hybrid composites of short banana fibre and glass...
TRANSCRIPT
HYBRID COMPOSITES OF SHORT BANANA FIBRE AND GLASS FIBRE
Part 3 - Chapter I
HYBRID COMPOSITES OF SHORT BANANA
FIBRE AND GLASS FIBRE:
MECHANICAL PROPERTIES,
STRESS RELAXATION AND
WATER ABSORPTION BEHAVIOUR
Part of this chapter had been pubiishec in Polimery nr 11-12.1999
1 Abstract
Variations in the tensile and impact
properties of banana fibre reinforced polyester composites caused by the addition of glass fibre have been
analysed. Banana fibre in combination
with glass has proved to be excellent for making cost effective composite materials. The effect of the arrangement of glass and banana fibre in the preparation of composites has also been studied. A volume fraction of 0.1 1 glass mixed with banana fibre has given 54.5
% increase in the tensile strength and 196 % increase in the impact strength of the composites. Linear increase in tensile strength has also been noted as a result of the increase of glass. The tensile strength has shown the highest value when a relative glass volume fraction of 0.17 is used and an interleaving arrangement of ghss and banana fibre is followed. However, when lower volume fraction of gbss is used, an intimate mixture of banana fibre and glass shows the highest tensile strength. The impact strength
shows the highest value when a relative glass volume fraction of 0.11 is used. Stress relaxation and water absorption behaviour of the hybrid composites were also investigated. Compared to the gum
sample, rate of stress relaxation of hybrid composites with very low and very high glass content was found to be much higher during the initial stages of relaxation. Water absorption behaviour showed a multistage mechanism in ail hybrid composites. The multistage mechanism was found to be associated with the delamination in hybrid composites.
-- - -- Hybrrd Composrtes of Short Banana . . .23 1
3.1.1. Introduction
Mu~ticom~onent composite materials comprising of two or more
families of fibres have been attracting the attention of researchers these years.
This is because, the usage of one type of fibre alone has proved to be inadequate
in satisfactorily tackling all the technical and economic problems confronted by
them while making fibre reinforced composites. These types of composites
introduce additional degrees of compositional freedom for its making and
provide yet another dimension to the potential versatility of fibre reinforced
composite materials. Therefore the ultimate strength of the system is the stress
level at which the elongation of the system has reached the ultimate elongation
of the fibre family. Attempts have been made by other researchers for the
preparation of hybrid composites of natural fibre and synthetic fibre to improve
the mechanical properties of the composites. In this chapter, attempts have been
made to improve the mechanical properties of the composite by the
incorporation of glass fibre, based on the reports of other researchers [I-91.
3.1.2. Results and Discussion
3.1.2a. Tensile stress-strain behaviour
In all the composite samples considered in the present cases, the total volume
hction of the fibres, namely, banana and glass were kept constant at 0.4. Relative
volume hctions of glass ranging born 0.03 to 0.17 were incorporated in the various
samples prepared
~
HybridComposires of Short Banana . .. ,232
Figure 3.1.1 Stress-strain behaviour of neat polyester and bananalpolyester with fibre volume fraction 0. 4
Figure 3.1.1 represents the tensile stress-strain behaviour of neat
polyester and banana polyester with fibre volume fraction 0.4.
Figure 3.1.2 delineates the tensile stress-strain behaviour of bananafglass
hybrid composites with constant fibre volume fraction (banana fibre and glass
fibre) of 0.4 (within experimental error) and varying glass volume fraction. The
details of samples marked A to F is shown in Table 2.2 (Section I; Chapter 2).
Figure 3.1.2 Stress-strain behaviour of banana-glass hybrid composites with constant fibre volume fraction (banana fibre~and glass-fibre) of 0 .4 (within experimental error) and varying glass volume fraction
--- -~ Hybrid Composilrs of Short Banana . . . ,233
Increase in glass volume fraction changes the general nature of the stress
strain curve. Aveston and Silwood [lo] studied the general nature of the stress-
strain curve of hybrids. Hybrid stress-strain curve, theoretically, has different
slope in the initial and final portion. When subjected to tension, the fibres
break over a range of stress instead of a single value predicted by theory. Stress-
strain curve ofthe hybrid composites with different glass volume fractions show an
inflection at a certain point. The point of inflection rises with an increase in glass
content except for samples with mark D. The point of inflection corresponds to the
limiting elongation of high modulus glass. Short and Summerscales [ l 11 observed
that the minimum strength of the hybrid is proportional to the critical content of the
low modulus fibres. If the content of the low modulus fibres in the composite is
greater than the critical content, an inflection occurs in the stress-strain curve
corresponding to the limiting elongation of the high modulus material. The
fibrehatrix interface has a lot to do with the form of the stress-strain curve [12].
Increase in glass content helps in the mutual reinforcement of strength
characteristics by the low elongation glass and high elongation banana fibre.
Figure 3.1.3 shows the variation of tensile modulus of the hybrid samples
with glass volume fraction. Fibre length, fibre aspect ratio, relative moduli of the
fibre and matrix. thermal expansion mismatch etc. are all-important variables that
control the performance of a composite. Stiffness of the material has been
calculated as tangent modulus at elongations of 2, 4 and 5%. The modulus values
increase with increase in glass volume fraction. Glass fibre has a higher tensile
modulus than banana fibre and incorporation of high modulus glass increases
the tensile modulus of the composite.
Hybrid Composites of Short Banana . . . .234
Glass volume fraction
Figure 3.1.3 Variation of tensile modulus with glass volume fraction
volume fractlon.of glass
Figure 3.1.4 Variation of tensile strength of the hybrid composites with variation of glass fibre volume fraction [total volume fraction of the two fibres is 0.41
Figure 3.1.4 shows the variation of tensile strength of the samples with
respect to the variation of glass fibre volume fraction when the total volume
fraction of the two fibres is kept constant. Tensile strength of the samples increases
linearly wit11 the increase in glass volume fraction. Hams and Runsell [13] have
noticed that occurrence of a hybrid effect, negative or positive will depend on
the relative volume fraction of the two fibres. As a result, the strength of thc
hybrid compositc in tension u~~iforinly increases wit11 glass content. FIowever,
at relativcly higher glass content, failure by delaminatiol~ occurs cmd the tensile
strcngth values show only a slight ellhancernent,
Figure 3.1.5 a,b,c Optical photographs of the failed samples with glass volume fraction 0.1 I
Optical photographs of the failed samples in figure 3.1. 5 a, b and c
show delamination between the fibre layers. 111 hybrids of carbon and glass the
presence of higher cxtcnsion glass fibre has bee11 found to reduce the probability
Hybrid Composites c?f Shorl Banana . . . ,236
of failure of the lower extension carbon fibre resulting in a higher breaking
strength of the carbon fibres [14].
In the present study, thc increased tensile strength of the hybrid
composite can be attributed to the presence of high modulus glass fibres. When
the volume fraction of glass is changed from 0.1 1 to 0.15, the increase in tensile
strength is marginal. At high glass volurne fraction, the fracture occurs in the
composite ~nainly by interlayer delamination.
Figure 3.1.6 a,b,c SEM photographs of the composites with glass volume fraction 0.03, 0.11 and 0.15
~~~~~~ ~- - Hybrid Composites ofshort Banana . ... 237
3.1.2.b. Effect of banana glass layering on the tensile strength
Different layering patterns were studied for composites marked A, C,
and F. Figure 3.1.7 represents the various tensile strength values of the different
layering patterns.
Figure 3.1.7 Tensile strength values of the different layering patterns
In composites marked A and C an intimate mixture of the two fibres
gave the highest tensile strength. Fischer et al. [I51 have found that when the
fibres are more intimately mixed. failure by delamination will be more difficult
because of the greater energy involved in creating the large amount of new
surface in an intimate mix than that required to cause delamination of a layered
hybrid. In composite marked F, the tensile strength for layering L2 and Lj are
almost similar in intimately mixed hybrids, the area of the high elongation
component to the low elongation component interface per unit volume will be
high compared to the composites where the fibres are not intimately mixed. In
an intimately mixed composite there will be only a small distance from the
failed fibre to the unfailed fibre. The full reinforcing strength therefore, will be
. . ~~~
Hybrid Composites of Short Banana .. . ,238
redeveloped in the failed fibre within a short distance of the fracture surface. When
individual glass and banana layers are made, the tensile strength values are found to
be lower than that in an intimate mixture for composites with low glass content.
Bader and Manders [I61 noted that the hybrid effect was maximum only when the
layer thickness had a certain minimum value. Mohan et al. [2] also noted that when
the glass fibre reinforced plastic shell thickness was small, the resistance to withstand
strain was insuficient and thus the specimen failed prematurely by fibre buckling.
3.1.2~. Impact strength of banana-glass hybrid composites
The impact performance of fibre-reinforced composites depends on
many factors including the nature of the constituents, fibrelmatrix interface, the
construction and geometry of the composite and test conditions. The impact
strength of the composites with varying glass volume fraction is shown in
Figure 3.1.8. lt is found to increase with increasing glass volume fraction.
A maximum value is observed around 0.1 1.
20 ! , . , . I . , . 0.00 0 05 0 10 0 15 0 20
volme fmason d glass
Figure 3.1.8 Impact strength of the composites with respect to glass volume fraction
.. Hybrid Composites ~/Short Banana .... 239
The impact energy of a composite occurs by factors like matrix Fracture,
fibretmatrix debonding and fibre pull out. Even though fibre pullout is believed
to be the important energy dissipation mechanism in long fibre reinforced
composites it occurs in short fibre composites as well [17]. The applied load,
transferred by shear to the fibres may exceed the fibretrnatrix interfacial bond
and debonding may occur. The frictional force along the interface may transfer
the stress to the debonded fibre. If the fibre stress level exceeds the fibre
strength, fibres may undergo fracture. The fractured fibres may be pulled out of
the matrix, which involves energy dissipation [18]. Many authors have stated
that the energy dissipated by fibre fracture is small [19]. The impact strength of
the composites increases linearly upto an optimum value of 0.11 and then
decreases slightly. The slight lowering of impact strength can be attributed to
the change in energy dissipation mechanism. At high glass fibre content, the
fracture mechanism is mainly fibre fracture, due to the brittle nature of glass.
However at lower glass volume fraction, the fracture mechanism is mainly by
fibre pullout due to the presence of higher volume fraction of banana fibre. A
synergistic effect of the two fibres leads to a linear increase initially.
3.1.2.d. Effect of glass-banana layering on the impact strength
Mallick and Broutman 1201 have reported that stacking sequence is more
important. than composition in determining toughness, and that different lay-
ups maxirnise different toughness parameters such as total energy, initiation energy or
propagation energy. Ln this study also, it is found that the arrangement of the fibre
---- ~. Hybrid Composites of Short Banana . . . .240
within the composite affects the value of impact strength. Figure 3.1.9 shows
the effect of layering on the impact strength.
Layering
Figure 3.1.9 Effect of layering on the impact strength
The highest value is obtained when banana and glass are kept as
interleaving layers. In this arrangement, the core thickness is very small. When
a crack tip approaches a fibre, the crack crosses the fibres and cuts them as well
as the matrix. Then the crack changes its direction and moves through the
matrix parallel to the fibres. Such debonding fracture consumes more energy by
creation of more surface area within the sample.
The impact strength shows a decrease with the decrease in the number of
layers. Unlike tensile strength, intimately mixed composites show the lowest
impact strength. Short and Summerscales [21] have reported a negative hybrid
effect in fracture tests of intimately mixed composites. Harris and Bunsell [13]
have reported that intimately mixed composites are inferior to interply lay-ups
in impact resistance because of the finer state of subdivision.
Hyhrid Conlposife.~ ufShor./ Banclnn . . . .24 1
Optical micrographs of the hybrid colnposite samples are givcn in
Figures 3.1.10a and b. Figures 3.1.10 a and b represents the crack propagation
at the fibrelmatrix interface.
x 100 x 100 a b
Figure 3.1.10 a,b Optical photographs of hybrid composite samples
3.1.2.e. Theoretical modelling
Hybrid reinforcing effect of the two fibres was theoretically calculated
using parallel and Hirsch model [22,23].
According to Parallel model
where X,, Xr, and X,,, are chai-acteristic strength property of composite, fibrc
and matrix respectively.
According to Hirsch's model,
..- Hybrid Composites ofshort Banana .. ..242
where x varies between 0 and 1. The value of x determines the stress transfer
behveen fibre and matrix. The value of x is the determining factor in describing
the real behaviour of short fibre composites [24]. The composite strength
calculated using the above model was incorporated in the additive rule of hybrid
mixtures,
where Xh is the characteristic property of the hybrid composites. Theoretically,
the increase in volume fraction of glass increases the tensile strength linearly
upto a certain volume fraction thereafter a slight decrease is predicted.
0.0 0.2 0.4 0.6 0.8 1 .O 1.2
Volume fraction of glass fiber
Figure 3.1.11 Comparison of the experimental and theoretical values
The experimentally determined tensile strength values are found to be
higher than the theoretical predictions emphasising a positive hybrid effect.
Figure 3.1.1 1 represents the comparison of the experimental and theoretical
values. The relative glass fibre volume fraction based on the total fibre content
is represented in the X-axis
~
Hybrid Composiles oJShorl Banana .. ..243
3.1.3. Effect of Hybridisation on Stress Relaxation
The physico chemical differences in different systems give rise to
difference in stress relaxation mechanism. The stress relaxation mechanisms are
complicated in short fibre composites, in that several factors are included. The
fibre length distribution, non-uniform bulk distribution of the fibres all affects
the stress relaxation mechanism. Relaxation in stress in a system can occur due
to a combination of factors namely molecular level arrangement in the fibre as
well as in the polymer. Chain scission between different molecular layers can
occur and also molecular slippage. Molecular rearrangement in turn can give
rise to an increase in crystallinity of the system.
0 1 2 3 4 log time (seconds)
Figure 3.1.12 Effect of hybridisation on the stress relaxation of the composites
Details of the samples A to F are given in the experimental chapter.
Samples refer to composites with different glass volume fractions and with
three layers.
-- ~ -- Hybrid Composites of Short Banana . . ..244
The amount of the amorphous and crystalline phase in the system also
affects the nature of the stress relaxation behaviour. In a randomly arranged
composite system, as the one under study, rearrangement of the fibres is also a
possibility when the system is subjected to stress.
Figure 3.1.12 shows the effect of hybridisation on the stress relaxation
of the composites.
Composites with glass volume fraction 0.03,0.15, and 0.17 were used in
the preparation of composites with glass mat as the core material. In all the
samples, there were three layers of the material, banana being the skin and glass
the core. The nature of the stress relaxation curve seems to be more or less the
same for composites with glass volume fraction 0.15 and 0.17. Non-linear
stress relaxation curve has been reported in thermosets such as polyester and
phenolics 1251. Stress relaxation behaviour reported in the case of oil palmlglass
hybrid composites also showed a reduced relaxation compared to the unhybridised
sample [26]. Composites with a glass fibre volume fraction of 0.03 have given a
stress relaxation curve with a change in the relaxation mechanism at around a time
span of 100 seconds. The difference in the relaxation curve could be explained
as due to the difference in the behaviour of the glass fibres, which form the
core. The intrinsic stress-relaxation behaviour of the reinforcing fibre plays an
important role in the relaxation process. Raman spectroscopy studies on the
deformation behaviour of cellulose fibres revealed that lignins are not the load
camers and that eventual failure of the fibre occurs due to slippage [27]. When
uniaxial tensile stress is applied, because the breaking strains of glass and
-- .- -- Hybrid Composites ~/Short Banana . . . .245
banana fibres are different, both the fibres behave differently. The difference in
response of the two fibres is felt prominently in the stress relaxation curve of
composites with fibre volume fraction 0.03 i.e. sample marked A. In this sample,
the volume of banana fibre is comparatively higher and the corresponding higher
critical defects occurring on the fibre surface also lead to a difference in
response from the other systems. Apart from the intrinsic properties of the
interface or interphase, the stress distribution along the embedded fibre also
plays an important role in the overall performance of the composite [28]. Glass
fibres being brittle and of low elongation, behave differently from the banana
fibre when subjected to tensile stress and the change in the slope of the
relaxation curve can be explained as due to this. The fast decay in stress at the
glass fibre volume fraction 0.03 can be explained as due to the weak interfacial
bonding. The amount of glass fibres is not enough to impart high strength or
better interaction because the volume fraction is very low. In addition, better
dispersion of the fibres is not possible when the volume fraction is low.
The rate of stress relaxation is found to decrease with increasing glass
fibre content the reason being the difference in the nature of the two fibres.
Elastic glass fibre on combination with viscoelastic cellulose fibre the overall
nature of the relaxation curves gets affected. Change in the shape of the fibre tip
affects the maximum stress. The point of maximum stress concentration in a
square ended fibre system is located at a short distance from the fibre end,
whilst that in a round-tip fibre system is at the fibre tip. Since the two fibres are
different in shape, it results in a difference in stress concentration between the two.
However, at higher glass volume fractions the effect of the fibres opposes each
~- ~ ~~
Hybrid Composites ofShort Banana . . . ,246
other and the slope of the stress relaxation curves are found to be in the same
range. It is interesting to note that the relaxation rate is almost the same for
composites with high glass fibre content. Composites with high cellulose
content however show a higher rate of stress relaxation with a greater slope. In
other words, the stress relaxation properties are believed to be controlled mainly
by the cellulose fibre component.
3.1.4. Water Absorption Behaviour of the Hybrid Composites
Figure 3.1.13 shows the moisture absorption curves for various hybrid
composites with different relative glass fibre volume fractions of banana-glass
at room temperature.
0 20 , 40 60 80 1W
Rod tirne(minutes)
Figure 3.1.13 Water absorption curves for various fibre volume fractions of banana-glass hybrid composites at room temperature
The water absorption curves show a multistage mechanism in the hybrid
composites studied. In all the samples, glass was kept as the core material and
banana as the skin. The peculiar sorption curves of the composite can be
attributed 10 the nature of the polymer as well as that of the fibre. The initial
-- ~
Hybrid Composites of Short Banana . . . ,247
absorption of water into the composite occurs mainly through the matrix
material and partly through the fibre. Based on the fibrelmatrix interaction, the
water diffusion occurs through the interface and from there to the bulk material.
In addition, transport of water takes place through micro cracks, which occur,
on the surface of the composite and also through micro channels, which occur
inside the material due to defects [29]. The diffusion of water through natural
fibres has been reported to be anomalous [30]. Reports are also there in the
literature on the penetration of solution through the polyester, which is
facilitated by capillary effects through the matrix [31] and wicking along the
polymerlglass interface [32]. Composites with higher glass content, 0.16,0.17 i t .
where banana fibre content is low is seen to give a two step water absorption curve
whereas composites with glass volume tiaction, 0.07,O.ll and 0.15 are found to give
a three stage water absorption mechanism. Figure 3.1.14 shows the variation in
equilibrium water content of the composites with increasing glass volume fraction.
0 6
13 7
' L - W 3 0 2 004 006 Glass 008 volume 010 012 fract~on 014 016 018 020
Figure 3.1.14 Variation in equilibrium water content of the composites with increasing relative glass volume fraction
-- ~ -- Hybrid Composiles ~/Short Banana . . . ,248
In all the samples, the same geometry of glass as the core and banana as
the periphery material was followed. The response of the addition of
impermeable fibres to permeable fibre composites is clear from the graph. The
maximum water content decreases with increase in glass volume. The
observation stems from the fibre nature. The difhsion mechanism is obviously
multistage in all the samples. The initial portion of the moisture absorption
curve is linear. The mechanism changes after that. In the hybrid composites, the
change in mechanism is attributed to the delamination occurring in the
composites. Penetration of water into the matrix causes absorption of water by
the fibres as well. The rate of absorption of water is different for the two fibres.
Glass fibres principally consist of silicates of various metals. When immersed in
water, the nucleophilic attack of the - O H at silicon takes place, with the
formation of a transition complex followed by the breakage of the bonds, and
formation of new bonds. In the case of cellulose fibres, hydrogen bonding
through the --OH of the glucose molecule is the principal water absorption
method. The absorption of water causes delamination of the two layers of the
fibre as well as the delamination of the fibre and matrix. This causes finther
absorption of water into the free voids. The steep change in the absorption curve
can be explained as due to the uptake of water into the free voids created by the
delamination.
The nature of water absorption through the polyester resin has been
reported to be due to various reasons by several authors. A polyester chain end
-OH, oxygen of the ester links or the residual cobalt ions are all sites for
hydrogen bond tbrmation [33].
- - -- Hybr~d Composrtes of Short Banana 249
The diffusion coefficient D can be calculated from the equation
where 0 is the slope of the linear portion of the sorption curves and h the initial
sample thickness. The diffusion co-efficient characterises the ability of the
solvent molecules to move among the polymer segments. The value of diffusion
coefficient in the case of samples with different glass fibre volume fraction is
considered.
The value of diffusion coefficient is found to be the lowest in the case of
samples with high glass fibre content at room temperature (Table 3.1.1).
However, at 50 and 90°C the value is found to be the highest at high glass
volume fraction. This may be due to the possible delamination and crack
formation. The moisture permeability in the case of natural fibres is
responsible for thahigh diffusion coefficient of the samples with high banana
fibre content. The maximum water uptake of the composites with high banana
is found to be the highest at 90°C. The higher water uptake at high
temperature can be attributed to the increased capillary action. In other words
diffusion of water through the capillaries of the natural fibre increments with
increase in temperature.
- Hybrid Composites of Short Banana . ... 250
Table 3.1.1 The values of the diffusion coefficient of the various hybrid samples at different temperature
Diffusion Sorption Permeability Temperature
Sample ("C)
coefficient, coefficient, coefficient, - D (cm2i') S(g/g) P (cm2 s-')
30 2.62E-10 0.16 4.2E-11
Increase in temperature opens up the pores of the natural fibre, thereby
increasing the water uptake. The strong sorption power is the physicochemical
peculiarity of natural fibre. A strong chemisorption of water molecules by
interaction with the hydroxyl groups of the polymer takes place, followed by a
multilayer sorption at medium relative humidity and a capillary condensation of
free water at high relative humidity.
Cellulose-water interaction depends strongly on the supramolecular
structure of the specimen in question, and on the factors such as temperature.
~
Hybrid Composires ~/Short Banana .... 25 1
For example, from this study it is clear that the maximum water uptake of the
sample A is only 0.01 at temperatures 30, 50 and 70°C. At 90°C, the
maximum water uptake is increased to 0.09 due to the activation of the
diffusion by temperature and also due to the delamination at high temperature.
The increased absorption at high temperature also points to the fact that curing
reaction was over in the resin. Increase in temperature gives rise to resin
cracking and thereby water absorption at higher temperature.
The Qt values have been determined in all hybrid composites systems.
At high glass volume content the value of Q, was found to be lowered with increase
of temperature. Difhsion coefficient of the samples were calculated based on Q,
values. No regular trend in diffusion coefficient could be observed. The sorption
coefficient of the composite also has been calculated using the equation 2.4.3.
The diffusion coefficient is related to the equilibrium sorption of the penetrant.
The permeabilities, P, of the composite samples to water molecules can be
expressed using the equation 2.4.4 (Section 11; Part 2, Chapter 4) [34].
Permeability therefore talks about the net effect of sorption and
diffusion. Ln most cases, value of the permeability coefficient is found tc
increase with temperature. Development of micro cracks on the surface and thc
bulk of the material as a result of the effect of high temperature and moisture
environment can be given as the reason for the increase. Moreover, the value
of the permeabil~ty coefficient is found to be the lowest in the case of samples
with the maximum glass fibre content Peeling and surface dissolution of the
composite take place as a result of these crack developments [35].
~ ~~
Hybrid Composites of Short Banana . . . ,252
'To understand the mechanism of sorption, the moisture uptake data of
bananalpolyester composites w a fitted to the equation 2.4.5.(Section 11; Part 2,
Chapter 4).
In this equation, n and k give an idea about the mechanism of diffusion
that takes place inside the composite. Table 3.1.2 gives the values of n and k for
various hybrid composites. It is interesting to note that the values of n and k
increase with increase of temperature. The maximum value of n and k are
obtained at 90°C. However the k value of the untreated composite shows some
variation. The increase of k at high temperature clearly shows the high extent
of interaction of water molecules at high temperature. The low value of n
(1 0.5) clearly shows that the mechanism of transport deviates from Fickian.
Hybrid Composites of Short Banana .... 253
Table 3.1 -2 Values of n and k for the various hybrid composites
Values of n and k for the various hybrid composites .
Sample TemperaturerC) n k (gig min-")
30 0.15 0.07
Untreated
- --- -- Kybr~d Cornposlrrs of Short Banana .... 254
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S. R. Harogoppad and T. M. Aminabhavi, J. Appl. Polym. Sci., 42,2329
(1991)
J. Zhou and .I. P. Lucas, Comp. Sci. and Tech., 53,57 (1995)
Part 3 - Chapter 2
DYNAMIC MECHANICAL ANALYSIS OF
BANANAIGLASS HYBRID FIBRE REINFORCED POLYESTER
COMPOSITES
Results of th~s chapter have beer communicated to the Journal of the Institute of Materials, Malaysia
Abstract I The importance of dynamic mechanical analysis as a tool in the study of the behaviour of composite structures is paramount. Hybrid composites of glass and banana fibre in polyester matrix were subjected to dynamic mechanical analysis over a range of temperature and frequencies. Parameters like storage modulus (E'), loss modulus (E") and loss factor or damping efficiency (tan 6) were determined in a resonant frequency mode. All the properties were compared with those of the gum samples and the un-hybridised composites. At temperatures above T,, the storage modulus values decrease even with the addition of glass fibre for the geometry where glass is the core material. The value of the storage modulus above the glass transition temperature is found to be still lower than that below the glass transition temperature in the particular geometry with glass as the core and banana as the skin. The reason for the peculiar behaviour can be associated with the difference in the nature of the interface of the two fibres with the polyester matrix and also with the delamination occurring in the material due to the particular geometry adopted. The loss modulus curves and the damping peaks were flattened by the addition of glass. Effect ~f the glass-layering pattern on the properties of the composite were also investigated. Layering pattern or the geometry of the composites was found to have a profound effect on the dynamic properties of the composite. An intimately mixed composite gave the highest storage modulus values in all compositions. The values are consistent #ith the results of tensile strength. The tan 6 curves were affected by the layering pattern followed and gave insiaht into the interactions in the
~- ~.~ Dynamic Mechanical Analysis of:. . .257
3.2.1. Introduction
Dynamic mechanical spectroscopy is an effective tool to understand the
structure-property relationship and interface in multiphase polymer systems [I-131.
Hybridisations of banana fibres with glass fibres have been proved to improve
the mechanical performance and the water absorption behaviour of the
composites [14]. We have reported on the mechanical behaviour of hybrid
composites in an earlier chapter. Relatively small volume fractions of glass
ranging from 0.03 to 0.17 were incorporated along with banana fibres in the
preparation of composites, keeping the total volume hction of the two fibres a
constant equal to 0.4. In addition, different layering patterns were followed to
understand the effect of layering patterns on the mechanical properties and water
uptake of the composites. The main objective of this chapter is to investigate the
performance of the hybrid composites under dynamic conditions. The effect of
the relative glass volume fraction as well as the layering patterns on the
properties of the composites like storage modulus, loss modulus and damping
peaks are proposed to be investigated. A literature search has shown that
investigation of this q p e has not been carried out in the last ten years for hybrid
composite systems of synthetic and natural fibres. The nature of the storage
modulus and damping peaks give an idea about the load transfer efficiency
between the polymer and the reinforcement and it is proposed to investigate and
choose the banana/glass ratio based on the nature of the results.
In this chapter we report on the dynamic mechanical behaviour of
bananalglass hybrid composites with special reference to the effect of relative
-- - - Dynamrc Mechanical Analysis of:. . ,258
glass volume fraction and layering pattern on the storage modulus, loss modulus
and the damping behaviour of the composites.
3.2.2. Results and Discussion
3.2.2a. Storage modulus
DMA is one of the best techniques that provide information regarding
the structure of the material as well as the quantitative data regarding the
modulus of the material. The effect of temperature on the storage modulus of
the various hybr~d samples with different glass fibre volume fraction, at a
frequency 10Hz. is given in Figure 3.2.1. The storage modulus values of the
hybrid composites have been compared with those of the unhybridised samples
as well as the gum samples. It may be noted that in all the samples referred to as
A to F in Figure 3.2.1, there are three layers with glass as the core and banana as
the skin. The details of the layering pattern and the volume of glass fibre used are
shown in the experimental chapter. The samples marked A to F shows composites
with increasing relative glass volume fraction, ranging from 0.03 to 0.17. Plots
of storage modulus allow for the direct comparison of a variety of materials that
may be considered as candidates for an application. Any drop in the storage modulus
points to the tnolecdar motions happening in a material. Addition of reinforcement
to a polymer increases the modulus of the system because the reinforcement
prevents the free molecular motions to an extent. The effect is usually found to
be noticeable at temperatures above the glass transition than below it, because
molecular motions become prominent above the glass transition. The plot of E'
over the whole range of temperature reflects the effectiveness of the stress
- ~~~ ~ -~ ~ .- Dynamic Mechanical Analvsis of.. . .259
transfer occumng between the fibre and the matrix. The value of the storage
modulus has been reported to be proportional to the interface bonding by other
authors [1 51.
Figure 3.2.1 Effect of temperature on the storage modulus values of hybrid composites
In all the samples considered in Figure 3.2.1, glass has been used as the
core material and banana fibre as the skin. The storage modulus values for
samples with glass as the core material is found to be lower than that of the gum
samples at temperatures above and below Tp. However, at the glassy region, of
all the hybrid composites considered, the storage modulus values are found to be the
highest for samples with the glass fibre volume fiaction, 0.16. i.e. samples marked E.
The modulus values however, drop steadily at a temperature around 55OC. The drop
in the storage modulus value at temperature below the glass transition
temperature of' polyester i.e. in the glassy region can be explained as due to the
.~-. ~ ~ Dynamic Mechanical Analysis af....260
following reasons. The glass fibre being the core material and the banana fibre
being the skin, the stress will be taken up by the low modulus banana fibre
initially. This leads to composite failure by the initial delamination between the
two fibre layers. After the initial drop in modulus value at the temperature
around 55"C, the composite samples show a second drop in modulus at the
temperature range of 120-150°C. Thereafter, the modulus values just level off.
The reason for the higher storage modulus at lower temperature in all
composite samples, compared to the values at higher temperature can be
attributed to the delamination occurring in the samples especially at higher
temperature. At higher temperature and also at dynamic loading conditions, the
bonding between the different fibre layers gets affected more. When the
temperature is increased, the difference in strength between the two fibres
becomes more. 7he main reason for the delamination between the two layers is
due to the difference in the ability to cany stress by the two fibres and also the
difference in the interfacial properties of the two fibres with the polyester matrix.
It has been reported that glass fibres have the highest strength when
absorption of moisture is eliminated at high temperatures, because on heating,
there is high elastic and plastic deformation which promotes healing of the
micro defects and micro cracks developed on the fibres [16]. This strength
difference gives rise to higher shear stresses in the composite and also to a
decrease in storage modulus value. In all the samples considered, a layer of glass
was kept in between the banana layers. Optical photograph of the composite with
glass as the core material is given in Figure 3.2.2.
Uyjnrnic Mechn17icul Analqsis of: . . .26 I
XlOO
Figure 3.2.2 Optical photograph of the failed composite with glass as the core material (sample A)
Dynamic loading and high temperature augments the incompatibility of
the libre layers, possibly due to the difference in the thermal expansion
coefficient of the two fibres. This also reduces the storage modulus values.
Delarninatiot~ of the material, under dynamic loading conditions, in addition
gives rise to the lowering of the storage modulus values. Another reason for
the lowering of the storage modulus value can be attributed to the difference in
the extensibility of the two fibres. The difference in the extensibility of the matrix
and fibre as well as that between the two fibres leads to unevenness of deformation.
Changes in the filler agglomerates and or breakage of filler polyrner bonds, all lead
to changes in dynamic properties. All these occur more at higher ten~perat~lres
and at higher glass fibre content except For the glass volulne fraction, 0.16. Tlle
seaon can very well be attributed to the uneven extensibility of the two tibres,
which becomes more prominent at higher glass fibre content. The storage n~odulus
values also give an insight into the nature of the interface bonding. However, the
.~ -- Dynamic Mechanical Analysis of: .. ,262
tensile strength values showed an increasing trend with the incorporation of glass
volume fraction.
3.2.2b. Loss modulus
The loss modulus curve is the contribution of the viscous component in
the polymer and is indicative of the energy dissipated by the system. The rapid
rise in loss modulus in a system indicates an increase in the structural mobility
of the polymer, a relaxation process that permits motions along larger portions
of the individual polymer chains than would be possible below the transition
temperature. During the glass transition, which is the largest and most important
of these relaxations, those regions within the polymer structure that are not
either crystallised or cross-linked, become capable of an increased degree of
fieedom. The variation of E" with temperature for the various hybrid composites
and the gum sample is shown in Figure 3.2.3. The maximum heat dissipation
occurs at the temperature where E" is maximum, indicating the T, of the system [17].
The peak of the loss modulus c w e is conventionally identified as the glass
transition temperature (Tg), even though the DMA plot clearly shows that the
transition is a process that spans a temperature range. The magnitude of the loss
modulus peak varies with the severity of the decline in the storage modulus.
During a transition, the loss modulus goes to a rise due to the sudden decline in
the storage modulus, which occurs due to the molecular motions occumng in
the polymer. However, the sharp drop in storage modulus in the present case is
expected to be more due to the delamination of the different layers than the
molecular motions.
.- Dynamic Mechanical Analysis of:.. ,263
-X- BananalPolyerter -X- Nest polyester
4 , . , , , , , , , , , , , , , , , , , , ~ 20 40 60 60 100 120 140 160 180 200 220
Temperature ("C)
Figure 3.2.3 Effect of temperature on the loss modulus curve
The reinforcement also acts as efficient energy transfer agent and the
loss modulus peak is found to be reduced pointing to the reduced effect the
glass transition has on the storage modulus of the material. It is also observed
that by the incorporation of fibres in the matrix, the T, is shifted to higher
temperature region. Increase in the relative glass volume fraction, shifts the
peak region positively. This points to the improved stress transfer at higher
glass fibre content. In addition, the loss modulus curves show an additional peak
when the glass volume iiaction is higher. The initial relaxation peak around 55OC
has also been found to be affected depending on the glass volume fraction.
Compared to the samples with no glass fibre, the relaxations are found to be
shifted to the higher temperature side. However, the loss modulus peaks are
found to be lowered by the incorporation of glass fibre. In addition to the
lowering, the loss modulus curves are also found to be flattened. Flattening of
the loss modulus curves point to an increased range of order. The second
relaxation peak around the temperature range 120°C has also been found to be
~~
Dynamic Mechanical Analysis of.. . ,264
affected by the incorporation of glass fibre. It has been reported by other authors
that in the case of hybrid composites, a change in the volume fraction ratio of the
two types of fibres leads to a change in their fibre lengths. The change in fibre
length arises due to the damage caused by the friction of the different fibres.
This r can occur during the processing of the composites. Even though this has
been suggested in processes like injection moulding, the likelihood of fibre
breakage cannot be ruled out in the present case also.
3.2.2~. Damping coefficient
The damping is a sensitive indicator of all kinds of molecular motions
that are going on in a material. The high damping peaks in a composite indicate
that once the deformation is induced in a material, the material will not recover its
original shape. In a composite, the molecular motions at the interface contribute to
the damping of the material. Fibdmatrix inter phase effects can also be understood
to a very good extent based on the damping curves. The lower tan delta values and in
particular the lower peak height associated with the glass transition, reflects the
improved load bearing properties of the system. Strong interactions of fibres and
matrix tend to reduce the mobility of the molecuIar chains at the interface and
therefore to reduce the damping. Figure 3.2.4 shows the effect of temperature on the
damping peaks of the composites with different relative glass volume fractions.
-- Dynamrc Mechanrcal Analysrs of ... 265
Figure 3.2.4 Effect of temperature on the tan Gcurve of different hybrid samples (Frequency 10Hz.)
Analysis of the damping curves (Figure 3.2.4) reveals that the damping
peaks have been lowered and that the relaxation peaks have been shifted to the
right. Both the lowering of the damping peaks and the shifting of the peak
heights point to the effective stress transfers between the fibre and the matrix.
The lowering of the damping peaks also occur due to the decrease in the amount
of the polymer due to fibre incorporation. The increased stress transfer can be
attributed to the increase in the high modulus glass fibre. Unlike the storage
modulus and the loss modulus curves where there is a lowering of the ultimate
values, due to the delamination between the different fibres, the damping peaks
point to the fibreirnatrix interaction alone.
- ~ -- Dynamic Mechanical Analysis of.. . ,266
3.2.2d. Effect of layering pattern
Table 3.2.1 Values of the tan 6 maximum and T, of neat polyester and banana
l;igure3.2.5 shows the effect of layering patterns on the storage modulus
values of the different composites.
fibre composites with relative glass 1 - - --
tan 6 ,, j _ _ Samples Frequency (Hz.)
volume fractions
T, from tan 6 ("C)
Frequency (Hz.)
{ p~ --;y ..
A i
B 0.24
C 1 0 . 1 5
D I 033
E 0.39
F 1 0.43
Ban/poly 0.21
0.1
107
117
122
119
122
126
106
104
1
0.22
0.17
0.12
0.28
0.25
0.32
0.24
1
116
128
125
127
126
127
115
114
10
0.20
0.20
0.11
0.27
0.21
0.30
0.22
Gm .]~.--!!: E" Max* 0.42 1 O.'(P~) 0.45
10
131
135
127
135
129
132
133
124
T, from E("C)
6.5
6.5
6.6
6.5
6.5
6.5
7.75
7.98
A 6.8
~ C I 6.7
j 6.4
E 1 6.6
F 6.5 i
Untreated 1 7.72
Gum 7.53 i ~
124
122
124
119
127
120
124
105
- 113
128
122
125
125
124
79
85
6.9
6.7
6.8
6.5
6.9
6.6
7.68
7.97
120
130
124
125
130
129
103
95
Dynam~c Mechanical Anolys~s of. .. . ,267 --. - -
Figure3.2.5 Effect of layering patterns on the storage modulus values of the composites with glass volume fraction 0.11
Table 3.2.2 gives details ofthe layering patterns followed.
Table 3.2.2 Details of layering pattern
Sample marking Layering pattern .-
LI G-B-G-B-G-B-G-B-G
L2 Intimate mixture of G and B
L3 G-B-G
4 G-B
L5 G-B-G-B-G - ~- G -glass, I3 -banana.
The storage modulus values of the composites with a glass fibre volume
fraction of 0.1 1 are given in Figure 3.2.5. The different layering patterns that
are followed are given by L,, L2 etc. and they are designated as CII, C12 etc in
samples with a glass volume fraction, 0.1 1. In all the cases, samples where an
intimate mixture of glass and banana has been used as the reinforcement is
found to have the highest tensile properties i.e. the samples marked C12. The
~ -- Dynamic Mechanical Analysis of.. . ,268
consistently high storage modulus value in the case of intimately mixed
composites can be attributed to the high elongation fibres acting as crack
arrestors in the case of a matrix failure. Unlike in the other geometries followed,
the fibres being intermingled, failure of the matrix or the low elongation fibre
will give way to crack arrest by the high elongation fibres. The high shear strain
stored in the interphase due to the mismatch between the fibre and the matrix
properties will also be minimised when the two fibres are intimately mixed. In
the case of composites with different layering arrangements, the stress
concentration at the crack tip induces interlaminar delamination. The material in
the periphery takes the stress and in composites where glass is kept in the
periphery, the high modulus glass fibres will take the stress and delamination
between the different layers is prevented to an extent. This leads to relatively
higher strength values compared to composites where banana is the skin.
In addition, the polymer chains immobilised on the fibre surface make a
link between the fibres, creating a flexible network whose properties are
dependent on the modulus of these chains. These additional networks serve as
supplementary cross-link points. The nature of the network is different in the
case of banana fibre and glass fibre. In dynamic experiments, the two networks
respond in a different way. But depending on the way in which the different
fibre layers are arranged, the responses of the materials differ, which is revealed
in the modulus values. The difference in response gets nullified based on the
fibre arrangement. Chazean et al. [I81 have suggested formation of networks on
the surface ot cellulosic fibres.
Dynamic Mechanical Analysis of. ... 269
3.2.2e. Damping coefficient
Figure 3.2.6 shows the effect of layering pattern on the damping curve
of the composites with glass volume fraction 0.1 1. All the damping curves show
two peaks irrespective of the layering pattern followed. The damping peaks
also get shifted depending on the layering patterns followed. The maximum
shifting of the damping peaks occur in the case of samples marked CIS and CIS.
In samples marked '213, banana forms the core material and glass the skin. In
samples marked CIS, there are altogether five layers, with glass as both the skin
and the core and banana layers in between. In both the samples, glass forms the
periphery. Moreover, the glass and banana layers are interdispersed.
O O O G I 0 50 100 150 2M)
Temperature("C)
Figure 3.2.6 Effect of layering on the damping curves of the composite with glass volume fraction 0.11
In the different layering patterns followed, the composite with five layers,
where glass forms both the core and the skin has given the maximum impact properties
as well [19]. The s h i h g of the damping peak to the high temperature region points to
the effective stress transfer between the fibre and the matrix in the particular geometry
Q)~nnntic Mecllanical Anaiy,si,s of:. . ,270
followed. Table 3.2.3 shows the values of thc clamping peaks obtained for composites
with a relative glass volume fiaction of 0.1 1 and wilh different layering patterns.
Table 3.2.3 Values of tan 6 max obtained for composites with relative glass volume fraction 0.11 and different layering pattern
Sample Tm 6 max Tg from tan 8 ,,'C C 0.127 123 C11 0.182 122 C12 0.193 117
For intimately mixed composites also, the damping peak values are
more or less the same as that of the composites with glass as the periphery
material. In intimately mixed composites, the high elongation cellulose fibres
serve as crack arrestors in a micro mechanical way, better than in layered
composites, and help in etlective stress transfer. Optical photograph of the intimately
rnixcd composite is given in Figure 3.2.7. The broken glass fibres and the cellulose
fibres, which act as blidges can very well be seen in the optical photographs.
Figure 3.2.7 Optical photographs of the failed composite with an intimate arrangement of glass and banana(relative glass volume fraction 0.1)
-. - . -- Dynamic Mechanical Analysis of.. . .27 I
'l'he three-layer composite samples, where glass forms the core material is
found to have a damping curve different fiom that of the other samples. The
difference in the nature of the damping curves can be attributed to the delamination
occurring in the composite. The two fibres take the stress applied on the
composite differently. 'The high modulus glass fibre being the core material, the
banana fibres will take the stress.
In all glasshanana combinations, there are two peaks visible. The additional
peak can be attributed to the micro mechanical transitions. The micro mechanical
transitions arise due to the presence of the immobilised polymer layer in between
the fibre and the matrix as explained in the earlier chapters.
Other authors have also reported on the additional peak due to the
presence of the immobilised polymer layer [20]. The intensity of the additional
tan 6 peak is found to be greater due to the difference in the nature of the
immobilised polymer layer on the two different fibres. The tan S peak also gets
shifted depending on the layering pattern.
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