Sulfur Free Nano Crystalline Cellulose

Published Date: 02 Nov 2017

Isolation of Nano Crystalline Cellulose (NCC) and Micro Crystalline Cellulose (MCC) from hardwood pulp, using 2-Hydroxy-1,2,3-propane tri-carboxylic acid (H3C6H5O7) (pKa1=3.09) was demonstrated. NCC with small particle size of 158±5 nm (L) and 10 nm (D), with an aspect ratio (L/D) of >15 were produced. Spent liquor obtained from the acid hydrolysis process contained 31.7-32.8% of total sugars, based on the pulp. A yield of NCC and MCC as residues on quantity of pulp were 16.6-18.4 %; 31.3-32.4 (%), respectively. Under an optical microscope, NCC suspension viewed in cross-polarized light showed birefringence characteristics and a spectrum of color. This process provides an integrated platform for easy isolation of NCCs, MCC, and recovery of sugar as well as H3C6H5O7 from spent liquor can be recycled. NCC and MCC obtained from low and high pulp to acid ratio of 1:20 g.mL-1, and 1:12 g.mL-1, respectively at same conditions were characterized. NCC prepared with low pulp to acid ratio showed better thermal stability up to 359°C and crystallinity index of 78% more than that of cellulose and others.

Keywords: Nano-crystalline cellulose (NCC), Hardwood pulp, Citric acid, Birefringence, Sugars, Crystallinity index.

Introduction

Cellulose is a naturally occurring polymer in plants and is abundant in nature. Cellulose has a heterogeneous morphology comprised of repeating glucose units joined by β-1-4 glycosidic bonds. Cellulose consists of bundles of microfibrils; these microfibrils are composed of repeated crystalline and amorphous regions within the structure [1]. (Gibson et.al. 2012). Cellulose microfibrils crystalline regions are highly stable, very strong, and are physiologically inert towards acid/alkali attacks. The crystalline region of microfibrils is termed as Nano Crystalline Cellulose (NCC) or Cellulose Nanocrystals (CNC's) which have a width of 4-20 nm. NCC has many potential applications such as optical coatings, sensors, rheological modifiers, tissue scaffolds, packaging purposes (barrier properties), etc. [2,3,4] (Beck et al. 2010; Jackson et al. 2011; Nogi et al. 2009). In bio-composite applications, NCCs are widely used as it exhibits strength higher than steel, and a higher stiffness than aluminum [5] (Nishino and Arimoto et al. 2007).

In recent years, extensive research has been done to prepare NCCs from different cellulosic materials such as cotton, wood pulp, and agricultural residues [6,7] (Bhatnagar and sain.2005; Orts et al. 2005). In addition, a platform of bio-refinery concepts to produce multiple products, such as fuels, precursors for biopolymer and valuable chemicals are demonstrated [8,9,10,11] (Amidon et al.2008; Marzialetti et al.2008; Leschinnsky et al.2009; Li et al.2010). Current drawbacks are that there are no proper developed means to produce standardized NCCs with uniform size, narrow particle size distribution, and aspect ratio [12] (Hamad and Hu. 2010). Another aspect is that the integration of "bio-refinery" concepts and production of NCCs, in more economical ways, are yet to be achieved [13] (Zhu et al. 2011).

Different methods like acid hydrolysis, enzymatic hydrolysis, TEMPO oxidation, ionic liquid, and high pressure mechanical shear techniques are extensively used for preparation of such NCCs [14] (Habibi et al.2005). Isolation of needle shape and rod-like structured crystalline particles by using concentrated acid methods from cellulosic materials is well established [15] (Dong et al. 1998). A concentrated sulfuric acid hydrolysis method produces a needle-shaped NCC particles with dimensions of length of 115 nm and a diameter of 7 nm [16,17] (Beck-Candanedo et al. 2005, Elazzouzi-Hafraoui et al. 2008). In contrast, hydrochloric acid/sulfuric acid hydrolysis or NaOH with ultrasonication produces spherical particles with a mean size of 40-80 nm and 20 nm respectively [18,19] (Chen et al. 2012; Zhang et al. 2007).

Acid hydrolysis involve heterogeneous activities such as diffusion of acid, followed by cleavage of glycosidic bonds [20,17] (Edgar et al. 2003; Elazzouzi-Hafraoui et al. 2008) which leaves a fraction of solid residue as NCC and MCC. The major challenges in the sulfuric acid H2SO4 hydrolysis process are highly concentrated acid is being used (64% w/v). In order to stop the hydrolysis reaction, quenching with an excessive amount of water is required. Centrifugation as well as continuous dialysis for several days are required to remove the excess acid thereby ensuring that the suspension is near neutral pH [16] (Beck-Candanedo et al. 2005). In general, strong acids are very corrosive, and recycling spent liquor is a very expensive as well as a tedious process [21] (Brinchi et al. 2012). NCC prepared from concentrated sulfuric acid hydrolysis has a high degree of substitution of anionic charged sulfate ester groups (SO4-) on their surface. Such anionic charged sulfate groups (84 mmol.kg-1) on the NCC surface induces a formation of negative electrostatic layer thereby a very poor thermal stability at elevated temperatures>200°C, which affects the use of NCC in certain applications such as bio composite and thermoplastic engineered materials [22] (Zhang et al. 2012; Brinchi et al. 2012). Other steps for achieving better thermal stability and intercalation properties, de-sulfonation and rendering of cationic charges on the surface of NCCs were suggested [23,24] (Zaman et al.2013; Jean et al.2008).

The improvement of previous mentioned process weakness by defining a straightforward economical way to scale up production of NCC and integration of bio refinery concepts within itself. In this context, the selection of weak polyprotic acid (Di or Tri carboxylic acid) is ideal to overcome the use of corrosive chemical and other limitations posed by the concentrated sulfuric acid process. As it allows an easy and effective way to fractionate biomass material into useable fractions such as monomeric sugar, NCC, and Micro-Crystalline Cellulose (MCC) [13] (Zhu et al. 2011). The fractionation of biomass using polyprotic acid mainly acetic acid, formic acid, and combination with peroxy formic acid was demonstrated [25] (Jahan et al. 2011). For example, polyprotic acid like acetic acid is widely used in the preparation of microcrystalline cellulose and cellulose acetate from wood pulp.

In this study, a weak polyprotic acid, i.e., 2-Hydroxy-1,2,3- propane tri-carboxylic acid (H3C6H5O7) and have a commercial name "Citric Acid"(CA), was used as a potential chemical agent for viscose pulp fractionation and isolation of NCCs. Citric acid is a weak acid; it is stable, and safe to use. After completion of hydrolysis reaction, using simple unit operations spent liquor can be easily recovered, recycling and reuse of acid in the process is possible [25] (Jahan et al. 2011).

The objective of this study includes 1) Method to prepare NCC and MCC with citric acid (pKa1=3.09) from viscose pulp; 2) analysis of fractionated components such as sugar, Nano crystalline cellulose, and MCC (solid residue) based on CA treatment process; and 3) characterization of NCC and MCC samples by Thermo Gravimetric Analysis (TGA), Fourier Transform Infrared (FTIR), X-ray diffraction, Optical microscopy, Scanning Electron Microscope (SEM), EDAX, and Transmission Electron Microscope (TEM) techniques.

Material and Methods

Materials

Air Dried (A.D) bleached hardwood viscose pulp, with α cellulose content of 95.0± 0.5% and viscosity of 14.0 cPs was collected from A.V Nackwick Mill, Eastern Canada. Typical chemical compositions were 0.5± 0.1% acetone extractive, 1.0± 0.2% lignin, 2±0.5 % pentosans, and < 0.5% ash. Citric acid anhydrous (crystalline) assay of >99% was obtained from Fisher Scientific Ltd. All water used was purified (Millipore Milli-Q purification system). All chemicals without further purifications were used.

Preparation of NCC

The citric acid hydrolysis of viscose pulp were conducted according to the method, with slight modifications [26] (Wood et al.1989).The changes made to the method were batch wise citric acid hydrolysis treatment using sealed tubular reactors, then followed by fractionation techniques, instead of proposed heterogeneous acid hydrolysis. The construction of reactors was made of Haste alloy C-276 tubing (internal volume 10ml) capped with Swagelok end fittings [27] (Xiang et al. 2003). Two different pulp to acid ratio, such as high (1:12 g.mL-1), and low (1:20 g.mL-1) was carried out. Before treatment using a Willey mill, viscose pulp was ground to pass through a 20 mesh screen. Both pulp and acid were loaded in the tubular reactor and placed carefully into the autoclave reactor. The samples were hydrolyzed with citric acid 65% (w/v) at 150°C for 180 min without any agitation in an autoclave reactor [28,29] (Harmer et al. (2009); Mosier et al. 2001). The hydrolyzed mixture was allowed to reach room temperature and then after washing with D.I water, and centrifuged at the acceleration of 4000 RPM for 15 min repeatedly for three times until the mixture became acid-free (the filtrate showed a near neutral pH). The recovered acid filtrate (spent liquor) was termed as filtrate-I and at near neutral, supernatant solution were collected for sugar analysis (filtrate-II). The solid residue remained was again mixed with D.I water, after centrifuge an aliquot of turbid suspension was collected and termed as NCC. Then, the finale left over white residue washed with D.I water to get a clear suspension of MCC. Both NCCs and MCC, using Branson's sonicator (300 watts) sonicated for 20 min in pulse mode prior to drying in a vacuum oven to a constant weight for 24h to obtain a dried material, so that can be further characterization studies can be carried out. Both NCC and MCC samples were dehydrated with acetone.

Sugar analysis

The determination of sugar concentrations was performed by using an Ion Chromatography unit equipped with a CarboPacTMPA1 column (Dionex-300, Dionex Corporation, Canada) and a pulsed amperometric detector [30] (Saeed et al.2011). For analysis, filtered, sufficiently diluted samples were used.

SEM and TEM analyses

Scanning Electron Microscope (SEM, JEOL 6400, Tokyo, Japan) was used in the micro-structural analysis of the MCC samples onto a carbon coated copper grid. The samples were sputter coated with gold palladium and all images taken at an accelerating voltage of 15 kV. Transmission Electron Microscope (TEM) observations performed by using JOEL 2011 STEM. For TEM, a 10 µL drop of diluted solution was pipetted out onto a glow discharged carbon coated copper grid to have better film formation and all images taken at an accelerating voltage of 200 KV. For each sample, more than hundred particles were randomly chosen and measured from several TEM images (N=10) using Image J 146r software.

Thermo gravimetric analysis

Using a Q series-500,Thermal Gravimetric Analysis (TGA) was performed with about 5-10 mg of air-dried sample and a heating rate of 10°C/min from room temperature to 600°C, under nitrogen atmosphere (N2 flow rate =100 mL/min) was carried out.

FT-IR analysis

The dried samples mixed with KBr powder, and analyzed by using a Perkin- Elmer spectrometer. The spectra recorded were in the transmittance band mode, in the range of 400-4000 cm-1.

X-ray diffractometry

Cellulose powder was reverse-packed in order to gain maximum randomization in particle orientation. NCC films were scraped from the vial sidewalls and methanol was used to "set" and lay flat on the glass slide surface. MCC suspension was prepared using methanol and smeared on a glass plate in order to obtain a uniform film upon drying. The X-ray powder patterns for viscose cellulose, NCC, and MCC were scanned and recorded using the Bruker D8 Advance spectrometer with a D-5000 rotating anode X-ray generator from 5 to 45° of 2-theta (scanning angle), Cu Kα radiation generated at 30 mA and 40 kV. The goniometer was computer controlled with independent stepper motors and optical encoders for the 0°and 2° circles with the smallest angular step size of 0.0001. No correction made for Kβ radiation. The raw data obtained from the spectrometer was analyzed and refined by the program EVA (Bruker) . The calculation of crystalline indices of the samples from the X-ray diffraction patterns was carried out based on the following equation [31] (Mihranyan.et.al 2004),

Xc = I002 - Iam / I002*100 .............................................(1)

Where, Xc is Crystallinity Index (CrI),

I002 is the peak intensity from the 002 lattice plane (2Ɵ = 22°) and

Iam is the peak intensity of the amorphous phase.

Optical microscopy

The microscopic study for understanding the optical birefringence characteristics was investigated with a Lecia digital light microscope, under the bright field transmittance mode and viewed in cross-polarized light condition. Around 10 µl of diluted NCC suspension placed over the glass slide, and a cover slip placed over to obtain a thin film. The prepared sample was viewed in a cross polarized light condition at a magnification of 40x.

Results and discussion

Fractionation

Shown in Table.1 are the results of viscose pulp fractionation after the citric acid process. The total NCC and MCC yield were around 48.8%.The much higher yield of NCC and MCC from citric acid hydrolysis is due to: (1) very high α-cellulose content in the initial raw material and (2) the crystalline region is highly protected in the CA process and (3) mainly removal of a large amount of amorphous regions. Especially, low pulp to acid ratio showed better yields of NCC and MCC. The yield of total sugar from low pulp to acid ratio is 1.5% (on a ratio basis) more than that of high pulp to acid ratio (Table1). This can be supported by FTIR results showing a strong peak at 930 cm-1 explaining that more glycosidic bonds are removed as in Fig. 6 (a) [32] ( Chen et al. 2012). Many research studies indicate that polyprotic acid such as acetic acid and formic acid can effectively remove hemicelluloses and amorphous part of cellulose from lignocellulosic materials [29] (Nathan etal. 2001). As reported, dilute sulfuric acid hydrolysis at high temperature and prolonged time can yield up to 35-50 % of sugars from the cellulosic material due to a slow decrease in the degree of polymerization [33](Chandan et al. 2007).

Table.1 Percentage of yield on fractionated materials (Based on cellulose quantity used)

Total fractionation (yield % on pulp)

Description

Total sugar

NCC

MCC

Low

32.2

18.4

32.4

High

31.7

17.8

31.6

Mass balance

The total recovery of fractionated material from the citric acid hydrolysis process was around 81-82.5%, which included 32.4% of MCC, 18.4% of NCC, and 32.2% of total dissolved sugars in the spent liquor (Table 1). The result concurs with previous studies that pulp to acid ratio at elevated temperature and optimum time can yield higher fractions of sugar and solid residues [34] (Torget et.al 2000). Based on kinetic analysis of mechano-enzymatic hydrolysis of cellulosic material, total sugar around 35-50% can be removed [35] (Chang et al. 1981). In another study, overall yields of glucose at extremely low acid conditions at high temperature were 70 % [36] (Gurgel et al.2012).

The unaccounted fractions are most likely due to acid hydrolysis at elevated temperature mainly degrades the long chain cellulose polymer into glucose and then to furfural as well as hydroxyl-methyl furfural that accounts for 10-12% [37,38] (Qian et.al. 2005; Shen et.al. 2013). Under experimental conditions, the observation of brown discoloration, the other possible reason for lower yield of material related to the formation of carbonaceous material from hydrolysis reaction. As shown in Fig.1, proposed overall process scheme of various fractions that can be isolated from raw material and converted into useful fractions. MCCs is preferred starting material that can be recycled back into the acid hydrolysis process for preparation of NCCs, an increased yield of NCCs and sugar fraction can be foreseen [39] (Ioelovich. 2012).

Flow_chart1.png

Figure1: Over all process flow chart of fractionated raw material, recovery of acid and recycle of streams

Table 2.Chemical composition of filtrate-I and filtrate -II

Chemical compositions of spent liquor (% on original weight of material)

Description

Arabinose

Galactose

Glucose

Xylose

Mannose

High

0.16

0.14

24.41

3.03

3.96

Low

0.13

0.15

27.33

2.04

2.55

The sugar compositions in the acid spent liquor (filtrate-I) and neutral spent liquor (filtrate -II) are shown in Table 2. Under these conditions, recovery of 31.7-32.4% of the total sugars from the viscose pulp in this process is possible. During the citric acid hydrolysis treatment, the total dissolved sugars were mainly glucose that accounted for nearly 77.75%, while xylose, mannose and others accounted for 9.55%, 12.6% and 1% in the spent liquor, respectively. Total recoverable sugar from CA hydrolyzed method (based on cellulose) concurs with previous research studies [40] (Kootstra et.al 2009). Glucose can be converted into fuels and production of 5-hydroxymethyl furfural (HMF) in the catalytic conversion process and other value added products [41] (Hu et.al.2012).

Proposed scheme for recovering of products from spent liquor stream

Fig.2 shows a proposed flow chart for recovering valuable products (total sugar), recycling of acid filtrate from spent liquor and wash liquid (solvent) from respective streams. In the literature, solvent extraction method to recover most of the dissolved organics from the acid hydrolysis filtrate been demonstrated. For example, solvent extraction technique by using ethanol (EtoH) as a medium could effectively precipitate most of dissolved hemicelluloses/sugar fraction in the spent liquor up to 85%, which could be further separated from the ethanol/ citric acid /water solution through filtration operations [42,43] (Li et.al 2011, Mao et al.2008). In the same study, it was reported that the optimum condition for high recovery rate of dissolved organic fractions from hydrolysis liquor, a liquid to solvent ratio of 1:4 (v/v). Amorphous sugar fractions can be converted into value-added products, as described earlier [44] (Marinova, et al .2010).

fig2.png

Figure 2: Flowchart for recovery of sugar fractions and recycle of spent liquor.

In order to recover dissolved organics from the spent liquor, counter current (II-stages) solvent extraction procedure is practical, since most of the spent liquor consists of citric acid and dissolved organics. As a note, citric acid is sparingly soluble in ethanol at 15°C (76g/100ml) at absolute conditions, because citric acid does not precipitate in organic solvent medium. Effectively, critic acid from II nd stage filtrate can be recovered by the evaporation technique or crystallization at less than 20°C [45,46] (Pazouki and Panda.1998; Soccol et al. 2006). The recovered solvent and citric acid can be recycled to their respective process streams, as shown in Fig.2.The advantage for such pure spent liquor recovery with solvent extraction process over other conventional method is that it avoids the use of lime and sulfuric acid and the contaminant problem of gypsum disposal [47] (Dhillon et al. 2011).

NCC/MCC characterization

SEM and TEM observations

NCC is known to have strong mechanical properties, moreover it can be used in the production of high-strength engineered materials, and low-abrasive products [48,49] (Eichhorn and Young 2001; Mathew et al.2005). As reported, a well accepted fact is that the dimensions of the NCC are vital in yielding these mechanical properties. The TEM results for NCC as shown in Fig.3 (a). They revealed that the NCC is needle shape and rod like structures, and the width was in the range of 10-12nm (Individual particles).

Fig3b.pngFig3a.jpg

Figure.3 (a) TEM image of NCC (200nm) Figure.3 (b) Particle size distribution of NCC

In Fig.4 (a), the SEM micrograph of the MCCs are prepared from viscose pulp provides an evidence that MCC particles have well-defined crystal structure. Dimensional characteristics of the NCC and MCC samples prepared from viscose pulp, based on the citric acid hydrolysis as shown in Fig.3 (b) and 4 (b). The average length of NCCs and MCC were 158±25 nm and 11.9±3µm, respectively.

Fig4b.pngFig4a.jpg

Figure.4 (a) SEM image of MCC Figure.4 (b) Particle size distribution of MCC

The obtained average diameters for the NCC and MCC are 10nm and 12µm respectively, which is in agreement with other studies [16, 50] Beck-Candanedo et al. (2005); Pranger and Tannenbaum (2008).

Elemental Analysis

Energy Dispersive X-ray Spectrum (EDAX) analysis confirms that there is no presence of the sulfate group in the prepared NCC/MCC sample. The EDAX showed elemental composition peaks of NCCs with the majority of it is being carbon (84.8%) and oxygen (5.6%) as determined, which was consistent with that of cellulose fiber and other studies presented [51] (Jia et al. 2011). The amount of copper peak accounted for 8.1%, which is artifacts, due to the sample placed over the carbon coated copper grid exposed to the X-ray beam. Detection of hydrogen peaks in the spectrum was not possible, because of equipment limitations.

Thermo Gravimetric Analysis

Thermal stability of filler materials is very critical factor for many composite applications, including the use of NCCs in the production of bio-composites, whereas the processing temperature in such processes may rise above 200°C. NCC prepared from concentrated sulfuric acid, there is a certain degree of substitution of negatively charged sulfate groups rendered into their outer surface of the cellulose crystallites, a significant decrease in their thermal stability were reported [52] (Roman and winter, 2004).

fig5a.png

Figure.5 (a) Percentage weight loss of NCC between 25 to 600°C

Many studies about the decomposition thermal degradation of NCCs, which related to its structure, as reported. For example, Moon et.al. (2011) [53] showed that in the thermal analysis, cellulose decomposition occurs at 200-300°C. The onset of thermal degradation (temperature at 5% weight loss) for CA treated NCC was found at 253°C which was much higher than the acid processed nanocrystals. The DTG peak implied (Fig.5b) that the peak maximum temperature for CA processed NCC occurred at 359°C comparable to that of bacterial cellulose nano crystals at 379°C and degradation temperature of native cellulose around 400°C [54,55] (George et.al 2007,2011). NCCs prepared from low pulp to acid ratio a maximum weight loss (>70%) was reached at 315-375°C. As shown in Fig.5 (a), almost all cellulose was pyrolyzed at 420°C, and the charred residues were relatively small (<15.0 wt %). No other degradation peaks other than that of crystalline regions as seen in Fig.5 (a). However, the high pulp to acid ratio NCC had higher residue indicating that the more nonvolatile carbonaceous material formed during the pyrolysis reaction that occurs above 550°C [56] (Sain et.al 2006).The available xylan and mannan undergoes thermal degradation forming unwanted side product(free chain) that reduces thermal stability of materials [57](Um et al. 2009).

fig5b.png

Figure.5 (b) DTG curves of NCC.

The obtained results conclude that the prepared NCCs with different pulp to acid ratio were pure, as the residue was less than 9-12% at 600°C. As seen in Fig 5 (b), the decomposition peaks for the maximum percentage of weight loss were at 345°, 359° and 353°C for cellulose, low, and high pulp to acid ratio, respectively. However, NCC with low and high pulp to acid ratio has better thermal stability compared to other NCCs that is prepared from concentrated sulfuric acid hydrolysis procedure. Based on the above results, we conclude that the sulfate free NCCs from citric acid hydrolysis have excellent thermal stability, and highly suitable in the production of bio-composite materials.

FT-IR

The FT-IR spectra of NCC and MCC sample as shown in Fig.6 (a) and 6 (b). The broad band at 3340-3420 cm-1 related to stretching of H-bonded OH groups, and 2900 cm-1 to the C-H stretching spectra. The absorbency at 658, 901, 1033, 1114, 1158 cm-1 increased weakly, while it caused a small change at 1373, 1431, 2900 cm-1 compared to cellulose spectra. A very strong shoulder at 1720 cm-1 in NCC and MCC spectra is the representation of carbonyl (C=O) groups as indicated by an arrow in Fig.6 (a) and 6 (b). As reported, Sun et al. (2008) [58], acid hydrolysis mainly removed the amorphous region on the surface, therefore indicating an increase in the stretching absorbency, and more C-OH, C-O-C and C-C bonds were exposed. Another strong shoulder at 1430 cm-1 is an indicative of symmetric CH2 bending vibration in the prepared NCC/MCC. This band is termed as "Crystallinity Band" indicating a higher degree of crystallinity or crystallinity index of sample [59] (Diana et al. 2010). The absorbance spectra at 1400-1600 and 1200 cm−1 gives an estimation of sulfate group those are present. Such peaks were not available as in Fig 6 (a). A sharp peak at 1720 cm-1 indicating a significant amount of carbonyl group's presence; a greater extent of oxidation reaction has occurred.

fig6.png

Figure. 6(a) Spectral analysis of NCC in between 500-4000 cm-1

fig6a.png

Figure 6(b). Spectra analysis of MCC in between 500-4000 cm-1

As shown in Fig.6 (a) and 6 (b), both NCC and MCC produced from low pulp to acid ratio had a higher transmittance of spectra peak at 1430 cm-1 indicating higher crystallinity index than that of high pulp to acid ratio. A strong absorption band at 1430 cm-1 for NCC, and medium band for MCC indicate higher crystallinity than amorphous region (indicated by an arrow in Fig.6 (a) and 6 (b). Such peaks were not present in the cellulose sample. Both NCC and MCC showed characteristics of a typical FTIR spectra band confirms transition of cellulose I to cellulose II form. The crystallinity proportion in the NCC and MCCs increased in the order of low > high > cellulose.

X-ray diffraction

Fig.7 shows the X-ray diffraction patterns of the NCC samples. Using origin Pro 9.0, de-convoluted X-ray diffraction cumulative fit for NCCs amorphous and crystalline peaks as in Fig.7. A highly native cellulose-I characteristics can be interpreted by peaks at 15, 16 and 22.5° corresponds to the 2-Ɵ value of 1-10,110 and 002, respectively. The peak at 2-Ɵ =22.5° is sharper for low pulp to acid ratio, an indicative of a higher crystallinity degree in the NCC structure. The crystallinity index of viscose pulp was 40.2%, which increased to 77.1% for prepared NCCs at low pulp to acid ratio. The crystallinity index of NCC, MCC, and cellulose are shown in Table 3. The FT-IR results with a strong peak at 1430 cm-1 confirm an increase in crystallinity index as in Fig.6 (a).

fig7a.png

Figure 7. De-convoluted X-ray diffraction cumulative fit spectra peaks of NCC at different pulp to acid ratio, and cellulose

The result concurs with other studies, indicating that the crystallinity index increased with the transformation of microfibrils to nano-fibrils and crystallites [60, 61] (Park et.al 2010; Lee et.al. 2009). The higher crystallinity index is associated with higher tensile strength of crystallite material. NCCs prepared in this process have a high crystallinity index more than that of cellulose indicating higher tensile strength of the material. The crystallinity index for different pulp to acid ratio increased in the order of low> high> cellulose.

Table 3: Crystallinity index of NCC, MCC and cellulose

Description

NCC

MCC

Amorphous

(2ÆŸ)

Crystalline

(2ÆŸ)

C.I

(%)

Amorphous

(2ÆŸ)

Crystalline

(2ÆŸ)

Low

15.2,16.2

22.4

74.0

15.2,16.4

22.5

High

15.2,16.1

22.6

77.1

15.2,16.0

22.4

Amorphous

(2ÆŸ)

Crystalline

(2ÆŸ)

C.I

(%)

Cellulose

16

22

40.2

Birefringence observations

Birefringence is a unique optical characteristic exhibited only by orderly oriented crystallite structure. Such iridescence characteristics can be used for certain applications, such as bank notes; ID cards; as well as passports with optical variable coating based on viewing angle [62] (Godbout et al. 1997). The study revealed that NCCs in suspension, under controlled drying, showed birefringence and nematic pitch at different wavelength spectra can be retained [2] (Beck et al.2010). Furthermore, NCC films produced from cellulosic material exhibit iridescence by reflecting left-handed circularly under polarized light in a narrow wavelength band, as reported. From the observations, NCCs film dried on the side of glass vial, such iridescence were present. After exposure for 15 min at 40% of sonication power (300watts), MCC aggregated particles in suspension were observed under an optical microscope (40x) for determining whether the particles are optically active or not. Fig.8 (a) shows birefringence characteristics of single as well as aggregated crystal of MCC in suspension, under a bright field transmittance mode, viewed in cross-polarized light (40x), confirms optically very active. The significance of MCC crystals exhibiting such iridescence is that exposure of the crystallite in the fibrils in this CA hydrolysis processes as in Fig. 8 (a) and 8 (b).

Fig8b.jpgFig8a.jpg

(a) X-ray diffraction patterns of MCC (b) 3D view of MCC aggregated particles

Figure 8: Birefringence characteristics of prepared MCC using an optical microscope (40x) (a)Bright field transmittance viewed in cross polarized light (left) (b) 3D view of MCC aggregated particles extracted using Image J (Right).

In Fig.8 (b), MCC aggregated particles (40x) viewed in cross polarized light, a 3-D spacing with X= 90µm,Y= 110µm, and Z =220 counts (intensity of the LUT scale), are extracted using Image J software. This confirms that NCC or MCC crystallites produced from this acid hydrolysis process have a significant amount of iridescence and a potential material for special application as mentioned earlier. MCC single particles on 2-dimensional space has a diameter of 10±2.5µm and a length of 50-75µm as in Fig. 8 (a). The extracted 3-D image of MCC aggregated as in Fig.8 (b) exhibits bluish color on the right half portion, and bright reddish color on the left half portion of Y-X co-ordinates are similar to optical microscopy image and is very accurate as seen in Fig.8 (a). As reported, post-sulfonated NCC suspensions formed a birefringent glassy phase having a crosshatch pattern, due to their low surface charge densities [63,14] (Arkai et al. 2000; Habibi et al. 2006). In contrast, MCC produced from citric acid hydrolysis showed birefringence characteristics is unique, compared to other weak acid hydrolysis isolation methods as mentioned elsewhere.

Conclusions

Citric acid treatment process can effectively remove the amorphous fraction from viscose pulp to produce a sulfate-free NCC and MCC. The proposed process requires much less water to achieve neutral pH, and further processing steps are not required. As per the proposed scheme, separation and recycling of spent liquor using the solvent extraction technique is highly possible. The total yield of NCC and MCC from this process was very high (48.8%), with a diameter of 10±2.0nm and 11.9 ±3.0µm, respectively. A large amount of sugars can be extracted (about 317-322 kgs /ton of pulp) in this process, which fits very well within the bio-refinery concept. An aggregated particle of MCC viewed under cross-polarized light exhibited optical birefringence characteristics and confirms produced crystallite is pure. NCCs produced from a low pulp to acid ratio showed an overall yield higher than that of a high pulp to acid ratio, and increased crystallinity index in the order of low pulp to acid ratio> high pulp to acid ratio> cellulose. Based on the results, CA process is efficient and easy to adopt for an economy of scale.

Acknowledgement

We thank Dr. Louise Weaver, Dr. Susan and Steven Cogswell of UNB for SEM and TEM observations.