Compositions: coating or plastic – Coating or plastic compositions – Marking
Reexamination Certificate
1998-03-31
2002-04-30
Bell, Mark L. (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Marking
C106S031600
Reexamination Certificate
active
06379440
ABSTRACT:
This invention relates to ink jet printer dispersion inks.
In ink jet printing, a fluid ink is forced under pressure, and sometimes at elevated temperature, through a very small nozzle in a printing head. In one kind of printer, known as a “continuous” printer, ink droplets which are produced continuously are passed through a charging area where individual droplets receive an electrical charge in response to a signal and are directed towards a substrate to be printed. The droplets then pass through an electrical field causing them to be deflected by an amount which is dependent on the intensity of the charge and the field. Droplets not required to form print on the substrate are directed to a by-pass gutter. Inks for use in such printers need to be conductive.
In another kind of printer, known as a “drop-on-demand” (DOD) printer, the ink droplets are expelled from the nozzle of a print head only when required during the printing process. Drop-on-demand printers can use an electrostatically accelerated ink jet or droplet sequences ejected by pressure impulse actuation. In the latter kind of DOD printer, each drop of ink is individually ejected from a nozzle by means of pressure pulses induced e.g. by use of a piezo-electric actuator acting on the ink in the channel supplying the nozzle or by generation of a vapour bubble in response to a thermal pulse.
In general, there are two classes of ink for use in such ink-jet printers, namely solvent inks in which the colourant, usually a dye, is dissolved in a solvent, and dispersion inks wherein the colourant, i.e. pigment, is dispersed in a diluent. The present invention is concerned with the latter class.
One particular problem that is encountered with continuous printing with an ink jet printer is that a small amount of the tail of the droplet being ejected tends to separate from the droplet and collect on the surface of that part of the area of the nozzle plate immediately surrounding the nozzle. In general, provided the surface energy of the plate is sufficiently low this ink deposit will simply recede into the nozzle and be ejected as part of a later droplet; however as the frequency of droplet ejection is increased, there is less opportunity for the ink to recede before ejection of the next droplet and a pool of ink commences to form on the surface of the nozzle plate the size of which can increase until it reaches a level at which it, or airborne dust or other contaminant which is attracted to the thus- wetted nozzle plate, interferes with droplet ejection and prevents reliable operation of the printer. The problem is further exacerbated by the trend towards increasing the number of nozzles per unit area of the nozzle plate and thus reducing internozzle spacing. Moreover any residue of ink on the nozzle plate surface can be disadvantageous where different coloured inks are being used because of the risk of contaminating one colour with another. It thus becomes necessary from time to time to clean the face of the nozzle plate, e.g. by wiping. This is undesirable because it means interrupting the printing run and because repeated wiping of the nozzle plate face can damage its surface.
Improved treatment of the relevant part of the surface of the nozzle plate of the printhead to reduce its surface energy, e.g. by provision of a so-called non-wetting coating e.g. of fluorocarbon or fluorosilane, can substantially reduce the problem and increase the duration of acceptable operation before cleaning or replacement of the nozzle plate is required; however, with commercial demands for ever increasing reliability and reduced down time for servicing, still further improvement would be welcomed and therefore the applicants have addressed not only the nature of the surface of the printhead nozzle plate but also the nature of the ink.
One class of inks which has proved particularly attractive for their print quality and performance is dispersion inks. Of particular interest are those which employ an ionic dispersant, more preferably a macromolecular polyionic dispersant, to improve the stability of the disperse phase. Ionic dispersants, as the term is used herein, describes dispersants comprising molecules having acid or basic groups, and macromolecular polyionic dispersants, as the term is used herein, comprise large molecules, e.g. oligomers or polymers, sometimes of imprecisely known size or structure, carrying a large number of either acidic or basic groups, eg. in the form of repeating units having acid or basic groups in the polymeric or oligomeric structure.
Ink jet printers employing dispersion inks based on ionic dispersants can produce excellent print reliability for substantial periods; however, a still further improvement in their performance, and in particular the duration of the periods of continuous printing that can be achieved before cleaning, eg. wiping, or replacement of the nozzle plate of the ink jet print head, would be desirable.
We have now found in accordance with the present invention that such an improvement may be obtained by including in the ink a non-destabilising amount of a compound (hereinafter referred to as a neutraliser) containing at least one group capable of neutralising the acidic or basic groups, respectively, of the dispersant. Moreover, when using the ink in an ink jet printer head of a drop-on-demand printer, greater negative ink supply pressure can be withstood without ingesting air and greater short term positive pressure surges, eg. due to acceleration or deceleration of the print head on ashuttle, can be tolerated without flooding.
As the amount of dispersant included in a dispersion ink is increased from zero, we have found from earlier work that the viscosity of the ink decreases until it reaches a minimum and then commences to increase again and that the optimum amount of dispersant from the point of view of overall properties of ink is approximately that which results in the minimum viscosity or slightly in excess thereof; e.g. from 100% to 200% of that amount, and more generally from 100% to 150% of that amount. This generally equates to about 0.1 to 1, more usually about 0.1 to 0.7 part, per part of pigment, by weight. While not wishing to be bound by this theory, it is believed that at this concentration of dispersant, the acidic or basic groups provided by the dispersant are in excess of those required to stabilise the dispersion of pigment in the diluent and that the excess increases the ability of the ink to wet a surface. In any event, it has been found that adding a small proportion of a compound containing a moiety or moieties capable of neutralising the acidic or basic groups of the dispersant increases the speed with which the ink deposit left by the droplet on the surface of the nozzle plate recedes into the nozzle and thus reduces the risk of forming a pool on the surface of the nozzle plate. A direct indication of this property can be obtained by measuring the dewetting or receding meniscus velocity (RMV) of the ink on a surface.
For a discussion of dewetting velocity and its measurement, reference is made to the article by Redon et al in Physical Review Letters, Vol 66, No. 6, Feb. 11, 1991, pages 715-718.
While the invention will be illustrated by reference to non-aqueous inks it is believed that it is generally applicable to both aqueous and non-aqueous inks. Particular attention, however, has been given to inks wherein the diluent is non-aqueous or substantially non-aqueous (that is to say, it contains less than 2% water by weight), and has a low polar solubility parameter, e.g. of not greater than 7.0 MPa
½
. References herein to polar solubility parameter are to the values obtained according to the method of Hansen, C. M. and Skaarup, K., Journal of Paint Technology, 39 No. 51, pp. 511-514 (1967) as detailed by Patton, T.C. “Paint Flow & Pigment Dispersion” 2nd Ed., Wiley Interscience, 1979. Particular examples of non-aqueous diluents with a low polar solubility parameter eg. a polar solubility parameter of 7.0 MPa
½
or less, are non-polar organic solvent
Griffin Mary Catherine Ambrose
Tatum John Philip
Woods Jill
Bell Mark L.
Faison Veronica F.
Marshall Gerstein & Borun
Xaar Technology Limited
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