Composites Science and Technology 64 (2004) 955–965
www.elsevier.com/locate/compscitech
Mechanical properties of sisal/oil palm hybrid fiber reinforced
natural rubber composites
Maya Jacoba, Sabu Thomasa,*, K.T. Varugheseb
a
School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills P.O. Kottayam, Kerala, 686 560, India
b
Thermal Research Center, KORADI, Nagpur, 441 111, India
Received 8 August 2002; received in revised form 19 June 2003; accepted 25 June 2003
Abstract
Natural rubber is reinforced with untreated sisal and oil palm fibers chopped to different fiber lengths. The effects of concentration and modification of fiber surface in sisal/oil palm hybrid fiber reinforced rubber composites have been studied. Increasing
the concentration of fibers resulted in reduction of tensile strength and tear strength, but increased modulus of the composites.
Composites were prepared using fibers treated with varying concentrations of sodium hydroxide solution and for different time
intervals. The vulcanisation parameters, processability characteristics, and stress–strain properties of these composites were analysed. The rubber/fiber interface was improved by the addition of a resorcinol-hexamethylene tetramine bonding system. The reinforcing property of the alkali treated fiber was compared with that of untreated fiber. The extent of fiber alignment and strength of
fiber-rubber interface adhesion were analysed from the anisotropic swelling measurements.
# 2003 Elsevier Ltd. All rights reserved.
Keywords: A. Natural rubber; Fiber composite; B. Polymer-matrix composite; C. Mechanical properties
1. Introduction
Fiber reinforced rubber composites are of tremendous
importance both in end-use applications and the area of
research and development. These composites exhibit the
combined behavior of the soft, elastic rubber matrix and
the stiff, strong fibrous reinforcement. The development
of fiber reinforced rubber composites has made available polymers that are harder than aluminum and stiffer
than steel. Generally short fiber reinforced rubber composites has become popular in industrial fields because
of the processing advantages and increase in strength,
stiffness, modulus and damping [1,2]. The design of a
short fiber reinforced rubber composite depends on
several factors such as the aspect ratio of the fiber, control of fiber orientation and dispersion and existence of
a strong interface between fiber and rubber.
Cellulosic fibers are derived from many renewable
resources and have many desirable properties for reinforcement of thermoplastics such as low density, high
* Corresponding author. Tel.: +91-481-730003; fax: +91-481561190.
E-mail address: [email protected], [email protected]
(S. Thomas).
0266-3538/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0266-3538(03)00261-6
stiffness and low cost [3–8]. Various natural fibers have
been used as reinforcement in NR viz. sisal and coir
fiber as reinforcing fillers for NR has been investigated
by Varghese et al. [9] and Geethamma et al. [10]. The
use of waste silk fiber and jute fiber in natural rubber
has also been studied [11,12]. Recently studies relating
to the incorporation of bamboo fiber in natural rubber
has been investigated by Ismail et al. [13]. Sisal fiber is
one of the strongest fibers, which can be used for several
applications [14]. Oil palm empty fruit bunch (OPEFB)
and oil palm mesocarp fibers are two important types of
fibrous materials left in the palm oil mill. Oil palm fibers
are hard and tough and have found to be a potential
reinforcement in phenol-formaldehyde resin [15].
Among the different natural fibers sisal and oil palm
fibers appear to be promising materials because of the
high tensile strength of sisal fiber and toughness of oil
palm fiber. Therefore any composite comprising of these
two fibers will exhibit the above desirable properties of
the individual constituents. The present study deals with
the optimization of sisal and oil palm fiber lengths in
natural rubber composites. The influence of fiber loading and fiber ratio on mechanical properties was
analyzed. Studies of equilibrium swelling in a hydrocarbon solvent were also carried out. The rubber/fiber
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M. Jacob et al. / Composites Science and Technology 64 (2004) 955–965
interface was improved by the addition of a resorcinolhexamethylene tetramine bonding system. The reinforcing property of the alkali treated fiber was also
compared with that untreated fiber.
2. Experimental
Sisal fiber was obtained from Sheeba Fibers, Poovancode, Tamil Nadu. Oil palm fiber was obtained from
Oil Palm India Limited. Natural rubber used for the
study was procured from Rubber Research Institute of
India, Kottayam. All other ingredients used were of
commercial grade.
the ingredients. The composite materials were prepared
in a laboratory two-roll mill (150300 mm). The nipgap, mill roll, speed ratio, and the number of passes
were kept the same in all the mixes. The samples were
milled for sufficient time to disperse the fibers in the
matrix at a mill opening of 1.25 mm. The bonding system consisting of resorcinol and hexamethylene tetramine was incorporated for mixes containing treated
fibers. The fibers were added at the end of the mixing
process, taking care to maintain the direction of compound flow, so that the majority of fibers followed the
direction of the flow.
5. Measurement of properties
3. Fiber preparation
The chemical constituent fibers of given in Table 1
[15]. The physical constituents of fibers were determined
and are given in Table 2. Sisal and oil palm fibers were
first separated from undesirable foreign matter and pith
material. Oil palm fiber was extracted from empty oil
palm fruit bunches by retting. The fibers were then
cleaned of undesirable foreign matter and pith followed
by repeated washing and drying in the oven. This ensured
the removal of the oily matter present in the fiber.
The fibers were then chopped to different lengths.
Sisal and oil palm fibers of lengths 10 mm and 6 mm
were treated for 1 h with sodium hydroxide solutions of
varying concentrations viz., 0.5, 1, 2 and 4 and 10% at
room temperature. Finally the fibers were repeatedly
washed with water and air-dried.
4. Preparation of the composite
Formulation of mixes is shown in Table 3. NR was
masticated on the mill for 2 min followed by addition of
Table 1
Properties of sisal fiber
The fiber breakage analysis was carried out by dissolving 1 g of the uncured composite in toluene, followed
by the separation of fibers from the solution. The distribution of fiber length was determined using a travelling microscope.
Curing properties were measured in a Monsanto R-100
rheometer, at a temperature of 150 C.
Stress–strain measurements were carried out at a
crosshead speed of 500 mm per minute. Tensile strength
and tear strength was measured according to ASTM
methods D412-68 and D624-54, respectively. The tensile
tests were done using dumbbell samples cut at different
angles with respect to the orientation of fibers.
Green strength measurements were carried out at a
stretching rate of 500% min1 using dumb-bell shaped
samples obtained from unvulcanized composites.
Scanning electron microscopic studies were conducted
to analyse the fracture behaviour of the composites. The
fracture ends of the tensile and tear specimens were
mounted on aluminium stubs and gold coated to avoid
electrical charging during examination.
Equilibrium swelling measurements of the vulcanizates was performed in toluene at room temperature.
Table 2
Properties of oil palm fiber
Chemical constituents (%)
Cellulose
Hemicellulose
Lignin
Wax
Ash
78
10
8
2
1
Chemical constituents (%)
Cellulose
Hemicellulose
Lignin
Ash content
65
19
2
Physical properties of sisal fiber
Diameter (mm)
Density ( g/ cm3)
Tensile strength ( MPa)
Young’s modulus (GPa)
Microfibrillar angle
Elongation at break (%)
120–140
1.45
530–630
17–22
20–25
3–7
Physical properties of oil palm fiber
Diameter mm
Density g/cc
Tensile strength (MPa)
Young’s modulus (MPa)
Elongation at break%
Microfibrillar angle ( )
150–500
0.7–1.55
248
6700
14
46
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M. Jacob et al. / Composites Science and Technology 64 (2004) 955–965
P
Ni Li2
Lw ¼ P
Ni L i
Circular shaped samples having a diameter of 2 mm,
punched out of tensile sheets were used. The samples
were allowed to swell for 72 h in toluene.
Anisotropic swelling studies were carried out using
rectangular samples cut at different angles with respect
to orientation of the fiber from the tensile sheets and
swollen in toluene at room temperature for 3 days.
where Ln is the number average fiber length, Lw the
weight average fiber length and Ni is the number of
fibers having length Li.The value of Lw/Ln, the polydispersity index, can be taken as a measure of fiber
length distribution. The values of Ln, Lw and Lw/Ln are
calculated based on 50 fibers for the chopped sisal and
oil palm fibers extracted from the mix. The fiber length
distribution index of untreated sisal and oil palm fibers
before and after mixing is given in Table 4. The value of
Lw/Ln remains about the same before and after processing, indicating that there is not considerable fiber
breakage.
6. Results and discussion
6.1. Evaluation of fiber breakage
The extent of fiber breakage during mixing was estimated by immersing 1 g of the unvulcanized mix containing individual sisal and oil palm fibers, in toluene to
dissolve the rubber component and thus separate the
fibers intact. The fibers were washed with toluene to
ensure complete removal of rubber from the surfaces.
The washed fibers were then collected and examined
under a traveling microscope.
The control of fiber length and aspect ratio of fibers in
the rubber matrix is difficult because of fiber breakage
during processing. The severity of fiber breakage
depends mainly on the type of fiber, the initial aspect
ratio and the magnitude of stress and strain experienced
by the fibers during processing [16,17]. The breakage of
fibers due to high shear forces caused during mixing can
be indicated by fiber length distribution curve. The distribution of fiber lengths can be represented in terms of
moments of the distribution. The number and weight
average fiber lengths can be defined as:
P
Ni Li
Ln ¼ P
Ni
ð2Þ
6.2. Processing characteristics
The processability of the compounds can be studied
from the rheographs. The minimum torque in the rheograph gives an indication of the filler content in the
rubber while the maximum torque in the rheograph is a
measure of crosslink density and stiffness in the rubber.
In general, for all the mixes the torque initially decreases, then increases and finally levels off. The initial
decrease in torque to a minimum value is due to the
Table 4
Fiber length distribution index
Sisal
Ln
Lw
Lw/ Ln
ð1Þ
Oil palm
Before mixing
After mixing
Before mixing
After mixing
10.28
8.688
0.845
9.89
8.4
0.849
11
10.2
0.927
10.8
9.8
0.9074
Table 3
Formulation of mixes (G to M)
Ingredients
G
B
C
A
D
E
I
J
K
L
M
NR
ZnO
Stearic acid
Resorcinol
Hexaa
TDQ
CBSb
Sulphur
Sisal fiber
Treated
Fiber length, (mm)
Oil palm fiber
Treated (NaOH)
Fiber length (mm)
100
5
1.5
–
–
1
0.6
2.5
–
100
5
1.5
–
–
1
0.6
2.5
5
–
10
5
–
6
100
5
1.5
–
–
1
0.6
2.5
10
–
10
10
–
6
100
5
1.5
–
–
1
0.6
2.5
15
–
10
15
–
6
100
5
1.5
–
–
1
0.6
2.5
20
–
10
20
–
6
100
5
1.5
–
–
1
0.6
2.5
25
–
10
25
–
6
100
5
1.5
7.5
4.8
1
0.6
2.5
21
0.5% 1 h
10
9
0.5% 1 h
6
100
5
1.5
7.5
4.8
1
0.6
2.5
21
1% 1 h
10
9
1% 1 h
6
100
5
1.5
7.5
4.8
1
0.6
2.5
21
2% 1h
10
9
2% 1 h
6
100
5
1.5
7.5
4.8
1
0.6
2.5
21
4% 1h
10
9
4% 1 h
6
100
5
1.5
7.5
4.8
1
0.6
2.5
21
10% 1 h
10
9
10% 1 h
6
a
b
–
Hexamethylene tetramine.
N-Cyclohexylbenzothiazyl sulphenamide.
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M. Jacob et al. / Composites Science and Technology 64 (2004) 955–965
softening of the rubber matrix while the increase in
torque is due to the crosslinking of rubber. The levelling
off is an indication of the completion of curing. It is
found that generally the presence of fibers increases the
maximum torque.
6.2.1. Effect of fiber loading
Fig. 1 shows the rheographs of mixes G, B, C, A, D,
E. The presence of fibers generates an increase in viscosity of the mixes. The increment in torque values with
increasing fiber loadings indicates that as more and
more fiber gets into the rubber matrix, the mobility of
the macromolecular chains of the rubber reduces
resulting in more rigid vulcanizates. The cure time was
found to be independent of fiber loading but the gum
compound exhibited higher cure time. The reduction in
cure time of the filled vulcanizates was attributed to the
higher time the rubber compounds remain on the mill
during mixing. As the fiber loading increases, the time
of incorporation also increases and consequently generates more heat due to friction. Table 5 shows the
maximum torque, minimum torque and cure time of
various mixes.
6.2.2. Effect of bonding agent
Fig. 2 shows the rheograph of mixes G, A, L. It is
seen that the mix containing bonding agent and chemically treated fiber has a maximum torque value indicating greater crosslinking. In Table 6 it can be seen that
Mix L shows a longer cure time than mix A. This can be
attributed to the direct involvement of the bonding
agents in the vulcanization resulting in the formation of
good bonding between fibers and rubber. Earlier studies
have indicated that longer cure time is due to better
binding between fiber and matrix [18].
6.3. Mechanical properties
6.3.1. Effect of fiber loading
The effect of fiber loading on tensile strength in shortfiber reinforced natural rubber composites has been
widely studied [19–21]. Generally, the tensile strength
initially drops up to a certain amount of fiber and then
increases.
Fig. 1. Torque–time curve at different fiber loadings.
Fig. 2. Torque–time curve of composite containing bonding agent and
treated fibers.
Table 5
Cure characteristics of the various mixes
Mix
Tmax (dNm)
Tmin (dNm)
T
t90
Gum
B
C
A
D
E
58
64
65.5
66
68
69
6
6
6.8
7
6.5
7.1
52
58
58.7
59
61.5
61.9
14
7.3
7.2
7
7.5
7.5
Table 6
Cure characteristic of mixes
Mix
Tmax (dNm)
Tmin (dNm)
T
t90
Gum
A
L
58
66
81
6
7
7.4
52
59
73.6
14
7
8.5
M. Jacob et al. / Composites Science and Technology 64 (2004) 955–965
959
In the present study the behavior of composites containing 10,20,30,40 and 50 phr untreated sisal and oil
palm fibers were analyzed. Sisal and oil palm fibers were
taken in the ratio 50:50. From Fig. 3 it is clear that
tensile strength increases up to 30 phr and then declines.
The tensile strength in the transverse direction also
shows the same trend.
Natural rubber inherently possesses high strength due
to strain-induced crystallization. When fibers are incorporated into NR, the regular arrangement of rubber
molecules is disrupted and hence the ability for crystallization is lost. This is the reason why fiber reinforced
natural rubber composites possess lower tensile strength
than gum compounds.
When fiber reinforced rubber composites is subjected
to load, the fibers act as carriers of load and stress is
transferred from matrix along the fibers leading to
effective and uniform stress distribution which results in a
composite having good mechanical properties. The uniform distribution of stress is dependent on two factors:
(a) population of fibers, (b) orientation of fibers.
At low levels of fiber loading, the orientation of fibers
is poor and the fibers are not capable of transferring
load to one another and stress gets accumulated at certain points of composite leading to low tensile strength.
At high levels of fiber loading the increased population
of fibers leads to agglomeration and stress transfer gets
blocked. At intermediate levels of loading (30 phr) the
population of the fibers is just right for maximum
orientation and the fibers actively participate in stress
transfer.
The variation of composite tensile strength with the
angle of orientation at different levels of fiber loading is
given in Fig. 4. It is seen that the longitudinal orientation of fibers results in maximum tensile strength and as
the angle of orientation of fibers increases, tensile
strength decreases. When fibers are longitudinally
oriented the fibers are aligned in the direction of force
and the fibers transfer stress uniformly. When transversely oriented, the fibers are aligned perpendicular to the
direction of load and they cannot take part in stress
transfer.
Fig. 5 shows the variation of tear strength with fiber
loading. As loading increases the tear strength at first
decreases and at 30 phr shows an increase. At high fiber
loadings tear strength is found to decrease as the
increased strain in the matrix between closely packed
fibers increases tearing and reduces the tear strength.
Similar results have been observed by Ismail et al. [22].
Fig. 6 presents the variation of elongation at break
with fiber loading. The value of elongation at break
shows a reduction with increasing fiber loading.
Increased fiber loading in the rubber matrix resulted in
composites becoming stiffer and harder. This will reduce
the composite’s resilience and toughness and lead to
lower elongation at break.
Fig. 3. Variation of tensile strength with fiber loading. (L0—longitudinally oriented; P30—oriented at 30 ; R45—oriented at 45 ; S60—
oriented at 60 ; T90—transversely oriented).
Fig. 4. Variation of tensile strength with angle of orientation at
different levels of fiber loading.
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M. Jacob et al. / Composites Science and Technology 64 (2004) 955–965
The variation of modulus at 100% with fiber loading
is given in Fig. 7. The values show a steady increase
with fiber loading. This is because at high fiber loadings the composites will be able to withstand more
loads.
Fig. 5. Variation of tear strength with fiber loading.
Fig. 6. Variation of elongation at break with fiber loading.
6.3.2. Effect of fiber ratio
Composites were prepared varying the ratio of fibers
while keeping the total fiber loading constant (30 phr).
Fig. 8 shows the variation of tensile strength with wt. of
sisal in total fiber content at different orientations. It is
clear that the composite containing 21 g sisal and 9 g oil
palm has maximum tensile strength. This suggests that
Fig. 7. Variation of modulus at 100% with fiber loading.
Fig. 8. Variation of tensile strength with fiber ratio.
M. Jacob et al. / Composites Science and Technology 64 (2004) 955–965
this is the optimum ratio. It is also seen that as the
concentration of sisal fiber increases the tensile strength
also increases. This suggests that the tensile strength of
the composite is dependent more on the weight of sisal
rather than oil palm, which could be due to the high
strength of sisal fiber.
6.4. Extent of fiber orientation from green strength
measurements
The green strength of short fiber reinforced composites depends upon the degree of fiber orientation and so
the latter can be obtained from the expression:
%
Fiber
SL =SG;L
orientation ¼
SL =SG;L þ ST =SG;T
ð3Þ
where S=green strength and subscripts G, L and T
represents gum, longitudinal and transverse respectively. The effect of fiber loading on percentage orientation is shown in Fig. 9. The percentage orientation is the
lowest when the fiber loading is small i.e. 10 phr. At low
fiber loading the fibers can randomly move around
leading to increased chaoticity and decreased levels of
orientation. As the fiber loading increases, percentage
orientation increases with the maximum value for composite containing 30 phr fiber. At 40 phr fiber loading,
the percentage orientation decreases indicating that the
fibers cannot orient themselves due to entanglement
caused by the increased population of fibers.
Fig. 9. Effect of fiber loading with on percentage orientation.
961
6.5. Equilibrium swelling studies
Equilibrium swelling measurements of the vulcanisates was performed in toluene at room temperature.
Circular shaped samples, having a diameter of 1.9 mm,
punched out of tensile sheets were used. The samples
were allowed to swell for 72 h in toluene.
6.5.1. Rubber–fiber interactions
The extent of interaction between rubber and fiber
can be analysed using Kraus [23,24] and Cunneen and
Russell and Lorentz Park [25] equation. The Kraus
equation is
Vro
f
¼1m
1f
Vrf
ð4Þ
where Vro=volume fraction of rubber gum vulcanizates, f=volume fraction of fiber, m=polymer - fiber
interaction parameter, f=volume of filler / total volume
of recipe, Vrf is the volume fraction of elastomer in the
solvent swollen filled sample and is given by the equation by Ellis and Welding [26].
Vrf ¼
ðd fwÞp1
p
ðd fwÞpp1 þ As p1
s
ð5Þ
where d=deswollen wt. of sample, f=volume fraction
of fiber, w=initial weight of sample, pp=density of
polymer, ps=density of solvent, As=amount of solvent
absorbed by sample, Vro=volume fraction of elastomer
in solvent swollen unfilled sample.
Since Eq. (7) is in the form of a straight line, a plot of
Vro/Vrf versus f/ 1–f should give a straight line, whose
slope (m) will be a direct measure of reinforcement of
the fiber. The ratio Vro/Vrf represents the degree of
restriction of the swelling of the rubber matrix due to
the presence of fiber.
According to the theory by Kraus, reinforcing fillers
will have a negative higher slope.
The Kraus plot and Vro/Vrf values are given in Fig. 10
and Table 7 respectively. In the present case it is
observed that as the fiber loading increases, the solvent
uptake of the sample decreases. This causes an increase
in Vrf values, which will decrease the ratio of Vro/Vrf
since Vro is a constant. It can be seen from the graph
that Vro/Vrf decreases with fiber loading. This behavior
leads to a negative slope indicating the reinforcement
effect of the fibers.
From Table 7, mix L has the lowest Vro/Vrf value.
This can be associated with enhanced fiber- rubber
adhesion in the composite due to the presence of bonding agent and alkali treated fiber. Since there is better
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M. Jacob et al. / Composites Science and Technology 64 (2004) 955–965
bonding between fiber and matrix, there is a strong
interface, which restricts the entry of solvent.
The Cunneen–Russell equation is
Vro
¼ aez þ b
Vrf
ð6Þ
where Vro and Vrf are the same, z is the weight fraction
of fiber, a and b are constants. Here a plot of Vro/Vrf
versus ez should give a straight line with a positive
slope as shown in Fig. 11. This shows that as fiber
loading increases the extent of reinforcement also
increases.
The Lorentz Park equation is
Qf
¼ aez þ b
Qg
ð7Þ
where Q is defined as the amount of solvent absorbed by
1 g of rubber and is given by the equation
.
Q¼
Swollen wt:-dried wt:
Original wt: x 100=formula
wt:
ð8Þ
The subscripts f and g refer to filled and gum vulcanisates respectively. Table 8 shows the Qf/Qg values at
variable fiber loading. The higher the Qf/Qg values, the
lower will be the extent of interaction between fiber and
matrix. It can be seen from Table 8 that as fiber loading
increases Qf/Qg decreases which indicates greater extent
of interaction between rubber matrix and fibers. The
ratio Qf/Qg has lowest value for mix L. This confirms
that maximum interaction between fiber and matrix
occurs in presence of bonding agent and alkali treated
fiber. A plot of Qf/Qg (Fig. 12) versus ez gives a
straight line with positive slope which confirms the
above findings.
6.5.2. Swelling index and crosslink density determination
The swelling index which is a measure of the swelling
resistance of the rubber compound, is calculated using
the equation:
Swelling
index
%¼
As
W1 100
ð9Þ
Table 8
Mixes
Qf/Qg
B
C
A
D
E
L
1.191
1.1128
1.054
1.00516
0.9289
0.4295
Fig. 10. Variation of Vro/Vrf with f / 1–f (Kraus plot).
Table 7
Mixes
Vro/Vrf
B
C
A
D
E
L
1.147
1.0915
1.0079
1.056
0.8581
0.4396
Fig. 11. Variation of Vro/Vrf with ez (Cunneen–Russel plot).
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M. Jacob et al. / Composites Science and Technology 64 (2004) 955–965
where As=amount of solvent absorbed by sample
W1=initial weight of sample before swelling.
The diffusion mechanism in rubbers is essentially
connected with the ability of the polymer to provide
pathways for the solvent to progress in the form of
randomly generated voids. As the void formation
decreases with fiber addition, the solvent uptake also
decreases. Table 9 shows the swelling index values of
composites at different fiber loadings and in presence of
bonding agent. Mix L shows the minimum swelling
index value indicating that the presence of bonding
agent binds the alkali treated fiber and rubber so that
swelling is highly restricted in the composite.
The extent of crosslinking can be established by
determining the Vrf of the composites. Table 10 shows
the Vrf values. The higher Vrf value for mix L indicates
better adhesion and higher degree of crosslinking. A
comparison of crosslink density has also been made from
the reciprocal swelling values 1/Q where Q is defined as
the amount of solvent absorbed by 1 g of rubber.
Table 11 shows the apparent crosslinking values in
various mixes. Mix L shows the maximum crosslink
density indicating maximum interaction between fiber
and matrix. The reinforcing effect of the fibers can be
supported by the cross-linked density values. It can be
seen in Fig. 13 that as the fiber loading increases the
cross-link density values are also found to increase. This
effect can be explained using the basic equations used
for swelling:
1=3
2
Mc ¼ r Vs Vrf =ln 1 Vrf þ Vrf þ Vrf
ð10Þ
Table 11
Apparent crosslink density values
Mixes
1/Q
Gum
B
C
A
D
E
L
0.1399
0.1432
0.1473
0.15015
0.1552
0.1766
0.388
Fig. 12. Variation of Qf/Qg with ez (Lorentz–Park plot).
Table 9
Samples
Swelling index
G
B
C
A
D
E
L
540.6
474.8
470.6
469.4
422.9
349.6
166
Table 10
Mixes
Vrf
B
C
A
D
E
L
0.1098
0.1102
0.1158
0.1196
0.1254
0.1473
Fig. 13. Variation of crosslink density values with fiber loading.
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M. Jacob et al. / Composites Science and Technology 64 (2004) 955–965
where Mc=molecular weight of polymer between
crosslinks, pr=density of polymer, Vs=molar volume
of solvent =interaction parameter is given by
Hildebrand equation
mum for composite at lower fiber loading and therefore
it absorbs a maximum quantity of solvent indicating
poor reinforcement.
6.6. Anisotropic swelling studies
2
Vs s p
¼þ
RT
ð11Þ
where =lattice constant, Vs=molar volume, R=universal gas constant, T=absolute temperature,
s=solubility parameter of solvent, p=solubility parameter of polymer, Vrf=volume fraction of elastomer in
the solvent swollen filled sample.
As loading increases the amount of solvent absorbed
by sample decreases which leads to increase in Vrf values
and this in turn increases cross-link density values as
shown in Fig. 13. The crosslink density value is mini-
For short fiber reinforced rubber composites, the
swelling ratio in any direction forming an angle y with
the fiber orientation is given by the expression:
ð12Þ
a2 ¼ a2T a2L sin2 þ a2L
where aL and aT are the dimensional swelling ratios in
the longitudinal and transverse directions, respectively.
Fig. 14 shows dimensional swelling variation with angle
in accordance with the above equation where various
values of were assumed. For all the mixes, the swelling
increases with and is found to be maximum when becomes 90 C. The line corresponding to gum compound
Table 12
Slope values from anisotropic swelling studies
Mixes
Slope values
Gum
B
C
A
D
E
0.1427
0.153
0.153
0.1983
0.1673
0.1522
Fig. 15. Effect of tensile strength on alkali concentration.
Fig. 14. Swelling variation as a function of the angle of measurement, .
Fig. 16. (a) SEM tensile fracture surface of sisal /oil palm fiber filled
natural rubber composites at 30 phr loading at magnification100. (b)
SEM tensile fracture surface of sisal /oil palm fiber filled natural rubber composites at 30 phr loading in presence of bonding agents and
alkali treated fibers at magnification100.
M. Jacob et al. / Composites Science and Technology 64 (2004) 955–965
which does not contain fibers is positioned above those
for the other mixes. This shows that short fibers restrict
the transport of solvent into the composite.
The extent of fiber alignment can be understood from
the slope values given in Table 12. It has been reported
[27] that the steeper the line the higher the degree of
fiber alignment. Mix A shows the maximum slope value
among the various mixes. It shows that the composite
containing 30 phr fiber has the highest degree of fiber
alignment.
6.7. Effect of alkali treatment
Sisal and oil palm fibers (of lengths 10 and 6 mm)
were subjected to alkali treatment. The fibers were treated with different concentrations of NaOH (0.5, 1, 2, 4
and 10%) for 1 h at room temperature. After treatment
the diameter and weight of the fibers were found to
decrease due to the removal of lignin.
Composites were prepared with sisal and oil palm
fibers (of lengths 10 and 6 mm) that were treated with
varying concentrations of NaOH solution for 1 h. The
variation of tensile strength with chemical treatment is
given in Fig. 15. It is seen that alkali treated composites
show an increase in tensile strength as compared to
untreated composites. Maximum tensile strength was
shown by composite containing fibers treated with 4%
NaOH. It has been reported that alkali treatment
increases the roughness of fiber surface, hence increasing
the surface area available for contact with the matrix [28].
The better adhesion between sisal and oil palm fibers
is evident in SEM micrographs. Fig. 16 a shows the
tensile fracture surface of sisal and oil palm filled natural rubber composites. It can be seen that there are a
few holes after the fibers are pulled out from the rubber
matrix indicating poor wetting between the fibers and
matrix. However for similar composites but with the
presence of bonding agents and treated fibers (Fig. 16b)
better adhesion between fiber and rubber matrix can be
seen. The fibers are well wetted by the rubber matrix
and there is fiber breakage due to strong adhesion.
the longitudinal direction are superior to those in the
transverse direction. Addition of sisal and oil palm
fibers led to increase of tensile strength and tear strength
but increased modulus. The extent of adhesion between
fiber and rubber matrix was found to increase on alkali
treatment of fibers. From the mechanical properties the
alkali treated fibers exhibited better tensile properties
than untreated composites. Processing characteristics
were found to be independent of fiber loading and
modification of fiber surface. Swelling studies revealed
that composites containing bonding agents and alkali
treated fibers showed higher crosslink density and betteradhesion. Anisotropic swelling studies indicated that
the presence of short fibers restricted the entry of solvent.
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