Precipitated calcium carbonate (PCC) is a synthetic mineral pigment. At Specialty Minerals Inc. (SMI), however, it’s much more than that. The technology used to produce PCC provides a platform to develop other useful technologies for the paper industry. There are methods to make PCC and there are methods to use PCC. There are also areas where production and use overlap. This is where SMI has developed some of the most valuable PCC technology.
PCC is an alkaline material and the laws of inorganic chemistry dictate that PCC dissolves in acids. Many grades of paper, especially those that use mechanical fiber as a raw material, are produced under acidic conditions. In the past, PCC could not be used as a filler in those grades because it would dissolve in the wet end of the paper machine. Dissolved calcium combines with dissolved anions in the water and causes machine deposits that are difficult to remove and adversely impact the quality of the paper. AT® PCC is Specialty Minerals’ patented technology for using PCC in acidic media. AT® PCC technology mediates the pH of the paper machine by using a weak multiprotic acid in combination with a chelating agent to control soluble calcium. AT® PCC has been used successfully and profitably on machines throughout the world to fill groundwood papers—such as supercalendered (SC) grades—often at loading levels of 35 percent or more.
Unlike mechanical papers, woodfree grades are produced to high-brightness specifications, relying on expensive chemical pulps to achieve such attributes. Since fillers are generally less expensive than fiber, papermakers try to add as much mineral filler as possible without compromising the mechanical properties of the sheet. Scientists at SMI have discovered that adding PCC fillers to pulp, at a stage where the pulp has not yet been highly diluted, allows some of the filler to enter the hollow spaces (called lumens) of the fibers. This "lumen loading" of filler is less disruptive to interfiber bonding in the final paper and affects the final strength of the paper to a lesser degree. By also adding part of the wet-end starch that is normally used in papermaking, the lumen loaded fillers can be fixed in place so that they do not escape from the lumens during dilution of the pulp. SMI has patented the process of splitting PCC and starch addition in the manner just described. We call it Dual Flow® PCC, and the process has been used commercially on machines worldwide. Dual Flow® PCC often allows for a 2 to 4 percentage-point increase in filler loading with no adverse impact on mechanical or optical properties of the paper.
This “systems approach” is what distinguishes SMI as the world’s premier supplier of PCC to the global paper industry. We are committed not only to supplying the highest-quality minerals but also to developing and patenting leading-edge technologies that benefit and advance the industry itself.
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