PL114234B1 - Copying material with own pressure-sensitive layer and method of making the same - Google Patents

Description

The invention relates to a method for producing a copying material with its own pressure-sensitive layer and a copying material with its own pressure-sensitive layer. In the widely known pressure-sensitive copying method, commonly known as the transfer method, the lower surface of the upper sheet is coated with microcapsules containing a colorless solution of a color-producing substance, and the upper surface of the lower sheet is coated with a color-developing substance, for example, acid clay or phenolic resin. In most cases, a series of intermediate sheets are also used. The lower surface of each sheet is coated with microcapsules, and the upper surface with a color-developing substance. Pressure exerted on the sheets during handwriting or typing causes the microcapsules to burst, releasing the color-developing solution and allowing it to come into contact with the color-developing substance of the next lower sheet, causing a chemical reaction that develops color from the color-developing substance. This method has the disadvantage of requiring the production of at least two, and usually three, types of coated sheets mentioned above: upper, lower, and intermediate. To overcome this disadvantage, a system of copying material with its own layer was proposed. In this system, microcapsules and a color-developing substance are applied to the same sheet surface, and writing by hand or type on another sheet placed above the sheet thus coated causes the capsules to burst, releasing the color-developing substance, which then reacts with the color-developing substance on the sheet, producing the color. The disadvantage of the self-coated material is that, until now, it has been necessary to produce the sheets in a two-stage coating operation. The first stage involves applying the mixture containing the microcapsules to the sheet, and the second stage involves applying a coating of the color-developing substance to the sheet produced in the first stage. Of course, it would be simpler, and therefore cheaper, to apply the microcapsules and the color-developing substance simultaneously in a single coating operation. Such proposals are given, for example, in U.S. Patent No. 4,431,391. No. 2,730,457. However, premature color development occurs if this operation is performed either by mixing the microcapsule suspension with the dispersed color developing substance before coating or after coating. The causes of premature color development are complex and not fully understood. It is believed that one reason is that, when manufactured, the microcapsule suspension usually contains "some colour-forming solution outside the capsules, which reacts violently with the colour developer present in the system, resulting in colour development. However, it has been found that premature colour development can also occur even if the colour-forming solution is not present outside the capsules, for example, if it is removed before being mixed with the colour developer, which would be prohibitively expensive on an industrial scale. * The reason for premature colour development in this way is not fully understood, and research indicates that it may be due to mutual attraction between the capsules and the particles of the color-develop- ing substance. It is not precisely known how such an attraction causes premature color development. As mentioned above, it has also been found that even if it is possible to obtain an uncolored coating composition, paper coated with this composition may become colored either immediately during drying or within a few days after coating. Again, the cause of premature color development is not completely understood. It has now been found that in the case of a particulate mineral color-develop- ing substance, the symptoms of premature color development described above can essentially be avoided or at least reduced by appropriate chemical treatment of the color-develop- ing substance and possibly also of the microcapsules. The substance used for such treatment will be referred to hereinafter as a protective agent. The subject of the invention is a method for producing a copying material with its own pressure-sensitive layer, comprising the step of coating a sheet material, for example paper, with a mixture which contains both a finely divided mineral for a color-developing substance and microcapsules containing a solution of a colorless color-developing substance which is reactive towards the color-developing substance to produce a colored product, wherein the particles of the color-developing substance, as well as the microcapsules if applicable, are treated with a protective agent in an amount sufficient to inhibit premature color development, which agent is compatible with the type of microcapsules and does not substantially reduce the developing properties of the color-developing substance. The protective agent according to the invention is A self-crosslinking polymer is used, for example a self-crosslinking agent that gives wet strength to paper. Such a polymer is an aminoplast, for example a urea-formaldehyde or melamine-formaldehyde polymer, or an epichlorohydrin polymer, for example amino-epichlorohydrin or an amide-amino-epichlorohydrin copolymer. An anionic surfactant containing a sulfate, for example a lauryl sulfate salt, or a sulfonate surfactant, for example a sulfonated derivative of ricinoleic acid, a dodecylbenzene sulfonic acid salt, or the dibutyl or dioctyl ester of sulfosuccinic acid, can also be used as a protective agent. A fluorine-containing paper sizing agent or a non-self-crosslinking agent giving wet strength to paper, for example a polyethylene imine resin, can also be used as a protective agent. The invention furthermore relates to a copying material with its own pressure-sensitive layer, which is a sheet material coated with both a finely divided mineral color-develop- ing substance and microcapsules containing a solution of a colorless color-forming substance which is reactive with a color-developing substance to produce a colored product, which comprises microcapsules and a color-developing substance disposed in a single superimposed layer, said layer also containing a protective agent in an amount sufficient to inhibit premature color development, which agent is compatible with the type of microcapsules and does not substantially diminish the developing properties of the color-developing substance. It has been found that some substances which can be used with certain types of microcapsules leach out the contents of other types of microcapsules. Therefore, it is important to ensure that the protective agent is compatible with the type of microcapsules used. Examples of mutually exclusive combinations of protective agent and microcapsules are given below. A preferred particulate mineral colour developing substance is an acid-treated clay, for example as described in UK Patent No. 759,800 p. 2 vols. 15-17, and UK Patent Nos. 1147,480, 1,213,835, . 1,232,208, 1,247,909, 1,307,319, 1,323,937, 1,379,254, 1,381,928, 1,383,376, 1,417,206, and 1,524,900. The particulate mineral color-developing substance may be treated with a protective agent by simply mixing a suspension of the substance with the protective agent. The mixture may then be aged, with optional agitation. The protective agent may be a self-crosslinking polymer. In this case, it may be desirable to heat the mixture of polymer and color-developing substance before mixing them with the microcapsules. It is advantageous to treat the color-developing substance and optional microcapsules before mixing them. However, in cases where premature color development does not occur immediately during mixing, it is possible to add a protective agent after mixing and still achieve the desired result. The expression "cross-linking polymer" as used in this specification includes not only polymers having a large number of repeat units, but also so-called prepolymers or precondensates which have only a small number of repeat units. The self-cross-linking polymer may be a resin or precondensate, such as is used in the paper industry to impart wet strength to paper rolls. Resins or precondensates of epichlorohydrin and aminoplasts are preferred. Examples of suitable epichlorohydrins are "Nadavin" FP and "Nadavin" FPN, both of which are polyamine-epichlorohydrin resins, and "Nadavin" LT, which is a polyamide/polyamine-epichlorohydrin resin. All of these are manufactured by Bayer U.K. Ltd. "Nadavin" FP is preferred. Examples of suitable aminoplasts are melamine-formaldehyde resins or precondensates or urea-formaldehyde resins or precondensates. Among the aminoplasts, melamine-formaldehyde substances have been found to be most effective in preventing premature color development. An example of a suitable melamine-formaldehyde resin is Beetle Resin GC 27 sold by British Industrial Plastics Ltd. This resin, as supplied, has a reactive resin content of approximately 100% and a solids content of approximately 93%. Examples of suitable melamine-formaldehyde precondensates are Beetle Resin BC 71, BC 309, and BC 336, all also sold by British Industrial Plastics Ltd. These are methylated modified melamine-formaldehyde precondensates. Precondensate BC 71, as supplied, has a reactive resin content of approximately 80% and a solids content of approximately 93%. BC 309, when delivered, has a reactive resin content of approximately 9 GP/o and a solids content of approximately 80 10/o. BC 336, when delivered, has a reactive resin content of approximately 76% and a solids content of approximately 71%. Other aminoplasts that can be used are urea-formaldehyde precondensates L 5084 and BC 6, both also sold by British Industrial Plastics Ltd. At the time of delivery, the former has a reactive resin content of about 70% and a solids content of about 68%, and the latter has a reactive resin content of about 50% and a solids content of about 45%. Other self-crosslinking polymers that can be used as protective agents are those sold by Hercules Powder Company as Kymene 709 and Kymene 557, and a polymer sold by Diamond Shamrock Corporation as Nopoobond SWS-10. When the protective agent is a self-crosslinking polymer, the method of treating the color-develop- ing agent depends largely on the type of polymer. Selection of appropriate treatment conditions is within the capabilities of a technologist experienced in this field. In the case of acid-treated clay-type color-develop- ing substances and the above-mentioned melamine-formaldehyde substances BC 27 and BC 71, a suitable treatment method is to heat the clay slurry with a 10% resin or precondensate solution (based on the weight ratio of resin to dry acid clay) at 90°C for about 2 hours. In the case of polyaminoepichlorohydrin resins and acid-treated clay-type color-develop- ing substances, a suitable treatment method is to mix the clay with a 10% aqueous resin solution and stir mechanically for a few minutes at a pH of 7-8. Heating is not necessary. In some cases, subjecting It may also be advantageous to treat the capsules with a self-crosslinking polymer. It is convenient to use the same polymer as was used for treating the color-develop- ing substance. Wet-strength agents which are not self-crosslinking polymers may also be used as protective agents, for example a polyethyleneimine resin such as that sold by BASF as Bolymin P. Other substances which may be used as protective agents are anionic surfactants, and among these in particular those containing sulfate or sulfonate groups. An example of an anionic surfactant containing a sulfate group is a salt of lauryl sulfate, such as sodium lauryl sulfate. Sulfonate surfactants include salts of dodecylbenzenesulfonic acid, sulfosuccinic acid dibutyl ester (for example, that sold by BDH Chemicals Ltd. as Manoxol IB), sulfosuccinic acid dioctyl ester (for example, that sold by BDH Chemicals Ltd. as Manoxol GT) or that sold by Rohm and Haas as Triton GR5 (and sulfonated derivatives of ricinoleic acid) for example, that sold by Zschimmor and Schwarz of Lahnstein, West Germany (as Glanzol CFD or Glanzol 100). Glanzol CFD is actually offered for sale as a plasticizer for coating compositions, but it is also a surfactant. An example of an anionic surfactant which can be used as a A protective agent that does not contain a sulfate or sulfonate group is ethylenediaminetetraacetic acid (EDTA). Plasticizers other than Glanzol CFD, which are 50 anionic surfactants, are also effective as protective agents, for example dibutyl phthalate, di-2-ethylhexyl adipate, dibutyl maleate, and tris trimellitate (livenol 7-9). However, these compounds have the disadvantage that their use causes "greasy" stains to form on the coated sheet and are therefore not recommended. Another group of substances that can be used as protective agents are fluorine-containing paper sizing agents, the purpose of which is to impart hydrophobic properties. Examples of such substances are Du Pont's Zonyl BP or Zonyl NF, and Minnesota Mining and Manufacturing Company's 65 Scotchban FC 807. If necessary, a mixture of nine different protective agents can be used. In certain cases, using a mixture offers advantages over using the individual components. Mixtures of fluorine-containing adhesives and Kymene resins, as mentioned above, have been found to have better protective properties than either adhesive or Kymene resin alone. The amount of protective agent to be used depends on the type of capsules and the color-developing agent used. The optimal amount can be readily determined by one skilled in the art. The quantities given in the examples below can be used as a guide. In conventional methods, the microcapsules used in the pressure-sensitive self-layer copying method had to have thicker, i.e., stronger, walls than the microcapsules used in the more widely used transfer system described above. Similarly, in this method, which is, of course, a self-layer method, the capsules would also have to be stronger than those normally used in the pressure-sensitive copying method if an adequate product was to be obtained. The method of producing capsules with strong walls for placement on copying paper with its own pressure-sensitive layer is well known in the art, but will be illustrated in each case. Although it is advantageous to use capsules with stronger walls than those normally used in the pressure-sensitive transfer-type copying method, a slight reduction in premature color development may be observed for capsules used in the transfer-type copying method if the particulate color-developer is treated with a self-crosslinking polymer prior to mixing with the microcapsule suspension during the coating mixture preparation. The resulting product, however, is probably too strongly colored to be used. Capsules for use in the present invention may have synthetic walls, for example of melamine-formaldehyde resin, urea-formaldehyde resin, and a copolymer of acrylic acid amide and acrylic acid as described in British Patent Application No. 48 616/75, or of a urea-formaldehyde polymer as described in KFN Patent Specification No. DOS 2 529 427. Other synthetic materials which may be used include polyacrylates, polyurethanes, polyureas or aminoplasts other than those previously cited. Alternatively, the capsules may have walls of coacervated hydrophilic colloids, for example of a mixture of gelatin, gum arabic or carboxymethyl cellulose (CMC) or a copolymer of polyvinyl methyl ether and maleic acid anhydride (PVM/MA) as described in UK Patent No. 870 476. In order for such capsules to be sufficiently strong for use on self-coated paper, the phase ratio at which they are made should preferably be lower than the phase ratio for capsules intended for use in a pressure-sensitive transfer-type copying method (the phase ratio being the weight ratio of the solution of the colour-forming substance to the substance forming the capsule wall in the aqueous solution from which the capsules are obtained). The use of a lower phase ratio results in thicker capsule walls than a higher phase ratio. If capsules of the transfer method are used, the use of the present invention results in less staining than if the protective agent is not present, however, the degree of staining is probably too great to be acceptable. Capsules having synthetic walls, for example of aminoplasts, are found to be stronger and less permeable than gelatin coacervate capsules. Therefore, they tend to stain less prematurely than gelatin coacervate capsules. However, it has been found so far that protection of synthetic-walled capsules is desirable, although the difference in whiteness between protected and unprotected copying paper may be small. Small differences in whiteness are noticeable to the naked eye and may significantly affect the commercial viability of the paper. Not all of the factors cited above apply. Protective agents can be used with all types of capsules. For example, anionic surfactants wash out the contents of capsules with gelatin coacervate walls, but they can be used with capsules with synthetic aminoplast walls. The skilled person will have no difficulty in selecting a protective agent that is compatible with the capsules they wish to use. Some substances that might be considered suitable for use as protective agents are, in fact, not suitable. Cationic and nonionic surfactants, which would be expected to have the same effect as anionic surfactants, have been found in practice to poison the color-producing substance, i.e., function as a color developer. Secondary retention agents used in papermaking, which in some respects are similar to wet strength agents, could be considered effective, but in practice to date they have been found to cause loculation of the coating composition. There may be methods by which the above difficulties could be overcome. The invention is not limited to the use of specific color-forming substances or their specific solvents. Examples of color-forming substances that can be used are phthalide derivatives such as crystal violet lactone, fluorane derivatives, diphenylamine derivatives, spiropyran derivatives, phthalimidine derivatives, and benzoyl leuco derivatives of various dyes. As is known in the art, mixtures of color-producing substances can be used. Examples of solvents that can be used are mixtures of partially hydrogenated terphenyls, chlorinated paraffins, diphenyl derivatives, diarylmethane derivatives, alkylnaphthalenes, mixtures of dibenzylbenzenes, phthalic or phosphoric esters, or linear alkylbenzenes having 10-14 carbon atoms. The solvents can be used in admixture with diluents such as kerosene. -^ If it is required to treat the capsules, as well as the color-producing substance, with a protective agent, the conditions of this treatment depend on the on the substances used in a particular case. The choice of these conditions is within the capabilities of an experienced technologist. The data contained in some of the examples below can serve as guidance. The coating mixture may also contain a filler to improve its rheological properties and increase the whiteness of the coated sheet. Suitable fillers include kaolin and calcium carbonate. Such fillers, as is well known, can sometimes produce a very slight color-developing effect. This effect may not be serious enough to attract attention, but if necessary, the filler can be treated with a protective agent in a similar way to the color-developing substance. For example, the processing conditions for kaolin would be almost the same as those for clay after acid-induced color-developing treatment. Selection The binder of the coating composition requires special attention. Some binders typically used for pressure-sensitive copying paper may cause color development or may have other undesirable properties and therefore would not be suitable. For example, a starch binder may reduce the reactivity of the color-developer. This may be because it forms a film on the surface of the color-developer particles. Highly hydrolyzed polyvinyl alcohol has been found to be particularly suitable as a binder for the coating composition of the invention. The coating composition may also contain a dispersant, preferably one that also serves to adjust the pH of the composition. Such dispersants for developing compositions used for pressure-sensitive copying paper are well known. Examples are sodium silicate and Sodium hydroxide, which also serve to adjust the pH. The optimal pH value is chosen to be most favorable for the color development reaction. For example, if one of the coloring agents is crystal violet lactone and the developing agent is acid-treated clay, a suitable pH value for the coating composition is about 10.0. The coating composition may also contain an agent to protect the capsules from premature rupture during storage and handling of the paper rolls. The use of such a protective agent (often known as a support material) is well known in the art of copying paper production on the principle of pressure sensitivity and will therefore not be described here. Examples of suitable support materials are cellulose fiber in floc form and starch from wheat granules. Typical dry coating density of papers coated by the process of the invention is 10-15 g/m². Coating techniques used in the production of this paper may be conventional, such as blade coating, air knife or roll coating. Until now, blade coating has not been conventionally used on an industrial scale for compositions containing capsules because this method could not apply an economically low density layer sufficiently uniformly. The present invention facilitates the use of blade coating because the presence of a color-inducing substance in a composition containing microcapsules means that the weight of the coating applied is higher than in a composition containing only microcapsules. The possibility of blade coating brings significant advantages in speed of operation and tonnage of copy paper that can be produced in the plant. The mechanism by which the protective agent inhibits premature color development in the self-coated paper mix is not fully understood. It is believed that this agent works by reducing the mutual attraction between the color-developing substance and the capsules. Figure 1 shows a large-scale schematic cross-section of a sheet of pressure-sensitive copy paper, and Figure 2 is a flow diagram illustrating an example method of producing the paper shown in Figure 1. Figure 1 shows that the self-coated pressure-sensitive copy paper sheet comprises a paper substrate. 1, supporting a coating 2 containing microcapsules 3 and color-developing particles 4. The remaining coating components are not shown to avoid obscuring the overall effect. One of these is a protective agent, but its precise location is unknown. From Figure 2, it can be seen that the ground developer, microcapsule suspension, filler, dispersant, binder, and pH adjusting agent are mixed to produce a coating mixture. Before mixing, the developer and optionally the microcapsules and/or filler are treated with the protective agent. If desired, the filler can be mixed with the developer before this treatment. In Figure 2 The alternative transitions are shown in dashed lines. The mixture is then applied to paper and dried to obtain paper sheets as shown in Fig. 1. The invention is illustrated by the following examples: * Example I. % a) Preparation of capsules "" 382 g of a 9.1% gelatin solution was placed at pH 6.4 in a Waring mixer. With the mixer running at low speed, 274 g of the internal phase, i.e. the substance to be enclosed in the capsules, was added. The internal phase was a 9:1 mixture of Dobane JN (mixtures of linear alkylbenzenes having 10-14 carbon atoms per molecule, sold by SheU) and Santicizer 711 (a phthalate-containing substance sold by Monsanto) containing 1.8% by weight of crystal violet lactone and 1.4% by weight of the benzoyl leuco derivative of methylene blue. Mixing was continued until the droplet size was less than 3 microns. To 218 g of the gelatin solution and the internal phase emulsion was added 58 g of a 17.61% by weight aqueous solution of gum arabic and 422 g of deionized water. The pH was adjusted to 8.7 by adding NaOH and 8 g of a 5% by weight aqueous solution of polyvinyl methyl ether and maleic anhydride (PVM/MA). 14.7% acetic acid was then used to uniformly reduce the pH to 4.3. During this operation, a coacervate separated and coated the internal phase droplets. The emulsion was then cooled to approximately 10°C and 3.3 ml of 50% glutaraldehyde was added to cross-link the coacervate. To prevent agglomeration of the capsules, a further 10 g of PVM/MA was added. 6.0 g of a 12.5% by weight aqueous solution of sodium carbonate was then added as a pH buffer, and the pH was then raised very slowly to 10.0 by adding sodium hydroxide. b) Preparation and application of the coating mixture 30 g of acid-treated clay (Silton M-AB brand, supplied by Mizusawa Industrial Chemicals Limited) developing color was dispersed in sufficient water to obtain a solids content of about 36%. 3 g of melamine-formaldehyde precondensate BC 71 was added as a protective agent. The resulting slurry was heated with stirring to 90°C, kept at this temperature for 2 hours, and then cooled. Then 10 g of kaolin (Dinkie brand, supplied by English China Clays Limited) was dispersed in sufficient water to obtain a solids content of about 36%. 1 g of g of melamine-formaldehyde precondensate as a protective agent, the slurry was heated to 90°C, held at this temperature for 2 hours, and then cooled. The prepared Silton clay and Dinkie kaolin were mixed, and the pH of the mixture was adjusted to 10 by adding NaOH solution. 132 ml of the capsule suspension prepared as described above was then added to the clay slurry, followed by 30 g of a 10% PVOH binder (MOVIOL 56-98 supplied by Harlow Chemical Co. Ltd.). The resulting mixture was applied using laboratory equipment at a rate of 9 g/m² onto a 49 g/m² paper substrate. After drying, the whiteness of the paper was checked. using an opacimeter (for example, the Bausch and Lomb opacimeter described in U.S. Patent No. 1950,975 or the Diano BNL 2 opacimeter). The whiteness test involves measuring the reflectance of a sheet and comparing the result with the reflectance of a standard white surface (a surface coated with magnesium oxide powder). The result is expressed as a percentage as the ratio of (sheet reflectance) x 100 to (reflectance of a standard white surface). Therefore, the higher the numerical result, the whiter the background. The test is carried out at several different points on the sheet, and the results are calculated as an average. A difference of a few percent may seem insignificant, but it is easily visible to the naked eye and can significantly affect the The commercial suitability of the paper was obtained. An average of 93% was obtained. The paper turned out to be white. When a normal sheet of paper was placed on the paper thus obtained and writing was done, clear blue writing appeared on the half-covered sheet. To demonstrate the effect of the melamine-formaldehyde precondensate, a similar experiment was carried out, but neither Silton clay nor Dinkie kaolin had been previously treated with the melamine-formaldehyde precondensate. 30 g of Silton M-AB clay and 10 g of Dinkie kaolin were dispersed in enough water to obtain a solids content of approximately 400/0. The pH of the mixture was adjusted to 10 by adding sodium hydroxide solution. Then 132 ml of the capsule suspension prepared as described above and 30 g of a PVOH binder (Gohsenol NH 26" supplied by Nippon Synthetic Chemical Industry Oo. Ltd., Japan) were added. The resulting mixture was pale blue in color and when applied to paper at a rate of 9 g/m², a pale blue sheet was obtained with an average reflectance of 81%. Example II. This example illustrates the use of various self-crosslinking polymers as a protective agent. The protective agent was obtained by dissolving 3 g of Nadavin FP in 60 g of water. In this solution 30 g of acid-treated clay developing the color of the Silton M-AB brand and 10 g of kaolin of the Dinkie'A brand were dispersed. To prevent flocculation, 2 g of Dispex N 40 (a dispersant supplied by Allied Collids Ltd. as a 1% solution) was added. The pH was slowly adjusted to 10 by adding sodium hydroxide solution, then 132 ml of the capsule suspension prepared as described was added. 132 ml (14 g) was mixed with 0.55 g of BC 71 melamine-formaldehyde resin as a preservative, and the mixture was heated to 70°C and held at this temperature for 1 hour, after which it was allowed to cool. The treated suspension of color-developing clay and the treated suspension containing the capsules were then mixed, and 15 milliliters of 10% polyvinyl alcohol ("Gohsenol NH 26") and, also with stirring, 15 g of kaolin (Dinkie's A brand) were added, followed by stirring in Example 1, followed by stirring in 30 g of 101% PVOH binder (MOVIL 56-98). The color of the mixture thus obtained was slightly pale blue, and when it was coated on paper at a density of 8 g/cm², sheets of pale blue color and a reflectance of 87% were obtained. This is an improvement over the result obtained when no Silton clay was used, which, as found in Example 1, gave a reflectance of 81%. Repeating the procedure, but using a different coloring agent solution, produced a sheet treated with Silton clay that had an average reflectance of 88%. When an ordinary sheet of paper was placed on top of the coated sheets prepared according to the procedures described above and written on in the normal manner, a distinct blue reflection was obtained on both coated sheets. Example III a) Preparation of capsules The following substances were mixed: — 19 g of cationic urea-formaldehyde resin BC 77 with a reactive resin content of about 45% and a solids content of about 35% (BC 77 is supplied by British Industrial Plastics Limited), — 42 g of R 1144 (a 20% solution of a copolymer of acrylic acid amide and acrylic acid with an average molecular weight of 40,000 and an acrylic acid content of 42%, supplied by Allied Colloids Limited), — 200 g of deionized water. The mixture was heated to 55°C and kept at this temperature for 45 minutes, after which 12 g of melamine-formaldehyde precondensate were added. BC 336 hydroxide solution was added, and the pH was lowered to 4.4 by adding acetic acid. 89 g of the color-forming substance solution was then added, and the dispersed mixture thus obtained was ground until the droplets had a size of 4-5 microns, after which 20 g of deionized water was added. The mixture was stirred for one hour at 35°C and for a further two hours at 55°C, after which it was allowed to cool to ambient temperature overnight. The next morning, the pH was raised to 10.0 by the addition of 10% sodium hydroxide solution. The solids content of the resulting suspension containing the capsules was approximately 30% and the phase ratio was 3.4:1. b) Preparation of the coating mixture and its application: 15 g of acid-treated Silton M-AB color-developing clay was dispersed in enough water to achieve a solids content of approximately 40%, and 1.5 g of melamine-formaldehyde precondensate was added as a protective agent. The mixture was heated to 90°C, held at this temperature for 2 hours, and then allowed to cool. The pH was then adjusted to 10 by adding sodium silicate. 20 milliliters of suspension containing capsules (dry weight of capsules 5.5 g/m²) The mixture obtained was then applied to a sheet of paper in the manner described in Examples I and II, with the difference that the density was 12.5 g/m². The whiteness of the sheet was checked, giving an average result of 93.5%. When a sheet of normal paper was placed on the copying sheet thus obtained and written on with normal writing pressure, a distinct blue imprint was obtained on the coated sheet. After 10 days, the whiteness of the sheet was measured again, giving an average value of 88%. Example IV. a) Preparation of capsules The preparation method was as in Example III, except that after adding and grinding the solution of the substance generating To obtain the desired color, 40 g of deionized water and 10 g of BC 336 were added. The next step was as described in Example III. b) Preparation of the coating mixture and its application 20 g of acid-treated color-developing clay, Silton M-AB, was dispersed in enough water to obtain a solids content of about 40%, and then 2.0 g of melamine-formaldehyde precondensate BC 71 was added as a protective agent. The mixture was heated to 90°C, kept at this temperature for 2 hours and then allowed to cool. The pH was then adjusted to 10 by adding sodium silicate. 15 g of kaolin (DinkieW brand) was dispersed in enough water to give a solids content of approximately 50%, and then 1.5 g of melamine-formaldehyde precondensate was added as a protective agent. The mixture was heated to 90°C, held at this temperature for 2 hours, and then allowed to cool. The treated color-developing clay and the treated kaolin slurry were then mixed. 25 milliliters of the capsule suspension (dry capsule weight 5.8 g) was mixed with 0.58 g of melamine-formaldehyde precondensate BC 71, serving as a protective agent, and the mixture was heated to 70°C, held at this temperature for 1 hour and allowed to cool. The treated suspension was then added to the treated clay suspension. 15 milliliters of 10% polyvinyl alcohol ("Gohsenol 26") was added with stirring. The resulting mixture was applied to a sheet of paper as described in the previous examples, except that the density was 14 g/m³. The sheet was tested for whiteness, obtaining an average result of 97%. When a sheet of ordinary paper was placed on the coated sheet and written on in the normal manner, a distinct blue imprint was obtained on the coated sheet. After 10 days, the sheet whiteness was repeated, obtaining an average value of 96%. Example V. In this example, capsules prepared as in Example IV were used, but they were not treated with the melamine-formaldehyde precondensate. Fifteen grams of acid-treated Silton M-AB color-developing clay were dispersed in enough water to obtain a clay content of approximately 40%, and then 1.5 grams of BC 27 melamine-formaldehyde resin was added as a protective agent. The mixture was heated to 90°C, held at this temperature for two hours, and then allowed to cool. The pH was adjusted to 10 by adding sodium silicate. 25 milliliters of a suspension containing capsules (dry weight of capsules 5.5 g) were added, followed by mixing 15 milliliters of 10% polyvinyl alcohol (CGohsenol NH 26") and 15 g of kaolin (Dinkie brand). The resulting mixture was then applied to a sheet of paper in the manner described in the previous examples, except that the coating density was 13.6 g/m2. The whiteness of the sheet was checked, obtaining an average result of 96.5%. When a sheet of ordinary paper was placed on the sheet thus coated and written on in the normal way, a distinct blue imprint was obtained on the coated sheet. After 10 days, the whiteness of the sheet was repeated, obtaining an average value of 96.5%. 96%. Example VI. In this example, 40 capsules prepared as in Example IV were used, but they were not subsequently treated with melamine-formaldehyde precondensate. 30 g of Silton M-AB clay were dispersed in enough water to obtain a solids content of about 40%. The pH was then adjusted to 10 by adding sodium silicate. The resulting mixture was divided into two parts. One of them was treated with melamine-formaldehyde precondensate as a protective agent, as described in Example IV. To both parts of the suspension, 22 ml of the suspension containing the capsules (dry capsule weight about 7 g) was added, and then mixing 15 milliliters of 10% PVOH ("Cohsenol NH 26"). The resulting colorless mixtures were applied to a sheet of paper as described in the previous examples, so that the coating density was 8 g/m2. The color of the sheet coated with the mixture treated with the precondensate was white, and the background reflectance was on average 95%, while for the second mixture a value of this coefficient was obtained of 91%. When sheets of ordinary paper were placed on both coated sheets and written on in the normal way, a clear blue reflection was obtained on both coated sheets, but on the sheet coated with the mixture containing the precondensate the writing was more distinct. After 10 days the reflectance measurements were repeated and an average of 95% was obtained. 92% for the sheet coated with the precondensate-treated mixture, while for the other sheet the value was 85%. Example VII. This example illustrates the use of amino-epichlorohydrin resin as a protective agent instead of melamine-formaldehyde resin or precondensate. The capsules used in this example were prepared as described in Example IV, but were not treated with melamine-formaldehyde precondensate. 15 g of acid-treated Silton M-AB color-developing clay were dispersed in a quantity of water containing 1.5 g of amino-epichlorohydrin resin as a protective agent to obtain a solids content of the mixture approximately 30%. The pH was adjusted to 8 by adding sodium hydroxide and the mixture was stirred at room temperature for a few minutes. The pH was then adjusted to 10 with sodium hydroxide. 22 milliliters of the capsule suspension (dry weight 7 g) were added, followed by 15 milliliters of 10% polyvinyl alcohol ("Gohsenol NH 26") with stirring. The resulting mixture was applied to a sheet of paper at a density of 8 g/m². The sheet was white, and whiteness measurements gave an average background reflectance of 93%. When a sheet of ordinary paper was placed on the coated sheet and written on in the normal manner, a distinct blue reflection was obtained on the coated sheet. After 10 days, the whiteness measurement was repeated, and an average value of 92.1%. It should be noted here that heating of the Nadavin resin is unnecessary, which is contrary to the previously described melamine-formaldehyde resins. Moreover, the possibility of formaldehyde vapors being released is minimal. Example VIII. This example illustrates the production of pressure-sensitive copying paper by the method according to the invention by means of a coating operation in an industrial-scale device.- a) Preparation of capsules The operation was carried out at a phase ratio of 2.7 : 1 as follows: 80.0 kg of hot deionized water was mixed with 16.8 kg of acrylic acid amide-acrylic acid copolymer R 1144 and the temperature was raised to 55°C. Then 7.6 kg of BC 77 urea-formaldehyde resin was added and the mixture was stirred for 40 minutes, maintaining the temperature at 55°C. 8.9 kg of BC 336 melamine-formaldehyde precondensate was added and the pH was adjusted to 4.4 by adding acetic acid. Then 36.4 kg of coloring matter solution was added and the resulting dispersed mixture was ground to a droplet size of 4-5 microns. The pH was adjusted to 4.0 with acetic acid and the mixture was heated to 55°C and maintained at this temperature for 3 hours. The pH was then raised to 10.0 and the mixture was left stirring overnight. The solids content of the resulting emulsion containing the capsules was 29.5%. b) Preparation of the coating mixture and its application: 100 liters of water were placed in a casein kettle and 50 kg of Silton M-AB clay was dispersed in it. Then 5 kg of melamine-formaldehyde precondensate was added as a protective agent and the mixture was kept at 90°C for 2 hours with constant stirring. The mixture was then transferred to a rod mill and the entire procedure was repeated, adding another 50 kg of Silton M-AB clay. 100 kg of Dinkie 'A* brand kaolin was treated in the same way as described for Silton M-AB clay and the resulting mixture was also transferred to the rod mill. 13 kg of 501% sodium silicate was then added and the resulting mixture transferred to a storage vat. 11 kg of PVOH ("Gohsonol NH 26") was added to 136 litres of water in the casein kettle. Water was heated to 90°C to dissolve the PVOH and the solution was transferred to another rod mill. 145 liters (47 kg dry capsule weight) of the capsule suspension prepared as described in (a) above was then placed in the casein kettle. 4.7 kg of melamine-formaldehyde precondensate BC 71 was added as a protective agent, mixed and the mixture was heated to 70°C for 1 hour with constant stirring. The mixture was then added to the PVOH in the rod mill. After homogenization, the resulting mixture was added to the clay slurry in the storage vat. After standing overnight, the mixture was applied to the paper using a coating device with a An air knife was used. A number of different coating densities were used. The results are shown in Table I. When a sheet of ordinary paper was placed over the coated sheet and written on in the normal manner, distinct blue reflections were obtained on the coated sheet. After 10 days, the whiteness measurement of a sample sheet with a coating density of 14.0 g/m² was repeated, and an average value of 94.01°/© was obtained. After 14 months, the whiteness measurement of the same sheet was repeated, and an average value of 93.4°/© was obtained. Example IX. This example illustrates the production of pressure-sensitive copying paper by the method of the invention using an industrial-scale coating apparatus, this time using a different cross-linking polymer than in the previous example. a) Preparation of Capsules The capsules used in this example were prepared as described in Example VIII. b) Preparation of the coating mixture and its application 19 kg of Nadevin FP, serving as a protective agent, and 50 g of Dispex N 40 were dissolved in 209 litres of water placed in a rod mill. In this water were dispersed 75 kg of Silton M-AB clay and 25 kg of Dinkie 'A' kaolin. The pH was adjusted to 10 by the slow addition of 14% sodium hydroxide solution. To this clay slurry was added approximately 178 kg (dry weight of capsules 55 kg) of the capsule suspension prepared as described in a) above. 30 kg of Dow 620 latex binder (dry weight 15 kg) was then added and after homogenization the mixture was transferred to a storage vat. This mixture was applied to the paper using an air knife coating machine. The results are shown in Table II below: T a b 1 i c a II 40 45 Coating density (g/m*) Average background reflectance, % 5 96 4 96 6 97 When a sheet of plain paper was placed on the coated sheet and written on in the normal manner, a distinct blue reflection was obtained on the coated sheet. "" After 12 days, the whiteness measurement was repeated for a sheet with a coverage density of 6 g/m2 and an average value of 96% was obtained. After 10 weeks, the whiteness measurement was repeated. Coverage density ¦fe/m«) Average background reflectance, «/o 17.0 95.9 17.0 36.2 Table 16.7 97.0 I 8.3 96.4 16.7 96.9 14.0 95.3 14.7 * 96.7f V id of the same sheet and an average result of 96% was obtained. Example X. 10 g of Silton M-AB clay and 3 g of Dinkie ! A' kaolin were dispersed in 20 ml of water and a protective agent was added. The pH was adjusted to 10 by slowly adding sodium hydroxide solution. Then 38 ml of a suspension containing capsules with a solids concentration of 17.5% (prepared similarly to Example 1) was added. 10 g of 10% PVOH binder (MOVIL 56-98) was also added. The resulting mixture was applied at a density of approximately 10 g/m² to a 49 g/m² paper substrate. After drying, the background reflectance was measured at various locations on the sheet using an opacimeter as previously described, and the average value was determined. The results, together with the results of a control experiment in which no protective agent was used, are presented in Table III. Table III Protective agent and its amount (relative to the mass of clay) Without protective agent (control) Nadavin FP (10%) Polyamin P (10%) Zonyl BP (10%) Menoxol IB (10%) Kymene 709 (5%) Zonyl RP (0.5%) Background reflectance (°/o) - initial 84.5 87.8 91.3 91.2 89.4 s 88.8 after 10 days 81.8 85.1 90.7 89.7 83.5 84.2 Each of the sheets with the protective coating was placed under a sheet of plain paper, which was then written on. In each case, a distinct blue reflection was obtained on the coated sheet. Example XI. a) Preparation of capsules 42 g of R 1144 copolymer were added to 170 g of deionized water and the mixture was heated to 55°C. To this was added 19 g of BC 77 resin and the mixture was kept at 55°C for 40 minutes. Then 105 g of deionized sodium and 179 g of a coloring matter were added. The resulting dispersed mixture was ground to a droplet size of 4-5 microns and cooled to 15°C. 40 g of BC 336 precondensate and 125 g of deionized water were then added, and the pH was adjusted to 4.15 by adding acetic acid. The temperature was maintained at 15°C for one hour, then raised to 55°C and maintained at this level for a further two hours. Finally, the pH was raised to 8.5 by adding sodium hydroxide solution. b) Preparation of the coating mixture and its application. 22.5 g of Silton M-AB clay and 7.5 g of Dinkie kaolin were dispersed in 40 g of water. 'A* and a protective agent were added. The pH was adjusted to 10 by the slow addition of sodium hydroxide solution. 65 ml of a capsule suspension at approximately 30% solids (prepared as described above) was then added, along with 22.5 g of granulated wheat starch (Keestar-328 supplied by Staley Starch Company) to protect the capsules from premature bursting, as mentioned above, along with 15 g of latex binder (620 Latex supplied by Dow Chemical). The resulting mixture was coated at a density of approximately 10 g/m² on a 49 g/m² paper substrate. After drying (at approximately 105°C for 20 seconds), the reflectance was measured. The background reflectance was measured using an opacimeter at various locations on the sheet and its average value was determined. The results, together with the result of the control experiment in which no protective agent was used, are presented in Table IV. Table IV Protective agent and its amount (relative to the mass of clay) Without protective agent (control) Nadavin FP (10%) Glanzol CFD (10%) Kymene 557 (5%) + Zonyl RP (0.5%) Polyethylene emulsion (10% polyethylene by mass) Background reflectance coefficient (%) initial 93.2 95.0 . 95.1 94.7 95.1 and after 10 days 92.6 93.9 94.1 93.3 93.9 Each sheet with a protective coating was placed under a sheet of plain paper, which was then written on. In each case, a distinct blue imprint was obtained on the coated sheet. Example XII. Example XI was repeated using a series of 80-amino surfactants as the protective agent. For comparison, the experiments using Nadanin FP and GlanzoF were repeated, and a control experiment was carried out in which no protective agent was used. The results are presented in Table V.21 - Table V 114 234 22 Table VI Protective agent and its amount (relative to the mass of clay) Without protective agent (control) Nadavin FP (10%) Glanzol CFD (10%) Sodium dodecylbenzenesulphonate (1010/o) EDTA (10%) Manoxol 1 B (10%) Manoxol 1 T (10%) Sodium lauryl sulphate (10%) 1 1 Triton GR 5 (10%) Nopcote C 104 (10%) Background reflectance (%) initial 93.3 95.3 95.0 95.1 95.9 ^ 95.9 95.1 95.3 96.3 94.7 after 10 days 92.1 94.5 94.5 94.4 94.3 95.3 94.5 94.7 95.9 | 94.1 Each of the sheets with the protective coating was placed under a sheet of plain paper, which was then written on. In each case, a distinct blue imprint was obtained on the coated sheet. Example XIII. 22.5 g of Silton M-AB clay and 7.5 g of Dinkie 'A' kaolin were dispersed in water and the protective agent was added. The pH was adjusted to 10 by slowly adding sodium hydroxide solution. 65 ml of a suspension containing microcapsules at a solids concentration of approximately 30% (prepared as described in Example 11) was then added, as well as 22.5 g of Keestar 328 wheat starch granules and 15 g of Dow 620 latex binder. The resulting mixture was then coated at a density of approximately 10 g/m² onto a 49 g/m² paper substrate. After drying, the background reflectance was measured at various locations on the sheet with an opacimeter as previously described and the average value was determined. The results, along with those of a control experiment in which no protective agent was used, are given in Table VI. Each sheet with a protective coating was placed under a sheet of plain paper, which was then written on. In each case, a distinct blue imprint was obtained on the coated sheet. Example XIV. In this example, capsules produced as described in German patent specification No. 2 529 427 were used. 22.5 g of Silton M--AB clay, 7.5 g of protective agent and its amount (based on the mass of clay) were mixed. Without protective agent (control) Nadavin FP (10%) Nadavin LT (10%) Kymene 70.9 (10%) Kymene 557 (15%) Kymene 557 (5%) + Zonyl RP (0.5%) 1 Glanzol CFD (10%) Kymene 557 (3%) + . Glanzol CFD (3%) + Zonyl RP (0.3%) Nopcobond SWS 10 (10%) Background reflectance (%) initial 95.2 96.3 95.9 95.9 95.9 95.7 96.1 95.9 95.8 after 10 days 94.3 95.8 94.9 95.1 95.2 94.9 95.3 | 95.4 | 95.2 Dinkie W brand kaolin, protective agent and 40 g water; the pH of the mixture was then adjusted to 0.10 by adding sodium hydroxide solution. Then 50 g of capsule suspension at a solids concentration of approximately 40% and 22.5 g of Keestar 328 wheat starch and 0 g of Dow 620 latex binder were added. The mixture was applied at a density of approximately 10 g/m2 to a 49 g/m2 paper support and dried at 105°C for approximately 20 seconds. The results, together with those of a control experiment in which no protective agent was used, are given in Table VII. Table VII. Protective agent and its amount (based on the weight of clay) No protective agent (control) Nadavin FP (10%) Polymin P (10%) 1 Zonyl RP (10%) | Glanzol CFD (10%) Sodium lauryl sulphate (10%) Teepol 610 (10%) | Teepol GD 53 (10%) Background reflectance (%) initial 76.4 90.0 84.8 85.7 85.7 87.0 86.4 85.8 after 10 days 64.3 85.9 ai.8- 79.5 79.9 1 72.3 78.0 | 73.0 | 23 114 234 24 Teepol 610 is an anionic surfactant supplied by Shell. It is the sodium salt of a secondary alkyl sulfate. Teepol GD 53 is also a surfactant sold by Shell. It is a mixture of sodium alkylbenzene sulfonate, sodium ethoxy sulfate, and nonionic alcohol ethoxylate. Each sheet with a protective coating was placed under a sheet of plain paper, which was then written on. In each case, a distinct blue imprint was obtained on the coated sheet. Example 15- The procedure of Example 14 was repeated, except that instead of the suspension containing one capsule used in Example 14, 65 ml of the suspension prepared as described in Example 11 was used. The results are given in Table VIII. Table VIII Protective agent and its amount (based on the mass of clay) 1 No protective agent (control) [ Nadavin FP (10%) | Glanzol CFD (10%) | Teepol 610 (10%) V Tergitol 7 (10%) Background reflectance (%) initial 94.9 95.4 96.1 96.2 96.5 after 10 days 94.3 95.0 95.6 95.5 95.8 Tergitol 7 is an anionic surfactant supplied by BDH Chemicals Limited. This is the sodium salt of a secondary alkyl sulfate containing 17 carbon atoms. Each sheet of the protectively treated coating was placed under a sheet of ordinary paper, which was then written on. In each case, a distinct blue imprint was obtained on the coated sheet. Example 16. This example illustrates the manufacture of paper according to the invention using a commercial blade-type coating apparatus. The capsules used herein were prepared generally as described in Example 11, except that the quantities used were increased to account for scale-up, and the mixture, after cooling to 15°C, was held overnight at 55°C. 22.5 kg of Nadavin FP and 296 liters of water were charged to a mixer, and 225 kg of Silton M-AB clay were dispersed in the resulting mixture. The pH was then raised to 8 by adding 13.7 g of 47% sodium hydroxide solution. Next, 500 kg (139 kg dry weight) of the capsule suspension was added. Dispersed in this mixture were 225 kg of Keestar 328 wheat starch and 75 kg of Dinkie 'A' kaolin, followed by 150 kg of Dow 620 latex binder (75 kg dry weight). The resulting mixture was applied at a density of approximately 14 g/m² onto a 47 g/m² paper substrate. The background reflectance of the coated sheet was 96.1% immediately after coating, 96.0% after 10 days, and 94.5% after 7 months. When a sheet of ordinary paper was placed on the coated sheet and written on in the normal manner, a distinct blue reflection was obtained on the coated sheet. Example 17. This example illustrates the application of the method of the invention to carbon-type capsules instead of self-layer capsules. As previously stated, the use of this type of capsule generally does not produce sufficiently bright sheets. However, the results show that the process of the invention gives increased resistance to premature color development for any capsule wall thickness used. The capsules used in this example were prepared similarly to the process described in Example I of British Patent No. 1,053,935, except that carboxymethyl cellulose was used instead of gum arabic. The suspension containing the capsules was divided into three parts a), b) and c). 10 - Part a) The capsule suspension was treated with a 13% solution of melamine-formaldehyde precondensate BC 71, serving as a protective agent. The operation was carried out at 60°C for 1/2 hour, after which the mixture was allowed to cool. 40 g of Silton M-AB clay were dispersed in enough water to obtain a 40% solids content in the mixture. The pH was adjusted to 10 by adding sodium silicate. Then, 100 ml of the capsule suspension were added with constant stirring, and the resulting mixture was applied to a sheet of paper. The resulting sheet was dark blue in color and had a reflectance of 19%. Part b) The suspension containing the capsules was treated as described for part a). 40 g of Silton M-AB clay was dispersed in enough water to bring the solids content of the mixture to 40%. 4 g of melamine-formaldehyde precondensate BC 71 was then added as a protective agent, the mixture was heated to boiling point and allowed to cool, after which the pH was adjusted to 10 by adding sodium silicate. 100 ml of the suspension containing the capsules was then added with constant stirring, and the resulting mixture was applied to a sheet of paper in the same manner as described above. The sheet was also blue in color. The reflectance was 30%, significantly higher than the 19% found when the clay was untreated. In this case, the suspension contained capsules which had not been treated before use. 4 g of Nadavin FP resin was dissolved in 60 g of water as a protective agent. 40 g of Silton M-AB clay was dispersed in this solution, giving a solids content of approximately 40%. The pH was adjusted with sodium hydroxide solution by stirring for a few minutes. 100 ml of the suspension containing the capsules was then added with constant stirring and the resulting mixture was applied to a sheet of paper as described above. The sheet was blue again. The reflectance was 3910/o, which is again significantly higher than the 19% found in the absence of clay treatment. Patent Claims 1. A method for producing a copying material with its own pressure-sensitive layer, comprising the step of coating sheets, for example sheets of paper, with a coating mixture which comprises both a particulate mineral colour-forming substance and microcapsules containing a solution of a colourless colour-forming substance which is reactive towards the colour-forming substance to produce a coloured product, characterised in that the particles of the colour-forming substance, as well as the optional microcapsules, are treated with a protective agent in an amount sufficient to inhibit premature colour development, which agent is compatible with with the type of microcapsules and does not substantially reduce the developing properties of the color developing substance. 2. A method according to claim 1, characterized in that the protective agent is a self-crosslinking polymer, for example a self-crosslinking wet strength agent for paper. 3. A method according to claim 2, characterized in that the crosslinking polymer is an aminoplast, for example a urea-formaldehyde or melamine-formaldehyde polymer. 4. A method according to claim 2, characterized in that the self-crosslinking polymer is an epichlorohydrin polymer, for example an aminoepichlorohydrin polymer or an amide-aminoepichlorohydrin copolymer. 5. A method according to claim 6. A method as claimed in claim 4, wherein the protective agent is an anionic surfactant. 7. A method as claimed in claim 5, wherein the protective agent is a sulfate-containing surfactant, for example a lauryl sulfate salt, or a sulfonate surfactant, for example a sulfonated derivative of ricinoleic acid, a dodecylbenzenesulfonic acid salt, or the dibutyl or dioctyl ester of sulfosuccinic acid. 8. A method as claimed in claim 1, wherein the protective agent is a fluorine-containing paper sizing agent. 9. The method according to claim 1, characterized in that the protective agent is a non-self-crosslinking agent that imparts wet strength to the paper, for example a polyethylene-imine resin. 10. The method according to claim 1, characterized in that the protective agent is a material that reduces the mutual attraction between the microcapsules and the particles of the color-develop- ing substance. 10. A copying material with its own pressure-sensitive layer, which comprises a sheet material coated with both a particulate mineral color-developing substance and microcapsules containing a solution of a colorless color-developing substance, which substance is reactive with the color-developing substance to produce a colored product, characterized in that the microcapsules and the color-developing substance are disposed in a single superimposed layer, said layer also containing a protective agent in an amount sufficient to inhibit premature color development, said agent being compatible with the type of microcapsules and not substantially reducing the developing properties of the color-developing substance. 11. 12. A material as claimed in claim 11, wherein the self-crosslinking polymer is an aminoplast, for example a urea-formaldehyde or melamine-formaldehyde polymer, as a protective agent. 13. A material as claimed in claim 11, wherein the self-crosslinking polymer is an epichlorohydrin polymer, for example an aminoepichlorohydrin polymer or an amide-aminoepichlorohydrin copolymer. 14. A material as claimed in claim 10, wherein the self-crosslinking polymer is an anionic surfactant. 15. A material as claimed in claim 14, wherein the surfactant is a sulfate-containing surfactant, for example a lauryl sulfate salt, or a sulfonate surfactant, for example a sulfonated ricinoleic acid derivative, a dodecylbenzene sulfonic acid salt, or the dibutyl or dioctyl ester of sulfosuccinic acid. 16. A material as claimed in claim 10, wherein the protective agent is a fluorine-containing paper sizing agent. 18. The material according to claim 10, characterized in that it contains as a protective agent a non-self-crosslinking agent which imparts wet strength to the paper, for example a polyethylene-imine resin. 18. The material according to claim 10, characterized in that it contains as a protective agent a substance which influences the reduction of mutual attraction between the microcapsules and the particles of the color-developing substance. xK FIG. 1. s* [, a . 4 \ , o. . o- '0 . °. . o 'ol v «,. TZZZZZZZZZZl FIG. 2. PROTECTIVE FACTOR, PARTICULAR COLOR DEVELOPER, FILLER, BINDER. SPREADER, REGULATING FACTOR, WHICH MIXES AND FORM THE COATING MIXTURE. COATING IN ONE OPERATION. SUSPENSION AND MICROCAPSULATES | DRYING | COPY PAPER WITH ITS OWN PRESSURE-SENSITIVE LAYER. Price: PLN 100. PL PL PL PL PL PL PL PL PL PL PL