Post on 13-Mar-2018
SHORT BANANA FIBRE REINFORCED POLYESTER COMPOSITES
Abstract
Part 2 - Chapter 1
MECHANICAL AND DYNAMIC
MECHANICAL ANALYSIS OF
SHORT BANANA FIBRE REINFORCED
POLYESTER COMPOSITES
The results of this chapter have beel communicated for publication in Composites Science and Technology.
The mechanical and dynamic mechanical
properties of banana fibre reinforced
polyester composites were studied with
special reference to the effect of fibre
loading, frequency and temperature. The
intrinsic properties of the components,
morphology of the system and the nature
of the interface between the phases
determine the dynamic mechanical
properties of the composite. At lower
temperatures, (in the glassy region) the
E' values are maximum for the neat
polyester whereas at temperatures
above T,, the E' values are found to be
maximum for composites with 40% fibre
loading, indicating that the incorporation of
banana fibre in polyester matrix induces
reinforcing effects appreciably at high
temperatures. The loss modulus and
damping peaks were fwnd to be lowered
by the incorporation of fibre. The height of
the damping peaks depends on the fibre
content. The glass transition temperature
associated with the damping peak was
lowered upto a fibre content of 30%. The
T, values were increased with higher
fibre content. A new micrornechanical
transition was observed at higher loading
which has been explained based on the
interlayer model. Cole-Cole analysis was
made to understand the phase behaviour of
the composite samples. Apparent activation
energy of the relaxation process of the
composites was also analysed. The value
MS found to be maximum for composites
with 40% fibre content.
- - -. Mechanical and Dynamic 140
2.1.1. Introduction
Dynamic mechanical analysis has been widely employed for
c----
investigating the viscoelastic behaviour of polymeric Aterials in terms of their
relevant stiffness (modulus) and damping characteristics for various applications.
The dynamic properties of polymeric materials are of considerable practical
significance when determined over a range of temperature and frequencies.
\ The details relating to terminology and measurement methodology have
been elaborately discussed [l-31. Composite damping property results from the
inherent damping of the constituents. This can be represented as [ l ]
where tan6,, tar16~ and tan&, are the damping values of the composite, the fibre
and the polymer respectively and Vf and V, are the volume fraction of the filler
and the matrix respectively. In the transition region, the molecular chains begin
to move and every time a frozen in segment begins to move its excess energy is
dissipated as heat. A segment, which is completely frozen-in or is not free to
move, gives rise to a low damping value. The rule of mixtures which is used in
predicting composite mechanical properties can be used in analysing the
composite damping property as well. According to Nielsen, [2] the damping of
a composite. tan 6, is given by the proportional contribution of the matrix
according to its relative content.
tan6 - tan6,j, (1 - V,)
~~ - ~ p ~ ~ -
Mechanical and Dynamic 14 1
where tanS, is the damping value of the matrix and Vt is the volume fraction of
the filler. Around Tg, the filler particles are not deformed and damping is purely
due to the polymer. Nevertheless, a specific interaction between the filler and
the polymer layer will create a layer of immobilised polymer in between the
filler and the polymer. In such cases equation 2.1.2 can be rewritten as
tan 6, = tan S,, (2.1.3)
In general, studies on the dynamic mechanical properties of fibrous
composite materials are carried out with the following main objectives.
(a) To study the chemical or physical modification of the matrix by the
introduction of fibre
(b) To study the behaviour as a function of the properties of each phase.
Gassan and Bledzki [4] carried out the dynamic mechanical analysis of
jutelepoxy composites and observed an improvement in the dynamic modulus
with the incorporation of treated jute fibre in epoxy. Finegan and Gibson [5] in
their investigation on the enhancement of damping in polymer composites have
suggested analysing different fibre matrix combinations. Saha et al. [6] have
made comparative studies on the damping of unmodified and chemically
modified jute-polyester composite samples. Valea et al. [7] have investigated
the influence of' cure conditions as well as the influence of an exposure to
various chemicals on the dynamic mechanical properties of several vinyl ester
and unsaturated polyester resins containing glass fibre. Exposure of these
materials to aromatic solvents was found to modify their viscoelastic character.
Amash and Lugenmaier [S] reported the effectiveness of cellulose fibre in
.- Mechanical and Dynamic 142
improving the stiffness and reducing the damping in polypropylene-cellulose
composites. The effect of adsorbed water on the storage modulus and loss
tangent of wood was analysed by Obataya et al. [9]. The adsorption of water in
amounts upto 8 % moisture content was found to increase the modulus value
followed by a decrease in value thereafter.
In the present chapter the influence of banana fibre on the viscoelastic
properties of polyester is discussed. The effect of fibre content, frequency and
temperature on the viscoelastic properties also is studied together with the effect
of fibre content on the T, values. The elevation of T, is taken as a measure of
the interfacial interaction. In this report the effect of fibre content on Tg has been
discussed. The T, is usually interpreted as the peak of the tan 6 or the loss
modulus curve that is obtained during a dynamic mechanical test conducted at a
low frequency. The T, values of the different samples and the shift in Tg were
determined from the loss modulus and the tan 6 curves to gain a better insight
into the effectiveness of load transfer between fibre and matrix. The effect of
interlayer on the micro mechanical transitions has also been reported.
2.1.2. Results and Discussion
2.1.2a. Mechanical properties
In our earlier publication, we have reported on the effect of the addition of
banana fibre as reinforcement in polyester matrix. &.A. Pothan, M.Phil. Thesis,
M.G. University, 1995) [lo]. Composites were prepared using banana fibre with
varying fibre length and fibre content, in polyester matrix. A fibre length of 30 mm and
- -- - - - - - Mechanical and Dynamic 143
a fibre content of 40% were found to be optimum for obtaining composites,
with overall better properties. Impact strength of the composites improved by
341% by the incorporation of 40% of fibre. The maximum tensile strength was
obtained for composites with a 40% loading. The specific tensile strength of the
composites with 40% fibre loading was found to be 57.52 whereas for the neat
resin, the value was 29.9.
2.1.2b. Dynamic mechanical properties
Figure 2.1.1 shows the effect of temperature on the dynamic modulus of the
neat polyester and banana fibre filled composite samples with varying fibre content
(in volume percent). Variation in modulus occurs due to the effect of the incorporated
fibres.
Figure 2.1.1 Effect of temperature on the dynamic modulus of the neat polyester and composite samples with varying fibre volume percent at O.1Hz.
In the case of neat isophthalic polyester cured with MEK peroxide and
cobalt naphthanate, it is seen that the dynamic modulus is higher than that of the
fibre filled system in the glassy region. However, the dynamic modulus curve
--- Mechanrcal and Dynamic 144
shows an ~ncrease in the E' value above the T, region in the rubbery plateau.
The increase of E' in the rubbery plateau is maximum for the composites with
40% fibre loading. The effectiveness of fillers on the moduli of the composites
can be represented by a coefficient C such as [ l I].
where E', and E', are the storage modulus values in the glassy and rubbery
regions respectively. The higher the value of the constant C, the lower the
effectiveness of the filler. The measured E' values at 45 and 130°C (for polyester)
were chosen as E', and E', respectively. The values obtained for the different
samples at frequency 0.1 Hz are given in Table 2.1.1.
Table 2.1.1 The values of constant C for different volume percent of fibre -
Sample (fibre volume) C --
10% 0.97
In this case the lowest value has been obtained for 40% fibre loading
and the highest value for 10% fibre loading. The effectiveness of the filler is the
highest at 40% fibre loading. It is important to mention that modulus in the
glassy state is determined primarily by the strength of the intermolecular forces
and the way the polymer chains are packed.
~ ~
Mechanical and Dynamic 145
There is a large fall in modulus with increasing temperature in the
unfilled system, the stiffness at high temperature being determined by the
amorphous regions, which are very compliant above the relaxation transition.
The drop in the modulus on passing through the glass transition temperature is
comparatively less for reinforced composites than for the neat resin. In other
words, banana fibres have a larger effect on the modulus above Tg, than below it.
However the difference between the moduli of the glassy state and rubbery state
is smaller in the composites than in the neat polyester. This can be attributed to
the combination of the hydrodynamic effects of the fibres embedded in a
viscoelastic medium and to the mechanical restraint introduced by the filler at the
high concentxations, which reduce the mobility and deformability of the matrix.
Other authors also have reported similar observations [12,13].
The tan 6 is a damping term that can be related to the impact resistance
of a material. Since the damping peak occurs in the region of the glass transition
where the material changes from a rigid to a more elastic state, it is associated
with the movement of small groups and chains of molecules within the polymer
structure, all of which are initially frozen in. In a composite system, damping is
effected through the incorporation of fibres. This is mainly due to shear stress
concentrations at the fibre ends in association with the additional viscoelastic
energy dissipation in the matrix material. Another reason could be the elastic
nature of the fibre. Figure 2.1.2 delineates the effect of temperature on tan delta.
Mecl~anical and Dvnrrrnic 1 46
Figure 2.1,2 Effect of temperature on the tan 8 curves of composites with different fibre loading (volume percent) at 0.1 Hz
Improvement in interfacial bonding in composites is suggested by the
lowering in tan delta values. The higher the damping at the interfaces, the poorer
the interface adhesion. SEM photographs of banana polyester composite with fibre
volume percent of l0,20 and 40% are shown in Figure 2.1.3a, b and c respectively.
Figure 2.1.3a,b,c SEM of the tensile failure surface banana /polyester composite with fibre volume percent 10,20 8 40
~ ~~
Mechanical and Dynamic 147
It is interesting to note that composites with 40% fibre loading (volume
percent) show better properties. Whereas fibrelmatrix de-bonding is evident in
composites with 10 and 20% fibre loading, composites with 40% fibre loading
show no gap between the fibre and the matrix even after failure. This is because
of the effectiveness of the load transfer. When the fibre concentration is lower,
the packing of the fibres behaves inefficiently in the composite. This leads to
matrix rich regions and consequently, an easier failure of the bonding at the
interfacial region. When there is closer packing of the fibres, crack propagation will
be prevented by the neighbouring fibres. The load transfer at the fibrelmatrix
interface is found to be most effective in the case of composites with the highest
loading in this particular study i.e. 40% (volume percent). The variation of tan
6 with temperature of the composites has been analysed with respect to fibre
loading and frequency. Incorporation of fibres reduces the tan delta peak height
by restricting the movement of the polymer molecules. Magnitude of the tan 6
peak is indicative of the nature of the polymer system. In an unfilled system,
the chain segments are free from restraints. Addition of fibres above 30%
(volume percent) show two peaks making the two phases, fibre and matrix
distinct. Addition of fibre decreases the T, value at low fibre loading, showing
that the addition of fibre below 30% (weight percent) has only a plasticising
effect. However, at 40% (volume percent) fibre loading, the T, values show a
positive shift, stressing the effectiveness of the fibre as a reinforcing agent i.e.
the effective stress transfer between the fibre and the matrix. The result is
consistent with the E' values obtained. The shifting of T, to higher temperatures
can be associated with the decreased mobility of the chains by the addition of fibres.
-. -- Mechanrcal and Dynamic 148
Elevation of 1, is taken as a measure of the interfacial interaction. In addition, the
stress field surrounding the particles induces the shift in T,. Chua [14] concluded
based on his studies that a composite with poor interface bonding tends to
dissipate more energy than that with good interface bonding. At high fibre content,
when strain is applied to the composite, the strain is taken mainly by the fibre in
such a way that the interface, which is assumed to be the more dissipative
component of the composite, is strained to a lesser degree [15]. The width of the
tan delta peak also becomes broader than that of the matrix. The behaviour
suggests that there are molecular relaxations in the composite that are not present in
the pure matrix. The molecular motions at the interfacial region generally contribute
to the damping of the material apart fiom those of the constituents [16]. Hence the
width of the tan delta peak is indicative of the increased volume of the interface.
Table 2.1.2 shows the peak width at half height of the samples from the
damping curve
The peak width is found to be maximum for composites with 40% fibre
loading. Increase in the concentration of the filler increases the interface
[17,18]. Stress induced motions may also occur in the composite.
Table 2.1.2 Peak height and peak width at half height of the Tan 6 curves
Sample Peak Height Peak Width (Volume percent) (cm) (cm)
Gum 10 6.4 10% 5.9 6.5 20% 6.3 6.5 30% 5.3 6.4 4n0/" 4 9
~ ~
Mechanical and Dynamic 149
The loss modulus E is a measure of the energy dissipated or lost and is
related to the sinusoidal deformation, when different samples are compared at the
same strain amplitude. It is in fact the viscous response of the material. Figure 2.1.4
shows the variation of loss modulus with temperature of composites with different
fibre loading.
Figure 2.1.4 Variation of loss modulus with temperature for composites with different fibre loading (0.1Hz.)
It can be seen from the figure that the loss modulus peak values decrease
with increase of fibre content at temperatures below the glass transition. The
effect of the filler is prominent above the glass transition temperature in this case
also. The modulus values increase with fibre content above the glass transition
temperature. Another interesting result that is observed is the broadening of the loss
modulus curve when the fibre content is increased to 40% (volume percent).
Figure 2.1.5 shows the plot of peak height vs. fibre volume fraction.
-- Mechanical and Dynamic 150
Figure 2.1.5 Plot of peak height vs. fibre volume fraction
I he peak height has been measured by finding the height of the E peak
from the centre. The peak height shows a regular decrease with increase of fibre
content and corresponding reduction in volume fraction. At a fihre loading of
40% (volume percent), the most pronounced effect of the filler has been the
broadening of the transition region as the fibre concentration increases. The
obserked broadening may be explained as due to the difference in the physical
state of the matrix surrounding the fibres compared to the rest of the matrix, and
immobilised polymer layer matrix [19]. As reported by other authors, a shell of
immobiliscd polymer surrounds the fibres [20]. Figure 2.1.6 shows a schematic
diagram ot'fibre, matrix and the immobilised polymer layer.
--- ~~ ~
Mechanical and Dynamic 15 1
lmmobiliied polymer layer
r . . . . . ~ . . ~ . - ~ , ~ . ~ . . , ~ . . Matrix
Figure 2.1.6 Schematic diagram of fibre, matrix and the immobilised polymer layer
When the volume fraction of the matrix is higher, there are more
restraints at the interface. The different physical state of the matrix surrounding
the fibre hinders the molecular motion. This can be taken as the inter layer
which causes the additional transition [21]. The increase in width of the loss
modulus curve is taken to represent the presence of an increased range of order.
The greater constraints on the amorphous phase could give rise to a higher or
broader glass transition behaviour. The peak width at half height of the E curves
of composites with different fibre volume fraction is given in Table 2.1.3.
Table 2.1.3 Peak height and peak width at half height of the E curves
Sample (volume percent) Peak Height (cm) Peak Width (cm)
Gum 7 8.5
The maximum peak width is found to occur for the composites with
maximum tibre content (40 Volume percent).
-. - -- - - Mechanical and Dynamic 152
The storage modulus, loss modulus and damping peaks have been found
to be affected by frequency. The variation of E' with frequency for neat
polyester, plotted as a function of temperature is shown in Figure 2.1.7.
Figure 2.1.7 Variation of E' with frequency of neat polyester as a function of temperature
Increase of frequency has been found to increase the modulus values.
Figure 2.1.8 shows the effect of frequency on the dynamic modulus of samples
with 40% fibre loading.
Frequency has a considerable impact on the dynamic modulus especially
at high temperatures. The modulus values are found to drop at a temperature of
around 60°C. The drop in modulus value continues steadily till a temperature of
120°C is reached. The molecular motion can be believed to be set in at 60°C.
-- Mechanrcal and Dynamrc 153
8 0 20 40 60 80 100 120 ?40 160
Temperature ('C)
Figure 2.1.8 Effect of frequency on the dynamic modulus of samples with 40% fibre loading (volume percent)
The glass transition temperature of the neat polyester is around 130°C,
(based on tan 6 at 10Hz.) and the modulus value remains unchanged after that.
The lowering of the modulus peak is maximum for the neat polyester due to the
development of microscopic cracks in the unfilled resins. Frequency is seen to
have a direct impact on the tan 6 values as well. The viscoelastic properties of a
material are dependent on temperature, time and frequency [IS]. If a material is
subjected to a constant stress, its elastic modulus will decrease over a period of
time. This is due to the fact that the material undergoes molecular rearrangement
in an attempt to minimise the localised stresses. Modulus measurements
performed over a short time (high frequency) result thus in higher values whereas
measurements taken over long times (low frequency) result in lower values. In this
system also, the modulus measurements over a range of frequencies have been
studied. Higher values were observed for measurements made over a short time.
The tan delta values measured over a range of frequencies for the neat polyester
samples are shown in Figure 2.1. 9 ,
Mechanical and Dynamic 154
Figure 2.1.9 tan 6 values of neat polyester samples at different frequencies
The tan delta peak, which is indicative of the glass transition temperature,
is found to shift to a higher temperature with an increase in frequency. The
damping peak is associated with large co-operative motions that are taking
place at the molecular level. Figure 2.1.10 shows the effect of frequency on the
tan 6 curve of composite samples with 40% loading
Figure 2.1.10 Effect of frequency on tan 6 curves of samples with 40% fibre loading
~
Mechanical and Dynamic 155
The nature of the tan 6 curve is affected by the incorporation of fibre.
Eklind and Maurer [22,23] have suggested an interlayer model to simulate the
dynamic mechanical properties of filled blends. The filler particles have been
reported to be surrounded by an interlayer attached to the filler surface. This
phenomenon could give rise to filler structure in the matrix able to alter the
dynamic mechanical modulus. The bound polymer results in a layer with
properties that are different from the bulk properties of the pure polymer. This
micro mechanical transition, caused by the interlayer resulted in a new tan 6 peak
occurring at a temperature lower than the T, of the pure matrix. The tan 6 curve in
this system is found to have two peaks at 40% fibre loading. This is believed to be
due to the micromechanical transition arising fiom the imrnobilised polymer layer,
which acts as the interlayer. The effect of the interlayer becomes prominent only at
high fibre loading and lower frequency. An increase in fkquency is found to have a
broadening effect on the tan delta curve. This broadening is more prominent in
composites with high fibre content (Figure 2.1.10). The addition of fibre increases
the free volume between monomeric units. The introduction of fibres, which in turn
affects the curing reaction, affects molecular motions and diffusion, as well.
Figure 2.1.1 1 shows the effect of frequency on the loss modulus values
of neat polyester samples.
- - -- Mechanrcal and Dynamic 156
Figure 2.1.11 Effect of frequency on the loss modulus values of neat polyester samples
The peak of the loss modulus curve is seen to have been shifted to a
higher temperature with increasing frequency. Figure 2.1.12 shows the effect of
frequency on the loss modulus curve of the samples with 40% fibre loading.
.. Temperature CC)
Figure 2.1.12 Effect of frequency on the loss modulus curve of samples with 40% fibre loading
The E" peak of the composite is broader than that of the neat polymer
revealing the morphological rearrangement resulting in a highly plasticised
amorphous region and also the improved interaction between the fibre and
matrix. This adequately supports the micro mechanical transition observed in
-- - Mechanical and Dynamic 157
the tan F peaks. Eklind and Maurer [23] have reported on glass transition peak
on the low temperature side for both loss modulus and tan 6 curves. It is interesting
to note that the micro mechanical transition becomes prominent only for relatively
high fibre content indicating the critical fibre volume hction and the related
interlayer responsible for the micro mechanical transition. Table 2.1.4 shows T,
values for different composite samples based on the tan6 max and E max values.
Table 2.1.4 Values of tan 6 maximum, E maximum and T, values of neat polyester and banana fibre reinforced polyester composites at different fibre loading and frequencies
The values of T, obtained from the loss modulus peaks are found to be lower
than those obtained from the damping peaks. Addition of fibre to the polyester matrix
tan 6 , ~ Fibre
loading Frequency (Hz) 1 10
Gum 0.421 0.423. 0.452 10% 0.248 0.241 0.253 20% 0.262 0.257 0.277 30% 0.228 0.207 0.233 40% --. 0.220 0.214 0.224
lo-7(pa) Gum --+7.982 7.979 8.005
has a plasticising efyect, which accounts for the decrease in Tg at low fibre loading.
T, from tan 6,, ("C) Frequency (Hz)
10% 20%
The loss modulus @ has a broadening nature when fibres are incorporated. With
0.1 104 99 102 99
.
118
T, 95 95 97 95 116
increase in frequency, the peak of the loss modulus curve, which corresponds to the
7.822 7.822
glass transition temperature, is also found to be shifted to higher temperature.
1 114 106 112 106 125
from E"('c) 105 102 102 103 122
30% / 7751 40% L 7 2 5 3 -.
10 125 119 112 120 133
108 103 108 101 124
7.802 7.768
7.825 7.762
7.717 7.683
7.713 7.729
Since the response of the sample changes with both temperature and
frequency of oscillation, it was decided to make a three dimensional thenno gram
for composites with different fibre loading. Figure 2.1.13 shows the three-
dimensional thermo gram for composites with 40% (volume percent) fibre. The
different peaks are clearly visible in the thermogram.
Figure 2.1.13 Three-dimensional thermo gram of composites with 40% fibre (volume)
Cole Cole Plots
Figure 2.1.14 Cole-cole plots
- - Mechanical and Dynamic 159
The nature of the cole-cole plot is reported to be indicative of the
homogeneity of the system. Homogeneous polymeric systems are reported to show
a semi circle diagam [I 81. The cole-cole diagrams presented in Figure 2.1.14 are
imperfect semi circles. However. the shape of the curve points towards the
relatively good fibrelmatrix adhesion.
Any increase in vibrational frequency causes the glass transition temperature
to rise and the amplitude of the damping peak to increase. The shift of the transition
temperature allows one to calculate the apparent activation energy of the relaxation
process for each of the samples assuming a linear equation of the type
log f = logf, - E
2.303RT
wherefo is an experimental constant, f and Tare the measuring frequency and
the temperature for the dispersion peak respectively, and R is a gas constant,
and E the activation energy.
Activation energy of the different composite samples was calculated from the
Arrhenius relationships 1241. The activation energy values are given in Table 2.1.5.
Table 2.1.5 Activation energy values of neat polyester and banana polyester composites under different loading
Fibre loading Activation Energy (kJImol) --
Gum 25.7
10% 21.0
20% 23.8
30% 22.7
- - Mechanrcal and Dynamrc 160
The actlvatlon energy values of the composites with 40% fibre loading
are the maximum. The activation energy values for neat polyester samples are
25.7 Wmol. At low fibre loading, as is evident from the SEM of the samples, the
stress transfer is low and the activation energy is also low. However, incorporation
of the critical fibre volume fraction brings about high interfacial interaction and
effective stress transfer. This increases the activation energy value.
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-- Mechanical and Dynamic 16 1
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Part 2 - Chapter 2
EFFECT OF FIBRE SURFACE
TREATMENTS ON THE MECHANICAL PROPERTIES OF SHORT BANANA
FIBRE REINFORCED POLYESTER
COMPOSITES
The results of this chapter have been accepted for publication in Composite Interface
Abstract
:ellulosic fibres have been used as cost-cutting
illers in plastic industry. Among the various
actors, the final performance of the composite
naterials depends to a large extent on the
adhesion between the polymer matrix and the
.einforcement and therefore on the quality of the
nterface. To achieve optimum performance of the
?nd product, sufficient interacfion between the
natrlx resin and the cellulosic material is desired.
rhis is often achieved by surface modification ol
he resin or the filler. Banana fibre, the cellulosic
ibres obtained from the pseudo-stem of banana
$ant (Musa sepientum) is a bast fibre with
.elatively g o d mechanical properties. The fibre
surface was modified chemically to bring aboui
mproved interfacial interaction between the fibre
and the polyester matrix. Various silanes and
alkali were used to modify the fibre surface.
Modified surfaces were characterised by SEM and
FTIR. Chemical modification was found to have a
profound effect on the fibrelmatrix interactions.
The improved Wmab ix intemtkm is e a t from
the enhanced tensile and Rexural properfies. The
bwer impact properties of the treated composites
mpared to the untreated composites furlher point
to the improved fibrelmatrix adhesion. Of the
various chemical treatments, simple alkali
treatment with NaOH of I-% mcentration was
Effect of Fibre Surface Treatments on. ... 163
2.2.1. Introduction
Many major issues have been identified in the processing of natural
fibre composites. One major issue is the lack of perfect bonding between the fibre
and the matrix, which ultimately leads to de-bonding with age. Chemical
modification of the matrix or the resin is one way in which higher interactions
between the fibre and the matrix can be brought about. In the case of
thermoplastic resins modification of the matrix has been carried out by
researchers [ I ] . Modification of the fibre by several chemical methods have
been suggested and carried out in the case of various polymer materials [2-61.
Treatments with alkali and also by other coupling agents like silanes, titanates etc.
have all been proved to be the best way to improve fibrelmatrix adhesion in
natural fibre polymer composites.
In this chapter, the effect of chemical modification of the fibres on the
macro-mechanical properties of the composites, namely tensile, flexural and
impact properties is investigated. The change in the surface morphology and the
polarity of the cellulose fibres after treatment with various chemicals has also
been analysed. The influence of fibrelmatrix adhesion on the behaviour has
been analysed by investigating the behaviour of the composite under tensile,
impact and flexural loading. The effect of the various chemical modifications on
the interfacial adhesion has been concluded based on the mechanical performance
of the material and also on the analysis of the failed surface.
- -- .- Efect of Fibre Surface Treatments on ... . 164
2.2.2. Results and Discussion
2.2.2a. Sodium hydroxide treatment
Banana fibre surface was treated with alkali of different concentration and
the treated fibres were used for the preparation of composites. Reports are already
in the literature regarding the effectiveness of NaOH in modifying the surface of
other natural fibres [2].
Figure 2.2.1 shows the tensile stresslstrain curves of the various treated
composites. It is observed that the ultimate tensile stress is found to be the
maximum for fibres treated with 1% alkali. The improvement in the tensile
strength in the case of fibres treated with alkali of higher concentration can be
attributed to the following reasons:
0 2 6 8 10 12
Strain
Figure 2.2.1 Stresslstrain curves of the various treated composites
1. Due to alkali treatment, the cementing material present in the fibre namely
lignin and hemicellulose get dissolved. This results in the interfibrillar region
becoming less dense and less rigid. The fibrils also become more capable of
-- Effect of Fibre Surface Treatments on .... 165
rearranging themselves along the tensile deformation [7]. Other authors have
also reported on the change in crystallinity of alkali treated fibres because of
the removal of the cementing materials, which leads to a better packing of
cellulose chains [81. More than that, increase in the surface area of the
fibre occurs due to the dissolution of lignin, hemicellulose and alien
substances associated with the fibre. This leads to a larger area of contact
between the fibre and the matrix leading to increased tensile strength [9].
2. Dissolution of waxy substances exposes the -OH and the -COOH groups
on the fibre surface leading to increased polarity and decreased acidity of
the fibre surface. This ultimately leads to increased polar-polar interaction
with the matrix leading to higher tensile strength.
3. The alkali sensitive bonds between the different components rupture
leading to the increased homogeneity of the fibre surface [lo].
4. Alkali treatment increases the yam toughness and affects the micro
fibrillax angle and other structural parameters [ l l ] . This is supported by
the SEM of the alkali treated fibre shown in Figure 1.2.8 (Section 11;
Part1 Chapter 2).
Other authors have also reported on the influence of alkali treatment in
improving the properties of natural fibre composites [12]. The tensile strength is
found to be the maximum for composites treated with NaOH. The tensile
strength of 58MPa for the untreated composite is enhanced to 70MPa for the
composites treated with I-% alkali. When alkali of a lower concentration is
used, the tensile strength is 65MPa.
Effect ofFibre Surface Trearmenls on .... 166
Figure 2.2.2 shows the effect of chemical treatment on the tensile
modulus.
T r e a r n
Figure 2.2.2 Variation of tensile modulus with fibre treatment
The tensile modulus is found to be the maximum for composites treated
with 0.5% NaOH. Treatment with NaOH helps in the removal of fractions of
cellulose of very low degree of polymerisation. The removal of low cellulose
fractions and cementing materials leads to better orientation and packing of
molecules. Figure 2.2.3 a and b shows the tensile fracture surface of the alkali
treated fibre composites. The polyester particles sticking on the fibre surface
and the broken fibres point to the improved fibrelmatrix adhesion. Dissolution
of the lignin effected by the alkali gives rise to free pores, which improves the
contact area between the fibre and the matrix.
Effect of' Fibre Slrrfuce Trt.ufnien/s on.. . . 1 67
Figures 2.2.3 a & b Tensile fracture surface of the alkali treated fibre composites
Figure 2.2.4 shows the variation of flexural modulus with the type of
chemical treatment. Flexural properties are reported in terms of the maximum stress
and strain that occur at the outside surface of the test bar. The flexural modulus is
.foimd to be the lowest for composites treated with 1 % NaOH. Thc stresses induced
due to the flexural load are a combination of compressive and tensile stresses.
Effect ojFibre Surface Treatments on .... 168
Treatment Figure 2.2.4 Variation of flexural modulus with fibre treatment
The performance of the alkali treated fibre composites can be firther
evaluated based on the enthalpy change involved based on the studies reported
earlier. The change in free enthalpy due to the acid-base interaction can be
calculated from the acid-base constants of the interacting phases by using the
theory of Guttman [13]. According to his theory, materials are characterised by
a donor number (DN) and acceptor number (AN) and the change in fke enthalpy is
given by equation 2.2.1 referred above. The values calculated for the alkali treated
fibres are found to be 0.01. The very low value obtained in the case of 1% NaOH is
associated with the high adhesive power of the fibre leading to high fibrelmattix
interactions.
Figure 2.2.5 shows the variation of impact strength with chemical
treatment. In the case of impact strength, the value is found to be the lowest in
the case of alkali treated fibre. Adhesion and strong interaction, however, are
not always necessary and advantageous to prepare composites of desired
~ - ~ - ~
Effect ofFibre Surface Treatments on. ... 169
properties. Plastic deformation of the matrix, fibre pull out etc. are the main energy
absorbing processes in impact, which decrease with increasing adhesion [14]. The
result is consistent with the observation of the improved fibrelmatrix interaction
in the case of alkali treated fibres.
Treatment
Figure 2.2.5 Variation of impact strength with chemical treatment
2.2.2.b. Silane treatment
The bonding of the organohnctional group of the silane with the polymer
can take place in several forms. It can form a copolymer, an interpenetrating
polymer network, or diffuse into the polymer matrix and cross-link at the
fabrication temperature [15]. Optimisation of the type and amount of the coupling
agent is crucial in the reactive treatment of fibre surfaces. Various silanes have been
tried to modify the surface of fibres. The general representation of the interaction of
silane with cellulose is given in Figure 1.2.12 (Section 11; Part 1 Chapter 2).
- - .- Effect of F ~ b r e Surface Treatments on 170
2.2.2131. Silane A151 (Vinyl triethoxy silane)
The tensile strength of the samples treated with the silane A151 is found
higher than the tensile strength of untreated samples. (Figure 2.2.6). In our earlier
chapter, we have indicated that the acceptor number, which is indicative of the
electron accepting ability, is found to be highest for fibres treated with the silane
A15 1 [lo]. (Section 11; Part1 Chapterl).
Figure 2.2.6 Tensile strength of the treated samples
The I.E.P. could be made use of for understanding the acid-base interactions
as reported by other authors [12]. The increased acidity values lead to increased
polarity and thereby improved interactions in the case of silane A151 treated
fibre composites. Fibres subjected to treatment with the silane A151 after pre
treatment with the alkali gave an IEP value 3.4. (Section 11; Part 1 Chapter 2).
Of the various silanes used, tensile strength is found to be the highest in
the case of silane A151 treated fibre composites. The reason can be attributed to
the increased polarity of the fibre surface and thereby the improved interaction
between the fibre and the matrix. SEM micrographs of the silane A1 5 1 treated fibre
Effect r f lFib~e Sat-foce Treatments on ... . 17 1
and the fracture surfaces of the co~nposite are shown in Figure 2.2.7a and b. The
improved adhesion in the case of treated fibre composites is evident from the
broken fibres, which are visible, and the river patterns, which are present on the
matrix. Unlike in the case of the alkali treated fibres, the tlexural yield strength
of the composite is found to be the highest in the case of silane A151 treated
composites (Figure 2.2.9). The flexural modulus is also found to be the highest
in the case of Silane A1 5 1 treated fibre composites (Figure 2.2.4). The impact
strength however is lower than the untreated fibre composites (Figure 2.2.5).
The lowering of the impact strength is consistent with the improved fibre/matrix
adhesion, which result in the high tensile strength values. Improved interaction
leads to a perfect bonding. This leads to the fBilure of the composites at low impact.
For most composites, including short fibre systems, a sometimes-espoused rule of
thumb is that as the strength increases, the toughness decreases. While this is true
for continuous libre reinforced brittle matrices, it is not always the case for short
fibre reinforced systems.
Figure 2.2.7a SEM of silane A151 treated fibre
Ejecf of Fibre Surface Treni~nents on ... . 1 72
Figure 2.2.7b SEM of the tensile failure surface of the A151 treated composite
2.2.2b2. Silane A174 (y-MethacryloxypropyItrimethoxysilane)
In the case of silane A174 treated fibres, the tensile stress-strain curve
shows a slight decrease in tensile strength than that of A15 1. Polarity parameter
measurements have shown that the hydrogen bond dodating acidity of the silane
A174 treated tibre is lowered compared to the untreated fibre as mentioned in
Section 11; Part 1 Chapter 1. This lowering of the polarity can be given as the reason
for the slight decrease in tensile strength of the composites prepared from silane
A 174 treated fibre as compared to the silane A 151 treated fibre (Table: 1.2.1,
Section 11; Part 1 Chapter 2). The SEM photographs of'the silane A174 treated fibre
and the failed composite upon tensile load are shown in Figure 2.2.8 a and b.
Figure 2.2,8a SEM photographs of the silane A174 treated fibre
Effect of Fibre Surface Treatlnents on. .. . 173
Figure 2.2.8b SEM photographs of the tensile failure surface of the silane A174 treated fibre composites
It has been observed by other researchers that maximum wetting tension
between an adhesive and a substrate is obtained when the surface energy of the
substrate is as high as possible [163.
The increased interaction can also be seen in terms of the improved
wetting between the silane A15 1 treated fibre and polyester. The tensile
modulus is also Found to be lower for composites made out of Silane A174
(y-Methacryloxypropyltrirnethoxysilane), treated fibres, compared to silane
A 1 5 1 treated fibres.
0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050 Strain
Figure 2.2.9 Flexural stresslstrain curves of the various treated composites
.-- -. -- -. Effect ofFihre Surface Treatments on .... 174
Figure 2.2.9 shows the flexural stresdstrain curves of the variously treated fibre
composites. The flexural yield strength values of the composites treated with silane
A151 and silane A174 are found almost the same. The flexural modulus is slightly
lower for the silane A174 treated fibre composites (Figure 2.2.4). However, the
impact strength is found to be almost the same as that of the untreated fibre
composite.
2.2.2.b3. Silane F 8261(1H, lH, 2H, 2H-Perfluorooctyl triethoxy silane)
Composites treated with the silane F8261 have given a comparatively
lower value of tensile strength (Figure 2.2.6). This points to the decreased
adhesion between the fibre and the matrix. The reduced interactions can be
expected to be due to the polarity values. The hydrogen bond donating acidity, a,
of materials is related to the surface polarity. Our earlier studies on the a value of
the F8261 fibre is low compared to that of the ones treated with A174 and A151.
(Section 11; Part 1 Chapter 1)The value of the n* parameter which represents the
overall polarity is also lower than that of the fibre treated with the silane Al74.
This low polarity value is also indicative of the lower surface fiee energy. This
leads to the reduced interaction with the polyester matrix. The tensile modulus is also
found to be unusually low. The polarity of the modified fibre surface is obviously
not capable of forming good fibreimabix interactions. The flexural modulus also is
found to be lower compared to the other silane treated fibre composites.
2.2.2b4. Silane A l l 00 (-y-Aminopropyltriethoxysilane)
The tensile strength of the composites made of silane A1 100 treated
fibre composite was observed to be comparable with that of the other silane treated
- Effect of Fibre Surface Treairnents on .... 175
composites. The probable scheme of reaction of the amino silane with the cellulose
fibre is represented schematically in (Fig.1.2.20) (Section 11; Part 1 ChapteR).
It is possible that silanols, formed by the hydrolysis of the alkoxy groups
of the silanes, self condense to form a thick layer of oligomeric silanol
deposition in the fibrelmatrix interface which can greatly reduce the efficiency
of bonding [17]. The flexural and impact strength however, gave values comparable
to the other silane treated composites. SEM of the surface of the silane A1 100
ireated fibre is shown in Figure 1.2.22 ( Section 11; Part 1 Chapter 2).
The fibre surface shows fibrillation and also regions where the
cementing material has been dissolved out. These crevices lead to better
wetting and regions of contact for the matrix material, leading to improved
tensile strength values.
2.2.2b5. Silane Si 69 bis(triethoxysilyl propyl) tetra sulphide
Composites made out of silane Si 69 treated fibres gave the lowest
mechanical properties. The reason can be attributed to the lower compatibility
between the organofunctional group and the polyester matrix. <-potential
measurements carried out on the fibre surface using the streaming potential
method has given indication of the relation between the pH and the surface
potential (Section 11; Part 1 Chapter 2). The iso-electric points are indicative of
the point where the surface charge is zero. The silane Si 69 treated fibres have
given an I.E.P. value of 3, slightly greater than that of the untreated fibre. The
overall polarity of a solid surface is given by the E T ( ~ ~ ) where T stands for the
Effect of Fibre Sz.trface Treal~nents on. .. . 1 76
transition energy. The El-(30) polarity parameter values are also approximately
equal to that of the untreated fibre. The observation concludes that treatment
with the silane Si 69 is not very effective for cellulose fibre.
2.2.2.b6. Acetylation
Acetylation has been suggested to be an effective method for the
modification of cellulose fibre surface by different researchers [I 81. Only the impact
properties of the composites have been investigated. The impact properties are found
higher than alkali treated fibres and lower than that of the silane treated fibres. The
higher impact strength values of the acetylated co~nposites than the alkali treated
Figure 2.2.10 a SEM of the impact fracture surface of the acetylated fibre composite
Figure 2.2.10b SEM of the of the acetylated fibre
~ ~~ -. Ejject of Fibre Surface Treatments on .... 177
fibre composite can be attributed to the comparatively lower fibrelmatrix
adhesion. (Figure 2.2.5) The probable reaction between the fibre and acetic
anhydride can be represented as
Figure 2.2.10a shows the SEM of the impact fracture surface of the acetylated
fibre composite and 2.2.10b, the SEM of the acetylated fibres. The high extent
of fibre pull out shows weak adhesion between fibre and matrix.
2.2.2.b7. Assessment of the effectiveness of different treatments
Silanes with different organohctional groups (vinyl, methacryloxy,
amine and fluorine) have been used to pre-coat the fibre in order to examine the
influence of silane treatment on mechanical properties of the composites. The
adhesion of the poly siloxane layer depends on the chemical composition of the
organo functional group of the coupling agent. The spectra of the different silane
treated and untreated fibres are given in Figures 1.2.3 1.2.7,1.2.13,1.2.14,1.2.17, and
1.2.21 (Section 11; Part 1 Chapter 2). Figurel.2.14 shows the spectrum of the
silane A1 74 treated fibre and 1.2.1 3 that of the silane A1 51 treated fibre.
The different chemical structures of the silanes lead to considerably
different absorptions. The inter difksion of the polymer layers on to the poly
siloxane network gives rise to entanglements creating strong adhesion. Of the
various silanes, silane A1 74, A1 5 1 and A1 100 have been proved to be reasonably
good in improving adhesion in cellulose/polyester systems. The reason for the
improved interaction can be attributed to the changed polarity values of the treated
~~~. ~ ~- ~ ~ Efect of Fibre Surface Treatments on ... . 178
fibres. The higher polarity values lead to more polar-polar interactions leading to a
strong interface. The tensile strength values of polyester filled with different silane
treated fibres (0.6O/0 silane) are given in Table 2.2.1.
Table 2.2.1 Tensile strength values of polyester filled with different silane treated fibres ~p
Silane Tensile Strength (MPa)
A174 60
A151 6 1
F8261 48
Si 69 45
At 100 58
Untreated 57
It is clear that the silane treatment produced an increase in tensile
strength as a result of improved adhesion between the fibre and matrix. Of the
various silane treatments, the highest improvement in tensile strength is found
to be for the silane with the vinyl functional group. The flexural and impact
properties were also found to be relatively high for the silane A151 treated fibre
composite compared to the other treated composites. However, of the various
chemical treatments, alkali treatment has been proved to be the best as far as
properties and cost are considered.
References
1. D. Maldas and B.V. Kokta, Comp. Interfaces,l, 87 (1993)
2. J. Wu. D. Yu, C. Chan, J. Kim and Y. Mai.,J Appl. Poly. Sci, 76,1000
(2000)
- ~- ~- -- Effect of Fibre Surface Trealments on... . 179
<:. i\. S. t l i l l and H. P. S. Abdul Khalil, .I Appl Poly. Sci.,77,1322
(2000)
A. K. Bledzki, Reihmane and J. Gassan, J. Appl. Poly. Sci., 59,1329
(1996)
A. V. tionzalez, J. M. Cervantes, R. Olayo and P. J. Herrera-Franco,
(,'omposi/e.\ Part B., 30,309 (1999)
J . (iassan and A. K. Bledzki Comp. Sci. and Tech., 59, 1303 (1999)
A. Mukherjee, P. K. Ganguly and D. Sur, J. Text. Inst., 84,348 (1993)
D. S . Varma, M. Varma and I. K. Varma, Text Res. Inst., 34,348 (1984)
E. '1'. N. Hisanda and M. P. Ansell Comp. Sci., and Tech.,41,165 (1991)
D. Klemm, B. Philipp, T. Heinze, U. Heinze and W. Wagenknecht
Comprehensive Cellulose Chemistry, Wiley VCH Verlag GmbH,
D-69469 Weinheim (1998) p.31
C . t'avithran, P. S. Mukherjee, M. Brahmakumar and A. D. Damodaran
J ,Muter Sci Lert., 7,825,(1988)
A. K . Mohanty, M. A. Khan and G. Hinrichsen Composites Part A,, 31
143 (2000)
H. Pukanszky and E. Fekete Adva. in Poly. Sci., 139, (1999)
13. Pukanszky and F. H. J . Maurer, Polymer, 36:1617 (1995)
Silane Coupling Agents; Pleuddemann E. P., Plenum Press, New York (1982)
h$, fipstcin, and R. L. Shishoo J Appl. Poly. Sci., 57,751 (1995)
S. Wu. Polymer Interface and Adhesion.; Marcel Dekker, mC. New York
(19821
I f S. A. Khalil, H. Ismail, H. D. Rozman and M. N. Ahmad, Eur.
I'O(VNI. ./ . 37,5(2001)
Part 2 - Chapter 3
DYNAMIC MECHANICAL ANALYSIS OF CHEMICALLY
MODIFIED SHORT BANANA FIBRE REINFORCED POLYESTER
COMPOSITES
Results of th~s chapter have be communicated for publication Composites Science and Technology
Dynamic mechanical properties of
composites made out of chemically
modified short banana fibre and polyester
were investigated and compared with
those of virgin fibre composites. The
dynamic modulus value and damping
parameter, used to quantify interfacial
interaction in composites were investigated
with special reference to the effect of
temperature and frequency. Increased
dynamic modulus values and low damping
value point to the improved interactions
between the fibre and the matrix. The
damping peaks were found to be dependent
on the nature of chemical treatment. Both
storage modulus and damping values
measured experimentally were consistent
with each other and point to the effectiveness
of Silane A174 (PAPS) coupling agent for
improving fibrelmatrix adhesion. Activation
energy values for the bnsitions of the
mposites were determined from m e n i
plots. ColeCole plots were made to evaluate
the heterogeneity of the system.
?en In
- -- Dynamic Mechanical Analysis of 18 1
2.3.1. Introduction
Investigation of dynamic mechanical properties, dynamic modulus and
internal friction over a wide range of temperature is useful in studying the
polymer composite structure [I-41. DMA has been mainly used as a technique
for evaluating the interfacial interactions in composite materials. Saha et al. [5]
carried out dynamic mechanical investigations of chemically modified jute fibre
and polyester composites. The data obtained from their study suggest that
storage modulus and thermal transition temperature of the composites improve
enormously due to chemical treatment of fibre. Bikiaris and Karayannidis [6]
carried out dynamic thermo mechanical and tensile properties of chain extended
polyethylene terephthalate afler chemical modification. The T, values determined
were found to be in good agreement with those obtained by differential scanning
calorimetry. Valea et al. [7] investigated the influence of cure conditions and the
exposure to different chemicals on the dynamic mechanical properties of several
vinyl ester and unsaturated polyester resins containing glass fibre. Exposure to
aromatic solvents was found to modify the viscoelastic character of these
materials. Finegan and Gibson [S] reported on the recent analytical and
experimental results regarding the improvement and optimisation of damping in
composites. I'hey have used dynamic modulus and damping values to quantify
fibreimatrix adhesion. Several authors have reported on the modification of
fibre surface to improve the interaction with the matrix [9-201.
In the present chapter, the influence of fibre surface treatment on the
visco-elastic properties of the composites is reported. The properties were
- -- - - Dynamic Mechanical Analysis of.... 182
found to be dependent on the type of chemical treatment. The loss modulus and
tan F curves were also found to be affected based on the chemical modification.
An insight into the fibrelmatrix interaction based on fibre treatment could be
evaluated from the damping peaks and the activation energy values. The
mechanical behaviour of the treated composites was found to be affected
considerably above the T, than below it by the incorporation of the fibre. SEM
studies have been made to understand more about the fibrelmatrix interaction
and the fibre surface topography.
2.3.2. Results and Discussion
2.3.2a. Storage modulus
In the earlier chapter we have reported on the effectiveness of banana
fibre as reinforcement in polyester matrix at temperatures above its T,. The
effectiveness of the reinforcement was also found to depend on the amount of
fibre incorporated in the matrix. The maximum improvement in storage
modulus was obtained for composites with a 40-volume percent fibre loading.
Figure 2.3.1. illustrates the temperature dependence of the storage modulus for
the various chemically treated composites. Figure 2.3.1 clearly illustrates that
the behaviour of the storage modulus vis-a-vis temperature for all the composites
are similar in nature i.e., the storage modulus initially remains almost constant at
lower temperatures, shows a steep drop with increasing temperature and then
levels off. In the present case, as the temperature is increased, the storage
modulus shows a sharp decrease at the temperature range around 80-9O0C.This
~ ~ -- Dynamic Mechanical Analysis of ... 183
is followed by a modulus plateau, at higher temperatures, where the polymer
behaves like a rubber.
t A l l 4 Acwalea
V - NsOH 1% t NsOH 0.5% - + - N e m Y
Temperature ('C)
Figure 2.3.1 The variation of dynamic modulus as a function of temperature (O.1Hz.)
In all the cases, a fibre loading of 40 volume percent has been used. It is
interesting to note that the storage modulus value is slightly lower for all the
composites compared to the neat polyester at a temperature range up to 60°C.
However, at temperatures above 85"C, while the dynamic modulus values of the
neat polyester sample is found to decrease considerably, the values of the
composite are found to remain much higher. The onset of the modulus drop
corresponds to molecular mobility. The lowering of the modulus value, at high
temperature, however, is reduced substantially by the incorporation of fibre. In
principle, E' is obviously influenced by fibre stiffness. 'The lowering of dynamic
modulus value_ on the other hand, occurs due to the micro Brownian movement
of the polymer chain as well as due to the short-range difisional motion of the
polymer [2]. 'lhe improvement in the dynamic modulus value at higher
temperatures is, however, also found to be dependent on the chemical modification
done on the fibre surface. Among the treated fibre composites, the improvement in
- ~.~ Dynamic Mechanicol Analysis of. ... I84
modulus at higher temperature is found to be the maximum for silane treated fibre
composites. specifically, for composites treated with silane A174 (y-Methacryloxy
propyl trimethoxy silane). All the other treatments have produced an improvement in
the modulus value almost to the same extent, above the glass transition temperature.
Composites made out of A174 treated fibre, which shows the highest modulus
at room temperature, shows the highest value at increased temperature as well.
The dynamic modulus curves of the treated and untreated composites present
three zones of abrupt modulus drops, which correspond to the respective
relaxations in the polymer matrix. The decrease in modulus value occurs at the
temperature range around 80°C and also around 120-130°C. The change in the
dynamic modulus value can be attributed to the changes in the molecular
dynamics which occur in the vicinity of T,. The organo functional group of the
silane forms interpenetrating polymer networks with the matrix resin [21] that
can be believed to cause the change in the polymer structure. The molecular
structure of the polymer profoundly affects Tg [I].
The E' curve shows an improved rubbery plateau, indicating that the
incorporation of fibre in polyester matrix induces reinforcing effects which
increase the thermal mechanical stability of the material at higher temperature.
In other words. in the glassy zone, the contribution from fibre stiffness to the
material modulus is minimal. When the temperature is increased, the abrupt
drop in the modulus of the matrix is compensated by fibre stiffness.
-- Dynamic Mechanical Analysis of. ... I85
2.3.2b. Loss modulus
Figure 2.3.2 shows the effect of temperature on the loss modulus values
of the treated and untreated fibre composites at frequency 1Hz.
Figure 2.3.2 Effect of temperature on the loss modulus values of the treated and untreated fibre composites (Frequency 1Hz.)
The neat polyester gives a broadened peak in the temperature range
80-120°C. Addition of fibre results in a broadening of the loss modulus peak. The
broadening of the loss modulus curve points to an increased range of order and the
width of the relaxation spectrum expresses the diversity of chain segments [22].
The loss modulus curve shows an interesting trend with a broadening of the
curves in all the fibre filled samples in this study.
'The height and area of the peak regions are also indications of the
energy absorbed by the system. Compared to the untreated composite, the peak
height is reduced for the A174 treated and acetylated fibre composites. For the
-~ ~ ~- Dynamic Mechanical Analysis of.. ,186
alkali treated fibre composites, however, the peak height is greater than that of
the untreated composites. The broadening of the loss modulus curve occurs near
the glass transition temperature of the material. The loss modulus values are a
measure of the viscous response of the material. The flattening of the loss
modulus curve in the case of the silane treated composites, again points to the
improved interaction between the fibre and the matrix. Depending on the nature
of the chemical treatment employed, the surface area of the fibre changes,
which in turn leads to improved fibrelmatrix interactions and thereby a shift in
the glass transition temperature. Poor bonding results in energy dissipation at
the interface 1221. The shifts in the Tg values for the various composites from
the loss modulus curves are given in Table 2.3.la.
Table 2.3.1a The shift in Tg for the various treated samples from the loss modulus curves
Sample Shift in T, ("C)
1 % NaOH treated 13
Neat polyester Nil
0.5% NaOH treated 20
A 1 5 1 treated 12
A1 74 treated 19
Acetylated 18
Untreated 19
The shift in the T,values is found to be the maximum for the 0.5% alkali
treated fibre composites. This can very well be related to the improved
interactions that occur between the fibre and the mapix.
p- -- - --. Dynamic Mechanical Analysis of. .... 187
2.3.2~. Damping curves
While the dynamic modulus is related to the interfacial adhesion, the
damping peaks are found to be inversely proportional. Damping is an important
parameter related to the study of dynamic behaviour of fibre reinforced
composite structures. Any change in the molecular mobility in the polymer
system will appear as a peak in the tan 6 curve. The magnitude of the tan 6 peak
is also an indication of the fibrelmatrix adhesion in the system. Change in
temperature affects the damping. In addition, the tan 6 peak gets shifted
depending on the chemical treatment. Figure 2.3.3 delineates the damping
curves of the treated and untreated fibre composites. Table 2.3.lb shows the
shift in the T, values of the various composites from the tan 6 curves.
Table 2. 3.lb Shift in T, values of the various composites from the tan Gcurve
Sample Shift in T, ( O C )
1 % NaOH treated 3
Neat polyester Nil
0.5% NaOH treated 12
A 15 l treated 3
A 174 treated 0
Acetylated 7
llntreated 7
-. Dynamic Mechanical Analysis o f . . . I88
The maximum shift in the peak of the tan 6 curve is also observed for
composites made out of' fibres treated with 0.5% NaOH. The shift in the curve is
indicative of the improved fibrelmatrix interaction. The tan 6 peak height also
gets affected depending on the chemical treatment. The lowest peak in this case
has been observed for silane A174 treated fibre composites as well as the alkali
treated composites. The lowering of the damping peak very well agrees with the
improved fibrelmatrix adhesion,
Figure 2.3.3 Damping curves of the treated and untreated fibre composites (O.1Hz.)
While the loss modulus curve showed broadening, the damping curves
show two peaks, one at the temperature around 80°C and the other around
130°C. In addition, the damping curves get lowered due to the incorporation of
fibre. The lowering of the damping curves, upon the addition of fibre, compared
to the neat polyester is also due to the decrease in the volume fraction of matrix.
The higher peak around 130°C is associated with the T, and that around 80°C
due to the micro mechanical transitions. Eklind and Maurer [23, 241 have
postulated an interlayer model to simulate the dynamic mechanical properties of
-- ~~ Dynamic Mechanical Analysis o/ ... 189
filled blends. 'The filler particles have been persumed to be surrounded by an
interlayer attached to the filler surface. This phenomenon could give rise to
filler structure in the matrix able to alter the dynamic mechanical modulus. The
bound polymer also results in a layer with properties different from the bulk
properties of the pure polymer. Schematic representation of the fibre, matrix and
the imrnobilised polymer layer is given in Figure 2.1.6 (Section 11; Part 2
Chapter 1). The effect of the interlayer becomes prominent only at high fibre
content, mainly because, the increase in the amount of the fibre increases the
area of the interface and thereby the interlayer.
The loss tangent peak height is found to depend on the nature of the
chemical treatment.
Figure 2.3.4 Plot of the storage modulus E' as a function of temperature for A174 treated fibres at different frequencies (40 volume percent)
Figure 2.3.4. shows the dependence of the storage modulus E' on
temperature for A174 (gamma methacryloxy propyl triethoxy silane) treated
fibres at different frequencies (40 volume percent).
~~p~~ .- - Dynamic Mechanical Analysis oJ ... 190
The viscoelastic properties of a material are dependent on temperature
and time (frequency). If a material is subjected to constant stress, its elastic
modulus will decrease over a period of time. Figure 2.3.5 shows the plot of loss
modulus E" as a function of temperature for A174 treated fibres at different
frequencies. Table 2.3.2 shows the En peak values observed and the
corresponding T,'s.
Figure 2.3.5 Plot of Loss Modulus curve as a function of temperature for A174 treated fibres
The peak around 80°C is found to be higher at lower frequencies and
the peak around 130°C is found to be higher at higher frequency. It has been
observed that the change in frequency affects the tan 6 peak height. Increase of
frequency shifts the peak height to higher temperatures.
~~--~~p . ~~~~
Dynamic Mechanical Analysis oJ ... 191
Table 2.3.2. Effect of chemical treatment on the E" maximum and T, Values of neat polyester and banana fibre reinforced polyester composites (40 volume percent)
Sample Frequency (Hz.)
A 151
A 174 7.75 7.68 7.67
7.74 7.72
1% NaOH 1 7.73 7.74 7.76
Acetylated ' / 7.66 7.51 I
7.59
Untreated 7.98 7.67 7.72
Neat 8.01 1 798 7.98 polyester
T, ("C) at E",,
Frequency (Hz.)
The change in the transition peak can be assigned to the change in the
initiation of segmental mobility. The presence of fibres restricts the segmental
mobility. and the transition peaks get affected. Table 2.3.3 shows the tan 6 peak
values and the corresponding Tg's of the various composites.
On treatment with alkali, the hemicellulose and lignin, which are the
main components in the banana fibre, get removed. The inter fibrillar region
becomes less dense and less rigid and makes the fibrils more capable of
rearranging themselves along the direction of tensile deformation [13].
Treatment of the cellulose fibres with alkali also brings about the process of
swelling and dissolution. This enhances the accessibility of the cellulosic
Dynamic Mechanical Analysis of.... 192
hydroxyl groups for a subsequent reaction 1151. All these factors lead to
improved interactions with the matrix material.
Table 2.3.3 Effect of chemical treatment on the tan 6 Maximum and T, Values of Neat Polyester and Banana Fibre Reinforced Polyester Composites
The rough surface topography of the alkali treated fibres points to the
removal of hemicellulose and lignin. In addition, the interface between the fibre
tan 8 ,, Sample i:l Frequency (Hz.)
and the matrix can be considered to be molecularly sharp giving rise to a
T, ("C )at tan F ,,
Frequency (Hz.)
mechanically strong interface [25]. Molecularly strong interfaces are known to
0.1
108
118
122
118
116
118
104
A 151 1 0.22
prevent interfacial slippage. The surface roughness of the fibres also increases the
1
0.22
0.16
0.20
0.19
0.16
0.21
0.42
A 174
0.5 % NaOH
1 % NaOH
Acetylated
adhesive bond, by mechanical interlocking. Dynamic modulus values of the fibres
10
0.24
0.16
0.22
0.21
0.18
0.22
0.41
1
117
125
126
. 122
124
125
104
0.16
0.20
0.18
0.16
treated with silane A1 74 and A151 are also found to be high (Figure 2.3.1). The
10
127
133
136
127
131
131
114
Untreated
Neat polyester
.. -~
highest storage modulus value shown by the silane A174 treated fibre composite
can be explained as due to the improved polar interaction between the fibre and
~ ~- ~~~ ~~ Dynamic Mechanical Analysis of ... 193
the matrix. Good adhesion between fibre and matrix leads to high values of
dynamic modulus 181. Solvatochromic measurements done earlier by us on the
fibres treated with the silane A174 have shown an acceptor number lower than
that of the untreated fibre [26]. The FTIR spectra of the treated fibre also shows
the presence of the absorption band at 765cm" corresponding to the -Six-
symmetric stretching band. (Figure 1.2.14, Section 11; Part 1 Chapter 2) The band
around 1150 cm-' can be attributed to the asymmetric stretching of the S i - O S i -
and or to the -SIX=- bonds [27]. The former bond is indicative of the poly
siloxanes deposited on the fibre and the latter confirms a condensation reaction
between the silane coupling agent and the fibre. Composites made out of acetylated
banana fibre show a dynamic modulus value almost similar to that of the
untreated banana fibre composites. The modulus values of the composite in the
temperature range 80-120°C is slightly lower than that of the untreated
composites. However, the modulus values of the acetylated and 1-% NaOH
treated composites merge together at frequency 1Hz. The modulus value of the
silane A151 treated fibre composite is also slightly lower than that of the
untreated composite.
Damping is an important parameter related to the study of dynamic
behaviour of tibre reinforced composite structures. The height and area under a
tan 6 curve give an indication of the total amount of energy that can be absorbed
by a material. A large area under the tan 6 curve indicates a great degree of
molecular mobility, which translates into better damping properties. The effects
of the various chemically treated fibres on the damping of the composites were
Dynamic Mechanical Analysis of .. ,194
analysed. The damping values in the case of composites treated with silane
A174, 0.5% NaOH and acetylated fibres have been found to be lower than that
of the untreated fibre composite. This result is in agreement with the
observation of the higher modulus values of the chemically modified fibre
composites. Cinquin et al. [28] observed a decrease in damping associated with
the improvement in interface bonding. Chua [29] concluded that a composite
with poor interface bonding tends to dissipate more energy than that with good
interface bonding. The major contribution to composite damping is due to
matrix [30]. A nature of the interface also affects the mechanical properties and
in turn the damping. In the case of Silane A151 treated fibre composite, the
damping peak is found to be almost the same as that of the untreated fibre
composite but slightly shifted to the negative side. When the damping curves
are compared, all the composites show two peaks, one around 8O0C and the
other around 12O0C, characteristic of the fibre and the matrix. The lowest damping
peak is obtained for composites treated with silane A174. The lowering of the
damping peak can very well be associated with the improved adhesion between
the fibre and the matrix and is consistent with the results of the mechanical
property measurements.
Adhesion promotion between glass and silanes is supported by various
theories. The chemical bonding theory applied to glass can be applied for
cellulose fibres as well. The general structure of organa silanes can be
represented as R-Si-X3. The X group hydrolyses to silanol groups, which then
react with the -OH group on cellulose fibre to form ether linkage. The
hydrolysed silane also self condenses to form poly siloxane. SEM photographs
- ~ Dynamic Mechanical Analys~s of ... 195
of the banana fibre surface treated with the two silanes, silane A151 and A174,
are shown in Figures 2.2.8a and 2.2.9a (Section 11; Part 2. Chapter 2). The silane
A174 treated fibres show a rough surface topography. It is clear that in addition
to the high polar-polar interactions, better interlocking occurs in the case of
A1 74 treated fibres.
The area under the tan 6 curve gives an indication of the total amount of
energy that can be absorbed by the material during an experiment. A large area
under the tan 6 curve indicates a great degree of molecular mobility, which
translates into better damping properties, meaning that the material can better
absorb and dissipate energy 181. The area under the tan 6 curve is found to be
dependent on the nature of chemical treatment.
Treatment with 1% NaOH has given the maximum value of the area
whereas treatment with silane A174 and acetylation has given the minimum value
(Table 2.3.4). This is consistent with the dynamic modulus values obtained for the
treated composites. All the treated fibre composites show an additional
shoulder unlike the neat polyester composite due to the micro mechanical
transitions of the immobilised polymer layer. (Figure 2.1.6. Section 11; Part 2
Chapter 1).
~~
Dynamic Mechanical Analysis o f . . ,196
Table 2.3.4 Effect of chemical treatment on the area under the tan 6 curve
Sample Area under the tan 6 Curve (cm2)
Gum (neat polyester) 20.7
Untreated composite 12.5
Acetylated 2.7
1 % NaOH treated 15.7
0.5 % NaOH treated 12.8
The better fibre-matrix adhesion reduces molecular mobility and thereby
the damping values. The SEM photographs of the acetylated fibres are also
shown in Figure 1.2.1 1 ( Section 11; Part 1 Chapter 2). Acetylation has made
the fibre surface rough, making sites for anchorage with the resin.
The Cole Cole plots of the various chemically modified composites are
given in Figure 2.3.6. The dynamic mechanical properties when examined as a
function of temperature and frequency are represented on the Cole Cole
complex plane.
--- -- -. - - Dynamic Mrchanfcal Anolysis of.... 197
Figure 2.3.6 fibres
* - 8 0
The Cole Cole plots are not perfect semi circles pointing to the
- +,+- +A~+++
+,
heterogeneity of the system. However, it is interesting to note that the nature of
the curve changes depending on the chemical treatment. The A174 treated
76 -
7 6 - +'
7. - + i
-. 7 2 - i a i 5 7 . 0 - 7
-A- k e w t e d -v- 1% NaOH t 5 % NnOH
6 4 - -+-Neslp* - X U m r t M
6 0 -
7 B 7.8 8 0 8 2 8.4 8.6 8 8 90
logE'(MPa)
Cole Cole plots of the various chemically modified
samples give the best semi circular curve, showing the intensity of the
fibrelmatrix interaction.
Any increase in vibrational energy causes the glass transition temperature to
rise. The shift of the transition temperature allows one to calculate the apparent
activation energy of the relaxation for each of the samples assuming a linear
equation of the type
H logf - logf,
2.303RT
where fo is an experimental constant, f and T are the measuring frequency and
the temperature for the dispersion peak respectively, and R is a gas constant,
and H the activation energy.
~ -- Dynamic Mechanical Analysis of.... 198
The activation energy of the various composites calculated from the
Arrhenius plots are shown in Table 2.3.5.
Table 2.3.5 Activation energy values of the various treated composites
Sample Activation energies (kJ1mol)
A 1 5 1 32
A 174 42
0.5 % NaOH treated 32
1 % NaOH treated 75
Acetylated 39
Untreated 41
The value fbr the activation energy is maximum for the alkali treated
fibre composite. The result is in agreement with the storage modulus values
where a high fibrelmatrix interaction is anticipated. However, acetylated
composites does not show a high activation energy value.
References
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Viscoelasticity, John Wiley (1983)
2. T. Murayama, Dynamic Mechanical Analysis of Polymeric Materials,
Elsevier (1978)
3. J. D. I'erry, Viscoelastic Properties of Polymers and Comp. Vol. 2, John
Wiley & Sons, New York (1980)
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6 . D. N. Bikiaris and G. P. Karayannidis, J. Appl. Poly. Sci., 70, 797 (1998)
Dynamic Mechanical Analysis OK ... 199
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I. C. Finegan and R. F. Gibson, Comp. Strs. Vol. 44,89 (1999)
L. A. Pothan, S. Thomas and N. R. Neelakantan, .I Reinf: Plast. and
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D. Klemm, B. Philipp, T Heinze, U. Heinze and W. Wagen Knecht
Comp. Cell. Chem. Vol. 2 Wiley, VCH (1998)
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N. Kelein, ti. Marom, A. Degoretti and C. Migliaresi, Composites, 26,
707 (1995)
p~~ . -- Dynamic Mechanical Analysis oJ ... 200
H. Eklind and F. H. J. Maurer, Polymer, 38, 1047 (1997)
H. Eklind and F. H. J. Maurer, .I Poly. Sci. Part B: Phys., 34, 1569
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Part 2 - Chapter 4
EFFECT OF CHEMICAL
MODIFICATION ON THE WATER
ABSORPTION BEHAVIOUR OF BANANA FIBRE REINFORCED POLYESTER
COMPOSITES
Results of th~s chapter have bee communicated for publication in the Journ: of Applied Polymer Science
Abstract I The water sorption characteristics of
banana fibre reinforced polyester
somposites were studied by immersion in
distilled water at 28, 50, 70 and 90°C.
The effects of chemical modification on
the water absorption of the composites
were also evaluated. Water uptake was
found to be dependent on the amount ol
banana fibre and also on the chemica
treatment involved. The water absorptior
showed a multistage mechanism in al
composites of untreated short fibre
where the interface was weak. The
multistage mechanism was found to be
associated with the interface failure
followed by collection of water in the
cracks thus created and also surfacc
blisters. Chemical modification was
found to affect the water uptake of thc
composites. Among the treatec
composites, the lowest water uptake was
observed for composites treated with
silane A1100. Finally, parameters like
diffusion, sorption and permeability
coefficients of the composites were also
determined.
. Effect of Chemical Modification .... 202
2.4.1. Introduction
For many applications, water absorption behaviour of fibre reinforced
systems is important. During its service, a composite material containing
fibrous reinforcement will absorb moisture from its surroundings. Moisture
substantially affects the properties of polymer matrix composites. Knowledge of
the properties such as permeability and conductivity in polymeric composite
systems is therefore essential. The diffusion properties in the composite are
accelerated when the reinforcement is hydrophilic. This hydrophilicity of the
reinforcement in turn affects the long-term mechanical properties of the
composite. Moisture diffuses into polymer in different degrees depending on a
number of molecular and micro structural aspects [I ] . The main factors, which
affect the diffusion process, are
1. The polarity of the molecular structure, presence of chemical groups
capable of forming hydrogen bonds with water.
2. The degree of cross-linking.
3. Presence of residual monomers or other water attacking groups.
4. Crystallinity and
5. Free volume.
'The permeability of overall composite is however decided mainly by
that of the fibres [2]. Many matrix resins also absorb moisture reversibly by
Fickian diffusion. but resin chemical structure and microstructure are complex, both
in cross-linking density and polarity. In addition, other impurities may also be
present in the polymer, which enhance the water uptake. These factors cause
- Effect ofChemica1 Modfication .... 203
non-Fickian processes to occur which may or may not lead to reversible effects.
Two mechanisms have been mainly suggested for the way in which the effect of
moisture affects composites generally [3,4]. One is that the absorbed water
causes plasticisation of the matrix, [5] and the other is the cracking of the
composite through swelling [6] . The way in which composite materials absorb
water depends upon many factors, such as temperature, fibre volume fraction,
orientation of reinforcement; fibre nature (i.e. permeable or impermeable), area
of exposed surface, diffusivity and the amount of surface protection. One major
mechanism of moisture penetration into composite materials is by diffusion.
This involves direct diffusion of water into the matrix and, to a much lesser
extent into the fibres. In addition, moisture penetration can also occur through
the fibre ends, which serve as conduits for water transport. The other common
mechanisms of' water diffusion are capillarity and transport by micro cracks.
The capillarity mechanism involves the flow of water molecules along the
fibrelmatrix interface, followed by diffusion from the interface into the bulk
matrix. Transport of moisture by micro cracks involves both flow and storage of
water in micro cracks or other forms of micro damage [7].
Expressions relating the composite diffusion coefficient to the fibre fraction
and its orientation have been given by investigators like Shen and Springer [S].
Rao et al. [9] presented a comprehensive moisture absorption analysis in jute-epoxy
composites. They showed that a Fickian diffusion model is valid for this type of
composite. In the case of Fickian diffusion, after a long period of time, the
M (moisture content) versus root t curves approach asymptotically the maximum
~ ~~ Effect of Chemical Modification .... 204
moisture content M,. The initial slope of the curve is proportional to the
diffusivity represented by
where MI and M2 stand for the moisture content at time tl and t* respectively.
Marais et al. [ I 01 have studied the diffusion and permeation properties of liquid
water through unsaturated polyester resins. They found that water absorption of
the resin results in a decrease in the glass transition temperature, leading to
diffusivity enhancement by the plasticisation effect . There are several methods by
which the absorption of water can be reduced in a composite. One is hybridisation
with glass and the other is by chemical modification of the fibre.
Composites made of natural fibres like oil palm and empty fruit bunch
fibre have shown a reduction in water uptake with the incorporation of glass
fibre [l 11. Banana fibre, the cellulosic fibres obtained from the pseudo-stem of
banana plant (Musa sepientum) is a bast fibre with relatively good mechanical
properties. It has been proved to be an excellent reinforcement in polymeric
matrix [12]. However, the water uptake of the composites increase with the
amount of fibre incorporated in the matrix. Chemical modification has been
found to reduce the water uptake in different composite systems as has been
reported by other researchers [13]. Chemical modifications like treatment with
alkali dissolves out the chief water absorbing components in the cellulose fibre
namely hemicellulose and lignin. This, other than reducing the water absorption
of the fibre reduces the water absorption of the composite as such. The reduced
~ ~ ~~
Effect ofChemical Modification .... 205
water uptake of the fibre and also the improved interfacial interaction between the
modified fibre and matrix lead to the low water absorption [14]. The diffusion
of water through natural fibres has been reported to be anomalous by other
researchers [15]. ?he diffusion of water in water-cellulose system has been
reported to be non-Fickian or anomalous by Newns [16] and Stamm [17]. The
microstructure of natural fibre is extremely complicated, in that it comprises
hierarchical microstructures [IS]. The rate of absorption of water is also dependent
on the fibre nature. In the natural fibre, the noncrystalline matrix phase of the cell
wall is very complex and consists of various compounds, including hemicellulose,
lignin and some pectin which all form complicated macromolecular networks. The
outer cell wall is porous and consists also of pectin and other non-structural
carbohydrates. The pores of the outer skin are the prime diffusion paths of
water through the material. The lumen in the centre of the fibre also contributes
to the water uptake properties of the composite [19]. The rate of absorption of
water is different for different fibres and is very much dependent on the surface
pores and other properties. Reports are there in the literature on the penetration
of solution through the polyester, which is facilitated by capillary effects
through the matrix [20] and wicking along the polymerlglass interface in hybrid
composites [21]. Detailed studies on the kinetics of water absorption and
influence of water on the interphase in plastics and rubber composites have been
reported by other authors [22, 23, 241.
Banana fibres having been proved to be effective reinforcement in polyester
matrix are susceptible to be in contact with water in various applications. The
present chapter aims at investigating the effect of the various chemical agents in
. . ~
Effect ~/Chemical Modification.. .. 206
controlling the water uptake of banana fibre composites, special emphasis is
given to the fibrelmatrix interface, which significantly controls the water
absorption.
2.4.2. Results and Discussion
0 50 100 150 200
Root tirne(Minutes)
Figure 2.4.1 Qt vs. root time curve of the various chemically treated composites at room temperature
Figure 2.4.1 shows the moisture absorption curves of the untreated and
variously treated fibre composites at room temperature. In the case of the
untreated composite, the water absorption is found to be following a clearly two
stage mechanism, unlike in other cases. The water absorption in the untreated
fibre occurs through the surface pores and also through the capillaries. The
nature of the water absorption curve seems to be different after the initial steep
rise in the absorption. The slope change after the initial fast absorption can be
attributed to the overall change in the absorption mechanism. The theory
suggested by Feughelman [25] on the formation of hydrogen bonds in the water
accessible regions to form a sol like structure, which is slowly converted to the
Eflecr of Chemical Modification .... 207
gel form, is quite acceptable in the present context. It is also clear that the
water sorption proceeds very quickly in the first stage for a specific time.
After the initial fast absorption, the rate of sorption seems to decrease, and
equilibrium seems to be achieved. The final equilibrium stage is found to be
comparatively slower in the case of the untreated fibre composites. Other
authors have reported on the slowing down of the final equilibrium water
uptake due to swelling stresses set up on the fibre [19]. After the initial
diffusion process, there is a second stage when the swelling stresses in the
fibre relax. Consequently, the attainment of the equilibrium moisture
condition is changed and the attainment of the final steady state becomes
delayed. Chemical modification, however, helps in lowering the building up
of the stresses. Lignin, one of the components of the natural fibres has been
reported to be the component responsible for restricting the hydrogen bond
between the fibres and thereby their swelling [26]. Chemical treatment helps
in removing this component and brings about a reduction in the swelling
stresses. In chemical modification, the cellulose fibrils are allowed to
rearrange and there is an increase in the surface area of the fibres. This also
lowers the equilibrium stresses involved. Among the treated composites, the
equilibrium water absorption is found to be the maximum in the case of
acetylated fibre composites and minimum in the case of silane A1 100 treated
fibre composites. Other authors have reported on the decreased water uptake
of acetylated samples of coir and oil palm fibrelpolyester composites [27].
The results we got however are different. The minimum water uptake is for
the silane A1 100 treated fibres. The treatment with the silane A1 100 makes
-~ ~ Effect of Chemical Modification .... 208
the surface more basic, as has been reported in our previous investigations [28].
The increased water uptake for the untreated composite can be attributed to
the increased capillary action and also due to the presence of free pores on the
surface. The fibre has a porous internal structure. In chemically modified
fibres, rearrangement of the cellulose fibrils leads to free spaces for the matrix
resin to squeeze in and lesser space for the water molecules. Figure 2.4.2
shows the water absorption curves of the variously treated composites at 50°C.
The nature of the curve seems to be slightly different in the case of the
acetylated and untreated composites. But at higher temperature also, the water
uptake is found to be the lowest for silane A1 100 treated fibre composite. The
maximum water uptake is found to be for the untreated fibre composites and
the acetylated composite. The change in the water absorption curve is found
to be different after a time span of 6400 minutes. The water absorption rises
steeply after that. The reason for the sudden increase in water uptake after the
time span of 6400 minutes can be associated with the dissolution of surface
materials after the immersion of the sample for a long time in water which
results in the increased number of pores and also the increase in area of the
existing ones. Moreover, the absorption of water through the interface is also
facilitated by the free space, which occurs due to the unequal expansion of
water, by the resin and the fibre.
*-.-.- 4
_*--X -x- -i --X
__--- -.- UnlrealedSO
-.- NaOHO 5 - 4 A174 -.- F8261 -x- A1 100 . . x. Acelylaled -
20 40 60 80 100 120 140
Root time (minutes)
Figure 2,4.2 Water absorption curve of the variously treated composites at 50°C
The SEM of the failed composite revealing the porous nature of the fibre
is shown in Figure 2.4.3.
Figure 2.4.3 SEM of the tensile failed composite showing the porous nature of the fibre
The cross sections of the fibres also become the main access to the
penetrating water. Unlike organic penetrants, the water molecule is small and
strongly associated through hydrogen bond formation. These water molecules
form strong localised interactions with hydroxyl groups available on cell~~lose
and lignin. Other than the uptake of water by the fibres, the not so strong
interfacial region also leads to the passage of water. Increase in temperature
~
Effect of Chemical Modi/icalion .... 2 10
leads to thernval expansions, which pave way for water absorption though the
micro cracks as well.
It has been observed in our earlier studies and reported in a previous
chapter that alkali treatment on the fibre surface reduces the polarity of cellulose
fibres [29]. The water absorption capacity is found to be lowered with increase
in the alkali concentration. Increased alkali concentration brings about more
crystallinity to the fibres, probably due to the arrangement of the fibrils. This reduces
the water sorption capacity of the fibre. In addition, chemical modification covers
some of the surface pores as well in the fibre. The low water uptake of the alkali
treated fibre composite (Figure 2.4.1) is due to the improved fibrelmatrix
adhesion. Water uptake and swelling of the fibre has been reported to reduce
the <-potential values, whereas greater accessibility of dissociated surface
functional groups has been reported to increase the <-potential [l3]. Moreover,
the resin flows to the regions where the cementing materials have been
dissolved giving rise to better interaction with the fibre. In the previous chapter,
(Section 11; Part I Chapter 2) we have reported on the positive <-potential value
of the alkali treated fibre. The positive <-potential value, measured after NaOH
treatment might be caused by alkali metal ions, 'strongly adsorbed' on the fibre
surface after alkali treatment process (possibly 40'-. . . ~ a + ) . The effectiveness
of the alkali treatment to increase the accessibility of surface groups is clear
from the results of water absorption measurements as well. In other words, the
low water uptake points to the improved fibrelmatrix adhesion. The diffusion
- - --- - Effect ofChemrcal Modification .... 2 1 1
co-efficient characterises the ability of the solvent molecules to move among
the polymer segments.
The diffusion co efficient D can be calculated from the equation
where O is the slope of the linear portion of the sorption curves and h the initial
sample thickness. The values of the diffusion coefficients for the various chemically
modified fibres are s h o w in Table 2.4.1. The sorption of water by the fibre
determines the permeability of water molecules through the composite sample. The
sorption coefficient of the composite has been calculated using the equation.
The difhsion coefficient is related to the equilibrium sorption of the penetrant.
The permeabilities, P, of the composite samples to water molecules can be
expressed by [2 11.
Permeability therefore talks about the net effect of sorption and
diffusion. Table 2.4.1 gives the values of the permeation coefficient and
diffusion coefficient of the various chemically modified composites.
-- - - - -- Eficr of Chemical Modificaiion 2 12
Table 2.4.1 Values of the diffusion coefficient and sorption coefficient of the various chemically modified composites
Diffusion Sorption Permeability Sample Temperature coefficient, coefficient, coefficient
("c) ~(cm's-I) S(g/g) p(cmZs-')
30 1.15E-09 0.21 2.41E-10
50 1.96E-09 0.23 4.50E-10 0.5 NaOH
70 885E-10 0.22 1.95E-10
-. . - 90 1.10E-10 0.18 1.98E-11 30 1 .ME-09 0.2 3.08E-10 50 1.76E-09 0.19 3.34E-10
1 %NaOH 70 2.03E-09 0.16 3.25E-10
~ 90 4.31 E-09 0.23 9.90E-10 30 3.55E-10 0.23 8.18E-11 SO 5.62E-10 0.24 1.35E-10
A174 70 2.61 E-08 0.15 3.92E-09 90 1.19E-09 0.11 1.31 E-10 30 3.55E-10 0.2 7.11E-11 50 1.39E-09 0.19 2.63E-10
A151 70 5.82E-10 0.17 9.89E-11
-- 90 6.54E-09 0.18 1.18E-09 30 2.44E-09 0.15 3.66E-10 50 1.66E-09 0.13 2.15E-10
A1 100 70 1.12E-09 0.15 1.68E-10 90 3.34E-09 0.39 1.30E-09 30 1.08E-09 0.25 2.69E-10 50 6.56E-09 0.24
F8261 1.57E-09
70 8.93E-09 0.24 2.14E-09
50 3.13E-09 0.21 6.57E-10 Acetylated
70 1.02E-09 0.01 1.02E-11
To understand the mechanism of sorption, the moisture uptake data of
bananalpolyester composites were fitted to the equation 2.4.5.
The diffusion coefficients of the samples were determined using
equation 2.4.2. I he value of the diffusion coefficient is found to be the
Effect of Chemical Modification .... 2 13
maximum tbr the composites made out of silane F8261 treated fibres at 50 and
70°C. However at room temperature and at 90°C the value is found to be
much lower. The lowest value of the diffusion coefficient is found to be for
composites made out of silane A174 treated fibre. Except at 70°C the value is
found to be uniformly low in all the cases. The value of the diffusion coefficient
is indicative of the diffusion barrier operating in the system. The d i f i i o n
coefficient is found to be high in the case of the untreated composites also. The
value of the maximum water uptake is found to be increased with increase of
temperature in the case of treated fibre composites. The value is found to be the
highest in the case of silane F8261 treated fibre composites.
To understand the mechanism of sorption, the moisture uptake data of
banana/polyester composites was fitted in the equation 2.4.5 to obtain n and k
values. Table 2.4.2 gives the values of n and k for the various treated
composites. The value of n clearly shows that the diffusion process deviates
from the Fickian mechanism.
Steady increase of the value of k with increase of temperature, indicates
the improved interaction of the polymer with the solvent. However, in the
present case, the value of k shows an irregular trend.
.. ~p Effecr of Chemical Modtficorion .... 214
Table 2.4.2 Values of n and k for various treated composites -
Values of n and k for the various chemically treated composites
S a m ~ l e Tem~erature("C) n k ( d ~ m i n -")
Untreated
Acetylated
- ~ -- ~~ ~
Effect of Chemical Modification ... . 2 15
Root time(minutes)
Figure 2.4.4 Effect of various silanes on the water absorption at 70°C
The effect of various silanes on the water absorption at 70°C has been
compared in Figure 2.4.4. The untreated composite shows a multistage
mechanism in which, at the first stage an apparent saturation level is reached.
Later the water uptake increase further and finally level off. Due to weak
interfacial interaction, the second stage is associated with interface failure and
crack formation. The water absorption is found to be the highest for the silane
F8261 treated composite and the A174 treated composite. The increased water
uptake for the composite can only be considered to be due to the poor adhesion
in the case of the silane F8261 treated composites. Silane F8261 is a
fluorinated-coupling agent and is a water repellent. The fluorinated surface
leads to poor wetting between the fibre and the matrix, which invariably leads
to water penetration due to the free voids present in between the fibre, and the
matrix. Our earller studies showed that in the case of silane A174 treated
fibres, the hydrogen bond donating acidity is found to be the lowest compared
to other silane treated fibres [29]. The increased water uptake in the case of
. . Effect o/Chemical Modification .... 2 16
silane A174 treated fibre composites could be attributed to the low hydrogen
bond donating ability, which ultimately leads to the increased interaction with
the polar water molecules thereby increasing the water uptake.
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~ ~~~ ~ Effect ofChernical Modificafion .... 2 17
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Part B Poiy Physrcs., 38, 19. 2546 (2000)
Stress relaxation behaviour of short
banana fibre reinforced polyester
composites in tension was investigated
with special reference to the effect of
fibre loading and fibre treatment. It has
Part 2 - Chapter 5 been observed that incorporation of fibre
in the polyester matrix reduces the rate
STRESS of relaxation and that the nature of the
RELAXATION relaxation curve depends on the amount
BEHAVIOUR OF of fibre incorporated. Chemical treatment
has also been found to have a profound
SHORT BANANA FIBRE REINFORCED
POLYESTER COMPOSITES made out of banana fibre treated with
Results of this chapter have been wrnmunicated for publication in Journal of Polymer Engineering
NaOH. Finally, it is important to mentionI
that the study on the stress relaxation
behaviour is an important tool to
understand the interaction at the
polymerlfibre interface.
r
~~ - ~ -- Stress Relaxafion Behwiour of....2 19
2.5.1. Introduction
Polymeric materials are replacing conventional engineering materials
and information about the response of the material over a long period of time is
important. A stress-relaxation study provides another route to study time
dependence shown by materials and helps to gain an understanding of their
viscoelastic behaviour. Meaningful data on the behaviour of the materials can
be obtained by accelerated testing methods. Creep and stress relaxation are the
widely employed testing methods for this. Since stress relaxation represents the
basic time-dependent response of the material from which other time-dependent
responses such as creep can be obtained, the measurement of stress relaxation is
considered very important [I] . Moreover, creep and stress relaxation are the
most fimdamental experiments used for characterising the viscoelastic
properties of materials. The stress decay with time when a solid is subjected to
constant strain can be measured using these experiments. The stress relaxation
modulus of polymers is increased by rigid fillers and decreased by elastomeric
ones up to the point where dewetting or crazing becomes pronounced. The rate
of stress relaxation for rigid and elastomeric fillers increases after the onset of
dewetting [2, 31. I'he stress relaxation rate has been chosen as a way of ranking
adhesion between fibres and matrix. The slope of the stress relaxation curve is
chosen as a measure of the level of adhesion between the fibre and matrix. Flink
and Stenberg [4] used stress relaxation experiments to measure the adhesive
strength of cellulose fibres with natural rubber by analysing the relaxation
mechanism. Bhagawan et al. [ S ] studied the stress relaxation behaviour of short
~ ~~ -. Stress Relaxation Behaviour of.. 2 2 0
jute fibre reinforced rubber composites. The stress relaxation behaviour of
polyacetal/polyurethane blends has been studied in detail by Kumar et al. [6] . Rate
of loss of the relaxation modulus was found to be a nonlinear function of time.
In the present chapter, we report on the stress relaxation behaviour of
short banana fibre reinforced polyester composites with special reference to the
effect of fibre content and fibre treatment.
2.5.2. Results and Discussion
2.5.2a. Effect of fibre loading
Figure 2.5. I shows the effect of fibre loading on the stress relaxation of
the gum sample and the composites with different fibre loading.
. - Gum "..<'. -..*
~---- 20 % loading **%. *4, <> -- 30% loading 0.5 --7- 40% loading
*%
0 i 2 i h Log time(sewnds)
Figure 2.5.1 Stress relaxation curves of composites with different fibre loading
The relaxation curve of the gum sample shows a decrease in stress with
time. Reports are there in the literature that unfilled materials usually have only
- ~- Stress Relaxation Behaviour oJ ... 22 I
one relaxation mechanism [4]. The rate of stress relaxation is found to be
maximum for composites with 20% fibre loading. The increased rate of stress
relaxation in the case of composites with low fibre volume fraction can be
attributed to the lower interaction between the resin and the fibre. With the
incorporation of fibre, the nature of the relaxation curves changes. In the case
of the gum sample, the maximum slope is observed in the final regions of the
curve. The slope values, however, are lower than that of the gum sample
especially during the initial and final stages. Introduction of fibre hinders the
speed of rearrangement of the viscoelastic polyester molecules. This leads to
lowering of the slope values especially at longer times. The change in the
nature of relaxation can be due to the nature of the fibre. as well as due to the
physical and chemical changes involved during the relaxation process. Fibres
are also viscoelastic with the elastic nature predominating. Incorporation of
fibre induces more of elastic nature into the material. The stress-induced decrease
of the viscoelastic relaxation times should be related to the increase of free volume
produced by the dilation accompanying a uniaxial tensile deformation [5]. The
relaxation curvc shows an abrupt change in the pattern in all the samples with
fibre incorporated. The change occurs after a time span of about 200 seconds in
all the cases. With the increase in fibre loading, the slope of the relaxation
curve decreases during the time span lo3 to lo4 secs. The rate of relaxation at
the initial stages is found to be lower for samples with 40% fibre loading
(volume percent) compared to the gum sample and also samples with lower
fibre content. The reason for the observation can very well be athibuted to the
constraints induced by the fibre to the flow behaviour of the matrix. When the
- Stress Relaration Behaviour oJ ... 222
composite sample is subjected to stress, both chemical and physical deformations
take place in the composite[6]. Upon the application of stress for a long time, the
polymer chains, which have been stressed, tend to unwind and that give rise to a
lowering of the initial stress. The polymer chains tend to get attached to the
filler particles as well. The change in the slope of the relaxation curve can be
attributed due to two reasons. One, the elastic nature of the fibre which induces
more stress relaxation effect and the other the improved stress transfer between
the fibre and the matrix. At higher fibre loading, (40 volume percent), the slope
of the curve is decreased considerably due to better stress transfer between fibre
and matrix, which is proved to be the optimum fibre loading in bananalpolyester
composites, in the present study [7]. At lower fibre loadings, fibres instead of
acting as reinforcements, act as flaws, which facilitates faster relaxation. This is
evident from the slope changes of the curves in Figure 2.5.1 and the rate of
relaxation of the respective composites, which is shown in Table 2.5.1. The rate
of relaxation of the gum sample and the different composites are compared in
Table 2.5.1.
In the case of the gum sample, the rate of relaxation is found to be
higher at longer times than during the initial stages. At lower fibre loading, i.e.
at a loading of 20 and 30%, the r.ate of relaxation is found to be more or less
the same during the initial as well as the final stages. At the optimum fibre
loading of 40%, (volume percent) however, the rate of relaxation is lower
towards the final stages than during the initial stages.
. - -- Stress Relmaflon Behoviour of ... 223
Table 2.5.1 Rate of relaxation of the gum sample and the different composites
7 Fibre Loading
(sec) Slope x 10 .~
Gum 20 30 40
0 10' 8.9 12.9 10 6.93 ..
12.43 18.6 13.3 11.17
16.20 8.48 11.53 7.78
2 1.28 11.9 12.75 4.93 -. ..
The lowering of the rate of relaxation during the later stages can be
attributed to the completion of the rearrangement of the molecules in the
composite. The relaxation mechanism can also be attributed to the intrinsic
stress relaxation behaviour of the fibre. Under the stressed condition, the
individual fibre can undergo various molecular and cellular rearrangements
within the three dimensional multicellular network. In the composites the fibres are
physically and chemically bound to the matrix and the independent behaviour of
the fibre is nullified to an extent. A similar trend in the rate of relaxation is
reported in the case of pineapplelpoly ethylene composites [ S ] . The modulus of
relaxation also shows the same trend and is shown graphically in Figure 2.5.2.
While the gum sample shows a 48% reduction in the stress relaxation modulus,
composites with 40% fibre loading shows a 31% reduction.
- -- - Slrrss Relaratrun Behavruur uf 224
log tima(sec)
Figure 2.5.2 Stress relaxation modulus of composites with different fibre loading
The stress relaxation modulus is found to be the highest for composites
with 40% fibre loading. The modulus values are found to be more or less the
same for composites with lower fibre loading as well as for the gum samples.
2.5.2b. Effect of fibre treatment
Figure 2.5.3 shows the effect of chemical treatment on the stress relaxation
curves. The fibres have been treated with various silanes and also with alkali. Silane
treatment of glass fibres have been found to reduce the relaxation rate especially after
longer times because of improved adhesion. The addition of coupling agents and the
treatment with alkali reduce the relaxation rate compared to the gum sample.
The initial relaxation is found to be lower for the alkali treated and the silane
A174 treated composites when considering the behaviour of untreated sample.
The final relaxation values however, are found to be the lowest for the untreated
libre composites. Of' the various chemical treatments, beatment with alkali and
with silane A 15 1 has given the lowest rate of stress relaxation at the final stages.
~~ .~ --- ~ - Stress Relaxation Behaviour oJ ... 225
J I 0 1 2 3 1
log time(seu)
Figure 2.5.3 Stress relaxation curves of composites with different fibre treatment
The SEM of the alkali treated fibre is given in Figure 1.2.8 (Section 11; Part 1,
Chapter 2). We have reported in an earlier paper regarding the chemical
modification and the surface characterisation of banana fibres [9,10]. The reduction
in the decay in stress in the initial stages can be attributed to the improved
fibrelmatrix adhesion in the case of the alkali treated fibres. Unlike treatment
with other coupling agents, treatment with alkali brings about the dissolution of
the lignin and the hemicellulose and thereby the availability of other replaceable
hydrogen atoms within the cellulose. In addition, treatment with alkali improves
the fibre surface area. This brings about better adhesion of the fibre and the
matrix leading to lower rate of relaxation at all stages. However, in the case of
composites made out of the silane F8261 treated fibres the bonding is less
effective and application of stress leads to the scission of bonds, established
between the fibre and the matrix. The reason for the breakage can be associated
with the presence of fluorine atoms in silane, which lowers the bonding. The
type of bonding in silane treated composites can be explained as due to silanol
from silane and the hydroxyl groups of the cellulose. Figure 1.2.12 (Section 11;
~. - -- Stress Relaxation Behuviour of ... 226
Part 1, Chapter 2) shows the schematic representation of the bonding between
the fibre and the matrix.
The stress decay at the initial portion is found to be the lowest for the
alkali treated and silane A174 treated composites, compared to the other treated
samples. The stress relaxation is affected only at longer times. At longer times,
on the application of the stress, bond scission occurs. Theories are there in the
literature regarding the formation of a flexible deformable phase between the
fibre and the matrix on silane treatment [l 11. The application of stress stretches
this deformable layer initially. After longer times, this deformable layer retracts
which is felt as the increased decay in shes?,. The response of all the silane
heated composites except the A151 and the A174 treated fibre composites is more
or less the same. The rate of relaxation of the differently heated composites is
compared in Table 2.5.2.
Table 2.5.2 Rate of relaxation of the differently treated composites
I I Fibre Treatment I
Among the treated composite, the stress retention however, is found to
be the maximum for composites treated with the silane A151 based on the rate
Stress Relaxation Behaviour 01....227
of relaxation of the final step. Our earlier studies on the determination of
polarity parameters of the various silane treated fibres have shown that the
ET(30) parameter which is indicative of the overall polarity is found to be
maximum for fibres treated with the silane A151 [9]. The increased polarity
leads to the better adhesion between the fibre and the matrix leading to
composites with better strength than the others, which leads to the higher stress
retention.
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Stress Relaxation Behmiour oJ ... 228
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