PROCESS FOR PRODUCING MICROCAPSULES

Abstract: The process of the present invention uses an aminoaldehyde polycondensate as capsule-wall and a hydrophobic core material dispersed in an aqueous acidic solution containing an anionic water-soluble colloidal substance, wherein the anionic water-soluble colloidal substance comprises a mixture of an anionic water-soluble substance having a high viscosity and that having a low viscosity. The process of the present invention provides a slight viscosity rise in the encapsulation, a capsule slurry with a high concentration, low viscosity, and a slight smudging in the application as a pressure-sensitive recording sheet and the like, compared with the conventional processes.

Description: The present invention relates to a process for producing microcapsules which use an aminoaldehyde polycondensate as a capsule-wall for the purpose of changing or controlling various properties of substances in such field as pharmaceuticals, agricultural chemicals, perfumes, liquid crystals, pressure-sensitive recording papers and heat-sensitive recording papers. More specifically, the present invention relates to a process for producing microcapsules which have an aminoaldehyde polycondensate wall effective for pressure-sensitive recording sheets. The process for the present invention is an in situ polymerization process, a chemical process in which a capsule wall composed of an aminoaldehyde polycondensate is formed by a reaction starting from the continuous phase. The use of amine/aldehyde polycondensate or urea/ aldehyde polycondensate for forming the wall film has already been disclosed in, for example, Japanese Patent Publication Nos. 37-12380, 44-3459 and 47-23165. This process has the defects that the emulsification or dispersion is not efficiently achieved, or that it is difficult to efficiently and stably deposit the polycondensate around the hydrophobic core material. In order to improve the above-mentioned defects, the use of acrylic acid polymers or copolymers as an anionic polyelectrolyte is disclosed in Japanese Patent Publication 54-16949, Japanese Patent Laid-Open Application Nos. 58-14942, 59-142836, 60-28819, 60-68045, 60-190227, 60-216838, 60-238140, 61-11138, 61-17491, 62-19238, 62-57645, 62-97638, 62-250943 and 63-134084. The processes of the above disclosures have the following problems requiring some improvements. Although the process of the Japanese Patent Publication 54-16949 improves the depositing efficiency of the polycondensate around the core material, it has the defects that the resultant capsule slurry exhibits a very high viscosity. Japanese Patent Laid-Open Application Nos. 58-14942, 59-142836 and 60-68045 disclose a capsule slurry having a high-concentration, a low viscosity and a good particle size distribution. However, they provide as a defect a capsule slurry having an undesirable high viscosity at a higher slurry concentration. Japanese Patent Laid-Open Application Nos. 60-190227, 60-216838, 60-238140, 61-11138, 61-17491, 62-19238, 62-57645, 62-97638, 62-250943 and 63-134084 provide a capsule slurry having a higher concentration and a lower viscosity in the use of the described anionic polyelectrolyte, but they provide as a defect a worse particle-size distribution. In using the capsules obtained by these conventional processes for pressure-sensitive recording sheets, the higher concentration and lower viscosity described above cause an economical advantage due to the high-speed coating, but the worse particle-size distribution has the defects of causing a smudging in the color-information and further a spot-like smudging on the surface of recording sheet. Furthermore, in a single-type pressure-sensitive recording sheet, a self-contained paper, a spot-like smudging appears remarkably. The present invention provides a process for producing microcapsules which comprises a) dispersing or emulsifying a hydrophobic core material in an aqueous acidic solution containing an anionic water-soluble colloidal substance which is an acrylic acid-base substance, a maleic anhydride-base substance or a styrene-sulfonic acid-base substance and which comprises a mixture of a high viscosity anionic water-soluble substance having a viscosity of at least 5 Pas (5,000 cps) and a low viscosity anionic water-soluble substance having a viscosity of at most 0.5 Pas (500 cps), the viscosity ratio of said high viscosity substance to said low viscosity substance being at least 10 : 1 at a concentration of 25 % by weight of each of the anionic water-soluble substances, and b) providing the core material with a coating of an aminoaldehyde polycondensate as a capsule wall. This process provides superior emulsifying ability, a good emulsion stability, a slight viscosity increase in a capsule-forming reaction and a capsule slurry with higher concentration, lower viscosity and narrow particle-size distribution. The resulting microcapsules have good capsule-film impermeability, superior runnability, slight smudging and reduced spot smudging during color-formation when using a pressure-sensitive recording sheet. As the anionic water-soluble colloidal substance of the present invention, there are used acrylic acid-base substances, maleic anhydride-base substances or styrene-sulfonic acid-base substances. The acrylic acid-base substances are preferred since they can be easily prepared from high viscosity to low viscosity in a constant condition. The acrylic acid base water-soluble polymers includes the polymers and copolymers of acrylic acid. The examples for the copolymers of acrylic acid include the salts (K, Na-salts) of acrylic acid/itaconic acid copolymer, acrylic acid/itaconic acid/alkyl acrylate copolymer, acrylic acid/alkyl acrylate/alkyl methacrylate copolymer, acrylic acid/maleic anhydride-copolymers, the salts (Li, Na, K, Ca-salts) thereof, acrylic acid/maleic anhydride/alkylacrylate/styrene copolymer, acrylic acid/ alkylacrylate/alkylmethacrylate/hydroxyalkylmethacrylate copolymer, acrylic acid/acrylonitrile copolymer, acrylic acid/acrylonitrile/alkylacrylate copolymer, acrylic acid/ acrylonitrile/acrylamide copolymer, acrylic acid/acrylamide copolymer, acrylic acid/acrylamide/alkylmethacrylate/alkylacrylate copolymer, acrylic acid/alkyl acrylate copolymer, acrylic acid/styrene-sulfonic acid copolymer, acrylic acid/styrenesulfonic acid/alkylacrylate copolymer, acrylic acid/styrenesulfonic acid/alkylacrylate/itaconic acid copolymer and acrylic acid/methacrylic acid/acrylonitrile/acrylamide/alkyl sulfonic acid copolymer. The examples for maleic anhydride-base water-soluble polymers include the copolymer of maleic anhydride with vinylmethyl ether, isobutylene, vinyl acetate and styrene. The examples for styrenesulfonic acid-base water-soluble polymers include the copolymers of styrenesulfonic-acrylic acid, styrenesulfonic-maleic anhydride, styrenesulfonic acid-acrylate, styrenesulfonic acid-vinyl acetate, styrenesulfonic acid-vinylpyrrolidone, styrenesulfonic acid-styrene and styrenesulfonic acid-vinylsulfonic acid. In combination with the mixture of anionic water-soluble substances, there can be used other anionic or nonionic water-soluble polymers such as carboxymethyl cellulose, hydroxy-ethyl cellulose, polyvinyl alcohol, cellulose-sulfonate, lignin sulfonate, gum arabic and polyethylene sulfonic acid, in an amount which does not decrease the function of the mixture of anionic water-soluble substances, for example in an amount of at most 20 - 30 weight-%. The anionic water-soluble substance used in the process of the present invention is produced preferably by a radical polymerization process using hydrogen gas, that is, by a conventional process for polymerizing an acrylic acid as follows. The monomer is placed in a vessel diluted with water to obtain an aqueous solution of an appropriate concentration. Then, the air in a vessel is substituted with nitrogen gas. An organic or inorganic peroxide or persulfate as a polymerization initiator is added to the solution, heated to carry out a polymerization reaction. However, an emulsion-polymerization process can be adopted for controlling a polymerization state. After the completion of a reaction, a part of the polymer is additionally neutralized with a hydroxide of alkali metal such as sodium hydroxide, etc. In general, the radical polymerization process in an aqueous system provides an aqueous polymer solution containing 5 - 30 weight-% of non-volatile components. Preferably, the composition of the monomer as a raw material of the acrylic acid-base anionic water-soluble polymers consists of 50 - 92% by weight of acrylic acid and an appropriate weight of other monomers in order to carry out a sufficient encapsulating reaction. (Hereinafter, % means % by weight.) For example, a composition ratio of acrylic acid/itaconic acid ranges 80 - 90%/20 - 5%, a ratio of acrylic acid/itaconic acid/alkylacrylate ranges 75 - 90%/1 - 10%/1 - 10%, a ratio of acrylic acid/styrene sulfonic acid/alkyl acrylate ranges 70 - 90%/2 - 15%/2 - 10% and a ratio of acrylic acid/methacrylic acid/acrylonitrile/acrylamide alkylsulfonic acid ranges 40 - 70%/1- 10%/5 - 3%/5 - 30%. Any of the above water-soluble copolymers can be easily encapsulated to obtain an aminoaldehyde polycondensate as a capsule wall. In order to perform the purpose of the present invention, it is required to use a mixture of a high viscosity water-soluble polymer with a low viscosity water-soluble polymer that is, a mixture of a high-molecular weight polymer and a low-molecular weight polymer. In the present invention, the term "high viscosity" means a viscosity of at least 5 Pas (5,000 cps), whereas the term "low viscosity" means a viscosity of at most 0.5 Pas (500 cps). The use of only a high viscosity water-soluble polymer in an amount sufficient for the capsule-forming reaction provides an undesirable high viscosity when emulsifying, in the encapsulating-reaction and in the resulting capsule-slurry, a worse runnability and a reduced yield. On the other hand, the use of only a low viscosity water-soluble polymer is undesirable because of troublesome capsule-formation, a wide particle-size distribution and non-uniform capsule particle-size. The use of only an intermediate viscosity water-soluble polymer, that is a polymer having a viscosity between that of the high viscosity and low viscosity polymers provides an undesirable high viscosity in encapsulating-reaction and in the resulting capsule-slurry, and a worse capsule-particle-size distribution. In an acrylic acid-base water-soluble polymer used in the present invention, the relation between the molecular weight (GPC-average molecular-weight) and the viscosity is as follows. A polymer having a high viscosity range of 5 to 1,500 Pas (5,000 - 1,500,000 cps) is calculated to have an average molecular weight range of circa 1,200,000 - 5,000,000, and a polymer having a low viscosity range of 0.1 to 0.5 Pas (100 - 500 cps) is calculated to have an average molecular weight range of circa 200,000 - 600,000. The viscosity of the water-soluble polymer used in the process of the present invention is measured at 25° C using a Brookfield viscometer on a solution containing 25% by weight of non-volatile components. The weight ratio of the high viscosity water-soluble polymer to the low viscosity water-soluble polymer preferably is 1 : 12 to 1 : 3. The viscosity after the mixing is preferably 0.5 to 2 Pas (500 - 2,000 cps). The water-soluble polymer can be used in form of the anhydride or partial metal salt, and lithium-, sodium-, potassium-, magnesium- or calcium-salts are preferred. The process of the present invention can be carried out as follows. (1) Preparation of the core material A hydrophobic liquid as the core material for the microcapsules is used as such or after dissolving other substances. (2) Preparation of the continuous phase The mixture of anionic water-soluble polymers is dissolved in city water at room temperature, and further, either an amine or aldehyde or a precondensate thereof is dissolved in the resultant solution to form a continuous phase. When a precondensate is used it can be modified with, for example, a phenolic substance or benzoguanamine. The weight-ratio (as solid) of the encapsulating material to the core material ranges from circa 1 : 1 to circa 1 : 20. Aminoaldehyde as an encapsulating material can be used alone, or in form of prepolymer, if they are dissolved in a continuous phase. Examples of suitable amines include urea, thiourea, melamine, their alkylates having 1 - 4 carbon atoms, methylolate, methylol alkylate having 1 - 4 carbon atoms, and mixtures thereof. Examples of suitable aldehydes include formaldehyde, paraformaldehyde, glyoxal and glutaraldehyde, with formaldehyde being preferred. (3) Emulsification The solution of core material is added to the continuous phase under stirring and emulsified with an emulsifying machine such as homogenizer or static mixer. Preferably, the emulsification is carried out at or below room temperature to obtain a good particle-size distribution. (4) Reaction for forming capsule walls After emulsified particles of predetermined particle-sizes are obtained, the pH is adjusted to a range suitable for polycondensing the capsule wall-forming material. With acidic pH-value, the impermeability of the capsule wall is increased. The smaller the pH-value, the faster the reaction rate. If the reaction is too fast, uniform capsule walls do not form. Taking into consideration this fact and the like, the pH-value should be adjusted to circa 3. The weight ratio of the amine to the aldehyde is not critical. Generally, the aldehyde is used in an amount of 0.5 - 5.0 moles, based on 1 mole of the amine. In using urea-formaldehyde system, formaldehyde is used preferably in an amount of 1.2 - 3.0 moles, based on 1 mole of urea. After the completion of emulsification or with the addition of aldehyde, the system is heated to 35 - 90°C. Usually, the temperature is adjusted to 50 - 75°C. After a set temperature is reached, the temperature should be maintained preferably for more than a predetermined period. A set temperature of 50 - 60°C should be maintained for a least one hour. (5) After-treatment After the reaction for forming capsule walls is completed, treatments such as a temperature control, a pH-control, a washing-out, a filtration, a drying and a pulverization can be carried out. When using formaldehyde as an aldehyde, excess formaldehyde remaining in the capsule slurry is removed. In emulsifying and encapsulating in accordance with the invention, the high-viscosity polymer provides a faster emulsifying rate, an improved particle-size distribution and a higher film impermeability. On the other hand, the low-viscosity polymer provides a low slurry viscosity during the emulsification and encapsulating process. As the result, there is obtained a capsule slurry having high-concentration, low-viscosity and narrow particle-size distribution. However, the use of only a high viscosity polymer provides a viscosity increase during the encapsulating reaction and in a capsule slurry, and worse capsule particle-size distribution. The reason for this undesired effect is not clear. Further, the use of a single polymer having the same viscosity as the mixture of polymers used in the process of the present invention exhibits both the remarkably undesired effect of using a high viscosity polymer and the remarkably undesired effect of using a low viscosity polymer, which cannot be explained. In the Examples which follow, parts means parts by weight as solid. Example 1 (1) Preparation of an anionic colloidal substance (A) Synthesis of the substance having a high viscosity 650 g of water were placed in a 2 litre-flask fitted with reflux cooler, thermometer, nitrogen introducing pipe, low dropping funnels were heated to 83 - 85° C under stirring. At this temperature, a mixed solution of 305 g of acrylic acid, 27 g of styrene-sulfonic acid, 27 g of butyl acrylate and 137 g of water was placed in a dropping funnel, and a solution containing 0.61 g of potassium persulfate dissolved in 150 g of water was placed in another dropping funnel. The mixed solution and the potassium persulfate solution were added dropwise into a flask for one hour, and were maintained at the same temperature for 2 hours. Further, a solution dissolving 1.44 g of potassium persulfate in 50 g of water was added thereto and maintained at the same temperature for 1 hour to complete the reaction. After cooling, 11 g of 48% aqueous solution of sodium hydroxide were added thereto neutralized, disluted with water to thereby obtain a 25% aqueous solution of an anionic water-soluble polymer composed of acrylic acid/sodium styrene sulfonate/butylacrylate of 85/7.5/7.5. The viscosity of obtained polymer was 170 Pas (170,000 cps) when it is measured at 25°C by using a Brookfield viscometer. (B) Synthesis of the substance having a low viscosity 600 g of water were placed in a 2ℓ separable round bottom flask equipped with a reflux cooler, a thermometer, a nitrogen inlet and two dropping funnels. The water was heated with stirring to 83 - 85°. A mixed solution containing 305 g of acrylic acid, 2.7 g of sodium styrene sulfonate and 100 g of water was charged in a dropping funnel, and a persulfate solution dissolving 9.55 g of potassium persulfate in 200 g of water was charged in another funnel. At the above temperature, the mixed solution and the persulfate solution were added dropwise into the flask for a long time, respectively, and were maintained for 3 hours, and cooled. Further, 11 g of 48% sodium hydroxide solution were added thereto, neutralized, diluted with water to obtain a 25% aqueous solution of anionic water-soluble polymer having acrylic acid/styrene sulfonate/butylacrylate of 85 : 7.5 : 7.5. The viscosity of the above polymer was 0.72 Pas (720 cps). (C) Anionic water-soluble polymer No.1 was prepared by mixing an anionic water-soluble polymer having a high viscosity prepared as described in (A) with that having a low viscosity prepared as described in (B) in a weight-ratio of 1/6. (2) Preparation of capsules having a wall film composed of urea/formaldehyde 28 parts of the above anionic water-soluble polymer No.1 were added to 72 parts of water and dissolved. Further, 10 parts of urea and 1.2 parts of resorcinol were added thereto to prepare an aqueous solution. The aqueous solution was adjusted to a pH-value of 3.4 by using a 40% citric acid solution. On the other hand, 6 parts of Crystal Violet Lactone as a colorless dye were added to 148 parts of a high-boiling solvent composed substantially of phenylxylylethane and dissolved under stirring while heated to 90°C to prepare a core material. The core material was mixed with the above prepared aqueous solution containing the anionic water-soluble polymer No.1, urea and resorcinol while paying attention to the phase inversion to prepare a mixture. The mixture was adjusted to a temperature of 30°C, emulsified to an average particle-size of 4.7 µm using a homomixer. Then 27.5 parts of water and 23.5 parts of 37% formaldehyde solution were added thereto, and heated to 55°C. The reaction for forming capsule walls was continued for 2 hours at 55°C to complete an encapsulating reaction. In order to remove residual formaldehyde, the mixture was cooled to 40°C, and 28% ammonia water was added until the pH was 7.5. Thus, a capsule slurry for a pressure-sensitive recording sheet was obtained. Examples 2 - 8 The water-soluble polymer having a high viscosity and that having a low viscosity were synthesized in the same manner as in Example 1 from the monomers in the composition-ratio as shown in Table 1. They were mixed in a ratio shown in Table 1 to obtain water-soluble polymers Nos. 2 - 8 used in Examples 2 - 8. Table 1 gives viscosities for the water-soluble polymers having a high viscosity, those having a low viscosity and the mixture of the two water-soluble polymers. The capsules were formed in the same manner as in Example 1, using the water-soluble polymers of samples Nos. 2 - 8. The viscosity rise during the encapsulation, the viscosity of the capsule slurry, the particle size-distribution of microcapsules and the smudging in color-formation were measured and summarized in Table 2. Comparative Examples 1 - 6 Anionic water-soluble polymers Nos. 9 - 14 in Table 1 were synthesized in the same manner as in Example 1. Using the above polymers Nos. 9 - 14, the capsule slurries of Comparative Examples 1 - 6 were prepared in the same manner as in Example 1. The test results of these capsules were summarized in Table 2. As will be obvious from Table 2, the single use of a water-soluble polymer having a medium viscosity or high viscosity, as in Comparative Examples 1, 2 and 4, provides a higher capsule-slurry-viscosity than the process of the present invention. The single use of the water-soluble polymer having a low viscosity, as shown in Comparative Examples 3 and 5, provides an inferior particle size-distribution and a worse smudging in the color formation. The use of mixed water-soluble polymers having a viscosity-ratio of less than 10, as shown in Comparative Example 6, exhibits an inferior particle-size distribution and a worse smudging in spite of a low slurry viscosity. The capsules were evaluated by the following methods. Viscosity: Viscosity of the capsule slurry was measured at 25°C by using a Brookfield Viscometer. Particle-size distribution: A product of 10 and a logaritm of a ratio in particle size between a point of 25% volume and a point of 75% volume as measured by the particle-size measuring device, Coulter Counter Model TA-II, was indicated as a particle-size distribution. A small value indicates a narrow particle-size distribution. Smudging: A capsule slurry was coated on a sheet. Then, the coated sheet was laid on the color-forming surface of a sheet, and treated at a pressure of 2 MPa (20 kg/cm²). The colored degree was measured by a Color-Difference Meter. The smaller value means a slight smudging. The process of the present invention provides a slight viscosity rise in the encapsulation, a capsule slurry with a high concentration and low viscosity, and a slight smudging in the application as a pressure-sensitive recording sheet, compared with the conventional processes. Further, owing to a coating material with a high concentration the process of the present invention provides a high-speed coating and a superior drying effect.

Claims: A process for producing microcapsules which comprises a) dispersing or emulsifying a hydrophobic core material in an aqueous acidic solution containing an anionic water-soluble colloidal substance which is an acrylic acid-base substance, a maleic anhydride-base substance or a styrene-sulfonic acid-base substance and which comprises a mixture of a high viscosity anionic water-soluble substance having a viscosity of at least 5 Pas (5,000 cps) and a low viscosity anionic water-soluble substance having a viscosity of at most 0.5 Pas (500 cps), the viscosity ratio of said high viscosity substance to said low viscosity substance being at least 10 : 1 at a concentration of 25 % by weight of each of the anionic water-soluble substances, and b) providing the core material with a coating of an aminoaldehyde polycondensate as a capsule wall. A process according to claim 1, wherein said anionic water-soluble substance is an acrylic acid polymer and/or an acrylic acid copolymer. A process according to claim 1 or 2, wherein the high viscosity anionic water-soluble substance has a viscosity of 5 to 1,500 Pas (5,000 to 1,500,000 cps), and the low viscosity anionic water-soluble substance has a viscosity of 0.1 to 0.5 Pas (100 to 500 cps). A process according to any one of claims 1 to 3, wherein the weight ratio of the high viscosity anionic water-soluble substance to the low viscosity anionic water-soluble substance is from 1 : 12 to 1 : 3. A process according to any one of the preceding claims, wherein said mixture of two anionic water-soluble substances has a viscosity of 0.5 to 2 Pas (500 to 2,000 cps). A process according to any one of the preceding claims, wherein said high viscosity anionic water-soluble substance and/or the low viscosity water soluble substance is used in the form of an anhydride or partial metal salt. A process according to claim 6, wherein said salt is at least one of lithium-, sodium-, potassium-, magnesium- or calcium-salts. A process according to any one of the preceding claims, wherein said anionic water-soluble substance is used in combination with at least one polymer which is an anionic and/or a nonionic water-soluble polymer. A process according to any one of the preceding claims which comprises the additional step of preparing a slurry of the microcapsules and coating a sheet of paper with the slurry.

Inventors: HARADA, SHOZO, SAKAMOTO, MASAHIRO, HARADA, SHOZO, C/O OISO BRANCH, SAKAMOTO, MASAHIRO, C/O OISO BRANCH
Applicants: JUJO PAPER CO., LTD.
Application year: 1990
Main classification: B01J13
IPC codes: B41M5, B01J13