Effect of Organoclay Reinforcement on Lignin-Based Carbon Fibers

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Effect of Organoclay Reinforcement on Lignin-Based Carbon Fibers W. Qin† and J. F. Kadla*,† †

Advanced Biomaterials Chemistry Lab, Faculty of Forestry, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada ABSTRACT: Organoclay reinforced carbon fibers were prepared from pyrolytic lignin isolated from a commercial bio-oil. Organoclay reinforcement improved the tensile strength of the pyrolytic lignin based carbon fibers by 12% at clay loadings below 2 wt %. Wide-angle X-ray diffraction analysis of the composite as-spun fibers revealed the successful intercalation of the organoclays (Cloisite 20A and 30B), which helped develop a more ordered structure after carbonization. However, only the 1.0 wt % Cloisite 30B as-spun fibers showed an increase in Young’s modulus, while all other as-spun fibers remained unchanged. By contrast, all of the carbon fibers showed a drop in young’s modulus (by 16 38%) upon addition of organoclay, decreasing as organoclay content increased. This is likely due to the presence of microvoids in the carbon fiber as well as the lack of preferred orientation of clay platelets along the fiber axis.

’ INTRODUCTION Carbon fibers are lightweight, high performance fibrous materials containing more than 90% carbon and exhibit imperfect graphite crystalline structures oriented along the fiber axis.1 Carbon fibers are generally stronger yet lighter than most of the other structural materials.2 Due to their superior properties, they are widely employed in areas where high strength and lightweight are required. The composite materials made from carbon fibers are used as parts in planes, wind turbines, and sports equipments. Due to the extremely low density, it is believed that the use of composites made from general performance carbon fiber in automotive parts could substantially decrease fuel consumption and environmental impact. However, the use of carbon fiber in the current automotive sector is limited due to the high costs associated with the price and limited supply of the precursor materials.3 The two major precursors for the production of carbon fibers, i.e., polyacrylonitrile (PAN) and petroleum pitch are not renewable materials. As a result, alternative precursors for general performance carbon fibers are under increasing investigation, with an emphasis on replacing the expensive petroleum based feedstock with low-cost renewable alternatives.3,4 Lignin has been employed as feedstock for carbon fiber production. The first and only commercially available lignin-based carbon fiber was the Kayacarbon manufactured on a pilot scale by Nippon Kayaku Co in 1967.5 7 These carbon fibers were made from lignosulfonate with polyvinyl alcohol (PVA) added as a plasticizer. In addition to lignosulfonate, other types of lignins have been investigated as precursors to make carbon fibers. These lignin feedstocks include steam explosion lignin,8,9 acetic acid lignin,10,11 Alcell lignin,12 and Kraft lignin.12,13 Because of their moderate price, lignin is currently regarded as a potential alternative precursor for carbon fiber production.3 Clay or layered silicate is a popular reinforcing agent for many polymeric systems.14 It is well suited for the design of hybrid composites, because of their lamellar elements have high in-plane strength and stiffness and a high aspect ratio.15 They exhibit a rich intercalation chemistry, which allows for chemical modifications for greater compatibility with organic polymers such that they r 2011 American Chemical Society

could be dispersed at the nanometer scale.16 Moreover, they are widely abundant in nature and can be obtained in mineralogically pure form at low cost. Adding small amounts of clay into most polymer/clay nanocomposite systems can produce dramatic improvements in mechanical, thermal, and barrier properties.14 Previously, we found that organosolv lignin fibers could be reinforced with organoclays.17 The intercalated hybrids exhibited a substantial increase in tensile strength and melt processability. In the present article we extend our efforts on the development of lignin-based composite fibers. Specifically, herein we report on the properties of organoclay reinforced pyrolytic lignin-based carbon fibers.

’ EXPERIMENTAL SECTION Materials. Pyrolytic lignin was isolated from bio-oil according to the literature.18 Bio-oil was obtained from Dynamotive Energy Systems Corp. and kept in the dark at 4 °C until utilized. During the isolation process, bio-oil was added dropwise to cold deionized water (kept in an ice water bath) with vigorous stirring using a homogenizer. The oil to water ratio was 1 to 30 (v/v). Phase separation of the bio-oil began immediately after introduction into water, with the water insoluble fraction precipitating from the system. The obtained suspension was then filtered, and the precipitate was thoroughly washed with deionized water. After freeze-drying, a light-brown fine powder (pyrolytic lignin) was obtained. The pyrolytic lignin was characterized by various methods giving structural information tabulated in Table 1. Two types of organically modified montmorillonite (MMT) organoclays, Cloisite 30B and Cloisite 20A, were purchased from Southern Clay Co. (Austin TX, USA) and used as received. Received: June 20, 2011 Accepted: September 30, 2011 Revised: September 27, 2011 Published: September 30, 2011 12548

dx.doi.org/10.1021/ie201319p | Ind. Eng. Chem. Res. 2011, 50, 12548–12555

Industrial & Engineering Chemistry Research

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Table 1. Pyrolytic Lignin Structural Informationa functional groups (mmol/g) hydroxyl elemental composition (%)

a

C

67.1

H

5.9

O

26.7

molecular mass

thermal analysis

aliphatic

phenolic

methoxyl

Mw

PD

Tg (°C)

Td (°C)

1.7

6.0

2.3

702

2.3

70

150

Mw: weight average molecular weight; PD: polydispersity; Tg: glass transition temperature; Td: decomposition temperature.

Table 2. Organoclay Structural Information 2 theta angle

interlayer

organoclay

organic modifiera

(degree)

spacing (nm)

Cloisite 20A

(CH3)2(HT)2N+

3.48

2.54

Cloisite 30B

(CH3)(T)-

4.67

1.89

(CH2CH2OH)2N+ a

T: tallow, HT: hydrogenated tallow.

The structures of the interlayer counterions of the two types of organoclays used in this study are listed in Table 2. Both are quaternary ammonium cations with slightly different organic components. The Cloisite 20A has 2 methyl and 2 hydrogenated tallow groups (hydrogenated tallow groups are saturated hydrocarbons with various number of carbon atoms from 14 to 18) making up its quaternary ammonium component. By contrast the Cloisite 30B has only one methyl group, one tallow group (unsaturated hydrocarbon chains with 14 18 carbon atoms), and two hydroxyethyl groups as part of its ammonium center. The two Cloisite organoclays contain clay platelets with an aspect ratio of 70 150 and a surface area g750 m2/g. The reported dry particle sizes are between 2 and 13 μm with a median of 6 μm. Preparation Methods. Prior to making the composite fibers, a thermal pretreatment of the pyrolytic lignin was performed in a modified gas chromatography oven (Hewlett-Packard HP 5890 Series II). Around 15 g of pyrolytic lignin powder was contained in a 250 mL sealed round-bottom flask connected to house vacuum (∼30 kPa). After evacuation of the flask, the temperature was increased at a rate of 30 °C/min to 160 °C for 1 h before compounding with organoclay. To reinforce pyrolytic lignin, the organoclay and pretreated lignin powder (10 g in total) were mechanically mixed in a Retsch PM 200 ball-mill, at 500 rpm for 30 min under an argon atmosphere. A 45 mL zirconium dioxide bowl with 6 stainless steel balls (1 cm in diameter) was used. These pyrolytic lignin/ organoclay mixtures were then extruded into pellets, which were then re-extruded and spun into the fiber form using a Dynisco Laboratory Mixing Extruder (Atlas Electric Devices Co.) equipped with a ca. 0.8 mm spinneret.17 The resultant extruded lignin fiber was collected on spools by a take-up device (Atlas Electric Devices Co.). For fiber spinning the pyrolytic lignin and corresponding organoclay mixtures were fed into the hot chamber (Zone 1) where it is heated to the predetermined spinning temperature (from 105 to 180 °C) and pumped into the spinneret (Zone 2) which was also heated at a predetermined temperature (Figure 1). The optimal conditions for fiber spinning were obtained by slowly increasing the extrusion temperature

Figure 1. Schematic of the melt spinning apparatus used in this work.

and adjusting the take-up speed until continuous fiber spinning was achieved.12,13 Thermostabilization was performed as per the literature12 using a modified gas chromatography oven (Hewlett-Packard HP 5890 Series II). Lignin fibers were heated in air at a rate of 0.5 °C/min to 250 °C and then held at 250 °C for an hour. In order to induce molecular orientation, the fibers were thermostabilized under tension by fixing the two ends of the fiber onto a clamping system. The thermostabilized fibers were then carbonized in a GSL1100X tube furnace (manufactured by MTI Corp.) under a nitrogen atmosphere by heating from 25 1000 at 3 °C/min and held for one hour at 1000 °C. During carbonization, fibers were not under tension due to the limitation of the carbonization equipment available. The yield after each processing step, except for the spinning yield, was determined by weighing the samples before and after the production step. However, due to the experimental setup it was not possible to use this protocol to measure the spinning yield; therefore, TGA was employed. In the TGA experiments, the corresponding lignin preparation was first heated to its spinning temperature (zone 1) in one minute and then heated to the zone 2 spinning temperature in 5 min (estimated resonance time) to simulate fiber spinning. Characterization Methods. The methoxyl and hydroxyl group contents were calculated from the quantitative 1H NMR spectrum of acetylated pyrolytic lignin. Acetylation was performed by dissolving 200 mg of pyrolytic lignin in 8 mL of pyridine/acetic anhydride (1:1, v/v) and stirring the reaction for 48 h at room temperature. The reaction solution was then added dropwise to 300 mL of stirred ice water. The precipitated lignin was collected by filtration through a Nylon membrane (0.45 μm, 47 mm), washed with 3 L ice-cold distilled water, and freezedried using a VirTis EX freeze-dryer. Complete acetylation was confirmed by FT-IR spectroscopy. Elemental analysis was performed by the UBC Mass Spectrometry and Microanalysis Centre using a Carlo Erba Elemental 12549

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Table 3. Spinning Properties of Pyrolytic Lignin with and without Organoclay Reinforcement spinning temperaturea (°C) organoclay loading

spinning zone 1

zone 2

speedb (m/min)

0 wt % organoclay

120

166

36

1.0 wt % (Cloisite 20A) 2.0 wt % (Cloisite 20A)

120 120

168 170

36 25

5.0 wt % (Cloisite 20A)

135

178

18

1.0 wt % (Cloisite 30B)

120

162

36


125

170

22


125

170

18

(type)

Recorded as the rate in which continuous fiber spinning was achieved. b Maximum spinning speed that could be obtained for each sample by adjusting the spinning temperature. a

Analyzer (EA 1108). C, H, N contents were directly determined, whereas the O content was calculated by difference. The relative average molecular weight distribution of the pyrolytic lignin preparations were determined by gel permeation chromatography (GPC; Agilent 1100, UV and RI detectors) using styragel columns (Styragel HR 4 and HR 2) at 35 °C with THF as the eluting solvent (0.5 mL min 1) and UV detection at 280 nm. Pyrolytic lignin concentration was 1 mg mL 1, and the injection volume was 100 μL. Polystyrene standards were used for the calibration. Thermal analyses of the lignin samples were performed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) using a TA Q500 TGA and TA Q1000 DSC, respectively. In thermogravimetric analysis, 5 10 mg samples were heated from 40 to 1000 °C under nitrogen at a heating rate of 20 °C/min. The decomposition temperature (Td) of all samples was determined as the temperature at which a 5% weight loss of the sample was measured. Values of Td are reported as the average of 3 replicates with a coefficient of variance less than 2%. DSC analyses were conducted under a nitrogen atmosphere using approximately 3 mg of sample in each run. The samples were heated from 20 to 120 °C at a rate of 10 °C/min (first heating run), then cooled to 0 at 5 °C/min (cooling run), and subjected to a seconding heating from 0 to 200 °C (second heating run) at a rate of 10 °C/min. The glass transition temperature (Tg) of each sample was measured at midpoint of the step change in heat capacity on the heat flow curve of the second heating run. Values of Tg are reported as the average of 3 replicates with a coefficient of variance less than 1%. Wide-angle X-ray diffraction (WAXD) experiments were performed using a Bruker D8 Discover X-ray diffractometer using a Cu Kα radiation source operated at 40 kV and 40 mA. Diffraction patterns were collected for well-aligned fiber bundles (each bundle consisted of approximately 50 fibers of 20 mm in length) using an area array detector. For pure organoclay, WAXD samples were prepared using a molding-press device; 200 mg organoclay powder was compressed at 8000 psi for 5 min into a thin disk. A collection time of 100 s was used for all samples. Micrographs of gold-coated fibers were taken on a Hitachi S-3000N scanning election microscope (SEM). The accelerating voltage was 20 kV and magnification varied from 100 to 1000.

Figure 2. SEM micrographs of fibers spun from pyrolytic lignin with and without organoclay reinforcement. (For all micrographs, magnification = 1000, bar = 50 μm).

The tensile strength, modulus, and elongation of the individual carbon fibers were measured according to the ASTM standard C1557-03 (2008) with an Instron Tension Tester (model 5565) using a gauge length of 25 mm. Data are reported as the average of 20 fibers per sample. Fiber diameters were determined using a calibrated optical microscope and are reported as an average of three measurements along the fiber. Tensile strength and modulus values are reported as averages (95% confidence interval using a T-statistic. One-way analysis of variance (ANOVA) was performed to the result of tensile test by Tukey multiple comparison at 5% significance level.

’ RESULTS AND DISCUSSION Effect of Organoclay on Lignin Spinning Properties. The incorporation of organoclay appeared to reduce the spinning properties of pyrolytic lignin; the addition of organoclay increased the temperature required to obtain continuous spinning (Table 3). This effect became pronounced for samples with higher organoclay content (5.0 wt %), regardless of organoclay type. Likewise, increasing the organoclay content also decreased the fiber-spinning rate that could be used to maintain continuous fiber take-up. However, the lignin preparation with the addition of 1.0 wt % organoclay slightly improved fiber spinning, particularly with Cloisite 30B organoclay. The higher loading of organoclay not only has a negative impact on the spinning properties of the lignin samples, leading to thicker fibers, but also decreased the quality of the as-spun fibers. As illustrated in Figure 2, the higher the organoclay loading, the rougher the fiber surface. For lignin compounded with 5.0 wt % organoclay loading, an obvious striated morphology on the fiber surface was observed regardless of organoclay type. Thermal Properties of Lignin/Organoclay Composites. Glass transition temperature (Tg) is the temperature at which polymer materials start to soften, transforming from the glassy state to the rubbery state. Above the Tg, some polymer materials are able to flow and spin into fibers.19 Thermal analysis of the composite powder and corresponding composite fibers reveals that Tg increases with increasing organoclay incorporation (Table 4). 12550



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Table 4. Glass Transition Temperatures of Lignin/Organoclay Composites composite powder

composite fiber

organoclay loading Cloisite 20A Cloisite 30B Cloisite 20A Cloisite 30B

a

0 wt %a

91

91

96

96

1.0 wt %

94

94

97

97

2.0 wt % 5.0 wt %

94 96

94 95

97 101

97 101

Pyrolytic lignin without organoclay loading.

Table 5. Decomposition Temperatures of Lignin/Organoclay Composites composite powder

composite fiber

organoclay loading Cloisite 20A Cloisite 30B Cloisite 20A Cloisite 30B

a

0 wt %a

235

235

223

223

1.0 wt %

229

228

217

221

2.0 wt %

217

229

223

223

5.0 wt %

198

218

197

210

Pyrolytic lignin without organoclay loading.

This is consistent with the increase in spinning temperature required for the lignin/organoclay composites as organoclay loadings increase (Table 3). These trends are maintained in the lignin/organoclay composite fibers after thermal extrusion and fiber spinning (Table 4). Thermal analysis can also provide information regarding the thermal stability of materials. This is typically performed using TGA and measuring the temperature at which a 5% weight loss occurs. Generally, the addition of organoclay into a polymer matrix will enhance thermal stability, in that the clay platelets act as a superior insulator and mass transport barrier to volatile products generated during thermal decomposition.14 However, the organoclay reinforced pyrolytic lignin powders and fibers exhibit lower decomposition temperatures than the control lignin (Table 5). In the composite powders, the decomposition temperature decreases with increasing organoclay loading, where the decrease is greater for the Cloisite 20A/lignin powders than those with Cloisite 30B. In the composite fibers, the decrease in decomposition temperatures are less severe, with those of 1.0 2.0 wt % appearing to show no change in decomposition temperatures over that of the pure lignin. However, at 5.0 wt % organoclay, the Cloisite 20A containing fibers are significantly lower than those using Cloisite 30B. Organoclay Morphology in As-Spun and Carbon Fibers. Wide-angle X-ray diffraction (WAXD) is commonly performed to estimate the degree of clay dispersion within polymer matrices.20 Based on Bragg’s law,21 this technique allows for the determination of the distances between the adjacent clay platelets. Figure 3 illustrates the 2θ (diffraction angle) plots obtained from the WAXD of as-spun fibers prepared from lignin/ Cloisite 20A and lignin/Cloisite 30B composites, respectively. According to Bragg’s law the relationship between the 2θ angles and the interlayer spacing of the clay platelets is given by λ = 2d•sin(θ), where λ is the wavelength of the X-ray radiation used in the WAXD experiment, d is the distance between the adjacent diffractional lattice planes, and θ is the measured

Figure 3. Wide-angle X-ray diffractograms of the organoclay Cloisite 20A and as-spun fibers prepared from lignin/Cloisite 20A composites with organoclay loading 1.0 wt %, 2.0 wt %, and 5.0 wt % (top) and organoclay Cloisite 30B and as-spun fibers prepared from lignin/Cloisite 30B composites with organoclay loading 1.0 wt %, 2.0 wt %, and 5.0 wt % (bottom).

Table 6. Comparison of d-Spacing of Clay Platelets in the Lignin/Organoclay Composite before and after Carbonizationb organoclay content

Cloisite 20A d-spacing (nm)

Cloisite 30B d-spacing (nm)

as-spun fiber

as-spun fiber

carbon fiber

a

1.0 wt %

3.94

4.13

-

2.0 wt %

3.94

4.21

4.13

5.0 wt %

3.94

4.37

4.03

100 wt %

2.54

carbon fiber 4.17 4.17 4.85

1.89

a

No discernible peak for d-spacing determination. b Values were obtained after peak deconvolution.

diffraction angle. For both organoclays, the d-spacing increased substantially after melt compounding (Table 6). For example, 12551



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Figure 5. Wide-angle X-ray diffraction patterns of the fiber samples with 5.0 wt % Cloisite 20A organoclay loading: (a) as-spun fiber; (b) thermostabilized fiber, and (c) carbon fiber. The black arrow in each pattern indicates the fiber direction during measurement. Figure 4. Wide-angle X-ray diffractograms (2 theta angle below 10 degree) of the organoclay and carbon fibers prepared from lignin/ organoclay composites for Cloisite 20A (top) and Cloisite 30B (bottom). Organoclay loadings are 1.0 wt %, 2.0 wt %, and 5.0 wt %.

the peak in the X-ray diffractograms shifted to lower 2θ, indicating the successful insertion of lignin molecules into the organoclay galleries. As expected, the chemistry of the organoclay modifier has a discernible effect on the extent of clay intercalation. For lignin reinforced with Cloisite 20A, the interlayer spacing expanded 55% (2.54 to 3.94 nm) regardless of organoclay loading, whereas the d-spacing of the lignin/Cloisite 30B more than doubled (1.89 nm to 4.03/4.13 nm) for all organoclay loadings examined in this study. One should note that the dspacing peak disappeared in the fiber with 1.0 wt % Cloisite 30B loading, an indication of exfoliation. To facilitate favorable mixing with polymer hosts, native clays are typically modified using organic quaternary ammonium compounds, i.e. organoclays. The organic counterions serve to lower the surface energy between the surface of the organoclay and that of the polymer matrix, thereby facilitating favorable mixing and polymer insertion into the clay galleries.22 The two organoclays used in this study were modified from the same native clay and differ only in their counterions (modifier).

Cloisite 20A organoclay has a dimethyl-dihydrogenated tallow ammonium counterion, which is more hydrophobic than the dihydroxyethyl-methyl-tallow ammonium counterion found in Cloisite 30B (Table 2). Although pyrolytic lignin should be considered hydrophobic in regard to water-soluble polymers, it exhibits a certain degree of polarity owing to its oxygen-containing groups. Moreover, since technical lignins form strong favorable hydrogen-bonding interactions with alcohols and ethers,23 we expect stronger interactions between pyrolytic lignin with Cloisite 30B than with Cloisite 20A. In fact, the higher degree of clay gallery expansion observed for the Cloisite 30B organoclay may be due to such strong interactions between pyrolytic lignin molecules and the hydroxyethyl moieties of the quaternary ammonium ions. WAXD analysis (Figure 4) of the corresponding carbon fibers depict a further expansion of the clay galleries in the Cloisite 20A composite carbon fibers of more than 4 nm, which increase with increased organoclay loadings. By contrast, the Cloisite 30B composite carbon fibers do not significantly change from that of the precursor fibers except at the highest organoclay loading (5 wt %) which expands to almost 5 nm in size (Table 6). It appears that during the manufacturing process of carbon fibers the d-spacing or degree of expansion of the organoclay galleries further increased. The diffractograms of the resulting carbon fibers also exhibit the presence of a broad 2θ angle peak at around 12552



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Table 7. Interlayer Spacing (in nanometers) of Graphitic Structure in the Carbon Fibers Made from Lignin/Organoclay Composite organoclay typeb organoclay loading

Cloisite 20A (nm)

Cloisite 30B (nm)

0.3990 ( 0.0609

0.3990 ( 0.0609

1.0 wt %

0.3544 ( 0.0086

0.3549 ( 0.0106

2.0 wt %

0.3516 ( 0.0110

0.3527 ( 0.0121

5.0 wt %

0.3516 ( 0.0110

0.3468 ( 0.0079

0 wt %

a

Pyrolytic lignin without organoclay loading. b (Values calculated from full width half-maximum of the respective peak; theoretical interlayer spacing of graphite: 0.3354 nm. a

Figure 6. Wide-angle X-ray diffractograms (2 theta angle above 10) of the organoclay and carbon fibers prepared from lignin/organoclay composites for Cloisite 20A (top) and Cloisite 30B (bottom). Organoclay loadings are 1.0 wt %, 2.0 wt %, and 5.0 wt %.

3.3° (∼2.6 nm) which can be assigned to microvoids in the carbon fibers.24 The microvoids are clearly visible in the 100% lignin carbon fibers and are believed to be responsible for the low tensile strength and modulus of lignin based carbon fibers.24 Orientation of Organoclay Platelets along the Fiber Axis. In addition to information on d-spacing and the expansion of the organoclay galleries, the X-ray diffraction patterns also provide information regarding the orientation of the organoclay platelets within the fibers. Figure 5 shows the X-ray diffraction patterns of the lignin/organoclay composite as-spun (Figure 5(a)) and thermostabilized fibers (Figure 5(b)). The arcs perpendicular to the fiber direction (indicated by the thick arrows) in the diffraction pattern is an indication of organoclay orientation along the fiber axis. The orientation is likely created during the fiber spinning process due to hot stretching and is believed to contribute positively to the tensile properties along the fiber axis. Comparison of the WAXD patterns for the as-spun, thermostabilized and carbon fibers reveals the sharp band pattern associated with the organoclay orientation decreases with increasing

thermal treatment. WAXD pattern associated with the carbon fibers appear to have no preferred clay platelet orientation, as demonstrated by the bright ring in the diffraction pattern (Figure 5(c)). However, this loss of orientation, or introduction of randomness may be a result of the increasing microvoids that are being generated and overlap this region of the diffractogram (Figure 4). In fact the 2θ plot of the WAXD pattern for thermostabilized fiber do not give discernible evidence of microvoid formation at around 3.3°, but a more randomly distributed 001 and 002 reflection pattern can be found in the corresponding WAXD diffractogram (Figure 5(b)). Thus it is more likely that the orientation is disrupted in the thermostabilization (fibers under tension) and carbonization process during which tension was not applied to the fibers. 3.2.5. Development of Ordered Structure in Carbon Fibers by Organoclay Addition. The tensile properties of carbon fibers rely heavily on their crystalline structure.25 Generaly, the more ordered the crystalline structure, the stronger the fibers. The interplanar spacing (d002) is an indication of the degree of alignment of the graphene planes within the graphitic structure. The theoretical value of this interplanar spacing for an ideal graphite crystal is 0.3354 nm.26 But for commercial carbon fibers, this value usually ranges from more than 0.3440 for PAN-based carbon fibers to around 0.3400 for those based on pitch.26 As the d002 of lattice plane decreases and approaches the theoretical value of 0.3354 nm, the turbostratic structure of the carbon fibers gets closer to the ideal structure of graphite. Generally, the shorter the interlayer spacing (d002 spacing) of the graphene planes perpendicular to the fiber axis, the higher the tensile strength and modulus of the carbon fibers.25 Figure 6 shows the 2θ plot for the various composite carbon fibers and the corresponding interplanar spacing. The pyrolyitc-lignin-based carbon fibers exhibit a broad peak at ∼22° in the diffractogram (Figure 6), indicating a broad interplanar spacing (d002) distribution centered at ∼0.3990 nm, significantly higher than that of the commercial carbon fibers. Organoclays have been reported to facilitate graphitization and assist in developing highly ordered graphite crystals for a number of polymer precursors.27 30 In this study, we also observed this catalytic effect in the diffractograms of carbon fibers made from lignin/organoclay composites. Both types of organoclay gave a sharper peak above 25° 2θ regardless of organoclay content (Figure 6). The corresponding interlayer spacing (d002) values were calculated and are listed in Table 7. All of the carbon fibers with organoclay loadings exhibit a shorter d002 and sharper signals (indicated by the full width half-maximum of the peak), thus resulting in a more ordered structure. 12553



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Table 8. Tensile Properties of As-Spun Fibers with and without Organoclay Reinforcementc organoclay loading

diameter

Young’s modulus

tensile strength

(type)

(um)

(GPa)

(MPa)

0 wt % organoclay

51 ( 4

4.9 ( 0.5

23 ( 3

1.0 wt %

51 ( 1

5.4a ( 0.3

27a ( 3

2.0 wt % (Cloisite 20A)

70 ( 4

5.3a ( 0.3

23a ( 2

5.0 wt %

74 ( 6

4.4a ( 0.4

17b ( 3

47 ( 2

6.5b ( 0.5

28b ( 2

71 ( 3

5.3a ( 0.2

26a ( 2

76 ( 4

5.3a ( 0.3

17b ( 2

(Cloisite 20A)

(Cloisite 20A) 1.0 wt % (Cloisite 30B) 2.0 wt % (Cloisite 30B) 5.0 wt % (Cloisite 30B) a

Values are not significantly different from those without organoclay loading. b Values are significantly different from those without organoclay loading. c Values are averages of 20 replicates; deviations are 95% confidence interval based on a T-statistic.

Table 9. Tensile Properties of Carbon Fibers with and without Organoclay Reinforcementc organoclay loading

diameter

Young’s modulus

tensile strength

(type)

(um)

(GPa)

(MPa)

0 wt % clay

49 ( 2

36 ( 1

370 ( 38

1.0 wt %

45 ( 1

32b ( 1

422b ( 24

50 ( 2

29b ( 1

307b ( 25

55 ( 2

26b ( 1

240b ( 17


47 ( 1

32b ( 1

438b ( 24

2.0 wt %

50 ( 1

30b ( 1

357a ( 29

57 ( 2

30b ( 1

255b ( 25

(Cloisite 20A) 2.0 wt % (Cloisite 20A) 5.0 wt % (Cloisite 20A)

(Cloisite 30B) 5.0 wt % (Cloisite 30B) a

Values are not significantly different from those without organoclay loading. b Values are significantly different from those without organoclay loading. c Values are averages of 20 replicates; deviations are 95% confidence interval based on a T-statistic.

3.2.6. Mechanical Properties and Yield. The addition of organoclay did not significantly affect the mechanical properties of the pyrolytic lignin fibers (Table 8). For both organoclays, the maximum tensile strength and Young’s modulus were observed for the 1.0 wt % organoclay loadings. The Cloisite 20A fibers showed a 10% and 17% increase in mean tensile strength and modulus at the 1.0 wt % level as opposed to 33% and 22%, respectively for Cloisite 30B; increasing the addition of organoclay decreases the mechanical properties. However, fiber diameter increased with organoclay beyond the 1.0 wt % level loading for both organoclays. Since tensile strength is negatively affected by increasing fiber diameter,31 the lower strength of the

fibers with higher organoclay loading may be partially attributable to their larger fiber diameters (Table 8). To determine if there were differences in mean tensile strength and modulus values between the lignin fibers with and without organoclay reinforcement an analysis of variance (ANOVA) was performed. From the ANOVA results it appears that only the 1.0 wt % Cloisite 30B lignin fibers show a significant increase in tensile strength and modulus as compared to the pure lignin fibers. As well, for both organoclays the higher loadings (5.0 wt %) significantly decreased tensile strength’s but did not impact modulus; however, in both cases the fiber diameters were significantly larger than the pure lignin control fibers. A similar trend in tensile strength is obtained for the corresponding carbon fibers (Table 9). Again, the 1.0 wt % Cloisite 30B organoclay loading fibers have the highest tensile strength, but so too were the 1.0 wt % Cloisite 20A carbon fibers significantly stronger in tensile strength than the pure lignin fibers. In both organoclay systems, tensile strength decreases with increasing organoclay loading. Unlike the precursor fibers, the carbon fiber modulus values dropped slightly upon incorporation of organoclay, and decreased with increasing organoclay content. This might be attributable to i) the loss of preferred orientation of the clay platelets along the fiber axis and ii) the presence of microvoids within the carbon fiber. Another important aspect of the commercial manufacturing of carbon fiber is carbon fiber yield; lower yield results in higher production costs per unit weight of the product. Regardless of organoclay type and loading, the addition of organoclay slightly changed the overall yield of carbon fiber production, from 45 wt % for samples without organoclay loading to 46 47 wt % for samples with organoclay regardless of the organoclay loading.

’ CONCLUSIONS The tensile strength of pyrolytic lignin based carbon fibers was improved (12%) by organoclay reinforcement at loadings below 1.0 wt %. Wide-angle X-ray diffraction analysis of the resulting composite as-spun fibers revealed the successful intercalation of the organoclay within the pyrolytic lignin matrix. Intercalation was achieved for both types of organoclays used in this study, with Cloisite 30B showing better dispersion in the pyrolytic lignin matrix as compared with Cloisite 20A. The intercalation was maintained after the composite fibers were made into carbon fibers. The incorporated organoclay also served as a catalyst during carbonization. Wide-angle X-ray diffraction revealed the development of a more ordered structure upon incorporation of both types of organoclay increased with organoclay loading. Increasing the organoclay content above 1.0 wt % resulted in a drop in tensile strength of both as-spun and carbon fibers, although this was also accompanied by an increase in fiber diameter, which is known to decrease tensile strength. As well, only the 1.0 wt % Cloisite 30B as-spun fibers showed an increase in Young’s modulus (increased to 6.5 GPa from 4.9 GPa for the neat lignin fiber). For all other as-spun fibers there was no change in Young’s modulus, while all of the carbon fibers dropped (by 16 38%) upon addition of both types of organoclays and decreased as organoclay content increased. This might be attributable to the presence of microvoids in the carbon fiber as well as the lack of preferred orientation of clay platelets along the fiber axis. Although the addition of organoclay to pyrolytic lignin can improve mechanical properties of the lignin-based fibers. 12554


Industrial & Engineering Chemistry Research Compared with commercial general performance carbon fibers (tensile strength: 1 GPa; modulus: 50 GPa), the organoclay reinforced carbon fibers based on pyrolytic lignin (with 1% loading of Cloisite 30B) are too weak (tensile strength: 438 MPa; modulus: 32 GPa) for commercial application.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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