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BIODEGRADABLE NANOCOMPOSITES FROM WHEAT STRAW
Mohini Sain, Ayse Alemdar
Center for Biocomposites and Biomaterials Processing, Faculty of Forestry,
University of Toronto, Toronto, ON, Canada
Abstract
We investigate the mechanical and thermal properties of cellulose nanofiber
reinforced starch based thermoplastic composites. Cellulose nanofibers were isolated from
wheat straw by a chemi-mechanical technique. Their morphology, physicochemical and
thermal properties were investigated to examine potential applications as reinforcement
fibers in biocomposites. Transmission electron microscopy results showed that almost 60%
of them have a diameter within a range of 30-40 nm and lengths of several thousand
nanometers. Their average aspect ratio was found to be 95. Chemical characterization of
the nanofibers confirmed that an applied alkali and acid treatment resulted in increased
cellulose content from 43% to 85%. FT-IR spectroscopic analysis of the fibers
demonstrated that this chemical treatment also led to partial removal of hemicelluloses and
lignin from the nanofibers’ structure. PXRD measurements revealed that this resulted in an
improved crystallinity of the fibers. After mechanical treatments, thermal properties of the
nanofibers were studied by the TGA technique and found to increase dramatically. The
nanocomposites were prepared by a solution casting method. The tensile strength and
modulus of the reinforced starch films were compared with those of pure starch and were
found to have considerably improved, even at 5% nanofiber loading.
Introduction
Ecological concerns and the impending depletion of fossil fuels are driving the
development of new bio-based, green products. Over the past few decades, research has
explored the possibility of exploiting natural fibers (bio-fibers) as load bearing constituents
in composite materials and produced encouraging results. Industrial use of these natural
fiber-reinforced composites is increasing due to their relative cheapness compared to
conventional materials and their potential to be recycled. The abundance of natural fibers is
evidently advantageous to manufacturing industry.
Generally, cellulose-based fibers (obtained from plants), including cotton, flax,
hemp, jute and sisal, and wood fibers are used to reinforce plastics. With their relative high
strength, high stiffness and low density, these reinforced plastics are of interest as a
replacement for synthetic fiber reinforced plastics in an increasing number of industrial
sectors including the automotive industry, packaging, and furniture production,
In the 1990s the novel idea was explored of manufacturing entirely natural and
biodegradable nanocomposites from cellulose nanofibers (rather than fibers) and
biopolymers (1,2). These nanocomposites have excellent properties of rigidity, strength and
resistance to high temperatures. The size of the fibers is crucial in allowing these
biocomposites to be as strong as a pure cellulose crystal. Theoretical calculations of the
elastic modulus of cellulose chains give a value of up to 250 GPa, which is to be compared
to bulk natural fibers having an elastic modulus of 10 GPa (3). Although, the technology of
extraction of the cellulose crystal from natural sources is deficient, it is still possible to
isolate nanofibers with a diameter between 5-80 nm by a chemi-mechanical treatment (1,2,
4,5).
In this work wheat straw was chosen as a nanofiber source due to its abundance in
agriculture. Tons of unused wheat straw residues are generated every year, with only a
small percentage being used in applications such as feedstock and energy production.
Biocomposites are a prospective commercial application that would unlock the potential of
these underutilized renewable materials and provide a non-food based market for
agricultural industry.
The main goal of the present work is to isolate cellulose nanofibers from the wheat
straw with a chemi-mechanical technique and characterize them in order to evaluate their
suitability as reinforcement for biocomposite applications. The structural and
physicochemical properties of the nanofibers were studied by scanning and transmission
electron microscopy, FTIR spectroscopy and powder X-ray diffraction (PXRD) techniques.
The thermal properties of the nanofibers were investigated by TGA. The mechanical
performance of the nanocomposites containing cellulose nanofibers and thermoplastic
starch were evaluated by tensile testing.
Experimental Procedure
Isolation of the Nanofibers
The nanofibers were isolated from wheat straw obtained from local sources
(Ontario farms). A patented method for the isolation of cellulose from crop-based fibers was
adopted (6). The wheat straw was cut into 4-5 cm lengths before a pre-treatment was
applied. The cut wheat straw was soaked in a concentrated sodium hydroxide solution and
then washed several times with distilled water. The pre-treated pulp was hydrolyzed by
diluted HCl and then washed with distilled water repeatedly. The pulp was treated once
more with the 2 % w/w of NaOH solution. The alkali treated pulp was washed several times
with distilled water until the pH of the fibers became neutral before being vacuum filtered
and dried at room temperature.
To individualize the microfibrils from the cell walls a mechanical treatment was
applied to the chemically treated fibers. The mechanical treatment procedure includes
cryocrushing, disintegration and defibrillation steps.
Processing of the Nanocomposites
The nanocomposites were prepared from the wheat straw nanofibers with
thermolastic starch polymer as the matrix and glycerol as the plasticizer. The wheat straw
nanofiber suspension was first mixed with thermoplastic starch, then cast in Petri dishes
and left to dry at room temperature for 2 days. The resulting films were placed in an oven at
37 °C for a week. The dry amount of the thermoplastic starch is 4g in each composite film.
The nanofiber content was varied between 0 and 10 % of the dry weight of the composites.
Characterization
The composition (α-cellulose, hemicelluloses and lignin contents) of the untreated
and chemically treated fibers was analyzed according to the procedure pioneered by Zobel
and McElvee, 1966 and the TAPPI standards. The holocellulose content (α-cellulose +
hemicelluloses) of the fibers was determined by treating them with a NaClO3 and NaOH
mixture solution (7). The cellulose content of the fibers was then determined after extracting
hemicelluloses from the holocellulose by NaOH treatment. The difference between the
values of holocellulose and α-cellulose gives the hemicelluloses content of the fibers. The
lignin content of the fibers was found by treating them with a sulphuric acid solution
according to TAPPI procedure 250UM-85 and TAPPI standard T222 om-83.
The fibers’ morphology was characterized using a scanning electron microscope
(SEM) and transmission electron microscope (TEM). Fibers were mounted on metal stubs
by double-faced tape and the surface was coated by carbon. Images were taken at 15 kV
by JEOL JSM-840 model SEM. A Philips CM201 model TEM operating at 60 kV was used.
FT-IR spectroscopy was used to examine any changes in the structure of the fibers
which arose after chemical treatment of the fibers. A Perkin Elmer spectrum 1000 was used
to obtain the spectra of each sample. Fibers were ground and mixed with KBr. They were
then pressed into transparent thin pellets. FT-IR spectra of each sample were obtained in
the range of 4000-400 cm-1. Spectral outputs were recorded in the transmittance mode as a
function of wave number.
The crystallinity of the untreated and chemically treated cellulose fibers was
examined using a Bruker AXS D8 Discovery Diffraction System equipped with a high power
point focus (1x1mm) Cu-kα target, graphite monochromator (26.53o) for elimination of Cu-
kβ lines, and a Hi-Star GADDS area detector for 2-D images. High-resolution frames
(1024x1024 pixels) for accurate integration of the diffraction images were recorded. The
frames from the analyzed cellulose materials were taken with a 0.5 mm pinhole collimator
at 600 s (5 min) using a transmission mode with 2θ = 21o and ω(θ) = 10.5o.
Thermogravimetric analysis was performed to compare the degradation
characteristics of the chemically treated fibers with the untreated ones. Thermal stability of
each sample was determined using a TGA Q 500 series Thermogravimetric analyzer (TA
Instruments) with a heating rate of 10 oC/min in a nitrogen environment.
Tensile testing of the composites was carried out using a Sintech-1 tensile machine
with a load cell of 50 lb and following ASTM D 638. The crosshead speed was set to 2.5
mm/min. The samples were cut in a dumbbell shape with an ASTM D 638 (type V) die and
at least 6 specimens were tested for each sample.
Results and Discussion
Chemical Composition and Morphology of Chemi-mechanically Treated Wheat Straw
Table 1 shows the chemical composition of the wheat straw after each stage of the
chemical treatment. It was found that at the end of the chemical treatment the α-cellulose
content was increased from 43.2 % to 84.6 % while hemicellulose and lignin content were
significantly decreased to 6% and 9.4 % respectively.
Table 1. Chemical composition of the wheat straw fibers after each step of the chemical treatment
Wheat straw fibers
α-cellulose
(%)
Hemicelluloses
(%)
Total lignin
(%)
Untreated fibers 43.2 34.1 22.0
Acid treated fibers 61.8 19.0 14.1
Acid and alkali treated fibers 84.6 6.0 9.4
* All values are reported as mean (N≥3)
The chemical treatments result in structural changes as well as chemical changes
to the fiber surfaces. SEM pictures of the wheat straw fibers were taken to investigate the
structure of these fibers and are shown in figure 1a. These pictures visually suggest the
partial removal of hemicelluloses, lignin and pectin after chemical treatment, which are the
cementing materials around the fiber-bundles. It is clear from the pictures that the average
diameter of the fibers is about 10-15 µm, which is lower than the average size of fiber
bundles, 25-125 µm (8) before chemical treatment.
(a) (b)
Figure 1. (a) SEM and, (b) TEM pictures of the wheat straw fibers
Figure 1b shows the TEM pictures for the wheat straw nanofibers obtained after the
chemi-mechanical treatment. The mechanical treatment of the fibers resulted in
defibrillation of nanofibers from the cell walls and the TEM picture of the cellulose fibers
shows the separation of the nanofibers from the micro-sized fibers. The diameters of the
fibers after chemical and mechanical treatments were calculated by an image processing
analysis program, UTHSCSA Image tool, using the SEM and TEM images. The diameters
of 75% of the wheat straw fibers obtained after chemical treatment were smaller than 9 µm.
The maximum diameters were between 6 and 7 µm, with 25% of the total in this range. It
was found that the diameters of the wheat straw fibers after mechanical treatment
decrease. Almost 60% of them have a diameter within a range of 30-40 nm and lengths of
several thousand nanometers. The aspect ratio of the wheat straw nanofibers was found to
be between 90 and 110.
Spectroscopic Analysis
Fig. 2 shows the FT-IR spectra of untreated and chemically treated wheat straw
fibers. The dominant peaks in the region between 3600 and 2800 cm-1 are due to stretching
vibrations of CH and OH. The prominent peak at 1737 cm-1 in the untreated wheat straw is
attributed to either the acetyl and uronic ester groups of the hemicelluloses or the ester
linkage of carboxylic group of the ferulic and p-coumeric acids of lignin and/or
hemicelluloses (9,10). This peak disappeared completely in the chemically treated wheat
straw because of the removal of most of the hemicelluloses and lignin from the wheat straw
by applied chemical extraction.
3500 3000 2500 2000 1500 1000 500
2925
1200
1436
896
3417
1058
1511
1385
1645
1737
Treated
Untreated
% Transmittance
Wave number (cm-1)
Figure 2. FT-IR spectrum of the untreated and treated wheat straw fibers
The peaks at 1507 and 1436 cm-1 in the untreated wheat straw represent the
aromatic C=C stretch of aromatic rings of lignin (9-11). The intensity of these peaks
decreased in the chemically treated wheat straw, which was attributed to the partial removal
of lignin. The sharp peak at 1385 cm-1 reflects C-H asymmetric deformations (10). The
peaks in the region 1200-950 cm-1 are due to C-O stretching (11). The increase of the band
at 896 cm-1 in the chemically treated wheat straw indicates the typical structure of cellulose.
Crystallinity of Untreated and Treated Fibers
The hydrogen bonds between cellulose molecules are arranged in a regular system
resulting in an ordered system with crystal-like properties. The crystalline lattice of cellulose
is monoclinic. Individual fibrillar units consist of long periods of ordered regions (crystallites)
interrupted by completely disordered regions. In native cellulose the length of the
crystallites can be 100-250 nm with cross-sections an average of 3 x 10 nm. The cellulose
molecule continues through several crystallites. Chemical and mechanical treatments affect
the crystallinity of the cellulosic fibers.
Crystallinity is commonly measured as a ratio between the diffraction portion from the
crystalline part of the sample, and the total diffraction from the same sample. Figure 3
shows the PXRD patterns of the untreated and treated cellulose fibers. The peak at 2θ =
22o is sharper for chemically treated wheat straw than untreated cellulose fibers. The
sharper diffraction peak is an indication of higher crystallinity degree in the structure of the
treated fibers. The crystallinity values were estimated as 57.5% and 77.8%, for the
untreated wheat straw and chemically treated wheat straw fibers respectively. In all cases
the portion of crystalline cellulose was found to be higher for chemically treated cellulose
fibers than for untreated fibers due to partial removal of the hemicelluloses and lignin during
chemical treatment. The increase in the number of crystallinity regions increases the rigidity
of cellulose. Higher crystallinity in the chemically treated cellulose fibers is associated with
higher tensile strength of the fibers.
20 30 40
200
400
600
800
1000
1200
1400
1600
1800
untreated fibers
chemically treated fibers
Intensity (counts)
2θ
Figure 3. PXRD diffraction patterns of the untreated and treated wheat straw fibers
Thermal Properties
Investigation of the thermal properties of the natural fibers is important in order to
gauge their applicability for biocomposite processing. Figure 4 and Table 2 show the TGA
results obtained from wheat straw fibers. These results clearly illustrate that the thermal
stability of the wheat straw fibers increases after chemical and further increases after
mechanical treatments. The degradation temperature of the nanofibers was increased.
There is also a distinction between the amounts of the residues of the fibers remaining after
550 ºC heating for untreated, treated, and nanofibers.
200 250 300 350 400 450 5
0
0
10
20
30
40
50
60
70
80
90
100
110
Weight loss (%)
Temperature (oC)
Untreated fiber
Chemically treated fiber
Nanofiber
Table 2. Degradation Characteristics of the
wheat straw fibers
Wheat straw
fibers
Onset of
degradation
(ºC)
Residue after
550 ºC (%)
Untreated
Chemically
treated
Nanofibers
215
232
296
22.3
13.6
10.2
Figure4. TGA thermograms of the wheat straw fibers.
Mechanical Properties of the Nanocomposites
Figure 5 shows the reinforcing ability of the wheat straw nanofibers by means of
modulus and tensile strength. The tensile strength and modulus of the nanocomposites
films were increased linearly while the nanofiber content was increased. The tensile
strength of the composite film with 10% nanofibers loading showed a 36% increase
compare to the pure thermoplastic starch polymer film. A two-fold increase in the modulus
was observed in the composite with 10% nanofiber loading.
Starch polymer 2% Nanofibre 5% Nanofibre 10%Nanofibre
0
1
2
3
4
5
6
7
8
9
10
Tensile Strength (MPa)
Starch polymer 2% Nanofibre 5% Nanofibre 10%Nanofibre
0
50
100
150
200
250
300
Modulus(MPa)
Figure 5. Mechanical performance comparison of nanocomposite films with different
loading
Conclusions
In this work, cellulose nanofibers were extracted from wheat straw by chemical
treatment followed by a mechanical treatment. Chemical composition, morphology and
physical and thermal properties of the nanofibers were characterized to investigate their
usability in biocomposite applications. Experimental results showed that the resulting wheat
straw nanofibers’ diameters are within the range of 10-80 nm with lengths of a few
thousand nanometers. Chemical analysis and FTIR measurements of the fibers revealed
the partial removal of hemicelluloses and lignin due to the success of the chemical
treatment applied. The crystallinity of the nanofibers was increased by 35% for the wheat
straw nanofibers relative to the untreated wheat straw. The nanofibers showed enhanced
thermal properties, with thermal degradation temperatures increased by 45%. The
mechanical tests showed that nanocomposites had slight improvement in tensile strength
and modulus, 2-fold increase, compared to pure thermoplastic starch. Results showed that
the cellulose nanofibers obtained from wheat straw can find potential application in
biocomposite production such areas of medical and automotive industry.
Acknowledgements
The authors are grateful to the Ontario Ministry of Agriculture and Food (OMAF) for the
financial funding of this research, Prof. N. Yang for providing the facility for
Thermogravimetric analysis, and Mahya Mokhtari for her assistance for chemical
characterization of the fibers.
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