THERMAL PROPERTIES -...

Chapter 4

THERMAL PROPERTIES

Abstract

This chapter deals with different thermal properties of the commingled

composites. The thermal degradation behavior of banana fiber and

polypropylene/banana fiber composites has been studied by

thermogravimetric analysis. Banana fiber was found to be decomposing

in two stages, first one around 320°C and the second one around 450°C.

For chemically treated banana fiber, the decomposition process has been

at a higher temperature, indicating thermal stability for the treated fiber.

Activation energy for thermal degradation of banana fiber was estimated

using Coats and Redfern method. Calorific value of the banana fiber was

measured using a constant volume isothermal bomb calorimeter.

Crystallization studies of the composites exhibited an increase in the

crystallization temperature and crystallinity of polypropylene upon the

addition of banana fiber. Investigation of the thermophysical properties of

the composites revealed that thermal conductivity and diffusivity of the

composites decreased with banana fiber loading. The chemically treated

banana fiber caused an increase of the thermophysical properties of the

composites. The use of a theoretical series conduction model allowed

estimating the transverse thermal conductivity of banana fiber.

A part of the results of this chapter have been published (on line) in Composites Part A (Article in press)

130 Chapter 4

4.1 Introduction

Thermal analysis is an important analytical method in understanding the

structure-property relationship and mastering the technology for

molecular design and industrial production of different polymeric

materials, especially fiber reinforced composites. Moreover, it is a useful

technique to determine the thermal stability of the materials. In addition, it

is possible to quantify the amount of moisture and volatiles present in the

composites that have a deteriorating effect.

An understanding of the thermal stability of the fibers and composites is

extremely important to assess the suitability of these materials in varied

applications. Thermal analysis can also help in evaluating the efficacy of

surface treatment and thermal decomposition kinetics. There have been

many studies on the thermal decomposition of biopolymers with special

emphasis to reaction kinetics [1-14]. Addition of fibers to a semicrystalline

polymer like polypropylene can affect the degree of crystallinity of the

polymer matrix which in turn can influence the eventual mechanical

properties of the composite. Thermal calorimetric techniques have been

used effectively as a tool to evaluate the crystallization of PP/biofiber

composites [6].

The interaction of thermal energy (i.e, heat) with the atoms which

constitute a material determines some of the most important physical

properties of the material. The properties describing this interaction at the

most fundamental level are often called thermophysical properties which

include heat capacity, thermal diffusivity, thermal conductivity etc. A

complete characterization of the thermal properties of materials requires

the determination of the thermal conductivity and thermal diffusivity.

Thermal conductivity is a property of materials that expresses the heat flux

that will flow through the material if a certain temperature gradient exists

Thermal Properties 131

over the material. Thermal diffusivity describes the time dependent non-

steady-state aspects of heat flow. Thermal conductivity is a bulk property

analogous to mechanical modulus. Moreover, it is well accepted that a

mathematical analogy exists between thermal conduction and elasticity of

fiber composites. Many theoretical and empirical models have been

proposed to predict the effective thermal conductivity of short fiber

composites [15-18]. In polymer composites, if the alignment of the fibers

are parallel to the direction of the thermal flux, the thermal conductivity will

be highest (parallel conduction) and if the alignment of the fibers are

perpendicular to the direction of the thermal flux, thermal conductivity will

be the lowest (series conduction). Fig. 4.1 shows the schematic

representation of parallel and series conduction.

Fig. 4.1 Schematic representation of parallel conduction and series conduction

Several studies have been reported in literature about the thermal conductivity

of fiber filled polymer composites. Kim et al. [19] studied the thermal

conductivity of thermoplastic composites composing of 50% PP and 50%

natural fiber irradiated by the electron beam (energy: 0.5 MeV, dose:

0–20 kGy). The results showed that the thermal conductivity became

132 Chapter 4

minimum when the dose of electron beam is 10 kGy. Li et al. [20]

investigated the thermal conductivity, thermal diffusivity, and specific heat

of flax fiber/high density polyethylene biocomposites in the temperature

range of 170–200°C. The fiber contents in the composites were 10%,

20%, and 30% by mass. It was found that the thermal conductivity,

thermal diffusivity, and specific heat decreased with increasing fiber

content, but thermal conductivity and thermal diffusivity did not change

significantly with temperature in the range studied. Agrawal et al. [21]

measured the thermal conductivity and thermal diffusivity of oil-palm-fiber

reinforced untreated and differently treated composites with the transient

plane source technique at room temperature and under atmospheric

pressure. The thermal conductivity of the treated fibers as well as of the

untreated fibers was calculated theoretically. The model results showed that

the thermal conductivity of untreated fiber was lower than the thermal

conductivity of the treated fibers. Idicula et al. [22] investigated the thermal

conductivity, diffusivity and specific heat of polyester/natural fiber

(banana/sisal) composites as a function of filler concentration and for several

fiber surface treatments. The thermophysical behaviour of hybrid pineapple

leaf fiber (PALF) and glass fiber reinforced polyester composites has been

also evaluated for a constant total fiber loading of 0.40 Vf by varying the ratio

of PALF and glass. The results showed that chemical treatment of the fibers

reduces the composite thermal contact resistance. Hybridisation of natural

fiber with glass allows a significantly better heat transport ability of the

composite.

During manufacturing of natural fiber plastic composites, heat transfer is

involved, but information about the thermal conductivity and thermal

diffusivity is not available. In this study, the thermal conductivity, thermal

diffusivity, and specific heat of PP/BF composites were determined in the

temperature range of 170–200°C. Processing method will have an


influence on the thermophysical, thermal degradation behaviour and

crystallization properties of the composites. No studies have reported in

literature where the above properties of the PP/BF commingled

composites were analyzed. In the present chapter, we report the thermal

degradation behavior, crystallization studies and thermophysical

properties of the PP/BF commingled composites.

4.2 Results and Discussion 4.2.1 Thermogravimetric analysis 4.2.1.1 Thermal degradation of banana fiber The thermograms from TG and DTA studies for BF are shown in Fig. 4.2.

Fig. 4.2 TG and DTA thermograms of banana fiber

It can be seen from the TG curve that the BF shows a weight loss of

around 6% before 100°C, due to moisture loss. However DTA does not

show any endotherm corresponding to evaporation of water during this

temperature range. Instead a broad, yet small exothermic peak beginning

around 60°C was observed in DTA. Similar observations have been

reported for sisal fibers which have similar chemical composition [4]. On

134 Chapter 4

the other hand, water volatilization has also been reported as a distinct

endothermic reaction in biopolymers in thyme [8]. On further heating the

thermal decomposition of the fiber was found to take place in two

successive stages. A weight loss of around 55% was observed in the first

stage (230-330°C) of the decomposition which was followed immediately

by the second stage (330-440°C). The DTA showed an exothermic peak

starting around 230oC with a peak temperature around 320°C

corresponding to the first stage decomposition. The second stage

decomposition following the first stage started around 340oC and

continued till around 450°C with a weight loss amounting to around 30%.

The DTA showed a sharp exotherm with a peak temperature of 450°C

corresponding to this stage of decomposition. The average weight of the

solid residue at 500°C from various trials on BF was in the range 6-8%.

Previous studies have shown that biomass (mainly consisting of cellulose,

hemicellulose and lignin) pyrolysis takes place in four stages. These

stages that take place in sequence while heating are moisture loss,

decomposition of hemicellulose, cellulose and lignin [8,19]. The first stage

decomposition observed after the moisture loss can be attributed to

hemicellulose decomposition which is reported to take place in the

temperature range 220-320°C. Hemicellulose mainly consists of various

saccharides and has a random amorphous structure, rich in branches

which can degrade to low molecular weight products at low temperature.

Cellulose, on the other hand, has a higher thermal stability as it is strong

and stable due to its polymeric chain consisting of thousands of glucose

units. The hemicellulose percentage in the BF under study is around 19%.

However, the average weight loss observed in the temperature range 220-

320°C, was more than 50% in the present studies. This suggests complex

reactions other than hemicellulose decomposition taking place at low

temperatures. Though cellulose pyrolysis is reported to takes place at a


higher temperature range 315-400°C with the evolution of various gases

and high boiling products, depolymerization and other degradation process

are likely to be initiated at temperatures as low as 250°C [4,9,24].

Obviously there is a strong overlap of thermal degradation of individual

constituents of the fiber. It must be remembered that the nature of thermal

decomposition of cellulose depends also on factors such as texture and

crystallinity which make thermal decomposition of biopolymer even more

complex. The second stage decomposition observed in the temperature

range 340-450°C is primarily due to cellulose decomposition and char

combustion [8,9,24]. It has been reported [4,8,9] from thermal studies on

cellulose decomposition that cellulose decomposition shows an endotherm

around 340°C due to depolymerization of cellulose with formation of

levoglucosan and its evaporation followed by an exotherm corresponding

to char formation. The endothermic decomposition process could have

been masked significantly by the exothermic reaction in the present case.

The sharp exothermic peak obtained around 450°C in the present studies

could hence be attributed to the char formation.

The decomposition of lignin, the third major component in the fiber

composition, is reported to be difficult and takes place slowly over a broad

range of temperature with decomposition starting from as low as 100oC.

Lignin is also reported to start decomposing only at 250°C [10, 25]. Since

content of lignin in the BF being less and the thermal decomposition is

believed to take place in a broad temperature range, it was difficult to

identify any particular stage to lignin decomposition. The overall thermal

behaviour of BF appears to be similar to that of other biopolymers [4,8]. In

general cellulose degrade to gases, primarily water and carbon dioxide,

high boiling products and char. In order to gain more insight into the

thermal decomposition mechanism of BF, pyrolyzed samples of fiber were

analyzed using FTIR spectroscopy. The IR spectra of BF, a sample that

136 Chapter 4

was heated upto 150°C and another sample that was heated upto 350°C

are shown in Fig.4.3-4.5 respectively. The IR spectrum of BF shows

characteristic absorptions of its constituents namely cellulose,

hemicellulose, lignin and moisture. The IR absorption peaks and its

relation to various functional groups in the constituents of the fiber are

shown in Table. 4.1

Fig.4.3 IR Spectrum of banana fiber

The IR spectra of the samples heated to different temperatures

(10°C/min heating rate) look more or less identical except for the

sharpening of some of the peaks in the spectra of the pyrolyzed samples.

While the peaks at 1738 (carbonyl group) and 1630 cm-1 (double bond)

became prominent, characteristic absorptions of the glucosidic structure,

(1169 cm-1, glucopyranose ring) decreased in intensity in the pyrolyzed

samples. It has been suggested that a cross linked unsaturated aliphatic


carbonylic structure is formed prior to charring. The charring of this

precursor is reported to take

Fig. 4.4 IR spectrum of banana fiber heated upto 150oC

place at 325-420°C. This explains the prominence of peaks at 1738 cm-1 in

pyrolyzed samples and exothermic peak in the temperature range 325 to

450oC in DTA thermograms. Besides, the characteristic absorptions of

aliphatic structures (3000-2800 and 1436 cm-1) became more

prominent in samples heated up to 350°C. The new band at 1515 cm-1

observed in pyrolyzed samples can been attributed to aromatic

semicircle stretching. Absorption at 1131 cm-1 in these samples may be

due to levoglucosan (C-O-C stretching), an intermediate product in the

138 Chapter 4

cellulose degradation. Similar observations have been made earlier in

studies on cellulose degradation [9]. Further analysis of the residue at still

higher temperatures was not attempted in this study. Comparing the

pyrolysis results with those reported in a recent study [10] wherein BF

thermal degradation have been studied at higher temperatures, it can be

inferred that the mechanism of thermal degradation may vary to some

extent depending upon the constitution of the biopolymer though the

general trend is the same. However, a more elaborate study involving

analysis of evolved gases, high boiling products and char is necessary

to elucidate a proper mechanism of decomposition. The thermogravimetric

data was used to calculate kinetic parameters like activation energy of

banana fiber decomposition.

Table 4.1 Assignment of IR absorption peaks

Wavenumber cm-1 Functional group

3100-3600 OH stretching

2800-3000 C-Hn stretching

1700-1740 C=O stretching

1637 OH (absorbed water)

1600-1650 C=C stretching

1505-1510 C=O stretching

1436 CH bending

1468 O-CH3

1230 C-O-C stretching

1207 C-O stretching

1169, 1048 C-O-C stretching

1106 OH

1048-1011 C-O stretching and C-O deformation


The method adopted by Coats and Redfern has been used in the present

study to calculate the activation energies of fiber degradation [26-27]. It

has been reported recently that among the various integral methods,

Coats and Redfern approximation may be preferred over other

approximations because the resulting errors are much smaller for the

entire range of E/RT [28]. According to Coats-Redfern (CR equation),

( ) ( )2In g / T = In AR/ E 1-2RT/E E/RTα ϕ − (4.1)

where g(α) = 1 – (1- α )1-n / 1-n for all values of n = 1 except for which g(α)

= -ln (1- α) α is the fraction decomposed, n is the order of reaction, T is the

temperature, R is the universal gas constant, φ is the heating rate. Linear

plots of ln g(α)/T2 vs 1/T are drawn for different values on n . The value of

n which gives the best fit with correlation coefficient nearest to unity is

chosen as the order of the reaction. The value of E can be obtained from

the slope of this line.

Fig. 4.5 IR spectrum of banana fiber residue heated upto 350°C

140 Chapter 4

The degradation kinetics of cellulose have been studied in detail in the

past and is generally accepted to be a first order reaction [1,8]. Unlike

cellulose and hemicellulose, lignin decomposition does not follow a first

order kinetics [8]. Since lignin percentage is not much compared to the

other two constituents of the fiber, first order kinetics have been followed

in the calculations involving determination of activation energy. Besides

the correlation coefficient calculated for the best fit curve was found to be

closest to the one for n =1.

Decomposition of a biopolymer is a continuous process. However, the

first two stages during the decomposition being somewhat distinct in

TG curves, activation energy was calculated separately for each

stage. Such calculations also have been reported for biopolymers [3].

The activation energy for the first stage and second stage

decomposition of banana fibers were calculated to be 132kJ/mol and

163kJ/mol respectively. The higher activation energy for the second

stage is because of the char formation. Though the values obtained are

in the range reported in the literature for cellulose decomposition in

general, it is difficult to compare the values as the constituents of the

fiber under study vary from other bio cellulose based samples and

decomposition mechanism itself is complex [14].

Thermal decomposition behaviour of BF at different heating rates (5,10,

15,20oC/min) was also carried out. Activation energy for the first stage

decomposition was calculated based on Flynn method [29]. This method

involves reading the temperature at a constant weight loss from several

integral thermograms at different heating rates. From the slope of log (β

heating rate) vs. 1/T, the activation energy may be closely approximated

by means of the equation

E ~ - 4.35 d log β /d 1/T (4.2)


The activation energy calculated (approximate) was 113 kJ/mol. Calorific

value of the BF was estimated based on heat of combustion. The heat of

combustion value is important to assess the combustion property of the

fiber. Heat of combustion usually expressed in cal/gm is the number of

heat units liberated by a unit mass of the substance when burnt in a

sealed enclosure of constant volume in an atmosphere of pure oxygen

gas. If W is the water equivalent of the calorimeter assembly, ∆T is the

rise in temperature, and ∆H is the heat of combustion of the material and

M is the mass of the sample burnt, then

W∆T = ∆HM (4.3)

From the water equivalent of the calorimeter (calculated using a standard

material, such as benzoic acid whose heat of combustion is known) and

rise in temperature, the heat of combustion can be calculated. Complete

combustion was ensured by the absence of solid residue in the bomb

after every measurement. The heat of combustion (∆H) was found to be

17.52 kJ/gm. This is almost equal to the heat of combustion of cellulose.

Heat of combustion of water bamboo husks, another biopolymer is

reported [3] to be slightly less than this value (15.69 kJ/mol).

Thermal analysis was carried out on different chemically treated BF. The

thermograms from TG and DTA studies are shown in Fig. 4.6. The

treated fibers were found to start decomposing around 240-260°C which

is around 10-30°C higher than the untreated fiber indicating a higher

thermal stability for the treated fibers. The alkali treated fiber seems to

have a slightly higher moisture loss below 100°C compared to BF. A

related observation was made in a separate study wherein the effect of

humidity on chemically treated fibers was carried out. The increase in

moisture content of alkali treated fibers was more pronounced than other

treated fibers when exposed to humidity. The alkali treatment effectively

142 Chapter 4

Fig. 4.6 TG and DTA curves of chemically treated banana fiber (1)

NaOH treated (2) KMnO4 treated (3) benzoylated.

removes a certain amount of lignin, wax and oils covering the external surface

of the fiber cell wall besides hemicellulose [30]. The removal of alkali soluble

hemicellulose results in a fibrillated structure. When hemicellulose is removed,

the inter fibrillar region is likely to be less dense and less rigid and thereby

makes the fibrils more capable of rearranging themselves. So the moisture

loss is easier and weight loss is higher around 100°C compared to the

untreated fibers. The onset of decomposition was found to have increased

significantly in alkali treated fibers. The decomposition was found to be taking

place only above 250°C. Similar thermal stability enhancement has been

reported for jute fibers after alkali treatment [30]. Though an increase in

thermal stability was observed in the alkali treated fibers the activation energy

calculated for the first stage decomposition was almost similar to that of the

BF. Apparently the surface treatment does not alter the decomposition

mechanisms. Another notable difference in the thermal behaviour of the

treated fiber was the absence of prominent exothermic peaks in the DTA


thermograms. Almost identical behaviour was observed for KMnO4 treated

fiber except that the decomposition temperature was slightly less than alkali

treated fibers and the exothermicity was visible in DTA thermograms.

Activation energy calculated for first stage decomposition was similar to that

obtained for alkali treated fiber. The formation of cellulose manganate

complex is believed to be responsible for the higher thermal stability of the

KMnO4 treated fiber. In benzoylated fiber, the weight loss at 100°C is found to

be lower indicating lower moisture content of the benzoylated fiber. This is due

to the presence of hydrophobic benzoyl groups, resulting from the

benzoylation reaction on the fiber surface. Lower moisture absorption was

observed for these fibers when exposed to different humidity levels. It is clear

from the thermogram that onset of degradation starts a bit early though

significant weight loss start taking place only above 240°C. The exothermicity

owing to the degradation also was evident in DTA thermograms. The

activation energy for first stage decomposition calculated for benzoylated

fibers however was higher (185 kJ/mol) when compared to BF. This is another

indication of higher thermal stability induced by chemical modification. A

similar observation including activation energy was made for stearic acid

treated fibers. However in this case the exothermicity in DTA thermograms

was completely missing. In general there is an increase in the onset of

decomposition temperature when the fibers are chemically modified; the

extent of increase depend upon the chemical treatment. The chemical

treatment induced by organic substrates also seems to enhance the activation

energy of the degradation of the biofiber.

4.2.1.2 PP/banana fiber composite Thermal degradation behaviour of polypropylene composites made

using (50%) BF was also studied. Typical TG and DTA curves of the

composites and PP are given in Figs.4.7 and 4.8 respectively. The DTA

of PP show an endotherm around 152°C corresponding to the melting of

144 Chapter 4

Fig. 4.7 TG curves of (1) Neat PP (2) PP/banana fiber composite (3)

banana fiber

Fig. 4.8 DTA thermograms of (1) Neat PP (2) PP/stearic acid treated

banana fiber composite (3) PP/benzoylated banana fiber composite (4) PP/ untreated banana fiber composite


polypropylene followed by another broad endotherm beginning around

340oC corresponding to the decomposition. No exothermic peaks were

found till the decomposition was over. The weight loss curve in TG studies

show a continuous weight reduction in the temperature range of 340 to

430°C. A complete weight loss was observed at 450°C. Activation energy of

decomposition was estimated to be 94 kJ/mol with an n value of 0.4 in this

temperature range using Coats-Redfern method. The thermal behavior of

PP observed in this study is similar to the reported results from thermal

studies on PP [31-32]. It can be seen from the thermograms of PP/BF

composites containing 50% fiber that there is no weight loss corresponding

to moisture, and decomposition sets in only above 250°C in composites.

Obviously the fiber decomposition which starts around 240°C initiates the

degradation of the composites. The thermograms from DTA studies of the

composite show endotherms corresponding to melting and degradation of

PP. Though fiber decomposition was exothermic in nature the composites

did not show any exothermic peak. The continuous weight loss pattern

observed in PP changed to a two stage decomposition, similar to fiber

decomposition, in composites. A 30% weight loss was observed in the first

stage in the temperature range 240 to 350°C. Unlike in the case of fiber

where the two stages of decomposition are somewhat distinct the

composites show a more or less merged weight loss stages. This could be

due to the fact that PP starts decomposing around 340°C where the second

stage of fiber decomposition begins. The weight of the final residue at 500°C

in each trial was around 10%. An activation energy estimation was carried

out for the decomposition in the temperature range 250-440°C and a value

of around 80-100 kJ/mol was obtained. This value was found to be more or

less same in all the fiber composites. Similar values of activation energy

have been reported for PP/sisal fiber composites [4]. The thermal

degradation kinetics of the composites apparently is guided by the PP matrix

146 Chapter 4

decomposition mechanism. It can also be seen from the Fig. 4.8 that the

surface treatment on the fiber does not have any significant influence in the

decomposition pattern of the composite. In general some of the notable

changes in the BF degradation behaviour with the incorporation of

polypropylene matrix are that the decomposition becomes endothermic,

moisture absorption becomes less and the decomposition kinetics shifts

more towards the PP decomposition pattern.

4.2.2 Crystallization studies For composites based on semicrystalline matrix polymers, the crystallinity is

an important factor that determines the stiffness and fracture behaviour of the

composite. The crystallinity depends on processing parameters; e.g.,

crystallisation temperature, cooling rate, nucleation density, and annealing

time [33]. It is known that a transcrystalline layer forms at the fiber/matrix

interface when fibers with high nucleating ability are employed [34-35].

Arbelaiz et al. [6] conducted the crystallization studies of short flax fiber

reinforced polypropylene composites and chemically treated flax fiber

composites. The results showed that addition of flax fiber increased

crystallization rate. Besides, depending on flax fiber surface treatment and

crystallization temperature, PP/flax fiber composites showed transcrystallinity.

In the present study, the effect of BF on the thermal properties of PP is

analysed by DSC. The thermal parameters such as crystallisation

temperature (Tc), melting temperature (Tm), crystallisation energy (∆Hc) and

melting enthalpy (∆Hm) of neat PP and PP/BF composites using both

untreated and treated BF are summarized in Table 4.2. The corresponding

DSC thermograms are shown in Figs. 4.9 and 4.10.

The crystallinity of PP component was determined using the relationship

Xc (% crystallinity) = f0

f

∆H ×100∆H ×W

(4.4)


where ∆ H0f is the heat of fusion of a hypothetical 100% crystalline PP

sample and W is the weight fraction of PP in the composite. A value of

∆H0f = 209 J/g was taken for calculations [6]. It can be seen from Table 4.2

that addition of BF has different effects on Tc and Tm. While Tm increased

marginally there is a relatively more pronounced increase in Tc with the

addition of fiber to the PP matrix. Tc increases with increase in BF loading

and is maximum for 50% BF composite. The addition of fibers results in a

decrease in the heat of reaction associated with both crystallization and

melting process. This could be primarily attributed to the dilution effect of PP

by the BF. All these observations are similar to what has been reported for

the PP/biofiber composites [6, 23]. It can be seen from the Table 4.2 that the

values increase significantly with the addition of fibers to PP matrix.

Obviously the fibers play a clear role in the nucleation and crystallization

50 100 150 200 250-35

-30

-25

-20

-15

-10

-5

0

5

exo down5

4

3

2

1

Hea

t flo

w (m

W)

Temperature (0C) Fig. 4.9 DSC curves for crystallisation of (1) neat PP (2) PP/banana fiber

(30%) composite (3) PP/banana fiber (50%) composite (4) PP/ NaOH treated banana fiber (50%) composite (5) PP/ benzoylated banana fiber (50%) composite

148 Chapter 4

process, and melting to some extent. However the effect of chemical

treatment on the fibers for increased compatibility between fibers and PP

matrix does not seem to have a significant role either in crystallization or

melting temperature. Though there has been reports of chemical

modification inducing changes in Tm, the findings here are similar to what

has been reported for PP/flax fiber composites [6].

Fig. 4.10 DSC curves corresponding to melting of PP, PP/banana fiber (10,

30,50%) composites and PP/chemically treated banana fiber (50%) composites

Temperature (°C)

100 150 200 250

-5

0

5

10

15

20

25

KMnO4 treated fiber

NaOH treated fiber

50%fiber 30% fiber

10% fiber PP

Hea

t flo

w (m

W)

endo up


Table 4.2 Thermal parameters from crystallization studies for PP and PP/banana fiber composites

Samples Tc

(oC)

∆Hc (J/g)

Tm (oC)

∆Hm (J/g)

Χc (%)

Tg (oC)

Neat PP 112.19 92.41 165 90.21 44.21 -8

PP/BF (10%)composite 114.24 89.43 165.8 87.23 47.54 -3.93

PP/BF (30%)composite 115.56 70.15 167.1 73.5 47.92 1.08

PP/BF(50%)composite 116.28 51.33 167.28 54.76 49.02 1.78

PP/BF(2% NaOH)

treated (50%)composite 117.58 55.21 166.73 58.45 52.83 4.2

PP/BF stearic acid treated (50%)composite 116.28 54.5 166.87 55.05 52.15 2.98

PP/ BF benzoylated (50%) composite 116.49 54.82 167.98 55.7 52.45 3.14

PP/BF KMnO4 treated (50%) composite 115.9 52.23 167.49 54.98 49.89 2.9

Though the increase in Tm values are only marginal, there is a large

decrease in the heat of melting when the fibers involved are chemically

modified. Obviously, the chemical treatment which decides the

fiber/matrix interface seems to play a facilitating role in the melting

process. It can also be seen from the Table 4.2 that there is a significant

increase in the Tg values with the addition of fibers to PP, more so when

the fibers were surface modified. This could be attributed to the restriction

of PP molecular chain by the introduction of cellulose fibers. As the fiber

concentration increases the restriction in the mobility of the of the PP

molecular chain also increases, resulting in higher Tg values at higher

150 Chapter 4

loading densities. It is also clear from Table 4.2 that the crystallinity of

neat PP is increased by the addition of BF and further by the addition of

surface treated BF. As the amount of BF increases, crystallinity is also

found to increase because the fibers act as nucleation sites for the

crystallisation and partial crystalline growth of PP. It may be assumed

that the nucleating effect considerably contributes to the occurrence of

transcrystalline layers around the fibers when a specimen of the PP/BF

composite is cooled from the melt. Microscopic analysis of the PP/BF

composites in polarized light also revealed a transcrystallinity of the

matrix around the fibers as shown in Fig. 4.11.

Fig. 4.11 Crystal growth of transcrystallinlty for KMnO4 treated banana

fiber Tc = 136oC under polarised light for 30 minutes (stress induced).

4.2.3 Thermophysical properties 4.2.3.1 Effect of banana fiber loading Incorporation of fillers and reinforcements changes the thermal properties

of the polymers. The dispersion of the filler particles, orientation, fiber

aspect ratio, relative modulus of the fiber and matrix, and thermal

expansion mismatch affect the thermal properties of the composite. The

measured thermal conductivity and diffusivity properties of PP/BF


composites with their associated uncertainties under two different fiber

loadings are given in Table 4.3. The density measurements and specific

heat calculations are also given in the same table. It can be seen from the

table that when the BF loading is increased, there is a significant decrease

of both thermal diffusivity and thermal conductivity (i.e., from 0.24 Wm-1K-1

for neat polypropylene matrix to 0.217 and 0.157 Wm-1K-1 for 10% and

50% respectively) of composites. Similar behavior was reported by Idicula

et al. [22]. When the fiber loading increases, due to the hollow cellular

structure of the fibers, they act as insulators, which causes a decrease of

conductivity. Thus, the addition of BF in the PP matrix decreases the heat

transport in the composite. Moreover, the polar cellulosic –OH groups are

involved in inter and intra molecular hydrogen bonding making them less

available for conduction when compared to the PP matrix.

Table 4.3 Thermal conductivity, thermal diffusivity, density and specific heat of PP/banana fiber composites.

Samples k (W.m-1.K-1)

a (×10-7 m2.s-1)

Cp (J.kg-1.K-1)

p (kg.m-3)

Neat PP 0.240 ± 0.001 1.66 ± 0.04 1588 ± 42 910 ± 10

10% BF composites 0.217 ± 0.002 1.43 ± 0.07 1642 ± 104 924 ± 36

50% BF composites 0.157 ± 0.002 1.05 ± 0.03 1522 ± 72 982 ± 35

The specific heat values (Table 4.3) for 10% and 50% fiber composites do not

have significant change due to the high uncertainty bounds obtained for this

parameter. However, density increases with the increase in fiber loading while

the thermal diffusivity behavior is similar to that of the thermal conductivity i.e,

decreases with the increase of the fiber loading.

152 Chapter 4

The effective thermal conductivity of polymers filled with natural fibers

can be represented using simple theoretical models. In fact, when the

ratio between the thermal conductivity values of the matrix and the fibers

is lower than 10, a first-order model can be used to estimate the effective

thermal conductivity of the composite. The simplest alternative is to

represent the composite material using two components arranged in

either parallel or series with respect to the heat flow (Fig.4.12). This gives

the upper (ksup) and lower (klow) bounds of effective thermal conductivity:

( )supParallel model 1P fk k kϕ ϕ= − + (4.5)

( )inf

11Series model P fk k kϕ ϕ−

= + (4.6)

where kp and kf are the thermal conductivities of the polymeric matrix and

the fibers respectively and ϕ is the fiber volume fraction.

0 10 20 30 40 50 60 70 80 90 1000.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

k (W

.m-1.K

-1)

ϕvol. (%)

Experimental Data Series model

kf = 0.122 +/- 0.005 W.m-1.K-1

Fig.4.12 Comparison of PP/banana fiber composites thermal conductivity with series conduction model.


In Fig.4.12, experimental data of PP filled with BF is plotted as a function

of BF loading. The use of the series conduction model (Eq. 4.6) allowed

to estimate the transverse thermal conductivity of BF. The value obtained

was: kf = 0.1166 ± 0.0001 Wm-1K-1. The curve corresponding to this

model was added on Fg.4.12. A good agreement between experimental

data and the fitting model is observed. It can be seen that both thermal

4.2.3.2 Effect of chemical treatment of the banana fiber The effect of chemical treatment of the BF on the thermophysical

properties and density values of the composites can be understood from

the values given in Table 4.4 with their uncertainties.

Table 4.4 Thermal conductivity, thermal diffusivity, specific heat and density of PP/banana fiber (50%) composites

Samples k (W.m-1.K-1)

a (×10-7 m2.s-1)

Cp (J.kg-1.K-1)

p (kg.m-3)

Untreated BF composites 0.157 ± 0.002 1.05 ±0.03 1522 ± 72 982 ± 35

Benzoylated BF

composites 0.182 ± 0.002 1.31 ±0.07 1370 ± 91 1012 ± 38

KMnO4 treated BF composites 0.168 ± 0.002 1.22 ±0.06 1265 ± 77 1088 ± 37

2% NaOH treated BF composites 0.163 ± 0.003 1.19 ±0.05 1352 ± 77 1013 ± 34

10% NaOH treated BF composites 0.178 ± 0.003 1.31 ±0.10 1305 ±114 1041 ± 40

Triethoxy octyl silane treated composites

0.160 ± 0.005 1.13 ±0.09 1417 ±132 999 ± 36

154 Chapter 4

conductivity and diffusivity values of the chemically treated fiber

composites (50%) are higher than that of the BF composites. This

indicates that chemical treatment of the BF increases the thermophysical

properties irrespective of the nature of the treatment. Another observation

is that NaOH concentration has an influence on the thermophysical

properties of the composites. The 10% NaOH treated composites

showed higher thermal conductivity and diffusivity than 2% NaOH treated

composites. The efficiency of different treatments on thermal diffusivity is

found in the following order: benzoylated >10% NaOH treated >KMnO4

treated >2% NaOH treated >silane treated >untreated composites.

However, it is important to note that a significant increase of the thermal

conductivity is observed only for benzoylated and 10% NaOH treated

composites. In fact, for this case the thermal diffusivity and thermal

conductivity of benzoylated composite is higher than the one of untreated

composites by 25% and 16% respectively. As a result of benzoylation,

the hydrophilicity of the fiber is reduced. This makes the fiber more

compatible with hydrophobic PP matrix, thereby increasing the properties

of the composite. The sodium hydroxide pretreatment before

benzoylation results in partial removal of hydrogen bonding in cellulosic

chains and make the fiber surface more reactive for benzoylation [36].

The improved thermophysical properties of the composite is attributed to

the presence of -C6H5 groups in benzoylated BF which favours the way

for better interaction with PP. Scheme of the reaction of the benzoylation

process on the surface of the BF is shown in [Chapter 3, Fig.3.9]. Upon

benzoylation, the fiber diameter decreases and hence the aspect ratio

increases. This may be due to the dissolution of alkali soluble fractions

like waxy layer, lignin etc. during alkali treatment and benzoylation.

Moreover, the treatment provides a number of small voids on the surface


of fiber that promote mechanical interlocking between the fiber and the

PP matrix [37].

The thermal conductivity and diffusivity of NaOH treated fiber composites

are higher than that of BF composites because the NaOH treatment

removes a certain amount of lignin, wax and oils covering the external

surface of the fiber cell wall. The effect of alkali on BF is a swelling

reaction, during which the natural crystalline structure of the cellulose

relaxes. When hemicellulose is removed, the inter fibrillar region is likely

to be less dense and less rigid and thereby makes the fibrils more

capable of rearranging themselves [37]. The fiber surface shows more

fibrillation which can be attributed to the removal of hemicellulose and

lignin. This effect becomes more pronounced as the concentration of

NaOH increases. 10% NaOH treated fiber shows more fibrillation than

2% NaOH treated fiber which is evident from the SEM microphotograph

as shown in Fig 4.13. As the concentration of NaOH is increased from 2

to 10%, the thermal conductivity and diffusivity of the composite are

increased by 9% and 10% respectively.

a b

Fig. 4.13 Scanning electro micrographs of alkali treated banana fiber (a) 2%NaOH treated (b) 10% NaOH treated

Thermal diffusivity of silane treated composite is 8% higher than that of

untreated composites. When BF is immersed in the aqueous solution of

156 Chapter 4

silane, the chemical bonds (R1 –Si-O) as well as hydrogen bonds are

established between the –OH group of the fiber surface and R1-Si- (OH)3

molecules. The long hydrophobic polymer chain of polymerized silane

can adhere to the matrix by Vander Waals attractive forces. As a result,

silane coupling agents form a bridge at the interface of the BF and PP

matrix and act like a compatibilizer, which binds two incompatible

polymers [38-39]. Fig. 4.14 shows the interface of the silane treated fiber.

Fig.4.14 Schematic representation of interface of the silane treated fiber

In the case of permanganate treated fiber composite, thermal diffusivity is

increased by 13% compared to untreated fiber composites. This can be

explained in terms of permanganate induced grafting of PP on to BF as

shown in scheme 4.1.

Cellulose─ H + Mn (111) → Cellulose─ H ─ Mn (111) (Complex)

Cellulose─ H ─ Mn (111) (Complex) → Cellulose. + H + ++ Mn (11)

Cellulose. + PP → Cellulose PP.

Cellulose PP. + n PP → Cellulose PP. n+1

Cellulose PP. n+1 + Mn (111) → Cellulose PP

n+1+ Mn (11)+ H+

Scheme 4.1 Reaction of permanganate with banana fiber


The highly reactive Mn3+ ion is responsible for the grafting reaction. Thus

all the chemical treatments allow a better contact between the

components of the composite i.e, (fiber/matrix) and reduce considerably

the thermal contact resistance between the fibers and the polymeric

matrix.

4.3 Conclusions TG curve of BF showed weight loss of around 6% before 100oC due to

moisture loss. The DTA showed exothermic peaks starting around 230oC

and 450oC with a peak temperatures around 320oC and at 500oC

corresponding to hemicellulose and cellulose decomposition. The

average weight of the solid residue at 500oC was in the range 6-8%.The

activation energy for the first stage and second stage decomposition of

BF was calculated to be 132 kJ/mol and 163 kJ/mol respectively. The

higher activation energy for the second stage was because of the char

formation. The chemically treated BF decomposed at a higher temperature

than untreated fiber indicating higher thermal stability for the treated fiber.

Thermal stability was found higher for benzoylated fiber followed by NaOH

treated fiber and KMnO4 treated fiber. The thermal degradation behavior of

the composites has a two stage decomposition pattern. Crystallization

studies revealed an increase in the crystallinity and crystallization

temperature of neat PP by the incorporation of BF and further by the

addition of chemically treated BF. Microscopic analysis of the PP/BF

composites with polarized light revealed a transcrystallinity of the matrix

around the fibers. The incorporation of BF into PP matrix induced a

decrease of the effective thermal conductivity of the composite. All the

chemical treatments enhanced both thermal conductivity and diffusivity of

the composite considerably to varying degrees. This indicated that the

chemical treatment allowed a better contact between the fiber and the

matrix and reduced considerably the thermal contact resistance.

158 Chapter 4

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