Radical coupling - A major obstacle to delignification in kraft pulping

Lignin dissolution is very slow in the final delignification phase of kraft pulping whereas carbohydrate degradation and dissolution proceeds at a rate that is about the same as that in the bulk phase. The reason for the change in lignin reactivity has been studied by the addition of selected phenols to laboratory kraft cooks of spruce. A substantial formation of new biphenyl bonds between the added phenol and lignin was found both in the residual fiber lignin and in the dissolved kraft lignin, as revealed by GC-MS of thioacidolysis products and complementary NMR analysis of isolated lignins. The formation of biphenyl structures can be explained by a radical coupling mechanism between the added phenol and free aromatic C-5 positions in phenolic lignin units, possibly involving sulfur/polysulfide as radical initiator. The possibility that similar radical coupling reactions may take place between dissolved low molecular mass phenols of lignin origin and polymeric lignin is discussed and it is suggested that this may be the reason for the slow delignification in the final phase.