Ink-jet recording and image formation methods and recording medium

Abstract: Provided is an ink-jet recording method employing a pigment-based ink, in which ink droplets are ejected from an orifice in a recording head in accordance with recording signals to conduct recording on a recording medium comprising a base material and a coating layer disposed thereon, wherein a surface pH of the recording medium is within a stable pH range of the pigment-based ink.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an ink-jet recording method employing a pigment-based ink, a method for the formation of an image and a recording medium preferably used for an ink-jet recording system. Related Background Art An ink-jet recording method is a method which performs recording by generating and ejecting ink droplets to apply a part or all of them to a recording medium such as paper or plastic films coated with an ink reception layer using various ink ejection methods, for example, electrostatic suction methods, methods using piezoelectric elements to apply mechanical vibrations and displacement to inks, or methods to heat a ink until it foams to generate a pressure. These ink-jet recording methods are gathering public attention due to their reduced noises and their ability to print in multiple colors at a high speed. Inks for the ink-jet recording method mainly comprise water to provide safety and good recording characteristics and contain polyhydric alcohol to prevent blocking of nozzles and improve an ejection stability. Such inks contain various water-soluble dyes dissolved in water or a mixture of water and an organic solvent. Due to its inherently low light fastness, the use of water-soluble dyes often results in a low light fastness of recorded images. In addition, since the ink is aqueous, recorded images often have a low water fastness. That is, if rain, sweat, or drinking water contacts recorded images, they may blot or be lost. On the other hand, those writing materials such as ball-point pens which use a dye have a similar problem, and various aqueous pigment-based inks for stationery have been proposed to solve the light and water fastness problems. Japanese Patent Applications Laid-Open No. 58-80368, No. 61-200182, No. 61-247774, No. 61-272278, No. 62-568, No. 62-101671, No. 62-101672, No. 1-249869, and No. 1-301760 attempt to provide a dispersion stability, to prevent an agglomeration of an ink at the tip of a pen, or to prevent a ball in a ball-point pen from being worn, in order to put aqueous pigment-based inks to practical use. Ball-point pens and markers employing an aqueous pigment-based ink have recently been on the market. In addition, as ink-jet inks making use of aqueous pigment-based inks, pigment-based inks containing a specific water-soluble solvent and a specific polymer dispersant have been proposed in Japanese Patent Application Laid-Open No. 56-147859 and No. 56-147860. As listed above, a large number of proposals have been made for ink-jet pigment-based inks. Recording media conventionally used for an ink-jet recording system consist of recording paper comprising base paper and a coating layer disposed thereon and containing fumed silica and a water-soluble binder such as polyvinyl alcohol, as described in Japanese Patent Publication No. 3-26665, glossy paper comprising cast-coated paper and a coating layer formed thereon and containing polyvinyl alcohol with a saponification degree of from 50 to 90% by mole and a crosslinking agent, as described in Japanese Patent Publication No. 3-25352, and recording paper for an overhead projector (OHP) comprising a polyester film and a hydrophilic coating layer of a water-soluble polyvinyl alcohol with a saponification degree of from 70 to 90% by mole, as described in Japanese Patent Application Laid-Open No. 60-220750. In recent years, more sophisticated and diversified characteristics of an ink-jet recording medium have been demanded due to the improved performance of ink-jet recording apparatus including recording at a higher speed and in multiple colors. That is, a recording medium should have characteristics of: (1) capable of performing an appropriate color mixing without a deviation from inherent color tones; (2) high in an ink absorption (a large absorption capacity and a fast absorption time); (3) high in an optical density of dots, and not gradated at a periphery of a dot; (4) capable of forming a dot near a true circle, and smooth at a periphery of a dot; (5) less change in their characteristics due to changes in temperature or humidity, and no curling; (6) no occurrence of blocking; (7) keeping stable and not deteriorated images over a long storage period (in particular, under hot and humid conditions); and (8) keeping itself stable and not deteriorated over a long storage period (in particular, under hot and humid conditions). In recording sheets for an OHP and color filters, the recording medium is required also to be sufficiently transparent. As described above, a large number of ink-jet recording methods in which pigment-based inks are used for an ink-jet system in order to improve water and light fastnesses of images on a recording medium have recently been reported. The pigment-based ink, however, is disadvantageous in that it rapidly starts to flocculate when its pH reaches a certain value. This may cause an advantage in that a color density of resultant images is increased when the images are printed on an opaque substrate such as paper to observe light reflected therefrom. However, depending on a pH of the recording medium, there has been a problem that the pigments may too rapidly be started to agglomerate, resulting in deviations of inherent color tones of the pigments, to disturb an appropriate color mixing. This significantly may raise a serious problem in recording when transparent substrates such as OHP films are used as the recording medium. In particular, there may be raised not only a problem, as to recording sheets for an OHP and color films that must be transparent, that, when a pigment may be strongly agglomerated, an appropriate color mixing cannot be performed due to deviations of inherent color tones of the pigments, but also a problem that a surface of the recording medium is blurred and then a printed region does not transmit light. For example, Japanese Patent Application Laid-open No. 56-89594 discloses a method for setting a difference between a pH of an ink and a surface pH of a recording sheet to be not more than 2 to suppress a deviation of color tones. With this method, however, there has not been solved a problem that a deviation from inherent color tones of pigment may be raised and then an appropriate color mixing cannot be performed, in case of using an ink for an ink-jet system comprising a pigment that brings a water fastness. As described above, a degradation of color tones and an image quality is a serious problem, as a higher recording speed, a higher image density, a higher water fastness, and a capability of using multiple colors are being required. SUMMARY OF THE INVENTION It is an objective of the present invention to provide an ink-jet recording method, a method for the formation of images and a recording medium that satisfies all of the above characteristics simultaneously with a well balance, that can mix colors appropriately without a deviation of the inherent color tones of pigments when a pigment that brings water and light fastnesses is used as described above, and that is superior in a light transparency at portions printed by an ink-jet recording system, even when a transparent base material such as an OHP film is used. The above object can be achieved by the present invention. According to the present invention, there is provided an ink-jet recording method employing a pigment-based ink, in which ink droplets are ejected from an orifice in a recording head in accordance with recording signals to conduct recording on a recording medium comprising a base material and a coating layer disposed thereon, wherein a surface pH of the recording medium is within a stable pH range of the pigment-based ink. According to the present invention, there is also provided a method for the formation of images by applying a pigment-based ink to a recording medium provided with a coating layer by means of an ink-jet recording system, wherein a surface pH of the recording medium is within a stable pH range of the pigment-based ink. According to the present invention, there is further provided a recording medium used for an ink-jet recording system employing a pigment-based ink, comprising a coating layer provided on its surface, the surface pH of which is adjusted so as to be within a stable pH range of the pigment-based ink. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a longitudinal cross section of a head section of an ink-jet recording apparatus. Fig. 2 is a transverse cross section of the head section of the ink-jet recording apparatus. Fig. 3 is a perspective view of a multi-head. Fig. 4 is a perspective view showing an example of the ink-jet recording apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In an attempt to develop ink-jet recording methods suitable for an ink-jet recording system and capable of providing high water and light fastnesses, as well as inks and recording media such as recording paper and transparent sheets for overhead projectors, the inventors have found that an ink-jet recording method and a recording medium that can mix the colors appropriately without a deviation of the original color tones of the pigments and is superior in a light transparency at portions printed by an ink-jet recording system, even when a transparent base material such as an OHP film is used, and then the inventors have thus completed this invention. The stable pH region of a pigment-based ink, as used herein, refers to a pH range in which a pigment in an ink does not agglomerate or precipitate. Next, the present invention will be described in detail by preferred embodiments. Pigment-based inks for an ink-jet recording system used in the present invention are not particularly limited, but the outline is as follows. In the present invention, an amount of pigment contained in a pigment-based ink is from 1 to 20% by weight, preferably from 2 to 12% by weight. Any pigments can be used in the present invention. The carbon black used in a black ink is prepared by a furnace method or a channel method, and has a primary particle size of from 15 to 40 mµ, a specific surface area of from 50 to 300 m 2 /g according to the BET method, a DBP oil absorption of from 40 to 150 ml/100 g, a volatile matter of from 0.5 to 10%, and a pH value of from 2 to 9. Commercially available carbon blacks of the above properties include No. 2300, No. 900, MCF 88, No. 33, No. 40, No. 45, NO. 52, MA 7, MA 8, No. 2200B (all trade-name, produced by Mitsubishi Kasei Co.), RAVEN 1255 (trade-name, produced by Columbia Co.), REGAL 400R, REGAL 330R, REGAL 660R, MOGUL L (all trade-name, produced by Cabot Co.), Color Black FW1, Color Black FW18, Color Black S170, Color Black S150, Printex 35, and Printex U (trade-name, produced by Degussa Co.). The pigments used in a yellow ink include C.I. Pigment Yellow 1, C.I. Pigment Yellow 2, C.I. Pigment Yellow 3, C.I. Pigment Yellow 13, C.I. Pigment Yellow 16, and C.I. Pigment Yellow 83. The pigments used as a magenta ink include C.I. Pigment Red 5, C.I. Pigment Red 7, C.I. Pigment Red 12, C.I. Pigment Red 48 (Ca), C.I. Pigment Red 48 (Mn), C.I. Pigment Red 57 (Ca), C.I. Pigment Red 112, and C.I. Pigment Red 122. The pigments used as a cyan ink include C.I. Pigment Blue 1, C.I. Pigment Blue 2, C.I. Pigment Blue 3, C.I. Pigment Blue 15:3, C.I. Pigment Blue 16, C.I. Pigment Blue 22, C.I. Pigment Blue 4, C.I. Vat Blue 4, and C.I. Vat Blue 6. The present invention, however, is not limited to the above pigments, and those pigments which are newly manufactured for the present invention can also be used. Any dispersant for the pigments can be used to the pigment-based inks according to the present invention so far as it is a water-soluble resin, but preferably has a weight average molecular weight of from 1,000 to 30,000, more preferably, from 3,000 to 15,000. Specifically, the dispersants include a block copolymer obtained from two or more types of monomers (at least one of them is a hydrophilic monomer) selected from the group consisting of styrene and derivatives thereof, vinylnaphthalenes and derivatives thereof, aliphatic alcohol esters of α,β-ethylenically unsaturated carboxylic acid, acrylic acid and derivatives thereof, maleic acid and derivatives thereof, itaconic acid and derivatives thereof, and fumaric acid and derivatives thereof; or a random copolymer, a graft copolymer; or salts thereof. These resins are alkali-soluble resins soluble in a solution, in which a base is dissolved. Furthermore, they may be homopolymers obtained from a hydrophilic monomer and their salts. The dispersants may also be water-soluble resins such as polyvinyl alcohol, carboxymethyl cellulose, condensates of a naphthalene sulfonate with formaldehyde. This dispersant may also be a cationic dispersant such as an acrylic copolymer containing monomer units formed by quaternizing N,N-dimethylaminoethyl methacrylate, N,N-dimethylaminoethyl acrylate, N,N-dimethyl acrylamide, N,N-dimethyl methacrylamide, N,N-dimethylaminopropyl acrylamide, N,N-dimethyl aminopropyl methacrylamide or the like with methyl chloride, dimethyl sulfate, benzyl chloride, or epichlorohydrin. Alkali-soluble resins, however, are rather advantageous in that a viscosity of a dispersant can be made lower and a dispersion can be performed easily. Furthermore, resins that start to agglomerate at a pH of 6 or less are particularly preferable for improving a recording density. An amount of the water-soluble resin contained in the ink is preferably from 0.1 to 5% by weight based on the total amount of the ink. Furthermore, the pigment-based ink used in the present invention preferably has its entirety regulated so as to be neutral or alkaline in order to improve the solubility of the water-soluble resin and to enable the ink to be stored for a longer period of time. In this case, however, since the ink may cause corrosion at various members in the ink-jet recording apparatus, it desirably has a pH value of from 7 to 10. In addition, as a pH regulator there may be exemplified various organic amines such as diethanol amine, triethanol amine and the like; inorganic alkali agents such as hydroxides of alkali metal such as sodium hydroxide, lithium hydroxide, potassium hydroxide and the like; organic acids; and mineral acids. As described above, the pigments and water-soluble resins are dispersed or dissolved in an aqueous medium. A preferable aqueous medium for the pigment-based ink used in the present invention is a mixed solvent of water and a water-soluble organic solvent, and it is preferably to use an ion exchanged water (deionized water) instead of a common water containing various ions. In addition, the water-soluble organic solvents used with water include alkyl alcohols having 1 to 4 carbon atoms such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol and the like; amides such as dimethyl formamide and dimethyl acetamide; ketone 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 whose alkylene moiety has 2 to 6 carbon atoms such as ethylene glycol, propylene glycol, butylene glycol, triethylene glycol, hexylene glycol and diethylene glycol; 1,2,6-hexanetriol; thiodiglycol; glycerol; lower alkyl ethers of polyhydric alcohol such as ethylene glycol monomethyl (or ethyl) ether, diethylene glycol methyl (or ethyl) ether, and triethylene glycol monomethyl (or ethyl) ether; N-methyl-2-pyrrolidone, 2-pyrrolidone, and 1,3-dimethyl-2-imidazolidinone. Among the above water-soluble organic solvents, polyhydric alcohols such as diethylene glycol and a lower alkyl ethers of polyhydric alcohol such as triethylene glycol monomethyl (or ethyl) ether are preferable. Furthermore, it is effective to add ethanol or isopropyl alcohol not less than 1% to the ink for obtaining an ejection stability. This may be considered because bubbling of a recording liquid can be performed more stably on a thin film resistor by the addition of these solvents. An amount of the water-soluble organic solvent contained in the pigment-based ink used in the present invention is within the range of from 3 to 50% by weight, preferably from 3 to 40% by weight based on the total weight of the recording liquid. An amount of water used is from 10 to 90% by weight, preferably from 30 to 80% by weight based on the total weight of the recording liquid. In addition to the above components, a surfactant, an antifoaming agent, or a preservative as well as a commercially available water-soluble dye can also be added to the pigment-based ink used in the present invention to provide a recording liquid having desired physical properties as required. The surfactant is not limited unless it affects the storage stability of the ink, and includes anionic surfactants such as fatty acid salts, higher alcohol sulfonate salts, liquid fatty oil sulfonate salts, and alkyl allyl sulfonate salts, and non-ionic surfactants such as polyoxy ethylene alkyl esters, polyoxy ethylene sorbitan alkyl esters, acetylene alcohol, and acetylene glycol. One or two or more of them can be selected as needed appropriately. An amount of the surfactant used in the present invention depends on the dispersant, but is preferably from 0.01 to 5% by weight based on the total amount of the ink. As to a method for the preparation of a pigment-based ink used in the present invention, a pigment is added first to a solution containing at least a disperse resin and water. The mixture is agitated, then dispersed using a dispersing means described below, and then centrifuged as needed to obtain a desired dispersion. The compounds used in the present invention and the components mentioned above are then added to the dispersion, and agitated to obtain a recording liquid. When an alkali-soluble resin is used, a base must be added to dissolve the resin. The solution containing the pigment can be effectively pre-mixed for thirty minutes or longer before a dispersing treatment. This pre-mixing operation improves a wettability of a surface of pigment in order to facilitate adsorption thereto. The base added to the dispersion in a case of the using an alkali-soluble resin is preferably an organic amine such as monoethanol amine, diethanol amine, triethanol amine, and aminomethyl propanol, or ammonia, or an inorganic base such as potassium hydroxide or sodium hydroxide. Dispersing devices for preparing a pigment-based ink used in the present invention may be any dispersing device generally used, and include a ball mill, a roll mill, or a sand mill. Among them, a high speed sand mill is preferable, and preferable sand mills include Super Mill, Sand Grinder, Beads Mill, Agitator Mill, Glenn Mill, Dyno Mill, Pearl Mill, and Cobol Mill (all of them are trade names). In the present invention, as to obtaining a pigment having a desired particle size distribution, it can be used certain procedures such as (1) to make a size of grinding media smaller in the dispersing device, (2) to make a filling rate of the grinding media larger, (3) to make a processing time longer, (4) to make a ejection speed slower, and (5) to classify the product by a filter or a centrifugal separator after grinding, and their combination may be used. Components used for the coating layer (the ink receiving layer) in the recording medium used in the present invention are not limited as long as it can produce the effects according to the present invention, that is, it can receive an aqueous ink, shows solubility to or affinity for the aqueous ink, and can maintain the surface pH of the recording medium within the stable pH region of the pigment-based ink in which the pigment in the ink does not precipitate when coated on the base material. Thus, a single material meeting these conditions may be used, or two or more types of materials may be mixed without causing a problem even if each of them does not meet the conditions, as long as they can be regulated so as to do so. Materials for the ink receiving layer used in the present invention include a synthetic resin such as polyvinyl alcohol, polyvinyl acetal, polyurethane, carboxyethyl cellulose, polyester, polyacrylic acid (ester), hydroxyethyl cellulose, melamine resin, or modifications thereof, or a natural resin such as albumin, gelatin, casein, starch, cationic starch, gum arabic, or sodium alginate, but is not limited to these resins. A plurality of them can also be used simultaneously. Furthermore, in a case of not satisfying conditions that the surface pH of the recording medium with the coating liquid coated thereon is maintained within the stable pH region of the pigment-based ink, i.e., the pigment in the ink does not precipitate, various cationic active agents and anionic active agents, various cationic or anionic polymers or oligomers, and various pH regulators can be mixed. These may be general and publicly known compounds. The anionic compounds are not particularly limited as long as they have an anionic part in its molecules. The anionic surfactants may include, for example, alkylbenzene sulfonic acid salts, alkyl sulfonate salts, alkylnaphthalene sulfonic acid salts, alkyl phosphates, alkyl sulfosuccinic acid salts, condensation products of naphthalene sulfonic acid with formalin, and polyoxyethylenealkyl phosphates. The surfactants may also be ampholytic surfactants of alkylbetaine-, imidazolimiumbetaine-, and alanine- series that include an anionic part. The anionic polymers and oligomers include those having in its molecules a sulfonic group, a carboxylic group, a sulfonate group, a phosphate group, a phenolic hydroxyl group, an alcoholic hydroxyl group. For example, it is preferably carboxylic radical terminated polyester obtained by reacting a polycarboxylic acid with a polyhydric alcohol, acidic cellulose derivatives modified with various polyhcarboxylic acids, a homopolymer of a vinyl ether ester monomer with a polycarboxylic acid or a copolymer of such a monomer with other general monomer, a homopolymer of (meth)acrylic acid or a copolymer of such a monomer with other general monomer, a homopolymer of α,β-unsaturated vinyl monomer with maleic anhydride or itaconic acid and a copolymer of such monomer with other general monomer, a sulfonic acid-modified polymer made from a polyvinyl alcohol or a vinyl alcohol copolymer using a sulfonic acid compound, a compound with a hydroxyl group such as ethyl cellulose, benzyl cellulose, hydroxyethyl cellulose, or hydroxypropyl cellulose, or any other compounds with a sulfonic group, a carboxylic group, a sulfonate group, a phosphate group, a phenolic hydroxyl group, or an alcoholic hydroxy group. The cationic compounds are not particularly limited as long as they include a cationic part in its molecules. For example, the cationic surfactants include quaternary ammonium salt type cationic surfactants such as monoalkyl ammonium chloride, dialkyl ammonium chloride, tetramethyl ammonium chloride, trimethyl phenylammonium chloride, and ethylene oxide-added ammonium chloride, or amine salt type cationic surfactants. The surfactant may also be ampholytic one of alkylbetaine-, imidazolimiumbetaine-, and alanine- series that include an anionic part. The cationic polymers or oligomers include cation-modified polyacrylamide or a copolymer of acrylamide with a cationic monomer, polyallyl amine, polyamine sulfone, polyvinyl amine, polyethylene imine, polyamide-epichlorohydrin resin, and polyvinyl pyridinium halide and the like. It may also be a homopolymer of a vinyl pyrrolidone series monomer or a copolymer of such a monomer with other general monomers, a homopolymer of a vinyl oxazolidone series monomer or a copolymer of such a monomer with other general monomers, or a homopolymer of a vinyl imidazole series monomer or a copolymer of such a monomer with other general monomers. The above general monomers include methacrylates, acrylates, acrylonitrile, vinylether, vinyl acetate, ethylene, styrene and the like. They may also be cation-modified polyvinyl alcohol or cellulose. Of course, they are not limited to these compounds. The pH regulators also include ammonia, various organic amines such as diethanol amine and triethanol amine; inorganic alkali agents such as hydrides of alkali metal such as sodium hydroxide, lithium hydroxide, and potassium hydroxide; and organic acids, as described above, as well as inorganic acids. A plurality of these compounds may be used simultaneously, and the pH regulators are not particularly limited as long as they satisfy conditions that the surface pH of the recording medium coated with the coating liquid thereon is maintained within the stable pH region of the pigment-based ink in which the pigment in the ink does not precipitate. In the present invention, a composition satisfying the above condition is applied on at least one surface of the base material to obtain a recording medium comprising a base material and an ink receiving layer disposed thereon, and various fillers and additives may be added to the composition to an extent that the object of the present invention is not prevented from being achieved. The fillers include silica, alumina, aluminum silicate, magnesium silicate, basic magnesium carbonate, talc, clay, hydrotalcite, calcium carbonate, titanium oxide, zinc oxide, and plastic pigments such as polyethylene, polystyrene, or polyacrylate, but are not limited to these compounds. The additives include various surfactants, dye fixing agents, antifoaming agents, antioxidants, optical whitening agents, ultraviolet ray absorbent, dispersants, viscosity regulators, mildewproof agents, and plasticizers. These additives can be arbitrarily selected from conventionally publicly known compounds. The base materials constituting the recording medium according to the present invention include paper such as wood free paper, printing paper, art paper, bond paper, recycled paper, baryta paper, cast-coated paper, and corrugated boards; plastic films and plates such as polyethylene terephthalates, diacetates, triacetates, cellophane, celluloid, polycarbonates, polyimides, polyvinyl chlorides, polyvinylidene chlorides, polyacrylates, polyethylene, and polypropylene; glass plates; cloths such as cotton, rayon, acrylic fiber, nylon, silk, and polyester. The base material can be selected from the above materials as appropriate depending on the object of the recording medium, the usage of recorded images, or the adhesion to the components coated on the recording medium. In the present invention, transparent substrates such as films or glasses are particularly preferable. To prepare a recording medium according to the present invention, the composition is first dissolved or dispersed in water, alcohol, polyhydric alcohol, or other adequate organic solvents together with other additives as needed, thereby forming a coating liquid. The coating liquid obtained is allied on the surface of the base material using, for example, a roll coater method, a blade coater method, an air knife coater method, a gate roll coater method, a bar coater method, a size press method, a spray coating method, a gravure coater method, or a curtain coater method. Thereafter, for example, a hot air drying furnace or a heat drum is used to dry the coating layer to obtain the recording medium according to the present invention. A super calendar processing can further be performed as required in order to smooth the ink receiving layer or increase the surface intensity. The total amount of the ink receiving layer on the medium is from 0.2 to 50 g/m 2 , preferably from 1 to 30 g/m 2 . If, however, this amount is small, a part of the base material may be exposed. If this amount is less than 0.2 g/m 2 , it is not effective in a coloring ability of a pigment as compared with a case of not providing an ink receiving layer, whereas if it is over 50 g/m 2 , the recording medium often curls under cold and dry conditions. The total coating amount is preferably from 0.5 to 100 µm in terms of thickness. The method for the recording by applying the ink to the recording medium is preferably an ink-jet recording method, and may be any ink-jet recording method as long as it enables inks to be effectively released from orifices and applied to a recording medium that is a target. In particular, the ink-jet method described in Japanese Patent Application Laid-Open No. 54-59936 in which an ink has its volume varied abruptly due to a thermal energy and is thus ejected from a nozzle can be effectively used. A preferred example of an ink-jet recording apparatus in which the recording medium according to the present invention is used for recording is described below. An example of the construction of a head, which is a main component of such an apparatus, is illustrated in Figs. 1, 2 and 3. A head 13 is formed by bonding a glass, ceramic or plastic plate having a groove 14 through which an ink is passed, to a heating head 15, which is used for thermal recording (the drawings show a head to which it, however, the present invention is not limited). The heating head 15 is composed of a protective film 16 made of silicon oxide, aluminum electrodes 17-1 and 17-2, a heating resistor layer 18 made of nichrome, a heat accumulating layer 19, and a substrate 20 made of alumina having a good heat radiating property. An ink 21 comes up to an ejection orifice (a minute opening) 22 and forms a meniscus 23 due to a pressure P. Now, upon application of electric signals to the electrodes 17-1 and 17-2, the heating head 15 rapidly generates heat at the region shown by "n" to form bubbles in the ink 21 which is in contact with this region. The meniscus 23 of the ink is projected by the action of the pressure thus produced, and the ink 21 is ejected from the orifice 22 to a recording medium 25 in the form of recording droplets 24. Fig. 3 illustrates a multi-head composed of an array of a number of heads as shown in Fig. 1. The multi-head is formed by bonding a glass plate 27 having a number of grooves 26 to a heating head 28 similar to the head illustrated in Fig. 1. Fig. 1 is a cross section of the head 13 taken along an ink flow passage, and Fig. 2 is a cutaway view taken along the line 2-2 of Fig. 1. Fig. 4 illustrates an exemplary ink-jet recording apparatus in which this head has been incorporated. In Fig. 4, reference numeral 61 designates a blade serving as a wiping member, one end of which is a stationary end held by a blade-holding member to form a cantilever. The blade 61 is provided at a position adjacent to a region in which the recording head operates, and in this embodiment, is held in such a form that it protrudes to the course through which the recording head is moved. Reference numeral 62 indicates a cap that is provided at a home position adjacent to the blade 61, and is so constituted that it moves in a direction perpendicular to a direction in which the recording head is moved and comes into contact with the face of the ink-ejecting openings to cap it. Reference numeral 63 denotes an ink-absorbing member provided adjacent to the blade 61 and, similar to the blade 61, held in such a form that it protrudes to the course through which the recording head is moved. The above blade 61, cap 62, and absorbing member 63 constitute an ejection recovery portion 64, where the blade 61 and absorbing member 63 remove water or dust from the face of the ink-ejecting openings. Reference numeral 65 designates the recording head having an ejection-energy-generating means and serving to eject the ink onto the recording medium set in an opposing relation with an ejection opening face provided with ejection openings to conduct recording. Reference numeral 66 indicates a carriage on which the recording head 65 is mounted so that the recording head 65 can be moved. The carriage 66 is slidably interlocked with a guide rod 67 and is connected (not illustrated) at its part to a belt 69 driven by a motor 68. Thus, the carriage 66 can be moved along the guide rod 67 and hence, the recording head 65 can be moved from a recording region to a region adjacent thereto. Reference numerals 51 and 52 denote a feeding part from which the recording media are inserted, and feed rollers driven by a motor (not illustrated), respectively. With such a construction, the recording medium is fed to the position opposite to the ejection opening face of the recording head, and discharged from discharge rollers 53 with the progress of recording. In the above constitution, the cap 62 in the head recovery portion 64 is receded from the moving course of the recording head 65 when the recording head 65 is returned to its home position, for example, after completion of recording, and the blade 61 remains protruded to the moving course. As a result, the ejection opening face of the recording head 65 is wiped. When the cap 62 comes into contact with the ejection opening face of the recording head 65 to cap it, the cap 62 is moved so as to protrude to the moving course of the recording head 65. When the recording head 65 is moved from its home position to the position at which recording is started, the cap 62 and the blade 61 are at the same positions as the positions upon the wiping as described above. As a result, the ejection opening face of the recording head 65 is also wiped at the time of this movement. The above movement of the recording head to its home position is made not only when the recording is completed or the recording head is recovered for ejection, but also when the recording head is moved between the recording regions for the purpose of recording, during which it is moved to the home position adjacent to each recording region at given intervals, where the ejection opening face is wiped in accordance with this movement. Embodiments The present invention is described below in more detail with reference to several embodiments, but is not limited by these embodiments. Parts or % in the following description is based on the weight unless otherwise stated. Examples 1 to 4 , and Comparative Example 1 In Examples 1 to 4, and Comparative Example 1, a Magenta Ink was used. Preparation of a pigment dispersed solution Components of: Styrene-acrylic acid-acrylic acid ethyl copolymer (Acid value: 140, weight average molecular weight: 5,000) 1.5 parts, Monoethanol amine 1 part, Ion exchanged water 81.5 parts, and Diethylene glycol 5 parts were mixed and heated to 70°C using a water bath to completely dissolve the resin. 10 parts of C.I. Pigment Red 112 and 1 part of isopropyl alcohol were added to the solution, pre-mixed for 30 minutes, and then dispersed under the following conditions. Dispersing device: Sand Grinder (produced by Igarashi Kikai Co), Grinding media: Zirconium beads (diameter: 1 mm), Filling rate of grinding media: 50% by volume, Grinding time: 3 hours. The solution was further centrifuged at 12,000 rpm for 20 minutes to remove coarse particles, thereby obtaining a dispersed solution. Preparation of an ink Components of: The above dispersed solution 30 parts, Glycerol 10 parts, Ethylene glycol 5 parts, N-methyl pyrrolidone 5 parts, Ethyl alcohol 2 parts, Acethylenol EH (by Kawaken Fine Chemical Kogyo) 0.2 part, and Ion exchanged water 47.8 parts were mixed to prepare the magenta ink. The stable pH region of this pigment-based ink was 7.2 or more. For the stable pH region of the pigment-based ink, a pH range of the pigment-based ink in which the pigment was sufficiently dispersed was measured by varying a pH value upon adding monoethanol amine and hydrochloric acid where neither precipitation nor agglomeration occurs. Preparation of a coating liquid, coating, and printing 20 parts of a mixture of (i) polyvinyl alcohol (PVA 217, trade-name, produced by Kuraray Co) and (ii) polyethylene imine (SP-200, trade-name, produced by Nihon Shokubai KK) was added and dissolved in a solution of 97 parts of water and 3 parts of methanol with stirring. A formulating ratio of the component (i) to the component (ii) is shown in Table 1. The coating liquid obtained was applied on polyethylene terephthalate films having each a thickness of 100 µm (Lumirror, trade-name, produced by Toray Co) using a wire bar so as to have a dry coating thickness of 10 µm. Thereafter, the films were then dried at 120°C for five minutes to prepare transparent recording media for the Examples 1 to 4 and Comparative Example 1. The surface pH of the ink receiving layer thus prepared was measured using a paper surface pH meter (produced by Kyoritsu Kagaku Kenkyusho KK). The surface pH values of are also shown in Table 1. Testing Example No. Formulating ratio Surface pH Component (i) Component (ii) Example 1 99.9 0.1 7.5 Example 2 99.5 0.5 8.7 Example 3 99 1 9.6 Example 4 95 5 10.2 Comparative Example 1 100 0 6.8 Recording was performed on the above recording media with inks having the above compositions by means of an ink-jet recording apparatus which ejects ink droplets by bubbling an ink upon an application of thermal energy, under the following conditions. Recording conditions Ejection frequency: 4 kHz Volume of ejected droplets: 45 pl Recording density: 360 DPI Maximum amount of single color ink applied: 8 nl/mm 2 Example 5 Recording media were prepared as in Example 1 except for the use of following ink, and recording was performed by means of an ink-jet recording apparatus which ejects ink droplets by bubbling an ink upon an application of thermal energy. In Example 5, a Cyan Ink was used. Preparation of a pigment dispersed solution Components of: Styrene-maleic acid-maleic acid half ester copolymer (Acid value: 70, weight average molecular weight: 12,000) 4 parts, Aminomethyl propanol 2 parts, Ion exchanged water 74 parts, and Diethylene glycol 5 parts were mixed and heated to 70°C using a water bath to completely dissolve the resin. 15 parts of C.I. Pigment Blue 22 was added to the solution, pre-mixed for 30 minutes, and then dispersed under the following conditions. Dispersing device: Pearl Mill (produced by Ashizawa Co); Grinding media: Glass beads; Filling rate of grinding media: 50% by volume; Ejection speed: 100 ml/min. The solution was further centrifuged at 12,000 rpm for 20 minutes to remove coarse particles, thereby obtaining a dispersed solution. Preparation of an ink Components of: the above dispersed solution 30 parts, Glycerol 80 parts, Ethylene glycol 5 parts, Ethanol 5 parts, and Ion exchanged water 56 parts were mixed to prepare the Cyan Ink. This pigment-based ink was stable in a pH range of 7.4 or more. Examples 6 to 9 , and Comparative Examples 2 and 3 In Examples 6 to 9 and Comparative Examples 2 and 3, Cyan Ink, Magenta Ink, Yellow Ink and Black Ink were used. Cyan Ink was prepared as in Example 5 except that the pigment was replaced with C.I. Pigment Blue 3. The stable pH region of this Cyan ink was 7.9 or more. Magenta Ink was prepared as in Example 1 except that the pigment was replaced with C.I. Pigment Red 7. The stable pH region of this Magenta Ink was 7.7 or more. Yellow Ink was prepared as in Example 5 except that the pigment replaced with C.I. Pigment Yellow 13. The stable pH region of this Yellow Ink was 8.0 or more. Black Ink was prepared as in Example 1 except that the pigment was replaced with carbon black (MCF88, trade-name, produced by Mitsubishi Kasei Co). The stable pH region of this Black Ink was 7.8 or more. Preparation of a coating liquid, coating, and printing 20 parts of a mixture of (i) polyvinyl alcohol (S-lec KW-1, trade-name, produced by Sekisui Chemical; degree of acetalization: 9% by mole) and (ii) polyallyl amine PAA-10C (trade-name, produced by Nittobo KK) was added and dissolved in a mixed solution of 95 parts of water and 5 parts of ethyleneglycol with stirring. A formulating ratio of the component (i) to the component (ii) is shown in Table 2. Testing Example No. Formulating ratio Surface pH Component (i) Component (ii) Example 6 99 1 8.2 Example 7 98 2 9.3 Example 8 90 10 10.4 Example 9 80 20 10.8 Comparative Example 2 100 0 5.7 Comparative Example 3 99.5 0.5 7.6 The coating liquid obtained was applied to polyethylene terephthalate films (Lumirror, trade-name, thickness: 100 µm, produced by Toray) using a wire bar so as to have each a dry thickness of 15 µm. The films were then dried at 120°C for five minutes to prepare transparent recording media for Examples 6 to 9 and Comparative Examples 2 and 3. The surface pH values of respective ink receiving layer thus prepared were measured using the paper surface pH meter (produced by Kyoritsu Kagaku Kenkyusho KK). The surface pH values are also shown in Table 2. Similar to Examples 1 to 4, recording was performed on the above recording media with four color inks having mentioned above by means of an ink-jet recording apparatus which ejects ink droplets by bubbling an ink upon an application of thermal energy. Examples 10 and 11 Glossy recording media according to the present invention were prepared as in Example 6 except for the use of art paper and wood free paper. The color print samples thus obtained were evaluated in terms of the following items. Evaluated items (1) Image density: A density of the black (Bk) solid paints obtained by the above printer (200% duty) was evaluated by means of a Macbeth densitometer RD-918. (2) Image irregularity: Image irregularity on each solid paint obtained by the above printer was measured visually and ranked as "A" where no image irregularity of was observed; "C" where image irregularity could be observed when the sample was seen 50 cm away from naked eyes; and "B" where an image irregularity was between "A" and "C". (3) Color tone and light transmittance of image: The paints obtained above were projected by a transparent-type overhead projector, and color tones and light transmittance of images were ranked as "A" where no problem was observed; "B" where color tone was somewhat changed against a color tone of used ink, or projected images were somewhat darkened; and "C" where color tone was evidently changed against a color tone of used ink, or projected images were darkened. The results of evaluation are shown in Table 3. Testing Example No. Evaluated items Image density Image irregularity Color tone and light transmittance of image Example 1 - A A Example 2 - A A Example 3 - A A Example 4 - A A Example 5 - A A Example 6 1.51 A A Example 7 1.49 A A Example 8 1.45 A A Example 9 1.44 A A Example 10 - A A (Tone only) Example 11 - A A (Tone only) Comparative Example 1 - B C Comparative Example 2 1.22 B C Comparative Example 3 1.35 B C As described above, the present invention provides an ink-jet recording method and a recording medium that can perform color-mixing appropriately without deviations of the inherent tones of pigments even when pigment-based inks are used for improving the water and light fastness of images and that are excellent in a light transparency of printed portions by an ink-jet recording system, even when a transparent base material such as an OHP film is used. Therefore, the present invention is significantly effective. Provided is an ink-jet recording method employing a pigment-based ink, in which ink droplets are ejected from an orifice in a recording head in accordance with recording signals to conduct recording on a recording medium comprising a base material and a coating layer disposed thereon, wherein a surface pH of the recording medium is within a stable pH range of the pigment-based ink.

Claims: An ink-jet recording method employing a pigment-based ink, in which ink droplets are ejected from an orifice in a recording head in accordance with recording signals to conduct recording on a recording medium comprising a base material and a coating layer disposed thereon, wherein a surface pH of the recording medium is within a stable pH range of the pigment-based ink. The ink-jet recording method according to Claim 1, wherein said pigment-based ink comprises a pigment, a dispersant, and a liquid medium. The ink-jet recording method according to Claim 2, wherein said dispersant is a water-soluble resin. The ink-jet recording method according to Claim 3, wherein said water-soluble resin has a weight average molecular weight ranging from 1,000 to 30,000. The ink-jet recording method according to Claim 3, wherein said water-soluble resin has a weight average molecular weight ranging from 3,000 to 15,000. The ink-jet recording method according to Claim 3, wherein said water soluble resin is a alkali-soluble resin. The ink-jet recording method according to Claim 1, wherein said pigment-based ink has a pH ranging from 7 to 10. The ink-jet recording method according to Claim 1, wherein said coating layer contains a resin exhibiting solubility to or affinity for an aqueous ink. The ink-jet recording method according to Claim 1, wherein said recording medium is transparent. The ink-jet recording method according to Claim 1, wherein said recording medium is glossy. The ink-jet recording method according to Claim 1, wherein said pigment-based ink is ejected by applying thermal energy.

Inventors: HIROSE MIFUNE, KASHIWAZAKI AKIO, SUZUKI EIICHI, HIROSE, MIFUNE, KASHIWAZAKI, AKIO, SUZUKI, EIICHI, HIROSE, MIFUNE, C/O CANON KABUSHIKI KAISHA, KASHIWAZAKI, AKIO, C/O CANON KABUSHIKI KAISHA, SUZUKI, EIICHI, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1995
Main classification: B41M5
IPC codes: B41M5