MICROCAPSULES AND PROCESS FOR PRODUCING THEM

Abstract: This process for producing microcapsules includes dissolving wax or a mixture of wax and protein in a mixed solvent consisting of an organic solvent and water, wetting, with the resulting solution, the surfaces of particles of a core substance at temperatures of 45 deg.C or below to selectively deposit the wax or the mixture of wax and protein on the particle surfaces and allow them to coat the particle surfaces. And then a slurry containing the particles are subjected to a solvent removal treatment.

Description: BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT The present invention relates to microcapsules and a process for producing them. More particularly, the present invention relates to microcapsules and a process for producing, by wax coating or coating a mixture of wax and protein, microcapsules which are preferably used in feeds (particularly, feeds for fries), medicines, foods, chemical products, etc. As described in "Technique for Surface Modification of Fine powder by Encapsulation", pages 547 to 551 (particularly page 548) of Chemical Engineering, Vol 46, No. 10 (1982), it is conventionally known that there are 14 processes for producing microcapsules, such as interfacial polymerization and in-liquid drying. The microcapsules produced according to these processes are being used in many industrial fields such as foods, medicines, agricultural chemicals, feeds, spices, enzymes and active carbon, as described in the above literature. Of the above 14 processes, there are known, as typical processes for coating particles of a core substance with a wax, a suspension-in-gas process (a coating-in-fluid-layer process) and a melting-dispersion-cooling process (a pelletization process by spraying and solidification). However, the above mentioned processes have defects in that the core substance particles tend to cause thermal denaturation, that the film formed on the particles becomes scaly and no uniform film is formed, and that a large amount of a wax is required to obtain a complete film on the particles. U.S. Patent No. 4,675,236 discloses mono-core type microcapsules and a process for producing them. The patent relates a wax-coated microcapsules in which wax coating is prepared by once melting the wax particles over the surface of the core particles and then resolidifying them. On the other hand, at present, it has not been known microcapsules and a process for producing them by simultaneously coating hydrophobic waxes (in particular, lipids) and hydrophilic proteins. The object of the present invention is to provide microcapsules and a process for producing them to solve the above mentioned defects associated with the conventional processes. The another object of the present invention is to provide microcapsules and a process for producing them on which waxes or a mixture of waxes and proteins are coated uniformly. SUMMARY OF THE INVENTION According to the present invention, there are provided microcapsules comprising particles of from 5 µm to 5 mm in size on which waxes or a mixture of wax and protein being coated as a layer of from 0.1 to 10 µm in thickness on a surface of a core substance particle. And further, there is provided a process for producing microcapsules by wax coating or coating a mixture of wax and protein, characterized by dissolving at least one wax or a mixture of wax and protein in a mixed solvent consisting of a water-miscible organic solvent and water, wetting, with the resulting solution, the surfaces of said particles of a core substance at a temperature of 45°C or below to selectively deposit the wax or the mixture of wax and protein on said particle surfaces and allow the wax or the mixture of wax and protein to coat the particle surfaces, and then subjecting a slurry containing the coated particles to a solvent removal treatment. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flow chart showing a typical embodiment of the present process. Fig. 2 is a photograph by scanning type electron microscope showing the structure of the microcapsules obtained in Example 1. Fig. 3 is a photograph by scanning type electron microscope showing the structure of the microcapsules after melting-cooling-solidification treatment, obtained in Example 1. Figs. 4 to 6 are photographs by scanning type electron microscope showing the structures of the microcapsules obtained in Examples 2 to 4, respectively. DETAILED DESCRIPTION OF THE INVENTION The organic solvent used in the present invention can be any as long as it is miscible with water and can dissolve the wax or the mixture of wax and protein used in the present invention, as mentioned above. The organic solvent is preferably selected from methanol, ethanol, propanol, acetone and methyl ethyl ketone, which are all polar solvents. Of these, ethanol and acetone are particularly preferred. The concentration of the organic solvent in the mixed solvent is ordinarily 65 to 95% by volume. It is decided in view of the type of wax and/or protein used and the amount of wax and/or protein required. The concentration of the organic solvent is, for example, 65 to 85% by volume, preferably 75 to 80% by volume in the case of methanol; 70 to 95% by volume, preferably 75 to 90% by volume in the case of ethanol; 80 to 95% by volume, preferably 85 to 90% by volume in the case of propanol; 70 to 95% by volume, preferably 75 to 90% by volume in the case of acetone; and 80 to 95% by volume, preferably 85 to 90% by volume in the case of methyl ethyl ketone. When the concentration of the organic solvent is lower than 65% by volume, the solubility of the wax decreases remarkably and it is necessary to increase the solution temperature to higher than 45 deg.C. When the concentration exceeds 95% by volume, the selective deposition of the wax and protein on the particle surfaces becomes difficult. Furthermore, production of microcapsules of a mono-core type or a multi-core type can be adjusted by selecting the concentration of the organic solvent in the mixed solvent. Where the concentration of the organic solvent in the mixed solvent is raised within said range, microcapsules of a mono-core type are produced. When, on the other hand, the concentration is lowered, microcapsules of a multi-core type are produced. To be concrete, the concentration of the organic solvent in the mixed solvent is 80 to 95% by volume, preferably 80 to 90% by volume when producing mono-core type microcapsules. The concentration of the organic solvent is 65 to 85% by volume, preferably 70 to 80% by volume when producing multi-core microcapsules. The operation of wetting the particle surfaces with the mixed solvent containing a wax is conducted at temperatures of 45 deg.C or below, ordinarily at 20 deg.C to 40 deg.C. When it is conducted at temperatures higher than 45 deg.C, the core substance causes thermal denaturation. In the microcapsules of the present invention, the core substance particles can be organic substance or an inorganic substance. It is preferable that these substances contain a water-soluble substance. Examples of the organic substance include not only those shown in Examples 1 to 4 which appear later, but also particles of foods, seasonings, medicines, additives for medicine, spices, enzymes and feeds. Examples of the inorganic substance include particles of oxide ceramics, non-oxide ceramics, inorganic salts, active carbon and zeolite. These core substance particles have particle diameters of 5 µm to 5 mm, preferably 5 to 500 µm. Preferably, they have a spherical shape or a shape close to it. As the wax, there are used those soluble in organic solvents. Specific examples include the followings. (1) Fatty acids Saturated fatty acids of carbon numbers 10 to 20 (C₁₀ ∼ C₂₀) and Unsaturated fatty acids of carbon numbers 18 to 24 (C₁₈ ∼ C₂₄). Examples: Capric acid, undecanoic acid, lauric acid, tridecylic acid, myristic acid, palmitic acid, stearic acid, elaidic acid, nonadecanoic acid, arachic acid, behenic acid, lignoceric acid, octadecenoic acid, vaccenic acid, erucic acid, brassidic acid, cerotic acid, montanic acid, hexadecenoic acid, eicosenoic acid, pentadecylic acid, heptadecylic acid, heptacosanoic acid, melissic acid, lacceric acid, undecylenic acid, cetoleic acid, arachidonic acid and octadecynoic acid. (2) Higher alcohols Higher alcohols of carbon numbers 10 to 20 (C₁₀ ∼ C₂₀) Examples: Undecanol, lauryl alcohol, tridecyl alcohol, myristyl alcohol, cetyl alcohol, stearyl alcohol, elaidic alcohol, nonadecyl alcohol, eicosyl alcohol, ceryl alcohol, pentadecyl alcohol, heptadecyl alcohol, melissyl alcohol, laccerol and undecyl alcohol. (3) Fatty acid glycerides Mono-, di-, and tri-glycerine ester of higher fatty acids of carbon numbers 10 to 24 (C₁₀ ∼ C₂₄). Examples: Palmitin, stearin, myristin, laurin. (4) Others Sterols (cholesterol and derivatives) Phospholipids (lecithin) These waxes can be used singly or in combination, or used in the mixture with the protein. In case of using the mixture of wax and protein, the amount of the wax to be used in generally from 1 to 30% by weight, preferably from 2 to 10% by weight, to 100% by weight of the core substance particle. Gliadin (wheat) belonging to the prolamin family, hordenine (barley) and zein (corn) are among the simple proteins which may be used. The amount of the protein to be used is generally from 0.5 to 8% by weight, preferably from 1 to 3% by weight, to 100% by weight of the core substance particle. As necessary, a water-soluble substance can be added to the wax or the mixture of wax and protein in order to control the collapse time of microcapsule film. Therefore, it is desirable that the type and amount of the water-soluble substance added be selected in view of the length of time of microcapsule use. As the water-soluble substance which can be added, there can be mentioned gelatin, gun arabic, sodium carboxymethylcellulose, polyvinyl alcohol (PVA), cellulose acetate phthalate (CAP) and Eudragit L30D-55 (brand name, methyl methacrylate - methacrylic acid copolymer). The microcapsules produced by depositing the wax or the mixture of wax and protein on the particle surfaces to coat the surfaces with the wax or the mixture of wax and protein are then subjected to a solvent removal treatment to obtain microcapsules of the present invention. The solvent removal treatment can be easily conducted according to a known treatment method such as spray drying or a combination of vacuum drying and disintegration. The process of the present invention exhibits the following surprising phenomena. In the conventional processes for microcapsule production, it is necessary to completely disperse particles of a core substance in order to obtain microcapsules which are separate from each other and not adherent to each other. In the present invention, however, such mono-core microcapsules can be easily obtained simply by wetting the surfaces of core substance particles with a wax solution or a solution of wax and protein, without the necessity of dispersing the particles. If desired, multi-core microcapsules can also be easily obtained. Further in the present invention, the total amount of the wax or the mixture of wax and protein used is deposited on the surface of the core substance and the deposition of wax or the mixture of wax and protein occurs even between the particles which are sticking to each other. Therefore, when the wax-coated particles or the particles coated by the mixture of wax and protein are disaggregated after the solvent removal treatment, each microcapsule can be obtained independently without causing any breakage of wax film or film of the mixture of wax and protein. Because of the above phenomena, the thickness of the film can be controlled easily in the process of the present invention. The average thickness t of the film is given by the following equation, based on assumptions that the total amount of the wax or the mixture of wax and protein used adhere to the core substance particles and the particles have a spherical shape. t = (R·ρ p /600·(1-ε)·ρ w ) x d₅₀ (µm) wherein R = weight ratio of wax or mixture of wax and protein to core substance, (%) ρ p = density of particles, (g/cc) ρ w = density of wax or mixture of wax and protein, (g/cc) d₅₀ = average diameter of core substance, (µm) ε = void of wax or mixture of wax and protein, (-) The film thickness t can easily be obtained from the above equation. The microcapsules produced according to the present process can further be subjected to a post-treatment, namely, a melting-cooling-solidification treatment, whereby microcapsules having a more dense surface can be obtained. In one example of the melting-cooling-solidification treatment, only the surface wax of the microcapsules produced according to the present process is momentarily melted with hot air in order not to cause thermal denaturation of the core substance and then cooled with cold air for solidification. In another example, the microcapsules are melted by a frictional heat in a vessel equipped with a high speed stirrer and then cooled for solidification. The microcapsules obtained as mentioned above, are particles of from 5 µm to 5mm in diameter on which waxes or a mixture of wax and protein are coated as a layer of 0.1 to 10 µm in thickness on the surface of the core substance particles. Examples The present invention will be explained in more detail by referring to Examples. Example 1 Explanation is made by referring to Fig. 1 showing a typical flow chart of the present process. A mixture X consisting of 3 g of palmitic acid and 2 g of lauric acid was dissolved in a mixed solvent Y consisting of 85 ml of ethanol and 15 ml of water, at 40 deg.C [step (a)]. The resulting solution was poured on 100 g of feed particles 1 of purified casein (particle diameters: 105 to 250 µm) heated to 40 deg.C on a hot water bath, to immerse the particles 1 in the solution [step (b)]. Promptly, stirring was initiated to make the whole system uniform, and the wax was deposited on the particles 1. In this case, the wax-coated particles caused agglomeration but the agglomerated microcapsules were disaggregated by the stirring [step (c)]. Next, the slurry containing the wax-coated particles were subjected to a solvent removal treatment (vacuum drying under condition of 20 Torr x 1 hour and 1 Torr x 12 hours in this order) [step (d)], subsequently were disintegrated softly to obtain microcapsules Z₁ of 30 deg.C which are not adherent to each other. The structure of the microcapsules Z₁ is shown in Fig. 2 of a photograph by scanning type electron microscope. It is seen from Fig. 2 that the wax is uniformly coated on the particle surfaces. The microcapsules Z₁ were subjected to a melting-cooling-solidification treatment (hot air temperature 220 deg.C, cold air temperature 20 deg., exhaust gas temperature 50 deg.C)[step (e)] to obtain microcapsules Z₂ with more dense surface which are not adherent to each other. The structure of the microcapsules Z₂ is shown in Fig. 3 of a photograph by scanning type electron microscope. Example 2 This Example is conducted in the same manner of Fig. 1. A mixture consisting of 6 g of palmitic acid, 4 g of lauric acid and 4 g of zein was dissolved in a mixed solvent consisting of 255 ml of ethanol and 45 ml of water, at 40 deg.C. The resulting solution was poured onto 100 g of particles of a water-soluble vitamin mixture (particle diameters: 10 to 30 µm) heated to 40 deg.C on a hot water bath, to immerse the particles in the solution. Promptly, stirring was initiated to made the whole system uniform. Next, the slurry containing the particles were subjected to a solvent removal treatment (spray drying at an inlet temperature of 70 deg.C and an outlet temperature of 50 deg.C) [step (d)], to obtain microcapsules of 30 deg.C which are not adherent to each other. The structure of the microcapsules obtained is shown in Fig. 4 of a photograph by scanning type electron microscope. Example 3 Microcapsules were produced in the same manner as in Example 1 except that there was used, as a wax, a mixture consisting of 2.5 g of stearic acid and 2.5 g of lauric acid, and as a solvent, a mixed solvent consisting of 90 ml of ethanol and 10 ml of water, and as core substance, 100 g of particles of a mixture of an agricultural chemical and lactose, having particle diameters of 44 to 63 µm. The structure of the microcapsules obtained is shown in Fig. 5 of a photograph by scanning type electron microscope. Example 4 Microcapsules were produced in the same manner as in Example 1 except that there was used, as a wax, a mixture consisting of 3 g of palmitic acid and 2 g of capric acid, and as a solvent, a mixed solvent consisting of 80 ml of ethanol and 20 ml of water, and as a core substance, 100 g of particles formed from 90% by weight of a mineral mixture and 10% by weight of dextrin, having particle diameters of 63 to 105 µm. The structure of the microcapsules obtained is shown in Fig. 6 of a photograph by scanning type electron microscope. According to the present process for producing microcapsules, there can be coated on core substance particles of any size without causing the thermal denaturation of the core substance.

Claims: A process for producing microcapsules from particles of a core substance, characterized by dissolving wax or a mixture of wax and protein in a mixed solvent consisting of a water-miscible organic solvent and water, wetting, with the resulting solution, the surfaces of said particles of a core substance at a temperature of 45 °C or below to selectively deposit the wax or the mixture of wax and protein on said particle surfaces and allow the wax or the mixture of wax and protein to coat the particle surfaces, and then subjecting a slurry containing the coated particles to a solvent removal treatment. A process according to claim 1, wherein the water-miscible organic solvent is a polar solvent. A process according to claim 2, wherein the polar solvent is methanol, ethanol, propanol, acetone or methyl ethyl ketone. A process according to any of the claims 1 to 3, wherein the concentration of the water-miscible organic solvent in the mixed solvent is 56 to 95 % by volume. A process according to any of the claims 1 to 4, wherein said core substance particles contain a water-soluble substance. A process according to any of the claims 1 to 5, wherein the wax is soluble in organic solvents. A process according to any of the claims 1 to 6, wherein the deposit layer of wax or mixture of wax and protein has a thickness of from 0.1 to 10 µm. Microcapsules obtainable by process according to any of the claims 1 to 7. Microcapsules according to claim 8, wherein the core particles have a diameter of from 5 µm to 5 mm. Microcapsules according to claims 8 or 9 which microcapsules are mono-core type or multi-core type microcapsules.

Inventors: AISHIMA, SHIZUO, KOSEMURA, MASAKI, OHKAWARA, MASAAKI, TAKAHASHI, YOSHITO, Aishima, Shizuo, c/o OHKAWARA KAKOHKI CO. LTD., Kosemura, Masaki, c/o OHKAWARA KAKOHKI CO. LTD., Ohkawara, Masaaki, c/o OHKAWARA KAKOHKI CO. LTD., Takahashi, Yoshito, c/o OHKAWARA KAKOHKI CO. LTD.
Applicants: OHKAWARA KAKOHKI CO., LTD.
Application year: 1987
Main classification: A61K9
IPC codes: A01N25, B01J13, A23K1, A61K9, A61J3, A23P1, A23L1