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Water-based Approach to High-Strength AllCellulose Material with Optical Transparency Xuan Yang, and Lars A. Berglund ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02755 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

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ACS Sustainable Chemistry & Engineering

Water-based Approach to High-Strength AllCellulose Material with Optical Transparency Xuan Yang and Lars A. Berglund* Wallenberg Wood Science Center, Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden KEYWORDS: ramie, all-cellulose composite, compression molding, mechanical strength, interface, high transparency, low moisture sorption

ABSTRACT. All-cellulose composites are usually prepared by a partial cellulose dissolution approach, using of ionic liquids or organic solvents. Here, an all-cellulose film based on moist ramie fibers was prepared by hot-pressing. The original ramie fiber was degummed, alkali treated, aligned and mounted into a specially designed mold. The wet ramie fiber “cake” was pressed into a transparent film. The structure, mechanical properties, moisture sorption and optical properties of the films were investigated using scanning electron microscopy (SEM), Xray diffraction, tensile tests, gravimetric method and integrating sphere devices. The all-cellulose films showed an ultimate strength of 620 MPa and a Young’s modulus of 39.7 GPa with low moisture sorption, and optical transmittance of 85%. These eco-friendly all-cellulose films are of interest for laminated composites, as coatings and in photonics applications.

ACS Paragon Plus Environment

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INTRODUCTION Green composites based on natural cellulosic fibers and biobased polymer matrices are of interest as sustainable and environmentally friendly composite materials.1,2 Although cellulosic composites are advantageous due to their origin from renewable plant resources, and biodegradability, there are limitations in terms of mechanical performance and moisture sensitivity. A critical consideration for failure properties and moisture durability is control of the interface between fiber and matrix.3,4 Challenges may include inhomogeneous fiber distribution, poor fiber-matrix adhesion, insufficient stress transfer, and moisture sorption. By using the same polymer as fiber and matrix, good interfacial compatibility is achieved.5,6 Cellulose, in the form of nanoscale cellulose fibrils, is a critical structural component of the native plant cell wall due to its high mechanical properties.7 Here, the term fiber refers to a single plant fiber sclerenchyma cell,8 whereas microfibrils or fibrils are reinforcing components of the cell wall in that fiber cell. There are several studies where cellulose fibers have been combined in such a way that the “matrix phase” or the “binder” between fibers also consists of cellulose, forming all-cellulose composites.9 Zelfo® is a commercially available material processed by the casting or molding and drying of a cellulosic wood fiber pulp. The material has a density of 0.5– 1.5 g/cm3, a Young’s modulus of 1.5–6.55 GPa, and a tensile strength of 7–55 MPa.10 Using this approach, Zelfo®-type composites with Young’s moduli and tensile strengths as high as 17 GPa and 120 MPa, respectively, have been obtained using either wood fiber pulp11,12 or flax fiber.13All-cellulose composites have also been prepared by first dissolving cellulose in ionic liquid, NaOH/urea solvent or dimethylacetamide/lithium chlorite (DMAC/LiCl) solvent to form the matrix phase. Different types of cellulose reinforcements have been used, including, cellulose nanofibrils,14,15 cellulose microfibers,16 and cellulose nanocrystals.17–19 The properties of the


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composites include Young’s moduli and tensile strengths are as high as 20 GPa and 210 MPa, respectively. However, due to the use of relatively short fiber elements, the tensile strength of these composites is limited. All-cellulose composites have also been obtained based on continuous regenerated cellulose fibers20,21 or pre-formed fiber network including papers22 and textiles,23 though the Young’s moduli and tensile strengths are below 10 GPa and 150 MPa, respectively. Zhu et al.24 prepared an all-cellulose film by compressing wood veneer, and obtained a tensile strength of 350 MPa. However, the mechanical properties of the final film are limited by the original structure of wood veneer. Composites based on ramie fibers, with longer fiber length and smaller microfibril angles (MFA), may potentially impart improved mechanical properties to all-cellulose composites. Ramie is an abundant fiber source in China, where it is harvested 2–3 times per year with high yield. A single ramie fiber cell, stripped from the outer culm of the ramie plant, has a typical length ranging from 60–250 mm and a diameter of approximately 30 µm. Ramie is one of the strongest plant fibers, with a single fiber tensile strength of 500–1000 MPa and a Young’s modulus of 20–60 GPa.25,26 Yang et al.27 reported a ramie fiber-reinforced regenerated cellulose film with a tensile strength of 124 MPa and a Young’s modulus of 6 GPa. Nishino et al.28 used dissolved kraft pulp fibers and regenerated the cellulose matrix in the presence of ramie fibers to achieve an all-cellulose composite. The tensile strength and Young’s modulus were as high as 400 MPa and 25 GPa, respectively. Qin et al.29 partially dissolved ramie fiber and then regenerated the cellulose in-situ to form a matrix surrounding the undissolved ramie. The composite had a tensile strength of 540 MPa and a Young’s modulus of 25 GPa. Nattakan et al.30 used a similar method and achieved a strength of 460 MPa and a Young’s modulus of 28 GPa.


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All these methods, however, use an environmentally hazardous solvent (DMAc/LiCl) to dissolve the ramie cellulose. In previous work,11–13 water-based processing was combined with hot-pressing for random-inplane short wood pulp fiber materials. This limits the modulus and strength. Here, we adapted and modified this method to prepare all-cellulose films based on ramie fibers with high fiber orientation, high mechanical properties, reduced moisture sorption and high optical transparency. Degumming treatment of the raw ramie give fibers with high cellulose purity (> 93 wt.%). This is expected to improve the cellulose-cellulose adhesion at the interfaces between neighboring fibers. Alkali treatment may further improve interfaces and strength by increasing the mobility of cellulose surface chains of each fiber during consolidation.31–34 Furthermore, high cellulose purity may improve transparency of the films.

EXPERIMENTAL SECTION Materials. Ramie fiber in pre-dried stem form was purchased from the local market in Hunan Province. Ethanol (EtOH, ≥99.8%), acetone (≥99.5%) and toluene (Analysis Emsure) were purchased from VWR, acetic acid (>90%) was purchased from Fisher Scientific, and sodium chlorite (NaClO2, 80% RT), sodium acetate (≥99%), potassium nitrate (KNO3, ≥99%) ethylenediaminetetraacetic acid (EDTA, ≥99%), and sodium hydroxide (≥98%) were purchased from Sigma Aldrich. Degumming of ramie fiber. Ramie fibers were peeled from the stem as strips, and then were delignified/degummed in an acetate buffer solution of NaClO2 (1 wt. %, pH = 4.6) for 3 hours at 80 °C. The fiber was then carefully washed in distilled water.


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Alkali treatment of ramie fiber. Degummed ramie fibers were fixed to the mold first at one end, 20 N of tension was applied to stretch the fibers before the other end was fixed to the mold (Figure 1). Then fibers were soaked in 15 wt.% NaOH (20 ml for 1 g of ramie fiber) at room temperature for 2 hours. Then fibers were carefully washed in distilled water containing acetic acid (1 wt. %) to neutralize any excess NaOH. This was followed by rinsing in distilled water. Chemical composition. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra were recorded at room temperature across the frequency range of 4000−600 cm-1 using a Spectrum System 2000 FT-IR spectrometer (Perkin Elmer). The resultant spectra were an average of 32 scans obtained at a resolution of 4 cm−1. Quantitative analyses of the different components were conducted as described below. First, wax was extracted using a mixed solution of EtOH/toluene (1:2 v/v, 3 h). Next, water-soluble components were extracted into boiling water (3h). Pectin was then extracted using EDTA (0.5 M) under steam for 1 h.25 The lignin (Klason lignin) content was determined using a TAPPI method (TAPPI T 222 om-02).35 Finally, carbohydrate analysis was conducted using a Dionex ICS-3000 ion chromatography system (Thermo Fisher Scientific Inc., USA), and the result was listed in Table S1. The hemicellulose and cellulose contents were back-calculated from the carbohydrate analysis results, considering the sugar components of ramie hemicellulose.36 Film preparation. Treated ramie strips were mounted onto a specially designed mold (80 × 10 mm) in the longitudinal direction, with tension applied to prevent shrinkage and distortion (Figure 1). A typical treated ramie strip was 100 ± 30 µm thick and 1–3 mm wide, where the variation in size was related to the use of different stems and stem parts. Generally, two layers of ramie strips (referred to as the “cake”) were deposited into the compression mold, followed by


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compression of the still wet “cake” under a fixed pressure (either 25, 75, 125 or 175 MPa) at 105 ºC for 25 min. X-ray diffraction (XRD). XRD measurements were carried out using an ARL™ X'TRA Powder Diffractometer (Thermo Fisher Scientific Inc., USA) with CuKα radiation generated at 45 kV and 44 mA. Scans were obtained from 5 to 50 degrees 2θ in 0.02 degree steps at a rate of 1 second per step. Two different methods were used to calculate the crystallinity index (CI).37,38 In the first method (Peak height), the CI was calculated from the height ratio between the intensity of the crystalline peak (I200 - IAM) and total intensity (I200). In the second method (Peak deconvolution), individual crystalline peaks were extracted and fitted by a curve-fitting process using a Gaussian function, while the broad peak located at approximately 21.5º was assigned to the amorphous contribution. Iterations were repeated until a R2 value of 0.997 was reached. Wide-angle X-ray Diffraction (WAXD). WAXD was performed using a single crystal X-ray diffractometer (Bruker D8 VENTURE, USA). Two-dimensional diffraction patterns were recorded by mounting the film perpendicularly with respect to the incident beam. From the azimuthal intensity distribution from the equatorial (200) reflection in the diffractograms, two orientation factors, namely the degree of orientation (П) and Hermans orientation parameter (),39 were calculated according to equations 1–3. The FWHM is the full width at half maximum, using Gaussian peak fitting. is the azimuthal angle (angle between the axis of drawing and the axis of the cellulose crystallites), and is the intensity along the Debye-Scherrer ring. The and were then used to calculate f value. = 1 corresponds to maximum orientation parallel to the direction of drawing, whereas = 0 corresponds to random orientation of the fibrils. П =

- Equation 1


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=

〈 〉 =

〈 〉

⁄ ∑$" ! % ⁄ ∑" $ ! %

- Equation 2

- Equation 3

Mechanical properties single fibers and all-cellulose films. The tensile strength and Young’s modulus of individual ramie fibers were tested in accordance with the following ASTM D3379-75 Standard Test Method.40 The diameter of the single fiber was measured by optical microscopy (Olympus, Japan) and is reported as an average of a minimum of 10 different areas for each individual fiber. For a single fiber, tensile tests were performed using an Instron 5944, equipped with a 250 N load cell, at a gauge length of 25 mm and a strain rate of 10% per min. For the all-cellulose films, tensile tests were performed using an Instron 5566, equipped with a 2 kN load cell, at a strain rate of 10% per min. These conditions are similar to Nishino et al.28 and Soykeabkaew et al.30 The width and gauge length were 3 mm and 25 mm for tests performed in the longitudinal direction, and 5 mm and 5 mm for tests in the transverse direction. Prior to testing, samples were conditioned at a relative humidity of 50% and 23 °C for at least 3 days. For each type of sample, more than 8 different fibers/films were tested. The Young’s modulus of each sample was calculated in elastic region (strain range of 0 to 0.2%). Scanning electron microscopy (SEM). The nanostructures of the single fibers and allcellulose films were probed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4300, Japan). Prior to imaging samples were sputter-coated (Cressington 208HR, UK) with a conductive palladium layer (30 s sputtering, ~ 5 nm layer). Moisture sorption. The moisture uptake was calculated based on a single measurement according to the equation 4, where &'() is the sample weight equilibrated at certain relative


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humidity (RH) and &%*+ is the dry weight of sample at 0 RH%. Dry samples were dried at 105 °C for 7 days and then directly stored in a vacuum desiccator (for two days) with silica gel, before measurement of weight. 50 RH% was obtained in a lab room with precise climate control. 95 RH% was obtained by placing saturated KNO3 solution in a sealed container. All the experiments were carried at 23 °C. Five samples were tested for each type of fiber or film, and the weight was measured using an analytical balance with an accuracy of 0.0001g. ,-./01 /2.341 =

567 89: 89:

- Equation 4

Optical Properties. According to the procedure described by Li et al.,41 transmittance and haze were measured using an integrating sphere, equipped with a high brightness light source, whose spectrum spanned the UV to near-IR wavelengths (170−2100 nm; EQ-99 from Energetiq Technology Inc.). The transmittance spectra (including both specular and diffuse transmittance) of the samples were recorded, and baseline corrected using a blank spectrum obtained without sample in the beam path. Haze measurements were performed according to the ASTM D1003 “Standard Method for Haze and Luminous Transmittance of Transparent Plastics”.42

RESULTS AND DISCUSSION Chemical Composition. The chemical composition of the original, degummed, and alkali treated fiber (NaOH-treated fiber) are presented in Table 1. The original ramie fiber had cellulose, hemicellulose, and lignin contents of 77.8%, 10.2%, and 2.6%, respectively. This is similar to values reported in other studies.36,43 After degumming, only cellulose (93.2%) and hemicellulose (6.8%) remained. The NaOH-treated ramie fibers had a similar composition, with a small decrease in the hemicellulose content. ATR-FTIR spectra (Figure S1) further support the observed changes in chemical composition after degumming and alkali treatment. The color of


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ramie strips turned from yellow to white, as shown in Figure 1, due to the removal of lignin. The high cellulose content is expected to improve cellulose-cellulose adhesion during hotpressing.11,44 We carefully prepared ramie fibers from whole stems to ensure minimal fiber damage.

Table 1. Chemical composition of original, degummed, and NaOH-treated ramie fibers

Original Degummed NaOH-treated

Wax

Water soluble

Pectin

Lignin

Hemicellulose

Cellulose

0.6% N/A N/A

7.7% N/A N/A

3.7% N/A N/A

2.5% N/A N/A

9.9% 6.8% 6.1%

75.6% 93.2% 93.9%

Crystallinity Peak Peak height deconvolution 0.87 0.65 0.92 0.72 0.90 0.70

Crystallinity. XRD measurements were conducted to evaluate and compare the crystallinities of the original, degummed, and NaOH-treated fibers. The results are shown in Table 1 and typical XRD curves are shown in Figure S2. The original fibers are highly crystalline prior to any pretreatment (0.87 using the peak height method and 0.65 using the peak deconvolution method), which is similar to other data.25,26,45 The crystallinity of the fibers increase after the degumming process, since amorphous components like waxes, pectin, and lignin are removed. NaOH treatment decreases the crystallinity slightly. During NaOH treatment, cellulose surface chains lose their crystalline order locally, and the degree of crystallinity is reduced.46–48 It’s also known that alkaline cellulose can return to cellulose I at some conditions.49,50 A mild NaOH treatment was employed (room temperature, short time) to promote cellulose-cellulose adhesion, but preserve the original crystalline bulk structure in the ramie fibers. Additionally, the ramie fibers were maintained under tension during NaOH treatment in order to retain the high


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crystallinity of the fibers and to minimize/prevent the transition from cellulose I to cellulose II.51,52

Film preparation. Never-dried treated (degummed- and NaOH-treated) ramie strips were longitudinally fixed in the mold, followed by hot pressing to give the final all-cellulose film (Figure 1). A temperature of 105 ºC and a processing time of 25 min facilitated efficient removal of water during the hot pressing. Higher temperatures or longer processing times will degrade the cellulose and hemicellulose, resulting in a yellowish color and reduced mechanical properties. The effect of increasing pressure from 25, 75, 125 to 175 MPa resulted in an increase in film density from 1.21 ± 0.06, 1.46 ± 0.08, 1.53 ± 0.03, to 1.54 ± 0.05 g/cm3, respectively. Since the density of pure cellulose is 1.5–1.6 g/cm3,53,54 a pressure of 125 MPa was sufficient to impart a good interface between different fibers while minimizing interface gaps. SEM micrographs also support the presence of very few interface debond gaps between different fibers, when 125 MPa pressure was used. The final films based on degummed and NaOH-treated fiber are highly transparent as evidenced by the appearance of the university logo below the film (Figure 1). The optical transmittance of the films will be discussed later.


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Figure 1. Photographs of ramie strips (original, degummed and NaOH-treated) and the corresponding hot pressed films placed on a KTH-logo surface. Schematic representation of the mold used in the hot pressing process (center).

Fiber Orientation. The diffraction patterns and corresponding azimuthal intensity distribution figures for WAXD measurements are presented in Figure 2. The calculated degree of orientation (Π) and the Hermans orientation parameter (f) are listed in Table 2. The calculated Π and f values of both films are higher than the original ramie fiber strips. The final films possess a degree of orientation (Π) above 0.9 and a Hermans orientation parameter (f) above 0.8. Data are significantly higher than in other reports of all-cellulose films (f = 0.29),55 oriented cellulose


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nanofiber papers (f = 0.72),39 and cellulose nanocrystal-cellulose acetate nanocomposites (f = ~0.7).56

Figure 2. WAXD data and the corresponding azimuthal intensity distributions of the original ramie strip, degummed fiber film, and NaOH-treated fiber film.

Table 2. Degree of Orientation (Π) and Hermans Orientation Parameter (f) of original ramie strip, degummed fiber film, and NaOH-treated fiber film.

FWHM (°) Degree of orientation, П Hermans orientation parameter, f

Strip (original) IP

Film (degummed) IP

Film (degummed) CS

Film (NaOH-treated) IP

Film (NaOH-treated) CS

18.4 0.90

12.7 0.93

12.9 0.93

13.8 0.92

13.9 0.92

0.77

0.86

0.85

0.83

0.82


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It is challenging to achieve high cellulose fibril orientation factors in materials based on plant fibers. There are f values as high as 0.96 in films based on highly oriented cellulose rod-like microcrystals prepared by alignment by rotation on a drum.57 The Hermans orientation parameter (f) describes the alignment of crystallites in the material. In the present case of ramie fibers, this corresponds to the alignment of cellulose nanofibrils in the fiber cell wall.58 In the present film, f values also depend on the native structure of the plant fiber, including the microfibril angle (MFA). But if we assume that the MFA is 7.5° for a ramie fiber,25,59 the value of the Hermans orientation parameter (f) of a ramie fiber is as high as 0.974 (Equation S7, with calculation details described in the supporting information). This means, that the present value of f = 0.86 could be improved even further by better fiber alignment. Few studies of all-cellulose composites have quantitatively reported fiber orientation, instead it is more common to present diffraction patterns. Furthermore, it is noted that for the present all-cellulose films, Π and f values were similar for in-plane (IP) and cross-sectional (CS) directions.

Morphology. SEM images show the morphology and nanostructure changes after degumming, NaOH treatment, and hot-pressing (Figure 3). Compared to the original ramie fiber, treated (degummed and NaOH-treated) ramie fibers have a significantly smoother surface (Figure 3AC; longitudinal section), with the gum between individual fibers removed (Figure 3A-C; at cross section). This observation confirms the successful degumming in which the wax, lignin, and pectin are removed. SEM images of films along the longitudinal fiber axis (Figure 3D and 3E) confirm that the fibers are highly oriented.


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Interface debond gaps were not so numerous in the longitudinal and cross sectional views of all-cellulose films (Figure 3D and 3E). Fibers appear well attached to one another with minimal gaps, despite the use of water rather than environmentally hazardous solvents such as DMAc/LiCl.28–30 Viewed along the long-axis of the fibers, the film prepared from degummed fibers appears to be composed of individualized fibers, whereas the fibers in the film prepared from NaOH-treated fibers appear better integrated. This indicates improved interface bonding achieved by NaOH treatment. We note that NaOH treatment will mobilize the cellulose surface chains of each fiber,29,31–34 and promote better adhesion between neighboring fibers. This is also supported by the cross-sectional image (Figure 3E).

Figure 3. SEM images of A) original ramie strip, B) degummed ramie strip, C) NaOH treated ramie strip, D) film prepared from degummed fibers, and E) film prepared from NaOH-treated fibers viewed in the longitudinal and cross directions.


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Mechanical properties. Films with high fiber orientation are expected to possess high axial mechanical properties. Figure 4A and 4B shows the typical stress–strain curves for different types of single fibers and films tested in the fiber direction, and Figure 4C shows the typical stress-strain curves of the films in the transverse direction. Mechanical properties are presented in Table 3. Fiber properties are of particular interest. The original fiber had a Young’s modulus of 33.5 GPa, an ultimate strength of 550 MPa, and a strain at break of 1.9%, similar to other reports.51,60 The degummed fiber had a Young’s modulus of 49.5 GPa and an ultimate strength of 760 MPa, representing significant improvements due to removal of non-structural components such as wax, pectin, and lignin. These data are similar to those for other chemically treated ramie fibers.28,30 Conversely, NaOH treatment caused a decrease in mechanical properties, with a lower Young’s modulus (32.8 GPa) and ultimate strength (570 MPa) but higher strain to failure (2.8%). This is different from other studies,29,51 where alkaline treatment could improve strength and Young’s modulus of fibers. In those studies, they use un-treated fibers as a starting point. Alkaline treatment will then remove non-cellulosic components. Cellulose content and crystallinity are increased, and fiber defects may to some extent be reduced. In our case, alkaline treatment was applied after degumming/delignification in order to improve the surface adhesion during solidification process, by swelling of the fibers.31–34 As an unwanted consequence, reduced crystallinity (Table 1) and associated structural changes46–48 may decrease the Young’s modulus and ultimate strength of NaOH-treated fiber.

Table 3. Mechanical properties of original, degummed, and NaOH-treated ramie single fibers, and two types of films prepared from either degummed fibers or NaOH-treated fibers (standard deviations reported in parentheses).


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Fiber (original) Fiber (degummed) Fiber (NaOH-treated) Film (degummed) Film (NaOH-treated)

Longitudinal Young’s modulus (GPa) 33.5 (5.4)

Longitudinal ultimate strength (MPa) 550 (70)

Longitudinal strain at break (%) 1.9 (0.6)

Transverse Young’s modulus (GPa) N/A

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Transverse ultimate strength (MPa) N/A

Diameter (µm)

Thickness (µm)

38 (8)

N/A

49.5 (2.1)

760 (50)

1.8 (0.6)

N/A

N/A

35 (5)

N/A

32.8 (7.2)

570 (80)

2.8 (0.6)

N/A

N/A

34 (7)

N/A

39.7 (3.9)

620 (60)

3.0 (0.4)

3.4 (1.0)

N/A

140*

30.5 (4.0)

550 (50)

4.0 (0.4)

4.6 (1.1)

N/A

140*

0.20 (0.04) 0.60 (0.05)

* The thickness variation within an individual sample was below 5 µm.


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Figure 4. Stress–strain curves of A) original, degummed, and NaOH-treated ramie single fibers, B) films tested in the fiber direction, C) films tested in transverse direction. D) Photograph of film fracture after tensile test at fiber direction. E) SEM images of film fracture surfaces after tensile tests in transverse direction.

Compared with the single fiber results, films prepared from these fibers had lower longitudinal mechanical properties and higher strains to failure. The probability of material defects is higher in a material with larger volume. This will lead to decreased strength. Also, imperfect alignment of fibers within the film will lead to decreased Young’s modulus and strength. The increased failure strain in films is probably a consequence of sequential fiber fractures, so that when final fracture is triggered, there are already fractured fibers in the material. The film prepared from degummed fibers had a Young’s modulus of 39.7 GPa, an ultimate strength of 620 MPa, and a strain to failure of 3.0%. The film prepared from NaOH-treated fibers had a Young’s modulus of 30.5 GPa, an ultimate strength of 550 MPa, and a strain to failure of 4.0%. We note films have more complex failure behavior than single fibers. Individual fibers, and possibly fiber bundles are fractured prior to final failure. This is manifested as a serrated appearance of the stress-strain curve and increased ductility (Figure 4B). This interpretation is supported by the jagged appearance of the fractured specimen, see Figure 4D. The present work has the highest mechanical properties reported for all-cellulose composites, see Table 4. The previous studies27–30 employ DMAC/LiCl to either prepare a cellulose solution as the matrix, or partially dissolve the ramie fibers. Thus, crystallinity decreases significantly, lowering the modulus of the final composite. In contrast, there is no obvious decrease in crystallinity for the present film (Figure S2). Moreover, composites in earlier studies27–30 have


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lower fiber volume fraction, below 85% (~100% for present study), which will lower the strength. The geometrical specimen size is included in Table 4, since it may influence mechanical properties. Specimen with longer gauge length may have lower strength, due to the higher probability of large fiber defects. Present data with 50 mm gauge length are still higher than previous data, see Table 4. The work of Wei et al.61 reported an all-cellulose composite based on straw cellulose fibers with a tensile strength of 650 MPa, but the strain rate in their experiment was 40% per min, which is significantly higher than the present (10% per min). The measured tensile strength is typically higher at higher strain rates,62,63 Indeed, the tensile strength for our film prepared from degummed fibers was 690 MPa at a strain rate of 40% per min. The use of a strain rate of 10% per min is suitable and used in the vast majority of tensile tests.64,65

Table 4. Mechanical properties, fiber volume, and thickness of all-cellulose composites based on ramie fibers.

Yang et al.27 Nishino et al.28 Qin et al.29 Nattakan et al.30 Present data

Longitudinal Young’s modulus (GPa)

Longitudinal ultimate strength (MPa)

6 20 ~20 28 39.7 37.8

124 480 540 460 620 590

Longitudinal strain at break (%) 3 ~4 3 3

Sample size at longitudinal direction Gauge Width length (mm) (mm) 50 10 20 50 20 3 25 3 50 5

Transverse ultimate strength (MPa) 15 22-40 3.4 3.4

Fiber volume

Thickness (µm)

Water-Based Approach to High-Strength All ... - ACS Publications

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