New Insights into Lignin Modification during Chlorine Dioxide Bleaching Sequences (IV): The Impact of Modifications in the (EP) and (EOP) Stages on the D 1 Stage*.

Peroxide‐reinforced extractions ((EP) or (EOP)) of D 0 pulps lower both post‐extraction kappa numbers and Klason lignins more than either E or (EO). An oxygen‐reinforced extraction (EO) results in lower kappa numbers, but affords comparable Klason lignins as an E stage (∼⃒1.2% lignin). Quinonoid structures in (EO) stocks are similar to those of the D 0 pulps (1.7 mmol/g lignin). Peroxide inclusion in EP and EOP appeared to eliminate both quinonoid and muconic acid moieties. These observations were confirmed with (EP) treatment of reference pulps (periodate or periodate‐peracetic acid) that contain significant levels of either quinones or muconic entities. The D 1 bleachability order, from easiest to hardest, was D 0 (EOP)>D 0 (EP)≈D 0 (EO)>D 0 E. An identical order was found for how quickly the yellow chromophores (b* value) are eliminated. All extracted pulps, regardless of the type of extraction employed, approach a common D 1 asymptotic brightness (∼⃒86 ISO) and b* value (∼⃒4.2) when excess ClO 2 is applied. The main difference among these pulps was how quickly the asymptotic limits are approached as a function of ClO 2 consumed. The bleachability study indicates that various extracted pulps have similar residual D 1 chromophores limiting the brightness ceiling. [ABSTRACT FROM AUTHOR]