Abstract:

Objective Lyocell fibers are recognized as the most promising cellulose-based materials in the 21st century because of their environmental profile and excellent mechanical and wear performance. However, the quality of pulp is one of the most critical factors that affect spinnability and fiber properties. In order to broaden the raw material sources for Lyocell spinning, hemp pulp intended for viscose was used as raw material, in which iron was removed using EDTA disodium salt (EDTA-2Na).

Method The influences of the EDTA-2Na mass concentration, reaction temperature, reaction time period, and solution pH value on iron ion removal rate was investigated. The changes in the microstructure, chemical structure, degree of polymerization, crystallinity, and solubility of the hemp pulp before and after the iron removal were evaluated. Finally, the influences of hemp pulp addition on the mechanical properties of Lyocell fibers was investigated through the standard Lyocell fiber spinning process.

Results The iron removal performance of EDTA-2Na under different reaction conditions was assessed by measuring the iron content in the pulp. The iron content in the pulp first decreased and then increased with the increase of reaction temperature, with the minimum at 50 ℃. As the mass concentration of EDTA-2Na increased, the iron content first decreased, but when the mass concentration was beyond 0.3%, iron in the pulp was almost completely chelated and the content became stabilized. With increasing solution pH value, the iron content decreased and then increased, reaching a minimum at pH value of 8. During the reaction, the iron content in the pulp first decreased, but increased when the reaction time period exceeded 1 h. Under the optimal conditions, the iron content in hemp pulp was reduced to 6.8 mg/kg. Comparison of hemp pulp before and after iron removal treatment revealed almost no changes occurred in its microstructure and chemical structure. The degree of polymerization decreased from 528 to 501, but remained above 500, meeting the requirement for Lyocell spinning. Crystallinity increased from 69.8% to 76.0%. Orientation improved and offset the potential impact of the lower degree of polymerization. After the iron removal treatment, the solubility of hemp pulp in NMMO solvent was improved. The mixing ratio of hemp pulp to wood pulp was adjusted and fibers were spun. Compared with Lyocell fibers prepared from pure wood pulp, fibers perpared from pure hemp pulp exhibited a decrease in crystallinity from 79.28% to 76.67%. The dry breaking strength decreased from 3.8 cN/dtex to 3.29 cN/dtex, and the wet breaking strength decreased from 3.58 cN/dtex to 3.31 cN/dtex. The mechanical properties declined slightly, but still met the requirements the industry standard FZ/T 52019—2018 Lyocell staple fiber.

Conclusion This study confirms the feasibility of using hemp pulp as a raw material for Lyocell fiber production. It also clarified the influence of EDTA-2Na iron removal treatment conditions on pulp properties and spinning quality. The optimal reaction conditions were established, where reaction temperature 50 ℃, EDTA-2Na mass concentration 0.3%, solution pH value 8, and reaction time period 1 h. These conditions reduced the iron content in hemp pulp to 6.8 mg/kg. Compared with the untreated hemp pulp, the de-ironed pulp shows almost no change in chemical structure. However, its degree of polymerization decreases from 528 to 501, and its crystallinity increases from 69.8% to 76%. Solubility also improves. These properties meet the requirements for Lyocell spinning. As the proportion of hemp pulp increases from 0% to 100%, the crystallinity of the spun Lyocell fibers decreases from 79.28% to 76.67%. The dry and wet breaking strengths decrease from 3.8 cN/dtex and 3.58 cN/dtex to 3.29 cN/dtex and 3.31 cN/dtex, respectively. The properties of the prepared Lyocell fibers decrease slightly but remain within the acceptable range.

Key words: hemp pulp, iron ion, dissolvability, Lyocell fiber, fiber property, cellulose fiber