Textile fibers prepared by combined alkali soaking, steam explosion and laccase/mediator treatments to bark of cotton stalks

Abstract: As we all know, burning of cotton stalks will bring serious environment pollution. In order to protect the environment and make efficient utilization of cotton stalks, this paper presented a new method for the preparation of cellulose textile fibers, which could obtain better fibers from bark of cotton stalks. It was reported that natural cellulose fibers from bark of cotton stalks had significantly better mechanical properties than those from other lignocellulosic agricultural byproducts such as rice and wheat straws. Fibers from bark of cotton stalks were used to reinforce thermoplastic composites, but they were difficult to be spun into textile yarns due to their high fineness value and rigidity. However, conventional method of using strong alkaline solution (30-100 g/L NaOH) for extraction of cellulose fibers from lignocellulosic byproducts led to environmental pollution and high cost. In this paper, the new method included 3 combined steps, i.e. alkali soaking, steam explosion and laccase mediator system treatment. Bark of cotton stalks were first soaked in NaOH solution at 20℃ for 24 h, steam-exploded with alkali within 0.0875 s and then the cotton stalk balk fibers obtained by steam explosion were treated in laccase mediator system to further degrade lignin. The effect of NaOH dosage on the separation of bark of cotton stalks was studied. The effects of the kind and dosage of the mediator, and the laccase dosage on the lignin mass fraction of cotton stalk bark fibers were also investigated. The chemical composition, morphology, mechanical property, hygroscopic property, crystalline structure and thermal stability of cotton stalk bark fibers before and after treated by steam explosion and laccase mediator system were comparatively investigated by chemical compositions analysis, scanning electron microscopy (SEM), tensile property test, moisture regain and water retention test, X-ray diffraction (XRD) and thermal stability analysis (TGA). The results showed the effect of each step in the combined method for fibers preparation from bark of cotton stalks. The optimized processing parameters were as follows: NaOH amount of 10 g/L, soaking at 20℃ for 24 h, ratio of liquor to fiber of 15 mL/g, steam pressure of 0.8 MPa, holding time of 180 s, laccase dosage of 600 U/g, ABTS mediator amount accounting for 1% of dry weight of cotton stalk bark fibers, temperature of 55℃, pH value of 3.5, and treatment time of 10 h. The results indicated that the cotton stalk bark fibers prepared by the combined method had cleaner surface, lower moisture regain and water retention, higher initial decomposition temperature and temperature for the maximum decomposition rate i.e. superior thermal stability. By the optimized combined method, the cotton stalk bark fibers had length of 55.7 mm, fineness of 28 dtex, aspect ratio of 1139, tensile strength of 2.97 cN/dtex, cellulose mass fraction of 78%, crystallinity index of 67.5, moisture regain of 10.7%, water retention of 104.4% and yield of 40%, which could be used for textile industry. The research can provide a reference for further research on the extraction of natural cellulose fibers from lignocellulose byproducts such as cotton stalks, wheat straws and rice straws.