Molecular Weight of the Residual Lignosulfonic Acid in Unbleached Sulfite Pulp.

Abstract

In the previous papers, it was reported that the residual lignosulfonic acid in unbleached sulfite pulp (RLS) has about half as much sulfonic acid group as the ordinary lignosulfonic acid (LSA) and has no chemical bond with carbohydrate.As this RLS was the lignin remained undissolved by cooking and subsequent refining of pulp, it was considered that there must exist any hinderance to dissolution because of the nature of the morphological structure of the cell wall and/or the higher degree of polymerisation of RLS than that of the LSA dissolved during the cooking.In this paper, the comparison of the molecular weights and of the functional groups of RLS and LSA has been made in order to know the reasons of difficulty of the dissolution of RLS from the unbleached sulfite pulp.The barium salts of LSA and RLS were fractionated into eight and six fractions respectively with ethanol by Schulz's method, and each fraction was analyzed as usual for methoxyl, sulfur, sulfur in sul-Ionic acid form, weakly acidic group, type I (guaiacyl type) phenolic hydroxl group, type II (condencedtype) phenolic hydroxyl group, total phenolic hydroxyl group and diffusion coefficient of each fraction was measured by McCarthy's method.The degree of sulfonation of RLS was distributed from 0.25 ot 0.40 per methoxyl and of LSA was 0.40-40.55 per methoxyl.Total phenolic hydroxyl group of RLS was 0.15-0.18 per methoxyl, being lower than that of LSA, and phenolic hydroxyl group of type I in RLS was 00.03 per methoxyl.The somewhat surprising fact is that the several fractions of RLS have almost no phenolic hydroxl group of type I.As diffusion coefficient of RLS measured by McCarthy's solution to gel method was about 3.8-5mm2/day and that of LSA was 6.528mm2/day, it will definitely be concluded that the molecular weight of RLS is higher than that of LSA.Molecular weight estimated roughly using McCarthy's relation between molecular weight and diffusion coefficient was about 60000-110000 for RLS and about 4000-40000 for LSA.It was thus confirmed that the reason for the difficulty of the dissolution of the RLS from the unbleached sulfite pulp may be due to too high molecular weight of the RLS to diffuse out from the cell wall.