The Precipitated Calcium Carbonate (PCC) Advantage for Supercalendered Papers
Supercalendered (SC) papers encompass a range of uncoated, highly filled, lightweight mechanical printing papers that are used to print a wide variety of magazines, advertising fliers and catalogs. These papers are ranked according to brightness namely, SC-A+, SC-A, SC-B, etc. Printed consumer papers are mass-produced everyday on modern high-speed heatset offset and rotogravure presses. Examples include department store catalogs and fliers, Sunday newspaper magazines, and a great many other advertising circulars, magazines and catalogues. Brightness, opacity, gloss and smoothness are key quality parameters, because they relate directly to the visual attributes most appreciated by consumers: high print contrast, low print-through, good print gloss and a uniform printed image.
SC paper is a relatively recent innovation, aimed at providing a cost-effective, high quality alternative to lightweight coated (LWC) papers. In the past 15 to 20 years, SC has undergone a rapid metamorphosis in terms of continually advancing manufacturing technology and improving quality. One of the most noteworthy innovations occurred in 1997 when Specialty Minerals Inc. (SMI) first introduced precipitated calcium carbonate (PCC) to SC manufacture. The replacement of traditional filler clay (kaolin) pigments by PCC led to significant improvements in print quality and economies in paper manufacturing, as well as opportunities to develop superior, cost-competitive paper grades.
In the case of SC printability, a picture is worth a thousand words. The above graph illustrates data developed from a worldwide survey of SC paper machines, in which paper from an appreciable fraction of the top North American and European machines currently producing SC-A and SC-A+ was sampled and tested. All the PCC-containing sheets were manufactured with SMI PCC products.
Each point on the graph represents the combination of brightness and opacity of 35 lb. (52 g/m2) paper from one paper machine. While it might not be evident that all clay or all PCC papers would tend to produce single, unique lines when plotted in this way, they apparently do; and the difference between clay and PCC is striking. Using PCC, producers are able to increase paper brightness to very high levels without sacrificing opacity, or are able to achieve high levels of opacity at standard brightness. Commercial print testing demonstrated that the increase in opacity is clearly visible as a reduction in print-through. If desired, intermediate levels of brightness and opacity can be achieved with PCC/clay blends as shown by the open diamond data points on the graph.
The uniform size, on the order of one micron, and the clustered nature of the individual PCC particles create a large number of sub-micron size pores within the paper structure. It’s these pores that maximize light-scattering surface area, achieving excellent paper optics. At the same time, the abundance of tiny pores within PCC-containing paper tends to increase the rate of fluid absorption, especially of the fountain solutions used in offset printing. For this reason, a few offset printers encountered issues with the introduction of this paper until techniques were developed to overcome them. SMI research has shown that issues such as these can easily be overcome with simple, cost-effective adjustments of the materials (fountain solution, ink, plates) used on the press.