AQUEOUS INK, INK JET RECORDING METHOD, INK CARTRIDGE, RECORDING UNIT AND INK JET RECORDING APPARATUS
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention
[0002] The present invention relates to an aqueous ink comprising a polymer and a pigment.
[0003] 2. Description of the Related Art
[0004] It has been known to use a pigment as a coloring material of an ink in order to make excellent the fastness properties, such as light fastness, gas fastness and water fastness, of an image obtained by an ink jet recording method. It has also been known to use in an ink a resin dispersion pigment that is dispersed with a polymer as a dispersant, or a self-dispersible pigment to the surface of which an hydrophilic group is bonded and a water-soluble polymer in order to improve the scratch resistance and highlighter resistance of an image (Japanese Patent Application Laid-Open No. H5-179183).
SUMMARY OF THE INVENTION
[0005] The present inventors have carried out a detailed investigation as to inks comprising a resin dispersion pigment with a view toward improving the fastness properties, such as light fastness, gas fastness and water fastness, of an image obtained by an ink jet recording method as well as the scratch resistance and highlighter resistance thereof. As a result, it has been found that the ejection characteristics of the inks are greatly affected by the type or properties of a polymer functioning as a dispersant.
[0006] Thus, the present inventors have carried out a further detailed investigation as to polymers used as dispersants for the purpose of making excellent the fastness properties, scratch resistance and highlighter resistance of images as well as the ejection characteristics of inks. As a result, it has been found that an ink in which a pigment is dispersed with a polymer having hydroxyl groups can solve the above-described problems.
[0007] However, it has been found that when an ink cartridge stored therein an ink in which a pigment is dispersed with a polymer having hydroxyl groups is installed in an ink jet recording apparatus and then is left to stand for a long period of time without ejecting the ink, the following new technical problem is raised. Namely, it has been found that the polymer in the ink seeps out through ejection orifices of a recording head, adheres in the vicinity of the ejection orifices and then sticks there, thereby the ejection characteristics are deteriorated.
[0008] Accordingly, it is an object of the present invention to provide an aqueous ink (hereinafter may also be referred to as "ink") capable of solving the following problems when the aqueous ink comprising a resin dispersion pigment that is dispersed with a polymer having hydroxyl groups as a dispersant. In other words, the object is to provide an aqueous ink that can provide images excellent in scratch resistance and highlighter resistance and can inhibit the seeping out phenomenon of the polymer and the deterioration of ejection characteristics even when it is stored for a long period of time.
[0009] Another object of the present invention is to provide an ink cartridge, a recording unit and an ink jet recording method using the above-described aqueous ink.
[0010] The above objects can be achieved by the present invention described below. More specifically, an aqueous ink according to the present invention comprises a polymer having hydroxyl groups, a pigment and a water-soluble organic solvent, wherein the water-soluble organic solvent comprises a polyhydric alcohol having a value of number of hydroxyl groups/number of carbon atoms in its molecule of 0.50 or less.
[0011] The ink jet recording method according to another embodiment of the present invention is an ink jet recording method comprising ejecting an ink by an ink jet method to conduct recording on a recording medium, wherein the ink is the aqueous ink of the above-described constitution.
[0012] The ink cartridge according to a further embodiment of the present invention is an ink cartridge comprising an ink storage portion storing an ink, wherein the ink is the aqueous ink of the above-described constitution.
[0013] The recording unit according to a still further embodiment of the present invention is a recording unit comprising an ink storage portion storing an ink and a recording head for ejecting the ink, wherein the ink is the aqueous ink of the above-described constitution.
[0014] The ink jet recording apparatus according to a yet still further embodiment of the present invention is an ink jet recording apparatus comprising an ink storage portion storing an ink and a recording head for ejecting the ink, wherein the ink is the aqueous ink of the above-described constitution.
[0015] According to the present invention, there can be provided an aqueous ink capable of solving the following problems when using the aqueous ink comprising a resin dispersion pigment that is dispersed with a polymer having hydroxyl groups as a dispersant. In other words, there can be provided an aqueous ink that can provide images excellent in scratch resistance and highlighter resistance and can inhibit the seeping out phenomenon of the polymer and the deterioration of ejection characteristics even when it is stored for a long period of time.
[0016] According to another embodiment of the present invention, there can be provided an ink cartridge, a recording unit and an ink jet recording method using the abovedescribed aqueous ink.
[0017] Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIGS. 1A, IB and 1C typically illustrate the condition that the state of an ink changes with time within a nozzle.
[0019] FIGS. 2A and 2B typically illustrate the relation among a free polymer having hydroxyl groups, a water molecule and a polyhydric alcohol.
[0020] FIG. 3 is a longitudinal cross-sectional view illustrating a recording head.
[0021] FIG. 4 is a transverse cross-sectional view of the recording head.
[0022] FIG. 5 is a perspective view of a multi-head composed of an array of a number of recording heads as shown in FIG. 3.
[0023] FIG. 6 is a perspective view illustrating an exemplary ink jet recording apparatus.
[0024] FIG. 7 is a longitudinal cross-sectional view illustrating an ink cartridge.
[0025] FIG. 8 is a perspective view illustrating an exemplary recording unit.
[0026] FIG. 9 typically illustrates the construction of an exemplary recording head.
DESCRIPTION OF THE EMBODIMENTS
[0027] The present invention will hereinafter be described in more detail by the best mode for carrying out the invention.
[0028] The present inventors have carried out a detailed investigation as to inks comprising a resin dispersion pigment with a view toward improving the scratch resistance and highlighter resistance of images obtained by an ink jet recording method. The inventors have supposed that the ejection characteristics can be improved by enhancing the hydrophilicity of a pigment, and thus carried out an investigation as to the use of a polymer having hydroxyl groups as a dispersant for the resin dispersion pigment further. However, it has been found that when an ink cartridge stored therein an ink containing a resin dispersion pigment dispersed with a polymer having hydroxyl groups is installed in an ink jet recording apparatus and then is left to stand for a long period of time without ejecting the ink, the following problem is raised. In other words, it has been confirmed that a phenomenon that the polymer in the ink seeps out through ejection orifices of a recording head occurs. Thus, the present inventors have carried out an investigation as to causes of the above phenomenon. As a result, the inventors have concluded that the following phenomenon caused within a nozzle of the recording head is one of the causes. The mechanism with which the polymer seeps out will hereinafter be described in detail with reference to FIGS. 1A, IB and 1C.
[0029] FIGS. 1A, IB and 1C typically illustrate the condition that the state of an ink containing a resin dispersion pigment dispersed by a polymer having hydroxyl groups changes with time within a nozzle.
[0030] FIG. 1A illustrates a state after the ink has been ejected from an ejection orifice 1 by thermal energy applied from a heater 2. As illustrated in FIG. 1A, water molecule 4, pigment particle 5 on which the polymer has been adsorbed, a water-soluble organic solvent 6, and a polymer having hydroxyl groups that is not adsorbed on the pigment (a free polymer 7 having hydroxyl groups) are present within the nozzle in a uniformly dispersed state. Incidentally, in the present invention, a polymer that is not adsorbed on the pigment is referred to as "free polymer", and a polymer having hydroxyl groups in a state not adsorbed on a pigment is referred to as "free polymer having hydroxyl groups".
[0031] FIG. IB illustrates a state after left to stand for a certain period of time with the state of FIG. 1A. As illus
trated in FIG. IB, water first evaporates from the ejection orifice 1 within the nozzle with time (water molecules 8 evaporated). When the time has further elapsed, the water molecule 4 within the nozzle transfers in the direction of the ejection orifice 1. At this time, the free polymer 7 having hydroxyl groups, which is forming a hydrogen bond with the water molecule 4, also transfers together with the water molecule 4 in the direction of the ejection orifice 1.
[0032] FIG. 1C illustrates a state after the time has still further elapsed. As illustrated in FIG. 1C, the free polymer 7 having hydroxyl groups, which is forming a hydrogen bond with the water molecule 4, seeps out through the ejection orifice 1 and adheres to an orifice face 3 centering on the ejection orifice 1. The free polymer 7 having hydroxyl groups right after having adhered to the orifice face 3 keeps a dissolved state by the bonded water molecule 4. Since the bonded water molecule 4 evaporates with time, however, the free polymer 7 having hydroxyl groups loses its solubility to stick as a deposit on the periphery of the ejection orifice 1.
[0033] The phenomenon that the polymer in the ink seeps out through the ejection orifice of the recording head occurs according to the above-described mechanism. As a result, it is considered that the ejection characteristics are deteriorated by the presence of the deposit.
[0034] Incidentally, whether all the free polymer having hydroxyl groups present in the ink according to the present invention forms the hydrogen bond with the water molecules or nor is unknown. It is however inferred that majority of the polymer having hydroxyl groups is present in the state of forming a hydrogen bond with the water molecule in order to more stably be present in the ink.
[0035] As described above, the polymer to cause seeping out is a polymer having hydroxyl groups that is not adsorbed on the pigment, i.e., a free polymer having hydroxyl groups. As a consequence, it is supposed that the deterioration of the ejection characteristics can be inhibited by reducing the content of the free polymer having hydroxyl groups in the ink.
[0036] Accordingly, it is supposed that the above-described phenomenon can be inhibited by taking the following inventive arrangement within such a range that the ejection characteristics are not deteriorated and the scratch resistance and highlighter resistance are sufficiently achieved. More specifically, it is supposed that the content of the free polymer having hydroxyl groups in the ink is reduced to the utmost, or the hydrophobicity of a monomer constituting a dispersant is made higher, whereby adsorbability between the pigment and the dispersant is enhanced to prevent the generation of the free polymer having hydroxyl groups to the utmost. However, it is very difficult to actually perform such arrangement as described above. Thus, the present inventors have carried out an investigation as to means for arranging the above-described phenomenon by devising the formulation of an ink.
[0037] As a result, it has been found that a polyhydric alcohol contained in an ink as a water-soluble organic solvent can inhibit the phenomenon that the free polymer having hydroxyl groups seeps out through the ejection orifice. The reason why the phenomenon that the free polymer having hydroxyl groups seeps out through the ejection orifice can be inhibited is not clearly known. However, the present inventors speculate as described below.
[0038] FIGS. 2A and. 2B typically illustrate the relation among a free polymer having hydroxyl groups, a water molecule and a polyhydric alcohol in an ink containing a resin dispersion pigment dispersed with a polymer having hydroxyl groups.
[0039] FIG. 2A illustrates a state that a free polymer 7 having hydroxyl groups has formed a hydrogen bond with a water molecule 4 in the case where the ink contains no polyhydric alcohol. The free polymer 7 having hydroxyl groups is composed of a segment 9 having hydroxyl groups and a segment 10 having no hydroxyl group. Incidentally, "segment" is defined as a repeating unit which constitutes the polymer. FIG. 2B illustrates states of the free polymer 7 having hydroxyl groups and a polyhydric alcohol 11 in the case where an ink contains the polyhydric alcohol.
[0040] As apparent from FIG. 2B, the free polymer 7 having hydroxyl groups, whose hydrophobicity is relatively high compared with the water molecule 4, tends to be present in the vicinity of the polyhydric alcohol 11, whose hydrophobicity is relatively high compared with the water molecule 4. As a result, the free polymer 7 having hydroxyl groups form a hydrogen bond selectively with the polyhydric alcohol 11, not with the water molecule 4. The free polymer 7 having hydroxyl groups, which does not form a hydrogen bond with the water molecule 4, does not cause such transfer to the ejection orifice attending on the evaporation of the water molecule as shown in FIGS. IB and 1C. As a result, it is supposed that the phenomenon that the free polymer having hydroxyl groups seeps out through the ejection orifice can be inhibited.
[0041] The present inventors have further carried out an investigation as to various polyhydric alcohols on the basis of the mechanism described above. As a result, it has been found that it is necessary to lower the hydrophilicity of a polyhydric alcohol to some extent in order to develop an interaction caused by efficiently forming a hydrogen bond between the polyhydric alcohol and a free polymer having hydroxyl groups. When the hydrophilicity of the polyhydric alcohol is high, such a polyhydric alcohol tends to reduce its proportion to form a hydrogen bond with the free polymer having hydroxyl groups because it can be completely dissolved in an aqueous medium. On the other hand, when the hydrophilicity of the polyhydric alcohol is low, such a polyhydric alcohol tends to increase its proportion to be present in the vicinity of the free polymer having hydroxyl groups, which is relatively hydrophobic, not the proportion to be dissolved in the aqueous medium. As a result, it is supposed that the hydrogen bond is easily formed between the polyhydric alcohol and the free polymer having hydroxyl groups.
[0042] In the present invention, thus, the value of number of hydroxyl groups/number of carbon atoms in its molecule (hereinafter referred to as "OH/C value") is used as an index for indicating the degree of hydrophilicity of a polyhydric alcohol. A higher OH/C value indicates that the number of hydroxyl groups in the molecule is large, or the number of carbon atoms is small, or the hydrophilicity of the polyhydric alcohol is high. On the other hand, a lower OH/C value indicates that the number of hydroxyl groups in the molecule is small, or the number of carbon atoms is large, or the hydrophilicity of the polyhydric alcohol is low.
[0043] The present inventors have found OH/C values of various polyhydric alcohols. Further, inks containing a free
polymer having hydroxyl groups and only the polyhydric alcohol as a water-soluble organic solvent have been used to confirm the extent of a difference in the effect to inhibit the phenomenon that the free polymer having hydroxyl groups seeps out through the ejection orifice. As a result, it has been confirmed that the effect to inhibit the seeping out phenomenon is attained when the OH/C value of the polyhydric alcohol is 0.50 or less. From the above circumstance, the present inventors have concluded that the polyhydric alcohol contained in an ink is required to have a value of number of hydroxyl groups/number of carbon atoms in its molecule, i.e., an OH/C value, of 0.50 or less. Incidentally, in order to more effectively inhibit the seeping out phenomenon, the OH/C value of the polyhydric alcohol is preferably 0.40 or less, more preferably 0.35 or less. The OH/C value is preferably 0.10 or more.
[0044] <Ink>
[0045] The components constituting an ink according to the present invention will hereinafter be described.
[0046] (Polyhydric Alcohol having a Value of Number of Hydroxyl Groups/Number of Carbon Atoms in its Molecule of 0.50 or less)
[0047] It is essential for the ink according to the present invention to contain a polyhydric alcohol having a value (OH/C value) of number of hydroxyl groups/number of carbon atoms in its molecule of 0.50 or less. In the present invention, the polyhydric alcohol means a compound in which saturated carbon atoms have plural hydroxyl groups.
[0048] Examples of the polyhydric alcohol having an OH/C value of 0.50 or less include the following compounds.
[0049] 1,2-butanediol, 1,3-butanediol, 1,4-butanediol,
2.3- butanediol and the like; 1,2-pentanediol, 1,3-pentanediol, 1,4-pentanediol, 1,5-pentanediol, 2,3-pentanediol,
2.4- pentanediol and the like; 1,2-hexanediol, 1,3-hexanediol, 1,4-hexanediol, 1,5-hexanediol, 1,6-hexanediol, 2,3-hexanediol, 2,4-hexanediol, 2,5-hexanediol, 3,4-hexanediol and the like; 1,2,3-hexanetriol, 1,2,4-hexanetriol, 1,2,5-hexanetriol, 1,2,6-hexanetriol, 1,3,4-hexanetriol, 1,3, 5-hexanetriol, 1,3,6-hexanetriol, 2,3,4-hexanetriol, 2,3,5hexanetriol, 3,4,5-hexanetriol and the like; 1,2-heptanediol, 1,3-heptanediol, 1,4-heptanediol, 1,5-heptanediol, 1,6-heptanediol, 1,7-heptanediol and the like; and 1,2-octanediol, 1,3-octanediol, 1,4-octanediol, 1,5-octanediol, 1,6-octanediol, 1,7-octanediol, 1,8-octanediol and the like.
[0050] Among the polyhydric alcohols having an OH/C value of 0.50 or less, those having a boiling point of 80° C. or more under ordinary pressure are particularly preferably used because they can avoid evaporation even under such an environment that water molecules evaporate from ejection orifices.
[0051] In the present invention, it is preferable to use a polyhydric alcohol that is a poor medium for the free polymer having hydroxyl groups among the polyhydric alcohols having a boiling point of 80° C. or more under ordinary pressure. When an ink contains the poor medium, the free polymer having hydroxyl groups tends to transfer from the ejection orifice to the interior of the nozzle (in the direction of the heater 2 in FIG. 1A), i.e., tends to cause a receding phenomenon. Therefore, even when the content of
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