PAPER TREATMENT

Abstract: The invention relates to apparatus and method for producing a deeply coloured paper product involving impregnating one side of the paper with colouring ink, allowing the ink to permeate into the paper product, and thereafter preferably applying a second printing application of the ink to the paper product.

Description: Paper Treatment This invention relates to colouring papers and cards particularly, but not exclusively, for use in book binding and the manufacture of games boards, presentation boxes and stationery. The cost of coloured dyes accounts for a significant proportion of the cost of producing coloured paper. This is particularly so for deep coloured papers which require high dye concentration. Attempts have been made in the past to print a layer of deeply coloured ink on to the entire surface of a sheet of lightly coloured paper to achieve a deep colour surface. The layer of ink printed on to the surface is made as thin as possible to minimise the amount of ink required. However, printing papers of this kind tend to scuff easily and are unsuitable for many purposes; the surface ink is easily damaged or scraped off, for example when the paper is scuffed or folded. This reveals an unsightly line and renders the colour of the base paper visible. It is also known to form a paper product of a dyed, deeply coloured surface ply and a lightly coloured backing ply. A disadvantage of such papers is the cost of producing same, given the multiply structure. An object of the present invention is to provide a deeply coloured paper that is durable, while still being cost effective to produce. This is achieved by impregnating the paper with ink or dye below the upper or outer surface of the paper, and using a technique to do this which optimises the concentration of the impregnated ink relative to the depth of penetration below the upper layer of the paper. According to a first aspect, the present invention provides a paper product comprising a ply of lightly coloured base paper having an upper surface in which a portion of the ply below the upper surface is impregnated with a colouring ink, wherein the colouring ink decreases in concentration in a direction through the ply moving away from the upper surface, characterised in that the rate of the decrease in concentration of the ink is highest just beneath the upper surface. The concentration of impregnated ink may decrease at a first substantially linear rate through a first portion of the paper product closest to the upper surface of the product, and at a second and lower substantially linear rate through a second portion of the paper product further away from the upper surface. The paper product may be a cellulosic fibre paper, a paper made of a synthetic fibre material or a paper made of mixed fibres. The colouring ink may extend substantially across the entire surface area of the paper product. The product may be capable or adapted to be folded so as to form a crease in the product such that the resulting damage to the outermost fibres towards the upper surface does not allow uncoloured fibres to show through. The lightly coloured paper base may be white or tinted. The paper product may have more than one ply of paper. The colouring ink may be present only in the top ply; it may extend substantially or wholly through the top ply. The presence of the colouring ink below the surface of the ply may allow the product to be folded without the base colour of the product showing through the surface of the top ply. The type of coloured ink is preferably chosen so as to have a good scuff resistance. The ink is preferably selected to have one or more of the following characteristics: a. high resistance to dry and moist rub; b. high resistance to colour change under pressure and temperature; c. flexibility of ink film for embossing and casing- in so as to prevent cracking of the ink layer; d. good foil stamping, blind blocking and screen printing properties; e. good lightfastness (ie resistance to fading, for example 3 min on the Blue Wool Scale); f. no set-off onto underside; g. no set-off onto design rolls or matrix bowls; h. good paper penetration characteristics. The ink may be solvent based but is preferably water based. The ink may penetrate to a depth of at least 30 or 35 microns below the upper surface of the paper product. It may penetrate to a greater depth. The ink may penetrate to a depth of substantially 30% of the depth of the ply below the upper surface of the paper product. It may have a penetration of at least 30% and may have a greater penetration. If the ink is solvent based then the penetration may be greater. The concentration of ink may decrease with depth of penetration. Traditional printing methods inevitably result in some ink penetrating into the paper. Nevertheless, printers generally seek to minimise the quantity of ink that is used. This is done by formulating the ink to minimise ink penetration into the paper and/or by coating, treating or selecting the paper so as to minimise ink penetration. A paper product in accordance with the present invention preferably has a greater ink penetration; in one of its aspects, the invention seeks to achieve a certain or minimum ink penetration to obtain the advantages discussed. According to a second aspect, the present invention provides a method of producing a paper product including the steps of: a) printing with ink a sheet surface of the paper product; b) allowing the ink to permeate into a portion of the paper product below the sheet surface; and c) reprinting the sheet with ink. The method may also include one or more of the following steps: a. unwinding the paper product; b. drying the ink between the two printing applications; c. applying a lacquer over the sheet surface; d. embossing the product; e. re-moisturising the underside of the product; f. rewinding the product; g. allowing a dwell time to re-stabilise the product, for example prior to rewinding it. The method may be performed on a paper making machine, for example, immediately after the drum reel operation. Alternatively, the method may be performed on a paper product removed from a paper making machine. An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings of which: Fig 1 is a cross section through a paper product; Fig 2 is a graph of ink concentration through the paper product; Fig 3 is a schematic cross section of an apparatus for producing the product; and Fig 4 is a graph of ink concentration through a paper product produced by the apparatus of Figure 3. The two ply paper product 10 of Fig 1 has been overprinted over its entire upper surface 20 with an ink 21. The ink penetrates from the top surface 20 substantially through the upper ply 11 but not into the lower ply 12. A thin layer of ink 21 is present at the top surface 20 and, as shown in Fig 2, the concentration of ink x decreases in a direction y moving away from the top surface. The plies 11,12 may be of a light colour having been dyed during manufacture or may be white; they may be of different colours. The plies 11,12 may be of a corresponding lighter colour to the ink; for example, the plies may be light blue when the ink is a deep or dark blue. The paper product 10 is sufficiently strong that it can be used as a binding paper for example for booking binding, games boards, box and stationery manufacturing. Its presentation surface has adequate scuff and water resistance so that it does not readily lose colour or deposit fibres when a contacting surface is rubbed against it. The paper accepts adhesives (for binding purposes) whilst not permitting the adhesive to saturate the sheet. Preferably, the scuff resistance is such that face to face abrasion resistance carried out on Martindale testing equipment is in excess of 1000 cycles. The ink is adequately absorbed into the paper such that when the paper is folded over an edge or corner, the resulting damage to the outermost layer of fibres does not allow uncoloured fibres to show through. The ink may be an aqueous based ink. The apparatus 30 of Fig 3 is arranged to treat a roll of paper 31 which has been manufactured in a separate process. The apparatus 30 comprises: a. an unwinding station 32; b. a first printing station 33 having a printing head for applying ink across the entire surface of the paper product; c. an infra-red or microwave drying station 34; d. a second printing station 35 having a printing head for applying ink across the entire surface of the paper product; e. a second infra-red or microwave drying station 36; f. an embossing nip 37; g. a station 38 for applying a layer of lacquer across the paper product; h. a backwet unit or re-moisturising station 39; i. an accumulator 40; and j. a rewind station 41. The paper product passes through a dwell portion 39,40 to allow the sheet adequate dwell time such that it re-stabilises before being rewound. The application of an aqueous based ink to one side of the paper tends to induce a "positive" curl ie the bias of the curve is towards the overprinted paper surface. If the paper was simply embossed and then rewound it would be likely to be unstable at the sheeting process. In order to alleviate the influence of the overprinted ink and thus the tendency to curl, the re-moisturising station 39 which may comprise a re-moisturising unit and a conditioning unit is stationed prior to rewinding. The re-moisturising unit applies water to the uncoated paper surface thereby contracting the influence of the water-based overprint ink. The conditioning unit allows the paper sufficient dwell time in the process to accept the moisture and re-stabilise. Whilst the ink may be applied to the paper product in a single printing operation it has been found advantageous to apply the ink in two separate passes with an intervening drying operation. The drying operation may be encouraged by use of an infra-red or microwave dryer (as illustrated) but such a dryer is not always necessary. Typically, 3 grams per m 2 dry ink may be applied to the paper product. In the embodiment of Fig 3 this may be achieved by applying 1.5 grams per m 2 at each of the two print heads. The use of two (or more) printing stages results in a different and indeed more preferable ink concentration distribution than that shown in Figure 2. Figure 4 graphs the concentration from a two print process. In likeness to Figure 2, the line AB represents the concentration of the ink relative to the depth of the paper below the upper sheet surface after the first print run. However, the very presence of the- impregnated ink from the first print run varies the rate of impregnation of the ink from the second print run, and this is depicted by line CD. The resultant aggregate concentration from the two print runs is therefore graphed by the line AD. This concentration is advantageous as the ink concentration is greatest, exponentially, where it is most wanted. It reduces the amount of ink needed to achieve a paper of suitable durability and this is beneficial in light of the aforementioned high cost of the ink itself. Additional passes of the paper through successive printing heads is also possible within the invention. Further modifications are also envisaged without departing from the scope of the invention herein intended.

Claims: CLAIMS : 1. A paper product comprising a ply of lightly coloured base paper having an upper surface in which a portion of the ply below the upper surface is impregnated with a colouring ink, wherein the colouring ink decreases in concentration in a direction through the ply moving away from the upper surface, characterised in that the rate of the decrease in concentration of the ink is highest just beneath the upper surface. 2. A paper product as claimed in Claim 1 wherein the concentration of impregnated ink may decrease at a first substantially linear rate through a first portion of the paper product closest to the upper surface of the product, and at a second and lower substantially linear rate through a second portion of the paper product further away from the upper surface. 3. A paper product as claimed in Claim 1 or Claim 2 made from cellulosic, synthetic or mixed fibres. 4. A paper product as claimed in any one of the preceding Claims wherein the colouring ink extends substantially across the entire surface area of the paper product. 5. A paper product as claimed in any one of the preceding Claims capable or adapted to be folded so as to form a crease in the product such that the resulting damage to the outermost fibres towards the upper surface does not allow uncoloured fibres to show through. 6. A paper product as claimed in any one of the preceding Claims wherein the lightly coloured paper base is tinted. 7. A paper product as claimed in any one of the preceding Claims having more than one ply of paper and wherein the colouring ink is present only in the top ply. 8. A paper product as claimed in any one of the preceding Claims wherein the colouring ink is selected to have one or more of the following characteristics: a. high resistance to dry and moist rub; b. high resistance to colour change under pressure and temperature; c. flexibility of ink film for embossing and casing-in so as to prevent cracking of the ink layer; d. good foil stamping, blind blocking and screen printing properties; e. good lightfastness (ie resistance to fading, for example 3 min on the Blue Wool Scale); f. no set-off onto underside; g. no set-off onto design rolls or matrix bowls; h. good paper penetration characteristics. 9. A paper product as claimed in any one of the preceding Claims wherein the colouring ink penetrates to a depth of at least 30 microns below the upper surface of the paper product. 10. A method of producing a paper product including the steps of: a) printing with ink a sheet surface of the paper product; b) allowing the ink to permeate into a portion of the paper product below the sheet surface; and c) reprinting the sheet with ink. 11. A method as claimed in Claim 10 also including one or more of the following steps: a. unwinding the paper product; b. drying the ink between the two printing applications; c. applying a lacquer over the sheet surface; d. embossing the product; e. re-moisturising the underside of the product; f. rewinding the product; g. allowing a dwell time to re-stabilise the product. 12. A method as claimed in Claim 10 or Claim 11 performed on a paper making machine immediately after the drum reel operation.

Inventors: FORBES STEPHEN, FORBES, STEPHEN
Applicants: FORBES STEPHEN, TULLIS RUSSELL GROUP LTD, FORBES, STEPHEN, TULLIS RUSSELL GROUP LIMITED
Application year: 1997
Main classification: B41M3
IPC codes: B41M1, B41M3, B41M7, D21H21