GB2412350A

GB2412350A - Ink jet recording using differential gloss inks.

Info

Publication number: GB2412350A
Application number: GB0406536A
Authority: GB
Inventors: Rebecca Louise Bugliarello; Alan Hodgson; William Edward Long
Original assignee: Ilford Imaging UK Ltd
Current assignee: Ilford Imaging UK Ltd
Priority date: 2004-03-24
Filing date: 2004-03-24
Publication date: 2005-09-28
Anticipated expiration: 2024-03-24
Also published as: GB0406536D0; GB2412350B

Abstract

To produce a copy resistant document which may be readily authenticated, an ink jet printing method includes printing with a first ink to provide a first printed area having a certain colour, diffuse optical density and gloss value, and printing a second area with a second ink to have the same colour and diffuse optical density as the first area, but with different gloss value. The two areas produce a distinctive specularly reflective mark to the eye by reason of the differential gloss. A standard commercial ink (gloss value 58.5%) may be used to print a mark (reference 5 in schematic figure). A non-glossy ink (gloss value 25.4%) may be used to print a background (reference 6 in schematic figure). The mark is visible to the unaided eye to facilitate document validation, but is not reproduced by photocopiers or scanners (figure 5). The ink may be chosen so that the mark is bronzed.

Description

24 1 2350 Ink Jet Recording Method

Field of the Invention

The present invention relates to a recording method using the ink jet printing process. In particular it relates to an ink jet recording method to enable rapid visual validation of an original document.

Background of the Invention

The ready availability of high quality colour photocopiers and, more recently, of scanners, digital printers, and image-editing software, have made it easy for just about anyone to reproduce or alter a document making it hard to tell legitimate documents from fakes.

Therefore a requirement has arisen for methods for authentication of documents in ways that can be readily observed and verified by a human reader without interfering with readability or viewing of the document, but which are not recognised by a conventional copier or scanner and thus protect against copying. Hereinafter the term print will be used to describe any printed material whether the printing carried by the material is in the form of text such as a document or is graphic or pictorial such as a print, sign, image, or the like.

One approach which has been used is to create a difference in specular reflected light and diffused light where an image is printed that can readily be discerned by a human reader by holding the print at an angle. This difference is visible to the eye as a difference in gloss: portions of an print which specularly reflect light appear glossy and portions of an print which diffuse light appear matt or dull. This effect is described on pages 110 to 112 of the book Handbook of Print Media, edited by H.Kipphan, English edition published by Springer-Verlag Berlin in 2001. A measure of specular reflected light is the measured gloss to DIN 67530 /ISO 2813, and a measure of the diffuse light is the optical density as perceived by a scanner or copier or by an observer under normal viewing circumstances.

Such differences cannot be detected by a conventional copier or scanner as such devices are designed to minimise the effects of specularly reflected light, generally by illuminating at an oblique angle and detecting normally to the print.

Hereinafter this effect will be described as differential gloss. As used herein, the term differential gloss is used to describe gloss differences observable by the eye on the print, for instance between the printed ink and the supporting medium, between regions printed using two or more different inks, or by printing with a varnish or laquer.

It is known to use differential gloss as a copy-prevention method. For example, United States Patent 5,695,220, describes a method for producing a validation mark on security paper by printing a mark on the paper using an ink that has the same colour as the paper but which has more uniform directional reflectance than the sheet of paper. The paper on which the ink is applied, because of its inherent glossiness, has a tendency to reflect light preferentially at an angle of reflectance which corresponds to the angle of incidence of the incident light. Accordingly, the brightness of the unprinted portions of the paper depends on the angle at which it is viewed. In contrast, the brightness of the printing is substantially independent of the angle at which it is viewed. As a result, at most viewing angles the validation message will be visible because it will appear either brighter or darker than the

unprinted paper background.

Similarly, United States Patent 6,108,512 discloses a copy prevention method in a xerographic printer wherein text is printed using clear plastic toner which reflects more light than the paper. Thus, the only optical difference between toner and non-toner portions of the page is in the reflectivity and a human reader can read the image by holding the page at such an angle that the eye will intercept the reflected light from the toner, producing a contrast between the lighter appearing toner and the darker appearing paper. On the other hand, a copier scanner will detect no difference and the copier will not be able to copy the original.

Further, United States Patent 6,664,017 discloses a xerographic method for placing permanent security marks on documents, which marks are formed by a composition comprised of a white pigment and a polymer which when fused form a white glossy mark which is visible to the eye at any angle, and an optional further security mark which is not visible to the naked eye, which mark is comprised, for example, of toners containing a component that fluoresces under an ultraviolet light.

Furthermore, United States Patents 6,245,136 and 6,626,985 disclose a matte ink and a method for preparing a matte - gloss contrast finish by first coating a printing substrate with a gloss film laminate, drying the gloss film, and then lithographically printing the matte ink to provide a differential gloss effect.

One limitation with these methods is that they are restricted in the printing media and printing techniques which may be used. They are not known for use with ink jet printers.

An additional limitation with many of these methods, and also by conventional watermarked paper, is that they require a clear or unprinted area of the print to work well, and this is particularly restrictive if the print is an image such as a photograph.

European Patent Applications 1 367 810 A, 1 370 062 A, and 1 377 007 A disclose methods for the control of differential gloss when halftone image data is printed into hard copy. In particular, perceived gloss in a halftone image is manipulated by splitting the image into two halftones having different anisotropic structure orientations. Two parts of the image having the same density but different anisotropic halftone dot orientations appear to the eye as showing differential gloss. For instance, the first halftone may have a 45 orientation to the right and the second halftone may have a 45 orientation to the left. This different orientation gives the appearance of differential gloss when the print is viewed as a specular reflection by eye and may be used as a security or validation mark. A limitation of this method is that it only works well in mid tones; also the need to use an ordered screen for the halftoning step may pose image quality problems for pictorial content.

Therefore, there still exists a need for a general method to control gloss and allow manipulation of differential gloss without requiring special papers or substrates. There is particular interest in such a method which makes use of the ink jet printing process as a result of the ready availability of ink jet printers and rapid development of ink jet techniques in recent years.

Ink jet recording is a non-impact printing method wherein droplets of a recording fluid, the ink, are formed by forcing the fluid through a tiny nozzle (or a series of nozzles) under computer control and deposited on a suitable recording material such as paper. There are several classes of ink jet printer, for instance thermal drop-on-demand printers, piezo dropon-demand printers, and continuous ink jet printers. Ink jet recording has undergone extensive development in recent years, and has found broad application as output for personal computers in the office and the home because of its accessible price, low noise characteristic, capability of high speed printing, facility of multi-colour printing, reliability, and achievable image quality.

Ink jet printers for the home or office generally use aqueous inks, that is to say inks wherein the carrier liquid is predominantly water, but may additionally comprise at least one water miscible organic cosolvent such as a polyhydric alcohol. In commercial ink jet printers, a set of four inks is typically provided for most uses comprising the three subtractive primary colours cyan, magenta, and yellow, together with a black ink (commonly referred to as a CMYK ink set). However it is also known to use ink sets comprising more than four inks, for example sets including dilute inks or additional coloured inks such as red, blue, orange, violet, or green inks.

Hitherto, ink jet inks have commonly employed dyes as colorants. By the term dye is meant a colorant which is present as a solution in the ink. Using dyed inks the printed ink jet image generally has essentially the same gloss value as the medium on which it is printed and thus there is little scope for the production of any differential gloss effects.

More recently, to overcome certain deficiencies to the use of dyed inks such as relatively poor light stability and fastness to gases, washing, and weathering, it has become more common to use pigments as the colorants for ink jet inks instead of, or sometimes in addition to, dyes. By the term pigment is meant a colorant which is provided in an insoluble particulate state. Using pigmented inks it is sometimes found that the printed ink jet image has a significantly different gloss value from that of the medium on which it is printed, and also that the different inks of an ink set have different gloss values from each other. Hitherto, this has been seen as a disadvantage, to the extent that United States Patents 6,598,967, 6,644,799, and 6,666,553 disclose methods for avoiding differential gloss in ink jet images printed using pigmented inks.

We have developed an ink jet printing method for providing visible marks on prints that are only visible as a gloss modulation and for using these marks as a method for authentication of documents, prints, and images.

Summary of the invention

According to this invention, there is provided an ink jet printing method comprising: 1) printing at least one first ink to provide a first printed area having a colour a diffuse optical density and a gloss value; and 2) printing at least one second ink to provide a second printed area having a colour and diffuse optical density the same as that of the first area of the print and a gloss value different from that of the first area of the print; wherein the first and second printed areas produce a distinctive specularly reflective mark to the eye by reason of differential gloss.

By colour and diffuse optical density is meant the diffusely reflected colour of the print as normally perceived by the eye. For the colour and optical density of the printed area of the second ink to be the same as that of the first ink is meant that the colour and density of the two inks are sufficiently similar that they are not perceived as different by eye or by a conventional copier or scanner. One of the advantages of the present invention is that it may be applied over a wide range of colour and optical density values.

On the basis of work reported by R. HoLmann et al on page 209 of the proceedings of the Society for Imaging Science and Technology NIP17 International Conference of Digital Printing Technologies, September 30 October 5, 2001, it is believed that a colour difference represented by a AE value greater than 10 will be visible by eye under normal viewing conditions. Therefore for the purpose of this invention, it is considered that the colour difference between the printed area of the first ink and that of the second ink should be represented by a AE value of less than 10. Colour and optical density is discussed in section 1.4 of the previously mentioned book Handbook of Print Media, edited by H.Kipphan, English edition published by Springer-Verlag Berlin in 2001; in particular AE values are defined on page 74.

By gloss is meant the degree to which the printed area specularly reflects light and appears visually glossy to the eye. The gloss value of the printed ink may be represented by the 60 gloss measured according to DIN 67530 / ISO 2813, usually reported by comparison with a black glass plate. The gloss value of the printed areas of the first and second inks is not restricted, provided there is sufficient difference in the gloss values of the two inks to provide a visually distinctive mark. A sufficient difference depends on factors such as the ink colour and the illumination conditions, and also on the actual gloss of the prints; that is to say that when the prints are glossier a larger gloss difference is required to be visible to the eye. For the purpose of this invention, it is considered on the basis of work reported by Y. Ng et al on page 88 of the Society for Imaging Science and Technology PICS Digital Imaging Conference, May 13 16, 2003 that a sufficient difference between the two inks to be noticeable is approximately 14% of the actual gloss value of the less glossy of the two printed inks.

However, it is preferable that the difference in gloss between the two printed areas is rather greater than this: depending on the medium in use it is especially preferred that the difference in gloss value be sufficient for an observer to consider one of the printed areas as glossy and the other as semi-matt, commonly described by terms such as pearl or eggshell; or for an observer to consider one of the printed areas as semimatt and the other as matt; or for an observer to consider one of the printed areas as glossy and the other as matt. A suitable difference in gloss values between the printed areas depends on factors such as the ink colour and the illumination conditions, but a suitable difference is at least 50% of the gloss value of the less glossy print.

Typically, the first ink of the invention is used to provide the main, or background printed area of the print, and the second ink is used to provide the visually distinctive mark.

However use of the inks may be reversed.

The formulations and components of the first and second inks are not specifically restricted as long as they are selected to provide different gloss values to the respective printed areas of the print and are compatible with the printer to be used and the medium to be printed.

Suitable components for the inks may be any of those commonly used in ink jet inks, but typically and preferably, the first ink is a conventional aqueous pigmented ink comprising at least one pigment in the form of fine particles which are stabilised by the presence of one or more dispersants, suspended in an aqueous carrier medium. Such inks are well known in the art. Depending on the medium on which it is printed, such an ink typically provides a moderate gloss value. The gloss value of a print from an ink of this type depends on the particular pigment and dispersant in use, the particle size and concentration of the pigment, the weight ratio of pigment to dispersant, the composition of the aqueous carrier medium of the ink, and the printing medium on which it is printed.

By an aqueous carrier medium is meant a liquid carrier vehicle which is water or is predominantly water. Deionised water is commonly used. The carrier vehicle may consist solely of water, but optionally and preferably the fluid vehicle is a mixture comprising water and at least one relatively involatile water-miscible organic cosolvent such as a polyhydric alcohol. More preferably the carrier vehicle comprises a plurality of additional water-miscible organic cosolvents. The additional water-miscible organic cosolvent or cosolvents are not particularly limited and may be any organic solvent which has sufficient solubility in water.

Preferred examples of water-miscible organic solvents that may be selected include: hydroxylic organic solvents, especially solvents having at least 2 hydroxyl groups, for example ethylene glycol, thiodiglycol, glycerol, 1,3-propane diol, 1,2-hexanediol, and 1,5-pentanediol; ethylene glycol condensates such as diethylene glycol, triethylene glycol, polyethylene glycol, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, and ethoxylated glycerol; nitrogen containing organic compounds such as 1,3-dimethyl imidazolidinone, urea, pyrrolidone, and N-methyl-2pyrrolidone; and mixtures comprising these solvents.

The solvent or solvents used and the concentration of organic solvent or solvents in the first ink may be selected according to the printer in use and other properties desired of the ink, but typical inks may comprise up to a total of approximately 50% of the organic cosolvent or solvents, preferably in the range of approximately 5% to 15% of each of a combination of at least two organic cosolvents. The organic solvent components generally serve as humectant to prevent drying out of the ink in the nozzles of the printer, but may also serve as cosolvent, viscosity regulating agent, ink penetration additive, wetting agent, levelling agent or drying agent.

The term pigment describes a colorant which is essentially insoluble in the aqueous ink medium. Many pigments are listed in the Pigments and Solvent Dyes section of the Colour Index International, published by the Society of Dyers and Colourists in 1997. Almost any suitable insoluble coloured compound may be used in the pigmented inks of the invention, but especially suitable pigments include those classified by the Colour Index as C.I.

Pigment Black 1; C.I. Pigment Black 7; C.I. Pigment Yellow 1, 3, 12, 13, 14, 17, 24, 55, 74, 81, 83, 87, 95, 97, 98, 100, 101, 104, 108, 109, 110, 117, 120, 138, 151, 154, 155, 180, and 213; C.I. Pigment Orange 5, 13, 16, 17, 36, 43, 51, 71, and 73; C.I. Pigment Red 1, 2, 3, 9, 17, 22, 23, 31, 38, 48:1, 48:2, 48:3, 48:4, 49:1, 52:2, 53:1, 57:1, 60:1, 63:1, 63:2, 64:1, 81, 83, 88, 112, 114, 122, 123, 146, 149, 166, 168, 170, 175, 176, 178, 179, 184, 185, 188, 190, 202, 207, 209, and 222; C.I. Pigment Violet 1, 3, 5:1, 19, 23, 35, and 37; C.I. Pigment Blue 1, 2, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 56, 60, 76, 79, and 80; and C.I. Pigment Green 1, 4, 7, 8, 10, and 36. It is also possible to mix more than one pigment in the ink.

Many of the above pigments are available commercially in finely divided forms which are specifically aimed at ink jet use. It is preferred to use such available forms in the inks of this invention.

The total pigment concentration of the ink may be up to approximately 30% by weight, but will generally be between about 1% and about 15%, preferably approximately 3 to 8%, by weight of the total ink composition. The concentration depends on the particular pigment used and the printer. The average particle size of the pigment is preferably up to lam, especially below 200nm.

Suitable dispersants for the first ink may be any dispersant as is known use in pigmented ink jet inks. The purpose of the dispersant is to stabilise the particles and prevent flocculation, aggregation, and settling of the ink.

The formulation of the second ink or inks is not particularly restricted as long as the second ink is selected to provide a gloss value different from that of the first ink. For example any or all of the factors listed above as affecting the gloss value from a conventional pigment ink, that is to say the particular pigment and dispersant in use, the particle size and concentration of the pigment, the weight ratio of pigment to dispersant, and the composition of the carrier medium, may be suitably adjusted in the formulation of the second ink to provide a different gloss value and achieve the differential gloss effect.

Thus according to one embodiment of the invention the second ink is an aqueous pigmented ink jet ink comprising at least one pigment in the form of fine particles which are stabilised by the presence of one or more dispersants, suspended in an aqueous carrier medium wherein the weight ratio of dispersant to pigment in the second ink is different from that in the first ink. In particular, the weight ratio of dispersant to pigment in the second ink may be greater than in the first ink. It is normally found that increasing the proportion of dispersant provides a more glossy image.

According to a second embodiment of the invention, the second ink is an aqueous pigmented ink jet ink comprising at least one pigment in the form of fine particles which are stabilised by the presence of one or more dispersants, suspended in an aqueous carrier medium wherein the pigment concentration in the second ink is different from that in the first ink. In particular, the pigment concentration may be reduced to provide a dilute or so-called "light" ink. It is normally found that decreasing the pigment concentration provides a more glossy image. However the maximum density available from the dilute ink restricts the density range over which this embodiment of the invention may be applied.

According to a third embodiment of the invention, the second ink is an aqueous pigmented ink jet ink comprising at least one pigment in the form of fine particles which are stabilised by the presence of one or more dispersants, suspended in an aqueous carrier medium wherein the particle size of the pigment in the second ink is different from that in the first ink. In particular, the particle size may be reduced. It is normally found that reducing the particle size of the pigment provides a more glossy image However the magnitude of the differential gloss effect which may be achieved by adjusting the above factors in the ink formulation is somewhat restricted. Alternatively and preferably therefore, the second ink may specifically formulated using different or additional components from those used in the first ink to provide a different gloss value and achieve the differential gloss effect.

Therefore, according to a fourth and preferred embodiment of the invention the second ink comprises one or more self-dispersing pigment instead of a conventional pigment and a dispersant. It is found that inks formulated with a self-dispersing pigment normally provide gloss values significantly lower than conventional pigmented inks.

By a seli:dispersing pigment is meant a surface processed pigment, the particles of which have been modified by having a surface treatment such as surface oxidation or the incorporation of solubilising groups such as, for example, carboxylic or sulphonic acid groups. Such surface modified pigments may be dispersed in the absence of conventional dispersants. Such modified pigments are known and commercially available from suppliers such as Cabot Corporation and Orient Corporation.

Certain commercial ink jet printers are provided with a set of at least five inks (commonly six or eight inks) made up of three inks comprising the subtractive primary colours cyan, magenta, and yellow, together with two black inks: a so-called photo black ink, which is a conventional pigmented ink which provides relatively glossy prints; and a second, socalled matt black ink which is formulated using a self-dispersing pigment. It is possible to use printers of this type in this fourth embodiment of the invention by using one of the black inks as the first ink of the invention, and the second black ink as the second ink to provide a black image having a different gloss value from that of the first ink. Thus with a suitable printer, this embodiment only requires suitable print driving software to control the printer, but can only be used to produce black or grey security marks.

According to a fifth embodiment of the invention, the second ink comprises at least one organic thermoplastic resin. It is found that inks formulated with organic thermoplastic resin components provide prints having gloss values significantly higher than conventional pigmented inks, especially if the polymer of the organic resin has a coalescing property in the presence of certain organic solvent components commonly used in ink jet ink formulations such as, for example, pyrrolidone or N-methyl pyrrolidone, which act as plasticisers for the organic resin.

By the term organic resin is meant an organic thermoplastic polymer which may be present in the ink as a solution resin or as an emulsion resin. Preferably according to this embodiment of the invention, the organic thermoplastic resin comprises an emulsion resin by which is meant an aqueous suspension comprising water or an aqueous solvent as a continuous phase and finely divided particles of an organic polymer; such may also be called a latex, an aqueous emulsion, or a polymer emulsion. The small particles of the emulsion resin are preferably characterized by an average particle size up to approximately m. The concentration of the emulsion resin in the ink may be up to approximately 10%, preferably between approximately 1% and 8%, dry solid content by weight of the total ink composition.

According to a sixth embodiment of the invention, the second ink comprises at least one dye as colorant instead of a pigment. It is found that inks formulated with a dye as colorant normally provide prints having gloss values substantially the same as the medium on which they are printed, and therefore according to this embodiment the first ink should provide a gloss value different from that of the unprinted medium.

Certain commercial ink jet printers are provided with a CMYK ink set made up of three dyed inks comprising the subtractive primary colours cyan, magenta, and yellow, together with a pigmented black ink. It is possible to use printers of this type in this embodiment of the invention by using the black ink as the first ink of the invention, and a combination of all the coloured inks as the second ink, a so-called 3K black, to produce a black image having a different gloss value from that of the first ink. This embodiment can only be used to produce black or grey security marks and is restricted to the printing medium which may be employed.

According to a seventh embodiment of the invention, the at least one second ink comprises at least two inks: one comprising a colorant and another comprising one or more organic thermoplastic resin as hereinbefore described. It is found that printing with a combination of an ink comprising a colorant and an ink comprising an organic thermoplastic resin provides prints having gloss values significantly higher than conventional pigmented inks, especially in the presence of certain organic cosolvents commonly used in ink jet inks such as, for example, pyrrolidone or N-methyl pyrrolidone, which act as plasticisers for the organic resin. In this embodiment, the second ink comprising the colorant may be the same as the first ink, but the differential gloss may be achieved by printing the ink comprising the organic resin in addition. This embodiment of the invention is particularly useful for providing differential gloss in regions of low colour density where there is normally insufficient ink coverage to provide a useful effect just using conventional coloured inks.

Clear inks comprising an organic thermoplastic resin are known, for example, according to United States Patent Application US 2003/189626 A and Japanese Patent Applications 2003-266912, 2003-266913, and 2003-266932.

According to an eighth embodiment of the invention, the second ink is formulated to comprise one or more colourless matting pigment or filler in addition to a colorant. It is found that inks formulated with a matting pigment or filler provide prints having gloss values significantly lower than conventional pigmented inks. Suitable matting pigments or fillers include, for example, white pigments such as titanium dioxide, alumina, silica, or zmc oxide.

Further, combinations of the above embodiments of the invention are of course possible.

Thus according to a ninth and preferred embodiment of the invention, the second ink is an aqueous pigmented ink jet ink comprising at least one pigment in the form of fine particles which are stabilised by the presence of one or more dispersants, suspended in an aqueous carrier medium wherein the pigment concentration in the second ink is lower than that in the first ink and wherein the second ink also comprises at least one organic thermoplastic resin as hereinbefore defined. Such an ink comprising a reduced pigment concentration and a fine particulate thermoplastic resin is known according to European Patent Application I 164 173 A. It is found that inks formulated with a low pigment concentration and an organic thermoplastic resin provide prints having gloss values significantly higher than conventional pigmented inks, especially in the presence of certain organic cosolvents commonly used in ink jet inks such as, forexample, pyrrolidone or N-methyl pyrrolidone, which act as plasticisers for the organic resin. However the maximum density attainable from the dilute ink restricts the density range over which this embodiment of the invention may be applied.

Certain commercial ink jet printers are provided with a set of at least five inks (commonly six or eight inks) made up of three inks comprising the subtractive primary colours cyan, magenta, and yellow, together with at least one additional coloured dilute ("light") ink and at least one black ink. Especially, a so-called CcMmYK ink set is provided wherein the light inks comprise additional dilute cyan and magenta inks which also comprise an organic thermoplastic resin, and these inks provide a higher gloss value than the full strength inks. It is possible to use printers of this type in this fifth embodiment of the invention by using suitable combinations of the full-strength inks as the first ink of the invention, and suitable combinations of the dilute inks as the second ink to provide an image having the same colour and density but a different gloss value from that of the first ink. Thus, with a suitable printer, this embodiment of the invention only requires suitable print driving software to control the printer, and can readily be used to produce security marks having a wide range of colour values. However the region of colour space to which this may be applied is restricted by the lack of a light yellow ink in the commercial printer.

According to a ninth embodiment of the invention, the first ink comprises at least one dye as colorant, and second ink comprises at least one dye as colorant and at least one organic thermoplastic resin as hereinbefore defined.

According to a tenth embodiment of the invention, the first ink comprises at least one dye as colorant, and second ink comprises at least one dye as colorant and at least one colourless matting pigment or filler.

According to a eleventh embodiment of the invention, the first ink comprises at least one organic thermoplastic resin as hereinbefore defined, and the second ink comprises at least one colourless matting pigment or filler. In this embodiment, the inks may be formulated as clear inks in the absence of colorant, or may additionally comprise at least one dye or pigment as colorant.

According to a further embodiment of the invention, the specularly reflective mark is bronzed. As used herein, the terms bronzed or bronzing describe an effect wherein the specularly reflective mark has a metallic appearance and wherein the specularly reflected light has a different colour from the diffused reflective light. Bronzing is a well known effect in digital printing and elsewhere, and is described by A. Hodgson on page 96 of the Conference Procedings of the second Institute of Physics on Preservation and Conservation Issues Relating to Digital Printing and Digital Photography, held at Heriot - Watt University, Edinburgh on 24th and 25th March 2003. Bronzing commonly occurs in areas of relatively heavy ink coverage, particularly from printing with black pigmented ink jet inks on certain glossy ink jet media, but may also be associated with inks which comprise an emulsion resin. Because bronzing is related to the specular reflection of the print, it is not picked up by conventional copiers or scanners. However bronzed prints are readily identified by the unaided eye.

Further embodiments of the invention will be apparent to those skilled in the art.

It is to be understood that the first and second inks suitable for the purpose the invention may also comprise other components which are advantageously added to aqueous ink jet inks, for example surfactants, biocides, pH buffers, thickeners, and sequestering agents such as EDTA. Such components are well known in the art.

The printing method of the invention is suitable for use with a variety of ink jet printers, for instance a thermal drop-on-demand printer, a piezo drop-on-demand printer, or a continuous ink jet printer. The printer may be a desk top ink jet printer or a wide format ink jet printer. The materials are suitable for use with inks comprising dyes or pigments as colorants. Preferably the ink is an aqueous ink, that is to say an ink in which the carrier liquid is predominantly water, but may also comprise one or more water soluble organic cosolvents such as a polyhydric alcohol.

The printing method of this invention is useful for preventing duplication of documents, prints, images, and the like by providing distinctive marks on the prints that are easily visible to the unaided eye as a gloss modulation. However the embedded information in the mark is not recognised by copying and scanning devices and therefore the mark cannot be reproduced using a conventional copier or scanner. Thus the presence of the mark on the document shows that the document is an original print, but if the document containing the visible mark has been scanned or copied, there results a copy without the mark and therefore, the absence of the visible mark on the copy indicates that it is not original, but a forgery, counterfeit, or other unauthorised copy.

The visible marks provided by the printing process of the invention have use as security marks, authenticating marks, validation marks, verification marks, and the like. The mark may just appear as a difference in gloss, but the mark may also be used a distinctive feature such as, for example, text or a signature, message, logo, design, symbol, graphic, image, or the like. The print marked may be: a text document; pictorial, for example a photographic image or artistic creation; or a high-value item such as a ticket, coupon, identification badge, pass, ID card, negotiable security, other valuable document, and the like. Such uses may be described as an Optical Variable Identification Device (OVID) or Digital Optical Variable Identification Device (DOVID).

Further, the method of the invention is also useful for providing special artistic effects or novel and eye-catching features in the advertising, marketing, and graphics arts industries, and for providing novelty items for printing applications such as greeting cards and the like.

It will be appreciated that the method of this invention may be applied to colour prints and also to monochromatic, so-called black and white prints. Further the method may be used for printing on a variety of printing media such as plain paper, glossy paper, polymer media and the like. However preferred media include in particular coated media designed for printing of images using ink jet printers, especially media described as photo-like, such as photo glossy media.

Description of the Figures

Figure 1 shows the measured density and the 60 gloss values of the step wedges printed in the following Example 1. In this figure, reference 1 shows the gloss values of areas printed with the non-glossy black ink formulated in the Example and reference 2 shows gloss values for the conventional commercial black ink.

Figure 2 shows the measured density and 60 gloss values of the step wedges printed in the following Example 2. In this figure, reference 3 shows the gloss values of areas printed with the combined 3K inks and reference 2 shows gloss values for the conventional commercial black ink as above.

Figure 3 shows the measured density and 60 gloss values of the step wedges printed in the following Example 3. In this figure, reference 4 shows the gloss values of areas printed with the dyed ink and reference 2 shows gloss values for the conventional commercial ink as above.

Figure 4 shows the image and security mark printed in the following Example 1. In this figure, reference 4 shows the mark and reference 5 shows the background.

Figure 5 shows a photocopy the first print of the image shown in Figure 4 produced in the following Example 1. It will be seen that the copier has completely failed to discern the differential gloss mark; thus if this mark were being used as an authentication mark the photocopy would be rapidly and easily recognised as non-authentic by eye.

It will be appreciated that the intention and effect of the invention is to produce prints which cannot be copied, and thus the actual prints produced in the Examples bearing the differential gloss marks are unsuitable as patent figures as they are not capable of being copied.

The following Examples show some of the effects which are possible with the printing method of the invention and will serve to illustrate the invention.

Example 1

A non-glossy black pigmented ink comprising a self-dispersing pigment was prepared according to the following formulation: Component Amount by Weight Black pigment dispersion 138 85 Diethylene Glycol 25 Polyethylene Glycol mw 300 50 Glycerol 50 Surfactant 0 8 Tri ethanol amine 0 7 Deionised water to 500 The self-dispersing black pigment was an 18% dispersion commercially available from Orient Corporation of America under the trade name Bonjet Black COO-2. The surfactant was a 70% solution of sodium dioctyl sulphosuccinate in propylene glycol and water. The pH of the ink was 8 45.

The ink was loaded in an ILFORD 700i wide format ink jet printer. A test chart comprising a step wedge of printed areas of increasing density was printed onto ILFORD STUDIO Glossy coated ink jet receiving medium. Another step wedge was printed with the same printer using the standard commercial ILFORD Archiva Extreme black pigmented ink.

which is a conventional aqueous pigmented ink comprising a pigment stabilised by a dispersant. The density of the steps was measured using a Gretag Macbeth Spectrolino reflectance calorimeter and the 60 gloss of the steps was measured using a Dr Lange gloss meter according to DIN 67530 / ISO 2813, and the results (shown as percentage reflection relative to a black glass plate) are plotted in Figure 1. In this figure, reference 1 shows the gloss values of areas printed with the ink formulated with the self-dispersed pigment and reference 2 shows gloss values for the conventional commercial ink. At zero density the measured gloss is that of the medium, but at any other density it is seen that the measured gloss of the commercial ink is greater than that of the experimental non-glossy ink. This difference in the gloss from the two inks is the differential gloss effect and is readily observable to the eye at densities greater than 0 5.

A test pattern comprising the design shown in Figure 4 was printed with the ILFORD 700i wide format ink jet printer loaded with the inks above on ILFOR1;STUDIO Glossy ink jet receiving medium as above. Firstly, the pattern was printed at a target density of 1 8 for both the background and the mark using the standard commercial black ink to print the mark area, reference 5, and the non-glossy ink to provide the background area, reference 6.

In the original print, the differential gloss effect is readily visible as a very distinct glossy

black mark against a semi-matt black background.

Next, the same test pattern shown in Figure 4 was printed at a target density of 1 8 for both the background and the mark, but using the nonglossy black ink to print the mark area, reference 5, and the standard commercial ink to provide the background area, reference 6.

Again, the differential gloss effect is clearly visible in the original print as a distinct

semi-matt black mark on a glossy black background.

The colour according to the CIELAB L*a*b* system and visual density of areas of the print printed with the commercial glossy ink and with the non-glossy ink were measured t:l peer using a Gretag Macbeth Spectrolino reflectance calorimeter and the gloss values were measured using the Dr Langegloss meter as above. The results are given in the table below.

Commercial ink Non-glossy ink Colour L* 12 04 14 42 Colour a* 0 5 -0 08 Colourb* -1 57 -1 85 Visual Density 1 85 1 75 Gloss 58 5% 25 4% The CIELAB L*a*b* colour system is discussed in section 1.4 of the previously mentioned book Handbook of Print Media, edited by H.Kipphan, English edition published by Springer-Verlag Berlin in 2001. The AE colour difference between the commercial ink and the non-glossy ink was calculated as 2 5.

For comparison, the gloss value of the unprinted medium was measured as 56 3%.

Next, both prints were copied using a conventional photocopier. The copies showed a featureless black print with no evidence of the marks at all. Thus the clearly visible mark on the original print was shown not to be recognised by the copier, and the copies could easily by seen as copies and not authentic. The photocopy of the first print is shown as the appended Figure 5. It is seen that no evidence of the mark is visible.

Next, the first print was scanned using an Epson 1680 pro scanner. When the scanned image was viewed on a computer screen there was no evidence of the mark; all that could be seen was a uniform black image showing that the mark was not recognised by the scanner. Further, the scanned image was printed using a Canon BJ S900 ink jet printer on ILFORD Galerie Smooth Gloss ink jet Paper. The print just showed a uniform black image with no evidence of a mark showing that the scan had completely failed to discern the security mark; thus if this mark were being used as an authentication mark the copy print would rapidly and easily be recognised as non-authentic by eye

Example 2

Two test charts comprising step wedges of printed areas of increasing density were printed onto ILFORD STUDIO Glossy media with an ILFORD 700i wide format ink jet printer.

The first wedge was printed using the standard ILFORD Archiva Extreme black pigmented ink (so-called 1K black) as in Example 1. The second wedge was printed using a combination of ILFORD Archiva Extreme yellow, magenta, and cyan pigmented inks balanced to provide a visually black image (so-called 3K black). The yellow, magenta, and cyan inks are conventional aqueous pigment inks each comprising fine pigment particles stabilised by a dispersant. The density and gloss of the steps were measured as in Example l, and are plotted in Figure 2. In this figure, reference 3 shows the gloss values of areas printed with the combined 3K inks and reference 2 shows gloss values for the conventional commercial black ink.

At zero density the measured gloss is that of the medium, but at any other density value it is seen that there is a difference between the gloss measured for the 1K black ink and that measured for the mixed 3K black inks. This is the differential gloss effect and is readily observable to the eye at densities greater than 0 8.

Example 3

The first wedge was printed using the standard commercial ILFORD Archiva Extreme black pigmented ink as above and the second wedge was printed using the standard commercial ILFORD Archiva dyed black ink. The density and gloss of the steps were measured as in example 1, and are plotted in Figure 3. In this figure, reference 4 shows the gloss values of areas printed with the dyed ink and reference 2 shows gloss values for the conventional commercial ink as above.

At zero density the measured gloss is that of the medium, and it is seen that the measured gloss for the dyed ink is always substantially the same as that of the medium, whereas the gloss measured for the pigmented black ink increases with density. The difference between the gloss measured for the two inks at the same density is the differential gloss effect and is readily observable to the eye at densities greater than 0 5.

Claims

Claims 1. An ink jet printing method comprising: 1) printing at least one

first ink to provide a first printed area having a colour a diffuse optical density and a gloss value; and 2) printing at least one second ink to provide a second printed area having a colour and diffuse optical density the same as that of the first area of the print and a gloss value different from that of the first area of the print; wherein the first and second printed areas produce a distinctive specularly reflective mark to the eye by reason of differential gloss.
2. A printing method according to claim I wherein the difference between the gloss values of the two inks to be is at least 14% of the actual gloss value of the less glossy of the two printed inks.
3. A printing method according to claim I or 2 wherein the first ink is an aqueous pigmented ink jet ink comprising at least one pigment in the form of fine particles which are stabilised by the presence of one or more dispersants, suspended in an aqueous carrier medium.
4. A printing method according to claim 3 wherein the second ink is an aqueous pigmented ink jet ink comprising at least one pigment in the form of fine particles which are stabilised by the presence of one or more dispersants, suspended in an aqueous carrier medium wherein the weight ratio of dispersant to pigment in the second ink is different from that in the first ink.
5. A printing method according to claim 3 wherein the second ink is an aqueous pigmented ink jet ink comprising at least one pigment in the form of fine particles which are stabilised by the presence of one or more dispersants, suspended in an aqueous carrier medium wherein the pigment concentration in the second ink is different from that in the first ink.
6. A printing method according to claim 3 wherein the second ink is an aqueous pigmented ink jet ink comprising at least one pigment in the form of fine particles which are stabilised by the presence of one or more dispersants, suspended in an aqueous carrier medium wherein the particle size of the pigment in the second ink is different from that in the first ink.
7. A printing method according to claim 3 wherein the second ink comprises one or more self-dispersing pigment.
8. A printing method according to claim 3 wherein the second ink comprises at least one organic thermoplastic resin.
9. A printing method according to claim 8 wherein the organic thermoplastic resin comprises an emulsion resin.
10. A printing method according to claim 3 wherein the second ink comprises at least one dye as colorant.
ll. A printing method according to claim 3 wherein the at least one second ink comprises one ink comprising a colorant and another ink comprising at least one organic thermoplastic resin.
12. A printing method according to claim 3 wherein the second ink comprises at least one colourless matting pigment or filler and at least one colorant.
13. A printing method according to claim 3 wherein the second ink is an aqueous pigmented ink jet ink comprising at least one pigment in the form of fine particles which are stabilised by the presence of one or more dispersants, suspended in an aqueous carrier medium wherein the pigment concentration in the second ink is lower than that in the first ink and wherein the second ink also comprises at least one organic thermoplastic resin.
14. A printing method according to either of claims 1 or 2 wherein the first ink comprises at least one dye as colorant, and second ink comprises at least one dye as colorant and at least one organic thermoplastic resin.
15. A printing method according to either of claims l or 2 wherein the first ink comprises at least one dye as colorant, and second ink comprises at least one dye as colorant and at least one colourless matting pigment or filler.
16. A printing method according to either of claims 1 or 2 wherein the first ink comprises at least one organic thermoplastic resin and the second ink comprises at least one colourless matting pigment or filler.
17. A printing method according to any of the preceding claims wherein the specularly reflective mark is bronzed.