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(copyright) 107 poly(vinyl alcohol) which is 98 to 98.8% hydrolyzed. The binders were xe2x80x9ccookedxe2x80x9d 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 Alcoholxe2x80x94Developments, 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, xe2x80x9cUsing Polyvinyl Alcohol in Ink-Jet Printing Paper,xe2x80x9d 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 m2/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 xc3x85, a pore volume of 0.05 to 3.0 cc/g, and an average particle size of 0.1 to 50 xcexcm. 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 xcexcm 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 inkjet paper coating compositions. The high viscosity in this low shear rate range presents problems in handling the dispersion during the coating process.
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 xcexcm 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 binders are needed in the final coating formulation; and
the poly(vinyl alcohol)/calcium carbonate coating formulation, when applied to a paper substrate as an ink jet paper coating, provides excellent ink jet printability.
The aqueous pigment dispersion typically consists of at least about 90% by weight fine particle size calcium carbonate at solids levels ranging from 10 to 50%; preferably 20 to 30%. Up to about 10% of other paper pigments such as clays, silica, and titanium dioxide may also be present.
The fine particle size calcium carbonate has a mean surface area of at least 50 m2/g; preferably at least 80 m2/g. Fine particle size calcium carbonate can be prepared by heat aging and/or milling precipitated calcium carbonate, such as the method described in U.S. Pat. Nos. 5,643,631 and 5,783,038. Calcium carbonate having a mean surface area of 80 m2/g is available commercially tinder the trademark JETCOA(trademark) 30 Specialty PCC from Specialty Minerals.
Suitable fine particle size, low molecular weight, partially hydrolyzed poly(vinyl alcohol) powder for use in this invention can be 70 to 90, preferably 85 to 90, and most preferably 87 to 89 mole % hydrolyzed, have a degree of polymerization (DPn) of 50 to 600, preferably 150 to 300, and an average particle size of 200 xcexcm or less; preferably, 180 xcexcm or less. An example of a preferred poly(vinyl alcohol) powder is Airvol(copyright) 203S poly(vinyl alcohol) supplied by Air Products and Chemicals, Inc. The poly(vinyl alcohol) used in this invention can be prepared by synthesis and saponification techniques well-known to those skilled in the art of manufacturing poly(vinyl alcohol). A fine particle size of the poly(vinyl alcohol) can be achieved by grinding the poly(vinyl alcohol) particles and passing the particles through a mesh.
The fine particle size, low molecular weight, partially hydrolyzed poly(vinyl alcohol) powder is slowly added to an agitated calcium carbonate slurry at a rate that does not cause clumping of the poly(vinyl alcohol). Typically, adding poly(vinyl alcohol) at a rate of 1% of poly(vinyl alcohol) in 10 seconds is sufficient to prevent clumping. Mixing is continued until the poly(vinyl alcohol) is solubilized; typically, mixing is continued at least 15 minutes. Mixing of the calcium carbonate slurry with the dry fine poly(vinyl alcohol) powder is preferably carried out at high shear rates. The amount of poly(vinyl alcohol) can range from 0.1 to 50 parts/100 parts of pigment; preferably 3 to 25 parts of poly(vinyl alcohol)/100 parts pigment. Amounts of 5 to 15 parts of poly(vinyl alcohol)/100 parts fine particle size calcium carbonate have been found to efficiently bind the pigment. Solubilization of the poly(vinyl alcohol) can be carried out at ambient temperature, i.e., 20xc2x0 C. Heating is not required to solubilize the poly(vinyl alcohol).
Low shear viscosity is the viscosity of a fluid (for example, calcium carbonate slurry containing 28 to 32% solids and 3 to 25 parts low molecular weight, partially hydrolyzed poly(vinyl alcohol) per 100 parts calcium carbonate) which results from the shear rate generated by a Brookfield viscometer (No.3 spindle at 100 rpm).
The high solids aqueous pigment dispersion containing poly(vinyl alcohol) can be used to prepare ink jet paper coating compositions or can be used directly as an ink jet paper coating composition. No additional binders or dispersants are needed in the coating composition. A typical coating composition for ink jet paper applications contains:
90 to 100 parts fine particle size calcium carbonate;
0 to 10 parts secondary pigment;
0.1 to 50 parts poly(vinyl alcohol);
0 to 3 parts cationic dye fixatives such as polyethyleneimine or poly(diallyldimethyl ammonium chloride); and
0 to 0.3 parts defoamer.
The invention will be further clarified by a consideration of the following examples, which are intended to be purely exemplary of the invention.