Coating processes – Transfer or copy sheet making – Coating opposite sides or forming plural or nonuniform coats
Reexamination Certificate
2001-03-05
2002-10-29
Hess, Bruce H. (Department: 1774)
Coating processes
Transfer or copy sheet making
Coating opposite sides or forming plural or nonuniform coats
C428S195100
Reexamination Certificate
active
06472013
ABSTRACT:
FIELD OF INVENTION
The present invention relates to multicolor ink jet recording. More particularly, it relates to ink-receptive materials having improved dimensional stability for use in wide format multicolor printing.
BACKGROUND OF THE INVENTION
The rapid growth of computers and their many and varied applications continue unabated. Multicolor ink jet printing has benefited from this growth aided by the low cost of color printers and improved quality of the recording media. The cost of color printers has become so low that they have become the preferred printing mode for home computer use.
The commercial market for ink jet color printing has likewise grown in such applications as presentation and computer-aided drafting. Contributing to this growth have been improvements in ink-receptive recording materials with regard to image density, feathering resistance, color brightness, color-to-color edge sharpness and water resistance. The graphic arts market, however, with its huge growth possibilities has yet to realize its potential, mainly because of its more demanding quality requirements. These include color brilliance and fidelity, freedom from surface dusting and color rub-off, instantaneous dry times and more consistent quality over the wide range of humidities normally encountered during printing. As often happens, one set of qualities can be obtained only at the expense of another, thereby making it difficult to obtain all of the requisite qualities.
The absorption of water into and desorption out of paper has produced some undesirable side effects in ink jet printing, such as strike through, paper cockles and paper curl. Paper curl is a function of the amount of aqueous liquid deposited on the recording medium. More printing on a page or the use of higher volumes of inks per unit area increases curl potential.
Depositing aqueous inks on paper causes an initial hydroexpansion of the fibers of the paper. This initial hydroexpansion causes an expansion curl away from the image which occurs typically right after printing. Steady state curl is toward the image, and typically occurs over a period of the time when the sheet tries to achieve a state of final stress release after being dried. Excessive drying in the process of coating accelerates steady state curl toward the image.
The amount of time that it takes for a sheet of paper to reach steady state depends on the type and the amount of the ink being used. Penetrants in the ink change the depth of its penetration and thus modify the amount of time taken to reach steady state curl. A co-solvent humectant such as ethylene glycol serves to keep a paper essentially flat. For example, ethylene glycol appears to produce a long term hydroexpansion that reduces or counteracts the stress relaxation of the sheet. This effect, however, is lost over time to produce a final curl toward the image.
Stiffness is an extremely important property for many paper products. It is one of the most important mechanical properties of paperboard used for packaging. On the other hand, there are a number of papers where a lack of stiffness is an important property. Tissue, toweling, and napkins all must be soft and pliable, which is just opposite of stiffness. Labels must often conform to a non flat surface in use, and a high stiffness would make this difficult.
The stiffness of paper is dependent on a number of factors. The most important are: thicknesses, the Young's modules of the paper, the amount of restraint applied during drying, moisture and surface treatment. Paper machine direction stiffness increases if additional tension on the web is applied during drying. A three-fold increase in the Young's modules occurs in going from zero restraint to high restraint. The paper moisture influences its Young's modules and consequence stiffness. For example, the Young's modules of a paper can be reduced by 50% in going from 22% to 85% relative humidity.
The hydroexpansion of paper is dependent on a paper sheet furnish. By changing the ratio of hardwood to softwood it is possible to achieve a paper with reduced curl, and minimal hydroexpansion. However, the end user has only a limited possibility to modify properties of already made paper.
Graphic art media designed for wide format color ink jet printers should give high quality images and, at the same time, have the capability to absorb high amounts of inks. In Hewlett Packard printers, such as HP 650, HP 750 and HP 2000 series printers, the printed edge is passed through a supporting drum and is held by a frame. The feeding and printing system keeps the media firmly in place and prevents paper surface distortion during printing. In Nova Jet printers, recording media are fed from the back and the printed edge falls down freely after printing. The differences obtained with both systems is hardly observable, for media such as vinyl, backlit film, and photobase. Photobase has a layer of extruded polyethylene on the surface. This layer prevents any paper surface distortion during printing. When paper bond is printed on EnCad printers the edge of the sheet does not band following the shape of printer feed plate but is nearly horizontal due to paper stiffness. At some moment of printing the edge falls abruptly, which creates head strike on the paper in the form of smearing and so called “horseshoes”. The problem is more severe for heavy weight papers (a high caliper) than for lightweight ones.
There is no paper base on the market suitable for absorbing a high amount of inks applied in wide format printing such as EnCad without paper surface distortion. The ink jet receiving layer improves the absorption ability of paper but on the other hand increases its stiffness.
It is well known that dimensional stable paper can be achieved by applying a liquid (i.e., water) impermeable coating to both sides of a base paper. Although the paper becomes impermeable to ink particularly to water and glycol based inks, curling due to shrinking and swelling of the functional coating still occurs. Moreover, the addition of this layer may greatly reduce the receptive ability for aqueous inks, which is seen in the form of a longer dry time that becomes similar to that of photobase. Furthermore, the hydrophobing surface layer of pretreated paper has low affinity to hydrophilic ink receiving layers. Consequently, such receiving layers have a tendency of flaking, peeling and very often have a low resistance to folding.
A paper recording medium should remain flat over a wide range of humidities both during and after printing. This ability to remain flat is especially important when the humidity in the surrounding environment changes.
It is known that curls of paper can be reduced or eliminated by the addition of a second coating to the backside of the recording sheet to balance the differential hydroexpansivity of paper. This coating is applied to counteract any shrinkage or swelling resulting from the ink jet receiving top coating. However, a backside layer may not reduce the curl to the desired extent in cases where precise amounts of the two coatings are needed. Suited compositions for such back-coating layers are well known to those skilled in the art. They may generally include the same coating as the top coat, e.g., polyvinyl alcohol, which is fully or partially hydrolyzed, various starches or carboxymethylcellulose. Although commonly used polymers can compensate curl relatively well, they make the paper stiffer. Further, the type of chemicals used in producing an anti-curl layer and a method of coating has to be selected with great caution to prevent deterioration of ink jet layer print quality.
In U.S. Pat. No. 4,853,255 to Onishi et al., a process for controlling curl in a web of coated paper is described. A paper web is unwound from a reel and coated with a coating composition applied to one side of the web. Water is applied to the opposite side of the coated paper to thereby control curl resulting from the coating composition previously applied.
U.S. Pat. No. 5,277,965 to Malhotra, describes a recording
Birch & Stewart Kolasch & Birch, LLP
Hess Bruce H.
Oce-Imaging Supplies
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