Paper making and fiber liberation – Processes and products – With coating after drying
Reexamination Certificate
2003-06-04
2004-01-20
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
With coating after drying
C162S181600
Reexamination Certificate
active
06679973
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
Applicant's invention relates to a purified natural zeolite pigment composition for filling and/or coating paper. More particularly, the present invention relates to a purified natural zeolite pigment composition that can be used for coating paper that produces a paper that exhibits improved characteristics over existing uncoated and coated papers made with other pigments.
2. Background Information
Pigments are used in papermaking and paper coating to improve the appearance, optical properties and printability of papers. Commonly used pigments include kaolin clay, calcium carbonate, titanium dioxide, alumina trihydrate and polystyrene. These pigments are useful in manufacture of conventional printing and writing papers and paperboards that are printed or imaged by common processes including offset lithography, gravure and xerography. Recently developed imaging technology has created needs for new types of coated and uncoated papers with properties not achievable with conventional pigments. Ink jet printing is a useful example.
Ink jet printing technology has undergone several changes in addressing the demands of existing and future digital printing applications that require high quality printed images. High quality ink jet printing typically occurs on coated paper; therefore, to produce such high quality printed images the coating composition and the ink formulation must be considered.
Current ink jet papers rely on the novel properties of the coating material to create desired properties to dry and set the ink solutions. Jet inks typically contain 2.5% by weight of organic dyes. The dye is fixed to the paper surface either by evaporation of a base such as ammonia, by migration of a base such as diethanolamine into the paper, or by changes in ionic environment when the ink meets the coating material layer.
The paper must exhibit unique properties in order to produce a high quality printed image when the ink is fixed to the paper surface. Once the ink drop is accepted by the paper, the ink must adhere to the paper and spread minimally in all directions to generate sharp edges for print contrast
1
and image fidelity. The paper must be smooth to give high print densities
2
. In addition, the paper should minimize bleeding
3
and wicking while promoting the absorption of ink to set the dye onto the coated surface since this promotes higher print densities. Ink jet droplets must be adsorbed quickly to avoid image smearing and multiple drop splatter. The dyes should be deposited near the paper surface to maximize color density and contrast while minimizing show through
4
.
1
Contrast is defined as the tonal change in color from light to dark.
2
Density is defined as the degree of color or darkness of an image.
3
Bleeding is defined as ink traveling into the sheet.
4
Show through is defined as printing that is visible from the backside of a sheet, or the next sheet, under normal lighting conditions.
Coating, which generally contains pigment, binders, and additives, is applied to the paper surface to improve the properties of the paper. The ink interacts with the coating to produce a high quality image. The coating prevents the ink from penetrating into the substrate. More specifically, the coating can optimize drying time for high water content dyes and separate the water-soluble organic dyes from the water vehicle and hold the dye on the surface so it doesn't strike through to the base sheet. Smoothness and thickness of the coating layer are two important physical properties that impact print quality. Pore structure and contact angle wettability effect print quality by preventing ink spreading. In order to prevent wicking and feathering
5
, it is important that the thickness of the coating layer be homogenous to a scale of a few microns in depth which also helps in the absorption of successive droplets of ink at high delivery rates and any water present.
5
Feathering is defined as the spreading of ink at the edges of printed type, caused by irregularities in the ink or its distribution.
Paper made for ink jet printing should have a hydrophilic, high porosity surface with no macroscopic structure in order to absorb ink jet droplets quickly with little spreading, wicking or dye penetration. Therefore the preferred coating for the paper surface should contain a highly porous, high surface area pigment that wets almost instantly with water. If the coating has sufficient thickness and void volume, it should be able to absorb successive droplets in multicolor printing at the highest delivery rates of commercial ink jet printing. The dye should react with the coating material to make it waterfast and rub resistant. The coating should have near neutral or alkaline pH to avoid shifts from the intended color of the dyes.
The rate of ink penetration has a large effect on final optical density through its effect on drying time and setting of the dye on the coated surface. The rate of ink penetration can be explained by the Lucas Washburn Equation of capillary flow:
I
2
=yr
(cos&thgr;)
t
/4
v
where I is the depth of ink penetration, r is the pore radius, t is time, y is the surface tension, &thgr; is the contact angle, and v is the viscosity of the ink. In generating high print quality, the rate of ink penetration must be modified to allow sufficient wetting to occur. The hydrophilic/hydrophobic surface chemistry of the coating plays an important role in the development of image quality through the control of dot gain. Sufficient dot gain requires the dot spreading on a smooth surface and is a function of contact angle. The contact angle is itself a function of the interactions between the surface tension of the liquid, surface vapor, and liquid vapor interfaces. The determination of sufficient dot gain can be characterized through the surface tension of the interfaces from Young's equation:
Y
slv
=Y
si+
Y
iv
cos&thgr;
This equation evaluates the development of the contact angle which controls spread of liquid through the surface tensions involved. If the contact angle is less than 90 degrees, surface roughness will reduce the contact angle even more. Whereas if the contact angle is greater than 90 degrees the surface roughness will increase the contact angle. Porosity also effects the measured contact angle.
The interactions between ink and the coated substrate play a vital role in producing images that are long lasting, well defined and of high strength regardless of printer application. The main interaction occurs at the surface of the substrate, where the type of bonding that occurs between the colorant and the media dictates the final print quality. The interactions that take place between the colorant and the plain paper are controlled by hydrogen bonding and Van der Waals forces, while ionic and electrostatic forces are responsible for the interactions between the colorant and the coated paper.
Hydrogen bonding is the most significant bonding that takes place between color and media, where cellulosic material is involved. For a large dye molecule, a large number of sites are available for hydrogen bonding which encourage the interaction between the colorant and the media. Hydrogen bonding between color and media increases the strength of the color binding on the media. Furthermore, the hydroxyl groups of the cellulose may interact with the &dgr; cloud of an aromatic group on the colorant by hydrogen bonding.
Van der Waals forces are very weak when the interacting groups are far apart and a weak repulsion typically exists between the media and anionic dyes. The interaction between colorant and media becomes strong as the dyes start penetrating into the base sheet.
Electrostatic forces occur due to coloumbic attraction. The cationic groups on the media, such as Ti
3+
, Al
3+
, and Ca
2+
attract anionic dyes, such as water-soluble groups of SO
3
2−
, COO , and PO
4
3−
. The result is strong attraction between these groups, which causes an effective immobilization of
Joyce Margaret K.
Klass Charles P.
Chin Peter
Evans Michelle
Guan, Lee & Hanor P.C.
Zo Resources, LLC
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