Stabilization of Brightness with H2O2

Brightness stability is affected by a number of parameters, and is typically analyzed

in tests using accelerated aging. The pulp samples are exposed to elevated

temperature under either dry or humid conditions. Brightness losses during dry

heating are normally less pronounced compared with humid reversion tests. A

standard procedure is to heat handsheets over boiling water for 1h. This test

method is described as E.4P method by Paptac [63]. The changes in light absorption

and scattering are measured as post color number [64]; the smaller the number,

the less reversion has taken place. Humid brightness reversion is thought to

correlate more with natural aging occurring in pulp bales [65].

7.6 Hydrogen Peroxide Bleaching 873

The intensity or aggressiveness of the bleaching process certainly has an

impact. Compounds and conditions that affect brightness stability include transition

metals, remaining lignin, hemicelluloses, and the pulping process used [66].

Past experience with hypochlorite pointed to rather negative effects of low pH,

high temperature and high charges of this chemical on brightness stability. Losses

could be attributed to oxidation of the cellulose chain, and were often very significant.

50 years ago kraft pulp was typically not bleached above about 80% ISO

brightness using a typical CEHH sequence. The situation has now changed with

the increasing use of chlorine dioxide, initially in a final stage (CEHD) and later

with -D1E2D2 or -D1EpD2 final bleaching.

Mill experience teaches that the higher the brightness, the greater the stability,

though this applies to the same sequence and moderate changes in bleaching conditions.

The effectiveness of lignin removal or impurities removal is important for

the stability of brightness, and therefore differences between TCF and ECF bleaching

can be expected. Indeed, in TCF bleaching of birch kraft pulp HexA were identified

as a source of high reversion [67]. Likewise, poor brightness stability was

found for ECF “light” bleached softwood pulp [68]. However, in “normal” ECF

bleaching HexA were found not to be the source for reversion [69], as it was

removed completely in the process. These variances explain the importance of

bleaching conditions. The TCF sequence used to bleach birch pulp with poor

brightness stability [67] was conducted exclusively with alkaline bleaching steps.

In order to attribute correctly the reversion to certain sources, it is important to

understand how complete or ineffective potential sources for the development of

colored compounds are destroyed.

A comparison of different ECF sequences, all using sufficient chemical for lignin

oxidation and HexA hydrolysis or destruction [69], showed that cellulose depolymerization

(apparent as a lower viscosity) has no direct influence on brightness

stability. Neither hot acid hydrolysis nor ozone nor aggressive conditions and a

very high temperature in the final peroxide stage had any significant impact on

reversion. The positive impact of using more bleaching chemical in a D0-Eop-D1

sequence is illustrated graphically in Fig. 7.130. A moderate input of chlorine

dioxide (in this example, 1%) provided a reasonable brightness close to 89% ISO,

but the brightness was not stable. With a loss of about 10 points of brightness in

humid reversion, the instability was pronounced. The use of additional chemical

improves bleached brightness, but not to any great degree. A doubling of the

active chlorine input (from 1% to 2%) added only one brightness point, yet reversion

losses decreased from 10 points to only 7 points. The more intense degradation

of lignin or other “impurities” was seen to improve brightness stability.

The advantage of an additional treatment stage to improve brightness and

brightness stability is illustrated in Fig. 7.131. A small amount of active chlorine

(0.5%) applied in the second D stage lifts brightness to 90% ISO, and reduces

losses in humid reversion to about 5 points of brightness. Even more pronounced

is the improvement of a stoichiometric substitution of chlorine dioxide by H2O2

(0.5% active Cl with 0.25% H2O2). Losses in reversion decrease at best to only 3.5

points.

874 7Pulp Bleaching

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