Mechanistic Differences Between Kraft and Soda/AQ Pulping. Part 2: Results from Lignin Model Compounds

Data accumulated in our laboratory from the cooking of chips and bleaching of resulting pulps consistently pointed to a higher rate of lignin condensation during soda-AQ (SAQ) as compared to kraft pulping. The data also appear to indicate that the rate of condensation was higher at lower alkalinity and that the higher concentration of condensed structures in SAQ pulps is likely responsible for their poorer bleachability. Alkaline condensation reactions were investigated with four commercially available lignin model compounds: apocynol (AP), ethylguaiacol (EG), homovanillyl alcohol (HVA), and vinylguaicol (VG). The investigation of lignin model compounds (LMCs) indicated mechanistic differences that would explain the higher rate of lignin condensation for SAQ pulping as compared to the kraft pulping. Several research groups have reported that the rate of formation of coniferyl alcohol (CA) is 4-5 times higher for SAQ or kraft/AQ as compared to kraft pulping. The present investigation showed that secondary quinone methides from CA degradation products, such as vinylguaicol, participate in condensation reactions. A higher rate of condensation was observed in 0.1 M NaOH as compared to 0.6 M NaOH. Evidence was also found for carbohydrate-rich materials condensing with quinone methides.