Abstract:
An ink jet printing process comprising the steps of; 
     a) providing an ink jet printer in which a continuous stream of ink jet ink is emitted from a nozzle that is responsive to digital data signals; 
     b) loading the printer with an ink jet recording element; 
     c) loading the printer with an inkjet ink comprising a thermally-responsive polymeric material; and 
     d) ejecting ink from a thermally-steered continuous ink jet print head onto one of the ink jet recording elements in response to the digital data signals.

Description:
FIELD OF THE INVENTION 
     This invention relates to an ink jet printing process that utilizes aqueous inks in the method of asymmetric heating drop deflection. 
     BACKGROUND OF THE INVENTION 
     Ink jet printers are well known in the printing industry. Ink jet printers are just one of many different types of printing systems that have been developed which include laser electrophotographic printers; LED electrophoto-graphic printers; dot matrix impact printers; thermal paper printers; film recorders; thermal wax printers; dye diffusion thermal transfer printers. Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfers and fixing. However, there is an ongoing demand for improved digitally controlled printing systems that are able to produce high color images at a high speed and low cost using standard paper. 
     One such improvement is disclosed in U.S. Pat. No. 6,079,821, the disclosure of which is hereby incorporated by reference. That patent discloses an apparatus for controlling ink in a continuous ink jet printer that includes an ink delivery channel; a source of pressurized ink communicating with the ink delivery channel; a nozzle bore which opens into the ink delivery channel to establish a continuous flow of ink in a stream, the nozzle bore defining a nozzle bore perimeter; and a droplet generator which causes the stream to break up into a plurality of droplets at a position spaced from the ink stream generator. The droplet generator includes a heater having a selectively-actuated section associated with only a portion of the nozzle bore perimeter, whereby actuation of the heater section produces an asymmetric patent of heat to the stream to control the direction of the stream between a print direction and a non-print direction. However, there is no disclosure of any specific inks in this application and the only specific fluid used in the example is water. 
     Another feature of that patent patent is a process for controlling ink in a continuous ink jet printer that includes establishing a continuous flow of ink in a stream which breaks up into a plurality of droplets at a position spaced from the ink stream generator; and asymmetrically applying heat to the stream before the position whereat the stream breaks up into droplets to thereby control the angle at which the ink deflects. 
     Along with the development of inkjet printing systems, such as the system employing asymmetric heating drop deflection, is the requirement of inks useful in ink jet printing. An ink composition must be capable of meeting very stringent criteria to be useful in ink jet printing. Such properties of major concern of the ink composition are viscosity, surface tension, pH, density, conductivity, adhesive characteristics, wetting characteristics, drying rate and shelf life. 
     It is an object of this invention to provide an ink jet printing process employing a particular ink useful in a thermally-steered, continuous ink jet print head which provides a greater difference in deflection angle. 
     SUMMARY OF THE INVENTION 
     This and other objects are achieved in accordance with the invention which comprises an ink jet printing process comprising the steps of; 
     a) providing an ink jet printer in which a continuous stream of ink jet ink is emitted from a nozzle that is responsive to digital data signals; 
     b) loading the printer with an ink jet recording element; 
     c) loading the printer with an ink jet ink comprising a thermally-responsive polymeric material; and 
     d) ejecting ink from a thermally-steered continuous ink jet print head onto one of the ink jet recording elements in response to the digital data signals. 
     By use of the process of the invention, ink jet prints are obtained using an ink in a thermally-steered, continuous ink jet print head which provides a greater difference in deflection angle. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In a preferred embodiment of the invention, the thermally-responsive material comprises at least one kind of block copolymer with at least one block comprising poly(ethylene oxide) or PEO. In another preferred embodiment of the invention, the thermally-responsive material comprises a tri-block copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), or PEO-PPO-PEO dissolved in an aqueous solution. This material is sold commercially under the trademark Pluronic® (BASF Corp.). 
     In another preferred embodiment of the invention, the ink jet ink contains about 1-40% of the thermally-responsive material and about 0.5-5% of a dye. 
     The viscosity of the ink increases dramatically when heated from room temperature to about 80° C. In a preferred embodiment of the invention, ink jet ink containing the thermally-responsive material has a viscosity of less than 10 centipoise at 20° C. and a viscosity of more than 100 centipoise upon heating. 
     For example, a 15% by weight aqueous solution of Pluronic® P85 has a viscosity of about 4 centipoise at 22° C. and a viscosity of about 3000 centipoise at 80° C. 
     When the ink is heated asymmetrically at the orifice of the nozzle plate, the thermally-responsive material causes the jet of ink to deflect at a greater angle than would otherwise be obtained with ink without any such material. It is believed that this occurs because the localized heating of the material near one side of the orifice causes the thermally-responsive material to temporarily form a plug, which causes the material to flow around it and be deflected in the opposite direction. 
     The viscosity change of the formulated solutions in response to a temperature change is entirely reversible as the ink solution returns to the original viscosity when cooled down to its initial temperature. 
     In another preferred embodiment of the invention, the continuous stream of ink has a deflection angle greater than that of ink without any such thermally-responsive material. 
     As noted above, a continuous ink jet printer system that employs the method of asymmetric heating deflection is disclosed in the above-referred to U.S. Pat. No. 6,079,821. Following is a general description of the process employed. For specific details, please referred to the above-referred to U.S. Pat. No. 6,079,821. The system includes an image source such as a scanner or computer which provides raster image data, outline image data in the form of a page description language, or other forms of digital image data. This image data is converted to half-toned bitmap image data by an image-processing unit that also stores the image data in memory. A plurality of heater control circuits read data from the image memory and applies time-varying electrical pulses to a set of nozzle heaters that are part of a print head. These pulses are applied at an appropriate time, and to the appropriate nozzle, so that drops formed from a continuous ink jet stream will form spots on a recording medium in the appropriate position designated by the data in the image memory. 
     Recording medium is moved relative to a print head by a recording medium transport system, which is electronically controlled by a recording medium transport control system, and which in turn is controlled by a micro-controller. In the case of page width print heads, it is most convenient to move a recording medium past a stationary print head. However, in the case of scanning print systems, it is usually most convenient to move the print head along one axis (the sub-scanning direction) and the recording medium along an orthogonal axis (the main scanning direction) in a relative raster motion. 
     Ink is contained in an ink reservoir under pressure. In the non-printing state, continuous ink jet drop streams are unable to reach a recording medium due to an ink gutter that blocks the stream and which may allow a portion of the ink to be recycled by an ink recycling unit. The ink-recycling unit reconditions the ink and feeds it back to a reservoir. Such ink recycling units are well known in the art. The ink pressure suitable for optimal operation will depend on a number of factors, including geometry and thermal properties of the nozzles and thermal properties of the ink. A constant ink pressure can be achieved by applying pressure to the ink reservoir under the control of an ink pressure regulator. 
     The ink is distributed to the back surface of a printhead by an ink channel device. The ink preferably flows through slots and/or holes etched through a silicon substrate of the printhead to its front surface, where a plurality of nozzles and heaters are situated. With a printhead fabricated from silicon, it is possible to integrate heater control circuits with the printhead. 
     In printing, an important system parameter is the angle at which the ink fluid deflects. This angle denoted by θ is the angle formed between a line connecting the deflected drops to the center of the nozzle bore on the surface of electrical insulating layers and a line normal to the electrical insulating layers centered at the nozzle bore. Greater drop deflection results in a more robust system. The larger the deflection angle θ, the closer the ink gutter may be placed to the printhead and hence the printhead can be placed closer to the recording medium resulting in lower drop placement errors, which will result in higher image quality. Also, for a particular ink gutter to printhead distance, larger deflection angles θ result in larger deflected drop to ink gutter spacing which would allow a larger ink gutter to printhead alignment tolerance. Larger deflection angles θ also allow larger amounts of (unintended) undeflected drop misdirection. Undeflected drop misdirection may occur, for instance, due to fabrication non-uniformity from nozzle to nozzle or due to dirt, debris, deposits, or the like that may form in or around the nozzle bore. 
     The ink used in the invention usually contains a colorant such as a pigment or dye. Suitable dyes include acid dyes, direct dyes, water soluble dyes or reactive dyes listed in the COLOR INDEX but is not limited thereto. Metallized and non-metallized azo dyes may also be used as disclosed in U.S. Pat. No. 5,482,545, the disclosure of which is incorporated herein by reference. Other dyes which may be used are found in EP 802246-A1 and JP 09/202043, the disclosures of which are incorporated herein by reference. 
     Any of the known organic pigments can be used to prepare ink jet inks used in the invention. Pigments can be selected from those disclosed, for example, in U.S. Pat. Nos. 5,026,427; 5,085,698; 5,141,556; 5,160,370 and 5,169,436, the disclosures of which are hereby incorporated by reference. The exact choice of pigment will depend upon the specific color reproduction and image stability requirements of the printer and application. For four-color printers, combinations of cyan, magenta, yellow and black (CMYK) pigments are used. An exemplary four color set is a cyan pigment, bis(phthalocyanylalumino)-tetraphenyldisiloxane, quinacridone magenta (pigment red 122), pigment yellow 74 and carbon black (pigment black 7). 
     In addition to the thermally-responsive material, a humectant may be employed in the ink jet compositions used in the invention to help prevent the ink from drying out or crusting in the orifices of the printhead. Examples of humectants which can be used include polyhydric alcohols, such as ethylene glycol, diethylene glycol(DEG), triethylene glycol, propylene glycol, tetraethylene glycol, polyethylene glycol, glycerol, 2-methyl-2,4-pentanediol, 2-ethyl-2-hydroxymethyl-1,3-propanediol(EHMP), 1,5 pentanediol, 1,2-hexanediol, 1,2,6-hexanetriol and thioglycol; lower alkyl mono- or di-ethers derived from alkylene glycols, such as ethylene glycol mono-methyl or mono-ethyl ether, diethylene glycol mono-methyl or mono-ethyl ether, propylene glycol mono-methyl or mono-ethyl ether, triethylene glycol mono-methyl or mono-ethyl ether, diethylene glycol di-methyl or di-ethyl ether, poly(ethylene glycol) monobutyl ether (PEGMBE), and diethylene glycol monobutylether(DEGMBE); nitrogen-containing compounds, such as urea, 2-pyrrolidinone, N-methyl-2-pyrrolidinone, and 1,3-dimethyl-2-imidazolidinone; and sulfur-containing compounds such as dimethyl sulfoxide and tetramethylene sulfone. 
     Penetrants may also be added to the inks employed in the invention to help the ink penetrate the receiving substrate, especially when the substrate is a highly sized paper. Examples of such penetrants include alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, iso-butyl alcohol, furfuryl alcohol, and tetrahydrofurfuryl alcohol; ketones or ketoalcohols such as acetone, methyl ethyl ketone and diacetone alcohol; ethers, such as tetrahydrofuran and dioxane; and esters, such as, ethyl lactate, ethylene carbonate and propylene carbonate. 
     Polymeric binders can also be added to the ink employed in the invention to improve the adhesion of the colorant to the support by forming a film that encapsulates the colorant upon drying. Examples of polymers that can be used include polyesters, polystyrene/acrylates, sulfonated polyesters, polyurethanes, polyimides and the like. The polymers may be present in amounts of from about 0.01 to about 15 percent by weight and more preferably from about 0.01 to about 5 percent by weight based on the total amount of components in the ink. 
     Surfactants may be added to the ink to adjust the surface tension to an appropriate level. The surfactants may be anionic, cationic, amphoteric or nonionic and used at levels of 0.01 to 1% of the ink composition. Preferred surfactants include Surfynol 465® (available from Air Products Corp.) and Tergitol 15-S-5® (available from Union Carbide). 
     A biocide may be added to the ink composition employed in the invention to suppress the growth of micro-organisms such as molds, fungi, etc. in aqueous inks. A preferred biocide for the ink composition employed in the present invention is Proxel® GXL (Zeneca Specialties Co.) at a final concentration of 0.0001-0.5 wt. %. 
     The pH of the aqueous ink compositions employed in the invention may be adjusted by the addition of organic or inorganic acids or bases. Useful inks may have a preferred pH of from about 2 to 10, depending upon the type of dye being used. Typical inorganic acids include hydrochloric, phosphoric and sulfuric acids. Typical organic acids include methanesulfonic, acetic and lactic acids. Typical inorganic bases include alkali metal hydroxides and carbonates. Typical organic bases include ammonia, triethanolamine and tetramethylethlenediamine. 
     A typical ink composition employed in the invention may comprise, for example, the following components by weight: colorant (0.05-20%), water (0-90%), a humectant (5-70%), the thermally-responsive material (1-40%), penetrants (2-20%), surfactant (0.1-10%), biocide (0.05-5%) and pH control agents (0.1-10%). 
     Additional additives which may optionally be present in the ink jet ink compositions employed in the invention include thickeners, conductivity enhancing agents, anti-kogation agents, drying agents, waterfast agents, dye solubilizers, chelating agents, binders, light stabilizers, viscosifiers, buffering agents, anti-mold agents, anti-rusting agents, anti-curl agents, dispersants and defoamers. 
     Examples of buffering agents include, but are not limited to sodium borate, sodium hydrogen phosphate, sodium dihydrogen phosphate, mixtures thereof and the like. 
     Ink-receptive substrates useful in ink jet printing are well known to those skilled in the art. Representative examples of such substrates are disclosed in U.S. Pat. Nos. 5,605,750; 5,723,211; and 5,789,070 and EP 813 978 A1, the disclosures of which are hereby incorporated by reference. 
     In traditional ink compositions for ink jet printer systems the solvents used function mainly to dissolve dyes and binders in the inks. It has been found that there is an improved operation in a continuous ink jet printer system employing the method of asymmetric heating drop deflection when using an aqueous ink comprising a thermally-responsive material. This improvement consists of a sizeable increase in deflection angle when compared to ink without any such thermally-responsive material or when compared to water. 
     The following examples are provided to illustrate the invention 
    
    
     EXAMPLES 
     Example 1 
     Viscosity vs. temperature of the thermally-responsive solutions 
     The thermally-responsive solutions were formulated by dissolving PEO-PPO-PEO in an aqueous solution. A series of the PEO-PPO-PEO tri-block copolymers were obtained from BASF under the product trade name of Pluronic®. A Rheometrics ARES Fluids Spectrometer (Rheometric Scientific, Inc.) equipped with a corvette geometry, was used to measure the oscillatory shear properties of Pluronic® solutions. Dynamic viscosity was measured continuous as the temperature was ramped from 20° C. to 80° C. The typical ramp rate was 1° C./minute. The fluids were initially characterized at 20° C. in a continuous shear experiment covering a typical range of shear rates from 1 to 100/second. All were found to have low viscosity and Newtonian response. For the temperature scan experiments, a monitoring frequency of 10 radians/second was used. 
     The results are shown in the following tables: 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Viscosity of 15% Pluronic ® P85 Solution 
               
             
          
           
               
                   
                 Temperature (° C.) 
                 Viscosity (cP) 
               
               
                   
                   
               
             
          
           
               
                   
                 20 
                 4.3 
               
               
                   
                 25 
                 3.7 
               
               
                   
                 30 
                 3.3 
               
               
                   
                 35 
                 3.1 
               
               
                   
                 40 
                 2.6 
               
               
                   
                 45 
                 2.2 
               
               
                   
                 50 
                 1.9 
               
               
                   
                 55 
                 1.6 
               
               
                   
                 60 
                 1.4 
               
               
                   
                 62 
                 1.6 
               
               
                   
                 64 
                 2.9 
               
               
                   
                 66 
                 6.5 
               
               
                   
                 68 
                 18.5 
               
               
                   
                 70 
                 79.2 
               
               
                   
                 72 
                 368.4 
               
               
                   
                 74 
                 1130.3 
               
               
                   
                 76 
                 2150.5 
               
               
                   
                 78 
                 2857.4 
               
               
                   
                 80 
                 3036.9 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Viscosity of 25% Pluronic ® L62 Solution 
               
             
          
           
               
                   
                 Temperature (° C.) 
                 Viscosity (cP) 
               
               
                   
                   
               
             
          
           
               
                   
                 20 
                 7.9 
               
               
                   
                 22 
                 7.2 
               
               
                   
                 25 
                 6.8 
               
               
                   
                 28 
                 6.9 
               
               
                   
                 30 
                 7.3 
               
               
                   
                 32 
                 8.1 
               
               
                   
                 34 
                 10.0 
               
               
                   
                 36 
                 13.6 
               
               
                   
                 38 
                 23.7 
               
               
                   
                 40 
                 44.0 
               
               
                   
                 42 
                 83.4 
               
               
                   
                 44 
                 97.6 
               
               
                   
                 46 
                 177.7 
               
               
                   
                 48 
                 586.4 
               
               
                   
                 49 
                 2670.4 
               
               
                   
                 50 
                 3710.7 
               
               
                   
                 52 
                 4067.7 
               
               
                   
                 54 
                 3504.5 
               
               
                   
                 56 
                 3124.5 
               
               
                   
                   
               
             
          
         
       
     
     The above results show that the 15% Pluronic® P85 solution has a viscosity increase of more than 3 orders of magnitude when the temperature increases from 60° C. to 80° C., and the 25% Pluronic® L62 solution has a viscosity increase of almost 3 orders of magnitude when the temperature increases from 30° C. to 50° C. The results also demonstrate that these Pluronic solutions have viscosities of less than 10 centipoise at 20° C. and are thermally-responsive. 
     Example 2 
     Pluronic® P85 and Pluronic® L62 vs. water 
     In the above-referenced U.S. Pat. No. 6,079,821, a print head with approximately 12 μm diameter nozzle was fabricated with a heater (resistance of about 380 ohms) surrounding one-half of the nozzle perimeter. An ink reservoir and pressure control was used to control the pressure of the ink stream. A fast strobe and a CCD camera were used to freeze the image of the drops in motion. 
     A heater power supply was used to provide a current pulse train to heater resulting in asymmetric heating of the fluid stream. The ink reservoir was first filled with water and a pressure of 135.0 kPa was applied forming a fluid stream. A series of 10 μs duration pulses at a repetition rate of 50 KHz was applied to the heater causing the stream to break into a series of regular drops and to cause the drops to deflect. 
     The resulting deflection angle was measured as a function of applied voltage. The deflection angle for water reached approximately 0.6 degrees at an applied voltage of 4.8 V. 
     The experiment was repeated filling the ink reservoir with an aqueous solution of 15% Pluronic® P85. The deflection angle at the same applied voltage is approximately 5.2 degrees, almost 9 times the deflection angle of water. 
     The experiment was then repeated filling the ink reservoir with an aqueous solution of 25% Pluronic® L62. The deflection angle at the same applied voltage is approximately 3.5 degrees, almost 6 times the deflection angle of water. 
     The above results show that for all applied voltages, the Pluronic® materials provide a larger deflection angle than water. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.