Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2000-02-23
2002-08-13
Green, Anthony (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S031600, C106S471000, C106S494000, C106S496000, C106S498000
Reexamination Certificate
active
06432192
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns methods for synthesizing and purifying organic pigments. The invention is further directed to methods for preparing inks containing pigments having fine particle size and very low levels of impurities.
BACKGROUND OF THE INVENTION
Printing inks for ink jet printers have become an important segment of the ink market. In general, the ink of ink jet printers is emitted through one or more small orifices or nozzles. In continuous ink jet printers, a continuous stream is ejected under pressure through the nozzles and broken up into electrically charged droplets at a fixed distance from the orifice. At the break-up point, the electrically charged ink droplets are passed through an electrical field that adjusts the trajectory of the droplets according to digital data signals to direct the droplets either to the printing medium to create an image or to a gutter for collection and re-circulation. In drop-on-demand ink jet printers, typically used for desktop printing with personal computers, the ink is emitted from the nozzles in droplets as needed to create an image on the printing medium. Desktop printers are commonly thermal ink jet, or bubble jet, printers. The thermal ink jet printer has an ink filled channel with a nozzle at one end and a heat generating resistor near the nozzle. The computer sends an signal that generates an electric current pulse in the resistor, heating the ink in the immediate vicinity of the resistor. The heat evaporates the ink to create a bubble. The ink at the orifice is forced out of the nozzle as a propelled droplet at high speed as the bubble expands.
Ink jet inks have historically been based on dyes, which are soluble colorants, because pigmented inks have had a tendency to clog the jet nozzles, which then must be cleaned before the printer can operate properly again. Dyes, however, have the shortcoming of being less colorfast than pigments. Printed dye-based aqueous ink jet inks can smear if they get wet. Thus, pigments would be desirable for ink jet inks, but the pigments must have very small particle sizes, typically less than 1 micron and usually less than about 0.4 micron, so that the pigment will not clog the ink jet nozzles.
Synthesis of pigments with very small particle sizes for ink jet printer inks has presented some problems. Syntheses of many organic pigments include a coupling step in a dilute aqueous medium to produce a slurry of the pigment product, which is followed by a step of filtering the slurry in a filter press or with a basket centrifuge to purify and concentrate the pigment by removing the water phase along with dissolved salt impurities. Very small pigment particles, however, are difficult to filter by conventional processes. For example, in filter presses such fine pigment particles “blind” the filter media; in other words, the fine pigment particles block the pores of the filter and prevent water from passing through the filter. Because the traditional filtering processes work so poorly for very small pigment particles, pigments for ink jet printer inks are presently manufactured as a flocculated pigment that has a larger particle size. The flocculated pigments can be filtered by conventional means but must then be dried, mechanically ground to again obtain the very small size needed for ink jet printer inks, and dispersed in a suitable ink vehicle. The added steps, particularly the mechanical grinding process, add considerable cost and, moreover, produce a pigment with a broader particle size distribution than is desirable.
Japanese application JP 10 183,008, published Jul. 7, 1998, describes purification of aqueous colorants using a charged, mosaic membrane containing particulate cationic and anionic polymers (i.e., an ion exchange membrane). Ion exchange membranes are costly, however, and must periodically be “recharged” by replacing the undesirable ions removed by the filtration. These shortcomings result in increased cost and decreased manufacturing flexibility.
It would be desirable to have a method for preparing synthetic organic pigments with the fine particle size and high purity necessary for ink jet inks that would overcome the problems of the present methods of synthesizing the pigment, flocculating the pigment for filtering, filtering the pigment, drying the pigment, grinding the pigment to again obtain a fine particle size, and re-dispersing the pigment.
The present invention concerns purification of aqueous pigment compositions with membrane separation processes. Membrane separation processes, also referred to as cross flow filtration, include (in increasing pore size order) nanofiltration, ultrafiltration, and microfiltration. Membrane separation processes have previously been used to purify dye solutions. One example of the dye solution purification process is described in Rebhahn et al., U.S. Pat. No. 4,560,746, incorporated herein by reference. The '746 reference discloses that purification of a dye requires a membrane having a nominal pore size of 5-15 Angstroms, falling in the range of nanofiltration. The dye solution is carefully pre-filtered to remove all particulate matter before it is introduced to a nanofiltration membrane so that the pores will not become blocked.
Japanese application 11 166,131, published Jun. 22, 1999, describes preparing a liquid dispersion of an oxidation-treated (acidified) carbon black, subjecting the dispersion to ultrafiltration and dialysis, and then further treating the dispersion with an ion exchange membrane. Carbon blacks are prepared by pyrolyzing (ashing) carbonaceous materials such as acetylene. The carbon black is obtained as a powder and may be surface treated, for example oxidized as described in the JP '131 reference. The JP '131 reference does not discuss pigment prepared by organic synthesis or methods for methods for rendering such organic synthetic pigments suitable for ink jet inks.
SUMMARY OF THE INVENTION
The present invention provides a simplified method for producing aqueous synthetic organic pigment dispersions for use in ink jet inks. Unlike previous methods, the invention is effective in preserving a narrow particle size distribution of the pigment as synthesized. In a first embodiment, the present invention provides a process for purifying pigment that is particularly suited to pigment having a very small particle size. Preferably, for ink jet applications, the pigment has a maximum particle size of about 400 nm. That is, the particle size distribution is such that 100% of the particles have a particle size of about 400 nm or less. For other kinds of inks, such as flexographic inks, the maximum particle size may be significantly greater, such as about 2 microns. The particle size distribution of the pigment should be narrow.
In a first step of the inventive method, a pigment is first synthesized in an aqueous medium and, in a second step, the aqueous dispersion of the synthesized pigment is purified using a cross flow filtration method in which the average pore diameter is from about 3 nanometers to about 3000 nanometers. The pigment is preferably prepared under conditions that preferably produce average particle sizes of up to about 1 micron. The aqueous purified pigment dispersion product preferably has a particle size range in which the minimum particle size is at least about 0.01 micron and the maximum particle size is up to about 1 micron. In a more preferred embodiment, the minimum particle size is at least about 0.03 micron and the maximum particle size is up to about 0.4 micron. At least one impurity, which may be a by-product of the synthesis, an unreacted starting material used for the pigment synthesis, or another impurity, is then removed from the aqueous pigment reaction mixture by the cross flow filtration.
The cross flow filtration of the inventive process, in which the average pore diameter is from about 3 nanometers to about 3000 nanometers, is often referred to as ultrafiltration (pore diameters of perhaps 3 nanometers up to about 300 nanometers) and microfiltration (pore
Cook Wayne L.
Gebler Douglas P.
Pratt Norman E.
Flint Ink Corporation
Green Anthony
Harness & Dickey & Pierce P.L.C.
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