Method of producing paper or coated paper carrying an unobtrusive image

Description

[0001] This invention relates to sheet material carrying an unobtrusive image, and to a method for its production. The unobtrusive image may for example be provided to afford an indication of origin. The invention is particularly, but not exclusively, concerned with paper sheet material.

[0002] Watermarking by means of a dandy roll and rubber-marking by means of a raised image on a press roll are well-known means of imparting a feint or unobtrusive image to paper so as to indicate its origin.

[0003] However, watermarking and rubber-marking add substantially to the cost of the paper, and hence they have been employed chiefly with high quality papers which command a premium price, such as paper for use in prestige stationery. Watermarking and rubber-marking are normally uneconomic for papers which are produced at high speed, and/or on wide deckle paper machines. A further problem is that in the region of the watermark or rubber-mark some properties of the paper are affected, for example density, absorbency, porosity and smoothness. This may be disadvantageous in some circumstances, for example if the paper is subsequently to be coated and a uniform surface for receiving the coating is desirable. In any event watermarking and rubber-marking are of limited value where coated papers are concerned since the watermark or rubber-mark may be obscured by the coating.

[0004] It has also been proposed to produce an unobtrusive image in a paper-sheet material by other means. For example DE-A-2043753 discloses the production of transparent symbols in paper by printing on to the paper a polymer- containing composition which has the effect of transparentizing the area of the paper with which it comes into contact. A further proposal is contained in GB-A-696673 which discloses the injection of "units" of a material into a paper web as it is formed in a cylinder mould type of paper-machine. This injected material serves to deposit an insoluble substance within the paper web, and may comprise a dye, pigment or ink. The injected material produces an identifying mark. The proposals in DE-A-2043753 and GB-A-696673 would each appear to be subject to at least some of the drawbacks similar to those outlined above in relation to true watermarking and rubber-marking.

[0005] It has now been found that printing techniques in which electrically charged marking material droplets are projected through an electric or magnetic field which guides their path on to a material to be printed afford a means of applying a feint or unobtrusive image to coated or uncoated paper or to other sheet material, even where high speeds of production or wide deckles are involved. Such printing techniques, which include ink-jet printing and electrostatic screen printing, will hereafter be referred to as charged droplet printing techniques. They are in themselves well-known. Ink-jet printing is described, for example, in an article entitled "Jet Set" in British Printer, June 1980, and in UK Patents Nos. 1 354 890, 1 432 366, 1 461 385, 1 462 193, 1 464 370, 1 479 963, 1 491 234, 1 500 908 and 1 533 659.

[0006] According to a first aspect of the invention, there is provided sheet material carrying an unobtrusive image applied by a printing process, characterized in that the printing process is a charged droplet printing process, preferably an ink-jet printing process.

[0007] According to a second aspect of the invention, there is provided a method of producing sheet material carrying a printed unobtrusive image, characterized in that the printed image is applied to the sheet material by means of a charged droplet printer, preferably an ink-jet printer.

[0008] By an "unobtrusive image" is meant an image which does not obtrude in relation to printed, typed or written matter applied to the sheet material before or after application of the unobtrusive image.

[0009] The sheet material may for example be paper or coated paper. In a preferred embodiment of the present method, the printer is mounted on the paper-making or paper-coating machine on which the paper or coated paper is produced, whereby the web speed through the printer is the same as the web speed through the machine. Normally, an array of printers will be provided across the width of the web.

[0010] The marking material used to produce the unobtrusive image may be an ink or any other material which affords a contrast with the sheet material, for example a resin or other substance which has the effect of transparentizing a paper sheet material to which it is applied.

[0011] The invention is particularly suitable for use with pressure sensitive copying material, since the copying material is not subjected to pressure during the printing operations (any alteration of local properties of the base material caused by water or rubber-marking could be particularly detrimental to the functional performance of a pressure-sensitive copying material). The copying material is preferably of the kind in which image-formation occurs as a result of reaction between two individually colourless reactants which are brought into contact with one another under the influence of imaging pressure to form a coloured product. In the most commonly used system of the kind just described, one of the reactants (the "colour former") is contained in solution in microcapsules which are rupturable under the influence of pressure to release their contents into contact with the other reactant (the "colour developer").

[0012] Pressure sensitive copying systems employing microcapsules in the manner just described may be of the self-contained or transfer type. In the transfer type, an upper sheet is coated on its lower surface with a coating of microcapsules containing colour former solution and a lower sheet is coated on its upper surface with colour developer. Intermediate sheets coated with colour developer on their upper surfaces and microcapsules containing colour former solution on their lower surfaces are provided if more than one copy is required. In the self-contained type, one surface of the sheet carries a coating composition containing both microcapsules and colour developer. Alternatively, the microcapsules and the colour developer may both be present within the thickness of the sheet. As an alternative to the use of microcapsules (except in the last-mentioned embodiment) the colour former solution may be present as globules in a pressure-rupturable coated matrix, for example of gelatine, or wax.

[0013] The present method may be used to apply a feint or unobtrusive image to either surface of the upper, lower or intermediate sheets of a transfer system as just described or to either surface of a self-contained sheet as just described.

[0014] The reactants used in the pressure sensitive copying material may be conventional, for example, the colour formers may be fluorans, phthalides or spirobipyrans and the colour developers may be, for example, acidic clays, phenolic resins or salts of certain aromatic acids. Alternatively, one reactant may be an aromatic solvent solution of N,N'-di-benzyl-di- thio-oxamide and the other reactant may be a nickel rosinate derivative. Since the nature of pressure sensitive copying material is now very well known and is described very extensively in the patent literature, further details will not be given herein.

[0015] Care must of course be taken to see that the ink or other marking material used for the unobtrusive image in a pressure-sensitive copying system does not de-sensitize the reactants or otherwise impair the functional performance of the copying system. Similar considerations of course apply to other sheet materials carrying functional coatings.

[0016] Although the application of the invention to pressure-sensitive copying materials has been stressed, it will be appreciated that the invention finds application in other fields where it is desirable for the origin or brand of a particular material to be apparent not only to the manufacturer's immediate customer but also to a subsequent purchaser (for example in order to increase the effectiveness of the manufacturer's advertising or to capitalise on goodwill achieved by the previous satisfactory use of the same manufacturer's product). It should be understood in this respect that because paper and similar products frequently reach the end user through the intermediary of a printer or other converter, rather than direct from the manufacturer, the end user may receive the product in the converter's packaging and labels rather than that of the manufacturer.

[0017] Examples of other products to which the present invention is applicable are heat_-sensitive recording material and numerous printing and writing papers; photographic print base; automotive air and oil filter base papers; and compressible gasket and jointing material.

[0018] The invention will now be illustrated by the following Examples:

Example 1

[0019] This illustrates the use of a static experimental ink-jet printing rig (shown in diagrammatic side view in Fig. 1) to produce unobtrusively-imaged sheets for use in a pressure-sensitive copying system.

[0020] The rig incorporated an ink gun 1 having a single nozzle 2 adapted to emit a stream of ink droplets 5 in conventional manner. The nozzle 2 was positioned generally above a paper support platform 3 which was movable in two directions x, y at right angles to one another, and to the direction Z of ink droplet emission. Movement of the paper support platform 3 was controlled by punched tape such that ink droplets impinging on the paper could be made to form a desired image. The nozzle had a diameter of about 75 microns and was arranged to be pulsed so as to effect ink droplet formation. Deflector plates 4 were positioned near the nozzle so as to deflect certain of the ink droplets 5 on to the paper (only one deflected drop 7 is shown). A gutter 6 was positioned beneath the nozzle for collecting undeflected ink droplets.

[0021] A sheet of clay-coated colour developing paper ("Idem"^* CF57 supplied by Wiggins Teape) was positioned (clay-coated side uppermost) on the paper support platform and the rig was operated to produce an unobtrusive image on the sheet. The ink used was a conventional ink jet printing ink except that it was of very much lighter intensity than is used for standard printing operations. The pulse frequency was 64 KHz, which gave ink droplets of approximately 140 microns diameter, and the deflecting voltage was adjusted until the desired angle of deflection was achieved. The deflection voltage was maintained constant so that each deflected droplet was deflected to the same extent. The arrangement was such that only about one droplet per thousand emitted droplets were actually deflected. The distance of travel before each deflected droplet impinged on the paper was about 70 mm. It was found that the resultant dot diameter on the imaged paper was about 400 microns.

[0022] The procedure was then repeated first with two microcapsule coated sheets ("Idem"^* CB56 supplied by Wiggins Teape), in one case with the uncoated surface uppermost and in the other with the coated surface uppermost and then with two sheets each coated on one surface with colour developing clay and on the other surface with microcapsules ("Idem"^* CFB53 supplied by Wiggins Teape). In one case the clay-coated surface was uppermost and in the other case the microcapsule coated surface was uppermost. The desired unobtrusive image was obtained on each occasion.

[0023] On assembling the sheets so as to form a pressure-sensitive copying set, and subjecting the set to normal use, the unobtrusive image was found not to impede the formation of good legible copies.

Example 2

[0024] This illustrates the use of an ink-jet printer on a paper-coating machine. The ink-jet printer described in Example 1 was mounted adjacent to the web path downstream of the drying station of the coater and close to a steam jet decurling station. The support platform 3 was of course omitted. A variable deflection voltage was used, so as to deflect the ink droplets to varying extents in order to form the desired image. The voltage variations were controlled by means of a punched tape. A range of coated and uncoated papers were run through the machine at speeds of up to 750 m.min-¹, and an unobtrusive image was successfully obtained in each case. It was found that even at web speeds of 750 m.min-¹, air entrained by the web did not affect the flight path of the droplets and so lead to distortion of the image, and that on reel-up, there was no set-off of ink on to the adjacent sheet. The presence of ambient steam was found not to affect the image, despite the ink being water based.

[0025] The papers unobtrusively printed were as follows:-

a) standard 49 gm-² base paper of the kind used commercially in the manufacture of "Idem" pressure-sensitive copying paper.

b) clay-coated colour developing paper ("Idem"^* CF 46 supplied by Wiggins Teape)-in some runs the coated surface was imaged and in others the uncoated surface.

c) "Idem"^* CF 46 coated on its surface not carrying the clay coating with colour-former containing microcapsules and particulate stilt material (ground cellulose fibre floc) serving to protect the microcapsules from premature rupture during handling and storage of the paper (the microcapsule coating was applied by the coating machine on which the printer was mounted)-in some runs the clay-coated surface was imaged and in others the microcapsule coated surface.

d) standard 49 gm-² base paper as described in (a) but coated on one surface with a microcapsule coating as described in (c)-in some runs the coated surface was imaged and in others the uncoated surface.

^*"Idem" is a Registered Trade Mark.
When the printed coated papers described above were assembled into a pressure-sensitive copying set, the set functioned satisfactorily.

Claims

1. Sheet material carrying an unobtrusive image applied by a printing process, characterized in that the printing process is a charged droplet printing process.

2. Sheet material as claimed in claim 1, wherein the printing process is an ink-jet printing process.

3. Sheet material as claimed in claim 1 or 2, wherein the sheet material is paper.

4. Sheet material as claimed in any preceding claim wherein the sheet material is pressure-sensitive copying material.

5. A method of producing sheet material carrying a printed unobtrusive image, characterized in that the printed image is applied to the sheet material by means of a charged droplet printer.

6. A method as claimed in claim 5, wherein the printed image is applied to the sheet material by means of an ink-jet printer.

7. A method as claimed in claim 5 or 6 wherein the sheet material is a web of paper or coated paper and the printer is mounted on the paper-making or paper-coating machine on which the web is produced, whereby the web speed through the printer is the same as the web speed through the machine.

Ansprüche

1. Blattförmiges Material, das mit einem durch ein Druckverfahren applizierten unauffälligen Bild versehen ist, dadurch gekennzeichnet, daß das Druckverfahren ein Druckverfahren auf der Basis geladener Tröpfchen ist.

2. Blattförmiges Material nach Anspruch 1, dadurch gekennzeichnet, daß das Druckverfahren ein Tintenstrahl-Druckverfahren ist.

3. Blattförmiges Material nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß das blattförmige Material Papier ist.

4. Blattförmiges Material nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß das blattförmige Material ein druckempfindliches Kopiermaterial ist.

5. Verfahren zur Herstellung eines blattförmigen Materials, das mit einem unauffälligen Bild versehen ist, dadurch gekennzeichnet, daß das gedruckte Bild mittels eines Druckers auf der Basis geladener Tröpfchen auf dem blattförmigen Material appliziert wird.

6. Verfahren nach Anspruch 5, dadurch gekennzeichnet, daß das gedruckte Bild auf dem blattförmigen Material mittels eines Tintenstrahl-Druckers appliziert wird.

7. Verfahren nach Anspruch 5 oder 6, dadurch gekennzeichnet, daß das blattförmige Material eine Bahn von Papier oder beschichtetem Papier ist und der Drucker an der das Papier erzeugenden oder das Papier beschichtenden Maschine angebracht ist, auf der die Bahn erzeugt wird, und wobei die Bahngeschwindigkeit durch den Drukker die gleiche ist wie die Bahngeschwindigkeit durch die Maschine.

Revendications

1. Matériau en feuille portant une image discrète appliquée par un procédé d'impression, caractérisé en ce que le procédé d'impression est un procédé d'impression par gouttelettes chargées.

2. Matériau en feuille suivant la revendication 1, caractérisé en ce que le procédé d'impression est un procédé d'impression par jet d'encre.

3. Matériau en feuille suivant la revendication 1 ou la revendication 2, caractérisé en ce que le matériau est du papier.

4. Matériau en feuille suivant l'une quelconque des revendications précédentes, caractérisé en ce que le matériau est un matériau pour copies sensible à la pression.

5. Procédé pour produire un matériau en feuille portant une image discrète imprimée, caractérisé en ce que l'image est appliquée au matériau en feuille à l'aide d'une impression par une imprimeuse par gouttelettes chargées.

6. Procédé suivant la revendication 5, caractérisé en ce que l'image est appliquée au matériau en feuille à l'aide d'une imprimeuse à jet d'encre.

7. Procédé suivant la revendication 5 ou la revendication 6, caractérisé en ce que le matériau en feuille est une feuille continue de papier ou de papier couché et l'imprimeuse est montée sur la machine à papier ou la coucheuse, sur laquelle la feuille continue est produite, de façon à assurer à la feuille continue une vitesse de défilement identique sur la machine à papier et devant l'imprimeuse.

Drawing