Stock material or miscellaneous articles – Ink jet stock for printing – Image viewable from either side
Reexamination Certificate
2000-05-18
2003-12-02
Schwartz, Pamela R. (Department: 1774)
Stock material or miscellaneous articles
Ink jet stock for printing
Image viewable from either side
C428S032250, C428S032290, C428S032300, C428S032340, C428S032380, C427S146000
Reexamination Certificate
active
06656545
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a low pH coating composition for a recording medium especially adapted for ink jet printing, and method for making the same.
Ink jet printers employ a plurality of jets connected to a supply of liquid based ink. The jets have small openings and can be energized to emit uniform liquid droplets of ink in a continuous stream upon demand. The jets are controlled to print characters or images on a moving flat surface, usually a sheet of paper.
In order to improve print quality and drying time, many proposals have been made for coatings on paper to improve ink reception. For example, it is well known to coat paper with various absorptive pigments, binders and sizing agents. An aqueous suspension of these agents are applied to a paper or other web using conventional coating methods.
An ideal ink receiving coating on paper will allow rapid absorption of the liquid component of the ink, which correlates to a rapid ink drying time, while preventing diffusion of the ink colors into the paper substrate. At the same time, the coating should cause the ink pigment or dye to be fixed on the sheet surface in the form of well defined dots of uniform size and shape. The coating, upon drying, should be non-dusting and non-toxic. A correct balance of these properties is very difficult to achieve, especially at higher printer resolutions and smaller dot diameters.
While a variety of acceptable coatings can be devised in theory, it is also imperative for the sake of economy that the coatings are capable of being applied uniformly to a base sheet at a high rate of speed using conventional coating equipment. Many of the known absorptive pigments, such as those based on powdered forms of silica, cannot be employed because an excessive amount of binder is required for processing at the solids content necessary to achieve the desired minimum coat weight. The suspensions become too thick or dilatant to allow pumping and uniform application using a conventional paper coater such as a blade coater. If lower binder levels are employed, this may also result in excessive dusting in the finished product.
SUMMARY OF THE INVENTION
An ink receptive coating is applied to one or both sides of a substrate. The substrate may be optionally precoated with a size solution before the ink receptive coating is applied. The ink receptive coating is an aqueous suspension comprising silica pigment dispersed at low pH with a cationic fixing agent. Coatings with pH values of 4.0 to 7.5 have been found to give improved ink holdout over conventional coating compositions having pH values in the range of 8.0-9.0. The improved ink holdout results in better color saturation, reduced bleed through, and better overall print quality.
Silica gels are preferred pigments. Silica gels are commercially available that have acceptably low pH values and high void volumes desirable for ink absorption.
Cationized pigments hold more dye at the surface than anionic pigments. Accordingly, the coating composition of the invention preferably includes a cationic fixing agent, such as polycationic quaternary ammonium polymer, to cationize the pigment.
The coating composition of the invention includes a binder. Although several suitable binders could be used, low molecular weight, partially hydrolyzed polyvinyl alcohol is preferred.
Alumina may be added to the coating composition as a whitening agent and to improve rheology. For applications where an FDA approved coating is not required, a fluorescent whitening agent may be added.
Thus, broadly the coating composition of the invention has a pH value of 4.0 to 7.5, and comprises, by one dry parts by weight:
50-100
parts absorptive silica pigment
0-50
parts alumina
30-50
parts polyvinyl alcohol
4-10
parts cationic fixing agent
The preferred embodiment comprises, approximately, in bone dry parts by weight:
75
parts silica gel
25
parts alumina trihydrate
40
parts low molecular weight, partially hydrolyzed
polyvinyl alcohol
10
parts polycationic quaternary ammonium polymer
2
parts fluorescent whitening agent
The coated ink jet medium of the present invention allows ink jet printing over a wide range of resolution with precise control of dot size (freedom from print mottle), dot size uniformity, and dot shape.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The coating composition of the invention comprises an aqueous suspension of an absorptive silica pigment, a binder and a cationic fixing agent, dispersed at an acidic pH. Although the precise technical effect is not fully known, it is believed that the acidic pH of the coating composition enhances the cationic function. The coating composition of the invention is applied to a substrate, for example paper. When an ink jet ink is applied to the coated paper, the absorptive silica particles absorb the ink solvent (water) leaving the dyes on the surface of the particles, and the cationic agent assists in fixing the dye on the surface. This results in rapid dry time and improved ink hold out and color density.
The pH value of the composition is between 4.0 and 7.5, but preferably in the range of 4.5 to 5.5. Coatings at these pH values have been found to have improved holdout of the ink jet ink dyes on the paper surface as compared to conventional alkaline coating compositions having pH values of 8.0 to 9.0. The improved holdout results in enhanced color saturation and better overall print quality. It also reduces dye penetration to the opposite side of the sheet. The technical cause of the improvements observed in the low pH coating compositions of the invention is not precisely known, however.
Tests have been conducted on coating formulations of varying pH values. Coating compositions were prepared in accordance with the invention using pigments and binders selected to result in low pH values. Acid was added to lower pH values. A base, sodium hydroxide, was added to increase pH. It was found that when the pH of the composition is reduced to a value below about 4.0 the composition becomes unstable—the ingredients become less compatible and viscosity increases with time. As the pH is increased above about 5.5, to beyond a pH value of 7.5, the dispersion becomes similarly unstable, the viscosity of the composition becoming undesirably high. Accordingly, it has been found that a pH value in the range of about 4.5 to about 5.5 is preferable.
Silica type pigments are well suited for ink jet printing due to their high absorption properties. It is believed that absorptive silica particles act like a “molecular sieve,” whereby the primary particle absorbs the aqueous ink medium, but holds the dye on the surface of the particle. This results in rapid dry time and improved ink hold out.
When evaluating the characteristics of voids in silica pigments, it is important to distinguish between primary particles, secondary or agglomerated particles and associated voids. The primary particles of some silicas agglomerate, that is, groups of primary particles cluster to form secondary particles. Other silicas lack agglomerates. Further, the structure of the primary particle may vary depending on the manufacturing process. Precipitated and gel processes produce porous primary particles, while the fumed silica process produces relatively small, glassy, non-porous particles. As a result, different void structures are created. Depending on the nature of the silica, there may be voids between secondary particles, voids between primary particles within the secondary particle structure, and voids within the primary particle. For purposes of the present application, the term “absorptive silica” means a silica where the primary particles are porous and capable of absorbing water. Also for the purposes of this application, the term “non-agglomerated” refers to a silica substantially free from agglomerates, i.e., the primary particles do not exhibit a tendency to cluster into secondary particles. Gel silicas are preferred as non-agglomerated, absorptive silicas, but some precipitated and hybrid silicas may also be non-agglomerated and abso
Schliesman Leonard J.
Spreda Karen K.
Tritz Leland O.
Greer Burns & Crain Ltd.
Schwartz Pamela R.
Stora Enso North America Corporation
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