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Chapter 3
SURFACE CHARACTERISTICS, SPECTROSCOPIC INVESTIGATIONS AND
THERMAL BEHAVIOUR OF BANANA FIBRES- MACRO TO NANO SCALE
Abstract
Surface chemistry and surface morphology of the reinforcement play an important role for the development of a strong interface between reinforcement and the matrix. In the present study, the surface of chemically modified and unmodified banana fibres with varying fibre diameters were investigated by atomic force microscopy, transmission electron microscopy, and scanning electron microscopy and environmental scanning electron microscopy. The crystallinity and diameter of the micro and nanofibres were compared using X-ray diffraction studies. In addition to the surface morphology, the acid-base property of a fibre surface determines its interaction with other materials with which they are in contact. The surface composition and surface polarity of the fibres with diameters in the macro and nano range were determined using solvatochromic measurements involving various probe dyes and also by electrokinetic studies. Zeta potential measurements carried out on the fibres confirmed the results obtained from solvatochromic measurements. The thermal behaviour of macro, micro and nanofibres were also compared. Substantial increase in thermal stability was observed from macro to nano fibres which proved the high thermal stability of nanofibres to processing conditions of biocomposite preparation. The composition of the fibres before, after steam explosion and acid hydrolysis were also analysed using FT-IR. The results of solvatochromism and zetapotential corroborate FTIR results.
The results of this paper have been submitted for publication in Colloid and Interface Science
134 Chapter 3
3.1 Introduction
Fibre reinforced thermosetting composites are highly beneficial because
the reinforced materials improve the strength and toughness of the plastics
(1-3). Natural fibres which are rich in cellulose, the most abundant
biopolymer, is sustainable, biodegradable and have low density.
Additionally these materials have low toxicity and abrasiveness. Natural
fibres find application in the production of automotive parts (4).
Nanofibrils separated from the natural fibres have also been used for
processing nanocomposites so that the mechanical properties may be
improved (5). The reinforcement is regarded as a nanoparticle when
atleast one of the dimensions is lower than 100nm. The nanocomposites
assume exemplorary and novel properties, unseen in conventional
macrocomposites, when this particular feature is attained (7). Gandini and
Belgacem (7, 8) illustrated that the use of cellulose nano crystals as a
reinforcing phase in nanocomposites has numerous well known
advantages. Recently Cherian et al. synthesized nanofibril whiskers from
banana fibres (9).
Electrokinetic phenomena can be observed by contacting a solid surface
with a polar liquid medium, because of the existence of an electrical
double layer at the solid-liquid interface, dispersion or acid-base interaction
(10). The surface polarity of grafted carbon fibres was determined by contact
angle measurements and confirmed by zeta potential measurements.
Bismarck et al. (11) reported on the characterization of modified jute fibres
using zeta potential measurements. Pothen et al. (12) studied the influence of
chemical treatments on the electrokinetic properties of cellulose fibres.
Bellmann et al (13) investigated the electrokinetic properties of natural fibres
Surface characteristics, spectral studies…
135
and concluded that this is suitable to analyse the swelling characteristics of
fibres. Measurements such as scanning probe microscopy and scanning
electron microscopy were employed to assist with the interpretation of
results.
The present work highlights the investigation results of the morphological
and surface properties of banana fibres in microfibrillated and
nanostructured forms. Careful analysis of the literature indicates that no
study has been reported on the systematic comparison between these
properties of microfibrilated and nanostructured cellulose fibres. Plant
based cellulose nanofibres have generated a great deal of interest as a
source of nanometer sized fillers because of their sustainability, easy
availability, and the related characteristics such as a very large surface to
volume ratio, high tensile strength, high stiffness, high flexibility, good
dynamic, mechanical, electrical and thermal properties as compared with
other commercial fibres (14-17). The use of nano reinforcements in the
polymer matrix has been predicted to give improved properties compared
to the neat polymer and micro composites based on the same fibres.
Therefore it is of great interest to examine the possibilities of cellulose
based nanofibres as reinforcing elements. Treating various biomass
resources by steam explosion has been studied by many researchers,
(18-20). During steam explosion process the raw material is exposed to
pressurised steam followed by rapid reduction in pressure resulting in
substantial break down of the lignocellulosic structure, hydrolysis of the
hemicelluloses fraction, depolymerisation of the lignin components and
defibrillisation (21). The effect of the difference in non-cellulosic
136 Chapter 3
composition and degree of structural disruption on the thermal stability is
an important issue to be investigated (22).
3.2 RESULTS AND DISCUSSION
3.2.1 Characterization of banana fibre
Electron Microscopical Analysis:
Banana fibres obtained from local sources was subjected to steam
treatment to obtain micro and nano fibres. The diameters of these three
were compared usig SEM. The fibre diameter of the raw sample was
observed to be in the range of 80µm while that for micro fibres are in the
range 10-15µm. The average fibre diameter was found to be much lower
for nanofibres. (5-15nm in diameter and 200-250nm in length). The fibre
diameter distribution curves are shown in Fig 3.1 (a, b and c). From the
distribution curves it is seen that there is a decrease in fibre diameter as
we move from macro to nano scale. In the case of macro, maximum
number of fibres have 80µm diameter. For micro, maximum number of
fibres have 10µm diameter and for nano fibres the number of fibres with
minimum fibre diameter (5nm) was found to increase.
Surface characteristics, spectral studies…
137
20 40 60 80 1000
5
10
15
20
25
30
35
Num
ber
of fi
bres
Fibre diameter (µm)
macro fibre
0 5 10 15 200
5
10
15
20
25
30
35
Num
ber
of fi
bres
Fibre diameter (µm)
micro fibre
(a) (b)
5 10 15 20 25 300
5
10
15
20
25
30
35
Num
ber
of fi
bres
Fibre diameter (µm)
nano fibre
(c)
Figure 3.1 Fibre diameter distribution curves of banana fibre (a) macro, (b) micro and (c) nano fibres
The structure and appearance of banana fibres in micro to nano-scale by
SEM is shown in Fig 3.2 (a, b and c). Steam treatment of macrofibre at high
pressure reduces the fibre diameter. It is clear from the SEM micrographs
that high pressure steam treatment helps in fibre separation and fibrillation
138 Chapter 3
(Figure 3.2 b). The tendency for fibre defibrillation was found to increase
during the transformation to nanoform. These conclusions were further
supported by the ESEM images shown in Fig 3.2 (c).
(a) (b)
(c)
Figure 3.2 SEM images of (a) macro banana fibre, (b) micro banana fibre, (c) ESEM image of banana nanofibre
It is evident from the ESEM images that the tendency for fibrillation
increased with chemical treatment and high pressure drop. The drop in
pressure facilitates the increase in the fibrillation process of the banana fibres
whose size range is in nanometer scale. It can be seen from ESEM image
shown in Fig 3.2 (c) that the cellulose nano fibres obtained from the banana
fibres are in the entangled fibril form and the length to diameter are in the
Surface characteristics, spectral studies…
139
range of 250 to 5 nm, having a wide range of aspect ratio (length/diameter),
the average value being 50.
Transmission electron microscopic analysis
The TEM investigation of the micro fibres (Fig 3.3 a) shows that the fibres
have a length of 250nm and a diameter of 10-15µm. The TEM of the
synthesised nanocellulose fibres (Fig 3.3 b) produced were in the form of
interconnected web like structure. The fibres were also found to have a
decrease in fibre diameter as well as a change in the composition (Table 3.1).
Most of the nanofibrils are agglomerates of hundreds of individual cellulose
nanocrystals.
(a) (b)
Figure 3.3 TEM of (a) micro and (b) nano banana fibres
Table 3.1 Chemical composition of macro, micro and nano fibre
Material α cellulose (%) hemicellulose (%) Lignin (%) moisture ( %)
Macro fibre 64.00 ± 2.82 18.60 ± 1.60 4.90 ± 0.70 12.50 ± 0.47
Micro fibre 82.40 ± 2.51 12.01 ± 0.38 3.64 ± 0.53 1.96 ± 0.36
Nano fibre 95.80 ± 0.58 0.40 ± 0.01 1.86 ± 0.39 1.94 ± 0.42
140 Chapter 3
Scanning probe microscopic studies
The surface roughness averages of the fibre samples were measured using
AFM and it was found to be decreased based on the fibre diameter from
macro fibre to nano fibre. Both micro and nano fibres resulted in the most
significant removal of noncellulosic components (Table 3.1). These results
(Fig 3.4) truly indicated that steam correlated acid treatment helped to
develop fibres of higher cellulosic component, and thus suggest a more
effective removal of the middle lamella and the primary cell wall and
therefore a more cellulose rich surface as supported by and effective
reduction of fibre dimension to nano range. Environmental scanning electron
and transmission microscopic studies corroborate the above results.
(a) (b)
Figure 3.4 SPM image of (a) macro banana fibre, (b) micro banana fibre
and (c) nano banana fibre
(a)
(b)
(a) (b)
Surface characteristics, spectral studies…
141
AFM and TEM suggested that only few lateral associations occur between
adjacent nanofibres. Nanofibres are much more clearly defined probably because
of the removal of amorphous zones and they seem to be more interwoven.
Optical analysis
The ocular polarized luminosity manifestation of the macro, micro and nano
banana fibres are shown in Fig 3.5 (a), (b) and (c). As it is seen from these
Figures, macro fibres are definitely not a monolithic and homogeneous single
fibre with a circular cross section but rather a bundle or a composite with an
elliptic or polygonal cross section consisting of several fibrous plant cells
(elementary fibrous cell). The high pressure chemical treatments results in
the structural changes as well as chemical changes on the fibre surfaces,
causing destruction of plant cell wall and helps in the isolation of the white
shining cellulose fibres.
Figure 3.5 Optical microscopic images of (a) macro (b) micro (c) and nano banana fibres
(a)
50 µm 10 µm
100 nm
142 Chapter 3
X-ray diffraction studies
XRD studies of the banana fibres were done to investigate the crystallinty
and the diameter of the fibres. From the XRD data, it is clear that the nano
fibres show a highly crystalline structure. Nanofibres exhibit a higher
crystallinity due to the efficient removal of noncellulosic polysaccharides
and dissolution of the amorphous zones by acid hydrolysis combined with
steam explosion (23, 24). The increase in the % crystallinity index of
microfibrils and nanofibrils occurs because of the removal of cementing
materals which leads to better packing of cellulose chains. Fig 3.6 shows the
XRD pattern of macro, micro and nano fibrils. The crystallinity is increased
for nano and gives a relatively intense peak at 2θ = 22.7o. For macro fibre the
crystallinity is very low and shows an amorphous nature. The sharp peak
observed in the case of nano fibres point to increased crystallinity. The
broadening of the peak at maximum 2θ proves the decrease in diameter. The
diameter is calculated using Scherrer formula (25)
D = K λ / B cos θ 3.1
K is constant (0.89), λ = X-ray wavelength, B = full width at half max and
θ = Bragg angle.
(a)
(b)
Surface characteristics, spectral studies…
143
Figure 3.6 XRD pattern of different stages of banana fibre (1) macro fibre, (2) micro fibre and (3) nanofibre
Table 3.2 Ratio of intensities of cellulose I and cellulose II crystallites
Material I 22.7o Crystallinity (%) I 22.7
o/I20.4o
Macro fibre I22.7o = Iamor=10.5 - -
Micro fibre 22.9 54.18 0.57
Nano fibre 39.8 73.62 0.93
By contrast, the crystalline part of nano fibre corresponds to cellulose I (26)
and shows highest scattering intensity at 22.7o. A comparison of θ/2θ scans
obtained for macro to nano fibres reveals differences in crystallinity and
cellulose I/cellulose II content. Fig 3.6 illustrates that macro fibre exhibits a
small shoulder at a scattering angle of 22.7o indicating the presence of
cellulose I. This shoulder becomes more prominent in micro, further more
144 Chapter 3
intensified and develops into an obvious intense peak in nano fibre. The ratio
of scattering intensity at 22.7o vs. intensity at 20.4o (I22.7o/I20.4
o) is indicative
of cellulose I vs cellulose II content. On going from macro to nano fibres,
this ratio increases from 0.43 to 0.57 and finally to 0.93 (Table 3.2).
Increasing amounts of cellulose I facilitate the elevated increase in crystallinity
of the overall material. When the value of cellulose I is more enhanced than
cellulose II, the overall crystallinity is amplified.
Fourier transform infrared spectrometry
The IR spectrum of the banana fibres is shown in Fig 3.7. The peaks in the
area 3420cm-1 arise due to –OH stretching vibrations of hydrogen bonded
hydroxyl group. The hydrophilic tendency of macro, micro and nano are
reflected in the broad absorption band at 3700-3100cm-1 region due to the
presence of –OH groups present as main component. The peak at 2921cm-1 is
due to aliphatic saturated C-H stretching vibration in hemicellulose and
cellulose. The peak at 1731cm-1 in macro fibre is due to acetyl (–C=O
stretching) and ester groups of hemicellulose, pectin and lignin (27). This
peak is absent in the micro and nano fibrils due to the removal of carboxylic
groups and ester groups due to the alkali treatment and sodium carboxylate
may be formed which decreases the intermolecular hydrogen bonds and
solubility of pectins. The peak at 1621cm-1 indicates the presence of lignin
by C=C vibration (28). The bands in the region 1250-1050cm-1 involve the
C-O stretching of primary and secondary alcohols in cellulose, hemicellulose
and lignin (29). The peak at 1430cm-1 is due to lignin components (30). The
intensity is decreased for micro and nano fibrils due the dissolution of
hemicellulose and lignin. The 1050cm-1 peak is assigned to the ether linkage
(C-O-C) in lignin and hemicellulose. A peak at 1430cm-1 is seen for
Surface characteristics, spectral studies…
145
microfibril showing that the removal of lignin is partial. The peak area is
decreased for nanofibrils. The narrowing of the peak at 3421cm-1 is due to
the formation of free hydroxyl groups by breaking up of hydrogen bonds.
The sharpening of the peak at 2921cm-1 reveals the increase of crystallinity
and thereby the increase of cellulose in the fibres (9). Table 3.3 also gives the
assignment of IR absorption peaks of macro, micro and nano fibres.
4000 3500 3000 2500 2000 1500 1000 500
0
50
321
10501370
1029
1621
1430
2911 1247
1750
3420
Tra
nsm
ittan
ce %
Wave number (cm -1)
1 Macro fibre2 Micro fibre3 Nano fibre
Figure 3.7 FTIR spectra of macro, micro and nano fibres
Table 3.3 Assignment of IR absorption peaks of macro, micro and nano fibres
Material ―OH
stretching (cm-1)
C―H stretching
(cm-1)
C = O stretching
(cm-1)
Absorbed water (cm-1)
C―H Stretching
(cm-1)
Aromatic ring
vibration of lignin
(cm-1)
C―O stretching
(cm-1)
Macro fibre 3420 2911 1750 1621 1370 1247 1050
Micro fibre 3429 2922 - 1627 1370 - 1050
Nano fibre 3430 2923 - - 1328 - 1050
146 Chapter 3
Solvatochromic measurements
Solvatochromic methods have been proved to be effective in characterising
lignocellulosic fibres. The correspondence of the empirical polarity
parameters determined has been discussed in relation to results from zeta
potential measurements and FTIR measurements.
400 500 600 7000.0
0.1
3
2
1
Ab
sorp
tion
(a.
u)
Wavelength (nm)
1.Macro fibre2.Micro fibre3.Nano fibre
Figure 3.8 UV/vis absorption spectra of furan dye loaded banana fibre (1) macro fibre (2) micro fibre and (3) nano fibre
Table 3.4 UV/Vis absorption maxima and values of the Kamlet-Taft polarity parameters for the three probe dyes used on the banana macro, micro and nano fibres
Samples ννννmax (1)
(10-3cm-1)
ννννmax (2)
(10-3cm-1)
ννννmax (3)
(10-3cm-1) α β ππππ* AN ET (30)
Macro fibre 19.65 26.6 19.2 1.54 0.47 0.46 60.7 59.8
Micro fibre 20.2 26.5 18.0 1.62 0.51 0.34 69.9 63.76
Nano fibre 20.6 27.1 18.6 2.0 0.57 0.27 74.2 64.42
Surface characteristics, spectral studies…
147
Fig 3.8 shows the UV/vis absorption spectra of furan dye loaded cellulose
fibres. The spectra show a definite change in the absorption peaks. The
hydrogen bond donating acidity and basicity of the micro and nanofibres
were determined using solvatochromic measurements using different probe
dyes. The α value (Table 3.4), which shows the surface acidity, has been
found to increase with reduction in fibre diameter. Chemical treatments
carried out on the macro fibre to reduce the fibre diameter, dissolve out
hemicelluloses and lignin, making available more hydroxyl groups on the
surface. In addition, it exposes the different acidic groups associated with the
natural fibre on the fibre surface. This gives rise to increased α value in the
case of nanofibres compared to the microfibres. The probe dyes used in
solvatochromic measurements detect those acidic hydrogen atoms and hence
the increased acidity in the case of nano fibres compared to microfibres. The
π* term for specific interaction in the case of cellulose fibre batches have
been reported by other researchers (31) because the HBD attack upon one of
the two lone pairs of the dimethyl amino group of the probe dye can take
place. In the present case also the nanofibres have the highest acidity value
compared to the untreated fibres and microfibrils. The interaction of the lone
pairs of electrons on the highly acidic sites of the nanofibres can be attributed
to the lower π* value in the case of the fibres. The overall polarity of the
environment given by ET (30) is also found to increase when the fibre
diameter is decreased. This can very well be explained based on the surface
groups which become exposed when the fibres are subjected to alkali
treatment and steam explosion.
148 Chapter 3
Zeta potential measurements
Zeta potential measurements were carried out to investigate the surface
properties and the possible interactions. Nature of the surface of banana
fibres and banana fibril can be understood through the studies of the
influence of pH on zeta potential. The pH that agrees with the zero of the
zeta potential (Iso electric point, IEP) goes to decide the acidity or the
basicity of the solid surfaces qualitatively. Thus at this pH the number of
negative charges equals the number of positive ones (37). It is the IEP that
characterises the acidity of the surface. When the IEP values are low there is
dominance for the number of acidic groups. Banana macro fibre holds the
IEP value 2.5, microfibre 2.1 and that of nanofibre is 1.6, thus indicating an
acidic surface (Fig 3.9). The results obtained from zeta potential
measurements are consistent with the solvatochromic measurement values.
The presence of carboxyl and OH groups which go to charge the natural
cellulose fibres –vely attribute to this acidic surface. Fig 3.9 portrays the pH
dependence on the zeta potential of macro, micro and nano cellulose fibres.
Macro fibre shows a zetapotential of -7.8mV, microfibre -21.3 mV and
nanofibre -27.5 mV. The chemical constitutions, polarity of the fibre surface,
porosity of the fibre and swelling behaviour in water happen to be the factors
for reckoning the electro kinetic potential of fibres if the liquid phase stands
constant. As per the findings of Kanamaru (33), the zeta potential of any
fibre comes down if there is more adsorption of water. The inner surface of
fibres get expanded due to the swelling processes. While the electrochemical
double layer is anticipated to shift in the swelling layer, the slipping plain is
seen to migrate towards the bulk electrolyte. There is a decrease of zeta
potential with increasing swelling time due to the potential drop in the
Surface characteristics, spectral studies…
149
electrochemical double layer. But the layer is influenced by the adsorption of
electrolyte ions. During the swelling process the amount of adsorbed
potential determining ions is seen to come down because of the competitive
adsorption of water. Correspondingly the zeta potential also records a
simultaneous decrease. The decrease in IEP value of the nanofibril depicts an
increase in surface acidity which leads to better adhesion properties with
resin matrix during composite formation.
1 2 3 4 5 6 7 8 9 10 11 12
-30
-25
-20
-15
-10
-5
0
5
Zet
a po
tent
ial (
mV
)
pH-Wert (gemessen in 10-3M KCl)
macro fibre micro fibre nano fibre
Figure 3.9 pH dependence of zeta potential of macro, micro and nano
cellulose fibres
Thermal properties
The natural fibres present three main weight loss regions (Fig 3.10). The
initial weight loss in the region 50–100oC is mainly due to moisture
evaporation. The temperature region ranging from 220-300oC is mainly
attributed to thermal depolymerisation of hemicellulose and the cleavage of
glycosidic linkages of cellulose (34). The degradation of cellulose take place
150 Chapter 3
between 275 and 400oC (35). The TGA and DTG curves of banana macro,
micro and nanofibrils are illustrated in Fig 3.10 and Fig 3.11 respectively.
The thermal degradation of all samples takes place between 275-480oC. The
fibres show a very small weight loss below 100oC as a result of evaporation
of moisture. Between 230-350oC the main degradation occurs. In the case of
macro fibre dehydration and degradation of lignin, hemicellulose and
cellulose occurs between 263 and 280oC. About 70% of degradation
occurred in this temperature range. At 263oC hemicellulose undergoes
degradation (36). The DTG curve of macro banana fibre shows a peak at
347oC (mass loss 51%) which is due to the thermal decomposition of α-
cellulose (37). The weight of the charred residue left was about 2.2%. During
the formation of microfibrils, hemicellulose, lignin, and pectin get dissolved
out partially in alkali and results in a fibrillated structure. The increase in the
% crystallinity index of microfibrils and nanofibrils reduces the moisture
absorption. The DTG curve of banana microfibril (Fig 3.11) exhibits two
peaks. The initial shoulder peak at about 60oC corresponds to a mass loss of
absorbed moisture and the major decomposition peak at about 356oC (mass
loss 51%) is attributed to α-cellulose decomposition (36,38). The differential
curve of microfibrils (Fig 3.11) shows a slight increase in the degradation
temperature (268oC) of hemicellulose which indicates the presence of trace
quantity of the hemicellulose. Fig 3.11 also represents the DTG curve of
banana nanofibrils in which we can see a major decomposition peak at 385oC
(mass loss 52%) due to α- cellulose decomposition. The main degradation
temperature gets shifted towards a higher temperature region. The shift has
been found to be higher for nanofibrils. From the above Figures it is clear
that there is a shift in the major decomposition temperature from 347oC to
385oC as we go from macro fibre to nanofibrils. About 85% decomposition
Surface characteristics, spectral studies…
151
occurred at 480oC. Dissolution of the various components leaves α-cellulose
as the residual material which has been reported to be crystalline (39). A
greater crystalline structure required a higher degradation temperature. The
increase in the degradation temperature in the nanofibrils occurs due to the
high crystallinity of the fibre structure. Therefore it can be concluded from
these results that the developed nanofibres exhibits enhanced thermal
properties compared to the macro fibre and micro fibre so that it can act as a
suitable reinforcing element in biocomposite preparation.
0 100 200 300 400 500 6000
20
40
60
80
100
nano fibremicro fibre
macro fibre
Wei
ght (
%)
Temperature (οC)
Figure 3.10 Thermograms of macro, micro and nano cellulose fibres
152 Chapter 3
0 100 200 300 400 500 600
1.0
0.5
0.0
micro fibre
nano fibre
macro fibre
DT
G (
%/ o
C)
Temperature (oC)
Figure 3.11: DTG curves of macro, micro and nano cellulose fibres
Table 3.5 shows the decomposition temperatures as well as the residual mass
of the banana fibres. From the Table, it is clear that the banana fibres become
more hydrophilic when converted to micro and nanofibrils. This is due to
the increase in fibre fineness, surface area and the increase in cellulosic
components obtained as a result of steam explosion followed by bleaching
which facilitates moisture evaporation at a higher temperature. It can be seen
from the Table 3.5, that the degradation temperature of macro fibre is lower
compared to that of micro and nano fibre. This is because in the raw banana
fibre cellulose is organized into fibrils, which are surrounded by a matrix of
lignin, hemicelluloses and pectins. Hemicelluloses are intimately integrated
into the structure of the cellulose, and located within and between the
cellulose fibrils. This strong association between the hemicellulose and
cellulose fibrils is believed to decrease the average crystallinity of the
cellulose fibrils (40). These impurities may initiate more active sites and
accelerate the beginning of thermal degradation. The fibre residue remaining
Surface characteristics, spectral studies…
153
after heating to 600oC in both micro and nano cellulosic fibres indicates the
presence of the carbonaceous materials in the banana fibre. Results show that
at 600oC, the highest residue was obtained for macro banana fibres and the
lesser residue was obtained for nano fibrils (1.8%). The relatively low
amount of residue in nanofibrils may be due to the removal of hemicelluloses
and lignin from the fibres. These results are very consistent with results
obtained from the chemical estimation, SPM and FTIR measurements.
Table 3.5 Residual mass and decomposition temperatures of macro, micro and nano banana fibres
Fibre Initial decomposition temperature (oC) Final decomposition
temperature (oC) Residue mass
(wt%)
Macro 250 337 2.2
Micro 290 356 2.0
Nano 320 385 1.8
3.3 Conclusion
Cellulose micro and nano fibrils of banana fibres were isolated using high
pressure hydrothermal process. Characterisation of the synthesised micro and
nanofibrils were done using AFM, TEM, SEM, ESEM, XRD, optical
microscopy, solvatocromism, electrokinetic studies and TGA. The chemical
composition of macro,micro and nano fibres were determined using ASTM
standards. From the chemical examination, major constituents of these fibres
were found to be cellulose. The percentage of cellulose components were
found to be increased during steam explosion and acid hydrolysis. The lignin
and hemicellulose components were found to be decreased from macro to the
nano fibres. The IR studies give evidence for the dissolution and chemical
154 Chapter 3
modification that occurred during steam explosion and further treatment of
the fibres for steam explosion in acidic medium.
The morphological structures of the macro to nano fibres were compared.
The observed fibre diameter of macro to nano fibres were respectively 80µm,
10µm and 50nm. XRD studies were done to investigate the fibre size and
percentage crystallinity of the modified fibres. The XRD studies also
revealed that there is a reduction in the size of fibres during steam explosion
in alkaline medium and reduction in size to the nanometer range during
repeated steam explosion in acidic medium. The percentage crystallinity of
the fibres was also found to increase from steam exploded fibres to repeated
steam explosion in acidic conditions. It was observed that the nano fibres
show a highly crystalline structure. The crystallinity was found to be
increased from micro to nano structure. This higher crystallinity was due to
the more efficient removal of noncellulosic polysaccharides and dissolution
of amorphous zones by acid hydrolysis combined with steam explosion. The
SPM analysis also showed that there was reduction in the size of banana
fibres to the nanometer range (below 40 nm). The TEM analysis also
supported the evidence for the formation of nanofibrils of banana fibres by
repeated steam explosion in acidic conditions. The average length and
diameter of the developed nanofibrils were found to be between 200-250nm
and 4-5nm respectively. Overall polarity (ET30) of the fibres was found to
increase when the fibre diameter was decreased. The surface acidity was
proved to be increased from macro to nano scale as the IEP value decreased.
The thermal stability of the nanofibres were found to be much higher than
that of the micro fibres. The increased thermal stability can be attributed to
the dissolution of less thermally stable components from the fibre surface
Surface characteristics, spectral studies…
155
when subjected to alkali and steam treatments. Substantial increase in
thermal stability was observed from macro to nano fibres which proved the
high thermal stability of nanofibres to processing conditions of biocomposite
preparation. The results of solvatochromism and zetapotential corroborate
FTIR results.
156 Chapter 3
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