This invention relates to novel paper products, and processes for the manufacture of such products by chemical treatment of paper. In particular, the invention relates to security paper products, eg banknotes.
Paper has been used for centuries, mainly for three purposes. First, paper is used to record thoughts or ideas by way of printing and writing. Secondly, paper is used as packaging or as packaging articles such as labels or cartons and bags of paper. Thirdly, paper is also used as a medium of exchange in the form of printed currency, banknotes, cheques etc.
For many paper products it is desirable that the lifetime of the paper should be as long as possible. One example is items such as banknotes which are frequently handled and folded or rolled. Such items come into contact with moisture (eg as perspiration) during normal usage. Another example is paper used in documents such as passports or share certificates which are valuable and contain information which must be preserved over lengthy periods of time.
There is therefore a need for long-lasting paper which will maintain its integrity over a long period of storage and/or after repeated handling. To date, however, this requirement has not been fully satisfied. Conventional papers normally absorb water or moisture to a considerable extent, with the result that the paper becomes soft and easy to tear, and also may spoil the contents of packaging in which it is used. The absorption of moisture also has an adverse effect on the other properties of the paper. For instance, it may be difficult or impossible to print or write on the paper and/or existing printed or written material may fade or become illegible. Paper used as packaging may fall apart or become torn; paper used in currency may have a short lifetime.
One approach to this problem which is used is to coat paper with a varnish. This gives the paper a shiny appearance but is not waterproof and also gives no additional strength to the papers. More recently, processes have been developed to wax papers. The quality of printing and writing on a waxed paper is of reduced quality, and bonding of such papers is also not good.
Another coating process comprises laminating the paper with plastic films such as PVC, polyester, HDP etc. However, these processes suffer from similar disadvantages to waxing. Lastly, plastic alternatives to papers are known which, whilst they are sturdy and waterproof and give good printing, are expensive and therefore not popular. Plastic alternatives to paper are costly and writing on them is not feasible.
GB-A-1095728 discloses surface treatment of materials with mixtures of polymers containing perfluorinated alkyl groups and any of a wide range of other polymers. Paper is one of the materials mentioned.
International Patent Application WO-A-98/14661 discloses a process for improving the water-repellency of papers and in particular to papers for use in banknotes. The process comprises the application of a formulation to the paper, the formulation being described only as a dispersion of a polymer or polymer precursor in organic solvents. The preferred polymers are graft copolymers, the precise nature of the copolymers not being specified.
GB-A-1217489 relates to the impregnation of paper with de-polymerised polybutadiene to increase the strength and water resistance of the paper. US-A-5,660919 discloses a printable paper sheet which is treated with a composition comprising an elastomeric material in order to reduce the water-absorptivity of the paper. US-A-5,480,941 discloses a process for preparing a masterbatch composition comprising an elastomer in an organic solvent with a high concentration of aramid fibers distributed throughout. Ullmann's Encyclopedia of Industrial Chemistry 5th Edition, vol. A 23, 1993, pages 424, 425, 429, 430 and 460 describes additives which provide rubber with various properties, solvents suitable for use with rubber and paper coated with rubber.
There have now been devised novel paper products, a process for the manufacture or treatment of such paper or paper products, and a chemical formulation for use in that process, which overcome or substantially mitigate the above disadvantages.
According to a first aspect of the present invention, there is provided a water-repellent and washable security paper or banknote, which has a Cobb value of between 0.01 and 20.
By the term "a washable and water-repellent paper" is meant a paper which is sufficiently water-repellent to survive exposure to moisture in normal usage and even a washing process.
Such a security paper or banknote is characterized by a very low degree of water absorption. This is expressed in terms of the parameter known as the Cobb value which is known from GB-A-2 052 587. Papers in accordance with the invention more preferably have Cobb values of between 0.01 and 15, and particularly between 0.01 and 10. This is in contrast with conventional security papers or banknotes, for which the Cobb value is typically 50 or more. Despite the very low water absorption and Cobb value of the security paper or banknote according to the invention, the security paper or banknote is nonetheless printable, as well as having other advantageous properties such as improved tensile strength, wet strength and "burst factor". Consequently, the products may be more durable than conventional security papers or banknotes and increased lifetime.
According to another aspect of the invention, there is provided a process for the manufacture or treatment of a security paper or banknote, which process comprises applying to paper, either during manufacture or subsequently, a solution or dispersion of poly(2-chlorobutadiene) or natural rubber in an organic solvent selected from the group consisting of xylene, toluene, methyl isobutyl ketone, mineral turpentine oil, turpentine substitute, white spirit and mixtures thereof, the solution or dispersion being at least substantially free of polymers containing perfluorinated alkyl groups.
The process according to the invention is advantageous in that the security paper or banknote produced by the process has the improved properties referred to above, notably reduced, water-absorbing characteristics compared with conventional security papers or banknotes.
The chemical formulation used in the process according to the present invention preferably comprises a solution or dispersion of poly(2-chlorobutadiene) or natural rubber in either an organic solvent selected from the group consisting of xylene, toluene, methyl isobutyl ketone, mineral turpentine oil, turpentine substitute, white spirit, and mixtures thereof, the solution or dispersion being at least substantially free of polymers containing perfluorinated alkyl groups.
By "substantially free of polymers containing perfluorinated alkyl groups" is meant that the solution or dispersion contains no such polymers or contains very little of such polymers, eg less than 1 % w/w, or more preferably less than 0. 1 % wlw. Preferably, the formulation is free of such polymers.
The poly(2-chlorobutadiene) or natural rubber is preferably present in the formulation at a level of from about 1 % to 20% w/w, more preferably from 1 % to 10% w/w, and most preferably about 5% w/w.
The poly(2-chlorobutadiene) or natural rubber is preferably dispersed or dissolved in a mixture of solvents. A wide variety of solvents may be used, including aromatic solvents such as xylene and toluene. Another example of a suitable solvent is methyl isobutyl ketone. Further components of the solvent system may be incorporated for reasons of economy, or to reduce the flash point of the formulation or to minimise noxious properties of the formulation. Such solvent components are mineral turpentine oil, turpentine substitute or white spirit, ie petroleum-derived solvents.
A particularly preferred solvent is xylene. The xylene may be mixed with mineral turpentine oil or turpentine substitute, most preferably in approximately equal proportions.
The formulation may also include further components which enhance the properties of the finished paper product or have other beneficial properties. Examples include preservatives or antioxidants (or optical whiteners) which prolong the life of the formulation. Such additives are generally present only at relatively low levels, typically less than 1 %. Another example of additives which may be incorporated are plasticizers. Particularly preferred plasticizers are dioctyl maleate and dioctyl phthalate. Such compounds are typically present at the level of a few %, most commonly between 1 % and 10% w/w, typically around 5% w/w.
The formulation may also include one or more components to increase the hydrophobicity of the paper surface and/or reduce aging. An example of such a material is paraffin wax, again typically at the level of a few %, most commonly between 1 % and 10% w/w, typically around 5% w/w.
Other additives which may be present in the formulation include polyvinyl acetate (typically 1 to 10% w/w), ethylene thiourea (typically 0.2 to 0.5%), sulfur (typically 0.1 to 2%), magnesium oxide (typically 1 to 4%), zinc oxide (typically 1 to 10%) and titanium dioxide (typically 1 to 10%). Numerous other additives, such as activators, catalysts; accelerators, cross-linking agents etc, may also be included in the formulation where appropriate.
The formulation may be prepared by dissolving or dispersing all of the components directly in the solvent. Alternatively, the poly(2-chlorobutadiene) or natural rubber may first be mixed or compounded with components such as the inorganic oxides, antioxidants etc and the mixture then dissolved or dispersed in the solvent. Components such as paraffin wax and polyvinyl acetate may be added to the solvent either before or after mixing of the solvent with the poly(2-chlorobutadiene) or natural rubber mixture.
Typically, the formulation has the following characteristics:
The process may be applied during manufacture of the security paper or banknote at any suitable stage. Alternatively, the process may be applied to the security paper or banknote at the end of the normal manufacturing process, or subsequently.
In less developed countries the lifetime of banknotes may be very short (eg no more than a few months) and the regular replacement of banknotes in circulation represents a very costly matter. By extending the lifetime of banknotes by treatment in accordance with the process of the invention very considerable savings may be made.
Security papers or banknotes subjected to the process of the invention may also incorporate other security features, eg watermarking, security threads etc. The water repellency of the paper in itself represents a security feature, being readily demonstrable and distinguishing banknotes made from paper in accordance with the invention from counterfeit notes.
The process of the invention may be applied to any kind of security paper or banknote made of any kind of pulp, cotton comber or rag, before calendering, before or after polyvinyl alcohol / gelatine bath or final processing. The process may be applied before or after application of a watermark, eg by cylinder mould, four dinier or other process. Similarly, the process may be applied before or after full or alternate embedding or window threading of any type of security thread, eg comet, continuous printed or unprinted, metallised or demetallised, positive text or negative text, magnetic or machine-readable threads etc. Coating by the process of the invention may also take place before or after incorporation of UV-detectable fibres. The process may also be applied after the security paper or banknote is calendered and ready for use or printing.
The process may be applied to all types of security paper and banknote, before or after printing by any type of printing ink, including intaglio, magnetic, short-wave, UV, etc.
The chemical formulation may be applied by various means, eg using a conventional varnishing machine, gravure machine, brush coating, dip coating or any other method of coating, or on-line during manufacture of the paper. The formulation may be applied either before or after printing, to paper sheets, paper web or any other paper article. Despite the reduced water absorptivity of the paper, it is nonetheless receptive to the printing process, eg by intaglio or dry offset printing.
The application of the process of the invention will now be illustrated by means of the following examples and accompanying drawing which shows schematically the stages in the production of a security paper containing a watermark and security thread.
The formulation is prepared by dissolving or dispersing the polychloroprene, paraffin wax, optical whitener (antioxidant) and dioctyl maleate (plasticizer) in the solvent mixture.
Prepared by the same method as Example 1.
Prepared by the same method as Example 1.
Prepared by the same method as Example 1.
The formulation is prepared by mixing the polychloroprene, titanium dioxide, zinc oxide, ethylene thiourea and optical whitener, and then dissolving or dispersing the mixture in the solvent, either after or before addition to the solvent of the paraffin wax, polyvinyl acetate and dioctyl maleate.
Prepared by the same method as Example 5, but omitting the polyvinyl acetate.
The formulation is prepared by mixing the rubber, sulfur and optical whitener, and then dissolving or dispersing the mixture in the solvent, either after or before addition to the solvent of the paraffin wax.
Referring to the drawing, a manufacturing line and process for a security paper incorporating a watermark and security thread (eg suitable for the printing of banknotes) contains the following stages:
In the arrangement illustrated, the formulation of the invention is applied at the point 10, using an on-line coating machine.
Alternatively, the process may be applied at other stages. For example, the treatment may be applied between stages 4 and 5, between stages 5 and 6, stages 8 and 9, stages 11 and 12, stages 12 and 13, stages 13 and 14, or subsequent to stage 14.
The process of the invention, comprising treatment of the paper with the chemical formulation of the invention, may also replace the PVA/gelatin bath at stage 7.
It will be understood that the process of the invention may be applied at any suitable stage of the paper manufacturing process. In other paper manufacturing processes the sequence and methods of operations may be different to that illustrated, and the precise point at which the treatment according to the invention is carried out will generally not be important.