HK1024142B - Novel water-in-oil microencapsulation process and microcapsules produced thereby - Google Patents

Description

Novel water-in-oil microencapsulation process and microcapsules prepared therefrom

The present invention relates to a novel process for the preparation of microencapsulated substances by a water-in-oil microencapsulation process, and to microcapsules comprising aqueous substances formed thereby.

There are many methods known in the art for preparing microencapsulated materials. Almost all of the microencapsulated substances prepared by these known processes contain substances that are immiscible or insoluble in water and are prepared by a process known as oil-in-water microencapsulation. Typically these methods involve preparing a dispersion of "oil" droplets or organic droplets that are substantially immiscible with water (the dispersed phase) in an aqueous medium (the continuous phase). The oil droplets contain one or more monomers or prepolymers, and the monomers or prepolymers present in the oil phase are polymerized by subjecting the resulting emulsion to conditions (e.g., temperature and/or pH and/or agitation) to form microcapsules having a polymeric shell encapsulating water-immiscible droplets. Such methods are described, for example, in U.S. Pat. Nos. 4,285,720 and 4,956,129. The former relates to the production of microcapsules of polyurea-based materials, while the latter relates to the production of microcapsules of etherified urea-formaldehyde polymers.

On the other hand, there is very little information on the preparation of microcapsules containing an aqueous substance by a water-in-oil microencapsulation method. One approach which in some respects is similar to the water-in-oil microencapsulation approach can be found in us patent 4,157,983. In this process, a mixture is first formed which contains an emulsifier, a water-immiscible liquid, a urea-formaldehyde prepolymer, a water-dispersible substance to be encapsulated and water; then stirring the resulting mixture to form a water-in-oil emulsion; the emulsion is then processed to produce microcapsules by curing the urea-formaldehyde prepolymer resin to form a matrix encapsulating the droplets and separating therefrom solid polymer capsules containing the water-dispersible material. The solidification or polymerization is carried out by using an amphiphilic catalyst, i.e. a catalyst which is soluble in both the aqueous and the oil phase of the emulsion. However, the product of this process is not a true microcapsule, but rather a matrix of urea/formaldehyde polymer containing a water-dispersible material.

Us patent 4,534,783 discloses a method of encapsulating an aqueous substance using two monomers or prepolymers.

It is an object of the present invention to provide a simple process for preparing true microcapsules containing aqueous liquid cores of relatively uniform and controlled size, whereby the microcapsules prepared can be used without further processing.

The present invention includes a process for preparing microcapsules encapsulating an aqueous material in a polymeric shell, said process comprising (a) providing an aqueous phase comprising the material to be encapsulated, having dissolved therein a urea-formaldehyde and/or melamine-formaldehyde prepolymer; (b) preparing an emulsion of said aqueous phase in a continuous organic liquid phase comprising one or more organic solvents and one or more surfactants, wherein said emulsion comprises dispersed droplets of the aqueous phase dispersed in the continuous organic liquid phase, thereby forming an interface between the dispersed droplets of the aqueous phase and the continuous organic liquid phase; and (c) in situ self-condensation of the prepolymer in the aqueous phase of the dispersed droplets adjacent said interface by heating said emulsion to a temperature of from about 20 ℃ to about 100 ℃ in the presence of a surface active proton transfer catalyst which is soluble in the organic liquid but only slightly soluble in the aqueous phase, the in situ condensation of the prepolymer being substantially complete over a sufficient period of time to convert the droplets of the aqueous medium into capsules comprised of a solid polymeric shell encapsulating the aqueous medium.

The invention also relates to microcapsules produced by the above process.

The process of the invention can be used to produce microcapsules comprising an aqueous medium in which the various ingredients are dissolved and/or dispersed, and even microcapsules containing only water, if there is a need to produce such capsules. The following is a description of the basic and optional features of the process of the invention and the products of the invention produced thereby:

aqueous medium

The aqueous medium encapsulated by the encapsulation may consist of water only, but is preferably an aqueous medium containing one or more ingredients to be microencapsulated, dissolved, dispersed and/or suspended in water.

The process of the invention is suitable for the production of microcapsules containing one or more ingredients dissolved, dispersed and/or suspended in an aqueous medium. The range of ingredients that can be encapsulated in the present invention is broad, so long as the materials do not react with each other or with the prepolymer or with any other components used in the overall encapsulation system.

The encapsulated material may be selected from a variety of water soluble materials suitable for encapsulation, such as water soluble pesticides and other biologically active materials, pigments, dyes, inks, and the like. For convenience, the invention will be described and illustrated with reference to pesticides.

The process of the invention is particularly useful for the production of microencapsulated pesticides such as herbicides, insecticides, fungicides, nematicides, bactericides, rodenticides, bactericides and the like, and non-pesticidal substances for pest control or similar activities such as agricultural and domestic, commercial or industrial pest control, such as biocides, animal, insect or bird repellents, plant or insect growth regulators, pheromones, fertilizers, sex attractants and odor compositions.

Some examples of water-soluble substances that can be encapsulated by the process of the present invention include the pesticides paraquat, diquat, glyphosate, dicamba, ioxynil, bromoxynil, bentazon, acifluorfen and fomesafen, all in the form of an acid or a salt. Also suitable as inclusions in the capsules of the invention are water dispersible high melting point pesticides such as atrazine and azoxystrobin.

Also urea-formaldehyde and/or melamine-formaldehyde prepolymers are contained in the aqueous medium. These prepolymers have a high solubility in water and a low solubility in the organic liquids used in the present invention. The molecular structure of said prepolymer contains a large number of hydroxymethyl (CH)₂OH). These prepolymers are usually sold in the form of aqueous solutions or water-soluble solids and are used as binders. Including products such as Cymel 401 and 408 from Cytec Industries and ResinCR-583 from Borden chemical company. The prepolymer may also be prepared by known techniques such as base-catalysed reaction between urea and formaldehyde or melamine and formaldehyde (weight ratio 0.6-1.3 parts formaldehyde to 1 part urea or melamine).

The concentration of prepolymer in the aqueous phase is not critical to the practice of the invention and can vary over a wide range depending on the strength of the capsule wall desired and the amount of aqueous liquid desired in the final capsule. However, the optimum concentration of prepolymer in the aqueous phase is from about 1 to 70% by weight, preferably from about 5 to 50% by weight.

The aqueous phase may contain, in addition to or in place of the dissolved active ingredient, one or more high melting solid active ingredients suspended or dispersed in the medium. For pesticides, these may be atrazine or azoxystrobin, for example. These compositions may also contain one or more dispersants.

In addition to the active ingredient contained in the aqueous phase, the active ingredient may also be contained in the organic phase, although such active ingredient is not encapsulated in any of the microcapsules formed. However, the inclusion of a second active ingredient (e.g. a second pesticide) in the organic phase allows the production of an organic suspension of microcapsules containing both pesticides in combination. For example, an oil soluble herbicide such as diuron contained in an oil phase may be used in combination with paraquat or diquat in an encapsulated water phase. Alternatively, an oil-soluble pesticide may be included in the oil phase, so that the entire microcapsule suspension produced contains the herbicide encapsulated with the pesticide and the pesticide unencapsulated.

Oil-soluble (equivalent to water-insoluble) pesticides that may be included in the unencapsulated organic phase of the capsule suspension include thiocarbamate herbicides such as dichlorfon, cyclometaldehyde, molinate or molinate; halogenated acetanilide herbicides such as acetochlor, metolachlor, alachlor, butachlor and propyzamide; nitroaniline herbicides such as trifluralin, organophosphorus insecticides such as parathion, malathion and fonofos; pyrethroid insecticides such as permethrin, lambda-cyhalothrin, deltamethrin, tralomethrin, cypermethrin and tefluthrin, and fungicides such as azoxystrobin.

In addition, a synergist, an activator, a safener and the like of the active pesticide can be contained in the whole preparation.

Emulsion formation

After the aqueous phase is prepared, an emulsion is formed by dispersing it in an organic or water-immiscible liquid.

The organic liquid typically contains one or more solvents, one or more surfactants, and a proton transfer catalyst as described below. The Solvent used in the process of the invention is an organic Solvent, preferably a hydrocarbon or a mixture of hydrocarbons, such as Solvent 450 (kerosene fraction from VWR), Union 7618-90 Oil (paraffinic hydrocarbon Solvent from Union Oil); diesel oil # 2; aromatic 100 and 200 solvents from Exxon; and Suresol 100 and 190 solvents from Koch Refinery.

The surfactant may be any known substance useful for reducing the surface tension of a fluid interface, and may be a nonionic or anionic type surfactant. Examples of nonionic surfactants are long chain alkyl and thiol polyethoxy alcohols, alkylaryl polyoxyethylene ether alcohols (e.g. polyoxyethylated nonylphenols), alkylaryl polyether alcohols, alkyl polyether alcohols, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene ethers and esters of polyalkylene glycols, especially polyethylene glycol, with fatty acids or rosin acids. Preferred anionic surfactants are the calcium, amine, alkanolamine and alkali metal salts of alkyl and alkylaryl sulfonates, Vegetable oil sulfonates (Vegetable oil sulfonates), polyoxyethylene or polyoxypropylene mono-or diethers of phosphoric acid. Blends of surfactants may also be used. The surfactants preferably used in the process of the present invention are polyoxyethylated nonylphenols.

The amount of surfactant is not critical to the present invention and can vary within wide limits. For convenience, the surfactant will generally be present in an amount of from about 0.1 to 5.0% by weight of the organic phase. The surfactant may be added before or after the emulsion is formed.

An effective amount of a proton transfer catalyst is also included in the organic liquid to catalyze the formation of a polymer wall. The proton transfer catalyst may be added before the polymerization and polymer wall formation, but at or before or after the emulsion formation stage. However, it is preferred to add it before the emulsion formation because the proton transfer catalyst has a surface activity that can be used in the emulsion formation step. When a proton transfer catalyst is present prior to emulsion formation, the resulting mixture temperature should be maintained at a maximum of about room temperature to avoid or minimize premature formation of polymer.

The proton transfer catalyst is an acidic substance that is substantially soluble in oil and at most slightly soluble in water. However, proton transfer catalysts have a molecular structure containing a large hydrophobic portion and an ionic portion capable of being transferred to the water side through an oil/water interface, with catalytic protons carried thereon capable of polymerizing the prepolymer in an aqueous medium, thereby forming a shell wall of the microcapsule at the oil/water interface. The hydrophobic portion of the proton transfer catalyst molecule is immobilized in a stationary manner in the oil phase, while the portion carrying protons is immobilized on the water side of the interface. This creates a fixed catalytic layer that limits the polymerization of the resin in the interfacial region, thereby creating a relatively thin shell wall around the water droplet. The amphiphilic catalyst of us patent 4,157,983, in contrast, is soluble in the aqueous phase, resulting in polymerization of the resin throughout the phase, thus forming a matrix rather than microcapsules.

Preferred proton transfer catalysts are sulfonic acids containing at least 20 carbon atoms in the molecule. Said sulfonic acid may be saturated or unsaturated, cyclic or acyclic; for example, it may be an alkyl, alkenyl, alkynyl, aryl, alkaryl, or other type of sulfonic acid. The sulfonic acid may have other substituents such as halogen atoms in its molecule as long as it has 20 or more carbon atoms, is substantially oil-soluble and is at most sparingly soluble in water. The most preferred proton transfer catalysts are long chain alkyl aryl sulfonic acids such as alkyl benzene sulfonic acids or alkyl naphthalene sulfonic acids wherein the alkyl group contains from about 16 to about 24 carbon atoms. The preferred proton transfer catalyst is didodecylbenzenesulfonic acid.

The size of the droplets in the emulsion is not critical to the invention. To make full use of the final product, the diameter size of the droplets should be in the range of about 0.5-4000 microns. The preferred droplet diameter range for most pesticide applications is about 1-100 microns. The emulsion is typically prepared using any conventional high shear mixing device. Once droplets of the desired size are obtained, moderate agitation during the remainder of the process is sufficient to prevent delamination of the sample.

Contact between reactive groups on the prepolymer becomes increasingly difficult as the polymer wall becomes stiffer. Thus, the in situ self-condensation polymerization reaction is self-terminating, and the reaction generally tends to end as such.

Acidity and temperature can increase the rate of the in situ self-condensation polymerization reaction. Thus, this reaction can be carried out at any temperature from about 20 to about 100 deg.C, preferably from about 40 to about 70 deg.C. At the low end of this temperature range, the polymer formation is so slow that it does not cause premature formation of the capsule wall. Thus, the reaction temperature is preferably maintained at about 20-25 ℃ until droplets of the desired size are obtained, and then the temperature is increased to accelerate the formation of a polymeric shell wall around the droplets. The reaction is usually complete within a few hours, but at high acid and high temperatures, the reaction can be complete within a few minutes.

Once the capsules are formed, they can be stored and used as a dispersion or filtered to obtain dry capsules. Both forms of capsules are very effective in the controlled release of core liquids. The dispersion is preferably stabilized by a thickener dissolved in the continuous phase. Any conventional thickener may be used. Typical thickeners include hydrogenated castor oil, organically treated clays, and silica.

It is a feature of the present invention that the capsules or capsule suspensions may be enclosed in a water-soluble packaging material such as a bag or pouch made of a water-soluble polymer such as polyvinyl alcohol or the like. Thus, the present invention essentially provides a method of delivering a water-soluble pesticide in a water-soluble package.

The following examples are illustrative of the process and products of the present invention and should not be construed as limiting in any way.

Example 1:

the organic phase was prepared by dissolving 12.0g of dodecylbenzenesulfonic acid (Aristol a, manufactured by Pilot chemical company) and 12.0g of a nonionic alkylarylphenol surfactant (Igepal CA-630, manufactured by Rhone-Poulenc) in 169.0g of a kerosene Solvent (Solvent 450, manufactured by VWR company). The aqueous phase was prepared by dissolving 100.0g of technical grade paraquat and 50.0g of urea/formaldehyde prepolymer (Casco SR-397C resin, manufactured by Borden chemical company) in 50.0g of water. The two phases are then mixed together by stirring to form a water-in-oil emulsion. Then, the temperature was raised to 40 ℃ and stirring was continued for 2 hours. The resulting product is cooled to produce a suspension of microcapsules in an organic phase (mainly solvent) in which the aqueous medium is encapsulated.

Similarly, other microcapsule formulations of paraquat were prepared as described in table 1 below. The materials used in the examples of Table 1 were, in addition to those mentioned above, WS-351-380 prepolymer (urea/formaldehyde, manufactured by Borden), Aristol E proton transfer catalyst (didodecylbenzenesulfonic acid, manufactured by Pilot chemical Co., Ltd.), surfactant Neodol 25-3 (linear primary alcohol polyoxyethylene ether, manufactured by Shell chemical Co., Ltd.), and Tergitol NP9 and NP13 (polyoxyethylated nonylphenol, manufactured by Union Carbide).

In all of the following examples (except examples 4 and 8) no compounding of the capsules occurred. Depending on the embodiment, the particle size ranges from about 30 microns or less to 300 microns or less. No coalescence was observed in any of the tests except example 7.

TABLE 1

Ingredient (g)	Examples
									2	3	4	5	6	7	8
Aqueous phase
								Paraquat, technical grade	100.0	150.0	150.0	150.0	150.0	150.0	150.0
WS-351-380 water-soluble prepolymer	50.0	50.0	50.0	50.0	50.0	50.0	50.0
								Organic phase
Union 76IV-90 solvent	173	169.0	-	169.0	169.0	169.0	169.0
								Solvent 450	-	-	169.0	-	-	-	-
Aristol A catalyst	12.0	12.0	12.0	12.0	-	-	-
								Aristol E catalyst	-	-	-	-	12.0	-	12.0
Aristol 360 catalyst	-	-	-	-	-	12.0	-
								Neodol 25-3 surfactant	8.0	-	-	-	-	-	-
Igepal CA-630 surfactants	-	12.0	8.0	-	-	-	-
								Tergigol NP13 surfactant	-	-	-	12.0	12.0	12.0	-
Tergitol NP9 surfactant	-	-	-	-	-	-	12.0

Claims (9)

1. A process for the preparation of microcapsules containing an aqueous material encapsulated within a polymeric shell, said process comprising (a) providing an aqueous phase containing the material to be encapsulated and a urea-formaldehyde prepolymer dissolved therein; (b) preparing an emulsion of the aqueous phase in a continuous organic liquid phase, wherein said continuous organic liquid phase comprises one or more organic solvents and one or more surfactants, wherein said emulsion comprises dispersed droplets of the aqueous phase dispersed in the continuous organic liquid phase, thereby forming an interface between the dispersed droplets of the aqueous phase and the continuous organic liquid phase; and (c) in situ self-condensation of the prepolymer in the aqueous phase of the dispersed droplets adjacent said interface by heating said emulsion to a temperature of from about 20 ℃ to about 100 ℃ in the presence of a surface active proton transfer catalyst which is soluble in the organic liquid but only slightly soluble in the aqueous phase, for a period of time sufficient to substantially complete the in situ condensation of the prepolymer, thereby converting the droplets of the aqueous phase into capsules comprised of a solid permeable polymer encapsulating an aqueous medium.

2. A process according to claim 1 wherein the aqueous material comprises a biologically active substance dissolved and/or dispersed therein.

3. The method of claim 2 wherein said biologically active substance is a pesticide.

4. The method of claim 3 wherein said pesticide is paraquat.

5. A process according to any one of claims 1 to 4, wherein the organic liquid comprises an active substance dissolved or dispersed therein.

6. Microcapsules prepared according to the process of any one of claims 1 to 4.

7. A suspension of microcapsules prepared according to the process of any one of claims 1 to 4 in an organic liquid.

8. The capsule suspension according to claim 7, wherein the organic liquid contains a substantially water-insoluble pesticide.

9. A packaged pesticide formulation comprising microcapsules or a capsule suspension according to any one of claims 6 to 8 encapsulated in a water-soluble packaging material.