Journal of King Saud University – Engineering Sciences xxx (2018) xxx–xxxContents lists available at ScienceDirect
Journal of King Saud University – Engineering Sciences
journal homepage: www.sciencedirect .comOriginal articleEffect of variation in frequencies on the viscoelastic properties of coir
and coconut husk powder reinforced polymer composites⇑ Corresponding author.
E-mail address: doobada@abu.edu.ng (D.O. Obada).
Peer review under responsibility of King Saud University.
Production and hosting by Elsevier
https://doi.org/10.1016/j.jksues.2018.10.001
1018-3639/
 2018 Production and hosting by Elsevier B.V. on behalf of King Saud University.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Obada, D.O., et al. Effect of variation in frequencies on the viscoelastic properties of coir and coconut husk powd
forced polymer composites. Journal of King Saud University – Engineering Sciences (2018), https://doi.org/10.1016/j.jksues.2018.10.001D.O. Obada a,⇑, L.S. Kuburi a, M. Dauda a, S. Umaru a, D. Dodoo-Arhin b,c, M.B. Balogun a, I. Iliyasu a,
M.J. Iorpenda a
aDepartment of Mechanical Engineering, Ahmadu Bello University, Zaria, Nigeria
bDepartment of Material Science and Engineering, University of Ghana, Legon, Ghana
c Institute of Applied Science and Technology, University of Ghana, Legon, Ghana
a r t i c l e i n f o a b s t r a c tArticle history:
Received 5 April 2018
Accepted 9 October 2018
Available online xxxx
Keywords:
Coir
Coconut husk particulate
Loss modulus
Storage modulus
Damping effect
Dynamic mechanical propertiesThis study describes an investigation into the physical, mechanical and dynamic mechanical properties of
coir (coconut fiber) and coconut husk particulates reinforced polymer composites which were prepared
by the hot press method. The impact of coir loading on the physical and mechanical properties of the
composites was examined in more detail. It was observed that the values of rigidity, flexural strength
and hardness were raised up to 7.1 MN/m2, 17.0 MPa and 92.5 MN/m2, with increase in coir length.
Impact energy reduced proportionately with increment in fiber length (from 0.78 J at no fiber inclusions
to 0.42 J at 30 mm fiber length). With the increment of fiber length from zero to 10 mm, a reduction in
density and increment in water absorption properties was observed. These properties (density and water
absorption) remained with increase in fiber length for a given structure of matrix/filler. The surface mor-
phology of composite with longest fiber length (30 mm) was investigated. Fiber pullout and little voids
on composite surfaces were seen. Fundamentally, these regions can encourage the matrix impregnation
onto the fiber. At the highest fiber loading and at highest frequency (10 Hz) used during the dynamic
mechanical examination, the tan d peak gets widened, underlining the enhanced fiber/matrix grip.
Also, extra and noticeable peaks were seen at higher frequency conditions in the tan d curves, because
of the interlayer phenomenon.

 2018 Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access
article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).1. Introduction
The interest for composite materials especially thermoplastic
matrix reinforced by natural fibers has improved remarkably for
several industrial applications. Composites with light-weight, high
strength to-weight ratio and other characteristics have made pro-
gress as compared to conventional materials like metals, wood
and so on. It is noteworthy that as far as composites are concerned,
the qualities of the completed product can be customfitted to a par-
ticular design prerequisite by the careful determination of base
material (matrix) and the reinforcing agents (Jahan et al., 2012;Kabir et al., 2014; Islam et al., 2015). In another sense, the amount
of waste is consistently expanding because of increase in popula-
tion, change in way of life etc. Water sachets made of polyethylene
are broadly utilized in developing nations, for instance, Nigeria.
Sachet water production lines are found in numerous areas, includ-
ing towns and villages of Nigeria. Recently, various types of waste
materials and natural fibers have been used as fillers in polymer
composites for different applications (Samotu et al., 2015a,b;
Dan-asabe et al., 2016; Kuburi et al., 2017). This decreases the man-
ufacturing cost as well as offers opportunities for the usage of waste
materials, which consequently reduces environmental pollution.
Reinforcing polymer, for example, polyethylene with natural fiber/-
particulates to produce polymer composites has gained significant
consideration because of their intrinsic properties (Kuburi et al.,
2017). Despite the fact that glass fiber reinforced plastics have high
quality, their fields of use are exceptionally restricted on account of
its inherent higher cost of production (Verma et al., 2013). Natural
lignocelluloses, for example, coconut shell powder, has remarkable
properties as reinforcement in plastics. Coconut shell has becomeer rein-
2 D.O. Obada et al. / Journal of King Saud University – Engineering Sciences xxx (2018) xxx–xxxessential as a filler material because of some natural properties like
high strength and highmodulus (Agunsoye et al., 2012). However, it
is worth noting that increment in coconut shell content improves
the mechanical strength and water absorption properties of the
composites, nonetheless, it lessens the stretching at break
(Husseinsyah and Mostapha, 2011). Natural fibers, for example,
flax, sisal, jute, coir etc, are of relatively low cost, and are readily
accessible which proves advantageous (Awwad et al., 2012; Yan,
2012; Lecompte et al., 2015). Natural fibers are also additionally
biodegradable, which makes them suitable materials for industrial
applications (Yan, 2016; Yan et al., 2015).
Among natural fibers, coir are essentially utilized in view of
their low cost, durability amongst other advantages (Zaman and
Beg, 2014). Coir has been accounted for as having the highest
extension at break among common natural fibers, which allows
it absorb strain more than other fibers (Satyanarayana et al.,
1981, 1990; Munawar et al., 2007).
The impact of fiber length on the mechanical properties of nat-
ural fiber reinforced polymer composites has gained significant
attention (Peltola, 2005; Kumar et al., 2014; Mohamed et al.,
2018). In this way, the critical fiber length (lc) is important because
it helps to comprehend the optimum mechanical properties of
fiber-reinforced composites (Bourmaud and Baley, 2010). At this
point, like many other natural fibers, coir also have some negative
attributes: they are not water resistant; their physico-mechanical
properties are subject to the atmosphere, and along these lines it
is hard to predict their composite properties (Yan et al., 2012).
Moisture intake of fibers like coir is quite high and prompts poor
interfacial adhesion with the water resistant matrix polymers
(Nam et al., 2011). It has been shown that the interface between
natural fibers and the polymer matrix is a problem for the proper-
ties of composites. It ought to be controlled to enhance themechan-
ical properties. In this way, it is important to reduce the moisture
absorption characteristics of fibers of this nature by appropriate
surface chemical adjustment (Nam et al., 2011; Yan, 2012).
A lot of researchers have reported on the viscoelastic properties
or dynamic mechanical analysis (DMA) of natural fiber reinforced
polymer composites (NFRPCs) and noted the acceptable viscoelas-
tic properties for industrial applications (Idicula et al., 2005;
Jawaid et al., 2013; Gupta and Gond, 2017). These DMA properties
of polymeric composites do not only depend on temperature and
frequency, but also on fiber length and loading, bond between
fibers and matrix e.t.c (Rana et al., 2017).
To date, to the best of our knowledge, a couple of works have
been reported on the chemical treatment of coir to enhance the
mechanical properties of coir reinforced polymer composites
(Yan, 2012; Kuburi et al., 2017). Moreover, seemingly, there appear
to be scanty reports which have featured the viscoelastic or DMA
properties of coir based polymer composites at different vibra-
tional frequencies. These dynamic mechanical properties have dif-
ferent responses under varying oscillating frequencies.
Therefore, in the current study, coir was chemically treated to
increase the compatibility of coir with coconut shell particles as
reinforcement of the matrix. The impact that the coir length has
on the physical and mechanical properties of the composites sam-
ples was investigated. Furthermore, the effect of different vibra-
tional frequencies on the dynamic mechanical responses of the
composite material developed with the highest coir length was
investigated.
2. Materials and methods
Coir obtained from coconut husk was washed with water to
expel contaminants and open air dried at room temperature
(27 C) in the laboratory for 48 h. The dried coir were soaked in
5 wt% NaOH solution at room temperature for 30 min. This wasPlease cite this article in press as: Obada, D.O., et al. Effect of variation in freque
forced polymer composites. Journal of King Saud University – Engineering Sciefollowed by washing with distilled water to enable retained alkali
to drain from the fibers. Next, these washed fibers were open air
dried in the laboratory for 24 h and afterwards dried at 60 C in a
hot air oven for 8 h. These fibers were then cut into an average
length of 10, 20 and 30 mm for investigation.
The coconut husk was dried for 72 h in open sun before grinding
into powder. The powder was sieved as per the BS 1377:1990 stan-
dard (Standard, 2004); utilizing a 425 mm sieve size. The polyethy-
lene was sun-dried and cut into small pieces using a plastic crusher
machine. The composition of coconut husk powder, coir and shred-
ded plastic water sachet (RLDPE) were weighted and melt mixed
for 30 min at temperature of 180 C utilizing a two-roll mill. The
composites were thermoformed by hot press machine with the
help of a steel mold. The working temperature was 80 C with
15 min of preheat, 2 min of applied compaction pressure and
5 min of cooling. Each sample of the composite fabricated con-
tained 70%, 15% and 15% by weight of recycled low density poly-
ethylene, coir and coconut husk powder respectively. Table 1
presents the nomenclature of fabricated composites and tests con-
ducted based on experimental design.
2.1. Characterization
2.1.1. Physico-mechanical properties of polymer materials
The densities of the compacted composite samples were
obtained from Eq. (1). The rate of water absorption was obtained
from the relationship in Eq. (2) after soaking the samples in water
for 24 h. The hardness characteristics of the samples produced was
resolved by using a hardness analyzer Model 5023-A with support-
ing table model 5019. A Charpy impact machine was utilized to
investigate how much impact the samples can absorb. Tensile test
of the composites was carried out using the Hounsfield Tensometer
tensile machine at a cross head speed of 5 mm/min according to
ASTM D 638 method (ASTM, 2008). The flexural tests were per-
formed in accordance to ASTM D790-00 standard. Utilizing Houns-
field Tensometer ‘‘w” S/No 9875 UK with a suitable fixture. The
flexural strength was calculated using Eq. (3).
q ¼ mv ð1Þ
where; ῥ = density (g/mm3)
m = mass of samples in grams (g)
v = volume of samples in mm3.
¼ Wf Wp i  100 ð2Þ
Wi
where; p = percentage increase in weight
Wi = initial weight of specimen (g)
Wf = final weight of specimen (g).
¼ 3PLFlexural Strength ðMPaÞ ð3Þ
2bd2
where, P = maximum load applied on test specimen (N)
L = support span/gauge length (mm)
b = width of specimen tested (mm)
d = thickness of specimen tested (mm).
2.1.2. Structural and morphological properties of polymer composite
with highest coir length (Polymer 30)
X-ray diffraction (XRD) examination of the composite samples
with 30 mm fiber length was conducted on a Shimadzu 6000
X-ray diffraction instrument. The X-ray beam was Copper Kancies on the viscoelastic properties of coir and coconut husk powder rein-
nces (2018), https://doi.org/10.1016/j.jksues.2018.10.001
D.O. Obada et al. / Journal of King Saud University – Engineering Sciences xxx (2018) xxx–xxx 3
Table 1
Nomenclature of fiber-powder reinforced polymer composites and tests carried out.
Symbol Composites Mechanical Tests Physical Tests Structural and
Morphological
Evaluation
Polymer 10 Composite reinforced by 15 wt% of coconut Tensile strength, hardness and Density and water absorption
powder and 10 mm coir length flexural strength
Polymer 20 Composite reinforced by 15 wt% of coconut Tensile strength, hardness and Density and water absorption
powder and 20 mm coir length flexural strength
Polymer 30 Composite reinforced by 15 wt% of coconut Tensile strength, hardness, flexural Density and water absorption XRD and SEM
powder and 30 mm coir length strength and DMA(k = 0.1540 nm) radiation, with operating conditions at 40 kV and
30 mA and a two theta range of 5–50 C. Scanning electron micro-
scopy examination was conducted to reveal the morphological fea-
tures of the polymer using a ultra-high vacuum and high resolution
FEI Xl-30 scanning electron magnifying lens furnished with an
EVEX EDS and operated at accelerating voltage of 15 kV. The sur-
faces of the composite were deposited with thin films of gold
before their morphological observations were made.
2.1.3. Dynamic mechanical properties of Polymer 30
Dynamic mechanical spectroscopy was performed on a DMA
242E equipment to get the glass transition temperatures of the
polymer samples with 30 mm fiber length (Polymer 30) due to
its comparative improvement in mechanical properties. Estima-
tions of the dynamic flexural strengths from room temperature
to 150 C at a rate of 5 K/min were made using the 3 point bending
mode on all the test samples with dimensions of (30  15  2)
mm3 according to the ASTM D 5023 (ASTM, 2007) standard. The
frequencies under which examinations were made were 2, 5 and
10 Hz. The temperature of transition from elastic to rubbery phase
was taken to be the tan d peak, while the loss and storage moduli in
addition to their ratios were analyzed.
3. Results and discussion
Fig. 1 reveals the thickness and water retention test results for
the composite samples produced. The results demonstrated that1E-6 10 mm, 13%
2
9,5E-7
9E-7
8,5E-7
8E-7
0 5 10 15
Coir length 
Fig. 1. Variations in density and wate
Please cite this article in press as: Obada, D.O., et al. Effect of variation in freque
forced polymer composites. Journal of King Saud University – Engineering Scie
Density (g/cm3)there is a decrease in density values with increment in fiber length
until the 10 mm mark and it was observed as nearly the same at
20 and 30 mm fiber length samples, which is by virtue of the same
volume of fillers. Density is just marginally affected by increment
of fiber length. Coir addition somewhat lessened the density of the
composites in light of a conceivable increase in porosity levels
stemming from fiber incorporation. The connection between
water intake and filler content of the composites at various fiber
lengths is shown also (see Fig. 1). It is observed that there is incre-
ment in water retention with incorporation of fiber because of
change in filler content. This observation was also noted by
Dhakal et al. (2007). This precisely describes an inverse relation-
ship between density and water intake as far as the polymer struc-
tures are concerned. It is imperative to note that that the fillers
used are natural fibers (coir) and these fibers are emphatically
hydrophilic materials with numerous hydroxyl groups in the
fibers’ structure and subsequently, the moisture absorption rate
is moderately high. The hydrophilic orientation of the fillers (coco-
nut husk powder and coir) causes the water uptake by these lingo-
cellulosic materials which are because of the creation of hydrogen
bonds amongst filler and molecules of water. It is notable that
these lingo-cellulosic materials (coir and coconut husk powder)
absorb water by developing hydrogen bonding between water
on the fillers’ biological cell walls. The biological cell walls provide
dimensional and biological stability after the chemical treatment.
With the creation of hydroxyl groups, a coconut shell which
clearly incorporates the coir removed from it tends to indicateDensity
 Water absorption
14
0 mm, 12%
30 mm, 11% 12
10
8
6
4
2
30mm, 8E-7
0
-2
20 25 30
(mm)
r absorption against coir length.
ncies on the viscoelastic properties of coir and coconut husk powder rein-
nces (2018), https://doi.org/10.1016/j.jksues.2018.10.001
 Water absorption (%)
4 D.O. Obada et al. / Journal of King Saud University – Engineering Sciences xxx (2018) xxx–xxxlow resistance to moisture. It is also worthy of mention that the
presence of miniaturized scale voids, pores, micro-cracks con-
tributes in the incorporation of water if present in the structure
of fabricated composites.
Fig. 2 shows the tensile and flexural strength in addition to
hardness test results for the composite samples produced. Coir
itself has a moderately low elasticity among the common fibers;
this affirmation is likewise buttressed by (Yan et al., 2016). The
tensile strength of the natural fiber reinforced polymer composite
is firmly reliant on the elasticity of the fiber. The higher the elastic-
ity of the fiber and the better the fiber/matrix grip, the shorter the
base fiber length required for the very effective transfer of stress
(Barkoula et al., 2010). The figure shows that the highest value of
tensile strength for fiber length of 30 mm was approximately
7.1 MN/m2. With the inclusion of fiber length, there is a drop in
tensile strength at 10 mm fiber length which starts increasing with
increase in fiber length. The decrease in elasticity in composite
structure with 10 mm fiber length can be by reason of insufficient
fillers to reinforce the matrix. As the filler content increases with a
comparative increment in fiber length, the elasticity of the com-
posites obviously increases. This is because the inclusion of fillers
strengthens the interface of bonding where it necessitates the abil-
ity of the filler to promote stress transferred from the matrix. It is
also noteworthy that the coconut husk has high lignin content
which was found to be 29.4% (results not shown). Lignin, which
is amongst the polymer network of hemi-cellulosic materials, not
only holds the composites together but also acts as a solidifying
agent for the cellulose molecules within wall of the composite cell
(Kim et al., 2007). Subsequently, the lignin and cellulose content of
coconut husk have increased the elasticity of the composites with
increasing fiber length addition. Fig. 2 likewise demonstrates the
shore A hardness value and the flexural strength of the composite
in terms of the fiber length. The result shows that there is a
decrease in hardness value with the inclusion of the shortest fiber
length (10 mm). An increase was observed with increase in fiber
length. Similar to the tensile properties, the increase in flexural
strength at 30 mm fiber length was 17.0 Mpa. The flexural strength
of the composites without the inclusion of coir was around
18.0 Mpa. Hence, as far as these result shows, the coir don’t offer
enough reinforcement to the polymer matrix due to the arbitrary
distribution of the fibers in the composite structure. The result Tensile Strength (MN/m2)
 Hardness Strength (MN/m2)
 Flextural Strength (Mpa)
7,2
30 
7,0
6,8
6,6
6,4
6,2
0 5 10 15
Coir length (m
Fig. 2. Variations in tensile strength, hardness
Please cite this article in press as: Obada, D.O., et al. Effect of variation in freque
forced polymer composites. Journal of King Saud University – Engineering Scie
Tensile Strength (MN/m2)for the flexural strength demonstrates that consideration of fiber
length in fiber and particulate filled composite lessens its flexural
strength at 10 mm fiber length, however, it increases with incre-
ment in fiber length. This demonstrates that coir has an effect on
the flexural strength because of good interfacial adhesion between
the filler and the matrix. Similar to composite elasticity, the flexu-
ral strength of the composites was found to be highest among the
composites fabricated with 30 mm fiber length, which can be
ascribed to fiber dispersion in the matrix.
Fig. 3 reveals the impact of fiber length on the Young’s modulus
of the polymer structure. It can be seen that the Young’s modulus
of the composites improved with an increment in fiber length and
this demonstrates strongly the effect of coir to provide significant
stiffness to the composites. At a point, after the 20 mm fiber length
mark, it is noticed that the modulus begins to decrease. It is real-
ized that the coir, which has a higher stiffness than the base
(matrix) can build the modulus of the composites. However, by
and large, it is an inverse of the elongation at break. It can also
be inferred that fabrication of the composites samples used in this
study, through compounding and hot pressing, led to a decrease in
impact strength, which can be ascribed to decreased energy
absorption through fiber crack, de-bonding attributes and fiber
removal.
As rightly mentioned, an ideal length of a natural fiber has a
high probability of existing for its composite produced through
hot pressing method as used in this investigation. This critical
length of the fiber in the composite is controlled by interfacial
bonding quality, interfacial de-bonding, and interface interactions.
In view of the tensile, hardness, flexural and stiffness of the com-
posite structures, in addition to the roughest surface as depicted
in the morphological features (see Fig. 5a & b), the 30 mm fiber
length can be selected as ideal fiber length in this study. Hence,
Polymer 30 was further evaluated for its structural, morphological
and dynamic mechanical properties.
Fig. 4 shows the structural orientation of Polymer 30. The reflec-
tions were found at 22.69, 25.76 and 42.36. The reflections are in
good agreement with the crystalline nature of reinforced poly-
olefin composites. At some point, the XRD reflections also exhibits
intense or clearly noticeable diffraction peaks, which indicate that
the synthesis of coir reinforced polymer composites treated chem-
ically, were present as high quality crystals.30 mm, 7.1 MN/m2
mm, 92.5 MN/m2 93 18
30 mm, 17 Mpa
92
17
91
90 16
89
15
88
87 14
86
13
85
20 25 30
m)
and flexural strength against coir length.
ncies on the viscoelastic properties of coir and coconut husk powder rein-
nces (2018), https://doi.org/10.1016/j.jksues.2018.10.001
Hardness Strength (MN/m2)
Flextural Strength (Mpa)
D.O. Obada et al. / Journal of King Saud University – Engineering Sciences xxx (2018) xxx–xxx 5
0,065
Young modulus 0,80
 Average impact energy 20 mm, 0.061 Gpa
0,060 0,75
30 mm, 0.06 Gpa
0,70
0,055 0,65
10 mm, 0.61 J 20 mm, 0.58 J 0,600,050
0,55
0,045 0,50
0,45
0,040 10 mm, 0.041 Gpa 30 mm, 0.42 J 0,40
0 5 10 15 20 25 30
Coir length (mm)
Fig. 3. Variations in Young modulus and impact energy against coir length.
(110)
12000
Polymer 30
10000
8000
6000
4000 Fibre (200)
2000
0
5 10 15 20 25 30 35 40 45 50
2 theta (o)
Fig. 4. XRD patterns of polymer 30.
Intensity (counts) Young modulus (Gpa)
Average impact energy (J)Fig. 5a & b show the SEM images of the fabricated compos-
ites with different magnifications. It can be noticed that there
is fiber delamination which can be due to inter-laminar stresses
as the fiber tend to pull-out near the small voids, leading to
small voids on composite surfaces. In principle, these aspects
can facilitate the matrix impregnation onto the fiber. Further-
more, it is clear that the composite surfaces have a rough frac-
ture surface, which enables a strong adhesion between the
filler and matrix. Therefore, the significant improvement of the
mechanical properties of the composites is expected owing toPlease cite this article in press as: Obada, D.O., et al. Effect of variation in freque
forced polymer composites. Journal of King Saud University – Engineering Sciethe good dispersion throughout the matrix and the strong inter-
action between the fillers and the matrix. Fig. 5c show a cross
section of coir used in this study. It is clear that the elementary
fibers produce amorphous shapes. Each elementary fiber is com-
prised of cell wall (primary and secondary walls), lumen and the
micro-fibrils. Generally, matrix cracking/splitting is typically
observed when there is a solid bond between the fiber and
the matrix (Johnson et al., 2012; Brighenti et al., 2013) and also
these trends may be observed at higher temperature
(Murayama, 1978).ncies on the viscoelastic properties of coir and coconut husk powder rein-
nces (2018), https://doi.org/10.1016/j.jksues.2018.10.001
6 D.O. Obada et al. / Journal of King Saud University – Engineering Sciences xxx (2018) xxx–xxx
Fig. 5. (a & b) SEM images of coir/polyethylene interface: (c) cross-section of coir.Fig. 6 shows the EDS spectrum (elemental composition of only
one region shown) of the composite surface. This spectrum reveals
that the composite is basically made out of carbon predominantly
due to the ligno-cellulosic materials utilized as fillers, and in addi-
tion oxygen.
The SEM images for untreated and treated coir which was filled
in the polyethylene matrix are shown in Fig. 7. For the untreated
fibre-formed composite (Fig. 7a), holes between the fiber and the
encompassing matrix are observed, showing the poor fiber/matrix
interfacial bond. Fig. 7(b) shows that the fiber/matrix interfacial
holes become very much smaller after the treatment, demonstrat-
ing a superior interfacial adhesion ability (Mohanty et al., 2001). As
shown in Fig. 7, a rougher fiber surface is accomplished after treat-
ment (Fig. 7b), which is valuable for coir /matrix interfacial attach-
ment since a rougher surface encourages fiber and matrix
mechanical grip.
Dynamic mechanical investigation can portray the viscoelastic
properties of the materials with a range of temperatures (Eklind
and Maurer, 1996; Wang et al., 2015). Representative curves ofFig. 6. SEM/EDS o
Please cite this article in press as: Obada, D.O., et al. Effect of variation in freque
forced polymer composites. Journal of King Saud University – Engineering Sciethe viscoelastic properties versus temperature at frequencies of
2, 5 and 10 Hz for polymer 30 are shown in Fig. 8. At all areas of
interest, the composites demonstrate the trademark drop in mod-
ulus around the transition stage from elastic to viscous states of
the material. It can likewise be seen that at the different vibrational
frequencies, there is a constant fall in the storage modulus with
temperature, which is connected with energy release with temper-
ature (Feng et al., 2011).
It is noted from Fig. 8 that the impact of frequency on E0 is more
noteworthy inside the low temperature region than that inside the
high temperature area. The values of E0 increase with the frequency
until 5 Hz and a slight decrease is observed at 10 Hz. This is on the
grounds that under cyclic loading at high frequency, the atoms of
the polymer won’t have enough time to undergo lasting deforma-
tion and afterwards, the material displays elastic tendencies. Thus,
the polymer behaviour is more similar to a solid and an increase
was observed for E0 values. This tendency of an increase in E0 under
a higher frequency in the high temperature area and comparatively
lower value of E0 for composite subjected to 10 Hz, can be ascribedf Polymer 30.
ncies on the viscoelastic properties of coir and coconut husk powder rein-
nces (2018), https://doi.org/10.1016/j.jksues.2018.10.001
D.O. Obada et al. / Journal of King Saud University – Engineering Sciences xxx (2018) xxx–xxx 7
Fig. 7. SEM images of coir/polyethylene interface: (a) un-cleaned b) cleaned.
Storage Modulus @ 2Hz
Storage Modulus @ 5Hz
Storage Modulus @ 10Hz
1000 o33.740 C, 913.18 Mpa
o
35.740 C, 979.94 Mpa
o 1000 1000
31.740 C, 984.18, Mpa
800
800 800
600
600 600
400
400 400
200 200 200
0 0 0
20 40 60 80 100 120 140
Temperature (oC)
Fig. 8. Storage modulus curves of Polymer 30 under varying frequencies.
Storage Modulus @ 2Hz
Storage Modulus @ 5Hz
Storage Modulus @ 10Hzto decrease in molecular mobility of the polymer chains above the
glass transition temperature under this frequency (10 Hz)
(Hameed et al., 2007). This assertion is also buttressed in the
results observed for loss modulus (E00) where a better thermal sta-
bility of composite was observed as a result of the glass transition
temperature displayed under 10 Hz as compared to under vibra-
tion frequencies (2 and 5 Hz) due to a decrease in molecular mobil-
ity of polymer chains
In Fig. 9, it is noticed that upon heat treatment, the loss modu-
lus (E00) decrease because a smaller force is required for deforma-
tion. In the area of the glass transition, sub-atomic segmental
movements are provoked, however these movements happen with
difficulties, portrayed as sub-atomic rubbing/friction that releases
a significant part of the force. As temperature keeps on increasing
over the glass change region, atomic frictions diminish, less energy
is released and the loss modulus again reduces. This higher tem-
perature decrease in loss modulus results in a peak which is
observed at all frequencies around 120 C. It can be highlighted
that the composite sample under frequency of 10 Hz showed more
resistance to deformation owing to the temperature (58.74 C) at
which the sharp decline was observed in the glassy region. ThisPlease cite this article in press as: Obada, D.O., et al. Effect of variation in freque
forced polymer composites. Journal of King Saud University – Engineering Scieexplains a better thermal stability of the composites at that fre-
quency comparatively, which can be ascribed to the decrease in
the molecular mobility of the polymer chains at 10 Hz frequency.
The change in loss factor of the polymer with different frequen-
cies is shown in Fig. 10. The composite response under 5 Hz fre-
quency displayed a higher tan d peak value than composites
investigated under the other frequencies. This can be credited to
energy losses by reason of the viscous damping. The temperature
at which tan d achieves the most extreme value can be referred
to as the transition phase from elastic to viscous (Shanmugam
and Thiruchitrambalam, 2013). A positive move in Tg can be seen
at all frequencies. It has been reported that Tg values obtained
from loss modulus (E00) curve peak are more realistic as compared
to those obtained from loss factor (tan d) (Akay, 1993; Murayama,
1978). Tg increased from 116.4 C for composite under 10 Hz to
118.74 C and 119.74 C for composites under the frequencies at
2, and 10 Hz respectively. This means that at 10 Hz frequency,
the polymers’ service temperature is prolonged due to the lower
tan d peak height shown by composite under a 10 Hz frequency.
The tan d peak height is minimum for the composite under
10 Hz vibrational frequency which indicates that at this frequencyncies on the viscoelastic properties of coir and coconut husk powder rein-
nces (2018), https://doi.org/10.1016/j.jksues.2018.10.001
8 D.O. Obada et al. / Journal of King Saud University – Engineering Sciences xxx (2018) xxx–xxx
Loss Modulus @ 2Hz
Loss Modulus @ 5Hz
Loss Modulus @ 10Hz
o
53.740 C, 109.43 Mpa o 120 12058.740 C, 107.62 Mpa
120 o
52.740 C, 113.32 Mpa
100 100
100
80 80
80
60 60
60
40 40 40
20 20 20
0 0 0
20 40 60 80 100 120 140
Temperature (oC)
Fig. 9. Loss modulus curves of Polymer 30 under varying frequencies.
tan delta @ 2Hz
tan delta @ 5Hz
tan delta @ 10Hz
o
o 119.74 C, 0.3313 Mpa 0.35
0.35 118.74 C, 0.3358 Mpa
o 0.30
116.74 C, 0.3213 Mpa 0.30
0.30
0.25
0.25 0.25
0.20 0.20
0.20
0.15 0.15
0.15
0.10
0.10
0.05
0.05 0.10
0.00
20 40 60 80 100 120 140
Temperature (oC)
Fig. 10. Tan delta curves of Polymer 30 under varying frequencies.
tan delta @ 2Hz Loss Modulus @ 2Hz (Mpa)
tan delta @ 5Hz Loss Modulus @ 5Hz (Mpa)
tan delta @ 10Hz Loss Modulus @ 10Hz (Mpa)the stress transfer and the interfacial bonding between the fiber-
powder/matrix is increased. Therefore, the higher the tan d peak
value, the greater is the degree of molecular mobility. In other
words, the sequence followed for the degree of molecular mobility
of polymer chains under the vibrational frequencies considered inPlease cite this article in press as: Obada, D.O., et al. Effect of variation in freque
forced polymer composites. Journal of King Saud University – Engineering Sciethis study is 2 Hz > 5 Hz > 10 Hz. In this way, the results obtained
for E0 and E00 are in line with values of tan d, where decreases in
the degree of molecular mobility of the polymer chains were
observed at 10 Hz. This finding is also corroborated by research
conducted by (Kuzak and Shanmugam, 1999)ncies on the viscoelastic properties of coir and coconut husk powder rein-
nces (2018), https://doi.org/10.1016/j.jksues.2018.10.001
D.O. Obada et al. / Journal of King Saud University – Engineering Sciences xxx (2018) xxx–xxx 94. Conclusions
In this investigation, coir and coconut shell powders were used
as reinforcement for polyethylene composites. Mechanical proper-
ties were determined by, tensile and flexural tests, impact, young
modulus, hardness and physical tests (density and water absorp-
tion). The maximum improvement in properties was observed for
composites fabricated with 30 mm fiber length. Hence, SEM stud-
ies were carried out to examine the morphology of fiber-powder/
polyethylene interfaces of Polymer 30. In addition, dynamic
mechanical properties were determined. The study reveals:
i. With introduction of coir length from zero to 10 mm,
decrease in density and increase in water absorption was
observed and this trend remained the same with increment
in coir (fiber) length (20 and 30 mm) for a given composition
of matrix/filler. This implies that the inclusion of fiber at the
lengths investigated in this study results in loss of weight of
the composites.
ii. The mechanical properties increase with increment in fiber
length (i.e. after reducing with inclusion of 10 mm fiber
length). There were improved mechanical properties at
30 mm coir length
iii. Young’s modulus has the highest value at 20 mm coir length
after rising with inclusion of coir length. This was ascribed to
the inverse relationship between the modulus and elonga-
tion at break.
iv. The sequence followed for the degree of molecular mobility
of polymer chains under the vibrational frequencies consid-
ered in this study is 2 Hz > 5 Hz > 10 Hz. In this way, the
results obtained for E0 and E00 are in line with values of tan
d, where decreases in the degree of molecular mobility of
the polymer chains were observed at 10 Hz.
v. A positive shift in Tg to higher temperature was clearly
observed for composites under the 10 Hz frequency i.e., Tg
increased from 116.4 C for composite under 10 Hz to
118.74 C and 119.74 C for composites under the frequen-
cies at 2, and 5 Hz respectively. This means that at 10 Hz fre-
quency, the polymers’ service temperature is prolonged due
to the lower tan d peak height shown by composite under a
10 Hz frequency, which is related with solid fiber/matrix
interfacial associations that confine the segmental move-
ment of the polymer chains.
vi. SEM studies revealed that the failure associated with the
fibers from the polymer matrix which were ascribed to
inter-laminar stresses as the fiber tend to pull-out near the
small voids needs to be further checked.
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