Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2000-05-26
2002-06-11
Marcheschi, Michael (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S465000, C423S430000, C423S432000
Reexamination Certificate
active
06402824
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the preparation of precipitated calcium carbonate (PCC) compositions and the resulting PCC composition products. More particularly, the invention relates to the preparation of precipitated calcium carbonate (PCC) compositions having reduced high-shear viscosity in aqueous slurry form, and containing PCC particles of narrower particle size distribution, while reducing requirements for relatively cost-intensive preparation steps such as evaporative-based dehydration procedures. The precipitated calcium carbonate composition products prepared by the invention are especially useful as a paper coating pigment and filler, among other things.
2. Description of the Related Art
Precipitated calcium carbonate is an extremely versatile coating aid, filler and pigment that is employed in a wide variety of commercial products including paper, paint, plastics, rubber, textiles, and printing inks. Precipitated calcium carbonate (PCC) is used on a large scale in paper filling and coating applications in particular. PCC is utilized to increase the opacity and brightness of paper, among other things. In addition to enhancing the opacifying and brightening characteristics of paper, PCC also imparts a high resistance to yellowing and aging of the paper. PCC offers cost advantages over other conventional additives used for increasing the opacity and brightness of paper, such as calcined clay or titanium dioxide.
As a practical matter, many contemporary paper coating machines operate at high running velocities, in which the paper coating composition is applied to a side of running base paper stock using a blade coater. As a consequence, the paper coating composition encounters high shearing forces during such blade coating, making it desirable to have a paper coating composition that displays low viscosity at high shear conditions so that it is sufficiently flowable to form a thin uniform coating when applied to paper base stock. An impediment to achieving such high-speed flowability is that paper coating compositions must employ a relatively high solids content in order to hold down the coating drying requirements, which tends to generally increase the viscosity. For instance, paper coating compositions comprised of aqueous slurries containing PCC have about 70 wt % or higher total solids content. However, once coated upon base paper, a paper coating composition needs to be relatively immobile so that it does not run or wick, to preserve the uniformly applied coating. Thus, it is also desirable that the paper coating composition be able to assume a relatively higher viscosity behavior immediately upon emerging from the high shear conditions associated with the blade coating. Thus, a challenge has existed to impart a stable, pseudoplastic-like rheological character in a paper coating slurry composition, as opposed to dilatant behavior, as those terms are customarily defined for non-Newtonian fluids. Namely, the apparent viscosity of a pseudo-plastic fluid material decreases with increasing shear rate, which is opposite to rheological behavior of a dilatant fluid material which has an apparent viscosity that increases with increasing shear rate. Moreover, the pursuit of the desired pseudo-plastic like rheological character for paper coating compositions cannot come at the expense of the light scattering properties required of the PCC. Ideally, enhancements to the optical performance properties of the PCC somehow would be provided together with the desired rheological improvements all in a single PCC containing composition.
The prior art has proposed manipulating the preparation procedures used to make PCC slurries in efforts to improve the ultimate properties of the resulting PCC composition used in paper coating applications. For example, U.S. Pat. No. 5,833,747 to Bleakley et al. describes a process scheme for making paper coating pigments in which a slurry of PCC particle aggregates made by carbonation of milk of lime is subjected to the steps of partial dewatering using a tube pressure filter in conjunction with a separate comminution step using a single pass attrition grinding mill fitted with a 250 horsepower (186 kW) motor and containing silica sand as grinding media such that at least 100 kW-hr of energy per dry ton of PCC is dissipated in the PCC suspension (where the sequence of the partial dewatering and comminution steps optionally can be switched). Bleakley et al. describe a PCC product provided having a particle size distribution such that 70% of the particles have an equivalent spherical diameter of less one micrometer and at least 50 percent of the particles have an equivalent spherical diameter of less than 0.5 micrometers.
U.S. Pat. No. 5,879,442 to Nishiguchi et al. teaches a method for making an aqueous slurry of PCC suitable for paper coating compositions in which a PCC slurry is subjected to a primary dispersion in the presence of a viscosity reducing agent using a mixer such as “Cowles” type mixers and agitating dispersers; and then dry ground calcium carbonate is added and the resulting slurry subjected to mixing again with “Cowles” type mixers and agitating dispersers as a secondary dispersing treatment; followed by a tertiary dispersing treatment in which the calcium carbonate slurry is subjected to a sand grinding treatment, preferably using 2-4 runs of the sand grinding treatment and also with admixture of a viscosity-reducing agent. The method described in U.S. Pat. No. 5,879,442 is used to provide an aqueous slurry of calcium carbonate particles (70 to 85 total wt % calcium carbonate) having a median particle size of 0.2 to 2.0 &mgr;m and a specific surface area of ranging from 5 to 25 m
2
/g, and the slurry has a viscosity not exceeding 1000 cP at 25° C. and a high shear viscosity not exceeding 200 cP at 25° C.
A need has existed for development of even higher quality aqueous slurries of PCC suitable for paper coating applications which can be made in a cost effective manner.
SUMMARY OF THE INVENTION
The present invention achieves the above and other objectives by providing a unique approach to preparing PCC compositions such that the resulting PCC compositions are endowed with reduced high-shear viscosity in aqueous slurry form at high solids content and contain PCC particles of narrower particle size distribution. As another aspect of the invention, the additional cost otherwise associated with the use of evaporative-based dehydration procedures to increase the PCC solids content in a PCC slurry or associated with the adding of dry CaCO
3
pigment to build slurry solids can be reduced or even eliminated by a facile inventive modification made in a decanting operation within the process flow according to this invention. The present invention is particularly well-suited for providing PCC slurries useful for paper coatings applications including medium and low coating weight, woodfree paper coatings, although it has wider applicability and is by no means limited thereto.
As a general embodiment of the invention, there is a process for the preparation of a precipitated calcium carbonate (PCC) composition, comprising the step of wet media mill grinding an aqueous slurry of PCC made by carbonation of lime milk, with the grinding being done at a low solids content in a plurality of separate grinding stages while essentially free of the presence of organic dispersant. After the multi-stage wet grinding step, the ground PCC slurry is partially dewatered in the presence of an organic dispersant in a solid bowl centrifuge. Then, the partially dewatered PCC slurry is subjected, in the presence of an organic dispersant, to fluid shearing forces created in a rotor/stator type mixer. While these three steps are advantageously used together within a single PCC process flow for optimal effect, the individual steps also can be incorporated singly or in lesser combinations thereof into different PCC manufacturing schemes to beneficial effect.
In one aspect of the invention, PCC particles in a crude, synthe
Freeman Gary M.
Jones William J.
Moller Kurt H.
Moritz Randal A.
J. M. Huber Corporation
Marcheschi Michael
Nieves Carlos
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