Concentrations of Phenolic Hydroxyl Groups in Secondary Wall and Middle Lamella of Spruce (Picea abies)

Abstract

A higher concentration of phenolic hydroxyl groups in the lignin of the secondary wall (SW) as compared to the middle lamella (ML) has long been used to explain the topochemical effect observed when spruce is pulped. However, a controversy exists as to whether or not there is a significant difference in phenolic hydroxyl concentration between the two regions. Fractions rich in SW and ML materials were obtained from thermomechanical pulps produced with high specific refining energies. Analysis by periodate oxidation showed that the SW lignin contained approximately 60% more phenolic hydroxyl groups than the ML lignin. However, p-hydroxyphenylpropane units, believed to be concentrated in the ML, were not detected. If a correction factor is applied, then the difference is 37% in favor of the SW lignin. Neutral sulfite treatment generated phenolic hydroxyls at a higher rate in the ML lignin but still sulfonated the SW lignin to a greater extent.