Abstract:
The present invention is directed to a method for debubbling an ink. The method comprises the steps of: providing an ink having an entrained gas; providing a membrane contactor comprising a plurality of integrally asymmetric hollow fiber microporous membranes; a membrane defining within the contactor a lumen side and a shell side; providing a vacuum source; passing the gas entrained ink through the shell side of the contactor; applying the vacuum source to the lumen side of the contactor; and debubbling the gas entrained ink across the membrane.

Description:
FIELD OF THE INVENTION 
     This invention is directed to a method of debubbling an ink by using a membrane contactor. 
     BACKGROUND OF THE INVENTION 
     It is known to use hollow fiber membrane contactors to degas liquids. See, for example, the LIQUI-CEL® SemiPer™ membrane contactor commercially available from Celgard Inc. of Charlotte, N.C. This contactor utilizes a homogeneous, nonskinned, symmetric, polypropylene microporous hollow fiber membrane coated with a fluoropolymer and has been used to remove gases from photoresist developer solutions, lithographic printing plate solutions, and photographic film and paper emulsions. In this contactor, the foregoing liquids flow over the exterior surfaces of the hollow fibers. 
     Inks, for example, inks for ink jet printers, are sensitive to bubble formation. Formation of the bubbles, as the ink is discharged, can be detrimental to, among other things, quality printing applications or cartridge filling operations. See, for example, European Publication 1,033,163, Paragraph 0014, which is incorporated herein by reference. 
     Several membrane-based solutions have been proposed for bubble-in-ink problems. See, for example, Japanese Kokai&#39;s 517712; 10-60339; 10-298470; European Publications 1,033,162; 1,052,011; and U.S. Pat. No. 6,059,405. Also, please note European Publication 1,033,162, Paragraph 0007 which categorizes additional techniques for removing dissolved gases from chemical liquids by use of a membrane. 
     Japanese Kokai 5-17712 discloses the use of membranes made from polyethylene, polypropylene, poly(tetrafluoroethylene), polystyrene, or polymethyl methacrylate resins (Paragraph 0008), and the ink flows on the lumen side of the membrane (Paragraph 0007). 
     Japanese Kokai 10-60339 discloses the use of membranes made from a fluororesin (claim 2), and the ink flows on the lumen side of the membrane (abstract). 
     Japanese Kokai 10-298470 (and its related case European Publication 1,052,011) discloses the use of composite (or conjugate or multi-layered) membranes with porous and nonporous layers, and suggests, among other things, the use of polymethylpentene (or PMP or poly(4-methylpentene-1)) (Paragraphs 0018-0020), and the ink flows on the lumen side of the membrane (abstract). 
     European Publication 1,033,162 discloses the use of composite membranes, with porous and nonporous layers and suggests, among other things, the use of PMP (Paragraphs 0026 and 0048) for both layers, and the ink flows on the lumen side of the membrane (Paragraph 0054). 
     U.S. Pat. No. 6,059,405 discloses the use of a membrane, a hollow fiber membrane, and the ink flows on the lumen side of the membrane (column 3, lines 55-65). 
     While each of the foregoing had a measured success in accomplishing the debubbling goal, there is still a need for a method of removing entrained gases from inks in a simple and cost effective manner. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method for debubbling (or degassing) an ink. The method comprises the steps of: providing an ink having an entrained gas; providing a membrane contactor comprising a plurality of integrally asymmetric hollow fiber microporous membranes; a membrane defining within the contactor a lumen side and a shell side; providing a vacuum source; passing the gas entrained ink through the shell side of the contactor; applying the vacuum source to the lumen side of the contactor; and debubbling the gas entrained ink across the membrane. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. 
     FIG. 1 is a schematic illustration of an ink debubbling system. 
     FIG. 2 is a schematic illustration of the first embodiment of a membrane contactor made according to the instant invention. 
     FIG. 3 is a schematic illustration of a second embodiment of the membrane contactor. 
     FIG. 4 is a schematic illustration of a third embodiment of the membrane contactor. 
     FIG. 5 is a graph illustrating the performance of the CELGARD SemiPer contactor to the instant invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein like numerals indicate like elements, there is shown in FIG. 1 an ink debubbling system  10 . Ink debubbling system  10  comprises an ink reservoir  12 . A membrane contactor  14  is in fluid communication with the reservoir  12 . An end use application  16  is in fluid communication with membrane contactor  14 . End use application may be, but is not limited to, an ink jet printing head (thermal or piezoelectric), an ink cartridge filling station, or the like. 
     Ink, as used herein, is a fluid containing pigments or dyes. Inks, preferably, have a surface tension less than water at room temperature (i.e., about 72.75 dynes/cm at 20° C. and 71.20 dynes/cm at 30° C.). These inks are, preferably, used in computer printers or other ink jet type printers. Such inks, preferably, have a viscosity of 0.8 to 10 centipoises (CPS), a specific gravity of 0.7 to 1.5 grams per milliliter (g/ml), and a surface tension of 20 to 40 dynes per centimeter (dynes/cm). 
     The membrane contactor  14 , which is discussed in greater detail below, is an external flow, hollow fiber membrane module. Hollow fiber membrane contactors are known. For example see: U.S. Pat. Nos. 3,228,877; 3,755,034; 4,220,535; 4,940,617; 5,186,832; 5,264,171; 5,284,584; 5,449,457, each is incorporated herein by reference. The membrane contactor  14  has a lumen side and a shell side. The lumen side, also known as the internal side, is defined, in large part, by the lumen of the hollow fiber. The shell side, also known as the external side, is defined, in part, by the external surface of the hollow fiber. The ink travels through the shell (or external) side, while the vacuum (or vacuum and sweep gas) is applied to the lumen (or internal) side. Thereby, entrained gases from the ink pass from the shell side through the membrane to the lumen side. The contactor  14  is made of components that are inert to or non-reactive with the ink (or other liquid). Preferably, these components are plastic, but metals may be used. 
     The membrane is preferably a semi-permeable, gas selective, heterogeneous, integrally asymmetric, and liquid impermeable membrane. The membrane has a permeability of less than 100 Barrers (10 −8  standard cm 3 .cm/sec.cm 2 .cm(Hg)). The membrane preferably has an active surface area of 0.1 to 20 meters 2 . The membrane is, preferably, a skinned membrane and the skin is on the shell side. The membrane is, preferably, a single layer membrane (e.g., not a composite or multi-layered membrane) and is made from a homopolymer of polymethylpentene. For example, see U.S. Pat. No. 4,664,681, incorporated herein by reference. 
     Referring to FIG. 2, ink  22  enters contactor  14  via ink inlet  24  of core tube  26 . Core tube  26  includes a perforated  28  area immediately ahead of block  30 . Ink  22  travels through the inlet  24  of core tube  26  and exits tube  26  via perforations  28  when it is diverted by block  30 . Ink  22  then travels over the exterior surfaces of hollow fibers  34 . Ink  22  re-enters core tube  26  via perforations  28  on the other side of block  30  and exits tube  26  via ink outlet  32 . The hollow fibers  34  surround core tube  26  and are maintained generally parallel to tube  26 &#39;s axis via tube sheets  36 . Hollow fibers  34  extend through tube sheet  36  and are in communication with headspaces  38  on either end of contactor  14 , so that vacuum  44  drawn at ports  40  and  42  is in communication with the lumen side via headspaces  38 . Port  40 , for example, may also be used to introduce a sweep gas, which facilitates entrained gas removal. 
     Referring to FIG. 3, contactor  14 ′ is the same as shown in FIG. 2 but for a flow diverting baffle  46  located within the shell side, and port  40  has been moved. The baffle  46  is added to promote distribution of ink over all exterior surfaces of the hollow fibers  34 . Port  40  is moved to illustrate the non-criticality of port location. 
     Referring to FIG. 4, contactor  14 ″ differs from contactors  14  and  14 ′ by moving ink outlet  32  from the terminal end of core tube  26  to the contactor shell, as illustrated. Vacuum  44  is in communication with headspace  38  which, in turn, is in communication with the lumens of hollow fibers  34 . The second headspace illustrated in the previous embodiments has been eliminated. Ink  22  enters ink inlet  24  of core tube  26 . Ink  22  exits tube  26  via perforations  28 , travels over the exterior surfaces of hollow fibers  34 , and exits the shell side via outlet  32 . Outlet  32  may be placed at other locations on the exterior of the contactor so that it maintains communication with the shell side. 
     In operation, entrained gases, which form bubbles, are removed from the ink by a concentration difference across the membrane, that is by diffusion. Vacuum, ranging from 25 to 200 torr, is placed on lumen side of the membrane, and the gas entrained ink is in contact with the shell side (or exterior surface) of the membrane. The concentration (partial pressure of the gas) difference drives the gas from the ink on the shell side, through the membrane to the lumen side. Furthermore, by routing the ink through the shell (or exterior) side, versus the lumen side, the pressure drop of the ink through the contactor is greatly reduced. This is because passage through the lumens provides a much greater resistance to flow than the shell side space. In FIG. 5, the performance of a contactor according to the present invention is compared to CELGARD&#39;s SemiPer contactor. The graph illustrates ‘Dissolved Oxygen (DO) Removal Efficiency’ (%) as a function of water flow rate (liters/minute) at 20° C. and 35 torr of vacuum. Water was used, instead of ink, but contactor performance is deemed analogous to the foregoing inks at the stated conditions. The upper curve represents performance of the instant invention (2.5″ diameter), and the lower curve represents performace of the SemiPer contactor (2.5″ diameter). 
     The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.