effect of fiber type on thermal and mechanical behavior of...
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806
ISSN 1229-9197 (print version)
ISSN 1875-0052 (electronic version)
Fibers and Polymers 2017, Vol.18, No.4, 806-810
Effect of Fiber Type on Thermal and Mechanical Behavior of
Epoxy Based Composites
Manish Kumar Lila*, Gaurav Kumar Saini1, M. Kannan
1, and Inderdeep Singh
Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India1Department of Mechanical Engineering, Quantum School of Technology, Roorkee 247667, India
(Received November 23, 2016; Revised February 12, 2017; Accepted February 14, 2017)
Abstract: Natural fibers are lignocellulosic and hollow in nature and having good mechanical, thermal and structuralproperties. In the present research endeavor, three types of natural fibers (namely Sisal, Hemp and Nettle fibers) are used asreinforcement in woven mat form to fabricate 4-ply laminate composites with epoxy resin as matrix using hand layupprocess. Atomic force microscopy (AFM) and scanning electron microscope (SEM) have been used to get the surfaceroughness of fiber, to study the fracture behavior of the developed composites and effect of inter surface bonding betweenfibers and matrix. Thermogravimetric analysis (TGA/DTA) has been performed to study the thermal behavior and inter-surface bonding among matrix and fibers for the developed composites. The results revealed that along with the appliedpressure and viscosity of the matrix, surface roughness of the fiber also plays a significant role in deciding the mechanicalproperties and natural fiber with high surface roughness exhibits better mechanical properties.
Keywords: Natural fibers, Polymer matrix composites, Mechanical properties, SEM, Thermal degradation
Introduction
Polymer matrix composites consist of thermoplastic or
thermoset resins matrix reinforced with fibers, which are
much stronger and stiffer than the matrix. Polymer matrix
composites are light in weight, stronger, and stiffer than the
unreinforced polymers, with additional advantage of easy
fabrication, suitable for mass production and their properties
as well as their form can be tailored to meet the needs of a
desired application, which depends also on the manufacturing
process [1]. Both synthetic and natural fibers can be used as
reinforcement in polymer matrices, but due to environment
and ecological consideration, natural fibers are now attracting
the researchers due to their low cost, carbon neutrality,
recyclability and degradable properties. Generally, mechanical
properties of natural fibers are lower than the synthetic
fibers, but can be made comparable by proper treatment of
fibers i.e. alkalization, acetone treatment etc. [2]. In terms of
geometry, natural fibers are not uniform monofilament
cylinders like carbon and glass, but are bundles of elementary
fibers, which consist of voids and defects with irregular
cross-sections. In terms of chemical structure, natural fibers
have varying surface energy and available bonding sites
along their fiber length due to the natural polymers i.e.
lignin, pectin etc., which create these bundles of elementary
fibers [3]. Various researchers have experimentally investigated
the mechanical properties of composites by using sisal,
hemp and nettle fiber as reinforcement. Singh et al. [4]
investigated the effect of several chemical treatments on the
physical and mechanical properties of sisal fiber reinforced
with polyester resin and reported an improvement in the
mechanical properties after treatment. A decrease of 30-
44 % in tensile strength and 50-70 % in flexural strength has
been reported under humid conditions. Joseph et al. [5]
investigated the influence of interfacial adhesion on the
mechanical and fracture behavior of sisal fiber with several
thermoset resin matrices (Polyester, Epoxy, Phenol
Formaldehyde) with respect to fiber length and fiber loading
and reported a general trend of increasing properties with
fiber loading in all the cases. Phenolic resin was reported as
the best matrix among all tested resins, when reinforced with
sisal fiber in terms of tensile and flexural properties due to
the high interfacial bonding.
Hautala et al. [6] fabricated 48 layered plywood-type
composites by reinforcing hemp fiber strips in epoxy resin
and reported the flexural strength as 140 MPa and a flexural
modulus of 6 GPa, comparable with 10 % Glass fiber
reinforced Epoxy composite. Rouison et al. [7] optimized
and investigated the mechanical properties of hemp fiber
reinforced polyester composites fabricated by resin transfer
molding (RTM) process. A linear increase in tensile strength
and tensile modulus was reported with increasing fiber
content above 11 % fiber volume fraction. A maximum
tensile strength of 60 MPa was achieved for fiber volume
fraction of 35 %.
Bodros et al. [8] studied that the mechanical behavior of
stinging nettle fibers and compared to flax and other
lignocellulosic fibers and reported that the stress/strain curve
of stinging nettle fibers shown a linear behavior with a
tensile modulus of 87 GPa and tensile strength as 1594 MPa.
Various researchers has experimentally investigated the
mechanical properties of the composites with change in fiber
volume fraction [9-12] fabrication processes [13-16] chemical
treatments [8,17,18] and degradation behavior [17,19-21] by
using sisal, hemp or nettle fiber as reinforcement and
reported different results. The difference may be attributed*Corresponding author: [email protected]
DOI 10.1007/s12221-017-1147-0
Fiber Effect on Epoxy Composites Fibers and Polymers 2017, Vol.18, No.4 807
to different processes, process variables, matrix and ecological
conditions, in which the fiber is grown.
The present paper analyses the mechanical properties of
the composites, fabricated by using similar matrix, process
and processing conditions as well as the fibers are produced
in same ecological zone. Surface roughness and thermal
degradation behavior has also been studied to check the
inter-surface bonding between the fiber and the matrix.
Experimental
The fiber in the woven mat form (Sisal, Hemp and Nettle)
were procured from Women development organization,
Dehradun. Araldite AW 106 resin along with HV 953U
hardener was used as matrix material to fabricate 4-ply
laminated composites. Hand lay-up process was used to
fabricate the composite laminates as it is simplest, versatile
and requires least infrastructure.
Initially the resin mixed with the hardener (1:1 ratio) in a
glass container and stirred thoroughly with a glass rod for
proper mixing of the components. Alternate layers of this
resinous mixture (5 layers) and woven mats (4 layers) were
placed over the mold and a roller was moved over the layers
to remove any air entrapment and excess polymeric resin.
The whole setup was then covered with second mold plate
and a uniform pressure of 1.225 N/cm2 (0.1225 bar) was
applied to provide compaction among the layers. The whole
setup was then kept idle for 12 Hours, for curing, as per the
conditions specified by the resin manufacturer. After curing,
the pressure was released and fabricated composite plates
were removed from the mold. An average thickness of 4 mm
was achieved for all the composite laminates. As per the
GSM weight of woven fiber mats, the calculated fiber
fraction has been found as 20 % (by weight). Specimen were
then cut (length 150 mm and width 15 mm) as per the
ASTM D-3039 and ASTM D-7264 standard for testing
purpose. Sides and edges of all the specimen were rubbed on
an emery paper (Grit size: 1200) to remove any notch or
irregularities, which might cause stress concentration during
testing.
Mechanical Testing
5 specimens of all type of composite were used for testing
tensile and flexural behavior of the developed composites.
The tests were carried out on the universal testing machine
(Make: INSTRON, Model: 5982), with a gauge length of
50 mm and crosshead speed of 2 mm/min and 1 mm/min for
tensile and flexural specimen respectively.
SEM Analysis
Microstructural examination was carried out at room
temperature using scanning electron microscope (SEM)
(Make: LEO, Model: 435VP). A very thin film of gold was
coated using Sputter Coater (BALTEC-SCD-005) onto the
specimen to enhance the conductivity before micrographs
were taken.
Thermal Analysis
Thermogravimetric analysis (TGA) and differential
thermal analysis (DTA) were carried on EXSTAR TG/ DTA
6300 in Aluminium pan with Nitrogen gas environment with a
heating rate of 10 oC/min.
Results and Discussion
Initially, Atomic force Microscopy was performed on
fibers to get the average surface roughness. The roughness
of Sisal fiber was measured between 200-220 nm ranges. It
was not possible to perform the same on Hemp and Nettle
fibers due to their small diameter (20-60 µm) as minimum
area required to perform should be in range of 50 µm.
Therefore SEM imaging of the fibers has also been
performed. The AFM image of Sisal fiber and SEM images
of all the fibers are shown in Figure 1, depicting their surface
morphology. The surface quality also leads to mechanical
bonding between fiber and reinforcement by means of
roughness. By the images, it can be observed that the surface
roughness of hemp is lesser than that of nettle and sisal fiber.
When loaded under tensile testing conditions, tensile
strength and tensile modulus have been calculated based on
the data for various specimens. Materials, when used in
structural applications, are prone to fail in bending and
therefore, the development of new composites with comparable
flexural characteristic is essential. The outcome of flexural
Figure 1. (a) AFM image of sisal fiber. SEM images of (b) sisal,
(c) nettle, and (d) hemp fiber.
808 Fibers and Polymers 2017, Vol.18, No.4 Manish Kumar Lila et al.
strength and flexural modulus of developed composite
showed a significant increase in both the values as compared
to neat epoxy. The average value for the tensile and flexural
properties are mentioned in Figure 2 and 3, for pure epoxy,
sisal epoxy composites (SEC), nettle epoxy composites
(NEC) and hemp epoxy composites (HEC).
An increase of 75-77 % in tensile strength has been
observed in case of SEC and NEC but 67 % in case of HEC.
Along with an increase in tensile modulus has been observed
as 87 %, 101 % and 83 % with sisal, nettle and hemp fiber
reinforcement respectively when compared with neat epoxy.
The possible cause for the difference in properties is due to
inter-surface bonding among fiber and matrix, individual
physical or mechanical properties of the fibers.
In SEM images, it is observed that the surface of hemp
fiber is smoother then the sisal and nettle fibers. Surface
roughness propagates the mechanical bonding among fibers
and matrix as well as helps in better mechanical properties.
The average values obtained are comparatively different
when compared to the work of other researches, which may
be attributed towards the applied load, fiber and mat
geometry and other processing conditions. From SEM
images of fractured surface under tensile loading, air
entrapment can be observed in the composite, which may
also leads to poor mechanical properties by reducing the
effective area to bear the tensile load. This air entrapment
may be due to the gap between the warp and fills of the
woven mat and/or the high viscosity of the epoxy resin. The
air entrapment (in form of voids) has been analyzed using
image analysis software (ImageJ, version: 1.51j) in terms of
void area and void fraction. The average results obtained are
shown in Table 1.
The maximum number of voids can be observed in pure
epoxy specimen with maximum void size and void fraction,
while minimum in NEC with lowest void fraction. The voids
fraction has been found as highest in HEC, when compared
with SEC and NEC, but the average void area is greater in
NEC as compared to SEC. Mechanical properties of pure
epoxy and developed composites are also showing the same
trend as of void presents in the specimen.
It can be observed from the Figure 3, that there is an
increase in flexural properties in all the cases. An increase of
159 %, 163 % and 143 % in flexural strength and an
increase of 103 %, 88 % and 57 % in flexural modulus is
observed for sisal, nettle and hemp fiber reinforced composites,
respectively.
Thermal stability is also one of the major factors in the use
of natural fibers as reinforcement for composites products,
therefore the thermal stability of developed composites has
been investigated. In particular, TG and DTA curves were
Figure 2. Mechanical strengths of developed composites.
Figure 3. Modulus properties of developed composites.
Figure 4. SEM images of fractured specimen for (a) SEC, (b) NEC,
and (c) HEC.
Table 1. Voids analysis in epoxy and developed composites
Material Pure epoxy SEC NEC HEC
No. of voids (per mm2) 20.7 8.41 3.86 15.3
Average void size (µm2) 9967 3514 5524 7334
Void fraction (%) 20.63 2.96 2.13 11.23
Fiber Effect on Epoxy Composites Fibers and Polymers 2017, Vol.18, No.4 809
used to study the degradation behavior and change in
entropy with neat resin and shown in Figure 5 and 6.
TGA curve shows that the onset of thermal degradation
for pure epoxy and developed composites fibers does not
occur before 300 oC, whilst the initial weight loss (0.6-
1.4 %) between 37 and 100 oC corresponds to the evaporation
of moisture from the surface.
Above this temperature, thermal stability is slightly
decreased up to 300 oC showing a weight reduction of 9.2-
10.48 % for all the samples. Pure epoxy shows highest
degradation rate of 1.28 mg/min at 357 oC, which can be
observed in the figure as a steep curvature at the temperature,
while the degradation rate for the developed composites has
been observed between 0.55-0.85 mg/min at same temperature.
It can be observed that the addition of fibers have a
significant effect on the degradation rate.
When curing epoxy, molecules get cross linked with each
other by strong covalent bonds which requires high energy
to break, while in composite the matrix and fibers are
attached with weak Van Der Waals forces. The peaks of the
DTA curves correspond to the decomposition temperature of
each constituent of the composites. Some researchers [22,
23] have reported that the addition of natural fibers causes
variation in the thermal stability of the composite due to
individual thermal properties of the fibers. DTA curve of
epoxy and developed composites shows three endothermic
peaks at 347-361 oC, 427-440 oC and 553- 561 oC. The peaks
are due to change in enthalpy (ΔH) corresponding to change
in thermocouple temperature and thus induced voltage. The
first peak in composites is mainly due to debonding between
the fiber and matrix by absorbing energy to break the weak
Van Der Waals bonds and de-polymerization in case of pure
epoxy. Second peak is due to thermal decomposition of
hemicellulose and lignin in natural fibers, and third peak is
due to burning and degradation of the material [24]. The
total change in enthalpy for the first two main peaks are
shown in Figure 7.
The change in enthalpy can be a treated as a measure of
the bonding between the fiber and reinforcement as the
energy is directly proportional to the number of bonds and
bond strength. High energy may represent more bonding
between the fiber and matrix. The total change in enthalpy
for the area up to 440 oC is 227 mJ/mg, 411 mJ/mg, 428 mJ/
mg and 263 mJ/mg for pure epoxy, sisal epoxy composites
(SEC), nettle epoxy composites (NEC) and hemp epoxy
composites (HEC) respectively. The same trend can be
observed in the mechanical properties of epoxy and the
developed composites in the same manner. Highest change
in enthalpy was observed in case of HEC in the temperature
range of 553-561 oC, which may be attributed towards the
lowest residues (2.9 %) in case of HEC when compared to
other specimen (5.16-5.95 %).
Conclusion
The study was carried to evaluate the mechanical
properties of natural fiber (sisal, nettle and hemp) reinforced
epoxy composite with same process, working conditions and
by using fibers grown in same ecological zone. By the
results, it can be concluded sisal and nettle fibers having
Figure 5. TGA curve for epoxy and developed composites.
Figure 6. DTA curves for epoxy and developed composites.
Figure 7. Change in enthalpy (ΔH) for epoxy and developed
composites.
810 Fibers and Polymers 2017, Vol.18, No.4 Manish Kumar Lila et al.
higher surface roughness thus better mechanical properties
than HEC and pure epoxy. It can be concluded that surface
roughness propagate more inter surface bonding, which
further can be observed in change in enthalpy during thermal
analysis. The change in enthalpy can also be correlated with
inter-surface bonding as it is showing the similar trends with
the mechanical properties of the developed composites. Sisal
and nettle fiber reinforced epoxy composites exhibited better
mechanical properties as compared to pure epoxy and hemp
reinforced epoxy composites. The mechanical properties can
also be correlated with the voids present due to air
entrapment in the composites.
Air entrapment can be reduced by applying higher
pressure during fabrication, so that the matrix can fill the gap
between the warp and fills of the woven mat. Also a low
viscosity resin can be used to improve surface wetting of the
fiber. When considering thermal stability, developed
composites shown better results in terms of lower degradation
rate at same temperature and higher enthalpy when compared
with pure epoxy Thus, the results obtained revealed that the
natural fibers reinforced epoxy composites are much better
than pure epoxy resin.
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