Security paper containing fused thermoplastic material distributed in a regular pattern

Abstract

A method of making security paper such as paper for currency notes and paper made by the method. The method comprises incorporating in the body of the paper thermoplastic material, such as polyethylene, polyamides, or polyvinylchloride, and raising the temperature of the thermoplastic material to a level at which the material fuses. Preferably the material is in the form of a nut which has been so stretched that the filaments between the nodes of the net have their molecules orientated so that on fusion the filaments part and draw back towards the nodes to leave islands. The material may also comprise single filaments or granules, and it may be incorporated into the paper sheet during deposition on a Fourdrinier or cylinder paper machine.

Parent Case Text

This is a continuation of copending application Ser. No. 199,130, filed Nov. 16, 1971, now abandoned.

Description

BACKGROUND OF THE INVENTION

This invention relates to the manufacture of security papers, that is to say papers which are used for the production of printed items which are of sufficient value to be a temptation to forging. The major use of such papers is in the production of currency notes but other uses include travel tickets and share certificates.

To detect forgeries, the genuine paper should have a feature or characteristic which can easily be detected by, for example, a bank cashier, but cannot be reproduced without great difficulty. Paper used for security papers is usually watermarked but it is possible for forgers to imitate watermarks sufficiently well to escape detection under casual scrutiny. Another problem with currency papers is that notes are sometimes split by delamination and the two halves are then used separately after further treatment. Bank notes are known with metal strips embedded in them but such strips are expensive and may even encourage delamination by providing a starting point for separation.

Accordingly, it is an object of the invention to provide a security paper which is difficult to reproduce. It is another object to provide a security paper which is difficult to delaminate.

SUMMMARY OF THE INVENTION

The present invention provides a method of making security paper which comprises incorporating in the body of the paper thermoplastic material and raising the temperature of the thermoplastic material to a level at which the material fuses.

When the material fuses it becomes amalgamated with the fibres adjacent to it, that is to say it impregnates these fibres, thus becoming inseparable from the paper. Delamination is thus made virtually impossible. Also, the presence of the thermoplastic material can, depending upon the particular properties of the material and of the body of the paper, be detected by examination under transmitted light, by direct light or by feel.

The invention also includes, according to another aspect, a security paper having non-fibrous material incorporated in the body of the paper and characterised in that the material is a thermoplastic material which has been fused in situ so as to mingle locally with the fibrous material of the paper.

Various thermoplastic materials may be employed. Suitable materials with appropriate fusion temperatures below that at which the paper is adversely affected include polyamides, polyvinylchloride and low melt polyethylene. Preferably the material fuses below 130.degree.C.

The thermoplastic material prior to incorporation in the paper may be in various forms and it may be incorporated in various ways. Preferably it is in continuous form such as a single filament or a sheet of parallel filaments or a foraminous sheet, i.e., a sheet having many holes therein, such as a net. The net may be of various types. For example it may be woven or knitted or it may be extruded as a net. The most preferred form of the thermoplastics material is a polymeric net which has been so stretched that the filaments between the nodes of the net have their molecules orientated. Such a net may be made by extruding a polymeric film, embossing it in such a manner with an embossed surface that when the embossed film is subsequently stretched in the cross and machine direction the film structure expands to produce a net like structure in which the cross over points or bosses are linked to other contiguous bosses by thinner strands of polymeric material. The arrangement of the bosses and their spacing are dependent upon the geometry of the embossed engraving on the surface of the embossing roll, together with the degree of stretch applied in the machine and cross directions. This method of making a net is described in greater detail in British Pat. No. 914,489. When a paper incorporating a net having filaments with orientated molecules is heated it is found that the filaments tend to shrink back towards the nodes to leave a series of islands of thermoplastic material arranged in a pattern determined by the form of the original net. The existence of the pattern can be detected and therefore it acts as a security check which can be reproduced only with the greatest difficulty. In most cases the pattern can be detected visually either in transmitted or reflected light or both.

Where the thermoplastic material is in continuous form prior to incorporation in the paper, it may be incorporated during the actual deposition of paper stuff to form the paper web. For example the material may be fed on the paper forming surface of a paper making machine (e.g., onto the cylinder of a cylinder paper machine or onto the wire of a Fourdrinier machine) in such manner that the material contacts the cylinder at a point at which a proportion, preferably about half, of the substance of the desired paper web has already been deposited on the surface. Thus the remainder of the substance of the sheet will be deposited over the material, drainage taking place through the material. In this way the material, e.g., a polymeric net, may be completely incorporated into and enclosed within the freshly formed sheet of paper. The resulting sheet normally can be processed on conventional vacuum boxes and drying systems and the waterleaf structure can normally be sized in the conventional manner.

Where the paper is made up of more than one super-imposed web of newly deposited fibrous material, as is the case in a board machine and in a tissue machine, the thermoplastic material may be introduced between two superimposed webs. Papers made from more than one layer have hitherto been unpopular for security use because of ease of delamination but the subsequent fusing of the thermoplastic material will bind the layers together to resist delamination.

Where the thermoplastic material is in continuous form such as a net it is preferably dispensed from a container in which it is arranged concertina fashion so that a further strip can be joined to the end of the strip being fed into the paper forming machine before it is exhausted so that paper making can proceed continuously. Strips can be joined for example by water-insoluble cements or by heat fusion.

The thermoplastic material may be incorporated in forms other than those already referred to. For example, the thermoplastic material may be in the form of powder or granules. In this case, the material is preferably added between two layers of paper material on, for example, a tissue machine. The powder may be of one colour or more than one colour randomly mixed or means may be provided for depositing the powder in geometric patterns such as stripes which may have different colours in different portions.

Various effects can be produced by appropriate treatment of the thermoplastic material prior to incorporation. For example, it may contain dyes or pigments to colour the material or it may contain opacifying pigments to make the material less transmissive to light than the surrounding paper or it may contain metallic particles or be metallised to reduce its transmissiveness or to permit the use of automatic detection means. Where the material is in sheet form, it may be treated by printing prior to incorporation in the paper. The material may be treated so that when incorporated in the paper and viewed by direct or transmitted light it has a distinctive outline such as a patterned outline.

The fusing may be complete so that the thermoplastic material becomes molten right through but this is not essential and in some cases softening of only parts of the thermoplastic material will be sufficient. For example, in the case of filaments it may not be necessary for the core of each filament to become softened while in the case of nets it may be satisfactory if only the connecting filaments are melted while the nodes are not fully softened. In any case, it is preferred for the fusing to be accompanied by pressure as by passing the paper between a pair of rolls; rolling may immediately follow heating or the rolls themselves may be heated so that heat and pressure are applied simultaneously. Fusing may be effected at any stage between formation of the final web and the point of use of the paper. Specifically, it may occur before, during or after a sizing and drying operation or before, during or after conditioning or before, during or after calendering.

As mentioned above, the presence of the fused polymer can normally be detected visually by observing the paper in either reflected or transmitted light. Such inspection will reveal the dots or other arrangements present inside the paper in characteristic patterns. In transmitted light uncoloured polymer appears as dots of enhanced transmission. Coloured nets produced by the incorporation of dyes or pigments in the polymer before extrusion or applied to the film by dyeing, printing, or metallising techniques, can, after the application of a thermal cycle such that the net is fused, be detected as a series of coloured dots.

The presence of the thermoplastic material will, in some cases, produce an enhanced tear as well as the enhanced delamination feature which has been described above. Another security feature which will normally be present is the enhanced thickness which the presence of the thermoplastic material produces in the paper. These enhanced thicknesses are readily detectable by touch. Such raised effect which will usually be on both sides of the paper cannot be simulated by embossing techniques.

The net structure, or thermoplastic material in other form, can be so incorporated that it is present in the form of single or multiple bands in the machine or in the cross direction of the paper, or can extend throughout the paper. The sides of the bands can be parallel or sinusoidal, or other regular or irregular patterns may be used.

Clearly, all these characterising effects such as bands of continuous colour, several colours, shape, and spacing of dots or bands can be used separately or jointly to produce discrete, characteristic, and readily recognised effects on security papers which cannot be readily simulated by unauthorised persons.

Incorporation of radioactive, metallic, ferromagnetic, fluorescent and other known security devices within or on the polymer can add yet other means of detection, recognition or counting for authorities who issue or inspect security documents.

Other features and advantages of the invention will be apparent from the following description in which the invention is illustrated with reference to the accompanying drawings and from the examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a purely diagrammatic side view of a single cylinder paper making plant;

FIG. 2 is a diagrammatic side view of a paper making plant incorporating two cylinder machines;

FIG. 3 is a diagrammatic side view of an air chamber paper making machine;

FIG. 4 is a diagrammatic side view of a four-vat tissue machine;

FIG. 5 is a diagrammatic side view of a Fourdrinier machine;

FIG. 6 is a diagrammatic plan view of a portion of paper incorporating a web prior to heating of the web;

FIG. 7 is a plan view similar to FIG. 6 of the paper after heating; and

FIG. 8 is cross-section to an enlarged scale of the paper shown in FIG. 7.

DESCRIPTION OF PREFERRED EMBODIMENTS AND EXAMPLES

in the plant shown in FIG. 1 the cylinder paper making machine 1 has a vat 2 with an inlet 3 for paper stock and an overflow outlet 4. Paper is formed on the periphery of a screen cylinder 5 from which it is removed by a couch roll 6 and carried away on a felt 7 to a drying zone 8 in which, for example, the paper may be dried between felts which pass over steam heated drums. After drying the paper is passed to a sizing facility 9 and then passes to a heating zone 10. A strip of thermoplastic net is incorporated in the paper as it is being formed on the cylinder 5. This is achieved by feeding the net 11 in continuous strip form from a package 12 into the vat 2 and onto the surface of the cylinder 5 at a point 12 where some paper-forming fibres have already been deposited on the cylinder. Adjustable guide rollers 13 direct the net web 11 onto the cylinder at the selected point. The package 12 contains the net strip in concertina folded form with a tail 15 which is joined by an adhesive or by welding to the head of a reserve package 12'. As soon as the package 12 is exhausted the package 12' begins to supply the net and a further package will be attached to the tail of the package 12'.

In the plant shown in FIG. 2 there are two cylinder machines 21 and 22 similar to the machines shown in FIG. 1 but connected by a single felt 23. The paper web formed by the machine 21 is carried by the felt 23 to the machine 22 where the web formed by this machine is superimposed on that formed by the machine 21. The combined sheet is conveyed through a drying zone 24, a sizing zone 25 and a heating zone 26 as in the plant shown in FIG. 1. Between the two machines 21 and 22 there is means for feeding a strip of thermoplastic net 27 onto the web formed by the machine 21 so that the net becomes sandwiched between the paper webs formed by the two cylinder machines.

In the plant shown in FIG. 3 the two cylinder machines of FIG. 2 are replaced by air layering machines 31 and 32, the first of which forms a web which is carried by a belt 33 past a net feeding means 34, the net becoming sandwiched between the web formed in the machine 31 and the web which is subsequently formed over the net in the machine 32.

The plant shown in FIG. 4 comprises a tissue machine of the four-cylinder kind having four vats 41, 42, 43 and 44, the webs from which are combined first into two webs at points 45 and 46 and then into a single web at point 47. Positioned above the web formed from the two webs from the machines 43 and 44 there is a hopper 48 from which particles of thermoplastic material 49 are deposited onto the web. It will be seen that these particles become sandwiched between the two two-ply webs at the point 47. The four-ply web then passes to a heating zone 48.

FIG. 5 shows a Fourdrinier machine having a stock inlet 51 which supplies paper stock to a Fourdrinier wire 52 from which the paper is passed through a press section 53, drying zones 54 and 55 and a calendering zone 56. Means 57 are provided for incorporating a thermoplastic net into the paper being formed on the wire 52, the distance along the wire at which the net is supplied to the wire being adjustable.

Various examples of security papers made on the machines described will now be set out in more detail.

EXAMPLE 1

A paper was made on the plant shown in FIG. 1 and incorporating a net prepared in the manner described above and in British Pat. No. 914,489. The net was of high density polythene coloured red by the inclusion of pigment during extrusion, and was so embossed that it contained an average of 97 bosses/sq.inch, and 30 yards of 38 inches wide net weighed 1.1 lb. The net was cut into tapes 5 cm wide and fed into the vat 2 of the cylinder forming machine 1 on which an 80 g.s.m. (grams per square metre) beaten cotton paper was being formed.

The angle and point of contact of the net band with the wire was adjusted by the guides 13, such that the amount of stock deposited on the wire before the net contacted the cylinder was approximately 40 g.s.m. It was observed that under these conditions sufficient drainage occurred through the net to result in a total deposition of 80 g.s.m. of paper fibres, with the paper formed through and around the net.

The web was couched on to the felt 7, dried over drying cylinders in the normal fashion in the drying zone 8, and then tub sized in the sizing zone 9. A diagrammatic plan view of a portion of the paper at this stage is shown in FIG. 6.

The web was then passed through the heating zone 10 which contained infra-red heaters by which the temperature of the paper was raised above the fusing temperature of the included net. After calendering the paper was examined, and it was found that on tearing no trace of the net structure could be detected. The red polythene dots which represented the residual bosses were plainly visible inside the paper by both transmitted light and reflected light. The presence of the security device, i.e., the dots, could be detected by feel. A diagrammatic plan view, similar to FIG. 6, of the finished paper is shown in FIG. 7 and a cross-section to an enlarged scale is shown in FIG. 8. It will be seen that the filaments between the nodes of the thermoplastic net have, under the influence of heat and because of the orientation of their molecules, broken and have drawn back into the nodes to leave a pattern of dots. These dots are faintly visible in direct light and are plainly visible in transmitted light. It will be seen from FIG. 8 that there is a slight elevation of both surfaces of the paper adjacent the nodes and these cna be detected by touch. It will also be seen from FIG. 8 that the thermoplastic material has mingled with the adjacent fibres of the paper.

The fused thermoplastic material thus gives the paper features which are very difficult for the forger to reproduce but whose presence can readily be detected; at the same time the material makes delamination of the paper extremely difficult.

EXAMPLE 2

A waterleaf sheet prepared as in Example 1 except that the heating zone 10 contained oil-heated rolls instead of infra-red heaters and the sizing zone 9 was placed after the heating zone 10. The paper was heated and compressed by passing it over the oil-heated rolls at 180.degree.C surface temperature at 5 m/min. The conditions of roll loading were varied until by examination of the waterleaf paper the net was found to be fused. The security aspects noted before were retained after sizing and the normal finishing treatments had been applied.

EXAMPLE 3

An unstretched but embossed polythene sheet was coloured by printing stripes of black, red, and yellow nitrocellulose lacquers extending in the cross direction. When stretched the net had stripes of black, red, and yellow bosses across it. Subsequently, 10 cm tapes were slit from the net and processed according to Example 2. The characteristic colours were readily detectable within the finished paper.

EXAMPLE 4

A coloured net 58 inches wide prepared as in Example 3 was fed onto the multivat cylinder machine shown in FIG. 2 in such a way that it contacted the first couched web before the second couched web was applied to it. After drying and fusing the net as in Example 2, the security effects described were readily detected. In addition the paper had an enhanced resistance to delamination.

EXAMPLE 5

A net prepared as in Example 3 was placed between two preformed and dried webs of waterleaf paper. Hot pressing as in Example 2 caused the plies mutually to adhere and produce the security effects described earlier. These effects were retained in other samples which were sized and calendered.

EXAMPLE 6

Shredded paper fibres were deposited onto the moving perforated belt or wire 33 of the machine shown in FIG. 3 to produce a 30 g.s.m. deposit in such a manner that a web of net prepared as in Example 1 could be overlaid by the means 34. The whole was then passed into the second air laying chamber 32 and a further 30 g.s.m. deposition of paper fibres achieved.

After passage through compression rolls in the zone 35 the web was saturated with a polyvinyl acetate aqueous binding and dried to 5 percent water content in the zone 36.

The resultant web was then passed through a heating zone 37 as in Example 2.

When examined the product was tough and paper like in handle, and clearly showed the security effects both by reflected and refracted light.

EXAMPLE 7

Paper was prepared on the tissue machine shown in FIG. 4 with each of the four cylinders producing a paper web of 20 g.s.m. The hopper 48 provided over one of the component webs contained granules of coloured polythene. The hopper was arranged to deposit the granules 49 in bands extending in the machine direction of the paper. When the component webs were combined the granules were between the two centre webs. The resultant web was passed through the heating zone 48 as in Example 2. The granules fused and bonded the component webs securely together so that delamination was impossible.

EXAMPLE 8

A 300 denier polyamide terpolymer containing 66 nylon, 12 nylon with a melting point of 120.degree.C was metallised by the vacuum deposition of aluminium. The monofil was then fed onto the cylinder paper making machine shown in FIG. 1 in such a manner that the thread contacted the cylinder after 40 g.s.m. of paper fibres had been deposited. In the residual rotation of the cylinder a further 40 g.s.m. of paper fibres were deposited. The paper web was couched and dried by passage over steam heated rollers in the zone 8. During this latter operation the polyamide thread fused so that the separate heating zone 10 could be omitted.

Inspection of the paper by transmitted light showed the presence of a metallised thread, but it was impossible to split the paper on this line, or to withdraw the thread.

EXAMPLE 9

A polythene monofil with a melting range starting at 120.degree.C was printed with nitrocellulose inks, by passing through a series of rollers each of which was synchronised to deposit a 2 mm long ink mark at 5 mm spacing onto the thread.

In this manner a series of colours were placed on the monofil.

The monofil was incorporated into a paper in the manner described in Example 8. On inspection the dried paper showed both by refracted and reflected light a series of spaced coloured dots inside the paper.

EXAMPLE 10

A paper was prepared as in Example 9 in which the monofil was pigmented with titanium dioxide to produce such an opacity that in the dried paper the thread was not detectable but only the coloured dots were visible.

EXAMPLE 11

Paper having a weight of 80 g.s.m. was prepared on the Fourdrinier machine shown in FIG. 5. The position of the net introducing means 57 along the wire was adjusted until the net was positioned in the central plane of the resulting paper. The temperature of the calendering rolls 56 was adjusted to produce melting of the filaments of the net while the bosses of the net were only softened on the surface. A paper having the structure shown in FIGS. 7 and 8 was obtained.

Claims

What we claim as our invention and desire to secure by Letters Patent is:

1. A method of making security paper comprising incorporating in the body of the paper a thermoplastic identification material in continuous form, said material comprising regions of enlarged cross-section connected by filaments of lesser cross-section, and raising the temperature of said thermoplastic material to a temperature at which said material fuses and said filaments part and draw back towards said regions of enlarged cross-section and the material is bonded to the fibres of the paper, whereby islands of thermoplastic material bonded to and within the paper are positioned in the paper in locations determined by the locations of the enlarged regions in the original thermoplastic material in continuous form.

2. A method according to claim 1, wherein, prior to incorporation of the identification material in the body of the paper, it is stretched to cause the molecules in the filaments to be oriented lengthwise thereof.

3. A method according to claim 2 wherein said thermoplastic identification material is in the form of a net which has been so stretched that the filaments between the nodes of the net have their molecules oriented.

4. A method according to claim 3 wherein said net is made by extruding a polymeric film, embossing the film and stretching the film in the cross and machine directions to produce a net-like structure having nodes interconnected by filaments having their molecules oriented.

5. A method acording to claim 1 wherein said thermoplastic identification material is in the form of continuous threads.

6. A method according to claim 1 wherein said thermoplastic identification material is fused at a temperature below 130.degree.C.

7. A method according to claim 1 wherein said thermoplastic identification material is incorporated during the actual deposition of the paper stuff to form a paper web.

8. A method according to claim 1 wherein a plurality of separately produced webs of newly deposited fibrous material and said thermoplastic identification material are superimposed on one another with said identification material being located between adjacent ones of said webs of newly deposited fibrous material.

9. A method according to claim 1 wherein said fusion of the thermoplastic identification material is accompanied by pressure applied by passing the paper between a pair of rolls.

10. A method according to claim 1 wherein said thermoplastic identification material is treated prior to incorporation in the paper with a coloring agent.

11. A method according to claim 1 wherein said thermoplastic identification material is treated prior to incorporation in the paper with an opacifying agent.

12. A method according to claim 1 wherein said thermoplastic identification material is treated prior to incorporation in the paper with a printing agent.

13. A method according to claim 1 wherein said thermoplastic identification material is treated prior to incorporation in the paper with a metallizing agent.

14. Security paper having within the body of the paper islands of thermoplastic identification material, each island being surrounded on all sides by and mingled locally with the fibrous material of the paper, the islands being distributed in a regular pattern and distributed in both the cross and machine directions of the paper.

15. Security paper according to claim 14 wherein said thermoplastic identification material has a color different from that of said paper.

16. Security paper according to claim 14 wherein said thermoplastic identification material is metallised.