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New crystal makes paper bright.

Materials scientists have created a new form of calcium carbonate to help whiten paper.

This printed page and any other paper product would look gray if it weren't for "filler," white minerals added to pulp during processing. Paper companies typically use mixtures of calcium carbonate and titanium dioxide as filler. As the industy's use of recycled paper fiber increases, so will the amount of filler needed per page, says June D. Passaretti of Pfizer, Inc., in Bethlehem, Pa.

Though much less expensive than titanium dioxide, calcium carbonate (limestone) whitens less. But adding too much filler weakens paper, forcing companies to balance many considerations in making their products, Passaretti adds.

To create a better filler, she and her colleagues worked to make calcium carbonate particles the optimal sized for light refraction -- between 0.2 and 0.4 micron across. Titanium dioxide crystals falls range already, but the 15 or so known types of calcium carbonate are too big or too small.

Te Pfizer researchers first tried to precipitate crystals of the right size by altering reaction conditions. But they succeeded in making crystals 0.3 to 0.35 micron across only by chemically modifying them as they develop, Passaretti reports. Each particle resembles a cube twisted in such a way that it contains no 90[degrees] angles. Most calcium carbonate crystals resemble prisms or rosettes, she notes.

Though more expensive than traditional calcium carbonate filler, this new form is "clearly better" and may decrease the overall cost ot whitening, Passaretti says.
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Title Annotation:calcium carbonate
Publication:Science News
Article Type:Brief Article
Date:May 9, 1992
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