Mechanism And Application Of Cellulose Protection In Oxidation Degumming Of Ramie

Ramie,commonly known as‘China grass'is a perennial herb whose fiber could be used as excellent material for textile industry.It is one of the main economic crops native to China,the production of ramie in China has accounted more than 90%of the total yield in the world.Ramie fiber is one of the strongest and longest plant fibers,lustrous with an almost silky appearance.The high length of ramie fiber makes it suitable for single fiber spinning,which is seldom seen in bast fiber.The production made of ramie fiber possesses many excellent properties such as favorable hygroscopicity,coolness,antibacterial,excellent thermal conductivity,ventilation function et al.Cellulose is the main component of ramie fibers,while the other components in ramie,such as pectin,lignin,water solubles,etc,are defined as gums.Degumming refers to the removal of gums from the cellulosic part of raw ramie,which is necessary prior to textile industrial utilization of fibers.There were mainly two approaches of ramie degumming,namely,traditional chemical degumming and bio-degumming.The energy and time consumption,chemical oxygen demand?COD value?of degumming wastewater in traditional chemical degumming was rather high,for cellulose fiber was extracted by scouring raw ramie in concentrated NaOH under high pressure for 6 h to 8 h.Bio-degumming is an eco-friendly way of ramie fiber extraction,however,the harsh reaction condition and sophisticated equipment inhibited its further industrial application.Under this circumstance,oxidation degumming with alkali peroxides showed good application foreground.Due to the strong oxidation ability of the peroxides,great amount of cellulose degradation may occur during the degumming process and large proportion of hydroxyl groups in cellulose were converted to acid groups?carboxyl groups and aldehyde groups?,which would cause great damage to fiber properties.That was the main problem existed in alkali peroxide oxidation degumming of ramie.In order to further improve the oxidation degumming technology of ramie,this study tried to protect cellulose by using anthraquinone?AQ?,sustained-release alkali source and CSBC?Carboxylated spherical bacterial cellulose,CSBC?in H₂O₂ oxidation degumming process,and studied the possibility of using 2,2,6,6-tetramethylpiperidine-1-oxyl radical?TEMPO?selective oxidation system?TSOS?for ramie degumming.?1?AQ is an important kind of celluloe protective reagent,who posscesses the ability of scavenging excessive free radicals,complexing Fe ions and holding back peeling reaction of cellulose and hemicellulose in H₂O₂ oxidation degumming process.When AQ concentration varied from 1 g/L to 4 g/L,polymerization degree?PD?value of cellulose increased by 12.50%to 19.65%,and residual hemicellulose contents increased with AQ concentration simultaneously.Retaining proper amounts of hemicellulose in fiber could boost the degumming yield without causing much damage to fiber properties.However,excessive amount of hemicellulose would cause the decreasing of DP value and tensile properties of fiber,for hemicellulose owned lower DP value and crystallinity.DP value and tensile properties of fiber showed an upward tendency with AQ concentration from 0 to 2 g/L,but decrease with further increasing of AQ concentration.With the optimal concentration of AQ?2 g/L?,the tenacity,enlongation,rupture,DP value,hemicellulose content and yield of fiber increased by33.21%,19.78%,41.0%,13.9%,16.71%,8.27%,respectively.To monitor the oxidation ability of degumming system,the reaction speed H₂O₂ was tested,results showed that when 2 g/L AQ was used,the reaction speed H₂O₂ decreased by 11%.That was because certain amounts of free radicals and Fe ions were scavenged by AQ.Moreover,COD value of degumming waste water decreased by 8.45%when 2 g/L AQ was used.?2?NaOH was commonly used as the main alkali source in oxidation degumming of ramie in the current studies.However,due to its strong alkalinity,the oxidation reaction speed of H₂O₂ was difficult to control and thus resulted in the great damage to the treated fibers.In this study,the mixture of Mg?OH?₂ and NaOH was selected as an effective sustained-release alkali source for H₂O₂ oxidation degumming.Due to the low solublility of Mg?OH?₂,it can dissolve gradually into the degumming solution with the consumption of OH^-and keep the pH value and oxidizing ability of degumming solution in a proper range.Substitution rate of Mg?OH?₂ was defined as the mole proportion of NaOH replaced by Mg?OH?₂ under the total alkali concentration of 1%.The pH value of degumming solution could be adjusted by changing the substitution rate?SR?and the pH value decreased with the increasing of SR.Residual gum anaylsis revealed that SR should be above 60%to prevent the adhesion of fiber.DP value and tensile properties of fiber increaed with SR from 0%to 20%,but decreased with further rising of SR.When SR was set at 20%,the pH value of degumming solution was 11.8;and the tenacity,enlongation,rupture,DP value,hemicellulose content yield of fiber increased by 39.82%,12.13%,46.15%,14.89%,5%,respectively.Moreover,COD value of degumming waste water decreased by 20%.When sustained-release alkali source was used for degumming,the decomposition speed of H₂O₂ was controlled by degumming temperature.In the initial period of degumming?0 to 60 min?,cellulose degradation seldom occurred,for it was covered by gums.Therefore,large amounts of free radicals were needed and the temperature should be set at 85?.60 min later,most of gums was removed and cellulose was exposed to the degumming solution and the temperature should be raised to 125?to slow down the free radical reaction speed and therefore prevent the degradation of cellulose.?3?Large amount of metal elements existed in ramie degumming system,these metal elements orginated from raw ramie and degumming water.Degumming water mainly contained Ca?3 mg/L⁴5 mg/L?and Mg?3 mg/L¹5 mg/L?.Raw ramie mainly contained six kinds of metal elements,including Ca,Mg,Fe,Mn,Cu,Pb?7403.3,1362.6,202,51.5,13.9,9.2 mg/g?.The decomposition speed and oxidation ability of H₂O₂ increased rapidly in the present of Ca,Fe,Mn,Cu,Pb in the degumming solution,and thus caused great damage to cellulose fiber and its mechanical properties.In previous studies,researchers tried to solve this problem by adding water soluble metal chelating reagent to the degumming solution to mask the negative effect of these metal ions.However,many kinds of chelates remained in the degumming solution still act as the catalytic of H₂O₂ and the use of chelating agent will cause serious environmental problems.In this study,CSBC was introduced into ramie degumming by acting as the metal adsorbent.Experiment results showed that optimal concentration of CSBC varied with the metal contents of degumming water.When water collected from Sichuan was used,the optimal dosage of CSBC was 4%,the tenacity,enlongation,rupture improved by 44.26%,26.47%,31.45%,respectively.When water collected from Hunan was used,the optimal dosage of CSBC was 1%,the tenacity,enlongation,rupture improved by 35.78%,20.97%,28.57%,respectively.In this study,CSBC was prepared by modifying spherical bacterial cellulose?SBC?in TEMPO selective oxidation system,the parameters of SBC culturing and TEMPO treatment was important to the adsorption ability of CSBC.The investigation of SBC culturing process revealed that flask volume,medium volume/flask volume value?M/F value?,shaking speed and culturing time was the decisive factors of SBC properties:?1?only when M/F value was 50%,the SBC was coherence in dimension,the further M/F value was away from 50%,the larger the diameter variation was;?2?when other parameters were maintained constant,the diameter of SBC decreased with the increasing of shaking speed,flask volume and M/F value;?3?the diameter of SBC increased with the culturing time from 0 to 120 h and maintained at constant value for 48 h,after that SBC bagan to combine with each other to form SBC with much higher diameter randomly;?4?when the other parameters were maintained constant,the SBC cultured in smaller flask got larger diameter and better adsorption ability.When both of SBC yield and adsorption ability was taken into consideration,the optimal culturing parameter was found as follow:250 mL flask,32?,150 rpm,120 h.In TEMPO treatment,metal adsorption ability of CSBC increased with NaClO concentration from 0 to 8 mmol/g,due to the increasing of acid group contents,however,further increasing of NaClO concentration would cause decreasing of CSBC adsorption ability,that was because the nanostructure of CSBC was damaged by the oxidation reaction.With the optimal concentration of NaClO,the adsorption capacity of CSBC for Pb²⁺?Fe²⁺?Mn²⁺can reach 370.37 mg/g?357.14 mg/g?85.47 mg/g,respectively.After immersing CSBC in 0.1 mol/L HNO₃,the metal ions adsorbed could be released and CSBC could be used for the second time.After three times of recycles,the metal releasing rate can be kept at levels above 85.6%.?4?In order to investigate the possibility of extracting ramie fiber with selective oxidation reagent,TEMPO selective oxidation system was used for degumming.TEMPO oxidation could be operated under mild condition and only C6 primary hydroxyl groups of cellulose could be convert to carboxyl groups and the secondary hydroxyl groups of cellulose were left unchanged.Results showed that ramie fiber with good quality could be extracted using a short TEMPO treatment followed by low concentration NaOH scouring.Larger NaClO concentration,higher pH value,longer oxidation time and higher NaOH concentration was helpful for removing gums,however,also aggravated cellulose degradation.This study tested the PD value of fiber after TEMPO oxidation and results showed that proper NaClO concentration and pH value was 1.5 mmol/g².5 mmol/g,10.5¹2.0,respectively.And the best degumming temperature was room temperature.In order to investigated the synthetic effect of pH value,concentration of NaClO,TEMPO oxidation time,concentration of NaOH on fiber tenacity,surface response design method was used in this study,the optimal operating parameters were 2.2 mmol/g NaClO,pH value 10.91,oxidation time 17.64min,NaOH 3.14 g/L.The properties of fiber?including yield of degumming,tensile properties and PD value?degummed in TEMPO selective oxidation system was slightly better than the ones degummed by H₂O₂ oxidation degumming,which was good enough to meet the requirement for spinning and other usage.The required time for TEMPO selective oxidation degumming was 127.64 mins,which was 72.36 mins less than H₂O₂oxidation degumming.The energy consumption for TEMPO oxidation degumming was much lower than H₂O₂ oxidation degumming,for the TEMPO degumming was processed under room temperature.Moreover,the COD value of wastewater in TEMPO oxidation degumming was 28%and 44%lower than that in alkali peroxide oxidation degumming and traditional chemical degumming.?5?Enlarge H₂O₂ oxidation degumming experiment and spinning trial was carried on in three enterprises.Results indicated that pressure scouring?0.6 kg,125??was necessary to ensure the uniformity of fiber properties.Defoamer reagent could reduce the concentration of H₂O₂ from 6 g/L to 4.5 g/L required for degumming.Weight of raw raime did not have much influence on the COD value of degumming waste water.Although tensile properties of fiber prepared by oxidation degumming was not lower than that of traditional degumming,the fiber and yarn prepared in oxidation degumming could meet the requirement of downstream application.