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Study on structural and thermal properties of cellulose microfibers isolated from pineapple leaves using steam explosion
Accepted Manuscript Title: Study on structural and thermal properties of cellulose microfibers isolated from pineapple leaves using steam explosion Authors: Supachok Tanpichai, Suteera Witayakran, Anyaporn Boonmahitthisud PII: DOI: Reference:
S2213-3437(18)30759-0 https://doi.org/10.1016/j.jece.2018.102836 JECE 102836
To appear in: Received date: Revised date: Accepted date:
28 August 2018 28 November 2018 14 December 2018
Please cite this article as: Tanpichai S, Witayakran S, Boonmahitthisud A, Study on structural and thermal properties of cellulose microfibers isolated from pineapple leaves using steam explosion, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.102836 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Study on structural and thermal properties of cellulose microfibers isolated from pineapple leaves using steam explosion
Learning Institute, King Mongkut’s University of Technology Thonburi, Bangkok, 10140,
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Supachok Tanpichai1, 2, 3*, Suteera Witayakran4 and Anyaporn Boonmahitthisud5, 6
Thailand.
NanotecKMUTT Center of Excellence on Hybrid Nanomaterials for Alternative Energy,
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Cellulose and Bio-based Nanomaterials Research Group, King Mongkut’s University of
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King Mongkut’s University of Technology Thonburi, Bangkok, 10140, Thailand.
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Technology Thonburi, Bangkok, 10140, Thailand.
Kasetsart Agricultural and Agro-Industrial Product Improvement Institute, Kasetsart
Department of Materials Science, Faculty of Science, Chulalongkorn University, Bangkok,
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10330, Thailand.
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University, Bangkok, 10900, Thailand.
Green Materials for Industrial Application Research Unit, Faculty of Science,
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Chulalongkorn University, Bangkok, 10330, Thailand
Corresponding author: Supachok Tanpichai Email: [email protected] Tel: +662-
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470-8385 Fax: +662-872-9082
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Abstract In Thailand, pineapple leaves are one of the most common agricultural wastes after cultivation. The aim of this research was to understand effects of the steam explosion treatment conditions (steam pressure and treatment cycle) on properties of cellulose microfibers extracted from pineapple leaves. The fibers were steam exploded at pressure of
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16 and 20 kgf cm-2 between 1 and 5 cycles. The steam explosion technique could partially eliminate hemicellulose and lignin and increase the cellulose content, leading to improvement
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of the thermal properties and crystallinity of the treated fibers. With increasing the steam pressure and treatment cycles, changes of the fiber morphology were observed. Smaller
widths and shorter lengths of the treated fibers could be obtained. Microfibers with widths of
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3 µm and lengths of 93 µm were extracted from fiber bundles with widths of 45.8 µm and
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lengths of ~2 cm with the pressure of 20 kgf cm-2 for 5 cycles. Also, the steam explosion
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method could possibly fibrillate cellulose nanofibers. With more cycles of the steam
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explosion treatment, large amounts of cellulose nanofibers could be found.
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1. Introduction
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Keywords: Pineapple leaf; steam explosion; thermal properties; fibrillation; nanofibers
The lignocellulosic materials from agricultural crops and residues have been
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increasingly attractive [1, 2]. Pineapple canned industry in Thailand is one of the World’s largest producers and exporters [3, 4]. In 2014 the annual pineapple production was 1.79
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million tons [5]. Normally, pineapple plants (Ananas comosus) take around a year to bloom, and another 6 – 8 months for fruits to be ready [6]. After cultivation, 30 – 50 pineapple leaves with lengths up to 180 cm with overall weight of 1 – 1.5 kg are left per plant as a waste from the pineapple cultivation [6]. The cellulose content in these pineapple leaf fibers is between 70 and 82 % while a content of lignin is as low as 5 – 12 % [7]. Pineapple leaf fibers with a 2
ribbon-like structure present high specific strength and stiffness because of a high cellulose content and low microfibrillar angle (14°) [4, 7, 8]. Due to these reasons, attempts have been made to extract pineapple leaf fibers from the pineapple leaves, and use in many applications such as composites, papers, textiles and cosmetics [7-9]. Currently, the use of the pineapple
reasonable price, abundance and superior mechanical properties [8].
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leaf fibers to replace glass fibers in building materials has been studied because of the
Generally, cellulose fibers are extracted from any lignocellulosic biomass such as
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wood or crops by the chemical treatment process [10]. Within this process, a series of the chemical treatments such as acidic or alkaline condition is introduced to lignocellulosic
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materials [11, 12]. For example, a time-consuming procedure of the chemical treatments
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(acidified sodium chlorite solution at 80 °C used for 1 h for 5 times and 4 wt% sodium
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hydroxide solution at 80 °C applied for 2 h for 2 times) was introduced to kenaf bast fibers, resulting in the accessibility of the cellulose due to the hydrolysis of hemicellulose to xylose
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and degradation of lignin [11-13]. The similar procedure has also been introduced to water
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hyacinth [14], wood [15], rice straw [15] and potato tuber [15]. However, this chemical
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treatment approach is expensive because of the requirement of the high processing costs of the chemical solvents, disposal of the used solvents, high energy consumption and long
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processing time [11, 12, 16].
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The steam explosion treatment has been reported to be a promising alternative process to extract cellulose fibers with a rapid process [16-19]. Fibers are saturated with high-
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pressure steam at high temperature for a specific period of time, and the pressure is immediately released. The steam explosion method could be acted as the thermomechanochemical process, consisting of three combined effects: heat from steam, shear force generated from moisture expansion and acetic acid form the hydrolysis of acetyl groups in hemicellulose obtained from cellulose fibers [10]. This leads to the rupture of the
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lignocellulosic structure, conversion of hemicellulose to water soluble oligomers and individual sugars and damage of the bonding between hemicellulose and lignin [17, 20-23]. The remaining part as a solid residue left is cellulose [24, 25]. It has been reported that hemicellulose in wood could be almost completely converted to oligosaccharides soluble in water after the introduction of the steam explosion treatment at 20 kgf cm-2 for 1 min [25].
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The steam explosion technique is also used to reduce non-cellulosic compounds which bind fibrils altogether [26]. Moreover, partial dissolution of lignin and hemicellulose using the
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steam explosion treatment has been found to be effectively similar to that treated with the alkaline treatment [27]. It is worth noted that the steam explosion treatment requires less
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hazardous chemical used, energy consumption, human toxicity and environmental impact in
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comparison with the alkaline treatment in order to gain higher fiber yielding [23].
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Additionally, this steam explosion treatment is possible to treat a large capacity of biomass or
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cellulose materials [26]. The steam explosion approach has been used to disintegrate individual fibers from cellulose materials such as rice straw [27], oak shell [28], kenaf [29],
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eucalyptus [30] and pineapple leaf fibers [4, 9, 31]. Effect of the steam explosion conditions
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has been previously studied to investigate the efficiency of this approach. For example, Han et al [32] studied effect of the steam temperature and retention time on properties of rice
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straw. When rice straw was steam exploded at higher steam temperature for the longer
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treatment time, more homogenous fiber-like materials were obtained. The surface wettability and high degree of crystallization were also improved after the steam explosion treatment
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[26]. Next, Boonterm et al [27] prepared cellulose fibers extracted from rice straw using the steam explosion method. With increasing the steam pressure, the significant reduction in fiber widths was found, resulting in a low aspect ratio of the treated fibers. Low fiber yielding was also observed with the high pressure due to fiber damages.
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Although researchers have recently studied effect of the steam explosion conditions and have used the steam explosion technique to disintegrate cellulose fibers from various cellulose sources for applications such as composites, pulp and paper and bioethanol production [4, 9, 29, 31, 33], scarce study of fiber properties after the introduction of the steam explosion treatment for several cycles, to authors’ knowledge, has been previously
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reported. Here, the aim of this research was to study effect of the steam pressure as a function of a number of the treatment cycles on properties of the treated pineapple leaf fibers. The
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structural and thermal properties of the steam exploded pineapple leaf fibers were studied using Fourier transform infrared (FTIR), Thermogravimetric analysis (TGA), X-ray
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diffraction (XRD) and scanning electron microscopy (SEM) techniques. Chemical
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components of the fibers after the introduction of the steam explosion method as a function of
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the treatment cycles were also analyzed.
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2. Materials and methods 2.1. Materials
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Pineapple leaf fibers, extracted by a decorticated machine, were supplied by Kasetsart
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Agricultural and Agro-industrial Product Improvement Institute. 2.2 Steam explosion treatment
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Pineapple leaf fibers were treated under the steam explosion conditions presented in
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Table 1. Briefly, dried pineapple leaf fibers with the weight of 150 g were transferred into a 2L batch steam explosion chamber, and saturated steam was admitted into the chamber. After
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the fibers were steam exploded at the specific pressure for 5 min (the retention time was counted when the steam reached the setting pressure), the steam pressure was immediately released. The fibers were fibrillated due to explosive decompression caused by the immediate pressure reduction. This process was repeated for 1, 3 and 5 times to study effect of the steam
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explosion treatment cycles on the isolation of the pineapple leaf fibers. The treated fibers were washed with distilled water and dried.
2.3 Chemical composition analysis The raw and steam-exploded fibers were analyzed to investigate the chemical
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compositions (cellulose, hemicellulose, lignin and ash). The chemical analysis of the fiber
samples was performed by the Technical Association of the Pulp and Paper Industry (TAPPI)
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test method [34]. Before chemical analysis, the samples were grinded to pass a 0.4-mm (40mesh) screen to permit the complete reaction of the samples with the reagents used in the
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analysis. About 50 g of the grinded specimen was then extracted successively with ethanol-
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benzene, ethanol and hot water, according to TAPPI T264 in order to remove the extractives
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before any chemical analysis and to determine the extractive fraction in the sample. After
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drying, 5 g of the extractive-free sample was used to determine the Klason lignin content (acid-insoluble lignin), according to TAPPI T222. Furthermore, approximately 5 g of the
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dried extractive-free specimen was used to determine the holocellulose content according to
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the acid chlorite method of Browing [35]. Then, 5 g of the holocellulose fraction was used to determine the α-cellulose content according to TAPPI T203. Based on these results, the
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hemicellulose content was determined by subtracting the α-cellulose content from the
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holocellulose content value. 2.4 Fourier transform infrared spectrophotometry
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Fourier transform infrared (FTIR) measurements were performed using a
spectrometer (Nicolet IR200, Thermo Fisher Scientific Co., Ltd., USA) with an attenuated total reflectance (ATR) mode. The original and treated pineapple leaf fibers were analyzed at wavenumbers of 400 – 4000 cm-1 with 4 cm-1 resolution and 64 repetitious scans. Prior to the sample investigation, the background was recorded.
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2.5 Thermogravimetric analysis The untreated and steam exploded pineapple leaf fibers were investigated by a Thermogravimetric analyzer (TGA/SDTA 851e, Mettler-Toledo LLC, USA) to study effect of the steam explosion conditions on thermal properties of the fibers. A dried sample with a weight of ~5 mg was placed on a platinum pan, and maintained at temperature of 110 °C for
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10 min to completely remove moisture. Later, the sample was heated to 600 °C with a heating rate of 10 °C min-1 under a nitrogen flow rate of 100 ml min-1.
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2.6 X-ray diffraction
The structures of the pineapple leaf fibers treated with the steam explosion method were studied using an X-ray diffractometer (D8DISCOVER, Bruker AXS, Germany)
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equipped with a Goebel mirror. The X-ray diffraction (XRD) profiles were recorded using the
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CuKα radiation with a wavelength of 0.1540 nm operating at the accelerating voltage of 40 kV and current of 40 mA. Samples were scanned with a step size of 0.02° and step speed of
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0.8 s over the 2θ range of 5 – 50°. The degree of crystallinity obtained from each sample was
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determined from the intensity of the crystalline peak located at 2θ of ~22° and minimum
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intensity of the peaks located between 16.5 and 22.6°, corresponding to the amorphous area, according to Segal’s method [36].
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2.7 Field emission scanning electron microscopy
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The surface morphologies of the pineapple leaf fibers after the steam explosion treatment were investigated using a field-emission scanning electron microscope (FEI Nova
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NanoSEM 450, FEI Co., Ltd., USA) with an acceleration voltage of 5 kV and a working distance of 10 mm. Before each measurement, a sample was coated with a thin layer of gold to avoid charging. 3. Results and discussion 3.1 Chemical composition analysis
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Effects of a number of the steam explosion treatment cycles and steam pressure on the chemical compositions of the pineapple leaf fibers are presented in Table 2. The results showed that chemical compositions of the fibers were changed after the introduction of the steam explosion treatment. In the raw pineapple leaf fibers, α-cellulose, hemicellulose, lignin and ash were 64.48, 20.89, 4.24 and 0.77 %, respectively. As expected, the content of
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hemicellulose and lignin were reduced significantly by passing the fibers through the steam
explosion treatment for only 1 time. The lignin content of the raw pineapple leaf fibers (4.24
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%) was dropped to 0.55 and 0.43 % for the 16/1 and 20/1 sample, respectively, and the
content of hemicellulose measured from the 16/1 and 20/1 sample were 12.6 and 9.5 % in comparison with the hemicellulose percentage of 20.89 obtained from the untreated fibers.
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On the other hand, the α-cellulose content increased from 64.48 to 76.6 and 81.3 %,
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respectively, when the steam pressure of 16 and 20 kgf cm-2 were applied. The similar
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increase of the cellulose content and reduction of hemicellulose and lignin content were reported for steam treated rice straw fibers [37]. Moreover, Jiang et al [29] reported the
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reduction of the hemicellulose, pectin and water-soluble matter with increasing the steam
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pressure. The decrease of the hemicellulose content has been previously observed with the severe steam explosion condition (high pressure and longer time) because of the
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depolymerization of the hemicellulose by hydrolysis of glycosidic linkages and acetyl groups
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to soluble saccharides [10]. Meanwhile, the degradation of lignin is caused by the cleavage of β-O-4ʹ linkages in the lignin structure [38]. More solubilization of hemicellulose and lignin
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has been found with the high pressure steam treatment due to the close contact between these components and steam [37]. The reduction of the hemicellulose and lignin content and the improvement of the cellulose percentage indicates that the steam explosion could possibly eliminate lignin and hemicellulose without any chemical solvents applied in the process, which can reduce expense of the fiber preparation [23, 39].
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When a higher number of the steam explosion cycles were applied, the slight decrease of the α-cellulose content was found for both steam pressure conditions (16 and 20 kgf cm-2). With 5 treatment cycles, the α-cellulose content of the 16/5 sample decreased to 73.62 % and that of the 20/5 material reduced to 73.02 %. This might be because of the depolymerization or degradation of cellulose caused by the severe condition. The degradation of the cellulosic
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structure has been reported for the steam explosion treatment at high pressure for a longer
time [39]. The cellulose degradation resulted in a higher portion of the hemicellulose with
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respect to a number of the treatment cycles, as presented in Table 2. Furthermore, slight improvement of the lignin content could be observed especially with the higher steam
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pressure treatment. The lignin content was found to be higher from 0.43 % for the 20/1 to
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0.81% for the 20/5 sample. It has been reported that with a longer treatment time at the high
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steam pressure, the repolymerization of lignin could occur by the condensation reaction of C-
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2 or C-6 in quaiacyl and syringyl rings [40]. Li et al [38] also stated that with the longer retention time, molecular weight (Mw) of the lignin shifted to a higher, and the
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repolymerization of lignin was more prominent.
3.2 Chemical properties
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The appearance of the cellulose, hemicellulose and lignin was investigated from the
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specific FT-IR peaks, as presented in Fig. 1. The adsorption peak located at 1050 cm-1, corresponding to a C-O-C glucopyranose unit in cellulose molecules [13, 27], was found
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from both original and steam exploded pineapple leaf fibers. This indicated the steam explosion treatment could not change the cellulose structure although the steam pressure of 20 kgf cm-2 was applied for 5 continuous treatment cycles. This result could be well supported by the XRD measurements, as discussed later. The appearance of the carbonyl groups of the hemicellulose (1640 cm-1) [41, 42] and phenolic compounds (1240 cm-1) [4, 41-
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43] was observed from the spectra of all untreated and steam exploded fiber samples. This could confirm the remaining contents of the lignin and hemicellulose in the fibers after the steam explosion treatment. This result was well agreed with results of the chemical composition measurements. The 3340 and 2920 cm-1 adsorption bands related to hydroxyl groups and C-H stretching of cellulose [11, 27] were also found from all fiber samples.
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3.3 Thermal stabilities
Thermal gravimetric (TG) and derivative Thermal gravimetric (DTG) curves of the
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steam exploded fibers are presented in Fig. 2, and the onset degradation temperature and the
maximum DTG peak temperature of the untreated and steam exploded fibers are summarized
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in Table 3. Generally, four transitional states (moisture evaporation and decomposition of
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hemicellulose, cellulose and lignin) are detected from cellulosic materials. In this study, no
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weight loss change at the temperature of approximately 100 °C, corresponding to moisture
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evaporation, could be observed from all cellulose samples due to the sample stabilization at 110 °C for 10 min prior to the measurements. Hemicellulose starts to degrade in the
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temperature range of 220 – 300 °C, and followed by the cellulose degradation at 275 – 400
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°C [42, 44-46]. Lastly, due to its complex structure, lignin decomposes in the wider region from 450 to 700 °C [42].
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The shift of the decomposition temperature to higher temperature could be detected
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from the steam exploded pineapple leaf fibers, indicating the higher thermal stabilities of the fibers after the steam explosion treatment. Similar TG curves were observed from the treated
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fibers with the different steam pressures and steam explosive cycles. The onset degradation temperature of the untreated pineapple leaf fibers was 330.5 °C while the onset degradation temperatures of the all steam exploded samples with the different steam pressures and treatment times were found to be around 337 °C. The improvement of the degradation temperature could be because of less quantities of hemicellulose located within and between
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cellulose fibrils [45]. The increase of the sharp DTG peaks, corresponding to a prominent cellulose decomposition, was also monitored for the steam exploded fibers. The DTG peak of the untreated fibers was found at temperature of 366.2 °C, and peak was shifted to a higher temperature range of around 370 °C with the use of the steam explosion method. The steam exploded fibers presented higher intensity of the DTG peak than the untreated did. The
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intensity of the DTG peaks could relatively imply the higher cellulose content in the steam
exploded fibers in comparison with the original pineapple leaf fibers, which was well agreed
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with the chemical composition results. Moreover, the higher percentage of lignin of the
original pineapple leaf fibers led to a higher char yield at the high temperature. A carbon
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residue of 11% was obtained from the original fibers at 600 °C while the steam exploded
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fibers had a char content of around 5 %. Pelissari et al [42] purposed that the low amount of
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carbon residues could be due to the removal of lignin and hemicellulose and the accessibility
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of cellulose in fibers. The lowest char yield has been found from the acid treated cellulose fibers in comparison with the raw fibers due to the disappearance of the noncellulosic
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components in the fibers after the acid treatment [45]. The higher thermal stability results of
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all steam exploded pineapple leaf fibers could confirm that the use of the steam explosion treatment could possibly remove hemicellulose and lignin from the fibers without the
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presence of the chemical treatment.
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3.4 Crystallinity Effects of a number of the steam explosion treatment cycles on crystallinity of the
treated fibers are presented in Fig. 3. All treated pineapple leaf fiber samples exhibited the outstanding peaks located at 14.8, 16.5, and 22.6°, corresponding to (1 0 1), (1 0 1̅) and (0 0 2) lattice planes of the typical cellulose I structure [47] while the peaks located at 2θ of
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12, 20 and 22° would be determined from the cellulose II structure [48, 49]. It is worthy noted that the steam explosion could not be able to change the crystal structure of the pineapple leaf fibers even steam treated for 5 continuous cycles. The change of crystallinity of the treated fibers, however, could be caused by the steam explosion treatment, as shown in Fig. 3(b). As always, the lowest crystallinity index was obtained from the untreated fibers.
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After the fibers were steam exploded for a single time with the steam pressure of 16 kgf cm-2, the crystallinity degree was improved from 82.2 to 86.7 %. A percentage of 87.8 for the
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crystallinity degree was obtained from the fibers steam-exploded with the pressure of 16 kgf cm-2 for 5 treatment cycles. This might be because the removal of the amorphous portion of
the treated fibers. No significant difference of the crystallinity degree could be seen from the
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treated fibers with the 16 and 20 kgf cm-2 steam pressure at the same treatment cycle. The
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similar improvement of the crystallinity of the mechanically treated banana fibers has been
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reported by Pelissari et al [42]. With increasing a number of the suspension of the treated cellulose fibers passing through a high pressure homogenizer, the crystallinity index of the
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treated nanofibers was found to be higher. This was in good agreement with the study of the
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ultrasonication output power level effect. The crystalline index value of the cellulose samples increased when the higher output power was applied due to the degradation of the amorphous
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cellulose [44]. However, the longer steam explosion treatment time have been reported to
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decrease the crystallinity of the treated fibers due to the cellulose degradation [37].
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3.5 Morphology
Changes of morphologies of the steam exploded fibers in comparison with the
original fibers are presented in Fig. 4, and Fig. 5 compares the fiber widths of the steam exploded fibers treated with the steam pressure of 16 and 20 kgf cm-2 as a function of the treatment cycles. In general, microfibers are bonded together with lignin and hemicellulose to
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from bundles. With the introduction of the steam explosion process for one cycle, microfibers with diameters of 4.7 ± 1.8 and 3.8 ± 1.5 µm obtained from the 16/1 and 20/1 fibers were extracted from the original fiber bundles with diameters of 45.8 ± 11.4 µm. The intensive reduction of the fiber diameter could confirm the efficiency of the steam explosion process [4]. With increasing a number of the steam explosion process, the average diameter of the
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fibers decreased slightly. The average fiber diameter of the 16/3 and 20/3 were 3.9 ± 1.5 and
3.2 ± 1.2 µm, respectively, and values of 3.1 ± 1.2 and 3.0 ± 1.3 µm were measured from the
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fibers steam exploded with the steam pressure of 16 and 20 kgf cm-2 for 5 continuous cycles.
When the fibers were steam exploded for 3 cycles with both pressures, considerable reduction
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of the fiber diameters was observed. However, with the continuous steam explosion for more than 3 times, no significant change of the fiber diameters was found with the higher steam
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pressure (20 kgf cm-2) while the reduction of the fiber diameters could be observed from the
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16 kgf cm-2 treatment. This indicated the use of the higher steam pressure could efficiently fibrillate microfibers from bundles with a few treatment cycles. However, the fibrillation of
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microfibers using this machine could reach the limitation after 3 cycles with the higher steam
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pressure.
Notably, the treatment of the steam explosion for many cycles not only affected fiber
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defibrillation, but also decreased fiber length. With increasing the treatment cycles, the length
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of fibers decreased tremendously when the steam pressure of 20 kgf cm-2 was applied to the fibers. With increasing the treatment cycle to 3 or 5 times, the fiber length of the 20/3 and
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20/5 material were significantly reduced to 142.5 ± 82.3 and 93.5 ± 52.5 µm, respectively, in comparison with the original fibers with lengths of up to 2 cm. The similar occurrence of the fiber length shortening has been also found by Boonterm et al [27]. The reduction of fiber diameters and lengths was observed when the steam pressure increased from 13 to 17 bar. With the pressure of 13 bar, fibers with diameters of 0.26 mm and lengths of 35.5 mm were
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obtained. When the steam pressure was increased to 17 bar, the diameters and lengths of the steam exploded fibers from 3-cm long rice straw fibers were significantly reduced to 0.11 and 8.1 mm, respectively. Interestingly, larger amounts of cellulose nanofiber fibrillated from microfibers could be seen when the fibers were shorter (Fig. 4). It could be said that using the steam explosion treatment can fibrillate nanofibers.
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4. Conclusions
Cellulose fibers were successfully extracted from pineapple leaf using the steam
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explosion process. Results showed that the steam explosion treatment could reduce partial contents of the hemicellulose and lignin and increase of the cellulose portion without any
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introduction of the chemical treatment, confirmed by chemical composition and FTIR
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measurements in comparison with the untreated pineapple leaf fibers. The decreased contents
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of the hemicellulose and lignin improved thermal properties of the steam exploded fibers.
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Also, the increase of the crystallinity index was found with increasing a number of the steam explosion treatment cycles. The defibrillation of individual microfibers from fiber bundles
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could be effectively investigated. The fibers with diameters of 3.8 µm were extracted from
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the original fiber bundles with diameters of 45.8 µm with the use of the steam explosion treatment with the pressure of 20 kgf cm-2 for 1 cycle. With the treatment of the pressure of
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20 kgf cm-2 for 5 cycles, the slight reduction of the fiber diameter to 3.0 µm was observed.
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When the fibers were treated with the severe steam explosion condition for a higher number of the treatment cycles, fibers were shortened. The average fiber length of 93.5 µm was
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investigated from the fibers steam exploded with the 20 kgf cm-2 pressure for 5 times in comparison with the original fibers with lengths up to 2 cm. Moreover, the use of the steam explosion treatment could fibrillate nanofibers from microfibers. The as-prepared steam exploded fibers could be beneficial for composite, packaging and food applications.
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Acknowledgements One of the authors (S.T.) would like to acknowledge the financial support of the Asahi Glass Foundation and King Mongkut’s University of Technology Thonburi for a research grant of 2017 and the Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program the Center of Excellence Network.
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Furthermore, S.T. was grateful to his wife and his little boy for endlessly valuable supports.
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[15] Abe K, Yano H. Comparison of the characteristics of cellulose microfibril aggregates of wood, rice straw and potato tuber. Cellulose. 2009;16(6):1017-23. [16] Phinichka N, Kaenthong S. Regenerated cellulose from high alpha cellulose pulp of steam-exploded sugarcane bagasse. J Mater Res Technol. 2018;7(1):55-65. [17] Karakoti A, Biswas S, Aseer JR, Sindhu N, Sanjay MR. Characterization of microfiber isolated from Hibiscus sabdariffa var. altissima fiber by steam explosion. J Nat Fibers. 2018:1-10. [18] Solikhin A, Hadi YS, Massijaya MY, Nikmatin S. Production of microfibrillated cellulose by novel continuous steam explosion assisted chemo-mechanical methods and its characterizations. Waste Biomass Valori. 2017:1-12. [19] Ge S, Chen X, Li D, Liu Z, Ouyang H, Peng W, et al. Hemicellulose structural changes during steam pretreatment and biogradation of Lentinus edodes. Arab J Chem. 2018;11(6):771-81. [20] Fernández-Bolaños J, Felizón B, Heredia A, Rodrı́guez R, Guillén R, Jiménez A. Steamexplosion of olive stones: hemicellulose solubilization and enhancement of enzymatic hydrolysis of cellulose. Bioresour Technol. 2001;79(1):53-61. [21] Cara C, Ruiz E, Ballesteros I, Negro MJ, Castro E. Enhanced enzymatic hydrolysis of olive tree wood by steam explosion and alkaline peroxide delignification. Process Biochem. 2006;41(2):423-9. [22] Deepa B, Abraham E, Cherian BM, Bismarck A, Blaker JJ, Pothan LA, et al. Structure, morphology and thermal characteristics of banana nano fibers obtained by steam explosion. Bioresour Technol. 2011;102(2):1988-97. [23] Sabiha-Hanim S, Azemi Mohd Noor M, Rosma A. Fractionation of oil palm frond hemicelluloses by water or alkaline impregnation and steam explosion. Carbohydr Polym. 2015;115:533-9. [24] Li J, Henriksson G, Gellerstedt G. Lignin depolymerization/repolymerization and its critical role for delignification of aspen wood by steam explosion. Bioresource Technology. 2007;98(16):3061-8. [25] M. T. Characterization and degradation Mechanisms of wood components by steam explosion and utilization of exploded wood. Wood Res. 1990;77:49-117. [26] Lorenzo-Hernando A, Martín-Juárez J, Bolado-Rodríguez S. Study of steam explosion pretreatment and preservation methods of commercial cellulose. Carbohydr Polym. 2018;191:234-41. [27] Boonterm M, Sunyadeth S, Dedpakdee S, Athichalinthorn P, Patcharaphun S, Mungkung R, et al. Characterization and comparison of cellulose fiber extraction from rice straw by chemical treatment and thermal steam explosion. J Clean Prod. 2016;134:592-9. [28] Yang J, Jiang J, Zhang N, Wei M, Zhao J. Effects of different pretreatment methods on the enzymatic hydrolysis of oak shell. Int J Green Energy. 2017;14(1):33-8. [29] Jiang W, Han G, Zhou C, Gao S, Zhang Y, Li M, et al. The Degradation of lignin, cellulose, and hemicellulose in kenaf bast under different pressures using steam explosion treatment. J Wood Chem Technol. 2017;37(5):359-68. [30] Feng YH, Zhong HT, Liang Y, Lei B, Chen H, Yin XC, et al. Structure and compositional changes of eucalyptus fiber after various cycles of continuous screw extrusion steam explosion. BioResources. 2018;13(2):2204-17. [31] Tanpichai S, Witayakran S. Mechanical properties of all-cellulose composites made from pineapple leaf microfibers. Key Eng Mat. 2015;659:453-7. [32] Han G, Deng J, Zhang S, Bicho P, Wu Q. Effect of steam explosion treatment on characteristics of wheat straw. Ind Crops Prod. 2010;31(1):28-33. [33] Niemi P, Pihlajaniemi V, Rinne M, Siika-aho M. Production of sugars from grass silage after steam explosion or soaking in aqueous ammonia. Ind Crops Prod. 2017;98:93-9.
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[34] Anonymous. TAPPI test method: The technical association of the pulp and paper industry. Atlanta, Georgia: TAPPI Press; 2002-2003. [35] Browing BL. Methods of Wood Chemistry. New York: Interscience Publisher; 1967. [36] Segal L, Creely JJ, Martin Jr. AE, Conrad CM. An empirical method for estimating the degree of crystallinity of native cellulose using the X-ray diffractometer Text Res J. 1959;29(10):786-94. [37] Chen X, Yu J, Zhang Z, Lu C. Study on structure and thermal stability properties of cellulose fibers from rice straw. Carbohydr Polym. 2011;85(1):245-50. [38] Li J, Henriksson G, Gellerstedt G. Lignin depolymerization/repolymerization and its critical role for delignification of aspen wood by steam explosion. Bioresour Technol. 2007;98(16):3061-8. [39] Maniet G, Schmetz Q, Jacquet N, Temmerman M, Gofflot S, Richel A. Effect of steam explosion treatment on chemical composition and characteristic of organosolv fescue lignin. Ind Crops Prod. 2017;99:79-85. [40] Li J, Gellerstedt G, Toven K. Steam explosion lignins; their extraction, structure and potential as feedstock for biodiesel and chemicals. Bioresour Technol. 2009;100(9):2556-61. [41] Wang B, Li D. Strong and optically transparent biocomposites reinforced with cellulose nanofibers isolated from peanut shell. Compos Pt A-Appl Sci Manuf. 2015;79:1-7. [42] Pelissari FM, Sobral PJdA, Menegalli FC. Isolation and characterization of cellulose nanofibers from banana peels. Cellulose. 2014;21(1):417-32. [43] Tibolla H, Pelissari FM, Martins JT, Vicente AA, Menegalli FC. Cellulose nanofibers produced from banana peel by chemical and mechanical treatments: Characterization and cytotoxicity assessment. Food Hydrocoll. 2018;75:192-201. [44] Khawas P, Deka SC. Isolation and characterization of cellulose nanofibers from culinary banana peel using high-intensity ultrasonication combined with chemical treatment. Carbohydr Polym. 2016;137:608-16. [45] Chirayil CJ, Joy J, Mathew L, Mozetic M, Koetz J, Thomas S. Isolation and characterization of cellulose nanofibrils from Helicteres isora plant. Ind Crops Prod. 2014;59:27-34. [46] Rout T, Pradhan D, Singh RK, Kumari N. Exhaustive study of products obtained from coconut shell pyrolysis. J Environ Chem Eng. 2016;4(3):3696-705. [47] Qing Y, Sabo R, Zhu JY, Agarwal U, Cai Z, Wu Y. A comparative study of cellulose nanofibrils disintegrated via multiple processing approaches. Carbohydr Polym. 2013;97(1):226-34. [48] Tanpichai S, Sampson WW, Eichhorn SJ. Stress-transfer in microfibrillated cellulose reinforced poly(lactic acid) composites using Raman spectroscopy. Compos Pt A-Appl Sci Manuf. 2012;43(7):1145-52. [49] Puttaswamy M, Srinikethan G, Shetty KV. Biocomposite composed of PVA reinforced with cellulose microfibers isolated from biofuel industrial dissipate: Jatropha Curcus L.seed shell. J Environ Chem Eng. 2017;5(2):1990-7.
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Fig. 1 FT-IR spectra of the steam-exploded and original pineapple leaf fibers.
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Fig. 2 (a) TG and (b) first derivative TG (DTG) curves of the pineapple leaf fibers
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with a number of the steam explosion treatment times.
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respect to a number of the steam explosion treatment cycles.
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Fig. 3 (a) XRD patterns and (b) degree of crystallinity of the pineapple leaf fibers with
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Fig. 4 Morphologies of the steam exploded fibers in comparison with the original
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fibers. Arrows present the isolation of the cellulose nanofibers.
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Fig. 5 Fiber widths of the steam treated fibers with the steam pressure of 16 and 20
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kgf cm-2 with respect to the treatment cycles.
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Table 1. Summary of the steam explosion conditions applied for the pineapple leaf fibers Steam
Steam explosion
Treatment time
(kgf cm-2)
temperature (°C)
(cycles)
(mins)
0
-
-
-
-
16/1
16
204
1
5
16/3
16
204
3
5
16/5
16
204
5
20/1
20
215
1
20/3
20
215
3
20/5
20
215
5
5 5 5 5
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Steam pressure
Materials
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Table 2. Chemical compositions of the pineapple leaf fibers after the steam explosion treatment. α-cellulose
Hemicellulose Lignin
Ash
(%)
(%)
(%)
(%)
(%)
0
85.66 ± 0.22
64.48 ± 0.49 20.89 ± 0.72
4.24 ± 0.25
0.77 ± 0.03
16/1
89.13 ± 0.03
76.57 ± 0.51 12.56 ± 0.49
0.55 ± 0.12
0.59 ± 0.01
16/3
89.74 ± 0.20
73.95 ± 0.27 15.79 ± 0.47
0.55 ± 0.17
0.39 ± 0.01
16/5
90.55 ± 0.05
73.62 ± 0.32 16.92 ± 0.37
0.61 ± 0.06
20/1
90.74 ± 0.14
81.25 ± 0.51 9.48 ± 0.37
0.43 ± 0.10
20/3
90.42 ± 0.05
74.06 ± 0.44 16.36 ± 0.45
20/5
90.37 ± 0.07
73.02 ± 0.40 17.34 ± 0.33
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Holocellulose
0.22 ± 0.03
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0.21 ± 0.02
0.64 ± 0.01
0.16 ± 0.05
0.81 ± 0.01
0.13 ± 0.01
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Materials
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Table 3. Summary of the onset degradation temperature and the maximum DTG peak temperature of the steam exploded fibers as a function of the steam pressures and a number of
DTG peak
temperature (°C)
temperature (°C)
0
330.5
366.2
16/1
336.8
367.6
16/3
336.7
368.9
16/5
335.7
369.3
20/1
336.6
370.3
20/3
337.8
372.1
20/5
337.0
371.6
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A
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Onset degradation
Materials
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the treatment cycles.
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S2213-3437(18)30759-0 https://doi.org/10.1016/j.jece.2018.102836 JECE 102836
To appear in: Received date: Revised date: Accepted date:
28 August 2018 28 November 2018 14 December 2018
Please cite this article as: Tanpichai S, Witayakran S, Boonmahitthisud A, Study on structural and thermal properties of cellulose microfibers isolated from pineapple leaves using steam explosion, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.102836 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Study on structural and thermal properties of cellulose microfibers isolated from pineapple leaves using steam explosion
Learning Institute, King Mongkut’s University of Technology Thonburi, Bangkok, 10140,
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Supachok Tanpichai1, 2, 3*, Suteera Witayakran4 and Anyaporn Boonmahitthisud5, 6
Thailand.
NanotecKMUTT Center of Excellence on Hybrid Nanomaterials for Alternative Energy,
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Cellulose and Bio-based Nanomaterials Research Group, King Mongkut’s University of
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King Mongkut’s University of Technology Thonburi, Bangkok, 10140, Thailand.
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Technology Thonburi, Bangkok, 10140, Thailand.
Kasetsart Agricultural and Agro-Industrial Product Improvement Institute, Kasetsart
Department of Materials Science, Faculty of Science, Chulalongkorn University, Bangkok,
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10330, Thailand.
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University, Bangkok, 10900, Thailand.
Green Materials for Industrial Application Research Unit, Faculty of Science,
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Chulalongkorn University, Bangkok, 10330, Thailand
Corresponding author: Supachok Tanpichai Email: [email protected] Tel: +662-
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*
470-8385 Fax: +662-872-9082
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Abstract In Thailand, pineapple leaves are one of the most common agricultural wastes after cultivation. The aim of this research was to understand effects of the steam explosion treatment conditions (steam pressure and treatment cycle) on properties of cellulose microfibers extracted from pineapple leaves. The fibers were steam exploded at pressure of
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16 and 20 kgf cm-2 between 1 and 5 cycles. The steam explosion technique could partially eliminate hemicellulose and lignin and increase the cellulose content, leading to improvement
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of the thermal properties and crystallinity of the treated fibers. With increasing the steam pressure and treatment cycles, changes of the fiber morphology were observed. Smaller
widths and shorter lengths of the treated fibers could be obtained. Microfibers with widths of
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3 µm and lengths of 93 µm were extracted from fiber bundles with widths of 45.8 µm and
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lengths of ~2 cm with the pressure of 20 kgf cm-2 for 5 cycles. Also, the steam explosion
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method could possibly fibrillate cellulose nanofibers. With more cycles of the steam
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explosion treatment, large amounts of cellulose nanofibers could be found.
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1. Introduction
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Keywords: Pineapple leaf; steam explosion; thermal properties; fibrillation; nanofibers
The lignocellulosic materials from agricultural crops and residues have been
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increasingly attractive [1, 2]. Pineapple canned industry in Thailand is one of the World’s largest producers and exporters [3, 4]. In 2014 the annual pineapple production was 1.79
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million tons [5]. Normally, pineapple plants (Ananas comosus) take around a year to bloom, and another 6 – 8 months for fruits to be ready [6]. After cultivation, 30 – 50 pineapple leaves with lengths up to 180 cm with overall weight of 1 – 1.5 kg are left per plant as a waste from the pineapple cultivation [6]. The cellulose content in these pineapple leaf fibers is between 70 and 82 % while a content of lignin is as low as 5 – 12 % [7]. Pineapple leaf fibers with a 2
ribbon-like structure present high specific strength and stiffness because of a high cellulose content and low microfibrillar angle (14°) [4, 7, 8]. Due to these reasons, attempts have been made to extract pineapple leaf fibers from the pineapple leaves, and use in many applications such as composites, papers, textiles and cosmetics [7-9]. Currently, the use of the pineapple
reasonable price, abundance and superior mechanical properties [8].
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leaf fibers to replace glass fibers in building materials has been studied because of the
Generally, cellulose fibers are extracted from any lignocellulosic biomass such as
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wood or crops by the chemical treatment process [10]. Within this process, a series of the chemical treatments such as acidic or alkaline condition is introduced to lignocellulosic
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materials [11, 12]. For example, a time-consuming procedure of the chemical treatments
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(acidified sodium chlorite solution at 80 °C used for 1 h for 5 times and 4 wt% sodium
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hydroxide solution at 80 °C applied for 2 h for 2 times) was introduced to kenaf bast fibers, resulting in the accessibility of the cellulose due to the hydrolysis of hemicellulose to xylose
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and degradation of lignin [11-13]. The similar procedure has also been introduced to water
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hyacinth [14], wood [15], rice straw [15] and potato tuber [15]. However, this chemical
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treatment approach is expensive because of the requirement of the high processing costs of the chemical solvents, disposal of the used solvents, high energy consumption and long
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processing time [11, 12, 16].
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The steam explosion treatment has been reported to be a promising alternative process to extract cellulose fibers with a rapid process [16-19]. Fibers are saturated with high-
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pressure steam at high temperature for a specific period of time, and the pressure is immediately released. The steam explosion method could be acted as the thermomechanochemical process, consisting of three combined effects: heat from steam, shear force generated from moisture expansion and acetic acid form the hydrolysis of acetyl groups in hemicellulose obtained from cellulose fibers [10]. This leads to the rupture of the
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lignocellulosic structure, conversion of hemicellulose to water soluble oligomers and individual sugars and damage of the bonding between hemicellulose and lignin [17, 20-23]. The remaining part as a solid residue left is cellulose [24, 25]. It has been reported that hemicellulose in wood could be almost completely converted to oligosaccharides soluble in water after the introduction of the steam explosion treatment at 20 kgf cm-2 for 1 min [25].
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The steam explosion technique is also used to reduce non-cellulosic compounds which bind fibrils altogether [26]. Moreover, partial dissolution of lignin and hemicellulose using the
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steam explosion treatment has been found to be effectively similar to that treated with the alkaline treatment [27]. It is worth noted that the steam explosion treatment requires less
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hazardous chemical used, energy consumption, human toxicity and environmental impact in
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comparison with the alkaline treatment in order to gain higher fiber yielding [23].
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Additionally, this steam explosion treatment is possible to treat a large capacity of biomass or
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cellulose materials [26]. The steam explosion approach has been used to disintegrate individual fibers from cellulose materials such as rice straw [27], oak shell [28], kenaf [29],
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eucalyptus [30] and pineapple leaf fibers [4, 9, 31]. Effect of the steam explosion conditions
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has been previously studied to investigate the efficiency of this approach. For example, Han et al [32] studied effect of the steam temperature and retention time on properties of rice
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straw. When rice straw was steam exploded at higher steam temperature for the longer
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treatment time, more homogenous fiber-like materials were obtained. The surface wettability and high degree of crystallization were also improved after the steam explosion treatment
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[26]. Next, Boonterm et al [27] prepared cellulose fibers extracted from rice straw using the steam explosion method. With increasing the steam pressure, the significant reduction in fiber widths was found, resulting in a low aspect ratio of the treated fibers. Low fiber yielding was also observed with the high pressure due to fiber damages.
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Although researchers have recently studied effect of the steam explosion conditions and have used the steam explosion technique to disintegrate cellulose fibers from various cellulose sources for applications such as composites, pulp and paper and bioethanol production [4, 9, 29, 31, 33], scarce study of fiber properties after the introduction of the steam explosion treatment for several cycles, to authors’ knowledge, has been previously
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reported. Here, the aim of this research was to study effect of the steam pressure as a function of a number of the treatment cycles on properties of the treated pineapple leaf fibers. The
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structural and thermal properties of the steam exploded pineapple leaf fibers were studied using Fourier transform infrared (FTIR), Thermogravimetric analysis (TGA), X-ray
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diffraction (XRD) and scanning electron microscopy (SEM) techniques. Chemical
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components of the fibers after the introduction of the steam explosion method as a function of
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the treatment cycles were also analyzed.
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2. Materials and methods 2.1. Materials
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Pineapple leaf fibers, extracted by a decorticated machine, were supplied by Kasetsart
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Agricultural and Agro-industrial Product Improvement Institute. 2.2 Steam explosion treatment
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Pineapple leaf fibers were treated under the steam explosion conditions presented in
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Table 1. Briefly, dried pineapple leaf fibers with the weight of 150 g were transferred into a 2L batch steam explosion chamber, and saturated steam was admitted into the chamber. After
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the fibers were steam exploded at the specific pressure for 5 min (the retention time was counted when the steam reached the setting pressure), the steam pressure was immediately released. The fibers were fibrillated due to explosive decompression caused by the immediate pressure reduction. This process was repeated for 1, 3 and 5 times to study effect of the steam
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explosion treatment cycles on the isolation of the pineapple leaf fibers. The treated fibers were washed with distilled water and dried.
2.3 Chemical composition analysis The raw and steam-exploded fibers were analyzed to investigate the chemical
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compositions (cellulose, hemicellulose, lignin and ash). The chemical analysis of the fiber
samples was performed by the Technical Association of the Pulp and Paper Industry (TAPPI)
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test method [34]. Before chemical analysis, the samples were grinded to pass a 0.4-mm (40mesh) screen to permit the complete reaction of the samples with the reagents used in the
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analysis. About 50 g of the grinded specimen was then extracted successively with ethanol-
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benzene, ethanol and hot water, according to TAPPI T264 in order to remove the extractives
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before any chemical analysis and to determine the extractive fraction in the sample. After
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drying, 5 g of the extractive-free sample was used to determine the Klason lignin content (acid-insoluble lignin), according to TAPPI T222. Furthermore, approximately 5 g of the
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dried extractive-free specimen was used to determine the holocellulose content according to
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the acid chlorite method of Browing [35]. Then, 5 g of the holocellulose fraction was used to determine the α-cellulose content according to TAPPI T203. Based on these results, the
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hemicellulose content was determined by subtracting the α-cellulose content from the
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holocellulose content value. 2.4 Fourier transform infrared spectrophotometry
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Fourier transform infrared (FTIR) measurements were performed using a
spectrometer (Nicolet IR200, Thermo Fisher Scientific Co., Ltd., USA) with an attenuated total reflectance (ATR) mode. The original and treated pineapple leaf fibers were analyzed at wavenumbers of 400 – 4000 cm-1 with 4 cm-1 resolution and 64 repetitious scans. Prior to the sample investigation, the background was recorded.
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2.5 Thermogravimetric analysis The untreated and steam exploded pineapple leaf fibers were investigated by a Thermogravimetric analyzer (TGA/SDTA 851e, Mettler-Toledo LLC, USA) to study effect of the steam explosion conditions on thermal properties of the fibers. A dried sample with a weight of ~5 mg was placed on a platinum pan, and maintained at temperature of 110 °C for
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10 min to completely remove moisture. Later, the sample was heated to 600 °C with a heating rate of 10 °C min-1 under a nitrogen flow rate of 100 ml min-1.
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2.6 X-ray diffraction
The structures of the pineapple leaf fibers treated with the steam explosion method were studied using an X-ray diffractometer (D8DISCOVER, Bruker AXS, Germany)
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equipped with a Goebel mirror. The X-ray diffraction (XRD) profiles were recorded using the
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CuKα radiation with a wavelength of 0.1540 nm operating at the accelerating voltage of 40 kV and current of 40 mA. Samples were scanned with a step size of 0.02° and step speed of
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0.8 s over the 2θ range of 5 – 50°. The degree of crystallinity obtained from each sample was
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determined from the intensity of the crystalline peak located at 2θ of ~22° and minimum
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intensity of the peaks located between 16.5 and 22.6°, corresponding to the amorphous area, according to Segal’s method [36].
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2.7 Field emission scanning electron microscopy
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The surface morphologies of the pineapple leaf fibers after the steam explosion treatment were investigated using a field-emission scanning electron microscope (FEI Nova
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NanoSEM 450, FEI Co., Ltd., USA) with an acceleration voltage of 5 kV and a working distance of 10 mm. Before each measurement, a sample was coated with a thin layer of gold to avoid charging. 3. Results and discussion 3.1 Chemical composition analysis
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Effects of a number of the steam explosion treatment cycles and steam pressure on the chemical compositions of the pineapple leaf fibers are presented in Table 2. The results showed that chemical compositions of the fibers were changed after the introduction of the steam explosion treatment. In the raw pineapple leaf fibers, α-cellulose, hemicellulose, lignin and ash were 64.48, 20.89, 4.24 and 0.77 %, respectively. As expected, the content of
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hemicellulose and lignin were reduced significantly by passing the fibers through the steam
explosion treatment for only 1 time. The lignin content of the raw pineapple leaf fibers (4.24
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%) was dropped to 0.55 and 0.43 % for the 16/1 and 20/1 sample, respectively, and the
content of hemicellulose measured from the 16/1 and 20/1 sample were 12.6 and 9.5 % in comparison with the hemicellulose percentage of 20.89 obtained from the untreated fibers.
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On the other hand, the α-cellulose content increased from 64.48 to 76.6 and 81.3 %,
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respectively, when the steam pressure of 16 and 20 kgf cm-2 were applied. The similar
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increase of the cellulose content and reduction of hemicellulose and lignin content were reported for steam treated rice straw fibers [37]. Moreover, Jiang et al [29] reported the
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reduction of the hemicellulose, pectin and water-soluble matter with increasing the steam
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pressure. The decrease of the hemicellulose content has been previously observed with the severe steam explosion condition (high pressure and longer time) because of the
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depolymerization of the hemicellulose by hydrolysis of glycosidic linkages and acetyl groups
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to soluble saccharides [10]. Meanwhile, the degradation of lignin is caused by the cleavage of β-O-4ʹ linkages in the lignin structure [38]. More solubilization of hemicellulose and lignin
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has been found with the high pressure steam treatment due to the close contact between these components and steam [37]. The reduction of the hemicellulose and lignin content and the improvement of the cellulose percentage indicates that the steam explosion could possibly eliminate lignin and hemicellulose without any chemical solvents applied in the process, which can reduce expense of the fiber preparation [23, 39].
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When a higher number of the steam explosion cycles were applied, the slight decrease of the α-cellulose content was found for both steam pressure conditions (16 and 20 kgf cm-2). With 5 treatment cycles, the α-cellulose content of the 16/5 sample decreased to 73.62 % and that of the 20/5 material reduced to 73.02 %. This might be because of the depolymerization or degradation of cellulose caused by the severe condition. The degradation of the cellulosic
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structure has been reported for the steam explosion treatment at high pressure for a longer
time [39]. The cellulose degradation resulted in a higher portion of the hemicellulose with
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respect to a number of the treatment cycles, as presented in Table 2. Furthermore, slight improvement of the lignin content could be observed especially with the higher steam
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pressure treatment. The lignin content was found to be higher from 0.43 % for the 20/1 to
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0.81% for the 20/5 sample. It has been reported that with a longer treatment time at the high
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steam pressure, the repolymerization of lignin could occur by the condensation reaction of C-
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2 or C-6 in quaiacyl and syringyl rings [40]. Li et al [38] also stated that with the longer retention time, molecular weight (Mw) of the lignin shifted to a higher, and the
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repolymerization of lignin was more prominent.
3.2 Chemical properties
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The appearance of the cellulose, hemicellulose and lignin was investigated from the
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specific FT-IR peaks, as presented in Fig. 1. The adsorption peak located at 1050 cm-1, corresponding to a C-O-C glucopyranose unit in cellulose molecules [13, 27], was found
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from both original and steam exploded pineapple leaf fibers. This indicated the steam explosion treatment could not change the cellulose structure although the steam pressure of 20 kgf cm-2 was applied for 5 continuous treatment cycles. This result could be well supported by the XRD measurements, as discussed later. The appearance of the carbonyl groups of the hemicellulose (1640 cm-1) [41, 42] and phenolic compounds (1240 cm-1) [4, 41-
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43] was observed from the spectra of all untreated and steam exploded fiber samples. This could confirm the remaining contents of the lignin and hemicellulose in the fibers after the steam explosion treatment. This result was well agreed with results of the chemical composition measurements. The 3340 and 2920 cm-1 adsorption bands related to hydroxyl groups and C-H stretching of cellulose [11, 27] were also found from all fiber samples.
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3.3 Thermal stabilities
Thermal gravimetric (TG) and derivative Thermal gravimetric (DTG) curves of the
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steam exploded fibers are presented in Fig. 2, and the onset degradation temperature and the
maximum DTG peak temperature of the untreated and steam exploded fibers are summarized
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in Table 3. Generally, four transitional states (moisture evaporation and decomposition of
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hemicellulose, cellulose and lignin) are detected from cellulosic materials. In this study, no
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weight loss change at the temperature of approximately 100 °C, corresponding to moisture
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evaporation, could be observed from all cellulose samples due to the sample stabilization at 110 °C for 10 min prior to the measurements. Hemicellulose starts to degrade in the
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temperature range of 220 – 300 °C, and followed by the cellulose degradation at 275 – 400
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°C [42, 44-46]. Lastly, due to its complex structure, lignin decomposes in the wider region from 450 to 700 °C [42].
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The shift of the decomposition temperature to higher temperature could be detected
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from the steam exploded pineapple leaf fibers, indicating the higher thermal stabilities of the fibers after the steam explosion treatment. Similar TG curves were observed from the treated
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fibers with the different steam pressures and steam explosive cycles. The onset degradation temperature of the untreated pineapple leaf fibers was 330.5 °C while the onset degradation temperatures of the all steam exploded samples with the different steam pressures and treatment times were found to be around 337 °C. The improvement of the degradation temperature could be because of less quantities of hemicellulose located within and between
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cellulose fibrils [45]. The increase of the sharp DTG peaks, corresponding to a prominent cellulose decomposition, was also monitored for the steam exploded fibers. The DTG peak of the untreated fibers was found at temperature of 366.2 °C, and peak was shifted to a higher temperature range of around 370 °C with the use of the steam explosion method. The steam exploded fibers presented higher intensity of the DTG peak than the untreated did. The
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intensity of the DTG peaks could relatively imply the higher cellulose content in the steam
exploded fibers in comparison with the original pineapple leaf fibers, which was well agreed
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with the chemical composition results. Moreover, the higher percentage of lignin of the
original pineapple leaf fibers led to a higher char yield at the high temperature. A carbon
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residue of 11% was obtained from the original fibers at 600 °C while the steam exploded
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fibers had a char content of around 5 %. Pelissari et al [42] purposed that the low amount of
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carbon residues could be due to the removal of lignin and hemicellulose and the accessibility
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of cellulose in fibers. The lowest char yield has been found from the acid treated cellulose fibers in comparison with the raw fibers due to the disappearance of the noncellulosic
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components in the fibers after the acid treatment [45]. The higher thermal stability results of
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all steam exploded pineapple leaf fibers could confirm that the use of the steam explosion treatment could possibly remove hemicellulose and lignin from the fibers without the
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presence of the chemical treatment.
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3.4 Crystallinity Effects of a number of the steam explosion treatment cycles on crystallinity of the
treated fibers are presented in Fig. 3. All treated pineapple leaf fiber samples exhibited the outstanding peaks located at 14.8, 16.5, and 22.6°, corresponding to (1 0 1), (1 0 1̅) and (0 0 2) lattice planes of the typical cellulose I structure [47] while the peaks located at 2θ of
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12, 20 and 22° would be determined from the cellulose II structure [48, 49]. It is worthy noted that the steam explosion could not be able to change the crystal structure of the pineapple leaf fibers even steam treated for 5 continuous cycles. The change of crystallinity of the treated fibers, however, could be caused by the steam explosion treatment, as shown in Fig. 3(b). As always, the lowest crystallinity index was obtained from the untreated fibers.
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After the fibers were steam exploded for a single time with the steam pressure of 16 kgf cm-2, the crystallinity degree was improved from 82.2 to 86.7 %. A percentage of 87.8 for the
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crystallinity degree was obtained from the fibers steam-exploded with the pressure of 16 kgf cm-2 for 5 treatment cycles. This might be because the removal of the amorphous portion of
the treated fibers. No significant difference of the crystallinity degree could be seen from the
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treated fibers with the 16 and 20 kgf cm-2 steam pressure at the same treatment cycle. The
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similar improvement of the crystallinity of the mechanically treated banana fibers has been
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reported by Pelissari et al [42]. With increasing a number of the suspension of the treated cellulose fibers passing through a high pressure homogenizer, the crystallinity index of the
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treated nanofibers was found to be higher. This was in good agreement with the study of the
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ultrasonication output power level effect. The crystalline index value of the cellulose samples increased when the higher output power was applied due to the degradation of the amorphous
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cellulose [44]. However, the longer steam explosion treatment time have been reported to
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decrease the crystallinity of the treated fibers due to the cellulose degradation [37].
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3.5 Morphology
Changes of morphologies of the steam exploded fibers in comparison with the
original fibers are presented in Fig. 4, and Fig. 5 compares the fiber widths of the steam exploded fibers treated with the steam pressure of 16 and 20 kgf cm-2 as a function of the treatment cycles. In general, microfibers are bonded together with lignin and hemicellulose to
12
from bundles. With the introduction of the steam explosion process for one cycle, microfibers with diameters of 4.7 ± 1.8 and 3.8 ± 1.5 µm obtained from the 16/1 and 20/1 fibers were extracted from the original fiber bundles with diameters of 45.8 ± 11.4 µm. The intensive reduction of the fiber diameter could confirm the efficiency of the steam explosion process [4]. With increasing a number of the steam explosion process, the average diameter of the
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fibers decreased slightly. The average fiber diameter of the 16/3 and 20/3 were 3.9 ± 1.5 and
3.2 ± 1.2 µm, respectively, and values of 3.1 ± 1.2 and 3.0 ± 1.3 µm were measured from the
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fibers steam exploded with the steam pressure of 16 and 20 kgf cm-2 for 5 continuous cycles.
When the fibers were steam exploded for 3 cycles with both pressures, considerable reduction
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of the fiber diameters was observed. However, with the continuous steam explosion for more than 3 times, no significant change of the fiber diameters was found with the higher steam
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pressure (20 kgf cm-2) while the reduction of the fiber diameters could be observed from the
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16 kgf cm-2 treatment. This indicated the use of the higher steam pressure could efficiently fibrillate microfibers from bundles with a few treatment cycles. However, the fibrillation of
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microfibers using this machine could reach the limitation after 3 cycles with the higher steam
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pressure.
Notably, the treatment of the steam explosion for many cycles not only affected fiber
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defibrillation, but also decreased fiber length. With increasing the treatment cycles, the length
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of fibers decreased tremendously when the steam pressure of 20 kgf cm-2 was applied to the fibers. With increasing the treatment cycle to 3 or 5 times, the fiber length of the 20/3 and
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20/5 material were significantly reduced to 142.5 ± 82.3 and 93.5 ± 52.5 µm, respectively, in comparison with the original fibers with lengths of up to 2 cm. The similar occurrence of the fiber length shortening has been also found by Boonterm et al [27]. The reduction of fiber diameters and lengths was observed when the steam pressure increased from 13 to 17 bar. With the pressure of 13 bar, fibers with diameters of 0.26 mm and lengths of 35.5 mm were
13
obtained. When the steam pressure was increased to 17 bar, the diameters and lengths of the steam exploded fibers from 3-cm long rice straw fibers were significantly reduced to 0.11 and 8.1 mm, respectively. Interestingly, larger amounts of cellulose nanofiber fibrillated from microfibers could be seen when the fibers were shorter (Fig. 4). It could be said that using the steam explosion treatment can fibrillate nanofibers.
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4. Conclusions
Cellulose fibers were successfully extracted from pineapple leaf using the steam
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explosion process. Results showed that the steam explosion treatment could reduce partial contents of the hemicellulose and lignin and increase of the cellulose portion without any
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introduction of the chemical treatment, confirmed by chemical composition and FTIR
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measurements in comparison with the untreated pineapple leaf fibers. The decreased contents
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of the hemicellulose and lignin improved thermal properties of the steam exploded fibers.
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Also, the increase of the crystallinity index was found with increasing a number of the steam explosion treatment cycles. The defibrillation of individual microfibers from fiber bundles
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could be effectively investigated. The fibers with diameters of 3.8 µm were extracted from
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the original fiber bundles with diameters of 45.8 µm with the use of the steam explosion treatment with the pressure of 20 kgf cm-2 for 1 cycle. With the treatment of the pressure of
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20 kgf cm-2 for 5 cycles, the slight reduction of the fiber diameter to 3.0 µm was observed.
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When the fibers were treated with the severe steam explosion condition for a higher number of the treatment cycles, fibers were shortened. The average fiber length of 93.5 µm was
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investigated from the fibers steam exploded with the 20 kgf cm-2 pressure for 5 times in comparison with the original fibers with lengths up to 2 cm. Moreover, the use of the steam explosion treatment could fibrillate nanofibers from microfibers. The as-prepared steam exploded fibers could be beneficial for composite, packaging and food applications.
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Acknowledgements One of the authors (S.T.) would like to acknowledge the financial support of the Asahi Glass Foundation and King Mongkut’s University of Technology Thonburi for a research grant of 2017 and the Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program the Center of Excellence Network.
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Furthermore, S.T. was grateful to his wife and his little boy for endlessly valuable supports.
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References
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[34] Anonymous. TAPPI test method: The technical association of the pulp and paper industry. Atlanta, Georgia: TAPPI Press; 2002-2003. [35] Browing BL. Methods of Wood Chemistry. New York: Interscience Publisher; 1967. [36] Segal L, Creely JJ, Martin Jr. AE, Conrad CM. An empirical method for estimating the degree of crystallinity of native cellulose using the X-ray diffractometer Text Res J. 1959;29(10):786-94. [37] Chen X, Yu J, Zhang Z, Lu C. Study on structure and thermal stability properties of cellulose fibers from rice straw. Carbohydr Polym. 2011;85(1):245-50. [38] Li J, Henriksson G, Gellerstedt G. Lignin depolymerization/repolymerization and its critical role for delignification of aspen wood by steam explosion. Bioresour Technol. 2007;98(16):3061-8. [39] Maniet G, Schmetz Q, Jacquet N, Temmerman M, Gofflot S, Richel A. Effect of steam explosion treatment on chemical composition and characteristic of organosolv fescue lignin. Ind Crops Prod. 2017;99:79-85. [40] Li J, Gellerstedt G, Toven K. Steam explosion lignins; their extraction, structure and potential as feedstock for biodiesel and chemicals. Bioresour Technol. 2009;100(9):2556-61. [41] Wang B, Li D. Strong and optically transparent biocomposites reinforced with cellulose nanofibers isolated from peanut shell. Compos Pt A-Appl Sci Manuf. 2015;79:1-7. [42] Pelissari FM, Sobral PJdA, Menegalli FC. Isolation and characterization of cellulose nanofibers from banana peels. Cellulose. 2014;21(1):417-32. [43] Tibolla H, Pelissari FM, Martins JT, Vicente AA, Menegalli FC. Cellulose nanofibers produced from banana peel by chemical and mechanical treatments: Characterization and cytotoxicity assessment. Food Hydrocoll. 2018;75:192-201. [44] Khawas P, Deka SC. Isolation and characterization of cellulose nanofibers from culinary banana peel using high-intensity ultrasonication combined with chemical treatment. Carbohydr Polym. 2016;137:608-16. [45] Chirayil CJ, Joy J, Mathew L, Mozetic M, Koetz J, Thomas S. Isolation and characterization of cellulose nanofibrils from Helicteres isora plant. Ind Crops Prod. 2014;59:27-34. [46] Rout T, Pradhan D, Singh RK, Kumari N. Exhaustive study of products obtained from coconut shell pyrolysis. J Environ Chem Eng. 2016;4(3):3696-705. [47] Qing Y, Sabo R, Zhu JY, Agarwal U, Cai Z, Wu Y. A comparative study of cellulose nanofibrils disintegrated via multiple processing approaches. Carbohydr Polym. 2013;97(1):226-34. [48] Tanpichai S, Sampson WW, Eichhorn SJ. Stress-transfer in microfibrillated cellulose reinforced poly(lactic acid) composites using Raman spectroscopy. Compos Pt A-Appl Sci Manuf. 2012;43(7):1145-52. [49] Puttaswamy M, Srinikethan G, Shetty KV. Biocomposite composed of PVA reinforced with cellulose microfibers isolated from biofuel industrial dissipate: Jatropha Curcus L.seed shell. J Environ Chem Eng. 2017;5(2):1990-7.
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Fig. 1 FT-IR spectra of the steam-exploded and original pineapple leaf fibers.
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Fig. 2 (a) TG and (b) first derivative TG (DTG) curves of the pineapple leaf fibers
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with a number of the steam explosion treatment times.
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respect to a number of the steam explosion treatment cycles.
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Fig. 3 (a) XRD patterns and (b) degree of crystallinity of the pineapple leaf fibers with
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Fig. 4 Morphologies of the steam exploded fibers in comparison with the original
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fibers. Arrows present the isolation of the cellulose nanofibers.
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Fig. 5 Fiber widths of the steam treated fibers with the steam pressure of 16 and 20
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kgf cm-2 with respect to the treatment cycles.
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Table 1. Summary of the steam explosion conditions applied for the pineapple leaf fibers Steam
Steam explosion
Treatment time
(kgf cm-2)
temperature (°C)
(cycles)
(mins)
0
-
-
-
-
16/1
16
204
1
5
16/3
16
204
3
5
16/5
16
204
5
20/1
20
215
1
20/3
20
215
3
20/5
20
215
5
5 5 5 5
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Steam pressure
Materials
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Table 2. Chemical compositions of the pineapple leaf fibers after the steam explosion treatment. α-cellulose
Hemicellulose Lignin
Ash
(%)
(%)
(%)
(%)
(%)
0
85.66 ± 0.22
64.48 ± 0.49 20.89 ± 0.72
4.24 ± 0.25
0.77 ± 0.03
16/1
89.13 ± 0.03
76.57 ± 0.51 12.56 ± 0.49
0.55 ± 0.12
0.59 ± 0.01
16/3
89.74 ± 0.20
73.95 ± 0.27 15.79 ± 0.47
0.55 ± 0.17
0.39 ± 0.01
16/5
90.55 ± 0.05
73.62 ± 0.32 16.92 ± 0.37
0.61 ± 0.06
20/1
90.74 ± 0.14
81.25 ± 0.51 9.48 ± 0.37
0.43 ± 0.10
20/3
90.42 ± 0.05
74.06 ± 0.44 16.36 ± 0.45
20/5
90.37 ± 0.07
73.02 ± 0.40 17.34 ± 0.33
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Holocellulose
0.22 ± 0.03
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0.21 ± 0.02
0.64 ± 0.01
0.16 ± 0.05
0.81 ± 0.01
0.13 ± 0.01
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Table 3. Summary of the onset degradation temperature and the maximum DTG peak temperature of the steam exploded fibers as a function of the steam pressures and a number of
DTG peak
temperature (°C)
temperature (°C)
0
330.5
366.2
16/1
336.8
367.6
16/3
336.7
368.9
16/5
335.7
369.3
20/1
336.6
370.3
20/3
337.8
372.1
20/5
337.0
371.6
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Onset degradation
Materials
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the treatment cycles.
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