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
1999-11-05
2002-07-02
Cain, Edward J. (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
Reexamination Certificate
active
06414065
ABSTRACT:
BACKGROUND OF THE INVENTION
Paper coating compositions are used by the paper industry to impart the desired strength and cosmetic properties to finished paper. The coating composition is typically an aqueous dispersion consisting mainly of mineral pigments, such as clay, calcium carbonate, silica, and titanium dioxide, and pigment binders, such as starch and synthetic polymer emulsions. Coating compositions may also contain low levels of additives, such as thickeners, humectants and lubricants.
The coating compositions are usually applied to a continuous web of cellulosic material, such as paper, by high speed coating machines, such as blade coaters, air knife coaters, rod coaters and roll coaters. There are trends to use faster coaters to increase productivity and to use higher solids coating compositions to decrease drying costs and improve binder distribution which enhances paper quality.
Coatings which contain fine particle size pigments, such as calcium carbonate, have been shown to be particularly useful in improving the properties of ink jet recording paper. U.S. Pat. No. 5,643,631 (Donigian et al., 1997) and U.S. Pat. No. 5,783,038 (Donigian, et al., 1998) disclose thermal ink jet recording paper, incorporating heat aged precipitated calcium carbonate and a binder, such as poly(vinyl alcohol), starches, and carboxymethyl cellulose. Treatment of paper with a coating composition of a slurry of fine particle size calcium carbonate in a poly(vinyl alcohol) or starch solution resulted in improved optical density of ink jet print. An example of an appropriate poly(vinyl alcohol) binder. was Airvol® 107 poly(vinyl alcohol) which is 98 to 98.8% hydrolyzed. The binders were “cooked” to obtain a solution prior to addition of the pigment slurry.
The use of poly(vinyl alcohol) and its derivatives as binders in ink jet coating systems are well known in the art. For example, an article by C. A. Finch in
Polyvinyl Alcohol—Developments
, Wiley, 1992, pages 555-556, describes the use of poly(vinyl alcohol) as a binder for ink-jet printing paper. Poly(vinyl alcohol), 98-99% hydrolyzed and a 4% viscosity of 25-31 cP (Poval-PVA-117) was reported to be generally used.
An article in
Tappi Journal
, Vol.80, No.1, January 1997, pp. 68-70, by John Boylan, entitled, “Using Polyvinyl Alcohol in Ink-Jet Printing Paper,” describes the use of various grades of poly(vinyl alcohol) for coating paper. It is noted that partially hydrolyzed grades of poly(vinyl alcohol) provide the best printability in terms of ink optical density and dry time when used with silica pigments in paper coatings. However, the final viscosity of poly(vinyl alcohol)/silica coatings increases sharply with small increases in solids. Because of the viscosity increase, the maximum solids is about 25 to 30%, depending on the grade of poly(vinyl alcohol). Partially hydrolyzed low/medium molecular weight grades allow for the highest level of coating solids.
There are many patents on the use of poly(vinyl alcohol)as a pigment binder for paper coatings. For example:
U.S. Pat. No. 4,478,910 (Oshima et al., 1984) discloses ink jet recording paper comprising a base sheet with a specific sizing degree having a coating layer comprising a water-soluble polymeric binder and fine silica particles. The silica particles have a specific surface area of more than 200 m
2
/g and poly(vinyl alcohol) or its derivatives are desired as binder because of their optical density. PVA 117, manufactured by Kuraray, was used in the examples.
U.S. Pat. No. 4,780,356 (Otouma et al., 1988) discloses a recording sheet comprising a sheet of paper with porous particles on the paper surface. The porous particles (e.g., silica, silica-alumina, alumina, and silica-boria) have an average pore size of 10 to 5000 Å, a pore volume of 0.05 to 3.0 cc/g, and an average particle size of 0.1 to 50 &mgr;m. Poly(vinyl alcohol) may be used as a binder for the particles in an amount of 5 to 60% (preferably 20 to 40%) by weight based on the total weight of binder and particles. PVA 117, manufactured by Kuraray, was used in the examples.
U.S. Pat. No. 5,057,570 (Miller et al., 1991) discloses a method of preparing a high solids, aqueous paper coating composition in which dry particulate solids of a partially hydrolyzed, low molecular weight poly(vinyl alcohol) is added to a high solids, aqueous pigment dispersion and mixed, without external heating, until dissolved. The aqueous pigment dispersion typically contains clay and/or calcium carbonate at solids levels of 70 to 76%.
U.S. Pat. No. 5,270,103 (Oliver, 1993) discloses a receiver sheet having a coating and suitable for printing with aqueous based inks, comprising a pigment, poly(vinyl alcohol) binder, and an additional binder component. The poly(vinyl alcohol) is at least 87 mole % hydrolyzed, preferably at least 99 mole % hydrolyzed.
JP 11-4983 (1999) discloses mixing poly(vinyl alcohol) with an organic and/or inorganic powder, and combining the mixture with water to obtain a non-lumping dispersion having a high concentration of poly(vinyl alcohol). The dispersion is reported to be useful adhesives and paints. The poly(vinyl alcohol) powder has an average particle of 500 &mgr;m or less, a degree of polymerization of 500 to 3000 (preferably 100 to 2500), and is 75 to 95 mole % (preferably 75 to 90 mole %) hydrolyzed. The two materials are blended in a volume ratio of 1/0.2 to 1/15 poly(vinyl alcohol)/organic and/or inorganic particles. Examples of inorganic particles are clays, silica, calcium carbonate, and barium sulfate.
As noted above, fine particle size calcium carbonate has been shown to be a particularly useful pigment in coating compositions for ink jet recording paper; however the fine particle size results in a very high viscosity in the low shear rate range after the particles are put into a slurry at the levels needed for ink jet paper coating compositions. The high viscosity in this low shear rate range presents problems in handling the dispersion during the coating process.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed to producing a paper coating composition having improved low shear viscosity at a high solids level of fine particle size calcium carbonate. The improvement in low shear viscosity is achieved by dissolving, without heating and without adding water, a fine particle size, partially hydrolyzed, low molecular weight poly(vinyl alcohol) powder in an aqueous slurry of pigment particles which is predominantly fine particle size calcium carbonate. The poly(vinyl alcohol) has an average particle size of 200 &mgr;m or less, is 85 to 90 mole % hydrolyzed, and has a degree of polymerization of 50 to 600. The slurry, containing 0.1 to 50 parts poly(vinyl alcohol) per 100 parts pigment particles, can then be formulated with other components to produce a paper coating composition for specific applications such as ink jet paper coatings.
There are several advantages to preparing a coating composition by first mixing fine particle size, partially hydrolyzed, low molecular weight poly(vinyl alcohol) powder directly to the fine particle size calcium carbonate slurry. They include:
the poly(vinyl alcohol) does not need to be solubilized prior to mixing with the calcium carbonate slurry, thus eliminating the problem of adding more water to the slurry and reducing the amount of solids;
the poly(vinyl alcohol) can be solubilized in the calcium carbonate slurry without heating;
the low shear viscosity of the calcium carbonate slurry is significantly reduced, thus allowing greater mixing efficiency, improved filterability, and improved pumping efficiency of the final coating formulation;
the solids level of the pigment slurry can be increased without increasing the shear viscosity, thus enabling easier handling of the final coating formulation;
binding of the calcium carbonate to a cellulosic substrate, despite its high surface area, is accomplished with a relatively small amount of poly(vinyl alcohol); e.g., as low as 5 to 15 parts of poly(vinyl alcohol) per 100 parts pigment;
no additional bind
Bongiorno Mary E.
Cain Edward J.
Celanese International Corporation
Lee Katarzyna W.
Spiering M. Susan
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