The present invention refers to surface treatment of paper, namely of a special security paper, which is suitable for manufacturing of banknotes or other security documents wherein such paper is characterized by its resistance to mechanical and meteorological influences and wherein said invention also refers to preparing of solution suitable for treatment of such paper.

According to the International Patent Classification such inventions belong to manufacturing of paper, namely to pulp composition and preparation, in particular to special paper, which is treated to obtain specific resistance against wear, weather and various other influences. Pursuant to said International Patent Classi fication such inventions belong to Class D 21 H 27/28.

The invention is rest on a problem how to formulate a solution, by which the paper, when manufactured in usual manner and by means of each existing equipment in a paper manufacturing industrial plant could be subsequently treated on the same paper making machine just in order to further improve its quality in particular its folding resistance, strength, tearing resistance, water resistance and grease resistance as well as resistance against washing, by simultaneously maintaining a suitable roughness porosity and pll.

As known, such security paper, in particular each paper for manufacturing of banknotes should consist of quality fibers, in particular cotton fibers and other processing ingredients in order to become resistant against wear during its whole lifetime. To this aim, such paper is usually treated with various coatings which can applied onto its surface prior to printing and/or after that. A method for surface treatment of paper for the purpose of improving its water resistance and resistance against weather influences is disclosed in DE 40 40 347 C2. In said method, a paper on the basis of cotton and other plant fibers is treated by resin in its neutral phase, by which its water resistance is improved, and besides to that, such paper is also treated in a bath containing polyvinyl alcohol and upon that calendered. Such treated paper becomes more resistant against moisture and grease, and is herewith during the use indirectly also better protected against impurities and moreover such paper retains a desired pH, porousness and also roughness which is required by printing or making certain security elements. On the other hand, such paper is relatively w eak in view' of mechanical damages like tearing or repeating bending i.e. folding. This is of course a crucial deficiency in each case, w'herc such paper is used in manufacturing of banknotes, namely as the so-called banknote paper, since in the practice such banknotes are then quickly destroyed and must be quite often withdrawn and replaced with new ones. fhe method according to the invention includes preparation of a solution for surface treatment of security paper and thereafter also application thereof by means of impregnation of each previously manufactured and dried paper of a required consistency and quality. In surface treatment of a security paper, in particular of a banknote paper, a solution of polyvinyl alcohol is used but the invention provides that a solution is prepared for said surface treatment of paper in which in addition to said polyvinyl alcohol also a polyurethane is used as the one of main ingredients. Combination of said main components in combination with a relatively small quantity of supporting additives enables that each paper, after being treated wdth such solution excels in enhanced grease resistance, waterproolhess as well as in essentially improved mechanical properties.

Consequently, the present invention is dealing with a process for manufacturing of a security paper, in particular of value or banknote paper, and simultaneously also with preparing of a solution, which is suitable for surface treatment of said paper, as w ell as applicability thereof. Thanks to the invention, such paper can therefore be manufactured and finalized on the same paper making machine and is ready for printing without any need to application of further coatings or any further surface treatment.

A clay-like solution for surface treatment of paper, in particular of security paper for manufacturing of banknotes or similar products on the basis of paper, namely a solution according to the invention which is suitable for applying on each previously manufactured paper, which is made of sufficiently strong libers, in particular cotton libers as well as of other processing ingredients, so that such paper is then wearing resistant during its lifetime and is prior to said surface treatment available in the form of a band is a water dispersion, comprising

- water solution of polyvinyl alcohol, in which the content of dry substance relative to the weight of the complete clay solution is 2 - 12 %. and preferably between 4 % and 10 % of dry substance;

- water dispersion of polyurethane, in which the content of dry substance relative to the weight of complete clay solution is 5 - 15 %, preferably between 6 5 % and 12 5 % of dry substance;

- supporting additives in particular clays, antilbaming agents curing agents and softening agents;

wherein the concentration of said clay-like solution in particular on the basis of polyvinyl alcohol and polyurethane as well as of said additives relative to the complete weight of solution is 5 - 15 %, preferably 6 - 14 % and most preferably 6.5 - 12.5 % of dry substance relative to the weight of complete solution while the viseosil _} according to ford (F4) at processing temperature is less than 40 s and preferably less than 30 s.

The invention also provides for a method for preparing of a clay-like solution with previously disclosed features wherein such method comprises the following steps: i ) preparing a water solution of polyvinyl alcohol, wherein said polyvinyl alcohol in granulated or powdered form with high, namely at least 90%, preferably at least 95%. degree of hydrolysis of polyvinyl acetate, of which it is synthetized. is solved in water by means of cooking said polyvinyl alcohol in water at 90°C in usual manner for such time which is sufficient for solving the complete quantity of polyvinyl alcohol in the water, upon which such obtained solution is cooled-down on temperature below 65 °C and thinned by adding water, so that concentration of polyvinyl alcohol expressed as % of dry substance relative to the weight of solution, is adjusted to 2 - 12 % and preferably 4 - 10 % of dry substance relative to the weight of die complete solution;

ii) preparing of aqueous dispersion of polyurethane at room temperature by means of usual mixing of commercially available polyurethanes with added water, wherein a concentration of water dispersion of polyurethane relative to the content of dry substance in the water solution is adjusted within the range 5 - 15 % and preferably 6,5 - 12,5 %;

iii) mixing of w ater solution of polyvinyl alcohol with aqueous dispersion of polyurethane at the temperature not exceeding 45°C and in such ratio relative to each other that a required ratio between the polyvinyl alcohol and polyurethane can be achieved and also the content of each of them relative to the weight of the complete solution;

iv) adding each supporting additives, in particular clays, anti foaming agents, curing agents and softening agents to such obtained mixture of aqueous solution of polyvinyl alcohol and aqueous dispersion of polyurethane;

v) transferring such obtained clay-like solution into a processing bath.

Viscosit ₎ of 4 % aqueous solution of polyvinyl alcohol at 20°C upon conclusion of said step i) is adjusted betw een 10 and 45 mPas. preferably between 20 and 35 mPas.

The invention also provides for a method for surface treatment of paper wherein such method comprises at least the following steps: i) manufacturing of paper in the form of a band in a wet area of the paper making machine wherein such manufactured paper consists mostly of quality libers in particular of properly treated, grinded and ibrillated cotton libers as well as of fillers and other additives like strength enhancing additives however without any clay content in the paper mass;

ii) drying the paper as obtained in step i).

wherein said steps i) and ii) are performed without any content of clays in the paper mass and moreover either immediately after performing said steps i) and ii), or subsequently after certain time period, said process is continued by adding the following steps:

iii ) surface treatment of paper as manufactured in step i) and dried in step ii) upon departure from a primary drying appliance by means of impregnating with clay- like solution according to anyone of Claims 1 or 2 in a suitable clay press for the purpose of enhancing quality of paper in course of achieving waterproolhcss, resistance to fats and washing, improving mechanical strength and reducing porousness, wherein said paper is transferred through a processing bath containing a previously clay-like solution, the viscosity of which is adjusted to less than 40 s. and preferably to less than 30 s, according to Ford (F4), wherein between 0,5 g/irT to 10 g/nr . and preferably 2 g/m ² to 8 g/m ² , of said clay-like solution is applied on said paper w hile the rest of said clay-like solution is returned back to said processing bath; and

iv) drying such obtained paper.

Upon that said paper when dried in step iv). is optionally cut into sheets of each desired format or and optionally alter that cut into sheets of each desired format. In one of the embodiments of the invention during said step iv) the paper is dried by means of infrared IR heaters in a secondary drying appliance, which is arranged w ithin the end area of each paper making machine, in w hich said method is applied. The invention will also be presented by embodiments as well as with praetieal inslruetions and information, which could be useful to each person skilled in the art by exploitation of the invention. Said clay-likc solution is prepared by means of aqueous solution of polyvinyl alcohol and by dispersion of polyurethane in water as follows.

Aqueous solution of polyvinyl alcohol in water is prepared in accordance with common technical standards namely by cooking polyvinyl alcohol in water at temperature above 90°C. Said cooking is performed until said polyvinyl alcohol is completely solved in the water. Upon said cooking, such obtained solution is thinned with water and cooled, so that each desired concentration of the solution is adjusted and the temperature is set below 65°C. Said concentration of the solution of polyvinyl alcohol, expressed as percentage of dry substance with regard to the weight of complete solution is adjusted into the area between 2 % and 12 %. in particular to still narrowed range between 4 % and 10 % of dry substance.

Water dispersion of polyurethane is performed at room temperature, wherein each desired concentration is achieved by mixing of commercially available polyurethane with water in order to assure each desired ratio between the quantity of polyurethane and quantity of water. L desired concentration of said polyurethane dispersion, expressed as the percentage of dry matter relative to the complete weight of the dispersion is set within the range between 5 % and 15 %, preferably between 6.5 % and 12.5% of dry substance.

As soon as both ingredients are prepared namely said aqueous solution of polyvinyl alcohol and said water dispersion of polyurethane said aqueous solution of polyvinyl alcohol and said water dispersion of polyurethane arc mixed with each other, wherein a required ratio between the quantity of polyvinyl alcohol and polyurethane is established, by which the temperature of such mixture should not exceed 45°C, upon which also each supporting additives like antifoaming agents curing agents. sofieners and clays in a relatively small quantity are gradually admixed, upon which then such obtained clay-like mixture is transferred into each disposable processing bath.

Upon that said clay-like solution as established during the previously disclosed process is applied in a practically the same manner like each usual polyvinyl alcohol solution namely by impregnating said solution into/onto the paper during manufacturing of each security paper in a suitable clay press within a paper making machine from which the excessive solution is returned back to the processing bath. Said processing bath is during manufacturing of paper somehow elevated, and the paper band is running through it and is continuously brought in contact with said clay-like solution. Said solution is impregnated within the paper in a clay press so that the excessive solution can drop back into the processing bath. In this, the paper is furnished with about 0.5 g/nr to 10 g/nr and preferably between 0.5 g/nr to 10 g _/'i rr of said solution. Upon leaving said clay press the paper is transferred along infrared ( I K ) healers and optionally also through a supplemental drying appliance, and is herew ith dried up to each desired level. Consequently by means of such impregnation a film is formed on the paper, which is however transparent and has no essential impact on optical properties of paper.

Said clay-like solution is therefore an aqueous solution of essentially polyvinyl alcohol and polyurethane in combination with a relatively small quantity of supporting additives. Concentration of said clay-like solution expressed as percentage of dry substance relative to the total weight of solution is between 5 % and 15 %. preferably between 6 % and 14 %, and most preferred between 6.5 % and 12.5 % of dry substance relative to the weight of the complete solution. Viscosit} of said solution pursuant to Ford (F4) at the processing temperature is less than 40 s and even better less than 30 s. Fach used polyvinyl alcohol must be suitable for use in paper making industry for the purpose of enhancing mechanical properties of paper. It is available either in granulated or powdered form. Polyvinyl alcohol with high degree of hydrolysis is suitable, which means with at least 90 % or even better at least 95 % of polyvinyl acetate, of which it is synlhetized. Thus, the viscosity of 4 % aqueous solution of polyvinyl alcohol at 20°C is between 10 and 45 mPas and still heller between 1 0 and 45 mPas.

Said ingredient on the basis of polyurethane is a dispersion of liquid polyurethane in the water. The aim of using said polyurethane is to assure improvement of mechanical strength the security paper, waterproolhess as well as resistance against fats. Concentration of polyurethane in the finalized solution expressed as a percentage of dry substance, is set between 2 % and 8 %, and preferably between 2 % and 8 % of dry substance d otal value of all supporting additives, which arc added in a relatively small quantity, should not exceed 20 % of complete dry substance in the clay -like solution, and is preferably less than 1 5 % of complete dry substance in the clay-i ikc solution.

A security paper, in particular banknote paper, which is manufactured by using the previously described method, and having the grammage of about 95 g/irf, is characterized by the following properties:

- average folding resistance is at least 5.500 double folds, measured according to Schoppcr. 1SO 5626. at 20°C and relative moisture 65 %. and normal ly at least 6.000 double folds;

- wnlerproofness i.e. absorption of water according to Cobb, ISO 535, is max. 25

-> p

g m . and normally less than 20 g/m ;

- pi 1 on the surface of paper is 6 to 8:

- porosity according to Bendtscn is max. 5 ml/min. ISO 5636-2, and normally less than 2 ml/min;

- roughness according to Bendtscn is max. 500 ml/min. ISO 879 1 , and normally less than 400 ml/min;

- grease resistance, as determined by KIT test, is at least 3 on both sides of paper, l Al ^J I 559.