Effect of NaClO dosage on the structure of degummed hemp fibers by 2,2,6,6-tetramethyl-1-piperidinyloxy-laccase degumming

Abstract

In this study, different dosages of NaClO were used in the 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)-laccase degumming system to remove non-cellulosic materials, and their effects on the structure of hemp fibers were analyzed and discussed. A scanning electron microscope was used to depict the surface morphology of fibers after oxidative degumming under various dosages of NaClO in the TEMPO-laccase degumming system. Chemical composition analysis was used to determine the content changes of the different components. Meanwhile, the content of functional groups was also discussed. Fourier transfer infrared spectroscopy, nuclear magnetic resonance spectroscopy, and X-ray diffraction were employed to evaluate the microstructural changes of degummed hemp fibers obtained from the degumming processes with different dosages of NaClO. The results showed that after the TEMPO-laccase system degumming process with a NaClO dosage of 16%, the cleanest and smoothest surface of degummed fibers could be observed and the non-cellulosic materials were significantly removed without any crystalline transformation or damage in the cellulose. This research could shed light on determining favorable operation parameters for hemp oxidation degumming and increasing the degumming efficiency, as well as in the oxidation control and quality assurance of hemp fibers for textile downstream end uses.

Keywords

hemp fibers 2 6 6-tetramethyl-1-piperidinyloxy-laccase oxidation degumming NaClO structure

Hemp, which originated in central Asia, is most likely the oldest cultivated fiber plant. Nowadays, the hemp plant is grown in many countries, such as China, France, Chile, Russia, Turkey, USA, and Canada,¹ as a kind of fibrous plant. With the increase of environmental consideration, hemp fibers have great potential in the textile industry owing to their special properties, such as high tensile strength, quick absorption of humidity accompanied with quick drying, good thermal and electrical properties, excellent antibacterial properties, and biodegradability.² However, it is well-known that the high content of non-cellulosic substances, such as lignin, hemicelluloses, and pectin, which exist in raw hemp make the fiber coarse and brittle, limiting the development and applications of hemp fibers in high-value textile uses. The main task of the hemp degumming was to prepare technical (multi-cellular) fibers.^3,4 These fibers demand a great removal of non-cellulosic materials to make hemp fiber cleaner, softer, and finer⁵ for high-value textile use. In the meantime, certain gummy materials should be kept in order to meet the requirement of further spinning processes.

Various efforts had been committed to exploring different techniques for hemp degumming, including chemical, mechanical, enzymatic, and ultrasonic methods.^6–8 However, these techniques have their limitations. Chemical degumming with high energy consumption proved to be heavily polluting to the environment.⁹ The enzymatic method required a long degumming time, and the quality of the degummed fibers was inferior.¹⁰ The low degumming efficiency of the ultrasonic method and extra fiber damage of the mechanical method blocked their development in hemp degumming.¹¹ In the current degumming industry, the chemical method is the most popular degumming process. The most common method used currently is as follows: raw hemp – scotching – acid soaking – washing – alkali boiling – washing – dehydration – bleaching – oil finishing – drying. This process requires a high consumption of alkali, high pressure, and high temperature, which leads to high environmental pollution and an energy cost that is not in accordance with the requirements of environment protection.

Recently, the 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)-mediated oxidation method used for the fabrication of cellulose nanofibers, which was pioneered by Isogai and co-workers,¹² has emerged and been developed as a new alternative degumming process. Most of the previous studies were focused on the research of oxidized cellulose, such as in cotton,¹³ sisal,¹⁴ kraft,¹⁵ and wood pulp, with the aim of nanofiber manufacturing and tracking temporal changes during the oxidation process.¹⁶ However, little effort had been done in the research of TEMPO-mediated oxidation used in the degumming process. The Milanovic group² studied the introduced functional groups that were generated during the TEMPO-mediated oxidation and existed on the crystal surfaces and in disordered regions of celluloses. The influences of the different catalytic amount of sodium bromide (NaBr) on the water uptake properties of hemp fibers were also discussed. The degummed fibers obtained in this research were unsuitable for the downstream textile procedure due to their higher content of non-cellulosic materials and the serious damage in the surface and mechanical properties. On the premise of satisficing the spinning demand, a degumming process that could provide degummed fibers with a lower content of gummy materials and better physical properties is needed in further researches. Laccase, a kind of multi copper-containing oxidase with phenoloxidase activity,¹⁰ is quite common in the use of delignification. Xia et al.¹⁷ and Yan et al.¹⁸ investigated the effect of laccase in the degumming process of jute and kenaf fibers, respectively. Both studies showed a considerable performance in improving the spinnable properties of natural fibers. To overcome the inferiority of the TEMPO-mediated oxidation in hemp degumming, the collaboration of TEMPO-mediated oxidation and laccase, which was referred to as the TEMPO-laccase system, were proposed and confirmed as an efficient degumming method for hemp fibers in our earlier research. Due to the oxidizing role of TEMPO-mediated oxidation and the collaboration with laccase, the gummy materials could be easily attacked by the functional oxidants and dissolved in solution under aqueous mild conditions during the degumming process. However, it should be noted that the TEMPO-laccase system not only promoted the rate of delignification but also selectively catalyzed the oxidation of cellulose, primarily hydroxyl groups to carboxyl groups. With previous studies, the primary role that NaClO played in the results of delignification during the TEMPO-mediated oxidation has already been discussed. However, the relationship between the dosage of NaClO and the degumming efficiency and lignin oxidation has not been examined. Under the condition of high dosage of NaClO, there was a possibility that the hemp fiber might be over-degummed and the cellulose could be damaged. Therefore, it is of great practical importance and highly desirable to characterize the performance of degummed hemp fibers and analyze the relationship of fiber quality with different NaClO dosages.

In this paper, based on the effect of the TEMPO-laccase degumming system in the removal of non-cellulosic materials from raw hemp fibers, different dosages of NaClO for the lignin oxidation in the TEMPO-laccase degumming system were studied and compared to obtain a relatively favorable and suitable NaClO dosage. The relationship between the dosage of NaClO and chemical compositions, as well as the measured content of aldehyde and carboxylate groups formed in oxidized hemp fibers, were discussed. Furthermore, microstructure and surface morphology were analyzed using nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and a scanning electron microscope (SEM). Moreover, the mechanical properties were tested to assess the fiber quality and its possibility for high-end uses. Understanding the influences of different oxidative degrees on degummed hemp fibers during the TEMPO-laccase degumming system operation would be useful in increasing the efficiency of hemp utilization and lay a theoretical foundation for the oxidative degumming process.

Experimental details

Materials

The hemp was planted in the Heilongjiang province of China, in 2015. The bast fibers were peeled manually from the hemp stem. The raw hemp fibers used in the study were cut of 100 cm from the middle section of the bast fibers. The chemical composition of raw hemp fibers was cellulose, 50.55%; lignin, 15.29%; hemicellulose, 19.45%; pectin, 4.92%; fats and waxes, 1.85%; and water-soluble matters, 7.94%.

The main chemicals used in this study were sodium hydroxide (NaOH), NaClO, NaBr, sodium pyrophosphate (Na₄P₂O₇₎, anhydrous sodium sulfate (Na₂SO₃₎, and TEMPO, which were purchased from Aladdin Biological Technology Co., Ltd (Shanghai, China). All the chemicals were of analytical reagent grade without further purification. Laccase was provided by Ruiyang Biotech Co., Ltd (Jiangsu, China). The laccase activity was measured as the amount of laccase transforming 1 µmol of 2,2-azinobis-(3-ethylbenzyl thiozoline-6-sulfonate) (ABTS) to its cation radical in 1 minute at the pH of 5 and the temperature of 25℃. The activity of laccase used in this study was 600 IU/L. Deionized water was used to prepare all solutions.

Degumming process

The constant humidity content of raw hemp fibers was ensured by drying the fibers at 40℃ for 24 hours in an oven before all the degumming treatments. The whole degumming process consisted of three parts described as follows.

TEMPO-mediated oxidation pretreatment.

Raw hemp fibers were immerged in a solution of 0.033 g/L TEMPO and 0.33 g/L NaBr at a liquor ratio of 1:75. The TEMPO-mediated oxidation was initiated by adding NaClO solution (containing 13% available chlorine). The pH value of the slurry was maintained at 10.5 at room temperature by adding 0.5 M NaOH solution until no further decrease in pH was observed for 4 hours.

Laccase treatment.

TEMPO-oxidized fibers were treated at the temperature of 45℃ for 4 hours in a solution of 12.8 g/L laccase at a liquor ratio of 1:20. The pH was adjusted between 4.5 and 5.5. In order to quench the reaction, the solution was boiled for 5 minutes.

Alkali boiling treatment.

Fibers were soaked in a water solution consisted of 10 g/L NaOH, 2 g/L Na₄P₂O₇, and 5 g/L Na₂SO₃ at a liquor ratio of 1:15 for 2 hours at a temperature of 100℃.

All the hemp fibers were washed with deionized water three times after every degumming part and dried at 40℃ for 24 hours.

Scanning electron microscopy

The degummed fibers morphologies were observed using a SEM Model JEOL (JSM-5600LV, Japan) at 10 kV after sputtering with gold under a temperature of 20℃ and a relative humidity (RH) of 65%.

Weight loss ratio and chemical composition

The weight loss ratio and chemical composition are introduced here to evaluate the degumming efficiency after different degumming processes. The weight loss ratio can be characterized as follows

weightlossratio = (w_{0} - w) / w_{0} \times 100 %

(1)

where w is the dry weight of the hemp fibers after treatments and w₀ is the dry weight of the raw hemp fibers before treatments.

The chemical composition tests adopted Chinese standard GB/T 5889-86 by averaging the experimental results of three samples. However, because of the heterogeneity of hemp fibers, we tested six samples in order to obtain more reliable results.

Each sample of 5 g raw hemp fibers was dried to constant weight in the oven. The samples were processed under boiling alkali solution (20 g/L, 150 mL) for 1 hour and re-boiled for 2 hours with the new alkali solution, then dried to constant weight after a washing process. The cellulose content of hemp fibers was calculated as follows

cellulosecontent = 1 - \frac{G_{0} - G}{G_{0}} \times 100 %

(2)

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

Raw hemp fibers were firstly degreased with alcohol-benzene to remove the resins, oils, fats, and waxes. Then three 1 g degreased raw hemp fibers were weighted in weighing bottles and dried in the oven for 3 hours at 105℃. The three weighted oven-dried specimens were then transferred to a 50 mL beaker with a glass cover, to which was added slowly 30 mL of H₂SO₄ (72%). The specimen was mixed well with the acid by constantly stirring for at least 1 minute and was allowed to stand for 24 hours. The materials were transferred into a 500 mL beaker, to which was added 270 mL of deionized water, and it was boiled for 1 hour. After allowing the insoluble material to settle, it was filtered into a weighted sand core funnel that had been dried at 105℃. The residue was washed free of acid with 500 mL of hot water, then dried in the sand core funnel with lignin in an oven for 3 hours at 105℃, and weighted. The lignin content of raw hemp fibers was calculated as follows

lignincontent = \frac{G_{L} - G_{F}}{G_{D}} \times 100 %

(3)

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.

Determination of carboxyl and aldehyde groups

The content of carboxyl groups was determined by the calcium-acetate titration method.¹⁹ The carboxyl groups could react with calcium acetate, which formed a salt and released an equivalent amount of a weaker acid.²⁰ A certain amount of degummed fibers (0.5 g) was mixed with 0.01 M hydrochloric acid for 1 hour, then they were washed thoroughly with distilled water. After this step, 50 ml of distilled water that contained oxidized degummed fibers was collected in a break and 30 mL of calcium-acetate solution (0.25 M) was added to the beaker. After shaking for 2 hours, 30 mL of the solution were titrated using a phenolphthalein indicator with 0.01 M NaOH. The carboxyl content was calculated by the volume of NaOH solution used for titration.

The aldehyde content was calculated according to the method described in the literature.^21,22 The aldehyde groups in the degummed fibers were further oxidized to carboxyl ones, and carboxyl content was determined by the above-mentioned calcium-acetate titration method. A slurry of degummed fibers with 10% consistency was prepared beforehand. A mixture containing 1.81 g of NaClO₂, 20 g of CH3COOH (5 M), and 57 mL of distilled water was added to 20 g of the slurry. Oxidation was carried out by stirring the mixture for 48 hours at room temperature. The carboxyl groups formed by the NaClO₂ oxidation were regarded as aldehyde groups present in the original degummed fibers.

Fourier transfer infrared spectroscopy

The chemical functional groups in degummed fibers were determined by FT-IR analysis. The spectra were recorded using a PerkinElmer spectrometer (Spectrum II, UK) with a resolution of 4 cm^–1. A total of 30 scans were selected and performed ranging from 400 to 4000 cm^–1. The FT-IR data was processed and analyzed using OMINC.

X-ray diffraction

The XRD patterns were performed on a Rigaku diffractometer (D/max-2550 PC, Japan). The X-ray source was Ni-filtered Cu kα radiation at 40 kV and 200 mA. The patterns were recorded in the 2θ range of 5^–60^o.

Degree of polymerization

The degree of polymerization (DP) was measured according to Chinese standard GB/T 5888-86. Before tests, all samples were degreased using a Soxhlet extractor with a benzene and ethyl alcohol mixture in a 2 : 1(v/v) ratio as the solution. Then an Ubbelohde capillary viscometer was used to measure the intrinsic viscosity of the degummed hemp fiber solution in copper ethylene-demined solvent.

Nuclear magnetic resonance spectroscopic analysis

An Avance 400 NMR spectrometer (Bruker, Switzerland) was used to obtain solid-state carbon-13 nuclear magnetic resonance (¹³C-NMR) spectra of the degummed fibers with cross-polarization-magic angle spinning (CP-MAS) operating at 400 MHz at room temperature. The spinning rate, pulse delay, and contact time were set at 5 kHz, 3 s and 1 ms, respectively. The data was processed and analyzed using MestReNova.

Mechanical treatment and properties tests

Before starting the mechanical properties tests, the degummed hemp fibers were firstly treated by the carding process, which is essential in the spinning process. Its primary function was to loosen the degummed fibers and convert them into uniform straight parallel and separate fibers in the yarn. The carding process was performed on a FA210B Flat Card with the speed of the cylinder, stripper and doffer of 330, 720, and 20 r/min, respectively.

Before the mechanical properties tests, all samples were balanced in standard atmospheric condition (temperature: 20 ± 2℃, RH: 65 ± 3%) for 24 hours. The linear density, tenacity, and breaking elongation were tested according to Chinese standards GB/T 18147.4 and GB/T 18147.5. Average values were obtained using results from 30 specimens.

Results and discussion

Surface morphology analysis

The SEM images shown in Figure 1 were used to study the changes in the surface morphology of the degummed fibers. As seen from Figure 1(a), there was still a certain amount of gummy substances covering the hemp fibers. As a result, the single fibers were still adhered to each other, indicating that the degumming process without NaClO was inefficient in removing non-cellulosic materials. This could also be explained as neither TEMPO nor NaBr could react with lignin or cellulose individually.²³ NaClO plays an important role in the removal of gummy materials, especially in delignification. The greater the addition of NaClO, the greater the removal of lignin and other gummy substances. As the NaClO dosage increased, the fiber surfaces became cleaner and smoother, which can be observed from Figures 1(b) and (c). When the dosage of NaClO reached 16%, the treated fibers exhibited a mostly clean surface, and the contour of individual fibers can be seen clearly from Figure 1(d). This could also be explained by the theory that the higher dosage of NaClO led to a greater degree of lignin oxidation, which resulted in an excellent performance of delignification. Meanwhile, as demonstrated in Figure 1(e), although the single fibers separated thoroughly from the bundle fibers, the fiber surface appeared to have a certain number of fine cracks and pits, and some fibrils were found to have peeled off from the surface of degummed hemp fibers. This was because the hemp fibers, which experienced an oxidative reaction with NaClO dosage of 24%, were over-oxidative. Moreover, the over-oxidation in the degumming process could damage the fibrous structure and induce irreversible loss to fiber properties. An adequate and proper oxidative condition could not only remove the gummy substances, which ensure excellent physical properties like linear density and length, but also not harm the fiber morphology and mechanical properties in order to facilitate the downstream textile processes. Under this research, the degumming process with a dosage of NaClO of 16% seemed to have the most desirable fiber morphology.

Figure 1.

Scanning electron microscope images of degummed fibers: (a)–(e) correspond to degummed fibers with NaClO dosages of 0%, 4%, 8%, 16%, and 24%, respectively.

Chemical composition

It is obviously seen from Figure 2 that the lignin content was decreased gradually with the increasing dosage of NaClO, while the weight loss ratio had the completely opposite trend. In the meantime, the content of cellulose increased at first and reached its peak at the NaClO dosage of 16%, then went down with the further increase of NaClO dosage. Similarly, when the dosage of NaClO was no more than 8%, these three parameters experienced relatively smooth changes due to the low concentration of NaClO and incomplete, mild oxidation of lignin. When the oxidative condition came to a stage like 16% dosage of NaClO in this study, great changes emerged in the cellulose and lignin content of degummed fibers, as well as the weight loss ratio. It is well-known that the weight loss of degummed fibers is mainly due to the removal of non-cellulosic components and to a smaller extent due to the dissolution of highly oxidized cellulose molecules and/or some low molecular weight products.²⁴ Generally speaking, a high weight loss ratio indicated a large removal of gummy materials, which led to a high content of cellulose. The results of the weight loss ratio and cellulose content of hemp fibers obtained through the severest oxidative condition (the dosage of NaClO was 24%) were abnormal. This phenomenon was because that, under severe oxidative conditions, the hemp fibers might be over-degummed into small pieces, which could be washed off during the process. These pieces of fiber were still being calculated as the weight loss, but the cellulose in the degummed fibers had already been damaged, inducing a decline in cellulose content. The lignin content in the fibers was of great importance from the application point of view of hemp, and was a vital parameter to evaluate the degumming efficiency. The lower content of lignin in degummed hemp fibers promised finer and less rigid fibers that could produce high-value textile produces. Overall, the oxidation degumming process with the TEMPO-laccase system was effective in lignin removal. In particular, considering the cellulose content, the degumming process with the NaClO dosage of 16% was most desirable and adequate.

Figure 2.

Weight loss ratio and the contents of cellulose and lignin of degummed fibers.

New functionalities in degummed fibers

The oxidized groups in degummed fibers were added with the increasing dosage of NaClO. To be specific, the greater the amount of NaClO employed, the greater the aldehyde and carboxylate contents, which can be easily seen from Figure 3. It should be noted that without and with a lower concentration of oxidative agent (0%, 4%, and 8% of NaClO), there were no significant changes in the aldehyde group content in degummed fibers. However, for higher concentrations of NaClO as the primary oxidant, plenty of the aldehyde groups presented due to the formation of intra- and intermolecular hemiacetals with hydroxyls that existed in cellulose, lignin, hemicellulose, and other accompanying components in hemp fibers.^21,25 Further oxidation from the aldehyde to carboxyl groups was realized by the transformation in situ from hypochlorite and bromide into hypobromite. It should be noted that carboxyl groups were formed not only from the C6 primary hydroxyl of cellulose, but also from hemicellulose and lignin.¹⁶ This fact gave a fundamental reason why the TEMPO-mediated oxidation could remove the non-cellulosic materials. Under a mild oxidative condition, a great part of the oxidizing agents was probably spent on oxidation and removal of non-cellulosic materials; the remaining amount of the oxidant was insufficient to enable significant conversion of hydroxyl groups to aldehyde and further to carboxyl groups. In the case of sufficient oxidant, the gummy substance that covered the cellulose of hemp fibers nearly disappeared and the cellulose was exposed to these oxidative agents. As a consequence, huge quantities of aldehyde and carboxylate groups immersed, inducing damage in cellulose that was in accordance with the former results. Combined with the results above, the degummed fibers with an NaClO dosage of 16% seemed to have relatively suitable content of the functional groups.

Figure 3.

Oxidized group contents of degummed fibers.

Chemical structure analysis (FT-IR)

FT-IR spectroscopy analysis of degummed fibers was performed to discuss the peak assignments and the spectral changes that occurred after oxidation by different dosages of NaClO and to further identify the changes in chemical composition of treated fibers.

As shown in Figure 4, the range between 3200 and 3500 cm^–1, which corresponded to –OH group stretching vibration, was attributed to the hydroxyl groups both in cellulose and lignin components in hemp fibers. In addition, the peak intensity in this region from 3200 to 3500 cm^–1 decreased gradually with the dosage of NaClO from 0% to 16%, indicating that the content of hydroxyl groups was reduced by oxidation. Combined with the results of chemical content, the oxidation mainly took place in the lignin component. A significant decline could be observed in the region of hemp fibers with an NaClO dosage of 24%, which was due to the excessive oxidation in cellulose, and the hydrogen bond structure in cellulose was partly broken. This might induce some loss in mechanical properties. Furthermore, the peaks at 2900 cm^–1(C-H bond vibrations), 1320 cm^–1 (C = O stretching vibration), and 1060 cm^–1(C = O stretching vibration) in the spectra curves of all degummed fibers were mainly attributed to the elemental functional groups in lignocellulosic materials.²⁶ Also, the 1507 cm^–1 peaks represented aromatic ring vibration of lignin.²⁷ The intensity of these peaks decreased gradually as the oxidative condition increased, which was in good accordance with the reducing lignin content demonstrated in Figure 2. This indicated that all degumming processes were efficient in lignin removal.

Figure 4.

Fourier transfer infrared spectra of degummed fibers with different dosages of NaClO.

Besides, the intensity of peaks at 1735 cm^–1 corresponding to C = O stretching from the ketones and/or esters of hemicellulose became weak with the increase of the degree of the oxidation and content of the oxidized group. This was related to the oxidation of hydroxyl groups and the introduction of aldehyde or carboxylate groups of hemp fibers. On the whole, cellulose was the most important component of hemp fibers, which created a structure of considerable tensile strength. The degumming process, which could remove lignin efficiently, should not harm the cellulose in hemp fibers. This suggested that the degumming process with an NaClO dosage of 16% was the best method to remove gummy materials in this study, compared with other processes.

Crystal structure analysis

The XRD patterns are shown in Figure 5. It could be observed that all the curves resembled each other, which meant that there was no transformation of crystal form in the degummed fibers and the crystal forms remained unchanged during the oxidation process under different dosages of NaClO. Major diffraction peaks for 2θ ranging between 22^o and 23^o were assigned to the crystallographic plans of cellulose I and peaks for 2θ ranging between 14.8^o and 16.4^o corresponded to the crystallographic plans of cellulose II. In addition, the peak intensity of the samples changed in the both diffraction peaks, which might be due to the oxidation of hydroxyl groups into aldehyde and carboxylate groups during the TEMPO pretreatment of the hemp fibers.

Figure 5.

X-ray diffraction patterns of degummed fibers with different dosages of NaClO.

From Table 1, it can be seen that with the increase of NaClO dosage, the crystallinity of degummed fibers increased at first and reached its summit at the value of 76.55%. Then, it dropped down with the increase of degree of oxidation. Due to an inadequate oxidation reaction and incomplete removal of non-cellulosic materials, the degummed fibers with a lower dosage of NaClO had inferior crystallinity. This was because the residual gums had a more amorphous region, which was a disadvantage for the crystallinity index. When the dosage of NaClO was 16%, the lignin and other amorphous materials in hemp fibers were mainly removed, and the content of cellulose was highest among the five samples, which can also be seen in Table 1. These two aspects induced the largest value of crystallinity index of 4# degummed fibers. Meanwhile, the absorption peaks of fibers with an NaClO dosage of 16% reflected a stronger intensity compared to other degummed fibers. The crystallinity index of fibers with an NaClO dosage of 24% declined from 76.55% to 72.16%, but the value was still larger than that of the sample with NaClO dosages of 0%, 4%, and 8%. In a mild oxidative condition, the oxidation took place largely in the gummy materials that were situated outside of the cellulose and easily accessible. Under the severe condition, the non-cellulosic materials were removed to a great extent, and the cellulose in hemp fibers became exposed to degumming solutions. The oxidation reaction that proceeded in the crystalline regions of cellulose eventually caused a decrease in the crystallinity index. This test result showed that the oxidative condition with an NaClO dosage of 16% was suitable for hemp degumming in this study.

Table 1.

Crystallinity index and degree of polymerization of degummed fibers with different dosages of NaClO

Dosage of NaClO (%)	0	4	8	16	24
Crystallinity (%)	66	68	69	77	72
DP	2075 ± 77	2103 ± 45	2217 ± 62	2452 ± 29	2211 ± 40

DP: degree of polymerization.

The DP of fibers after degumming, which could reflect the degumming efficiency, was also explored and is shown in Table 1. It could be seen that the DP increased with the dosage of NaClO from 0% to 16%, but decreased with further increase of the dosage of NaClO. The reason for the upward trend was that with a mild condition, the cellulose was less damaged during TEMPO-laccase degumming. The gummy materials that covered the cellulose in hemp fibers were reacted with the oxidant before the cellulose. When a higher dosage of NaClO (24%) was added to the solution, the oxidative reaction occurred not only in the gummy materials, but also in cellulose, which could lead to a change in the cellulose macromolecule chains, and induce a declining trend of DP.²⁸

¹³C-NMR

Considering the significant changes in crystallinity of the hemp fibers, it would be of interest to compare their structural features through ¹³C-NMR spectra.²⁹ The solid-state ¹³C-NMR spectra of degummed fibers prepared with different amounts of oxidant (0%, 4%, 8%, 16%, and 24% NaClO solution, 13% active chlorine) are shown in Figure 6. The singlet of chemical shifts at 105 ppm was assigned to the carbon of glucose residue as C1.³⁰ The chemical shifts of C2, C3, and C5, which were not linked to the glycosidic bonds, were overlapped partially in the range of 67–70 ppm. The peaks, observed at 88.6 and 83.6 ppm, were assigned to C4 of glucose in the crystal and non-crystal structures, respectively.³¹ Similarly, the peaks that were observed at 64.7 and 62.2 ppm were assigned to C6 of the primary hydroxyl groups in the crystal and non-crystal structures, respectively. Previous research has shown that the resonances of C1, C4, and C6 that were observed in ¹³C-NMR spectra represented the crystalline structure of cellulose I.³²

Figure 6.

Carbon-13 nuclear magnetic resonance spectra of degummed hemp fibers.

Despite different oxidative conditions, the carbon resonance positions were similar and the chemical shifts had small differences between different degummed fibers. It should be noted that the peak intensities of C4 at 83.6 ppm and C6 at 62.2 ppm decreased along with the increase of NaClO dosage. This indicated that the crystalline structure of degummed fibers was affected by the degree of oxidization. In regard to the hemp fibers treated with relatively higher dosages of NaClO of 8%, 16%, and 24%, the shapes and intensities of peaks in C2, C3, and C5 moieties experienced a distinct change, which also can be seen from Figure 5, that was due to the oxidation of primary hydroxyl groups evoked by the effect of NaClO.

Mechanical properties

Figure 7 illustrated the tenacity and breaking elongation of degummed hemp fibers. Seen in Figure 7, the tenacity increased sharply with increasing the dosage of NaClO from 0% to 4%, but with further increase of the NaClO dosage from 4% to 16%, the tenacity showed little difference and reached its peak at the 8% dosage of NaClO. A further sharp decrease could be seen when the dosage of NaClO was 24%. The upward trend of the tenacity is due to the increased amounts of aldehyde and carboxyl and the removal of non-cellulosic materials. However, under severe conditions, the depolymerization of cellulose during the oxidative treatment and the damage of cellulose may influence the reduction of tenacity, which could explain the decrease in the tenacity of the degummed fibers with the NaClO dosage increased from 16% to 24%. It also can be seen from Figure 7 and Table 1 that the DP and the tendency of tenacity were in good agreement with each other. Besides, the breaking elongation of degummed fibers showed almost the same trend with the fiber tenacity. However, the peak of breaking elongation took place when the NaClO dosage was 16%, which showed little difference with the tenacity. Overall, the mechanical properties of the degummed hemp fibers obtained with NaClO dosages of 8% and 16% were similar and superior to the fibers with other dosages.

Figure 7.

Tenacity and breaking elongation of degummed hemp fibers.

In order to meet the needs of downstream processing, such as yarn spinning, certain tenacity and breaking elongation of hemp fibers are desired after the degumming treatment. The greater tenacity the degummed fibers have, the better quality of hemp yarn can be obtained. Combined with the results of chemical composition, the range of best NaClO dosage had been narrowed down between 16% and 24%, which reached a balance among all properties of degummed hemp fibers. In further work, more effort should be made to draw out more information on the influences of different dosages and try to explore a more precise NaClO dosage.

Conclusion

In conclusion, the dosage of NaClO for the lignin oxidation in the TEMPO-laccase degumming system was investigated on the structure of hemp fiber after degumming in this paper. The results showed that the degummed fibers obtained from the degumming process with an NaClO dosage of 16% had the greatest cellulose content. The large removal of non-cellulosic materials without damage in cellulose was further proved by the discussion of the relationship between the dosage of NaClO and the measured content of aldehyde and carboxylate groups formed in TEMPO-laccase system degumming. Moreover, the results based on the SEM images showed that under a process with an NaClO dosage of 16% in the TEMPO-laccase degumming system, the degummed hemp fibers appeared to have the smoothest and cleanest surface without any visible damage. FT-IR analysis showed that the non-cellulosic materials were greatly being removed, especially lignin and hemicellulose, by the TEMPO-laccase system degumming process with all dosages of NaClO. However, considering the content of the cellulose component, the process of NaClO dosage of 16% was the most effective and reasonable among all the degumming processes. The results of XRD analysis indicated that the crystal forms were nearly unchanged during the four TEMPO-laccase system degumming processes. In addition, the highest crystallinity index of treated hemp fibers confirmed that the effective process for removing non-cellulosic materials without further damage in cellulose was a TEMPO-laccase degumming process with an NaClO dosage of 16%. Furthermore, the findings in this study may be instructive in determining favorable operation parameters for hemp oxidation degumming and increasing the degumming efficiency, as well as in the oxidation control and quality assurance of hemp fibers for downstream processing.

Experiments are now under way to estimate the impact of dosage of TEMPO, dosage of laccase, and oxidation time on the properties of degummed hemp fibers. Further insight into the selection and combinative usage of enzymes for the fungal degumming may further improve the treatment efficiency and mechanical properties of treated fibers. Further study will also be required to acquire a better understanding of the mechanisms behind non-cellulosic material removal and cellulose protection in the TEMPO-laccase degumming process with a view to preventing fiber strength losses and maximizing the benefits derived from the oxidative effects in view of textile end uses.

Footnotes

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 received no financial support for the research, authorship, and/or publication of this article.

References

Batog

Przepiera

. Enzymatic processing of lignin in the production of lignocellulosic composites. Cellulose Chem Technol 2011; 45: 503–503.

Milanovic

Kostic

Milanovic

et al.

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

Zhang

Yan

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

Zheng

Zhang

Luo

. Optimizing the complex formulation of boiling-off liquor for ramie chemical degumming. Text Res J 1988; 58: 663–666.

Zhang

. Effect of refined processing on the physical and chemical properties of hemp bast fibers. Text Res J 2010; 80: 744–753.

Liu

. Studies on hemp retting by enzyme. J Text Res 1999; 20: 286–288.

Renouard

Hano

Doussot

et al.

Characterization of ultrasonic impact on coir, flax and hemp fibers. Mater Lett 2014; 129: 137–141.

Liu

You

Jin

et al.

Influence of alkali treatment on the structure and properties of hemp fibers. Fibers Polym 2013; 14: 389–395.

Wang

Postle

Kessler

et al.

Removing pectin and lignin during chemical processing of hemp for textile applications. Text Res J 2003; 73: 664–669.

10.

Buschle-Diller

Fanter

Loth

. Structural changes in hemp fibers as a result of enzymatic hydrolysis with mixed enzyme systems. Text Res J 1999; 69: 244–251.

11.

Riddlestone

Stott

Blackburn

et al.

A technical and economic feasibility study of green decortication of hemp fibre for textile uses. J Ind Hemp 2006; 11: 25–55.

12.

Isogai

Saito

Fukuzumi

. TEMPO-oxidized cellulose nanofibers. Nanoscale 2011; 3: 71–71.

13.

Saito

Shibata

Isogai

et al.

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

14.

Aracri

Vidal

. Enhancing the effectiveness of a laccase–TEMPO treatment has a biorefining effect on sisal cellulose fibres. Cellulose 2012; 19: 867–877.

15.

Dang

Zhang

Ragauskas

. Characterizing TEMPO-mediated oxidation of ECF bleached softwood kraft pulps. Carbohydr Polym 2007; 70: 310–317.

16.

Okita

Saito

Isogai

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

17.

Xia

Liu

et al.

Effect of physical modification on properties of jute fibers. J Text Res 2008; 29: 27–29.

18.

Yan T, Fu J and Yu C. Research on laccase degumming of kenaf fiber. In: proceedings of the fiber society 2009 spring conference, (ed Qiu Yiping), Shanghai, China, 27 May–29 May, 2009, pp. 334–337.

19.

Kostic

Potthast

Popov

et al.

Sorption properties of TEMPO-oxidized natural and man-made cellulose fibers. Carbohydr Polym 2009; 77: 791–798.

20.

Kumar

Yang

. HNO 3 /H 3 PO 4 –NANO 2 mediated oxidation of cellulose — preparation and characterization of bioabsorbable oxidized celluloses in high yields and with different levels of oxidation. Carbohydr Polym 2002; 48: 403–412.

21.

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: 1983–1983.

22.

Parks

Hebert

. Thermal analysis of ion exchange reaction products of wood pulps with calcium and aluminum cations. Tappi Tech Ass Pulp Pap Indus 1972; 55: 1510–1514.

23.

Zhai

et al.

Influence of TEMPO-mediated oxidation on the lignin of thermomechanical pulp. Bioresour Technol 2012; 118: 607–607.

24.

Liu

. Mechanical modification of degummed jute fibre for high value textile end uses. Ind Crops Prod 2010; 31: 43–47.

25.

Saito

Isogai

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

26.

et al.

Characteristics and properties of cellulose nanofibers prepared by TEMPO oxidation of corn husk. Bioresources 2016; 11: 5276–5284.

27.

Sain

Panthapulakkal

. Bioprocess preparation of wheat straw fibers and their characterization. Ind Crops Prod 2006; 23: 1–8.

28.

Le 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.

29.

Varma

Chavan

Rajmohanan

et al.

Some observations on the high-resolution solid-state CP-MAS 13 C-NMR spectra of periodate-oxidised cellulose. Polym Degrad Stabil 1997; 58: 257–260.

30.

Varma

Chavan

. A study of crystallinity changes in oxidised celluloses. Polym Degrad Stabil 1995; 49: 245–250.

31.

Jie

et al.

Impact of degree of oxidation on the physicochemical properties of microcrystalline cellulose. Carbohydr Polym 2017; 155: 483–490.

32.

Keshk

SMAS

. Effect of different alkaline solutions on crystalline structure of cellulose at different temperatures. Carbohydr Polym 2015; 115: 658–662.

Effect of NaClO dosage on the structure of degummed hemp fibers by 2,2,6,6-tetramethyl-1-piperidinyloxy-laccase degumming

Abstract

Keywords

Experimental details

Materials

Degumming process

Scanning electron microscopy

Weight loss ratio and chemical composition

Determination of carboxyl and aldehyde groups

Fourier transfer infrared spectroscopy

X-ray diffraction

Degree of polymerization

Nuclear magnetic resonance spectroscopic analysis

Mechanical treatment and properties tests

Results and discussion

Surface morphology analysis

Chemical composition

New functionalities in degummed fibers

Chemical structure analysis (FT-IR)

Crystal structure analysis

13C-NMR

Mechanical properties

Conclusion

Footnotes

Declaration of conflicting interests

Funding

References

¹³C-NMR