Characterization and control of oxidized cellulose in ramie fibers during oxidative degumming

Abstract

Oxidative degumming with hydrogen peroxide provides an efficient pathway and new alternative for natural fiber extraction. In this work, the oxidized cellulose introduced into ramie fibers during the oxidative degumming process was systematically characterized in terms of x-ray photoemission spectroscopy, nuclear magnetic resonance, Fourier transform infrared spectroscopy and scanning electron microscopy. The differences of chemical components, chemical shift, surface structure and surface morphology were analyzed and compared within oxidized cellulose fibers with different oxidation degrees. In addition, the relationship between oxidized cellulose contents and mechanical properties of degummed fibers were further discussed. The results show that the number of oxidized groups increases with increasing oxidation. In addition, the greater presence of oxidized cellulose contributes to a larger loss of tenacity, breaking elongation, flexibility and degree of polymerization in degummed fibers. This study could offer useful information in better understanding the reaction characteristics of oxidative degumming and better control of degummed fiber quality. The contents of oxidized cellulose in ramie fibers could be an effective indicating factor to demonstrate oxidative degumming efficiency and fiber properties.

Keywords

ramie oxidative degumming oxidized cellulose hydrogen peroxide fiber properties

Ramie, commonly known as “China grass”, is mainly grown in temperate or tropical areas. It is one kind of important textile raw material with distinctive characteristics. Ramie produces one of the strongest and longest natural plant fibers, which is lustrous with an almost silky appearance. This type of fiber possesses many excellent properties, such as high tensile strength, high moisture absorption, good thermal conductivity, outstanding antibacterial function and favorable air permeability.¹ Given these performances, ramie fibers could be used as an excellent textile material for clothing fabrics, industrial packaging, car accessories, fiber reinforced composites, etc.^2,3 Raw ramie is in the form of fiber bundles consisting of many individual fibers adhesive to each other. The gummy contents are required to be moved to further improve fibers' downstream processing ability. The removal of heavily coated gummy materials from the cellulosic part of plant fibers is significant and necessary prior to textile industrial utilization.

Considerable efforts have been committed to develop various techniques for natural fiber degumming, including chemical, biological (enzymatic and microbic), ultrasonic and mechanical methods.^4–6 However, wide adoption of these techniques may be shadowed by limitations, such as sophisticated equipment, time-consuming procedures, high environmental pollution, high energy consumption and high operating cost. The technical route of the traditional chemical degumming method used in the industry is as follows: stamping – acid picking – washing – alkali boiling – dehydration – oil finishing – drying. In addition to high energy consumption, traditional chemical degumming with a hot alkaline solution has proved to be heavily polluting to the environment. With a collection of compelling features, oxidative degumming with hydrogen peroxide provides an efficient pathway and new alternative to traditional chemical degumming.^7,8

Hydrogen peroxide, which could generate hydroxyl radicals in alkaline solution, can be used as an effective degumming agent.^9,10 Gummy contents have a relatively lower degree of polymerization and are easily attacked by the highly reactive radicals, while cellulose content is comparatively resistant to such attacks. Compared with other chemical processes, the oxidation process could shorten reaction time, improve production efficiency, save energy consumption and produce less pollution to the environment.¹¹ However, the oxidizing capacity of hydrogen peroxide is so strong that it may cause damage to the fiber properties, owing to an excessive or intense oxidation reaction, if the operation parameters cannot be properly controlled. A large number of hydroxyl groups make the macromolecules very reactive and sensitive to the oxidizing reagents. Cellulose can been oxidized to the point at which all or most glucose residues have been converted to a carboxylic acid group, which is called “oxidized cellulose”. Hydroxyl groups are then converted into the corresponding carbonyl structure, resulting in the formation of aldehyde (–CHO) and carboxyl (–COOH) groups.^12–15 The presence of oxidized groups is not desirable, since several parameters for evaluating the quality of degummed fibers are affected by their occurrence. Therefore, it is of great practical importance and highly desirable to characterize the performance of oxidized cellulose and analyze the dependence of fiber properties on oxidized cellulose, so as to understand and control the formation of oxidized cellulose, as well as to evaluate the fiber quality after oxidative degumming.

In this paper, nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) analyses were employed to determine the existence and changes of oxidized groups in degummed ramie fibers with different oxidation degrees. A scanning electron microscope (SEM) was used to depict the surface morphological changes of cellulose fibers in different oxidation degrees. Moreover, the relationship between the oxidized cellulose contents and mechanical properties of degummed fibers were further discussed and explored. The contents of oxidized cellulose in ramie fibers could be presented as an effective indicating factor to demonstrate oxidative degumming efficiency and degummed fiber quality.

Experimental details

Material

The raw ramie (60–120 cm in length) used in this study was planted in Yuanjiang, Hunan, China, in 2013. The bast fibers were decanted by hand. The main chemicals used in this study were hydrogen peroxide, sodium polyphosphate, carbamide and acetanilide. Sodium polyphosphate and carbamide were used as the surface active agent and the fiber expansion agent, respectively. Hydrogen peroxide and acetanilide were respectively as the oxidizing agent and stabilizer. In addition, the pH of the solution was adjusted by using 0.1 mol/L NaOH. All chemicals were supplied by Sinopharm Chemical Reagent Co. Ltd (Shanghai, China).

Experimental set-up and degumming process

The experiments were conducted in a 500 mL beaker filled with 10.0 g raw ramie fiber and 100 mL distilled water, continuously mixed at 200 r/min with a magnetic stirred bar. The whole set-up was placed in a water bath. The pH was adjusted to the desired value by using the 0.1 mol/L NaOH. The reaction temperature was raised and controlled to the desired value with a water bath. A certain amount of H₂O₂ was added to the reactor and mixed well with the solution. Then the oxidation reaction process was conducted at some specific time. The treated fibers were thoroughly washed with deionized water. Finally, they were squeezed and properly dried in an oven (100℃, 3 h) for the subsequent characterization and measurement.

Traditional alkaline degumming

The technical route of the traditional chemical degumming method used in the industry is shown in Table 1.

Table 1.

Conditions of traditional alkaline degumming

	Pretreatment	First boiling	Second boiling
Chemical dosage	H₂SO₄ 2%	NaOH 5% Na₂SiO₃2% Na₂SO₃2.5%	NaOH 15% Na₂SiO₃ 2.5% Na₅P₃O₁₀ 2%
Temperature	50℃	100℃	100℃
Treatment time	60 min	120 min	180 min

Determination of aldehyde and carboxylate groups

The aldehyde and carboxylate contents were determined by the electric conductivity titration method.^16–18 For that purpose, 0.3 g of dried fiber sample was added with water (55 mL) and 0.01 M NaCl (5 mL). The mixture was sufficiently stirred to prepare 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. 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 was determined from the conductivity and pH curves, and expressed as an average value of three measurements. The aldehyde content was measured according to the following procedure. The water-insoluble degummed fibers were further oxidized with NaClO₂ at a pH value of 4–5 for selective conversion of the aldehyde groups to carboxylate ones, and carboxylate content was determined by the above-mentioned method. The carboxylate groups formed by the NaClO₂ oxidation were regarded as aldehyde groups present in the original degummed fibers.

Dyeing performance

The degummed ramie fibers either by oxidative degumming or traditional alkaline degumming were dyed with C.I. reactive red 2 at a liquor of 1:20 using a cold pad-batch process. A total of 0.5 g fiber was soaked in a dyeing bath composed of 0.1% aqueous solution of the dye. The temperature of the dyeing bath was gradually raised to 30℃ over 10 min, and the sodium chloride was added to accelerate dyeing. After the sample was kept in the dye bath for 30 min, sodium bicarbonate was added to the mixture. After 30 min, the dyed ramie fibers were squeezed, rinsed thoroughly with water and dried at 60℃. The color strength and the dye uptake were measured with ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrometer (model Lambd 950, PerkinElmer Co. Ltd, USA). The color strength, expressed as the K/S value, was employed to evaluate the dyeing performance of the treated fibers.

XPS

XPS (ESCA LAB250, Thermo Fisher Scientific) measurements were performed on a Kratos Axis Ultra spectrometer with a monochromatic Al Kα X-ray source. The atomic (C, O) composition and O/C ratio were determined by the XPS spectra analysis. Quantitative elemental compositions were determined from peak areas using experimentally determined sensitivity factors and spectrometer transmission function.

NMR

The solid-state¹³([A-Z])-NMR spectra were recorded on a Bruker MSL-300 spectrometer at room temperature using the true 90° pulse calibration time of 6 µs, the proton transmitter dead time of 2 µs and the contact time for polarization transfer with a Hartmann–Hahn match of 3 ms. The data acquisition time was 29 µs and a line broadening of 100 Hz was applied to the spectra.

FT-IR

FT-IR analysis was used to determine the chemical functional groups in the treated ramie fiber. The fibers were analyzed using a Nicolet 6700 Spectrometer (Thermo Fisher, USA). All samples were prepared and mixed with KBr in a sample/KBr ratio of 1/100. The spectra obtained were the result of 30 scans over a range from 400 to 4000 cm⁻¹ with a resolution of 8 cm⁻¹, followed by baseline correction and smoothing prior to further analysis.

SEM

SEM images of fiber surface topography of treated fibers were taken using a SEM Model JEOL (JSM-5600LV, Japan). It was operating at 10 kV, temperature 20℃ and relative humidity (RH) 65%. Prior to SEM evaluation, the samples were coated with a thin layer of gold by means of a plasma sputtering apparatus.

Mechanical and physical tests

All samples were conditioned in the standard atmospheric condition (temperature: 20 ± 2℃, RH: 65 ± 3%) for 24 hours before testing.¹⁹ The flexibility was characterized by the number of turns until the fibers were broken with the Y331A yarn twist tester. Each sample was measured 50 times and the average of them was taken as the final result. The fineness of fibers was tested according to the Chinese national test standard GB/5884-86. Tensile properties, such as tenacity and breaking elongation, were carried out using an instrument XQ-1A testing machine. The gauge length and drawing speed were, respectively, kept at 20 mm and 20 mm/min. Average values were obtained using results from 50 specimens.

Degree of polymerization

The degree of polymerization of all samples was determined by the viscosity method using an Ubbelohde capillary viscometer as the instrument and copper ethylene-diamine solution as the solvent. Prior to testing, the ramie fibers were degreased with benzene and ethyl alcohol mixture in 2:1 (v/v) ratio using a Soxhlet extractor. Then the samples were kept in a controlled-humidity atmosphere in a closed weighing container until they reached equilibrium water content. All samples of cellulose (0.1 g) were suspended in copper ethylene-diamine solution (50 mL).

Results and discussion

Factors that influence the contents of oxidized groups

The effects of hydrogen peroxide dosage, oxidation temperature, pH value and reaction time on oxidized group variations were systematically investigated. Various amounts of aldehyde and carboxylate group contents in oxidized cellulose fibers were formed under different operating conditions. It is demonstrated in Table 2 that the number of oxidized groups increased as the oxidation degree became more severe. Specifically, both aldehyde and carboxylate content values increased with an increase of hydrogen peroxide dosage, oxidation temperature, pH values and oxidation time. Also, the residual gum content in treated fibers continued decreasing with increasing oxidation.

Table 2.

Operation parameters and corresponding oxidized group contents in degummed fibers

Degummed fibers	Operation parameters				Oxidized group contents		Residual gum content (%)
Degummed fibers	Chemical dosage (%)	Reaction temperature (℃)	pH value	Reaction Time (min)	Aldehyde (μmol/g)	Carboxylate (μmol/g)	Residual gum content (%)
1#	4	80	10.0	60	10	98	10.64
2#	5	85	11.0	60	28	160	4.17
3#	6	90	12.0	60	39	197	3.58
4#	7	90	13.0	60	50	240	3.09
Traditional degumming	20	100	14.0	300	4	32	2.80

Note: experimental codes 1#, 2#, 3#, 4# stand for the fiber samples to varying oxidation degrees after oxidative degumming.

When the reaction condition was mild in the solution, namely there was less hydrogen peroxide dosage, lower oxidation temperature and lower pH value, the treated fibers could not react completely with the oxidant, which resulted in producing fewer oxidized groups. The residual gum content still accounted for a large proportion in treated fibers. However, when the reaction condition became strong, namely higher hydrogen peroxide dosage, higher oxidation temperature and higher pH value, it could generate a large amount of oxidized cellulose in terms of increased contents of aldehyde and carboxylate.^20,21 In addition, the treated fibers exhibited a lower amount of residual gum content. Therefore, the operation parameters should be kept within certain and appropriate limits, in order to lessen the damage of cellulose and achieve better degumming efficiency.

Process control of oxidation degumming by oxidized group contents

During the fiber extraction process, a large number of hydroxyl groups in cellulose macromolecules can be inevitably oxidized by the oxidizing reagent. When the oxidized group contents are few, the degumming efficiency is poor and the oxidation reaction is inadequate. Correspondingly, the tested mechanical properties of treated fibers are low. When the contents of the oxidized group increase within a certain range, the degumming efficiency is enhanced and, thus, the mechanical properties are improved as well. However, when the oxidized group contents are too many, the oxidation reaction during degumming is intense, excessive and violent, which facilitates the deterioration of the mechanical properties of treated fibers. This means that when the aldehyde and carboxylate contents in treated fibers vary from a certain range, a better degumming performance could be achieved. Under this range, the residual gum percentage of treated fibers is relatively lower and the mechanical properties are better compared with other oxidized group contents.

It is worth noting that tenacity is largely affected by the combined two opposite factors, namely breaking strength and linear density or fineness. When the gummy materials have been gradually removed from the ramie, the linear density of treated fibers continues to decrease. Hence, it generally happens that after the removal of gummy materials by partial oxidative degumming, the effective value of tenacity (breaking strength/linear density) of treated fibers accordingly appears to be apparently increased, even though the breaking strength may remain less affected or unaffected during the degumming process.

The variations of oxidized group contents can serve as the main technical specification for quality monitoring and control in fiber preparation. Figure 1 shows the effect of different amounts of oxidized group contents on tenacity and breaking elongation. As can be seen, both measured tenacity and breaking elongation values are enhanced stepwise when the aldehyde content increased from 5 to 26 μmol/g or when the carboxylate content increased from 60 to 142 μmol/g. However, when the aldehyde content exceeded 26 μmol/g or the carboxylate content exceeded 142 μmol/g separately, the tenacity and breaking elongation declined due to the damage of cellulose fibers caused by the excessive oxidation reaction.

Figure 1.

Dependence of oxidized group contents on mechanical properties in degummed fibers: (a) aldehyde content on tenacity and elongation; (b) carboxylate content on tenacity and elongation.

On the basis of these testing results, it was concluded that when the aldehyde content varied from 20 to 35 μmol/g or carboxylate content varied from 120 to 180 μmol/g, raw ramie could achieve a desired and reasonable degumming result. Under these ranges, both tenacity and breaking elongation of treated fibers were relatively better compared with other amounts of oxidized group contents in degummed fibers. The degree of oxidation and content of oxidized cellulose could be used as an effective indicating factor to demonstrate the oxidative degumming efficiency and fiber quality. The measured aldehyde and carboxylate content values could be adjusted by changing the operation conditions, such as hydrogen peroxide dosage, oxidation temperature, pH value and reaction time.

XPS

XPS is currently and commonly employed to determine the quantitative elemental compositions, the bonding state of the atoms and the location of atom types in cellulose fibers.^22,23 In this study, XPS was performed to trace the formation of aldehyde (–CHO) and carboxyl (–COOH) groups, as well as to compare the structure differences within oxidized cellulose fibers with different oxidation degrees.

The treated degummed fiber samples were firstly scanned in a low-resolution mode performed on a Kratos Axis Ultra spectrometer. The wide-scan XPS spectra of degummed fibers to varying oxidation degrees are plotted in Figure 2. The presented atomic percentages of elements were derived from a spectra run of the region of interest. The degummed fibers exhibited very simple spectra containing two characteristic peaks of carbon (binding energy = 284.5–288.9 eV) and oxygen (binding energy = 532.3–533.3 eV), while some weaker peaks were associated with the existence of N and S, as expected from natural cellulose fibers.²⁴ According to peak areas, the relative atomic percentage for each element can be calculated using experimentally determined sensitivity factors and the spectrometer transmission function. Table 3 summarizes the element compositions acquired for degummed fibers with different oxidation degrees. As demonstrated, the treated cellulose fibers mainly consisted of electrons from carbon (C_1s) and oxygen (O_1s) atoms, as well as a small quantity of electrons from nitrogen (N_1s) and sulfur (S_1s) atoms. The combined contents of carbon and oxygen elements quantitatively occupied more than 98% of the total elemental composition.

Figure 2.

Wide-scan X-ray photoelectron spectroscopy (XPS) spectra of degummed fibers to varying oxidation degrees.

Table 3.

Element compositions and O/C ratio in degummed fibers to varying oxidation degrees

Degummed fibers	Relative concentration (%)				O/C
Degummed fibers	C	O	N	S	O/C
1#	71.13	27.15	1.21	0.51	0.3817
2#	69.61	28.69	1.18	0.52	0.4122
3#	68.15	30.10	1.25	0.50	0.4417
4#	66.73	31.56	1.20	0.51	0.4730
Traditional degumming	76.36	22.67	1.12	0.39	0.2969

Note: experimental codes 1#, 2#, 3#, 4# stand for the fiber samples to varying oxidation degrees after oxidative degumming.

It is worth noting that the O/C ratio is shown to be proportional to the amount of oxidized cellulose. Hence, the ratio values can be used for quantitatively determining the oxidized cellulose contents in degummed fibers. A low O/C ratio reflects lower oxidized cellulose/cellulose, while a high O/C suggests the presence of more oxidized cellulose.²⁵ On this account, it is possible to monitor the amount of oxidized cellulose by measuring the elemental compositions and O/C ratio from XPS spectra. Just as predicted, the O/C ratio had a rising trend with increasing the oxidation degree exhibited in Table 3. For the degummed fibers with the traditional alkaline method, the value of the O/C ratio was smaller than the treated fibers with oxidative degumming. XPS measurement revealed the presence of oxidized group contents on the treated fibers during the oxidative degumming process. In order to obtain more information on the nature of oxidized cellulose, these spectra regions were further run in a high-resolution mode.

The high-resolution C_1s peak in the XPS spectra of the degummed fibers gives detailed information on the surface chemistry. The C_1s binding energy depends on the number of bonds between the carbon and oxygen. Figure 3 demonstrates the peak assignment of C moieties in treated fibers. Generally, the chemical shifts for carbon (C_ls) in cellulose fibers can be deconvoluted into four categories: C₁ (CH₂–CH₂), C₂ (C–O), C₃ (O–C–O) and C₄ (–COOH, R). These four C moieties exhibited correspond to peaks at 284.5, 286.5, 287.8 and 288.9 eV, respectively. It can be seen that the peak densities of C moieties experienced slight change with the increase of oxidation degrees. Table 4 summarizes the peak areas and relative atomic percentage of C moieties in degummed fibers with different oxidation degrees. The amount of C₁ and C₂ appeared to have a decreasing trend, while the amount of C₃ and C₄ showed an increasing trend. More oxidized cellulose could be generated on the fibers as the oxidative degumming reaction becomes more severe.

Figure 3.

High-resolution X-ray photoelectron spectroscopy spectra of C_1s peaks in degummed fibers to varying oxidation degrees. (a)–(d) correspond to degummed fibers 1#–4# in Table 2.

Table 4.

Peak areas and relative atomic percentage of C moieties in degummed fibers

Degummed fibers	Peak areas of C_1s peaks
Degummed fibers	C₁(CH₂–CH₂) 284.5 eV	C₂(C–O) 286.5 eV	C₃(O–C–O) 287.8 eV	C₄(–COOH,R) 288.9 eV
1#	14,120.3 (25.36%)	33,214.6 (59.66%)	6895.1 (12.39%)	1442.0 (2.59%)
2#	14,101.6 (25.21%)	33,200.4 (59.35%)	6989.5 (12.49%)	1647.2 (2.94%)
3#	13,984.0 (25.22%)	32,598.4 (58.78%)	7001.7 (12.62%)	1874.8 (3.38%)
4#	13,903.5 (25.13%)	32,145.9 (58.09%)	7145.6 (12.91%)	2139.4 (3.87%)
Traditional degumming	15,988.0 (28.96%)	32,947.72 (58.01%)	6540.8 (11.02%)	1320.1 (2.01%)

Note: experimental codes 1#, 2#, 3#, 4# stand for the fiber samples to varying oxidation degrees after oxidative degumming.

In terms of O_1s peaks, O₁ (C–OH, C–O–C) and O₂ (C=O, COOR) were generally involved. These two oxygen moieties exhibited corresponding peaks at 532.3 and 533.3 eV, respectively. Figure 4 illustrates the high-resolution XPS spectra of O_1s peaks in degummed fibers to varying oxidation degrees. As demonstrated, the peak area, peak width and peak height were apparently changed with the increased quantity of oxidized cellulose. This suggests that more aldehyde (–CHO) and carboxyl (–COOH) groups can be produced or introduced on the fibers with the increase of oxidation levels. The observed phenomenon is in good accordance with the Table 3 experimental results.

Figure 4.

High-resolution X-ray photoelectron spectroscopy spectra of O_1s peaks in degummed fibers to varying oxidation degrees. (a)–(d) correspond to degummed fibers 1#–4# in Table 2.

The relative atomic percentage of oxygen is given in Table 5, as calculated according to Gaussian/Lorentzian fitting of the corresponding peaks. It was obvious that the intensity of the O₁ peak showed a decreasing trend as the oxidation degree increased. In contrast, the intensity of O₂ peak experienced an increasing trend at the same time. The O₂/O₁ ratio had a significant increase from 0.691 in fiber 1# to 0.808 in fiber 4#. O₁ mainly assigns to oxygen in the hydroxyl group (–OH) or a carbon–oxygen single bond (C–O), while O₂ mainly comes from the oxygen in the carbonyl group (–C=O) or carbon–oxygen double bond (C=O). As the reaction conditions become more and more intense, a growing number of hydroxyl groups are converted into the corresponding carbonyl structure, resulting in the increasing formation of aldehyde (–CHO) and carboxyl (–COOH) groups on the degummed fibers. Correspondingly, for the degummed fibers with the traditional alkaline method, the value of the O₂/O₁ ratio was smaller than that of the treated fibers with oxidative degumming.

Table 5.

Peak areas of O moieties and O₂/O₁ ratio in degummed fibers

Degummed fibers	Peak areas of O_1s peaks and O₂/O₁ ratio
Degummed fibers	O₁ (C–OH, C–O–C) 532.3 eV	O₂ (C=O, COOR) 533.3 eV	O₂/O₁
1#	28,970.2	20,018.6	0.691
2#	28,184.1	20,979.0	0.744
3#	27,361.8	21,156.2	0.773
4#	26,990.3	21,814.7	0.808
Traditional degumming	30,964.4	18,021.3	0.582

Note: experimental codes 1#, 2#, 3#, 4# stand for the fiber samples to varying oxidation degrees after oxidative degumming.

NMR

The changes in the chemical structure of ramie fibers by hydrogen peroxide oxidation were examined by solid-state carbon-13 nuclear magnetic resonance (¹³C-NMR) spectra, which are shown in Figure 5. For degummed fiber 1#, we can observe that the signals of carbons exhibited the chemical shift of C1 at 102.9 ppm, C2 at 73.6 ppm, C3 at 75.6 ppm and C4 at 81.1 ppm. The signals of C5 exhibited at 79.2 ppm and C6 exhibited at 60.8 ppm in the ¹³C-NMR spectra.^26–28 Due to an inadequate and incomplete reaction, the cellulose in the fibers was slightly affected and partially oxidized.

Figure 5.

Carbon-13 nuclear magnetic resonance spectra of degummed fibers to varying oxidation degrees. (a)–(d) correspond to degummed fibers 1#–4# in Table 2.

Comparatively, degummed fiber 2# demonstrated an obvious chemical shift change due to oxidation. The signals of C1 possessed at 103.1 ppm, C2 at 73.5 ppm, C3 at 75.0 ppm and C4 at 81.7 ppm in oxidized cellulose. It should be noted that the C5 and C6 signals were particularly observed at 76.1 and 65.3 ppm. In this sort of situation, the degumming efficiency was complete and more oxidized groups were introduced on the macromolecules in the fibers.

In regard to degummed fibers 3# and 4#, the signals of carbons from glucose moiety are strongly shifted by the oxidation. The shapes and intensities of signals in C2, C3 and C5 moieties experienced distinct change with the increase of oxidation levels. The signals of carbon in chemical shifts were deeply affected by the oxidation of primary hydroxyl groups, as compared to the first two samples. Specifically, for degummed fiber 4#, the cellulose was nearly totally oxidized and it presented the highest degree of oxidation among the four samples. On the whole, the ¹³C-NMR measurement results were in good accordance with the above XPS scanning results.

FT-IR

The oxidation reaction would cause damage to the cellulose macromolecule structure to some extent. The FT-IR analysis of the degummed samples was performed in order to identify the change in chemical composition of treated fibers.²⁹ As presented in Figure 6, the range from 3300 to 3600 cm⁻¹ corresponded to -OH stretching vibrations, which were mainly attributed to the large number of hydroxyl groups in cellulose fibers. After reaction at different levels and varying degrees of oxidation, the relative intensities at 3300–3600 cm⁻¹ decreased, which suggested that the content of hydroxyl groups was significantly reduced by the oxidation. The hydrogen bond structure in cellulose was considerably broken and, consequently, induced the loss of mechanical properties in the degummed fibers.

Figure 6.

Comparisons of Fourier transform infrared spectra of degummed fibers with different oxidized group contents. ((a)–(d) correspond to degummed fibers 1#–4# in Table 2).

Furthermore, with the increase of degree of oxidation and content of oxidized groups, a slight increase in the intensity of the band at around 1735 cm⁻¹ can be observed, corresponding to the C=O stretching vibrations. This kind of change was related to the formation or introduction of aldehyde or carboxylate groups on fibers during the oxidation degumming process. The spectra of the treated ramie fiber exhibited O–H stretching absorption around 3420 cm⁻¹, C–H stretching vibration around 2940 cm⁻¹, CH₂ symmetric bending around 1420 cm⁻¹ and C–O–C stretching around 1060 cm⁻¹. These corresponding intensities in the spectra were also slightly changed due to the oxidation effect of hydrogen peroxide.

SEM

The change in the surface morphology of the treated fiber was studied by scanning electron microscopy.³⁰ There was one-to-one correspondence between Figures 7(a)–(d) and the sample numbers shown in Table 2. Examination of the feature in Figure 7(a) showed that a certain amount of waxy substances still covered the surface, which prevented fibers from separating fully. Due to inadequate oxidation reaction, the degumming efficiency was poor and the treated fibers exhibited rough and coarse morphologies. Figure 7(b) reveals that the gummy components were removed completely and the treated fibers appeared mostly clean with a smooth surface. The contour characteristics of individual fibers could be observed clearly. This proved this kind of oxidation process was effective and the degumming result was desired and reasonable. As demonstrated in Figure 7(c), the bundle fibers could be separated thoroughly from each other. However, minor and slight cracks within fibers could be observed clearly under the microscope. The excessive and intense degumming reaction would inevitably result in a high degree of oxidation in treated fibers. In regard to sample 4# in Figure 7(d), the fiber surface appeared to have many fine pits and plaques, and some fine particles were found to be stripped off from the surface. The oxidation-destructive degumming process could produce irreversible loss to the fibrous structure. The active oxidant generated a violent reaction and, hence, facilitated the deterioration of fiber morphology along with the mechanical properties.

Figure 7.

Comparison of scanning electron microscope examination micrographs of degummed fibers with different oxidized group contents (magnification × 1000). (a)–(d) correspond to degummed fibers 1#–4# in Table 2.

Physical and mechanical properties of degummed fibers

Larger and better tenacity, breaking elongation and flexibility are desired fiber properties after the degumming process.^31,32 Actually, at the beginning of oxidative degumming, the degumming efficiency was low and the bound fibers could not be separated completely and this, in turn, affected the mechanical properties, such as tenacity and breaking elongation. At this stage, the generated aldehyde and carboxyl contents were also very few. As the reaction continued, the gum components in ramie fibers were removed continually; thus, the mechanical properties of degummed fibers improved. Meanwhile, the generated aldehyde and carboxyl content kept increasing. In other words, under a certain range, degummed fibers' tenacity and breaking elongation were increasing due to the increased amounts of aldehyde and carboxyl, to some extent. As can be seen from Table 6, these kinds of properties first increased and then dropped down with the increase of oxidation degree and content of oxidized groups. It was confirmed that a greater presence of aldehyde and carboxylate groups in cellulose contributed to a larger loss of physical and mechanical properties of degummed fibers.

Table 6.

Physical and mechanical properties of degummed fibers

Sample no.	Tenacity (cN/dtex)	Breaking elongation (%)	Flexibility (number/10cm)	Degree of polymerization	Linear density/ fineness (dtex)	Residual gum content (%)	Dye uptake (%)	K/S value
Raw ramie	—	—	—	1846	13.87	25.98	—	—
1#	5.85	3.05	134	2004	8.21	10.64	42.3	10.2
2#	6.90	3.53	143	2151	6.03	4.17	48.8	15.3
3#	5.32	2.74	125	1923	5.64	3.58	53.0	19.8
4#	4.51	2.50	112	1810	5.42	3.09	55.7	22.3
Traditional degumming	6.42	3.21	152	2652	6.19	2.80	52.5	18.9

Note: experimental codes 1#, 2#, 3#, 4# stand for the fiber samples to varying oxidation degrees after oxidative degumming.

More specifically, the physical and mechanical properties of sample 1# were smaller than those of sample 2#. This was because the oxidation reaction of sample 1# was relatively inadequate and the degumming efficiency was unavoidably low. Some residual gums still existed inside the fibers, which had an adverse effect on the performance of fibers. Overall, sample 2# showed the maximum values in tenacity, breaking elongation and flexibility compared with other samples. This was attributed to the complete removal of non-cellulosic substances during degumming. For samples 3# and 4#, the values of physical and mechanical properties dropped gradually. When applying more severe conditions, the oxidation reaction became more intense and inevitably led to greater loss of fibrous structure as well as mechanical properties. This implies that these two degumming processes produced lower quality fiber products, in contrast with sample 2#.

Furthermore, the value of degree of polymerization (DP) increased from 2004 in sample 1# to 2151 in sample 2#. This resulted from the removal of low molecule weight of gummy components in fibers. Moreover, when exposed in a highly oxidizing environment, degummed fibers experienced a distinct decrease of DP values, which were 1923 and 1810 in samples 3# and 4#, respectively. This observation might possibly be attributed to the strong oxidation and degradation effect of hydrogen peroxide on cellulose macromolecule chains. The active oxidant could decompose large molecules into smaller ones, which induced the declining trends of DP. Comparatively, the degummed fibers with the traditional alkaline method possessed better flexibility and DP and less residual gum content than the treated fibers with oxidative degumming.

The dyeability of the degummed fibers with the C.I. reactive red 2 was measured by color strength (K/S value) and dye uptake. As shown in Table 6, for the fibers by oxidative degumming, the K/S value increased gradually with an increase in oxidation. This may be explained as follows. Dye molecules are combined mainly from physical adsorption or weak chemical interactions, besides which a small quantity of dye molecules can combine with the treated fiber via a covalent bond.³³ As a result of swelling during oxidation, more sites for chemical and physical bonding of dye molecules are created, which results in the K/S value being enhanced. Comparatively, for the degummed fibers with the traditional alkaline method, the K/S value was a little smaller than for treated fiber samples 3# and 4# with oxidative degumming.

Conclusion

Oxidized cellulose can be produced from the oxidation reaction of peroxide hydrogen in the degumming process of ramie fibers. In this work, the oxidized cellulose introduced into ramie fibers was systematically characterized in terms of XPS, NMR, FT-IR and the SEM. XPS demonstrated that the O/C ratio had a rising trend with increasing the oxidation degree, which revealed the increasing presence of oxidized group contents in treated fibers. ¹³C-NMR spectra indicated that the shapes, intensities and chemical shifts of signals in C2, C3 and C5 moieties experienced distinct changes with increasing oxidation levels. FT-IR analysis supported that the content of hydroxyl groups in cellulose samples was significantly reduced with the increase of degree of oxidation. SEM examination micrographs showed that the treated fibers appeared most clean and with the smoothest surface when the degumming efficiency was effective and the degree of oxidation was reasonable. In addition, further effort was committed to investigate the dependence of oxidized cellulose contents on the mechanical properties of degummed fibers. In addition, the greater presence of oxidized cellulose contributed to a larger loss of tenacity, breaking elongation, flexibility and degree of polymerization. When the aldehyde content varied from 20 to 35 μmol/g or carboxylate content varied from 120 to 180 μmol/g, raw ramie could achieve the desired and reasonable degumming result. This study could offer useful information in better understanding the reaction characteristics of oxidative degumming and better control of fiber quality. The contents of oxidized cellulose in ramie fibers could be used as an effective indicating factor to demonstrate oxidative degumming efficiency and fiber properties.

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 disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the earmarked fund for Modern Agro-industry Technology Research System (grant number CARS-19-E25), the China Scholarship Council (CSC) and the Doctorial Innovation Fund of Donghua University (grant number 15D310111).

References

Choi

Lee

. Effects of surface treatment of ramie Fibers in a ramie/poly (lactic acid) composite. Fiber Polym 2012; 13: 217–223.

Omar

Andrzej

Hans-Peter

et al.

Biocomposites reinforced with natural fibers: 2000–2010. Prog Polym Sci 2012; 37: 1552–1596.

Rebenfeld

. Fibers: the old and the new. J Text Inst 2001; 92: 1–9.

Zhang

Yan

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

Fan

Liu

et al.

A novel chemical degumming process for ramie bast fiber. Text Res J 2010; 80: 2046–2051.

Zheng

Zhang

Luo

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

. Effect of peroxide and softness modification on properties of ramie fiber. Fiber Polym 2014; 15: 2105–2111.

. The effect of oxidation–reduction potential on the degumming of ramie fibers with hydrogen peroxide. J Text Inst 2015; 106: 1251–1261.

Guo

Zou

et al.

An effective degumming enzyme from bacillus sp.Y1 and synergistic action of hydrogen peroxide and protease on enzymatic degumming of ramie fibers. Biomed Res Int 2013; 2013: 1–9.

10.

Jabli

Touati

Kacem

et al.

New chitosan microspheres supported [bis(2-methylallyl)(1,5-cyclooctadienne)ruthenium(II)] as efficient catalysts for colour degradation in the presence of hydrogen peroxide. J Text Inst 2012; 103: 434–450.

11.

Saravanan

Vasanthi

Raja

et al.

Bleaching of cotton fabrics using hydrogen peroxide produced by glucose oxidase. Ind J Fibre Text Res 2010; 35: 281–283.

12.

Luiz

CAB

Célia

RAM

Antônio

et al.

A rapid method for quantification of carboxyl groups in cellulose pulp. BioResources 2013; 8: 1043–1054.

13.

Darja

Robert

Vanja

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

14.

Hejlová

Milichovský

. Oxidized cellulose with different carboxyl content: Structure and properties before and after beating. Physics Procedia 2013; 44: 256–261.

15.

Sergiu

Gabriela

Bogdan

et al.

Oxidized cellulose-Survey of the most recent achievements. Carbohyd Polym 2013; 93: 207–215.

16.

Saito

Shibata

Isogai

et al.

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

17.

Saito

Isogai

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

18.

Saito

Isogai

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

19.

Meng

. Analysis of oxidized cellulose introduced into ramie fiber by oxidation degumming. Text Res J 2015; 85: 2125–2135.

20.

Saito

Hirota

Tamura

et al.

Individualization of nano-sized plant cellulose fibrils by direct surface carboxylation using TEMPO catalyst under neutral conditions. Biomacromole 2009; 10: 1992–1996.

21.

Kim

U-J

Kuga

Wada

et al.

Periodate oxidation of crystalline cellulose. Biomacromole 2000; 1: 488–492.

22.

Zafeiropoulos

Vickers

Baillie

. An experimental investigation of modified and unmodified flax fibres with XPS, ToF-SIMS and ATR-FTIR. J Mater Sci 2003; 38: 3903–3914.

23.

Matuana

Balatinecz

Sodhi

RNS

et al.

Surface characterization of esterified cellulosic fibers by XPS and FTIR spectroscopy. Wood Sci Technol 2001; 35: 191–201.

24.

Belgacem

Czeremuszkin

Sapieha

. Surface characterization of cellulose fibres by XPS and inverse gas chromatography. Cellulose 1995; 2: 145–157.

25.

Stenstad

Andresen

Tanem

et al.

Chemical surface modifications of microfibrillated cellulose. Cellulose 2008; 15: 35–45.

26.

Varma

Chavan

Rajmohanan

et al.

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

27.

Kumar

Yang

. Analysis of carboxyl content in oxidized celluloses by solid-state ¹³C CP/MAS NMR spectroscopy. Int J Pharm 1999; 184: 219–226.

28.

Saito

Isogai

. Wet strength improvement of TEMPO-oxidized cellulose sheets prepared with cationic polymers. Ind Eng Chem Res 2007; 46: 773–780.

29.

Perez

Montanari

Vignon

. TEMPO-mediated oxidation of cellulose III. Biomacromole 2003; 4: 1417–1425.

30.

Liu

Duan

Sun

et al.

A rapid process of ramie bio-degumming by Pectobacterium sp. CXJZU-120. Text Res J 2012; 82: 1553–1559.

31.

Kang

Epps

. Effects of souring and enzyme treatment on moisture regain percentage of naturally colored cottons. J Text Inst 2009; 100: 598–606.

32.

Chen

Zhou

et al.

High-efficiency ramie fiber degumming and self-powered degumming wastewater treatment using triboelectric nanogenerator. Nano Energy 2016; 22: 548–557.

33.

Liu

Z-T

Yang

Zhang

et al.

Study on the performance of ramie fiber modified with ethylenediamine. Carbohydr Polym 2008; 71: 18–25.