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From waste cotton linter—a renewable environment friendly biomass based carbon fibers preparation Xin Zhou, Penglei Wang, Yagang Zhang, Xuemin Zhang, and Yingfang Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01408 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 11, 2016

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ACS Sustainable Chemistry & Engineering

From waste cotton linter—a renewable environment friendly biomass based carbon fibers preparation

Xin Zhou,a,b Penglei Wang,a,b Yagang Zhang,a,b,c* Xuemin Zhang,a,b and Yingfang Jiang,a,b a

Center for Green Chemistry and Organic Functional Materials Laboratory, Xinjiang Technical Institute of Physics and Chemistry, The

Chinese Academy of Sciences, Urumqi 830011, China b University

of the Chinese Academy of Sciences, Beijing 100049, China

c Department

of Chemical & Environmental Engineering, Xinjiang Institute of Engineering, Urumqi 830023, China

*Corresponding

Author, E-mail: [email protected]

ABSTRACT: As one of high performance fiber materials, carbon fibers are practically important in various applications. Traditional methods for the preparation of carbon fibers are based on fossil fuels using poly(acrylonitrile) and mesophase pitch as starting materials. The preparation of carbon fibers based on renewable low-cost biomass is alternative and sustainable approach in green chemistry. Herein, an environment friendly low-cost approach is established to prepare carbon fibers from waste cotton linter though CarbaCell method using wet-spinning technology and carbonization process. The crude cellulose carbamate (CC) fibers and the one treated with dibasic ammonium phosphate (DAP) were investigated by TGA and DTG. The prepared carbon fibers were characterized with SEM, IR and contact angle measurement. Thermal behavior analysis indicated that the carbon yield increased by 133% with the use of DAP as impregnant. The SEM images showed these carbon fibers had relative smooth surface and approximate round compact morphology in cross-section. Without hotstretching and post-thermal treatments steps, the as–prepared carbon fibers carbonizing at 900°C reaches tensile strength around 0.72 Gpa and with the carbon yield up to 36.4%. This process provide a green approach for the preparation of carbon fibers based on a renewable resource.

KEYWORDS: Renewable carbon fiber, Cotton linter, CC, CC fiber, Thermal properties, Tensile strength

INTRODUCTION As one of high performance fibers, carbon fibers (CFs) are used in a broad variety of applications in composites, military aircraft, aerospace and sports industries due to the superior properties such as high strength and modulus, mechanical flexibility, light weight, chemical inertness, electrical conductivity, good anti-fatigue and corrosion resistance.1-7 Currently, although the commercial CFs are produced using precursors such as poly(acrylonitrile) (PAN), mesophase pitch (MP) and cellulose, most CFs production are dominantly based on fossil fuels (PAN and MP). For example, the CFs based on PAN precursor are most important one in the market due to their good mechanical property and high yield.8 However, with the rapidly consumption of fossil energy, not only the cost of CFs based on non-

ACS Paragon Plus Environment

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renewable energy increase, the environmental pollution due to the produce of toxic gases (e.g. hydrogen cyanide) using PAN as precursor

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cannot be omitted.9 As a result, it is urgent and practically important to develop a process preparing low-cost CFs based on renewable, non-toxic and sustainable resources in the view of sustainable development and environment protection. Bio-mass resource such as cellulose, lignin and wood are promising precursors to prepare CFs because of the advantages of renewable and low-cost features.10,11 As one of the most abundant renewable biomass resources in nature, cellulose, which has a long history being used as a precursor of carbon material. The first electrical light-bulb filament, using cotton and bamboo fibers as raw material for the production of carbon fibers, was prepared by Thomas Edison in 1880.12 However, the development of carbon filament based on cellulose had been in limbo later because of the invention of tungsten filament by Coolidge in 1910, which has superior properties such as simple processing technology, high productivity and longer service life. 13 Not until in late 1950s the research interest on carbon fibers were raised again due to its excellent mechanical properties.14 The high modulus carbon fibers based on viscose fibers by a post-carbonization stretching treatment became commercial available developed by Union Carbide in 1965.15 With the advent of PAN based CFs, the mechanical properties and overall performances are significantly improved.16 However CFs based on cellulose are still used due to the particular properties of lower thermal conduction coefficient and excellent ablation resistance resulted from the disorder crystal structure. 17 For cellulose based CFs industry, the traditional viscose process also is long, tedious and quite complex. Furthermore, it generates several environmentally hazardous substances including CS2, H2S and heavy metals.10,18 It is therefore desirable to find an economical and environmentally benign system to prepare regenerated cellulose fibers. Attempts have been made to prepare CFs with regenerated cellulose fibers, such as Lyocell fibers, CarbaCell process. Lyocell process uses N-methylmorpholine-N-oxide (NMMO) as solvent for cellulose resulting in improved properties of fibers such as higher tenacity, higher crystallinity and higher modulus.19 Technical problems associated with this process are high-spinning temperature and solvent recovery.20 CarbaCell process is an alternative method that has been developed to reduce hazardous substances, in this process, cellulose carabamate (CC) is stable enough at room temperature and can be stored for more than a year without loss of quality.10,21,22 It needs to be pointed out that the carbonization yield is crucial for CFs manufacturing. Cellulose, which has an ordered crystalline structure due to intra and inter molecular hydrogen bonds resulting from hydroxyl groups, is a glucose based, linear polymer connected by β-(1-4) glycosidic linkages.15 Although the theoretical carbon yield of carbonization process of cellulose structure is 44.4% according to the molecular stoichiometry (C6H10O5)n, the mass lose is up to 70-90%.8 This is due to the pyrolysis of cellulose, which is accompanied by depolymerisation reactions, leading to the formation of carbon monoxide, carbon dioxide, organic acids, tars alcohols, ketones and other carbon-containing low-molecular-weight substances.8,23 Some efforts have been made to understand the mechanism of cellulose pyrolysis and study the way to improve carbon yield. Carbon yield can be increased by slowing down heating rates during carbonization while with increased manufacturing cost.24,25 In addition to the carbon yield, molecular orientation, crystallite size and porosity will also influence the modulus and tensile strength of CFs.26 It was reported that the tensile strength and modulus of CFs based on rayon can be increased in magnitude by a graphitized treatment involving hot stretching above 2500 °C.27 However, this suffers being an energy consuming process. Yao et al.28 improved the orientation, tensile strength and carbon yield of CC by adding graphene oxide (GO). They reported that the Young’s modulus and tensile strength of the CC/GO composite increased by 280% and 180% respectively with the addition of 2 wt% of GO to the pure CC.


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ACS Sustainable Chemistry & Engineering It is important to develop cost effective, green and clean process for CFs manufacturing with improved mechanical properties and

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performance. To the best of our knowledge, little attention has been paid to the preparation of carbon fibers based on regenerated cellulose fibers derived from cellulose carbamate. Here we report an environment friendly biomass based carbon fibers preparation process using waste cotton linter—an abundant renewable nature resource. In our work, CC was produced by a green and recyclable system. Experimental conditions were optimized to improve the properties of CFs (e.g. tensile strength and yield of carbon fibers).

EXPERIMENTAL SECTION

Raw material and Chemicals Waste cotton linters (from cotton seed) were supplied by Xinjiang Aoyang Company. CC was made by an established procedure. Sodium hydroxide, sodium hypochlorite, xylene, hydrochloric acid, sulfuric acid (98%), aluminum sulfate, sodium sulphate, diammonium hydrogen phosphate (DAP) were all analytical grade.

The purification of waste cotton linter First, the waste cotton linters were washed, filtered and dried. Then they were blended in 5 wt% sodium hydroxide aqueous solution. The mixture was stewed in autoclave at 160 °C for 1.5 h. The cotton linter was bleached and treated with sodium hypochlorite (the chlorine content is about 1.5% of cotton linter) and dilute hydrochloric acid dosage (the content is about 2% of cotton linter) respectively to remove ash. The treated cotton linters were washed, dried, and then hydrolyzed with dilute hydrochloric acid. The average degree of polymerization (DP) of the refined cotton linter was determined in copper hexamethy-lenediamine solution using Ubbelohde viscometer, and the DP for qualified products was determined to be 1300-1600. CC fiber preparation Figure 1 exhibits different derivate methods for the preparation of regenerated fibers based on cellulose. The CarbaCell process is similar to the viscose method, the regenerated cellulose fibers were mainly prepared though alkalization, derivatization, dissolution and spinning process. However, compared to the viscose technologies, the CarbaCell procedure is relatively simple without ageing, blending and ripening procedures. Overall, CarbaCell method which can be done on the viscose spinning machines,10,22 is promising to prepare regenerated fibers. In this work, the qualified cotton linter was mixed with sodium hydroxide solution (mass ratio of 1:1), and the mixture was stirred at 20 °C for 4 h, and the alkali cellulose was obtained after filtration and washing to remove free of sodium hydroxide solution. The activated alkali cellulose was loaded into reactor of Oil-water separator, urea was added to the reactor with the mass ratio of 12:1 (alkali cellulose : urea), followed by addition of xylene with the mass ratio of 1:1.5 (alkali cellulose : xylene). After the dissolving of urea, the solution was heated up to 80 °C for 1 h under the stirring. Then the solution was quickly heated up to 130 °C after the removing of water in solution. The reaction was carried out at 130 °C for 5 h. After xylene was recycled at the end of the reaction, product was washed with hot water for 6 times. The gray white solid was obtained after filtering and drying. Finally, the obtained solid CC particles were shattered into small particles.


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The CC (6.5 g) were added into 9.1 wt% NaOH aqueous solution (100 g) at -10 °C, the mixture was stirred vigorously for 10 minutes.

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Mixed CC/NaOH solution was obtained, then followed by filtration with 300 mesh cloth, and were subjected to vacuum degasification at 0-5 °C for 4 h. Finally, the CC based regenerated fibers were obtained through wet-spinning procedure according to our patent.29 OH O HO

O

OH

OR O

Alkalization

O

RO

Alkalization

OR

(R:H or Na) Shredding OR O

Ageing

O

RO CS2 Wet chun

Derivatization

R:H or

OR

O C S- Na+

O C NH2

R:H or

Derivatization

Urea Xylene

Shredding Dissolution Dissolution CS2

Blending, Ripening Filtration, Deaeration

OH O HO

H2SO4

Spinning

OH

Filtration, Deaeration O

Spinning

H2SO4

CS2

viscose method

Carbamate method

Figure 1 Process for preparing regenerated cellulose: left, traditional viscose technologies; right, carbamate technologies.

As shown in figure 2, a self-designed wet-spinning apparatus with a spinneret of 500 orifices (diameter, 0.05 mm) was used to prepare CC fibers. Firstly, the degassed solution was passed through a filter controlled with a metering pump under the nitrogen atmosphere (0.2Mpa), and the spinning dope was spun into a coagulation bath composed of 14.4 wt% H2SO4, 3.6% Al2(SO4)3, 17.8 wt% Na2SO4 aqueous solution at 40°C. The multi-filaments from the spinneret were rapidly passed through coagulation bath, wash tank (40 °C) and regeneration bath (0.5 wt% NaOH aqueous solution, 95 °C) to improve the molecular orientation of CC fibers. Finally, the CC fibers were obtained after drying and rolling.

Figure 2 Schematic process to synthesis CC fibers via spinning method.

Carbon preparation


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Figure 3 shows the preparation process of CFs based on CC fibers. The CC fibers were washed with absolute ethyl alcohol and ultra pure

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water to remove impurities, and then dried at 120 °C for 2 h. After that, the fibers were impregnated into a 5 wt% DAP aqueous solution for 30 minutes, followed by wring, then drying under vacuum at 60 °C for 10 h. The pre-treated CC fibers were put into an oven under air atmosphere and kept for 30 minutes at each of the following temperature stages: 150 °C, 220 °C and 260 °C. During the process of oxidation, a heating rate of 5 and 1 °C /min were set from room temperature to 220 °C and 220 °C to 260 °C, respectively. Finally, the oxidized CC fibers were obtained.

Figure 3 The process of carbon fibers preparation using CC fiber as starting materials

During the carbonization, the oxidized CC fibers were heated at 300 °C for 30 min under nitrogen with the heating rate of 4 °C/min for the purpose of further dehydration, and then carbonized at 700, 900, 1100 °C respectively under nitrogen with the same heating rate. The final carbon fibers were obtained after washing and drying. During oxidation and carbonization process, all samples were wound onto a porcelain holder with the aim of constraining them to prevent physical shrinkage and to prevent the loss of molecular orientation. The carbon yield was calculated by dividing the sample’s mass measured after vacuum drying (Wa) by the dry mass measured after drying at 120 °C (Wb) according to the following Eq. (1). 𝐶𝑎𝑟𝑏𝑜𝑛 𝑦𝑖𝑒𝑙𝑑(%) =

𝑊𝑏 𝑊𝑎

× 100

Eq. (1)

Thermal Behaviour Analysis The thermal behavior analysis of CC fibers with and without treating with DAP were carried out by a thermal gravimetric analysis (TGA) instrument (STA449F3, Netzsch, Germany) under nitrogen atmosphere. The temperature scans were set in the range from ambient temperature to 800 °C at an increasing rate of 10 °C/min. SEM A field-emission scanning electron microscope (SEM) was used to observe the morphologies and microstructures of the CC fibers, oxidized fibers and CFs. The samples were imaged using a field-emission scanning electron microscope (SUPRA 55VP, Zeiss, Germany). The CC fibers and the soaked fibers were treated by a sputter coater instrument (E1045, Hitachi, Japan) prior to analysis. FTIR The fibers were characterized by Fourier Transform Infrared Spectroscopy (FTIR) (FTS165, BIO-RAD, USA) to monitor the surface functional groups using KBr plate. The specimens were scanned for 16 times in the wave number range of 400-4000 cm-1.


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X‐ray diffraction

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The CC fibers and CFs were evaluated by using a X-ray diffraction (XRD) to determine the crystal structure. Diffraction patterns of samples were recorded by the X-ray diffraction instrument (D8-ADVANCE, Bruker AXS, Germany). The samples were scanned in the angular range of 5-60° (2θ) using Cu Kα radiation (50KV, 40mA), and the scan rate was 2°/min. Raman spectra The Raman spectroscopy (Horiba Scientific, France, excitation-beam wavelength = 532nm) was used to characterize the properties of CFs. Single-fiber tensile test The mechanical property test of single fiber was performed using a testing machine (C43.104, MTS Criterion, USA) equipped with a 10 N load cell following the method of ASTM C1557-03. Single fiber was chosen randomly from a set of samples, separated from the bundle, mounted on a card table and fixed with ethyl cyanoacrylate superglue. The diameters of the fibers were measured at three points along the gauge length by optical microscopy (ECLIPSE LV100ND, Nikon, Japan). Both sides of tab were cut very carefully at mid-gage after the specimens had been fixed on the test machine. Over 20 fibers were tested for each set of fiber samples with a cross-head speed of 2 mm/min and a gauge length of 10 mm. Specimens fractured very close to the sample grips were discarded. The results of mechanical test were analyzed using a two-parameter Weibull distribution model. Contact angle test The contact angle measurement on single carbon fiber was carried out on Contact Angle Meter (JC2000DM, Shanghai Zhongchen Digital technical apparatus Co., Ltd, China) using ultra-pure water. Results are summarized and are average values based on 10 specimens for each sample, and each specimen was tested for 5 times.

RESULTS AND DISCUSSION Thermal behavior and yield The thermal properties of the precursors are crucial in the production of CFs. Generally, the deeper and narrower peaks of DTG curve indicate harsh conditions accelerating the side-reaction to release volatile carbon-containing substances, which are likely to lead to the poor and low carbon fibers yield. While more smooth pyrolytic peaks reflect stable reactions, which results in high quality and improved carbon yields.30 Figure 4 shows TGA and DTG curves of prepared CC fibers. The curves show that the precursor degrade rapidly at about 328 °C, and the DTG curve exhibits a sharp pyrolytic peak with the maximum mass loss rate (25.34 %/min weight loss at 343 °C), indicating drastic thermal decomposition reactions take place at this temperature, and the solid residue remains only 15 wt% of its initial mass at 800 °C. More rapid the decomposition, the greater amount of volatiles will be produced. The low carbon yield is mainly due to the formation of levoglucose and tar formation.9,31 As a result, it is very crucial for the preparation of CFs to tune the pyrolytic mechanism by treating raw CC fibers with different impregnant which would influence the quality and yield of the process. 32


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120

0

80 60

-10

TG

40

DTG

residue at 800 ℃: 15%

-20

20

Weight loss rate(dw/dt)

Weight(%)

10

TG onset: 328 ℃ maximum mass loss rate at 343 ℃

100

-30

0 0

200 400 600 Temperature (℃)

800

Figure 4 TGA and DTG curves of CC fibers measured under nitrogen atmosphere

DAP is an inorganic salt containing N, P elements which can effectively increase carbon yield as catalyst.30,31 In this work, the CC fibers were treated with DAP as the impregnant to investigate its effect on pyrolytic process.

120

0

80 60

-10

40

TG

20

DTG

-20

residue at 800 ℃: 35 wt%

Weight loss rate(dw/dt)

10

TG onset: 228 ℃ maximum mass loss rate at 253 ℃

100

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-30

0 0

200 400 600 Temperature (℃)

800

Figure 5 TGA and DTG curves of CC fibers pre-treated with DAP under nitrogen atmosphere

TG and DTG results of CC fibers treated by 5 wt% DAP are shown in figure 5. These curves indicate that the thermal behaviour of DAP treated CC fibers are different from untreated ones. It shows relatively flat pyrolytic peak with the maximum mass loss rate (7.54 %/min weight lose) at 253 °C compared with untreated CC fibers. In addition, the onset temperature of rapidly degradation occurs near 228 °C and the solid residue remains 35 wt% at 800 °C compared with untreated CC fibers. These results imply that the pyrolytic mechanism of impregnated DAP is different from the untreated ones. The possible explanation could be that the acids were generated with the decomposition of DAP, the phosphorylation reactions occurred and accelerated the dehydration of cellulose (it can catalyze cellulose to dehydrate). Use of DAP can reduce the formation of levoglucose and enhance the formation of char at the same time.31,33 Therefore, the yield of carbon can be effectively improved by using of DAP as impregnant. Table 1 summarizes the char yields of CFs prepared at different carbonization temperature with DAP as impregnant. It showed that the char yield gradually decreased with the temperature increase. The actually char yields are higher than the one from TG data, this can be


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attributed to the slow heating rate (4 °C/min) compared to the heating rate of 10 °C/min in TG test.9,32 Impressively, the char yield at 1100 °C is about 34.7 wt%, which is much higher than that of untreated CC fibers and is close to the theoretical carbon yield (44.4%). It can be concluded that DAP is essential as impregnant to prepare the CFs based on CC precursor.

Table 1 Summary of yield of CC fibers based CFs at different carbonization temperature Sample CF-700 CF-900 CF-1100

Carbonization Temperature (°C) 700 900 1100

Yield（%） 37.8 ± 0.9 36.4 ± 0.7 34.7 ± 0.6

FT-IR Figure 6 shows FTIR spectra of untreated CC fibers and the fibers treated by DAP and Oxidation. All the samples exhibit a broad peak between 3100 and 3600 cm-1 which is attributed to the stretching vibrations of hydroxyl groups.9 Noticeably, an absorption peak at 1711 cm-1 can be seen on the curve in figure 6, which is assigned to the stretching vibration of the carbonyl of urethane. 21,34 This indicates that the urethane still remains in CC fibers after regeneration. And the same peak at 1711 cm-1 also appears for fibers treated with DAP and Oxidation, which implies the oxidation process have no obvious effect on the urethane group.

a Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

b 1711

c

1711 1711

4000

3400

2800 2200 1600 Wavenumber (cm-1)

1000

400

Figure 6 FT-IR spectra of CC fiber (a), DAP treated fiber (b) and oxidized fiber (c)

Figure 7 shows FTIR spectra of CFs prepared at different carbonization temperature. All samples exhibit a broad peak from 3200 to 3600 cm-1 which can be assigned to the stretching vibration of O-H, however, the peak is weakened compared to ones in figure 6 and these peaks are gradually diminishing with the increase of carbonizing temperature. The stretching vibration peak of the carbonyl at 1711 cm-1 is not observed in all CFs after carbonization, which is probably due to the pyrolysis of CC fibers. The peaks at 2855 and 2925 cm-1 are assigned to the asymmetric and symmetric stretching vibration of CH2 bands respectively. They also diminished with the increase of carbonizing temperature due to the decomposition and loss of CH2 of the fiber at elevated temperature.


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CF-1100 CF-900

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4000

CF-700

3400

2800 2200 1600 1000 Wavenumber (cm-1)

400

Figure 7 FT-IR spectra of CFs prepared at different carbonization temperatures

Morphology analysis by SEM

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Figure 8 SEM image of CC fibers (a), DAP treated fibers (b), oxidized fibers (c) and CFs carbonized at 700 °C (d and g), 900 °C (e and h), 1000 °C (f and i)


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The morphology of those fibers was visualized by SEM. Figure 8a-8c show the exterior surface micrographs of untreated, impregnated and oxidized CC fibers, as can be seen, those fibers are in regular shape and present a smooth outer surface. Noticeably, some floccules can be observed on the surface of impregnated CC fibers (figure 8b), which indicates the DAP have adhered onto the surface of CC fibers. While the amount of floccules decreased obviously after oxidation due to the decomposition of DAP during this process. 33 At the same time, the diameter of oxidized fibers (8-11μm) decreased compared to that of the CC fibers (10-15μm). The carbon fiber with good mechanical properties is known to have characteristics of defect-free, compact microstructure and low porosity.15,25,35 The surface morphology and cross-section microstructure of CFs are shown in figure 8d-8i. It can be seen clearly in figure 8d-8f, the prepared CFs are overall smooth with only a few stripe texture observed. Micrographs (g-i) showed that those CFs were approximately circular in cross-sections, and most importantly, carbon fibers prepared in this exhibit solid structure and compact morphology, which are in favor of the forming high quality carbon fibers.35 These structure is dramatically different from rayon based CFs (skin-core structure with many cracks and grooves on the surface) which can lead to poor tensile strength.15, 36 From the above results, it can be concluded that the CC fiber is a promising precursor for preparing carbon fiber under appropriate conditions. XRD XRD was employed to analysis the crystal structure of CC fibers (figure 9) and CFs (figure 10). As depicted in figure 9, the CC fibers produced from cotton linter exhibit diffraction peaks at 2θ=12.2° (110), 2θ=20.4° (110) and 2θ=21.9° (200), corresponding to the crystallographic form of cellulose II, which indicates the prepared CC fibers have structure of cellulose II.20

(110) (200)

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(110)

5

15

25

35

45

55

2θ (degree) Figure 9 XRD spectra of prepared CC fiber

Figure 10 shows X-ray diffraction spectra of CFs carbonized at 700 °C (CF-700), 900 °C (CF-900), 1100 °C (CF-1100). As can be seen, the appearance of a broad and low diffraction peak at 2θ=23.5°, which is assigned to the (002) plane, indicates an amorphous phase of carbon materials. And all the carbon fiber samples exhibit a weak and broad peak at around 2θ=44.5°, which is attributed to the (100) plane associating with the turbostratic structure of disordered carbon materials.37


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(002)

Intensity(a.u.)

(100) CF-1100

CF-900

CF-700

5

15

25 35 2θ (degree)

45

55

Figure10 XRD spectra of carbon fibers carbonized at different temperatures

Raman Raman was conducted to further characterize the structure of CFs. A typical Raman spectroscopy of carbon material is shown in figure 11. As shown, all CFs present two Raman bands centred at about 1350 cm-1 (the D band) and at about 1590 cm-1 (the G band), which are considered to the disordered carbon structure and ordered carbon structure with sp2 hybridised C atoms, respectively.38-40 In addition, the appearance of a broad and weak peek from 2400 to 3100 cm-1 in Raman spectroscopy, which is assigned to the 2D Raman band of CFs, confirms the amorphous structure of the CFs at low carbonization temperatures.9 On the other hand, the intensity ratio of D band to G band (ID/IG) is used to describe the degree of graphitization of carbon fibers. As marked in figure 11, the ID/IG value of CFs increased from 0.98 to 1.07 with increasing carbonization temperature, indicating an increase in the structural order of the fibers.

D G

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

ID/IG 1.07

1200

1900

2D CF-1100

1.04

CF-900

0.98

CF-700

2600

Raman shift

3300

4000

(cm-1)

Figure 11 Raman spectra of prepared CFs at different temperatures

The tensile strength of carbon


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As one of brittle solid materials, the strength of carbon fibers usually present a high scatter distribution, thus, Weibull distribution function based on the assumption of “the weakest link theory are often used for the statistically analysis of the tensile strength.41 Two-parameter Weibull model is used to analyze the probability of failure of single fiber pull, which was expressed by Eq. (2). 𝜎 𝑚

𝐿

𝑃 = 1 − 𝑒𝑥𝑝 (− 𝐿 (𝜎 ) ) 0

Eq. (2)

0

Where P is the probability of failure, m is the Weibull modulus, L0 is the largest length that contains only one flaw, σ0 is the characteristic strength at L0, σ is the strength at L, L is the length of fiber. m is the shape parameter and σ0 is the scale parameter. The bigger m is the narrower the data points and the flaws will distribute, whereas σ0 is related to the severity of the distribution. The P value could be estimated by Eq. (3). 𝑖

𝑃 = 𝑛+1

Eq. (3)

Where n is the total number of data points and i is the counter of the data point. All the data are sorted in ascending order and marked each number as counter i. For simplicity, L0 is taken as unity then Eq. (2) is derived to be Eq. (4). 1

𝑙𝑛 (𝑙𝑛 (1−𝑃)) = 𝑚 𝑙𝑛(𝜎) − 𝑚 𝑙𝑛(𝜎0 ) + 𝑙𝑛(𝑙)

Eq. (4)

Where l is dimensionless which equal to the ratio L/L0.

2 1

Ln(Ln(1/(1-P)))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0 -1 -2

CF-700 CF-900

-3

CF-1100

-4 5.2

5.6

6

6.4

6.8

7.2

lnσ Figure 12 Weibull plots of tensile strength for CFs prepared at different temperatures

Figure 12 shows the linear Weibull plots of CFs. The corresponding Weibull parameters, tensile strength and the extension at break of the samples are listed in table 2. Results showed that the regression coefficients R2 of all samples were close to unity for conditions studied, which indicates the validity of this fitting method. It is noted that the CF-900 sample exhibits superior tensile strength, m value and elongation at break among all carbon fiber samples, indicating that the CF-900 sample has less severe flaws and a higher homogeneity.


12

Page 13 of 18

ACS Sustainable Chemistry & Engineering Table 2 Weibull parameters and tensile prooerties of prepared CFs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Regression Sample

m

Fiber diameter(μm)

Extension

Tensile strength (MPa)

(mean ± one std deviation)

(%)

(mean ± one std deviation)

σ0 (MPa)

coefficient, R2 CF-700

0.9795

5.5745

357.23

9.5 ± 0.7

1.32 ± 0.44

330.79 ± 60.01

CF-900

0.9628

7.2366

764.99

9.0 ± 1.1

1.63 ± 0.44

719.08 ± 103.27

CF-1100

0.9599

6.9014

543.56

8.7 ± 0.8

1.56 ± 0.51

509.51 ± 77.78

Usually the mechanical properties of CFs are depends on the properties of precursor, spinning technology, carbonization process and post-treatment conditions. Compared to other renewable precursors based carbon fibers (table 3), including Rayon fibers,16-17,42,43 Lyocell fibers,16,17,44,45 Sisal fibers,46 lignin-based fibers47-52 and wool fibers.37,53 CF-900 based on CC fibers exhibits decent tensile strength. Although the tensile strength of CF-900 is not as high as Lyocell fibers and Rayon fibers based CFs16,17,42,44, the energy consumptions for producing CF-900 are much lower compared to these CFs manufacturing process due to the relative low carbonization temperature and the high carbon yield. Moreover, the mechanical property of cellulose based carbon fibers could be improved by spinning process and carbonization procedures such as filling with nano-carbon black or GO or treating with organosilicon composite catalyst.28,42,45

Table 3 Comparison of tensile strength among carbon fibers based on various renewable precursors

Precursors

Tensile strength (Gpa)

Carbon Temp (°C)

Mass yield (%)

Reference

1

Rayon fibers

0.82

1300

--

16

2

Lyocell fibers

0.94

1300

--

16

3

Lyocell fibers

1.07

1300

--

16

4

Rayon fibers

0.94

1300

--

42

5

Rayon fibers

1.54

1300

--

42

6

Rayon fibers

0.02

1000

33.7

43

7

Rayon fibers

0.81

1300

--

17

8

Lyocell fibers

0.9

1300

--

17, 44

9

Lyocell fibers

0.49

1300

--

45

10

Lyocell filled with 5 wt% carbon black

0.57

1300

--

45

11

Lyocell filled with 10 wt% carbon black

0.71

1300

--

45

12

Sisal fibers

0.82

1040

--

46

13

CC fibers

0.72

900

36.4

This work

14

Pyrolytic lignin

0.37

1000

46

47

15

Lignin/Poly(lactic acid) (PLA) blends

0.29

1000

--

48

16

Hardwood lignin

0.61

1000

--

49

17

PAN/Lignin/CNT

0.72

1100

--

50

18

Hardwood kraft lignin

0.42

1000

45.7

51

19

Hardwood lignins

0.52

1000

38.5

52

20

wool fibers

0.22

800

20.5

37

21

Wool Fibers

0.16

800

22.8

53

Entry


13


Contact angle measurements The surface properties of CFs are critical for the preparation of carbon fiber composites, while the contact angle with water can reflect the surface wettability of solid materials, so contact angle test with water was conducted. CF-700

CF-900

CF-1100

Figure13 Photographs of water droplets on the surface of CFs at different carbonizing temperatures

Figure 13 shows the photographs of water droplets on the surface of CFs. The contact angles of all samples are greater than 90°, which indicates the obtained carbon fibers exhibit hydrophobic surface.

130

110

Contact angle /°

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 18

90

106.1

106.2

109.0

CF-700

CF-900

CF-1100

70

50

Figure 14 Static contact angle with water of CFs prepared at different carbonizing temperatures

The contact angle of CFs are very close, as show in figure 14, indicating those samples have uniform structure and similar surface properties. And the CF-1100 sample exhibits the largest contact angle at 109.0° in all samples. Although all samples exhibit hydrophobic nature, the contact angle rises slowly with the increase of carbonizing temperature. This is probably due to the release of more non-carbon atoms with increasing temperature.9 Then it will form more carbon materials so the hydrophobicity of surface increased. These results are also consistent with the IR data shown in figure 6. IR results showed that the bands of O-H stretching vibrations at 32003600 cm-1 were diminishing and the band around 1711 cm-1 attributed to C=O stretching almost disappeared with the increase of carbonizing temperature, which indicates the formation of hydrophobic carbon materials and decrease of non-atoms.

CONCLUSIONS


14

Page 15 of 18


In summary, we developed an environmentally friendly approach preparing CFs based on sustainable and low-cost cotton linter with

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

decent tensile strength reaching 0.72Gpa and the carbon yield up to 36.4%. It was found that the char yield can be effectively improved by using DAP as impregnant. The SEM micrographs show the prepared CFs have smooth surface, approximately circular and compact transverse texture in cross section, which indicates CC fibers is a promising precursors for the preparation of quality CFs. The obtained CC fibers present the structure of cellulose II and exhibit turbostratic structure. The contact angle measurements indicate the obtained CFs featuring hydrophobic surface, which can be utilized in hydrophobic resin composites manufacturing.

 AUTHOR INFORMATION Corresponding Authors Prof. Yagang Zhang Tel:+86-18129307169 Fax: +86-991-3838957 E-mail: [email protected]

 ACKNOWLEDGMENTS This work was financially supported by the “One Hundred Talents” and “One Thousand Talents” Program (Y32H291501) of China, National Natural Science Foundation of China (21472235, 21464015), and Returned overseas Young Talents Program (2014), Ministry of Human Resources and Social Security of the People's Republic of China. Special Support Program for National High Level Talent Candidate in Xinjiang.

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Page 18 of 18

For Table of Contents Use Only From waste cotton linter—a renewable environment friendly biomass based carbon fibers preparation Xin Zhou, Penglei Wang, Yagang Zhang, Xuemin Zhang, and Yingfang Jiang

The process of continue carbon fibers based on waste cotton linter — a low-cost, sustainable and non-toxic nature resource


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From Waste Cotton Linter: A Renewable ... - American Chemical Society

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