The difference between never-dried and dried chemical pulps.

Application: Pulps should only be dried if storage, transport, or use of wet pulps is uneconomical or impractical

Most market chemical pulps are dried to approximately 85-90% solids. Where economical, market pulps are also sold in wet laps dewatered to approximately 45% solids. Integrated mills use never-dried fibers in their papermaking operations, resorting to dry- and wetlap pulps during the occasional pulp mill outage. The physical properties and the response to refining of never-dried, wet-lapped, and once-dried fibers are different. Therefore, papermakers have to adjust operating conditions to compensate for these differences.

Study objective

Laboratory studies were carried out to understand the differences in the performance of never-dried and dried kraft pulps. Several species of softwoods and a mixed hardwood were examined. In each case, comparison between never-dried and dried pulps was made on fibers of the same origin.

Discussion

Although never-dried and once-dried fibers are chemically similar, they differ greatly in their physical properties. Never-dried fiber walls contain much more water per unit dry mass than those of dried fibers after rewetting. Being more swollen, the never-dried walls are more flexible or conformable. In contrast, the walls of dried (and rewetted) fibers are stiff. Significant changes in the papermaking properties of fibers occur with water removal as the walls become progressively more rigid and less conformable.

Dried fibers can be made more conformable by mechanical treatment, such as beating or refining, which flexes fibers repeatedly in water. Unfortunately, this requires a considerable expenditure of energy. Moreover, refining of fibers introduces undesirable effects such as fiber shortening and the production of fines, which increase the drainage resistance of the pulp. Furthermore, the changes brought about by drying are not entirely reversible. The photomicrographs in Fig. 1 show that never-dried fibers fibrillate to a much greater extent than once-dried fibers. Figure 2 shows that at a given level of relining energy, the swelling or WRV (water retention value) in dried fibers remains lower than in never-dried fibers.

[FIGURE 1-2 OMITTED]

Clearly, what distinguishes never-dried fibers from dried ones at any beating or refining level is fiber swelling and the resultant conformability of the fiber wall. Both factors greatly influence papermaking and sheet properties. The higher conformability and fibrillation of never-dried fibers translate to lower refining costs to achieve a given tensile strength.

Conclusion

Energy is required to dry pulps. The drying process stiffens fibers, and refining restores only a portion of fiber conformability. Therefore, pulps should only be dried if storage, transport, or use of wet pulps is uneconomical or impractical.

Seth is a scientist at PAPRICAN, 3800 Wesbrook Mall, Vancouver, BC, Canada V6S 2L9. Address correspondence to Seth by email at [e-mail]