Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2001-08-27
2004-03-30
Shosho, Callie (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S503000, C106S483000
Reexamination Certificate
active
06713550
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to coating compositions and ink recording media, especially adapted for ink jet printing, and methods for making the coating composition and recording media.
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 ink drying time, many proposals have been made for coatings on paper to improve ink reception. For example, it is well known that paper can be coated with various absorptive pigments, binders and sizing agents. An aqueous suspension of these agents is applied to a paper or other substrate 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. 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” (defined infra) in the finished product.
An ideal ink jet recording medium requires a finished product with a smooth, uniform finish, free of defects. Scratches, streaks, and other defects are commonly caused by grit (particles greater than 40 microns in diameter) in the coating composition. Grit also causes undesirable wear on coating blades and other application equipment. Grit is formed in the coating composition due to agglomeration of pigment particles and other ingredients. The particles tend to agglomerate under various conditions, including the presence of agents that are reactive in some manner with the pigment or other coating ingredients, such as cationic fixing agents and sizing agents. However, the use of such agents is desirable for overall product quality.
Another condition that tends to increase the occurrence of agglomeration and, hence, grit is increasing solids concentration in the coating composition. In many prior art formulations, solids concentrations of above 15-25% were often not possible due to unacceptable levels of agglomeration.
In many commercial applications, however, a high solids concentration is needed to produce a finished coat weight of about 7-13 gm/m
2
(5-8 lbs/3300 ft
2
). Coat weights of this magnitude are difficult, and may even be impossible, to achieve using low solids content (15% -25%) coating compositions with standard coating application equipment.
Low solids concentration coatings also have correspondingly high water contents, resulting in relatively large energy requirements for drying.
For the foregoing reasons, an unfulfilled industry need exists for a high solids, i.e. 30% or greater, coating composition which remains relatively free from grit and which may be applied using standard coating equipment.
OBJECTS OF THE INVENTION
It is an object of the invention to provide a method for making a coating composition and a recording medium adapted for ink jet printing that will rapidly absorb the liquid component of the ink, thereby providing rapid ink drying time.
It is an additional object of the invention to provide a method for making an ink jet coating composition and recording medium that is interactive with ink jet dyes to fix the ink on the surface of the sheet without objectionable dye penetration or “bleeding” (erratic migration in the X-Y plane).
It is a further object of the invention to provide a method for making an ink jet coating composition that has a high solids content, is substantially free of grit, and which may be readily applied to a base sheet with conventional coating equipment.
A final object of the invention is to provide a sequential make down process whereby reactive agents may be added to the composition under conditions that prevent or minimize undesirable reactions with other agents and grit formation.
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 generally is comprised of a binder, a cationic fixing agent, an absorptive pigment, and a sizing agent. The coating contains a high concentration of solids, above 30% and most preferably in the range 35-38%, and is capable of being uniformly applied using standard blade type coating equipment. The ink jet recording medium of the invention provides superior ink jet printing quality. The invention further comprises methods for making the coating formulation of the invention.
A preferred embodiment of a method of the invention begins with the dispersion of a suitable binder in water. Preferably the binder should not only provide strength in the applied coating, but also function as a protective colloid in the coating makedown process. As a protective colloid, it prevents interaction of the various coating ingredients, and thereby allows the coating formulation of the invention to achieve high solids concentrations with substantially no grit.
Although several suitable binders could be used, low molecular weight, partially hydrolyzed polyvinyl alcohol is preferred. A preferred dispersion process consists of dispersing the polyvinyl alcohol in water at 15% solids, heating the dispersion to 95° C., holding at that temperature (for at least approximately 30 minutes) until the polyvinyl alcohol has substantially dispersed, and then cooling the dispersion.
Preferably, a cationic fixing agent is then added to the binder/water dispersion. A cationic charge is desirable in the coating composition to charge the pigment. Cationically charged pigments hold more dye at their surfaces than pigments with anionic charges. A preferred cationic fixing agent is polycationic quaternary ammonium polymer.
Preferably an absorptive pigment, or pigments, is/are then added to the binder/water/fixing agent dispersion. Preferred pigments are comprised of dry powders. The previously added cationic fixing agent acts as a dispersing aid, with the result that no dispersing agent is needed for combination with the pigment. The preferred pigment is an absorptive silica dispersed at a low pH, with silica gels being preferred. Coating compositions having a pH value of 4.0-7.5 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 to the backside, less dark fade (defined infra), and better overall print quality. As will be appreciated by those knowledgeable in the art, silica gels with low pH values and high void volumes desirable for ink absorption are commercially available.
An alternate absorptive pigment is calcined diatomaceous earth. Coating compositions with diatomaceous earth pigments can be made with high solids concentrations, i.e. greater than 30%, and with good inkjet printing qualities. Although diatomaceous earth has good ink solvent absorptive properties, its use r
Schliesman Leonard J.
Spreda Karen K.
Tritz Leland O.
Greer Burns & Crain Ltd
Shosho Callie
Stora Enso North America Corporation
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