Coating For Ink-Jet Paper And Methods Of Manufacture

Abstract

Disclosed an ink jet printing paper and a coating color used in preparing the paper, and associated methods for manufacturing the paper and its coating. The coating comprises the following components as raw materials: an anionic pigment, an adhesive and a cationic polymer, wherein the adhesive comprises a silane-modified polyvinyl alcohol. In the inventive ink jet printing paper coating, the raw material cost can be relatively low, the coating color solids content can reach 50% in many embodiments, and the coating can have a good water-retaining property. It can be suitable for large-scale production. In ink jet printing, it can give clear images with high color density, and can meet demanding requirements for high speed commercial ink jet printing.

Claims

1. A coating color for ink-jet paper, comprising on a bone-dry weight basis: 100 parts pigment particles, 5 to 12 parts adhesive, 0.5 to 10 parts cationic polymer, and an anionic dispersing agent, wherein the adhesive comprises a silane-modified polyvinyl alcohol, the pigment particles comprise one or more of precipitated calcium carbonate, ground calcium carbonate, kaolin, calcined clay, titanium dioxide and talcum powder, the pigment particle surface charge is within the range of −100 to 0 μeq/L, and the pigment has a D50 particle size in the range of 0.4 to 0.8 μm.

2. The coating color of claim 1, wherein at least 60% by weight of the pigment particles have a particle diameter of at least 0.2 and wherein at least 50% by mass of the pigment particles have a particle diameter in the range of 0.2 to 2 μm.

3. The coating color of claim 1, wherein the anionic dispersing agent is selected from one or more of an acrylic acid copolymer aqueous dispersion, acrylic polymers, poly dimethyl diallyl ammonium chloride, and polyamine quaternary ammonium salts.

4. The coating color of claim 1, wherein the silane modified polyvinyl alcohol has a degree of polymerization from 500 to 2000.

5. The coating color of claim 1, wherein the cationic polymer is selected from of one or more of the group consisting of polyethyleneimine, poly hydroxypropyl dimethyl ammonium chloride, poly quaternary ammonium salts, polyvinylpyridine, poly amine sulfones, poly (2-hydroxyethyl methacrylate), poly acrylic acid dialkyl amino ethyl acrylate, poly (2-hydroxyethyl) methacrylamide, poly (2-hydroxyethyl) acrylamide, poly epoxy amine, polyamide, dicyandiamide-formaldehyde condensates, polyethylene amine, poly allyl amine and salts thereof, poly diallyl dimethyl ammonium chloride, a copolymer formed by diallyl dimethyl ammonium chloride and acrylamide monomers, poly diallylmethylamine hydrochloride, dimethyl amine-epichlorohydrin condensation polymers, polycondensates formed by diethylene triamine-epoxy chloropropane polycondensate aliphatic mono amines or aliphatic polyamine with epoxy halogen propane compounds.

6. The coating color of claim 5, wherein the cationic polymer has a weight average molecular weight of from 400 to 100,000.

7. The coating color of claim 1, further comprising a water resistant agent comprising one or more of epoxies, amino zirconium carbonate and poly ammonia polyureas.

8. The coating color of claim 7, wherein the relative amount of water resistant agent is 0.2 to 3 parts on a bone dry basis, relative to said 100 parts of pigment.

9. An ink-jet printing paper comprising: a paper basesheet having two sides, at least one side of which having a surface coating, wherein the surface coating is made from a coating color comprising the following raw material components, on a bone-dry basis: 100 parts of pigment, 5 to 12 parts of adhesive and 0.5 to 10 parts of cationic polymer; and further comprising an anionic dispersant, wherein the adhesive comprises a silane modified polyvinyl alcohol, the pigment comprises at least one of precipitated calcium carbonate, ground calcium carbonate, porcelain clay, calcined clay, titanium dioxide and talcum powder, and wherein the pigment has a D50 particle size from 0.4 to 0.8 μm, and wherein at least 60% by weight of the pigment particles have a particle diameter of 0.2 μm or greater, and at least 50% by weight of the pigment particles have a particle diameter from 0.2 μm to 2 μm; and wherein the ink-jet printing paper under the condition of solid color block printing has a black color density greater than 1.1.

10. The ink-jet printing paper of claim 9, wherein solid color blocks printed on the ink-jet printing paper have a Gap Test Grayscale Value of at least 180.

11. The ink-jet printing paper of claim 9, wherein solid color blocks printed on the ink-jet printing paper have a Gap Test Grayscale Value of at least 200.

12. The ink-jet printing paper of claim 9, wherein the pigment particle surface charge of the pigments used to prepare the coating color is in the range of −100 to 0 μeq/L.

13. The ink-jet printing paper of claim 9, wherein the anionic dispersing agent comprises one or more of an acrylic acid copolymer aqueous dispersion, acrylic polymers, poly dimethyl diallyl ammonium chloride and polyamine quaternary ammonium salts.

14. The ink-jet printing paper of claim 9, wherein the silane modified polyvinyl alcohol has a polymerization degree ranging from 500 to 2000, and wherein at least 70% by weight of the pigment particles have a particle diameter from 0.2 μm to 2 μm.

15. The ink-jet printing paper of claim 9, wherein the cationic polymer is selected from one or more of the group consisting of: polyethyleneimine, poly hydroxypropyl dimethyl ammonium chloride, poly quaternary ammonium salt, polyvinylpyridine, poly amine sulfones, poly (2-hydroxyethyl methacrylate), poly acrylic acid dialkyl amino ethyl acrylate, poly (2-hydroxyethyl) methacrylamide, poly (2-hydroxyethyl) acrylamide, poly epoxy amine, polyamide, dicyandiamide-formaldehyde condensates, polyethylene amine, poly allyl amine and its hydrochloride salt, poly diallyl dimethyl ammonium chloride, a copolymer formed by diallyl dimethyl ammonium chloride and acrylamide monomers, poly (diallylmethylamine hydrochloride, dimethyl amine-epichlorohydrin condensation polymers, polycondensates formed by diethylene triamine-epoxy chloropropane polycondensate aliphatic mono amine or aliphatic polyamine with epoxy halogen propane compound.

16. The ink-jet printing paper of claim 15, wherein the cationic polymer has a weight average molecular weight of from 400 to 100,000.

17. The ink-jet printing paper of claim 9, wherein the coating color further comprises a water resistant agent comprising at least one of epoxy resin, amino zirconium carbonate and poly ammonia polyurea.

18. The ink-jet printing paper of claim 17, wherein the relative amount of water resistant agent in the coating color is from 0.2 to 3 parts on a bone dry basis, relative to said 100 parts of pigment.

19. The ink-jet printing paper of claim 9, wherein the paper basesheet comprises a base paper or a coated paper, and the roughness of the basesheet prior to application of the surface coating is from 4.0 to 8.0 μm.

20. The ink-jet printing paper of claim 19, wherein when the paper basesheet is an uncoated base paper, the coating amount of the surface coating is from 20 gsm to 100 gsm, and when the paper basesheet is a coated paper, the coating amount of the surface coating is from 8 gsm to 15 gsm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0076] FIG. 1 is a schematic of cross-section of the coated ink-jet paper depicting several versions of the invention.

[0077] FIG. 2 shows a printed paper of Example 1 showing the damage that can be caused by friction (rubbing) on the printed image.

[0078] FIG. 3 shows a printed image on the paper of Comparative Example 3 showing the damage that can be caused by friction (rubbing) on the printed image.

[0079] FIG. 4 shows a printed reference image on the paper of Example 3.

[0080] FIG. 5 shows a printed reference image on the paper of Comparative Example 1.

[0081] FIG. 6 shows a printed reference image on the paper of Example 3.

[0082] FIG. 7 shows a printed reference image on the paper of Comparative Example 1.

[0083] FIG. 8 shows part of the original reference pattern from the original reference image file.

[0084] FIG. 9 shows a close up of portion of the reference image used in the Gap Test to obtain the Gap Test Grayscale Value, here showing the region to be selected for measurement of the mean grayscale value.

[0085] FIG. 10 shows a portion of the printed reference image from Example 3 showing a selection being made for measurement.

[0086] FIG. 11 shows a an enlarged view of FIG. 10.

[0087] FIG. 12 shows a portion of the printed reference image from a comparative example with a commercial ink-jet paper from New Page Corp. believed to have been manufactured with nanoparticles in the coating.

[0088] FIG. 13 shows a portion of the printed reference image from a comparative example with a commercial ink-jet paper from Mitsubishi Paper.

[0089] FIG. 14 shows a portion of the printed reference image from an early ink jet coating formulation at Gold East Paper.

[0090] FIG. 15 shows a portion of the printed reference image from Example 3.

[0091] FIG. 16 shows a portion of the printed reference image from Comparative Example 1.

[0092] FIG. 17 shows a portion of the printed from Example 3.

[0093] FIG. 18 shows a portion of the printed from Comparative Example 1.

[0094] FIG. 19 shows a portion of the printed from Example 3.

[0095] FIG. 20 shows a portion of the printed from Comparative Example 1.

DETAILED DESCRIPTION

[0096] Below various embodiments are disclosed based on the drawings in order to illustrate further details. The invention as claimed is not meant to be limited by the scope or nature of the specific embodiments described.

[0097] In the following embodiments, experimental methods without specific conditions are selected according to conventional methods and conditions, or according to well-known standard specifications.

[0098] In following examples, the solids content of the coating color can be correspondingly adjusted according to the coating method or coating apparatus. For example, for optimum coating operation with a blade coater the solids content of the coating color can be adjusted to above 50%, as needed. For air knife type or curtain coating, the solids content of the coating color may be adjusted to below 50%, such as from 40% to 50%.

[0099] Examples according to some embodiments of the present invention, as discussed below, show a Gap Test Grayscale Value of at least 180 (on a scale where 0 is black and 255 is white).

Examples 1-3

[0100] Examples 1 to 3 were prepared from paper and coating colors as listed in Table 1 below. The pigment was selected from ground calcium carbonate and precipitated calcium carbonate or combinations thereof. The coating was prepared using conventional mixing technology to prepare a uniform coating color from individual components.

[0101] The printing surface of coated ink jet paper was coated using precipitated calcium carbonate pigment combined with suitable anionic dispersant added in quantities sufficient to adjust the PCD values between −100 to 0 μeq/L. The pigment particle size parameter DP50 based on mass fraction was between 0.4 to 0.8 μm, with more than 70% of the pigment particles by mass having an effective diameter greater than 0.2 microns.

[0102] In some embodiments the fraction of particles between 0.2 to 2 microns in effective diameter can have a mass cumulative percentage greater than 50%, greater than 70%, greater than 80% (as was the case here, as indicated below), or greater than 90%. (In some embodiments, the pigment is substantially free from added nanoparticles, and more specifically substantially free from added nanometer size silica, alumina or other inorganic nanoparticles, with “nanometer size” as used herein referring to particles with a DP50 parameter less than 0.1 microns.)

[0103] In Examples 1 to 3, both sides of the basesheet were coated to give a structure similar to FIG. 1A, which shows a basesheet 3 with a first coated side on the upper surface comprising a base coating layer 2 and a top coating layer 1, or simply the topcoat, which serves as an ink receiving layer. The second side of the basesheet 3 has its own respective base coating layer 4 and a topcoat layer 5. The coating were applied in multiple steps, then given final drying and then calendaring to form coated ink jet paper. For Examples 2 and 3, the coated ink-jet printing paper preparation method was the same as in Example 1.

[0104] The topcoat formulation in Examples 1 to 3 comprised silane-modified polyvinyl alcohol with a viscosity (in a 4% aqueous solution at 20° C.) of 20.0 to 30.0 mPa s, a hydrolysis degree of 98.0 to 99.0%, non-volatiles comprising 98 to 100% of the PVA, an the ash content less than or equal to 0.2%, and a pH value of 5 to 7, with about 0.2% sodium acetate.

[0105] The silane modified polyvinyl alcohol can be prepared by mixing vinyl acetate with vinyl-tri-alkyloxy silane and copolymerizing. The copolymer is then directly hydrolyzed in alkali solution. Wacker Chemical's patents CN 101180330B and U.S. Pat. No. 7,052,773 disclose a silane-modified polyvinyl alcohol preparation technology. The resulting silane modified polyvinyl alcohol may contain from about 0.5 to 1.0 mol % of silyl group as vinyl silane units, the degree of polymerization is about 1700, and the degree of hydrolysis for the vinyl acetate units can be greater than 99%.

[0106] The cationic polymer prior to combining with the other ingredients of the coating color had a solids content of about 40%. a viscosity (4% aqueous solution at 20° C.) of 2740 cps, and a pH of 2.2.

Example 4

[0107] Example 4 was prepared in the same way as Example 1, but with a structure generally following FIG. 1B, with a first topcoat 6, a basesheet 7, and a second topcoat 8 on the opposing side of the basesheet. Compared to Example 1, the base coat was left out. Here the coating basis weight was 24 g/m.sup.2 on each side.

[0108] The basis weight of the basesheet in this example was 55 g/m.sup.2, the ash content was 14%, and the Cobb value was 21 g/m.sup.2.

Example 5

[0109] Example 5 was prepared in the same way as Example 1, but with a structure corresponding generally to FIG. 1C, such that the basesheet is only coated on nested embossing side. FIG. 1C shows a basesheet 10 with coated side on the upper surface comprising a top coating layer 9. The basis weight of the coating was 24 g/m.sup.2, and the basis weight of the basesheet was of 55 g/m.sup.2. The ash content of the base sheet was 14%, and the Cobb value was 21 g/m.sup.2.

Example 6

[0110] Example 6 was generally prepared in the same way as Example 1, but with a structure corresponding generally to FIG. 1D. Thus, the paper is coated on only one side of the basesheet 13, with a base coating 12 and a top coat 11.

TABLE-US-00001 TABLE 1 Comp. Comp. Reference Example 1 Example 2 Example 3 Ex. 1 Ex. 2 Base- Coated sheet 63 85 143 128 157 sheet basis weight, gsm Base Pigment PCC, 100 PCC, 100 PCC, 90 parts + — — coat parts parts GCC, 10 parts Silane- Parts 8 6 3 — — mod. Deg. of 1400-1800 1400-1800 1400-1800 — — PVA Polym. SB Parts 0 5 9 — — latex Gel — 70-80% 70-80% — — content Solids content, % 55 58 62 — — Coating weight, gsm 8 12 15 — — Top coat Pigment Pigment PCC 100 parts PCC 100 PCC 100 parts — — type parts Dispersant One or more of acrylic copolymer aqueous — — type dispersion, poly dimethyl diallyl ammonium and chloride, polyamines, quaternary ammonium dosage salts of acrylic polymer, with a dosage of 15-20 kg/ton of pigment (dry weight) Silane- Parts 6 8 9 — — mod. Deg. of 1400 to 1800 1400 to 1400 to 1800 — — PVA Polym. 1800 Cationic Type poly diallyl poly diallyl poly quaternary — — polymer and dimethyl dimethyl ammonium salt, 5 dosage ammon. ammon. parts chloride, 2 chloride, 3.5 parts parts Mol. wt. 400 to 100000 400 to 400 to 100000 — — 100000 Wear-resistant Epoxy resin, Epoxy resin, PAPU, 1.3 parts — — agent 0.6 parts 1.0 parts Solids content, % 55 53 52 — — Coating weight, gsm 15 12 10 — — Ink jet Drying behavior Immediate Immediate Immediate drying Immed. Dried printing drying after drying after after printing after slowly printing printing printing (over 60 sec.) Color density: K 1.25 1.27 1.27 1.22 0.98 Color density: M 1.13 1.15 1.14 1.11 0.87 Color density: C 0.88 0.90 0.89 0.84 0.65 Color density: Y 0.96 0.96 0.97 0.98 0.90

[0111] Remarks: (1) In evaluating the drying behavior after printing, a finger was used to lightly rub the printed image. If the ink was already dry, then there would be no ink on the paper after rubbing. (2) In Table 1, for the base coat and top coat formulations, the listed parts of the components are relative to 100 parts dry pigment in the coating colors, all on a dry basis. (3) The color density test device is an X-Rite 528 spectroscopic density meter (Estelle Corp., Shanghai, affiliated with Danaher in the U.S.), used according to test standard GB/T 35390-2017. (4) For Examples 1 to 3, the pigment particle surface charge was −50 mu eq/L. (5) For Examples 1 to 3, the pigment had a D50 particle size of 0.4-0.8 μm. (6) For Examples 1-3, the roughness of the coated paper substrate prior to applying the ink-receiving coating was 4.0 to 8.0 (7) The color density printing uses solid color block printing. Since color density may vary depending on the model of inkjet printer used, the printer employed herein is a commercially available EPSON L810 type ink jet printer with continuous ink supply using 6 color cartridges. (8) After printing, allow the sheet to dry for 30 seconds or more before further processing or testing.

[0112] For the topcoat coating color of Examples 1-3, using an X-ray gravity sedimentation pigment particle size detection method, the data in Table 2 was obtained.

TABLE-US-00002 TABLE 2 Diameter Diameter Cumulative Incremental upper lower limit, Mean mass mass limit, μm μm diameter, μm distribution, % distribution, % 10.0 8.0 8.944 99.6 0 8.0 6.0 6.928 99.4 0.2 6.0′ 5.0 5.477 99.0 0.4 5.0 4.0 4.472 98.7 0.3 4.0 3.0 3.464 98.3 0.4 3.0 2.0 2.449 96.6 1.7 2.0 1.5 1.732 94.1 2.5 1.5 1.0 1.225 88.1 6.0 1 0.8 0.894 81.9 6.2 0.8 0.6 0.693 67.1 14.9 0.6 0.5 0.548 53.6 13.5 0.5 0.4 0.447 36.8 16.8 0.4 0.3 0.346 20.4 16.4 0.3 0.2 0.245 16.3 4.1

[0113] Remarks: In Table 2, each incremental range considered for particle diameter has an upper limit and a lower limit. About 16.3% by weight of the particles are below 0.3 microns, and about 12.2% by weight of the particles are below 0.2 microns (16.3% cumulative at 0.3 microns minus the 4.1% weight fraction between 0.2 and 0.3 microns) so it can be said that over 70% of the particles are greater than 0.2 microns in diameter, or over 80% are greater than 0.2 microns in diameter, since about 88% by weight as measured were actually above 0.2 microns in diameter.

Comparative Example 1

[0114] Nanomater silicone dioxide was the ink-jet paper coating pigment from a commercial ink jet paper.

Comparative Example 2

[0115] Ordinary offset printing coated paper was used.

Comparative Example 3

[0116] Comparative Example 3 generally followed Example 1, but the topcoat formulation employed an ordinary polyvinyl alcohol instead of silane-modified polyvinyl alcohol.

Comparison of Color Density Values

[0117] When ink jet droplets contact a paper, there is a combination of infiltration into the paper and surface diffusion of the wet ink. For best results (minimum penetration into the paper and low spreading of the ink), the ink should dry quickly and be retained on the upper surface of the topcoat. An important measure of image quality of particular importance in ink jet printing is the color density. If diffusion of the ink occurs to a high degree, the resulting image will not be clear due to the spreading of the ink. To examine the quality of the printing for Examples 1-3 and for the comparative examples, the color density of a printed image was measured. The specific test employed a commercially available EPSON L810 type ink jet printer with continuous ink supply using 6 color cartridges, ink model T674. Printing was based on the KCMY color model using inks that respectively are black, cyan, magenta, and yellow (4 colors). Color density was evaluated using an X-Rite® color density instrument (Estelle Corp. 528 spectroscopic density meter), according to the Chinese test standard GB/T35390-2017). Results for each of the 4 colors are shown in Table 1.

[0118] As can be seen from Table 1, inventive Examples 1-3 provide similar good results in printing. After printing and drying, the overall color density for Examples 1-3 was higher than that of Comparative Example 1 and for Comparative Example 2 with an ordinary offset printing coated paper. In Comparative Example 2, the ink-jet printing did not dry rapidly, allowing high diffusion and penetration into the sheet, with a resulting low value for color density, significantly lower than for the inventive Examples 1-3.

Wet Friction Resistance

[0119] Example 1 and Comparative Example 3 were printed and subjected to a printing paper wet friction resistance test, using the following method: The printed sheets from Example 1 and Comparative Example 3 each received a drop of water (estimated 0.5 ml) applied after printing. After allowing the ink to absorb for 10 seconds, an average-sized human index finger pad is placed flatly on the paper providing a contact area of about 1 square cm, and with an applied load of about 0.5 MPa of pressure, based on measurement with a laboratory balance. During the test, the finger is moved back and forth over a span of 5 cm for a total of 5 times (forward, backward, forward, backward, then forward again), maintaining substantially constant force, at a uniform speed over a total time of about 5 seconds. Similar treatment is applied to each sample tested for wet friction resistance.

[0120] FIG. 2 shows the effect of rubbing a wet spot for the printing paper of Example 1, and FIG. 3 shows the results after rubbing the wet spot for the paper of Comparative Example 3. It can be seen that the paper of inventive Example 1 not only has improved absorptivity and fixation of ink-jet ink, and but also has better wet friction resistance.

Analysis of Print Clarity Using Gap Test Grayscale Values

[0121] From Example 3 and Comparative Example 1, the inkjet printing papers were evaluated in terms of printing clarity and performance. Related results are provided in FIG. 4 to FIG. 19.

[0122] FIG. 4 shows the printing of a portion of a reference image from Example 3. FIG. 5 shows the same image for Comparative Example 1. As can be seen, Example 3 shows relatively higher values of color density (i.e., the density value of the solid regions are relatively high). Further, the dot density (generally refers to a halftone region reflection density) are superior to those of comparative Example 1's ink jet paper.

[0123] FIG. 6 shows Example 3's printing performance relative to a grid of black and gray squares from a reference image.

[0124] FIG. 7 is a related portion of a printed reference image from the printed sheet of Comparative Example 1.

[0125] FIG. 8 is shows the electronic image from the original reference image. FIG. 8 is used as a standard to evaluate the quality of the test patterns printed. The grid of dark blocks is formed from square blocks each with sides about 4.72 mm in length, and the gap between the blocks is about 0.32 mm wide. The width of the top two rows of 8 squares from edge to edge is 40 mm. The color of the squares varies, with CMY values for each column listed above the columns, and K values (black intensity) listed for each tow in labels at the left.

[0126] FIG. 9 depicts the portion of the original reference image used for evaluation of the Gap Test. The test is conducted by analyzing a particular section of the printed reference grid, corresponding to the location of a white region on the reference image between the second and third rows from the top and extending from the left side of the grid to the beginning of the third column of squares. The target region for measurement, the “measurement zone,” is shown in the rectangle filled with hatched lines over a portion of the reference grid, as shown in FIG. 9. On printed images, it corresponds to an area of 0.32 mm tall by 10.08 mm wide. On an image scanned at 600 dpi, the measurement zone corresponds to a region of about 7.8 pixels by 245 pixels, but to use a whole number of pixels and avoid requiring the measurement area even with perfect printing to overlap with some of the black ink of the ideal reference image, the pixel height is rounded down to 7 pixels instead of being rounded up to 8. Thus, on 600 dpi scanned images, a measurement zone of 7 by 245 pixels is a reasonable approximation of the idealized measurement zone as defined on the reference image.

[0127] FIG. 10 shows the relevant portion of the printed reference image on the paper of Example 3.

[0128] FIG. 11 is an enlarged view of the measurement region of FIG. 10 for Example 3.

[0129] FIG. 12 shows a portion of a reference image printed on a commercial New Page ink-jet paper.

[0130] FIG. 13 shows a portion of a reference image printed on a commercial Mitsubishi ink-jet paper.

[0131] FIG. 14 shows a portion of a reference image printed on an early experimental form of ink-jet paper from Gold East Paper.

Determining Gap Test Grayscale Value

[0132] After printing the reference image onto a selected paper substrate, the printed image is scanned electronically to obtain an image file. A Hewlett-Packard LaserJet M1213nf multi-functional printer is used, which has a scanner integrated in the system. Scanning is done at a resolution of 600 dpi. Image files are stored in TIFF format to preserve the quality of the scanned image without loss from compression.

[0133] Measurement of the gray-scale color of the scanned printed image in the above-mentioned measurement zone is made using a public domain image analysis tool, ImageJ (Wayne Rasband, National Institutes of Health, United States, version 1.52d), processed on a Windows 7 machine. Using a scanned image with x and y axes aligned the x and y axes of the computer screen running ImageJ (image rotation of up to 1 degree may be needed to ensure good alignment, given natural variability that can occur in scanning images), the average gray-scale color in the measurement zone is obtained by simply applying the “Measurement” function for the selected measurement zone. A value of 255 corresponds to pure white, the ideal result that can be obtained from a perfectly printed reference image on pure white paper. A value of 0 corresponds to pitch black.

[0134] The measurement area is vertically centered over the boundary between the two rows of black squares, and if there is doubt over the exact vertical location due to blurring, it can be adjusted up or down by one pixel and the location that gives the lightest average gray-scale value is selected.

[0135] FIG. 10 shows a portion of the scanned printed image for the inventive paper of Example 3. Small squares indicate “handles” on the ImageJ screen on the selected measurement zone. The measurement zone has dimensions of 7×245 pixels. It can be seen that the content of the selected measurement zone is largely white. The ImageJ measurement gave a mean value of 217.

[0136] The New Page ink jet sheet used for the printed squares in FIG. 12 shows more blurring in the gap between the squares, and gave a grayscale value of 164 for the measurement zone, indicating that more black ink had entered the measurement zone due to blurring in the printing. The New Page sheet had a basis weight of 118 gsm and is believed to have a coating rich in nanoparticles.

[0137] The results with the Mitsubishi ink jet paper shown in FIG. 13 were similar, with a gray scale value of 168 for the measurement zone. The Mitsubishi sheet had a basis weight of 128 gsm and is believed to have a coating rich in nanoparticles.

[0138] A high level of blurring was seen in the early Gold East ink jet prototype in FIG. 14, where the Imagej image analysis measurement of the average grayscale value in the measurement zone gave 107, reflecting a high amount of black ink that had spread laterally into the measurement zone in the gap between two rows of squares in the printed reference image. The early Gold East ink jet prototype had a basis weight of 118 gsm, and calcium chloride and calcium carbonate were used in the coating. Fortunately, the poor results in the early trial were overcome with the discoveries leading to the present invention, where excellent print quality can be obtained without the need for using nanoparticles or high levels of aluminum salts, which can be undesirable for environmental or other reasons.

[0139] In general, the ink jet papers of the present invention may provide a Gap Test Grayscale Value of 180 or greater, 190 or greater, 200 or greater, or 210 or greater may be achieved with the inventive method. Note that the upper limit of the test inherently is 255, so it is understood that “180 or greater” means “from 180 to 255.” In practice, the unprinted portions of a scanned paper sheet will rarely be at maximum whiteness, and thus in practice the upper limit for regions completely free of ink may be less than 255, such as about 235, about 240, about 245 or about 250. Thus, in practice, a paper within the scope of the present invention may display a Gap Test Grayscale Value from 180 to about 250 or from 180 to 245.

[0140] Further comparisons in print quality can be seen in additional portions of the printed reference image as we compare the printed results for Example 3 in FIGS. 15, 17, and 19 with the same portions of the printed reference image for Comparative Example 1 in FIGS. 16, 18, and 20, respectively. In each case, the inventive ink-jet paper of Example 3 gives superior printing results, and achieves this improvement in color density, clarity, color saturation, etc., without relying on expensive nanoparticles.

CONCLUDING REMARKS

[0141] Although the invention has been described with reference to particular embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments as well as alternative embodiments of the invention will become apparent to persons skilled in the art. It is therefore contemplated that the appended claims will cover such modifications or embodiments that fall within the scope of the invention.

[0142] The dimensions and values disclosed herein are not to be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each such dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as “40 mm” is generally also intended to mean “about 40 mm.”

[0143] All documents cited in the specification are, in relevant part, incorporated herein by reference to the extent it is not contradictory herewith. The citation of any document is not to be construed as an admission that it is prior art with respect to the present invention. To the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern.