8.1. ABSTRACT - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/13586/16/16_chapter 8.pdf ·...
Transcript of 8.1. ABSTRACT - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/13586/16/16_chapter 8.pdf ·...
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8.1. ABSTRACT
Nanowhiskers obtained from microcrystalline cellulose (MCC) are of huge interest
due to their good mechanical strength. The present study deals with preparation of
cellulose nanowhiskers (CNW) from microcrystalline cellulose using high pressure
homogenizer. Microcrystalline cellulose (MCC) was prepared by acid hydrolysis of
short staple cotton fibers using hydrochloric acid. To achieve better homogenization,
MCC was passed through homogenizer repeatedly till 15 passes; and after every 3
passes was characterized using scanning electron microscopy (SEM), atomic force
microscopy (AFM), X-ray diffractometer (XRD) and viscometer. From SEM it was
found that the average diameter of MCC decreased significantly from 5-10 μm to
about 60 nm (also confirmed by AFM) after 15 passes. Force distance (FD) curve
analysis demonstrated that the Young’s modulus of CNW was about 452 MPa.
Prepared CNW was used as a reinforcing agent in chitosan. Tensile strength and
Young’s of the composite increased by 33.3% and 52.3% respectively, whereas %
elongation at break decreased by 69.6%, at 3% loading of CNW. Enthalpy of melting
increased with increase in concentration of CNW in Chitosan but no significant
change was observed in the melting temperature. CNW was found to have uniformly
distributed at 3% concentration, above which it started aggregating as depicted by
SEM.
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8.2. INTRODUCTION
Biocomposites are composite materials comprising one or more phase(s) derived from
a biological origin. Bionanocomposites are plastics from renewable raw material
reinforced with either Nanocellulose extracted from pulp fibers or some other
nanomaterials. Bionanocomposites have established themselves as a promising class
of hybrid materials derived from natural and synthetic biodegradable polymers and
organic/inorganic fillers (Hule et al, 2007). Attempts to improve the properties of
materials through the addition of reinforcing fillers, either inorganic or organic, are
not new. For years, synthetic polymer composites have been developed and applied in
various industrial fields, domestic equipment, the automotive industry, and even in
aerospace. However, these synthetic materials come from limited, non-renewable
sources, and are not easily decomposed (Lu et al, 2009 and Tokiwa et al, 2009) by
microorganism present in nature. In addition, global warming, caused in part by
carbon dioxide released by the combustion of fossil fuels, has become an increasingly
important problem and the disposal of items made of petroleum-based plastics such as
fast-food utensils, packaging containers, and carrier bags also creates a serious
environmental problem (Wool and Sun, 2005). As we begin the 21st century, there is
an increased awareness that non renewable resources becoming scarce and
dependence on renewable resources is growing (Rowell et al, 2008). A growing
environmental awareness all over the world and the pressures to use evermore
“greener” technologies (Bismarck et al, 2008) have encouraged researchers and
industrialists to consider natural plant fibers as an alternative reinforcing agent or
filler to produce composite materials known as biobased composites (Thakur et al,
2010). Natural fiber composites are also claimed to offer environmental advantages
such as reduced dependence on non-renewable energy/material sources, lower
pollutant emissions, lower greenhouse gas emissions, enhanced energy recovery etc.
(Joshi et al, 2004) There are various types of the natural fibers and are classed
according to their source; plants, animals or minerals (Eichhorn et al, 2001).
By far the most abundant are the wood fibers (Siqueira et al, 2010) from trees;
however other fiber types are emerging. A common choice for reinforcement of bio-
based composites is cellulose. It is well known that cellulose fiber networks–as in the
case of paper – provides good mechanical properties because of the degree of
hydrogen bonding obtained between the fibers in the network. The greater the
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hydrogen bonding, the stronger the composite material and nature of hydrogen
bonding depends on the size of the cellulose fibers or whiskers (figure 8.1)
Figure 8.1. Comparison of hydrogen bonded cellulose network in nano (A) and micro
(B) size.
Renewable biomaterials offer a broad range of commodities, including forests, crops,
and farm and marine animals; all of which have many uses. The idea of using plant-
based fiber as reinforcement has been around since the beginning of human
civilization. The use of straw-reinforced clay for making stronger bricks is one of the
earliest composites referred to in the literature (Prassad, 1992). Plenty of examples
can be found of plant-based fibers being used for reinforcement of petrobased
thermoplastic polymers such as polypropylene (PP), polyethylene (PE), nylon and
polyvinylchloride (PVC). Plant fibers have also been used to reinforce biodegradable
polymers such as cellulose ester, polyhydroxybutyrate (PHB), polyester amide,
polylactic acid (PLA) and starch derivatives and blends (Peijs, 2000). These types of
composites can be used for packaging (Kumar et al, 2010, Bucci et al, 2005,
Savenkova et al, 2000, and Sorrentino et al, 2007) agriculture (Dilara et al, 2000 and
Dave et al, 1999), and biomedical (Kamel S., 2007 and Kim et al, 2005) applications.
Microcrystalline cellulose (MCC), bundles of crystallites, has attracted attention as
the starting material for preparing biocomposites (Mathew et al, 2005). The source of
raw material and manufacturing process decisively influence the characteristics of the
MCC (Douglas et al, 2008).
In the present work, cellulose nanowhiskers (CNW) were prepared using high
pressure homogenizer, in which combination of shear, cavitational and impact forces
acted on MCC. CNW prepared by the homogenization process were characterized by
Scanning electron microscopy (SEM), Atomic force microscopy (AFM), X-ray
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diffraction (XRD) and viscometer. The cellulose nanowhiskers produced by above
method were used as reinforcing agent in chitosan to enhance its performance. The
chitosan/CNW nanocomposites were prepared using solution casting process and
characterized for mechanical, morphological, thermal, optical and barrier properties.
8.3. EXPERMINTAL WORK
8.3.1. Materials and Methods
Short staple cotton fibers were used as the starting material for preparing MCC was
procured from Fem Cotton Pvt. Ltd., Rajkot, India. Sodium hydroxide and hydrogen
peroxide were purchased from Thermo Fisher Scientific India Pvt. Ltd., Mumbai,
India. Sodium silicate (meta) nonhydrate and hydrochloric acid (11.6N) were supplied
by S. D. Fine-Chem Ltd., Mumbai, India. Nonyl phenol ethylene oxide (wetting
agent) was supplied by Amrutlal Industrial products, Mumbai, India. All chemicals
were used as supplied without any modification or further purification.
8.3.2. Process Flow Diagram and Experimental Work:
Figure 8.2 depicts the typical route followed to synthesize CNW and its application in
chitosan nanocomposites. The bleached cotton fibers were used as the raw material
for production of MCC. MCC was produced by acid hydrolysis using 4N HCl
followed by cooling, neutralization and drying. The dried MCC was used as the
starting material in homogenization process to obtain CNW. Initially, MCC was
dispersed in distilled water (10 g MCC in 1l water) and passed through the
homogenizer at a pressure of 35000 psi. Uniform nanowhiskers were obtained by
repeated passing of solution through the homogenizer (max 15 passes) and after every
3 passes were characterized for settling, degree of polymerization, AFM and SEM.
The prepared CNW were used as a reinforcing agent in chitosan to improve its
performance properties. The chitosan/CNW composites were prepared by solution
casting process. The composites were characterized for mechanical, thermal, barrier,
optical and morphological properties.
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Figure 8.2. Process Flow Diagram for Preparation of CNW and CNW/Chitosan
Nanocomposite
8.4. CHARACTERIZATION
8.4.1. Characterization of cellulose nanowhiskers
8.4.1.1. Settling Study
Settling of the suspension depends upon the particle size. Particle size was expected to
decrease with increase in no of passes of MCC through the homogenizer. For this
study, 50 ml of homogenized suspension after every 3 passes was added to a
Cotton fibers
Kier boiling and
Bleaching
Drying at 40°C
Hydrolysis with
HCl
Neutralization,
Filtration and Drying
Homogenization
Centrifugation and
Lyophilization
Characterization
(Settling study, SEM,
AFM, XRD, DP)
Application in
Chitosan by Solution
Casting
Characterization(Mechanic
al, XRD, Thermal, Optical,
Barrier and Morphological
Properties)
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measuring cylinder, opening of which was closed by using parafilm. The suspensions
were allowed to settle for 24 hrs and then their settling height was recorded.
8.4.1.2. Scanning Electron Microscopy (SEM)
A Phillips® scanning electron microscope operated at 15 kV was used to take
micrographs of MCC samples before and after homogenization. A drop of diluted
(1ml homogenized sample diluted in 100 ml deionized water) sample was taken on a
mica sheet and was dried at room temperature. Dried samples were then vacuum
sputtered with gold-palladium mixture to improve their conductivity.
8.4.1.3. Atomic Force Microscopy (AFM)
A scanning probe microscope (Veeco®) was used to characterize the surface
morphology of CNW. AFM height images of the MCC samples after homogenization
were taken in a tapping mode with silicon cantilever at a frequency of 253Hz.
The force distance analysis was also carried out in order to evaluate the strength of a
single CNW. Imaging was done in a contact mode using silicon nitride tip at different
magnifications. 0.5µm magnification was selected for the measurement of modulus of
elasticity. Before starting the analysis, spring constant was measured by calibrating
the tip. After completion of the calibration force distance analysis was carried out by
indenting the tip on the sample surface with the help of point spectroscopy. The
voltage data obtained by point spectroscopy was analyzed using SPIP (scanning probe
image processor) software which converts the voltage into force-distance relation.
There are different models18-19
such as DMT and Sneddon which describes about the
force distance curve analysis. The DMT model assumes that the probe has some
radius of curvature i.e. spherical in nature at end where as Sneddon model assumes
that the probe has conical shape. In the present study DMT theory has been used for
the analysis. From this we can get Youngs modulus which gives an idea about the
mechanical strength of the sample.
8.4.1.4. X-ray diffraction (XRD) analysis
XRD measurements were performed on a Rigaku® Wide Angle X-ray diffractometer
(WAXD). The XRD of CNW and chitosan-CNW nanocomposites were measured in a
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2θ range from 4° to 40°. Crystallinity of the samples was measured by using the
equation 8.1.
% Crystallinity ={[Ic-Ia]/Ic} x 100………………………………………………..(8.1)
Ic is the peak intensity of crystal plane at 2θ=22 which is also called as principle peak
and Ia is the peak intensity of amorphous phase at 2θ=18.
8.4.1.5. Degree of polymerization (DP)
Degree of polymerization was measured with the help of Ubbelohde viscometer. 50
mg of sample was dissolved in 100 ml of cupra ammonium hydroxide. The mixture
was vigorously shaked and kept in a water bath at 25°C. Viscosity was measured only
when the sample got completely dissolved in the solvent. The viscosity was
calculated from the efflux time of the cellulose solution and pure solvent. Degree of
polymerization was calculated from equation 8.2 and 8.3,
DP = [{(2000*ηspec)/(c*(1+0.29* ηspec)}].................................................................(8.2)
ηspec = [(η/η0)-]…………………………………………………………………….(8.3)
Where, ηspec is specific viscosity, η/η0 is relative viscosity, c is concentration (g/l), t0 is
the efflux time of the solvent and t is the efflux time of the sample.
8.4.2. Characterization of the Chitosan/CNW Nanocomposites
8.4.2.1. Mechanical Properties
The tensile strength and percent elongation at break of the films was determined using
Universal Testing Machine (LR-50K, LLOYD instrument, UK) using 500N load cell
in accordance to ASTM D 882.
8.4.2.2. Differential Scanning Calorimeter (DSC)
DSC was used to measure thermal transitions of the chitosan/CNW nanocomposite
films. The test was performed using Q100 DSC (TA Instruments) equipment, fitted
with an nitrogen-based cooling system. The samples were weighed in aluminium pans
whereas an empty pan was used as the reference pan. All the measurements were
performed in the temperature range from -40 to 200ºC at a heating rate of 10ºC/min.
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8.4.2.3. X-ray Diffraction (XRD) Analysis
X-ray Diffraction (XRD) patterns were obtained using a Rigaku Miniflex X-ray
diffractometer using Cu target and having X-ray wavelength of 1.54 A through 4 to
40° angle.
8.4.2.4. Optical Properties
The light transmittance of the chitosan and chitosan/CNW films having thickness of
about 70 µm was measured using an ultraviolet–visible (UV–Vis) spectroscope (UV-
160A, Shimadzu, Japan) in a wavelength range from 200–800 nm.
8.4.2.5. Water Vapour Transmission Rate (WVTR)
Water Vapour Transmission Rate (WVTR) of the films was determined
gravimetrically in accordance to ASTM E96. The composite films were cut into
circles of 90 mm diameter and then were sealed on the permeation cells, containing
calcium chloride, using paraffin wax. The permeation cells were placed in a
desiccator in which RH was maintained at 71%. The water transferred through the
film gets absorbed by the desiccant which is determined from the weight of the
permeation cell. Each permeation cell was weighed at an interval of 24 hrs. The
WVTR was expressed in g/h.m2 per day.
8.4.2.6. Morphological Properties (SEM)
The morphology of the nanocomposite films was observed under a scanning electron
microscope (SEM). SEM analysis was carried out using Philips® XL30 (Netherland)
Scanning Electron Microscope. Samples were fractured under liquid nitrogen to avoid
any disturbance to the molecular structure. The specimens were then coated with gold
and palladium using sputter coater before imaging.
8.5. RESULTS AND DISCUSSIONS
8.5.1. Characterization of cellulose Nanowhiskers
8.5.1.1. Settling study
In order to understand the effect of number of passes on the size of the obtained
whiskers, settling study was performed. Settling depends on the particle size of the
sample. Bigger size particles settles faster compared to the smaller size particles.
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Figure 8.3. Settling study of homogenized cellulose suspension after 3, 6, 9, 12 and
15 passes
Figure 8.3 shows the settling study of the MCC before and after (3, 6, 9, 12, 15
passes) homogenization. It was observed that as the number of passes increased,
stability of the cellulose suspensions also increased. This can be attributed to the size
reduction taking place which enhanced the surface area of the whiskers. Settling was
observed up to 9 passes but after 12 the suspension was stable which indicates that all
MCC is converted in to CNW.
8.5.1.2. Scanning Electron Microscopy Analysis
Surface morphology of the CNW was done using SEM. Figure 8.4 shows
micrographs of MCC and homogenized MCC after 3 and 15 passes.
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Figure 8.4. Scanning electron micrographs of (a) MCC (2000X), (b) MCC (15000X),
(c) whiskers after 3 passes and (d) CNW after 15 passes
It was observed that the initial diameter of the MCC was about 6 µm however got
reduced to 50 nm after 15 passes in homogenizer. Even after 3 passes MCC was
converted into CNW but the amount produced was very less and also lacked
uniformity. After 15 passes MCC got completely converted in to CNW with uniform
size distribution. This can be due to the continuous exposure of MCC to the severe
forces like shearing, cavitational and impact which acted on it during the
homogenization process.
8.5.1.3. Atomic Force Microscopy Analysis
Surface morphology and strength of CNW was analyzed using atomic force
microscopy. Figure 8.5 depicts the AFM image of the homogenized microcrystalline
cellulose (MCC) after 15 passes.
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Figure 8.5. AFM height image of CNW obtained after 15 passes in the
homogenization process
Figure 8.5 depicts the AFM height image of CNW obtained after 15 passes in the
homogenization process. AFM height image of the CNW, obtained after 15 passes,
was taken in a tapping mode at a frequency of 253Hz using silicon nitride cantilever.
It can be observed from the height image that the diameter of CNW was about 40 nm.
The force distance analysis was also carried out in order to evaluate the strength of a
single CNW. The Youngs modulus was found to be about 452.39 MPa.
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8.5.1.4. X-ray diffraction (XRD) analysis
Figure 8.6. X-ray crystallograph of the MCC before and after (3, 6, 9, 12 and 15
passes) homogenization
Figure 8.6 illustrates X-ray crystallographs of the MCC before and after (3, 6, 9, 12
and 15 passes) homogenization. Crystallinity of MCC before homogenization was
found to be 93.23% whereas after 3, 6, 9, 12 and 15 passes in homogenizer was found
to be 90.71, 90.39, 90.37, 90.34 and 89.84% respectively. Homogenization process
did not appreciably influence the crystallinity of the cellulose whiskers. Mechanical
forces exerted in homogenization process were unable to disturb the structural
arrangement of the cellulose even after 15 passes due to its high crystallinity.
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8.5.1.5. Degree of Polymerization
Table 8.1. Degree of polymerization of the homogenized MCC after 3, 6, 9, 12 and
15 passes
No. of Passes Degree of Polymerization
0 1528.17±26.74
3 1262.31±26.97
6 1032.76±27.15
9 1013.56±27.15
12 936.66±27.2
15 917.39±27.24
Degree of polymerization (DP) is the number of repeating units present in a polymer
chain. Mechanical stresses generated due to shearing, cavitational and impact forces
immensely influenced the chain scission process of cellulose and hence it’s DP.
Homogenization resulted in significant reduction of DP of the cellulose. Table 8.1
signifies that the DP decreased by 17.4, 32.4, 33.7, 38.7 and 39.9% for 3, 6, 9, 12 and
15 passes respectively in the homogenization process.
8.5.2. Characterization of the Chitosan/CNW Nanocomposites
8.5.2.1. Mechanical properties
Table 8.2. Mechanical Properties of the Chitosan/CNW Composite
Sample Tensile Strength
(MPa)
Youngs Modulus
(MPa)
Elongation
at Break (%)
Control Chitosan 27.44±2.63 572.83±21.37 43.91±0.97
1%CNW Chitosan 33.73±2.09 1201.6±44.7 36.81±2.18
3%CNW Chitosan 41.12±1.46 1360.22±63.2 13.35±1.92
5%CNW Chitosan 37.87±1.77 1059.62±81.62 16.73±6.82
Table 8.2 depicts the mechanical properties like of the tensile strength, Youngs
modulus and percentage elongation at break of control chitosan and chitosan/CNW
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nanocomposites. Tensile strength and Youngs modulus increased up to 3%
concentration of CNW in chitosan, above which they started decreasing. Tensile
strength & Youngs modulus were found to have increased by 33.3 and 52.3%
respectively, whereas, percentage elongation at break reduced down by 69.6%. As the
concentration of CNW increased above 3% it might have started forming aggregates
resulting in more number of stress concentration points.
8.5.2.2. X-ray diffraction (XRD) analysis
Figure 8.7. X-ray crystallographs of the control cellulose (CNW), control chitosan
and chitosan/CNW Nanocomposites
Figure 8.7 indicates the X-ray crystallographs of the control cellulose (CNW), control
chitosan and chitosan/CNW nanocomposite. It was observed that as the concentration
of the CNW increased, the crystallinity of the nanocomposite also increased.
Crystallinity of the control chitosan was 2.05% but increased to 5.9, 6.3 and 7.1%
after incorporation of 1, 3 and 5% of CNW in chitosan respectively. Increase in the
crystallinity of the sample was attributed to incorporation highly crystalline cellulose
nanowhiskers (CNW).
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8.5.2.3. Differential Scanning Calorimetry
Figure 8.8. DSC thermograms of the control Chitosan and Chitosan/CNW
Nanocomposite
Figure 8.8 depicts the DSC thermograms of the control chitosan and chitosan/CNW
nanocomposite. During the analysis the samples were heated from -40 to 210°C at a
heating rate of 10°C/min. Melting points of the control chitosan, 1, 3 and 5% CNW
loaded chitosan were 107.7, 107.1, 111.3 and 106.9°C respectively. Enthalpy of
melting (∆H) for control chitosan, 1, 3 and 5% CNW loaded chitosan were 65.2, 68.6,
70.9 and 62.2 respectively. ∆H might have increased due to the increase in
concentration of highly crystalline CNW in chitosan, which required more energy for
melting.
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8.5.2.4. Optical Properties
Figure 8.9. Effect of CNW concentration on the Transparency of the Chitosan/CNW
Nanocomposite
It was observed from figure 8.9 that control chitosan films were more transparent and
it started reducing as the concentration of CNW increased from 1% to 5%. Addition
of CNW in chitosan increased its crystallinity providing barrier to the transmission of
light, thus increasing haziness of the composite film.
8.5.2.5. Water vapour transmission rate (WVTR)
Figure 8.10. Water vapour transmission rate of chitosan/CNW Nanocomposites
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Figure 8.10 depicts the WVTR of control chitosan and chitosan/CNW
nanocomposites. It was observed that as the concentration of CNW increased, WVTR
decreased up to 3% CNW, above which it started increasing. As compared to control
chitosan WVTR decreased by 18% and 32% for 1 and 3% CNW loaded chitosan
respectively. Addition of CNW in chitosan increased its crystallinity providing barrier
to the transmission of water vapour, thus increasing the barrier property of the
composite film. Increase in WVTR for 5% CNW concentration was due to the
formation of aggregates as evident from SEM micrographs.
8.5.2.6. Scanning electron microscopy analysis
Scanning electron microscopy (SEM) analysis was done in order to understand the
correlation between the dispersion behavior of CNW in to chitosan films and their
performance.
Figure 8.11. SEM micrographs of the chitosan/CNW nanocomposite taken at 15KV
and 10000X. (a) 3% CNW and (b) 5% CNW
Figure 8.11 depicts SEM micrographs of the chitosan/CNW nanocomposite for (a)
3%CNW and (b) 5% CNW loaded chitosan. It was observed that at 3% concentration
of CNW, dispersion was uniform but as the concentration of the CNW increased to
5%, presences of aggregates were observed. Formation of aggregates in 5% CNW
loaded chitosan (as evident from Figure 10 (b)) can be a probable reason for overall
reduction in the properties above 3% CNW concentration.
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8.6. CONCLUSIONS
CNW from MCC were successfully produced using homogenization process. Settling
study showed that the cellulose suspension was stable above 12 passes. SEM
micrographs depicted that some amount of MCC were converted into CNW after 3
passes of homogenization but were not of uniform size, but after 15 passes all
whiskers were very fine and had uniform size disribution. Youngs modulus obtained
from force distance curve using AFM was about 452 MPa. Degree of polymerization
decreased by 39.9% after 15 passes of homogenization. Crystallinity of CNW was
insignificantly affected even after 15 passes in the homogenization process. Tensile
strength and Youngs modulus of the chitosan/CNW nanocomposite improved by 33.3
and 52.3% respectively, whereas % elongation at break decreased by 69.6%.
Crystallinity of the nanocomposite increased from 2% (control chitosan) to about 7%
after incorporation of CNW in chitosan. DSC analysis showed that there was no
significant change in the melting temperature but enthalpy of melting increased with
increase in concentration of CNW. WVTR decreased by 32% for 3% CNW loaded
chitosan. SEM micrographs of the nanocomposites depicted that uniform dispersion
was observed at 3% concentration of CNW, but as the concentration increased CNW
started forming aggregates.