Improvements in or relating to writing surfaces

Abstract

 A method is described for providing a substrate with a writing surface which will accept slow-drying and/or water-based inks which cannot be easily smudged. The substrate is provided with a writing surface by applying to it a coating comprising an adhesive medium and a pigment in a conventional manner, but before the coating has dried or set, a dry, powdery, absorbent material is applied to the coating by spraying, puffing, or brushing or by roller or other means so as to provide the coating with an absorbent layer which readily accepts slow-drying or water-based inks.

Description

[0001] This invention relates to the provision on a substrate, such as a plastics material or other non-­absorbent material, of a surface which can be written upon.

[0002] A number of materials, in particular plastics such as polyethylene or very high absorbency paper, are often required to have an area where it is possible to write, type, draw or stamp information. This requirement is of particular importance where the material is in the form of a bag or envelope and the information relates to the contents or addressee of the bag or envelope.

[0003] For many years now such areas have been provided by applying to the substrate a writing surface comprising a pigmented adhesive medium where it is required to mark the substrate with the required information. The writing surface, which is usually white, may be applied by hot die stamping, by printing or otherwise.

[0004] Such writing surfaces are generally satisfactory for accepting writing in pencil or in the types of inks used in ball-point and like pens. However, the previously used writing surfaces present a problem where slow-drying and/or water based inks are used, since unless a relatively long time is allowed for drying the inks, the inks may smudge and the information may be rendered incomprehensible.

[0005] This problem has been found to be of particular significance for the clearing banks and the Post Office which each year use a considerable number of plastics bags which require a fast drying surface when using relatively slow drying inks. One way of solving this problem has been to use absorbent paper labels but if these are used in banking then special security adhesives are required and an extra operation in manufacture is also required.

[0006] It is an object of the invention to provide on a substrate an improved writing surface which will accept a relatively slow drying ink such that very shortly after the application of the ink serious smudging cannot occur.

[0007] According to the present invention there is provided a method of providing a substrate with a writing surface, wherein a coating comprising an adhesive medium and a pigment is applied over a region of the substrate, which is to constitute the writing surface, and the applied coating is allowed or caused to dry or set, and wherein after the coating has been applied and before it has dried or set a dry, powdery absorbent material is applied over the coating. The invention also resides in the writing surface so provided.

[0008] The coating of pigment and adhesive medium may be similar to or the same as the coatings which are conventionally employed to provide writing surfaces. In these coatings, while the pigment itself may be absorbent, the amount of adhesive medium required to retain the pigment in the coating is such that most of the medium has been absorbed by the pigment. Thus the coating as a whole is of reduced absorbency and does not readily accept a slow drying ink.

[0009] However, with the method of the present invention the application of a dry absorbent coating to the coating of pigmented adhesive medium before the latter has dried or set or is still tacky, provides the latter coating with an absorbent layer so that a slow drying ink is readily accepted and absorbed so that it cannot be easily smudged.

[0010] The dry, powdery absorbent material may be fine chalk or powdered paper or other absorbent material and can be applied as a spray, or by a roller, brush or other means of laying down a controlled surface layer. The resulting writing surface may then be subjected to a final drying step and if required subjected to a rolling or compression operation.

[0011] In order to enable the invention to be more readily understood, reference will now be made to the accompanying drawing, which illustrates diagrammatically and by way of example a plant showing one way of carrying out the method of the present invention.

[0012] Referring now to the drawings, a substrate 1 is unwound from a storage reel 2. The substrate may be in the form of a strip of polyethylene which is subsequently to be formed into bags or in the form of joined bags which can subsequently be cut along joining lines to form individual bags.

[0013] The substrate 1 is passed to a first flexographic printing station 3 comprising an ink trough 4, containing ink 5, a pick-up roller 6 rotating in contact with ink 5 in the trough, a transfer roller 7, a flexographic impression roller 8 and a backing roller 9. At the printing station, ink 5 is picked up from the trough 4 and transferred by the roller 7 to the impression roller 8 which prints on to the substrate as it passes between the rollers 8 and 9.

[0014] The printed substrate is then passed to a second flexographic printing station 10, which comprises a trough 11 containing coating material 12, a pick-up roller 13 rotating in contact with the coating material in the trough, a transfer roller 14, a flexographic impression roller 15 and a backing roller 16. At the second printing station, coating material 12 is picked up from the trough 11 and transferred to the impression roller 15 which coats an area of the substrate 1 to form on the substrate a writing surface.

[0015] After changing direction around the backing roller 16, the substrate is passed beneath an electrostatic charging device 17 where a negative electrostatic charge is imparted to the substrate or at least to that region of the substrate to which the coating material was applied by the roller 15. The substrate then passes beneath a powdering device 18 where a powder bearing a positive electrostatic charge is sprayed on to the substrate or at least into the coated region of the substrate. Any residual positive charge on the substrate as the result of spraying the powder thereon is neutralized by a further electrostatic charging device 19 beneath which the substrate passes before it passes beneath a hood 20 connected to a source of vacuum for removing loose powder.

[0016] The substrate is then passed beneath a hood 21 containing ultra-violet lamps 22 for curing and drying the coating composition. The substrate is then led over an idle roller 23 and is passed between a pair of calender rollers 24. The finished substrate is then wound up for storage or transferred to a subsequent process stage.

[0017] It will be appreciated that many variations of the plant shown in the drawing are possible. For example, if it is desired to print the substrate in several colours, an appropriate number of first printing stations will be provided. If necessary, means for drying the ink may be provided between each such station as well as between the last of such stations and the second printing station where the coating is applied.

[0018] As just indicated, the number of printing stations may be varied as required and the order in which coating material and the printing ink(s) are applied to the substrate can also be varied so that the writing surface may contain, for example, printed instructions or guides.

[0019] The ultra-violet lamps 22 are provided where the coating material 12 contains an ultra-violet curable resin, but if the coating material 12 does not contain such a resin other heating and drying means may be provided in the hood 22, or the heating and drying station could be omitted entirely if there were sufficient length of travel between the second printing station and the calendar rollers to ensure that the coating material was dry by the time it had reached the calendar rollers 24.

[0020] The substrate 1 may consist of any material which is to be provided with a writing surface such as non-­absorbent plastics materials, or even highly absorbent materials. As indicated above, the substrate preferably consists of polyethylene bags which are to contain banknotes and the bags may be present as a strip or web of bags joined to one another or temporarily attached to a carrier. The individual bags can subsequently be formed by severing them from the strip or removing them from the carrier. Alternatively, the substrate may consist of a continuous web of polyethylene which, after printing and coating in appropriate areas, can be formed into bags intended to contain banknotes.

[0021] The inks used at the first printing station(s) are conventional printing inks suitable for printing on polyethylene or other material of which the substrate is made.

[0022] Various pigmented compositions are suitable for use as the coating material which is applied to form the writing surface. A suitable composition is one which is curable by ultra-violet radiation and which comprises an epoxy acrylate prepolymer, tripropylene glycol diacrylate, benzophenones N-methyl diethanolamine and a pigment. A preferred composition comprises in proportions by weight:-
epoxy acrylate prepolymer      28
tripropylene glycol diacrylate      9
benzophenone      28
N-methyl diethanolamine      19
talc (pigment)      16.

[0023] This composition will cure under ultra-violet radiation and will firmly adhere to a wide variety of surfaces. In its unpigmented form, this composition is transparent when cured and can be printed over other printing on the substrate to provide a high-security coating over the usual printing inks which are solvent removable.

[0024] An alternative coating composition is one which comprises whiting or titania as pigment and an adhesive medium comprising a mixture of 60% ethylene vinyl acetate, 30% ethylene glycol and 10% water. This composition does not require ultra-violet radiation to cure it.

[0025] In either case, the viscosity of the coating material can be adjusted for different methods of application and may contain anti-foaming agents if required.

[0026] The pigment employed in the coating composition may be any conventional pigment suitable for producing a writing surface and which is compatible with the other ingredients of the composition. White pigments, such as whiting, chalk, talc or titania are preferred, but coloured pigments or pigmented powders can be used, bearing in mind that the coating material is to provide a surface which is intended to be written on. Where pigmented powders are used with an ultra-violet curable resin, the refractive index of the powder should be the same as or close to that of the resin in order that ultra-­violet curing can be achieved.

[0027] The coating composition is still wet, uncured and/or tacky as the substrate passes beneath the powdering device which applies a thin layer of dry powder to the surface of the coating composition. The powdering device may be arranged to puff or spray the powder on to the composition but other means of applying the powder may be employed, such as, for example, by brushing or by means of a roller.

[0028] In order to ensure that electrostatic charges generating by the movement of the substrate do not lead to repulsion of the powder as it is applied, it is preferred to apply a negative electrostatic charge to the substrate and a positive electrostatic charge to the powder. This promotes the application of the powder to the coating composition and ensures good adhesion of the powder.

[0029] The powder which is applied is conveniently the same material as that used to pigment the coating composition. Powdered paper is also a suitable material. The powder is preferably white, but may be coloured, and the use of different colours for the pigments of the coating composition and the powder may constitute an additional security feature. The powder is finely divided and of microsize and will generally be of the same order of size as the pigment in the coating composition.

[0030] Any loose powder which has not adhered to the coating composition is removed as by a vacuum or otherwise before the coating is subjected to ultra-violet curing. However, if an air-drying resin or heat-curable resin is employed, the non-adhered powder can be removed after drying or curing.

[0031] In a comparative test, a writing surface which had been produced in accordance with the present invention was compared with one which had been produced in the same manner but with the omission of the application of a powder layer to the uncured composition. Each writing surface was stamped with a rubber stamp address using a normal rubber stamp and ink pad, and a thumb was immediately rubbed over the stamped surface. No smudging of the address was noted in the writing surface produced in accordance with the present invention while the writing surface produced without the absorbent surface layer of powder was so badly smudged that the address was virtually illegible.

Claims

1. A method of providing a substrate with a writing surface, wherein a coating comprising an adhesive medium and a pigment is applied over a region of the substrate which is to constitute the writing surface, and the applied coating is allowed or caused to dry or set, characterised in that, after the coating has been applied and before it has dried or set, a dry, powdery, absorbent material is applied over the coating.

2. A method as claimed in Claim 1, wherein the dry, powdery, absorbent material is chalk, whiting, a pigment or powdered paper.

3. A method as claimed in Claim 1 or 2, wherein the substrate is a plastics material or other non-absorbent material, preferably a plastics bag or envelope.

4. A method as claimed in any one of Claims 1 to 3, wherein the adhesive medium comprises an ethylene-vinyl acetate copolymer, polyvinylacetate or an ultra-violet curable resin.

5. A method as claimed in any one of Claims 1 to 4, wherein the pigment is white or coloured and is preferably whiting, chalk, titania or talc.

6. A method as claimed in any one of Claims 1 to 5, wherein the coating is applied by hot die stamping, printing or spraying.

7. A method as claimed in any one of Claims 1 to 6, wherein the dry, powdery, absorbent material is applied by spraying, puffing, roller, brushing or other means of laying down a controlled surface layer.

8. A method as claimed in any one of Claims 1 to 7, wherein before the dry powdery absorbent material is applied, the substrate and coating thereon are given an electrostatic charge and the powder is given an opposite electrostatic charge as or before it is applied.

9. A substrate provided with a writing surface by the method claimed in any one of Claims 1 to 8.

10. A substrate as claimed in Claim 9 in the form of a plastics bag or envelope.

Drawing