Chips of Japanese red pine and beech woods were cooked with NaOH to a relatively high residual lignin content, and the partially delignified chips were then defiberized mechanically. The obtained first stage pulps were further delignified, in the second stage, by cooking with NaOH and O2 to obtain the unbleached two stage soda oxygen pulps (OAP). The conditions of pulping were shown in Table 1.
OAP and unbleached kraft pulps (KP) from pine and beech woods were bleached by the methods listed in Table 3 to prepare the bleached OAP and KP of brightness 87-89%. The bleached OAP were studied for the yield, brightness, viscosity, copper number, and strength properties in order to compare with the bleached KP on these properties. The results were given in Table 4 and Table 5.
The yields, percent based on wood, of OAP were somewhat less than those of the bleached KP. The intrinsic viscosities (SCAN) of bleached OAP were lower than these of the bleached KP. The addition of MgCO3 in the oxygen-alkali cooking of the pulping stage had no effect on the viscosity of the bleached OAP. The copper numbers of the bleached OAP were in the range of 0.49-1.00 and were on a similar level to those of the bleached KP.
The bleached OAP required less refinining energy with PFI mill to reach a given freeness than did the bleached KP. The bleached pine OAP had practically the same strengths as the corresponding bleached pine KP. The bleached beech OAP were comparable to the bleached beech KP on the breaking length and burst factor, however, the formers were somewhat inferior on tear strength to the latter.