PROCESS FOR FORMALDEHYDE CONTENT REDUCTION IN MICROCAPSULE FORMULATIONS

Abstract: A process is disclosed for the reduction of the formaldehyde content of a formulation of amino resin microcapsules which comprises treating the microcapsule formulation with from about 0.03 wt. % to about 0.75 wt. % of ammonia. Also disclosed are the microcapsules formed by the process.

Description: PROCESS FOR FORMALDEHYDE CONTENT REDUCTION IN MICROCAPSULE FORMULATIONS BACKGROUND OF THE INVENTION Field of the Invention This invention relates to microcapsules and to a process for reducing the formaldehyde content of formulations containing microcapsules. 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 an amino resin and preferentially from a modified urea-formaldehyde prepolymer. 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, bacteri- cides, and fertilizers. Non-agricultural uses include encap¬ sulated dyes, inks, pharmaceuticals, flavoring agents, and fragrances. The most common forms of controlled release mater¬ ials are coated droplets or microcapsules, coated solids including both porous and non-porous particles, and coated aggregates of solid particles. In some instances, a water- soluble encapsulating film is desired, which releases the encapsulated material when the capsule is placed in contact with water. Other coatings are designed to release the entrapped material when the coating is ruptured by external force. Still further coatings are porous in nature and release 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 matter 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 soil. This is described in U.S. Patents 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. Patents 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 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 poly¬ merize. 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 Patents 952,807 and 965,074 describe a process whereby a solid such as wax or a thermoplastic resin is melted, dispersed and cooled to form an encapsulating film around liquid droplets. U.S. Patent 3,111,407 (Lindquist et al., November 19, 1963) describes a spray drying method which forms encapsu¬ lated 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 settling agents, and percent microcapsule phase. U.S. Patent 4,956,129, the disclosure of which is incorporated herein by reference, teaches the process in which a liquid material which is substantially insoluble in water can be microencapsulated within a porous poly(urea-formalde¬ hyde) shell. However, formaldehyde develops when the methylol groups present in the urea-formaldehyde resin react with each other. Formaldehyde is the by-product of the resin polymeri¬ zation. It is, therefore, an object of the present invention to provide a simple, inexpensive method for reducing the formaldehyde content of formulations of microcapsules. Other objects of the invention 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 amino resin shell wherein the formal¬ dehyde content formed as a by-product of the microencapsula- tion process is no greater than 0.1% by a process which comprises: (a) Providing an organic solution comprising said liquid 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. 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 interf ce between the discrete droplets of organic solution and the surrounding continuous phase aqueous solu¬ tion; (c) causing in situ self-condensation and curing of said amino resin prepolymers in the organic phase of said discrete droplets adjacent to said interface by simul¬ taneously heating said emulsion to a temperature between about 20°C to about 100°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 substan¬ tial completion of in situ condensation of said resin pre¬ polymers to convert the liquid droplets of said organic solution to capsules consisting of solid permeable polymer shells enclosing said liquid material; and (d) post-treating the formulation of amino resin capsules with from about 0.03 wt. % to about 0.75 wt. % of ammonia. The present invention resides in both the process described above and the microcapsules thus formed. Description of the Preferred Embodiment In the process of forming microcapsules from a urea formaldehyde prepolymer, such as that disclosed in U.S. Patent 4,956,129, formaldehyde is formed as a by-product of the polymerization reaction of the urea formaldehyde prepolymer. Typically, the resulting microcapsule formulation contains about 0.5% of free formaldehyde. The process of this invention comprises post- treating the formulation of amino resin microcapsules with from about 0.03 wt. % to about 0.75 wt. % of ammonia, effec¬ tively reducing the free formaldehyde content of the micro¬ capsule formulation to no greater than 0.1% formaldehyde. The treatment is preferably carried out by mixing a concentrated ammonia solution with the microcapsule formula¬ tion. 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. How¬ ever, it has been discovered that treatment with significantly higher amounts of ammonia, in the range of 1.5 wt. %, severely damages the microcapsule wall and modifies the release characteristics of such a microcapsule formulation. As taught in U.S. Patent No. 4,956,129, it is essential that the organic solution which forms the interior of the microcapsules (i.e., the core liquid) be substantially insoluble in water. The organic solution may consist of a single liquid material or one or more active liquid or solid materials dissolved in an inert solvent which has a slight solubility in water. In the latter case, the liquid or solid solute must reside preferentially in the organic phase when the two phases are in equilibrium. A wide variety of liquids can be encapsulated by the present process. Any liquid which is not reactive with the amino resin prepolymer and which will diffuse through the shell membrane is suitable. Liquids suitable for encapsula¬ tion include chemical-biological agents such as herbicides, insecticides, fungicides, nematocides, bactericides, rodenti- cides, molluscides, acaricides, larvicides, animal, insect and bird repellantε, plant growth regulators, fertilizers, phero- mones, sex lures and attractantε, and flavor and odor composi¬ tions. The microcapsules of the present invention are particularly well adapted to pesticides, including thiocarba¬ mates, dithiocarbamates, acetamides, anilides, sulfonamides, triazineε, organophosphorus compounds and pyrethroids. Examples of these compounds are disclosed in U.S. Patent No. 4,956,129. 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 subεtituted thiocarbamates, particularly those thiocarbamates known commonly as: molinate (S-ethyl hexahydro-lH-azepine-1-carbothioate) ; cycloate (S-ethyl N-ethylthiocyclohexanecarbamate) ; EPTC (S-ethyl dipropylthiocarbamate) ; butylate (S-ethyl diisobutylthiocarbamate) ; diallate (S-(2,3-dichloroallyl)diisopropylthiocarbamate) and vernolate (S-propyl dipropylthiocarbamate) . Another class of pesticide which is particularly preferred is that of the anilide herbicides, preferably the subclass of these known in the art as α-haloacetanilide, or more specifically α-chloroacetanilide herbicides. Some specific compounds of this class include: metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy- 1-methylethyl) acetamide) ; butachlor (N-butoxymethyl-2-chloro-2',6'-diethylacetanilide) ; alachlor (2-chloro-2' ,6'-diethyl-N-(methoxymethyl) acetanilide) ; acetochlor (2-chloro-N-(ethoxymethyl)-6 '-ethyl-O-aceto- toluidide) ; metazochlor (2-chloro-N-(2,6-dimethylphenyl)-N-(lH-pyrazol-l- ylmethyl) acetamide) pretilachlor (2-chloro-2',6'-diethyl-N-(2-propoxyethyl) acetanilide) ; dimethachlor (2-chloro-N-(2-methoxyethyl)acet-2 , ,6'-xylidide) ; and propachlor (2-chloro-N-isopropyl acetanilide) . Many other compounds of this type are disclosed in numerous patents. As is well known in the art, the utility of many pesticides can be broadened by the inclusion of antidotes or εafeners (See U.S. Patent No. 4,021,224 and 4,7708,735), and synergists in the composition. Classes of antidotes include N,N-disubstituted haloacetamides, oxazc idines, thiazolidines, εulfonamideε, various halogenated esters, J ilogenated ketoneε, disulfides, imidazolineε, oximeε and pyridyloxy alkonic acid amides. The prepolymerε suitable to the present invention are not limited to the etherified urea-formaldehyde pre- polymerε aε deεcribed in the prior art, but can broadly include other amino resin prepolymers with a high solubility in the organic phase and a low solubility in water. 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 subclasses: urea-formaldehyde, melamine-formaldehyde, benzoguanamine-formaldehyde and glycoluril-formaldehyde. The first two mentioned are preferred in this invention, with urea-formaldehyde prepoly ers being most preferred. The prepolymers melamine-formaldehyde, benzoguan¬ amine-formaldehyde and glycoluril-formaldehyde are available commercially under the trademark CYMEL and sold by American Cyanamid Co. The amino resin prepolymers themselves can be prepared by known techniques, by the reaction between the amine and formaldehyde. A water soluble urea-formaldehyde or melamine-for¬ maldehyde prepolymer can alεo be uεed aε the polymer forming material in an aqueous phase separation microencapsulation proceεε. The prepolymer iε added to an aqueous medium and polymerization proceeds under acidic conditionε. A water soluble urea-formaldehyde or melamine-formaldehyde polymer precipitateε and the polymer depositε on the oil droplets as disclosed in U.S. Patent No. 3,516,846. Formaldehyde is produced as a by-product of the wall formation process. The process hereinabove described comprising post-treating the formulation of the amino resin microcapsules with ammonia will be effective in reducing the formaldehyde of the microcapsule formulation when the prepolymer is used as the polymer forming material. The following examples are offered as illustrative of both the process and product of the present invention, and are intended neither to define nor limit the invention in any manner. MOLINATE TREATMENT Microcapsule Formulation - Treatment 1A, IB and 1C. An organic solution was prepared, comprising 6.37 kg of a fully butylated urea-formaldehyde prepolymer (Beetle 80, obtained from American Cyanamid) and 0.64 kg of Mercaptate Q-43 pentaerythritol tetrakis (mercaptopropionate) , sold by Cincinnati Milacron Chemicals in 84.2 kg S-ethyl hexahydro-lH- azepine-l-carbothioate ( olinate) technical grade. This solution was emulεified with a high shear stirrer in 75.955 kg of water containing 2.374 kg of sodium lignosulfonate (Daxad 23) and 0.798 kg of a sodium dialkylnaphthalene sulfonate (Petro BAF) . The pH of the dispersion was lowered to 2.0 with 1.0 kg of cone, sulfuric acid. The temperature was raised to 50 C and low shear stirring was continued for three hourε. The diεperεion waε then allowed to cool to room temperature. In the untreated εa ple (Trt. 1A) the pH waε raised to 7.0 with 1.0 kg of a 50% sodium hydroxide solution. In the treated samples (Trt. IB and IC) ammonia was added as 30% aqueous ammonia solution. Microscopic observation of the dispersion revealed fully formed, discrete roughly spherical microcapsules. The capsules were an average of 12 microns in diameter. Formaldehyde Content and Release Rate of Molinate Capsules: The formaldehyde content of these formulations was measured using a chromatographic column equipped with an amperometric detector. The results are reported in Table 1. In a thermostated stoppered flask, a solution of 1.67 g molinate (4 lbs molinate/gal) microcapsule formulation in 1000 g water was stirred at 25 C for two hours. After two hours, two 10 ml samples were taken and consecutively passed through a 0.2 micron filter to remove the microcapsules. One ml of the second filtrate was extracted with 20 ml of toluene, The toluene was then analyzed by gaε chromatography. The results are reported as ppm molinate in the aqueous filtrate in Table 1. TABLE 1 TRT Post-Treatment Formaldehyde Molinate % NH added Content % Release 1A 0.44 98.0 ppm IB 0.73 0.09 103.5 ppm 1C 1.50 0.03 543.3 ppm When the microcapsule formulation was post-treated with 0.73 % ammonia (Trt. IB) , the formaldehyde content was reduced to 0.09 %, and the core liquid releaεe rate waε similar to the untreated formulation (Trt. 1A) . While the formaldehyde content of the 1.50 % ammonia post-treatment was only 0.03%, the release rate was five times higher than that of the 0.73 % treatment. ACETOCHLOR TREATMENT Microcapsule Formulation - Treatment 2A, 2B and 2C. The organic solution consisted of 50.2 g of Beetle 1050-10 resin and 378.8 g of acetochlor (96.1% purity). Beetle 1050-10 iε a partially butylated (approximately 80 %) liquid urea-formaldehyde resin obtained from American Cyanamid Company. The organic solution was dispersed in a high shear stirrer in a solution 4.0 g of sodium lignosulfonate (Reax 100M) , 1.8 g sodium dialkylnaphthalene sulfonate (PETRO BAF) and 347.2 g water. The pH of the resulting emulsion was lowered to 2.0 with 0.8 g of concentrated sulfuric acid. The emulsion temperature was then raised to 50 C. Low shear stirring was continued at this temperature for 3 hourε. The reεulting diεperεion waε cooled. The formulation was post- formulated by stirring in 0.32 g Xanthum gum (Kelzan) , 3.2 g attapulgite clay (Attagel 40), and 0.8 g Proxel C XL (biocide) . The pH of the untreated sample (Trt. 2A) was raised to 7.0 with 50% sodium hydroxide solution. In the ammonia treated sample 2B, the pH waε raiεed to 7.0 with 0.3 g 30% ammonia εolution, and then raiεed to 8.0 with additional 1.3 g 30% ammonia εolution. The pH waε then raised to 9.0 and maintained at this pH with additional 0.5 g 30% ammonia solution. In the ammonia treated sample 2C, the pH was raised to 10.0 with 1.5 g 30% ammonia solution and maintained at pH 10.0 with additional 1.0 g 30% ammonia solution. Microscopic observation revealed fully, well formed discrete spherical microcapsules. Formaldehyde Content and Release Rate of Acetochlor Capsuleε: The formaldehyde content of these formulations was measured using a chromatographic column equipped with an amperometric detector. The results are reported in Table 2. In a thermostated stoppered flask, a solution of 0.178 g acetochlor (4 lb acetochlor/gal) microcapsule formula¬ tion in 1000 g water was stirred at 25 C for two hours. After two hours, two 10 ml εampleε were taken and consecutively passed through a 0.2 micron filter to remove the microcap¬ sules. One ml of the second filtrate was extracted with 20 ml of toluene. The toluene was then analyzed by gas chromato¬ graphy. The resultε are reported aε ppm acetochlor in the aqueouε filtrate in Table 2. TABLE 2 TRT Poεt-Treatment Formaldehyde Acetochlor % NH-. added content % release 2A 0.0 0.50 10.55 ppm 2B 0.63 0.09 14.05 ppm 2C 0.75 0.03 9.90 ppm The post-treatment additions of ammonia signifi¬ cantly reduced the formaldehyde content of the formulation below that of the untreated sample. Additionally, the integ¬ rity of the microcapsule waε maintained, aε demonstrated by the acetochlor release rate for each treatment. The release rate for each treatment was the same within experimental error.

Claims: WHAT IS CLAIMED IS: 1. A process for reducing the free content of formaldehyde for a microencapsule formulation of a substan¬ tially water-insoluble liquid material within a porous amino resin shell comprising: (a) Providing an organic solution comprising said material and an etherified amino resin prepolymer dis¬ solved therein in which from about 50% to about 98% of the methylol groups of said prepolymer have been etherified with a C4-C1,0- 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 phaεe aqueous solution, there being formed thereby an interface between the discrete droplets of organic solution and the surrounding continuous phase aqueous solu¬ tion; (c) causing in situ self-condensation and curing of said amino resin prepolymers in the organic phase of said discrete droplets adjacent to said interface by simul¬ taneously heating said emulsion to a temperature between about 20°C to about 100°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 substan¬ tial completion of .in situ condensation of said resin pre¬ polymers to convert the liquid droplets of said organic solution to capsuleε consisting of solid permeable polymer shells enclosing said liquid material; and (d) post-treating the formulation of the amino resin capsules with from about 0.03 wt. % to about 0.75 wt. % of ammonia. 2. A procesε according to Claim 1 wherein the amino resin prepolymer is a urea-formaldehyde, melamine-formalde¬ hyde, benzoguanamine-formaldehyde or glycoluril-formaldehyde prepolymer. 3. A procesε according to Claim 1 wherein the amino reεin prepolymer is a urea-formaldehyde or melamine-formalde¬ hyde prepolymer. 4. A procesε according to Claim 1 wherein the amino resin prepolymer is a urea-formaldehyde prepolymer. 5. A process according to Claim 1 in which the herbicide is acetochlor, metazochlor, pretilachlor, metoa- chlor, butachlor, alachlor, dimethachlor or propachlor. 6. A procesε according to Claim 5 in which the herbicide is acetochlor. 7. A procesε according to Claim 1 in which the herbicide iε a thiocarbamate. 8. A process according to Claim 7 in which the thiocarbamate is EPTC. 9. A procesε according to Claim 7 in which the thiocarbamate iε molinate. 10. A proceεε according to Claim 1 which further comprises a herbicide antidote suitable for use with a thio¬ carbamate or haloacetanilide herbicide. 11. In the process of microencapsulation of core material by interfacial polymerization or aqueous phase separation with an amine resin prepolymer, the improvement comprising reacting the resulting amino polymer with from about 0.03 wt. % to about 0.75 wt. % of ammonia. 12. A method of reducing residual free formaldehyde concentration in urea formaldehyde microcapsuleε comprising post-treatment of the microcapsule formulation with from about 0.03 wt. % to about 0.75 wt. % of ammonia. 13. Improved amino resin microcapsules having reduced free formaldehyde content prepared by post-treatment of the microcapsuleε with about 0.03 wt. % to about 0.75 wt. % ammonia.

Inventors: BAKER DON R, DAVIS RICHARD W, KEZERIAN CHARLES, RODSON MARIUS, SCHER HERBERT B, BAKER, DON, R., DAVIS, RICHARD, W., KEZERIAN, CHARLES, RODSON, MARIUS, SCHER, HERBERT, B.
Applicants: BAKER DON R, DAVIS RICHARD W, KEZERIAN CHARLES, RODSON MARIUS, SCHER HERBERT B, BAKER, DON, R., DAVIS, RICHARD, W., KEZERIAN, CHARLES, RODSON, MARIUS, SCHER, HERBERT, B.
Application year: 1993
Main classification: B01J13
IPC codes: A01N25, A01N47, B01J13, C08L61