Microencapsulation by Interchange of Multiple Emulsions

1. Field of the Invention

The present invention relates to microcapsules and methods of microencapsulating a core of fill material. The resulting microcapsules are adaptable to a variety of applications, but particularly for use in carbonless copying systems.

2. Background of the Invention

Microcapsules generally comprise a core of fill material surrounded by a wall or shell of polymeric material. The fill material may be either gaseous, liquid, or solid, and may be composed of a signal substance, a solution, a suspension or a mixture of substances. The wall surrounding the core of fill material acts to isolate the fill material from the external environment. When it is desirable to release the fill material, the capsule wall may be ruptured by mechanical pressure, for example, thereby introducing the fill material into its surroundings. Generally, microcapsules comprise separate and discrete capsules having non-interconnecting hollow spaces for a fill material. The fill material is thus enveloped within the generally continuous polymeric walls of the microcapsules, which may range from 0.1 to approximately 500 microns in diameter.

Uses for microcapsules are as varied as the materials that can be microencapsulated. Of particular importance are the uses of microcapsules in medicinal and biological preparations, fertilizers, flavorings, deodorizers, adhesives, xerographic toners, and carbonless copying systems.

Though microcapsules and microencapsulation techniques are applicable to a wide variety of products, one of the most significant applications is their use in carbonless copying systems. The present invention is particularly adaptable to carbonless copying systems and will be discussed primarily in connection with such systems. However, it should be understood that the invention is not limited to carbonless copy applications and may be used wherever the use of microcapsules is beneficial.

Carbonless copying systems usually include a plurality of paper sheets arranged in a manifold set, each sheet of the set having one or more coatings on its surfaces. The manifold set is designed so that when a marking pressure caused by a typewriter, pen, or other instrument is applied to the outermost sheet, a colored mark will be formed on at least one surface of each sheet of the manifold set.

To this end, the top sheet of the manifold set to which the marking pressure is applied is provided with a coating on its back surface. This coated back surface includes microcapsules containing an initially colorless chemically reactive color-forming dye precursor as the fill material. The upper surface of the next sheet, which is adjacent to the back surface of the top sheet, is coated with a material containing a component, such as phenolic resin or reactive clay, that is capable of reacting with the colorless dye precursor contained in the microcapsules to produce a color. Thus, a marking pressure on the upper surface of the top sheet will rupture the microcapsules on the bottom surface and release the colorless dye precursor. The colorless dye precursor then chemically reacts with the reactive component of the coated front of the lower sheet to produce a colored mark corresponding to the area of marking pressure. In similar fashion, colored marks are produced on each succeeding sheet of the manifold set by the marking pressure rupturing the microcapsules carried on the lower surfaces of each sheet.

The sheets of the manifold set in carbonless copying systems are designated in the art by the terms CB, CFB, and CF, which stand respectively for "coated back," "coated front and back," and "coated front." The CB sheet is usually the top sheet of the manifold set and the sheet upon which the marking pressure is applied. The CFB sheets are the intermediate sheets of the manifold set, each of which is able to have a mark formed on its front surface by a marking pressure and each of which also transmits the contents of ruptured microcapsules from its back surface to the front surface of the next sheet. The CF sheet is the bottom sheet and is only coated on its front surface so that an image may be formed thereon.

While it is customary to have the coating containing the microcapsules on the back surface of the sheets and to have the coating containing the reactive component for the capsules on the front surface of each of the sheets, a reverse arrangement is also possible. In addition, one or more of the reactive ingredients may be carried in the sheets themselves, rather than applied as surface coatings. Furthermore, the reactive component for the colorless dye precursor may be microencapsulated. Patents illustrative of the various kinds of systems that may be used in the production of manifold carbonless copying systems include by way of example, U.S. Pat. Nos. 2,299,694 (Green); 2,712,507 (Green); 3,016,308 (Macauley); 3,429,827 (Ruus); and 3,720,534 (Macauley et al).

The literature also contains many methods and techniques for preparing microcapsules, whereby two or more reactive components are brought together to form a microcapsular wall. A majority of these methods form the encapsulating walls by providing minute discrete droplets containing the intended fill material dispersed within a continuous phase that contains at least one of the reactive components. In one class of microencapsulation technique, the walls of the microcapsules are formed from reactive components that are present only in the continuous phase and not within the dispersed droplets. Examples of such microencapsulation methods are the urea-formaldehyde polymerization technique disclosed in U.S. Pat. No. 3,016,308 (Macauley) and the coacervation methods described in U.S. Pat. No. Re. 24,899 (Green). The Macauley patent teaches the formation of a high molecular weight urea-formaldehyde condensate wall from a urea-formaldehyde precondensate that is present in the continuous, aqueous phase. The reaction is carried out by adjusting the pH of the continuous, phase. The Green patent discloses forming a gelatinous coating around oil droplets containing the fill material. This coating is then hardened into microcapsule walls by cross-linking agents present in the aqueous continuous phase.

A second class of microencapsulation is interfacial polycondensation exmplified by U.S. Pat. No. 3,429,827 (Ruus). The method taught by Ruus includes producing an aqueous dispersion of a water immiscible organic liquid containing one of the reactive components. A second reactant is then added to the aqueous phase whereupon the reactants form a polymer wall at the interface between the aqueous and organic phases. For example, the organic dispersed phase may contain compounds such as diacid chloride or mixtures of diacid chloride and disulfonyl chloride, and the aqueous continuous phase may contain compounds such as hexamethylenediamine, ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, or mixtures of a polyamine and polyol, such as bisphenol A, thus forming microcapsules having polyamide or copolyamide walls.

One possible disadvantage with the interfacial polycondensation method taught by Ruus is that at least one of the reactive compounds must be soluble in the aqueous phase. Thus, for example, the formation of a microcapsule through the reaction of an acid chloride with an aromatic amine, rather than an aliphatic amine, has not been possible via interfacial condensation since aromatic amine compounds are generally insoluble in aqueous solutions. The use of an acid chloride/aromatic amine pair is not feasible with coacervation techniques because they are not oppositely charged polyelectrolytes. Thus, there is a need in the art for a microencapsulation technique that would allow the use of two or more highly reactive components that are both substantially insoluble in aqueous media.

It is an object of the present invention to provide methods of producing microencapsules through the interchange of a plurality of emulsions. It is believed that the present invention is directed to an entirely novel class of microencapsulation techniques. Specifically, two organic-in-aqueous emulsions are prepared, each containing at least one oil soluble reactive compound that will react to form polymeric microcapsular walls when brought in contact with each other. The first organic-in-aqueous emulsion comprises a first organic solution having a first oil soluble reactive material dissolved therein. This first organic solution is then emulsified within a first aqueous emulsification solution to form the first organic-in-aqueous emulsion. The second organic-in-aqueous emulsion includes a second oil soluble reactive material dissolved in a second organic solution. The second organic solution is likewise emulsified within a second aqueous emulsification solution such that the second organic-in-aqueous emulsion is formed.

Microencapsulation in accordance with the present invention is obtained by mixing the two organic-in-aqueous emulsions for a time and temperature sufficient to permit the emulsified organic droplets of each emulsion to collide with one another. Collision of two or more emulsion droplets causes the emulsified droplets to exchange at least a portion of their contents. This is believed to occur either through the merger or coalescence of multiple droplets into a single droplet following a collision, or through the exchange of the contents of the droplets during an elastic collision. Regardless of the precise mechanism, however, the contents of colliding droplets are transferred to some extent such that reactive materials are brought into reactive contact with each other. Thus, collisions between the droplets of the two emulsions initiate chemical reactions between the reactive materials such that a generally continuous polymeric microcapsular wall is formed around an emulsion droplet. In the case of an elastic collision, two or more separate microcapsules may be formed, while in the case of a merging collision, only one microcapsule results.

In the context of the present invention, the fill material may be dissolved in either the first or the second organic-in-aqueous emulsion or both emulsions. Alternatively, the fill material may be present in a third emulsion that may or may not contain a reactive material dissolved in the third organic solution. A plurality of fill materials may also be used separated between the various emulsions. As an example, when two fill materials are placed in separate organic-in-aqueous emulsions that are mixed together, microcapsules containing proportions of both fill materials result.

The organic solvents used to dissolve the reactive and fill materials with the present invention may be the same for the various emulsions of the present invention or they may be different. Likewise, the aqueous emulsification solutions may be identical for the various organic-in-aqueous emulsions or they may be different. The reactive materials used should be oil soluble and should react to form a polymeric substance suitable for forming a generally continuous microcapsular wall. Many suitable reactive components are well known in the prior art. Though the present invention is useful with any combination of oil soluble reactive compounds, the present invention is particularly useful where neither of the reactive compounds are sufficiently soluble in aqueous solution to be used with the prior art interfacial condensation or coacervation techniques described above. The present invention may use two or more emulsions, though the use of more than four emulsions would be unnecessary in most instances.

Further objects and embodiments of the present invention will become evident in the following description of the preferred embodiments and claims.

Many compounds can be used as the reactive material to form polymeric capsule walls according to the present invention. In theory, any combination of oil soluble reactive materials that yields a generally continuous polymeric wall suitable for microencapsulation may be used. Table I below lists some examples of combinations of oil soluble reactive compounds contemplated by the present invention and the type of polymeric wall formed by their reaction.

Among the acid chlorides that are particularly useful in connection with the present invention are the following: azelayl dichloride, 1.4-cyclohexanedicarbonyl chloride, sebacyl dichloride, phthaloyl chloride, isophthaloyl chloride, terephthaloyl chloride (TCL), tetrachloroterephthaloyl chloride, 4.4'-biphenyldicarbonyl chloride, naphthalene dicarbonyl chloride, and 1.3.5-benzenetricarboxylic acid chloride.

Examples of sulfonyl chlorides that are particularly useful in connection with the present invention are 4.4-sulfonyldibenzoyl chloride, 1.3 benzenedisulfonyl chloride, 1.4-benzenedisulfonyl chloride, 1.5 naphthalene disulfonyl chloride, 2.7-naphthalene disulfonyl chloride, 4.4'-bisphenyldisulfonyl chloride, methylene bis(4-benzenesulfonyl chloride), and sulfonyl bis(4-benzenesulfonyl chloride).

Examples of isocyanate compounds that are particularly useful with the present invention are the following: toluene diisocyanate (TDI), 1.4-cyclohexylenediisocyanate, 4.4'-bisphenylene diisocyanate, 4-methyl-1.2-phenylenediisocyanate, 3.3'-dimethyl-4.4'-biphenylenediisocyanate, 3.3'-dimethoxy-4.4'-biphenylenediisocyanate, 1.4 phenylenediisocyanate, hexamethylenediisocyanate, octamethylenediisocyanate, p.p'-diphenylmethane diisocyanate, and polymethylene polyphenylisocyanates.

Examples of bischloroformate compounds that may be used in connection with the present microencapsulation techniques are the following: ethylenebischloroformate, tetramethylene bischloroformate, 1.4-cyclohexylene bischloroformate, hexamethylene bischloroformate, and 2.2-dimethyltrimethylene bischloroformate. Examples of epoxy compounds useful as oil soluble reactive materials in connection with the present invention are the following: methylenedianiline based epoxy resin, bisphenol based epoxy resin, methylolated bisphenol A based epoxy resin, p-aminophenol based epoxy resin, 1.1.2.2-(p-hydroxyphenol) ethane based epoxy resin, phenol novolac epoxy resin, and cresol novolac epoxy resin.

Among the amine compounds that are useful in connection with the present invention are the following: bis(4-aminophenyl) methane, phenylenediamines, naphthalene diamines, 2.2-bis(4-aminophenyl) propane, 2.4-bis(p-aminobenzyl) aniline (BABA), bis(p-aminocyclohexyl) methane, bishexamethylenetriamine (BHMT), bis(4-aminophenyl) ketone, bis(4-aminophenyl) ether, and bis(4-aminophenyl) sulfone. Examples of bisphenol compounds useful in connection with the present invention are as follows: 2.2-bis(4-hydroxyphenyl) propane, 2.2-bis(4-hydroxyphenyl) butane, 1.6-dihydroxynaphthalene, 2.7-dihydroxynaphthalene, 4.4'-dihydroxybiphenyl, bis(4-hydroxy-3-methyl phenyl) methane, 1.1 bis(4-hydroxyphenyl) ethane, 3.3-bis(4-hydroxyphenyl) pentane, and bis(4-hydroxyphenyl) sulfone.

In connection with carbonless copy systems, the fill material to be encapsulated within the inventive microcapsules will usually be a colorless dye precursor such as crystal violet lactone (CVL, benzoylleucomethylene blue (BLMB), rhodamine lactam, p-toluenesulfinate of Michler's hydrol (PTSMH), or any of the various chromogenic compounds that are capable of changing from a colorless to a colored form on contact with reactive substances, such as phenolic resins or reactive clays.

When a colorless dye precursor is used as the fill material, an organic solvent capable of dissolving or suspending the dye precursor must be used. Suitable organic solvents include benzylbutylphthalate (BBP), dibutylphthalate (DBP),toluene, various xylenes, alkylbenzenes, alkylnaphthalenes, and biphenyls. Aqueous emulsification solutions that are useful with respect to the present invention include emulsifiers such as polyvinylpyrrolidone, polyvinyl alcohol, polyethylene glycol, starch, carboxymethylcellulose, and hydroxyethylcellulose, dissolved in water.

Of course, the novel methods of microencapsulation and microcapsules disclosed here are not limited to use on carbonless copying systems. The fill material could comprise pesticides, insecticides, flavors, fragrances, colored dye solutions, oils, solvents, xerographic toners, plasticizers, or any other materials where microencapsulation would be beneficial. For example, capsules prepared by this invention would be useful for slow release applications.

In general, the fill material to be microencapsulated and a first reactive material are both dissolved within a mutual organic solvent to form the first organic solution. Additional fill material, which may be identical to the first fill material or different, is similarly mixed and dissolved with a second reactive material in a mutual organic solvent, which may or may not be identical to the organic solvent used with the first fill material. The resulting organic solutions are then separately emulsified into organic-in-aqueous emulsions in the presence of aqueous emulsification solutions. Preferably, the organic droplets formed have sizes in the range of 1 to 20 microns. Different emulsification solutions may be used for the various emulsions or the same solution may be used. The two emulsions are then mixed together and stirred for approximately four to twenty-four hours at room temperature. Alternatively, the two emulsions are mixed together and heated to 30.degree.-80.degree. C. to complete the reaction between the two reactive materials. During the time that the two emulsions are mixed, droplets from each emulsion collide with droplets of the other emulsion and transfer or merge their contents to some extent. This initiates the reaction between the two reactive materials such that generally continuous polymeric walls are formed surrounding emulsion droplets. The resulting microcapsules are generally within the range of 1 to 20 microns and have 5 to 30% of the total microcapsule weight constituting wall material.

The proper ratio of the two reactants may be determined by using approximately equal equivalent weights. However, ratios of equivalent weights that are greater or less than one may produce better quality or yield of microcapsules. The most effective ratio may be determined by routine experiments.

In another embodiment of the present invention, the fill material is not present in the emulsions that contain the reactive materials. For example, the fill material can be placed in its own organic-in-aqueous emulsion. In addition, more than two reactive materials may be present in two or more organic-in-aqueous emulsions. Furthermore, a plurality of fill materials may be used and, if desirable, may be separated into various emulsions. This separation of distinct fill materials results in composite microcapsules containing some combination of the plurality of the fill materials.

8.65 parts of 1.1.2.2-(p-hydroxyphenol) ethane based epoxy resin, commerically available as Ciba-Geigy epoxy resin 0163 from Ciba-Geigy Corporation, and 2.4 parts of PTSMH were heat dissolved in 30 parts of BBP. The solution was afterwards brought to room temperature.

2.36 parts of BABA was heat dissolved in 30 parts of BBP and the resulting solution was cooled to room temperature.

Organic Solutions 1 and 2 were each emulsified in a Waring blender using 65 parts of 3% Vinol 540 aqueous solution as an emulsifier (Vinol 540 is a partial hydrolyzed polyvinyl alcohol, commercially available from Air Products & Chemicals, Inc.) until organic droplet sizes on the order of about 1-20 microns were obtained.

The above two emulsions were poured into a glass jar container and stirred at low speed at 45.degree. C. for 4 hours, after which time the mix was stirred at room temperature for another 16 hours to complete the microencapsulation reaction. A slurry containing the above microcapsules was coated on a paper substrate at a coating weight of about 3.3 grams per square meter. This CB coating produced a very distinctive blue image upon impact on an active clay coated CF sheet. Under a scanning electron microscope (SEM), spherical individual capsules werre noted on the CB sheet.

In these examples, the procedures described above in Example 1 were repeated except that differing compositions used in preparing the two emulsions, as noted below.

The microcapsules prepared in accordance with Examples 2-8 were coated onto a paper substrate at a coating weight of approximately 3.0-3.5 g/m.sup.2. The coated paper, upon impact on a clay coated receiving sheet, produced a very good blue image. SEM micrographs of the CB sheets of Examples 3, 6, and 7 were taken and showed good microcapsular formation.

A first organic solution consisting of 17.5 parts of pine oil, a fragrance material, and 2.24 parts of BHMT was prepared. A second organic solution consisting of 17.5 parts of pine oil mixed with 2.72 parts of TDI was also prepared. These two solutions were each emulsified in 50 parts of a 2% Vinol 540 aqueous solution until the sizes of the microdroplets were in the 1-20 microns range. The two emulsions were then mixed together and stirred at room temperature for about 8 hours to complete the microencapsulation reaction. The capsules, upon breaking under the pressure of a fingernail, released a strong odor of pine oil. Under SEM, good microcapsular formation was observed.

In this example, microcapsules were prepared by mixing three emulsions together. The first emulsion included 2.4 parts of PTSMH dissolved in 20 parts of DBP and emulsified in 66.7 parts of a 2% Vinol 540 aqueous solution. The second organic-in-aqueous emulsion included 4.06 parts of BABA dissolved in 20 parts of DBP and emulsified in 66.7 parts of 2% Vinol 540 aqueous solution. The third emulsion included 4.08 parts of TCL dissolved in 20 parts of DBP and emulsified in 66.7 parts of 2% Vinol 540 aqueous solution. The above three emulsions were each prepared in a 1-20 microns range. The emulsions were then mixed together and 2.13 parts of sodium carbonate dissolved in 20 parts of water was added into the slurry to neutralize the reaction by-product, HCl. The slurry was then stirred at room temperature for approximately 20 hours to complete the microencapsulation reaction. Under SEM observation, good microcapsules were noted.

In this example, three emulsions were mixed to form microcapsules.

Organic Solution 1: 9.1 parts of Apogen 101 was dissolved in 20 parts of DBP.

Organic Solution 2: 2.1 parts of BHMT was mixed with 20 parts of BBP.

Organic Solution 3: 2.4 parts of PTSMH was dissolved in 20 parts of diisopropyl naphthalene.

These three organic solutions were each emulsified in 66.7 parts of a 2% Vinol 540 aqueous solution until the sizes of the emulsions were in the 1-20 microns range. The emulsions were then mixed together and stirred at approximately 60.degree. C. for 2 hours and at room temperature for 16 hours to complete the microencapsulation reaction. The resulting microcapsules were coated on a paper substrate at about 3.3 g/m.sup.2 coating weight. This coated sheet was found to produce a very good blue image upon writing against a clay coated receiving sheet. Under SEM, good spherical microcapsules were noted.

It is to be understood that the above specification emphasizes certain embodiments and features of the present invention and that many embodiments not specifically described above may come within the spirit and scope of the present invention as claimed hereafter.