Paper currency printed with ink and coated with a protective layer, and process for producing it

Abstract: PAPER PRINTED WITH INK THAT CONTAINS MICRONIZED WAX IS COATED WITH CELLULOSE ESTER OF ETHER. THEREBY IT BECOMES SOIL-REPELLING AND ITS USEFUL LIFE-TIME INCREASES CONSIDERABLY.ESPECIALLY APPLICABLE TO BANK NOTES

Description: This invention relates to paper currency printed with ink and coated with a protective layer, and to a process for producing the same. Paper currency is subject to intensive circulation and frequent use and therefore it is desired to coat it with a protective layer to counteract soiling so that it may have a longer useful life. As the useful life-time of paper currency is mainly determined by the soiling of the bank notes in circulating it is quite worth-while to make such paper currency more or less dirt-repelling. The coating of paper currency with a thin layer of polyamide is known; since 1955 Dutch bank notes carry a coating of Ultramid®. However, the effect of this polyamide coating is not yet satisfactory; it does add to the strength of the paper and to keeping the paper clean to enhance its life-time but these effects are quite limited and it seems that dirt of a lipophilic nature may even be attracted rather than repelled. Moreover coating printed paper creates the problem that the coating does not adhere well to the printed parts, especially if the printing ink contains wax. Nowadays wax is often incorporated in printing ink to improve the rheological properties thereof and also because then the ink will not come off onto the next sheet in a pile of freshly printed paper. In this connection it should be noticed that waiting until the ink is thoroughly dry is not consistent with the high production rates applied nowadays. Said not coming-off seems to be due to the protrusion of the wax particles from the ink, because of which the printed sheet makes no real contact with the next sheet. However, the varnish applied to thus printed paper is removed easily, even by rubbing with the fingers only. Said wax is incorporated into the ink by stirring in a wax dispersion. This wax dispersion can be made in several ways; the most common way is melting the wax with the appropriate amount of hydrocarbon oil or mixture of oil and binder at elevated temperatures under vigorous agitation and slowly cooling down the blend. Another method is to dissolve the wax in a real solvent for the wax at elevated temperatures. Then mixing with a co-solvent in which the wax is less soluble, and then cooling the mixture to room temperature. The necessary temperature increase depends in both cases on the nature of the wax component; typical values are from 80° to 140°C. In this manner very fine dispersions of wax in the liquid phase (particles < 10 micrometer) can be obtained. Now we have found that paper currency printed with wax-containing ink may be coated with a well adhering layer of varnish which will repel dirt and add to the useful life of the thus treated paper if the wax in the ink consists of particles having a specific particle size and if the varnish is a cellulose ester or ether. Thus the present invention provides paper currency printed with ink and coated with a protective layer, having as characterizing features that the printing ink contains 1-10% by weight of micronized wax consisting for the greater part of particles of 30 µm and smaller and for a minor part of particles smaller than 10 µm and that the protective coating consists for a major part of cellulose ester or ether. The micronized wax to be used according to the present invention is generally obtained by a special grinding treatment. This special grinding treatment, called "Micronizing of wax" is described by T.C. Patton in "Paint flow and Pigment Dispersion", 2nd Ed. (John Wiley & Sons, New York, 1979) page 378 and by H.F. Payne in "Organic coating Technology", vol. II (John Wiley & Sons, New York, 1960) pages 715 and 777, and the product may be obtained from E.G. Hoechst AG in Germany or from Cera Chemie at Deventer (the Netherlands). The micronized wax may have various compositions and be of various origin. E.g. it may be polyethylene, polypropene, ethene-propene copolymer, hydrocarbon resin or montan wax, or it may be an animal or vegetable wax such as bees wax, carnauba wax or candellila wax, etc., by preference it is polyethylene. Said micronized wax is not dispersed into the ink by melting it with mineral oil or a mixture of oil and binder but by kneading or blending it with the oil, the ink binder or mixture thereof at room temperature or slightly elevated temperature, in such a way that the particle size is modified hardly or not at all. The ink contains 1-10% by weight of micronized wax and by preference it contains 1-5% by weight of micronized wax. The ink to be used according to the invention may contain, apart from the micronized wax, all the ingredients that are usual for printing inks. The drying of the ink may be caused by absorption by the paper fibres and/or penetration of the liquid constituents in the paper fibres, or by evaporation of solvent or in any other way. The invention is particularly useful for paste ink which dries by reaction with oxygen from the air. As binder and pigment any composition known therefor is applicable, and in physically drying inks any usual solvent may be applied. Moreover the ink may contain any of the known and usual additives, such as fillers and drying agents and mixtures thereof. The protective coating has to consist substantially of cellulose ester or ether. Additionally it will generally contain plastifier, generally 1 part by weight for 5 to 20 parts by weight of cellulose derivative. Examples of suitable plastifiers are the alkyl esters of dicarboxylic acids such as di-octyl and dibenzyl esters of succinic, adipic and phthalic acids, low-molecular acryl derivatives, diacyl derivatives of sucrose, sorbitan and oxyalkylated sorbitan such as sucrose acetate, propionate, campher, etc. Moreover minor amounts of other constituents such as optical brighteners may be present. The cellulose ester has to contain 1-40% by weight of acetyl groups, 0-50% by weight of propionyl groups and 0-60% by weight of butyryl groups, the hydroxyl group content being from 0 to 5.5% by weight. By preference the cellulose derivative is a mixed ester containing both acetyl groups and propionyl and/or butyryl groups, as such esters are soluble in alcohol/water mixtures whereas cellulose acetate requires ketones like acetone and butanone for dissolving. Consequently a preferred composition is 5-20% by weight of acetyl groups and 20-40% of propionyl and/or butyryl groups, the hydroxyl group content being above 3%, by preference 4-5.5%. With higher hydroxy-group levels the contents of propionyl and/or butyryl groups also may be somewhat higher, up to 41%, resp. 44%. If the cellulose derivative is an ether it may be any cellulose ether that is not water soluble, e.g. any methyl cellulose, ethyl cellulose or butyl cellulose of sufficiently high substitution degree. Processes for preparing the above-mentioned cellulose derivatives are sufficiently known and need not be discussed here in detail. Moreover all these cellulose derivatives may be obtained commercially, e.g. from Eastman Kodak and Du Pont de Nemours in the US, Bayer AG in Germany, Gevaert in Belgium, and others. The invention also comprises a process for producing the above-described paper currency having a protective coating, wherein paper is printed with an ink which comprises binder, pigment and optionally solvent, drying agent and/or other auxiliaries, and subsequently is covered with a layer of varnish. The process according to the invention is characterized in that the ink contains 1-10% by weight of micronized wax consisting for the greater part of particles of 30 µm and smaller and for a minor part of particles smaller than 10 µm and in that the protective coating consists for a major part of cellulose ester or ether. The composition of the ink and the micronized wax may be selected along the above-discussed lines. The ink may be applied by any of the known printing techiques, e.g. by a relief, intaglio or silk screen process or by planography. The composition of the coating has been discussed above. As the cellulose ester or ether coating will generally be applied as a solution the cellulose derivative is by preference a mixed ester containing both acetyl and propionyl and/or butyryl groups, as such mixed esters are soluble in alcohol/water mixtures and do not require ketones for dissolution as does cellulose acetate. Alcohol/water mixtures are preferred above ketones because they are scarcely inflammable or not at all and because their toxicity is generally lower. The actual way in which the coating solution is applied is not critical and it may be done by spraying, dipping, roller coating or whatever technique that is convenient under the prevailing conditions. During or after applying the coating solution, the solvent is evaporated and it may be recovered, both for economical and environmental considerations. Applicant does not want to be bound by any theory, but he believes the favourable results of the combination of micronized-wax-containing ink and cellulose derivatives to be due to a better wetting of the wax particles and the ink-binder phase by the cellulose derivative. The repellance of dirt by the cellulose ester or ether may be due to the rather hydrophilic nature of said cellulose derivatives but, again, these are just theoretical considerations. The invention will now be illustrated by a few non-restricting examples. Example 1 Paper having a thickness of 100 g/m² was printed with an ink having the following composition: Alkyd resin (binder) 59% by weight Carbon black (pigment) 20% by weight Silica gel (filler) 10% by weight Wax dispersion 10% by weight Drying agent mixture 1% by weight The composition of the wax dispersion was: Micronized polyethylene (Ceridust 3620 from Hoechst AG) 30% by weight Alkyd resin 10% by weight Mineral oil 60% by weight It will be appreciated that this ink contains 3% by weight of micronized wax. The printed sheets were stacked without any difficulty, no coming off of the black ink onto the next sheets was observed. Next they were coated by spraying at short distance with a coating solution having the following composition in a rate to give a coating weight of 2 g/m²: Cellulose acetate propionate 13% by weight Sucrose acetate isobutyrate 2% by weight Isopropanol 73% by weight Water 12% by weight The cellulose acetate propionate was the product "CAP 504/0.2" from Eastman Kodak, having an acetyl group content of 2.5%, a propionyl group content of 40% and a hydroxyl group content of 5.0% and having a viscosity of 0.2 seconds (estimated according to ASTM D-1343, in the solution described as Formula A, ASTM D-817). The coated paper was dried with a current of air of about 80-90°C. It looked excellent and kept well when folded and handed over frequently. Example 2 The process of Example 1 was repeated, except that the composition of the coating solution was now as follows: Cellulose acetate butyrate 5% by weight Cellulose acetate butyrate 10% by weight Dioctylphthalate 1% by weight Ethyl acetate 10% by weight Butyl acetate 15% by weight Ethanol 59% by weight Example 3 Paper having a thickness of 100 g/m² was printed with an ink having the following composition: Cellulose acetate propionate (binder) 5% by weight Phthalocyanine blue (pigment) 20% by weight Ethanol 52% by weight Ethyl acetate 22% by weight Wax dispersion (Ceridust VP 3910 from Hoechst AG) 1% by weight The printed sheets were dried to remove the solvent and were then stacked without difficulty, no coming off being observed at all. After the stack of printed paper was fully dry, the sheets were coated in the way described in Example 1 with a varnish composition as specified in Example 1. The coated sheets were then dried with air of about 90°C. The finished product looked excellent and kept well when folded and handed over frequently.

Claims: Paper currency printed with ink and coated with a protective layer, characterized in that the printing ink contains 1-10% by weight of micronized wax consisting for the greater part of particles of 30 µm and smaller and for a minor part of particles smaller than 10 µm and in that the protective coating consists for a major part of cellulose ester or ether. Paper currency according to claim 1, characterized in that the protective layer consists substantially of cellulose ester containing 1-40% by weight of acetyl groups, 0-50% by weight of propionyl groups and 0-60% by weight of butyryl groups, the hydroxyl group content being from 0 to 5.5% by weight. Paper currency according to claim 2, characterized in that the protective layer consists substantially of cellulose ester containing 5-20% by weight of acetyl groups and 20-40% by weight of propionyl and/or butyryl groups, the hydroxyl group content being 4-5.5% by weight. Paper currency according to any of the preceding claims, characterized in that the protective coating contains 1 part by weight of plastifier for 5 to 20 parts by weight of cellulose derivative. Paper currency according to any of the preceding claims, characterized in that the printing ink contains 1-5% by weight of said micronized wax. Paper currency according to any of the preceding claims, characterized in that said micronized wax is micronized polyethene. A process for producing paper currency printed with ink and coated by a protective layer, wherein paper is printed with an ink which comprises binder, pigment and optionally solvent, drying agent and other auxiliaries, and subsequently is covered with a layer of varnish, characterized in that the ink contains 1-10% by weight of micronized wax consisting for the greater part of particles of 30 µm and smaller and for a minor part of particles smaller than 10 µm and in that the protective coating consists for a major part of cellulose ester or ether.

Inventors: MULDER GERARDUS JOHANNES, PLANTENGA TJALLING MEINOUD, WOTTE JOHAN, MULDER, GERARDUS JOHANNES, PLANTENGA, TJALLING MEINOUD, WOTTE, JOHAN
Applicants: ENSCHEDE & ZONEN GRAFISCH, JOH. ENSCHEDE EN ZONEN GRAFISCHE INRICHTING B.V.
Application year: 1986
Main classification: B41M7
IPC codes: C09D11, B41M7