New insights into lignin modification during chlorine dioxide bleaching sequences (IV):: The impact of modifications in the (EP) and (EOP) stages on the D1 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 (similar to 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) approximate to D-0(EO) > D0E. 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 (similar to 86 ISO) and b* value (similar to 4.2) when excess ClO2 is applied. The main difference among these pulps was how quickly the asymptotic limits are approached as a function Of ClO2 consumed. The bleachability study indicates that various extracted pulps have similar residual D-1 chromophores limiting the brightness ceiling.