohio. This solution was emulsified in an aqueous εolution consisting of 194.6 g of water, 3.9 g of Gelvatol ® 40-20, and 7.8 g of Darvan #2, to a droplet size of 1 to 40 microns. The pH was adjusted to 2.0 with a 5% solution of hydrochloric acid and the temperature was raised to 50°C with continued stirring for three hours. The disperεion waε then allowed to cool to room temperature and the pH was raised to 9.0 with caustic εolution. SUBSTITUTESHEET Microεcopic obεervation revealed fully formed, diε- crete capεules as in Example 1. EXAMPLE 5 An aqueous εolution conεisting of 251.6 g of water, 5 g of Gelvatol ® 40-20, and 2.5 g of Ta ol ® SN was heated to 50°C. Tamol ® SN is a dispersant identified as a sodium salt of a condensed naphthalene sulfonic acid, obtained from Rohm and Haas Company, Philadelphia, Pennsylvania. To this heated aqueous solution was added an organic solution consiεting of 173.4 g of S-ethyl diiεobutylthiocarbamate (butylate) , 7.5 g of N,N-diallyl dichloroacetamide, and 22.5 g of Resimene ® X-918. The thiocarbamate/acetamide combination is a known herbicide/antidote combination—see U.S. Patent No. 4,021,224, issued May 3, 1977, to Pallos et al. An emulεion was formed by means of a high-speed stirrer as in the above examples, to a droplet size of 1 to 40 micronε. The high temperature waε maintained and the pH waε lowered to 2.0 with 5% hydrochloric acid. After three hourε of additional stirring, the diεper¬ εion waε cooled to room temperature and the pH waε raiεed to 9.0 with cauεtic solution. Microscopic observation revealed fully formed, dis¬ crete capsules aε in Example 1. EXAMPLE 6 In this example, an additional feature is demon- εtrated—the inclusion of an organic solvent (kerosene) in the organic phase, the solvent thus becoming part of the en¬ capsulated liquid. The aqueous solution was prepared with 177.12 g of water, 2 g of Gelvatol ® 40-20, and 2 g of Darvan ≠2. The organic solution was prepared with 132.74 g of S-ethyl hexa- hydro-lH-azepine-1-carbothioate (a commercial herbicide known by the common name "molinate") , 44.25 g of kerosene, and 35.48 g of Beetle ® 1050-10. The latter iε a 60% n-butanol solution of a partially butylated urea-formaldehyde prepolymer in which the degree of butylation is approximately 70-90%, obtained from American Cyanamid Company, Reεinε Department, Wayne, New Jerεey. The organic solution was emulsified in the aqueouε solution by means of a high shear stirrer to an average drop¬ let diameter of 18 micronε, and 19.68 g of water containing 2 g of DAXAD ® LAA waε εlowly added, lowering the pH of the emulsion to 1.7 DAXAD ® LAA is a diεperεant in acidic form, identified as a polymerized alkyl naphthalene sulfonic acid, a product of W. R. Grace and Company, Organic Chemicals Divi¬ sion, Lexington, Massachusetts. The emulsion temperature waε then raiεed to 50°C for three hours with continued stirring. The dispersion thus formed was cooled to room temperature and the pH was raised to 7.5 with cauεtic εolution. Microscopic observation revealed fully formed, dis¬ crete capsules as in Example 1. EXAMPLE 7 In this example, two additional features are demon¬ strated—the inclusion of kerosene aε in Example 6 and the addition of a wall-modifying component (castor oil) to the prepolymer. The aqueous solution was prepared with 181.6 g of water, 2 g of Gelvatol ® 40-20, and 2 g of Darvan #2. The or¬ ganic solution was prepared with 132.7 g of S-ethyl hexa- hydro-lH-azepine-1-carbothioate, 44.25 g of kerosene, 22.97 g of Beetle ® 1050-10, and 6.9 g of castor oil. An emulsion with an average droplet diameter of 18 microns was formed, and 20.2 g of water containing 2 g of DAXAD ® LAA was added, lowering the pH to 1.7. The emulsion temperature was then raiεed to 50°C for three hourε with continued εtirring. The resulting dispersion was then cooled to room temperature and the pH was raised to 7.5 with caustic solution. Microscopic observation revealed fully formed, dis¬ crete capsuleε aε in Example 1. EXAMPLE 8 The organic solution consisted of 154 g of butyl- ate, 6.7 g of N,N-diallyl dichloroacetamide, and 47.6 g of Resimene ® X-918 (εame ingredientε as Example 5) . This solu¬ tion was emulsified in 197.8 g of a 4.0% (by weight) aqueous εolution of Darvan ≠2 to a droplet εize of 1 to 40 micronε. The pH of the dispersion was then a juεted to 2.0 with a 5% εolution of hydrochloric acid and the temperature was raised to 50°C with continuous stirring for three hours. The dis¬ persion was then allowed to cool to room temperature and the pH was raised to 9.0 with cauεtic εolution. Microεcopic obεervation of the dispersion revealed fully formed, discrete capsules as in Example 1. The following examples demonstrate production of microencapsulated products containing the anilide herbicide acetochlor [α-chloro-2'-methyl-6'-ethyl-N(ethoxymethyl)acetan¬ ilide]. EXAMPLE 9 The organic solution consiεted of 189.4 g of aceto¬ chlor (95.9% purity), 1.3 g of Mercaptate Q-43 Eεter and 12.7 g of Beetle ® 80 reεin. Mercaptate Q-43 Eεter is penta- erythritol tetrakis (mercaptopropionate) , εold by Cincinnati Milacron Chemicalε. Beetle ® 80 resin is a highly butylated (about 95%) liquid urea-formaldehyde resin obtained from American Cyanamid Company, Resins Department, Fort Wayne, New Jersey. The organic solution was emulsified with continuouε stirring in an aqueous solution consiεting of 5.7 g of εodium lignoεulfonate (DAXAD ® 23, obtained from W. R. Grace & Co.), 0.9 g of the sodium dialkylnaphthalenesulfonate (Petro BAF ® , obtained from Petrochemicalε Company, Inc. , Fort Worth, Texas) and 185.1 g of water. The pH of the resulting emulsion was lowered to 2.0 with 1.5 g of concentrated sulfuric acid. The emulsion temperature was then raised to 50°C for three hours with continuous εtirring. The reεulting dispersion was cooled to 25°C and the pH was raised to 7 with 50% caustic solution. Microscopic observation revealed fully formed dis¬ crete capsules as in Example 1. EXAMPLE 10 The organic solution consisted of 25.1 g of Beetle ® 1050-10 resin and 189.4 g of acetochlor (95.9% purity). It was emulsified in an aqueous solution consiεting of 5.4 g of DAXAD ® 23, 0.9 g of Petro BAF ® and 173.2 g of water. The pH of the reεulting emulsion waε lowered to 2.0 with 1.4 g of concentrated εulfuric acid. The emulsion tem¬ perature was raised to 50°C for three hours with continuouε εtirring. The diεperεion was cooled to 25°C and the pH raised to 7.0 with caustic εolution. Microscopic observation revealed fully formed dis¬ crete capsules aε in Example 1. EXAMPLE 11 The organic εolution consisted of 13.4 g of Beetle ® 80 resin, 1.4 g of Mercaptate Q-43, 27.9 g of N,N-diallyl dichloroacetamide (95% purity) and 171.6 g of acetochlor (92.6% purity). It was emulsified in an aqueous solution consisting of 1.84 of REAX ® 100 M (sodium lignosulfonate ob¬ tained from Westvaco Company, Charleston Heights, South Carolina), 0.9 g of Petro BAF ® , and 180 g of water. The pH of the resulting emulsion waε lowered to 2.0 with 0.5 g of concentrated εulfuric acid. The emulεion tem¬ perature was raised to 50°C for three hours with continuous stirring. The resulting dispersion was cooled to 25°C and the pH was raised to 7.0 with caustic solution. Microscopic observation revealed fully formed diε- crete capsuleε as in Example 1. EXAMPLE 12 The organic solution consiεted of 250 g Beetle ® 1050-10 reεin, 27.9 g of N,N-diallyl dichloroacetamide (95% purity) and 171.6 g of acetochlor (92.6% purity). It waε emulεified in an aqueous solution consiεting of 1.84 g of REAX ® 100 M, 0.9 g of Petro BAF and 180.96 g of water. The pH of the emulsion was lowered to 2.0 with 0.5 g of concentrated sulfuric acid. The emulsion temperature was raised to 50°C for three hours with continuous stirring. The resulting dispersion was allowed to cool to 25°C and the pH waε raiεed to 7.0 with cauεtic εolution. Microεcopic obεervation revealed fully formed dis¬ crete capsules as in Example 1. EXAMPLE 13 This example demonstrateε the production of micro¬ capsules using a melamine-formaldehyde prepolymer. The organic solution consisted of 12.7 g of butylated melamine-formaldehyde resin prepolymer (Cymel ® 1156, from American Cyanamid Co.), 184.4 g of acetochlor (95.9% purity) and 1.3 g of pentaerythritol tetramercaptopro- pionate (Evans Che etics Corp., New York, New York). It was emulεified in an aqueous εolution consisting of 5.7 g of sodium lignosulfonate (DAXAD ® 23), 0.9 g of Petro BAF ® , and 185.1 g of water. The pH of the emulsion was lowered to 2.0 with 1.4 g of concentrated sulfuric acid. The emulsion temperature was raised to 50°C for three hours with continuous stirring. The reεulting diεperεion waε allowed to cool to 25 β C and the pH waε raised to 7.0 with caustic εolution. Microεcopic obser- vation revealed fully formed discrete capsules as in Example 1. BIOLOGICAL EVALUATION Greenhouse evaluation The compositions of Examples 9 and 10 were evaluated in the greenhouse by pre-emergence surface treatment for in¬ jury to corn, in comparison with compositions containing acetochlor alone, and acetochlor with the herbicide antidote N,N-diallyl dichloroacetamide. The compositions of Examples 9 and 10 do not contain an antidote. The comparison composi- tionε were: A) an emulεifiable concentrate containing 6.4 poundε acetochlor per gallon in Aromatic 100 εolvent (Exxon Chemical Co., Houston, Texas). B) Technical grade material containing acetochlor and N,N-diallyl dichloroacetamide in a 6:1 weight ratio. The corn varieties utilized were Garst 8711 and Pioneer 3475. Corn seeds were planted in rows in a flat in 63-18-9% sand-εilt-clay εoil. Before planting, the flatε were εprayed with the teεt compoεitionε, appropriately diluted with water (Compoεition A and Exampleε 9 and 10) or dissolved in a water-acetone mixture (Composition B) to provide application rates of 1 and 2 kg/ha acetochlor. The flats were rated 8 and 16 days after treatment. Rating was performed visually using a εcale of 0-100 with 0 repreεenting no control as com¬ pared to an untreated test flat, and 100 to complete kill. The results are expressed in Table 1 below. E TABLE 1: Greenhouse Bioassav of Acetochlor Formulations -- Corn Inlurv The performance, i.e., ability to reduce phytotox- icity to corn, of the microencapsulated compoεitions of this invention, without an antidote, at timeε approached that of the nonencapsulated antidoted acetochlor (Composition B) , particularly in the 8-day ratings. Field evaluation The compositions of Examples 9 and 10 were tested in compariεon to an emulsifiable concentrate containing only acetochlor (Compoεition A) , in a field near Viεalia, California. The εoil composition was 60-26-14% sand-silt- clay. Weeds which appeared and were evaluated were wild proso millet (Panicum miliaceum) ("WPM") and Rox-orange εorghum (Sorghum vulgare v. Rox-orange) ("ROS") . The corn variety utilized waε Pioneer 3475. Application of the compoεitions was by pre-emergence surface treatment, at the indicated rates. Evaluation was done 7 and 15 days after treatment, using the same scale. The resultε are contained in Table 2, below. A s The resultε of theεe teεts demonstrate that the compositions of this invention exhibited lesε injury to corn than the acetochlor emulεifiable concentrate, while maintain¬ ing weed control. SUBSTITU

Claims: WHAT IS CLAIMED IS: 1. A microencapεulated herbicidal composition com¬ prising acetochlor within a porous shell, having a slow re¬ lease of said herbicide through said shell, εaid composition having been prepared by a process comprising: a) providing an organic solution comprising acetochlor and an etherified amino resin prepolymer disεolved therein in which from about 50% to about 98% of the methylol groupε of εaid prepolymer have been etherified with a c ~ c 10 alcohol; b) creating an emulεion of εaid organic solu¬ tion in a continuous phaεe aqueous solution comprising water and a surface-active agent, wherein said emulsion compriseε discrete dropletε of εaid organic solution dispersed in εaid continuous phase aqueous solution, there being formed thereby an interface between the discrete droplets of organic solu¬ tion and the εurrounding continuous phase aqueous solution; and c) causing in situ self-condenεation and cur¬ ing of said amino resin prepolymer in the organic phase of εaid discrete droplets adjacent to said interface by simulta¬ neously heating said emulsion to a temperature between about 20°C to about 100°C, and adding to said emulεion an acidify¬ ing agent and maintaining εaid emulεion at a pH of between about 0 to about 4 for a εufficient period of time to allow substantial completion of in εitu condenεation of said resin prepolymers to convert the liquid droplets of εaid organic solution to capsules consisting of solid permeable polymer shells enclosing acetochlor. 2. A compoεition according to Claim 1 which fur¬ ther comprises a herbicide antidote suitable for use with acetochlor. 3. A compoεition according to Claim 2 in which the antidote iε a dichloroacetamide antidote, a dioxolane anti¬ dote or 1,8-naphthalic anhydride. 4. A compoεition according to any of Claimε 1-3 wherein the amino resin prepolymer is a urea-formaldehyde, melamime- formaldehyde, benzoguanamine-for aldehyde or glycoluril-formaldehyde prepolymer. 5. A composition according to Claim 4 wherein the amino resin prepolymer iε a urea-formaldehyde prepolymer. 6. A compoεition according to Claim 4 wherein the amino reεin prepolymer is a melamine-formaldehyde prepolymer. 7. A composition according to any of Claims 1-6 in which from about 70% to about 90% of the methylol groups of the prepolymer have been etherified. 8. A composition according to Claim 7 in which the alcohol with which the methylol groupε of the prepolymer have been etherified iε n-butanol or isobutanol. 9. A composition according to any of Claimε 1-8 in which the organic solution containε a wall-modifying agent which εerveε to modify the character of the wall by varying itε permeability to the acetochlor. 10. A proceεs for the microencapsulation of aceto¬ chlor within a porous εhell to effect a εlow release of εaid herbicide through εaid shell which comprises: a) providing an organic εolution compriεing said acetochlor and an etherified amino resin prepolymer dis¬ solved therein in which from about 50% to about 98% of the methylol groups of εaid prepolymer have been etherified with a C4.-C1, ft 0 alcohol; b) creating an emulεion of said organic solu¬ tion in a continuous phase aqueous εolution compriεing water and a εurface-active agent, wherein said emulsion comprises discrete droplets of said organic solution dispersed in said continuous phase aqueous εolution, there being formed thereby an interface between the discrete droplets of organic εolu¬ tion and the surrounding continuous phase aqueous solution; and c) causing in situ self-condensation and cur¬ ing of said amino resin prepolymer in the organic phase of said discrete droplets adjacent to εaid interface by simulta- neously heating εaid emulεion to a temperature between about 20°C to about 100°C, and adding to εaid emulsion an acidify¬ ing agent and maintaining said emulsion at a pH of between about 0 to about 4 for a sufficient period of time to allow subεtantial completion of in situ condensation of said resin prepolymers to convert the liquid droplets of said organic solution to capsules conεiεting of εolid permeable polymer shells enclosing acetochlor.

Inventors: RODSON MARIUS, SCHER HERBERT B, RODSON, MARIUS, SCHER, HERBERT, B.
Applicants: ICI AMERICA INC, ICI AMERICAS INC.
Application year: 1991
Main classification: A01N25
IPC codes: A01N25, A01N37, A01P13