High yield pulp contains more lignin than the conventional chemical pulp. Therefore, refining and beating have little effect on fibrillation of pulp fiber, and the strength of paper therefrom is relatively low. Consequently, a process for making high yield pulp, which is easily refined and beaten to produce a high strength paper, have been desired. This paper discusses a new pulping process, so-called sulfomathylation process, in which sodium sulfite and formaldehyde are used as the cooking agents of wood chips. The properties of this pulp (Abbrev. SM pulp) were compared with those of pulp cooked with sodium sulfite alone (Abbrev. S pulp). The results are summarized as follows :
(1) The amounts of sulfonate group and water retention value of SM pulp are higher than those of S pulp (Table 2). This shows that lignin in SM pulp is sulfonated and sulfomethylated (Fig. 3).
(2) SM pulp is beaten easier than S pulp as a result of increased flexibility of pulp fiber (Fig. 1 and 2).
(3) Strength of SM pulp sheet is improved, especially in breaking length and burst factor (Table 4).
(4) In order to obtain more flexibilty of pulp fiber, two-stage cooking, first with sodium sulfite and second with sodium sulfite and formaldehyde, was studied. Breaking length of two-stage pulp sheet is somewhat higher than that of single stage one (Table 6).