3 CONCLUSIONS

THE RESULTS IN FIGURES 1, 2, 4, 6, and 7 demonstrate that, regardless of whether darkening or bleaching took place during exposure to visible or near-ultraviolet radiation, papers darkened upon subsequent storage in the dark over three-month's time at normal room conditions. For a high-lignin-content (29.1%) groundwood pulp, the extent of post-irradiation darkening after 90 days' storage increased with increased initial exposure in a xenon-arc Fade-ometer� (Pyrex� filters) (Figure 3). In contrast, post-irradiation darkening was little affected by the amount of initial exposure in the Fade-ometer� in the case of test sheets made from pulps that contained 4.4% lignin or less.

In a series of experiments in which exposures were carried out at temperatures of about 25 to 28�C, the extent of initial darkening or bleaching and also the extent of post-irradiation darkening was found to be influenced by the pH of the test papers. Acid papers tended to darken the most upon subsequent storage and highly alkaline papers the least (Figure 4). The post-irradiation darkening of filter paper that had been previously thermally aged was scarcely affected by the pH of the test sheets (Figure 5).

In an especially-prepared series of pulps, post-irradiation darkening was found to be directly related to lignin content over the range between 0.9 to 29.5% lignin (Figure 8). This result confirms the trend of relative darkening noted in papers containing 29.1, 4.4 and less than 0.24% lignin which were exposed in a Fade-ometer� (Figures 3 and 5).

The presence of varying amounts of HAS matter was found to have no positive correlation with post-irradiation darkening (Figure 9). With a series of test sheets containing little lignin but varying amounts of gamma cellulose (Figure 6), little darkening was induced subsequent to exposure to near-ultraviolet radiation.