MICROENCAPSULATION

Abstract: Biologically active material is encapsulated by first blending such material with hardenable liquid, e.g., hardenable by polymerization, followed by dispersing the resultant composition in a non-flowable but stirrable aqueous gel, and hardening the dispersed droplets containing the biologically active material to form a stable dispersion of microcapsules containing such material in the gel.

Description: TITLE MICROENCAPSULATION This invention relates to the microencapsulation of biologically active material within polymer particles,- which is useful for slow release of the biologically active material. Handjani et al., U.S. 5,139,783, discloses alginate capsules which may contain a cosmetic active agent such as a biological compound, and which may be contained in an external gel phase. The present compositions differ from those of Handjani et al. in that the biologically active material is introduced during the formation of the microcapsules, not afterwards. SUMMARY OF THE INVENTION The present invention provides a process for encapsulating biologically active material in which equipment costs and power needs are low, effective encapsulation of insoluble biological active material is accomplished, microcapsule dispersions of long-term stability are obtained, and such dispersions can be advantageously used. One embodiment of the present invention is-the process for microencapsulating biologically active material, comprising forming a thixotropic gel which is essentially nonflowable but is stirrable without fracturing, forming a substantially homogeneous carrier composition of said biologically active material and hardenable immiscible organic liquid, combining said gel and said homogeneous carrier composition one with the other in a proportion in which said gel is the continuous phase and said homogeneous carrier composition is the dispersed phase in the resultant combination in the presence of stirring of said gel to finely disperse said homogeneous carrier composition as droplets in said gel without causing appreciable fracture of said gel, and hardening said droplets to obtain as a result thereof a stable dispersion of microencapsulated biologically active material. Another embodiment of the present invention is the composition comprising a continuous phase of essentially non-flowable aqueous thixotropic gel and fine microcapsules of polymer stably dispersed therein, said microcapsules containing a pesticide. DETAILED DESCRIPTION OF THE INVENTION In the process and product of the present invention, the aqueous gel is formed by mixing sufficient water-soluble gelation agent with water to form the gel in the viscous, essentially non-flowable form desired by the present invention. Insufficient amounts of gelation agent merely thicken the resultant solution, such as by forming a colloid, so that the solution is still readily pourable. In such thickened solution r unlike gels, suspended droplets or suspended solids eventually settle upon standing over time, e.g., one day or one month. In greater detail, the essentially non-flowable state of the gel desired for the present invention can be defined with reference to the pouring characteristics of the gel when housed in a cylindrical glass container having an inner diameter of about 5 cm. When the container is inverted after having stood undisturbed for at least 1/2 hour, the essentially non- flowable gel will not pour for a period of at least 3 seconds, preferably at least 5 seconds. When the gel contents of the container have been disturbed, such as by stirring of the gel, the gel may be pourable as a slow-moving mass resembling partially crystallized honey. Preferably, even immediately after such disturbance, the gel remains nonpourable, i.e., essentially rigid for at least 5 seconds, upon inversion of the container holding the gel. While essentially non-flowable, the gel is also stirrable without causing fracture of the gel, i.e., the gel can be stirred without causing it to break into pieces. Fracture detracts from the ability of the gel to disperse the hardenable organic liquid as will be described later herein. The non-flowable and stirrable natures of the present gels preferably exist at room temperature, e.g, 15° to 25°C. The gels of the present invention are thixotropic in that they are rigid and non-pourable, thin appreciably upon stirring, but then become rigid and non-flowable again upon standing. Examples of gelation agents include clays, cellulose derivatives, polyacrylic acids, poly- saccharides, gelatins, gums, starches, alginates, ___: polyvinyl alcohols, sodium stearate polyethylene glycol and ethylene maleic anhydride copolymer. Preferred gelation agents include carboxymethyl cellulose and ethylene maleic anhydride copolymer after having been treated (hydrolyzed) with aqueous base such as NaOH to form the salt of the copolymer. Without the aqueous base treatment, the copolymer may thicken water, but tends not to gel it unless used in very large amounts, e.g., 25% by weight based on the weight of the water in the gel. The amount of gelation agent required to make the essentially non-flowable, stirrable gel will depend on the gel-effectiveness of the particular agent. Generally the effective amount to provide these characteristics will fall within 1 to 15% based on the weight of the water plus gelation agent. When the agent is carboxy ethyl cellulose salt, the preferred amount is from 8 to 12% by weight, and when the copolymer salt is the agent, the preferred amount is from 1 to 5% by weight. The preferred aqueous gels are 10% by weight aqueous metal salts of carboxymethyl cellulose and gels derived from hydrolysis of crosslinked ethylene-maleic anhydride copolymers (EMA ® -91 and EMA ® -61, Monsanto Chemical Co.) . The dispersion or solution of biologically active material in the hardenable organic liquid is formed by mixing powdered or liquid active material with the hardenable liquid. When the material is soluble in the liquid, the carrier composition is a solution, and when the material is insoluble in the liquid, the carrier composition is a dispersion. In the latter case, a dispersing agent may be used. The proportions of material and hardenable liquid can vary widely to obtain the ultimate result of forming microcapsules encapsulating the biologically active material. The selection of organic liquid depends on which embodiment of hardening is employed. The organic liquid includes a dissolved polymer or a polymer precursor. The hardening of the liquid involves either (1) precipitating the polymer out of its solution, e.g., by removing the solvent, and having the active ingredient dissolved or dispersed within the polymer, or (2) polymerizing the polymer precursor, to form polymer microcapsules which contain the biologically active material in the gel as is hereinafter described. The carrier composition is a liquid by virtue of the presence of the hardenable organic liquid and is mixed with the gel to form a fine dispersion of droplets of the carrier composition within the gel. The fine dispersion can be achieved by stirring the gel as the liquid composition is added. Preferably, slow speed stirring is all that is necessary. The liquid carrier composition rapidly disperses into fine droplets containing the biologically active material within the gel. The stirring does not force the active material out of the hardenable liquid when the hardenable liquid is a dispersion rather than a solution of the biologically active material. The gel is much more viscous than the liquid carrier composition, the latter of which preferably has a viscosity ranging from that of about 1 to 600 centepoises (cps) at 25°C. The viscosity of the gel used in the present invention is about at least two times the viscosity of the liquid composition (i.e., about 2-1200 cps at 25°C) . The viscosities described herein are measured using the Brookfield viscometer. Preferably, the stirring to form the gel composition is accomplished by a simple paddle mixing blade operating at a low rotational speed, e.g., as low as 60 rpm, and generally not higher than 150 rpm. Higher speeds can be used but without achieving any advantage and at the disadvantage of increasing power expenditure, foaming, and the risk of separating dispersed biologically active material, if present, from the droplets of hardenable organic liquid. In small batches, the dispersion of droplets can be usually obtained by hand mixing by using a spatula. The proportions of the homogeneous carrier composition and aqueous gel are such that the gel is present in an amount sufficient to form the continuous phase and the carrier composition forms the dispersed phase. Generally, this requires that the dispersed phase not be more than at least 50% by weight of the total composition (continuous phase plus dispersed phase) . The proportion of homogeneous carrier composition present as droplets depends on the concentration of bio ' logically active material desired. Generally, at least 0.5%, and preferably at least 1% of biologically active material based on the weight of the total composition is desired. The high viscosity of the gel, on the order of at least 1000 centapoises, keeps the droplets separated, unaggregated, and well dispersed in the gel. The dispersion facilitates the conversion of the droplets to microcapsules by the hardening of the organic liquid in the droplets. The average particle size of the microcapsules obtained by the process of the present invention and present in the product of the present invention is from about 1 to 1000 microns and desirably is in the range of 1 to 200 microns in average diameter. The smaller capsule sizes are desirable when the preferred application technique is to dilute the gel/microcapsule composition; followed by spraying. The biologically active material can be present as a core within the microcapsules, i.e., surrounded by polymer shell, as would occur if the active material is present as a dispersion within the original hardenable organic liquid. If the active material is present in solution in the organic liquid, the hardening of the liquid by polymer precipitation often creates microcapsules within which the active material is homogeneously dispersed in a polymer matrix. The method by which the organic liquid hardens, e.g., interfacial polymerization, solvent evaporation, can also influence the physical structure of the microcapsules. Microcapsules formed by interfacial polymerization usually have a polymer shell encapsulating the active ingredient. On the other hand, microcapsules formed by evaporation of a solvent tend to be uniform dispersions of polymer and active ingredient. To describe the hardenable liquid in greater detail, in a preferred embodiment the hardening is achieved by interfacial polymerization. In this embodiment, the organic liquid comprises at least one water-insoluble polyfunctional, polycondensation monomer, which contains at least two isocyanate or acid chloride groups or mixtures of such polyfunctional compounds and, optionally, a water-insoluble organic solvent. The organic liquid/active material composition is then mixed with the aqueous gel as described above to give droplets of microencapsulated active material suspended in the gel matrix. Then, a water-soluble complementary monomer or mixture of monomers is added, by stirring into the gel, each containing at least two primary or secondary amino or hydroxyl groups, capable of undergoing interfacial polymerization with the monomer system in the droplets. Continued stirring of the gel after the dispersion is formed and during the interfacial polymerization reaction surprisingly does not cause agglomeration of the dispersed droplets. The preferred water-soluble complementary amine is diethylene triamine. Examples of polyfunctional monomers include toluene diiso¬ cyanates, sebacoyl chloride, hexamethylene diiso- cyanate, * and terephthaloyl chloride. The preferred polyisocyanate is polymethylene polyphenyl isocyanate available as PAPI ® (Dow Chemical Co.). In another embodiment of interfacial polymerization to harden the liquid monomer present in the dispersed droplets in the gel, an interfacial polymerization process is carried out using at least one polyfunctional isocyanate without subsequent addition of a complementary water-soluble monomer system. In this case, some of the isocyanate groups hydrolyze in the aqueous gel to give amino groups which produce polyurea by self-condensation. In still another embodiment, the organic liquid is a solution of polymer in water-insoluble solvent and active material is dissolved or dispersed therein. upon dispersion of this organic liquid in the gel, the liquid takes the form of discrete droplets containing solvent, polymer, and active material suspended in the gel. The droplets are hardened by evaporating the solvent by heating of the gel composition, producing hard microcapsules. The preferred polymers or this process are poly ethyl ethacrylate, polylactic acid, or lactic acid/glycolic acid copolymers. The preferred polymer solvent is methylene chloride. The methods of interfacial polymerization and precipitation from solution to form polymer capsules follow the methods related to those disclosed in U.S. 3,577,515, 4,285,720, and 3,523,906. These references do not disclose the present gel dispersion compositions, the manner of making them, or the advantages of the present invention described herein. In another embodiment of hardening of the organic liquid in the dispersed droplet by precipitation of the dissolved polymer, the organic solvent for the polymer is selected to be slowly water soluble, whereby the solvent slowly migrates into the gel matrix, spontaneously liberating microcapsules containing active material without heating. Examples of such water-soluble solvents include ethyl acetate, tetrahydrofuran, and cyclohexanone. While these solvents may be miscible in water and thereby in the aqueous gel, when present in solution with the water- insoluble polymer, the droplets are immiscible with the aqueous gel, which enables the active/organic liquid carrier composition to form as dispersed droplets in the aqueous gel. When fast dissolving solvents which are miscible with water in all proportions are used (e.g., methanol, acetone, etc.), agglomeration of the droplets tends to occur during stirring. The stability of the droplet dispersion in the aqueous gel is maintained when the droplets harden to microcapsules. The resultant product of the present invention, i.e., the dispersion of microcapsules containing the biologically active material in the aqueous gel, is very stable as indicated by the lack of settling during three months in storage at room temperature as determined by visual inspection. With conventional aqueous suspensions, settling generally occurs overnight shortly after preparation, unless a thickener is employed, and even then settling often occurs on prolonged storage. The biologically active material and the organic liquid, whether monomer, polymer, solvent or any of these present in the substantially homogeneous carrier composition added to the gel should be inert to one another. Preferably the active material is sufficiently inert (non-reactive) to water that even if active material is present at the surface of the dispersed droplets and microcapsules form therefrom, there is no appreciable reaction between the active material and the surrounding gel. Within this characteristic of non-reactivity, a wide variety of biologically active materials can be used in the present invention to provide therapeutic benefit to the subject to be contacted with the microcapsules. Examples of biologically active materials include agrichemicals such as herbicides, fungicides, insecticides, nematicides, acaricides, miticides, virusides, algicides, bactericides, plant growth regulants, and mixtures thereof and pharmaceuticals. The gel composition product of the present invention is especially useful as a way of handling agrichemicals in that leakage from exterior packaging is minimized. The gel composition can be packaged in cartridges, toothpaste-type tubes, and bags, and can be directly applied from these containers. The gel composition when applied as such have several advantages over conventional agricultural formulations. They may be directly applied to trees or foliage or crevices where insects may enter and because of the thixotropy of gels, be expected to remain in place longer than conventional agricultural sprays. These gels could be applied by direct injection into furrows. Also, these formulations can be mechanically injected into or mixed into water for dilution prior to application by spraying. The polymer component controls the release of the active material present in the microcapsules. As is the case with controlled release microcapsules of the prior art, the rate of release can be adjusted over a wide range by the amount of polymeric barrier used for a given weight of active, the size of the micro¬ capsules, the type of polymer used and use of optional adjuvants to control the porosity of the polymer. The advantages cited in the literature for conventional controlled release systems, such as reduced mammalian toxicity, reduced worker exposure, and prolonged activity, are also true for the gel compositions of the present invention. This invention provides low cost controlled release compositions comprising controlled release microcapsules suspended in gels. The process for preparing these compositions is unusually simple and uses conventional, low-intensity mixing equipment such as paddle stirrers, kneaders, and the like. The low shear mixing permits the use of not only solutions of active ingredients, but also dispersions of active ingredients, without production of unencapsulated active. EXAMPLE 1 To 50 g of the 10% by weight sodium salt of carboxymethyl cellulose, low viscosity range, food grade, and 0.7 degrees of substitution (CMC LF-7) aqueous gel, which was non-flowable but stirrable, was added with hand stirring for about 30 seconds, a mixture of 6 g of PAPI ® 901 and 0.3 g powdered bensulfuron methyl herbicide to produce a fine dispersion of droplets of PAPI ® and herbicide mixture in the gel. Then with constant hand stirring (using a spatula) was added 0.2 g diethylene triamine, generating crosslinked 2-50 micron diameter polyurea microcapsules. The PAPI ® and herbicide mixture was allowed to stand overnight in the gel. A sample of the microcapsule-gel mixture aged in excess water released only about 20% of the biologically active material (hereinafter "active") in 40 hours and 75% in 120 hours. Release rates were measured by dispersing the final composition in sufficient pH 7 water to obtain a 3-30 ppm concentration of active once all the active ingredient is released. Percent active was determined at the indicated times by High Pressure Liquid Chromatography (HPLC) . EXAMPLE 2 To 50 g of the 10% by weight CMC, as defined in Example 1, aqueous gel was added with hand stirring for about 15 seconds, a mixture of 6 g polymethylene polyphenylisocyanate (PAPI ® 901, Dow Chemical Co.) containing 0.1% dibutyltin dilaurate catalyst and 0.3 g of powdered N-( ( (4,6-dimethoxypyrimidin-2-yl)amino- carbonyl)-l-methyl-4-(2-methyl-2H-tetrazol-5-yl)-IH- pyrazole-5-sulfonamide herbicide to form a fine dispersion of droplets of the mixture in the gel. The mixture was allowed to stand overnight in the gel, whereby the droplets converted to polyurea shells around each of the droplets of PAPI ® mixture. The capsules, containing 5% by weight N-( ( (4,6-dimethoxy- pyrimidin-2-yl)aminocarbonyl)-l-methyl-4-(2-methyl-2H- tetrazol—5-yl)-lH-pyrazole-5-sulfonamide, released 41% of the active in water in 24 hours, compared to 100% for the powdered, unencapsulated compound control. Release rates were obtained as in Example 1. The control sample was prepared as described in Example 2 without the addition of PAPI ® and dibutyltin dilaurate. EXAMPLE 3 To 40 g of the 10% by weight CMC, as defined ~~" above, aqueous gel was added with hand stirring for about 10 seconds a mixture of 6 g PAPI ® 901, 1.0 g of liquid organophosphorous insecticide, 0,0-diethyl O- (l,2,2,2-tetrachloroethyl)phosphorothioate, and 0.01 g dibutyltin dilaurate to form a fine dispersion of droplets of the mixture in the gel. The mixture was allowed to stand overnight in the gel, whereupon the fine droplets polymerized by reaction with water in the gel to crosslinked polyurea microcapsules with an average particle size (diameter) of about 25 microns. Upon aging in an oven at 45°C for 24 hours, the loss of active was 29%. A control dispersion of unencapsulated active in the same gel lost 51% of the active, under the same conditions as determined by HPLC. The control sample was prepared as described in Example 3 except dibutyltin dilaurate and PAPI ® were omitted. EXAMPLE 4 A non-flowable but stirrable ethylene/maleic anhydride copolymer gel was prepared by mixing 5.0 g EMA ® -91 polymer with 500 g water and treating with 2.3 g (50% by weight) sodium hydroxide. A mixture of 0.2 g of powdered N-( ( (4, 6-dimethoxypyrimidin-2- yl)aminocarbonyl)-l-methyl-4-(2-methyl-2H-tetrazol-5- yl)-lH-pyrazole-5-sulfonamide herbicide and 1.0 g PAPI ® 901, catalyzed with dibutyltin dilaurate, was added with stirring to 7.0 g of the EMA ® gel to form droplets of the mixture finely dispersed in the gel. Another gel mixture was prepared comprising 0.2 g diethylene triamine dispersed in 8.6 g of EMA ® gel prepared as described above. The two gel mixtures were hand-blended, and allowed to stand 20 minutes to form microcapsules by interfacial polymerization. To recover the microcapsules from the gel, the gel was treated with 50 g of concentrated sodium chloride solution, which destroyed the gel structure liberating miσroencapsulated N-( ( (4, 6-dimethoxypyrimidin-2- yl)aminocarbonyl)-l-methyl-4-(2-methyl-2H-tetrazol-5- yl)-lH-pyrazole-5-sulfonamide, average diameter 25 microns. ^ EXAMPLE 5 To 75 g of 10% by weight CMC, as defined in Example 1, aqueous gel was added a solution of 15 g 10% Elvacite ® 2008 poly(methylmethacrylate) (Du Pont Company) in methylene chloride and 0.3 g of powdered N-( ( (4, 6-dimethoxy-pyrimidin-2-yl)aminocarbonyl)-1- methyl-4-(2-methyl-2H-tetrazol-5-yl)-lH-pyrazole-5- sulfonamide herbicide, with mild mechanical stirring for 15 seconds. After a good dispersion of the methylene chloride solution was obtained, the mixture was stirred an additional 3.0 hours at 30-35°C to remove the solvent, giving hard 5-60 micron microcapsules. Water (500 g) was then added with stirring, enabling the capsules to be isolated from the gel by centrifugation and dried at 40-50°C. The microcapsules contained 16.6% by weight active. The microcapsules released 15% of the active in 20.5 hours at 30°C in excess pH 7 buffered water as determined by HPLC. EXAMPLE 6 To 60 g of a gel consisting of 10% by weight CMC, as defined in Example 1, in water was added by hand stirring a mixture of 15 g 10% Elvacite ® 2008 poly(methylmethacrylate) (Du Pont Co.) in ethyl acetate and 0.3 g of powdered N-(( (4,6-dimethoxypyrimidin-2- yl) minocarbonyl)-l-methyl-4-(2-methyl-2H-tetrazol-5- yl)-lH-pyrazole-5-sulfonamide herbicide. Stirring was continued for about 30 seconds until a white opaque mass consisting of the gel and droplets of the Elvacite ® solution containing the active compound, was uniformly and finely dispersed in the gel. The gel composition was allowed to stand 3.5 hours whereupon most of the ethyl acetate migrated into the continuous aqueous gel phase, leaving hard 5-45 μm capsules containing the active. The gel composition was diluted with 400 mL of water with stirring and the capsules were filtered off using a basket centrifuge and dried. The capsules contained 16.6% by weight active.

Claims: _______ What is claimed is: 1. Process for microencapsulating biologically active material, comprising forming a thixotropic gel which is essentially nonflowable but is stirrable without fracturing, forming a substantially homogeneous carrier composition of said biologically active material and a hardenable immiscible organic liquid, combining said gel and said homogeneous carrier composition one with the other in a proportion in which said gel is sufficient to form the continuous phase and said homogeneous carrier composition becomes the dispersed phase in the resultant combination as the result of stirring of said gel which finely disperses said homogeneous composition as droplets in said gel without causing appreciable fracture of said gel, and then hardening said droplets to obtain as a result thereof a stable dispersion of microencapsulated biologically active material. 2. Process of Claim 1 wherein said substantially homogeneous carrier composition is a dispersion of said biologically active material in said hardenable organic liquid. 3. Process of Claim 1 wherein said hardenable liquid comprises polymerizable monomer and said hardening comprises polymerizing said monomer to form said encapsulated biologically active material. 4. Process of Claim 1 wherein said hardenable liquid comprises polymer dissolved in organic solvent and said hardening comprises removing said solvent from said droplets to precipitate said polymer and form said microencapsulated biologically active material. 5. Process of Claim 1 wherein the viscosity of said gel is from 2 to 1200 centepoises (cps) . 6. Process of Claim 3 wherein the polymerizable monomer is polymethylene polyphenylisocyanate and polymerizing said monomer comprises adding diethylene triamine. 7. Process of Claim 4 wherein the polymer is poly(methylmethacrylate) , the solvent is ethyl acetate, and removing said solvent comprises allowing the product to stand and allowing the solvent to jnigrate into the aqueous gel. 8. Composition comprising an effective micro- encapsulating amount of a continuous phase of a non- flowable aqueous gel and microcapsules of polymer less than 1000 microns in diameter, stably dispersed therein as indicated by lack of settling during three months at room temperature., said microcapsules containing a pesticide. 9. The composition of Claim 8 wherein said microcapsules containing said pesticide are formed in situ within said aqueous gel.

Inventors: TOCKER STANLEY, TOCKER, STANLEY
Applicants: DU PONT, TOCKER STANLEY, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1993
Main classification: A01N25
IPC codes: A01N25, A61K9, B01J13