Coating processes – Foraminous product produced – Microporous coating
Reexamination Certificate
2001-03-12
2004-02-10
Pianalto, Bernard (Department: 1762)
Coating processes
Foraminous product produced
Microporous coating
C427S256000, C427S336000, C427S352000, C427S373000, C427S385500
Reexamination Certificate
active
06689421
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to microporous films and methods of manufacturing thereof, and to image receptors which are particularly suitable for use in ink-jet imaging using either aqueous or nonaqueous inks.
BACKGROUND OF THE ART
Microporous films and membranes are well-known materials, and may be regarded as polymer films in which a significant proportion of the volume of the film comprises microscopic pores and voids. The pores and voids normally connect with each other and with at least one surface of the film. Microporous films find use in a wide range of fields, including filtration, reverse osmosis, battery separators, breathable fabrics, surgical dressings and, when the pores have been filled with a suitable substance, sustained-release systems. Recently, there has been growing interest in the use of microporous films as printable surfaces, and in particular as receptors for ink jet printing.
The manufacture and uses of microporous films are reviewed, for example, in
Synthetic Polymeric Membranes
(R. E. Kesting), McGraw-Hill, 1971, and described in numerous patents.
In a widely-used manufacturing process, a mixture comprising a thermoplastic polymer and a particulate filler and/or a second polymer, which is incompatible with the first polymer, is melt extruded as a heterophasic film, then stretched in at least one direction so as to form voids at the phase boundaries.
This method is described, for example, in U.S. Pat. Nos. 3,903,234; 3,967,978; 4,472,328; 4,585,604; and 4,613,643.
In another widely-used process, a blend of two incompatible polymers, or a mixture of a polymer and a low molecular weight additive, often in combination with a filler, is melt extruded as a film and then washed with a suitable solvent so as to remove one of the polymers or the additive, thereby creating voids in the film. This is described, for example, in U.S. Pat. Nos. 3,351,495; 3,729,332; 3,862,030; 4,237,083; 4,335,193; 5,326,391 and WO 97/20885. A combination of washing and stretching may be employed, e.g., as disclosed in U.S. Pat. Nos. 4,833,172 and 4,861,644.
Methods involving casting of the film from a solution or dispersion are also known. For example, U.S. Pat. No. 5,374,475 and EP 555,398 disclose the formation of microporous films by coating dispersions of polymer microparticles on a support, followed by drying in a manner which does not cause coalescence of the particles. EP 156,532 and U.S. Pat. No. 5,374,475 disclose formation of microporous films by the process known as polymer phase inversion. In this method, a polymer is dissolved in a solvent mixture comprising a good solvent for the polymer and a less-volatile poor solvent for the polymer, then coated on a support and dried. Voids are formed in the resulting film as the residual poor solvent is driven off.
Formation of microporous polyester membranes by “polymer assisted phase inversion” is described in the above-referenced book
Synthetic Polymeric Membranes
and in U.S. Pat. No. 3,957,651. In this process, a polyester derived from an aromatic dibasic acid and an “assisting” polymer are dissolved in a mutual solvent, then coated on a support and dried. The resulting film is washed in a solvent capable of selectively dissolving the assisting polymer, thereby forming voids in the film. Only expensive and/or noxious fluorinated solvents such as hexafluoroisopropanol and trifluoroacetic acid are said to be suitable as the mutual solvent.
There is a growing interest in the use of ink jet printing for the output of high quality images, e.g., of a quality rivalling conventional photographic images. In ink jet printing, tiny droplets (e.g., of a volume measured in picolitres) of colored ink are expelled from an array of nozzles onto a receptor, in accordance with digitally-stored image information. The process may be repeated several times using inks of different colors to build a full color image, and has the potential to provide images of photographic quality in a cost-effective manner using relatively simple apparatus suitable for use in the home or office environment, without the need for light-sensitive materials or the use of processing chemicals. However, the quality and throughput obtainable is largely dependent on the properties of the receptor on which the image is printed. The jetting process generally demands the use of low viscosity inks with a low solids content, and so large volumes of liquid must be absorbed by the receptor, especially in the case of 4-color imaging. The liquid must be absorbed rapidly, to enable successive printing of the different colors without bleeding, and to provide an image that resists smearing, without extended drying times. Furthermore, the ink must not diffuse sideways after contacting the receptor, otherwise the resolution is degraded. Finally, so as to obtain bright, dense images, the dyes or pigments in the ink preferably should be retained at or near the surface of the receptor, but in a manner which does not render the image susceptible to physical damage.
Two techniques are currently employed in the art in pursuit of higher quality images. One is to jet a normal density ink and a low density ink (from separate printheads) so that the number of available grey levels is increased. The second is to jet multiple droplets of ink from a single nozzle into a particular pixel area in order to obtain multiple grey levels. A combination of the two techniques can provide even more grey levels, and hence even higher image quality. However, such techniques place great demands on the image receptor. Even larger volumes of ink must be absorbed, and the rate of absorption must be commensurate with the ever-increasing firing rate of commercial ink jet printheads, otherwise the image-forming process becomes limited by the absorption rate and capacity of the receptor.
Piezoelectric ink jet printheads are most suitable for multiple droplet jetting, owing to the superior control they afford over the droplet size and placement. In particular, the shared-wall type of piezoelectric ink jet printheads, as exemplified by the Xaarjet™ printheads supplied by Xaar Ltd., Cambridge, UK and described in U.S. Pat. Nos. 4,887,100 and 4,879,568, have advantages in this respect. Such printheads are not limited to the use of aqueous inks (which are used by most, if not all, of the commercially-available ink jet printers), and indeed many advantages are to be gained by the use of oil-based inks. These include the ease with which pigmented inks can be designed, since pigmented inks can provide a more accurate and stable color rendition than their dye-based counterparts.
Although a large body of literature exists on the design and manufacture of receptors for ink jet printing, there is still a need for improved receptors, suitable for use in high quality multilevel printing. In particular, Applicants are unaware of any published art addressing the particular demands of multilevel printing using oil-based inks.
In the context of conventional ink jet printing, many patents describe the use of microporous films as receptors. The pores of such materials potentially provide a means of absorbing substantial amounts of liquid, and at the same time channelling it downwards (away from the surface) rather than sideways. Patents disclosing microporous receptors include WO97/29916; WO97/20885; EP 555,398; EP 409,440; EP 156,532; and U.S. Pat. Nos. 3,729,332; 4,460,637; 4,481,244; 4,496,629; 4,780,356; 4,861,644; 5,002,825; 5,104,730; 5,326,391; 5,374,475; 5,445,868; 5,635,291 and 5,647,935. Many patents teach the use of a two-layer system in which an ink-absorbing layer is overlaid by an ink-transporting layer, one or both of said layers being microporous (see, for example, WO97/33758; Japanese Patent Publication Nos. 61-035276; 61-035277; 61-035278 and 05-051470; and U.S. Pat. Nos. 4,785,313; 4,832,984; 4,954,395; 5,027,131; 5,059,983; 5,275,867; 5,374,475; 5,463,178 and 5,605,750. A variety of different microporous layer compositions are disclosed, including many composed predominantly of inorganic mater
Hobson Rachel J.
Lindsay Alan J.
Parsons Keith P.
Patel Ranjan C.
Spence Graham
Faegre & Benson LLP
Kodak Polychrome Graphics, Inc.
Pianalto Bernard
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