Paper coating pigments, their production and use

Details Paper coating pigments, their production and use Paper coating pigments, their production and use

2000-11-13

2003-11-11

Marcheschi, Michael (Department: 1755)

Compositions: coating or plastic

Materials or ingredients

Pigment, filler, or aggregate compositions, e.g., stone,...

C106S465000, C423S432000

Reissue Patent

active

RE038301

ABSTRACT:

We claim priority to GB
9
,
520
,
703
issued Oct.
10
,
1995
.
The present invention relates to paper coating pigments and their production and use.
In particular, the invention concerns an improved precipitated calcium carbonate product for use as a paper coating pigment, a process for preparing the same and paper coating compositions containing such pigment.
Coated paper and coated paperboard is used for a large range of products including packaging, art paper, brochures, magazines, catalogues and leaflets. Such coated paper and paperboard is required to give a range of properties, including brightness, opacity and sheet gloss, as well as printing performance.
In an effort to attain the required properties, many paper makers use small proportions of calcined clay and/or titanium dioxide (TiO
2
) in their coating formulations. Such additives have the advantage that they strongly scatter light, and thus give good opacity and brightness, but their drawback is their relatively high cost.
The general principle of using a precipitated calcium carbonate (PCC) to replace partly or wholly such expensive additives has been recognised before [J. N. Ishley, E. J. Osterhuber & N. Roman, 1992 TAPPI Coating Conference Proceedings, 335-348 (1992)].
Calcium carbonate can be precipitated from aqueous solution in three different principal crystal forms: the vaterite form which is thermodynamically unstable, the calcite form which is the most stable and the most abundant in nature, and the aragonite form which is metastable under normal ambient conditions of temperature and pressure, but converts to calcite at elevated temperatures.
The aragonite form crystallises as long, thin needles having a length:diameter ratio of about 10:1, but the calcite form exists in several different shapes of which the most commonly found are the rhombohedral shape in which the length and diameter of the crystals are approximately equal, and the crystals may be either aggregated or unaggregated; and the scalenohedral shape in which the crystals are like double, two-pointed pyramids having a length:diameter ratio of about 4:1, and which are generally aggregated. All these forms of calcium carbonate can be prepared by carbonation of milk of lime by suitable variation of the process conditions.
The work of Ishley et al. reported in the reference specified above refers to the use of rhombohedral calcitic PCC. The use of aragonitic PCC in paper coating has also been reported [D. B. Crawshaw, C. H. Kahn-Schneider & P. C. Clark, 1982 TAPPI Coating Conference Proceedings, 143-164 (1982); and G. Engstrom & M. Rigdahl, Nordic Pulp and Paper Research Journal, 90-101 (1992)], although this work does not refer specifically to light scattering performance.
One of the problems with aragonitic PCC, produced by the reaction of carbon dioxide with slaked lime, is that the reaction product consists of aggregates of needle shaped particles. The aggregated structure results in poor
Theological

rheological
behaviour and poor paper coating performance (e.g. sheet and print gloss). A similar but less pronounced problem can occur with scalenohedral PCC.
According to the present invention in a first aspect there is provided a method for the preparation of a precipitated calcium carbonate (PCC) for use as a pigment in paper coating compositions, the method comprising the steps of (a) carbonating an aqueous lime-containing medium to produce an aqueous suspension of a PCC predominantly in a selected crystal form, (b) at least partially dewatering the PCC-containing suspension; and (c) subjecting the PCC-containing suspension to comminution by high shear attrition grinding with an attrition grinding medium.
Steps (b) and (c) may be applied in either order, ie. (b) followed by (c) or alternatively (c) followed by (b). Where step (b) is applied before step (c) a dispersing agent (as described below) is likely to be required prior to application of step (c).
The dewatering step (b) is preferably carried out using a pressure filter device operating at a pressure of at least 5 MPa, preferably at least 10 MPa. Such a device may conveniently be of the known tube press type wherein a material is pressure filtered between two co-axially disposed tubular bodies. Such devices are described for example in GB 907,485 and in GB 1,240,465. In GB 907,485 for example, the tube pressure filter essentially comprises an upright annular chamber formed between two co-axially disposed tubular bodies, which chamber is divided into inner and outer non-intercommunicating compartments by an impermeable elastic sleeve, the arrangement being such that, in use, a material to be pressure filtered is introduced into the compartment formed between one side of the elastic sleeve and one of the tubular bodies, the one tubular body supporting a filter element, and a hydraulic fluid is introduced into the compartment formed between the other side of the elastic sleeve and the other tubular body so as to compress the material to be pressure filtered against the filter element.
The comminution step (c) is preferably carried out such as to dissipate in the suspension in which the PCC is formed at least 100 kilowatt hours of energy per dry tonne of PCC. The dissipated energy may be
200 kwhr

200
kWhr
or more per tonne.
The grinding medium employed in step (c) may comprise one of the hard, inorganic materials well known in the grinding of particulate materials. For example, silica sand having a median particle diameter in the range from about 0.1 mm to 4 mm, eg. 0.2 mm to 2 mm, is a preferred grinding medium. The grinding medium could alternatively be aluminium oxide, zirconium oxide, hard steel or a mixture of any of these materials.
Preferably, step (a) is carried out in an known manner by carbonating a lime containing aqueous medium. Carbonation is desirably carried out using a carbon dioxide containing gas.
We have found unexpectedly that by use of the method according to the first aspect of the present invention PCC products can be formed which have improved optical properties when compared with those prepared in the conventional manner. Such products are therefore especially suitable for producing paper coatings with improved performance. Examples of such improvements are given hereinafter.
Particles obtained in a PCC produced as in step (a) in the method according to the first aspect of the present invention will comprise aggregates as described hereinbefore. We have found unexpectedly that substantial breaking down of such aggregates occurs in both steps (b) and (c) in the method according to the first aspect of the present invention. The contribution to breakdown of the aggregates by step (b) is greater when step (b) precedes step (c) and this is one of the factors which may lead an operator to choose to apply step (b) before step (c).
We have found that when the particle aggregates are broken down in steps (b) and (c) the pH of the aqueous suspension being treated rises. We believe that the reason for this is that when PCC is formed as in step (a) unconverted lime becomes entrapped in the PCC crystal aggregates. When the aggregates are broken down this free lime is released and dissolves in the host aqueous medium. The PCC produced as in the prior art or in step (a) may for example contain by weight up to 5% free lime, eg. 0.2% to 2% free lime. The pH may rise to pH11 or more after the application of the first of step (b) and step (c). Such a pH level is undesirable in the paper coating applications in which the PCC may be employed, as described hereinafter, because it is potentially harmful to machinery and to operators who have to process the suspension.
Desirably, an additional step (d) to reduce the pH of the aqueous PCC-containing suspension is applied preferably after both steps (b) and (c) have been applied although it could be applied after the first of these two steps. The additional step (d) may be applied until the pH falls to a suitable level, eg. below pH 9 preferably to or below pH 7.5. The additional step (d) may comprise further

Affiliated with

Also associated with

Rating

Paper coating pigments, their production and use does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Paper coating pigments, their production and use, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Paper coating pigments, their production and use will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3146064