Structure and properties of high quality natural cellulose nano fibrils from a novel material Ficus natalensis barkcloth

Abstract

We report for the first time the extraction of cellulose nano fibrils from Ficus natalensis barkcloth fibers by means of chemical treatments and catalytic oxidation of cellulose fibers by using 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO). After every stage of treatments, the structural properties of barkcloth fibers powder (BC-PW), cellulose fibers (BC-NB) and cellulose nano fibrils (BC-CNF) were carefully characterized by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for morphological analysis, Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction techniques for chemical analysis, and thermal properties were done by thermo-gravimetric analysis. FTIR results revealed the progressive removal of non-cellulosic contents and X-ray diffraction (XRD) analysis showed that the crystallinity increased with the successive alkali and bleaching treatments. Finally, by evaluating the TEM images, the average diameter of the nano-cellulose fibrils from Ficus natalensis barkcloth was also confirmed as 28 ± 0.6 nm and the length in hundred nano meters was recorded. The resultant cellulose nano fibrils maintaining the cellulose I structure had dimensional properties in nano-scale, higher crystallinity (68.5), and better thermal stability (305.62°C). The barkcloth cellulose nano fibrils can be used in nano technology like food packing material, nano composites and medical textiles.

Keywords

Materials properties structure properties fibrous materials cellulose nano fibrils

Introduction

Environmental fortification has become an important and challenging issue in the recent years [1]. The reason is the formation of non-biodegradable materials like fossil fuels and the byproducts of petroleum refineries [2]. Additionally, these caused severe issues in the health of humans and marine life [3]. Therefore, due to the increasing contemplations of the United Nation Organization (UNO) environmentalist and World Health Organization (WHO) attention, recently, a lot of studies have focused on the conservation and environment preservation [4,5]. The key advantage of these investigations is the use of natural polymers from natural resources like plants and animal matter.

Cellulose is the most abundant natural biopolymer in the world [6]. It is widely present in plants (approximately 40–55 wt.%) and bacteria [7] and considered to be the lasting renewable biomass, nontoxicity, biodegradability, stability, low cost and consequently a good substitute to synthetic products [8 –10]. Cellulose and their derivatives are utilized in various applications such as coating development, food packing, electrical elements, papermaking and textures [11]. Furthermore, it has a unique structure which comprised of highly stereo-regular linear homo-polysaccharide of B-1, 4-linked anhydro-D-glucose units with a high aspect ratio, i.e. 2–50 nm wide and hundreds of nanometer long which endows the cellulose with tensile stiffness (2–6 GPa) [12 –14]. The cellulose nanomaterial exhibited considerable features and characteristics, such as hydrophilicity, high elastic modulus (140–150 GPa), lightweight (1.5 g/cm3), low thermal expansion coefficient in the axial direction, broad chemical-modification capacity, crystallinity character and high surface area [15 –19].

The extraction of cellulose fibers to produce polymeric materials is a complex phenomenon [20]. In the learning resources, several methods (acid hydrolysis, mechanical defibrillation and TEMPO oxidation) have been applied to break the inter-fibrillar hydrogen bonds between the hydroxyl groups of cellulose for the extraction of nano-materials or nano-crystals [14]. Amongst all, acid hydrolysis (late 1940s) is described as the primary method which yielded crystalline rod-like cellulose nano-crystals (CNC) [21]. In the mechanical defibrillation method, the nano cellulose fibrils (CNF) are isolated from the constituent of fiber matrix [22]. The 2,2,6,6-tetramethylpiperidin-1-oxyl- (TEMPO) mediated oxidation method liberated the nano cellulose fibrils by mechanical disintegration treatment [23]. TEMPO oxidation method which is widely used is advantageous over other methods since it generated efficiently CNF with a uniform size [24]. Additionally, TEMPO does not change the crystallographic forms of the cellulosic fibers, which is beneficial in the application of nano-composite materials [25].

In the recent years, researchers and technical experts have focused on the agricultural wastes (cotton stalk, Ammophila arenaria, napier grass, rice husk, etc.) for the CNF production instead of wood fibers [15,26 –28], as they have shorter growth cycles, lower lignin contents, biodegradable, environmental friendly and also disintegrates into CNF easily [29]. Thus, the production of CNFs from broad sources of biomasses, including wood, non-woody plants and agricultural residues has become the subject of interest for the investigation. Ficus natalensis is a deciduous large tree of 20 m height, whose inner bark is a naturally biodegradable material which can be freely unwrapped from the stem, and most interestingly it takes few months to regenerate a fresh bark which can be used multiple times, and these trees are extensively distributed in Kenya, Zambia, Uganda, Malawi and South Africa [30]. Anciently, the barkcloth is used in cultural inheritance like shoes, clothes and bags, but now a days many researchers are trying to evaluate the effectiveness of bark cloth extracted from Ficus natalensis species, as Rwawiire et al. [19] used barkcloth in the development of a bio-composite, which is based on green epoxy polymer and natural cellulose fabric (bark cloth) for the automotive instrument panel applications [19]. Similarly, morphology, thermal, and mechanical characterization of barkcloth from Ficus natalensis were also investigated [31]. Meanwhile, the low thermal conductivity of nonwoven barkcloth fabric having good absorption behavior as acoustic and thermal properties was evaluated by Rwawiire et al. [32].

However, as natural polymers are attracting much attention [33] especially nano cellulose fibrils (CNF) from plants which are in high demand in global market expected at USD 1.08 billion by 2020 due to their interestingly increasing demand in food and pharmaceutical industries [3]. But, barkcloth generated from Ficus natalensis trees is not evaluated by the extraction and characterization techniques from a biodegradable, highly available and naturally occurring material. Besides various applications in cultural outfits of the recent studies show that Ficus natalensis barkcloth is rich in cellulose (68.69%) content [32] which is the cause of inspiration of this study to extract and characterize the nano cellulose fibrils. The aim of the present work is to use barkcloth (natural reinforcing component) as a precursor for the production of CNF. For this purpose, cellulose fibers were firstly isolated including chemical treatments as alkalization, bleaching and TEMPO oxidation. Characterization of the CNF has been investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and thermo gravimetric analysis (TGA).

Experimental works

Materials

Ficus natalensis plants are observed in almost all regions in Kenya and Uganda. Bio-degradable barkcloth from Ficus natalensis tree was obtained from Kenya, after extraction techniques as mentioned in Rwawiire et al. [34]. All the chemicals sodium bromide (NaBr, 99%), sodium chlorite (NaClO_2, 80%), sodium hydroxide (NaOH, 98%), sodium hypochlorite (NaClO, solution, 15%) and hydrochloric acid (HCl, 37%) were purchased from Sinopharm Chemical Reagent Co., Ltd, Shanghai, China. Deuterated dimethyl sulfoxide (DMSO), ethanol (CH₃CH₂OH, solution, 98%) and 2,2,6,6-Tetramethylpiperidin-1-yl oxyl (TEMPO, 98%) were obtained from Aladdin Chemistry Co. Ltd, Shanghai, China.

Extraction of cellulose fibers

Material preparation

Barkcloth material was cut into an approximate length of 2–4 cm and washed thoroughly with purified water at 25°C for 30 min and then dried out in an oven for maximum moisture elimination at a temperature of 80°C for 24 h. After fully drying, residual was ground into powder form by using a DJ-04 grinder (Shanghai Dianjiu Traditional Medicine Machinery Manufacture Co., Ltd, Shanghai, China) to pass through a 60-mesh sieve, so that non cellulosic materials can easily be removed [35]. Figure 1 shows the schematic diagram of cellulose nano fibrils.

Figure 1.

Schematic diagram of cellulose nano fibrils extraction.

NaOH treatment

A total of 50 g/l cleaned and dried barkcloth residual fibers in powder form was taken and then mixed with 1.0 M NaOH solution for 2 h at 80°C under constant stirring by keeping the beaker on a magnetic stirrer. Alkali-treated samples were washed with deionized water for several times, filtered and dried at 40°C for 6 h in the vacuum oven.

Bleaching

Alkali treated samples were bleached as mentioned in [36] briefly by treating with 2.5% w/v sodium chlorite using material to liquor ratio of 1:20 for 1 h at pH 4.5 under continuous stirring. After that the mixture was thoroughly washed in a beaker with deionized water. At the end of the process, the left out mixture was thoroughly washed with deionized water and dried out at 60°C in an oven until the constant weight was achieved. Then the obtained cellulose fibers were ground into powder form.

Preparation of cellulose nano fibrils

Cellulose nano fibrils preparation was performed according to the procedure described by Cao et al. [37] and during this process, cellulose fibers mainly C6 primary hydroxyls groups oxidize to C6 carboxylate groups by TEMPO/NaBr/NaClO in the water at pH 10–10.5 [38]. Briefly, the above-mentioned cellulose fibers from barkcloth (10 g) were distributed in the 190 g water for 6 h and later a fixed amount of NaBr 0.20 g and TEMPO 0.02 g was added to the solution. By adding drop wise 12% NaClO (18 g) solution The reaction was started under constant stirring by adding 12% NaClO (18 g) solution dropwise. The pH (10–10.5) was monitored continuously with the pH meter and adjusted with sodium hydroxide aqueous solution (0.5 mol/L). After that 5 mL ethanol was added to stop the reaction followed by stirring for another 20 min. The obtained product was washed several times with deionized water to remove the chemicals and centrifuged at 1960g for 15 min; 1.0 g nano cellulose slurry was dispersed into 100 g of water and placed in an ice bath for 5 min sonication at 10,000 r/min.

Characterization

Composition analysis

The composition (cellulose, lignin, and ash content) of barkcloth was determined by using the standard methods. Calorimetric method was used to determine the cellulose content by using anthrone reagent [39]. For lignin content, APPITA P11s-78 method was employed as mentioned in [40], while TAPPI method 211-om 93 as reported in [41] was used to measure the ash content at 525°C.

Scanning electron microscopy analysis

Scanning electron micrographs of CNF were carried out to observe the surface morphology of the samples using an ultra-high resolution scanning electron microscopy JSM-6360LA instrument (Jeol, Japan) using an accelerating voltage of 15 kV. Before the SEM analysis of the specimens, a very thin film of gold coating on the surface of all the samples with vacuum sputter coater (model SC 500) was done.

Transmission electron microscopy analysis of CNF

Transmission electron microscopy (TEM) (model JEM-2100 TEM (JEOL Ltd, Tokyo, Japan) was used to determine the dimensions of cellulose nano fibrils obtained from Ficus natalensis bark cloth fibers. Using a pipette, a drop of diluted suspension of 1 wt.% CNF dispersion was deposited on a glow-discharged carbon-coated Cu grid. The redundant liquid solution was drained with filter paper. The grid was left to be dried at room temperature and then tested the dimensions of the imaged cellulose nano fibrils with an accelerating voltage of 80 kV.

Fourier-transformed infrared spectroscopy

The FTIR spectra of samples Ficus natalensis barkcloth fibers powder, cellulose fibers and cellulose nano fibrils were recorded by grinding and mixing all the samples with potassium bromide, KBr and then firmly compressing into pill form. The FTIR spectra were recorded using an instrument (Thermo Fisher Scientific Co., Ltd, Waltham, MA, USA) in the range of 400–4000 cm⁻¹ with a resolution of 4 cm¹ and 64 scans.

Thermo gravimetric analysis

The dried samples were analyzed with Perkin Elmer TGA-7 thermo-gravimetric analyzer. Each sample (0.4–0.7 mg) was put into a platinum sample pan and heated from 30°C to 600°C under a nitrogen (N₂) atmosphere with a gas flow rate of 20 mL/min at a heating rate of 10°C/min.

X-ray diffraction analysis

XRD data were obtained using the instrument D500 diffractometer (SIEMENS) to determine the crystallinity index CrI of Ficus natalensis barkcloth fibers raw material, cellulose fibers and cellulose nano fibrils. Each sample was milled into powder form to obtain a uniform X-ray exposure, using a monochromatic Cu Ka radiation source in the step-scan mode of 2θ angle ranging from 10 to 60° at a scanning time of 5.0 mint with a resolution of 0.02° using the 200 mA current and 40 kV operating voltage. To characterize the crystallinity, the crystallinity index CrI of every sample was calculated as mentioned in equation (1) by Segal method mentioned in [42] using the maximum intensity of the diffraction from the 002 plane (I₀₀₂), and the minimum intensity between the 002 and 110 peaks (I_am).

CrI (%) = 100 \times \frac{I_{002} - I_{a m}}{I_{002}}

(1)

Results and discussion

Chemical composition

A comprehensive analysis of content of different constituents of the barkcloth was determined by using different standard methods. To get an average value, three samples were taken for all components of barkcloth material. The samples had average value of α-cellulose, lignin and hemicellulose of 34.5 ± 1%, 19.5 ± 0.9% and 23.5 ± 0. 8%, respectively, but could be lower than the report mentioned in Rwawiire et al.[32]. Barkcloth α-cellulose (34.5 ± 1%) contents are higher as compared to various cereal straws, non woody and perennial sources of cellulose as reported in literature [43 –47], for example wheat straw [48], miscanthus cane [49], rye husk [50] and giant reed [47] contain 30%, 31%, 26% and 29.18–32.93% cellulose, respectively. The importance of the barkcloth material from Ficus natalensis tree is obvious mainly due to its regrowing ability after several harvests and admirable cellulose contents present in it.

Surface morphology by SEM analysis

Before SEM images analysis, it is important to assess the visual macroscopic evolution at each steps of CNF isolation. Figure 2 illustrates that the chemical treatments changed the appearance of the untreated barkcloth material (Figure 2(a) and (b)). Figure 2(c) shows the bleached material which is white brown in color and obviously different from the untreated barkcloth fibers powder due to the removal of non-cellulosic constituents such as lignin, hemicelluloses, wax and pectin [51]. The observed white color of the final product (Figure 2(c)) is an indication of almost pure cellulosic material as reported in Taflick et al. [52]. The yield of the extracted CNF from dried cellulose fibers was found to be 50%.

Figure 2.

(a) Photographs of untreated cut pieces of Ficus natalensis barkcloth material (b) alkali-treated samples (c) final bleached product.

Figure 3 shows the micrographs of raw material and it was observed that the barkcloth raw material (Figure 3(a)) is composed of individual fibers which are adhered naturally into bundles that give strength to each other. Figure 3(b) shows the conversion of barkcloth fibers into powder form (BC-PW) after passing through the sieve mess 60 and the BC-PW samples show an unaligned and random structure.

Figure 3.

(a) Barkcloth raw material, (b) barkcloth fibers powder (BC-PW).

As shown in Figure 4(a), the applied chemical treatments (alkali treatment and bleaching) on BC-PW, caused to align and isolate the fibers by eliminating the non-cellulosic fractions and impurities presented in the Ficus natalensis barkcloth fibers powder. Figure 4(b) shows freeze dried needle shaped, closely linked and soft foam like cellulose nano fibrils, when water was transfered out from the fibrous network by freeze drying method. The freeze-dried CNF consisted of thin film pieces inter-laced with sub-micron fibrils.

Figure 4.

(a) cellulose fibers (BC-NB), (b) freeze dried cellulose nano fibrils.

Transmission electron microscopy analysis

As TEMPO oxidation at controlled conditions expected to reduce the amorphous region of cellulose fibers and transversely increase the crystalline domains of the treated sample [53], finally a reduction is noticed in the size of cellulose fibers from the micron to nano meter scale. Figure 5(a) provides the TEM images of CNF fibrils which show that the samples fined down to the smallest elements next to the cellulose molecules after the TEMPO oxidation. TEM micrograph at 500 nm of individual cellulose exhibits high length/width nano fibrils ratios and the obtained structures were relatively isolated with defined shapes. To better understand the dimension of CNF samples, the TEM image was evaluated by using the image J software to get the diameter frequency distribution data. The diameter distribution through image J software of 100 samples of CNF extracted from Ficus natalensis barkcloth fibres was taken to better understand the dimensions and shape as shown in Figure 5(b), and the diameter was distributed mainly from 15 to 41 nm, average diameter of about 28 ± 0.6 nm and length in several hundred nano meters.

Figure 5.

(a) Transmission electron micrograph from diluted suspension of CNF extracted from Ficus natalensis barkcloth fibers (b) diameter distribution of cellulose nano fibrils.

Fourier-transformed infrared spectroscopy

Figure 6 shows the FTIR spectra recorded for barkcloth powder, alkali treated and bleached fibers cellulose nano fibers obtained after TEMPO oxidation. The main peaks are reported in Table 1. The spectrum of all the samples has broad absorption bands having peaks around 3306–3401 cm⁻¹, which is corresponded to –OH stretching vibrations of methyl groups [11] and peaks at 2900 cm⁻¹ are ascribed to the C–H stretching vibrations of methylene groups. It is worth noting that these two characteristics peaks are assigned to stretching of hydrogen bonds and bending of hydroxyl (OH) groups due to cellulose existence in all three samples, indicating that cellulose was not removed during alkalization, bleaching and TEMPO oxidation of all the samples. A shoulder was observed around 1165 cm⁻¹ in the spectrum corresponding to the BC-PW ascribed to C–C stretching vibration. Similarly, the broad absorption band at around 1028 cm⁻¹ is due to C–O–C pyranose ring stretching vibration. Meanwhile, the BC-PW shows absorption peaks at 1603 cm⁻¹ which corresponds to O–C = O asymmetric stretching, but in case of BC-CNF, the peaks moved to the point 1606 cm⁻¹ which is bit sharper than BC-PW, attributed to carboxylate groups caused by TEMPO oxidation process [12].

Figure 6.

FTIR spectra for BC-PW, BC-NB and BC-CNF.

Table 1.

FTIR spectra of all the samples at different stages of treatment.

Barkcloth fibers			Groups
BC-PW	BC-NB	BC-CNF
3305	3300	3306	–OH stretching
2902	2900	2900	C–H vibration
1603	1603	1606	O–C=O asymmetric
1165	1163	1165	C–C band
1028	1028	1028	Broad absorption band

Thermal analysis

The thermal stability of both untreated and chemically treated Ficus natalensis barkcloth fiber samples was done by thermogravimetric analysis (TGA). Figure 7 graphically represented the TGA and derivative thermogravimetry (DTG) curves obtained for BC-PW, BC-NB and TEMPO-oxidized barkcloth cellulose nano fibers BC-CNF which occurs over a wide range of temperature according to the degradation of all the samples constituents [54]. Regardless of their treatment, an initial weight loss of the samples occurs below 100°C. The initial weight reduction is attributed to the amount of water moisture content presented in each sample. Compared to BC-PW, the sample BC-NB has higher thermal stability because of the presence of hemicellulose, lignin, and pectin in untreated sample (BC-PW) [55]. That’s why BC-PW shows onset degradation temperature at about 251°C and final degradation temperature was at 363 °C as similar finding was reported by Tendo et al. [56]. But in the case of alkali and bleach-treated sample, BC-NB shows higher onset degradation temperature of 310.62 °C and degrading ends at 342°C, close to the literature which recommends that cellulose degradation takes place between 275 and 400°C [35,57,58]. Meanwhile, cellulose nano fibrils show degradation peaks lower than those of barkcloth powder (BC-PW) and cellulose fibers (BC-NB). Decrease of decomposition temperatures of CNF is due to the introduction of unstable sodium carboxylate groups during TEMPO oxidation [59]. Similarly, Figure 7 shows the DTG curves for every sample; BC-PW shows two degradation peaks which is due to decomposition of non-cellulosic materials and the other peak shoulder is possibly ascribed to the decomposition of cellulosic compounds. Meanwhile, BC-NB shows one sharp DTG curve which gave significance evidence of the removal of non-cellulosic impurities after the alkali treatment and bleaching of barkcloth fibers. Conversely, the sample of BC-CNF shows reduction in the early onset degradation temperature with respect to other samples, as TEMPO oxidation caused the destruction and conversion of macro cellulose into micro cellulose chains and converted into low molecular weight chains so the activation energy decreases making the sample less thermal resistant.

Figure 7.

TGA and DTG curves of barkcloth fiber powder (BC-PW), alkali treated, bleached fibers (BC-NB) TEMPO oxidized cellulose nano fibrils (BC-CNF).

X-ray diffraction

XRD evaluation was completed to investigate the crystalline structures of the fibers and relationship between fiber structures and properties. The X-ray diffractograms of barcloth fibers powder BC-PW, alkali and bleached treated fibers BC-NB and cellulose nano fibrils BC-CNF are shown in Figure 8. The XRD diffractrograms indicated that the diffraction peaks at around 16.02°, 22.25° and 34.68° have been observed for all the samples, and similar results were also observed in Haafiz et al. [14]. Figure 8 has illustrated that the diffraction peaks at around 16.02° (broad) corresponding to 110, 22.25° (sharp intense) corresponding to 002 and 34.68° (small) corresponding to 004 [23] have been observed for the crystallographic planes of the cellulose I. But the diffraction peak at 22° of cellulose nano fibrils (BC-CNF) is sharper than the other samples due to higher crystallinity value.

Figure 8.

X-ray diffraction patterns of BC-PW, BC-NB and BC-CNF.

Chemical purification and treatments affect the crystallinity of natural fibers like alkali, and bleaching treatment can increase the stiffness of plant fibers by removing the impurities present in the fibers. The amorphous structure maintained by intermolecular and intramolecular hydrogen bonding can evaluate the effectiveness of chemical treatments on the extraction of nano cellulose fibrils from Ficus natalensis barkcloth fibers, and the crystallinity of BC-PW, chemically treated samples BC-NB and BC-CNF was determined and compared as summarized in Table 2. Increased crystallinity of BC-NB (62.3%) as compared to BC-PW (36.8%) is attributed to the progressive reduction of amorphous region (maintained by intermolecular and intramolecular hydrogen bonding [33]) and non-cellulosic materials inclined to remove. Similarly from Table 2, the increased CrI% of cellulose nano fibrils BC-CNF (68.5%) isolated by the controlled TEMPO-oxidation is associated with the removal of remaining hemicellulose and amorphous regions in cellulose samples [52].

Table 2.

Crystallinity index (CrI) of barkcloth fibres at different stages of treatment.

Samples	Cr_I (%)
BC-PW	36.8
BC-NB	62.3
BC-CNF	68.5

Conclusion

To our knowledge, the Ficus natalensis barkcloth fibers are the first reported material for the simultaneous cellulose extraction and derivatization of nano cellulose fibrils directly from the TEMPO oxidation which can be an efficient material for nano technology. Chemical purification for cellulose extraction by means of alkali treatment and bleaching was done, obtained cellulose fibers were TEMPO oxidized to get the cellulose nano fibrils (CNF). The infrared spectroscopy, morphological investigation, and X-ray diffraction analyses confirmed the removal of non-cellulosic materials. An increase of crystallinity index induced by the chemical treatments from 36.8% to 68.5% was examined, while the diameter reduced to 20–25 nm having length in hundred nano meters. The overall yield of cellulose fibers based on the raw material was comparatively good as 34.5 ± 1%. So, the cellulose and nano cellulose fibrils from the barkcloth materials could act as an excellent filler in the biocomposites.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The work was supported by “National Key R&D Program of China (2018YFC2000900)” and “Fundamental Research Funds for the Central Universities” (2232018A3-04)”.

ORCID iDs

Amjad Farooq

Muhammad A Naeem

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