There is a fundamental difference between bleaching of chemical pulps and mechanical
pulps. In the bleaching of chemical pulps lignin is oxidized, decomposed and finally eliminated
from the pulp fibres. This results in less chromophores in the pulp. In the bleaching of mechanical
pulps on the other hand the lignin is not decomposed and eliminated from the fibre. Instead
only the chromophores in the lignin are attacked by oxidizing or reducing chemicals. Thus, the
major amount of the lignin is still present but it is brighter and the pulp brightness is therefore
higher.