WO1993025391A1

WO1993025391A1 - Receiver sheet and a method for the production thereof

Info

Publication number: WO1993025391A1
Application number: PCT/GB1993/001238
Authority: WO
Inventors: Kenneth West Hutt; Ian Richard Stephenson
Original assignee: Imperial Chemical Industries Plc
Priority date: 1992-06-15
Filing date: 1993-06-10
Publication date: 1993-12-23
Also published as: GB9212628D0

Abstract

Receiver sheets useful as security cards are disclosed which have dye pixels defining a security image and a personal image wherein substantially all of the pixels are applied to the receiver sheet in a single stage printing operation by light-induced thermal transfer printing. A method for the production of such sheets is also disclosed.

Description

Receiver Sheet and a Method for the Production thereof This invention relates to a receiver sheet, in particular to an imaged receiver sheet for example a tamper evident receiver sheet suitable for use as a high security card and a thermal transfer printing method for the production thereof.

Receiver sheets are known for use in security applications including identification cards, credit cards, charge cards, driving licences and the like. It is a requirement in such applications that the receiver sheet is secure, not easily forgeable and that the information contained on the sheets is not easily replaceable.

Typically cards for security applications comprise security or reference information by which the issuer of the card comprising the sheet may identify the card and personal information, for example the name, address, signature or photograph of the card holder. Hitherto the security information and personal information of the card holder have been applied to the receiver sheet in separate stages. US Patent No 4738949 discloses a high security identification card which has background security information comprising a series of printed lines and personal information. The background image is applied to provide a stock of cards which may then be subjected to a thermal transfer printing (TTP) process to provide an image comprising personal information. Off-set printing is disclosed as a means of applying the background image to the card; in such a process the printing ink is typically deposited on the surface of the receiver sheet.

US4738949 discloses using a programmable thermal print head in a TTP process to provide the personal information. The dye transferred in this process diffuses into the receiver sheet, thus the dye providing the personal image and the ink providing the security image may be located in different layers of the receiver sheet.

Thermal transfer printers having programmable print heads may provide a photographic reproduction of an image but do not allow for very high resolution images to be produced which are typically required for the security image. Thus, where a high resolution security image is required and a thermal transfer printer having programmable print head is to be employed, it is necessary to produce a receiver using a two stage process. Two stage processes involve a security risk as the cards may be misappropriated between the two printing stages and false personal information applied to the card having the security image.

We have now found that a receiver sheet having a high resolution image comprising security and personal information which information is applied to the sheet in a single stage process may be provided by employing a light-induced thermal transfer (LITT) process to effect transfer of the dye to the receiver thus avoiding the security risk associated with security cards of the prior art.

Accordingly, a first aspect of the invention provides an receiver sheet having high resolution dye pixels on at least one surface thereof said dye pixels defining an image comprising security information and an image comprising personal information wherein the dye pixels are applied to the receiver sheet in a single stage printing operation by light induced thermal transfer of the dye to the receiver sheet.

By "high resolution" is meant a resolution greater than that presently achievable by using a programmable thermal print head preferably greater than 300 dots per inch (equivalent to a pixel size of less than 80 microns), more preferably at least 600 dots per inch (equivalent to a pixel size of less than 40microns), particularly 1000 dots per inch (equivalent to a pixel size of less than 25 microns), especially 2000 dots per inch (equivalent to a pixel size of less than about 10 to 15 microns) , for example up to about 4000 dots per inch (equivalent to a pixel size of less than about 5 to 10 microns). It is to be understood that "resolution" refers to the size of the dye pixels and not to the size of the image defined by them.

By "single stage printing operation" is meant a printing process in which pixels defining the security image and the personal image are applied to the receiver sheet simultaneously as the inducing light scans the dye sheet or a process in which pixels defining the security image and the personal image are applied to the receiver sheet sequentially as the inducing light scans the dye sheet in the pre-determined area of either the security image or the personal image and then scans the dye sheet in the pre-determined area of the other image.

Receiver sheets according to the first aspect of the invention are advantageous over receiver sheets of the prior art in that by employing

LITT, higher resolution pixels may be applied to the sheet, for example at a resolution up to about 4,000 dots per inch to provide an image which may, if desired be of high resolution and in full colour. Moreover, the pixels, by being applied to the receiver sheet in a single stage printing operation, provide an image comprising security information (security image) and an image comprising personal information (personal image) which are formed in the same layer of the receiver sheet, thus tampering with either the security image or the personal image will affect the other image and leave clear evidence that tampering has occurred.

According to a second aspect of the invention there is provided, a tamper evident receiver sheet having high resolution dye pixels on at least one surface thereof, the dye pixels defining an image comprising security information and an image comprising personal information wherein said pixels are applied to the sheet in a single printing operation by light induced thermal transfer of the dye to the receiver sheet and wherein substantially all of said pixels are applied to the sheet in a single layer such that tampering with the pixels defining one of said images causes a detectable alteration of the other image.

The tamper evident nature of the receiver sheets according to the second aspect of the invention significantly reduces the risk of such sheets being altered or forged once the image has been applied to the sheet.

By "detectable alteration" we mean that the tampering affects the other image in such a way that it is possible to detect by appropriate means that the pixels on the receiver sheet have been tampered with. Detection of tampering may be achieved using any desired means including by detection of radiation - for example visible and ultra-violet light, from the pixels.

Suitably, the evidence of tampering is immediately apparent to the naked human eye.

As the dye pixels are applied to the receiver sheet in a single stage printing operation, the dye pixels defining both the personal image and the security image are suitably in substantially the same layer of the receiver sheet as each other. Thus, in cases where the receiver sheet comprises a substrate having a separate receiver sheet coated onto it, an attempt to replace either the security or personal image by for example peeling one image from the other to allow replacement of the removed image with a bogus image, will inevitably lead to an uncontrolled fracture of the receiver sheet thereby destroying both images contained therein.

According to a further aspect of the invention there is provided a high security card which comprises a substrate having at least one dye-receiving surface having high resolution dye pixels on the said at least one surface, the dye pixels defining an image comprising security information and an image comprising personal information wherein said pixels are applied to the surface in a single printing operation by light induced thermal transfer of the dye to the receiver sheet and wherein substantially all of said pixels are applied to the sheet in a single layer such that tampering with the pixels defining one of said images causes a detectable alteration of the other image.

By "dye-receiving surface" we mean the volume defined by at least part of the surface of the card and the depth into the surface which is penetrable by a dye in a light induced thermal transfer printing process.

The security card may comprise a back coat on the opposite side of the substrate to the dye-receiving surface to impart desirable properties for example, to improve handling characteristics and to aid adhesion of a protective cover sheet to the card.

Suitably a receiver sheet or card according to the present invention is laminated with a cover sheet on both sides to provide protection for the images on the sheet. The cover sheet may be the same or different on the different sides of the sheet and is preferably transparent on at least one side of the sheet. The cover sheet suitably comprises a thermoplastic film, for example polyvinyl chloride, polyethylene terephthalate and polycarbonate compositions.

The cover sheet can be a supportive card-like sheet and if desired may itself be a laminate suitably where a functional feature is to be retained between the layers of the laminate. Such sheets are particularly suitable for stand-alone uses for example credit cards, security cards and card-keys where a suitable thickness may about 200 μm for the cover sheets and about 50μjn for the receiver sheet. For security card applications, it is particularly desirable to provide a finished card wj=hich conforms to the ISO standard thickness

For other applications, much thinner cover sheets may be preferred for example pouch laminates in which both cover sheets on a receiver sheet extend beyond the edge of the sheet and are bonded together around their periphery.

A further aspect of the invention provides a method of producing a receiver sheet having high resolution dye pixels defining a first pre-determined security image and a second pre-determined personal image which method comprises providing a receiver sheet adapted to receive a thermally transferable dye, locating a thermal transfer printing dyesheet comprising a thermally transferable dye and an absorber for the inducing light in intimate contact with the receiver sheet, heating by light, first and second pre-determined areas of the dye sheet in a single printing operation to cause light-induced thermal transfer of the dye pixels to the receiver sheet in the said areas thereby to produce high resolution dye pixels on the receiver sheet defining the said pre-determined images.

Desirably, in cases where the security image of a receiver sheet according to the invention comprises text, the security image itself is of high resolution to render forging of it more difficult. At present, TTP using a programmable print head does not provide sufficiently high resolution text for the security image and is, in this respect inferior to LITT.

Receiver sheets according to the present invention suitably comprise a substrate having a dye-receiving surface on one side.

Substrates which are themselves dye-receiving materials, for example polyvinyl chloride may be adapted by the provision of a smooth surface texture. I most cases, however, receiver sheets comprise a substrate having a receiver layer on one side of the substrate, which layer comprises a dye-receptive composition into which thermally transferable dyes can readily pass in a TTP process. The substrate may also have a back coat on the opposite side to the dye-receiving surface if desired. Receiver sheet substrates known in the art may be employed in the present invention including cellulose fibre paper desirably with a polymer coating, thermoplastic films for example polyethylene terephthalate (desirably biaxially orientated), filled and/or voided thermoplastic films for example pearl film, and laminates of two or more substrate materials.

The receiver layer preferably comprises at least one dye-receptive polymer which is an amorphous polyester, polyvinyl chloride. The polymer may comprise other polymers for example polyvinyl alcohol/polyvinyl chloride copolymer as desired.

Commercially available examples of suitable amorphous polyesters include VITEL PE200 Goodyear) and VYLON polyesters (Toyobo) especially grades 103 and 200. Different grades of polyester may be mixed to provide a suitable composition as desired.

If desired, the receiver layer may also comprise a release agent. A preferred release agent is the thermoset reaction product of at least one silicone having a plurality of hydroxyl groups per molecule and at least one organic polyfunctional N-(alkoxymethyl) a ine resin which is reactive with the hydroxyl groups under acid catalysed conditions.

Suitably, the back coat, if present, comprises a cross-linked polymer binder and is provided to impart desirable properties to the receiver sheet for example improved handling characteristics and reduced tendency to retransfer the dye at low temperatures. If desired, the back coat may have a textured surface which may be imparted by a filler material or by the polymer per se.

Dye sheets typically employed for LITT may be employed to produce a receiver sheet according to the present invention. Dye sheets conventionally comprise a substrate sheet having on one side thereof, a dye coat layer comprising a thermally transferable dye dissolved and/or dispersed in a polymeric binder. If desired, the opposite side of the substrate has a back coat to fulfill a variety of functions for example to improve handling properties. In order to allow absorption of the inducing light, it is necessary to employ a light-absorbing material which may be present in the dye coat or may be present in a separate layer preferably located between the dye coat and the substrate.

The inducing light is desirably a laser, for example Nd.YAG, Argon ion and Ti:sapphire and preferably a laser diode.

For lasers operating in the near infra-red, there are a number of organic materials known to absorb at the laser wavelengths. Examples of such materials include the substituted phthalocyanines described in EP-B-157,568, which can readily be selected to match laser diode radiation at 750-900 nm, for example and carbon black pigment which has a broad absorption spectrum and is thus useful for a wide range of visible light and infra red emitting lasers.

Also of importance is the provision of sufficient absorber for the system used. It is desirable to use sufficient to absorb at least 50Z of the incident inducing light. We prefer to use sufficient to absorb at least 90Z of the inducing light, to obtain an optical density of 1 in transmission, although higher proportions may be used if desired.

A variety of materials can be used for the substrate, including transparent polymer films of polyesters, polyamides, polyimides, polycarbonates, polysulphones, polypropylene and cellophane, for example. Biaxially orientated polyester film is the most preferred, in view of its mechanical strength, dimensional stability and heat resistance. The thickness of the substrate is suitably 1-50 μm, and preferably 2-30 μ .

The dyecoat is formed by coating the substrate or, if present, the interlayer absorber coat with an ink prepared by dissolving or dispersing one or more thermal transfer dyes and a binder resin to form a coating composition; then removing any volatile liquids. Any dye capable of being thermally transferred in the manner described above, may be selected as required. Dyes known to thermally transfer, come from a variety of dye classes, eg from such nonionic dyes as azo dyes, anthraquinone dyes, azomethine dyes, methine dyes, indoaniline dyes, naphthoquinone dyes, quinophthalone dyes and nitro dyes. The dyecoat binder can be selected from such known polymers as polycarbonate, polyvinylbutyral, and cellulose polymers, such as methyl cellulose, ethyl cellulose and ethyl hydroyethyl cellulose, for example, and mixtures of these. A preferred dyecoat is one comprising one or more thermally transferable dyes dispersed throughout a polymeric binder comprising a mixture of polyvinylbutyral and cellulosic polymer, wherein the percentage by weight of polyvinylbutyral in the mixture lies within the range 65-852, the range 70-852 being particularly preferred.

The ink may also include dispersing agents, antistatic agents, antifoaming agents, and oxidation inhibitors, and can be coated onto the absorber layer as described for the formation of the latter. The thickness of the dyecoat is suitably 0.1-5 μm, preferably 0.5-3 μm.

The dyesheet may be elongated in the form of a ribbon and housed in a cassette for convenience, enabling it to be wound on to expose fresh areas of the dyecoat after each print has been made.

Dyesheets designed for producing multicolour prints have a plurality of panels of different uniform colours, usually three: yellow, magenta and cyan, although the provision of a fourth panel containing a black dye, has also previously been suggested. When supported on a substrate elongated in the form of a ribbon, these different panels are suitably in the form of transverse panels, each the size of the desired print, and arranged in a repeated sequence of the colours employed. During printing, panels of each colour in turn are held against a dye-receptive surface of the receiver sheet, as the two sheets are imagewise selectively irradiated, the first colour being overprinted by each subsequent colour in turn to make up the full colour image.

In applying a coating to a substrate, either for the dye sheet or the receiver sheet, various coating methods may be employed including, for example, roll coating, gravure coating, screen coating and fountain coating. After removal of any solvent, the coating can be cured for example by heating or by irradiation with for example ultra violet light, electron beams and gamma rays.

The invention will now be illustrated by way of the following non-limiting examples.

Example 1

A selection of dye sheets were prepared as follows: Dyesheets 1. 2. 3

6μm polyester film (Toray) having a back coat and a sub-coat was coated to a dry coat thickness of approximately lμm by gravure printing with the following dye compositions (amounts are in kg unless otherwise stated) and the compositions were dried by heating in air for about 15 seconds at 110°C:

Dyesheet 1 Dyesheet 2 Dyesheet 3

Yl was Cl solvent yellow 1.41; Y2 was Cl disperse yellow 126; Ml was 3-methyl-4(3-methyl-4-cyanoisothiazol-5-ylazo)-N-ethyl-N-acetoxyethyl aniline; M2 was Cl disperse red 60; Cl was

3-acetylamino-4-(3-cyano-5-phenylazothiophenyl-2-ylazo)-N,N-diethyl aniline; C2 was Cl solvent blue 63; PVB-BX1 was polyvinyl butyral BX1 from Sekisui; EC-T10 and EC-T200 were ethyl cellulose grade T10 and T200 respectively from Hercules.

Dye sheets 4. 5. 6

23μm S grade Melinex (ICI trade mark) polyester film was coated to a dry coat thickness of approximately lμm and optical density of 0.8 at

807nm using a No 2 Meier bar with the following absorber composition which was prepared by milling in a sand mill with zirconium oxide beads for 15 minutes:

Carbon Black (SP250 - Degussa) 20g Cellulose Acetate Phthalate 40g

Dowanol PM 180g

Methyl ethyl ketone 125g

Methanol 75g

Cymel 303 4g (hexamethoxymethylmelamine from American Cyanamid)

PTSA 2g

(Amine blocked p-toluene sulphonic acid)

The polymeric binder absorber coating was cured and dried by heating at 110°C for 5 minutes. Dyesheets 4, 5 and 6 were produced by coating onto a sample the absorber coated substrate, one of the following dye compositions (amounts in g) to a dye coat thickness of l.Sμm using a No2 Meier bar and the compositions were dried by heating in air for about 30 seconds at 110°C;

Dyesheet 4 Dyesheet 5 Dyesheet 6 Yellow Magenta Cyan TTD Yl 6.37 - -

TTD Ml - 0.833 TTD Cl TTD C2

Binder: PVB-BX1 EC-T10

Tetrahydrofuran

A receiver sheet was prepared which comprised a web of biaxially orientated polyester film (Melinex 990 from ICI) having a receiver layer provided from the following coating composition (parts by weight); Vylon 200 500 Tegomer HSi 2210 0.65

((bis-hydroxyalkyl polydimethyl- siloxane from Goldschmidt) Cymel 303 4.0

Nacure 2530 1.0

(amine blocked toluene-sulphonic acid catalyst)

Tinuvin 234 5.0

(UV stabiliser) Toluene/Methyl ethyl ketone 60/40 solvent mixture This coating composition was made by mixing three functional solutions immediately prior to coating, one containing the dye receptive Vylon and the Tinuvin UV absoiber, a second containing the Cymel cross-linking agent and a third containing both the Tegomer silicone release agent and the Nacure catalyst in the solvent mixture. Sufficient solvent was used to provide a final composition having a solids content of 12 2. The composition was coated onto the substrate using a bead coating method, and then dried and cured by heating at 140°C for 30 seconds.

The prepared dyesheets were each brought into contact with a sample of the receiver sheet by application of 1 atmosphere pressure. An STC LT-100 laser diode operating at 807nm was collimated and then focused using a 160mm achromat lens. The incident laser power at the dyesheet was about 60mW and the laser spot (full width at half maximum power) was about 30x20μm. The laser spot was scanned by a galvanometer scanner. The dyesheet and receiver sheet were held on an arc which allowed focus to be retained throughout the scan length. The scanning equipment addressed the laser to locations 20xl0μm apart giving a good overlap of adjoining pixels. At each pixel the laser was pulsed for a a specific time of between 100 and 600μs according to the desired optical density of the pixel to provide high resolution dye pixels on the receiver sheets.

The above test demonstrated that well defined pixels of high resolution 20μm and well-controlled shade could be produced and hence, that high resolution dye pixels defining a security image and personal image may be applied to a receiver sheet in a single stage printing operation to produce a receiver sheet and security card according to the present invention.

Example 2 The imaged receiver sheets of Example 1 were laminated to produce a tamper-evident sheet by placing the imaged receiver sheet between two sheets of adhesive coated laminate to produce a composite assembly. Laminates used were DDOT and DD3, both polyester coated polyester terephthalate film (available from Transilwrap US). The composite assemblies were fed through a laminator (Type 5026 from Morane Ltd UK) which was set at 150°C. The assemblies were allowed to cool prior to removal from the laminator.

The laminated sheets were then tampered with by way of trying to remove the laminate without destroying the image on the receiver sheet. In all cases the image on the receiver sheet was completely destroyed by being removed with the laminate from the receiver sheet due to fracture of the receiver coating on the receiver substrate.

Claims

1. A receiver sheet medium having high resolution dye pixels on at least one surface thereof said dye pixels defining an image comprising security information and an image comprising personal information wherein the dye pixels are applied to the receiver sheet in a single stage printing operation by light induced thermal transfer of the dye to the receiver sheet.

2. A tamper evident receiver sheet having high resolution dye pixels on at least one surface thereof, the dye pixels defining an image comprising security information and an image comprising personal information wherein said pixels are applied to the sheet in a single printing operation by light induced thermal transfer of the dye to the receiver sheet and wherein substantially all of said pixels are applied to the sheet in a single layer such that tampering with the pixels defining one of said images causes a detectable alteration of the other image.

3. A high security card which comprises a substrate having at least one dye-receiving surface having high resolution dye pixels on the said at least one surface, the dye pixels defining an image comprising security information and an image comprising personal information wherein said pixels are applied to the surface in a single printing operation by light induced thermal transfer of the dye to the receiver sheet and wherein substantially all of said pixels are applied to the sheet in a single layer such that tampering with the pixels defining one of said images causes a detectable alteration of the other image.

4. A method of producing a receiver sheet having high resolution dye pixels defining a first pre-determined security image and a second pre-determined personal image which method comprises providing a receiver sheet adapted to receive a thermally transferable dye, locating a thermal transfer printing dyesheet comprising a thermally transferable dye and an absorber for the inducing light in intimate contact with the receiver sheet, heating by light, first and second pre-determined areas of the dye sheet in a single printing operation to cause light-induced thermal transfer of the dye pixels to the receiver sheet in the said areas thereby to produce high resolution dye pixels on the receiver sheet defining the said pre-determined images.