EP0829374A2

EP0829374A2 - Immobilization of cationic and anionic dyes and pigment dispersions with insoluble metal salts

Info

Publication number: EP0829374A2
Application number: EP97306764A
Authority: EP
Inventors: Betty Yu; Michael Sklarewitz
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1996-09-16
Filing date: 1997-09-02
Publication date: 1998-03-18
Also published as: JPH10100533A; EP0829374A3; KR19980024075A

Abstract

A specially prepared ink-jet paper comprising fibers containing pits filled with a slightly water-soluble metal salt by precipitation is disclosed for use with thermal ink-jet color printers, such as Hewlett-Packard Company's DeskJet® printers. An ink-jet ink containing a colorant, such as a dye or pigment, and a dispersant for dispersing the colorant when necessary, is printed onto the specially prepared paper. The dispersing ability of the dispersant is capable of being vitiated by particles of the metal salt precipitated in the pits contained in the ink-jet paper, thereby indirectly causing the colorant to precipitate on the surface of the paper when the ink-jet ink is printed on the ink-jet paper. In the case of dye-based inks the metal salt directly causes the dyes to precipitate. As a consequence, the ink-jet color printer is able to produce high-quality color images in a reliable manner with improved color gamut and water-resistance and reduced bleed between adjacent-printed colors.

Description

TECHNICAL FIELD

The present invention relates to ink-jet printing, and, more particularly, to a paper for use in ink-jet printing in which the pulp fibers contained in the paper have been modified by filling them with precipitated slightly soluble metal salts.

BACKGROUND ART

In commercially-available thermal ink-jet color printers, such as the DeskJet® printer available from Hewlett-Packard Company, a color spectrum is achieved by combining yellow, magenta, and cyan inks in various proportions. The color spectrum achievable by combining a set of colored inks is called the color gamut of the ink set. The color gamut is always a subset of the theoretically available color spectrum.

One reason for not achieving the theoretically available spectrum is that the intensity of a particular ink in the ink set, as it appears on the print medium, cannot effectively be raised above a particular level. Colors in the spectrum which need a higher intensity of a particular colorant than can be delivered to the print medium, therefore, cannot be produced. The total amount of colorant which can be delivered depends on a number of factors, including the concentration of the colorant in the ink and the deliverable volume of the ink. Typically, the concentration of the colorant in the ink cannot be arbitrarily raised to increase the intensity on the paper without sacrificing other important aspects of the ink formulation.

Another reason for reduced color gamut is that some of the ink colorant penetrates into the print medium, especially when the medium is paper. When this happens, the apparent intensity of the colorant in the printed region is lower than expected based on the amount of colorant delivered to the surface of the print medium. A way of alleviating this problem is to ensure that the colorant remains on the surface of the paper or, phrased differently, to control penetration of the colorant into the surface of the paper.

In ink-jet ink compositions, ink is formulated by dispersing dye or pigment in an ink vehicle. For example, a cyan ink would comprise a cyan dye dissolved in an ink vehicle. The dye molecules employed in ink-jet ink compositions are often in the form of dye salts comprising a dye anion and a cation such as sodium. In the case of pigment-based colorants, the ink vehicle typically includes a charged dispersant.

Methods for controlling penetration of the colorant into the surface of the paper rely on interfering with the dispersing character of the vehicle once the ink has been applied to the paper. One way to stop the dispersant action of the vehicle is to evaporate the ink solvent. Alternatively, penetration of the colorant into the surface of the paper can be decreased by destroying the dispersing ability of the dispersant in the ink after the colorant has been applied. A method that destroys the action of the dispersant which operates more quickly than evaporation of the solvent will better decrease the penetration of the colorant into the surface of the paper.

U.S. Patent No. 4,694,302, entitled "Reactive Ink-Jet Printing' and assigned to the same assignee as the present application, discloses a print method for increasing the water-fastness and print quality of an ink. In this patent, a reactive species that chemically links the dye in the ink to the paper substrate is applied to the print medium either before or after the ink is applied to the print medium. This method, however, requires two separate printheads to achieve improvements in print quality.

U.S. Patent No. 4,740,420, entitled "Recording Medium for Ink-Jet Printing", and U.S. Patent No. 4,554,181, entitled "Ink Jet Recording Sheet Having a Bicomponent Cationic Recording Surface", disclose recording media which have been modified by surface treatments containing soluble metal salts to aid in insolubilization of the colorant in the ink. These methods suffer from complex and expensive, post-manufacture, surface modification of the print media. Furthermore, none of these methods particularly addresses the need to reduce penetration of the colorant into the surface of the print medium. Improvement of color gamut calls for a reduction in penetration.

U.S. Patent No. 5,126,010, entitled "Ink-Jet Recording Sheet", discloses a recording sheet in which water-insoluble calcium carbonate is mixed into the paper and serves as a filler pigment. Additional treatment of the paper surface allows for the absorption of water-based ink into the paper. This print medium is not designed to retain the ink on the surface of the paper.

Accordingly, a need remains for a method of ink-jet printing that conveniently and economically reduces colorant penetration into the medium's surface and thereby increases the gamut of colors the ink set can reproduce.

DISCLOSURE OF INVENTION

In accordance with the invention, a print method is provided which substantially reduces penetration of the ink colorant into the print medium (paper) by conveniently supplying a metal salt within the pits of the pulp fiber that causes the dispersant action of the ink vehicle to cease solubilizing or disbursing the ink colorant. Specifically the ink-jet print method comprises the steps of:

(a) providing an ink-jet paper comprising pulp fibers which include pits filled with particles of a precipitated slightly soluble metal salt, with the pits small enough to mechanically retain the particles but large enough so that ions from an ink vehicle can migrate to them;

(b) providing at least one ink-jet ink comprising:

(1) at least one colorant that can be precipitated by particles of the precipitated slightly soluble metal salt in the ink-jet paper when the ink-jet ink is printed onto the ink-jet paper, and

(2) a vehicle that contains at least one dispersant when the colorant is pigment-based or dissolves the colorant without a dispersant when the colorant is dye-based; and

(c) printing the ink-jet ink onto the surface of the ink-jet paper, thereby causing the colorant to precipitate onto the surface of the ink-jet paper, thereby improving color gamut and retention and decreasing penetration and bleed of the colorant.

Further in accordance with the present invention, an ink-jet paper is provided which improves color gamut and retention in ink-jet printing, comprising pulp fibers which include pits therein. The pits are filled with particles of the precipitated slightly soluble metal salt, the pits being small enough to mechanically retain the particles and large enough so that ions from the ink vehicle can migrate to the pits.

The use of ink-jet paper which incorporates precipitated slightly soluble metal salts to precipitate the colorant on the surface of the paper improves color gamut and retention, decreases penetration of the colorant and reduces bleed between adjacent-printed colorants.

BEST MODES FOR CARRYING OUT THE INVENTION

The invention described herein is directed to a method and a paper for use with ink-jet color printers, particularly thermal ink-jet printers such as Hewlett-Packard's DeskJet® printers. The paper enables an ink-jet color printer to produce high-quality images with increased color gamut and color retention and decreased penetration and bleed of printed ink-jet inks by inducing precipitation of the colorant of the ink-jet ink. Specifically, the paper contains metal salts that were precipitated in the pits in the fibers of the paper pulp prior to the paper sheet being laid down. The metal salt causes precipitation of the colorant in the ink-jet ink by destroying the dispersing or dissolving ability of the ink vehicle. This dispersing ability comes from the interaction of the ink vehicle with the dye of a dye-based ink and from a dispersant molecule/pigment interaction when the colorant is pigment-based. When the dispersing ability of the ink vehicle or pigment molecule is destroyed by interaction with the metal salt, the ink vehicle is no longer capable of dissolving the colorant and, therefore, the colorant precipitates. The precipitation reaction occurs more rapidly than penetration into the paper and, therefore, the colorant remains substantially on the surface of the paper. Since there is no longer any component present which can substantially dissolve the colorant in water, the resulting printed color is extremely water resistant. The printed color resists water better than the base paper does.

The size of the pits in the paper pulp fibers plays an important role in capturing the precipitated salts once they have been precipitated in the pits, but perhaps more crucial is the use of softwood pulp in the papermaking process. Pit size, a physical characteristic, depends upon the type of wood used for pulp and not the pulp treatment process. Softwoods, the type used in the blotter paper (described below), differ from hardwoods in that hardwoods contain vessel elements whose pits cannot contain the precipitated material because the vessel elements consist of open ended structures. The tracheids and parenchyma of softwoods contain pits which are not open ended and thus can contain the precipitated salts. Accordingly, softwoods are preferred in the practice of the invention.

The metal salt must be insoluble enough in water to avoid its removal during the processing of the pulp fibers to paper, yet soluble enough to provide free ions to interact with the ink vehicle or dispersant molecules. It is contemplated that the metal salt may comprise various slightly soluble metal salts with low solubility product constants. Preferably, the metal salts employed in the practice of the present invention have a solubility product of less than about 5 x 10^-5.

Suitably-employed cations for the inorganic salt include, but are not limited to, alkaline earth metals of Group 2A of the Periodic Table (e.g., magnesium, barium and calcium). Metal salts which may be effective in carrying out this invention are magnesium oxalyte, calcium sulfate, calcium tartarate, barium oxylate, and barium fluoride. Preferably, magnesium is employed as a cation in the practice of the invention. Suitably-employed anions for the metal salt include, but are not limited to, sulfate, carbonate and oxylate. The most preferred anion associated with magnesium is carbonate.

It is contemplated that the performance of the color and black ink sets of the DeskJet 850C and DeskJet 500C ink-jet printers will all be improved when printed on the paper described in this invention. Hewlett-Packard part numbers for these ink sets are given in Table I below. Additionally, Canon BJC 600 ink sets were also tested; results in terms of penetration were similar to those found with Hewlett-Packard inks. Canon part numbers are also given in Table I.

PART NO.	PRINTER	INK TYPE
HP 51645A	DeskJet 850C	Black Pigment
HP 51641A	DeskJet 850C	Cyan, Magenta, Yellow Dye
HP 51625A	DeskJet 500C	Cyan, Magenta, Yellow Dye
HP 51626A	DeskJet 500C	Black Dye
BJI-201Bk	BJC 600	Black
BJI-201c	BJC 600	Cyan
BJI-201m	BJC 600	Magenta
BJI-201y	BJC 600	Yellow

The purity of all components is that employed in normal commercial practice for paper making. All concentrations are expressed in weight percent unless other indicated.

In addition to the metal salt, the paper may comprise components as found in commercial paper manufacture. The wood pulp used in this invention is common papermaking pulp such as that prepared by bleaching wood pulp. The wood pulp may also be combined with other sources of pulp provided that the pit sizes in the pulp fibers are of an appropriate range.

Weight percentages of components of commercial paper
Component	Parts	Weight %
Hardwood Pulp	66	20.58833
Softwood Pulp	80.4	25.08033
Eucalyptus Pulp	120.6	37.62049
Internal Size	2	0.623889
Retention Aid	1	0.311944
Wet End Starch	2	0.623889
Alum	0.3	0.093583
Precipitated Calcium Carbonate	15	4.679165
Biocide	0.05	0.015597
Size Press Starch	13.37	4.170696
Surface Size	0.24	0.074867
Optical Brighteners	0.61	0.190286
MgCO₃	19	5.926943
TOTAL	320.57	100

The concentration of metal salt within the paper can range from 5 to 15 wt%, and preferably is between about 7 to 12 wt%. The weight percent of magnesium carbonate is determined by a thermogravimetric analyzer (TGA, Perkin Elmer TGA7). With the TGA, the paper is ashed and the resulting material is assumed to be magnesium oxide, the mineral form of magnesium carbonate. That weight percentage is converted to grams of MgCO₃ using stoichiometry.

The size of pits in the pulp fibers can range from about 1 to 50 microns, with the preferred average size being about 15 to 30 microns.

In a preferred embodiment, the paper contains magnesium carbonate, at about 5.9 wt%, precipitated in the pits of about 15 to 30 microns of dried paper pulp present at about 83.3 wt% and also containing the ingredients listed in Table II. The paper is printed with any of the inks described above.

EXAMPLES

Results for all examples and comparative examples are included after a description of the examples.

EXAMPLE 1

The paper formed in this example is referred to as in situ loaded paper. An example was performed in which the paper of the invention was prepared by soaking blotter papers to reform pulp, rather than using undried pulp. Blotter papers were used because they lack any kind of chemical treatment. The blotter paper used came from James River; its type is Blotting Verigood 19"x24"-160M Basis 80, 411-01-11. Four blotter papers were soaked overnight in 500 ml of saturated magnesium sulfate solution. The excess solution was removed from the soaked reformed pulp. The pulp fibers remained damp with the magnesium sulfate solution. These fibers were then placed in 500 ml of saturated sodium carbonate and disintegrated with a Hamilton Beach blender for five minutes at high speed. The fibers were removed from the sodium sulfate solution and rinsed until the pH of the rinsing water was neutral. Paper was laid down into sheets by allowing the fibers to settle onto a screen. Waxpaper was placed onto the fibers and the screen was rolled with a ten pound roller to remove excess water. The sheets were then dried at 105°C until all the water had evaporated.

The amount of magnesium carbonate present in the paper was calculated by gravimetric methods and found to be 9 wt%.

The in situ paper was printed with inks from a Canon BJC 600, a DeskJet 850C and a DeskJet 500C ink-jet printer, after which the penetration depths of the inks were measured (See Table III, heading MgCO₃ (in situ)). Penetration depth was measured by taking cross sections of printed media and measuring the penetration depth of the ink. Samples were prepared by setting the samples in epoxy and placing microtomed slices on a microscope glass. Penetration depth was measured from the microscopy photos and appropriate conversions were applied to obtain the actual penetration depths of the inks.

Cross-sectional micrographs of the paper showed a layer of ink coating the surface, thus illustrating the immobilization of the coloring agent and prevention of penetration as expected. Penetration tests performed on the inks showed decreased penetration (an improvement) for the dyes in the 500 series printers, and the dye-based color ink set of the DeskJet 850C printer (see Table III). Microscopic examination of this paper showed that the fibers of this paper were relatively long. The pits appeared physically different compared to pits in paper made from hardwood. To further emphasize the differences, hardwood contains fewer sites for any precipitate to remain lodged. The primary softwood structures are the tracheid and parenchyma cells which both contain enclosed pits within the cell walls to capture precipitated salts. The hardwoods, however, also contain vessel elements as a major component. Vessel elements, as mentioned previously, are open ended, allowing the precipitated material to escape during the pulp rinsing process. The size of the pits in both cases can range from about 1 to 50 microns.

To test ink stability, the paper was printed using a Hewlett-Packard DeskJet 850C printer. After the paper was soaked and agitated for three days, the fibers were used to reform paper using the method described above. The reformed paper was blue, indicating that the inks had been retained on the fibers during the reformation process. Also, printed sheets were placed in water but still retained their deep, rich colors after almost a month.

Color gamut describes the saturation, hue, and brightness of a particular color. Herein, increased color gamut was qualitatively determined by an observer who compared conventional paper to the magnesium carbonate loaded hand sheets. Optical density values were obtained using the Macbeth RD 913 with a 2mm aperture. The precipitated magnesium carbonate sheets of this example performed better than all other samples except for Hewlett-Packard Premium Inkjet paper on the DeskJet 850C printer. Other metal salts of this invention were tested as well. With the DeskJet 560C and 500C printers, the precipitated magnesium carbonate followed HP Premium Inkjet paper in optical density for cyan, yellow, and black. Calcium carbonate precipitated sheets performed well with magenta. On the Canon BJC 600 printer, precipitated magnesium carbonate produced higher optical densities than Eureka!35 (described below) for all colors.

COMPARATIVE EXAMPLE 1

To prepare the conventionally loaded paper, magnesium carbonate was mixed with the fibers. Paper was laid down into sheets by allowing the fibers to settle onto a screen. Waxpaper was placed onto the fibers and the screen was rolled with a ten pound roller to remove excess water. The sheets were then dried at 105°C until all the water was evaporated. Magnesium carbonate was found to be in the conventionally-treated sheets at 2 wt%. In the in situ method, metal salts are precipitated in the fiber walls whereas for the conventionally-treated sheets, metal salts are mixed in with pulp so that mechanical entrapment occurs between pulp fibers. For the case of the precipitated material, the salt is trapped within the cell wall of the fiber.

The conventionally loaded paper was printed with inks from a Canon BJC 600, a DeskJet 850C and a DeskJet 500C ink-jet printer. Optical density values were measured and are listed in Table IV, under the heading MgCO₃ conventional.

COMPARATIVE EXAMPLE 2

To prepare the plain paper, the method described above for the "in situ" treated paper was followed except no treatment of salt solution occurred. The blotter paper was dispersed into pulp and then deposited on a screen and dried as described above.

The plain paper was printed with inks from a Canon BJC 600, a DeskJet 850C and a DeskJet 500C ink-jet printer. Optical density values were measured and are listed in Table IV, under the heading Blotter Paper.

COMPARATIVE EXAMPLE 3

Two commercially available papers were used as controls. These papers are called Eureka!35 brand paper (James River Corporation) and Hewlett-Packard Premium Inkjet Paper part number HP 51634Y. These papers were printed with inks from a Canon BJC 600, a DeskJet 850C and a DeskJet 500C ink-jet printer, after which the penetration depth of the inks was measured. The penetration depths are listed in Table III, under the headings of Eureka! (James River) and HP Premium Ink Jet Paper respectively. Optical density values were measured and are listed in Table IV, under the headings Eureka! and HP Premium Ink Jet, respectively.

RESULTS OF EXAMPLES

Penetration Data
Media	Printer	Color	Penetration Depth (mm)	Magnification	Actual Depth (µm)
MgCO₃ (in situ)	DJ850C	Black	1.5	100	15
	DJ850C	Cyan	2	100	20
	DJ850C	Magenta	1	100	10
	DJ850C	Yellow	5	100	50
	DJ560C	Black	1	100	10
	DJ560C	C. Black	1.5	100	15
	DJ560C	Cyan	1	100	10
	DJ560C	Magenta	1	100	10
	DJ560C	Yellow	3	100	30
	Canon BJC 600	Black	1	100	10
	Canon BJC 600	Cyan	1	100	10
	Canon BJC 600	Magenta	2	100	20
	Canon BJC 600	Yellow	1	100	10
HP Prem. Ink Jet Paper	DJ850C	Black	3.5	200	17.5
	DJ850C	Cyan	4	200	20
	DJ850C	Magenta	4	200	20
	DJ850C	Yellow	6	200	30
	DJ560C	Black
	DJ500C	C. Black	3	200	15
	DJ500C	Cyan	4	200	20
	DJ500C	Magenta	4	200	20
	DJ500C	Yellow	2.5	200	12.5
	Canon BJC 600	Black	4	200	20
	Canon BJC 600	Cyan	4	200	20
	Canon BJC 600	Magenta	3	200	15
	Canon BJC 600	Yellow	4	200	20
Eureka! (James River)	DJ850C	Black	2	200	10
	DJ850C	Cyan	12	200	60
	DJ850C	Magenta	12	200	60
	DJ850C	Yellow	10	200	50
	DJ560C	Black	10	200	50
	DJ560C	C. Black	8	200	40
	DJ560C	Cyan	3	100	30
	DJ560C	Magenta	5	200	25
	DJ560C	Yellow	3	200	15
	Canon BJC600	Black	10	200	50
	Canon BJC600	Cyan	6	200	30
	Canon BJC600	Magenta	8	200	40
	Canon BJC600	Yellow	5	200	25
Note: C. Black = composite black

Optical Density Values
Media	Printer	Black	Cyan	Magenta	Yellow
HP premium Ink Jet	DJ500C	1.08	0.37	1.32	1.09
MgCO₃ conventional	DJ500C	1.07	0.36	0.97	0.84
MgCO₃ ppt in situ	DJ560C	1.1	0.38	0.97	0.92
CaCO₃ ppt in situ	DJ560C	0.93	0.33	1.04	0.88
Mg(OH)₂ in situ	DJ500C	0.92	0.35	0.98	0.83
HP Premium Ink Jet	DJ850C	1.44	0.53	1.76	1.33
MgCO₃ ppt in situ	DJ850C	1.3	0.5	1.48	1.26
MgCO₃ conventional	DJ850C	1.24	0.4	1.03	0.98
Blotter Paper	DJ850C	1.19	0.42	1.2	1.05
Eureka!	DJ850C	1.17	0.48	1.23	1.09
CaCO₃ ppt	DJ850C	1.14	0.4	1.19	0.9
HP Premium Ink Jet	Canon BJC600	1.67	0.4	1.54	1.05
MgCO₃ ppt in situ	Canon BJC600	1.38	0.4	1.23	1
Eureka!	Canon BJC600	1.2	0.39	1.09	0.92

INDUSTRIAL APPLICABILITY

The method of employing a precipitated, slightly soluble metal salt in paper to cause precipitation of colorants in inks disclosed herein is expected to find commercial use in all ink-jet color printers, especially thermal ink-jet printers.

Thus, there has been disclosed a method of employing a precipitated, slightly soluble metal salt in paper to cause precipitation of colorants in inks for use in thermal ink-jet color printing. It will be readily apparent to those skilled in the art that various changes and modifications of an obvious nature may be made without departing from the spirit of the invention; all such changes and modifications are considered to fall within the scope of the invention as defined by the appended claims.

Claims

An ink-jet paper which improves color gamut and retention in ink-jet printing, comprising pulp fibers which include pits therein, said pits filled with particles of a precipitated slightly soluble metal salt, said pits being small enough to mechanically retain said particles and large enough so that ions from an ink vehicle of an ink used in said ink-jet printing can migrate to said pits.
The ink-jet paper of Claim 1 wherein said particles comprise a metal salt comprising at least one cation and at least one anion and wherein said salt has a solubility product less than about 5 x 10^-5.
The ink-jet paper of Claim 2 wherein said at least one cation is selected from the group consisting of magnesium, barium, and calcium and at least one anion is selected from the group consisting of sulfate, carbonate, and hydroxide.
The ink-jet paper of Claim 3 wherein said slightly soluble metal salt is selected from the group consisting of magnesium carbonate, magnesium hydroxide, calcium carbonate, calcium sulfate, calcium hydroxide, barium sulfate, barium hydroxide, and barium carbonate.
The ink-jet paper of Claim 1 wherein said precipitated slightly soluble metal salt has a concentration in said paper in a range of about 5 to 15 wt%.
The ink-jet paper of Claim 1 wherein the size of said pits are within the range of about 1 to 50 microns and are closed ended in nature.
The ink-jet paper of Claim 1 wherein said particles consist essentially of magnesium carbonate present in about 5 to about 15 wt% and said pits have a size within the range of about 1 to about 50 microns.
A method of improving color gamut and retention in ink-jet printing, comprising the steps of:

(a) providing said ink-jet paper of Claim 1;

(b) providing at least one ink-jet ink comprising:

(1) at least one colorant,

(2) a vehicle,

(i) wherein said vehicle contains at least one dispersant with dispersant ability to disperse said at least one colorant in said vehicle when said colorant is pigment-based, or

(ii) wherein said vehicle dissolves said colorant without a dispersant when said colorant is dye-based, and

(3) wherein said colorant can be precipitated by particles of said precipitated slightly soluble metal salt in said ink-jet paper when said ink-jet ink is printed onto said ink-jet paper; and

(c) printing said ink-jet ink onto a surface of said ink-jet paper,

thereby causing said at least one colorant to precipitate on said surface of said ink-jet paper, thereby improving color gamut and retention and decreasing penetration and bleed of said at least one colorant.