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

0001The present invention relates to microcapsules useful in particular for the production of carbonless pressure-sensitive paper, containing an organic solution of a hydrophobic dye, the solvent of which is at least partly of vegetable origin.

0002The 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.

0003A bundle of carbonless pressure sensitive paper includes:<ul id="ul0001" list-style="dash"><li>an emitting sheet called CB, obtained by coating on the back of microcapsules containing a solution of hydrophobic chromogenic agents,</li><li>a receiving sheet called CF, obtained by coating the front with an absorbent and reactive layer vis-à-vis chromogenic agents,</li><li>one or more intermediate sheets comprising both the microcapsules on the back and the receiving layer on the front called CFB.</li></ul>

0004There are also sheets, called "autonomous", obtained by coating the front of a mixture of microcapsules and receiving layer.

0005The 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.

0006The solvent used to prepare the internal phase plays a key role in the final quality of the product. It must meet the following requirements:<ul id="ul0002" list-style="none"><li>. be colorless and odorless,</li><li>. have good solvent power with respect to chromogenic agents,</li><li>. be chemically inert vis-à-vis the chromogenic agents and vis-à-vis the materials used to form the walls of the microcapsules,</li><li>. have a high boiling point and a very low vapor pressure at room temperature,</li><li>. be liquid at the temperature of use of carbonless paper (down to -10 ° C see - 20 ° C in winter),</li><li>. have the lowest possible viscosity in order to facilitate the flow of the internal phase during the rupture of the capsules,</li><li>. have good heat and light stability,</li><li>. allow rapid and intense development of chromogenic agents on the reagent of the CF sheet,</li><li>. give good light fastness to the image obtained on the CF sheet,</li><li>. be practically insoluble in water in order to be encapsulated,<ul id="ul0003" list-style="dash"><li>be safe for the human body and the environment,</li><li>exhibit high biodegradability,</li><li>have a low price compatible with current prices for carbonless carbon paper.</li></ul></li></ul>

0007Currently, among the solvents commonly used, there may be mentioned hydrogenated terphenyls, alkylnaphthalenes, alkylbiphenyls, diarylmethane derivatives, dibenzylbenzene derivatives, chlorinated paraffins, etc.

0008These solvents are most often mixed with diluents such as kerosene, light mineral oils or alkylbenzenes (for example dodecylbenzene) etc.

0009All these products fairly well meet all the requirements set out above except the last concerning biodegradability and non-toxicity towards the environment hence a risk of pollution during the use and destruction of paper. chemical carbonless.

0010To overcome these shortcomings, consideration has been given to using solvents of natural origin such as vegetable oils (soy, 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.

0011None of these vegetable oils gives satisfactory results, because in all cases we observe:<ul id="ul0004" list-style="none"><li>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,</li><li>2) too high a viscosity (poor flow of the internal phase resulting in a loss of yield),</li><li>3) insufficient revelation of the chromogenic agents in the presence of vegetable oil (low intensity duplication), in particular for fluorane or phthalide derivatives.</li><li>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),</li><li>5) insufficient hold in the light of the colored image on CF,</li><li>6) often insufficient biodegradability.</li></ul>

0012Various improvements have been proposed in order to try to use vegetable oils while minimizing their drawbacks, in particular the European patent n ° 520 639 recommends mixtures of vegetable oils with mono- or di-esters of fatty acids. This solution proves insufficient for points 1), 2) and 3) and brings little improvement on points 4) and 5).

0013It has also been proposed in patent EP 593 192 to use a saturated or unsaturated functional mono-, di- or tri-ester as solvent. This achievement is of a very high cost on the industrial level.

0014The 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.

0015Among the microcapsules, advantageously, but not exclusively, those formed from a wall of crosslinked gelatin.

0016By "ester" is generally meant the mixture obtained by transesterification, comprising at least 95% of fatty acid esters of natural origin.

0017Transesterification 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.

0018It 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.

0019This is all the more advantageous since the manufacturing cost of such a product is much lower than that of synthetic esters.

0020This 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.

0021It 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. .

0022The solvent must contain sufficient ester obtained by transesterification of a vegetable oil to meet the conditions set out above.

0023Among 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.

0024Of course, the abovementioned oils can be used alone or as a mixture.

0025As 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.

0026Preferably, the alcohol residue is a C en to C₈ residue.

0027Among 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.

0028Mention may be made, among the chromogenic agents used, of derivatives of the phthalic type, 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'-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. This list is non-limiting and can be extended to all the chromogenic substances commonly used in the art considered.

0029The solution of chromogenic agent in the solvent is around 5% by weight.

0030According to a preferred variant, the solvent comprises at least 30% by weight of ester obtained by transesterification of a vegetable oil.

0031In addition to said esters, the solvent may contain mineral oils.

0032Among 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.

0033Preferably light naphthenic or paraffinic oils are used.

0034The 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.

0035According to a preferred embodiment, the microcapsule is characterized in that the solvent comprises in percentage by weight:<ul id="ul0005" list-style="dash"><li>mixture obtained by transesterification of a vegetable oil 30 to 100%</li><li>mineral oil 0 to 70%</li></ul>

0036More preferably, the solvent comprises in percentage by weight:<ul id="ul0006" list-style="dash"><li>mixture obtained by transesterification of a vegetable oil 30 to 50%</li><li>mineral oil 50 to 70%.</li></ul>

0037According to an advantageous embodiment, mineral oils are preferred.

0038The invention also relates to a process for the preparation of such microcapsules.

0039According to a general method, an emulsion of a hydrophobic phase consisting of an organic solution as described above of a chromogenic substance is formed, 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 copolymer of maleic anhydride such as a copolymer of methyl ethervinyl anhydride maleic (PVMMA) or a copolymer of ethylene and maleic anhydride (EMA).

0040The 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.

0041A curing agent such as formalin or glutaraldehyde is then added in order to crosslink said solid coacervate walls and to obtain the desired microcapsules.

0042It 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.

0043According 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:<ul id="ul0007" list-style="dash"><li>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.</li><li>cooling to solidify the wall,</li><li>crosslinking of gelatin to fix the structure and make the process irreversible,</li><li>warming and neutralization.</li></ul>

0044It has been 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 curve than the emulsions prepared with solvents based on virgin oil.

0045The 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).

0046Of course, other methods can also be used without departing from the scope of the present invention.

0047Among these other processes, mention may be made of:<ul id="ul0008" list-style="dash"><li>A gelatin-based process by complex coacervation such as that described in Example 1 or in US Patents 4,402,856, FR 2,458,313, EP 339866,</li><li>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,</li><li>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,</li><li>or any other known encapsulation process for the manufacture of microcapsules used for the manufacture of chemical carbonless paper.</li></ul>

0048The 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.

0049They are then laid down on a paper support.

0050The subject of the invention is also a pressure-sensitive paper coated on one side with a layer of microcapsules as described above.

0051This paper support constituting the emitting sheet, called CB, with a grammage generally between 40 to 90 g / m 3 was coated by coating the suspension of microcapsules, then these microcapsules were dried to obtain the paper according to the invention.

0052The method of coating and the formulation of the coating bath are not critical to the invention.

0053The 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.

0054The 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.

0055The 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.

0056In addition, the accelerated aging tests of the CB sheets, either in dry heat or in wet heat are improved.

0057The invention is illustrated by the following examples:

Example 1

0058In 1.2 l of deionized water at 40 ° C., 55 g of gelatin with a bloom value of approximately 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.

0059In parallel, a hydrophobic organic phase is prepared, by heating between 100 and 130 ° C for one hour, of the following mixture:<ul id="ul0009" list-style="dash"><li>500 g rapeseed methyl ester manufactured by the company Novamont (MATROL BI 141), obtained by transesterification,</li><li>345 g light naphthenic oil,</li><li>21 g of black ODB2</li><li>9 g of black S205 (Yamada)</li><li>3.8 g CVL blue</li><li>3.5 g blue Pergascript SRB (Ciba Geigy)</li><li>4.2 g of orange Pergascript I5R (Ciba Geigy).</li></ul>

0060The organic phase and the aqueous phase are mixed with stirring and the product is emulsified using an Ultra-TURRAX type apparatus until particles having an average diameter of between 5 and 6 μm are obtained (the diameter is measured using a Coulter LS 100 laser granulometer).

0061Then 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.

0062The temperature is raised to 60 ° C and acetic acid is added over 30 minutes to adjust the pH to 4.3.

0063The coacervate is cooled to 8 ° C. and this temperature is maintained for 10 hours.

0064The hardening of the walls is carried out in two stages:<ul id="ul0010" list-style="none"><li><u>1st step</u> : addition of 17 g of 50% glutaraldehyde with vigorous stirring,</li><li><u>2nd stage</u> : 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 stirring before raising the pH to 7 at the temperature of 20 ° C.</li></ul>

Example 2

0065Example 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

0066Example 2 is reproduced but with the methyl ester of rapeseed produced by the company Henkel.

Example 4

0067Example 1 is reproduced by replacing the rapeseed methyl ester with an isopropyl ester of soybean obtained by transesterification manufactured by the company Stéarinerie Dubois et Fils.

Example 5

0068Example 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

0069Example 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

0070Example 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)

0071Example 1 is reproduced by replacing the rapeseed methyl ester with refined virgin rapeseed oil (produced by the company CEREOL).

Results

0072The checks carried out on the microcapsules as well as on the CB papers obtained by coating these microcapsules are collated in the table below.<ul id="ul0011" list-style="none"><li>. 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.</li><li>. The viscosity is measured using a HAAKE VT 181 viscometer with mobile MV1,</li><li>. The dry matter rate using an infrared desiccator (the internal phase is counted as dry matter),</li><li>. 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.</li></ul>

0073These checks are supplemented by a series of additional tests:<ul id="ul0012" list-style="none"><li>. 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.</li><li>. 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.</li><li>. 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.</li><li>. Accelerated aging tests for CB sheets<ul id="ul0013" list-style="dash"><li>either in dry heat: 72 hours at 140 ° C under a weight of 5 kg,</li><li>either in humid heat: 5 days at 80 ° C and 80% relative humidity under a weight of 5 kg; CB slip versus CF slip.</li></ul></li></ul>

0074In 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.<tables id="tabl0001" num="0001"><img file="EP0697293A1_D0001.tif" /></tables>