Aqueous ink for ink-jet heads

An ink composition, a printer and a method for ejecting and aqueous fluid having a viscosity above about 1 Pa-sec at 25° C. from a fluid ejection head. The ink composition, printer and method include an aqueous fluid that contains an aqueous carrier component, solid particles ranging from about 8 wt. % to about 25 wt. % based on a total weight of the aqueous fluid, and a rheology modifier. A heater pulse signal applied to ejection heaters on a fluid ejection head for a period of time is sufficient to sputter fluid from fluid ejection nozzles associated with the ejection heaters and to shear the fluid thereby reducing the viscosity of the fluid from above about 1 Pa-sec to less than about 0.1 Pa-sec. A firing signal subsequently applied to the ejection heaters on the ejection head provides steady state fluid ejection from the ejection head.

TECHNICAL FIELD

The disclosure is directed to methods for reliably jetting fluids onto a substrate, into the atmosphere or into a gas, into a liquid, or onto a solid material and in particular to methods for improving the reliability of jetting fluidic quantities of fluids using fluid thermal jet heads.

BACKGROUND AND SUMMARY

Ink-jet technology using aqueous inks is very well understood when jetting fluids of that exhibit Newtonian fluid dynamic behavior. However, high solids containing fluid formulations are particularly troublesome during an initial fluid jetting startup after a period of non-ejection of fluid. For example, ink-jet inks may contain high density particles such as colorant and/or pigments which may settle from the bulk fluid quickly in the fluid ejection heads. Settling of such high density particles causes a concentration gradient which significantly increases the fluid density and viscosity in the microscopic flow features of a fluid ejection head.

Accordingly, such fluids will often fail to eject from fluid jet heads when the jet head ejection nozzles or jet heads are idle for a period of time, thus resulting in poor startup of fluid ejection, poor fluid jetting, and in the case of colorants and/or pigments, poor color consistency. While the bulk fluid may be stirred or agitated to increase the dispersion of particles in the fluid, such stirring or agitation is not effective for the flow feature areas and nozzles in the ejection head. Accordingly, the initial ejection of high solids containing fluids from an ejection head is a challenge.

By “high solids” is meant fluid formulations that contain from about 8 wt. % to about 25 wt. % solid particles. Such fluids may be used in ink formulations for cosmetic ink applications, for example, and thus may have a relatively high non-volatile particle settling rate over time and a viscosity of above about 1 Pa-sec at 25° C.

Embodiments of the disclosure provide an ink composition, a printer and a method for ejecting and aqueous fluid having a viscosity above about 1 Pa-sec at 25° C. from a fluid ejection head. The ink composition, printer and method include an aqueous fluid that contains an aqueous carrier component, solid particles ranging from about 8 wt. % to about 25 wt. % based on a total weight of the aqueous fluid, and a rheology modifier. A heater pulse signal applied to ejection heaters on a fluid ejection head for a period of time is sufficient to sputter fluid from fluid ejection nozzles associated with the ejection heaters and to shear the fluid thereby reducing the viscosity of the fluid from above about 1 Pa-sec to less than about 0.1 Pa-sec at 25° C. A firing signal subsequently applied to the ejection heaters on the ejection head provides steady state fluid ejection from the ejection head.

In one embodiment, there is provided an ink composition that includes water, pigment particles, and a rheology modifier. The ink composition has a pigment content ranging from about 8 wt. % to about 25 wt. % based on a total weight of the ink composition. The rheology modifier is an alkali swellable acrylic polymer emulsion having a solids content of about 30 wt. % and a pH below about 4 and the rheology modifier is effective to provide a pseudo-plastic aqueous ink formulation.

A further embodiment provides a printer that contains an aqueous ink composition having a viscosity of greater than about 1 Pa-sec at 25° C. The aqueous ink composition includes, an aqueous carrier component, solid color particles ranging from about 8 wt. % to about 25 wt. % based on a total weight of the aqueous ink composition, and a rheology modifier. A heater pulse signal applied to ejection heaters on an ejection head of the printer for a period of time is sufficient to sputter ink from ejection nozzles associated with the ejection heaters and to shear the aqueous ink composition thereby reducing the viscosity of the ink composition from above about 1 Pa-sec to less than about 0.1 Pa-sec at 25° C. A firing signal applied to the ejection heaters on the ejection head provides steady state ejection of the aqueous ink composition from the ejection head.

In some embodiments, the rheology modifier is an alkali swellable acrylic polymer emulsion having a solids content of about 30 wt. % and a pH below about 4. In other embodiments, the aqueous fluid or ink contains from about 0.2 to about 1.0 percent by weight of the rheology modifier based on a total weight of the aqueous fluid or ink. In some embodiments, the rheology modifier provides a pseudo-plastic ink-jet ink formulation that, upon shearing, has a reduction in viscosity from about 1 Pa-sec to less than about 0.1 Pa-sec at 25° C. Such fluids may be hereinafter referred to as “thixotropic” fluids. Such fluids may be characterized by a thixotropic index. The thixotropic index represents the degree of thixotropy and is determined by taking a ratio of static viscosity of the fluid to dynamic viscosity of the fluid.

In some embodiments, the heater pulse signal is at a frequency ranging from about 2 to about 5 KHz for a period of three to five seconds. In other embodiments, the heater pulse signal has a pre-heat pulse of about 200 to about 400 nanoseconds (nsec), a dead time of about 1200 nsec and a firing pulse ranging from about 900 to about 1000 nsec.

In some embodiments, the heater pulse signal is effective to provide a shear rate of the aqueous fluid or ink within flow features of the ejection head ranging from about 2 to about 3×106per second.

In other embodiments, fluid or ink ejection is initiated in the absence of a pre-heat pulse used to lower the viscosity of the fluid or ink.

The foregoing methods are particularly suitable for the initial ejection of fluids having a high solids content when the ejection head has been idle or unused for a period of time. Such fluids typically contain thickeners or emulsifiers that tend to keep the solids in suspension. However, such thickeners or emulsifiers tend to greatly increase the viscosity of the fluids while keeping the particles in suspension in the fluids. Accordingly, it is difficult to refill the ejection heads after long idle times due to the size of flow features in the ejection head and the high viscosity of such fluids. The foregoing embodiments enable start up and continued operation of ejection heads ejecting such fluids without the need for a preheat pulse or step that may be harmful to the fluids and may accelerate drying of the fluid in or on the nozzles causing plugged nozzles.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

A plan schematic view of a portion of a thermal fluid ejection head10is illustrated inFIG. 1. The ejection head10includes a silicon substrate12and a nozzle plate14attached to the substrate12. The substrate12may include a single fluid feed slot or multiple fluid feed slots16and18. A plurality of ejection devices, such as resistor heaters20are adjacent the slots16and18. Upon activation of the ejection devices20, fluids are ejected through nozzle holes22in the nozzle plate14.

A cross-sectional view, not to scale, of a portion of the thermal fluid ejection head10is illustrated inFIG. 2. The silicon substrate12includes a plurality of layers24on the device side thereof defining the plurality of heater resistors20. The nozzle plate14includes nozzle holes22, a fluid chamber28and a fluid channel30, collectively referred to as flow features, in fluid flow communication with the slot16for providing fluid to the heater resistor20.

A plan view, not to scale of the flow feature of a single heater resistor20and fluid chamber28is illustrated inFIG. 3. The fluid channel30providing fluid from the fluid feed slot16may include a column32and/or other pinch points34that may inhibit or reduce the flow of fluid to the fluid chamber28. Accordingly, for fluids that contain a high solids content, plugs of fluid may accumulate adjacent the columns32and/or pinch points34in the fluid channel30, in the fluid chambers28, and/or nozzles22preventing fluid from flowing into the fluid chamber28and being ejected through the nozzles22.

As shown inFIG. 4, a fluid having a solids content of 20 wt. % was compared to the same fluid containing a rheology modifier according to the disclosure. After 10 days, the solids concentration at the bottom, middle and top of a jar containing the fluids was determined.FIG. 4shows the results of the variation in solids content for a control fluid A (solid line) without the rheology modifier and a fluid B (dashed line) containing the rheology modifier. As shown byFIG. 4, fluid A had significant variation in solids content from top to bottom indicating that significant solids settling took place over the 10 day period. By contrast, fluid B had a relatively constant concentration of solids from top to bottom over the 10 day period. The rheology modifier that may be used to improve the suspension of solids in the fluid may be selected from a wide variety of rheology modifier or thickeners used for aqueous fluids including, but not limited to casein, alginates, modified cellulosic materials, acrylic polymers, maleic anhydride copolymers, polyamide resins, alkoxylated aliphatic amine diols, and alkali swellable acrylic polymer emulsions. A particularly suitable rheology modifier is the alkali swellable acrylic polymer emulsion sold under the tradename ACRYSOL EXCEL.

Since the flow features of a fluid ejection head10are in the micron range, solids settling over a period of time creates significant blockage in the ejection head10. Blockage of flow feature in the ejection head10reduces the fluid refill time for the fluid chambers28and thus reduces the ejection head frequency. A typical fluid ejection head10operates with a frequency of about 25 to 50 kilohertz or higher. However, pluggage of the flow features of the ejection head10may reduce the frequency to below acceptable limits.

A high solids fluid also has a higher viscosity than a conventional aqueous fluid due to the presence of the rheology modifier used to keep the solids in suspension. Accordingly, the viscosity of the high solids fluid may be more than 1 Pa-sec at 25° C. The higher viscosity of the fluid also reduces the flow rate of fluid through the flow features of the ejection head10thereby reducing ejection frequency and ejection head performance. Longer refill times for fluid to the ejection head10may result in misfiring from a nozzle, reduced fluid droplet volumes, low fluid ejection velocity, fluid droplet misdirection, and the like. The amount of a typical fluid droplet for an ink formulation may range from about 2000 picograms for color inks to about 16,000 picograms for black inks. Corresponding fluid droplet diameters may range from about 14 μm to about 29 μm. Other fluids may have droplet amounts above or below the foregoing amounts depending on the viscosity of the fluid.

Use of from 0.2 to 1.0 percent by weight such as from 0.2 to 0.6 percent by weight of the alkali swellable acrylic polymer emulsion in the aqueous fluids of the disclosure provide a composition having a thixotropic index of from about 1 to about 50 such as from about 5 to about 20. The thixotropic index of the fluid may be estimated from the graph inFIG. 5showing the viscosity at 25° C. of the fluid at shear rates ranging from 10−1per second to 103per second.

The viscosity characteristics of a silicone oil standard fluid is shown as line C inFIG. 6. Such a fluid has very little change in viscosity with shear rate. However, a fluid composition containing the rheology modifier according to the disclosure has a significant change in viscosity with shear rate as evidenced by line D inFIG. 6.

In view of the foregoing viscosity characteristics of a rheology modified fluid according to the disclosure, a procedure was developed to initiate flow of fluid in the flow features of a fluid ejection head10after particle settling in the flow features has occurred. According to the procedure, a heater pulse signal at a frequency ranging from about 2 to about 5 KHz is applied to the heater resistors20for a period of three to five seconds. This pulse signal has a pre-heat pulse of about 200 to about 400 nanoseconds (nsec), a dead time of about 1200 nsec and a firing pulse ranging from about 900 to about 1000 nsec. Since the rheology modified fluid is sensitive to shear as shown inFIG. 6, the bursts of energy provided by the heater pulse signal is effective to shear the fluid and thus cause a significant decrease in fluid viscosity. Accordingly, a shear rate for the aqueous fluid within flow features of the ejection head10ranges from about 2 to about 3×106per second in order to decrease the viscosity of the fluid. The foregoing heater pulse signal causes fluid to sputter from the nozzles22. Nozzle sputtering loosens up any viscous plugging material that may be in the flow feature areas of the ejection head10thereby starting flow of fluid through the flow features of the ejection head10. Once the plugging fluid has been loosed up by the foregoing procedure, continuous ejection of fluid from the ejection head may be obtained using a pulse train that includes a pre-fire pulse of 200 to 300 nsec, a dead time of 1200 nsec and a jetting pulse of 700 to 950 nsec.

The foregoing procedure is thus able to obtain thixotropic behavior for a stabilized fluid composition containing a high solids content so that the fluid exhibits pseudo plastic characteristics upon shearing. Since significant heating of the fluid is avoided by the foregoing procedure, deterioration of the fluid is avoided as well as drying of the fluids in the nozzles22.

It is contemplated, and will be apparent to those skilled in the art from the preceding description and the accompanying drawings, that modifications and changes may be made in the embodiments of the disclosure. Accordingly, it is expressly intended that the foregoing description and the accompanying drawings are illustrative of exemplary embodiments only, not limiting thereto, and that the true spirit and scope of the present disclosure be determined by reference to the appended claims.