Abstract:

Objective As a popular type of urban landscaping tree, camphor trees (cinnamomum camphora) generate substantial amounts of discarded trunks and branches annually. However, China has long relied on imports to meet its demand for high-grade dissolving pulp used in textile fiber production. To achieve value-added utilization of camphor wood resources and develop novel raw material sources for producing dissolving pulp, this study investigates the preparation process of camphor wood dissolving pulp using discarded trunks and branches as feedstock.

Method The camphor wood dissolving pulp was prepared by the pre-hydrolysis kraft process. First, camphor wood chips were subjected to hot water pre-hydrolysis to remove most hemicellulose, with single factor experiments to investigate how different process conditions would affect the pre-hydrolysis. Subsequently, the pre-hydrolyzed wood chips underwent kraft cooking using NaOH and Na₂S to eliminate lignin, and orthogonal experiments was employed to determine optimal cooking parameters. Finally, the obtained pulp was bleached through a four-stage elemental chlorine-free (ECF) bleaching sequence, whick are ClO₂ bleaching (D₁), alkali refining treatment (E), ClO₂ bleaching (D₂), and H₂O₂ bleaching (P), to improve pulp quality. The prepared dissolving pulp was then tested for degree of polymerization, α-cellulose content, and brightness, while its surface morphology was examined by scanning electron microscopy.

Results The hot water pre-hydrolysis process was found effective in removing a substantial portion of hemicellulose from camphor wood. The single-factor variable study revealed that the contents of pentosan and holocellulose exhibited non-monotonic trends with increasing pre-hydrolysis temperature or prolonged holding time, while variations in the solid-to-liquid ratio showed no significant effect on their contents. For the kraft cooking process, given the stringent requirements for dissolving pulp regarding degree of polymerization, α-cellulose content, and brightness, a comprehensive evaluation through orthogonal experimental analysis indicated the following order of parameter influence: alkali charge > sulfidity > holding time > cooking temperature. The optimal conditions were determined as 22% alkali charge, 25% sulfidity, and a holding time of 120 min at 160 ℃. Under these conditions, the resulting camphor wood pulp exhibited a polymerization degree of 1 035.85, an α-cellulose content of 90.63%, and a brightness of 21.35%. During ECF bleaching, the stepwise removal of lignin through the four-stage D₁ED₂P sequence increased the pulp brightness to 78.91%, while simultaneously elevating the α-cellulose content and reducing the polymerization degree. SEM observations indicated that the original camphor wood surface contained abundant tubular pores, facilitating the penetration of pre-hydrolysis and cooking liquors. After the pre-hydrolysis kraft treatment, complete fiber separation was achieved due to the extensive removal of lignin and hemicellulose. The liberated fibers displayed a flattened morphology with surface wrinkles and grooves.

Conclusion Pre-hydrolysis effectively removes hemicellulose, with the contents of pentosan and holocellulose showing non-monotonic variations depending on pre-hydrolysis temperature and duration. Orthogonal experiments revealed that the influencing order of kraft cooking parameters was alkali charge > sulfidity > holding time > cooking temperature. The optimal conditions were determined as: 22% alkali charge, 25% sulfidity, 120 min holding time, and 160 ℃ cooking temperature. After four-stage ECF bleaching (D₁ED₂P sequence), the resulting dissolving pulp exhibited satisfactory properties, with 914.75 of polymerization degree, 94.17% α-cellulose content, 78.91% brightness, 0.42% ash content, and 10.62 mg/kg iron content, meeting essentially the requirements for regenerated cellulose fiber pulp. This indicates that camphor wood can serve as a high-quality raw material for producing dissolving pulp for spinning. However, further process optimization is necessary to enhance pulp brightness and reduce ash content. This study demonstrates a viable approach for the value-added utilization of camphor wood waste resources, with the prepared dissolving pulp satisfying the requirements for textile fiber applications and showing promising potential for industrial-scale implementation.

Key words: camphor wood, pre-hydrolysis, kraft, dissolving pulp, chemical pulp, pulp, cellulose fiber