Degumming Of Apocynum Venetum Fibers By Biochemical Treatment In Supercritical CO₂

In order to make better use of Apocynum venetum bast fiber resources in the textile industry, it is necessary to degum Apocynum venetum bast fiber, and the degumming quality is crucial to the subsequent processing of Apocynum venetum bast fiber. Currently, among the Apocynum degumming processes, natural water retting process shows advantage of low cost while it has disadvantages of incomplete retting, long cycle and water pollution. Chemical degumming process realizes more thorough degumming, but havs damaging effects on the properties and smoothness of Apocynum fiber, with other disadvantages such as high water and energy consumption as well as serious environmental pollution. Microbial degumming process is environment-friendly, but the quality of degummed Apocynum venetum bast fiber is unstable. In addition, flavonoid compounds in Apocynum venetum bast fiber are all released into the degumming waste solution andare not well utilizedamong the above degumming processes, thus resulting in wasting of resources.To solve the above problems, supercritical CO₂ synergistic biochemical degumming process for Apocynum venetum bast fiber is proposed according to the demands of current environmental situation. In the supercritical state, CO₂ fluid is used to carry bio-enzyme and chemical reagents to degum Apocynum venetum bast fiber. The swelling ability, high diffusivity and high solubility of supercritical CO₂ fluid are used to accelerate the reaction speed, improve mass transfer, increase the selectivity and yield of reaction product, promote product separation, and achieve supercritical CO₂ degumming of Apocynum venetum bast fiber. Moreover, flavonoids in Apocynum venetum bast fibers extracted to improve the overall utilization value of Apocynum venetum bast fiber.（1） Supercritical CO₂ degumming pretreatment process of Apocynum venetum bast fiber was first studied. In order to reduce the burden of supercritical CO₂ degumming of Apocynum venetum bast fiber and accelerate the rate of degumming, it is necessary to carry out supercritical CO₂ degumming pretreatment for Apocynum venetum bast fiber. The pretreatment process will cause the swelling of Apocynum venetum bast fiber, increase the chances of bio-enzyme combining with noncellulosic materials in Apocynum, accelerate the mass transfer rate, quickeing the degumming process. Influences of time, entrainer content, temperature, pressure and CO₂ flow rate on the swelling of Apocynum in supercritical state were analyzed. The study results showed that the entrained certain proportion of ethanol solution in degumming pretreatment of Apocynum venetum bast fibercould enhance the elasticity and flexibility of fiber and improve the swelling ability of Apocynum venetum bast fiber in supercritical CO₂. The swelling ability of Apocynum venetum bast fiber constantly increased along with the increase of temperature and pressure. However, no significant change of swelling ability of Apocynum venetum bast fiber was detected after temperature and pressure exceeded 60 °C and 20 MPa. CO₂ flow rate had low influence on the swelling ability of Apocynum venetum bast fiber, so a lower CO₂ flow rate could be selected in order to reduce CO₂ consumption. After Apocynum venetum bast fiber was treated in supercritical CO₂ at 58 °C, 21 MPa with 70% ethanol solutionfor 63 min, its density was reduced from 1.183 g/cm3 to 1.085 g/cm3 with porosity increased. The test results showed that supercritical CO₂ degumming pretreatment could remove partial fat & wax and other impurities and cause the swelling of fiber, result in the displacement and rearrangement of fiber macromolecular chains, increasing crystallization index and grain size, and improving the thermal stability.（2） When supercritical pretreatment was conducted for Apocynum venetum bast fiber, supercritical CO₂ extraction of flavonoids in Apocynum was also performed. The test results showed that the polarity of supercritical CO₂ was significantly enhanced along with the increase of entrainer content, while the polarity enhancement of supercritical fluid would improve the solubility of flavonoid in supercritical CO₂ and increase the extraction rate of flavonoids. In addition, the increase of entrainer content, especially water insupercritical state which appears to be weak-acidic, would destroy the interaction between flavonoid molecules and Apocynum venetum bast fiber, thereby facilitating the separation of flavonoids, and improving the extraction rate of flavonoids. When the entrainer content was too high, the increase of moisture led to the formation of the two-phase solvent in extraction kettle, affecting the extraction process. When the entrainer was 70% ethanol solution and entrainer content was 2%, the extraction results of flavonoids were satisfactory. The extraction rate of flavonoids increased firstly and then decreased along with the increase of pressure and temperature, which was higher at 45 °C and 20 MPa. The flow rate of supercritical CO₂ had a positive impact on extraction of flavonoids. On that basis, a mass transfer model of supercritical fluid extraction of flavonoids was established to work out the mass transfer coefficient Kf·a and Ks·a under various conditions, and to reflect the effect of each operating factor on the extraction process; the calculated values of this model tally with trial values, which means that the model established is able to reflect extraction process well, while playing an important guiding role in the industrial amplification of extraction process.（3） How supercritical state affects enzyme stabilityis conducted: when processing time is less than 90 min, the activity of pectinase, xylanase and laccase is relatively high, and their relative activity ratesare 90%, 82.5% and 90.4%, respectively. With time increase, the vitality of pectinase and xylanase drops quickly, while that of laccase remains unchanged. Temperature experiment on enzyme stability shows that pectinase and xylanase present better heat stability in supercritical CO₂ system below 60 °C. Pressure exerts a limited impact on enzyme stability; pectinase and laccase show good compressive stability, while that of xylanase is slightly inferior. When the test on bio-enzyme degumming is conducted under supercritical CO₂ conditions, the concentration rate of pectinase, xylanase and laccase are 1%, 0.6% and 1%, respectively. Using a cycle adding approach for enzyme, that is, adding enzyme for 10 min and then stopping it for 20 min before next round of adding cycle; and adding enzyme in this way allows faster degumming rate and desirable degumming effect. Temperature rise brings two effects on supercritical degumming: first of all, degumming rate will rise with temperature increase; secondly, enzymatic inactivation and denaturation occur. In 50 °C, pectinase and xylanase enjoy the biggest weight-loss ratio and the best degumming effect with the least temperature effect on laccase degumming; below 60 °C, degumming weight-loss ratio remains unchanged. Pressure is another important factor influencing enzymatic degumming: on the one hand, pressure built-up results in increased density of supercritical CO₂, shortened mass-transfer distance between molecules, improved mass-transfer efficiency and enhanced reaction rate; on the other hand, fluid viscosity increases with rising of pressure built-up; mass-transfer effect becomes worse, thus reaction rate is lowered; thirdly, pressure change will possibly lead to the change onenzyme conformation, thus affecting enzymatic activity; when pectinase is in 20 Mpa and xylanase is in 25 MPa, their degumming weight-loss ratios are the highest. As for the effect of p H value on degumming, the weight loss ratio of pectinase and xylanase is the biggest in p H5 buffer solution, while laccase has the biggest weight loss ratioin p H6 buffer solution. On the basis of single-factor experiment, degumming process is optimized according to Box-Behnken center’s combinational design. The optimal technological parameters was obtained at 51 °C, 20 MPa, p H5 and 140 min（degumming time）. It turns out that degumming weight-loss ratio is 30.9%, showing ideal optimized outcome. Compared with enzyme degumming under normal pressure, bio-enzyme degumming under supercritical CO₂ conditions consumes less water and time.（4） Another research is conducted on the chemical degumming of apocynum venetum bast fiber under supercritical state, by analyzing the temperature, pressure and time under different supercritical treatment conditions, as well as the effect of chemical reagent concentration on degumming. The optimization on the degumming process research is also conducts, with optimum craft parameters of 100 °C, 20 MPa, 80 min, and 10 g/l Na OH（for degumming reagent）. Under such degumming conditions, the residual gum rate for apocynum venetum bast fiber is 9.08%. All factors are arranged in a descending order in accordance to their degree of impact on degumming, that is, temperature﹥Na OH concentration﹥time﹥pressure. The results show that degumming temperature exerts the biggest impact on fiber residual gum rate, followed by Na OH concentration and time. Pressure exerts the least impact. Infrared spectroscopic analysis shows that, through bio-enzymatic and chemical treatments under supercritical CO₂ conditions, the lignin and hemicellulose in apocynum have been effectively removed; XRD test shows thatboth its crystallization index and grain size have been increasedafter supercritical treatment, partly because non-cellulosic material has been removed during the treatment. The other reason is the displacement of fiber macromolecular chain under supercritical CO₂ conditions, resulting in the rearrangement of macromolecular chain and change on crystallization index. Thermogravimetric analysis shows that the heat stability of apocynum venetum bast fiber has been improved with supercritical treatment; SEM test shows that the gum wrapped around apocynum is basically removed after chemical degumming under supercritical CO₂ conditions, with smooth and cleanfiber surface. The fiber obtained with biochemical degumming under supercritical CO₂ conditions is superior in single-fiber strength and length in comparasion with traditional chemical method, but its fineness and residual gum rate are inferior to that of traditional chemical method. In general, the biochemical degumming under supercritical CO₂ conditions is able to basically meet degumming purpose and requirement.