MONO-CORE TYPE MICROCAPSULES AND PROCESS FOR PRODUCING THEM

Abstract: Mono-core type microcapsules comprising core particles with a particle size from 5 µm to 5 mm on which waxes are coated as a layer on the surface of the particles and/or penetrated to the inside of the particles, in which said wax coating is prepared by once melting the wax particles over the surface of the core particles and then re-solidifying them. The core material of the microcapsule undergoes less thermal degradation and can be formed with a dense and thin membrane profiling the surface layer thereof with the use of lesser amount of waxes.

Description: This invention concerns mono-core type microcapsules coated with waxes and a process for producing them. Mono-core type microcapsules coated with waxes in accordance with this invention can be utilized in all sorts of industrial fields including medicines, foodstuffs, feeds, fertilizers and chemical products. Microcapsules have hitherto been produced by using different methods as described in "Chemical Engineering of Japan", vol. 46, No. 10 (1982), on page 548. They are utilized in all sorts of industrial fields to encapsulate, for example, industrial foodstuffs, medicines, agricultural agents, feeds, perfumes, enzymes and activated carbons as described in the literature cited above. Among the production processes for the microcapsules aforementioned, an aerial suspension process (fluidizing bed coating) and a melt-dispersion-cooling process (spray and coagulating pelletization) are typical examples for depositing waxes on particles. However, since in the former process a fluidizing bed is used, particles as the core material are liable to be destroyed and intact membranes can be difficult to form because of the flaky shape of the membrane. Further, a large amount of wax has to be used to form an intact membrane. In the latter process, since the core material is added to a molten wax, the core material is thermally denatured and a lot of wax is required as well. Furthermore, the core material is localized in the wax or the core material is contained in the form of a plurality of particles in a wax particle to form multi-cored capsules. Accordingly, adequate microcapsule function cannot be attained depending on the application uses, for example, in the case of fish feeds. This invention provides mono-core type microcapsules coated with waxes and a process for producing them capable of overcoming the following problems in the conventional microcapsules as described below: (a) The core material is destroyed. (b) The core material is thermally denatured. (c) A lot of wax is required. (d) The core material forms a multi-cored and/or matrix structure or is localized in the wax. (e) No dense and thin membrane can be formed on the surface of particles, if waxes are coated, in the case of using the fluidizing bed. In order to overcome the foregoing problems in the prior art, this invention employs the following means. (1) At first, fine particles less soluble to an organic solvent (hereinafter referred to as primary particle 3) are immersed in the organic solvent 7 containing waxes 6 dissolved therein. (2) After immersing the primary particles into the wax solution of an organic solvent for a predetermined time, they are taken and vacuum-dried to form the primary wax membrane 9 thereon. The primary membrane 9 forms a coarse and not dense surface membrane, since the solvent is released, therethrough. Further, a portion of primary particles 3, 3 are agglomerated with each other by means of the dried primary membrane 9 into lumps. (3) Then, lumped primary particles 3 are disintegrated into the assembly of the capsule semi-product 11 comprising mono-core type particles formed with the primary membrane 9 thereon. (4) Then, since the capsule semi-product 11 is instantaneously melted and then quenched in a melting and quenching chamber 13, the coarse primary membrane 9 turns into the final membrane 16 which is highly smooth at the surface and in the shape profiling the surface layer of the particles. Accodingly, the core material of the capsule product 17 undergoes less thermal degradation and can be formed with a dense and thin membrane profiling the surface layer thereof with a use of lesser amount of waxes. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, as well as features of this invention will now be described by way of preferred embodiments thereof while referring to the accompanying drawings, wherein Figure 1(a) is an explanatory view for the step of forming the primary particles 3 by a spray dryer 2 ; Figure 1(b) is an explanatory view for the step of forming the primary particles 3 from dry lump starting material 4 by means of a crusher 5; Figure 2(a) is an explanatory view for the solution of waxes 6 in an organic solvent 7; figure 2(b) is an explanatory view for the step of forming the primary membrane 9 on the primary particles 3; Figure 2(c) is an explanatory view for the lump of the primary particles 3 agglomerated by means of the primary membrane 9; Figure 3(a) is an explanatory view for the operation of an disintegrator 10; Figure 3(b) is an explanatory view for the capsule semi-product 11; Figure 4(a) is an explanatory view for the operation of the melting and cooling chamber; Figure 4(b) is an explanatory view for the microcapsule product 17; Figure 5 shows another embodiment of the primary particles; Figure 6 is a graph showing the result of the test in Example 1; Figure 7 is a graph showing the result of the test in Example 2; and figure 8 is a graph showing the result of the test in Example 3. DETAILED DESCRIPTION OF THE INVENTION This invention will now be described more in details. Description for the Production Process (1) Preparation of primary particles : Both solid material and liquid material can be used as the starting material for the primary particles 3 as the core material. The primary particles 3 can be formed by a suitable process such as granulation, crushing and sieving, etc. As an embodiment using liquid material as the starting material, the starting material is supplied to a spray drier 2 through a material feed pipe 1 in Figure 1. The primary parti­ cles 3 can thus be formed by spray drying the starting material for the primary particles. Examples of the materials and the properties of the primary particles 3 will be described later. In another process for forming the primary particles 3, the primary particles 3 can also be obtained by preparing start­ ing lump material 4 through drying in a drier which are then pulverized in a crusher 5 as shown in Figure 1 (b). Further, the primary particles 3 can also be obtained by a fluidized bed-granulation process, a stirring-granulation process and a coating-granulation process etc. in which a powder material is used as the starting material and granulated by spraying an appropriate binder. (2) Preparation of primary-wax membrane : Waxes 6 are previously dissolved in an organic solvent 7 to prepare a wax solution which is stored in a container 7 as shown in Figure 2(a). The concentration of the wax solution is generally from 0.01 to 40 wt%, preferably from 0.5 to 30 wt% and more preferably from 2 to 20 wt%. The primary particles 3 prepared as described above are immersed into the wax solution for 1 - 60 minutes as shown in Figure 2(a). Then, when the primary particles 3 are vacuum-dried. In this case, the vacuum degree is high at first and the final vacuum degree is generally 0.5 to 100 Torr, preferably 1 to 30 Torr and more preferably 1 to 10 Torr. The organic solvent 7 is evaporated from the solution containing the waxes at the surface of the primary particles 3 to form the primary membrane 9. It is of course necessary for the primary particles 3 that they are less soluble to the organic solvent 7 or should be surrounded with a less soluble substance. Since the organic solvent 7 has been released through the primary membrance 9, the surface thereof becomes not dense but rough. Further, a portion of the primary particles 3 is agglomerated with each other into a lump form due to the presence of the film-like primary membrane 9 as shown in Figure 2(c). (3) Preparation of capsule semi-product 11 : The agglomerated particles are disintegrated in a disintegrator 10 as shown in Figure 3(a) to obtain a capsule semi-product 11 formed with the primary membrane 9 as shown in Figure 3(b). It is desired for the waxes that they have such a crystalline structure as enabling this disintegration. (4) Completion of microcapsule product : The capsule semi-product 11 is supplied together with air (a) or nitrogen gas into a melting and cooling chamber 13. (Figure 4) The primary membrane 9 on the semi-product particles 11 supplied into the melting and cooling chamber 13 is instantaneously melted in a hot gas zone 14 with a hot gas (h) at a temperature generally higher than the melting point by 30 - 200°C, preferably by 50 - 150°C and more preferably by 75 - 100°C, as shown in Figure 4(a). Then, the molten primary membrane 9 is cooled to solidify again with a cold air (c) at a temperature below the melting point by 10 - 200°C, preferably by 30 - 150°c and more preferably by 50 - 100°C, in the cooling zone 15. In this way, the final membrane 16 with a highly smooth surface and in a shape profiling the surface layer of particles is formed. The microcapsule product 17 can thus be produced in a short time continuously. Waxes : The waxes usable in this invention can include the followings. 1. Natural oil and fat and hardened oils : Hardened rape oil, hardened castor oil, hardened beef tallow, palm oils, and the like. 2. Wax : Carnauba wax, bleached bees wax, paraffin wax, montan wax, ceresine wax, candelilla wax, shellac wax, and the like. 3. Fatty acid (C₁₀ or higher) Capric acid, undedanoic acid, lauryl acid, tridecyl acid, myristic acid, palmitic acid, stearic acid, elaidic acid, nonadecanoic acid, arachic acid, behenic acid, lignocerinic acid, octadecenoic acid, vaccenic acid, ercic acid, brassidic acid, cerotic acid, montanic acid, hexadecenic acid, eicosenic acid, pentadecylic acid, heptadecylic acid, heptacosanoic acid, melissic acid, racceric acid, lenolic acid, linolenic acid, undecylenic acid, cetoleic acid, arachidolinic acid, octadecylic acid, oleic acid, and the like. 4. Alcohols : 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, undecyl alcohol, linoleyl alcohol, olein alcohol, linolenyl alcohol, and the like. 5. Fatty acid glycerine ester : Caprin, myristine, palmistin, laurin, stearin, elaidin, brassidin, linolein, linolenin, olein, and the like. 6. Others : cholresterol, lecithin. That is, waxes means those organic materials melted under heating and solidified by cooling as described above, either solely or in admixture. It is desired that the waxes have such a crystalline structure as can be separated with no stickings upon disintegration in the disintegrator 10. Organic solvent : Organic solvent usable herein can include usual low boiling solvents such as ethanol, acetone, chloroform, carbon tetrachloride, benzene and n-hexane. Those solvents dissolving the primary particles 3 have to be avoided. Examples of the material for the primary particle 3 and particle size of the microcapsules : All sort of materials including both organic and inorganic materials can be used as the primary particles 3. Examples of the organic material are shown in Examples (1) and (2). Examples of inorganic materials include ceramic particles, activated carbon and the like in addition to those shown in Example (3). The microcapsules used herein are defined as those having particle size from 5 µm to 5 mm. In the process according to this invention, since the primary membrane 9 is heat-melted instantaneously in the hot blow zone 14 and then quenched to re-solidify in the cooling zone 15, the particle size of the microcapsules is defined as within the above specified range. If the particles size is too small, the particles are suspended and not cooled immediately and, while on the other hand, if it is too large no uniform heat melting is possible. Addition of water soluble material : The collapsing time of the final film-like membrane 16 (or the release rate of core material) can be controlled by adding water soluble material to some extent to the waxes. Accordingly, it is desired to add water soluble material depending on the time required for using the microcapsules. The water soluble material that can be added herein includes gelatin, gum arabic, sodium carboxymethylcellulose (CMC), polyvinyl alcohol (PVA), cellulose acetate phthalate (CAP), methyl methacrylic-methacrylic acid copolymer (Eudragit L30D-55, trade name). Other examples of primary particles : As shown in Figure 5, for instance, porous material such as activated carbon is used as core material carrier 18, which is immersed in a solution contained the core material dissolved therein and adsorbed to penetrate and dried to form primary partaicles 20 carrying the core material 19 thereon. The production step for the primary particles can thus be simplified. The primary particles using the carrier 18 is suitable for the catalyst or the like. In the case if the core material is soluble to the organic solvent, a primary particles less soluble to the organic solvent may be prepared by mixing a water soluble high molecular material, for example, sodium casein, skim milk powder and cyclodextrin. Example 1 This example shows the case where this invention is applied to fry young fish feeds. (1) The following formulations are used for the fry young fish feeds. : Egg yolk 24 wt% Shellfish (small clam) extracts 8 wt% Bonite extracts 5 wt% Egg albumine 10 wt% Cuttle liver oil 2 wt% Opossum shrimp (okiami) extract oil 2 wt% Vitamin E 1 wt% Oil and fat yeast 12 wt% Vitamin mixture 8 wt% Mineral mixture 6 wt% were dissolved in water added to form an emulsion containing 40 % solid component. The emulsion was spray-dried using a spray drier to obtain particulates comprising primary particles of about 100 µm particle size. (2) While on the other hand, 20 g of waxes comprising, 40 % palmitic acid and 60 % lauryl acid were dissolved in 80 g ethanol and kept at 40°C. The particulates comprising the primary particles 3 were immersed for 5 minutes into the wax solution as described above. (3) Then, the primary particle 3 were taken out from the wax solution and directly vacuum-dried. In this case, the vacuum degree was controlled to 10 Torr during the period while a great amount of the solvent was remained and to 2 Torr when the solvent was reduced to a smaller amount. In this way, the primary membrane 9 was formed and, at the same time, the primary particles were agglomerated with each other by means of the primary membrane 9 into lumps. (4) Then, the agglomerated primary particles 3 were disinte­ grated in a disintegrator 10 to obtain the assembly of capsule semi-product 11. (5) Then, the assembly of the capsule semi-product 11 was supplied to a melting and cooling chamber 13 in which the primary membrane 9 was instantaneously melted at the hot gas zone 14 with a hot gas at a temperature of 200°C and, thereafter, rapidly cooled to solidify in the cooling zone 15 with a cold gas at 20°C to obtain microcapsule product 17 having the final membrane 16. The microcapsule product prepared as described above was poured into water and the leaching amount of vitamin B₂ contained therein was tested to obtain the curve B as shown in Figure 6. In Figure 6, the abscissa indicates the time after pouring the microcapsule product and the ordinate indicates the leaching amount of vitamin B₂. In Figure 6, the curve A shows the result of the test where no wax coating was applied at all. Further, the curve (C) shows the result of the test carried out for the microcapsule product 17 obtained by immersing the primary particles 3 into a solution of 10 g of bleached bees wax as waxes dissolved in 90 g of chloroform. The curve (D) shows the result for the test of the capsule product prepared by settling the formulation of the primary particles 3 as described above with a binder into fry yound fish feeds. Furthermore, the curve (E) shows the result of the test in a case where the water soluble material is added to the waxes shown by the curve (C). Addition of the water soluble material is attained by dispersing 5 g of fine powder prepared by spray drying Eudragit L-30D-55 into the chloroform solution containing bleached bees wax dissolved therein. Example 2 In this example, this invention was applied to medicines. (1) Preparation of primary particles 3 100 g of a mixture comprising 70 wt% of sodium casein and 30 wt% of aspririn were dissolved into water incorporated with an emulsifier to obtain an emulsified suspension containing 20 % solid component. The emulsified liquid suspension was spray dried in a spray drier to obtain primary particles 3 with 50 µm average particle size. (2) Wax solution 10 g of carnauba wax were dissolved in 90 g of chloroform and kept at 45°C. (3) Immersion The primary particles 3 as described above were immersed in the wax solution as described above for 5 minutes. (4) Vacuum drying The immersed primary particles 3 were taken out and directly vacuum-dried under 2 Torr of vacuum degree for one hour. (5) Disintegration The dried primary particles were disintegrated in the disintegrator 10 to form the assembly of the capsule semi-­ product 17. (6) Melting and cooling The primary membrane 9 was instantaneously melted in the hot gas zone 14 (hot gas temperature at 180°C) in the melting and cooling chamber 13 and, thereafter, quenched to solidify in the cooling zone 15 to obtain the microcapsule product 17. (7) Test result In Figure 7, the curve (G) shows the result of the test for the microcapsule product 17 as described above and the curve (F) shows the result of the test for the aspririn powder not applied with the wax coating. Example 3 This example shows the case where this invention was applied to agricultural agents. (1) Primary particle 3 Copper sulfate (CuSO₄.5H₂O) was dissolved in water and spray liquid with 40 % solid contents was spray - dried in a spray drier to obtain particulates of 50 µm average particle size. (2) Wax solution 5 g of carnauba wax were dissolved in 95 g of chloroform and kept at 40°C. (3) Immersion The primary particles 3 as described above were immersed in the wax solution as described above for 5 minutes. (4) Vacuum drying The primary particles 3 immersed as described above were instantly vacuum - dried for one hour under the vacuum degree of 2 Torr to form the lump product comprising particles formed with the primary membrane 9. (5) Disintegration The lump product was disintegrated to obtain the assembly of the capsule semi-product 11. (6) Melting and cooling The primary membrane 9 of the capsule semi-product 11 was instantaneously melted in the hot gas zone 14 with a hot gas at 180°C. Then, it was quenched to solidify in the cooling zone 15 with a cold gas at 15°C to obtain the microcapsule 17. (7) Test result In Figure 8, the curve (I) shows the result of the test for the microcapsule products 17 and the curve (H) shows result of the test for those not applied with wax coating. As mentioned above, this invention has the following advantages and effects. (a) Since the microcapsules according to this invention undergo no impact shock during the production step therefor, different from the fluidizing layer coating process for instance, the primary particles 3 as the core material are free from damages. (b) Since the capsule semi-product 11 is merely exposed to the high temperature only instantaneously and within a range not reaching the core material in the hot gas zone 14 of the melting and cooling chamber 13, the core material free from heat denaturation. This is extremely important in the case where the microcapsules according to this invention is served for such application uses where the core material has to be stored, for example, in foodstuffs, medicines and feeds. (c) Since the final membrane 16 comprises the membrane with a highly smooth surface and in a shape profiling the surface layer of individual primary particles 3 and since a portion thereof penetrates to the inside of the core material, a satisfactory hydrophobic membrane can be obtained with a lesser amount of waxes. (d) The strength of the membrane can be adjusted in the mono-core type microcapsules according to this invention by controlling the coating amount of the final membrane 16. In the conventional multi-cored capsules, the strength of the membrane can not be adjusted. For instance, in the case of using the microcapsules to the fry young fish feeds as in Example 1, adjustment for the strength of the membrane is necessary. (e) Although the mono-core type particles may also be formed by using the fluidizing coating process, the particles thus formed show much scattering in the membrane strength and are inferior in the moisture proofness, fluidizing property, filling performance and handlability as compared with this invention. (f) When fatty acid type waxes are used, microcapsules show good intra-intestinal solubility and can comply the requirements for the medicines.

Claims: 1. Microcapsules of mono-core type comprising core particles of from 5 µm to 5 mm particle size and a wax coating, characterized in that said wax coating has been prepared by melting a primary wax coating on the surface of the core particles and then re-solidifying the coating. 2. A process for producing mono-core type microcapsules, characterized by immersing core particles having a particle size from 5 µm to 5 mm in a solution comprising at least one wax dissolved in an organic solvent, taking out and then vacuum-drying the resulting wax-coated core particles to form primary particles comprising a primary wax coating on the core particles, a portion of the primary particles being agglomerated by adhesions of the primary wax coating, disintegrating said agglomerated primary particles into a capsule semi-product comprising mono-core type particles, introducing said capsule semi-product to a hot gas zone to melt the primary wax coating, and quenching to solidify said molten primary membrane in a cooling zone to obtain a mono-core type microcapsule product coated with a final wax coating. 3. A process according to claim 2, wherein water soluble materials are added to the waxes. 4. A process according to claim 2 or 3 wherein the primary particles are vacuum-dried at a pressure of from 0.067 to 13 kPa (0.5 to 100 Torr). 5. A process according to claim 2, 3 or 4 wherein a hot gas having a temperature 30 - 200°C above the melting point of the primary wax coating is introduced into the hot gas zone. 6. A process according to any of claims 2 to 5, wherein a cold gas having a temperature 10 - 200°C below the melting point of the primary wax coating is introduced into the cooling zone.

Inventors: MIYAHARA, MASAYUKI, OHKAWARA, MASAAKI, ONO, YOSHITAKA
Applicants: OHKAWARA KAKOHKI CO., LTD.
Application year: 1986
Main classification: A61K9
IPC codes: A23K1, B01J13, A61K9, A01N25