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

Abstract

Peroxide‐reinforced extractions ((EP) or (EOP)) of D₀ 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 D₀ 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 D₁ bleachability order, from easiest to hardest, was D₀(EOP)>D₀(EP)≈D₀(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 D₁ asymptotic brightness (～86 ISO) and b* value (～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 D₁ chromophores limiting the brightness ceiling.