The Canadian Co-operative Permanent Paper Research examined, besides others, the impact of air pollutants on the permanence of handsheets from six pulp types and four alkaline commercial papers. The pollution results confirm that sheets from mechanical pulps absorb more air polllutants and show a greater brightness loss than lignin-free sheets. However, this does not necessarily result in greater deterioration of strength properties. A four-day exposure to 10 ppm SO2 and 20 ppm NOx causes a significant acidification and degradation of all papers containing no alkaline reserve. The addition of calcium carbonate as an alkaline reserve is very effective in reducing the negatives effect of air pollutants on both lignin-containing and lignin-free sheets.
