D2 P D2 P D2 P

brightness (%ISO) T260 UM200 bleached

Fig. 7.131 Analysis of brightness reversion of eucalyptus kraft

pulp after four bleaching stages with D0-Eop-D1-D2, respectively

D0-Eop-D1-P.

This advantage of an application of H2O2 can be attributed to the destruction of

carbonyl groups and quinoid structures remaining in pulp after the D stage.

Using UV Raman spectroscopy, Jaaskelainen [70] detected these intermediates (or

end products) of chlorine dioxide bleaching [9] in kraft pulp bleached with a final

D stage. In peroxide-bleached pulp such structures were absent, the reason being

the rapid reaction of alkaline peroxide with quinones.

7.6 Hydrogen Peroxide Bleaching 875

The importance of the remaining quinoid structures, respectively their elimination

by alkaline H2O2, explains the rather good brightness stability of TCF or ECF

“light” bleached pulp. Despite their higher lignin residual, these pulps normally

show very moderate reversion losses in accelerated aging. The same applies to

bleached mechanical pulp, which is rather stable during heat-induced aging. Its

sensitivity against light-induced yellowing has phenols as the main source, and

follows a different reaction pathway [71].

The conclusion drawn regarding the source for reversion of the TCF-bleached

birch pulp mentioned above [67] can now be seen from a different angle. This

pulp had been subjected to a very intense peroxide treatment, but not to any acid

stage capable of removing HexA. Therefore, the source for the remaining reversion

of this pulp was HexA. It is not appropriate to generalize this specific finding,

however. In ECF bleaching with sufficient hydrolysis or oxidation of HexA,

other compounds are responsible for brightness losses in aging. The best brightness

stability results from the most effective removal of all impurities. This

includes sufficient bleaching chemical with different reactivities towards the

impurities and definitively sufficient washing. The combination of a very high

temperature in the D0 and the last D stage (90 °C) with a final peroxide stage provides

access to the brightness range above 93% ISO, and simultaneously to

extreme brightness stability. The increase in brightness, and the development of

brightness stability in humid reversion, is shown graphically in Fig. 7.132. The

peroxide-bleached pulp loses less than one point of brightness during the aggressive

aging treatment.

D1 D2 P

brightness (%ISO)

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