Circulating solution for the degumming and modification of hemp fiber by the laccase–2,2,6,6-tetramethylpiperidine-1-oxyl radical

Abstract

Hemp fiber, as a kind of renewable textile fiber, has evoked much interest due to its excellent properties in various fields. It has been investigated in just one step to finish the process of chemo-enzymatic degumming and modification of hemp fiber, which is done in acetic acidsodium acetate buffer solution containing laccase (6.0 g), hemicellulase (0.3 g) and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (0.3 g) under a mild condition (pH 5, 50C). The degumming effects of different batches of raw hemp fibers were also discussed. We found that the residual rate, fineness, length and tensile strength of degummed fiber were 15–27%, 9.00–24.00 dtex, 14.00–19.00 cm and 3.8–5.1 cN/dtex, respectively. Meanwhile, some of the oxidized functional groups were introduced into degummed fiber, which improved the moisture absorption and water retention value of the hemp fiber. The hemp fiber treated by this process could be further improved and then applied in the textile industry.

Keywords

hemp fiber circulating degumming 2 6 6-tetramethylpiperidine-1-oxyl radical laccase hemicellulase

Hemp, as the oldest cultivated fiber plant, has failed to become one of the major textile crops mainly due to its complex degumming process. In view of its excellent properties, such as high tensile strength, good thermal and electrical properties, antibacterial properties and biodegradability, there is an urgent need to simplify the degumming process of hemp fiber, considering its product and environmental quality as well as economic concerns.¹ Nowadays, the chemical degumming process used in the industry is as follows: raw hemp, scutching, acid soaking, washing, alkali boiling, washing, dehydration, bleaching, oil finishing and drying.² Chemical degumming for hemp fiber requires much more time and a large amount of acids and alkali. Thus, the process not only causes higher cost and serious environmental pollution, but also leads to great damage to degummed hemp fiber.³ Considering all of this, the substitution of hazardous chemicals such as strong acid or alkali at the source and/or shortening the preparation steps are potential options.⁴

It was well known that the composition of raw hemp is several elementary fibers held together with 20–55% of non-cellulosic gummy compounds.⁵ The length of a single totally degummed hemp fiber is too short to meet the requirements of the follow-up spinning process. During the degumming process, it is necessary to retain some gummy compounds, where they are mainly pectin and hemicellulose, to keep the degummed hemp fiber in the form of technical (multi-cellular) fibers. In the hemp degumming field, the enzymatic degumming technology, as an environment-friendly and highly effective method, is receiving more and more attention. Ding et al.⁶ put forward to an optimization of the enzyme mixture for degumming ramie. The degummed ramie fiber they produced fulfilled the requirements of the following textile process. Zhang and Yan⁷ managed to degum ramie with the employment of compounded enzyme preparate, and the fibers could be directly applied in the textile industry. However, the raw ramie fiber needed to be pre-treated with 1% Na₂CO₃ at 100℃ for 20 min, after which it should be treated with a solution containing pectinase, hemicellulase and laccase at 37℃ for 48 h. The degummed fiber would finally be rinsed and dried again. The whole degumming process took a long time, and the multiple steps made it a complicated method to operate. Besides, the repetitive rinsing could do some damage to the fiber.

Yu et al.⁸ studied hydrophobic cotton fabrics, which were prepared by grafting octadecylamine (ODA) onto cotton fiber surfaces via the laccase/2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) treatment. The cotton fibers were oxidized by laccase/TEMPO to introduce aldehyde groups, which reacted with the amino groups of ODA to form a Schiff base. Liu et al.⁹ studied and discussed the effects on the structure of hemp fibers of different dosages of NaClO used in the TEMPO–laccase degumming system, demonstrating that this degumming system was efficient in removing non-cellulosic materials. Liu et al.² studied hemp fiber that needed to be pre-treated with the laccase–TEMPO system, and then the pre-treated hemp fibers were subjected to a further alkali boiling treatment.

The traditional degumming methods mainly use high solubility sodium hydroxide or hydrochloric acid. There is no doubt that the use of sodium hydroxide and hydrochloric acid would pollute water and soil. In this study, we provided an environmentally friendly system in the degumming of hemp fiber without using strong acid and alkali. The advantages of the hemp fiber degummed by the laccase/TEMPO/hemicellulase (LTH) system is a short process, simple operation, short processing time, low energy consumption (mild processing conditions) and low fiber damage as compared with the traditional degumming process. The degumming solution could be continuously used to degum five different batches of raw hemp fibers under a mild condition. Besides, this experiment introduced and tuned the content of the aldehyde (CHO) and carboxyl (COOH) groups on the hemp fibers to improve the water retention and moisture sorption. Hemp fibers so treated could meet the spinning requirement after further post-processing.

Experimental details

Materials

Raw hemp fibers used in the study were cultivated in Heilongjiang Province,China. Laccase (activity =600 U/g, pH = 4.5–6.5, T = 30–65℃) and hemicellulase (activity = 200,000 U/g, pH = 4.0–5.5, T = 30–60℃) were purchased from Jiangsu Ruiyang Biological Technology Co., Ltd. TEMPO and sodium chlorite (NaClO₂) were purchased from Aladdin Industrial Corporation. Sodium acetate anhyfrums (CH₃COONa) and acetic acid (CH₃COOH) were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. All the chemicals were of analytical reagent grade without further purification. Deionized water was used to prepare all solutions.

Preparation of degummed hemp fibers

The constant humidity content of raw hemp fibers was insured by drying the fibers at 40℃ for 24 h in an oven before all the degumming treatments. The raw hemp fibers (25 g of dry fibers) were immersed in the degumming solution an acetic acid–sodium acetate buffer solution (0.2M, pH 5) containing laccase, hemicellulase and TEMPO at 50℃ for 8 h, and the solution was be stirred for 1 min every hour. Under this mild condition, the activity of both enzymes can achieve high activity at the same time. The bath ratio of the degumming solution versus raw hemp fibers was 20:1. The treated fibers were taken out immediately from the degummed solution after 8 h, and then a new batch of raw hemp was put into this solution until the fifth batch of raw hemp was taken out. The treated fibers were washed thoroughly with deionized water and then dried at 105℃ in a blast oven for 3 h.

Scanning electron microscopy

The morphology of hemp fibers was observed using a scanning electron microscope (SEM, VEGA 3, TESCAN Ltd, Czech Republic) at 20℃, 60% relative humidity (RH); samples were mounted on a copper plate and sputter-coated with gold layer prior to imaging.

Determination of the aldehyde and carboxylate groups

The aldehyde (-CHO) and carboxylate (-COOH) contents of the degummed fibers were determined by the electric conductivity titration method.^10–12 For that purpose, 0.3 g of dried fiber sample was added to water (55 mL) and 0.01 M NaCl (5 mL). The mixture was stirred to prepare a well-dispersed slurry. Then 0.1 M HCl was added to the mixture to set the pH value in the range of 2.5–3.0 after stirring for 1 h in a magnetic stirrer in a sealed environment. A 0.04 M NaOH solution was added at the rate of 0.1 mL/min up to pH 11 by using a pH stat. The carboxylate content of the sample was determined from the conductivity and pH curves and expressed as an average value of three measurements.

The aldehyde content in the treated fibers was measured according to the following procedure. The degummed fibers were further oxidized with 0.2 M sodium chlorite at pH 4–5 for selective conversion of the aldehyde groups in the samples to carboxylate ones, and carboxylate content was determined by the electric conductivity titration method mentioned above. The carboxylate groups formed by the NaClO₂ oxidation were regarded as aldehyde groups present in the original degummed fibers.

Chemical components analysis

The chemical composition of hemp fibers was tested according to Chinese national test standard of GB/T 5889-86 (Method of quantitative analysis of ramie chemical components).

A total of 5 g of raw hemp fibers for each sample were dried in the oven for 3 h. Then, the dried fibers were processed under boiling alkali solution (150 mL, 20 g/L) for 1 h and re-boiled for 2 h with the new alkali solution. After the boiling treatment and the washing process, the fibers were dried to constant weight. The cellulose content of hemp fibers was calculated as follows

Cellulosecontent (%) = 1 - \frac{G_{0} - G}{G_{0}} \times 100

(1)

where G₀ is the dry weight of the raw hemp sample and G is the dry weight of the sample after treatment.

To remove the resins, oils, fats and waxes, 5 g raw hemp fibers were firstly degreased with 150 mL benzene–ethanol mixed solution (2:1 by volume). Then 1 g degreased raw hemp fibers were weighed for each sample in weighing bottles and dried at 105℃ for 3 h in the oven. After weighting the oven-dried specimens, they were transferred to a beaker of 50 mL with a glass cover. H₂SO₄ (72%), 30ml, was added slowly, the specimen was mixed well with the acid by constantly stirring for at least 1 min and then allowed to stand for 24 h. The materials were transferred into a beaker of 500 mL, deionized water of 270 mL was added, followed by boiling for 1 h. After allowing the insoluble material to settle, it was filtered into a weighted sand core funnel that had been dried. It was washed free of acid residue with hot water, then dried in the sand core funnel with lignin at 105℃ for 3 h, and weighed. The lignin content of raw hemp fibers was calculated as follows

Lignincontent (%) = \frac{G_{L} - G_{F}}{G_{D}} \times 100

(2)

where G_D is the dry weight of degreased raw hemp fibers; G_L is the dry weight of the sand core funnel with lignin; G_F is the dry weight of the empty sand core funnel.

Fourier transform infrared spectroscopy

The Fourier transform infrared spectroscopy (FT-IR) spectra were examined using a Nicolet 6700 spectrometer (Thermo Fisher Scientific, Waltham, USA). A total of 30 scans were taken over a range from 4000 to 600 cm⁻¹ with a resolution of 0.4 cm⁻¹. The baseline correction and smoothing were accomplished prior to further analysis.

Mechanical property test

All samples were conditioned in the standard atmospheric condition (temperature: 20 ± 2℃, RH: 65 ± 3%) for 24 h before testing. The breaking tenacity of fibers was carried out using the XQ-1A fiber tensile tester (New Fiber Instrument, Shanghai, China). The gauge length and drawing speed were kept at 20 mm and 20 mm/min, respectively. Average values were obtained using results from 100 specimens.

Physical property test

Before starting the mechanical property tests, the degummed hemp fibers were firstly treated by a carding process, which is essential in the spinning process. Its primary function is to loosen the degummed fibers and convert them into uniform, straight, parallel and separate fibers in the yarn.¹³ The carding process was performed on an FA210B Flat Card (Hongda, Qingdao, China) with the speed of the cylinder, stripper and doffer of 330, 720 and 20 r/min, respectively. Before the mechanical property tests, all of the samples were balanced in standard atmospheric condition with a temperature of 20℃ ± 2℃ and RH of 65% ± 3% for 24 h. The linear density, tenacity and elongation at break were tested in accordance with the Chinese standards GB/T 18147.4 (2000) and GB/T 18147.5 (2000). The average values were obtained out of the results from 30 specimens.

Determination of moisture sorption and the water retention value

The moisture sorption of degummed fibers was determined according to the standard ASTM D 2654-76:1976. Fibers were exposed to standard atmosphere (temperature: 20 ± 2℃, RH: 65 ± 3%) for 24 h. Moisture sorption was calculated as a weight percentage of absolute dry material and expressed as an average value of three measurements. The water retention of cellulose fibers was determined as the average of three parallel determinations by the standard centrifuge method (ASTM D 2402-78:1978). According to the standard (ASTM D 2402-78:1978), the centrifuge speed (CT14RDII, Shanghai Tian Mei Science and Technology Industry Co., Ltd) was set up to 400 rpm/min.

Results and discussion

Surface morphology analysis

A SEM was used to examine the effects of the chemo-enzymatic modification and degumming upon the surface of hemp fiber. Figures 1(a)–(e) present the micrographs of five batches of hemp fibers by circulating degumming of the same solution. Figure 1(f) presents the raw hemp fibers that were wrapped in non-cellulose compounds, such as the surface of pine bark. We could find that the surface of treated fibers was smoother compared with the untreated raw hemp fiber (Figure 1(f)). Nevertheless, the smoothness of the degummed fiber decreased as the treatment solution cycles increased. It could be clearly seen that the single fiber was divided in Figures 1(a)–(d), and the degree of separation of the fiber in Figure 1(a) was the largest. Figure 1(e) shows that the fibers were still wrapped by non-cellulose compounds and there was no significant separation among single fibers, which implied that the activity of enzymes had been reduced and some enzymes had even lost their activity. The concentration of the enzymes in the degumming solution decreased, leading to a negative effect for degumming.

Figure 1.

Scanning electron microscopy pictures of different batches of hemp fibers: (a) first batch of treated hemp fibers; (b) second batch of treated hemp fibers; (c) third batch of treated hemp fibers; (d) fourth batch of treated hemp fibers; (e) fifth batch of treated hemp fibers; (f) raw hemp fibers.

In order to describe the mechanism of the LTH degumming process of hemp fiber, we introduced simulation diagrams, which are shown in Figure 2. The raw hemp fibers (Figure 2(a)) consisted of a cortex, bast fiber layer and formation layer. Cellulose mainly existed in the bast fiber layer that was sandwiched between the cortex and the formation layer. This conjecture could be supported by Figure 1(f). The raw hemp fibers were treated with acetic acid–sodium acetate buffer solution containing laccase, hemicellulase and TEMPO for a certain period. The cortex and formation layer attached to the surface of the hemp fiber firstly underwent swelling, and then fell off after stirring (Figure 2(b)). This phenomenon was consistent with the observation of Figure 1(e). At the same time, the LTH played a synergistic role in the decorticated hemp fibers (Figure 2(c)). A large amount of gum that adhered on the surface of hemp fibers was degraded. After being washed with deionized water, the degummed fibers (Figure 2(d)) still had a very small amount of gum glued among the single fiber to form the technical fiber, which indicated a good agreement with Figure 1(a).

Figure 2.

The mechanism of the laccase/2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)/hemicellulase system degumming process. (a) Raw hemp fiber. (b) Cortex and formation layer swelling and shedding. (c) The synergism of Laccase/TEMPO/Hemicellulase. (d) Degummed hemp fiber.

Chemical compositions

A shown in Table 1, the percentage of non-cellulosic compounds was about 46.33%. The main non-cellulosic components in raw hemp fiber were hemicellulose (19.36%) and lignin (13.04%).

Table 1.

Chemical components of raw hemp fiber (%)

	Cellulose	Hemicellulose	Pectin	Lignin	Wax	Water soluble
Raw hemp	53.67	19.36	4.40	13.04	1.49	8.04

The changes of the residual rate of degummed hemp fiber with the increase of circulating times are shown in Figure 3. When the circulating times increased, the residual glue rate gradually increased, indicating that the degumming effect was getting worse. This implied that the enzyme in the solution was still active to degrade the non-cellulosic compounds. The residual rate of the 5# sample was nearly 10% higher than that of the 1# sample and nearly 20% lower than that of the raw hemp fiber. From the third to the fourth, the residual rate was increased. The possible reasons were that the process of the replacement of the raw hemp would, on the one hand, contribute to the dilution of the content of enzymes and TEMPO with the previous hemp being taken away and, on the other hand, the pH of the solution would change, which would cause a negative effect for the activity of the enzymes, and ultimately decrease the efficient expression of the enzymes. We also speculated that some of the enzymes had even died during the constant reactive process. Those reasons would result in poor degumming effect.

Figure 3.

The residual rate of different batches of hemp fibers.

Chemical structure analysis (FT-IR)

The FT-IR spectra of the raw hemp fiber and hemp fiber treated by the LTH system are shown in Figure 4. FT-IR analysis was carried out to verify the changes, compared to raw hemp fiber (6#), of hemp fiber treated by the LTH system. The differences in FT-IR mainly occurred in the region of 600–2000 cm⁻¹. These peaks at 670 cm⁻¹ (C-OH out-of-plane bending), 1160 cm⁻¹ (C-O-C asymmetrical stretching mode) and 1210 cm⁻¹ (cellulose OH plane bending vibration) were sharp. All of these peaks are the main characteristic peaks of cellulose.¹⁴ This indicated that the content of the cellulose increased after the LTH system degumming treatment. The peak at 1650 cm⁻¹ occurred because the bonded water in hemicellulose disappeared. This indicated the removal of bonded water from the hemicellulose. The peak at 1730 cm⁻¹ was considered to be due to the absorption of carbonyl (C=O) stretching of (-COOH) or (-CHO) present in the oxidized fiber. These observations proved that most of non-cellulosic compounds in the hemp fiber were removed. No peak disappeared or appeared dramatically and the intensity of peaks was different compared with the different batches of hemp fiber, which indicated that the degumming effect of the LTH system was obvious, but that the degree of the degumming effect deteriorated as the number of cycles of the solution increased.

Figure 4.

Fourier transform infrared spectra of raw hemp fiber and hemp fiber treated by the laccase/2,2,6,6-tetramethylpiperidine-1-oxyl radical/hemicellulase system.

Analysis of factors that influence the contents of oxidized functional groups

The hydroxyl (-OH) group in the raw hemp fiber could be converted to aldehyde (-CHO) and carboxyl (-COOH) groups during the degumming process. The two functional groups were the main factors that determined the macroscopic and chemical behavior of the cellulosic material.¹⁵ Figure 5 shows the content of the -CHO and -COOH of different batches of hemp fibers. In the case of lignocellulosic fibers, carboxyl groups were formed not only from the C6 primary hydroxyls of cellulose and hemicelluloses but also from lignin.^16,17 The content of the aldehyde group of the treated hemp fiber was very high—nearly three times higher than that of raw hemp fibers. This indicated that the degumming solution could convert the hydroxyl group to an aldehyde group in hemp fiber. Aldehyde groups formed in TEMPO-oxidized fibers mainly because of the presence of C6 aldehydes as the intermediate structure during the TEMPO-mediated oxidation.¹⁸ The content of the aldehyde group continuously decreased with the increase of cycling times for different batches of raw hemp fibers. The possible reason was that some enzymes had lost their activity. The TEMPO oxidation rate depended on the laccase concentration.¹⁹ As we had studied before, the conclusion was that laccase may oxidize a certain amount of hydroxyl groups to aldehyde groups.⁹

Figure 5.

The content of the carboxylate and aldehyde groups for different batches of hemp fibers.

The content of carboxyl groups in the raw hemp was higher than that of the degummed hemp fibers. With the increasing of replacing times for different batches of raw hemp, the content of carboxyl groups was increasing, which was consistent with the trend of the residual rate. Meanwhile, in this chemical conversion, the oxidation from the hydroxyl to the aldehyde was affected by the nitroxyl radical (N=O+) formed from oxidized TEMPO. The further oxidation of the aldehyde to the carboxyl groups was determined by auto-oxidation using air–oxygen, while the effect of this process was not particularly obvious.¹⁹ The results further illustrated that carboxyl groups were formed not only from the C6 primary hydroxyls of cellulose and hemicelluloses, but also from lignin in the lignocellulosic fibers.^16,17 Compared with the ratio of the residual rate, the content of the aldehyde group obtained by this method was significantly increased, while the carboxyl group content did not change significantly.

Physical and mechanical properties

The result was of great importance to the mechanical characterization of the hemp fibers. The physical and mechanical properties of different batches of treated hemp fiber are shown in Table 2. It could be seen that as the number of cycles increased, the linear density of the fiber increased gradually, which indicated that the degree of separation of fibers was decreased. The higher the residual rate was, the more “bonding points” there were among fibers. Therefore, the mutual cohesions between the single fibers were increased. The length of the degummed fibers showed the same trend with the linear density. As the degree of separation of fibers decreased, the head-to-tail connection of the fibers increased, which resulted in a high value of length of degummed hemp fibers.

Table 2.

Physical and mechanical property test results

Circulating times	Linear density (dtex)	Tenacity (cN/dtex)	Length (cm)
1	9.92 ± 0.44	5.03 ± 0.13	14.15 ± 0.13
2	14.45 ± 0.38	4.08 ± 0.18	16.93 ± 0.16
3	16.03 ± 0.32	4.81 ± 0.12	17.19 ± 0.22
4	20.73 ± 0.25	3.67 ± 0.25	18.21 ± 0.26
5	23.03 ± 0.22	3.96 ± 0.28	18.51 ± 0.21

As the number of cycles increased, the tenacity of the fibers fluctuated. The tenacity was directly related to the linear density of the fibers. The tenacity was affected by the fiber itself as well as the test operation. Thus, the diameter of the single hemp fiber itself was not uniform. By observing the hemp fiber after degumming from Figure 1, we found that the degree of unevenness of the single fiber was different in the vertical view. The mature hemp fibers had thick and compact cell walls that almost fill the lumen.²⁰ These factors would affect the test results of tenacity.

The single hemp fiber was very short. Thus, certain amounts of gummy components must be retained as the “bonding points” among the single fibers to extend a certain length as technical fiber in order to meet the spinning requirements. The length increased because the difference in length was directly related to the residual rate. The more gummy components were retained, the more “bonding points” or the possibility that the single fibers were wrapped by the gum. It can be seen from Figure 3 and Table 2 that the length of the treated fiber was related to the residual rate. Thus, the length of the treated fibers met the textile field requirements.

Sorption properties of degummed and oxidized hemp fibers

Generally, the water retention values (WRVs) were influenced by the components of fibers as well as the swelling ability and degrees of fibrillation of the long fiber.^12,21 As shown in Figure 6, compared with the raw hemp fiber, the treated hemp fibers slightly enhanced the WRV. As the number of cycles increased, the WRVs of the fibers decreased. The WRV of the first round was higher than others treated with LTH, which was mainly ascribed to the changing in morphological structures and chemical components, as well as the effect of the non-crystalline area. The finer the single fibers were, the fluffier the fiber mass was, in which more water molecules could be contained. Comparing Figure 5 with Figure 6, it could be found that the WRVs were related to the content of aldehyde group (-CHO). The LTH system could oxidize the hydroxyl group on the cellulose C6 to form the carboxyl group that existed in the amorphous region and the surface of the crystallization region. The binding ability of carboxyl groups to water molecules was stronger than that of the hydroxyl groups. Therefore, the more functional groups that were related to WRV exposed on the surface of the fiber, the greater the force of water retention between molecules.

Figure 6.

Water retention value and moisture sorption for different batches hemp fibers.

Moisture sorption values (MSVs) for hemp fibers are presented in Figure 6. There was no significant change in MSVs in oxidized fibers compared to raw hemp fibers (6#). Comparing Figure 5 with Figure 6, it is possible to clarify that the moisture sorption ability was in direct proportion to the content of the carboxyl group. The content of the hydrophilic groups had great influence on the MSV. A large number of hydrogen bonds between water and the COOH groups were formed. In addition, the intermolecular hydrogen bonds between the molecular chains simultaneously increased, bonding large amounts of water and retaining them in a spatial network.²² In addition, the removal of non-cellulosic compounds would decrease the MSV of hemp fiber.²³ Comparing Figure 3 with Figure 6, there was a positive correlation between the MSVs and the residual rate, which indicated that the content of gum would affect the MSV of the hemp fiber. This is because the increase of the content of gum would lead to the increase of proportion of the amorphous regions in the hemp fiber, and that the content of hydrophilic groups, such as hydroxyl and carboxyl groups, that existed in the amorphous regions was relatively large, which would improve the MSVs of fibers.

Conclusions

This experiment introduced simulation diagrams of the degumming process of hemp in solution. As an ecologically friendly system, the LTH system employed in the paper was used as a substitute for traditional chemical systems for degumming raw hemp fiber. It was also used to introduce and tune the content of the aldehyde (CHO) and carboxyl (COOH) groups on the hemp fibers to improve the water retention and moisture sorption. The degumming solution could be continuously used to degum five different batches of raw hemp fibers. We found the degumming effects of different batches of hemp fiber were obvious when the residual rate, fineness, length and tensile strength of degummed fiber were 15–27%, 9.00–24.00 dtex, 14.00–19.00 cm and 3.8–5.1 cN/dtex, respectively. In summary, this study demonstrated an environmentally friendly method for the degumming and modification of hemp fiber. Even though the effect of degumming and modification become worse as the number of cycles increased, there were more hopeful prospects for degumming hemp fiber.

Footnotes

Acknowledgment

Xiang Yeping and Liu Liu contributed to this work equally and should be regarded as co-first authors.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by “the Fundamental Research Funds for the Central Universities” (Project No. EG2018006), the Shanghai Municipal Natural Science Foundation (Grant No. 14ZR1401000) and the Open Fund of Key Laboratory of Textile Science & Technology (Donghua University) (KLTST 201609).

References

Milanovic J, Kostic M, et al. Influence of TEMPO-Mediated Oxidation on Properties of Hemp Fibers[J]. Industrial & Engineering Chemistry Research 2012; 51(29): 9750–9759.

Liu

Xiang

Zhang

, et al. TEMPO-media oxidation combined with laccase for effective degumming pretreatment of hemp fibers. Bioresources 2017; 12: 8848–8861.

Wang

Sun

, et al. Research on alkaline pectinase for hemp degumming. Adv Mater Res 2013; 821-822: 355–359.

Ibrahim

Khalil

Eid

. A cleaner production of ultra-violet shielding wool prints. J Clean Prod 2015; 92: 187–195.

Bergfjord

Holst

. A procedure for identifying textile bast fibres using microscopy: flax, nettle/ramie, hemp and jute. Ultramicroscopy 2010; 110: 1192–1197.

Ding

Zhang

. Optimization of enzyme mixture degumming of ramie fiber. J Nat Fiber 2014; 11: 13–24.

Zhang

Yan

. Degumming of ramie bast fibers by Ca2+-activated composite enzyme. J Text Inst 2013; 104: 78–83.

Wang

Yuan

, et al. Hydrophobic modification of cotton fabric with octadecylamine via laccase/TEMPO mediated grafting. Carbohydr Polym 2016; 137: 549–555.

Liu L, Xiang Y, Zhang R, et al. Effect of NaClO dosage on the structure of degummed hemp fibers by 2,2,6, 6-tetramethyl-1-piperidinyloxy-laccase degumming. Text Res J 2019; 89(1): 76–86.

10.

Saito

Shibata

Isogai

, et al. Distribution of carboxylate groups introduced into cotton linters by the TEMPO-mediated oxidation. Carbohydr Polym 2005; 61: 414–419.

11.

Saito

Isogai

. Introduction of aldehyde groups on surfaces of native cellulose fibers by TEMPO-mediated oxidation. Colloid Surf A Physicochem Eng Aspect 2006; 289: 219–225.

12.

Saito

Isogai

. TEMPO-mediated oxidation of native cellulose. The effect of oxidation conditions on chemical and crystal structures of the water-insoluble fractions. Biomacromolecules 2004; 5: 1983–1989.

13.

Harwood J, Mccormick P, Waldron D, et al. Evaluation of flax accessions for high value textile end uses[J]. Industrial Crops and Products 2008; 27(1): 22–28.

14.

Troedec

Sedan

Peyratout

, et al. Influence of various chemical treatments on the composition and structure of hemp fibres. Composites Part A 2008; 39: 514–522.

15.

Potthast

Rosenau

Kosma

Analysis of oxidized functionalities in cellulose. In: Klemm

(ed). Polysaccharides II, Berlin, Heidelberg: Springer Berlin Heidelberg, 2006, pp. 1–48.

16.

Okita

Saito

Isogai

. TEMPO-mediated oxidation of softwood thermomechanical pulp. Holzforschung 2009; 63: 529–535.

17.

Tsuguyuki

Satoshi

Yoshiharu

, et al. Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose. Biomacromolecules 2007; 8: 2485–2491.

18.

Milanovic

Kostic

Milanovic

, et al. Influence of TEMPO-mediated oxidation on properties of hemp fibers. Ind Eng Chem Res 2012; 51: 9750–9759.

19.

Jaušovec

Vogrinčič

Kokol

. Introduction of aldehyde vs. carboxylic groups to cellulose nanofibers using laccase/TEMPO mediated oxidation. Carbohydr Polym 2015; 116: 74–85.

20.

Placet

Méteau

Froehly

, et al. Investigation of the internal structure of hemp fibres using optical coherence tomography and focused ion beam transverse cutting. J Mater Sci 2014; 49: 8317–8327.

21.

Saito T, Yanagisawa M, and Isogai A. TEMPO-mediated Oxidation of Native Cellulose: SECMALLS Analysis of Water-soluble and -Insoluble Fractions in the Oxidized Products[J]. Cellulose 2005; 12(3): 305–315.

22.

Pei

Yin

Shen

, et al. Oxidation of primary hydroxyl groups in chitooligomer by a laccase-TEMPO system and physico-chemical characterisation of oxidation products. Carbohydr Polym 2016; 135: 234–238.

23.

Pejic

Kostic

Skundric

, et al. The effects of hemicelluloses and lignin removal on water uptake behavior of hemp fibers. Bioresource Technology 2008; 99: 7152–7159.

Circulating solution for the degumming and modification of hemp fiber by the laccase–2,2,6,6-tetramethylpiperidine-1-oxyl radical–hemicellulase system

Abstract

Keywords

Experimental details

Materials

Preparation of degummed hemp fibers

Scanning electron microscopy

Determination of the aldehyde and carboxylate groups

Chemical components analysis

Fourier transform infrared spectroscopy

Mechanical property test

Physical property test

Determination of moisture sorption and the water retention value

Results and discussion

Surface morphology analysis

Chemical compositions

Chemical structure analysis (FT-IR)

Analysis of factors that influence the contents of oxidized functional groups

Physical and mechanical properties

Sorption properties of degummed and oxidized hemp fibers

Conclusions

Footnotes

Acknowledgment

Declaration of conflicting interests

Funding

References