GB2192169A - Method of forming polymeric microcapsules containing oily liquid - Google Patents

Abstract

Microcapsules containing an oily liquid are formed by polymerising melamine and formaldehyde or an initial precondensate thereof at the surface of droplets of a hydrophobic oily liquid (e.g. containing a color-forming agent in a biphenyl) containing (a) a monoamine (aliphatic) or a polyamine (e.g. an aliphatic or aromatic diamine or a polymeric amine) and (b) a polyisocyanate or prepolymer thereof; e.g. the oily mixture is emulsified in an aqueous solution of a water-soluble high polymer, e.g. gelatin; melamine and aqueous formaldehyde or an initial condensate thereof is added, and the pH is made acidic and the temperature raised to cause formation of microcapsules of melamine-formaldehyde containing the oily liquid. The amine prevents the capsules from fusing or forming a film of the oil. The capsules can be used to make pressure-sensitive recording paper. <IMAGE>

Description

SPECIFICATION Method of forming polymeric microcapsules containing oily liquid The present invention relates to a method of forming microcapsules. More particularly, it relates to a method of forming microcapsules in which certain components are incorporated into a hydrophobic oily liquid to be encapsulated, and the liquid is emulsified in an aqueous solution of water-soluble high polymer, whereby the stability of the emulsion in the process of forming microcapsules by polymerizing melamine and formaldehyde, or initial condensates of melamine and formaldehyde is increased. This prevents the oily liquid droplets from being united or coming to the emulsion surface in a form of oil film.

Methods of forming a capsule wall of melamine-formaldehyde resin are described in U.S. Patents 4,100,103 and 4,233,178.

These methods comprise the three steps of (1) emulsifying hydrophobic oily liquid in an aqueous solution of a copolymer of maleic anhydride and an ethylenic monomer or polyacrylic acid, (2) mixing the emulsified oily liquid with an aqueous solution containing melamine and formaldehyde, or an initial condensate of melamine and formaldehyde, and (3) encapsulating said mixture by controlling the pH and raising the temperature.

Water-soluble high polymers used therein, e.g., a copolymer of maleic anhydride and an ethylenic monomer, or polyacrylic acid, to have four roles, acting as (1) an accelerator for emulsifying an oily liquid to be encapsulated, (2) a stabilizer for the emulsion formed, (3) an accelerator for forming the capsule wall, and (4) a dispersant of the capsules formed. However, such polymers are not able to fulfil all those roles, particularly the role of an emulsion stabilizing agent.Therefore, the methods suffers from the defects that the oily liquid is not completely emulsified in the emulsifying step (1), to remain in part as an oily liquid film at the surface of the emulsion; and that the oil droplets in the emulsion combine with one another to form a film of the oily liquid at the surface of the reaction solution or to produce giant drops in the wall material mixing step (2) and in the encapsulating step (3). In producing, e.g., pressure-sensitive recording paper, by coating the capsule solution on paper, the oily liquid film which remains unencapsulated in the capsule solution is responsible for repuision of the capsule solution toward the paper upon coating, and makes it impossible to produce a uniformly coated material.

On the other hand, giant drops of the oily liquid are encapsulated as they are, and cause serious defects, e.g., such that the encapsulated giant drops are ruptured by low pressure to give rise to stains when such capsules are used in producing pressure-sensitive recording paper.

In addition, there is a problem that the capsule solution prepared has high viscosity. The viscosity of the capsule solution depends mainly on that of the water-soluble high polymer used. However, using a water-soluble high polymer having a lower molecular weight and reducing the addition amount thereof in order to lowerthe viscosity of the capsule solution results in decreased stability of the emulsion and facilitates the generation of film or giant drops of the oily liquid. Therefore, these means cannot be completely satisfactorily employed.

This undesirable tendency is aggravated by prolonging the storage of the emulsion, and raising the temperature upon encapsulation.

A primary object of the the present invention is to provide a method of forming a microcapsule solution of excellent quality, which allows improvement in stability of the emulsion and thereby overcomes the above-described various problems arising from the instability of the emulsion.

Another object of the present invention is to provide a method of forming capsules having a wall of melamine resin by undergoing a polycondensation reaction of melamine with formaldehyde, or a reaction of initial condensates of melamine and formaldehyde, at the surface of oily liquid droplets to be encapsulated, which enables expansion of the kind and the concentration range of water-soluble high polymers usable for emulsification and encapsulation, and provides resistance against influences of changes in reaction conditions, e.g., temperature and pH of the reaction system, kinds and quantities of additives, and soon, that is to say, ensures stable formation of microcapsules.

The above-described objects are attained by a method of forming microcapsules by polymerizing melamine and formaldehyde or polymerizing an initial condensate of melamine and formaldehyde at the surface of droplets of hydrophobic oily liquid containing a polyisocyanate or a prepolymer thereof, wherein at least one of a monoamine compound and a polyamine compound is incorporated into said hydrophobic oily droplets prior to microencapsulation.

The term "polyisocyanates or prepolymers thereof" which can be used in the method of the present invention refers to compounds containing two or more isocyanate groups, and specific examples thereof include diisocyanates such as m-phenylenediisocyanate, p-phenylenediisocyanate, 2,6-tolylenediisocyanate, 2,4-tolylenediisocyanate, naphthalene-1 ,4-diisocyanate, 4,4'-diphenylmethanediisocyanate. 3,3'-dimethoxy4,4'-biphenyldiisocyanate, 3,3'-dimethyldiphenylmethane-4,4'-diisocyanate, xylylene-1 ,4-diisocyanate, xylylene-1 ,3-diisocyanate, 4,4'-diphenylpropanediisocyanate, trimethylenediisocyanate, hexamethylenediisocyanate, propylene-1 ,2-diisocyanate, butylene-1 ,2-diisocyanate, ethylidinediisocyanate, cyclohexane 1 ,2-diisocyanate, cyclohexylene-1 ,4-diisocya nate, etc.; triisocyanates such as 4,4',4"triphenylmethanetriisocyanate, toluene-2,4,6-triisocyanate, polymethylenepolyphenyltriisocyanate, etc.; tetraisocyanates such as 4,4'-dimethyldiphenylmethane-2,2',5,5-tetraisocyanate, etc.; polyisocyanate prepolym ers such as polymethylenepolyphenylpolyisocyanate, addition products of hexamethylenediisocyanate and hexanetriol, addition products of 2,4-tolylenediisocyanate and Brenzcatechin, addition products of tolylene diisocyanate and hexanetriol, addition products of tolylenediisocyanate and trimethylolpropane, addition products of xylylenediixocyanate and trimethylolpropane, addition products of hexamethylenediisocyanate and trimethylolpropane, etc.; and any appropriate polyisocyanates which are analogous to the abovedescribed ones. Of these compounds, the prepolymers are particularly preferred over others. It is also possible to employ a combination of two or more of the foregoing polyisocyanates.

Specific examples of monoamine compounds which can be employed include aliphatic amines such as dodecylamine, hexadecylamine, stearylamine, oleylamine, distearylamine, dicyclohexylamine, etc., and aromatic gamines such as aniline, p-aminoethylbenzene, o-aminoacetophenone, 1-aminoanthracene, 2aminodiphenylmethane, 2-aminobiphenyl, etc.The term polyamines refers to compounds having not less than two -NH2 or-N H groups in a molecule, with specific examples including aliphatic diamines such as dodecamethylenediamine, hexadecyldiamine, octadecamethylenediamine, stearylpropylenediamine, alkylpropylenediamines of beef tallow, etc.; aromatic diamines such as diaminobenzene, diaminotoluene, diaminonaphthalene, etc.; diamines such as diaminodiphenylmethane, diaminodiphenylether, diaminopyrimidine, diaminoazpbenzene, diaminoanisidine, diaminobenzidine, diaminophenyltriazine, etc.; triamines such as triaminodiphenyl, diaminophenylamine, tricarbinol, triamino-pyrimidine, bis(5-aminopentyl)amine, etc.; denatured polyamines such as Eponote B-001 (trademark for product produced byAjinomoto Co., Inc.) and Epomate ATU (trademarkfor product produced by Ajinomoto Co., Inc.), which are known as hardeners of epoxy resins; and polymers having not less than two amino groups in their structures, such as homo- and co-polymers of aminostyrenes. Of these compounds, oil-soluble diamines are particularly preferred over others. The monoamines and polyamines set forth above may be used as a mixture of two or more thereof.

In addition, a polyhydroxy compound, such as an aliphatic or aromatic polyhydric alcohol, a hydroxypolyester, a hydroxypolyalkylene ether, an adduct of an alkylene oxide and a polyamine, etc., may be incorporated in hydrophobic oily droplets together with an amine compound.

These wall film-forming substances may be added directly to the hydrophobic oily liquid, or they may be previously dissolved in an organic solvent and then added to hydrophobic oily liquid. There is not any restriction as to the addition manner, except that they must be added to the hydrophobic oily liquid prior to the emulsification step.

It is generally appropriate to use a polyisocyanate or a prepolymer thereof in an amount of from about 0.005 to 3 parts by weight, and preferably from 0.01 to 2 parts by weight, per 100 parts by weight of hydrophobic oily liquid used.

A more preferred amount thereof ranges from 0.2 to 2 parts by weight, and the most preferred one is within the ranges of from 1 to 1.5 parts by weight.

On the other hand, the amount of a monoamine or polyamine to be used generally ranges from about 5 to 100 parts by weight, particularly preferably is from 20 to 50 parts by weight, per 100 parts by weight of the polyisocyanate or the polyisocyanate prepolymer used.

The encapsulation process employed in the present invention is described in detail below.

The method of the present invention basically comprises the steps of: (1) preparing an aqueous solution system containing a water-soluble high polymer, (2) emulsifying hydrophobic oily liquid, to which a polyisocyanate or a prepolymer thereof and monoamine or/and polyamine compounds are added in advance, in the aqueous solution prepared in step (I), (3) adding melamine and an aqueous solution of formaldehyde, or an initial condensation product of melamine and formaldehyde to the emulsion prepared in the step (2), (4) adjusting the pH of the resulting emulsion to the acidic side, and raising the temperature with stirring, thereby forming an inner wall of polyurethane-urea resin from the inside of the dispersed oily liquid, and at the same time, forming an puter wall of melamine-formaldehyde resin through polycondensation reaction of melamine and formaldehyde, and (5) removing residual formaldehyde from the reaction system after pH adjustment, if necessary, by adding a formaldehyde scavenger thereto.

Moreover, the use of an ammonium salt of an acid at the time of polycondensation reaction of melamine and formaldehyde can promote the reaction. Especially favorable ammonium salts for such a purpose are, e.g., ammonium chloride, ammonium sulfate, ammonium nitrate and ammonium dihydrogen phosphate.

Examples of materials to be enclosed in the nucleus of the individual microcapsules in the present invention includes natural mineral oils, animal oils, vegetable oils, synthetic oils, and so on. Specific examples of mineral oils that can be used include petroleum and petroleum fractions such as kerosene, gasoline, naphtha and paraffin oil. Specific examples of animal oils which can be used include fish oils, lard oil, and the like.

Specific examples of vegetable oils which can be used include peanut oil, linseed oil, soy bean oil, castor oil, corn oil, and the like. Specific examples of synthetic oils which can be used include biphenyl compounds (such as isopropyl-substituted biphenyls, isoamyl-substituted biphenyls, etc.), terphenyl compounds (e.g., those described in German Patent Application (OLS) No. 2,153,635), phosphate compounds (such as triphenyl phosphate), naphthalene compounds (e.g., those described in German Patent Application (OLS) No.

2,141,1S4), methane compounds (e.g. those described in German Patent Application (OLS) No. 2,153,634), phthaloylated compounds (e.g., diethylphthalate, dibutyl phtahlate, dioctyl phthalate) and salicyclic acid derivatives (such as ethyl salicylate).

With these natural mineral oils, animal oils, vegetable oils and synthetic oils, agricultural chemicals, medical supplies, perfumes, chemicals, adhesives, liquid, crystals, foods, detergents, dyestuffs, dye precursors, color developing agent, catalysts, rust inhibitors, or so on can be properly admixed depending upon the particular end use intended.

A suitable amount of a water-soluble high polymer to be used in the present invention corresponds to 0.5 to 30 wt%, preferably 1 to 20 wt%, of the hydrophobic oily liquid to be encapsulated. The expression "water-soluble high polymer" is intended to include water-soluble anionic high polymers, nonionic high polymers and amphoteric high polymers. Anionic high polymers which can be used include both natural and synthetic ones, for example, those containing -COO- group, -SOB- group, or the like. More specifically, natural anionic high polymers include gum arabic, alginic acid, and soon, and semisynthetic ones include carboxymethyl cellulose, phthaloylated gelatin, sulfonated starch, cellulose sulfate and lignin sulfonate.

In addition, synthetic anionic high polymers which can be used include maleic anhydride copolymers (including hydrolysis products thereof), acrylic acid (including methacrylic acid) homo- and co-polymers, vinylbenzene sulfonic acid homo- and co-polymers and carboxy-denatured polyvinyl alcohol.

Amphoteric high polymers which can be used include gelatin and the like.

Specific examples of maleic anhydride copolymers (including hydrolysis products thereof) include methyl vinyl ether/maleic anhydride copolymer, ethylene/maleic anhydride copolymer, vinyl acetate/maleic anhydride copolymer, methyacrylamide/maleic anhydride copolymer, isobutylene/maleic an hydroxide copolymer and styrene/maleic anhydride copolymer. A preferred molecular weight of such copolymers is within the range of 5,000 to 2,000,000.

Specific examples of acrylic acid copolymers which can be used include methylacrylate/acrylic acid copolymer, ethylacrylate/acrylic acid copolymer, methylacrylate/methacrylic acid copolymer, methylmethacrylate/acrylic acid copolymer, methylmethacrylate/methacrylic acid copolymer, methylacrylatel acrylamide/acrylic acid copolymer, acrylonitrile/acrylic acid copolymer, acrylonitrile/methacrylic acid copolymer, hydroxyethylacrylate/acrylic acid copolymer, hydroxyethylmethacrylate/methacrylic acid copolymer, vinylacetate/acrylic acid copolymer, vinylacetate/methacrylic acid copolymer, acrylamide/acrylic acid copolymer, acrylamide/methacrylic acid copolymer, methacrylamide/acrylic acid copolymer, methacrylamidel methacrylic acid copolymer, and alkali metal salts thereof.

A preferred molecular weight if such acrylic acid copolymers is within the range of 20,000 to 1,000,000, and a preferred fraction of acrylic acid or methacrylic acid (or a salt thereof) in the copolymers as set forth above is from 5 to 100 mol%.

Specific examples of vinylbenzenesulfonic acid copolymers include methylacrylate/vinylbenzenesulfonic acid (or a salt thereof) copolymer, vinylacetate/vinylbenzenesulfonic acid copolymer, acrylamide/ vinylbenzenesulfonic acid copolymer, acryloylmorpholine/vinylbenzenesulfonic acid copolymer and vinylpyrrolidone/vinylbenzenesulfonic acid copolymer.

A preferred molecular weight of vinylbenzenesulfonic acid copolymers as set forth above ranges from 5,000 to 2,000,000, more preferably from 10,000 to 1,000,000 and particularly preferably from 20,000 to 500,000.

Suitable carboxy-denatured polyvinyl alcohols are those having a molecular weight of 1,000 to 300,000, a saponification degree of 90% or more, and a carboxy group content of from about 1 to 40 mol%. A carboxy-denatured polyvinyl alcohol can be prepared using (1) a method which involves copolymerizing a carboxy group-containing vinyl or vinylidene monomer (such monomers are described in Canadian Patent 929,430) and a vinyl ester (e.g., CwC5 acid ester of vinyl alcohol), and hydrolyzing the resulting copolymer; (2) a method which involves reacting polyvinyl alcohol with a polybasic acid derivative (e.g., acid an hydride, acid halide, etc.) to bind a carboxy group-containing unit to the side chain (as described in J. Polym. Sci. 3,880 (1948)).Carboxy-denatured polyvinyl alcohols prepared by any of the above-described methods can be used in the present invention.

Nonionic high polymers which can be used include both natural and synthetic ones. For example, -OH group-containing high polymers can be cited as described in Japanese Patent Application No. 102935/81 (the term "OPI" as used herein means an unexamined publication application).

Specific examples of semisynthetic non ionic high polymers include hydroxyethyl cellulose, pullulan (non-crystalline, highly water-soluble macromolecular polysaccharide prepared from starch using a microbial fermentation method), soluble starch, oxidized starch and so on.

Polyvinyl alcohol is a typical representative of synthetic nonionic high polymers.

A preferred polyvinyl alcohol has a molecular weight of from about 10,000 to about 200,000 and a saponification degree of 90% or more.

In order to effect the polycondensation reaction of melemine and formaldehyde for the purpose of formation of the outer wall film of microcapsules in the present invention, it is necessary to control the pH of the system as a whole to 7.0 or less. Desirably the system is adjusted to a pH of 6.5 or less.

As a starting material for forming melamine-formaldehyde resin to function as the outer wall film of the microcapsules, an aqueous solution of the melamine-formaldehyde mixture, or methylolmelamine is employed. Methylolmelamine can be easily prepared by heating a mixture of melamine and formaldehyde to a temperature not lower than 40"C (preferably within the range of 50 to 700C) under a weak alkalinity condition.

In addition, commercial methylolmelamine may be used as the starting material.

The molar ratio of formaldehyde to melamine has a great influence on denseness, strength, and shape of the microcapsule wall formed; it generally ranges from about 1.5 to 4, and preferably from 2 to 3.

In the aqueous solution of a melamine-formaldehyde mixture melamine present in a dissolved condition is preferable to melamine dispersed in a solid condition.

Microencapsulation is based in the simultaneous formation of the inner wall of polyurethaneurea and the outer wall of melamine-formaldehyde resin under heat-applied condition. A suitable reaction time depends on the reaction temperature. For instance, one hour suffices for the completion of microencapsulation at a temperature of 60"C or higher. A suitable reaction temperature is 40"C or higher, and particularly preferred is a temperature from 50 to 95"C.

It is important from a hygienic point of view to dispose of residual free formaldehyde. For this purpose, a formaldehyde scavenger is added to the finished microcapsule slurry.

Examples of formaldehyde scavengers which can be used include urea, sulfites, hydrogen sulfites, ethyleneurea, hydroxyamine hydrochloride, and so on. In order to make these scavengers function under the optimal reaction condition, it is necessary to control the pH of the microcapsule slurry. For example, it is effective to adjust the pH of the system to the acidic region when urea or methyleneurea is employed as scavenger. In particular, a pH of 4 or less is desirable. The microcapsule slurry of the present invention does not undergo an increase in viscosity, nor does it suffer from condensation even in a low pH region. Therefore, removal of residual formaldehyde can be carried out efficiently.

The size of the microcapsules can be suitably selected depending on the intended end use. When microcapsules are used in pressure-sensitive recording paper, a suitable number average size thereof is from 1 to 20,am, preferably from 1.5 to 10 Calm, and particularly preferably from 2 to 8 lim.

The present invention is particularly effective in producing microcapsules to be employed for pressuresensitive recording paper.

According to the method of the present invention, pressure-sensitive recording paper free from stain and local coloration, or having no coloration-impossible area resulting from coating marlcs or so on, that is to say, having uniform quality, can be produced.

The present invention is illustrated in greater detail by reference to the following examples. However, the present invention should not be construed as being iimited to the following examples.

Example 1 Partial sodium salt of polyvinylbenzenesulfonic acid (VERSA TL 500, produced by National Starch Co., mean molecular weight: 500,000) was used as a water-soluble high polymer. 5 g of VERSA TL 500 was added with stirring to 95 g of water heated to about 80"C to dissolve therein, and then cooled. The pH of the water solution was 2 to 3, and it was changed to 6.0 by adding a 20 wt% of sodium hydroxide aqueous solution.

Separately, 4 g of Crystal Violet lactone (CVL) as color forming agent was dissloved in 100 g of KMC-1 13 (trademark of an alkylnaphthalene containing diisopropyl-naphthalene as a main component, produced by Kureha Chemical Industry Co., Ltd.) under heating to obtain a hydrophobic solution to be encapsulated. Into the hydrophobic solution after cooling to 20"C. 0.3 g of a mixture of 4,4'-diphehylmethanediisocyanate (MDI) polymers (Millionate MR 100, produced by Nipon Polyurethane Industry Co., Ltd.) as a polyisocyanate, and 0.1 g of stearylamine (Farmine-80, produced by Kao Soap Co., Ltd.) as an amine compound were dissolved. This solution was added to the above-described aqueous solution of water-soluble high polymer with vigorous stirring to produce an oil-in-water (O/W) type emulsion.Stirring was stopped when the oil droplet size became 5.0 pWm on the number average.

Further, a transparent aqueous mixture of melamine, formaldehyde and a melamine-formaldehyde initial condensate was prepared by mixing 6 g of melamine, 11 g of a 37 wt% water solution of formaldehyde and 83 g of water, and heating the mixture up to 60"C with stirring over a period of 30 minutes. The pH of this aqueous mixture was 6-8. It was admixed with the foregoing emulsion, and adjusted to pH 6.0 by addition of 10 wt% water solution of phosphoric acid with stirring. The mixture was heated up to 650C, and the stirring was further continued for about 1 hour to complete microencapsulation.

The thus obtained capsule solution was cooled to room temperature, and adjusted to pH 9.0 using a 20 wt% of sodium hydroxide solution.

Example 2 Another microcapsule solution was prepared in the same manner as in Example 1 except that 0.2 g of dodecamethylenediamine was used as an amine compound to be added to the hydrophobic solution.

Example 3 Still another microcapsule solution was prepared in the same manner as in Example 1 except that a mixture of 4.2 g of 3,6-bis-diphenylaminofluoran, 0.2 g of 3-diphenylamino-6-(N-phenyl-N isopropylphenyl)aminofluoran, 0.2 g of 3,6-biethylaminofluoran-p-nitroanilinoactam and 1 g of 2,2,4 trimethyl-1 ,2-dihydroxyquinoline was used in place of CVL dissolved in the hydrophobic oily liquid, an amount of the 4,4'-diphenylmethanediisocyanate (MDI) polymer mixture used as the polyisocyanate to be added to the hydrophobic solution was changed to 1.2 g, 0.4 g of 2,4'-diaminobiphenyl was used as the amine compound, and 0.3 g of butylene oxide adduct of ethylenediamine (mole number of butylene oxide added to 1 mole of ethylenediamine: 12, molecular weight: 924) was further added as a polyhydroxy compound to be added to the hydrophobic solution.

Example 4 Afurther microcapsule solution was prepared in the same manner as in Example 3, except that 0.25 g of dicyclohexylamine was employed as the amine compound to be added to the hydrophobic solution.

Example 5 Another microcapsule solution was prepared in the same manner as in Example 1, except that a mixture of 3.6 g of [2-anilino-6-(N-ethyl-N-isopentylamino)-3-methyl]xanthene-9-spiro-1 '-(3'-isobenzofuranone), 1.0 g of 3-diethylamino-7-dibenzylaminofluoran, 0.8 g of 3,6-bis-diphenylaminofluoran and 0.7 g of 3-chioro-6-Ncyclohexaylaminofluoran was used in place of CVL dissolved in the hydrophobic oily liquid, and 0.1 g of alkylpropylenediamines of beef tallow (Nissan Amine DT, produced by Nippon Oils & Fats Co., Ltd.) was used as an amine compound in place of 0.1 g of stearylamine.

Example 6 Still another microcapsule solution was prepared in the same manner as in Example 5 except that 0.15 g of bis(5-aminopentyl)amine was used as the amine compound to be added to the hydrophobic solution.

Comparative Example 1 A microcapsule solution was prepared in the same manner as in Example 1 except that no polyisocyanate and no amine compound were added to the hydrophobic solution.

Comparative Example 2 A microcapsule solution was prepared in the same manner as in Example 1 except that no polyisocyanate was added to the hydrophobic solution.

Comparative Example 3 A microcapsule solution was prepared in the same manner as in Example 1 except that amine compound was not added to the hydrophobic solution.

Comparative Example 4 A microcapsule solution was prepared in the same manner as in Example 3 except that amine compound was not added to the hydrophobic solution.

Comparative Example 5 A microcapsule solution was prepared in the same manner as in Example 5 except that amine compound was not added to the hydrophobic solution.

These samples (Examples 1 to 6, and Comparative Examples 1 to 5) were each examined as to whether an oily liquid film remained unencapsulated or not, and giant capsules were present or not, in the following manners, respectively.

Confirmation of Presence of Oily Liquid Film A 100 g portion of each capsule solution was diluted three times with water, and ailowed to stand for one hour. Then, the surface of the solution was dipped up, coated on a sheet covered with a color developing agent (zinc salicylate and activated clay), and dried. The condition of stains generated on the coated sheet was examined.

Confirmation of Presence of Giant Capsules 100 g portions of each capsule solution were passed through a 200 mesh screen, a 400 mesh screen, and a 500 mesh screen, respectively. The residue left on each screen was spread over on a sheet covered with a color developing agent, and dried. The resulting sheet was passed between pressure-applying rolls, and examined for the degree of spot-shaped coloration. Therein, spot-shaped coloration was caused by giant capsules resulting from combination of oil droplets.

Evaluation of the presence of oily liquid film and the presence of giant capsules were made by grouping the samples according to the extent of stains and spot-shaped coloration into the following four ranks A to D.

A: Stains and spot-shaped coloration were not observed at all and the sample is practically useful.

B: Stains and spot-shaped coloration were observed to a slight extent and the sample was practically acceptable.

C: Stains and spot-shaped coloration were observed to a considerable extent and the sample was practically unacceptable.

D: Stains and spot-shaped coloration were observed all over the surface and the sample was practically useless.

Results of the above-described examination are shown in Tabie 1. It can be seen from the results that the microcapsules of the present invention were free from giant capsules and the oily liquid film which remained unencapsulated, in contrast to those obtained in the comparative examples, so they were well suited for production of pressure-sensitive recording paper.

Table 1 Presence of Giant Example No. Color Forming Agent Polyisocyanate Amine Compound Oil Film Capsules Example 1 Crystal Violet 4.0 g 4,4'-diphenylmethane- 0.3 g stearylamine 0.1 g B A lactone diisocyanate polymer mixture Example 2 same as in Example 1 same as in Example 1 dodecamethyl- 0.2 g A A enediamine Example 3 3,6-bis-diphenyl- 4.2 g 4,4'-diphenulmethane- 1.2 g 2,4-diamino- 0.4 g A A amimofluoran diisocyanate polymer biphenyl mixture 3-diphenylamino- 0.2 g 6-(N-phenyl-Nisopropylphenyl)aminofluoran 3,6-bis-diethyl- 0.2 g aminofluoran-pnitroanilinolactam Example 4 same as in Example 3 same as in Example 3 dicyclo- 0.25 g B A hexylamine Table 1 (cont'd) Presence of Giant Example No. Color Forming Agent Polyisocyanate Amine Compound Oil Film Capsules Example 5 2-anilino-6-(N- 3.6 g same as in Example 1 beef tallow 0.1 g A A ethyl-N-iso- alkylpropylpentylamino)-3- enediamine methyl xanthene9-spiro-1'-(3'isobenzofuranone) 3-diethylamino- 1.0 g 7-dibenzylaminofluoran 3,6-bis-diphenyl- 0.8 g aminofluoran 3-chloro-6-N- 0.7 g cyclohexylaminofluoran Example 6 same as in Example 5 same as in Example 5 bis (5-amino- 0.15 g A A pentyl)amine Comparative same as in Example 1 - - D D Example 1 Comparative same as in Example 1 - stearylamine 0.10 g C D Example 2 Comparative same as in Example 1 same as in Example 1 - C C Example 3 Comparative same as in Example 3 same as in Example 3 - C B Example 4 Comparative same as in Example 5 same as in Example 5 - C D Example 5

Claims (14)

1. A method of forming microcapsules by polymerizing melamine and formaldehyde or polymerizing an initial condensate of melamine and formaldehyde at the surface of droplets of hydrophobic oily liquid containing a polyisocyanate or a prepolymerthereof, wherein at least one of a monoamine compound and a polyamine compound is incorporated into said hydrophobic oily droplets prior to microencapsulation.

2. A method of forming microcapsules as in Claim 1, wherein said polyisocyanate or prepolymerthereof is used in an amount of from 0.01 to 2 parts by weight per 100 parts by weight of hydrophobic oily liquid used.

3. A method of forming microcapsules as in Claim 2, wherein said polyisocyanate or prepolymer thereof is used in an amount of from 0.02 to 2 parts by weight per 100 parts by weight of hydrophobic oily liquid used.

4. A method of forming microcapsules as in Claim 3, wherein said polyisocyanate or prepolymerthereof is used in an amount of from 1 to 1.5 parts by weight per 100 parts by weight of hydrophobic oily liquid used.

5. A method of forming microcapsules as in any preceding claim, wherein the monoamine or polyamine is used in an amount of from 20 to 50 parts by weight of polyisocyanate or the polyisocyanate prepolymer used.

6. A method as claimed in any one of Claims 1 to 5, wherein an aliphatic monoamine is used.

7. A method as claimed in any one of Claims 1 to 5, wherein an aliphatic or aromatic diamine is used.

8. A method as claimed in any one of Claims 1 to 5, wherein a polymer having not less than two amino groups is used as the amine.

9. A method as claimed in any preceding claim, wherein said oily liquid is emulsified in an aqueous solution of a water-soluble high-polymer, before addition of melamine and formaldehyde.

10. A method as claimed in any preceding claim, wherein a polyhydroxy compound is also incorporated also in the capsules.

11. A method of forming microcapsules, substantially as hereinbefore described with reference to any of Examples 1 to 6.

12. Microcapsules formed by the method of any preceding claim.

13. Microcapsules as claimed in Claim 12, having a number average size of 1 to 20 ijm.

14. Pressure-sensitive recording paper having a layer containing microcapsules as claimed in Claims 12 or 13.