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Mechanical and Thermal Properties of Cellulose Nanofibers Reinforced Epoxy Polymer Nanocomposites

Authors:

Abstract

Polymers are being continuously modified with nanoparticles to enhance their mechanical, visco-elastic and thermal properties. In this work, cellulose nano-fibers (CNFs) from renewable and biodegradable sources were extracted and incorporated in an epoxy resin system at 1 wt. %, 2 wt. % and 3 wt. % to fabricate nanocomposites. Nanocomposites were then subjected to flexure testing, dynamic mechanical analysis and thermogravimetric analysis. Morphological study was conducted with scanning electron microscope. Addition of CNFs in epoxy showed significant improvement in thermal and mechanical properties compared to those of control ones. Best results were obtained for 2 wt. % addition of CNFs in epoxy. Flexural strength and modulus for 2 wt. % cellulose nanofibers modified polymers depicted about 15 and 34% improvement compared to control ones. An increase of about 14 °C in decomposition temperature was also observed. From viscoelastic properties analysis, the storage modulus and glass transition temperature were improved by 16 and 17% as compared with neat sample. Therefore, an enhancement of mechanical, and thermal properties was achieved by incorporating CNFs to polymers.
Thermal and Mechanical Properties of Cellulose Nanofibers Reinforced
Polyvinyl Alcohol Composite Films
Md. Nuruddin
1
, Raju Gupta
2
, Alfred Tcherbi-Narteh
1
, Mahesh Hosur
1
, Shaik Jeelani
1
ABSTRACT: This research was aimed to use ball milling method to extract cellulose nanofibers (CNFs) from a
bio-waste (i.e. wheat straw), and also to use the extracted cellulose nanofibers as reinforcing materials in polyvinyl
alcohol (PVA) thin film. To study the effect of cellulose nanofibers (CNFs) on mechanical and thermal properties of
polyvinyl alcohol (PVA) nano-composite films, thin film nano-composites were loaded with different loading of
cellulose nanofibers (CNFs) by weight percent (i.e. 1,3,5 and 7% loading). As a result of the research, we found that
the tensile and thermal properties of PVA thin composite increased up to 5% loading of cellulose nanofibers
(CNFs). In contrast, the tensile as well as thermal properties of PVA nano-composite film degraded because of poor
dispersion and agglomeration of CNFs.
KEYWORDS: Extract, hydrolysis, nanofibers, nano-composite, agglomeration
1.
1
INTRODUCTION
Cellulose is a natural polymer consisting of linear
homo polysachharide β-(1,4)-D-glucose units linked
together by β-1-4-linkages [1]. It is used for various
applications since its most abundantly found in
natural resources, environment friendly and bio-
compatible in nature. Cellulose nanofibers (CNFs)
are extracted from cellulose using different methods
like high pressure homogenizer [2,3], enzyme
assisted hydrolysis [4,5], and acid hydrolysis
treatment process [6-8]. Cellulose nanofibers (CNFs)
provides excellent mechanical properties [9], large
specific area, low coefficient of thermal expansion,
low cost and availability [10], better biodegradability,
high aspect ratio (L/D), biocompatibility and
renewability [11].
Polyvinyl alcohol (PVA) is a water soluble synthetic
polymer, widely used as a matrix for fabrication of
biodegradable polymer composites due to its
biodegradability, biocompatibility, high tensile
strength, excellent resistance and adhesive properties
[12].
In this study, wheat straw was used to extract
cellulose nanofibers (CNFs) by organic acid
treatment followed by ballmilling process to use it as
reinforcing materials. PVA nanocomposite thin films
1
Department of Materials Science & Engineering
2
Department of Aerospace Engineering
were prepared by reinforcing CNFs at various
loading and these different CNFs loaded thin
composite films were evaluated and compared
against their mechanical, morphological and thermal
properties.
2. EXPERIMENTAL
2.1 MATERIALS
Wheat straw was purchased from Home Depot, USA.
Polyvinyl alcohol (PVA, 98-99 % hydrolyzed,
Mw 31000-50000), hydrogen peroxide (30 wt.% in
H
2
O), ethanol (≥ 99.5%), formic acid (≥ 95%),
sodium hydroxide pellets and glycerol (≥ 99%)
were purchased from SigmaAldrich (St. Louis,
MO, USA).
2.2 ISOLATION OF CELLULOSE
NANOFIBERS (CNFS)
Pretreatment of wheat straw was done using formic
acid treatment followed by hydrogen peroxide
bleaching [13-14]. CNFs were obtained from
pretreated wheat straw by ball milling treatment
process according to Nuruddin et.al. [15]. For this
process, a mixture of approximately 10 gm of
bleached cellulose and 10 ml of 80% ethanol was
prepared and was allowed to mill for 120 minutes in
a Mixer/Mill 8000D
TM
(SPEX Sample Prep, USA)
using zirconia ceramic grinding vial and ball of
diameter 0.5 inch. The mixture was then washed
repeatedly with distilled water and centrifuged to
bring the pH value of cellulose between 6 and 7.
2.3 PREPARATION OF POLYVINYL
ALCOHOL NANOCOMPOSITE FILMS
PVA granules and distilled water was mixed along
with 30% glycerol to prepare 10 wt.% PVA solution,
which was then heated and magnetically stirred to
completely dissolve the polymer. The desired CNFs
suspension (0.5%) was then added and sonicated for
uniform dispersion of CNFs in PVA solution. The
mixture was then poured in petri dishes to allow
water to evaporate then the film was demolded and
stored.
3. CHARACTERIZATION
3.1 SCANNING ELECTRON MICROSCOPE
(SEM)
JEOL JSM-6400 scanning electron microscope
(SEM) was used at 20kV accelerating voltage to take
scanning electron micrograpth of wheat straw,
cellulose, and CNFs while morphological study of
fracture surface of PVA and CNFs was done at 10 kV
accelerating voltage.
3.2 DIFFERENTIAL SCANNING
CALORIMETER (DSC)
DSC analysis was performed at a heating rate of
10ºC/min from -10 to 240 ºC with two heating and
one cooling scan. The crystallinity degree was
calculated by:




] ×100%
(1)
Where ΔH, is the enthalpy of melting, ΔH
o
is
enthalpy of melting for 100% crystalline PVA and (1
mf) is the weight fraction of PVA in the sample.
3.3 TENSILE TEST
MTS 809 Axial/Torsional Test system was used to
perform tensile test of neat PVA and CNFs
reinforced PVA films according to ASTM D882.
Five samples for each nanocomposites each with size
of 5mm x 80mm rectangular strips and thickness of
200-300 μm were tested at 30 mm grip separation,
500 mm/min crosshead speed and 100 N load cell.
4. RESULT AND DISCUSSIONS
4.1 MORPHOLOGICAL
CHARACTERIZATION OF CNFS AND
NANOCOMPOSITE FILMS
Morphological study was done by observing fracture
surface of neat PVA and CNFs modified PVA
nanocomposites using Scanning Electron Microscope
(SEM) to analyze the effect of CNFs incorporation
into the polymer matrix.
Figure 1 shows the SEM image of tensile specimen
of nanocomposite films which shows smooth and
uniform fracture surface of neat PVA while
comparatively rougher fracture surface of the
specimen that were incorporated with CNFs and well
dispersed. The rough surface is caused by the
restriction of propagation of crack by the presence of
better interaction (hydrogen bonding) between CNFs
and PVA polymer matrix. So, roughest surface can
be observed in SEM image at 5% loading of CNFs
showing the maximum restriction by CNFs. In
contrast, SEM image of fractured surface of 7%
loaded CNFs shows agglomeration of CNFs.
Figure 1: SEM images of fracture surfaces of (a) neat
PVA, (b) PVA/1% B-CNFs, (c) PVA/3% B-CNFs, (d)
PVA/5% B-CNFs and (e) PVA/7% B-CNFs
4.2 THERMAL STABILITY OF
NANOCOMPOSITE FILMS
Differential scanning calorimetry (DSC) analysis was
performed to study the crystallization and melting
phenomena of neat PVA and CNFs modified PVA in
order to evaluate the thermal properties, which is
shown in Figure 2.
a
b
%
c
d
FIGURE 2: DSC thermograms of neat PVA and B-CNFs
reinforced PVA composite films
DSC thermograms from figure 2 shows that from
cooling scan addition of CNFs increases the
crystallinity and crystallization temperature of PVA
till 5% loading of CNF while both crystallinity and
crystallization temperature decreases for 7% loading.
Heating scan shows similar trend for melting
temperature, heat of fusion and crystallinity.
4.3 MECHANICAL PERFORMANCE OF
NANOCOMPOSITE FILMS
Tensile test was performed to analyze the mechanical
performance of the nanocomposite films. The data
for tensile strength, tensile modulus and elongation at
break was obtained from the tensile test for both PVA
thin film and CNFs reinforced PVA thin films and
were compared as shown in figure 3. A significant
improvement in tensile strength and modulus was
observed with increased CNFs loading, which was
found highest for 5% loading of CNFs where
improvement in tensile strength was about 41-49 %
and elastic modulus was about 258-267 %.
FIGURE 3: Stress-strain curves of neat PVA and ball
milled cellulose nanofibers (B-CNFs) reinforced PVA
composite films.
5. CONCLUSION
The aim of this study was to prepare CNFs reinforced
PVA nanocomposite and see analyze the effect on
mechanical and thermal properties. Reinforcement of
CNFs increases the hydrogen bonding between the
fibers and polymers which resulted in better
improvement in mechanical and thermal properties of
the PVA nanocomposite thin films as observed from
tensile test and DSC analysis. At the same time,
loading of CNFs more than 5% (i.e. 7% CNFs
loading) into PVA matrix system causes the
degradation in mechanical and thermal properties due
to the formation of agglomeration. Ball milling
causes better interaction of CNFs with PVA polymer
matrix by exposing more hydroxyl groups on the
surface.
ACKNOWLEDGEMENT
The authors are grateful to the NSF-CREST (grant
no. 1137681) and NSF-EPSCoR (grant no. 1158862)
for the financing support to carry out this research.
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... Viscoelastic properties of neat epoxy, and epoxy/CNFs nanocomposites Figure 11. (a) Storage modulus vs. temperature, and (b) tan δ vs. temperature curves of nanocompositesOn the other hand, the addition of CNFs increases the crosslink density and hence improves T g value.Nuruddin et al. reported that addition of CNFs up to 2 wt.% enhances the glass transition temperature36 ...
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