Hexamethylolmelamine was used as a cross-linking agent to graft 3-(dimethylphosphono)-N-methylolpropionamide (MDPA) onto Lyocell fibers in a posttreatment process to reduce flammability. The effects of processing conditions on the mechanical properties and P content of the fibers were examined. The results show that increasing the MDPA concentration, microwave treatment time, baking temperature, and baking time increases the P concentration in the fibers, although the mechanical properties of the fibers are reduced to varying degrees. The optimal processing conditions comprise a 40% MDPA concentration, a 320 W microwave treatment for 3 minutes, and baking at 160 C for 5 minutes. Under these conditions, flame-retardant Lyocell fibers with a breaking strength of 2.47 cNÁdtex À1 and a limiting oxygen index of 34.6% were obtained, which meet the performance requirements for textile fibers. Thermogravimetric data indicate that the residual fiber mass after heating increased from 4.7% before treatment to 21.5% after treatment. There was only a minimal change in the hygroscopicity of the treated Lyocell fibers. In addition, after 30 washes, the limiting oxygen index was decreased to 26.5% while maintaining a good flame-retardant effect.
In order to overcome the disadvantage of Lyocell fiber flammability, two types of flame-retardant finishing liquids, 2-carboxyethyl phenylphosphic acid (CEPPA) and N-hydroxymethyl-3-dimethoxyphosphoacyl propanamide (MDPA), were used in this study to treat Lyocell fiber in two different states: never-dried and dry. The results showed that CEPPA and MDPA can react with the hydroxyl groups of the cellulose and graft onto the Lyocell fiber under appropriate conditions, resulting in increased flame-retardant performance of the fiber, a slight reduction in crystallinity, and a significant decline in mechanical properties. Compared with the dry fiber, the P content and LOI of the fiber obtained by treating the as-spun never-dried Lyocell fiber rose significantly: the P content was higher by 38.9% (for CEPPA) and 20.5% (for MDPA), respectively, while the LOI increased by 6.0% (for CPPA) and 4.0% (for MDPA), respectively, which means that the fiber had better flame-retardant performance. Although the breaking strength of the fiber decreased, it still met the requirements for textiles. In addition, direct flame-retardant treatment of never-dried wet fiber can reduce energy consumption by avoiding repeated drying. Furthermore, the results of this study also have guiding significance for other post-processing procedures for Lyocell fibers, such as dyeing, catalyst infiltration during carbon fiber preparation etc
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