Abstract
A NaOH/urea (or thiourea) solvent system capable of dissolving cellulose at lower temperatures is a breakthrough in cellulose chemistry, and it was reported that cellulose rapidly dissolved when it was added to a precooled aqueous solution of sodium hydroxide (NaOH) and additives. In fact, upon initiating this work, we plan to fabricate pulp fibers based cushioning packaging material based on part dissolution of dissolving pulp in aqueous NaOH/additives solutions, and found certain dissolving pulp added to a precooled aqueous solution did not show better dissolution. Therefore, this work compared the effectiveness of the direct dissolution method and freezing-thaw method in dissolving pulp fiber and pure cellulose. Three aqueous solutions were examined: 7% NaOH/12% urea, 9.5% NaOH/4.5% thiourea, and 8% NaOH/8% urea/6.5% thiourea. The dissolving capacity of three NaOH/additives aqueous solutions was analyzed by polarized optical microscopy and the dissolved cellulose proportion was determined. The results showed that the never-dried softwood dissolving pulp and bamboo dissolving pulp achieved better dissolution using freezing-thaw method than using direct dissolution method in the three aqueous solutions. The dissolving method had a negligible effect on the dissolution of each dissolving pulp in the 8% NaOH/8% urea/6.5% thiourea solution. It seems that the direct dissolution method was more suitable for oven-dried microcrystalline cellulose with a low degree of polymerization (DP) and the freezing-thaw method was more suitable for never-dried pulp cellulose fibers with a higher DP.
Highlights
Aqueous NaOH/additives solutions for the dissolution of pulp cellulose have attracted wide attention from researchers because they are inexpensive, eco-friendly, and dissolve rapidly
The degree of polymerization (DP) of the hardwood dissolving pulp (HDP) and bamboo dissolving pulp (BDP) was provided by the manufacturer, while the other parameters were measured in the laboratory
The images clearly showed that the 8% NaOH/8% urea/6.5% thiourea solution had a better dissolving capacity than the 7% NaOH/12% urea and 9.5% NaOH/4.5% thiourea solutions for each dissolving pulp with a DP higher than 740, regardless of which dissolving method was used
Summary
Aqueous NaOH/additives solutions for the dissolution of pulp cellulose have attracted wide attention from researchers because they are inexpensive, eco-friendly, and dissolve rapidly. The direct dissolution of cotton linter pulp cellulose in any precooled aqueous solution of. LiOH/urea, NaOH/urea, or NaOH/thiourea has been found to produce a stable cellulose solution [1, 5,6,7]. In a study regarding the direct dissolution of cotton linter pulp cellulose in an 8% NaOH/8%. Urea/6.5% thiourea aqueous solution precooled to -12 °C, it was found that untreated(never dried) or inactivated cellulose was able to dissolve directly and quickly [8,9]. A stable inclusion complex formed by cotton linter pulp cellulose in a NaOH/urea aqueous solution precooled to -12 °C was studied by Qin et al [10]. The dissolving behavior of cotton linter pulp cellulose in a 7%
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