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

1. J. J . Aklonis and W. J. Macffiight, Introduction to Polymer

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)

4. L. E. Nielsen, Mech. Props of Polymers and Comp. Vol. 2, Dekker, New

York (1974)

5. A. K. Saha, S. Das, D. Bhatta and B. C. Mitra, J. Appl Poly. Sci.,71,

1 505 (1999)

6 . D. N. Bikiaris and G. P. Karayannidis, J. Appl. Poly. Sci., 70, 797 (1998)

Dynamic Mechanical Analysis OK ... 199

A. Valea, M. L. Gonzalez and I. Mondragon, J. Appl. Poly. Sci., 71, 21

(1999)

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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J. Gassan and A. K. Bledzki, Proc. International Conference on

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K. Joseph, S. Thomas and C. Pavithran, Comp. Sci. and Tech., 53, 99

(1995)

C. Joly, R. Gauthem and B. Chabert, Comp. Sci. and Tech., 56, 761

(1996)

J. Gassan and A. K. Bledzki, Comp. Sci. and Tech., 59, 1303 (1999)

A. V. Gonzalez and J. M. Ce~antes-Uc, R. Olayo and P. J. Herrera

Franco, Composites: Part B., 30,321 (1999)

D. Klemm, B. Philipp, T Heinze, U. Heinze and W. Wagen Knecht

Comp. Cell. Chem. Vol. 2 Wiley, VCH (1998)

K. Joseph, C. Pavithran and S. Thomas, Polymer, 37,23,5139 (1996)

J. George, S. S. Bhagawan and S. Thomas, Comp. Interfaces., Vo1.5, 3,

201 (1998)

N. Chand and P. K. Rohatgi, Poly. Comp., 27,157 (1986)

E. T. N . Bisanda and M. P. Ansell, Comp Sci. and Tech., 41, 165 (1991)

M. S. Sreekala, S. Thomas and M. G. Kumaran, Comp. Sci. and Tech.,

(In press)

C. J. Broutman and R. H. Krock, Comp. Structures, Academic Press,

New York Vol. 6 (1974)

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

(1996)

S. Wu, Polymer Interface and Adhesion, Marcel Dekker Inc. New York

(1982)

L. A. Pothan, Y. Zimrnennann, S. Thomas and S. Spange, J. Poly. Sci.

Part B: Poljl. Phys., 38, 19,2546, (2000)

L. Britcher, D. Kehoe, J. Matisons and G. Swincer, Siloxane Coupling

Agents, Macromolecules, 28,3 1 10 (1995)

J. Cinquin, B. Chabert, J. Chauchard, E. Morel and J. D. Trotignon,

composite.^. Part A,, 2 1, 141, (1990)

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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.

References

1. F. R. Jones, editor Hand Book of Polymer-fibre Composites., lSt ed.

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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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