Feasibility of ramie fibers as raw material for the isolation of nanofibrillated cellulose

Journal Pre-proof Feasibility of ramie fibers as raw material for the isolation of nanofibrillated cellulose Nelson Potenciano Marinho, Pedro Henrique Gonzalez de Cademartori, Silvana Nisgoski, Valcineide Oliveira de Andrade ˆ Bolzon ´ de Muniz ˜ Tanobe, Umberto Klock, Graciela Ines

PII:

S0144-8617(19)31247-0

DOI:

https://doi.org/10.1016/j.carbpol.2019.115579

Reference:

CARP 115579

To appear in:

Carbohydrate Polymers

Received Date:

1 August 2019

Revised Date:

19 October 2019

Accepted Date:

6 November 2019

Please cite this article as: Marinho NP, de Cademartori PHG, Nisgoski S, de Andrade Tanobe ˜ GIB, Feasibility of ramie fibers as raw material for the isolation of VO, Klock U, de Muniz nanofibrillated cellulose, Carbohydrate Polymers (2019), doi: https://doi.org/10.1016/j.carbpol.2019.115579

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Feasibility of ramie fibers as raw material for the isolation of nanofibrillated cellulose

Nelson Potenciano Marinhoa, Pedro Henrique Gonzalez de Cademartoria,b, Silvana Nisgoskia,b, Valcineide Oliveira de Andrade Tanobec, Umberto Klocka,b, Graciela Inês Bolzón de Muñiza,b

a Programa de Pós-Graduação em Engenharia Florestal (PPGEF), Universidade

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Federal do Paraná, Curitiba 80210 170, Brazil. [email protected]

b Departamento de Engenharia e Tecnologia Florestal (DETF), Universidade Federal do Paraná, Curitiba 80210 170, Brazil. [email protected]; [email protected];

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[email protected]; [email protected].

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c Engenharia de Bioprocessos e Biotecnologia, Universidade Federal do Paraná,

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Centro Politécnico, Curitiba 80050-540. [email protected]

Corresponding author: Nelson Potenciano Marinho - Programa de Pós-Graduação em

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Engenharia Florestal (PPGEF), Universidade Federal do Paraná, Curitiba. 632, Lothário

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Meissner Avenue, 80210-170, Brazil. [email protected] / + 55 41 33624974.

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Highlights

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Ramie fibers were used to produce cellulose nanofibers.

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The alkaline organosolv method and a bleaching step were applied to prepare the pulps.

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Effect of mechanical defibrillation was systematically studied. 1

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CNF films were characterized in terms of physical and mechanical properties.

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Ramie CNF films with remarkable barrier properties were produced.

Abstract In this study, a strategy was adopted to enhance the use of ramie fibers as raw material

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for isolation of cellulose nanofibers (CNFs). Ramie pulp was produced by alkaline organosolv followed by bleaching. CNFs were produced by mechanical defibrillation, and films were fabricated via casting. Effects of number of passes in the mechanical

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grinding on physical and mechanical properties of CNF films were comprehensively

studied. Potential of ramie fibers was proved by fabricating homogeneous nanofibers

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with average thickness of 8.72 nm, which led to CNF films with dense and non-porous networks, and crystallinity index of 76-78%. Tensile strength (42-82 MPa) and dynamic

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mechanical (9-11 GPa) performance were good only for less severe mechanical defibrillation. Lower solubility (1.85-2.43%). and activity (0.69) in water, and outstanding

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barrier properties against water vapor and oxygen make ramie suitable for more sustainable extraction of cellulose nanofibers and production of CNF films for diverse

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applications.

Keywords: Boehmeria nivea; nanocellulose; physical durability; alkaline organosolv

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method; biorefinery.

1. Introduction The wide use of lignocellulosic materials has been revolutionizing the industrial sector for the development of new materials based on a sustainable economy. These aspects are in agreement of principles of bioeconomics, which suggest the use of 2

renewable sources – such as natural fibers – with minimum or absence of environmental damage (Bracco, Calicioglu, San Juan, & Flammini, 2018). Among lignocellulosic materials, ramie (Boehmeria nivea) fibers stand out due to their excellent mechanical resistance, with tensile strength varying from 400-1600 MPa (Jose, Rajna, & Ghosh, 2016), and higher concentration of α-cellulose (80-85%) (Satyanarayana, Arizaga, & Wypych, 2009) compared to cotton and silk fibers (Smole, Hribernik, Kleinschek, & Kreze, 2013; Pickering, Aruan Efendy, & Le, 2016). From a morphological point of view, these fibers’ length typically varies from 120 to 150 mm, but

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can reach 620 mm, whereas their diameter can vary from 40-60 μm to 126 μm

(Kozlowski, Rawluk, & Barriga-Bedoya, 2005). Global production of ramie was 102,466 thousand tons in 2016, of which China accounted for 97.38% (FAOSTAT, 2017). These

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fibers are widely applied to manufacture many products, like filters, textiles, biomedical

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devices and paper money (Sen & Reddy, 2011; Jose, Rajna, & Ghosh, 2016; Kandimalla et al., 2016).

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Cellulose can be extracted from any plant and is one of the most abundant and inexpensive biodegradable sources in the world (Abdul Khalil, Bhat, & Yusra et al.,

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2014). However, the main technologies for cellulose processing usually apply chemicals with high toxicity, like sulfite, sulfate and soda (Chen et al., 2017). Ecofriendly

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processes have been tested as viable alternatives for cellulose extraction, such as organosolv delignification. This alternative uses organic solvents and water as agents

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for pulp delignification, with lower environmental impact (Brosse, Hussin, & Rahim, 2017; Rodrígez, Espinosa, Juan, & Sánchez, 2018). Compared to conventional processes – sulfite and Kraft, for example - organosolv delignification has the advantage of no odor (Rodrígez, Espinosa, Juan, & Sánchez, 2018), ability to recover hemicelluloses during the processing (Chirayil, Mathew, & Thomas, 2014), higher rates of delignification and isolation of lignin in purer form (Rodrígez, Espinosa, Juan, & 3

Sánchez, 2018; Cybulska, Brudecki, Schmidt, & Mette 2015), easy adaption for bleaching without non-chlorinated reagents (Shatalov & Pereira, 2008), higher process yields (Rodrígez, Espinosa, Juan, & Sánchez, 2018), and recovery of solvents by evaporation and distillation (González, Álvaro, Cristina, & Labidi, 2008). All these advantages perfectly fit the principles of biorefining (Borand & Karaosmanoglu, 2014). Cellulose nanofibers (CNFs), a product obtained from cellulosic fiber processing, have been used as a source for development of biobased materials with high performance (Dufresne, 2013). CNFs have many interesting and useful properties, such

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as optical transparency and high thermal stability, low weight, electrical conductivity, thermal insulation and mechanical resistance (Beecher, 2007; Flauzino Neto et al.,

2013; Nogi et al., 2013; Usov et al., 2015; Jose, Rajna, & Ghosh, 2016; Kovalov et al.,

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2017). Their extraction and production help to minimize environmental problems

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(Berglund et al., 2017) because they can be made from many renewable sources and/or agroindustrial wastes (Widiarto, Yuwono, Rochliadi, & Arcana, 2016). One of the most

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attractive products developed with CNFs is thin films (Azeredo, Rosa, & Mattoso, 2017; Hubbe, Ferrer, Tyagi, & Yin, 2017). Because of characteristics like high mechanical

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resistance and barrier properties (Gamelas & Ferraz, 2015), these thin CNF films have been applied as membranes for cellular encapsulation (Park, Shin, Cheng, & Hyun,

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2017), smart films in food packaging (Missio et al., 2018) green composites (Kumode, Bolzon, Magalhães, & Satyanarayana, 2013; Kwak, Lee, Lee, & Joon Jin, 2018) and

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films with good structural and thermal properties (Adu et al., 2018). In recent decades, technologies for CNF production have been improved, and new

raw materials have been discovered (Nechyporchuk, Belgacem, & Bras, 2016). CNFs can be produced by mechanical, biological and chemical methods. Characteristics like morphology and mechanical properties vary essentially as a function of the source and method of production (Chirayil, Mathew, & Thomas, 2014; 4

Moon, Martini, Nairn, Simonsen, & Youngblood, 2011). Among the technologies for CNF production, the mechanical process by ultrafine grinding has been considered an efficient and low-cost alternative, especially due to the absence of chemicals (Nechyporchuk, Belgacem, & Bras, 2016). Despite their high potential, there is a lack of information about the characteristics of CNFs extracted from ramie fibers, and consequently, nanostructured products like films for practical applications. Compared to other natural fibers, only a few investigations have explored the potential of ramie fibers as raw material for producing CNFs to make added-value products.

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Therefore, this study investigated the characteristics and properties of ramie

CNFs to produce high-value nanostructured films. The CNF films were produced using alkaline organosolv pulping, mechanical defibrillation by ultrafine grinding and solvent

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casting/filtration under different conditions. The effects of different conditions to produce

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CNFs and their films were investigated by morphological, physical and mechanical

2. Materials and Methods

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techniques.

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of all steps.

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The schematic flowchart shown in Figure 1 explains the chronological sequence

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Fig. 1. Schematic flowchart of key steps for the research methodology.

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2.1 Raw Materials

Strings of ramie (Boehmeria nivea, Gaud) fibers were acquired from a textile company. The strings were cut with scissors into small samples with 1-3 cm length.

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These strings of ramie have 71.09% of α-cellulose, 12.11% of hemicelluloses, 1.06% of

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acid insoluble lignin and 8.55% ethanol-toluene extractives (Marinho et al. 2018).

2.2 Production and delignification of alkaline organosolv pulps Alkaline organosolv pulp (AOP) was produced in a RegMed AU/E-20 rotary reactor (volume of 4 liters) through the method described by Marton & Granzou (1982). The cooking liquor was composed of ethanol (C2H5OH) and distilled water at different concentrations (50-50%, 60-40%, 70-30% and 80-20%), and 6% (v/v) of pure sodium 6

hydroxide (NaOH in pellets, CAS Number 1310-73-2) based on total volume of ethanol and water solution. The temperature was 160°C, time was 60 minutes and hydromodulus was 6:1 (mass of fibers: liquor), as proposed by Díaz et al. (2004) and Akgul, & Kirci (2009). The ramie fibers were sequentially depured as follows: i) 2 liters of the solution (1:1, ethanol, distilled water and 1% NaOH) at ~50°C; ii) 2 liters of distilled water at ~40°C; and iii) 4 liters of distilled water at room temperature (~25°C). The resulting pulp was centrifugated for 5 minutes. The Kappa number (TAPPI T236 om-13) and the yield were used to determine the best conditions of the cooking liquor. Based

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on this preliminary experiment, we selected 50:50 ethanol-water concentration due to the absence of statistical differences in the yield and Kappa number for all conditions. Delignification of ramie AOP (50:50 ethanol-water concentration) was performed in

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two steps. The process was performed at 80°C in a thermostatic bath for 60 minutes.

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The first step (Brch-1) used two buffer solutions in equal parts, one composed of 30g of NaCH3COO and 60 mL of CH3COOH diluted in 1000 mL of distilled water (pH 5.5), and

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the other (Brch-2) composed of 2% NaClO2 (w/v). The second step was carried out

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using a solution of 2% NaOH (w/v).

2.3 Morphology and chemical composition of the delignified ramie fibers

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The morphology of the delignified fibers was characterized with a Quanta FEG 450 FEI scanning electron microscope (SEM). The fibers were coated with a thin layer of

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gold using a Quick Coater Metalizer (SC-701, Sanyu Electron). High-resolution images were acquired in different magnifications at accelerating voltage of 5 kV. Holocellulose content (TAPPI T249 cm-00 method), alpha cellulose content

(TAPPI T203 cm-99) mean and hemicelluloses content (by difference) of delignified fibers were determined in triplicate. The insoluble acid-lignin content was determined by gravimetry using the NREL/TP-510-42618 method (Sluiter et al., 2012), and the soluble 7

acid-lignin was determined with a UV-Vis spectrophotometer (Varian, model Cary 100) to measure the absorbance at 240 nm based on the NREL/TP-510-42617 method (Hymanet al., 2008).

2.4 Production of cellulose nanofibers CNF production was performed using a mechanical process in a Masuko Sangyo Super Masscolloider (model MKCA6-3, Masuko Sangyo Co, Ltd., Japan). The equipment was set at 1500 rpm as recommended by Iwamoto, Abe, & Yano (2008) and

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Ifuku et al. (2010). Bleached ramie pulp was hydrated for seven days to swell the cell walls of the fibers, presenting 1% consistency (w/v). CNF suspensions were produced

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as a function of pass number through the grinder (1, 3, 5 and 10 passes).

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2.5 Transmission electron microscopy (TEM) of CNFs

The morphology of ramie CNFs was investigated using a JEOL JEM 1200EX-II

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transmission electron microscope. Samples were diluted in 1.5 mL of distilled water. Subsequently, 0.05 mL of this solution was put on the surface of a sample support and

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oven-dried at 30°C for 24 h. High-resolution images were acquired and processed using

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the software ImageJ v.1.5 to measure the diameter of the nanofibers.

2.6 Preparation of CNF films

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CNF films were prepared by casting and filtration methods, with a nominal

grammage of ~50 g/m2 or 100g per sample. CNF pulp with ~97% moisture was diluted in distilled water to reach a concentration of 3.10-3 g.mL-1, and homogenized in a magnetic stirrer for 20 seconds. The produced material was homogenously dispersed under a polyester circular film (125 μm) with 150 mm diameter placed in a Petri dish. CNF films were kept in a climatic chamber (23 °C temperature and 50% relative 8

humidity) for 48 h and subsequently dried in a Rapid-Köethen apparatus as recommended by TAPPI T205 sp-95 and ISO 5269-2. Thickness of CNF films was determined in a thickness tester (ME-1000, Regmed) as described in TAPPI T411 om97.

2.7 Characterization of CNF films X-ray diffraction patterns of the CNF films were determined with a Bruker AXS diffractometer. The equipment was operated at 40kV, 20 mA, with a Cu-Kα radiation

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source (ʎ= 0.15418 Å) and scanning rate of 5°.min-1. Diffractograms were smoothed through the Savitsky-Golay method (polynomial order = 2, smoothing points = 50). The crystallinity index was calculated from the peak height ratio between the intensity of the

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crystalline peak (I002-IAm) and the total intensity (I002) after subtracting the

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background signal (non-crystalline), measured without cellulose according to the method developed by Segal, Creely, Martin & Conrad (1959) and described by Polleto,

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Ornaghi & Zattera (2014).

Static and dynamic mechanical behavior of ramie nanocellulose films was

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investigated by tensile strength and elongation at break tests. Static tensile strength and elongation at break were determined with a Brookfield CT3 texture analyzer (USA) as

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described by ASTM D882-02. Five samples with size of 80 x 25 mm (height x width) for each treatment were cut from the rami nanocellulose films. The equipment was set to

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operate with span of 20 mm and velocity of 1 mm.s-1. Dynamic mechanical analysis (DMA) was performed using a DMA Q800 (TA Instruments) in tensile mode. The equipment was set to operate at 1 Hz frequency over the temperature range of 20250°C at a heating rate of 3°C.min-1. Samples with nominal dimensions of 37 x 6 mm (length x width) were cut from the rami nanocellulose films. The tests were performed in triplicate. 9

2.8 Physical durability of CNF films Solubility in water (Ws) was measured following the method described by Gontard, Duchez, Cuq & Guilbert (1994) and Nazan Turhan & Sahbaz (2004). Samples with 20 mm diameter were oven dried at 105°C for 24 h followed by immersion in 50 mL of distilled water and stirring (150 rpm) at 25 °C using a Tecnal TE-421orbital shaker. Subsequently, the samples were oven dried again at 105°C for 24 h to determine the solubility. Moisture content (Mc) was determined with an infrared moisture meter (Marte

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model ID-200). Water activity (Aw) was measured with an AquaLab CX-2 device. CNF films were placed inside a chamber containing a stainless-steel mirror and were

repeatedly cooled and heated at 25 °C to reach the equilibrium by mass. Barrier

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properties were investigated by water vapor (Wvp) and oxygen permeabilities (Op). The

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WVP was determined as described by ASTM E96/E96M and adapted by Nazan Turhan & Sahbaz (2004). The Op was measured using a Gurley porosimeter (Regmed, model

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PGH-T) in three distinct positions. The passing range for air permeability measurements

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was 5-1800 s for 100 mL of air volume, recommend by TAPPI T460 om-02 (2006).

3. Results and Discussion

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3.1 Optimal conditions for organosolv alkaline pulp (AOP) production Preliminary studies of cooking liquor composition indicated that the most favorable

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condition for organosolv delignification was 50% ethanol, 50% of a mixture of distilled water with 6% NaOH, considering a hydromodulus of 6:1 (mass of fibers:liquor). The processing conditions for AOP production were temperature of 160°C and cooking pass of 60 minutes. The production of AOP resulted in a pulp yield of 73.5% and Kappa number of 12.5.

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3.2 Chemical composition and morphology of delignified ramie fibers In general, the chemical composition of ramie fibers presented higher values than other commercial fibers, which mean interesting properties like elevated mechanical resistance, thermal resistance and durability (Sarkar et al., 2010). Regarding the chemical composition of delignified ramie fibers sued in this study, the holocellulose content was 96.6 ± 0.5%, α-cellulose content was 79.4 ± 1.7%, hemicellulose content was 17.2 ± 0.8, and soluble and insoluble acid-lignin were 0.3 ± 0.2% and 0.4 ± 0.2%, respectively.

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Chemical processes for bleaching pulps in general promote changes in their

constituents. In treatments performed with NaOH, for example, the removal of lignin is less severe as it preserves the constituents of the fiber. Sarkar et al. (2010) and Li,

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Zhaoling, Ding & Yu (2015) found a chemical composition of raw ramie fibers of 86.9%

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(α-cellulose), 5.0% (β-cellulose), 14.0-17.9% (hemicelluloses) and 0.8-1.5% (lignin). Compared with those values, the values found in the present study are within the

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expected indexes.

The presence of hemicellulose in larger amounts in the cell wall is a positive

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aspect, since it leads to the swelling of the fibers and a consequent increase of contact areas (Modenbach & Nokes, 2014; Pickering, Aruan Efendy, & Le, 2016). According to

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Iwamoto, Abe & Yano (2008), hemicellulose inhibits the coalescence of microfibrils, contributing positively to the processes of nanofibrillation. In their experiments with

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pulps with high concentration of hemicellulose, they obtained CNFs with around 10-20 nm diameter through only one pass in the mill. In contrast, in the case of xylans, one of the most common components in hemicellulose, low concentrations favor the formation of fibril networks that can result in better, i.e., more structured, lignocellulose-based products (Arola et al., 2013). Taking this into account, among many natural fibers, ramie

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fibers present the lowest concentration of xylose, around 4.3% (Pandey, 2007), which can be a positive aspect to develop added-value products. The analyses of the microstructure performed by SEM illustrate details of the surfaces at different stages. Figures 2A and B show the surface of the fibers after organosolv alkaline pulping and Figures 2C and D show the fibers after the bleaching

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processes.

Fig. 2. SEM micrography of ramie fibers. (A) Fiber from AOP at 1000x; (B) Fiber wall of

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AOP at 15000x; C) Bleached fibers at 1000x; (D) Wall of bleached fiber of AOP at 15000x.

The morphological aspects of ramie fibers (Fig. 2A and 2B) corroborate the results previously described by Hearle & Greer (1970). In other words, ramie fibers present a rough surface with small grooves and fractures, easily identified through their dense cell wall. Fig. 2 shows the partially cleaned fibers, but still grouped in bundles. 12

Chemical processing for pulp purification promotes changes in the fibers’ constituents. Alkaline pre-treatment with NaOH is the most common and advantageous treatment because it exposes the maximum cellulose content, resulting in biomass swelling (Fig. 2D) (Nakano, Tanimoto, & Hashimoto, 2013; Modenbach & Nokes, 2014). The use of NaOH for delignification has been found to be efficient. Hydroxy ions from NaOH attack ester bonds present between lignin and cellulose or hemicelluloses (Lee, Hamid, & Zain, 2014). The effects of the delignification process can be observed in Fig. 2C and 2D (area

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depicted by white box), especially the rupture of cell walls, probably due to the NaOH penetration through the inner wall of the ramie fibers. However, many other aspects are commonly visualized, such as fiber degradation (Fig. 2C) due to the cellulose

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depolymerization and changes in crystallinity index (Icr); reduction of fibers’ length due

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to changes of linear chains and increase of amorphous regions; and swelling of the fibers (Fig. 2D) (Nakano, Tanimoto, & Hashimoto, 2013). Pickering, Aruan Efendy & Le

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(2016) observed that alkaline treatments and bleaching processes improve the

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accessibility to the fibers, as shown in our study in Fig. 2C.

3.3 Transmission electron microscopy (TEM) of ramie cellulose nanofibrils

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TEM images of cellulose nanofibrils produced with 1, 3, 5 and 10 passes are shown in Fig. 3-A, B, C and D, respectively. The cellulose nanofibrils did not present a

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homogeneous structure, being arranged as individual and/or a group of fibers enclosed by a cellulose matrix.

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passes; (C) 5 passes; (D) 10 passes.

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Fig. 3. TEM micrographs of ramie cellulose nanofibrils produced with (A) 1 pass; (B) 3

Quantitative analysis of the TEM images showed similar average values for the

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diameter of the nanofibrils. The diameter of nanofibrils was from 4 nm to 23 nm, which is similar to the values found by Sun et al. (2018). These results demonstrated that

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regardless the number of passes, the mechanical defibrillation had sufficient force to defibrillate the ramie cellulose fibers. Previous studies have illustrated the same

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behavior, showing the presence of nanofibrils from different renewable sources with 4100 nm diameter (Chirayil, Mathew, & Thomas, 2014; Nechyporchuk, Belgacem, & Bras, 2016). From an industrial and economic point of view, only one pass was enough to produce ramie cellulose nanofibrils. The same network with heterogeneous structure was previously observed by Zimmermann, Pohler & Geiger (2004). In general, cellulose nanofibrils have different 14

thickness and size, forming a reticulated structure (Kalia et al., 2011), and their aqueous suspensions can contain large fragments or incompletely defibrillated material (Andresen & Stenius, 2007). This heterogeneous structure observed in our study may be due to the high density of hydroxyl groups on the microfibrils’ surface (Zimmermann, Pohler, & Geiger, 2004), in which the quantity is related to the renewable source used for nanocellulose extraction (Moon, Martini, Nair, Simonsen, & Youngblood, 2011). Morphological characteristics of CNF obtained by mechanical defibrillation are strongly related to intrinsic nature of the raw material. Taniguchi & Okamura (1998)

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investigated the characteristics of CNF-based films produced by casting from wood pulp fibers, cotton fibers, tunicin cellulose, chitosan, silk fibers and collagen mechanically defibrillated with 10 passes. They observed a large range – from 20 to 90 nm - of

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thickness of nanofibers and attributed this variation to intrinsic characteristics of each

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raw material. On the other hand, Bufalino et al. (2015) obtained CNF from three Amazonian wood species and Eucalyptus grandis with smaller thickness, ranging from

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5 to 9 nm. They investigated the CNF production by mechanical defibrillation with 10, 20, 30 and 40 passes, and found a tendency to thickness homogenization of CNF using

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at least 20 passes in the grinder. Claro et al. (2018) needed 30 passes in the mechanical defibrillation to stabilize the thickness between 17 and 34 nm of CNF from

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Eucalyptus and Curaua fibers. Therefore, the degree of defibrillation is directly related to

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intrinsic characteristics of the fibers and the parameters adopted for CNF production.

3.4 X-ray diffraction of CNF films The patterns presented in Fig.4 A-B illustrate three crystalline peaks for the ramie

nanocellulose films, which are well defined as crystalline peak of cellulose I due to the presence of peaks around 16°, 22° and 34°, corresponding to (110), (002) and (004) in the crystalline planes. Crystallinity changed to 82.9% for AOP after pulping, and 85.2% 15

for 1-Brch and 85.8% for 2-Brch after the bleaching process (Fig. 4A). Such crystallographic planes are characteristic components of crystalline cellulose Iβ (Peng et al., 2013; French & Cintrón, 2013; French, 2014). Since NaClO2 (Brq-1) and NaOH (Brq-2) are oxidizing agents, they break down glycoside bonds of the crystalline cellulose chains, promoting the dissolution of amorphous substances such as pectin, lignin and hemicellulose, as well as the removal of extractive soluble in hot water. Such reactions are directly linked to the increase in Icr in the ramie fibers. This increase was also found by Xu et al. (2015) for nanofibrils

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isolated from coconut fibers.

In contrast, the increase of grinding passes to obtain CNF did not promote

significant changes in crystallinity. CNF films presented crystallinity indexes of 77.9%,

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76.5%, 77.0% and 76.3% for 1, 3, 5 and 10 passes, respectively (Fig. 4B). Variation in

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crystallinity index can be related to grinding passes applied to the pulp (Xu et al., 2015), influencing the tensile resistance (Bhatnagar & Sain, 2005). Since the preparation

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process is purely mechanical, such changes may be linked in part to hemicellulose

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decomposition and chain breakage of the crystalline regions.

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Fig. 4. X-ray Diffraction (XRD) analysis in: (A) Alkaline Organosolv Pulp-AOP, 1st Breanch-Brch-1, 2nd Breanch- Brch-2 and (B) CNF films with 1 Pass (1T), 3 Passes (3T), 5 Passes (5T) and 10 Passes (10T).

3.5 Mechanical properties and physical durability of CNF films Tensile strength and elongation at break decreased with increasing number of grinding passes (Fig. 5). Tensile strength declined by up to 53.7% and elongation at break diminished by up to 35.3% between 1 and 10 passes. Previous studies have

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observed that a greater number of grinding passes can reduce the aspect ratio of CNFs, and consequently negatively affect their properties (Chun, Lee, Doh, Lee, & Kim, 2011;

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Iwamoto, Abe, & Yano, 2008), as illustrated in our study (Fig. 5A).

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Fig. 5. (A) Tensile strength and (B) Elongation at break of ramie CNF films.

Mechanical resistance of CNF films depends on many factors, such as the

methods and parameters adopted in the production. For example: Claro et al. (2018) produced CNF films by continuous casting using Eucalyptus and Curaua nanofibers defibrillated in a Supermass colloider grinder with 30 passes at 5000 rpm. They found lower mechanical performance than our study, in which tensile strength ranging from 9 17

to 18 MPa. On the other hand, Chun, Lee, Doh, Lee, & Kim (2011) produced CNF in a high-pressure homogenizer at 1400 bar. The CNF nanopapers produced by vacuum filtration showed tensile strength from 71 to 135 MPa. Fig. 6A illustrates brittle fractures and absence of surface elastic deformations, i.e., abrupt rupture in the CNF films, while Fig. 6B shows rupture of the nanofibrils with strong surface anchoring of nanofibrils onto the cellulose matrix, presenting only internal fractures formed during the tensile tests. The decrease of elongation at break and the morphological pattern observed in Fig. 6A suggest loss of strength due to the rupture of

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fibrils and break of hydrogen bonds between the fibrils, especially in the middle of the films. These aspects were also observed by Siró & Plackett (2010) and Mao et al.

(2017). During the drying step, the energy of hydrogen bonds on the surface is higher

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than in the middle of the film, producing elevated surface tensions and rigidity (Mao et

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al., 2017). This suggests lower and weaker bonding forces between fibrils in internal

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areas of the films (Fig. 6A).

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Fig. 6. SEM micrographs of the fracture after tensile tests: (A) Rupture of the film; (B)

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CNF fracture on the surface; and morphology of the CNF films at 500x (C) and 8000x (D) of magnification. CNF films produced with 3 passes. White arrow refers to internal

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fracture and black arrows refer to anchoring of nanofibrils.

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Likewise, hemicelluloses degradation during grinding with larger number of passes may be related to the reduction of tensile resistance. According to Molin & Teder (2018),

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the presence of hemicelluloses is important to maintain high values of stiffness and tensile strength of paper sheets. Hemicelluloses contribute to the adhesion between cellulose nanofibrils in a dry state, improving stiffness and resistance of the films (Iwamoto, Abe, & Yano, 2008). Brittle fractures are commonly observed in the rupture of fibers for pulps with higher concentration of hemicelluloses (Molin & Teder, 2002). As observed in our study, many other researchers have reported the positive contribution 19

of hemicelluloses to stronger mechanical properties of cellulosic fibers and their products, in which greater concentrations can result in better fiber-fiber binding (Hannunksela & Holmbom, 2004; Schönberg, Oksanen, Suurnäkki, Kettunen, & Buchert, 2001). To further examine the effects of number of passes on the mechanical behavior of CNF films, they were subjected to dynamic mechanical analysis (DMA). The storage modulus (E’) first slightly increased due to the water loss of the films and then decreased with increasing temperature. This decrease of E’ as a function of

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temperature is related to the softening of the material and the loss of the ability to maintain the connections between the fibrils (Sheykhnazari, Tabarsa, Ashori, & Ghanbari, 2016).

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The E’ of CNF films (Fig. 7) decreased with increasing number of passes, showing

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lower elasticity and energy loss, especially for 1 and 3 passes. This behavior corroborates the pattern found in the elongation at break parameter (Fig. 6B). Average

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values of E’ were 10.78±1.94 GPa, 9.13±2.49 GPa, 8.50±0.56 GPa and 5.79±0.71 GPa for 1, 3, 5 and 10 passes, respectively. This suggests higher hemicelluloses

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degradation found in CNF produced with 5 and 10 passes, and consequently poorer interconnected network (fiber-fiber interactions) of ramie cellulose nanofibrils in the

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films, which may contribute to create a material with undesirable viscoelastic properties. Thus, CNF films produced with greater number of passes, especially 10 passes, could

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be unuseful in real-life conditions that require high mechanical performance due to their higher brittleness.

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Fig. 7. Illustration of storage modulus (E’) measured by tension test in DMA of CNF

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films produced under different conditions.

Practical tests are useful to investigate the durability of nanostructures, providing

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more information about their behavior under real-life conditions (Table 1).

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The moisture content of CNF films was between 10.9% and 12.4%, slightly above results found by Kumar et al. (2014), 9.72%. According to Hubbe, Ferrer, Tyagi & Yin

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(2017) and Wang et al. (2017), satisfactory moisture properties normally do not provide superior oxygen barrier properties, a fact also observed in our study.

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Ramie CNF films presented similar water solubility (Ws) regardless of the number of grinding passes, ranging from 1.85±0.57 to 2.43±1.17%. The Ws is useful to estimate

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the degree of protection of products like foods, especially when the films have intensive water activity (Aw) (Machado et al., 2014). The Aw at 25°C presented average value of

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0.69, which is in agreement to the range (0.6-0.7) to avoid microbial growth in most types of foods (Bell & Labuza, 1992). The CNF films presented average oxygen permeability (Op) of 1.8 x 103

cm3.μm(m-2.day-1. atm), which, according to the TAPPI T460-om 02 standard characterizes them as impermeable material. This also corroborates the observations of Hult, Lotti & Lene (2010), since values less than 75 cm3.μm/(m-2.day.atm) correspond to 21

a material with high oxygen barrier properties. Syverud & Stenius (2009) reported oxygen transmission rates of 17.75 ±0.75 cm3.μm/(m-2 day-1.atm). Regardless of the degree of defibrillation, the results of resistance to Op are always high (Aulin, Gällstedt, & Lindström, 2010). All mechanical conditions used to produce nanocellulose films resulted in material with lower Op. This can be partially explained by the morphological pattern (Fig. 6C-D), a uniform, non-porous and compact surface. Likewise, water vapor permeability (Wvp) presented very low average values, ranging from 51 to 67 g.H2O m-2.day-1.mmHg-1. Aulin, Salazar-Alvarez & Lindström

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(2012) reported Wvp rate of 70.3 g.H2O m-2.day-1 at 50% moisture content.

These excellent barrier properties found by us agree with previous studies that have illustrated outstanding oxygen barrier performance (Cheng, Zhang, Cha, Yang, &

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Jiang, 2016; Rojo et al., 2015; Dufresne, 2013; Spence, Venditti, Habibi, Rojas,

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&Pawlak, 2010; Spence et al., 2011) and water vapor barrier performance (Baiet al., 2015; Ferrer, Salas, & Rojas, 2016; Lundahl et al., 2016; Rojo et al., 2015; Spence,

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Venditti, Rojas, Pawlak, & Hubbe 2011) for other lignocellulosic raw materials. These barrier properties and lower water solubility found in our study may be related to the

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dense, uniform and non-porous network formed by the ramie cellulose nanofibrils, as observed in SEM images (Fig. 6C and 6D), and as evidenced for other materials by

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Aulin, Gällstedt, & Lindström (2010) and Belbekhouche et al. (2011). Moreover, hornification of cellulose fibers during water desorption can contribute to notable barrier

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properties of CNF films (Fernandes Diniz, Gil, & Castro, 2004). Overall, the ramie fibers can be considered as a potential raw material to produce

high-quality cellulose nanofibrils and their derivative nanomaterials, by a simple and more sustainable method of organosolv pulping and mechanical defibrillation.

4. Conclusions 22

The alkaline organosolv pulping and mechanical defibrillation are simple and effective methods for ramie nanofibrillated cellulose manufacture, resulting in higher yields and satisfactory production of nanostructures. Production of ramie alkaline organosolv pulp with low lignin content and higher hemicelluloses content was a positive aspect, indicated by the production of CNF films with a dense and non-porous network. Unlike the crystallinity index behavior, severe grinding conditions (5 and 10 passes) significantly influenced the static and dynamic mechanical performance of CNF

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films. Tensile strength, elongation at break and storage modulus decreased with

increasing number of grinding passes during the defibrillation, suggesting poorer fiberfiber interactions, reduction of hydrogen bonds and degradation of hemicelluloses.

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Excellent barrier properties, like lower water vapor and oxygen permeability, were

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achieved in all grinding conditions, highlighting ramie’s potential for production of CNF films and broad applications like food packaging and reinforcing agents for

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Acknowledgements

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biodegradable polymers.

We are grateful to the company Itimura Têxtil S/A (Londrina, Brazil), for supplying

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the raw materials and infrastructure, the Pulp and Paper Laboratory, Electron Microscopy Center (CME-UFPR), Multi-user Center for Characterization of Materials

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(CMCM-UTFPR), Center for Research and Processing of Foodstuffs (CEPPA-UFPR), Laboratory for Wood Anatomy and Quality (LANAQM-UFPR), X-Ray Optics and Instrumentation Laboratory (LORXI-UFPR) and Embrapa Florestas for the technical support. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) – Finance Code 001.

23

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Figure captions

Fig. 1. Schematic flowchart of key steps for the research methodology.

Fig. 2. SEM micrography of ramie fibers. (A) Fiber from AOP at 1000x; (B) Fiber wall of AOP at 15000x; C) Bleached fibers at 1000x; (D) Wall of bleached fiber of AOP at 15000x.

Fig. 3. TEM micrographs of ramie cellulose nanofibrils produced with (A) 1 pass; (B) 3 passes; (C) 5

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passes; (D) 10 passes.

Fig. 4. X-ray Diffraction (XRD) analysis in: (A) Alkaline Organosolv Pulp-AOP, 1st Breanch-Brch-1, 2nd

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Breanch- Brch-2 and (B) CNF films with 1 Pass (1T), 3 Passes (3T), 5 Passes (5T) and 10 Passes (10T).

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Fig. 5. (A) Tensile strength and (B) Elongation at break of ramie CNF films.

Fig. 6. SEM micrographs of the fracture after tensile tests: (A) Rupture of the film; (B) NFC fracture on the surface; and morphology of the CNF films at 500x (C) and 8000x (D) of magnification. CNF films

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nanofibrils.

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produced with 3 passes. White arrow refers to internal fracture and black arrows refer to anchoring of

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Fig. 7. Illustration of storage modulus (E’) measured by tension test in DMA of CNF films produced under different conditions.

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Table 1. Mean values of moisture content, water activity, water solubility, oxygen permeability and water vapour permeability for ramie nanocellulose films produced under different conditions. Water vapour Water Number of

Moisture

passes

content (%)

Water permeability (Wvp)

activity

solubility at (g.H2O mm- 2.day-

(Aw)

25°C (%) 1.mmHg-1)

10.9 (2.03)

0.64 (0.03)

1.85 (0.57)

5.1 (0.14)

3

10.6 (0.77)

0.70 (0.03)

2.07 (0.76)

5.7 (0.15)

5

12.3 (1,22)

0,71 (0.01)

2.18 (1.06)

6.2 (0.24)

10

12.4 (1.75)

0.72 (0.02)

2.43 (1.17)

6.7 (0.11)

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1

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Values between parenthesis are the standard deviation.

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