EP0697293A1 - New organic solvent for microcapsules useful notably for pressure-sensitive copy paper and pressure-sensitive copy coated with such microcapsules - Google Patents

Abstract

Microcapsule useful for pressure sensitive copying paper contains an organic soln. of a hydrophobic chromogen. The microcapsule is characterised in that the solvent is a mixt. obtd. by transesterification of a vegetable oil. Also claimed is the pressure sensitive paper coated on one side with the microcapsule.

Description

The present invention relates to microcapsules useful in particular for making carbonless pressure-sensitive paper, containing an organic solution of a hydrophobic dye, the solvent of which is at least partly of vegetable origin.

The invention also relates to pressure-sensitive paper coated on one side with a layer of such microcapsules and to the bundle of pressure-sensitive paper comprising at least one paper according to the invention.

• une feuille émettrice appelée CB, obtenue par enduction au verso de microcapsules contenant une solution d'agents chromogènes hydrophobes,
• une feuille réceptrice appelée CF, obtenue par enduction au recto d'une couche absorbante et réactive vis-à-vis des agents chromogènes,
• une ou plusieurs feuilles intermédiaires comprenant à la fois les microcapsules au verso et la couche réceptrice au recto que l'on appelle CFB.

A bundle of carbonless pressure sensitive paper includes:

• an emitting sheet called CB, obtained by coating on the back of microcapsules containing a solution of hydrophobic chromogenic agents,
• a receiving sheet called CF, obtained by coating the front with an absorbent and reactive layer vis-à-vis chromogenic agents,
• one or more intermediate sheets comprising both the microcapsules on the back and the receiving layer on the front called CFB.

There are also sheets, called "autonomous", obtained by coating the front of a mixture of microcapsules and receiving layer.

The invention relates generally to CB, CFB and autonomous sheets. The operating principle of carbonless pressure-sensitive paper consists in bursting the microcapsules under the pressure of a pen or under the shock caused by a typewriter strike or by the needles of a dot-matrix printer or more generally by all the so-called "impact" printing processes. The internal phase contained in the microcapsules thus released flows on the CF receiving layer and the chromogenic agents react with the developer to form the colored image.

• . être incolore et inodore,
• . présenter un bon pouvoir solvant vis-à-vis des agents chromogènes,
• . être inerte chimiquement vis-à-vis des agents chromogènes et vis-à-vis des matériaux utilisés pour former les parois des microcapsules,
• . avoir un point d'ébullition élevé et une très faible tension de vapeur à température ambiante,
• . être liquide à la température d'utilisation du papier autocopiant (jusqu'à -10°C voir - 20°C en hiver),
• . présenter la viscosité la plus faible possible afin de faciliter l'écoulement de la phase interne lors de la rupture des capsules,
• . avoir une bonne stabilité à la chaleur et à la lumière,
• . permettre un développement rapide et intense des agents chromogènes sur le réactif du feuillet CF,
• . donner une bonne solidité à la lumière de l'image obtenue sur le feuillet CF,
• . être pratiquement insoluble dans l'eau pour pouvoir être encapsulé,
  • être sans danger pour le corps humain et pour l'environnement,
  • présenter une biodégradabilité élevée,
  • avoir un prix faible compatible avec les prix actuels du papier autocopiant chimique.

The solvent used to prepare the internal phase plays a key role in the final quality of the product. It must meet the following requirements:

• . be colorless and odorless,
• . have good solvent power with regard to chromogenic agents,
• . be chemically inert vis-à-vis the chromogenic agents and vis-à-vis the materials used to form the walls of the microcapsules,
• . have a high boiling point and a very low vapor pressure at room temperature,
• . be liquid at the temperature of use of carbonless paper (down to -10 ° C see - 20 ° C in winter),
• . have the lowest possible viscosity in order to facilitate the flow of the internal phase during the rupture of the capsules,
• . have good heat and light stability,
• . allow rapid and intense development of chromogenic agents on the reagent of the CF sheet,
• . give good light fastness to the image obtained on the CF sheet,
• . be practically insoluble in water so that it can be encapsulated,
  • be safe for the human body and the environment,
  • exhibit high biodegradability,
  • have a low price compatible with current prices for carbonless carbon paper.

Currently, among the solvents commonly used, mention may be made of hydrogenated terphenyls, alkylnaphthalenes, alkylbiphenyls, diarylmethane derivatives, dibenzylbenzene derivatives, chlorinated paraffins, etc.

These solvents are most often mixed with diluents such as kerosene, light mineral oils or alkylbenzenes (for example dodecylbenzene) etc.

All these products fairly well meet all the requirements set out above except the last concerning biodegradability and non-toxicity vis-à-vis the environment hence a risk of pollution during the use and destruction of paper chemical carbonless.

To overcome these shortcomings, consideration has been given to using solvents of natural origin such as vegetable oils (soya, rapeseed, olive, peanut, palm kernel, corn, sunflower, copra, sesame, castor, palm, babassu, jojoba, etc. ) in particular in American patents No. 2,712,507, 2,730,457, 3,016,308, 4,783,196, 4,923,641 and European patent applications No. 86,636, 155,593, 262,569.

• 1) une émulsification, de la phase interne dans l'eau, très difficile avec en final une courbe granulométrique très étalée (présence de particules fines ≦ 1 µm et de particules grossières de diamètre supérieur à 8 µm).
  Ce défaut se traduit en pratique par une perte de rendement en duplication liée aux petites capsules et un papier trop sensible au stockage en pile et aux différentes manipulations à cause des grosses capsules (bleuissement ou noircissement prématuré du papier avant utilisation) et en particulier lors du passage sur les machines d'impression,
• 2) une viscosité trop élevée (mauvais écoulement de la phase interne d'où une perte de rendement),
• 3) une révélation insuffisante des agents chromogènes en présence d'huile végétale (duplication d'intensité faible), notamment pour les dérivés de fluorane ou de phtalide.
• 4) des tenues au vieillissement insuffisantes des papiers CB, CFB et autonomes en particulier en atmosphère humide et chaude (coloration bleue ou noire des papiers avant utilisation),
• 5) une tenue insuffisante à la lumière de l'image colorée sur CF,
• 6) une biodégradabilité souvent insuffisante.

None of these vegetable oils gives satisfactory results, because in all cases we observe:

• 1) an emulsification, of the internal phase in water, very difficult with in the end a very spread particle size curve (presence of fine particles ≦ 1 μm and coarse particles of diameter greater than 8 μm).
  This defect is reflected in practice by a loss of duplication yield linked to small capsules and a paper that is too sensitive to stack storage and to various manipulations because of large capsules (premature blueing or blackening of the paper before use) and in particular during passage on printing machines,
• 2) too high a viscosity (poor flow of the internal phase resulting in a loss of yield),
• 3) insufficient revelation of the chromogenic agents in the presence of vegetable oil (low intensity duplication), in particular for fluorane or phthalide derivatives.
• 4) insufficient aging resistance of CB, CFB and autonomous papers in particular in a humid and hot atmosphere (blue or black coloring of the papers before use),
• 5) insufficient resistance to light in the colored image on CF,
• 6) often insufficient biodegradability.

Various improvements have been proposed to try to use vegetable oils while minimizing their drawbacks, in particular European patent n ° 520 639 recommends mixtures of vegetable oils with mono- or di-esters of fatty acids. This solution is insufficient for points 1), 2) and 3) and brings little improvement on points 4) and 5).

It has also been proposed in patent EP 593 192 to use a functional saturated or unsaturated mono-, di- or tri-ester as solvent. This achievement is of a very high cost on the industrial level.

The microcapsules according to the invention make it possible to overcome the drawbacks mentioned above and are characterized in that the solvent for the hydrophobic chromogenic agent comprises an ester obtained by transesterification of a vegetable oil.

Among the microcapsules, advantageously, but not exclusively, those formed from a wall of crosslinked gelatin.

By "ester" is generally meant the mixture obtained by transesterification, comprising at least 95% fatty acid esters of natural origin.

Transesterification is the chemical operation which consists in exchanging glycerol in an acidic or basic medium with a monoalcohol, generally with a short chain, which leads to the formation of an ester, then eliminating the glycerol.

It has been found that this chemical transformation of vegetable oils, although not eliminating the impurities present in the original oils, and in no way modifying the fatty acid composition of these same oils, makes it possible to achieve performance levels. synthetic esters as described in patent EP 593,192.

This is all the more interesting since the manufacturing cost of such a product is much lower than that of synthetic esters.

This is particularly true for the mixtures commonly called "diester", developed to be used as fuels, and therefore available in large quantities at a competitive price.

It has been found that this chemical transformation of vegetable oils made it possible to obtain microspheres and subsequently carbonless pressure-sensitive papers having appreciably improved properties compared to those comprising microcapsules as described in the prior art at an acceptable industrial cost. .

The solvent must contain sufficient ester obtained by transesterification of a vegetable oil to meet the conditions set out above.

Among the starting vegetable oils which are suitable in the case of the present invention, there may be mentioned by way of nonlimiting indication, palm kernel, rapeseed, soybean, flax, copra, palm, corn, sunflower oils, olive and peanut. It is also possible to use the triglycerides called from the English term "tall oil" comprising a high proportion of glycerol trioleate.

Of course, the abovementioned oils can be used alone or as a mixture.

As indicated above, it is desirable for the alcohol part of the ester resulting from the transesterification to be a lower linear or branched alcohol residue, that is to say of which the carbon number ranges from C₁ to C₁₀.

Preferably, the alcohol residue is a C en to C₈ residue.

Among the alcohol residues, mention may be made of the residues of the following alcohols: methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, t-butanol, 2-ethylhexanol.

Mention may be made, among the chromogenic agents used, of phthalic type derivatives such as 3,3-bis (4-dimethylaminophenyl) -6-dimethylamino-phthalide (CVL) and 3,3-bis (1-octyl- 2-methylindole-3-yl) phthalide or fluorane derivatives such as 2-anilino-3 methyl-6 dialkylamino -2 '- (N'-eryth-N-phenylamino-4'-methylfluorane) or 2'-anilino-3' methyl-6 diethylamino fluorane, 6'-dimethylamino-2 '- (N-ethyl-N-phenylamino-4'-methylfluorane), 3'-chloro-6'-cyclohexylaminofluorane or 3,7-bis (dimethylamino) -10-benzoylphenotiazine (BLMB ) and bisarylcarbazolylmethane compounds. This list is non-limiting and can be extended to all the chromogenic substances commonly used in the art considered.

The solution of chromogenic agent in the solvent is around 5% by weight.

According to a preferred variant, the solvent comprises at least 30% by weight of ester obtained by transesterification of a vegetable oil.

In addition to said esters, the solvent may contain mineral oils.

Among the mineral oils which it is possible to use in combination with the esters, mention may be made, for example, of kerosene, paraffinic or naphthenic oil.

Preferably light naphthenic or paraffinic oils are used.

The alcohol part of these esters is most of the time chosen from the alcohol residues already mentioned above for the esters obtained by transesterification of a vegetable oil.

• mélange obtenu par transestérification d'une huile végétale   30 à 100 %
• huile minérale   0 à 70 %

According to a preferred embodiment, the microcapsule is characterized in that the solvent comprises in percentage by weight:

• mixture obtained by transesterification of a vegetable oil 30 to 100%
• mineral oil 0 to 70%

• mélange obtenu par transestérification d'une huile végétale   30 à 50%
• huile minérale   50 à 70%.

More preferably, the solvent comprises in percentage by weight:

• mixture obtained by transesterification of a vegetable oil 30 to 50%
• mineral oil 50 to 70%.

According to an advantageous embodiment, mineral oils are preferred.

The invention also relates to a process for the preparation of such microcapsules.

According to a general method, an emulsion of a hydrophobic phase is formed consisting of an organic solution as described previously of a chromogenic substance, in a basic aqueous phase comprising several colloids including gelatin and one or more other anionic colloids among which mention may be made of carboxymethylcellulose (CMC) and a maleic anhydride copolymer such as a copolymer of l 'ethyl ethervinyl and maleic anhydride (PVMMA) or a copolymer of ethylene and maleic anhydride (EMA).

The temperature is then raised and the emulsion is coacerved by adding an appropriate acid, in particular acetic acid, in order to adjust the pH to around 4. Liquid-walled microcapsules are thus formed by formation of the coacervate around the droplets of emulsified oil. Cooling the mixture to about 10 ° C causes the walls of liquid coacervate to solidify.

A curing agent such as formalin or glutaraldehyde is then added in order to crosslink said solid walls of coacervate and to obtain the desired microcapsules.

It is then possible to add binders and other conventional ingredients suitable for the suspension of microcapsules to obtain a coating composition for pressure-sensitive paper.

• formation d'une suspension dans une phase aqueuse acide de particules constituées de gouttelettes de la solution organique décrite précédemment, lesdites gouttelettes étant revêtues d'un coacervat formé de gélatine, d'un premier colloïde anionique constitué par l'acide polyacrylique et d'un second colloïde anionique constitué par de la carboxyméthylcellulose, et lesdites particules ayant un diamètre de 1 à 12 µm environ, de préférence de 3 à 8 µm.
• refroidissement pour solidifier la paroi,
• réticulation de la gélatine pour fixer la structure et rendre le processus irréversible,
• réchauffement et neutralisation.

According to a particular process described in application FR 94 03 838 filed on March 31, 1994, the content of which is incorporated by reference, the following steps are carried out:

• formation of a suspension in an acidic aqueous phase of particles consisting of droplets of the organic solution described above, said droplets being coated with a coacervate formed of gelatin, a first anionic colloid consisting of polyacrylic acid and a second anionic colloid consisting of carboxymethylcellulose, and said particles having a diameter of from 1 to 12 μm approximately, preferably from 3 to 8 μm.
• cooling to solidify the wall,
• crosslinking of gelatin to fix the structure and make the process irreversible,
• warming and neutralization.

It was noted that with the organic solutions of chromogenic agents according to the invention, the emulsions obtained were more stable and had a narrower particle size distribution than the emulsions prepared with solvents based on virgin oil.

The emulsion obtained is of the oil in water type and comprises droplets of the hydrophobic phase whose diameter is between 1 and 12 micrometers (preferably 3 to 8 micrometers).

Of course, other methods can also be used without departing from the scope of the present invention.

• Un procédé à base de gélatine par coacervation complexe tel que celui décrit dans l'exemple 1 ou dans les brevets US 4 402 856, FR 2 458 313, EP 339866,
• un procédé à base de résines mélamine-formol comme ceux décrits dans les brevets US 4 406 816, 4 444 699, 4 898 696, ou EP 319 337 et 444559,
• un procédé à base de polyurées ou polyuréthanes comme ceux décrits dans les brevets FR 2 591 124 ou US 4 668 580, 4 785 048, 4 898 780 et 5 075 279,
• ou tout autre procédé d'encapsulation connu pour la fabrication de microcapsules servant à la fabrication du papier autocopiant chimique.

Among these other processes, there may be mentioned:

• A method based on gelatin by complex coacervation such as that described in Example 1 or in US Patents 4,402,856, FR 2,458,313, EP 339866,
• a process based on melamine-formaldehyde resins such as those described in US Patents 4,406,816, 4,444,699, 4,898,696, or EP 319,337 and 444,559,
• a process based on polyureas or polyurethanes such as those described in patents FR 2 591 124 or US 4 668 580, 4 785 048, 4 898 780 and 5 075 279,
• or any other known encapsulation process for the manufacture of microcapsules used for the manufacture of chemical carbonless paper.

The microcapsules obtained are mixed with starch binders or latexes, a spacer, generally calibrated wheat starch and various additives such as optical brightener, water retention etc.

They are then laid down on a paper support.

The subject of the invention is also a pressure-sensitive paper coated on one side with a layer of microcapsules as described above.

This paper support constituting the transmitting sheet, called CB, with a grammage generally between 40 to 90 g / m 2 was coated by coating the suspension of microcapsules, then these microcapsules were dried to obtain the paper according to the invention.

The method of coating and the formulation of the coating bath are not critical to the invention.

The invention also relates to a bundle of pressure-sensitive paper comprising, as has been described in the preamble to the present description, a transmitting sheet and a receiving sheet, and optionally one or more intermediate sheets (CFB) and also so-called autonomous sheets obtained by coating the front of a mixture of microcapsules and receiving layers.

The CF receiving paper associated with the CB sheet is preferably of the "activated clay" type as described in French patents 2,581,350 or US 4,422,670, but it is also possible to use a phenolic type CF such as those described in US Patents 4,559,242 or 4,769,305, or a CF of the zinc salycilate type.

The use of the solvent according to the invention makes it possible to improve, in addition to the viscosity and the quality of the emulsion, the degree of encapsulation in a completely surprising manner but also the intensity and the light fastness of the 'writing.

In addition, the accelerated aging tests of the CB sheets, either in dry heat or in wet heat are improved.

The invention is illustrated by the following examples:

Example 1

In 1.2 l of deionized water at 40 ° C., 55 g of gelatin with a bloom value of about 160 are introduced with stirring. The mixture is heated between 40 and 50 C until the gelatin is completely dissolved and then added 18 g of a 50% solution of sodium polyacrylate (molar mass ≅ 1800) in a drop of soda.

• 500 g d'ester méthylique de colza fabriqué par la société Novamont (MATROL BI 141), obtenu par transestérification,
• 345 g d'huile naphténique légère,
• 21 g de noir ODB2
• 9 g de noir S205 (Yamada)
• 3,8 g de bleu CVL
• 3,5 g de bleu Pergascript SRB (Ciba Geigy)
• 4,2 g d'orange Pergascript I5R (Ciba Geigy).

In parallel, a hydrophobic organic phase is prepared, by heating between 100 and 130 ° C for one hour, of the following mixture:

• 500 g of rapeseed methyl ester manufactured by the company Novamont (MATROL BI 141), obtained by transesterification,
• 345 g of light naphthenic oil,
• 21 g ODB2 black
• 9 g of black S205 (Yamada)
• 3.8 g CVL blue
• 3.5 g blue Pergascript SRB (Ciba Geigy)
• 4.2 g of orange Pergascript I5R (Ciba Geigy).

The organic phase and the aqueous phase are mixed with stirring and emulsified using an Ultra-TURRAX type apparatus until particles having an average diameter between 5 and 6 μm are obtained (the diameter is measured using a Coulter LS 100 laser granulometer).

Then the emulsion obtained above and a solution of 17 g of carboxymethylcellulose in 625 ml of deionized water are mixed in a thermostatic reactor, equipped with a stirrer. Carboxymethylcellulose has a degree of substitution of the order of 0.8 and a viscosity in 3% aqueous solution at 20 ° C of between 60 and 100 mPas measured using a Haake VT 181 viscometer with MVI coaxial cylinders at 180 rpm.

The temperature is raised to 60 ° C and acetic acid is added over 30 minutes to adjust the pH to 4.3.

The coacervate is cooled to 8 ° C. and this temperature is maintained for 10 hours.

• 1ère étape : addition de 17 g de glutaraldéhyde à 50 % sous forte agitation,
• 2ème étape : addition 4 heures plus tard de 66 g d'une solution à 26 % d'alun de chrome. On maintient la température à 8°C pendant 16 heures ainsi qu'une bonne agitation avant de remonter le pH à 7 à la température de 20°C.

The hardening of the walls is carried out in two stages:

• 1st stage : addition of 17 g of 50% glutaraldehyde with vigorous stirring,
• 2nd stage : addition 4 hours later of 66 g of a 26% solution of chromium alum. The temperature is maintained at 8 ° C for 16 hours and good agitation before raising the pH to 7 at the temperature of 20 ° C.

Example 2

Example 1 is reproduced by replacing the rapeseed methyl ester obtained by transesterification produced by Novamont with the same type of product produced by the company Robbe under the brand Estorob or Lubrirob 926.

Example 3

Example 2 is repeated but with the methyl ester of rapeseed produced by the company Henkel.

Example 4

Example 1 is reproduced by replacing the rapeseed methyl ester with an isopropyl soy ester obtained by transesterification manufactured by the company Stéarinerie Dubois et Fils.

Example 5

Example 4 is reproduced by replacing the soybean oil with a "tall oil" (product manufactured by the company Stéarinerie Dubois et Fils) from wood.

Example 6

Example 1 is repeated, replacing the solvent mixture with 400 g of Novamont rapeseed methyl ester + 445 g of purified isopropyl palmitate (Kessco IPP brand from Akzo).

Example 7

Example 1 is repeated, replacing the solvent mixture with 400 g of Novamont rapeseed methyl ester + 445 g of purified methyl stearate (brand Edenol W750 from Henkel).

Example 8 (comparative)

Example 1 is reproduced by replacing the rapeseed methyl ester with refined virgin rapeseed oil (produced by the company CEREOL).

Results

• . Le contrôle de la granulométrie est effectué à l'aide d'un granulomètre à laser (Coulter LS 100). On détermine le diamètre moyen et la dispersion en taille caractérisée par la variance.
• . La viscosité est mesurée à l'aide d'un viscosimètre HAAKE VT 181 avec mobile MV1,
• . Le taux de matières sèches à l'aide d'un dessicateur à infrarouge (la phase interne est comptée comme matière sèche),
• . Le taux d'encapsulation est mesuré par couchage des microcapsules sur un papier CF puis écrasement sous 500 bars de pression sur la moitié de la feuille. On mesure ensuite la différence de densité optique entre la partie écrasée et la partie non écrasée à l'aide d'un densitomètre MACBETH RD 914.

The checks carried out on the microcapsules as well as on the CB papers obtained by coating these microcapsules are collated in the table below.

• . The particle size control is carried out using a laser granulometer (Coulter LS 100). The mean diameter and the size dispersion characterized by the variance are determined.
• . The viscosity is measured using a HAAKE VT 181 viscometer with mobile MV1,
• . The dry matter rate using an infrared desiccator (the internal phase is counted as dry matter),
• . The degree of encapsulation is measured by coating the microcapsules on CF paper and then crushing under 500 bar of pressure on half of the sheet. The difference in optical density between the crushed part and the non-crushed part is then measured using a MACBETH RD 914 densitometer.

• . La sensibilité au maculage sous faible pression. On écrase une liasse (CB + CF) sous 50 bars puis on mesure la perte de blancheur du CF à l'aide d'un réflectomètre Docteur Lange.
• . La réactivité du CB par écrasement à la calandre d'une liasse (CB + CF) et mesure de la densité optique de la coloration obtenue après une heure d'attente à l'abri de la lumière.
• . La résistance à la lumière de l'écriture par exposition aux UV (lampe à mercure de 400 W). On mesure la chute de la densité optique après 3 heures d'exposition.
• . Les tests de vieillissement accélérés des feuillets CB
  • soit en chaleur sèche : 72 heures à 140°C sous un poids de 5 kg,
  • soit en chaleur humide : 5 jours à 80°C et 80 % d'humidité relative sous un poids de 5 kg ; feuillet CB contre feuillet CF.

These checks are supplemented by a series of additional tests:

• . Sensitivity to smearing under low pressure. A bundle (CB + CF) is crushed under 50 bars and then the loss of whiteness of the CF is measured using a Doctor Lange reflectometer.
• . The reactivity of the CB by crushing with a calender of a bundle (CB + CF) and measurement of the optical density of the coloration obtained after an hour of waiting in the dark.
• . The resistance to light of writing by UV exposure (400 W mercury lamp). The drop in optical density is measured after 3 hours of exposure.
• . Accelerated aging tests for CB sheets
  • either in dry heat: 72 hours at 140 ° C under a weight of 5 kg,
  • either in humid heat: 5 days at 80 ° C and 80% relative humidity under a weight of 5 kg; CB slip versus CF slip.

In both cases, the loss of reactivity of the CB is measured by crushing with the calender before and after aging. In addition, in the case of the moist heat test, the loss of whiteness of the CF due to the migration of part of the chromogenic agents present in the microcapsules is measured compared to a blank test on the same CF.

Claims (11)

Microcapsule useful in particular for pressure-sensitive copying paper, containing an organic solution of a hydrophobic chromogenic agent, characterized in that the solvent comprises a mixture obtained by transesterification of a vegetable oil. Microcapsule according to claim 1, characterized in that the vegetable oil is chosen from the group consisting of rapeseed, soybean, flax, copra, palm, corn, sunflower, olive, peanut, palm kernel oils. Microcapsule according to claim 1, characterized in that the alcohol residue of the ester of the mixture is a linear or branched alcohol residue from C₁ to C₁₀. Microcapsule according to claim 3, characterized in that the alcohol residue is the rest of the following alcohols: methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, t-butanol. Microcapsule according to one of claims 1 to 4, characterized in that the solvent comprises at least 30% by weight of ester obtained by transesterification of a vegetable oil. Microcapsule according to claim 5, characterized in that the solvent comprises, in percentage by weight: - mixture obtained by transesterification of a vegetable oil 30 to 100% - mineral oil 0 to 70% Microcapsule according to claim 6, characterized in that the solvent comprises, in percentage by weight: - mixture obtained by transesterification of a vegetable oil 30 to 50% - mineral oil 50 to 70% Microcapsule according to claim 6 or 7, characterized in that the mineral oil is chosen from kerosene, paraffinic or naphthenic oil. Microcapsule according to one of Claims 1 to 8, characterized in that the chromogenic agent is chosen from the group consisting of derivatives of the phthalic type such as 3,3-bis (4-dimethylamino-phenyl) -6-dimethylamino- phthalide (CVL) and 3,3-bis (1-octyl-2-methylindole-3-yl) phthalide or fluorane derivatives such as 2-anilino-3 methyl-6-dialkylamino -2 '- (N'- ethyl-N-phenylamino-4'-methylfluorane) or 2'-anilino-3 ' 6-methyl diethylamino fluorane, 6'-dimethylamino-2 '- (N-ethyl-N-phenylamino-4'-methylfluorane), 3'-chloro-6'-cyclohexylaminofluorane or 3,7-bis (dimethylamino) -10-benzoylphenotiazine (BLMB) and bisarylcarbazolylmethane compounds. Pressure-sensitive paper coated on one side with a layer of microcapsule according to one of claims 1 to 9. Bundle of pressure-sensitive paper comprising at least one paper according to claim 10.