méthode d'éclaircissement

CONT: Lignin Preserving or Lignin Removal? Two approaches are used in the chemical bleaching of pulps. One approach is to utilize chemicals that selectively destroy some of the chromophoric groups, but do not materially attack lignin. The other approach is to almost totally remove residual lignin. The selective approach (often referred to as ""brightening"" to distinguish it from true bleaching) is used for high-yield pulps with significant lignin content. This methodology is limited in many cases to achieving brightness values in the 70's; but under favorable circumstances, values above 80 can be obtained. Although the brightness gains can be substantial, no known method of selective brightening produces a permanent effect; exposure to light and atmospheric oxygen causes the lignin to rapidly discolor, as can be readily observed with old newspapers.