Process for producing banknotes

Abstract: Paper printed with ink is covered with a protective coating whose composition essentially consists of water, 15-45 wt% (preferably 20-30 wt%) of at least one emulsified acrylic (co)polymer of the air-drying or UV-curing type, and when using an air-drying acrylic (co)polymer 0.8-2.0 wt% (preferably 1.2-1.6 wt%) of wax (preferably polyethylene wax), in addition to the usual additives. Said composition is applied on printed paper (particularly on thin paper, i.e. 100 g/m2 or lower, generally 75 to 95 g/m2) in an amount such as to obtain a dry film weight of 1.5-5 g/m2 (preferably 2-3 g/m2) then subjected to air-drying followed where applicable by UV-curing. Acrylic (co)polymers of the UV-curing type are preferred because they provide more resistant coatings.

Description: This invention relates to a process for producing paper printed with ink and covered by a protective coating. The invention particularly relates to the production of paper currency, more particularly banknotes, and of documents and other printed paper that are intended to be subjected to intensive use over a long period of time. The invention also relates to the use of water-based emulsion varnishes for coating printed paper. It is known that printed matter intended for intensive circulation and frequent use can be coated with a protective layer to counteract soiling so that they may have a longer useful life. Indeed, circulating paper currency for example has a useful life which is mainly determined by the soiling of the banknotes. The technique of coating printed paper to improve its durability is not to be confused with the technique of coating paper to improve its printability, particularly to reduce "feathering", which generally does not improve the durability. Moreover, coating printed paper creates the problem that the coating has to adhere to the printed parts, particularly when the printing ink contains wax. Indeed, wax is often incorporated in printing ink to improve the rheological properties thereof and to prevent the ink from coming off onto the next sheet in a pile of freshly printed paper. Since the latter effect is due to the migration of the wax to the surface of the printed ink, care must be taken that the varnish applied thereto adheres as strongly as to the non-printed areas of the paper. Another required property for the protective coating is solvent resistance. US-A-4170681 discloses a method of applying a layer of protective varnish on a hydrophilic surface carrying at least one area of hydrophobic ink, wherein the varnish is an aqueous dispersion of a film-forming polymer which contains 0.5 - 10 % of an auxiliary film-forming material capable of raising the critical surface tension of the hydrophobic inked area. EP-A-543385 discloses a water-based printing ink composition, particularly suitable for gold and silver colours. US-A-5384160 discloses a method for depositing an aqueous coating composition on a printed surface, using viscosity and temperature controls. DE-A-4336214 discloses specific binder combinations that can be used directly to coat printed substrates. They contain either (a) a binder having a film-forming temperature above 60 °C, a wax or paraffin having a melting temperature above 60 °C, except combinations containing 3 - 10 wt % (of the non-volatiles) of wax or paraffin, or (b) combinations containing 3 - 10 wt % of wax or paraffin but containing < 2 wt % (of the non-volatiles) of emulsifiers. DE-A-3225875 discloses copolymers of 37-45 wt % of (meth)acrylic acid, 55-63 wt % of styrene and 0 - 10 wt % of other hydrophobic olefinically unsaturated monomers, prepared by continuous mass polymerisation at 200 - 400 °C, and their use to prepare water-based emulsion for coating paper. US-A-1575940 discloses paper currency, bonds or other instruments of value comprising paper printed with an ink comprising cellulose ester material and incorporated oily softening material, with a thin substantially transparent protective coating. The disclosed protective coating compositions are all based on thinly fluent volatile solvents for pyroxylin or cellulose ester. EP-B2-256170 discloses paper objects printed with ink containing 1-10 wt% of wax consisting for the greater part of particles of 0.03 mm and smaller and for a minor part of particles smaller than 0.01 mm, and coated with a protective coating consisting for a major part of cellulose ester or ether. Those protective coatings however require a large amount of solvent in which the cellulose ester or ether is dissolved for application to the printed paper (respectively 73 and 84 wt% in the examples). The protection of the environment has recently prompted concerns that the level of solvent emission in the atmosphere should be reduced to the absolute minimum. There is thus a need in the art for low-solvent or even solvent-free coating compositions for protecting printed paper, particularly banknotes. It is an object of the invention to provide a low-solvent coating composition for protecting printed paper. Another object of the invention is to provide a low-solvent composition for making coatings that adhere to the printed areas of the printed paper. A further object of the invention is to provide a low-solvent composition for making coatings that are solvent-resistant. The Applicants have now unexpectedly found that printed paper, particularly when printed with wax-containing ink, may be coated with a strongly-adhering layer of low-solvent varnish which will protect the printed paper, thereby repelling dirt and increasing the useful life of the coated printed paper. The process of the invention for producing paper printed with ink and covered with a protective coating is characterised by the use of a coating composition essentially consisting of: (i) water; (ii) 15 to 45 wt% of at least one emulsified acrylic (co)polymer selected from the group consisting of air-drying (co)polymers and UV-curing (co)polymers; and (iii) when air-drying (co)polymers are selected, 0.8 to 2.0 wt% of wax; in addition to the usual additives, said composition being applied in an amount such as to obtain a dry film weight of 1.5 to 5 g/m2 then subjected to air-drying followed where applicable by UV-curing. It was surprising that water-based compositions could be used at all, considering that paper (particularly thin paper, i.e. 100 g/m2 or lower, generally 75 to 95 g/m2) is very sensitive to the presence of water. The binder consists of at least one essentially acrylic (co)polymer in the form of an emulsion. The acrylic (co)polymers may be of the air-drying type, but they should preferably be of the UV-curing type which provides more resistant coatings. The composition comprises from 15 to 45 wt% of acrylic (co)polymers, preferably from 20 to 30 wt%. The selection of acrylic monomers is known in the art and need not be described herein (see e.g. Surface Coatings, Vol. I, Raw materials and their usage, Chapter 17, pages 164-70, Chapman and Hall, 1983). When the acrylic (co)polymers are of the air-drying type, the composition further comprises from 0.8 to 2.0 wt% (preferably from 1.2 to 1.6 wt%) of wax, preferably polyethylene wax. Such wax is well-known in the art and need not be described here. It is preferred to use micronised wax, as described in EP-B2-256170 and in the references cited therein. When the acrylic (co)polymers are of the UV-curing type, the use of such a wax is neither essential nor detrimental. The composition to be used according to the invention may further contain any and all additives that are usual for water-based coating compositions, such as for example biocides, fungicides, defoamers, emulsifiers and flow agents, or any and all agents that are usual for security or banknote paper, such as for example agents preventing chemical falsification and authentication agents. A particularly desirable additive is polysiloxane polyether copolymer, up to about 0.5 wt%. The compositions to be used according to the invention are easily prepared by mixing the desired components. The preferred method is to put some water in a tank (preferably in a dissolver), then to add the other components ending with the acrylic emulsion(s), and finally to adjust the pH to light basic (preferably about 8.5) and the viscosity to the value required by the application method (usually to a viscosity of 15 to 30 s, preferably of 20 to 25 s, measured at 23°C according to the DIN 4 standard method). The protective coating according to the invention is preferably applied using roller-coating. The composition is applied in an amount such as to obtain a dry film weight of 1.5 to 5 g/m2, preferably of 2 to 3 g/m2, by air-drying (preferably using hot air at a temperature of 60 to 90°C, optionally combined with infra-red drying) followed where applicable by UV-curing. The process of the invention is particularly useful for producing paper printed with wax-containing ink and covered with a protective coating. The invention is particularly well adapted to the production of paper currency, more particularly banknotes, and of documents and other printed paper that are intended to be subjected to intensive use over a long period of time. Comparative example A (reference) Example 1 according to EP-B2-256170 was repeated. The composition was applied by means of a roller coater on printed polyvinylalcohol-treated paper having a thickness of 80 g/m2, and dried using hot air (about 85°C) to form a 1.5 g/m2 dry film. Example 1 A protective coating composition was prepared which had the following composition: (parts by weight) water 73.72 (total) acrylic copolymer 20.24 (as 49 wt% emulsion in water) polyethylene wax 1.60 additives 1.95 bases 0.77 glycols 1.72 The composition was prepared by introducing 47 parts of water in a dissolver, adding the wax and the additives (defoamer, biocide, flow agent) then the acrylic (co)polymers. The main acrylic component was a commercially available anionic modified acrylic copolymer having a Tg (glass transition temperature) of 9°C and a good resistance to blocking; the emulsion had a Brookfield viscosity of 400 mPa.s at 25°C and a minimum film-forming temperature below 0°C. The glycols were not essential in the composition but were present in the components as commercially available. The bases were used to adjust the pH to about 8.5. The composition was applied by means of a roller coater on printed polyvinylalcohol-treated paper having a thickness of 80 g/m2, and dried using hot air (about 85°C) to form a 3 g/m2 dry film. The resistance of the coated printed paper was evaluated according to the usual practice in the art. The chemical resistance was evaluated by dipping the coated printed paper in various media under set conditions (including ethyl acetate for 10 minutes at 30°C, ethanol for 20 minutes at 45°C, 4% Cl bleach water for 15 minutes at 45°C) and visually determining the chemical resistance. It was as good as that of the comparative example. To evaluate the dirt resistance, the coated printed paper and the reference were subjected under set conditions to a proprietary composition of artificial dirt essentially consisting of sand, oil, carbon black and artificial sweat; after removal of the dirt by brushing, the paper was compared visually with the reference. The dirt resistance was similar. Example 2 Example 1 was repeated using a commercially available UV-curing acrylic copolymer and 2-hydroxy 2-methyl 1-phenyl 1-propanone as photoinitiator. The coating composition was as follows: (parts by weight) water 73.00 (total) acrylic copolymer 24.00 (as 38 wt% emulsion in water) additives 0.66 bases 0.54 glycols 1.60 photoinitiator 0.20 Curing of the dry film was made using a standard high pressure mercury lamp. The dry film had a thickness of 2 g/m2. The dirt resistance was similar to that of the reference while the chemical resistance was slightly better.

Claims: Process for producing paper printed with ink and covered with a protective coating, characterised by the use of a coating composition essentially consisting of (i) water, (ii) 15 to 45 wt% of at least one emulsified acrylic (co)polymer selected from the group consisting of air-drying (co)polymers and UV-curing (co)polymers; and (iii) when air-drying (co)polymers are selected, 0.8 to 2.0 wt% of wax, in addition to the usual additives, said composition being: (a) applied in an amount such as to obtain a dry film weight of 1.5 to 5 g/m2; then (b) subjected to air-drying followed where applicable by UV-curing. Process according to claim 1 , wherein the composition has a viscosity of 15 to 30 s, measured at 23°C according to the DIN 4 standard method. Process according to claim 2, wherein the composition has a viscosity of 20 to 30 s. Process according to any one of claims 1 to 3, wherein the paper has a weight of 100 g/m2 or lower. Process according to claim 4, wherein the paper has a weight of 75 to 95 g/m2. Process according to any one of claims 1 to 5, wherein the acrylic (co)polymers are of the UV-curing type. Process according to any one of claims 1 to 6, wherein the wax is polyethylene wax. Process according to any one of claims 1 to 6, wherein the wax is micronised wax. Process according to any one of claims 1 to 8, wherein the composition contains up to 0.5 wt% of polysiloxane polyether copolymer as additive. Process according to any one of claims 1 to 9, wherein the composition is applied using roller-coating. Use of the composition described in any one of claims 1 to 10 as protective coating for printed paper.

Inventors: AKKERMAN MATTHIAS ANTHONIE JOH, MULDER GERHARDUS JOHANNIS, AKKERMAN, MATTHIAS ANTHONIE JOHANNES, MULDER, GERHARDUS, JOHANNIS
Applicants: ATOFINA RES, ATOFINA RESEARCH
Application year: 1998
Main classification: B41M7
IPC codes: B41M7