Chapter 7 Electrical and Thermophysical Properties of Short Banana/Sisal Hybrid Fibre Reinforced Polyester Composites

Transcription

1 Chapter 7 Electrical and Thermophysical Properties of Short Banana/Sisal Hybrid Fibre Reinforced Polyester Composites Chapter 7 is divided into two parts. The first part; chapter 7.1 deals with the electrical properties of the hybrid composites. The second part; chapter 7.2 consists of the thermophysical properties of the above composites.

2 Chapter 7.1 Electrical Properties of Banana/sisal Hybrid Fibre Reinforced Polyester Composites Abstract The dielectric properties such as dielectric constant and volume resistivity of banana/sisal hybrid fibre reinforced polyester composites were evaluated as a function of fibre loading, relative volume fraction of the two fibres, frequency and chemical modification of fibres. The dielectric constant values of fibre-reinforced system were found to be higher than that of neat polyester due to polarization exerted by the incorporation of lignocellulosic fibres. With the increase in fibre loading there is an increase in the dielectric constant values and a decrease in volume resistivity. Composite having 100% sisal fibre showed the highest volume resistivity, compared to 100% banana fibre and hybrid composites. Chemical modification of the fibres resulted in decrease in dielectric constant and an increase in volume resistivity values. Results in this chapter have been submitted to Journal of Composite Materials

3 254 Chapter Introduction Polymer composite materials are widely used in electrical applications because of their good dielectric or mechanical properties. For a given polymer type, the electrical properties are determined by the amount and type of conductive additives. Such electrically conductive composite materials are widely used in the areas of electrostatic discharge dissipation, electromagnetic interference shielding and various other electronic applications. Nevertheless, the durability of insulating materials is linked to the presence of trapped electric charges. Storage and transport of these charges are related to their trapping and detrapping process. Indeed it was confirmed that the breakdown fracture and wear are the consequence of dielectric relaxation which follows charge detrapping 1-2 Over the last few years, natural fibres have emerged as the main reinforcement for high performance composite materials. 3-7 The main attraction of bio-fibre reinforced composites lie in their low density, high strength and their positive environmental impact. The hollow cellular structure provides excellent insulation against heat and noise. Composite as a dielectric is becoming more popular and studies of electrical properties of natural fibre reinforced composites are therefore very important. The electrical properties of some natural fibres, such as volume resistivity and dielectric strength have been studied by Kulkarni et al. 8 and there have been many studies of electrical properties of composites. 9,10 The electrical properties of sisal fibre reinforced LDPE have been studied by Paul et al. 11 Pinto and Maaroufi 12 investigated the electrical conductivity of tin-filled urea formaldehyde cellulose composites. The electrical conductivity of the composites was observed to be less than orders of magnitude. Soloman et al. 13 reported that the addition of magnets had a

4 Electrical Properties of Banana/Sisal Hybrid Fibre 255 profound influence on the dielectric properties of natural fibre composites containing strontium ferrate. Recently, Fraga et al. 14 conducted the dielectric measurements of composites with glass, jute and washed jute fibres to monitor the water uptake in composites. The results showed that the dielectric constant of the jute fibre composite is higher than that of the composites based on glass fibres. Hong and Wool 15 developed a new low dielectric constant material suited to electronic material applications using hollow keratin fibres and chemically modified soybean oil. The unusual low value of is due to the air present in the hollow microcrystalline keratin fibres and the triglyceride molecules. The authors suggest that the low cost composite made from avian sources and plant oil has the potential to replace the dielectrics in microchips and circuit boards in the ever-growing electronics materials field. In an extension of the above study, the authors have also observed that the coefficient of thermal expansion (CTE) of the developed composite was low enough for electronic applications and similar to the value of silicon materials or polyimides used in printed circuit boards. 16 Xua et al. 17 studied the electrical properties of vapour grown carbon fibre (VGCF) reinforced vinyl ester composites. The electrical properties exhibited percolation behaviour with a sharp drop in resistivity occuring between 2 and 3 weight % VGCF loading. Cabral et al. 18 reported that a change in the dependence of dielectric properties with fibre loading at critical fibre content was observed in short jute fibre reinforced polypropylene composites. The electrical properties of sisal fibre reinforced epoxy composites were studied by Chand and Jain. 19 The authors found that dielectric constant and tan of the epoxy, 0 and 90 0 oriented sisal fibre epoxy composites

5 256 Chapter 7.1 decreased with increasing frequency. The mechanical and electrical anisotropy of pineapple leaf fibres was investigated by Dutta et al. 20 They observed a sharp increase of dielectric constant and fall of loss factor along the fibre direction compared with transverse direction and they established that crystallinity was greater along the fibre direction. Studies involving dielectric properties and biodegradability of protein filled polymer composites were carried out by Tchmutina et al. 21 The composites comprised of a metallocene-based linear low-density polyethylene and a biopolymer, hydrolysed protein, which was isolated from the chrometanned solid leather waste of the tanning industry by enzymatic hydrolysis. The enzyme Aspergillus oryzae was used for the studies. The authors observed that the biodegradation coefficients determined using dielectric measurements were in good agreement with the weight loss of the composites. Electrical conduction in composites can be explained by the three main theories. Conduction path theory 22 The conducting filler forms few continuous chains in the matrix. Electrons move through this continuous net work from one place to another causing electrical conduction. Tunnel effect theory 23 Electrical conduction is assumed to take place not only by interparticle contact but also by the ability of electrons to jump across gaps between conducting elements in the polymer matrix. There is a threshold value of these gaps, which is equivalent to inter particle contact.

6 Electrical Properties of Banana/Sisal Hybrid Fibre 257 Electric field radiation theory 24 It is assumed that an emission current is caused to flow by the high electric field being generated between conducting elements separated by a gap of few nm. The researchers have reported the mechanical aspects of banana and sisal hybrid fibre reinforced polyester composites. However, there has not been a systematic study of the electrical properties of hybrid bio-fibre reinforced polyester composites till date. This chapter deals with the influence of total fibre loading, relative volume fraction of the two fibres, frequency and chemical modification of fibres on the dielectric properties of short intimately mixed banana/sisal hybrid fibre reinforced polyester composites Results and Discussion Dielectric constant The dielectric constant or static permittivity ( ) of a material is defined as the ratio of the capacitance of a condenser containing the material to that of the same condenser under vacuum. The capacitance of a condenser measures the extent of which it is able to store charges. Dielectric constant ( ) can be calculated from the capacitance using the equation: = Ct/ E 0 A (7.1.1) where C capacitance of the material, t- thickness of the sample, E 0 - permittivity or capacitivity of air ( 8.85 x F m -1 ) A - area of sample under electrode.

7 258 Chapter Effect of fibre loading Figure shows the effect of fibre loading on the dielectric constant values as a function of frequency of banana/sisal hybrid fibre reinforced polyester composites. The minimum dielectric constant is exhibited by the gum (neat polyester) compound and maximum by the composite having 0.50 V f of fibre. It can be observed that dielectric constant increases with fibre loading at all frequencies. Dielectric constant Neat polyester 0.20 V f 0.30 V f 0.40 V f 0.50 V f Log frequency (Hz) Figure Variation of dielectric constant with frequency as a function of fibre loading. (banana : sisal = 1:1) The dielectric constant values decrease with increase in frequency for all the composites, which is more prominent at low frequencies. The dielectric constant of the material depends upon the polarizability of the molecules. The polarizability of non-polar molecules arises from electronic polarization (in which the application of applied electric field causes a displacement of the electrons relative to the nucleus) and atomic polarization (in which the application of the applied electric field causes a displacement of the atomic nuclei relative to one another). In the case of polar molecules a third factor

8 Electrical Properties of Banana/Sisal Hybrid Fibre 259 also comes into play, which is orientation polarization (in which the application of applied electric field causes an orientation of dipoles). The addition of lignocellulosic fibres (banana and sisal) in polyester leads to an overwhelming presence of polar groups giving rise to dipole or orientation polarizability. The over all polarizability of a composite is therefore the sum of electronic, atomic and orientation polarization giving rise to higher dielectric constant. Hence the dielectric constant increases with fibre loading at all frequencies. The decrease in dielectric constant with frequency is due to the decrease in orientation polarization at high frequencies. At low frequencies, complete orientation of the molecule is possible while at medium frequencies there is only a little time for orientation. Orientation of the molecules is not possible at all at very high frequencies Effect of varying the relative volume fraction of the two fibres The dielectric properties as a function of frequency of unhybridized and hybrid composites having a volume fraction 0.40V f can be observed in figure The symbol B represents 100 % banana and S, 100 % sisal fibre reinforced composite. Composites having banana: sisal in the ratio 3:1, 1:1 and 1:3 are also presented. It is interesting to note that banana fibre reinforced polyester composite shows the highest dielectric constant values at all frequencies. However, sisal reinforced composite is having the lowest dielectric constant values. It is due to the air present in the high lumen size of sisal fibre compared to that of banana fibre. The lumen size of sisal is 11 m, while that of banana is 5 m (see table 1.4., chapter 1). Similar results were obtained in earlier studies by Hong et al. 15 They reported that the unusual low value of of keratin fibre reinforced composite was due to the air present in the hollow microcrystalline keratin

9 260 Chapter 7.1 fibres and the triglyceride molecules. The graph shows that in all cases, dielectric constant decreases with frequency, which is more predominant at lower frequencies. This is due to the lack of orientation of fibre at higher frequencies as explained above. Dielectric constant B B:S = 3:1 B:S = 1:1 B:S = 1:3 S Log frequency (Hz) Figure Variation of dielectric constant with frequency as a function of varying the volume fraction of the two fibres. (V f = 0.40) Another reason for the low dielectric constant of sisal fibre is the low hydrophilicity of sisal fibre, compared to that of banana fibre. It was already proved by the water sorption studies of sisal, banana and banana/sisal hybrid fibre reinforced polyester composites. 27 The Q value of banana/polyester composite is higher than that of sisal/polyester composite. It was also observed that as the volume fraction of sisal fibre increased in the composite, the hydrophilicity decreased. The decrease in hydrophilicity of the fibres leads to the lowering of orientation polarization and subsequently the dielectric constant.

10 Electrical Properties of Banana/Sisal Hybrid Fibre Effect of chemical modification of fibres Figure delineates the effect of chemical modifications on banana and sisal fibres on dielectric constant values of banana/sisal hybrid fibre reinforced polyester composites. The dielectric constant of chemically modified fibre composites decreased to a considerable extent when compared to the untreated fibre composites. It is due to the decrease of orientation polarization of composites containing treated fibres. 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. A decrease in water uptake was also observed in chemically treated fibre composites. The resultant decrease of hydrophilicity of the fibres leads to the lowering of orientation polarization and subsequently dielectric constant. It is observed from the figure that the dielectric constant of 10% NaOH treated fibre composite is lower than that of 1% NaOH treated fibre composite. Besides the removal of hemicellulose and waxes, the treatment with NaOH solution promotes the activation of hydroxyl groups of cellulose unit by breaking the hydrogen bond. In a previous study, the authors observed enhanced tensile properties in 10% NaOH treated composite compared to 1%. 28 The NaOH treatment degrades and removes practically all noncellulose components. Due to alkali treatment, the cementing materials, lignin gets dissolved and leads to fibrillation. All these factors provide a large surface area and give a better mechanical interlocking between the fibre and matrix and thus reduce water absorption. This results in orientation polarization and consequently dielectric constant of the composites. The structures of various silanes used for modification ie. A1100 ( -amino propyl triethoxy silane, Silane 1 (trimethyl ethoxy silane) and Silane 2.

11 262 Chapter 7.1 (triethoxy octyl silane are depicted in scheme 2.1., chapter 2. The dielectric constant of silane treated fibre composites reduced dramatically when compared to the untreated fibre composites. The potential advantages of using silane-coupling agents are their inherent natural attraction with both the natural fibre and resin matrix. Figure 5.1.2, in chapter.5.1 shows the general representation of the interaction of silane with cellulose. Silanes undergo hydrolysis to form silanols, which further react with the OH group of the cellulose. The organofunctional group of the silanes in turns form interpenetrating polymer networks, with the polyester. The interfacial adhesion will be stronger in the composite, leading to decreased hydrophilicity and the lowering of dielectric constant. Dielectric constant Untreated 1 % NaOH 10 % NaOH 10 % NaOH-60hrs Silane 1 Silane 2 A1100 PSMA Log frequency (Hz) Figure Variation of dielectric constant with frequency as a function of chemical Modification( V f = 0.40, banana: sisal = 1:1) PSMA treated fibre composite also showed the same trend of dielectric properties. The structure of PSMA is also given in scheme 2.1., chapter 2. The hydrophilic fibre OH groups are replaced by the hydrophobic moieties by the hydrogen bonding between the PSMA and OH groups of

12 Electrical Properties of Banana/Sisal Hybrid Fibre 263 the fibre. This increases the strength of the chemical interlocking of the hydrophobic centres of the polyester resin and the fibres. A schematic representation of the interaction between PSMA and fibre can be observed in figure in chapter 5.1. Scanning electron micrographs (SEM) of the tensile fracture surfaces of the chemically modified fibre composites show the improved interfacial adhesion. Figure (a) and (b) in (chapter 5.2) depict the SEM of the tensile fracture topography of untreated and A1100 treated fibre composites. The SEM of the tensile fracture surfaces of silane 1, silane 2, 10% NaOH and PSMA treated fibre composites can be seen in figures 5.2.9, , and (chapter 5.2) respectively. Compared to the fracture surface of the untreated composite, more brittle failure can be observed and matrix particles are found to adhere to the surface of the treated fibre composites. Fibre pealing and fibrillation can also be seen. Surface coating can be seen on the fibre due to the possible reaction between the cellulose fibre and PSMA. All these factors are indications of good fibre/matrix adhesion Volume resistivity The study of the volume resistivity of an insulating material is important because the most desirable property of an insulator is its ability to resist the leakage of electric current. The insulation resistance of a material depends on its volume resistance, thus the volume resistivity ( ) can be calculated by using the equation = RA/t (7.1.2) where R is the volume resistance, A is the area of cross section and t is the thickness of the sample.

13 264 Chapter Effect of fibre loading Figure shows the effect of fibre loading with volume resistivity as a function of frequency. It can be observed that volume resistivity decreases with frequency. The graph shows that by the addition of fibres, a drastic fall in volume resistivity takes place in the composites. A gradual decrease in volume resistivity can be observed when the volume fraction increases from 0.30 to 0.50 V f. This implies that the conductivity increases upon addition of lignocellulosic fibres. This is due to the polar groups, which facilitate the flow of current. In polymers it is well known that the most of the current flows through the crystalline regions and the passage of current in the amorphous regions is due to the presence of moisture. The presence of two lignocellulosic fibres increases the moisture content resulting in the increased conductivity of the system. Volume resistivity x 10 4 (ohm.cm) Neat polyester 0.30 V f 0.40 V f 0.50 V f Log frequency (Hz) Figure Effect of fibre loading on volume resistivity as a function of frequency. (banana: sisal = 1:1)

14 Electrical Properties of Banana/Sisal Hybrid Fibre Effect of varying the relative volume fraction of the two fibres The effect of varying the relative volume fraction of the two fibres with volume resistivity as a function of frequency can be delineated in figure The volume resistivity of 100% sisal fibre reinforced polyester composite is very high compared to banana fibre reinforced polyester composites and hybrid composites. Banana fibre reinforced polyester composite shows the lowest volume resistivity. In the case of dielectric constant, the trend was reverse. The volume resistivity values of the hybrid composites are in between that of banana and sisal fibre composites. The high lumen size as well as the low hydrophilicity of sisal fibre are the main reasons for this behaviour, which is explained in section Volume resistivity x 10 4 (ohm.cm) B : S = 1:1 B S B : S = 1:3 B : S = 3: Log(frequency(Hz) Figure Effect of varying the relative volume fraction of fibres on volume resistivity as a function of frequency (V f = 0.40)

15 266 Chapter Effect of chemical modification Figure shows the variation of volume resistivity with frequency as a function of chemical modification of the fibres. Here also volume resistivity decreases with increasing frequency, which is predominant at lower frequencies. It is obvious from the figure that volume resistivity increases upon chemical treatment of fibres. As described earlier chemical modification of fibres result in lowering of moisture content and increased interfacial adhesion leading to increased resistivity values. Chemically modified sisal/oil palm hybrid fibre reinforced natural rubber composites also showed similar results. 29 The volume resistivity of 1% NaOH treated fibre is comparatively less than that of the other treatments. The reason is the low interfacial adhesion in 1% NaOH treated fibre composite compared to other treatments. 28. A1100 and silane 2 treated fibre composites show very high volume resistivity values. The high tensile properties, low impact strength and low hydrophilicity of these composites indicate an improved fibre/matrix adhesion, which lead to higher volume resistivity values 28. Volume resistivityx10 4 (ohm m) Log frequency (Hz) Untreated 1 % NaOH 10 % NaOH 10 % NaOH -60 hrs Sil.1 Sil.2 A1100 PSMA Figure Effect of chemical modification on volume resistivity as a function of Frequency (banana: sisal 1:1, V f = 0.40)

16 Electrical Properties of Banana/Sisal Hybrid Fibre 267 The resistivity is also dependent on dielectric constant by the equation Log R 10 (298 0 K) = 23-2 (298 0 K) (7.1.3) where R is the resistivity and is the dielectric constant. According to this equation the electric resistance of the composites decreases exponentially with increasing dielectric constants. As seen earlier, chemical modification had resulted in lowering of dielectric constant, which would automatically lead to an increase of resistivity values Conclusions The dielectric properties such as dielectric constant and volume resistivity were evaluated as a function of fibre loading, fibre ratio, frequency and chemical modification. As the fibre loading increased, the dielectric constant also increased. This was attributed to the increase in orientation polarization of the polar groups present in the lignocellulosic fibres. Sisal/polyester composite showed higher volume resistivity and lower dielectric constant compared to banana/polyester and hybrid composites, which is due to the low hydrophilicity of sisal fibre compared to banana fibre. Chemical modification resulted in lowering of dielectric constant due to decrease of orientation polarization. The volume resistivities were seen to decrease with fibre loading and increase with chemical modification, which implies that incorporation of fibres increased the conductivity of composites. The increased volume resistivity upon chemical modification is due to the improved fibre/matrix adhesion and low water uptake of treated fibre composites.

17 268 Chapter 7.1 References 1. Berriche Y, Vallayer J, Trabelsi R, Treheux D. J. Eur. Ceram. Soc., 20, Vallayer J, Pauhle O, Juve D, Vernet J M, Treheux D. Materax et techniques., 7-8, Rouison D, Sain M, Couturier M. Comp. Sci. Tech., 66, 7-8, Arbelaiz A, Fernandez B, Valea A, Mondragon I. Carbohydr. Polym., 64, 2, Martins M. A, Forato L.A, Mattoso L.H.C, Colnago L.A. Carbohydr. Polym., 64, 1, Pickering K.L, Beckermann G.W, Alam S.N, Foreman N.J. Comp. Part A (in press) 7. Antich P, Vazquez A, Mondragon I, Bernal C. Comp. Part A., 37, 1, Kulkarni A. G, Satyanarayana K. G, Rohatgi P. K. J. Mater. Sci., 18, Lin Y. S, Chiu S.S. J. Appl. Polym. Sci., 93, 5, Nashar D.E.E, Abd-El-Messieh S.L, Basta A. H. J. Appl. Polym. Sci., 91, Paul A, Joseph K, Thomas S. Comp. Sci. Tech., 51, Pinto G, Maaroufi A. K. Polym. Comp., 26, 3,

18 Electrical Properties of Banana/Sisal Hybrid Fibre Soloman M. A, Kurien P, Anantharaman M. R, Joy P. A. J. Elast. Plast., 37, Fraga A. N, Frulloni E, delaosa O, Kenny J.M, Vazquez A. Polym. Test., 25, 2, Hong C. K, Wool R. P. J. Nat. Fib., 1 2, 83, Hong C. K, Wool R. P. J. Appl. Polym Sci., 95, 6, Xua J, Donohoeb J. P, Pittman Jr. C. U. Comp. Part A., 35, Cabral H, Cisneros M, Kenny J. M, Vasquez A, Bernal C. R. J. Comp. Mater., 39, 1, Chand N, Jain D. Comp. Part A., 36, Dutta A. K, Samantary B. K, Bhattachatterjee. J. Mater. Sci. Lett., 3, Tchmutina I, Ryvkinaa N, Saha N, Saha P. Polym. Deg. Stab., 86, Calleja B. F. J, Ezquetva T. A, Rueda D. R. J. Mater. Res., 3, Pooley M. H, Boonstra B. B. T. Rubb. Chem. Tech., 30, Van Beek L. K. H. J. Appl. Polym. Sci., 6, 24, Idicula M, Neelakantan N. R, Oommen Z, Joseph K, Thomas S. J. Appl. Polym. Sci., 96, Idicula M, Malhothra S. K, Oommen Z, Joseph K, Thomas S. Comp. Sci. Tech., 65, 7-8,

19 270 Chapter Idicula M, Malhothra S. K, Joseph K, Thomas S. J. Appl. Polym. Sci., 97, 5, Idicula M, Joseph K, Thomas S. J. Adh. Sci.Tech., (communicated). 29. Jacob M, Varghese K.T, Thomas S. J. Mater. Sci. (in press)

20 Chapter 7.2 Thermophysical Properties of Banana/Sisal Hybrid Fibre Reinforced Polyester Composites Abstract This chapter deals with the thermophysical properties of banana/sisal hybrid fibre reinforced polyester composites. Thermal conductivity, diffusivity and specific heat of the composites were investigated as a function of filler concentration and fibre surface treatments. The results show that chemical treatment of the fibres reduces the composite thermal contact resistance. The thermal conductivity measured in the direction transverse to the plane of composite plate could be well represented by a series prediction model. The results in this chapter have been accepted for publication in Composites Science and Technology.

21 272 Chapter Introduction Fibre reinforced polymer composites are considered as replacements for metal materials where the association of metallic fibre with polymeric matrix results in attractive material for electronic packaging applications. The combination of reinforcement with high thermal conductivity embedded in a resin matrix with low thermal conductivity is desirable to dissipating the heat flux for electronic packaging components. Studies on the mechanical properties of short fibre reinforced polymer composites have shown that both fibre length distribution and fibre orientation distribution play very important role in determining the mechanical properties. 1-4 A number of analytical models have been proposed to predict the thermal conductivity of short fibre composites. 5-8 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 fibre composites. Mai and Fu 9 studied the effects of fibre length and fibre orientation angle on the thermal conductivity of short (carbon) fibre reinforced polymer composites. It is observed that the thermal conductivity of the composite increases with mean fibre length but decreases with mean fibre orientation angle with respect to the measured direction. Agarwal et al. 10 analyzed the variation of thermal conductivity and thermal diffusivity of banana-fibre reinforced polyester composites caused by the addition of glass fibre. They observed that the thermal conductivity of composites increased as compared to the matrix. However, the thermal conductivity of the composites with increased percentage of glass fibre decreases when compared to composite of pure banana fibre. Thermal and crystallization studies of short flax fibre reinforced polypropylene matrix composites were studied by Arbelaiz et al. 11 The effect of thermal ageing on long term mechanical behaviour and strength

22 Thermophysical Properties of Banana/Sisal Hybrid Fibre 273 of carbon fibre epoxy composites was studied by Leveque et al. 12 Researchers 13 recently developed a novel composite material consisting of polypropylene (PP) fibres in a random poly (propylene-co-ethylene) (PPE) matrix and its thermal properties were evaluated with reference to the fibre concentration. It was seen that increasing PP fibre concentration in PPE resulted in no significant difference in melting and crystallization temperatures of the PPE. In recent years there is a growing interest in the hybridizing of different natural fibres in order to produce high performance composite materials. Haseena et al. studied the mechanical, electrical and water sorption characteristics of sisal/coir/nr composites. 14 Thomas and co-workers 15 studied the mechanical properties and cure characteristics of sisal and oil palm hybrid fibre reinforced natural rubber composites. In a previous work, the static and dynamic mechanical properties of banana/sisal hybrid fibre reinforced polyester composites were analysed. A synergistic strengthening of fibres was observed in the case of banana/sisal hybrid fibre composites, where tensile and flexural properties showed a positive hybrid effect. 16,17 But, no work has been done for investigating the thermophysical properties of the hybrids of two natural fibres. In this chapter, the thermophysical properties of short intimately mixed banana/sisal hybrid fibre reinforced composites were studied as a function of fibre volume fraction and fibre surface modification. The results will be an indication for the improvement of the process and also for the utilisation of natural fibre composite materials for their low or high thermal properties. The thermal conductivity measurement values are compared to models based on an electrical analogy.

23 274 Chapter Example of measurement The experimental set up for the thermal measurements can be observed in figure Conditioning modules T rear Second metallic plate Sample First metallic plate Tfront Microcomputer with Multifunction Analog/Digital I/O card Power amplifier Thermoelectric cooler Turbomolecular pump Rough pump Figure Thermophysical measurements set up. The measurements of composite samples were achieved for a large frequency range (0.5 mhz < f < 32 mhz). The choice of the excitation frequencies is largely empirical but remains limited by the heating source, the thermal resistance of sample and the temperature variation generated inside the sample. The thermophysical properties of the sample can be

24 Thermophysical Properties of Banana/Sisal Hybrid Fibre 275 considered as constants during the experiment because the total amplitude of the temperature variation is lower than 6 C. Transfer function phase (rad) Calculated transfer function Experimental transfer function Frequency (Hz) Transfer function modulus Figure Theoretical and experimental heat transfer functions (modulus and phase) for the composite sample prepared with 0.60 V f of polyester and 0.40 V f of banana/sisal fibre Figure shows an example of the identification result for a composite sample prepared with 0.60 V f of polyester and 0.40 V f of banana/sisal fibre. The experimental transfer function modulus and phase angle is calculated using thermal conductivity and diffusivity identified values and compared to the experimental ones. A good agreement between theoretical and experimental heat transfer functions is observed. A study of the sensitivity of the heat transfer function to the identified parameters was performed in order to determine the optimal frequency domain of excitation. It is shown in figure that for the low frequency range, the heat transfer function

25 276 Chapter 7.2 modulus is very sensitive to the thermal conductivity of the sample and little sensitive to the thermal diffusivity H modulus sensitivity k.( H / k) a.( H / a) k.( (H)/ k) a.( (H)/ a) H phase sensitivity (rad) Frequency (Hz) 0 Figure Sensitivity to identified parameter of heat transfer function modulus and phase for the composite sample prepared with 0.6 V f of polyester and 0.4 V f of banana/sisal fibre The heat transfer function phase is sensitive to the thermal conductivity at low frequencies and to thermal diffusivity only at high frequencies. It is noted that for the frequency range used during experiments, the variation of the sensitivity curves are not the same but the thermal model remains sensitive to both k and a. Thus, these two thermophysical properties in the frequency range chosen can be estimated correctly Specific heat capacity determination The specific heat capacity (Cp) values of the composite samples were determined using thermal conductivity and diffusivity values and knowing the density :

26 Thermophysical Properties of Banana/Sisal Hybrid Fibre 277 k Cp (7.2.1) a where k is the thermal conductivity, a; the thermal diffusivity and the density of the composite. The density measurements were achieved using the square plates samples used for thermal measurements. A Mettler-Toledo TM AT61 delta range balance was used to measure samples weight. The sample sizes were measured using a caliper square Results and discussion Effect of fibre volume fraction In order to explain the behaviour of the effective thermal conductivity of the composite, we need thermal conductivity values of its constituents i.e. fibre and matrix. According to the literature, the effective thermal conductivity of a composite or a blend depends upon the conductivity of the individual components. 10 Fibre length, fibre aspect ratio, relative modulus of the fibre and matrix, thermal expansion mismatch are all-important variables that control the performance of a composite. 18,19 The thermal conductivity and diffusivity measurements are presented in table with their associated uncertainties for the polyester and the composite samples loaded with banana/sisal fibres. The specific heat measurements are also given in the same table.

27 278 Chapter 7.2 Table Thermal conductivity, thermal diffusivity and specific heat of banana/sisal hybrid fibre composites k (W m -1 K -1 ) a (m 2 s -1 ) 10-7 Cp (J kg -1 K -1 ) Polyester only Polyester V f Polyester V f Polyester V f treated with NaOH Polyester V f treated with PSMA The results show that the addition of banana and sisal fibre in the polyester matrix induces a decrease of the effective thermal conductivity of the composite: i.e. from Wm -1 K -1 for polyester matrix to Wm -1 K -1 and Wm -1 K -1 for 0.20 and 0.40 V f respectively. Nevertheless, the specific heat and thermal diffusivity values given in table have not a significant variation. This is mainly due to high uncertainty bounds obtained for these two thermal parameters Effect of fibre treatment The results of the thermophysical and density measurements of the composites prepared with chemically treated fibres are given in table with their uncertainties. We notice that the variation of Cp and a are not significant, as their uncertainties remain important. Nevertheless, the thermal conductivity of sodium hydroxide treated fibre composite is 43% higher than the untreated fibre composite. The NaOH treatment removes practically all non cellulose components except waxes.

28 Thermophysical Properties of Banana/Sisal Hybrid Fibre 279 By the dissolution of lignin by alkali, some pores are formed on the fibre surface, which improves the contact area between the fibre and the matrix. Figure shows the scanning electron micrographs of the fracture surfaces of untreated (7.2.4.a) and NaOH treated (7.2.4.b) fibre composites. Compared to the fracture surface of the untreated composite, more brittle failure can be observed and matrix particles are found to adhere on the surface of the NaOH treated fibre composite. (a) (b) Figure Scanning electron micrographs of the fracture surfaces of (a) untreated and (b) NaOH treated fibre composites In the case of PSMA treated fibre composite, thermal conductivity increased by 52 % compared to untreated composite. In the case of PSMA coating, maleic anhydride group of PSMA form hydrogen bonding with the hydroxyl group of the fibre and the polystyrene segments present in the PSMA improves the compatibility between the fibre and the polyester matrix. As a result of this, the interfacial interaction between the fibre and matrix increases. A hypothetical model of interface of polyester-psma treated fibre is shown in figure 5.1.4; chapter 5.1. The hydrophilic fibre OH groups are replaced by the hydrophobic moieties by the hydrogen bonding between the PSMA and OH groups of the fibre. This increases the strength of the chemical interlocking of the hydrophobic centres of the polyester resin and

29 280 Chapter 7.2 the fibres. Figure shows a scanning electron micrograph of fracture surface of PSMA treated fibre composite. Fibrillation can be observed in the fracture plane due to strong fibre-matrix adhesion. Surface coating can be seen on the fibre due to the possible reaction between the cellulose fibre and PSMA. Figure Scanning electron micrograph of fracture surface of PSMA treated fibre composite Thermal conductivity and the first order model Many theoretical and empirical thermal models have been developed to predict the effective thermal conductivity of polymer composite materials. 20 The simplest alternatives would be with the materials arranged in either parallel or series with respect to heat flow (see figure 7.2.6), which gives the upper or lower bounds of effective thermal conductivity.

30 Thermophysical Properties of Banana/Sisal Hybrid Fibre 281 Thermal Flux Thermal Flux Fibre Fibre Polymer Polymer Series model (k Inf ) Parallel model (k Sup ) Figure Thermal conductivity first order models of fibre composites. Parallel model k sup k (7.2.2) 1 1 k kn n Series model 1 k k k k inf 1 2 n... (7.2.3) 1 2 n Where k i and i are the thermal conductivity and volume fraction of the i th material respectively.

31 282 Chapter 7.2 Untreated fibre Figure Evaluation of relative thermal conductivity versus relative density for all composite studied. Figure shows the variation of the thermal conductivity k C as function of density C for all composites studied. These two parameters are normalized to the corresponding polyester measured values (k Matrix and Matrix ). There is a correlation between these two physical properties for the untreated composites. So, a simple relationship between these two physical properties can be deduced: k k C Matrix C 1 1 (7.2.4) Matrix The value of the coefficient obtained from a linear regression is = , with a correlation coefficient r = Nevertheless, the fibre treatment with NaOH and PSMA allows a higher thermal conductivity for a given concentration and leads to an increase in the interfacial interaction between fibre and matrix.

32 Thermophysical Properties of Banana/Sisal Hybrid Fibre Conclusions Short randomly oriented intimately mixed banana/sisal hybrid fibre reinforced polyester composites were processed by varying the fibre volume fraction. The thermophysical properties of the above composites were studied. The incorporation of the banana/sisal fibre induces a decrease of the effective thermal conductivity of the composite. This shows that thermal conductivity of polymeric matrix seems to be more important than the banana/sisal fibre. Using NaOH and PSMA chemical treatments of fibre allow a significant increase of thermal conductivity values of banana/sisal fibre composites. This shows that the chemical treatment allows a better contact between the components (fibre/matrix) and reduces considerably the thermal contact resistance. References 1. Foulk J. A, Chao W. Y, Akin D. E, Dodd R. B, Layton P. A. J. Polym. Environ., 14, 1, Li Z, Wang X, Wang L. Comp. Part A., 37, 3, Reis J.M.L. Constr. Build. Mater., 20, 9, Jacob M, Varghese K. T, Thomas S. J. Comp. Mater., 40, Wills J. R. J. Mech. Phys. Solids., 25: Normura S, Chou T.W. J. Comp. Mater., 14, Hatta H, Taya M. J.Appl. Phys., 58: Chen C. H, Wang Y. C. Mech. Mater., 23: Fu S.Y, Mai Y.W. J. Appl. Polym. Sci., 88,

33 284 Chapter Agarwal R, Saxena N. S, Sharma K. B, Thomas S, Pothan L. A. Ind. J. Pure. Appl. Phys. 41, Arbelaiz A, Fernandez B, Ramos J. A, Mondragon I. Thermochimica Acta., 440, 2, Leveque D, Schieffer A, Mavel A, Maire J.F. Comp. Sci. Tech., 65, Houshyar S, Shanks R. A, Hodzic A. J. Appl. Polym. Sci., Haseena A.P, Dasan K.P, Namitha R, Unnikrishnan G, Thomas S. Comp. Interf., 117, Jacob M, Varghese K. T, Thomas S. J. Appl. Polym. Sci., 93, 5, Idicula M, Neelakantan N. R, Thomas S. Proceedings USM JIRCAS Joint International Symposium, Penang Idicula M, Malhothra S. K, Joseph K, Thomas S. Comp. Sci. Tech., 65, Boudenne A, Ibos L, Gehin E, Candau Y. J Phys D: Appl. Phys., 37, Boudenne A, Ibos L, Gehin E, Fois M, Majeste J.C. J. Polym. Sci. Part B : Polym. Phys., 42, Mottram J. T, Taylor R. Thermal Transport Properties, International Encyclopedia of composites (Ed.) S. M. Lee, Vol. 5, Vch Publishers New York, 1991.