GB2029791A - Method of making microcapsules - Google Patents

Abstract

In a method of making oil- containing microcapsules by coacervation in an aqueous system at a temperature above the gelation points of the hydrophilic colloid materials to form a coacervate suspension in which each of the oil droplets, which include at least one polyvalent isocyanate, is surrounded by a coacervate, and cooling said coacervate suspension to a temperature below the gelation points of said hydrophilic colloid materials to form multi-nucleus capsules, during the step of cooling the coacervate suspension, aggregation of particles of oil droplets each having a coacervate therearound is controlled by agitation so as to allow formation of multi- nucleus microcapsules having an average diameter within the range of 3 to 20 microns, for example by rotating at least one agitator in a vessel containing the coacervate suspension under specific conditions.

Description

SPECIFICATION Method of making microcapsules BACKGROUND OF THE INVENTION This invention relates to a method of making microcapsules containing water-immiscible oil droplets and more particularly to a method of making multi-nucleus microcapsules formed by aggregation of such mono-nucleus microcapsules and which are useful for the manufacture of pressuresensitive copying papers.

Pressure-sensitive copying papers and heat-sensitive recording papers which utilize the color developing reaction between electron donating organic chromogenic material (hereinafter referred to as "colour former") and electron accepting acidic reactant material (hereinafter referred to as "acceptor") are now widespread. In pressure-sensitive copying paper at least one of the colour former and the acceptor is contained in microcapsules so as to be isolated from the other and they come into contact with each other by rupturing such microcapsules to develop a colour. In a most typical type of pressuresensitive copying paper minute oil droplets in which the colour former is dispersed or dissolved are encapsulated and coated onto the paper.

Pressure-sensitive copying papers have found usefulness in a variety of commercial material applications. For example, they are very useful as computer output recording papers, business forms and copying slips. In these arts one of the requirements in quality is that the copying can be done at one time and under usually applied pressure for as many superposed sheets of paper as possible. This requirement, however, involves a disadvantage that paper sheets are easily smudged by inadvertent pressure during storage, handling and shipping. There has been proposed pressure sensitive copying paper having a coating layer of multi-nucleus microcapsules, e.g., as disclosed in US Patent Specification No. 3,041,289 and Japan Kokai (Laid Open Patent Publication) No. 118,509 of 1976.The conventional technique of making the multi-nucleus microcapsules has never been able to resolve the antinomic problem that the copying capacity of the multi layered copying paper sheets should be increased while preventing the copying paper sheets should be increased while preventing the copying paper sheets from being smudged by inadvertent pressure.

The most typical method for making oil-containing microcapsules is to utilize the complex coacervation technique. For example, according to the disclosure in US Patent Specification No.

2,800,457 oil-containing microcapsules are made by the following steps: 1) A mixture of two different hydrophilic colloid sols in which oil droplets are dispersed is prepared. The mixture may be made by forming an aqueous sol of one colloid material, emulsifying the selected oil therein, and mixing the emulsion with an aqueous sol of another colloid material, or the two sols may be made and mixed and the oil emulsified therein. The two colloid materials have opposite electric charges and at least one of them is gellable.

2) Coacervation is caused by dilution and/or by adjusting the pH of the mixture to form and adhere a coacervate around each of the oil droplets.

3) The coacervate around each of the oil droplets is gelled by cooling; and 4) The coacervate is further hardened by addition of a hardening agent and if necessary by adjusting the pH to an alkaline value.

It is considered that in order to resolve the before-mentioned antinomic problem it is imperative to obtain oil-containing microcapsules having selected and generally uniform particle sizes within a limited range. The particle size of the microcapsules obtained by the above mentioned coacervation technique depends on various factors such as the temperature and the pH of the system, the colloid concentration in the system, the kinds of colloid materials used and the composition ratio of the oil and the colloid materials. It is our conclusion that after substantial investigations and experiments that it would be impossible or extremely difficult to obtain microcapsules having desired and generally uniform particles sizes with chemical adjustment of any of the above mentioned factors alone.

The inventors found that if aggregation of particles of oil droplets with coacervates is precisely controlled by a specified agitated flow applied to the system for an appropriate balance between the adhesion force of coacervates and the separation force given by the controlled agitated flow, multinucleus capsules having desired diameters and desired particle size uniformity are obtained. This agitated flow control technique is disclosed in Japanese Patent Application No. 9022 of 1 977 filed 28th January 1 977.

The inventors have continued a further study and research of the above mentioned agitated flow control technique in making microcapsules and found that if the oil includes at least one polyvalent isocyanate, multi-nucleus capsules having selected and generally uniform particle sizes as desired and improved water and solvent resistances.

It is well known to utilize polyvalent isocyanates in the production of microcapsules. For example, Japanese Patent Publication No. 771 of 1967 discloses a method for making microcapsule by emulsifying an oily material dissolving a polyvalent isocyanate therein into water to which a polyvalent amine is added. Japanese Patent Publication No. 13,508 of 1977 discloses another method for making microcapsules by emulsifying an oily material dissolving a polyvalent isocyanate therein into water which includes a polyhydroxylated polymer such as polyvinyl alcohol. Further, Japanese Laid Open Patent Publication No. 36,097 of 1973 discloses a method for encapsulating the oil dissolving a polyvalent isocyanate therein through the utilization of the complex coacervation of hydrophilic colloid materials.In any of those known methods for making microcapsules utilizing polyvalent isocyanates it was impossible to obtain multi-nucleus microcapsules having selected and generally uniformed particle sizes as desired since the encapsulation process is highly affected by a strong reaction of the polyvalent isocyanate used. Particularly in case where encapsulation of the oil dissolving a polyvalent isocyanate is carried out through the utilization of the complex coacervation of hydrophilic colloid materials, it was extremely difficult to have the coacervation caused under control as desired since interfacial conditions are excitedly changed by the reaction of the polyvalent isocyanate used.

An object of the invention is to provide an improved method of making multi-nucleus oil-containing microcapsules having selected and generally uniform particle sizes as desired and improved water and solvent resisting qualities.

The invention provides, at least in its preferred embodiments, a control of the particle size of the muiti-nucleus oil-containing microcapsules and the uniformity of the particle size as desired through the utilization of a specified agitated flow during the making of oil-containing microcapsules according to the coacervation technique.

The invention also provides, at least in its preferred embodiments, a control of the particle size of the multi-nucleus oil-containing microcapsules and the uniformity of the particle size with use of the oil including at least one polyvalent isocyanate in addition to the utilization of the above mentioned agitated flow.

SUMMARY OF THE INVENTION According to the invention in the method of making oil-containing microcapsules comprising the steps of (1) preparing an aqueous system comprising an aqueous solution of at least two kinds of hydrophilic colloid materials with opposite electric charges and oil droplets dispersed in said solution, said oil droplets including at least one polyvalent isocyanate, (2) causing coacervation in said aqueous system at a temperature above the gelation points of said hydrophilic colloid materials to form a coacervate suspension in which each of said oil droplets is surrounded by a coacervate and 3) cooling said coacervate suspension to a temperature below the gelation points of said hydrophilic colloid materials to form multi-nucleus capsules, aggregation of particles of oil droplets each having a coacervate therearound is controlled, during the step of cooling the coacervate suspension, by agitated flow so as to allow formation of multi-nucleus microcapsules having an average diameter within the range of 3 to 20 microns.

In a preferred embodiment of the invention, aggregation of particles of oil droplets each having a coacervate therearound is controlled by an agitated flow produced by rotating at least one agitator having at least one vane in a vessel containing the coacervate suspension under the following conditions: 2a 0.3 < < 1 D Sp 0.05 S ---- < 1 ST 5n6 30a wherein D is the maximum inner diameter of the vessel, a is the maximum radius of gyration of the agitator, ST is the maximum vertical sectional area of the coacervate suspension in the vessel, Sp is the maximum vertical sectional area of the volume swept by said at least one agitator vane and n is the number of revolutions of the agitator per minute.If the agitator has two or more radially extending vanes or two or more agitators are used in a single vessel, the value Sp should be the total of the respective maximum vertical sectional areas of the volume swept by the vanes.

DETAILED DESCRIPTION OF THE INVENTION The first step of the method of making oil-containing microcapsules according to the invention is to prepare an aqueous system comprising an aqueous solution of at least two kinds of hydrophilic colloid materials with opposite electric charges and oil droplets dispersed therein. Such the aqueous system like this may be prepared either by forming the aqueous sol of one colloid material, emulsifying the selected oil therein, and mixing the emulsion with an aqueous sol of another colloid material, or, by forming a mixed sol of the two colloid materials and emulsifying the oil therein.

According to the invention the oil droplets which are dispersed in the solution includesat least one polyvalent isocyanate. The polyvalent isocyanate containing oil may be prepared by dissolving at least one polyvalent isocyanate in the oil. Among the useful polyvalent isocyanates there are included diisocyanates such as m-phenylene diisocyanate, p-phenylene diisocyanate, 2,6-tolylene diisocyanate, 2,4-tolylene diisocyanate, naphtha lene-1 ,4-diisocyanate, diphenylmetha ne-4,4'-diisocyanate, 3,3'- dimethoxy-4,4'-biphenyl diisocyanate, 3,3' -dimethyl-phenylmethane-4,4'-diisocyanate, xylylene-1,4diisocyanate, xylylene- 1 ,3-diisocyanate, 4,4-diphenylpropane diisocyanate, trimethylene diisocyanate, hexamethylene diisocyanate, propylene- ,2-diisocyanate, butylene- 1 ,2-diisocyanate, ethylidyne diisocyanate, cyclohexylene-l ,2-diisocyanate and cyclohexylene- 1 ,4-diisocyanate; triisocyanates such as 4,4',4"-triphenylmethane triisocyanate and toluene-2,4,6-triisocyanate; polyisocyanate monomers such as 4,4'-dimethyldiphenylmethane-2 ,2',5.5' -tetraisocyanate; and adducts of the above di-, tri- or polyisocanate with a compound having a hydrophilic group such as polyamines, poiycarboxylic acids, polythiols, polyhydroxyl compounds and epoxy compounds.

In case of the manufacture of microcapsules for use in pressure-sensitive copying paper a colour former and/or an acceptor is dispersed or dissolved in the oil droplets.

The oil may be conventional oil. For example, animal oils such as fish oil and lard oil; vegetable oils such as castor oil, soybean oil, linseed oil, earth-nut oil and corn oil; mineral oils such as kerosene, naphtha and paraffin oil; synthetic oils such as alkylated naphthalene, alkylated biphenyl, hydrogenated terphenyl and alkylated diphenylmethane may be used either solely or in combination.

Among the useful colour former compounds there may be included triarylmethane derivatives such as 3,3-bis(p-dimethylarninophenyl)-6-dimethylaminophtha lide (CVL), 3,3-bis(p- dimethylaminophenyl)phtha lide, methyl 3-(p-dimethylaminophenyl)-3-( 1 ,2-dimethylindole-3-yl)phthalide, 3-(p-dimethylaminophenyl)-3-(2-methylindole-3-yl)phthalide, 3,3-bis-( 1 ,2-dimethylindole-3-yl)- 5-dimethyla minophthalide, 3,3-bis( 1 ,2-dimethylindole-3-yl)-6-dimethylaminophthalide, 3,3-bis( 9-ethylca rbazole-3-yl)-5-dimethylaminophthalide, 3,3-bis-(2-phenylindole~T3-vl)-5- dimethylaminophthalide and 3-p-dimethylaminophenyl-3-( 1 -methylpyrrole-2-yl)-6- dimethylaminophthalide, diphenylmethane derivatives such as 4,4'-bis dimethylaminobenzhyd rinebenzylether, N-haiophenyl-ieucoau ramine and N-2,4,5-trichlorophenyl leucoauramine, xanthene derivatives such as rhodamine-B-anilinolactam, rhodamine-(p nitroanilino) lactam, rhodamine-(p-chloroa nilino)lactam, 3-dimethyla mino-7-methoxyfluoran, 3 diethylamino-6-methoxyfluoran, 3-diethylamino-7-methoxyfluoran, 3-diethylamino-7- chlorofluoran, 3-diethylamino-7-chloro-6-methylfluoran, 3-diethylamino-6,8-dimethylfluoran, 3- diethylamino-7-(acetylmethylamino)fluoran, 3-diethylamino-(7-methylamino)fluoran, 3,7- diethylaminofluoran, 3-diethylamino-7-(dibenzylamino)fluoran, 3-diethylamino-7- (methylbe nzyla min o)fluora n, 3-diethylamino-7-(chloroethylmethylamino)fluoran and 3- diethylamino-7-(diethylamino)fluoran, thiazine derivatives such as benzoylleuco-methylene blue and p-nitrobenzyl-leucomethylene blue, and spiro-compounds such as 3-methyl-spiro dinaphthopyrane, 3-ethyl-spiro-dinaphthopyrane, 3,3'-dichloro-spiro-dinaphthopyrane, 3 benzylspi ro-dinaphthopyrane, 3-methyl-naphtho-(3-methoxy-benzo)-spi ropyrane and 3-propyl spiro-dibenzopyrane.

These compounds may be used either solely or in combination.

Among the useful acceptor compounds there may be included inorganic acid materials such as acid clay, activated clay, attapulgite, silica, zeolite, bentonite and aluminum silicate, and organic acceptors such as phenolic compounds and phenol resins, e.g., phenol-aldehyde polymers and phenolacetylene polymers and polyvalent metal salts of phenol resins as described in US Patent Specifications Nos. 3,51 6,845 and 3,732,1 20;; aromatic carboxylic acids, e.g., benzoic acid, salicylic acid, 3,5-di-ter- butyl salicylic acid, 3-phenyl-5-(a,a-dimethylbenzyl)salicylic acid, 3,5-di(a-methyibenzyl)salicylic acid, and 1 -hydroxyl-2-carboxynaphthalene, and polyvalent metal salts of aromatic carboxylic acids as disclosed in US Patent Nos. 3,864,146,3,924,027 and 3,983,292; and aromatic carboxylic acidaldehyde polymers, aromatic carboxylic acid-acetylene polymers, and their polyvalent metal salts as disclosed in US Patent Specifications Nos. 3,767,449 and 3,772,052. These compounds may also be used either solely or in combination.

The colloid materials may also be of any known type. For example, among cationic colloid materials there may be included gelatin, casein, albumin, fibrinogen, hemoglobin, polyvinylpyrrolindone, copolymers of vinylpyridine derivatives with stylene or methylacrilite, piperidine-acetyl cellulose and benzylamino-starch. Among anionic colloid materials there may be included cellulose derivatives such as carboxymethylcellulose, carboxyethylcellulose, carboxymethyl-ethylcellulose, carboxymethylhydroxyethylcellu lose and sulfonated cellulose, gum arabic polyvinylsulfonic acid, copolymers of vinyl monomers with maleic anhydride or phthalic anhydride. These are hydrophilic polymers having anionic groups.

The particle size of the emulsified oil droplets as mono-nuclei for microcapsules depends on the polyvalent isocyanate and hydrophilic colloid materials used. It may be within the range of 1 to 1 5 microns, preferably, within the range of 105 to 5 microns in terms of the diameter measured by Coulter Counter, Model TA manufactured by Coulter Electronics Inc., USA.

The emulsifying step may be effectively carried out by using a conventional agitator or stirrer such as a homogenizer, a propeller mixer or a Warren blender.

In some cases a surfactant such as sodium alkylbenzenesulfonate, sodium salt of naphthalenesulfonic acid-formaldehyde condensate, sodium alkylsulfate, potassium ligninsulfanate, sodium oleate orsulfonate oil may be used in the emulsifying step. It is also preferred to use protective colloid such as polyvinyl alcohol in the emulsifying step.

The next step to make microcapsules is to cause coacervation in the aqueous system thus prepared to form a coacervate suspension in which a coacervate is deposited on and around each of the oil droplets. the coacervation may be caused by dilution and/or by adjusting the pH of the system.

The amount of water to be added for dilution may be decided as desired according to the coacervation conditions required, but usually water is added in such an amount as to reduce the colloid concentration in the system to 2 to 5% by weight.

The pH value for causing coacervation depends on the isoelectric point of the colloid material to be used. For example, if an acid-treated gelatin having an isoelectric point of about 8.0 is used, the pH of the system may be adjusted within the range of 4.0 to 6.0.

It is not always desirable to cause coacervation at the optimum conditions. According to the invention coacervation may preferably be caussed not at the optimum conditions. This can be achieved by any of reducing the amount of water added, adjusting the pH so as not to be an optimum value and changing the composition ratio of the colloid material to form coacervates.

Sufficient agitation during the coacervation step is also preferred for the purpose of preventing formation of coacervate aggregation masses of undesirably great dimensions.

It must be noted that the temperature of the system must be maintained above the gelation points of the colloid materials throughout the emulsifying and coacervation steps. Usually the temperature of the system may preferably be maintained at a constant temperature fairly above the gelation points of the colloid mateerials.

The coacervate suspension is then cooled with a gentle temperature gradient until a temperature below the gelation points of the colloid materials to gel the coacervate deposited around each of the oil droplets to obtain fixed capsules. In this process individual coacervates tend to adhere to each other. In some cases many individual coacervates are adhered to each other to form multi-nucleus capsules of undesirably great diamaters. In another some cases formation of multi-nucleus capsules is prevented by an undesirably strong external force applied.

According to the invention, aggregation of particles of oil droplets with coacervates is precisely controlled by a specified agitated flow applied to the system so that multi-nucleus capsules having desired diameters and desired particle size uniformity may be obtained. This is owing to an appropriate balance between the adhesion force of coacervates and the separation force given by the controlled agitated flow. At least all during the step of cooling the coacervate suspension the agitated flow applied must be so controlled as to only allow formation of microcapsule aggregation masses having an average particle diameter of 3 to 20 microns, preferably, 6 to 1 5 microns.The average particle diameter mentioned in this specification means the particle size in terms of an average value of the particle diameters calculated from the particle size distribution measured by Coulter Counter, Model TA manufactured by Coulter Electronics Inc., USA.

It has been observed and confirmed that if the average particle diameter of each of the microcapsule aggregation masses is smaller than 3 microns the colour developing ability is insufficient and if the average diameter of each of the microcapsule aggregation masses is greater than 20 microns the copying paper sheets are easily smudged by inadvertent pressure applied.

According to the invention the above mentioned specifically controlled agitated flow may preferably be produced by rotating at least one agitator having at least one vane in a vessel containing the coacervate suspension under the following conditions: 2a 0.3 < < 1 D Sp 0.05 < ~ ST 5 < n < 300 In the above formulas D is the maximum inner diameter of the vessel, a is the maximum radius of gyration of the agitator, ST is the maximum vertical sectional area of the coacervate suspension in the vessel, Sp is the maximum vertical sectional area of the volume swept by said at least one agitator vane and n is the number of the revolution of the agitator per minute.If the agitator has two or more radially extending vanes or two or more agitators are used in a single vessel, the value Sp should be the total of the respective maximum vertical sectional areas of the volume swept by the vanes. In the calculation of Sp the vertical sectional area of the rotating shaft may be excluded.

Some of actually useful agitator assemblies are illustraterd by way of examples in the accompanying drawings in which: Figures 1 A, 2A, 3A and 4A are schematic vertical sectional views of agitation vessels which are used in the examples hereinafter described and Figures 1 B, 2B, 3B and 4B are plan views of the vessels illustrated in Figures 1 A, 2A, 3A and 4A, respectively.

Referring now to the drawings, throughout Figures 1 to 4 the reference numeral 31 indicates vessels, the reference numeral 32 generally indicates agitators rotatable in the respective vessels 31.

Each of the vessels may preferably be provided with suitable cooling means. Each of the agitators 32 comprises a rotating shaft 33 and at least one agitation vane 34 fixed to the shaft 33. The reference numeral 40 indicates the level of the liquid in the vessel.

The vane 34 of the agitator 32 may be shaped in any form as desired. For example, the vane 34 is shaped in any flat plate form (Figures 1 A and 1 B, and, 3A and 3B) in any straight bar form (Figures 4A and 4B), in any frame form (Figures 2A and 2B) or in any screw form (not shown).

The vessel 31 may be provided with at least one projection or stationary vane which cooperate with the vane 34 of the agitator 32 to effectively produce a turbulent flow. In the embodiments illustrated in Figures 4A and 4B such projections or stationary vanes fixed to the inner wall of the vessel are indicated with the reference numeral 37.

In Figures 2A, 2B, 3A, 3B, 4A, and 4B dimensional ratios of various parts are indicated.

According to the invention multi-nucleus oil-containing capsules can have desired and generally uniform particle sizes owing to the fact that coacervates deposited on the oil droplets are prevented from being adhered to each other to an appropriate extent by the force of the agitated flow under control. It has been observed that the uniformity of the particle size of the multi-nucleus capsules obtained according to the invention is so good that more than 70% of the particles belong to the channel of the peak and its adjoining channels in the volume integration of particles having different particle diameters measured by Coulter Counter. In addition the multi-nucleus oil-containing microcapsules produced according to the invention have improved water and solvent resisting qualities.

It might be assumed that these unexpected good results can be obtained by cooperation of the wall film of a synthetic polymer formed by the reaction of the polyvalent isocyanate with the wall film formed by the complex coacervation of hydrophilic colloid materials. Although we do not wish to limit the mechanism for formation of capsules to any theory it may be probable that the isocyanate resolved in the oil droplets first reacts with water and/or hydrophilic colloid materials in water to form capsule wall film when the core material is emulsified and then another capsule wall film is formed thereon by the coacervation of hydrophilic colloid materials with or without the isocyanate being involved into reaction.

During the cooling step for forming the cluster by aggregation of microcapsules a further coacervation of hydrophilic colloid materials may be occurred for forming a further overcoating film.

When the technique according to the invention is utilized for the production of pressure sensitive copying paper it has been found that inadvertent rupture of microcapsules and development of smudges during a printing operation on pressure sensitive copying paper can be desirably and unexpectedly prevented.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The following examples serve to illustrate the invention in more detail although the invention is not limited to the examples. Unless otherwise indicated, parts and % signify parts by weight and % by weight, respectively.

EXAMPLE 1 3 parts of crystal violet lactone and 1 part of benzoyl leucomethylene blue were added to 90 parts of kerosene, dissolved by heeating and then cooled. To the mixture, a solution prepared by dissolving 20 parts of an adduct of trimethylolpropane with tolylene diisocyanate (Coronate L manufactured by Nippon Polyurethane Kogyo Kabushiki Kaisha) in 10 parts of dimethylphthalate was added to prepare an oily solution. On the other hand, 20 parts of an acid-treated gelatin having an isoelectric point of 8.0 was added to 130 parts of water, dissolved by heating and then cooled to 350C to prepare a gelatin solution. The gelatin solution was added to the above oily solution and the mixture was emulsified with a homomixer to obtained mono-nucleus capsules having an average particle size of 2.8 microns. All of thus obtained system was poured into an agitator vessel equipped with cooling means as shown in Figure 1. Then 100 parts of warm water was added into the system maintained at 55 0C and the system was thoroughly agitated at 300 r.p.m.

The pH of the system was adjusted to 5.4 with an aqueous solution of sodium hydroxide and then 75 parts of 3% aqueous solution of carboxymethylcellulose (having an average polymerization degree of 160 and an etherification degree of 0.75) and 400 parts of warm water at 550C. The system in the vessel was cooled under the following condition.

2a = 0.75 D SP = 0.24 ST n = 100 The system in the vessel was gradually cooled from the initial temperature of 550C with a temperature drop gradient 1 OC/min. An adhesion between capsules was initiated with reducing the liquid temperature and substantially completed at about 200 C. The liquid temperature was cooled until 100C which was less than the gel point of gelatin. Then the particle size distribution of the obtained multi-nucleus capsules was measured by Coulter Counter manufactured by Coulter Electronics Inc., USA. The average particle size of the capsules obtained was 9.9 microns and the volume percentage of the particles contained in the unit channel including the peak and its adjoining unit channels in the chart of Coulter Counter was 84%.

To the system thus obtained 50% of aqueous solution of glutaric aldehyde was added in an amount of 1 part per 10 parts of gelatin included in the system at 1 OOC and then the pH of the system was adjusted to 7.0 with an aqueous solution of sodium hydroxide to complete the hardening of capsules. To the hardened capsules-containing liquid thus obtained, pulp powder was added in an amount of 30 parts per 100 parts of oily core material included in the system to prepare a coating composition. The coating composition was coated on one surface of paper sheet of 40 g/m2 in an amount of 5 g/m2 on dry basis to obtain a capsule coated sheet.

EXAMPLE 2 1 5 parts of an acid-treated gelatin having an isoelectric point of 8.0 was added into 130 parts of water, dissolved by heating and then cooled to 350C to prepare a gelatin solution. The gelatin solution was added into the same oily solution as prepared in Example 1 and the mixture was emulsified with a homomixerto obtain mono-nucleus capsules having an average particle size of 3.1 microns. All of thus obtained system was poured into same vessel as in Example 1. Then 1 50 parts of warm water was added into the system maintained at 55 OC and the mixture was completely agitated at 300 r.p.m. After the pH of the system was adjusted to 4.2 with an aqueous solution of sodium hydroxide, 75 parts of 15by6 aqueous solution of gum arabic and 350 parts of warm water at 550C were added to the system.

The system was agitated and cooled until 1 OOC C under same conditions as in Example 1.

The obtained multi-nucleus capsules had an average particle size of 9.8 microns and the volume percentage of the particles contained in the unit channel including the peak and its adjoining unit channels in the chart of Coulter Counter was 70%. A capsule coated sheet was prepared with the use of the system thus obtained in the same manner as in Example 1.

EXAMPLE 3 1 5 parts of an acid-treated gelatin having an isoelectric point of 8.0 and 5 parts of polyvinylalcohol were added into 130 parts of water, dissolved by heating and then cooled until 350C to prepare an aqueous solution. The aqueous solution was added into the same oily solution as prepared in Example 1 and the mixture was emulsified with a homomixer to obtain mono-nucleus capsules having an average particle size of 4.2 microns. All of thus obtained system was poured into the same vessel as in Example 1. 150 parts of warm water was added into the system maintained at 550C and the mixture was completely agitated at 300 r.p.m. After the pH of the system was adjusted to 5.3 with an aqueous solution of sodium hydroxide, 75 parts of 2.2% aqueous solution of carboxymethylcellulose and 250 parts of warm water at 550C were added to the system.The system was agitated and cooled until 1 00C under the same conditions as in Example 1.

The obtained multi-nucleus capsules had an average particle size of 7.0 microns and the volume percentage of the particles contained in the unit channel including the peak and its adjoining unit channels in the chart of Coulter Counter was 72%. A capsule coated sheet was prepared with the use of the system thus obtained in the same manner as in Example 1.

EXAMPLE 4 Example 1 was repeated except that an adduct of trimethylolpropane with hexamethylenediisocyanate (Coronate HL manufactured by Nippon Polyurethane Kogyo Kabush iki Kaisha) was used instead of an adduct of trimethylolpropane with tolylene diisocyanate (Coronate L manufactured by Nippon Polyurethane Kogyo Kabushiki Kaisha) to prepare a capsule coated sheet.

The mono-nucleus capsules obtainerd after emulsifying had an average particle size of 3.0 microns and the multi-nucleus capsules after cooling the liquid temperature had an average particle size of 10.1 microns and the volume percentage of the particles contained in the unit channel including the peak and its adjoining unit channels in the chart of Coulter Counter was 81%.

EXAMPLE 5 Example 1 was repeated except that tolylene diisocyanate was used instead of the adduct of trimethylolpropane with tolylene diisocyanate (Coronate L manufactured by Nippon Polyurethane Kogyo Kabushiki Kaisha) to obtain a capsule coated sheet.

The mono-nucleus capsules after emulsifying had an average particle size of 2.9 microns and the multi-nucleus capsules after cooling had an average particle size of 9.8 microns and the volume percentage of the particles contained in the unit channel including the peak and its adjoining unit channels in the chart of Coulter Counter was 84%.

EXAMPLE 6 Example 1 was repeated except that diphenylmethane-4,4'-diisocyanate was used instead of an ,adduct of trimethylolpropane with tolylene diisocyanate to prepare a capsule coated sheet.

The mono-nucleus capsules after emulsifying had an average particle size of 3.1 microns and the multi-nucleus capsules had an average particle size of 10.2 microns and the volume percentage of the particles contained in the unit channel including the peak and its adjoining unit channels in the chart of Coulter Counter was 83%.

CONTROL 1 0.15 parts of hexamethylenediamine and 6 parts of polyvinylalcohol were dissolved in 140 parts of water and cooled at 250C. Thus obtained aqueous solution was added into the same oily solution as prepared in Example 1 and the mixture was emulsified with a homomixer. As the result, the viscosity of the mixture system extremely increased and the system was coagulated not to form an emulsion. Thus obtained capsules were observed under a microscope. The capsule-forms were very irregular and the average particle size was about 26 microns.

CONTROL 2 6 parts of polyvinylalcohol dissolved in 140 parts of water by heating and then cooledd to 250C.

The obtained aqueous solution was added to the same oily solution as prepared in Example 1 and the mixture was emulsified with a homomixer to obtain mono-nucleus capsules having an average particle size of 6.0 microns. The emulsion was vigorously agitated with a usual propeller mixer at 1 800 r.p.m.

and simultaneously heated to 60 C to complete the capsule formation. Then the system was diluted by 250 parts of water to obtain a capsule dispersion. Pulp powder was added to the capsule dispersion to prepare a coating composition in the same manner as in Example 1 and a capsule coated sheet was prepared with the use of the coating composition in the same manner as in Example 1.

CONTROL3 Example 1 was repeated except that the system in the vessel was cooled by vigorous stirring of a usual propeller mixer at 2000 r.p.m. to obtain a capsule coated sheet When the system was cooled until 1 OOC, the obtained capsules had an average particle size of 6.5 microns. Thus obtained capsules were observed under a microscope. They are not substantially coagurated and almost took the form of mono-nucleus capsules.

CONTROL 4 Example 1 was repeated except that dimethylphthalate solution of an adduct of trimethylolpropane with tolylene diisocyanate was not used to obtain a capsule coated sheet. The average particle size of the emulsified oil droplets was 3.0 microns and the average particle size of the obtained multi-nucleus capsules after cooling until 1 OOC was 9.1 microns.

EXAMPLE 7 Example 1 was repeated except that an agitating vessel as shown in Figures 2A and 2B was used and the system in the vessel was cooled with the agitation under the following condition to obtain a capsule coated sheet: 2a =0.97 D Sp =0.32 ST n= 100 The average particle size of the obtained multi-nucleus capsules was 9.0 microns and the volume percentage of the particles contained in the unit channel including the peak and its adjoining unit channels in the chart of Coulter Counter was 85Q/o.

EXAMPLE 8 Example 1 was repeated except that an agitating vessel as shown in Figure 3A and 3B was used and the system in the vessel was cooled with the agitation under the following condition to obtain a capsule coated sheet: 2a = 0.76 D SP =0.18 ST n = 220 The average particle size ofthe obtained multi-nucleus capsules was 9.2 microns and the volume percentage of the particles contained in the unit channel including the peak at its adjoining unit channels in the chart of Coulter Counter was 82%.

EXAMPLE 9 Example 1 was repeated except that an agitating vessel as shown in Figures 4A and 4B was used and the system in the vessel was cooled with the agitation under the following condition to obtain a capsule coated sheet: 2a = 0.63 D Sp =0.16 ST n=35 The average particle size of the obtained multi-nucleus capsules was 11.2 microns and the volume percentage of the particles contained in the unit channel including the peak and its adjoining unit channels in the chart of Coulter Counter was 87%.

PREPARATION OF AN ACCEPTOR SHEET 8 parts of zinc 3,5-di-(a-methylbenzyl)salicylate and 2 parts of styrene--methylstyrene copolymer was melted to obtain a homogeneous mixture. The obtained mixture was then finely divided.

12 parts of the finly divided mixture, 53 parts of aluminium hydroxide, 20 parts of activated clay, 1 5 parts of zinc oxide, 30 parts of styrene-butadiene copolymer latex (solid amount of 50%), 6 parts of 10% aqueous solution of modified polyvinyl alcohol and 300 parts of water were mixed to make a coating composition. The coating composition was applied on one surface of a base sheet of 40 gIm2 in an amount of 6 g/m2 on dry basis and dried to obtain an acceptor sheet.

TEST FOR THE PROPERTIES Each capsule coated sheet obtained in Examples and Controls was put on an acceptor sheet so that the coated layers were close to each other to provide samples for the following tests.

On the other hand, each coating composition obtained in Examples and Controls was coated on the acceptor coating layer of the above acceptor sheet in an amount of 5 g/m2 on dry basis and dried to obtain the so-called self contained pressure sensitive copying sheet.

(1) Color formability: The above samples were pressed by a test machine to form a color image. The color density of the image on the acceptor coated surface after 20 hours was measured by Macbeth densitometer RD-lOOR (manufactured by Macbeth Corporation, USA). The larger the number, the more superior the color formability.

(2) Smudging by press: The above samples were left standing under a pressure of 1 5 kg/cm2 for 30 seconds at atmospheric temperature and humidity and then the color density on the acceptor coated surface was measured in the same manner as in the above test (1). The lower the number, the fewer the smudging.

(3) Humidity resistance: The above samples were left standing under the condition of 50 C,90% RH for 100 hours and then the color density on the acceptor coated surface was measured in the same manner as in the above test (1). The lower the number, the stronger the humidity resistance.

(4) Solvent resistance: The above self contained pressure sensitive copying sheet was left standing in a vessel saturated with trichloroethylene vapor for 3 minutes, and then the color density on the surface was measured in the same manner as in the above test (1 ). The lower the number, the more superior the solvent resistance.

The test results are shown in Table 1.

TABLE 1

Color Smudging Humidity Solvent Formability by Press Resistance Resistance Example 1 0.75 0.09 0.05 0;06 Example 2 0.74 0.08 0.06 0.07 Example 3 0.72 0.10 0.08 C.10 Example 4 0.75 0.08 0.05 0.08 Example 5 0.74 0.09 0.06 0.07 Example 6 0.73 0.10 0.06 0.09 Control 1 * * * * Control 2 0.70 0.14 0.06 0.30 Control 3 0.67 0.13 0.08 0.31 Control 4 0.76 0.10 0.34 0.10 Example 7 0.72 0.08 0.06 0.06 Example 8 0.74 0.09 0.05 0.06 Example 9 0.76 0.10 0.06 0.06 Note: *) A capsule coated sheet could not be obtained.

Claims (9)

1. A method of making oil-containing microcapsules comprising (1) preparing an aqueous system comprising an aqueous solution of at least two kinds of hydrophillic colloid materials with opposite electric charges and oil droplets dispersed in said solution, said oil droplets including at least one polyvalent isocyanate, (2) causing coacervation in the aqueous system at a temperature above the gelation points of the hydrophilic colloid materials to form a coacervate suspension in which each of the oil droplets is surrounded by a coacervate and (3) cooling the coacervate suspension to a temperature below the gelation points of the hydrophilic colloid materials, during the- cooling of the coacervate suspension, aggregation of particles of oil droplets each having a coacervate therearound being controlled by agitated flow so as to allow formation of multi-nucleus microcapsules having an average diameter within the range of 3 to 30 microns.

2. A method according to Claim 1, in which the average diameter of said oil droplets is within the range of 1 to 1 5 microns.

3. A method according to either preceding claim, in which said coacervation is caused by dilution.

4. A method according to Claim 3, in which water is added so as to reduce the colloid concentration in said system to 2 to 5% by weight to cause coacervation.

5. A method according to claim 1 or 2, in which said coacervation is caused by adjustment of the pH of said system.

6. A method according to any preceding claim, in which said agitated flow is produced by rotating at least one agitator having at least one vane in a vessel containing said coacervate suspension under the following conditions: 2a 0.3 < 1 D Sp 0.05 < < 1 ST 5 < n < 300 wherein D is the maximum inner diameter of the vessel, a is the maximum radius of gyration of the agitator, ST is the maximum vertical sectional area of the coacervate suspension in the vessel, Sp is the total of the maximum vertical sectional areas of the volume swept by said at least one agitator vane and n is the number of revolutioin of the agatitor per minute.

7. A method according to Claim 1, wherein the agitated flow is effected in an agitation vessel substantially as described herein with reference to and as illustrated in Figures 1A and 1 B, 2A and 2B, 3A and 3B or 4A and 4B of the accompanying drawings.

8. A method according to claim 1 , substantially as described herein in any of Examples 1 to 9.

9. Pressure-sensitive copying paper comprising a paper sheet having applied thereto oilcontaining microcapsules made buy a method according to any preceding claim, a colourformerand/or acceptor being dispersed or dissolved in the oil products.