Compositions: coating or plastic – Coating or plastic compositions – Marking
Reexamination Certificate
2000-06-07
2003-07-22
Bell, Mark L. (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Marking
C106S031640, C106S478000
Reexamination Certificate
active
06596068
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an aqueous ink composition having dispersed therein carbon black or an organic pigment as a colorant. More particularly, it relates to an improvement of an aqueous ink composition containing a surface-modified pigment capable of dispersing and/or dissolving in water.
BACKGROUND OF THE INVENTION
Conventional aqueous inks used in writing pens or ink jet printers comprise water-soluble dyes. In order to improve image quality or image preservability, pigments such as carbon black have also come to be used as a colorant. For example, JP-A-64-6074 and JP-A-64-31881 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) disclose aqueous pigment inks having carbon black dispersed with the aid of a surface active agent or a polymeric dispersant.
However, known aqueous inks containing pigments steeply increase the ink viscosity where the amount of the colorant is increased to increase the recording density. Further, the surface active agent or the polymeric dispersant which is added in excess so as to stabilize the disperse system tends to cause foaming or reduction of defoaming properties, which may result in printing instability particularly in use in an ink jet printer.
To solve these problems, JP-A-8-3498 and JP-A-10-120958 teach that carbon black modified with a specific amount of surface active hydrogen or a salt thereof can be dispersed spontaneously without using a surface active agent or a polymeric dispersant. JP-A-10-95941 proposes an ink jet ink comprising the above-described surface-modified carbon black and a glycol ether. JP-A-10-212426 proposes adjusting each of the total sulfur content and the total chlorine content of carbon black to 0.1% by weight or less in order to secure ionization of the functional group on the surface-modified carbon black.
The ink comprising the surface-modified carbon black that is capable of spontaneous dispersion is extremely effective in maintaining the ink viscosity and printing stability. However, this type of ink turned out to have the following disadvantage. In order to make the pigment capable of spontaneous dispersion, the pigment is surface-treated with an oxidizing agent, etc. The oxidizing agent, etc. tends to decompose to produce ionic substances. Ionic substances also generate from sulfur, etc. released from the pigment itself during the oxidation. If these ionic substances are produced in quantity, the ink undergoes an increase of viscosity, resulting in reduced storage stability. Such a problem does not arise with the aqueous pigment inks having carbon black dispersed with the aid of a surface active agent or a polymeric dispersant as disclosed in JP-A-64-6074 and JP-A-64-31881 supra.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an aqueous ink composition which comprises a surface-modified pigment capable of dispersing and/or dissolving in water with no aid of a dispersant and exhibits improved printing stability while securing high storage stability.
The present invention provides an aqueous ink composition comprising an aqueous medium and a surface-modified pigment capable of dispersing and/or dissolving in water without a dispersant, wherein the composition has a free monovalent anion concentration of 250 ppm or less.
DETAILED DESCRIPTION OF THE INVENTION
The aqueous ink composition according to the present invention is a dispersion in an aqueous medium a surface-modified pigment capable of dispersing and/or dissolving in water without a dispersant, such as a surface active agent or a water-soluble resin, which is characterized in that the free monovalent anion concentration is 250 ppm or less. The language “capable of dispersing and/or dissolving in water without a dispersant” as used herein is intended to mean that a pigment can exist in water stably with a dispersible minimal particle size without using a dispersant. By the term “dispersible minimal particle size” as used herein is meant the smallest particle size that cannot be reduced further even if the dispersing time is extended. It is assumed that carbon black or an organic pigment having a large amount of a hydrophilic functional group introduced onto the surface thereof is spontaneously dispersed in water with a dispersible minimal particle size just like a water-soluble dye dissolving in water in the form of monomolecules.
Surface modification for making the pigment capable of dispersing and/or dissolving in water without a dispersant can be carried out by bonding a hydrophilic functional group, such as a carboxyl group, a carbonyl group or a hydroxyl group, or a salt thereof to the surface of the pigment either directly or via an alkyl group, an alkyl ether group, an aryl group, etc. Specifically, such surface modification can be effected by oxidizing the pigment surface with an oxidizing agent in water by chemical treatment to introduce a carboxyl group, a carbonyl group, a hydroxyl group, etc. or by grafting such a functional group or an active species containing the functional group to the pigment surface. It is particularly preferred that surface modification be carried out by oxidizing the pigment with a halogenic acid compound, such as an aqueous solution of sodium hypochlorite, an aqueous solution of calcium hypochlorite, an aqueous solution of sodium hypobromite, or an aqueous solution of sodium hypoiodite. Note that hypofluorite is practically unsuitable for surface modification because of its extreme instability.
The aqueous ink composition of the invention preferably contains the surface-modified pigment in a concentration of 1 to 15% by weight, more preferably 2 to 10% by weight, to secure a sufficient image density.
With the free monovalent anion concentration being adjusted to 250 ppm or less, preferably 150 ppm or less, the aqueous ink composition undergoes no increase in viscosity and exhibits excellent storage stability even when allowed to stand for a long period of time. The monovalent anion species the concentration of which should be controlled include halide ions, a nitrate ion, and a low-molecular weight carboxylate ion. Attention should particularly be paid to halide ions which are very likely to be present in a considerable amount as a result of decomposition of a halogenic acid salt used in wet oxidation for surface modification of the pigment. Such halide ions include a chloride ion originated in hypochlorites, a bromide ion originated in a hypobromites, and an iodide ion originated in a hypoiodite. These halide ions should be reduced to 250 ppm or less. A nitrate ion and a low-molecular weight carboxylate ion are present because the substances adsorbed on the pigment surface are released and oxidized with the oxidizing agent. It is also necessary to reduce these ions to 250 ppm or less.
In order not to deteriorate the disperse state of the dispersion, the control of the free monovalent anion concentration is preferably achieved by preparing the ink composition by using the surface-treated pigment dispersion having been purified by reverse osmosis, ultrafiltration, electro-dialysis, or a like technique. It is also possible to add an alkali, such as sodium hydroxide, to the dispersion before purification to form a salt with the existing monovalent anions.
The free monovalent anion concentration can be determined by, for example, separating the ink composition by ultrafiltration into a solid phase and a liquid phase, diluting the liquid phase with ultrapure water, and then analyzing by ion chromatography.
In addition to the free monovalent anions, the ink composition has monovalent anions inside the pigment particles or monovalent anions which are so strongly adsorbed on the surface of the pigment particles as not to be released even when allowed to stand. Such non-free monovalent anions, which may make the total monovalent anion concentration greater than 250 ppm, do not adversely affect the long-term storage stability of the ink as far as the free monovalent anion concentration is 250 ppm or less.
The pigment which can be
Ito Hiroshi
Momose Masayuki
Bell Mark L.
Faison Veronica F.
Ladas & Parry
Seiko Epson Corporation
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