Indian Journal of Pure & Applied Physics Vol. 41 , June 2003, pp. 448-452

Thermal conduction and diffusion through glass-banana fiber polyester composites

Rajni Agarwal 1, N . S. Saxena2, K . B. Sharma2

, S. Thomas3 & Laly A Pothan4

1Department of Physics, Government College, Dausa-303303 2Department of physics, University of Rajasthan, Jaipur 302004, India

3School of Chemical Sciences, M .G. University, Kottayam 686560, Kerala, India 4Department of Chemistry, Bishop Moore College, Mavelikara 690 II 0, Kerala, India

Received 3 February 2003; accepted 6 M ay 2003

The analysi s of variation in thermal conductivity and them1 al diffusivity of Banana fiber reinforced po lyester composites caused by the addition .of glass fiber have been presented in thi s paper. The composite shows an increase in therm al conductivity in compari son to matrix . However, the thermal conductivity of the composites with increased percentage of glass fiber, decreases in comparison to composite of pure banana fiber. It is minimum when g lass fiber fraction is II % in the compos ite. The decrease in thermal conducti vity values with increasing percentage of glass fiber from 3% to II % in the composite is due to fiber/ matri x de- bonding, fiber pull out and matrix fracture. Increase in thermal conductivity at 15% of glass fiber can be attr ibuted to a change in the energy dissipation mechanism . Y. Agari mode l is used to evaluate the thermal conductiviti es of the fibers in the hybrid

composi te .

[Keywords: Banana fibre , Glass fibre, Polyester composites , Thermal conduction]

1 Introduction Natural fiber reinforced thermoset composites

have received considerable attention , as they demonstrate exce ll e nt mech anica l properties , dimensional stab ility and remarkable economical advantages. The use of one type of fiber alone is inadequate in tackling all the technical and economic problems confronted with in making fiber reinforced composites. Multi component composite material s comprising two or more families of fibers introduce additional degree of compositional freedom and provide yet another dimension to the potential versatility of fiber reinforced composite material s. Combination of a high performance fiber with a low performance fiber provides versati lit y in the performance of the product. In such composites, cost of the material is reasonably low in comparison to high performance synthetic fiber, due to low cost of the natural fibers used. Investigations on ligno-

cellulosic fiber composites have shown that the properties of the fiber can be better uti lized in hybrid composites 1•6•

Composites having two or more fillers contained in the same matrix results to the hybrid composites7·

8 In hybrid composite formation each fiber has been selected generally to off set the poor qualities of the other fiber. For example, tough-brittle, high- low modulus , high-low strength combinations. The properties of such mixtures can adequately be preqicted by the ru le of mixtures. Although in some composites, a synergistic stre ng thenin g o r weakening occurs. This is known as hybrid effect. The major reinforcing fillers used with thermosetting resins to produce hybrid composites are E-type glass fibers. The reinforcement of the glass in polyester matrix produces composites with impact strength comparable to reinforced thermoplastics'~. In this paper, we have taken the glass and banana fiber

AGARWAL et a/.: GLASS BANANA FIBRE POLYESTER COMPOSITES 449

Tab le 1- Chemical and mechanical characteristics of banana fiber

Cellulose Hemi-ce llulose Lignin Moisture Initial modulus Tensi le strength Density

63-64 % 19 % 5 % 10- 11 % 20-51 MPa 520-750 MPa I .35 glee

Table 2- Characteristics of isophthalic polyester

Viscosity Specific Gravity at 25°C Tensile Strength Flexural strength Water absorption at 25°C (28 days) Gel time

650 cps 1.11 9000 psi 16,000 psi 0 .65 %

25 minute

re inforced hybrid polyester compos ites for the thermal study. Glass fiber has been reinforced in the polyester matri x to increase the toughness.

2 Materials

Banana fiber was obtained from Sheeba Fiber and Handic raft, Poovancode, Tamil Nadu . The chemi cal and mechanical characteristics of the ban ana fiber are g ive n in Tabl e I . Unsaturated isophthalic pol yester HSR 8131 obtained from

MIS Bakelite Hylam, Hyderabad, India, was used as matrix . The important characteri stics of the polyester resin are given in Table 2.

Multidirectional g lass strand mat used for the study was supplied by Ceat Ltd., Hyderabad, India. Methyl ethyl ketone peroxide and cobalt naphthenate were of commercial grade supplied by Sharon Enterprise, Cochin.

3 Preparation of Composites

Randomly oriented g lass mats and neatly separated banana fiber cut at a uniform length of 30 mm were evenly arranged in a mould measuring

Table 3- Experimental results of thermal conducti vity and thermal diffusivity of the compos ites .

Comp GF BF poly A. ,W/mK ?

x,mm-/s Matrix 0 0 100 0 .1 8 0. 155 U-40 0 40 60 0. 195 0.20

A 3 37 60 0. 1864 0.1935 B 7 33 60 0. 1839 0. 1867 c II 29 60 0.1822 0.1209 D 15 25 60 0. 188 1 0. 1635

where, Comp=composite, GF=Giass fiber, BF=B anana fiber.

poly=polyester, e= thermal conducti vity, 7= thennal diffusivity

!SOx 150x3 mm. Composite sheets were prepared by impregnating the fiber with the polyes ter resin to which 0.9 volume percent cobalt naphthenate and I % methyl ethyl ketone peroxide were added. The resin was degassed before pouring and air bubbles were removed carefully with a roller. The closed mould was kept under pressure for 12 hours, samples were post cured and test specimens of the requi red size were cut out from sheets. Different volume fractions of glass were used for the preparation of samples as detailed in Table 3. In a ll these samples, glass was used as the core materi a l.

4 Thermal Conductivity and Thermal Diffusivity of Hybrid Polyester Composites

Thermal conductivity of glass and banana fiber reinforced hybrid polyester composites have been measured using TPS technique at room temperature and normal pressure. Experimental results have been shown in Table 3.

The experimental values of therma l conductivity of the hybrid composites are plotted as a function of volume percentage of the fibers in Fig. I. Effective thermal conductivity first increases (from the thermal conductivity value of the matrix) with the volume percentage of the filler the n the re is a decreasing trend up to V = 0. 1 L and again increases

g

and becomes maximum at V = 0.15 . g

In order to explain the behav iour of the effective thermal conductivity of the composite , we need

450 INDIAN J PURE & APPL PHYS, VOL 41, JUNE 2003

0 .196 •

c .J 8 0.184 / - . <0 E a:; 0 .18 • -<>-- Glass

~ 0. 176 tl--~-~-~~==;;:==::;-' 0 10 20 30 40 50

Volume %of fiber

Fia !-Effecti ve them1al conducti vity of the po lyester hybrid o·

compos ites with vo lu me percentage of the fibers

thermal conducti vity values of its constituents i.e. g lass fiber, banana fiber and matri x. According to the literature 10

, the effecti ve thermal conducti vity of a compo s ite o r a bl e nd de pe nd s upo n th e conducti vity of the indi vidual components . Fiber length , fiber aspect ratio, re lati ve modulus of the fiber and matri x, thermal expansion mi smatch are all important vari ables that contro l the performance of a compos ite ' '-'2.

The trend of thermal diffusivity with fiber percentage (Fig. 2) is also the same as that of the thermal conducti vity.

5 Theoretical considerations

The thermal conducti vity of glass and banana fiber has been evaluated using Y. Agari model 13

.

COnductivity of glass (?g) and of banana fibers (?b) comes out to be 0. 374 W /mK and 0 .235 W /mK respecti ve ly. Thermal conductivity of composites in parall e l and seri es conducti on can be estimated respective ly by the fo ll owing equations :

Parallel conduction

? = V? + ( I - V ) ? . . 2 . I ( 1)

Series conduction

.. . (2)

u

"' ~.!!!.

E E

0.25

0.2

-~· 0.15

.::: ., ~ 0.1

0

e o.os Q; .r:. 1-

0 10 20 30 40 50

Volume %of fiber

Fig. 2- Effecti ve thermal diffus ivity of the polyester hybrid compos ites wi th volume percentage of the fibers .

Where A= thermal conductivity of the composite, \ = thermal conductivity of polymer, A

2 = thermal

conductivity of fibers, and V = vo lume content of fibers.

The effect of non-unifo rm di spers ion of fillers in continuous phase of matrix, effect of fill ers on crystallinity and crystal size of the polymer results in c ha ng in g th e the rm a l co nd uc ti v ity of the composite. Considering these effects, the modified equation 13 can be written as

log[-1

] = V.Cr log[_!::_] Cp.AI Cp.AI

.. . (3)

Further, Eq. (4) can be rearranged to

log? = A.V +B

and B =lou (C .A ) ... (4) 1:> p I

Eq. (4) means that logarithms of the thermal conductiviti es of composites are linearl y re lated to the volume contents of fillers .

Results and Discussion Resul ts of Y.Agari mode l are shown in Table

4. Figs 3 and 4 show the curve of log A versus volu me percentage of g lass and banana f ibers respectively.

AGARWAL et al.: GLASS BANANA FIBRE POLYESTER COMPOSITES 451

::G E

-0.6

~ _-0 .8

.< e.o ,g

0 4 8 12 16

Volume % of Glass Fibe r

Fig. 3- log A versus volume % of glass fiberf

-0.6

0

- 1 +-----.-----~-----.-----.-----.

20 25 30 35 40 45

Volrnne % of Banana fiber

Figure 4-- log A vs volume % of Banana fiber.

From the results, it can be concluded that g lass fibers contribute more to the thermal conductivity of the composite compared to banana fibers . Hence , incorporation of glass fiber along with banana fiber in the composite should increase the thermal conductivity of the composite. But experimental results show different trend (Table 3). Thermal conductivity of hybrid composites is less th an the pure untreated banana fiber composite, while tota l fiber loading is same in both type of composites . Reason for thi s type of behaviour may be given as fo llows:

It has been found that the surface of the banana fiber exhibits the presence of the waxy layer 14 which may reduce the adhesion of the fibers with the matrix res in . But the surface of synthetic fibers is smooth , which g ives less opportunity to the matrix resin to adhere to the fiber firmly. There are reports' 5· 16 that the fiber/matrix adhesion plays an important role in

Table 4- Parameters obtained from the Y Agari model

Thermal Conductivity (W/mK)

\ =0.374

~ = 0.235

Cr 3.447

0.826

CP 0.992

0.988

the overall performance of the composite. Due to the above reasons and the higher values of thermal conductivity of the fibers in comparison to that of the resin, any combination of the fibers will increase the thermal conductivity of the composite. But the thermal conductivity of the hybrids is less than the thermal conductivity of pure banana fiber composite. Keeping the above facts in mind, the behaviour of thermal conductivity of the composite can be explained in the light of thermal conductivity of the individual components. Initi al increase in the thermal conductivity of the composite from that of the matrix is due to the partial replacement of the matrix resin by the more conductive fibers. In the beginning, the relative percentage of the banana fiber is higher than the percentage of the g lass fiber in the composite.

As volume percentage of g lass increases in the composite, its thermal conductivity decreases. It may depend on many factors including the nature of the constituents, fiber/matrix interface, the construction and geometry of the composite". The decrease in the thermal conductivity values with increasing percentage of glass fiber from 3% to II % in the composite is due to fiber/matrix de-bonding, fiber pullout and matrix fracture. Increase in the thermal conductivity at 15% glass fiber can be attributed to a change in the energy dissipation mechanism. At high g lass contents , the fracture mechani sm is mainly fiber fracture , due to the brittle nature of g lass. However, at lower glass volume fractions , the fracture mechanism is mainly ·fiber pullout due to the presence of a higher volume fraction of banana fiber. A synergistic effect of the two fibers leads to initial increase in the thermal conductivity.

452 INDIAN J PURE & APPL PHYS, VOL 41, JUNE 2003

Fig. 5 - Scanning Electron Mi crographs (SEM) o f the compos ites havi ng g lass vo lume fracti on (a) 0.03, (b) 0. 11 and (c) 0. 15

Scanning e lectron micrographs (SEM) of the composites, the glass volume fraction of 0 .03, 0.11 and 0.15 are shown in Fig. 5 (a, band c) respectively. At a high glass volume fraction , the fracture occurs in the composite mainly by interlayer de-lamination. At relatively lower volume fraction s of glass fiber, fiber pullout is believed to be the important energy dissipation mechanism in long fiber reinforced composites. It occurs in short fibers composites as well 17

• This may be the reason for decrease in thermal conductivity up to 11 % of glass fiber.

(a)

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