GB2319267A - Microcapsules for pressure-sensitive paper - Google Patents

Abstract

Microcapsules of mean particle particle diameter of 3-10 ~m and narrow particle size distribution containing colour formers are used in pressure-sensitive recording papers the microcapsules have a cumulative particle size distribution taken from the side of larger particles, the ratio of the diameter at 25% value to the diameter at 50% value is not more than 1.25 and the ratio of the diameter at 75% value to the diameter at 50% value is not less than 0.75 and the standard deviation of the particle size distribution is in the range of 1 to 2 ~m. The microcapsules are produced by (1) dissolving a colour former in a hydrophobic solvent, (2) forming an aqueous solution of isobutylene/maleic anhydride copolymer, (3) dispersing solution 1 in solution 2 by stirring to form an o/w emulsion with particle sizes set out above, 4 forming an amino resin prepolymer and 5 adding prepolymer 4 to dispersion 3 and lent to polymerise 4 and form microcapsules of specified particle size. Alternatively 4 can be added to solutions 1 and 2 and then heated to form microcapsules.

Description

MICROCAPSULES FOR PRESSURE-SENSITIVE RECORDING PAPER AND THE METHOD FOR PRODUCING THE SAME BACKGROUND OF THE INVENTION (1) Field of the Invention This invention relates to microcapsules for use in the preparation of pressure-sensitive recording paper and a method for producing the same. More particularly, the invention relates to the microcapsules and the method for producing the same, which microcapsules have relatively large particle diameters and a narrow particle size distribution and are excellent in color developing property.

(2) Description of the Prior Art Various kinds of pressure-sensitive recording materials are hitherto known well. For example, a colorless or a light colored electron-donative color forming agent (hereinafter referred to as "color former") is dissolved in a solvent and the obtained solution is formed into microcapsules. One surface of a sheet of paper is applied with these microcapsules. The surface of another sheet of paper is applied with a electron-acceptive substance (hereinafter referred to as "color developer") ) such as an acidic inorganic substance, a polymeric material, and aromatic carboxylic acids which produce a color upon reacting with the above color former. When the recording material is used, the treated surfaces of the above set of paper sheets are put together face to face and local pressures are applied to the paired sheets, thereby obtaining desired duplicate impressions.

The recording mechanism in the pressure-sensitive recording material of this type is such that the microcapsules applied on the surface of a sheet of paper are ruptured by the pressure of handwriting or typewriting to release the color former solution from microcapsules. The solution containing a color former comes into contact with the color developer on the opposing surface of the other sheet of paper to produce a color.

There is also know anther type of recording paper in which the respective coating materials providing such a color forming mechanism are applied together to one surface of a substrate paper.

Various methods for producing the microcapsules for use in the preparation of pressure-sensitive recording materials are hitherto proposed. For example, a solution of a color former in a hydrophobic solvent is dispersed with a dispersing agent and membranes of synthetic resin are formed over the dispersed particles.

More particularly, in the method disclosed in Japanese Laid-Open Patent Publication No. 54-85184, an aqueous solution of a dispersing agent containing urea is mixed with a color former solution to disperse the color former solution and formaldehyde is added to the obtained dispersion and the urea and formaldehyde are polymerized by heating to form resin membranes over the dispersed particles. Furthermore, disclosed in Japanese Laid-Open Patent Publication No. 58-216737 is a method of producing microcapsules in which a membrane forming agent and a dispersing agent are dissolved in water to prepare an aqueous solution of them, a hydrophobic solvent solution of a color former is mixed into the above solution to be dispersed and the obtained dispersion is heated to cause thermal polymerization to form resin membranes over the dispersed particles. In addition, there is a method of producing microcapsules in which a primary condensation product is obtained by reacting urea, melamine and formaldehyde and a hydrophobic solvent containing this condensation product and a color former are mixed with an aqueous solution of dispersing agent to produce a dispersion and the obtained dispersion is heated to form resin membranes over the dispersed particles. In any of the methods, the so-called in-situ polymerization is employed.

However, the particle diameters of microcapsules prepared in the conventional methods are too small for use as microcapsules in the preparation of pressure-sensitive recording paper. Furthermore, the particle size distribution of the microcapsule is too wide, so that the color developing property as the pressure-sensitive recording paper is not always satisfactory.

Meanwhile, in order to prepare microcapsules having a narrow particle size distribution, the diameter of dispersed particles in the dispersed liquid of hydrophobic solvent must be limited. However, the conventional methods have not always been satisfactory in this regard.

BRIEF SUMMARY OF THE INVENTION The present invention is intended to solve the above-described problems in the prior art involved in the pressure-sensitive recording material.

The principal objects of the present invention are, therefore, to narrow the range of particle size distribution of microcapsules and to provide the microcapsules for use in the preparation of pressure-sensitive recording paper which is excellent in the color developing property.

Another object of the invention is to provide an improved method for producing the above-mentioned microcapsules for the pressure-sensitive recording paper.

Still a further object of the present invention is to provide an O/W type emulsion of color former solution which is suitable for preparing microcapsules for pressuresensitive recording paper and which is large in particle diameter and narrow in particle size distribution.

In view of the above objects according to the present invention, the inventors have carried out intensive investigations and found out specific dispersing agents, proper dispersing conditions and membrane forming agents which are compatible with the dispersing agents. As a result, it was made possible to improve the particle size distribution which has been one of the problems in the conventional method for producing the pressure-sensitive recording material, thereby reducing the range of particle size distribution, enlarging the particle diameter, and providing microcapsules for preparing a pressure-sensitive recording material having excellent color developing rate and color density.

It is, therefore, a first aspect of the present invention to provide microcapsules for pressure-sensitive recording paper which is characterized in that the arithmetical mean particle diameter of the microcapsules is in the range of 3 to 10 pm and, in the cumulative particle size distribution taken from the side of larger particles, the ratio of the diameter at 25% value to the diameter at 50% value is not more than 1.25 and the ratio of diameter at 75% value to the diameter at 50t value is not less than 0.75 and the standard deviation of the particle size distribution is in the range of 1 to 2 pm.

"The diameter at 50% value in the cumulative particle size distribution taken from the side of larger particles" is the value on volume basis distribution and it is hereinafter referred to simply as "50% diameter".

A second aspect of the present invention is the microcapsules in the above first aspect in which the microcapsules are those prepared by forming synthetic resin membranes over the particles which are prepared by dispersing a colorless or light colored electron-donative die solution using a dispersing agent.

A third aspect of the present invention is the microcapsules in the above second aspect in which the dispersing agent is an isobutylene-maleic anhydride copolymer or its modified product and the synthetic resin is an amino resin.

The fourth aspect of the present invention is that, in a method for producing microcapsules for pressuresensitive recording paper comprising the steps of dispersing a colorless or light colored electron-donative color former solution into water using a dispersing agent and forming membranes of synthetic resin over the obtained dispersed particles, the improvement in that the membranes are formed over said dispersed particles which do not contain substantially a membrane forming agent.

A further aspect of the present invention is an O/W type emulsion of a solution of a colorless or light colored electron-donative color former in a hydrophobic solvent, which is characterized in that the arithmetical mean particle diameter of said emulsion particles is in the range of 3 to 10 um and, in the cumulative particle size distribution taken from the side of larger particles, the ratio of the diameter at 25% value to the diameter at 50% value is not more than 1.25 and the ratio of the diameter at 75% value to the diameter at 50% value is not less than 0.75 and the standard deviation of the particle size distribution is in the range of 1 to 2 um.

Still a further aspect of the present invention is a method for producing an O/W type emulsion of the solution of a colorless or light colored electron-donative color former in a hydrophobic solvent, which comprises the steps of mixing a colorless or light colored electron-donative color former in a hydrophobic solvent with an aqueous solution of dispersing agent to give an apparently uniform state of mixture in a preliminary dispersing step, and then vigorously stirring the mixture.

DETAILED DESCRIPTION OF THE INVENTION This invention will be described in more detail in the following.

The color former solution used in the method of the present invention is prepared by dissolving an electrondonative color former into a hydrophobic solvent such as aromatic hydrocarbon solvent. The color former is not especially limited, so that known compounds can be used.

For example, triarylmethane color formers, diphenylethane color formers, thiazine color formers, spiropyran color formers, and xanthene color formers, are used. More particularly, the color former is exemplified by Crystal Violet Lactone and benzoyl Leucomethylene Blue.

The aromatic hydrocarbon solvents in the above hydrophobic solvents are exemplified by diarylalkanes such as phenylxylylethane, phenylethylphenylethane and butyldiphenylethane; alkyl naphthalenes such as diisopropylnaphthalene; and alkyl biphenyls such as monoisopropyl biphenyl. Besides them, usable solvents are aliphatic hydrocarbon solvents such as normal paraffin and isoparaffin; alicyclic hydrocarbon solvents having a naphthene ring, animal and vegetable oils such as fish oils, lard oil, peanut oil, linseed oil, soybean oil, castor oil, and corn oil. These materials can be used singly or in a mixture of two or more.

The concentration of the color former is not especially limited, however, it is generally in the range of 0.01 to 20% by weight. If necessary, heating can be done in the dissolving of a color former.

The color former solution prepared by dissolving a color former is then dispersed in water with a dispersing agent. As the dispersing agent, a maleic anhydride copolymer is used. The comonomers which are copolymerized with maleic anhydride are exemplified by linear or branched olefinic hydrocarbons such as ethylene, propylene, 1-butene, 1-pentene, 2-methyl-l-butene, 1-hexene, 2-methyl-l-pentene, 3-methyl-l-pentene, 2-ethyl-1-butene, and diisobutylene.

Among them, a-olefins are mainly used. Besides them, the copolymers of styrene and methyl vinyl ether are also usable. These materials are used either singly or in a mixture of them.

In the present invention, it is preferable that isobutylene-maleic anhydride copolymer is used as a dispersing agent. The molecular weight of the copolymer is in the range of about 50,000 to 300,000, and preferably in the range of 10,000 to 200,000. The ratio of maleic anhydride in the co-monomers is in the range of 20 to 60 molar & , preferably in the range of 40 to 50 molar %. When isobutylene and maleic anhydride are used, the equimolar ratio is preferable. As far as the isobutylene and maleic anhydride are copolymerized, modified compounds may be used through a known process.

For the modification of these compounds, esterification, amine modification and amide modification are employed. Furthermore, it is possible to copolymerize another co-monomer in the range that the water solubility of the isobutylene-maleic anhydride copolymer is not impaired.

The co-monomers for this purpose are exemplified by the above-mentioned styrene, methyl vinyl ether, and in addition, acrylic acid and methacrylic acid and esters of them with methanol, ethanol or butanol and amine- or amidemodified co-monomer. In the use of these copolymers, the quantity of maleic anhydride is preferably in the abovedescribed range.

In the dispersing process using isobutylene-maleic anhydride copolymer, an aqueous solution of the isobutylenemaleic anhydride copolymer is prepared. This aqueous solution can easily be prepared by heating or by alkalifying it. The concentration of the aqueous solution is in the range of 1 to 20t by weight. If the concentration is lower than 1% by weight, the dispersion takes a long time resulting in a low production efficiency. On the other hand, if the concentration is higher than 20% by weight, the viscosity of the aqueous dispersing agent is raised which causes difficulty in the formation of larger particles and a narrow particle size distribution. Therefore, it is not acceptable in view of the economy in production.

By mixing and stirring the color former solution containing a dissolved color former and the aqueous solution of isobutylene-maleic anhydride copolymer as a dispersing agent, the O/W type emulsion can be prepared by dispersing the color former solution. The mixing ratio of the color former solution to the isobutylene-maleic anhydride solution is preferably in the range of 20:1 to 5:1 by weight as solid contents. If the ratio of color former solution is larger than the above value of 20:1, the dispersing takes longer time and the stability of dispersion is lowered. On the other hand, if the ratio of color former solution is smaller than the above value of 5:1, the dispersing effect is too large which makes the controlling of particle size difficult. In addition, a larger dispersing device must be employed which increases the cost for the processing.

The temperature of dispersing is one of important factors. The temperature is preferably in the range of 10 to 50do, and more preferably 15 to 40"C. If the dispersing temperature is too low, the fluidity of solutions in a dispersing vessel is lowered, which exerts a bad influence relative to the particle distribution. Therefore, it is not possible to control the particle size and to narrow the particle size distribution. In addition, if the temperature is too high, the controlling of particle size is difficult because the dispersing action is accelerated to excess and bubbles are taken into the liquid, accordingly, the stability of the dispersion is lost and a narrow particle size distribution cannot be obtained.

The pH value in the dispersion depends upon the pH value of the aqueous solution of dispersing agent, however, a value in the range of 2.5 to 4.0 is preferable in the usual operation.

The preferable stirrers for use in the dispersing process are conventionally used homogenizers because the handling of them is not difficult. As the homogenizer of this sort is a cylindrical vessel having a certain volume which is provided with an agitating blade, if necessary, a plurality of agitating blades, in which the rotational speed of the agitating blades and other operation conditions can be optionally controlled. The shape of blade may be in the form of a flat plate, a twisted plate, or in any other appropriate shape.

With regard to the dispersing vessel, the shearing diameter (the diameter of agitating blade) to the inner diameter of the dispersing vessel, is preferably in the range of 1:4-8. It is necessary to select a proper value concerning the diameter of dispersing vessel. For example, if the diameter is too large, the turbulence of fluid is lowered and the particles in the vessel flow through constant passages, as a result, large dispersion particles in a wide particle size distribution are sometimes formed.

However, even when a large diameter vessel is used, particles of proper diameters can be formed by additionally using an auxiliary stirrer so as to attain uniform stirring.

On the other hand, when the diameter of a dispersing vessel is too small, the turbulence of fluid is too large and air bubbles are taken into the liquid phase.

Furthermore, the formation of dead spaces, i.e., the portions not moving, are liable to occur in the corner of bottom portion of the dispersing vessel during the stirring. In order to avoid the formation of such dead spaces, it is desirable to make the bottom portion of dispersing vessel protruded downwards, such as in the shape of an inverted cone or an inverted hemisphere, thereby enabling to insert the stirring blades into the portion near the bottom.

With regard to the rotational speed of the stirrer in the dispersing vessel, the peripheral speed of stirring blades is important. In general, in order to narrow the particle size distribution and to reduce the time length of operation, the peripheral speed is preferably in the range of 12 to 22 m/sec. The peripheral speed can be calculated with an equation of: S = dnn/60 in which S = peripheral speed (m/sec), d = diameter of stirring blade (m) and n = rotational speed (rpm). In the case of a small size apparatus, the rotational speed of a stirrer is preferably in the range of 3,000 to 15,000 rpm.

As described above, the high speed rotation is necessary for the dispersing operation. However, it is desirable that a preliminary dispersing with a low speed stirring is done in the initial stage with low speed stirring because the two liquids are unevenly mixed in the initial state. After that, high speed stirring is carried out. When the preliminary dispersing is not sufficient, it is impossible to obtain a narrow particle size distribution.

The preliminary dispersing is carried out with a rotational speed generally in the range of 1,000 to 2,000 rpm and must be continued to the condition that two liquids are roughly mixed together. The time length for the preliminary dispersing is generally 1 to 10 minutes and when the aqueous solution and the solvents which were separated in the initial stage, are mixed in an apparent state of almost uniform dispersion, it is regarded that the preliminary dispersing is finished.

After the preliminary dispersing, the color former solution is further dispersed in water phase by the abovementioned high speed stirring. The time length for the stirring is not especially limited, however, it is generally in the range of 1 to 30 minutes.

Through the above-described process, the color former solution is dispersed and the dispersion particles of the color former solution are formed.

The method to disperse by mixing and stirring the color former solution and the aqueous solution of dispersing agent was described above. Besides the above method, it is possible to employ optionally a process, for example, when an aqueous solution of dispersing agent is prepared, a color former solution is simultaneously added and stirred together to prepare a dispersion. In any method, the color former solution is dispersed in the first place with the aid of the dispersing agent of isobutylene-maleic anhydride copolymer to obtain an aqueous dispersion. As far as such an aqueous dispersion is prepared, any of the methods can be employed.

The preferable and inevitable point in the method for producing microcapsules for pressure-sensitive recording paper according to the present invention is that the membrane forming material as described later does not coexist substantially in the above procedure to prepare the dispersed particles. In the conventional art, as disclosed in the examples of the foregoing two patent gazettes, at least a part of membrane forming material coexists already in the dispersed particles in the procedure to disperse the color former solution for the reasons to simplify the process and so forth. Especially, such a measure is often taken when the dispersing agent of isobutylene-maleic anhydride copolymer is used. It should be noted, however, that if the membrane forming material exists in the stage to obtain the dispersion particles, there is a possibility that the polycondensation of the membrane forming material is caused to proceed by the heat generated in the dispersing.

The occurrence of such polycondensation is not desirable because the polymerization of the membrane forming material itself and capsules containing gas bubbles occur. In addition, the stability of dispersion particles is impaired and it is sometimes difficult to prepare microcapsules having a narrow particle size distribution by regulating the particle diameter. Accordingly, it is desirable to prepare an aqueous dispersion of color former solution without substantially containing the membrane forming material.

Through the above procedure, it is possible to prepare the O/W type emulsion of the hydrophobic solvent solution of a colorless or light colored electron-donative color former, in which the arithmetical mean particle diameter is in the range of 3 to 10 urn and, in the cumulative particle size distribution taken from the side of larger particles, the ratio of the diameter at 25 value to the diameter at 50% value is not more than 1.25 and the ratio of the diameter at 75% value to the diameter at 50t value is not less than 0.75 and the standard deviation of the particle size distribution is in the range of 1 to 2 urn.

In the next step, synthetic resin membranes are formed over the dispersion particles. The synthetic resins used for this purpose are preferably the amino resins such as melamine-formaldehyde resin and urea-formaldehyde resin.

The so-called in-situ polymerization is employed as the method for the formation of membranes.

The amino resin can be prepared by the polycondensation of formaldehyde and an amine having two amino groups in the molecule such as urea, melamine, guanidine, N-methyl urea, and thiourea. Those properly modified with polyphenols such as resorcinol, catechol and pyrogallol and alcohol such as butanol, can also be used.

In order to form the resin membranes over dispersion particles, it is possible to use the method to polymerize the above synthetic resin membrane forming material by mixing it into the above aqueous dispersion.

The usable membrane forming materials of synthetic resin are exemplified by monomers of urea, melamine, formaldehyde. In addition, the precondensates or modified products of urea-formaldehyde (methylolurea), melamine-formaldehyde (methylolmelamine) or urea-melamine-formaldehyde can also be used. In the preparation of an aqueous solution of membrane forming agent, a proper quantity of the dispersing agent of above-mentioned isobutylene-maleic anhydride copolymer can be added. The addition of this isobutylene-maleic anhydride copolymer to the aqueous solution of membrane forming agent is desirable for the reason that the formation of resin membranes is accelerated and a part of the copolymer serves as a membrane forming component.

The above-described polycondensation of an amine with formaldehyde can be carried out through a well known method. For example, it is preferable that the reaction system of amine and formaldehyde is made alkaline. The reaction temperature is in the range of 10 to 95"C, and preferably 25 to 85"C. The molar ratio of formaldehyde to amine such as urea is in the range of 0.6 to 5.0, preferably 1.0 to 4.0. The reaction time is selected from the range of 10 minutes to 10 hours.

Through the above-described process, the membranes of synthetic resin are formed over the dispersion particles of color former solution, thereby obtaining the microcapsules for the preparation of pressure-sensitive recording paper.

The particle size distribution of microcapsules which have hitherto been used for the pressure-sensitive recording paper, is considerably wide. Accordingly, there exist the capsules of larger particle size and those of smaller particle size in comparison with a desirable range in particle size. The capsules of excessively small particle sizes are buried in the spaces among fibers of substrate paper, in the binder for fixing capsules to the substrate paper, or in the buffering agent to protect capsules from the pressure of weight load or friction, so that the smaller capsules cannot be ruptured or even when they are ruptured, the color former solution does not run out of capsules, therefore the excessively small capsules cannot contribute to the development of color sometimes.

These cause the lowering of the rate and density of color development. Furthermore, if the quantity of small capsules is large, the use quantity of the membrane forming agent must be increased because the surface area of capsules increases.

On the other hand, in the case of capsules having too large particle diameters, the resisting property to the pressures of weight load and friction is lowered, so that stains are caused to occur due to the partial color development in the surface applied with a color developer, which is difficulty used sometimes as a pressure-sensitive recording paper.

It is generally accepted that the mean particle diameter of capsules to be applied to upper paper of a pressure-sensitive recording paper is preferably large. For example, when microcapsules of large diameter containing a color former solution of high concentration is used, much color former is rapidly released to the surface of color developer, so that the color developing is rapid and dense to enable to prepare a pressure-sensitive recording paper of high color density. Accordingly, it is considered that microcapsules having a larger particle diameter is desirable, however, if the particle diameter is too large, the hydrophobic solvent containing a color former runs widely to produce large blots, so that it is impossible to produce clear images of characters. Accordingly, the arithmetical mean particle diameter of the microcapsules for pressure-sensitive recording paper in the present invention is in the range of 3 to 10 urn, preferably in the range of 4 to 8 ,um. When color developing property is regarded as important, the particle diameter of capsules is made large.

Meanwhile, when the prevention of stain is regarded as important, the particle diameter of capsules is made small.

Furthermore, an especially important point is that the particle size distribution of microcapsules is narrow (sharp). When the particle size distribution is narrow, the a buffer agent suitable for the groups of particles is easy, the use quantities of buffer agent and binder are small, and the staining of under paper is reduced, thereby enabling to produce a pressure-sensitive recording paper with excellent color developing property.

In view of the above facts, the following standards are set, that is, in the cumulative particle size distribution taken from the side of larger particles, the ratio of the diameter at 25% value to the diameter at 50% value is not more than 1.25, that is in the range of 1.00 to 1.25 and the ratio of the diameter at 75% value to the diameter at 50% value is not less than 0.75, that is 0.75 to 1.00, and the standard deviation of the particle size distribution is in the range of 1 to 2 urn. When these conditions are met, the particle size distribution is narrow and it is possible to produce microcapsules for pressuresensitive recording paper which is economical and is excellent in color developing property. The above particle diameters corresponding to the respective cumulative percentages are measured by particle groups using Coulter Counter (trademark, made by Coulter K.K.) The value of standard deviation is also obtained by the same test machine.

The microcapsules for pressure-sensitive recording material consisting of the particle groups having a narrow particle size distribution can easily be prepared through a method using a dispersing agent of isobutylene-maleic anhydride copolymer.

That is, when the O/W type emulsion of color former solution is prepared, the obtained emulsion produces basically an influence to the final shape of microcapsules.

Accordingly, when the above microcapsules are produced, those having an arithmetical mean particle diameter in the range of 3 to 10 urn and, in the cumulative particle size distribution taken from the side of larger particles, the ratio of the diameter at 25% value to the diameter at 50% value of not more than 1.25 and the ratio of the diameter at 75% value to the diameter at 50% value of not less than 0.75 and the standard deviation of the particle size distribution in the range of 1 to 2 urn, must be used.

A proper quantity of an ordinary binder is added to the thus obtained dispersion of microcapsules and the dispersion is applied to a substrate paper and it is then subjected to drying to produce pressure-sensitive recording paper with microcapsules.

In the following, the present invention is described in more detail with reference to several examples.

EXAMPLE Microcapsules were prepared using several kinds of dispersing agents in the following preparation examples. The particle size distributions of the microca

< Preparation Example 1 > (Dispersing Step) The preparation of a hydrophobic solution of color former and an aqueous solution of dispersing agent was carried out through the following method.

(1) Preparation of Hydrophobic Solution of Color former A solvent mixture (100 weight parts) of phenylxylyl phenylethane and phenylethyl phenylethane as a hydrophobic solvent and 5 weight parts of Crystal Violet Lactone (CVL) as a color former were put into a beaker. The contents were heated to 1200C with stirring to dissolve the color former. After that, the solution was cooled to a temperature of 35"C to obtain a color former solution.

(2) Preparation of Aqueous Solution of Dispersing Agent A ca. 22 wt. aqueous solution (261 weight parts) of isobutylene-maleic anhydride copolymer (trade name: CHIOPET E-20, made by Chiyoda Petroleum Co., Ltd.) as a dispersing agent and 528 weight parts of water were mixed together to obtain an aqueous solution of dispersing agent.

(3) Dispersing Operation A separable flask of about 2,000 ml was put in a water bath and it was maintained at 40"C. A high speed stirrer (trade name: TK HOMOMIXER MARK, made by Tokushu Kika Kogyo Co., Ltd.) was put in the separable flask. The whole quantities of previously prepared hydrophobic solution of color former and aqueous solution of dispersing agent were fed into the separable flask and a flat plate was set near the liquid surface to avoid the splashing. After the liquid temperature was raised to 38"C, the stirring was continued for a while at a rotation rate of 1,800 rpm. After the preliminary stirring in which a turbid state was observed by naked eyes, the rotational speed was raised to 10,000 rpm without delay. The temperature of the water bath was controlled so as to prevent the temperature of dispersed liquid did not rise above 40"C. Through the above-described operation, a dispersed liquid of about 5 urn in mean particle diameter was prepared.

A small quantity of the obtained dispersed liquid was sampled for test and it was diluted to 1/10 to 1/100 concentration in an electrolytic solution. The particle size distribution was measured using a Coulter counter TA-II (trade name, made by Coulter K.K.) with an orifice of 50 urn in diameter. The results are shown in Table 1.

(Capsulation Step) (1) Preparation of Membrane Forming Agent To a 300 ml beaker were added 44 weight parts of urea (reagent grade, above 99 purity) and 19 weight parts of melamine (reagent grade, above 99% purity). Then, 143 weight parts of 37% aqueous solution of formaldehyde was added and the reaction was carried out for 20 minutes in a water bath at 60"C with stirring. After the reaction, the reaction mixture was cooled to 35"C to obtain a preliminary condensation product as a membrane forming agent.

(2) Capsulation Operation The previously prepared dispersed liquid was put into the separable flask which was used in the dispersing operation and it was stirred using an ordinary stirrer. The whole of the membrane forming agent was poured gradually into the flask along it inlet wall. The separable flask was put in a warm water bath at 60 C. The preliminary condensation product of melamine-urea-formaldehyde was reacted for 3 hours after the liquid temperature was raised to 60"C to form melamine-urea-formaldehyde resin membranes over the dispersion particles. After stopping the heating, the stirring was continued for about 10 hours. After that, about 24 weight parts of 28% aqueous ammonia was added with stirring and about 7.5 weight parts of 208 aqueous solution of sodium hydroxide to adjust the pH to 9 so as to prepare microcapsules. The particle size distribution of the obtained microcapsules was measured, the result of which is shown in Table 1.

< Preparation Example 2 > (Dispersing Step) (1) Preparation of Hydrophobic Solution of Color Former A hydrophobic solution of color former was prepared in the like manner as in Preparation Example 1.

(2) Preparation of Aqueous Solution of Dispersing Agent An aqueous solution of dispersing agent was prepared in the like manner as in Preparation Example 1 except that 129 weight parts of CHIOPET E-20 and 458 weight parts of water were used.

(3) Dispersing Operation A dispersed liquid was prepared in the like manner as in Preparation Example 1 using the above obtained hydrophobic solution of color former and the aqueous solution of dispersing agent, in which the mean particle diameter was about 5 urn. Its particle size distribution was measured, the results of which are shown in Table 1.

(Capsulation Step) (1) Preparation of Membrane Forming Agent To a 500 ml beaker were added 203 weight parts of 60 wt.% aqueous solution of methylolmelamine (trade name: SUMITEX RESIN M-3, made by Sumitomo Chemical Co., Ltd.), 45 weight parts of CHIOPET E-20 and 167 weight parts of water, and they were mixed together with stirring to obtain a membrane forming agent.

(2) Capsulation Operation Through the process in the like manner as in Preparation Example 1, microcapsules which was finally adjusted to pH 9 was obtained. The particle size distribution of the obtained microcapsules was measured, the results of which are shown in Table 1.

< Preparation Example 3 > (Dispersing Step) (1) Preparation of Hydrophobic Solution of Color Former A hydrophobic solution of color former was prepared in the like manner as in Preparation Example 1.

(2) Preparation of Aqueous Solution of Dispersing Agent An aqueous solution of dispersing agent was prepared by mixing 260 weight parts of methyl vinyl ethermaleic anhydride copolymer (trade name: AN-119 BF, made by GAF Corporation) and 530 weight parts of water.

(3) Dispersing Operation A dispersed liquid was prepared in the like manner as in Preparation Example 1 except that the rotational speed in dispersing was 7,500 rpm and the particle size distribution was measured, the results of which are shown in Table 1.

(Capsulation Step) (1) Preparation of Membrane Forming Agent Using melamine-urea-formaldehyde preliminary condensation product in the like manner as in Preparation Example 1 and 6 weight parts of resorsinol (made by Kishida Chemical Co., Ltd.) was mixed to obtain a membrane forming agent.

(2) Capsulation Operation Through the process in the like manner as in Preparation Example 1, microcapsules which was finally adjusted to pH 9 was obtained. The particle size distribution of the obtained microcapsules was measured, the results of which are shown in Table 1.

Table 1 Dispersing Mean Standard Ratio of Preparation Agent Particle Diameter Examples ------ Diameter Deviation 25%D. 758D.

Kind MFA (urn) (urn) 50%D. 50%D.

[After Emul sification] (2) Prep.Ex. 1 IB-MA No 5.0 1.3 1.18 0.82 Prep.Ex. 2 IB-MA No 5.0 1.2 1.16 0.82 (3) Prep.Ex. 3 MVE-MA No 4.9 2.2 1.33 0.69 [After Capsulation] Prep.Ex. 1 -- -- 5.3 1.3 1.16 0.81 Prep.Ex. 2 -- -- 5.2 1.2 1.15 0.82 Prep.Ex. 3 -- -- 5.4 2.9 1.37 0.67 Notes: (1) MFA: Existence of membrane forming agent (2) IB-MA: Isobutylene-maleic anhydride copolymer (3) MVE-MA: Methyl vinyl ether-maleic anhydride copolymer As will be understood in view of Table 1, the particle size distributions of the dispersed liquids and the microcapsules in Preparation Examples 1 and 2 were narrower than those in Preparation Example 3. Furthermore, the agglomeration was not observed in Preparation Examples 1 and 2 because the compatibility between the dispersing agent and membrane forming agent was good, so that the values in particle size distributions of the dispersed liquids and the microcapsules were the same and desirable microcapsules were prepared.

In the following, the cases in which the membrane forming agent was added during the dispersing step will be described.

< Preparation Example 4 > (Dispersing Step) (1) Preparation of Hydrophobic Solution of Color former A hydrophobic solution of color former was prepared in the like manner as in Preparation Example 1.

(2) Preparation of Aqueous Solution of Dispersing Agent An aqueous solution of dispersing agent was prepared in the like manner as in Preparation Example 1.

(3) Preparation of Membrane Forming Agent A membrane forming agent was prepared in the like manner as in Preparation Example 1.

(4) Dispersing Operation A separable flask of about 2,000 ml was put in a water bath and it was maintained at 15"C. A high speed stirrer (trade name: TK HOMOMIXER MARK, made by Tokushu Kika Kogyo Co., Ltd.) was put in the separable flask. The whole quantities of previously prepared hydrophobic solution of color former and the aqueous solution of dispersing agent were fed into the separable flask and the membrane forming agent was further added. A flat plate was set near the liquid surface to avoid the splashing. After the liquid temperature reached 15"C, the stirring was continued for a while at a rotation rate of 1,800 rpm. After the mixing, the rotational speed was raised to 10,000 rpm rapidly. The temperature of the water bath was controlled so as to prevent the temperature of dispersed liquid did not rise above 15"C. Through the above-described operation, a dispersed liquid of about 5 pm in the mean particle diameter was obtained.

With regard to the obtained dispersed liquid, the particle size distribution was measured in the like manner as in Preparation Example 1. The results are shown in Table 2.

(Capsulation Step) (1) Capsulation Operation The previously prepared dispersed liquid containing the membrane forming agent was put into a separable flask and it was stirred using an ordinary stirrer. The separable flask was put in a warm water bath at 60"C. The reaction was carried out for 3 hours with regulating the temperature to 60"C to form melamine-urea-formaldehyde resin membranes over the dispersion particles. After stopping the heating, the stirring was continued for about 10 hours.

After that, about 24 weight parts of 28t aqueous ammonia was added with stirring and about 7.5 weight parts of 20% aqueous solution of sodium hydroxide was added to adjust the pH to 9 so as to prepare microcapsules. The particle size distribution of the obtained microcapsules were measured, the result of which is shown in Table 2.

< Preparation Example 5 > (Dispersing Step) A hydrophobic solution of color former, a hydrophobic solution of dispersing agent and a membrane forming agent were prepared in the like manner as in Preparation Example 2. Then, the dispersing operation was carried out in the co-existence of the membrane forming agent in the like manner as in Preparation Example 4 to obtain a dispersed liquid. With regard to the obtained dispersed liquid, the particle size distribution was measured and the results are shown in Table 2.

(Capsulation Step) The capsulation was carried out in the like manner as in Preparation Example 4 to prepare microcapsules that was finally adjusted to pH 9. The particle size distribution of the obtained microcapsules were measured, the result of which is shown in Table 2.

< Preparation Example 6 > The preparation process was carried out in the like manner as in Preparation Example 4 except that a dispersing agent of methyl vinyl ether-maleic anhydride copolymer (trade name: AN-119 BF, made by GAF Corporation) was used to prepare microcapsules that was finally adjusted to pH 9. The particle size distribution of the obtained microcapsules were measured, the result of which is shown in Table 2.

< Preparation Example 7 > The preparation process was carried out in the like manner as in Preparation Example 4 except that a dispersing agent of a-methylstyrene-maleic anhydride copolymer (trade name: SUMIREZ RESIN DS-40K, made by Sumitomo Chemical Co., Ltd.) was used to prepare microcapsules that was finally adjusted to pH 9. The particle size distribution of the obtained microcapsules were measured, the result of which is shown in Table 2.

Table 2 Dispersing Mean Standard Ratio of Preparation Agent Particle Diameter Examples Diameter Deviation 25%D. 75%D.

(1) Kind MFA (urn) (urn) 50%D. 50D.

[After Emulsification] (2) Prep.Ex. 4 IB-MA Yes 5.0 1.6 1.22 0.77 Prep.Ex. 5 IB-MA Yes 5.1 1.6 1.22 0.76 (3) Prep.Ex. 6 MVE-MA Yes 5.0 2.1 1.31 0.70 (4) Prep.Ex. 7 St-MA Yes 5.8 2.4 1.27 0.73 [After Capsulation] Prep.Ex. 4 -- -- 5.2 1.6 1.21 0.78 Prep.Ex. 5 -- -- 5.2 1.7 1.22 0.77 Prep.Ex. 6 -- -- 5.0 2.2 1.32 0.69 Prep.Ex. 7 -- -- 5.9 2.3 1.29 0.75 Notes: (1) MFA: Existence of membrane forming agent (2) IB-MA: Isobutylene-maleic anhydride copolymer (3) MVE-MA: Methyl vinyl ether-maleic anhydride copolymer (4) St-MA: a-Methylstyrene-maleic anhydride copolymer As will be understood in view of Table 2, the particle size distributions of the dispersed liquids and the microcapsules in Preparation Examples 4 and 5 were very narrow as compared with those in Preparation Examples 6 and 7. Furthermore, the agglomeration or partial separation was not observed because the compatibility between the dispersing agent and the membrane forming agent is good, so that no difference in the particle size distributions between the dispersed liquid and the microcapsule was observed and desirable microcapsules were prepared.

The influence to the color developing property which was brought about by the difference in particle diameters of microcapsules was tested.

< Preparation Example 8 > A sizing agent and a protective agent were added to microcapsules of about 5 urn in particle size as prepared in the same method as in Preparation Example 1 and it was applied on a quality paper using a Mayer bar to prepare an upper sheet of pressure-sensitive recording material.

In order to test the color developing property of the pressure-sensitive recording paper, the above upper sheet was put on a commercially available lower sheet to which a color developer of salicylic acid compound was previously applied. The color development was done using an impact printer. The reflectance ratios of the lower sheet were measured using a spectroreflectometer at 3 seconds, 20 seconds and 60 minutes after the color development. The results of the measurement are shown in Table 3.

cPreparation Example 9 > Using the microcapsules of about 3 urn in particle size as prepared in the like manner as in Preparation Example 1 and an upper sheet of pressure-sensitive recording material was prepared in the like manner as in Preparation Example 8, and also in the similar manner, the color developing property was tested. The results of the test are shown in Table 3.

Table 3 Mean Color Density (t) Preparation Particle Examples Diameter After After After (urn) 3 sec. 20 sec. 60 min.

[After Capsulation] Prep.Ex. 8 5.4 79 85 87 Prep.Ex. 9 2.9 76 82 85 As will be understood in view of results in Table 3, the microcapsules having a larger mean diameter in Preparation Example 8 excel in the color developing property because both the initial color density and the final color density of them are high as compared with those in Preparation Example 9.

In addition, it was ascertained that the upper sheet prepared with the microcapsules obtained in Preparation Example 8, did not stain the lower sheet by the rupture of microcapsules and used as a pressure-sensitive recording paper of good quality.

Furthermore, in view of the results in Tables 1 and 2, it was understood that the microcapsules of Preparation Examples 1 and 2 in which the coexistence of membrane forming agent did not adopted during the dispersing operation, had narrower particle size distributions as compared with those in Preparation Examples 4 and 5 in which the membrane forming agent was used together in the dispersing. Accordingly, it is desirable that the dispersing operation is carried out without the addition of the membrane forming agent.

The microcapsules according to the present invention are characterized in the following points: (1) The strength of microcapsules is large and the microcapsules are hardly ruptured by pressures of loads and friction in the uses.

(2) The agglomeration of microcapsules hardly occurs.

(3) The color is white or slightly colored.

(4) The particle size distribution is narrow.

(5) The particle diameter is relatively large.

(6) The storage stability of microcapsules is high.

(7) The color developing reaction in pressuresensitive recording paper is not impaired and the rate of color development is high.

(8) The blurring of developed image in pressuresensitive recording paper does not occur and clear images can be produced after the storage of a long period.

(9) Capsulation can be done without difficulty.

(10) Inexpensive.

Claims (8)

WHAT IS CLAIMED IS:

1. Microcapsules for pressure-sensitive recording paper which is characterized in that the arithmetical mean particle diameter of said microcapsules is in the range of 3 to 10 urn and, in the cumulative particle size distribution taken from the side of larger particles, the ratio of the diameter at 25% value to the diameter at 50% value is not more than 1.25 and the ratio of the diameter at 75 value to the diameter at 50% value is not less than 0.75 and the standard deviation of the particle size distribution is in the range of 1 to 2 urn.

2. The microcapsules for pressure-sensitive recording paper as claimed in Claim 1, wherein said microcapsules are prepared by forming synthetic resin membranes over the particles which are prepared by dispersing a colorless or light colored electron-donative die solution using a dispersing agent.

3. The microcapsules for pressure-sensitive recording paper as claimed in Claim 2, wherein said dispersing agent is an isobutylene-maleic anhydride copolymer or its modified product and said synthetic resin is an amino resin.

4. In a method for producing microcapsules for pressure-sensitive recording paper comprising the steps of dispersing a colorless or light colored electron-donative color former solution into water using a dispersing agent and forming membranes of synthetic resin over the obtained dispersion particles, the improvement comprises that said membranes are formed over said dispersion particles which do not contain substantially a membrane forming agent.

5. An O/W type emulsion of solution of a colorless or light colored electron-donative color former in a hydrophobic solvent, which is characterized in that the arithmetical mean particle diameter of dispersion particles is in the range of 3 to 10 urn and, in the cumulative particle size distribution taken from the side of larger particles, the ratio of the diameter at 25 value to the diameter at 50t value is not more than 1.25 and the ratio of the diameter at 75 value to the diameter at 50% value is not less than 0.75 and the standard deviation of the particle size distribution is in the range of 1 to 2 urn.

6. The O/W type emulsion as claimed in Claim 5, wherein said emulsion is dispersed with a dispersing agent of an isobutylene-maleic anhydride copolymer or its modified product.

7. A method for producing O/W type emulsion of the solution of a colorless or light colored electrondonative color former in a hydrophobic solvent, which comprises the steps of mixing a colorless or light colored electron-donative color former in a hydrophobic solvent with an aqueous solution of dispersing agent to give an apparently uniform mixed state in a preliminary dispersing step, and then vigorously stirring the mixture.

8. The method for producing O/W type emulsion of the solution of a colorless or light colored electrondonative color former in a hydrophobic solvent as claimed in claim 7, wherein said aqueous solution of dispersing agent is an aqueous solution of isobutylene-maleic anhydride copolymer or its modified product.