Holocellulose fibers and Kraft fibe

Holocellulose fibers and Kraft fibers have similar lengths and diameter (Table 2).The distribution of lengths and diameters for 200 measurements for each type of fiber are listed in Figure S2. Typical optical and SEM images are presented in Figure 5. As apparent in Figure5A, holocellulose fibers are straight and rod-like without noticeable kinks or collapsed segments,while Kraft fibers are curved or kinked. Both fibers have not been beaten, and are freeze-dried from never-dried suspension using same procedure. The present laboratory scale Kraft-processinvolves stronger mechanical treatment, since fibers are separated in a water jet defibrator (see experimental section). SEM images provide a more detailed view of fiber structures. The cross-sections of both fibers show a hollow, quasi-cylindrical structure (Figure 5B) with apparentedges, as expected from the cell structure in wood. Both fibers are earlywood tracheids with thin cell walls. Interestingly, the pit structure of the holocellulose fiber is well-preserved, whereas it is collapsed in the Kraft fibers (Figure 5C and 5D). In summary, the data presented indicate that mild PAA treatment under minimal mechanical stirring and agitation results in less fiber damage as compared with lab-scale Kraft pulping.