GB2115372A - Process for producing microcapsules - Google Patents

Abstract

Microcapsules resistant to heat, solvents, and impact and capable of preserving even a labile substance without substantial change are obtained by a process for producing microcapsules containing a hydrophobic substance as core material by dispersing or emulsifying the hydrophobic substance in an aqueous acidic solution of a styrene-maleic anhydride copolymer to form discrete particles in the acidic solution, and adding to the resulting dispersion or emulsion a melamine- formaldehyde precondensate to form a capsule wall membrane of melamine- formaldehyde resin, the wall- membrane-forming temperature being varied during the process.

Description

SPECIFICATION Process for producing capsules This invention relates to a process for producing microcapsules and, more particularly, to a process for producing unprecedented microcapsules remarkably resistant to heat, solvents, and impact and capable of preserving even labile substances (reactive or liquid substances) without substantial change in quality.

As generally known processes for producing microcapsules, mention may be made of those of physical encapsulation, coacervation, interfacial polymerization, and in situ encapsulation.

Although adaptable to the production of microcapsules intended for use in certain areas such as agricultural chemicals, the physical process is unsatisfactory in preserving the capsule contents owing to an imperfect capsule wall. The coacervation process is widely utilized in the encapsulation of colorless dye precursors for carbonless paper, adhesives, liquid crystals, and so on. The capsules produced by this process, however, are not sufficiently resistant to solvents and, as a consequence, are not suitable for uses where contact with a solvent is anticipated. In the interfacial polymerization process, encapsulation is performed by formation of a wall material such as a polyamide, epoxy, or polyurethane resin at the interface between a hydrophobic liquid and water.It is possible by the proper selection of wall material to produce microcapsules excellent in preserving the capsule contents. In this process, however, the high reactivity or high toxicity of the membrane-forming materials such as acyl chlorides, isocyanates, or epoxy compounds gives rise to disadvantages of difficult control of the reaction in operation, impracticability of the encapsulation of compounds having an active hydrogen atom, and expensiveness of the materials. The in situ process employing an aminoplast (amino resin) wall material is also currently in practical use and many improvement proposals are disclosed in the patent literature (for example, Japanese Patent Post Examination Publication Nos. 12,380/62, 12,518/63,10,780/72 and 23,165/72).This process has also disadvantages in that the capsule wall is not sufficiently impervious and the satisfactory emulsification or dispersion of a hydrophobic substance is difficult to attain. In order to overcome these difficulties a proposal has been made to use as modifier an ethylene-maleic anhydride copolymer, a methyl vinyl ether-maleic anhydride copolymer, or a polyacrylic acid, as described in Japanese Patent "Kokai" (Laid-open) No.

9,079/76.

In conventional encapsulation, urea-formaldehyde resins have been predominantly used, while melamine-formaldehyde resins are seldom used. It is only recently that a melamine resin has been proposed as a modifier for urea-formaldehyde resins [Japanese Patent "Kokai" (Laid-open) No. 66,878/77]. Although encapsulation with melamine-formaldehyde resins has been described in Japanese Patent Post Examination Publication Nos. 12,380/62 and 12,518/63, detailed description of the process cannot be found therein so that it is difficult to obtain satisfactory microcapsules by following the procedure described in said patent literature. The present inventors also disclosed in Japanese Patent Application "Kokai" (Laid-open) No. 49,984/79 a process of encapsulation with a melamine-formaldehyde resin.The disclosed procedure provides comparatively good microcapsules under the reaction temperature conditions of 50"C or above, preferably 60 to 80"C. Such reaction conditions, however, are apt to produce microcapsules of the characteristics which fall short of the full advantages of a melamine-formaldehyde resin over a urea-formaldehyde resin, such as a higher rate of hardening, higher tensile and compressive strengths, a higher heat tensile and compressive strengths, a higher heat resistance, and a lower water absorption.

Consequently, such microcapsules are not sufficiently resistant to heat, solvents, and impact to meet the requirements for uses under severe conditions, though suitable for use in fields where the required values of the above-noted resistances are not so high. As examples of cases where high resistances are required, mention may be made of those top sheets of carbonless copying papers which are manufactured by a paper-making process from a mixture of an aqueous dispersion of microcapsules and a pulp slurry, and printing inks manufactured by kneading dry microcapsules obtained by the spray-drying of an aqueous dispersion of microcapsules together with a wax for use in back-coated carbon paper or by dispersing an aqueous dispersion of microcapsules in a solvent such as alcohol ortoluene.

A primary object of this invention is to improve a conventional process for producing microcapsules, in which a melamine-formaldehyde resin is used as the capsule wall material, and thus to provide a process for producing microcapsules remarkably resistant to heat, solvents, and impact and capable of preserving even a labile substance without substantial change.

It has now been found that the above object is achieved by varying the wall-forming reaction temperature, preferably in two or more stages, in producing microcapsules containing a hydrophobic substance as core material by dispersing or emulsifying said hydrophobic substance in an aqueous acidic solution of a styrene-maleic an hydride copolymer so as to form discrete particles in said acidic solution, and then adding to the resulting dispersion or emulsion a melamine-formaldehyde precondensate to form a capsule-wall membrane of melamine-formaldehyde resin. It is also possible to vary the reaction temperature steplessly (continuously), as described later.

In a preferred embodiment of the present invention, microcapsules are produced through the following steps in sequence: (1) A step of emulsifying a hydrophobic substance in an aqueous acidic solution of a styrene-maleic anhydride copolymer; (2) a step of preparing a melamine-formaldehyde precondensate; (3) a step of encapsulating the hydrophobic substance by adding the precondensate prepared in (2) to the emulsion prepared in (1) and allowing the melamine-formaldehyde resin to form at a temperature of at least 50"C up to 80"C; and (4) a step of strengthening the capsule wall of the melamine-formaldehyde resin formed in (3) by heating at a temperature exceeding 80"C.

In step (1) the pH of the emulsion should be 7 or below. The amount to be used of the styrene-maleic anhydride copolymer is preferably about 2 to about 20 parts by weight for 100 parts by weight of the hydrophobic substance. In step (2) the molar ratio of melamine to formaldehyde should be 1:1.5 or more, preferably in the range of from 1:2 to 1 :3.5. The precondensate is easily prepared in a customary manner.

For instance, it is formed in a short period of time (e.g. 15 to 30 minutes) by heating the reactant mixture at a temperature of 50"C or above in an alkaline medium (pH about 8 - 10). A certain grade of the commercial melamine resin precondensate may be used as such. In step (3) the pH is maintained at a level of 3.5 to 7.0, preferably 4.0 to 6.5, most preferably 5.5 to 6.5.A suitable reaction temperature is in a range of from 50 to 80"C. The reaction time should be 30 minutes or more. in step (4) the capsule wall of melamineformaldehyde resin formed in step (3) is strengthened by heating at a temperature of 80"C or above, preferably 90" to 100"C. If the temperature is below 80"C, it is difficult to obtain those microcapsules which are remarkably resistant to heat, solvents, and impact, even if the pH or the reaction time is controlled in various ways.If the encapsulation is carried out at a temperature exceeding 80"C throughout the encapsulation period from the beginning to the end instead of varying the temperature in two or more stages, the stability of the emulsified particles will be injured, causing the break-up of emulsion, and the growth of emulsified particles will take place, resulting in defective microcapsules; in some cases even entire or partial gelation of the reaction system will take place, presumably because of an accelerated rate of reaction.The microcapsules unprecedently resistant to heat, solvents, and impact are obtained only by the process of this invention, in which the melamine-formaldehyde capsule wall is formed at first at a lower reaction temperature (preferably 50 to 80"C) and the capsule wall is then strengthened at a higher temperature (preferably higher than 80"C). It is also possible and is even desirable in practice to start the wall-forming reaction at 50 - 80"C and to elevate the reaction temperature gradually to a level exceeding 80"C.

It is unobjectionable to add in any of the steps (1) to (4) those substances which form a resin upon reaction with the formaldehyde, such as, for example, urea, thiourea, guanidine, and resorcinol in an amount of preferably below one-half of the amount of melamine used.

The invention is illustrated below with reference to examples.

Example 1 A hydrophobic substance was prepared by dissolving with heating 3 g of Crystal Violet Lactone (CVL) in 97 g of KMC-1 13 (tradename for an oil produced by Kureha Chemical Co.). This hydrophobic substance was emulsified in 100 g of a 5-% aqueous solution (adjusted to pH 5.3) of Scripset 520 (a styrene-maleic anhydride copolymer of Monsanto Chemical Co.). A mixture of 10 g of melamine, 25 g of 37-% formalin, and 65 g of water was adjusted to pH 9.0 and heated at 60"C to form a solution of melamine-formaldehyde precondensate. The resulting solution was added to the emulsion prepared above and stirred for 30 minutes while the temperature being maintained at 600C. The temperature of the mixture was then elevated to and maintained at 90"C for 30 minutes with continued stirring.Thereafter, the mixture was cooled down to room temperature to complete the encapsulation.

Comparative Example 1 Encapsulation was carried out by the same procedure as in Example 1, except that the reaction temperature was maintained at 60"C throughout the encapsulation period.

Comparative Example 2 Encapsulation was carried out by the same procedure as in Example 1, except that the reaction temperature was maintained at 90"C throughout the encapsulation period.

Comparative Example 3 Encapsulation with a urea-formaldehyde resin as wall material by the conventional in situ process.

Into 100 g of a 5-% aqueous solution (adjusted to pH 4.0) of EMA-31 (an ethylene-maleic anhydride copolymer of Monsanto Chemical Co.), was emulsified 100 g (3 g of CVL and 97 g of KMC-1 13) of the same hydrophobic substance as used in Example 1. To the resulting emulsion, was added a solution of lOg of urea, 1 g of resorcinol, and 25 g of 37-% formalin in 100 g of water. The mixture was heated at 600C and stirred for one hour to prepare a dispersion of microcapsules.

Comparative Example 4 Encapsulation by the conventional coacervation process.

Into 100 g of a 10-% aqueous gelatine solution, was emulsified 100 g (3 g of CVL and 97 g of KMC-113) of the same hydrophobic substance as used in Example 1 followed by the addition of 600 g of a 1.6-% aqueous gum arabic solution. To the emulsion, after having been adjusted to pH 4.5 and cooled from 50"C down to 10"C, was added 10 g of 37-% formalin. The mixture was stirred for 24 hours and adjusted to pH 10 to yield a dispersion of microcapsules.

Comparative Example 5 Encapsulation with a polyurea resin as wall material by the interfacial polymerization process.

An internal-phase oil prepared by adding 2.3 g of Coronate HL (an aliphatic isocyanate of Nippon Polyurethane Co.) to 100 g (3 g of CVL and 97 g of KMC-1 13) of the same hydrophobic substance as used in Example 1 was emulsified into 100 g of a 0.5-% aqueous solution of Gosenol NM-300 (tradename of a polyvinyl alcohol produced by Nippon Synthetic Chemical Co.). To the resulting emulsion, was added 100 g of an aqueous solution containing 1 g of hexamethylenediamine and 1.8 g of sodium hydroxide dissolved therein. The mixture was adjusted to pH 9.5 and heated at 60"C for one hour to yield a dispersion of microcapsules with a polyurea as wall material.

The six microcapsule dispersions obtained above were dried by means of a spray drier (Type DL-21 made by Yamato Kagaku Co.) at an exit temperature of 80"C to give dried microcapsules in powder form. A 5-% solution of a p-phenylphenol-formaldehyde resin in toluene was added dropwise to each of the 6 spray-dried capsule samples to observe the color reaction.

Sample Color reaction Example 1 The microcapsules in white powder form remained as such without ex hibiting any change in color.

Comparative Example 1 Blue coloration.

Comparative Example 2 Blue coloration.

Comparative Example 3 Deep blue coloration.

Comparative Example 4 Deep blue coloration.

Comparative Example 5 Deep blue coloration.

The above experiment had been designed to test simultaneously the thermal and impact resistances by spray drying and the solvent resistance by contact with toluene. The microcapsule sample obtained in Example 1 was found to be excellent in all of the resistances to heat, solvents, and impact, indicating no damage in capsule wall, whereas blue coloration of the samples obtained in Comparative Examples 1 to 5 indicated that at least one of the heat, solvent, and impact resistances became inferior due to the damage in capsule wall.

In another experiment, each of the 6 spray-dried microcapsule samples was kneaded together with a paraffin wax (melting point, 70'C) and coated on a paper sheet to obtain a top sheet of the carbonless copying paper. The top sheet was superposed to a commercial undersheet and impression was made by means of a typewriter.

Sample Result ofimpression Example 1 Deep blue coloration comparable in color density to that obtained with a commerical top sheet.

Comparative Example 1 Substantially no coloration; the impressed letters were almost illegible because of insufficiency of the color density.

Comparative Example 2 Ditto.

Comparative Example 3 Entirely no coloration.

Comparative Example 4 Ditto.

Comparative Example 5 Substantially no coloration.

From the above results, it may be presumed that in the cases of samples of Comparative Examples 1 to 5, the internal phase of the microcapsules had been exuded through the damaged capsule wall and lost the color forming ability due to the desensitizing action of the wax. On the contrary, the microcapsules obtained in Example 1 were unprecendently excellent in resistances to heat, solvents, and mechanical impact. In the above experiment, the microcapsules were coated on a paper sheet. This is only an example of the uses of the present microcapsules and not to be construed to limit the invention.

Claims (10)

1. A process for producing microcapsules containing a hydrophobic substance as core material by dispersing or emulsifying said hydrophobic substance in an aqueous acidic solution of a styrene-maleic anhydride copolymer so as to form discrete particles in said acidic solution, and then adding to the resulting dispersion or emulsion a melamine-formaldehyde precondensate to form a capsule-wall membrane of melamine-formaldehyde resin, wherein the improvement comprises varying the wall membrane-forming temperature.

2. A process according to Claim 1, wherein the wall membrane-forming temperature is varied in two stages, the temperature being 80"C or below in the first stage and above 80"C in the second stage.

3. A process according to Claim 2, wherein the temperature is 50 to 80"C in the first stage and 90" to 100"C in the second stage.

4. A process according to Claim 1, wherein the wall-membrane-forming temperature is varied steplessly.

5. A process according to Claim 4, wherein the wall-membrane-forming temperature is 50 - 70"C at the start and steplessly elevated to above 80"C at the end.

6. A process according to any one of Claims 1 to 5, wherein the duration of the membrane formation is varied in accordance with the membrane-forming temperature.

7. A process according to Claim 1, wherein the amount of the styrene-maleic anhydride copolymer used is 2 to 20 parts by weight for 100 parts by weight of the hydrophobic substance.

8. A process according to Claim 1, wherein the melamine-formaldehyde precondensate is obtained from melamine and formaldehyde in a ratio of 1:1.5 or more.

9. A process according to Claim 1, wherein at least one member selected from the group consisting of urea, thiourea, guanidine, and resorcinol is used as wall-forming material in addition to melamine and formaldehyde.

10. Microcapsules obtained by the process according to any one of Claims 1 to 9.