Pressure-sensitive recording sheet with microcapsules having polyurea walls

Abstract

A pressure-sensitive recording sheet having reduced color fog which comprises a base sheet and (1) a layer of microcapsules containing a color former in oil droplets surrounded by the wall of a polyurea obtained by the reaction of a polyisocyanate or isocyanate-containing polyisocyanate adduct with a polyamine or polyamine adduct, and (2) a layer of an adsorbent or color reactive substance capable of producing a distinct color upon contact with the color former. Layers (1) and (2) are applied to the same surface of the base material.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to pressure-sensitive recording sheets.

2. Description of the Prior Art

A pressure-sensitive recording sheet capable of recording on itself by the application of pressure comprising a base material and (a) microcapsules containing oils having dissolved therein a color former and (b) a developer, both (a) and (b) being on the same surface of the base material, has been produced by using ordinary microcapsules obtained by, for example, the complex coacervation of gelatin and gum arabic. However, remarkable color fog occurs over the total surface of such a pressure-sensitive recording paper utilizing microcapsules, and the resulting product has a poor appearance and is of low commercial value. In order to reduce the color fog which occurs with ordinary microcapsules, it has been necessary, for example, to provide a protective layer between a capsule layer and a layer of a developer, or to envelope the ordinary capsules with a further coating to form double-walled microcapsules.

Microcapsules obtained by normal methods cannot contain a color former in high concentration because high color former concentrations in the oil droplets results in the exudation of the color former out of the capsules. Therefore, a pressure-sensitive recording sheet prepared by using such microcapsules cannot give a high color density upon the application of writing pressure.

SUMMARY OF THE INVENTION

The present invention relates to a pressure-sensitive recording sheet having extremely reduced color fog, which comprises a base material and (1) oil droplets containing dissolved therein a colorless or almost colorless compound (color former) having color reactivity, said droplets being enclosed in walls of a polyurea obtained by the reaction of a polyisocyanate or isocyanate-containing polyisocyanate adduct with a polyamine or polyamine adduct, and (2) a developer capable of giving a colored substance by reaction with said color former, (1) and (2) being on the same surface of said base material.

The invention is characterized especially by using polyurea as the capsule wall forming material to encapsulate the oil droplets containing the color former therein.

The most important feature of the invention is that the wall of the microcapsules is formed by the reaction of a compound having isocyanate groups with a compound having amine groups. The two compounds are preferably a polyisocyanate adduct having uncombined isocyanate groups, most preferably having more than two uncombined isocyanate groups, and a polyamine adduct having uncombined amino groups, most preferably having more than two uncombined amino groups.

The isocyanate containing polyisocyanate adduct is a polyisocyanate adduct having uncombined isocyanate groups which are not combined with the other compound but which will be combined with the amine to take part in the formation of the microcapsules. The above explanation can be represented as follows: ##SPC1##

The compound having amine groups acts in a similar manner.

One object of this invention is to provide a pressure sensitive recording sheet of high commercial value which has reduced color fog and reduced extraneous coloration, said sheet comprising a base material with both a microcapsular layer containing an oily liquid having dissolved therein a color former and a developer on the same surface of said base material.

Extensive research conducted to achieve the above object led to the discovery that good results can be obtained by using a polyurea, obtained by the reaction of a polyisocyanate or isocyanate-containing polyisocyanate adduct with a polyamine or polyamine adduct, to form the microcapsule walls containing the oily liquid having dissolved therein a color former.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures represent plots of color fog density at various wavelengths for products both within and outside the present invention as explained in the Examples.

DETAILED DESCRIPTION OF THE INVENTION

The polyurea referred to in the present invention is one having a plurality of urea linkages in the molecular structure, and includes those having groups other than urea groups, which groups are originally present in the isocyanate-containing polyisocyanate adduct.

Microcapsules containing the polyurea walls described above are prepared, for example, by dissolving a polyisocyanate or isocyanate-containing polyisocyanate adduct in an oily liquid having dissolved therein a color former, emulsifying the solution in a polar liquid, and then adding a polyamine or amino-containing polyamine adduct as a reaction promotor, or first adding a polyamine or amino-containing polyamine adduct to a polar liquid and then emulsifying an oily solution of a color former in which a polyisocyanate or isocyanate-containing polyisocyanate adduct is dissolved, in the polar liquid.

By effecting polymerization from the exterior of the individual oil droplets, microcapsules containing an oil solution of a color former covered with the polyurea are obtained.

The addition of a polyamine or polyamine adduct, especially the polyamine adduct, is preferably performed after emulsification since this procedure lends itself to easy emulsification without forming large oil drops or causing multi-nuclear capsules as a result of the aggregation of several oil drops. The use of the resulting microcapsules provides a pressure sensitive recording sheet capable of recording on itself which stable to pressure or friction and which has reduced contamination.

When the polyamine or addition product thereof is added before emulsification, faults are encountered such as polynuclear microcapsules are produced, and, in some cases, they coagulate.

The encapsulation operation described above may be carried out with good results at room temperature, but when the formation of polyurea walls is carried out at elevated temperatures, the reaction time required for the polyurea wall formation can be shortened, and firmer microcapsules of lower permeability can be obtained within shorter periods of time.

In general, as the temperature of reaction is elevated, the time for reaction is lessened, and as the temperature is lowered, the time is increased. In the examples it will be seen that the temperature is 70.degree.-80.degree.C, and the reaction time is less than 30 minutes. However, the exact relationship between the temperature and time can easily be decided by one skilled in the art as it is desirably selected in accordance with the scale and type of apparatus used. It is important that the temperature be lower than the boiling point of the polar solvent capable of forming a continuous phase, however.

Each operation of the microcapsule preparation described above can be performed using ordinary state of the art techniques, there being no significant reaction restrictions. One preferred condition, however, is that the temperature not be higher than 30.degree.C until the formation of the capsule walls, the temperature then being raised to a point not higher than the boiling point of the polar solvent used in order to harden the capsules within short periods of time.

Usually from about 1/16 to 1 wt part of isocyanate is used per 1 weight part of oil, preferably 1/8-1/2 part of isocyanate per 1 weight part of oil.

As indicated, a polar liquid is used as a dispersion medium for forming a continuous phase, and this term includes materials defined as such in the chemical arts. The relationship between the oily liquid and the polar liquid is such that the former forms a disperse phase and the latter forms a dispersion medium.

The most typical polar liquid which forms a continuous phase used in the preparation of microcapsules is water but, for example, ethylene glycol, glycerol, butyl alcohol, and octyl alcohol can also be used as polar liquids in the present invention. In order to emulsify an oily liquid in the polar liquid, it is desirable to use a protective colloid or a surface active agent. Examples of protective colloids that can be used are natural or synthetic hydrophilic polymeric substances such as gelatin, gum arabic, casein, carboxymethyl cellulose, starch or polyvinyl alcohol. Examples of surface active agents include anionic compounds such as the alkylbenzenesulfonates, alkylnaphthalene sulfonates, polyoxyethylene sulfates or Turkey red oil and nonionic compounds such as the polyoxyethylene alkyl ethers, polyoxyethylene alkylphenol ethers, or sorbitan fatty acid esters.

When the polyisocyanate or isocyanate-containihg polyisocyanate adduct is not soluble in the oily liquid, an auxiliary solvent such as acetone, tetrahydrofuran, ethyl acetate, butyl acetate, dioctyl phthalate, dibutyl phthalate, diethyl phthalate, toluene, xylene, benzene, chloroform, methylethylketone, or dimethyl formamide may be mixed in the oily liquid.

Examples of the polyisocyanates used for preparing the polyurea capsular walls are diisocyanates and dithioisocyanates such as m-phenylene diisocyanate, p-phenylene diisocyanate, 2,6-tolylene diisocyanate, 2,4-tolylene diisocyanate, naphthalene-1,4-diisocyanate, diphenylmethane-4,4'-diisocyanate, 3,3'- dimethoxy-4,4'-biphenyl-diisocyanate, 3,3'-dimethyl diphenylmethane-4,4'-diisocyanate, xylylene-1,4-diisocyanate, xylylene-1,3-diisocyanate, 4,4'-diphenylpropane diisocyanate, trimethylene diisocyanate, hexamethylene diisocyanate, propylene-1,2-diisocyanate, butylene-1,2-diisocyanate, ethylidine diisocyanate, cyclohexylene-1,2-diisocyanate, cyclohexylene-1,4-diisocyanate, xylylene-1,4-diisothiocyanate, ethylidine diisothiocyanate, or hexamethylene dithioisocyanate, triisocyanates such as 4,4',4"-triphenylmethane triisocyanate, toluene-2,4,6-triisocyanate or polymethylene polyphenyl isocyanate, polyisocyanate monomers such as 4,4'-dimethyldiphenyl methane-2,2',5,5'-tetraisocyanate, and adducts of these polyisocyanates with compounds containing a hydrophilic group such as polyamines, polycarboxylic acids, polyols, polythiols, polyhydroxy compounds or epoxy compounds.

As examples of polyamines to which the polyisocyanate can be added there are aromatic polyamines such as o-phenylene diamine, p-phenylene diamine or 1,5-diaminonaphthalene, and aliphatic polyamines such as 1,3-propylene diamine, 1,4-butylene diamine or hexamethylene diamine.

Examples of such polycarboxylic acids are pimelic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, terephthalic acid, 4,4'-biphenyldicarboxylic acid or 4,4'-sulfonyldibenzoic acid.

Examples of such polythiols are condensation products of thioglycol or reaction products between polyhydric alcohols and suitable thioglycol ethers.

The polyhydroxy compounds include aliphatic and aromatic polyhydric alcohols, hydroxy polyesters, and hydroxy polyalkylene ethers. Examples of the aliphatic and aromatic polyhydric alcohols which can be used are catechol, resorcinol, hydroquinone, 1,2-dihydroxy-4-methylbenzene, 1,3-dihydroxy-5-methylbenzene, 3,4-dihydroxy-1-methylbenzene, 3.5-dihydroxy-1-methylbenzene, 2,4-dihydroxyethyl benzene, 1,3-naphthalene diol, 1,5-naphthalene diol, 2,7-naphthalene diol, 2,3-naphthalene diol, o,o'-biphenol, p,p'-biphenol, 1,1'-bi-2-naphthol, bisphenol A, 2,2'-bis(4-hydroxyphenyl) butane,2,2'-bis (4-hydroxyphenyl)-isopentane,1,1'-bis (4 -hydroxyphenyl)-cyclopentane, 1,1'bis(4-hydroxyphenyl)-cyclohexane, 2,2'-bis(4-hydroxy-3-methylphenyl)-propane, bis-(2-hydroxyphenyl)-methane, xylylene diol, ethylene glycol, 1,3- propylene glycol, 1,4-butylene glycol, 1,5-pentane diol, 1,6-heptane diol, 1,7-heptane diol, 1,8-octane diol, 1,1,1-trimethylol propane, hexane triol, pentaerythritol, glycerol, and sorbitol.

Examples of hydroxy polyesters which can be used as those obtained from polycarboxylic acids and polyhydric alcohols. As the polycarboxylic acids for the production of such hydroxy polyesters, there are malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, maleic acid, isophthalic acid, terephthalic acid or gluconic acid. Examples of the polyhydric alcohols used are the same as already exemplified above.

Hydroxy polyalkylene ethers which may be used are, for example, a condensation product between an alkylene oxide and a polyhydric alcohol. Examples of the alkylene oxide used for the production of the hydroxy polyalkylene ether are butylene oxide or amylene oxide. As the polyhydric alcohol, those exemplified above can be used.

Examples of the epoxy compounds which can be used are aliphatic diglycidyl ethers such as diglycidyl ether, glycerine triglycidyl ether and polyallyl glycidyl ethers, having a molecular weight of 150 to 5,000, aliphatic glycidyl esters such as diglycidyl ester of rinoleic dimeric acid, aromatic glycidyl ethers such as diglycidyl ether of bisphenol A, triglycidyl ether of trihydroxyphenyl propane, triglycidyl ether of trihydroxyphenylpropane and tetraglycidyl ether of tetraphenylene ethane, and glycidyl ether/ester mixtures such as diglycidyl ether ester of 4,4-bis(4-hydroxyphenyl)pentanoic acid.

Microcapsules with walls having lower permeability than those obtained by using a polyisocyanate monomer can be obtained by using an isocyanate-containing polyisocyanate adduct obtained by adding a polyisocyanate monomer to a compound having a hydrophilic group such as a polyamine, polycarboxylic acid, polythiol, polyhydroxy compound or epoxy compound.

Examples of the polyamine or polyamine adduct used to promote the polymerization of the polyisocyanate or isocyanate-containing polyisocyanate adduct and to form the polyurea walls are aromatic polyamines such as o-phenylene diamine, p-phenylene diamine or diamino-naphthalene, aliphatic polyamines such as 1,1,3-propylene diamine or hexamethylene diamine, and adducts formed between these aliphatic or aromatic polyisocyanates and epoxy compounds. Furthermore, compounds containing many amino groups in the molecule, such as gelatin, can also be used. In other words, any compounds having two or more amino groups in the molecule can be used as a polymerization promotor in the present invention.

Microcapsules best suited to meet the object of this invention, which have capsular walls of especially reduced permeability and are very stable to pressure or friction, can be obtained most easily when an oily liquid having dissolved therein an adduct formed between an isocyanate-containing polyisocyanate and a compound containing a hydrophilic group is dispersed and emulsified in a polar solvent and then a polyamine or polyamine adduct is added to the solution.

By coating a mixture of the microcapsules thus obtained (which contain an oil solution of a color former) with a developer on a base material, or by first coating the microcapsules on the base material and then coating the developer thereon, or first coating a developer on the base material and then coating the microcapsules thereon, there can be obtained a record sheet capable of recording on itself having reduced contamination as compared with conventional pressure-sensitive record sheets.

Examples of the color former to be incorporated in the capsules as a solution in an oily liquid are crystal violet lactone, benzoyl leucomethylene blue, malachite green lactone, Rhodamine B Lactam, 2,4-dialkyl-7-dialkylaminofluoranes, 3-methyl-2,2'-spirobi(benzo[f] chromene), and mixtures thereof, all of which are standard color formers for the preparation of pressure-responsive record sheets. Additional useful color formers are disclosed in, e.g., U.S. Pat. Nos. 2,730,457, 3,293,060, 3,501,331, 3,514,311, 3,554,781, 3,619,238 and 3,619,338.

On the other hand, examples of the developer which on reaction with the color former gives a distinctive color are activated clay substances such as attapulgite, zeolite or bentonite, organic acidic substances such as succinic acid, tannic acid, gallic acid, pentachlorophenol or phenolic resins, metal compounds of aromatic acids such as zinc salicylate, tin salicylate, zinc 1-hydroxy-3-naphthoate, tin 2-hydroxy-3-naphthoate or zinc 3,5-di-tert. butylsalicylate, and mixtures of aromatic acids and metal compounds such as a mixture of 2-hydroxy-3-naphthoic acid and zinc oxide or a mixture of p-tert.-butylsalicylic acid and zinc hydroxide, all of which are generally used as developers for pressure-responsive record sheets. The amount of color former used is in accordance with amounts as are used in the prior art; giving due consideration to the solubility of the color former in the oil, the color density to be developed by the acid substance, etc.

Examples of the oily liquid used for dissolving the color former include natural oils such as cotton seed oil or soy bean oil, and synthetic oils such as chlorinated diphenyl, chlorinated paraffin, alkyl naphthalenes, hexahydroterphenyl or alkyl diphenyl methanes.

The amounts of these compounds are in accordance with ordinary prior art microencapsulation techniques.

As previously stated, polyureas as obtained in the present invention provide superior microcapsule walls. However, a polyurea obtained by the reaction of phosgene with a polyamine, and a polyurea obtained by the reaction of a polyisocyanate with water do not bring about good results. It is only by the use of polyureas prepared from polyisocyanates or polyisocyanate adducts and polyamines or polyamine adducts that good results can be obtained. Satisfactory results are obtained expecially when polyisocyanate adducts and polyamines or polyamine adducts are used.

The polyurea walls of the microcapsules obtained in the present invention have very low permeability, and it is not necessary therefore to provide a protective coating layer between the developer annd microcapsules containing the color former in order to prevent contamination leading to extraneous coloration. This is very advantageous in the manufacturing process. Furthermore, if microcapsules having polyurea walls as described above are used, a pressure sensitive recording sheet can be obtained by only one coating of a mixture of such microcapsules containing an oily liquid having dissolved therein a color former with a developer on a base material without causing color fog.

The invention will now be described by the following Examples. In the Examples, solutions of acid clay, phenolic resin, and zinc 3,5-di-tert.-butyl salicylate were produced in the following manner.

Solution of acid clay: To 300 g of water were added 8 cc of a 20% aqueous solution of sodium hydroxide, and 100 g of acid clay were added to the mixture. With vigorous stirring, the acid clay was dispersed in the mixture. As a binder, 40 g of a styrene/butadiene rubber latex (SBR Latex No. 636, tradename, Chemical Corporation Co., Ltd.) were added to form a solution containing acid clay as a developer.

Solution of zinc 3,5-di-tert.-butylsalicylate: 10 g of polyvinyl alcohol (Kurare Poval 205, tradename, Kuraray Co., Ltd.) as a binder were dissolved in 90 g of water. To the solution, 60 g of zinc 3,5-di-tert.-butylsalicylate powder were added, and with vigorous stirring, dispersed therein to form a solution containing zinc 3,5-di-tert.-butylsalicylate as a developer.

Solution of a phenolic resin: To 200 g of water were added 20 g of a condensate of p-phenylphenol and formalin and 8 g of a styrene/maleic anhydride copolymer (Seripset 500, tradename, Monsanto Company) as a binder. Using a ball mill, the solid matter was pulverized for one day and one night to form a solution containing the phenol resin as a developer.

EXAMPLE 1

Crystal violet lactone (0.7 g) and 0.6 g of benzoyl leuco methylene blue as color formers were dissolved in 30 g of dipropylnaphthane to form an oily solution of the color formers. In this oily solution were dissolved 6 g of an isocyanate-containing adduct of hexamethylene diisocyanate and trimethylol propane (Coronate HL, trademark, Nippon Polyurethane Kogyo K.K.). With vigorous stirring, the mixed solution was added to 55 g of water in which 3 g of carboxymethyl cellulose and 3 g of polyvinyl alcohol were dissolved to form oil droplets with a diameter of 4 to 10 microns, followed by adding 100 g of water to dilute the solution. Then, 40 g of water containing 6 g of an aliphatic polyamine adduct (Epicure U, tradename, Shell Chemical Company) were added. During the above procedure, the temperature of the system was maintained at not higher than 25.degree.C.

In order to promote the curing of the pre-adduct of the polyisocyanate, the system was heated to 70.degree.C, and encapsulation was finished.

As a result, walls of polyurea were formed around the oil droplets containing the color formers dissolved therein to produce microcapsules. The microcapsule solution obtained was coated on a sheet of paper, and dried by heating. Then, a solution containing acid clay as described was coated thereon and dried to form a multi-layer pressure-sensitive record sheet which could be recorded merely by applying pressure thereto. No color fog was observed in the resulting pressure sensitive record sheet. The sheet was white, and developed a blue color when a writing pressure was applied. The color fog density on this sheet was measured using a spectrophotometer. The results are shown in FIG. 1a.

With reference to FIG. 1a, the color fog density at a wavelength corresponding to a blue color at 610 .mu. was as low as 0.05, which means that the record sheet of this invention hardly suffers from color fog.

On the other hand, a pressure-sensitive recording sheet capable of recording on itself which was obtained by the process described below but using microcapsules formed by the complex coacervation of gelatin and gum grabic (used widely in the art of pressure-responsive duplicating sheets) was colored blue over its entire surface, and could not be practically used.

Specifically, 0.7 g of crystal violet lactone and 0.6 g of benzoyl leucomethylene blue were dissolved in 30 g of dipropylnaphthalene. The resulting oily solution was emulsified with vigorous stirring in 30 g of water at 50.degree.C in which 6 g of acid-processed gelatin and 6 g of gum arabic were dissolved, to form oil droplets with a diameter of 4 to 10 microns. The emulsion was diluted by addition of 200 g of water at 50.degree.C, and 80 percent acetic acid was added to adjust the pH of the emulsion to 4.5. At this time, coacervation of gelatin and gum arabic began to take place, and films of gelatin and gum arabic began to form around the oil droplets. The temperature of the system was lowered to 10.degree.C. The complex coacervation was allowed to proceed further, and the filmy gelatin was gelled. In order to harden the gelatin, 2 cc of 37% formalin were added, and to promote the hardening, a 20% aqueous solution of sodium hydroxide was added dropwise to reduce the pH of the system to 11. For thorough hardening, the system was further heated up to 60.degree.C to finish the production of a capsular solution. Microcapsules containing walls of gelatin and gum arabic which contained an oily solution having dissolved therein the color formers were thus obtained.

The microcapsular solution obtained was coated on a sheet of paper, and dried by heating. A solution of acid clay as described above was coated on the same surface on which the microcapsules were applied, and this solution then dried by heating. The record sheet so obtained was colored blue all over the sheet surface immediately after drying, and proved impractical. The color fog density of this sheet was measured using a spectrophotometer, and the results are shown in FIG. 1b. From FIG. 1b, it can be seen the color fog density at a wavelength corresponding to blue (610 .mu.) was 0.54, which was more than 10 times as large as that of the record sheet obtained by the method of this invention (0.05).

Each of the microcapsule solutions obtained above was mixed with the solution of acid clay, and the mixed solution was coated on a base material. When the microcapsules containing the polyurea walls were used, the resulting record sheet did not undergo blue fog, but when the microcapsules containing the gelatin-gum arabic walls were used, the entire surface of the record sheet was colored blue and became impractical to use. Furthermore, when the microcapsule solution containing microcapsules with the polyurea walls was coated on top of a dry layer of a developer which had been applied onto a base sheet, no color fog occurred on the resulting record sheet. When the microcapsule solution containing gelatin-gum arabic walls was coated on the same developer-coated surface, however, the entire surface of the sheet was colored blue, and became impractical to use.

EXAMPLE 2

The microcapsule solution containing microcapsules with the polyurea walls as obtained in Example 1 was coated on a sheet of paper and dried. Then, a solution containing zinc 3,5-di-tert.-butyl salicylate as heretofore described was coated on the same surface on which the microcapsule solution had been coated. The salicylate solution was then dried. The resultant pressure-sensitive record sheet did not undergo color fogging at all; it was white, and developed a blue color only when writing pressure was applied. The color fog density of this record sheet was measured using a spectrophotometer, with the results being shown in FIG. 2a. Referring to FIG. 2a, the color fog density at a wavelength corresponding to blue (600 .mu.) was as low as 0.045, which means that the record sheet of this invention is almost free of color fog. On the other hand, a record sheet obtained in the same manner as above using the microcapsule solution containing gelatin-gum arabic walls obtained in Example 1 was colored blue all over the surface, and proved impractical to use. The color fog density of this record sheet was measured using a spectrophotometer, and the results are shown in FIG. 2b, where the density of maximum absorption at 600 .mu. was as high as 0.66, more than 14 times that of the record sheet of this invention.

When a mixture of each of these capsule solutions with the developer solution was applied to a base material or when the developer solution was first applied to a base material and then each of the capsule solutions was coated on top of the dried developer solution, the color fog of the sheet coated with the microcapsules containing the polyurea walls in accordance with this invention was far smaller than that of the sheet coated with the microcapsules containing ordinary gelatin-gum arabic walls.

EXAMPLE 3

The microcapsule solution containing microcapsules with polyurea walls obtained in Example 1 was coated on a sheet of paper and dried. The developer solution containing the phenol resin as heretofor described was coated on the same surface of the paper to which the microcapsule solution was coated, followed by drying. The resultant pressure-sensitive record sheet was completely free from fog, and was white. It developed a blue color only when writing pressure was applied. The color fog density of this record sheet was measured using a spectrophotometer, and as shown in FIG. 3a, the fog density at a wavelength corresponding to blue (600 .mu.) was as low as 0.03, which shows that the record sheet of this invention hardly undergoes fogging.

On the other hand, a sheet obtained in the same manner as above using the microcapsule solution containing the gelatin-gum arabic walls obtained in Example 1 was colored blue, and proved impractical for use. The color fog density of this record sheet was measured using a spectrophotometer and, as shown in FIG. 3b, the density of the absorption maximum at 600 .mu. was 0.13, more than 4 times that of the record sheet of this invention.

When a mixture of each of these capsule solutions with the developer solution was applied to a base material or when the developer solution was first applied to a base material and then each of these capsule solutions was coated on top of the dried developer solution, the color fog of the sheet coated with the microcapsules containing the polyurea walls in accordance with this invention was far smaller than that of the sheet coated with the microcapsules containing ordinary gelatin-gum arabic walls.

EXAMPLE 4

Crystal violet lactone (0.7 g) and 0.6 g of benzoyl leucomethylene blue as color formers were dissolved in 35 g of trichlorodiphenyl to form an oily solution of color former. To the solution were added 6 g of an isocyanate-containing adduct of tolylene diisocyanate and trimethylpropane (Takenate D-103, tradename, Takeda Chemical Co., Ltd.), and 6 g of acetone were then added in order to dissolve the adduct in toluene. The oily solution thus obtained was added with vigorous stirring to 30 g of water containing 3 g of starch and three drops of Turkey red oil. After forming oil droplets with a diameter of 5 to 10 microns, 150 g of water were added to dilute the emulsion. To the resulting dispersion, 4 g of an aliphatic polyamine adduct (as used in Example 1) and 0.5 g of hexamethylene diamine were added, and the temperature was raised to 80.degree.C to form a polyurethane wall around the oil droplets, whereafter encapsulation was finished. The supernatant liquid was removed by decantation, and excess amines were removed. Then, 100 g of water having dissolved therein 3 g of polyvinyl alcohol were added to serve as a binder. The capsule solution obtained was coated on a series of base sheets and dried, and then each of the developer solutions containing acid clay, the developer solution containing zinc 3,5-di.tert-butylsalicylate and the developer solution containing the phenol resin was coated onto a different base sheet coated with the capsule solution. On the other hand, a mixture of each of these developer solutions with the above capsule solution was applied on a base sheet. Furthermore, each of these developer solutions was first coated on a base sheet, and then the capsule solution was coated thereon. In all cases, the sheet obtained was free from color fog, and very good results were obtained.

EXAMPLE 5

Example 4 was repeated except that 3 g of an isocyanate-containing adduct of xylylene diisocyanata (Takenate D-110 N, tradename, Takeda Chemical Co., Ltd.) and 3 g of hexamethylene diisocyanate were used instead of 6 g of Takenate D-103. Good results similar to those obtained in Example 4 were obtained.

EXAMPLE 6

A microcapsule solution containing microcapsules with polyurea walls containing an oily solution of a color former was prepared by the same procedure as in Example 1 except that 7 g of the following compound: ##SPC2##

(Millionate MR, tradename for a mixture of n=0, 1, 2 and more, Nippon Polyurethane Kogyo Kabushiki Kaisha) were used instead of 6 g of Coronate HL. The resulting capsule solution was coated on a series of sheets of paper, and dried, followed by coating each sheet thus formed with the heretofore described developer solutions containing acid clay, zinc 3,5-di.tert.butyl salicylate and the phenol resin, to yield three separate sheets. Furthermore, a mixture of each of these developer solutions and the microcapsule solution was applied onto its own base sheet. Also, each of the developer solutions was first coated on a base sheet, and then the microcapsular solution was coated thereon. In any case, the resulting sheet was almost free from color fog, and good results were obtained.

EXAMPLE 7

A microcapsule solution containing microcapsules with polyurea walls containing an oily solution of a color former was obtained by the same procedure as in Example 1 except that 3 g of o-phenylene diamine was used instead of 6 g of Epicure U. The microcapsule solution obtained was coated onto three sheets of paper and dried, and then each sheet coated with one of the heretofore described developer solutions containing acid clay, zinc 3,5-di.tert.butyl salicylate and the developer solution containing the phenol resin. Furthermore, a mixture of each of these developer solutions with the microcapsule solution was coated on a sheet of paper, and dried. Also, each of the developer solutions was first coated on a sheet of paper and dried, and then the microcapsule solution was coated thereon. In any case, the resulting sheet was completely free from color fog, and very good results were obtained.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims

What is claimed is:

1. A pressure-sensitive recording sheet which comprises a base material and (1) a layer of a colorless or substantially colorless color-forming substance rendered colored upon reaction with a developer, encapsulated in walls of a polyurea obtained by the reaction of a polyisocyanate or isocyanate-containing polyisocyanate adduct with a polyamine or polyamine adduct, and (2) a layer of developer capable of giving a colored substance by reaction with said color-forming substance, layers (1) and (2) being on the same surface of said base material.

2. The pressure-sensitive recording sheet of claim 1, wherein said isocyanate-containing polyisocyanate adduct is an adduct formed between a polyisocyanate and a compound containing a hydrophilic group.

3. The pressure-sensitive recording sheet of claim 2, wherein said compound containing a hydrophilic group is selected from the group consisting of polyamines, polycarboxylic acids, polyols, polythiols, polyhydroxy compounds and epoxy compounds.

4. The pressure-sensitive recording sheet of claim 1, wherein said polyamine adduct is an adduct formed between a polyamine and an epoxy compound.

5. The pressure-sensitive recording sheet of claim 1, wherein the polyurea is obtained from an isocyanate containing polyisocyanate adduct which has at least two uncombined isocyanate groups.

6. The pressure-sensitive recording sheet of claim 1, wherein the polyurea is obtained from a polyamine adduct which has at least two uncombined amine groups.

7. The pressure-sensitive recording sheet of claim 1, wherein the polyisocyanate is selected from the group consisting of dithioisocyanates, diisocyanates and triisocyanates.

8. The pressure-sensitive recording sheet of claim 1, wherein the polyisocyanate is a polyisocyanate monomer.

9. The pressure-sensitive recording sheet of claim 1, wherein the polyisocyanate per se or polyisocyanate used to form the adduct is selected from the group consisting of m-phenylene diisocyanate, p-phenylene diisocyanate, 2,6-tolylene diisocyanate, 2,4-tolylene diisocyanate, naphthalene-1,4-diisocyanate, diphenylmethane-4,4'-diisocyanate, 3,3'-dimethoxy-4,4'-biphenyl-diisocyanate, 3,3'-dimethyl diphenylmethane-4,4'-diisocyanate, xylylene-1,4-diisocyanate, xylylene-1,3-diisocyanate, 4,4'-diphenylpropane diisocyanate, trimethylene diisocyanata, hexamethylene diisocyanate, propylene-1,2-diisocyanate, butylene-1,2-diisocyanate, ethylidine diisocyanate, cyclohexylene-1,2-diisocyanate, cyclohexylene-1,4-diisocyanate, xylylene-1,4-diisothiocyanate, ethylidinne diisothiocyanate, hexamethylene dithioisocyanate, 4,4',4"-triphenylmethane triisocyanate, toluene-2,4,6-triisocyanate, polymethylene polyphenyl isocyanate, and 4,4'-dimethyldiphenyl methane-2,2',5,5'-tetraisocyanate.

10. The pressure-sensitive recording sheet of claim 1, wherein the polyamine per se or polyamine used to form the adduct is an aromatic amine having at least two free amino groups in the molecule.

11. The pressure-sensitive recording sheet of claim 1, wherein the polyamine per se or polyamine used to form the adduct is an aliphatic amine having at least two free amino groups in the molecule.

12. The pressure-sensitive recording sheet of claim 1, wherein the polyurea is obtained from a polyamine per se or the polyamine used to form the adduct is selected from the group consisting of o-phenylene diamine, p-phenylene diamine, diamino naphthalene, 1,1,3-propylene napthalene or hexymethylene diamine.

13. The pressure-sensitive recording sheet of claim 1, wherein a polyisocyanate is reacted with the polyamine or the polyamine adduct.

14. The pressure-sensitive recording sheet of claim 1, wherein a isocyanate-containing polyisocyanate adduct is reacted with the polyamine or polyamine adduct.

15. The pressure-sensitive recording sheet of claim 1, wherein a polyamine is reacted with the polyisocyanate or the isocyanate-containing polyisocyanate adduct.

16. The pressure-sensitive recording sheet of claim 1, wherein a polyamine adduct is reacted with the polyisocyanate or isocyanate-containing polyisocyanate adduct.

17. The pressure-sensitive recording sheet of claim 3, wherein the compound is a polyamine.

18. The pressure-sensitive recording sheet of claim 3, wherein the compound is a polycarboxylic acid.

19. The pressure-sensitive recording sheet of claim 3, wherein the compound is a polythiol.

20. The pressure-sensitive recording sheet of claim 3, wherein the compound is a polyhydroxy compound.

21. The pressure-sensitive recording sheet of claim 3, wherein the compound is an epoxy compound.

22. The pressure-sensitive recording sheet of claim 3, wherein the compound is a polyol.

23. Pressure sensitive microcapsules containing a colorless color forming substance in an oily liquid encapsulated in walls of a polyurea produced by the reaction of a polyisocyanate or isocyanate-containing polyisocyanate adduct with a polyamine or polyamine adduct comprising:

a. dissolving the polyisocyanate or isocyanate-containing polyisocyanate adduct in the oily liquid having dissolved therein the color forming substance;

b. emulsifying the product of step (a) in a polar liquid;

c. adding to the product of step (b) the polyamine or polyamine adduct; and

d. subjecting the system of step (c) to polymerization conditions, whereby the oily liquid having dissolved therein the color forming substance is encapsulated in the polyurea product formed.