Production method of microcapsules

Abstract: A process for producing microcapsules comprises forming microcapsules having wall films of an amine-aldehyde condensation polymer in an emulsion of a hydrophobic core substance in an acid aqueous solution containing an anionic water-soluble polymer which is a copolymer of acrylic acid, an acrylamidealkylsulfonic acid and an alkyl acrylate.

Description: This invention relates to a process for producing microcapsules having amine-aldehyde condensation polymer wall films used for converting or controlling properties in the areas of, for example, pharmaceuticals, agrochemicals, perfumes, liquid crystals, pressure-sensitive copy paper and heat-sensitive copy paper These microcapsules are particularly effective for use in a pressure-sensitive paper. This invention concerns a chemical method to form microcapsule wall films comprising an amine-aldehyde condensation polymer from only one continuous phase, that is, an in-situ polymerisation method. Methods in which amines and aldehydes are used as wall film formation materials are described, for example, in JP-C-37-12380, JP-C-44-3495 and JP-C-47-23165. However, these methods involve disadvantages in that emulsification and dispersion are not efficient, or it is difficult to build up a polymer around a hydrophobic substance efficiently and stably. As an anionic polymer which improves such problems, use of an acrylic polymer or an acrylic acid copolymer is disclosed in JP-B-54-16949, JP-A-58-14942, JP-A-59-142836, JP-A-60-28819, JP-A-60-68045, JP-A-60-190227, JP-A-60-216839, JP-A-60-238140, JP-A-61-11138, JP-A-61-17491, JP-A-62-19238, JP-A-62-57645, JP-A-62-97638, JP-A-62-250943 and JP-A-63-134084. However, these technologies have the following problems which have yet to be improved. In JP-B-54-16949, the build-up efficiency of a condensation polymer around a core substance has been improved, but the resulting capsule slurry has a very high viscosity. JP-A-58-14942, JP-A-59-142836 and JP-A-60-68045 disclose the production of a slurry which has a high concentration, low-viscosity and good particle size distribution. However, the slurry viscosity in a high concentration is not sufficiently low to be satisfactory. When the anionic polymers disclosed in JP-A-60-28819 (EP-A-133,295), JP-A-60-190227, JP-A-60-216838, JP-A-60-238140, JP-A-61-11138, JP-A-61-17491, JP-A-62-19238, JP-A-62-57645, JP-A-62-97638, JP-A-62-250943 and JP-A-63-134084 are used, high-concentration, low-viscosity capsule slurries can be obtained in all cases. However, when capsules obtained by these methods are used in a pressure-sensitive copy paper, they are defective in that the staining degree is high with respect to colour development, because the particle size distribution of the capsules is broad, or, if not, they are still dissatisfactory in terms of staining (spot staining) generated on the pressure-sensitive copy paper. In particular, spot staining is notable in self-contained pressure-sensitive copy paper. The present invention seeks to provide an improvement in a production method of microcapsules by providing an in-situ polymerization method to form microcapsule wall films comprising an amine-aldehyde condensation polymer by a reaction from only one continuous phase, to reduce spot staining when used in pressure-sensitive copy paper by reducing traces of coarse particles present in the capsule slurry, and to provide a production method of microcapsules in which the emulsification power is high, the emulsified particles during the reaction have good stability, and the resulting microcapsules have good particle size distribution, good density of capsule walls, good workability and superior overall properties. The present invention provides a process for producing microcapsules comprising forming microcapsules having wall films of an amine-aldehyde condensation polymer in an emulsion or dispersion of a hydrophobic core substance in an acid aqueous solution containing an anionic water-soluble polymer which is a copolymer consisting of 50 to 90, excluding 50, weight % of acrylic acid, 2 to 40 weight % of an acrylamidealkylsulfonic acid and 2 to 15 weight % of an alkyl acrylate, and optionally, 0 to 15 weight % of at least one of methacrylic acid, alkylmethacrylate hydroxyalkylacrylate, hydroxyalkylmethacrylate, acrylamide, methacrylamide, succinic acid ester of hydroxyakylacrylate, succinic acid ester of hydroxyalkylmethacrylate and vinylactate. The monomer composition of the water-soluble polymer used in the present invention is generally 50-90 weight % of acrylic acid, 2-40 weight % of acrylamidealkylsulfonic acid, 2-15 weight % of alkylacrylate and 0-15 weight % of at least one monomer selected from methacrylic acid, alkylmethacrylate, hydroxyalkylacrylate, hydroxyalkylmethacrylate, acrylamide, methacrylamide, succinic acid ester of hydroxyalkylacrylate, succinic acid ester of hydroxyalkylmethacrylate and vinylacetate, more preferably, 60-80 weight % of acrylic acid, 5-30 weight % of acrylamidealkylsulfonic acid, 3-10 weight % of alkylacrylate and 2-10 weight % of at least one monomer selected from methacrylic acid, alkylmethacrylate, hydroxyalkylacrylate, hydroxyalkylmethacrylate, acrylamide, methacrylamide, succinic acid ester of hydroxyalkylacrylate, succinic acid ester of hydroxyalkylmethacrylate and vinylacetate. The copolymer may be in the form of non-salt or part salt, preferably a sodium, potassium or lithium salt. To obtain the copolymer it is preferable to use a radical polymerization method in an aqueous medium, which is a conventional polymerization method of acrylic acid in which a mixture of acrylic acid, acrylamidealkylsulfonic acid and alkylacrylate, and as required at least one monomer selected from methacrylic acid, alkylmethacrylate, hydroxyalkylacrylate, hydroxyalkylmethacrylate, acrylamide, methacrylamide, succinic acid ester of hydroxyalkylacrylate, succinic acid ester of hydroxyalkylmethacrylate and vinylacetate, is placed in a vessel, after air in the vessel has been purged with nitrogen gas, an organic or inorganic peroxide or persulfate is added as a polymerization initiator, and the mixture heated to carry out polymerization. After completion of the reaction, part of the copolymer may be neutralized with an alkali such as caustic soda as required. When radical polymerization is carried out in an aqueous medium, the copolymer is generally obtained as an aqueous solution containing 5-30 weight % of non-volatiles. It is practically convenient to handle the aqueous solution in the form of a composition containing normally 20 weight % of non-volatiles and having a pH of 1 to 4. Normally, the viscosity is preferably 100-5,000 cps for a solution of 20 weight % of non-volatiles at 25°C and a pH of 3.1, measured by a B-type viscometer, more preferably 300-1,000 cps. If the viscosity is less than 100 cps, the particle size distribution tends to become broader and the content of coarse particles tends to increase. On the other hand, if the viscosity is higher than 5,000 cps, since the viscosities during the reaction and that of the capsule slurry are high, it is not suitable for use in pressure-sensitive copy paper for which a low-viscosity slurry is favoured. The acrylamidealkylsulfonic acid used in the present invention can be, for example, acrylamideethylpropane-sulfonic acid, acrylamidepropanesulfonic acid, 2-acrylamide-2-methylpropanesulfonic acid, acrylamidebutane-sulfonic acid or acrylamideethansulfonic acid. Of these, it is preferable to use 2-acrylamide-2-methylpropanesulfonic acid (hereinafter referred to as AMPS) in view of the availability of the monomer and its copolymerizability. The alkylacrylate is generally one in which the alkyl has a straight or branched chain and has 1 to 10 carbon atoms. More preferably the alkyl acrylate is methylacrylate, ethylacrylate, propylacrylate, butylacrylate, amylacrylate or hexylacrylate. One embodiment of the process for producing microcapsules according to the present invention basically comprises the following steps: (1) Preparation of the core substance A hydrophobic liquid for the core substance of the capsules is used alone, or as needed in combination with other substances dissolved in the hydrophobic liquid by heating and stirring, to prepare the core substance. (2) Preparation of continuous phase The water-soluble polymer is dissolved in water at room temperature, in which an amine or an initial condensate of amine and aldehyde is dissolved to obtain a continuous phase. The initial condensate may be modified with, for example, phenols or benzoguanamine. The ratio of the wall film material and the core substance is 1 : 1-20 in solid weight, and the water-soluble polymer of the present invention is used in an amount of 0.5 to 7 weight % of the entire capsule production system. Depending on the type of the water-soluble polymer, the type of the wall film formation material, the type of the core substance, and the purpose of encapsulation, the water-soluble polymer is preferably used in an amount of 1 to 5 weight % of the entire capsule production system. The amine and aldehyde as wall film formation materials can be used alone or as a prepolymer dissolved in the continuous phase. The amine is selected from urea, thiourea, and melamine, alkylated products of these with an alkyl of 1-4 carbon atoms, methylolated products of these, alkylated methylolated products of these with an alkyl of 1-4 carbon atoms and mixtures thereof. Of these substances, it is preferable to use at least one of melamine, methylolmelamine, methylated methylolmelamine, urea, dimethylolurea and methylated methylolurea. (3) Emulsification While agitating the continuous phase, the core substance solution is added to the continuous phase and emulsified by an emulsifying machine such as a homogenizer or a stationary mixer. Better particle size distribution is obtained by carrying out the emulsification at room temperature or a lower temperature. (4) Capsule wall formation reaction When emulsified particles of a predetermined particle diameter ale obtained, the pH of the emulsified solution is adjusted to a range for condensation polymerisation of the wall film materials used. In general, this reaction is preferably carried out in an acid condition to obtain a dense wall film. In view of the fact that the lower the pH, the faster the reaction, but too violent reaction disturbs the formation of a uniform capsule wall, the pH is adjusted to approximately 3. The proportions of the amine and aldehyde depend on the types of the individual materials and cannot be determined wholly, but normally 0.5 to 5.0 moles of aldehyde are used to 1 mole of amine. For the case of urea and formaldehyde, it is preferable to use 1.2 to 3.0 moles of formaldehyde to 1 mole of urea. Heating is started simultaneously with the completion of emulsification or the addition of the aldehyde. Heating is carried out 35 to 90°C, and the temperature is normally adjusted to 50 to 80°C. After reaching the setting temperature, it is necessary to maintain the setting temperature for more than a predetermined period of time. For a reaction at 50 to 60°C, it is preferable to maintain the temperature for at least 1 hour. (5) Post-treatment After the completion of encapsulation, concentration adjustment, pH adjustment, washing, filtration, drying and pulverization, for example, are performed if required. When formaldehyde is used as the aldehyde, treatment is performed to reduce excess formaldehyde which is usually present in the slurry. This is achieved by using a substance having active hydrogen such as ammonia, urea, sulfite, hydrogensulfite, ethyleneurea, hydroxyamine salt or methylacetoacetate, alone or in combination. Furthermore, chemical removing methods using many types of carbonates in combination, and physical removing methods such as steam distillation and ultrafiltration, are proposed, which may be used appropriately Although the detailed behaviors of the individual monomers are unknown, in the polymerisation, the acrylic acid enhances the stability of the emulsified particles in the emulsification stage and the capsule wall formation, reaction (hereinafter referred to as emulsification stability) and promotes the capsule wall formation reaction to give a capsule slurry of reduced agglomeration. However, use of a single monomer is insufficient in terms of its action, resulting in a capsule curry of a relatively high viscosity containing coarse particles. The capsule wall film is dense but relatively soft. The acrylamidealkylsulfonic acid promotes the capsule wall formation reaction and gives a capsule slurry with a narrow particle size distribution and low viscosity, but the capsule wall film is relatively hard and low in density. The alkylacrylate does not promote the capsule wall formation reaction, but does improve the emulsification and dispersing effect, and is effective to reduce coarse particles. However, use in excess tends to broaden the particle size distribution, increase the capsule slurry viscosity, and cause agglomeration of capsules during the capsule wall formation reaction. When acrylic acid, acrylamidealkylsulfonic acid, and alkylacrylate are used in combination, the reduction in coarse particles is remarkable as compared to the use of two of these components. This is considered as due to a comprehensive balance between the effect of hydrophilicity of the carboxyl group of acrylic acid and the sulfonic group of acrylamidealkylsulfonic acid and the effect of hydrophobic groups such as alkyl and acrylamidealkyl. Furthermore, by using at least one monomer selected from methacrylic acid, alkylmethacrylate, hydroxyalkylacrylate, hydroxyalkylmethacrylate, acrylamide, methacrylamide, succinic ester of hydroxyalkylacrylate, succinic ester of hydroxyalkylmethacrylate and vinylacetate as needed, an improvement in capsule wall formation rate, particle diameter distribution adjustment of emulsified particles, adjustment of density and rigidity of capsule wall films and adjustment of capsule slurry viscosity, for example can be achieved. The present invention is now further described in the following Examples and Comparative Examples. Parts and % used in the Examples indicate parts by weight and % by weight, respectively. Example 1 550 g of water was charged in a 2-liter flask provided with a reflux cooler, a thermometer, a nitrogen inlet tube, and two dropping funnels, and heated to 85 to 88°C under stirring. Separately, 36 g of 2-acrylamide-2-methylpropanesulfonic acid was dissolved in 150 g of water, mixed with 14.5 g of 48% NaOH to neutralize, and a mixture of 216 g of acrylic acid and 25 g of butylacrylate (solution A) and 2.22 g of potassium persulfate dissolved in 100 g water (solution B) were prepared. The solution A and the solution B were individually charged in the separate dropping tunnels and dropped at the same temperature over a period of 1.5 hours. After that, the reaction mixture was maintained at the same temperature for 4 hours, and then cooled. Further, 7.5 g of 48% NaOH and water was added to obtain an anionic water-soluble polymer having a composition of acrylic acid/2-acrylamide-2-methylpropanesulfonic acid/butylacrylate = 78/13/9 and a concentration of 20%. The product had a viscosity of 710 cps. 35 parts of the water-soluble polymer obtained as above was diluted with 80 parts of water, and 10 parts urea and 1.2 parts of resorcinol were mixed. The pH of the mixture was adjusted to 3.4 with an aqueous caustic soda solutions Separately, 135 parts of a high-boiling solvent (Nippon Petrochemicals, Hisol SAS-296) mainly comprising phenylxylylethane was mixed with 5 parts of Crystal Violet Lactone and dissolved by heating and stirring at 100°C. After dissolution, the solution was cooled to room temperature, mixed into the previously prepared water-soluble polymer-containing aqueous solution taking care of phase transition, and emulsified by agitating with the Homomixer Model M (Tokushukika) at 10,000 rpm for 3 minutes to obtain a stable O/W emulsion with an average particle diameter of 3.6µm. The emulsion was mixed with 70 parts of dilution water and 23.5 parts of 37% formaldehyde, heated, and after the temperature reached 55°C, the capsule film formation reaction was continued for 2 hours. When a small amount of the resulting capsule solution was put into a 20% aqueous resorcinol solution and stirred to observe the completeness of the capsule wall, no change was noted (incomplete capsule wall formation is indicated by a blue colour) and dense wall films were formed. To reduce residual formaldehyde, after cooling to 30°C, the capsule solution was mixed with 29% ammonia water until the pH was 7.5 to obtain a capsule slurry for a pressure-sensitive copy paper. Example 2 A water-soluble polymer was prepared using the same procedure as in Example 1, except that 9 parts of ethylacrylate was used in place of 9 parts of butylacrylate. A capsule slurry was produced using this water-soluble polymer and the same procedure as in Example 1. Example 3 A water-soluble polymer was prepared using the same procedure as in Example 1, except that 9 parts of propylacrylate was used in place of 9 parts of butylacrylate. A capsule slurry was produced using this water-soluble polymer and the same procedure as in Example 1. Example 4 A water-soluble polymer was prepared using the same procedure as in Example 1, except that 65 parts of acrylic acid, 30 parts of AMPS, and 5 parts of butylacrylate were used in place of 78 parts of acrylic add, 13 parts of AMPS and 9 parts of butylacrylate. A capsule slurry was produced using this water-soluble polymer and the same procedure as in Example 1. Example 5 A water-soluble polymer was prepared using the same procedure as in Example 1, except that 88 parts of acrylic acid, 7 parts of AMPS, and 5 parts of butylacrylate were used in place of 78 parts of acrylic acid, 13 parts of AMPS, and 9 parts of butylacrylate. A capsule slurry was produced using this water-soluble polymer and the same procedure as in Example 1. Example 6 A water-soluble polymer was prepared using the same procedure as in Example 1, except that 78 parts of acrylic acid, 13 parts of AMPS, 6 parts of butylacrylate and 3 parts of 2-hydroxyethylmethacrylate were used in place of 78 parts of acrylic acid, 13 parts of AMPS, and 9 parts of butylacrylate. A capsule slurry was produced using this water-soluble polymer and the same procedure as in Example 1. Example 7 A water-soluble polymer was prepared using the same procedure as in Example 1, except that 78 parts of acrylic acid, 13 parts of AMPS, 6 parts of butylacrylate, and 3 parts of methylmethacrylate were used in place of 78 parts of acrylic acid, 13 parts of AMPS, and 9 parts of butylacrylate. A capsule slurry was produced using this water-soluble polymer and the same procedure as in Example 1. Example 8 A water-soluble polymer was prepared using the same procedure as in Example 1, except that 78 parts of acrylic acid, 13 parts of AMPS, 3 parts of butylacrylate, 3 parts of 2-hydroxyethylmethacrylate, and 3 parts of methylmethacrylate were used in place of 78 parts of acrylic acid, 13 parts of AMPS, and 9 parts of butylacrylate. A capsule slurry was produced using this water-soluble polymer and the same procedure as in Example 1. Comparative Example 1 A water-soluble polymer was prepared using the same procedure as in Example 1, except that 90 parts of acrylic acid and 10 parts of butylacrylate were used in place of 78 parts of acrylic acid, 13 parts of AMPS, and 9 parts of butylacrylate. A capsule slurry was produced using this water-soluble polymer and the same procedure as in Example 1. Comparative Example 2 A water-soluble polymer was prepared using the same procedure as in Example 1, except that 87 parts of acrylic acid and 13 parts of AMPS were used in place of 78 parts of acrylic acid, 13 parts of AMPS, and 9 parts of butylacrylate. A capsule slurry was produced using this water-soluble polymer and the same procedure as in Example 1. Comparative Example 3 A water-soluble polymer was prepared using the same procedure as in Example 1, except that 84 parts of acrylic acid, 8 parts of styrenesulfonic acid, and 8 parts of butylacrylate were used in place of 78 parts of acrylic acid, 13 parts of AMPS, and 9 parts of butylacrylate. A capsule slurry was produced using this water-soluble polymer and the same procedure as in Example 1. Comparative Example 4 A water-soluble polymer was prepared using the same procedure as in Example 1, except that 75 parts of acrylic acid, 13 parts of styrenesulfonic add, 6 parts of 2-hydroxyethylmethacrylate, and 6 parts of methylmethacrylate were used in place of 78 parts of acrylic acid, 13 parts of AMPS, and 9 parts of butylacrylate. A capsule slurry was produced using this water-soluble polymer and the same procedure as in Example 1. (Test methods) The capsule slurries obtained in the above Examples and Comparative Examples were tested by the following test methods, and the test results are shown in Table 1. Viscosity: Viscosity of the capsule slurry was measured at 25°C by a B-type rotary viscometer. Emulsification property: Emulsification was carried out under the same conditions, and the average volume particle diameter after 3 minutes of emulsification was measured by an ELZONE Particle Counter Model 80XY particle diameter measuring instrument. The smaller the value, the batter the emulsification property. Emulsion stability: The average volume particle diameter of the capsule particles after encapsulation was measured by an ELZONE Particle Counter Model 80XY particle diameter measuring instrument, and the emulsion stability was evaluated from the difference between the average volume particle diameter of the capsule particles and the average volume particle diameter of the emulsified particles. The smaller the value, the better the emulsion stability. Particle size distribution: Emulsification and encapsulation were carried out under the same conditions. The average volume particle diameter was measured by an ELZONE Particle Counter Model 80XY particle diameter measuring instrument, and the particle size distribution was represented by 10 times the logarithm of the particle diameter ratio the 25% volume point to the 75% volume point. The smaller the value, the narrower the particle size distribution. Coarse particles: The produced capsule slurry was diluted to 20%, coated on a coated front (hereinafter referred to as CF) paper of a pressure-sensitive copy paper (Jujo Paper, CCP ACE W-40BL) using a No.20 meyer bar, dried, and the number of spot-like coloured stains appearing on the CF paper in an area of 15 x 20 cm was counted. Coarse particles of greater than 20µm appear as spot-like staining on the CF paper. Film density: The capsule slurry was diluted to 20%, coated on a CF paper of a pressure sensitive copy paper (Jujo Paper, CCP ACE W-40BL) using a No.14 meyer bar, dried in a 150°C oven for 1 minute, allowed to stand for 1 hour, and reflectance of the coated surface was measured by a Hunter reflectometer (Toyo Seiki Seisakusho). The difference in reflectance between the uncoated CF paper and the coated CF paper was used as an index of film density. The value is small for a good film density since the reduction in reflectance is small. For practical use, it is preferable that the value is 5% or less. Water-soluble polymer vis. 25°C,cps Slurry vis. 25°C.cps Emulsified solution av. particle diameter µm Capsule av. particle diameter µm Particle size distribution Spot-like staining (pieces) Film density % Example 1 710 436 3.6 3.8 1.86 3 3.0 2 680 413 4.1 4.2 1.98 4 3.3 3 662 380 4.1 4.3 1.99 4 3.8 4 460 555 3.8 4.0 2.01 4 3.3 5 705 310 4.2 4.3 2.03 4 3.9 6 600 328 4.0 4.0 1.81 3 3.2 7 580 270 4.1 4.2 1.92 3 3.7 8 800 224 4.3 4.5 1.83 4 2.6 Comp. Example 1 890 230 3.5 4.0 2.32 over 50 3.9 2 490 1100 6.3 6.5 2.16 47 7.6 3 1050 260 3.3 3.4 2.32 14 3.1 4 1325 310 5.8 6.2 1.76 11 3.0 It can be seen from Table 1 that in the Examples using the anionic water-soluble polymer as used in the present invention, almost no coarse particles evaluated as spot-like staining are present as compared to Comparative Examples 1, 3, and 4 which use prior art water-soluble polymers. From a comparison of Comparative Examples 1 and 2 with Example 1, it is clearly shown that, whereas Comparative Examples 1 and 2 using acrylic acid and butylacrylate or AMPS have interior spot-like staining, Example 1 which uses the three substances in combination has remarkably improved in spot-like staining, conspicuously showing the effect of the use of three in combination. In the Comparative Examples, the anionic water-soluble polymers used in Comparative Examples 1, 3, and 4 are individually those disclosed in the inventions of JP-A-60-68045, JP-A-62-19238 and JP-A-58-14942. By virtue of use of the anionic water-soluble polymer used in the present invention, microcapsules can be easily produced with almost no coarse particles evaluated as spot staining. The emulsification property, particle size distribution, and slurry viscosity are satisfactory there is an improved density of capsule wall films.

Claims: A process for producing microcapsules comprising forming microcapsules having wall films of an amine-aldehyde condensation polymer in an emulsion or dispersion of a hydrophobic core substance in an acid aqueous solution containing an anionic water-soluble polymer which is a copolymer consisting of 50 to 90, excluding 50, weight % of acrylic acid, 2 to 40 weight % of an acrylamidealkylsulfonic acid and 2 to 15 weight % of an alkyl acrylate, and, optionally, 0 to 15 weight % of at least one of methacrylic acid, alkylmethacrylate, hydroxyalkylacrylate, hydroxyalkylmethacrylate, acrylamide, methacrylamide, succinic acid ester of hydroxyalkylacrylate succinic acid ester of hydroxyalkylmethacrylate and vinylacetate. A process according to claim 1 wherein said amine-aldehyde condensation polymer comprises at least one of melamine, methylolmelamine, methylated methylolmelamine, urea, dimethylolurea and methylated dimethylolurea, and formaldehyde. A process according to claim 1 or 2 which comprises the additional step of coating the microcapsules on a substrate to provide a pressure sensitive copy paper.

Inventors: MURANO TSUTOMU, NISHIKUBO TOSHIFUMI, MURANO, TSUTOMU, NISHIKUBO, TOSHIFUMI, Murano, Tsutomu, c/o Res. of Product Dev., Nishikubo, Toshifumi, c/o Res. of Product Dev.
Applicants: JUJO PAPER CO LTD, NIPPON PAPER INDUSTRIES CO., LTD.
Application year: 1993
Main classification: B01J13
IPC codes: A61K9, B01J13, B41M5, A61K47