Chapter 9 A COMPARATIVE STUDY OF BANANA FIBRE/PF COMPOSITES ON THE MECHANICAL, THERMAL AND VISCOELASTIC PROPERTIES- FROM MACRO TO NANO SCALES

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1 Chapter 9 A CMPARATIVE STUDY F BANANA FIBRE/PF CMPSITES N THE MECHANICAL, THERMAL AND VISCELASTIC PRPERTIES- FRM MACR T NAN SCALES Abstract The present chapter deals with the extraction of cellulose nanofibrils and microfibrils from banana fibres and their potential use as reinforcement in phenol formaldehyde resin matrix. The effects of modified fibre loading on mechanical, thermal and viscoelastic behaviour of the composites have been investigated. It was observed that the tensile and flexural properties increased with increasing fibre loading upto 40wt% for macro, 20wt% for micro and 10wt% for nano composites above which there was a significant deterioration in the mechanical strength. The improved mechanical properties of nano composites are distinctively better than the other two composites. Nano composites showed better properties at a lower weight percentage than macro composite. The fibre-matrix morphology of the composites was confirmed by scanning electron microscopic analysis of the tensile fractured specimens and by atomic force microscopy. Dynamic mechanical analysis of the composites showed an increase in the storage modulus and glass transition temperature for nano composite compared to micro and macro composites. fibre treatment as well as fibre loading were found to influence the tan δ curves. Storage modulus and T g obtained from tan δ curves have been found to be increased with fibre loading. The ultimate properties of nano composites were improved compared to macro and micro composites due to the net work formed between the nano fibres. The results of this chapter have been communicated to Polymer Composites.

2 330 Chapter Introduction The load bearing constituent of natural fibres has been a lively topic of discussion and the positive impact of this in developing new and inexpensive reinforced polymers is emphatically considered now (1,2). Fibre reinforced thermosetting composites are highly beneficial because the reinforced materials improve the strength and toughness of the plastics (3-5). The cellulose fibres are biodegradable and sustainable and have low density, toxicity and abrasiveness. ver last decades new composite materials using banana fibres have been increasingly developed. Some recent studies also report the possibility of using banana fibres as a source of cellulose nanoparticles like whiskers or microfibrillated cellulose (6). Banana plant waste, as lignocellulosic fiber, was treated with alkaline pulping and steam explosion to produce banana fibers and banana microfibrils. Composite materials were processed from these natural unmodified and maleated lignocellulosicfibers using polyethylene as the polymeric matrix. The thermal and mechanical properties were studied by differential scanning calorimetry (DSC) and tensile tests, respectively. Better compatibility and enhanced mechanical properties were obtained when using banana microfibrils. The chemical composition of fibers, in terms of lignin and cellulose, as well as their degree of crystallinity, were found to have a strong influence on the mechanical properties of the composites (7). Cellulose nanoparticles can be obtained from plant cell walls by mechanical or chemical treatments. The preparation of such nanoparticles is becoming an important topic for researchers working on cellulose. n one hand stable aqueous suspensions of cellulose nanocrystals can be obtained by acid hydrolysis of the biomass [8]. This hydrolysis treatment, generally performed

3 A comparative study of banana fibre/pf composites 331 using oxalic acid and bleached fibres, consists in the disruption of amorphous cellulosic regions surrounding and embedding cellulose microfibrils while leaving the crystalline segments intact. The main drawback of cellulosic nanoparticles is their hydrophilic nature, which inhibits their homogeneous dispersion in non-polar polymer matrices and limits the compatibility between the reinforcing phase and the matrix. Kaushik et al. (9) worked on green nanocomposites based on thermoplastic starch and steam exploded cellulose nanofibrils from wheat straw. The surface chemical modification is a typical and classical approach to transform the polar hydroxyl groups on the surface of cellulosic particles into moieties able to enhance interactions with the matrix. [10]. The aim of this work is to increase the compatibility of cellulose particles with the matrix. Indeed, the present study also aims at evaluating the influence of the nature of the cellulose particles to be reinforced in the PF matrix. Dynamic mechanical and thermal analysis has become a widely used technique for determining the interfacial characteristics of heterogeneous polymeric systems. (11). The dynamic modulus, storage modulus, loss modulus and mechanical damping factor tan δ provides an insight into the level of interactions between the polymer matrix and fibre reinforcement. In general, because of the ultra fine phase dimensions, nanocomposites exhibit new and improved properties in comparison with their microcomposite and macrocomposite counterparts (12-18). The present work highlights the investigation results of the mechanical properties, dynamic mechanical properties of banana fibre (microfibrilated and nanostructured form) reinforced phenol formaldehyde composites. Careful analysis of the literature indicates that no study has been reported on the

4 332 Chapter 9 systematic comparison between these microfibrilated and nanostructured composites. 9.2 Results and discussion Scanning electron microscopy analysis Composites prepared from macro, micro and nano cellulose are compared in Fig. 9.1 to study the compatibility and the surface morphology of the polymer and the fibre. (a) (b) (c) Figure 9.1 SEM image of (a) macrocomposite (b) microcomposite (c) ESEM image of nanocomposite

5 A comparative study of banana fibre/pf composites 333 ESEM is employed for the more extensive morphological inspection of cellulose nano particle reinforced polymers. This allows conclusions about the homogeneity of the composite, presence of voids, dispersion level of the fibres within the continuous matrix, presence of aggregates, sedimentation and possible orientation of the fibres. The SEM comparison (fig.9.1) between macro, micro and nano composites demonstrated that micro and nano displayed more aggregation than macro composite. ESEM topograph of the compression moulded cellulose-pf nanocomposites (Fig.9.1 (c)) shows well embedded nano banana fibrils in the resin offering well defined reinforcement ability in the hydrophilic PF resin, supporting effective compatibility of the cellulose fibrils and the PF resin. The figure shows the zigzag crosslinking of the fibres inside the resin offering maximum strength to the amalgamated complex. In addition to SEM, TEM analysis was carried out to investigate the nano composites. This technique is difficult to perform especially in the case of macro and micro composites because it involves the use of specimens around 50 nm thick prepared by ultramicrotomy. Challenging analysis was carried out for TEM analysis of the developed nanocomposite structure. The major challenge to be faced was lack of contrast observed between the whiskers and the matrices. It can be seen in some areas the staining appeared to be more concentrated allowing structural insight (Fig. 9.2).

6 334 Chapter 9 Figure 9.2 TEM of nanocomposite These areas were observed to have a slightly higher concentration of nanofibres, i.e., some local areas with a tendency of agglomeration. The staining in these areas made a continuous background, giving reasonable contrast between the cellulose and the surroundings. In areas where the whiskers were more evenly dispersed, the contrast between the whiskers and the matrix was low and almost insufficient for imaging. TEM analysis allowed the determination of the whisker length in the matrix. It is found that the size of the whiskers was of the same range as before processing, i.e., compression or injection moulding process does not affect the fibre geometry. TEM analysis is done by cutting sheets of approximately 50nm using ultra microtome. During the trimming process the probability of the fibre break is expected to be increased which pave way for the underestimation of length of the cellulose fibres in the TEM analysis of the composite samples. From the bright field TEM images of Cellulose-PF composite the agglomeration was found to be reduced due to the high compatibility between the fibre and the matrix.

7 A comparative study of banana fibre/pf composites 335 Morphological analyses of cellulose nanofibres reinforced composites were also carried out using AFM. Figure 9.3 gives an obvious picture of surface roughness of the pure PF and nanofibrils incorporated PF composites. Roughness is a factor which influences the adhesion between polymer and fibre. The AFM topography for the pure phenol formaldehyde matrix shows high roughness which proves the characteristic property the high brittleness and cure shrinkage of pure matrix. The nanofibrils inclusion to PF matrix reduces the surface roughness (Fig. 9.3) which proves the achievement of compatibility in the system between hydrophilic vegetable fibre and hydrophilic polymer. PF-nanobanana fibril composite (fibre loading 10 wt%) appears as a compact surface, where both the phases contact with each other to form a continuous peak-valley structure, and many separated peaks with height of about 50 nm connecting together in the fibre matrix. To get an idea of the surface roughness of the fibres, atomic force microscopy in force modulation mode was used. Figure 9.3 3D tapping mode AFM images of nanofibril/pf composite AFM images have been used to visualize the initial surface roughness of the reinforced fibrous composites. Compared with the pure matrix surface, which is rather rough, the fibre reinforced composites show smoother

8 336 Chapter 9 surfaces. AFM images of the composites reinforced with untreated cellulose fibre in force modulation mode (Fig. 9.3) show that the surface roughness of the composite is decreased when the percentage of fibre is increased which is mainly due to the increase in the fibre-matrix interactions which leads to the development of strong interfacial bonds. The RMS roughness of pure PF, 4wt% nanofibril reinforced PF and 10wt% nanofibril reinforced PF were found to be 356, 243 and 15 respectively showing a sharp decrease in the surface roughness of the composites which strengthens the maximum reinforcing ability of the developed nanofibrils with hydrophilic polymeric matrices. A significant increase in surface roughness is observed in the case of microfibrilated composites which are evidenced from Fig The projections of the surface irregularities of the fibre particles on the matrix are displayed in the figure. In the case of 4 and 10wt% microfibril reinforced PF composites the RMS value is found to be 310 and 20 respectively. The higher value confirms the presence of filler agglomerates on the surface. The roughness increases by 28% and 35% respectively for 4wt% and 10wt% microfibril reinforced PF composites respectively. This RMS roughness analysis gives the primary information about the fine scale fluctuations in the effective surface height. The AFM study suggests that in nanofibril composites the fibre matrix interaction increases significantly.

9 A comparative study of banana fibre/pf composites 337 Figure 9.4 3D tapping mode AFM images of microfibril/pf composite Mechanical performance Stress-strain curves of microcellulose reinforced phenolics at different fibre loadings (4-25 wt%) are illustrated in Fig The curves showed a linear increase at low stress level. At lower fibre loading fibres act as flaws and the fibre content is not enough to impart high strength to plastic (19). The tensile stress is found to increase with fibre loading, the value being a maximum for 20wt% loading. The plasticizing effect starts at low strain for 4 wt% to 15wt%. At low fibre loading the matrix is not restrained by enough fibres and highly localised strain occurs in the matrix at low stresses, causing the bond between matrix and fibre to break leaving the matrix diluted by non reinforcing debonded fibres. As the fibre concentration increases the stress is more evenly distributed and the strength of the composite increases. The slope of the curve increased and the plastic deformation occurred at a higher

10 338 Chapter 9 stress level for 20wt% micro composite. At a loading of 20-25wt% the stress-strain curves seem to overlap up to a strain of 3% emphasizing the maximum allowable fibre content. Thereafter the stress level for 20 wt% increases. The fibre matrix interaction is more for 20wt% as evidenced from the curve. There is 157% increase in stress for 20 wt% composite compared to neat PF. Increase in fibre loading to 25 wt% produces no appreciable change at low strain level but decreases above 3% strain. Young s modulus values were obtained from the initial slope of the tensile curves. Table 9.1 describes the tensile properties of microfibril reinforced PF composites at different fibre loading. The Young s modulii of the composites were found to increase. The modulus which denotes the stiffness of the material, reached a maximum value at 20wt% and then decreased. 20wt% microfibre composite increased the modulus by 282% than pure PF. The % elongation at break increased with increasing the fibre content. The % elongation at break of pure PF, which is quite brittle, is significantly increased by 366% by the addition of 20wt% microfibre. An increase of mechanical properties with fibre loading was observed and attributed to cellulose/pf interactions. Mechanical properties of microcomposites were found to increase upto 20 wt% fibres and to decrease with further fibre loading. Agglomeration of fibres upon fibre addition and fibre/fibre interaction were proposed to explain this observation. Flexural properties showed the same trend.

11 A comparative study of banana fibre/pf composites Stress (MPa) Strain(%) 4% 7% 10% 15% 20% 25% 0% Figure 9.5 Stress Strain curves of tensile strength for microcellulose reinforced phenolics at different fibre loading Table 9.1 Tensile properties of microfibril reinforced phenolics at different fibre loadings Composite (wt%) Tensile strength (MPa) Young s Modulus (MPa) Elongation at break (%) Impact strength (MPa) Flexural strength (MPa) Flexural modulus (MPa) PF 7 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 100.3

12 340 Chapter 9 The tensile and flexural strength of composites of banana nano cellulose /PF composites containing 2 to 12wt % fibre are shown in the Fig Variation of Young s modulus and flexural modulus of banana nanocellulose reinforced phenolics at different fibre loadings are given in Fig For these composites, the tensile strength and Young s modulus are found to increase with increase in fibre loading upto a 10wt% and a decrease is observed at 12wt% fibre which is due to the increase in the possibility of agglomeration, which negatively adds to the property of the composite. The tensile strength and Young s modulus are found to have 160 and 576% increase with increasing nanofibre loading to 10wt% when compared to neat resin. It can be seen that an approximately linear relationship between the tensile modulus and the fibre content is obtained over the whole range of fibre loading. The presence of cellulose whiskers in the PF matrix contributes more effectively in enhancing the tensile modulus of the PF than in enhancing the tensile strength. Improvement in modulus is attributed to the better adhesion between the fibre and matrix.

13 A comparative study of banana fibre/pf composites Tensile strength (MPa) Flexural strength (MPa) Fibre loading (wt%) Figure 9.6 Variation of Tensile strength (1) and Flexural Strength (2) of nanocellulose banana fibre reinforced phenolics at different fibre loadings. Young's modulus (MPa) Flexural modulus (MPa) Fibre loading (wt%) 1700 Figure 9.7 Variation of Young s modulus (1) and Flexural Modulus (2) of nanocellulose banana fibre reinforced phenolics at different fibre loadings

14 342 Chapter 9 Table 9.2 explains tensile properties of macro fibre, microfibril, and nanofibril/pf composites at 4wt% fibre loading. Stress strain behaviour obtained from tensile tests for 4% macro fibre/pf composites, microfibril/pf composite and nanofibril/pf composites is illustrated in Fig The behaviour of neat PF is also presented in this figure. The brittle nature of the resin is clear from the tensile stress strain curve in figure. The stress strain curves of all the composites show a linear behaviour at low strain followed by a significant change in slope showing a non linear behaviour up to complete failure of the composite. The second stage of the curve leading to decrease in slope corresponds to the plastic deformation of matrix and to micro crack initiation in matrix. Randomly oriented fibres inhibit crack propagation. Gradual debonding of the fibres from the matrix occurs during plastic deformation. Unstable propagation of the initiated cracks through the matrix occurs and the strength decreases abruptly to an almost zero value. Depending upon the nature of fibres, the mechanical properties differ. The compatibilizing effect of nano particles and increased surface area induce better dispersion within the matrix (20). Nano composites have maximum tensile strength compared to micro composites and macro composites (Fig.9.8). Nano, micro and macro fibre composites having 4wt% fibre loading increase the tensile strength by a factor of 85, 29 and 7% respectively compared to the neat PF. With 12 wt% loading tensile strength was increased by 157, 54 and 46% respectively. At higher fibre loading there is a strong tendency for fibre/fibre interaction which leads to poor wetting of fibres and fibre dispersion. The crack initiation and its propagation will be easier at higher loading and results in a decrease in tensile strength. In the case of micro and nano composites at

15 A comparative study of banana fibre/pf composites 343 higher loadings the dispersion of the fibrils is poor due to agglomeration and entanglements. Thus stress transfer does not occur and crack initiation followed by matrix failure occurs in the composite. Young s modulus of the three different composites displayed the same trend. Figure 9.9 shows the variation of Young s modulus of microcellulose reinforced phenolics at different fibre loadings. The Young s modulus increased upto 20wt% and then decreased. The increase is due to the efficient stress transfer between the fibre and the matrix. But at higher fibre loadings the degree of fibrillation is less. With 10wt% fibre composites the modulus increased by 233% and 130% respectively for nanocomposites and microcomposites compared to macro fibre composites. The stiffness also increases with fibre content (21,22). It is also noteworthy that the elongation at break for the composites increases with fibre content Stress (MPa) PF 2 Macro 3 Micro 4 Nano Strain (%) Figure 9.8 Stress strain curves for 4wt% macro, micro and nano fibre composites and neat PF resin

16 344 Chapter 9 Table 9.2 Tensile properties of cured PF, macro fibre, microfibril, and nanofibril reinforced phenolics at 4wt% fibre loading Composite Tensile strength (MPa) Young s Modulus (MPa) Elongation at break (%) PF 7 ± ± ± 0.4 macro 7.5 ± ± ± 0.19 micro 9 ± ± ± 0.7 nano 13 ± ± ± Young's Modulus (MPa) Weight (%) of fibre Figure 9.9 Variation of Young s Modulus of microcellulose reinforced phenolics at different fibre loadings Figure 9.10 shows the variation of elongation at break of PF and nanocellulose/pf biocomposites with different fibre contents. The percent elongation at break largely increases with increasing the fibre content. The

17 A comparative study of banana fibre/pf composites 345 elongation of neat PF matrix significantly increaed by 261% with the addition of 10wt% cellulose nanofibres. The percentage elongation at break is very low in pure PF. The brittle nature of PF resin decreased even with the addition of trace quantities of micro and nanofibres. At 12wt% loading of nanocellulose the elongation was observed to be about three times that of the resin. Figure 9.8 describes the stress-strain curves for 4wt% microfibrilated and nanofibrilated PF resin composites. The elongation at break of the nano fibril composite is greater than the micro composite. Nano composites have better properties than the micro composites which is evidenced from the Fig Cellulose nanocrystals reinforced polymer composites (Fig. 9.8) displayed outstanding mechanical properties (23,24) Elongation at break (%) Weight (%) of fibre Figure 9.10 Variation of elongation at break of nanocellulose banana fibre reinforced phenolics at different fibre loadings

18 346 Chapter 9 Generally the interaction of cellulose fibre with PF resin is good due to the hydrophilic nature of cellulose and PF resin. This is shown schematically in Fig Hydrophilicity of the cellulose crop up from the free cellulosic hydroxyl groups. These can easily form hydrogen bonds with the methylol and phenolic hydroxyl groups of the resole in the partial curing stage at 50 o C. n curing at 100 o C these groups can undergo cross condensation reaction leading to three-dimensional network between the fibre and matrix. This increases the strength of the chemical interlocking at the hydrophilic centres of the phenol formaldehyde resin. Thus the effective stress transfer efficiently takes place at the interface which leads to the debonding of the fibres only at very high tensions causing rupture of the composite structure. CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 H H H H H H H H H 2C CH 2 CH 2 CH 2 o CH 2H o CH 2H HH 2C H o H HH 2C H o Curing o HH 2C CH 2H o HH 2 C o o CH 2H o HH 2C CH2H o H H H 2C CH 2 H 2C H CH 2 CH 2 H H CH 2 CH 2 CH 2 CH 2 Resole PF Cured Composite Figure 9.11 Schematic representation of interaction of cellulose fibre with PF resin Although many fibres are perpendicular to the plane of the surface, pull-out lengths are extremely short. The cellulose fibres are found to be swollen in

19 A comparative study of banana fibre/pf composites 347 phenolics and formaldehyde. The polymerisation is then carried out, creating a molecular cell wall/pf composite in the outer layers of the cellulose fibres and an extremely strong fibre-matrix interface. This supports that the cellulose nanofibril reinforced PF composite exhibited enhanced strength compared to microfibril-pf composites which can be attained by inducing smaller volume fraction of nanofibrils. When 10wt% nanofibres were used as reinforcement, tensile strength of the composite increased to 18MPa (Fig.9.6) when compared to 7MPa of nonreinforced pure phenol formaldehyde. This implies that in practical applications, the materials made from these nanocomposites can withstand much greater stresses without undergoing irreversible deformation. It is predicted that cellulose nanofibres appreciably reinforce the phenolic matrix. Since phenol formaldehyde is a hydrophilic polymer, there is a strong interface bonding between hydroxyl groups of nanofibres with polymer, resulting in an increase in tensile strength of the composite film. Three to five fold increases in Young s modulus (25) was observed in nanofibres reinforced composite compared to nonreinforced pure polymer (Fig. 9.7). The chemo-mechanical defibrillation at submicron level led to isolation of nanofibres and that induced an increase in Young s modulus. This is probably due to the fact that high pressure chemo mechanical treatments form a network of nanofibres as displayed in ESEM, AFM and TEM.

20 348 Chapter Flexural strength (MPa) Weight % of fibre Figure 9.12 Variation of flexural strength of banana microcellulose reinforced phenolics at different fibre loadings Flexural modulus (MPa) Weight % of fibre Figure 9.13 Variation of flexural modulus of banana microcellulose reinforced phenolics at different fibre loadings

21 A comparative study of banana fibre/pf composites 349 The strength of composite materials depends on the failure behaviour of the fibrematrix interface. In general, strong interfacial adhesion tends to favour high strength. Weaker interfacial adhesion facilitates fibre pull-out and increased energy absorption through this particular mechanism of failure. Strength, toughness and durability all depend on interfacial adhesion in a complex manner. Figures 9.6 and 9.12 show the variation of flexural strength of nanofibrils and microfibril composites. It can be seen that as the fibre loading is increased an increase in the flexural strength is observed. Dissimilarity is observed in the flexural modulus of the composites (Fig. 9.7and 9.13). The flexural modulus of 2-8wt% nanofibrils and microfibrils loaded composites is found to be less than the neat resin (26, 27). 2% and 8% composite samples have shown comparable flexural modulus where as in the case of 10%, it is very high due to better interaction between fibre and matrix. Higher flexural modulus indicates better stress transfer from fibre to resin as a result of which material can withstand higher load. Compared with 10%, 2 and 8% composites have higher amount of brittle phenolic resin in the composite. The insufficient filling of the molten PF matrix resin into the surrounding fibres during composite processing at about this wt% loading is also contributes to this condition. The flexural strength and modulus ( Fig.9.6 and 9.7 ) are found to have 333% and 21% increase with increasing nanofibre loading to 12 wt% when compared with neat resin. In the case of microfibril composite 20wt% shows 164% increase in flexural strength and 82% increase in flexural modulus (Fig and 9.13) when compared with neat resin. Higher fibre loadings show a decrease in properties. Here the dispersion of the fibrils will be poor and the stress transfer does not occur and crack initiation followed by matrix failure occurs in the composite. When compared with nanocomposites the flexural strength is lower and the flexural modulus is higher for microcomposites. Combining the results in Fig. 9.5 to

22 350 Chapter , it is noted that the tensile and flexural strengths and Moduli of PF matrix gradually increase with the incorporation of nano fibres up to 10 wt% and microfibril upto 20wt%. The smaller depression of the strength at higher loading may be attributed to the competitive phenomenon between the fibre reinforcement effect and the micro crack initiation as a result of relatively high loading of cellulose fibres. The tensile and flexural moduli also increase remarkably with increasing the fibre content, showing a greater improvement than that seen in the tensile and flexural strength. It is known that the tensile modulus of a short fibre reinforced polymer composite mainly depends on the modulus of the fibre and the resin matrix, the fibre content and orientation, and fibre length. In specific, nanofibres can be associated with higher degree of orientation in the matrix resin at an increasing fibre loading, leading to an increase of tensile modulus in a composite system. It is stressed that the optimum loading is 10wt% nano and 20wt% micro cellulose fibres for successfully fabricating a cellulose/pf biocomposite using the present processing technique and also for obtaining the highest mechanical properties. The mechanical properties such as tensile and flexural properties strongly depend on several experimental factors like constitutive materials and processing parameters. Among them, the properties of the biofibre reinforcement and matrix, fibre content, fibre length, fibre orientation, and processing method are critically important. In general, biofibres exhibit considerable variation in fibre diameter along the length of individual filaments. Indirect comparisons among the composite materials processed at a corresponding weight fraction of biofibres with a similar type of reinforcement often provide useful information on understanding their performances and potential.

23 A comparative study of banana fibre/pf composites Micro Nano 26 Impact strength (KJ/m 2 ) Weight (%) of fiber Figure 9.14 Variation of Impact Strength of micro and nanocellulose reinforced phenolics at different fibre loadings The impact performance of fibre-reinforced composites depends on many factors including the nature of the constituent, fibre/matrix interface, the construction and geometry of the composite and test conditions. The impact failure of a composite occurs by factors like matrix fracture, fibre/matrix debonding and fibre pull out. From Fig it is seen that for nano composites a gradual increase in the impact strength is observed as the fibre loading is increased from 4wt% to 12wt% (28). The maximum impact strength is observed for 12wt% fibre loading. Figure 9.14 also illustrates the impact strength of microfibril composites. The impact strength increases with fibre loading. But the value is less than that of the corresponding nanofibril composites.

24 352 Chapter 9 (a) (b) (c) 20nm Figure 9.15 SEM of tensile fracture surface of (a) macro fibre/pf composite (b) microfibril /PF composite and (c) ESEM fractograph of cellulose-pf nanocomposite Figures 9.15 (a), (b) and (c) show the tensile fracture surface of (a) macro fibre/pf composite (b) microfibril /PF composite and (c) ESEM fractograph of cellulose-pf nanocomposite. Figure 9.15 (a) shows matrix rich phase and fibres pulled out of the matrix. The applied load is transferred from the matrix to the fibre through the interface. Since the interfacial area is lower, efficient stress transfer does not occur and matrix failure occurs easily. If a crack develops at the interface, it propagates easily as in PF resin composite with no obstructions posed by fibre content since the fibre/matrix adhesion is low. Composite failures

25 A comparative study of banana fibre/pf composites 353 take place with least resistance and rapid easiness. Figure 9.15 (c), nano fibre composite show the minimum fibre pullout from the resin during mechanical testing which proves the strong interfacial bond between the hydrophilic cellulose fibre and the PF resin when compared with 9.15 (b) which shows less pull out than 9.15 (a) Thermal properties The characterisation of the thermal properties of materials is important to determine the temperature range. Thermal decomposition of banana/pf composites were studied in helium environment. Figure 9.16, shows the TGA thermogarm of pure matrix only one main degradation peak is observed at a temperature of 489 C in which 99% of the specimen degardes which is mainly imparted by the degradation of the resin itself. Figure 9.16 TG and DTG curves of PF resin The degradation behaviour of the representative composites is presented in Fig and The micro and nanofibrillated phenolics were found to have a significant effect on the degradation temperatures.

26 354 Chapter Weight (%) macro micro nano Figure 9.17 TG curves of macro fibre, micro and nano cellulose reinforced phenolics DTG (%/min) macro 2.micro 3.nano Temperature ( o C) Figure 9.18 DTG curves of macro fibre, microfibril and nano fibril reinforced phenolics

27 A comparative study of banana fibre/pf composites 355 Table 9.3. The percentage weight loss and Peak temperatures obtained from the DTG curves of PF resin, banana macro, micro and nano fibre/pf composites Sample % wt loss at temperature ( o C) Peak temperature ( o C) Peak1 Peak II PF macro micro nano In banana fibre reinforced phenol formaldehyde composites two degradation peaks are observed (Fig. 9.18). The first one at C is associated with the degradation of cellulose fibre in the banana fibre used as strengthening phase. In the case of fibre present in composites, the part of fibre, which is not well impregnated by resin, can degrade very easily, while the interfacial region fully wetted by the resin degrades at a higher temperature. So the initiation of thermal degradation of phenol formaldehyde and the fibre portions well impregnated by the resin will occur at C. The peak at C is solely associated with the degradation of PF resin matrix. The percentage weight loss at different temperatures and the peak temperatures obtained from DTG curves of PF resin, macro, micro and nano cellulose reinforced phenolics are given in Table 9.3. The inclusion of biofibres decreases the thermal stability of the resin as revealed by the weight loss at two pronounced temperatures. For micro and nano the percentage weight loss is decreased. Both steam explosion and acid treatment increase the thermal stability of the composites. The resin is well penetrated into the fibrillated cellulose, resulting in a strong fibre/matrix interface. This will increase the

28 356 Chapter 9 stability of the composite. The degradation peaks obtained from DTG curves also support the earlier observation. The increase of crystallinity as a result of treatment of the banana fibre improves the interfacial bonding in the composite as understood from the weight loss and peak temperatures Dynamic mechanical analysis The storage modulus (E ) and the dissipation factor (tan δ) curves are presented in Fig to 9.22 for the representative samples of micro and nano composites respectively. It is apparent that the addition of micro and nano fibrils to the composites leads to an increase in the storage modulus consistent with the reinforcing action of the fibrils (Fig and 9.21). The storage modulus increases with the fibre content. For example, 4 wt% microcomposite has a modulus value of 1898 MPa, which is increased to 2358 MPa with the addition of 20 wt% of microfibre at 50 C. In the case of nano composite 4 wt% composite has a modulus 2458 MPa which is increased to 2873 MPa for a 10 wt% nanocomposite at 50 C. Ma et al. (29) observed similar trend of an increase of the storage modulus in rectorite/linear thermoplastic PU nanocomposites. The storage modulus of the microcomposite at 20wt% loading at 50 C is increased by 24% over 4wt%. The enhancement is lower at higher concentration of the fibrils and can be attributed to agglomeration of the fibrils. T g is observed to increase in an approximately linear fashion with the increased addition of micro and nano fibrils. (Fig and 9.22). The results are in agreement with those reported by Maiti and Bhowmick (30) in the fluoroelastomer nanocomposites. For example, T g for the 4 wt% microcomposite is 116 C, whereas for 20wt % loading is 129 C. When the fibre content of the nano composite was increased from 4wt% to 10wt% there was a spurting of T g shift by 21 C. An increase of 13 C T g shift was observed with 20 wt% microfibre

29 A comparative study of banana fibre/pf composites 357 loading. The magnitude of the tan δ peak max also came down with an increase in the concentration of fibre for both micro and nano composites, shown in the same figure. Mishra et al. (31) and Xiong et al. (32) reported similar trend for the linear PU nanocomposites. The tan δ peak for 4 wt% microcomposite was observed to be at 0.118, which was then reduced to for 20 wt% microcomposite. This is mainly attributed to good adhesion between the matrix and the fibre. In nano composites the shifting was more prominent and the tan δ peak max shift was from to This is applicable for 8, 10, and 15 wt% microcomposites and 4,6 and 10 wt% nano composites. It is evident from the Fig 9.20 and 9.22 that the storage modulus and T g values increased and tan δ peak max decreased with fibre content upto 20 wt% for micro composite and 10 wt% for nanocomposite as manifested from the Fig.9.19 to Thereafter the trend was altered due to agglomeration and weak fibre/matrix adhesion. log E'(Pa) 2.80E E E E E E E E E E E E E E E Temperature ( o C) 1. 0% 2. 4% 3. 8% 4. 10% 5. 15% 6. 20% 7. 25% Figure 9.19 Storage modulus vs temperature curves of micro cellulose reinforced phenolics

30 358 Chapter % 2. 4% 3. 8% 4. 10% 5. 15% 6. 20% 7. 25% Temperature ( o C) Figure 9.20 tan δ vs temperature curves of micro cellulose reinforced phenolics log E' (Pa) 3.20E E E E E E E E E E E E E E E E E Temperature ( o C) % 2. 4% 3. 6% 4. 8% 5. 10% 6. 12% Figure 9.21 Storage modulus vs temperature curves of nano cellulose reinforced phenolics

31 A comparative study of banana fibre/pf composites Tan δ Temperature ( o C) 1. 0% 2. 4% 3. 6% 4. 8% 5. 10% 6. 12% Figure 9.22 tan δ vs temperature curves of nano cellulose reinforced phenolics Storage modulus curves for macro,micro and nano fibre/pf composites are presented in Fig Details are furnished in the Table 9.4 and 9.5. Conversion of macro fibres to micro and nano fibrils as reinforcement in PF led to an increase in the storage modulus consistent with the reinforcing action. This increase is more significant with nano composite. This result is also in agreement with tensile tests. Macro composite(40wt%) has a modulus value of 2059 MPa, which is increased to 2371 MPa for micro (20wt%) at 50 o C. A still higher storage modulus (2879 MPa) is obtained for nano composite (10wt%). As the temperature increases all the three composites show a gradual drop in the storage modulus. The storage modulus was increased by 15% for micro and 40% for nano over the macro composites.

32 360 Chapter E E PF 2 macro (40wt%) 3 micro (20wt%) 4 nano (10wt%) 2.00E log E' (Pa) 1.50E E E E Temperature ( o C ) Figure 9.23 Storage modulus vs temperature curves of macro fibre (40wt%), micro(20wt%) and nano(10wt%) cellulose reinforced phenolics Table 9.4 Values of the constant C Composites C Macro (40wt%) Micro (20wt%) Nano (10wt%)

33 A comparative study of banana fibre/pf composites 361 Table 9.5 Variation of storage modulus and normalized storage modulus of banana/pf composites with fibre loading at different temperatures Material Storage modulus (MPa) Normalized storage modulus (MPa) 50 0 C C C 50 0 C C C Cured PF macro micro nano It was observed that the T g values increased from macro to nano scale reinforced composites (Fig. 9.24). The maximum Tg was observed for the nano composites. T g is an important parameter which controls different properties of the composite such as its mechanical behaviour, matrix chain dynamics and swelling behaviour. The enhancement of crystallinity of the nano particle probably results in the improvement of the stiffness of these nano composites (20). The stiffness of the material was due to infinite aggregates of cellulose nano particles. The cellulosic nano particles can connect and form a 3D continuous pathway through the nano composite film. The formation of this cellulose network was supposed to result from strong interaction between the cellulose, like hydrogen bonds (33). A modification of T g values was obtained when we move from macro to nano level composites. During composite formation nano particles have enough surface area to interact and connect to the hydrophilic PF matrix and form a percolating net work which is the basis of their reinforcing effect. The dissipation factor (tan δ ) curve is shown in figure Tg is observed to increase in an approximately linear fashion for macro,

34 362 Chapter 9 micro and nano composites. Tg for macro composite was 125 o C, for micro 129 o C and for nano 137 o C. The observed shift in temperature is 4 o C for macro to micro and 12 o C for macro to nano composite. The tan δ peak max decreased from macro composite to nano composite (Fig.9.24). The tan δ peak max for macro was at and decreased to for micro and for nano composite. This was attributed to good adhesion between fibre and matrix, as a result of which the nano meter sized particles, can restrict the segmental motion near the interface. Figure 9.25 illustrates the loss modulus vs temperature curves of macro, micro and nano fibre/pf composites. The decreasing order of T g values was nano > micro > macro composite. The T g obtained from loss modulus was found to be less than that obtained from tan δ curve for all the three composites Tan delta PF macro 3. micro 4. nano Temperature ( o C ) Figure 9.24 Tan δ vs temperature curves of macro fibre, micro and nano cellulose reinforced phenolics

35 A comparative study of banana fibre/pf composites macro micro nano Loss modulus (MPa) Temperature ( o C) Figure 9.25 Loss modulus vs temperature curves of macro fibre (40wt%), micro(20wt%) and nano(10wt%) cellulose reinforced phenolics 9.3 Conclusion Cellulose nano particles are inherently a low cost, sustainable and eco friendly material which is available from a variety of natural sources and in a wide variety of aspect ratios nm long and 5-15nm in diameter. They are attractive nano materials for multitudes of applications in a diverse range of fields. Micro and nanofibres were separated from banana fibre by steam explosion method. Aspect ratios of macro fibre are cm long and µm in diameter and that of micro are µm long and 10-15µm in diameter. Composites were compounded with macro, micro, nano fibre and PF resin. The effects of fibre loading were investigated. Mechanical properties of micro and nano composites were improved with the incorporation of fibres and the nano composites exhibited mechanical properties greater than micro and macro composites and may lead to improved light weight PF composites for application in automobile parts. Mechanical properties of the nano composites

36 364 Chapter 9 showed significant increase in tensile and modulus values when only small quantities (10% by weight) of nanofibres were added. This could be due to good interfacial adhesion between the fibres and matrix. At low fibre loading, uniform dispersion is possible whereas at high fibre loading chances of agglomerationare there due to hydrogen bonding. The stress increases linearly with strain at low elongation. More than 10 fold increase in strength and modulus is realized by the addition of only 10 wt % of the nanofibres. For 12wt% nanocomposites, the impact strength of the system was 123% higher than that of the pure matrix system. However the tensile strength, Young s modulus and elongation at break of the nano composite exhibits a maximum for 10 wt% composite. The nanofibres improve their compatibility with PF resin matrix. The dispersibility of micro and nano fibrils decreased at higher wt% due to agglomeration. The stiffness and ductility of the micro and nano composites also were found to increase. The reinforcement effect depends largely on matrix ductility. ie, the resistance to crack the propagation. Above 40wt% for macro, 20wt% for micro and 10wt% for nano, the mechanical properties decreased due to fibre fibre contact that occurred when the fibre mat impregnated with resin was pressed in the mould to prepare composite laminates. The crystallinity and elongation at break were lower for micro composites than nano composites due to entanglements of the microfibrils which were more pronounced at higher fibre content. The thermal stability of the composites has been investigated by TGA. The thermal stability is more pronounced for nano composites. All properties like mechanical, thermal, dynamic mechanical and fracture properties are affected by the reinforcing particles. It can be concluded that properties of composites made by nano particle reinforcement are much better than the composites made by micro and macro particles. This is due to the net work formation of nano fibres by hydrogen bonding.

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