Production of Microcapsules

The present invention relates to a process for the production of oil-containing microcapsules; and the product produced by that process. More particularly, the present invention relates to the production of microcapsules which have hard, thick capsule walls which readily lend themselves to separation and use in press-applied printing inks.

In the manufacture of pressure-sensitive recording papers, better known as carbonless copy papers, a layer of pressure-rupturable microcapsules containing a solution of colorless dyestuff precursor is normally coated on the back side of the front sheet of paper of a carbonless copy paper set. This coated backside is known as the CB coating. In order to develop an image or copy, the CB coating must be mated with a paper containing a coating of a suitable color developer, also known as dyestuff acceptor, on its front. This coated front color developer coating is called the CF coating. The color developer is a material, usually acidic, capable of forming the color of the dyestuff by reaction with the dyestuff precursor.

Marking of the pressure-sensitive recording papers is effected by rupturing the capsules in the CB coating by means of pressure to cause the dyestuff precursor solution to be exuded onto the front of the mated sheet below it. The colorless or slightly colored dyestuff, or dyestuff precursor, then reacts with the color developer in the areas at which pressure was applied, thereby effecting the colored marking. Such mechanism for the technique of producing pressure-sensitive recording papers is well known.

Among the well known color developers used on CF record sheets are phenolic-type resins, such as acetylated phenolic resins, salicylic acid modified phenolics and, particularly, novolac type phenolic resins.

Among the well known basic, reactive, colorless chromogenic dye precursors useful for developing colored marks when and where applied to a receiving sheet coated with such color developers are Crystal Violet Lactone (CVL), the p-toluenesulfonate salt of Michler's Hydrol or 4,4'-bis(diethylamino)benzhydrol, Benzoyl Leuco Methylene Blue (BLMB), Indolyl Red, Malachite Green Lactone, 8'-methoxybenzoindoline spiropyran, Rhodamine Lactone, and mixtures thereof.

Microencapsulation has been used in the production of carbonless copy papers for some time. One of the major techniques involves phase separation from an aqueous solution. The complex coacervation process (U.S. Pat. No. 2,800,457 and others) falls into this category. In such a process, a phase separation into a liquid condensed colloid phase and a dilute colloid phase results from two oppositely charged condensed colloids neutralizing each other. Under appropriate conditions, the condensed colloid phase can be induced to first surround and envelope the oil droplets, and then be hardened to form the microcapsules.

Polymer insolubilization by pH adjustment is another implementation of this general technique. The polymer must become insoluble in water within a specific pH range. Casein is a high molecular weight (33,600-375,000) globular protein with pendant amino, hydroxyl, and carboxylic acid groups, and as such possesses this property. Alone, the polymer is essentially insoluble in water, and this insolubility is at a maximum at its isoelectric point of pH 4.6. However, through salt formation with the amino and carboxylic acid groups, casein will dissolve in bases and in strong acids pH 8, pH 3). Thus, the solubility of casein in water can be controlled by pH adjustment. This property has been used in the prior art to encapsulate oils and solids. Japanese patents 37/7727, 37/7731, and 37/9681, disclose the microencapsulation of dye solutions by precipitation. The dye solution is first dispersed into an alkaline solution of casein wherein the casein (actually sodium caseinate) acts as a polymeric emulsifier. Then, the pH is gradually decreased to 4.6, causing the casein to precipitate onto and envelop the droplets. The resulting capsules were stabilized and strengthened/hardened by insolubilizing the casein with formaldehyde or by drying the capsules through spray drying. A similar preparation of capsules is disclosed in French Patent 1470724. In all cases, as a result of the chemistry of aldehyde hardeners, a structurally strong capsule that can withstand moderate amounts of shear and pressure commonly incurred during handling was not achieved until the capsules were completely dry.

This encapsulation technique has several disadvantages:

(1) The precipitation of casein as a solid does not uniformly occur at the interface of the oil droplet. The nucleation and growth of the solid occurs in the bulk of the water, creating particles of casein of varying size, some of which may or may not collect at the oil-water interface. The result is a non-uniform layer of casein around the droplets. This capsule can have varying thicknesses around the circumference, with numerous cracks or holes in coverage. As a result, the capsules have a great tendency to leak, producing discoloration when used in carbonless papers, and limited shelf life.

(2) The capsules are extremely fragile, and must be spray dried before they are sufficiently strong to withstand normal handling. Thus, they are essentially useless in applications where strong wet capsules are required, as in the case of applying them to paper.

(3) The process uses a high casein:oil ratio, and must be performed at a low solids content (10%) in order to maintain a workable viscosity when the casein is precipitated. This makes subsequent coating or processing (drying) expensive due to the large amounts of water that must removed.

In response to these shortcomings, interfacial polymerization or interfacial crosslinking was developed. In U.S. Pat. No. 4,138,362, an amine containing polymer is crosslinked or polymerized at the interface by a polyisocyanate dissolved in the dye solution, which is emulsified in the polymer. In the case of casein, the capsules produced by this method are extremely poor. The capsules are very fragile, due to a very thin wall, and exhibit poor aging as a result of a steady release of the solution they contain.

This problem apparently stems from (1) the colloidal nature of casein solutions (they exhibit the Tyndall effect), (2) the high molecular weight of casein, and (3) the anionic (charged) nature of sodium or alkali metal salt solutions of casein. The alkali salt of casein functions as a high molecular weight polyanionic emulsifier and protective colloid. When an oil is emulsified in an alkaline casein solution, a thin layer is formed around the droplet. The polyisocyanate can crosslink this small amount of casein absorbed on the surface, but additional casein cannot reach the isocyanate. The very properties that make sodium caseinate a good emulsifier and protective colloid inhibit further reaction. Namely, the negative charge around the droplet created by the thin molecular layer creates the familiar double layer that functions as a barrier to further penetration of ionized casein to the droplet surface. The high molecular weight also sterically hinders the diffusion of additional casein to the reaction zone near the oil-water interface. The fragile capsules produced by the simple interfacial polymerization/crosslinking are inadequate for the processing procedures needed for preparing carbonless papers, namely, filtration and dispersion into ink vehicles.

Another method to improve the strength of capsule walls is shown in U.S. Pat. No. 4,404,251, which discloses printing ink containing microcapsules containing dye precursors. The microcapsules are made by polyaddition of a polyisocyanate and a polyamine, and the aqueous phase may contain protective colloids and emulsifiers. The formed microcapsules are formulated into the printing ink composition by stirring the aqueous dispersion of microcapsules directly into the binder and subsequently removing the water in vacuo. Alternatively, the microcapsules may be spray dried and then added to the binder. Dispersing aids, preferably cationic surfactant, may be added to the ink to prevent agglomeration of the microcapsules.

Reference is also made to U.S. Pat. No. 4,193,889 which discloses microcapsules and a process for the production of microcapsules the walls of which consist of polycondensates of a film-forming aliphatic polyisocyanate containing at least one biuret group, or polyaddition products thereof, with a chain extending agent. The chain extending agent is preferably either water, a polyol or a polyamine. It is stated in that patent that the so-produced microcapsules have improved toughness, show adequate crosslinking density, and, therefore, are only slightly permeable to easily volatile encapsulated substances.

Another method for dealing with the problems of fragile microcapsules is disclosed in U.S. Pat. No. 4,435,340, wherein an isocyanate is used in the hydrophobic phase and a polyamine, such as a low molecular weight polyamine, is used in the hydrophilic phase. Microcapsules are formed by interfacial polymerization.

U.S. Pat. No. 4,356,108 discloses an encapsulation process by interfacial reaction of an isocyanate and a low molecular weight polyamine.

Even with these various improved microcapsules, processes for printing microcapsules in coating compositions on offset printing machines or even book printing machines were heretofore regarded as unworkable because both in the production of the printing ink and in the distributor rollers of the printing machine and during the printing process, shearing and compressive forces would destroy most of the microcapsules. A process for accomplishing such printing inks is described in U.S. Pat. No. 4,404,251 in which formed microcapsules are formulated into the printing ink composition either by stirring the aqueous dispersion of microcapsules directly in the binder and subsequently removing the water in vacuo (the so-called flushing process), or by spray-drying the microcapsules and then adding to the binder. These processes require special equipment and are not entirely satisfactory. In the former process, the hydrophilic nature of the microcapsules may make direct incorporation into the binder very difficult. The spray-drying technique is very costly. Furthermore, during spray-drying some capsules inevitably aggregate which results in a large particle size distribution. The aggregates can easily reach 100 microns or more, and once formed are virtually impossible to break up non-destructively to the capsule. Such large particles are quite unsuitable for inks.

Accordingly, the need remains for improved microcapsules which are sufficiently strong to be dispersed in an ink vehicle and press applied.

That need is met by the present invention, where the object is to eliminate the deficiencies of the prior art, such as those set forth hereinabove.

It is a further object of the present invention to provide improved microcapsules.

It is another object of the present invention to provide an improved process for preparing microcapsules used in carbonless copy paper technology, including for use in preparing CB carbonless copy sheets.

It is yet another object of the present invention to provide microcapsules for use in carbonless copy paper technology which have strong thick walls and thus permit their application by printing press without the destruction of the microcapsules.

These and other objects, features and attendant advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of preferred embodiments and the appended claims.

In summary, the present invention includes a method for forming microcapsules that are sufficiently strong to be dispersed in an ink vehicle, either an aqueous one or an oil-based one, and spread onto a substrate.

The microcapsules are formed using a polyelectrolyte complex or a polysalt consisting of (1) a high molecular weight polyanion, i.e., an alkali-soluble polymer with repeating units containing carboxylic, phosphoric, or sulfonic acid groups and/or amino acid groups, such as casein, sodium caseinate, zein, soya protein, polyacrylic acids, acrylic acid copolymers, acid copolymers, and maleic anhydride copolymers and (2) a low molecular weight polycation having a molecular weight of less than 1200 such as polyamine with a functionality of at least 3. The preferred polycation is a polycationic polyamine such as diethylene triamine and the preferred polyanion is casein. An oil solution of dye precursor containing a crosslinking agent is dispersed into an aqueous solution of the polyanion. The crosslinking agent may be a polyisocyanate, a polyacid chloride, a polyanhydride, a polyepoxide, a polychlorofoamate, or a polyaldehyde. The preferred crosslinking agent is a polyisocyanate. The polycation may be added before or after such dispersing step. The crosslinking agent reacts with the polyamine-polyanion complex to form a strong, thick-walled capsule. Heat treatment accelerates the crosslinking. A denatured crosslinked layer of polyanion builds up around the droplet, thus producing a tough, thick capsule wall.

The present microencapsulation process is not a coacervation one as such. This is an advantage in that it is not necessary to chill the mixture, add a hardener, adjust the pH, or otherwise conduct traditional post-coacervation steps in order to produce high strength microcapsules. Rather, the present process use a low molecular weight polycationic polyamine and a polyanion in combination with a crosslinking agent to produce capsule by a non-coacervation process. A key to avoiding coacervation in the present process is the use of a polycationic polyamine having a molecular weight of less than 1200.

The present microencapsulation process is actually an improvement of interfacial polymerization ones. It is an improvement in that while the crosslinking agent reacts with the polycation portion of the polysalt in much the same manner a typical interfacial polymerization reaction, the polyanion portion of the polysalt precipitates out of the solution and builds up around the droplet. All of this takes place at the interface of capsule wall formation and, as mentioned, result in a microcapsule which is thicker and stronger than microcapsules produced by ordinary interfacial polymerization.

CB coated papers can be readily made by coating paper with an aqueous or oil-based dispersion of the microcapsules of the present invention. Since the microcapsules are relatively strong, a dispersion of the microcapsules can be coated onto paper using a standard printer. To make microcapsules according to the process of the present invention, an oily solution of dye precursor in an oily solvent and containing a crosslinking agent is emulsified into an aqueous solution of casein or other polyanion that has been dissolved using a weak base such as borax (sodium tetraborate). A polyamine preferably with a functionality of 3 or more, and preferably having a molecular weight of less than 1,200, is added, either before or after emulsification.

With the preferred polycationic polyamines it has been found that a molecular weight of 1,200 or more may cause coacervation to occur immediately, with the result that a polycationic (polyamine)/polyanionic(casein) complex separates as a distinct liquid phase. This is undesirable. Accordingly, the preferred polycationic polyamines have a molecular weight of less than 1200. The preferred polyamine also preferably has at least one active hydrogen available on at least two of the three amino groups. Examples of suitable polyamines are diethylenetriamine, triethylenetetramine, iminobispropylamine, bis(hexamethylene)triamine, polyoxypropylenetriamines, polyoxypropylenepolyamines, and amine epoxy adducts (hydroxyalkyl-substituted polyamines).

Any crosslinking agent which reacts with both segments of the polyamine-polyanion complex simultaneously, or with the polyamine segment causing the polyanion segment to precipitate concurrently, or both, will be operable in the present invention. Examples of appropriate crosslinking agents are polyisocyanates, polyacid chlorides, polyanhydrides, polyepoxides, polychloroformates, or polyaldehydes. It has been found that the crosslinking agent works best in the system of the present invention when it contains a functionality of at least three.

Isocyanates suitable for this purpose are polyisocyanates and polyisocyanate prepolymers. The term "polyisocyanate" includes polyisocyanates and polyisothiocyanates. Examples of suitable oleophilic polyisocyanates include diisocyanates such as m-phenylenediisocyanate, p-phenylenediisocyanate, 2,6-tolyenediisocyanate, 2,4-tolylenediisocyanate, naphthalene-1,4-diisocyanate, diphenylmethane-4,4'-diisocyanate, 3,3'-dimethoxy-4,4'-biphenyldiisocyanate, 3,3'-dimethyldiphenylmethane-4,4'-diisocyanate, xylylene-1,4-diisocyanate, xylylene-1,3-diioscyanate, 4,4'-diphenylpropanediisocyanate, trimethylenediisocyanate, hexamethylenediisocyanate, propylene-1,2-diisocyanate, butylene-1,2-diisocyanate, ethylidynediisocyanate, cyclohexylene-1,2-diisocyanate, cyclohexylene-1,4-diisocyanate, xylylene-1,4-diisothiocyanate, ethylidynediisothiocyanate, hexamethylenediisocyanate, etc.; triisocyanates such as 4,4',4"-triphenylmethanetriisocyanate, toluene-2,4,6-triisocyanate, etc; and tetraisocyanates such as 4,4'-dimethyldiphenylmethane,2,2',5,5'-tetraisocyanate, etc. Particularly preferred are the addition product of hexamethylene diisocyanate and hexane triol, the addition product of 2,4-tolylene diisocyanate with pyrocatechol, the addition product of tolylene diisocyanate with hexane triol, the addition product of tolylene diisocyanate with trimethylol propane, or suitable polyisocyanates analogous to the compounds mentioned, or methylene(polyphenyl isocyanate).

Other usable modified isocyanates are those based on hexamethylene-1,4-diisocyanate, m-xylylene diisocyanate, 4,4'-diisocyanato-dicyclohexyl methane and isophorone diisocyanate which contain at least two functional isocyanate groups per molecule. Polyisocyanates based on derivatives of hexamethylene-1,6-diisocyanate having a biuret structure, as disclosed in U.S. Pat. No. 4,051,165, are also comprehended.

Concentrations of the components in the encapsulation process of the present invention are suitable as follows, expressed in parts by weight:

The microcapsules of the present invention may be dispersed in an non-aqueous ink vehicle as disclosed and claimed in copending application Ser. No. 866,224, filed May 22, 1986 and now U.S. Pat. No. 4,729,792, to form a CB coating material. As described in that application, this is accomplished by adding a lipophilizing agent to the microcapsules to render them non-polar. This renders the microcapsules readily filterable from the aqueous solution and dispersible in a non-aqueous coating vehicle such as an oil-based one. As disclosed in Ser. No. 866,224, and now U.S. Pat. No. 4,729,792, a wetting agent may be added to aid in the preparation of the oil-based CB coating material.

The microcapsules of the present invention may also be dispersed in an aqueous ink vehicle to form a high solids CB printing ink as disclosed and claimed in copending application Ser. No. 141,632, filed on an even date herewith. As described in that application, this is accomplished by using a non-volatile diluent as a co-solvent with water to provide the aqueous solution for the encapsulation reaction. The resulting microcapsule slurry is itself a high solids aqueous CB printing ink because the aqueous solution alone serves as the aqueous ink.

In either event, an advantage of the present invention is that dryers are not needed when the microcapsule-containing printing ink of the present invention is used. This is true even when the printing ink is an aqueous one; whereas, conventional aqueous inks require a substantial amount of drying by external dryers after application of the ink to the paper web substrate and prior to wind-up. As mentioned previously, no additional water need be added when the microcapsule filter cake is placed in the ink vehicle. Rather, as mentioned with the aqueous based ink the aqueous solution (water and non-volatile diluent in that instance) of the filter cake becomes a part of the ink vehicle. The result is a high solids printing ink which may be press applied at low coat weights and which requires minimal amounts of drying.

There is one overall constraint on the present invention: no material in contact with the capsules can possess significant mutual, simultaneous, oil and water miscibility. Capsule leakage or destruction may occur if one does not adhere to this restriction.

The following examples will more clearly define the invention.

In a 2 liter beaker, 36.4 grams of Crystal Violet Lactone and 3.6 grams Copiken XX (a color precursor manufactured by Hilton-Davis of Cincinnati, Ohio) are dissolved in 960 grams of di-isopropyl naphthalene. At room temperature 98.5 grams Desmodur L2291A (a biuret-containing polyisocyanate made by Mobay Chemical Corp., Pittsburgh, Pa.) are added and stirred until a clear solution is obtained.

In a four liter beaker, 135 grams casein are added to 1.7 liters water. The solution is heated to 80-85 degrees C., held for 30 minutes, and then cooled to 60 degrees C. 20.25 grams of borax are added, the solution stirred for 15 minutes, then cooled to room temperature.

Solution B is placed in a Waring blender connected to a variac. With the blender set on high and the variac at 50%, solution A is slowly poured into the vortex of Solution B within a period of two minutes. After the addition of A is complete, the variac is set to 100%, and the blender is allowed to run for only 30 seconds. The emulsion is transferred to the four liter beaker, and stirred moderately to produce a slight vortex. Then, 15 grams diethylene triamine in 15 grams water is added to the emulsion. The mixture is heated to 60.degree. C. and held at that temperature for about two hours. The capsule slurry is then cooled to room temperature. The average capsule size is about 7 microns.

Example I is repeated using a solution containing 130 grams of soya protein rather than casein. The average capsule size is about 7 microns.

Example I is repeated using a solution containing 140 grams of zein instead of the casein and with the addition of sufficient sodium hydroxide to raise the pH to 11.5 and thus cause dissolution of the zein. Microcapsules are formed having an acceptably tough outer skin.

Example I is repeated using a polyacrylic acid (Carboset 525 from B. F. Goodrich Chemical Company) as a replacement for the casein to form acceptable microcapsules.

In Example I, the polysalt is formed after the emulsion. Alternatively, the polysalt may be formed initially and used directly.

Solution A is prepared as in Example I.

In a four liter beaker, 35 grams casein is added to 1.7 liters water, heated to 80.degree.-85.degree. C. and held for about 30 minutes. After cooling to 60.degree. C., 15 grams diethylene triamine is added. The solution is stirred until the casein dissolves, and then cooled to room temperature.

As in Example I, the Waring blender is charged with B, and then A is added over a two minute period with the blender set on high and the variac at 50% power. After all of A is added, the power is adjusted to 100% for 30 seconds. The mixture is transferred to the four liter beaker, and heated for two hours at 60.degree. C. with stirring.

Eight grams of CVL and 13.15 g PA-18 resin (a polyanhydride crosslinker from Gulf Oil Chemicals Co.) are dissolved in Sure-Sol 290 (a biphenyl solvent mixture from Koch Chemical Company, Wichita, Kans.).

In a separate beaker, 18 g casein is added to 280 ml water. The mixture is heated to 80.degree.-85.degree. C. and held for 30 minutes. After cooling to 60.degree. C., 2.7 g of borax (sodium borate decahydrate) is added. The resulting mixture is stirred for 15 minutes, and then cooled to room temperature.

Solution B is placed in a Waring blender connected to a variac. With the blender set on high and the variac at 70%, solution A is poured into the vortex of B within a period of 11/2 minutes. On completion of this addition, the variac is set to 100%, and the blender is allowed to run for 30 seconds. The emulsion is transferred to a 1 L beaker, and stirred moderately to produce a slight vortex. Then 2 g diethylene triamine in 2 g water is added to the emulsion. The mixture is heated to 55.degree.-60.degree. C. and held there for 2 hours. The average capsule size is 3 microns.

A 13.15 g quantity of PA-18 resin (a polyanhydride crosslinker from Gulf Oil Chemicals Co.) is dissolved in 192 g of Sure-Sol 290 (a biphenyl solvent mixture from Koch Chemical Company, Wichita, Kans.) by heating to 120.degree. C. The solution is cooled to 90.degree. C., and 7.4 g of CVL is added. The mixture is then cooled to room temperature.

In a separate beaker, 18 g casein is added to 300 ml water. The mixture is heated to 80.degree.-85.degree. C. and held for 30 minutes. After cooling to 60.degree. C., 2 g diethylene triamine is added. The mixture is stirred for 15 minutes, and then cooled to room temperature.

Solution B is placed in a Waring blender connected to a variac. With the blender set on high and the variac at 70%, solution A is poured into the vortex of B within a period of 55 seconds. On completion of this addition, the variac is set to 100%, and the blender is allowed to run for 15 seconds. The emulsion is transferred to a 1 L beaker, and stirred moderately to produce a slight vortex. The emulsion is heated to 60.degree. C. and held for 2 hours. The capsule slurry is then cooled to room temperature. The average capsule size is 9 microns.

A mixture of 20 g EPN 1138 (an epoxy novolac crosslinker from Ciba-Geigy), 9 g Pergascript I-BR, and 3.1 g Pergascript I-36 (both color former compounds from Ciba-Geigy) is dissolved in 168.2 g Benzoflex 9-88 (an organic solvent from Velsicol Chemical Co.).

A 27 g quantity of casein is dissolved in a solution of 4.27 g Hardener HY 943 (an aliphatic/polyamine-epoxy adduct from Ciba-Geigy) in 269 ml water.

Solution B is placed in a Waring blender connected to a variac. With the blender set on high and the variac at 35%, solution A is poured into the vortex of B within a period of 1 minute. On completion of this addition, the variac is set to 80%, and the blender is allowed to run for 30 seconds. The emulsion is transferred to a 1 L beaker, and stirred moderately to produce a slight vortex. A solution of 1.35 g borax in 70 ml water is added. The emulsion is heated to 65.degree.-70.degree. C. and held for 3 hours. When the temperature reaches 60.degree. C., 1 g of Accelerator DY064 (tri(dimethylaminoethyl)phenol [DMP-30] from Ciba-Geigy) in 9 ml water is added.

A mixture of 3.6 g CVL and 12 g iso-phthaloyl chloride is dissolved in 110 g Meflex DC029 (a chlorinated paraffin solvent from ICI America, Inc.).

In a separate beaker, 18 g casein is added to 300 ml water. The mixture is heated to 80.degree.-85.degree. C. and held for 30 minutes. After cooling to 60.degree. C., 2.7 g borax is added. The mixture is stirred for 15 minutes, and then cooled to room temperature.

Additional borax may be added to solution B if the iso-phthaloyl chloride of solution A contains significant amounts of HCl.

Solution B is placed in a Waring blender connected to a variac. With the blender set on high and the variac at 100%, solution A is poured into the vortex of B within a period of 1 minute. On completion of this addition, the emulsion is quickly transferred to a 1 L beaker, and stirred moderately to produce a slight vortex. A solution of 6.83 g diethylene triamine and 3.7 g sodium carbonate in 30 ml water is immediately added. The dispersion is stirred for 3 hours. The average capsule size is 9 microns.

The dispersion gells in a couple of minutes due to hydrolysis of the acid chloride-generation of HCl. The polyamine should therefore be added before significant hydrolysis has occurred.

It will be obvious to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is described in the specification.