Abstract:

Objective Lyocell fiber direct web formation is an innovative method of preparing nonwoven materials by drafting a cellulose solution through a liquid stream and directly webbing the filaments by a dry-spray wet method using air gap cooling. It has the advantages of short preparation process, low investment in equipment and small footprint. The prepared non-woven material has high strength, good softness, good moisture absorption and air permeability, and low flaking rate, and can be applied in the fields of beauty mask, medical gauze, high-end wiping paper, and tea bags.

Method In this research, the liquid flow rate of the liquid flow drafting device under different structural parameters was investigated, and the crystallinity, breaking strength, liquid absorption rate, permeability and softness of the materials prepared under different liquid flow rates, different liquid flow NMMO concentrations and temperatures were studied comparatively, so as to determine the optimal structural parameters of the device and the liquid flow process parameters.

Results In the case of constant accelerating fluid flow rate, the total fluid rate increased with the increase of the ratio of the accelerating runner inlet and outlet widths (d₀/d₁). However, if d₀/d₁ value is too large, the accelerating fluid flow outlet pressure would be too large, which is prone to cause the non-uniformity of the fluid velocity. As the width of the bottom plate increased, the liquid flow velocity was decreased, and the width of the bottom plate was preferably 3-4 mm. As the liquid flow rate increased, the diameter of the fiber gradually became finer, and the finest was 9 μm in diameter, and the crystallinity and orientation of the fiber gradually increased with the increase of the liquid flow rate. The longitudinal and transversal breaking strengths of the nonwoven materials showed a tendency to increase with the increase of the liquid flow velocity. With the increase of the liquid flow rate, the liquid absorption rate and air permeability of the nonwoven materials demonstrated a decrease, and the softness of the nonwoven materials in both dry and wet states was deteriorated. With the increase of the concentration of NMMO, the crystallinity of the nonwoven materials was increased, and with the increase of the liquid flow temperature, the crystallinity of the nonwoven materials was decreased. The longitudinal and transverse breaking strength of the nonwoven materials was increased as the concentration of NMMO in the liquid flow increased, and the longitudinal and transverse breaking strength of the nonwoven materials was decreased as the temperature of the liquid flow increased. As the concentration of NMMO in the liquid flow increased, the softness of the nonwoven material was deteriorated, and as the temperature of the liquid flow increased, the softness of the nonwoven material was improved. Under the optimal process parameters, the prepared material had the longitudinal breaking strength of 35-40 N, transverse breaking strength of 18-25 N, liquid absorption rate of 1 100%-1 200%, air permeability of 4 600-4 900 mm/s, and dry state longitudinal softness 240-280 mN, dry state transverse softness 60-70 mN, wet state longitudinal softness 80-100 mN, and wet state transverse softness 49-56 mN.

Conclusion By optimizing the ratio of the inlet and outlet widths of the accelerating runner and the width of the base plate, the flow rate is maximized, resulting in improved fiber drafting. An increase in flow rate leads to a reduction in fiber diameter and an increase in crystallinity, which in turn enhances the longitudinal and transverse breaking strength of the nonwoven material. An increase in the liquid flow rate also leads to a decrease in the liquid absorption rate and air permeability of the material, and a deterioration in softness. As the concentration of liquid rate NMMO increases, the crystallinity and breaking strength of the material increase, and the softness decreases; while the increase of liquid rate temperature leads to the decrease of crystallinity, the decrease of breaking strength, and the improvement of softness. By precisely controlling the structural and process parameters of the liquid flow drafting device, the properties of Lyocell nonwoven materials can be effectively regulated, and nonwoven materials with high strength, good moisture absorption and air permeability, and suitable softness can be prepared. Future research can further explore the specific requirements of material properties in different application scenarios and achieve more refined process control and product customization.

Key words: cellulose solution, Lyocell fiber, direct web formation, fluid flow drafting, moisture absorption and permeability, softness