INK JET RECORDING METHOD

Abstract: There is disclosed an ink jet recording method which performs recording by attaching liquid droplets of a recording liquid onto a recording medium, wherein recording is performed by elevating the temperature of the recording liquid having the viscosity of 2 centipoise or higher and the surface tension of 40 dyne/cm or higher at 25°C to form recording liquid droplets having the viscosity less than 2 centipoise.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an ink jet recording method in which recordings can be made not only on specially designated paper but also on papers commonly used in offices or homes e.g copying paper, report paper, bond paper, slip paper and various cloths. The method can enable the image to be fixed onto the recording medium rapidly, to have a high optical density and to exhibit relatively little feathering. Related Background Art The leading method of ink jet recording uses as recording liquid or ink a dye dissolved in water or a water-soluble organic solvent. In such methods, although particular requirements may differ slightly from system to system the important and commonly demanded features are as follows: 1. there should be produced a recorded image having good quality and of high optical density without significant feathering; 2. the ink should become fixed rapidly; 3. the resulting printed matter should be durable, for example it should be resistant to water and should be light fast; 4. the ink should not clog the ink passageway and nozzles of the printer; 5. stable ejection of the ink should be available over a long period of operation; and 6. the ink should be stable to storage over a long period without significant change. Extensive research has been carried out into both the printer and the ink in order to satisfy some or all of the above objects. However, even in the case of commercially available ink jet printer systems, there is no recording method which completely satisfies all the above objects. For example, when there is used an ink which contains in addition to the dye and water a low-volatile polyalkylene glycol or glycerine in order to prevent clogging of the nozzles, because of the increased viscosity the ink it only weakly penetrates normal papers having effective sizing so that several minutes may be taken for the image to become fixed. Although the problem of clogging of the nozzles is avoided this is achieved at the expense of an impractically long fixing time. Japanese Laid-Open Patent Publication No 29546/1980 proposes to solve the above problem by adding a surfactant to the ink. In this case, although the rate of fixing can be increased to a satisfactory extent depending on the kind and the amount of the surfactant added, the surface tension of the ink is reduced and feathering is increased so that the quality of the resulting printed image is significantly reduced. Unexamined Japanese Patent Publication No 57862/1981 discloses replacing the surfactant by a strongly basic substance which destroys an internal sizing agent present in the paper, typically rosin. Fixing is rapid and printed letters with little feathering can be obtained. However, substantially no effect is obtained when the substrate is neutral paper because of a difference in the sizing agent employed and inks having high pH are dangerous when used in the home or in the office. Unexamined Japanese Patent Publication No 49762/1983 discloses a method in which recording is carried out using an ink having a dynamic viscosity of 4 to 8 cst at 35°C and maintaining the ink with a content of water of one-quarter or less at 30°C or higher. In this method although satisfactory results can be obtained to some extent with respect to clogging and maintenance of stable high speed recording, problems arise, particularly with respect to the fixing characteristics and feathering on normal papers because the content of solvent in the ink is so high. One route to improving print quality could be to increase the dye concentration in the ink. However, this increases the risk of clogging of the orifices of the printer. SUMMARY OF THE INVENTION The present invention provides a method in which recording is carried out by causing droplets of a recording liquid to become attached to a recording medium, the recording liquid having a viscosity of 2 centipoise or above and a surface tension of 40 dyne/cm or above at 25°C characterised in that the recording liquid formed into the droplets is heated to a temperature such that its viscosity is less than 2 centipoise. The use in ink jet printing of inks having low viscosity at ambient temperatures is disclosed in EP-A-0085869. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventors have found that, among various items of requirements as described above in the ink jet recording technique, particularly both of good printing quality without feathering and rapid fixing characteristic can be satisfied by controlling the viscosity and the surface tension of the ink to certain specific values. More specifically, it has been found that both printing quality and fixing characteristic can be satisfied by defining the physical property values of the ink upon performing printing on normal papers which are subjected to sizing and have fibers exposed on the surface to be recorded as the viscosity less than 2 centipoise at 25 °C, and as the surface tension of 40 dyn/cm or more, preferably 70 dyn/cm or less. When the viscosity of the ink upon printing exceeds 2 centipoise, fixing characteristic is insufficient, which becomes particularly in a recording medium with high degree of sizing. On the other hand, when the surface tension of the ink is smaller than 40 dyn/cm, feathering is liable to be generated, and lowering in printing quality by feathering is particularly marked in a recording medium with low degree of sizing. For the method for setting the physical property values of the ink during printing within the ranges as specified above, namely at a viscosity less than 2 centipoise and a surface tension of 40 dyn/cm or more, it is effective to increase the water content and reduce the amount of solvent used in ink so far as possible, and to select the solvent to be used from those having lower viscosities, namely, having generally higher vapor pressures. However, in the recording method by use of such ink, evaporation of the ink solvent is great, whereby clogging at the nozzles,etc., of the printer is generated to pose a problem with respect to reliability. Accordingly, in the present invention, to solve first the problem of clogging which is the essential requisite item in the ink jet recording method, as the first specific feature, an ink with the viscosity at 25 °C of 2 centipoise or higher and the surface tension of 40 dyn/cm or higher is to be used. For prevention of clogging, it is beneficial to increase the amount of the solvent or to use a low vapor pressure solvent. More specifically, when ink design is performed by use of an aqueous solution of a water-soluble acidic dye, direct dye or a dye for food, the amount of the solvent to be used in view of the solubility of the dye may be, for example, about 26 parts by weight for diethylene glycol frequently used in the ink for ink jet recording, about 20 parts by weight for triethylene glycol, and about 14 parts by weight for polyethylene glycol (average molecular weight 300) as at least essential amount, and the viscosity of the ink in any of these cases becomes about 2 centipoise. In the ink thus prepared, no clogging is generated, whereby reliability becomes good. However, since the viscosity of the ink exceeds 2 centipoise, fixing characteristics becomes insufficient as described above, particularly there is the problem in fixing characteristic on a recording medium with high degree of sizing. Thus, the present invention has been accomplished as the result of intensive studies about the method satisfying simultaneously the requisite items antagonistic to each other in ink design, namely the item of "generation of no clogging" and the item of "obtaining good printing quality with rapid fixing speed and without feathering". That is, the ink jet recording method capable of satisfying simultaneously clogging and fixing characteristic and printing quality has been accomplished by the method of performing recording by elevating the temperature of the ink with the viscosity at 25 °C of 2 centipoise or more to make the viscosity less than 2 centipoise, preferably less than 1.8 centipoise, which is the second primary specific feature of the present invention. It has been well known in the art that the viscosity of ink is lowered by elevation in temperature. Its surface tension also tends to be lowered with temperature elevation. Accordingly, with respect to this point, the changes occur in the direction not preferable in aspect of printing quality, but as compared with the abrupt viscosity change by temperature, influence on the printing quality by surface tension change is small, and the object of the present invention can be sufficiently accomplished with an ink with the surface tension of 40 dyn/cm or more at 25 °C. Also, by lowering in ink viscosity accompanied by ink temperature elevation during recording, generally the ejected amount of ink is increased and the ink spread area on the recording medium becomes larger, and therefore the recording method practiced with temperture elevation of ink is also effective for densifying optical densityof the printed matter. In this case, since the dye concentration in ink is not particularly required to be made greater, densification can be accomplished without accompaniment of the problem of geneation of clogging. Such densification of optical density of printed matter is further effective, particularly when recording paper for ink jet recording or various cloths are used as the recording medium. When the increase in the ejected amount of ink by temperature elevation is a problem, for example, when influence by the increase in ejected amount is remarkably increased as compared with the effect of improvement of fixing characteristic due to viscosity lowering on normal paper with high degree of sizing and fixing badness is generated, and when the ejected amount of the ink is remarkably greater than the ink absorption amount of the recording medium to generate overflowing of ink, it is possible to cope with such problem by lowering in driving voltage or driving frequency and change in printing pattern, etc. Next, the constitution of the present invention is described in more detail. The ink jet recording system to be used in the present invention which characterizes the present invention may be any system which permits ink to be released effectively from a nozzle and imparts the ink to the recording medium which is a shooting target, and typical examples of such systems are described in IEEE Transactions on Industry Applications Vol. JA-13, No. 1 (February, March, 1977); Nikkei Electronics, April 19, 1976, January 29, 1973 and May 6, 1974. The methods described in these References may include, firstly the electrostatic attracting system, and secondly the system in which fine ink particles are ejected forcibly by applying high pressure on ink by a small pump and vibrating mechanically the nozzle by quartz oscillator, etc. As another system of the second system, there is the system called the microdot ink jet system. Also, as the third system, there is the system in which ink is ejected from a nozzle by applying pressure to the ink by giving electrical signals to a piezoelectric element to generate mechanical displacement. Further, it is possible to use effectively the ink jet system as disclosed in Unexamined Japanese Patent Publication No. 59936/1979, in which the ink receiving the action of heat energy is abruptly changed in volume, and the ink is ejected from the nozzle by the active force through this change in state. The above systems are specific examples of preferable recording methods in the present invention. Accordingly, the present invention is not limited to these, but it is also applicable for other ink jet recording methods than those as mentioned above. Next, the constitution of the ink ejected by various system as described above is to be explained. The dye itself to be used in the ink according to the present invention may be known per se , and for example, insoluble dyes such as pigments or disperse dyes can be also used, but preferable dyes are water-soluble dyes typically direct dyes, acidic dyes, basic dyes, reactive dyes, dyes for foods, etc. Particularly, those which are suitable as the ink for ink jet recording system and satisfy performances such as color forming property, sharpness, stability, lightfastness and others may include: direct dyes such as C.I. Direct Black 17, 19, 32, 51, 62, 71, 108, 146, 154; C.I. Direct Blue 6, 22, 25, 71, 86, 90, 106, 199; C.I. Direct Red 1, 4, 17, 28, 83; C.I. Direct Yellow 12, 24, 26, 44, 86, 98, 100, 142; C.I. Direct Orange 34, 39, 44, 46, 60; C.I. Direct Violet 47, 48; C.I. Direct Brown 109; C.I. Direct Green 59, etc.; acidic dyes such as Acid Black 2, 7, 24, 26, 31, 52, 63, 112, 118; C.I. Acid Blue 9, 22, 40, 59, 93, 102, 104, 113, 117, 120, 167, 229, 234, 254; C.I. Acid Red 1, 6, 8, 32, 37, 51, 52, 80, 85, 87, 92, 94, 115, 180, 256, 317, 315; C.I. Acid Yellow 11, 17, 23, 25, 29, 42, 49, 61, 71; C.I. Acid Orange 7, 19; C.I. Acid Violet 49, etc.; reactive dyes such as C.I. Reactive Yellow 2, 3, 13, 15, 17, 18, 23, 24, 37, 42, 57, 58, 64, 75, 76, 77, 79, 81, 84, 85, 87, 88, 91, 92, 93, 95, 102, 111, 115, 116, 130, 131, 132, 133, 135, 136, 137, 139, 140, 142, 143, 144, 145, 146, 147, 148, 151, 162, 163; C.I. Reactive Orange 5, 7, 11, 12, 13, 15, 16, 35, 45, 46, 56, 70, 72, 74, 82, 84, 87, 91, 92, 93, 95, 97, 99; C.I. Reactive Red 3, 13, 16, 21, 22, 23, 24, 29, 31, 33, 35, 45, 49, 55, 63, 85, 106, 109, 111, 112, 113, 114, 118, 126, 128, 130, 131, 141, 151, 170, 171, 174, 176, 177, 183, 184, 186, 187, 188, 190, 193, 194, 195, 196, 200, 201, 202, 204, 206, 218, 221; C.I. Reactive Violet 1, 4, 5, 6, 22, 24, 33, 36, 38; C.I. Reactive Blue 2, 3, 5, 8, 10, 13, 14, 15, 18, 19, 21, 25, 27, 28, 38, 39, 40, 41, 49, 52, 63, 71, 72, 74, 75, 77, 78, 79, 89, 100, 101, 104, 105, 119, 122, 147, 158, 160, 162, 166, 169, 170, 171, 172, 173, 174, 176, 179, 184, 190, 191, 194, 195, 198, 204, 211, 216, 217; C.I. Reactive Green 5, 8, 12, 15, 19, 23; C.I. Reactive Brown 2, 7, 8, 9, 11, 16, 17, 18, 21, 24, 26, 31, 32, 33; C.I. Reactive Brown 1, 5, 8, 13, 14, 23, 31, 34, 39, etc., and otherwise C.I. Basic Black 2; C.I. Basic Blue 1, 3, 5, 7, 9, 24, 25, 26, 28, 29; C.I. Basic Red 1, 2, 9, 12, 13, 14, 37; C.I. Basic Violet 7, 14, 27; C.I. Food Black 1, 2. The above examples of dyes are particularly preferable for the ink to be used in the present invention, and the dyes to be used for the ink of the present invention are not limited to these dyes. Such dye may be used generally at a ratio of about 0.5 to 30 wt.% in the ink, preferably 1 to 25 wt.%, more preferably 1 to 10 wt.%. The solvent used in the ink to be used in the present invention is water or a solvent mixture of water with a water-soluble organic solvent, more preferably a solvent mixture of water with a water-soluble organic solvent, which contains a polyhydric alcohol having the drying-preventing effect of ink as the water-soluble organic solvent. Also, as the water, it is preferable to use deionized water rather than water in general containing various ions. Examples of the water-soluble organic solvent to be used mixed with water may 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, isobutyl alcohol, etc.; amides such as dimethylformamide, dimethylacetamide, etc.; ketone or ketoalcohols such as acetone, diacetone alcohol, etc.; ethers such as tetrahydrofuran, dioxane, etc.; polyalkylene glycols such as polyethylene glycol, polypropylene glycol, etc.; alkylene glycols with alkylene group containing 2 to 6 carbon atoms such as ethylene glycol, propylene glycol, butylene glycol, triethylene glycol, 1,2,6-hexane triol, thiodiglycol, hexylene glycol, diethylene glycol, etc.; glycerin; lower alkyl ethers of polyhydric alcohols such as ethylene glycol methyl (or ethyl) ether, diethylene glycol methyl (or ethyl) ether, triethylene glycol monomethyl (or ethyl) ether, etc.; N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, triethanolamine, sulforane, etc. Among these many water-soluble organic solvent, polyhydric alcohols such as diethylene glycol, etc., lower alkyl ethers of polyhydric alcohols such as triethylene glycol monomethyl (or ethyl) ether, etc., are preferred. The content of the above water-soluble organic solvent in the ink may be generally 2 to 50 wt.% based on the total weight of the ink, preferably 5 to 40 wt.%, more preferably 10 to 30 wt.%. The content of water in the ink, particularly in view of fixing characteristic on normal paper and feathering of dots, may be 50 wt.% or more, preferably 60 wt.% or more, more preferably 70 wt.% or more. The essential components in the ink composition in the present invention are as described above. Various other dispersing agents, surfactants, viscosity controllers, surface tension controllers, fluorescence brighteners, etc., known in the art can also be added if necessary. For example, there may be included viscosity controllers such as polyvinyl alcohol, celluloses, water-soluble resins, etc.; various surfactants such as cationic, anionic, or nonionic surfactants; surface tension controllers such as diethanolamine, triethanolamine, etc.; pH controllers with buffer solution; antifungal agent, etc. Also, for making up the ink to be used in the ink jet recording method of the type which charges ink, specific resistance controllers of inorganic salt, etc., such as lithium chloride, ammonium chloride, sodium chloride may be added. When applied for the ink jet system of the type in which ink is ejected through the action of heat energy, thermal, physical property values (e.g. specific heat, coefficient of thermal expansion, thermal conductivity, etc.) may be sometimes controlled. The ink to be used in the present invention comprises basically the above components, and the present invention is characterized in that the recording is performed by use of such recording ink according to the ink jet recording system as described above, by allowing the ink with the viscosity at 25 °C of 2 centipoise or higher and the surface tension of 40 dyn/cm or higher to be an ink with the viscosity less than 2 centipoise, preferably less than 1.8 centipoise by temperature elevation. For satisfying the above conditions, the ink to be used in the present invention should be desirably controlled to the viscosity at 25 °C of 15 centipoise or lower, preferably 10 centipoise or lower, more preferably 5 centipoise or lower. According to such ink jet recording method of the present invention, on papers used in office and various cloths, etc., other than the papers for ink jet recording on which no image of good quality could be formed due to feathering during printing in the prior art, images of high quality and high optical density without feathering can be formed, and the recording method remarkably excellent in drying and fixing characteristics can be realized. Accordingly, according to the recording method of the present invention, since not only designated paper specially prepared for ink jet recording as a matter of course, but also papers widely utilized in office or home such as copying paper, report paper, bond paper, slit paper, etc., can be sufficiently utilized as the recording medium for the ink jet recording system, it becomes possible to accelerate further prevailing of the ink jet recording system broadly. Also, it becomes possible to obtain a recording method having excellent reliability not only in ejecting stability, storage stability, frequency response of ink as a matter of course, but also in respect of clogging of ink at nozzles, etc., which is particularly deemed to be important in ink jet recording. Further, since no use of a basic substance, etc., is required, a recording method extremely excellent in safety for use in home or office can be realized. As the temperature elevation method of the ink in the present invention, any means may be employed. For example, temperature elevation can be freely made according to , e.g., a method of providing a resistant heating generating element capable of controlling the amount of heat generation electrically in the ink liquid chamber in the vicinity of nozzles. Further, the control of ink temperature is also possible by employment of a temperature sensor such as a thermistor, etc., in the ink liquid chamber, and the temperature control for setting to a desired viscosity can be freely effected. In the present invention, temperature elevation may be effected to a temperature of about 30 °C to 80 °C, which may differ depending on the ink employed. The values of viscosity and surface tension in the specification were obtained by the measuring instruments shown below. Viscosity: VISCONIC ELD, produced by Tokyo Keiki Surface tension: Kyowa CBVP SURFACE TENSIOMETER A-1, produced by Kyowa Kagaku The present invention is described in more detail by referring to the following Reference examples, Examples and Comparative examples. In the sentences, parts and % are based on weight, unless otherwise particularly noted. Reference examples Preparation of inks A-G After the following components were stirred for 5 hours, each mixture was controlled to pH 7.5 with 0.1% aqueous sodium hydroxide solution, and further subjected to filtration under pressurization through a membrane filter (trade name: Fluoropore Filter, produced by Sumitomo Denko) with a pour size of 0.45 micron to prepare inks of the present invention. Examples 1 to 5 and Comparative examples 1 to 6 By use of the Inks A-G obtained, and by use of an ink jet recording device which is an ink jet printer utilizing heat-generating element as the ejecting energy source of ink (orifice size 40 x 50 µm, heater size 50 x 150 µm, the number of nozzles 24, driving voltage 24.5 V, frequency 3 KHz), printing was performed on a commercially available continuous slip paper and copying paper, and fixing characteristic and the extent of feathering of the printed matter obtained were evaluated. In Examples and a part of Comparative examples there was mounted a thermistor having positive characteristics around the ink liquid chamber (trade name: Posistor, Curie point 30°, 40°, 45°, 60°, produced by Murata Seisakusho) to apply temperature elevation operation. Also, the printing head was left to stand at 25 °C for 24 hours without capping operation, and again printing operation was performed, and the power source of the printer was turned off when three lines of printing were performed, and then the clogging states of the 24 nozzles were observed. The above results, physical property values of ink at 25 °C, elevated temperature during recording and the viscosity of the ink are shown in Table 1. Table 1. *1 Fixing characteristic evaluation After 10 seconds and 30 seconds after printing on a continuous slip paper (trade name: Dia 64Gr, produced by Mitsubishi Paper Mills, Ltd.) and copying paper (trade name: 9-50015HI, produced by Hammermill), the printed portion was rubbed with a filter paper (trade name: No. 5C, produced by Toyo Kagaku Sangyo K.K.) (evaluated under the environmental conditions of 25 °C, 60%RH). O ○ ...... no scratch after 10 seconds O ...... small scratch after 10 seconds Δ ...... small scratch after 30 seconds x ...... great scratch after 30 seconds *2 Measurement of feathering After printing 300 dots on the continuous slip paper as described above so that the dots were not continuous to one another, the printed matter was left to stand for one hour or longer and thereafter the number of dots having generated feathering was counted by a microscope and feathering was represented in terms of % (printed under the envirounmental conditions of 25 °C, 60%RH). O ○: 10% or less O: 11 to 20% Δ: 21 to 30% x: 31% or higher *3 Clogging evaluation This was evaluated in terms of the number of the nozzles suffered from clogging among 24 nozzles (evaluated under the environmental conditions of 25 °C, 30%RH) O ○: 0 O: 1 or 2 Δ: 3 to 5 x: 6 or more Examples 6 - 7 By use of the ink D and the ink jet printer (BJ-80, produced by Cannon K.K.), solid printing was performed at 40 °C, 60 °C on a commercially available coated paper for ink jet recording (Matcoat NM, produced by Mitsubishi Seishi) by use of a thermistor of positive characteristic (trade name: Posistar, Curie point 40°, 60°), and the optical density (O.D.) of the printed matter obtained was evaluated. The evaluation results are shown in Table 2. Comparative examples 7 - 10 By use of the ink D, and the inks H, I, J prepared by increasing the dye concentration in the ink D to 7%, 9% and 11%, solid printing was performed on the coated paper for ink jet recording as described above at 25 °C, and the optical density (O.D.) of the printed matter obtained was evaluated. Also, clogging evaluation was conducted similarly as in Example 1 for the inks D and H-J. The evaluation results are shown in Table 2. According to the ink jet recording method of the present invention, highly reliable recording with good fixing characteristic, printed letter quality and further without generation of nozzle clogging has been rendered possible not only on a designated paper specially prepared for ink jet recording as a matter of course, but also on other normal papers which are subjected to sizing and have fibers exposed on the surface to be recorded, typically copying paper, report paper, bond paper, continuous slip paper, etc. Further, since no strongly basic substance, etc., is used in the ink, the ink jet recording method also excellent in safety could be realized.

Claims: An ink jet recording method in which recording is carried out by causing droplets of a recording liquid to become attached to a recording medium, the recording liquid having a viscosity of 2 centipoise or above and a surface tension of 40 dyne/cm or above at 25°C characterised in that the recording liquid formed into the droplets is heated to a temperature such that its viscosity is less than 2 centipoise. A method according to claim 1, wherein the recording liquid formed into the droplets is heated to a temperature such that its viscosity is less than 1.8 centipoise. A method according to claim 1 or 2, wherein the viscosity of the recording liquid at 25°C is 10 centipoise or below. A method according to claim 1 or 2, wherein the viscosity of the recording liquid at 25°C is 5 centipoise or below. A method according to any preceding claim, wherein the recording liquid formed into the droplets is heated to a temperature of 30-80°C. A method according to any preceding claim, wherein there is provided a recording liquid chamber leading to a plurality of nozzles from which the ink is ejected and the recording liquid in the chamber is heated by means of a resistance heater. A method according to any preceding claim, wherein the recording liquid contains 50 wt.% or more of water. A method according to any preceding claim, wherein the recording liquid droplets thus formed are attached onto the surface of a sized recording medium having fibers exposed at its surface. A method according to any preceding claim, wherein the recording liquid comprises a water-soluble dye, a water-soluble organic solvent and water as main components. A method according to claim 9, wherein the content of said dye is from 0.5 to 30 wt.%. A method according to claim 9 or 10, wherein the content of said organic solvent is from 2 to 50 wt.%. An ink jet recording method according to any of claims 7 to 11, characterised in that the content of water in said recording liquid is 60 wt.% or more. A method according to any of claims 7 to 11, characterised in that the content of water in said recording liquid is 70 wt.% or more.

Inventors: KOIKE, SHOJI, TOMIDA, YASUKO
Applicants: CANON KABUSHIKI KAISHA
Application year: 1987
Main classification: C09D11
IPC codes: B41M5, C09D11