Ink, and Recording Process Making Use of It

1. Field of the Invention

The present invention relates to an ink, and a recording process making use of it. More particularly, it relates to a water-based ink capable of giving a black image improved in indoor color change resistance, and a recording process, in particular, ink-jet recording process, making use of it.

2. Related Background Art

Water-based inks comprising a water-soluble dye dissolved in a water-based medium have been hitherto used as inks used in fountain pens and felt pens and inks used for ink-jet recording. In these water-based inks, water-soluble organic solvents are commonly added so that pen points or ink ejection nozzles can be prevented from being clogged with ink.

It is required for these conventional inks to give an image with a sufficient density, not to cause any clogging at pen points or nozzles, to have good drying properties on recording mediums, to cause less feathering, to have excellent shelf stability, and, particularly in ink-jet recording systems utilizing heat energy, to have excellent thermal resistance. It is also required for the image formed to have a satisfactory light-fastness and water fastness.

Inks with various hues are also prepared from dyes with various hues. Of these, black inks, which are used in both monochromatic and full-color images, are most important inks. As dyes for these black inks, C.I. Food Black 2 has been mainly used taking account of various performances (see Japanese Laid-Open Patent Application No. 59-93766 and No. 59-93768).

Among the various required performances, what is particularly important is the fastness of the images formed.

In regard to the fastness of images, hitherto mainly questioned is the color fading due to direct sunlight or every kind of illumination light. Such a problem of color-fading has been attempted to be settled by the selection of dyes having superior light-fastness.

Recently, however, a problem of color changes of images has become important in addition to the above color fading. Namely, images formed by conventional inks have not only the problem of color fading but also the problem of color changes. The color changes refer to changes in hues with, however, less changes in density, and what is important in black inks particularly used in a largest quantity is a problem of the browning that black turns brown. In particular, in the instance of full-color images, this browning results in a great lowering of image quality.

This problem of browning also occurs indoors without exposure to direct sunlight. The color change is also accelerated depending on the types of recording mediums on which images are formed, and this problem of browning has been unavoidable in respect of the C.I. Food black 2 that has been hitherto widely used.

In particular, so-called coated papers comprises a substrate such as paper and formed thereon an ink-receiving layer containing a pigment and a binder, for the purpose of improving the color-forming performance of ink and the image quality such as sharpness and resolution. In such an instance, the browning may seriously occur even with use of inks that may cause less problem of color change in the instance of plain papers. This problem has been unsettled by the mere selection of dyes having superior light-fastness.

Accordingly, an object of the present invention is to provide an ink that can satisfy the performances commonly required as mentioned above and also may not cause browning even on the coated papers, and a recording process making use of this ink.

The above object can be achieved by the present invention as described below.

The present invention is an ink, and a recording process making use of the ink, containing at least a dye and a liquid medium, wherein said dyes is a dye represented by the following Formula (I): ##STR2## wherein R.sub.1 and R.sub.2 each represent hydrogen atom, a methyl group, a methoxy group or --SO.sub.3 M, R.sub.3 and R.sub.4 each represent a methyl group, methoxy group or an acetylamino group; X represents an acetyl group, a benzoyl group, --SO.sub.2 C.sub.6 H.sub.5, --SO.sub.2 C.sub.6 H.sub.4 --CH.sub.3, or ##STR3## where R.sub.5 and R.sub.6 each represent a hydrogen atom, or --C.sub.2 H.sub.4 OH; and M represents an alkali metal, an ammonium group, or an organic ammonium group.

FIGS. 1(a) and 1(b) are respectively a longitudinal sectional view and a cross-sectional view of the head part of an ink jet recording device;

FIG. 2 is a perspective view of the appearance of a multiple head which comprises the head shown in FIG. 1;

FIG. 3 is a perspective view of an example of an ink jet recording apparatus;

FIG. 4 is a longitudinal sectional view of an ink cartridge; and

FIG. 5 is a perspective view of an ink jet device.

Use of the dye of the above Formula (I) as the dye for the ink makes it possible to provide a black ink capable of giving an image that may not cause the indoor color changes, i.e., the browning, even when the coated papers are used.

In the recording process of the present invention, use of the above ink makes it possible to provide a black image that may cause less browning on the coated papers.

The present invention will be described below in greater detail by giving preferred embodiments.

Black dyes used in the present invention may all commonly comprise sodium salts of water-soluble groups such as a sulfonic acid group. In the present invention, however, they are not limited to the sodium salts, and the same effect can be obtained also when the counter ion thereof is potassium, lithium, ammonia, organic amine, or the like. Thus, the dyes containing any of these other counter ions are also included in the present invention.

Examples of the dye represented by the above Formula (I) include the following dyes, but are by no means limited to these. ##STR4##

In the above dyes, particularly preferable dyes are the dyes wherein R.sub.1 and R.sub.2 each are selected from the group consisting of a methyl group, a methoxy group and --SO.sub.3 M respectively, R.sub.3 and R.sub.4 each are selected from the group consisting of a methyl group, acetyl group and amino group respectively, X is selected from the group consisting of benzoyl group, --SO.sub.2 C.sub.6 H.sub.5 and ##STR5## and R.sub.5 and R.sub.6 each are selected from the group consisting of a hydrogen atom and --C.sub.2 H.sub.4 OH.

The dyes as exemplified in the above can be prepared following the syntheses of azo dyes known in the art. An example of the synthesis of the above No. 2 dye will be described below.

In 100 ml of water, 0.1 mol of o-anilinesulfonic acid is diazotized according to a conventional method, and then coupled with 0.1 mol of 1-naphthylamine-7-sulfonic acid. The product is further diazotized using sodium nitrite. In a mixed solution of 100 ml of water and 0.2 mol of hydrochloric acid, 0.1 mol of 5-acetylamino-o-anisidine is dissolved, and the above diazotized solution is added thereto to carry out reaction at a pH of 5 to 6 and a temperature of 5.degree. to 10.degree. C. Thereafter sodium chloride is added to effect salting out, followed by filtration to give a paste. The paste is diazotized in water and, at pH 8 to 9, reacted with 0.1 mol of N-p-toluenesulfonyl H-acid prepared following the method as disclosed in Yutaka Hosoda, SENRYO KAGAKU (Dye Chemistry), Gihodo, p. 536. Next, lithium chloride is added to effect salting out, and a precipitate of the dye is filtered. This operation of salting-out is repeated several times to remove impurities and at the same time convert the sulfonic acid group of the dye to a lithium salt type, followed by desalting purification by the use of an ultrafiltration machine (manufactured by Saltrius Co.). The above No. 2 dye is thus obtained.

There are no particular limitations on the amount of the dye to be used in the ink of the present invention. In general, however, it may be an amount that holds from 0.1 to 15% by weight, preferably from 0.3 to 10% by weight, and more preferably from 0.5 to 6% by weight, based on the total weight of the ink.

The aqueous medium preferably used in the ink of the present invention is water, or a mixed solvent of water with a water-soluble organic solvent. Particularly preferably used is the mixed solvent of water with a water-soluble organic solvent, containing as the water-soluble organic solvent a polyhydric alcohol having the effect of preventing the ink from drying. As the water, it is preferred not to use commonly available water containing various ions, but to use deionized water.

The water-soluble organic solvent used by mixture with the water includes, for example, alkyl alcohols having 1 to 5 carbon atoms, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, isobutyl alcohol, and n-pentanol; amides such as dimethylformamide and dimethylacetamide; ketones or ketoalcohols such as acetone and diacetone alcohol; ethers such as tetrahydrofuran and dioxane; polyalkylene glycols such as polyethylene glycol and polypropylene glycol; alkylene glycols comprising an alkylene group having 2 to 6 carbon atoms, such as ethylene glycol, propylene glycol, butylene glycol, triethylene glycol, 1,2,6-hexanetriol, thiodiglycol, hexylene glycol, and diethylene glycol; glycerol; lower alkyl ethers of polyhydric alcohols, such as ethylene glycol monomethyl or monoethyl ether, diethylene glycol monomethyl or monoethyl ether, and triethylene glycol monomethyl or monoethyl ether; lower dialkyl ethers of polyhydric alcohols, such as triethylene glycol dimethyl or diethyl ether and tetraethylene glycol dimethyl or diethyl ether; sulfolane, N-methyl-2-pyrrolidone, and 1,3-dimethyl-2-imidazolidinone.

Suitable solvents are selected from the organic solvents as described above and put into use. Particularly important from the view point of preventing the clogging with ink is glycerol or a polyethylene oxide with a degree of polymerization of 2 to 6. Taking account of image density and ejection stability, preferred are nitrogen-containing cyclic compounds such as N-methyl-2-pyrrolidone or 1.3-dimethyl-2-imidazolidinone or monoether compounds of polyalkylene oxides such as diethylene glycol monomethyl ether and triethylene glycol monomethyl ether. Further taking account of frequency response, it is preferred to use lower alcohols such as ethyl alcohol, isopropyl alcohol or surface active agents. Hence, the solvent composition preferred in the present invention is the composition in which all the components as described above are contained in addition to the water.

The above water-soluble organic solvent may be contained in the ink in an amount of generally from 2 to 80% by weight, preferably from 3 to 70% by weight, and more preferably from 4 to 60% by weight, based on the total weight of the ink.

The water to be used may be in a proportion such that it holds not less than 35% by weight, and preferably not less than 45% by weight, of the whole ink. An excessively small amount of water may result in a large quantity of a low-volatile organic solvent remaining in the image formed, undesirably causing the problems of migration of dyes, feathering of images, and so forth.

In addition to the above components, the ink of the present invention may also optionally contain pH adjustors, viscosity modifiers, surface tension modifiers, and so forth. The pH adjustors used in the above ink include, for example, all sorts of organic amines such as diethanolamine and triethanolamine, inorganic alkali agents such as oxides of alkali metals as exemplified by sodium hydroxide, lithium hydroxide and potassium hydroxide, organic acid salts such as lithium acetate, organic acids, and mineral acids.

The ink of the present invention, as described above, may preferably have physical properties of a viscosity at 25.degree. C., of from 1 to 20 cP, and preferably from 1 to 15 cP; a surface tension of not less than 30 dyne/cm, and preferably not less than 40 dyne/cm; and a pH of approximately from 4 to 10.

The recording process of the present invention is a recording process characterized by using the ink described above, and there are no particular limitations on the recording system and recording medium. In particular, however, particularly effective are methods in which an ink-jet system is used as the recording system and a coated paper is used as the recording medium.

The ink-jet system may include any conventionally known systems, without any particular limitations. In the present invention, however, the system as disclosed in Japanese Laid-Open Patent Application No. 54-59936 is particularly useful, which is a system in which heat energy is acted on an ink to cause therein an abrupt volume change and the ink is ejected from a nozzle by the force of action attributable to this change in state.

As the recording medium used in the present invention, any recording mediums can be used such as commonly available plain papers such as wood free papers, coated papers, and plastic films for OHP or the like. A remarkable effect can be exhibited particularly when the coated papers are used.

The coated papers refer to those which are comprised of wood free paper used as a substrate, and provided on the surface thereof an ink-receiving layer comprising a pigment and a binder, aiming at improvements in the color-forming properties attributable to ink, sharpness, and dot shapes.

In the case of these coated papers, those which employ as the pigment a fine pigment such as synthetic silica having a BET specific surface area of from 35 to 650 m.sup.2 /g can provide images having excellent color-forming properties and sharpness. When conventional inks are used, however, the image formed particularly with black ink may seriously cause the problem of browning with lapse of time though its theoretical reasons are unknown, and great problems are also caused in not only black monochromatic images but also full-color images. Similar problems are also caused in recording mediums comprised of, like these coated papers, a paper substrate and provided thereon a thin layer comprising a pigment and a binder, where fibers of the paper that constitutes the substrate are present in this layer in a mixed state.

Use of the ink of the present invention does not cause the problems of browning as discussed above even when monochromatic images or full-color images are formed on the coated papers as mentioned above. Thus, the process according to the present invention can provide recorded images that may not bring about any indoor color change for a long period of time, when using not only the coated papers employing the pigment having a BET specific surface area of from 35 to 650 m2/g, but also coated papers employing a pigment having a BET specific surface area smaller than that, and also plain papers and any other recording mediums.

The recording processes according to the ink-jet system and the various recording mediums are known in the art, or proposed in variety by the present applicants and others. These recording processes and the recording mediums can all be used in the present invention as they are.

The ink of the present invention is preferably used in an ink jet recording method in which ink droplets are discharged by employing thermal energy. However, the recording solution can also be used for general writing utensils.

An example of the recording apparatus which is preferable for recording by using the ink of the present invention is an apparatus in which ink droplets are produced by applying heat energy to the ink in the chamber of a recording head in correspondence with a recording signal.

FIGS. 1(a), 1(b) and 2 show examples of the structure of a head, which is a principal part of an ink jet recording apparatus.

In the drawings, a head 13 is formed by bonding a glass, ceramic or plastic plate, which has a groove 14 for allowing ink to pass therethrough, and a heating head 15 used for heat-sensitive recording. Although a thin film head is shown in the drawings, the head is not limited to such an embodiment. The heating head 15 comprises a protective film 16 made of silicon oxide or the like, aluminum electrodes 17-1, 17-2, a heating resistor layer 18 made of nichrome or the like, a heat-accumulating layer 19 and a substrate 20 made of aluminum or the like and having good heat radiation properties.

Ink 21 reaches a discharging orifice (micropore) 22 and forms a meniscus 23 at pressure P.

When an electrical signal is applied to the electrodes 17-1, 17-2, a region off the heating head 15, which is denoted by n, rapidly generates heat so as to generate air bubbles in the ink 21 which contacts with the region. The meniscus 23 is projected by the pressure generated, and the ink 21 is discharged as a jet of ink droplets 24 from the orifice 22. The droplets 24 are propelled toward a recording material 25. FIG. 2 shows a multiple head comprising a plurality of the heads shown in FIG. 1(a) which are arranged in parallel. The multi-head is formed by bonding a glass plate 27 having a plurality of grooves 26 and a heating head 28, which is the same as that shown in FIG. 1(a).

FIG. 1(a) is a sectional view taken along the ink flow channel of the ink, and FIG. 1(b) is a sectional view taken along the line A-B in FIG. 1(a).

FIG. 3 shows an example of an ink jet recording apparatus in which the head shown in FIG. 1 is incorporated.

In FIG. 3, reference numeral 61 denotes a blade serving as a wiping member in the form of a cantilever in which one end is a fixed end held by a blade holding member. The blade 61 is disposed at a position adjacent to a region of recording by a recording head. In this example, the blade 61 is held in a position in which it projects in the path of the movement of the recording head. Reference numeral 62 denotes a cap which is disposed at a home position adjacent to the blade 61 and which is moved in the direction vertical to the moving direction of the recording head so as to contact with the orifice surface for the purpose of capping. Reference numeral 63 denotes an ink absorber which is disposed at a position adjacent to the blade 61 and which is held in a position in which it projects in the path of the movement of the recording head in the same way as the blade 61. The blade 61, the cap 62 and the absorber 63 form a discharging recovery part 64. Moisture and dust on the ink orifice surface are removed by the blade 61 and the absorber 63.

Reference numeral 65 denotes the ink jet device which has a means for generating discharging energy so as to record an image by discharging the ink to the recording material opposite to the orifice surface having orifices. Reference numeral 66 denotes a carriage for moving the ink jet device 65 which is loaded thereon. The carriage 66 is slidably engaged with a guide shaft 67 and is partially connected (not shown) to a belt 69 which is driven by a motor 68. This permits the carriage 66 to move along the guide shaft 67 and move in the region of recording by the ink jet device 65 and the region adjacent thereto.

Reference numeral 51 denotes a sheet feeding part, and reference numeral 52 denotes a sheet feeding roller which is driven by a motor (not shown). This arrangement allows the recording paper to be fed to a position opposite to the orifice surface of the recording head and to be delivered to a take-off part having a take-off roller 53 during the progress of recording.

In the aforementioned arrangement, when the ink jet device 65 is returned to the home position at the end of recording, the cap 62 is retracted from the path of the movement of the ink jet device 65, while the blade 61 is projected in the path of the movement. As a result, the orifice surface of the ink jet device 65 is wiped by the blade 61. When the cap 62 contacts with the orifice surface of the recording head 65 so as to cap it, the cap 62 is moved so as to project in the path of the movement of the ink jet device 65.

When the ink jet device 65 is moved from the home position to the recording start position, the cap 62 and the blade 61 are at the same positions as the above-described positions in wiping. As a result, the orifice surface of the ink jet device 65 is wiped even during the movement of the ink jet device 65.

The recording head 65 is moved to the home position adjacent to the recording region not only at the end of recording and during the recovery of discharging (the operation of sucking an ink from an orifice in order to recover the normal discharge of an ink from an orifice), but also at predetermined intervals when it is moved in the recording region for the purpose of recording. This movement causes the above-described wiping.

FIG. 4 is a drawing which shows an example of an ink cartridge 45 for containing the ink to be supplied to the head through an ink supply tube. In the drawing, reference numeral 40 denotes an ink bag for containing the ink to be supplied which has a rubber stopper 42 at its one end. When a needle (not shown) is inserted into the stopper 42, the ink contained in the ink bag 40 can be supplied to the ink jet device 65. Reference numeral 44 denotes an ink absorber for absorbing waste ink. As the ink bag in the present invention, there may preferably be used ones of which the surface coming into contact with the ink is formed from polyolefins, in particular polyethylene.

The ink jet recording apparatus used in the present invention is not limited to an apparatus in which a device and an ink cartridge are separately disposed, as described above. The ink jet device shown in FIG. 5 in which a device and an ink cartridge are integrated can be preferably used in the present invention.

In FIG. 5, reference numeral 70 denotes an ink jet device which contains an ink storing member impregnated with ink. The ink in the ink storing member is discharged as ink droplets from a head part 71 having a plurality of orifices. Further, as the ink storing member, there may be used an ink absorber on an ink bag. The head is the same as those referred to in FIGS. 1 and 2.

Reference numeral 72 denotes a communicating hole for allowing the inside of the device 70 to communicate with the atmosphere. As a material for the ink absorber in the present invention, there may preferably be used polyurethanes.

The ink jet device 70 is used in place of the ink jet device 65 shown in FIG. 3 and is detachably provided on the carriage 66.

The present invention will be described below in detail by giving Examples and Comparative Examples. In the following, "%" is by weight unless particularly mentioned.

Components as shown below were mixed, thoroughly stirred and dissolved, followed by pressure filtration using Fluoropore Filter (trademark; available from Sumitomo Electric Industries, Ltd.) with a pore size of 0.45 .mu.m, to prepare inks of the present invention.

The inks of Examples 1 to 6 were each set on an ink-jet printer BJ-80A (a manufacture of Canon Inc.; nozzle size: 50.times.40 .mu.m; nozzle number: 24) that utilizes a heating element as an ink ejection energy source, and printing was carried out on the following recording mediums A to C, under which evaluation was made on the clogging observed when the printing was stopped for a while and then again started, the performance of recovery from the clogging, observed when the printing was stopped for a long term and then again started, and the color change resistance.

Recording medium A: Ink-jet coated paper, NM (trade name; available from Mitsubishi Paper Mills, Ltd.)

Recording medium B: Ink-jet coated paper, FC-3 (trade name; available from Jujo Paper Co., Ltd.

Recording medium C: Copy paper, Canon PAPER DRY (trade name; available from Canon Sales Inc.)

(i) Clogging observed when the printing was stopped for a while and then again started:

Judgement was made on whether defective prints such as blurs and chips of characters are seen or not, when the printing was stopped after alphanumeric characters were continuously printed on the recording medium C for 10 minutes using the printer filled with a given ink, and then the alphanumeric characters were again printed after the ink was left to stand for 10 minutes without capping on the nozzle or the like (which was left to stand at 20.degree..+-.5.degree. C. under 50.+-.10% RH). As a result, no defective prints were seen.

(ii) Performance of recovery from clogging observed when the printing was stopped for a long term and then again started:

Judgement was made on how many times the operation for recovery had to be repeated to enable normal printing free from blurs or chips of characters, when the printing was stopped after alphanumeric characters were continuously printed on the recording medium C for 10 minutes using the printer filled with a given ink, and the operation for recovery of the clogging of nozzles were carried out after the ink was left to stand for 7 days without capping on the nozzle or the like (which was left to stand at 60.degree. C. under 10.+-.5% RH). As a result, normal printing became possible after the recovery operation was made once to five times.

(iii) Color change resistance:

Black solid patterns of 10 mm.times.30 mm were each printed on the recording mediums A, B and C. Thereafter, as a color change promotion means, the print was left to stand for 30 minutes in a light-intercepted chamber in which the density of ozone was always kept within the range of 0.1.+-.0.05% by volume, and the color differences .DELTA.E*ab after and before the test were measured (according to JIS Z8730). As a result, the .DELTA.E*ab was found to be not more than 5 in all instances.

The above Examples were repeated to prepare 6 kinds of inks, except that the dyes used in the above Ink Formulation Examples 1 to 6 were replaced with C.I. Food Black 2, C.I. Direct Black 62, C.I. Direct Black 118, C.I. Acid Black 24, C.I. Acid Black 26, and C.I. Acid Black 60, respectively. Then the above Use Example was repeated using the recording apparatus to give black solid prints on the recording mediums A and B. Using the resulting prints as test pieces, similar tests were carried out using the above ozone test chamber. As a result, the .DELTA.E*ab was found to be not less than 15 in all instances.

As described in the above, the present invention has made it possible to form an image not only having superior performances such as clogging resistance of inks, as generally required, but also having superior color change resistance.

Further, with respect to each ink in Examples 1 to 6, the ink absorber of the ink-jet device as shown in FIG. 5 was impregnated with the ink. Then the ink jet apparatus as shown in FIG. 3 was allowed to carry the ink-jet device. By use of the ink jet apparatus, recording was performed. As a result, good recording which was excellent in a discharge property could be realized.