TWIN-PLY PAPER

Abstract: A twin ply uncoated paper for ink jet processes comprised of a supporting paper substrate sheet as a first ply, and thereover as a second ply a paper sheet with filler additives attached to the fibers thereof, which additives are, for example, selected from the group consisting of amorphous synthetic silicas, inorganic silicates, metal alumino-silicates, and inorganic oxides. Three ply papers are also illustrated wherein there is situated between two second plies a supporting substrate sheet.

Description: The invention is generally directed to uncoated papers, and more specifically, the present invention is directed to economical twin-ply papers useful in ink jet processes. Thus, in one embodiment the present invention relates to uncoated twin-ply papers containing, for example, various liquid-absorbing fillers, inclusive of specific silicas, which papers enable the rapid absorption drying of ink jet compositions, and wherein the images obtained are comparable with those on silica-coated ink jet papers, and are of superior image quality compared with uncoated papers, as illustrated in M. Lyne and J.S. Aspler, "Paper for Ink Jet Printing", TAPPI Journal 68 (5) 1985, pp. 106-110 . Also, the twin-ply papers of the present invention exhibit improved drying in that, for example, no heating is needed, as is the case with some known coated and uncoated papers presently used for ink jet processes. In addition, the uncoated twin-ply papers of the present invention have substantially no undesirable show-through and strike-through characteristics, and images generated thereon are sharp, that is for example there is negligible image raggedness as compared with many known uncoated and coated ink jet papers. Furthermore, the uncoated papers of the present invention are similar to ordinary paper in feel, appearance and receptiveness to lead pencil marking, while simultaneously possessing printing performances comparable with premium grade ink jet coated papers. Additionally, the twin-ply papers of the present invention are also useful in other liquid development systems, such as electrostatic xerography and direct electrography. Pigment-coated papers developed for ink jet processes are well known. These papers are usually comprised of a supporting substrate and thereover high surface area hydrophilic pigment, such as colloidal silicas dispersed in an appropriate organic binder system. Disadvantages associated with these papers, especially the coatings thereof, include their high process costs, relatively poor substrate adhesion, and the need for special coating processes to circumvent the peculiar rheology associated with the high surface area pigments selected. Additionally, the known coated papers do not have the feel, appearance and pencil marking receptivity of ordinary paper, and therefore are undesirable to some users. On the other hand, TAPPI Journal, Vol. 68, no. 3, March 1985, pages 102-106, Atlanta, US; B. Alince et al.: "Effect of pigment location on paper brightness - prediction and reality", discloses an uncoated paper sheet comprising a first fibrous layer and thereover a second layer of paper fibres with pigments, e.g. clay or titanium dioxide. As indicated herein, the uncoated papers of the present invention are especially useful in ink jet processes. Marking compositions which are useful in such processes are well known, and generally contain water-soluble dyes. There is thus described, for example, in US-A- 3,846,141, an ink composition useful in jet printing, comprised of an aqueous solution of a water-soluble dye and a humectant material formed of a mixture of a lower alkoxy triglycol; and at least one other compound such as a polyethylene glycol, a lower alkyl ether of diethylene glycol, or glycerol. These printing inks have the desired viscosity for use in jet printing, in that the viscosity of the composition is subjected to little variation with use as water is lost by evaporation during recirculation of the ink composition through the jet printer. Further, apparently the humectant system disclosed in this patent substantially prevents or minimizes drying of the printing ink in the orifice or nozzle during down-time of the printer. As further disclosed in this patent, the basic imaging technique in ink jet printing involves the use of one or more ink jet assemblies connected to a pressurized source of ink. Each individual ink jet includes a very small orifice, usually of a diameter of 50 µm, which is energized by magneto-strictive piezo-electric means for the purpose of emitting a continuous stream of uniform droplets of ink at a rate of 33 to 75 thousand per second. This stream of droplets is then directed onto the surface of a moving web of, for example, paper, and is controlled to form printed characters in response to video signals derived from an electronics character generator, and as a result of an electrostatic deflection system. In addition, there are disclosed in US-A- 4,279,653 ink jet compositions containing water-soluble wetting agents, a water-soluble dye and an oxygen absorber. Similarly, US-A- 4,196,007 describes an ink jet printing composition containing an aqueous solution of a water-soluble dye and a humectant consisting of at least one water-soluble unsaturated compound. Other prior art disclosing aqueous inks for ink jet printing include US-A- 4,101,329; 4,290,072; 4,383,859; 4,235,773; 4,279,814; 4,443,371; 4,286,989 and 4,299,630. Additionally, there is disclosed in US-A-4,197,135 ink compositions with improved waterfastness comprised of at least one water-soluble dye, and a polyamine with seven or more nitrogen atoms per molecule. Therefore, there is a need for uncoated papers that are useful in ink jet processes. In addition, there is a need for economical uncoated ink jet papers providing image performance comparable with silica-coated papers; and further the papers of the present invention have the feel and appearance of ordinary uncoated paper. Additionally, there is a need for uncoated papers that will enable the rapid drying of inks on the surface thereof, which papers also permit developed images of high resolution, and superior density when compared, for example, with many known papers. Accordingly, the present invention provides uncoated twin-ply papers, preferably for ink jet processes, as claimed in the appended claims. The present invention provides uncoated twin ply papers comprised of a supporting substrate sheet, and thereover a second sheet formulated from, for example, a blend of a pulp suspension and synthetic filler components. Therefore, in one specific embodiment of the present invention, there are provided uncoated papers comprised of a supporting substrate of paper obtained from, for example, bleached hardwoods and softwood fibers; and a second ply of paper with, for example, fillers of colloidal silicas attached to the fibers thereof, and wherein the aforementioned second ply can be formulated from a blend of paper pulp and filler. More specifically, the second ply is formulated by first blending a filler such as a colloidal silica with an agitated pulp suspension, for example, of bleached hardwoods and/or softwoods, cotton, eucalyptus, or synthetic fiber blends enabling a paper with discrete plies formulated on a laboratory centrifugal-former type paper machine, sucha as a Formette Dynamique, by means of applying the second paper ply to a previously formed base paper ply, to produce a two-ply paper of a total basis weight of, for example, approximately 75 g/m², and wherein the two plies are initially maintained as a wet fiber slurry, and thereafter are de-watered. Depending upon the end-use application, the second ply composition may be comprised of from 25/75 to 75/25 filler/pulp ratios, and the thickness thereof can be from 5 to 50 µm. Also, depending upon the filler concentration, various types of natural and synthetic binder resins may be utilized to permit adequate end-use sheet integrity and acceptable image waterfastness. In one important embodiment of the present invention, there are provided uncoated papers comprised of a base sheet of paper obtained from bleached hardwood and softwood fibers; and thereover a second paper ply, with a thickness of from 5 to 50 µm obtained from bleached hardwood and softwood fibers; and wherein there are attached to the paper ply fibers of the second ply fillers of synthetic amorphous silicas, such as Syloid 74 available from the Grace-Davison Company; calcium silicates, inclusive of XP974 available from Huber Corporation; surface chemically modified sodium aluminum silicates, including CH-430-106-1 available from Huber Corporation; calcium fluoride/silicas, available from Opalex, C, Kemira Oy, Finland; or like materials. Another embodiment of the present invention relates to a twin- ply uncoated paper for ink jet processes comprised of a supporting paper substrate sheet as a first ply having a thickness of 50 to 90 µm, and thereover as a second ply a paper sheet with a thickness of from 5 to 50 µm and filler additives, for example from 25 to 75 percent by weight, attached to the fibers thereof, which additives are synthetic silicas or inorganic silicates, such as sodium alumino-silicates, yielding a composite sheet with excellent drying, high image resolution, that is for example images with high edge definition, and wherein the aforementioned sheet also possesses excellent waterfastness with certain colored aqueous anionic dye-based ink jet compositions. In addition, in another embodiment of the present invention there is provided a three-ply uncoated paper for ink jet processes comprised of a support paper substrate sheet as a first ply situated between a second ply paper sheet, with a thickness of from 5 to 50 µm with filler additives, for example, from 25 to 75 percent by weight, attached to the fibers thereof, which additives are synthetic silicas or inorganic silicates, such as sodium alumino-silicates, and a third ply paper sheet with a thickness of from 5 to 50 µm with filler additives, for example, from 25 to 75 percent by weight, attached to the fibers thereof, which additives are synthetic silicas or inorganic silicates, such as sodium aluminosilicates Examples of the first plies, or support substrates, with a thickness of from 50 100 µm, include paper obtained from (1) bleached hardwood and softwood fibers; (2) cotton fibers; and the like, which papers are commercially available as, for example, softwood (Domtar Q90), hardwood (Domtar Seagul 'W') and cotton linters (Buckeye 513). The second and third plies can be comprised of the same paper as the first ply containing the additives indicated herein, such as amorphous silicas or inorganic silicates such as metal alumino-silicates. The second and third plies, which are of a thickness of from 5 to 50 µm, can be prepared by mixing from 25 to 75 percent by weight of additives with from about 75 to 25 percent by weight of paper pulps. The base sheet mixture is maintained as a wet fiber slurry prior to de-watering on the forming wire of the paper machine. Thereafter, the second ply can be applied to the first partially de-watered ply, and then vacuum de-watered to enable formation of a paper structure with discrete pies. Any third ply may be formulated in a similar manner. Other components may be added to the second and/or third ply to improve certain characteristics of the resulting papers. Thus, for example, there can be added in the pulp stock or via size press treatment, dry strength synthetic or natural product binders, such as an anionic polyacrylamide, starch, and the like in an amount of from about 0.5 to about 7 percent, primarily for the purposes of achieving ply-to-ply, and fiber-to-filler adhesion without adversely effecting the ink-absorbing properties of the final papers. Cationic polymers or surfactant type materials may similarly be incorporated to enhance dye waterfastness. Alternatively, as indicated herein, the papers of the present invention can comprise a supporting substrate sheet situated between two individual plies, to form a three-ply structure with printing performance on both sides comparable to the aforementioned two-ply papers. Compared with commercially-available ink jet papers coated with colloidal silicas, the uncoated twin-ply papers of the present invention, when used with a selected glycol/water based ink jet composition in either text or full color graphics printing processes, have comparable image circularity, acceptable spreading characteristics, excellent drying times, negligible image show-/strike-through, improved waterfastness, and acceptable color rendition, that is for example, negligible diminution of potential dye color, and comparable image optical densities for black and primary colors. The following Examples are provided, which illustrate, but are not intended to limit the scope of, the present invention. Parts and percentages are by weight unless otherwise indicated. EXAMPLE I There were prepared three twin-ply papers on a laboratory centrifugal paper machine former "Formette Dynamique", (see "Simulation of Fourdrinier Paper Machine Forming in the Laboratory," Pulp & Paper Canada 84 (12) 1983, pp. T283-286). The base sheets were comprised of a fine paper furnish containing a 75/25 percent bleached hardwood (Domtar Seagul 'W') and softwood fibers (Domtar Q90) beaten to a Canadian Standard Freeness value of approximately 400 to 450; and the second ply in each instance was comprised of the same furnish blended with the high surface area colloidal silica pigment filler, Syloid 74, available from Grace-Davison. More specifically, a first pulp suspension for the base ply was supplied from stock tank A at 0.4 percent consistency to produce a base sheet of 65 grams/meter² basis weight onto a forming wire via a vertically-oscillating nozzle. The second agitated pulp suspension (stock tank B), containing a blend of pulp and 50 percent silica filler, was applied to the first pre-formed ply (which was maintained as a wet fiber slurry on the wire) by an oscillating nozzle, and then drained to form a paper structure with two discrete plies having total basis weight of about 75 grams/meter². The thickness of the base or second ply may be varied by altering the number of nozzle passes. In this example the number of nozzle passes was computed so as to result in twin-ply sheets with a top ply 8, 14 and 20 percent of the total sheet thickness (designated Samples 1, 2 and 3, respectively). After draining the sheets to about 20 percent dryness, that is a level at which it possessed adequate wet strength, they were stripped from the wire, further de-watered on a single nip wet press with the second (top) ply sandwiched against a smooth Teflon surface and the base ply against a press felt, and then dried on a photographic-type drum dryer. The thickness of the first and second plies together was about 100 µm. The performance of the resultant papers was evaluated on a Xerox Corporation Diablo Model C150 color ink jet printer using a print test pattern composed of: solid areas (inch square), text, and various pixel width lines in primary as well as mixed colors. The optical density of the printed papers was measured with a Tobias RX densitometer, and selected test pattern features were analyzed in an optical microscope. Ink absorption drying characteristics were evaluated with a Bristow type liquid absorption apparatus (Svensk Papperstidning 70,623 (1967)). The optical density of ink jet prints on paper Samples 1, 2 and 3 are provided in Table 1, along with the uncoated (Sample A) and commercial-coated (Sample B) ink jet papers supplied with the aforementioned printer. Compared with Sample A, the image optical density of the above prepared twin-ply papers 1, 2 and 3 was increased in all situations. Optical density (back side), which is indicative of the extent of image show-through, also significantly improves as the thickness of the top ply increases. Image resolution data are summarized in Table 2. These data indicate that as the ply thickness decreases, the single pixel line width decreases to a value below Sample A and slightly higher than the value for Sample B, that is the coated paper control which has a non-fibrous surface. Similar trends are evident for the yellow and magenta primary colors which are superimposed on the papers of Table 2 to produce red images. Compared with Sample B, the line width data show that the thicker top ply thickness, Sample 3, has comparable absorption drying capacity. This trend was further established by print rub tests (finger rubbing of the solid print area of the test pattern) which revealed no image smear on multicolored solid areas 2 seconds after printing. Finally, the image circularity (as measured by the technique described in US-A-4,361,843) of a single drop of a black ink (viscosity 2.7 centipoises, and surface tension 57 dynes/cm) was less than about 0.7 on Sample 2, compared with less than 0.8 for Sample B. Corresponding drop-spreading factors, that is the ratio of image spot size to drop size, were 2.1 and 1.9 on Samples 2 and B, respectively. EXAMPLE II Twin-ply papers were prepared by repeating the procedure of Example I with the exception that calcium silicate filler (XP974, Huber Corp.) was incorporated in the top ply. Table 3 summarizes the optical density for the three uncoated papers, Samples 4, 5 and 6, and the control papers, Samples A and B. These results demonstrate that the calcium silicate filler was about as effective in the top ply layer as the aforementioned silica insofar as accomplishing optical densities comparable to or better than the control papers. Similarly, the data presented in table 4 indicate that the image resolution for the twin-ply uncoated papers achieved, for example, the single pixel line width, was superior to Sample A; and for the double pixel multi-color line employing twice the ink volume/unit area. Also, the image resolution for the twin-ply uncoated papers achieved was very comparable to the uncoated Sample B. The Bristow absorption data recited in Table 5 reveal the high ink absorption rate and capacity for the twin-ply uncoated papers compared with the control papers; and as a result rapid drying for the twin-ply uncoated papers was achieved. In addition, application of this paper for use in a liquid electrophotographic type of printing process, as described in US-A- 3,084,043, proved highly effective. For example, an ink comprising 8 percent by weight of carbon black dispersed in a blend of natural and synthetic oils yielding a viscosity of 300 centipoises and a surface tension of 38 dynes/cm, was used in a Cheshire Addressograph DI 785 machine to print test pattern images on Sample 5 and a xerographic 4024 bond paper. Absorption drying of images, as determined by thumb rub resistance, was nearly instantaneous for Sample 5, compared with 3 minutes for the latter. Furthermore, Sample 5 showed a marked improvement in line pair resolution with line raggedness, see "The Raggedness of Edges", J. Opt. Soc. Am. 71, 285-288 (1981), approaching 10µm compared with 25µm for the xerographic bond control paper. The superior print quality characteristics of the twin-ply paper are believed to be attributable to efficient interfiber filling by the microscopically fine high surface area filler particles, which diminish the tendency for ink wicking along fibers. In contrast, the surface of a more conventional, bulk-filled, bond paper possesses much less filler; and consequently produces prints with more fiber wicking, and hence inferior image edge raggedness. EXAMPLE III Twin-ply papers incorporating 50 percent of a surface chemically modified sodium silicate (CH430-106-1, Huber Corp.) in the top ply with varying percentage thickness of the total sheet, were prepared by repeating the procedure of Example I. Table 6 illustrates optical density data of black solid area prints generated in the Diablo C150 printer, measured before and after 10 minute immersion in water, followed by air drying for Sample 7 with a top ply 14 percent of the total sheet thickness, and Sample B, the control coated paper. A significant improvement in waterfastness of the black ink results from the incorporation of this type of filler in the paper. EXAMPLE IV A dry strength synthetic and natural product binder was incorporated into the second ply furnish to achieve ply-to-ply and fib er-to-filler adhesion without negating ink absorptive properties and print quality. For example, a 10 percent aqueous starch solution (Cato 72, National Starch & Chemical Corp.) precooked 30 minutes at 98°C, was applied hot onto the twin-ply sheet of Sample 2, prepared in accordance with the aforementioned Examples, on a KRK (Japan) Laboratory size press operating at a pressure of 245kPa and speed of 40 meters/minute. At a size press pick-up solids of approximately 7 percent, the surface strength of paper (TAPPI Standard T459-OM-83) increased from a wax pick value of 2, without size treatment, to 5 for Sample 2. Alternatively, similar size treatment with a 10 percent solution of starch (Cato 72), 10 parts, and anionic polyacrylamide (Accostrength 85, American Cyanamid Co.), 1 part, at approximately 7 percent solids pick-up increased the wax pick value from 2, without treatment, to 6 for Sample 2. EXAMPLE V As an alternative approach to that outlined in Example III, a specific binder chemistry was selected to enhance subsequent image permanence of ionic ink jet dyes, reference US-A-4,554,181. For example, in the aforementioned size press application, a twin-ply sheet (Sample 2) of a 10 percent aqueous solution of starch (Cato 72), 1 part, alum 0.2 part, and cationic polyamine (Cypro 514, American Cyanamid Corp.), 0.2 part, at a size pick-up of approximately 4 percent solids was selected. This paper was evaluated for waterfastness (10 minutes water immersion of this sample printed with an ink loading of 8.5 milliliters/meter² administered by a Bristow absorption apparatus) with an anionic (Acid Yellow 34) dye. As a result of the aforementioned size press treatment, waterfastness was increased from 15 percent without treatment to approximately 80 percent for the treated Sample 2. EXAMPLE VI Modification to form a three-ply structure with the base ply appropriately sandwiched between the two outer plies of similar composition to that of Example I, was prepared. Thus, there was formed a three-ply sheet of approximately 75 grams/meter² total basis weight consisting of a base sheet of approximately 18 grams/meter² formed from a 100 percent bleached groundwood pulp (Acadia Forest Products Ltd.), second outer ply of approximately the same basis weight as the base sheet and comprised of equal parts calcium silicate (XP974, Huber Corp.), and a 75/25 blend of bleached hardwood (Domtar SEAGUL 'W'), and bleached softwood fibers (Domtar Q90). When partially de-watered, this structure was removed from the wire of the Formette Dynamique and laminated against an identical, previously-formed, wet structure in a wet press, as indicated herein, to form a three-ply structure. Print quality and ink drying performance of this three ply-sheet was similar to the twin ply paper of Example II. Table 7 indicates line width resolution data for three samples of three-ply papers prepared with different outer ply thicknesses. There was considerable improvement in the net opacity of the base groundwood sheet because of the high filler content, and light scattering of the outer plies compared with that of the base groundwood sheet alone.

Claims: An uncoated paper comprising a support ply of paper fibres and a surface ply of paper fibres with additives attached to the fibres, characterised in that the additives are synthetic silicas, synthetic inorganic silicates or synthetic sodium alumino-silicates. Uncoated paper as claimed in claim 1, wherein the thickness of the surface ply is from 5 to 50 µm. Uncoated paper as claimed in claim 1 or 2, wherein the thickness of the support ply is from 50 to 90 µm. Uncoated paper as claimed in any preceding claim, wherein the support ply and/or the surface ply is made from bleached hardwood and softwood fibres. Uncoated paper as claimed in any preceding claim, wherein the support ply and/or the surface ply consists of, or includes, cotton fibres. Uncoated paper as claimed in any preceding claim, wherein the surface ply has a binder incorporated therein. Uncoated paper as claimed in any preceding claim, wherein the surface ply has incorporated therein cationically-charged natural and synthetic polymer materials. Uncoated paper as claimed in any preceding claim, wherein the support ply is sandwiched between two surface plies. Uncoated paper as claimed in claim 8, wherein the surface plies are each of paper with additives attached to the fibres thereof.s claimed in any preceding claim, wherein the surface ply has a binder incorporated therein.

Inventors: JONES, ARTHUR Y., OLIVER, JOHN FREDERICK
Applicants: XEROX CORPORATION
Application year: 1987
Main classification: D21H27
IPC codes: D21H27, B41M5, D21H17, B41M1