Molecular Mass Distribution of Lignin from the Alkaline Pulping of Hardwood, Softwood, and Wheat Straw

The behavior of lignin during kraft (hardwood, softwood, and wheat straw) and soda-AQ (wheat straw) pulping was studied, mainly in terms of delignification degree and molecular mass distribution (MMD). In the initial delignification phase (at 140 degrees C for 15-60 min), a prominent part of the dissolved softwood kraft lignin (18-25 g/L, MM mostly > 3,000 Da) was found in the liquid phase of chip cavities, rather than in the external bulk black liquor (5-7 g/L, MM mostly < 3,000 Da). The maximum weight average MM values (<(M)over bar>(w)) of the soluble lignin under conventional cooking conditions were detected for the kraft softwood (4,100 Da), and kraft birch (3,400 Da) when the degree of delignification was 65-75%, corresponding to a residual lignin content of 5-10%. The maximum (M) over bar (w) of the dissolved wheat straw kraft (5,050 Da) and soda-AQ (5,900 Da) lignins was clearly higher than that of wood-based kraft lignins (2,950-4, 100 Da).